Characterization of minerals and organic phosphorus species in marine sediments using soft X-ray fluorescence spectromicroscopy

摘要

Phosphorus Near Edge X-ray Fluorescence Spectroscopy (P-NEXFS) data were collected on phosphorus containing phases including organic and inorganic compounds and minerals. Although phases containing P in the plus five oxidation state P(Ⅴ) in a tetrahedral PO_4 structure have similar primary fluorescence peak positions, the size, shape, and positions of secondary spectral features are diagnostic for different compounds and minerals. In particular, calcium phosphates exhibited a notable post-peak shoulder at 2154.5 eV, while oxidized iron phosphates had a distinctive pre-peak feature at 2148 eV. Polyphosphates have a broad secondary peak located approximately 2 eV higher in energy than a similar feature in phosphate esters and diesters. Compounds containing P(Ⅴ) in structures other than PO_4 tetrahedra such as phosphonates have a primary peak shifted about 1 eV lower than corresponding organo-phosphates. Organo-phosphates with P in the plus three oxidation state P(Ⅲ) such as phosphines had primary fluorescence peaks shifted still further down in energy (2-3 eV). The substitution of aromatic carbon groups in close proximity to P structures in organic compounds generated both pre- and post-peak features as well as a number of secondary peaks. In addition, X-ray fluorescence mapping of P, Si, Al, Mg, and Na was conducted on a marine sediment sample with sub-micron spatial resolution. Phosphorus was heterogeneously distributed in the sample and not correlated on a broad scale with any other element examined. Much of the P present in the sample was located in small, 0.6—8 μm size, P-rich domains. Several P-rich regions were examined with P-NEXFS using a focused beam with 60 nm resolution and were found to consist of either calcium phosphate or polyphosphate phases. The presence of significant polyphosphate-dominated regions in a marine sediment sample supports the recent observations that such phases can play an important role in marine P cycling. The combination of fluorescence mapping and P-NEXFS data collection on fine particles provides a powerful new tool for environmental phosphorus studies.