Solidification Of Heavy Metals Research Articles

Study on the Properties and Heavy Metal Solidification Characteristics of Sintered Ceramsites Composed of Magnesite Tailings, Sewage Sludge, and Coal Gangue.

With the rapid development of industry, the disposal of industrial solid waste needs to be solved urgently in China. Thus, an effective disposal method should be proposed to recycle these solid wastes in an environmentally friendly and sustainable manner. In this paper, ceramsite was prepared from sewage sludge (SS), magnesite tailings (MTs), and coal gangue (CG). The influence of the material ratio and sintering temperature on the properties of the ceramsite was investigated. The results show that the ceramsite had better properties when the following parameters were used: a ratio of SS: CG: MT of 4.5:4:1.5; a sintering temperature of 1250 °C; a compressive strength of 11.2 MPa (or it can be rounded to 11; our major remark relates to significant figures, and they should be up to 2–3 figures, according to measurement errors); a water absorption of 3.54%; and apparent and bulk densities of 1.19 and 0.81 g/cm3, respectively. The strength was superior to more than twice the 900-density grade prescribed by the Chinese national standard. After sintering, most of the heavy metals in the ceramsite mainly existed in the form of residue state (FD), meaning that they were highly stable. The leaching concentrations of Zn and Ni from the ceramsite were 0.72 and 0.25 mg/L lower than the prescribed regulatory limits (2.0 and 0.1 mg/L). The overall pollution toxicity index (OPTI) was only 240, less than that of raw pellets, indicating that the environmental risk is low. Not only did the ceramsite, prepared from SS, CG, and MT, exhibit excellent chemical properties, but it also proved to be an environmentally safe material. Therefore, it is an effective approach to realize the collaborative treatment of SS, CG, and MT by preparing ceramsite.

Co-vitrification of municipal solid waste incinerator fly ash and bottom slag: Glass detoxifying characteristics and porous reformation

Safety and efficient dispose of municipal solid waste incineration (MSWI) fly ash with high toxicity has emerged as a worldwide challenge. Vitrification provides the advantages of capacity reduction, detoxification, and solidification of heavy metals, which has the potential to dispose of hazardous waste on a large scale. Herein, co–vitrification of MSWI fly ash and bottom slag has been accomplished based on the characteristics of calcium and silicon composition. A novel approach for producing glass ceramic foams by alkaline activation–crystallization was developed to realize the disposal of the obtained glass. The effect of MSWI fly ash/bottom slag ratios on the glass network, crystallization ability of the basic glass, pore structure, and physical properties of the porous green body was investigated. The results revealed that with increasing MSWI fly ash proportion, the Si–O of [SiO4] in the basic glass changed significantly and the crystallization ability steadily reduced. Si–O and Al–O in basic glass are easy to corrode under alkaline conditions, releasing Ca2+ and forming a low solubility product, calcium silicate hydrate. When the crystallization temperature increases from 950 ℃ to 1150 ℃, it is more conducive to the precipitation of the gehlenite phase. Extending the crystallization time promotes three–dimensional growth of crystals that are coupled with each other to form a network structure and a multi–stage pore structure. The pore structure was developed with the help of NH3 and H2 generated by the secondary aluminum ash (SAA). Through the preparation of glass ceramic foams, the raw materials were detoxified. The toxic heavy metals showed extremely low leaching concentrations, which were smaller than the limit of TCLP. The prepared samples had 70.22–80.61% of porosity, 0.78–1.19 g/cm3 of low bulk density, and 0.54–7.86 MPa of compressive strength.

Biochar as additive for improved building performances and heavy metals solidification of sediment-based lightweight concrete.

The sustainable disposal of large volumes of contaminated dredged river sediment has become a challenge for municipal management. In this study, a cutting-edge biochar application method was innovated, which converted the polluted dredged sediment into a low-carbon and environmentally friendly building material through an autoclave-free method. As the amount of biochar addition increased from 0 to 2% (w/w), the compressive strength of the dredged sediment-based lightweight concrete (DS-LC) increased from 3.92 to 4.61 MPa. Accordingly, the thermal conductivity decreased from 0.237 to 0.222 W/(m K), the water absorption decreased by 6%, and the water resistance coefficient increased by 33%. Results of X-ray diffraction (XRD) and thermogravimetric (TG) analysis showed that biochar promoted the hydration reaction and the carbonation process. Scanning electron microscopy (SEM) attached with energy-dispersive X-ray spectroscopy (EDX) showed that biochar addition changed the microstructure of the DS-LCs, which made the pore distribution more uniform and densified. Biochar addition also strengthened the immobilization of heavy metals (Cu, Zn, Cr, and As) by approximately 18-27% and combination of biochar and silica fume could increase the heavy metal immobilization by 28-44%. Compared with the traditional concrete material, the DS-LC with biochar addition could not only reduce the carbon emission but also has potential economic benefit for the treatment and utilization of dredged sediment.

Study on the accelerated carbonation of MSWI fly ash under ultrasonic excitation: CO2 capture, heavy metals solidification, mechanism and geochemical modelling

The current carbonation technology of municipal solid waste incineration fly ash (FA) has disadvantages such as low carbon sequestration efficiency, harsh conditions, and complicated equipment. An ultrasonic chemical method coupled with wet-carbonization technology is proposed. The effects of the FA ultrasonic carbonation (UC) method on CO2 capture, heavy metals (HMs) release and solidification were investigated, and geochemical model of FA was explored. The results showed that UC improved the CO2 capture capacity, and the carbonation efficiency increased from 14.1 % to 18.8 % because the ultrasonic physical and chemical excitation improved the mass transfer efficiency and strengthened the gas–liquid-solid three-phase carbonation reaction. UC inhibited the release of Pb and Zn into the liquid phase (0.002 mg/L and 0.004 mg/L) and increased the solidification efficiency of Pb and Zn in FA (99.99 % and 100 %) because free Pb and Zn converted carbonates completely. UC FA had a single component, small particle size, high degree of homogenization, and contained slender nano-CaCO3 crystals. The acid neutralization capacities of original FA, FA4, and FA9 were 10, 8, and 3 meq/g because the carbonation reaction neutralized the alkaline substance. The content of carbonate minerals was the highest in UC FA and the pH of carbonate-minerals controlled dissolution/precipitation was mainly between 6 and 10. The HMs were in a soluble state (pH < 6) and the amphoteric HMs were affected by soluble hydroxides, the leaching concentration increasing (pH > 10). Therefore, UC FA had the potential of economical, environmentally friendly, and resource-based applications when avoiding strong acid and alkali application scenarios.

Effects of curing pathways and thermal-treatment temperatures on the solidification of heavy metal in fly ash by CaCO3 oligomers polymerization

The solidification of the heavy metals (HMs) of municipal solid waste incineration fly ash (MSWI FA) by a calcium carbonate polymer is a novel approach. The effects of different curing pathways and thermal-treatment temperatures on the leaching concentrations of the HMs were investigated by conducting different leaching tests and performing an overall pollution toxicity indices (OPTI) analysis of the leaching data, respectively. The results showed that the solidification efficiencies of the Pb and Zn of FA samples, which could form carbonate-bound states, from the fly ash-CaCl 2 ▪2H 2 O wet carbonation pathway were above 99.5%. The samples from the pathway that involved mixing FA with calcium carbonate oligomers (CCOs) had lower leaching concentrations of Cd and Cr (0 and 0.1112 mg▪L −1 ). With the thermal treatment temperature increasing from 105 °C to 500 °C, the treated FA samples had larger particle sizes, fewer pores, and more compact surface structures covering much more calcite , and the solidification efficiencies of Pb and Zn increased from 64.3% to 98.8% and from 60.2% to 98.9%, respectively. The calcium carbonate structure was decomposed in an acidic leaching agent, so the solidification effects were weakened and even negative. In contrast, the treated FA samples achieved effective solidification of HMs by physical packaging and chemical precipitation in the HT/J 557–2010 standard and the OPTI decreased from 8126.39 to 44.88. Therefore, a nonacidic environment is suitable for the storage or resource utilization of FA samples prepared by this method. • Two different curing methods of MSWI fly ash using CaCO 3 polymer were studied. • Inhibition on heavy metals leaching of 500 °C heat treatment was better than 105 °C. • The leaching characteristics of the samples were evaluated by different standards. • Fly ash samples were subjected to comprehensive heavy metal toxicity assessments.

Effect of zeolite contents on mineral evolution and heavy metal solidification in alkali-activated MSWI fly ash specimens

Municipal solid waste incineration fly ash (MSWIFA) with a high heavy metal concentration must be consolidated before disposal in a landfill. Alkali-activated technique contributes significantly to boosting the degree of heavy metal solidification and usage ratio on MSWIFA material. In this work, zeolite, a new additional siliceous material, was employed to improve the properties of a pure alkali-activated MSWIFA (FZ0) matrix. When compared to other FZ specimens with varying zeolite concentrations (0–50 wt%), the compressive strength of alkali-activated MSWIFA with 40 wt% zeolite substitution (FZ40) after 3, 7, 14 d curing was the highest. The compressive strength of partial FZ specimens reduced somewhat after 28 d of curing due to dry shrinkage. According to magic angle spinning nuclear magnetic resonance (MAS NMR) data, the predominant amorphous products in FZ specimens were C-(A)-S-H. Based on X-ray diffraction (XRD) data, new minerals (tobermorite, tilleyite, and calcite) were discovered and quantified in the FZ matrix. Fourier transform infrared spectroscopy (FT-IR) findings revealed partial Na2SO4 in FZ0 specimens. Notably, the leaching concentrations of Pb, Zn, Cu, Cr, Ni, and Cd in FZ0 and other FZ specimens that met Chinese criteria were significantly lower than in the original MSWIFA. Meanwhile, additional FZ specimens showed superior solidification tendencies on Pb, Zn, Cr, and Cd than FZ0 specimens. The inclusion of zeolite may increase the solidification efficiency of heavy metals in the MSWIFA matrix and has a promising application.

Novel hydroxyapatite (HAP)-assisted hydrothermal solidification of heavy metals in fly ash from MSW incineration: Effect of HAP liquid-precursor and HAP seed crystal derived from eggshell waste

Novel hydroxyapatite (HAP)-assisted hydrothermal methods were applied to solidify heavy metals in fly ash (FA) from solid waste incineration to create hydrothermally-treated (HT) products that meet landfilling standards. The leaching behavior, speciation distribution of heavy metals, and characteristics of HT products were analyzed by XRD, FT-IR, SEM-EDX, and XPS to explore the solidification mechanism. Large amounts of Ca2+, HPO42−, and PO43− in the HAP liquid-precursor (LP-HAP) directly formed hydrothermally-synthesized HAP and reacted with heavy metal ions to form stable compounds. Adsorption and ion exchange occurred between HAP and metals (i.e., Pb2+, Cd2+, and Zn2+), in which heavy metals occupied the Ca2+ sites. For one-step hydrothermal method with LP-HAP, a lower temperature of 35 °C resulted in better solidification efficiency than a subcritical temperature of 140 °C. By adding hydroxyapatite seed crystal (C-HAP) during the second step of two-step hydrothermal method, solidification was improved because more HAP or metal-exchanged HAP was formed via seed-induced mechanism. Under hydrothermal conditions, LP-HAP and C-HAP additives reduced the leaching toxicity but also inhibited the transfer of heavy metals from FA to the liquid phase. The overall pollution toxicity index of HT products from the optimized two-step hydrothermal method with LP-HAP and C-HAP was reduced by 96.3%.

Heavy metal solidification and CO2 sequestration from MSWI fly ash by calcium carbonate oligomer regulation

The efficiencies of heavy metals (HMs) solidification and CO2 sequestration from current municipal solid waste incineration fly ash (MSWI FA) carbonation technology are low, and the reaction conditions are complicated. A novel MSWI FA carbonation treatment, which was carried out under room temperature and pressure and controlled by calcium carbonate oligomers, is proposed. Through orthogonal tests, the reaction time (t), temperature (T), CO2 flow rate (V), magnetic stirring rate (W), and liquid-solid ratio (L) of the MSWI FA carbonation treatment were investigated. The results showed that the five influential factors were W>T>L>t>V, and the optimal parameters were 60 min, 20 °C, 200 ml min−1, 600 rpm and 10 L kg−1 (the carbonation efficiency (CE) was 13.8%). When alkaline additives (NaHCO3 or NaOH) were added, the CE increased to 16.5% and 15.1%, respectively, due to the increase of CO32− ions in the solution. The leaching concentrations of Zn, Pb, Cu, and Ni were greatly reduced in the treated samples, and the solidification efficiency reached to 99.1%, 100.0%, 91.3%, and 95.6%, respectively, because they were converted to carbonates and were adsorbed and wrapped by calcium carbonate polymer. The pores on the surface of the treated sample were reduced, and the particle sizes increased. The pH of the leachate of the treated samples decreased to 9.0–11.5 due to the carbonation reaction. This study provides an effective MSWI FA treatment method that has a highly efficient use of HMs and the potential for CO2 sequestration and resource utilization.

Solidification of heavy metals in lead smelting slag and development of cementitious materials

Lead smelting slag (LSS) is a hazardous waste containing heavy metals (Cr, Ni, Cu, Zn, As, and Pb) and its improper disposal may cause irreparable damage to the ecosystem. Cementitious materials prepared with LSS can be used in construction fields and hazardous heavy metals are also solidified in prepared cementitious materials. In this study, the effects of particle sizes of LSS (8.30, 12.54, and 19.04 μm) on the compressive strength of prepared with high-volume LSS multiple coupling excitation cementitious materials were explored. According to the composition of LSS, activators were set as 6% MgO, 6% bischofite, 7% CaO, 1% CaCl2, and 8% cement, respectively. Decreasing the particle size of LSS could significantly improve the compressive strength of cementitious materials, especially in the early stage. The sample containing LSS with the particle size of 8.30 μm showed the highest compressive strength (47.1 MPa) and activity index (78.2%) among the samples containing LSS with different particle sizes. LSS with three particle sizes was analyzed with Fourier transform infrared spectroscope (FTIR) and X-ray diffractometer (XRD). The analysis results indicated that decreasing the particle size of LSS increased the specific surface areas (SSAs) and decreased the degree of crystallinity . Moreover, the microscopic analysis of cementitious materials implied that decreasing the particle size enhanced the degree of reaction and gelation , thus resulting in the denser structure of blocks. The results confirmed the compressive strength of LSS-based cementitious materials. • Heavy metals in LSS were effectively solidified in cementitious materials. • A hydration model for LSS-based cementitious materials is proposed. • The paper presents early strength could be significantly improved with decreasing particle size. • LSS-based cementitious materials can be used in the field of prefabricated construction industrial.

Mechanism of Different Passivating Agents on the Solidification of Heavy Metals in Soil

Through the mechanism study, it can be seen that the three kinds of passivating agents can increase the soil pH value and cation exchange capacity, which is beneficial to the transformation of heavy metals from ion exchange state to residue state. The addition of organic fertilizer can increase the activities of urease and acid phosphatase, and montmorillonite is beneficial to activate the activity of catalase in the soil. MO treatment can simultaneously increase the activities of the three enzymes and increase the absorption of soil carbon, nitrogen and organic phosphorus. Electron microscope scanning results showed that MO effectively increased the connectivity between soil particles and increased the specific surface area of soil. Therefore, the synergistic effect of MO treatment on the passivation of heavy metals is obvious, and the passivation effect is the best.

Heavy metal residues, releases and food health risks between the two main crayfish culturing models: Rice-crayfish coculture system versus crayfish intensive culture system

High-density culturing with excessive feeding of commercial feed has caused heavy metals pollution to agricultural production system. In this study, the dynamic changes and transfer of heavy metals in rice-crayfish coculture system (RCCS) and crayfish intensive culture system (CICS) within a completed culture cycle were systematically quantified. Our results showed that Cd in feed represented more than 50% of the total Cd input, and the inputs of As and Cr were mainly from irrigation. The residues of As and Pb in RCCS were slightly higher than those in CICS, while the residues of Cd and Cr in RCCS were far fewer than those in CICS. Moreover, the metal pollution index in CICS was 0.781, while it was 0.543 in the RCCS. Furthermore, a large proportion of the Cd and Pb in CICS was released into the external environment through drainage. Notably, the absorption and solidification of heavy metals by straw did not increase the residues of As and Pb in the major components of RCCS in the second year. Compared to CICS, RCCS did not produce many heavy metal residues or cause heavy metal discharge pressure on the external environment, and its food product had a low risk of heavy metal contamination.

Converting wastes to resource: Utilization of dewatered municipal sludge for calcium-based biochar adsorbent preparation and land application as a fertilizer

Pyrolysis combined with land application for dewatered municipal sludge disposal revealed advantages in heavy metals solidification and resource utilization compared with other disposal technologies. In this study, utilizing dewatered municipal sludge for calcium-containing porous adsorbent preparation via pyrolysis was proposed and verified. After pyrolyzing at 900 ° C (Ca-900), the dewatered sludge obtained maximum adsorption capacity (83.95 mg P⋅ g−1) and the adsorption process conformed to the pseudo-second-order model and double layer model. Characteristic analysis showed the predominant adsorption mechanism was precipitation. Continuous column bed experiment indicated 2 g adsorbent could remove 4.27 mg phosphorus from tail wastewater with the initial phosphorus concentration of 1.03 mg ⋅ L−1. No heavy metals leaching was observed from Ca-900 adsorbent with pH value exceeding 1.0, and merely 1% addition of Ca-900 adsorbent (after actual water phosphorus adsorption) with soil could extremely promote the early growth of seedlings. Economic estimates demonstrated that this cost-effective modification could generate the most add-on value production. Based on these results, the strategy of ‘one treatment but two uses’ was proposed in this study, converting the wastes to resource and providing a native strategy for sludge disposal and resource recovery.

The use of contaminated soil and lithium slag for the production of sustainable lightweight aggregate

The contaminated soil (CS) and lithium slag (LS) contain amounts of hazardous ingredients which may move into the ground water and local cropland. In order to prevent CS and LS from damaging the environment, an investigation on the utilization of CS and LS for the production of lightweight aggregate (LWA) was presented in this article. The basic properties of LWA were evaluated. Furthermore, the crystalline phases of the produced LWA were analyzed, and the microstructure of LWA was analyzed by X-ray micro-tomography. The experimental results showed that the LWA produced from the CS and LS had good performances with density of 0.5–1.5 g/cm3 and compressive strength of 1.0–8.7 MPa, which met the required standard of LWA. Meanwhile, it can be revealed that the LS can reduce the sintering temperature of LWA, thus exhibiting low energy consumption. The analysis of crystal phases indicated that the mineral compositions of LWA contained spinel phases, which provided a possibility for the solidification of heavy metals. In addition, the evaluation of environmental effect revealed the leaching concentrations of hazardous metals were much lower than the critical limits of nonhazardous materials. Finally, the economic benefits were analyzed in the entire production process of LWA, which indicated that using CS and LS to produce LWA can lower the raw material cost by 85–98%. This work will create a more eco-friendly and economical method for the management of solid wastes (CS and LS). Meanwhile, this study can provide theoretical basis for the sustainable production of LWA.

Co-pyrolysis of industrial sludge and rice straw: Synergistic effects of biomass on reaction characteristics, biochar properties and heavy metals solidification

Co-pyrolysis of sludge and biomass was conducted to produce biochar with heavy metals solidification. The synergistic coupling mechanism and the surface functional groups and pore structure characteristics of co-pyrolytic biochar were studied. The solidification characteristics and mechanism of heavy metals were also investigated. Biomass composition in the co-pyrolysis system provided energy, solidified heavy metals and improved product quality. Compared with the weighted calculated value of two individual materials, the secondary decomposition process of co-pyrolysis was prolonged by approximately 80 °C and enhanced by 29%. OH bond, CH bond, and CO bond of co-pyrolytic biochar obtained at 500 °C increased significantly, and the carbon network structure and skeleton were enhanced, and the specific surface area increased by 102.8%, comparing with that of sludge biochar. The solidification effect of Cu and Cd in co-pyrolytic biochar was 29% and 50% higher than that in sludge biochar, respectively. The leaching rates of Cu and Cd in co-pyrolytic biochar were only 38.22% and 39.54% of that from sludge biochar. This study demonstrated that high-quality biochar with low heavy metal risk can be obtained by co-pyrolysis technology, which provides a potential way for the comprehensive utilization of industrial sludge and biomass.

Characteristics of MSWI fly ash and its resource transformation by road engineering: Mechanical and environmental considerations

This research investigated the possibility of resource transformation of municipal solid waste incineration fly ash (IFA) in road engineering. The thermal stabilities, metallic elements, chemical compositions, morphological characteristics and physical properties of IFA were detected by means of simultaneous thermal analyzer, inductively coupled plasma mass spectrometry, X-ray fluorescence spectrometer and scanning electron microscope, the results showed that IFA had the potential to be reused as road substitute material. Based on this, the mechanical properties and heavy metal leaching toxicities of IFA in asphalt mixture and cement stabilized macadam were studied. Results showed that the asphalt mixture with SMA gradation type could digest more IFA than the asphalt mixture with AC or ATB gradation type because of its high strength from skeleton-dense structure, high asphalt content and the toughening and anti-cracking effect of lignin fiber. And the asphalt binder could effectively bond and stabilize the heavy metals of IFA. The applicability of IFA in cement stabilized macadam was stronger and the digestibility was more, and it could promote the increase of compressive strength when the content of IFA was suitable. When the content of IFA was high, the application of IFA could be realized by adjusting the parameters of cement dosage and gradation type. The solidification of heavy metals in cement stabilized macadam was achieved by the hydration products such as C-S-H gels, Ca(OH)2, ettringite and so on, which had a series of physical chemistry effects on heavy metals. The application of IFA in road engineering was technologically sound and environmentally safe.

Preparation of composite micro-slag based on the application of tailings slag in cement and concrete

To improve the application performance and utilization of tailings slag in building materials, a composite micro-slag composed of ultrafine tailings particles was prepared based on the chemical composition, phase composition, and surface morphologies of three tailings slags. Using of ball mill and horizontal sander combined with gradient grinding technology, the utilization rate of the composite micro-slag in the cement raw meal was as high as 60%, and the raw meal particle uniformity was good. When it was used as a concrete admixture, it can effectively reduce the amount of cement by 40%. The results showed that the grinding process improves the particle morphology of the tailings slag, thereby affecting the interaction between the particles, which not only promotes the particles to receive uniform energy during the calcination process, reduces the energy consumption of clinker firing, but also stimulates the activity of tailings slag so that improves the strength and workability of concrete. Moreover, the combination of cement kiln calcination and heavy metal solidification technology or hydration reaction solidification mechanism allows the heavy metal elements contained in the composite micro-slag to be solidified in cement and concrete, ensuring its safety in use.

Supercritical CO2 coupled with mechanical force to enhance carbonation of fly ash and heavy metal solidification

Accelerated carbonation of fly ash is a potential way to achieve CO2 emission reduction and heavy metal solidification. Slow conversion in the diffusion control stage is the bottleneck of the carbonation technical route. Based on the strong diffusion and permeability of supercritical CO2, and the modification of mechanical force to produce more fresh surfaces and pores, a method consisting of supercritical CO2 coupled with mechanical force was carried out to strengthen the carbonation of fly ash. Research results show that the carbonation efficiency of fly ash under supercritical CO2 is generally higher than under low-pressure conditions, and carbonation under supercritical CO2 can effectively stabilize heavy metals in fly ash. In this work, the optimal amount of carbon sequestration under low-pressure was found to be 42.3 g-CO2/kg-fly ash (g/kg), with a carbonation efficiency of 18.65%. Under the supercritical condition of 8 MPa, the maximum carbon sequestration by fly ash is 54.9 g/kg, and the carbonation efficiency is 24.20%. In experiments with mechanical ball milling modification, the order of carbonation efficiency is: wet milling modified ashes > dry milling modified ashes > raw ashes. Additionally, carbonation has an obvious inhibitory effect on the leaching of Pb, Cr and Cd from fly ash.

Solidification of Heavy Metals in Lead-Zinc Smelting Slag by Multi-Component Synergistic Activation

Solidification of Heavy Metals in Lead-Zinc Smelting Slag by Multi-Component Synergistic Activation

Synthesis and Influencing Factors of High Performance Concrete Based on Copper Tailings for Efficient Solidification of Heavy Metals

Synthesis and Influencing Factors of High Performance Concrete Based on Copper Tailings for Efficient Solidification of Heavy Metals

Thermodynamic evaluation on chemical looping conversion of Cd- and Zn-contained phytoremediation plant with different CaO pathways

With the development of phytoremediation for soil contamination, disposal of phytoremediation plant becomes a serious problem. Thermochemical conversion of phytoremediation plant can greatly reduce the volume and mass, meanwhile the clean and reusable utilization is realized. As one of the thermochemical conversion technologies, chemical looping (CL) offers a carbon negative way for clean utilization of biomass. In this technology, CaO has binary roles of heavy metal solidification and CO2 sorption for gasification enhancement. To assess the CaO pathway in CL of phytoremediation plant, two different CL processes are constructed and comparatively studied based on thermodynamic evaluation. The effects of different operating parameters on the products of gasifier (GR) and reduction reactor (RR) are compared and discussed. Results demonstrated that the CaO addition in GR is beneficial to the production of pure combustible gases. Increasing RR temperature can promote the chemical looping reactions in RR. Under lower temperature, CaO in RR can consume more CO2 leading to CO2 free environment. When it is higher than 850 °C, there is no effect of CaO in RR. Increasing the amount of OC in system can enhance the conversion of combustible gases. When αOC is higher than 0.3, the OC is reduced to a mixed state of Fe3O4 and FeO. When the CaO circulates only between GR and calciner, pure CO2 can be captured at the outlet of calciner. Existence of CaO is beneficial to retain Cd and Zn in solid phases. When the gasification temperature increases from 500 °C to 800 °C, the Cd(g) increases while Cd decreases in both CL1 and CL2. For a long lifetime of OC, CaO is suggested to circulates between GR and calciner.