A 31P and 1H MAS NMR study of phosphate sorption onto calcium carbonate

Sorption of phosphate onto calcite; results from batch experiments and surface complexation modeling

“…The reactions in model 2 are analogous to our model for sorption of arsenate onto calcite (Sø et al, 2008). The use of a protonated adsorbed phosphate species, HPO 2À 4 and CaHPO 0 4 , is not consistent with the 31 P and 1 H MAS NMR study by Hinedi et al (1992), but using solely unprotonated CaPO À 4 as the adsorbed phosphate species resulted in a poor model fit to the experimental data (R 2 = 0.988).…”

“…With respect to pH, House and Donaldson (1986) observed that phosphate sorption increases with increasing pH, whereas Cowan et al (1990) reported the opposite. Such contradictions may be caused by the experimental design, as only some investigators (de Kanel and Morse, 1978;Suzuki et al, 1986;Cowan et al, 1990;van der Weijden and Comans, 1997;Millero et al, 2001;Sawada et al, 2003) have used pre-equilibrated calcite solutions, while others added calcite to a phosphate containing solution which was not in equilibrium with calcite (Griffin et al, 1973;Freeman and Rowell, 1981;House and Donaldson, 1986;Giannimaras and Koutsoukos, 1987;Hinedi et al, 1992;Wang and Tzou, 1995). Using pre-equilibrated calcite solutions minimizes the risk of calcite dissolution or precipitation, and concomitant changes to the calcite surface, while studying phosphate sorption.…”

“…However, the application of these previous models to our experimental data was unsuccessful. Possible reasons include that a) the model was only calibrated on one sorption isotherm (Hinedi et al, 1992), b) the model did not include the surface area of calcite (Millero et al, 2001), c) the model was based on very low total phosphate concentrations (<0.125 lM PO 4 ) where the sorption isotherms were linear (van der Weijden and Comans, 1997) or d) the model was based on one total phosphate concentration (100 lM) assuming linear sorption isotherms (Cowan et al, 1990).…”

“…Different types of models for sorption of phosphate onto calcite exist in the literature, including different types of Langmuir models, anion-exchange models, and the constant capacitance model (House and Donaldson, 1986;Cowan et al, 1990;Hinedi et al, 1992;van der Weijden and Comans, 1997;Millero et al, 2001). Most of these models are based on a calcite surface site density of around 8.2 lmol/m 2 and two adsorbing phosphate species with monodentate complexation.…”

Phosphate defects and apatite inclusions in coral skeletal aragonite revealed by solid-state NMR spectroscopy

“…Phosphate 57 co-precipitation, sorption, and desorption studies at phosphate solution concentrations less than 58 100 M and near neutral pH suggest that inorganic phosphate incorporation in calcite can be 59 modeled by adsorption and subsequent incorporation of protonated phosphate groups (House and 60 Donaldson, 1986;Hinedi et al, 1992;Millero et al, 2001). It has also been shown in certain 61 cases that calcite with 60-100 g/g P can contain crystalline Ca-phosphate inclusions that are 62 difficult to detect by microanalytical methods, suggesting that surface precipitates can form and 63 be subsequently encapsulated by continued crystal growth (Mason et al, 2007).…”