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Abstract\nSTATEMENT OF NOVELTYSoil heavy metal contamination and sludge disposal have become globally environmental issues problems of great concern. Utilizing sludge pyrolysis to produce biochar for remediating heavy metal-contaminated soil is an effective strategy to solve these two environmental problems. In this study, municipal sewage sludge and papermaking sludge were used as feedstock to prepare co-pyrolyzed biochar, which was then applied to reduce the toxicity of Cd in soil. The results indicated that the application of co-pyrolyzed biochar significantly increased soil pH, CEC, and enzyme activity, while decreasing the content of available Cd in the soil. Following the application of 3% co-pyrolyzed biochar, the proportion of acid-soluble Cd in the soil decreased to below 46%, as the biochar facilitated the conversion of leachable acid-soluble Cd to stable oxidizable and residual forms through precipitation and complexation. The DFT computational results indicate that the aromatics in co-pyrolyzed biochar can adsorb Cd ions through cation-π interactions, while carboxyl, hydroxyl, aldehyde, and amide groups can provide more electrons for the adsorption of Cd ions, resulting in stronger adsorption capacities. The study findings provide a feasible solution for the resourceful treatment of sludge and the remediation of heavy metal-contaminated soil.This study is the first to investigate the remediation potential of municipal sludge and papermaking sludge co-pyrolyzed biochar for Cd-contaminated soil. Additionally, density functional theory (DFT) calculations were employed to elucidate the microscopic adsorption mechanisms of Cd ions by the co-pyrolysis biochar. These findings provide a theoretical foundation for the resource utilization of sludge and the development of cost-effective soil remediation agents. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heavy metals can controlled well in solid waste/sludge pyrolysis by vermiculite. • Interlayer spacing of vermiculite is positively correlated with heavy metal retention. • Heavy metals except Cd are at low potential ecological risk and 3IV reduces this risk. • Heavy metal oxides react more readily with vermiculite flakes than their chlorides. • The reactivity of Cr/Zn with vermiculite flakes is stronger than those of Cd/Pb/Cu. Pyrolysis is effective in reducing the volume of solid waste and sludge, and produces less pollutants than incineration and landfill, but the process still suffers from heavy metal pollution. Four types of intercalated-exfoliated modified vermiculite (UIV, DIV, TIV and 3IV) were prepared using urea, dimethylsulfoxide, tributyl phosphate and 3-aminopropyltriethoxysilane as intercalators for the control of Cd, Cr, Cu, Pb and Zn in municipal sewage sludge (MSL), paper mill sludge (PML), municipal domestic waste (MWA) and aged refuse (AFE). The larger the interlayer spacing of the vermiculite, the more favorable the retention of heavy metals. 3IV was the most effective additive, with an average retention of more than 75 % of all heavy metals at 450 ℃ for the four raw materials. Cr, Cu, Pb and Zn were all at low potential ecological risk (Pr), while Cd was moderate or considerable Pr, and the addition of 3IV reduced the Pr. Distribution of intercalators between vermiculite interlayers was haphazard, and interlayer spacing results were close to those of the experiment (except for tributyl phosphate). The reactive electrons mainly flowed from the Highest Occupied Molecular Orbital (HOMO) of vermiculite flakes to the Lower Unoccupied Molecular Orbital (LUMO) of heavy metal chlorides. In contrast, the reactive electrons mostly flowed from the HOMO of heavy metal oxides to the LUMO of vermiculite flakes. Heavy metal oxides were more readily adsorbed on vermiculite flakes than heavy metal chlorides, and the adsorption capacity of Cr and Zn was stronger than that of Cd, Pb and Cu. [ABSTRACT FROM AUTHOR]

• Adding kaolin reduces the volatility and leachability of heavy metals. • Xerogel with high porosity was successfully synthesized for heavy metal adsorption. • Silica-alumina xerogel can effectively adsorb heavy metals in pyrolysis gas. • Heavy metal species can form ionic or covalent bonds with kaolinite. It is of great significance to reduce the secondary risk of heavy metals during the pyrolysis of paper sludge. This study used kaolin and alumina-silica-based xerogels to control heavy metals released during sludge pyrolysis. Pyrolyzing a mixture of sludge and 7% kaolin at 400 °C achieved high retention rates for Cu (95.85%), Zn (95.97%), Pb (97.15%), Cd (84.23%), and Cr (84.05%) when the pyrolysis tail gas was treated with 9 g of xerogel. The addition of kaolin facilitated the transformation of Cu, Zn, Pb, and Cr from the unstable fraction to the stable fraction in pyrolysis biochar, reducing their leachability. The xerogels also played a crucial role in adsorbing and stabilizing the heavy metals. The results of thermodynamic equilibrium calculations showed that Pb(g), PbS(g), PbCl 2 (g), PbCl(g), Zn(g), ZnCl 2 (g), and Cd(g) were the main gaseous products of Zn, Pb, and Cd during paper sludge pyrolysis. The Pb atoms in PbCl 2 and PbS, and the Zn atoms in ZnCl 2 bond with the oxygen atoms on the kaolin surface by covalent bonds, while the Cl atoms in PbCl and the Pb atoms of elemental lead form ionic bonds with H and O atoms on the kaolinite surface, respectively. These experimental and simulation results offer new ideas for controlling heavy metals during sludge pyrolysis. [ABSTRACT FROM AUTHOR]

The co-pyrolysis of municipal sludge (MS) and papermaking sludge (PS) is beneficial for improving the quality of co-pyrolysis biochar and the stability of heavy metals in it. This study investigated the synergistic effect of co-pyrolysis of MS and PS, as well as the influence of different mass ratios (1:9, 3:7, 5:5, 7:3, and 9:1; MS:PS) and pyrolysis temperatures (400–600 ℃) on the physicochemical properties of pyrolysis char, chemical speciation of heavy metals, and their potential ecological risk index. Furthermore, thermodynamic equilibrium calculations were used to study the transformation and stabilization mechanisms of heavy metals during co-pyrolysis. The results indicated that the addition of PS increased the comprehensive pyrolysis index of MS (by 2.09–16.72 times) and the specific surface area of co-pyrolysis char. The inclusion of MS effectively promoted the transformation of heavy metals from unstable fractions to stable fraction, significantly reducing the potential ecological risk index of pyrolysis biochar (by 21.93–41.58 %). The combination of thermodynamic equilibrium calculations and experimental data demonstrated that heavy metals in sludge could be stabilized through the formation of stable heavy metal compounds or their encapsulation within the lattice of inorganic components during co-pyrolysis. These findings suggest that co-pyrolysis of MS and PS is a feasible and promising approach for producing high-quality pyrolysis char with low risk of heavy metals. [Display omitted] • The co-pyrolysis of municipal sludge and papermaking sludge was researched. • Adding municipal sludge elevates the stability of heavy metals in biochar. • Adding papermaking sludge improved the pyrolysis characteristics of mixed sludge. • The inorganic components play a key role in stabilizing heavy metals. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thermal activated-acid/alkali modification greatly enhanced S BET and V of kaolin. • AlIII-O formed by dealuminization in H-kaolin owned higher adsorption ability. • Unsaturated SiIII-O group provided a pathway for adsorbing HMs in OH-kaolin. • PbCl 2 adsorption efficiency of H-kaolin was significantly higher than OH-kaolin. • Derived chars were safe to be used for soil improvement in gardens or parks. The mandated waste classification poses challenges for the eco-friendly disposal of combustible high-organic solid waste (HSW), characterized by elevated heavy metal content and a low ash proportion. Fortunately, pyrolysis technology for HSW is emerging as a promising direction for achieving waste-to-resource conversion. To further mitigate secondary pollution, thermal activated-acid modified kaolin (H-kaolin) and thermal activated-alkali modified kaolin (OH-kaolin) were separately prepared, and their enhanced effects of immobilization and stabilization on heavy metals during pyrolysis were contrastively explored. Characterization results demonstrated that dehydroxy-dealuminization and zeolite NaA formation were intrinsic reasons for improvements in physical properties (specific surface area, pore volume) of H-kaolin and OH-kaolin, respectively. Experimental results revealed that H-kaolin outperformed OH-kaolin, showcasing its potential as a high-performance in-furnace adsorbent. Compared with other natural mineral-based sorbents reported in the literature, H-kaolin exhibited leading levels of immobilization, stabilization, and adsorption efficiency for heavy metals. Theoretical calculations illustrated that AlIII-O generated via dealuminization and SiIII-O in zeolite NaA were the main adsorption sites of H-kaolin and OH-kaolin, respectively, correspondingly binding with heavy metals to form oxides and silicates. The differences in active sites, electron transfer directions and bonding types during adsorption of H-kaolin and OH-kaolin were the intrinsic reasons for their distinct adsorption efficiency. Furthermore, derived chars with H-kaolin were deemed safe for application in pollutant adsorption in industrial field and soil improvement in gardens or parks. This research presented new ideas for developing high-performance in-furnace sorbents during pyrolysis, and explored the feasible safety application fields for pyrolytic derived-char. [ABSTRACT FROM AUTHOR]

An intercalation-exfoliation method is applied to modify the natural kaolin mineral, so that to improve the enrichment effects on heavy metals (Zn, Pb, Cr & Cd) during coal combustion. The modified kaolin is scanned by electron microscope (SEM), X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and Brunner-Emmett-Teller (BET), which indicate that the natural kaolin is peeled off to form fine flakes and the interlayer spacing is significantly increased. The coal-kaolin combustion tests were performed in a tube furnace from 900°C to 1300°C. It is found that the enrichment of heavy metals is enhanced obviously during the coal combustion, especially when the raw kaolin has high activity. Besides, the adsorption effects on the above four heavy metals are different. To be specific, the kaolin modified by potassium acetate has a better performance for Zn and Pb, but that intercalated by dimethyl sulfoxide shows better influences on Cd and Cr. The modified kaolin can provide more active sites for the adsorption of heavy metals, enhance chemical adsorption, and fix heavy metals in the form of aluminosilicates, silicates and aluminates. These founding could reduce the pollutant emissions of coal combustion in industrial applications. [ABSTRACT FROM AUTHOR]

Increasing amounts of solid waste and sludge have created many environmental management problems. Pyrolysis can effectively reduce the volume of solid waste and sludge, but there is still the problem of heavy metal contamination, which limits the application of pyrolysis in environmental management. The intercalated-exfoliated modified vermiculite (IEMV) by intercalators of sodium dodecylbenzene sulfonate, hexadecyltrimethylammonium bromide and octadecyltrimethylammonium bromide were used to control the release of Cd, Cr, Cu, Zn and Pb during pyrolysis process of sludge or solid waste. The retention of heavy metals in sludge was generally better than that in solid waste. The IEMV by octadecyltrimethylammonium bromide as the intercalator calcined 800 °C (STAB-800) was the best additive for heavy metal retention, and the retention of Cr, Cu and Zn was significantly better than that of Pb and Cd. Cr, Cu, Zn and Pb were at low risk, while Cd had considerable risk under certain circumstances. New models were proposed to comprehensively evaluate the results of the risk and forms of heavy metals, and the increasing temperature was beneficial in reducing the hazards of heavy metals by the addition of STAB-800. The reaction mechanism of heavy metals with vermiculite was revealed by simulation of reaction sites, Fukui Function and Frontier Molecular Orbital. Thermal activation-intercalated-exfoliated modified vermiculite (T-IEMV) is more reactive and had more active sites for heavy metals. Mg atoms and outermost O atoms are the main atoms for T-IEMV to react with heavy metals. The Cr, Cu and Zn have better adsorption capacity by T-IEMV than Pb and Cd. This study provides a new insight into managing solid waste and sludge and controlling heavy metal environmental pollution. [Display omitted] • Vermiculite can stabilize five heavy metals during pyrolysis of four raw materials. • New models are proposed to comprehensively evaluate the hazards of heavy metals. • Intercalation-exfoliation and calcination improve the reactivity of vermiculite. • Mg and the outermost O are main atoms of vermiculite to react with heavy metals. • Vermiculite has better adsorption capacity for Cr, Cu and Zn than for Pb and Cd. [ABSTRACT FROM AUTHOR]

The high-organic solid waste (HSW), collected in compliance with China's mandatory waste classification policy, exhibits elevated levels of heavy metals and a low ash composition. Effective management of HSW has become a contemporary focal point, with pyrolysis emerging as a viable method to mitigate secondary pollution and achieve waste-to-resource conversion. To address metallic contamination, the utilization of Si/Al-based and Ca-based additives was investigated to immobilize and stabilize heavy metals in situ during HSW pyrolysis. Experimental results demonstrated that Si/Al-based minerals, notably kaolin, effectively immobilized Cr, Cu, Pb, and Zn, while CaO enriched As and Cd better. The employment of additives enhanced heavy metal stabilization, thereby reducing ecological risks of derived chars. This comparative study could also provide principle for sorbent selection during other solid waste disposal containing heavy metals. Computational simulations explored the active sites and morphological alterations of additives at varying temperatures, establishing a theoretical basis for selecting appropriate pyrolytic parameters. The adsorption processes of kaolin and CaO for heavy metals exhibited opposite electron transfer directions, resulting in the formation of distinct types of covalent bonds. This disparity accounted for the differential adsorption selectivity of Si/Al-based and Ca-based additives towards various heavy metals. This research presented new ideas for developing targeted in-furnace sorbents during pyrolysis. [Display omitted] • Kaolin and CaO had a better adsorption selectivity for Pb and As, respectively. • Additive addition and higher pyrolysis temperature stabilized heavy metals better. • Chemical speciation of heavy metals significantly correlated with char properties. • Exposed O and unsaturated Al of kaolin were active sites for heavy metal chlorides. • The directions of electron induced transfer in kaolin and CaO were opposite. [ABSTRACT FROM AUTHOR]

Na+/K+/Mg2+/Ca2+ expansion–modified vermiculite and calcination expansion (700 °C, 800 °C and 900 °C)–modified vermiculite (700-Mg-V, 800-Mg-V and 900-Mg-V) were prepared as additives to control the emission of five heavy metals (Zn, Cr, Cu, Pb, and Cd) during the pyrolysis of municipal sewage sludge, paper mill sludge, municipal domestic waste, and aged refuse. Mg2+-Modified vermiculite obtained via thermally activated calcination at 800 °C retained 65% of heavy metals from all raw materials at 450 °C. Zn, Cr, and Cu retained nearly 90%. Although modified vermiculite could reduce the ecological risk, Cd had an ecological risk level higher than Zn, Cr, Cu, and Pb. The fine textural properties, laminated morphology, and expansion capacity of modified vermiculite were positively correlated with its retention of heavy metals. Heavy metals interacted with the (002) surface of vermiculite, and the reactions were mainly concentrated near the 17-O and surrounding atoms. The heavy-metal monomers were less capable of binding to the (002) surface of vermiculite than the oxides and chlorides of heavy metals. The effect of heavy-metal oxides and chlorides binding to the (002) surface of vermiculite was related to heavy metals. [Display omitted] • Vermiculite effectively controls heavy metal emission in solid waste/sludge pyrolysis. • Expansion of vermiculite is positively correlated with retention of heavy metals. • Rational calcination of vermiculite can further improve the retention of heavy metals. • Heavy metals mainly interact with No.17-O atom on the (002) surface of vermiculite. • The interaction between heavy metal monomers and vermiculite is relatively weak. [ABSTRACT FROM AUTHOR]

Heavy metals emitted from coal combustion can easily pose many threats. In this study, five modified additives based on kaolin were prepared for reducing the emission of heavy metals from high temperature coal combustion and characterized by XRD, FT-IR, XRF, BET, SEM and zeta potential. The heavy metal retention characteristics of the additives under high temperature (900–1300 °C) coal combustion were investigated by static and dynamic experiments, and the results showed that pseudo-boehmite-metakaolin (BMK) was the most effective additive, and the ability of heavy metal capture was Cr > Zn > Pb > Cd, and the dynamic experiments were better than the static experiments. The quantum chemical models of heavy metals (monomers, oxides and chlorides), kaolin and adsorbent-active substances were developed and optimized, and the adsorption energy was calculated. The morphological changes of the active substances were taken into account in the simulation, and new mathematical models for adsorption energy calculation were proposed, which effectively solved the extreme results of adsorption energy calculation and made a comprehensive evaluation of heavy metal adsorption for the whole studied temperature range system. The mechanism of heavy metal adsorption on the active substance was further investigated by the simulations. Heavy metal atoms can form covalent or ionic bonds with O atoms of the active substance, Cl atoms of heavy metal chlorides can be covalently bonded or non-bonded with Al atoms of the active substance, and O atoms of heavy metal oxides can combine with Al atoms of the active substance to form ionic bonds. This study sheds new light on the control of heavy metals in high-temperature coal combustion, the methods for the optimization of simulation and the adsorption mechanism of heavy metals on relevant reactive substances. The method has a promising potential for large-scale application due to its simple preparation, low cost and significant effect. [Display omitted] • Effective retention of heavy metals by adding modified kaolin into coal combustion. • The retention capacity of heavy metals was generally ranked as Cr > Zn > Pb > Cd. • New methods for adsorption energy calculation is proposed to solve extreme results. • Improvement of heavy metal chlorides adsorption is the key for capturing heavy metals. • Heavy metals can form ionic or covalent bonds with different reactive substances. [ABSTRACT FROM AUTHOR]

In order to recover valuable pyrolytic oils, mixed municipal solid waste was pyrolyzed in a fluidized bed reactor. Results showed that liquid products yielded among 38.4-56.5 wt% and separated into water-soluble phases and organic phases. Moisture was concentrated in the water-soluble phases (39.4-57.3 wt%), making them low in carbon content and heating value. On the other hand, the higher carbon content and lower oxygen content of organic phases make their heating value (27.5-32.1 MJ/kg) and quality higher than bio-oils. Water-soluble phases mainly included acids, carboxylics, phenols, and sugars, which could be used as chemical feedstocks and substantial fuel. Organic phases mostly contained aromatics and phenols and could be used as fossil fuels directly or as chemical materials. Heavy metals of Cd and Pb were proved to be poor in both water-soluble phases and organic phases. As for Zn, it was found to be higher in the water-soluble phases at 450 and 550 °C with quartz sand as bed material than that in crude oils. However, Zn content in organic phases was comparable to crude oils. High-aluminum bauxite and attapulgite as bed materials increased heating value of water-soluble phases and organic phases respectively, and both performed well in reducing the Zn content of water-soluble phases. This work proved that it was an operative way to produce valuable pyrolytic oils by pyrolysis of mixed municipal solid waste. [ABSTRACT FROM AUTHOR]

Thermal treatment is one of the most promising disposal techniques for heavy metal- (HM)-enriched hyperaccumulators. However, the thermal characteristics and fate of HMs during thermal treatment of hyperaccumulator biomass need to be known in detail. A horizontal tube furnace was used to analyze the disposal process of hyperaccumulator biomass derived from a phyto-extracted field in which the soil was moderately contaminated with heavy metals. Different operational conditions regarding temperature and gas composition were tested. A thermo-dynamic analysis by advanced system for process engineering was performed to predict HM speciation during thermal disposal and SEM-EDS, XRD and sequential chemical extraction were used to characterize the heavy metals. The recovery of Zn, Pb and Cd in bottom ash decreased with increasing temperature but recovery increased in the fly ash. Recovery of Zn, Pb and Cd fluctuated with increasing air flow rate and the metal recovery rates were higher in the fly ash than the bottom ash. Most Cl, S, Fe, Al and SiO2were found as alkali oxides, SO2, Fe2(SO4)3, iron oxide, Ca3Al2O6, K2SiO3and SiO2instead of reacting with HMs. Thus, the HMs were found to occur as the pure metals and their oxides during the combustion process and as the sulfides during the reducing process. [ABSTRACT FROM AUTHOR]

With the new municipal solid waste classification policy implemented in China, attention on achieving the waste-to-energy disposal of "dry waste" has been growing. Pyrolysis conversion of organic waste into value-added chemicals is a promising method to treat solid waste. However, after removing the non-combustible components of "dry waste", the obtained high-organic solid waste (HSW) contains various heavy metals, which requires urgent attention during thermochemical conversion. To mitigate heavy metals risk, kaolin was employed as additive during HSW pyrolysis, and intercalation–exfoliation and thermal activation modifications were performed on the kaolin to further immobilize and stabilize heavy metals in the derived chars. The characterization results illustrated that the interlayer spacing, pore volume and diameter of kaolin were expanded after intercalation–exfoliation modification, providing more opportunities for the adsorption of metals. The thermal activation method favored the transformation of kaolin into metakaolin via dehydroxylation to enhance its nonhexacoordinated Al proportion and chemisorption. During 450–650 °C, kaolin exhibited an effective solid enrichment performance for targeting heavy metals, and the intercalation–exfoliation and thermal activation modification further enhanced the adsorption capacity of the kaolin for Cd, Cr, Pb and Cr, Cu, Pb, Zn, respectively. Compared with Cu and Zn, additives demonstrated better stabilization effects for Cd, Pb, and Cr, transforming more bioavailable fractions to the residual speciation. Overall, a higher pyrolytic temperature (650 °C) and the addition of effective additives could simultaneously increase the residual fraction and decrease the bioavailable fraction of heavy metals in HSW-derived chars, reducing the potential ecological risk. [Display omitted] • Fixation of heavy metals during high-organic solid waste pyrolysis was studied. • Intercalation-exfoliation expanded kaolin interlayer spacing, pore volume/diameter. • Moderate thermal activation temperature increased specific surface area of kaolin. • Additive addition and higher pyrolysis temperature stabilized Cd, Cr and Pb better. • Pyrolysis at 650 °C with FA-kaolin was optimal to reduce ecological risks of chars. [ABSTRACT FROM AUTHOR]

Experimental and theoretical studies are combined to analyze the adsorption properties of modified kaolin for heavy metal (Pb, Cd, Zn and Cr) from coal combustion. The results indicate that the retention effect of kaolin for Pb, Cd, Zn and Cr has been significantly enhanced after intercalation-exfoliation combined with acid/alkali modification, which is mainly attributed to more active sites for adsorption, richer porosity and more effective in retarding coking of coal ash. The higher oxygen concentration is positive to the enrichment of heavy metals at 900–1200 ℃, while the coking of coal ash and the thermal conversion of additives become the main factors affecting the absorption at 1200–1300 ℃. The acid/alkali modification effectively promotes the inductive effect of electron transfer between modified kaolin and heavy metals to form stable chemical adsorption. The electron transfer induction of modified kaolin for Pb, Cd is higher than Zn, Cr at 900–1000 ℃, while the adsorption activity of mullite and cristobalite for Zn, Cr is stronger than Pb, Cd at 1200–1300 ℃. In addition, Pb, Cd and Zn are more readily adsorbed as oxides by additives at 900–1300 °C. The results shed new light on strengthening the adsorption activity of kaolin to Pb, Cd, Zn and Cr in high temperature. [Display omitted] • The thermal conversion of additive has a significant effect on the adsorption activity. • The higher oxygen concentration is positive to the enrichment of heavy metals. • Modified kaolin has selectivity for the adsorption of Pb, Cd, Zn and Cr at 900 ℃–1300 ℃. • The acid/alkali modified KAc-kaolin effectively promotes the nucleophilic sensitivity. • Pb, Cd and Zn are more readily adsorbed as oxides by additives at 900 °C–1300 °C. [ABSTRACT FROM AUTHOR]

Co-combustion of coal and wheat straw (WS) was conducted in a lab-scale BFB combustor. Fuel composition (coal, 70%coal+30%WS), temperature (750, 800, 850, 900, 950 °C), secondary air ratio (0, 10%, 20%, 30%) were varied to on the release of gaseous pollutant was studied. CO, NO x and SO 2 concentration in flue gas (FG) were measured on-line by a flue gas analyzer. Fly ash (FA), bottom slag (BS) and bed material (BM) were collected, digested and analyzed by ICP-OES to determine the distribution of heavy metals (e.g. Pb, Zn, Cr and Cd). Results indicated that co-combustion could improve the combustion of coal alone by reducing CO, NO x and SO 2 emission and carbon content in fly ash effectively. In co-combustion the increasing secondary air could reduce CO emission and SO 2 by enhancing disturbance and promoting sulfation respectively while the minimum NO emission was reached at the ratio of 20%. Co-combustion restrained the release of Zn, Cd and Pb compared with coal combustion alone. In co-combustion, high temperature increased their portion in the flue gas. For Zn, Pb and Cd, their content in the bottom solids increased while the portion of Cr decreased. Secondary air decreased their content in fly ash and transferred into flue gas significantly and in bottom solids content of Zn and Pb decreased while Cd increased. • Co-combustion decreased SO 2 , NO and CO emission and carbon content in fly ash. • Releases of Pb, Zn and Cd are restrained in co-combustion. • High temperatures promote the transfer of Zn, Pb and Cd in bottom solids. • Melting in the slag affected the release of Cd and Zn. • Secondary air promote the release of heavy metals. [ABSTRACT FROM AUTHOR]

Raw montmorillonite (MMT) was modified by intercalation-exfoliation method to improve the capture capacity of heavy metals (Pb, Zn, Cr and Cd) during coal combustion. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) Spectroscopy and Brunner-Emmett-Teller (BET) indicated that intercalation agents cetyltrimethylammonium bromide (CTAB) and sodium dodecyl benzene sulfonate (SDBS) could be successfully inserted into the interlayer of MMT during the intercalation step, additionally exfoliation process could peel off the MMT into fine flakes. The co-combustion of coal and MMTs was conducted in a tube furnace from 900 to 1300 °C. Results indicated that with the increase of temperature, the retention ratio of Pb, Zn and Cd gradually decreased, especially higher than 1200 °C with a great capture capacity reduction, whereas for Cr, it rose first and then decreased. The capture performance of modified MMT on Pb, Zn and Cr was obviously improved. However, for Cd, adverse effect was observed, of which the capacity was even worse than that of raw MMT in some cases. According to the mechanism analysis, the intercalation-exfoliation modification of MMT primarily enhances the chemisorption of heavy metals by exposing more active sites, which facilitates the fixation of heavy metals in the form of aluminosilicates. [ABSTRACT FROM AUTHOR]