Structural characterization of arsenate ion exchanged MgAl-layered double hydroxide

Abstract

This paper describes a study on the adsorption and exchange of toxic and carcinogenic arsenate (AsO 4 3−) ions on positively charged surface of MgAl layered double hydroxides (LDH). The basal spacing of arsenate-free LDH containing nitrate and carbonate ions were d 003 = 0.863 and d 006 = 0.435 nm, which decreased to d 003 = 0.778 and d 006 = 0.387 nm, after ion-exchange adsorption of arsenate, an ion that is smaller than nitrate. After heating up to 350 °C the basal spacing d 003 of arsenic-containing LDH is reduced due to the dehydration of arsenate ions, and the 006 reflection disappears. The X-ray reflections of arsenic-free LDH remain nearly unchanged, because the adsorption of interlamellar water is reversible. Thermo-analytical measurements on LDH samples with identical initial nitrate and carbonate ion ratios but diverse arsenic contents reveal that the extent of mass loss decreases with increasing arsenic content up to 1000 °C. Thermally unstable arsenic oxides are stabilized between the lamellae and are converted to form mixed oxides with the ions constituting the layers of the LDH. TG and DSC measurements reveal that dehydration of LDH samples is shifted to lower temperatures, and water elimination increases proportionally with arsenic content up to 300 °C. This increase is also confirmed by the calculated values of specific enthalpy changes. The values obtained were 2504 J/g for arsenic-free LDH and 2589, 2736 and 3367 J/g for LDH samples containing 9.8, 26.8 and 54.9 mg As(V)/g, respectively. This excess originates from the conversion of arsenate ions to oxides. The peak near 830 cm − 1 appearing in the Raman spectra of LDH samples with relatively high arsenic contents is due to the symmetric vibration of the coordinative bond between arsenate and Mg 2+ and/or Al 3+ ions (As–OMe, where Me = Mg 2+, Al 3+). When the relative area of peak characteristic of the As–OMe bond increases, that of the peaks associated with nitrate and carbonate ions (1045–1070 cm − 1 and ~ 714 cm − 1 ) decrease, which means that, in addition to ionic interactions, single coordinative bonds were also formed.