『Abstract
The adsorption of phosphate onto calcite was studied in a series
of batch experiments. To avoid the precipitation of phosphate-containing
minerals the experiments were conducted using a short reaction
time (3 h) and low concentrations of phosphate (≦50μM). Sorption
of phosphate on calcite was studied in 11 different calcite-equilibrated
solutions that varied in pH, PCO2,
ionic strength and activity of Ca2+, CO32-
and HCO3-. Our results show strong
sorption of phosphate onto calcite. The kinetics of phosphate
sorption onto calcite are fast; adsorption is complete within
2-3 h while desorption is complete in less than 0.5 h. The reversibility
of the sorption process indicates that phosphate is not incorporated
into the calcite crystal lattice under our experimental conditions.
Precipitation of phosphate-containing phases does not seem to
take place in systems with ≦50μM total phosphate, in spite of
a high degree of super-saturation with respect to hydroxyapatite
(SIHAP≦8.83). The amount of phosphate adsorbed
varied with the solution composition, in particular, adsorption
increases as the CO32- activity
decreases (at constant pH) and as pH increases (at constant CO32- activity). The primary effect of
ionic strength on phosphate sorption onto calcite is its influence
on the activity of the different aqueous phosphate species. The
experimental results were modeled satisfactorily using the constant
capacitance model with ＞CaPO4Ca0
and either ＞CaHPO4Ca+ or ＞CaHPO4- as the adsorbed surface species.
Generally the model captures the variation in phosphate adsorption
onto calcite as a function of solution composition, though it
was necessary to include two types of sorption sites (strong and
weak) in the model to reproduce the convex shape of the sorption
isotherms.』
1. Introduction
2. Experimental procedures
3. Results
3.1. Sorption and desorption kinetics
3.2. Precipitation interferences
3.3. Sorption of phosphate
3.4. Ionic strength
3.5. Sorption edges
4. Modeling
4.1. Models with two types of sites
4.2. Heterogeneity of the calcite surface
4.3. Modeling previously published phosphate adsorption data
5. Discussion
5.1. Implication for the mobility of phosphate in the environment
6. Conclusions
Acknowledgements
Appendix A. Supplementary data
References