Evaluating the phosphate adsorption on goethite and substituted Al-goethite from a theoretical perspective

PATRICIA G. BELELLI; SILVIA A. FUENTE; MARÍA M. BRANDA; NORBERTO J. CASTELLANI

Granada

Congreso; XXXIX Congreso Internacional de Químicos Teóricos de Expresión Latina (QUITEL); 2013

Instituto Andaluz de Ciencias de la Tierra (IACT)

Phosphate is one of the main nutrients for plants and it is therefore a fundamental part of fertilizers used with agrochemical purpose in crop and fields. Its indiscriminate use is the main reason of the eutrophication arising in nearby water bodies1. In the specialized literature, some experimental investigations have examined the ability of certain mineral surfaces, such as those of oxides and oxohydroxides of iron (Fe), to retain these nutrients. Goethite is the oxo-hydroxide of Fe most studied, due to its crystallinity and stability in natural systems. The obtained results have failed to discriminate the type of phosphate complexes formed on these materials and, in this sense, it is of significance the implementation of theoretical modeling to provide information to the experimentalists. The adsorption modes of the phosphate ion on goethite and Al-goethite surfaces with Alsubstitution of 12.5 mol % were evaluated. The corresponding calculations were performed at the spin-polarized DFT level with the Vienna Ab-Initio Simulation Package (VASP)2,3. The Dudarev approach was used with the U parameter set at 6 eV4. The phosphate was adsorbed on 1 × 2 cells of both (110) goethite and (110) Al-goethite surfaces. Four different coordination modes of the phosphate adsorbed species modes were evaluated: bidentate-binuclear (BB), bidentate-mononuclear (BM), monodentate-mononuclear (MM) and monodentate-binuclear (MB). Besides, for each of these modes, two surface complexes were considered: protonated (H1) and non-protonated (H0), which are the most common species found in the experimental conditions. Our geometrical parameters for the BBH0, BBH1, MMH0 and MMH1 modes are in agreement with the experimental value of 1.54 Å for the mean distance in PO4 minerals (ICSD). Taking into account the P-Os bonds, which are the phosphate links with the surface, we observed that the length of these bonds can be associated with the magnitude of the reaction energies. When the P-Os bond is shortened, more stable becomes the phosphate complex formed on the goethite surface. In all cases, the reaction energy values of phosphate complexes formed on the goethite surface are indicative of a strong interaction (about 3 eV). The most stable complexes show hydrogen bonding between the O atoms of the phosphate and the H atoms of the hydroxyl groups on the surface; thus, this property can be the reason for the significant reaction energies obtained. In addition, the vibrational frequencies of the phosphate complexes and their corresponding normal modes were compared with the experimental FTIR spectra extracted from the literature.