Oxidation of Ferrous Sulfate Hydrolyzed Slurry—Kinetic Aspects and Impact on As(V) Removal

Abstract

In this paper the air sparging-oxidation kinetics of hydrolyzed iron(II) sulfate slurries in connection to acidic effluent treatment by neutralization for the removal of contaminants like arsenic(V) were investigated. It was shown in the absence of As(V) and forced air sparging that Fe(II) sulfate of initial concentration 75 mmol/L hydrolyzed completely between pH 7.5 and 8.5 at 22 °C. Subsequent oxidation by forced air sparging of the ferrous hydroxide slurry was found to proceed via a series of transformations starting from ferrous hydroxide to green rust, to lepidocrocite or magnetite depending on the pH and rate of oxidation, and finally to goethite. The oxidation kinetics at pH 8 or higher were governed by oxygen mass transfer while at pH 6 they were chemically controlled with autocatalytic behavior arising from the effect of in situ formed ferric oxyhydroxide. In the presence of As(V), both Fe(II) and As(V) precipitated from solution starting at pH 4 with the latter ultimately dropping below 1 mg/L past pH 6.5 via the apparent formation of ferrous arsenate compound. Subsequent oxidation by air sparging of the Fe(II)–As(V) slurry at constant pH 8 led to destabilization of the arsenate-carrying phase resulting in partial release of As(V). The bulk control of As(V) in the latter case appears to switch from ferrous arsenate to arsenate adsorption on in situ formed iron(III) oxyhydroxide.