An EXAFS study of arsenic bonding on amorphous aluminium hydroxide

Arsenic-contaminated waters can be remediated by adsorption of the As onto an amorphous Al(OH)3 sludge precipitated using poly-aluminium chloride, which is a widely used, very effective polymeric coagulant. Investigation of the atomic near-range order and structure of As adsorbed to this sludge using X-ray Absorption Spectroscopy showed that the EXAFS spectrum of the sludge samples is very similar to that of mansfieldite (AlAsVO4·2H2O), indicating that As in the sludge is present as AsV, and that the atomic near-range structure of As is similar in the two materials. In the sludge, the first As–O coordination shell is populated with ∼4 oxygen neighbours at 1.70 ± 0.01 Å, and As has ∼2 Al neighbours at 3.18 ± 0.02 Å, consistent with bidentate binuclear As–O–Al bonding (two O atoms from an AsO4 unit bonded individually to two Al atoms). Previous studies have shown that As bonding on crystalline Al oxides and hydroxides is also bidentate binucleate, contrasting with the variable As bonding on ferric oxy-hydroxides. In particular, the bidentate mononucleate bonding reported for As sorption on amorphous ferric oxy-hydroxide sludges suggests that As is more strongly bonded to Al(OH)3 sludges. The latter may, therefore, have an advantage over ferric oxy-hydroxide coagulants for treatment of As-enriched waters.

► Aluminium hydroxide sludge generated using poly-aluminium chloride adsorbs arsenic. ► XAS showed that arsenic is sorbed to the sludge through a strong bidentate–binucleate bond. ► The strong bonding indicates that the sorbed arsenic is chemically stable.