Efficiency And Mechanism Of Recovering Heavy Metals In Waste Incineration Fly Ash And Degrading Organic Matter By Na₂EDTA Leaching And Electrochemistry-Fe³⁺/Oxone

The proportion of incineration treatment of municipal solid waste in China is increasing year by year,and the fly ash produced by incineration,a hazardous waste,has strong enrichment for heavy metals and contains a large amount of heavy metals.How to realize the harmlessness and resource utilization of MSWI fly ash has been receiving much attention.Na₂EDTA leaching is often used in the treatment of soil.This paper aimed to recover heavy metals from municipal solid waste incineration fly ash.Heavy metals from incineration fly ash were removed by Na₂EDTA was proposed,and then the optimal concentration of Na₂EDTA was determined by heavy metals removal rates and toxicity leaching concentration.The EDTA was recovered by the decomposition of Fe³⁺on the heavy metals-EDTA,and the availability of recycled Na₂EDTA（re-Na₂EDTA）was examined.The electrochemistry-Fe³⁺/Oxone system was used to enrich and recycle and degradate organics from the solution that EDTA had been recovered under different conditions,including current intensity,Oxone dosage and initial pH of solution,were described.At the same time,the distribution of heavy metals and removal mechanism of organics by capturing free radicals were explored.Finally,another alkali precipitation method was used to remove heavy metals in the solution that EDTA had been recovered,and the two precipitates produced during the reaction were subjected to toxicity leaching analysis.Then the electrochemistry-Fe³⁺/Oxone system was used to remove organics and remanent heavy metals from the solution.At the same time,the optimal Oxone dosage was explored.The main findings are as follows:（1）Ca,O,Cl and C were the main elements of MSWI fly ash,and accounted for 21.97%,21.47%,16.84%and 10.92%,respectively.Moreover,the incineration fly ash had a great enrichment for heavy metals,the concentrations of Cu,Pb and Zn were 1367.2 mg/kg,1079.7 mg/kg and 8752.5 mg/kg,respectively,and their leaching concentrations were 132.8 mg/L、19.54 mg/L和502 mg/L,respectively.They exceed the soil background values and the limit values of national toxicity identification standards in China,indicating that incineration fly ash had great potential harm to the environment.（2）It had good practical application value that heavy metals such as Cu,Pb and Zn in MSWI fly ash were leached by Na₂EDTA.When the concentration of Na₂EDTA solution was 0.1 mol/L,the leaching concentrations of Cu,Pb and Zn in the incineration fly ash residue were 65.9 mg/L,3.29 mg/L and 85.8mg/L,respectively,which were lower than the limit value of the toxicity identification standard.And the removal rates of the three heavy metals were 69.7%,72.7%,and 66.5%,respectively.（3）The leaching concentrations of Cu,Pb and Zn in PBET increased with respect to leaching toxicity,but were still within the standard limits.In the sequential extraction procedure,heavy metals of the exchanged state and the carbonate-bound state in the incineration fly ash were mainly leached by Na₂EDTA,and the residue state leaching amount was the least.The cumulative leaching contents of Cu,Pb and Zn in multiple extraction procedure were higher than that of toxicity characteristic leaching,indicating that the incineration fly ash had long-term potential hazard in the environment.It was also proved that the toxicity risk of incineration fly ash to the environment required comprehensive evaluation of various methods.（4）The decomplexation of Fe³⁺on heavy metal-EDTA made the maximum recovery of EDTA reach89.67%.In addition,heavy metals in incineration fly ash can be effectively leached by recycled Na₂EDTA,but due to its own structural changes and a certain amount of heavy metals,the leaching concentration of Cu,Pb and Zn in the remaining residues increased,and were 70.5 mg/L,4.08 mg/L and 92.1 mg/L,respectively,but still less than the limit value of Chinese national toxicity identification criteria.（5）In the wastewater treatment by using electrochemistry-Fe³⁺/Oxone system,when the current intensity was 0.5 A,the Oxone dosage was 0.5 g,and the initial pH of the solution was 7,after 24 hours of electrolysis,the Cu,Pb,Zn and COD contents in the solution were reduced to 0.41 mg/L,0.052 mg/L,0.82mg/L and 47.7 mg/L,respectively.At this time,Cu,Pb,and Zn were mainly distributed on the cathode iron plate,which are 90.8%,79.8%,and 93.2%,respectively.At the same time,the degradation of organics was mainly caused by?OH was discovered by trapping free radical,and it accounted for 54.86%.（6）A large amount of heavy metals in the solution that EDTA had been recovered were removed by Ca（OH）₂ during alkali precipitation.And after that,the concentrations of Cu,Pb and Zn were only 1.65mg/L,2.46 mg/L and 2.57 mg/L.When the current intensity was 0.5 A and the Oxone dosage was 0.5 g,after 12 h of electrolysis,the contents of Cu,Pb,Zn and COD in the solution were reduced to 0.178 mg/L,0.091 mg/L,0.29 mg/L and 36.8 mg/L,respectively.Moreover,the leaching concentrations of the three heavy metals in the two precipitates produced during the alkali precipitation process were lower than the limit values of the national toxicity identification standards.