Study On Thermal Desorption And Combined Treatment Of Chlorinated Organic Ontaminated Soil

Recently,soil pollution is becoming more and more serious,organic pollutants in soil have received extensive attention.Thermal desorption is an effective technology for remediating volatile or semi-volatile organic compounds contaminated soil(e.g.petroleum hydrocarbon,pesticide,PCBs,et.al.),however,the technology also has limitations due to its high costs and second pollutants.Especially for the chlorinated organics,PCDD/Fs are formed during the process.In the present study,fundamental research of some special organics desorption properties during thermal remediation of contaminates soil was conducted.Besides,PCDD/F formation and effects of some key factors during thermal desorption were also investigated.Three kinds of inhibitors were added to study the suppression effects of PCDD/F formation transforming from PCBs.Combinedmechanochemical and thermal treatment of PCB contaminated soil was carried out and its feasibility and economy were also discussed.Some primary conclusions obtained are as follows:Heating temperature and retention time were the two most critical factors during thermal desorption process.The targeted pollutants began to evaporate from the temperatures even much lower than the boiling points.A considerable removal effects were obtained if the retention time of contaminated soil in the heating region was long enough.The desorption efficiency of HCBz approached 100 percent when heated at 400? for 40 min or at 500 ? for 20 min.For p,p'-DDT,an optimum combination of temperature and time was 300 ? with 40 min or 400 ? with 10 min.The molecular weight of PCBs was much higher and a combination of heating temperature over 500 ? and retention time higher than 60 min was enough to attain a considerable removal efficiency.Destruction and dechlorination of contaminants were also occurred during the thermal treatment.The first-order kinetic model was used to describe the evapotation of HCBz and the desorption rate increased with rising temperature.PCDD/Fs were detected both in the treated soil and off gas during thermal treatment of PCB,CBz and p,p'-DDT contaminated soil and the sequence of their possibility transforming to PCDD/Fs was:PCBs>HCBz>p,p'-DDT>1,2-DiCBz.PCDFs were the main contributors of all PCDD/Fs generated during thermal desorption of PCB contaminated soil since PCBs was one of the precursors of PCDFs.The largest amount was observed at 300 ?,with the PCDDs and PCDFs concentration of 8600 and 61500 pg/g,respectively.Low chlorinated PCDD/Fs dominated the homologues of PCDD/Fs.At low temperatures,almost all generated PCDD/Fs remained in the solid phase,the G/S value was only 0.04 at 300 ? and 0.52 at 400 ?.The desorption of 1,2-DiCBz clearly gave rise to the formation of PCDD/Fs,with the maximum output at 250?.PCDFs were also the main contributor and most reported to the gas phase.OCDD dominated the homologues of PCDDs while TeCDFs were the main contributors among PCDFs homologues.The yields of PCDD/Fs from HCBz looked quite different from those arising from 1,2-DiCBz.HCBz converted into PCDD/Fs even more actively,of which PCDDs were the main contributors and most PCDD/Fs stayed adsorbed on the matrix,rather than reporting to the carrier gas.When it came to p,p'-DDT,the cleavage of 9,7-positions or 1,7-positions C-C bonds led to the formation of chlorophenyl radicals,which were converted to PCDD/Fs finally.The formation of PCDFs was favoured over the formation of PCDDs.Low chlorinated PCDD/Fs dominated the homologues of PCDD/Fs and most existed in the soild phase.The quantities of generated PCDD/F after thermal desorption correlated positively with the amounts of p,p'-DDT added to soil before thermal desorption.The total concentration of PCDD/Fs both in the flue gas and soil decreased when CaO,CO(NH2)2 or(NH4)2SO4 was added.Compared with(NH4)2SO4,CaO and CO(NH2)2 were more effective in inhibiting the PCDD/Fs formation reaction,among which CaO had the highest suppression efficiency.The efficiency of PCDD/Fs suppression reached 95.6%with 1 wt.%addition of CaO.The inhibition effects promoted when more amount of CaO was added.(NH4)2SO4 was considered the best option as suppressant,as it combined two inhibiting elements.But in this study,the inhibition efficiency of(NH4)2SO4 was the lowest.The main function of(NH4)2SO4 was to convert metal chlorides into sulphates(oxidising conditions),thus poisoning the active catalyst.So it was more effective to suppress PCDD/Fs formed through de novo synthesis rather than the precursors.A combination of the mechanochemical pre-treatment and thermal desorption was a feasible alternative for remediating PCB contaminated soil.The first few hours was a period of very rapid increases in PCBs destruction efficiency.After 1 h of grinding,the total amount of PCBs in soil decreased from 505 to 215 ?gg/g,and the destruction efficiency reached 57.4%.The addition of SiO2 could improve the grinding conditions and promote the mechanical chemical reaction.The removal effects of PCBs could be promoted by longer grinding time or heating time and higher heating temperature.Moisture in raw soil could be absorbed by CaO to form Ca(OH)2.Furthermore,the excess CaO and generated Ca(OH)2 could facilitate dechlorination and destruction of PCBs.The grinding process facilitated consequent thermal desorption.After ball milling for a few hours,all soil particles were ground into very fine particles.Compared with coarse particles,the ground fine particles had a higher pollutant desorption efficiency.PCBs molecules were likewise degraded into light masses by long time grinding,which easily desorbed from the solid matrix when heated.