Asymmetric flow field-flow fractionation as a new approach to analyse iron-(hydr)oxide nanoparticles in soil extracts

• Iron-(hydr)oxide nanoparticles are important in soils.
• The size of the nanoparticles determines it’s specific surface area.
• We dispersed Fe-(hydr)oxide nanoparticles from the soil.
• We analysed the nanoparticles using AF4 coupled to HR-ICP-MS.
• We provide direct evidence for the presence of Fe-(hydr)oxide nanoparticles.

Iron-(hydr)oxide nanoparticles are important for the sequestration of organic carbon because of their small size and consequently large specific surface area. Therefore, there is an increasing interest in analytical techniques such as asymmetric flow field-flow fractionation (AF4) that allow for a direct measurement of the size distribution of nanoparticles (1–150 nm). We used AF4 coupled to high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) to analyse the size distribution and elemental composition of nanoparticles dispersed from three horizons of a podzol. We tested three extractants for the amount of dispersed Fe-(hydr)oxide nanoparticles. No Fe-(hydr)oxide nanoparticles were dispersed in 5 mM NaCl. In a 1 mM NaOH extraction (pH 9.0), the amount of Fe dispersed in the form of Fe-(hydr)oxide nanoparticles amounted to 0.2–0.8 g kg− 1, which corresponded to 2–13% of the Fe content as extracted with ammonium oxalate (Fe-ox). Pyrophosphate was found to be the most effective extractant for Fe-(hydr)oxide nanoparticles and extracted 1.0–4.7 g kg− 1 Fe as Fe-(hydr)oxide nanoparticles, corresponding to 16–47% of the Fe-ox content. These Fe-(hydr)oxide nanoparticles were 2–20 nm in size and maximum concentrations were found at a particle diameter of 5 nm. The dispersion of Fe-(hydr)oxide nanoparticles in pyrophosphate coincided with the extraction of a large fraction of the soil organic carbon content (55–69%) which shows that dispersion of organo-mineral aggregates results in the release of Fe-(hydr)oxide nanoparticles from the soil. The amount of Fe-(hydr)oxide nanoparticles extracted from the soil did not increase after ultrasonic treatment of the pyrophosphate suspension. Since not all Fe-(hydr)oxides can be dispersed from the soil as primary particles, AF4 cannot be used as a tool to analyse the specific surface area of the Fe-(hydr)oxides in the soil. Instead, AF4 should be considered as a complementary technique providing a direct measurement for the size of the Fe-(hydr)oxide nanoparticles in soil extracts.

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