Feasibility of stabilizing Cd, Pb, and Ni in MSWI fly ash using leachate as an alternative stabilizer

Fugeng Zha; Wenjin Qi; Xin Chen; Siqi Yue; Xuanxuan Xue; Yuxiao Li; Juan Xu; Fuxin Liu; Xingming Wang; Duoxi Yao; Shiwen Zhang

doi:10.4491/eer.2023.765

Environ Eng Res > Volume 30(1); 2025 > Article

Zha, Qi, Chen, Yue, Xue, Li, Xu, Liu, Wang, Yao, and Zhang: Feasibility of stabilizing Cd, Pb, and Ni in MSWI fly ash using leachate as an alternative stabilizer

Research

Environmental Engineering Research 2025; 30(1): 230765.

Published online: May 31, 2024

DOI: https://doi.org/10.4491/eer.2023.765

Feasibility of stabilizing Cd, Pb, and Ni in MSWI fly ash using leachate as an alternative stabilizer

Fugeng Zha^†

, Wenjin Qi, Xin Chen, Siqi Yue, Xuanxuan Xue, Yuxiao Li, Juan Xu, Fuxin Liu, Xingming Wang, Duoxi Yao, Shiwen Zhang

School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, Anhui, China

^†Corresponding author: E-mail: pgzha@aust.edu.cn, Tel: +865546668430, ORCID: 0000-0002-1869-8799

Received December 20, 2023 Revised March 18, 2024 Accepted May 30, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

For the synergistic disposal of municipal solid waste incineration (MSWI) fly ash and leachate, MSWI fly ash was stabilized using leachate, sodium sulfide (Na₂S), and sodium diethyldithiocarbamate (DDTC) separately or simultaneously. Results show that the leaching concentration of Cd, Ni, and Pb can be reduced when leachate is used alone but exceeds the landfill admission contents. The leaching concentration of heavy metals were lower than the threshold of the pollution control standard of Chinese garbage landfill (GB 16889-2008) after curing for 14d when DDTC was beyond 1.5%, and Na₂S more than 10% except for Pb. The results of orthogonal experiment showed that the lowest leaching concentration was got at 5-fold dilution of leachate, 3% Na₂S and 0.8% DDTC, and DDTC was the most influential factor. The leachate can be utilized as a partial stabilizer for stabilization of fly ash. After chemical stabilization, the crystal structure of fly ash changed little, and the surface was densified. The analysis of two-dimensional correlation spectroscopy results showed that the different order between the major function groups of fly ash and stabilizers was got, may be related to chelation, chemical reactions, and precipitation.

Keywords: Fly ash, Heavy metal, Leaching concentration, Municipal solid waste incineration, Stabilization

Graphical Abstract

Keywords: Fly ash, Heavy metal, Leaching concentration, Municipal solid waste incineration, Stabilization

1. Introduction

Municipal solid waste incineration (MSWI), which respectively accounted for 18.8% to 72.5% of the total municipal solid waste in 2010 and 2021, has been widely adopted for the utilization of waste resource and effectively reducing the volume solid waste [1, 2]. However, secondary pollutants, such as leachate and fly ash, are inevitably generated during MSWI [3]. Leachate usually contains high concentrations of organic matter, inorganic matter and toxic elements [4, 5], which have a harmful impact on the ecosystem and the biological chain [6, 7], and thus requires proper disposal. Fly ash, which needs harmless treatment before landfill [8], accounts for approximately 2%–5% of the total residue of MSWI and contains toxic pollutants such as heavy metals, soluble salts, and dioxins [9–12], posing threats to human health and the environment [13].

At present, the disposal technologies for fly ash are mainly divided into four categories: thermal treatment, extraction and separation, solidification, and chemical stabilization. Thermal treatment solidifies heavy metals into a crystal structure and difficult to be leached by applying high temperature [14], but it needs high energy consumption and may produce secondary pollution [15,16]. The separation process uses a leachant or an electrodialytic to separate heavy metals, which is efficient but requires post-processing for the resulting wastewater and has high treatment costs [17]. Cement solidification technology can reduce the mobility of heavy metals but increases the volume of the final product, and the chlorine compounds in the fly ash hinder the cement hydration [15, 18, 19]. Among the disposal technologies, chemical stabilization is commonly used to stabilize fly ash due to its economic efficiency and reliability; this method immobilizes hazardous substances by adding different chemicals that promote chemical interactions, converting them into low-solubility, migration, and toxic substances [20–22], which are highly adaptable and minimize the potential leaching risk for subsequent resource utilization [23, 24].

The chemical agents used are mainly divided into two categories: inorganic and organic agent [9]. Inorganic agents mainly include silica fume [25], polysulfide [26], phosphates [27], and silicates [28]. Sulfide has a strong affinity for heavy metals, which can be converted into insoluble sulfide, Na₂S is one of the main sulfur-containing compounds used on a commercial scale in China [17]. The commonly used organic chemicals include thiourea [29], disodium ethylene diaminetetracetic acid [30], tetrathio bicarbamic acid [31], diethyldithiocarbamate (DDTC) [32], which form stable compounds by complexing the functional groups of polymer chains with heavy metal ions through valence bond complexation [24]. Due to the effective coordination between heavy metals and organic sulfide groups, dithiocarbamate is frequently used for heavy metal stabilization [33]. Organic agents are more effective at lower dosages than inorganic agents, while inorganic agents are less expensive. To combine the advantages of both, some studies have focused on mixture stabilizer, Zhu et al. [20] used Na₂S, NaH₂PO₄, and ammonium dibutyl dithiophosphate as a mixture reagent to stabilize fly ash which could effectively stabilize heavy metals and had a lower cost.

Recently, with the widespread attention on waste resource utilization, some scholars have used types of waste for fly ash stabilization, such as MSWI bottom ash (BA) [34], the mixture of MSWI BA, flue gas desulphurization (FGD) residues and coal fly ash [35], the mixture of MSWI BA, FGD residues and silica fume [36], etc., after using these types of waste for fly ash stabilization, the leaching concentration of Pb and Zn in the fly ash was reduced. Leachate generated during the waste stockpiling process has been used as a resource due to the high concentration of humic substances and metal elements [37], some scholars [38, 39] have studied the resource utilization of leachate, such as a substrate for bio-electrochemical systems, anaerobic digestion into biogas, recycling as fertilizers, etc., however, fewer studies have used leachate for fly ash stabilization.

This study explored the synergistic disposal of leachate and fly ash, MSWI fly ash was stabilized using leachate, Na₂S, and DDTC separately or simultaneously for reducing the leaching concentration of heavy metal to meet the landfill admission contents. The changes in heavy metal leaching concentration and speciation distribution were compared, and the feasibility of stabilizing the heavy metals in fly ash using leachate as an alternative stabilizer was analyzed.

2. Materials and Methods

2.1. Materials and Reagents

In this work, fly ash and leachate were obtained from the Renewable Energy Company in Anhui, China, which handles 2000 tons of municipal solid waste per day and uses a mechanical grate incinerator for incineration, while a large amount of leachate is generated during waste stockpiling process and collected through a leachate collection system. The leachate and fly ash used in this study was obtained randomly from the leachate collection system and dust-collection system, respectively. The sampling method refers to the Chinese standard (HJ/T 20-1998), the collected fly ash was dried in a vacuum drying oven to constant weight before use, the leachate was kept at 4°C in refrigerator before use.

The contents of Cd, Ni and Pb in fly ash were 113.56±2.45, 132.52±1.87, 548.72±14.59 mg/Kg respectively. The leachate adopted in this research contained a high concentration of organic matter (COD 60825±1835 mg/L) and alkalinity (1057±81 mg/L), its pH was 8.73±0.11.

The reagents used in the experiments were analytically pure and purchased from Aladdin^® Reagent Ltd.

2.2. Experiment Procedures

The MWSI fly ash was separately treated with leachate, Na₂S, and DDTC. Different dosages of leachate, Na₂S, and DDTC, which were dissolved or diluted with deionized water, were mixed with the MSWI fly ash at a liquid to solid ratio of 1 L/kg. The samples were cured at room temperature for varying durations, then taken out at a pre-selected time. The orthogonal experiments listed in Table 1 were conducted at room temperature and a liquid to solid ratio of 1 L/kg, and then the samples were cured for 14 days. The treated fly ash was dried, ground, and sieved for the leaching concentration. Then, the chemical speciation distribution of heavy metals, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) were determined.

2.3. Analytical Methods

The leaching concentration of Cd, Ni, Pb in the fly ash was measured based on the Chinese standard (HJ/T300-2007). The determination of heavy metal speciation distribution was consistent with peer research [40, 41]. The raw and chelated fly ash samples were analyzed by XRD (Smartlab SE, Rigaku, Japan), with the Cu/Ka radiation scanning of the samples within 10° < 2θ < 80° at 5°/min after drying, grinding, and sieving, and the resulting data were analyzed using the jade software. The microscopic morphologies of the fly ash samples were obtained by SEM (FlexSEM1000, Hitachi, Japan). FTIR spectroscopy (Nicolet IS50, Thermo Fisher Scientific, USA) was used to scan between 400 and 4000 cm⁻¹, with a spectral resolution of 4 cm⁻¹, to analyze the changes of the functional groups in the fly ash.

2.4. Statistical Analysis

The results of the orthogonal stabilization experiment were analyzed using the comprehensive balance analysis method, with the stabilization rate as the test index for selecting the most effective combination [20]. The two-dimensional correlation spectroscopy (2D-COS) results were plotted adopting the 2D Shige software (Kwansei Gakuin University, Japan) to analyze the changes of the functional groups in the fly ash [42].

3. Results and Discussion

3.1. Effect on Fly Ash Stabilization by Different Stabilizers

3.1.1. Leachate

As shown in Fig. 1 (a), the leaching concentration of the three heavy metals exceeded the threshold of GB 16889-2008 in the raw fly ash. After stabilization by leachate, the leaching concentration of the three heavy metals decreased, which may be due to the combination of heavy metal ions with coordination atoms containing N and O (shown in Fig. 7 (a)) in the leachate to form cyclic complexes, thus reducing the leaching concentration of heavy metals [29]. The leaching concentration of heavy metals was related to the curing time. The prolongation of curing was conducive to the stabilization of heavy metals. However, the leaching concentration of Cd and Pb did not meet the landfill admission contents when leachate used alone. The speciation distribution of the heavy metals changed after stabilization. The percentage of residual states increased slightly except for Ni, but the morphology of Cd was unstable and dominated in the reducible state, which was probably affected by the pH [43].

3.1.2. Na₂S

After treatment by Na₂S, the leaching concentration of the heavy metals showed a decreasing trend, which was due to the strong affinity of Na₂S for metals, reducing heavy metal leaching by converting soluble forms into insoluble sulfides [26]. The leaching concentration of Cd and Ni satisfied the landfill admission contents after curing for 14 days when Na₂S exceeded 10%. However, the stabilization of Na₂S on Pb was ineffective, even when the dosage exceeded 15%, which is consistent with the results of Zhang [44]. This may be due to the strong alkaline of Na₂S hydrolysis and the high solubility of Pb at high pH conditions [45]. As shown in Fig. 2 (b), the speciation distribution of heavy metals changed after stabilization, the percentage of reducible and acid-soluble states decreased, and the residual state increased; especially for Ni, the residual state accounted for more than 80%, resulting in a low leaching potential [17].

3.1.3. DDTC

As shown in Fig. 3 (a), the leaching concentration of the three heavy metals decreased significantly after the stabilization by DDTC, and the leaching concentration was positively related to the DDTC dosage. When the proportion of DDTC exceeded 1.5%, the leaching concentration of the three heavy metals was lower than the threshold of GB16889-2008 after curing for 14 days. The heavy metal leaching concentration of Pb was lower than that of inorganic agents and can reach the landfill admission contents after stabilization with DDTC, maybe because the organic chelating agent has two or more coordination atoms (generally including S, N, and O) that can combine with heavy metal through covalent bonds to form stable compounds [29]. As shown in Fig. 3 (b), all three heavy metals were stabilized by DDTC, and the proportion of oxidizable and residual states increased, probably due to the strong affinity of DDTC and heavy metals [32]. The difference in speciation distribution of the three heavy metals may be associated with the metal competition during the chelating reaction, and the order of the three heavy metals from stable to unstable was Ni > Pb > Cd [46], so more than 50% of Ni existed in the residual state. Overall, DDTC exhibited a favorable stabilization property for heavy metals in fly ash.

3.2. Orthogonal Experiments

To discuss the synergic relationship of the leachate, Na₂S, and DDTC on the stabilization of fly ash, orthogonal experiments were conducted at different dosages. The lowest leaching concentration of Cd, Ni, and Pb was obtained at the 5-fold dilution of leachate, 0.8% DDTC, and 3% Na₂S. According to the comprehensive balance analysis [20], DDTC was the most influential factor on the stabilization of heavy metals among the three agents, and the stabilizing effect of Cd and Pb followed the order DDTC > Na₂S > leachate, and that of Ni followed the order DDTC > leachate > Na₂S. As shown in Fig. 4, the leaching concentration decreased significantly after the treatment with the mixture stabilizer and the chemical speciation was transformed to the residual and oxidizable states. After treatment with the mixture stabilizer, the pH of the system was higher than 9, and the Ca²⁺ generated by hydrolysis can form CaCO₃ with the CO₃²⁻, the decrease in heavy metal leaching concentration may be related to the control of the leaching behavior of heavy metals by the carbonate minerals [17]. Physical encapsulation might occur to immobilize precipitates containing heavy metals on the surface of the fly ash during the calcium carbonate precipitation process [47], thus changing the speciation distribution of heavy metals. The differences in heavy metal morphology changes may be related to the heavy metal competition and solubility of the precipitates. The mixture stabilizer could resourcefully utilize the leachate and reduce the dosages of stabilizers, providing an economical and feasible method for stabilizing fly ash.

3.3. Changes of Microstructure

3.3.1. XRD

The XRD pattern (Fig. 5) shows that the main crystalline compounds in the raw fly ash were CaCO₃, CaClOH, Ca(OH)₂, KCl, and NaCl, which is similar to the results reported by other researchers [22, 48]. The presence of small and broad peaks in the XRD spectrum indicates that the fly ash contained a large amount of amorphous material. After stabilization by different chemicals, the crystalline phase material in the fly ash remained basically the same as the raw fly ash. The changes in the characteristic peaks of the soluble salts in the system may be due to the dissolution and elemental migration [26]. During the reaction process, the hydrolysis of the chelating agents and the dissolution of soluble compounds produce a large amount of OH⁻ and CO₃²⁻, which can easily react with Ca²⁺ to form precipitates, as evidenced by the increased intensity of the characteristic peaks of CaCO₃ in the XRD pattern [49].

3.3.2. SEM

Fig. 6 (a) shows the SEM images of the raw fly ash, which had spherical and elongated agglomerated particles with a rough and unevenly distributed surface and a loose structure with high porosity and a large specific surface area. A high porosity will easily lead to the enrichment of heavy metals on the fly ash surface, and the large specific surface area also increased the leaching of heavy metals, which will lead to environment pollution [50]. After stabilization using stabilizers, the surface of the fly ash densified, and the fine particles were replaced by agglomerated particles, probably due to the dissolution of the attachments or the deposition of the precipitates attached to the surface; a dense structure may impede the leaching of heavy metals from the fly ash [51].

3.3.3. FTIR

The FTIR spectra of raw fly ash, leachate, and treated fly ash were measured as shown in Fig. 7 (a). The absorption peaks near 3430, 600–900, and 1430 cm⁻¹ in the leachate are related to the stretching vibration of O-H, C-H, and carboxylic acid group, respectively, and the bending transformations of N-H in the amide groups leads to strong absorption peak near 1560 cm⁻¹ [52, 53]. The peaks near 2940–2960 and 1407 cm⁻¹ correspond to the stretching vibration of C-H bonds and the asymmetric C-O stretching of the carboxyl groups or the deformation of the O-H and C-O-H bonds, respectively [54]. The absorption peaks of the raw fly ash are mainly distributed near 874, 1430, 1630, and 3430 cm⁻¹, the sharp peak around 874 cm⁻¹ indicates the presence of C-O, and the absorption peak near 1630 cm⁻¹ may be attributed to the C=O stretching vibration [51]. The strong peaks at 1430 and 3430 cm⁻¹ in the fly ash correspond to the stretching vibrations of the C-N and O-H bonds, respectively [55]. After stabilization, the main absorption peaks in the fly ash, did not change significantly, but absorption peaks near 2350 cm⁻¹ and 2510 cm⁻¹ appeared, which might be related to the products formed after the chemical reaction between the fly ash and the stabilizers. In addition, the infrared peak near 1160 cm⁻¹ in the raw fly ash shifted slightly toward a lower wavenumber after the treatment using leachate and the mixture stabilizer, which may be due to interactions between these agents, whereas no obvious change was observed after the treatment with Na₂S and DDTC.

The variation sequence of major characteristic peaks in fly ash utilizing 2D-COS and the FTIR spectra was exploited (Fig. 7 (b)). The asynchronous spectrum showed more complex variations than the other experimental results, probably due to the complexity of fly ash and leachate. The intersect peaks in the synchronous spectrum were positive in the fly ash treated with Na₂S and DDTC, indicating that the spectrum intensities were increased (or decreased) simultaneously by the external perturbations, and the sequential order of the signal changes could be determined based on the signs of the asynchronous peaks [56]. As shown in Fig. 7 (b), the order of the functional groups was different from the asynchronous spectrum after stabilization with different stabilizers, probably due to the different mechanisms between the fly ash and the stabilizers. The peak changes followed the order 3430 cm⁻¹→1630 cm⁻¹→1430 cm⁻¹→874 cm⁻¹ after stabilization by Na₂S. Therefore, the order of structural changes between Na₂S and fly ash may be O-H > C=O > C-N > C-O. The sequence of peak changes after stabilization by DDTC followed 1630 cm⁻¹→3430 cm⁻¹→874 cm⁻¹→1430 cm⁻¹, and the order of interaction between DDTC and fly ash may be C=O > O-H > C-O > C-N. By contrast, the intersect peaks appeared negative after treatment with leachate and the mixture stabilizers, the sequence of the peak changes followed 874 cm⁻¹→1430 cm⁻¹→3430 cm⁻¹→1630 cm⁻¹ for leachate, the order of structural changes between leachate and fly ash may be C-O > C-N > O-H > C=O. The results show that the absorption peak at 874 cm⁻¹ changed last after the stabilization using a mixture stabilizer, but the sequence of the three absorption peaks near 1630, 1430 and 3430 cm⁻¹ was contradictory, probably because the reaction process involved not only chelation but also precipitation and other chemical reactions.

4. Conclusions and Outlook

4.1. Conclusions

In this study, the chemical stabilization method was adopted for the harmless treatment of MSWI fly ash to investigate its stabilizing effect on heavy metals with different chelators. The main research results are as follows:

Leachate can reduce the leaching concentration of heavy metals, but Cd and Pb cannot meet the landfill admission limit standards. The leaching concentration of heavy metals is correlated with the stabilizer dosage and the curing time. After 14 days of curing time, when DDTC exceeded 1.5% and Na₂S exceeded 5% (except for Pb), the threshold of GB16889-2008 can be satisfied.
The results of the orthogonal experiments show that all three heavy metals satisfy the landfill admission contents, obtaining the lowest leaching concentration at the 5-fold dilution of the leachate, 3% Na₂S, and 0.8% DDTC. Mixture stabilizers can use leachate as part of the stabilizers to resourcefully utilize and save the dosage of other stabilizers, such as DDTC.
The XRD and SEM results show that the stabilizers had little effect on the crystal structure of the fly ash but resulted in surface densification. The analysis of the 2D-COS results revealed the possible order between fly ash and different stabilizers, and the different orders may be related to chelation, chemical reactions, and precipitation.

4.2. Outlook

This research has explored the possibility of synergistic disposal of leachate and fly ash, but the leaching concentration of dioxins in fly ash, the impact on fly ash of pollutants in leachate and the long-term environmental effects are not clear and should be discussed in future.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China [grant number 51608006 and 51878004], the National Key Research and Development Program of China [grant number 2020YEC1908601], and the Innovation Fund Designated for Graduate Students of Anhui University of Science and Technology [grant number 2022CX2001].

Notes

Conflict of Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

C.X. (Postgraduate) conducted part of the experiments. Q.W.J. (Postgraduate) supplemented relevant experiments and wrote the first manuscript of the article. X.X.X. (Postgraduate), Y.S.Q. (Postgraduate), L.Y.X. (Postgraduate), X.J. (Postgraduate) and L.F.X. (Postgraduate) assisted in the experiments. Z.F.G. (Associate Professor) supervised the planning and implementation of research activities, provided financial support for the project, and gave critical comments and revisions to the manuscript. W.X.M. (Associate Professor) Z.S.W. (Professor) and Y.D.X. (Professor) participated in the coordination of the study and reviewed the manuscript.

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Fig. 1

(a) Leaching concentration of fly ash stabilized by leachate, (b) Speciation distribution of fly ash stabilized by leachate. (Red horizontal dotted lines represent the threshold of “Standard for pollution control on the landfill site of municipal solid waste (GB 16889-2008)”).

Fig. 2

(a) Leaching concentration of fly ash stabilized by Na₂S, (b) Speciation distribution of fly ash stabilized with Na₂S.

Fig. 3

(a) Leaching concentration of fly ash stabilized by DDTC, (b) Speciation distribution of fly ash stabilized by DDTC.

Fig. 4

(a) Leaching concentration of fly ash stabilized by mixture stabilizer, (b) Speciation distribution of fly ash stabilized by mixture stabilizer (The orthogonal experiment numbered 0 was the raw fly ash).

Fig. 5

XRD patterns of raw fly ash and fly ash stabilized by different stabilizers.

Fig. 6

SEM of (a) raw fly ash, fly ash stabilized by (b) Na₂S, (c) DDTC, (d) leachate, and (e) mixture stabilizer.

Fig. 7

(a) FTIR spectra of leachate and fly ash, (b) 2D-FTIR-COS maps of interaction between fly ash and Na₂S (1, 2), DDTC (3, 4), leachate (5, 6), mixture stabilizer (7, 8).

Table 1

Orthogonal experiments design

No.	DDTC (%)	Leachate (dilution factor)	Na₂S (%)
1	0	5	0
2	0	10	1.5
3	0	50	3
4	0	100	5
5	0.4	5	1.5
6	0.4	10	0
7	0.4	50	5
8	0.4	100	3
9	0.8	5	3
10	0.8	10	5
11	0.8	50	0
12	0.8	100	1.5
13	1.2	5	5
14	1.2	10	3
15	1.2	50	1.5
16	1.2	100	0

Feasibility of stabilizing Cd, Pb, and Ni in MSWI fly ash using leachate as an alternative stabilizer

Abstract

Graphical Abstract

1. Introduction

2. Materials and Methods

2.1. Materials and Reagents

2.2. Experiment Procedures

2.3. Analytical Methods

2.4. Statistical Analysis

3. Results and Discussion

3.1. Effect on Fly Ash Stabilization by Different Stabilizers

3.1.1. Leachate

3.1.2. Na2S

3.1.3. DDTC

3.2. Orthogonal Experiments

3.3. Changes of Microstructure

3.3.1. XRD

3.3.2. SEM

3.3.3. FTIR

4. Conclusions and Outlook

4.1. Conclusions

4.2. Outlook

Acknowledgements

Notes

References

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Table 1

3.1.2. Na₂S