Speciation analysis and leaching behaviors of selected trace elements in spent SCR catalyst

Extraction of W, V, and As from spent SCR catalyst by alkali pressure leaching and the pressure leaching mechanism

Spent selective catalytic reduction (SCR) catalysts are environmentally hazardous and resource-enriching. In this work, V, W, and As in a spent SCR catalyst was extracted by alkali pressure leaching. Results showed that the V, W, and As were loaded on the anatase TiO2 crystal grains as amorphous oxides. The optimum pressure leaching conditions were NaOH concentration of 20 wt%, reaction temperature of 180 °C, reaction time of 120 min, L/S of 10 mL/g, and stirring speed of 300 rpm. The leaching efficiency of W, V, and As reached 98.83%, 100%, and 100%, respectively. The experiment revealed the preferential leaching of V and As rather than W, and the leaching mechanisms of V, W, and As were studied through experiment and density functional theory (DFT). The leaching kinetics of W conformed to a variant of the shrinking core model and the leaching process of W is controlled by both chemical reactions and diffusion processes. During the leaching process, Na2Ti2O4(OH)2 product powder layer was generated, which affects the mass transfer of W. The destruction of the TiO2 skeleton in the spent SCR catalyst is essential for adequate W extraction, especially for the extraction of W embedded in the TiO2 lattice. The DFT simulation result indicated that the V and As loaded onto the TiO2 support are easier to absorb hydroxide ions rather than W, and the leaching reaction energy of V and As was lower than W, As, and V has leaching priority over the leaching of W. Furthermore, an anatase TiO2 photocatalyst with the {001} crystal surface exposed was successfully prepared from the alkali pressure leaching residue. This work provides theoretical support for the metal leaching and utilization of spent SCR catalysts via alkali pressure leaching.

Recent advances toward structural incorporation for stabilizing heavy metal contaminants: A critical review

Heavy metal pollution has resulted in serious environmental damage and raised significant public health concerns. One potential solution in terminal waste treatment is to structurally incorporate and immobilize heavy metals in some robust frameworks. Yet extant research offers a limited perspective on how metal incorporation behavior and stabilization mechanisms can effectively manage heavy metal-laden waste. This review sets forth detailed research on the feasibility of treatment strategies to incorporate heavy metals into structural frameworks; this paper also compares common methods and advanced characterization techniques for identifying metal stabilization mechanisms. Furthermore, this review analyses the typical hosting structures for heavy metal contaminants and metal incorporation behavior, highlighting the importance of structural features on metal speciation and immobilization efficiency. Lastly, this paper systematically summarizes key factors (i.e., intrinsic properties and external conditions) affecting metal incorporation behavior. Drawing on these impactful findings, the paper discusses future directions in the design of waste forms that efficiently, effectively treat heavy metal contaminants. By examining tailored composition-structure-property relationships in metal immobilization strategies, this review reveals possible solutions for crucial challenges in waste treatment and enhances the development of structural incorporation strategies for heavy metal immobilization in environmental applications.

Co-pyrolysis of sewage sludge as additive with phytoremediation residue on the fate of heavy metals and the carbon sequestration potential of derived biochar

Considering the characteristics of municipal sewage sludge (MS) and Sedum alfreddi L. (SA, a hyperaccumulator plant), we attempted to use MS to enhance the enrichment and stability of heavy metals (HMs) in pyrolysis residue during SA pyrolysis. The effects of pyrolysis temperature (400-800 °C) and co-pyrolysis on migration behavior, chemical speciation, long-term leaching toxicity of HMs, and the environmental risk and carbon sequestration potential of biochar were systematically investigated. Besides, thermodynamic equilibrium simulations were performed to study the transformation of HM compounds during pyrolysis. When the pyrolysis temperature increased from 400 °C to 800 °C, the unstable fractions (F1+F2) of Cd, Pb, Cu, and Cr in MS1SA3 800 had decreased to less than 6% and Zn to 20.4%, and long-term leachability of HMs decreased continuously. Meanwhile, biochar's ecological risk was reduced to a low level, while its carbon sequestration potential improved with little released HMs. Compared with SA pyrolysis alone, adding MS increased the relative residue content of Cd and Zn in biochar, whereas no apparent effect on Pb, Cu, and Cr, and the proportion of stable fractions (F3+F4) increased. Co-pyrolysis enhanced the carbon sequestration potential of biochar, attributed to the inherent minerals of MS. Equilibrium calculations showed that the influence of MS on the fate of HMs during SA pyrolysis is mainly attributed to its high sulfur content, while Si and Al preferentially combine with alkali metal (K)/alkaline earth metal (Ca) and then interact with Zn. The findings in this paper suggest that co-pyrolysis of MS as an additive with hyperaccumulator plants is a feasible proposal, and the co-pyrolysis biochar obtained at suitable temperatures has the potential for safe application.

Effect of Al modification on the adsorption of As2O3 on the CaSiO3(001) surface: A DFT study

CaSiO₃ is highly resistant to sintering and can trap arsenic at high temperatures in the boiler furnace. However, the trapping capacity of CaSiO₃ for arsenic does not meet the requirements of practical applications, and it is easy to react with acidic gases, which significantly affects the adsorptive property of arsenic. In this paper, the effect of Al modification on the As₂O₃ adsorption behaviour on the CaSiO₃(001) surface was systematically investigated using a density functional theory. By comparing the magnitude of adsorption energy of different sites, the active site of As₂O₃ adsorbed on the surface of CaSiO₃(001) was determined to be Ca, and the adsorption activity of As₂O₃ by the silicon oxygen chain composed of [SiO₄] tetrahedron is deficient. The Si atoms in the [SiO₄] tetrahedral structure are directly replaced by Al atoms, the difference in bond length and bond energy between Al-O bond and Si-O bond is used to promote the redistribution of surface charge and the increase of local structural bond angle of CaSiO₃(001), leading to the exposure of new active sites (Si-top and Al-top sites) on the silicon oxygen chain. The new active site can realize the chemical adsorption of As₂O₃, the higher adsorption energy of the Al-top site is attributed to the stronger s-p orbital hybridization between Al and O atoms after doping, which is more conducive to the charge transfer between As₂O₃ and the adsorbent surface. In this work, influence of SO₂ and HCl gases on the adsorption of As₂O₃ by modified silicon oxygen chains was also discussed. The results show that SO₂ and HCl in the flue gas may occupy the Al-top site on the silicon oxygen chain through chemical adsorption, and reduce the activity of this site, thereby affecting the adsorption of As₂O₃. However, the exposed Si-top sites owing to Al doping show good acidic gas resistance, which in turn help the surface of Al-CaSiO₃(001) can also maintain stable adsorption of As₂O₃ in SO₂ and HCl atmosphere.

Comprehensive characterization and environmental implications of spent telecommunication printed circuit boards: Towards a cleaner and sustainable environment

The management and prevention of environmental risks associated with spent telecommunications printed circuit boards (STPCBs) is a concerning issue worldwide. Recycling might be proposed as a proper method to overcome this issue. Despite knowing that, choosing a sustainable method is challenging because of STPCBs complexity. This problem was overcome by analyzing STPCBs using different analytical methods and metal speciation. Understanding these data is essential in selection strategies to maximize selective recycling of metals and to minimize environmental impact. This research focused on characterizing STPCBs based on their structural, morphological, physiochemical, surface, and thermal properties. The accurate measurement of metal contents, indicating 187,900 mg kg^-1 Cu, 22,540 mg kg^-1 Pb, 1320 mg kg^-1 Ag, and 205 mg kg^-1 Au elements, plus other base metals, revealed a remarkable potential value in STPCBs. The results of structural analyses indicated that the powder has a crystalline structure and consists of Cu, Sn and Pb phases as well as different functional groups. In addition, after evaluating the zeta potential of the sample, the isoelectric pH of the sample was observed to be 5.6, which indicates that the powder particles have a negative surface in an environment with a pH higher than this value. Further, the metal speciation via sequential extraction procedure was performed, which showed that a unique harsh recycling strategy is required due to the stable structure of STPCBs. According to the results of this analysis, the global contamination factor (GCF) value was 83.48, which indicates STPCBs have a high degree of contamination. Leaching tests and environmental criteria were also conducted on this waste. The findings suggest that STPCBs needs pretreatments before landfilling to lower the concentration of toxic metals. Also, waste extraction test was the most aggressive procedure to assess mobility. Achieving this information is considered an essential step to choosing the most efficient recycling methods that minimize environmental impact while maximizing selective recycling of metals.

Multifaceted evaluation of distribution, occurrence, and leaching features of typical heavy metals in different-sized coal gasification fine slag from Ningdong region, China: A case study

The coal gasification fine slag (CGFS) from the entrained-flow coal gasification unit faces the challenge of safe disposal and clean utilization in the Ningdong region, China. This study aims to provide complete and thorough understanding of the distribution features, chemical speciation, environmental impact, and leaching behavior of typical heavy metals (i.e., V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb) in the CGFS with different size fractions. The results show that the distribution of selected heavy metals in the CGFS has evident particle size dependence. Except for Zn, the other heavy metals in different size fractions mainly exist in chemical speciation of residual form with the ratio of 50.11-86.69 wt%. Moreover, it is found that the heavy metals in the different-sized CGFS show different RAC (risk assessment code) environmental risk levels and TCLP (Toxicity Characteristic Leaching Procedure) leaching concentrations. Especially, Zn in SGFS-C and SGFS-D posed a high-risk level to the environment, while the heavy metal elements of Cr, Mn, Ni, Zn, and Ba in other size fractions are classified as a medium environmental risk. In addition, the TCLP test results indicate that the leaching concentration of Cr, Mn, Ni, Zn, Ba, and Pb exceeds the groundwater-related regulatory limit in China. The pH-dependent leaching experiments suggest that Pb shows the amphoteric behavior, while the leaching mode of other heavy metals seems to be the cationic pattern. Furthermore, the leachability of the selected heavy metals in small-size fractions of the CGFS should be given more consideration at both acid and alkaline pH ranges. The leaching kinetic results demonstrate that the most effective mechanism to describe the leaching process of Cr, Ni, Zn, and Pb in different CGFS size fractions is the diffusion-controlled theory, which is supported by the different morphological traits of spherical mineral particles and carbon particles in the CGFS.

Spent sulfuric acid plant catalyst: valuable resource of vanadium or risky residue? Process comparison for environmental implications

The enormous amount of spent catalysts generated worldwide may pose a risk to the environment because of their high load of metals, including vanadium. The latter may be mobilized and released to the environment if managed improperly. Moreover, the catalysts could be considered as secondary resources rather than waste. This study aimed at the efficient extraction of vanadium from spent desulfurization catalyst (SDC) from a sulfuric acid production plant. The raw SDC and the post-extraction residues were characterized in terms of their chemical and phase composition. The metal mobility from the materials was examined with both single-step and multi-step extractions. The environmental risk assessment was performed using sequential extraction. The study revealed that both tested methods (citric acid leaching and bioleaching with Acidithiobacillus thiooxidans) enable the extraction of nearly 96% of V from SDC with a simultaneous reduction of metal mobility. However, the bacterial treatment was found more suitable. The leached residue was mostly (> 90%) composed of SiO2, which makes it a potential candidate for application in construction (e.g., concrete mixtures) after additional examinations. The study highlights the need to develop a metal extraction process for SDC in a way that metal-free residue could be a final product.

Penetration of Arsenic and Deactivation of a Honeycomb V2O5-WO3/TiO2 Catalyst in a Glass Furnace

Deactivation of honeycomb V2O5-WO3/TiO2 catalysts by arsenic has been studied widely in coal-fired power plants but rarely in glass furnaces. In this paper, deactivated catalysts that had been used for more than 4000 h were analyzed. We maintained the catalysts in their original monolith shape to retain their adhered substance and used appropriate methods to strip the substance layer by layer. With various characterization techniques, it was determined that the adhered substance was composed almost entirely of Na2SO4 and CaSO4. We also quantified the penetration depth of arsenic visually, which was more than 370 μm. A three-stage penetration and deactivation process induced by arsenic was proposed. It was pointed out that molten and volatile As2O3 played a key role in the deactivation process, while substances in the solid state had little impact on the deep bulk of the catalyst. In this study, we proposed an integrated deactivation process consisting of adhesion, penetration, and deactivation in a honeycomb V2O5-WO3/TiO2 catalyst by arsenic in a glass furnace. Finally, we also provided guidance on alleviating the deactivation caused by arsenic. The key is to convert molten and volatile As2O3 to solid-state substances before it contacts the catalyst.

Resource Recycling of Mn-Rich Sludge: Effective Separation of Impure Fe/Al and Recovery of High-Purity Hausmannite

Groundwater treatment sludge is a Fe/Mn-rich waste generated in mass production in a groundwater treatment plant for potable water production. The conventional disposal of sludge, such as direct discharge into river/lake, sea, and landfill, is not environmentally sustainable. Herein, a novel method was proposed to effectively separate Fe/Al and recover Mn via a combined hydrochloric acid leaching and hydrothermal route. The sludge contained 14.6% Fe, 6.3% Mn, and 11.5% Al and was first dissolved in 5 M HCl to prepare a leaching solution. Second, the leaching solution was hydrothermally treated, in which 97.1% Fe and 94.8% Al were precipitated as hematite and boehmite and more than 98% Mn was kept. Increasing the reaction temperature to 270 °C was beneficial for Fe/Al removal. With the consumption of abundant H+, the reaction of added glucose and nitrate accelerated as the temperature increased. An optimal pH was utilized for Fe/Al hydrolysis and crystallization, leading to extensive removal of Fe/Al. Third, the residual solution was adjusted to pH 8.3 with NaOH, and approximately, 99.2% Mn was removed as hausmannite with a Mn content of 63.6%. This method exhibited efficient separation of impure Fe/Al from Mn-rich groundwater treatment plant iron mud, and the recycled high-purity hausmannite was a marketable active pharmaceutical ingredient.

Comprehensive characterization and environmental risk assessment of end-of-life automotive catalytic converters to arrange a sustainable roadmap for future recycling practices

Environmentally appropriate economic recycling of spent automotive catalytic converters (SACCs) is difficult due to their complexity. The prominent reason is the lack of knowledge and comprehensive characterization of SACCs. This study focused on the characterization of SACCs in terms of their structural, morphological, physiochemical, surface, and thermal properties. The accurate determination of metals content, including 4975 mg/kg platinum group metals, 42,119 mg/kg rare earth elements, and other base metals, showed a great potential wealth in SACCs. Besides, the sequential extraction method was applied for metals fractionation, which represents a unique harsh recycling approach needed due to the stable structure of SACCs, metals embedded in silicate phases, and the presence of barely soluble metal phosphates. This waste was also examined for environmental criteria and leaching tests, including Toxicity Characteristic Leaching Procedure, Waste Extraction Test, and Synthetic Precipitation Leaching Procedure. The findings declare that Waste Extraction Test was the most aggressive procedure to assess mobility. The contamination indexes, such as risk assessment code, contamination factor, and global contamination factor, were also investigated, which show SACCs must be regarded as hazardous waste. As an example, the global contamination factor of 11.87 depicts SACCs have a moderate contamination degree.

Re-using of coal-fired fly ash for arsenic vapors in-situ retention before SCR catalyst: Experiments and mechanisms

Arsenic is easily evaporated with coal combustion, which not only causes serious environmental issues but also leads to the deactivation of selective catalytic reduction (SCR) catalyst. This study focused on the re-using of coal-fired fly ash for arsenic vapors in-situ retention before SCR catalyst in the furnace. Experiments were carried out to estimate the effects of typical fly ash compounds (Ca-, Fe-, and Al-bearing components) as well as acid gases (SO₂/NO) on arsenic capture at temperatures of high-temperature furnace stage (900 °C) or SCR system entry stage (450 °C). The results demonstrated that, regardless of collection plants, all the ash samples showed certain capacity in arsenic vapors retention and the capture performance was enhanced at 900 °C than at 450 °C. Both physical and chemical adsorptions occurred for arsenic capture at low temperature, while chemical adsorption by effective mineral components dominated at high temperature. The role of Ca-compounds was more remarkable than Fe/Al-compounds and CaSO₄/calcium silicates were identified as the key calcium compounds that acted on arsenic adsorption by fly ash. Insignificant effects were found regarding the acid gases (SO₂ and NO) on arsenic retention by fly ash owing to the high resistance of CaSO₄ and calcium silicates to acid gases. These findings provided reference for the in-situ retention of arsenic by reusing fly ash that enriched in specific compositions.

Stabilization/Solidification of Zinc- and Lead-Contaminated Soil Using Limestone Calcined Clay Cement (LC3): An Environmentally Friendly Alternative

Due to increased carbon emissions, the use of low-carbon and low-cost cementitious materials that are sustainable and effective are gaining considerable attention recently for the stabilization/solidification (S/S) of contaminated soils. The current study presents the laboratory investigation of low-carbon/cost cementitious material known as limestone-calcined clay cement (LC3) for the potential S/S of Zn- and Pb-contaminated soils. The S/S performance of the LC3 binder on Zn- and Pb-contaminated soil was determined via pH, compressive strength, toxicity leaching, chemical speciation, and X-ray powder diffraction (XRPD) analyses. The results indicate that immobilization efficiency of Zn and Pb was solely dependent on the pH of the soil. In fact, with the increase in the pH values after 14 days, the compressive strength was increased to 2.5–3 times compared to untreated soil. The S/S efficiency was approximately 88% and 99%, with increase in the residual phases up to 67% and 58% for Zn and Pb, respectively, after 28 days of curing. The increase in the immobilization efficiency and strength was supported by the XRPD analysis in forming insoluble metals hydroxides such as zincwoodwardite, shannonite, portlandite, haturite, anorthite, ettringite (Aft), and calcite. Therefore, LC3 was shown to offer green and sustainable remediation of Zn- and Pb-contaminated soils, while the treated soil can also be used as safe and environmentally friendly construction material.

Power production waste

In this paper, the research literatures published in 2018 about power production waste generated from the coal-fired power plants and nuclear power plants are reviewed. The wastes from coal-fired plants include fly ash, flue gas desulfurization gypsum, spent selective catalytic reduction catalyst, hazardous trace elements, and the management, reuse, and disposal of these wastes are discussed. The treatment and disposal of wastes generated from nuclear power plants is also considered. PRACTITIONER POINTS: Reuse and disposal of coal-fired and nuclear power plant waste is discussed in this annual literature review. Emission of hazardous trace elements from coal-fired power plant is summarized. Radiological contaminant removal and radionuclides waste disposal is essential for nuclear power plant.