Silicate production and availability for mineral carbonation

Insight into selective removal of trace thallium (Tl(Ι)) by novel chitosan adsorbents encapsulating low-cost silicate mineral wastes

Recently, thallium (Tl) contamination at trace levels has gained worldwide attention, particularly in the remote ore-smelting regions of China. To effectively eliminate the residual target Tl(I) ions, one of the best strategies is to develop novel adsorbents with high selectivity. In this study, we selected silicate mineral waste (SMW) and chitosan (CTS) to synthesize a low-cost composite adsorbent for the removal of trace Tl(I). The results show that a mass ratio for SMW to CTS was optimized as 3:1 (denoted as SMW@CTS), and the preferred SMW@CTS exhibited a 37% higher removal efficiency compared to pure CTS beads (19%), underscoring the critical role of SMW in enhancing trace-level adsorption of Tl(I). Under the optimal conditions of 25 °C, pH = 5, and 6 g/L dosage, SMW@CTS achieved a Tl(I) removal efficiency of approx. 89% for an initial concentration of 100 μg/L. Moreover, the thermodynamics for the adsorption of Tl(I) onto SMW@CTS were investigated, and the structural characteristics of SMW@CTS were also characterized in detail. Furthermore, the mechanism for the high selectivity of Tl(I) onto SMW@CTS has been explored in the presence of humic acid and a series of competing ions. These findings highlight the novel SMW@CTS as a promising candidate for eliminating trace-level Tl(I) contamination from co-existing substances.

Evolution of silicate coordination in architected amorphous and crystalline magnesium silicates during carbon mineralization

Advancing durable solutions for carbon storage and removal at the gigaton scale to produce solid carbonates via carbon mineralization requires harnessing earth abundant magnesium silicate resources. Calibrated insights linking the structural and morphological features of earth abundant amorphous and crystalline magnesium silicate phases to their reactivity are essential for scalable deployment but remain underdeveloped. To resolve the influence of silica coordination and mass transfer on carbon mineralization behavior, magnesium silicates bearing amorphous and crystalline phases (AC Mg-silicate) are synthesized. The structural and morphological transitions starting from colloidal precursors to their final synthesized form on heating are delineated using operando ultra small/small/wide angle X-ray scattering (USAXS/SAXS/WAXS) measurements. The evolution of the silicate phases on carbon mineralization of AC Mg-silicate is contrasted with that of highly crystalline Mg-silicate (HC Mg-silicate) when reacted at 200 °C and a CO2 partial pressure of 20 atm in water and 1 M NaHCO3 solution in stirred and unstirred environments. These experimental conditions are analogous to those of the water-gas-shift reaction for sustainable recovery of H2 with inherent carbon mineralization. Enhancement in the extent of carbon mineralization by 13.3-19.5% noted in the presence of NaHCO3 compared to water in AC and HC Mg-silicate with and without stirring, is attributed to the buffering effect which aids simultaneous silicate dissolution and carbon mineralization. Enhanced extents of carbon mineralization in the presence of NaHCO3 correspond to the formation of MgSiO3 and SiO2 phases from the starting Mg2SiO4 precursors in AC and HC Mg-silicate. Unlocking these silicate transformations during carbon mineralization by harnessing architected Mg-silicate precursors reveals the feasibility of integrating these chemical pathways with sustainable H2 conversion pathways with inherent carbon mineralization.

Carbon capture potential and environmental impact of concrete weathering in soil

The enhanced weathering of concrete in soil has potential to capture atmospheric CO2. The objective of this research was to conduct a laboratory experiment and evaluate the environmental impacts and carbon capture potential of concrete as an enhanced weathering material in soil. A column study was conducted with four treatments comprised of: 1) 100 % soil (S treatment), 2) 90 % soil and 10 % concrete by weight of 0.25-0.71 mm diameter fragments (S + Cfine treatment), 3) 90 % soil and 10 % concrete by weight of 8-25 mm diameter fragments (S + Ccoarse treatment), and 4) 100 % concrete composed of 8-25 mm diameter fragments (C treatment). Deionized water was added to the columns for 16 weeks. The S + Cfine treatment experienced a significant increase in soil pH (8.0 ± 0.07) compared to the S (6.9 ± 0.22) and S + Ccoarse (7.1 ± 0.08) treatments. The C treatment experienced a significant increase in leachate pH. Leachate NO3- concentrations in the S + Cfine (33 ± 18 mg L-1) samples were significantly greater than those in other treatments. Soil microbial community concentrations were significantly less in the S + Cfine treatment. The S + Cfine treatment had a calculated average HCO3- concentration of 350 ± 120 mg L-1 which was significantly greater than the S (230 ± 100 mg L-1), C (270 ± 170 mg L-1), and S + Ccoarse (260 ± 50 mg L-1) treatments. Increased concentrations of Ca2+, SO42-, H4SiO4, and HCO3- in the mixed concrete and soil samples are evidence that chemical reactions occurred due to the soil-concrete interaction and are likely capturing atmospheric CO2.

The Utilization of Carbonated Steel Slag as a Supplementary Cementitious Material in Cement

Carbon emission reduction and steel slag (SS) treatment are challenges in the steel industry. The accelerated carbonation of SS and carbonated steel slag (CSS) as a supplementary cementitious material (SCM) in cement can achieve both large-scale utilization of SS and CO2 emission reduction, which is conducive to low-carbon sustainable development. This paper presents the utilization status of CSS. The accelerated carbonation route and its effects on the properties of CSS are described. The carbonation reaction of SS leads to a decrease in the average density, an increase in the specific surface area, a refinement of the pore structure, and the precipitation of different forms of calcium carbonate on the CSS surface. Carbonation can increase the specific surface area of CSS by about 24-80%. The literature review revealed that the CO2 uptake of CSS is 2-27 g/100 g SS. The effects of using CSS as an SCM in cement on the mechanical properties, workability, volume stability, durability, environmental performance, hydration kinetics, and microstructure of the materials are also analyzed and evaluated. Under certain conditions, CSS has a positive effect on cement hydration, which can improve the mechanical properties, workability, bulk stability, and sulfate resistance of SS cement mortar. Meanwhile, SS carbonation inhibits the leaching of heavy metal ions from the solid matrix. The application of CSS mainly focuses on material strength, with less attention being given to durability and environmental performance. The challenges and prospects for the large-scale utilization of CSS in the cement and concrete industry are described.

The Impact Ground Phonolite Rock's Potassium Solubilization in Tropical Soil Depends on the Cultivated Forage Species

Cover crops can be used to accelerate the solubilization process of low-solubility fertilizers; thus, the aim of this study was to evaluate the potential of grasses in solubilizing potassium from phonolite rock powder. With a 2 × 5 factorial scheme, two doses of phonolite rock powder, equivalent to 0 and 8 t ha-1, were combined with four grass species (Urochloa ruziziensis, U. decumbens, U. humidicola, and Andropogon gayanus), besides a control treatment without any cover crop. The dry matter production of the aerial parts of the plants was evaluated at days 40 and 70 post-emergence, and then the concentration of potassium in the plants and the soil was evaluated (exchangeable, non-exchangeable, structural, and total potassium contents). In the soil, the phonolitic rock powder increased the exchangeable, non-exchangeable, structural, and total K contents, favoring the absorption of K and the production of the dry mass of the three Urochloa, but U. decumbens stood out because it promoted greater availability of K in the system compared to the cultivation of other plant species. This research proposes the inclusion of U. decumbens in production systems that receive phonolitic rock, constituting a sustainable strategy to improve its agronomic efficiency.

The enhanced weathering potential of a range of silicate and carbonate additions in a UK agricultural soil

Enhanced weathering (EW) is a carbon dioxide removal (CDR) technology which aims to accelerate silicate and/or carbonate weathering in agricultural land. At present, the rate and magnitude of CDR from EW remains uncertain. In this study, soil cores extracted from a typical UK agricultural site in Oxfordshire were used to geochemically assess the efficacy of EW while simulating field conditions. Six material "treatments" were applied to soil cores at a rate equivalent to 50 t ha-1: agricultural lime (aglime), basalt, cement kiln dust (CKD), olivine, steel slag, and volcanic ash. A range of chemical measurements were used to constrain the rate of dissolution, fate of dissolution products, and the CDR potential and environmental impact of treatment. After a single application, the CDR rates were, in decreasing order: steel slag (20 ± 4 kgCO2 ha-1 yr-1) > CKD (16 ± 4 kgCO2 ha-1 yr-1) > basalt (5 ± 3 kgCO2 ha-1 yr-1) > volcanic ash (3 ± 3 kgCO2 ha-1 yr-1) > aglime (2 ± 1 kgCO2 ha-1 yr-1) > olivine (0 ± 2 kgCO2 ha-1 yr-1). Despite its drawdown potential, steel slag addition is not advised because application raised the dissolved concentration of heavy metals in the soil. CKD application will be limited due to availability of this material. Liming of agricultural soil is normally considered a source of CO2, but this study indicates liming could cause CDR in some UK soil conditions. Extrapolating from this site to a wider scale supports the conclusions of recently published research which suggests 10 years of basalt application over UK cropland could remove 1.8 ± 0.9 MtCO2 yr-1. Given the low rates of CDR observed in this study, EW is only likely to be worthwhile, at least in soil and climate conditions common to SE England, where there are co-benefits beyond carbon uptake.

Improved net carbon budgets in the US Midwest through direct measured impacts of enhanced weathering

Terrestrial enhanced weathering (EW) through the application of Mg- or Ca-rich rock dust to soil is a negative emission technology with the potential to address impacts of climate change. The effectiveness of EW was tested over 4 years by spreading ground basalt (50 t ha-1  year-1 ) on maize/soybean and miscanthus cropping systems in the Midwest US. The major elements of the carbon budget were quantified through measurements of eddy covariance, soil carbon flux, and biomass. The movement of Mg and Ca to deep soil, released by weathering, balanced by a corresponding alkalinity flux, was used to measure the drawdown of CO2 , where the release of cations from basalt was measured as the ratio of rare earth elements to base cations in the applied rock dust and in the surface soil. Basalt application stimulated peak biomass and net primary production in both cropping systems and caused a small but significant stimulation of soil respiration. Net ecosystem carbon balance (NECB) was strongly negative for maize/soybean (-199 to -453 g C m-2  year-1 ) indicating this system was losing carbon to the atmosphere. Average EW (102 g C m-2  year-1 ) offset carbon loss in the maize/soybean by 23%-42%. NECB of miscanthus was positive (63-129 g C m-2  year-1 ), indicating carbon gain in the system, and EW greatly increased inorganic carbon storage by an additional 234 g C m-2  year-1 . Our analysis indicates a co-deployment of a perennial biofuel crop (miscanthus) with EW leads to major wins-increased harvested yields of 29%-42% with additional carbon dioxide removal (CDR) of 8.6 t CO2  ha-1  year-1 . EW applied to maize/soybean drives a CDR of 3.7 t CO2  ha-1  year-1 , which partially offsets well-established carbon losses from soil from this crop rotation. EW applied in the US Midwest creates measurable improvements to the carbon budgets perennial bioenergy crops and conventional row crops.

Arabidopsis seedlings respond differentially to nutrient efficacy of three rock meals by regulating root architecture and endogenous auxin homeostasis

BACKGROUND

Plants show developmental plasticity with variations in environmental nutrients. Considering low-cost rock dust has been identified as a potential alternative to artificial fertilizers for more sustainable agriculture, the growth responses of Arabidopsis seedlings on three rock meals (basalt, granite, and marlstone) were examined for the different foraging behavior, biomass accumulation, and root architecture.

RESULTS

Compared to ½ MS medium, basalt and granite meal increased primary root length by 13% and 38%, respectively, but marlstone caused a 66% decrease, and they all drastically reduced initiation and elongation of lateral roots but lengthened root hairs. Simultaneous supply of organic nutrients and trace elements increased fresh weight due to the increased length of primary roots and root hairs. When nitrogen (N), phosphorus (P), and potassium (K) were supplied individually, N proved most effective in improving fresh weight of seedlings growing on basalt and granite, whereas K, followed by P, was most effective for those growing on marlstone. Unexpectedly, the addition of N to marlstone negatively affected seedling growth, which was associated with repressed auxin biosynthesis in roots.

CONCLUSIONS

Our data indicate that plants can recognize and adapt to complex mineral deficiency by adjusting hormonal homeostasis to achieve environmental sensitivity and developmental plasticity, which provide a basis for ecologically sound and sustainable strategies to maximize the use of natural resources and reduce the production of artificial fertilizers.

Microstructural analysis of slag properties associated with calcite precipitation due to passive CO2 mineralization

CO2 mineralization in slag has gained significant attention since it occurs with minimal human intervention and energy input. While the amount of theoretical CO2 that can be captured within slag has been quantified based on slag composition in several studies, the microstructural and mineralogical effects of slag on its ability to capture CO2 have not been fully addressed. In this work, the CO2 uptake within legacy slag samples is analyzed through microstructural characterization. Slag samples were collected from the former Ravenscraig steelmaking site in Lanarkshire, Scotland. The collected samples were studied using X-ray Computed Tomography (XCT) to understand the distribution and geometry of pore space, as well as with scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements within the studied samples. Electron backscatter diffraction (EBSD) was used to study the minerals distribution. The samples were also characterized through X-ray diffraction (XRD) and X-ray fluorescence (XRF), and the amount of captured CO2 was quantified using thermogravimetric analysis (TGA). Our results demonstrate that CO2 uptake occurs to the extent of ∼9-30 g CO2/ kg slag. The studied samples are porous in nature, with pore space occupying up to ∼30% of their volumes, and they are dominated by åkermanite-gehlenite minerals which interact with the atmospheric CO2 slowly at ambient conditions. EDS and EBSD results illustrate that the precipitated carbonate in slag is calcite, and that the precipitation of calcite is accompanied by the formation of a Si-O-rich layer. The provided analysis concludes that the porous microstructure as well as the minerals distribution in slag should be considered in forecasting and designing large-scale solutions for passive CO2 mineralization in slag.

Mineralogical and chemical characterization of mining waste and utilization for carbon sequestration through mineral carbonation

Mining activities have often been associated with the issues of waste generation, while mining is considered a carbon-intensive industry that contributes to the increasing carbon dioxide emission to the atmosphere. This study attempts to evaluate the potential of reusing mining waste as feedstock material for carbon dioxide sequestration through mineral carbonation. Characterization of mining waste was performed for limestone, gold and iron mine waste, which includes physical, mineralogical, chemical and morphological analyses that determine its potential for carbon sequestration. The samples were characterized as having alkaline pH (7.1-8.3) and contain fine particles, which are important to facilitate precipitation of divalent cations. High amount of cations (CaO, MgO and Fe2O3) was found in limestone and iron mine waste, i.e., total of 79.55% and 71.31%, respectively, that are essential for carbonation process. Potential Ca/Mg/Fe silicates, oxides and carbonates have been identified, which was confirmed by the microstructure analysis. The limestone waste composed majorly of CaO (75.83%), which was mainly originated from calcite and akermanite minerals. The iron mine waste consisted of Fe2O3 (56.60%), mainly from magnetite and hematite, and CaO (10.74%) which was derived from anorthite, wollastonite and diopside. The gold mine waste was attributed to a lower cation content (total of 7.71%), associated mainly with mineral illite and chlorite-serpentine. The average capacity for carbon sequestration was between 7.73 and79.55%, which corresponds to 383.41 g, 94.85 g and 4.72 g CO2 that were potentially sequestered per kg of limestone, iron and gold mine waste, respectively. Therefore, it has been learned that the mine waste might be utilized as feedstock for mineral carbonation due to the availability of reactive silicate/oxide/carbonate minerals. Utilization of mine waste would be beneficial in light of waste restoration in most mining sites while tackling the issues of CO2 emission in mitigating the global climate change.

Geochemical carbon dioxide removal potential of Spain

Many countries have made pledges to reduce CO₂ emissions over the upcoming decades to meet the Paris Agreement targets of limiting warming to no >1.5 °C, aiming for net zero by mid-century. To achieve national reduction targets, there is a further need for CO₂ removal (CDR) approaches on a scale of millions of tonnes, necessitating a better understanding of feasible methods. One approach that is gaining attention is geochemical CDR, encompassing (1) in-situ injection of CO₂-rich gases into Ca and Mg-rich rocks for geological storage by mineral carbonation, (2) ex-situ ocean alkalinity enhancement, enhanced weathering and mineral carbonation of alkaline-rich materials, and (3) electrochemical separation processes. In this context, Spain may host a notionally high geochemical CDR capacity thanks to its varied geological setting, including extensive mafic-ultramafic and carbonate rocks. However, pilot schemes and large-scale strategies for CDR implementation are presently absent in-country, partly due to gaps in current knowledge and lack of attention paid by regulatory bodies. Here, we identify possible materials, localities and avenues for future geochemical CDR research and implementation strategies within Spain. This study highlights the kilotonne to million tonne scale CDR options for Spain over the rest of the century, with attention paid to chemically and mineralogically appropriate materials, suitable implementation sites and potential strategies that could be followed. Mafic, ultramafic and carbonate rocks, mine tailings, fly ashes, slag by-products, desalination brines and ceramic wastes hosted and produced in Spain are of key interest, with industrial, agricultural and coastal areas providing opportunities to launch pilot schemes. Though there are obstacles to reaching the maximum CDR potential, this study helps to identify focused targets that will facilitate overcoming such barriers. The CDR potential of Spain warrants dedicated investigations to achieve the highest possible CDR to make valuable contributions to national reduction targets.

Carbon dioxide sequestration of iron ore mining waste under low-reaction condition of a direct mineral carbonation process

Mining waste that is rich in iron-, calcium- and magnesium-bearing minerals can be a potential feedstock for sequestering CO₂ by mineral carbonation. This study highlights the utilization of iron ore mining waste in sequestering CO₂ under low-reaction condition of a mineral carbonation process. Alkaline iron mining waste was used as feedstock for aqueous mineral carbonation and was subjected to mineralogical, chemical, and thermal analyses. A carbonation experiment was performed at ambient CO₂ pressure, temperature of 80 °C at 1-h exposure time under the influence of pH (8-12) and particle size (< 38-75 µm). The mine waste contains Fe-oxides of magnetite and hematite, Ca-silicates of anorthite and wollastonite and Ca-Mg-silicates of diopside, which corresponds to 72.62% (Fe₂O₃), 5.82% (CaO), and 2.74% (MgO). Fe and Ca carbonation efficiencies were increased when particle size was reduced to < 38 µm and pH increased to 12. Multi-stage mineral transformation was observed from thermogravimetric analysis between temperature of 30 and 1000 °C. Derivative mass losses of carbonated products were assigned to four stages between 30-150 °C (dehydration), 150-350 °C (iron dehydroxylation), 350-700 °C (Fe carbonate decomposition), and 700-1000 °C (Ca carbonate decomposition). Peaks of mass losses were attributed to ferric iron reduction to magnetite between 662 and 670 °C, siderite decarbonization between 485 and 513 °C, aragonite decarbonization between 753 and 767 °C, and calcite decarbonization between 798 and 943 °C. A 48% higher carbonation rate was observed in carbonated products compared to raw sample. Production of carbonates was evidenced from XRD analysis showing the presence of siderite, aragonite, calcite, and traces of Fe carbonates, and about 33.13-49.81 g CO₂/kg of waste has been sequestered from the process. Therefore, it has been shown that iron mining waste can be a feasible feedstock for mineral carbonation in view of waste restoration and CO₂ emission reduction.

Nano- to Global-Scale Uncertainties in Terrestrial Enhanced Weathering

Enhanced weathering (EW) is one of the most promising negative emissions technologies urgently needed to limit global warming to at least below 2 °C, a goal recently reaffirmed at the UN Global Climate Change conference (i.e., COP26). EW relies on the accelerated dissolution of crushed silicate rocks applied to soils and is considered a sustainable solution requiring limited technology. While EW has a high theoretical potential of sequestering CO₂, research is still needed to provide accurate estimates of carbon (C) sequestration when applying different silicate materials across distinct climates and major soil types in combination with a variety of plants. Here we elaborate on fundamental advances that must be addressed before EW can be extensively adopted. These include identifying the most suitable environmental conditions, improving estimates of field dissolution rates and efficacy of CO₂ removal, and identifying alternative sources of silicate materials to meet future EW demands. We conclude with considerations on the necessity of integrated modeling-experimental approaches to better coordinate future field experiments and measurements of CO₂ removal, as well as on the importance of seamlessly coordinating EW with cropland and forest management.

Carbon mineralization with concurrent critical metal recovery from olivine

Carbon dioxide utilization for enhanced metal recovery (EMR) during mineralization has been recently developed as part of CCUS (carbon capture, utilization, and storage). This paper describes fundamental studies on integrating CO₂ mineralization and concurrent selective metal extraction from natural olivine. Nearly 90% of nickel and cobalt extraction and mineral carbonation efficiency are achieved in a highly selective, single-step process. Direct aqueous mineral carbonation releases Ni²⁺ and Co²⁺ into aqueous solution for subsequent recovery, while Mg²⁺ and Fe²⁺ simultaneously convert to stable mineral carbonates for permanent CO₂ storage. This integrated process can be completed in neutral aqueous solution. Introduction of a metal-complexing ligand during mineral carbonation aids the highly selective extraction of Ni and Co over Fe and Mg. The ligand must have higher stability for Ni-/Co- complex ions compared with the Fe(II)-/Mg- complex ions and divalent metal carbonates. This single-step process with a suitable metal-complexing ligand is robust and utilizes carbonation processes under various kinetic regimes. This fundamental study provides a framework for further development and successful application of direct aqueous mineral carbonation with concurrent EMR. The enhanced metal extraction and CO₂ mineralization process may have implications for the clean energy transition, CO₂ storage and utilization, and development of new critical metal resources.

Image-Based Analysis of Weathered Slag for Calculation of Transport Properties and Passive Carbon Capture

Weathering of silicate-rich industrial wastes such as slag can reduce emissions from the steelmaking industry. During slag weathering, different minerals spontaneously react with atmospheric CO2 to produce calcite. Here, we evaluate the CO2 uptake during slag weathering using image-based analysis. The analysis was applied to an X-ray computed tomography (XCT) dataset of a slag sample associated with the former Ravenscraig steelworks in Lanarkshire, Scotland. The element distribution of the sample was studied using scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy (EDS). Two advanced image segmentation methods, namely trainable WEKA segmentation in the Fiji distribution of ImageJ and watershed segmentation in Avizo ® 9.3.0, were used to segment the XCT images into matrix, pore space, calcite, and other precipitates. Both methods yielded similar volume fractions of the segmented classes. However, WEKA segmentation performed better in segmenting smaller pores, while watershed segmentation was superior in overcoming the partial volume effect presented in the XCT data. We estimate that CO2 has been captured in the studied sample with an uptake between 20 and 17 kg CO2/1,000 kg slag for TWS and WS, respectively, through calcite precipitation.

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.

Remineralizing soils? The agricultural usage of silicate rock powders: A review

Soil nutrient depletion threatens global food security and has been seriously underestimated for potassium (K) and several micronutrients. This is particularly the case for highly weathered soils in tropical countries, where classical soluble fertilizers are often not affordable or not accessible. One way to replenish macro- and micronutrients are ground silicate rock powders (SRPs). Rock forming silicate minerals contain most nutrients essential for higher plants, yet slow and inconsistent weathering rates have restricted their use in the past. Recent findings, however, challenge past agronomic objections which insufficiently addressed the factorial complexity of the weathering process. This review therefore first presents a framework with the most relevant factors for the weathering of SRPs through which several outcomes of prior studies can be explained. A subsequent analysis of 48 crop trials reveals the potential as alternative K source and multi-nutrient soil amendment for tropical soils, whereas the benefits for temperate soils are currently inconclusive. Beneficial results prevail for mafic and ultramafic rocks like basalts and rocks containing nepheline or glauconite. Several rock modifications are highly efficient in increasing the agronomic effectiveness of SRPs. Enhanced weathering of SRPs could additionally sequester substantial amounts of CO₂ from the atmosphere and silicon (Si) supply can induce a broad spectrum of plant biotic and abiotic stress resistance. Recycling massive amounts of rock residues from domestic mining industries could furthermore resolve serious disposal challenges and improve fertilizer self-sufficiency. In conclusion, under the right circumstances, SRPs could not only advance low-cost and regional soil sustaining crop production but contribute to various sustainable development goals.

Mechanism of aging of biochars obtained at different temperatures from sewage sludges with different composition and character

The aim of this study was to determine the effect of abiotic aging of sewage sludge (SSL)-derived biochars on their physicochemical properties and in consequence on their stability. Biochars produced at 500 or 700 °C from SSLs with a different composition and properties were incubated at different temperatures (-20, 4, 20, 60, and 90 °C) for 6 and 12 months. Pristine and aged biochars were characterized in terms of their composition and properties using a range of complementary methods. The results showed that SSL-derived biochars will not be as stable as previously thought in the long term. The stability of the SSL-derived biochars was closely related to the content and character of C. The biochars that had more C in their composition and, apart from aromatic C, also aliphatic matter/carbon substances deposited in surface pores (i.e. those produced from SSL with a lower initial ash content and a lower degree of aromaticity) were less stable than the biochars with a lower C content and a typically aromatic character of C (i.e. those derived from SSL with a higher initial ash content and a higher degree of aromaticity). Their oxidation led to partial mineralization of aliphatic chains or organic surface film and manifested itself in a greater changes in their properties. The low-temperature biochars (BC-500) with lower aromaticity were found to be more susceptible to oxidation than the high-temperature ones (BC-700) with higher aromaticity. The more aromatic structure of C limited access of O₂ molecules to biochar interior, due to which the processes occurring during aging were concentrated in their surface layer and their properties were less change. It can therefore be concluded that pyrolysis of SSL with higher aromaticity and a lower organic content and higher pyrolysis temperatures will lead to obtaining more stable SSL-derived biochars.

Environmentally safe release of plant available potassium and micronutrients from organically amended rock mineral powder

The staggering production of rock dusts and quarry by-products of mining activities poses an immense environmental burden that warrants research for value-added recycling of these rock mineral powders (RMP). In this study, an incubation experiment was conducted to determine potassium (K) and micronutrients (Zn, Cu, Fe and Mn) release from a quarry RMP to support plant nutrition. Four different size fractions of the RMP were incubated with organic amendments (cow dung and legume straw) under controlled conditions for 90 days. Samples were collected at different intervals (7, 15, 30, 45, 60 and 90 days) for the analysis of available K and micronutrients in the mineral-OM mixtures and leachates. There was a significant (p <0.05) increase in pH of leachates from the mineral-OM mixtures. The K release was significantly higher from the finer size fraction of RMP. About 18.7% Zn added as RMP was released during the incubation period. Zn release increased from 4.7 to 23.2% as the particle size of RMP decreased. Similarly, Cu release from RMP increased from 2.9 to 21.6%, with a decrease in the particle size. Fe and Mn recovery from RMP recorded 11.2 and 6.6%, respectively. Combined application of OM and RMP showed significantly higher nutrient release than other treatments. This study indicates that effective blending of RMP with organic amendments could be a potential source of K and micronutrients in agriculture without posing a risk of toxic element contamination to the soil.

CO2 utilization in built environment via the PCO2 swing carbonation of alkaline solid wastes with different mineralogy

Carbon mineralization to solid carbonates is one of the reaction pathways that can not only utilize captured CO2 but also potentially store it in the long term. In this study, the dissolution and carbonation behaviors of alkaline solid wastes (i.e., waste concrete) was investigated. Concrete is one of the main contributors to a large carbon emission in the built environment. Thus, the upcycling of waste concrete via CO2 utilization has multifaceted environmental benefits including CO2 emission reduction, waste management and reduced mining. Unlike natural silicate minerals such as olivine and serpentine, alkaline solid wastes including waste concrete are highly reactive, and thus, their dissolution and carbonation behaviors vary significantly. Here, both conventional acid (e.g., hydrochloric acid) and less studied carbonic acid (i.e., CO2 saturated water) solvent systems were explored to extract Ca from concrete. Non-stoichiometric dissolution behaviors between Ca and Si were confirmed under far-from-equilibrium conditions (0.1 wt% slurry density), and the re-precipitation of the extracted Si was observed at near-equilibrium conditions (5 wt% slurry density), when the Ca extraction was performed at a controlled pH of 3. These experiments, with a wide range of slurry densities, provided valuable insight into Si re-precipitation phenomena and its effect on the mass transfer limitation during concrete dissolution. Next, the use of the partial pressure of CO2 for the pH swing carbon mineralization process was investigated for concrete, and the results were compared to those of Mg-bearing silicate minerals. In the PCO2 swing process, the extraction of Ca was significantly limited by the precipitation of the carbonate phase (i.e., calcite), since CO2 bubbling could not provide a low enough pH condition for concrete-water-CO2 systems. Thus, this study showed that the two-step carbon mineralization via PCO2 swing, that has been developed for Mg-bearing silicate minerals, may not be viable for highly reactive Ca-bearing silicate materials (e.g., concrete). The precipitated calcium carbonate (PCC) derived from waste concrete via a pH swing process showed very promising results with a high CO2 utilization potential as an upcycled construction material.

Low-Temperature Pretreatment of Organic Feedstocks with Selected Mineral Wastes Sustains Anaerobic Digestion Stability through Trace Metal Release

A low-cost approach for enhancing mesophilic (37 °C) anaerobic digestion (AD) of organic waste using a low-temperature (37 °C) pretreatment with different mineral wastes (MW) was investigated. A higher and stable methane production rate, in comparison to MW-free controls, was achieved for 80 days at organic loading rates of 1-2 g VS/L·d, using a feed substrate pretreated with incinerator bottom ash (IBA). The boiler ash and cement-based waste pretreatments also produced high methane production rates but with some process instability. In contrast, an incinerator fly ash pretreatment showed a progressive decrease in methane production rates and poor process stability, leading to reactor failure after 40 days. To avoid process instability and/or reactor failure, two metrics had to be met: (a) a methanogenesis to fermentation ratio higher than 0.6 and (b) a cell-specific methanogenic activity to cell-specific fermentation activity ratio of >1000. The prevalence of Methanofastidiosum together with a mixed community of acetoclastic (Methanosaeta) and hydrogenotrophic (Methanobacterium) methanogens in the stable IBA treatment indicated the importance of Methanofastidiosum as a potential indicator of a healthy and stable reactor.

Optimizing Inorganic Carbon Sequestration and Crop Yield With Wollastonite Soil Amendment in a Microplot Study

Carbon dioxide (CO₂) is a major greenhouse gas, and its concentration in the atmosphere is increasing continuously, hence there is an urgent need to reduce its level in the atmosphere. Soils offer a large natural sink to store CO₂. This study focuses on sequestering CO₂ in the agricultural soils as inorganic carbon, which can be accomplished by adding alkaline-earth silicates. Wollastonite is used in this study as a soil amendment, to sequester CO₂ via the geochemical route of mineral carbonation. The first objective of the present study was to evaluate the effect of mixing a wide range of dosages of wollastonite, as a soil amendment, on the growth performance of two leguminous plants frequently used in agricultural sector: soybean and alfalfa. The plants were grown with different wollastonite dosages (3-20 kg·m^-2 for soybean and 3-40 kg·m^-2 for alfalfa), for a duration of 14 weeks in a microplot experiment in Ontario, Canada. The second objective was to find evidence of enhanced weathering of wollastonite in soil, in addition to the augmentation of inorganic carbon content in soil. For this, mineralogical assessment of the soils was performed using XRD and SEM-EDS analyses. Wollastonite increased the soybean yield by two-fold in the plot amended with 10 kg·m^-2. At all dosages, wollastonite increased the alfalfa growth in terms of height and above-ground biomass dry weight, as well as root biomass. The rate of CO₂ sequestration, at optimum wollastonite dosage, reached 0.08 kg CO₂·m^-2·month^-1. XRD and SEM-EDS analyses indicated accumulation of calcite in wollastonite-amended soil and formation of other weathering products. The results obtained from this study help to understand the impact of wollastonite soil amendment on agronomy, and will aid in implementing such negative emissions technology by informing farmers and industry alike that the use of wollastonite contributes toward global climate change mitigation while supporting crop yield. The findings of this study add to the existing body of knowledge on enhanced weathering as an atmospheric CO₂ removal technology, providing further evidence that wollastonite weathering in agricultural soils can lead to significant capacity for CO₂ sequestration as inorganic carbon, while concurrently promoting plant growth.

Assessment of Sectoral Greenhouse Gas Emission Reduction Potentials for 2030

The aim of this article is to provide an overview of greenhouse gas emission reduction potentials for 2030 based on the assessment of detailed sectoral studies. The overview updates a previous assessment that dates back more than ten years. We find a total emission reduction potential of 30–36 GtCO2e compared to a current-policies baseline of 61 GtCO2e. The energy production and conversion sector is responsible for about one third of this potential and the agriculture, buildings, forestry, industry, and transport sectors all contribute substantially to the total potential. The potential for 2030 is enough to bridge the gap towards emissions pathways that are compatible with a maximum global temperature rise of 1.5–2 °C compared to preindustrial levels.

Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste

Legacy iron (Fe) and steel wastes have been identified as a significant source of silicate minerals, which can undergo carbonation reactions and thus sequester carbon dioxide (CO₂). In reactor experiments, i.e., at elevated temperatures, pressures, or CO₂ concentrations, these wastes have high silicate to carbonate conversion rates. However, what is less understood is whether a more "passive" approach to carbonation can work, i.e., whether a traditional slag emplacement method (heaped and then buried) promotes or hinders CO₂ sequestration. In this paper, the results of characterization of material retrieved from a first of its kind drilling program on a historical blast furnace slag heap at Consett, U.K., are reported. The mineralogy of the slag material was near uniform, consisting mainly of melilite group minerals with only minor amounts of carbonate minerals detected. Further analysis established that total carbon levels were on average only 0.4% while average calcium (Ca) levels exceeded 30%. It was calculated that only ∼3% of the CO₂ sequestration potential of the >30 Mt slag heap has been utilized. It is suggested that limited water and gas interaction and the mineralogy and particle size of the slag are the main factors that have hindered carbonation reactions in the slag heap.

Negative emissions technologies: A complementary solution for climate change mitigation

Carbon dioxide (CO₂) is the main greenhouse gas (GHG) and its atmospheric concentration is currently 50% higher than pre-industrial levels. The continuous GHGs emissions may lead to severe and irreversible consequences in the climate system. The reduction of GHG emissions may be not enough to mitigate climate change. Consequently, besides carbon capture from large emission sources, atmospheric CO₂ capture may be also required. To meet the target defined for climate change mitigation, the removal of 10 Gt·yr^-1 of CO₂ globally by mid-century and 20 Gt·yr^-1 of CO₂ globally by the end of century. The technologies applied with this aim are known as negative emission technologies (NETs), as they lead to achieve a negative balance of carbon in atmosphere. This paper aims to present the recent research works regarding NETs, focusing the research findings achieved by academic groups and projects. Besides several advantages, NETs present high operational cost and its scale-up should be tested to know the real effect on climate change mitigation. With current knowledge, no single process should be seen as a solution. Research efforts should be performed to evaluate and reduce NETs costs and environmental impact.

Role of policy in managing mined resources for construction in Europe and emerging economies

Rapid urbanisation, with associated housing and infrastructure demands, leads to increased mining and use of non-renewable mineral raw materials needed for the construction industry including concrete and cement. In an emerging economy, like Thailand, which is part of Association of Southeast Asian Nations (ASEAN), current environmental management policies are insufficient to reduce raw material requirements or waste from demolition by generating inputs to construction through reuse or recycling. As part of the European Union (EU), Great Britain has successfully implemented integrated policies and achieved high rates of recycled aggregates in construction (29%) and a 70% reuse and recycling target for construction and demolition (C&D) waste. In this paper, Material Flow Analysis (MFA) of cement/concrete materials is combined with an interpretation of related policies to provide a deeper understanding how to achieve more sustainable management of natural resources. A comparative MFA for the construction industry in Great Britain and Thailand (representing an ASEAN country) has been developed that quantifies raw material inputs, buildings and infrastructure outputs, so that the practices in the two countries can be contrasted. We report domestic cement production and import/export data, and calculate the raw materials needed for cement and its calcination process for concrete production. Considering the most relevant policies and taxation in Great Britain, we identify possible ways forward for Thailand by introducing new policies and taxation that will have positive effects on raw material extraction, processing, construction and disposal practices and disposal behaviors. Following the MFA and policy analysis, we believe that similar benefits apply to other emerging economies.

The negative emission potential of alkaline materials

7 billion tonnes of alkaline materials are produced globally each year as a product or by-product of industrial activity. The aqueous dissolution of these materials creates high pH solutions that dissolves CO₂ to store carbon in the form of solid carbonate minerals or dissolved bicarbonate ions. Here we show that these materials have a carbon dioxide storage potential of 2.9–8.5 billion tonnes per year by 2100, and may contribute a substantial proportion of the negative emissions required to limit global temperature change to <2 °C.

The potential of biomass energy carbon capture and storage is unclear. Here the authors estimated the negative emissions potential from highly alkaline materials, by-products and wastes and showed that these materials have a CO₂ storage potential of 2.5–7.5 billion tonnes per year by 2100.

Co-digestion of organic and mineral wastes for enhanced biogas production: Reactor performance and evolution of microbial community and function

Mineral wastes (MWs) from municipal solid waste incineration plants and construction demolition sites are rich in minerals, heavy metals and have acid neutralising capacity. This renders such MWs a promising source of bulk and trace elements to enhance and stabilize biogas production in anaerobic processes. However, finding a MW with typical heavy metal concentrations, which promotes anaerobic digestion (AD) without adverse effects on the microbial community of the reactor is of major importance. To investigate the impact of several MW additives (1. incineration bottom ash; 2. fly ash; 3. boiler ash; 4. cement-based waste) as AD co-substrates, six 5 L single stage mesophilic, continuously stirred tank reactors (CSTR) were setup. Two different feeding regimes were employed including: (a) a liquid-recycled feeding method (LRFM); (b) a draw-and-fill feeding method (DFFM). Under the LRFM regime, one gram MW per gram organic waste enhanced process stability (pH), increased methane production (25-45% increase), and yielded (450-520 mL CH₄/g VS); DFFM enhanced digestibility to a lesser degree. Illumina HiSeq 16S rRNA community sequencing of reactors showed that the microbial community compositions were unaffected by the presence of MW additives in comparison to unamended controls, but MW amendment accelerated bacterial growth (determined by qPCR). In contrast, different feeding regimes altered the microbial communities; Methanoculleus (hydrogenotrophic) and Methanosaeta (acetoclastic) were the most abundant methanogenic genera in the LRFM reactors, and the more metabolically versatile Methanosarcina genus dominated under DFFM.

Negative emissions technologies and carbon capture and storage to achieve the Paris Agreement commitments

How will the global atmosphere and climate be protected? Achieving net-zero CO₂ emissions will require carbon capture and storage (CCS) to reduce current GHG emission rates, and negative emissions technology (NET) to recapture previously emitted greenhouse gases. Delivering NET requires radical cost and regulatory innovation to impact on climate mitigation. Present NET exemplars are few, are at small-scale and not deployable within a decade, with the exception of rock weathering, or direct injection of CO₂ into selected ocean water masses. To keep warming less than 2°C, bioenergy with CCS (BECCS) has been modelled but does not yet exist at industrial scale. CCS already exists in many forms and at low cost. However, CCS has no political drivers to enforce its deployment. We make a new analysis of all global CCS projects and model the build rate out to 2050, deducing this is 100 times too slow. Our projection to 2050 captures just 700 Mt CO₂ yr^-1, not the minimum 6000 Mt CO₂ yr^-1 required to meet the 2°C target. Hence new policies are needed to incentivize commercial CCS. A first urgent action for all countries is to commercially assess their CO₂ storage. A second simple action is to assign a Certificate of CO₂ Storage onto producers of fossil carbon, mandating a progressively increasing proportion of CO₂ to be stored. No CCS means no 2°C.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.

Negative emissions technologies and carbon capture and storage to achieve the Paris Agreement commitments

How will the global atmosphere and climate be protected? Achieving net-zero CO₂ emissions will require carbon capture and storage (CCS) to reduce current GHG emission rates, and negative emissions technology (NET) to recapture previously emitted greenhouse gases. Delivering NET requires radical cost and regulatory innovation to impact on climate mitigation. Present NET exemplars are few, are at small-scale and not deployable within a decade, with the exception of rock weathering, or direct injection of CO₂ into selected ocean water masses. To keep warming less than 2°C, bioenergy with CCS (BECCS) has been modelled but does not yet exist at industrial scale. CCS already exists in many forms and at low cost. However, CCS has no political drivers to enforce its deployment. We make a new analysis of all global CCS projects and model the build rate out to 2050, deducing this is 100 times too slow. Our projection to 2050 captures just 700 Mt CO₂ yr^-1, not the minimum 6000 Mt CO₂ yr^-1 required to meet the 2°C target. Hence new policies are needed to incentivize commercial CCS. A first urgent action for all countries is to commercially assess their CO₂ storage. A second simple action is to assign a Certificate of CO₂ Storage onto producers of fossil carbon, mandating a progressively increasing proportion of CO₂ to be stored. No CCS means no 2°C.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.

Farming with crops and rocks to address global climate, food and soil security

The magnitude of future climate change could be moderated by immediately reducing the amount of CO2 entering the atmosphere as a result of energy generation and by adopting strategies that actively remove CO2 from it. Biogeochemical improvement of soils by adding crushed, fast-reacting silicate rocks to croplands is one such CO2-removal strategy. This approach has the potential to improve crop production, increase protection from pests and diseases, and restore soil fertility and structure. Managed croplands worldwide are already equipped for frequent rock dust additions to soils, making rapid adoption at scale feasible, and the potential benefits could generate financial incentives for widespread adoption in the agricultural sector. However, there are still obstacles to be surmounted. Audited field-scale assessments of the efficacy of CO2 capture are urgently required together with detailed environmental monitoring. A cost-effective way to meet the rock requirements for CO2 removal must be found, possibly involving the recycling of silicate waste materials. Finally, issues of public perception, trust and acceptance must also be addressed.

Climate change mitigation: potential benefits and pitfalls of enhanced rock weathering in tropical agriculture

Restricting future global temperature increase to 2°C or less requires the adoption of negative emissions technologies for carbon capture and storage. We review the potential for deployment of enhanced weathering (EW), via the application of crushed reactive silicate rocks (such as basalt), on over 680 million hectares of tropical agricultural and tree plantations to offset fossil fuel CO₂ emissions. Warm tropical climates and productive crops will substantially enhance weathering rates, with potential co-benefits including decreased soil acidification and increased phosphorus supply promoting higher crop yields sparing forest for conservation, and reduced cultural eutrophication. Potential pitfalls include the impacts of mining operations on deforestation, producing the energy to crush and transport silicates and the erosion of silicates into rivers and coral reefs that increases inorganic turbidity, sedimentation and pH, with unknown impacts for biodiversity. We identify nine priority research areas for untapping the potential of EW in the tropics, including effectiveness of tropical agriculture at EW for major crops in relation to particle sizes and soil types, impacts on human health, and effects on farmland, adjacent forest and stream-water biodiversity.

Carbon dioxide mineralization process design and evaluation: concepts, case studies, and considerations

Numerous carbon dioxide mineralization (CM) processes have been proposed to overcome the slow rate of natural weathering of silicate minerals. Ten of these proposals are mentioned in this article. The proposals are described in terms of the four major areas relating to CM process design: pre-treatment, purification, carbonation, and reagent recycling operations. Any known specifics based on probable or representative operating and reaction conditions are listed, and basic analysis of the strengths and shortcomings associated with the individual process designs are given in this article. The processes typically employ physical or chemical pseudo-catalytic methods to enhance the rate of carbon dioxide mineralization; however, both methods have its own associated advantages and problems. To examine the feasibility of a CM process, three key aspects should be included in the evaluation criteria: energy use, operational considerations as well as product value and economics. Recommendations regarding the optimal level of emphasis and implementation of measures to control these aspects are given, and these will depend very much on the desired process objectives. Ultimately, a mix-and-match approach to process design might be required to provide viable and economic proposals for CM processes.

Enhancement of accelerated carbonation of alkaline waste residues by ultrasound

The continuous growth of anthropogenic CO2 emissions into the atmosphere and the disposal of hazardous wastes into landfills present serious economic and environmental issues. Reaction of CO2 with alkaline residues or cementitius materials, known as accelerated carbonation, occurs rapidly under ambient temperature and pressure and is a proven and effective process of sequestering the gas. Moreover, further improvement of the reaction efficiency would increase the amount of CO2 that could be permanently sequestered into solid products. This paper examines the potential of enhancing the accelerated carbonation of air pollution control residues, cement bypass dust and ladle slag by applying ultrasound at various water-to-solid (w/s) ratios. Experimental results showed that application of ultrasound increased the CO2 uptake by up to four times at high w/s ratios, whereas the reactivity at low water content showed little change compared with controls. Upon sonication, the particle size of the waste residues decreased and the amount of calcite precipitates increased. Finally, the sonicated particles exhibited a rounded morphology when observed by scanning electron microscopy.

The role of forest trees and their mycorrhizal fungi in carbonate rock weathering and its significance for global carbon cycling

On million-year timescales, carbonate rock weathering exerts no net effect on atmospheric CO2 concentration. However, on timescales of decades-to-centuries, it can contribute to sequestration of anthropogenic CO2 and increase land-ocean alkalinity flux, counteracting ocean acidification. Historical evidence indicates this flux is sensitive to land use change, and recent experimental evidence suggests that trees and their associated soil microbial communities are major drivers of continental mineral weathering. Here, we review key physical and chemical mechanisms by which the symbiotic mycorrhizal fungi of forest tree roots potentially enhance carbonate rock weathering. Evidence from our ongoing field study at the UK's national pinetum confirms increased weathering of carbonate rocks by a wide range of gymnosperm and angiosperm tree species that form arbuscular (AM) or ectomycorrhizal (EM) fungal partnerships. We demonstrate that calcite-containing rock grains under EM tree species weather significantly faster than those under AM trees, an effect linked to greater soil acidification by EM trees. Weathering and corresponding alkalinity export are likely to increase with rising atmospheric CO2 and associated climate change. Our analyses suggest that strategic planting of fast-growing EM angiosperm taxa on calcite- and dolomite-rich terrain might accelerate the transient sink for atmospheric CO2 and slow rates of ocean acidification.

Synthesis of highly efficient CaO-based, self-stabilizing CO2 sorbents via structure-reforming of steel slag

Capturing anthropogenic CO2 in a cost-effective and highly efficient manner is one of the most challenging issues faced by scientists today. Herein, we report a novel structure-reforming approach to convert steel slag, a cheap, abundant, and nontoxic calcium-rich industrial waste, as the only feedstock into superior CaO-based, self-stabilizing CO2 sorbents. The CO2 capture capacity of all the steel slag-derived sorbents was improved more than 10-fold compared to the raw slag, with the maximum uptake of CO2 achieving at 0.50 gCO2 gsorbent(-1). Additionally, the initial steel slag-derived sorbent could retain 0.25 gCO2 gsorbent(-1), that is, a decay rate of only 12% over 30 carbonation-calcination cycles, the excellent self-stabilizing property allowed it to significantly outperform conventional CaO, and match with most of the existing synthetic CaO-based sorbents. A synergistic effect that facilitated CO2 capture by CaO-based sorbents was clearly recognized when Mg and Al, the most common elements in steel slag, coexisted with CaO in the forms of MgO and Al2O3, respectively. During the calcium looping process, MgO served as a well spacer to increase the porosity of sorbents together with Al2O3 serving as a durable stabilizer to coresist the sintering of CaCO3 grains at high temperatures.

Rapid Removal of Atmospheric CO2 by Urban Soils

The measured calcium carbonate content of soils to a depth of 100 mm at a large urban development site has increased over 18 months at a rate that corresponds to the sequestration of 85 t of CO2/ha (8.5 kg of CO2 m(-2)) annually. This is a consequence of rapid weathering of calcium silicate and hydroxide minerals derived from the demolition of concrete structures, which releases Ca that combines with CO2 ultimately derived from the atmosphere, precipitating as calcite. Stable isotope data confirm an atmospheric origin for carbonate carbon, and 14C dating indicates the predominance of modern carbon in the pedogenic calcite. Trial pits show that carbonation extends to depths of ≥1 m. Work at other sites shows that the occurrence of pedogenic carbonates is widespread in artificially created urban soils containing Ca and Mg silicate minerals. Appropriate management of fewer than 12000 ha of urban land to maximize calcite precipitation has the potential to remove 1 million t of CO2 from the atmosphere annually. The maximal global potential is estimated to be approximately 700-1200 Mt of CO2 per year (representing 2.0-3.7% of total emissions from fossil fuel combustion) based on current rates of production of industry-derived Ca- and Mg-bearing materials.

Steel slag carbonation in a flow-through reactor system: the role of fluid-flux

Steel production is currently the largest industrial source of atmospheric CO2. As annual steel production continues to grow, the need for effective methods of reducing its carbon footprint increases correspondingly. The carbonation of the calcium-bearing phases in steel slag generated during basic oxygen furnace (BOF) steel production, in particular its major constituent, larnite {Ca2SiO4}, which is a structural analogue of olivine {(MgFe)2SiO4}, the main mineral subjected to natural carbonation in peridotites, offers the potential to offset some of these emissions. However, the controls on the nature and efficiency of steel slag carbonation are yet to be completely understood. Experiments were conducted exposing steel slag grains to a CO2-H2O mixture in both batch and flow-through reactors to investigate the impact of temperature, fluid flux, and reaction gradient on the dissolution and carbonation of steel slag. The results of these experiments show that dissolution and carbonation of BOF steel slag are more efficient in a flow-through reactor than in the batch reactors used in most previous studies. Moreover, they show that fluid flux needs to be optimized in addition to grain size, pressure, and temperature, in order to maximize the efficiency of carbonation. Based on these results, a two-stage reactor consisting of a high and a low fluid-flux chamber is proposed for CO2 sequestration by steel slag carbonation, allowing dissolution of the slag and precipitation of calcium carbonate to occur within a single flow-through system.

Calcium silicates synthesised from industrial residues with the ability for CO2 sequestration

This work explored several synthesis routes to obtain calcium silicates from different calcium-rich and silica-rich industrial residues. Larnite, wollastonite and calcium silicate chloride were successfully synthesised with moderate heat treatments below standard temperatures. These procedures help to not only conserve natural resources, but also to reduce the energy requirements and CO2 emissions. In addition, these silicates have been successfully tested as carbon dioxide sequesters, to enhance the viability of CO2 mineral sequestration technologies using calcium-rich industrial by-products as sequestration agents. Two different carbon sequestration experiments were performed under ambient conditions. Static experiments revealed carbonation efficiencies close to 100% and real-time resolved experiments characterised the dynamic behaviour and ability of these samples to reduce the CO2 concentration within a mixture of gases. The CO2 concentration was reduced up to 70%, with a carbon fixation dynamic ratio of 3.2 mg CO2 per g of sequestration agent and minute. Our results confirm the suitability of the proposed synthesis routes to synthesise different calcium silicates recycling industrial residues, being therefore energetically more efficient and environmentally friendly procedures for the cement industry.

Long-term fertilization modifies the structures of soil fulvic acids and their binding capability with Al

The binding characteristics of organic ligands and minerals in fulvic acids (FAs) with Al are essential for understanding soil C sequestration, remain poorly understood. In this study, Fourier transform infrared (FTIR) spectroscopy combined with two-dimensional correlation spectroscopy (2DCOS) analysis was applied for the first time to explore the binding of Al with organic ligands and minerals in soil FAs. For these analyses, two contrasting treatments were selected from a long-term (i.e., 22-year) fertilization experiment: chemical (NPK) fertilization and swine manure (SM) fertilization. The results showed that the long-term application of organic and inorganic fertilizers to soils had little effect on the compositions of the fluorescent substances and organic ligands in the soil FAs. However, long-term SM fertilization increased the weathered Al and Si concentrations in the soil FAs compared with long-term chemical fertilization. Furthermore, organic ligands in the soil FAs were mainly bound with Al in the NPK treatment, whereas both organic ligands and minerals (Al-O-Si, Si-O) were bound with Al under the M fertilization conditions. Both transmission electron microscopy (TEM) images and X-ray diffraction spectra demonstrated that amorphous and short-range-ordered nanominerals were abundant in the soil FAs from the SM plot in contrast to the soil FAs from the NPK plot. This result illustrates the role nanominerals play in the preservation of soil FAs by during long-term organic fertilization. In summary, the combination of FTIR and 2D correlation spectroscopy is a promising approach for the characterization of the binding capability between soil FAs and Al, and a better understanding FA-Al binding capability will greatly contribute to global C cycling.