Original paper

Influence of calcium precipitation on copper sorption induced by loosely bound extracellular polymeric substance (LB-EPS) from activated sludge

Kang, Fuxing; Hamilton, Paul B.; Long, Jian; Wang, Qian


The behavior and fate of copper binding with extracellular polymeric substance (EPS) from activated sludge, under varying calcium concentrations, was studied. Results showed that the degree of LB-EPS sequestering with calcium had a strong differential effect on copper sorption. At lower Ca2+ concentrations (≤ 80 mg · l-1), the Langmuir equation was able to model adsorptive processes between Cu2+ and EPS, which showed competitive adsorption between free Cu2+ and Ca2+ ions. However, at higher Ca2+ concentrations (> 80 mg · l-1), the competitive sorption between Cu2+ and Ca2+ was replaced by calcium precipitation which isolated Cu2+ from EPS. Three-dimensional fluorescence spectrum and Fourier transform-infrared spectra analysis of loosely bound extracellular polymeric substance (LB-EPS) indicated that the EPS sample was composed of humic acid-like and proteins-like complexes containing proteins, polysaccharides, and lipids that reacted with the investigated metal ions. Microscopic images of crystal structure and configuration, with the increase of Ca2+ concentrations, showed that calcification needles gradually grew and the aggregating EPS inchmeal dissolved. In essence, EPS are potential good for sequestering free Cu2+ from freshwaters. However in the presence of Ca2+, LB-EPS also acts as a nucleation site for precipitation “marl” deposition which reduces Cu2+ adsorption. Bio-lithification layer formation in combination with high calcium bio-adsorption levels significantly reduced, but did not eliminate, copper adsorption in EPS.