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

Highlights

• 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.

Abstract

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.

Introduction

Molybdenum (Mo) is widely present in natural environments, and serves as an important micronutrient for living organisms because of its important role in enzymatic redox reactions and biochemical processes (Liu et al., 2018). Due to the strong complexation of thiomolybdate (MoS₄²⁻) with Cu²⁺, an essential trace element providing the critical enzymatic function of connective tissues (Suttle, 1991), the long-term exposure to elevated levels of Mo (VI) could pose a serious threat to living organisms (Vyskocil and Viau, 1999). In natural environment, Mo commonly occurs in various minerals such as molybdenite, jordicite, ferrimolybdate, and wulfenite (Xu et al., 2013). Mo is present mainly as Mo (VI) (MoO₄²⁻) form in natural waters, at an extremely low concentration (<70 μg/L). In hydrometallurgical operations, Mo(VI) could release into the mineral processing solutions and effluents during the extraction of metals by oxidation and acid leaching of the Mo-containing minerals (Migeon et al., 2018; Pestryak et al., 2013; Xu et al., 2013; Zeng et al., 2015). Discharging high-concentration Mo(VI) wastewater may lead to Mo contamination to the surrounding environments such as soils, groundwater, and surface waters (Das and Jim Hendry, 2013). Because the well-known toxicity of the elevated concentration of environmental Mo to living organisms, most Mo in the industrial effluents must be removed and disposed safely for the prevention of contamination to surrounding soils and waters.

The common methods employed to remove the Mo(VI) from wastewater include adsorption (biosorption, Fe-based adsorbents, Al oxide or hydroxide adsorbent, etc), Al-flocculation, wetland technology, and Fe(III)–Mo(VI) coprecipitation, etc (Gary et al., 1975; Gustafsson, 2003; Hua et al., 2018, 2019; Kafshgari et al., 2013; Kim and Zeitlln, 1969; Lin et al., 2014; Zhang et al., 2016). Among these methods, Fe(III)–Mo(VI) coprecipitation is widely practiced in industrial operation around the world owing to its high-efficiency and low-cost (Gary et al., 1975; Liang et al., 2012; Ramadorai and Hanten, 1986; Zhang et al., 2015, 2016). Particularly the source of Fe(III) solutions can be acquired from the leachate or added as ferric sulfate (Bhappu et al., 1965; Langmuir et al., 1999). For example, at JEB uranium mill in northern Saskatchewan, Canada, ferric sulfate (Fe₂(SO₄)₃) was added into the mineral processing effluents to coprecipitate the contaminants including Mo, As etc. and subsequently disposed into tailings ponds ((Blanchard et al., 2015). In Mo-bearing ores mining area, the acid mine drainage (AMD) usually contains molybdate and ferric iron generated from weathering of the ores. Either natural elevation of pH or manual addition of lime to treat the AMD would initiate coprecipitation of Mo(VI) with Fe(III). Hence, our interest of this work focused on understanding the fundamental processes of molybdate removal in this environmentally important Fe(III)–Mo(VI)–SO₄^2- system.

There have been some studies on the coprecipitation of Mo(VI) with Fe(III) regarding the process parameters and the speciation of Mo(VI). The optimal pH for the removal of Mo(VI) lies in a mildly acidic region (i.e. 3.5–4.5) after which the concentration of dissolved Mo(VI) increases with increasing pH (Gary et al., 1975; Kim and Zeitlln, 1969; Lin et al., 2014; Ramadorai and Hanten, 1986; Zhang et al., 2015, 2016). Higher amount of ferric sulfate leads to better molybdenum removal and more than 95% Mo can be removed on the bench scale at pH 4.5 and Fe/Mo > 5 (Liang et al., 2012; Ramadorai and Hanten, 1986).

Considering the actual operation, CaO is generally used as neutralizing reagent (Ramadorai and Hanten, 1986) due to its lower unit price, although most of previous laboratory coprecipitation studies used NaOH rather than CaO as base for the neutralization of acidic Fe(III)–Mo(VI) solutions. In the studies on Fe(III)–As(V) coprecipitation using CaO as base, the increased uptake of arsenate was observed compared to using NaOH, which was explained by the formation of a calcium-iron-arsenate phase (yukonite-like phase) (Jia and Demopoulos, 2008). As an analog to oxyanionic metalloid As(V) tetrahedron, the Mo(VI) metal oxyanion is tetrahetral as well in a wide range of pH values and probably behaves in a similar way. In addition, the influence of coexisting heavy metal cations (Ni²⁺, Zn²⁺, Cu²⁺ etc.) should also be considered on the removal of Mo(VI) because these ions are commonly co-occurring with Mo(VI) in AMD and hydrometallurgical effluents (Ramadorai and Hanten, 1986). It has been reported that the removal of arsenate can be significantly improved by Fe(III)–As(V) coprecipitation at circum-neutral pH in the presence of divalent metal cations such as Cu²⁺, Pb²⁺, Zn²⁺, Ni²⁺ (Emett and Khoe, 1994; Harris and Monette, 1988). By analogy, this may also be the case for Mo(VI) removal during the Fe(III)–Mo(VI) coprecipitation process.

The speciation of Mo(VI) in Fe(III)–Mo(VI) coprecipitates is also an important issue since it controls the stability and geochemical cycling of molybdenum. It has been suggested that the mechanism of iron coagulation removing molybdenum is the surface electrochemical adsorption process on iron oxyhydroxide (Jones, 2006; Lin et al., 2014). In addition, it was proposed that better removal efficiency was achieved at low pH value due to the adsorption of polyoxomolybdate species based on aqueous Mo(VI) speciation modelling (Liang et al., 2012). In a laboratory and pilot scale study on the removal of molybdate and heavy metals from mining wastewaters by coprecipitation with ferric iron, it was thought that molybdate was precipitated as a ferric-molybdenum (Fe–Mo) insoluble complex at acidic pH (Ramadorai and Hanten, 1986). In the JEB mill tailing ponds which contain both flotation tailing and coprecipitates generated in effluents treatment, Mo(VI) was proposed to occur primarily as powellite (CaMoO₄), ferrimolybdite (Fe₂(MoO₄)₃·8H₂O), and molybdate adsorbed on ferrihydrite (Fe(OH)₃–MoO₄) based on Mo K-edge X-ray absorption near edge structure (XANES) analysis (Blanchard et al., 2015). It has been demonstrated that the speciation of metal(loid) oxyanions such as arsenate in the coprecipitates with ferric iron strongly depends on pH and co-occurring metal cations. In the Fe(III)–As(V) coprecipitates, the formation of amorphous ferric arsenate was observed at acidic pH, while As(V)–Fe(III)–Ca(II) association and/or As(V)–Me(II) surface precipitates (Me(II): divalent heavy metal cations, e.g. Ni²⁺, Zn²⁺) occurred in the coprecipitates at circum-neutral pH in the presence of calcium and heavy metal cations (Chen et al., 2009; Jia and Demopoulos, 2008). Here, by analogy we hypothesize that similar mechanisms could also exist in Fe(III)–Mo(VI) coprecipitation system.

In summary, the influence of neutralizing reagent (NaOH vs. CaO) and common coexisting heavy metal cations such as Cu²⁺, Zn²⁺, Ni²⁺ on the removal of molybdenum in the Fe(III)–Mo(VI) coprecipitation process and the speciation of molybdenum in coprecipitates are not well understood. The objectives of this work are (i) to investigate the effect of lime as opposed to sodium hydroxide as base and the coexisting Zn²⁺, Cu²⁺, and Ni²⁺ ions in solution on the coprecipitation of Mo(VI) with Fe(III) at different Fe/Mo ratios and pHs (3–8), and (ii) to characterize the speciation of Mo(VI) in the coprecipitates. The study emphasizes high Mo(VI) concentrations relevant to the treatment of industrial hydrometallurgical effluents.