Solidification/stabilization and optimization of municipal solid waste incineration fly ash with aluminosilicate solid wastes.

Alkali-activation is an effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. However, the characteristics of calcium-rich silica-poor aluminum phase in MSWIFA easily cause the structural instability and contamination of alkali activated MSWIFA S/S bodies. Therefore, the aluminosilicate solid wastes are used in this work to optimize the immobilization and structural properties. Results showed that incorporation of aluminosilicate solid wastes significantly improved the compressive strength and heavy metals pollution toxicity of MSWIFA S/S bodies. Compared to alkali activated MSWIFA, the compressive strength of S/S bodies with addition of coal fly ash, silica fume and granulated blast furnace slag improved by 31.0%, 47.6% and 50.8% when the curing time was 28 days, respectively. Leachability of Pb, Zn and Cd in these alkali activated MSWIFA S/S bodies was far below the threshold value specified in Standard GB16889. Aluminosilicate solid wastes provided abundant Si/Al structural units, and some new phases such as ettringite(AFt, 3CaO⋅Al2O3⋅3CaSO4⋅32H2O), calcium sulfoaluminate hydrate (3CaO⋅Al2O3⋅CaSO4⋅12H2O) and Friedel's salt (CaO⋅Al2O3⋅CaCl2⋅10H2O) can be detected in S/S matrix with aluminosilicate solid wastes, along comes increased the amount of the amorphous phases. Lower Ca/Si molar ratio tended to form the network structure gel similar to tobermorite with higher polymerization degree. Meanwhile, the silica tetrahedron of the gels changed from the oligomerization state like island to the hyperomerization state like chain, layer network or three-dimensional structure, and average molecular chain length increased. These findings provide theoretical basis for structural properties optimization and resource utilization of MSWIFA S/S matrices.

Analysis of composition characteristics and treatment techniques of municipal solid waste incineration fly ash in China.

The rapid development of the economy and society is causing an increase in the amount of municipal solid waste (MSW) produced by people's daily lives. With the strong support of the Chinese government, incineration power generation has steadily become the primary method of treating MSW, accounting for 79.86%. However, burning produces a significant amount of municipal solid waste incineration fly ash (MSWI-FA), which contains heavy metals, soluble chlorine salts, and dioxins. China's MSWI-FA yield increased by 8.23% annually to 7.80 million tons in 2022. Besides, the eastern region, especially the southeastern coastal region, has the highest yield of MSWI-FA. There are certain similarities in the chemical characteristics of MSWI-FA samples from Northeast, North, East, and South China. Zn and CaO have the largest amounts of metals and oxides, respectively. The Cl content is about 20 wt%. This study provides an overview of the techniques used in the thermal treatment method, solidification and stabilization, and separation and extraction of MSWI-FA and compares their benefits and drawbacks. In addition, the industrial applications and standard requirements of landfill treatment and resource utilization of MSWI-FA in China are analyzed. It is discovered that China's resource utilization of MSWI-FA is insufficient through the study on the fly ash disposal procedures at a few MSW incineration facilities located in the economically developed Guangdong Province and the traditional industrial city of Tianjin. Finally, the prospects for the disposal of MSWI-FA were discussed.

Valorization of steelmaking slag and coal fly ash as amendments in combination with Betula pubescens for the remediation of a highly As- and Hg-polluted mining soil.

Soil pollution by As and Hg is a pressing environmental issue given their persistence. The intricate removal processes and subsequent accumulation of these elements in soil adversely impact plant growth and pose risks to other organisms in the food chain and to underground aquifers. Here we assessed the effectiveness of non-toxic industrial byproducts, namely coal fly ash and steelmaking slag, as soil amendments, both independently and in conjunction with an organic fertilizer. This approach was coupled with a phytoremediation technique involving Betula pubescens to tackle soil highly contaminated. Greenhouse experiments were conducted to evaluate amendments' impact on the growth, physiology, and biochemistry of the plant. Additionally, a permeable barrier made of byproducts was placed beneath the soil to treat leachates. The application of the byproducts reduced pollutant availability, the production of contaminated leachates, and pollutant accumulation in plants, thereby promoting plant development and survival. Conversely, the addition of the fertilizer alone led to an increase in As accumulation in plants and induced the production of antioxidant compounds such as carotenoids and free proline. Notably, all amendments led to increased thiolic compound production without affecting chlorophyll synthesis. While fertilizer application significantly decreased parameters associated with oxidative stress, such as hydrogen peroxide and malondialdehyde, no substantial reduction was observed after byproduct application. Thermal desorption analysis of the byproducts revealed Hg immobilization mechanisms, thereby indicating retention of this metalloid in the form of Hg chloride. In summary, the revalorization of industrial byproducts in the context of the circular economy holds promise for effectively immobilizing metal(loid)s in heavily polluted soils. Additionally, this approach can be enhanced through synergies with phytoremediation.

Leachability of hexavalent chromium from fly ash-marl mixtures in Sarigiol basin, Western Macedonia, Greece: environmental hazard and potential human health risk.

Hexavalent chromium (Cr (VI)) is an environmental contaminant brining high concerns due to its higher toxicity and mobility in comparison with trivalent chromium Cr(III). Cr (VI) has been linked with several adverse health effects, including respiratory diseases, lung cancer, and skin irritation. The primary sources of it in the environment are industrial activities.Most of the time, fly ash made of lignite can release Cr(VI) when it comes into contact with water in an aquatic environment. The objective of this study is the investigation of Cr (VI) concentration in leachates of fly ash and marl mixtures and the determination of its solubility under different pH conditions. Samples of fly ash were collected from the Power Plant of Agios Dimitrios. Additionally, samples of marl were collected from the mine of South Field, and both samples were mixed and prepared in in different proportions (% w.t.). The leaching experiments were carried out according to the EN-12457/1-4 (2003) standard under different pH conditions and chemical analysis of the leachates were performed by spectrophotometry with diphenylcarbazide (DPC). The environmental footprint of Cr (VI) in the study area was significant, especially in mixtures containing higher concentrations of fly ash. A critical pH range between 6 to 12 is observed. At acidic pH values, a high release of Cr (VI) was observed, while at the mentioned critical values (pH 10-12), a gradual decrease in its leachability was noticed. The high concentrations of Cr (VI) in the industrial area studied require immediate actions in terms of managing and limiting the potential hazardous impacts on the environment and by extension on the public health by developing appropriate prevention strategies.

Neighborhood-scale lead (Pb) speciation in Akron, Ohio (USA) soils: primary sources, post-deposition diagenesis, and high concentrations of labile Pb.

Lead (Pb) poses a significant risk to infants and children through exposure to contaminated soil and dust. However, there is a lack of information on Pb speciation and distribution at the neighborhood-scale. This work aimed to determine: (1) the distribution of acid-extractable (labile) Pb and other metals ([M]AE) in two neighborhoods in Akron, Ohio (USA) (Summit Lake and West Akron; n = 82 samples); and (2) Pb speciation and potential sources. Total metal concentration ([M]T) and [M]AE was strongly correlated for Pb and Zn (R2 of 0.66 and 0.55, respectively), corresponding to 35% and 33% acid-extractability. Lead and Zn exhibited a strong positive correlation with each other (R2 = 0.56 for MT and 0.68 for MAE). Three types of Pb-bearing phases were observed by electron microscopy: (1) galena (PbS)-like (5-10 µm); (2) paint chip residuals (10-20 µm); and (3) Pb-bearing Fe-oxides (20 µm). Isotope ratio values for PbAE were 1.159 to 1.245 for 206Pb/207Pb, and 1.999 to 2.098 for 208Pb/206Pb, and there was a statistically significant difference between the two neighborhoods (p = 0.010 for 206Pb/207Pb and p = 0.009 for 208Pb/206Pb). Paint and petrol are the dominant sources of Pb, with some from coal and fly ash. Lead speciation and distribution is variable and reflects a complex relationship between the input of primary sources and post-deposition transformations. This work highlights the importance of community science collaborations to expand the reach of soil sampling and establish areas most at risk based on neighborhood-dependent Pb speciation and distribution for targeted remediation.

Novel method for recovering valuable metals from Sn ash: Vacuum carbothermal reduction-directional condensation.

Sn ash recycling is an industry with positive development prospects, as it provides better-protected resources, promotes sustainable development, and lays a solid foundation for future development. In this study, an innovative vacuum carbothermal reduction-directional condensation process was developed. The thermodynamic analysis results indicated that the initial reaction pressure and temperature for the carbothermal reduction of the system was 1-10 Pa and 998-1063 K, respectively. The saturation vapor pressure, separation coefficient, and condensation temperature of Sn, Pb, and Zn in the reduced products differed significantly, and their separation could be achieved by controlling the volatilization and condensation temperatures. A single-factor experiment investigated the effects of carbon ratio, temperature, and time on the reduction efficiency, direct yield, and recovery rate. The optimal experimental conditions were the ratio of MeO to C of 4:1, temperature of 1373 K, and time of 120 min. Sn, Pb, and Zn products were obtained at different positions. This process shortens the traditional process, reduces the reduction cost of Sn, and enables the implementation of the process, making it environmentally friendly.

Periodic DFT calculations for the heterogeneous formation of 2-chlorothiophenoxy radical from 2-chlorothiophenol on Cu(111) surface in fly ash.

Copper plays a crucial role in the heterogenous dissociation of chlorothiophenols (CTPs) to form chlorothiophenoxy radicals (CTPRs), which is the initial and critical step in the formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs). Here, first-principles calculations were performed to investigate the activity of Cu(111) surface towards the formation of adsorbed 2-CTPR from 2-CTP. The interaction between 2-CTP and Cu(111) surface was explored to find stable adsorption configurations. Besides, the decomposition routes of 2-CTP on the Cu(111) surface were further explored. Moreover, the effects of water on the formation of absorbed 2-CTPR on the Cu(111) surface were examined. Our results demonstrate that the flat adsorption of 2-CTP on the surface with adsorption energy in the range of -33.21 kcal/mol to -28.37 kcal/mol is more stable than the vertical adsorption with adsorption energy ranging from -23.53 kcal/mol to -13.38 kcal/mol. The Cu(111) surface catalyzes the conversion of 2-CTP into the adsorbed 2-CTPR with a modest energy barrier of 9.46 kcal/mol. Furthermore, water molecules exhibit stronger catalytic activity in this process with a decreased energy barrier of 5.87 kcal/mol through "water bridge" and hydrogen bonding. Specifically, the water accepts the hydrogen atom from 2-CTP and donates another hydrogen to the surface via "water bridge". This research provides a molecular-level understanding of the heterogeneous formation of PCTA/DTs by fly ash, suggesting novel approaches for control strategy and legislation of dioxin analogues.

Insights into Full-congener Profiles of Chlorinated Benzenes in Fly and Bottom Ash: Case Study in Vietnamese Industrial and Municipal Waste Incinerators.

Chlorinated benzenes (CBzs) are a group of organic pollutants, which have been industrially or unintentionally produced through various chemical and thermal processes. Studies on full congener profiles of CBzs in waste and environmental samples are relatively limited and not updated. In the present study, concentrations of 12 CBzs were determined in fly ash (FA) and bottom ash (BA) samples collected from one municipal waste incinerator (MWI) and one industrial waste incinerator (IWI) in northern Vietnam. Levels of Σ12CBzs were higher in bottom ash (median 25.3; range 1.59-45.7 ng/g) than in fly ash (median 7.30; range 1.04-30.0 ng/g). The CBz profiles were dominated by di- and tri-chlorinated congeners with the major congeners as 1,2,4-TCB, 1,2,3-TCB, 1,2-DCB, and 1,3-DCB. However, CBz profiles varied greatly between sample types and incinerators, implying differences in input materials, formation pathways, and pollutant behaviors. Incomplete combustion is possibly responsible for high levels of CBzs in industrial bottom ash. The emission factors of Σ12CBzs ranged from 21 to 600 µg/ton for fly ash and from 190 to 4570 µg/ton for bottom ash, resulting in annual emissions of about 6 and 3 g/year for the IWI and MWI, respectively. Our results suggest additional investigations on industrial emission and environmental occurrence of all 12 CBzs rather than solely focusing on regulated congeners like hexachlorobenzene and pentachlorobenzene.

Formation of Pre-PCTA/DT Intermediates from 2-Chlorothiophenol on Silica Clusters: A Quantum Mechanical Study.

Silica (SiO2), accounting for the main component of fly ash, plays a vital role in the heterogeneous formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) in high-temperature industrial processes. Silica clusters, as the basic units of silica, provide reasonable models to understand the general trends of complex surface reactions. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. By employing density functional theory, this study examined the formation of 2-chlorothiophenolate from 2-CTP adsorbed on the dehydrated silica cluster ((SiO2)3) and the hydroxylated silica cluster ((SiO2)3O2H4). Additionally, this study investigated the formation of pre-PCTA/DTs, the crucial intermediates involved in PCTA/DT formation, from the coupling of two adsorbed 2-chlorothiophenolates via the Langmuir-Hinshelwood (L-H) mechanism and the coupling of adsorbed 2-chlorothiophenolate with gas-phase 2-CTP via the Eley-Rideal (E-R) mechanism on silica clusters. Moreover, the rate constants for the main elementary steps were calculated over the temperature range of 600-1200 K. Our study demonstrates that the 2-CTP is more likely to adsorb on the termination of the dehydrated silica cluster, which exhibits more effective catalysis in the formation of 2-chlorothiophenolate compared with the hydroxylated silica cluster. Moreover, the E-R mechanism mainly contributes to the formation of pre-PCTAs, whereas the L-H mechanism is prone to the formation of pre-PCDTs on dehydrated and hydroxylated silica clusters. Silica can act as a relatively mild catalyst in facilitating the heterogeneous formation of pre-PCTA/DTs from 2-CTP. This research provides new insights into the surface-mediated generation of PCTA/DTs, further providing theoretical foundations to reduce dioxin emission and establish dioxin control strategies.

Dysfunction of Nrf2-regulated cellular defence system and JNK activation induced by high dose of fly Ash particles are associated with pulmonary injury in mouse lungs.

The mechanisms of the exposure to fine particulate matter (PM) as a risk factor for pulmonary injury are not fully understood. The transcription factor, NF-E2-related factor 2 (Nrf2), plays a key role in protection lung against PM insult and cancer chemoprevention. In this study, F3-S fly ash particles from a municipal waste incinerator were evaluated as a PM model. We found that F3-S triggered hierarchical oxidative stress responses involving the prolonged activation of the cytoprotective Nrf2 transcriptional program via Keap1 Cys151 modification, and c-Jun NH2-terminal kinase (JNK) phosphorylation at higher doses. In mouse lungs exposed to fly ash particles at a low dose (10-20 mg/kg), Nrf2 signalling was upregulated, while in those exposed to a high fly ash particle dose (40 mg/kg), there was significant activation of JNK, and this correlated with Nrf2 phosphorylation and the downregulation of antioxidant response element (ARE)-driven genes. The JNK inhibitor, SP600125, reversed Nrf2 phosphorylation, and downregulation of detoxifying enzymes. Silencing JNK expression in mouse lungs using adenoviral shRNA inhibited JNK activation and Nrf2 phosphorylation, promoted ARE-driven gene expression, and reduced pulmonary injury. Furthermore, we found that the 452-515 amino acid region within the Neh1 domain of Nrf2 was required for its interaction with P-JNK. We demonstrated that Nrf2 was an important P-JNK target in fly ash-induced pulmonary toxicity. JNK phosphorylated Nrf2, leading to a dysfunction of the Nrf2-mediated defence system.

Preparation of phosphorus containing products by co-incineration of sludge ash and calcium-based additives: Focusing one-step and multi-step method.

Due to the irreplaceable nature of phosphorus (P) in biological growth and the shortage of P rock, it is necessary to recover P from waste, such as sludge ash. P-containing products were prepared using sludge ash and calcium-based additives (CaCO3 and eggshell). In addition, the effects of different incineration methods (one-step method (OSM) and multi-step method (MSM)), additive doses, and incineration temperature on the P content and species in the products were investigated. The results indicated that as the dose of calcium-based additives increased, total P (TP) content in P-containing products reduced, apatite P (AP) content increased, non-apatite P (NAIP) content declined, and P solubility in citric acid content decreased. The amount of AP increased, NAIP reduced, and P solubility in citric acid decreased as the incineration temperature climbed. Although P in P-containing products prepared by OSM and MSM changed in a similar way at different additive doses and temperatures, P-containing products prepared by MSM had at least a 6.1% increase in P solubility in citric acid. Compared with OSM, MSM could save 10% of calcium-based additives when reaching the maximum AP value. Additionally, pure materials were employed to investigate how P species changed during the incineration procedure. The advantage of the MSM-prepared product over the OSM-prepared product may be explained by the high concentration of Ca3(PO4)2 and low concentration of amorphous calcium bound P (Ca-P). Overall, MSM is an effective method to reduce the dose of calcium-based additives and increase the bioavailability of P in P-containing products.

Medical waste incineration fly ash-based magnesium potassium phosphate cement: Calcium-reinforced chlorine solidification/stabilization mechanism and optimized carbon reduction process strategy.

The traditional solidification/stabilization (S/S) technology, Ordinary Portland Cement (OPC), has been widely criticized due to its poor resistance to chloride and significant carbon emissions. Herein, a S/S strategy based on magnesium potassium phosphate cement (MKPC) was developed for the medical waste incineration fly ash (MFA) disposal, which harmonized the chlorine stabilization rate and potential carbon emissions. The in-situ XRD results indicated that the Cl- was efficiently immobilized in the MKPC system with coexisting Ca2+ by the formation of stable Ca5(PO4)3Cl through direct precipitation or intermediate transformation (the Cl- immobilization rate was up to 77.29%). Additionally, the MFA-based MKPC also demonstrated a compressive strength of up to 39.6 MPa, along with an immobilization rate exceeding 90% for heavy metals. Notably, despite the deterioration of the aforementioned S/S performances with increasing MFA incorporation, the potential carbon emissions associated with the entire S/S process were significantly reduced. According to the Life Cycle Assessment, the potential carbon emissions decreased to 8.35 × 102 kg CO2-eq when the MFA reached the blending equilibrium point (17.68 wt.%), while the Cl- immobilization rate still remained above 65%, achieving an acceptable equilibrium. This work proposes a low-carbon preparation strategy for MKPC that realizes chlorine stabilization, which is instructive for the design of S/S materials.

Short-term residual characteristics of ambient-cured green geopolymer mortar exposed to elevated temperatures.

Every structure might be exposed to fire at some point in its lifecycle. The ability of geopolymer composites to withstand the effects of fire damage early before it is put out is of great importance. This study examined the effects of fire on geopolymer composite samples made with high-calcium fly ash and alkaline solution synthesised from waste banana peduncle and silica fume. A ratio of 0.30, 0.35, and 0.4 was used in the study for the alkaline solution to fly ash. Also used were ratios of 0.5, 0.75, and 1 for silica oxide (silica fume) to potassium hydroxide ratio. The strength loss, residual compressive strength, percentage strength loss, relative residual compressive strength, ultrasonic pulse velocity, and microstructural properties of the thirteen mortar mixes were measured after exposure to temperatures of 200, 400, 600, and 800 °C for 1 h, respectively. The results reveal that geopolymer samples exposed to elevated temperatures showed great dimensional stability with no visible surface cracks. There was a colour transition from dark grey to whitish brown for the green geopolymer mortar and brown to whitish brown for the control sample. As the temperature rose, weight loss became more pronounced, with 800 °C producing the most significant weight reduction. The optimum mixes had a residual compressive strength of 25.02 MPa after being exposed to 200 °C, 18.72 MPa after being exposed to 400 °C, 14.04 MPa after being exposed to 600 °C, and 7.41 MPa after being exposed to 800 °C. The control had a residual compressive strength of 8.45 MPa after being exposed to 200 °C, 6.67 MPa after being exposed to 400 °C, 3.16 MPa after being exposed to 600 °C, and 2.23 MPa after being exposed to 800 °C. The relative residual compressive strength decreases for green geopolymer mortar are most significant at 600 and 800 °C, with an average decrease of 0.47 and 0.30, respectively. The microstructure of the samples revealed various phase changes and new product formations as the temperature increased.

Experimental study on municipal solid waste incineration bottom ash as a component of alkali-activated coal gangue-based geopolymer.

This study explores the potential of municipal solid waste incineration bottom ash (MSWI BA) and coal gangue as precursors for alkali-activated cementitious materials (CG-MBA). An examination of the impact of MSWI BA content, NaOH/Na2SiO3 ratio, liquid-solid ratio, and NaOH concentration on strength and reaction through the application of diverse analytical methodologies. Results demonstrate that CG-MBA offers significant environmental benefits compared to conventional cement. When used as a construction filling material, CG-MBA exhibits a remarkable 74.5 ~ 79.2 wt% reduction in carbon dioxide emissions and 70.6 ~ 77.0 wt% reduction in energy consumption. Additionally, CG-MBA effectively immobilizes heavy metal ions in MSWI BA, with a fixation efficiency exceeding 56.0%. These findings suggest that CG-MBA is a promising sustainable solution for waste management, offering significant environmental benefits while demonstrating effective heavy metal immobilization. This approach contributes to pollution control and promotes environmental sustainability in the construction industry.

Characterization of novel polysulfide polymer coated fly ash and its application in mitigating diffusion of contaminants.

Fly ash consists of a considerable amount of hazardous elements with high mobility, posing substantial environmental risks during storage in surface impoundments and landfills. This hinders its efficient reuse in construction or material industries. To enhance the versatility of fly ash applications, a novel surface modification technique, termed SuMo, has been developed to create a hydrophobic polysulfide polymer coating on the surface of fly ash particles. The physicochemical properties of SuMo fly ash samples were examined using atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM), thermal gravimetric analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR), and leaching of hazardous elements was tested under practical environmental conditions (pH 4-12) based on the EPA's leaching environmental assessment framework (LEAF). The successful coating of polysulfide polymer on fly ash surface was verified through an increased percentage of C, S, and O in elemental mapping, coupled with the identification of S-O, CO, and C-H functional groups consistent with the chemical structure of polysulfide polymer. While the SuMo fly ash particles maintained their spherical shape, they exhibited increased surface roughness, robust hydrophobicity, and thermal stability up to 250 °C. Notably, owing to the coating's resilience against water leaching, the SuMo fly ash demonstrated a substantial reduction (up to 60-fold) in leachate concentrations of multiple concerning elements, including B, Be, Ba, Mn, Zn, As, Cr, Hg, etc., under various pH conditions compared to the uncoated fly ash. Furthermore, the polysulphide polymer coating effectively prevented Hg volatilization from fly ash below 163 °C. This study highlights the efficacy of the developed polysulfide polymer coating in mitigating the diffusion of hazardous elements from fly ash, thereby enhancing its potential reutilization in material, construction, and agriculture industries.

Characterization of lightweight aerated mortars using waste-to-energy bottom ash (WtE-BA) as aerating agent.

The management of Waste-to-Energy Bottom Ash (WtE-BA), generated during the incineration of waste, poses a global challenge. Presently, the majority of WtE-BA is disposed of in landfills due to the lack of alternatives. Meanwhile, the construction industry remains the primary consumer of raw materials and significantly contributes to Greenhouse Gas Emissions. This study attempts to address these issues by utilizing the fine fraction of WtE-BA (<2 mm) as a raw material for aerated mortar production. Thanks to its metallic aluminum content, WtE-BA is utilized as an aerating agent. The study investigates how the quantities of water and WtE-BA, as well as its granulometric sub-fractions, impact the properties of the final product. An analysis of properties such as density, compressive strength, and thermal conductivity was conducted. Additionally, the environmental impact of each raw material (i.e. WtE-BA, cement and sand) was assessed through leaching tests and elemental content analysis enabling the determination of their individual contribution to the presence of trace elements in the produced mortars. The aforementioned properties are discussed using microstructure and porosity analyses. The findings demonstrate that the quantity of water is a crucial factor in controlling the aeration of mortars, whereas the granulometry of the WtE-BA particles did not significantly affect their macro-properties. Furthermore, this study highlights that WtE-BA based mortars has the potential to exhibit better environmental and insulating performances than standard aerated mortar of equal density and strength. The differences in pore size and type between WtE-BA and aerated mortars can account for the variation in performance. Thus, WtE-BA proves to be an effective substitute for aerating agent in the production of aerated mortars.

Environmental performance of unfired bricks produced from co-disposal of mine tailings and municipal solid waste incineration fly ash based on comprehensive leaching tests.

The potential leaching of heavy metals is a crucial concern for construction materials produced from solidification/stabilization (S/S) treatment of wastes. This study comprehensively evaluated the leaching characteristics of heavy metals from the unfired bricks produced from co-disposal of Pb-Zn mine tailings and municipal solid waste incineration fly ash using batch, sequential, and semi-dynamic leaching tests. The results show that S/S treatment drastically reduced the leachability of heavy metals from the unfired bricks through lowering their distribution in the acid-soluble fraction. The effective diffusion coefficients of heavy metals within unfired bricks were all below 1.55 × 10-13 cm2/s, which is indicative of low mobility in the environment. The release of heavy metals from the unfired bricks was primarily governed by diffusion and dissolution. Slaking treatment of fly ash significantly reduced the leaching of heavy metals from the unfired bricks due to their improved structural integrity and compactness, which minimizes the surface area in the solid matrix accessible by the leaching medium. The leachability indices of heavy metals within the unfired bricks ranged from 13.12 to 18.10, suggesting that they are suitable for "controlled utilization" in specific scenarios. Compared to untreated mine tailings, converting them into unfired bricks could reduce the releases of heavy metals by several to hundreds of folds. These findings demonstrate that S/S can be an effective and sustainable strategy for co-disposal of mining tailings and incineration fly ash to produce construction materials with sound long-term environmental performance.

Systematic review of alternative materials that improve retention of potentially toxic metals in soil/clay liners in waste disposal areas.

When soils available for the construction of liners do not display the characteristics necessary for a good performance, mixtures with other materials can be employed for achieving the desired quality. Several researchers have addressed those mixtures from either a geotechnical or a gas diffusion perspective, emphasizing low hydraulic conductivity. However, in recent years, growing attention has been drawn to the ability of liners to mitigate contamination. The literature lacks studies on the use of amendments for soil liners or cover systems to retain potentially toxic metals, which are important inorganic contaminants. This paper provides a systematic review of the literature considering publications available on Web of Science and SpringerLink databases between January 1st, 2012, and December 5th, 2022. The aim of the review was to identify the types of soils and amendments studied as liners or cover systems for such retention of potentially toxic metals, the methodologies of application of the alternative materials in the soils, and the research gaps and perspectives in the field. Seventeen papers that addressed 31 materials as amendments were retrieved. The most studied amendment was coal fly ash, and 17 amendments were residues or by-products, which indicates concerns over waste destination and sustainability. Among the potentially toxic metals analyzed are Pb, Cu, and Cd. Gaps such as lack of pilot, field-scale, and long-term studies, as well as perspectives for future research (e.g., different liner configurations, concomitant mixtures of two or more materials in the soil, and focus on the sustainability of amendments), were identified.

Effect of long-term dry-wet circulations on the Solidification/stabilization of Municipal solid waste incineration fly ash using a novel cementitious material.

Solidification/stabilization (S/S) is a typical technique to immobilize toxic heavy metals in Municipal solid waste incineration fly ash (MSWI FA). This study utilized blast furnace slag, steel slag, desulfurization gypsum, and phosphoric acid sludge to develop a novel metallurgical slag based cementing material (MSCM). Its S/S effects of MSWI FA and long-term S/S effectiveness under dry-wet circulations (DWC) were evaluated and compared with ordinary Portland cement (OPC). The MSCM-FA block with 25 wt.% MSCM content achieved 28-day compressive strength of 9.38 MPa, indicating its high hydration reactivity. The leaching concentrations of Pb, Zn and Cd were just 51.4, 1895.8 and 36.1 µg/L, respectively, well below the limit standard of Municipal solid wastes in China (GB 16889-2008). After 30 times' DWC, leaching concentrations of Pb, Zn and Cd for MSCM-FA blocks increased up to 130.7, 9107.4 and 156.8 µg/L, respectively, but considerably lower than those for OPC-FA blocks (689, 11,870.6 and 185.2 µg/L, respectively). The XRD and chemical speciation analysis revealed the desorption of Pb, Zn and Cd attached to surface of C-S-H crystalline structure during the DWC. The XPS and SEM-EDS analysis confirmed the formation of Pb-O-Si and Zn-O-Si bonds via isomorphous replacement of C-A-S-H in binder-FA blocks. Ettringite crystalline structure in OPC-FA block was severely destructed during the DWC, resulting in the reduced contents of PbSO4 and CaZn2Si2O7·H2O and the higher leachability of Pb2+ and Zn2+.

Preparation of functional geopolymers from municipal solid waste incineration fly ash: An approach combining experimental and computational simulation studies.

This study aims to evaluate the feasibility and safety of using municipal solid waste incineration fly ash (MSW-IFA) in the development of geopolymer-based solidification/stabilization (S/S) treatments. Geopolymers have garnered attention as a sustainable alternative to traditional cement, owing to their high strength, stability, and minimal CO2 emissions. In this study, a combination of experimental and simulation calculations was used to investigate the setting time, mechanical properties, environmental risks, hydration mechanisms and processes of municipal solid waste incineration fly ash-based polymeric functional cementitious materials (GFCM). The results demonstrate that the mechanical properties of GFCM are related to the changes in the mineral phases and the degree of compactness. Quantum chemical calculations indicate that the hydration products may be [Si(OH)4], [Al(OH)3(OH2)] and [Al(OH)4]-. It is possible that the heavy metals are embedded in the hydrated silica-aluminate by electrostatic interaction or chemisorption. Heavy metals may be embedded in hydrated silica-aluminate by electrostatic action or chemisorption. This study provides a feasible method for resource utilization and heavy metal stabilization mechanism of MSW-IFA.