REMOVAL OF HEAVY METALS FROM INDUSTRIAL WASTEWATERS USING MICROBIAL BIOSORBENTS

Abstract

Industrial effluents contain heavy metals. These are toxic. When released in environment these affect public health. When present in low concentration, conventional treatment technologies have limited capacity to remove them. In addition these are not cost effective. Microbial biosorbents may be used as an economical option. The objective of this research work was to study the potential of various biosorbents for the removal of heavy metals. The biomass of three bacterial strain of Bacillus sp. and three yeast strain of Candida sp was produced in laboratory. The biomass of each strain was then immobilized separately using calcium alginate. This process resulted in beads which were dried at 70 oC. This improved their mechanical properties. Three biosorbents were prepared from Bacillus sp. i.e. (1) immobilized Bacillus circulans beads (IBCB), (2) immobilzed Bacillus licheniformis beads (IBLB) and (3) immobilized Bacillus subtilis beads (IBSB). From Candida sp., the three biosorbent prepared were (1) immobilized Candida lypolytica beads (ICLB) (2) immobilized Candida tropical beads (ICTB) and (3) immobilized Candida utilus beads (ICUB). Removal of cadmium, lead, nickel and zinc ions was studied for batch and continuous flow process. The interaction between the biosorbent and metal ions was confirmed using FTIR and SEM analysis. FTIR analysis showed that the functional groups like hydroxyl, carboxyl, amines, amides and alcohol were mainly involved in the binding of metal ions on the biosorbent. The SEM micrographs revealed changes in the surface morphology of the biosorbents. After the biosorption of heavy metal ions cavities and fissions appeared showing adsorption of heavy metals ions. Cadmium biosorption process was investigated using the ICTB, ICUB, IBLB and IBSB. The optimum value of pH for the ICTB and ICUB was 5.17 whereas for IBLB and IBSB ranged from 5.18 to 5.92. Among these four biosorbents the IBSB showed the better performance for the removal of cadmium ions with maximum biosorption capacity of 225.56 mg g-1 at 25 oC, optimum pH and equilibrium time of 3 h. The significance of different parameters for the biosorption process of cadmium ions was analyzed using ANOVA (analysis of variance). It was found out that initial concentration of metal ions was most important parameter affecting biosorption ~ ii ~ Lead biosorption process was investigated using the ICTB, ICUB and IBSB. Batch studies showed that the optimum value of the pH for ICTB and ICUB was 4.85 whereas for IBSB it ranged from 4.85 to 5.78. Among the three biosorbents tested IBSB showed the highest efficiency with maximum biosorption capacity of 663.41 mg g-1 at 25 o C, optimum pH and equilibrium time of 2 h. Batch studies for nickel ions showed that the optimum pH value for the ICLB, ICTB and ICUB was 6.62 whereas for the IBCB, IBLB and IBLB was 6.04. Among these six biosorbents ICTB showed the best performance with maximum biosorption capacity of 160.49 mg g-1 at 25 oC, optimum pH and equilibrium time of 3 h. Zinc biosorption process was investigated using the ICLB, ICTB, ICUB and IBSB. The optimum value of the pH for ICLB, ICTB and ICUB was 5.17 and for IBSB was 6.35. Batch studies showed that among the four biosorbents ICUB showed best performance with biosorption capacity of 149.70 mg g-1 at 25 oC, optimum pH value and equilibrium time of 4 h. Removal of cadmium, lead, nickel and zinc, in continuous flow, was studied using three columns with internal diameter of 1.5, 2.4 and 3.0 cm. Depth of each column was varied from 20 to 50 cm. Best biosorbent for each metal, resulting from batch studies, was used for the dynamics studies. The dynamics biosorption data was investigated using the various column models. The uptake capacity for the cadmium ions, in column studies, was 48.93 mg g-1of IBSB. It was achieved in fixed bed with column having internal diameter of 2.4 cm, bed depth 20 cm, influent concentration 30 mg l-1 and flow rate of 20 ml min-1. Under similar operating parameters the uptake capacities for the lead ions was 178.57 mg g-1 of IBSB, for nickel ions was 31.28 mg g-1 of ICTB and for zinc ions was 29.50 mg g-1 of ICUB. The maximum uptake capacity of the biosorbents increased with the increase in the bed depth of the column, decrease in flow rate and decrease in internal column diameter for all the four heavy metal ions. In both the batch and continuous biosorption process the metal ions loaded biosorbents were successfully regenerated using the 0.1 M HCl solution. About 99 percent of the adsorbed metal ions were recovered in concentrated solution form. The regeneration proved successful for five consecutive cycles. This could be a major breakthrough in cyclic/commercial use of these immobilized biosorbents (IBs).