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