1.1 Significance of the Study
The Pasig River runs through Metro Manila in the Philippines. It is approximately 32 Km long and serves as the drainage outlet for most of Metro Manila. It is fed by several tributaries like the San Juan and Marikina Rivers, and the outlet for the 700,000-acre Laguna Lake. In the 1980’s and 1990’s, it is one of the most polluted rivers in the Philippines. In 1998, the population of Metro Manila increased to 11 million, and there were around 12,000 industries using the Pasig River and 315 of these were considered as major water polluters (Bautista, 1998).
Presently, the banks of Pasig River are lined with industries, commercial establishments, institutions, residential areas, and open spaces. Most firms have been intensively using the river as their dumping ground of solid, liquid, and toxic waste. Only few industries practice waste treatment process before discharging waste into the river. Effluent standards do not establish a limit on the total load released in a given time period. This allows some establishments to dilute their waste to meet effluent concentration standards and encourage excessive use of water.
Presently, the river has fallen below a “Class D” category . According to DENR studies, during the dry season many portions of the river are considered biologically dead.
Some pollutants, particularly oxygen-demanding organic wastes and nutrients, are so common and have a profound impact on almost all types of rivers that they deserve special emphasis. This is not to say that they are always the most significant pollutants in any river, but rather that no other pollutant category has as much overall effect on rivers (Davis, 1998).
Activated carbon (AC) can be prepared from a large variety of carbon-containing feedstocks by the activation of pyrolyzed char. The most common feedstocks for the commercial production of activated carbons are anthracite and bituminous coal, lignite, peat, and the lignocellulosic materials wood and coconut shells (Polard et al., 1992). Plentiful agricultural by-products such as sugarcane bagasse, rice straw, soybean hulls, rice hulls, and nutshells are lignocellulosic wastes that may offer an inexpensive and renewable additional source of activated carbons. Such carbons may have the potential to replace existing carbons, especially coal-based carbons used in many industrial applications including the removal of color and odor compounds in aqueous systems and removal of colorants from raw sugar (Ahmedna, et al., 2000a).
Activated Carbon treatment of wastewater is usually thought of as a polishing process for water that has already received normal biological treatment. The carbon in this case is used to remove a portion of the remaining dissolved organic matter.
Complete treatment with activated carbon is also being studied as a possible substitute for biological treatment of municipal wastewater when site limitations or industrial waste components pose problems for biological processes (Tchobanoglous, 1991).
Philippine coconut production reported from 1990-1996 ranges from 11M- 12M metric tons per year, around 1-2 M of which are shells that are made into charcoal and AC (as cited in Arquero, 2002).
This study evaluated the locally made coconut shell activated carbon, which we also export today. Their export potential is huge. The results help us in determining if its quality is competitive compared to the AC made abroad.
Plans of using the Laguna Lake as the source of potable water for Metro Manila are ongoing. The need to clean our water resources and the abundant supply of coconut shells justified the conduction of this study.
1.2 Objectives
The general objective was to lower the COD of the samples taken from the Pasig River using local coconut shell activated carbon.
The specific objectives of the study were:
1. to characterize the Pasig River water samples in terms of pH, turbidity, color, chloride content, and COD;
2. to characterize the AC (iodine no., moisture content, and bulk density);
3. to determine the equilibrium contact time;
4. to determine the pH necessary for maximum adsorption;
5. to determine the rate of adsorption of solute onto AC;
6. to determine the minimum amount of AC that will have satisfactory COD reduction.
1.3 Date and Place of the Study
The study was done from June 2003 to October 2003. Characterization and pretreatment of AC were done at the Dept. of Chemical Engineering, CEAT, UP Los Banos. The characterization of the Pasig River water sample and the batch adsorption experiment were conducted at the Regional Standards and Testing Laboratory of DOST Regional Office No. IV located in Los Banos, Laguna.
1.4 Limitations of the Study
The effect of temperature on the COD reduction of the sample by activated carbon was not determined, as the study was done using ambient temperature only. No effort was made to make the Pasig River water samples as the representative samples of that part of river as they were taken only from the surface and near the banks
PAC
powdered activated carbon
Wednesday, April 8, 2009
Tuesday, April 7, 2009
REVIEW OF LITERATURE
2.1 Chemical Oxygen Demand (COD)
Anything that can be oxidized in the receiving water with the consumption of dissolved molecular oxygen is termed oxygen-demanding material. This material is usually biodegradable organic matter but also includes certain inorganic compounds. Oxygen in water is consumed by the oxidation of organic matter, {CHO}:
{CH2O} + O2 microorganism CO2 + H2O
Unless the water is reaerated efficiently, as by turbulent flow in a shallow stream, it rapidly becomes depleted in oxygen and will not support higher forms of aquatic life.
In addition to microorganism-mediated oxidation of organic matter, oxygen in water may be consumed by the bioxigenation of nitrogenous material,
NH4+ + 2O2 microorganism 2H+ + NO3- + H2O
and by the chemical or biochemical oxidation of chemical reducing agent:
4F2+ + O2 + 10H2O 4Fe(OH)3 (s) + 8H+
All these processes contribute to the deoxygenation of water.
The degree of oxygen consumption by microbially mediated oxidation of contaminants in water is called the biological oxygen demand (or biochemical oxygen demand), BOD. Although BOD is a reasonably realistic measure of water quality insofar as oxygen is concerned, the test for determining it is time-consuming and cumbersome to perform. Total Organic Carbon (TOC) and COD have become popular alternatives for BOD since these are easily performed instrumentally. (Manahan, 1994)
DENR studies in 1990-1993 showed that the total daily BOD load discharged into the Pasig River was 327 metric tons/day. In 1997, it has dropped to 230 metric tons/day.
Generally, the minimum dissolved oxygen (DO) level needed to support a diverse population of fish is 5 mg/L. However, studies in July 1996 showed that the DO (mg/L) of the Pasig River near the Vargas Bridge and P. Sanchez Bridge are 1.8 and 0.8 respectively. Also, in that period, BOD for the Vargas sample is 41 mg/L and 38mg/L for the Sanchez sample. The Vargas Bridge is in western Makati while the Sanchez Bridge is along the Shaw Boulevard that traverses the San Juan River in Mandaluyong. (See Appendix A for other data)
The chemical oxygen demand (COD) is used as measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant.
The dichromate reflux method is preferred over procedures using other oxidants because of superior oxidizing ability, applicability to a wide variety of samples, and ease of manipulation. Oxidation of most organic compounds is 95 to 100% of the theoretical value. Pyridine and related compounds resists oxidation and volatile organic compounds are oxidized only to the extent that they remain in contact with the oxidant. Ammonia, present either in the waste or liberated from nitrogen–containing organic matter, is not oxidized in the absence of significant concentration of free chloride ions (APHA, 1995).
2.2 Adsorption
Adsorption involves the accumulation of solute molecules at an interface (Perry, 1997). The adsorbed solutes are referred to as adsorbate, whereas the solid material is the adsorbent.
Separation occurs because differences in molecular weight, shape, or polarity cause some molecules to be held more strongly on the surface than others or because the pores are too small to admit the larger molecules. In many cases, the adsorbing components (or adsorbates) are held strongly enough to permit complete removal of that component from the fluid with very little adsorption of other components.
Depending upon the type of forces between the fluid molecules and the molecules of the solid, adsorption is classified as physical adsorption (Van der Waals adsorption) or chemisorption (activated adsorption).
Physical adsorption, which may be a monomolecular (unimolecular) layer, or may be two, three or more layers thick (multimolecular) occurs rapidly. If unimolecular, it is reversible; if multimolecular, such that capillary pores are filled, hysteresis may occur. The density of the adsorbate is of the order of magnitude of the liquid rather than the vapor state. As physical adsorption takes place, it begins as a monolayer, becomes multilayered, and then, if the pores are close to the size of the molecules, capillary condensation occurs, and the pores fill with adsorbate. Accordingly, the maximum capacity of a porous adsorbent can be more related to the pore volume than to the surface area.
In contrast, chemisorption involves the formation of chemical bonds between the adsorbent and adsorbate in a monolayer, often with a release of heat much larger than the heat of vaporization. Commercial adsorbents rely on physical adsorption; catalysis relies on chemisorption.
When porous particles of adsorbent are immersed in a liquid mixture, the pores, if sufficiently larger in diameter than the molecules in the liquid, fill with liquid. At equilibrium, because of differences in the extent of physical adsorption among the different molecules of the liquid mixture, the composition of the liquid in the pores differs from that of the bulk liquid surrounding the adsorbent particles. The observed exothermic heat effect is referred to as the heat of wetting, which is much smaller than the heat of adsorption from the gas phase. As with gases, the extent of equilibrium adsorption of a given solute increases with concentration and decreases with temperature. Chemisorption can also occur with liquids.
For the adsorption of a solute onto a porous surface of an adsorbent, the following steps are required:
1. External (interphase) mass transfer of the solute from the bulk fluid by convection, through a thin film or boundary layer, to the outer solid surface of the adsorbent.
2. Internal (intraphase) mass transfer of the solute by pore diffusion from the outer surface of the adsorbent to the inner surface of the internal porous structure.
3. Surface diffusion along the porous surface.
4. Adsorption of the solute onto the porous surface.
For chemisorption, which involves bond formation, the rate of the fourth kinetic step may be slow and even controlling; for physical adsorption, however, step 4 is almost instantaneous because it depends only on the collision frequency and orientation of the molecules with the porous surface.
During regeneration of the adsorbent, the reverse of the fourth step occurs, where the rate of physical desorption is instantaneous. Adsorption and desorption are accompanied by heat transfer because of the exothermic heat of adsorption and the endothermic heat of desorption (Seader and Henly, 1998).
2.2.1 Slurry Adsorption ( Contact Filtration)
Three modes of adsorption from a liquid in an agitated vessel are of interest. These are the batch mode, continuous mode, and the semibatch or semicontinuous mode. In the batch mode, a batch of liquid is contacted with a batch of adsorbent for a period of time, followed by discharge of the slurry from the vessel, and a filtration step to separate the solids from the liquid.
In the batch mode, the rate of adsorption of solute, as controlled by external mass transfer, is
where c is the concentration of solute in the bulk liquid, ce is the concentration in equilibrium with the loading on the adsorbent, q; kL is the external liquid-phase mass transfer coefficient ; and a is the external surface area of the adsorbent pler unit volume of liquid (Seader and Henly, 1998).
2.2.2 Adsorption Isotherms
The adsorption isotherm is the equilibrium relationship between the concentration in the fluid phase and the concentration in the adsorbent particles at a given temperature. For liquids, the concentration is often expressed in mass units, such as part per million. The concentration of the adsorbate on the solid is given as mass adsorbed per unit mass of original adsorbent.
For the design and operation of all sorption systems, the adsorption isotherm is of great importance because it relates, at equilibrium, the concentration of the solute in the fluid to its loading as a sorbate in and/or the sorbent. Most commonly, the overall rate of adsorption is expressed in the form of a linear driving force (LDF) model, where the driving force is the difference between the bulk concentration and the concentration in equilibrium with the loading. The coefficient in the LDF equation is a combined overall mass transfer coefficient and area for sorption. (Seader and Henly, 1998)
2.2.3 Theory on Isotherms
The adsorbability of a contaminant is determined by its physical and chemical properties. The adsorption capacity of an adsorbate on carbon is related to the adsorbate’s concentration in solution and to other competing solutes. The objective of the isotherm test is to determine the relationship between the concentration of the adsorbate and the capacity of the carbon. Equations that are often used to describe the experimental isotherm data were developed by Freundlich, Langmuir and by Brunauer, Emmet, and Teller (BET isotherm). Other isotherms were developed by Redlich-Peterson (1959), Radke-Prrausnitz (1972), Jossens (1978), and Weber- Vliet (1980).
The equilibrium between the concentration of a solute in the fluid phase and its concentration on the solid resembles somewhat the equilibrium solubility of a gas in a liquid. Data are plotted as adsorption isotherms as shown in Fig.1. The concentration in the solid phase is expressed as q, kg adsorbate (solute)/ kg adsorbent (solid), and in the fluid phase (gas or liquid) as c, kg adsorbate/m3 fluid.
q, kg adsorbate/
kg adsorbent
c, kg adsorbate /m3 fluid
Fig. 2.1 Common types of adsorption isotherm
The linear isotherm is uncommon, but in the dilute region it can be used to approximate data of many systems.
The empirical Freundlich isotherm equation approximates data for many physical adsorption systems and is particularly useful for liquids.
Where K and n are constants and must be determined experimentally. If a log-log plot of q versus c is made, the slope is the dimensionless exponent n. The dimensions of K depend on the value of n. This equation is sometimes used to correlate data for hydrocarbon gases on activated carbon.
The Langmuir isotherm has a theoretical basis and is given by the following, where qo (kg adsorbate/kg solid), and K (kg/m3) are empirical constants.
This equation was derived assuming that there are only a fixed number of active sites available for adsorption, only a monolayer is formed, and the adsorption is reversible and reaches equilibrium condition. By plotting 1/q versus 1/c, the slope is K/qo and the intercept is 1/qo.
Almost all adsorption systems show that as temperature is increased the amount adsorbed by the adsorbent decreases strongly. Only the temperature scale shifts from system to system. This is useful since adsorption is normally at room temperatures and desorption can be attained by raising the temperature (Geankoplis, 1993).
2.3 Activated Carbon
Typical commercial adsorbents, which may be granules, spheres, cylindrical pellets, flakes, or powders, have size ranging from 50 μm to 1.2 cm. They have specific surface areas of 300 to 1200 m2/g. Such a large area is made possible by a particle porosity from 30 to 85 vol% with average pore diameters from 10 to 200 Å (Seader and Henly, 1998).
Activated Carbon is a microcrystalline material made by thermal decomposition of wood, vegetable shells, coal, etc., and has surface area of 300 to 1200 m2/g with average pore diameter of 10 to 60 Å (Geankoplis, 1993).
The characteristics of AC depend in the parent or precursor material, activation method, and activation process. Some precursors allow production and activation of the granules in existing form. Others, which are softer, require that the activation take place in a powder form and are then pressed or extruded into pellets.
Higher activation levels (increased surface areas) generally result in a softer, more powdery carbon unless compacted into pellets.
The most common precursors are wood (35%), coal-bituminous or anthracite (28%), lignite (14%), coconut shell (10%), peat (10%) and 3% other. The other group may include other fruit and nut stones (olive, macadamia nut, almond), coal liquids, petroleum pitches, or even blood and bone chars.
Activation, changing a low-porosity solid into a high porosity solid, can be accomplished either thermally or chemically. The thermal and most common route involves a two-step process of carbonization (removal of most non-carbon atoms) followed by activation through partial gasification (reaction at high temperature with an oxygen donor such as carbon dioxide or steam). This tends to open up the internal porosity of the material while retain the blueprint pore structure of the original material.
The second route to activation involves the reaction of the parent material with a chemical agent, which allows lower temperature activation. The two most common chemical agents used presently are zinc chloride and phosphoric acid (Andrews, 2003).
Banzai (as cited in Ahmedna, et al., 2000b) states that the adsorption capacity of AC is determined by its total surface area, by its internal porous structure, and the presence of functional groups on the pore surface. Carbon surfaces have a pore size that determines its adsorption capacity, a chemical structure that influences its interaction with polar and nonpolar adsorbates, and active sites that determine the type of chemical reactions with other molecules (Ahmedna, et al., 2000b).
2.4 Wastewater Treatment by Activated Carbon Adsorption
Even after filtration, coagulation, sedimentation, and secondary treatment, soluble organic materials that are resistant to biological breakdown will persist in the effluent. The persistent materials are often referred to as refractory organics. Refractory organics can be detected in the effluent as soluble COD. Secondary effluent values are often 30 to 60 mg/L (Davis, 1998).
To date, carbon adsorption is used for the removal of the refractory organic compounds in wastewater. Both granular and powdered carbons appear to have a low adsorption affinity for low-molecular weight polar organic species. If biological activity is low in the carbon contactor or in other biological unit processes, these species are difficult to remove with activated carbon. Under normal conditions, after treatment with carbon the effluent BOD ranges from 2 to 7 mg/L. Under optimum conditions, it appears that the effluent COD can be reduced to about 10 mg/L (Tchobanoglous, 1991).
AC adsorption, when used with other treatment methods is very effective in the removals of organics in wastewater. Ozonization or oxygenation of secondary effluents before treatment by granular AC extended the life of adsorbent from 70 to 480 days and was effective in removing C, COD, BOD, and N compounds. The effluent from the treatment met wastewater quality standard (Netzer, 1985).
Treatment of oil refinery, pulping, dyeing, and fungicide manufacturing wastewaters by ozonization and biological AC adsorption showed that the total organic carbon was removed 10, 20, 30, and 15 % respectively, from those wastewaters. The primary mechanism for pollutant removal in the wastewaters was by adsorption, biodegradation, oxidation, and oxidation- biodegradation, respectively (Schwarrz, 1981).
The effect of magnetic charcoal activated carbon on water-soluble organic dyes was also investigated. The adsorbent was prepared by entrapment of fine charcoal into the structure of magnetic ion oxides. For the adsorption experiments, water–soluble organic dyes belonging to the triphenylmethane, heteropolycyclic and azodye groups were used as model compounds. Maximum adsorption capacities ranged between 132.5- 265 mg of dyes per one gram of the dried adsorbent. (Safarik et. al., 1997)
Lignosulfate–derived AC was effective in the removal of phenol from aqueous solutions. The temperature dependence of phenol sorption on the AC had an external character. At phenol concentrations 0-10 mg/L the sorption isotherms could be described by Freundlich’s equation (Buzanova, et.al.1994).
Nase (2001) investigated the effectiveness of US-made Calgon™ powdered AC in reducing the COD of water samples from Tadlac Lake in Laguna. It was shown that the adsorption of COD required 30 min of contact time to reach the equilibrium residual concentration. It was observed that adsorption was most favored at pH 7.0. The study shows that at pH 7.0, using 0.0687 g, the initial COD of 366.7 mg/L can be reduced to 66.05%. Data from the batch adsorption experiment did not fit the Freundlich and Langmuir equations.
Suarez (1999) studied the ability of technical grade powdered AC to adsorb nitrate-contaminated wastewater. It was found out that at 27C, pH 5.5, 61.5901 g AC contacted with a liter of contaminated water for 30 minutes could remove 82.25% of the nitrate. However, results of AC testing showed that AC is not effective in removing methylene chloride (I) from the title wastewater, and that air –stripping would be more suitable. Whereas, the combined air-stripping and AC treatment effectively removes volatile organics, AC alone is highly effective in removing the phthalates. (Whittaker, 1984)
Activated Carbon is applicable in treating volatile and highly toxic organics, pesticides, and explosives in aqueous wastes. It is potentially applicable in treating metals in aqueous wastes, but it is not applicable in treating radioactive materials, corrosives, cyanide, and asbestos (Kent, ed., 1992).
Some compounds are poorly adsorbed that it appears that carbon has no capacity. A good example of this phenomenon would be isopropyl alcohol (IPA). Although IPA is infinitely soluble and is a small, simple molecule, carbon has very little capacity for it. On the other hand, carbon has tremendous capacity for a compound such as naphthalene. Naphthalene is poorly soluble in water and compared to IPA, it is a very large molecule with many carbon-carbon bonds (www.envirotrol.com).
2.5 Activated Carbon from Coconut Shell
Coconut (cocos mucifera) is the common name for the fruit of a palm belonging to family Arecacea (formerly Palmae). It is widely distributed in tropical regions.
Coconut shell charcoal activated at atmospheric condition can produce an iodine number of 192.45 to a maximum of 233.97. The 30-minute activation at elevated temperature of coconut charcoal by steam can produce AC with an iodine adsorption no. of 689.85, while 10 minute activation can give an iodine no. of 456.48 (Trivino, 1993).
Perry (1994) demonstrated the efficacy of North Dakota lignite towards the potassium hydroxide mediated production of granular AC using proven technology, and compared the experimental North Dakota lignite based carbon with commercial coconut shell, peat and lignite based AC. Table 2.1 shows the comparison.
Wang, et al. (1997) studied the adsorption of phenol onto granular coconut shell activated carbon in liquid- solid fluidized bed. Table 2.2 shows the properties of the coconut shell activated carbon used. Appendix C summarizes the isotherm equations and constants for phenol adsorption in Wang’s study.
Table 2.1 Comparative Data for North Dakota lignite based and Commercial AC
Value ( Dry Basis )
Parameter
Units DARCO
12 x 20
(coconut shell) PICA
G210 AS
(Peat) NORIT
R03515
(lignite) North Dakota
CCV/94/295
Mean Diameter mm granular granular 1.4 1.62 ± 0.07
Length of extrudate mm granular granular 4 4 ± 2
Hg (0.003-414 Mpa) Pore intrusion
volume/surface area
cm3/g
m2/g
0.6838
162.441
n.a.
n.a.
0.3556
83.3950
Apparent density
Kg/m3
430
n.a.
n.a.
762
Stirring abrasion number
%
89
100
99
99
pH
6.0
10.3
10.1
6.8
CO2 surface area
m3/g
740
1080
1150
1300
CO2 micropore volume
cm3/g
0.20
0.29
0.31
0.35
N2 surface area
m2/g
655
1290
1080
1325
Non-micropore surface area
m2/g
220
625
380
500
N2 pore volume
cm3/g
0.79
0.63
0.60
0.72
Iodine number
mg I2/g
620
1180
1050
1400
Benzene index
%
n.a.
40
36
39
Carbon tetrachloride index
%
n.a.
n.a.
70
66
Methylene Blue number
mg/g
150
338
275
425
Moisture
% ar
8.2
2.8
2.6
5.8
Ash
%ar
32.5
1.9
4.8
5.9
Fixed Carbon
%ar
63.4
95.6
94.2
90.9
Au adsorption capacity constant
(Gold loading capacity)
Kg/ton
19
24
22.8
Apparent kinetic activity
1/hr
n.a.
880
594
Relative kinetic activity1
%
125
100
80
n.a. = not available
ar = As Received , 1Relative to NORIT R03515
Table 2.2 Properties of ACs used in Wang’s study (1997)
Type of activated carbon Parameter Range
Coconut-shell
Darco 20-40d
Darco 20-40d Particle diameter (mm)
N2 –BET surface area (m2/g)
True density x 10-3 (g/dm3)
Apparent density x 10-3 (g/dm3)
Iodine no.
Pore volume x 10-3 (mm3/g)
(150 m-60 A)
N2 –BET surface area (m2/g)
N2 –BET surface area (m2/g) 0.600, 0.937 , 1.524
1500-1200a
2.1
0.4 –0.5b
1050- 1100b
2.2 0.1 c
650a
450a
a From experiment
b Provided by Taiwan Chemistry Industry Co.
c From CTCI/CPDC Catalyst Research
d Manufactured by Aldrich Chemical Company, Inc.
Pimonsree (1998) studied the effects of phenol concentration and pH on phenol adsorption by Coconut Shell Granular Activated Carbons (CSGAC) and Coal Granular Activated Carbons (CGAC. Results from the batch experiments indicated that variation of initial phenol concentration did not affect the equilibrium contact time. The equilibrium contact time of both CSGAC and CGAC of 4 hours were obtained. It was observed that adsorptive capacity increased as the initial phenol concentration raised
In the batch system, equilibrium contact times of 4 hours were same at the pH 3-9 but that of 6 hours were obtained at pH 11. The highest adsorptive capacity could be observed at the pH 9 and 7, which were slightly lower than the pKa value of phenol, for CSGAC and CGAC, respectively. In addition, it could be concluded that the adsorptive capacity of CSGAC was higher than that of CGAC in both the batch and the fixed bed-continuous flow experiments.
Saejen (1998) studied the adsorption of Arsenic (III) and Arsenic (V) by activated carbon prepared from coconut shell and rice husk. The results showed that the adsorption capacity constants (K) for Arsenic(III) calculated according to Freundlich equation were 0.01, 0.44, 0.03 and 0.21 mg As/g activated carbon, respectively and those for Arsenic(V) were 0.01, 3.26, 0.01 and 2.92 mg As/g activated carbon, respectively. It was concluded from the adsorption isotherm that arsenic was better adsorbed by FeCl3-activated carbon than by NaCl-activated carbon
The adsorptions of iron (III) from aqueous solution at room temperature on activated carbons obtaining from bagasse, pericarp of rubber fruit and coconut shell have been studied by atomic absorption spectrophotometry. The adsorption behavior of iron (III) on these activated carbons could be interpreted by Langmuir adsorption isotherm. The values of Xmax calculated from slopes of the Langmuir plots for all activated carbons obtained from bagasse, rubber fruit pericarp and coconut shell at room temperature are in the range 0.25 - 0.66 mmol/g, 0.11 - 0.41 mmol/g and 0.12 - 0.19 mmol/g, respectively.
The maximum amounts of iron (III) adsorbed per gram of these activated carbons were 0.66 mmol/g, 0.41 mmol/g and 0.18 mmol/g, respectively. Study of the temperature dependence on these adsorptions has revealed them to be exothermic processes with the heats of adsorption of about -8.9 kJ/mol , -9.7 kJ/mol and -5.7 kJ/mol for bagasse, pericarp of rubber fruit and coconut shell, respectively. (Sirichote, 2002)
Anything that can be oxidized in the receiving water with the consumption of dissolved molecular oxygen is termed oxygen-demanding material. This material is usually biodegradable organic matter but also includes certain inorganic compounds. Oxygen in water is consumed by the oxidation of organic matter, {CHO}:
{CH2O} + O2 microorganism CO2 + H2O
Unless the water is reaerated efficiently, as by turbulent flow in a shallow stream, it rapidly becomes depleted in oxygen and will not support higher forms of aquatic life.
In addition to microorganism-mediated oxidation of organic matter, oxygen in water may be consumed by the bioxigenation of nitrogenous material,
NH4+ + 2O2 microorganism 2H+ + NO3- + H2O
and by the chemical or biochemical oxidation of chemical reducing agent:
4F2+ + O2 + 10H2O 4Fe(OH)3 (s) + 8H+
All these processes contribute to the deoxygenation of water.
The degree of oxygen consumption by microbially mediated oxidation of contaminants in water is called the biological oxygen demand (or biochemical oxygen demand), BOD. Although BOD is a reasonably realistic measure of water quality insofar as oxygen is concerned, the test for determining it is time-consuming and cumbersome to perform. Total Organic Carbon (TOC) and COD have become popular alternatives for BOD since these are easily performed instrumentally. (Manahan, 1994)
DENR studies in 1990-1993 showed that the total daily BOD load discharged into the Pasig River was 327 metric tons/day. In 1997, it has dropped to 230 metric tons/day.
Generally, the minimum dissolved oxygen (DO) level needed to support a diverse population of fish is 5 mg/L. However, studies in July 1996 showed that the DO (mg/L) of the Pasig River near the Vargas Bridge and P. Sanchez Bridge are 1.8 and 0.8 respectively. Also, in that period, BOD for the Vargas sample is 41 mg/L and 38mg/L for the Sanchez sample. The Vargas Bridge is in western Makati while the Sanchez Bridge is along the Shaw Boulevard that traverses the San Juan River in Mandaluyong. (See Appendix A for other data)
The chemical oxygen demand (COD) is used as measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant.
The dichromate reflux method is preferred over procedures using other oxidants because of superior oxidizing ability, applicability to a wide variety of samples, and ease of manipulation. Oxidation of most organic compounds is 95 to 100% of the theoretical value. Pyridine and related compounds resists oxidation and volatile organic compounds are oxidized only to the extent that they remain in contact with the oxidant. Ammonia, present either in the waste or liberated from nitrogen–containing organic matter, is not oxidized in the absence of significant concentration of free chloride ions (APHA, 1995).
2.2 Adsorption
Adsorption involves the accumulation of solute molecules at an interface (Perry, 1997). The adsorbed solutes are referred to as adsorbate, whereas the solid material is the adsorbent.
Separation occurs because differences in molecular weight, shape, or polarity cause some molecules to be held more strongly on the surface than others or because the pores are too small to admit the larger molecules. In many cases, the adsorbing components (or adsorbates) are held strongly enough to permit complete removal of that component from the fluid with very little adsorption of other components.
Depending upon the type of forces between the fluid molecules and the molecules of the solid, adsorption is classified as physical adsorption (Van der Waals adsorption) or chemisorption (activated adsorption).
Physical adsorption, which may be a monomolecular (unimolecular) layer, or may be two, three or more layers thick (multimolecular) occurs rapidly. If unimolecular, it is reversible; if multimolecular, such that capillary pores are filled, hysteresis may occur. The density of the adsorbate is of the order of magnitude of the liquid rather than the vapor state. As physical adsorption takes place, it begins as a monolayer, becomes multilayered, and then, if the pores are close to the size of the molecules, capillary condensation occurs, and the pores fill with adsorbate. Accordingly, the maximum capacity of a porous adsorbent can be more related to the pore volume than to the surface area.
In contrast, chemisorption involves the formation of chemical bonds between the adsorbent and adsorbate in a monolayer, often with a release of heat much larger than the heat of vaporization. Commercial adsorbents rely on physical adsorption; catalysis relies on chemisorption.
When porous particles of adsorbent are immersed in a liquid mixture, the pores, if sufficiently larger in diameter than the molecules in the liquid, fill with liquid. At equilibrium, because of differences in the extent of physical adsorption among the different molecules of the liquid mixture, the composition of the liquid in the pores differs from that of the bulk liquid surrounding the adsorbent particles. The observed exothermic heat effect is referred to as the heat of wetting, which is much smaller than the heat of adsorption from the gas phase. As with gases, the extent of equilibrium adsorption of a given solute increases with concentration and decreases with temperature. Chemisorption can also occur with liquids.
For the adsorption of a solute onto a porous surface of an adsorbent, the following steps are required:
1. External (interphase) mass transfer of the solute from the bulk fluid by convection, through a thin film or boundary layer, to the outer solid surface of the adsorbent.
2. Internal (intraphase) mass transfer of the solute by pore diffusion from the outer surface of the adsorbent to the inner surface of the internal porous structure.
3. Surface diffusion along the porous surface.
4. Adsorption of the solute onto the porous surface.
For chemisorption, which involves bond formation, the rate of the fourth kinetic step may be slow and even controlling; for physical adsorption, however, step 4 is almost instantaneous because it depends only on the collision frequency and orientation of the molecules with the porous surface.
During regeneration of the adsorbent, the reverse of the fourth step occurs, where the rate of physical desorption is instantaneous. Adsorption and desorption are accompanied by heat transfer because of the exothermic heat of adsorption and the endothermic heat of desorption (Seader and Henly, 1998).
2.2.1 Slurry Adsorption ( Contact Filtration)
Three modes of adsorption from a liquid in an agitated vessel are of interest. These are the batch mode, continuous mode, and the semibatch or semicontinuous mode. In the batch mode, a batch of liquid is contacted with a batch of adsorbent for a period of time, followed by discharge of the slurry from the vessel, and a filtration step to separate the solids from the liquid.
In the batch mode, the rate of adsorption of solute, as controlled by external mass transfer, is
where c is the concentration of solute in the bulk liquid, ce is the concentration in equilibrium with the loading on the adsorbent, q; kL is the external liquid-phase mass transfer coefficient ; and a is the external surface area of the adsorbent pler unit volume of liquid (Seader and Henly, 1998).
2.2.2 Adsorption Isotherms
The adsorption isotherm is the equilibrium relationship between the concentration in the fluid phase and the concentration in the adsorbent particles at a given temperature. For liquids, the concentration is often expressed in mass units, such as part per million. The concentration of the adsorbate on the solid is given as mass adsorbed per unit mass of original adsorbent.
For the design and operation of all sorption systems, the adsorption isotherm is of great importance because it relates, at equilibrium, the concentration of the solute in the fluid to its loading as a sorbate in and/or the sorbent. Most commonly, the overall rate of adsorption is expressed in the form of a linear driving force (LDF) model, where the driving force is the difference between the bulk concentration and the concentration in equilibrium with the loading. The coefficient in the LDF equation is a combined overall mass transfer coefficient and area for sorption. (Seader and Henly, 1998)
2.2.3 Theory on Isotherms
The adsorbability of a contaminant is determined by its physical and chemical properties. The adsorption capacity of an adsorbate on carbon is related to the adsorbate’s concentration in solution and to other competing solutes. The objective of the isotherm test is to determine the relationship between the concentration of the adsorbate and the capacity of the carbon. Equations that are often used to describe the experimental isotherm data were developed by Freundlich, Langmuir and by Brunauer, Emmet, and Teller (BET isotherm). Other isotherms were developed by Redlich-Peterson (1959), Radke-Prrausnitz (1972), Jossens (1978), and Weber- Vliet (1980).
The equilibrium between the concentration of a solute in the fluid phase and its concentration on the solid resembles somewhat the equilibrium solubility of a gas in a liquid. Data are plotted as adsorption isotherms as shown in Fig.1. The concentration in the solid phase is expressed as q, kg adsorbate (solute)/ kg adsorbent (solid), and in the fluid phase (gas or liquid) as c, kg adsorbate/m3 fluid.
q, kg adsorbate/
kg adsorbent
c, kg adsorbate /m3 fluid
Fig. 2.1 Common types of adsorption isotherm
The linear isotherm is uncommon, but in the dilute region it can be used to approximate data of many systems.
The empirical Freundlich isotherm equation approximates data for many physical adsorption systems and is particularly useful for liquids.
Where K and n are constants and must be determined experimentally. If a log-log plot of q versus c is made, the slope is the dimensionless exponent n. The dimensions of K depend on the value of n. This equation is sometimes used to correlate data for hydrocarbon gases on activated carbon.
The Langmuir isotherm has a theoretical basis and is given by the following, where qo (kg adsorbate/kg solid), and K (kg/m3) are empirical constants.
This equation was derived assuming that there are only a fixed number of active sites available for adsorption, only a monolayer is formed, and the adsorption is reversible and reaches equilibrium condition. By plotting 1/q versus 1/c, the slope is K/qo and the intercept is 1/qo.
Almost all adsorption systems show that as temperature is increased the amount adsorbed by the adsorbent decreases strongly. Only the temperature scale shifts from system to system. This is useful since adsorption is normally at room temperatures and desorption can be attained by raising the temperature (Geankoplis, 1993).
2.3 Activated Carbon
Typical commercial adsorbents, which may be granules, spheres, cylindrical pellets, flakes, or powders, have size ranging from 50 μm to 1.2 cm. They have specific surface areas of 300 to 1200 m2/g. Such a large area is made possible by a particle porosity from 30 to 85 vol% with average pore diameters from 10 to 200 Å (Seader and Henly, 1998).
Activated Carbon is a microcrystalline material made by thermal decomposition of wood, vegetable shells, coal, etc., and has surface area of 300 to 1200 m2/g with average pore diameter of 10 to 60 Å (Geankoplis, 1993).
The characteristics of AC depend in the parent or precursor material, activation method, and activation process. Some precursors allow production and activation of the granules in existing form. Others, which are softer, require that the activation take place in a powder form and are then pressed or extruded into pellets.
Higher activation levels (increased surface areas) generally result in a softer, more powdery carbon unless compacted into pellets.
The most common precursors are wood (35%), coal-bituminous or anthracite (28%), lignite (14%), coconut shell (10%), peat (10%) and 3% other. The other group may include other fruit and nut stones (olive, macadamia nut, almond), coal liquids, petroleum pitches, or even blood and bone chars.
Activation, changing a low-porosity solid into a high porosity solid, can be accomplished either thermally or chemically. The thermal and most common route involves a two-step process of carbonization (removal of most non-carbon atoms) followed by activation through partial gasification (reaction at high temperature with an oxygen donor such as carbon dioxide or steam). This tends to open up the internal porosity of the material while retain the blueprint pore structure of the original material.
The second route to activation involves the reaction of the parent material with a chemical agent, which allows lower temperature activation. The two most common chemical agents used presently are zinc chloride and phosphoric acid (Andrews, 2003).
Banzai (as cited in Ahmedna, et al., 2000b) states that the adsorption capacity of AC is determined by its total surface area, by its internal porous structure, and the presence of functional groups on the pore surface. Carbon surfaces have a pore size that determines its adsorption capacity, a chemical structure that influences its interaction with polar and nonpolar adsorbates, and active sites that determine the type of chemical reactions with other molecules (Ahmedna, et al., 2000b).
2.4 Wastewater Treatment by Activated Carbon Adsorption
Even after filtration, coagulation, sedimentation, and secondary treatment, soluble organic materials that are resistant to biological breakdown will persist in the effluent. The persistent materials are often referred to as refractory organics. Refractory organics can be detected in the effluent as soluble COD. Secondary effluent values are often 30 to 60 mg/L (Davis, 1998).
To date, carbon adsorption is used for the removal of the refractory organic compounds in wastewater. Both granular and powdered carbons appear to have a low adsorption affinity for low-molecular weight polar organic species. If biological activity is low in the carbon contactor or in other biological unit processes, these species are difficult to remove with activated carbon. Under normal conditions, after treatment with carbon the effluent BOD ranges from 2 to 7 mg/L. Under optimum conditions, it appears that the effluent COD can be reduced to about 10 mg/L (Tchobanoglous, 1991).
AC adsorption, when used with other treatment methods is very effective in the removals of organics in wastewater. Ozonization or oxygenation of secondary effluents before treatment by granular AC extended the life of adsorbent from 70 to 480 days and was effective in removing C, COD, BOD, and N compounds. The effluent from the treatment met wastewater quality standard (Netzer, 1985).
Treatment of oil refinery, pulping, dyeing, and fungicide manufacturing wastewaters by ozonization and biological AC adsorption showed that the total organic carbon was removed 10, 20, 30, and 15 % respectively, from those wastewaters. The primary mechanism for pollutant removal in the wastewaters was by adsorption, biodegradation, oxidation, and oxidation- biodegradation, respectively (Schwarrz, 1981).
The effect of magnetic charcoal activated carbon on water-soluble organic dyes was also investigated. The adsorbent was prepared by entrapment of fine charcoal into the structure of magnetic ion oxides. For the adsorption experiments, water–soluble organic dyes belonging to the triphenylmethane, heteropolycyclic and azodye groups were used as model compounds. Maximum adsorption capacities ranged between 132.5- 265 mg of dyes per one gram of the dried adsorbent. (Safarik et. al., 1997)
Lignosulfate–derived AC was effective in the removal of phenol from aqueous solutions. The temperature dependence of phenol sorption on the AC had an external character. At phenol concentrations 0-10 mg/L the sorption isotherms could be described by Freundlich’s equation (Buzanova, et.al.1994).
Nase (2001) investigated the effectiveness of US-made Calgon™ powdered AC in reducing the COD of water samples from Tadlac Lake in Laguna. It was shown that the adsorption of COD required 30 min of contact time to reach the equilibrium residual concentration. It was observed that adsorption was most favored at pH 7.0. The study shows that at pH 7.0, using 0.0687 g, the initial COD of 366.7 mg/L can be reduced to 66.05%. Data from the batch adsorption experiment did not fit the Freundlich and Langmuir equations.
Suarez (1999) studied the ability of technical grade powdered AC to adsorb nitrate-contaminated wastewater. It was found out that at 27C, pH 5.5, 61.5901 g AC contacted with a liter of contaminated water for 30 minutes could remove 82.25% of the nitrate. However, results of AC testing showed that AC is not effective in removing methylene chloride (I) from the title wastewater, and that air –stripping would be more suitable. Whereas, the combined air-stripping and AC treatment effectively removes volatile organics, AC alone is highly effective in removing the phthalates. (Whittaker, 1984)
Activated Carbon is applicable in treating volatile and highly toxic organics, pesticides, and explosives in aqueous wastes. It is potentially applicable in treating metals in aqueous wastes, but it is not applicable in treating radioactive materials, corrosives, cyanide, and asbestos (Kent, ed., 1992).
Some compounds are poorly adsorbed that it appears that carbon has no capacity. A good example of this phenomenon would be isopropyl alcohol (IPA). Although IPA is infinitely soluble and is a small, simple molecule, carbon has very little capacity for it. On the other hand, carbon has tremendous capacity for a compound such as naphthalene. Naphthalene is poorly soluble in water and compared to IPA, it is a very large molecule with many carbon-carbon bonds (www.envirotrol.com).
2.5 Activated Carbon from Coconut Shell
Coconut (cocos mucifera) is the common name for the fruit of a palm belonging to family Arecacea (formerly Palmae). It is widely distributed in tropical regions.
Coconut shell charcoal activated at atmospheric condition can produce an iodine number of 192.45 to a maximum of 233.97. The 30-minute activation at elevated temperature of coconut charcoal by steam can produce AC with an iodine adsorption no. of 689.85, while 10 minute activation can give an iodine no. of 456.48 (Trivino, 1993).
Perry (1994) demonstrated the efficacy of North Dakota lignite towards the potassium hydroxide mediated production of granular AC using proven technology, and compared the experimental North Dakota lignite based carbon with commercial coconut shell, peat and lignite based AC. Table 2.1 shows the comparison.
Wang, et al. (1997) studied the adsorption of phenol onto granular coconut shell activated carbon in liquid- solid fluidized bed. Table 2.2 shows the properties of the coconut shell activated carbon used. Appendix C summarizes the isotherm equations and constants for phenol adsorption in Wang’s study.
Table 2.1 Comparative Data for North Dakota lignite based and Commercial AC
Value ( Dry Basis )
Parameter
Units DARCO
12 x 20
(coconut shell) PICA
G210 AS
(Peat) NORIT
R03515
(lignite) North Dakota
CCV/94/295
Mean Diameter mm granular granular 1.4 1.62 ± 0.07
Length of extrudate mm granular granular 4 4 ± 2
Hg (0.003-414 Mpa) Pore intrusion
volume/surface area
cm3/g
m2/g
0.6838
162.441
n.a.
n.a.
0.3556
83.3950
Apparent density
Kg/m3
430
n.a.
n.a.
762
Stirring abrasion number
%
89
100
99
99
pH
6.0
10.3
10.1
6.8
CO2 surface area
m3/g
740
1080
1150
1300
CO2 micropore volume
cm3/g
0.20
0.29
0.31
0.35
N2 surface area
m2/g
655
1290
1080
1325
Non-micropore surface area
m2/g
220
625
380
500
N2 pore volume
cm3/g
0.79
0.63
0.60
0.72
Iodine number
mg I2/g
620
1180
1050
1400
Benzene index
%
n.a.
40
36
39
Carbon tetrachloride index
%
n.a.
n.a.
70
66
Methylene Blue number
mg/g
150
338
275
425
Moisture
% ar
8.2
2.8
2.6
5.8
Ash
%ar
32.5
1.9
4.8
5.9
Fixed Carbon
%ar
63.4
95.6
94.2
90.9
Au adsorption capacity constant
(Gold loading capacity)
Kg/ton
19
24
22.8
Apparent kinetic activity
1/hr
n.a.
880
594
Relative kinetic activity1
%
125
100
80
n.a. = not available
ar = As Received , 1Relative to NORIT R03515
Table 2.2 Properties of ACs used in Wang’s study (1997)
Type of activated carbon Parameter Range
Coconut-shell
Darco 20-40d
Darco 20-40d Particle diameter (mm)
N2 –BET surface area (m2/g)
True density x 10-3 (g/dm3)
Apparent density x 10-3 (g/dm3)
Iodine no.
Pore volume x 10-3 (mm3/g)
(150 m-60 A)
N2 –BET surface area (m2/g)
N2 –BET surface area (m2/g) 0.600, 0.937 , 1.524
1500-1200a
2.1
0.4 –0.5b
1050- 1100b
2.2 0.1 c
650a
450a
a From experiment
b Provided by Taiwan Chemistry Industry Co.
c From CTCI/CPDC Catalyst Research
d Manufactured by Aldrich Chemical Company, Inc.
Pimonsree (1998) studied the effects of phenol concentration and pH on phenol adsorption by Coconut Shell Granular Activated Carbons (CSGAC) and Coal Granular Activated Carbons (CGAC. Results from the batch experiments indicated that variation of initial phenol concentration did not affect the equilibrium contact time. The equilibrium contact time of both CSGAC and CGAC of 4 hours were obtained. It was observed that adsorptive capacity increased as the initial phenol concentration raised
In the batch system, equilibrium contact times of 4 hours were same at the pH 3-9 but that of 6 hours were obtained at pH 11. The highest adsorptive capacity could be observed at the pH 9 and 7, which were slightly lower than the pKa value of phenol, for CSGAC and CGAC, respectively. In addition, it could be concluded that the adsorptive capacity of CSGAC was higher than that of CGAC in both the batch and the fixed bed-continuous flow experiments.
Saejen (1998) studied the adsorption of Arsenic (III) and Arsenic (V) by activated carbon prepared from coconut shell and rice husk. The results showed that the adsorption capacity constants (K) for Arsenic(III) calculated according to Freundlich equation were 0.01, 0.44, 0.03 and 0.21 mg As/g activated carbon, respectively and those for Arsenic(V) were 0.01, 3.26, 0.01 and 2.92 mg As/g activated carbon, respectively. It was concluded from the adsorption isotherm that arsenic was better adsorbed by FeCl3-activated carbon than by NaCl-activated carbon
The adsorptions of iron (III) from aqueous solution at room temperature on activated carbons obtaining from bagasse, pericarp of rubber fruit and coconut shell have been studied by atomic absorption spectrophotometry. The adsorption behavior of iron (III) on these activated carbons could be interpreted by Langmuir adsorption isotherm. The values of Xmax calculated from slopes of the Langmuir plots for all activated carbons obtained from bagasse, rubber fruit pericarp and coconut shell at room temperature are in the range 0.25 - 0.66 mmol/g, 0.11 - 0.41 mmol/g and 0.12 - 0.19 mmol/g, respectively.
The maximum amounts of iron (III) adsorbed per gram of these activated carbons were 0.66 mmol/g, 0.41 mmol/g and 0.18 mmol/g, respectively. Study of the temperature dependence on these adsorptions has revealed them to be exothermic processes with the heats of adsorption of about -8.9 kJ/mol , -9.7 kJ/mol and -5.7 kJ/mol for bagasse, pericarp of rubber fruit and coconut shell, respectively. (Sirichote, 2002)
Monday, April 6, 2009
MATERIALS AND METHODS
3.1 Materials
The adsorbent used was the Mapecon( TM) activated carbon in powdered form. It is of technical grade (mesh 50 - ) and was solicited from the Mapecon Plant in Alaminos, Laguna. For the separation of AC from the sample, Whatman™ no.1 filters were used.
3.2 Equipment
For pH measurements in the laboratory, the pH/mV/Temperature meter and the pH 600 pen type meter were used. Oven and top loading balance (Fig. 3.1) were used in the drying and weighing of AC, respectively. Dessicator was used in cooling and storing AC. For the batch treatments, the equipment used was the Gerhardt RO™ Rotary Shaker (Fig. 3.2). Merck- SQ™ Spectrophotometer (Fig. 3.3) was used in the color and turbidity analyses. The reflux apparatus (Fig. 3.4) was used in the COD tests.
Fig. 3.1 Top Loading Balance
Fig. 3.2 Gerhardt RO™ Rotary Shaker
Fig. 3.3 Merck- SQ™ Spectrophotometer
Fig. 3.4 Reflux apparatus
3.3 Methods
3.3.1 Sample Collection and Preservation
Water samples were collected from a part of Pasig River System near the P. Sanchez Bridge in Mandaluyong City.
The samples were placed in opaque plastic containers. Before filling, the plastic containers were rinsed twice with the water collected. Significant amounts of suspended matter were separated by decantation, or by cloth that acted as filter.
The samples will be worthless if the physical, chemical, and biological integrity of the samples are not maintained during the interim periods between sample collection and sample analysis. Considerable research on the problem of sample preservation has failed to perfect a universal treatment or method or to formulate a set of fixed rules applicable to samples of all types.
Since delay before analysis is unavoidable, the samples were stored immediately at low temperature (4C) after collection.
3.3.2 Characterization of the Pasig River Water Samples
The pH, turbidity, color, chloride content, and COD of the samples were measured. Turbidity was measured using the nephelometric method, while the spectrophotometric method was used in the color analysis. The Argentometric method was used in the chloride determination. The open reflux method was used in all COD tests. The procedure is outlined in Appendix C.
3.3.3 Pretreatment and Characterization of Activated Carbon
a. Drying of AC
The AC was pretreated before every test. It was subjected in an oven at 110 C for 3 hours to remove the inherent moisture from its surface. Then, the dried AC was kept in a dessicator for cooling and storage.
b. Determination of Physico–Chemical Characteristics of the AC
The obtained AC was in powdered form, labeled as technical grade, mesh size of 50 pass, and iodine number of 968 mg/g. Thus, its other properties were determined experimentally. The iodine number of the AC was also empirically validated.
i. Moisture Content
The determination of moisture content was followed from ASTM standards of 1975-D 2867. 5 g of AC was heated in an electric oven at 150 C for 3 hours. Heating was continued until the weight of the sample becomes constant. The moisture content was computed by multiplying by 100 the quotient between weight lost and initial weight.
ii. Bulk Density
The bulk density is also known as apparent density. It is required by engineers to know the volume of containers that will store AC. Bulk density is affected by the raw material used and the degree of activation.
The standard procedure was followed from Beg and Usmani (1985) as cited from Encyclopedia of Industrial and Chemical Analysis. 5.5 g of AC were placed in a pre-weighed 10-mL graduated cylinder. The cylinder was tapped slightly to give the closest possible packing. Then, the volume occupied by the carbon and the added weight in the cylinder were determined.
The bulk density was computed as grams C per mL volume occupied in the graduated cylinder.
vi. Iodine Number
Iodine no. indicates the ability of particular carbon to adsorb small molecules like nitrates and other organic substances. This represents the number of mg iodine per gram of AC at a specified equilibrium concentration.
The procedure for the determination of iodine no. was based from Culp and Culp Method of 1971 outlined in Appendix D.
3.3.4 Equilibrium Contact Time Determination
The contact time determination was performed as a preliminary experiment. The initial COD concentration of the solution was determined and the samples were contacted with fixed weights of carbon for 0.5, 1, 2, 3 , 4, and 5-hour period. For this purpose, 100 mg AC was contacted with 100 mL of test solution. The samples were agitated using the rotary shaker at 155 rpm. The contact time was determined in order to know the time in which concentration equilibrium occurs. The residual CODs were determined. Statistical analysis was used to determine if there was no significant change in the residual COD.
There were two trials and the pHs were recorded.
3.3.5 Determination of Optimum pH for Maximum Adsorption
The effect of pH on the adsorption was investigated. The initial COD concentration of the solution was determined without altering its pH. This served as the control and the pH was recorded. This 100-ml control sample was contacted with 200 mg of AC at the chosen contact time. The sample was agitated using the rotary shaker at 155 rpm.
Four (4) other 100-ml test solutions were contacted at the chosen contact time with 200 mg AC at pHs higher or lower than the pH of the control sample. There were two trials and the pH was adjusted without diluting the test solutions using NaOH pellets and concentrated H2SO4. The final pHs were recorded to determine the change in pH.
The adsorbent used was the Mapecon( TM) activated carbon in powdered form. It is of technical grade (mesh 50 - ) and was solicited from the Mapecon Plant in Alaminos, Laguna. For the separation of AC from the sample, Whatman™ no.1 filters were used.
3.2 Equipment
For pH measurements in the laboratory, the pH/mV/Temperature meter and the pH 600 pen type meter were used. Oven and top loading balance (Fig. 3.1) were used in the drying and weighing of AC, respectively. Dessicator was used in cooling and storing AC. For the batch treatments, the equipment used was the Gerhardt RO™ Rotary Shaker (Fig. 3.2). Merck- SQ™ Spectrophotometer (Fig. 3.3) was used in the color and turbidity analyses. The reflux apparatus (Fig. 3.4) was used in the COD tests.
Fig. 3.1 Top Loading Balance
Fig. 3.2 Gerhardt RO™ Rotary Shaker
Fig. 3.3 Merck- SQ™ Spectrophotometer
Fig. 3.4 Reflux apparatus
3.3 Methods
3.3.1 Sample Collection and Preservation
Water samples were collected from a part of Pasig River System near the P. Sanchez Bridge in Mandaluyong City.
The samples were placed in opaque plastic containers. Before filling, the plastic containers were rinsed twice with the water collected. Significant amounts of suspended matter were separated by decantation, or by cloth that acted as filter.
The samples will be worthless if the physical, chemical, and biological integrity of the samples are not maintained during the interim periods between sample collection and sample analysis. Considerable research on the problem of sample preservation has failed to perfect a universal treatment or method or to formulate a set of fixed rules applicable to samples of all types.
Since delay before analysis is unavoidable, the samples were stored immediately at low temperature (4C) after collection.
3.3.2 Characterization of the Pasig River Water Samples
The pH, turbidity, color, chloride content, and COD of the samples were measured. Turbidity was measured using the nephelometric method, while the spectrophotometric method was used in the color analysis. The Argentometric method was used in the chloride determination. The open reflux method was used in all COD tests. The procedure is outlined in Appendix C.
3.3.3 Pretreatment and Characterization of Activated Carbon
a. Drying of AC
The AC was pretreated before every test. It was subjected in an oven at 110 C for 3 hours to remove the inherent moisture from its surface. Then, the dried AC was kept in a dessicator for cooling and storage.
b. Determination of Physico–Chemical Characteristics of the AC
The obtained AC was in powdered form, labeled as technical grade, mesh size of 50 pass, and iodine number of 968 mg/g. Thus, its other properties were determined experimentally. The iodine number of the AC was also empirically validated.
i. Moisture Content
The determination of moisture content was followed from ASTM standards of 1975-D 2867. 5 g of AC was heated in an electric oven at 150 C for 3 hours. Heating was continued until the weight of the sample becomes constant. The moisture content was computed by multiplying by 100 the quotient between weight lost and initial weight.
ii. Bulk Density
The bulk density is also known as apparent density. It is required by engineers to know the volume of containers that will store AC. Bulk density is affected by the raw material used and the degree of activation.
The standard procedure was followed from Beg and Usmani (1985) as cited from Encyclopedia of Industrial and Chemical Analysis. 5.5 g of AC were placed in a pre-weighed 10-mL graduated cylinder. The cylinder was tapped slightly to give the closest possible packing. Then, the volume occupied by the carbon and the added weight in the cylinder were determined.
The bulk density was computed as grams C per mL volume occupied in the graduated cylinder.
vi. Iodine Number
Iodine no. indicates the ability of particular carbon to adsorb small molecules like nitrates and other organic substances. This represents the number of mg iodine per gram of AC at a specified equilibrium concentration.
The procedure for the determination of iodine no. was based from Culp and Culp Method of 1971 outlined in Appendix D.
3.3.4 Equilibrium Contact Time Determination
The contact time determination was performed as a preliminary experiment. The initial COD concentration of the solution was determined and the samples were contacted with fixed weights of carbon for 0.5, 1, 2, 3 , 4, and 5-hour period. For this purpose, 100 mg AC was contacted with 100 mL of test solution. The samples were agitated using the rotary shaker at 155 rpm. The contact time was determined in order to know the time in which concentration equilibrium occurs. The residual CODs were determined. Statistical analysis was used to determine if there was no significant change in the residual COD.
There were two trials and the pHs were recorded.
3.3.5 Determination of Optimum pH for Maximum Adsorption
The effect of pH on the adsorption was investigated. The initial COD concentration of the solution was determined without altering its pH. This served as the control and the pH was recorded. This 100-ml control sample was contacted with 200 mg of AC at the chosen contact time. The sample was agitated using the rotary shaker at 155 rpm.
Four (4) other 100-ml test solutions were contacted at the chosen contact time with 200 mg AC at pHs higher or lower than the pH of the control sample. There were two trials and the pH was adjusted without diluting the test solutions using NaOH pellets and concentrated H2SO4. The final pHs were recorded to determine the change in pH.
Sunday, April 5, 2009
RESULTS AND DISCUSSION
4.1 Characteristics of the Pasig River Water Samples
The characteristics of the Pasig river water samples taken from July- September 2003 near the Sanchez Bridge in Mandaluyong City are presented in Table 4.1. The samples were filtrated and the suspended solids were allowed to settle before measuring the characteristics shown in Table 4.1.
The characterization was made to know the characteristics of the samples before contacting it with AC. Preliminary COD measurement was made to determine the most suitable method for COD test. The chloride content was determined because the COD reflux method is not suitable for samples containing 2000 mg Cl-/L. Techniques designed to measure COD in saline waters are available. On the contrary, ammonia will not be oxidized in the absence of significant concentration of free chloride ions. (APHA-AWWA, 1995).
Table 4.1 Characteristics of the Samples taken near the P. Sanchez Bridge
COD (ppm) 77.90 - 83.23
pH 7.1- 7.61
Turbidity (NTU) 14 – 26
Color (PCU) 35 – 40
odor objectionable
Chloride content (mg Cl-/L) 22.96 ± 0.85
Note: COD values are the COD of the samples after filtration using Whatman no.1 filters
NTU = Nephelometric Turbidity Unit
PCU= Potassium Chloroplatinate Color Unit
Chloride content is for Sept. 2003 samples only
4.2 Characteristics of Activated Carbon
The physical and chemical properties of ACs may not relate directly to their effectiveness, but they are important for their commercial utilization. As a result, they are usually given as a part of the commercial specification of industrial carbons. Table 4.2 shows the characteristics of the AC obtained from Mapecon Company.
Table 4.2 Physical Characteristics of AC
Moisture content
5.98 % ± 0.66
Bulk Density
0.579 g/mL ± 0.007
Iodine Number (given by the Mapecon)
968
experimental result
841 ± 22
The moisture content is 6%. The bulk density is 0.579 g/mL. The bulk density does not affect the effectiveness of the AC measured in adsorption per unit weight, but will have an effect on adsorption per unit volume.
The Iodine number given by the Mapecon Co. as printed in the label of the AC solicited was 968 mg/g. However, experimental results show that it is 841 mg/g ± 22. Mapecon AC’s empirical iodine number of 841 mg/g is near to that of AC made in Arquero’s study (2002).
In that study, coconut shell charcoal was activated at 800 °C which gives an iodine number of 809.92 mg/g
Also, the Mapecon AC’s empirical iodine number of 841 mg/g is higher than that of the Darco Coconut Shell AC which has an Iodine number of 620 mg/g (See Table 2.1). However, it is still lower than the iodine number of Pica peat AC and Norit lignite AC which is 1180 mg/g and 1050 mg/g, respectively.
4.3 Equilibrium Contact Time
Contact time was determined in order to know the time in which COD concentration equilibrium occurs. In the COD reduction, equilibrium occurs when there is no considerable change in the residual COD. The time where insignificant changes in COD reduction was utilized as the appropriate contact time. Table 4.3 shows the % COD reduction at different contact time. Figure 4.1 shows the equilibrium contact time curve.
The chosen contact time is 0.5 hour. By statistical analysis (Appendix E), it was determined that the different contact time have no significant difference in the % COD reduction.
With good agitation and small particles, the external resistance to mass transfer from the bulk liquid to the external surface of the adsorbent particles is small. For small adsorbent particles, the internal resistance to mass transfer within the pores of the particles is also small.
Accordingly, the rate of adsorption is rapid. The required residence time of the slurry in a well-mixed agitated vessel is determined by how fast equilibrium is approached. In this study, most of the adsorption occurred within the 0 – 30 minute contact time.
Table 4.3 also shows the chloride content of the samples at different contact time. It shows that there is no significant reduction of the chloride content of the samples after contacting it with AC.
Table 4.3 Effect of Contact Time in % COD Reduction
Contact time
COD (ppm) % COD reduction Chloride content
(mg Cl-/L)
0 h
80.84 - 22.86
0.5 h
51.58 35.26
1 h
45.99 42.19
2 h
41.56 47.80
3h
44.31 45.19
4h
42.96 46.86 22.75
5h
40.28 50.17 19.08
4.4 Optimum pH for Maximum Adsorption
The pH necessary for maximum adsorption was determined. Table 4.4 shows the results of the optimum pH determination. They also show the change of pH after the contacting of carbon.
The electrical charge of the surface groups in AC may enhance or hinder the adsorption of the target molecules on the carbon surface. If the adsorbate has the same electrostatic charge as that of the carbon surface, repulsion will occur, thereby inhibiting the process if adsorption.
However, the adsorption of such molecules will be enhanced if they and the carbon surface carry opposite charges.
Studies showed that coconut shell AC adsorbed better in low pH. In the study of Pimonsree (1998) about the phenol adsorption onto AC, equilibrium contact times of 4 hours were same at the pH ranged between 3-9 but that of 6 hours were obtained at the pH of 11.
In this study, the pH of the control sample is 7.1. The highest COD reduction was found at pH 5.5 (See Figure 4.2). Lowering the pH of the samples will increase the positively charged (+) ions in the adsorbate, therefore, it can be assumed that the AC has more negatively charged surface areas.
Table 4.4 Effect of Initial pH in % COD Reduction
sample
pH initial COD (ppm) % COD reduction pH final
C0
7.1 81.57 7.1
C1
7.1 50.68 37.87 7.3
L1
6.5 45.38 44.00 7.1
L2
5.5 26.24 68.08 6.4
U2 8.5 47.14 42.66 7.8
Except for the initial pH of 8.5, the pH of the samples increases after the contacting of carbon. Studies have determined that the adsorptive forces in the AC are the most probable cause of pH rise. Leading scientist in the carbon industry have theorized that the pH rise effect was caused by the ash in the carbon washing off and recombining to create basic compounds that increase the pH of the effluent stream. However, if the pH rise was due to high ash content, the ash concentration of the AC should have reduced substantially after the pH rise. Studies in 1999 revealed that the ash levels were identical for the carbon both prior to start-up anf after the pH rise (www.envirotrol.com).
4.5 Kinetics of Adsorption of Organics
The COD of the samples as a function of time is presented in Table 4.5 and Figure 4.3.
Table 4.5 COD of the samples as a function of Time
Time (h) COD (ppm)
0 80.84
0.5 51.58
1 45.99
3 44.31
4 42.96
5 40.28
The rate of adsorption of organics can be described as
where –r is the rate of disappearance of COD in the liquid, [COD] is the concentration of organics in the liquid, [CODe] is the equilibrium concentration of organics in the liquid, k is the coefficient of the adsorption, and n is the order of the reaction. Substituting C for [COD], the above equation will be
Taking the natural logarithm of both sides, it will be
Setting up tangential lines along the points of the curves in figure 4.3, and solving for their slope will give the values of dC/dt. Getting the COD of these points and subtracting it by the equilibrium COD of 40.28 ppm will give the values of [C-Ce]. Using regression analysis, the corresponding slope will be equal to n. The y- intercept will be equal to ln k. Figure 4.4 shows the curve of ln [C-Ce] vs ln dC/dt and the corresponding slope and intercept. Table 4.6 shows the computed values of k, and n.
Table 4.6
The computed adsorption coefficient and order of reaction
k 0.72088 h-1
n 1.3445
From these values, the adsorption of organics can be described by the equation
4.6 Minimum amount of AC that will give satisfactory COD Reduction
Adsorption is not a chemical reaction between the media and the compounds; rather it is a physical reaction, like gravity. The forces that bind organic molecules to AC are a weak form of gravity known as Van Der Waals Forces. While there are many interpretations as to why some compounds adsorb to carbon and others do not, the fact is that activated carbon has a limited capacity for all organics compounds.
In addition, carbons with small pore size will not trap large adsorbate molecules and those, with large pores may not be able to retain small adsorbate molecules.
This is maybe the reason why a complete removal of organics was not attained in this study. Table 4.8 shows that even at using 200 mg AC, the % organic removal is only 86.71. In the optimum pH determination, at 200 mg AC, the average COD removal is only 68.08 %. Maybe the samples have high concentration of organics similar to isopropyl alcohol. Although isopropyl alcohol is infinitely soluble and is a small, simple molecule, carbon has very little capacity for it.
The capacity or “loading“ is the amount of adsorbate taken up by the adsorbent per unit mass or volume of the adsorbent. This is considered the most important attribute of the adsorbent. It dictated capital cost because it determines the amount of adsorbent required and accordingly, the volume of adsorber vessels.
As shown in Table 4.7, it was found that at pH 5.5, using 22 mg AC, the initial COD of 83.23 ppm can be lowered by 61.42 %. This gives the highest loading of 2.32 ppm organics adsorbed per mg of AC.
Table 4.7 Minimum amount that will give satisfactory COD reduction
mg AC
% COD Reduction
0
0
22
61.42
76
62.04
105
68.38
200
86.71
The characteristics of the Pasig river water samples taken from July- September 2003 near the Sanchez Bridge in Mandaluyong City are presented in Table 4.1. The samples were filtrated and the suspended solids were allowed to settle before measuring the characteristics shown in Table 4.1.
The characterization was made to know the characteristics of the samples before contacting it with AC. Preliminary COD measurement was made to determine the most suitable method for COD test. The chloride content was determined because the COD reflux method is not suitable for samples containing 2000 mg Cl-/L. Techniques designed to measure COD in saline waters are available. On the contrary, ammonia will not be oxidized in the absence of significant concentration of free chloride ions. (APHA-AWWA, 1995).
Table 4.1 Characteristics of the Samples taken near the P. Sanchez Bridge
COD (ppm) 77.90 - 83.23
pH 7.1- 7.61
Turbidity (NTU) 14 – 26
Color (PCU) 35 – 40
odor objectionable
Chloride content (mg Cl-/L) 22.96 ± 0.85
Note: COD values are the COD of the samples after filtration using Whatman no.1 filters
NTU = Nephelometric Turbidity Unit
PCU= Potassium Chloroplatinate Color Unit
Chloride content is for Sept. 2003 samples only
4.2 Characteristics of Activated Carbon
The physical and chemical properties of ACs may not relate directly to their effectiveness, but they are important for their commercial utilization. As a result, they are usually given as a part of the commercial specification of industrial carbons. Table 4.2 shows the characteristics of the AC obtained from Mapecon Company.
Table 4.2 Physical Characteristics of AC
Moisture content
5.98 % ± 0.66
Bulk Density
0.579 g/mL ± 0.007
Iodine Number (given by the Mapecon)
968
experimental result
841 ± 22
The moisture content is 6%. The bulk density is 0.579 g/mL. The bulk density does not affect the effectiveness of the AC measured in adsorption per unit weight, but will have an effect on adsorption per unit volume.
The Iodine number given by the Mapecon Co. as printed in the label of the AC solicited was 968 mg/g. However, experimental results show that it is 841 mg/g ± 22. Mapecon AC’s empirical iodine number of 841 mg/g is near to that of AC made in Arquero’s study (2002).
In that study, coconut shell charcoal was activated at 800 °C which gives an iodine number of 809.92 mg/g
Also, the Mapecon AC’s empirical iodine number of 841 mg/g is higher than that of the Darco Coconut Shell AC which has an Iodine number of 620 mg/g (See Table 2.1). However, it is still lower than the iodine number of Pica peat AC and Norit lignite AC which is 1180 mg/g and 1050 mg/g, respectively.
4.3 Equilibrium Contact Time
Contact time was determined in order to know the time in which COD concentration equilibrium occurs. In the COD reduction, equilibrium occurs when there is no considerable change in the residual COD. The time where insignificant changes in COD reduction was utilized as the appropriate contact time. Table 4.3 shows the % COD reduction at different contact time. Figure 4.1 shows the equilibrium contact time curve.
The chosen contact time is 0.5 hour. By statistical analysis (Appendix E), it was determined that the different contact time have no significant difference in the % COD reduction.
With good agitation and small particles, the external resistance to mass transfer from the bulk liquid to the external surface of the adsorbent particles is small. For small adsorbent particles, the internal resistance to mass transfer within the pores of the particles is also small.
Accordingly, the rate of adsorption is rapid. The required residence time of the slurry in a well-mixed agitated vessel is determined by how fast equilibrium is approached. In this study, most of the adsorption occurred within the 0 – 30 minute contact time.
Table 4.3 also shows the chloride content of the samples at different contact time. It shows that there is no significant reduction of the chloride content of the samples after contacting it with AC.
Table 4.3 Effect of Contact Time in % COD Reduction
Contact time
COD (ppm) % COD reduction Chloride content
(mg Cl-/L)
0 h
80.84 - 22.86
0.5 h
51.58 35.26
1 h
45.99 42.19
2 h
41.56 47.80
3h
44.31 45.19
4h
42.96 46.86 22.75
5h
40.28 50.17 19.08
4.4 Optimum pH for Maximum Adsorption
The pH necessary for maximum adsorption was determined. Table 4.4 shows the results of the optimum pH determination. They also show the change of pH after the contacting of carbon.
The electrical charge of the surface groups in AC may enhance or hinder the adsorption of the target molecules on the carbon surface. If the adsorbate has the same electrostatic charge as that of the carbon surface, repulsion will occur, thereby inhibiting the process if adsorption.
However, the adsorption of such molecules will be enhanced if they and the carbon surface carry opposite charges.
Studies showed that coconut shell AC adsorbed better in low pH. In the study of Pimonsree (1998) about the phenol adsorption onto AC, equilibrium contact times of 4 hours were same at the pH ranged between 3-9 but that of 6 hours were obtained at the pH of 11.
In this study, the pH of the control sample is 7.1. The highest COD reduction was found at pH 5.5 (See Figure 4.2). Lowering the pH of the samples will increase the positively charged (+) ions in the adsorbate, therefore, it can be assumed that the AC has more negatively charged surface areas.
Table 4.4 Effect of Initial pH in % COD Reduction
sample
pH initial COD (ppm) % COD reduction pH final
C0
7.1 81.57 7.1
C1
7.1 50.68 37.87 7.3
L1
6.5 45.38 44.00 7.1
L2
5.5 26.24 68.08 6.4
U2 8.5 47.14 42.66 7.8
Except for the initial pH of 8.5, the pH of the samples increases after the contacting of carbon. Studies have determined that the adsorptive forces in the AC are the most probable cause of pH rise. Leading scientist in the carbon industry have theorized that the pH rise effect was caused by the ash in the carbon washing off and recombining to create basic compounds that increase the pH of the effluent stream. However, if the pH rise was due to high ash content, the ash concentration of the AC should have reduced substantially after the pH rise. Studies in 1999 revealed that the ash levels were identical for the carbon both prior to start-up anf after the pH rise (www.envirotrol.com).
4.5 Kinetics of Adsorption of Organics
The COD of the samples as a function of time is presented in Table 4.5 and Figure 4.3.
Table 4.5 COD of the samples as a function of Time
Time (h) COD (ppm)
0 80.84
0.5 51.58
1 45.99
3 44.31
4 42.96
5 40.28
The rate of adsorption of organics can be described as
where –r is the rate of disappearance of COD in the liquid, [COD] is the concentration of organics in the liquid, [CODe] is the equilibrium concentration of organics in the liquid, k is the coefficient of the adsorption, and n is the order of the reaction. Substituting C for [COD], the above equation will be
Taking the natural logarithm of both sides, it will be
Setting up tangential lines along the points of the curves in figure 4.3, and solving for their slope will give the values of dC/dt. Getting the COD of these points and subtracting it by the equilibrium COD of 40.28 ppm will give the values of [C-Ce]. Using regression analysis, the corresponding slope will be equal to n. The y- intercept will be equal to ln k. Figure 4.4 shows the curve of ln [C-Ce] vs ln dC/dt and the corresponding slope and intercept. Table 4.6 shows the computed values of k, and n.
Table 4.6
The computed adsorption coefficient and order of reaction
k 0.72088 h-1
n 1.3445
From these values, the adsorption of organics can be described by the equation
4.6 Minimum amount of AC that will give satisfactory COD Reduction
Adsorption is not a chemical reaction between the media and the compounds; rather it is a physical reaction, like gravity. The forces that bind organic molecules to AC are a weak form of gravity known as Van Der Waals Forces. While there are many interpretations as to why some compounds adsorb to carbon and others do not, the fact is that activated carbon has a limited capacity for all organics compounds.
In addition, carbons with small pore size will not trap large adsorbate molecules and those, with large pores may not be able to retain small adsorbate molecules.
This is maybe the reason why a complete removal of organics was not attained in this study. Table 4.8 shows that even at using 200 mg AC, the % organic removal is only 86.71. In the optimum pH determination, at 200 mg AC, the average COD removal is only 68.08 %. Maybe the samples have high concentration of organics similar to isopropyl alcohol. Although isopropyl alcohol is infinitely soluble and is a small, simple molecule, carbon has very little capacity for it.
The capacity or “loading“ is the amount of adsorbate taken up by the adsorbent per unit mass or volume of the adsorbent. This is considered the most important attribute of the adsorbent. It dictated capital cost because it determines the amount of adsorbent required and accordingly, the volume of adsorber vessels.
As shown in Table 4.7, it was found that at pH 5.5, using 22 mg AC, the initial COD of 83.23 ppm can be lowered by 61.42 %. This gives the highest loading of 2.32 ppm organics adsorbed per mg of AC.
Table 4.7 Minimum amount that will give satisfactory COD reduction
mg AC
% COD Reduction
0
0
22
61.42
76
62.04
105
68.38
200
86.71
Saturday, April 4, 2009
V. SUMMARY AND CONCLUSIONS
The capability of the Mapecon AC to lower the COD of water samples from a part of Pasig River was studied at ambient temperature. The kinetics of adsorption of organics was also computed. The characteristics of the AC were also determined and the Iodine number was validated.
Results showed that the AC has moisture content of 6%, bulk density of 0.579 g/mL, and Iodine number of 841 ± 22.
It was determined that the contact time required to reach the concentration equilibrium is 30 minutes. The maximum adsorption can be attained at pH 5.5 compared to pH 6.5, pH 7.1, and pH 8.5. This implies that the AC has more negatively charged surface areas.
The rate of adsorption of organics onto AC can be described by the equation –r = 0.72088[C-Ce]1.3445 .
it was found that at pH 5.5, using 22 mg AC, the initial COD of 83.23 ppm can be lowered by 61.42 %
Results showed that the AC has moisture content of 6%, bulk density of 0.579 g/mL, and Iodine number of 841 ± 22.
It was determined that the contact time required to reach the concentration equilibrium is 30 minutes. The maximum adsorption can be attained at pH 5.5 compared to pH 6.5, pH 7.1, and pH 8.5. This implies that the AC has more negatively charged surface areas.
The rate of adsorption of organics onto AC can be described by the equation –r = 0.72088[C-Ce]1.3445 .
it was found that at pH 5.5, using 22 mg AC, the initial COD of 83.23 ppm can be lowered by 61.42 %
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