| Municipal solid waste incinerations(MSWIs) fly ash is classified as a hazardous solid waste since it contains toxic substances such as heavy metals. Fly ash occupies about 3%-5% of the total amount of incinerated waste. It is predicted that there will be 400 thousand t/y of fly ashes at the end of 2015 in China. Two conflicts has become bottlenecks and restrict the development of fly ash disposal, includ ing conflicts between increasing amount of fly ash and lacking of disposal facilities; and increasing demand of recycle and lacking of technical standards and principles.One key issue is the potential for human and environment risks caused by pollutants(heavy metals, dioxins etc.) in the fly ash, which precludes recycling of MSWI fly ash. Nowadays, there is still no such system for risk assessment of MSWI fly ash recycling in China. The objective of this study was to evaluate the chemical distributions and enrichment characteristics of heavy metals and dioxins in MSWI fly ashes in China. It also discussed toxicity characteristics of raw and washed fly ashes. Then, we compared heavy metal stabilizations of raw, washed and stabilized fly ashes at long-time leaching. Based on these results, a complete assessment procedure was developed to assess the potential risk of fly ash recycling, and three conceptual site models were assessed according to the risk assessment procedure. Main conclusions are as follows.Characteristics of fly ashes sampled from 15 typical MSWI plants in Chinese mainland were analyzed. The results showed that total content of heavy metals decreased in the order Zn > Pb > Cu > Cr > N i > Cd in these samples. The contents of PCDD/Fs and dl-PCB were 0.034-2.5 ng-TEQ/g, and the main contributors were 1,2,3,7,8-Pe CDD and 2,3,4,7,8-Pe CDF. Furthermore, heavy metals were enriched in fine particles(≤ 10μm), which should be considered when accessing inhalation risks. TCLP and PBET were used to evaluate the toxicity of fly ash and the factors affecting the toxicity were also investigated. The great relevance was found between leaching behavior and acid soluble fraction of heavy metals. This was the reason why total amount of Cd was very low, but its leaching amount was the highest in all detected heavy metals. Compared to raw fly ash, the content of Zn and Cr in washed ash showed increasing tendency, but decreasing leaching toxicity. Especially for Pb, its leaching amount decreased from 1.1-76.3 mg/L to 0.8-5.12 mg/L. Result of risk assessment code(RAC) suggested that Cd and Pb showed a very high risk to the environment, and high risk samples were obviously reduced after washing. Bioavailability of heavy metals in fly ashes were all far lower than TDI(To lerable daily intake), which should be considered when accessing ingestion risk.Long time stability of heavy metal is the focus when recycling of fly ash. Column leaching test(CLT), multiple extraction procedure(MEP) and simulated natural exposure experiment were adopted to study the leaching behavior of heavy metals in different pretreated fly ashes, including raw, washed and stabilized fly ashes. Several phenomena were obtained under long time acid rain: For Pb, only the leaching concentration of the stabilized washed fly ash was under V class standard of GB/T 14848-93; Zn and Cd leaching from various fly ashes were lower than the standard above. The cumulative amount of heavy metals in different leaching methods increased in the order CLT<TCLP<MEP. It was found p H at the end of experiment greatly affected the leaching amount of heavy metals. It was suggested that heavy metal leaching amount of MEP was conservative to evaluate the leaching risk to the soil. The simulated natural exposure experiment showed that wash and stabilization controlled the leaching of Pb, but wash did not control the leaching of Cd. It was found that different from raw fly ash, leaching behavior of heavy metals in stabilized fly ash had no correlation with their chemical speciations.Large volume of fly ash is recycled for area of road construction. A risk assessment procedure was developed for the use of fly ash in road construction. And based on the results above, three conceptual scenarios, including fly ash natural storage,fly ash used in road paving and road used fly ash using, were assessed according to the developed procedure. At the same time, this procedure was also further verified. As a result, in the natural storage scenario, the main risk was posed by inhalation of PM10 enriched with heavy metals and dioxins. The cumulative cancer risk values for the staff on site and maximum particulate precipitation site were 4.96E-04and9.17E-04, higher than the threshold value of 1.0E-04.Correspondingly,non-cancer risks were 2.92 and 1.97, higher than threshold value of 1.The contribution percentages of PM2.5 to cancer and non-cancer risk were 27% and 20%,respectively. In the road paving scenario, the cumulative cancer risk values of PM10 for the staff on site was 6.57E-04, corresponding non-cancer risks 3.95, both of which were higher than threshold values. In this scenario inhalation risk also was the main risk pathway, and the PM2.5 risk was very lower, accounted for only 5%。In these two scenarios above, the main substance causing cancer risk was Cr and non-cancer risk was Cd. In road using scenario, with the amount 2% or 5% of raw or washed fly ash added in asphalt concrete, only the leaching of Zn was safe for soil and groundwater. And with the amount 10% or 20% of washed fly ash added in cement concrete, the leaching of Zn was also safe for soil and groundwater. But Pb and Cd were potentially dangerous to soil or groundwater. It was analyzed that if the leaching concentration could protect groundwater and it was also safe for soil. Thus, it was suggested that the maximum allowable amount of Zn, Pb and Cd in the asphalt concrete were 216,1.16 and1.13 mg/kg, and in the cement concrete were 158,0.85 and 0.82 mg/kg, respectively. When adding 30% of raw fly ash in subbase materials, the concentration of Pb at the target site reached 0.042 mg/L, higher than the Chinese SDWQ 0.01 mg/L. However, concentration of Pb reached 0.028 mg/L when 30% of washed ash was added. |
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