Manganese (Mn) removal in passive mine water treatment remains a challenge due to its slow oxidation kinetics, requiring specific biogeochemical conditions. Constructed wetlands are often the key functional units enabling Mn removal in full-scale passive treatment plants. This study examines the key biogeochemical factors influencing Mn removal in a full-scale passive mine water treatment plant located in Alès (South-East France). Over one year, monitoring of physicochemical parameters, microbial communities, and Mn speciation in solid phases was conducted every two months. Results highlight temporal variations in Mn removal efficiency, with two main mechanisms identified: (1) Mn carbonate (MnCO₃) precipitation, likely influenced by high carbonate concentrations in mine water, and (2) Mn oxide (δ-MnO₂) formation, mainly associated with reed rhizosphere, where it accumulates as mineral plaque. In mine water, Mn removal correlates with Fe particle concentrations, suggesting a catalytic effect, as well as with alkalinity and the abundance of microorganisms affiliated to Alteromonadaceae, suggesting a microbial influence. Mn removal appears to be primarily abiotic, driven by favourable pH and alkaline conditions that promote Mn carbonate precipitation, by autocatalytic oxidation reactions occurring on rhizosphere surfaces and by plant's design including surface area and hydrological conditions. Microbial communities may facilitate certain Mn removal processes depending on environmental conditions.

Passive mine water treatment is widely used to remove metals from mine water, but manganese (Mn) remains a challenge due to its slow oxidation kinetics at neutral pH. This study investigated the mechanisms of Mn removal and factors influencing its efficiency at the passive treatment plant of Messeix, central France, which integrates a settling pond, wetlands and pozzolana biofilters. Temporal monitoring over one year encompassed water chemistry, Mn precipitate mineralogy, and microbial community analysis. Results showed that the settling pond and wetlands achieved 28-64% Mn removal efficiency depending on season which was insufficient to consistently meet regulatory discharge limits, whereas the pozzolana biofilters achieved 98-100% Mn removal efficiency in all seasons. Mn attenuation was primarily explained by abiotic processes controlled by physicochemical parameters such as pH, dissolved oxygen, redox potential, and iron content. The biofilter provided surfaces for catalytic Mn removal mechanisms on pre-existing Mn oxides and Fe oxyhydroxides. 16S rRNA gene profiles and PICRUSt2 predictions revealed bacterial genera previously reported in Mn-rich environments (Hydrogenophaga, Sideroxydans, Flavobacterium, Aquabacterium, Bacteriovorax, and Leptothrix) together with predicted Mn-oxidase-related genes. Although no direct evidence of bacterial Mn oxidation is provided, the spatial and temporal organisation of microbial communities in relation to physicochemical gradients within the treatment plant is consistent with a potential contribution to Mn transformations.This study highlights the interplay between geochemical factors and microbial community structure during the passive treatment of Mn in mine water and offers insights for optimising biofilter design and operation.

Understanding Cr, Fe, and Mn speciation is essential to evaluate Cr stability and transformation in highly weathered red soils, which are vital for agriculture and elemental cycling in southern China. Here, XANES spectroscopy was applied to investigate Cr, Fe, and Mn speciation in two Fe-Mn nodule (FMN)-rich red soil profiles. Cr primarily exists as Cr(III)-Fe(III) co-precipitates, accounting for ∼90 % of total Cr in FMNs and 60-70 % in surrounding soils, with increasing proportions of Cr-bearing silicates in the latter. Goethite and magnetite are the main Fe species in FMNs, while lepidocrocite dominates in surrounding soils. Mn(III) hydroxides and organically complexed Mn dominate in FMNs, contrasting with tectomanganate and physisorbed Mn in surrounding soils. The Mn average oxidation state is higher in non-paddy soil than in paddy soil, but increases in the bottom horizons of the paddy profile. Pre-edge analysis indicates negligible Cr(VI) in FMNs; however, its presence in surrounding soils is not removable by a KH2PO4 solution, suggesting limited mobility. Integrated with previously published Cr isotope data, the results suggest that Cr(VI) released through oxidative weathering is largely reduced and incorporated into Fe (hydr)oxides within FMNs, whereas in surrounding soils, it undergoes incomplete local reduction with isotopic fractionation, migrates downward, and is captured within Fe (hydr)oxides. In deeper horizons, the coexistence of amorphous Fe (hydr)oxides, elevated Cr concentrations, and high-valent Mn (hydr)oxides implies that Cr(III) may be reoxidized under redox fluctuations. The combined XANES and Cr isotope approach clarifies vertical Cr transformation mechanisms and informs environmental risk assessment in red soils.

Natural environments subjected to hydrologically driven redox fluctuations are regarded as propitious to contaminant degradation since they favor the cyclic oxygenation of Fe(II) minerals, which produces oxygen reactive species (ROS), such as the hydroxyl radical, OH center dot. However, the identity of these reactive species may vary as a function of physicochemical conditions and remains a matter of research. Here, using spin-trapping electron paramagnetic resonance (EPR) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, we show that a non-hydroxyl reactive species is produced in significant amounts upon air-oxidation of alluvial soil suspensions (Seine River Basin, France). Indeed, among the DMPO-OH center dot, DMPO-CO2 center dot-, and DMPO-alkyl(center dot) adducts observed, the latter dramatically increases with the addition of ethanol or, to a lesser extent, tert-butanol, especially with phosphate buffer. This result reveals a dominant non-hydroxyl species, which we interpret as Fe(IV) since it is known to oxidize alcohols to alkyl(center dot) radicals and is favored by phosphate ligands. With phosphate buffer and ethanol, the DMPO-alkyl(center dot) production correlates with the initial reduced-state iron pool in the samples, as determined using Fe K-edge X-ray absorption spectroscopy (XAS). Fe(II) phyllosilicates, Fe(0) in one soil core and, to a lesser extent, vivianite, are found to be the most significantly oxidized iron phases upon soil oxygenation, and pyrite appears less reactive. Hence, we show that a significant reactive species, differing from OH center dot, forms upon oxygenation of soil Fe(II) minerals, especially in the presence of soil-sourced phosphate. Our results may therefore call to further directly identify this putative Fe(IV) species and to investigate its ability to degrade organic contaminants in natural environments.

Studying the interactions between metals and thiol moieties in natural systems is challenging, although they are of major importance for some (ultra)trace elements (e.g., Hg, Cu, Pt). A major current bottleneck is the development of accurate preservation and detection methods. Based on our current knowledge, thiol moieties are abundant in reduced organic waters, where thiolation of natural organic matter (NOM) occurs, as well as in metal-enriched environments, where organisms secrete thiol moieties. Depending on their affinity and their redox potential, metals complexed to thiolated NOM, can be reduced, and even transformed into sulphur nanoparticles over time. Such mechanisms are not properly considered in currently used biogeochemical models, explaining why the fate of metals in the environment is not well predicted.

Manganese oxide minerals are among the most powerful oxidizers on the Earth's surface. They are therefore key minerals both for the origin of life and exobiology issues but also for those concerning current biogeochemical cycles. Most of these manganese oxides are formed by biomineralization processes carried out by microorganisms that must be deciphered to better understand the fate of metals and metalloids in subsurface environments. A recent study showed that liquid-cell scanning transmission electron microscopy (LC-STEM) enables to monitor in situ the growth of Mn-bearing minerals onto Escherichia coli cells. This study has also highlighted the critical role of the chemical functions carried by cell surfaces and exopolymers during biomineralization. However, the contribution of the different functional groups associated to these biopolymers during mineral nucleation and growth remains poorly defined. In order to better assess the role played by these different chemical functions during biomineralization, functionalized polystyrene beads were used here as analogs of biological surfaces. In addition to control beads without functionalization, nine representative types of functionalization were selected, ranging from simple carboxylic and amino groups, to strong chelating agents such as nitrilotriacetic acid (NTA), or more complex proteins such as streptavidin and collagen. Each bead type was exposed to Mn(II)-bearing solution, and mineralization dynamics was continuously monitored in situ by LC-STEM. Mn mineralization was observed for all ten bead types with the formation of pyrolusite (MnO2) at the bead surfaces, as the result of changes in Mn redox state in solution triggered by radiolysis resulting from water and electron beam interactions. For all bead types, mineralization can be described as a nucleation step followed by the formation of larger dendritic structures. However, nucleation site densities, precipitates morphologies, as well as the overall mineral growth kinetics were found to vary significantly between the different grafted chemical functions. The bead surface charge, estimated by electrophoretic mobility, only partly explains these differences in mineralization dynamics. Steric effects, hydrophobicity as well as Mn affinity for the functional groups are certainly important parameters for Mn mineralization. As a result, this study brings interesting constraints on biomineralization processes driven by microorganisms.

Motivation for detecting engineered nanoparticles (ENPs) in the environment comes from a need to understand fate and behavior of these materials in natural matrices. The difficulty lies in the low expected ENP particle number concentration (PNC) and the presence of a large and variable background concentration of natural NPs. We report the PNCs and characteristics of cerium-bearing nanoparticles (Ce-NPs) and titanium-bearing nanoparticles (Ti-NPs) in an aquatic matrix (the Seine River and three of its tributaries) with the use of single particle ICP-MS (spICPMS) and electron microscopy (FEG-SEM). Ce-bearing and Ti-bearing particles were observed in suspended particulate matter collected onto 0.2 μm and 1 kDa filters, using FEG-SEM imaging. At Marnay-sur-Seine, the upstream point, PNCs for Ce-NPs and Ti-NPs were 0.47 ± 0.07 × 106 and 1.35 ± 0.17 × 106 particles as measured by spICPMS. The maximum PNC for both Ce-NPs and Ti-NPs, 1.59 ± 0.10 × 106 particles mL−1 and 5.89 ± 0.10 × 106 particles mL−1, respectively, were found in the Marne River, a major tributary to the Seine. It was shown that downstream of each confluence, an increase in the PNC of the Seine is observed, suggesting a significant contribution of the different tributaries. Mass balance of particles flows and elemental ratios of Ce/La showed that in the Marne and the Oise River, a contribution of natural CeO2 NPs exists. The anthropogenic contribution in TiO2 ENPs for the Marne River was further assessed with Ti/Al, Ti/V, and Ti/Y elemental ratios. Near constant element ratios in the Seine below the Orge River and Paris city suggest neither contribute significantly to Ce or Ti NP concentrations. The study provides further investigation of the strengths and limitations of the application of spICPMS to natural samples and contributes data to the currently highly-limited dataset on natural NP backgrounds in rivers, information that is key to assessing the potential for quantifying the input of ENPs to surface waters. Of the total mass of Ce and Ti, 83 and 90%, respectively, could be detected as particles by spICPMS.

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Electron transfer in the critical zone is driven by biotic and abiotic mechanisms and controls the fate of inorganic and organic contaminants, whether redox-sensitive or not. In these environments, Fe- and Mn-bearing minerals, as well as organic matter, are key compounds. They interact with each other and constitute important electron shuttles. As a result, not only their solubility but also their structure controls the mobility of many essential and toxic elements. In addition, microorganisms that form hot spots and are widespread in environmental systems are also primordial players in electron transfer processes by acting as a catalyst between an electron donor and an acceptor, and through their contaminant detoxification metabolism.

Predicting the fate of engineered nanoparticles (ENPs) once they are released in the environment is essential to evaluate their impacts to ecosystems. Microbial biofilms, as highly reactive compartments in soils and sediments, have the potential to impose strong controls on ENPs life cycle in natural settings. However, information regarding impacts of biofilms toward ENPs environmental fate are not easily accessible, and such evidences are collected and discussed in this review, in order to identify common trends and to better constrain the role played by these microbial structures. Biofilms are reported to exhibit important ENPs accumulation capacities, and short to long-term ENPs immobilization can thus be expected. Mechanisms that govern such accumulation and ENPs migration within biofilms depend strongly on electrostatic and hydrophobic interactions, as well as biofilm structural properties, such as density and permeability. They are a combination of key parameters that include ENPs size and surface properties, mineral substrate reactivity, ability to develop organic corona around ENPs, or formation of aggregates within the biofilm thickness. In addition, these microbial structures exhibit highly reactive microenvironments, and are consequently able to impose major ENPs transformations such as dissolution, through ligand- or redox-mediated pathways, as well as passivation or stabilization processes. Interestingly, exposure to toxic ENPs can even trigger a response from micro-organisms biofilms which has the potential to strongly modify ENPs speciation. Promising approaches to investigate the role of microbial biofilms for ENPs cycling in realistic systems are introduced through the use of mesocosms, medium-size replicated ecosystems that allow to integrate the complexity of natural settings. Finally, biofilm-mediated nanoparticles synthesis in man-impacted systems is presented. This raises important questions regarding biofilms role as secondary sources of nanoparticles.

The increasing use of Quantum Dots (QDs) - nanoparticles exhibiting unique optical properties - and their incorporation in multiple engineering products is likely to result in the release of this new class of contaminants into natural systems. In soils, bacterial biofilms and mineral surfaces form highly reactive interfaces, which may control QDs' environmental fate. However, little is known regarding QDs' stability in, and modes of interactions with, biofilm/mineral interfaces. This study examines the interactions, distributions and stability of thioglycolic acid-capped CdSe/ZnS QDs at the corundum (alpha-Al2O3)/Shewanella oneidensis MR-1 interface, for exposure times ranging between 1 h to 24 h. Long Period - X-ray Standing Wave - Fluorescence Yield spectroscopy and Grazing Incidence - X-ray Absorption Spectroscopy were used. Results indicate increases in Zn and Se concentrations within the biofilm/crystal system with time, demonstrating its high accumulation capacity over 24 h. In addition, dissolution of a part of the ZnS shell occurs within 1 h, highlighting the potential degradation of QDs when exposed to the biofilm/crystal compartment. Once released, Zn(II) migrates toward the biofilm-crystal interface and interacts preferentially with the crystal surface. In contrast, the remaining CdSe core is mostly preserved, and stays within the biofilm thickness. However, at 24 h, Se and Zn present similar distribution profiles indicating a general reduction in ZnS shell dissolution at this longer exposure time.

The need to quantify engineered nanoparticles (ENPs) in the environment is due to the increasing incorporation of these particles in the daily products, which threatens human health and can possibly impact natural systems. Ceria NPs (CeO2NPs) and titanium dioxide NPs (TiO2NPs) are two of the most used ENPs in the world. In this study their occurrence was determined in river waters with accurate and relevant techniques such as single particle ICP-MS (spICPMS). In the Loire River (France), the variation of both CeO2NPs and TiO2NPs could be assessed locally, with an increase of the concentrations near a wastewater treatment plant (WWTP) outlet as well as in a lake connected to the river and dedicated to outdoor activities. In the upstream river water, supposedly less impacted by NPs, 6.4 ± 1.2 × 104 part mL–1 Ce-bearing and 13.4 ± 1.8 × 104 part mL–1 Ti-bearing particles were measured. These values increased to 33.9 ± 3.4 × 104 part mL–1 Ce-bearing and 80.3 ± 3.4 × 104 part mL–1 Ti-bearing particles near the WWTP outlet. Equivalent size for sphere distributions ranged from 24 nm to 70 nm for CeO2 and from 80 nm to 500 nm for TiO2 in the river water. In the lake, a raise of the concentrations has been observed with 38.3 ± 2.0 × 104 part mL–1 and 71.6 ± 2.1 × 104 part mL–1 containing Ce and Ti, respectively, with similar size distributions. FEG-SEM imaging confirms the occurrence of Ce- and Ti-bearing particles in the water samples. On the contrary, NPs seem to undergo strong heteroaggregation in the Loire river water. The Ce/La elemental ratios does not evolve from upstream to downstream the WWTP outlet, suggesting that a natural origin cannot be excluded to explain the increase observed in NPs number concentration. On the contrary, the Ce/La ratio increases in the outdoor activities center, which suggests the contribution of NPs potentially related to the cars parked nearby. Besides, elemental ratios Ti/V and Ti/Y have been assessed to highlight an anthropogenic source of Ti in both sampling sites, possibly to the sunscreens used during the summer.

This study focuses on the determination of field solid/liquid ratios (Rd) values of trace element (TE) and radionuclide (RN) in the Seine River (France) during a concerted low radioactivity level liquid regulatory discharge performed by a Nuclear Power Plant (NPP) and their confrontation with Kd values calculated from geochemical modeling. This research focuses on how field Rd measurements of TE and RN can be representative of Kd values and how Kd models should be improved. For this purpose 5 sampling points of the Seine River during a NPP's liquid discharge were investigated: upstream from the discharge in order to assess the natural background values in the area of effluent discharge, the total river water mixing distance (with transect sampling), and 2 points downstream from this last area. The main parameters required determining field Rd of TE and RN and their geochemical modeling (Kd) were acquired. Filtered waters were analyzed for alkalinity, anions, cations, dissolved organic carbon (DOC), TE, and RN concentrations. Suspended particulate matter (SPM) was analyzed for particulate organic carbon (POC), TE and RN concentrations and mineralogical composition. Field Rd and Kd values are in good agreement for stable Cd, Cu, Ni, Pb and Zn and for 7Be. Conversely, measured field Rd for stable Ag, Ba, Sr, Co and Cs are systematically higher than modeled Kd values. Even if only the lowest possible values were obtained for 137Cs and 60Co Rd measurements, these estimated limits are higher than calculated Kd for 137Cs and in good agreement for 60Co. Finally, only two RN exhibit field Rd lower than calculated Kd: 234Th and 210Pb. Comparison of field Rd vs. modeled Kd values for TE and RN allows the identification, for each element, of the main involved adsorption phases and geochemical mechanisms controlling their fate and partitioning in river systems.

Surface Enhanced Raman Scattering (SERS) has been widely praised for its extreme sensitivity but has not so far been put to use in routine analytical applications, with the accessible scale of measurements a limiting factor. We report here on a frugal implementation of SERS dedicated to the quantitative detection of Zn 2+ in water, Zn being an element that can serve as an indicator of contamination by heavy metals in aquatic bodies. The method consists in randomly aggregating simple silver colloids in the analyte solution in the presence of a complexometric indicator of Zn 2+ , recording the SERS spectrum with a portable Raman spectrometer and analysing the data using multivariate calibration models. The frugality of the sensing procedure enables us to acquire a dataset much larger than conventionally done in the field of SERS, which in turn allows for an in-depth statistical analysis of the analytical performances that matter to end-users. In pure water, the proposed sensor is sensitive and accurate in the 160–2230 nM range, with a trueness of 96% and a precision of 4%. Although its limit of detection is one order of magnitude higher than those of golden standard techniques for quantifying metals, its sensitivity range matches Zn levels that are relevant to the health of aquatic bodies. Moreover, its frugality positions it as an interesting alternative to monitor water quality. Critically, the combination of the simple procedure for sample preparation, abundant SERS material and affordable portable instrument paves the way for a realistic deployment to the water site, with each Zn reading three to five times cheaper than through conventional techniques. It could therefore complement current monitoring methods in a bid to solve the pressing needs for large scale water quality data.