The rapid redox transformation of arsenic(As)species in waters presents a great environmental challenge in accurate determination of its concentration and toxicity.As exists primarily in two inorganic species,As(Ⅴ)predominating in oxygenated waters and As(Ⅲ)prev ailing in reducing conditions.As(Ⅲ)is more toxic and mobile than As(Ⅴ),and can be readily oxidized to As(Ⅴ)upon exposure to air.Thus,the oxidation state of As may change during sample collection,storage,and transfer to the laboratory,and could lead to biased conclusions about its toxicity.Therefore,easy and fast on-site separation of As(Ⅲ)and As(Ⅴ)in waters is of great importance.The motivation for our study is therefore to develop a method for rapid on-site separation of As(Ⅴ)and As(Ⅲ)in various aqueous matrices.We present a thiol-modified sand(T-sand),which selectively removed As(Ⅲ)but did not adsorb As(Ⅴ).The novel application of this T-sand in a disposable cartridge could successfully separate As(Ⅴ)(37-970 μg L-1)and As(Ⅲ)(N.D.-488 μg L-1)in 23 groundwater samples collected in areas with naturally occurring As.The As speciation results determined with T-sand separation in the field were consistent with those obtained using HPLC-AFS.Moreover,the separation efficiency was not affected by changes in pH(4-11)and temperature(10-90oC).In addition,the T-sand cartridge was applicable in a wide variety of matrices including groundwater,leachants of the toxicity characteristic leaching procedure,and extracts from the California waste extraction test,sequential extraction test,and in vitro gastrointestinal extraction.This easy-to-use separation method is especially suitable for routine field monitoring of As speciation.