| 1. |
- Wang, Xiaoming, et al.
(författare)
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Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide
- 2019
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Ingår i: Soil Systems. - : MDPI. - 2571-8789. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Aluminum (Al) oxides are important adsorbents for phosphate in soils and sediments, and significantly limit Phosphate (P) mobility and bioavailability, but the speciation of surface-adsorbed phosphate on Al oxides remains poorly understood. Here, phosphate sorption speciation on amorphous Al hydroxide (AAH) was determined under pH 3-8 and P concentration of 0.03 mM-15 mM using various spectroscopic approaches, and phosphate precipitation mechanisms were discussed as well. AAH exhibits an extremely high phosphate sorption capacity, increasing from 3.80 mmol/g at pH 7 to 4.63 mmol/g at pH 3. Regardless of reaction pH, with increasing P sorption loading, the sorption mechanism transits from bidentate binuclear (BB) surface complexation with d(P-Al) of 3.12 angstrom to surface precipitation of analogous amorphous AlPO4 (AAP), possibly with ternary complexes, such as (equivalent to Al-O)(2)-PO2-Al, as intermediate products. Additionally, the percentage of precipitated phosphate occurring in AAP linearly and positively correlates with P sorption loading. Compared to phosphate reaction with ferrihydrite, phosphate adsorbs and precipitates more readily on AAH due to the higher solubility product (K-sp) of AAH. The formation of AAP particles involves Al-III release, which is promoted by phosphate adsorption, and its subsequent precipitation with phosphate at AAH surfaces or in the bulk solution.
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| 2. |
- Wang, Xiaoming, et al.
(författare)
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Binding geometries of silicate species on ferrihydrite surfaces
- 2018
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Ingår i: ACS Earth and Space Chemistry. - : AMER CHEMICAL SOC. - 2472-3452. ; 2:2, s. 125-134
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Tidskriftsartikel (refereegranskat)abstract
- Silicate sorption on ferrihydrite surfaces, as monomers, oligomers, and polymers, strongly affects ferrihydrite crystallinity, thermodynamic stability, and surface reactivity. How these silicate species bind on ferrihydrite surfaces is, however, not well understood. We have determined silicate binding geometries using a combination of X-ray absorption spectroscopy (XAS), differential atomic pair distribution function (d-PDF) analysis, and density functional theory (DFT) calculations. Silicon K-edge absorption pre edges and DFT-predicted energies indicate that silicate forms monomeric monodentate mononuclear (MM) complexes at low silicate sorption loadings. With increasing silicate loading, the pre-edge peak shifts to higher energies, suggesting changes in the silicate binding geometry toward multidentate complexation. The d-PDF analysis determines the Si Fe interatomic distance to be 3.25 A for the high-loading samples. The DFT calculations indicate that such distance corresponds to an oligomer in the bidentate binuclear (BB) binding geometry. The transition of the silicate sorption geometry accompanied by polymerization can affect stability of ferrihydrite and its adsorption and redox reactivity and increase the degree of Si isotopic fractionation upon silicate sorption on Fe oxides. MM monomeric complexes and BB oligomeric complexes should be used for surface complexation models predicting silicate sorption on Fe oxide surfaces.
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