Complete Removal of Organoarsenic by the UV/Permanganate Process via HO^• Oxidation and in Situ-Formed Manganese Dioxide Adsorption

The UV/permanganate process is an innovative advanced oxidation process (AOP) that generates oxidative species, such as hydroxyl radicals (HO^•) and reactive manganese species (RMnS), and adsorptive species of in situ formed MnO₂. The cooperation of oxidation and adsorption has the potential to remove organic metals from water. Herein, the removal of organoarsenic compounds of roxarsone and p-arsanilic acid by UV/permanganate was investigated. The pseudo-first-order reaction rate (k_ox′) of roxarsone was 0.0375 min^–1 at a permanganate dose of 100 μM and pH 7.0 in the UV/permanganate process, which was mainly attributable to HO^•. The transformation of roxarsone was initiated with a HO^• attack on the C–As bond, releasing As­(V), which was then removed from the aqueous phase by MnO₂ adsorption. Additionally, hydroxylated products were generated by HO^• oxidation and subsequently oxidized into nitro-1,4-benzoquinone and ring cleavage products. Under neutral conditions, the removal rate of total arsenic reached 73% at 20 min, 50% of roxarsone was transferred to As­(V) via oxidation, and the in situ formed MnO₂ could remove 78% of the remaining roxarsone and 70% of As­(V) via adsorption. With an increase in the pH from 3.0 to 9.0, the k_ox′ of oxidation decreased by 68%, whereas the removal rate of total arsenic increased from just 20% to 73%. This is the first study to use the UV/permanganate process to completely remove organic heavy metals with the combination of oxidative and adsorptive species, which demonstrates that this process is promising in water treatment.