Controls on arsenate, molybdate, and selenate uptake by hydrotalcite-like layered double hydroxides

Mill-scale neutralization of sulfatic and carbonic magnesium–aluminum–iron residues enriched with trace metal oxyanions often leads to the formation of mixed oxides, such as hydrotalcite-like layered double hydroxides (HT-LDHs). These compounds exert control on the migration of contaminants via sorption and are thus of environmental importance. This study evaluated various controls on arsenate, molybdate, and selenate uptake by HT-LDHs. Arsenate has a stronger affinity for the HT-LDHs than molybdate or selenate. HT-LDHs with both SO42 − and CO32 − as interlayer anions and Fe3 + as a substituting cation act as a better scavenger than CO32 −- and Al3 +-containing HT-LDHs. Greater uptake occurred when excess sorbates were made available, but the percent total uptake was reduced once surface saturation was attained. Sorbate uptake was negatively influenced by increasing pH and the presence of competing anions; the least uptake was observed in the presence of the large charge density PO43 − anion and at pH 10. X-ray diffractogram and Raman spectral studies provided evidence of interlayer occupancy of sorbed oxyanions. X-ray absorption and photoelectron spectroscopic data showed that the redox state of all sorbates remained unchanged and that there is strong binding between ferric iron and adsorbed arsenate that is absent between iron and either adsorbed molybdate or selenate. Partial oxyanion desorption occurred in a reverse order to their affinity for uptake, resulting in a 1.4–8.5% fractional release after 30 days.