Toxicity Characteristic Leaching Procedure Research Articles

Solidification/stabilization of lead-contaminated soil using alkali-activated volcanic ash

The bioaccumulation of lead in soil poses a significant human health risk. The solidification/stabilization (S/S) technique, employing binders like Portland cement or lime, is a common method for remediating lead-contaminated soil. However, cement production has adverse environmental impacts, prompting the exploration of eco-friendly alternatives like alkali-activated materials (AAMs). This study assesses AAM efficacy in the S/S of lead-contaminated soil. The effects of several factors, including varying amounts of volcanic ash (VA), lead concentration, curing temperatures, and curing times are investigated. Unconfined compressive strength (UCS), toxicity characteristic leaching procedure test (TCLP), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and field emission scanning electron microscope-energy-dispersive spectroscopy-mapping analyses (FESEM/EDS/mapping) analyses are used to study the specimens. The findings indicated a substantial increase in the UCS of lead-contaminated soil treated with 15% VA (under oven curing (OC) conditions), and 10% VA (under ambient curing (AC) conditions) exhibited remarkable increases of up to 600% and 458%, respectively. Moreover, the leaching of Pb2+ ions from samples contaminated with 10,000 mg/kg (under OC conditions) and 2500 mg/kg (under AC conditions) experienced significant reductions of 87% (from 135.14 to 13.36 ppm) and 91% (from 26.32 to 2.21 ppm), respectively. The S/S process in these samples operated through three primary mechanisms of chemical bonding, physical encapsulation, and the formation of insoluble silicate. The formation of N-A-S–H and hydroxy sodalite structures played a vital role in facilitating these mechanisms. Therefore, alkali-activated VA demonstrated excellent performance in the remediation of lead-contaminated soil.Graphical

Evaluation of asphalt film thickness and heavy metal leaching of oxidizing slag used as an aggregate material in dense-graded asphalt concrete.

Electric-arc-furnace (EAF) steelmaking uses scrap iron and steel as raw materials. Scrap iron and steel originate from complex sources and may contain heavy metal components which can leach into the environment over time due to wear-and-tear. A by-product of the EAF steelmaking process is oxidizing slag, and approximately 1.2 million metric tons is produced every year in Taiwan alone. This study investigated substitution of natural aggregates with oxidizing slag in dense-graded asphalt concrete. We evaluated the water resistance and asphalt film thickness of the oxidizing slag substituted asphalt concrete and further explored the performance of oxidizing slag as paving material. We determined the dissolved and total amounts of heavy metals in the oxidizing slag, comparing these results with current regulatory controls to assess the environmental compatibility of the oxidizing slag. We found that due to the complicated sources of oxidizing slag, the basic properties should be analyzed on a batch-to-batch basis. Furthermore, we recommend trial mixing before upscaling the production of oxidizing slag substituted dense-graded asphalt concrete to confirm the mixing time required to achieve uniformity. The results also show that in comparison to natural aggregates used in asphalt concrete, oxidizing slag exhibits superior performance in terms of increased asphalt film thickness and improved water resistance. Furthermore, oxidizing slag as an aggregate material was associated with decreased heavy metal leaching and reduced environmental pollution. The results of the toxicity characteristic leaching procedure (TCLP) met regulatory requirements. However, the microwave-assisted aqua-regia digestion procedure showed heavy metal concentrations exceeding the monitoring standards for food crops. Considering environmental compatibility, it is recommended that controlling the total amount of heavy metals in oxidizing slag should be included in regulatory requirements. Furthermore, we should prohibit the use of materials such as oxidizing slag and other steel furnace slag in the roadways adjacent to edible crop farmlands.

Microbiologically induced calcite precipitation (MICP) in situ remediated heavy metal contamination in sludge nutrient soil

Microbiologically induced calcite precipitation (MICP), as a newly developing bioremediation technology, could redeem heavy metal contamination in diverse scenarios. In this study, MICP bacterium Sporosarcina ureilytica ML-2 was employed to suppress the pollution of Pb, Cd and Zn in municipal sludge nutrient soil. After MICP remediation, the exchangeable Cd and Zn in sludge nutrient soil were correspondingly reduced by 31.02 % and 6.09 %, while the carbonate-bound Pb, Cd and Zn as well as the residual fractions were increased by 16.12 %, 6.63 %, 13.09 % and 6.10 %, 45.70 %, 3.86 %, respectively. In addition, the extractable Pb, Cd and Zn either by diethylenetriaminepentaacetic acid (DTPA) or toxicity characteristic leaching procedure (TCLP) in sludge nutrient soil were significantly reduced. These results demonstrated that the bio-calcite generated via MICP helped to immobilize heavy metals. Furthermore, MICP treatment improved the abundance of functional microorganisms related to urea cycle, while reduced the overall abundance of metal resistance genes (MRGs) and antibiotic resistance genes (ARGs). This work confirmed the feasibility of MICP in remediation of heavy metal in sludge nutrient soil, which expanded the application field of MICP and provided a promising way for heavy metal pollution management.

Sustainable Application of Pyrolytic Oxygen Furnace Slag in Cement-Stabilized Macadam: Volume Stability, Mechanical Properties, and Environmental Impact

As an industrial waste, basic oxygen furnace (BOF) slag is an ideal substitute for natural rubble and sand. However, its potential instability of volume restricts the application of the BOF slag in engineering. This study aims at investigating the volume stability and mechanical properties of BOF slag and its application as an aggregate in cement-stabilized macadam. As part of this research, the physicochemical properties, especially the volume stability, of two types of BOF slags and andesite were first studied. Then, mechanical properties, volume stability, and an environment analysis are used to evaluate the application of pyrolytic BOF slag in cement-stabilized macadam. The experimental results show that different types of BOF slags have similar thermal expansion coefficients, which are higher than andesite. The free CaO content of pyrolytic BOF slag is much lower than that of ordinary BOF slag and the volume expansion of pyrolytic BOF slag is less than 0.5%. The unconfined compressive strength (UCS) of cement-stabilized macadam using pyrolytic BOF slag is about 30% higher than that of andesite. Although the water loss rate is higher than a natural aggregate, dry shrinkage of pyrolytic BOF slag cement-stabilized macadam is about 30–50% less than that of a natural aggregate. Meanwhile, its shrinking speed is also slower than that of a natural aggregate. The micro-expansion properties of pyrolytic BOF slag could effectively partially offset the shrinkage characteristics of cement-stabilized macadam. Finally, the Toxicity Characteristic Leaching Procedure (TCLP) test results indicated that the metal leaching concentration meets the Chinese environmental standards. This study provides a direction for the large-scale and effective sustainable application of pyrolytic BOF slag.

A comparative life cycle assessment (LCA), life cycle cost analysis (LCCA), mechanical and long-term leaching evaluation of road pavement structures containing multiple secondary materials

Oil-Based Drill Cuttings (OBDC) are one of the hazardous wastes of the oil and gas industry. In this study, Reclaimed Asphalt Pavement (RAP), OBDC and Fly Ash (FA) geopolymer were used in Cold Central Plant Recycling (CCPR) mixtures. The dynamic modulus test was used to appraise the viscoelastic properties of CCPR mixtures. The Toxicity Characteristic Leaching Procedure (TCLP) test was also utilized to investigate the long-term contaminants leaching. In addition, a cradle-to-grave Life Cycle Assessment (LCA) and Life Cycle Cost Analysis (LCCA) were conducted to evaluate the potential environmental burdens and economic costs of pavement structures containing CCPR mixtures. Moreover, Multi-Criteria Decision-Making (MCDM) analysis using four different methods was performed to determine the optimal sample. The results revealed that the recycling of OBDC and RAP in stabilized CCPR mixtures with FA geopolymer can improve the viscoelastic properties and prevent the leaching of contaminants from these waste materials. Furthermore, recycling of the mentioned waste materials in CCPR mixtures has caused a significant reduction in global warming, acidification, eutrophication, smog, respiratory effects, Human toxicity, ecotoxicity, and cumulative energy demand up to 41%, 36.6%, 73%, 43%, 47%, 34%, 73%, and 13%, respectively. Additionally, using CCPR mixtures containing the OBCD, RAP, and FA geopolymer in the pavement structure has led to economic benefits of up to 31% compared to the conventional pavement system.

Co-pyrolysis of alkali-fused fly ash and corn stover to synthesize biochar composites for remediating lead-contaminated soil

Fly ash (FA) is mainly composed of silica, alumina, and other metal oxide components, and has a positive stabilizing effect on soil heavy metals. Biochar composites produced from FA and corn stover (CS) can improve its remediation performance. Therefore, a batch of biochar composites (alkali-fused FA-CS biochars, ABs), synthesized via co-pyrolysis of CS and alkali-fused FA (AFFA) at different temperatures of 300, 500, and 700 °C (AB300-1, AB500-1, and AB700-1) and CS to AFFA mass ratios of 10:1, 10:2, and 10:5 (AB500-1, AB500-2, and AB500-5), was used to remediate lead (Pb)-contaminated soil. Compared with pristine biochars (BCs), ABs were enriched with oxygen-containing functional groups (Si–O–Si and Si–O) and aromatic structures. The ABs prepared at lower pyrolytic temperature (≤500 °C) and lower ratio of CS to AFFA (10:1) showed higher yield and stability. The contents of Toxicity Characteristic Leaching Procedure (TCLP)-extractable Pb and DTPA-CaCl2-triethanolamine (DTPA)-extractable Pb were generally lower in the soils amended with ABs than BCs. Compared with other ABs such as AB300-1, AB500-2, AB500-5, and AB700-1, the soil amended with AB500-1 had lower contents of TCLP and DTPA-extractable Pb (24% reduction), exhibiting superior performance in stabilizing Pb in the soil. The gradual decrease of DTPA-extractable Pb content in the soil with increasing dosage of AB500-1 amendments suggests that AB500-1 facilitated the conversion of bioavailable Pb to the stable and less toxic residual fractions. Specifically, the highest percentage of residual fraction of Pb in soil amended with AB500-1 was 14%. Correlation analyses showed that the soil DTPA-extractable Pb content decreased with the increase of soil pH and cation-exchange capacity (CEC) value. ABs stabilize Pb in the soils mainly via electrostatic attraction, precipitation, cation-π interaction, cation exchange, and complexation. These findings provide insights for producing functionalized biochar composites from industrial waste like FA and biomass waste for remediating the soils polluted by heavy metals.

Evaluating the effectiveness of sulfidated nano zerovalent iron and sludge co-application for reducing metal mobility in contaminated soil

Sewage sludge has long been applied to soils as a fertilizer yet may be enriched with leachable metal(loid)s and other pollutants. Sulfidated nanoscale zerovalent iron (S-nZVI) has proven effective at metal sorption; however, risks associated with the use of engineered nanoparticles cannot be neglected. This study investigated the effects of the co-application of composted sewage sludge with S-nZVI for the stabilization of Cd, Pb, Fe, Zn. Five treatments (control, Fe grit, composted sludge, S-nZVI, composted sludge and S-nZVI), two leaching fluids; synthetic precipitation leaching procedure (SPLP) and toxicity characteristic leaching procedure (TCLP) fluid were used, samples were incubated at different time intervals of 1 week, 1, 3, and 6 months. Fe grit proved most efficient in reducing the concentration of extractable metals in the batch experiment; the mixture of composted sludge and S-nZVI was the most effective in reducing the leachability of metals in the column systems, while S-nZVI was the most efficient for reducing about 80% of Zn concentration in soil solution. Thus, the combination of two amendments, S-nZVI incorporated with composted sewage sludge and Fe grit proved most effective at reducing metal leaching and possibly lowering the associated risks. Future work should investigate the longer-term efficiency of this combination.

An environmentally benign process for effective recovery and solidification of Cr from stainless-steel slag

Stainless-steel slag is a hazardous solid that has caused severe chromium contamination globally. This study proposes an environmentally benign vitrification process for efficient recycling and solidification of Cr from stainless-steel slag using waste copper slag and coal tar residue. The transformation behaviors of heavy metals (Cr, Cu, Pb, Zn) and the Cr toxic leaching mechanism are also determined in this work. The findings indicated that compared with Cu and Fe phases, the Mg(Al,Cr)2O4 and chemically dissolved Cr phases were not easily reduced via coal tar residue, leading to the leaching of highly toxic Cr3+/Cr6+ during the toxicity characteristic leaching procedure (TCLP). Under optimal conditions, 95.56% Cr, 97.71% Fe, and 99.71% Cu could be recovered in the form of high value-added crude stainless-steel, and 99.62% Pb and 99.19% Zn could be enriched in the dust. The furnace slag TCLP result indicated that the leaching concentrations of Cr, Cu, Pb and Zn were 1.82 mg/L, 0.1 mg/L, 0.38 mg/L, and 2.58 mg/L, respectively. More importantly, the concentrations of residual Cr, Cu, Pb and Zn in the furnace slag were only 0.095 wt%, 0.028 wt%, 0.0192 wt%, and 0.011 wt%, respectively. These findings indicate that treated slag can be categorized as general solid waste, minimizing the environmental risks of heavy metals. The results of this study are significant for the sustainable development of the stainless-steel industry and copper metallurgy.

Remediation of Cd(II), Zn(II) and Pb(II) in contaminated soil by KMnO4 modified biochar: Stabilization efficiency and effects of freeze–thaw ageing

Biochar has been widely used in soil remediation because of its porosity and impressive efficiency. Many studies have demonstrated the good affinity of biochar for heavy metals (HMs), but efficient modification methods to increase the stability of multiple HMs are lacking. Moreover, few studies focus on biochar’s potential ability to ageing resistance and changes in its physicochemical properties during freeze–thaw (F/T) cycles. Here, KMnO4-modification biochar (MBC) was prepared and the immobilization and anti-freeze performance of MBC on Cd, Zn and Pb in soil were investigated. The addition of 10 % MBC decreased concentrations of toxicity characteristic leaching procedure (TCLP) by 66.80 %, 25.39 % and 99.68 % for Cd, Zn and Pb, respectively, after 28 days, accompanied by a significant decrease in the phytoavailability and bioaccessibility of HMs in soil. Soil fertility was also improved by MBC application. Furthermore, frequent F/T cycles obviously induced the fragmentation of soil particles and enhanced the activity of HMs. Conversely, MBC particles became finer, accompanied by further oxidation and higher alkalinity but degraded pore structures during F/T cycles. The oxidation of lignocellulose and changes in C-O stretching were greatest among all functional groups in biochar. Therefore, MBC could suppress the migration of HMs through precipitation, cohesion of soil particles, and stronger complexation by oxygen-containing functional groups. Moreover, fractions of HMs were also transferred from exchangeable to potentially labile or nonlabile species. Overall, MBC displayed advantages in the stabilization of HMs and showed higher resistance to F/T cycles and promise for long-term remediation of HM-contaminated soils in cold regions.

Removal of heavy metals in water-extracted solution through adsorption by palygorskite and stabilization by comilling.

Removing water-soluble chlorides (WSCs) through water extraction is a common pretreatment technology for recycling municipal solid waste incineration (MSWI) fly ash (FA). However, the extracted solution often contains heavy metals, the concentrations of which exceed standards for effluent. This study aims to investigate the adsorption of heavy metals by palygorskite in water-extracted solution and explore the feasibility of stabilizing heavy metals through comilling palygorskite-adsorbed heavy metals (PAHMs) with water-extracted fly ash (WFA). The experimental parameters include: two-stage water extraction with a liquid-to-solid ratio of 5, adding 0, 0.125, 0.25, 0.5, 1, 2 or 3 g of palygorskite to 100 mL of water-extracted solution, and comilling the mixture of PAHMs and WFA for 0, 0.5, 1, 2, 4, 8, 12, 24 or 96 hours. The experimental results revealed that 3 g of palygorskite in 100 mL of extracted solution could absorb Pb, Cd, Cr, Cu and Zn, meeting the effluent standards. The total amount of Pb, Cd, Cr, Cu and Zn removal rate reached 99.7%. Moreover, 98.44% of the WSCs were not adsorbed, the water extraction process for removing WSCs was not compromised. After the comilling of PAHMs and WFA, the distribution of the heavy metals in the milled blended powder was greater than 99.44%; moreover, toxicity characteristic leaching procedure concentrations were determined to conform to regulatory standards, and the sequential extraction procedure revealed that the heavy metals tended to be in stable fractions. This achieves the goal of preventing secondary pollution from heavy metals during the MSWI FA recycling process.

Solidification and stabilization of heavy metals in medical waste ash through alkali activation

In the present work, medical waste ash was used as an aluminosilicate precursor to produce a geopolymer binder with stabilised heavy metals. The ash was initially calcined at a temperature of 800 °C for two hours to reduce the organic content. Calcined kaolin and medical waste ash were used as precursors to produce the geopolymer binder and sodium silicate and calcium oxides were used as alkaline activators. The effect of curing temperature (23, 60, and 100 °C) and the calcium oxide addition (0, 5, and 10%) on the geopolymer properties were evaluated. Eighteen geopolymer samples were prepared and evaluated for immobilizing heavy metals through the Toxicity Characteristic Leaching Procedure (TCLP). When compared to raw materials, TCLP results indicated that geopolymerization can significantly reduce the concentration of heavy metals in the medical waste ash leachate. The metal fixation percentages of the geopolymers ranged from 70 to 100%. The sequential extraction test results showed that geopolymerization (stabilization/solidification) is very beneficial in reducing the bioavailable fraction in the solid waste and maximizing the difficulty of fraction extraction.

Assessment of the stabilization effect of ferrous sulfate for arsenic-contaminated soils based on chemical extraction methods and in vitro methods: Methodological differences and linkages

Stabilization of arsenic-contaminated soils with ferrous sulfate has been reported in many studies, but there are few stabilization effects assessments simultaneously combined chemical extraction methods and in vitro methods, and further explored the corresponding alternative relationships. In this study, ferrous sulfate was added at FeAs molar ratio of 0, 5, 10 and 20 to stabilize As in 10 As spiked soils. Stabilization effects were assessed by 6 chemical extraction methods (toxicity characteristic leaching procedures (TCLP), HCl, diethylenetriamine pentaacetic acid (DTPA), CaCl2, CH3COONH4, (NH4)2SO4), and 4 in vitro methods (physiologically based extraction test (PBET), in vitro gastrointestinal method (IVG), Solubility Bioaccessibility Research Consortium (SBRC) method, and the Unified Bioaccessibility Research Group of Europe method (UBM)). The results showed that the HCl method provides the most conservative assessment results in non-calcareous soils, and in alkaline calcareous soils, (NH4)2SO4 method provides a more conservative assessment. In vitro methods provided significantly higher As concentrations than chemical extraction methods. The components of the simulated digestion solution as well as the parameters may have contributed to this result. The small intestinal phase of PBET and SBRC method produced the highest and lowest ranges of As concentrations, and in the range of 127–462 mg/kg and 68–222 mg/kg when the FeAs molar ratio was 5. So the small intestinal phase of PBET method may provide the most conservative assessment results, while the same phase of SBRC may underestimate the human health risks of As in stabilized soil by 51 %(at a FeAs molar ratio of 5). Spearman correlation analysis indicated that the small intestinal phase of PBET method correlated best with HCl method (correlation coefficient: 0.71). This study provides ideas for the assessment of stabilization efforts to ensure that stabilization meets ecological needs while also being less harmful to humans.

Proposal of an Encapsulation-led Disposal Method for Tannery Sludge – A Circularity Approach

In the context of achieving sustainable development goals, continuously pushing possible extended research and attempting to implement the respective outcomes in expanding a circular economy for a broad range of products are high priorities. In this paper, we considered disposable tannery sludge as a matter of concern and proposed an innovative framework for placing tannery sludge in the economic value chain via an encapsulation technique-based construction product development. We used polypropylene plastic and a cement-water matrix as encapsulation materials, and the encapsulated tannery sludge bodies were tested for their drop strength, water absorbing behaviour, ability to leach chromium and tendency to lose weight upon ignition. Value of water absorption for the prepared double layered encapsulated tannery sludge bodies was 1.332%, the drop strength performance index arrived was 90% and 0.0001 µg/g of Chromium leaching was found in Toxicity Characteristic Leaching Procedure (TCLP) and shown 67% weight loss in thermogravimetric (TGA) analysis. The results confirmed the possibility of ecocompatible disposal and recirculation of tannery sludge for the sustainable production of building blocks in the form of encapsulated bodies. The outcomes of our work add upon a new perspective to the existing literature regarding the environmentally positive utilization of tannery sludge in the production of building blocks.

3D printed concrete with sewage sludge ash: Fresh and hardened properties

In this study, the possibility of application of sewage sludge ash (SSA) in 3D printed concrete (3DCP) technology was explored. The replacement of partial cement with SSA in the concrete mix design was investigated, followed by a comprehensive analysis of its impact on the workability, printability, and mechanical properties of the 3DCP. The findings indicated that the incorporation of SSA accelerated the loss of workability of 3DCP, such as fluidity and consistency. However, it significantly improved the green strength and buildability. Further investigation revealed that the addition of SSA hampered the hydration process of cement, resulting in a reduction in the mechanical properties of the hardened material. CT scanning indicated that the addition of SSA increased the interlayer porosity, leading to further degradation of the mechanical properties. The toxicity characteristic leaching procedure demonstrated that the maximum leaching concentration of 3DCP incorporating SSA was 0.14 mg/L. This result confirmed the effective immobilization of heavy metals, fulfilling the prerequisites for the application of 3DCP incorporating SSA.

Biogenic calcium improved Cd2+ and Pb2+ immobilization in soil using the ureolytic bacteria Bacillus pasteurii

Bioremediation based on microbial-induced carbonate precipitation (MICP) was conducted in cadmium and lead contaminated soil to investigate the effects of MICP on Cd and Pb in soil. In this study, soil indigenous nitrogen was shown to induce MICP to stabilize heavy metals without inputting exogenous urea. The results showed that applying Bacillus pasteurii coupled with CaCl2 reduced Cd and Pb bioavailability, which could be clarified through the proportion of exchangeable Cd and Pb in soil decreasing by 23.65 % and 12.76 %, respectively. Moreover, B. pasteurii was combined separately with hydroxyapatite (HAP), eggshells (ES), and oyster shells (OS) to investigate their effects on soil heavy metals' chemical fractions, toxicity characteristic leaching procedure (TCLP)-extractable Cd and Pb as well as enzymatic activity. Results showed that applying B. pasteurii in soil significantly decreased the heavy metals in the exchangeable fraction and increased them in the carbonate phase fraction. When B. pasteurii was combined with ES and OS, the content of carbonate-bound Cd increased by 114.72 % and 118.81 %, respectively, significantly higher than when B. pasteurii was combined with HAP, wherein the fraction of carbonate-bound Cd increased by 86 %. The combination of B. pasteurii and biogenic calcium effectively reduced the leached contents of Cd and Pb in soil, and the TCLP-extractable Cd and Pb fractions decreased by 43.88 % and 30.66 %, respectively, in the BP + ES group and by 52.60 % and 41.77 %, respectively, in the BP + OS group. This proved that MICP reduced heavy metal bioavailability in the soil. Meanwhile, applying B. pasteurii and calcium materials significantly increased the soil urease enzyme activity. The microstructure and chemical composition of the soil samples were studied, and the results from scanning electron microscope, Fourier transform infra-red spectroscopy, and X-ray diffraction demonstrated the MICP process and identified the formation of CaCO3, Ca0.67Cd0.33CO3, and PbCO3 in heavy metal-contaminated soil.

Bioleaching of sewage sludge for copper extraction using Acidithiobacillus thiooxidans: Optimization and ecological risk assessment

In this study, Acidithiobacillus thiooxidans was used for the bioleaching of copper (Cu) from sewage sludge. In order to find optimization conditions, three factors including solid-to-liquid ratio (S/L) (0.01-0.2 %(w/v)), initial element sulfur (S0) (1–10 g/L), and initial pH (1–3) have been investigated. Based on response surface methodology (RSM) determined a significant reduced quadratic model with a p-value of 0.0022 (<0.05 significant level). The maximum Cu recovery was 85.3% in the optimum condition of S/L = 0.16% (w/v), S0 = 8.2 g/L, and pH = 1.4. Furthermore, a kinetic study based on a shrinking core model was performed and the result showed that chemical reaction was rate limiting in the extraction. Toxicity Characteristic Leaching Procedure (TCLP) results after bioleaching showed the bioleaching process detoxified sludge and the bioleached sludge residue was well within the regulatory limits for disposal. The germination seed with adding bioleached and unbioleached sludge to the soil was determined. Various parameters such as Germination Index (GI), Tolerance Index (TI), Vigor Index (VI), and stem length showed that the sewage sludge indices significantly increased than the sample soil with unbioleached sludge.

Pyrolysis of exhausted biochar sorbent: Fates of cadmium and generation of products

Biochar is a promising sorbent for Cd removal from water, while the disposal of the exhausted Cd-enriched biochar remains a challenge. In this study, pyrolysis was employed to treat the exhausted biochar under N2 and CO2 atmospheres at 600–900 °C, and the fate of Cd during pyrolysis and characteristics of high-valued products were determined. The results indicated that higher temperature and CO2 atmosphere favored the volatilization of Cd. Based on the toxicity characteristic leaching procedure (TCLP) results, the pyrolysis treatment under both atmospheres enhanced the stability of Cd, and the leached Cd concentration of regenerated biochar obtained at high temperatures (>800 °C) was lower than 1 mg/L. Compared with the pristine biochar, the regenerated biochar demonstrated higher carbon content and pH, whereas the contents of oxygen and hydrogen declined, and exhibited promising sorption properties (35.79 mg/g). The atmosphere played an important role in modifying biochar properties and syngas composition. The N2 atmosphere facilitated CH4 production, whereas the CO2 atmosphere increased the proportion of CO. These results implied that pyrolysis can be a valuable and environmental-friendly strategy for the treatment and reuse of exhausted biochar sorbent.

Sustainable use of ferrochrome slag and quarry dust composites as cement-bound layers of flexible pavement

&lt;p&gt;In this study, an attempt was made to characterize ferrochrome slag as cement-bound layers of flexible pavement in view of its leaching behavior. Toxicity Characteristic Leaching Procedure (TCLP) test was conducted to assess the mobility of toxic elements in ferrochrome slag. Ferrochrome slag aggregate was classified as a non-hazardous material that has good physical and mechanical properties. The standards for cement stabilization set forth by the MoRTH can be met by two gradations, ferrochrome slag(F) and quarry dust(Q) in the ratios of 70/30 and 60/40(F/Q) respectively. Cylindrical specimens of cement contents (2%, 4%, and 6% dry weight, respectively) were cast at optimum moisture content and maximum dry density for 7 and 28 days of curing to perform unconfined compressive strength. The cylindrical specimens are subjected to a series of cycles consisting of alternative wetting and drying, and the corresponding weight loss for each cycle is recorded in order to evaluate the effect of subsequent cycles. The strength of cement-treated ferrochrome slag and quarry dust mixtures was measured before and after they were subjected to durability testing. The cement-treated F60Q40 mix showed better performance than the F70Q30 mix and it can be efficiently utilized as a replacement for granular sub-base and base courses of flexible pavement, thereby reducing leaching and environmental pollution.&lt;/p&gt;&#x0D;

Utilization of phosphoric acid-modified biochar to reduce vanadium leaching potential and bioavailability in soil

Remediating vanadium (V) polluted soil has garnered widespread attention over the past decade. Yet, few research projects have investigated the stabilization of soil V using modified biochar, so the effects and interacting mechanisms between soil properties and modified biochar for V immobilization and stabilization remain unclear. Hence, this gap is addressed by determining the leaching behavior and mechanisms of soil V on different dosages of phosphoric acid (H3PO4) impregnated biochar (MLBC, 0.5%–4%). The applicability and durability in soil V immobilization was investigated under acid precipitation. The MLBC effect on V bioavailability and mobility was assessed first by CaCl2, Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) extractions in different periods. The V concentrations significantly reduced in CaCl2, TCLP, and SPLP extract with MLBC at each dosage (30 d), while slight to significant increase in SPLP and TCLP extract V was recorded in a long-term incubation (90 d). Column leaching test further demonstrated the high durability of 4% MLBC in V stabilization under continuous acid exposure. Compared to the control (no-biochar), the accumulated V content in the leaching solution significantly decreased in MLBC-amended soil. Acid soluble fraction of V showed significant negative correlation with both soil organic matter (SOM) and available P, which was positively correlated with pH, suggested that pH, available P and SOM were key factors affecting the bioavailability of V in soil. Moreover, combining with the characterization results of MLBC and amended soil, the results revealed that H3PO4 modified biochar played a vital role on V immobilization and soil improvement by forming electrostatic adsorption, ion exchange, redox reaction or complexation with the increase of functional groups. These revealed an efficient and steady development of soil quality and treatment for soil V contamination, under MLBC operation to soil polluted with exogenous V.

Strength and leaching behavior of tailing-based paste backfill at high water content amended with lime activated ground granulated blast furnace slag and flocculant.

Flocculent is commonly used in mining activities to improve the concentration of tailing slurry by enhancing the sedimentation process of small tailings particles. The presence of flocculent in thickened tailings is unavoidable, and it affects the heavy metal leaching performances and mechanical and rheological characteristics of tailing-based cemented paste backfill (CPB). This study is carried out to investigate the physicochemical and leachability of CPB amended with flocculants and lime-activated ground granulated blast-furnace slag (GGBS). The stabilized samples were subjected to a series of model tests, including toxicity characteristics leaching procedure (TCLP) and pH, unconfined compressive strength (UCS), scanning electron microscopy (SEM), and X-ray diffraction. Moreover, the CPB amended with anionic polyacrylamide (APAM) demonstrated better performance in terms of a decrease in heavy metal leachability besides higher mechanical strength than poly aluminum chloride (PAC) and poly ferric chloride (PFC) samples. Furthermore, the UCS results showed that increasing binder content up to 15% negatively influences strength improvement of all stabilized samples because of weak connections between soil particles and cementitious material, resulting in high leachability of heavy metals. The analysis of XRD and SEM showed that anionic polyacrylamide (APAM) cases exhibited more voluminous hydration products, resulting in a compact stabilized matrix and substantially reduced heavy metal leachability.