Integration of •SO4- -based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II

The sulfate radical (center dot SO4-)-based advanced oxidation processes (AOPs) for the degradation of refractory organic pollutants consume a large amount of persulfate activators and often generate toxic organic byproducts. In this study, we proposed a novel iron-cycling process integrating center dot SO4--based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II completely. The rusted waste iron particles (Fe-0@FexOy), which contained Fe-II/Fe-III oxides (FexOy) on the shell and zero-valent iron (Fe-0) in the core, efficiently activated persulfate to produce center dot SO4- and hydroxyl radicals (center dot OH) to degrade over 95% of Orange II within 120 min. Both center dot SO(4)(-)and center dot OH destructed Orange II through a sequence of electron transfer, electrophilic addition and hydrogen abstraction reactions to generate several organic by-products (e.g., aromatic amines and phenol), which were more toxic than the untreated Orange II. The AOP-generated organic by-products were further mineralized and detoxified in a sulfidogenic bioreactor with sewage treatment together. In a 170-d trial, the organic carbon removal efficiency was up to 90%. The inhibition of the bioreactor effluents on the growth of Chlorella pyrenoidosa became negligible, due to the complete degradation and mineralization of toxic AOP-generated byproducts by aromatic-degrading bacteria (e.g., Clostridium and Dechloromonas) and other bacteria. The sulfidogenic process also well recovered the used Fe-0@FexOy particles through the reduction of surface Fe(III )back into Fe-II by hydrogen sulfide formed and iron-reducing bacteria (e.g., Sulfurospirillum and Paracoccus). The regenerated Fe-0@FexOy particles had more reactive surface Fe ll sites and exhibited much better reactivity in activating persulfate in at least 20 reuse cycles. The findings demonstrate that the integrated process is a promising solution to the remediation of toxic and refractory organic pollutants because it reduces the chemical cost of persulfate activation and also completely detoxifies the toxic byproducts. (C) 2020 Elsevier Ltd. All rights reserved.