Assessing The Impacts of Wet Treatment On Leaching Toxicity of Municipal Solid Waste Incineration Bottom Ash


 Wet-treatment is usually employed to recover metals from bottom ash (BA). However, its effectiveness on regulating the leaching behavior of BA and minimizing environmental impact is still unquantified when BA is used as engineering materials. This study investigated the leaching behavior of targeted pollutants including Cu, Zn, Ni, SO42- and Cl- in fresh bottom ash (FBA) and treated bottom ash (TBA) using batch, standard column up-flow leaching and simulated rainfall down-flow leaching tests. It was firstly noted by the batch leaching that the potential ecological risk of MSWI bottom ash could not be ignore during its reutilization, and wet treatment enabled reduce the leaching concentration of Cu, Zn, Cl- and SO42- by 7.1%, 33.8%, 46.3% and 18.9%, respectively. The leaching concentrations of the targeted ions in FBA are generally higher than those of TBA. Furthermore, when BA was applied in water dynamic system such as rainfall environment, its toxicity leaching should be paid more attention onto the early leaching stage with a very high water dissolved salts release, such as Cl- and SO42-. The leaching concentration of Cu particularly exceeded the limit value before L/S=1. Meanwhile, it was also found that the leaching of SO42-, Ni, Cu and Zn in water dynamic system were directly proportionate to the liquid-to-solid (L/S) ratio. The leaching concentration of the pollutants obtained from the simulate rainfall down-flow tests was usually 4-6 times higher than those from the standard up-flow column when at the same ratio of liquid and solid.


Introduction
Many countries suffer from the surge of municipal solid waste (MSW) generation due to the increasing consumption level and rapid urbanization (Barisa et al. 2015). To counter this issue, municipal solid waste incineration (MSWI) has become a viable alternative due to its effectiveness in reducing the volume up to 90% and the mass up to 70% of solid waste, while at the same time producing energy and destroying organic pollutants (Del Valle-Zermeño et al., 2014; Sabbas et al., 2003). Most solid residues of MSWI remains as bottom ash (BA), which accounts for 80-85% of the total ash residues (Wiles 1996). It brings an impetus to the safe disposal, or preferably, reutilization of BA. The reutilization of BA in road pavement, glasses and ceramics, adsorbent for dyes, and aggregate in concrete are considered safer as reported (Yin et al. 2018). One of the main concerns is the possibility of material leaching when exposed to rainwater, which could lead to the contamination of nearby sensitive recipients such as water bodies, groundwater systems, as well as fauna and ora (Shih and Ma 2011). Therefore, pre-treatment processes were usually employed to reduce the leaching of heavy metals and dissolved salt from reutilized BA. In southern China, the combination of wet treatment technologies using only water for separating and washing such as wet eddy current separator, jig, and wet shaker are often used to extract metals from bottom ash. Detailed information about wet treatment process of BA in China has been provided in our previous work (Hu et al. 2021).
In the last few decades, various leaching testing methods were developed and standardized in order to evaluate leaching behavior of certain materials from solid waste and their by-products when exposed to water (Kosson et al. 2002 important environmental factor, the in uence of the rainwater leaching due to rainfall acidity on eldscale leachate should be studied. Another factor not considered in the standard column up-ow leaching test is the compaction of BA applied in road construction. Therefore, the leaching results obtained from the standardized column leaching test were different from practical application due to the differences in pH and compaction. Standardized up-ow columns also use the saturated ow condition with constant ow rate, while the eld-scale utilization of BA in construction works is more likely to experience intermittent down-ow conditions with varying ow rates. It is therefore important to understand the similarities and disparity between the various types of leaching tests to provide better tools for environmental evaluation. While eld conditions are unlikely to be exactly duplicated in a laboratory, column leaching tests seem to be the best t to re ect the eld-scale application of BA. In this study, the leachability of MSWI bottom ash before and after wet treatment was investigated using batch leaching, standardized column up-ow leaching, and the simulated rainfall down-ow leaching tests. The leaching concentration of the targeted ions (Cu, Zn, Ni, Cl − and SO 4 2− ) were compared with the limit value of surface water of Class V in China, in order to evaluate their environmental impact. The obtained results could be used to assess the leaching characteristics of the targeted pollutants when reuse BA as engineering materials.

Materials
MSW BA samples were collected from a MSW incineration plant in Shanghai. The grate furnace incinerator operates between 850 ℃ to 1000 ℃ with a capacity of 750 tons/day. The samples were classi ed into fresh bottom ash (FBA) and the treated bottom ash (TBA) after wet treatment. FBA refers to a mixture of grate ash, grate sifting, boiler ash, and economizer ash. After incineration, water quenching and water washing were used to recycle metals from FBA and additionally make the resulted bottom ash usable as engineering materials. FBA and TBA samples were sieved to obtain samples with particles size less than 5mm. Large particles such as ceramics, metal, and glass were manually separated. X-ray uorescence (XRF) was used to measure the chemical composition of major elements in the samples, and the results are presented in Table 1. The results revealed that the most abundant elements, present as oxides, were as following: SiO 2 (> 30 wt%), CaO (> 30 wt%), Al 2 O 3 (> 8 wt%), and Fe 2 O 3 (> 6 wt%). Other than the listed oxides, it is also necessary to pay attention to the heavy metal elements such as Zn, Cu and Ni because of their potential leaching toxicity.

Batch leaching test
The leaching toxicity tests of the two kinds of materials described in Sect. 2.1 were performed using HJ557-2009 leaching test method. The analytical procedure is as follows: bottom ash samples with a dry basis weight of 100g was weighed, then mixed with deionized water with the liquid-solid ratios of 10 (L/kg). The mixture was then placed in sealed bottles and shaken. The mixing process was done using a mechanical shaker set at 110 ± 10 rpm, for 8 hours. The leachates were then left to settle in the bottles for 16 hours. After that, all leachates were ltered through 0.45 µm polypropylene membrane lters and stored at 4 ℃ for further analyses.

Standardized column leaching test
The percolation column tests were performed in Plexiglas columns with the height of 40 cm and the inner diameter of 5 cm, based on the speci cations of the standard method CEN/TS 14405, 2017. The bottom of the columns was equipped with perforated plates. The columns were closed with threaded connection and stuffed with box packing. A 2.5 cm thin layer of non-reactive quartz sand was placed in the top and bottom sections of the column. TBA was packed into the columns under light tamping with a plastic rod to level the material between layers. The experiment setting is depicted in Fig. 1a.
Before starting the test, the lled columns were saturated with deionized water and left for three days in order to equilibrate the system. After the equilibration, the column test is initiated by starting the pump again to ow water at a rate of around 12 mL/h. Each run was conducted until 10 L of deionized water has been owed through the column, giving an L/S ration of 10 L/kg. During the test, seven kinds of distinct leachate were collected at different cumulative L/S ratios including 0.1, 0.2, 0.5, 1, 2, 5, and 10 L/kg of dry BA. The eluates were collected in bottles covered with lm in order to minimize the effects of carbonation prior to further analysis.

Simulated rainfall down-ow leaching test
The experiment setting for rainwater leaching test is illustrated in Fig. 1b. The plexiglass column has an inner diameter of 9 cm and length of 75 cm. It is lled with sand and BA layers similar to the standard column up-ow test with different amount of BA such as depicted in Fig. 1b. The liquid used was a solution with pH = 5, which was adjusted using appropriate volumes of deionized water and nitric acid to simulate acid rain water. A constant ow rate of 3.18ml/min simulating the maximum 24-hour rainfall rate in southern China was maintained by the peristaltic pump. The column test was done for 72 hours each run. A threshold of 85% of the minimum compaction speci cation requirements for road pavement (JTGF10, 2006) was adopted.
Prior to the experiment, the column was rinsed with acid solution and then cleaned with deionized water. Two ltration layers of acid-washed quartz sand were then placed at the top and bottom of the sample layer. The sample was then placed and compacted every 5 cm, to 85% of the minimum compaction speci cation requirements for road pavement (JTGF10-2006). Then the soil surface was roughened, and the next lling was continued. The total loading height of slag was 50 cm. Depending on the material, the total weight of slag was between 5 kg and 6 kg. Before the experiment, the soil column was saturated slowly from top to bottom with leaching liquid, before the peristaltic pump was started. The leaching solution was collected at the lower outlet at certain time intervals. All leachates obtained from the tests were preserved with HNO 3 (2% volume), sealed, and stored at 4℃ before analysis for dissolved elements.
To quantify the pollution level of leachate, the release data were compared with the limit value of surface water of Class V in China (GB3838-2002), which corresponds to agricultural and general surface water   Meanwhile, SO 4 2− leaching level is also found to be high at the beginning leaching stage, with 2020mg/L and 3960 mg/L for TBA and FBA, respectively. This raises a concern because large amounts of soluble salts in the leachate would deteriorate the environment when it is applied. As shown in Fig. 4, Cl − evidently exceeds the limit value before L/S = 1, while SO 4 2− exceeds the limit value almost in the entire leaching process. This phenomenon correlates the high leaching of the dissolved salt in the BA. Moreover, it is compounded by the lengthy equilibrium period at the beginning of the up-ow percolation tests. The leaching concentration of Cu also exceeds the limit value, with the highest concentration of 7.92mg/L. Zn concentration generally falls below the limit value, with only a data point exceeding the limit. The above results indicate that the adverse effect of Cu and Zn leaching behavior could be ignored after L/S = 1, with all concentrations below the safe value. Comparing the obtained results on the leachability of different types of BA, an obvious difference between FBA and TBA is noted. The leaching concentration of the targeted pollutants in FBA is generally higher than that of TBA. Especially for SO 4 2− and Cl − , their leachability in FBA is far higher than that in TBA.
The most obvious disparity regarding leaching concentration between FBA and TBA could be noted in

Leachability of BA by Simulated rainfall down-ow leaching test
The ow rate is designed according to the recorded maximum South China's natural rainfall intensity of 0.5mm/min. Accordingly, the ow rate of peristaltic pump is set to be 3.18ml/min, under which the amount of water owed through the sample per hour is equivalent to 9.7 days of water exposure to continuous natural rainfall. The leaching experiments were conducted for FBA and TBA, and the leachability of Cl − , Cu, and Zn were detected for their high concentrations in the leachate as found in the standard tests. Figure 5 shows the leaching concentrations of Cl − , Cu 2+ and Zn 2+ along time.
According to the leaching characteristics of the targeted pollutants shown in Fig. 5, the leaching process of Cl − , Cu 2+ and Zn 2+ could be divided into three stages, viz. a continuous and rapid decrease, a slow decrease, and a stable leaching in the 1st-7th month, 7th-13th month, and 13th-24th month, respectively. After washing, however, the Cu 2+ leaching levels falls below the safe limit for a very short period at the beginning of experiment. The comparison also shows that wet treatment can improve the environmental friendliness by reutilizing MSWI bottom ash. The nal values of the Cl − , Cu 2+ and Zn 2+ release from FBA is 3.5, 11.8 and 11.3 times of those of TBA, respectively.

Comparison among various leaching tests
The experimental conditions for rainwater leaching test are more appropriate for simulating eld scale due to the consideration of pH value and compaction, while the factors are not considered in the standard column up-ow tests. Figure 6 presents the different leaching concentrations at the beginning and the end stage of all types of columns leaching tests. It should be noted that the initial leachate was sampled at the same L/S ratio of 0.1. The results indicates that the leaching values of the targeted pollutants using simulated rainfall down-ow leaching test are usually 4-6 times of standard column up-ow tests. Especially, the comparison of Cu is most prominent with a factor of 35.
Despite the leaching differences, some similar conclusions could still be drawn. Firstly, the targeted heavy metals in this study follow a cationic leaching pattern. Luo et. al. reported that the concentrations of leached elements decrease with the increased pH values of the rainwater solution's (Luo et al. 2019b). Secondly, the compaction in the simulated rainfall down-ow leaching tests affects the reaction time between water and ash particles. A compacted BA layer can prolong the retention time of rainwater, thus increasing the leaching concentrations. The differences in leaching concentrations are important for evaluating environmental characteristics of BA during its reutilization. Figure 7 shows the comparison between cumulative leaching of the targeted pollutants from the simulated rainwater down-ow leaching and standardized up-ow column leaching tests. Results of cumulative leaching amounts from standard up-ow column are still lower than that of standard column up-ow tests, except for the leaching mass of Cu in the FBA. As illustrated by the correlation, it could be deduced the leaching tests method applied has an evident effect on the nal assessment of the expected leaching release. For instance, the cumulative release of the two leaching tests is signi cantly different. The results also give an important information that the initial and nal cumulative release of the targeted elements ions are highly consistent in the simulated rainwater rainfall down-ow leaching tests, indicating that the environmental impact of BA is underestimated for standardized column leaching test.

Conclusions
The toxic leaching characteristics of fresh and treated bottom ash were investigated using batch leaching and standard column up-ow leaching and simulated rainfall down-ow leaching tests. Cu, Zn, Ni, Cl − and SO 4 2− were chosen to be the targeted pollutants in this study. The batch leaching results showed that the potential ecological risk of MSWI bottom ash could not be ignore during its reutilization, and wet treatment enabled reduce the leaching concentration of Cu, Zn, Cl − and SO 4 2− by 7.1%, 33.8%, 46.3% and 18.9%, respectively. The leaching concentrations of all the targeted ions exceeded the limits speci ed in the standard of the surface water class V. Especially, Zn had the highest concentration with 210 and 139 mg/L for FBA and TBA, respectively. The results obtained from standard column up-ow leaching indicated that the Cu Zn Cl − and SO 4 2− in leachate evidently exceeded limit value before L/S = 1, while the environmental pollution of Cu and Zn could be ignored after L/S = 1 for a low extraction toxicity. Based on the results obtained from the two column leaching tests, the leaching concentration and cumulative release amount of the target pollutants in the leachate of FBA was generally higher than that of TBA. Furthermore, this indicated that wet-treatment was appropriately used to improve the environmental friendliness with the resource utilization process of MSWI bottom ash.

Declarations
Ethics approval and consent to participate: No applicable.
Consent for publication: No applicable.
Availability of data and materials: The datasets generated and analysed during the current study are not publicly available due the nature of this research but are available from the corresponding author on reasonable request. Results of pH (a) and conductivity (b) pattern over L/S ratio from standardized column leaching test Figure 4 The leachability of Cl-, SO42-, Cu, Zn and Ni from the up-ow percolation tests as a function of L/S ratio Leaching concentrations at the beginning and the end stage of leaching tests Figure 7 Cumulative leaching of the targeted pollutant from the tests of simulated rainwater leaching and standardized column leaching