Abstract

Arsenic is known around the globe in recent history due to the consequence of mass poisoning through the exposure of drinking water. Due its carcinogenic and many other adverse health effects, the regulatory authorities like the world health organization (WHO), United States environmental protection agency (USEPA) have reduced the maximum contamination level (MCL) of total arsenic in drinking water from 50 og/L to 10 og/L. Biosorption, a process of passive sequestration of contaminant materials by some dead and inactive biomass, has been preferred in this study to remove arsenic from contaminated water due to its eco-friendly nature and cost-effectiveness in comparison to the conventional technologies. An anaerobic digestion sludge obtained from a wastewater treatment plant was introduced to remove inorganic arsenic from contaminated water. The biomass was prepared in a granular form by drying, grinding and sieving. This granular biomass was investigated in equilibrium batch experiments and used in a continuous flow fixed-bed column operation. The biomass was also treated with KH 2 PO 4 and KCl to improve the sorption capacity but treatment did not contribute to the improvement as anticipated. Removal of arsenate [As (V)] was found pH dependent with the maximum removal at a pH range of 5 to 6, whereas arsenite [As (III)] was almost insensitive to pH over a range of 3 to 10. Initial arsenic concentration and contact time, in addition to pH, affected the biosorption capacity. The biosorption capacity of arsenate [As (V)] was 152 og/g at a pH of 5 and that of arsenite [(As (III)] was 60 og/g at a pH of 8 at an initial arsenic concentration of 2000 og/L for both cases. Adsorption data fitted with Langmuir isotherm model. Kinetic data followed pseudo-second-order model. A 40-minute contact time was sufficient to complete almost 95% of the total biosorption. In column operation, at a pH value of 5, 90 and 220 bed volumes of contaminated water with the respective arsenate concentration of 500 og/L and 200 og/L were treated by bringing the concentrations down to the regulatory limit of 10 og/L. Desorption of almost 40% arsenate was achieved using 0.5M NaCl solution. Protein/amino acid-arsenic interaction was proposed as the dominant mechanism in the biosorption process