Schwertmannite and jarosite are naturally occurring iron (Fe) oxyhydroxysulfates with strong sorption capacities for hexavalent uranium [U(VI)] in various acidic sulfate-rich environments. These metastable minerals commonly undergo recrystallization, particularly in the presence of dissolved Fe2+ [Fe(II)aq], which may influence the fate of associated U(VI). Here, we quantified molecular-level changes in U repartitioning and speciation when U(VI)-sorbed schwertmannite and jarosite reacted with Fe(II)aq under near-neutral and anaerobic conditions over 2 weeks. The results show that Fe(II)aq additions promoted rapid mineral transformation to goethite via a dissolution–reprecipitation pathway, proceeding (near-completely) for schwertmannite but slowly and incompletely for jarosite. Importantly, even at early transformation stages when goethite likely only started forming on the surface of the transforming minerals, the recrystallization process led to near-complete retention of U, predominantly as U(VI), within the structure of the neo-formed goethite. Subsequent U reduction to U(V) increased with time but remained incomplete, even after extensive mineral transformation in the presence of 1–50 mM Fe(II)aq for 2 weeks. The results demonstrate that Fe(II)-promoted recrystallization of Fe-oxyhydroxysulfates can rapidly and persistently lock both U(VI) and U(V) into chemically stable goethite, with important implications for predicting U behavior and designing remediation strategies in various acidic and U-contaminated environments.

There are numerous per- and polyfluoroalkyl substances (PFAS)-contaminated areas worldwide, with aqueous film-forming foams (AFFFs) frequently representing a significant contaminant source. This research reviews nine remediation methods and their efficacy and suitability under different environmental conditions and various PFAS. The evaluation included the remediation methods' suitability for soil or water remediation; in situ or ex situ use; effectiveness for different PFAS; cost-effectiveness; regenerative capabilities; susceptibility to organic material, other substances, or pH; potential for PFAS-concentrated waste production; and past use in large-scale remediation projects. The study concludes that the methods have varying advantages and disadvantages, depending, for example, on the specific PFAS that require remediation and the type of media such as soil or water.

Fine-grained hypermonosulfidic sediments are widespread on the coastal plains of the northern Baltic Sea that when drained, cause the formation and dispersion of acid and toxic-metal species. In this study, a 30-month laboratory oxidation experiment with such a sediment was performed in incubation cells. To minimize or prevent acidification, limestone was applied in two grain sizes: agricultural limestone with particles that were all <3.15 mm and half of them <0.80 mm, and fine-grained limestone with a median grain size of 2.5 mu m. The amount of limestone applied corresponded to the theoretical acidity contained in the sulfides, as well as four times that amount. Another treatment included addition of peat to the low limestone dose to test its effects on immobilizing sufhur and metals. The pH of the drainage water and solid phase decreased to pH <4.0 in the control, and to pH <5.0 in the coarse-grained low-limestone treatment, but remained near-neutral in the other treatments. Hence, the fine-grained limestone effectively hindered acidity formation in contrast with the coarse-grained limestone when applied in amounts corresponding to the potential acidity held in the sulfides. The limestone treatments further overall decreased the rate of pyrite oxidation, slowed down the movement of the oxidation front, strongly minimized the formation of dissolved and solid-phase labile Al, and caused formation of gypsum as well as more labile secondary Fe(III) phases than corresponding Fe phases formed in the control. The limestone and peat treatments also caused shifts in the 16S rRNA gene-based microbial communities, where the control developed acidophilic iron and sulfur oxidizing communities that promoted acidity and metal release. Instead, the limestone-treated unacidified incubations developed acid tolerance to neutrophilic communities of iron and sulfur oxidizers that promoted sulfate formation without acidity release. The results showed that limestone treatments have several biogeochemical effects, and that using a fine-grained limestone as amendment was favourable in terms of minimizing acidity formation and metal release.

Acid sulfate soils impact surrounding ecosystems with pronounced environmental damage via leaching of strong acidity along with the concurrent mobilization of toxic metals present in the soils and, in consequence, they are often described as the nastiest soils on Earth. Within Sweden, acid sulfate soils are distributed mainly under the maximum Holocene marine limit that stretches the length of the country, some 2000 km north to south. Despite only minor geographical differences in the geochemical composition of the Swedish acid sulfate soils, their field oxidation zone microbial community compositions differ along a north-south regional divide. This study compared the 16S rRNA gene amplicon-based microbial community compositions of field oxidation zones (field tested pH _ 4.0) with reduced zone samples (field tested pH _ 6.5) collected from the same field sites throughout Sweden that had acidified (final pH _ 4.0) after laboratory incubation at approximately 20 degrees C. The previously identified regional differences observed in field oxidation zone microbial compositions were notably absent in the laboratory incubation samples. Instead, a commonly shared community was selected for with few statistically significant differences regardless of regional origin. For instance, the potential eurypsychrophilic Baltobacteraceae family was found in higher relative abundances in the northerly region of the field oxidation zone samples than the southern regions and was notably absent from the laboratory incubation samples. Furthermore, the microbial communities of the laboratory incubation samples were dominated by acidophilic autotrophic Acidithiobacillaceae and chemoheterotrophic Rhodanobacteraceae and Burkholderiaceae that have optimal growth temperatures (_= 20 degrees C) greater than what was experienced by the field oxidation zone samples when sampled (similar to 2 degrees C-9 degrees C). These data suggested that in the absence of significant geochemical differences, temperature was the predominant driver of microbial community composition in Swedish acid sulfate soil materials.

Estuaries play a vital role in the coastal environment by filtering pollutants and nutrients from catchment runoff. In areas where acid sulfate (AS) soils are abundant, the importance of the estuary as a coastal filter is heightened as AS soils typically stress the marine environment with acidic metal-laden drainage waters. In this study, we took sediment cores from a shallow estuary in Western Finland and used geochemical and palaeoecological methods to investigate how the estuary is affected by loading from AS soils. An overall decrease in diatom species richness and diversity in the estuarine sediments was found, with a clear change from species preferring pelagic conditions to species indicative of more eutrophic conditions. The change coincides with human disturbance during the early 20th century when extensive drainage and rework of forests and peatlands into agricultural use increased. Geochemical analyses show a significant enrichment of Cd, Ni, Co, Zn and Al in the estuarine sediments which correspond to the metal loads originating from the catchment AS soils. Our calculations, however, show that in comparison to the total load of soluble metals from the catchment area, more than 80% of chalcophiles and 70% of Al are transported further out to sea. We hypothesised that a precipitation gradient driven by changes in pH and salinity due to seawater mixing would form along a transect towards the estuary outlet. Instead, we found that physical sedimentation processes are stronger drivers for element transport, as enrichment takes place only in low-energy hydrodynamic conditions at greater water depths. Glacioisostatic land uplift and significant particle transport from the catchment area are further isolating the estuary, effectively moving the saline gradient seawards and diminishing the role of the estuary as a coastal filter. We also found that the estuarine sediments are hypersulfidic and contain stores of potential acidity significantly larger than conventional AS soils. Without proper management, disturbance of the estuarine sediments can cause disastrous consequences at a local level.

Recognising the need for robust models in predicting groundwater contamination risks from metal(loid)s in contaminated topsoil, this study focuses on the geochemical behaviour of As, Cd, Cu, Pb, Sb and Zn in one of Sweden’s most heavily contaminated areas. Samples were collected from the waste zone and underlying subsoil down to 5 meters and batch experiments were carried out to assess pH-dependent solubility. The results indicate that Cd, Cu, Pb and Zn are efficiently immobilized in the waste zone, while As(V) and Sb(V) are more easily leached. With the exception of Pb and Cu at high pH, the mobilized metals appear to be predominantly in a truly dissolved state, as confirmed by ultrafiltration at 10 kDa.

Speciation modelling using Visual MINTEQ did not suggest a significant role of precipitates such as Zn or Pb arsenates and phosphates, although their involvement could not be ruled out. To better understand sorption/desorption patterns, a multi-surface geochemical model was established, drawing on the Stockholm Humic and CD-MUSIC models for organic matter and Fe/Al (hydr)oxide sorption. However, when default parameters were used, the model consistently overestimated the solubility of Cd, Cu, Pb and Zn in both the waste zone and the uncontaminated subsoil. In contrast, As(V) solubility was generally underestimated, also when the reactive surface area of the Fe- and Al (hydr)oxides was decreased in the model. The model's performance was better for Sb(V), though not without imperfections. When the parameters for organic matter were adjusted such that 100% of the solid-phase organic matter was active with respect to ion binding, but only 25% of the dissolved organic matter, the model description improved considerably for Pb and Cu in the upper soil layers. The model revealed distinct differences in the adsorption behaviour of the metal cations, with Pb being sorbed mostly to Fe/Al (hydroxides), whereas a considerable part of Cu was sorbed to organic matter, particularly in the waste zone.

Possibly, the dissolution of easily weatherable metal-containing mineral phases may have contributed to the poor model performance for Cd, Zn and for Cu in the deeper soil layers, although other factors, such as a contribution of hydrous SiO2 or Mn oxides to metal binding, could not be ruled out. Metal sorption to carbonate phases may also have been a contributing factor in the waste zone. Lastly, the reactivity of Fe- and Al (hydr)oxides may have been overestimated by oxalate extraction when default parameters for high-surface-area ferrihydrite were applied. 

These findings provide valuable insights for environmental management and underscore the need for a more detailed characterization of metal(loid) sorption in contaminated soils, as well as the development of improved modelling strategies to enhance solubility predictions.

Acid sulfate soils are found globally and have significant environmental impact as a source for metals and acidity to surrounding streams that can cause, for example, large-scale fish kills. In the face of changing climate and its effect on groundwater fluctuations, the environmental risk associated with these soils needs to be thoroughly investigated. This study examined the water-soluble concentrations of multiple elements from the oxidized, transition and reduced zones of acid sulfate soil profiles situated on the Swedish coastal plains. By comparing untreated (naturally oxidized in field) and incubated samples from these zones, we gain insight into the current and near-future mobilization and leaching of acidity and metals that occur in these soils. The results showed that concentrations of Al, Cd, Co, Mn, Ni, S and Zn mobilized from incubated samples were about an order of magnitude higher than from the untreated samples. Notably, the concentrations of mobilized Co, Mn and Ni were higher than released by 1 M HCl at the same sites, highlighting the particularly high mobility of these metals from in situ oxidation of acid sulfate soils. Conversely, Fe and Cu showed lower than expected water-soluble concentrations and were also low compared to the 1 M HCl-extractable element concentrations, likely due to rapid re-mobilization of secondary Fe minerals. Arsenic, Cr and Pb showed overall low water-soluble concentrations in both the incubated and untreated samples, consistent with these elements not being abundantly leached from acid sulfate soils. This observation was further supported by the retention of these metals in secondary Fe-mineral phases such as jarosite and schwertmannite as reported in previous studies. A strong correlation between acidity and near-total S indicated that S can serve as an indicator for the acidification risks associated with acid sulfate soil oxidation. Overall, the findings demonstrated that even a small lowering of the groundwater table can lead to significant mobilization of metals and acidity. This highlights the increased risks of environmental degradation in the face of climate change and intensified drainage operations and, thus, the need for proper management to reduce the risks.

As critical transition zones between the land and the sea, estuaries are not only hotspots of hydrogeochemical and microbial processes/reactions, but also play a vital role in processing and transferring terrestrial fluxes of metals and nutrients to the sea. This study focused on three estuaries in the Gulf of Bothnia. All of them experience frequent inputs of acidic and Mn/metal-rich creek waters due to flushing of acid sulfate soils that are widespread in the creekś catchments. Analyzing existing long-term water chemistry data revealed a strong seasonal variation of Mn loads, with the highest values in spring (after snow melt) and autumn (after heavy rains). We sampled surface waters, suspended particulate matter (SPM), and sediments from the estuarine mixing zones and determined the loads and solid-phase speciation of Mn as well as the composition and metabolic potentials of microbial communities. The results showed that the removal, cycling, and lateral transport of Mn were governed by similar phases and processes in the three estuaries. Manganese X-ray absorption spectroscopy data of the SPM suggested that the removal of Mn was regulated by silicates (e.g., biotite), organically complexed Mn(II), and MnOx (dominated by groutite and phyllomanganates). While the fractional amounts of silicate-bound Mn(II) were overall low and constant throughout the estuaries, MnOx was strongly correlated with the Mn loadings of the SPM and thus the main vector for the removal of Mn in the central and outer parts of the estuaries, along with organically complexed Mn(II). Down estuary, both the fractional amounts and average Mn oxidation state of the MnOx phases increased with (i) the total Mn loads on the SPM samples and (ii) the relative abundances of several potential Mn-oxidizing bacteria (Flavobacterium, Caulobacter, Mycobacterium, and Pedobacter) in the surface waters. These features collectively suggested that the oxidation of Mn, probably mediated by the potential Mn-oxidizing microorganisms, became more extensive and complete towards the central and outer parts of the estuaries. At two sites in the central parts of one estuary, abundant phyllomanganates occurred in the surface sediments, but were converted to surface-sorbed Mn(II) phases at deeper layers (>3–4 cm). The occurrence of phyllomanganates may have suppressed the reduction of sulfate in the surface sediments, pushing down the methane sulfate transition zone that is typically shallow in estuarine sediments. At the outermost site in the estuary, deposited MnOx were reduced immediately at the water–sediment interface and converted most likely to Mn carbonate. The mobile Mn species produced by the Mn reduction processes (e.g., aqueous Mn(II) and ligand complexed Mn(III)) could partly diffuse into the overlying waters and, together with the estuarine Mn loads carried by the surface waters, transfer large amounts of reactive Mn into open coastal areas and subsequently contribute to Mn shuttling and inter-linked biogeochemical processes over the seafloor. Given the widespread occurrence of acid sulfate soils and other sulfidic geological materials on many coastal plains worldwide, the identified Mn attenuation and transport mechanisms are relevant for many estuaries globally.

Acid sulfate soils are frequently described as the nastiest soils on Earth, and they pose environmental risks associated with their strong acidity and consequential mobilization of toxic metals present in the soils. Within Sweden, acid sulfate soils have been extensively studied around the northern Baltic coastline and have now been found to occur throughout the area below the maximum Holocene marine limit that stretches for some 2000 km from North to South. This study investigated 20 active acid sulfate soils (field tested pH < 4.0) collected throughout this area that were tested for microbial community composition using 16S rRNA gene amplicons, representing a novel study of microbial communities in ripening zones across a broad regional scale. The microbial community compositions exhibited a north (boreal zone) to south (hemiboreal zone) regional divide, primarily within the oxidized zone (pH < 4.0), to a lesser degree in the transition zone (steep pH gradient), while little differences were observed in the reduced zone (near-neutral pH). For instance, a higher relative abundance of Ktedonobacteraceae was identified in the northern boreal and Gallionellaceae in the southern hemiboreal oxidized zones. In addition, microbial taxa associated with iron and sulphur oxidation and reduction were identified, such as Acidobacteriaceae, Gallionellaceae and Koribacteraceae that have been previously identified in other acid sulfate soils and acid mine drainage settings. The predominant controls of the microbial community differences were the north-south divide indicative of a strong soil-temperature effect followed by soil zones suggesting an influence of the soils' pH and/or redox conditions.

In a boreal acidic sulfate-rich subsoil (pH 3–4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers (“macropore surfaces”) are strongly enriched in 1 M HCl-extractable reactive iron (2–7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.