Characterization of colloidal Fe from soils using field-flow fractionation and Fe K-edge X-ray absorption spectroscopy.

Colloids may facilitate the transport of trace elements and nutrients like phosphate in soil. In this study, we characterized soil colloids (<0.45 µm), extracted from four agricultural soils by Na-bicarbonate and Na-pyrophosphate, by two complementary analytical techniques; asymmetric flow field-flow fractionation (AF4) and X-ray absorption spectroscopy (XAS). The combined results from AF4 and XAS show that colloidal Fe is present as (i) free Fe-(hydr)oxide nanoparticles, (ii) Fe-(hydr)oxides associated with clay minerals, and (iii) Fe in clay minerals. Free Fe-(hydr)oxide nanoparticles, which can be as small as 2-5 nm, are extracted with Na-pyrophosphate but not with Na-bicarbonate, except for one soil. In contrast, Fe-(hydr)oxides associated with clay minerals are dispersed by both extractants. XAS results show that the speciation of Fe in the colloidal fractions closely resembles the speciation of Fe in the bulk soil, indicating that dispersion of colloidal Fe from the studied soils was rather unselective. In one Fe-rich soil, colloidal Fe was dominantly dispersed in the form of free Fe-(hydr)oxide nanoparticles. In the other three soils, dispersed Fe-(hydr)oxides were dominantly associated with clay minerals, suggesting that their dispersion as free nanoparticles was inhibited by strong attachment. However, in these soils, Fe-(hydr)oxides can be dispersed as oxide-clay associations and may as such facilitate the transport of trace elements.

Characterization of colloidal phosphorus species in drainage waters from a clay soil using asymmetric flow field-flow fractionation.

Phosphorus transport from agricultural land contributes to eutrophication of surface waters. Pipe drain and trench waters from a grassland field on a heavy clay soil in the Netherlands were sampled before and after manure application. Phosphorus speciation was analyzed by physicochemical P fractionation, and the colloidal P fraction in the dissolved fraction (<0.45 µm) was analyzed by asymmetric flow field-flow fractionation (AF4) coupled to high-resolution inductively coupled plasma-mass spectrometry and ultraviolet diode array detector. When no manure was applied for almost 7 mo, total P (TP) concentrations were low (<21 µmol L), and TP was almost evenly distributed among dissolved reactive P (DRP), dissolved unreactive P (DUP), and particulate P (PP). Total P concentrations increased by a factor of 60 and 4 when rainfall followed shortly after application of cattle slurry or its solid fraction, respectively. Under these conditions, DRP contributed 50% or more to TP. The P speciation within the DUP and PP fractions varied among the different sampling times. Phosphorus associated with dissolved organic matter, probably via cation bridging, comprised a small fraction of DUP at all sampling times. Colloidal P coeluted with clay particles when P application was withheld for almost 7 mo and after application of the solid cattle slurry fraction. At these sampling times, PP correlated well with particulate Fe, Al, and Si, indicating that P is associated with colloidal clay particles. After cattle slurry application, part of DUP was probably present as phospholipids. Physicochemical fractionation combined with AF4 analysis is a promising tool to unravel the speciation of colloidal P in environmental water samples.

Iron-rich colloids as carriers of phosphorus in streams: A field-flow fractionation study.

Colloidal phosphorus (P) may represent an important fraction of the P in natural waters, but these colloids remain poorly characterized. In this work, we demonstrate the applicability of asymmetric flow field-flow fractionation (AF4) coupled to high resolution ICP-MS for the characterization of low concentrations of P-bearing colloids. Colloids from five streams draining catchments with contrasting properties were characterized by AF4-ICP-MS and by membrane filtration. All streams contain free humic substances (2-3 nm) and Fe-bearing colloids (3-1200 nm). Two soft water streams contain primary Fe oxyhydroxide-humic nanoparticles (3-6 nm) and aggregates thereof (up to 150 nm). In contrast, three harder water streams contain larger aggregates (40-1200 nm) which consist of diverse associations between Fe oxyhydroxides, humic substances, clay minerals, and possibly ferric phosphate minerals. Despite the diversity of colloids encountered in these contrasting streams, P is in most of the samples predominantly associated with Fe-bearing colloids (mostly Fe oxyhydroxides) at molar P:Fe ratios between 0.02 and 1.5. The molar P:Fe ratio of the waters explains the partitioning of P between colloids and truly dissolved species. Waters with a high P:Fe ratio predominantly contain truly dissolved species because the Fe-rich colloids are saturated with P, whereas waters with a low P:Fe ratio mostly contain colloidal P species. Overall, AF4-ICP-MS is a suitable technique to characterize the diverse P-binding colloids in natural waters. Such colloids may increase the mobility or decrease the bioavailability of P, and they therefore need to be considered when addressing the transport and environmental effects of P in catchments.

Ni adsorption and Ni-Al LDH precipitation in a sandy aquifer: an experimental and mechanistic modeling study.

Mining activities and industries have created nickel (Ni) contaminations in many parts of the world. The objective of this study is to increase our understanding of Ni adsorption and Nickel-Aluminium Layered Double Hydroxide (Ni-Al LDH) precipitation to reduce Ni mobility in a sandy soil aquifer. At pH ≥ 7.2 both adsorption and Ni-Al LDH precipitation occurred. In batch experiments with the sandy soil up to 70% of oxalate-extractable Al was taken up in LDH formation during 56 days. In a long term column experiment 99% of influent Ni was retained at pH 7.5 due to Ni adsorption (≈ 34%) and Ni-Al LDH precipitation (≈ 66%) based on mechanistic reactive transport modeling. The subsequent leaching at pH 6.5 could be largely attributed to desorption. Our results show that even in sandy aquifers with relatively low Al content, Ni-Al LDH precipitation is a promising mechanism to immobilize Ni.