Influence of Humic Substances on the Arsenate Immobilization Performance by Magnesium Oxide

Abstract

The objective of this study was to investigate the influence of humic substances on the arenate (As(V)) immobilization performance by magnesium oxide (MgO). To achieve the objective, a humic substance-kaoliniteassociate (HSKA) was artificially synthesized as a model for clays containing organic matter and a series of experiments was performed to obtain insights into the behavior of As(V) in HSKA during leaching tests. Our experimental results showed that the influence of humic substances on the As(V) immobilization performance by MgO depended on the type of humic substances. In particular, water-insoluble humin which contains low concentrations of carboxyl and phenol groups had little influence on the As(V) immobilization performance. On the other hand, humic acids which are alkali-soluble and contains relatively high concentrations of these acid functional groups significantly influenced the As(V) immobilization performance, but the way and magnitude of the influence were different depending on the MgO dosage and As(V) content in HSKA. When the MgO dosage was low, humic acids in HSKA positively influenced As(V) immobilization efficiencies by inhibiting soil pH increase and corresponding decrease in the amount of As(V) desorption from HSKA. However, when the MgO dosage was relatively high, humic acids in HSKA negatively influenced As(V) immobilization efficiencies because humic acidsMg²⁺-As(V) complex were desorbed into the aqueous phase and the complex were poorly sorbed by MgO particles. It was also demonstrated that the magnitude of the influence was larger for the HSKA having lower As(V) content.