Microbially Accelerated Carbonate Mineral Precipitation as a Strategy for in Situ Carbon Sequestration and Rehabilitation of Asbestos Mine Sites

Weathering and cementation of historic kimberlite residues from South Africa: Implications for residue stabilization and CO2 sequestration

Enhanced weathering and carbon dioxide (CO2) mineralization of ultramafic mine wastes, including kimberlite residues from diamond mining, provides secure storage of this greenhouse gas and may physically stabilize mine impoundments. Yet, the outcomes of these processes over extensive periods (i.e., decades) remain relatively unknown. This study examined coarse residues from historic impoundments at the Cullinan and Voorspoed diamond mines in South Africa that have weathered over 50 to more than 100 years to investigate weathering and cementation pathways. Cemented residues (n = 7) were mainly composed of lizardite (9.6-43.2 wt.%), saponite (10.2-34.7 wt.%), and augite (6.6-27.8 wt.%), and had minor abundances of calcite (1.7-8.8 wt.%). Electron microscopy and Raman spectroscopy revealed that three plausible pathways contributed to cementing residues: (1) secondary clay precipitation, (2) carbonate precipitation and recycling, and (3) particle entrainment and infilling. Quantitative mineralogical analyses of the cement (<63 μm) and clast (>63 μm) fractions showed that the abundances of most minerals were similar between fractions, indicating that infilling of pore spaces with fine-grained particles contributed substantially to residue cementation. Stable and radiogenic carbon isotope (13C, 14C) analyses of carbonates indicated limited incorporation of organic matter and atmospheric CO2, an indication of surface weathering. Residue cementation led to some strength development (unconfined compressive strengths = 0.1-0.2 MPa), demonstrating the potential for mineral weathering to stabilize mine residues without chemical additives. Modifying residue management practices during the operational, closure, and reclamation phases at Cullinan, Voorspoed, and other mines, such as through residue co-disposal or implementing enhanced weathering practices, may improve residue stabilization and CO2 sequestration..

The interplay between microbial communities and soil properties

In recent years, there has been considerable progress in determining the soil properties that influence the structure of the soil microbiome. By contrast, the effects of microorganisms on their soil habitat have received less attention with most previous studies focusing on microbial contributions to soil carbon and nitrogen dynamics. However, soil microorganisms are not only involved in nutrient cycling and organic matter transformations but also alter the soil habitat through various biochemical and biophysical mechanisms. Such microbially mediated modifications of soil properties can have local impacts on microbiome assembly with pronounced ecological ramifications. In this Review, we describe the processes by which microorganisms modify the soil environment, considering soil physics, hydrology and chemistry. We explore how microorganism-soil interactions can generate feedback loops and discuss how microbially mediated modifications of soil properties can serve as an alternative avenue for the management and manipulation of microbiomes to combat soil threats and global change.

Combination of magnetic field and ultraviolet for fouling control in saline wastewater distribution systems

Fouling is a significant challenge for recycling and reusing saline wastewaters for industrial, agricultural or municipal applications. In this study, we propose a novel approach of magnetic field (MaF) and ultraviolet (UV) combined application for fouling mitigation. Results showed, combination of MaF and UV (MaF-UV) significantly decreased the content of biofouling and reduced the complexity of microbial networks, compared to UV and MaF alone treatments. This was due to MaF as pretreatment effectively reduced the water turbidity, improve the influent water quality of UV disinfection and increases UV transmittance, eliminating the adverse impacts of UV scattering and shielding, hence increased the inactivation effectiveness of UV disinfection process. MaF assisted UV also reduced the abundance of UV-resistant bacteria and inhibited the risk of bacterial photoreactivation and dark repair. Meanwhile, MaF-UV drastically reduced the contents of precipitates and particulate fouling by accelerating the transformation rate of CaCO3 crystal from compact calcite to loosen hydrated amorphous CaCO3, and enhancing the flocculation process. These findings demonstrated that MaF-UV is an effective anti-fouling strategy, and provide insights for sustainable application of saline wastewaters.

The Mining Industry's Role in Enhanced Weathering and Mineralization for CO2 Removal

Enhanced weathering and mineralization (EWM) aim to remove carbon dioxide (CO2) from the atmosphere by accelerating the reaction of this greenhouse gas with alkaline minerals. This suite of geochemical negative emissions technologies has the potential to achieve CO2 removal rates of >1 gigatonne per year, yet will require gigatonnes of suitable rock. As a supplier of rock powder, the mining industry will be at the epicenter of the global implementation of EWM. Certain alkaline mine wastes sequester CO2 under conventional mining conditions, which should be quantified across the industry. Furthermore, mines are ideal locations for testing acceleration strategies since tailings impoundments are contained and highly monitored. While some environmentally benign mine wastes may be repurposed for off-site use─reducing costs and risks associated with their storage─numerous new mines will be needed to supply rock powders to reach the gigatonne scale. Large-scale EWM pilots with mining companies are required to progress technology readiness, including carbon verification approaches. With its knowledge of geological formations and ore processing, the mining industry can play an essential role in extracting the most reactive rocks with the greatest CO2 removal capacities, creating supply chains, and participating in life-cycle assessments. The motivations for mining companies to develop EWM include reputational benefits and carbon offsets needed to achieve carbon neutrality.

Mineral extraction from asbestos-containing waste (ACW) and changes in its morphology during treatment with ammonium salts

Asbestos is a known carcinogen and a banned hazardous material. However, the generation of asbestos-containing waste (ACW) is increasing because of the demolition of old constructions, buildings, and structures. Therefore, asbestos-containing wastes need to be effectively treated to render them harmless. This study aimed to stabilize asbestos wastes by using for the first time three different ammonium salts at low reaction temperatures. The treatment was performed with ammonium sulfate (AS), ammonium nitrate (AN), and ammonium chloride (AC) at concentrations of 0.1, 0.5, 1.0, and 2.0 M and reaction times of 10, 30, 60, 120, and 360 min intervals at 60 °C. Asbestos waste samples were treated in both plate and powder form during the experiment. The results demonstrated that the selected ammonium salts could extract the mineral ions from asbestos materials at a relatively low temperature. Concentrations of the minerals extracted from powdered samples were higher than those extracted from plate samples. AS treatment demonstrated better extractability compared to that of AN and AC, based on the concentrations of magnesium and silicon ions in the extract. The results implied that among the three ammonium salts, AS had better potential to stabilize the asbestos waste. This study demonstrated the potential of ammonium salts for treating and stabilizing asbestos waste at low temperatures by extracting the mineral ions from the asbestos fibers.Implications: This study aims to establish an effective treatment to stabilize the hazardous asbestos waste to harmless forms. We have attempted treatment of asbestos with three ammonium salts (ammonium sulfate, ammonium nitrate, ammonium chloride) at relatively lower temperature. The selected ammonium salts could extract the mineral ions from asbestos materials at a relatively low temperature. These results suppose that asbestos containing materials could change the harmless state by using simple method. Among the ammonium salts, especially, AS has better potential to stabilize the asbestos waste.

Microbially mediated carbon dioxide removal for sustainable mining

The climate crisis and rising demand for critical minerals necessitate the development of novel carbon dioxide removal and ore processing technologies. Microbial processes can be harnessed to recover metals from and store carbon dioxide within mine tailings to transform the mining industry for a greener and more sustainable future.

Removal of atmospheric CO2 by engineered soils in infrastructure projects

The use of crushed basic igneous rock and crushed concrete for enhanced rock weathering and to facilitate pedogenic carbonate precipitation provides a promising method of carbon sequestration. However, many of the controls on precipitation and subsequent effects on soil properties remain poorly understood. In this study, engineered soil plots, with different ratios of concrete or dolerite combined with sand, have been used to investigate relationships between sequestered inorganic carbon and geotechnical properties, over a two-year period. Cone penetration tests with porewater pressure measurements (CPTu) were conducted to determine changes in tip resistance and pore pressure. C and O isotope analysis was carried out to confirm the pedogenic origin of carbonate minerals. TIC analysis shows greater precipitation of pedogenic carbonate in plots containing concrete than those with dolerite, with the highest sequestration values of plots containing each material being equivalent to 33.7 t C ha^-1 yr^-1 and 17.5 t C ha^-1 yr^-1, respectively, calculated from extrapolation of results derived from the TIC analysis. TIC content showed reduction or remained unchanged for the top 0.1 m of soil; at a depth of 0.2 m however, for dolerite plots, a pattern of seasonal accumulation and loss of TIC emerged. CPTu tip resistance measurements showed that the presence of carbonates had no observable effect on penetration resistance, and in the case of porewater pressure measurements, carbonate precipitation does not change the permeability of the substrate, and so does not affect drainage. The results of this study indicate that both the addition of dolerite and concrete serve to enhance CO₂ removal in soils, that soil temperature appears to be a control on TIC precipitation, and that mineral carbonation in constructed soils does not lead to reduced drainage or an increased risk of flooding.

Kinetics and mechanisms of cyanobacterially induced precipitation of magnesium silicate

The biomineralization of CO₂ , in the form of carbonate minerals, is considered as one of the efficient solutions of atmospheric CO₂ removal, allowing stable and sustainable storage of this greenhouse gas. Cyanobacteria are among the most powerful microorganisms capable of precipitating carbonate minerals, both in the present and in the past. In the modern environments, high Si concentration during geoengineering biomineralization could occur due to dissolution of Mg-bearing primary silicates such as olivine. However, most of experimental studies aimed to understand the formation of these carbonates were performed in Si-poor solutions. Thus, experimental characterizations of the nature, rate, and stoichiometry of precipitated minerals in Si-rich solutions in the presence of bacteria are lacking. The present study attempted to reproduce, in controlled laboratory experiments, the processes of biomineralization in a carbonate- and Mg-bearing medium having high Si concentrations (2-4 mM, which is below the saturation with respect to amorphous silica). These experiments have been carried out in the presence of three contrasting cyanobacteria: Synechococcus sp., Chroococcidiopsis sp. and Aphanothece clathrata in order to characterize the rate of formation, stoichiometry and mineralogical nature of precipitates. The results demonstrated significant role of cyanobacteria in the precipitation of carbonate and silicate minerals by increasing the pH of the medium during photosynthesis. Magnesium precipitation rates measured between 50 and 150 h of reaction time ranged from 0.05 to 0.5 mmol h^-1 g_dry¹ and decreased (Synechococcus sp. and Chroococcidiopsis sp.) or increased (A. clathrata) with an increase in the Si:Mg ratio in solution. The abiotic instantaneous rates of Mg and Si removal from alkaline solutions were similar to those in the presence of cyanobacteria at the same pH value suggesting that photosynthetically induced pH rise was the main factor of mineral formation. The transmission electron microscopy (TEM) and spectroscopic observations and associated analyses identified an amorphous magnesium silicate together with hydrous Mg carbonates (hydromagnesite). The formation of carbonate solid phase at high Mg: Si ratios indicated the potential for the removal of inorganic carbon at pH > 10. The difference in the degree of C removal between different species was primarily linked to different degree of pH rise during photosynthesis. Taken together, the results obtained in this study allowed an efficient reproduction of combined magnesium hydroxo-carbonates and hydrous silicates precipitation under cyanobacterial activity, suitable for geoengineering of biologically controlled CO₂ sequestration in Si-Mg-carbonate-bearing solutions.

The utilization of alkaline wastes in passive carbon capture and sequestration: Promises, challenges and environmental aspects

Alkaline wastes have been the focus of many studies as they act as CO₂ sinks and have the potential to offset emissions from mining and steelmaking industries. Passive carbonation of alkaline wastes mimics natural silicate weathering and provides a promising alternative pathway for CO₂ capture and storage as carbonates, requiring marginal human intervention when compared to ex-situ carbonation. This review summarizes the extant research that has investigated the passive carbonation of alkaline wastes, namely ironmaking and steelmaking slag, mine tailings and demolition wastes, over the past two decades. Here we report different factors that affect passive carbonation to address challenges that this process faces and to identify possible solutions. We identify avenues for future research such as investigating how passive carbonation affects the surrounding environment through interaction with the biosphere and the hydrosphere. Future research should also consider economic analyses to provide investors with an in-depth understanding of passive carbonation techniques. Based on the reviewed materials, we conclude that passive carbonation can be an important contributor to climate change mitigation strategies, and its potential can be intensified by applying simple waste management practices.

Kinetics-informed global assessment of mine tailings for CO2 removal

Chemically reactive mine tailings are a potential resource for drawing down carbon dioxide out of the atmosphere in mineral weathering schemes. Such carbon dioxide removal (CDR) systems, applied on a large scale, could help to meet internationally agreed targets for minimising climate change, but crucially we need to identify what materials could react fast enough to provide CDR at relevant climate change mitigation timescales. This study focuses on a range of silicate-dominated tailings, calculating their CDR potential from their chemical composition (specific capacity), estimated global production rates, and the speed of weathering under different reaction conditions. Tailings containing high abundances of olivine, serpentine and diopside show the highest CDR potential due to their favourable kinetics. We conclude that the most suitable tailings for CDR purposes are those associated with olivine dunites, diamond kimberlites, asbestos and talc serpentinites, Ni sulphides, and PGM layered mafic intrusions. We estimate the average annual global CDR potential of tailings weathered over the 70-year period 2030-2100 to be ~93 (unimproved conditions) to 465 (improved conditions) Mt/year. Results indicate that at least 30 countries possess tailings materials that, under improved conditions, may offer a route for CDR which is not currently utilised within the mining industry. By 2100, the total cumulative CDR could reach some 33 GtCO₂, of which more than 60% is contributed by PGM tailings produced in Southern Africa, Russia, and North America. The global CDR potential could be increased by utilization of historic tailings and implementing measures to further enhance chemical reaction rates. If practical considerations can be addressed and enhanced weathering rates can be achieved, then CDR from suitable tailings could contribute significantly to national offset goals and global targets. More research is needed to establish the potential and practicality of this technology, including measurements of the mineral weathering kinetics under various conditions.

Multiple fouling dynamics, interactions and synergistic effects in brackish surface water distribution systems

Dissolved salts, colloidal particles, and active microorganisms in brackish surface water distribution systems (BSWD) cause multiple fouling, poses potential threat to the environmental pollution, and raising technical and economic issues as well. So far, the co-occurrence and interactions of multiple fouling remains largely unknown. Multiple fouling behaviors were assessed in agriculture BSWD under different nitrogen (N) fertilizers. X-ray diffraction, Rietveld refinement analysis, 16S rRNA, and microbial network analysis were conducted to determine the fouling characteristics. Statistical analysis was applied to reveal the relative contributions and interaction of multiple fouling. Our results demonstrated, multiple fouling of precipitates, particulates and biofoulings were co-occurred. Fouling growth was largely attributed to the strong interactions of different fouling. The binary interactions of precipitates - particulates contributed 51.1%, and ternary interactions of precipitates - particulates - biofouling contributed 25.4% to explain the decline of system performance, while the contribution of each single type fouling was minimal. Thereby indicating the significant role of calcium silica, biomineralization and bio-silicates in fouling. The lower acid N fertilizer broken the interaction of multiple fouling by increasing the precipitate crystal parameters and repulsive forces amongst particulates, as well as destroyed microbial interactions in biofouling. Overall, this study open frontier for multiple fouling in-depth profiling and antifouling guidance for effective utilization of BSWD.

Attachment on mortar surfaces by cyanobacterium Gloeocapsa PCC 73106 and sequestration of CO2 by microbially induced calcium carbonate

Cyanobacterial carbonate precipitation induced by cells and extracellular polymeric substances (EPS) enhances mortar durability. The percentage of cell/EPS attachment regulates the effectiveness of the mortar restoration. This study investigates the cell coverage on mortar and microbially induced carbonate precipitation. Statistical analysis of results from scanning electron and fluorescence microscopy shows that the cell coverage was higher in the presence of UV-killed cells than living cells. Cells are preferably attached to cement paste than sand grains, with a difference of one order of magnitude. The energy-dispersive X-ray spectroscopy analyses and Raman mapping suggest cyanobacteria used atmospheric CO2 to precipitate carbonates.

Carbonation, Cementation, and Stabilization of Ultramafic Mine Tailings

Tailings dam failures can cause devastation to the environment, loss of human life, and require expensive remediation. A promising approach for de-risking brucite-bearing ultramafic tailings is in situ cementation via carbon dioxide (CO₂) mineralization, which also sequesters this greenhouse gas within carbonate minerals. In cylindrical test experiments, brucite [Mg(OH)₂] carbonation was accelerated by coupling organic and inorganic carbon cycling. Waste organics generated CO₂ concentrations similar to that of flue gas (up to 19%). The abundance of brucite (2-10 wt %) had the greatest influence on tailings cementation as evidenced by the increase in total inorganic carbon (TIC; +0.17-0.84%). Brucite consumption ranged from 64-84% of its initial abundance and was mainly influenced by water availability. Higher moisture contents (e.g., 80% saturation) and finer grain sizes (e.g., clay-silt) that allowed for a better distribution of water resulted in greater brucite carbonation. Furthermore, pore clogging and surface passivation by Mg-carbonates may have slowed brucite carbonation over the 10 weeks. Unconfined compressive strengths ranged from 0.4-6.9 MPa and would be sufficient in most scenarios to adequately stabilize tailings. Our study demonstrates the potential for stabilizing brucite-bearing mine tailings through in situ cementation while sequestering CO₂.

Using electromagnetic fields to inhibit biofouling and scaling in biogas slurry drip irrigation emitters

Reusing biogas slurry (BS) in agricultural drip irrigation systems may provide a solution to deal with the adverse environmental impacts of applying BS. Biofouling and scaling are two leading issues in drip irrigation emitters. This study investigated a practice that applied electromagnetic fields (EMFs) to control biofilms and scales. The bacterial communities and mineral precipitations in the clogging substances of emitters were determined. Results showed that EMFs inhibited the growth of microbes, and influenced BS physicochemical parameters. Consequently, EMFs shifted the bacterial communities with reduced diversities. Network analyses revealed that bacterial species under EMFs treatments showed lower average connectivities and simpler interactions, which were responsible for the decreases of extracellular polymers substances (EPS). Moreover, EMFs treatments not only reduced the carbonates in emitters, but also prevented the depositions of phosphates, silicates, and quartzes. EMFs also had impacts on the lattice parameters and crystal volumes of carbonates. In addition, the changes in bacterial communities and EPS contents were associated with the reductions of various minerals. Accordingly, EMFs effectively mitigated biofilms and scales with the fixed clogging substances reduced by 29.1-53.8 %. These findings demonstrated that applying EMFs is an effective anti-biofouling and anti-scaling treatment with potential applications in BS irrigation systems.

Mitigation of biofouling in agricultural water distribution systems with nanobubbles

Biofouling poses considerable technical challenges to agricultural irrigation systems. Controlling biofouling with strong chemical biocides is not only expensive and sometimes ineffective, but also contributes to environmental pollution. This study investigated the application of nanobubbles (NBs) on minimizing biofouling in agricultural irrigation water pipelines. Treatment performances were assessed using low concentration bubbles (LCB) and high concentration bubbles (HCB) together with a negative control (CK: no-NBs). 16 s rRNA gene sequencing and X-ray diffraction were used to characterize the microbial community and mineral compositions of biofilms in water emitters. Results demonstrated that NBs effectively mitigated biofouling through reducing fixed-biomass by 31.3-52.1%. A significantly different microbial composition was found in the biofilm community with reduced biodiversity. Molecular ecological network analysis revealed that NBs were detrimental to the mutualistic interactions among microbial species - destabilizing the network complexity and size, which was expressed as decreasing in extracellular polymers and biofilm biomass. Furthermore, NBs significantly decreased the deposition of carbonate, silicate, phosphate, and quartz on the pipe surfaces, leading to reductions of total content of minerals in biofilms. Therefore, this study demonstrated that NBs treatment could be an effective, and eco-friendly solution for biofouling control in agricultural water distribution systems.

Opportunities for Mineral Carbonation in Australia’s Mining Industry

Carbon capture, utilisation and storage (CCUS) via mineral carbonation is an effective method for long-term storage of carbon dioxide and combating climate change. Implemented at a large-scale, it provides a viable solution to harvesting and storing the modern crisis of GHGs emissions. To date, technological and economic barriers have inhibited broad-scale utilisation of mineral carbonation at industrial scales. This paper outlines the mineral carbonation process; discusses drivers and barriers of mineral carbonation deployment in Australian mining; and, finally, proposes a unique approach to commercially viable CCUS within the Australian mining industry by integrating mine waste management with mine site rehabilitation, and leveraging relationships with local coal-fired power station. This paper discusses using alkaline mine and coal-fired power station waste (fly ash, red mud, and ultramafic mine tailings, i.e., nickel, diamond, PGE (platinum group elements), and legacy asbestos mine tailings) as the feedstock for CCUS to produce environmentally benign materials, which can be used in mine reclamation. Geographical proximity of mining operations, mining waste storage facilities and coal-fired power stations in Australia are identified; and possible synergies between them are discussed. This paper demonstrates that large-scale alkaline waste production and mine site reclamation can become integrated to mechanise CCUS. Furthermore, financial liabilities associated with such waste management and site reclamation could overcome many of the current economic setbacks of retrofitting CCUS in the mining industry. An improved approach to commercially viable climate change mitigation strategies available to the mining industry is reviewed in this paper.

Variance in bacterial communities, potential bacterial carbon sequestration and nitrogen fixation between light and dark conditions under elevated CO2 in mine tailings

This study is the first to show the response of bacterial communities with primary carbon and nitrogen fixers to elevated CO₂ (eCO₂) in light and dark conditions based on 6 months of culture growth. Carbon sequestration and nitrogen fixation were analyzed by ¹³C and ¹⁵N isotope labeling using ¹³C-labeled CO₂ and ¹⁵N-labeled N₂, followed by pyrosequencing and DNA-based stable isotope probing (SIP) to identify carbon fixers and nitrogen fixers. The results indicated that eCO₂ decreased the Chao 1 richness, and the eCO₂-light treatment exhibited the highest Shannon diversity. In addition, eCO₂ (in either light or dark conditions) greatly increased the relative abundances of bacteria belonging to the classes Betaproteobacteria and Alphaproteobacteria. The ¹³C atom % in the mine tailings increased from 1.108 to 1.84 ± 0.11 under light conditions and 1.52 ± 0.17 under dark conditions after 6 months of culture growth. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) form I-coding gene (cbbL) copy numbers were 164.30-fold and 40.36-fold higher than RubisCO form II-coding gene (cbbM) copy numbers in the heavy fractions with a buoyant density of 1.7388 g·mL^-1 relative to the buoyant density gradients of DNA fractions obtained under eCO₂-light and eCO₂-dark treatment, respectively. The Proteobacteria-like cbbL genes were dominant in the carbon fixers. In addition, the ¹⁵N atom % in the mine tailings increased from 0.366 to 0.454 ± 0.021 in light conditions and 0.437 ± 0.018 in dark conditions. Furthermore, uncultured nitrogen-fixing bacteria were the dominant nitrogen fixers in light conditions, and bacteria harboring the Bradyrhizobium-like nifH and Leptospirillum-like nifH genes were the dominant nitrogen fixers in dark conditions. These first data for a mine tailing ecosystem are inconsistent with those obtained for a range of other ecosystems, in which the effects of CO₂ were limited to several nonphotoautotrophic communities and different nitrogen fixers.