Solidification/Stabilization Treatment And Assessment Methods Of Heavy Metal In Municipal Solid Waste Incineration Fly Ash

In this paper,on the current issues of serious exceeding criteria and high re-release risk of heavy metals in China,in the solidification/stabilization(S/S)treatment of municipal solid waste incineration fly ash(hereinafter referred to as "fly ash"),an organic/inorganic solidified system was designed,and the S/S effect and mechanism of fly ash were investigated under the invariability of capacity-increase ratio in fly ash.Based on the ecological risk issue of sodium dimethyldithiocarbamate(SDTC)chelator during the S/S process of fly ash,the model organism Caenorhabditis elegans(C.elegans)was selected to perform the ecological toxicology assessment.Meanwhile,a cycle assessment method with multiple factors was designed based on the process of freeze-thawing,dry-wet and carbonation to evaluate the release risk and migration and transformation rule of heavy metals in fly ash solidified blocks during the following landfill disposal.This study presented the innovative technology and assessment method focused on the issues in the fly ash treatment and disposal in China,which has some theoretical and practical value.The main results of the research are as follows:(1)Referred to the technical specification for pollution control of fly ash(HJ 1134-2020),the pollution situations and traceability of fly ash site samples in several regions of China were investigated.The results show that,Pb and Cd leaching concentrations of fly ashes and solidified samples in various regions exceeded criteria seriously,which was associated with the mixing of part industrial waste into the source.Based on the situations of serious exceeding criteria of heavy metals in fly ash,the process optimization methods of 6 MPa compaction-forming and water-wash treatments were performed.The results show that,Cd leaching concentration and capacity-increase ratio of compaction-forming treatment were significantly reduced by 60.6%～70.2%and 53.4～74.1%,respectively.After water-wash treatment,Pb,Cd leaching concentrations in fly ash solidified sample were significantly reduced and immobilization efficiency of Pb was up to above 98%,which improved the compliance rate of fly ash landfilling.This paper proposed an emergency disposal plan of"water-wash +S/S+forming treatment" for high-pollution fly ash.(2)Based on the issue of limited effect on the prescribed low-dosage ordinary Portland cement solidifying fly ash,the 0.5-2.0%gelatinized sticky rice slurry was added into the system of 10%cement+fly ash,and it was found that the initial and final setting time in this solidified system were reduced by 18.7%～25.3%and 18.0%～26.8%,respectively,the compressive strength after curing 28 days was increased by 22.9%～67.3%.Besides,the Pb leaching concentrations were significantly reduced in the solidified system,and the immobilization efficiency of Pb is up to above 99%after 28 days of curing,which much below the limit value in the criteria of landfill,however,the Pb leaching concentrations of the control group of cement treatment always exceeded the criteria of landfill.The reasons are that addition of SR could restrict the growth of hydration particles,and the solidified structure became more compact,so that Pb in fraction of Fe-Mn oxides was more easily transformed into the residual and organic matter structure.Furthermore,based on the complex fly ash solidified system,the mechanism of gelatinized sticky rice starch/calcium silicate hybrid materials solidifying Pb was investigated,and it is found that incorporating sticky rice starch could accelerate the induction of calcium silicate solidifying Pb to yield the new crystalline phase mineral,which reduced the content of mobility state fraction of Pb.(3)The research demonstrated that the acute 24 h half-lethal concentration(LC50)value of SDDC in C.elegans was 139.4 mg/L,and SDDC was found that the concentration of 1.0 mg/L is a safe limit value in the water environment.The concentration above 1.0 mg/L caused significant adverse effects of C.elegans on the survival rate,growth,locomotion behaviors and reactive oxygen species(ROS)production,as well as expression of hsp-16.2,ctl-2 and cat-1 genes,which are associated that higher concentration of SDDC has the neurotoxicity and causes the disorder of in vivo dopamine metabolism.Besides,the Cu,Pb chelates of SDTC had obviously lower toxic effect than untreated Cu,Pb metals,but these chelates still perturbed the early growth development in C.elegans.(4)The long-term cycles assessment method of "freezing-thawing+drying-wetting+accelerated carbonation" was established.The physical-chemical properties and the release behavior of heavy metals in the fly ash solidified blocks were studied,and the release risk of heavy metals was evaluated by the overall pollution toxicity index(OPTI)and risk assessment code(RAC).The results of long-term stability performances indicated that:SDTC chelator treated<cement+chelator treated<cement+chelator+gelatinized sticky rice starch.The cumulative mass release rate of chelator treated was up to 42.8%after cycles of 6,and then the disintegration of chelator treated sample had occurred;The organic sulphur chelates of Pb were decomposed and transferred to PbCO3 and PbO2 in chelator treated samples after cycles of 12.After addition of cement and gelatinized sticky rice starch,the cumulative concentrations of Pb,Cd and release of inorganic salts were obviously reduced,and cumulate OPTI and RAC risk grade always maintained in the low-risk level.During the cumulate tank leaching of carbonated water,the dissolution of CaClOH and the decalcification and decomposition of hydrate products made the pore structure became deteriorate.The correlation analysis demonstrated that the dissolution of salts and increase of pore volume are the main factors affecting the release of heavy metal.