Geochemistry of vanadium in hyperalkaline environments

Hyperalkaline environments associated with leaching of industrial wastes such as steel slags can have a number of deleterious effects on their local environment including rapid carbonate precipitation and mobilisation of potentially toxic trace metals. Steel slag is the primary byproduct of steelmaking and is enriched in trace elements such as Al, Cr and V. V is of particular concern due to its high concentration in steel slag, its potential toxicity and its mobility in high pH leachate as the vanadate oxyanion (VO43-). This study used aerated and air-excluded batch leaching tests, aerated leachate neutralisation tests, field observations, X-ray microanalysis and electron microscopy techniques to investigate the effects of environmental conditions on V mobility, leachate chemistry and secondary phase formation during weathering of steel slags. Scanning electron microscopy/energy- dispersive X-ray spectroscopy (SEM/EDX) identified four principal mineral phases in unweathered slag comprising dicalcium silicate (Ca2SiO4), dicalcium aluminoferrite (Ca2(Al,Fe)2O5), a CaO-rich phase, and a Wüstite-like solid solution ((Fe, Mn, Mg, Ca)O). V was primarily incorporated in the dicalcium aluminoferrite (~1.1 wt%) and dicalcium silicate phases (~0.4 wt%). During leaching, alkalinity was rapidly produced by dissolution of dicalcium silicate and free lime resulting in a high pH (11.5 – 12.5) leachate. Dicalcium silicate dissolution leads to oversaturation with respect to calcium silicate hydrate (Ca-Si-H) phases and their subsequent precipitation. Under aerated conditions, in-gassing of atmospheric CO2 resulted in CaCO3 precipitation which consumed OH- ions and subsequently lowered solution pH to ~8.0. Micro-focus X-ray absorption spectroscopy (µXAS) analysis showed that V was released as V5+ during dicalcium silicate dissolution. V release was significantly higher under aerated conditions than under air-excluded conditions (~850 ppb and 490 ppb respectively in block leaching tests). Aqueous V concentrations were influenced by Ca3(VO4)2 solubility limits which imposed an inverse relationship on Ca and V concentrations. In air-excluded systems, leachate reaches saturation with respect to Ca3(VO4)2 which precipitates thus limiting aqueous V concentrations. Under aerated conditions, precipitation of CaCO3 provided a sink for aqueous Ca. Leachate therefore remained undersaturated with respect to Ca3(VO4)2, allowing higher concentrations of V to accumulate in solution. Leachate neutralisation experiments showed that some V was incorporated into neo-formed CaCO3 at high pH. V removal was enhanced in the presence of goethite (α-FeOOH). Extended X-ray absorption fine structure (EXAFS) analysis showed that vanadate adsorbed to goethite by formation of inner-sphere complexes indicating that Fe (oxy)hydroxides provide an important environmental sink for V in steel slag leachates. This study highlights the importance of leaching environment for V mobility during weathering of steel slags and suggests that prospects for slag reuse and storage may be improved by the formation of a weathered region containing secondary phases including Ca-Si-H and CaCO3. Leachate neutralisation results demonstrate removal mechanisms for aqueous V which will assist with environmental risk assessment and remediation at legacy sites where leachate has been released to the local aquatic environment.