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

The sulfate radical (•SO)-based advanced oxidation processes (AOPs) for the degradation of refractory organic pollutants consume a large amount of persulfate activators and often generate toxic organic by-products. In this study, we proposed a novel iron-cycling process integrating •SO-based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II completely. The rusted waste iron particles (Fe@FeO), which contained Fe/Fe oxides (FeO) on the shell and zero-valent iron (Fe) in the core, efficiently activated persulfate to produce •SO and hydroxyl radicals (•OH) to degrade over 95% of Orange II within 120 min. Both •SO and •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 by-products by aromatic-degrading bacteria (e.g., Clostridium and Dechloromonas) and other bacteria. The sulfidogenic process also well recovered the used Fe@FeO particles through the reduction of surface Fe back into Fe by hydrogen sulfide formed and iron-reducing bacteria (e.g., Sulfurospirillum and Paracoccus). The regenerated Fe@FeO particles had more reactive surface Fe 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 by-products.