Role of low molecular weight organic acids on pyrite dissolution in aqueous systems: implications for catalytic chromium (VI) treatment

A systematic study combining batch experiments with spectroscopic analyses was carried out to better understand the effects of various organic acids on pyrite dissolution and subsequent Cr(VI) removal in aqueous systems. Our results suggest that organic acids had no effect on total Fe dissolution from pyrite relative to systems containing no acid. However, while nearly 100% of total Fe dissolved from pyrite was in Fe(II) form in the absence of ligands, the addition of organic acids led to significant oxidation of Fe(II) species to Fe(III). The degree and extent of Fe(II) oxidation increased in the order: tartrate < salicylate < oxalate ≈ citrate < EDTA. Except for salicylate (an aromatic compound), this stimulatory effect observed in Fe(II) oxidation was well correlated with the strength of Fe-ligand complexes. In systems containing Cr(VI), the amount of Fe dissolved increased significantly relative to non-Cr(VI) containing system, and the ligands enhanced the dissolution of surface oxidation products from pyrite. Overall, it is clear that the dissolution of pyrite with organic acids had very little effect on solution phase Cr(VI) removal, but significantly stimulated surface phase Cr(VI) reduction by removing surface oxidation products, and thus creating new surface sites for extended Cr(VI) removal.