Speciation of sulphur in soils and soil particles by X‐ray spectromicroscopy

Abstract

Summary Current wet chemical methods for the speciation of sulphur (S) in soils are inaccurate and do not allow one to assess the S speciation of individual soil particles and colloids. X‐ray microscopy and Near Edge X‐ray Absorption Fine structure Spectroscopy (NEXAFS) can be used to study individual species of S at the K‐adsorption edge. We have used these techniques to identify and quantify S species in bulk soil, soil particles and colloids from Oh and Bh horizons of two forested Podzols. The partitioning of soil sulphur as determined on bulk samples of the Oh horizons by X‐ray spectromicroscopy agreed fairly well with the results of a conventional S speciation for the soil at Schluchsee, and reasonably well for that at Rotherdbach. The NEXAFS analyses on individual soil particles revealed that they are richer in reduced organic sulphur than the bulk soil for the Schluchsee Oh and richer in sulphate for Rotherdbach Oh. The techniques can be used reliably to separate and quantify sulphur species with different oxidation states in the soil. The combination of X‐ray transmission and sulphur fluorescence images with unfocused and focused NEXAFS spectra at the K‐adsorption edge of sulphur at specific microsites allowed us to compare the distribution of S species in bulk soil with that of distinct soil particles and soil colloids. Moreover, we can use it to assess the spatial distribution of different S species on soil particles on a scale of a few hundred nanometres.

Synchrotron‐Based Near‐Edge X‐Ray Spectroscopy of Natural Organic Matter in Soils and Sediments

“…The discrepancy in these sets of values could be caused by limitations in the comparability of the two sulfur speciation techniques for different subgroups of organic sulfur fractions. Prietzel et al (2003) and Solomon et al (2003) suggested that the wet-chemical technique relies on the differential reduction of organic sulfur compounds to H 2 S and thus it might not recover all the organic sulfur fractions compared to NEXAFS, which is a more direct and nondestructive technique. The shortcoming of sulfur K-edge NEXAFS spectroscopy, however, is its inability to discriminate precisely between soil organic sulfur forms with an oxidation state of 0 to +1 (polysulfides, disulfides, thiols, monosulfides, and thiophenes).…”

“…A generally greater amount of reduced organic sulfur was also found in temperate forest soils (Prietzel et al, 2003) or wetlands (Jokic et al, 2003) than in well-drained agricultural soil (Solomon et al, 2003(Solomon et al, , 2005b. Forms of organic sulfur appear to react significantly to the oxygen environment, with the lowest degree of oxidation in submerged soils and the highest in agricultural soils (Zhao et al, 2006).…”

“…Microorganisms can very often be measured without sectioning in their hydrated stage (Thieme et al, 1994). Individual colloids or soil particles can in some instances also be measured by transmission without pretreatment (Prietzel et al, 2003). More difficult is the measurement by transmission of mixtures of NOM and minerals or particulate NOM in their natural assemblage.…”

“…An additional advantage is the ability of NEXAFS to directly interrogate the composition of minor or 1 Q PRINCIPLES 725 trace elements in NOM without extraction or chemical modifications (Brandes et al, 2007). For example, NEXAFS has provided an entirely new approach to the study of organic sulfur compounds in soils that was not possible before (Xia et al, 1998;Prietzel et al, 2003;Solomon et al, 2003).…”

Redox dynamics of sulphur with Ni/GDC anode during SOFC operation at mid- and low-range temperatures: An operando S K-edge XANES study

“…Depending on the SOFC temperature under investigation, the establishment took several hours at start up to 15e30 min at lower temperatures. S K-edge X-ray absorption near edge structure spectroscopy (XANES) at the sulphur K-edge can be used to identify and quantify sulphur atoms exhibiting different oxidation states and molecular structure and those being part of different functional groups [44]. A powerful statistical tool facilitating the speciation analysis is principal component analysis (PCA), allowing estimating the minimal number of independent signal patterns ("principal components", PCs) needed to reproduce a set of data, here the in-total eight operando-recorded S K-edge XANES spectra.…”