Simultaneous removal of iron and manganese from acid mine drainage by acclimated bacteria

Optimization of operating parameters and microbiological mechanism of a low C/N wastewater treatment system dominated by iron-dependent autotrophic denitrification

Ferrous iron (Fe2+) reduces the amount of external carbon source used for the denitrification of low-C/N wastewater. The effects of key operating parameters on the efficiency of ferrous-dependent autotrophic denitrification (FDAD) and the functioning mechanism of the microbiome can provide a regulatory strategy for improving the denitrification efficiency of low C/N wastewater. In this study, the response surface method (RSM) was used to explore the influence of four important parameters-the molar ratio of Fe2+ to NO3--N (Fe/N), total organic carbon (TOC), the molar ratio of inorganic carbon to NO3--N (IC/N) and sludge volume (SV, %)-on the FDAD efficiency. Functional prediction and molecular ecological networks based on high-throughputs sequencing techniques were used to explore changes in the structure, function, and biomarkers of the sludge microbial community. The results showed that Fe/N and TOC were the main parameters affecting FDAD efficiency. Higher concentrations of TOC and high Fe/N ratios provided more electron donors and improved denitrification efficiency, but weakened the importance of biomarkers (Rhodanobacter, Thermomonas, Comamonas, Thauera, Geothrix and unclassified genus of family Gallionellaceae) in the sludge ecological network. When Fe/N > 4, the denitrification efficiency fluctuated significantly. Functional prediction results indicated that genes that dominated N2O and NO reduction and the genes that dominated Fe2+ transport showed a slight decrease in abundance at high Fe/N levels. In light of these findings, we recommend the following optimization ranges of parameters: Fe/N (3.5-4); TOC/N (0.36-0.42); IC/N (3.5-4); and SV (approximately 35%).

Effect of biopolymer chitosan on manganese immobilization improvement by microbial‑induced carbonate precipitation

Microbially induced carbonate precipitation (MICP), as an eco-friendly and promising technology that can transform free metal ions into stable precipitation, has been extensively used in remediation of heavy metal contamination. However, its depressed efficiency of heavy metal elimination remains in question due to the inhibition effect of heavy metal toxicity on bacterial activity. In this work, an efficient, low-cost manganese (Mn) elimination strategy by coupling MICP with chitosan biopolymer as an additive with reduced treatment time was suggested, optimized, and implemented. The influences of chitosan at different concentrations (0.01, 0.05, 0.10, 0.15 and 0.30 %, w/v) on bacterial growth, enzyme activity, Mn removal efficiency and microstructure properties of the resulting precipitation were investigated. Results showed that Mn content was reduced by 94.5 % within 12 h with 0.15 % chitosan addition through adsorption and biomineralization as MnCO3 (at an initial Mn concentration of 3 mM), demonstrating a two-thirds decrease in remediation time compared to the chitosan-absent system, whereas maximum urease activity increased by ∼50 %. Microstructure analyses indicated that the mineralized precipitates were spherical-shaped MnCO3, and a smaller size and more uniform distribution of MnCO3 is obtained by the regulation of abundant amino and hydroxyl groups in chitosan. These results demonstrate that chitosan accelerates nucleation and tunes the growth of MnCO3 by providing nucleation sites for mineral formation and alleviating the toxicity of metal ions, which has the potential to upgrade MICP process in a sustainable and effective manner. This work provides a reference for further understanding of the biomineralization regulation mechanism, and gives a new perspective into the application of biopolymer-intensified strategies of MICP technology in heavy metal contamination.

Turning harmful Mn2+ to treasure: In-situ formed ε-MnO2 for removing heavy metals from acid mine drainage

Acid mine drainage (AMD) contains high concentrations of heavy metals, causing serious environmental pollution. Current neutralization techniques fail to recover and utilize valuable heavy metals, and generate large quantities of hazardous sludge. Manganese (Mn) is generally present at high levels in AMD. Therefore, this paper proposed a technology to recover Mn from AMD, by adding KMnO4 to converting Mn into ε-MnO2. Ultra-Violet C (UVC) was used to photolyze the residual KMnO4. The study then evaluated the processes and mechanisms involved in the technology. The photolysis of KMnO4 in strong acidic conditions was determined, and new mechanisms were proposed. MnO2 produced by the photolysis process was formed through the reaction between Mn(III) and KMnO4. In the absence of KMnO4, Mn(III) underwent further photolysis and was reduced to Mn2+. The maximum adsorption capacities of in-situ formed ε-MnO2 for Pb2+, Cd2+, and Fe3+ were 449.80, 122.05, and 779.88 mg/g, respectively. Higher Mn-OH levels and MnO2 regeneration were crucial in improving adsorption performance. Proton exchange and inner-circle complexation were the main pathways for Pb2+ and Cd2+ adsorption by in-situ formed ε-MnO2. A phase transformation occurred when a substantial amount of Fe3+ was adsorbed, leading to the gradual transformation to MnFe binary oxides. When applying in-situ formed ε-MnO2 technology for actual AMD treatment, 98.62 % of Mn in AMD was recovered within 24 h in the presence of ε-MnO2 for possible further reuse in industries, with a final recovery of 0.76 kg/m3. Further, this technique removed other heavy metals and reduced the sludge volume by 20.99 % when used as a pre-treatment step for neutralization. These results demonstrated the broad potential of this treatment technology.

Driving mechanism of different nutrient conditions on microbial mediated nitrate reduction in magnetite-present river infiltration zone

Significant research is focused on the ability of riparian zones to reduce groundwater nitrate contamination. Owing to the extremely high redox activity of nitrate, naturally existing electron donors, such as organic matter and iron minerals, are crucial in facilitating nitrate reduction in the riparian zone. Here, we examined the coexistence of magnetite, an iron mineral, and nitrate, a frequently observed coexisting system in sediments, to investigate nitrate reduction features at various C/N ratios and evaluate the response of microbial communities to these settings. Additionally, we aimed to use this information as a foundation for examining the effect of nutritional conditions on the nitrate reduction process in magnetite-present environments. These results emphasise the significance of organic matter in enabling dissimilatory nitrate reduction to ammonium (DNRA) and enhancing the connection between nitrate reduction and iron in sedimentary environments. In the later phases of nitrate reduction, nitrogen fixation was the prevailing process in low-carbon environments, whereas high-carbon environments tended to facilitate the breakdown of organic nitrogen. High-throughput sequencing analysis revealed a robust association between C/N ratios and alterations in microbial community composition, providing insights into notable modifications in essential functioning microorganisms. The nitrogen-fixing bacterium Ralstonia is more abundant in ecosystems with scarce organic matter. In contrast, in settings rich in organic matter, microorganisms, such as Acinetobacter and Clostridia, which may produce ammonia, play crucial roles. Moreover, the population of iron bacteria grows in such an environment. Hence, this study proposes that C/N ratios can influence Fe(II)/Fe(III) conversions and simultaneously affect the process of nitrate reduction by shaping the composition of specific microbial communities.

Removal of cadmium and cobalt from water by Slovak bentonites: efficiency, isotherms, and kinetic study

Slovak bentonite was used as an effective natural adsorbent for the removal of Cd(II) and Co(II). Characterization of the samples was conducted using X-ray diffraction (XRD), high-resolution scanning electron microscopy with an X-ray energy dispersion spectrometer (SEM-EDS), and infrared spectroscopy (FT-IR). Adsorption experiments were carried out for pure water and artificial seawater, each containing cobalt and cadmium cations within the concentration range of 5-60 mg/L. The highest bentonite adsorption capacities of the tested bentonites were 23.5 (Cd) and 32.2 (Co) mg g-1. The kinetics data revealed that, in addition to chemisorption, intraparticle diffusion contributes to metal removal. The physical and structural properties of bentonites play an important role in adsorption. Bentonite P 135 from the Lieskovec deposit showed the highest efficiency for removing both ions, with removal percentages exceeding 90% and 77.5% for pure water and artificial seawater, respectively. The results indicate the suitability of using Slovak bentonites as an alternative sorbent for both metal extractions. The mechanism of metal ion adsorption on bentonite clay can be understood through surface complexation and ion exchange. The examined bentonite deposits show potential as promising natural sorbents for the removal of cobalt and cadmium cations from polluted waters.

Microbial-induced MnO2 precipitation in a carbonate coastal aquifer

A study was carried out to identify biogeochemical reactions along a transect of a coastal dolomitic aquifer. In this transect, the physicochemical parameters of the groundwater as well as the microbial composition of samples taken at different depths and salinities were measured. Many of the dissolved ions measured in the groundwater follow a pattern that reflects the distribution of the water masses (fresh, interface and salt) in the aquifer, while others such as Ca and Mg ions deviate from this trend by identifying the zones of maximum dissolution of the carbonate matrix. The concentrations of minor ions, such as Fe and Mn, also follow a singular pattern, with maximum concentrations in the reducing zones of the aquifer and lower values in the oxidizing zones. Precipitates of Mn oxides along with other metals, such as Fe, Ba, Zn and Ni, were observed in the saline zone displaying oxidizing conditions close to the coastline, where a continuous core was recovered. This zone, which is located below the freshwater-seawater mixing zone and features percentages of seawater higher than 80 %, is characterized by the presence of Marinobacter as the predominant genus. These bacteria are also related to the formation of Mn-rich polymetallic oxides in other contexts such as the ocean floor (Wang et al., 2012; Cao et al., 2021). All in all, a biogeochemical reaction model is proposed that describes the formation of these oxides in areas close to the discharge zone of coastal aquifers. To do this, it has been necessary to integrate the results obtained from geochemical, hydrogeological and microbiological information.

Application of biological soil crusts for efficient cadmium removal from acidic mine wastewater

Utilizing an acid-resistant biological soil crust (BSC) species that we discovered, we developed a device capable of efficiently removing cadmium (Cd) from mine wastewater with varying levels of acidity. Our research has demonstrated that this particular BSC species adapts to acidic environments by regulating the balance of fatty acids and acid-resistant enzymes. At a Cd concentration of 5 mg/L, the BSC grew well. When the initial Cd concentration was 2 mg/L, and the flow rate was set at 1 mL/min (at pH levels of 3, 4, and 5), BSC had a high removal rate of Cd, and the removal rate increased with the increase of pH (from 90% to 97%). Chemisorption is the primary removal mechanism in the initial stage, where the functional groups and minerals on the surface of the BSC play a significant role. In addition, BSC also adapts to Cd stress by changing bacterial community structure. It was discovered through infrared spectroscopy and two-dimensional correlation analysis that hydrophilic groups, specifically phosphate and carboxyl groups, exhibited the highest reactivity during the Cd binding process. Protein secondary structure analysis confirmed that as the pH increased, the adsorption capacity of the BSC increased; making biofilm formation easier. This study presents a novel approach for the treatment of acidic wastewater.

Sulfur-manganese carbonate composite autotrophic denitrification: nitrogen removal performance and biochemistry mechanism

A novel composite sulfur-manganese carbonate autotrophic denitrification (SMAD) system was developed to reduce sulfate production and provide pH buffer function while improving denitrification efficiency without external organics. The average removal efficiency of total nitrogen (TN) was 98.09% and 96.29%, and that of NO3--N was 99.53% and 97.77%, respectively, in the SMAD system with a hydraulic retention time (HRT) of 6 h and 3 h. They were significantly higher than that in the controls (quartz sand, manganese carbonate ore, and sulfur systems). The H+ produced by the sulfur autotrophic denitrification (SAD) process promoted the release of Mn2+ in the SMAD system. And this system had a stable pH with no accumulation of NO2--N. The decrease of sulfate and formation of Mn oxides through Mn2+ electron donation confirmed the presence of the manganese autotrophic denitrification (MAD) process in the SMAD system. Dominant functional bacteria in the SMAD system were Thiobacillus, Chlorobium, and Sulfurimonas, which were linked to nitrogen, sulfur, and manganese conversion and promoted denitrification. Meanwhile, Flavobacterium participating in Mn2+ oxidation was found only in the SMAD system. The SMAD system provided a new strategy for advanced tailwater treatment.

Mobility, speciation of cadmium, and bacterial community composition along soil depths during microbial carbonate precipitation under simulated acid rain

An overexploitation of earth resources results in acid deposition in soil, which adversely impacts soil ecosystems and biodiversity and affects conventional heavy metal remediation using immobilization. A series of column experiments was conducted in this study to compare the cadmium (Cd) retention stability through biotic and abiotic carbonate precipitation impacted by simulated acid rain (SAR), to build a comprehensive understanding of cadmium speciation and distribution along soil depth and to elucidate the biogeochemical bacteria-soil-heavy metal interfaces. The strain of Sporosarcina pasteurii DSM 33 was used to trigger the biotic carbonate precipitation and cultivated throughout the 60-day column incubation. Results of soil pH, electrical conductivity (EC), and quantitative CdCO3/CaCO3 analysis concluded that the combination of biotic and abiotic soil treatment could reinforce soil buffering capacity as a strong defense mechanism against acid rain disturbance. Up to 1.8 ± 0.04 U/mg urease enzyme activity was observed in combination soil from day 10, confirming the sustained effect of urease-mediated microbial carbonate precipitation. Cadmium speciation and distribution analyses provided new insights into the dual stimulation of carbonate-bound and Fe/Mn-bound phases of cadmium immobilization under microbially induced carbonate precipitation (MICP). As confirmed by the microbial community analysis, outsourcing urea triggered diverse microbial metabolic responses, notably carbonate precipitation and dissimilatory iron metabolism, in both oxygen-rich topsoil and oxygen-depleted subsurface layers. The overall investigation suggests the feasibility of applying MICP for soil Cd remediation under harsh environments and stratagem by selecting microbial functionality to overcome environmental challenges.

Magnetite-equipped algal-rich sediments for microbial fuel cells: Remediation of sediment organic matter pollution and mechanisms of remote electron transfer

Using the bio-electrochemical methods for the restoration of high algae sediments is full of potential and challenges. How to promote extracellular electron transfer (EET) process in microbial fuel cells (MFC) is the key bottleneck. The study had explored the potential application of magnetite on accelerating electron transfer for improving the output of MFC and sediment pollution remediation. The results indicated that the organic matter degradation rate showed a remarkable increase of 27.45 %, and the voltage output was approximately 1.68 times higher compared to the MFC configured with regular sediment. Abundant electroactive bacteria (EABs), such as Geobacter and Burkholderiaceae, and fermentative bacteria were responsible for these results, accompanied by the enhanced fluorescence of humic substances (HS), increased concentration and activity of cytochrome C (25.05 % and 21.12 %), as well as elevated extracellular polymeric substance content. Moreover, the intrinsic EET mechanisms among Fe-oxides, HS, and EABs were explored. According to the electrochemical analysis and substance transformation, the EET process involved four stages: magnetite-enhanced direct electron transfer via strong conductivity, iron respiration mediating electron transfer to the electrode, the model quinone substance acting as an electron shuttle facilitating EET and iron reduction, and iron cycling mediating electron transfer. This study provides an effective strategy for pollution remediation in algal-rich sediment, which was beneficial for the harmless treatment and resource utilization of both algae and sediment, simultaneously.

Removal of thallium by MnOx coated limestone sand filter through regeneration of KMnO4: Combination of physiochemical and biochemical actions

Treatment of industrial thallium(Tl)-containing wastewater is crucial for mitigating environmental risks and health threats associated with this toxic metal. The incorporation of Mn oxides (MnOx) into the filtration system is a promising solution for efficient Tl(I) removal. However, further research is needed to elucidate the underlying mechanism behind MnOx-enhanced filtration and the rules of its stable operation. In this study, limestone, a cost-effective material, was selected as the filter media. Raw water with Mn(II), Tl(I), and other pollutants was prepared after a thorough investigation of actual industrial wastewater conditions. KMnO4 was added to induce the formation of MnO2 on limestone surfaces, while long-term operation led to enrichment of manganese oxidizing microorganisms (MnOM). Results revealed a dual mechanism. Firstly, most Mn(II) were oxidized by KMnO4 to form MnO2 attaching to limestone sands, and both Tl(I) and residual Mn(II) were adsorbed onto the newly formed MnO2. Subsequently, enzymes secreted by MnOM facilitated oxidation of remaining Mn(II), resulting in the generation of biogenic manganese oxides (BioMnOx) with numerous vacancies during long-term operation. The generated BioMnOx not only adsorbed Mn(II) and Tl(I) but also promoted their oxidation process. This approach offers an effective and sustainable method for removing both Mn(II) and Tl(I) from industrial wastewater, thereby addressing the challenges posed by thallium-contaminated effluents.

Chemical Mineralization of AMD into Schwertmannite Fixing Iron and Sulfate Ions by Structure and Adsorption: Paving the Way for Enhanced Mineralization Capacity

Abundant iron and sulfate resources are present in acid mine drainage. The synthesis of schwertmannite from AMD rich in iron and sulfate could achieve the dual objectives of resource recovery and wastewater purification. However, schwertmannite cannot emerge spontaneously due to the Gibbs free energy greater than 0. This results in the iron and sulfate in AMD only being able to use the energy generated by oxidation in the coupling reaction to promote the formation of minerals, but this only achieved partial mineralization, which limited the remediation of AMD through mineralization. In order to clarify the mechanism of iron and sulfate removal by the formation of schwertmannite in AMD, kinetic and thermodynamic parameters were crucial. This work used H2O2 oxidation of Fe2+ as a coupling reaction to promote the formation of schwertmannite from 64.4% of iron and 15.7% of sulfate in AMD, and determined that 99.7% of the iron and 89.9% of sulfate were immobilized in the schwertmannite structural, and only a small fraction was immobilized by the adsorption of schwertmannite, both of which were consistent with second-order kinetics models. The thermodynamic data suggested that reducing the concentration of excess sulfate ions or increasing the energy of the system may allow more iron and sulfate to be immobilized by forming schwertmannite. Experimental verification using the reaction of potassium bicarbonate with the acidity in solution to increase the energy in the system showed that the addition of potassium bicarbonate effectively promoted the formation of schwertmannite from Fe3+ and SO42-. It provided a theoretical and research basis for the direct synthesis of schwertmannite from Fe3+ and SO42- rich AMD for the removal of contaminants from water and the recovery of valuable resources.

Linking groundwater microbiome and functional ecological clusters to geogenic high hexavalent chromium from deep aquifers in a loess plateau

Geogenic high hexavalent chromium [Cr(Ⅵ)] in groundwater is a global environmental problem. However, the groundwater microbiome and its linkage to geogenic high Cr(Ⅵ) from deep aquifers still need to be elucidated. Here, we evaluated geogenic Cr(Ⅵ), groundwater microbiome with featured functional ecological clusters and their interactive responses in groundwater from a deep aquifer in a loess plateau of Northern Shaanxi, China. We found that the compositions and structures of microbial communities in groundwater from the deep aquifer were significantly different between low Cr(Ⅵ) (LCG, < 50 μg/L) and high Cr(Ⅵ) groundwater (HCG, > 50 μg/L), with higher microbial diversity and richness in HCG (p < 0.05). Functional "specialists" related to Cr biotransformation, including Cr(Ⅵ) reducing bacteria (CRB) Rhodococcus, Nocardioides, Novosphingobium, and Acidovorax and Mn-oxidizing bacteria (MnOB) Sphingobium, and Ralstonia were positively correlated to total Cr and Cr(VI) concentrations in groundwater. Moreover, these CRB and MnOB were dominant in high Cr(VI) groundwater and associated by strong interspecific relation in an ecological cluster (p < 0.05), suggesting their indicator roles for high Cr(Ⅵ) and the contribution of MnOB mediated Cr(III) oxidation to Cr(VI) enrichment. RDA and path analysis further revealed that the geogenic Cr(Ⅵ) directly promoted the key Cr-related functional cluster with the groundwater depth, dissolved oxygen, and total dissolved solids as the cofactors indirectly influencing Cr(Ⅵ) and the functional clusters (p < 0.05). Collectively, our results highlight the significant roles of microbial ecological clusters especially functional "specialists" MnOB and CRB in groundwater Cr(Ⅵ) from deep aquifers in the loess plateau and provide a basis for sustainable management of high Cr(Ⅵ) groundwater.

Enhancing the Mn-Removal Efficiency of Acid-Mine Bacterial Consortium: Performance Optimization and Mechanism Study

In this study, an acclimated manganese-oxidizing bacteria (MnOB) consortium, QBS-1, was enriched in an acid mine area; then, it was used to eliminate Mn(Ⅱ) in different types of wastewater. QBS-1 presented excellent Mn removal performance between pH 4.0 and 8.0, and the best Mn-removal efficiency was up to 99.86% after response surface methodology optimization. Unlike other MnOB consortia, the core bacteria of QBS-1 were Stenotrophomonas and Achromobacter, which might play vital roles in Mn removal. Besides that, adsorption, co-precipitation and electrostatic binding by biological manganese oxides could further promote Mn elimination. Finally, the performance of the Mn biofilter demonstrated that QBS-1 was an excellent inoculant, which indicates good potential for removing Mn contamination steadily and efficiently.

Immobilization of Cd in the soil of mining areas by FeMn oxidizing bacteria

Microorganisms are widely used in large-scale pollution remediation due to their rapid reproduction and low cost. In this study, bioremediation batch experiments and characterization methods were adopted to investigate the mechanism of FeMn oxidizing bacteria on the immobilization of Cd in mining soil. The results showed that the FeMn oxidizing bacteria successfully reduced 36.84 % of the extractable Cd in the soil. The exchangeable forms, carbonate-bound forms, and organic-bound forms of Cd in the soil decreased by 11.4 %, 8 %, and 7.4 %, respectively, due to the addition of FeMn oxidizing bacteria, while FeMn oxides-bound and residual forms of Cd increased by 19.3 % and 7.5 %, as compared to the control treatments. The bacteria promotes the formation of amorphous FeMn precipitates such as lepidocrocite and goethite, which have high adsorption capacity on soil Cd. The oxidation rates of Fe and Mn in the soil treated with the oxidizing bacteria reached 70.32 % and 63.15 %, respectively. Meanwhile, the FeMn oxidizing bacteria increased soil pH and decreased soil organic matter content, further decreasing the extractable Cd in the soil. The FeMn oxidizing bacteria have the potential to be used in large mining areas to assist in the immobilization of heavy metals.

Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Detoxification and metabolism of glyphosate by a Pseudomonas sp. via biogenic manganese oxidation

Biogenic manganese oxides (BMO) are widely distributed in groundwater and provides promise for adsorbing and oxidizing a wide range of micropollutants, however, the continuous biodegradation and bioavailability of micropollutants via cycle biogenic Mn(II) oxidation remains to be elucidated. In this study, glyphosate was degraded and to serve as the nutrient source by a Pseudomonas sp. QJX-1. The addition of glyphosate will not affect the Mn(II) oxidation function of the strain but will affect its Mn(II) oxidation process and effect. The glyphosate degradation products could further be used as the C, N and P sources for bacterium growth. Analysis of the RNA-seq data suggested that Mn(II) oxidation driven by oxidoreductases for glyphosate degradation. The long-term column experiments using biological Mn(II) cycling to realize continuous detoxification and metabolism of glyphosate, and thus revealed the synergism effects of biological and chemical conversion on toxic micropollutants and continuous metabolism in an aquatic ecosystem.

Aerobic denitrification of oligotrophic source water driven by reduced metal manganese

The lack of organic electron donors limits the potential utility of aerobic denitrification in treatment of oligotrophic source water. Here, reduced manganese (Mn) was used as an inorganic electron donor to improve the denitrification of oligotrophic source water under the high dissolved oxygen condition (7-9 mg L^-1). Over 30 days, the total nitrogen removed by the treatment with reduced Mn was 76.21 ± 2.11% (maximum), substantially higher than that of the control treatment, which was 41.48 ± 2.33%. Furthermore, the addition of Mn resulted in the directional evolution of the microbial community. Water samples with Mn added showed a higher abundance of Limnohabitans, the dominant denitrifying genus, reaching 51.02%, 36.79%, and 20.19% (with 30, 50, and 70 g Mn, respectively), versus only 5.54% in the control. In biofilm, Mn promoted Hydrogenophaga and Brevundimonas growth while Pseudarthrobacter growth was promoted by 30 and 50 g Mn, but inhibited by 70 g Mn. This study demonstrates an improved performance in aerobic denitrification of water sources through the use of inorganic electron donors.

Immobilization of metal(loid)s from acid mine drainage by biological soil crusts through biomineralization

Acid mine drainage is harmful to the environment. Bioremediation based on biological soil crusts (BSCs) can be used as a new method to alleviate metal pollution in acid mine drainage. In this study, we found that BSCs can survive in a strongly acidic environment (pH = 3.28) and have a high metal(loid)s accumulation ability. The algae of genera Fragilaria, Klebsormidium, Cymbella, Melosira, Microcystacea, and Planctonema a're the main components of BSCs. These organisms in the BSCs regulated fatty acids and produced acid-resistant enzymes. The bioconcentration factors for As, Cd, Pb, Zn, and Cu were as high as 16,000, 200, 50, 26, and 400, respectively. The concentration of As and Cd in acid mine drainage decreased from 7.1 μg and 350 μg/L to 1.9 μg and 110 μg/L, respectively. In total, 56% of As, 73% of Cd, 88% of Pb, 85% of Zn, and 92% of Cu were present in BSCs as residual or mineral-bound forms. The XRD results (e.g., quarartz and phyllosilicates), SEM results (e.g., phylosilicates and diatom shells) and correlation results show that these metal(loid)s are immobilized by Cymbella (diatoms) during the deposition of silica in the acidic environment. Furthermore, adsorption and co-precipitation are other ways that metal(loid)s could have been bound. These findings provide new insights into the removal of metals (loid) in acidic water.

Heavy metal bioremediation using microbially induced carbonate precipitation: Key factors and enhancement strategies

With the development of economy, heavy metal (HM) contamination has become an issue of global concern, seriously threating animal and human health. Looking for appropriate methods that decrease their bioavailability in the environment is crucial. Microbially induced carbonate precipitation (MICP) has been proposed as a promising bioremediation method to immobilize contaminating metals in a sustainable, eco-friendly, and energy saving manner. However, its performance is always affected by many factors in practical application, both intrinsic and external. This paper mainly introduced ureolytic bacteria-induced carbonate precipitation and its implements in HM bioremediation. The mechanism of HM immobilization and in-situ application strategies (that is, biostimulation and bioaugmentation) of MICP are briefly discussed. The bacterial strains, culture media, as well as HMs characteristics, pH and temperature, etc. are all critical factors that control the success of MICP in HM bioremediation. The survivability and tolerance of ureolytic bacteria under harsh conditions, especially in HM contaminated areas, have been a bottleneck for an effective application of MICP in bioremediation. The effective strategies for enhancing tolerance of bacteria to HMs and improving the MICP performance were categorized to provide an in-depth overview of various biotechnological approaches. Finally, the technical barriers and future outlook are discussed. This review may provide insights into controlling MICP treatment technique for further field applications, in order to enable better control and performance in the complex and ever-changing environmental systems.

Direct observation of Mn distribution/speciation within and surrounding a basidiomycete fungus in the production of Mn-oxides important in toxic element containment

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn²⁺ absorption by a basidiomycete fungus, Coprinopsis urticicola, isolated from Ningyo-toge mine water samples, was investigated in the laboratory under controlled conditions utilizing electron microscopy, synchrotron-based X-ray analysis, and fluorescence microscopy with a molecular pH probe. The fungus' growth was first investigated in an agar-solidified medium supplemented with 1.0 mmol/L Mn²⁺, and Cu²⁺ (0-200 μM), Zn²⁺ (0-200 μM), or diphenyleneiodonium (DPI) chloride (0-100 μM) at 25 °C. The results revealed that Zn²⁺ has no significant effects on Mn oxide formation, whereas Cu²⁺ and DPI significantly inhibit both fungal growth and Mn oxidation, indicating superoxide-mediated Mn oxidation. Indeed, nitroblue tetrazolium and diaminobenzidine assays on the growing fungus revealed the production of superoxide and peroxide. During the interaction of Mn²⁺ with the fungus in solution medium at the initial pH of 5.67, a small fraction of Mn²⁺ infiltrated the fungal hyphae within 8 h, forming a few tens of nm-sized concentrates of soluble Mn²⁺ in the intracellular pH of ∼6.5. After 1 day of incubation, Mn oxides began to precipitate on the hyphae, which were characterized as fibrous nanocrystals with a hexagonal birnessite-structure, these forming spherical aggregates with a diameter of ∼1.5 μm. These nanoscale processes associated with the fungal species derived from the Ningyo-toge mine area provide additional insights into the existing mechanisms of Mn oxidation by filamentous fungi at other U mill tailings sites under circumneutral pH conditions. Such processes add to the class of reactions important to the sequestration of toxic elements.

Coupling and environmental implications of in situ formed biogenic Fe-Mn minerals induced by indigenous bacteria and oxygen perturbations for As(III) immobilization in groundwater

Iron (Fe)-manganese (Mn) minerals formed in situ can be used for the natural remediation of the primary poor-quality groundwater with coexistence of arsenite [As(III)], Mn(II), and Fe(II) (PGAMF). However, the underlying mechanisms of immobilization and coupling of As, Mn, and Fe during in-situ formation of Fe-Mn minerals in PGAMF remains unclear. The simultaneous immobilization and coupling of arsenic (As), Mn, and Fe in PGAMF during in-situ formation of biogenic Fe-Mn minerals induced by O₂ perturbations and indigenous bacteria (Comamonas sp. RM6) were investigated at the different molar ratios of Fe(II):Mn(II) (1:1, 2:1, and 3:1). Compared with systems without Fe(II) in the presence of Mn(II), the coexisted Fe(II) significantly enhanced Mn(II) bio-oxidation and mineral precipitation, resulting in As immobilization increased by 5, 7, and 7 times at initial Fe(II) concentration of 0.3, 0.6, and 0.9 mM, respectively. Moreover, the As(III) immobilization efficiencies in Mn(II) and Fe(II) mixed system at initial Fe(II) concentration of 0.3, 0.6, and 0.9 mM were 73%, 91%, and 92%, respectively, that were significantly higher than those of single Fe(II) system (30%, 59%, and 74%) and those of single Mn(II) system (12%), indicating that Fe(II) and Mn(II) oxidation synergically enhanced As(III) immobilization. This was mainly attributed to the formation and As adsorption capacity of biogenic Fe-Mn minerals (BFMM). The formed BFMM significantly facilitated simultaneous immobilization of Fe, Mn, and As in PGAMF by oxidation, adsorption, and precipitation/coprecipitation, a coupling of biological, physical, and chemical processes. Fe component was mainly responsible for As fixation, and Mn component dominated As(III) oxidation. Based on the results from this work, biostimulation and bioaugmentation techniques can be developed for in-situ purification and remediation of PGAMF. This work provides insights into the simultaneous immobilization of pollutants in PGAMF, as well as promising strategies for in-situ purification and remediation of PGAMF.

A Review of Manganese-Oxidizing Bacteria (MnOB): Applications, Future Concerns, and Challenges

Groundwater serving as a drinking water resource usually contains manganese ions (Mn2+) that exceed drinking standards. Based on the Mn biogeochemical cycle at the hydrosphere scale, bioprocesses consisting of aeration, biofiltration, and disinfection are well known as a cost-effective and environmentally friendly ecotechnology for removing Mn2+. The design of aeration and biofiltration units, which are critical components, is significantly influenced by coexisting iron and ammonia in groundwater; however, there is no unified standard for optimizing bioprocess operation. In addition to the groundwater purification, it was also found that manganese-oxidizing bacteria (MnOB)-derived biogenic Mn oxides (bioMnOx), a by-product, have a low crystallinity and a relatively high specific surface area; the MnOB supplied with Mn2+ can be developed for contaminated water remediation. As a result, according to previous studies, this paper summarized and provided operational suggestions for the removal of Mn2+ from groundwater. This review also anticipated challenges and future concerns, as well as opportunities for bioMnOx applications. These could improve our understanding of the MnOB group and its practical applications.

Role of intracellular energy metabolism in Mn(Ⅱ) removal by the novel bacterium Stenotrophomonas sp. MNB17

Microorganism-mediated Mn(Ⅱ) removal has gained increasing attention as a valuble bioremediation approach. In this study, a novel strain Stenotrophomonas sp. MNB17 - obtained from marine sediments - was found to show Mn(Ⅱ) removal efficiencies of 98.51-99.38% within 7 days and 92.24% within 20 days at Mn(Ⅱ) concentrations of 10-40 mM and 50 mM, respectively. On day 7, 80.44% of 50 mM Mn(Ⅱ) was oxidized to Mn(Ⅲ/Ⅳ), whereas only 2.11-2.86% of 10-40 mM Mn(Ⅱ) was oxidized. This difference in the proportion of Mn-oxides suggested that the strain MNB17 could remove soluble Mn(Ⅱ) via distinct mechanisms under different Mn(Ⅱ) concentrations. At 10 mM Mn(Ⅱ), indirect mechanisms were employed by strain MNB17 to remove Mn(Ⅱ). The sufficient energy generated by increased cellular respiration led to enhanced ammonification, and MnCO₃ was the main component of the Mn-precipitates (97.27%). Meanwhile, intracellular fatty acids were degraded and served as an important carbon source for respiration. At 50 mM Mn(Ⅱ), most of the soluble Mn(Ⅱ) was oxidized, and Mn-oxides dominated the Mn-precipitates (80.44%). Mn(Ⅱ) oxidation likely contributed to electrons for energy production, as the down-regulation of respiratory pathways resulted in a deficit of electron supply, which warrants futher study. The exogenous addition of tricarboxylic acid cycle substrates (malate, α-ketoglutarate, oxaloacetate, succinate, and fumarate) was found to accelerate Mn(Ⅱ) removal as MnCO₃ at a concentration of 50 mM. Overall, this study reports a novel strain MNB17 with the biotechnological potential of Mn(Ⅱ) removal and elucidates the function of cellular energy metabolism during the Mn(Ⅱ) removal process. In addition, it demonstrates the potential of aerobic respiration-related substrates in accelerating the removal of high concentrations of Mn(Ⅱ) for the first time.

Algae, biochar and bacteria for acid mine drainage (AMD) remediation: A review

Acid mine drainage (AMD) is a global issue and causes harmful environmental impacts. AMD has high acidity and contains a high concentration of heavy metals and metalloids, making it toxic to plants, animals, and humans. Traditional treatments for AMD have been widely used for a long time. Nevertheless, some limitations, such as low efficacy and secondary contamination, have led them to be replaced by other methods such as bio-based AMD treatments. This study reviewed three bio-based treatment methods using algae, biochar, and bacteria that can be used separately and potentially in combination for effective and sustainable AMD treatment to identify the removal mechanisms and essential parameters affecting AMD treatment. All bio-based methods, when applied as a single process and in combination (e.g. algae-biochar and algae-bacteria), were identified as effective treatments for AMD. Also, all these bio-based methods were found to be affected by some parameters (e.g. pH, temperature, biomass concentration and initial metal concentration) when removing heavy metals from AMD. However, we did not identify any research focusing on the combination of algae-biochar-bacteria as a consortium for AMD treatment. Therefore, due to the excellent performance in AMD treatment of algae, biochar and bacteria and the potential synergism among them, this review provides new insight and discusses the feasibility of a combination of algae-biochar-bacteria for AMD treatment.

The “Infernaccio” Gorges: Microbial Diversity of Black Deposits and Isolation of Manganese-Solubilizing Bacteria

Simple Summary

“Infernaccio” gorges are one of the Earth’s hidden habitats in Central Italy. Beyond the deep incisions and high slopes, these gorges are characterized by black deposits in gorge walls and covering rock surfaces. Several geological events have shaped these unique geological formations and their microbiota. This study investigated microbial contribution to black deposit formation and isolating Mn-oxide-solubilizing bacteria. Our results provided evidence of the putative role of Bacteria and Archaea in forming manganese oxide deposits. Findings also showed that these deposits are a source of valuable strains with manganese oxide bioleaching properties, essential for bioremediation and metal recovery.

Abstract

The present study explored the microbial diversity of black deposits found in the “Infernaccio” gorge. X-ray Powdered Diffraction (XRPD) was used to investigate the crystallinity of the samples and to identify the minerals. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDS) were used to detect the bacterial imprints, analyze microbe–mineral interactions, and highlight the chemical element distribution in the black deposits. 16S rRNA gene metabarcoding allowed the study of Archaea and Bacteria communities. Mn-oxide-solubilizing isolates were also obtained and characterized by culturable and molecular approaches. The multidisciplinary approach showed the occurrence of deposits composed of birnessite, diopside, halloysite, and leucite. Numerous bacterial imprints confirmed the role of microorganisms in forming these deposits. The Bacteria and Archaea communities associated with these deposits and runoff waters are dynamic and shaped by seasonal changes. The uncultured and unknown taxa are the most common and abundant. These amplicon sequence variants (ASVs) were mainly assigned to Proteobacteria and Bacteroidetes phyla. Six isolates showed interesting Mn solubilization abilities under microaerophilic conditions. Molecular characterization associated isolates to Brevibacterium, Bacillus, Neobacillus, and Rhodococcus genera. The findings enriched our knowledge of geomicrobiological aspects of one of the Earth’s hidden habitats. The study also unveiled the potential of this environment as an isolation source of biotechnologically relevant bacteria.

New insight into ammonium oxidation processes and mechanisms mediated by manganese oxide in constructed wetlands

Manganese oxide (MnOx) mediated ammonium (NH₄⁺) oxidation in wetlands is receiving increased interest; however, the biochemical mechanisms of this process are vague due to only few studies have focused on terrestrial ecosystems. In this study, three subsurface flow constructed wetlands (CWs), high/low content of Mn-sand CW (HMn-CW/LMn-CW) and quartz sand CW (C-CWs), were set up to explore the extent of ammonium nitrogen (NH₄⁺-N) removal and underlying mechanisms. According to the surface characteristics of Mn-sand, MnOx nanospheres were loaded as birnessite on the sand, while changes of the Mn/N contents indicated involvement of Mn-sand in NH₄⁺-N removal. During the 120-day operation, higher extent of NH₄⁺-N removal with decreased nitrous oxide (N₂O) emission was achieved in the HMn-CW (76%) than in the LMn-CW (73%) and C-CW (67%). According to the distribution of nitrogen compounds and Mn²⁺, Mn-sand in the HMn-CW delayed oxidation of NH₄⁺ and production of nitrate and nitrite. High abundance of Zooloea and Psychrobacter was observed in the Mn-sand layer of HMn-CW, corresponding to a higher observed NH₄⁺-N removal. NH₄⁺ oxidation to hydroxylamine and then to nitrite was enhanced in HMn-CW due to ammonia monooxygenase genes being promoted. The decrease of N₂O emission was closely related to the genus TM7a, verified by Pearson correlation analysis. Our findings expand the knowledge of MnOx-mediated NH₄⁺ oxidation in wetlands and support the potential application of manganese oxide for effective nitrogen removal in CWs.

What is the relationship between hypoxia, water chemistry and otolith manganese content?

Previous studies have shown an increase in otolith Mn caused by exposure to hypoxic water masses. The mechanism leading to the increases in otolith Mn is still unclear, but might possibly be due to the larger amount of available Mn left in the water column under hypoxia. Thus, this study aimed to examine the relationship between hypoxia, water Mn and otolith Mn through marbled flounder (Pseudopleuronectes yokohamae, Günther) captured from Tokyo Bay and reared under different water Mn at laboratory. Otoliths from the Bay showed a higher (Mn/Ca)_otolith than outside, together with a seasonal trend of high (Mn/Ca)_otolith at the start of translucent zones (which form in the summer), supporting the occurrence of summer hypoxia in Tokyo Bay. Nonetheless, juveniles reared under control (Mn 0.50 μmol l^-1 ), middle (Mn 6.94 μmol l^-1 ) and high (Mn 10.4 μmol l^-1 ) treatments of water Mn concentrations showed a disproportional smaller increase in (Mn/Ca)_otolith . Comparing the laboratory experiment with the field data, (Mn/Ca)_water under hypoxia in Tokyo Bay could reach a low level similar to control treatment, yet (Mn/Ca)_otolith of the Bay showed a higher value than the high treatment. These results revealed an elevated (Mn/Ca)_otolith towards hypoxia, but also suggested that changes in water Mn might not be directly recorded by otolith Mn.

Influence of Fe2+ and Fe3+ on the Performance and Microbial Community Composition of a MFC Inoculated with Sulfate-Reducing Sludge and Acetate as Electron Donor

A sulfidogenic sludge supplemented with acetate was evaluated in the anodic chamber of microbial fuel cells (MFCs) in the presence of sulfate (SO4-2)/Fe3+ and sulfate (SO4-2)/Fe2+ to investigate the MFC performance and the effect of the iron ions on the composition of the microbial community since sulfate and iron ions are frequently present in wastewater derived from several anthropogenic activities. The current densities were up to 0.025 mA/cm2 and 0.017 mA/cm2 for MFCs with Fe2+ and Fe3+, respectively. Accordingly, the redox activity was slightly higher in the presence of Fe2+ than Fe3+. In general, the metabolic activity of the MFC supplemented with Fe2+ was higher than the system with Fe3+ reaching a percentage of sulfate reduction (% SR), sulfide concentration (mg/L HS-), and removal of chemical oxygen demand (% COD removal) of $35.2\pm 0.75$ , $450.3\pm 3.6$ , and $50.05\pm 0.24$ for % SR, HS-, and % COD, respectively, whereas in the MFC with Fe3+, the percentages were of $30.1\pm 1.076$ , $220.6\pm 2.0$ , and $11.78\pm 10.81$ for % SR, HS-, and % COD, respectively. The microbial population determined in each system was also correlated to the metabolic activity. Rhodospirillales, Caulobacterales, and Burkholderiales were the most abundant orders of bacteria in the MFC with Fe3+, whereas with Fe2+, Rhodobacterales, Sphingomonadales, and Rhizobiales. Desulfohalobiaceae and Desulfovibrionaceae were identified in the presence of Fe2+. Unexpected interactions and combinations of microorganisms were observed in a relatively short culturing time, demonstrating the importance of characterizing the anode biofilm prior to shifts in iron ion concentrations on a long-term basis.

Ultrasound-assisted adsorption on porous ceramic for removal of iron in water

This study proposes the use of an ultrasound-assisted adsorption system coupled to porous ceramic fragments to improve the removal of iron from FeSO₄ aqueous solution. Ultrasound was applied using an ultrasound bath at a low frequency (37 kHz, 330 W). The optimized conditions for Fe removal were achieved by 7 g of adsorbent, 40 min of sonication, 20 mg L^-1 of initial Fe concentration, and 30 °C of reaction temperature. After optimizing the conditions, the method was applied for the removal of iron in groundwater. A central composite design and response surface methodology were used to evaluate the degree to which different variables had a significant effect on iron removal. The efficiency of iron removal using the selected conditions for FeSO₄ solution was near to 100%. However, for groundwater samples, the maximum iron removal efficiencies of the system with and without ultrasound were 80.7% and 51.1%, respectively, indicating that the adsorption with ultrasound was significantly higher than that without ultrasound. It was shown that the proposed ultrasound-assisted adsorption system can be used to enhance the removal of inorganic iron from groundwater.

Microbial Consortia Are Needed to Degrade Soil Pollutants

Soil pollution is one of the most serious environmental problems globally due to the weak self-purification ability, long degradation time, and high cost of cleaning soil pollution. The pollutants in the soil can be transported into the human body through water or dust, causing adverse effects on human health. The latest research has shown that the clean-up of soil pollutants through microbial consortium is a very promising method. This review provides an in-depth discussion on the efficient removal, bio-adsorption, or carbonated precipitation of organic and inorganic pollutants by the microbial consortium, including PAHs, BPS, BPF, crude oil, pyrene, DBP, DOP, TPHP, PHs, butane, DON, TC, Mn, and Cd. In view of the good degradation ability of the consortium compared to single strains, six different synergistic mechanisms and corresponding microorganisms are summarized. The microbial consortium obtains such activities through enhancing synergistic degradation, reducing the accumulation of intermediate products, generating the crude enzyme, and self-regulating, etc. Furthermore, the degradation efficiency of pollutants can be greatly improved by adding chemical materials such as the surfactants Tween 20, Tween 80, and SDS. This review provides insightful information regarding the application of microbial consortia for soil pollutant removal.

Experimental study on treatment of heavy metal-contaminated soil by manganese-oxidizing bacteria

There are many studies on the treatment of heavy metals by manganese-oxidizing bacteria and the reaction is good; the problem of compound pollution of heavy metals in soil has been difficult to solve. In this study, the application of manganese-oxidizing bacteria in soil was studied. The tolerance of manganese-oxidizing strains (Pseudomonas taiwanensis) to environmental conditions and the treatment effect of heavy metals As, Pb, and Cd in aqueous solution were investigated, and the effect of iron-manganese ratio on the treatment effect was discussed. The results showed that the suitable pH conditions for the growth of P. taiwanensis were 5-9, and the salt tolerance was 6% (by sodium chloride). The tolerant concentrations for heavy metals As(V) and Mn(II) were 500 mg L^-1 and 120 mg L^-1, respectively. The strains were enriched by nutrient broth medium. After the logarithmic phase, the bacterial suspension was mixed with ATCC#279 medium at a ratio of 1:10, and a certain amount (10 mg L^-1) of Mn(II) was added. The results of As, Pb, and Cd removal in the composite polluted water phase were 22.09%, 30.75%, and 35.33%, respectively. The molar ratio of manganese and iron affected the removal efficiency of single arsenic, the highest efficiency is 68%, and the ratio of iron to manganese is 1:5. However, when the soil was treated by the same method, the results showed that not all metals were passivated, such as Cu. At the same time, for As, Pb, and Cd, the treatment effects in soil were worse than those in water, perhaps more consideration should be given to environmental conditions, such as soil moisture and temperature, when manganese-oxidizing bacteria are used to treat soil.

Study of the preparation of Maifan stone and SRB immobilized particles and their effect on treatment of acid mine drainage

The problems of acid mine drainage (AMD) in coal mine acidic wastewaters arise from a range of sources, including severe pollution with heavy metals and SO₄²⁻ and difficulties during treatment. Based on the ability of Maifan stone to adsorb heavy metals and the dissimilatory reduction of SO₄²⁻ by sulfate-reducing bacteria (SRB), Maifan stone–sulfate-reducing bacterium-immobilized particles were prepared via immobilization techniques using Shandong Maifan stone as the experimental material. A single factor experiment was used to investigate the influences of the dosage of Maifan stone, the particle size of Maifan stone and the dosage of SRB on the pH improvement effect and the removal rates of SO₄²⁻, Fe²⁺ and Mn²⁺. The Box–Behnken response surface method was used to determine the optimal preparation conditions for the Maifan stone and SRB immobilized particles in accordance with the ion removal rate and pH improvement effect when dealing with AMD. The results show that: (1) the optimal preparation conditions for Maifan stone synergistic SRB immobilized particles are determined by single factor experiment: the dosage of Maifan stone is 5 g, the particle size of Maifan stone is 0.075–0.106 mm, and the dosage of SRB is 25 mL per 100 mL; the removal rates of SO₄²⁻, Fe²⁺ and Mn²⁺ from AMD by the Maifan stone and SRB immobilized particles prepared under these conditions were 92.22%, 95.41% and 86.05%, and the pH was increased from 4.08 to 7.45. (2) From the variance analysis of the response surface model, it can be seen that the model effectively predicts the SO₄²⁻ removal rate, Fe²⁺ removal rate, Mn²⁺ removal rate and pH change. (3) After further optimization using the response surface method, the optimal preparation conditions of Maifan stone and SRB immobilized particles are determined as follows: Maifan stone dosage is 5 g, Maifan stone particle size is 0.075–0.106 mm, and SRB dosage is 25 mL per 100 mL. Through experiments, the removal rates of SO₄²⁻, Fe²⁺ and Mn²⁺ from AMD by the Maifan stone and SRB immobilized particles prepared under these conditions were 92.12%, 95.93% and 87.14%, respectively, and the pH was increased from 4.08 to 7.49.

Based on the ability of Maifan stone to adsorb heavy metals and the dissimilatory reduction of SO₄²⁻ by sulfate-reducing bacteria (SRB), Maifan stone–sulfate-reducing bacterium-immobilized particles were prepared via immobilization techniques.

Inhibition of organosilane/ATP@HQ self-healing passivator for pyrite oxidation

Organosilane, with functional organic groups attached to inorganic silicon atoms, exhibits excellent passivation performance for pyrite. However, a considerable number of micro-cracks will gradually appear on the surface of passivation film under long-term corrosion of oxidizing medium, resulting in a significant decrease of passivation effect. To improve the stability and long-term performance of organosilane coating, a novel passivator (PT-ATP@HQ) with self-healing function was prepared to inhibit the oxidation of pyrite. We chose 3-mercaptopropyltrimethoxysilane (Prop-SH) and tetraethoxysilane (TEOS) as the host coating (PT), and attapulgite clay (ATP) loaded with 8-hydroxyquinoline (8-HQ) was used to endow the coating with better passivation and self-healing performance. The electrochemical and chemical leaching results showed that the addition of ATP@HQ greatly improved the passivation performance of PT coating. The passivation efficiencies of total Fe and SO₄^2- reached to 88.1% and 79.2%, respectively. We also found that the protective capability of the scratched PT-ATP@HQ coating can be recovered automatically through 8-HQ release from ATP. The passivation and self-healing mechanisms were investigated by FT-IR, XPS, ²⁹Si NMR, and other characterization methods, which were as follows: firstly, the organosilanes hydrolyzed to form highly active silanol groups, then dehydration condensation reaction occurred between silanol molecules and ATP@HQ to obtain cross-linked network structure connected by Si-O-Si bonds. After that, Si-OH groups reacted with the hydroxyl groups of pyrite to form Fe-O-Si bonds, thereby an inert and dense passivation film attached to the surface of pyrite. Once the passivation film is locally damaged, 8-HQ will automatically release to repair the cracks.

Manganese Stress Adaptation Mechanisms of Bacillus safensis Strain ST7 From Mine Soil

The mechanism of bacterial adaption to manganese-polluted environments was explored using 50 manganese-tolerant strains of bacteria isolated from soil of the largest manganese mine in China. Efficiency of manganese removal by the isolated strains was investigated using atomic absorption spectrophotometry. Bacillus safensis strain ST7 was the most effective manganese-oxidizing bacteria among the tested isolates, achieving up to 82% removal at a Mn(II) concentration of 2,200 mg/L. Bacteria-mediated manganese oxide precipitates and high motility were observed, and the growth of strain ST7 was inhibited while its biofilm formation was promoted by the presence of Mn(II). In addition, strain ST7 could grow in the presence of high concentrations of Al(III), Cr(VI), and Fe(III). Genome-wide analysis of the gene expression profile of strain ST7 using the RNA-seq method revealed that 2,580 genes were differently expressed under Mn(II) exposure, and there were more downregulated genes (n = 2,021) than upregulated genes (n = 559) induced by Mn stress. KAAS analysis indicated that these differently expressed genes were mainly enriched in material metabolisms, cellular processes, organism systems, and genetic and environmental information processing pathways. A total of twenty-six genes from the transcriptome of strain ST7 were involved in lignocellulosic degradation. Furthermore, after 15 genes were knocked out by homologous recombination technology, it was observed that the transporters, multicopper oxidase, and proteins involved in sporulation and flagellogenesis contributed to the removal of Mn(II) in strain ST7. In summary, B. safensis ST7 adapted to Mn exposure by changing its metabolism, upregulating cation transporters, inhibiting sporulation and flagellogenesis, and activating an alternative stress-related sigB pathway. This bacterial strain could potentially be used to restore soil polluted by multiple heavy metals and is a candidate to support the consolidated bioprocessing community.

Simultaneous sorption of heavy metals on Algerian Bentonite: mechanism study

Industrialization has induced tremendous environmental problems, particularly surface and underground water pollution by harmful heavy metals. In this work, we attempt to eliminate simultaneously iron and cadmium from aqueous solutions by adsorption on Algerian Bentonite. This abundant material was used without any modification. The sorption mechanism was investigated, and effects of pH, initial adsorbate and adsorbent concentrations were studied. The kinetics study revealed that equilibrium was reached after 120 min for both metals, and maximal adsorbed quantities of cadmium (76 mg/g) and iron (55 mg/g ) were obtained at pH = 10 and 8 respectively. Different kinetic models were tested, such as pseudo first-order, pseudo second-order, film and intraparticle diffusion models. The pseudo second-order model fits well the experimental data. Biot number values (2-39) show that the simultaneous sorption process is controlled by film diffusion due to electrostatic interactions between negatively charged surface and cations, then by the internal diffusion and formation of bonds between cations and adsorbent constituents. Also, compared to iron, the high and rapid sorption of cadmium reflects the selective nature of the sorption process. These results show that raw bentonite could be used without further treatment as an efficient sorbent for heavy metal mixtures.

The effects of Fe(III) and Fe(II) on anammox process and the Fe-N metabolism

This study aims to compare the effects of different Fe stress on anammox (anaerobic ammonium oxidation) process, therefore seven identical reactors were operated under different Fe(II)/Fe(III) concentrations. After 38 days of operation, the anammox activity was highest (10.49 ± 0.41 mg-TN/(g-VSS·h)) under conditions of 5 mg/L-Fe(II), while under 30 mg/L-Fe(III) displayed severe inhibition. The results showed that continuous addition of 30 mg/L-Fe(III) would damage the composition of EPS (extracellular polymeric substances) and make anammox bacteria more sensitive to environmental stress. While high Fe(II) concentrations could result in precipitates encasing granular sludge, affecting substrate utilization. Moreover, the results of ΔNO₃^--N/ΔNH₄⁺-N indicated that Fe(II)-dependent nitrate reduction was induced in reactors added with Fe(II). OM27_clade and norank_f__Burkholderiaceae might be candidates for this process according to the correlation of genera and functional genes (based on the PICRUSt 2 functional prediction). Overall, this research is expected to provide new ideas to the effects of Fe(II)/Fe(III) on anammox and to the practical application of coupled system based on anammox in wastewater treatment.

Performance and Mechanisms of PropS-SH/HA Coatings in the Inhibition of Pyrite Oxidation

Acid mine drainage (AMD) entering the environment will cause long-term environmental pollution and ecological damage, the treatment or remediation for which has become a difficult worldwide problem. To control AMD at the source, a novel composite coating, hydroxyapatite (HA) as the filler embedded in a γ-mercaptopropyltrimethoxysilane (PropS-SH) coating, was introduced in this study. The performance and mechanisms of PropS-SH/HA coatings in the inhibition of pyrite oxidation were investigated by chemical leaching testing and material structure characterization. The results of the investigations revealed that the addition of an appropriate amount of HA can enhance the passivation efficiency of the PropS-SH coating. The best coating was obtained from 3% (v/v) of PropS-SH solution with 16 wt % HA, as this coating decreased pyrite oxidation by 78.7% (based on total Fe release). The main mechanism of PropS-SH/HA for the inhibition of pyrite oxidation involved the generation of a PropS-SH network through a polycondensation reaction. The addition of HA increased the stability of the passivation film composed of PropS-SH as well as the combining capacity of PropS-SH/HA through the formation of Si-O-Si and Fe-O-Si bonds, respectively.

The removal of arsenic and metals from highly acidic water in horizontal subsurface flow constructed wetlands with alternative supporting media

A laboratory-scale horizontal subsurface flow constructed wetland system was used to quantify the arsenic removal capacity in the treatment of highly acidic, arsenic and metal-rich water: pH ≈ 2, Fe ≈ 57 mg/L, Pb ≈ 0.9 mg/L, Zn ≈ 12 mg/L. The system was operated in two stages, being As ≈ 2.1 mg/L in stage one, and ≈ 3.7 mg/L in stage 2. Limestone and zeolite were employed as main supporting media to build non-vegetated and vegetated cells with Phragmites australis. The system was very effective in the removal of arsenic and iron (> 96%), and lead (> 94%) throughout the whole experimental period, having the four treatment types a similar performance. The main effect of the media type was on the pH adjustment capacity: limestone cells were able to raise the pH to ≈ 7.1, whereas zeolite cells raised it to ≈ 3.8. The contribution of plant uptake to the overall removal of As, Fe and Zn was minor; accounting for less than 0.02%, 0.07% and 0.7% respectively. As such, pollutants were mainly retained in the wetland beds. Our results suggest that limestone is recommended over zeolite as wetland medium mainly due to its neutralization capacity.

The adsorption of Mn(II) by insolubilized humic acid

The eco-friendly and non-toxic natural organic substance, insolubilized humic acid (IHA), was used to remove Mn(II) from aqueous solutions. The adsorption characteristics were studied through a series of static adsorption tests. The results show that conditions such as the dose, the pH of the solution and the initial concentration of Mn(II) all affect removal efficiency, and the optimal pH value was 5.5. The sorption process for Mn(II) on IHA conforms to the pseudo-second-order adsorption kinetic model and intra-particle diffusion is not the only factor affecting the adsorption rate. Both Langmuir and Freundlich models can describe this adsorption behavior, and the experimental maximum adsorption capacity of IHA was 52.87 mg/g under optimal conditions. The thermodynamic analysis of adsorption shows that the adsorption process is a non-spontaneous endothermic physical reaction. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to characterize the samples, it was found that as IHA successfully adsorbed Mn(II), the surface morphology of IHA changed after the adsorption reaction. The adsorption mechanism for Mn(II) on IHA is to provide electron pairs for carboxyl, phenolic hydroxyl and other functional groups to form stable complexes with Mn(II).