Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/20/1997
Publication Date: N/A
Citation: Interpretive Summary: Arsenic (As) is a toxic trace element which occurs naturally in soils and soil parent materials. Characterizing the adsorption of arsenic on soils, and the effects of speciation between arsenite (As(III)) and arsenate (As(V)) on adsorption, are critical for developing a comprehensive understanding of As mobility. In addition, soil properties such as %clay and %iron oxide (%FeO) content are important for assessing the retention of As in soil. This study systematically described the adsorption of As(III) and As(V) on three California soils which had been thoroughly characterized in a previous paper. In general, the soils had a higher affinity for As(V) than As(III). However, As(III) was more strongly bound than As(V) on the soils at pH greater than 8 and when total As adsorbed was less than 20 micromole kg 1. Both As(III) and As(V) adsorption depended on %FeO and were highest in the soil with the highest level of dithionite extractable-FeO. This information will eventually be used to develop predictive modeling for As(III) and As(V) transport in soil columns.
Technical Abstract: Adsorption of arsenic(III) and arsenic(V) on three soils was investigated under conditions of variable As concentration, pH, and ionic strength. The objectives were to characterize and compare As(III)and As(V) adsorption on three arid-zone soils from California with known soil properties (surface area, dithionite extractable-Fe,%clay). We employed a novel and sensitive HPLC method to directly determine As(III)/(V) speciation in reaction solutions which verified that the soils contained low levels of background As(V). Oxidation of added As(III) to As(V) was not detectable in soil suspensions during 16 h adsorption experiments below pH 9. The soil with the highest dithionite extractable-Fe and % clay (Wyo) had the highest affinity for As(III) and As(V) and displayed adsorption behavior characteristic of ferric oxides. Adsorption isotherms indicated that As(V) species adsorbed more strongly than As(III)under most conditions. However a reversal in the relative affinity of As(III) and As(V) for the soils was observed at pH greater than 8, especially in the high Fe (Wyo) soil.