Reductive solidification/stabilization of chromate in municipal solid waste incineration fly ash by ascorbic acid and blast furnace slag

Ladle slag-based binder for the solidification/stabilization of heavy-metal-rich industrial waste

Heavy metal pollution from industrial sources is a major environmental and health hazard on a global scale. This study introduces a solidification/stabilization method of industrial waste using a waste-based, ettringite-rich solid binder from ladle slag and gypsum for the immobilization of an industrial waste material with extremely high contents of several heavy metals. The importance of sulfate and water content on the immobilization efficiency and the use of citric acid to increase the processing time of the binder were studied. The leaching of Pb, Hg, Se, As, Cd, Cu, and Ni was measured, and X-ray powder diffraction, field-emission scanning electron microscopy, and field-emission electron probe microanalysis combined with wavelength-dispersive X-ray spectroscopy were used to analyze the structure of the hardened binder and the location of the heavy metals within. The study shows that the ladle slag/gypsum binder is suitable for the solidification/stabilization of heavy-metal-rich solid industrial waste. Hg, As, Cd, Cu, and Ni were fully immobilized in all scenarios covered in the study, whereas Pb and Se showed more complicated behaviors. The main immobilization method was encapsulation, and partial Se incorporation into ettringite was observed. The presence of citric acid increased the processing time of the binder without harming the immobilization, unless combined with low sulfate content.

Solidification of heavy metals in municipal solid waste incineration washed fly ash by asphalt mixture

Using asphalt mixture to solidify heavy metals in municipal solid waste incineration fly ash can reduce pollution and realize resource utilization. In this study, the physical and chemical properties of washed fly ash were analyzed, and washed fly ash was added to asphalt mixture as filler instead of mineral powder. The study involved analyzing the mechanical attributes of asphalt mixtures containing washed fly ash, along with examining the characteristics of asphalt binder that incorporates the washed fly ash. Subsequently, assess the potential leaching hazards associated with asphalt mixture incorporating washed fly ash. The test results showed that washed fly ash was a Si-Al-Ca system material, which had small particle size, large specific surface area and many pores. It increased the contact area with asphalt, which improved encapsulation of asphalt and aggregates. The optimal dosage of washed fly ash is 2.5%. At this dosage, the mixture attains optimal high-temperature performance, while both low-temperature performance and the characteristics of washed fly ash asphalt binder align with requirements. Asphalt mixture has solidification on heavy metals, with strongest solidification for Zn, followed by Cu, Cr. A prediction model of leaching amount versus time was constructed for Pb, Ba and Ni, which have weak solidified ability. The cumulative leaching amount of the road within 15 years of service life was calculated through the model, and it was obtained that the addition of washed fly ash will not cause pollution to environment. Overall, this study showed that asphalt mixtures can be used for stabilization/solidification of washed fly ash while saving natural mineral, providing a theoretical basis for the resource application of washed fly ash in asphalt road construction.

Probing the aging process and mechanism of microplastics under reduction conditions

Microplastics (MPs) are becoming a class of pollutants with high global concerns. Research on the aging of MPs has focused on oxidative environments, it is of great interest to study the aging of MPs under reduction conditions. In this study, a reduction environment was constructed by purging nitrogen and adding reducing agents (NaBH₄, VC, Na₂S, C₂Na₂O₄) to understand the aging behavior and mechanism of MPs. The results proved that PVC occurred aging under four reduction conditions, and the aging degree was the strongest under NaBH₄ reduction condition. The aged PVC became broken, particle size decreased, and dechlorination phenomenon was observed. These phenomena were more obvious under the reduction condition in light, which was the superposition of photo-aging and reduction aging. The functional group components of PVC changed (C-C/CC increased, and oxygen-containing functional groups decreased) under reduction conditions, but photo-aging was dominant in the light system. Electron transfer occurred during the reduction process, and the EDC of PVC aged increased and EAC decreased. This study may shed light on a highly efficient aging pathway of MPs that is often overlooked in nature, contributing to understanding the aging behavior of MPs in complex environments.

Low Temperature Thermal Treatment of Incineration Fly Ash under Different Atmospheres and Its Recovery as Cement Admixture

Municipal solid waste incineration fly ash is classified as hazardous waste because it contains dioxins and a variety of heavy metals. It is not allowed to be directly landfilled without curing pretreatment, but the increasing production of fly ash and scarce land resources has triggered consideration of the rational disposal of fly ash. In this study, solidification treatment and resource utilization were combined, and the detoxified fly ash was used as cement admixture. The effects of thermal treatment in different atmospheres on the physical and chemical properties of fly ash and the effects of fly ash as admixture on cement properties were investigated. The results indicated that the mass of fly ash increased due to the capture of CO₂ after thermal treatment in CO₂ atmosphere. When the temperature was 500 °C, the weight gain reached the maximum. After thermal treatment (500 °C + 1 h) in air, CO₂, and N₂ atmospheres, the toxic equivalent quantities of dioxins in fly ash decreased to 17.12 ng TEQ/kg, 0.25 ng TEQ/kg, and 0.14 ng TEQ/kg, and the degradation rates were 69.95%, 99.56%, and 99.75%, respectively. The direct use of fly ash as admixture would increase the water consumption of standard consistency of cement and reduce the fluidity and 28 d strength of mortar. Thermal treatment in three atmospheres could inhibit the negative effect of fly ash, and the inhibition effect of thermal treatment in CO₂ atmosphere was the best. The fly ash after thermal treatment in CO₂ atmosphere had the possibility of being used as admixture for resource utilization. Because the dioxins in the fly ash were effectively degraded, the prepared cement did not have the risk of heavy metal leaching, and the performance of the cement also met the requirements.

Study on combined technology of glutathione reduction and alkali solidification of chromium-containing sludge

Chromium-containing sludge (CCS) is a solid hazardous waste containing various heavy metals, threatening the human body and the environment. In this study, CCS was solidified/stabilized by a combined method of glutathione (GSH) prereduction of Cr(VI) and alkali-excited blast furnace slag (BFS) preparation of geopolymer/gel. To explore the prospect of glutathione treatment of solid waste and combination with solidification/stabilization technology. The appropriate dosage and initial pH value of GSH were obtained through experiments (the dosage of GSH was 0.5%, and the initial pH value was 1). The preparation parameters (content of alkali activator 13%, modulus of alkali activator 1.7, initial curing temperature 25 ℃, liquid-solid ratio 0.26) and mechanism of geopolymer/gel were studied. GSH pre-reduction technology significantly increases the maximum dosage of CCS in the solidified body, and increases the proportion of heavy metals in the residual state. The solidified product meets landfill requirements and holds promise as a building material. According to XRD, FTIR, SEM-EDS, Geopolymers/gels solidify heavy metals utilizing physical encapsulation, chemical encapsulation, charge balance, and bonding.

Roasting mechanism of lightweight low-aluminum-silicon ceramisite derived from municipal solid waste incineration fly ash and electrolytic manganese residue

Municipal solid waste incineration (MSWI) fly ash and electrolytic manganese residue (EMR) belong to hazardous waste, and must be disposed of before processing. It was found that the low content of silicon and aluminum at low roasting temperature can meet the expansion mechanism of lightweight aggregates. A low-aluminum-silicon lightweight ceramisite was successfully prepared from MSWI fly and EMR, the formation mechanism of which was that the viscosity of molten stuffs in pellet was the function of temperature and chemical composition and had enough capacity of capturing the emerged gas over roasting. The resulting ceramisite met with the requirement of Lytag commercial lightweight aggregate. The content of heavy metal in ceramisite accorded with the requirement of soil environmental quality for development GB 36600-2018 Class I, and PCDD/Fs in ceramisite was 2.0 ng I-TEQ/kg, which was safe. The collaboration of thermal simulation and characterization (SEM-EDS, FTIR and XRD) elaborated the formation mechanism of ceramisite, with six stages provided.

A Multi-Commodity Mathematical Modelling Approach—Hazardous Waste Treatment Infrastructure Planning in the Czech Republic

This paper presents an analysis of infrastructure for the processing of Czech hazardous waste and pays attention to predictions of waste management development in the upcoming years. For this purpose, a unique complex approach to modelling future waste management changes is applied. The method uses a multi-commodity network flow model with reverse flows between treatment facilities to consider complete waste management of hazardous waste. The future outlook (2030) for the forecasted generation of different types of hazardous waste in the Czech Republic requires decisions on waste treatment facility infrastructure. The uniqueness lies in using real data for such a wide scope of a task, further enhanced by concurrent analysis of more types of waste interconnected through limited processing capacities. The results indicate the insufficiency in hazardous waste thermal treatment and stabilization. A suggestion is to extend the incineration capacity because it influences the stabilization units, which must process the remaining waste. The recommended increase is 100% with different proportions in individual regions.

The fate of heavy metals in the co-processing of solid waste in converter steelmaking

The improper disposal of large amounts of solid waste (SW) has led to serious ecological and environmental problems, especially heavy metal (HM) pollution. Converter steelmaking has the potential to co-process SW, but the distribution of heavy metals (HMs) during converter steelmaking is unclear. In this study, the effects of smelting temperature and slag alkalinity on the distribution of typical HMs in the SW of steel samples, steel slag, and the gas phase were investigated in a specially-made induction furnace. The results showed that upon increasing the smelting temperature, As (As2S3) was mainly distributed in the steel sample, and the HM-containing compounds Cr2O3, CrCl3, ZnCl2, ZnS, ZnO, PbCl2, PbS, and PbO were mainly distributed in the gas phase. Upon increasing the alkalinity within a certain range, the distribution of HMs in steel samples and steel slag increased gradually, while their distribution in the gas phase decreased. Thermodynamic calculations, Eh-pH diagrams, XRD patterns, and XPS spectra indicated that impurity elements in the hot metal and the CaO content affected the chemical reactions by which HM-containing compounds in the steel sample formed elemental HMs and those in steel slag existed as oxides; therefore, it is necessary to choose a suitable temperature and alkalinity for slag when disposing of different types of SW.

Phase changes during various treatment processes for incineration bottom ash from municipal solid wastes: A review in the application-environment nexus

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.

A microwave irradiation-persulfate-formate system for achieving the detoxification and alkali-activated composite geopolymerization of the chromate-contaminated soil

A microwave (MA) irradiation-persulfate-formate system was constructed to detoxify Cr contamination and solidify the geopolymerization of the alkali-activated composite material. Three series of experiments were correspondingly conducted to evaluate the treatment for the chromate-contaminated soil. The changes in the molar ratios of formate to persulfate and the mass rates of fortifier to soil led to a significantly greater reduction of Cr^VI in the detoxification experiments. The increase of blast furnace slag from 50% to 80% in the composite cementitious materials (CCM) intensified the immobilization efficiencies of chromate and the compressive strengths of geopolymer blocks. MA irradiation potentially enhanced the binding of Ca cations to the aluminosilicate compounds. The degree of reaction in the phenomenological kinetics model mathematically verified the geopolymerization process. Ettringite was formed within the structure of the geopolymer in the coupling system. Sulfate radicals released from persulfate not only contributed to the detoxification process but also strengthened the immobilization process.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

Detoxification and immobilization of chromite ore processing residue using the alkali-activated cementitious materials mixed with ascorbic acid

The existence of leachable Cr(Ⅵ) in chromite ore processing residue (COPR) makes it hazardous waste. Therefore, resourceful utilization of COPR is necessary to protect the ecosystem and living biota from hazardous effect of Cr(Ⅵ) caused by its leaching. In this study, detoxification and immobilization of COPR was carried out through introduction of ascorbic acid (AA) in alkali-activated cementitious materials. Several dosages of AA were treated with water extractable/soluble Cr(Ⅵ) to achieve the optimum dosage which could be further utilized in solidification process. While, the compressive strength was developed through utilizing different modulus of water glass, liquid to solid ratios and curing temperatures. The results showed that 0.3% of AA was enough to reduce the Cr(Ⅵ) into Cr(Ⅲ), and highest compressive strength of 120 MPa was achieved after using the modulus of 1.6, liquid to solid ratio of 0.24 and curing temperature of 30 °C. The solidified samples having AA had not exceeded the toxicity limit up to 60% addition of COPR, and samples without addition of AA were effective for solidification of 20% COPR. Regarding mechanism, the compressive strength, leaching behavior and microscopic analysis i.e. X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR) and scanning electron microscope with energy dispersive spectrometry (SEM-EDS) showed that immobilization of chromium was carried out through physical and chemical means.

Human exposure and risk assessment of recycling incineration bottom ash for land reclamation: A showcase coupling studies of leachability, transport modeling and bioaccumulation

Incineration bottom ash (IBA) faces challenges for its sustainable recycling due to the absence of scenario-specific risk assessment. Environmental risk assessment was carried out via a case study incorporating key factors to dominate human exposures during IBA utilization in land reclamation. Three research components echoing respective IBA leaching, exposures, and consequences were performed under a supportive framework to elaborate these interlinked key factors and unveil the potential environmental risks. IBA leachability was firstly investigated using various laboratory standard leaching methods while conducted a large-scale field trial experiment for mutual confirmation, suggesting that maximum leached amounts may be achieved when liquid to solid (L/S) ratio increases to 10. Dilution and transportation models were both developed to discriminate the mitigation of IBA leachate between two periods i.e. during and after land reclamation, suggesting that dilution rather than transportation may dominate the environmental impact for metal exposures. Metal bioaccumulation from a typical mollusk species was performed coupling the calculated dietary safety limits based on Singaporean diet intake for development of the threshold of toxicology concerns on human exposures. With such, IBA benign usage in land reclamation was also conferred in the form of distance and dilution factor.

Emerging trends in municipal solid waste incineration ashes research: a bibliometric analysis from 1994 to 2018

The rapidly increasing generation of municipal solid waste (MSW) threatens the environmental integrity and well-being of humans at a global level. Incineration is regarded as a technically sound technology for the management of MSW. However, the effective management of the municipal solid waste incineration (MSWI) ashes remains a challenge. This article presents the global dynamics of MSWI ashes research from 1994 to 2018 based on a bibliometric analysis of 1810 publications (research articles and conference proceedings) extracted from the Web of Science database, followed by a comprehensive summary on the research developments in the field. The results indicate the rapid growth of annual publications on MSWI ashes research, with China observed as the most productive country within the study period. Waste Management, Journal of Hazardous Materials, Chemosphere and Waste Management & Research, which accounted for 35.42% of documents on MSWI research, are the most prominent journals in the field. The most critical thematic areas on this topic are MSWI ashes characterisation, dioxin emissions from fly ash, valorisation of bottom ash and heavy metal removal. The evolution of MSWI ashes treatment technologies is also discussed, together with the challenges and future research directions. This is the first bibliometric analysis on global MSWI ashes research based on a sufficiently large dataset, which could provide new insights for researchers to initiate further research with leading institutions/authors and ultimately advance this research field.

Microwave irradiation assisted sodium hexametaphosphate modification on the alkali-activated blast furnace slag for enhancing immobilization of strontium

An inadvertent leakage of ⁹⁰Sr into the environment can induce an easy accumulation in biosphere and cause a continuous radiation to the surrounding ecosystem. In this study, sodium hexametaphosphate (Na₆O₁₈P₆) was employed to modify the blast furnace slags (BFS) to enhance the chemical stabilization of Sr²⁺ ions in the BFS-based cementitious materials. Microwave irradiation (MW) was used to further increase the binder activity of BFS samples and strengthened the mechanical strengths and durability of BFS-based blocks. A combination of experimental factors including the mass ratio of Na₆O₁₈P₆ to BFS-Sr_0.1 of 15%, the ratio of solid to liquid of 1:4 mg/L, the output power of 650 W, and the activation time of 3 min was most conductive to achieving an optimal microwave-irradiation process. Four extraction solutions were sorted by their leaching abilities following as MgSO₄ solution > H₂SO₄ solution > CH₃OOH solution > deionized (DI) water based on their leaching results. Compared with microwave irradiation, an addition of Na₆O₁₈P₆ to BFS samples obtained a better compressive strength for BFS-based blocks. However, a microwave-irradiation treatment was more effective in improving the resistances of blocks to gamma irradiation and thermal-thaw changes. Exposing to gamma irradiation over 6 months and enduring to thermal-thaw tests over 15 cycles, the microwave-treated blocks only lost 3.29% and 2.23% of leaching removal efficiencies in deionized water, respectively. Microwave irradiation increased the mechanical strengths of BFS-based blocks and inhibited leaching of Sr²⁺ ions from matrices mainly by strengthening hydration reactions and Sr²⁺ encapsulation.

Catalytic pyrolysis of swine manure using CO2 and steel slag

Pyrolysis of swine manure (SM) was conducted as a case study to establish an environmentally sound management of livestock manure. To build a more renewable pyrolysis platform for SM, this study selected carbon dioxide (CO₂) as the reaction medium. In addition, CO₂ was used in pyrolysis of SM to restrict the formation of toxic compounds, such as benzene derivatives and polycyclic aromatic hydrocarbons (PAHs). A series of thermo-gravimetric analysis (TGA) tests was done to understand the thermolysis of SM in the CO₂ environment. The TGA tests elucidated no occurrence of heterogeneous reactions between the SM sample and the CO₂. Moreover, the TGA tests of SM suggested that SM contains more volatile matter (VM) than lignocellulosic biomass. Non-catalytic transesterification of SM lipids confirmed that the dried SM sample contained 8.85 ± 0.05 wt% of lipids. This study also confirmed that the mechanistic role of CO₂ was realized through the gas phase reactions between volatile pyrolysates evolved from the thermolysis of SM and CO₂. In summary, CO₂ donates O, enhancing the generation of CO through homogeneous reactions. In parallel, this study confirmed that CO₂ suppress dehydrogenation. Therefore, the identified gas phase reactions between volatile pyrolysates and CO₂ led to the compositional modifications in the condensable pyrolysates. However, such mechanistic features arising from CO₂ only initiated at ≥520 °C. To expedite the reaction kinetics of the homogeneous reaction triggered by CO₂, steel slag (SS) was used as a catalyst. Hence, the reaction kinetics associated with the mechanistic role of CO₂ were substantially enhanced (up to 80%) when SS was used as a catalyst. Therefore, all experimental findings strongly suggest that CO₂ can be utilized as a raw material in a thermo-chemical process. More importantly, all observations suggest that CO₂ lopping can also be achieved in a thermo-chemical process. Lastly, this study shows that the high Cu content in SM was effectively immobilized through pyrolysis. Conclusively, this study experimentally proved that CO₂ could be promising for restricting the formation of toxic pollutant in the thermo-chemical treatment in that CO₂ offers an innovative and strategic means for controlling the ratio of C to H. Note that aromaticity and toxicity of chemical compounds are highly contingent on the ratio of C to H.

An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

Toxicity characteristic leaching procedure (TCLP) is a prevalent way to evaluate the treatment effectiveness for Cr(Ⅵ)-bearing solid matrices (CBSM). But when a certain amount of residual reductants are present in the treated CBSM, Cr(Ⅵ) leaching concentration rebound phenomenon (CLCRP) occurs, which invalidates the TCLP. This study explores the microstructure of ferrous-reduced CBSM and proves that the residual Cr(Ⅵ), Fe_xCr_1-x(OH)₃ precipitate and residual ferrous are separately distributed in a three-layer structure. In natural scenarios, the residual ferrous in the out-layer is firstly flushed away by rainfall and groundwater or oxidized by dissolved oxygen, resulting in the decrease of ferrous with time. Residual Cr(Ⅵ), due to being blocked by precipitate layer, is less flushed away. While in TCLP, all of released residual ferrous and Cr(Ⅵ) are in the leachate and react till one of them is almost exhausted, resulting in the underestimation of Cr(Ⅵ) leaching concentrations. The longer the samples experience the natural scenarios, the less of the residual ferrous, resulting in the decline of underestimation of Cr(Ⅵ) leaching concentrations with time. This study also provides a pretreatment which can effectively reduce the residual ferrous, achieving more accurate Cr(Ⅵ) leaching concentrations and eliminating CLCRP.

Immobilization of trace heavy metals in the electrokinetics-processed municipal solid waste incineration fly ashes and its characterizations and mechanisms

The mass production of municipal solid waste incineration (MSWI) fly ashes has caused severe concerns in regarding to the inherently ecological harm and the decreasing landfilling space. To relieve the heavy metal contamination into the environment and complete a closed-loop electrokinetics (EK), the sequential effects of electrokinetics and alkali activating solidification/stabilization (S/S) technique on the immobilization of HMs in the remnant of MSWI fly ashes were investigated in the research. The compressive strength for the remnant-based solidified product at the proposing time of 18 days was 2.58 MPa, 104.76% higher than that of the MWSI-based specimen with the optimal curing parameters being adopted for the production of the solidified matrices. The leaching concentrations of Zn, Pb, and Cu were 9.74, 0.21, and 0.75 mg/L, correspondingly reduced by 57.84%, 80.19%, and 17.58%, respectively. The opposite correlation between the compressive strength and the leaching concentration was demonstrated for both two groups of the curing samples. The mechanism results showed that the electrokinetics had significantly changed the micromorphology characteristics of MSWI fly ashes and distinctly affected the whole curing process. The cementation and geo-polymerization were comprehensively determined as two of main mechanisms for the immobilization of HMs in the curing matrices.

Characteristics of the cement-solidified municipal solid waste incineration fly ash

Cement solidification is an important pre-treatment technology for municipal solid waste incineration (MSWI) fly ash into landfill. The physicochemical properties and leaching characteristics are the foundation for assessing the long-term stability of the fly ash solidified with benchmark cement in landfills. The leaching performances of bulk components (Na, K, Ca, Cl, CO₃^2-, and SO₄^2-) and heavy metals (Cu, Zn, Cr, Pb, and Zn) were analyzed based on the percolation column test and pH dependent test respectively. The research showed that in the cement-solidified fly ash, Na and K were mainly in the form of soluble chloride salts and would be washed out severely at the initial leaching stage due to the weak fixation effect of cement. Moreover, a considerable amount of Ca was washed out simultaneously with Na and K, causing a temporary increase in pH value, and then Ca leaching was controlled by the solubility of minerals, mainly calcium carbonate, ettringite formed with CO₃^2- and SO₄^2-. Cement solidification reduced the cumulative release of mobile Cu, Zn, Cr, Pb, and Cd contained in MSWI fly ash. In the cement-solidified fly ash, the leaching of Cu and Zn was controlled by mineral solubility under alkaline conditions, Cr was dependent on the redox conditions, and Pb was related to the complex structures formed with Si-O bonds of silicates. A further research on the long-term stability of the cement-solidified fly ash in landfills was needed.

Co-disposal of MSWI fly ash and electrolytic manganese residue based on geopolymeric system

MSWI fly ash (MSWI FA) and electrolytic manganese residue (EMR) were successfully co-disposed by use of a geopolymeric system. Alkaline products of MSWI FA and NaOH were used to elicit chemical reactions to promote solidification. The best performing formulation of EMR-based geopolymer for immobilization of heavy metals was composed of 75 wt% MSWI FA and 25 wt% EMR with NaOH solution (7.5 M)/solid of 0.5. Solidification was most effective for the heavy metals: Pb > Cu > Cr > Zn > Mn, respectively. The EMR-based geopolymer had high structural stability likely due to the high ratio of SiO₂/Al₂O₃. The Solidification/Stabilization (S/S) mechanism for heavy metals of geopolymers is likely due to alkaline conditions and geopolymeric encapsulation, highlighting the utility and feasibility of this approach.

Destruction and formation of polychlorinated dibenzo-p-dioxins and dibenzofurans during pretreatment and co-processing of municipal solid waste incineration fly ash in a cement kiln

During a three-day industrial trial, municipal solid waste incineration fly ash (FA) was co-processed in a cement kiln after water-washing pretreatment for waste-to-resource conversion. All inputs and outputs were sampled to obtain the dioxin fingerprints. During washing, the relative contents of polychlorinated dibenzo-p-dioxins and dibenzofurans in FA, washed FA and sludge were basically the same and only a simple physical migration resulted. During drying, only physical processes resulted, which included volatilization and migration. Minimal dioxins residue remained in the clinker, cement kiln dust and flue gas, and the dioxins degraded completely. Through co-processing, the dioxins degraded obviously. The main compounds synthesized include 1,2,3,4,7,8-hepta-chlorodibenzo-p-dioxin, 2,3,7,8-tetra- chlorodibenzofuran and octa-chlorodibenzofuran. A comparison of dioxins fingerprints in the clinker, cement kiln dust and flue gas under baseline and co-processing conditions showed that co-processing had no effect on the cement kiln production. The baseline sample also contained a certain amount of dioxins, possibly because of the 'memory effect' and heterogeneous formations. The dioxins concentrations in the clinker and FA were far lower than the national standards. Thus, no environmental risk results during co-processing.

Characteristics and Treatment Methods of Medical Waste Incinerator Fly Ash: A Review

Medical waste incinerator fly ash (MWIFA) is quite different from municipal solid waste incinerator fly ash (MSWIFA) due to its special characteristics of high levels of chlorines, dioxins, carbon constituents, and heavy metals, which may cause irreversible harm to environment and human beings if managed improperly. However, treatment of MWIFA has rarely been specifically mentioned. In this review, various treatment techniques for MSWIFA, and their merits, demerits, applicability, and limitations for MWIFA are reviewed. Natural properties of MWIFA including the high contents of chlorine and carbonaceous matter that might affect the treatment effects of MWIFA are also depicted. Finally, several commendatory and feasible technologies such as roasting, residual carbon melting, the mechanochemical technique, flotation, and microwave treatment are recommended after an overall consideration of the special characteristics of MWIFA, balancing environmental, technological, economical information.

Leachability and Stability of Hexavalent-Chromium-Contaminated Soil Stabilized by Ferrous Sulfate and Calcium Polysulfide

Ferrous sulfate (FeSO4) and calcium polysulfide (CaS5) stabilization are practical approaches to stabilizing hexavalent chromium (Cr(VI))-contaminated soil. The leachability and stability of Cr(VI) and Cr are important factors affecting the effectiveness of stabilized Cr(VI)-contaminated soil. This study compared the leachability and stability of Cr(VI) and Cr in Cr(VI)-contaminated soil stabilized by using FeSO4 and CaS5. The contaminated soil was characterized before and after stabilization, and the effectiveness of FeSO4 and CaS5 stabilization was assessed using leaching, bioaccessibility, alkaline digestion, sequential extraction, and X-ray diffraction tests. Results showed that FeSO4 and CaS5 significantly reduced the leachability and Cr(VI) content in the contaminated soil. The acid-buffering capacity and stability (leachability, bioaccessibility, speciation distribution, and mineral composition) of the Cr(VI)/Cr and Cr(VI) content of CaS5 were better than those of FeSO4. This study demonstrated that CaS5 had a better effect than FeSO4 on the stabilization of Cr(VI) in Cr(VI)-contaminated soil. The CaS5 significantly enhanced the stabilization and immobilization of Cr(VI) and reduced its leachability and toxicity.

Green remediation of contaminated sediment by stabilization/solidification with industrial by-products and CO2 utilization

Navigational dredging is an excavation of marine/freshwater sediment to maintain channels of sufficient depth for shipping safety. Due to historical inputs of anthropogenic contaminants, sediments are often contaminated by metals/metalloids, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and other contaminants. Its disposal can present significant environmental and financial burdens. This study developed a novel and green remediation method for contaminated sediment using stabilization/solidification with calcium-rich/low-calcium industrial by-products and CO₂ utilization. The hydration products were evaluated by quantitative X-ray diffraction analysis and thermogravimetric analysis. The incorporation of calcium carbide residue (CCR) facilitated hydration reaction and provided relatively high 7-d strength. In contrast, the addition of Class-F pulverized fly ash (PFA) and ground granulated blast furnace slag (GGBS) was beneficial to the 28-d strength development due to supplementary pozzolanic and hydration reactions. The employment of 1-d CO₂ curing was found to promote strength development (98%) and carbon sequestration (4.3wt%), while additional 7-d air curing facilitated cement rehydration and further carbonation in the sediment blocks. The leachability tests indicated that all studied binders, especially CCR binder, effectively immobilized contaminants in the sediments. The calcium-rich CCR and GGBS were regarded as promising candidates for augmenting the efficacy of CO₂ curing, whereas GGBS samples could be applicable as eco-paving blocks in view of their superior 28-d strength. This study presents a new and sustainable way to transform contaminated sediment into value-added materials.

Binders alternative to Portland cement and waste management for sustainable construction - Part 2

The paper represents the "state of the art" on sustainability in construction materials. In Part 1 of the paper, issues related to production, microstructures, chemical nature, engineering properties, and durability of mixtures based on binders alternative to Portland cement were presented. This second part of the paper concerns the use of traditional and innovative Portland-free lime-based mortars in the conservation of cultural heritage, and the recycling and management of wastes to reduce consumption of natural resources in the production of construction materials. The latter is one of the main concerns in terms of sustainability since nowadays more than 75% of wastes are disposed of in landfills.

Physicochemical properties of stabilized sewage sludge admixtures by modified steel slag

Cement stabilization is used extensively to solidify sludge. However, previous studies suggest that the organic acids in sludge can restrain the hydration processes of cementitious materials. This study presents a new binder (steel slag based materials) whose effectiveness of stabilization was investigated on sewage sludge. Several series of tests for solidified sludge were conducted including the moisture content, compressive strength, shear force, permeability, and toxicity characteristic leaching (TCLP). The result shows that the stabilization effect of physicochemical properties of stabilized sewage by S1* (content 90% steel slag) were better than that of cement. With 20% S1, the moisture content of the sludge could be reduced to 30% at 20+1°C for 7 days; The compressive strength was 74.5 kappa and the permeability coefficient was 5.03 × 10–5 (cm/s); The TCLP was below the limit value. The hydration products of paste and solidified sludge at 1, 3, 14, 28, and 90 days were analyzed by XRD, SEM, and energy spectroscopy to distinguish ettringite/iron ettringite in S1 solidified sludge. the result shows the Needle-like ettringite of S1 possess high iron content and may be Fe–Aft.