Influence of organic acids on selenite sorption by poorly ordered aluminum hydroxides

Competition between tow molecular weight (LMW) organic acids and selenite for sorption sites on hydrous metal oxides is an important factor affecting the dynamics of Se in soils and associated environments. Equilibrium and kinetic studies on the competition between selenite and 12 LMW organic acids (acetic, aspartic, citric, formic, glycine, glycolic, p-hydroxybenzoic, malic, oxalic, salicylic, succinic, and tartaric) for the sorption sites on short-range ordered Al hydroxides were investigated using the batch method. The equilibrium and kinetic results showed that the influence of the organic acids on selenite sorption by Al hydroxides depended on the concentration, structure, and functionality of the organic acids. The competitiveness of the organic acids with selenite for the sorption sites increased with increasing organic acid concentration. Generally, the larger the stability constant of the Al-organic solution complexes (KAl-L), the more effective the organic acid was in competing dth selenite for the sorption sites of the Al hydroxides. However, some of the organic acids competed less successfully than expected based on their KAl-L values. This is attributed to the stereochemical and electrostatic effects originating ham both the surface of the Al hydroxides and the organic acids, which lowered the ability of some organic acids to compete with selenite far the sorption sites of the Al hydroxides. Moreover, the kinetic results revealed that the organic acids competed with selenite for the sorption sites primarily in the early reaction period (<3 h), and subsequently, it was this period that was largely responsible for the equilibrium level of selenite sorbed by the Al hydroxides. The findings are of fundamental significance in understanding the role of organic acids in influencing the fate of Se in the environment.