Stabilization-solidification-utilization of MSWI fly ash coupling CO2 mineralization using a high-gravity rotating packed bed

CO2 sequestration in microbial electrolytic cell-anaerobic digestion system combined with mineral carbonation for sludge hydrolysate treatment

The combination of microbial electrolytic cells and anaerobic digestion (MEC-AD) became an efficient method to improve CO2 capture for waste sludge treatment. By adding CaCl2 and wollastonite, the CO2 sequestration effect with mineral carbonation under 0 V and 0.8 V was studied. The results showed that applied voltage could increase dissolved chemical oxygen demand (SCOD) degradation efficiency and biogas yield effectively. In addition, wollastonite and CaCl2 exhibited different CO2 sequestration performances due to different Ca2+ release characteristics. Wollastonite appeared to have a better CO2 sequestration effect and provided a wide margin of pH change, but CaCl2 released Ca2+ directly and decreased the pH of the MEC-AD system. The results showed methane yield reached 137.31 and 163.50 mL/g SCOD degraded and CO2 content of biogas is only 12.40 % and 2.22 % under 0.8 V with CaCl2 and wollastonite addition, respectively. Finally, the contribution of chemical CO2 sequestration by mineral carbonation and biological CO2 sequestration by hydrogenotrophic methanogenesis was clarified with CaCl2 addition. The chemical and biological CO2 sequestration percentages were 46.79 % and 53.21 % under 0.8 V, respectively. With the increased applied voltage, the contribution of chemical CO2 sequestration rose accordingly. The findings in this study are of great significance for further comprehending the mechanism of calcium addition on CO2 sequestration in the MEC-AD system and providing guidance for the later engineering application.

Transformation behavior of heavy metal during Co-thermal treatment of hazardous waste incineration fly ash and slag/electroplating sludge

In this study, the behavior of heavy metal transformation during the co-thermal treatment of hazardous waste incineration fly ash (HWIFA) and Fe-containing hazardous waste (including hazardous waste incineration bottom slag (HWIBS) and electroplating sludge (ES)) was investigated. The findings demonstrated that such a treatment effectively reduced the static leaching toxicity of Cr and Pb. Moreover, when the treatment temperature exceeded 1000 °C, the co-thermal treated sample exhibited low concentrations of dynamically leached Cr, Pb, and Zn, indicating that these heavy metals were successful detoxified. Thermodynamic analyses and phase transformation results suggested that the formation of spinel and the gradual disappearance of chromium dioxide in the presence of Fe-containing hazardous wastes contributed to the solidification of chromium. Additionally, the efficient detoxification of Pb and Zn was attributed to their volatilization and entry into the liquid phase during the co-thermal treatment process. Therefore, this study sets an excellent example of the co-thermal treatment of hazardous wastes and the control of heavy metal pollution during the treatment process.

Synthesis of metakaolin-based geopolymer foamed materials using municipal solid waste incineration fly ash as a foaming agent

The existence of metallic aluminum in municipal solid waste incineration fly ash (MSWIFA) makes it challenging to recycle MSWIFA into cement materials because expansion occurs in the resultant matrices. Geopolymer-foamed materials (GFMs) are gaining attention in the field of porous materials due to their high-temperature stability, low thermal conductivity and low CO2 emission. This work aimed to utilize MSWIFA as a foaming agent to synthesize GFMs. The physical properties, pore structure, compressive strength and thermal conductivity were analyzed to assess different GFMs which were synthesized with various MSWIFA and stabilizing agent dosages. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis were conducted to characterize the phase transformation of the GFMs. Results showed that when MSWIFA content was increased from 20 to 50%, the porosity of GFMs increased from 63.5 to 73.7%, and bulk density decreased from 890 to 690 kg/m3. The addition of stabilizing agent could trap the foam, refine the cell size, and homogenize the cell size range. With the stabilizing agent increase from 0 to 4%, the porosity increased from 69.9 to 76.8%, and the bulk density decreased from 800 to 620 kg/m3. The thermal conductivity decreased with increasing MSWIFA from 20 to 50%, and stabilizing agent dosage from 0 to 4%. Compared with the collected data from references, a higher compressive strength can be obtained at the same level of thermal conductivity for GFMs synthesized with MSWIFA as a foaming agent. Additionally, the foaming effect of MSWIFA results from the H2 release. The addition of MSWIFA changed both the crystal phase and gel composition, whereas the stabilizing agent dosage had little impact on the phase composition.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

Submicron tourmaline enhanced the solidification of municipal solid waste incineration fly ash by chemical structure reorganization and stabilized heavy metals

Municipal solid waste incineration fly ash (MSWIFA) exsits in large quantitities and contains pollutants such as heavy metal. While solidification is one of the most effective methods for treating MSWIFA, this application is limited by cost, subsequent treatment, and simultaneous immobilization of anions and cations. This research demonstrated that under a certain initial pressure (20 MPa), a gelation reaction involving ball milling-modified tourmaline powder, a small amount of cement clinker, and MSWIFA forms a stable consolidated body and significantly reduces the risk of heavy metal dissolution. The consolidated MSWIFA can easily be formed into unfired bricks in large-scale pilot production, and a response surface model was used to optimize the experimental parameters. When the mass ratio of tourmaline: cement clinker: MSWIFA was 15:15:200 (mixed with a moisture content of 13 to 15 %), the compressive strength of the consolidated body reached 13 MPa, and the amounts of Cr and Pb leached decreased from 12 mg/L to 0.1 mg/L and 25 mg/L to 0.3 mg/L, respectively. The consolidated form contained a new mineral phase (Ca₃Si₂O₇·3H₂O, Ca₁₀Mg_0.8(SiO₄)_0.6O₂Cl, and CaCl₂∙Ca(OH)₂·H₂O) with a high compressive strength. Notably, the soluble PbSO₄ in the MSWIFA was converted into relatively stable PbSiO₃, and Cr(VI) was lattice-wrapped. This study was the first to demonstrate that tourmaline synchronously passivates Pb(II) and Cr(VI) in fly ash in the solid phase, with a low cost and requires no subsequent treatment. This study provided a novel technical path for recycling MSWIFA. Eventually, leaching of the heavy metals Pb, Cr, Cu, Cd, and Zn from the solids achieved concentrations less than 0.25, 1.5, 0.5, 0.15, and 100 mg/L.

Accelerated and natural carbonation of a municipal solid waste incineration (MSWI) fly ash mixture: Basic strategies for higher carbon dioxide sequestration and reliable mass quantification

The carbonation of alkaline wastes is an interesting research field that may offer opportunities for CO₂ reduction. However, the literature is mainly devoted to studying different waste sequestration capabilities, with lame attention to the reliability of the data about CO₂ reduction, or to the possibilities to increase the amount of absorbed CO₂. In this work, for the first time, the limitation of some methods used in literature to quantify the amount of sequestered CO₂ is presented, and the advantages of using suitable XRD strategies to evaluate the crystalline calcium carbonate phases are demonstrated. In addition, a zero-waste approach, aiming to stabilize the waste by coupling the use of by-products and the possibility to obtain CO₂ sequestration, was considered. In particular, for the first time, the paper investigates the differences in natural and accelerated carbonation (NC and AC) mechanisms, occurring when municipal solid waste incineration (MSWI) fly ash is stabilized by using the bottom ash with the same origin, and other by-products. The stabilization mechanism was attributed to pozzolanic reactions with the formation of calcium silicate hydrates or calcium aluminate hydrate phases that can react with CO₂ to produce calcium carbonate phases. The work shows that during the AC, crystalline calcium carbonate was quickly formed by the reaction of Ca(OH)₂ and CaClOH with CO₂. On the contrary, in NC, carbonation occurred due to reactions also with the amorphous Ca. The sequestration capability of this technology, involving the mixing of waste and by-products, is up to 165 gCO₂/Kg MSWI FA, which is higher than the literature data.

Converting industrial waste into a value-added cement material through ambient pressure carbonation

Converting industrial wastes into value-added building products in an environmental management strategy is a challenging yet vital component of the industrial process. Steel slag (SS), an industrial waste by-product from the steel-making process, is typically disposed of in landfill which consumes land resources and pollutes the environment. This paper explores the possibility of a closed-loop system to convert steel slag into a cement material through carbonation activation, thereby significantly reducing the amount of steel slag waste sent to landfills across Canada. The production of this cementing material can occur next to the steel mill, utilizing steel slag and carbon dioxide collected on-site to fabricate carbon-negative products. To save energy and allow production to be feasible on an industrial scale, ambient pressure (AP) carbonation is developed to reduce carbon emissions while improving their performance. High pressure (HP) carbonation curing and normal hydration (NH) references were also implemented at the same time to justify the application of AP carbonation in reducing CO₂ emission. The results of this study found AP carbonation-activated SS compacts have comparable CO₂ uptake (about 7.5 tons CO₂/100 tons slag) and mechanically compressive strength values as those subjected to HP carbonation, suggesting that AP could be used to replace HP in carbonation curing to ensure a lower energy input. Additionally, AP seemed to possess as effective carbonation as HP. The studies investigated by multiple techniques including X-ray diffractometer (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopic analysis, and scanning electron microscopy (SEM) aim to identify the microstructure development of carbonated SS paste to assess carbonation results. Developed with life cycle assessment (LCA), environmental impact evaluation shows that AP presents a smaller global warming potential (GWP) value than HP. The comparable CO₂ sequestration, satisfactory engineering properties, enhanced microstructure and lesser environmental impact in AP carbonation confirm the feasibility of replacing high pressure with extremely low pressure to cure concrete products. The use of AP carbonation for cement material created using steel slag reduces carbon emissions, energy usage, and natural resource consumption.

Co-stabilization/solidification of heavy metals in municipal solid waste incineration fly ash and electrolytic manganese residue based on self-bonding characteristics

Municipal solid waste incineration (MSWI) fly ash and electrolytic manganese residue (EMR) were classified as hazardous waste, must be harmlessly processed prior to subsequent treatment or disposal. The competition between massive free manganese ions of raw EMR and other heavy metals was found, thus raw EMR was pretreated by calcining to eliminate competition of manganese with other heavy metals for stabilizer complexation. MSWI fly ash was successfully solidified with 6% NaH₂PO₄, 6% H₂NCSNH₂ and 20% sintered EMR (800 °C). The addition of sintered EMR enhanced solidification/stabilization of heavy metals in fly ash and the resulting product had a higher compressive strength for further reutilization like trench backfilling, structural fill and void filling. The stabilization/solidification mechanism of heavy metals was attributed to the combined interaction of heavy metal precipitation in stabilizers and ion exchange or physical encapsulation in silicate compounds like calcium silicate, which is a feasible and valuable approach to co-disposal of MSWI fly ash and EMR.

Stabilization of Waste-to-Energy (WTE) fly ash for disposal in landfills or use as cement substitute

This study investigated two approaches for managing Waste-to-Energy (WTE) fly ash (FA): (i) phosphoric acid stabilization of FA and disposal in non-hazardous landfills, so that it can pass the U.S. TCLP procedure and meet the U.S. Resource Conservation and Recovery Act (RCRA) standards; (ii) use of FA or phosphoric acid stabilized fly ash (PFA) as cement substitute in construction for avoiding disposal in landfills and reducing the consumption of Portland cement. The effect of stabilization was identified by TCLP tests and XRD quantification (QXRD), which showed that the economically optimal concentration for PFA to pass the RCRA was 1 mol/L H₃PO₄ (equivalent to 0.4 mol of H₃PO₄/kg of FA). Zn/Pb-phosphates were formed in treated ash by using high concentration H₃PO₄ (e.g., 3 mol/L). Thus, the hazardous FA was chemically stabilized to PFA, that were both discussed as cement substitute. QXRD and SEM results showed that both FA and PFA (1 mol/L H₃PO₄) chemically reacted with cement and water. Up to 25 vol% of the cement can be replaced by FA or PFA, with similar mechanical performance of cement mortars than that of reference. Testing by LEAF Method 1313-pH dependence showed that the FA and PFA cement mortars exhibited the same leachability of heavy metals; therefore, this study demonstrated the technical feasibility of utilizing either raw FA or stabilized PFA as supplementary cementitious material. The leachability of heavy metals in optimal FA or PFA 25 vol% cement mortar was under the U.K. WAC non-hazardous limits.

Improving consistency at testing cementitious materials in the Dynamic Surface Leaching Test on the basis of the European technical specification CEN/TS 16637-2 - Results of a round robin test

The environmental impact assessment of materials is usually based on laboratory tests, mostly in combination with models describing the longterm fate of the substances of interest in the targeted environmental compartment. Thus, laboratory tests are the fundamental link to achieve appropriate assessment conclusions which makes it essential to generate consistent results. This just as applies to the leaching of cementitious materials. In Europe, the leaching behavior of monolithic building materials is tested in the Dynamic Surface Leaching Test following the specification CEN/TS 16637-2. An interlaboratory comparison on European level regarding this technical specification showed relatively high intra- and interlaboratory variations for the tested materials (monolithic copper slag and cement stabilized coal fly ash). Therefore the German Committee for Structural Concrete (DAfStb) framed a guideline to specify additional testing conditions for cementitious materials. To assess the possible improvement by this guidelines measures, a round robin test with 11 participants from Germany and the Netherlands was conducted. This work aims to provide insight into the factors to be considered in the testing of alkaline materials, including sample preparation, and highlights crucial procedures and their manifestation in the results. All evaluated parameters showed improved results compared to the earlier round robin test. The relative standard deviations for repeatability (RSD_r) and reproducibility (RSD_R) of the elements calcium, barium, antimony, chromium, molybdenum and vanadium, which are the parameters evaluated in both round robin tests, were RSD_r = 4%, 4%, 2%, 5%, 5%, and 5% respectively (4% in average) for this work, in comparison to the European round robin test with an average RSD_r of 29% (17%, 17%, 20%, 40%, 36%, and 42%). The RSD_R improved from 41% (30%, 36%, 29%, 57%, 40%, and 56%) to 14% (12%, 8%, 6%, 28%, 15%, and 12%). CO₂ ingress during testing and the inaccuracy of eluate analytics for concentrations close to the determination limits were identified as the main sources of error.

Risk assessment of lead and cadmium leaching from solidified/stabilized MSWI fly ash under long-term landfill simulation test

The long-term effectiveness concern of municipal solid waste incineration (MSWI) fly ash (FA) disposal has been placed more emphatic recently, however, few studies worked on the control of leaching risk of heavy metals under the long-term stability. In this study, the leaching properties and risk assessment of two representative solidified/stabilized (S/S) FA wastes, i.e., sodium dithiocarbamate (DTC) chelator treated and Portland cement + chelator combining treated, were evaluated by a long-term cycles assessment method which coupled multifaceted environmental stresses (e.g., freezing-thawing, drying-wetting, accelerated carbonation). The results showed that the cement/chelator had a better long-term stability and exhibited ~55% lower cumulative overall pollution toxicity index (OPTI) than chelator treatment after the test, which was always rated as "low risk" during the cycles. In addition, the cement/chelator exhibited ~23.3% smaller cumulative mass release rate than the chelator treatment after 6 cycles and restrained the transformation of Pb and Cd from stable states to removable fractions, which attributes to its great erosion resistance and compact pore structure. Under the cumulative external factors and carbon dioxide attacks, the decalcification of hydrate products (e.g., C-S-H, hydrocalumite), as well as deterioration of pore structure are the critical factors increasing the local erosion, cracking and heavy metals release. Thus, the optimization of S/S waste microstructure (e.g., enhancing binder system) and landfill site conditions (e.g., reducing rainfall impact) could be propitious to the S/S waste risk control and management.

Solidification of heavy metals and PCDD/Fs from municipal solid waste incineration fly ash by the polymerization of calcium carbonate oligomers

Calcium carbonate oligomers are gel-state precursors that can be crystallized by low-temperature heat treatments to form an inorganic material with a monolithic and continuous structure, this material can effectively solidify/stabilize heavy metals in municipal solid waste incineration fly ash (MSWI FA). Calcium chloride addition achieves FA stabilization/solidification by the formation and polymerization of calcium carbonate oligomers. The effects of calcium, triethylamine (TEA), and water-washing pretreatment on the solidification of heavy metals by the polymer were studied. Consequently, as more calcium was added, the solidification improved. When the ratio of TEA/Ca²⁺ was increased from 2:1 to 3:1, the solidification efficiency of As and Cd increased, but it decreased when the ratio was continuously increased to 4:1. After the water-washing pre-treatment, the MSWI FA had a significantly improved solidification effect on the heavy metals, and the solidification efficiencies of zinc, copper, cadmium, chromium, lead, and arsenic were 81.9%, 90.0%, 93.5%, 91.8%, 99.6% and 95.5%, respectively. Additionally, the solidification efficiency of PCDD/Fs was 56.5%. The heavy metals and PCDD/Fs in MSWI FA solidified by physical adsorption, wrapping and chemical precipitation. The continuous calcium carbonate structure adsorbed and encased the MSWI FA, and the heavy metals in the MSWI FA were converted from a free state to carbonate precipitates through carbonation, and the carbonate precipitate was more likely to be physical solidification by calcium carbonate.

Environmental benefit assessment of steel slag utilization and carbonation: A systematic review

The rapid increase in steel slag generation globally highlights the urgent need to manage the disposal or utilization processes. In addition to conventional landfill disposal, researchers have successfully reused steel slag in the construction, chemical, and agricultural fields. With the large portions of alkaline silicate mineral content, steel slag can also be used as a suitable material for carbon capture to mitigate global warming. This article comprehensively reviews the environmental performance of steel slag utilization, especially emphasizing quantitative evaluation using life cycle assessment. This paper first illustrates the production processes, properties, and applications of steel slag, and then summarizes the key findings of the environmental benefits for steel slag utilization using life cycle assessment from the reviewed literature. This paper also identifies the limitations of quantifying the environmental benefits using life cycle assessment. The results indicate steel slag is largely utilized in pavement concrete and/or block as a substitution for natural aggregates. The associated environmental benefits are mostly attributed to the avoidance of the large amount of cement utilized. The environmental benefits for the substitution of traditional energy-intensive material and carbonation treatment are further discussed in detail. Due to the presence of heavy metals, the potential risks to human and ecological health caused by the manufacturing process and usage stage are examined. Finally, the current challenges and global social implications for steel slag valorization are summarized.