Study On Harmlessness Of Pahs And Heavy Metals In Fly Ash Under Fe₃O₄/H₂O₂ Hydrothermal System

With the wide application of incineration technology in the field of solid waste treatment,the amount of by-product fly ash increased by municipal solid waste incineration?MSWI?treatment.Fly ash is included in the National Hazardous Waste List?HW18?and need to be treated harmlessly because it contains high leaching concentration of heavy metals and highly toxic"triad"organics?such as polycyclic aromatic hydrocarbons?PAHs?,Chlorinated biphenyls?PCBs?,dioxins?PCDD/Fs?,etc.?.This paper carried out a series of studies on the degradation of PAHs and the harmlessness of heavy metals in MSWI fly ash under Fe₃O₄/H₂O₂ hydrothermal system.The main contents and conclusions are as follows:?1?By studying the effects of two-step hydrothermal on the degradation of PAHs and the harmlessness of heavy metals in fly ash,it was found that the total concentration and toxicity equivalent?TEQ?of PAHs in solid phase products were significantly reduced after hydrothermal oxidation?H₂O₂ as oxidant?.The total concentration is mainly 2⁴ ring PAHs,and the TEQ is mostly contributed by 5-ring PAHs.Since the·OH produced by the excessive concentration of the oxidant is easily trapped by H₂O₂,the total concentration and the toxicity equivalent of the PAHs increased at first and then decreased with the increase of the oxidant concentration.Among them,the contribution of the first step of oxidant to the degradation of total concentration and TEQ of PAHs exceeded 69%,and gradually increased with the increase of oxidant concentration.Under the condition of equal oxidant addition,the two-step hydrothermal treatment has higher total concentration and TEQ degradation efficiency of PAHs in fly ash than one-step hydrothermal oxidation,reaching the highest at the oxidant concentration of 1 mol/L,respectively 94.50%and 96.63%.In addition,there is no synthetic aluminosilicate mineral in the hydrothermal oxidation process,which has a good stabilizing effect on heavy metals,so the stability of heavy metals is relatively poor.The leaching concentration of Pb in hydrothermal solid phase products decreased,and the leaching concentrations of Zn,Cu,Cd and Cr were lower than the standard of identification of hazardous waste.Some of the heavy metals transferred from the solid phase to the liquid phase.?2?The fly ash was pretreated by a water-washing process.The results indicated that the total concentration of PAHs in the solid phase products decreased from 2185.20?g/kg to 1496.11?g/kg with the increase of liquid-solid ratio?L/S?after water-washing because of different hydrophobicity(logK_OW)in each homologue monomer of PAHs.The TEQ of PAHs increased to some extent after water-washing.When the L/S was 6ml/g,it gained an increase of 19.87%at 31.75?g·TEQ/kg.This was mainly due to the5-6 ring PAHs,which was highly toxic and extremely low solubility.With the increase of L/S,a large amount of soluble chloride salts gradually dissolved and diffused into the aqueous solution,and the dissolved amount increased until it was completely dissolved.When the L/S was 6 ml/g,the chlorine removal rate reached 84.75%.Continued increase in L/S has little effect on chlorine removal.The removal of Na and K is mainly related to the dissolution of easily soluble components?NaCl,KCl and K₂Ca?SO₄?₂·H₂O,etc.?,and the dissolution effects of Ca and S are relatively poor.The total content of Pb,Zn,Cu,Cd and Cr in solid phase products after water-washing is slightly higher than raw fly ash due to the mass loss of soluble chloride salt.The order is:Zn>Pb>Cu>Cr>Cd.The leaching concentration of each heavy metal was lower than the standard of hazardous waste identification.Due to the dilution of water,the concentration of heavy metals in the liquid phase decreased with the increase of L/S.?3?This paper further studied the catalytic oxidation of PAHs and the harmlessness of heavy metals in fly ash under Fe₃O₄/H₂O₂ hydrothermal system.It was found that magnetite can catalyze the production of more·OH by H₂O₂ and promote further oxidative degradation of PAHs,thereby increasing the total concentration and the TEQ degradation rate of PAHs in solid phase products.With the increase of the amount of magnetite addition,the total concentration degradation rate of PAHs in hydrothermal solid phase products increased and then decreased,reaching the highest when the amount of magnetite was 15 wt.%,owing to a large amount of high-ring PAHs are converted into low-ring PAHs which were not completely degraded;at the same time,the TEQ of PAHs tends to decrease linearly and the degradation rate was up to 97.76%.Compared with the one-step hydrothermal catalytic oxidation,the total concentration and TEQ of PAHs in solid phase products after two-step hydrothermal catalytic oxidation were significantly reduced.The contribution rate of the first step of hydrothermal catalytic oxidation to the total concentration and TEQ degradation of PAHs were higher than 89%.In addition to the adsorption of heavy metals by magnetite,some heavy metals can replace the iron in magnetite and thus be stabilized in the crystal structure of magnetite.Therefore,under the conditions of hydrothermal catalytic oxidation,the leaching concentrations of Pb,Zn,Cu,Cd and Cr in the solid phase products are all reduced to the below of identification standard of hazardous waste.Due to the competition between the heavy metals,the leaching concentration of heavy metals in solid phase products tends to fluctuate.Among the three processes,the two-step hydrothermal catalytic oxidation process has the best effect on the stability of heavy metals.In this study,the effective degradation of PAHs and the significant reduction of heavy metal leaching concentration were achieved by studying the harmlessness of PAHs and heavy metals under different hydrothermal processes.Meanwhile,the process parameters were optimized.This study provides data and technical support for the hydrothermal treatment of PAHs and heavy metals in MSWI fly ash.