X - R A Y N A N O P R O B E

S C I E N T I F I C H I G H L I G H T S

8 8 H I G H L I G H T S 2 0 2 1 I

the P is associated with organic matter, clay minerals or hydrous oxides of aluminium (Al) and iron (Fe), a distinction critical for the mobility, bioavailability and reactivity of the mobilised P [4]. Sulfur is mostly released from soils to aquatic environments in organic forms, and S transformations may induce P eutrophication under conditions of high sulfate leaching [5]. The speciation of S in particles leached from the soil carries information on processes relevant for both S itself and for P. Thus far, no study has examined the molecular speciation of P and S simultaneously in colloids and nanoparticles leached from agricultural land.

Synchrotron X-ray fluorescence microscopy (XRF) and X-ray absorption near-edge structure (XANES) spectroscopy were used to reveal the speciation of these elements in two fractions of naturally leached particles

( micro- and nanoparticles , i.e., > 0.45 and < 0.45 µm, respectively) collected from four tile-drained agricultural sites in Sweden. The measurements were possible thanks to excellent optimisation of the X-ray microscopy beamline ID21 to operate at submicron resolution in the 2 to 9.5 keV energy range, and a dual-energy beam setup involving the selection of both the first and the third harmonic in order to allow the simultaneous P K-edge XANES and mapping of both low- and high-Z elements.

Results across all the sites showed organic P and P bound to Al and calcium (Ca) as the predominant P forms in the microparticles. P bound to Fe appeared as the least abundant P species and was detected only in certain areas of the samples from two of the four sites (Figure 69). The nanoparticles were generally richer in organic P, which accounted for 74% of the total P in one site, Krusenberg

Fig. 70: a) Microscale variation in the distribution of phosphorus species and (b) average phosphorus and sulfur speciation in nanoparticle (< 0.45 μm) leached from Krusenberg.