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 (kox′) 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 kox′ 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.