Formation Of Iron (Hydr)oxides In The Presence Of Arsenic And Investigation Of Arsenic Immobilization Mechanisms

Oxidation of ferrous ion is an important pathway for the formation of iron(hydr)oxides in natural environments, and less attention has been paid to the presence of arsenic on the formation of iron(hydr)oxides and the mechanism of arsenic immobilization during this process is also not clear. In this study, the oxidation and hydrolysis of ferrous in the presence of arsenic at near neutral pH, similar to natural water was carried out. XRD, FT-IR and TEM were used for the determination of the structure and morphology of iron(hydr)oxides, as well as the formation mechanisms of these iron(hydr)oxides. In additon, removal kinetics of arsenate, arsenite, ferrous and ferric were also analyzed to investigate the fate and transport of arsenic during this process.Under pH neutral conditions, lepidocrocite is the most commonly formed iron(hydr)oxides during the abiotic oxidation of Fe(?) ion. In the absence of arsenate, a small amount of goethite forms at pH 6.0 and 7.0, whereas magnetite is the dominant specie at pH 8.0. In the presence of arsenate, dense-structured ferric arsenate forms at pH 6.0, hollow-structured iron(hydr)oxide forms at pH 7.0, and loose-structured ferric arsenate forms at p H 8.0.Arsenite inhibits the formation of goethite and magnetite, and shortens the length of lepidocrocite. It also prolongs the time required for the formation of iron(hydr)oxides. The Fenton-like reaction initiated by Fe(?) in solution results in the oxidation of arsenite to arsenate, and the newly formed arsenate shows different immobilization behaviors under different pH values. Arsenate preferentially forms ferric arsenate at pH 6.0, while favors to adsorb on lepidocrocite at pH 7.0 and 8.0. The presence of arsenate and arsenite on the formation of iron(hydr)oxides is largely due to the formation rate and relative amounts of Fe(O, OH)6 monomers, dimmers and trimers, as well as the competitive connection patterns between arsenate/arsenite and Fe(O, OH)6.The oxidative flocculation of Fe(?) ion shows high removal efficiencies in both arsenate and arsenite. With the initial concentrations of 0.5 mmol·L-1 arsenate and arsenite and at As/Fe molar ratio of 0.5, the removal percentage of arsenate and total arsenic could reach 87.5% and 82.4%, respectively. The key factor during arsenic removal is the initial pH of arsenic solution before Fe(?) addition. The optimal initial pH was 9.0 and 11.0 as to arsenate and arsenite removal, respectively.The significance of this study is to investigate the effect of arsenic on the formation of iron(hydr)oxides and explore the formation mechanism of different iron(hydr)oxides. In addition, this study opens a door for efficient removal of arsenic in aqueous environments, and provides a suitable solution for the remediation of arsenic contaminated sites.