Study On Technology And Mechanism Of PAEs In Leachate By Coagulation-biological-advanced Oxidation Combination Processes

Landfill leachate with high concentrations refractory wastewater and complex composition,extremely difficult to degradate by using of a single biological,physical and chemical treatment methods,and can achieve more satisfactory results by using a combination of technology and depth of treatment.Phthalates?PAEs?as a plasticizer are one of the main pollutants in landfill leachate.Due to the lack of PAEs analytical methods,the pollution characteristics of PAEs in landfill leachate is still very unclear.Hydrogen peroxide?H₂O₂?and potassium persulfate?K2S₂O8?are mostly available green oxidant but don't show great potential in treating pollutants due to their relatively weak oxidation ability.In this regard,the development of their related oxidation technologies,which can enhance these oxidants to decompose to highly reactive oxidant,has received great interests.Therefore,in this study,we studied PAEs removal mechanism in landfill leachate by a combined coagulation-anoxic/oxic?A/O?-Fenton treatment processes,and further investigated the oxidation mechanism of PAEs in Fenton,UV/oxidant system.First,47 kinds of organic compounds were identified in landfill leachate by GC-MS,mainly containing six kinds PAEs,alcohols,phenols,ketones,amines,olefins,naphthalene,indole,ethers and other compounds.Only using Fenton method can effectively remove the organic compounds in water phase of landfill leachate,but the five kinds of substances such as phthalate?2-ethylhexyl?phthalate?DEHP?detected in the precipitated iron sludge,suggesting that PAEs were widespread in landfill leachate and difficult to degradation by a single treatment technology.Under optimized Fenton process conditions,COD and TOC removal efficiency were 65% and 42%,respectively,including 20% COD removal and 21% TOC removal by iron sludge coagulation process.To further study the environmental behavior of PAEs in landfill leachate,6 PAEs?dimethyl phthalate?DMP?,diethyl phthalate?DEP?,Diethyl phthalate?DEP?,dibutyl phthalate?DBP?,DEHP,di-n-octyl phthalate?Dn OP??were selected.We used the solid phase extraction?SPE?and soxhlet extraction to analyze 6 PAEs in landfill leachate,surface water and sludge,and studied the removal behavior of PAEs by combined coagulationA/O-Fenton treatment processes.The method performance was satisfactory,with results optimizing the recoveries in landfill leachate and surface water were 89 109%,82 127% and 68 117.3% in sludge.The concentrations of DMP,DEHP and DBP in landfill leachate were higher than other three PAEs.The occurrence of PAEs could be seasonal,which is related with climate and physical structure.The concentrations of PAEs in Chongqing landfill sites were higher than those in Wuhan and Yichang City,and higher about one order of magnitude than those in Japan.The concentration of DMP in leachate was higher 20 times than that in surface water,while concentrations of other PAEs is higher 0.5⁵ times than those in surface water.The total removal efficiency of PAEs by coagulation-A/O-Fenton treatment processes was 93%,and 88% 100% of PAEs be removed in the aqueous phase.The total concentration of PAEs in sludge or sediments in coagulation unit,stripping unit,anoxic unit and oxic unit were 33.5 ?g/g,d.w.?dry weight?,85.5 ?g/g,d.w.,59.6 ?g/g,d.w.and 52.9 ?g/g,d.w.,indicating that the adsorption is an important way for PAEs removal.Therefore,this paper confirmed that the biodegradation and sludge adsorption is the main removal mechanism of PAEs.The activated sludge was a huge reservoir of PAEs,emphasizing the importance of sludge management.In order to investigate the oxidation effectiveness and mechanism of Fenton method,the DEP selected as a target compound.The results showed that DEP degradation in Fenton process was found to occur in two stages,with an extremely fast stage and then a slow one.Based on the intermediate assumptions and pseudo-steady-state hypothesis of hydroxyl radical,the oxidation kinetics of various initial PAEs concentrations under the same experimental conditions followed the pseudo-first-order kinetics and BehnajadyModirshahla-Ghanbery?BMG?kinetic model.Suitable [Fe²⁺]/[H₂O₂] molar ratio would improve DEP degradation efficiency.The removal efficiency of DEP was enhanced or inhibited with different background components,such as humic acid,surfactant and DMP would reduce the degradation efficiency of the target compound,while hydroxylamine hydrochloride by accelerating Fe???/Fe???cycle,promoting the degradation efficiency.During the degradation process,seven degradation intermediates were detected out by means of GC/MS,including ethyl 2-hydroxybenzoate,ethyl ₃^-hydroxybenzoate,phthalic anhydride,benzoic acid ethyl ester,malonic acid,oxalic acid,and acetic acid.Inhibitory effects of DEP and intermediate products were investigated in aqueous solution with Photobacterium phosphoreum.Findings indicated that the solution was not completely detoxified even if DEP completely disappeared.All these observations have significant potential applications and require further investigation.Finally,we studied the oxidative behavior of DEP using UV/H₂O₂ and UV/S₂O₈^2-treatments at various conditions,focusing on the oxidant,pH,dissolved oxygen?DO?and the background matrices in real waters?CO₃^-,Cl^-,NO₃^-,NO₂^-and humic acid?.The results showed that DEP could be degraded effectively using both processes,and followed the pseudo-first-order kinetics.Regardless of the oxidant species,the pseudo-first-order rate constants?kobs?increased linearly with increasing initial oxidant concentration.Because the higher DO would inhibit the photochemical reactions,the removal efficiency of DEP was enhanced as the concentration DO decreased by bubbling with nitrogen in both systemes.The effect of background matrices on degradation efficiency of DEP depends on their concentrations.NO₃^-?concentration range: 0.01 M⁰.5 M?has little effect on the removal of DEP in UV/H₂O₂ system,while NO₃^-?concentration: 0.05 M⁰.5 M?has a role in promoting DEP degradation in UV/S₂O₈^2-system.Radical inhibition experiments shown that O2?-is active species in both systemes,and the intermediates were consistent with that detected in Fenton method.This thesis studied the environmental behavior of PAEs in landfill leachate by biological and advanced oxidation processes,and explorated the DEP degradation using Fenton and UV/oxidants treatment in practical water.