Geochemical Fate Research Articles

Chromium(VI) Adsorption and Reduction in Soils under Anoxic Conditions: The Relative Roles of Iron (oxyhr)oxides, Iron(II), Organic Matters, and Microbes.

Chromium (Cr) transformation in soils mediated by iron (Fe) (oxyhr)oxides, Fe(II), organic matter (OM), and microbes is largely unexplored. Here, their coupling processes and mechanisms were investigated during anoxic incubation experiments of four Cr(VI) spiked soil samples with distinct physicochemical properties from the tropical and subtropical regions of China. It demonstrates that easily oxidizable organic carbon (EOC, 55-84%) and microbes (16-48%) drive Cr(VI) reduction in soils enriched with goethite and/or hematite, among which in dryland soils microbial sulfate reduction may also be involved. In contrast, EOC (38 ± 1%), microbes (33 ± 1%), and exchangeable and poorly crystalline Fe (oxyhr)oxide-associated Fe(II) (29 ± 3%) contribute to Cr(VI) reduction in paddy soils enriched with ferrihydrite. Additionally, exogenous Fe(II) and microbes significantly enhance Cr(VI) reduction in ferrihydrite- and goethite-rich soils, and Fe(II) greatly promotes but microbes slightly inhibit Cr passivation. Both Fe(II) and microbes, especially the latter, promote OM mineralization and result in the most substantial OM loss in ferrihydrite-rich paddy soils. During the incubation, part of the ferrihydrite converts to goethite but microbes may hinder the transformation. These results provide deep insights into the geochemical fates of redox-sensitive heavy metals mediated by the complicated effects of Fe, OM, and microbes in natural and engineered environments.

Mercury isotopic compositions of iron oxide‑copper‑gold (IOCG) hydrothermal systems: Deep Hg cycling in intracontinental settings

Mercury isotopes display unique mass-independent fractionation (MIF), typically expressed as Δ199Hg, which relates to photochemical reactions occurring at Earth's surface. Pronounced Hg-MIF signals have been observed in various hydrothermal systems in both convergent margins and intracontinental settings, highlighting the recycling of Hg from marine or terrestrial reservoirs into shallow continental hydrothermal systems. However, the geochemical fate of Hg in deep continental environments of intracontinental settings remains poorly understood. Iron oxide‑copper‑gold (IOCG) hydrothermal systems typically involve the circulation of fluids from both deep magmatism and continental basins, providing an opportunity to investigate this issue. Here, we present the Hg isotopic compositions of ore minerals from five representative IOCG deposits in the Kangdian region, South China. All the studied ore samples display large mass-dependent fractionation with δ202Hg of −3.23 to 1.06‰, but a relatively narrow range of Δ199Hg of −0.17 to 0.11‰. These isotopic signatures support the hypothesis of binary mixing of Hg from two sources: (1) a magmatic endmember (∼two thirds) derived from the lithospheric mantle with low δ202Hg (−3 to −1‰) and near-zero Δ199Hg, and (2) a crustal source (∼one third) originating from the basement rocks with relatively higher δ202Hg (−1 to 1‰) and negative Δ199Hg (−0.2 to 0‰). Compared to the shallow Pb-Zn-Au-Sb hydrothermal systems which formed in intracontinental basins through the circulation of fluids in the upper crust, the Hg components (and by analogy, the ore metals) in the Kangdian IOCG system are dominated by a lithospheric mantle source. Our study sheds light on deep Hg cycling and confirms the Hg isotope tracer as a tool for revealing metal sources in hydrothermal ore systems.

Photochemical transformation of terrestrial dissolved organic matter derived from multiple sources in tropical plantations

Photochemical transformation is a critical geochemical fate of terrestrial dissolved organic matter (DOM) that drains from forests to downstream water bodies. However, the photo-degradability and photo-transformation of terrestrial DOM from various sources are yet to be explored. Here, optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry were used to analyze photochemical change in spectroscopic and molecular-level signatures of DOM derived from throughfall, stemflow, runoff, and soil pore water at depths of 20 and 40 cm in tropical typical Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) plantations. The dissolved organic carbon concentrations were generally higher in the EU plantation than in the AA plantation and higher in stemflow than in other sample types. The greatest DOM degradation occurred in soil pore water and the least degradation occurred in stemflow. Similarly, the common photo-transformation of DOM, including decreased aromaticity and unsaturation degrees, was greatest for soil pore water and the least for stemflow DOM. Most of the photochemical products were high H/C and O/C molecules for throughfall, stemflow, and pore water, but high H/C and low O/C molecules for runoff because of strong decarboxylation. These findings reveal the highly varied molecular composition and photo-degradability of DOM from different terrestrial sources and highlight that stemflow is an important contributor to photo-resistant DOM among terrestrial DOM sources.

Fate of arsenic during the interactions between Mn-substituted goethite and dissolved Fe(II)

Geogenic arsenic has become a globally-distributed groundwater contaminant, liberated from the weathering of arsenic-bearing sulfide minerals and often transported to aquifer sediments adsorbed to iron oxides. Among the iron oxides, goethite (α-FeOOH) is uniquely important for the fate of arsenic because of its widespread abundance, stability, and high affinity for binding arsenic. Goethite is ubiquitous in soils and sediments and often contains substituted elements, including manganese. Structural manganese may affect the surface reactivity and redox capacity of goethite and alter the mechanisms of recrystallization catalyzed by dissolved Fe(II), potentially affecting arsenic adsorption. This study examined the fate of As(V) during the interactions between dissolved Fe(II) and Mn-substituted goethites at pH 4 and 7 as well as associated changes in arsenic speciation. At pH 7, the addition of dissolved Fe(II) initially increases the adsorption of As(V) onto Mn-bearing and Mn-free goethites. For the Mn-substituted goethites, the adsorbed As(V) slowly releases to solution at longer aging times. Fe(II) addition at pH 4 slightly increases As(V) uptake by Mn-substituted goethites, with differences in total sorption correlating with the Mn content in goethite. The addition of Fe(II) releases substantial dissolved manganese but the amount solubilized is higher at pH 4 compared to 7, suggesting that the presence of adsorbed As(V) may substantially promote the Mn release at pH 4. X-ray absorption fine structure spectroscopy shows that arsenic is stabilized as As(V) in all the samples and adsorbed on goethite via a bidentate binuclear mechanism. Fitting results show that the binding distance and coordination numbers are stable in Mn-free goethite and Mn-substituted goethite samples; the effect of substituted Mn on the surface complex structure is minor. High resolution transmission electron microscopy and X-ray diffraction confirm that no secondary ferrous arsenate minerals precipitate under both pH conditions. This study improves our understanding of the Fe(II)-As(V) interactions on iron oxides, and demonstrates that the substituted cations such as manganese may quantitatively alter the geochemical fate of arsenic during the reaction of dissolved Fe(II) with Fe(III) oxides.

Novel biological aqua crust enhances in situ metal(loid) bioremediation driven by phototrophic/diazotrophic biofilm

BackgroundUnderstanding the ecological and environmental functions of phototrophic biofilms in the biological crust is crucial for improving metal(loid) (e.g. Cd, As) bioremediation in mining ecosystems. In this study, in combination with metal(loid) monitoring and metagenomic analysis, we systematically evaluated the effect of biofilm in a novel biological aqua crust (biogenic aqua crust—BAC) on in situ metal(loid) bioremediation of a representative Pb/Zn tailing pond.ResultsWe observed strong accumulation of potentially bioavailable metal(loid)s and visible phototrophic biofilms in the BAC. Furthermore, dominating taxa Leptolyngbyaceae (10.2–10.4%, Cyanobacteria) and Cytophagales (12.3–22.1%, Bacteroidota) were enriched in biofilm. Along with predominant heterotrophs (e.g. Cytophagales sp.) as well as diazotrophs (e.g. Hyphomonadaceae sp.), autotrophs/diazotrophs (e.g. Leptolyngbyaceae sp.) in phototrophic biofilm enriched the genes encoding extracellular peptidase (e.g. family S9, S1), CAZymes (e.g. CBM50, GT2) and biofilm formation (e.g. OmpR, CRP and LuxS), thus enhancing the capacity of nutrient accumulation and metal(loid) bioremediation in BAC system.ConclusionsOur study demonstrated that a phototrophic/diazotrophic biofilm constitutes the structured communities containing specific autotrophs (e.g. Leptolyngbyaceae sp.) and heterotrophs (e.g. Cytophagales sp.), which effectively control metal(loid) and nutrient input using solar energy in aquatic environments. Elucidation of the mechanisms of biofilm formation coupled with metal(loid) immobilization in BAC expands the fundamental understanding of the geochemical fate of metal(loid)s, which may be harnessed to enhance in situ metal(loid) bioremediation in the aquatic ecosystem of the mining area.BQhiNsEmXgEaKyMcCjHWdrVideo

Formation and Dynamic Behavior of Macroscopic Aluminum-Based Silica Gardens.

Chemical gardens are self-assembled structures with intricate plant-like morphologies and consist of mineralized membranes, which form spontaneously at interfaces between compartments with dissimilar chemical composition, most typically acidic metal salt and alkaline sodium silicate solutions. While this phenomenon is thought to occur in a number of practical settings, it has also proven to be valuable for investigating transport characteristics in distinct applied systems. For example, coupled diffusion and precipitation processes were monitored in silica gardens based on calcium and iron salts, considered to be models for cement hydration and steel corrosion, respectively. Here we extend these studies to the case of aluminum-based silica gardens, one of the so far less frequently investigated examples of silica gardens. To this end, single macroscopic tubes were prepared in a reproducible way by the controlled addition of sodium silicate solution to a pellet of pressed aluminum nitrate. Continued sampling of the volumes enclosed by and surrounding the formed membraneous structure allowed the time-dependent development of ion concentration gradients to be tracked over extended periods of time, while both the pH and electrochemical potential differences across the membrane were recorded online by immersed probes. The dynamic behavior revealed in this way was finally complemented by ex-situ analyses of the composition of the formed tubes. The collected data shows that the as-prepared tubular structures consist of sodium aluminosilicate phases with certain similarities to zeolites and geopolymers. The emerging tube wall was further found to be permeable to all ionic species present in the system, allowing significant electrochemical potential to be sustained over tens of hours until diffusion had eventually diminished the initially generated gradients. The findings of this work may have important implications for the geochemical fate of natural aluminosilicate sources, the use of such geopolymers in construction applications, and the synthesis and properties of zeolites.

Physical transport of magmatic sulfides promotes copper enrichment in hydrothermal ore fluids

AbstractLoss of magmatic sulfides to the mantle is posited to explain the copper deficit of evolved arc magmas and the depleted Cu/Ag ratio of the continental crust. We address the question of whether saturating sulfides may instead be mechanically entrained with rising magmas, and how this would affect their geochemical fate in the upper crust. Entrainment is plausible considering sulfide wetting properties and settling velocities relative to magma ascent velocities. Entrained sulfide increases the pressure at which magmas become saturated with respect to H-O-S fluids in the upper crust by 10–100 MPa, with the pressure difference increasing with temperature, water content, and oxidation. Bubbles are likely to nucleate on sulfide particles, allowing transfer of S and Cu from the sulfide to the fluid over a small crystallization interval without limitations by diffusion through the silicate melt. This sequence of processes gives magmatic sulfides an active role in ore metal transport and enrichment to form porphyry copper deposits, and may have global implications for crustal Cu budgets.

Mercury isotopic compositions of the Precambrian rocks and implications for tracing mercury cycling in Earth's interior

Mercury isotopes undergo unique mass-independent fractionation (MIF) during photochemical processes on Earth's surface. Studies have observed pronounced Hg-MIF signals in sedimentary and magmatic rocks, suggesting recycling of Hg from Earth's surface systems into the lithosphere via sedimentation and magmatism. However, the isotopic signature of Hg in metamorphic rocks and the geochemical fate of Hg during metamorphism remain unclear. Precambrian basements are important components of cratons or orogenic belts on Earth. Here, we study the Hg concentration and isotopic composition of Precambrian metamorphic and sedimentary rocks from the eastern Central Asian Orogenic Belt, and North and South China cratons. Metamorphic rocks show much lower Hg contents (0.21–7.8 ppb) than sedimentary rocks (2.6–694 ppb), indicating a substantial loss of Hg during metamorphism. The lack of correlation between δ202Hg values (–2.41 to 0.18‰) and metamorphic grades indicates no systematic mass-dependent fractionation (MDF) of Hg isotopes during metamorphism. The Δ199Hg/Δ201Hg ratios of ∼ 1.0 for both metamorphic and sedimentary rocks indicate Hg was sourced from Earth's surface systems. The coupling of Hg-MIF signals between the metasedimentary rocks and the sedimentary settings of their protolith suggests no Hg-MIF during metamorphism. The negative Δ199Hg values (–0.30 to –0.02‰) in the Precambrian coastal sedimentary rocks imply the input of Hg into coastal regions via soil erosion. The positive Δ199Hg values (0.06 to 0.31‰) in the Precambrian marine sedimentary rocks suggest deposition of atmospheric Hg(II) to open oceans via wet deposition. The lack of significant Hg-MIF during metamorphism and other underground geological processes shows that Hg-MIF signals can work as a reliable tracer for indicating material cycling in Earth's interior.

Simultaneous Kinetics of Selenite Oxidation and Sorption on δ-MnO2 in Stirred-Flow Reactors.

Selenium (Se) is an essential and crucial micronutrient for humans and animals, but excessive Se brings negativity and toxicity. The adsorption and oxidation of Se(IV) on Mn-oxide surfaces are important processes for understanding the geochemical fate of Se and developing engineered remediation strategies. In this study, the characterization of simultaneous adsorption, oxidation, and desorption of Se(IV) on δ-MnO2 mineral was carried out using stirred-flow reactors. About 9.5% to 25.3% of Se(IV) was oxidized to Se(VI) in the stirred-flow system in a continuous and slow process, with the kinetic rate constant k of 0.032 h−1, which was significantly higher than the apparent rate constant of 0.0014 h−1 obtained by the quasi-level kinetic fit of the batch method. The oxidation reaction was driven by proton concentration, and its rate also depended on the Se(IV) influent concentration, flow rate, and δ-MnO2 dosage. During the reaction of Se(IV) and δ-MnO2, Mn(II) was produced and adsorbed strongly on Mn oxide surfaces, which was evidenced by the total reflectance Fourier transform infrared (ATR-FTIR) results. The X-ray photoelectron spectroscopy (XPS) data indicated that the reaction of Se(VI) on δ-MnO2 produced Mn(III) as the main product. These results contribute to a deeper understanding of the interface chemical process of Se(IV) with δ-MnO2 in the environment.

Influence of humic acids on fungal laccase-initiated 17α-ethynylestradiol oligomerization: Transformation kinetics and products distribution

Fungal laccase has aroused great concern in rapidly removing estrogens because of its ability to accelerate humification and oligomerization. Here, the effect of two humic acids (HAs) on the reaction kinetics and products distribution of 17α-ethynylestradiol (EE2) in laccase-initiated humification and coupling was systematically elucidated. Laccase from Trametes versicolor exhibited over 98.3% removal rate for EE2 at pH 5.0 within 120 min, while HAs invariably restrained EE2 transformation by competing with target-substrate for the enzymatic catalytic center. EE2 removal followed pseudo-first-order kinetics, and the rate constant was decreased markedly with increasing concentration of two HAs (0–60 mg L−1). Additionally, laccase heightened the aromaticity and humification degrees (A250 nm/A365 nm ratio) of HAs probably due to the formation of new humic polymers such as (HA)m and/or (HA)m-(EE2)n (m and n represent the number of HA and EE2 units, respectively). Three major EE2 oligomers were identified, in accordance with a mechanism involving the phenoxy radical-driven polymerization to yield a wide variety of self-coupling products. Notably, HAs diminished the extent of EE2 self-coupling but aggrandized the cross-coupling between EE2 and HAs, and the inhibition degree of EE2 self-coupling increased with the concentration of HAs. One major reason is EE2 could be covalently incorporated into humic molecules to produce (HA)m-(EE2)n cross-coupling products via radical-caused C–C, C–O–C, and/or C–O–C bonds, thereby reducing EE2 self-oligomerization. These findings highlight that HAs play a vital role in the fungal laccase-induced humification and oligomerization of EE2, which obviously alter the geochemical fate and transport of EE2 in natural aquatic ecosystems.

Learned Discourses: Timely Scientific Opinions.

The fate of a chemical in the environment is primarily controlled by the physico-chemical properties of the substance, prevailing environmental conditions, and patterns of use (Mackay et al. 1997). Thus, accurate determination of physico-chemical properties is critical to the formulation of valid environmental models and assessments (Bennett et al. 2001). As litigation becomes an increasingly common aspect of environmental science, such studies may be challenged and even refuted. Recent legislation in the United States mandates that federal agencies must ensure the quality of data upon which regulatory action and policy decisions are founded (Reichhardt 2002). Some observers have suggested that because this legislation provides for unprecedented scrutiny of agency data quality, regulatory agencies might face a marked increase in the number of legal challenges. Two of the most important physico-chemical properties relating to the environmental behavior of hydrophobic organic compounds are aqueous solubility (S w ) and the octanol-water partition coefficient (K ow ). Much of the interest in these properties comes from the fact that they can be used to estimate or predict other properties of more immediate environmental and ecotoxicological interest. For this reason, a primary focus of research in the last 20 years has been development of linear regression models in which S w and K ow (or more accurately, their logarithms) are correlated with parameters such as the organic carbon-normalized partition coefficient, bioconcentration and bioaccumulation factors, and indices of biodegradability or toxicity. These, in turn, can be incorporated into models that attempt to characterize the equilibrium distribution and transport rates of organic contaminants among environmental media or to predict impacts on biota. The predictions of such multimedia models have taken on greater immediacy with the recognition that certain persistent organic pollutants, such as DDT and DDE, are globally distributed and may be exerting adverse biological effects far from their original source areas (Wania and Mackay 1996). Our interest in the K ow and S w values of DDT and DDE arose in the context of research we were conducting on the geochemical fate of these compounds in sediments of the Palos Verdes Shelf, California. We expected that studies reporting S w and K ow values for DDT and DDE would be well documented because DDT has a very long history, and its widespread application and biological effects are well known. Results of our initial literature survey, however, were unsatisfactory due to a large spread in data values. We decided to undertake an exhaustive review to identify all measured and estimated values of S w and K ow for these compounds. It soon became apparent that a number of serious problems severely limited use of the database and that our initial expectations had been naive. A complete description of the study discussed here can be found in a USGS report (Pontolillo and Eganhouse 2001) available in PDF format on the web (http://pubs.water.usgs.gov/ wri01-4201/). We examined more than 700 publications including databases, handbooks, review articles, and bibliographies for data, methodological details, and pertinent literature citations. The initial survey was augmented by computerized searches of the scientific literature using the Chemical Abstracts Search Service Index (CASSI) database and the World Wide Web using a variety of Internet searchengines. Several hundred potential articles were generated in this fashion. Each article was retrieved and scrutinized, and additional relevant leads were identified from the reference lists. The search progressed until no further leads could be identified (approximately 2 years). Two problems are evident with the available database: egregious errors in reporting data and references, and poor data quality and/or inadequately documented procedures. The published literature is characterized by a preponderance of unnecessary data duplication. Numerous data and citation errors are also present. The percentage of original S w and K ow data for DDT

Geochemical behavior and fate of trace elements in naturally contaminated soils under projected land-use changes

PURPOSE: This paper focuses on determining the geochemical fractionation pattern of trace elements (As, Cd, Cu, Pb, Tl, and Zn) naturally occurring at elevated levels in chestnut grove soils of SW Spain. The goal was to explore how environmental changes triggered by land use and management decisions might affect the resilience and adaptive capacity of soil to retain geogenic trace elements. MATERIALS AND METHODS: Two plausible scenarios were considered: conversion of forestland to cropland (scenario I) and mining area (scenario II). The potential for trace element removal under the assumed scenarios was assessed by chemical extraction procedures designed to simulate the combined effects of experimentally induced pH and redox changes. Trace elements were partitioned into residual and labile fractions using a five-step sequential extraction scheme optimized for soils enriched in well-crystallized Fe oxides, and their concentrations in the soil extract solutions were measured by inductively coupled plasma mass spectrometry. RESULTS AND DISCUSSION: Most metals are tightly bonded to residual and reducible phases, indicating that silicate minerals and Fe oxy-hydroxides, respectively, played a remarkable role in the metal geo-accumulation. Limited mobilization and dispersion of exchangeable and acid-soluble contaminants would be expected to occur through releases or accidental spills from hazardous wastes. An increase in the oxidation state of the soil environment would affect the stability of the organic matter involving the release of the associated trace elements, particularly Cu. Upon reducing conditions induced by land-degradation processes, reductive dissolution of Fe oxy-hydroxides could release large proportions (45–60%) of adsorbed and occluded potentially harmful elements, notably As, Pb, and Cd. CONCLUSIONS: The increasing abandonment of the chestnut groves constitutes a driving force for environmental changes that might affect the geochemical status of the trace elements stored in the soil. Soil could shift from a sink to a source of harmful contaminants over time. This fact should be considered by local stakeholders engaged in planning and decision-making on future land uses.

Assessment of clay mineral selectivity for adsorption of aliphatic/aromatic humic acid fraction

Understanding the interactions of humic acids with clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soils and natural waters. This study assesses the selectivity of the clay minerals kaolinite and montmorillonite for the adsorption of the leonardite humic acid (LHA) fractions. The whole LHA was fractionated by ultrafiltration into five fractions of different molecular size (F1, >0.2 μm; F2, 0.2 μm–300,000 Da; F3, 300,000–50,000 Da; F4, 50,000–10,000 Da; F5, 10,000–1000 Da). Characterization by solid-state 13C nuclear magnetic resonance spectroscopy (NMR) indicated that the fraction >0.2 μm contains primarily aliphatic carbon, while the smaller molecules of the fraction <10,000 Da consists of predominantly aromatic carbon species. The adsorption characteristics of humic acid fractions onto kaolinite and montmorillonite at pH = 6 and 0.01 mol dm−3 NaCl ionic strength, were also evaluated by solid-state 13C NMR and UV–visible spectroscopy.UV–visible and 13C NMR spectra demonstrated preferential sorption by the clay minerals for different carbon types; kaolinite preferentially sorbed the aromatic carbon types, while montmorillonite preferentially sorbed the aliphatic carbon at the first point of LHA adsorption (low coverage). However, this preferential effect wasn't pronounced at the complete surface coverage (high coverage).

Weathering in a regolith on the Werenskioldbreen Glacier forefield (SW Spitsbergen). 2. Speciation of Fe, Mn, Pb, Cu and Zn in the chronosequence

The evolution of chemical speciation of Fe, Mn, Pb, Cu, and Zn was investigated in the chronosequence of young sediments, exposed by a currently retreating Arctic glacier on Spitsbergen. Werenskioldbreen is a 27 km 2 subpolar, land-terminated, polythermal glacier in recession, located near the SW coast of West Spitsbergen. Three samples of structureless till were collected at locations exposed for 5, 45 and 70 years. Four grain-size fractions were separated: &gt; 63, 20–63, 2–20, and &lt; 2 μm. Speciation of Fe, Mn, Pb, Cu, and Zn was determined using a 6-step sequential chemical extraction method: 1) 1 M sodium acetate, 2) 1 M hydroxylamine hydrochloride in acetic acid, 3) sodium dithionite in buffer, 4) acid ammonium oxalate, 5) boiling HCl, 6) residuum. The weathering in the proglacial area of the retreating glacier is very fast. The geochemical fates of the metals in question correlate with each other, reflecting a) the geochemical similarities between them, b) the similarities of their primary mineral sources, c) the significant role of incongruent dissolution. The weathering processes dominating the system are redox reactions and incongruent dissolution, followed by precipitation of secondary phases and partial sorption of aqueous species. As a result, the elements released from weathering minerals are only partially transported away from the system. The remaining part transforms by weathering from the coarse-grained fraction (dominated by fragments of primary minerals) into the fine-grained fraction (in the form of secondary, authigenic minerals or as species sorbed onto a mineral skeleton). This is very strongly pronounced within the chronosequence: the content of each of the metals studied correlates identically with the grain size, despite the differences in their chemical character and affinities. The microscope study presented herein indicates that the role of incongruent dissolution previously was underestimated. Also, the formation of coatings of secondary phases on primary mineral surfaces was observed. All these rapid weathering processes affect the mineral speciation of initial soils as well as the composition of mineral suspensions transported away by rivers to the nearby ocean.

Towards improving the electroanalytical speciation analysis of indium

The geochemical fate of indium in natural waters is still poorly understood, while recent studies have pointed out a growing input of this trivalent element in the environment as a result of its utilisation in the manufacturing of high-technology products. Reliable and easy-handling analytical tools for indium speciation analysis are, then, required. In this work, we report the possibility of measuring the total and free indium concentrations in solution using two complementary electroanalytical techniques, SCP (Stripping chronopotentiometry) and AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) implemented with the TMF/RDE (Thin Mercury Film/Rotating Disk Electrode). Nanomolar limits of detection, i.e. 0.5 nM for SCP and 0.1 nM for AGNES, were obtained for both techniques in the experimental conditions used in this work and can be further improved enduring longer experiment times. We also verified that AGNES was able (i) to provide robust speciation data with the known In-oxalate systems and (ii) to elaborate indium binding isotherms in presence of humic acids extending over 4 decades of free indium concentrations.The development of electroanalytical techniques for indium speciation opens up new routes for using indium as a potential tracer for biogeochemical processes of trivalent elements in aquifers, e.g. metal binding to colloidal phases, adsorption onto (bio)surfaces, etc.

Nucleation and growth of feitknechtite from nanocrystalline vernadite precursor

ernadite is a nanocrystalline manganese oxide, which controls the fate of many trace elements in soils and sediments through sorption and oxidative-degradation mechanisms. This exceptional reactivity directly results from its crystal structure, which may however evolve upon contact with redox-sensitive species. Understanding these changes is a prerequisite to predict and model the geochemical cycle of trace elements in the environment. Here, the structural and morphological modifications affecting synthetic nanocrystalline vernadite (δ-MnO2) upon contact with increasing concentrations of Mn2+ were investigated using wet chemistry, synchrotron X-ray diffraction and transmission electron microscopy. Fresh δ-MnO2 crystals had an Mn oxidation state of 3.94 ± 0.05 and a ~10 A layer-to-layer distance. Crystal size was ~10 nm in the layer plane, and ~1 nm perpendicular to that. Upon contact with aqueous Mn2+ under anoxic conditions, δ-MnO2 crystals underwent several morphological and mineral evolutions, starting with the stacking, perpendicular to the layer plane, of d -MnO2 crystals to form crystals ~10 nm 2 nm which were then subjected to oriented aggregation both along and perpendicular to the layer plane to form lath-like crystals with dimensions o ~100 nm 20 nm. Finally, these laths stacked perpendicular to the layer plane to form synthetic feitknechtite (β-MnOOH) crystals with sizes up to ~100 nm 500 nm when the Mn2+ loading reached 31.9 mmol g1. Structural transformation from d -MnO2 to synthetic feitknechtite was detected at Mn2+ loading equal to or higher than 3.27 mmol g-1. These mechanisms are likely to influence the geochemical fate of trace elements in natural settings where Mn2+ is abundant. Firstly, the systematic increase in crystal size with increasing Mn2+ loading may impact the sorption capacity of vernadite and feitknechtite by reducing the density of reactive edge sites. Secondly, the fate of trace elements initially sorbed at the vernadite surface is unclear, as they could either be released in solution or incorporated into the feitknechtite lattice.

Geochemical fates and unusual distribution of arsenic in natural ferromanganese duricrust

Preferential enrichment of arsenic in iron oxides relative to manganese oxides has been well documented. In this study, however, a distinct arsenic enrichment is revealed in natural ferromanganese duricrusts, which are commonly found in natural weathering profiles of manganese-bearing carbonate rocks. In the studied ferromanganese duricrust covering Carboniferous carbonates at Qixia Mountain in eastern China, stromotalite-like structures composed by hematite, goethite, pyrolusite and hetaerolite have been observed. Electron microprobe analysis (EMPA) mapping and synchrotron-based micro-scanning X-ray fluorescence (μ-XRF) analyses reveal that the arsenic content in manganese oxides is elevated with respect to iron oxide phases. For example, the arsenic content of pyrolusite is approximately 5 times as much as that of hematite or hetaerolite. However, the highest arsenic content (0.58 wt% As2O5) occurs in 2.75 (±0.96, ±σ) μm micro-bands of hematite ((FexMnIII1-x)2O3, 0.75 < x < 0.83). Although arsenic contents in the Mn-rich hematite micro-bands are extraordinarily high, the amount of hematite with a high Mn content is very low in the duricrust. Hence manganese oxides are suggested to be the major arsenic sink in the ferromanganese duricrust. Extended X-ray absorption fine structure spectra (EXAFS) further shows that all arsenic is present as oxidized As(V) and are bound to Fe/Mn oxides in bidentate binuclear bridging complexes with AsFe and AsMn bond distances of 3.24 Å and 3.23 Å, respectively. In addition, it is found that zinc is also more enriched in Mn oxides (besides hetaerolite) than in Fe oxides. The fine hematite crust with low contents of heavy metals could act as a protective seal to separate Mn oxides core with high Zn and As from environmental fluids. This separation could reduce the interaction between them and decrease the release of Zn and As from this ferromanganese duricrusts, which ensures long-term sequestration of heavy metals. The unique structural and mineralogical constraints on the distribution of heavy metals can provide insights into novel strategies for environmental remediation of heavy metals contaminations.

Adsorption of methylantimony and methylarsenic on soils, sediments, and mine tailings from antimony mine area

Biogeochemical processes of antimony (Sb) – a chemically similar element as arsenic (As) – in a variety of environment is largely unexplored. Soil and sediment environment are known to control the geochemical fate of several contaminants via a multitude of processes, primarily adsorption. Here adsorption of organic Sb/As on soils, sediments, and mine tailings was investigated. Sorption studies showed that the methylarsenic sorption capacity and rate were greater than methylantimony sorption. For TMSb, MMA, and DMA sorption samples, the highest sorption capacity occurred in the Ferralsol, which contained the highest amount of Fe/Al-oxyhydroxides. The sorption capacity was proportional to the Fe/Al concentration in the environmental matrices (TMSb-Fe: R2=0.52, TMSb-Al: R2=0.69; MMA-Fe: R2=0.61, MMA-Al: R2=0.76; DMA-Fe: R2=0.42, DMA-Al: R2=0.59). The highest amount of TMSb, MMA, and DMA sorption rate was also observed for the Ferralsol with 82%, 97%, and 87%, respectively and that of the lowest values were the mine tailings. The other samples had similar amounts of TMSb/MMA/DMA sorption rate ranging from 5 to 60% of initially added organic species.

Subsurface geochemical fate and effects of impurities contained in a CO2 stream injected into a deep saline aquifer: What is known

Geological storage of anthropogenic CO2 captured from large emitters has been identified and demonstrated on a pilot and commercial scale as a viable means for reducing atmospheric CO2 emissions from fossil-fuel based power generation and other industrial processes such as cement and steel making. In particular, CO2 storage in deep saline aquifers has been identified as having the greatest capacity and widest global distribution. Although the captured stream will be “overwhelmingly CO2”, any number of source and/or process associated impurities may be co‑injected, with the potential of additional impurities initially present in the aquifer being incorporated in the plume of stored CO2 post-injection. Impurities may make the injected phase more reactive than pure CO2 and will also change the physical properties of the injected CO2 stream. This paper documents the progress since the IPCC Special Report on CCS in understanding the fate and geochemical effects of these impurities when co-injected in an impure CO2 stream into deep saline aquifers.The presence of inert or non-condensable impurities in the injected CO2 stream, such as Ar, N2 and CH4, will have no or negligible geochemical effects in the subsurface, notwithstanding a reduction in CO2 storage capacity. The presence of acid gases such as SOx (and NOx) will likely have a negative effect particularly in the near-well region due to their tendency to accumulate in acidic forms in the water (and mineral phases) around the injection well. Other impurities that may be present in the CO2 stream, such as O2 and H2S, may or may not have a negative effect, depending on aquifer’s mineral composition. The severity of these effects should be evaluated through appropriate modelling of the impure-CO2/water/rock system. In extreme cases this may impose more severe limitations than those imposed by transportation systems on the type and amount of certain impurities present in the CO2 stream. In general, further development of the work performed to date will continue to yield valuable results dealing with impure-CO2/ aquifer interactions. Future research will benefit from closer integration between the various research communities to better define the myriad chemical environments which may develop due to the injection of impure CO2 streams. Exploration of the post-injection interactions in the vicinity of injection wells is one avenue which should be explored more vigorously.

2-Nitrophenol reduction promoted by S. putrefaciens 200 and biogenic ferrous iron: The role of different size-fractions of dissolved organic matter

The reduction of nitroaromatic compounds (listed as a priority pollutant) in natural subsurface environments typically coexists with dissimilatory reduction of iron oxides effected by dissolved organic matter (DOM). Investigating the impact of the DOM that influences those reduction processes is crucial for understanding and predicting the geochemical fate of these environmental species. This study investigated the impact of different molecular weight DOM fractions (DMWDs) on the 2-nitrophenol (2-NP) reduction by S. putrefaciens 200 (SP200) and α-Fe2O3 with lactate (excluding electron donor interference). Kinetic measurements demonstrated that 2-NP reduction rates were affected by the redox reactivity of active species under DMWDs (denoted as L-DOM, M-DOM, and H-DOM). The enhanced reduction rates are consistent with the negative shifts in peak oxidation potential values, the increases in HA-like/FA-like values, aromaticity index values and electron transfer capacity values. L-DOM acted mainly as ligands to complex Fe(II), whereas the significant role of H-DOM in reductive reactions should be acting as an electron shuttle, transferring electrons from SP200 to Fe(III) and 2-NP and from biogenic Fe(II) to 2-NP, further accelerating the 2-NP reductions. Those observations provide valuable insights into the role of DOM in the biogeochemical redox processes and the remediation of contaminated soil in a natural environment.