Sulphate-arsenate exchange properties of Zn-Al layered double hydroxides: preliminary data

"Layered double hydroxides" (LDHs), a class of compounds with general formula M(1-x)(2+)M(x)(3+)(OH)(2)(A(n-))(x/n)center dot yH(2)O, have a high anionic exchange capacity, which can widely vary as a function of their composition. Such property is at the base of their use as catalysts and contaminant removers. In this study we investigated the capacity of Zn(Cu)-Al sulphate LDHs to remove As from solution. Previous field investigations coupled with preliminary laboratory experiments showed that natural LDHs of such composition (glaucocerinite and zincowoodwardite) are effective in the attenuation of As content in circum-neutral surface waters. Our laboratory studies confirm that these phases, both synthetic and natural, possess an excellent capacity to reduce As concentration in solution. The main mechanism suggested is a sulphate-arsenate exchange process in the interlayer, possibly with a contribution of surface adsorption. The efficiency of As removal increases with Zn/Al ratios, because the reduced ionic charge of the brucite-like layer facilitates release of sulphate. The removal potential shown by Zn(Cu)-Al sulphate LDHs encourage further investigation of more complex aqueous systems.