As(III, V) Uptake from Nanostructured Iron Oxides and Oxyhydroxides: The Complex Interplay between Sorbent Surface Chemistry and Arsenic Equilibria

Iron oxides/oxyhydroxides, namely maghemite, iron oxide-silica composite, akaganeite, and ferrihydrite, are studied for As-V and As-III removal from water in the pH range 2-8. All sorbents were characterized for their structural, morphological, textural, and surface charge properties. The same experimental conditions for the batch tests permitted a direct comparison among the sorbents, particularly between the oxyhydroxides, known to be among the most promising As-removers but hardly compared in the literature. The tests revealed akaganeite to perform better in the whole pH range for As-V (max 89 mg g(-1) at pH(0) 3) but to be also efficient toward As-III (max 91 mg g(-1) at pH(0) 3-8), for which the best sorbent was ferrihydrite (max 144 mg g(-1) at pH(0) 8). Moreover, the study of the sorbents' surface chemistry under contact with arsenic and arsenic-free solutions allowed the understanding of its role in the arsenic uptake through electrophoretic light scattering and pH measurements. Indeed, the sorbent's ability to modify the starting pH was a crucial step in determining the removal of performances. The As-V initial concentration, contact time, ionic strength, and presence of competitors were also studied for akaganeite, the most promising remover, at pH(0) 3 and 8 to deepen the uptake mechanism.