Effect of oxide coatings, pH and competing anion on the sorption of arsenic species onto Light Expanded Clay Aggregates (LECA’s)

Abstract

Abstract Arsenic (As) is a highly toxic metalloid that it causes globally serious groundwater problems. In aquatic environment it exists as arsenate (AsV) or arsenite (AsIII) which decisively differ in their p K a values and chemical behaviour. Sorption is one of the most efficient technologies applied to remove As from water. However, its efficiency depends on the chemical composition of the sorbent. In this study we compared the ability of Light Expanded Clay Aggregates (LECAs) and LECA coated with aluminium oxide (Al-LECA) or iron oxide (Fe-LECA) to remove As species from water. To unravel the optimal pH conditions for removal, sorption envelopes for AsV were prepared at pH 2–9 and those for AsIII at pH 5–11. Furthermore, effect of phosphate competing with AsV for sorption sites on LECA sorbents was studied at pH 6. The peak sorption of AsV onto LECA, Al-LECA and Fe-LECA was obtained at pH 2, 4 and 6, respectively. As for AsIII, the corresponding peak sorptions were recorded at pH 10, 9 and 10, respectively. The actual sorption capacity relevant in practice was determined on the basis of the As concentration 10 μ g L-1 ( X 10 ) in the equilibrium solution, which corresponds to the upper limit of As in water. The X 10 values estimated at varying solution pHs were decisively lower than the sorption maximum ( X m ). The oxide-coated LECAs were decisively more efficient in As removal than LECA. Furthermore, phosphate decreased the sorption of AsV onto Al-LECA more than that onto Fe-LECA. Although LECA had a lower As sorption capacity than the oxide-coated LECAs, the sorption onto it was less affected by phosphate than that onto the other sorbents. Arsenic is a highly toxic metalloid that it causes globally serious groundwater problems. Sorbent containing Al or Fe oxides as LECA can be used as sorbent for arsenic ions. Coating LECA with Al or Fe oxides decisively increased the removal efficiency.