Environ. Sci. Technol. 2008, 42, 766–770
Removal of Endosulfan and Methoxychlor from Water on Carbon Slurry V I N O D K . G U P T A * ,† A N D I M R A N A L I ‡ Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247 667, India, and Department of Chemistry, Jamia Millia Islamia (A Central University), New Delhi, 110 025, India
Received October 5, 2007. Revised manuscript received November 13, 2007. Accepted November 19, 2007.
A carbon slurry, produced in generators of fuel-oil-based industrial generators was converted into an effective and efficient adsorbent for the removal of endosulfan and methoxychlor from aqueous solution. The adsorbent was chemically treated, activated, characterized, and used for the adsorption of endosulfan and methoxychlor pesticides. The maximum adsorption was found at 90 min, 6.5 pH, 0.025 g/L dose, and 25 °C temperature. Langmuir and Freundlich adsorption models were applied to analyze adsorption data, and the former was found applicable to this adsorption system in terms of relatively high regression values. The thermodynamic aspect of the process was also investigated by evaluating certain important parameters (enthalpy, free energy, and entropy of system). Kinetics of adsorption was found to follow the pseudo second order rate equation. The diffusion of pesticides into carbon slurry pores was suggested to be the rate controlling step by applying Bangham’s equation. Adsorption on a column was also investigated in a continuous flow system. Adsorption efficiencies of endosulfan and methoxychlor were 34.11 and 36.06 mg/g in batch processes and 32.62 and 33.52 mg/g in column operations, respectively.
Introduction Organochlorine pesticides have been widely used to control pests in agriculture, forestry, and some household activities in India. But nowadays, these pesticides are banned because of their long persistence and toxicities. India is the largest consumer of pesticides in South Asian countries with 44.5% consumption of the total pesticides for the cotton crop alone (1). Substantive application of pesticides may cause accumulation in the hydrological systems (2) in food crops (3, 4). The leaching runoff from agricultural and forest lands, deposition from aerial applications, and discharge of industrial wastewater are responsible for water contamination (5). Pesticides are considered to be potential chemical mutagens and potential chronic health hazardous (6). Considerable evidence has been accumulated indicating the contamination of natural water resources globally, including India (7). Endosulfan and methoxychlor are widely found in both surface water and groundwater throughout the world, including India (7, 8). World Health Organization drinking * Corresponding author phone: +91-1332-285801; +91-1332273560; e-mail:
[email protected]. † Indian Institute of Technology Roorkee. ‡ Jamia Millia Islamia. 766
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water quality standards permits a maximum 20 µgL-1 for both endosulfan and methoxychlor (9). Therefore, effective methods of pesticide removal from water are urgently needed. But the wide ranges of pesticides or herbicides in use make research extremely difficult for producing a single method of their removal. Several methods, either independent or in conjunction with others, have been used for the removal of pesticides, and these include chemical oxidation with ozone (10), photo degradation (11), combined ozone and UV irradiation (8), biological degradation (8), and adsorption (12–20). Adsorption on activated carbon is the most widespread technology used for water treatment (21, 22). However, the commercially available carbon is not economically feasible at large scale due to its high cost, and that is why many industrial, agricultural, natural, and synthetic materials such as bagasse fly ash, carbon cloth, porous polymeric adsorbents, wheat-residue black carbon, resin, lignin, wood charcoal, waste tire rubber granules, etc. have been converted into inexpensive activated carbons for this purpose (21, 22). In view of all these facts, attempts have been made to convert carbon slurry, a fuel-oil-based generated waste material, into an inexpensive and effective adsorbent. The adsorption of endosulfan and methoxychlor has been investigated in detail on the prepared adsorbent to (i) assess its potentiality for the removal of pesticides from aqueous solutions and (ii) to elucidate the mechanisms of adsorption on this adsorbent.
Materials and Methods High purity (99.1%) endosulfan and methoxychlor were obtained from Sigma Co. (St. Louis, MO). Carbon slurry is a fuel-oil-based generated waste material; it was collected from National Fertilizer Limited (NFL), Panipat, India. Deionized water was prepared using the Millipore water purification system All reagents used in the present study were of analytical grade. A gas chromatograph (model Nucon, 5700, India) with electron capture detector (ECD) was used to detect the pesticides. The column used was equity (30 m × 0.25 mm, id) and obtained from Sigma. pH measurements were made using a pH meter (Hach, Loveland, CO). Carbon content of adsorbent was measured by Elemental CHNS analyzer model Vario EL III. I.R. spectra of the samples were recorded on a Perkin-Elmer FTIR spectrophotometer (model 1600). Deionized water was prepared using a Millipore Milli-Q (Bedford, MA) water purification system. X-ray measurements were performed by using a Philips X-ray diffractometer employing Ni-filtered Cu KR radiation and Ni filters. Preparation of Adsorbents. The raw material was in the form of small, spherical, carbonaceous, black, greasy granules. Initially, the sample of the waste slurry was treated with hydrogen peroxide to oxidize the adhering organic material and then heated to 200 °C in air until the emission of black soot was completely stopped. The heated product was then cooled and activated in air. The activation was performed by heating the sample for 1 h in a muffle furnace at 450 °C in the presence of air. The material was further treated with 1.0 M HCl solution to remove remaining ash contents and then washed with deionized water. It was dried at 100 °C for 24 h, and finally, the product was stored in vacuum desiccators until required. The yield of activated product was about ∼90%, and the cost of prepared adsorbent in India comes out to be 0.2 U.S. $/kg (including transportation, processing, and chemical treatment). GC Analysis. Remaining concentrations of endosulfan and methoxychlor were extracted from the water sample by using n-hexane. Water samples were extracted with 10 mL of n-hexane. The n-hexane layer was separated, and the same 10.1021/es7025032 CCC: $40.75
2008 American Chemical Society
Published on Web 01/04/2008
procedure was repeated three times. The three fractions of extracted n-hexane were mixed together, and a total volume of extracted n-hexane obtained was 30 mL. The combined n-hexane (30 mL) extract was concentrated to 1.0 mL using rotary evaporation apparatus and was analyzed by GC as described above. The temperatures of the column, injector and detector were 250, 280, and 300 °C, respectively. The flow of carrier gas (nitrogen) was kept constant at 2.0 mL/ min with 25 mL/min as make up gas flow. Adsorption Studies. Adsorption was determined by batch method, which permits convenient evaluation of parameters that influence the adsorption process. In the batch method, a fixed amount of the adsorbent (0.01 g) was added to 10 mL of pesticide solution of varying concentrations taken in stoppered glass tubes, which were placed in thermostat cum shaking assembly. The solutions were stirred continuously at constant temperatures for 90 min to achieve equilibration. The concentration of the pesticides in the solution after equilibrium was determined by the GC method as described above. The experiments were repeated five times, and average values were reported. The standard deviation of the experiments was (0.10, while the values of correlation coefficient were in the range of 0.9997-0.9998 with 99.3 as the confidence limit. Further, the error bars for the figures were so small as to be smaller than the symbols used to plot the graphs and, hence, not shown. Kinetic studies of adsorption were also carried out at various concentrations of the adsorbates wherein the extent of adsorption was investigated as a function of time. The pH of the solution was adjusted with concentrated hydrochloric acid and sodium hydroxide. Column Studies. The column for these studies was made of Pyrex glass (30 × 2 cm). The column on the right-hand side was attached with a manometer by the two pressure points to monitor the introduction of air in the column, if any. 250-BBS (British Standard Size) mesh wire gauge was fitted at the pressure points, which prevented entry of adsorbent particles into these points. The upper end of the column was covered with a cover containing a tube connection to remove any air bubbles. The upper end is also attached with a head tank from which the flow of wastewater is regulated. The flow of the wastewater is also controlled by the stopper point at the lower end of the column. The supporting media, glass wool, was packed in the column by hydraulic filling. The weighed adsorbent material was kept for 12 h in deionized water, and then it was used to pack the column. The column was kept undisturbed overnight for its full settlement and saturation. The flow rate was varied from 0.1 to 2.5 mL/min to achieve the maximum removal of the pesticides.
Results and Discussion Characterization of the Prepared Adsorbents. For characterization, a 1.0 g sample of the material was stirred with 100 mL of deionized water at a pH 6.8 for 2 h and left for 24 h in an airtight stoppered conical flask, and a minute lowering of pH was observed. As a result, carbon may be considered as L-type in nature. Besides, physicochemical analyses of the filtrate after 24 h were carried out and no change in water composition was observed indicating no possibility of secondary contamination due to adsorbent. The surface area of the sample, as determined by a Quantasorb surface area analyzer, was 629 m2g-1. X-ray diffraction spectrum pattern of the sample did not show any peak, thereby indicating the amorphous nature of the product. The chemical analysis of the adsorbent was carried out as per the standard procedures (23). Analysis of the product gave carbon, aluminum, and iron contents as 92.0, 0.45, and 0.6%, respectively. The loss on ignition was 7% with amounts of silica and ash negligible. The density and porosity of the sample as determined by the standard methods were 1.30 g cm-3 and 78%, respectively.
FIGURE 1. Effect of contact time on the uptake of endosulfan and methoxychlor at different initial concentration. (Temperature: 25 °C, particle size: 200–250 mesh, pH 6.5). Analysis of Pesticides. The values of retention times of endosulfan and methoxychlor were 6.8 and 15.0 min under the reported experimental conditions. The values of capacity factor (k) were 5.8 and 14.0, respectively, while the values of separation (R) and resolution (Rs) factors were 2.41 and 1.85, indicating a good resolution. The qualitative analysis of the pesticides from water was carried out by comparing their retention times and separation factors with the retention times and separation factors of standard ones. The quantitative analysis was ascertained by comparing the peak areas of sample pesticides with the peak areas of standard pesticides. The detection limit of the reported gas chromatographic method was at nano level. Effect of Contact Time and Concentration. The results of contact time (90 min) are shown in Figure 1, which indicates that the rate of uptake of each pesticide was rapid in the beginning and decreased slowly as equilibrium condition was reached. An increase in adsorption was observed from 14.73 to 22.66 mgg-1 for endosulfan and from 15.56 to 23.79 mgg-1 for methoxychlor with 1.0 × 10-6 M and 2.0 × 10-6 M initial concentration of both pesticides, respectively. The time of equilibration adsorption (maximum adsorption) was unaffected with initial concentration, but the amount adsorbed increased by increasing concentration of the pesticides. Effect of Particle Size on Adsorption. The adsorption of both the pesticides was investigated at three particle sizes 100–150, 150–200, and 200–250 BSS mesh, respectively. It was found that the adsorption capacity increases to some extent with a decrease in particle size of the adsorbent. This was only due to substantial increase in surface area (24). But it may be possible that pesticide molecules are not able to penetrate to some of the interior pores of the particles, especially when their sizes are large. The access to all pores is facilitated in small size particles, which is supported by the work of Pontius et al. (25). The maximum adsorption was achieved on 200–250 mesh particle size, and hence, all the studies were carried out with this fraction only. Effect of Adsorbent Dose. The optimization of adsorbent dose was also carried out by varying amount of adsorbent with fixed concentrations for pesticides in the experiments. The adsorbent doses were varied from 0.005 to 0.035 gL-1 at 3.0 × 10-6 M fixed concentrations of endosulfan and methoxychlor each. The results of dose–effect are shown in Figure 2, which indicate a good amount of adsorption at 0.025 g/L. Further increase of adsorbent dose resulted in very little increase in adsorption, and hence, 0.025 g/L was considered the optimum dose. VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effect of adsorbent dose on the uptake of endosulfan and methoxychlor on carbon slurry. (Temperature: 25 °C, particle size: 200–250 mesh, initial concentration of both pesticide: 3.0 × 10-6 M, pH 6.5).
FIGURE 4. Effect of temperature on the adsorption of endosulfan on carbon slurry at 6.5 pH.
FIGURE 5. Langmuir adsorption isotherm of endosulfan on carbon slurry at different temperatures. FIGURE 3. Effect of solution pH on the uptake of pesticides on carbon slurry. (Temperature: 25 °C, particle size: 200–250 mesh.) Effect of pH. To observe pH effects, the adsorption of the pesticides was studied over a pH range of 2.0-11.0 with a fixed concentration (2.0 × 10-6 M) of each pesticide and the results are plotted in Figure 3. Over the range of experimental pH, from 2.2 to 6.6, surface functional groups of carbon slurry (including carbonyl, hydroxyl, etc.) become deprotonated and the extent of deprotonation increases with an increase in pH. This deprotonation results in a less positively or more negatively charged carbon surface at higher pH than at the lower one. An increase in pH from 2.0 to 7.7 for endosulfan and from 2.0 to 6.6 for methoxychlor, resulted in a small decrease (18 and 17%, respectively) in adsorption (Figure 3). But it has been observed that with further increase in pH up to 11.0, the extent of adsorption becomes poor. As a result of exhaustive experimentation, 6.5 pH was selected as the optimum and used through out the study. Effect of Temperature. The experiments were conducted at 25, 35, and 45 °C temperatures, and the results are shown in Figure 4 for endosulfan with similar trend of methoxychlor (Figure is not given). A perusal of these isotherms indicated that maximum adsorption decreased from 34.62 to 29.51 mgg-1 and from 36.08 to 31.67 mgg-1 in case of endosulfan and methoxychlor, respectively, with an increase in temperature. Further, a comparison of adsorption at 25-45 °C for each pesticide showed that adsorption decreased with an increase in temperature, indicating that the process was apparently exothermic for both the pesticides. 768
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Modeling of Adsorption Isotherms. The results obtained from adsorption isotherms were analyzed with Langmuir and Freundlich models as discussed earlier (16). The values of regression coefficients obtained from these models were used as fitting criteria to find out these isotherms. It was found that the data followed the Langmuir equation more precisely than Freundlich calculations. Figure 5 indicates the linear plots of 1/qe vs 1/Ce for endosulfan adsorption showing applicability of the Langmuir equation. The same type of plots were obtained for methoxychlor. The values of monolayer capacity (Q0) and Langmuir constant (b) have been evaluated from the intercept, and the slopes of these plots are given in Table 1. The “b” values reflected equilibrium constant for the adsorption process, which indicated more affinity for endosulfan than for methoxychlor. Q0 shows the monolayer capacity of adsorbent for pesticides. Thermodynamic Study. To study the thermodynamics of adsorption of pesticides on carbon slurry, three basic thermodynamic parameters, free energy (∆G °), enthalpy (∆H°), and entropy (∆S °), were calculated using equations described in our earlier work (16). The values of these parameters are compiled in Table 2, which indicates negative value of free energy of pesticides related to feasibility of adsorption of each pesticide on carbon slurry. High negative values of ∆G ° were observed with a decrease in temperature (from 25 to 45 °C). This was due to less adsorption of pesticides at higher temperature. ∆H° values were calculated by considering minimum and maximum temperature studied
TABLE 1. Langmuir Constants for Adsorption of Endosulfan and Methoxychlor on Carbon Slurry b pesticides Endosulfan Methoxychlor
Qo
temp. (°C)
L · mg-1
L · mol-1(103)
mg · g-1
mol · g-1(10-4)
R2
25 35 45 25 35 45
4.31 × 10-6 3.65 × 10-6 2.99 × 10-6 12.04 × 10-6 10.6 × 10-6 9.87 × 10-6
1.71 1.46 1.20 4.14 3.65 3.40
38.10 34.04 28.94 34.91 33.67 30.50
0.90 0.84 0.70 1.00 0.95 0.86
0.980 0.975 0.970 0.997 0.992 0.992
Plots were made for Lagergren’s (not shown) and pseudo second order model for both the pesticides. The first-order model data do not fall on straight lines indicating that this model is less appropriate. The Lagergren’s first-order rate constant (k1,ads) and qe determined from the model are presented in Table 3 along with the corresponding correlation coefficients. However, qe estimated by this model differs substantially from those measured experimentally; suggesting that the adsorption is not a first-order reaction. By plotting t/q against t for the studied temperatures, straight lines were obtained in both the cases (Figures are not given) and the second-order rate constant (k2,ads) and qe values were determined from the plots and given in Table 3. Good correlation coefficients and the theoretical values of qe (Table 3) were in agreement with experimental ones suggesting a second-order kinetic model for endosulfan and methoxychlor pesticides. The kinetic data were further used to learn about the slow step occurring in the present adsorption system. The applicability of the following Bangham’s equation (31) was tested:
TABLE 2. Thermodynamic Parameters for Adsorption of Endosulfan and Methoxychlor on Carbon Slurry pesticides Endosulfan Methoxychlor
temp. (°C)
∆G° (KJ.mol-1)
25 35 45 25 35 45
-30.94 -30.47 -29.90 -33.66 -32.80 -32.04
∆H°a (KJ · mol-1)
∆S° (KJ mol-1 · K-1) 55.76 52.42 49.00 86.75 81.12 76.22
-14.31 -7.80
a ∆H° was measured between 45 and 25 °C (whole temperature range studied).
(i.e., 25 and 45 °C). Negative values were obtained due to the release of extra energy during interaction of pesticides and the adsorbent surface. As a matter of fact, ∆H° values reflected the combined effect of endothermic hydrogen bond breaking and exothermic adsorption processes, respectively. The exothermic process predominated the endothermic adsorption process, which gave rise to negative ∆H° values. Positive values of entropy (∆S°) were obtained indicating the increased randomness at the solid-solution interface during the fixation of both pesticides on the active sites of the adsorbent. Dynamic Modeling. The kinetics of adsorption is important as it controls the process efficiency. There have been several reports, where the adsorption has been treated as pseudo first order (26, 27) and pseudo second order processes (28). For evaluating the adsorption kinetics of pesticides, two models, i.e., pseudofirst-order Lagergren’s and pseudosecond-order kinetic models, were used. The pseudo first-order rate Lagergren‘s (29) model: log ( qe - q ) ) logqe -
k1, ads t 2.303
(
loglog
)
(3)
where C ′0 is the initial concentration of adsorbate in solution (mmol L-1), V is the volume of solution (mL), m is the weight of adsorbent used per liter of solution (g L-1), q′ (mM g-1) is the amount of adsorbate retained at time t, and R (
