The effects of pH, neutralizing reagent and co-ions on Mo(VI) removal and speciation in Fe(III)–Mo(VI) coprecipitation process

Coprecipitation of molybdate (Mo(VI)) with ferric iron (Fe(III)) is usually employed for the treatment of Mo-bearing acid mine drainage (AMD) and mineral-processing effluents. However, the speciation of Mo(VI) in the Fe(III)–Mo(VI) coprecipitates and the roles of process parameters in Mo(VI) removal remain unclear. In this work, the effects of pH, neutralization reagents (CaO vs NaOH) and co-ions (Zn2+, Cu2+ and Ni2+) on the removal and speciation of Mo(VI) were investigated. It was found that Mo(VI) removal was significantly enhanced at circum-neutral pH by using CaO as base instead of NaOH or in the presence of Ni2+ ions. X-ray diffraction, Fourier transform infrared, Raman and linear combination fitting of Mo K-edge X-ray absorption near edge structure (XANES) spectra results indicated that amorphous ferric molybdate was the major Mo(VI) phase in the coprecipitates formed at acidic pH regardless of the base used. While at circum-neutral pH, Mo(VI) mainly existed as surface adsorbed form on ferrihydrite in NaOH neutralized coprecipitates, and as calcium molybdate in CaO neutralized coprecipitates. The pre-edge features of Mo K-edge and L3-edge XANES features indicated the formation of polymeric molybdate in the acidic coprecipitates. This work may have implications to Mo(VI) immobilization via coprecipitation with Fe(III) and the fate of Mo(VI) in tailings. [Display omitted] •Effects of pH, base, co-cations on Fe–Mo coprecipitation and speciation were studied.•Use of CaO as base and coexisting Zn, Cu, Ni ions enhanced Mo removal at pH ≥ 6.•Coprecipitated Mo was mainly Fe2(MoO4)3 at acidic pH and adsorbed Mo(VI) at pH ≥ 6.•Polymeric Mo(VI) formed in the Fe(III)–Mo(VI) coprecipitates at pH 3–6.