A spectra metrology insight into the binding characteristics of Cu2+ onto anammox extracellular polymeric substances

Extracellular polymeric substance (EPS) of anaerobic ammonium oxidation (anammox) sludge plays an important role in resisting the toxicity when treating heavy metal containing wastewater. Understanding the interaction mechanism between EPS and heavy metal is essential for stable and optimal operation of such technology. In this work, the identification and preferential response of the binding sites, as well as the binding thermodynamic and the conformational changes of EPS to Cu2+ were explored. Unlike the activated sludge system, functional groups in EPS would coordinate with Cu2+ to form inner sphere complexes, which is an endothermic and entropy-increasing process. Low concentration of Cu2+ was found to result in the reduced electrostatic repulsion and bridging effect, which promoted the compaction and agglomeration of EPS colloid. Two-dimensional correlation spectroscopy analysis confirmed that carboxyl groups of proteins were preferentially responded to Cu2+ compared to polysaccharides and hydrocarbons. More importantly, the secondary structure of the protein in EPS was disrupted and changed into loose state after combining with Cu2+. This work is of great significance for understanding the binding properties of anammox sludge EPS with Cu2+ and may provide theoretical support for elucidating the resist mechanism of anammox bacteria to heavy metals.