Molecular Environmental Sciences
Dalton Abdala's webpage
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. Experimental (solid line) and best fit (dashed line) Fourier transformed spectra of the phosphate surface complexes formed at the goethite/water interface at pH 4.5. A change in spectrum shape (R-space) followed by an increase in the phosphate loading indicates that the phosphate surface speciation changes with surface loading. Braces are intended to show the approximate region where the P – O, multiple scattering (MS) and P – Fe shells most significantly contribute in radial distance in the Fourier transformed spectra.
Surface Loading Effects on Orthophosphate Surface Complexation at the Goethite/Water Interface as Examined by Extended X-Ray Absorption Fine Structure (EXAFS) Spectroscopy
ABDALA, Dalton Belchior, NORTHRUP, Paul Andrew, ARAI, Yuji, SPARKS, Donald Lewis
Abstract
To investigate the effect of P surface loading on the structure of surface complexes formed at the goethite/water interface, goethite was reacted with orthophosphate at P concentrations of 0.1, 0.2, and 0.8 mmol L-1 at pH 4.5 for 5 days. The P concentrations were chosen to ensure that P loadings at the surface would allow one to follow the transition between adsorption and surface precipitation. Extended X-ray Absorption Fine Structure (EXAFS) spectra were collected in fluorescence mode at the P K-edge at 2,150 eV. The structural parameters were obtained through the fits of the sorption data to single and multiple scattering paths using Artemis. EXAFS analysis revealed a continuum among the different surface complexes, with bidentate mononuclear (2E), bidentate binuclear (2C) and monodentate mononuclear (1V) surface complexes forming at the goethite/water interface under the studied conditions. The distances concerning the P – O (1.51 – 1.53 Å) and P – Fe (3.2 – 3.3 Å for bidentate binuclear and around 3.6 Å for mononuclear surface complexes) shells observed in our study were consistent with distances obtained via other spectroscopic techniques. The shortest P – Fe distance of 2.83 – 2.87 Å was indicative of bidentate mononuclear bonding configuration. The coexistence of different surface complexes or the predominance of one sorption mechanism over others was directly related to surface loading.
Environmental Significance of our Findings
In the highly weathered agricultural soils of the tropics, P is arguably the major limiting factor for crop production due to the high sorption capacity of these soils together with its strong binding to mainly Al- and Fe-(hydr)oxide soil minerals. On the other hand, over-application of P fertilizers, particularly via application of organic amendments, has led to the build up of soil P to levels at which P loss potential can be significantly increased (Abdala et al., 2012). Addressing how P surface complexation (SC) is affected by environmental conditions such as surface loading in acidic tropical soils represents a true challenge in terms of analytical methods in view of the limitations imposed by the techniques that have traditionally been employed, e.g., FTIR, for which utilization is constrained under pHs lower than 4.5 (Li et al., 2010) (which is a soil pH range commonly found under tropical conditions) and 31P NMR analysis in Fe-rich soils due to Fe paramagnetism (Kim & Kirkpatrick, 2004).
Showing the feasibility of performing P-EXAFS experiments to probe mineral/water interfaces is itself a great achievement. Therefore, our P-EXAFS results represent a breakthrough over the analytical limitations imposed by the above-mentioned techniques and provide direct evidences on the molecular basis for the low P availability in acidic soils low in P as well as for the greater cycling potential of P in soils high in this element. In addition, it shows the suitability of the EXAFS technique to address P surface complexation at mineral/water interfaces under conditions typically found in tropical soils, that is, at relatively low P concentrations (2.5 μmol L-1, i.e., 77.5 mg kg-1) and at low pHs, as EXAFS is insensitivity to the later.
Our results indicated that P was rapidly (< 5 days) sorbed at the goethite surface, even at surface loadings beyond the P loadings predicted for monolayer coverage on this mineral. Regardless of the P surface loadings employed in this study, P sorbed on goethite via a ligand exchange mechanism, that is, a quite stable surface complex generally presenting covalent bonding character, thus not in equilibrium with soil’s solution phase and not readily available to plants nor easily desorbable. It was also observed that surface loading has a rather determining effect on SC, which transitioned from bidentate binuclear into bidentate mononuclear or monodentate with increases in surface loading (Figure 1). This continuum between binding mechanisms corroborates the vast literature indicating the thermodynamic feasibility for the formation of more stable structures at low surface coverages, where P availability is constrained due to the much higher binding energy involved (Parfitt et al.,m 1975; Parfitt, 1975; 1976; 1977). In most acidic soils, the available P pool associated with soil minerals is usually low and only a small fraction of sorbed P is readily desorbable, most likely from solid phases formed from recent additions of fertilizer P or physically sorbed phosphate by less energetic binding. Phosphorus over fertilization may, therefore, enhance P availability and mobility due to formation of monodentate surface complexes, with a less energetic character, favored at high surface coverages, as was the case at higher P surface loadings employed in this study.
