*3.2. Compositional Validation of Suspended Sediment Using X-Ray Fluorescence (XRF) and X-ray Absorption Near-Edge Structure (XANES)*

The elemental compositions of these sediments as detected by X-ray fluorescence (XRF) (Figure 4) also show that the BB sediment has a much higher Ca concentration, while the TA sediment has a relatively higher Fe content. This is in general agreement with results from sequential chemical P fractionations which indicated more Ca-associated P within BB sediments and more Fe and/or redox associated P with TA sediments (Section 3.1; Figure 3). Previous studies have used synchrotron XRF coupled with operationally defined chemical P fractionation results to establish the dominant elements governing sediment composition [40–44].

**Figure 4.** X-ray fluorescence spectra of bulk TA and BB sediments. The intensity was calibrated relative to the scattered X-ray (to an intensity of 10000 at 7000 eV), so that the comparison of elemental intensities is possible. The incident photon energy is 7200 eV.

Figure 5 shows the P K-edge spectra of (a) bulk TA and BB sediments and in comparison to (b) the spectra of selected P reference compounds. The Fe phosphate-related compounds have a unique pre-edge peak, as indicated by the arrow in the spectrum of FePO4.2H2O shown in Figure 5b, while the Ca phosphate compounds, such as apatite, have several distinct shoulders in their P K-edge XANES [21,25]. P K-edge spectra of bulk sediment samples were dominated by featureless post-edge peak, similar to that of phytic acid [45]. This indicates the relatively significant presence of organic P (POrg) in these samples, which may suggest the POrg fraction of the chemical P fractionation results underestimate the significance of this P pool (Figure 3). The pre-edge peak, as indicated by the arrow, is clearly resolved in the spectrum of the TA sediment (Figure 5a), suggesting the presence of Fe-P in the bulk of TA sediment. This is in agreement with the chemical extraction and bulk XRF results (Figures 3 and 4). However, for Ca-rich BB sediment, no Ca-P related resonances are observed in the spectrum of BB sediment.

**Figure 5.** P K-edge XANES spectra of (**a**) bulk TA and BB sediments and (**b**) reference compounds.

*Soil Syst.* **2020**, *4*, 51

This lack of Ca-P in the bulk spectrum of BB sediment implies the dominance of organic P and the inhomogeneous distribution of Ca-P in the BB sediment, which could be revealed by the microanalysis of these sediments. Chemical P fractionation results indicated a significant Ca-P pool normally associated with a recalcitrant mineral such as apatite. The tricolor μ-XAF maps of two sediments are shown in Figure 6. In this work, we chose to focus on the correlations between P, Fe, and Ca, as Fe oxides have been shown to complex with organic matter in sediments, thus impacting P mobility and surface water eutrophication [10–12]. The important role of Ca-P species in sediment weathering, P transformation in alkaline soil, and biogeochemical P cycling has been demonstrated by microscale XRF mapping, together with 31P NMR and chemical extraction [27,42,43]. In Figure 6, elemental correlation maps or hotspot maps down to the micron scale are shown for P (blue), Ca (green), and Fe (red). These are elemental correlation maps with comparison of relative elemental concentration within a specified hotspot/area and not absolute concentrations. Where necessary, the hotspot can be identified down to 10 microns for μ-XANES. It is obvious that there are more Ca-rich hotspot correlations with P in the BB catchment. The BB sample also has very few and weak Fe spots, but there are identifiable Ca- and P-correlated spots (such as *A*). The TA sediment is generally Fe-rich, with only one Ca-rich hotspot. This is consistent with the XRF analysis and the mineralogy of these sites. A few hotspots (*A*, *B*, *C* as P-rich, Fe-rich, and Ca-rich for BB sediment and *D*, *E* as Fe-rich and Ca-rich for TA sediment, respectively) are selected for P, Ca, and Fe μ-XANES.

**Figure 6.** Elemental mapping for P (blue), Ca (green) and Fe (red) of BB and TA sediments with selected spots.

Figure 7a,b present the P K-edge μ-XANES of hotspots or region of interests (ROI) for BB and TA sediments. The P spectra for TA sediments (*D* and *E*) were quite noisy and without distinct features associated with inorganic P (Figure 7b), confirming its weak and significant organic P (Figure 3). The μ-XRF map of the TA sample revealed that ROI *D* had high Fe and P correlation (Figure 6); unfortunately, Fe-P could not be resolved in its μ-XANES (Figure 7b), likely due to its low P concentration and interference from the high Si content (Figure 4). For the Ca- and P-rich hotspots of BB sediment, the post-edge resonances were clearly resolved (Figure 7a, spots *A* and *C*), indicating the high Ca-P in the BB site. This is in agreement with the chemical extraction result, demonstrating the advantage of the μ-XANES, as no Ca-P was detected in the bulk P K-edge (Figure 5a). On the contrary, no Ca-P is detected in the μ-XANES of spot B, as it shows high Fe and P correlation (Figure 6). There might be a hint of the pre-edge peak in the P K-edge of spot B, implying the presence of Fe-P. Results of Ca K-edge μ-XANES (Figure 7c) are also in agreement with the P K-edge μ-XANES, as Ca in spot A matches well with that of apatite, spots *B* and *C* being mostly calcite, and spots *D* and *E* being mostly organic Ca [25]. The Fe K-edge μ-XANES of all spots (Figure 7d) are similar to each other and to those of bulk samples, as they are dominated by Fe hydroxyl oxide species [11,45]. No Fe-P can be resolved in the Fe K-edge μ-XANES due to its relatively low concentration.

**Figure 7.** μ-XANES of selected spots identified in Figure 5: (**a**,**b**) P K-edge; (**c**) Ca K-edge and (**d**) Fe K-edge.
