Comparison of MSWI fly ash from grate-type and circulating fluidized bed incinerators under landfill leachate corrosion scenarios: the long-term leaching behavior and speciation of heavy metals

Solidification/stabilization pre-treatment coupled with landfill disposal of heavy metals in MSWI fly ash in China: A systematic review

The growth in municipal solid waste incineration (MSWI) has resulted in a substantial rise in the production of fly ash in China. It is anticipated that during the "14th Five-Year Plan", the accumulated amount of fly ash stocked and disposed of at landfills will surpass 100 million tons. With the development of the economy and the implementation of garbage classification relevant policies, the pollution characteristics of heavy metal change in spatiotemporal distribution. Solidification/stabilization (S/S) pre-treatment coupled with landfill disposal is the mainstream method for fly ash. This study provides a systematic overview and comparison of the current application status and research on the mechanism of S/S technology, and the long-term stability of solidified/stabilized fly ash is a crucial factor in controlling the risks of landfills. Subsequently, it examines the influencing factors and mechanisms associated with heavy metals leaching under different environmental scenarios (meteorological factors, leachate and acid rain erosion, and carbonation, etc.), and concludes that single stabilization technology is difficult to meet long-term landfill requirements. Finally, the limits of heavy metal leaching toxicity evaluation methods and landfilled fly ash supervision were discussed, and relevant suggestions for future development were proposed. This study can provide theoretical instruction and technical support for the risk control of potential environmental risks of heavy metals in solidified/stabilized fly ash from landfills in China.

Leaching behavior and comprehensive toxicity evaluation of heavy metals in MSWI fly ash from grate and fluidized bed incinerators using various leaching methods: A comparative study

Municipal solid waste incineration fly ash (MSWI FA) is a kind of hazardous waste that contains a substantial amount of heavy metals. To facilitate the appropriate treatment of MSWI FA, the leaching behavior of heavy metals was evaluated in MSWI FA from various sources using different leaching methods. Nine kinds of MSWI FA were investigated using three kinds of batch leaching tests (TCLP, HJ/T 300, and EN12457-2). The chemical form distributions of heavy metals in MSWI FA were obtained by sequential extraction procedures (SEPs) and the environmental risk posed by MSWI FA was comprehensively evaluated. The results showed that the grate and fluidized bed MSWI FA performed differently in various leaching methods, which was mainly dependent on the leachate pH and the chemical form distributions of the heavy metals. In addition, the BCR SEP was more suitable for the fractionation of heavy metals and the environmental risk assessment of MSWI FA when compared with Tessier's SEP. The overall pollution toxicity index allowed a comprehensive risk assessment specific to the leaching environment, thereby offering valuable guidelines for the stabilization or resource-based treatment of MSWI FA.

Density functional theory study on the formation mechanism of CaClOH in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is defined as a kind of hazardous waste because of its high levels of multiple pollutants. The main component of MSWI fly ash is CaClOH, and the characteristics have not achieved consensus. And density functional theory (DFT) was used to calculate the formation process of CaClOH in this study, which mainly included HCl adsorption on CaO (0 0 1) surface and Ca(OH)2 (0 0 1) surface and the surface reaction process. The reaction mechanism was investigated. The results showed that the maximum adsorption energies of HCl on CaO and Ca(OH)2 surfaces reached - 195.17 kJ/mol and - 83.48 kJ/mol, respectively, representing strong chemisorption. The chemisorption process was shown as the adsorption of H atom on O site, and the adsorption capacity was reflected in the adsorption range of O site. The significant electron density overlap between O site and H atom meant that a new chemical bond formed, which made the adsorption structure stable. The adsorption energy of multi-HCl adsorption on the crystal surfaces was not proportional to the number of HCl molecule, indicating that the adsorption processes were influenced by each other. After surface reaction, the H-Cl bond was broken completely, and the structure of CaO and Ca(OH)2 changed to new structures. According to transition state (TS) search, the formation of CaClOH had a higher priority, easier than that of CaCl2, explaining the presence of CaClOH in fly ash. The study provides helpful information for the solidification treatment of fly ash.

Footprints in compositions, PCDD/Fs and heavy metals in medical waste fly ash: Large-scale evidence from 17 medical waste thermochemical disposal facilities across China

Chemical compositions, polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) profiles and heavy metals (HMs) leachability of medical waste fly ash (MWFA) from 17 thermochemical treatment facilities in eight Chinese provinces were first investigated. Large-scale and extended monitoring revealed high chloride and Zn contents and similar PCDD/Fs congener profiles in MWFA. Particularly, the PCDD/Fs and HMs concentrations implied greater toxicity than that observed for municipal solid waste incinerator fly ash (MSWIFA). The maximum international toxic equivalent value of PCDD/Fs in MWFA was 40 times that of MSWIFA, and the leaching concentrations of Zn and Hg were 15 and 4 times those of MSWIFA, respectively. Notably, MWFA characteristics suggest the possibility of recycling and sustainable disposal solutions owing to the high Cl and Zn content with good recovery instead of landfill disposal. Similarities in chemical composition, PCDD/Fs homolog distribution, and water-solubility of chloride salts allows co-processing of MWFA and MSWIFA via water-washing detoxification and thermal treatment, such as that used in cement kilns. This study supplements existing literature on the characteristics and risk management of MWFA.

Zeolite NaP1 synthesized from municipal solid waste incineration fly ash for photocatalytic degradation of methylene blue

The disposal of hazardous municipal solid waste incineration (MSWI) fly ash is a challenge nowadays. Recently, the re-utilization of MSWI fly ash by converting it to useful zeolite-containing materials has attracted attention. However, the zeolitic products fabricated from MSWI fly ash are usually of low quality and rarely reported to be applied for photocatalysis. In this study, valuable zeolites (e.g., NaP1) are synthesized from MSWI fly ash via a modified microwave-assisted hydrothermal method. The key parameters for the hydrothermal method including temperature, duration, the amount of additive, and water volume, are investigated and optimized. Specifically, increasing the hydrothermal temperature can promote the synthesis of zeolitic materials; a relatively long hydrothermal duration is essential to accomplish the assembly of zeolites; the addition of Na₂SiO₃ can increase the precursor for the fabrication of zeolites; the water volume makes little influence on the crystal style of products. Eventually, the hydrothermal condition of 180 °C, 1 h, 0.5 g Na₂SiO₃, and 10 mL water is suggested based on the energy consumption and the quality of zeolites. The product containing zeolite NaP1 from such a condition is further applied to degrade methylene blue by photocatalysis. The removal rate has reached 96% within 12 h, which dramatically surpasses that of the raw fly ash (38%). Such excellent photocatalytic performance is attributed to the 10-fold increased surface area (24.864 m² g^-1) and active metal elements embedding in the zeolite structures.

Improving stabilization/solidification of MSWI fly ash with coal gangue based geopolymer via increasing active calcium content

Municipal solid waste incineration fly ash (MSWI FA) is categorized as a hazardous waste, which demands environmentally acceptable treatment due to its easy leachability toxic of heavy metals. This study investigated an innovative and improved method for stabilization/solidification (S/S) of MSWI FA with coal gangue based geopolymer by the addition of active calcium content. The specimen with addition of calcium oxide up to 10 % reached the compressive strength of 2.14 MPa at 28 d. The addition of 30 % calcium oxide resulted in the highest immobilization efficiencies of Cd (98.96 %) and Pb (99.19 %). X-ray Diffraction (XRD), Fourier Transform Infrared Spectrometry (FTIR), Scanning Electron Microscope (SEM), and thermogravimetric (TG) analysis indicated the generation of calcium-containing hydration products was promoted after the improvement of calcium content in binder. Heavy metals were stabilized through the chemical adsorption and ions exchange of amorphous hydration products. On the whole, this study illustrated that the incorporation of active calcium content can improve efficiently S/S of hazardous ash waste such as MSWI FA.

Chemical stabilization of heavy metals in municipal solid waste incineration fly ash: a review

Sufficient attention should be attached to the large amount of fly ash containing high levels of toxic heavy metals generated after municipal solid waste incineration. Because heavy metals could be leached out of the fly ash under specific conditions, it is necessary to stabilize the heavy metals in fly ash before landfill disposal. Processing technologies of incineration fly ash include solidification/stabilization technology, thermal treatments, and separation processes. This study reviewed the current treatment technologies of municipal solid waste incineration (MSWI) fly ash, with the main focus on the treatment of heavy metals in fly ash with chemical stabilization. Chemical stabilization processes involve chemical precipitation of heavy metal and chelation of heavy metals. In multiple studies, chemical stabilization technology has shown practical feasibility in terms of technology, economy, and effect. In addition, the combination of two or more stabilization agents broadens the general applicability of the agents to heavy metals and reduces the cost. The application of joint processing technology realizes the remove of soluble salt from fly ash. To minimize pollutants while increase their usable value, effective use of waste and co-disposal of several kinds of wastes have gradually become the research hotspots. New developments in chemical stabilization are progressively moving towards the sustainable direction of harmlessness and resource utilization of MSWI fly ash.