Adsorption of Cu²⁺ and Zn²⁺ by extracellular polymeric substances (EPS) in different sludges: Effect of EPS fractional polarity on binding mechanism

Extracellular polymeric substances (EPS) in sludge samples played a major role in heavy metals removal during wastewater treatment. In this study, the binding quality, adsorption mechanism, as well as the chemical fractional contribution of the sludge EPS from activated sludge, anaerobic granular sludge and anaerobic flocculent sludge to the adsorption of Zn²⁺ and Cu²⁺ was investigated. For all three sludge samples, Cu²⁺ could be more easily adsorbed than Zn²⁺, and EPS extracted from the anaerobic granular sludge exhibited a relatively higher adsorption capacity than that of anaerobic flocculent sludge and activated sludge. Specifically, hydrophobic EPS of the activated sludge and anaerobic flocculent sludge was more efficient in adsorbing Cu²⁺ and Zn²⁺ than that of the hydrophilic EPS. However, hydrophilic EPS in anaerobic granular sludge played a greater role in heavy metals removal. The adsorption of those two heavy metals onto the unfractionated and hydrophobic EPS could be better described by the Langmuir isotherm, while Freundlich models fitted hydrophilic EPS. In addition, the effect of the heavy metals adsorption on the spectrum characteristics of the sludge EPS was also explored by analysis of FT-IR and fluorescent spectra.