Fungal biotransformation of vanadinite (Pb5(VO4)3Cl)

Bioweathering effects of different saprotrophic fungi on the vanadium- and lead-containing insoluble apatite group mineral, vanadinite (Pb5(VO4)3Cl) were investigated. Despite the insolubility of vanadinite, fungi exerted both biochemical and biophysical effects on the mineral including etching, penetration and formation of new biominerals. Lead oxalate was the main precipitated biomineral by Aspergillus niger during bioleaching of natural and synthetic vanadinite. Some calcium oxalate monohydrate (whewellite) was also formed with natural vanadinite because of the presence of associated ankerite (Ca(Fe2+,Mg)(CO3)2). A. niger also mediated the precipitation of lead oxalate during growth in the presence of lead carbonate, vanadium(V) oxide and ammonium metavanadate. Abiotic tests confirmed the efficacy of oxalic acid in solubilizing vanadinite and precipitating lead as lead oxalate. Geochemical modelling by PHREEQC and Phreeplot confirmed the complexity of vanadium speciation, and the significant effect of oxalate. Oxalate-vanadium complexes markedly reduced the vanadinite stability field, with cationic lead(II) and lead oxalate also occurring. In all treatments and geochemical simulations, no other lead vanadate, or vanadium minerals were detected. This research highlights the importance of oxalate in vanadinite bioweathering. The findings are also relevant to remedial treatments for lead/vanadium contamination, and novel approaches for vanadium recovery.