Incineration with energy recovery is recommended as a preferred option for dealing with municipal solid waste (MSW) to effectively reduce the original waste volume and mass by approximately 90% and 70%, respectively, and generate electricity and heat [1
]. However, a considerable amount of residual material is still generated after the incineration process: typically MSW incineration fly ash (MSWI-FA) and MSW incineration bottom ash (MSWI-BA) [4
]. MSWI-BA accounts for nearly 80% of the total residual by mass and is complex, consisting of combustion residue and non-combustible constituents of the waste feed [6
]. In Europe and Asia, MSWI-BA is often classified as non-hazardous waste [9
]. For this reason, MSWI-BA is commonly discarded in landfills.
In 2017, 84.63 million tons of MSW were treated by incineration in China, accounting for nearly 39% of the total mass [10
], which produced approximately 20.31 million tons of MSWI-BA. It is reported that the amount of MSWI-BA generated is expected to exceed 28 million tons by 2020 according to the National Thirteenth Five-Year Plan of China [11
]. If all these MSWI-BAs are to be disposed of in landfill sites; around 12.73 million m3
of landfill space (calculated by the density of 2.20 g/cm3
]) will be needed. Therefore, MSWI-BA treatment has been a tremendous challenge to most Chinese cities with high population densities and limited land resources.
Traditionally, landfilling is regarded as the most convenient and inexpensive approach for disposal of MSWI-BA. However, it results in significant environmental problems. Studies indicate that MSWI-BA contains various heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and chromium (Cr), which are present in high concentrations [3
]. The leaching of these metals from MSWI-BA when exposed to rainwater can seriously contaminate the surrounding sensitive recipients, including soil, surface and sub-surface water bodies [5
]. Environmental safety, thus, has become a great concern for MSWI-BA management.
The recycling of MSWI-BA as road construction material, especially considering the reduction of natural aggregate usage, is recommended as an important management option and has gained worldwide attention. Numerous studies were conducted on the physicochemical and engineering properties of MSWI-BA. It is concluded that MSWI-BAA made by fresh MSWI-BA after being pretreated, has a good particle size distribution and similar properties to natural aggregate and is suitable to be used in asphalt mixtures and cement concrete [4
]. Some studies have consequently used MSWI-BAA to partially replace natural aggregate in asphalt mixtures. It is reported that the substitution of MSWI-BAA with coarse and fine natural aggregates in traditional, dense hot-mix asphalt (HMA) can meet the technical requirements, but the characteristics of being lightweight, and having a smaller specific weight, compared to natural aggregate, must be seriously considered [14
]. It is, therefore, suggested that MSWI-BAA is more suitable for use in low-traffic-volume roads [23
Rapid urbanization coupled with climate change is placing increasing pressures on urban stormwater management [24
]. The increase in impervious surfaces with asphalted roads and rooftops significantly increases stormwater volumes and peak flowrate, while also decreasing stormwater quality and impeding groundwater recharge [25
]. Permeable asphalt (PA) pavement is a popular and practical, low-impact development (LID) technology that serves as an ideal alternative to conventional low-traffic-volume pavement because it can help to address the issues stated above by providing in situ restoration of the urban hydrological cycle and reducing the need for traditional stormwater facilities [28
]. PA pavement consists of various layers with porous materials, which not only allow stormwater to infiltrate into the ground, unlike conventional asphalt pavement, but can also simultaneously remove pollutants (e.g., total suspended solids (TSS) and heavy metals) from stormwater runoff on site, thus raising their value as a LID option in current urban development [25
]. As the PA mixture (the surface material of the PA pavement) requires relatively low strength compared to conventional dense asphalt mixtures, researchers have explored the feasibility of recycling MSWI-BA aggregate (MSWI-BAA) in PA mix designs. The results are encouraging, indicating that PA mixtures containing MSWI-BAA have better performance than those without, and the replacement ratio can be up to 80% [23
On the other hand, the potential environmental impacts associated with the use of MSWI-BAA in asphalt mixtures are of great concern [21
]. Results indicate that the leaching concentrations of heavy metals (e.g., Pb, Zn, Cu, Cr and Cd) from HMAs containing MSWI-BAA are significantly reduced compared to unbounded MSWI-BAA. It has been concluded that there is very little environmental risk for substituting MSWI-BAA for natural aggregate in dense asphalt mixtures. Previous investigations have provided valuable information for the leaching characteristics of heavy metals from dense asphalt mixtures containing MSWI-BAA; however, there are few studies focusing on PA mixtures containing MSWI-BAA. High voids in PA mixtures provide benefits for stormwater management in urban areas, but it can also lead to an increase in contact between the PA material and the infiltrated stormwater during wet weather.
To encourage the utilization of MSWI-BAA in PA mixtures, investigating the leaching behavior of unbounded MSWI-BAA and within PA mixtures to identify environmental consequences is warranted. The objectives of this study are to investigate and compare the heavy metal leaching of unbounded MSWI-BAA and PA mixtures containing both the coarse and fine MSWI-BAAs at optimal replacement ratios. In this study, heavy metals present in MSWI-BAAs with three particle sizes (0.075–2.36 mm, 2.36–4.75 mm and 4.75–9.5 mm) were identified based on X-ray Fluorescence (XRF) tests. The leaching behaviors of heavy metals in MSWI-BAA with three particle sizes were investigated with HVEP tests and simulation experiments. The leaching behaviors of PA mixtures containing these MSWI-BAAs were examined by simulation experiments. The difference in leaching behavior between unbounded MSWI-BAA and PA with MSWI-BAA were compared and analyzed. The leaching data were also compared to limit values in Chinese standards for surface water, subsurface water and irrigation water, to assess the environmental risk for the utilization of MSWI-BAA in PA mixtures.
This study investigated and compared leaching behaviors for four heavy metals (Zn, Pb, Cu and Cr) from unbounded MSWI-BAA and PAC-13 mixtures containing MSWI-BAA. The HVEP test and simulated leaching experiments were conducted on MSWI-BAAs of three particle sizes (0.075–2.36 mm, 2.36–4.75 mm and 4.75–9.5 mm), but only the simulated leaching experiment was conducted on PAC-13 mixtures with and without MSWI-BAA. Leaching data were also compared to the regulatory limit values to evaluate the toxicity of leachate.
The HVEP data from unbounded MSWI-BAA showed that the type of leaching heavy metal directly affected the leaching. MSWI-BAA particle size was proven to have very good linear relationships with the leaching concentrations of Cr, Zn and Pb, which indicates that the leaching levels of Cr, Zn and Pb are strongly correlated to MSWI-BAA particle size.
Regarding the simulated leaching experiment for unbounded MSWI-BAA, the leaching concentrations of four heavy metals showed different trends over contact time. Increase in leaching time led to regular increases in Cr and Cu and regular decreases in Pb and Zn, indicating that contact time is another factor affecting heavy metal leaching.
For the PAC-13 mixture with MSWI-BAA, in general, the leaching concentration of Cu remained stable over time, Pb and Zn concentrations showed a small fluctuation and Cr showed a continuously increasing trend throughout the experiment. Similar tendencies in leaching concentrations over the contact time were also observed for the control mixture. This indicates that the presence of MSWI-BAA in a PAC-13 mixture does not necessarily change the basic tendency of heavy metal leaching over time, but it is able to lead to an increase in Cr and Zn in leachate overall.
The Cr and Cu concentrations in leachate from PAC-13 containing MSWI-BAA showed an overall decrease compared with unbounded MSWI-BAA, which suggests that the encapsulation of MSWI-BAA particles by asphalt binder is effective for reducing Cr and Cu concentrations in leachates.
All leaching data indicate that the leaching process of heavy metals was comprehensively influenced by contact time, the metal species leaching and MSWI-BAA particle size whether MSWI-BAA is used alone or mixed with a natural aggregate in PAC-13 mixtures. According to the results of environmental risk assessment, both unbounded MSWI-BAA and PAC-13 mixtures containing MSWI-BAA have very little negative impact on surrounding surface and subsurface water quality, and the leachate is safe for irrigation. But the leaching of Cr and Pb should be monitored and mitigated.