Secondary Iron Minerals Research Articles

Influences and mechanisms of iron input for methane productions in peatlands.

Atmospheric deposition provides a stable iron source for peatlands. The influences of Fe input on methane (CH4) productions and the underlying mechanisms remain unclear. We conducted a microcosm experiment with peat sediments collected from the Qinghai-Tibet Plateau of China to explore the effects of ferrihydrite reductionfor CH4 productions in peatlands by using geochemical analyses including 57Fe Mössbauer spectroscopy and three-dimensional fluorescence spectroscopy (3D-EEM) in combination with high-throughput sequencing of 16S rRNA and real-time fluorescence quantitative PCR (qPCR). Results showed that ferrihydrite reduction significantly increased CH4 production, being 30 times of that under the control. Selective extractions for iron oxides and 57Fe Mössbauer spectroscopy measurements revealed that no crystalline secondary iron minerals were formed during the ferrihydrite reduction process. The addition of ferrihydrite enhanced the degradation of dissolved organic matter (DOM) in peat soil, resulting in a reduction in the concentration of dissolved organic carbon (DOC). Furthermore, the relative abundance of typical fermentative microorganisms in peat sediments, including Acidobacteriota and Bacteroidota, significantly increased. Such a result indicated that reduction of ferrihydrite accelerated organic matter decomposition and increased substrate concentration required for methanogenesis. Furthermore, a co-increase in relative abundance of Geobacter, Geothrix, and Methanobacterium in the ferrihydrite-amended group suggested a potential synergistic interaction that may promote the CH4 production. Our results demonstrated that ferrihydrite reduction could significantly enhance CH4 production and play a vital role in regulating CH4 emissions in peatlands.

Effects of pH on coprecipitation of As(III) with biogenic synthesized schwertmannite and jarosite

ABSTRACT Secondary iron minerals play significant roles in the immobilization of As under acidic conditions, such as acid mine drainage. However, previous research works have not clarified the effect of pH on As(III) removal through coprecipitation with secondary minerals. Therefore, in this study, we aimed to investigate the discrepancy in As(III) coprecipitation with biogenic synthesized schwertmannite (Sch) and jarosite (Jar) at different pH values. For this, concentrations of Fe2+, TFe, S O 4 2 − , and As(III) in shake flasks were monitored during an overall incubation period of 83 h at initial pH of 1.5, 2.0, and 2.5. In addition, the physicochemical properties of collected minerals after incubation were identified using scanning electron microscopy, X-ray diffraction, pore size distribution, and Brunauer – Emmett – Teller surface area analyses. Our results showed that almost no mineral synthesis and no As(III) removal were detected in coprecipitated schwertmannite (Co-Sch) system and coprecipitated jarosite (Co-Jar) system at an initial pH of 1.5. The TFe precipitation efficiencies and As(III) removal efficiencies increased considerably and morphologies of Co-Sch and Co-Jar improved significantly when the initial pH value increased from 2.0–2.5. The maximum TFe precipitation efficiency and As(III) removal efficiency reached 30.8% and 89.6%, respectively, for the Co-Sch system, and were 47.5% and 37.4%, respectively, for the Co-Jar system. The overall results show that pH significantly affects the formation of Co-Sch and Co-Jar and the behaviour of As(III) coprecipitation.

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.

Nitrate and nitrite reduction by adsorbed Fe(II) generated from ligand-promoted dissolution of biogenic iron minerals in groundwater

Chemical denitrification by redox-active Fe(II) species is pivotal in the coupled iron and nitrogen cycles. The reductive dissolution of ferric minerals by ligand can generate Fe(II)-ligand complexes, but their reducing capability for electrophilic pollutants like nitrate and nitrite remains uncertain. Here, biogenic secondary iron minerals (SIM) after dissimilatory iron reduction were reductively dissolved by oxalate and the siderophore desferrioxamine B, and subsequently the partially-dissolved SIM (SIMD) effectively removed NO2− from groundwater via reduction, while exhibiting much lower reactivity towards NO3−. The dissolution and removal processes were well-fitted with the Kabai model and the pseudo-second-order adsorption model, respectively. The equilibrium NO2− removal capacity (qe) of SIMD reached 0.146–0.223 mmol/g, accompanied with the rate constants as 0.433–0.810 g/(mmol·h). The emission of N2O and NO verified the occurrence of chemical denitrification during NO2− removal by SIMD. From the perspective of Fe(II) reactivity, SIMD exhibited higher densities of surface Fe(II) and more negative Eh values than SIM, and these two indicators showed linear correlations with the removal rates. Combined with microscopic, electrochemical and spectral analysis, our results indicated the redox reaction of adsorbed Fe(II)-complexes with NO2− on SIMD surface. The concurrent substance biochar was also considered, as it indirectly influenced dissolution and pollutant removal by shifting the iron mineral phase in SIM from magnetite to goethite. These findings highlight the significant role of reductive dissolution of iron mineral in N transformation, expand the electron pool available to support chemical denitrification, and have implications for Fe and N cycling coupling with pollutant reduction.

Sequestration of Labile Organic Matter by Secondary Fe Minerals from Chemodenitrification: Insight into Mineral Protection Mechanisms.

Labile organic matter (OM) immobilized by secondary iron (Fe) minerals from chemodenitrification may be an effective way to immobilize organic carbon (OC). However, the underlying mechanisms of coupled chemodenitrification and OC sequestration are poorly understood. Here, OM immobilization by secondary Fe minerals from chemodenitrification was investigated at different C/Fe ratios. Kinetics of Fe(II) oxidation and nitrite reduction rates decreased with increasing C/Fe ratios. Despite efficient sequestration, the immobilization efficiency of OM by secondary minerals varied with the C/Fe ratios. Higher C/Fe ratios were conducive to the formation of ferrihydrite and lepidocrocite, with defects and nanopores. Three contributions, including inner-core Fe-O and edge- and corner-shared Fe-Fe interactions, constituted the local coordination environment of mineral-organic composites. Microscopic analysis at the molecular scale uncovered that labile OM was more likely to combine with secondary minerals with poor crystallinity to enhance its stability, and OM distributed within nanopores and defects had a higher oxidation state. After chemodenitrification, high molecular weight substances and substances high in unsaturation or O/C ratios including phenols, polycyclic aromatics, and carboxylic compounds exhibited a stronger affinity to Fe minerals in the treatments with lower C/Fe ratios. Collectively, labile OM immobilization can occur during chemodenitrification. The findings on OM sequestration coupled with chemodenitrification have significant implications for understanding the long-term cycling of Fe, C, and N, providing a potential strategy for OM immobilization in anoxic soils and sediments.

Kinetics of ferrihydrite reduction in a biofilm system

Microorganisms can exist in both suspended and attached forms in natural and engineered systems. While extensive research has been performed to investigate the bioreduction kinetics of iron oxides by suspended microorganisms, the kinetics of iron oxide reduction by biofilms has not been well studied. This study investigated the rates of ferrihydrite bioreduction by biofilms of Shewanella oneidensis MR-1 with variable concentration ratios of electron donor to acceptor, and different biofilm thickness. Results indicated that the bioreduction rate of ferrihydrite by biofilms increased with increasing biofilm surface area and was faster than that by suspended microorganisms for the same biomass amount, apparently as a result of the biofilm surface area facilitating the direct contact between ferrihydrite and microorganisms for electron transfer. Increasing biofilm thickness decreased bioreduction rate for the same total biomass as a result of the decrease of the ratio of the biofilm surface area to total biomass. The bioreduction rate increased with increasing electron donor and ferrihydrite inputs, consistent with the saturation type of kinetic models. The different rates of ferrihydrite reduction in the biofilm system resulted in different rates of Fe(II) production as a function of electron donor, acceptor, and biofilm surface area, which in turn affected the type and concentrations of the secondary iron minerals, such as lepidocrocite and goethite. A kinetic model was developed that integrates biofilm surface area, and electron donor and acceptor concentrations that well described the measured rates of ferrihydrite bioreduction. The results emphasized the importance of the biofilm surface area in understanding and modeling iron redox biogeochemistry and iron-related biogeochemical processes.

Antimony isotope fractionation during Sb(V) and Sb(III) adsorption on secondary Fe-minerals (schwertmannite, ferrihydrite) typical of mine waters

The application of antimony (Sb) isotopes as tracers for Sb environmental cycling is currently limited. Indeed, there is a lack of knowledge of isotope fractionation factors associated with key (bio-) geochemical processes controlling its behaviour in surface environments. This study investigated the equilibrium isotope fractionation generated by Sb(V) and Sb(III) sorption on two iron minerals typical of acid mine drainage (AMD) impacted streams, ferrihydrite and schwertmannite, under controlled conditions of pH and solid to liquid ratio. Sorption behaviour and Sb isotope fractionation were similar for the different mineral phases and Sb oxidation degrees, with fractionation factors Δ123Sbsolid-solution of −0.25 ± 0.08 ‰ for Sb(III) and −0.34 ± 0.08 ‰ for Sb(V) adsorbed on ferrihydrite and −0.36 ± 0.06 ‰ for Sb(III) and −0.27 ± 0.03 ‰ for Sb(V) adsorbed on schwertmannite. The pH and initial Fe:Sb ratio did not significantly affect the Δ123Sbsolid-solution value under the experimental conditions. The light 121Sb isotope was preferentially adsorbed on the mineral phases, following an equilibrium closed system between dissolved and adsorbed Sb species. This fractionation may be related to the apparition of iron as the second closest neighbour which distorts the Sb(OH)3 or Sb(OH)6- atomic polyhedron. This study confirms that Sb equilibrium isotope fractionation occurs during sorption of Sb(III) and Sb(V) onto secondary iron minerals and suggests that the pH and redox of Sb do not exert significant effect. pH and redox conditions are important parameters which control Sb mobility in AMD streams, therefore, the study provides data for interpreting Sb isotope signatures in mine waters.

Antimony(V) behavior during the Fe(II)-induced transformation of Sb(V)-bearing natural multicomponent secondary iron mineral under acidic conditions

Fe(II)-induced phase transformations of secondary iron minerals have attracted considerable attention due to their influence on antimony (Sb) mobility. However, Fe(II)-induced natural multicomponent secondary iron mineral (nmSIM) transformations and the corresponding repartitioning of Sb on nmSIM under acidic conditions upon Fe(II) exposure have not been systematically examined. Herein, we investigated the effect of Fe(II) on nmSIM mineralogy and Sb mobility in Sb(V)-bearing nmSIM at pH 3.8 and 5.6 at various Fe(II) concentrations over 15 d. The Sb(V)-bearing nmSIM phase transformation occurred under both strongly and weakly acidic conditions without Fe(II) exposure, while the presence of Fe(II) significantly intensified the transformation, and substantial amounts of intermediary minerals, including jarosite, ferrihydrite, lepidocrocite and fougerite, formed during the initial reaction stage, especially at pH 5.6. X-ray diffraction (XRD) analyses confirmed that goethite and hematite were the primary final-stage transformation products of Sb(V)-bearing nmSIM, regardless of Fe(II) exposure. Throughout the Sb(V)-bearing nmSIM transformation at pH 3.8, Sb release was inversely related to the Fe(II) concentration in the initial stage, and after maximum release was achieved, Sb was gradually repartitioned onto the nmSIM. No Sb repartitioning occurred in the absence of Fe(II) at pH 5.6, but the introduction of Fe(II) suppressed Sb release and improved Sb repartitioning on nmSIM. This transformation was conducive to Sb reimmobilization on Sb(V)-bearing nmSIM due to the structural incorporation of Sb into the highly crystalline goethite and hematite generated by the Sb(V)-bearing nmSIM transformation, and no reduction of Sb(V) occurred. These results imply that Fe(II) can trigger mineralogical changes in Sb(V)-bearing nmSIM and have important impacts on Sb partitioning under acidic conditions. These new insights are essential for assessing the mobility and availability of Sb in acid mine drainage areas.

Sorption of antimony(V) to naturally formed multicomponent secondary iron minerals: Sorption behavior and a comparison with synthetic analogs

Antimony (Sb) pollution in water has attracted extensive attention due to the biotoxicity of Sb. Secondary iron minerals readily sorb heavy metal(loid)s and critically affect their cycling in terrestrial environments. However, compared with synthetic pure iron mineral phases, little is known about the Sb sorption behavior and mechanism on natural secondary iron minerals (nSIMs) composed of various mineral phases. In this study, sorption experiments were conducted to investigate the Sb(V) sorption properties of nSIMs from an acid mine drainage zone and corresponding single-component synthetic secondary mineral phases and to compare their sorption behaviors and mechanisms. Spectroscopic analyses indicated that the nSIMs structurally resembled a hybrid of schwertmannite, jarosite and goethite. Sb(V) sorption on nSIMs, schwertmannite, goethite and jarosite was controlled by chemisorption, with maximum Sb(V) sorption capacities of 217.39, 233.65, 32.17 and 35.61 mg/g, respectively. nSIMs demonstrated an excellent Sb(V) sorption capacity equivalent to or greater than that of the single-component phases. XRD, FTIR and Raman analyses indicated that Sb(V) was immobilized on nSIMs mainly through ion exchange with structural SO42− and complexation interactions with surface FeO and FeOH; then, an FeOSb surface phase formed during the dissolution and further transformation of schwertmannite and jarosite into goethite. SEM revealed that nSIMs had an advantage in surface microstructure over the single components. These results suggested that despite the similarities in Sb(V) binding mechanism between nSIMs and schwertmannite, nSIMs might be more reactive for Sb(V) sorption since the nSIM components could mutually influence each other and facilitate Sb(V) sorption. This research suggests that nSIMs have potential for Sb(V) removal and helps elucidate the environmental behavior of Sb(V) associated with nSIMs.

Cadmium immobilization during nitrate-reducing Fe(II) oxidation by Acidovorax sp. BoFeN1: Contribution of bacterial cells and secondary minerals

Microbially nitrate-reducing Fe(II) oxidation (NRFO) is widespread in anoxic environments at circumneutral pH, in which Fe(II) oxidation is considered to be mainly catalyzed by nitrite via chemodenitrification after microbial nitrate reduction. Cadmium (Cd) is toxic to organisms, and its availability is strongly affected by iron (oxyhydr)oxides and/or bacteria. However, it remains unclear how NRFO affects the fate of Cd with environmentally relevant concentrations, and the relative contributions of microbes and secondary minerals to Cd immobilization during NRFO. Here, Acidovorax sp. BoFeN1, a typical NRFO bacterium, was used to investigate the Cd immobilization during NRFO at pH 7.0 with 0.1, 0.5, and 1.0 mg/L Cd, respectively. Higher initial Cd concentration resulted in a higher amount of Cd immobilized in precipitates, but showed stronger inhibition on nitrate reduction, acetate metabolism, and cell growth. The presence of Cd barely influenced the Fe(II) oxidation during NRFO or chemodenitrification, but retarded the mineral transformation to secondary crystalline minerals, i.e., lepidocrocite and goethite. The extracellular polymeric substances deposited in the bacteria-mineral precipitates were rich in tyrosine-like and tryptophan-like proteins with negatively charged amino and carboxyl groups. The STEM-HAADF images with EDX mapping demonstrated that Cd was strongly co-localized with Fe on the cell surface and in the periplasm. Cadmium immobilization experiments with bacterial cells or during chemodenitrification indicated that the bacterial cells and secondary minerals formed during chemodenitrification are comparably important for Cd immobilization in the Cd0.1 treatment, while the secondary minerals played a predominant role in Cd immobilization in the Cd0.5 and Cd1.0 treatments. These results suggested that Cd was likely associated with the secondary iron minerals, which precipitated with the proteins on the cell surface and in the periplasm. Our findings provide a comprehensive understanding of the roles of Cd, bacterial cells, and secondary minerals formed via chemodenitrification, as well as their interactions during the complicated NRFO process at circumneutral pH.

Impact of Fulvic Acid and Acidithiobacillus ferrooxidan Inoculum Amount on the Formation of Secondary Iron Minerals.

The catalytic oxidation of Fe2+ by Acidithiobacillus ferrooxidan (A. ferrooxidans) and the synthesis of iron sulfate-based secondary minerals is considered to be of great significance to the treatment of acid mine drainage (AMD). Along these lines, in this work, the shaker experiment was carried out to study the underlying mechanism of the inoculation amount of fulvic acid (FA) and A. ferrooxidans on the synthesis process of secondary minerals. From the acquired results, it was demonstrated that the oxidation rate of Fe2+ increased with the increase in the concentration of fulvic acid in the range of 0.1-0.2 g/L. On top of that, the concentration of fulvic acid in the range of 0.3-0.5 g/L inhibited the activity of A. ferrooxidans. However, A. ferrooxidans retained its activity, and the complete oxidation time of Fe2+ was delayed. When the concentration of fulvic acid was 0.3 g/L, the TFe (total iron) precipitation efficiency was 30.2%. Interestingly, when 0.2 g/L fulvic acid was added to different inoculum systems, the incorporation of a higher inoculum amount of A. ferrooxidans led to an increased oxidation rate. On the contrary, the lower inoculum amount yielded a more obvious effect of the fulvic acid. From the mineralogical characteristics, it was also revealed that a fulvic acid concentration of 0.2 g/L and different inoculation amounts of A. ferrooxidans did not change the mineral facies, whereas pure schwertmannite was obtained.

Effects of Initial pH and Carbonate Rock Dosage on Bio-Oxidation and Secondary Iron Mineral Synthesis.

The effect of pH is a key factor in biomineralization mediated by Acidithiobacillus ferrooxidans to promote the transformation of Fe into secondary iron minerals. This study aimed to investigate the effects of initial pH and carbonate rock dosage on bio-oxidation and secondary iron mineral synthesis. Variations in pH and the concentrations of Ca2+, Fe2+, and total Fe (TFe) in the growth medium of A. ferrooxidans were examined in the laboratory to determine how they affect the bio-oxidation process and secondary iron mineral synthesis. The results showed that in systems with an initial pH of 1.8, 2.3, and 2.8, the optimum dosages of carbonate rock were 30, 10, and 10 g, respectively, which significantly improved the removal rate of TFe and the amount of sediments. At an initial pH of 1.8 and a carbonate rock dosage of 30 g, the final removal rate of TFe reached 67.37%, which was 28.03% higher than that of the system without the addition of carbonate rock, and 36.9 g·L-1 of sediments were generated, which was higher than that of the system without the addition of carbonate rock (6.6 g·L-1). Meanwhile, the number of sediments generated by adding carbonate rock were significantly higher than those without the addition of carbonate rock. The secondary minerals were characterized by a progressive transition from low crystalline assemblages composed of calcium sulfate and subordinated jarosite, to well crystal-line assemblages composed of jarosite, calcium sulfate, and goethite. These results have important implications for comprehensively understanding the dosage of carbonate rock in mineral formation under different pH conditions. The findings help reveal the growth of secondary minerals during the treatment of AMD using carbonate rocks under low-pH conditions, which offers valuable information for combining the carbonate rocks with secondary minerals to treat AMD.

Iron (oxyhydr)oxides are responsible for the stabilization of Cu and Zn in AMD after treatment with limestone.

The formation and transformation of secondary iron (oxyhydr)oxides and their role in the stabilization of copper (Cu) and zinc (Zn) in acid mine drainage (AMD) after limestone treatment are worth studying to better understand the impacts of limestone AMD treatment. In this study, the wastewater from a copper mine ditch was sampled. Two different doses of limestone (S: 5.33 g L-1 and SS: 8.00 g L-1) were applied to adjust the pH range of the sampled AMD. The concentrations of Fe, Cu and Zn in the supernatant and the levels of iron (oxyhydr)oxides and heavy metals in AMD sediments were dynamically monitored for 300 days to analyze the transformation of the secondary iron mineral phase and the role iron (oxyhydr)oxides play in the removal and stabilization of Cu and Zn. The results showed that the pH rose rapidly to 6.82, decreased to 5.82 on the 150th day, and finally decreased to approximately 4.63 by the 300th day, when the dosage of limestone (S) was 5.33 g L-1. Goethite was the main form of iron oxides in the sediments. As the incubation time increased, so did the content of crystalline Fe (oxyhydr)oxides. In addition, the Cu and Zn content in the fraction of crystalline Fe (oxyhydr)oxides increased as the corresponding iron (oxyhydr)oxide increased. When the high dosage of limestone (8.00 g L-1 or SS) was applied, the pH remained at approximately at 7.46 during the whole period and goethite and lepidocrocite were present in the sediment. Amorphous/ poorly crystalline Fe-oxyhydroxide was the main product after SS limestone dosage, indicating that the risk of Cu and Zn reactivation in the sediment was higher with a higher limestone treatment dosage.

Arsenic mobilization in nZVI residue by Alkaliphilus sp. IMB: Comparison between static and flowing incubation

Arsenate reducing bacteria (AsRB) enhance arsenic (As) release via reducing As(V) to As(III), and As mobility is usually controlled by As(III) re-uptake on in-situ formed secondary iron minerals. The re-uptake of As(III) under groundwater flow conditions significantly impacts the fate and transport of As. Herein, a novel As(V)-reducing bacterium Alkaliphilus IMB was isolated in an As-contaminated soil. Scanning transmission X-ray microscopy showed that dissolved As(V) was mainly bound to the cell walls whereas dissolved As(III) was homogeneously distributed around IMB, indicating that As(V) reduction occurs outside the cell membrane. To explore the effect of IMB on As mobility, IMB was incubated with As-loaded nanoscale zero-valent iron (nZVI) residues under static and flowing conditions. IMB reduced 100% dissolved As(V) to As(III) even in a short contact time (∼1 h) during flowing incubation. The formation of As(III) did not influence As mobility under static condition as evidenced by the comparable concentrations of released As in the presence of IMB (8.5% to total As) and the abiotic control (10% to total As). Biogenic As(III) was re-adsorbed on the solids as shown by the higher ratio of solid-bound As(III) to total As in the presence of IMB (54%) than that in the abiotic control (12%). By contrast, the degree of As(III) re-adsorption was inhibited in the flowing environment, as suggested by the lower As(III) ratio in the solid (31%). This inhibition can be ascribed to the relatively slow adsorption of As(III) compared with the quick reduction of As(V) (∼1 h). Thus, IMB significantly enhanced As release during flowing incubation as shown that 9.8% As was released in the presence of IMB while 2.1% As in the abiotic control. This study found the contrary effect of AsRB on As mobility in static and flowing environments, highlighting the importance of re-adsorption rate of As(III).

Metagenomic Influential Insights in the Formation of Biogenic Iron Hydroxysulfate Precipitates by Ferrous Oxidative Microbial Consortia

The formation of iron hydroxysulfate precipitated minerals by microbial bioleaching is an undesired dominant process because it decreases the solubilization efficiency of targeted metals. To date, the microbial role in forming these precipitated minerals is unclear. Herein, we compared two microbial community diversity (consortia M1-18 and M8-15) with the distinct formation of secondary iron minerals. In the case of M1-18, a 16S rRNA metagenomic analysis revealed a higher microbial diversity and a prevalence of Ferrimicrobium and Acidimicrobiacea. This consortium also showed a slightly higher oxidation capacity and a remarkably higher particulate precipitate capacity of secondary minerals. In contrast, the consortium M8-15 showed lower diversity and poor precipitate capacity. XRD, EDS, and FE-SEM analysis of secondary iron minerals showed that the microbial consortia promote the mineralogical formation of the secondary iron precipitates in the form of schwertmannite, an early metastable phase that can be easily transformed to jarosite.

Experimental insights into the formation of secondary minerals in acid mine drainage-polluted karst rivers and their effects on element migration

Acid mine drainage (AMD) threatens the water quality and safety of karst river water (KRW), and the formation of secondary iron or aluminum-bearing minerals during the mixing of AMD with KRW plays a crucial role in the migration of elements. However, the variations in the mineralogical, morphological and elemental compositions of secondary minerals and their influences on the migration of elements during AMD–KRW mixing have not been systematically studied. In this study, we mixed different proportions of AMD and KRW in a laboratory experiment to simulate seasonal hydrological conditions in a river to understand the major and trace elemental distributions in the mixed water and in precipitates and we discuss the formation process for the secondary minerals. The results showed that AMD can lead to a decrease in pH and DO and an increase in heavy metals and rare earth elements (REEs) in KRW. With the biological or chemical oxidation of Fe2+, Fe3+ combines with SO42− to form schwertmannite or hydrolyzes to form Fe(OH)3(s) and FeOOH(s), accompanied by the formation of amorphous Al hydroxide, resulting in a decrease in pH and an increase in Eh. Schwertmannite had strong adsorption and coprecipitation effects on Mn, Cr, Cu and As, so the adsorption and coprecipitation effects of schwertmannite on REEs were inhibited, while the migration of REEs were mainly affected by Al hydroxides. Therefore, after the AMD mixes with KRW, it not only causes severe water and sediment pollution but also adsorbs and enriches high concentrations of heavy metals in the secondary minerals formed during the mixing process, creating a major ecological hazard that requires further attention.

Enhanced cadmium immobilization by sulfate-mediated microbial zero-valent iron corrosion

A biotic iron (Fe0) treatment system combined with mixed microorganisms was applied to remediate cadmium (Cd)-contaminated groundwater under the intervention of sulfate. Due to hydrogenotrophic desulfuration effect, severe iron corrosion was observed in this microbe-collaborative Fe0 system according to surface morphology analysis as lots of secondary minerals (e.g. magnetite, green rust and lepidocrocite) were generated, which was essential for Cd(II) adsorption and immobilization. The sulfate-mediated biotic Fe0 system thereafter achieved a significantly enhanced Cd(II) removal efficiency of 86.1%, over 3.3 times than that in the abiotic Fe0 system. Increasing initial sulfate concentration could improve the removal of cadmium, which further proved that hydrogenotrophic desulfuration played a key role for enhanced Cd removal. According to the experimental results and current reports, the mechanism of Cd(II) removal was revealed into three pathways including adsorption to secondary iron minerals, co-precipitation with iron (hydr)oxides and formation of cadmium sulfide precipitation. Increasing Fe0 dosages showed positive correlation to Cd(II) removal and neutral pH was preferred to sulfate-mediated biotic Fe0 corrosion. These results indicated that sulfate-mediated biotic Fe0 corrosion could greatly relieve the limitation of Fe0 in Cd(II) immobilization, which could be a promising method to eliminate Cd(II) pollution from groundwater.

Stabilization of cadmium in contaminated sediment based on a nanoremediation strategy: Environmental impacts and mechanisms

Nanomaterials have great application potential for the remediation of heavy metal contaminated sediments, but their environmental impacts are still limited. Herein, graphene oxide-supported nanoscale zero-valent iron (GNZVI) was synthesized to explore its role in mediating the immobilization of cadmium (Cd) from contaminated river sediments, with the consideration of the potential impacts on sediment enzyme activities and bacterial community. Compared to NZVI and GO, GNZVI could more effectively promote the transformation of mobile Cd into stable speciation with a maximum residual percentage increasing by 64.82% after 56 days of treatment. The activities of urease, catalase and sucrase were gradually increased and stabilized with the prolongation of treatment time, indicating that the metabolic function of sediments was recovered. 16 S rRNA gene sequencing results confirmed that the application of GNZVI increased the abundance of some Fe(III)-reducing bacteria, further stimulating the bioavailability of organic matter. Additionally, the properties of GO were gradually changed via microbial reduction and finally showed similar properties to rGO. The critical role of rGO as an electrical conductor was to promote the electron transfer process of microbial Fe(III) mineral reduction, which redistributes part of the Fe(III) mineral-associated Cd to more stable secondary iron minerals, thereby further improving the stabilization efficiency of r-GNZVI for Cd.

Activated carbon as an insoluble electron shuttle to enhance the anaerobic ammonium oxidation coupled with Fe(III) reduction process

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is an autotrophic biological nitrogen removal (BNR) technique in treating low-C/N wastewater. However, the nitrogen removal rate of Feammox is limited by the extracellular electron transfer. In this study, wood activated carbon (AC) was chosen as electron shuttle to enhance the start-up of the Feammox process. Within an operational period of 150 days, the NH4+-N removal efficiency reached 97.9–99.5% with a volumetric loading rate (VLR) of 0.04–0.06 kg N m−3 d−1. Batch experiments indicated that compared with Fe2O3-AQDS and Fe2O3 groups, Fe2O3-AC group showed higher catalytic performance and TN removal efficiency reached 85.7%. Quinone (CO) and phenolic (−OH) chemical groups of AC were equipped with electron transfer capacity (76.51 ± 9.27 μmol e− g−1). Moreover, Fe(II)/Fe(III) species and the secondary iron minerals were found in our system. Microbial analysis showed that Proteobacteria and Acidobacteriota, which observed with relatively high abundance, were played an important role in the integrated Feammox system. This study demonstrates the significant influence of AC on Feammox process and provides an enhanced biological nitrogen removal strategy for practice engineering application.

A new hybrid method for epithermal gold exploration using multi-sensor satellite data in Sistan and Baluchestan Province (Iran)

Remote sensing data provide a useful piece of information for initial identification of alteration zones, lithological units and structures in relation to mineralization, saving time and cost. In this research, silicic, argillic and propylitic alteration zones with gold mineralization in Taftan area in Sistan and Baluchestan province (Iran) were identified by thermal infrared (TIR), shortwave infrared (SWIR) and visible-near infrared (VNIR) bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat-8 data. Detecting altered and mineralized zones of epithermal gold were conducted using a conventional hybrid method of band ratio in false coloured composite (BR-FCC), and a new hybrid method including, constrained energy minimization in false coloured composite (CEM-FCC). The hybrid methods of the BR-FCC (BRs in RGB) identified the silicic area and places, containing clay and secondary iron minerals. Furthermore, the advanced argillic, argillic and propylitic alteration zones detected and derived in altered areas specifically to the northwest of Taftan, in Kharestan and Bidester deposits using the hybrid methods of the CEM-FCC (CEMs in RGB). Results of image processing showed two new prospective altered places to the northeast of Kharestan deposit and to the northwest of Taftan volcano.