Mutual enhancement of metal coagulant and peroxymonosulfate for ciprofloxacin removing and the multiple-mode mechanisms

Abstract

Metal coagulants, which are widely employed in water and wastewater treatments, have been recently found to have catalyzation effect on oxidation, while chemical oxidation, in turn, could possibly impose impacts on coagulation process. In this study, a hybrid coagulation/oxidation process was developed using four metal-based coagulants, i.e., aluminum sulfate (Al2(SO4)3), ferric chloride (FeCl3), titanic sulfate (Ti(SO4)2), zirconium oxychloride (ZrOCl2) and peroxymonosulfate (PMS) as the oxidant, for removing ciprofloxacin (CIP), which is a representative emerging organic contaminant (EOC) with small molecular weight (MW). The results showed PMS (0.2 mM) improved the CIP removals by Al2(SO4)3 and FeCl3 from 13% and 21% respectively to around 92% at the coagulant dose of 0.02 mM, and for Ti(SO4)2 and ZrOCl2, the CIP removal was increased from about 21% to 70∼75%, which was also more efficient than separate PMS oxidation (around 42%). The results of zeta potential, dynamic floc growth and residual coagulants demonstrated PMS led to the destabilization of coagulation suspensions, where lag time for floc formation was largely shortened and also, residual metal coagulants in terms of dissolved and colloids/particles were decreased. PMS alleviated the negative effect of crystallization on colloids and clots aggregation via reducing the quantity and crystallinity of nanocrystal according to the high-resolution transmission electron microscopy (HR-TEM) analysis, which facilitated the flocculation. The results of radicals scavenging experiments verified metal coagulants could activate PMS in turn, where •OH played a dominant role in CIP degradation. While, the four kinds of coagulants behaved significantly different when combined with PMS. Due to the different hydrolysis and polymerization properties, coupling PMS with aluminum/iron salt were more efficient than with titanium/zirconium. Nevertheless, the synthetic effect was suppressed as the coagulant dose increased to 0.1 mM and above, where restabilization occurred. Therefore, coagulation played a major role in coagulant/PMS process, which could provide a promising technology for CIP removals.