Environ. Sci. Technol. 2004, 38, 300-306
Bioaccumulation of Chromium from Tannery Wastewater: An Approach for Chrome Recovery and Reuse RATHINAM ARAVINDHAN, BALARAMAN MADHAN, JONNALAGADDA RAGHAVA RAO,* BALACHANDRAN UNNI NAIR, AND THIRUMALACHARI RAMASAMI Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India
The presence of chromium in the effluent is a major concern for the tanning industry. Currently, chemical precipitation methods are practiced for the removal of chromium from the effluent, but that leads to the formation of chrome-bearing solid wastes. The other membrane separation and ion exchange methods available are unfeasible due to their cost. In this study, the removal of chromium from tannery effluent has been carried out using abundantly available brown seaweed Sargassum wightii. Simulated chrome tanning solution was used for the standardization of experimental trials. Various factors influencing the uptake of chromium, viz., quantity of seaweed, concentrations of chromium, pH of the chrome-bearing wastewater, and duration of treatment, have been studied. Chemical modification of the seaweed through pretreatment with sulfuric acid, magnesium chloride, and calcium chloride showed improved uptake of chromium. Langmuir and Freundlich isotherms have been fitted for various quantities of seaweed. The dynamic method of treatment of protonated seaweed with simulated chrome tanning solution at a pH of 3.5-3.8 for a duration of 6 h gave the maximum uptake of about 83%. A similar uptake has been established for commercial chrome tanning wastewater containing the same concentration of chromium. The Sargassum species exhibited a maximum uptake of 35 mg of chromium per gram of seaweed. Fourier transform infrared spectroscopy, energy-dispersive X-ray analysis, and flame photometry studies have been carried out to understand the mechanistic pathway for the removal of chromium. The potential reuse of chromium-containing seaweed for the preparation of basic chromium sulfate (tanning agent) has been demonstrated.
Introduction Leather processing involves conversion of putrescible hide or skin into leather. Tanning agents bring about permanent stabilization of the skin matrix against biodegradation. This industry has gained a negative image in society with respect to its pollution potential and therefore is facing a severe challenge. The unit processes that cause tanners the most difficulty with regard to perceived environmental impact are unhairing and chrome tanning (1). Basic chromium sulfate (BCS) is a tanning agent, which is employed by 90% of the * Corresponding author phone: +91 44 2441 1630; fax: +91 44 2491 1589; e-mail:
[email protected]. 300
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tanning industry. Conventional chrome tanning results in wastewater containing as high as 1500-3000 ppm (parts per million) of chromium (2); however, the present day highexhaust chrome tanning methods lead to a wastewater containing 500-1000 ppm of chromium. Chromium has low, acute and chronic toxicity to humans at high doses. Chromium in its trivalent form is an essential trace element when present at the micro level (3), whereas the same when present in excess is proven to be a potential soil, surface water, groundwater, and air contaminant under specific conditions (4). BCS solution is known to contain a mixture of several molecular species of chromium varying in charge structure and reactivity (5). The currently used BCS salts and methods show an uptake of 60-70% of the chromium used for tanning (6). This lower uptake was attributed to high kinetic lability and poor thermodynamic affinity of tetra-positive tetrameric chromium species present in the BCS (4). The uptake behavior of chrome is influenced by both the manufacturing conditions of BCS and the tanning conditions employed (7). Although the oxidation state of chromium in the BCS is only trivalent, discharge norms do not specify the redox states, because of concerns about possible conversion of the trivalent state to the more toxic hexavalent form (8, 9). This has called for technological interventions for skirmishing chromium pollution. Recovery of the chromium present in tannery wastewater is necessary for environmental and economic reasons. At present, two methods of chromium recovery are practiced. The first is a direct recycling approach, which involves filtration of the waste liquor followed by chemical replenishment (10). The second method is the precipitation of chromium in the waste liquor as chromic trihydroxide, filtration, and subsequent dissolution in sulfuric acid to form BCS solution for reuse in the tanning process (11). Both the methods offer significant advantages and disadvantages depending on the volume of production of the tanning industry (12). Other methods followed for the removal or recovery of chromium from other industrial wastewaters are chemical reduction followed by precipitation, ion exchange (13), membrane technologies (14), and adsorption by several types of adsorbents, such as activated carbon, bone charcoal, and waste activated sludge (15, 16). Most of these materials and methods suffer from drawbacks such as high capital or operational costs. Therefore, there is a need for the development of a methodology with low-cost, easily available materials by which chromium can economically be removed from tannery wastewaters. The use of low-cost natural resources for the removal of chromium is being looked upon by researchers in preference to the other conventional methods. The application of water hyacinth weeds, treated sawdust, brown seaweed biomass, and coconut shell as adsorbents for removal of chromium from industrial wastewaters was reported earlier (17-19). The potency for accumulating metal ions by certain types of dead biomass was well established over the last two decades. Biological materials, both living and dead, are capable of removing heavy metal ions from solutions through a process involving a number of diverse mechanisms collectively known as biosorption (20). This work aims at the bioremoval of chromium from the tannery effluent by commonly available seaweed, Sargassum wightii. The present study also aims to develop a suitable methodology to maximize the uptake of chromium by the seaweed from the tannery effluent and also to find a suitable means to utilize the treated seaweed containing chromium. 10.1021/es034427s CCC: $27.50
2004 American Chemical Society Published on Web 11/14/2003
Materials and Methods Beach-dried brown seaweed S. wightii Greville was obtained from the Department of Algology, Centre for Advanced Studies, Madras University, Chennai. The beach-dried seaweed was washed with distilled water thoroughly to remove sand and other debris materials. The washed seaweed was then shade-dried and stored in an airtight pack to prevent moisture absorption. The moisture content of the dried seaweed was estimated and was found to be 5 ( 1%. Doubledistilled water was used throughout the work. Commercial BCS supplied by Golden Chemicals Ltd., India, was used for the preparation of synthetic chrome tanning wastewater. All other reagents used were of analytical grade. Preparation of Synthetic Chrome Tanning Solution. A stock solution containing 1000 ppm of chromium was prepared using commercial BCS. The pH was adjusted to be 3.0-3.5, and the solution was aged for 12 h and stored in a cool room at 4 °C. For further experiments, the requisite quantity was taken from the stock solution and made up to the required volume. The pH of the synthetic chrome solution was adjusted accordingly by using either 0.1 N H2SO4 or 0.1 N NaOH. pH versus Uptake. The effect of pH on chromium uptake was studied using a synthetic chromium solution of 150 ppm chromium concentration. The pH of the simulated wastewater was adjusted to 2.0, 3.5, and 4.5. The solution at the respective pH was treated with the protonated seaweed for a period of 6 h in a rotary mechanical shaker. The chromium content in the supernatant liquor was estimated by standard procedure (21). Pretreatment of Seaweed. The seaweed was subjected to chemical pretreatment prior to treatment with synthetic chromium solution in order to increase the uptake efficiency. The air-dried seaweed was treated with 0.4 N sulfuric acid, a 0.25 g/L solution of calcium chloride, and a 0.25 g/L solution of magnesium chloride for a period of 10 min under stirring. The acid-, calcium-ion-, and magnesium-ion-treated seaweeds were then washed twice with double-distilled water. Finally, the washed materials were air-dried and kept in an airtight pack for further experiments with chromium. The pretreated seaweed was used to study the effect of pretreatment upon time of treatment, quantity of seaweed, pH, and initial concentration of chromium. Experiments with Pretreated Seaweed. Time versus Uptake. To optimize the time required for maximum uptake of chromium, five experimental trials were carried out for each pretreated sample at varying time intervals, between 15 min and 6 h. A 50 mL aliquot of 150 ppm simulated chromium effluent was taken in a 100 mL conical flask for each trial. The pH of the synthetic chrome effluent was maintained at 3.5-3.8. Pretreated seaweed (500 mg) was added to each flask and agitated in a rotary mechanical shaker. The amount of chromium remaining in the solution was determined using standard procedure (21). Quantity of Seaweed versus Uptake. To evaluate the effect of the quantity of pretreated seaweed upon the uptake of chromium from synthetic chromium solution, about 50 mL of synthetic chromium solution containing 150 ppm of chromium at pH 3.5-3.8 was taken in five different conical flasks of 100 mL capacity. Seaweed (0.25, 0.5, 0.75, 1.0, and 2.0 g, respectively) was added to each flask. Subsequently the flasks were agitated in the rotary mechanical shaker for a period of 6 h, after which the chromium content in the supernatant liquor was estimated by standard procedure (21). Initial Concentration of Chromium versus Uptake. To study the effect of the initial chromium concentration upon the uptake of the chromium by pretreated seaweed, the following experiments were carried out. Six conical flasks were taken to which 50 mL of 150, 300, 400, 500, 750, and 1000 ppm
synthetic chromium solution was added. The pH of the solution was maintained at 3.5-3.8, and the flasks were agitated in a rotary mechanical shaker for a period of 6 h after which the chromium content in the supernatant liquor was estimated by standard procedure (21). Equilibrium Studies. Generally, adsorption isotherm equations are derived on the basis of the relation between the amount of the solute (metal ion) in the solution and the amount that is adsorbed onto the solid phase when the two phases are in equilibrium, at a given temperature and pressure. An adsorption isotherm was used to characterize adsorption capacity or the maximum uptake capacity of the given material. The results of experimental measurements can be expressed in the form of adsorption isotherms. In the present work, two types of isotherms, viz., Freundlich and Langmuir adsorption isotherms, have been tested for the validity of the metal uptake behavior of the seaweed. Adsorption isotherms were determined by the treatment of 0.5 g of protonated seaweed with 50 mL of chromium effluent at concentrations of 150, 300, 400, 750, and 1000 ppm for a contact time of 6 h in a mechanical agitator. After agitation, the contents of the flasks were decanted. The concentration of chromium remaining in the solution was determined by standard procedure (21). The experiments were repeated using 0.25 and 1.0 g of seaweed for treatment. Studies on the Mechanism of Accumulation of Chromium by Seaweed. To gain mechanistic insights into the bioremoval of chromium by the seaweed, the seaweed before and after treatment with chromium solution was characterized using several techniques, such as Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), flame photometry, and UV-vis spectrophotometry. The FT-IR spectra of untreated, pretreated, and chromiumtreated seaweed were obtained using the KBr disk technique. The seaweed was ground in a mortar for 5 min after drying it for a period of 2 h at 80 °C. Dilution and homogenization to 0.01% (w/w) with KBr (spectroscopic grade) were carried out with additional grinding. The disks were pressed in a hydraulic KBr press. The transmission FT-IR spectra were then recorded between 400 and 4000 cm-1 using a PerkinElmer Spectrum RX I FT-IR system. Sample specimens were coated with gold using an Edwards E306 sputter coater. An INCA 200 energy-dispersive X-ray microanalyzer equipped with a LEO-stereo scan 440 scanning electron microscope was used for the analysis. The EDAX graphs of seaweed, for both untreated and metalloaded samples, were obtained by operating the EDAX apparatus. The uptake of chromium by seaweed was analyzed using standard procedure (21). The seaweed was digested using concentrated nitric acid and made up to a known volume. The made-up solutions were analyzed for other metal ion constituents (calcium, potassium, and magnesium) present in untreated, pretreated, and chromium-treated seaweed using flame photometry. An ELICO CL 22D flame photometer was used for the measurement of metal ion contents in the seaweed. Utilization of Chromium-Treated Seaweed. It is not sufficient to identify and develop technology for the removal of chromium. Proper disposal and utilization of the metalbearing seaweed is as important as the removal of chromium from the effluent liquor. Hence, desorption studies were carried out for the removal of chromium from the seaweed for a possible reuse of seaweed. Studies were also carried out to find utility for the chromium-containing seaweed as a reductant in the manufacture of BCS. Experimental trials were carried out using 1 g of chromeloaded seaweed treated with 50 mL of 1, 2, 3, and 4 N sulfuric acid for a period of 3 h in a rotary mechanical shaker. After VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Uptake of chromium from simulated chrome tanning solution by Sargassum seaweed at various pH values. 3 h, the chromium concentration in the seaweed was estimated by standard procedure (21).
Results and Discussion The amount of seaweed used for the chromium uptake studies was based on the dry weight of the same. The pH of simulated chromium solution prepared using the commonly used BCS tanning salt was found to be around 3.5. pH versus Uptake. The removal of chromium from synthetic chrome effluent with seaweed is highly dependent on the pH of the solution, which affects the uptake capacity of the seaweed. The pH can significantly influence the bioremoval behavior of heavy metals, and there will be an optimum pH for maximum uptake, below or above which a decrease in uptake could occur. Hence, experiments have been carried out to study the uptake of chromium by seaweed at varying pH of the synthetic chromium solution (150 ppm). The chromium uptake (as milligrams of chromium per gram of seaweed) by the seaweed at different pHs is shown in Figure 1. It is noticed from the graph that the chromium uptake capacity of seaweed is very low at a pH of 2.0. A maximum chromium uptake of 12.2 mg/g of Sargassum species occurs at a pH of around 3.5. With a further increase in pH to about 4.5, the chromium uptake of seaweed decreases to about 10.7 mg/g of Sargassum seaweed. The bioremoval of chromium from aqueous solution by seaweed is more efficient at a pH around 3.5-3.8. The efficiency of uptake decreases as the pH is shifted above or below this value. The investigation at pH values above 4.5 was not possible since chromium precipitation occurred. These results show the suitability of the seaweed for treatment of chrome-bearing wastewater generated from the commercial chrome tanning stream. Thus, for further experiments a pH range of 3.5-3.8 was maintained. Pretreatment of Seaweed to Improve the Uptake Efficiency of Chromium. Earlier, Volesky et al. (22) showed that pretreatment of brown seaweed with protons and calcium ions resulted in an increase of chromium uptake by seaweed. Hence, in the present study the Sargassum species has been pretreated with sulfuric acid, calcium chloride, and magnesium chloride, respectively, before treatment with simulated chrome tanning solution. Experiments with Pretreated Seaweed. Time versus Uptake. The effect of contact time on percentage uptake of chromium by several pretreated Sargassum samples with the chrome tanning solution, with agitation, is shown in Figure 2a. It is clearly observed that uptake of chromium by Sargassum is saturated after 6 h for all pretreatments. The maximum percentage chromium uptake for protonated, 302
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FIGURE 2. (a) Effect of time of treatment on the percentage uptake of chromium for different pretreated seaweeds: (9) protonated, (b) calcium treated, and (2) magnesium treated. (b) Effect of time of treatment on the maximum uptake capacity of different pretreated seaweeds: (9) protonated, (b) calcium treated, and (2) magnesium treated. calcium-treated, and magnesium-treated seaweed is found to be 83, 80, and 79%, respectively. The graph shown in Figure 2b represents the amount of chromium taken up by Sargassum seaweed. It could be observed from the figure that 12.2, 11.7, and 11.6 mg of chromium are being accumulated per gram of protonated, calcium-treated, and magnesiumtreated seaweed, respectively, after 6 h of treatment. Quantity of Seaweed versus Uptake. The effect of the amount of seaweed on the percentage uptake of chromium for the pretreated seaweed is shown in Figure 3. The amount of seaweed used for the treatment studies is an important parameter as it determines the potential of the material to accumulate chromium for a given initial concentration of the solution. The simulated chrome tanning solution containing effective chromium of 7.5 mg in 50 mL was treated with varying quantities of seaweed (0.25-2.0 g). The results as seen from Figure 3 demonstrate that an increase in the amount of seaweed increases the percentage uptake of chromium. When the quantity of protonated Sargassum species is increased from 0.25 to 0.5 g, the uptake increases from 71 to 84%. With a further increase in the quantity of protonated seaweed, the corresponding increase in the observed uptake of chromium drops off, leveling off at a maximum of 87% uptake with 2 g of protonated seaweed. Calcium- and magnesium-treated seaweed revealed a trend similar to that of the protonated seaweed. However, they showed a lower percentage uptake of chromium of 78 and 79% for calcium- and magnesium-treated seaweed, respectively. Initial Concentration of Chromium versus Uptake. The effect of the initial concentration of chromium on the
FIGURE 3. Percentage uptake of chromium at pH 3.5 at various concentrations of the seaweed for the three pretreated seaweeds: (9) protonated, (b) calcium treated, and (2) magnesium treated.
FIGURE 5. Effect of the quantity of protonated seaweed used on Freundlich adsorption isotherms: (9) using 0.25 g of seaweed, (b) using 0.5 g of seaweed, and (2) using 1.0 g of seaweed.
TABLE 1. Comparison of Percentage Uptake of Chromium by Different Pretreated Seaweeds from 150 ppm Synthetic Chrome Tanning Liquor pretreatments
% uptakea
pretreatments
% uptakea
H2SO4 treated Ca(Cl)2 treated
83 79
MgCl2 treated untreated
78 67
a
6 h treatment in a mechanical agitator.
TABLE 2. Freundlich Parameters for Chromium Adsorption onto Protonated Seaweed
FIGURE 4. Effect of the initial concentration of chromium in effluent on the maximum uptake of chromium for various pretreated seaweeds: (9) protonated, (b) calcium treated, and (2) magnesium treated. maximum uptake capacity of seaweed is shown in Figure 4. Maximum uptake in terms of percentage removal of chromium has been found to be higher at a lower concentration of the effluent liquor (150 ppm), which exhibited a maximum of 83, 79, and 79% removal of chromium for protonated, calcium-treated, and magnesium-treated seaweed, respectively, at an initial effluent concentration of 150 ppm of chromium. As the concentration increases, the percentage uptake capacity of seaweed is found to decrease. However, the effective amount of chromium taken up by the seaweed in terms of milligrams of chromium per gram of seaweed increases with increasing initial concentration of chromium, which is shown in Figure 4. For an initial concentration of 1000 ppm, the maximum uptake capacity of seaweed is 35.2, 34.6, and 32.6 mg/g of protonated, CaCl2-treated, and MgCl2treated Sargassum seaweed, respectively. This clearly indicates that the accumulation of chromium by the seaweed is driven by the concentration of chromium in the effluent liquor. Standardization of Pretreatment. The uptake of chromium by the protonated seaweed has been found to be better than that of other pretreated seaweeds. It is clearly seen from Table 1 that pretreatment of seaweed follows the order sulfuric acid > calcium chloride > magnesium chloride > untreated seaweed for the uptake of chromium. Hence, seaweed pretreated with sulfuric acid (protonated) has been chosen for the equilibrium studies.
quantity used
K
1/n
correlation coefficient (R 2)
0.25 0.5 1.0
0.764 67 1.474 42 0.782 25
0.422 68 0.321 2 0.361 98
0.999 0.998 0.894
Equilibrium Studies. Freundlich Adsorption Isotherms. The values obtained from the experiments have been analyzed using the Freundlich equation. In the Freundlich adsorption isotherm, the model assumes that the adsorbent consists of a heterogeneous surface composed of different classes of adsorption sites. The Freundlich constants n and k were obtained from the linear regression analysis of the equation
log q ) log k + 1/n log Ce
(1)
where q is the maximum uptake capacity and Ce is the equilibrium concentration. A plot of log q versus log Ce should give a straight line with a slope of 1/n and an intercept of log k. The values of n and k were calculated from the slope and intercept of the plots shown in Figure 5. A linear relation was observed among the plotted parameters at different quantities of seaweed used, which indicates the applicability of the Freundlich equation. The k and 1/n values and the correlation coefficients (R2) for the adsorption of chromium ion from the synthetic chrome effluent for different quantities of the protonated Sargassum seaweed are given in Table 2. The value of k, which is the representation of the capacity of seaweed to adsorb chromium, increased from 0.764 67 to 1.474 42 when the quantity of seaweed increased from 0.25 to 0.5 g. At a higher dosage, when 1 g of seaweed was used, the capacity decreased to 0.782 25. Thus the k value indicates that 0.5 g of seaweed used as the initial dosage showed a VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Effect of the quantity of protonated seaweed used on Langmuir adsorption isotherms: (9) using 0.25 g of seaweed, (b) using 0.5 g of seaweed, and (2) using 1.0 g of seaweed.
TABLE 3. Langmuir Constants for Chromium Adsorption onto Protonated Seaweed quantity
q0 (mg/g)
b (L/mg)
correlation of deviation (R 2)
0.25 0.5 1.0
81.7 38.0 21.1
0.024 0.063 0.108
0.972 0.990 0.999
TABLE 4. Relation between RL Value and the Type of Isotherm
better uptake behavior. This was further substantiated by the value of 1/n, the representation of the affinity of chromium toward seaweed. When 1/n is lower, affinity is better, and vice versa. Table 2 clearly shows that the 1/n value is lower when 0.5 g of seaweed was used for the experiment. Hence, when 0.5 g of seaweed was used, a better uptake and a better affinity of chromium were obtained. Langmuir Adsorption Isotherms. The Langmuir adsorption isotherm model assumes that the adsorbed layer will be only one molecule thick. All sides of the absorbent will have equal affinities for molecules. Thus, the presence of adsorbed molecules at one side will not affect the adsorption of molecules at an adjacent side. Langmuir constants q0 and b can be determined from the linear plot of Ce/q versus Ce, which has a slope of 1/q0 and an intercept of 1/q0b. The linear form of the Langmuir plot is given as
Ce/q ) 1/q0b + 1/q0Ce
(2)
The constant q0 signifies the adsorption capacity (mg/g), and b signifies the energy of adsorption. Ce is the equilibrium concentration of the chromium ion. The values q0 and b along with the correlation coefficient (R2) are given in Table 3. Figure 6 shows the Langmuir plots for protonated Sargassum species. A linear relation is observed among the plotted parameters, which indicates the applicability of the Langmuir model. From the table, the sorption capacity (q0) value of Sargassum is 82 mg/g when 0.25 g of seaweed was used. The values decrease as the dosage of seaweed increases to 0.5 and 1.0 g. A higher value of b implies a strong binding of chromium ions with the seaweed. The essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant, separation factor or equilibrium parameter RL, which is defined as
RL ) 1/(1 + bC0)
(3)
where b is the Langmuir constant (L/mg) and C0 is the initial concentration of chromium ion (mg/L). RL values between 304
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FIGURE 7. (a) EDAX spectrum of Sargassum seaweed (total counts ) 120). (b) EDAX spectrum of Sargassum seaweed treated with synthetic chromium solution (total counts ) 420).
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RL value
type of isotherm
RL value
type of isotherm
R>1 R)1
unfavorable linear
R)0 0
