Characterization and identification of mixed-metal phosphates in soils: the application of Raman spectroscopy

Abstract

The development of protocols for the identification of metal phosphates in phosphate-treated, metalcontaminated soils is a necessary yet problematical step in the validation of remediation schemes involving immobilization of metals as phosphate phases. The potential for Raman spectroscopy to be applied to the identification of these phosphates in soils has yet to be fully explored. With this in mind, a range of synthetic mixed-metal hydroxylapatites has been characterized and added to soils at known concentrations for analysis using both bulk X-ray powder diffraction (XRD) and Raman spectroscopy.Mixed-metal hydroxylapatites in the binary series Ca –Cd, Ca –Pb, Ca –Sr and Cd –Pb synthesized in the presence of acetate and carbonate ions, were characterized using a range of analytical techniques including XRD, analytical scanning electron microscopy (SEM), infrared spectroscopy (IR), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and Raman spectroscopy. Only the Ca –Cd series displays complete solid solution, although under the synthesis conditions of this study the Cd5(PO4)3OH end member could not be synthesized as a pure phase. Within the Ca –Cd series the cell parameters, IR active modes and Raman active bands vary linearly as a function of Cd content. X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) suggest that the Cd is distributed across both the Ca(1) and Ca(2) sites, even at low Cd concentrations.In order to explore the likely detection limits for mixed-metal phosphates in soils for XRD and Raman spectroscopy, soils doped with mixed-metal hydroxylapatites at concentrations of 5, 1 and 0.5 wt.% were then studied. X-ray diffraction could not confirm unambiguously the presence or identity of mixed-metal phosphates in soils at concentrations below 5 wt.%. Raman spectroscopy proved a far more sensitive method for the identification of mixed-metal hydroxylapatites in soils, which could positively identify the presence of such phases in soils at all the dopant concentrations used in this study. Moreover, Raman spectroscopy could also provide an accurate assessment of the degree of chemical substitution in the hydroxylapatites even when present in soils at concentrations as low as 0.1%.

Bone char effects on soil: sequential fractionations and XANES spectroscopy

“…Due to elevated pH surrounding the BC particles, dissolved P can be resorbed to maintain solubility and the Ca-P equilibrium constant, which likely resulted in a decreased proportion of soluble CaHPO 4 and possibly transformation into the relatively insoluble Ca hydroxyapatite fraction. In contrast, if BC plus particles were applied to soils, larger proportions of CaHPO 4 at the expense of Ca hydroxyapatite could be explained by soil acidification through the microbial oxidation of released S (Lee, et al 1987;Fan et al, 2002). This effect was more pronounced over the longer time period in the ryegrass cropping pot experiment, favoring a greater CaHPO 4 than Ca hydroxyapatite fraction.…”

“…Morshedizad et al: Bone char effects on soil: sequential fractionations and XANES spectroscopy rock phosphates (Powers, 1923;Lee et al, 1987;Fan et al, 2002) or meat and bone ashes (Schnug et al, 2003).…”

Soil Chemical Insights Provided through Vibrational Spectroscopy

“…Since then, the technique has been widely used in many disciplines to study bacteria (Huang et al, 2004;Xie and Li, 2003), fish (Ikoma et al, 2003), ceramics (Durand et al, 2012), and soils (Lanfranco et al, 2003). While, no sample preparation is required.…”

“…This research demonstrates the importance of considering divalent metals, such as Cd II , on the fate of phosphate in soil environments. Lanfranco et al (2003) successfully utilized Raman microspectroscopy to determine the presence of mixed-metal hydroxylapatites (Ca and Cd substituted) in manually doped soil at concentrations as low as 0.1% despite fluorescence interference. They were also able to determine the relative composition of the mixed-metal hydroxylapatites due to variance in Raman active bands as a function of the Cd content.…”

Re-examining the chemical evaluation of diagenesis in human bone apatite

“…The effects of these substitutions have been described in detail (e.g. Lanfranco et al, 2003;Thomas et al, 2007), with changes based primarily on distortion of the crystal lattice in substitution. This distortion of the crystal lattice causes changes to the characteristic wavenumber of the peaks associated with the bone apatite ( Fig.…”