A critical review on bioremediation technologies of metal(loid) tailings: Practice and policy.

Globally, most high-grade ores have already been exploited. Contemporary mining tends to focus on the extraction of lower-grade ores thereby leaving large stored tailings open to the environment. As a result, current mines have emerged as hotspots for the migration of metal(loid)s and resistance genes, thereby potentially contributing to a looming public health crisis. Therefore, the management and remediation of tailings are the most challenging issues in environmental ecology. Bioremediation, a cost-effective solution for the treatment of multi-element mixed pollution (co-contamination), shows promise for the restoration of mine tailings. This review focuses on the bioremediation technologies developed to untangle the issues of non-ferrous metal mine tailings. These technologies address the environmental risks of multi-element exposure to the ecosystem and human health risks. It provides a review and comparison of current bioremediation technologies used to mineralize metal(loid)s. The role of plant-microorganisms and their mechanisms in the remediation of tailings are also discussed. The importance of "treating waste with wastes" is crucial for advancing bioremediation technologies. This approach underscores the potential for waste materials to contribute to environmental cleanup processes. The concept of a circular economy is pertinent in this context, emphasizing recycling and reuse. There's an immediate need for international collaboration. Collaboration is needed in policy-making, funding, and data accessibility. Sharing data is essential for the growth of bioremediation globally.

Considering the bioavailability and bioaccessibility of metal(loid)s for risk assessment of soils affected by different non-ferrous metal activities in Southwest China.

Toxic metal(loid)s released into the soil by non-ferrous metal mining and smelting activities pose a serious threat to residents and the surrounding ecosystem. Considering only total metal(loid) concentrations likely overestimates routine (eco)toxicological risk assessment of soil. We hypothesize that considering metal(loid) bioavailability/accessibility will improve the accuracy of risk assessment. To test this hypothesis, four mining areas in Southwest China, including mining and surrounding sites, were studied. Bioavailability was determined considering metal(loid)s leached by a simulated strong acid rain (SSAR) treatment. In the four areas, the mining site showed higher cumulative releases of metal(loid)s under SSAR treatment than the agricultural field located in the surrounding sites. Thus, the bioavailable metal(loid)s contents were continuously being released during SSAR treatment and likely increased the environmental risk. Ecological and health risk assessment of soil, calculated using total metal(loid)s content, was corrected considering bioavailable/accessible metal(loid)s, which was determined by the heavy metal(loid)s forms and in vitro simulated intestinal stages. Although the corrected indices indicated that the risk of metal(loid)s-contaminated soil was reduced, unfavorable ecological and health risks remained in the four areas. Our study provides new perspectives to better predict the risk of bioavailable/accessible metal(loid)s in non-ferrous metal contaminated and surrounding soils.

Effects of soil metal(loid)s pollution on microbial activities and environmental risks in an abandoned chemical smelting site.

Abandoned chemical smelting sites containing toxic substances can seriously threaten and pose a risk to the surrounding ecological environment. Soil samples were collected from different depths (0 to 13 m) and analyzed for metal(loid)s content and fractionation, as well as microbial activities. The potential ecological risk indices for the different soil depths (ordered from high to low) were: 1 m (D-1) > surface (S-0) > 5 m (D-5) > 13 m (D-13) > 9 m (D-9), ranging between 1840.65-13,089.62, and representing extremely high environmental risks, of which Cd (and probably not arsenic) contributed to the highest environmental risk. A modified combined pollution risk index (MCR) combining total content and mobile proportion of metal(loid)s, and relative toxicities, was used to evaluate the degree of contamination and potential environmental risks. For the near-surface samples (S-0 and D-1 layers), the MCR considered that As, Cd, Pb, Sb, and Zn achieved high and alarming degrees of contamination, whereas Fe, Mn, and Ti were negligible or low to moderate pollution degrees. Combined microcalorimetry and enzymatic activity measurements of contaminated soil samples were used to assess the microbial metabolic activity characteristics. Correlation analysis elucidated the relationship between metal(loid)s exchangeable fraction or content and microbial activity characteristics (p < 0.05). The microbial metabolic activity in the D-1 layer was low presumably due to heavy metal stress. Enzyme activity indicators and microcalorimetric growth rate (k) measurements were considered sensitive indicators to reflect the soil microbial activities in abandoned chemical smelting sites.

Bioindicator responses to extreme conditions: Insights into pH and bioavailable metals under acidic metal environments.

Acid mine drainage (AMD) caused environmental risks from heavy metal pollution, requiring treatment methods such as chemical precipitation and biological treatment. Monitoring and adapting treatment processes was crucial for success, but cost-effective pollution monitoring methods were lacking. Using bioindicators measured through 16S rRNA was a promising method to assess environmental pollution. This study evaluated the effects of AMD on ecological health using the ecological risk index (RI) and the Risk Assessment Code (RAC) indices. Additionally, we also examined how acidic metal stress affected the diversity of bacteria and fungi, as well as their networks. Bioindicators were identified using linear discriminant analysis effect size (LEfSe), Partial least squares regression (PLS-R), and Spearman analyses. The study found that Cd, Cu, Pb, and As pose potential ecological risks in that order. Fungal diversity decreased by 44.88% in AMD-affected areas, more than the 33.61% decrease in bacterial diversity. Microbial diversity was positively correlated with pH (r = 0.88, p = 0.04) and negatively correlated with bioavailable metal concentrations (r = -0.59, p = 0.05). Similarly, microbial diversity was negatively correlated with bioavailable metal concentrations (bio_Cu, bio_Pb, bio_Cd) (r = 0.79, p = 0.03). Acidiferrobacter and Thermoplasmataceae were prevalent in acidic metal environments, while Puia and Chitinophagaceae were identified as biomarker species in the control area (LDA>4). Acidiferrobacter and Thermoplasmataceae were found to be pH-tolerant bioindicators with high reliability (r = 1, P < 0.05, BW > 0.1) through PLS-R and Spearman analysis. Conversely, Puia and Chitinophagaceae were pH-sensitive bioindicators, while Teratosphaeriaceae was a potential bioindicator for Cu-Zn-Cd metal pollution. This study identified bioindicator species for acid and metal pollution in AMD habitats. This study outlined the focus of biological monitoring in AMD acidic stress environments, including extreme pH, heavy metal pollutants, and indicator species. It also provided essential information for heavy metal bioremediation, such as the role of omics and the effects of organic matter on metal bioavailability.

Effects of three plant growth-promoting bacterial symbiosis with ryegrass for remediation of Cd, Pb, and Zn soil in a mining area.

The quality of soil containing heavy metals (HMs) around nonferrous metal mining areas is often not favorable for plant growth. Three types of plant growth promoting rhizobacteria (PGPR)-assisted ryegrass were examined here to treat Cd, Pb, and Zn contaminated soil collected from a nonferrous metal smelting facility. The effects of PGPR-assisted plants on soil quality, plant growth, and the migration and transformation of HMs were evaluated. Results showed that inter-root inoculation of PGPR to ryegrass increased soil redox potential, urease, sucrase and acid phosphatase activities, microbial calorimetry, and bioavailable P, Si, and K content. Inoculation with PGPR also increased aboveground parts and root length, P, Si, and K contents, and antioxidant enzyme activities. The most significant effect was that the simultaneous inoculation of all three PGPRs increased the ryegrass extraction (%) of Cd (59.04-79.02), Pb (105.56-157.13), and Zn (27.71-40.79), compared to CK control (without fungi). Correspondingly, the inter-root soil contents (%) of total Cd (39.94-57.52), Pb (37.59-42.17), and Zn (34.05-37.28) were decreased compared to the CK1 control (without fungi and plants), whereas their bioavailability was increased. Results suggest that PGPR can improve soil quality in mining areas, promote plant growth, transform the fraction of HMs in soil, and increase the extraction of Cd, Pb, and Zn by ryegrass. PGPR is a promising microbe-assisted phytoremediation strategy that can promote the re-greening of vegetation in the mining area while remediating HMs pollution.

Physiological and biochemical responses of Leersia hexandra Swartz to nickel stress: Insights into antioxidant defense mechanisms and metal detoxification strategies.

Phytoremediation is widely considered as a cost-effective method for managing heavy metal soil pollution. Leersia hexandra Swartz shows a promising potential for the remediation of heavy metals pollution, including chromium (Cr), copper (Cu), and nickel (Ni). It is vital to understand the physiological and biochemical responses of L. hexandra to Ni stress to elucidate the mechanisms underlying Ni tolerance and accumulation. Here, we examined the metabolic and transcriptomic responses of L. hexandra exposed to 40 mg/L Ni for 24 h and 14 d. After 24-h Ni stress, gene expression of glutathione metabolic cycle (GSTF1, GSTU1 and MDAR4) and superoxide dismutase (SODCC2) was significantly increased in plant leaves. Furthermore, after 14-d Ni stress, the ascorbate peroxidase (APX7), superoxide dismutase (SODCP and SOD1), and catalase (CAT) gene expression was significantly upregulated, but that of glutathione metabolic cycle (EMB2360, GSTU1, GSTU6, GSH2, GPX6, and MDAR2) was downregulated. After 24-h Ni stress, the differentially expressed metabolites (DEMs) were mainly flavonoids (45%) and flavones (20%). However, after 14-d Ni stress, the DEMs were mainly carbohydrates and their derivatives (34%), amino acids and derivatives (15%), and organic acids and derivatives (8%). Results suggest that L. hexandra adopt distinct time-dependent antioxidant and metal detoxification strategies likely associated with intracellular reduction-oxidation balance. Novel insights into the molecular mechanisms responsible for Ni tolerance in plants are presented.

Unraveling assemblage of microbial community dwelling in Dabaoshan As/Pb/Zn mine-impacted area: A typical mountain mining area of South China.

Microbial community assemblage includes microorganisms from the three domains including Bacteria, Archaea, and Eukarya (Fungi), which play a crucial role in geochemical cycles of metal(loid)s in mine tailings. Mine tailings harbor vast proportions of metal(loid)s, representing a unique source of co-contamination of metal(loid)s that threaten the environment. The elucidation of the assembly patterns of microbial communities in mining-impacted ecospheres has received little attention. To decipher the microbial community assembly processes, the microbial communities from the five sites of the Dabaoshan mine-impacted area were profiled by the MiSeq sequencing of 16S rRNA (Bacteria and Archaea) genes and internal transcribed spacers (Fungi). Results indicated that the coexistence of 31 bacterial, 10 fungal, and 3 archaeal phyla, were mainly dominated by Mucilaginibacter, Cladophialophora, and Candidatus Nitrosotalea, respectively. The distribution of microorganisms was controlled by deterministic processes. The combination of Cu, Pb, and Sb was the main factor explaining the structure of microbial communities. Functional predicting analysis of bacteria and archaea based on the phylogenetic investigation of communities by reconstruction of unobserved states analyses revealed that the metabolic pathways related to arsenite transporter, arsenate reductase, and FeS cluster were important for metal detoxification. Furthermore, the ecological guilds (pathogens, symbiotrophs, and saprotrophs) of fungal communities explained 44.5 % of functional prediction. In addition, metal-induced oxidative stress may be alleviated by antioxidant enzymes of fungi communities, such as catalase. Such information provides new insights into the microbial assembly rules in co-contaminated sites.

Contrasting response strategies of sulfate-reducing bacteria in a microbial consortium to As3+ stress under anaerobic and aerobic environments.

The sulfate-reducing efficiency of sulfate-reducing bacteria (SRB) is strongly influenced by the presence of oxygen, but little is known about the oxygen tolerance mechanism of SRB and the effect of oxygen on the metalliferous immobilization by SRB. The performance evaluation, identification of bioprecipitates, and microbial and metabolic process analyses were used here to investigate the As3+ immobilization mechanisms and survival strategies of the SRB1 consortium under different oxygen-containing environments. Results indicated that the sulfate reduction efficiency was significantly decreased under aerobic (47.37%) compared with anaerobic conditions (66.72%). SEM analysis showed that under anaerobic and aerobic conditions, the morphologies of mineral particles were different, whereas XRD and XPS analyses showed that the most of As3+ bioprecipitates under both conditions were arsenic minerals such as AsS and As4S4. The abundances of Clostridium_sensu_stricto_1, Desulfovibrio, and Thiomonas anaerobic bacteria were significantly higher under anaerobic than aerobic conditions, whereas the aerobic Pseudomonas showed an opposite trend. Network analysis revealed that Desulfovibrio was positively correlated with Pseudomonas. Metabolic process analysis confirmed that under aerobic conditions the SRB1 consortium generated additional extracellular polymeric substances (rich in functionalities such as Fe-O, SO, CO, and -OH) and the anti-oxidative enzyme superoxide dismutase to resist As3+ stress and oxygen toxicity. New insights are provided here into the oxygen tolerance and detoxification mechanism of SRB and provide a basis for the future remediation of heavy metal(loid)-contaminated environments.

Mining-related multi-resistance genes in sulfate-reducing bacteria treatment of typical karst nonferrous metal(loid) mine tailings in China.

Management of tailings at metal mine smelter sites can reduce the potential hazards associated with exposure to toxic metal(loid)s and residual organic flotation reagents. In addition, microbes in the tailings harboring multi-resistance genes (e.g., tolerance to multiple antimicrobial agents) can cause high rates of morbidity and global economic problems. The potential co-selection mechanisms of antibiotic resistance genes (ARGs) and metal(loid) resistance genes (MRGs) during tailings sulfate-reducing bacteria (SRB) treatment have been poorly investigated. Samples were collected from a nonferrous metal mine tailing site treated with an established SRB protocol and were analyzed for selected geochemical properties and high throughput sequencing of 16S rRNA gene barcoding. Based on the shotgun metagenomic analysis, the bacterial domain was dominant in nonferrous metal(loid)-rich tailings treated with SRB for 12 months. KEGGs related to ARGs and MRGs were detected. Thiobacillus and Sphingomonas were the main genera carrying the bacA and mexEF resistance operons, along with Sulfuricella which were also found as the main genera carrying MRGs. The SRB treatment may mediate the distribution of numerous resistance genes. KOs based on the metagenomic database indicated that ARGs (mexNW, merD, sul, and bla) and MRGs (czcABCR and copRS genes) were found on the same contig. The SRB strains (Desulfosporosinus and Desulfotomaculum), and the acidophilic strain Acidiphilium significantly contributed to the distribution of sul genes. The functional metabolic pathways related to siderophores metabolism were largely from anaerobic genera of Streptomyces and Microbacterium. The presence of arsenate reductase, metal efflux pump, and Fe transport genes indicated that SRB treatment plays a key role in the metal(loid)s transformation. Overall, our findings show that bio-treatment is an effective tool for managing ARGs/MRGs and metals in tailings that contain numerous metal(loid) contaminants.

Microbial metabolic activity in metal(loid)s contaminated sites impacted by different non-ferrous metal activities.

Many non-ferrous metal mining and smelting activities have caused severe metal(loid) contamination in the local soil environment. The metabolic activity of soil microorganisms in four areas affected by different metallurgical activities (production vs. waste disposal) was characterized using a contamination gradient from the contaminated site to the surrounding soils. Results indicated that the soil microcalorimetric and enzyme activities were correlated with the fractionated metal(loid) properties (p < 0.05). All four areas had high total As, Cd, Pb, Sb, and Zn concentrations, of which mobile As, Cu, Ni, Pb, Sb, and Zn were higher in the contaminated sites than the surrounding sites, reflecting an elevated environmental risk. Three contaminated site areas had lower microbial activities than their surrounding sites suggesting that high metal(loid) concentrations inhibited soil microbial communities. Interestingly, the fourth area (tailing pond) showed an opposite trend (i.e., increased microbial activity in contaminated vs. surrounding areas). The microbial thermodynamic parameters of this contaminated site were higher than its surrounding sites, suggesting that the selected microbial communities can develop a functional resistance to metal(loid)s stress. This study provides a theoretical basis for ecological prevention and control of metal-polluted areas.

High-resolution imaging of O2 dynamics and metal solubilization in the rhizosphere of the hyperaccumulator Leersia hexandra Swartz.

The mobilization of trace metals in the rhizosphere can be affected by the redox potential, which is closely related to the O2 dynamics. This study examined the distributions of O2 and trace metals in the rhizosphere of the subaquatic hyperaccumulator Leersia hexandra Swartz under chromium (Cr) stress using planar optodes and the diffusive gradients in thin films technique coupled with laser ablation inductively coupled plasma mass spectrometry. The O2 concentrations and oxidized areas in the rhizosphere significantly increased with increases in the light intensity, air humidity, and atmospheric CO2 concentrations (p < 0.05). The O2 concentration first increased with increasing ambient temperatures, then decreased when the temperature increased from 25 to 32 â„ƒ. The O2 concentration in the rhizosphere was significantly decreased under Cr stress (p < 0.05), with a prolonged response time to the altered ambient temperature. Cr stress led to decreased mobilities of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, V, W, and Zn in the rhizosphere, which were negatively correlated with the concentrations of O2. These results provide new insights into the role of changes in the O2 concentration induced by the roots of hyperaccumulator plants in controlling the mobility of trace metals in soils.

New insights on the effect of non-ferrous metal mining and smelting activities on microbial activity characteristics and bacterial community structure.

Mining and smelting activities have brought potentially serious heavy metal(loid)s pollution to their surrounding locale. However, studies on microbial metabolic activities, community structure, and adaptation in soils proximal to non-ferrous metal mining and smelting areas are still lacking. Here the effects of biotic and abiotic characteristics of soil taken from sites surrounding inactive and active non-ferrous metal mine smelting facilities on microbial enzyme activity, microcalorimetry, and high-throughput sequencing of 16S rRNA gene barcoding were studied. Data indicated that the soils were heavily polluted by toxic metal(loid)s, of which As and Cd were the main contaminants. Microbial acid phosphatase activity and microcalorimetric total heat value were sensitive metabolic indicators in the studied areas. Actinobacteriota had the highest relative abundance, followed by Proteobacteria, Chloroflexi, and Acidobacteria. Microbial metabolic activity, bacterial community structure and phenotype varied between inactive and active sites (p < 0.05). Such analyses indicated that electrical conductivity, total As, Cu, and Mn contents, and bioavailable As, Cu, Cd, and Mn concentrations were key factors determining microbial activities, bacterial community structure, and phenotypes. Knowledge of microbial adaptation to heavy metal stressors is important for better understanding the aerial transfer of fugitive heavy metal(loid)s (and possibly microbes) and for designing future strategies for improved soil bioremediation.

Effect of air pollution-controlled residue of a sewage sludge incinerator on the drying shrinkage and the pore structure of alkali-activated materials.

Recycling air pollution-controlled residues (APCR) generated from sewage sludge incinerators can be used for waste management, but the leaching of potentially toxic heavy metals from APCR poses environmental and human health issues. The present paper describes a procedure using APCR to produce alkali-activated materials and thereby realize their disposal. The effect of APCR on the compressive strength and drying shrinkage of the alkali-activated slag/glass powder was investigated. The pore structure characteristics were analyzed for clarifying its relationship with drying shrinkage. The results indicated that the drying shrinkage of the alkali-activated material was related to the mesopore volume. The drying shrinkage was slightly increased after the incorporation of the 10 % APCR, which was likely attributed to the high volume of mesopores compared to the 20 % APCR that lowered the drying shrinkage and compressive strength. This decrease in drying shrinkage was due to the recrystallization of sodium sulfate in the pore solution that can act as expansive agents and aggregates. The growth stress of the crystalline sodium sulfate within the matrix can offset the tension stress caused by the water loss. In addition, leaching studies using the SW-846 Method 1311 showed that recycling APCR into the alkali-activated system did not present a toxicity leaching risk or release unacceptable concentrations of heavy metals. The incorporation of waste APCR and waste glass can make AAMs a very promising and safe environmental technology.

Assembly processes of bacterial and fungal communities in metal(loid)s smelter soil.

There are few studies on concurrent bacterial and fungal community assembly processes that govern the metal(loid)s biogeochemical cycles at smelters. Here, a systematic investigation combined geochemical characterization, co-occurrence patterns, and assembly mechanisms of bacterial and fungal communities inhabiting soils around an abandoned arsenic smelter. Acidobacteriota, Actinobacteriota, Chloroflexi, and Pseudomonadota were dominant in bacterial communities, whereas Ascomycota and Basidiomycota dominated fungal communities. The random forest model indicated the bioavailable fractions of Fe (9.58%) were the main positive factor driving the beta diversity of bacterial communities, and the total N (8.09%) was the main negative factor for fungal communities. Microbe-contaminant interactions demonstrate the positive impact of the bioavailable fractions of certain metal(loid)s on bacteria (Comamonadaceae and Rhodocyclaceae) and fungi (Meruliaceae and Pleosporaceae). The fungal co-occurrence networks exhibited more connectivity and complexity than the bacterial networks. The keystone taxa were identified in bacterial (including Diplorickettsiaceae, norank_o_Candidatus_Woesebacteria, norank_o_norank_c_AT-s3-28, norank_o_norank_c_bacteriap25, and Phycisphaeraceae) and fungal (including Biatriosporaceae, Ganodermataceae, Peniophoraceae, Phaeosphaeriaceae, Polyporaceae, Teichosporaceae, Trichomeriaceae, Wrightoporiaceae, and Xylariaceae) communities. Meanwhile, community assembly analysis revealed that deterministic processes dominated the microbial community assemblies, which were highly impacted by pH, total N, and total and bioavailable metal(loid) content. This study provides helpful information to develop bioremediation strategies for the mitigation of metal(loid)s-polluted soils.

Quantifying ecological and human health risks of metal(loid)s pollution from non-ferrous metal mining and smelting activities in Southwest China.

The environmental release and transfer of heavy metal(loids) from natural and anthropogenic sources to neighboring habitats can pose an ecological threat to the exposed biota and habitat, as well as a human health risk to the residents. However, analytical tools to identify the potential contamination source(s) and assess the impact of this transfer have not been well described. Soil samples were collected from affected areas proximal to non-ferrous metal(loid)s mining and smelting facilities. Two integrated assessment methods, based on soil total metal(loid) content, included: (1) the potential ecological risk index combined with positive matrix factorization (PMF) and (2) human health risk assessment combined with PMF. Results indicated that there were four generic sources of pollution (based on PMF analyses of 115 replicated samples collected from four study areas): agricultural and industrial activities, traffic emissions, and natural sources. For ecological risk, the contribution of these metal(loid)s pollution sources were industrial activities (20.34-70.76 %), traffic emissions (18.73-56.93 %), natural sources (3.69-27.02 %), and agricultural activities (3.79-21.43 %). Health risks were higher for children than for adults. Industrial activity was a major source of non-carcinogenic risk to children (32.10-74.62 %) and adults (31.33-73.78 %), and carcinogenic risk to children (22.53-67.27 %) and adults (20.69-64.76 %). Total metal analysis indicated that As and Cd were highly enriched in the soil, but chemical fractionation revealed low As mobility. Total Cd and possibly As were the main pollutants causing the ecological risks at these contaminated sites. This study demonstrates that ecological and human health risks could be quantified to prioritize the pollution sources for reasonable contaminated site risk management.

Integrated transcriptome and metabolome analysis reveals the mechanism of tolerance to manganese and cadmium toxicity in the Mn/Cd hyperaccumulator Celosia argentea Linn.

Understanding the molecular mechanism of tolerance to heavy metals in hyperaccumulators is important for improving the efficiency of phytoremediation and is interesting for evolutionary studies on plant adaption to abiotic stress. Celosia argentea Linn. was recently discovered to hyperaccumulate both manganese (Mn) and cadmium (Cd). However, the molecular mechanisms underlying Mn and Cd detoxification in C. argentea are poorly understood. Laboratory studies were conducted using C. argentea seedlings exposed to 360 µM Mn and 8.9 µM Cd hydroponic solutions. Plant leaves were analyzed using transcriptional and metabolomic techniques. A total of 3960 differentially expressed genes (DEGs) in plants were identified under Cd stress, among which 17 were associated with metal transport, and 10 belonged to the ATP transporter families. Exposures to Mn or Cd led to the differential expression of three metal transport genes (HMA3, ABCC15, and ATPase 4). In addition, 33 and 77 differentially expressed metabolites (DEMs) were identified under Mn and Cd stresses, respectively. Metabolic pathway analysis showed that the ABC transporter pathway was the most affected in Mn/Cd exposed seedlings. Conjoint transcriptome and metabolome analysis showed that the glutathione (GSH) metabolic pathway was over-represented in the KEGG pathway of both DEGs and DEMs. Our results confirm that the ABC transporter and GSH metabolic pathways play important roles in Mn and Cd detoxification. These findings provide new insight into the molecular mechanisms of tolerance to Mn and Cd toxicity in plants.

Novel microbial consortia facilitate metalliferous immobilization in non-ferrous metal(loid)s contaminated smelter soil: Efficiency and mechanisms.

Exposure to toxic metals from nonferrous metal(loid) smelter soils can pose serious threats to the surrounding ecosystems, crop production, and human health. Bioremediation using microorganisms is a promising strategy for treating metal(loid)-contaminated soils. Here, a native microbial consortium with sulfate-reducing function (SRB1) enriched from smelter soils can tolerate exposures to mixtures of heavy metal(loid)s (e.g., As and Pb) or various organic flotation reagents (e.g., ethylthionocarbamate). The addition of Fe2+ greatly increased As3+ immobilization compared to treatment without Fe2+, with the immobilization efficiencies of 81.0% and 58.9%, respectively. Scanning electronic microscopy-energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed that the As3+ immobilizing activity was related to the formation of arsenic sulfides (AsS, As4S4, and As2S3) and sorption/co-precipitation of pyrite (FeS2). High-throughput 16S rRNA gene sequencing of SRB1 suggests that members of Clostridium, Desulfosporosinus, and Desulfovibrio genera play an important role in maintaining and stabilizing As3+ immobilization activity. Metal(loid)s immobilizing activity of SRB1 was not observed at high and toxic total exposure concentrations (220-1181 mg As/kg or 63-222 mg Pb/kg). However, at lower concentrations, SRB1 treatment decreased bioavailable fractions of As (9.0%) and Pb (28.6%) compared to without treatment. Results indicate that enriched native SRB1 consortia exhibited metal(loid) transformation capacities under non-toxic concentrations of metal(loid)s for future bioremediation strategies to decrease mixed metal(loid)s exposure from smelter polluted soils.

Transcriptomic response of Pseudomonas nicosulfuronedens LAM1902 to the sulfonylurea herbicide nicosulfuron.

The overuse of the herbicide nicosulfuron has become a global environmental concern. As a potential bioremediation technology, the microbial degradation of nicosulfuron shows much promise; however, the mechanism by which microorganisms respond to nicosulfuron exposure requires further study. An isolated soil-borne bacteria Pseudomonas nicosulfuronedens LAM1902 displaying nicosulfuron, chlorimuron-ethyl, and cinosulfuron degradabilities in the presence of glucose, was used to determine the transcriptional responses to nicosulfuron exposure. RNA-Seq results indicated that 1102 differentially expressed genes (DEGs) were up-regulated and 702 down-regulated under nicosulfuron stress. DEGs were significantly enriched in "ABC transporters", "sulfur metabolism", and "ribosome" pathways (p ≤ 0.05). Several pathways (glycolysis and pentose phosphate pathways, a two-component regulation system, as well as in bacterial chemotaxis metabolisms) were affected by nicosulfuron exposure. Surprisingly, nicosulfuron exposure showed positive effects on the production of oxalic acid that is synthesized by genes encoding glycolate oxidase through the glyoxylate cycle pathway. The results suggest that P. nicosulfuronedens LAM1902 adopt acid metabolites production strategies in response to nicosulfuron, with concomitant nicosulfuron degradation. Data indicates that glucose metabolism is required during the degradation and adaptation of strain LAM1902 to nicosulfuron stress. The present studies provide a glimpse at the molecular response of microorganisms to sulfonylurea pesticide toxicity and a potential framework for future mechanistic studies.

Advances in the use of recycled non-ferrous slag as a resource for non-ferrous metal mine site remediation.

The growing global demand for non-ferrous metals has led to serious environmental issues involving uncovered mine site slag dumps that threaten the surrounding soils, surface waters, groundwater, and the atmosphere. Remediation of these slags using substitute cement materials for ordinary Portland cement (OPC) and precursors for alkali-activated materials (AAMs) can convert hazardous solid wastes into valuable construction materials, as well as to attain the desired solidification and stabilization (S/S) of heavy metal(loid)s (HM). This review discusses the current research on the effect of non-ferrous slags on the reaction mechanisms of the OPC and AAM. The S/S of HM from the non-ferrous slags in AAM and OPC is also reviewed. HM can be stabilized in these materials based on the complex salt effect and isomorphic effects. The major challenges faced in AAMs and OPC for HM stabilization include the long-term durability of the matrix (e.g., sulfate attack, stability of volume). The existing knowledge gaps and future trends for the sustainable application of non-ferrous slags are also discussed.

Evidence that nano-TiO2 induces acute cytotoxicity to the agronomically beneficial nitrogen-fixing bacteria Sinorhizobium meliloti.

When nano-sized titanium dioxide (nano-TiO2) absorbs ultra-violet (UV-A) radiation, it produces reactive oxygen species that can be toxic to bacteria. We used the agronomically beneficial nitrogen-fixing bacterium Sinorhizobium meliloti strain 1021 as a model microorganism to detect nano-TiO2 toxicity. Sinorhizobium meliloti was exposed to aqueous dispersions of micrometer-sized TiO2 (micron-TiO2, 44 µm) or nanometer-sized TiO2 (nano-TiO2, 21 nm) at nominal concentrations of 0, 100, 300, 600, 900, and 1800 mg TiO2/L. There were fewer viable S. meliloti cells after exposure to nano-TiO2 under dark and UV-A light conditions. Nano-TiO2 was more toxic to S. meliloti with UV-A irradiation (100% mortality at 100 mg TiO2/L) than under dark conditions (100% mortality at 900 mg TiO2/L). Micron-TiO2 concentrations less than 300 mg TiO2/L had no effect on S. meliloti viability under dark or UV-A light conditions. Exposure to 600 mg/L or more of micron-TiO2 under UV-A light could also photo-kill S. meliloti cells (100% mortality). Further studies are needed to ascertain whether nano-TiO2 interferes with the growth of N2-fixing microorganisms in realistic agricultural environments.