CO2 sequestration and simultaneous zeolite production by carbonation of coal fly ash: Impact on the trapping of toxic elements

Highly efficient carbonation and dechlorination using flue gas micro-nano bubble for municipal solid waste incineration fly ash pretreatment and its applicability to sulfoaluminate cementitious materials

Cement production is a primary source of global carbon emissions. As a hazardous waste, municipal solid waste incineration fly ash (MSWI-FA) can be pretreated as a cementitious and effective carbon capture material. This study proposes an efficient carbonation dechlorination pretreatment and resource recovery strategy using flue gas micro-nano bubble (MNB) to wash MSWI-FA. The results showed that the flue gas MNB water washing reaction solution inhibited CaCO3 boundary layer blocking and adsorption on NaCl and KCl leaching. Under low water-to-solid ratio and CO2 concentration conditions, two-step washing reduced the MSWI-FA chlorine content to <1%, improving the dechlorination effect by 19.72% compared to conventional carbonation. The flue gas MNB water accelerated the precipitation of Ca2+ and Ca(ClO)2 in the form of calcite. The higher the CO2 concentration in the flue gas MNB, the better the fragmentation and purification of the MSWI-FA shell, leading to improved dechlorination and CO2 fixation. Under optimized conditions, the mean particle size of MSWI-FA decreased by 47.82%, and the CO2 fixation rate reached 73.80%, with a 58.35% increase in the washing carbonation rate. MSWI-FA pretreated by flue gas MNB washing was used as both the raw material and supplementary cementitious material for sulfoaluminate cementitious (SAC) material, exhibiting excellent compressive strength and heavy metal stabilization. The maximum compressive strength of the MSWI-FA-based SAC material cured for 28 d reached 130 MPa. Cr leaching was inhibited with increased hydration time, and the leaching concentration was far below the standard limit.

Is the co-application of self-produced compost and natural zeolite interesting to reduce environmental and toxicological availability in metal-contaminated kitchen garden soils?

Composting can turn organic waste into a valuable soil amendment that can improve physical, chemical, and biological soil quality. Compost amendments can also contribute to the remediation of areas anthropogenically degraded by metals. However, it is well known that compost, particularly self-produced compost, can show enrichment in metals. An experimental study was conducted to examine the short- and long-term distribution and the mobility of metals in soils amended with a self-produced compost when it was added alone or in combination with different doses of a natural zeolite to soil. The aim was also to study the interest of managing moderately metal-contaminated kitchen garden soils by assessing the chemical extractability, phytoavailability, and oral bioaccessibility of metals. When zeolite was added to compost alone, it had the tendency to better reduce extractability of Cd and Zn at 25%, and those of Pb at 15%. When the self-produced compost alone or in co-application with zeolite at these doses was applied to soils, the results showed (1) a decrease of NH4NO3-extractable Zn; (2) a reduction of Pb environmental availability, but not Pb bioaccessibility, and (3) an increase of ryegrass biomass. Nevertheless, the risk posed by the self-produced compost was minimal when applied at the proper rate (0.6% w/w). In the selected experimental conditions, the study recommends that self-produced compost be mixed with 15% zeolite to maximize vegetal biomass and minimize environmental risk. The question of sustainability of the results with repeated compost addition is also raised.

A green and multi-win strategy for coal fly ash disposal by CO2 fixation and mesoporous silica synthesis

Coal combustion provides plenty of energy, along with enormous coal fly ash (CFA) and CO₂ emission. CFA could be recycled for mesoporous silica synthesis, but expensive templates are usually needed. In this work, we proposed a multi-win strategy using CO₂ as the precipitator and template. Mesoporous silica powders, with a maximum specific surface area of 355.45 m²/g, a pore volume of 0.73 cm³/g, and an average pore size of around 7.67 nm, were synthesized. The influences of silicon concentration, CO₂ flow rate, and ultrasound were investigated. In addition, the Na₂CO₃ by-product was produced with a purity of over 92 %. By averagely calculating, 1 ton CFA could generate 285 kg mesoporous silica and 1.02 t crude Na₂CO₃. Around 433 kg of CO₂ could be absorbed. Therefore, multi-goals of CFA disposal, CO₂ storage, and valuable silica materials production were realized, and the study could pave the way for large-scale industrial applications.

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.

Techno-Economic Optimization of Multistage Membrane Processes with Innovative Hollow Fiber Modules for the Production of High-Purity CO2 and CH4 from Different Sources

Within the current climate emergency framework and in order to avoid the most severe consequences of global warming, membrane separation processes have become critical for the implementation of carbon capture, storage, and utilization technologies. Mixtures of CO2 and CH4 are relevant energy resources, and the design of innovative membranes specifically designed to improve their separation is a hot topic. This work investigated the potential of modified polydimethylsiloxane and ionic liquid-chitosan composite membranes for separation of CO2 and CH4 mixtures from different sources, such as biogas upgrading, natural gas sweetening, or CO2 enhanced oil recovery. The techno-economic optimization of multistage processes at a real industrial scale was carried out, paying special attention to the identification of the optimal configuration of the hollow fiber modules and the selection of the best membrane scheme. The results demonstrated that a high initial content of CH4 in the feed stream (like in the case of natural gas sweetening) might imply a great challenge for the separation performance, where only membranes with exceptional selectivity might achieve the requirements in a two-stage process. The effective lifetime of the membranes is a key parameter for the successful implementation of innovative membranes in order to avoid severe economic penalties due to excessively frequent membrane replacement. The scale of the process had a great influence on the economic competitiveness of the process, but large-scale installations can operate under competitive conditions with total costs below 0.050 US$ per m3 STP of treated feed gas.

Behavior of potentially toxic elements from stoker-boiler fly ash in Interior Alaska: paired batch leaching and solid-phase characterization

Despite significant investigation of fly ash spills and mineralogical controls on the release of potentially toxic elements (PTEs) from fly ash, interactions with the surficial environment remain relatively poorly understood. We conducted 90-day batch leaching studies with paired analysis of supernatant and solid-phase mineralogy to assess the elemental release and transformation of fly ash upon reaction with aquatic media (18 MΩ cm^-1 water and simulated rainwater). The fly ash in this study, collected from the University of Alaska Fairbanks stoker-boiler power plant, is high in unburned carbon (~20% LOI) and highly enriched in several PTEs relative to the upper continental crust. Supernatant concentrations of oxyanion-forming elements (e.g., As, Se, Mo, Sb) remained relatively low and constant, suggesting equilibrium with the solid phase, possibly ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂•26H₂O], which is known to incorporate and sorb oxyanion-forming PTEs and was identified by X-ray diffraction. Synthetic precipitation leaching procedure (SPLP) results failed to capture important temporal trends. Lead and Ba supernatant concentrations consistently exceeded drinking water standards, as well as others upon exposure to simulated physiological solutions. Seven-day experiments with dissolved organic matter-isolate solutions indicated that for certain elements, liberation was influenced by carbon concentration and/or the identity of the isolate. Overall, this paired approach can serve as a model for future studies, bridging existing gaps between batch leaching and single-element mineralogical, sorption, or speciation studies.

Carbon dioxide emission control of a vermicompost process using fly ash

Composting and vermicomposting generate a valuable product rich in plant nutrients and at the same time, reduce environmental pollution. However, along with these processes and in relation to the metabolism of the microorganism and the worms present in the vermicomposting, CO₂ is emitted to the atmosphere, contributing to the greenhouse effect. Taking these issues into account, different masses of fly ash were used to study the control of the CO₂ of the gas coming from a vermicomposting process and to evaluate the possibility of using the adsorbent as fertilizer in the culture of lettuce Lactuca sativa. Along the vermicomposting process, an increase in the concentration of CO₂ emissions was observed, with a maximum level of emission at the day 20 of the process and an average of 770 mg/L in air. After the adsorption process, the CO₂ concentration was lower due to the effect of the fly ash that was able to trap the emitted gas. The percentage of CO₂ adsorption shows maximum values of 55.5, 58.1 and 63.8% with 0.5, 1 and 1.5 kg of fly ash, respectively. The CO₂ uptake capacities of the different loads of fly ash used were 3.39, 7.03 and 6.84 mmol CO₂/g sorbent with 0.5, 1 and 1.5 kg of fly ash, respectively. After five weeks of sowing L. sativa, it was observed that when no fly ash was used in the soil, the length of the stem was 10.2 cm. Then, the length of the stem was 22 cm, and 16 cm when 10% of fly ash was applied and not applied in the adsorption process, getting a significant correlation between the load of fly ash and the length of the stem. The r when fly ash was used in the adsorption process was 0.9817 and 0.9811 when no ash fly was used in the process.

Use of CO2 to Cure Steel Slag and Gypsum-Based Material

To improve the utilization of steel slag (SS) in CO2 capture and making building materials, the paper mainly discussed the effects of desulphurization gypsum (DG) and w/s ratio on strength development and CO2 capture capability of high Al content SS. It showed that 10 wt% DG enhanced the strength of hydration-curing SS by 262% at 28 days. Similarly, adding 6 wt% DG in carbonation-curing SS contributed to increases in strength and CO2 uptake by 283% and 33.54%, reaching 42.68 MPa and 19.12%, respectively. Strength decreases and CO2 uptake increases with w/s. Microanalysis (QXRD, SEM-EDS, TG-DTG, FTIR, XPS, and MIP) revealed that the main hydration products of SS were C-S-H gel and C4AH13, which transformed to ettringite with DG addition. The carbonation products were mainly calcite and aragonite. Additionally, the amount of aragonite, mechanically weaker than calcite, decreased and calcite increased significantly when DG was added in carbonation-curing samples, providing a denser structure and higher strength than those without DG. Furthermore, high Al 2p binding energies revealed the formation of monocarboaluminate in the DG-added carbonation samples, corresponding to higher CO2 uptake. This study provides guidance for the preparation of SS-DG carbide building materials.

Study on Mineral Compositions of Direct Carbonated Steel Slag by QXRD, TG, FTIR, and XPS

Steel slag CO2 sequestration helps mitigate global warming and decrease the stockpile of steel slag (SS). Through orthogonal design tests and single-factor tests, this paper evaluated the effects of the water/solid mass ratio (w/s), gypsum ratio (G/SS), molding pressure, and curing duration on uniaxial compressive strength (UCS) and CO2 uptake of the compacts. The results indicated that high w/s enhanced both strength and CO2 capture ability. The proper addition of gypsum helps promote UCS increase and CO2 uptake of steel slag. In addition, increasing the molding pressure can significantly improve UCS without reducing CO2 uptake. The optimum conditions in the study were a w/s of 0.20, G/SS of 1/16, and molding pressure of 27 MPa, under which conditions 1 d UCS and CO2 uptake were 55.30 MPa and 12.36%, respectively. Microanalyses showed that gypsum activates mainly mayenite in steel slag. An increase in water addition also increased the hydration and carbonation products greatly, and the strengthened molding pressure had a significant densification effect on micro-pore structures. The study gives guidance in the application of steel slag in CO2 capture and manufacturing green construction material.

Influencing Factors of the Mineral Carbonation Process of Iron Ore Mining Waste in Sequestering Atmospheric Carbon Dioxide

Mining waste may contain potential minerals that can act as essential feedstock for long-term carbon sequestration through a mineral carbonation process. This study attempts to identify the mineralogical and chemical composition of iron ore mining waste alongside the effects of particle size, temperature, and pH on carbonation efficiency. The samples were found to be alkaline in nature (pH of 6.9–7.5) and contained small-sized particles of clay and silt, thus indicating their suitability for mineral carbonation reactions. Samples were composed of important silicate minerals needed for the formation of carbonates such as wollastonite, anorthite, diopside, perovskite, johannsenite, and magnesium aluminum silicate, and the Fe-bearing mineral magnetite. The presence of Fe2O3 (39.6–62.9%) and CaO (7.2–15.2%) indicated the potential of the waste to sequester carbon dioxide because these oxides are important divalent cations for mineral carbonation. The use of small-sized mine-waste particles enables the enhancement of carbonation efficiency, i.e., particles of <38 µm showed a greater extent of Fe and Ca carbonation efficiency (between 1.6–6.7%) compared to particles of <63 µm (0.9–5.7%) and 75 µm (0.7–6.0%). Increasing the reaction temperature from 80 °C to 150–200 °C resulted in a higher Fe and Ca carbonation efficiency of some samples between 0.9–5.8% and 0.8–4.0%, respectively. The effect of increasing the pH from 8–12 was notably observed in Fe carbonation efficiency of between 0.7–5.9% (pH 12) compared to 0.6–3.3% (pH 8). Ca carbonation efficiency was moderately observed (0.7–5.5%) as with the increasing pH between 8–10. Therefore, it has been evidenced that mineralogical and chemical composition were of great importance for the mineral carbonation process, and that the effects of particle size, pH, and temperature of iron mining waste were influential in determining carbonation efficiency. Findings would be beneficial for sustaining the mining industry while taking into account the issue of waste production in tackling the global carbon emission concerns.