Coexisting Phosphate Controls Arsenate Speciation and Partitioning during Fe(II)-Catalyzed Ferrihydrite Transformation.

Arsenic immobilization in soils and sediments is primarily controlled by its sorption onto or incorporation into reactive soil minerals, such as iron (oxyhydr)-oxides. However, coexisting ions (e.g., dissolved bicarbonate, phosphate, silica, and organic matter) can negatively impact the interaction of the toxic arsenate species with iron (oxy)-hydroxides. Of special note is inorganic phosphate, which is a strong competitor for sorption sites due to its analogous chemical and structural nature to inorganic arsenate. Much of our understanding of this competing nature between phosphate and arsenate focuses on the impact on mineral sorption capacities and kinetics. However, we know very little about how coexisting phosphate will alter the stability and transformation pathways of arsenate-bearing Fe (oxyhydr)-oxides. In particular, the long-term fate and behavior regarding arsenate immobilization are unknown under anoxic conditions. Here, we document, through mineral transformation reactions, the immobilization of both phosphate (P) and arsenate [As-(V)] in secondary mineral products and characterize their changing compositions during the transformations. We did this while controlling the initial P/As-(V) ratios. Our results document that, in the absence or at low P/As-(V) ratios, the initial ferrihydrite rapidly transforms to green rust sulfate (GR