Phosphate sorption by synthetic amorphous aluminium hydroxides: a ²⁷Al and ³¹P solid‐state MAS NMR spectroscopy study

Abstract

The sorption of phosphate on amorphous aluminium hydroxides was investigated using 27Al and P solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, following the effect of different exposures to soluble phosphate. The spectra obtained were compared with the spectrum of amorphous aluminium phosphate. Aluminium i n the unreacted hydroxide had a 100% octahedral co-ordination. When dried at 200°C and exposed to soluble phosphate, very little (maximum 0.1 %) amorphous aluminium hydroxide transformed to a tetrahedral co-ordination (A1 bound by oxygen bridges to four P atoms), even after 120 d. The tetrahedral co-ordination exists in aluminium phosphate gel, although most of its A1 atoms exhibit an octahedral co-ordination. For the aluminium hydroxide dried at 200"C, no formation of aluminium phosphate in which aluminium is in octahedral co-ordination could be detected, not even when the aluminium hydroxide was exposed to a phosphate solution for 120 d. We concluded that the formation of aluminium phosphate is restricted to the surface of the hydroxide. Most of the phosphate which is bound to the aluminium oxide however may not have formed a 'bulk solid' aluminium phosphate, but is adsorbed on the internal and external surface of the oxide. The same amorphous aluminium hydroxide, dried at 70°C instead of 2OO"C, is converted much more rapidly to aluminium phosphate when exposed to soluble phosphate. We propose a P-induced weathering mechanism to describe P sorption on amorphous aluminium hydroxides at high P concentrations. In addition to NMR, phosphate adsorption experiments conducted on aluminium hydroxides dried at different temperatures produced evidence that the porosity of the aluminium hydroxide aggregated particles can also be a factor controlling the rate of phosphate uptake from solution, if the aggregate is stable (is not resuspended) in solution. 71

Molecular Speciation of Phosphorus Present in Readily Dispersible Colloids from Agricultural Soils

“…Beauchemin et al (2003) reported that Al-P was present only in the soil with the highest ammonium oxalate extractable Al content (673 mmol kg −1 ). The lack of detected Al-P by XANES in soils may result from its relatively low abundance due to inhibition of the formation of Al-P by Fe (hydr)oxides (Lookman et al, 1994). Khare et al (2004) reported that P preferentially adsorbed on ferrihydrite, with the adsorbed PO 4 concentration <100 mmol kg −1 (?3100 mg P kg −1 ) in a model system mixed ferrihydriteboehmite (1:1 mass ratio) suspensions.…”

Measuring Surface Chemical Properties of Soil Using Flow Calorimetry 1

“…This slow uptake is thought to be the result of one or more of the following processes: i) diffusion to less accessible sites within pores of solid aggregates (Willet et al, 1988;Cabrera et al, 1981;Parfitt, 1989;Madrid and de Arambarri, 1985;Lookman et al, 1994); ii) penetration into the amorphous Fe and/or Al oxides by solid state diffusion (van Riemsdijk and de Haan, 1981;van Riemsdijk et al, 1984;Barrow, 1983); and iii) pre cipitation with metals derived by dissolution of the soil matrix (Aulenbach and Meisheng, 1988;Lookman et al, 1994;Martin et al, 1988;Nanzyo, 1984Nanzyo, , 1986Pierzynski et al, 1990). Malati et al (1993) reported that phosphate sorption on kaolinite, mica, and anatase at pH Ͻ 5 was endothermic and generally in the range of ϩ10 to ϩ15 kJ mol Ϫ1 .…”

Role of montmorillonite for enhancing fire retardancy of intumescent PLA

“…The room temperature PLAMel5APP25C30B spectrum had one band centered at 8 ppm, which can be assigned to Al in octahedral coordination [25,26]. Another signal from the OMMT should be observed at 68 ppm corresponding to Al in tetrahedral coordination [25][26][27] but since the amount of OMMT is very low, that one is not observed. At 120 s the spectrum presents two peaks, a broad one at14ppm and second one which is low intense and broad at 8 ppm.…”

“…Phosphoric acid (H3PO4) in aqueous solution (85 %) was used as a reference. 27 Al NMR measurements were carried out on the same spectrometer as above at 24.5 MHz (9.4 T) with MAS of 15 kHz). The repetition time was fixed at 1 s due to the quick relaxation time of the quadripolar nucleus, and a minimum of 1024 scans was necessary to obtain a satisfactory signal to noise ratio.…”

“…Since we investigated the mechanism of action of the intumescent formulations based on APP and OMMT, it is of interest to run 27 Al solid state NMR (Fig. 9).…”