Multiple roles of Ca2+ in the interaction of ciprofloxacin with activated sludge: Spectroscopic investigations of extracellular polymeric substances

Calcium ion is an important cation influencing the binding of recalcitrant organic contaminants with activated sludge during wastewater treatment process, but there is still unknown about its role in amphoteric fluoroquinolones binding. Binding experiments show that Ca2+ markedly inhibited binding of ciprofloxacin (CIP) onto sludge, causing 7-203 times of CIP release. Multi-spectroscopic examinations indicate that tryptophan-like and tyrosine-like proteins in extracellular polymeric substances (EPS) were dominant components for CIP binding by static quenching and forming CIP-proteins complexes. Addition of Ca2+ into EPS and CIP binding systems induced increase of association constants (from 0.024-0.064 to 0.027-0.084 L/mu mol) and binding constants (from 0.002-0.039 to 0.012-0.107) and decrease of binding sites number (from 0.893-2.007 to 0.721-1.386). Functional groups of EPS and secondary structure of proteins were remarkably changed upon reactions with CIP and Ca2+. Calcium ion interacted with EPS and CIP binding system in two distinct ways: Ca2+ shielded Cdouble bondO in amide I in EPS for CIP binding, whereas strengthened binding between CIP and functional groups including Cdouble bondO in carboxyl groups in extra-microcolony polymers and Osingle bondH in extra-cellular polymers by forming ternary complexes. Cation competition for Cdouble bondO in amide I is responsible for Ca2+ induced CIP release from the sludge. Results suggest the highly potential release of CIP from high saline wastewater and cation-conditioned sludge which needs further monitoring and evaluation. (C) 2020 Elsevier B.V. All rights reserved.