Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate

Schwertmannite effectively sorbs chromate (Cr(VI)), yet the sorption mechanisms remain elusive. We determined the Cr(VI) sorption mechanisms on schwertmannite at pH 3.2 and 5 using combined macroscopic sorption experiments with molecular-scale characterization and by comparing them to arsenate (As(V)) sorption. Cr(VI) adsorbs as bidentate-binuclear (BB) inner-sphere complexes through exchanging more sulfate and less >Fe-OH/OH₂, with 0.59-0.71 sulfate released per Cr(VI) sorbed. While As(V) also forms BB complexes, it exchanges sulfate and >Fe-OH/OH₂ equally with 0.49-0.52 sulfate released per As(V) sorbed. At high As(V) loadings, As(V) precipitates as amorphous FeAsO₄, particularly at low pH. The abovementioned differences between Cr(VI) and As(V) can be related to their different ionic radii and binding strength. Moreover, Cr(VI) and As(V) preferentially exchange sulfate inner-sphere complexes, increasing the proportion of sulfate outer-sphere complexes in schwertmannite. In turn, the concentration of sulfate outer-sphere complexes increases and then decreases with increasing Cr(VI) loading. Results suggest that an oxyanion, which would form inner-sphere complexes on a mineral surface, preferentially exchanges inner-spherically bound oxyanions than outer-spherically bound ones on the surface, even though both are exchanged. This study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.