The role of silicate in the adsorption/desorption of arsenite on goethite

Abstract

The importance of anion displacement as a mechanism for arsenic release was examined through the competitive adsorption and desorption of arsenite on goethite in the presence of silicate. Arsenite remained the only arsenic species throughout the adsorption studies. Oxyanion surface concentration versus time plots developed from adsorption experiments were analyzed with a kinetic rate equation to determine the apparent rate coefficients for arsenite and silicate alone and arsenite in the presence of pre-equilibrated silicate. Silicate adsorption on goethite was significantly slower than arsenite. FTIR analysis indicated that silicate polymerization on goethite may be related to the slow adsorption kinetics. Under all conditions pre-equilibrated silicate reduced the rate and total quantity of arsenite adsorbed. The percent increase in the final aqueous arsenite concentration ranged from 0.25 to 30% depending on the concentration of the adsorbed silicate and aqueous arsenite. Increases in the aqueous arsenite were greatest after 30 min before reaching a steady state after 150 min. Desorption experiments where silicate was introduced to previously adsorbed arsenite indicated that silicate was able to irreversibly displace between 0.3 and 1.5% of the adsorbed arsenite resulting in an increase in solution concentrations ranging from 9 to 266 μg L − 1 of arsenite. Experimental results demonstrate the importance of anion displacement as a mechanism for arsenic release. This is readily seen through the ability of silicate, a naturally occurring and ubiquitous oxyanion, to: 1) reduce arsenite adsorption rates, 2) block potential adsorption sites thereby reducing the total quantity of arsenite adsorbed, and 3) displace adsorbed arsenite. These three processes may ultimately result in an increase in the mobility and potential bioavailability of arsenite.