The enhancement of tetracycline degradation through zero-valent iron combined with microorganisms during wastewater treatment: Mechanism and contribution.

Ttetracycline (TC) posed potential threats to human health and ecological environment due to its mutagenicity, deformity and strong toxicity. However, few researches focused on the mechanism and their contribution of TC removal through microorganisms combined with zero-valent iron (ZVI) in wastewater treatment field. In this study, three groups of anaerobic reactors, added with ZVI, activated sludge (AS), ZVI coupled with activated sludge (ZVI + AS), respectively, were performed to explore the mechanism and the contribution of ZVI combined with microorganisms on TC removal. The results showed that the additive effects of ZVI and microorganisms improved TC removal. In ZVI + AS reactor, TC was mainly removed by the ZVI adsorption, chemical reduction and microbial adsorption. At the initial period of the reaction, microorganisms played a major role in the ZVI + AS reactors, contributing 80%. The fraction of ZVI adsorption and chemical reduction were 15.5% and 4.5%, respectively. Afterwards, the microbial adsorption gradually reached saturation and the chemical reduction as well as the adsorption of ZVI did their stuff. However, iron-encrustation covered on the adsorption sites of microorganisms and the inhibitory effect of TC on biological activity led to the decreasing TC removal in the ZVI + AS reactor after 23 h 10 min. The optimum reaction time for TC removal in ZVI coupling microbial system was about 70 min. In 1 h 10 min, the TC removal efficiencies were 15%, 63% and 75% in ZVI, AS and ZVI + AS reactors, respectively. Finally, in order to relieve the influence of TC on activated sludge and the iron cladding, a two-stage process was proposed to be explored later in the future.