Mechanisms of Arsenic Adsorption in Calcareous Soils

Arsenic (As) contamination in soil and water is a serious environmental issue. Release of As in soil solution as bioavaiolable form is controlled by its retention mechanisms. The factors controlling As adsorption need to be reviewed in the light of recent advancements in understanding the processes operative in calcareous soil and sediments. The dominant As ionic species in the soil environment are As(III) (H2AsO3 -) or As(V) (HAsO4 2-) depending upon soil pH. Arsenic is retained in soil by adsorption on surface of iron and aluminum oxides and hydroxides and by ligand exchange. Silicate clays (kaolinite, illite and montmorilonite) also adsorb As by coprecipitation with silicates. Soil calcite adsorbed As by forming precipitation. Organic matter, PO4, SO4 and MoO4 affects As adsorption as humate and phosphate ion behave like arsenate and compete for adsorption on adsorption sites. Dominant mechanismof As retention may be due to well crystallized goethite. This paper represents dominant mechanisms of As retention in calcareous soils and sediments.