Optimization of the Fe(II)/O2/TPP system for ciprofloxacin degradation using response surface methodology

Ciprofloxacin (CIP) was used as the target pollutant to examine its degradation by the Fe(II)/O2/sodium tripolyphosphate (TPP) system. Based on single-factor experiments, a central composite design combined with response surface methodology was employed to systematically investigate the effects of Fe(II) concentration, TPP/Fe(II) molar ratio, and pH on CIP removal efficiency, and a quadratic polynomial regression model was established. The results showed that the order of influence of the three factors on CIP removal efficiency was TPP/Fe(II) molar ratio > pH > Fe(II) concentration. The model exhibited a good fit (R2 = 0.9563) and, thus, demonstrated accurate predictive capability. The optimized conditions were determined as follows: a TPP/Fe(II) molar ratio of 2.45, a pH of 7.0, and a Fe(II) concentration of 15.8 mmol l−1. Under these conditions, a CIP removal rate of 91.7% was achieved after 20 min of reaction. The experimentally optimized TPP/Fe(II) molar ratio closely agreed with the theoretical stoichiometric ratio (2:1) required for the formation of the dominant complex [Fe(II)(TPP)2]4−. Quenching experiments indicated that the reaction follows a consecutive single-electron reduction pathway (O2 → ·O2− → H2O2 → ·OH), with ·OH being the dominant reactive species for CIP degradation, as evidenced by inhibition rates of 91.9%, 91.7%, and 96.9% upon addition of tert-butanol, benzoquinone, and catalase, respectively. This study provides both a theoretical basis and a technical reference for the treatment of CIP-containing wastewater using the Fe(II)/O2/TPP system.