Abstract
Calcium phosphate minerals are typically the solubility-limiting phase for phosphate in calcareous soils. Magnesium (Mg), despite being present in high concentrations in calcareous soils, has been largely neglected in the study of formation and stabilization of soil phosphate minerals due to the high solubility of pure Mg phosphate phases. In this study, a series of four common calcium and magnesium phosphate minerals, hydroxyapatite/bobierrite and brushite/newberyite were synthesized in the presence of widely varying Mg concentrations to examine the effects of Mg substitution upon the local bonding environment and overall structure of the precipitates. Phosphorus K-edge X-Ray absorption near edge structure (XANES) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) provide insight into the local coordination environment, whereas synchrotron powder X-Ray diffraction (SP-XRD) and transmission electron microscopy (TEM) were used for structural analysis. In acidic to neutral pH, Mg-bearing brushite phases formed over a wide range of Ca:Mg ratios. In neutral to high pH systems, a short-range order amorphous calcium phosphate (ACP) with a local structure analogous with hydroxyapatite precipitated for a wide range of Ca to Mg ratios. It can be inferred that the presence of Mg in soils leads to stabilization of metastable phases: via cation substitution in brushite and via poisoning of crystal growth propagation for hydroxyapatite.
Highlights
Phosphorous (P) is a macronutrient for plant and microbial growth and is integral to all life and is tightly coupled to global food demands
Brushite with increasing Mg substitution was synthesized with starting pH of 5.4, whereas newberyite synthesis with increasing Ca substitution occurred at pH 6.0–7.0. (Results from the newberyite series is found in Supplemental Information)
Low Mg substitution does not induce localized structural changes: X-ray absorption near edge spectroscopy, ATR-FTIR, and XRD spectra and patterns are all in agreeance with a structure that is unchanged from a pure Ca brushite mineral
Summary
Phosphorous (P) is a macronutrient for plant and microbial growth and is integral to all life and is tightly coupled to global food demands. Overapplication of phosphate to soils can lead to environmental consequences such as eutrophication and algal blooms when soil P forms are mobilized via leaching, erosion, and runoff. There is a need to fully understand the chemical fate of P minerals in the environment to enhance bioavailability for crops and to minimize losses to surface waters. To be effective in P nutrient stewardship, scientists require detailed information about the chemical forms of soil P so that they can link speciation to plant uptake and environmental fate. The solubility-limiting solid-phase phosphate mineral is often predicted to be hydroxyapatite (HAP; Ca5 (PO4 ) OH). A number of analogues exist with varying degrees of crystallinity and solubility: amorphous, calcium deficient, and carbonate substituted [1]; all are strongly favored to form at neutral to basic pH system and a Ca:P ratio greater than 1.67:1
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