Biochar is a porous carbonaceous material with high alkalinity and rich minerals, making it possible for CO2 capture. In this study, biochars derived from pig manure, sewage sludge, and wheat straw were evaluated for their CO2 sorption behavior. All three biochars showed high sorption abilities for CO2, with the maximum capacities reaching 18.2–34.4 mg g−1 at 25 °C. Elevating sorption temperature and moisture content promoted the transition of CO2 uptake from physical to chemical process. Mineral components such as Mg, Ca, Fe, K, etc. in biochar induced the chemical sorption of CO2 via the mineralogical reactions which occupied 17.7%–50.9% of the total sorption. FeOOH in sewage sludge biochar was transformed by sorbed CO2 into Fe(OH)2CO3, while the sorbed CO2 in pig manure biochar was precipitated as K2Ca(CO3)2 and CaMg(CO3)2, which resulted in a dominant increase of insoluble inorganic carbon in both biochars. For wheat straw biochar, sorbed CO2 induced CaCO3 transformed into soluble Ca(HCO3)2, which led to a dominant increase of soluble inorganic carbons. The results obtained from this study demonstrated that biochar as a unique carbonaceous material could distinctly be a promising sorbent for CO2 capture in which chemical sorption induced by mineralogical reactions played an important role.

In recent years, slagging-gasification technology has received increasing attention in treating municipal solid waste (MSW). Compared with conventional incineration, the higher temperature in the slagging-gasification process optimizes its residue composition, and gasification fly ash (GFA) is the only unreused solid residue. Although GFA is a potential civil engineering material, its high content of heavy metals, chlorides, and sulfates hinders its practical use. Moreover, although carbonation has proven to immobilize heavy metals in incineration fly ash, the conventional gas carbonation method cannot remove chlorides and sulfates. In this study, sodium bicarbonate (NaHCO₃) treatment was studied to treat GFA for the first time, and sodium carbonate (Na₂CO₃) was used for comparison. Different concentrations of NaHCO₃ and Na₂CO₃ solutions were used to treat the GFA, and comprehensive tests were conducted on the treated samples. The results indicated that NaHCO₃ treatment was effective in immobilizing Pb, Zn, Cu, and Ni in GFA, while Na₂CO₃ treatment could not effectively immobilize Pb and Zn. Both NaHCO₃ and Na₂CO₃ promoted the removal of chlorides and sulfates in GFA. The wastewater from the NaHCO₃ treatment contained fewer heavy metals compared with those from water washing or Na₂CO₃ treatment, benefitting its treatment or reuse.

Carbon capture has become an important technology to mitigate ever-increasing CO₂ emissions worldwide, and alkali waste is a potential source of CO₂ capture material. Slagging-gasification is a novel technology for treating municipal solid waste (MSW), and the gasification fly ash (GFA) is the only solid residue that is not reused at present due to its high heavy metal content. GFA contains high amounts of Ca(OH)₂ and Ca(OH)Cl, making it protentional for CO₂ capture. In this study, GFA and washed gasification fly ash (WGFA) were treated with CO₂ for different treatment periods. Weight changes of samples were recorded to evaluate the efficiency of CO₂ capture. To assess the properties of treated GFA, pH value, leached heavy metal concentration, mineral composition, and microscopic morphology were studied. The results revealed that GFA and WGFA could adsorb 18.8% and 23.7% CO₂ of their weights, respectively. Carbonation could immobilize heavy metals including Pb, Zn, and Cu when a proper treatment period was applied. An excessive treatment period decreased the efficiency of heavy metal immobilization. Pre-washing is recommended as a pre-treatment method for GFA carbonation, which increased the efficiency to adsorb CO₂, improved the pH of carbonated GFA, and enhanced the effect to immobilize heavy metals.

Incineration technology has been widely employed, as an effective method to decrease the volume of waste disposal. In this review, relationships between municipal solid waste (MSW) inputs and residues after combustion―specifically, the incineration bottom ashes (IBA) of MSW, were discussed, with an emphasis on the geoenvironmental impacts of IBA associated with the complex crystal and amorphous phase reactions and changes during combustion and from their downstream treatments, whereas, their influences on IBA leaching behaviors are considered as another focus. This review summarizes the IBA leaching behaviors based on literature, showing the leaching variabilities induced by natural weathering and artificial intervention conditions, such as accelerated carbonation, washing treatment, stabilization/solidification, and thermal treatments, all of which can be attributed to changes of mineral phases and microstructure. It helps to understand IBA characteristics and transitions in application-environment nexus, and better reuse it for multiple applications.

Cement-solidification and chelator-stabilisation of municipal solid waste incineration fly ash (MSWI-FA) are two main treatment techniques to immobilise heavy metals. Differences in the long-term stabilities of those two methods of heavy-metal immobilisation were explored to aid in determining the better MSWI-FA treatment. However, few comparative studies have been conducted on 6-year-old cement-solidified FA (Ce-6-FA) and chelator-stabilised FA (Ch-6-FA). In this study, we compared the physicochemical and heavy metal leaching characteristics of Ce-6-FA and Ch-6-FA. The chemical speciation of heavy metals was modelled using geochemical software to assess long-term stability. The results showed weaker long-term stability in Pb immobilisation under the chelating system. The leaching concentrations of target heavy metals, acetic acid leaching tests, acid neutralising capacity, and pH-dependent leaching results indicated that Ce-6-FA had higher long-term stability than Ch-6-FA. A column experiment indicated that the cumulative release rates of Pb in Ce-6-FA and Ch-6-FA were 2.49% and 4.72%, respectively. The phase-controlled leaching of Pb in Ce-6-FA mainly occurred through Pb2(OH)3Cl and chloropyromorphite (Pb5(PO4)3Cl), whereas that in Ch-6-FA mainly occurred through Pb5(PO4)3Cl. The decomposition of heavy metal chelates in Ch-6-FA and salt generation in this process led to the release of Pb via the inorganic complex.

MgO-based absorbent has been recognized as a promising CO₂ absorbent at intermediate temperature, though the carbonation performance of pure MgO is poor. Researchers have been devoted to optimize the CO₂ absorption ability via introducing alkali metal carbonates or nitrates. In this paper, the absorption performance of MgO-based absorbents promoted by alkali metal carbonates or modified by alkali metal nitrates has been summarized, and the affecting mechanism has been concluded. Alkali metal nitrates are essential for high absorption ability, and alkali metal carbonates facilitate high-temperature carbonation. Effects of fuel gas conditions and absorbent pelletization are also mentioned for practical applications. H₂O can accelerate the carbonation rate effectively, but the influencing mechanism of H₂O and the CO₂ absorption stability in presence of H₂O was not clearly reported. Further investigations on pelletized MgO-based absorbents modified by alkali metal salts considering regeneration conditions of high CO₂ concentration are proposed based on the recent research findings.

Zinc tailing waste is a type of mine waste generated during the extraction of zinc metal. Disposal of a huge amount of mine tailing waste is an open area and tailing dam causing a negative impact on the natural ecosystem and human health. In this research study, the mechanical properties and durability performance of concrete containing zinc mine tailing waste was investigated through an experimental and statistical analysis. The mechanically treated and untreated zinc tailing waste was used as a cement substitute in concrete production. Concrete specimens were fabricated by replacing cement (0%, 5%, 10%, 15%, and 20%) with the mechanically treated and untreated zinc mine tailing waste. The effect of the zinc mine tailing waste was investigated by conducting the various mechanical (compressive strength and elastic modulus) tests, durability (ultrasonic pulse velocity, water absorption, chloride penetration, carbonation, sulfate attack) tests. The X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) on concrete samples were also conducted for microstructure analysis. According to the various tests conducted, all concrete properties showed comparable results at the 5% cement substitution in concrete by mechanically treated zinc tailing waste. However, the zinc tailing waste concrete was shown to be more sulphate resistance than the control concrete. Test findings suggest that it is feasible to use 10% mechanically treated and 5% untreated zinc tailing waste as a substitute for cement in concrete to reduce the adverse effect on the environment.

CO₂ uptake due to carbonation is an important issue for sustainability in the concrete industry. This study presents an analysis model of CO₂ uptake of slag-blended concrete considering the service stage and the recycling stage. First, a slag-blended cement hydration model is used to evaluate the content of carbonatable substances, porosity, and diffusivity. Regarding the service stage, a one-dimensional carbonation model is proposed to evaluate carbonation depth. For the recycling stage, an unreacted core model is proposed to evaluate the carbonation fraction of crushed, spherical concrete. Second, CO₂ uptake in the service stage and recycling stage is determined based on the carbonated fraction, shape of the concrete element, concrete component, and exposure conditions. The total CO₂ uptake ratio is determined based on the content of CO₂ uptake and CO₂ emissions. Third, the analysis results show that for concrete with a water-to-binder ratio of 0.3, as the slag replacement ratio increases from 0 to 50%, the total CO₂ uptake ratio increases from 21.43 to 28.87%. For concrete with 50% slag as the binder, as the water-to-binder ratio increases from 0.30 to 0.35, the total CO₂ uptake ratio increases from 28.87 to 30.59%. The sizes and types of the structural elements and the diameter of the crushed concrete can impact the rate of CO₂ uptake, but do not modify the total CO₂ uptake ratio.

Municipal solid waste incineration (MSWI) generates bottom ash, fly ash (FA), and air pollution control (APC) residues as by-products. FA and APC residues are considered hazardous due to the presence of soluble salts and a high concentration of heavy metals, and they should be appropriately treated before disposal. Physicochemical characterization using inductively coupled plasma mass spectroscopy (ICP-MS), X-ray diffraction (XRD), and X-ray fluorescence (XRF) have shown that FA and APC have potential for reuse after treatment as these contain CaO, SiO₂, and Al₂O₃. Studies conducted on treatment of FA and APC are categorized into three groups: (i) separation processes, (ii) solidification/stabilization (S/S) processes, and (iii) thermal processes. Separation processes such as washing, leaching, and electrochemical treatment improve the quality and homogeneity of the ash. S/S processes such as chemical stabilization, accelerate carbonation, and cement solidification modify hazardous species into less toxic constituents. Thermal processes such as sintering, vitrification, and melting are effective at reducing volume and producing a more stable product. In this review paper, the treatment processes are analyzed in relation to ash characteristics. Issues concerning mixing FA and APC residues before treatment, true treatment costs, and challenges are also discussed to provide further insights on the implications and possibilities of utilizing FA and APC as secondary materials.

Carbonic anhydrase (CA) has been immobilized on chitosan stabilized iron nanoparticles (CSIN) for the biomimetic carbonation reaction. CSIN was characterized using scanning electron microscope, energy dispersive X-ray, X-ray diffraction spectroscopy, and Fourier transform infrared analysis. The effect of various parameters such as pH, temperature and storage stability, on immobilized CA was investigated using a p-NPA assay. Kinetic parameters of immobilized and free CA (K ₘ and V ₘₐₓ values) were also evaluated. The K ₘ and V ₘₐₓ for immobilized CA was 1.727 mM and 1.189 μmol min⁻¹ ml⁻¹, respectively, whereas for free enzyme the K ₘ and V ₘₐₓ was 1.594 mM and 1.307 μmol min⁻¹ ml⁻¹, respectively. It was observed that the immobilized enzyme had longer storage stability and retained 50 % of its initial activity upto 30 days at room temperature. CA immobilized on CSIN has been used for hydration of CO₂, and the results were validated by using a gas chromatographic method. Proof of concept has been established for the biomimetic carbonation reaction. Immobilized CA show reasonably good CO₂ sequestration capacity of 21.55 mg of CaCO₃/mg of CA as compared to CO₂ sequestration capacity of 34.92 mg of CaCO₃/mg of CA for free CA respectively, under a limiting concentration of CO₂ (14.5 mg of CO₂/10 ml).