Solid-State NMR Spectroscopy Investigation of Phosphorus Incorporation In Calcium Carbonate

Abstract

Phosphorus is an essential element to Earth's ecosystems its availability to organisms can depend on precipitation as a stable phosphate mineral, alone, as well as within other minerals as trace chemical or structural defects. Of these, calcium carbonate minerals are among the most reactive and abundant in many low temperature geochemical settings and often contain significant amounts of phosphorus. The concentration in which phosphorus is found in natural samples such as speleothems, corals and marine sediments has been used to create paleohydrology and paleonutrient proxies. Through the use of highly sensitive instrumental techniques it is now possible to identify individual phosphorus species in the presence of carbonate minerals to better understand phosphorus mobility and create paleoclimate proxies. This work focuses on the uptake of phosphate during calcite precipitation using solid-state nuclear magnetic resonance (NMR) spectroscopy. The first study focuses on the speciation of phosphorus in a natural speleothem from Christmas Island, investigating the effect of phosphorus extraction from calcite. Three phosphorus species were observed with 31P NMR spectroscopy in the speleothem. The digestion of calcite by application of various acids, bases and chelating agents was used to selectively remove calcium carbonate and its effect on the distribution of the phosphate forms in the speleothem. Through the use of NMR it is possible to investigate if phosphorus species are preserved, specifically targeted, or re-precipitated as a secondary phosphorus species during dissolution treatments. Throughout these studies it was determined that none of the phosphorus species in the speleothem were specifically removed, nor were any secondary phosphorus forms precipitated; during treatment phosphorus bearing calcite appears to be removed along with phosphate-free calcite. The second study focuses on the effects of temperature, initial phosphate concentration, pH, rate of reactant addition, and magnesium concentration on the uptake of dissolved phosphate during calcite precipitation in synthetic samples. It was determined with 31P NMR spectroscopy that some variables did influence the solid in phosphorus species formed, their proportions, and the amount of phosphate included during calcite precipitation. As samples were synthesized with higher phosphate concentrations, faster reactant addition rates, higher magnesium concentrations and lower temperatures the proportion of crystalline calcium phosphates (monetite and an unidentified phase) to phosphate coprecipitated in the calcite structure decreased. Precipitation of crystalline calcium phosphates, including additional unidentified phases, and the number of species present increased at higher temperatures and slower reaction addition rates. These results suggest that the distribution of phosphate species in calcite deposits might provide insight into the conditions of calcite/phosphate coprecipitation in natural systems.