Influences of co-existing species on the sorption of toxic oxyanions from aqueous solution by nanocrystalline Mg/Al layered double hydroxide

Abstract

The influences of common oxyanions (i.e. nitrate, silica, sulfate, carbonate, and phosphate) and natural organic matter (NOM) on the sorption of arsenate, chromate, bromate and vanadate (toxic oxyanions) by nanocrystalline Mg/Al layered double hydroxide (LDH) were investigated. Besides the type and concentration of the co-existing species, sorption competition was greatly dependent on the solution pH. In general, based on their sorption competitiveness with the toxic oxyanions, the co-existing common oxyanions were ranked in the order of nitrate < silica ≤ sulfate < carbonate < phosphate, while the toxic oxyanions could be ranked in the order of bromate < arsenate ≈ chromate ≤ vanadate, indicating the oxyanions with lower ionic potentials were less preferably sorbed by the LDH. Based on the comprehensive study on sorption of arsenate by the LDH, the decrease in sorption of toxic oxyanions could be attributed to the competition of common oxyanions for common sorption sites on the LDH which predominantly occurred via ion exchange mechanism. NOM inhibited toxic oxyanion sorption through shielding LDH surface sites, directly competing for sorption sites of LDH, and co-precipitating the LDH. The presence of common oxyanions and NOM generally did not affect the mineral stability of the nanocrystalline LDH.