Heavy Metal Leaching Behavior Research Articles

Unlocking the potential: Transforming hazardous waste into solid cementitious materials

Incineration fly ash and mineral slag represent hazardous and industrial solid wastes necessitating immediate remediation in China. The liberation and stabilization of heavy metals have erected formidable barriers to their reclamation and repurposing. This study presents a pivotal technology for creating all-solid waste cementitious materials, with the optimal parameters consisting of a dosage of 55.773% incineration fly ash, 38.822% cementitious materials dosage, and 5.405% adhesive powders dosage. Furthermore, a comprehensive evaluation model has been developed to assess the performance of these materials. Several extraction procedure experiments were conducted to investigate the leaching behavior of heavy metals under various leaching scenarios, which suggests the diffusion coefficients of heavy metals under the optimal ratio are all less than 3 × 10–13 m2/s, affirming the effective stabilization and the absence of potential migration risks. The primary mechanisms of solidification involved include ion exchange reactions for charge balance and the encapsulation of precipitates through cementation, both of which contribute to the solidification of heavy metals. The findings will illuminate advanced strategies for reducing waste, enhancing resource efficiency, and promoting environmentally friendly building practices, as well as provide a promising avenue for achieving sustainability goals in construction projects, leading to reduced environmental impact and improved long-term sustainability. The integration of ACM-SW can pave the way for transformative advancements in sustainable building practices, ultimately leading to a more environmentally friendly and resilient built environment.

Converting municipal solid waste incineration bottom ash into the value-added artificial lightweight aggregates through cold-bonded granulation technology

To effective recycle the Municipal solid waste incineration bottom ash (MSWIBA), this study adopts cold consolidation granulation technology to develop eco-friendly artificial lightweight aggregates (ALCBAs), and the materials are over 50 % dosage of MSWIBA, less 50 % fly ash (FA) or ground granulated blast furnace slag (GGBFS), and 10 % ordinary Portland cement (OPC). This study investigated the effects of different binder types and dosages on the properties of ALCBAs, including compressive strength, water absorption rate, bulk density, microstructures, pore development, heavy metal leaching behavior, and sustainability. The feasibility of ALCBAs to design green concrete was also explored. The results show that with higher content of MSWIBA and binders of FA, GGBFS and OPC, the ALCBAs with compressive strength greater than 2.5 MPa, bulk density of about 1000 kg/m3 and water absorption rate greater than 12 % was successfully developed. Increasing the content of FA/GGBFS is beneficial to improve the compressive strength and reduce the water absorption rate and the total porosity and lower the air bubbles percentage of ALCBAs. More importantly, both FA and GGBFS are mixed into ALCBAs, the compressive strength is at 2.5 MPa-3.0 MPa, the water absorption rate is significantly reduced, and the energy consumption, carbon footprint and cost are also significantly reduced. As the content of ALCBAs increases, although the compressive strength and splitting tensile strength are not as good as concrete with natural aggregates, the density of low-carbon concrete with ALCBAs gradually decreases, and the interface transition zone is gradually improved. Overall, ALCBAs can be beneficial to the conservation of natural resources (natural aggregates) and effectively solve the shortage of natural aggregates, and can be used to deal with the accumulation of solid waste in a green way, and it also can open up new channel for solid waste recycling.

The long-term leaching behavior of heavy metals in geopolymers and Portland cement bricks containing MSWI fly ash: A comparative study

Geopolymers was considered as an ideal alternative to cement solidification/stabilization for municipal solid waste incineration fly ash (MSWI FA). However, the long-term leaching behavior of heavy metals in geopolymers had attracted less attention. In this study, the long-term leaching behavior of heavy metals in geopolymers containing MSWI FA was investigated and compared with Portland cement bricks under various corrosion scenarios. The findings revealed a consistent trend of "slowly increasing - slowly decreasing - gradually stabilizing" in the pH changes in the effluent of all samples and the absence of corrosive risk in the utilization of geopolymers. The maximum pH values of the effluent were G2 (12.00) > G1 (11.90) > G3 (10.83) > C1 (9.67), suggesting that geopolymers were not corrosive in resource utilization. The long-term leaching behavior of heavy metals under different corrosion conditions could be divided into two stages, which included quick release in stage Ⅰ and gradual stabilization in stage Ⅱ. Compared with Portland cement bricks, the cumulative leaching concentration of heavy metals in geopolymers was lower, indicating less environmental risk. Due to the strongest corrosiveness from simulated acid rain (SAR), C2 and G2 exhibited significant compressive strength loss rates of 20.2% and 9.25%, respectively. In the leaching of CO2-saturated water (CSW), the leaching behavior of heavy metals was mainly regulated by the nucleation-dissolution trade-off of carbonates, however the dissolution mechanism of heavy metals in SAR could be summarized as follows: (1) corrosion/replacement of H+, and (2) counter-ion effect. this study will promote the resource utilization of MSWI FA. In a word, this work systematically studied the long-term environmental risk of geopolymers, which was beneficial to the resource utilization of MSWI FA.

Distribution, occurrence, and leachability of typical heavy metals in coal gasification slag

Coal gasification slag (CGS) contains variable amounts of heavy metals, which can negatively impact the environment. The mineral composition, element distribution, occurrence, and leaching characteristics of heavy metals in coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) are studied to explain the leaching behavior of heavy metals in CGS. The movable components of heavy metals in CGFS (0.06 %–63.03 %) are significantly higher than those in CGCS (0 %–18.72 %). Leaching Environmental Assessment Framework 1313 data shows that heavy metals Zn, Cr, Cd, As, Pb, Ni, and Cu exhibit high leaching rates at low pH conditions, with Zn leaching concentrations as high as 2.11 mg/L at pH 2. Zn, Cr, and As exhibit obvious amphoteric leaching characteristics, and the leaching concentration of As at high pH (1.34 mg/L) even exceeds that at low pH (1.31 mg/L). Except for Cu, all heavy metals in CGS exceed the class III groundwater standard in some cases. Therefore, evaluation is needed before resource utilization of CGS due to potential leaching of some heavy metals.

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.

Geopolymerization of MSWI fly ash and coal fly ash for efficient solidification of heavy metals: Insights into stabilization mechanisms and long-term leaching behavior

High levels of heavy metals (HMs) are present in municipal solid waste incineration fly ash (MSWI-FA), which requires proper management before landfilling. The aim of this study is to investigate the feasibility of co-disposal of MSWI-FA and coal fly ash (Coal-FA) using geopolymerization technology to solidify heavy metals and to analyze the long-term leaching behavior of heavy metals (HMs). The active silicon, aluminum, and calcium present in MSWI-FA and Coal-FA are essential for the synthesis of geopolymers. The geopolymer with 20 wt% MSWI-FA (M20) achieved the maximum compressive strength of 25.73 MPa, mainly due to the formation of C-A-S-H gels. Heavy metals were effectively immobilized and met the limits of GB18598–2019. The compressive strength of M20 was decreased by 1.6% in sulphuric acid-nitric acid leaching solution for 64 days, but decreased by 57.4% in the acetic acid leaching solution. The geopolymer was more stable in the acid rain environment, resulting in less leaching of heavy metals. The bulk diffusion model (BDM) and Finite Element Method (FEM) could satisfactorily describe the leaching behavior of HMs. The results provide valuable guidance for future waste management practices and the development of solidification processes for MSWI-FA.

Experimental assessment of utilizing copper tailings as alkali-activated materials and fine aggregates to prepare geopolymer composite

Globally, there is an increasing attention drawn by copper tailings (CT) due to the environmental problems caused by its tremendous storage volume and low recycling rate. With the purpose of promoting the sustainability and value-added utilization of CT, this study used CT powder with particle size ≤ 0.075 mm, granulated blast furnace slag (GBFS), and fly ash (FA) to prepare CT/FA-GBFS geopolymer paste composites. Also, 0.075–0.6 mm CT sand was also used as a replacement of natural fine aggregates. The effect of CT powders on the mechanical strength of the geopolymer paste was assessed, as well as the influence of CT sand on the mechanical strength and the heavy metal leaching behavior of CT-GBFS geopolymer mortar. The obtained results from the study demonstrate that CT powder exhibits partial participation in the geopolymerization reaction within an alkaline environment. Additionally, it manifests a filling effect when incorporated as micro powders. It is noteworthy that the compressive strength of the geopolymer paste improved by 18% and the flexural strength by 74% when the CT content reached 45%. Furthermore, in the context of CT-GBFS geopolymer mortar, the 20% replacement ratio of CT sand yields a more favorable interfacial transition zone (ITZ) between the geopolymer paste and the mixed sand. This improved ITZ improves the mechanical properties of the mortar by about 10% compared to other experimental groups. Finally, when CT were immobilized in the CT-GBFS geopolymer mortar, the leaching of heavy metals from copper tailings was significantly reduced, eliminating the impact of CT utilization on the environment.

Immobilization enhancement of heavy metals in lightweight aggregate by component regulation of multi-source solid waste

Integrated recycling of solid waste containing heavy metals is a critical environmental challenge. In this study, a green solution to reduce heavy metal leaching from solid waste is demonstrated by combining contaminated soil, industrial sludge and lithium slag in pairs to produce lightweight aggregates (LWAs). The physical properties and heavy metal leaching behavior of LWA samples were systematically investigated and characterized. The results showed that industrial sludge reduced the density and water absorption of LWA, while the high content of lithium slag was detrimental to the physical properties. LWA containing 80% contaminated soil and 20% lithium slag had the lowest particle density of 1.47 g/cm3 due to the hollow structure caused by the low viscosity and violent generation of SO2. LWAs with lithium slag leached excessive Cu and Cr relatively, while heavy metals were immobilized well in LWAs with contaminated soil and industrial sludge as the main components. Because the flux components of industrial sludge could enhance the encapsulation of heavy metals by glass phase. In addition, the co-immobilization of multiple heavy metals was observed in the spinel phase. This study provides an efficient and safe method for the synergistic recycling of solid waste.

The performance and microstructure of alkali-activated artificial aggregates prepared from municipal solid waste incineration bottom ash

In order to reduce the negative environmental impact of municipal domestic waste incineration bottom ash (MSWIBA), it is converted into lightweight aggregates by cold bonding technology. To enhance the utilization of waste materials, minimize cement consumption, and fabricate artificial aggregates with superior comprehensive performance, we designed and synthesized alkali-activated aggregates (AAAs) in this study. This study used MSWIBA and ground granulated blast furnace slag (GGBFS) as precursors for alkali-activated aggregates (AAAs), activated by a mixture of sodium hydroxide solutions and sodium silicate. The physical and mechanical properties of AAAs, including water absorption, apparent density, crushing strength and carbon-resistance, were investigated at different NaOH concentrations. In addition, the microstructure, mineral composition, aggregation, porosity and heavy metal leaching behavior of AAAs were analyzed by SEM, XRD, FTIR, TGA, MIP and ICP techniques. All AAAs exhibited a consistent particle size distribution, characterized by apparent densities spanning the range of 1633–1940 kg/m3. The leaching levels of heavy metals align with the standards established by Chinese regulations. Among the different activator concentrations tested, aggregate (M4) displayed superior single-particle crushing strength (3.90 MPa) and better carbonization resistance. It also used a lower concentration of NaOH, which enabled more cost-effective and eco-friendly production. Overall, the findings suggest that MSWIBA can be a viable precursor for producing sustainable lightweight concrete, with 4 M NaOH being the optimal activator concentration for achieving the best performance.

Utilization of manganese-rich sludge in concrete: Mechanical, nanoscale characteristics and leaching behavior

The management and recycling of heavy metal-rich sludge have posed serious environmental challenges. High levels of potassium permanganate are often used to treat eutrophic water bodies, which produce manganese-rich drinking water treatment sludge (DWTS). Although there have been some studies on recycling sludge in cement-based materials, little attention has been paid to the multiscale behavior of concrete with manganese-rich sludge. In this study, eco-friendly concrete was produced by substituting part of the cement with calcined DWTS (CWTS), and its mechanical properties, drying shrinkage, microstructure, and environmental characteristics were systematically investigated. The results showed that the incorporation of an appropriate amount of CWTS improved the mechanical properties and volume stability of the concrete, and the best performance was achieved when the substitution level was 10%. The 90-d compressive strength of 10% CWTS-modified concrete was relatively increased by 7.91%, while the 150-d drying shrinkage was relatively reduced by 14.84% compared with plain concrete. The pozzolanic action of CWTS consumed part of Ca(OH)2 and increased the degree of reactivity of the paste at appropriate substitution levels. From the perspective of nanoscale characteristics, the incorporation of 10% CWTS reduced the content of the pore phase and clinker and increased the content of the C-S-H phase relative to plain concrete, where high-density C-S-H and low-density C-S-H increased by 12.5% and 6.8%, respectively. Moreover, incorporating an appropriate amount of CWTS also reduced the width of the interfacial transition zone (ITZ), but with the further increase of its content, the width of the ITZ increased. In addition, CWTS-modified concrete not only exhibited safe heavy metal leaching behavior but also had lower costs and carbon emissions. This study provides new insights into the management and recycling of DWTS, especially manganese-rich sludge obtained from the treatment of algal blooms.

환경인자 변화에 따른 폐광산 부지 토양의 중금속 용출 특성

Improper management of abandoned mine sites can cause heavy metal contamination of down-gradient agricultural soil, groundwater, and surface water. In this study, we investigated the effects of changes in environmental factors on the leaching behavior of the heavy metals and suggested future research direction for better risk management of the abandoned mine sites under climate change. Numerous studies showed that the concentration of heavy metals in the leachate frequently exceed the environmental criteria even though the leachable heavy metal was negligible (< 1 % of the total concentration), which indicates that mine soils can be long-term resources of contamination. From the result of batch and column experiments, it has been found that soil properties (e.g., soil pH, redox potential, etc.) affect the leaching behavior by solubility change, complex formation, precipitation, etc. Two-site kinetic leaching models were mainly applied for understanding nonequilibrium leaching behavior of the metals due to heterogeneous physical structure and various binding sites of the soil. Meanwhile, it was also reported that the change in hydrodynamic properties due to rainfall pattern can influence leaching behavior by changing the interfacial interaction between soil and water. Several researchers recognized that extreme weather condition (high temperature and increased drought period) due to climate change can elevate initial leaching concentration of the heavy metal. In reality, climate change can cause the nonequilibrium leaching of the metals by influencing hydrodynamic condition and chemical stability of the soil system. Therefore, future works to precisely predict the heavy metal leaching behavior across abandoned mine sites are necessary and risk management of these sites in response to climate change should be designed.

The effect of sulfur on the leaching of Cr3+, Cr6+, Pb2+ and Zn2+ from fly ash glass

This work assessed the capture and subsequent release of potentially harmful Cr(VI), Cr(III), Pb(II) and Zn(II) ions in and from dechlorinated fly ash glass. Differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy – energy dispersive spectroscopy and inductively coupled plasma spectrometry along with other analytical techniques were used to explore the mechanism by which sulfur affected the immobilization and long-term leaching behavior of heavy metals in fly ash glass. Working with a CaO–MgO–Al2O3–SiO2–SO3 system, increasing the sulfur content was found to promote the leaching of Cr but had only a minimal effect on the loss of Pb and Zn. The concentrations of Pb and Zn in the leachate were found to remain at essentially nil over time while the Cr level increased up to 64 h and then decreased. The presence of Sulfur ions degraded the glass network and this promoted the leaching of S2−, Cr3+/Cr6+, Pb2+ and Zn2+. In addition, the S2− ions reacted with Pb2+ and Zn2+ to form needle-shaped and flocculent sulfide precipitates, thus trapping the Pb2+ and Zn2+. Si4+, Ca2+, Al3+ and Fe3+ were also found to migrate into the leaching solution where they combined to form a dendritic flocculent that adsorbed and encapsulated Cr. This phenomenon greatly reduced the concentration of Cr in the leachate. Thus, sulfur prevented the leaching of Cr, Pb and Zn via different mechanisms.

The behavior of heavy metal release from sulfide waste rock under microbial action and different environmental factors.

The dissolution of heavy metals from the waste rock is controlled by many factors. Herein, we investigated the release behavior of iron (Fe), chromium (Cr), copper (Cu), and zinc (Zn) from sulfide waste rock under the actions of microorganisms and different environmental factors (solution pH value, particle size of waste rock, temperature, Fe3+ concentration). The release quantity of heavy metals was negatively correlated with pH and particle size and positively correlated with ambient temperature and Fe3+ concentration. Under the experimental conditions of pH value of 3.0, temperature of 35°C, and waste stone particle size of less than 0.075 mm,, the release quantity of Fe, Cr, Cu, and Zn reached 3680, 18.32, 132.20, 26.60 mg·kg-1 after 20 days of leaching, respectively. Rising the temperature to 45 °C, Fe, Cr, Cu, and Zn release quantities increased to 89.30, 5.81, 105.08, and 28.00 mg·kg-1. Six hundred milligrams per liter Fe3+ increased the release of heavy metals considerably (2.63-65.48 folds). The presence of microorganisms can significantly facilitate the release of heavy metals. Compared to the control group, the release quantities of Fe, Cr, Cu, and Zn increased 4.29, 3.17, 1.54, and 2.39 times, respectively. In addition, the waste rock under microbial action was more seriously corroded than that under chemical factors. The release behavior of these four heavy metals was consistent with the interfacial chemical reaction control model, indicating that the reactions mainly occurred on the surface of the waste rock. This study provides an essential reference for the study of heavy metal leaching behavior.

Preparation of artificial lightweight aggregate using alkali-activated incinerator bottom ash from urban sewage sludge

The preparation of lightweight aggregates (LWAs) using sewage sludge ash (SSA) by cold bonding technology can reduce the negative impact of SSA on the environment, it is an efficient solution to dispose of SSA. In this study, the granulated blast furnace slag (GBFS) was mixed with SSA to improve the performance of the prepared LWAs, with a dosage of 15% (SSA-G15) and 30% (SSA-G30) SSA by weight. Moreover, the 15% cement and 15% GBFS (SSA-G15C15) were added to the SSA as the third batch for comparison. The physical and mechanical characteristics of the LWAs were investigated, including particle size distribution, density, water absorption, compressive strength, and freeze-thaw resistance. Besides, the mineral composition, bonding type, microstructure, porosity, and heavy metal leaching behavior of the LWAs were evaluated by XRD, FTIR, SEM, MIP, and ICP, respectively. Results showed that all LWAs exhibited a uniform particle size distribution, a bulk density below 700 kg/3, and an acceptable leaching value of harmful metals as recommended by NEN-EN 6966, 2005. The SSA-G15C15 aggregates had higher cylinder compressive strength (1.8 MPa) and lower mass-loss rate (62.06%) under freeze-thaw cycles due to the dense microstructure compared to other batches. The water absorption value of SSA-G15C15 was 24.58%, within the acceptable limits as recommended by ACI-213R. It can be concluded that the application of SSA to produce LWAs is feasible, with a promising potential in sustainable lightweight concrete.

New insight into the role of FDOM in heavy metal leaching behavior from MSWI bottom ash during accelerated weathering using fluorescence EEM-PARAFAC

Fluorescence excitation-emission matrix (EEM) spectroscopy is a powerful tool to characterize DOM that interacts with heavy metals in MSWI bottom ash (IBA). Here, two fresh IBA samples collected from large MSWI plants were subjected to 33 days of accelerated weathering. Carbon content and fluorescence characterization of DOM and leaching behavior of heavy metals (Cu, Ba, Cr, Ni, and oxyanions) were monitored during the weathering. The mineralogical and chemical properties of IBA during the weathering process were also characterized. EEM combined with parallel factor analysis showed that fluorescent DOM could be decomposed into humic-like (C1, C2) and tryptophan-like substances (C3), while the accelerated weathering process can be further divided into three phases. Fitted cubic polynomials described well the changes in the specific intensity of fluorescence components. Humification and freshness indexes and SUVA results suggested the leached DOM contained a higher proportion of condensed aromatic structures and/or conjugation of aliphatic chains post-weathering. The results also revealed that adsorption of humic-like substances onto neo-formed reactive surfaces occurred quickly in the early stage of accelerated weathering; thereafter, biodegradation of lower molecular mass-hydrophilic organic carbon fraction plays a vital role in further reduction of Cu and Cr leaching in subsequent weathering. Oxyanions (Mo and Sb) became more mobile after 3 days of accelerated weathering, but their leaching was effectively reduced after the weathering process. A novel method for an IBA weathering treatment combined with enhanced microbial degradation is proposed. These findings provide new and inspiration for improving accelerated weathering technology.

Release of heavy metals from conventional and reflective cool cement pavements

Cool cement blocks are used for pavement construction to mitigate the heat island effect. Cement block pavements in contact with water will produce leachates, which may affect soil, surface water and groundwater quality. Therefore, the objective of this work was to determine and compare the leaching behavior of heavy metals (Cd, Co, Cr, Fe, Mn, Ni, Pb, Zn) from conventional and reflective cool cement blocks using the mass transfer tank leaching test, according to USEPA method 1315 and the batch leaching test as a function of pH, according to method CEN/TS 14429. Comparison of the results of the two tests indicates that equilibrium was not reached during the mass transfer test. In all cases, the Leachability Index values are higher than 8, indicating limited mobility. Observed diffusivity values differ by several orders of magnitude (10−11 – 10−20 m2/s) and are very small, indicating very slow diffusion. Leaching curves as a function of pH were very consistent and close to each other, indicating that the leaching mechanisms were the same for all cement blocks. Α two-domain one-dimensional mathematical model describing heavy metal diffusion from the pavements to the underlying soil was developed and tested. Simulations with this model showed that emissions from reflective cool cement pavements to the underlying soil are not different from the conventional ones in the long run.

Leachability of cement mortars containing sewage sludge ash

Currently, the use of wastes and by-products as cementitious additions has become a necessity in the cement production sector. Because of their technical benefits and low cost. However, because of the pollutant load of these materials, it is essential to study the environmental behavior and the leachability of these cement mixtures before their use into buildings.This paper investigates the leachability of cement-based mortars containing various percentages of the waste material obtained from the incineration of sewage sludge, i.e. sewage sludge ash (SSA). The SSA and SSA- mortars was characterized by classical, analytical and instrumental measurement. In addition, the leaching behavior of heavy metals in SSA and SSA- mortars were assessed using NF EN12457-2 and NF PX31-211 leaching tests. Results illustrate that the leaching behavior of SSA shows an important release of Cr, Mo, Ba, Sr, V, Zn and Al. While, the leaching behavior of mortars containing different amounts of SSA showed that heavy metals were successfully immobilized in cement matrix.

Mechanical property and heavy metal leaching behavior enhancement of municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

The conventional sintering process of municipal solid waste incineration (MSWI) fly ash is always energy intensive. The process forms a cracked structure because of the difficulty in forming the liquid phase to enhance the mass transfer process. Therefore, exploring a new disposal method to simultaneously decrease the sintering temperature and improve the mechanical and heavy metal leaching properties of sintered samples is necessary. In this study, a pressure-assisted sintering treatment was introduced to dispose fly ash by varying the chemical composition and mechanical pressure at relatively low temperatures (300–500 °C). The results revealed that the compressive strength of treated samples increased with the CaO/SiO2 molar ratio increasing from 0.5 to 1.0, and a maximum value of 238.28 ± 8.50 MPa was obtained. The heavy metal leaching concentration results demonstrated a low risk of contamination in the treated samples. Microstructure analyses suggested that the densification process was enhanced with increased mechanical pressure, and the formed calcium silicates and aluminosilicates positively affected the compressive strength. Moreover, smaller crystal lattices were observed during aggregation formation, suggesting the restraint of anomalous crystal growth, which accelerated the densification process and increased the compressive strength. Moreover, the mass transfer process during the pressure-assisted sintering process was enhanced compared with the conventional thermal process, which was reflected by the transformation of elements from homogeneous to heterogeneous distribution. Therefore, the improved mechanical properties and leaching behavior of heavy metals were attributed to the densified microstructure, formation of new minerals, and enhanced driving force during the pressure-assisted sintering process. These findings suggest that pressure-assisted sintering is a promising method for maximizing the reutilization and minimizing the energy consumption simultaneously to dispose fly ash.

Leaching characteristics of heavy metals in MSW and bottom ash co-disposal landfills

Bottom ash (BA) management is often implemented through its co-disposal with municipal solid waste (MSW) in landfills. However, BA co-disposal may lead to heavy metal leaching in landfills. In this study, the effect of BA co-disposal on heavy metal leaching behavior under different scenarios, specifically, MSW, low BA co-disposal (BA_L), high BA co-disposal (BA_H), and BA monofill were investigated. The heavy metal concentrations in the leachate decreased in landfills over time. The leached metals primarily included Zn, Cu, Mn, Pb, Cr, and Cd. The discharge concentration ratio of heavy metals in the leachates exhibited the following decreasing order: MSW, BA_L, BA_H, and BA. In particular, the discharge concentration ratio of Cu in the MSW, BA_L, BA_H, and BA cases ranged from 7.1 × 10−3 to 8.8 × 10−1 (mean = 3.0 ×10−1), 2.8 × 10−4 to 2.0 × 10−1 (mean = 5.4 ×10−2), 9.1 × 10−5 to 3.0 × 10−2 (mean = 5.9 ×10−3), and 4.4 × 10−4 to 7.9 × 10−3 (mean = 1.8 ×10−3), respectively. Moreover, the leaching of the heavy metals could be attributed to waste contents, properties of the heavy metals, and leachate characteristics, such as the pH, chemical oxygen demand (COD), alkalinity, and Cl- content. The presented findings can help clarify the leaching characteristics of heavy metals in BA co-disposal landfills, thereby facilitating the optimization of practical landfills.

Leaching behavior of copper and chromium in the mortar containing artificial fine aggregate prepared by contaminated soil

The feasibility of recycling contaminated soil by preparing artificial fine aggregate (AFA) and applying in artificial fine aggregate mortars (AFAM) was investigated systematically. Properties and heavy metal immobilization status of AFA, compressive strength and heavy metal leaching behavior of AFAM were tested. Results showed that Cu reduced the melting and crystallization temperatures of AFA, whereas Cr improved that. Heavy metal addition had a negative impact on the compressive strength of AFAM, but the toxicity characteristic leaching procedure (TCLP) test indicated that the AFAM system was is suitable for solidification of heavy metals. Moreover, it was observed that Cu leaching almost disappeared in the initial periods, while it tended to dissolve in the terminal periods. The leaching mechanism of AFAM with Cr at the initial periods conformed to Fick's law of diffusion, and it tended to deplete in the terminal periods. These results are practically instructive the recycling of heavy metal contaminated solid waste in the field of construction materials.