1 Introduction

The toxicity of Municipal Solid Waste Incineration (MSWI) fly ash is because of heavy metals (HMs) and persistent organic contaminants present in it, which deteriorate the ecological environment and human health (Lin et al., 2021; Wang et al., 2022a; Zhang et al., 2022). The MSWI fly ash contains teratogenic and carcinogenic contaminants such as lead, zinc, copper, cadmium, and dissolved salts. The environmental factor is the discharge of heavy metals from mishandled MSWI fly ash in the environment (Li et al., 2016; Pan et al., 2022). Several methods are used to treat MSWI fly ash, including landfill, chemical separation, sintering, fusion, solidification/stabilization (S/S) (Guo & Shi, 2013; Zheng et al., 2022). The landfill is not economical due to the utilization of more land (Huang et al., 2015). The chemical separation process extracts higher concentrations of heavy metals using chemical extraction and biological leaching techniques and allows them to be recycled after treatment. The solidification/stabilization process mainly uses cement, asphalt, melting (high-temperature treatment), chemicals, etc., to immobilize heavy metals (Bashar et al., 2014; Hwang & Huynh, 2015; Leong et al., 2016; Xu et al., 2022). Solidification of MSWI fly ash immobilizes heavy metals and other pollutants by adding chemically active substances (Quina et al., 2018; Wang et al., 2015a; Xue et al., 2012). According to US Environmental Protection Agency solidification/stabilization is best technology for treatment of toxic and hazardous waste (Asavapisit et al., 2005; Malviya & Chaudhary, 2006; Yoon et al., 2010). So this method is of great interest for researchers and scholars all over the world (Bai et al., 2022; Chen et al., 2022; Wang et al., 2022b).

A lot of work has been conducted on the solidification of heavy metals in fly ash using cement or slag and achieved good results, but the gel-like characteristics of fly ash were ignored for a long. Using slag or cement to solidify fly ash is not only a compaction problem but also a large volume of land for its disposal (Poon et al., 2006; Zheng et al., 2011). Due to gelatin activity, fly ash on hydration reaction generates ettringite, which enhances the compressive strength of the body (Zhao et al., 2002). Moreover, Fly ash with pozzolanic activity contains a certain amount of active silica, alumina, and other components. Adding an alkali activator in fly ash generates hydrated calcium silicate and aluminate or hydrated calcium aluminate reaction products (Wei et al., 2011). These properties and reactions provide a theoretical basis for the solidification of fly ash. The solidification of the fly ash by the alkali activator has no problem with compaction (Wang et al., 2016). Using fly ash as a solidification/stabilization of heavy metals is an economical and simple process and has a great advantage over other techniques (Wang et al., 2015b).

In this paper, a composite alkali-activator is selected to immobilize the HMs in MSWI fly ash. The gel properties of MSWI fly ash are similar to those found in coal ash. So fly ash is solidified and stabilized with the moderate addition of sodium silicate (i.e., Na2SiO3) and sodium hydroxide (i.e., NaOH) in the current experiment. A single-factor experiment is conducted to determine the appropriate combination of alkali-activators. Orthogonal tests are designed to analyze the optimal set of the experimental parameters; (a) mass ratio of alkali-activators and fly ashes (A/M ratio), (b) Na2SiO3 to NaOH ratio (i.e., Na2SiO3/NaOH) and (c) proportion of water to the solid mixture (i.e., L/S ratio). The X-ray diffraction (XRD), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) are used to understand the solidification mechanism.

2 Materials and Experimental Methods

2.1 Materials Preparation

Municipal solid waste incineration fly ash used in this study was obtained from a waste incineration power generation plant in Chongqing, China. Fly ash samples were sieved through 200-mesh after being dried at 60 ℃ for 6 h. All experiments were performed using triplets of samples from the same batches of materials. Distilled water was used throughout the experiment. The raw materials of chemical composition were analyzed using X-ray fluorescence (XRF Shimadzu, PerkinElmer), and the results are listed in Table 1. The particle size distribution of the fly ash specimen is shown in Fig. 1.

Table 1 Chemical compositions of the raw MSWI fly ash (w/%)
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Fig. 1
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Particle size distribution of raw fly ashes

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2.2 Preparation of Solidified Body

The certain proportion of the MSWI fly ash, alkali-activators, and distilled water was mixed, and was evenly stirred until cooled to room temperature. The slurry was then shifted into a 20 mm × 20 mm × 20 mm steel mold and vibrated for 10 min. The preparation process of solidified body is shown in Fig. 2. The solid specimens were removed from the mold after 24 h and then restored to the indoor environment for 28 days.

Fig. 2
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The preparation process of solidified body

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2.3 Leaching and Compressive Strength Tests

The leaching toxicity test of solidified samples were prepared according to Chinese standard HJ/T300-2007 entitled “The leaching toxicity of solid waste-Acetic acid buffer solution method.” The pulverized sample particles were sieved (Φ 9.5 mm) and added to the prepared chemical regents (glacial acetic acid solution) at a mass-liquid ratio of 20:1 (g/mL). The mixture was shaken and tumbled at a speed of 30 \(\pm\) 2 rpm for 18 \(\pm\) 2 h at 23 \(\pm\) 2℃ in an oscillating device. Then leachate of the samples was filtrated through a micro-porous filter membrane (Φ 0.8 μm), and leaching concentrations of HMs (Zn, Pb, Cu, and Cd) were detected using an atomic absorption spectrophotometer (TAS-999). The compressive strengths of solidified specimens were measured according to Chinese standard GB/T 17671–1999 using a universal testing machine (AGN-250) with a 10% standard deviation. All experiments were conducted with triplicate specimens after 28 days.

2.4 Analytical Methods

The raw samples and solidified bodies were scanned by the X-ray diffraction (PANalytical B.V., Holland) at CuK \(\mathrm{\alpha }\) radiation generated at 30 mA and 40 kV with 2θ ranging from 10° to 90°. Morphology of samples was obtained using Scanning Electron Microscopy (SEM, Carl Zeiss AG, Germany) at an accelerating voltage of 20 kV. Fourier transform infrared spectroscopy analysis was performed using Fourier Transform Infrared spectroscopy (Nicolet5DXC FT-IR) in the range of 400–4000 cm−1.

3 Results and Discussions

3.1 Alkali-activator Comparison

The alkali-activators used in solidifying wastes include Na2SiO3, K2SiO3, NaOH, and KOH. Single-factor experiments were designed to select suitable alkali activators for immobilizing heavy metals in fly ash using Na2SiO3, NaOH, KOH, Na2SiO3:NaOH(1:1, g/g), and Na2SiO3:KOH(1:1, g/g). A total of 100 g alkali-activator and 40 mL distilled water were used in each set of experiments with three replicates specimens. The influence of different alkali-activators on the leaching concentration of HMs presented in Fig. 3. It concluded from Fig. 3 that in the case of Na2SiO3-NaOH; the leaching concentration of Cu, Zn, and Cd was lower than the other four alkali-activators. While minimum leaching concentration of Pb ions was observed by using NaOH as an alkali-activator in contrast to other activators. Since there is no significant difference in the leaching concentration of Pb resulting from NaOH and Na2SiO3, the lowest leaching concentration may be obtained by adjusting the mass ratio of Na2SiO3 and NaOH in Na2SiO3-NaOH.

Fig. 3
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The effects of different alkali-activators on the leaching concentration of HMs

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3.2 Optimization Analysis

The fixation rate of the HMs and the compressive strength of the solidified bodies measured in orthogonal experiments were used to quantify the influence of A/M, Na2SiO3/NaOH, and L/S ratios on the S/S performance of fly ashes. The fixation rate of the HM directly reflects the decrease in the leaching concentration from the solidified body as compared to the original sample, which is expressed in the following equation:

$${\text{y}}=({\text{U}}1-{\text{U}}2)/{\text{U}}1\times 100\mathrm{\%}$$
(1)

where y is the fixation rate of the heavy metal and \({\text{U}}1\) and U2 denote the leaching concentrations from the raw sample and solidified body, respectively. The design of orthogonal experiments and the fixation results are shown in Tables 2 and 3, respectively. The leaching concentration of HMs in the raw fly ashes and the solidified body samples in the orthogonal tests are presented in Table 4.

Table 2 Orthogonal experiment matrix
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Table 3 The range analysis of fixation rate and compressive strength
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Table 4 The leaching concentration (mg/L) of HMs
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It was seen from Table 3 that the maximum fixation rate of elements Cu, Zn, Cd, and Pb in 9 groups of tests was 86.96%, 22.35%, 61.31%, and 64.91%, respectively, and the corresponding leaching concentrations were 2.60 mg/L, 18.58 mg/L, 1.48 mg/L, and 7.22 mg/L, respectively. The optimum solidification of Cu was observed at A3B3C3; A3 (9.1% A/M ratio), B3 (7/3 Na2SiO3/NaOH ratio) and C3 (0.47 L/S ratio). The highest fixation rates of Zn and Cd were examined at A2B3C1; the highest fixation rate of Pb was examined at A1B1C1. Based on the ranking of three factors, A, B, and C, for the four HM elements and the R-value analysis, the best parameters combination achieved at A1B3C1; A/M ratio of 14.2% (A1), Na2SiO3/NaOH ratio of 7/3 (B3) and L/S ratio of 0.43 (C1). In the current study, the highest compressive strength of solidified body was 1.025 MPa. The optimum combination regarding compressive strength (0.68 MPa) was observed at A3B3C3. The difference in the compressive strength of the solidified body was not significant at L/S ratios of 0.47 vs. 0.43 and A/M ratios of 14.2% vs. 9.1%.

3.3 Mineral phase analysis

Hydration products were determined by XRD (Fig. 4) and were one of the main factors in determining the leaching properties of heavy metals in the solidified body. The main phases in the original samples were calcium carbonate (CaCO3) and silicon chloride (SiCl4). While in the case of the solidified body, the main mineral phases were calcium silicoaluminate hydrate (2CaO·Al2O3·SiO2·8H2O), denoted as C-A-S–H, and the calcium silicate hydrate (CaO·SiO2·nH2O) represented as C-S–H.

Fig. 4
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The XRD patterns of the original sample and the solidified body

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Some aluminum atoms in C-A-S–H bonded with silicon and formed a two-dimensional network structure, which had a large specific surface area and pore volume. The greater specific surface area led to an increase in the cation exchange capacity of C-A-S–H. It ultimately enhanced heavy metals’ solidification/stabilization ability in the fly ash grains. At the same time, C-S–H has a stratified structure with lower degrees of polymerizations for the silicon and has a great specific surface area and pore volume. In addition, it has a high unsaturated surface potential which strongly bound water molecules, and the high density of irregular hydrogen bonding caused the stronger adsorption of HMs on the polymer surface.

3.4 Morphology analysis

As shown in Fig. 5a, the original MSWI fly ashes consist of fine particles with a hollow and sparse appearance. The flocculation of particles resulted in amorphous and polycrystalline aggregates. Moreover, it also contained tabular and layered structures. The surface of the particles was not smooth, with many raised and hollow structures indicating clear network structures. While in the case of the solidified body, dense grid structures were observed as a result of Si–O, Ca-O, and H–O reactions (Fig. 5b). Therefore, the solidified body has a smaller surface area and lower permeability, indicating that contaminants were more strongly adsorbed and difficult to be leached.

Fig. 5
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The morphology of the raw MSWI fly ash and the solidified body (a a raw MSWI fly ash; b solidified body)

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3.5 FTIR analysis

The infrared spectrum of the solidified body shown in Fig. 6 indicates that each hydration product exhibited a similar absorption band. The absorption peaks were observed at 3441 cm–1 (bending vibration), 1622 cm–1 (bending vibration), and 1444 cm–1 (stretching vibration), which ensured the presence of O–H (\(\upgamma\) OH), middle water H–O-H (\(\upgamma\) 2H2O), and O-C-O ions respectively in the solidified body. The absorption peaks at 873 cm–1 were due to Al–OH symmetric structure. Si–O (γ3) presence is observed at 1000 cm–1 (asymmetric stretching vibration) absorption peaks. While the absorption bands at 458 cm–1, 1121 cm–1, and 1156 cm–1 correspond to Si–O inner surface bending vibration. The above analysis confirmed that the solidified body’s hydration reaction occurred under the calcium aluminosilicate hydrate wave (C-A-S–H).

Fig. 6
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The FTIR spectra of the original sample (a) and the solidified body (b)

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3.6 Mechanism exploration

XRD, SEM, and FTIR analyses indicate that the alkali-activator significantly influenced the immobilization of the HMs in MSWI fly ash. The pozzolanic behavior of coal ash is mainly due to CaO-SiO2. During the alkali activation, the fly ash particles rapidly dissolved, resulting in active components; SiO2 and Al2O3 released. This mechanism leads to the encapsulation of heavy metals by C-S–H and C-A-S–H present in a hard solid matrix. Furthermore, heavy metals combined either with OH− or silicate to form calcium salts, which adsorbed on C-S–H and became components of crystal structure. In the current study, Zn2+ and Cu2+ replaced the Ca2+ of C-S–H or reacted on the surface of particles, forming the oxides of Ca2+ and Zn2+ or Cu2+. In the meantime, Cd2+ would be precipitated into Ca(OH)2. The immobilization process of Pb2+ can be described as follows:

$$\begin{array}{c}\text{Adsorption}:\text{C}-\text{S}-\text{H}+\text{Pb}^{2+}\rightarrow\text{Pb}-\text{C}-\text{S}-\text{H}\\\mathrm{Isomorphous}\;\mathrm{substitution}:\text{C}-\text{S}-\text{H}+\text{Pb}^{2+}\rightarrow\text{Pb}-\text{C}-\text{S}-\text{H}+\text{Ca}^{2+}\\\mathrm{Precipitation}\;\mathrm{reaction}:\text{Pb}^{2+}+2\text{OH}^-+\text{Ca}^{2+}+\text{SO}_4^{2+}\rightarrow\mathrm{double}\;\mathrm{salt}\end{array}$$

4 Conclusions

The medium diameter of the particle size for the MSWI fly ash is approximately 73.6 μm, with a range varying between 50 and 500 μm. The heavy metals detected in the raw samples are trace elements of Pb, Zn, Cd, and Cu. The main components responsible for pozzolanic activity are CaO-SiO2 in MSWI fly ash. The Municipal solid waste incineration fly ash potentially has tephra properties, which possess some specific gelling properties. Their gelling properties are activated by adding alkali-activators, and thus fly ashes solidified. In this paper, alkali-activator Na2SiO3 + NaOH (1:1, g/g) was selected for further investigation of the immobilization of HMs due to their lower leaching concentration. Based on results, fixation rate, and compressive strengths, the optimum selected parameters were; A1 (A/M ratio of 14.2%), B3 (Na2SiO3/NaOH ratio of 7/3), and C1(L/S ratio of 0.43) forming a combination of A1B3C1. The difference in compressive strength of solidified bodies was not significant between A1 (14.2%) and A3 (9.1%); the difference in compressive strength of solidified bodies was not significant between C1 (0.43) and C3 (0.47). The main hydration products of the solidified body of the fly ashes were C-S–H and C-A-S–H. The immobilization mechanism of the four HM elements was as follows; Zn2+ and Cu2+ replaced Ca2+ or reacted with Ca2+ on the surface of C-S–H to form the oxides of calcium, zinc, or copper in the hydration process. While Cd is incorporated into the gel of the calcium hydroxide (i.e., Ca(OH)2) through co-precipitation and Pb solidified in the C-S–H gel through a combined process of the adsorption, isomorphic substitution, and precipitation reaction. This study demonstrated that the solidification of heavy metals in municipal solid waste incinerators flies ash is achieved by alkali activations.