Electrolysis-Assisted Mn(II)/Sulfite Process for Organic Contaminant Degradation at Near-Neutral pH

Abstract

Manganese-catalyzed sulfite activation (i.e., Mn(II)/sulfite) has emerged as an advanced oxidation process to produce sulfate radical (SO4•−) for water treatment. However, to maintain the catalytic activity of Mn(II) ion, solution acidity has to be kept below pH 4, which is difficult to maintain in practice. Moreover, Mn(II)/sulfite reaction is a strongly oxygen-dependent process, and purging air into reaction solution is another extra cost. To solve the above issues, we devised to implement electrolysis into Mn(II)/sulfite (i.e., electro/Mn(II)/sulfite process) for organic compound (bisphenol A, BPA) oxidation. It was revealed that, under near-neutral conditions (pH 6), the removal rate of 10 μM BPA was increased from 46.3%, by Mn(II)/sulfite process, to 94.2% by electro/Mn(II)/sulfite process. The enhancement of BPA removal after implementation of electrolysis to Mn(II)/sulfite process was investigated, and concluded to be a result of several pathways. In detail, the produced oxygen from water electrolysis, direct sulfite oxidation on anode, and local acidic pH at anode vicinity together play a role in promoting SO4•− production and, therefore, contaminant removal. Radical-scavenging assays confirmed the dominant role of SO4•− in electro/Mn(II)/sulfite process.

Mechanistic Insight into the Heterogeneous Electro-Fenton/Sulfite Process for Ultraefficient Degradation of Pollutants over a Wide pH Range

“…Sulfite has low toxicity, and its median lethal dose (LD 50 ) to animals can reach 3560 mg/kg, which is 3 times that of persulfate (802 mg/kg). 24 Second, as a byproduct of desulfurization, 25 sulfite is inexpensive, which can save cost in practical applications. Studies have shown that using transition metals (Cu, 26 Cr, 27 Fe, 28,29 Co, 30 Mn, 31 etc.)…”

“…Finding a new environmentally friendly and inexpensive alternative to replace persulfate has become a new research focus. Sulfite has low toxicity, and its median lethal dose (LD 50 ) to animals can reach 3560 mg/kg, which is 3 times that of persulfate (802 mg/kg) . Second, as a byproduct of desulfurization, sulfite is inexpensive, which can save cost in practical applications.…”

First evidence of a key role of the peroxymonosulfate radical (SO5•−) in a Fenton-like process based on metabisulfite

“…1d). MBS may react as SO3 2− under Fenton conditions (the Fentonlike reactivity of sulfite is shown in several studies) (Jia et al 2019), but MBS is a more effective way to add sulfite to the system. Sulfite is actually unstable in aqueous solution in the presence of oxygen; therefore, the presence of high concentrations of SO3 2− from the very start could trigger degradation/auto-oxidation reactions.…”