African Journal of Biotechnology Vol. 7 (21), pp. 3887-3891, 5 November, 2008 Available online at http://www.academicjournals.org/AJB ISSN 1684–5315 © 2008 Academic Journals
Full Length Research Paper
COD removal from industrial wastewater using activated carbon prepared from animal horns Emmanuel O. Aluyor and Olalekan A. M. Badmus Department of Chemical Engineering, University of Benin, Benin City, Nigeria. Accepted 14 August, 2008
The present study was undertaken to compare the adsorption efficiency of activated carbon prepared from animal horns (AHC), which is both a waste and a pollutant and a commercial activated carbon (CAC) with respect to uptake of the organic components responsible for the chemical oxygen demand (COD) of industrial wastewater. The adsorption process was examined in terms of its equilibria and its kinetics. The effect of pH, contact time and adsorbent dose were investigated. The most effective pH was found to be 5 for AHC and 6 for CAC. The equilibrium data for COD removal fitted the Linear, Langmuir and the Freundlich models. The mechanisms of the rate of adsorption of COD were analysed using the pseudo-second-order model. The model provided a very high degree of correlation of the experimental adsorption rate data suggesting that this model could be used in design applications. Key words: Adsorption, batch, kinetics, Langmuir, Freundlich, adsorbent. INTRODUCTION Pollution of water by organic and inorganic chemicals is of serious environmental concern. Industrial wastewater differs in characteristics from the domestic wastewater. Industrial wastewaters result from spills, leaks, and product washing and water resulting from cooling processes. The organic content of wastewater is traditionally measured using lumped parameters such as biological oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS) and total organic carbon (TOC). These parameters as such do not show any chemical identity of organic matter. In recent years, increasing awareness of the environmental impact of COD has prompted a demand for the purification of industrial wastewaters prior to discharge into natural waters. This has led to the introduction of more strict legislation to control water pollution, such as the Environmental Quality (Scheduled Wastes) Regulation 1989 in Malaysia (Quek et al., 1998). This effect is likely to be even pronounced for small and medium scale industries where profit is small and expertise on wastewater treatment is unlikely to be available.
*Corresponding author. E-mail:
[email protected] Tel: +2348023350667, +2348055657745.
A number of conventional treatment technologies have been considered for treatment of wastewater contaminated with organic substances. Among them, adsorption process is found to be the most effective method. Adsorption as a wastewater treatment process has aroused considerable interest during recent years. Commercial activated carbon is regarded as the most effective material for controlling the organic load. However, due to its high cost and about 10 - 15% loss during regeneration, unconventional adsorbents like fly ash, peat, lignite, bagasse pith, wood, saw dust, periwinkle shells, etc. have attracted the attention of several investigations and adsorption characteristics have been widely investigated for the removal of refractory materials (Pandey et al., 1985; Badmus et al., 2007; Mall et al., 1994) for varying degree of success. This study is aimed at analyzing the adsorption capacity of activated carbon prepared from animal horns on industrial wastewater effluent using a brewing industry as a case study; and also to demonstrate the use of activated carbon prepared from animal horns as an alternative media over conventional activated carbon. This paper deals with the results of the batch adsorption tests to establish adsorption isotherms and adsorption capacity of the activated carbon prepared from animal horns (AHC) for the removal of COD in wastewater.
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Table 1. Sample wastewater characterization.
MATERIALS AND METHODS The wastewater sample used was collected at the point of discharge from the industry. Materials used for sample collection were pretreated by washing the container with dilute hydrochloric acid and later rinsed with distilled water. The containers were later dried in an oven for 1 h at 110 ± 5oC and allowed to cool to ambient temperature. At the collection point, containers were rinsed with samples thrice and then filled with sample, corked tightly and taken to the laboratory for treatment and analysis. The method of analysis was consistent with the standard methods (APHA, 1985; Goltermann, 1978). The pH of the sample was measured on the site and other parameters were measured in the laboratory. Samples were stored at a temperature below 3oC to avoid any change in physic-chemical characteristics. The COD of the samples were estimated before and after adsorption giving different treatment
Parameter
8.7
BOD5 (mg/l)
112.49
COD (mg/l)
692.57
TSS (mg/l)
44
BOD, Biological oxygen demand; COD, chemical oxygen demand; TSS, total suspended solids.
Adsorption studies
95
AHC
94
CAC
93 % Removal
All the experiments were carried out at ambient temperature in batch mode. Batch mode was selected because of its relative simplicity. The batch experiments were run in different glass flask of 250 ml capacity using average speed shaker. Prior to each experiment, a predetermined amount of adsorbent was added to each flask. The stirring was kept constant for each run throughout the experiment ensuring equal mixing. The desired pH was maintained using dilute NaOH/HCl solutions. Each flask was filled with a known volume of sample having desired pH commenced the stirring. The flask containing the sample was withdrawn from the shaker at the predetermined time interval, filtered through whatmann No. 44 filter paper. The experiments were carried out under different experimental conditions.
Value
pH
92 91 90 89 88 87 0
30
60
90
120 150 180 210 240 270
Adsorbent dose (g/l).
Adsorbent dose The studies were conducted by varying the amount of adsorbent. 100 ml of wastewater sample was treated with different doses of prepared periwinkle carbon, 30 – 240 g/l. The samples were agitated for 60 min, filtered and then analyzed.
Figure 1. Effect of adsorbent dose on COD removal using activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
Contact time
moval. The result shows that optimum dosage of adsorbents for COD was 90 and 60 g/l of wastewater for activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC), respectively. About 90.62 and 94.45% removal was achieved for AHC and CAC, respectively.
These studies were conducted by agitating 100 ml sample with best adsorbent dose in and agitating it for different time period, 30 - 300 min. After the predetermined time intervals, the samples were withdrawn, filtered and analyzed. pH pH effect was performed taking a specific concentration, adsorbent dose, and contact time and varying the pH values from 1 - 8 using dilute NaOH/HCl solutions. The samples were agitated for specific time, filtered and then analysed.
RESULTS AND DISCUSSION The wastewater sample was characterized in terms of the parameters pH, COD and BOD (Table 1). Effect of adsorbent dose Figure 1 shows the effect of adsorbent dose on COD re-
Effect of pH Figure 2 depicts the effect of pH on percent removal of COD. For AHC, at pH 5, the COD removal was 90%, which then decreased as the pH was increased. The same trend was observed for CAC. About 94% COD removal was achieved for CAC at pH 6. The reason for the better adsorption capacity observed at pH 5 and pH 6 may be attributed to the larger number of H+ ions present, which in turn neutralize the negatively charged adsorbent surface, thereby reducing hindrance to the diffusion of organics at higher pH. At higher pH, the capacity of the adsorbent recessed. The reduction in adsorption may be possible due to the abundance of OH
Aluyor and Badmus
95
97
AHC
96
CAC
94
AHC
93
94
92
93
% Removal
% Removal
95
91 90
3889
CAC
92 91 90 89
89
88
88 0
1
2
3
4
5
6
7
8
87
9
0
pH
ions, causing increased hindrance to diffusion of organics contributing to COD) ions. Similar observations have also been reported by the workers (Mohan and Karthikeyan, 1997; Liskowitz et al., 1980; Mott and Weber, 1992; Pandey et al., 1985). Effect of contact time The result obtained shows that the mixing time had greater impact on COD removal (Figure 3). At an optimum time of 150 min, about 95.67% COD removal was achieved for AHC, while about 96. 4% COD removal was achieved for CAC at 180 min. The smooth and independent nature of curve indicates formation of monolayer cover of the adsorbate on the outer surface of the adsorbents. Adsorption isotherms Two important physiochemical aspects for the evaluation of the adsorption process as a unit operation are equilibria of the adsorption and the kinetics. Equilibrium studies give the capacity of the adsorbent. The equilibrium relationships between adsorbent and adsorbate are described by adsorption isotherms, usually the ratio between the quantity adsorbed and that remaining in solution at a fixed temperature at equilibrium. The equations for the three types of adsorption isotherms are expressed by:
Langmuir isotherm
x = k pCe m
C e Ce 1 = + qe qo qo k
100
150
200
250
300
Contact time (min.)
Figure 2. Effect of pH on COD removal activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
Linear isotherm qe =
50
(1)
(2)
Figure 3. Effect of contact on COD removal activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
Where Ce is the equilibrium concentration of COD in water sample (mg/l), x/m is the amount of COD adsorbed per unit of adsorbent (mg/g), qo is a constant related to the area occupied by a monolayer of adsorbate, thus reflecting the adsorption capacity (mg/g), k is a direct measure of the intensity of the adsorption process, energy of adsorption (l/mg). Freundlich isotherm log
x 1 = log k + log C e m n
(3)
Where k is the quantity of COD adsorbed in mg/g adsorbent for a unit equilibrium concentration of the sample, that is, Ce = 1 and 1/n is a measure of the adsorption intensity. For n = 1, the partition between the two phases is independent of the concentration. In this case, k kp (linear isotherm). A value of 1/n < 1 shows a normal Langmuir isotherm, while 1/n > 1 is indicative of a cooperative adsorption (Atkins, 1970; Mohan and Karthikeyan, 1997; Badmus, et al., 2007). In order to decide which type of isotherm fits better the adsorption experimental data, a plot of x/m vs Ce was made for the linear isotherm (Figure 4); Ce/qe vs Ce was made for the Langmuir isotherm (Figure 5) and log (x/m) 2 vs log Ce for the Freundlich isotherm (Figure 6). The r values (goodness fit criterion) were computed using linear regression for the three types of isotherms. The results obtained show that the three types of adsorption isotherms are best suited for adsorption of COD. The 2 coefficients of determination (r ) and the isotherm constants are given in Table 2. The high values of r – squared (> 95%) for the three isotherms indicate that the adsorption of COD could be well described by the Linear, Langmuir and Freundlich isotherms.
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Table 2. Adsorption isotherm constants and coefficient of determination for different adsorbents.
Langmuir isotherm constants 2 qo (mg/g) k (l/mg) r 6.46 0.248 0.9998 10.11 0.412 0.9999
Adsorbent AHC CAC
12
AHC
1.1
CAC
11 10
1
AHC
LOG(x/m)
x/m (mg/g)
Freundlich isotherm constants 2 k (mg/g) 1/n r 9.863 - 11.820 0.9933 13.712 - 15.576 0.9946
9 8
CAC 0.9
7 6 20
30
40
50
60
70
80
90
0.8
Ce (mg/l)
1.2
Figure 4. Adsorption isotherm of COD on activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
1.4
1.6
1.8
2
LOGCe
Figure 6. Comparison of Freundlich isotherms of activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
14 12
log (qe − qt ) = log qe −
Ce/qe
10
AHC CAC
8 6 4 2 0 20
30
40
50
60
70
80
90
Ce (mg/l)
Figure 5. Comparison of Langmuir isotherms of activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
Adsorption kinetics The study of the adsorption kinetics is the main factor for designing an appropriate adsorption system and quantifying the changes in adsorption with time requires that an appropriate kinetic model is used. In order to consider the kinetics effects, the following Lagergren pseudo-first order equation can be used to determine the rate constants (Mohan et al., 2002; Badmus et al., 2007).
K1, ad 2.303
(t )
(4)
Where qe is the amount of COD adsorbed at equilibrium (mg/g); qt is the amount of COD adsorbed at time t (mg/g); K1,ad is the pseudo-first order rate constant (min 1 ); and t is the time (min). In many cases, the Equation 4 can not be used to describe the kinetics of the adsorption process. In such cases, a pseudo-second order expression may be used and this is more appropriate for describing this type of adsorption. Hence, this model reduces to:
t 1 1 = + (t ) 2 qt K 2, ad qe qe
(5)
Where K2,ad is the pseudo–second order rate constant -1 -1 (gmg min ). In this study, second order kinetics model was considered for the adsorption process of COD. Figure 7 shows the linearised form of the pseudo-second order model for the adsorption of COD onto AHC and CAC. 2 The correlation coefficients, R , and the pseudo-second order rate parameters are shown in Table 3. The agreement between the sets of data reflects the extreme high correlation coefficient obtained and shown
Aluyor and Badmus
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Table 3. Pseudo-second order parameters and coefficient of determination for different adsorbents.
Adsorbent
qe 0.7224 11.1732
AHC CAC
Pseudo-second order equation k 3.5266 0.0499
2
r 0.9997 0.9999
50
REFERENCES
t/qt (min.g/mg)
40
30 AHC CAC
20
10 0 0
40
80
120
160
200
240
280
320
Time (min.)
Figure 7. Pseudo-second order adsorption kinetics of COD on activated carbon prepared from animal horns (AHC) and commercial activated carbon (CAC).
in Table 3. In order words, the data also show good compliance with the proposed pseudo-second order equation. Indeed, the regression coefficients for the linear plots were higher than 0.999 for both systems in these studies. Due to the correlation of the experimental results with the pseudo-second order model, the main adsorption mechanism is probably a chemisorption reaction. Conclusion The following conclusions were drawn from the present study. Activated animal horn carbon competed favourably with commercial activated carbon. The results obtained showed that AHC can be used in the removal of COD from industrial wastewaters. Trend of COD removal by AHC, 95.67%, is comparable to that of CAC with 96.34% efficiency. These results show that granular activated carbons made from agricultural waste (animal horn) can be used with greater effectiveness for organic matter removal from industrial wastewater. This would be of benefit not only to the manufacturing industry in terms of minimising cost of COD treatment, but also to minimise the impact on the environment. The adsorption data followed both Langmuir and Freundlich isotherms. In addition, a pseudo-second order kinetics appears to model the rate of adsorption.
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