Enhanced removal of organoarsenic by chlorination: Kinetics, effect of humic acid, and adsorbable chlorinated organoarsenic

In this study, the effective removal of organoarsenic by the combined process of chlorination + Fe(II) was achieved. Chlorine could effectively degrade roxarsone (ROX) over pH from 5 to 10. The fitting results of acidbase protonation model proved that the degradation of ROX was mainly attributed to the reaction of HOCl and deprotonated ROX. The transformation of arsenic species conformed to the fitting results of two-channel kinetic model, in which 32.4% of ROX was oxidized to As(V) via electron transfer pathway (ii) and the rest was converted into monochloro-ROX via electrophilic substitution pathway (i). Humic acid inhibited the degradation of ROX due to the competitive consumption of chlorine and the restraint on the pathway ii. Subsequently, an enhanced removal of total arsenic achieved after chlorination, due to that the generating As(V) and monochloroROX were easier adsorbed compared with ROX, over 97.8% of total arsenic was removed by ferric (oxyhydr) oxides which in-situ formed from the oxidation of Fe(II). Additionally, toxicity studies indicated that the acute toxicity was significantly eliminated by adding Fe(II) after chlorination, likely due to the removal of As(V) and chlorinated products. Furthermore, organoarsenic was also effectively removed by the combined process of chlorination + Fe(II) in real water.

Automated summary

In this study, effective removal of organoarsenic ROX was achieved by the combined process of chlorination + Fe(II), with chlorine degrading ROX effectively at different pH levels. The degradation of ROX was mainly attributed to the reaction between HOCl and deprotonated ROX, while humic acid inhibited this degradation. Furthermore, an enhanced removal of total arsenic was observed after chlorination, with over 97.8% of total arsenic removed.