Dissolution, adsorption and phase transformation in the fluorapatite–goethite system

An aqueous system containing fluorapatite (Ca 5(PO 4) 3F), (FAP) and varying amounts of goethite (α-FeOOH) has been investigated. Batch experiments and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to monitor the dissolution products of FAP, as well as the adsorption, and phase transformation of phosphate at the goethite surface over a period of 129 days. The results show that the presence of goethite increases dissolution of FAP, mainly due to the high affinity of phosphate for the goethite surface: Ca 5 ( PO 4 ) 3 F(s) + 3 FeOH + ( 3 + 3 n ) H + ↔ 3 FeH n PO 4 ( n - 2 ) - + 3 H 2 O + 5 Ca 2 + + F - . Besides monitoring the pH changes associated with this reaction, the concentrations of Ca 2+ and fluoride were determined. Furthermore, the amount of phosphate adsorbed was quantified from ATR-FTIR spectra. In addition to adsorbed phosphate, phase transformations of goethite into a Fe phosphate phase (FePO 4(s)) are seen in the samples with relatively high phosphate to goethite ratios (excess phosphate to available surface sites) equilibrated for 15–129 days. An equilibrium model that takes into account (i) FAP dissolution, (ii) solution complexation, (iii) surface complexation of phosphate species onto goethite and (iv) possible phase transformation Ca 5(PO 4) 3F–CaF 2 and FeOOH–FePO 4 was designed. This model was found to be in very good agreement with experimental observations and could thus be used to give qualitative and quantitative information about goethite promoted dissolution of FAP under other pH conditions than those studied in the present work.