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Bisulfite-activated permanganate (PM/BS) process has shown extremely great potential for the rapid degradation of organic contaminants while the active oxidants involved in this process have been under disputation. In this work, the active oxidants in PM/BS process were re-identified and the contributions of different active oxidants to the degradation of various organic contaminants were investigated with a competition kinetics method. Many lines of evidence indicated that both radical species, including hydroxyl radical (HO) and sulfate radical (SO4-), and reactive manganese species (RMnS), including Mn(VI), Mn(V), and possible Mn(III), contribute to the degradation of contaminants in PM/BS process. SO4- contributed much more than HO to the degradation of all contaminants of interest except nitrobenzene in PM/BS process. RMnS were very selective and showed high reactivity to phenolic and sulfoxide compounds as well as carbamazepine. With the increase of initial pH and the [bisulfite]/[permanganate] molar ratio, a mechanistic changeover from RMnS to radicals occurred during phenol degradation in PM/BS process. The generation of RMnS and radical species, especially SO4-, could be well explained by the proposed mechanism consisting of radical chain reactions and manganese reduction reactions via both electron transfer and oxygen transfer steps.

In this study, Fe2+ addition was employed to overcome the negative effects of humic acid (HA) on contaminant removal by zerovalent iron (ZVI), and its feasibility to improve electron efficiency of ZVI was also tested. HA at high concentrations suppressed the removal of 4-nitrophenol (4-NP) by ZVI, while the addition of 0.25–1.0 mM Fe2+ could greatly mitigate this inhibitory effect and enhance 4-NP reduction. Specifically, with a mixed-order model, global fitting results showed that the addition of Fe2+ increased the rate constant from 0.124 × 10−2–0.219 × 10−2 mM/min to 0.227 × 10−2–0.417 × 10−2 mM/min and shortened lag period from 19.7–47.9 min to 8.0–15.2 min for 4-NP removal. The mechanistic investigation revealed this trend could be explained by the following aspects: i) Fe2+ can facilitate the generation of Fe(II)-containing oxides, which can act as an electron mediator or direct electron donor for 4-NP reduction; ii) the presence of Fe2+ could lead to aggregation of HA particles and accordingly reduced its coverage on ZVI surface. But the results of respike experiments indicate that Fe2+ addition did not show remarkable effect on the electron efficiency of 4-NP by ZVI, which should be associated with that Fe2+ was not able to favor the enrichment of 4-NP on ZVI surface.

Fenton and Fenton-like reactions employed for oxidative treatment of a typical industrial wastewater generate a large amount of ferric hydroxide sludge which has to be properly disposed at a high cost. This paper presents a simple and cost-effective method for recovering the iron catalyst from the iron hydroxide sludge for oxidative treatment of industrial wastewaters. The sludge was dewatered, dried and baked at 350-400 degrees C for 20-30 min; the residual solids were dissolved in sulfuric acid to form the reusable catalyst for Fenton and Fenton-like reactions. The recovered catalyst was highly effective for the oxidative pretreatment of a fine chemical wastewater to improve its biodegradability; the resulting COD removal and BOD(5)/COD ratio of the treated stream remained nearly unchanged during the time period when the regenerated catalyst was reused six times. The simple and effective catalyst regeneration method will make Fenton and Fenton-like oxidation a more cost-effective wastewater treatment alternative.