Adsorption, desorption and oxidation of arsenic affected by clay minerals and aging process

Adsorption/desorption and oxidation/reduction of arsenic at clay surfaces a re very important to the natural attenuation of arsenic in the subsurface e nvironment. Although numerous studies have concluded that iron oxides have high affinities for the adsorption of As(V), very little experimental work has addressed the arsenic attenuation capacities of different clay minerals and aging process affecting the transformation of arsenic. The abundance o f day minerals in a variety of geochemical environments and their influence on adsorption of contaminants suggests a need for more experimental work t o characterize the adsorption desorption, and oxidation of arsenic on clay minerals. In this investigation three types of clay mineral were studied: t he 1:1 layer clays [halloysite (IN), sedimentary M-kaolinite, and weathered EPK-kaolinite]; the 2:1 layer clays [illite (MT) and illite/montmorillonit e (MT)]; the 2:1:1 layer clay [chlorite (CA)]. The halloysite and the chlor ite had much greater As(V) adsorption (25-35 folds) than the other clay min erals. The clay minerals had lower As(III) adsorption than As(V) adsorption , and the adsorption was affected by pH. Desorption of arsenic from the cla y minerals was significantly influenced by the aging process. The quantitie s of extractable As(III) and As(V) decreased with increasing aging time. Th e results demonstrated that oxidation of As(III) to As(V) occurred on the c lay surfaces, whereas reduction of As(V) to As(III) was not found in any of the clay minerals studied. The oxidation of As(III) was affected by the ty pes of clay and aging time.