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

An aqueous system containing fluorapatite (Ca5(PO4)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: image Besides monitoring the pH changes associated with this reaction, the concentrations of Ca2+ 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 (FePO4(s)) are seen in the samples with relatively high phosphate to goethite ratios (excess phosphate to available surface sites) equilibrated for 15–129 days.