Effect of Phosphate on the Sorption of Arsenite onto Aluminum– or Iron–Oxide Coated Light Expanded Clay Aggregate (LECA)

Abstract

Based on human epidemiology, arsenic (As) is classified as a first group carcinogen and lots of efforts have been made to reduce its intake by humans. Nevertheless, more than 120 million people in Bangladesh and West Bengal are still exposed to groundwater where As concentration is above the WHO maximum permissible limit. In groundwater the conditions favor the occurrence of arsenite (AsIII) that is a more toxic species than arsenate (AsV). Moreover, AsIII has a lower sorption tendency than aluminium (Al) and iron (Fe) oxides. The presence of phosphate in groundwater further reduces the sorption tendency of AsIII. This study was undertaken to unravel the ability of Light Expanded Clay Aggregate (LECA) without and with Al or Fe oxide coating to remove AsIII as well as the impact of phosphate on the removal efficiency. The oxide coatings enhanced decisively the sorption. The sorption of AsIII increased with elevated pH, at pH 7 the apparent maximum sorption (Xm) being in LECA 68 µg g-1, in Al-LECA 114 µg g-1 and in Fe-LECA 96 µg g-1. At pH 10, on the contrary, Xm for Fe-LECA was greater than for Al-LECA. The competition by phosphate affected less the sorption onto LECA than onto the oxide-coated LECAs. At pH 7 the highest P addition (100 µg L-1) diminished the AsIII removal by LECA by 4 times, the corresponding decreases in Al-LECA and Fe-LECA being 6– and 5– fold, respectively. At pH 3 and 5, the competition by phosphate was more marked in Fe-LECA than in Al-LECA, while at pH 7 an opposite trend was detected. In general, the uncoated LECA showed to be superior to the oxide-coated LECAs in the AsIII removal from groundwater.