The coupling reaction of Fe2+ bio-oxidation and resulting Fe3+ hydrolysis drastically improve the formation of iron hydroxysulfate minerals in AMD

Abstract

The oxidation of Fe²⁺ by Acidithiobacillus ferrooxidans (A. ferrooxidans) in acid mine drainage (AMD) is often accompanied by formation of iron hydroxysulfate minerals, such as schwertmannite and jarosite. This study reported that 80 mmol L^-1 of Fe²⁺ could be completely oxidized by A. ferrooxidans LX5 within 48 h, but only 27.7% of the resultant Fe³⁺ precipitated to form schwertmannite. However, the conversion efficiency to jarosite was much higher (54.5%). The formation of jarosite lasted 120 h, while only 24 h when conversed to schwertmannite. By constructing a cyclic process of 'Cu-reducing coupled with bio-oxidization', the total Fe in AMD could be fully converted into mineral precipitates. The resultant mineral specie could be regulated simply by control the K⁺ concentration. Thermodynamically, Fe³⁺ cannot hydrolyze spontaneously to form schwertmannite due to the positive Gibbs free energy (${\Delta }_r{G}_m^{\circ }$ = 6.63 kJ mol^-1) of the reaction. However, if Fe²⁺ were biologically oxidized by A. ferrooxidans, the resultant Fe³⁺ could spontaneously form schwertmannite because the aforementioned coupling reaction has a negative Gibbs free energy (${\Delta }_r{G}_m^{\circ }$ = -34.12 kJ mol^-1). Even though Fe³⁺ itself could hydrolyze to form jarosite spontaneously with ${\Delta }_r{G}_m^{\circ }$ = -22.20 kJ mol^-1, the coupling reaction of Fe²⁺ bio-oxidation followed by Fe³⁺ hydrolysis in the presence of K⁺ could easily promote the formation of jarosite, which exhibited a great negative Gibbs energy (${\Delta }_r{G}_m^{\circ }$ = -67.45 kJ mol^-1).

Keywords: Acidithiobacillus ferrooxidans; acid mine drainage; coupling; iron hydroxysulfate minerals; thermodynamics.