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Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by zeta-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions.
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Adsorption and precipitation of myo ‐inositol hexakisphosphate onto kaolinite
Abstract Sorption ofmyo‐inositol hexakisphosphate (IHP), a common type of organic phosphorus in soils, largely controls its mobility and bioavailability. Research on the interaction between IHP and phyllosilicate minerals such as kaolinite, which is commonly present in highly weathered soils, has often been neglected, probably due to the common assumption that negatively charged phyllosilicate minerals have low sorption capacity and binding affinity to IHP and thus do not play any significant role in its fate. Here, the interaction between IHP and poorly crystallized kaolinite (KGa‐2) was investigated in batch experiments using Zeta (ζ) potential measurement and31P nuclear magnetic resonance (NMR) spectroscopy. The results showed that dissolved Al(III) concentration at the adsorption initiation stage increased with increasing IHP concentration at pH 4.0. From pH 2.5 to 9.0, IHP presented a maximum sorption capacity (50 μmol g−1) at pH 4.0 at 24 hr. With IHP sorption, theζpotential of kaolinite first decreased sharply to a negative value, then gradually increased with resorption of Al(III) released from kaolinite dissolution at acidic pH, and finally approached the original value of the pure kaolinite.31P NMR spectroscopy andζpotential analyses revealed that IHP formed inner‐sphere surface complexes and aluminium phytate precipitated on kaolinite at low pH (2.5 and 4.0), whereas the formation of inner‐sphere surface complexes was the dominant sorption mechanism at pH ≥ 5.5. This study implies that various mechanisms, depending on ambient pH condition, can dominate the IHP sorption onto kaolinite, which impacts the mobility and bioavailability of phosphorus in highly weathered soils. HighlightsIHP promotes the dissolution of kaolinite mainly through the formation of aluminium phytate complex.IHP sorption presents a sharp maximum at pH 4.0.IHP forms inner‐sphere complexes at the surface of kaolinite.Formation of aluminium phytate surface precipitates is favourable at relatively low pH.
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- Award ID(s):
- 1709724
- PAR ID:
- 10454508
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- European Journal of Soil Science
- Volume:
- 71
- Issue:
- 2
- ISSN:
- 1351-0754
- Format(s):
- Medium: X Size: p. 226-235
- Size(s):
- p. 226-235
- Sponsoring Org:
- National Science Foundation
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