Geochemical factors governing vanadium speciation and mobilisation in technosol eco-engineered from bauxite residue...

The high solubility of oxyanions, such as vanadium (V), in alkaline (pH > 9) bauxite residue (BR) generated from alumina production is one of the major environmental risks associated with BR management. Ecological engineering of BR into soil-like growth media (i.e., technosol) offers a promising solution by irreversibly neutralising alkalinity and improving geochemical stability. However, the mechanisms governing the V fractionation, potential mobility, and bioavailability during this transformation remain poorly understood. Here, we show that in a long-term field lysimeter trial, V in BR technosol is primarily hosted in Fe oxide minerals, particularly hematite, of residual phase, followed by organic and oxide-bound phases. The pH was the key driver controlling V solubility, suggesting the importance of irreversible neutralisation of alkalinity in BR using effective eco-engineering inputs (e.g., organic matter and fertiliser). Co-amendments with organic matter and superphosphate fertiliser were most effective in achieving pH neutralisation and V immobilisation. Organic-V complexation in the OM-treated BR can act as an important sink to immobilise V. Consequently, the long-term pollution risk associated with V in BR technosol is expected to be minimal as the technosol system progressively develops into a stable, circumneutral pH, with soil-like properties capable of supporting long-term vegetation growth.