Experimental And Theoretical Studies On The Adsorption Performance Of Modified Kaolin On Heavy Metals In The Thermal Conversion Of Fuels

In recent years,with the rapid socio-economic development of China,the demand for coal energy has been growing and the production of municipal solid waste has been rising significantly.Coal-fired power generation and thermal treatment of waste can pose a number of environmental problems that cannot be ignored.Apart from some conventional pollutants such as SOx,NOx,HCl,dioxins,PM2.5,etc.,heavy metals are gradually gaining attention due to their high migration,non-degradable and hazardous characteristics.Compared to traditional control technologies,the application of mineral additives in coal combustion and waste pyrolysis for heavy metal abatement is economical,efficient and operational,especially the advantages of silica-alumina based additive kaolin have been proven by numerous researchers.In this paper,in order to further enhance the adsorption and enrichment capacity of kaolinite for heavy metals Pb,Zn,Cd,Cr and As,a series of experimental and mechanistic studies were conducted on the adsorption of heavy metals by modified kaolinite during coal combustion and waste pyrolysis,aiming to find economical and efficient kaolinite modification methods with excellent adsorption performance,and to clarify the intrinsic logic and microscopic mechanism of the adsorption of heavy metals by kaolinite and modified kaolinite,which provide theoretical guidance for the control of heavy metals in coal combustion and waste pyrolysis.The characteristic peaks of both the loadingγAl₂O₃ and the building blockγAl OOH were observed in the XRD patterns,which improve the thermal stability and adsorption capacity of the adsorbent under high temperature conditions to a certain extent.Four methods were used to modify kaolin clay,while characterization was performed to analyze the modification effect and the potential beneficial factors for enrichment of heavy metals.The results show that the surface properties and pore structure ofγAl OK and PBK composites are greatly improved compared with the original kaolinite,and the specific surface area is increased by about 5～8 times,which provides a rich physical space for the adsorption of heavy metals.The specific surface area and pore structure of Al SK and Ca HPK did not visibly change,and their surfaces exhibited scattered and finely ground loadings,which were presumed to be alumina and Ca₂P₂O₇ in combination with XRD patterns.In addition,from the FTIR results,Si-O-Si,Al-O-Si,and O-Si-O inγAl OK and PBK were significantly weakened,indicating that these two modification methods produced noticeable damage to the structure of kaolinite,while only the surface hydroxyl groups in Al SK and Ca HPK were obviously diminished and did not affect the internal structure too much,which corresponds to the SEM image.The prediction of the adsorption reactivity of the modified kaolin fraction based on quantum chemical simulations showed that both the modification process and the introduction of active substances were theoretically beneficial for the control of heavy metals.Tube and sinker furnaces were employed to experimentally study the adsorption enrichment of Pb,Cd,Zn and Cr by combustion of kaolinite and modified kaolinite blended coal at 900℃～1300℃.In the tube furnace experiments,the adsorption and retention of the four heavy metals were enhanced to varying degrees after kaolinite was modified.γAl OK and PBK have selective ability to enrich Pb,Zn and Cd,Cr respectively,and the retention rate of Pb is improved by10%～22%compared with the original kaolinite,which can still be maintained at 65%～70%when the temperature is increased to 1100℃.Combined with the results of characterization analysis,the better performance may be attributed to the introduction ofγAl₂O₃ andγAl OOH.In the sinker,the shorter residence time weakens the effect of thermal decomposition of the adsorbent on the adsorption activity of heavy metals,and the kaolin fraction undergoes flash calcination,which removes some of the hydroxyl groups and exposes the active sites in a limited time,and the introduction of the fusion-blocking active speciesγAl₂O₃ leads to the high adsorption activity ofγAl OK even at higher temperatures,and the retention rate of Pb in the sinker under mixed coal combustion conditions is up to 64%.Solid-phase enrichment of Pb,Cd,As,Zn and Cr during waste pyrolysis at 450℃～650℃ with the addition of kaolinite and modified kaolinite using tube furnace and small fluidized bed experiments.The retention rates of PBK andγAl OK for Pb,Zn,and Cr were maintained above60%,with excellent selectivity for Zn and Cr adsorption and retention,respectively.γAl OK and Ca HPK modifications were able to increase the solid-phase enrichment rate of As up to 30%,while PBK was capable of increasing the retention rate of As and Cr up to 35%and 25%,respectively.The modification effect of Al SK on Cd can reach up to 30%or more.In the fluidized bed experiment,two mixing methods,γAl OK:Al SK=1:1 and PBK:γAl SK=1:1,were chosen because of its excellent ability of retention,and although the advantages of different heavy metals were complementary to a certain extent,they did not have a synergistic effect.Therefore,the stepwise modification experiments usingγAl OK+Al SK showed a certain synergistic effect on the retention of Pb,Cd and As and achieved a retention rate of more than60%for all five heavy metals.The microscopic mechanism of PbO and PbCl₂ adsorption by kaolin and modified kaolin is investigated by quantum chemical calculations based on the front-line orbit and Fukui function theory.The hydroxyl group inγAl OOH during waste pyrolysis promotes the conversion of Pb Cl₂ to Pb O to the extent that the solid-phase enrichment of Pb by PBK is greater than that byγAl OK in the range of 450℃～550℃.The key to the enhanced retention of Pb O and Pb Cl₂ byγAl OK and PBK over kaolinite is attributed to the stronger electron transfer induced interactions between the front orbitals contributing to the formation of stable chemical bonds.The HOMO of Pb Cl₂ and Pb O are most likely to interact with the LUMO of kaolinite（De OH001）.Correlation analysis showed a significant correlation between△Eg（Kaolin）and△E（Product-Pb Cl₂/Pb O）.Pearson’s R values reached 0.978 and 0.970,respectively,and kaolinite dehydroxylation and thermal decomposition lead to variations in its own reactivity,i.e.,the ability to gain and lose electrons,which in turn affects the adsorption for Pb Cl₂/Pb O.Pb O and Pb Cl₂ formed bonding and antibonding orbitals,respectively,when adsorbed on the surface of kaolinite（De OH 001）,which microscopically explains that Pb O is the most stable form of adsorption on the surface of kaolinite（De OH 001）.The competitive adsorption of PbCl₂ and CdCl₂ by kaolinite dehydroxylation models were simulated and calculated based on molecular dynamics theory using Monte Carlo method,while microscopic mechanisms such as the strength of electron transfer induction and electrophilic/nucleophilic sensitivity sites were obtained using frontline orbitals and Fukui function theory.The dehydroxylation of kaolinite obviously enhanced the electron transfer induction with Pb Cl₂,and its effect was larger than the effect of temperature on Pb Cl₂volatilization,which facilitated the enrichment of Pb in the bottom residue.The active adsorption sites exposed after dehydroxylation of kaolinite for Cd Cl₂ adsorption and retention cannot overcome the dominant effect of Cd Cl₂ volatilization due to temperature.The most reasonable adsorption reaction path after calculation and analysis is that the surface of Al ring（001）of kaolinite after dehydroxylation firstly adsorbs Pb Cl₂/Cd Cl₂ molecules under van der Waals forces,while the Al in IV and V coordination and the reactive oxygen atoms on the surface after dehydrogenation act as electron acceptors and electron donors,respectively,and then electron transfer occurs under the induction of the energy gap difference of the front orbitals,which leads to a more stable chemical adsorption.