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Dalton Abdala synchrotron environmental molecular LNLS
Figure 1. Conceptual model depicting the surface loading effect of P on surface complexation at the goethite/water interface as determined by P-EXAFS analysis of sorption data. 
Figure 2. Fourier transformed spectra of experimental (solid line) and best fit (dashed line) of the phosphate surface complexes formed at the goethite/water interface at pH 4.5 at 5 and 18 days reaction time. Braces are intended to show the approximate region where the P – O, MS and P – Fe shells most significantly contribute in radial distance in the Fourier transformed spectra.

Residence Time and pH Effects on the Bonding Configuration of Orthophosphate Surface Complexes at the Goethite/Water Interface as Examined by Extended X-Ray Absorption Fine Structure (EXAFS) Spectroscopy

 

ABDALA, Dalton Belchior, NORTHRUP, Paul Andrew, VICENTIN, Flávio César, SPARKS, Donald Lewis 

 

Abstract

 

Identifying the mechanisms by which P is bound to soils and soil constituents is ultimately important as they provide information on the stability of bound species and their reactivity in the environment. EXAFS studies were carried out to provide information on how the local chemical environment of sorbed P changes as an effect of pH and time. Goethite was reacted with orthophosphate at a P concentration of 0.8 mmol L-1 P at pH 3.0, 4.5 and 6.0. The residence time effect on the mechanisms of P sorption on goethite was also evaluated for two different reaction times, 5 and 18 days, on goethite suspensions reacted at pH 4.5. The objective of this study was to understand how P sorption mechanisms change over a wide pH range when subjected to P concentrations above the P saturation ratio of goethite. Phosphorus K-edge EXAFS spectra were collected at 2,150 eV in fluorescence mode and the structural parameters were obtained through the fits of sorption data using Artemis. The monodentate surface complex was shown to be the predominant mechanism by which P sorbs at the goethite surface under the experimental conditions. The lack of a discrete Fe – P shell and the presence of highly disordered structures, particularly, at R-space ≥ 4 suggested the formation of P surface precipitates at the goethite/water interface.

Environmental Implications of our Findings

 

The establishment of environmentally sound P management practices relies on the identification of soil P retention and release mechanisms. Molecular descriptions of phosphate retained on mineral surfaces is important because they allow one to precisely model the P sorption process, to determine the stability of the surface complex being formed and, ultimately, to predict the fate of P in the environment.

 

Observations in this study show that the local coordination environment of P is sensitive to changes in soil pH and reaction time, lending further evidence that at lower pHs, P is more tightly retained by goethite via a bidentate bridging surface complex, exhibiting the shortest bond lengths at the lowest pH studied and that bond lengths progressively increase as an increase in pH occurs. These findings corroborate macroscopic observations that in more acidic soils P availability is limited, presumably due to energetic constraints imparted by the formation of a more thermodynamically stable surface complex even at high P surface loadings, exceeding monolayer formation (0.8 µmol m-2). On the other hand, a monodentate configuration would be the predominant surface complex at higher pHs, particularly at higher P loadings (Abdala et al., 2014).

 

Therefore, in a typical agricultural scenario, where soils are consecutively fertilized with P, whether in organic forms, e.g., manures, or in inorganic forms, e.g., mineral fertilizers, soil pH is generally raised due to liming for crop cultivation and, under such circumstances, the higher pHs may favor the release of P (Abdala et al., 2012). In addition, our observations suggest that P loss potential should be greater following fertilization and, as time proceeds, attenuation in P loss should be expected as a result of longer contact between P and the soil mineral surfaces.

 

 

 

 

Figure 3. Fourier transformed spectra of experimental (solid line) and best fit (dashed line) of the phosphate surface complexes formed at the goethite/water interface at pH 4.5 at 5 and 18 days reaction time. Braces are intended to show the approximate region where the P – O, MS and P – Fe shells most significantly contribute in radial distance in the Fourier transformed spectra.
 
 
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