Submitted to: Clays and Clay Minerals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 13, 1996
Publication Date: N/A
Interpretive Summary: Arsenic (As) is a toxic trace element which can occur in saline agricultural drainage waters and ground waters of semi-arid regions of the southwestern United States. The mobility of As in the environment depends on such factors as soil texture and mineralogy, as well as physicochemical parameters including redox potential, pH, and the presence of competing anions. The objectives of this study were to (i) determine the effects of two solution properties (pH and competing ions) on arsenate (As(V)) adsorption on three phyllosilicate soil minerals (kaolinite, montmorillonite, and illite), and (ii) use a mechanistic surface complexation model (the constant capacitance model) to compare As(V) adsorption data from laboratory experiments with model predictions based on our current understanding of mineral surface chemistry. Arsenate adsorption was strongly dependent on pH, being highest on all three minerals in the pH range 5-7 with sharp decreases in adsorption at either lower or higher pH. Phosphate (P) competed strongly with As(V) for binding sites on the minerals whereas molybdate (Mo) affected As(V) adsorption only at low pH (<5). Competition was highest between As(V) and P due to the similarities in chemical properties between the two ions (e.g., ionic size, valence, and acidity) Arsenate and Mo, though both tetrahedral, are chemically quite different which resulted in less competition between these ions.
Technical Abstract: The adsorption of arsenate (As(V)) on the clay minerals kaolinite, montmorillonite, and illite was investigated at varying pH and in the presence of competing anions. Arsenate adsorption envelopes (adsorption vs. solution pH) were determined at constant As(V) concentration (6.7 10-7 M), clay suspension density (2.5 g L-1), and ionic strength (0.1 M NaCl). The effects of two concentrations of phosphate (P) or molybdate (Mo) (6.7 10-7 and 6.7 10-6 M) on the As(V) adsorption envelopes were interpreted as either evidence for competitive adsorption (in the case of As(V)+P) or possibly site-specific adsorption (As(V)+Mo). Arsenate adsorption increased as pH decreased to a point where adsorption maxima were reached at approximately pH 5.0, 6.0, and 6.5 for kaolinite, montmorillonite, and illite, respectively. The relative As(V) adsorption capacity at the adsorption maxima was kaolinite > illite > montmorillonite and ranged from 0.15 to 0.22 mmol As(V) kg-1. When both As(V) and P were present at equimolar concentrations (6.7 10-7 M), As(V) adsorption decreased slightly, whereas As(V) adsorption substantially decreased in binary As(V)/P systems when P was present at Concentrations greater than As(V). The constant capacitance surface complexation model was applied to As(V) adsorption data and used to predict As(V) adsorption at varying P concentrations. The model successfully described As(V) adsorption on the three clay minerals at varying pH and in the presence of co-adsorbing oxyanions, indicating that surface complexation modeling may be useful in predicting As(V) adsorption in soils.