Influence of arsenate species on the formation of Fe(III) oxyhydroxides and Fe(II–III) hydroxychloride

Iron(III) oxyhydroxides were prepared by oxidation of aerated aqueous suspensions of Fe(II) hydroxide. The effects of arsenate species on their formation were studied by mixing FeCl₂·4H₂O, NaOH and Na₂HAsO₄ solutions. The intermediate and final products of the oxidation processes were characterised by X-ray diffraction, Infrared and Raman spectroscopy. Arsenate species were not reduced during the process but they influenced both oxidation stages, that is the formation of the intermediate Fe(II–III) compound and its subsequent oxidation into Fe(III) compounds. Arsenate species clearly inhibited the growth and hindered the crystallisation of GR(Cl^?), the Fe(II–III) hydroxychloride that would have formed in the experimental conditions considered here. For the largest arsenate concentrations, the intermediate product was nanocrystalline and more likely consisted of clusters showing an ordering of atoms similar to that of GR(Cl^?), isolated from each other by adsorbed arsenate species. The adsorption of As(V) prevented growth of these clusters into well-crystallised GR(Cl^?). The arsenate species influenced similarly the second reaction stage by inhibiting the formation of well-ordered and crystallised Fe(III) compounds. Lepidocrocite, the final product in the absence of arsenate, was replaced by “6-line” ferrihydrite with increasing As(V) concentration, then “6-line” ferrihydrite was replaced by another poorly ordered compound, feroxyhite. These crystallised compounds were obtained together with an increasing part of nanocrystalline Fe(III) ox(yhydrox)ide(s).