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Removal of heavy metals using a plant biomass with reference to environmental control
Int. J. Miner. Process. 68 (2003) 37 – 45 www.elsevier.com/locate/ijminpro
Removal of heavy metals using a plant biomass with reference to environmental control K. Chandra Sekhar a,*, C.T. Kamala a, N.S. Chary a, Y. Anjaneyulu b a
Analytical Chemistry Department, Indian Institute of Chemical Technology1, Hyderabad 500 007, India b Centre for Environment, Jawaharlal Nehru Technological University, Hyderabad 500 028, India Received 25 March 2001; received in revised form 30 April 2002; accepted 30 April 2002
Abstract Heavy metal pollution has become one of the most serious problems today, and the use of microbial and plant biomass for the detoxification of industrial effluents for environmental protection and recovery of valuable metals offers a potential alternative to existing treatment technologies. In the present study, the biosorption capacity of a plant biomass was studied for different toxic metals and the removal was found to be higher for Pb, Zn and Cr among the 11 metals studied (As, Se, Zn, Fe, Ni, Co, Pb, Mn, Hg, Cr and Cu). The results of the biosorption studies revealed higher Pb removal followed by Cr and Zn at lower metal concentrations, less than 250 ppm and with biomass concentrations above 2 g. The results of shake flask experiment revealed enhanced metal removal with 15 min agitation for Pb and 180 min for Zn and Cr removal. Metal removal was higher at lower pH for Cr and Zn and increased pH decreased the percentage metal removal. Lead removal was unaffected by pH changes. The presence of co-ions (As, Se, Hg, etc.) did not affect Pb removal by biomass, but on the other hand, Zn and Cr uptakes decreased. For the reuse of biomass, the used biomass was subjected to desorption studies using HNO3. The retention capacity of the biomass was almost constant after three cycles of chelation – desorption, suggesting that the lifetime cycle was sufficiently long for continuous industrial application. The suggested process can be used as an alternative to the classical technologies for effluent decontamination and would also be efficient for polishing effluents treated by other methods. The biosorption model developed was applied to a ‘‘real life system’’ successfully. D 2003 Elsevier Science B.V. All rights reserved. Keywords: plant biomass; biosorption; co-ion effect; desorption
*
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Chandra Sekhar). 1 IICT Communication Number: 4627. 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 4 7 - 9
38
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
1. Introduction The discharge of heavy metals into aquatic systems has been a matter of worldwide concern over the last few decades. These pollutants are introduced into aquatic system significantly as a result of various industrial operations. Over a few decades, community is devoting concentrated efforts for the treatment and removal of heavy metals in order to combat this problem. The commonly used procedures for removing metal ions from dilute aqueous streams include chemical precipitation, reverse osmosis and solvent extraction (Rich and Cherry, 1987). However, these techniques have certain disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal. Heavy metals such as lead, zinc and chromium have a number of applications in basic steel works, paper and pulp, leather tanning, organochemicals, petrochemicals, fertilizers, etc. Major lead pollution is through automobiles and battery manufacturers. For zinc and chromium, their major application is in fertilizer and leather tanning, respectively (Trivedi, 1989). The search for alternate and innovative treatment techniques has focussed attention on the use of biological materials for heavy metal removal and recovery technologies (Biosorption) and has gained important credibility during recent years because of the good performance and low cost of this complexing material (Scott, 1992). The natural affinity of biological compounds for metallic elements could contribute to economically remediating heavily metal-loaded wastewater (Gadd, 1988). Among the various resources in biological wastes, both dead and live biomass of microorganisms (bacteria, yeasts, fungi and algae) and some plants exhibit particularly interesting metal-binding capacities. The use of dead biomass eliminates the problem of toxicity and the economic aspects of nutrient supply and culture maintenance. The biosorption of metals using nonliving biomass has been comprehensively reviewed by Modak and Natarajan (1995). As an alternative to the existing methods, an inexpensive, naturally available plant biomass can be used as a material for removal of toxic heavy metal ions (Reddy et al., 1997; Gaballah et al., 1997; Tiwari et al., 1999; Villaescusa et al., 2000) from synthetic solutions and industrial effluents. In the present investigation, the potential of a plant biomass has been assessed for the removal of metal ions such as lead, zinc and chromium. The effects of various parameters have been studied and the results are presented in this paper.
2. Experimental 2.1. Metal cations The metal cations studied were chromium [Cr(VI)], lead [Pb(II)] and zinc [Zn(II)]. The stock solutions of Pb, Zn and Cr (10,000 ppm) were prepared from lead nitrate [Drug houses (India), Graham Road, Bombay-1], zinc acetate (Glaxo Laboratories India, Bombay) and potassium dichromate (Ranbaxy Laboratories, India) in deionized water. All AR grade reagents and deionized double distilled water were used for all the experiments. The pH adjustments were done using a pH meter (Elico, India). The solutions were agitated using a rotary shaker (Techno India). In all the experiments, the biomass-contained
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
39
solutions were filtered using no. 41 Whatman filter papers. The concentration of metals in the filtered solutions was estimated using Varian Ultra Mass 700 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Australia. 2.2. Biomass In this process, a plant biomass, the root bark of Indian Sarsaparilla was used as biosorbent for removal of Pb, Cr and Zn from aqueous solutions. Indian Sarsaparilla (Hemidesmus indicus) has a wide distribution and taxanomically classified as a member of family Asclepiadacea (Warrier, 1995). The biomass was oven dried at 100 jC and was used in our studies. 2.3. Biosorption and analytical procedures A known weight (2.0 g) of biomass was added to a solution containing 100 ppm of metal ion under investigation and the pH was measured as it is. The solution was shaken well in a rotary shaker at 250 rpm at room temperature (30 F 2 jC) for 30 min. The slurry, after equilibration, was filtered through Whatman 41 filter paper. The clear filtrate was analyzed for metal concentration using ICP-MS. The amount of metal taken up by plant biomass was calculated as the difference between the initial and final concentrations of metal in the aqueous solution.
3. Results and discussion 3.1. Effect of shake time on metal removal This experiment was conducted to enhance metal removal by biomass, simultaneously decreasing the incubation period. The results of the above experiment are represented in Fig. 1. The agitation of metal solutions with biomass was effective for all the metals. Within 15 min of shaking, the percentage removal for Pb reached 100%, whereas for Zn and Cr, the maximum percentage removal was at 180 min. 3.2. Effect of pH on metal removal Experimental results showing adsorption of Pb, Cr and Zn by H. indicus at varying pH are presented in Fig. 2. The removal of Pb was unaffected with increase in pH but the removal of Cr and Zn by H. indicus decreased marginally. The reduction in metal removal with increasing pH beyond its optimum values has been attributed to reduced solubility and precipitation (Harris and Ramelone, 1990; Zhou and Kliff, 1991). 3.3. Effect of biomass concentration on metal removal The results of the experiment with varying biomass concentrations are presented in Fig. 3. With increase in biomass concentration, the percentage removal increases as the number
40
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Fig. 1. Effect of shake time on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH: Pb = 2.0, Zn = 4.0, Cr = 2.0; biomass loading: 2.0 g.
of possible binding sites are increased. The difference in percentage Pb removal with 0.2 g of biomass (94.35%) and 2 g of biomass (100%) was very little, whereas a prominent increase in adsorption of Zn and Cr is observed by increasing the concentration of biomass from 0.2 g (Zn 44.3%; Cr 55%) to 2 g (Zn 86.9%; Cr 81.9%). 3.4. Effect of initial metal concentration on percentage metal removal The results of the experiment are shown in Fig. 4. The results revealed maximum metal removal with lower initial concentrations. The percentage Pb uptake was 100% for metal solutions till 250 ppm metal. Later, an increase in initial concentration decreased the
Fig. 2. Effect of pH on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH range: 2 – 9; biomass weight: 2.0 g.
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
41
Fig. 3. Effect of biomass concentration on metal removal. pH: Pb = 2.0, Zn = 4.0, Cr = 2.0, concentration of metal ions: 500 ppm, volume of solution: 100 ml, biomass loading: 0.2 – 2 g.
percentage binding. The observations are the same for Zn (70%) and Cr (75.3%) at 250ppm concentration. These observations can be explained by the fact that at very low concentrations of metal ions, the ratio of sorptive surface area to the total metal ions available is high and thus, there is a greater chance for metal removal. Thus, at low initial metal ion concentrations, the removal capacity is higher. When metal ion concentrations are increased, binding sites become more quickly saturated as the amount of biomass concentration remained constant. During the saturation trial, the pH of the solution decreases. This suggests that metal binding to the biomass is associated with the release of H + ions into the solution. This clearly indicates that the mechanism of metal binding involves a cation exchange
Fig. 4. Effect of initial concentration on metal removal. pH: Pb = 2.0, Zn = 4.0, Cr = 2.0; concentration range: 50 – 5000 ppm; volume of solution: 100 ml; biomass loading: 2.0 g.
42
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Fig. 5. Effect of multi-component system on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH: Pb + Zn + Cr = 3.0; biomass loading: 2.0 g.
mechanism, possibly, the metal ions react with hydroxyl or carboxylic groups of the biomass’ organic compounds releasing H + ions into the solution. 3.5. Multicomponent systems The results of the experiment are presented in Fig. 5. The results revealed that Pb removal was not effected by any of the other cations, however, the presence of co-ions enhanced Zn removal but decreased Cr removal. In the presence of co-ions in solution, chemical interactions between the ions themselves, as well as with the biomass, take place resulting in site competition (Sommers, 1963; Khovrychev, 1973). Many of the functional groups present on the cell wall and the membrane are nonspecific and different cations compete for the binding sites. It has been reported that metal removal is increased as the ionic radii of metal cations affect the ion exchange and adsorption process (Marcus and Kertes, 1969). The differences in the sorption affinities may also be attributed to differences in the electrode potentials of the various ions. The greater the electrode potential, the greater is the affinity for biomass (Mattuschka and Straube, 1993). The ionic radii and the electrode potentials of the metal ions are summarized in Table 1. In multicomponent systems, the complex interactions of several factors such as ionic charge, ionic radii and electrode potential will account for the differences in the metal removal
Table 1 Ionic charge, ionic radii and electrode potentials of metal ions Metal ion
Ionic charge
˚) Ionic radii (A
Electrode potential (V)
Pb2 + Zn2 + Cr3 +
+2 +2 +3
1.32 0.74 0.64
0.126 0.763 0.744
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
43
Fig. 6. Effect of HCl concentration on percentage desorption. Range of acid concentration: 5 – 20%; volume of solution: 100 ml; biomass loading: 2.0 g; concentration of metal ions: 100 ppm.
capacity of the biomass. As a result, ordering of the metal ions based on a single factor is very difficult. 3.6. Desorption of metals from the loaded biomass A few cursory experiments were conducted to desorb the metal ions from the loaded biomass as a function of acid concentration and the results are shown in Figs. 6 and 7. The preloaded biomass was subjected to a comparative desorption study using HNO3 and HCl in the concentration range from 5% to 20%. It is evident that 100% desorption of metal is achieved in HNO3 than in HCl. The desorption was maximum at higher acid concentrations (20%). Retention capacity of the biomass for metal ions and its regeneration trials
Fig. 7. Effect of HNO3 concentration on percentage desorption. Range of acid concentration: 5 – 20%; volume of solution: 100 ml; biomass loading: 2.0 g; concentration of metal ions: 100 ppm.
44
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Table 2 Analytical application S. no.
Sample label and composition
Pb percentage removal
Zn percentage removal
Cr percentage removal
1
Syn-1 composition (ppm) Pb = 100; Zn = 500; Cr = 100 Syn-2 composition (ppm) Pb = 250; Zn = 250; Cr = 50 Syn-3 composition (ppm) Pb = 500; Zn = 100; Cr = 250 Real-1 composition (ppm) Pb = 26.04; Zn = 0.41 Real-2 composition (ppm) Pb = 19.27; Zn = 0.94 Real-1 + Cr Syn composition (ppm) Pb = 26.04; Zn = 0.41; Cr = 100
100
92
80
100
90
86.5
96.02
72.3
82.3
75.27
52.99
–
95.87
69.56
–
76.38
50.33
56.5
2 3 4 5 6
Average of five readings.
was carried out for three cycles and these results indicate the possibility of regenerating the plant biomass for its reuse.
4. Analytical application The studies reported here are conducted using synthetic metal ion solutions. However, the metal ions at the range of concentrations chosen are representative of typical waste effluents emanating from various nonferrous processing industries. Therefore, the results of this work have considerable practical implications. The results shown in Table 2 represent the comparison of metal removal in synthetic co-ion solutions and as well in real effluent collected from a nonferrous metal industry. As we did not get ‘‘real-life’’ effluent samples for Cr, we have spiked sample 1 with Cr and estimated the percentage removal of Cr. The lower percentage removal in case of real life sample can be attributed to the complex nature of the effluents. These results show that this method can be used as an alternative to the conventional wastewater treatment methods.
5. Conclusions This work shows the interest of a concept, based on the waste to treat another waste or to resolve an environmental problem. This method can be employed for the recovery of metals and/or polishing of treated effluents. Recycling of heavy metal-bearing residues could allow the elimination of pollution by them and not its transfer from effluents to the soil through its waste disposal. The results obtained in this study clearly demonstrate the potential of plant biomass for the removal of Pb, Zn and Cr from aqueous solutions. The metal removal reached 100% for Pb within 15 min of contact time but for Zn and Cr, maximum metal uptake is attained within 180 min of the contact.
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A primary study on using this plant for phytoremediation of contaminated soils (with Pb, Zn and Cr) was successful at laboratory scale and allows the elimination of more than 90% of these metal ions. The results of these findings will be published soon.
Acknowledgements Authors are grateful to Dr. K.V. Raghavan, Director, Indian Institute of Chemical Technology (IICT), and Dr. M. Vairamani, Head, Analytical Chemistry Division, IICT for their encouragement and providing facilities for carrying out this work.
References Gaballah, I., Goy, D., Alliain, E., Kilbertus, G., Thauront, J., 1997. Recovery of copper through decontamination of synthetic solution using modified barks. Metal. Metall. Trans. B 28, 13 – 23. Gadd, G.M., 1988. Accumulation of metals by microorganisms and algae. Biotechnology 6b, 401 – 433. Harris, P.O., Ramelone, G.J., 1990. Binding of metal ions by particulate biomass derived from Chlorella vulgaris and Scenedesmus quadacauda. Environ. Sci. Technol. 24, 220 – 228. Khovrychev, M.P., 1973. Absorption of copper ions by cells of Candida utilis. Microbiology 42, 745 – 749. Marcus, Y., Kertes, A.S., 1969. Ion Exchange and Solvent Extraction of Metal Complexes. Wiley, New York. Mattuschka, B., Straube, G., 1993. Biosorption of metals by waste biomass. J. Chem. Technol. Biotechnol. 58, 57 – 63. Modak, J.M., Natarajan, K.A., 1995. Biosorption of metals using nonliving biomass—a review. Min. Metall. Proc. 12, 189 – 195. Reddy, B.R., Mirghaffari, N., Gaballah, I., 1997. Removal and recycling of copper from aqueous solutions using treated Indian barks. Resour., Conserv. Recycl. 21, 227 – 245. Rich, G., Cherry, K., 1987. Hazardous Waste Treatment Technologies. Pudvan Publishers, New York. Scott, C.D., 1992. Removal of dissolved metals by plant tissues. Biotechnol. Bioeng. 39, 1064 – 1068. Sommers, E., 1963. The uptake of copper by fungal cells. Ann. Appl. Biol. 51, 425 – 437. Tiwari, Diwaker, Mishra, Shuddhodan, Mishra, Manisha, Dube, R.S., 1999. Biosorption behaviour of mango (Mangifera indica) and neem (Azardiracta indica) barks for Hg +2, Cr +3 and Cd +2 toxic ions from aqueous solutions. Appl. Radiat. Isotopes 50 (4), 631 – 642. Trivedi, R.K., 1989. Pollution Management in Industries. Environmental Publications, Karad, New Delhi, p. 170. Villaescusa, I., Martinez, M., Miraes, N., 2000. Heavy metal uptake from aqueous solutions by cork and yohimbe bark wastes. J. Chem. Technol. Biotechnol. 75 (9), 812 – 816. Warrier, P.K., 1995. Indian Medicinal Plants, a Compendium of 500 sps., vol. 3. Orient Longman, Madras, pp. 141 – 145. Zhou, J.L., Kliff, R.J., 1991. The uptake of copper from aqueous solutions by immobilized fungal biomass. J. Chem. Technol. Biotechnol. 52, 317 – 340.
Removal of heavy metals using a plant biomass with reference to environmental control K. Chandra Sekhar a,*, C.T. Kamala a, N.S. Chary a, Y. Anjaneyulu b a
Analytical Chemistry Department, Indian Institute of Chemical Technology1, Hyderabad 500 007, India b Centre for Environment, Jawaharlal Nehru Technological University, Hyderabad 500 028, India Received 25 March 2001; received in revised form 30 April 2002; accepted 30 April 2002
Abstract Heavy metal pollution has become one of the most serious problems today, and the use of microbial and plant biomass for the detoxification of industrial effluents for environmental protection and recovery of valuable metals offers a potential alternative to existing treatment technologies. In the present study, the biosorption capacity of a plant biomass was studied for different toxic metals and the removal was found to be higher for Pb, Zn and Cr among the 11 metals studied (As, Se, Zn, Fe, Ni, Co, Pb, Mn, Hg, Cr and Cu). The results of the biosorption studies revealed higher Pb removal followed by Cr and Zn at lower metal concentrations, less than 250 ppm and with biomass concentrations above 2 g. The results of shake flask experiment revealed enhanced metal removal with 15 min agitation for Pb and 180 min for Zn and Cr removal. Metal removal was higher at lower pH for Cr and Zn and increased pH decreased the percentage metal removal. Lead removal was unaffected by pH changes. The presence of co-ions (As, Se, Hg, etc.) did not affect Pb removal by biomass, but on the other hand, Zn and Cr uptakes decreased. For the reuse of biomass, the used biomass was subjected to desorption studies using HNO3. The retention capacity of the biomass was almost constant after three cycles of chelation – desorption, suggesting that the lifetime cycle was sufficiently long for continuous industrial application. The suggested process can be used as an alternative to the classical technologies for effluent decontamination and would also be efficient for polishing effluents treated by other methods. The biosorption model developed was applied to a ‘‘real life system’’ successfully. D 2003 Elsevier Science B.V. All rights reserved. Keywords: plant biomass; biosorption; co-ion effect; desorption
*
Corresponding author. E-mail addresses: [email protected], [email protected] (K. Chandra Sekhar). 1 IICT Communication Number: 4627. 0301-7516/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 4 7 - 9
38
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
1. Introduction The discharge of heavy metals into aquatic systems has been a matter of worldwide concern over the last few decades. These pollutants are introduced into aquatic system significantly as a result of various industrial operations. Over a few decades, community is devoting concentrated efforts for the treatment and removal of heavy metals in order to combat this problem. The commonly used procedures for removing metal ions from dilute aqueous streams include chemical precipitation, reverse osmosis and solvent extraction (Rich and Cherry, 1987). However, these techniques have certain disadvantages such as incomplete metal removal, high reagent and energy requirements, generation of toxic sludge or other waste products that require disposal. Heavy metals such as lead, zinc and chromium have a number of applications in basic steel works, paper and pulp, leather tanning, organochemicals, petrochemicals, fertilizers, etc. Major lead pollution is through automobiles and battery manufacturers. For zinc and chromium, their major application is in fertilizer and leather tanning, respectively (Trivedi, 1989). The search for alternate and innovative treatment techniques has focussed attention on the use of biological materials for heavy metal removal and recovery technologies (Biosorption) and has gained important credibility during recent years because of the good performance and low cost of this complexing material (Scott, 1992). The natural affinity of biological compounds for metallic elements could contribute to economically remediating heavily metal-loaded wastewater (Gadd, 1988). Among the various resources in biological wastes, both dead and live biomass of microorganisms (bacteria, yeasts, fungi and algae) and some plants exhibit particularly interesting metal-binding capacities. The use of dead biomass eliminates the problem of toxicity and the economic aspects of nutrient supply and culture maintenance. The biosorption of metals using nonliving biomass has been comprehensively reviewed by Modak and Natarajan (1995). As an alternative to the existing methods, an inexpensive, naturally available plant biomass can be used as a material for removal of toxic heavy metal ions (Reddy et al., 1997; Gaballah et al., 1997; Tiwari et al., 1999; Villaescusa et al., 2000) from synthetic solutions and industrial effluents. In the present investigation, the potential of a plant biomass has been assessed for the removal of metal ions such as lead, zinc and chromium. The effects of various parameters have been studied and the results are presented in this paper.
2. Experimental 2.1. Metal cations The metal cations studied were chromium [Cr(VI)], lead [Pb(II)] and zinc [Zn(II)]. The stock solutions of Pb, Zn and Cr (10,000 ppm) were prepared from lead nitrate [Drug houses (India), Graham Road, Bombay-1], zinc acetate (Glaxo Laboratories India, Bombay) and potassium dichromate (Ranbaxy Laboratories, India) in deionized water. All AR grade reagents and deionized double distilled water were used for all the experiments. The pH adjustments were done using a pH meter (Elico, India). The solutions were agitated using a rotary shaker (Techno India). In all the experiments, the biomass-contained
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
39
solutions were filtered using no. 41 Whatman filter papers. The concentration of metals in the filtered solutions was estimated using Varian Ultra Mass 700 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Australia. 2.2. Biomass In this process, a plant biomass, the root bark of Indian Sarsaparilla was used as biosorbent for removal of Pb, Cr and Zn from aqueous solutions. Indian Sarsaparilla (Hemidesmus indicus) has a wide distribution and taxanomically classified as a member of family Asclepiadacea (Warrier, 1995). The biomass was oven dried at 100 jC and was used in our studies. 2.3. Biosorption and analytical procedures A known weight (2.0 g) of biomass was added to a solution containing 100 ppm of metal ion under investigation and the pH was measured as it is. The solution was shaken well in a rotary shaker at 250 rpm at room temperature (30 F 2 jC) for 30 min. The slurry, after equilibration, was filtered through Whatman 41 filter paper. The clear filtrate was analyzed for metal concentration using ICP-MS. The amount of metal taken up by plant biomass was calculated as the difference between the initial and final concentrations of metal in the aqueous solution.
3. Results and discussion 3.1. Effect of shake time on metal removal This experiment was conducted to enhance metal removal by biomass, simultaneously decreasing the incubation period. The results of the above experiment are represented in Fig. 1. The agitation of metal solutions with biomass was effective for all the metals. Within 15 min of shaking, the percentage removal for Pb reached 100%, whereas for Zn and Cr, the maximum percentage removal was at 180 min. 3.2. Effect of pH on metal removal Experimental results showing adsorption of Pb, Cr and Zn by H. indicus at varying pH are presented in Fig. 2. The removal of Pb was unaffected with increase in pH but the removal of Cr and Zn by H. indicus decreased marginally. The reduction in metal removal with increasing pH beyond its optimum values has been attributed to reduced solubility and precipitation (Harris and Ramelone, 1990; Zhou and Kliff, 1991). 3.3. Effect of biomass concentration on metal removal The results of the experiment with varying biomass concentrations are presented in Fig. 3. With increase in biomass concentration, the percentage removal increases as the number
40
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Fig. 1. Effect of shake time on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH: Pb = 2.0, Zn = 4.0, Cr = 2.0; biomass loading: 2.0 g.
of possible binding sites are increased. The difference in percentage Pb removal with 0.2 g of biomass (94.35%) and 2 g of biomass (100%) was very little, whereas a prominent increase in adsorption of Zn and Cr is observed by increasing the concentration of biomass from 0.2 g (Zn 44.3%; Cr 55%) to 2 g (Zn 86.9%; Cr 81.9%). 3.4. Effect of initial metal concentration on percentage metal removal The results of the experiment are shown in Fig. 4. The results revealed maximum metal removal with lower initial concentrations. The percentage Pb uptake was 100% for metal solutions till 250 ppm metal. Later, an increase in initial concentration decreased the
Fig. 2. Effect of pH on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH range: 2 – 9; biomass weight: 2.0 g.
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
41
Fig. 3. Effect of biomass concentration on metal removal. pH: Pb = 2.0, Zn = 4.0, Cr = 2.0, concentration of metal ions: 500 ppm, volume of solution: 100 ml, biomass loading: 0.2 – 2 g.
percentage binding. The observations are the same for Zn (70%) and Cr (75.3%) at 250ppm concentration. These observations can be explained by the fact that at very low concentrations of metal ions, the ratio of sorptive surface area to the total metal ions available is high and thus, there is a greater chance for metal removal. Thus, at low initial metal ion concentrations, the removal capacity is higher. When metal ion concentrations are increased, binding sites become more quickly saturated as the amount of biomass concentration remained constant. During the saturation trial, the pH of the solution decreases. This suggests that metal binding to the biomass is associated with the release of H + ions into the solution. This clearly indicates that the mechanism of metal binding involves a cation exchange
Fig. 4. Effect of initial concentration on metal removal. pH: Pb = 2.0, Zn = 4.0, Cr = 2.0; concentration range: 50 – 5000 ppm; volume of solution: 100 ml; biomass loading: 2.0 g.
42
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Fig. 5. Effect of multi-component system on metal removal. Concentration of metal ions: 500 ppm; volume of solution: 100 ml; pH: Pb + Zn + Cr = 3.0; biomass loading: 2.0 g.
mechanism, possibly, the metal ions react with hydroxyl or carboxylic groups of the biomass’ organic compounds releasing H + ions into the solution. 3.5. Multicomponent systems The results of the experiment are presented in Fig. 5. The results revealed that Pb removal was not effected by any of the other cations, however, the presence of co-ions enhanced Zn removal but decreased Cr removal. In the presence of co-ions in solution, chemical interactions between the ions themselves, as well as with the biomass, take place resulting in site competition (Sommers, 1963; Khovrychev, 1973). Many of the functional groups present on the cell wall and the membrane are nonspecific and different cations compete for the binding sites. It has been reported that metal removal is increased as the ionic radii of metal cations affect the ion exchange and adsorption process (Marcus and Kertes, 1969). The differences in the sorption affinities may also be attributed to differences in the electrode potentials of the various ions. The greater the electrode potential, the greater is the affinity for biomass (Mattuschka and Straube, 1993). The ionic radii and the electrode potentials of the metal ions are summarized in Table 1. In multicomponent systems, the complex interactions of several factors such as ionic charge, ionic radii and electrode potential will account for the differences in the metal removal
Table 1 Ionic charge, ionic radii and electrode potentials of metal ions Metal ion
Ionic charge
˚) Ionic radii (A
Electrode potential (V)
Pb2 + Zn2 + Cr3 +
+2 +2 +3
1.32 0.74 0.64
0.126 0.763 0.744
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
43
Fig. 6. Effect of HCl concentration on percentage desorption. Range of acid concentration: 5 – 20%; volume of solution: 100 ml; biomass loading: 2.0 g; concentration of metal ions: 100 ppm.
capacity of the biomass. As a result, ordering of the metal ions based on a single factor is very difficult. 3.6. Desorption of metals from the loaded biomass A few cursory experiments were conducted to desorb the metal ions from the loaded biomass as a function of acid concentration and the results are shown in Figs. 6 and 7. The preloaded biomass was subjected to a comparative desorption study using HNO3 and HCl in the concentration range from 5% to 20%. It is evident that 100% desorption of metal is achieved in HNO3 than in HCl. The desorption was maximum at higher acid concentrations (20%). Retention capacity of the biomass for metal ions and its regeneration trials
Fig. 7. Effect of HNO3 concentration on percentage desorption. Range of acid concentration: 5 – 20%; volume of solution: 100 ml; biomass loading: 2.0 g; concentration of metal ions: 100 ppm.
44
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
Table 2 Analytical application S. no.
Sample label and composition
Pb percentage removal
Zn percentage removal
Cr percentage removal
1
Syn-1 composition (ppm) Pb = 100; Zn = 500; Cr = 100 Syn-2 composition (ppm) Pb = 250; Zn = 250; Cr = 50 Syn-3 composition (ppm) Pb = 500; Zn = 100; Cr = 250 Real-1 composition (ppm) Pb = 26.04; Zn = 0.41 Real-2 composition (ppm) Pb = 19.27; Zn = 0.94 Real-1 + Cr Syn composition (ppm) Pb = 26.04; Zn = 0.41; Cr = 100
100
92
80
100
90
86.5
96.02
72.3
82.3
75.27
52.99
–
95.87
69.56
–
76.38
50.33
56.5
2 3 4 5 6
Average of five readings.
was carried out for three cycles and these results indicate the possibility of regenerating the plant biomass for its reuse.
4. Analytical application The studies reported here are conducted using synthetic metal ion solutions. However, the metal ions at the range of concentrations chosen are representative of typical waste effluents emanating from various nonferrous processing industries. Therefore, the results of this work have considerable practical implications. The results shown in Table 2 represent the comparison of metal removal in synthetic co-ion solutions and as well in real effluent collected from a nonferrous metal industry. As we did not get ‘‘real-life’’ effluent samples for Cr, we have spiked sample 1 with Cr and estimated the percentage removal of Cr. The lower percentage removal in case of real life sample can be attributed to the complex nature of the effluents. These results show that this method can be used as an alternative to the conventional wastewater treatment methods.
5. Conclusions This work shows the interest of a concept, based on the waste to treat another waste or to resolve an environmental problem. This method can be employed for the recovery of metals and/or polishing of treated effluents. Recycling of heavy metal-bearing residues could allow the elimination of pollution by them and not its transfer from effluents to the soil through its waste disposal. The results obtained in this study clearly demonstrate the potential of plant biomass for the removal of Pb, Zn and Cr from aqueous solutions. The metal removal reached 100% for Pb within 15 min of contact time but for Zn and Cr, maximum metal uptake is attained within 180 min of the contact.
K. Chandra Sekhar et al. / Int. J. Miner. Process. 68 (2003) 37–45
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A primary study on using this plant for phytoremediation of contaminated soils (with Pb, Zn and Cr) was successful at laboratory scale and allows the elimination of more than 90% of these metal ions. The results of these findings will be published soon.
Acknowledgements Authors are grateful to Dr. K.V. Raghavan, Director, Indian Institute of Chemical Technology (IICT), and Dr. M. Vairamani, Head, Analytical Chemistry Division, IICT for their encouragement and providing facilities for carrying out this work.
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