Adsorption of arsenic with struvite and hydroxylapatite in phosphate-bearing solutions

Arsenic sorption at above neutral pH is relevant when considering contaminant mobility in alkaline, phosphorus-bearing wastewaters, and may be viable in the presence phosphate minerals. Arsenic adsorption on struvite (MgNH4PO4 · 6H2O, MAP) and hydroxylapatite (Ca5(PO4)3OH, HAP) was evaluated at pH 8-11 from solutions with 2.7-0.125 mM phosphate and 0.05 mM As(III) or As(V). Over 7 d, As(III) removal from solution was minimal, but As(V) removal increased with pH, and was higher in the presence of MAP compared to HAP with a maximum of 74% removal in pH 11 MAP-bearing solutions. X-ray absorption fine structure spectroscopy (XAFS) analysis of solids recovered from pH 10-11 solutions revealed different mechanisms of As(V) sorption with MAP and HAP. Arsenic forms monodentate mononuclear surface complexes with MAP through the formation of a Mg-O-As bond, but is incorporated at the near-surface of HAP forming a johnbaumite-like (Ca5(AsO4)3OH) structure. Experiments using radioactive (33)P at pH 10-11 revealed faster exchange of P at the HAP surface, which could promote more facile As incorporation. Near-surface incorporated As in HAP may be less susceptible to remobilization compared to surface adsorbates formed with MAP. Overall, both MAP and HAP may sorb As at high pH in the presence of phosphate. This is relevant to the fate of As in alkaline contaminated waters in contact with phosphate mineral phases.