Abiotic Oxidation Research Articles

Polyethylene and related hydrocarbon polymers (“plastics”) are not biodegradable

Research on the biodegradation of polyethylene (PE), polystyrene (PS) and related polymers has become popular and the number of publications on this topic is rapidly increasing. However, there is no convincing evidence that the frequently claimed biodegradability of these so-called “plastics” really exists. Rather, a diffuse definition of the term “biodegradability” has led to the publication of reports showing either marginal weight losses of hydrocarbon polymers by the action of isolated bacterial strains or mechanical disintegration and polymer surface modification in case of hydrocarbon polymer-consuming insect larvae. Most of the data can be alternatively explained by the utilization of polymer impurities/additives, by the utilization of low molecular weight oligomers, and/or by physical fragmentation and subsequent loss of small fragments. Evidence for a (partial) biotic and/or abiotic oxidation of the amorphous polymer fraction and of surface-exposed hydrocarbon side chains is not sufficient to claim that PE is biodegradable. To the best of my knowledge, no report has been so far published in which substantial biodegradation and mineralization of PE or related (long chain length) hydrocarbon polymers to carbon dioxide has been convincingly demonstrated by the determination of the fate of carbon atoms in isotope-labeled polymers. It is disappointing that publications with a critical view on biodegradation of hydrocarbon polymers are not cited in most of these reports. The possibility should be considered that the rapidly expanding research field of hydrocarbon polymer biodegradation is chasing rainbows.

Iron minerals enhance Fe(II)-mediated abiotic As(III) oxidation

The abiotic oxidation of As(III) is simultaneously mediated by the oxidation of Fe(II) in microaerobic environment, but the role of Fe minerals in the Fe(II)-mediated As(III) oxidation have been neglected. This work mimicked the microaerobic environment and examined the mechanisms of Fe(II) mediated the As(III) oxidation in the presence of Fe minerals using a variety of iron minerals (lepidocrocite, goethite, etc.). The results indicated the Fe(II) and As(III) oxidation rate were improved with Fe minerals, while As(III) oxidation efficiency increased by 1.3–1.8 times in comparison to that without minerals. Fe(II) mediated the As(III) oxidation happened on Fe minerals surface in the presence of Fe minerals. The As(III) oxidation efficiency increased with increasing Fe mineral concentrations (from 0.5 to 2 g L−1) but decreased with increasing pH values. Reactive oxygen species (ROS) that play a crucial role in As(III) oxidation were Fe(IV) and ·O2−, accounting for 42.7%–47.9% and 24.1%–29.8%, respectively. The Fe minerals facilitated the oxidation of As(III) by ROS and stimulated the release of ROS through the adsorbed-Fe(II) oxidation, both of which favored As(III) oxidation. This work highlighted the potential mechanisms of Fe minerals in promoting Fe(II) mediated the As(III) oxidation in microaerobic environment, especially in terms of As(III) oxidation efficiency, shedding a valuable insight on optimization of arsenic contaminated wastewater treatment processes.

A comprehensive investigation of the continuous oxidation of pyrrhotite by micromolar hydrogen peroxide in near neutral and acidic solutions

Pyrrhotite oxidation poses a big threat to water environment duo to its high potential for generating pollutants. Hydrogen peroxide, commonly found in natural water at micromolar concentrations, possesses much more aggressive oxidation ability than oxygen and can complicate the pyrrhotite oxidation process. Here, the effects of micromolar H2O2 on the biotic and abiotic oxidation of pyrrhotite were examined at pH 1.93 and 6.45, respectively. Pyrrhotite oxidation was much more severe in acidic solutions compared to near neutral solutions. Jarosite with a high Fe/S molar ratio was widely detected in the precipitate collected in acidic solutions, and the introduction of external H2O2 influenced the crystallinity of jarosite. A layer of iron-deficient iron-sulfur oxide formed on the surface of pyrrhotite prevents its continuous oxidation, and the presence of Acidithiobacillus ferrooxidans enhanced this situation by promoting the release of Fe from the pyrrhotite. Additionally, the presence of external micromolar H2O2 also determined the elemental state on pyrrhotite surface, as it found that the contribution of Fe3+ and S (S4+ and S6+) species on pyrrhotite surface increased with the increase of H2O2 concentration in the solutions, especially in the presence of Acidithiobacillus ferrooxidans.

Degradation efficiency of biodegradable plastics in subtropical open-air and marine environments: Implications for plastic pollution

Bioplastics are increasingly used as a solution to tackle plastic pollution problems. However, their degradability in natural environments is currently under debate. To evaluate their degradation efficiencies, we conducted in-situ degradation experiments in an open-air and two marine environments in Hong Kong. Three groups of biodegradable plastic were tested, namely (1) additive-modified low-density polyethylene (LDPE), labelled as oxo-biodegradable or photodegradable plastics, (2) polylactic acid (PLA), and (3) polyvinyl alcohol (PVA)/starch blends. Most biodegradable plastics fail to completely degrade but remain visually present after six months of exposure. Only PLA is able to demonstrate 100 % disintegration in one to three months in marine settings, suggesting that subtropical marine environments may favor PLA degradation. Biodegradable plastics that are bio-based (PLA and PVA/Starch blends) show notably larger mass losses by 23–100 % than the fossil-based ones (modified-LDPE). Our results reveal higher degradation efficiencies of PLA and PVA/Cassava starch blend in marine than open-air settings (with mass losses larger by 50 %, and by 39–41 %, respectively), potentially via biodegradation and hydrolysis. Meanwhile, modified-LDPE and PVA/Corn starch blends in general show higher degradation efficiencies in open-air than marine settings (with mass losses larger by 2 %, and by 17–33 %, respectively), potentially via abiotic oxidation. Since all tested biodegradable plastics exhibit potential fragmentation signs, further investigation is needed to characterize the behaviours of the microplastics generated. The current labelling on biodegradable bags fails to provide comprehensive information regarding their actual environmental degradation behaviours, especially considering their fragmentation risk and limited degradation exhibited in this study. This highlights the imperative for improved messaging to ensure consumers are better informed about these products.

Treating waste with waste: Lignin acting as both an effective bactericide and passivator to prevent acid mine drainage formation at the source

Acid mine drainage (AMD) induced by pyrite oxidation is a notorious and serious environmental problem, but the management of AMD in an economical and environmentally friendly way remains challenging. Here, lignin, a natural polymer and abundant waste, was employed as both a bactericide and passivator to prevent AMD formation. The addition of lignin to a mimic AMD formation system inoculated with Acidithiobacillus ferrooxidans at a lignin-to-pyrite weight ratio of 2.5: 10 reduced the combined abiotic and biotic oxidation of pyrite by 68.4 % (based on released SO42−). Morphological characterization of Acidithiobacillus ferrooxidans revealed that lignin could act on the cell surface and impair the cell integrity, disrupting its normal growth and preventing biotic oxidation of pyrite accordingly. Moreover, lignin can be used alone as a passivator to form a coating on the pyrite surface, reducing abiotic oxidation by 71.7 % (based on released SO42−). Through multiple technique analysis, it was proposed that the functional groups on lignin may coordinate with iron ions on pyrite, promoting its deposition on the surface. In addition, the inherent antioxidant activity of lignin may also be actively involved in the abatement of pyrite oxidation via the reduction of iron. Overall, this study offered a “treating waste with waste” strategy for preventing AMD formation at the source and opened a new avenue for the management of AMD.

Algal Extracellular Organic Matter Induced Photochemical Oxidation of Mn(II) to Solid Mn Oxide: Role of Mn(III)-EOM Complex and Its Ability to Remove 17α-Ethinylestradiol.

Photosensitizer-mediated abiotic oxidation of Mn(II) can yield soluble reactive Mn(III) and solid Mn oxides. In eutrophic water systems, the ubiquitous algal extracellular organic matter (EOM) is a potential photosensitizer and may have a substantial impact on the oxidation of Mn(II). Herein, we focused on investigating the photochemical oxidation process from Mn(II) to solid Mn oxide driven by EOM. The results of irradiation experiments demonstrated that the generation of Mn(III) intermediate was crucial for the successful photo oxidization of Mn(II) to solid Mn oxide mediated by EOM. EOM can serve as both a photosensitizer and a ligand, facilitating the formation of the Mn(III)-EOM complex. The complex exhibited excellent efficiency in removing 17α-ethinylestradiol. Furthermore, the complex underwent decomposition as a result of reactions with reactive intermediates, forming a solid Mn oxide. The presence of nitrate can enhance the photochemical oxidation process, facilitating the conversion of Mn(II) to Mn(III) and then to solid Mn oxide. This study deepens our grasp of Mn(II) geochemical processes in eutrophic water and its impact on organic micropollutant fate.

Compartmentalization transformation of Fe2+, Mn2+ and NH4+ in groundwater: A comparative research containing experimental medium and functional genera profiling derived from experiment data

Contamination of soluble iron (Fe2+), manganese (Mn2+), and ammonium (NH4+) in groundwater is prevalent, particularly in Northeast China. This study aims to elucidate the intricate biological and physical-chemical reactions involved in the sequential transformation of three target contaminants. Two dynamic column simulations were used to compare quartz sand filtration (QSF) system with riverbank filtration (RBF) system to assess the interactions and respective contributions of the reactions involved in transforming process. Similar pattern of transformation vertical compartmentalization in two systems along the filter depth were founded, and the Fe2+ transformation was mainly distributed in the upper part, while NH4+ and Mn2+ occurred primarily in the downstream sections. The average transformation rate of Fe2+, Mn2+ and NH4+ in RBF system was 84.52 %, 47.68 % and 67.95 %, respectively, which was 5.20 %, 11.43 % and 23.06 % higher than in QSF system. The dominant transformation process for Fe2+ in RBF involved chemical oxidation, bio-oxidation and adsorption. The lower transformation rate for Mn2+ was attributed to abiotic homogenous oxidation limitation and negative affect derived from high concentration of NH4+ with high nitrification rate in RBF system. Nitrification by Nitrosomonas and Nitrospira was critical NH4+ transformation mechanism in RBF system, while the adsorption by iron and manganese oxide and aerobic denitrification were contributed strongly to NH4+ transformation in QSF system. Furthermore, the spatial distribution of functional bacterial genera was in good consistent with the transformation compartmentalization of three target contaminants in both systems, and similar microbial community structure reflected the domestication effect of groundwater chemistry on functional bacteria.

Aerobic iron-oxidizing bacteria secrete metabolites that markedly impede abiotic iron oxidation.

Iron is one of the Earth's most abundant elements and is required for essentially all forms of life. Yet, iron's reactivity with oxygen and poor solubility in its oxidized form (Fe3+) mean that it is often a limiting nutrient in oxic, near-neutral pH environments like Earth's ocean. In addition to being a vital nutrient, there is a diversity of aerobic organisms that oxidize ferrous iron (Fe2+) to harness energy for growth and biosynthesis. Accordingly, these organisms rely on access to co-existing Fe2+ and O2 to survive. It is generally presumed that such aerobic iron-oxidizing bacteria (FeOB) are relegated to low-oxygen regimes where abiotic iron oxidation rates are slower, yet some FeOB live in higher oxygen environments where they cannot rely on lower oxygen concentrations to overcome abiotic competition. We hypothesized that FeOB chemically alter their environment to limit abiotic interactions between Fe2+ and O2. To test this, we incubated the secreted metabolites (collectively known as the exometabolome) of the deep-sea iron- and hydrogen-oxidizing bacterium Ghiorsea bivora TAG-1 with ferrous iron and oxygen. We found that this FeOB's iron-oxidizing exometabolome markedly impedes the abiotic oxidation of ferrous iron, increasing the half-life of Fe2+ 100-fold from ∼3 to ∼335 days in the presence of O2, while the exometabolome of TAG-1 grown on hydrogen had no effect. Moreover, the few precipitates that formed in the presence of TAG-1's iron-oxidizing exometabolome were poorly crystalline, compared with the abundant iron particles that mineralized in the absence of abiotic controls. We offer an initial exploration of TAG-1's iron-oxidizing exometabolome and discuss potential key contributors to this process. Overall, our findings demonstrate that the exometabolome as a whole leads to a sustained accumulation of ferrous iron in the presence of oxygen, consequently altering the redox equilibrium. This previously unknown adaptation likely enables these microorganisms to persist in an iron-oxidizing and iron-precipitating world and could have impacts on the bioavailability of iron to FeOB and other life in iron-limiting environments.

Gallionellaceae in rice root plaque: metabolic roles in iron oxidation, nutrient cycling, and plant interactions.

In waterlogged soils, iron plaque forms a reactive barrier between the root and soil, collecting phosphate and metals such as arsenic and cadmium. It is well established that iron-reducing bacteria solubilize iron, releasing these associated elements. In contrast, microbial roles in plaque formation have not been clear. Here, we show that there is a substantial population of iron oxidizers in plaque, and furthermore, that these organisms (Sideroxydans and Gallionella) are distinguished by genes for plant colonization and nutrient fixation. Our results suggest that iron-oxidizing and iron-reducing bacteria form and remodel iron plaque, making it a dynamic system that represents both a temporary sink for elements (P, As, Cd, C, etc.) as well as a source. In contrast to abiotic iron oxidation, microbial iron oxidation results in coupled Fe-C-N cycling, as well as microbe-microbe and microbe-plant ecological interactions that need to be considered in soil biogeochemistry, ecosystem dynamics, and crop management.

Mercury deposition and redox transformation processes in peatland constrained by mercury stable isotopes

Peatland vegetation takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remains, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg redox transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that above the water table, dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations. Below the water table, supersaturation of gaseous Hg is likely created more by direct photoreduction of rainfall rather than by reduction and release of Hg from the peat soil. Identification and quantification of these light-driven and dark redox processes advance our understanding of the fate of Hg in peatlands, including the potential for mobilization and methylation of HgII.

Abiotic aerobic oxidation pathways of stibnite revealed by oxygen and sulfur isotope systematics of sulfate

The environmental threat posed by stibnite is an important geoenvironmental issue of current concern. To better understand stibnite oxidation pathways, aerobic abiotic batch experiments were conducted in aqueous solution with varying δ18OH2O value at initial neutral pH for different lengths of time (15-300 days). The sulfate oxygen and sulfur isotope compositions as well as concentrations of sulfur and antimony species were determined. The sulfur isotope fractionation factor (Δ34SSO4-stibnite) values decreased from 0.8‰ to -2.1‰ during the first 90 days, and increased to 2.6‰ at the 180 days, indicating the dominated intermediate sulfur species such as S2O32−, S0, and H2S (g) involved in Sb2S3 oxidation processes. The incorporation of O into sulfate derived from O2 (∼100%) indicated that the dissociated O2 was only directly adsorbed on the stibnite-S sites in the initial stage (0-90 days). The proportion of O incorporation into sulfate from water (27%-52%) increased in the late stage (90-300 days), which suggested the oxidation mechanism changed to hydroxyl attack on stibnite-S sites promoted by nearby adsorbed O2 on stibnite-Sb sites. The exchange of oxygen between sulfite and water may also contributed to the increase of water derived O into SO42−. The new insight of stibnite oxidation pathway contributes to the understanding of sulfide oxidation mechanism and helps to interpret field data.

Influence of natural cellulose on hydroxyl radical generation by abiotic oxidation of pyrite under acidic conditions

Natural cellulose is one of the most important substances coexisting on the surface of pyrite. Oxidation of pyrite produces hydroxyl radicals (•OH). In this study, a pyrite-cellulose binary system was constructed with natural cellulose to investigate the effect of cellulose on the mechanism of •OH generation via oxidation of pyrite, and the mechanism for abiotic oxidative •OH production by pyrite under the influence of cellulose was investigated with oxidation and quenching experiments and characterization techniques. It was demonstrated that cellulose was chemisorbed onto the pyrite surface and some of the Fe(II) on the pyrite surface was masked, thus inhibiting the reaction between pyrite and O2 and decreasing the •OH production level from 33.54 to 22.48 μM under oxic conditions. In addition, the cellulose caused SS bond breakage, resulting in defects on the pyrite surface, which oxidized H2O to produce •OH in anoxic conditions. Therefore, under anoxic conditions, cellulose promoted the production of •OH and increased the •OH content from 11.85 to 14.78 μM. In addition to •OH, pyrite oxidation also produced SO42−. The amount of SO42− produced by a single pyrite crystal was less than that produced in the pyrite-cellulose system in all cases, and the amount produced under oxic conditions was approximately 10 times greater than that produced under anoxic conditions. More sulfate production indicated more sulfur intermediates during the reaction, such as S2O32−, which may decompose to produce S0 or Sn2− adsorbed on pyrite and decrease the amount of •OH produced. During the oxidation of pyrite by H2O2, cellulose competed with pyrite to react with H2O2, which inhibited the reaction between pyrite and H2O2 and decreased •OH production. Therefore, natural cellulose influenced the abiotic oxidation of pyrite to produce •OH.

Burning question: Are there sustainable strategies to prevent microbial metal corrosion?

The global economic burden of microbial corrosion of metals is enormous. Microbial corrosion of iron-containing metals is most extensive under anaerobic conditions. Microbes form biofilms on metal surfaces and can directly extract electrons derived from the oxidation of Fe0 to Fe2+ to support anaerobic respiration. H2 generated from abiotic Fe0 oxidation also serves as an electron donor for anaerobic respiratory microbes. Microbial metabolites accelerate this abiotic Fe0 oxidation. Traditional strategies for curbing microbial metal corrosion include cathodic protection, scrapping, a diversity of biocides, alloys that form protective layers or release toxic metal ions, and polymer coatings. However, these approaches are typically expensive and/or of limited applicability and not environmentally friendly. Biotechnology may provide more effective and sustainable solutions. Biocides produced with microbes can be less toxic to eukaryotes, expanding the environments for potential application. Microbially produced surfactants can diminish biofilm formation by corrosive microbes, as can quorum-sensing inhibitors. Amendments of phages or predatory bacteria have been successful in attacking corrosive microbes in laboratory studies. Poorly corrosive microbes can form biofilms and/or deposit extracellular polysaccharides and minerals that protect the metal surface from corrosive microbes and their metabolites. Nitrate amendments permit nitrate reducers to outcompete highly corrosive sulphate-reducing microbes, reducing corrosion. Investigation of all these more sustainable corrosion mitigation strategies is in its infancy. More study, especially under environmentally relevant conditions, including diverse microbial communities, is warranted.

Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system.

Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0 ) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0 in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated by Beggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related to Sulfurovum, Halothiobacillus, Thiofaba, Thiovirga, Thiobacillus, and Desulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae. Desulfocapsa and Sulfurovum populations accounted for 12%-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0 disproportionation in pure culture. Gibbs energy (∆Gr ) calculations revealed that S0 disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed that Beggiatoa mats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0 generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.

Interactive Effects of Ceftriaxone and Chitosan Immobilization on the Production of Arachidonic Acid by and the Microbiome of the Chlorophyte Lobosphaera sp. IPPAS C-2047.

Pharmaceuticals including antibiotics are among the hazardous micropollutants (HMP) of the environment. Incomplete degradation of the HMP leads to their persistence in water bodies causing a plethora of deleterious effects. Conventional wastewater treatment cannot remove HMP completely and a promising alternative comprises biotechnologies based on microalgae. The use of immobilized microalgae in environmental biotechnology is advantageous since immobilized cultures allow the recycling of the microalgal cells, support higher cell densities, and boost tolerance of microalgae to stresses including HMP. Here, we report on a comparative study of HMP (exemplified by the antibiotic ceftriaxone, CTA) removal by suspended and chitosan-immobilized cells of Lobosphaera sp. IPPAS C-2047 in flasks and in a column bioreactor. The removal of CTA added in the concentration of 20 mg/L was as high as 65% (in the flasks) or 85% (in the bioreactor). The adsorption on the carrier and abiotic oxidation were the main processes contributing 65-70% to the total CTA removal, while both suspended and immobilized cells took up 25-30% of CTA. Neither the immobilization nor CTA affected the accumulation of arachidonic acid (ARA) by Lobosphaera sp. during bioreactor tests but the subsequent nitrogen deprivation increased ARA accumulation 2.5 and 1.7 times in the suspended and chitosan-immobilized microalgae, respectively. The study of the Lobosphaera sp. microbiome revealed that the immobilization of chitosan rather than the CTA exposure was the main factor displacing the taxonomic composition of the microbiome. The possibility and limitations of the use of chitosan-immobilized Lobosphaera sp. IPPAS C-2047 for HMP removal coupled with the production of valuable long-chain polyunsaturated fatty acids is discussed.

Oxidation rates and redox stabilization of ferrous iron in trioctahedral smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L−1 nitrite was negligible for the moderate-iron smectite and yielded only ∼17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.

Redox Oscillations Activate Thermodynamically Stable Iron Minerals for Enhanced Reactive Oxygen Species Production.

Reactive oxygen species (ROS) play key roles in driving biogeochemical processes. Recent studies have revealed nonphotochemical electron transfer from redox-active substances (e.g., iron minerals) to oxygen as a new route for ROS production. Yet, naturally occurring iron minerals mainly exist in thermodynamically stable forms, restraining their potential for driving ROS production. Here, we report that tide-induced redox oscillations can activate thermodynamically stable iron minerals for enhanced ROS production. •OH production in intertidal soils (15.8 ± 0.5 μmol/m2) was found to be 5.9-fold more efficient than those in supratidal soils. Moreover, incubation of supratidal soils under tidal redox fluctuations dramatically enhanced •OH production by 4.3-fold. The tidal hydrology triggered redox alternation between biotic reduction and abiotic oxidation and could accelerate the production of reactive ferrous ions and amorphous ferric oxyhydroxides, making thermodynamically stable iron minerals into redox-active metastable iron phases (RAMPs) with reduced crystallinity and promoting surface electrochemical activities. Those RAMPs displayed enhanced redox activity for ROS production. Investigations of nationwide coastal soils verified that tide-induced redox oscillations could ubiquitously activate soils for enhanced ROS production. Our study demonstrates the effective formation of RAMPs from redox oscillations by hydrological perturbations, which provides new insights into natural ROS sources.

Formation and Degradability of Per- and Polyfluoroalkyl Substances in River Soils around a Fluoropolymer-Manufacturing Plant in Osaka, Japan.

Our previous studies reported that perfluorooctanoic acid (PFOA) contamination decreased in well, tap, and surface water around a fluoropolymer plant in Osaka, Japan, between 2003 and 2016. In this study, we evaluated the degradability of PFOA and perfluorohexanoic acid in river soils to identify the influence of the degradation on the perfluorocarboxylic acids (PFCAs) in the Yodo River Basin. We also investigated the influence of abiotic oxidation on the formation of PFCAs in soils and measured the fluorotelomer alcohols (FTOHs) as precursors of PFCAs in the soil and air samples collected at Osaka and Kyoto. No major degradations were observed in soils contaminated with PFCA during the 24-week experimental period, while the PFOA levels increased only in the control group. The PFCA levels significantly increased after oxidation in this group. The dominant FTOH in soils was 10:2 FTOH, whereas 6:2 FTOH was dominant in the air samples. These findings suggest that PFOA was rapidly removed from water system but persist in soils. Moreover, the results indicate the need to evaluate not only the PFCAs, but also the FTOHs and other precursors for the accurate prediction of PFCA accumulation and fates in the environment.

Kinetics of coupled sorption and abiotic oxidation of antimony(III) in soils

A quantitative evaluation of the dynamics of antimony (Sb) species transformation when Sb(III) enters the soil matrix would facilitate the understanding of the speciation and environmental fate of Sb in natural environments. Towards this end, the coupled sorption and abiotic oxidation kinetics of Sb(III) were examined across 12 soils with a 24 h timeframe. The soil-solution distribution coefficients (Kd) of Sb ranged between 214 and 5965 L kg−1. Both apparent and mechanistic kinetic models were used to described the process. The apparent kinetic model indicated a two-rate oxidation and associated stabilization behavior of Sb in soils. The fast oxidation rate constants ranged from 0.147 to 0.763 h−1, which were one to two orders of magnitudes higher than the slow rate constants (0.002–0.039 h−1). Iron and Mn oxides were identified as the key soil components associated with adsorptive-oxidative sites (SOH) and oxidative sites (Mn-OH) of soils for Sb species, and these were subsequently integrated into the mechanistic kinetic model. This model included the adsorption–desorption of Sb(III) as well as the oxidation product, Sb(V) on the SOH, the oxidation of Sb(III) by SOH and Mn-OH, and the stabilization of both Sb(III) and Sb(V). The model produced a global fit for dissolved and adsorbed Sb species across the 12 soils. This study provides new information for predicting the dynamics of Sb in soils when multiple adsorption–desorption, oxidation and stabilization reactions are coupled in natural environments.

Controls on the Generation and Geochemistry of Neutral Mine Drainage: Evidence from Force Crag Mine, Cumbria, UK

Neutral mine drainage (NMD) at Force Crag mine in north-west England has a circumneutral pH and high levels of Zn contamination. A long-term geochemical and hydrological dataset from this site was analysed using a novel molar mass balance approach, which demonstrated that the water chemistry is dominated by species released by the oxidation of sulphides: sphalerite (Zn, Cd, Ni), galena (Pb, mostly removed by adsorption to ferrihydrite) and pyrite (Fe, mostly precipitates as ferrihydrite). The calculations show that the sphalerite:galena:pyrite oxidation ratio is ~1:2:1, but the mine water chemistry is dominated by Zn due to the removal of Pb and Fe by adsorption/precipitation. The acidity released by pyrite oxidation is neutralised by the dissolution of calcite and, to a lesser extent, chlorite. The presence of pyrite is responsible both for the release of acidity and the removal of some contaminant metals by adsorption on ferrihydrite. The concentrations of sulphate, Zn, Cd and Ni in the mine water decrease with increasing flow due to dilution; modest increases in metal flux with flow probably reflect increased oxidation due to greater amounts of oxygenated water flowing through the mine. In contrast, Al, Pb and Cu concentrations are positively correlated with flow due to the flushing of these metals adsorbed to ferrihydrite particles. The influence of temperature is relatively subtle; metal fluxes are a balance between abiotic oxidation (which increases at higher temperatures and flows) and bacterially mediated oxidation (which is depressed at high flow rates when temperatures decrease below 10 °C). These conclusions apply to NMD mine water throughout the UK and elsewhere in the world, including mines hosted in both limestone and silicate rocks. The molar mass balance approach, together with synchronous flow and geochemistry data, provides crucial information for effective mine-water-treatment system design by elucidating the critical roles of flow rate and temperature in determining contaminant concentrations and loads.