Adsorption and Stability of Arsenic(III) at the Clay Mineral−Water Interface

TLDR

Adsorption and oxidation of arsenite (As(III)) at mineral–water interfaces govern arsenic mobility, yet As(III) is known to be more soluble and mobile than arsenate (As(V)) and its interfacial reactivity remains poorly characterized. This study examined As(III) adsorption on kaolinite, illite, montmorillonite, and amorphous Al(OH)₃ across pH and ionic strength and compared it to As(V) adsorption. Adsorption was quantified by HPLC–HGAAS and interpreted with surface complexation modeling to describe As(III) and As(V) interactions on the four minerals. Alkaline solutions (pH > 9) promoted homogeneous oxidation of As(III) to As(V), while heterogeneous oxidation on kaolinite and illite surfaces further enhanced As(III) conversion to As(V).

Abstract

Adsorption and oxidation reactions of arsenite (As(III)) at the mineral−water interface are two important factors affecting the fate and transport of arsenic in the environ ment. Numerous studies have concluded that As(III) is more soluble and mobile than arsenate (As(V)) in soils, though very little experimental work has demonstrated the differences in reactivity and stability of As(III) and As(V) at the mineral−water interface. In this investigation, As(III) adsorption on kaolinite, illite, montmorillonite, and amorphous aluminum hydroxide (am-Al(OH)3) was studied as a function of pH and ionic strength and was compared with As(V) adsorption. High-performance liquid chromatog raphy−hydride generation atomic absorption spectrophotometry (HPLC−HGAAS) was employed for direct determination of As(III) and As(V). In addition, surface complexation modeling was used to describe As(III) and As(V) adsorption on the four minerals. It was revealed that alkaline solutions (pH > 9) without mineral solids caused homogeneous oxidation of As(III) to As(V). In addition, recovery of adsorbed As from As(III)-treated clay mineral solids showed that oxidation of As(III) to As(V) was enhanced by heterogeneous oxidation on kaolinite and illite surfaces.