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Biosorption of aqueous chromium(VI) by Tamarindus indica seeds
Bioresource Technology 97 (2006) 949–956
Biosorption of aqueous chromium(VI) by Tamarindus indica seeds G.S. Agarwal a, Hitendra Kumar Bhuptawat b, Sanjeev Chaudhari
b,*
a
b
MP Public Health Engineering Department, Bhopal, MP, India Centre for Environmental Science and Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India Received 28 January 2005; received in revised form 6 April 2005; accepted 8 April 2005 Available online 16 June 2005
Abstract The effectiveness of low cost agro-based materials namely, Tamarindus indica seed (TS), crushed coconut shell (CS), almond shell (AS), ground nut shell (GS) and walnut shell (WS) were evaluated for Cr(VI) removal. Batch test indicated that hexavalent chromium sorption capacity (qe) followed the sequence qe(TS) > qe(WS) > qe(AS) > qe(GS) > qe(CS). Due to high sorptive capacity, tamarind seed was selected for detailed sorption studies. Sorption kinetic data followed first order reversible kinetic fit model for all the sorbents. The equilibrium conditions were achieved within 150 min under the mixing conditions employed. Sorption equilibria exhibited better fit to Freundlich isotherms (R > 0.92) than Langmuir isotherm (R 0.87). Hexavalent chromium sorption by TS decreased with increase in pH, and slightly reduced with increase in ionic strength. Cr(VI) removal by TS seems to be mainly by chemisorption. Desorption of Cr(VI) from Cr(VI) laden TS was quite less by distilled water and HCl. Whereas with NaOH, maximum desorption achieved was about 15.3%. When TS was used in downflow column mode, Cr(VI) removal was quite good but head loss increased as the run progressed and was stopped after 200 h. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Chromium(VI); Adsorption; Tamarindus indica; Low cost biosorbent
1. Introduction Heavy metals such as chromium, copper, lead, cadmium, etc., in wastewater are hazardous to the environment. Because of their toxicity, their pollution effect on our ecosystem presents a possible human health risk (Nourbakhsh et al., 1994). In recent years, increasing awareness of water pollution and its far reaching effects has prompted concerted efforts towards pollution abatement. Among the different heavy metals, chromium is a common and very toxic pollutant introduced into natural waters from a variety of industrial wastewaters (Donmez and Aksu, 2002). The two major sources of contamination are electroplating, metal finishing indus-
*
Corresponding author. Tel.: +91 22 25767855; fax: +91 22 25723480. E-mail address: [email protected] (S. Chaudhari). 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.04.030
tries (hexavalent chromium) and tanneries (trivalent chromium). Chromium occurs most frequently as Cr(VI) or Cr(III) in aqueous solutions (Dakiky et al., 2002). Both valences of chromium are potentially harmful but hexavalent chromium poses a greater risk due to its carcinogenic properties (Dakiky et al., 2002). Hexavalent chromium, which is primarily present in the form 2 of chromate ðCrO2 4 Þ and dichromate ðCr2 O7 Þ, poses significantly higher levels of toxicity than the other valency states (Sharma and Forster, 1995). Conventional methods for removing Cr(VI) ions from industrial wastewater include reduction (Kim et al., 2002), reduction followed by chemical precipitation (Ozer et al., 1997), adsorption on the activated carbon (Lotfi and Adhoum, 2002), solvent extraction (Mauri et al., 2001) cementation, freeze separation, reverse osmosis (Padilla and Tavani, 1999), ion-exchange (Rengaraj et al., 2003) and electrolytic methods (Namasivayam and Yamuna, 1995). These methods have found
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limited application because they often involve high capital and operational costs. Adsorption is an effective and versatile method for removing chromium. Natural materials that are available in large quantities, or certain waste products from industrial or agricultural operations, may have potential as inexpensive sorbents. Due to their low cost, after these materials have been expended, they can be disposed of without expensive regeneration. Most of the low cost sorbents have the limitation of low sorptive capacity and thereby for the same degree of treatment, it generates more solid waste (pollutant laden sorbent after treatment), which poses disposal problems. Therefore, there is need to explore low cost sorbent having high contaminant sorption capacity. Several recent publications utilized locally available adsorbents e.g. fly ash, peat, microbial biomass (Bai and Abraham, 2003; Nourbakhsh et al., 1994) and agricultural byproducts (Bailey et al., 1999) for heavy metal removal. However, the literature is still insufficient to cover this problem and more work and investigations are needed to deal with other locally available and cheap adsorbents to eliminate Cr(VI) from industrial compositions and characteristics. Tamarind (Tamarindus indica) is a common tree in tropical countries. It is grown mainly for its sour fruits pulp. Tamarind seed, a by product of the tamarind pulp industry, is an underutilized or waste material (Bhattacharya et al., 1997). Literature survey by the author in most of the peer reviewed journals indicated that adsorption study of Cr(VI) with Tamarind seed as an adsorbent has not been investigated and this is the first such study undertaken by the authors. This study aims at comparative evaluation of agrobased materials, namely T. indica (tamarind) seed (TS), almond shell (AS), ground nut shell (GS), walnut shell (WS), and crushed coconut shell (CS) for removal of Cr(VI) from simulated industrial wastewater. Based on their efficacy, tamarind seed was selected for further study. The effect of pH, contact time, adsorption equilibria and temperature were investigated. Desorption of Cr(VI) from Cr(VI) laden TS was also attempted. Further continuous wastewater treatment was done in column mode to assess the viability of TS for continuous operation.
tion. The pH of solution was maintained using acetate and sodium tetra borate buffer. The initial Cr(VI) concentration was maintained by adding stock solution of chromium. 2.1. Preparation of biosorbents The sorbents used were crushed coconut shell, almond shell, ground nut shell, walnut shell and tamarind seed. All the materials were obtained from local market; materials were washed, dried and then pulverized in pulverizer and air-dried in the sun for two days. After drying, the materials were kept in air tight plastic bottles. All the materials were used as such and no pretreatment was given to the materials. The particle size was maintained in the range of 212–300 lm (geometric mean size: 252.2 lm). 2.2. Screening of biosorbents For the comparative evaluation of different sorbents for Cr(VI) removal capacity, the experiments were conducted in 150 ml double stoppered polyethene bottles at 28°C on a rotary shaker at 70 rpm. Experiments were conducted at pH 4, 6 and 9 and initial Cr(VI) concentration of 10 mg/l was maintained. After 4 h of mixing, the samples were filtered through Whattman No. 42 filter paper, and filtrates were analysed for residual chromium concentration. 2.3. Sorption kinetics Sorption studies were conducted in 150 ml polyethene bottles at 27–28 °C on a rotary shaker at 70 rpm. The chromium concentration was maintained by adding appropriate quantity of stock solution of K2Cr2O7. The ionic strength of the aqueous phase was adjusted by adding appropriate quantity of KNO3. The sample bottles were taken out at different time intervals and samples were filtered through Whattman No. 42 filter paper. The filtrates were analysed for residual chromium concentration and pH. Sorption experiments were conducted for TS, GS, WS at pH of 4, 6 and 9 for initial chromium concentration of 10 mg/l. 2.4. Sorption equilibria
2. Methods All the chemicals used were of analytical grade. Cr(VI) concentrations in aqueous phase were estimated by diphenyl carbazide method as per standard methods (APHA, 1985). AIMIL colorimeter with green filter (540 nm) was used for colorimetric measurement. The Cr(VI) loadings on sorbents were computed based on mass balance through loss of metal from aqueous solu-
Batch sorption experiments were conducted in 150 ml polyethene double stoppered bottles at 28 °C on a rotary shaker (70 rpm) for mixing. The pH values were adjusted using acetate buffer or sodium tetra borate buffer before addition of biosorbent and were maintained throughout the experiments. The sorption equilibria experiments were performed for different pH conditions and for different sorbents. The aqueous solution was mixed till equilibrium time of 180 min. After equilibrium
G.S. Agarwal et al. / Bioresource Technology 97 (2006) 949–956
time, the sample bottles were taken out for the analysis of residual chromium concentration in the solution.
3. Results and discussion 3.1. Screening of biosorbents Preliminary experiments were conducted with initial Cr(VI) concentration of 10 mg/l and biosorbents dose of 2 g/l at different pH. The results are presented in Fig. 1. At pH 4, the Cr(VI) removal was 30%, 36%, 35%, 40%, and 80% for CS, AS, GS, WS, and TS respectively, which at pH 6 decreased to 22%, 24%, 23%, 27% and 64%. It further decreased at pH 9. These preliminary results indicated the higher Cr(VI) removal capacity of tamarind seeds in the pH range of 4–9, as compared to other biosorbents. The sequence of Cr(VI) removal was TS > WS > AS > GS > CS. Though percentage removal of Cr(VI) decreased with increase in pH, but the sequence remained same. 3.2. Sorption kinetics To evaluate sorption as a unit operation, it requires consideration of two important physico-chemical aspects of the process, the kinetics and the equilibria of sorption. Kinetics of sorption describing the solute uptake rate, which in turn governs the contact time, is one of the important characteristics defining efficiency of sorption. The study of the equilibrium established in any liquid–solid system is important in determining distribution of the solute between the solid and liquid phases and determining feasibility and capacity of the sorbent for sorption. The rate at which dissolved heavy metal ions are removed from dilute aqueous solution by solid sorbents
is a significant factor for application in water quality control. The rate at which sorption proceeds is important in terms of the contact time to be provided between the solution and the sorbent. The capacity of the sorbent for the uptake of sorbate i.e., the position at which equilibrium is attained determines the useful life of sorbent to a large extent. Fig. 2 shows the adsorption of Cr(VI) by tamarind seed, walnut shell, almond shell and groundnut shell biosorbents as a function of time at pH of 4.0–4.25 for initial Cr(VI) concentration of 10 mg/l. Tamarind dose of 2 g/l was used while 3 g/l dose of other sorbents were used. Fig. 2 shows rapid adsorption in the initial 20 min for all biosorbent. The initial high adsorption rate decreased gradually as the equilibrium approached. Equilibrium time was approximately 60 min for groundnut, while for walnut and tamarind seed, it was 120 and 150 min respectively. Cr(VI) removal was found to be 90% at 60 min, 95% at 120 min and 98% at 180 min for tamarind seed. The effect of pH on the rate of Cr(VI) sorption was investigated at different pH for tamarind seed as indicated in Fig. 3. It is interesting to note that rate of sorption is more at lower pH. This may be due to difference in the speciation of Cr(VI) and pore density (morphology) of sorbents at different pH. The likely effect of pH on Cr(VI) adsorption has been discussed under Section 3.4. The rate constant for the sorption of Cr(VI) on tamarind seed was evaluated using the Lagergen equation (Orhan and Buyukgungor, 1993; Selvaraj et al., 2003) lnð1 U ðtÞÞ ¼ K a t
where U ðtÞ ¼ is called the fractional attainment of equilibrium. qe and q are the amount of Cr(VI) adsorbed (mg/g) at equilibrium and at time t, respectively, ka is the first order rate constant and t is the time (min). The plots
140
pH = 4
60
pH = 6
pH = 9
Walnut shell Almond shell
120
GMS = 252.2 µ; Initial Cr(VI) conc. = 10 mg/l; Sorbent dose = 2 g/l; Temp. = 26-27 °C;
% of Cr(VI) sorbed
Fraction Sorbed (%)
70
ð1Þ
q qe
90 80
951
contact time = 3 hrs 50 40 30
Groundnut shell Tamarind seed
100 80
60 40
20 20
10 0
0
CS
AS
GS Sorbents
WS
TS
Fig. 1. Comparative evaluation of different biosorbents.
0
50
100
150
200
250
300
350
Time (min)
Fig. 2. Sorption kinetics of Cr(VI) for TS, AS, GS, and WS.
400
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measure of adsorption capacity and (1/n) is a measure of adsorption intensity. The linear form of the Langmuir adsorption isotherm is represented as
120 pH = 4
Sorbent dose = 2 g/l
100
% of Cr(VI) sorbed
pH = 9
pH = 6
Ce 1 Ce þ ¼ Q0 b Q0 qe
80
60
40
20
0 0
100
200
300
400
Time (min)
Fig. 3. Sorption kinetics of Cr(VI) at different pH for tamarind seed.
of ln(qe q) versus t were linear (not shown) for the biosorbents and indicated that the adsorption can be approximated to first order reversible kinetics. The rate constants determined from the slopes of the plots were obtained as TS: 0.0767 min1, GS: 0.0433 min1, WS: 0.0142 min1, AS: 0.0135 min1. 3.3. Adsorption isotherms The adsorption data were fitted by least square method to linearly transformed Freundlich and Langmuir adsorption isotherms. The linearised Freundlich equation is mentioned below. log qe ¼ log K f þ
1 log C e n
ð2Þ
where qe is the amount of Cr(VI) adsorbed per unit mass of biosorbent (mg/g), Ce is the residual concentration of Cr(VI) in solution (mg/l), Kf is a constant which is a
ð3Þ
where Ce is the equilibrium concentration (mg/l), and qe is the amount of Cr(VI) adsorbed per gram at equilibrium (mg/g). Q0 (mg/l) and b (l/mg, i.e., l of adsorbent per mg of adsorbate) are Langmuir constants related to the adsorption capacity and energy of adsorption, respectively. The data obtained from the adsorption experiments conducted at 29 °C were fitted to Eqs. (2) and (3), linear plot (not shown) were obtained for log(qe) versus log(Ce) and Ce/qe versus Ce. The isotherm parameters for both equation along with the values of coefficient of correlation (R) and standard error of estimate (r) are presented in Table 1. Table shows that data better fits to Freundlich equation than Langmuir equation, which is indicated from the higher values of R and lower values of r. 3.4. Effect of pH on sorption Aqueous phase pH governs the speciation of metals and also the dissociation of active functional sites on the sorbent. Hence, metal sorption is critically linked with pH. Not only different metals show different pH optima for their sorption but may also vary from one kind of biomass to the other (Tiwari et al., 1995; Ucun et al., 2002). Fig. 4 shows the sorption isotherms at different pH. It can be observed from the figure that the uptake of Cr(VI) decreases with increase in pH. In general, the Cr(VI) adsorption by different biosorbents have shown similar trend and the optimum pH 2 has been reported (Donmez and Aksu, 2002; Dakiky et al., 2002; Selvaraj et al., 2003; Yu et al., 2003; Ucun et al., 2002; Hu et al., 2003; Gupta et al., 2001). The highest sorption
Table 1 Estimated Freundlich and Langmuir isotherm parameters and relevant statistical information for Cr(VI) sorbents system Sorbent
pH
Ionic strength
Initial solute concentration C0 (mg/l)
Sorbent dose (g/l)
Freundlich isotherm Kf
1/n
r
R
1/Q0
1/bQ0
r
R
0.170 0.06
2.852 5.100
0.1173 0.1694
0.9879 0.9915
Langmuir isotherm
GS
4.0 4.0
0.01 0.01
10 10
3 5
0.3239 0.1978
0.8730 1.0541
0.0648 0.0870
0.9759 0.9639
WS
4.0 4.0
0.01 0.01
10 10
3 5
0.9866 1.0873
0.4392 0.2924
0.0667 0.0307
0.9550 0.9790
0.4384 0.05421
0.341 0.2137
0.1629 0.1038
0.9168 0.9214
AS
4.0 4.0
0.01 0.01
10 10
3 5
0.6134 1.3326
0.6020 0.3022
0.1025 0.0387
0.9200 0.9716
0.04536 0.4152
0.8560 0.2329
0.2509 0.04382
0.8761 0.9877
TS
2.0 4.0 6.0 8.0
0.01 0.01 0.01 0.01
50 50 50 50
0.5–3.5 0.5–3.5 0.5–3.5 0.5–3.5
4.9910 2.5320 2.0982 3.0615
0.2403 0.3830 0.7591 0.4419
0.02348 0.0165 0.02963 0.01253
0.9905 0.9964 0.9782 0.9821
0.0102 0.0181 0.0125 0.04343
0.0079 0.0177 0.1175 0.8563
0.00149 0.00395 0.00392 0.0025
0.9683 0.9523 0.9768 0.9785
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the Cr(VI) interaction with binding sites of the biosorbents by greater attractive forces. A sharp decrease in adsorption above pH 4 may be due to occupation of the adsorption sites by anionic species like 2 2 HCrO 4 ; Cr2 O7 ; CrO4 , etc., which retards the approach of such ions further toward the sorbent surface (Das et al., 2000; Donmez and Aksu, 2002). The decrease in adsorption at high pH values may be due to the competitiveness of the oxyanion of chromium and OH ions in the bulk. As the pH increased, the overall surface charge on tamarind seed became negative and biosorption decreased. Marathe et al. (2002) have reported that tamarind seed have excellent stability over acidic pH range.
mg of Cr(VI) removal/sorbent wt, qe, mg/g
100 90
pH = 2
pH = 4
pH = 7
pH = 8
953
pH = 6
80 70 60 50 40 30 20 10 0 0
10
20
30
40
50
3.5. Effect of ionic strength
Equilibrium Concentration (mg/l)
capacity of TS for Cr(VI) was at pH 2 and the decrease in sorption capacity with increase in pH may be attributed to the changes in metal speciation and the dissociation of functional groups on the sorbent. Ucun et al. (2002) have reported that the pH dependence of metal uptake could be largely related to the various functional groups on the adsorbent surface along with metal solution chemistry. The pH of zero point charge (pHZPC) of tamarind seed was determined by fast alkalimetric titration method. In absence of specific chemical interaction between the single electrolyte and the surface, the net titration curves usually meet at a point that is defined as the pHZPC (pH of zero point charge). The net titration curves for tamarind seed were plotted to obtain pH of zero point charge. The value of pHZPC of tamarind seeds was found to be 6.00. The pHZPC (zero point of charge) of tamarind seed is 6.0, i.e., below this pH, the surface charge of the adsorbent is positive and above pH 6, tamarind seed would have a net negative charge. The composition of tamarind seeds powder as reported by Siddhuraju et al. (1995) is as: crude protein 11.9–14.1%; crude lipid 6–7.74%; crude fibre 12.7– 15.3%; moisture 7–9.14%; and ash 3.45–4.7%. It is hydrophilic in nature. Tamarind seeds are a rich source of protein and amino acids (Marathe et al., 2002; Siddhuraju et al., 1995). Some functional groups, such as amines, are positively charged when protonated and may electrostatically bind with negatively charged metal complexes. The decrease in the adsorption with increase of pH may be due to the decrease in electrostatic force of attraction between the sorbent and sorbate ions. At lower pH ranges, due to the high electrostatic force of attraction, the percentage of Cr(VI) removal is high. At very low pH value, the surface of sorbent would also be surrounded by the hydronium ions which enhance
Ionic strength, besides pH is also one of the important factors that influence the equilibrium uptake. Effect of ionic strength on Cr(VI) sorption is shown in Fig. 5. It is clear from the figure that Cr(VI) removal decreases with increasing ionic strength though the decrease is insignificant at lower ionic strength. But, it is slightly significant at ionic strength greater than 0.5 M. In general, adsorption decreases with increasing ionic strength of the aqueous solution (Donmez and Aksu, 2002). This behaviour may be due to the competition between anions of salt with chromate anions sorbed on the active centre of tamarind seed. This may also be due to the lowering in chromate anions ðCrO2 4 Þ, a reduction in columbic attraction, for chromate species on solid surfaces and/or to the presence of competing anions. The results indicate the possibility that adsorption of Cr(VI) on tamarind seed is mainly by chemisorption rather
80
mg of Cr(VI) removal/sorbent wt, qe, mg/g
Fig. 4. Effect of pH on Cr(VI) sorption by tamarind seed.
70 60 50 40 30 IS = 0.001 M IS = 0.5 M
20
IS = 0.1 M IS = 1.0 M
Initial Cr(VI) conc. = 50 mg/l pH = 4, Temp. = 24-25 °C
10 0 0
10
20
30
Equilibrium Concentration (mg/l) Fig. 5. Effect of ionic strength on Cr(VI) sorption by tamarind seed.
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shown in Table 2. It is evident from the table that particle size of sorbents has a significant effect on Cr(VI) sorption. The larger sorbent size showed lesser Cr(VI) removal as compared to the smaller sorbent size. The reason may be that surface area available for adsorption decreases with the increase of particle size for the same dose of sorbent, providing less active surface sites for adsorption of sorbate. The reduction in Cr(VI) removal capacity with increase in sorbent size gives an idea about the porosity of sorbent i.e., if the sorbent is highly porous then it would not have significant effect on Cr(VI) removal at equilibrium. The results obtained are in accordance with adsorption/ion-exchange processes, where smaller particles of sorbents/ion exchanger enhance the rate of metal uptake.
than physical sorption as effect of ionic strength is not so significant on the Cr(VI) removal capacity. 3.6. Effect of temperature Temperature dependence of the adsorption process is associated with several thermodynamic parameters. Fig. 6 shows an increasing trend of Cr(VI) removal with the rise in temperature from 12 to 58 °C. The reason may be that, at high temperature some polymers might have released from the sorbent which assist in adsorption, because the tamarind seed contains some free sugars. The increase in Cr(VI) uptake may also be due to creation of some new sorption sites on the sorbent surface or the increased rate of intraparticle diffusion of sorbate ions into the pores of adsorbent at higher temperature, as diffusion is an endothermic process (Das et al., 2000; Guo et al., 2002). Up to certain extent, enhancement of adsorption capacity of tamarind seed at higher temperatures may be attributed to enlargement of pore size and/ or activation of the adsorbent surface (Namasivayam and Yamuna, 1995).
3.8. Cyclic sorption (continuous loading) Experiments were conducted with TS to observe the possibility of removal of Cr(VI) by spent sorbent. It is obvious from the sorption equilibria plots that sorption capacity qe decreases with equilibrium concentration Ce. The regulatory Ce is quite low. Therefore, when the biosorbent is used in both batch mode or in a CSTR (continuous flow stirred tank reactor) for removal of a contaminant, the qe is quite less and thereby if counter current system is designed then spent sorbent can be reused. To assess the Cr(VI) removal capacity of the spent sorbent, cyclic sorption test was conducted. For the cyclic loading of Cr(VI), 500 mg of TS was contacted with 100 ml of Cr(VI) solution having concentration of 50 mg/l for 1 h. The cycle was repeated for 10 times at pH: 4 and temperature: 24°C. At the end of every cycle, metal solution was filtered and the metal concentration determined. The total amount of Cr(VI) sorbed on TS (in mg/g of TS) was 16.0, 31.0, 41.0, 44.5, and 45.0 after 2nd, 4th, 6th, 8th and 10th cycle, respectively. This observation indicates that TS was able to adsorb Cr(VI) ions even up to the 9th cycle, though the extent of metal sorption has gradually decreased. The results of the above experiments indicate the possible reusability of spent sorbent without regeneration. However, the tamarind seed is a low cost material and can be used on use and throw basis. Moreover, regeneration of sorbent might not be economical as the cost of regeneration chemical might be significant. Thereby, if Cr(VI) sorp-
3.7. Effect of particle size Effect of particle size on Cr(VI) sorption capacity of tamarind seed, walnut shell and groundnut shell are
mg of Cr(VI) removal/sorbent wt, qe, mg/g
100 Initial Cr(VI) conc. = 50 mg/l pH = 4, Ionic strength = 0.01 M
80
60
40
20 T = 10 °C T = 28 °C
T = 20 °C T = 48 °C
0 0
5
10
15
20
Equilibrium Concentration (mg/l)
Fig. 6. Effect of temperature on Cr(VI) sorption by tamarind seed.
Table 2 Effect of particle size on Cr(VI) removal Particle size range (l)
GMS (l)
Tamarind seed (%)
Walnut shell (%)
Groundnut shell (%)
Remarks
150–212 212–300 300–425
178.0 252.2 357.0
96.75 90 56.5
82.5 50 40
50 45 23
Initial Cr(VI) = 10 mg/l, pH = 4, temperature = 29–30 °C, sorbent dose, TS = 2 g/l, WS = 3 g/l, and GS = 3 g/l
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tion is maximized then it would be advantageous for the system operation. 3.9. Column experiments Downflow column studies were conducted to evaluate the sorption behaviour of TS for chromium removal in a continuous mode. The time to breakthrough as well as final effluent concentration at four different bed heights of TS viz. 20, 35, 50 and 65 cm were studied and the breakthrough time were 12 h, 28 h, 42 h and 60 h respectively. After 200 h of column operation, the head loss across the sorbent bed was increased to quite high values and then the operation was stopped due to choking of TS bed. The overall system was capable of treating 195 l of 10 mg/l of chromium solution before any choking at initial flow rate of 0.76 m3/m2/h. This indicates that the tamarind seed can be effectively used for chromium(VI) removal in column operation also. 3.10. Desorption and regeneration Sorption of solute on any sorbent can either be by physical bonding, ion-exchange or combination of both. If the adsorption is by physical bonding then the loosely bound solute can be easily desorbed with distilled water in most of the cases. However, if the mode of sorption is by chemical bonding or ion-exchange or combination of the both, then the desorption can be effected by stronger desorbents like acid or alkali solutions. Attempts were made to desorb Cr(VI) from previously Cr(VI) loaded tamarind seed using double distilled water (pH 6.6). Cr(VI) recovery was not observed. Hence, experiments were conducted with acid and alkali solutions to desorb Cr(VI) ions. The results obtained (not shown) indicate that the desorption of Cr(VI) ions with acid was not achieved even when 0.1 N HCl and 0.2 N HCl were used. However, there was very little desorption with alkali solutions. It was found that Cr(VI) desorption was 2.7%, 4.5% and 6.7% with 0.1 N NaOH at 1 h, 4 h and 24 h contact time respectively, whereas with 0.2 N NaOH, the percentage desorption of chromium(VI) was 4.5%, 11% and 15.3% respectively at 1 h, 4 h and 24 h contact period. From the results of desorption studies, the following inference may be made. The negligible desorption of Cr(VI) with double distilled water indicates the predominance of chemical bonding between tamarind seed and Cr(VI). This implies that physical adsorption is not playing significant role in Cr(VI) removal by TS. The results of alkali desorption of metal suggest either chemisorption or ion-exchange as the possible mechanism of metal solution (0.2 N NaOH), about 84.7% of Cr(VI) still remained on sorbent, which indicates that most of the Cr(VI) ions are able to form strong bonds with TS.
955
3.11. Comparison with other sorbents In the present study, TS has been compared with other sorbents based on their maximum sorptive capacity for Cr(VI). From Fig. 6, it can be observed that Cr(VI) sorption capacity for tamarind seed is 90 mg/g at equilibrium Cr(VI) concentration of 2.5 mg/l at pH 2. Hu et al. (2003) reported Cr(VI) sorptive capacity in the range of 30–40 mg/g for three different commercial activated carbon at equilibrium Cr(VI) concentration of 3–10 mg/l at pH 3. Granulated activated carbon and fibrous activated carbon have approximately 10 mg/g of Cr(VI) at equilibrium Cr(VI) concentration of 35 mg/l (Aggarwal et al., 1999). Lotfi and Adhoum (2002) have reported a Cr(VI) removal capacity of 6.84 mg/g for modified activated carbon (sodium diethyl dithiocarbamate immobilized at the surface), which was almost two times that of plain activated carbon. The maximum adsorption capacities of Cr(VI) removal reported by Bailey et al. (1999) are 16.05 mg/g for sawdust, and 0.65 mg/g for zeolite. Donmez and Aksu (2002) have reported a maximum Cr(VI) removal capacity of 17.7 mg/g for hazelnut shell biomass at pH 2.0. Accordingly, it can be stated that TS has high sorptive capacity in comparison to other available sorbents. 4. Conclusions Following conclusions are made based on the results of present study and scientific information derived from literature: 1. The biosorbents evaluated viz. CS, AS, GS, WS and TS can remove hexavalent chromium from aqueous phase. Tamarind seed has high Cr(VI) removal capacity than the other sorbent and can reduce aqueous phase Cr(VI) concentration up to non-detectable level. 2. The removal of Cr(VI) ions by tamarind seed significantly decreased with increase in pH, slightly decreased with increase in ionic strength and increased with increase in temperature. The Cr(VI) removal appears to be mainly by chemisorption. 3. Desorption of Cr(VI) from TS was partially achieved under alkaline conditions. 4. The use of tamarind seed as an adsorbent seems to be an economical and worthwhile alternative over conventional methods.
References Aggarwal, D., Goyal, M., Bansal, R.C., 1999. Adsorption of chromium by activated carbon from aqueous solution. Carbon 37 (12), 1989–1997.
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Biosorption of aqueous chromium(VI) by Tamarindus indica seeds G.S. Agarwal a, Hitendra Kumar Bhuptawat b, Sanjeev Chaudhari
b,*
a
b
MP Public Health Engineering Department, Bhopal, MP, India Centre for Environmental Science and Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India Received 28 January 2005; received in revised form 6 April 2005; accepted 8 April 2005 Available online 16 June 2005
Abstract The effectiveness of low cost agro-based materials namely, Tamarindus indica seed (TS), crushed coconut shell (CS), almond shell (AS), ground nut shell (GS) and walnut shell (WS) were evaluated for Cr(VI) removal. Batch test indicated that hexavalent chromium sorption capacity (qe) followed the sequence qe(TS) > qe(WS) > qe(AS) > qe(GS) > qe(CS). Due to high sorptive capacity, tamarind seed was selected for detailed sorption studies. Sorption kinetic data followed first order reversible kinetic fit model for all the sorbents. The equilibrium conditions were achieved within 150 min under the mixing conditions employed. Sorption equilibria exhibited better fit to Freundlich isotherms (R > 0.92) than Langmuir isotherm (R 0.87). Hexavalent chromium sorption by TS decreased with increase in pH, and slightly reduced with increase in ionic strength. Cr(VI) removal by TS seems to be mainly by chemisorption. Desorption of Cr(VI) from Cr(VI) laden TS was quite less by distilled water and HCl. Whereas with NaOH, maximum desorption achieved was about 15.3%. When TS was used in downflow column mode, Cr(VI) removal was quite good but head loss increased as the run progressed and was stopped after 200 h. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Chromium(VI); Adsorption; Tamarindus indica; Low cost biosorbent
1. Introduction Heavy metals such as chromium, copper, lead, cadmium, etc., in wastewater are hazardous to the environment. Because of their toxicity, their pollution effect on our ecosystem presents a possible human health risk (Nourbakhsh et al., 1994). In recent years, increasing awareness of water pollution and its far reaching effects has prompted concerted efforts towards pollution abatement. Among the different heavy metals, chromium is a common and very toxic pollutant introduced into natural waters from a variety of industrial wastewaters (Donmez and Aksu, 2002). The two major sources of contamination are electroplating, metal finishing indus-
*
Corresponding author. Tel.: +91 22 25767855; fax: +91 22 25723480. E-mail address: [email protected] (S. Chaudhari). 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.04.030
tries (hexavalent chromium) and tanneries (trivalent chromium). Chromium occurs most frequently as Cr(VI) or Cr(III) in aqueous solutions (Dakiky et al., 2002). Both valences of chromium are potentially harmful but hexavalent chromium poses a greater risk due to its carcinogenic properties (Dakiky et al., 2002). Hexavalent chromium, which is primarily present in the form 2 of chromate ðCrO2 4 Þ and dichromate ðCr2 O7 Þ, poses significantly higher levels of toxicity than the other valency states (Sharma and Forster, 1995). Conventional methods for removing Cr(VI) ions from industrial wastewater include reduction (Kim et al., 2002), reduction followed by chemical precipitation (Ozer et al., 1997), adsorption on the activated carbon (Lotfi and Adhoum, 2002), solvent extraction (Mauri et al., 2001) cementation, freeze separation, reverse osmosis (Padilla and Tavani, 1999), ion-exchange (Rengaraj et al., 2003) and electrolytic methods (Namasivayam and Yamuna, 1995). These methods have found
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limited application because they often involve high capital and operational costs. Adsorption is an effective and versatile method for removing chromium. Natural materials that are available in large quantities, or certain waste products from industrial or agricultural operations, may have potential as inexpensive sorbents. Due to their low cost, after these materials have been expended, they can be disposed of without expensive regeneration. Most of the low cost sorbents have the limitation of low sorptive capacity and thereby for the same degree of treatment, it generates more solid waste (pollutant laden sorbent after treatment), which poses disposal problems. Therefore, there is need to explore low cost sorbent having high contaminant sorption capacity. Several recent publications utilized locally available adsorbents e.g. fly ash, peat, microbial biomass (Bai and Abraham, 2003; Nourbakhsh et al., 1994) and agricultural byproducts (Bailey et al., 1999) for heavy metal removal. However, the literature is still insufficient to cover this problem and more work and investigations are needed to deal with other locally available and cheap adsorbents to eliminate Cr(VI) from industrial compositions and characteristics. Tamarind (Tamarindus indica) is a common tree in tropical countries. It is grown mainly for its sour fruits pulp. Tamarind seed, a by product of the tamarind pulp industry, is an underutilized or waste material (Bhattacharya et al., 1997). Literature survey by the author in most of the peer reviewed journals indicated that adsorption study of Cr(VI) with Tamarind seed as an adsorbent has not been investigated and this is the first such study undertaken by the authors. This study aims at comparative evaluation of agrobased materials, namely T. indica (tamarind) seed (TS), almond shell (AS), ground nut shell (GS), walnut shell (WS), and crushed coconut shell (CS) for removal of Cr(VI) from simulated industrial wastewater. Based on their efficacy, tamarind seed was selected for further study. The effect of pH, contact time, adsorption equilibria and temperature were investigated. Desorption of Cr(VI) from Cr(VI) laden TS was also attempted. Further continuous wastewater treatment was done in column mode to assess the viability of TS for continuous operation.
tion. The pH of solution was maintained using acetate and sodium tetra borate buffer. The initial Cr(VI) concentration was maintained by adding stock solution of chromium. 2.1. Preparation of biosorbents The sorbents used were crushed coconut shell, almond shell, ground nut shell, walnut shell and tamarind seed. All the materials were obtained from local market; materials were washed, dried and then pulverized in pulverizer and air-dried in the sun for two days. After drying, the materials were kept in air tight plastic bottles. All the materials were used as such and no pretreatment was given to the materials. The particle size was maintained in the range of 212–300 lm (geometric mean size: 252.2 lm). 2.2. Screening of biosorbents For the comparative evaluation of different sorbents for Cr(VI) removal capacity, the experiments were conducted in 150 ml double stoppered polyethene bottles at 28°C on a rotary shaker at 70 rpm. Experiments were conducted at pH 4, 6 and 9 and initial Cr(VI) concentration of 10 mg/l was maintained. After 4 h of mixing, the samples were filtered through Whattman No. 42 filter paper, and filtrates were analysed for residual chromium concentration. 2.3. Sorption kinetics Sorption studies were conducted in 150 ml polyethene bottles at 27–28 °C on a rotary shaker at 70 rpm. The chromium concentration was maintained by adding appropriate quantity of stock solution of K2Cr2O7. The ionic strength of the aqueous phase was adjusted by adding appropriate quantity of KNO3. The sample bottles were taken out at different time intervals and samples were filtered through Whattman No. 42 filter paper. The filtrates were analysed for residual chromium concentration and pH. Sorption experiments were conducted for TS, GS, WS at pH of 4, 6 and 9 for initial chromium concentration of 10 mg/l. 2.4. Sorption equilibria
2. Methods All the chemicals used were of analytical grade. Cr(VI) concentrations in aqueous phase were estimated by diphenyl carbazide method as per standard methods (APHA, 1985). AIMIL colorimeter with green filter (540 nm) was used for colorimetric measurement. The Cr(VI) loadings on sorbents were computed based on mass balance through loss of metal from aqueous solu-
Batch sorption experiments were conducted in 150 ml polyethene double stoppered bottles at 28 °C on a rotary shaker (70 rpm) for mixing. The pH values were adjusted using acetate buffer or sodium tetra borate buffer before addition of biosorbent and were maintained throughout the experiments. The sorption equilibria experiments were performed for different pH conditions and for different sorbents. The aqueous solution was mixed till equilibrium time of 180 min. After equilibrium
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time, the sample bottles were taken out for the analysis of residual chromium concentration in the solution.
3. Results and discussion 3.1. Screening of biosorbents Preliminary experiments were conducted with initial Cr(VI) concentration of 10 mg/l and biosorbents dose of 2 g/l at different pH. The results are presented in Fig. 1. At pH 4, the Cr(VI) removal was 30%, 36%, 35%, 40%, and 80% for CS, AS, GS, WS, and TS respectively, which at pH 6 decreased to 22%, 24%, 23%, 27% and 64%. It further decreased at pH 9. These preliminary results indicated the higher Cr(VI) removal capacity of tamarind seeds in the pH range of 4–9, as compared to other biosorbents. The sequence of Cr(VI) removal was TS > WS > AS > GS > CS. Though percentage removal of Cr(VI) decreased with increase in pH, but the sequence remained same. 3.2. Sorption kinetics To evaluate sorption as a unit operation, it requires consideration of two important physico-chemical aspects of the process, the kinetics and the equilibria of sorption. Kinetics of sorption describing the solute uptake rate, which in turn governs the contact time, is one of the important characteristics defining efficiency of sorption. The study of the equilibrium established in any liquid–solid system is important in determining distribution of the solute between the solid and liquid phases and determining feasibility and capacity of the sorbent for sorption. The rate at which dissolved heavy metal ions are removed from dilute aqueous solution by solid sorbents
is a significant factor for application in water quality control. The rate at which sorption proceeds is important in terms of the contact time to be provided between the solution and the sorbent. The capacity of the sorbent for the uptake of sorbate i.e., the position at which equilibrium is attained determines the useful life of sorbent to a large extent. Fig. 2 shows the adsorption of Cr(VI) by tamarind seed, walnut shell, almond shell and groundnut shell biosorbents as a function of time at pH of 4.0–4.25 for initial Cr(VI) concentration of 10 mg/l. Tamarind dose of 2 g/l was used while 3 g/l dose of other sorbents were used. Fig. 2 shows rapid adsorption in the initial 20 min for all biosorbent. The initial high adsorption rate decreased gradually as the equilibrium approached. Equilibrium time was approximately 60 min for groundnut, while for walnut and tamarind seed, it was 120 and 150 min respectively. Cr(VI) removal was found to be 90% at 60 min, 95% at 120 min and 98% at 180 min for tamarind seed. The effect of pH on the rate of Cr(VI) sorption was investigated at different pH for tamarind seed as indicated in Fig. 3. It is interesting to note that rate of sorption is more at lower pH. This may be due to difference in the speciation of Cr(VI) and pore density (morphology) of sorbents at different pH. The likely effect of pH on Cr(VI) adsorption has been discussed under Section 3.4. The rate constant for the sorption of Cr(VI) on tamarind seed was evaluated using the Lagergen equation (Orhan and Buyukgungor, 1993; Selvaraj et al., 2003) lnð1 U ðtÞÞ ¼ K a t
where U ðtÞ ¼ is called the fractional attainment of equilibrium. qe and q are the amount of Cr(VI) adsorbed (mg/g) at equilibrium and at time t, respectively, ka is the first order rate constant and t is the time (min). The plots
140
pH = 4
60
pH = 6
pH = 9
Walnut shell Almond shell
120
GMS = 252.2 µ; Initial Cr(VI) conc. = 10 mg/l; Sorbent dose = 2 g/l; Temp. = 26-27 °C;
% of Cr(VI) sorbed
Fraction Sorbed (%)
70
ð1Þ
q qe
90 80
951
contact time = 3 hrs 50 40 30
Groundnut shell Tamarind seed
100 80
60 40
20 20
10 0
0
CS
AS
GS Sorbents
WS
TS
Fig. 1. Comparative evaluation of different biosorbents.
0
50
100
150
200
250
300
350
Time (min)
Fig. 2. Sorption kinetics of Cr(VI) for TS, AS, GS, and WS.
400
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measure of adsorption capacity and (1/n) is a measure of adsorption intensity. The linear form of the Langmuir adsorption isotherm is represented as
120 pH = 4
Sorbent dose = 2 g/l
100
% of Cr(VI) sorbed
pH = 9
pH = 6
Ce 1 Ce þ ¼ Q0 b Q0 qe
80
60
40
20
0 0
100
200
300
400
Time (min)
Fig. 3. Sorption kinetics of Cr(VI) at different pH for tamarind seed.
of ln(qe q) versus t were linear (not shown) for the biosorbents and indicated that the adsorption can be approximated to first order reversible kinetics. The rate constants determined from the slopes of the plots were obtained as TS: 0.0767 min1, GS: 0.0433 min1, WS: 0.0142 min1, AS: 0.0135 min1. 3.3. Adsorption isotherms The adsorption data were fitted by least square method to linearly transformed Freundlich and Langmuir adsorption isotherms. The linearised Freundlich equation is mentioned below. log qe ¼ log K f þ
1 log C e n
ð2Þ
where qe is the amount of Cr(VI) adsorbed per unit mass of biosorbent (mg/g), Ce is the residual concentration of Cr(VI) in solution (mg/l), Kf is a constant which is a
ð3Þ
where Ce is the equilibrium concentration (mg/l), and qe is the amount of Cr(VI) adsorbed per gram at equilibrium (mg/g). Q0 (mg/l) and b (l/mg, i.e., l of adsorbent per mg of adsorbate) are Langmuir constants related to the adsorption capacity and energy of adsorption, respectively. The data obtained from the adsorption experiments conducted at 29 °C were fitted to Eqs. (2) and (3), linear plot (not shown) were obtained for log(qe) versus log(Ce) and Ce/qe versus Ce. The isotherm parameters for both equation along with the values of coefficient of correlation (R) and standard error of estimate (r) are presented in Table 1. Table shows that data better fits to Freundlich equation than Langmuir equation, which is indicated from the higher values of R and lower values of r. 3.4. Effect of pH on sorption Aqueous phase pH governs the speciation of metals and also the dissociation of active functional sites on the sorbent. Hence, metal sorption is critically linked with pH. Not only different metals show different pH optima for their sorption but may also vary from one kind of biomass to the other (Tiwari et al., 1995; Ucun et al., 2002). Fig. 4 shows the sorption isotherms at different pH. It can be observed from the figure that the uptake of Cr(VI) decreases with increase in pH. In general, the Cr(VI) adsorption by different biosorbents have shown similar trend and the optimum pH 2 has been reported (Donmez and Aksu, 2002; Dakiky et al., 2002; Selvaraj et al., 2003; Yu et al., 2003; Ucun et al., 2002; Hu et al., 2003; Gupta et al., 2001). The highest sorption
Table 1 Estimated Freundlich and Langmuir isotherm parameters and relevant statistical information for Cr(VI) sorbents system Sorbent
pH
Ionic strength
Initial solute concentration C0 (mg/l)
Sorbent dose (g/l)
Freundlich isotherm Kf
1/n
r
R
1/Q0
1/bQ0
r
R
0.170 0.06
2.852 5.100
0.1173 0.1694
0.9879 0.9915
Langmuir isotherm
GS
4.0 4.0
0.01 0.01
10 10
3 5
0.3239 0.1978
0.8730 1.0541
0.0648 0.0870
0.9759 0.9639
WS
4.0 4.0
0.01 0.01
10 10
3 5
0.9866 1.0873
0.4392 0.2924
0.0667 0.0307
0.9550 0.9790
0.4384 0.05421
0.341 0.2137
0.1629 0.1038
0.9168 0.9214
AS
4.0 4.0
0.01 0.01
10 10
3 5
0.6134 1.3326
0.6020 0.3022
0.1025 0.0387
0.9200 0.9716
0.04536 0.4152
0.8560 0.2329
0.2509 0.04382
0.8761 0.9877
TS
2.0 4.0 6.0 8.0
0.01 0.01 0.01 0.01
50 50 50 50
0.5–3.5 0.5–3.5 0.5–3.5 0.5–3.5
4.9910 2.5320 2.0982 3.0615
0.2403 0.3830 0.7591 0.4419
0.02348 0.0165 0.02963 0.01253
0.9905 0.9964 0.9782 0.9821
0.0102 0.0181 0.0125 0.04343
0.0079 0.0177 0.1175 0.8563
0.00149 0.00395 0.00392 0.0025
0.9683 0.9523 0.9768 0.9785
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the Cr(VI) interaction with binding sites of the biosorbents by greater attractive forces. A sharp decrease in adsorption above pH 4 may be due to occupation of the adsorption sites by anionic species like 2 2 HCrO 4 ; Cr2 O7 ; CrO4 , etc., which retards the approach of such ions further toward the sorbent surface (Das et al., 2000; Donmez and Aksu, 2002). The decrease in adsorption at high pH values may be due to the competitiveness of the oxyanion of chromium and OH ions in the bulk. As the pH increased, the overall surface charge on tamarind seed became negative and biosorption decreased. Marathe et al. (2002) have reported that tamarind seed have excellent stability over acidic pH range.
mg of Cr(VI) removal/sorbent wt, qe, mg/g
100 90
pH = 2
pH = 4
pH = 7
pH = 8
953
pH = 6
80 70 60 50 40 30 20 10 0 0
10
20
30
40
50
3.5. Effect of ionic strength
Equilibrium Concentration (mg/l)
capacity of TS for Cr(VI) was at pH 2 and the decrease in sorption capacity with increase in pH may be attributed to the changes in metal speciation and the dissociation of functional groups on the sorbent. Ucun et al. (2002) have reported that the pH dependence of metal uptake could be largely related to the various functional groups on the adsorbent surface along with metal solution chemistry. The pH of zero point charge (pHZPC) of tamarind seed was determined by fast alkalimetric titration method. In absence of specific chemical interaction between the single electrolyte and the surface, the net titration curves usually meet at a point that is defined as the pHZPC (pH of zero point charge). The net titration curves for tamarind seed were plotted to obtain pH of zero point charge. The value of pHZPC of tamarind seeds was found to be 6.00. The pHZPC (zero point of charge) of tamarind seed is 6.0, i.e., below this pH, the surface charge of the adsorbent is positive and above pH 6, tamarind seed would have a net negative charge. The composition of tamarind seeds powder as reported by Siddhuraju et al. (1995) is as: crude protein 11.9–14.1%; crude lipid 6–7.74%; crude fibre 12.7– 15.3%; moisture 7–9.14%; and ash 3.45–4.7%. It is hydrophilic in nature. Tamarind seeds are a rich source of protein and amino acids (Marathe et al., 2002; Siddhuraju et al., 1995). Some functional groups, such as amines, are positively charged when protonated and may electrostatically bind with negatively charged metal complexes. The decrease in the adsorption with increase of pH may be due to the decrease in electrostatic force of attraction between the sorbent and sorbate ions. At lower pH ranges, due to the high electrostatic force of attraction, the percentage of Cr(VI) removal is high. At very low pH value, the surface of sorbent would also be surrounded by the hydronium ions which enhance
Ionic strength, besides pH is also one of the important factors that influence the equilibrium uptake. Effect of ionic strength on Cr(VI) sorption is shown in Fig. 5. It is clear from the figure that Cr(VI) removal decreases with increasing ionic strength though the decrease is insignificant at lower ionic strength. But, it is slightly significant at ionic strength greater than 0.5 M. In general, adsorption decreases with increasing ionic strength of the aqueous solution (Donmez and Aksu, 2002). This behaviour may be due to the competition between anions of salt with chromate anions sorbed on the active centre of tamarind seed. This may also be due to the lowering in chromate anions ðCrO2 4 Þ, a reduction in columbic attraction, for chromate species on solid surfaces and/or to the presence of competing anions. The results indicate the possibility that adsorption of Cr(VI) on tamarind seed is mainly by chemisorption rather
80
mg of Cr(VI) removal/sorbent wt, qe, mg/g
Fig. 4. Effect of pH on Cr(VI) sorption by tamarind seed.
70 60 50 40 30 IS = 0.001 M IS = 0.5 M
20
IS = 0.1 M IS = 1.0 M
Initial Cr(VI) conc. = 50 mg/l pH = 4, Temp. = 24-25 °C
10 0 0
10
20
30
Equilibrium Concentration (mg/l) Fig. 5. Effect of ionic strength on Cr(VI) sorption by tamarind seed.
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shown in Table 2. It is evident from the table that particle size of sorbents has a significant effect on Cr(VI) sorption. The larger sorbent size showed lesser Cr(VI) removal as compared to the smaller sorbent size. The reason may be that surface area available for adsorption decreases with the increase of particle size for the same dose of sorbent, providing less active surface sites for adsorption of sorbate. The reduction in Cr(VI) removal capacity with increase in sorbent size gives an idea about the porosity of sorbent i.e., if the sorbent is highly porous then it would not have significant effect on Cr(VI) removal at equilibrium. The results obtained are in accordance with adsorption/ion-exchange processes, where smaller particles of sorbents/ion exchanger enhance the rate of metal uptake.
than physical sorption as effect of ionic strength is not so significant on the Cr(VI) removal capacity. 3.6. Effect of temperature Temperature dependence of the adsorption process is associated with several thermodynamic parameters. Fig. 6 shows an increasing trend of Cr(VI) removal with the rise in temperature from 12 to 58 °C. The reason may be that, at high temperature some polymers might have released from the sorbent which assist in adsorption, because the tamarind seed contains some free sugars. The increase in Cr(VI) uptake may also be due to creation of some new sorption sites on the sorbent surface or the increased rate of intraparticle diffusion of sorbate ions into the pores of adsorbent at higher temperature, as diffusion is an endothermic process (Das et al., 2000; Guo et al., 2002). Up to certain extent, enhancement of adsorption capacity of tamarind seed at higher temperatures may be attributed to enlargement of pore size and/ or activation of the adsorbent surface (Namasivayam and Yamuna, 1995).
3.8. Cyclic sorption (continuous loading) Experiments were conducted with TS to observe the possibility of removal of Cr(VI) by spent sorbent. It is obvious from the sorption equilibria plots that sorption capacity qe decreases with equilibrium concentration Ce. The regulatory Ce is quite low. Therefore, when the biosorbent is used in both batch mode or in a CSTR (continuous flow stirred tank reactor) for removal of a contaminant, the qe is quite less and thereby if counter current system is designed then spent sorbent can be reused. To assess the Cr(VI) removal capacity of the spent sorbent, cyclic sorption test was conducted. For the cyclic loading of Cr(VI), 500 mg of TS was contacted with 100 ml of Cr(VI) solution having concentration of 50 mg/l for 1 h. The cycle was repeated for 10 times at pH: 4 and temperature: 24°C. At the end of every cycle, metal solution was filtered and the metal concentration determined. The total amount of Cr(VI) sorbed on TS (in mg/g of TS) was 16.0, 31.0, 41.0, 44.5, and 45.0 after 2nd, 4th, 6th, 8th and 10th cycle, respectively. This observation indicates that TS was able to adsorb Cr(VI) ions even up to the 9th cycle, though the extent of metal sorption has gradually decreased. The results of the above experiments indicate the possible reusability of spent sorbent without regeneration. However, the tamarind seed is a low cost material and can be used on use and throw basis. Moreover, regeneration of sorbent might not be economical as the cost of regeneration chemical might be significant. Thereby, if Cr(VI) sorp-
3.7. Effect of particle size Effect of particle size on Cr(VI) sorption capacity of tamarind seed, walnut shell and groundnut shell are
mg of Cr(VI) removal/sorbent wt, qe, mg/g
100 Initial Cr(VI) conc. = 50 mg/l pH = 4, Ionic strength = 0.01 M
80
60
40
20 T = 10 °C T = 28 °C
T = 20 °C T = 48 °C
0 0
5
10
15
20
Equilibrium Concentration (mg/l)
Fig. 6. Effect of temperature on Cr(VI) sorption by tamarind seed.
Table 2 Effect of particle size on Cr(VI) removal Particle size range (l)
GMS (l)
Tamarind seed (%)
Walnut shell (%)
Groundnut shell (%)
Remarks
150–212 212–300 300–425
178.0 252.2 357.0
96.75 90 56.5
82.5 50 40
50 45 23
Initial Cr(VI) = 10 mg/l, pH = 4, temperature = 29–30 °C, sorbent dose, TS = 2 g/l, WS = 3 g/l, and GS = 3 g/l
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tion is maximized then it would be advantageous for the system operation. 3.9. Column experiments Downflow column studies were conducted to evaluate the sorption behaviour of TS for chromium removal in a continuous mode. The time to breakthrough as well as final effluent concentration at four different bed heights of TS viz. 20, 35, 50 and 65 cm were studied and the breakthrough time were 12 h, 28 h, 42 h and 60 h respectively. After 200 h of column operation, the head loss across the sorbent bed was increased to quite high values and then the operation was stopped due to choking of TS bed. The overall system was capable of treating 195 l of 10 mg/l of chromium solution before any choking at initial flow rate of 0.76 m3/m2/h. This indicates that the tamarind seed can be effectively used for chromium(VI) removal in column operation also. 3.10. Desorption and regeneration Sorption of solute on any sorbent can either be by physical bonding, ion-exchange or combination of both. If the adsorption is by physical bonding then the loosely bound solute can be easily desorbed with distilled water in most of the cases. However, if the mode of sorption is by chemical bonding or ion-exchange or combination of the both, then the desorption can be effected by stronger desorbents like acid or alkali solutions. Attempts were made to desorb Cr(VI) from previously Cr(VI) loaded tamarind seed using double distilled water (pH 6.6). Cr(VI) recovery was not observed. Hence, experiments were conducted with acid and alkali solutions to desorb Cr(VI) ions. The results obtained (not shown) indicate that the desorption of Cr(VI) ions with acid was not achieved even when 0.1 N HCl and 0.2 N HCl were used. However, there was very little desorption with alkali solutions. It was found that Cr(VI) desorption was 2.7%, 4.5% and 6.7% with 0.1 N NaOH at 1 h, 4 h and 24 h contact time respectively, whereas with 0.2 N NaOH, the percentage desorption of chromium(VI) was 4.5%, 11% and 15.3% respectively at 1 h, 4 h and 24 h contact period. From the results of desorption studies, the following inference may be made. The negligible desorption of Cr(VI) with double distilled water indicates the predominance of chemical bonding between tamarind seed and Cr(VI). This implies that physical adsorption is not playing significant role in Cr(VI) removal by TS. The results of alkali desorption of metal suggest either chemisorption or ion-exchange as the possible mechanism of metal solution (0.2 N NaOH), about 84.7% of Cr(VI) still remained on sorbent, which indicates that most of the Cr(VI) ions are able to form strong bonds with TS.
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3.11. Comparison with other sorbents In the present study, TS has been compared with other sorbents based on their maximum sorptive capacity for Cr(VI). From Fig. 6, it can be observed that Cr(VI) sorption capacity for tamarind seed is 90 mg/g at equilibrium Cr(VI) concentration of 2.5 mg/l at pH 2. Hu et al. (2003) reported Cr(VI) sorptive capacity in the range of 30–40 mg/g for three different commercial activated carbon at equilibrium Cr(VI) concentration of 3–10 mg/l at pH 3. Granulated activated carbon and fibrous activated carbon have approximately 10 mg/g of Cr(VI) at equilibrium Cr(VI) concentration of 35 mg/l (Aggarwal et al., 1999). Lotfi and Adhoum (2002) have reported a Cr(VI) removal capacity of 6.84 mg/g for modified activated carbon (sodium diethyl dithiocarbamate immobilized at the surface), which was almost two times that of plain activated carbon. The maximum adsorption capacities of Cr(VI) removal reported by Bailey et al. (1999) are 16.05 mg/g for sawdust, and 0.65 mg/g for zeolite. Donmez and Aksu (2002) have reported a maximum Cr(VI) removal capacity of 17.7 mg/g for hazelnut shell biomass at pH 2.0. Accordingly, it can be stated that TS has high sorptive capacity in comparison to other available sorbents. 4. Conclusions Following conclusions are made based on the results of present study and scientific information derived from literature: 1. The biosorbents evaluated viz. CS, AS, GS, WS and TS can remove hexavalent chromium from aqueous phase. Tamarind seed has high Cr(VI) removal capacity than the other sorbent and can reduce aqueous phase Cr(VI) concentration up to non-detectable level. 2. The removal of Cr(VI) ions by tamarind seed significantly decreased with increase in pH, slightly decreased with increase in ionic strength and increased with increase in temperature. The Cr(VI) removal appears to be mainly by chemisorption. 3. Desorption of Cr(VI) from TS was partially achieved under alkaline conditions. 4. The use of tamarind seed as an adsorbent seems to be an economical and worthwhile alternative over conventional methods.
References Aggarwal, D., Goyal, M., Bansal, R.C., 1999. Adsorption of chromium by activated carbon from aqueous solution. Carbon 37 (12), 1989–1997.
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