Removal and environmentally safe disposal of As(III) and As(V)-loaded ferrihydrite/biosilica composites.

Pure ferrihydrite and ferrihydrite-biosilica composite were synthesized and studied for the removal of As(III) and As(V). The synthesized materials have an adsorption capacity higher than some reported materials in the literature - 140 and 90 mg g-1 for As(III) and As(V), respectively. The pH of the solution was shown to impact greatly on As(V) adsorption, but not on As (III), which is stable as a protonated, uncharged oxyanion, at pH < 9.2. The adsorption products were subjected to thermal treatment (500 °C for 2 h), promoting ferric arsenate formation. The adsorbed As on ferrihydrite (Fh) was shown to inhibit the phase transformation of Fh to hematite. More so, thermal treatment was shown to oxidize As(III) to As (V). The changes in the adsorption residues after thermal treatment also had an impact on As mobility. The As (III) associated with the Fh phase increased from 42 to 95%, according to a sequential extraction protocol. Therefore, this work presents a process for As removal, followed by thermal treatment of arsenic-loaded ferrihydrites which enables environmentally safe disposal of As residues.