Study On The Stabilization Control And Mechanism Of Typical Semi-Volatile Heavy Metals In The Thermochemical Disposal Process Of Municipal Sludge

Sludge drying and incineration technology as the quickest and most thorough final disposal technology to achieve sludge reduction,stabilization and harmlessness,has developed into one of the mainstream mature technologies in foreign countries.However,the emission of fine particles enriched with volatile and semi-volatile heavy metals in high-temperature incinerators will cause serious hazard to the surrounding ecosystem and human health,which requires targeted removal.In this paper,the typical heavy metals(Cr,Cu,Pb,Zn,and Ni)in municipal sludge with two different sludge were taken as the research objects,and the retention performance of four silica-aluminabased adsorbents(kaolinite,montmorillonite,attapulgite,and iron-rich attapulgite)on typical heavy metals in the incinerator was comparatively investigated.In addition,based on density functional theory and VASP theoretical calculation,the surface adsorption model was established.Combined with adsorbent characterization results before and after the reaction,the adsorption curing mechanism and reaction path of PbCl2 vapor on the surface of attapulgite in different flue gas atmosphere were explored at the atomic level.A fixed-bed experimental system was used to investigate the effects of incineration temperature and silica-alumina-based mineral additives on the retention of typical heavy metals in municipal sludge incineration,and the heavy metal distribution and leaching toxicity of incineration products were analyzed.The results show that attapulgite had better comprehensive retention effect on typical heavy metals in sludge incineration than kaolin and montmorillonite.The curing rate of Cr,Cu,Pb,Zn and Ni could reach up to 96.75%,80.72%,93.12%,93.27%and 76.02%.Its physicochemical interaction with heavy metals during incineration forms more stable eutectic,resulting in a shift in the fugitive form of heavy metals in the product to a stable state with lower leaching toxicity,indicating the high adsorption performance and low risk for heavy metals in the gas phase of the high-temperature furnace.The BET,SEM and XRD results show that the 2:1 chain-layered structure of(Fe-rich)graptolite results in less crystal structure deformation at high temperatures,higher thermal stability,and retention of more nanopores and surface active adsorption sites,with both external and internal diffusion,which is more favorable to retain more adsorption active sites at high temperatures and thus have better curing performance.Based on the preferred attapulgite adsorbent,it was modified by acid activation and loading of magnetic nanoferrite,and its adsorption performance on lead chloride vapor was tested by a two-stage vertical fixed-bed furnace,and various characterization methods such as BET,FTIR,XRD and SEM were used to study the physical and chemical properties of the adsorbent before and after the reaction.It was found that the composite modification resulted in a modified attapulgite with substantially increased PbCl2 vapor adsorption capacity.Acid activation substantially increased the specific surface area and thus the percentage of active sites on the surface of attapulgite by decomposing impurity cements and carbonates in the original ore,depolymerizing mineral aggregates and etching intercrystalline pore channels.The double active adsorption sites formed by the surface-loaded iron-based oxides and the lattice oxygen of attapulgite will further increase its adsorption capacity for PbCl2 vapor.In addition,the modified attapulgite also possessed water and sulfur resistance properties,which would promote its adsorption of PbCl2 vapor when the flue gas atmosphere contained less than 20%O2,400 ppm SO2 and less than 10%H2O.The Fe/HFP2 modified sample that showed optimal adsorption in different flue gas atmospheres were preferentially selected after a series of experiments.Both surface acid activation and ferrite loading modification resulted in a decrease of zeolite water and hydroxyl groups in the attapulgite crystals.XPS results of the adsorbent after adsorption indicate that some of the active sites of the adsorbent existed in the form of divalent iron and were converted to trivalent iron compounds after the reaction.At the same time,the low concentration of water vapor in the reaction atmosphere will have a protective effect on the Fe(Ⅱ)active sites on the surface of the attapulgite,allowing more Fe(Ⅱ),Fe(Ⅲ)and lattice oxygen active sites on the surface to be effective at the same time to maximize the adsorption capacity.Based on density functional theory(DFT),the VASP software was used to perform theoretical calculations on the adsorption of PbCl2 on the surfaces of the modified Attapulgite adsorbents ATT(110),Fe(II)/ATT(110)and Fe(Ⅲ)/ATT(110)before and after modification to understand the adsorption mechanism of the adsorbents on the gaseous semi-volatile heavy metal PbCl2 at the atomic level and the possible solidification reaction pathway.The results showed that the adsorption energy of heavy metals on the surface of ATT(110)was consistent with the experimentally obtained results of heavy metal curing properties,and it was also found that heavy metal chlorides were more difficult to be removed than heavy metal oxides.On the surface of ATT(110),chlorine atoms in PbCl2 preferentially undergo a desorption reaction with hydrogen atoms in adsorbed hydroxyl groups(OH*)to generate hydrogen chloride by the following path:PbCl2+H2O→PbCl2ads+OHads+Hads→PbClads+Oads+Hads+HClfree→Pbads+Oads+2HClfree.Furthermore,the double active adsorption sites formed by surface-loaded iron-based oxides and lattice oxygen in attapulgite could significantly improve the coadsorption capacity of the modified adsorbent for H2O and PbCl2 gas,and explained that the presence of H2O promoted the adsorption of PbCl2 from the perspective of adsorption energy change.On the Fe(Ⅲ)/ATT(110)surface,the pathway is as follows:PbCl2+H2O→PbCl2-H2Oads→PbCl-HO-HClads→PbClO-HClads+Hfree→PbO-HClads+HClfree→Pb-Oads+2HClfree.On the Fe(II)/ATT(110)surface,the paths are as follows:(ⅰ)PbCl2+H2O→PbCl2-H2Oads;(ⅱ)PbCl2-H2Oads→PbCl2-HOads+Hfree→PbCl-HOads+HClfree→PbCl-Oads+Hfree+HClfree→Pb-Oads+2HClfree at the double adsorption oxygen sites(Fe(Ⅱ)-O and Olatt);(ⅲ)the transition from Fe(Ⅱ)-O to Fe(Ⅲ)-O at the reactive oxygen site Pb-Oads+2HClfree→Pbads+2HClfree.