Effect of Ionic Composition on Arsenate Adsorption to Clay Minerals

Abstract:

Adsorption of arsenate on clay minerals can control the mobility of As and subsequent contamination of groundwater. While the effect of ionic strength on As adsorption to phyllosilicate minerals has been evaluated for various clay minerals, the specific ionic composition of the surrounding pore water can play a critical role in promoting adsorption (or desorption) of As(V). We conducted a series of isotherms to evaluate the adsorption of As(V) to various phyllosilicates in the presence of monovalent (K⁺), divalent (Ca²⁺, Mg²⁺) and trivalent (La³⁺) cations while maintaining constant ionic strength and pH. Adsorption isotherms were combined with surface complexation modeling to describe retention mechanisms of As(V) as a function of ionic composition in the surrounding solution. When ionic strength is supplied by higher valency cations, the ions in the surrounding pore water possess higher charger densities. Consequently, these higher valency cations form stronger outer sphere bridging complexes between negatively charged phyllosilicate mineral surfaces and negatively charged arsenate oxyanions. We find that higher valency cations greatly enhance the propensity for As(V) adsorption on phyllosilicate minerals.