Effect of Low Cadmium Concentration on the Removal Efficiency and Mechanisms in Microbial Electrolysis Cells

Abstract

Microbial electrolysis cells (MECs) can be used to remove cadmium (Cd²⁺) via three removal mechanisms: electrodeposition; chemical precipitation; and biosorption. Here, we investigated how cadmium concentration affects its removal mechanisms and efficiency in lab-scale MECs. For 10, 50, and 100 μg-Cd/L, cadmium was removed by electrodeposition and biosorption without chemical precipitation. The total amount of cadmium removed by electrodeposition increased from 0.96 to 7.7 μg with the increasing cadmium concentration while its fractional contribution was stationary at 8–10 %. The fractional contribution of biosorption dropped from 59 % to 4 % with the increasing cadmium concentration, but the mass removed by biosorption (3-6 μg) was relatively unaffected by the initial cadmium concentration. For the low concentrations, the cadmium removal was not sufficiently high, varying 13 to 69 %. However, at 2.5 mg-Cd/L, effective removal of cadmium (93 % removal in 7 days) was observed and electrodeposition made the largest contribution to cadmium removal (68 %) while chemical precipitation (18 %) and biosorption (14 %) was relatively minor. These findings showed that cadmium concentration governs the removal mechanisms as well as the removal efficiency in MECs.