Redox potential EH [Se(VI)/Se(IV)] of aqueous extractof soils from Yongshou Kaschin-Beck diseaseregion and the influence by humic acid

The relation between E_H [Se(VI)/Se(IV)] and pH of soil aqueous extract in Kaschin-Beck disease region and the effects of natural redox agents, namely humic substances, MnO₂ and Fe²⁺, on the redox property of the system were studied. The results indicated that both humic acid and Fe²⁺ could enhance the reducibility of Se(VI) and MnO₂, a limited oxidizability for Se(IV). Fe²⁺ showed a weak reducibility only at low pH value. The reducibilities of three sulfur-containing compounds for Se(VI) were in following order:thioglycollic acid > L-cysteine > sulfide