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Perfluorooctane sulfonate (PFOS) is an emerging contaminant frequently detected in subsurface environments, raising significant concern due to its environmental persistence, mobility, and potential human health impacts. This study examines PFOS adsorption onto a range of solid substrates, including pure minerals, mineral assemblages, and natural soils. Specifically, the adsorption behavior of 2-line ferrihydrite, ferrihydrite-coated sand, and soil collected from a PFOS-impacted site in Killingworth, Connecticut was investigated to evaluate their capacity to retain PFOS under varying geochemical conditions. By integrating batch adsorption experiments with surface complexation modeling (SCM) and applying the component additivity approach, this study elucidates the reactive transport mechanisms governing PFOS behavior under a range of geochemical conditions. Our findings demonstrate that PFOS adsorption occurs significantly on both ferrihydrite and quartz surfaces, with the ferrihydrite-coated sand and soil exhibiting retention behavior attributable to contributions from both mineral phases. At lower pH values, sorption is predominantly governed by outer-sphere complexation driven by the surface charge characteristics of ferrihydrite. Specifically, under acidic conditions (pH < 5.5 for ferrihydrite-coated sand and pH < 6.0 for soil), PFOS retention is primarily facilitated through an outer-sphere hydrogen-bonded complex at ferrihydrite’s surface, while a secondary outer-sphere complex involving Na + co-adsorption contributes to a lesser extent. At elevated pH levels, however, electrostatic interactions become less favorable, and non-electrostatic hydrophobic interactions with quartz surfaces become increasingly dominant, highlighting the transition in sorption mechanisms from charge-driven to hydrophobic partitioning under neutral to alkaline conditions. A comparison with traditional partitioning coefficients (K d ) revealed that their variability closely corresponds with changes in dominant surface complexes across different pH conditions. Given the critical role of solid-phase partitioning in governing PFAS transport in the subsurface, enhanced predictive capabilities are essential for advancing site-specific risk assessments and informing management strategies aimed at protecting both public and private water resources. Graphical abstract

Weathering of ultramafic rocks has been linked to the occurrence of elevated concentrations of hexavalent chromium (Cr(VI)) in soils, sediments, and groundwater. Ultramafic rocks and the derived serpentine soils and sediments are encountered in populated areas around the world and present high Cr concentrations, with an average of 2200 and 2650 mg/kg for rocks and soils, respectively. Groundwater concentrations between 0.2 and 180 μg/L have been reported for Cr(VI) in ultramafic areas, exceeding occasionally the most prevalent drinking water limit of 50 μg/L Cr tot , the 5 μg/L Cr(VI) limit established in Italy, and the 10 μg/L Cr(VI) limit proposed in California. Cr release in groundwater occurs through the dissolution of trivalent chromium (Cr(III)) from its mineral hosts, followed by sorption of Cr(III) onto high-valence Mn oxides and oxidation to Cr(VI), which desorbs and is mobile at alkaline pH. Recent findings indicate that hydrogen peroxide and birnessite produced on the surface of Cr(OH) 3 by heterogeneous oxidation are two additional potential mechanisms. Thus, groundwater concentrations are controlled by a variety of geoenvironmental factors, including climate, soil mineralogy, pH, organic matter, and others. To provide a basis for the evaluation of Cr mobility in ultramafic environments, this paper presents an overview of the mineralogy and geochemistry of Cr-rich rocks, sediments, and soils, along with the weathering and geochemical processes that control the fate and transport in the subsurface.

Urban agriculture is a sustainable practice for communities to have access to healthy and affordable produce by reducing the energy costs of food production and distribution. While raised beds are often used in community gardens to ensure that soil quality meets proper standards, the use of existing urban soils is desired for economic and sustainability purposes. The main objective of this study is to evaluate a methodology to test soil health parameters using in situ screening methods. Soil testing was conducted at three urban lots in Hartford, CT, that were candidates for community gardens. In situ measurements of metals were taken with a pXRF instrument in all three lots, and an additional 30 samples were tested in the laboratory, both on pressed pellets via pXRF and with acid digestion and ICP-MS analysis. Ultimately, in situ pXRF measurements were comparable to pelletized pXRF and ICP-MS measurements for elements of interest, and pXRF is shown to be a reliable screening tool to evaluate exceedances for metal regulatory thresholds exceeding 100 ppm (e.g., Pb, Cu, Ni, Zn, and Se), although soil moisture content exceeding 5% is shown to have a dilution effect on in situ results up to about a 30% difference. The current study serves as a case study in Hartford, CT, for the evaluation of in situ pXRF analysis as a rapid soil screening tool, and further research will be needed to extend the current recommendations to a general rapid soil assessment methodology.

Iron oxides and hydroxides are highly reactive mineral phases in natural systems since they interact with pollutants, controlling their fate and transport in the environment. Goethite (GH) and hematite (HT) are the most abundant iron minerals in nature, while ferrihydrite (FH) is a nanomineral with high surface reactivity. Subsurface transport modeling has usually represented the adsorption processes by empirical relationships, such as distribution coefficients (Kd) or isotherm equations. However, empirical approaches cannot account for variable geochemical conditions. These effects can be addressed by the mechanistic surface complexation models (SCMs). So far, the application of SCMs has been limited mostly to the description of laboratory experiments, resulting in highly variable parameters even when a pure sorbent–ligand system is described. This limits their usefulness and transferability in reactive transport models.This study is an attempt to bridge the gap between laboratory and field studies, but keep the predictive power of SCMs. The latter is achieved by analyzing several adsorption datasets systematically to extract unified parameters, and understand the driving forces leading to parameter variability. The optimization process is a problem itself that may lead to non-unique parameters. With this in mind, a hybridized optimization approach (MUSE algorithm), based on a multi–start algorithm combined with a local optimizer, has been developed to allow the simultaneous optimization of SCM parameters. A unified model for surface charge was developed to simulate the variable charging behavior of FH. The model was able to capture differences in both surface charge magnitude and points of zero net proton charge (PZNPCs). Finally, the ultimate purpose of this work was to study the adsorption of one ligand (i.e. chromate) on a group of iron oxides (FH, HT, and GH), and examine whether the complexation parameters can be represented by a unified framework. The results of this analysis showed that thermodynamic constants are highly dependent on the surface properties, an effect that can be quantified by the model calculations, while differences in adsorption energetics are also present under different surface coverages. The latter is reflected in thermodynamic parameters and added to the complexity of the model.