Kinetics and Mechanisms of Copper Sorption by Loosely Bound Extracellular Polymeric Substances (LB-EPS) from Activated Sludge under Simulating Bio-Calcification Conditions

The behavior and fate of copper binding with extracellular polymeric substances (EPS) from activated sludge, under simulating bio-calcification, was studied. Results showed that the degree of EPS sequestering with calcium had a strong differential affect 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 bio-calcification which isolated Cu²⁺ from EPS. Three-dimensional fluorescence spectrum and Fourier transform-infrared spectra analysis of 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 bio-calcification "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 bio-adsorption in EPS.