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Abstract
This study investigated the biosorption kinetics and equilibrium of cadmium, nickel and zinc by activated sludge from a biological nutrient removal (BNR) process. A series of batch experiments with different initial metal concentrations were conducted to determine the metal removal in BNR sludge. The harvested sludges were from a continuous-flow anaerobic-anoxic-oxic (A2O) system operated under a 10 days of sludge retention time. Batch tests were specially designed to isolate the precipitation effects of metal ions in solution so as to compare the isotherm constants of biosorption with and without precipitation isolation. Experimental results revealed that BNR sludge exhibited two stages of biosorption, i.e., passive and then active uptake, for all three metals. The biosorption kinetic data of three tested metals can be effectively simulated by pseudo-second-order rate equations. Besides, the biosorption isotherm showed that metal biosorption behavior was statistically in agreement with the Freundlich model. The capacity constants k of the Freundlich model for Cd, Ni and Zn are 0.50, 0.23 and 0.41; the affinity constants 1/n are 0.96, 0.81 and 0.31, respectively. Additionally, precipitation behaviors of metals obviously should be carefully examined during biosorption batch tests with activated sludge; otherwise the biosorption effect could be significantly overestimated.