| Tibet is an important ecological security barrier in China and is listed as a highland for the construction of national ecological civilization.The altitude here is high,the temperature is low,and the ecological environment is fragile.With the rapid economic and social development and population increase in Tibet,the output of municipal solid waste in Tibet is also increasing.The efficient treatment and disposal method of municipal solid waste is not only an important guarantee for the healthy and sustainable development of plateau cities,but also an important basis for the ecological security of Tibet under the current and future development scenarios.Because incineration has the advantages of reduction,harmlessness and resource utilization,incineration technology has become one of the mainstream technologies for urban domestic waste treatment in my country.However,incineration will produce 3%to 5%of incineration fly ash.The fly ash contains a lot of persistent toxic pollutants such as heavy metals and dioxins.China has stipulated the fly ash as hazardous solid waste and standardized management of it.Due to the special plateau natural environment characteristics of Tibet,especially the characteristics of low air pressure,low temperature and low atmospheric oxygen content,there are significant differences in the characteristics of domestic waste and waste incineration conditions between the plateau and the mainland,which may make the characteristics of the fly ash from waste incineration in Tibet different from other regions in China.In the end,the characteristics of waste incineration fly ash under plateau conditions and how to achieve the maximum stabilization effect have not yet been reported.This paper takes the fly ash from a domestic waste incineration plant in Lhasa as the research object,and the heavy metal such as Pb,Cd,Zn,Cu,Cr,Ni,As and Hg in the fly ash is taken as the monitoring items,the characteristics and heavy metal leaching characteristics of raw fly ash in Lhasa were tested and analyzed by the standard analysis methods.The leaching toxicity of heavy metals in the stabilized products of fly ash in Lhasa was analyzed,and the risk of heavy metal pollution was evaluated using the potential ecological pollution risk assessment method and the health risk assessment method.Aiming at the fly ash in Lhasa Plateau,comparing the treatment effects of different chemicals on heavy metals in the fly ash,combined with the orthogonal experimental design,the stabilizing agent suitable for stabilizing the fly ash in Lhasa City was optimized,and a fly ash stabilization suitable for the fly ash in Tibet Plateau was proposed plan.The results show:（1）The raw fly ash from waste incineration in different seasons in Lhasa is strongly alkaline with low moisture content.The content of various elements in fly ash varies greatly,among which the content of Zn is the highest,followed by Pb and Cu,and the contents of Cd,Cr,As,Ni and Hg are all lower.The content of Zn in the rainy season is higher than that in the dry season,and the content of Cu,Pb,Cd,Cr,Ni,As and Hg in the rainy season is lower than that in the dry season.Compared with other cities,the content of Zn,Pb,and Cu is at a moderate level,the content of Cd is comparable to that of the other six cities,and the content of Cr and Ni in the fly ash of Lhasa is lower than that of the other six cities.The leaching concentrations of Pb and Cd elements in the original fly ash in Lhasa exceeded the standard limit of hazardous waste identification and the standard limit of pollution control of domestic waste landfills to varying degrees,while the leaching concentrations of Cu,Zn,Cr,Ni,As and Hg elements did not exceed the relevant standard limits.（2）The stabilization technology of fly ash in Lhasa is not effective in stabilizing heavy metal elements,and the leaching concentration of Pb and Cd elements after stabilization treatment still exceeds the control standard limit of the landfill.Environmental risk assessment found that heavy metals in fly ash have strong potential environmental risks to the environment,of which Cd is the main contributor.The health risk assessment found that the non-carcinogenic risk values of heavy metals for adults and children are 8.8794 and 20.006,respectively.Heavy metals Cd and Pb were the main contributors.The non-carcinogenic risk values for adults are 6.1258 and 2.7229,respectively.They are 13.802 and 6.1350,respectively,which are much higher than the non-carcinogenic risk level recommended by the US Environmental Protection Agency（EPA）（1）,and the non-carcinogenic risk to children is higher than that of adults.Cd element has a higher carcinogenic risk,while the carcinogenic risk of Cr,Pb and As is within an acceptable range,and the carcinogenic risk to adults is higher than that of children.The total carcinogenic risk values of heavy metals in adults and children are 8.14×10 and 4.59×10,respectively,of which Cd is the main contributor,and the RISK value is greater than the carcinogenic risk level of 1.0×10,with a high risk of carcinogenicity to humans.However,the carcinogenic risks of Cr and Pb elements are between the EPA-recommended carcinogenic risk levels of1.0×10 and 1.0×10,and the carcinogenic risks to humans are within an acceptable range.The carcinogenic risk value of As is less than1.0×10,and there is no carcinogenic risk to humans.And the risk of cancer of heavy metals in adults is higher than that in children.（3）Aiming at the fly ash of Lhasa in dry season,nine main stabilizing agents such as Na2S,sodium thiosulfate,Na2HPO4,Na H2PO4,disodium ethylenediaminetetraacetate（EDTA）,ethyl disulfide,sodium dimethyldithiocarbamate（DTC）,nine main stabilizing agents of hydroxyethylidene diphosphonate（HEDP-2Na）and butylammonium black medicine were used for stabilization experiments.The results showed that under the same addition amount（5%）,the removal rate of Pb element in Lhasa fly ash by the agents ethionine,butylammonium black medicine and Na2HPO4is very high,which are 99.3%,99.6%and 98.8%,respectively.Elements also have certain processing effects.DTC has a good stabilization effect on Cd elements.The HEDP-2Na,EDTA,Na2S,Na H2PO4,and sodium thiosulfate have poor treatment effects on Pb and Cd.Therefore,the chemical with better treatment effect is further selected,and the treatment effect of the chemical on the heavy metals in fly ash is studied under different dosages.The results show that ethionidine,butylammonium black medicine and Na2HPO4 have good stabilization effect on Pb element,and the minimum addition amount of 1%,1%and 4%can make the leaching concentration of Pb element lower than the landfill limit.value,and the addition of DTC is 10%,the leaching concentration of Pb element still could not reach the landfill limit.The leaching concentration of Cd element can be lower than the limit value of landfill site when the addition amount of ethionine,butylammonium black medicine and DTC is 2%.When the amount of Na2HPO4 used is 10%,the leaching concentration of Cd element still cannot reach the landfill pollution control.（4）In order to improve the stabilizing effect of the chemicals,the effects of three factors,such as the type of the agent,the amount of the agent added,and the stabilization time,on the stabilizing effect of the heavy metals were studied through orthogonal experiments,and the optimal combination was selected.The results show that the influence degree of different factors on Pb element is in the order of ethyl thionitride>Na2HPO4>stabilization time,and the optimal level combination is 2.5%ethyl sulfide nitrogen+4%Na2HPO4+5d stabilization time;the degree of influence on Cd element is as follows:Ethyl thionitride>stabilization time>Na2HPO4,the optimal level combination is 2.5%ethionine+5%Na2HPO4+3d stabilization time.Taking into account the stabilizaing effect on Pb and Cd elements,the cost of the agent and the stabilization time,the optimal process combination was obtained as 2.5%ethyl sulfide nitrogen+4%Na2HPO4+3d stabilization time.The research of this paper provides scientific basis and technical scheme for the harmless treatment and disposal of waste incineration fly ash in Lhasa,and also provides reference for other cities in the plateau. |
