Effective bioleaching of chromium in tannery sludge with an enriched sulfur-oxidizing bacterial community

Acidithiobacillia class members originating at sites within the Pacific Ring of Fire and other tectonically active locations and description of the novel genus 'Igneacidithiobacillus'

Recent studies have expanded the genomic contours of the Acidithiobacillia, highlighting important lacunae in our comprehension of the phylogenetic space occupied by certain lineages of the class. One such lineage is 'Igneacidithiobacillus', a novel genus-level taxon, represented by 'Igneacidithiobacillus copahuensis' VAN18-1T as its type species, along with two other uncultivated metagenome-assembled genomes (MAGs) originating from geothermally active sites across the Pacific Ring of Fire. In this study, we investigate the genetic and genomic diversity, and the distribution patterns of several uncharacterized Acidithiobacillia class strains and sequence clones, which are ascribed to the same 16S rRNA gene sequence clade. By digging deeper into this data and contributing to novel MAGs emerging from environmental studies in tectonically active locations, the description of this novel genus has been consolidated. Using state-of-the-art genomic taxonomy methods, we added to already recognized taxa, an additional four novel Candidate (Ca.) species, including 'Ca. Igneacidithiobacillus chanchocoensis' (mCHCt20-1TS), 'Igneacidithiobacillus siniensis' (S30A2T), 'Ca. Igneacidithiobacillus taupoensis' (TVZ-G3 TS), and 'Ca. Igneacidithiobacillus waiarikiensis' (TVZ-G4 TS). Analysis of published data on the isolation, enrichment, cultivation, and preliminary microbiological characterization of several of these unassigned or misassigned strains, along with the type species of the genus, plus the recoverable environmental data from metagenomic studies, allowed us to identify habitat preferences of these taxa. Commonalities and lineage-specific adaptations of the seven species of the genus were derived from pangenome analysis and comparative genomic metabolic reconstruction. The findings emerging from this study lay the groundwork for further research on the ecology, evolution, and biotechnological potential of the novel genus 'Igneacidithiobacillus'.

Vanadium extraction from steel slag: Generation, recycling and management

The metal vanadium has superior physical and chemical properties and has a wide range of applications in many fields of modern industry. The increasing demand for vanadium worldwide has led to the need to guarantee sustainable vanadium production. The smelting process of vanadium and titanium magnetite produces vanadium-bearing steel slag, a key material for vanadium extraction. Herein, vanadium production, consumption, and steel slag properties are discussed. A detailed review of methods for extracting vanadium from vanadium-bearing steel slag is presented, including the most commonly used roasting and leaching method, and direct leaching, bioleaching and enhanced leaching methods are also described. Finally, the rules and regulations of steel slag management are introduced. In general, it is necessary to further develop environmentally friendly vanadium extraction methods and technologies from vanadium containing solid wastes. This study provides research directions for the technology of vanadium extraction from steel slag.

Sources, impacts, factors affecting Cr uptake in plants, and mechanisms behind phytoremediation of Cr-contaminated soils

Chromium (Cr) is released into the environment through anthropogenic activities and has gained significant attention in the recent decade as environmental pollution. Its contamination has adverse effects on human health and the environment e.g. decreases soil fertility, alters microbial activity, and reduces plant growth. It can occur in different oxidation states, with Cr(VI) being the most toxic form. Cr contamination is a significant environmental and health issue, and phytoremediation offers a promising technology for remediating Cr-contaminated soils. Globally, over 400 hyperaccumulator plant species from 45 families have been identified which have the potential to remediate Cr-contaminated soils through phytoremediation. Phytoremediation can be achieved through various mechanisms, such as phytoextraction, phytovolatilization, phytodegradation, phytostabilization, phytostimulation, and rhizofiltration. Understanding the sources and impacts of Cr contamination, as well as the factors affecting Cr uptake in plants and remediation techniques such as phytoremediation and mechanisms behind it, is crucial for the development of effective phytoremediation strategies. Overall, phytoremediation offers a cost-effective and sustainable solution to the problem of Cr pollution. Further research is needed to identify plant species that are more efficient at accumulating Cr and to optimize phytoremediation methods for specific environmental conditions. With continued research and development, phytoremediation has the potential to become a widely adopted technique for the remediation of heavy metal-contaminated soils.

Bioleaching performance of vanadium-bearing smelting ash by Acidithiobacillus ferrooxidans for vanadium recovery

The bioleaching process is widely used in the treatment of ores or solid wastes, but little is known about its application in the treatment of vanadium-bearing smelting ash. This study investigated bioleaching of smelting ash with Acidithiobacillus ferrooxidans. The vanadium-bearing smelting ash was first treated with 0.1 M acetate buffer and then leached in the culture of Acidithiobacillus ferrooxidans. Comparison between one-step and two-step leaching process indicated that microbial metabolites could contribute to the bioleaching. The Acidithiobacillus ferrooxidans demonstrated a high vanadium leaching potential, solubilizing 41.9% of vanadium from the smelting ash. The optimal leaching condition was determined, which was 1% pulp density, 10% inoculum volume, an initial pH of 1.8, and 3 Fe²⁺g/L. The compositional analysis showed that the fraction of reducible, oxidizable, and acid-soluble was transferred into the leaching liquor. Therefore, as the alternative to the chemical/physical process, an efficient bioleaching process was proposed to enhance the recovery of vanadium from the vanadium-bearing smelting ash.

The occurrence of "yellowing" phenomenon and its main driving factors after the remediation of chromium (Cr)-contaminated soils: A literature review

Chromium (Cr) is a highly toxic element, which is widely present in environment due to industrial activities. One of most applicable technique to clean up Cr pollution is chemical reduction. However, the Cr(VI) concentration in soil increases again after remediation, and meanwhile the yellow soil would appear, which is commonly called as "yellowing" phenomenon. To date, the reason behind the phenomenon has been disputed for decades. This study aimed to introduce the possible "yellowing" mechanism and the influencing factors based on the extensive literature review. In this work, the concept of "yellowing" phenomenon was explained, and the most potential reasons include the reoxidation of manganese (Mn) oxides and mass transfer were summarized. Based on the reported finding and results, the large area of "yellowing" is likely to be caused by the re-migration of Cr(VI), since it could not sufficiently contact with the reductant under the effects of the mass transfer. In addition, other driving factors also control the occurrence of "yellowing" phenomenon. This review provides valuable reference for the academic peers participating in the Cr-contaminated sites remediation.

A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Impact of physiochemical properties, microbes and biochar on bioavailability of toxic elements in the soil: a review

The pollution of toxic elements (TEs) in the ecosystem exhibits detrimental effects on the human health. In this paper, we debated remediation approaches for TEs polluted soils via immobilization methods employing numerous amendments with reverence to type of soil and metals, and amendment, immobilization competence, fundamental processes and field applicability. We argued the influence of pH, soil organic matter, textural properties, microbes, speciation and biochar on the bioavailability of TEs. All these properties of soil, microbes and biochar are imperative for effective and safe application of these methods in remediation of TEs contamination in the ecosystem. Further, the application of physiochemical properties, microbes and biochar as amendments has significant synergistic impacts not only on absorption of elements but also on diminution of toxic elements.

Quantitative Analysis of the Research Development Status and Trends of Tannery Wastewater Treatment Technology

In order to better grasp the development and trends of tannery wastewater (TWW) treatment research, this paper provides a review of the TWW treatment research dynamics based on the Web of Science (WoS) database and using CiteSpace software. The research dynamics, hot topics, evolutionary history and research trends in this field are revealed. The results showed that research related to TWW treatment has shown a high growth trend in the number of articles in recent years, and India was outstanding in terms of influence in this area. The keyword clustering analysis showed that the main research hotspots in the field of TWW treatment were biological treatment processes (phytoremediation, constructed wetlands, anaerobic treatment and biofilm reactors) and chemical treatment processes (coagulation and flocculation, and advanced oxidation processes). The analysis of new research frontiers showed that the bioremediation and the application of biofuel cells in TWW will become important research directions in the future.

Bioleaching of Heavy Metals by Streptomyces avermitilis - BBA4 Isolate from Coal Mine Soil of Tamil Nadu

Printed Circuit Board (PCB) includes precious metals, heavy metals and brominated flame retardants. The usage of PCB is increasing exponentially, generated in vast quantities, and considered an E-waste contaminating the environment. The bioleaching potential of a novel isolated Streptomyces BBA4 strain from a coal mine soil was studied and discussed here. A total of twelve isolates were isolated from coal mine soil (Neyveli area). The metal-resistant activity of the isolated cultures was examined using a dot plot and agar well diffusion assay. Both assays revealed that the isolate BBA4 showed resistance and survival ability in the copper (Cu) and chromium (Cr)-rich environment up to 1000 mg/kg and in the iron (Fe) and silver (Ag)-rich environment up to 500 mg/kg, respectively. Based on the phenotypic, physiological, biochemical and molecular characteristics, the novel and potential strain (BBA4) isolated from coal mine soil matched with Streptomyces avermitilis. The strain showed bioleaching ability to recover 73.3% Cobalt and 36.4% Chromium within 120 h under pH 5 (in vitro conditions) from the PCB. The heavy metal content in the PCB was analyzed using quantitative analytical techniques, such as Atomic Absorption Spectroscopy (AAS) and Powder X-ray diffraction (XRD). The analysis of XRD peaks exhibited an increase in crystallinity with the Cr, associated with the accumulation of Cr in the bioleached residues. Thus, generated data from this investigation reveal that BBA4 is an effective bioleaching strain on PCB over other environmentally hazardous chemical leaching methods.

A critical review on microbes-based treatment strategies for mitigation of toxic pollutants

Contamination of the environment through toxic pollutants poses a key risk to the environment due to irreversible environmental damage(s). Industrialization and urbanization produced harmful elements such as petrochemicals, agrochemicals, pharmaceuticals, nanomaterials, and herbicides that are intentionally or unintentionally released into the water system, threatening biodiversity, the health of animals, and humans. Heavy metals (HMs) in water, for example, can exist in a variety of forms that are inclined by climate features like the presence of various types of organic matter, pH, water system hardness, transformation, and bioavailability. Biological treatment is an important tool for removing toxic contaminants from the ecosystem, and it has piqued the concern of investigators over the centuries. In situ bioremediation such as biosparging, bioventing, biostimulation, bioaugmentation, and phytoremediation and ex-situ bioremediation includes composting, land farming, biopiles, and bioreactors. In the last few years, scientific understanding of microbial relations with particular chemicals has aided in the protection of the environment. Despite intensive studies being carried out on the mitigation of toxic pollutants, there have been limited efforts performed to discuss the solutions to tackle the limitations and approaches for the remediation of heavy metals holistically. This paper summarizes the risk assessment of HMs on aquatic creatures, the environment, humans, and animals. The content of this paper highlights the principles and limitations of microbial remediation to address the technological challenges. The coming prospect and tasks of evaluating the impact of different treatment skills for pollutant remediation have been reviewed in detail. Moreover, genetically engineered microbes have emerged as powerful bioremediation capabilities with significant potential for expelling toxic elements. With appropriate examples, current challenging issues and boundaries related to the deployment of genetically engineered microbes as bioremediation on polluted soils are emphasized.

Complexation behaviour and removal of organic-Cr(III) complexes from the environment: A review

Chromium (Cr) is mainly found in the form of organic-Cr(III) complexes in the natural environment and industrial waste. The widespread existence of composite contaminants composed of organic matter (OM) and Cr pose a serious ecological threat, and its potential interaction and removal need to be further summarised. Organic ligands, such as carbohydrates, nitrogen compounds, phenolic compounds, humus substances (HS), and low molecular weight organic acids (LMWOAs), play an important role in governing the speciation, mobility, and absorption and desorption of Cr in the environment. Moreover, growing evidence indicates that oxygen-containing functional groups (e.g., carboxyl, hydroxyl, and phosphate) are closely related to the complexation of Cr(III). Advanced oxidation processes (AOPs) are efficient and widely applicable technologies. However, the re-complexation of oxidation intermediates with Cr(III) and the formation and accumulation of much more toxic Cr(VI) species hinder the possible utilisation of AOPs. In this paper, the sources and harmful effects of organic-Cr(III) complexes are reported in detail. The complexation behaviour and structure of the organic-Cr(III) complexes are also described. Subsequently, the application of AOPs in the decomplexation and degradation of organic-Cr(III) complexes is summarised. This review can be helpful for developing technologies that are more efficient for organic-Cr(III) complex removal and establishing the scientific background for reducing Cr discharge Cr into the environment.

Co-inoculation with beneficial microorganisms enhances tannery sludge bioleaching with Acidithiobacillus thiooxidans

Bioleaching of tannery sludge is an efficient and environmentally friendly way for chromium (Cr) removal, which supports the sustainable development of the leather industry. Acidithiobacillus thiooxidans has been reported effective in Cr bioleaching of tannery sludge. However, little is known about whether the presence of other benefiting species could further improve the Cr leaching efficiency of A. thiooxidans. Here, we studied the enhancing roles of four species namely Acidiphilium cryptum, Sulfobacillus acidophilus, Alicyclobacillus cycloheptanicus, and Rhodotorula mucilaginosa in chromium bioleaching of tannery sludge with A. thiooxidans by batch bioleaching experiments. We found that each of the four species facilitated the quick dominance of A. thiooxidans in the bioleaching process and significantly improved the bioleaching performance including bioleaching rate and efficiency. The bioleaching efficiency of Cr in the tannery sludge could reach 100% on the sixth day by co-inoculating A. thiooxidans and four auxiliary species. The achievements shed a light on the role of the community-level interactions on bioleaching and may also serve as guidance for managing bioleaching consortiums for better outcomes.

Indirect bioleaching recovery of valuable metals from electroplating sludge and optimization of various parameters using response surface methodology (RSM)

Electroplating sludge contains amounts of valuable/toxic metals as a typical hazardous solid waste, but existing technology is hard to simultaneously gain the high recovery of valuable metals and its convert into general solid waste. In this study, indirect bioleaching process was optimized by using RSM for high recovery of four valuable metals (Ni, Cu, Zn and Cr) from electroplating sludge and its shift into general waste. The results showed that the maximum leaching rate respectively was 100% for Ni, 96.5% for Cu, 100% for Zn and 76.1% for Cr at the optimal conditions. In particular, bioleaching saw a much better performance than H₂SO₄ leaching in removal of highly toxic Cr (76.1% vs. 30.2%). The extraction efficiency of Cr by H₂SO₄ leaching sharply rose to 72.6% in the presence of 9.0 g/L Fe³⁺, suggesting that Fe³⁺ played an important role in the bioleaching of Cr. Based on bioleaching dynamics analysis, it was speculated that Fe³⁺ passes through the solid shell and enter inside the sludge to attack Cr assisting by extracellular polymeric substances (EPS), leading to high extraction and low residue of Cr. Meanwhile, due to high-efficient release and removal of valuable/toxic metals by bioleaching, the bioleached residues successfully degraded into general based on TCLP test and can be reused as construction material safely.

Extraction of Metals from Polluted Soils by Bioleaching in Relation to Environmental Risk Assessment

Environmental pollution has particular implications for the whole geosystem and increases the global risk to human and ecological health. In this regard, investigations were carried out on soil samples to perform the quality status assessment by determining: pH, texture, structure and metal concentration, as well as carrying out an assessment of anthropogenic activity by determining pollution indices: C_f (contamination factor), C_d (degree of contamination), PLI (pollution load index), E_r (ecological risk index) and PERI (potential ecological risk index). Analyses on soil samples showed high concentrations of metals (Cu: 113-2996 mg kg^-1; Pb: 665-5466 mg kg^-1; Cr: 40-187 mg kg^-1; Ni: 221-1708 mg kg^-1). The metal extraction experiments were carried out by bioleaching using Thiobacillusferrooxidans, microorganisms at different amounts of bioleaching solution (20 mL and 40 mL 9K medium) and a stirring time of up to 12 h. The results on the degree of contamination, pollution loading index PLI (2.03-57.23) and potential ecological risk index PERI (165-2298) indicate that the soils in the studied area have a very high degree of pollution. The decontamination procedure by bioleaching showed a decrease, but at the end of the test (12 h), the followed indices indicate high values, suggesting that bioleaching should continue. The depollution yield after 12 h of treatment is, however, encouraging: Cu 29-76%, Pb: 10-32%, Cr: 39-72% and Ni 44-68%. The use of yield-time correlation equations allows the identification of the optimal exposure time on the bioleaching extraction process to obtain optimal results. The aim of the research is to determine the soil quality, soil environmental risk, extraction of metals from polluted soils by bioleaching and to identify influencing factors in achieving high remediation yields.

Tannery wastewater treatment: conventional and promising processes, an updated 20-year review

Mismanagement of various wastes especially waste water produced by tanning processes has caused serious environmental problems and ultimately impaired human health. Constant efforts have been making to alleviate the pollution of tannery wastewater (TWW), yet terminal treatment still takes dominance. In this review, research on TWW treatment from 2000 to 2021 was summarized, and main methods such as coagulation and flocculation, adsorption, biological treatment, membrane filtration, advanced oxidation process were briefly discussed. More detailed introduction was given to the method of electrochemical treatment since it has excellent performance such as environmental friendliness and high efficiency, hence attracting more and more research attention in recent years. In view of the harsh physi-chemical conditions of TWW, integrated or combined treatment methods are accordingly recommended with better performance and multi-function, however comprehensive studies on optimization of methods combination and cost-effectiveness are needed. The certain issues that the residue Cr in treatment sludge and high salinity in effluent still remain were put forward in this work and potential solutions were provided. Moreover, this review proposed the perspective that realizing multi-function, recycling, and intensification should be the developing direction for future TWW treatment. This review is expected to provide a general guide for researchers who aspire to ameliorate TWW pollution problems and understand various methods utilized in this field.

Graphical abstract

Copper and Zinc Recovery from Sulfide Concentrate by Novel Artificial Microbial Community

Exploring efficient methods to enhance leaching efficiency is critical for bioleaching technology to deal with sulfide concentrate. In our study, a novel artificial microbial community was established to augment the bioleaching efficiency and recovery of copper (Cu) and zinc (Zn). The optimum parameters in bioleaching experiments were explored according to compare a series of conditions from gradient experiments: the pH value was 1.2, temperature was 45 °C, and rotation speed was 160 r/min, which were different with pure microorganism growth conditions. Under optimal conditions, the result of recovery for Cu and Zn indicated that the average leaching rate reached to 80% and 100% respectively, which almost increased 1.8 times and 1.2 times more than control (aseptic condition) group. Therefore, this method of Cu and Zn recovery using a new-type artificial microbial community is expected to be an environmentally-friendly and efficient bioleaching technology solution, which has the potential of large-field engineering application in the future.

Bioleaching of vanadium by Acidithiobacillus ferrooxidans from vanadium-bearing resources: Performance and mechanisms

Bioleaching is promising to meet the demand of strategic vanadium both economically and environmentally. Whereas the combination of bioleaching with traditional techniques is of great interest, little is known on bioleaching of vanadium from abundant vanadium-bearing resources utilized/produced in existing processes. This study investigated the bioleaching of vanadium from vanadium-titanium magnetite, steel slag, and clinker, which are common raw mineral and intermediates used in conventional vanadium extraction process. Clinker had greater leachability by Acidithiobacillus ferrooxidans, compared to vanadium-titanium magnetite and steel slag. Pulp density, inoculum volume, initial pH and initial Fe²⁺ concentration had influencing effects on this bioleaching process. Under optimal condition with 3% pulp density, 10% inoculum volume, initial pH at 1.8, and 3 g/L initial Fe²⁺ concentration, the bioleaching of clinker achieved the maximum vanadium leaching efficiency of 59.0%. Both X-ray fluorescence and energy dispersive spectroscopy analysis confirmed the reduction of vanadium content in the solid residues after leaching. The results of Community Bureau of Reference sequential extraction suggested that vanadium in acid-soluble and oxidizable phase was more easily leachable. This study is helpful to develop sustainable and practical techniques for vanadium extraction from abundant raw materials and step forward in combining bioleaching with traditional process.

Investigating the chromium status, heavy metal contamination, and ecological risk assessment via tannery waste disposal in sub-Saharan Africa (Kenya and South Africa)

To mitigate the negative environmental and public health concerns associated with increased industrial discharges amid the rapid industrial growth, sub-Saharan African countries have instituted several environmental regulations and policies. However, a paradox of good environmental policies but inadequate enforcement exists in most of these countries, with information on the pollution level unknown due to poor monitoring resources. In this study, potentially toxic heavy metal contamination and physicochemical characteristics in effluents and dumpsite soils of two tanneries in Kenya (DB) and South Africa (BO) were evaluated during the dry and rainy season of 2018. Pollution levels and ecological risk in the dumpsite soils were assessed by adopting geoaccumulation index (I_geo), contamination factor (CF), pollution load index (PLI), and ecological risk index (PRI). The results showed that the mean final effluent concentration for total dissolved solids (TDS), chemical oxygen demand (COD), and Cr, Cu, Fe, Ni, Zn, and Cd for BO (2127, 890, 1.82, 1.38, 1.96, 0.60, 1.21, and 1.16, respectively) and DB tanneries (8157, 1369, 7.90, 0.69, 1.05, 0.60, 1.72 mg/L, respectively) were above the limits of emission guidelines. The mean Cr and Cr(VI) concentrations in tannery dumpsite soils (204.9 ± 29.1 and 0.31 ± 0.01 and 943 ± 29.8 and 0.4 ± 0.07 mg/kg for BO and DB, respectively) and Fe (2498 ± 62 mg/kg in DB) exceeded acceptable thresholds of the World Health Organization (WHO), Food and Agriculture Organization (FAO), and local background levels. A positive strong correlation was observed between Cr and organic matter, OM (r > 0.7, p < 0.001), electrical conductivity, EC (r = 0.99, p < 0.05), and As (r = 0.62; p < 0.05), suggesting a common anthropogenic point source. The mean PLI values of 5.3 and 1.6 for DB and BO dumpsites indicated significant pollution of the soils with heavy metals, specifically Cr (I_geo = 18 and 2.4 for DB and BO, respectively). Similarly, PRI values of 174.8 and 57.4 indicated a moderate and low potential ecological risk for DB and BO tannery dumpsite, respectively, with several plants sampled within the two sites exhibiting elevated levels of Cr contamination. In summary, these results provide scientific insights on the need for both improved effluent management and treatment technologies of tannery wastes, coupled with the strengthening of continuous monitoring and enforcement for compliance of industrial discharges in sub-Saharan countries.

Progress, Challenges, and Perspectives of Bioleaching for Recovering Heavy Metals from Mine Tailings

The accumulation of mine tailings on Earth is a serious environmental challenge. The importance for the recovery of heavy metals, together with the economic benefits of precious and base metals, is a strong incentive to develop sustainable methods to recover metals from tailings. Currently, researchers are attempting to improve the efficiency of metal recovery from tailings using bioleaching, a more sustainable method compared to traditional methods. In this work, the research status of using biological leaching technologies to recover heavy metals from tailings was reviewed. Furthermore, CiteSpace 5.7.R2 was used to visually analyze the keywords of relevant studies on biological leaching of tailings to intuitively establish the current research hotspots. We found that current research has made recent progress on influencing factors and microbial genetic data, and innovations have also been made regarding the improvement of the rate of metal leaching by biological leaching combined with other technologies. This is of great significance for the development of bioleaching technologies and industrial production of heavy metals in tailings. Finally, challenges and opportunities for bioleaching provide directions for further research by the scientific community.

A continuous flow membrane bio-reactor releases the feedback inhibition of self-generated free organic carbon on cbb gene transcription of a typical chemoautotrophic bacterium to improve its CO2 fixation efficiency

Since the free organic carbon (FOC) generated by chemoautotrophic bacterium self has a feedback inhibition effect on its growth and carbon fixation, a continuous flow membrane bio-reactor was designed to remove extracellular FOC (EFOC) and release its inhibition effect. The promotion effect of membrane reactor on growth and carbon fixation of typical chemoautotrophic bacterium and its mechanism were studied. The accumulated apparent carbon fixation yield in membrane reactor was 3.24 times that in the control reactor. The EFOC per unit bacteria and cbb gene transcription level in membrane reactor were about 0.41 times and 11.18 times that in control reactor in late stage, respectively. Membrane reactor separated out EFOC, especially the small molecules, which facilitated the release of intracellular FOC, thereby releasing the inhibition of FOC on cbb gene transcription, thus promoting growth and carbon fixation of the typical chemoautotrophic bacterium. This study lays a foundation for enhancing carbon fixation by chemoautotrophic bacteria and expands the application field of membrane reactor.

Advances in microbial remediation for heavy metal treatment: a mini review

Abstract

In recent years, microbiological treatment to remediate contamination by heavy metals has aroused public attention as such pollution has seriously threatens ecosystems and human health and impedes sustainable development. However, the aspect of actual industrial wastewater and solid waste remediation by microorganisms is not explored sufficiently. And what we focus on is technical field of microbial remediation. Therefore, in this review, we discuss and summarize heavy metal treatment via microbiological approaches in different media, including wastewater, solid waste from industrial factories and polluted sites. We also clarify the technical applicability from the perspective of biosorption, bioleaching, biominerization, etc. In particular, the exploration of the combination of microbiological approaches with chemical methods or phytoextraction are scrutinized in this review relative to real waste heavy metal remediation. Furthermore, we highlight the importance of hyperaccumulator endophytes.

Graphical abstract

Influence of Sludge Initial pH on Bioleaching of Excess Sludge to Improve Dewatering Performance

pH has an important effect on the physiological activity of eosinophilic microorganisms. Therefore, this study used excess sludge produced by the mixed treatment of leachate and municipal sewage to explore the impact of different sludge initial pH on microbial biochemical reactions associated with the performance of excess sludge dehydration. Shake-flask tests were performed using inoculated microorganisms and fresh excess sludge in 500 mL Erlenmeyer flasks at a ratio of 1:4, with the addition of 2 g/L S0 and 6 g/L FeS2 as energy sources. Erlenmeyer flasks were shaken for 72 h at 180 rpm and 28 °C, in a reciprocating constant homeothermic oscillating water-bath. Results show that the specific resistance to filtration (SRF) of the bioleached excess sludge decreased from (1.45~6.68) × 1012 m/kg to (1.21~14.30) × 1011 m/kg and the sedimentation rate increased from 69.00~73.00% to 81.70~85.50%. The SRF decreased from 1.45 × 1012 m/kg to 1.21 × 1011 m/kg and the sedimentation rate increased from 69.00% to 85.00%, which both reached the highest level when the initial pH of the excess sludge was 5 and the bioleaching duration was 48 h. At this time, the rates of pH reduction and oxidative redox potential (ORP) reached the highest values (69.67% and 515 mV, respectively). Illumina HiSeq PE250 sequencing results show that the dominate microbial community members were Thiomonas (relative abundance 4.59~5.44%), which oxidize sulfur and ferrous iron, and Halothiobacillus (2.56~3.41%), which oxidizes sulfur. Thus, the acidic environment can promote microbial acidification and oxidation, which can help sludge dewatering. The presence of dominant sulfur oxidation bacteria is the essential reason for the deep dehydration of the bioleached sludge.

Leaching Behaviors of Chromium(III) and Ammonium-Nitrogen from a Tannery Sludge in North China: Comparison of Batch and Column Investigations

Tannery sludge usually has high content of trivalent chromium (Cr(III)) and ammonium-nitrogen (NH4+-N). It is important to make a critical evaluation of the releasing behaviors of Cr(III) and NH4+-N from tannery sludge before its use on improving soil fertility in agricultural applications. For this purpose, static batch and dynamic leaching experiments with different mathematical models were carried out to simulate the Cr(III) and NH4+-N releasing kinetics from tannery sludge sampled in a typical tannery disposal site in North China, and their influencing factors were also discussed. The results showed that a larger solid-liquid ratio, a higher temperature, and a lower pH value of the leaching solution were beneficial for the release of Cr(III) and NH4+-N from the tannery sludge. The release kinetics of Cr(III) and NH4+-N followed parabolic diffusion and simple Elovich models both in the static and dynamic leaching conditions, indicating that the release was a complex heterogeneous diffusion process. The NH4+-N was easy to be leached out and its released amount reached 3.14 mg/g under the dynamic leaching condition (pH 7), whereas the released amount of the Cr(III) was only 0.27 μg/g from the tannery sludge. There was a positive correlation coefficient between dissolved Fe and Cr(III) in the leachate under different leaching conditions, and the calculated average ratio of Fe/Cr(III) concentration was 3.56, indicating that the small amount of the released Cr(III) came from the dissolution of Cr0.25Fe0.75(OH)3 minerals in tannery sludge.

New Plant Growth-Promoting, Chromium-Detoxifying Microbacterium Species Isolated From a Tannery Wastewater: Performance and Genomic Insights

Hexavalent chromium [Cr(VI)], widely generated by tannery activities, is considered among the most toxic substances and causes a serious damage for the environment and for human health. Interestingly, some microorganisms have a potential of bioremediation of chromium-contaminated wastewaters and soils through the reduction of Cr(VI) (soluble and harmful form) into Cr(III) (stable and non-toxic form). Here, we present the full genome sequence of a novel heavy-metal-resistant, plant growth-promoting bacterium (PGPB), Microbacterium metallidurans TL13, which was isolated from a Tunisian leather industry. The strain TL13 was resistant to many heavy metals, such as chromium, copper, nickel, cobalt, and arsenic. The 50% TL13 growth inhibitory concentration (IC₅₀) values of HgCl₂, CoCl₂, K₂Cr₂O₇, CuSO₄, NiCl₂, FeSO₄, and Na₂HAsO₄ are 368, 445, 676, 1,590, 1,680, 4,403, and 7,007 mg/L, respectively, with the following toxicity order: HgCl₂ > CoCl₂ > K₂Cr₂O₇ > CuSO₄ > NiCl₂ > FeSO₄ > Na₂HAsO₄. This new strain was also able to promote the growth of the hybrid tomato (Elika F1) under chromium metal stress. Its whole genome sequence length was estimated to be 3,587,460 bp (3,393 coding sequences) with a G + C content of 70.7%. Functional annotation of the genome of TL13 revealed the presence of open reading frames (ORFs) involved in adaptation to metal stress, such as the chromate transport protein, cobalt–zinc–cadmium resistance protein, copper resistance protein, copper responsive transcriptional regulator, multidrug resistance transporters, arsenical resistance operon repressor, arsenate reductase, arsenic resistance protein, mercuric resistance operon regulatory protein, mercuric ion reductase, and organomercurial lyase. Moreover, genes for the production of glutathione peroxidase, catalase, superoxide dismutase, and thioredoxin reductase, which confer a higher tolerance to oxidative/metal stresses, were identified in TL13 genome. In addition, genes for heat shock tolerance, cold shock tolerance, glycine-betaine production, mineral phosphate solubilization, ammonia assimilation, siderophores, exopolysaccharides, polyketides, and lytic enzymes (cellulase, chitinase, and proteases) production that enable bacteria to survive biotic/abiotic stress and to promote plant growth and health were also revealed. Based on genome analysis and experimental approaches, strain TL13 appears to have evolved from various metabolic strategies and could play a role in ensuring sustainable environmental and agricultural systems.

Isolation and Identification of Chromium Reducing Bacillus Cereus Species from Chromium-Contaminated Soil for the Biological Detoxification of Chromium

Chromium contamination has been an increasing threat to the environment and to human health. Cr(VI) and Cr(III) are the most common states of chromium. However, compared with Cr(III), Cr(VI) is more toxic and more easily absorbed, therefore, it is more harmful to human beings. Thus, the conversion of toxic Cr(VI) into Cr(III) is an accepted strategy for chromium detoxification. Here, we isolated two Bacillus cereus strains with a high chromium tolerance and reduction ability, named B. cereus D and 332, respectively. Both strains demonstrated a strong pH and temperature adaptability and survival under 8 mM Cr(VI). B. cereus D achieved 87.8% Cr(VI) removal in 24 h with an initial 2 mM Cr(VI). Cu(II) was found to increase the removal rate of Cr(VI) significantly. With the addition of 0.4 mM Cu(II), 99.9% of Cr(VI) in the culture was removed by B. cereus 332 in 24 h. This is the highest removal efficiency in the literature that we have seen to date. The immobilization experiments found that sodium alginate with diatomite was the better method for immobilization and B. cereus 332 was more efficient in immobilized cells. Our research provided valuable information and new, highly effective strains for the bioremediation of chromium pollution.

Hydrometallurgical Recovery of Cr(III) from Tannery Waste: Optimization and Selectivity Investigation

Tanneries produce significant quantities of hazardous waste/waters and, according to international environmental organizations, their further recycling/reuse/exploitation in the context of circular economy is considered mandatory. A relevant case study is the recovery of Cr(III), used in large amounts during the tanning process, which can be recovered from the corresponding industrial waste/sludge. The aim of this work is to optimize the Cr(III) hydrometallurgical recovery from the tannery sludge by examining the major experimental conditions that affect the efficiency of its different sequential steps, namely extraction/leaching and precipitation. The chemical characterization revealed that tannery sludge contains high amounts of Cr(III) (14.1%), Ca (14.8%), and organic matter (22%). The extraction of Cr(III) was examined by applying various common acids (H2SO4, HNO3, HCl) in the concentration range of 0.02–2 N. The highest selectivity and efficiency (93%) were obtained by the addition of H2SO4 as the insoluble CaSO4 and soluble CrSO4+ species were formed. Regarding the experimental conditions, only the increase in temperature showed a positive effect on Cr recovery. The Cr(III) can be further precipitated from the resulting leaching solution by increasing the equilibrium pH; the precipitate with the higher purity in Cr(OH)3 (70%) was obtained by the application of NaOH and could possibly be reused by tanneries.

A review of chromite mining in Sukinda Valley of India: impact and potential remediation measures

Sukinda Valley, one of the highly polluted areas of the world is generating tons of mining waste and causing serious health and environmental issues in its surroundings. Several reports are available reporting the severity of hexavalent chromium, yet little efforts have been made to address the pollution and its remediation due to a lack of proper remedial measures. The review highlights the pros and cons of various physical, chemical and biological techniques used worldwide for the treatment of chromium waste and also suggests better and reliable bioremediation measures. Microbes such as Acidophilium and Acidithiobacillus caldus (Bioleaching), Pseudomonas, Micrococcus and Bacillus (Bioreduction), Aereobacterium and Saccharomyces (Biosorption), are widely used for bioremediation of hexavalent chromium owing to their unique metabolic activities, ionic movement through an extracellular membrane, and other cellular adsorptions and reduction properties. The use of native and hybrid combinations of microbes supported by organic supplements is projected as a fast and efficient technique that not only reduces chromium quantity but also maintains the integrity of the microbial sources. Innovation and emphasis on nano-based products like nanocomposite, nano adsorbent, nanoscale zerovalent iron (nZVI) particles and multifunctional plant-growth-promoting bacteria (PGPB) will serve as the next generation environmental remediation technologies in the near future.

Cleaner utilization of electroplating sludge by bioleaching with a moderately thermophilic consortium: A pilot study

The semi-pilot scale bioleaching of electroplating sludge by the moderately thermophilic acidophilic consortium was carried out for the first time. During the microbial cultivation, Leptospirillum ferriphilum CS13, Acidithiobacillus caldus S2, and Sulfobacillus acidophilus CS5 could grow rapidly in a 300 L aeration packed reactor, in which the total suspended cell concentration could fluctuate around 3 × 10⁸ cells/mL and the community structure remained relatively stable. During the bioleaching process, the microbial stock solution could effectively leach heavy metals from electroplating sludge in a stirred reactor within a few hours. Meanwhile, the effects of pH, temperature, the quantity of active culture, and liquid-solid ratio on the bioleaching behavior were also investigated. The optimal conditions for electroplating sludge bioleaching were pH 1.5, temperature 45 °C, bacterial liquid ratio 40%, liquid-solid ratio 4:1 L kg^-1, and leaching time 5 h. The total removal rate of various heavy metals in electroplating sludge was over 99%. The bioleaching residue was successfully passed the TCLP test, and the total contents of heavy metals in the residue were also well below the regulatory criteria. In addition, the XRD analysis of the bioleaching residue was also confirmed that the moderately thermophilic consortium bioleaching provided a cleaner process than chemical leaching on the removal of the residual fraction metals, which was feasible and attractive for industrial treatment of electroplating sludge.

Influence of initial pH on bioleaching of river sediments to achieve deep dehydration

Acidification is one of the important methods of bioleaching and a rate-limiting steps in the reaction, with initial sediment pH having an important influence on bioleaching efficiency. Therefore, in this study, bioleaching treatments were assessed on river sediments with initial pH ranging from 3.0 to 11.0. Shake-flask tests were performed, using inoculated microorganisms and fresh river sediments in 500 mL Erlenmeyer bottles at a ratio of 1:9, with the addition of 5 g/L FeS₂ and 2 g/L S⁰ as energy sources. Erlenmeyer flasks were shaken for 72 h at 180 rpm and 28 °C, in a homeothermic oscillating water-bath. Results show that the bioleached sediment CST decreased from 39.10 s to between 10.3 s and 13.0 s. Sediment dewaterability reached a maximum level when the initial pH was 5 and the bioleaching duration was 36 h, with CST decreasing from 39.10 s to 10.30 s, a decrease in the CST rate by 82.25%, and moisture content was reduced to 57.5%. Illumina high-throughput sequencing results showed that the relative abundance of dominant microorganisms under different initial pH conditions had no significant difference at the phylum level. However, significant differences were apparent at the genus level. When the initial pH value increased from 3 to 5 in sediments, the relative abundances of the autotrophic bacteria Alicyclobacillus and Ferritrophicum and the heterotrophic bacteria Acidocella, which oxidize S⁰ and Fe²⁺, increase and dominate the microbial community, reaching abundances of 8.88%, 5.11%, and 4.87%, respectively. The heterotrophic bacteria Acidocella can biodegrade CLs products and eliminate their inhibitory effect on autotrophic bacteria Alicyclobacillus and Ferritrophicum with this synergistic effect improving the sediment dewatering performance. When the initial pH increased further to 9, the relative abundances of dominant bacteria decreased to varying degrees. In addition, variation in the initial pH conditions had no effect on the elimination efficiencies of pathogenic microorganism which were constantly > 98%. Graphical abstract .

Metal biorecovery and bioremediation: Whether or not thermophilic are better than mesophilic microorganisms

Metal mobilization and immobilization catalyzed by microbial action are key processes in environmental biotechnology. Metal mobilization from ores, mining wastes, or solid residues can be used for recovering metals and/or remediating polluted environments; furthermore, immobilization reduces the migration of metals; cleans up effluents plus ground- and surface water; and, moreover, can help to concentrate and recover metals. Usually these processes provide certain advantages over traditional technologies such as more efficient economical and environmentally sustainable results. Since elevated temperatures typically increase chemical kinetics, it could be expected that bioprocesses should also be enhanced by replacing mesophiles with thermophiles or hyperthermophiles. Nevertheless, other issues like process stability, flexibility, and thermophile-versus-mesophile resistance to acidity and/or metal toxicity should be carefully considered. This review critically analyzes and compares thermophilic and mesophilic microbial performances in recent and selected representative examples of metal bioremediation and biorecovery.

Soil Pollution Characteristics and Microbial Responses in a Vertical Profile with Long-Term Tannery Sludge Contamination in Hebei, China

An investigation was made into the effects of tannery sludge on soil chemical properties and microbial communities in a typical soil profile with long-term tannery sludge contamination, North China. The results showed that trivalent chromium (Cr(III)), ammonium, organic nitrogen, salinity and sulfide were the predominant contaminants in tannery sludge. Although the tannery sludge contained high chromium (Cr, 3,0970 mg/kg), the proportion of mobile Cr forms (exchangeable plus carbonate-bound fraction) only accounted for 1.32%. The X-ray diffraction and X-ray photoelectron spectroscopy results further demonstrated that the Cr existed in a stable state of oxides and iron oxides. The alkaline loam soil had a significant retardation effect on the migration of salinity, ammonium, Cr(III) and sulfide, and the accumulation of these contaminants occurred in soils (0⁻40 cm). A good correlation (R² = 0.959) was observed between total organic carbon (TOC) and Cr(III) in the soil profile, indicating that the dissolved organic matter from sludge leachate promoted the vertical mobility of Cr(III) via forming Cr(III)-organic complexes. The halotolerant bacteria (Halomonas and Tepidimicrobium) and organic degrading bacteria (Flavobacteriaceae, Tepidimicrobium and Balneola) became the dominant microflora in the soil profile. High contents of salinity, Cr and nitrogen were the main environmental factors affecting the abundance of indigenous microorganisms in soils.

Traits of Exogenous Species and Indigenous Community Contribute to the Species Colonization and Community Succession

Introducing exogenous species into an environment is an effective method to strengthen ecological functions. The traits of the exogenous species and the indigenous communities, as well as the resistance and subsequent succession of the community to exogenous species, are not well-understood. Here, three different functional consortia were introduced into two extremely acidic systems, leaching heap (LH) and leaching solution (LS), derived from the Zijin copper mine in China. The results showed that the structures of both LS and LH communities were affected by the three consortia, but not all the structural changes were in line with variations of community function. Among the three consortia, only the complementary sulfur oxidizers greatly enhanced copper extraction efficiency of LS (by 50.42%). This demonstrated that functional niche novelty gave exogenous species an advantage to occupy an ecological niche in a complementary manner, thus leading to successful colonization. The resistance to, and subsequent succession by, exogenous organisms varied between the two indigenous communities. More specifically, the LS community with low community diversity and simple composition was susceptible to exogenous species, and the community structural changes of LS were both divergent and irreversible. In comparison, the LH community with greater community diversity and more complex composition was more resistant to exogenous species, with the community structures showing a convergent trend over time despite different species being introduced. Therefore, we propose that diverse communities compete for resources more intensely with exogenous species and resist their introduction, and that communities with complex composition are able to cope with exogenous disturbances.

Intensified bioleaching of chalcopyrite by communities with enriched ferrous or sulfur oxidizers

The chalcopyrite bioleaching by enriched ferrous or sulfur oxidizers was investigated. The bioleaching was also intensified three times by the enriched communities. The results indicated that copper recoveries extracted by the enriched ferrous and sulfur oxidizers (Fe-O and S-O) were 38.87% and 43.13%, compared with that by the original community (35.35%). The positive effects of re-introducing S-enriched community to Fe-O and S-O groups were observed with copper extraction rates up to 41.67% and 46.45%. CCA indicated that the community dynamics intensified by S-enriched community was closer to that of the no re-inoculated one, but the Fe-enriched community drove a great fluctuation. A mechanism model for S-enriched community intensifying chalcopyrite bioleaching was proposed. More sulfur oxidizers in community slowed down jarosite formation and maintained lower ORP, which was propitious to chalcopyrite dissolution. Meanwhile, they accelerated S⁰ decomposition and decreased pH, which promoted acid leaching of chalcopyrite at a low cost.

Bioleaching of heavy metals from pig manure with indigenous sulfur-oxidizing bacteria: effects of sulfur concentration

The purpose of this work was to study the sulfur concentration on bioleaching of heavy metals from pig manure employing indigenous sulfur-oxidizing bacteria. Also, the variations in physicochemical properties of pig manure before and after bioleaching were investigated. The results showed that sulfur concentration significantly affected the rate of acidification, sulfate production and metal solubilization during pig manure bioleaching process. A Michaelis–Menten type equation was utilized to interpret the relationships between sulfur concentration, sulfate production and metal solubilization in the bioleaching process. The rates of metal solubilization during pig manure bioleaching were well described by a first order kinetic equation related to time. After 12 days of bioleaching, 93%–97% of Zn, 96%–98% of Mn and 48%–94% of Cu were leached out from pig manure, respectively. The metals remaining in the pig manure residual were mainly existed in stable forms. In addition, elemental analysis showed that bioleaching process could significantly modify the dewaterbility and organic composition of pig manure. However, fertility analysis found that 9.0%–19.1% of nitrogen, 68.5%–71.0% of phosphorus, 76.5%–78.8% of potassium and 47.5%–49.4% of the total organic carbon (TOC) were lost from pig manure in the bioleaching process. Therefore, bioleaching process used in this study could be applied to remove heavy metals effectively from the pig manure, but more detailed studies need to be done to decrease the nutrients loss from pig manure.

Biochar addition for accelerating bioleaching of heavy metals from swine manure and reserving the nutrients

Biochar was applied during the bioleaching of heavy metals (HMs) from swine manure (SM), in an attempt to accelerate the HMs removal rates and to reduce the losses of nutrient elements (nitrogen and phosphorus). Results showed that the addition of biochar (5gL^-1) could not only significantly shorten the leaching time of HMs (Cu, Zn, Mn and Cd) from 10 (control) to 7days with a high solubilization efficiency of 90%, but also decrease the total nitrogen loss efficiency by 42.7% from 180.3 (control) to 103.3mgL^-1 in the leachate. In addition, biochar addition facilitated Fe²⁺ oxidation rate, achieving much better pH and ORP conditions. Electronic conductivity and adsorption properties of biochar with changed microbial community probably contributed a lot to the enhanced HMs solubilization and reduced nitrogen loss during bioleaching. Although the addition of biochar only slightly reduced the total amount of phosphorus loss, the bioavailable phosphorus in SM after bioleaching was markedly increased by 13.7%.

Chromium speciation, bioavailability, uptake, toxicity and detoxification in soil-plant system: A review

Chromium (Cr) is a potentially toxic heavy metal which does not have any essential metabolic function in plants. Various past and recent studies highlight the biogeochemistry of Cr in the soil-plant system. This review traces a plausible link among Cr speciation, bioavailability, phytouptake, phytotoxicity and detoxification based on available data, especially published from 2010 to 2016. Chromium occurs in different chemical forms (primarily as chromite (Cr(III)) and chromate (Cr(VI)) in soil which vary markedly in term of their biogeochemical behavior. Chromium behavior in soil, its soil-plant transfer and accumulation in different plant parts vary with its chemical form, plant type and soil physico-chemical properties. Soil microbial community plays a key role in governing Cr speciation and behavior in soil. Chromium does not have any specific transporter for its uptake by plants and it primarily enters the plants through specific and non-specific channels of essential ions. Chromium accumulates predominantly in plant root tissues with very limited translocation to shoots. Inside plants, Cr provokes numerous deleterious effects to several physiological, morphological, and biochemical processes. Chromium induces phytotoxicity by interfering plant growth, nutrient uptake and photosynthesis, inducing enhanced generation of reactive oxygen species, causing lipid peroxidation and altering the antioxidant activities. Plants tolerate Cr toxicity via various defense mechanisms such as complexation by organic ligands, compartmentation into the vacuole, and scavenging ROS via antioxidative enzymes. Consumption of Cr-contaminated-food can cause human health risks by inducing severe clinical conditions. Therefore, there is a dire need to monitor biogeochemical behavior of Cr in soil-plant system.

Catalytic potential of selected metal ions for bioleaching, and potential techno-economic and environmental issues: A critical review

Bioleaching is considered to be a low-cost, eco-friendly technique for leaching valuable metals from a variety of matrixes. However, the inherent slow dissolution kinetics and low metal leaching yields have restricted its wider commercial applicability. Recent advancements in bio-hydrometallurgy have suggested that these critical issues can be successfully alleviated through the addition of a catalyst. The catalyzing properties of a variety of metals ions (Ag⁺, Hg⁺⁺, Bi⁺⁺⁺, Cu⁺⁺, Co⁺⁺ etc.) during bioleaching have been successfully demonstrated. In this article, the role and mechanisms of these metal species in catalyzing bioleaching from different minerals (chalcopyrite, complex sulfides, etc.) and waste materials (spent batteries) are reviewed, techno-economic and environmental challenges associated with the use of metals ions as catalysts are identified, and future prospectives are discussed. Based on the analysis, it is suggested that metal ion-catalyzed bioleaching will play a key role in the development of future industrial bio-hydrometallurgical processes.