Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate
- Huazhong Agricultural Univ., Wuhan (China); Univ. of Wyoming, Laramie, WY (United States)
- Huazhong Agricultural Univ., Wuhan (China)
- Argonne National Lab. (ANL), Lemont, IL (United States)
- Univ. of Wyoming, Laramie, WY (United States)
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 molecularscale 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 innersphere 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 outerspherically bound ones on the surface, even though both are exchanged. Furthermore, this study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.
- Research Organization:
- Argonne National Laboratory (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES); National Natural Science Foundation of China; Fundamental Research Funds for the Central Universities; National Science Foundation (NSF)
- Grant/Contract Number:
- AC02-06CH11357; AC02-76SF00515
- OSTI ID:
- 1819669
- Alternate ID(s):
- OSTI ID: 1854158
- Journal Information:
- Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 9 Vol. 55; ISSN 0013-936X
- Publisher:
- American Chemical Society (ACS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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