Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process

Simultaneous removal of cadmium and tetracycline from aqueous solutions by oxalic acid and pyrite co-modified biochar: Performance and mechanism

The remediation of combined contamination with heavy metals and antibiotics in soil and aqueous environments represents an ongoing challenge. In this study, a novel highly functionalized biochar-based composite (FeS2@OA-BC) was synthesised by combining oxalic acid (OA) pre-treatment with ball-milling of FeS2 for the simultaneous removal of cadmium (Cd2+) and tetracycline (TC) from aqueous solutions. FeS2@OA-BC demonstrated exceptional performance in simultaneously removing 74.7 % of Cd2+ and 95.8 % of TC from the binary systems, meanwhile the degradation rate of TC reached up to 64.8 %. Moreover, no significant competitive or promoting effects between Cd2+ and TC removal were observed by FeS2@OA-BC in binary systems. The adsorption of Cd2+ was primarily governed by three mechanisms: complexation with functional groups, Cd-π conjugation and cation exchange. Meanwhile, TC degradation relied on reactive oxygen species (ROS), where hydroxyl radicals (•OH) and hydrogen peroxide (H2O2) played dominant roles, with singlet oxygen (1O2) contributing minimally. The co-modification of OA and FeS2 synergistically introduces abundant exogenous defect sulphur vacancies (SVs), enhancing molecular oxygen activation and stimulating more ROS for TC degradation, as well as promoting more functional groups as adsorption sites for Cd2+ complexation. This therefore ultimately led to the reinforcement of the concurrent removal of Cd2+and TC. Overall, FeS2@OA-BC shows great promise for addressing combined pollution involving heavy metals and antibiotics in environmental systems.

Insights into the Restoration of Micropolluted Groundwater by Pyrite: The Contribution of Fe(IV) and Outer-Sphere Electron Transfer

Pyrite has been investigated for its potential to modulate the redox microenvironment of groundwater porous media through self-activation. However, the self-purification process of the contaminants by pyrite after their migration from surface water to groundwater has been neglected. This process is accompanied by a decrease in pollutant concentration with a transition from aerobic to anaerobic environments. Here, we selected sulfamethoxazole (SMX), a micropollutant frequently detected in groundwater, as a modeled micropollutant for the investigation. The findings indicate that pyrite could degrade micropollutants SMX (20 μg/L) by self-activation with nearly 100% degradation efficiency under anaerobic conditions. It was also found that •OH was not the primary reactive oxygen species (ROS), but rather the longer-lived and more stable Fe(IV) generated by •OHad-mediated oxidation of structural Fe(III). Additionally, SMX can be degraded by outer-sphere electron transfer with dissolved Fe3+ in the system. Whereas, the reduction product Fe2+ facilitates the supply of electrons to pyrite and promotes the production of Fe (IV). The contributions of Fe(IV) and outer-sphere electron transfer to SMX degradation were 67.5% and 32.5%, respectively. Furthermore, pyrite self-activation exhibited selective oxidation of electron-rich pollutants under anaerobic conditions. This finding provides a new insight into the self-purification of micropollutants in groundwater environments.

Coexisting ferrihydrite-enhanced contaminant degradation during pyrite oxygenation

Oxygenation of pyrite (Py) is known to mediate generation of reactive oxygen species (ROS) with these species capable of inducing contaminants degradation, whereas the possible participation of coexisting Fe(III) minerals in these processes is still unclear. This study finds that freshly formed ferrihydrite (Fh) significantly enhances the Py-mediated sulfamethoxazole (SMX) degradation process. Through the 56Fe isotope tracer experiment and a series of control experiments, Fh is found to be reduced by Py to form secondary solid-phase Fe(II) species (Fe(II)RF) which in turn facilitates generation of H2O2 from the O2 reduction pathway. However, Py was found to mediate rapid structural transformation of Fh to form more thermodynamically stable goethite and hematite with these less redox active minerals unable to sustainably promote the Py-mediated SMX degradation process. Therefore, the improvement of Fh on Py-mediated SMX degradation process is not readily observable in reaction systems with low concentrations of coexisting Fh. In comparison, continuing input of 10 mM Fh increased the degradation efficiency of SMX by 60 % over three days. Our results demonstrate that the oxidative degradation of organic contaminants over the oxygenation of Py when coexisting with Fh can be more significant but currently underestimated.

Boosted H2O2 utilization and selective hydroxyl radical generation for water decontamination: Synergistic roles of dual active sites in H2O2 activation

H2O2 as a green oxidant plays a crucial role in numerous green chemical reactions. However, how to improve its activation and utilization efficiency as well as regulate the distribution of ROS remains a pressing challenge. In this work, a sulfur quantum dots (SQDs) modified zero-valent iron (SQDs@ZVI) was delicately designed and prepared, whose iron sites can coordinate with strongly electronegative sulfur atoms to construct highly reactive Fe-S dual active sites, for high-efficient selective H2O2 activation and utilization with potent •OH production. Experimental tests, in situ FTIR/Raman spectra and theoretical calculations demonstrated that SQDs modulates the local coordination structure and electronic density of iron centers, thus effectively enhancing its Fenton reactivity and promoting the rate-limiting H2O2 adsorption and subsequent barrierless dissociation of peroxyl bonds into •OH via the formation of bridged S-O-O-Fe complexes. Consequently, substantial generated surface-bound •OH induced by the highly reactive Fe-S dual sites enabled excellent degradation of miscellaneous organic pollutants over a broad pH range (3.0-9.0). The developed device-scale Fenton filter realized durable performance (up to 200 h), verifying the vast potential of SQDs@ZVI with diatomic sites for practical application. This work presents a promising strategy to construct metal-nonmetal diatomic active sites toward boosting selective activation and effective utilization of H2O2, which may inspire the design of efficient heterogeneous Fenton reaction for water decontamination.

Redox Oscillation-Driven Production of Reactive Oxygen Species from Black Carbon

Wildfire and stubble burning release substantial amounts of black carbon (BC) into natural environments that experience frequent redox oscillations, such as wetlands and farmlands. Here, we report that redox oscillations can effectively drive ROS production from BC. Following sequential microbial reduction and air exposure, 6.5 ± 0.2 μM/gC hydrogen peroxide (H2O2) and 285.3 ± 9.5 nM/gC hydroxyl radical (•OH) were produced from BC. Moreover, BC derived from various biomass sources, temperatures, and particle sizes exhibits 111.5-fold variations in ROS production. Electrochemical analyses revealed that both the electron transfer capacity and the ROS production selectivity are critical determinants of ROS generation under redox oscillations. The variation in electron transfer capacity (0.3-5.7 mmol e-/gC) is primarily governed by the abundance of electron-storing moieties such as quinones, while the ROS generation selectivity (26.2-72.0%) is influenced by the presence of competitive sites for oxygen reduction reactions, such as carbon defects. These findings provide insights into ROS production from BC under fluctuating redox conditions, with potential implications for elemental cycles and pollutant dynamics in regions prone to wildfire and stubble burning events and substantial BC deposition (e.g., wetlands and rice paddies).

Overlooked pyrite-mediated heterogeneous Fenton processes: Mechanisms of surface hydroxyl radical generation and associated decontamination performance

Pyrite-based Fenton-like processes have been extensively studied for wastewater decontamination; however, most relevant studies placed excessive emphasis on the homogeneous Fenton reaction mediated by aqueous Fe2+, resulting in the proposed technologies facing issues such as additional acid requirements for pH adjustment and excessive iron sludge production. Herein, through in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), custom dual-chamber reactor experiments, and a series of control experiments, significant hydroxyl radical generation was identified during the pyrite/H2O2 process, while the dominant reactive iron species was verified to the structural Fe sites on the pyrite surface, rather than structural Fe(II) in secondary iron minerals and surface adsorbed Fe2+. Consequently, even with significant suppression of the homogeneous Fenton pathway, the pyrite/H2O2 process exhibited significant degradation efficiency for sulfamethoxazole (SMX) at pH 4. Moreover, the pyrite/H2O2 process was found to selectively remove 50 μM of pollutants with high affinity for pyrite (bisphenol A, carbamazepine, nitrobenzene, and SMX), even in the presence of 50-100 mM methanol. Compared to the typical iron-based reductive catalyst (zero-valent iron, ZVI), pyrite mediated a Fenton process with greater potential for practical applications at pH 4, achieving a 43.75-fold reduction in iron sludge production and almost doubling the H2O2 utilization efficiency. Additionally, in contrast to ZVI, minimal iron oxide formed on the pyrite surface during the oxidation process. Thus, after seven cycles of degradation experiments, the decontamination efficiency of the pyrite/H2O2 process remained stable. These findings are crucial for understanding the complex environmental behavior of pyrite in both natural and engineering processes and provide a new perspective for the efficient utilization of pyrite resources as well.

Perfluorooctanoic acid transport and fate difference driven by iron-sulfide minerals transformation interacting with different types of groundwater

Perfluorooctanoic acid (PFOA) is an emerging persistent organic pollutant that threatens human health and ecosystems. However, the intricate mechanism of the change in PFOA transport behavior that interacts with FexSy minerals under groundwater-type differences is not clear. To address this knowledge gap, multi-scale experiments and multi-process reaction models were constructed to investigate the underlying mechanisms. The results showed that different groundwater (NO3-, Cl--Na+, SO42-, and HCO3- types) had significant effects on PFOA transport. NO3-, Cl--Na+, SO42-, and HCO3- decreased the retardation effect of PFOA in the FexSy media. Compared to other groundwater types, the adsorption sites of FexSy were the least occupied in the NO3- groundwater. This observation was supported by the least inhabition of λ in FexSy-NO3- interaction system, which demonstrated that more PFOA was in a high reaction zone and electrostatic repulsion was weakest. The surface tension of different ion types in groundwater provided evidence explaining the lowest inhibition in the FexSy-NO3- system. The 2D spatiotemporal evolution results showed that in FexSy with NO3- system, the pollutant flux (6.00 ×10-5 mg·(m2·s)-1) was minimal. The pollutant flux in the SO42- groundwater system was 9.95-fold that in FexSy with the NO3- groundwater. These findings provide theoretical support for understanding the transport and fate of PFOA in FexSy transformations that interact with different types of groundwater.

Synergistic promotion of antimony transformation in the interaction of Acidithiobacillus ferrooxidans and pyrite by driving the formation of reactive oxygen species and secondary minerals

As one of the important microorganisms in the mining area, the role of iron-sulfur oxidizing microorganisms in antimony (element symbolized as Sb) migration and transformation in mining environments has been largely neglected for a long time. Therefore, the processes of the typical iron-sulfur oxidizing bacterium Acidithiobacillus ferrooxidans (A. ferrooxidans) and pyrite interaction coupled with the migration and transformation of Sb were investigated in this paper. The bio-oxidation process of pyrite by A. ferrooxidans not only accelerates the oxidation rate of Sb(III) to Sb(V) (62.93% of 10 mg L-1 within 4 h), but also promotes the adsorption and precipitation of Sb (32.89 % of 10 mg L-1 within 96 h), and changes in the dosage of minerals, Sb concentration, and pH value affect the conversion of Sb. The characterization results show that the interaction between A. ferrooxidans and pyrite produces a variety of reactive species, such as H2O2 and •OH, resulting in the oxidation of Sb(III). In addition, A. ferrooxidans mediates the formation of stereotyped iron-sulfur secondary minerals that can act as a major driver of Sb (especially Sb(V)) adsorption or co-precipitation. This study contributes to the further understanding of the diversified biogeochemical processes of iron-sulfur oxidizing bacteria-iron-sulfur minerals-toxic metals in mining environments and provides ideas for the development of in-situ treatment technologies for Sb.

Sulfur Vacancies in Pyrite Trigger the Path to Nonradical Singlet Oxygen and Spontaneous Sulfamethoxazole Degradation: Unveiling the Hidden Potential in Sediments

Pharmaceutical residues in sediments are concerning as ubiquitous emerging contaminants. Pyrite is the most abundant sulfide minerals in the estuarine and coastal sediments, making it a major sink for pharmaceutical pollutants such as sulfamethoxazole (SMX). However, research on the adsorption and redox behaviors of SMX on the pyrite surface is limited. Here, we investigated the impact of the nonphotochemical process of pyrite on the fate of coexisting SMX. Remarkably, sulfur vacancies (SVs) on pyrite promoted the generation of nonradical species (hydrogen peroxide, H2O2 and singlet oxygen, 1O2), thereby exhibiting prominent SMX degradation performance under darkness. Nonradical 1O2 contributed approximately 73.1% of the total SMX degradation. The SVs with high surrounding electron density showed an advanced affinity for adsorbing O2 and then initiated redox reactions in the sediment electron-storing geobattery pyrite, resulting in the extensive generation of H2O2 through a two-electron oxygen reduction pathway. Surface Fe(III) (hydro)oxides on pyrite facilitated the decomposition of H2O2 to 1O2 generation. Distinct nonradical products were observed in all investigated estuarine and coastal samples with the concentrations of H2O2 ranging from 1.96 to 2.94 μM, while the concentrations of 1O2 ranged from 4.63 × 10-15 to 8.93 × 10-15 M. This dark-redox pathway outperformed traditional photochemical routes for pollutant degradation, broadening the possibilities for nonradical species use in estuarine and coastal sediments. Our study highlighted the SV-triggered process as a ubiquitous yet previously overlooked source of nonradical species, which offered fresh insights into geochemical processes and the dynamics of pollutants in regions of frequent redox oscillations and sulfur-rich sediments.

Soil carbon and nitrogen cycles driven by iron redox: A review

Soil carbon and nitrogen cycles affect agricultural production, environmental quality, and global climate. Iron (Fe), regarded as the most abundant redox-active metal element in the Earth's crust, is involved in a biogeochemical cycle that includes Fe(III) reduction and Fe(II) oxidation. The redox reactions of Fe can be linked to the carbon and nitrogen cycles in soil in various ways. Investigating the transformation processes and mechanisms of soil carbon and nitrogen species driven by Fe redox can provide theoretical guidance for improving soil fertility, and addressing global environmental pollution as well as climate change. Although the widespread occurrence of these coupling processes in soils has been revealed, explorations of the effects of Fe redox on soil carbon and nitrogen cycles remain in the early stages, particularly when considering the broader context of global climate and environmental changes. The key functional microorganisms, mechanisms, and contributions of these coupling processes to soil carbon and nitrogen cycles have not been fully elucidated. Here, we present a systematic review of the research progress on soil carbon and nitrogen cycles mediated by Fe redox, including the underlying reaction processes, the key microorganisms involved, the influencing factors, and their environmental significance. Finally, some unresolved issues and future perspectives are addressed. This knowledge expands our understanding of the interconnected cycles of Fe, carbon and nitrogen in soils.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Improvement of Fe(Ⅲ)/percarbonate system by molybdenum powder and tripolyphosphate: Co-catalytic performance, low oxidant consumption, pH-dependent mechanism

The homogeneous sodium percarbonate (SPC) systems are limited by narrow pH range, ineffective consumption of oxidant, and weak reusability of catalyst. Herein, molybdenum (Mo) powder and sodium tripolyphosphate (STPP) were selected to overcome these challenges. Sulfamethoxazole (SMX), as a model contaminant, was almost completely degraded in 60 min with higher removal rate (0.1367 min-1) than the Mo or STPP-absent system. In addition, Mo/STPP-Fe(Ⅲ)/SPC system was cost-effective in terms of oxidant consumption, requiring only 0.2 mM SPC. About activation mechanism, the main active species for SMX degradation was pH-dependent, with hydroxyl radical (·OH) as the dominant active species at pHi = 7 and ·OH, carbonate radical (CO3·-), and superoxide radical (O2·-) derived from a series of chain reaction at pHi = 10, respectively. Due to the generation of various electrophilic free radical, the system exhibited excellent performance towards electron-rich pollutants under a wide pH range. Furthermore, Mo exhibited excellent stability and reusability. SMX was degraded through hydroxylation, N-S cleavage, amino and sulfanilamide oxidation into intermediates whose toxicities were evaluated by Toxicity Estimation Software Tool (T.E.S.T.) software. This work provided new insights to Fe/SPC system towards high-efficiency and low consumption treatment of practical application.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation

Reactive oxygen species (ROS) are widespread in nature and play central roles in numerous biogeochemical processes and pollutant dynamics. Recent studies have revealed ROS productions triggered by electron transfer from naturally abundant reduced iron minerals to oxygen. Here, we report that ROS productions from pyrite oxidation exhibit a high facet dependence. Pyrites with various facet compositions displayed distinct efficiencies in producing superoxide (O2• -), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). The 48 h •OH production rates varied by 3.1-fold from 11.7 ± 0.4 to 36.2 ± 0.6 nM h-1, showing a strong correlation with the ratio of the {210} facet. Such facet dependence in ROS productions primarily stems from the different surface electron-donating capacities (2.2-8.6 mmol e- g-1) and kinetics (from 1.2 × 10-4 to 5.8 × 10-4 s-1) of various faceted pyrites. Further, the Fenton-like activity also displayed 10.1-fold variations among faceted pyrites, contributing to the facet depedence of •OH productions. The facet dependence of ROS production can greatly affect ROS-driven pollutant transformations. As a paradigm, the degradation rates of carbamazepine, phenol, and bisphenol A varied by 3.5-5.3-fold from oxidation of pyrites with different facet compositions, where the kinetics were in good agreement with the pyrite {210} facet ratio. These findings highlight the crucial role of facet composition in determining ROS production and subsequent ROS-driven reactions during iron mineral oxidation.

Amendment of organic acids significantly enhanced hydroxyl radical production during oxygenation of paddy soils

Recently, hydroxyl radical (^•OH) production during soil redox fluctuations has been increasingly reported, but the low efficiency of contaminant degradation is the barrier for engineering remediation. The widely distributed low-molecular-weight organic acids (LMWOAs) might greatly enhance ^•OH production due to their strong interactions with Fe(II) species, but it was less investigated. Herein, we found that LMWOAs amendment (i.e., oxalic acid (OA) and citric acid (CA)) significantly enhanced ^•OH production by 1.2 -19.5 times during oxygenation of anoxic paddy slurries. Compared with OA and acetic acid (AA) (78.4 -110.3 μM), 0.5 mM CA showed the highest ^•OH accumulation (140.2 μM) due to the elevated electron utilization efficiency derived from its strongest capacity for complexation. Besides, increasing CA concentrations (within 6.25 mM) dramatically enhanced the ^•OH production and imidacloprid (IMI) degradation (increased by 48.6%), and further decreased due to the extensive competition from excess CA. Compared to 0.5 mM CA, the synergistic effects of acidification and complexation induced by 6.25 mM CA rendered more formation of exchangeable Fe(II) that easily coordinated with CA, and thus significantly enhanced its oxygenation. This study proposed promising strategies for regulating natural attenuation of contaminants using LMWOAs in agricultural fields, especially soils with frequent occurrence of redox fluctuations.

Microbe Regulates the Mineral Photochemical Activity and Organic Matter Compositions in Water

Photochemical reactions that widely occur in aquatic environments play important roles in carbon fate (e.g., carbon conversion and storage from organic matter) in ecosystems. Aquatic microbes and natural minerals further regulate carbon fate, but the processes and mechanisms remain largely unknown. Herein, the interaction between Escherichia coli and pyrite and its influence on the fate of carbon in water were investigated at the microscopic scale and molecular level. The results showed that saccharides and phenolic compounds in microbial extracellular polymeric substances helped remove pyrite surface oxides via electron transfer. After the removal of surface oxides on pyrite, the photochemical properties under visible-light irradiation were significantly decreased, such as reactive oxygen species and electron transfer capacity. Unlike the well-accepted theory of minerals protecting organic matter in the soil, the organic matter adsorbed on minerals preferred degradation due to the enhanced photochemical reactions in water. In contrast, the minerals transformed by microbes suppressed the decomposition of organic matter due to the passivation of the chemical structure and activity. These results highlight the significance of mineral chemical activity on organic matter regulated by microbes and provide insights into organic matter conversion in water.