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 27C, 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)