Kinetics and modeling of trace metal leaching from bottom ashes dominated by diffusion or advection

Effect of 60Co Irradiation on Boehmite Dissolution in Caustic Solutions

Here, we examine how radiation impacts the dissolution behavior of boehmite by subjecting dry nanoparticles of different sizes to 60Co γ radiation and subsequently analyzing their dissolution behavior in caustic solutions as a function of temperature. The measured kinetics show that irradiation with an amount 228.24 Mrad significantly slows the dissolution rate, particularly for smaller sizes at lower temperatures. Specifically, the temperature-dependent dissolution rates of irradiated 20 nm boehmite versus pristine material in 3 M NaOH solutions were several times lower (e.g., rate constant of 0.026 vs 0.075 h-1 at 60 °C), with an apparent activation energy 40 kJ mol-1 higher. Although various imaging techniques and X-ray diffraction measurements consistently revealed no obvious differences between pristine and irradiated samples, after irradiation significant binding energy shifts were detected in the X-ray photoelectron Spectroscopy peaks of Al 2p and O 1s, and a change in their relative intensities indicated a lower O/Al ratio. This suggests that γ-irradiation may stabilize boehmite particle surfaces by driving their chemistry and structure toward more stable aluminum oxide forms. This finding may help explain slower dissolution rates of boehmite in nuclear waste and may be useful for the development of more robust predictive models and effective strategies for waste processing.

Leaching law of heavy metals in coal gangue: A combination of experimental optimization and simulation

Coal gangue, a solid waste generated during coal mining and washing processes, has caused significant environmental burdens in China. This study aims to optimize and investigate the leaching mechanisms of heavy metals, such as Pb, Zn, and Cu, in coal gangue. The effectiveness of different eluents in removing heavy metals from coal gangue was evaluated by combining experimental methods with software simulations. The leaching conditions (EDTA-2Na concentration of 5 g/L, pH 3, solidliquid ratio of 1:10, leaching time of 4 h, 300 r/min) were optimized to achieve efficient and economical removal of heavy metals. Box-Behnken Design was used to show the key factors of eluant concentration and solid-liquid ratio. The leaching amounts of Pb, Zn, and Cu from coal gangue using EDTA-2Na as a leaching agent were 86 mg/kg, 430 mg/kg, and 66 mg/kg, respectively. The release mechanism and kinetic behavior of heavy metals in the leaching process were studied. The study provided information about leaching mechanisms of heavy metals from coal gangue by experiments and simulations of Visual MINTEQ and DFT that EDTA-2Na enhanced the leaching of heavy metals from coal gangue by enhancing ion exchange and complexation.

Leaching behavior and kinetics of beryllium in beryllium-containing sludge (BCS)

Beryllium-containing sludge (BCS) is a byproduct of the physicochemical treatment of beryllium smelting wastewater. The pollutant element beryllium within BCS is highly unstable and extremely toxic, characterized by its small ionic radius and low charge density, resulting in a high risk of leaching and migration. This study is the first to investigate the leaching behavior, influencing mechanisms, and kinetic processes of beryllium in BCS under various environmental conditions. The results indicate that, under national standard conditions, beryllium exhibits a rapid leaching phase within the first 5 h, which then stabilizes after 10 h, with the total leached content significantly exceeding the leaching toxicity identification standards. Under mildly acidic (pH ≤ 5) or highly alkaline (pH = 14) conditions, beryllium demonstrates pronounced leaching and migration behaviors. Notably, in acidic conditions, the leaching rate exceeds 80% within 5 h. Combining the treatment process of beryllium-containing wastewater with analytical methods such as SEM, XPS, ToF-SIMS, and FTIR, it is revealed that due to the heterogeneous nature of BCS, the particle aggregates dissociate over time under acidic conditions. The particle surfaces become increasingly rough, leading to dissolution and the emergence of more reactive sites, resulting in a high proportion of beryllium leaching. Under these conditions, the gradual reaction of Be(OH)2 in BCS to form soluble Be2+ and its hydrolytic complexes is identified as the primary mechanism for extensive beryllium migration. The process encounters minimal diffusion resistance and is classified as reaction-controlled. In acidic conditions with pH = 4, the leaching rate of beryllium significantly increases with rising temperature. The leaching kinetics equation is [(1-x)-0.44]=e(18.26-53050RT)·t, with an apparent activation energy of 53.05 kJ mol-1.

Optimizing MSW incineration bottom ash reuse: A study on treated wastewater washing and leaching control

The current study introduces an innovative methodology by utilizing treated wastewater (TWW) from an effluent treatment plant as a washing agent to enhance the characteristics of incineration bottom ash (IBA). This approach addresses sustainability concerns and promotes the circular economy by reusing wastewater generated in municipal solid waste incineration facilities. Previous research has underscored the challenges of open IBA reuse due to elevated leaching of chlorides, sulfates, and trace metal(loid)s. Thus, the experimental setup explores various combinations of washing, with or without screening, to optimize the properties of soil-like material (SLM < 4.75 mm) and overall material (OM < 31.5 mm) fractions of IBA for unrestricted applications. Batch leaching tests were conducted on treated samples, and leaching characteristics were evaluated in accordance with regulatory standards, primarily the Dutch standard for unrestricted IBA reuse. The findings reveal that washing in isolation proves insufficient to enhance IBA properties; however, washing followed by screening, specifically for removing fines (<0.15 mm), proves effective in reducing contamination. The study identifies that multiple steps of washing and screening (with recirculation) process render OM and SLM fractions suitable for unrestricted reuse with a cumulative liquid-to-solid ratio of 6 L/kg and a total washing time of 15 min. The multi-step treatment was found effective in reducing sulfate contamination by 65-74 % and chloride contamination by 83-89 % in IBA fractions. This approach offers a promising solution for overcoming the limitations associated with IBA leaching, thereby promoting sustainable waste reuse practices.

Kinetics and toxicological potential of heavy metal leaching from asphalt pavements

The leaching of heavy metals from asphalt pavement has attracted increasing attention due to its associated environmental risks. Comprehending the leaching process is crucial for ensuring the safe utilization of asphalt pavement. This study investigates heavy metal leaching kinetics from asphalt pavements using tank-leaching tests and dynamic simulations employing both first and second-order kinetic models. Furthermore, this study reveals the toxicological potential of heavy metal leaching from asphalt pavement by assessing its temporal metal accessibility based on the obtained kinetic attributes. Six distinct asphalt mixtures were prepared and tested, each exhibiting two different gradations. The findings demonstrated that both kinetic models effectively elucidated the leaching process. Notably, the relatively stable final leaching stages primarily adhered to first-order kinetics, while the second-order kinetics provided a superior description of the more intricate initial leaching stages. In terms of toxicological potential, the results indicated that recycled waste-incorporated asphalt pavements, specifically bottom ash-incorporated asphalt and asphalt rubber, exhibited excessive heavy metal leaching for varying durations, ranging from several days to months under specific conditions. This study has provided valuable insights into the metal leaching kinetics of asphalt pavements and their associated toxicological impact, significantly advancing the current understanding of the consequences of heavy metal leaching from asphalt pavements.

New insight into the geochemical mechanism and behavior of heavy metals in soil and dust fall of a typical copper smelter

The desorption mechanism of heavy metals (HMs) in soil around the mining region are complex and affected by multiple pollution sources, including sewage discharge and atmospheric deposition. Meanwhile, pollution sources would change soil physical and chemical properties (mineralogy and organic matter), thus affecting the bioavailability of HMs. This study aimed to investigate the pollution source of HMs (Cd, Co, Cu, Cr, Mn, Ni, Pb, and Zn) in soil near mining, and further evaluate influence mechanism of dust fall on HMs pollution in soil by desorption dynamics processes and pH-dependence leaching test. Result presented that dust fall is the primary pollution source to HMs accumulation in soil. Additionally, the result of mineralogical analysis in dust fall revealed that quartz, kaolinite, calcite, chalcopyrite, and magnetite are the major mineralogical phases by XRD and SEM-EDS. Meanwhile, the abundance of kaolinite and calcite in dust fall is higher than in soil, which is the primary reason of higher acid-base buffer capacity of dust fall. Correspondingly, the weakened or disappeared of hydroxyl after the adding acid extraction (0-0.4 mmol· g-1) demonstrated that hydroxyl is the main participants of HMs absorption in soil and dust fall. These combined findings suggested that atmospheric deposition not only increases the pollution loading of HMs in soil, but also changes the mineral phase composition of soil, which would increase the adsorption capacity and bioavailability of HMs in soil. This is very remarkable that heavy metals in soil influenced by dust fall pollution could be released preferentially when soil pH is changed. The present results of this study would provide efficient and scientific targeted strategies for pollution control of HMs in soil near mining areas.

Leaching risks of antibiotic resistance genes in urban underlying surface sediments during the simulated stormwater runoff and its controls

Recently, increasing attention has been paid to antibiotic resistance genes (ARGs) in urban stormwater runoff. However, no available literature could be found on ARGs leaching from urban underlying surface sediments during stormwater runoff. In this study, surface sediments from commercial public squares around Nanjing (China) was selected for the investigation of target ARGs leaching kinetics, showing that absolute abundances of target ARGs desorption reached at the equilibrium during leaching time of 120-240min with all of the desorption efficiencies about 30%, indicating that there would be low proportion of leaching ARGs in the total ARGs migrating with runoff during rainfall events. Five target ARGs leaching including intI1 (clinic), strA, strB, tetM and tetX can be better described by the pseudo-second-order equation, while qacEdelta1 leaching can be better described by the pseudo-first-order equation, and the leaching for both sul1 and sul2 can be well described by the pseudo-first-order and pseudo-second-order equations. The effects of environmental factors including S/L ratios, pH values and water temperatures indicated that leaching efficiencies of target ARGs enhanced significantly with the increase of S/L ratios and water temperatures, but decreased with the increase of pH values. The transmission experiments after the simulated solar irradiation and heat implied that both large solar UV irradiation (30W/m²) and high temperature (40℃) were conducive to conjugation and transformation frequencies of ARGs. Furthermore, both high Cao and medium PAM levels could be effective for blocking ARGs transmission in the leachate from underlying surface sediments.

The geochemical behaviors of potentially toxic elements in a typical lead/zinc (Pb/Zn) smelter contaminated soil with quantitative mineralogical assessments

This study comprehensively investigated the potential roles of soil mineralogy identified by the automated mineral liberation analysers (MLA) in the prediction of geochemical behavior of toxic metals in the smelter polluted soils. The results from modal mineralogy revealed that the non-reactive silicate phases such as quartz (42.05%) and biotite (40.43%) were the major mineralogical phases. The element deportment showed that fayalite, lead oxide, apatite, galena and wollastonite were identified as the dominant As, Cd, Pb and Zn bearing minerals. Furthermore, MLA analysis also confirmed that Pb was most concentrated in the smaller particles of lead oxide, which significantly enhanced Pb release in reaction with the chemical extractant during chemical kinetic tests. The results from pH-dependent leaching tests indicated that the leaching concentrations of As, Pb and Zn increased at low and high pH values, but were lowest at the neutral pH range. In addition, the results from the kinetic study demonstrated that the second order model provided the best description for the release patterns of the main metal contaminants in the bioavailability and bioaccessibility tests. The integrated geochemical analysis demonstrated that among these studied elements, As showed a typical geochemical pattern, which was predominantly controlled by 90.09% of fayalite. The above study results would have significant implications for soil remediation and risk management of smelter contaminated sites.

Desorption and Migration Behavior of Beryllium from Contaminated Soils: Insights for Risk-Based Management

Factors influencing the desorption, distribution, and vertical migration behavior of Be in contaminated soils are not fully understood. This study examined the desorption and migration of Be in a soil profile from a legacy radioactive waste disposal site using different batch leaching [monofilled waste extraction procedure (MWEP); synthetic precipitation leaching procedure (SPLP); simulated acid rain solution (SARS); and toxicity characteristic leaching procedure] and sequential leaching [community bureau of reference (BCR)] methods for insights relevant to the application of risk-based management. The results showed that Be desorption was higher in the presence of organic than the inorganic leachate composition (MWEP < SPLP < SARS < TCLP < BCR first-step). The desorption followed three diffusion control mechanisms, which resulted in three desorption rate constant estimates of 157, 87.1, and 40.4 Be/kg.h^0.5, and the estimated desorption maximum was 556 μg/kg. The desorption process was, spontaneous (ΔG > 0), enthalpically and entropically influenced. Increasing the incubation period and heat treatment resulted in a decrease of Be desorption and migration. The soil clay content and pH were the primary factors influencing Be desorption, and the results suggested that Be was desorbed from metal oxyhydroxides and surfaces of silicates (e.g., reactive surfaces of clay minerals), organic matters, and soil pores. Because of high K _d values, the mobility of Be was limited, and no exceedances of ecological or human health risk index or guidelines were determined for the current contamination levels at the site. However, Be released from the waste trenches has the ongoing potential to increase Be concentration in the soil.

Reliable environmental trace heavy metal analysis with potentiometric ion sensors - reality or a distant dream

Over two decades have passed since polymeric membrane ion-selective electrodes were found to exhibit sufficiently lower detection limits. This in turn brought a great promise to measure trace level concentrations of heavy metals using potentiometric ion sensors at environmental conditions. Despite great efforts, trace analysis of heavy metals using ion-selective electrodes at environmental conditions is still not commercially available. This work will predominantly concentrate on summarizing and evaluating prospects of using potentiometric ion sensors in view of environmental determination of heavy metals in on-site and on-line analysis modes. Challenges associated with development of reliable potentiometric sensors to be operational in environmental conditions will be discussed and reasoning behind unsuccessful efforts to develop potentiometric on-site and on-line environmental ion sensors will be explored. In short, it is now clear that solely lowering the detection limit of the ion-selective electrodes does not guarantee development of successful sensors that would meet the requirement of environmental matrices over long term usage. More pressing challenges of the properties and the performance of the potentiometric sensors must be addressed first before considering extending their sensitivity to low analyte concentrations. These are, in order of importance, selectivity of the ion-selective membrane to main ion followed by the membrane resistance to parallel processes, such as water ingress to the ISM, light sensitivity, change in temperature, presence of gasses in solution and pH and finally resistance of the ion-selective membrane to fouling. In the future, targeted on-site and on-line environmental sensors should be developed, addressing specific environmental conditions. Thus, ion-selective electrodes should be developed with the intention to be suitable to the operational environmental conditions, rather than looking at universal sensor design validated in the idealized and simple sample matrices.

Leaching behavior of polycyclic aromatic hydrocarbons (PAHs) from oil-based residues of shale gas drill cuttings

Cuttings are the main solid residues which are generated from drilling operations. Due to the presence of heavy and radioactive elements, the environment risk posed by cuttings has attracted increasing attention. In this work, a short-term static immersion experiment was carried out to investigate the leaching of polycyclic aromatic hydrocarbons (PAHs) from oil-based residues of shale gas drilling cuttings. Furthermore, the effects of some relevant environmental factors controlling the leaching behavior were evaluated, including the different particle sizes, pH, extraction time, solid-to-liquid (S/L) ratio and dissolved organic matter (DOM) concentration. The results showed that (1) the concentrations of leached PAHs gradually increased with prolonged leaching time, but the cumulative amount of PAHs released during leaching was less than 3% of the total. (2) The Elovich, parabolic diffusion and power function models were found to fit the experimental data better than the first-order kinetic equation, indicating that the leaching of PAHs was controlled by the coupling of diffusion and chemical reactions at the source surface. (3) Different environmental factors had different impacts on the leaching of PAHs: the shaking time and presence of DOM increased leachability, the particle size and S/L ratio decreased leachability, and the pH did not affect the leachability of PAHs. Therefore, PAHs leaching was a complex process, and it is of scientific and environmental interest to conduct the leaching tests under the simulated environmental conditions.

Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials

Initial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the necessary insight to understand the complexity of column leaching tests especially when heterogeneous samples are concerned. By means of numerical experiments, we illustrate the initial concentration distribution inside the column after the first flooding and how this impacts leaching concentrations. Steep concentration gradients close to the outlet of the column have to be expected for small distribution coefficients (Kd<1 L kg-1) and longitudinal dispersion leads to smaller initial concentrations than expected under equilibrium conditions. In order to elucidate the impact of different mass transfer mechanisms, film diffusion across an external aqueous boundary layer (first order kinetics, FD) and intraparticle pore diffusion (IPD) are considered. The results show that IPD results in slow desorption kinetics due to retarded transport within the tortuous intragranular pores. Non-linear sorption has not much of an effect if compared to Kd values calculated for the appropriate concentration range (e.g., the initial equilibrium concentration). Sample heterogeneity in terms of grain size and different fractions of sorptive particles in the sample have a strong impact on leaching curves. A small fraction (<1%) of strongly sorbing particles (high Kd) carrying the contaminant may lead to very slow desorption rates (because of less surface area)-especially if mass release is limited by IPD-and thus non-equilibrium. In contrast, mixtures of less sorbing fine material ("labile" contamination with low Kd), with a small fraction of coarse particles carrying the contaminant leads to leaching close to or at equilibrium showing a step-wise concentration decline in the column effluent.

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.

Impact of bottom ash co-disposed with municipal solid waste on geotextile clogging in landfills

Co-disposal of bottom ash (BA) with municipal solid waste (MSW) in landfills is commonly used for BA management. However, BA co-disposal may cause clogging of geotextiles in MSW landfills. This study investigated the effect of different BA co-disposal ratios on geotextile clogging, including MSW, low ash co-disposed (BA_L), high ash co-disposed (BA_H) landfills, and BA mono-fill. Results showed that the BA_L group increased the geotextile clogging by 0.1-0.6 times, compared to that in the MSW landfill. In contrast, the geotextile clogging of the BA_H and BA groups was reduced than that in the MSW landfill. The clogging was in a dynamic process during the experimental period in all the conditions, including chemical clogging and bio-clogging. Moreover, bio-clogging was the main contributor to the geotextile clogging, accounting for 64-83% of the total clogging mass. The BA co-disposal affected the leachate characteristics, such as pH, calcium concentration, and alkalinity, resulting in chemical clogging. When pH was above 7.0, calcium concentration and alkalinity were limiting factors for the calcium carbonate formation. In terms of the bio-clogging, the microbial analysis indicated that different BA co-disposal ratios influenced the diversity and structure of microbial community. These findings could help clarify the effect of BA co-disposal on geotextile clogging, thus useful to landfill operation in practice.

Eco-friendly geopolymer prepared from solid wastes: A critical review

Solid wastes are generated from human activities which could cause damage to the ecological environment and human beings. In recent years, there has been extensive research on solid wastes utilized as precursors, aggregates, fibers, etc. to prepare the geopolymers, which has invariably been a research hotspot. This review classifies the solid wastes utilized for geopolymers into three main categories: industrial waste, agricultural waste, and municipal waste. Accordingly, we systematically dissert solid wastes-based geopolymer from the perspectives of structure, properties, and application. The chemical composition, morphology, particle size, thermal conductivity, and other characteristics of solid wastes can trigger changes in the specific properties of geopolymers. On this account, solid wastes-based geopolymers have great potential in the domain of concrete, fireproof materials, impermeable materials, catalysts, adsorbents, and energy storage materials, etc. More importantly, geopolymers have obvious advantages in immobilizing heavy metals in solid wastes. Therefore, it can demonstrate geopolymer is a sustainable and environmentally friendly "green material". However, it still confronts the challenges of solid wastes utilized in geopolymer (technology, economy, administration). It requires the government, enterprises, and the public to work together for co-governance to accomplish industrialization and commercialization of solid wastes-based geopolymer.

Assessment of the long-term leaching characteristics of cement-slag stabilized/solidified contaminated sediment

The use of stabilized/solidified (S/S) soils and sediments as sustainable construction materials is a global concern due to the potential risk of contaminant leaching including potentially toxic elements. The long-term leachability of four metals (Zn, Pb, Cd and As) in sediments mixed with OPC (Ordinary Portland Cement) and OPC/GGBS (Ground Granulated Blast Furnace Slag) binders were investigated through a combination of tank leaching tests and kinetic leaching models, with varying ranges of curing ages and ambient pH. The leaching data revealed that both binder compositions offer an excellent immobilization capacity for the four metals, while their releases are strongly pH-dependent and are a complex function of curing time. The partial replacement of OPC by GGBS is more effective for fixing Zn and As at pH of 1, Pb at pH of 7, Zn and Pb at pH of 10. Controlling leaching mechanisms and leachability indices were determined using nonlinear regression analysis and kinetic leaching models. The first-order diffusion model (FRDM) was the most applicable model for describing the leaching characteristics of these metals under the investigated cases, the leaching rate is controlled by surface wash-off initially and then by diffusion. The leachability indices indicate that the cement-slag S/S sediment can be regarded as an environmentally sustainable material with potential beneficial uses in construction.

Environmental impact analysis of municipal solid waste incineration in African countries

Globally, proper management of solid waste has been a massive issue. Incineration is popularly used in waste treatment worldwide due to its ability to minimize waste volume and generate electricity. Despite its advantages, incineration of waste can still generate large amounts of flue gas, which can be harmful if not handled properly. Therefore, the present study seeks to examine the environmental consequences of incineration in Africa. The study used the most accurate secondary data on the municipal solid waste incineration in 56 African countries from literature for the analysis. Due to data availability, the years 2012 and 2025 are considered for the study. The environmental analysis was based on global warming, acidification, and dioxin emission potentials. The emission of greenhouse gases from the incineration plant was estimated based on the method from the 2006 Intergovernmental Panel on Climate Change Guidelines for National Greenhouse Gas Inventories. The acid gases and dioxins emissions were evaluated based on the United States Environmental Protection Agency "Compilation of Air Pollutant Emission Factors" (Acidification Potential-42). Key findings show that the global warming potential of Seychelles is expected to decline from 200.10 kton CO₂eq to 196.18 kton CO₂eq in 2025, while that of Saint Helena will be constant at 7.85 kton CO₂eq. The study found that except for Seychelles and Saint Helena, the global warming potential of incineration projects in all the countries is expected to increase in 2025 compared to 2012. It was realized that the acidification potential of the project in 2012 ranges from 740.56 kg SO₂eq to 4,297,839.96 kg SO₂eq, and that of 2025 is expected to be from 740.56 kg SO₂eq to 9,449,175.32 kg SO₂eq. The results further indicate that South Africa, Egypt, Algeria, Morocco, and Nigeria have higher dioxin emission potential in 2012 and 2025 compared to other countries in this study. This study will guide decision-making on the environmental sustainability of waste-to-energy incineration projects in Africa.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

A componential approach for evaluating the sources of trace metals in municipal solid waste

Trace metals concentrations of 25 elements were determined for 22 subcomponents of biodegradable and non-biodegradable waste samples representing the United States municipal solid waste (MSW) stream collected during three separate waste sorts. The subcomponent trace metal concentrations and estimated composition results were used to predict trace metal concentrations present in the overall MSW stream along with MSW compost and waste to energy (WTE) ash, which were compared to health-based standards (i.e., US EPA regional screening levels) and to values previously reported in the literature. These estimates for potentially problematic elements like As and Sb could be attributed to abundant base materials in MSW, while other elements, such as Pb, were calculated at much lower concentrations than other published studies. This suggests that trace metals measured in actual MSW compost and WTE ash could originate not only from MSW base components but also from other sources, such as highly concentrated low-mass wastes (e.g., e-waste). While the removal of small quantity components with high metal concentrations may reduce concentrations of some potentially problematic metals (e.g., Pb), others (e.g., As and Sb) are likely to persist in quantities that impede reuse and recycling since they are present in the more abundant base MSW components (e.g., papers, plastics, organics). Promoting meaningful reductions in potentially problematic trace metals in MSW-derived materials may require reevaluating their presence in higher-volume, lower-concentrated MSW components such as paper, plastics, and organics.