Study On The Gas Phase Reaction Mechanism Of Four Heavy Metals And Typical Organics In Sewage Sludge Incineration

In current decades,heave metals in sludge cause serious environmental pollution,which is harmful to human’s health and production activities.One of the most popular treatment of sludge is incineration.With many heavy metal and organics contained in sludge,especially in industrial sludge,the transformation of heavy metal and the reaction mechanism of heavy metal with organics are essential to understand the progress of sludge incineration,and further treatment after incineration to reduce environmental pollution.In this thesis,using quantum chemistry methods,four validated heave metals,i.e.,Hg,Cd,Sn and Pb,and six typical simple organics,i.e.,CH₄,CH₃OH,HCHO,HCOOH,C₆H₆ and C₆H₅OH,were chosen to simulate the reaction mechanisms and products in sludge incineration.The main conclusions are as follows:（1）The Hg with six typical organics reactions are determined by Hg-inserted into C-H,C-O/C=O and H-O bond mechanism to form CH₃HgH,CH₃OHgH,CH₃HgOH,HHgCH₂OH,HCOHgH,CH₂HgO,HHgCOOH,HOCHHgO,HCOOHgH,HCOHgOH,C₆H₅HgH,C₆H₅OHgH,C₆H₅HgOH,m-,o-,p-HHgC₆H₄OH.The relative energy of all transition states involved in these reaction very high around several hundred k J/mol,and all products are less stable than initial reactants thermodynamically,thus,Hg will be discharged into atmosphere as elemental form.With extra oxygen supplied Hg will be oxidized to be HgO,which cannot react with CH₄.For rest five organics only one channel was located with HgO insertion into C-H or O-H bond.The final products are CH₃OHgOH,HCOHgOH,HOCOHgOH,C₆H₅HgOH and o-HOHg C₆H₄OH;the relative energy of transition state varies from 4.9 to 36.4 k J/mol,and the formation of final product is highly exothermic.Therefore,in presence of O₂,Hg will be more active to react with organics.（2）The reactions of Cd with six organics take place via the same manner with that of Hg,and to generate similar products.The order of relative stability（the relative energy（35）E）of all products from the most stable to the least stable is CH₃CdOH>C₆H₅OCd H>C₆H₅CdOH>HCOOCd H>CH₃OCd H>HCd COOH>HCOCd H>o-,m-,p-HCd C₆H₄OH>C₆H₅Cd H>CH₃Cd H>HCOCdOH>HCd CH₂OH>HOCHCdO>HCHCdO.Moreover,the barrier involved varies from about 180 to 630k J/mol.While CdO reacts with these organics via one channel surmounts a lower barrier in each reaction,except for methane,from-6.0 to 82 k J/mol,therefore,supplied oxygen will be benefit for organic cadmium compounds gathered in incineration ash,and the secondary treatment of the ash will reduce pollution of Cd in environment.（3）The reactions of Sn with six organics take place via the same manner with that of Hg and Cd,and to generate similar products.However,the products are more stable,and the barriers are lower than that with Hg and Cd.As for Sn with C₆H₆ and C₆H₅OH reactions,with the corresponding transition state is 27 and 8.5 k J/mol,respectively,these reactions readily to take place.With in presence of oxygen,the reaction of SnO with C₆H₆ will surpass high barriers;while SnO with CH₃OH,HCHO,HCOOH and C₆H₅OH reaction occurs via lower barriers.Therefore,organic tin compounds rather than elemental tin will be gathered in incineration bottom slag or fly ash.（4）The reactions of lead with six selected organics take place via insertion mechanisms to form similar products as that in reactions of Sn,and all products are rather stable.With lower barrier heights involved in Pb with C₆H₆ and C₆H₅OH,these reactions are easier to undergo in free of oxygen.With extra oxygen supplied,the barrier heights in the reactions of PbO with organics expect for CH₄ are in the order of CH₃OH<HCOOH<HCHO<C₆H₅OH<C₆H₆ from the lowest to the highest.The order of stability for products agrees with the energy barriers.Moreover,compared with the stability of all products and energy barriers in the reactions of Pb with organics,it is obviously PbO is favorable to react with CH₃OH,HCOOH and HCHO,it is therefore with the three organics contained Pb could be disposed in fly ash or bottom slag as Pb-contained organic compounds after incineration of sludge when extra O₂ is provided.（5）The relative energy of products formed in the reactions of HgO,CdO,SnO and PbO with six organics are in the order of Sn>Pb>Cd>Hg from the most stable to the most unstable.The energy barriers to form the above compounds from highest to lowest are in the order of Hg>Cd>Pb>Sn,which is in accordance with the stability of products.While in presence of oxgyen,the stability of CH₃OMOH,C₆H₅MOH and o-HOMC₆H₄OH（M=Hg,Cd,Sn,and Pb）is in the order of Cd>Hg>Sn>Pb from the most stable to the most unstable.While for HCOMOH and HOCOMOH（M=Hg,Cd,Sn,and Pb）,the stability is in the order of Cd>Hg>Pb>Sn from the most stable to the most unstable.Moreover,HgO and CdO will be more difficult to form CH₃OMOH,HCOMOH and HOCOMOH（M=Hg and Cd）,while SnO and PbO will be more difficult to react with C₆H₆ and C₆H₅OH.（6）The verification experiment shows that the quantitative trend of substances determined by GC-MS is basically consistent with the reaction energy barrier in quantum calculation,that is,the lower reaction energy barrier corresponds to the higher content determined by GC-MS.The most important thing is that the residual amount of gaseous reactant benzene is higher after incineration,which is consistent with the quantum calculation that the reaction energy barrier of benzene is much higher than that of methanol.After incineration,the combined organic matter（OM）content of heavy metals increased significantly compared with that before incineration,indicating that heavy metal organic compounds may be formed during the incineration process.At the same time,the increment in organic matter combined state（OM）of Cd was slightly greater than that of Pb,which agrees with the energy barriers in the calculation,i.e.,lower reaction energy barrier produces more heavy metal organic compounds.