Synthetic hydrotalcite-type and hydrocalumite-type layered double hydroxides for arsenate uptake

Abstract

The objectives of this study were to (i) synthesize cost-effective layered double hydroxides (LDH) or anionic clays to remove arsenate from water and (ii) quantify arsenate uptake by LDH and understand the mechanisms of uptake. Hydrotalcite and hydrocalumite-type LDHs were synthesized by coprecipitation at room temperature or higher with different compositions of layers and interlayers. The kinetics study showed that anion exchange apparently attained a steady-state in the range of 8–16 h. The arsenate removal was 100% and 99.9% with nitrate form of hydrotalcite and hydrocalumite, respectively. Carbonate and chloride forms of hydrotalcite and chloride form of hydrocalumite removed 50–90% of arsenate from solution. The uptake capacities of hydrotalcites synthesized by different methods were also compared. Hydrotalcite-type LDH synthesized by coprecipitation method had greater uptake capacity than those synthesized by hydrothermal method because of smaller crystal size in the former. The uptake of oxyanions with calcined hydrotalcite-type LDH was higher than with commercially available uncalcined carbonate form of hydrotalcite-type LDH, as expected. Calcination of hydrotalcite-type LDH produced intermediate non-stoichiometric oxides, which underwent rehydration and regeneration of the structure with the incorporation of these oxyanions. In the presence of much larger concentrations of other anions, the uptake of arsenate was reduced but it was still selective on LDH. The results of uptake were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM).