Sorption mechanisms of chromate with coprecipitated ferrihydrite in aqueous solution

Abdullah Al Mamun; Masao Morita; Mitsuaki Matsuoka; Chiharu Tokoro

doi:10.1016/j.jhazmat.2017.03.058

Sorption mechanisms of chromate with coprecipitated ferrihydrite in aqueous solution

J Hazard Mater. 2017 Jul 15:334:142-149. doi: 10.1016/j.jhazmat.2017.03.058. Epub 2017 Mar 27.

Authors

Abdullah Al Mamun¹, Masao Morita², Mitsuaki Matsuoka³, Chiharu Tokoro⁴

Affiliations

¹ Graduate School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Electronic address: mamun@toki.waseda.jp.
² Graduate School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Electronic address: masao.swimer@akane.waseda.jp.
³ Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Electronic address: m-matsuoka@aoni.waseda.jp.
⁴ Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Electronic address: tokoro@waseda.jp.

PMID: 28407541
DOI: 10.1016/j.jhazmat.2017.03.058

Abstract

Hexavalent chromium (Cr(VI)) attracted researchers' interest for its toxicity, natural availability and removal difficulty. Nevertheless, its sorption mechanism is not clearly understood yet. In this work, we elucidated the sorption mechanism of the co-precipitation of chromates with ferrihydrite through quantitative analysis. The influence of Cr/Fe molar ratio on sorption was investigated by zeta potential measurements, X-ray diffraction (XRD) and X-ray adsorption fine-structure analysis (XAFS). Coprecipitation at pH 5 showed almost twice the sorption density of adsorption at pH 5. In co-precipitation, a shift of the XRD peak due to inner-sphere sorption of chromate was observed at Cr/Fe molar ratio 0.5. For adsorption, the same peak shift was confirmed at Cr/Fe molar ratio of 1. Zeta potential at pH 5 suggested that the sorption mechanism changed at Cr/Fe molar ratio 0.25 for coprecipitation and at Cr/Fe molar ratio of 1 for adsorption. Fitting of Cr and Fe K-edge extended X-ray adsorption fine-structure suggested that ferrihydrite immobilized Cr(VI) via outer sphere surface complexation for lower Cr/Fe ratios and via inner-sphere surface complexation for higher molar ratios. At higher molar ratios, bidentate binuclear CrFe bonds were well established, thus resulting in the expansion of the ferrihydrite structure.

Keywords: Chromate; Coprecipitation; Extended X-ray absorption fine structure; Ferrihydrite; Inner sphere.