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Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide

1
Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture and Rural Affairs, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
2
Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY 82071, USA
3
Department of Geosciences, Stony Brook University, Stony Brook, New York, NY 11794-2100, USA
4
Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden
5
Canadian Light Source, University of Saskatchewan, Saskatoon, SK S7N 2V3, Canada
6
X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL60439, USA
*
Author to whom correspondence should be addressed.
Soil Syst. 2019, 3(1), 20; https://doi.org/10.3390/soilsystems3010020
Received: 14 January 2019 / Revised: 27 February 2019 / Accepted: 13 March 2019 / Published: 20 March 2019
(This article belongs to the Special Issue Using Synchrotron Radiation to Perform Phosphate Speciation in Soils)
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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 dP-Al of 3.12 Å to surface precipitation of analogous amorphous AlPO4 (AAP), possibly with ternary complexes, such as (≡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 (Ksp) of AAH. The formation of AAP particles involves AlIII release, which is promoted by phosphate adsorption, and its subsequent precipitation with phosphate at AAH surfaces or in the bulk solution. View Full-Text
Keywords: phosphate; speciation; sorption; precipitation; Al hydroxide phosphate; speciation; sorption; precipitation; Al hydroxide
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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MDPI and ACS Style

Wang, X.; Phillips, B.L.; Boily, J.-F.; Hu, Y.; Hu, Z.; Yang, P.; Feng, X.; Xu, W.; Zhu, M. Phosphate Sorption Speciation and Precipitation Mechanisms on Amorphous Aluminum Hydroxide. Soil Syst. 2019, 3, 20.

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