Biomineralization based remediation of As(III) contaminated soil by Sporosarcina ginsengisoli

For aqueous/soil cadmium immobilization under acid attack, does the hydroxyapatite converted from Pseudochrobactrum sp. DL-1 induced vaterite necessarily show higher stability?

Microbial induced carbonate precipitation (MICP) technology was widely applied to immobilize heavy metals, but its long-term stability is tough to maintain, particularly under acid attack. This study successfully converted Pseudochrobactrum sp. DL-1 induced vaterite (a rare crystalline phase of CaCO3) to hydroxyapatite (HAP) at 30 ℃. The predominant conversion mechanism was the dissolution of CdCO3-containing vaterite and the simultaneous recrystallization of Ca4.03Cd0.97(PO4)3(OH)-containing HAP. For aqueous Cd immobilization, stability test at pH 2.0-10.0 showed that the Cd2+ desorption rate of Cd-adsorbed vaterite (3.96-4.35 ‱) were 7.13-20.84 times greater than that of Cd-adsorbed HAP (0.19-0.61 ‱). For soil Cd immobilization under 60 days of acid-rain erosion, the highest immobilization rate (51.00 %) of exchangeable-Cd and the lowest dissolution rate (-0.18 %) of carbonate-Cd were achieved with 2 % vaterite, while the corresponding rates were 16.78 % and 1.31 % with 2 % HAP, respectively. Furthermore, vaterite outperformed HAP in terms of soil ecological thorough evaluation. In conclusion, for Cd immobilization by MICP under acid attack, DL-1 induced vaterite displayed direct application value due to its exceptional stability in soil and water, while the mineral conversion strategy we presented is useful for further enhancing the stability in water.

Bioremediation of uranium enriched coal fly ash based on microbially induced calcite precipitation

Coal fly ash (CFA) is an essential raw material in brickmaking industry worldwide. There are some coal mines with a relatively high content of uranium (U) in the Xinjiang region of China that are yet understudied. The CFA from these coal mines poses substantial environmental risks due to the concentrated uranium amount after coal burning. In this paper, we demonstrated a calcifying ureolytic bacterium Halomonas sp. SBC20 for its biocementation of U in CFA based on microbially induced calcite precipitation (MICP). Rectangle-shaped CFA bricks were made from CFA using bacterial cells, and an electric testing machine tested their compressive strength. U distribution pattern and immobility against rainfall runoff were carefully examined by a five-stage U sequential extraction method and a leaching column test. The microstructural changes in CFA bricks were characterized by FTIR and SEM-EDS methods. The results showed that the compressive strength of CFA bricks after being cultivated by bacterial cells increased considerably compared to control specimens. U mobility was significantly decreased in the exchangeable fraction, while the U content was markedly increased in the carbonate-bound fraction after biocementation. Much less U was released in the leaching column test after the treatment with bacterial cells. The FTIR and SEM-EDX methods confirmed the formation of carbonate precipitates and the incorporation of U into the calcite surfaces, obstructing the release of U into the surrounding environments. The technology provides an effective and economical treatment of U-contaminated CFA, which comes from coal mines with high uranium content in the Xinjiang region, even globally.

Effect of phosphate-mineralized bacteria on multi-metals migration behavior in vanadium tailing slags: Coexistence of immobilization and mobilization

Biomineralization techniques have been utilized to remediate heavy metals (HMs) contaminated environments. However, the effect of microbial-induced phosphate precipitation (MIPP) on HMs behavior in vanadium tailing slags has not been revealed. This study is the first to report the influence of MIPP on multiple HMs including Cd, Cu, Pb and Zn in the slags with and without soil mixing. The results showed that MIPP exhibited excellent ability for Cd immobilization, Cd immobilization rate reached 43.41 % under the optimal parameters within 7 days. Cd immobilization performance was significantly improved and sustained after the slags were covered with soil, resulting from better colonization of phosphate mineralizing bacteria in slag-soil mixtures. Surprisingly, DTPA-Cu, Zn and Pb contents in slags were all increased to varying degrees after MIPP treatment. Leaching solution mineralization tests further suggested that MIPP significantly reduced the concentration of Cd2+, Pb2+, Ca2+, Mg2+ and Al3+, but barely changed Cu2+ and Zn2+ concentrations. Characterization analysis confirmed that formation of phosphates including Cd(PO4)2 and dissolution of minerals including PbZnSiO2 were the reason for HMs immobilization and mobilization in vanadium tailing slags. This study provides new insights for understanding biomineralization technology and using MIPP to remediate HMs contaminated mine waste.

Molecular and geochemical basis of microbially induced carbonate precipitation for treating acid mine drainage: The case of a novel Sporosarcina genomospecies from mine tailings

Microbially induced carbonate precipitation (MICP) immobilizes toxic metals and reduces their bioavailability in aqueous systems. However, its application in the treatment of acid mine drainage (AMD) is poorly understood. In this study, the genomes of Sporosarcina sp. UB5 and UB10 were sequenced. Urease, carbonic anhydrases, and metal resistance genes were identified and enzymatic assays were performed for their validation. The geochemical mechanism of precipitation in AMD was elucidated through geo-mineralogical analysis. Sporosarcina sp. UB5 was shown to be a new genomospecies, with an average nucleotide identity < 95 % (ANI) and DNA-DNA hybridization < 70 % (DDH) whereas UB10 is close to S. pasteurii. UB5 contained two urease operons, whereas only one was identified in UB10. The ureolytic activities of UB5 and UB10 were 122.67 ± 15.74 and 131.70 ± 14.35 mM NH4+ min-1, respectively. Both strains feature several carbonic anhydrases of the α, β, or γ families, which catalyzed the precipitation of CaCO3. Only Sporosarcina sp. UB5 was able to immobilize metals and neutralize AMD. Geo-mineralogical analyses revealed that UB5 directly immobilized Fe (1-23 %), Mn (0.65-1.33 %) and Zn (0.8-3 %) in AMD via MICP and indirectly through adsorption to calcite and binding to bacterial cell walls. The MICP-treated AMD exhibited high removal rates (>67 %) for Ag, Al, As, Ca, Cd, Co, Cu, Fe, Mn, Pb, and Zn, and a removal rate of 15 % for Mg. This study provides new insights into the MICP process and its applications to AMD treatment using autochthonous strains.

Two decades of research trends in microbial-induced carbonate precipitation for heavy metal removal: a bibliometric review and literature review

Amidst the increasing significance of innovative solutions for bioremediation of heavy metal removal, this paper offers a thorough bibliometric analysis of microbial-induced carbonate precipitation (MICP) for heavy metal removal, as a promising technology to tackle this urgent environmental issue. This study focused on articles published from 1999 to 2022 in the Scopus database. It assesses trends, participation, and key players within the MICP for heavy metal sequestration. Among the 930 identified articles, 74 countries participated in the field, with China being the most productive. Varenyam Achal, the Chinese Academy of Sciences, and Chemosphere are leaders in the research landscape. Using VOSviewer and R-Studio, keyword hotspots like "MICP", "urease", and "heavy metals" underscore the interdisciplinary nature of MICP research and its focus on addressing a wide array of environmental and soil-related challenges. VOSviewer emphasises essential terms like "calcium carbonate crystal", while R-Studio highlights ongoing themes such as "soil" and "organic" aspects. These analyses further showcase the interdisciplinary nature of MICP research, addressing a wide range of environmental challenges and indicating evolving trends in the field. This review also discusses the literature concerning the potential of MICP to immobilise contaminants, the evolution of the research outcome in the last two decades, MICP treatment techniques for heavy metal removal, and critical challenges when scaling from laboratory to field. Readers will find this analysis beneficial in gaining valuable insights into the evolving field and providing a solid foundation for future research and practical implementation.

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Microbiologically induced calcite precipitation (MICP) in situ remediated heavy metal contamination in sludge nutrient soil

Microbiologically induced calcite precipitation (MICP), as a newly developing bioremediation technology, could redeem heavy metal contamination in diverse scenarios. In this study, MICP bacterium Sporosarcina ureilytica ML-2 was employed to suppress the pollution of Pb, Cd and Zn in municipal sludge nutrient soil. After MICP remediation, the exchangeable Cd and Zn in sludge nutrient soil were correspondingly reduced by 31.02 % and 6.09 %, while the carbonate-bound Pb, Cd and Zn as well as the residual fractions were increased by 16.12 %, 6.63 %, 13.09 % and 6.10 %, 45.70 %, 3.86 %, respectively. In addition, the extractable Pb, Cd and Zn either by diethylenetriaminepentaacetic acid (DTPA) or toxicity characteristic leaching procedure (TCLP) in sludge nutrient soil were significantly reduced. These results demonstrated that the bio-calcite generated via MICP helped to immobilize heavy metals. Furthermore, MICP treatment improved the abundance of functional microorganisms related to urea cycle, while reduced the overall abundance of metal resistance genes (MRGs) and antibiotic resistance genes (ARGs). This work confirmed the feasibility of MICP in remediation of heavy metal in sludge nutrient soil, which expanded the application field of MICP and provided a promising way for heavy metal pollution management.

Perspectives of nanomaterials in microbial remediation of heavy metals and their environmental consequences: A review

Nanomaterials (NMs) have diverse applications in various sectors, such as decontaminating heavy metals from drinking water, wastewater, and soil. Their degradation efficiency can be enhanced through the application of microbes. As microbial strain releases enzymes, which leads to the degradation of HMs. Therefore, nanotechnology and microbial-assisted remediation-based methods help us develop a remediation process with practical utility, speed, and less environmental toxicity. This review focuses on the success achieved for the bioremediation of heavy metals by nanoparticles and microbial strains and in their integrated approach. Still, the use of NMs and heavy metals (HMs) can negatively affect the health of living organisms. This review describes various aspects of the bioremediation of heavy materials using microbial nanotechnology. Their safe and specific use supported by bio-based technology paves the way for their better remediation. We discuss the utility of nanomaterials for removing heavy metals from wastewater, toxicity studies and issues to the environment with their practical implications. Nanomaterial assisted heavy metal degradation coupled with microbial technology and disposal issues are described along with detection methods. Environmental impact of nanomaterials is also discussed based on the recent work conducted by the researchers. Therefore, this review opens new avenues for future research with an impact on the environment and toxicity issues. Also, applying new biotechnological tools will help us develop better heavy metal degradation routes.

Enhanced Strontium Removal through Microbially Induced Carbonate Precipitation by Indigenous Ureolytic Bacteria

Microbial ureolysis offers the potential to remove metals including Sr2+ as carbonate minerals via the generation of alkalinity coupled to NH4+ and HCO3- production. Here, we investigated the potential for bacteria, indigenous to sediments representative of the U.K. Sellafield nuclear site where 90Sr is present as a groundwater contaminant, to utilize urea in order to target Sr2+-associated (Ca)CO3 formation in sediment microcosm studies. Strontium removal was enhanced in most sediments in the presence of urea only, coinciding with a significant pH increase. Adding the biostimulation agents acetate/lactate, Fe(III), and yeast extract to further enhance microbial metabolism, including ureolysis, enhanced ureolysis and increased Sr and Ca removal. Environmental scanning electron microscopy analyses suggested that coprecipitation of Ca and Sr occurred, with evidence of Sr associated with calcium carbonate polymorphs. Sr K-edge X-ray absorption spectroscopy analysis was conducted on authentic Sellafield sediments stimulated with Fe(III) and quarry outcrop sediments amended with yeast extract. Spectra from the treated Sellafield and quarry sediments showed Sr2+ local coordination environments indicative of incorporation into calcite and vaterite crystal structures, respectively. 16S rRNA gene analysis identified ureolytic bacteria of the genus Sporosarcina in these incubations, suggesting they have a key role in enhancing strontium removal. The onset of ureolysis also appeared to enhance the microbial reduction of Fe(III), potentially via a tight coupling between Fe(III) and NH4+ as an electron donor for metal reduction. This suggests ureolysis may support the immobilization of 90Sr via coprecipitation with insoluble calcium carbonate and cofacilitate reductive precipitation of certain redox active radionuclides, e.g., uranium.

Kinetic analysis and mechanism of nitrate, calcium, and cadmium removal using the newly isolated Pseudomonas sp. LYF26

The co-existence of heavy metals and nitrate (NO3--N) pollutants in wastewater has been a persistent global concern for a long time. A strain LYF26, which can remove NO3--N, calcium (Ca(II)), and cadmium (Cd(II)) simultaneously, was isolated to explore the properties and mechanisms of synergistic contaminants removal. Different conditions (Cd(II) and Ca(II) concentrations and pH) were optimized by Zero-, Half-, and First-order kinetic analyses to explore the environmental parameters for the optimal effect of strain LYF26. Results of the kinetic analyses revealed that the optimal culture conditions for strain LYF26 were pH of 6.5, Cd(II) and Ca(II) concentrations of 3.00 and 180.00 mg L-1, accompanied by Ca(II), Cd(II), and NO3--N efficiencies of 53.10%, 90.03%, and 91.45%, respectively. The removal mechanisms of Cd(II) using strain LYF26 as a nucleation template were identified as biomineralization, lattice substitution, and co-precipitation. The differences and changes of dissolved organic matter during metabolism were analyzed and the results demonstrated that besides the involvement of extracellular polymeric substances in the precipitation of Cd(II) and Ca(II), the high content of humic acid-like species revealed a remarkable contribution to the denitrification process. This study is hopeful to contribute a theory for further developing microbially induced calcium precipitation used to treat complex polluted wastewater.

Microbial induced carbonate precipitation for remediation of heavy metals, ions and radioactive elements: A comprehensive exploration of prospective applications in water and soil treatment

Improper disposal practices have caused environmental disruptions, possessing by heavy metal ions and radioactive elements in water and soil, where the innovative and sustainable remediation strategies are significantly imperative in last few decades. Microbially induced carbonate precipitation (MICP) has emerged as a pioneering technology for remediating contaminated soil and water. Generally, MICP employs urease-producing microorganisms to decompose urea (NH2CONH2) into ammonium (NH4+and carbon dioxide (CO2), thereby increasing pH levels and inducing carbonate precipitation (CO32-), and effectively removing remove contaminants. Nonetheless, the intricate mechanism underlying heavy metal mineralization poses a significant challenge, constraining its application in contaminants engineering, particularly in the context of prolonged heavy metal leaching over time and its efficacy in adverse environmental conditions. This review provides a comprehensive idea of recent development of MICP and its application in environmental engineering, examining metabolic pathways, mineral precipitation mechanisms, and environmental factors as well as providing future perspectives for commercial utilization. The use of ureolytic bacteria in MICP demonstrates cost-efficiency, environmental compatibility, and successful pollutant abatement over tradition bioremediation techniques, and bio-synthesis of nanoparticles. limitations such as large-scale application, elevated Ca2+levels in groundwater, and gradual contaminant release need to be overcome. The possible future research directions for MICP technology, emphasizing its potential in conventional remediation, CO2 sequestration, bio-material synthesis, and its role in reducing environmental impact for long-term economic benefits.

Immobilization of Cd2+ in an aqueous environment using a two-step microbial-induced carbonate precipitation method

Previous researches indicate that the potent toxicity of cadmium hinders the efficacy of the microbial-induced carbonate precipitation (MICP) process for bioremediation of Cd2+ in aqueous environment. Increasing urea and calcium resource doses, introducing synergists, and utilizing urease-producing consortia can improve bio-immobilization performance of MICP. However, such measures may incur cost increases and/or secondary contamination. This study first verifies the substantial biotoxicity of Cd2+ for urease activity and then analyzes the practical limitation of traditional MICP using Bacillus pasteurii for bioremediation of Cd2+ in an aqueous environment containing 1-40 mM Cd2+ by a series tube tests and numerical simulation. Subsequently, a two-step MICP method, which separates urea hydrolysis and heavy metal precipitation, is introduced in this study to eliminate the inhibitory effect of heavy metal on urease activity. The concentrations of ammonium, Cd2+, and pH were monitored over time. The results indicate that the urease expression in B. pasteurii can be significantly inhibited by Cd2+ particularly at the concentration ranging from 10 to 40 mM, leading to pretty low efficacy of traditional MICP for bioremediation of Cd2+ (Cd2+ removal rate as low as 21.55-38.47% when the initial Cd2+ concentration = 40 mM). In contrast, when the two-step MICP method is applied, the Cd2+ can be almost completely immobilized, even though the concentration ratio of urea to Cd2+ is as low as 1.5:1.0, which is close to the theory minimum concentration ratio for the complete precipitation of carbonate to cadmium ions(1.0:1.0). Therefore, the cost-effective, environmentally sustainable, and straightforward two-step MICP method holds great potential for application in the bioremediation of Cd2+-contaminated solutions in high concentration.

Microbially Induced Calcium Carbonate Precipitation as a Bioremediation Technique for Mining Waste

Mining waste represents a global issue due to its potential of generating acidic or alkaline leachate with high concentrations of metals and metalloids (metal(loid)s). Microbial-induced calcium carbonate precipitation (MICP) is an engineering tool used for remediation. MICP, induced via biological activity, aims to precipitate calcium carbonate (CaCO3) or co-precipitate other metal carbonates (MCO3). MICP is a bio-geochemical remediation method that aims to immobilize or remove metal(loid)s via enzyme, redox, or photosynthetic metabolic pathways. Contaminants are removed directly through immobilization as mineral precipitates (CaCO3 or MCO3), or indirectly (via sorption, complexes, or inclusion into the crystal structure). Further, CaCO3 precipitates deposited on the surface or within the pore spaces of a solid matrix create a clogging effect to reduce contaminant leachate. Experimental research on MICP has shown its promise as a bioremediation technique for mining waste. Additional research is required to evaluate the long-term feasibility and potential by-products of MICP-treated/stabilized waste.

Enrichment, characterization, and sand consolidation application of urease active calcite-producing bacteria

Minerals such as calcium carbonate, which is prevalent in marble and limestone, are present naturally in rocks. Both physicochemical processes and microbial processes can result in the creation of calcium carbonate in nature, as is well documented. In this study, microbiologically induced calcite precipitation potential of three different Travertine-type water sources (Pamukkale Travertine Spring (PTS), Pamukkale Travertine Terraces (PTT), and Red Travertine of Karahayit (RTK)) using three different incubation media (NB, NB3, and ATCC1832) were investigated. After enrichment with ATCC1832 media, urease assays were positive for all of the microbial sources. The PTS and PTT were cultured with ATCC1832 medium for 48 h, which showed the best results for urease activity and microbial growth among other samples. Metagenome analyses indicated that PTT enriched with ATCC1832 media contains > 99% Firmicutes, while PTS enriched with ATCC1832 contains > 99% Proteobacteria at the Phylum level. Results from SEM-EDX and XRD analysis revealed that calcite and/or vaterite were the minerals that emerged from the mineralization of the PTS and PTT during incubation. The type of calcium carbonate crystals tended to change from one form to another when the incubation period extends from 72 to 120 h. Both the PTS and the PTT were able to precipitate calcite within the sand column. However, the bacteria from the PTT (26% CaCO3) outperformed those from the PTS (18% CaCO3) in terms of calcium carbonate deposition on the 21st day of incubation.

Comparison of the Intestinal Bacterial Communities between Captive and Semi-Free-Range Red-Crowned Cranes (Grus japonensis) before Reintroduction in Zhalong National Nature Reserve, China

The wild populations of red-crowned cranes (Grus japonensis) in west China are gradually decreasing, necessitating the optimization of reintroduction measures. This study used 16S rRNA high-throughput sequencing technology to compare the gut microbiota communities of cranes living in two modes (captive and semi-free-range) before their reintroduction in Zhalong National Nature Reserve, Heilongjiang Province, China. The results showed that Proteobacteria (74.39%) and Firmicutes (25.29%) were the dominant gut bacterial phyla inhabiting these cranes. Significant differences were found in the gut microbiota community composition between semi-free-range and captive cranes (p < 0.01). Psychrobacter, Sporosarcina, and Lactococcus were significantly enriched in captive cranes (p < 0.05), while Pseudomonadaceae_Pseudomonas, Pantoea, Lysobacter, and Enterobacteriaceae_Pseudomonas were more abundant in semi-free-range cranes (p < 0.05). The functions and community structure of gut microbiota were affected by feeding patterns (p < 0.05). The metabolic pathways of ethylbenzene degradation, PPAR signaling pathway, betalain biosynthesis, systemic lupus erythematosus, and shigellosis were up-regulated in semi-free-range cranes (p < 0.05).

Research progress and hotspots on microbial remediation of heavy metal-contaminated soil: a systematic review and future perspectives

Microbial remediation technology has received much attention as a green, ecological, and inexpensive technology, and there is great potential for the application of microbial remediation technology for heavy metals (HMs) contaminated soil alone and in conjunction with other technologies in environmental remediation. To gain an in-depth understanding of the latest research progress, research hotspots, and development trends on microbial remediation of HMs-contaminated soil, and to objectively reflect the scientific contributions and impacts of relevant countries/regions, institutions, and individuals of this field, in this manuscript, ISI Web of Knowledge's Web of Science™ core collection database, data visualization, and analysis software Bibliometrix, VOSviewer, and HistCite Pro were used to collect and analyze the relevant literature from 2000 to 2022, and 1409 publications were subjected to scientometric analyses. It involved 327 journals, 5150 authors, 75 countries/regions, and 2740 keywords. The current progress and hotspots on microbial remediation of HMs-contaminated soil since the twenty-first century were analyzed in terms of the top 10 most productive countries (regions), high-yielding authors, source journals, important research institutions, and hotspots of research directions. Over the past 22 years, China, India, and the USA have been the countries with the most articles. The institution and author with the most publications are the Chinese Acad Sci and Zhu YG, respectively. Journal of Hazardous Materials is the most productive journal. The keywords showed 6 co-occurrence clusters. These findings revealed the research hotspots, knowledge gaps, and future exploration trends related to microbial remediation of HMs-contaminated soil.

Accelerated Reinforcement of Calcareous sand via Biomineralization with Aluminum Ion Flocculant

Microbially induced calcium carbonate precipitation (MICP) is an immensely growing technique that utilizes the metabolic pathways of bacteria to form calcite precipitation throughout the soil matrix, leading to improve geotechnical engineering properties. However, the excessive number of treatments limited the application of MICP for strengthening calcareous sand. To reduce the number of treatments and develop efficiencies, this paper investigates the optimized treatment protocol of adding aluminum ion flocculants to the cementing solution to accelerate the curing rate of the MICP and its effect. The results show that adding a certain concentration of AlCl3 to the cementing solution can resulted in a rapid increase in strength of the calcareous sand column. When 0.02 M aluminum chloride was added to the cementing solution, the unconfined compressive strength of the sand column reached 827 kPa after three treatments, and it reached 2 MPa after five treatments, while the control group needed to be treated 10 and 15 times, respectively, to reach equivalent strengths. In this paper, the unconfined compressive strength of the sand column formed using the proposed method was 27-40 times that of the control group at the same calcium carbonate content. The presented experimental approach can be used as a tool to design the treatment protocol for the engineering application of MICP-reinforced calcareous sand in practice.

Removal of cadmium and arsenic from water through biomineralization

Due to anthropogenic activities, heavy metals such as cadmium (Cd) and arsenic (As) are one of the most toxic xenobiotics contaminating water, thus affecting human health and the environment. The objective of the present investigation was to study the effect of ureolytic bacteria Bacillus paramycoides-MSR1 for the bioremediation of Cd and As from contaminated water. The B. paramycoides showed high resistance to heavy metals, Cd and As, with minimum inhibitory concentration (MIC) of 12.84 μM and 48.54 μM, respectively. The urease activity and calcium carbonate (CaCO3) precipitation were evaluated in artificial wastewater with different concentrations of Cd (0, 10, 20, 30, 40, 50, and 60 μM) and As (0, 20, 40, 60, 80, and 100 μM). The maximum urease activity in Cd-contaminated artificial wastewater was observed after 96 hours, which showed a 76.1% decline in urease activity as the metal concentration increased from 0 to 60 μM. Similarly, 14.1% decline in urease activity was observed as the concentration of As was increased from 0 to 100 μM. The calcium carbonate precipitation at the minimum inhibitory concentration of Cd and As-contaminated artificial wastewater was 189 and 183 mg/100 ml, respectively. The percentage removal of metal from artificially contaminated wastewater with varied concentrations was analyzed using atomic absorption spectroscopy (AAS). After 168 hours of incubation, 93.13% removal of Cd and 94.25% removal of As were observed. Microstructural analysis proved the presence of calcium carbonate in the form of calcite, confirming removal of cadmium and arsenic by microbially induced calcium carbonate precipitation (MICCP) to be promising technique for water decontamination.

Fungal biorecovery of cerium as oxalate and carbonate biominerals

Cerium is the most sought-after rare earth element (REE) for application in high-tech electronic devices and versatile nanomaterials. In this research, biomass-free spent culture media of Aspergillus niger and Neurospora crassa containing precipitant ligands (oxalate, carbonate) were investigated for their potential application in biorecovery of Ce from solution. Precipitation occurred after Ce3+ was mixed with biomass-free spent culture media and >99% Ce was recovered from media of both organisms. SEM showed that biogenic crystals with distinctive morphologies were formed in the biomass-free spent medium of A. niger. Irregularly-shaped nanoparticles with varying sizes ranging from 0.5 to 2 μm and amorphous biominerals were formed after mixing the carbonate-laden N. crassa supernatant, resulting from ureolysis of supplied urea, with Ce3+. Both biominerals contained Ce as the sole metal, and X-ray diffraction (XRD) and thermogravimetric analyses identified the biominerals resulting from the biomass-free A. niger and N. crassa spent media as cerium oxalate decahydrate [Ce2(C2O4)3·10H2O] and cerium carbonate [Ce2(CO3)3·8H2O], respectively. Thermal decomposition experiments showed that the biogenic Ce oxalates and carbonates could be subsequently transformed into ceria (CeO2). FTIR confirmed that both amorphous and nanoscale Ce carbonates contained carbonate (CO32-) groups. FTIR-multivariate analysis could classify the biominerals into three groups according to different Ce concentrations and showed that Ce carbonate biominerals of higher purity were produced when precipitated at higher Ce3+ concentrations. This work provides new understanding of fungal biotransformations of soluble REE species and their biorecovery using biomass-free fungal culture systems and indicates the potential of using recovered REE as precursors for the biosynthesis of novel nanomaterials.

Reduction of bioavailability and phytotoxicity effect of cadmium in soil by microbial-induced carbonate precipitation using metabolites of ureolytic bacterium Ochrobactrum sp. POC9

The application of ureolytic bacteria for bioremediation of soil contaminated with heavy metals, including cadmium (Cd), allows for the efficient immobilization of heavy metals by precipitation or coprecipitation with carbonates. Microbially-induced carbonate precipitation process may be useful also in the case of the cultivation of crop plants in various agricultural soils with trace but legally permissible Cd concentrations, which may be still uptaken by plants. This study aimed to investigate the influence of soil supplementation with metabolites containing carbonates (MCC) produced by the ureolytic bacterium Ochrobactrum sp. POC9 on the Cd mobility in the soil as well as on the Cd uptake efficiency and general condition of crop plants (Petroselinum crispum). In the frame of the conducted studies (i) carbonate productivity of the POC9 strain, (ii) the efficiency of Cd immobilization in soil supplemented with MCC, (iii) crystallization of cadmium carbonate in the soil enriched with MCC, (iv) the effect of MCC on the physico-chemical and microbiological properties of soil, and (v) the effect of changes in soil properties on the morphology, growth rate, and Cd-uptake efficiency of crop plants were investigated. The experiments were conducted in soil contaminated with a low concentration of Cd to simulate the natural environmental conditions. Soil supplementation with MCC significantly reduced the bioavailability of Cd in soil with regard to control variants by about 27-65% (depending on the volume of MCC) and reduced the Cd uptake by plants by about 86% and 74% in shoots and roots, respectively. Furthermore, due to the decrease in soil toxicity and improvement of soil nutrition with other metabolites produced during the urea degradation (MCC), some microbiological properties of soil (quantity and activity of soil microorganisms), as well as the general condition of plants, were also significantly improved. Soil supplementation with MCC enabled efficient Cd stabilization and significantly reduced its toxicity for soil microbiota and plants. Thus, MCC produced by POC9 strain may be used not only as an effective Cd immobilizer in soil but also as a microbe and plant stimulators.

Optimization of biocementation responses by artificial neural network and random forest in comparison to response surface methodology

In this article, the optimization of the specific urease activity (SUA) and the calcium carbonate (CaCO₃) using microbially induced calcite precipitation (MICP) was compared to optimization using three algorithms based on machine learning: random forest regressor, artificial neural networks (ANNs), and multivariate linear regression. This study applied the techniques in two existing response surface method (RSM) experiments involving MICP technique. Random forest-based models and artificial neural network-based models were submitted through the optimization of hyperparameters via cross-validation technique and grid search, to select the best-optimized model. For this study, the random forest-based algorithm is aimed at having the best performance of 0.9381 and 0.9463 in comparison to the original r² of 0.9021 and 0.8530, respectively. This study is aimed at exploring the capability of using machine learning-based models in small datasets for the purpose of optimization of experimental variables in MICP technique and the meaningfulness of the models by their specificities in the small experimental datasets applied to experimental designs. This study is aimed at exploring the capability of using machine learning-based models in small datasets for experimental variable optimization in MICP technique. The use of these techniques can create prerogatives to scale and mitigate costs in future experiments associated to the field.

Immobilizing lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and featured with high risk of secondary pollution. In the past few years, the microbial-induced carbonate precipitation (MICP) is considered as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitate (EICP) has attracted attention because bacterial cultivation is not required prior to catalyzing urea hydrolysis. This study compared the performance of lead (Pb) and copper (Cu) remediation using MICP and EICP respectively. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the remediation efficiency was investigated. Results indicated that ammonium ion (NH4+) concentration reduced with the increase in lead ion (Pb2+) or copper ion (Cu2+) concentration, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than EICP. Further, the remediation efficiency against Cu2+ is approximately zero, which is way below that against Pb2+ (approximately 100%). The Cu2+ toxicity denatured and even inactivated the urease, reducing the degree of urea hydrolysis and the remediation efficiency. Moreover, the reduction in the remediation efficiency against Pb2+ and Cu2+ appeared to be due to the precipitations of cotunnite and atacamite respectively. Their chemical and thermodynamic properties were not as good as calcite, cerussite, phosgenite, and malachite. The findings shed light on the underlying mechanism affecting the remediation efficiency against Pb2+ and Cu2+.

Soil heavy metal pollution from Pb/Zn smelting regions in China and the remediation potential of biomineralization

Smelting activities pose serious environmental problems due to the local and regional heavy metal pollution in soils they cause. It is therefore important to understand the pollution situation and its source in the contaminated soils. In this paper, data on heavy metal pollution in soils resulting from Pb/Zn smelting (published in the last 10 years) in China was summarized. The heavy metal pollution was analyzed from a macroscopic point of view. The results indicated that Pb, Zn, As and Cd were common contaminants that were present in soils with extremely high concentrations. Because of the extreme carcinogenicity, genotoxicity and neurotoxicity that heavy metals pose, remediation of the soils contaminated by smelting is urgently required. The primary anthropogenic activities contributing to soil pollution in smelting areas and the progressive development of accurate source identification were performed. Due to the advantages of biominerals, the potential of biomineralization for heavy metal contaminated soils was introduced. Furthermore, the prospects of geochemical fraction analysis, combined source identification methods as well as several optimization methods for biomineralization are presented, to provide a reference for pollution investigation and remediation in smelting contaminated soils in the future.

Immobilizing copper in loess soil using microbial-induced carbonate precipitation: Insights from test tube experiments and one-dimensional soil columns

Biomineralization as an alternative to traditional remediation measures has been widely applied to remediate copper (Cu)-contaminated sites due to its environmental-friendly nature. Immobilizing Cu is, however, a challenging task as it inevitably causes inactivation of ureolytic bacteria. In the present work, a series of test tube experiments were conducted to derive the relationships of Cu immobilization efficiency versus pH conditions. The Cu speciation transformation that is invisible in the test tube experiments was investigated via numerical simulations. Apart from that, the one-dimensional soil column tests, accompanied by the X-ray diffraction (XRD) and Raman spectroscopy analysis, mainly aimed not only to investigate the variations of Cu immobilization efficiency with the depth but to reveal the underlying mechanisms affecting the Cu immobilization efficiency. The results of the test tube experiments highlight the necessity of narrowing pH ranges to as close as 7 by introducing an appropriate bacterial inoculation proportion. The coordination adsorption of Cu, while performing the one-dimensional soil column tests, is encouraged by alkaline environments, which differs from the test tube experiments where Cu²⁺ is capsulized by carbonate precipitates to prevent their migration. The findings highlight the potential of applying the microbial-induced carbonate precipitation (MICP) technology to Cu-rich water bodies and Cu-contaminated sites remediation.

Effect of phosphate on cadmium immobilized by microbial-induced carbonate precipitation: Mobilization or immobilization?

Microbial-induced carbonate precipitation (MICP) is a promising technology to immobilize/remediate heavy metals (HMs) like cadmium (Cd). However, the long-term stability of MICP-immobilized HMs is unclear, especially in farmland where chemical fertilization is necessary. Therefore, we performed MICP treatment on soils contaminated with various Cd compounds (CdCO₃, CdS, and CdCl₂) and added diammonium phosphate (DAP) to explore the impact of phosphate on the MICP-immobilized Cd. The results showed that MICP treatment was practical to immobilize the exchangeable Cd but to mobilize the carbonate and Fe/Mn oxide-bound Cd. After applying DAP, soil pH declined due to ammonium nitrification. At high P/Ca molar ratios (1/2 and 1), partial previously immobilized Cd was released due to the carbonate dissolution. Contrarily, exchangeable Cd transformed to less mobilizable Fe/Mn oxide-bound at low P/Ca molar ratios (1/4 and 1/8). Meanwhile, other treatments were also helpful in avoiding the release of immobilized Cd, such as applying non-ammonium phosphate and adding lime material after soil acidification. Our investigation suggested that the long-term stability of HMs in remediated sites should be carefully evaluated, especially in agricultural areas with phosphate and nitrogen fertilizer input.

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

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

Immobilization of Cd2+ and Pb2+ by biomineralization of the carbonate mineralized bacterial consortium JZ1

Microbially induced carbonate precipitation (MICP) has been proven to effectively immobilize Cd²⁺ and Pb²⁺ using a single bacterium. However, there is an urgent need for studies of Cd²⁺ and Pb²⁺ immobilized by a bacterial consortium. In this study, a stable consortium designated JZ1 was isolated from soil that was contaminated with cadmium and lead, and the dominant genus Sporosarcina (99.1%) was found to have carbonate mineralization function. The results showed that 91.52% and 99.38% of Cd²⁺ and Pb²⁺ were mineralized by the consortium JZ1 with 5 g/L CaCl₂ at an initial concentration of 5 mg/L Cd²⁺ and 150 mg/L Pb²⁺, respectively. The bioprecipitates were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Moreover, the kinetic studies indicated that the urea hydrolysis reaction fit well with the Michaelis-Menten equation, and the kinetic parameters K_m and V_max were estimated to be 38.69 mM and 58.98 mM/h, respectively. When the concentration of urea increased from 0.1 to 0.3 M, the mineralization rate increased by 1.58-fold. This study can provide a novel microbial resource for the biomineralization of Cd and Pb in soil and water environments.

Microbial repairing of concrete & its role in CO2 sequestration: a critical review

Background

Being the most widely used construction material, concrete health is considered a very important aspect from the structural point of view. Microcracks in concrete cause water and chlorine ions to enter the structure, causing the concrete to degrade and the reinforcement to corrode, posing an unacceptable level of structural risk. Hence repair of these cracks in an eco-friendly and cost-effective way is in the interest of various researchers. Microbially induced calcite precipitation (MICP) is an effective way considered by various researchers to heal those concrete cracks along with an important environmental contribution of CO2 (carbon dioxide) sequestration in the process.

Main content

As the current concentration of CO2 in the earth’s atmosphere is about 412 ppm, it possesses a deadly threat to the environmental issue of global warming. The use of bacteria for MICP can not only be a viable solution to repairing concrete cracks but also can play an important role of CO2 arrestation in carbonate form. This will help in carbon level management to lessen the adverse effects of this greenhouse gas on the atmospheric environment, particularly on the climate. To overcome the insufficiency of studies concentrating on this aspect, this review article focuses on the metabolic pathways and mechanisms of MICP and highlights the value of MICP for CO2 arrestation/sequestration from the atmosphere during the process of self-healing of concrete cracks, which is also the novelty of this work. An overview of recent studies on the implementation of MICP in concrete crack repair is used to discuss and analyse the factors influencing the effectiveness of MICP in the process, including various approaches used for CO2 sequestration. Furthermore, this investigation concentrates on finding the scope of work in the same field for the most effective ways of CO2 sequestration in the process of self-healing cracks of concrete.

Conclusion

In a prospective study, MICP can be an effective technology for CO2 sequestration in concrete crack repair, as it can reduce adverse environmental impacts and provide greener environment. This critical study concludes that MICP can bear a significant role in arrestation/sequestration of CO2, under proper atmospheric conditions with a cautious selection of microorganisms and its nutrient for the MICP procedure.

Graphical Abstract

Microbiologically induced calcite precipitation for in situ stabilization of heavy metals contributes to land application of sewage sludge

Microbiologically induced calcite precipitation (MICP) has shed new light on solving the problem of in situ stabilization of heavy metals (HMs) in sewage sludge before land disposal. In this study, we examined whether MICP treatment can be integrated into a sewage sludge anaerobic digestion-land application process. Our results showed that MICP treatment not only prevented the transfer of ionic-state Cd from the sludge to the supernatant (98.46 % immobilization efficiency) but also reduced the soluble exchangeable Pb and Cd fractions by up to 100 % and 48.54 % and increased the residual fractions by 22.54 % and 81.77 %, respectively. In addition, the analysis of the stability of HMs in MICP-treated sludge revealed maximum reductions of 100 % and 89.56 % for TCLP-extractable Pb and Cd, respectively. Three-dimensional fluorescence, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy analyses confirmed the excellent performance of the ureolytic bacteria Sporosarcina ureilytica ML-2 in the sludge system. High-throughput sequencing showed that the relative abundance of Sporosarcina sp. reached 53.18 % in MICP-treated sludge, and the urease metabolism functional genes unit increased by a maximum of 239.3 %. The MICP technology may be a feasible method for permanently stabilizing HMs in sewage sludge before land disposal.

Synergistic mechanism and application of microbially induced carbonate precipitation (MICP) and inorganic additives for passivation of heavy metals in copper-nickel tailings

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

Adverse health effects of emerging contaminants on inflammatory bowel disease

Inflammatory bowel disease (IBD) is becoming increasingly prevalent with the improvement of people's living standards in recent years, especially in urban areas. The emerging environmental contaminant is a newly-proposed concept in the progress of industrialization and modernization, referring to synthetic chemicals that were not noticed or researched before, which may lead to many chronic diseases, including IBD. The emerging contaminants mainly include microplastics, endocrine-disrupting chemicals, chemical herbicides, heavy metals, and persisting organic pollutants. In this review, we summarize the adverse health effect of these emerging contaminants on humans and their relationships with IBD. Therefore, we can better understand the impact of these new emerging contaminants on IBD, minimize their exposures, and lower the future incidence of IBD.

Microbial‑induced carbonate precipitation (MICP) technology: a review on the fundamentals and engineering applications

The microbial‑induced carbonate precipitation (MICP), as an emerging biomineralization technology mediated by specific bacteria, has been a popular research focus for scientists and engineers through the previous two decades as an interdisciplinary approach. It provides cutting-edge solutions for various engineering problems emerging in the context of frequent and intense human activities. This paper is aimed at reviewing the fundaments and engineering applications of the MICP technology through existing studies, covering realistic need in geotechnical engineering, construction materials, hydraulic engineering, geological engineering, and environmental engineering. It adds a new perspective on the feasibility and difficulty for field practice. Analysis and discussion within different parts are generally carried out based on specific considerations in each field. MICP may bring comprehensive improvement of static and dynamic characteristics of geomaterials, thus enhancing their bearing capacity and resisting liquefication. It helps produce eco-friendly and durable building materials. MICP is a promising and cost-efficient technology in preserving water resources and subsurface fluid leakage. Piping, internal erosion and surface erosion could also be addressed by this technology. MICP has been proved suitable for stabilizing soils and shows promise in dealing with problematic soils like bentonite and expansive soils. It is also envisaged that this technology may be used to mitigate against impacts of geological hazards such as liquefaction associated with earthquakes. Moreover, global environment issues including fugitive dust, contaminated soil and climate change problems are assumed to be palliated or even removed via the positive effects of this technology. Bioaugmentation, biostimulation, and enzymatic approach are three feasible paths for MICP. Decision makers should choose a compatible, efficient and economical way among them and develop an on-site solution based on engineering conditions. To further decrease the cost and energy consumption of the MICP technology, it is reasonable to make full use of industrial by-products or wastes and non-sterilized media. The prospective direction of this technology is to make construction more intelligent without human intervention, such as autogenous healing. To reach this destination, MICP could be coupled with other techniques like encapsulation and ductile fibers. MICP is undoubtfully a mainstream engineering technology for the future, while ecological balance, environmental impact and industrial applicability should still be cautiously treated in its real practice.

The global research trend on microbially induced carbonate precipitation during 2001-2021: a bibliometric review

Microbially induced carbonate precipitation (MICP) is a remarkable method that creates sustainable cementitious binding material for use in geotechnical/structural engineering and environmental engineering. This is due to the increasing demand for alternative environmentally friendly technologies and materials that result in minimal or zero carbon footprint. In contrast to the previously published literature, through bibliometric analysis, this review paper focuses on the current prospects and future research trends of MICP technology via the Scopus database and VOSviewer analysis. The objective of the study was to determine the annual publications and citations trend, most contributing countries, the leading journals, prolific authors, productive institutions, funding sponsors, trending author keywords, and research directions of MICP. There were a total of 1058 articles published from 2001 to 2021 on MICP. The result demonstrated that the volume of publications is increasing. China, Construction and Building Materials, Satoru Kawasaki, Nanyang Technological University, and the National Natural Science Foundation of China are the leading country, journal, author, institution, and funding sponsor in terms of total publications. Through the co-occurrence analysis of the author keywords, MICP was revealed to be the most frequently used author keyword with 121 occurrences, a total link strength of 213, and 152 links to other author keywords. Furthermore, co-occurrence analysis of text data revealed that researchers are concentrating on four important research areas: precipitation, MICP, compressive strength, and biomineralization. This review can provide information to researchers that can lead to novel ideas and research collaboration or engagement on MICP technology.

The role of Fe-oxidizing bacteria (FeOB) and organic matters in As removal in the heavy-polluted arid soil

The bio-remediation of As-polluted farmlands in the arid area is seldomly reported. This study aimed at understanding the impact of DOM, Fe-oxides, and FeOB biogeochemical processes on As remediation. The approaches used included: FeOB strain Pseudomonas flavescens LZU-3; Batch-experiment. Our results showed that all FeOB tested effectively immobilized As (>95%) during microbial mineralization; DOM play an important role in the reduction of Fe(III)(hydr)oxides and As(V); Less-crystallized ferrihydrite transform to more-crystallized goethite and secondary minerals; Under the reaction of FeOB and DOM, the As-Fe-OM ternary compound were formed, containing N, S, C and O functional group; The addition of OM can clearly reduce soil Eh, promoting dissolution of As in bound to iron oxides, co-precipitation of the amorphous iron oxide in Fe(III)-OM-FeOB, closely related to As in bound to insoluble organics and sulfides and mineral residues, which plays an important role in controlling the mobilization of As. This study provides controlling of As transportation and transformation in the As-DOM-Bio-Fe ternary system as As-remediation technology in the arid soil.

Catalyzing urea hydrolysis using two-step microbial-induced carbonate precipitation for copper immobilization: Perspective of pH regulation

Microbial induced carbonate precipitation (MICP) has recently applied to immobilize heavy metals toward preventing their threats to public health and sustainable development of surrounding environments. However, for copper metallurgy activities higher copper ion concentrations cause the ureolytic bacteria to lose their activity, leading to some difficulty in forming carbonate precipitation for copper immobilization (referred to also as “biomineralization”). A series test tube experiments were conducted in the present work to investigate the effects of bacterial inoculation and pH conditions on the copper immobilization efficiency. The numerical simulations mainly aimed to compare with the experimental results to verify its applicability. The copper immobilization efficiency was attained through azurite precipitation under pH in a 4–6 range, while due to Cu²⁺ migration and diffusion, it reduced to zero under pH below 4. In case pH fell within a 7–9 range, the immobilization efficiency was attained via malachite precipitation. The copper-ammonia complexes formation reduced the immobilization efficiency to zero. The reductions were attributed either to the low degree of urea hydrolysis or to inappropriate pH conditions. The findings shed light on the necessity of securing the urease activity and modifying pH conditions using the two-step biomineralization approach while applying the MICP technology to remedy copper-rich water bodies.

E-waste scenario in South-Asia: an emerging risk to environment and public health

Over the past decade, e-waste generation has been accelerated in the world as never before, particularly South-Asia is confronted with an enormous risk of e-waste intensification owing to both locally generated and internationally imported. There has been a gradual increase of e-waste generated in South-Asia and in 2019, 4,057 Kilo tons (kt) of e-waste was generated, which is about 16% of the Asian region. Though there is an urgent requirement to rectify the catastrophic accumulation of e-waste and for its effective eco-friendly management, inadequate legal implementation and poor enforcement, lack of awareness, weak formal e-waste collection and recycling process allow for escalating problems associated with e-waste, particularly towards the environmental and public health concern. Under these circumstances, this paper has been written by reviewing the available research findings, since 2000 to find out the current scenario of South-Asia. Unfortunately, the problem is also not seen as a hot topic to address by the researchers, there are only 106 research studies conducted in South-Asia. Out of that, a considerable number of studies were conducted only in India (54%), Bangladesh (23%), and Pakistan (16%). Sri Lanka, Nepal, and Bhutan shared the rest. As a matter of fact, many more studies are needed on environmental and human health effects, legal implementations, awareness and novel managerial strategies etc. to assist policymakers and other relevant authorities in making their decisions. Similarly, rather than facing threats alone, fighting against as a region would be ideal, which also helps to restrict intra movement of e-waste among the South-Asian countries.

Micro-dynamic process of cadmium removal by microbial induced carbonate precipitation

Microbially induced carbonate precipitation (MICP) is a technique used extensively to address heavy metal pollution but its micro-dynamic process remains rarely explored. In this study, A novel Cd-tolerant ureolytic bacterium DL-1 (Pseudochrobactrum sp.) was used to study the micro-dynamic process. With conditions optimized by response surface methodology, the removal efficiency of Cd²⁺ could achieve 99.89%. Three components were separated and characterized in the reaction mixture of Cd²⁺ removal by MICP. The quantitative-dynamic distribution of Cd²⁺ in different components was revealed. Five synergistic effects for Cd²⁺ removal were found, including co-precipitation, adsorption by precipitation, crystal precipitation on the cell surface, intracellular accumulation and extracellular chemisorption. Importantly, during Cd²⁺ removal by MICP, the phenomenon that crystalline nanoparticles adhere to the cell surface, but without any micrometer-sized precipitation encapsulated bacterial cells was observed. This indicated that the previously studied model of bacterial cells as nucleation sites for metal cation precipitation and crystal growth is oversimplified. Our findings provided valuable insights into the mechanism of heavy metals removal by MICP, and a more straightforward method for studying biomineralization-related dynamic process.

Effects of urease-producing bacteria and eggshell on physiological characteristics and Cd accumulation of pakchoi (Brassica chinensis L.) plants

Soil cadmium (Cd) contamination resulting from anthropogenic activity poses severe threats to food safety and human health. In this study, a pot experiment was performed to evaluate the possibility of using urease-producing bacterium UR21 and eggshell (ES) waste for improving the physiological characteristics and reducing Cd accumulation of pakchoi (Brassica chinensis L.) plants. UR21 has siderophore and IAA production ability. The application of UR21 and ES individually or in combination could improve the root and shoot length, and fresh and dry weight of pakchoi plants under Cd stress. In Cd + ES + UR21-treated plants, the dry weight of shoot and root were increased by 61.54% and 72.73%, respectively. The chlorophyll a, chlorophyll b, and carotenoid content were increased by 52.19%, 42.95%, and 95.56% in Cd + ES + UR21-treated plants. Meanwhile, the H₂O₂ and MDA content were decreased while the SOD and POD activity were increased, and an increase of soluble protein level in pakchoi plants was observed under Cd + ES + UR21 treatment. Importantly, eggshell and UR21 alone or in combination induced a decline of Cd content in pakchoi plants, especially that Cd + ES + UR21 treatment decreased Cd content in shoot and root by 26.96% and 42.91%, respectively. Meanwhile, the soil urease and sucrase activities were enhanced. Generally, the combined application of ureolytic bacteria UR21 and eggshell exhibited better effects than applied them individually in terms of alleviating Cd toxicity in pakchoi plants. Our findings may give a unique perspective for an eco-friendly and sustainable strategy to remediate heavy metal-polluted soils.

Bio-enrichment of heavy metals U(VI) in wastewater by protein DSR A

Uptaking U(VI) from the environment by biological method is an environmental friendly and efficient way. In this work, we have acquired and isolated the protein DSR A by genetic engineering, then assessed its capacity and mechanisms to absorb U(VI) from wastewater. As results, we proved that protein DSR A can precisely recognize, enrich and remove uranyl ions in simulated wastewater solution. Its great removal potential was demonstrated in the adsorption experiments, the adsorption capacity of protein DSR A can reach 182.3 mg/g in the condition at 10 mg/L U(VI) and pH = 6. The Langmuir isotherm model and the pseudo-first-order kinetic equation were used to better describe the absorption process. We can confirm that Na⁺, Sr²⁺ and K⁺, these three metal ions have less effect on the enrichment of U(VI) by protein DSR A compared with other common cations. Besides, we can educe some mechanisms for the removal of U (VI) by protein DSR A from the results of FTIR, SEM-EDS, XPS (binding energy = 2.0 ~ 4.0ke V), MAP and XRD analysis before and after adsorption. This work has demonstrated the great potential of genetic engineering and biological methods in dealing with environmental heavy ion pollution.

Microbial Remediation: A Promising Tool for Reclamation of Contaminated Sites with Special Emphasis on Heavy Metal and Pesticide Pollution: A Review

Heavy metal and pesticide pollution have become an inevitable part of the modern industrialized environment that find their way into all ecosystems. Because of their persistent nature, recalcitrance, high toxicity and biological enrichment, metal and pesticide pollution has threatened the stability of the environment as well as the health of living beings. Due to the environmental persistence of heavy metals and pesticides, they get accumulated in the environs and consequently lead to food chain contamination. Therefore, remediation of heavy metals and pesticide contaminations needs to be addressed as a high priority. Various physico-chemical approaches have been employed for this purpose, but they have significant drawbacks such as high expenses, high labor, alteration in soil properties, disruption of native soil microflora and generation of toxic by-products. Researchers worldwide are focusing on bioremediation strategies to overcome this multifaceted problem, i.e., the removal, immobilization and detoxification of pesticides and heavy metals, in the most efficient and cost-effective ways. For a period of millions of evolutionary years, microorganisms have become resistant to intoxicants and have developed the capability to remediate heavy metal ions and pesticides, and as a result, they have helped in the restoration of the natural state of degraded environs with long term environmental benefits. Keeping in view the environmental and health concerns imposed by heavy metals and pesticides in our society, we aimed to present a generalized picture of the bioremediation capacity of microorganisms. We explore the use of bacteria, fungi, algae and genetically engineered microbes for the remediation of both metals and pesticides. This review summarizes the major detoxification pathways and bioremediation technologies; in addition to that, a brief account is given of molecular approaches such as systemic biology, gene editing and omics that have enhanced the bioremediation process and widened its microbiological techniques toward the remediation of heavy metals and pesticides.

Bioremediation of stainless steel pickling sludge through microbially induced carbonate precipitation

In this study, microbial induce carbonate precipitation (MICP) was introduced to immobilize chromium (Cr) in stainless steel pickling sludge (SSPS). Two methods were utilized to conduct the MICP process - Bacteria lysis liquor (BLL)-based MICP and bacteria-based MICP. BLL was obtained by breaking the cell walls with ultrasonic treatment. The urea hydrolyzation test illustrated that the BLL was better than bacteria solution. Both the treatments of bacteria lysis liquor-based MICP and bacteria-based MICP process can effectively entrap the Cr into mineral lattices, that reduce the potential environmental risk of SSPS. With 30 g/L urea and 7 days' treatment, BLL-based MICP presented better immobilization performance than bacteria-based MICP by lowering the bacteria concentration (OD₆₀₀) from 0.8 to 0.7. The excellent biosorption of BLL contributed to Cr removal. Nevertheless, the addition of calcium (Ca) significantly enhanced the immobilization performance of bacteria-based MICP process rather than BLL-based MICP process. pH-dependent leaching tests illustrated the leaching of Cr followed an amphoteric pattern, while the leaching of Ni and Ca followed the cation pattern. Geochemical modeling revealed that the leaching of Cr from bio-mineralized products was solubility-controlled by Cr(OH)₃ and Cr₂O₃.

Plants-Microorganisms-Based Bioremediation for Heavy Metal Cleanup: Recent Developments, Phytoremediation Techniques, Regulation Mechanisms, and Molecular Responses

Rapid industrialization, mine tailings runoff, and agricultural activities are often detrimental to soil health and can distribute hazardous metal(loid)s into the soil environment, with harmful effects on human and ecosystem health. Plants and their associated microbes can be deployed to clean up and prevent environmental pollution. This green technology has emerged as one of the most attractive and acceptable practices for using natural processes to break down organic contaminants or accumulate and stabilize metal pollutants by acting as filters or traps. This review explores the interactions between plants, their associated microbiomes, and the environment, and discusses how they shape the assembly of plant-associated microbial communities and modulate metal(loid)s remediation. Here, we also overview microbe-heavy-metal(loid)s interactions and discuss microbial bioremediation and plants with advanced phytoremediation properties approaches that have been successfully used, as well as their associated biological processes. We conclude by providing insights into the underlying remediation strategies' mechanisms, key challenges, and future directions for the remediation of metal(loid)s-polluted agricultural soils with environmentally friendly techniques.

Remediation of arsenic-contaminated paddy field by a new iron oxidizing strain (Ochrobactrum sp.) and iron-modified biochar

Iron-oxidizing strain (FeOB) and iron modified biochars have been shown arsenic (As) remediation ability in the environment. However, due to the complicated soil environment, few field experiment has been conducted. The study was conducted to investigate the potential of iron modified biochar (BC-FeOS) and biomineralization by a new found FeOB to remediate As-contaminated paddy field. Compared with the control, the As contents of G_B (BC-FeOS), G_F (FeOB), G_FN (FeOB and nitrogen fertilizer), G_BF (BC-FeOS and FeOB) and G_BFN (BC-FeOS, FeOB and nitrogen fertilizer) treatments in pore water decreased by 36.53%-80.03% and the microbial richness of iron-oxidizing bacteria in these treatments increased in soils at the rice maturation stage. The concentrations of available As of G_B, G_F, G_FN, G_BF and G_BFN at the tillering stage were significantly decreased by 10.78%-55.48%. The concentrations of nonspecifically absorbed and specifically absorbed As fractions of G_B, G_F, G_FN, G_BF and G_BFN in soils were decreased and the amorphous and poorly crystalline hydrated Fe and Al oxide-bound fraction was increased. Moreover, the As contents of G_B, G_F, G_FN, G_BF and G_BFN in rice grains were significantly decreased (*P < 0.05) and the total As contents of G_FN, G_BF and G_BFN were lower than the standard limit of the National Standard for Food Safety (GB 2762-2017). Compared with the other treatments, G_BFN showed the greatest potential for the effective remediation of As-contaminated paddy fields.

Effects of the Urease Concentration and Calcium Source on Enzyme-Induced Carbonate Precipitation for Lead Remediation

Heavy metal contamination during the rapid urbanization process in recent decades has notably impacted our fragile environments and threatens human health. However, traditional remediation approaches are considered time-consuming and costly, and the effect sometimes does not meet the requirements expected. The present study conducted test tube experiments to reproduce enzyme-induced carbonate precipitation applied to lead remediation under the effects of urease concentration and a calcium source. Furthermore, the speciation and sequence of the carbonate precipitation were simulated using the Visual MINTEQ software package. The results indicated that higher urease concentrations can assure the availability of CO3 2- during the enzyme-induced carbonate precipitation (EICP) process toward benefiting carbonate precipitation. The calcium source determines the speciation of carbonate precipitation and subsequently the Pb remediation efficiency. The use of CaO results in the dissolution of Pb(OH)2 and, therefore, discharges Pb ions, causing some difficulty in forming the multi-layer structure of carbonate precipitation and degrading Pb remediation. The findings of this study are useful in widening the horizon of applications of the enzyme-induced carbonate precipitation technology to heavy metal remediation.

Indigenous microbial populations of abandoned mining sites and their role in natural attenuation

Environmental contamination by toxic effluents discharged by anthropogenic activities including the mining industries has increased extensively in the recent past. Microbial communities and their biofilms inhabiting these extreme habitats have developed different adaptive strategies in metabolizing and transforming the persistent pollutants. They also play a crucial role in natural attenuation of these abandoned mining sites and act as a major driver of many biogeochemical processes, which helps in ecological rehabilitation and is a viable approach for restoration of wide stretches of land. In this review, the types of mine wastes including the overburden and mine drainage and the types of microbial communities thriving in such environments were probed in detail. The types of biofilms formed along with their possible role in metal bioremediation were also reviewed. This review also provides an overview of the shift in microbial communities in natural reclamation process and also provides an insight into the restoration of the enzyme activities of the soils which may help in further revegetation of abundant mining areas in a sustainable manner. Moreover, the role of indigenous microbiota in bioremediation of heavy metals and their plant growth-promoting activity weres discussed to assess their role in phytoremedial processes.

Bio-Mediated Method for Immobilizing Copper Tailings Sand Contaminated with Multiple Heavy Metals

Microbial induced carbonate precipitation (MICP) is a natural bio-mediated process that has been investigated for soil stabilization and heavy metal immobilization in soil and groundwater. This study analyzed the effect and mechanism of MICP for the solidification/stabilization of tailings sand with multi-heavy metals. When the concentration of cementation solution (CS) is 1.0 mol/L and the optical density(OD600) is 1.6, the unconfined compressive strength of tailings sand treated by MICP is the largest, and the solidification efficiency of heavy metals in tailings sand is also the highest. The macroscopic and microscopic observations reveal that the mechanism of MICP solidification of tailings is bacterial outer oxide, hydroxide, alkaline carbonate, and carbonate precipitation.

Effects of bacterial inoculation and calcium source on microbial-induced carbonate precipitation for lead remediation

Heavy metal contamination has caused serious threats to surrounding fragile environments and human health. While the novel microbial-induced carbonate precipitation (MICP) technology in the recent years has been proven effective in improving material mechanical and durability properties, the mechanisms remedying heavy metal contamination still remain unclear. In this study, the potential of applying the MICP technology to the lead remediation under the effects of urease activity and calcium source was explored. The values of OD₆₀₀ corresponding to the ureolytic bacterial activity, electrical conductivity (EC), urease activity (UA) and pH were applied to monitor the degree of urea hydrolysis. Further, the carbonate precipitations that possess different speciations and cannot be distinguished through test tube experiments were reproduced using the Visual MINTEQ software package towards verifying the validity of the proposed simulations, and revealing the mechanisms affecting the lead remediation efficiency. The findings summarised in this work give deep insights into lead-contaminated site remediation engineering.

A review on the applications of microbially induced calcium carbonate precipitation in solid waste treatment and soil remediation

Improper disposal and accumulation of solid waste can cause a number of environmental problems, such as the heavy metal contamination of soil. Microbially induced calcium carbonate precipitation (MICP) is considered as a promising technology to solve many environmental problems. Calcium-based solid waste can be utilized as an alternative source of calcium for the MICP process, and carbonate-based biominerals can be used for soil remediation, solid waste treatment, remediation of construction concrete, and generation of bioconcrete. This paper describes the metabolic pathways and mechanisms of microbially induced calcium carbonate precipitation and highlights the value of MICP for solid waste treatment and soil remediation applications. The factors affecting the effectiveness of MICP are discussed and analyzed through an overview of recent studies on the application of MICP in environmental engineering. The paper also summarizes the current challenges for the large-scale application of this innovative technology. In prospective study, MICP can be an effective alternative to conventional technologies in solid waste treatment, soil remediation and CO₂ sequestration, as it can reduce negative environmental impacts and provide long-term economic benefits.

Arsenic Transformation in Soil-Rice System Affected by Iron-Oxidizing Strain (Ochrobactrum sp.) and Related Soil Metabolomics Analysis

Iron-oxidizing bacteria (FeOB) could oxidize Fe(II) and mediate biomineralization, which provides the possibility for its potential application in arsenic (As) remediation. In the present study, a strain named Ochrobactrum EEELCW01 isolated previously, was inoculated into paddy soils to investigate the effect of FeOB inoculation on the As migration and transformation in paddy soils. The results showed that inoculation of Ochrobactrum sp. increased the proportion of As in iron-aluminum oxide binding fraction, which reduced the As bioavailability in paddy soils and effectively reduced the As accumulation in rice tissues. Moreover, the inoculation of iron oxidizing bacteria increased the abundance of KD4-96, Pedosphaeraceae and other bacteria in the soils, which could reduce the As toxicity in the soil through biotransformation. The abundance of metabolites such as carnosine, MG (0:0/14:0/0:0) and pantetheine 4'-phosphate increased in rhizosphere soils inoculated with FeOB, which indicated that the defense ability of soil-microorganism-plant system against peroxidation caused by As was enhanced. This study proved that FeOB have the potential application in remediation of As pollution in paddy soil, FeOB promotes the formation of iron oxide in paddy soil, and then adsorbed and coprecipitated with arsenic. On the other hand, the inoculation of Ochrobactrum sp. change soil microbial community structure and soil metabolism, increase the abundance of FeOB in soil, promote the biotransformation process of As in soil, and enhance the resistance of soil to peroxide pollution (As pollution).

Investigating the potential for microbially induced carbonate precipitation to treat mine waste

In this study, the feasibility of promoting microbially induced carbonate precipitation (MICP) in mine waste piles by using an environmental bacterial enrichment is explored, with goals to reduce metals and acid leaching. MICP has been explored for remediation applications and stabilization of mine waste. Here, we utilize a native bacterial enrichment to promote MICP on seven mine waste samples with variability in acid production and extent of toxic metal leaching. During fifteen applications of MICP solutions and bacteria on waste rock in bench-scale columns, calcium carbonate formed on grain surfaces within all waste samples, though microscopy revealed uneven distribution of CaCO3 coating. The effluent from acid-producing wastes increased in pH during MICP treatment. MICP performance was evaluated with humidity cell and synthetic precipitation leaching procedure (SPLP) tests. Leaching tests revealed reductions in Cd, Pb and Zn concentrations in leachate of all but one sample, mixed results for Cu, and As increasing in all but one leachate sample after treatment. MICP technology has potential for coating mine waste and reducing release of acid and some metals. This study provides a laboratory assessment of MICP feasibility for stabilizing mine waste in situ and mitigating release of toxic metals into the environment.