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

Abstract

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 Ca 2+ concentrations (≤ 80 mg · l ―1 ), the Langmuir equation was able to model adsorptive processes between Cu 2+ and EPS, which showed competitive adsorption between free Cu 2+ and Ca 2+ ions. However, at higher Ca 2+ concentrations (> 80 mg · l ―1 ), the competitive sorption between Cu 2+ and Ca 2+ was replaced by calcium precipitation which isolated Cu 2+ 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 Ca 2+ concentrations, showed that calcification needles gradually grew and the aggregating EPS inchmeal dissolved. In essence, EPS are potential good for sequestering free Cu 2+ from freshwaters. However in the presence of Ca 2+ , LB-EPS also acts as a nucleation site for precipitation marl deposition which reduces Cu 2+ adsorption. Bio-lithification layer formation in combination with high calcium bio-adsorption levels significantly reduced, but did not eliminate, copper adsorption in EPS.