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Extracellular superoxide radical (O 2 •−) is ubiquitous in microbial environments and has significant implications for pollutant transformation. Microbial extracellular O 2 •− can be produced through multiple pathways, including electron leakage from the respiratory electron transport chain (ETC), NADPH oxidation by the transmembrane NADPH oxidase (NOX), and extracellular reactions. Extracellular O 2 •− significantly influences the geochemical processes of various substances, including toxic metals and refractory organic pollutants. On one hand, extracellular O 2 •− can react with variable-valence metals and detoxify certain highly toxic metals, such as As(III), Cr(VI), and Hg(II). On the other hand, extracellular O 2 •− can directly or indirectly (via Bio-Fenton) degrade many organic pollutants, including a variety of emerging contaminants. In this work, we summarize the production mechanisms of microbial extracellular O 2 •−, review its roles in the transformation of environmental pollutants, and discuss the potential applications, limiting factors, and future research directions in this field. [Display omitted] • ETC and NOX are the main production pathways of microbial extracellular O 2 •−. • Environmental stresses usually increase the levels of microbial extracellular O 2 •−. • Extracellular O 2 •− can oxidize or reduce variable metals (Fe, Mn, Cr, Ag, Sb, etc.). • Extracellular O 2 •− can degrade refractory organic pollutants (plastics, PPCPs, etc.). [ABSTRACT FROM AUTHOR]

Biogenic calcium carbonate (bio-CaCO 3) cementing tailings is an efficient technology to immobilize heavy metals in waste tailings. However, the underlying mechanism of interface cementation has not yet been clearly established, which limits the technological development. In this study, we used advanced techniques, including atomic force microscopy-based Lorentz contact resonance (AFM-LCR) spectroscopy, AFM-based nanoscale infrared (AFM-IR) spectroscopy, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy, to reveal the structural, mechanical, and chemical properties of the interface on the nanoscale. Ureolytic bacteria produced bio-CaCO 3 to fill in pore space and to bind cement tailings particles, which prevented the formation of leachate containing heavy metals. After cementation, a strong 40–300 nm thin interface was formed between the taillings and bio-CaCO 3 particles. Unlike chemically synthesized CaCO 3 , bio-CaCO 3 is strongly negatively charged, which gives it better adhesion ability. Fourier transform infrared (FTIR), AFM-IR, and 29Si ssNMR spectra indicated that the Si–OH and Si–O–Si groups on the silicate surface were converted to deprotonated silanol groups (≡Si–O-) at a high pH and they formed strong chemical bonds of Si–O–Ca on the interface through a Ca ion bridge. In addition, hydrogen bonding with Si–OH also played a role at the cementation interface. These findings provide the nano-scale interfacial structure and mechanism of bio-CaCO 3 cementing silicate tailings and accelerate the development of tailings disposal technology. [Display omitted] • Bio-CaCO 3 cementing tailings is an efficient way to control heavy metal pollution in tailings. • An interface with greater mechanical strength was formed between the tailings and bio-CaCO 3. • A strong chemical bond Si–O–Ca was an important chemical structure on the cemented interface. • Intermolecular forces and hydrogen bonding also played important roles during cementation. [ABSTRACT FROM AUTHOR]

Uranium (U) contamination often occurs in the topsoil (arable layer), and is a serious threat to crop growth. However, conventional microbial reduction methods are sensitive to oxygen and cannot be used to treat aerobic topsoils. In this study, phosphate-solubilizing microorganisms (PSM) were isolated from U-contaminated topsoil and used for soil remediation. Microbial metabolites and products were analyzed, and the pathways and mechanisms of PSM immobilization were revealed. The results showed that strain PSM8 had the highest phosphate-solubilizing capacity (dissolved P was 208 ± 5 mg/L) and the highest U removal rate (97.3 ± 0.1%). Multi-technical analyses indicated that bacterial surface functional groups adsorbed (UO 2)2+ ions on the cell surface, glycolysis produced 3–10 mg/L of lactic acid (pH 4.7–6.0), and lactic acid solubilized Ca 3 (PO 4) 2 to form stable chernikovite (a type of uranyl phosphate) on the cell surface. The coupled application of Ca 3 (PO 4) 2 and strain PSM8 significantly reduced the bioavailability of soil U (62 ± 11%), converting U from the exchangeable to the residual phase and P from the steady to the available form. In addition, pot experiments showed that soil remediation promoted crop growth and significantly reduced U uptake and toxicity to photosynthetic systems. These findings demonstrate that PSM and Ca 3 (PO 4) 2 are good coupled fertilizers for U-contaminated agricultural soil. [Display omitted] • Ca 3 (PO 4) 2 stimulated PSM8 producing lactic acid which lowered solution pH < 5.0. • Ca 3 (PO 4) 2 was recrystallized to stable chernikovite in a "selective pH range" of 3.0–6.0. • Uranium immobilization rate reached 97.3 ± 0.1% in solution and 62 ± 11% in soil. • Soil remediation twice promoted pakchoi growth and reduced U accumulation by > 50%. [ABSTRACT FROM AUTHOR]

Metal and mineral transformations are a cornerstone of geomicrobiological processes. Of central importance are the mechanisms by which microorganisms alter the speciation and mobility of metals, including their properties of mineral dissolution or mineral formation. Such processes underpin natural biogeochemical cycles for metals, and related elements such as metalloids, and metal radionuclides, as well as those elements that may be commonly associated with metal compounds and minerals, e.g., P and S. They may also determine the fate of contaminant toxic metals and radionuclides when introduced or redistributed in the environment as a result of natural or anthropogenic activities. Such contamination is an increasing problem, and microbe-based systems are increasingly viewed as potentially useful approaches for bioremediation, as well as for element biorecovery. Biomineralization is a general term for the processes by which living organisms form minerals, and this can result in metal removal from solution providing a means of detoxification as well as biorecovery. Dead biota and derived products may also provide a template for mineral deposition, the reversibility, or otherwise, of the reaction depending on physico-chemical conditions. The most common biominerals precipitated by microbes include oxides, phosphates, sulfides and oxalates, and these can have special chemical properties such as high metal sorption capacities and redox catalysis. Furthermore, some biominerals may be deposited in nanoscale dimensions, providing further significant physical, chemical and biological properties of applied significance. The purpose of this Special Issue of theGeomicrobiology JournalonBiomineralization, Bioremediation and Biorecoveryis to bring together and highlight some specific examples of metal and radionuclide biomineralization by microorganisms from the different major microbial groups such as cyanobacteria, bacteria, microalgae and fungi. A range of elements and organisms are covered together with a variety of environmental and industrial contexts where a biomineralization approach may assist bioremediation, metal biorecovery or substrate stability. [ABSTRACT FROM AUTHOR]

Heavy metal contamination of mine tailings is one of the most serious environmental challenges facing the mining industry worldwide. Conventional technologies used for the treatment of such tailing soils are expensive both in terms of operation and capital costs as well as being not so effective. In the present study, an indigenous calcifying urease producing bacterial strain was isolated from copper mine tailing soil and used for bioimmobilization of copper, lead and cadmium in contaminated mine tailing soils. Phylogenetic analysis of the 16S rRNA gene sequence identified it asBacillus firmus. The efficiency of metal bioimmobilization was based on microbially induced calcite precipitation (MICP). A five-stage sequential soil extraction procedure was carried out to obtain distribution patterns for the different metals. The mobility of the toxic metals was found to be significantly reduced in the exchangeable fraction with their concentrations markedly increasing in the carbonated fraction after bioremediation. Scanning electron microscopy showed the precipitation of calcite in association with the bacterial cells. Calcium carbonate minerals such as calcite, gwihabaite and aragonite were identified in the bioremediated tailings soils using X-ray diffraction (XRD). It is concluded that MICP holds considerable promise for the remediation of mine tailing soils by efficient immobilization of toxic metals such as copper, lead and cadmium. [ABSTRACT FROM AUTHOR]

Highlights: [•] Urban street dust samples were collected in Taipei City. [•] Concentrations of Cr, Pb, Hg, Ni, Cd, Zn, Cu, Sr, As and Sb in street dusts were measured. [•] Sources and potential health risks to humans were evaluated. [•] Determinant role of anthropogenic factors in forming street dust were highlighted. [Copyright &y& Elsevier]

Abstract: Chromium (Cr) is a toxic metal that poses a great threat to aquatic ecosystems. Information is limited on coinstantaneous responses of photosystems I (PSI) and II (PSII) to Cr(VI) stress due to lack of instruments that can simultaneously measure PSI and PSII activities. In the present study, responses of quantum yields of energy conversion and electron transport rates of PSI and PSII in Microcystis aeruginosa cells to Cr(VI) stress were simultaneously analyzed by a DUAL-PAM-100 system. Quantum yield of cyclic electron flow (CEF) under Cr(VI) stress and its physiological role in alleviating toxicity of Cr(VI) were also analyzed. At 5mgL−1 Cr(VI), quantum yield and electron transport rate of PSII decreased significantly, and light-induced non-photochemical fluorescence quenching lost. Cr(VI) also inhibited efficiency of PSII to use energy under high light more than of PSI. PSII showed lower maximal electron transport rate and light adaptability than PSI. Electron transport rate of PSI was higher and decreased less than that of PSII, implying less sensitivity of PSI to high light and Cr(VI). Energy dissipation through non-light-induced non-photochemical fluorescence quenching increased with increasing Cr(VI) concentration. CEF was stimulated under Cr(VI) treatment and made a significant contribution to quantum yield and electron transport of PSI, which was essential for protection of PSI from stresses of Cr(VI) and high light. [Copyright &y& Elsevier]

The chitosan–EDTA modified magnetic biochar (E–CMBC) was successfully used as a novel adsorbent to remove heavy metals. The adsorption behaviour and mechanisms of E–CMBC to Cd2+, Cu2+ and Ni2+ were performed in single and binary system in aqueous solutions. In single–metal system, the adsorption process of Cd2+, Cu2+ and Ni2+ on E–CMBC fitted well with the Avrami fractional–order kinetics model and the Langmuir isotherm model. The measured maximum adsorption capacities were 61.08 mg g−1, 48.36 mg g−1 and 41.17 mg g−1 for Cd2+, Cu2+ and Ni2+, respectively. In binary–metal system, coexisting ions have obvious competitive adsorption behaviour on E–CMBC when the concentration of heavy meal beyond 20 mg L−1. The maximum adsorption capacities of the heavy metals were found to be lower than that in single–metal system. The order of the competitive adsorption ability was Cu2+ > Ni2+ > Cd2+. Interestingly, in Cd2+–Cu2+ system the earlier adsorbed Cd2+ could be completely replaced by Cu2+ from the solution. Different competitive adsorption ability of those heavy metal were due to the characteristics of heavy metal and resultant affinity of the adsorption sites on E–CMBC. The adsorption mechanism indicated that chemical adsorption played a dominating role. Therefore, E–CMBC could be a potential adsorbent for wastewater treatment. [ABSTRACT FROM AUTHOR]

Wastewater containing selenium (Se) and soil contaminated by mercury (Hg) are two environmental problems, but they are rarely considered for synergistic treatment. In this work, anaerobic granular sludge (AnGS) was used to address both of the aforementioned issues simultaneously. The performance and mechanisms of Se(IV) removal from wastewater and Hg(II) immobilization in soil were investigated using various technologies. The results of the reactor operation indicated that the AnGS efficiently removed Se from wastewater, with a removal rate of 99.94 ± 0.05%. The microbial communities in the AnGS could rapidly reduce Se(IV) to Se0 nanoparticles (SeNPs). However, the AnGS lost the ability to reduce Se(IV) once the Se0 content reached the saturation value of 5.68 g Se/L. The excess sludge of Se0-rich AnGS was applied to remediate soil contaminated with Hg(II). The Se0-rich AnGS largely decreased the percentage of soil Hg in the mobile, extractable phase, with up to 99.1 ± 0.3% immobilization. Soil Hg(II) and Hg0 can react with Se (-II) and Se0, respectively, to form HgSe. The formation of inert HgSe was an important pathway for immobilizing Hg. Subsequently, the pot experiments indicated that soil remediation using Se0-rich AnGS significantly decreased the Hg content in pea plants. Especially, the content of Hg decreased from 555 ± 100 to 24 ± 3 μg/kg in roots after remediation. In summary, AnGS is an efficient and cost-effective material for synergistically treating Se-contaminated wastewater and Hg-contaminated soil. [Display omitted] • A novel synergistic method was proposed to treat wastewater Se(IV) and soil Hg(II). • AnGS removed 99.9% of Se from wastewater and immobilized 99.1% of Hg in soil. • Red Se0-rich AnGS could decrease Hg accumulation and increase Se content in plants. • Formation of inert HgSe through two pathways facilitated soil Hg(II) immobilization. [ABSTRACT FROM AUTHOR]

Tailings are one of the largest quantities of hazardous waste in the world, and their treatment is difficult and expensive. In this work, a new, low-cost technique coupling microbially induced carbonate precipitation (MICP) and inorganic additives was proposed, optimized, and applied. The results showed that CaO was the best additive among the six tested, with an optimum dosage of 5%. A 90-day experiment indicated that the MICP-CaO coupled technique was highly effective for all the concerned heavy metals (Cu, Ni, Pb, and Cr) in the Cu–Ni tailings. During the stabilization period (20–90 days), the passivation rates were stable at 78.8 ± 2.9% (Cu), 78.1 ± 1.0% (Ni), 89.2 ± 1.0% (Pb), and 97.8 ± 0.5% (Cr), 2%–866% higher than the single technique of either MICP or CaO. Multiple analyses demonstrated that the synergistic effect of MICP and CaO produced a large amount of calcite (1.5% of the tailings). This calcite cemented the tailings particles, sequestrated heavy metal ions into the lattices, and played a key role in heavy metal passivation. Moreover, CaO and MICP improved the strength and compactness of solidified body, respectively. This work demonstrates the feasibility of the MICP-CaO coupled technique in tailings solidification, which can be applied in practical projects. [Display omitted] • A new technique coupling MICP and CaO showed the best results for tailings solidification. • The MICP-CaO technique was long-term effective (>85% for 90 days) for all tested heavy metals. • The synergism of MICP and CaO produced a large number (1.5%) of submicron calcite particles. • Calcite cementation and heavy metal sequestration in lattices were the key ways of passivation. [ABSTRACT FROM AUTHOR]

Application of the conventional bioreactors based on biosorption for removal of heavy metals is limited because they can be broken through by heavy metals. The novel calcifying cyanobacteria reactors based co-precipitation of heavy metals with carbonate minerals induced by cyanobacteria may be a promising alternative technology to overcome the shortcomings of biosorption. In this study, removal of cadmium (Cd) from wastewater using three calcifying bacterium Nostoc calcicola reactors was investigated. N. calcicola can significantly increase aquatic phase pH in the reactor, associated with a decrease of dissolved inorganic carbon (DIC) content in the effluent. During the two-month operation of the bioreactors, over 98% of Cd removal was obtained with an initial Cd 2+ concentration of 2.5 μM. Cd 2+ was mainly sequestrated in organic-bound fraction, followed by a small amount of carbonate and exchangeable fractions in the N. calcicola cell. A crystalline compound of Ca, C, and O, as well as a small amount of Cd on the surface of N. calcicola cell, was observed. The two-month high Cd removal efficiency of the N. calcicola reactors shows the calcifying cyanobacteria reactors have advantages over the conventional biosorption system, which can be shortly broken through for removal of heavy metals. Although the results revealed that calcifying cyanobacterium reactor is a promising way to remove Cd from water, assessment of its longer-term performance of heavy metal removal is needed. [ABSTRACT FROM AUTHOR]

Microbial extracellular polymeric substances (EPS) may flocculate or be decomposed when environmental factors change, which significantly influences nutrient cycling and transport of heavy metals. However, little information is available on the stability of EPS in natural environments. Fluorescence and flocculation properties of EPS from Chroococcus minutus under different irradiation and pH conditions were studied. Two aromatic protein-like fluorescence peaks and one tyrosine protein-like peak were identified from the excitation–emission-matrix (EEM) fluorescence spectra of EPS. UVB (ultraviolet B) and solar irradiation increased the fluorescence intensity of all the three peaks while UVC (ultraviolet C) irradiation had little effect. EPS formed unstable flocs after exposure to UV (ultraviolet) irradiation and formed stable flocs under solar irradiation. EPS were prone to flocculation under highly acidic conditions and minimal fluorescence of peaks was observed. The fluorophores in EPS were relatively stable under neutral and alkaline conditions. These findings are helpful for understanding the behavior of EPS in aquatic environments and their role in biogeochemical cycles of the elements. [ABSTRACT FROM AUTHOR]

As, Sb, and Cr are redox-sensitive and toxic heavy metal(loid)s, and redox reactions are usually involved in the treatment of substrates containing these elements. In this study, manganese-oxidizing aerobic granular sludge (Mn-AGS) was obtained by continuously adding Mn(II) to the sludge in a sequencing batch reactor (SBR). Morphological observations, and analyses of extracellular polymeric substances (EPS), Mn valence-states, and microbial communities were performed on the resulting sludge. After 50 days of cultivation, biogenic Mn(III,IV) oxides (bio-MnO x) accumulated up to approximately 25 mg Mn/g suspended solids (SS). X-ray photoelectron spectroscopy (XPS) revealed that the percentage of Mn(III,IV) was 87.6%. The protein (PN) component in EPS increased from 80.3 to 87.8 mg/g volatile suspended solids (VSS) during cultivation, which might be favorable for sludge granulation and heavy metal(loid) removal. Batch experiments showed that Mn-AGS was better at oxidizing As(III)/Sb(III) into less toxic As(V)/Sb(V) than traditional AGS. Remarkably, the results indicated that Mn-AGS did not oxidize Cr(III) but was able to reduce Cr(VI) into relatively harmless Cr(III). This work provided a new promising method with which to treat As(III), Sb(III), and Cr(VI) in wastewaters. Image 1 • Manganese-oxidizing aerobic granular sludge (Mn-AGS) was successfully obtained. • A high level of biogenic MnO x accumulation was observed in aerobic granules. • Oxidation of As(III)/Sb(III) into less toxic forms was largely improved by Mn-AGS. • Cr(III) was not oxidized but Cr(VI) was reduced into harmless Cr(III) by Mn-AGS. [ABSTRACT FROM AUTHOR]