Fundamental insights into ciprofloxacin adsorption by sulfate-reducing bacteria sludge: Mechanisms and thermodynamics

Adsorption plays an important role on ciprofloxacin (CIP) removal in biological wastewater treatment systems. In this study, mechanisms and thermodynamics of CIP adsorption by sulfate-reducing bacteria (SRB) sludge were examined via a series of batch studies. CIP adsorption by SRB sludge was a pH-dependent process and was strongly correlated with CIP speciation and SRB sludge properties via multiple adsorption mechanisms including electrostatic attraction, cation exchange and bridging, π-π interaction and hydrogen bond. The presence of cations such as Cu(II) and Fe(III) significantly improved the CIP adsorption onto SRB sludge via surface complexation (coordination and chelation) at neutral (7.0) and acidic (4.0) pH conditions. The functional groups C–O, C–O–C, N–H, O–H and COOH on SRB sludge surface provided binding sites for CIP adsorption via π-π interaction, hydrogen bond and surface complexation as evident from Fourier Transform Infrared (FTIR) spectroscopy analysis. The adsorption of CIP onto SRB sludge was a spontaneous, exothermic and linear adsorption process that included both physisorption and chemisorption. The findings of this study provided insights into the mechanisms and thermodynamics of CIP adsorption by SRB sludge, and have significant potential of applying SRB sludge system for the treatment of CIP-laden wastewaters.