Stabilization Effect of Combined Stabilizing Agent on Heavy Metals in Hazardous Waste Incineration Fly Ash and Effect on Solidification Volume

Abstract

Based on the need for safe disposal of hazardous waste incineration fly ash (HFA), this study evaluated the solidification/stabilization (S/S) performance of silicate cement, sodium dihydrogen phosphate (NaH₂PO₄), and sodium dimethyl dithiocarbamate (SDD) used individually and in combination. The raw HFA failed the leaching test for Pb, Zn, Cd, and Ni, with their concentrations exceeding the GB16889-2024 limits by factors of 3.1, 2.45, 1.67, and 1.1, respectively. While cement (150% dosage) effectively immobilized Pb, and Cd with >90% leaching reduction, it resulted in significant volume expansion (2.7-fold). NaH₂PO₄ excelled in Pb stabilization (100% efficiency at 20% dosage) via insoluble phosphate formation but required high doses. SDD effectively chelated Zn (63.4% efficiency at 5% dosage) but was less effective for Pb and costly. A synergistic combination of 5% cement, 15% SDD, and 10% NaH₂PO₄ was identified as the possible optimal formulation, successfully immobilizing all heavy metals within regulatory limits. This combined approach minimized dosage, controlled volume expansion ratio (R_VE) (~1.31), and reduced cost. The low initial dioxin content (7.6 ng TEQ/kg) was unaffected by S/S treatments and remained compliant. Mechanistic analyses (XRD, FTIR, SEM-EDS) confirmed the formation of C-S-H gels from cement, insoluble phosphates from NaH₂PO₄, and metal chelates from SDD, collectively transforming the HFA into a compact, low-porosity matrix conducive to safe disposal. This stabilization and solidification strategy not only achieves the safe disposal of hazardous waste incineration fly ash but also contributes to the goals of sustainable waste management by reducing the environmental footprint of treatment processes and minimizing the final disposal volume.

1. Introduction

The escalation in hazardous waste generation due to rapid industrialization necessitates diverse waste treatment methods, with hazardous waste incineration being a widely employed approach [1]. Incineration involves subjecting hazardous waste to high temperatures, reducing its volume rapidly by destroying toxic components and converting them into ash and flue gases [2]. However, the process leads to the evaporation of heavy metals, which are then carried by flue gases to the air pollution control system (APC), where hazardous waste incineration fly ash (HFA) is collected. HFA is enriched with numerous pollutants, including heavy metals, posing potential environmental and human health risks [3,4]. Classified as hazardous solid waste, HFA can inflict harm if its harmful constituents leach out. Therefore, proper treatment and safe disposal of hazardous waste incineration fly ash are imperative.

Currently, HFA disposal comprises two primary categories: heat treatment and non-heat treatment. Heat treatment methods, including sintering, melt vitrification, and hydrothermal techniques, are energy-intensive and less prevalent. Non-thermal disposal includes cement solidification and chemical stabilization, biological and chemical leaching, and mechanochemical approaches. Among these, cement solidification and chemical stabilization are widely adopted for their affordability, low operational costs, effective heavy metal immobilization, and mature technology [5,6,7]. This method employs cement as a stabilizing and solidifying agent, encapsulating heavy metals in HFA through cement hydration reactions to minimize heavy metal leaching [8,9]. However, cement stabilization leads to volume expansion. Although higher cement amounts decrease heavy metal leaching, excessive volume expansion hinders landfill disposal. Furthermore, some heavy metals in hydrofluoric acid impede cement hydration reactions, decreasing the flexural and compressive strength of cement [10,11]. Additionally, silicate cement production releases substantial CO₂ into the environment. Consequently, researchers are exploring alternative and economical stabilizing agents [12,13,14,15].

Chemical stabilization is a widely employed technique for stabilizing HFA. It involves the precipitation of heavy metals in HFA through the addition of chemicals. In contrast to cement stabilization, the chemical stabilization process does not result in an increase in the solidification volume. Chemical stabilization can be categorized into inorganic reagents, organic reagents, and their combinations [16,17]. Inorganic reagents include substances such as gypsum, sulfates, phosphates, and carbonates. They operate on the principle of precipitation-dissolution equilibrium, wherein heavy metals are coupled with insoluble reactants, for example, phosphates and carbonates. Na₂S has proven to be effective in reducing the leaching rates of heavy metals like Cr, Cd, Ni, and Pb. Phosphates also exhibit strong stabilization effects on Cd, Pb, and Zn [18,19,20,21]. Organic reagents, including dithiocarbamates, tetrathiodicarbamate (TBA), thiourea, and thiols, chelate heavy metals via coordination reactions with ligand groups (-NH₂, -SH, -SCN, etc.). However, the use of organic reagents comes with drawbacks, such as higher costs and potential toxicity issues, as well as concerns about the long-term stabilization effects on heavy metals [22,23,24]. Currently, a noteworthy research focus involves the combination of inorganic reagents and organic chelating agents in specific ratios to reduce the quantity of additives [25]. Some researchers have successfully utilized combinations like dithiocarbamate chelating resin agent with calcium superphosphate, Na₂S, NaH₂PO₄, DDTP, or NaH₂PO₄ and dithiocarbamate to effectively stabilize heavy metals like Pb, Cu, Cd, and Ni in waste incineration fly ash at a relatively lower treatment cost [26].

In practical hazardous waste disposal, HFA is typically stabilized using a significant amount of cement along with a smaller quantity of organic reagents known for their effective metal chelating properties [27]. Despite its cost-effectiveness, the overreliance on cement raises concerns about limited landfill space as hazardous waste disposal demands increase.

Aligning with the principles of sustainability, which emphasize the minimization of environmental impact and the conservation of resources, there is a pressing need to develop treatment technologies that are not only effective in hazard reduction but also efficient in material usage and land conservation. The present study therefore aims to develop a novel combined stabilization/solidification (S/S) formulation that addresses the technical challenges while explicitly considering the sustainability metrics of treatment volume expansion and chemical consumption. By optimizing the synergistic effects of cement, phosphate, and chelating agents, this research seeks to transform hazardous fly ash into a safe and stable matrix, thereby supporting the transition towards a more sustainable and circular economy in waste management.

This approach involves the initial fixation of heavy metals in HFA by organic chemicals through ligand chelation, followed by reinforcement using inorganic chemicals through precipitation. The addition of cement enhances the strength of the solidified product, reduces stabilization time, and improves the efficiency of heavy metal stabilization, thereby reducing the number of reagents used, and consequently cutting costs. The incorporation of limited amounts of cement prevents excessive expansion of the solidified product volume. This three-step combined stabilization approach enhances the stabilization of heavy metals while minimizing the increase in solidification volume in an economically viable manner. By exploring different types of reagents and adjusting various combinations of addition ratios, we have investigated the performance of different ratio combinations in the stabilization of heavy metals in HFA for practical applications. The method developed in this study offers a feasible solution for the treatment of hazardous waste incineration fly ash generated through novel processes in hazardous waste companies.

2. Materials and Methods

2.1. Materials

The Hazardous waste incineration fly ash samples (HFA) were obtained from a hazardous waste disposal company in Nanjing, Jiangsu, China. These samples originated from a novel hazardous waste co-disposal process that combines various sources of complex hazardous wastes collected by a multi-source hazardous waste co-disposal system. This system incorporates physical, incineration, and landfill methods to manage these hazardous materials. The HFA is a byproduct of incinerating industrial hazardous solid wastes and waste liquids with intricate compositions. The flow of HFA generated by the co-disposal process and its solidifying and stabilization treatment are shown in Figure 1.

Figure 1. HFA generation and its solidification/stabilization disposal process.

Reagents: All chemical reagents employed in this study were of analytical or chemical grade. The cement used in the experiments was 32.5# ordinary cement, primarily composed of CaO, Al₂O₃, and SiO₂. Ultra-pure water was utilized in all experiments.

2.2. Methods

2.2.1. Characterization of Materials

The characterization of HFA involved several analytical techniques: The particle size distribution of HFA was determined using a laser particle size analyzer (HELOS-OASIS, Hydro 2000MU(A), Sympatec GmbH Company, Clausthal-Zellerfeld, Lower Saxony, Germany). The specific surface area of the HFA samples was measured using a fully automated specific surface and porosity analyzer (BET, TriStar II 3020, Micromeritics Instrument Corporation, Norcross, GA, USA). The chemical elements present in the HFA samples were analyzed using an X-ray fluorescence probe (XRF, Thermo Scientific ARL Perform’X, Thermo Fisher Scientific, Waltham, MA, USA). Scanning electron microscopy (SEM, Regulus 8100, Hitachi, Ltd., Kokubunji-shi, Tōkyō-to, Japan) and energy dispersive X-ray (EDX) were employed to analyze the microstructure of the samples and the distribution of specific elements. The mineralogical composition of the HFA and solidified samples was evaluated using an X-ray diffractometer (XRD, Bruker D8 Advance, Bruker AXS Company, Karlsruhe, Bavaria, Germany) with a scan rate of 5°/min and 2θ of 10–80°. Fourier infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20, Thermo Fisher Scientific, Waltham, MA, USA) was utilized to identify the functional groups present in the HFA and solidified samples. Heavy metal concentrations were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher-ICAP6300, Thermo Fisher Scientific, Waltham, MA, USA). The analysis for dioxin content followed the Chinese standard (HJ 773-2008) titled “Solid Waste Determination of Polychlorinated Dibenzo-p-dioxins (PCDDs) and Polychlorinated Dibenzofurans (PCDFs) Isotope Dilution HRGC-HRMS” [28] (HRGC-HRMS, Thermo Scientific Trace1310 DFS IE-3867, Thermo Fisher Scientific, Waltham, MA, USA).

2.2.2. Solidification and Stabilization Experiment

Conventional stabilization experiments were conducted using cement for HFA samples at various cement-to-HFA mass ratios (1%, 5%, 10%, 20%, 30%, 40%, 50%, 100%, 150%, and 200%). Solidification and stabilization experiments were also carried out using the organic agent sodium dimethyl dithiocarbamate (SDD) and the inorganic agent sodium dihydrogen phosphate (SDP), with cement serving as the stabilizing agent. Initially, SSD or SDP was applied to HFA samples at mass ratios of 5%, 10%, 15%, 20%, and 25% for individual agent solidification and stabilization. Subsequently, at a 5% wt. cement ratio, combinations of SSD and SDP at 5%, 10%, 15%, and 20% were added for compound agent solidification and stabilization. After thorough mixing with the fly ash, the solidified products were left to naturally set at room temperature for 7 days. The solidified products were then subjected to tests for leaching heavy metal toxicity.

2.2.3. Heavy Metal Ablation and Heavy Metal Leaching Tests

The HFA was digested using the HNO₃-HF-HClO₄ method. This involved the addition of concentrated nitric acid dissolved in distilled water, followed by the addition of hydrofluoric acid and subsequent boiling. Perchloric acid was then added, and the liquid mixture was evaporated to near dryness after cooling. Finally, 1% nitric acid was added, boiled, and digested for measurement.

Heavy metal leaching tests were performed using the Chinese standard HJ/T 300–2007 acetic acid leaching method [29], which is suitable for assessing the long-term leachability potential of hazardous elements. The extraction solution was prepared by adding 17.25 mL of glacial acetic acid to 1 L of deionized water, maintaining a solution pH of 2.64 ± 0.05 as per the HJ/T 300–2007 procedure. The HFA solidified product samples were ground into pieces smaller than 9.5 mm, and the sample pieces were mixed with the extraction solution at a liquid-solid ratio of 20:1 (L/kg). The mixture was stirred at 30 rpm for 18 h at room temperature using an overturning stirring device. The resulting leachate was collected by filtration through a 0.45 μm filter, and the concentrations of heavy metal ions in the leachate were measured by inductively coupled plasma spectrometry (ICP-OES, Agilent-5900, Agilent Technologies Inc., Santa Rosa, CA, USA). Tests were conducted in duplicate, and the results are reported as the mean value ± the absolute error.

2.2.4. Solidification Volume Expansion Ratio Detection

The volume expansion of solidified waste is a critical issue that must be thoroughly considered after solidification. Currently, there is no standardized method for measuring the volume of solidified monoliths. The Archimedes drainage method, while commonly used for volume measurement, is unsuitable for incineration fly ash and its solidified products due to their water absorption characteristics. In this study, the sand replacement method, similar to Archimedes drainage method, was employed for volume determination, using quartz sand with fine particle size and fluid-like properties as the displacement medium. The measurement procedure is as follows:

(1)

The quartz sand was first sieved to a particle size smaller than 60 mesh to ensure uniformity.

(2)

A standard mold with a volume of 0.001 m³ was used to contain a known volume of quartz sand, from which the bulk density of the sand was determined to be 1397.5 kg/m³.

(3)

Two identical 250 mL molds of cuboid shape (labeled as mold 1 and mold 2) were used. Mold 1 was completely filled with quartz sand, ensuring the sand was heaped above the rim before surface-leveling. The volume and mass are labeled as V₁ and m₁.

(4)

Approximately one-third of the sand from beaker 1 was transferred to the empty mold 2. The solidified specimen was then placed into mold 2, and additional quartz sand in mold 1 was poured in until mold 2 was full with surface leveling. The quartz sand remaining in mold 1 and the excess sand scraped off represent the volume of the solidified HFA product (the volume and mass are labeled as V and m₂).

(5)

The mass of the remaining quartz sand was measured. The volume of the solidified specimen was calculated using the density formula and the bulk density value obtained in Step 2.

This experiments on the volume increase in the solidified matrix were performed in triplicate, and the results are given as the mean value ± the standard deviation.

V = m₂/m₁*V₁

(1)

3. Results

3.1. Original HFA Properties

3.1.1. Appearance and Particle Size Distribution of HFA

The visual examination of HFA, as depicted in Figure 2a, reveals a light brown coloration with fine sand-like particles. The particle size distribution of HFA, as demonstrated in Figure 2b, follows an approximate normal distribution, featuring an average particle size of 41.74 µm and a median particle size of 39.81 µm. The particle size distribution is notably concentrated within the range of 10–100 µm, surpassing the dimensions of conventional incineration fly ash [30,31,32]. Additionally, this larger particle size results in a relatively diminished specific surface area of HFA (1.18 m²/g) and a reduced pore volume (0.01 cm³/g).

Figure 2. (a) Appearance, (b) particle size distribution, and (c) XRD pattern of HFA.

3.1.2. XRD Analysis of Original Fly Ash

Figure 2c presents the XRD findings for HFA. The primary crystalline compounds identified within HFA include KCl, NaCl, CaCO₃, CaSO₄, Ca(OH)₂, and Fe₂O₃. Cl predominantly exists in the form of alkali metal chlorides, specifically NaCl and KCl. The intensity of characteristic peaks for NaCl and KCl is notably pronounced, indicating a high chlorine salt content in HFA [33,34]. Calcium (Ca) primarily appears as Ca(OH)₂, CaCO₃, and CaSO₄. The presence of Fe₂O₃ in the crystallographic analysis indicates a substantial Fe content, contributing to the observed dark gray-brown coloration of HFA. These outcomes distinguish HFA from conventional incineration fly ash [35,36].

3.1.3. XRF Analysis of HFA

Table 1 provides the major chemical composition of HFA as determined through XRF analysis, with elemental compositions expressed in oxide form, except for Cl. The principal chemical constituents within HFA include Ca, Cl, S, Na, K, Si, Fe, P, Al, Zn, etc. Among these elements, calcium accounts for 32.55%, while Cl represents 15.76%, establishing them as the two primary elemental constituents of HFA [1,37]. HFA exhibits a notable Fe content of 6.12%, which contributes to its characteristic light brown coloration. Furthermore, HFA displays an elevated Zn content, distinguishing it from conventional incineration fly ash [38,39].

Table 1. XRF analysis results of the HFA. (wt.%).

Composition	wt./%	Composition	wt./%
CaO	32.55	Na2O	3.94
Cl	15.76	ZnO	3.54
SO3	9.97	Al2O3	2.13
SiO2	9.04	MgO	1.32
K2O	6.63	PbO	0.07
Fe2O3	6.12	CuO	0.11
P2O5	5.64	Others	3.18

3.1.4. Heavy Metal Content and Leaching Analysis

The heavy metal content within HFA follows this order: Zn (24,516 mg/kg), Cu (625 mg/kg), Cr (158 mg/kg), Ni (73 mg/kg), Pb (45 mg/kg), and Cd (6.53 mg/kg). Notably, Zn and Cu exhibit higher concentrations compared to other heavy metals and constitute the majority of the heavy metal content in HFA. Zn, in particular, is alarmingly elevated at 24,156 mg/kg, warranting attention in the context of stabilization.

The leaching concentrations (Leaching threshold in GB 16889-2024) [40] of Cr, Cu, Zn, Ni, Cd, and Pb are 0.18 (4.5) mg/L, 8.78 (40) mg/L, 244.75 (100) mg/L, 0.55 (0.5) mg/L, 0.25 (0.15) mg/L, and 0.78 (0.25) mg/L, respectively. This surpassed the threshold specified in GB16889-2024 by a factor of 3.1, 2.45, 1.67, and 1.1, respectively. Notably, the cumulative leaching of Pb greatly exceeded that of other heavy metals. This suggests that Zn in HFA exhibits significant environmental leaching, a phenomenon linked to the specific forms of Zn present in HFA. In Figure 2, it is evident that the acid-soluble fraction of Zn in HFA predominates over that of other heavy metals. This acid-soluble fraction is highly unstable and exhibits pronounced migratory tendencies. Additionally, the total Zn content is substantial, further contributing to the extensive leaching, rendering Zn the most concerning heavy metal in HFA. Cr and Cu remained below the threshold. Elevated leaching concentrations of Pb, Zn, Cd, and Ni exceed the established thresholds, posing a potential environmental threat. These heavy metals necessitate proper solidification and stabilization treatment before landfill disposal [41,42].

3.2. Analysis of Heavy Metal Stabilization Effect and Mechanism of Different Stabilizing Agents

3.2.1. Heavy Metal Stabilization by Stabilizing Agent Alone

Table 2 shows the results of experiments involving heavy metal stabilization, utilizing varying proportions of cement, NaH₂PO₄, and SDD added individually. (Stabilization efficiency is calculated by dividing the reduced heavy metal leaching value of the HFA after stabilization treatment by the leaching value of the original HFA.) As the cement content increased, the leaching quantities of the six heavy metals decreased. Since Pb, Zn, Cd, and Ni are elements exceeding standard Threshold, the following discussion will focus on these four metals. Cement demonstrated higher efficacy in stabilizing Pb, and Cd. Pb, and Cd exceeded 90% stabilization efficiency with cement additions, reaching 150%. Meanwhile, Ni exhibited a stabilization efficiency of approximately 40% at 150% cement addition, while Zn and Cr’s stabilization efficiency appeared to correlate positively with cement addition, with no discernible stable trend at 200% cement addition, although Zn’s leaching content met standard requirements at 150% cement addition. The solidification and stabilization of heavy metals using cement primarily hinge on the reaction products of cement hydration. These products facilitate physical fixation, chemical adsorption, isomeric replacement, and composite precipitation for the immobilization of heavy metals [7]. The cement hydration reaction is described in Equations (2)–(7). The hydration reaction products of cement, notably C–S–H colloids, AFt, and AFm, possess a high specific surface area and strong hydrogen bonding, enabling them to securely bind heavy metals through chemisorption. AFt and AFm exhibit a robust lattice binding effect on heavy metal ions, immobilizing them via isomeric substitution and chemisorption [43,44,45]. However, relying solely on cement stabilization presents evident drawbacks. To meet heavy metal leaching standards, an additive amount exceeding 150% is necessary, leading to significant volume expansion. At 150% cement addition, the ratio of volume change (R_vc) reaches up to 2.7 (the R_vc is calculated by dividing the volume of the solidified product by the volume of the original HFA).

$$C_{3}S+H_{2}O\to CaO\cdot Si{O}_2\cdot Y{H}_{2}O{(C-S-H)}_3+Ca{(OH)}_2(CH)$$

(2)

$$C_{2}S+H_{2}O\to CaO\cdot Si{O}_2\cdot Y{H}_{2}O{(C-S-H)}_3+Ca{(OH)}_2(CH)$$

(3)

$$C_{3}A+6H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 6H_{2}O(CAH)$$

(4)

$$C_{3}A+3CaS{O}_4\cdot 2H_{2}O+26H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 3CaS{O}_4\cdot 32H_{2}O(AFt)$$

(5)

$$3CaO\cdot A{l}_2{O}_3\cdot 3CaS{O}_4\cdot 32H_{2}O+2C_{3}A+4H_{2}O\to 3[3CaO\cdot A{l}_2{O}_3\cdot CaS{O}_4\cdot 12H_{2}O](AFm)$$

(6)

$$C_{4}AF+7H_{2}O\to 3CaO\cdot A{l}_2{O}_3\cdot 6H_{2}O(CAH)+CaO\cdot F{e}_2{O}_3\cdot H_{2}O(CFH)$$

(7)

Table 2. Individual stabilizing agent experimental results.

No. 1	Stabilizing Agent	Heavy Metal Leaching Concentration (mg/L)						RVE
		Pb	Zn	Cd	Ni	Cu	Cr
01	-	0.78	244.75	0.25	0.55	8.78	0.18	1
1	1% Cement	0.71 ± 0.048	236.51 ± 5.754	0.23 ± 0.030	0.53 ± 0.059	5.84 ± 0.555	0.17 ± 0.019	1.13 ± 0.017
2	5% Cement	0.56 ± 0.069	200.84 ± 7.530	0.20 ± 0.004	0.51 ± 0.071	5.05 ± 0.559	0.15 ± 0.018	1.17 ± 0.038
3	10% Cement	0.34 ± 0.044	190.64 ± 7.425	0.16 ± 0.055	0.47 ± 0.029	4.29 ± 0.104	0.13 ± 0.015	1.26 ± 0.029
4	50% Cement	0.05 ± 0.006	146.62 ± 5.870	0.05 ± 0.011	0.36 ± 0.066	1.32 ± 0.241	0.14 ± 0.032	1.69 ± 0.040
5	100% Cement	n.d. 3	107.42 ± 3.443	0.01 ± 0.001	0.31 ± 0.017	0.24 ± 0.037	0.13 ± 0.027	2.14 ± 0.035
6	150% Cement	n.d.	68.72 ± 4.048	0.02 ± 0.004	0.30 ± 0.029	0.14 ± 0.014	0.12 ± 0.010	2.7 ± 0.158
7	200% Cement	n.d.	39.16 ± 3.038	0.01 ± 0.002	0.32 ± 0.074	0.11 ± 0.004	0.10 ± 0.009	3.17 ± 0.125
8	5% NaH2PO4	0.27 ± 0.017	122.8 ± 7.075	0.09 ± 0.014	0.39 ± 0.038	3.79 ± 0.597	0.11 ± 0.008	1.16 ± 0.019
9	10% NaH2PO4	0.03 ± 0.004	111.2 ± 7.262	0.09 ± 0.011	0.36 ± 0.054	2.75 ± 0.214	0.08 ± 0.012	1.23 ± 0.006
10	15% NaH2PO4	0.01 ± 0.002	98.47 ± 7.510	0.05 ± 0.011	0.32 ± 0.041	2.20 ± 0.213	0.08 ± 0.016	1.28 ± 0.029
11	20% NaH2PO4	n.d.	80.13 ± 7.669	0.05 ± 0.010	0.31 ± 0.051	1.67 ± 0.224	0.11 ± 0.017	1.28 ± 0.009
12	25% NaH2PO4	n.d.	78.90 ± 5.060	0.07 ± 0.013	0.27 ± 0.019	1.33 ± 0.200	0.11 ± 0.003	1.28 ± 0.022
13	5% SDD	0.52 ± 0.031	89.33 ± 0.565	0.21 ± 0.011	0.45 ± 0.010	1.35 ± 0.208	0.12 ± 0.012	1.19 ± 0.010
14	10% SDD	0.35 ± 0.061	47.35 ± 4.952	0.17 ± 0.006	0.37 ± 0.069	0.8 ± 0.173	0.11 ± 0.018	1.26 ± 0.007
15	15% SDD	0.21 ± 0.022	17.60 ± 1.385	0.12 ± 0.012	0.30 ± 0.012	0.45 ± 0.102	0.10 ± 0.016	1.31 ± 0.012
16	20% SDD	0.15 ± 0.020	14.15 ± 2.249	0.07 ± 0.007	0.25 ± 0.038	0.2 ± 0.022	0.13 ± 0.022	1.31 ± 0.010
17	25% SDD	n.d.	11.25 ± 1.249	0.05 ± 0.008	0.15 ± 0.035	0.2 ± 0.026	0.1 ± 0.026	1.33 ± 0.017
Limit 2		0.25	100	0.15	0.5	40	4.5

¹: Leaching concentration of original HFA according to the national standard (GB 16889-2024); ²: Leaching threshold in GB 16889-2024; ³ n.d.: Below the instrumental quantification limit.

Stabilization experiments were performed using NaH₂PO₄, and the results indicated that the addition of NaH₂PO₄ did not lead to reduced leaching for most heavy metals. However, it is noteworthy that NaH₂PO₄ exhibited substantial effectiveness in stabilizing Pb. The stabilization efficiency for Pb reached 96.1% with a 10% addition and achieved full stabilization (100%) with a 20% addition. The optimal stabilization efficiencies for Cd and Zn were observed at a 20% addition of NaH₂PO₄, with stabilization efficiencies of 80% and 67.3% for Cd and Zn, respectively. Furthermore, the stabilization efficiency for Ni consistently improved as the NaH₂PO₄ addition increased. At a 20% NaH₂PO₄ addition, heavy metal leaching met pollution control standards. The stabilizing effect of NaH₂PO₄ on heavy metals in HFA primarily results from the reaction of phosphate with calcium ions in HFA, leading to the formation of hydroxyapatite (Ca₅[PO4]₃OH). This process replaces chloride ions, which are responsible for the formation of chlorinated apatite (Ca₅[PO4]₃Cl), effectively capturing and stabilizing heavy metals. Heavy metals also react with phosphate and chloride ions, resulting in lead chlorophosphate (Pb₅[PO4]₃Cl), as well as other phosphates like (Cd₅[PO4]₃Cl), (Zn₅[PO4]₃Cl), and (Cu₅[PO4]₃Cl). These reactions contribute to the reduction in heavy metal leaching [46,47]. The specific reaction equations are provided in Equations (8)–(13). Despite phosphate’s effective stabilization of lead, and zinc, it is worth noting that its addition must exceed 20% to meet standard requirements, which may not be economically practical in practice.

$$5C{a}^{2+}+3P{O}_4^{3-}+H_{2}O\to C{a}_5{(P{O}_4)}_{3}OH+H^{+}$$

(8)

$$5C{a}^{2+}+3P{O}_4^3+Cl\to C{a}_5{(P{O}_4)}_{3}Cl$$

(9)

$$5P{b}^{2+}+3P{O}_4^{3-}+C{l}^{-}\to P{b}_5{(P{O}_4)}_{3}Cl$$

(10)

$$5C{d}^{2+}+3P{O}_4^{3-}+C{l}^{-}\to C{d}_5{(P{O}_4)}_{3}Cl$$

(11)

$$5Z{n}^{2+}+3P{O}_4^{3-}+C{l}^{-}\to Z{n}_5{(P{O}_4)}_{3}Cl$$

(12)

$$5C{u}^{2+}+3P{O}_4^{3-}+C{l}^{-}\to C{u}_5{(P{O}_4)}_{3}Cl$$

(13)

In the stabilization experiments involving SDD, it demonstrated a notable stabilizing effect on Zn and Cu. When added at a level of only 5%, the leaching concentration of Zn reduced from 244.75 mg/L to 89.33 mg/L, achieving a stabilization efficiency of 63.4%. Furthermore, the stabilization efficiency for Cu reached 84.6%, and the leaching levels of Zn and Cu stabilized at an additional 15% addition of SDD. The primary mechanism for heavy metal stabilization by SDD relies on the coordination reaction between the functional groups of dithiocarbamate and heavy metals, allowing for effective chelation of heavy metals [48,49]. The chelation reaction of functional groups is depicted in Figure 3. However, the stabilization efficiency for Pb was relatively weak, and the standard requirement for Pb was only met with a 15% addition.

Figure 3. Chemical reaction of Sodium dimethyl dithiocarbamate with heavy metals.

Analysis of the experimental data reveals that exclusive cement stabilization requires an addition of 150% or more to effectively stabilize heavy metals in HFA. However, incorporating a substantial amount of cement leads to significant volumetric expansion, exerting considerable pressure on landfill disposal [50]. Inorganic salt stabilizing agents, on the other hand, demonstrate potent stabilization effects primarily for select heavy metals, implying notable limitations. Conversely, organic stabilizing agents fulfill the requirements for the stabilization of most heavy metals, albeit their effectiveness is influenced by the dosage and hampered by high costs, making them unsuitable for widespread use. Combining inorganic salt and organic stabilizing agents gives rise to challenges such as prolonged solidification time and compromised solidified product strength, which may impact subsequent landfill procedures.

3.2.2. Synergistic Solidification Effect of the Combined Stabilizing Agent on Heavy Metals

Due to the ineffective disposal of HFA by stabilizing agents alone, the experiments utilized a combination of organic chelator, phosphate, and cement stabilizing agents for synergistic stabilization of heavy metals in HFA, with the stabilization efficiency illustrated in Table 3. The 5% addition of cement primarily enhances the strength of the solidified product, reduces the solidification time by minimizing the volume expansion caused by the addition of cement, and also assists in the leaching of solidified heavy metals. A dosage of 10% NaH₂PO₄ effectively controls Pb leaching to 0.05 mg/L, well below the 0.25 mg/L threshold with a substantial safety margin. Similarly, a 10% SDD dosage reduces Zn leaching to a maximum of 22.45 mg/L, far complying with the 100 mg/L limit. Cd, however, requires more precise management, as its stabilization is influenced by both SDD and NaH₂PO₄. The stabilization efficiency of Ni appears to be enhanced by the combination of the three stabilizing agents, surpassing the sum of the stabilization efficiencies of the three agents individually.

Table 3. Synergistic stabilization achieved by combining stabilizing agents.

No. 1	Stabilizing Agent			Heavy Metal Leaching Amount (mg/L)						RVE
	Cement	SDD	NaH2PO4	Pb	Zn	Cd	Ni	Cu	Cr
01	-	-	-	0.78	244.75	0.25	0.55	8.78	0.18	1
1	5%	5%	5%	0.27 ± 0.007	59.09 ± 2.440	0.12 ± 0.020	0.30 ± 0.035	0.25 ± 0.020	0.1 ± 0.009	1.30 ± 0.019
2	5%	5%	10%	0.05 ± 0.007	43.25 ± 1.967	0.10 ± 0.004	0.15 ± 0.014	0.11 ± 0.009	0.12 ± 0.023	1.32 ± 0.012
3	5%	5%	15%	n.d. 3	39.35 ± 0.756	0.15 ± 0.036	0.09 ± 0.010	0.20 ± 0.015	0.1 ± 0.019	1.33 ± 0.013
4	5%	5%	20%	n.d.	36.15 ± 1.135	0.11 ± 0.021	0.10 ± 0.007	0.05 ± 0.011	0.1 ± 0.016	1.35 ± 0.008
5	5%	10%	5%	0.15 ± 0.005	22.45 ± 0.644	0.16 ± 0.037	0.25 ± 0.048	0.20 ± 0.002	0.1 ± 0.007	1.31 ± 0.010
6	5%	10%	10%	0.08 ± 0.004	23.35 ± 0.996	0.14 ± 0.036	0.20 ± 0.030	0.15 ± 0.026	0.12 ± 0.010	1.32 ± 0.014
7	5%	10%	15%	0.03 ± 0.005	9.90 ± 0.216	0.15 ± 0.009	0.15 ± 0.009	0.13 ± 0.029	0.13 ± 0.004	1.34 ± 0.012
8	5%	10%	20%	n.d.	13.81 ± 0.196	0.05 ± 0.004	0.11 ± 0.009	0.10 ± 0.007	0.13 ± 0.008	1.36 ± 0.014
9	5%	15%	5%	0.05 ± 0.004	15.05 ± 0.541	0.05 ± 0.006	0.22 ± 0.019	0.25 ± 0.024	0.05 ± 0.004	1.33 ± 0.006
10	5%	15%	10%	0.02 ± 0.002	13.51 ± 0.705	0.07 ± 0.011	0.10 ± 0.021	0.15 ± 0.026	0.02 ± 0.002	1.31 ± 0.013
11	5%	15%	15%	n.d.	14.60 ± 0.615	0.04 ± 0.003	0.11 ± 0.016	0.15 ± 0.006	0.05 ± 0.006	1.32 ± 0.009
12	5%	15%	20%	n.d.	14.65 ± 0.060	0.05 ± 0.004	0.10 ± 0.009	n.d.	n.d.	1.35 ± 0.015
13	5%	20%	5%	0.03 ± 0.004	10.30 ± 0.558	0.05 ± 0.004	0.15 ± 0.005	0.15 ± 0.008	0.1 ± 0.012	1.31 ± 0.018
14	5%	20%	10%	0.01 ± 0.002	12.45 ± 1.051	0.07 ± 0.003	0.12 ± 0.019	0.15 ± 0.023	0.15 ± 0.014	1.32 ± 0.010
15	5%	20%	15%	n.d.	16.58 ± 0.575	0.04 ± 0.002	0.10 ± 0.017	0.15 ± 0.002	0.15 ± 0.010	1.34 ± 0.016
16	5%	20%	20%	n.d.	14.70 ± 1.246	0.05 ± 0.07	0.13 ± 0.019	0.10 ± 0.012	0.12 ± 0.012	1.38 ± 0.013
Limit 2				0.25	100	0.15	0.5	40	4.5

¹: Leaching concentration of original HFA according to the national standard (GB 16889-2024); ²: Leaching threshold in GB 16889-2024; ³ n.d.: Below the instrumental quantification limit.

Among the tested combinations, the formulation with 5% cement, 15% SDD, and 10% NaH₂PO₄ (No. 10) emerges as the possible optimal solution. While lower-cost alternatives exist, such as the 5% cement + 5% SDD + 10% NaH₂PO₄ formulation (No. 2), they result in higher residual Zn (43.25 mg/L) and narrower Cd safety margins. Similarly, the 5% cement + 10% SDD + 20% NaH₂PO₄ formulation (No. 8), though achieving excellent results, uses excessive NaH₂PO₄ without significant additional benefit. Therefore, the 5% cement + 15% SDD + 10% NaH₂PO₄ formulation could be the most reliable and economically ensures all critical heavy metals remain within safe limits while maintaining adequate safety margins, particularly for cadmium which poses the greatest stabilization challenge.

Utilizing a combination of stabilizing agents reduces the required dosage while meeting pollution control standards. Since only a small amount of cement is added to the combined stabilizing agent, the R_vc of the solidified product remains at approximately 1.3, equivalent to the volume change when about 10% cement is added alone. The formulation with 5% cement, 15% SDD, and 10% NaH₂PO₄ (No. 10) achieves a relatively smaller R_vc at about 1.31. The R_vc for the formulation with 5% cement, 5% SDD, and 10% NaH₂PO₄ (No. 2) is about 1.32. The inclusion of cement in the combined stabilizing agent enhances the strength of the solidified product, reduces solidification time to a certain extent through the hydration reaction of the cement, and reduces disposal costs due to the use of organic chelating agents in smaller doses.

3.3. Change in Dioxin Content

Dioxins are highly toxic substances commonly generated during hazardous waste incineration. Therefore, the dioxin content is a subject of concern in the treatment of hazardous waste. We conducted an analysis of dioxin content in the original HFA, conventional cement-solidified HFA, and the best ratio combination of solidified HFA, with a total of 17 dioxin compounds detected, as illustrated in Figure 4. The HFA exhibited lower levels of dioxins, with a total toxic equivalent of 7.6 nanograms TEQ/kg. The dioxin levels in HFA were considerably lower compared to those found in other incineration fly ash, as reported in prior studies [51,52]. It is worth noting that dioxin production is significantly influenced by factors such as the incineration material type, incineration temperature, and flue gas treatment [53,54]. The lower dioxin levels in HFA may primarily result from the incineration temperature. In hazardous waste co-disposal systems, which handle multi-source hazardous wastes, the incineration temperature is approximately 1100 °C, significantly higher than the 850 °C used for municipal solid waste incineration. This higher incineration temperature greatly reduces dioxin production in HFA. Additionally, variations in the types of hazardous waste incinerated in the hazardous waste co-disposal system and different flue gas treatment methods also affect dioxin production. Comparing the dioxin content in HFA solidified with conventional 150% cement and HFA solidified with a combined stabilizing agent, we observed minimal change in the total toxic equivalent of dioxin in the treated HFA. This is due to the limited capacity of cement and composite stabilizers to degrade or react with the dioxin in HFA [13,14].

Figure 4. Influence of stabilizer type and combination on dioxin concentration during heavy metal stabilization.

It is noteworthy that the total dioxin toxicity equivalents in HFA treated with 150% cement stabilization are slightly lower than those in HFA treated with combined stabilizing agents, likely because of the high cement content. A substantial amount of cement undergoes a hydration reaction, resulting in effective physical encapsulation of HFA. This denser and more stable solidified product structure provides some adsorption or encapsulation of a small portion of dioxin, thereby reducing the diffusion of dioxin. Additionally, due to the low dioxin content in HFA, the dioxin content in HFA after treatment with the combined stabilizing agent is well below the limit value of 3000 ng TEQ/kg stipulated in GB16889-2024.

3.4. Solidification Product Analysis

3.4.1. XRD Analysis

In previous experiments evaluating the stability of cement, NaH₂PO₄, SDD, and their combinations, optimal additive ratios for different stabilizers were determined. Subsequently, samples with these optimal additive ratios were selected for further characterization and analysis of the stabilization mechanism. The XRD analysis profiles for samples treated with 150% cement, 20% NaH₂PO₄, 15% SDD, and the combined solidifying treatments are depicted in Figure 5a. The XRD patterns reveal significant changes in the predominant crystalline phases of untreated HFA and the four groups of solidified products. In the solidified products, KCl, NaCl, CaCO₃, Fe₂O₃, and SiO₂ become dominant, while the characteristic peaks of Ca(OH)₂ and CaSO₄ observed in the untreated HFA disappear due to reactions during the stabilization process. The cement hydration reaction also generates Ca(OH)₂, and the enhanced presence of CaCO₃ in the XRD pattern of the sample treated with 150% cement is attributed to the carbonization of Ca(OH)₂. Additionally, the high cement content leads to the detection of Al₂O₃, which is absent in the untreated HFA. The amorphous nature of the hydration product C-S-H colloids is reflected in the lack of distinct diffraction peaks [55,56]. The characteristic peaks of soluble salts in the solidified HFA samples also undergo changes, with significant weakening of the KCl characteristic peak. The NaCl characteristic peak varies notably, being considerably weakened in the 150% cement-treated sample. In the samples treated with 15% SDD, the NaCl characteristic peak remains relatively unchanged, but in the 20% NaH₂PO₄-treated and combined stabilizing agent-treated samples, the NaCl characteristic peak is enhanced due to the inclusion of inorganic sodium salts, compound dissolution, and element migration within the fly ash sample caused by the stabilizing agents’ addition [57].

Figure 5. (a) XRD patterns, (b) FTIR spectra, (c) SEM images, and (d) elemental distribution of samples with different stabilization treatments.

The absence of crystalline heavy metal phases in both untreated HFA and the four groups of solidified products is attributed to several factors. The primary form of heavy metals in fly ash is amorphous [58]. In addition, the content of any potential independent mineral phases of heavy metals may be below the conventional detection limit of XRD (~1–5 wt%) [59]. The concentration of heavy metals in the solidified matrix was diluted by the solidification agents, falling below the conventional detection limit of XRD. More importantly, the solidification process successfully transformed heavy metals into stable forms that are ‘invisible’ to XRD. Specifically, heavy metals reacted with phosphates (from SDP) or dithiocarbamates (from SDD) to form amorphous precipitates. Simultaneously, they were effectively immobilized through adsorption and micro-encapsulation by calcium silicate hydrate (C–S–H) gel produced during cement hydration. This also prevented the heavy metals from forming long-range ordered crystal structures detectable by XRD.

3.4.2. FTIR Analysis

An examination of the interaction between heavy metals and chemical stabilizers was carried out using infrared spectroscopy. Figure 5b displays the FTIR spectra of untreated HFA, HFA solidified with 150% cement, HFA solidified with 20% NaH₂PO₄, HFA solidified with 15% SDD, and the samples treated with a combination of stabilizers. From the FTIR spectra, it is evident that the primary functional groups of untreated HFA and the four different groups of solidified products exhibited minimal changes. They all displayed similar absorption peaks at 3430, 1635, 1415, 1156, 874, 660, and 600 cm⁻¹. Among these, the peaks at 3430 and 1415 cm⁻¹ correspond to the stretching vibrations of O–H and C–N bonds, respectively. The absorption peaks at 1635 and 874 cm⁻¹ result from the asymmetric stretching vibration of C = O in CO₃²⁻. The absorption peaks at 1156, 660, and 600 cm⁻¹ arise from the asymmetric stretching vibrations of Al-O or Si–O, indicating the presence of CaCO₃ and SiO₂ [60]. Discrepancies were observed in the functional groups of samples treated with different stabilizing agents. For example, absorption peaks at 1090 and 713 cm⁻¹, appearing in the spectra of HFA treated with 150% cement, are attributed to Si–O–Si bending vibrations. Similarly, the absorption peaks at 1000 and 460 cm⁻¹ may be attributed to the asymmetric stretching vibrations of Si–O in C–S–H [61,62]. The symmetric stretching vibration peak of PO₄³⁻ at 1030 cm⁻¹ and the characteristic peak of P₂O₇⁴⁻ at 556 cm⁻¹ were observed in the sample treated with 20% NaH₂PO₄. The sample treated with 15% SDD exhibited a characteristic peak at 960 cm⁻¹, indicating the presence of S–CS–N functional groups [17]. Even in the case of the combined stabilizing agent treatment, corresponding absorption peaks of both phosphate and dithiocarbamate functional groups were evident in the spectra, underscoring their significant role in heavy metal stabilization.

3.4.3. Micromorphology and Element Distribution

Figure 5c illustrates the SEM images and elemental distribution of untreated HFA, HFA solidified with 150% cement, HFA solidified with 20% NaH₂PO₄, HFA solidified with 15% SDD, and the combined stabilization treated HFA. Untreated HFA was observed to be loosely agglomerated into spherical particles with a wide range of particle sizes and morphologies. These agglomerates displayed uneven surfaces with loosely dispersed structures and large voids, leading to higher porosity, which facilitated the accumulation of heavy metals on the surface, consequently contributing to environmental pollution [63,64]. In contrast, the surfaces of the treated samples exhibited denser characteristics with fewer voids, signifying a notable reduction in the porosity of the fly ash particles subjected to compound stabilization. The structure became more compact and dense, thereby decreasing the risk of heavy metal leaching [65]. Additionally, flakes and fine particles adhered to the surface, suggesting that the stabilizing agent reacted with displaced heavy metals, leading to precipitation or chelation and subsequent adsorption onto the surface and within the voids. The surface may also bind with Al, Fe, and other substances, resulting in surface densification of the fly ash particles [66]. The EDS elemental energy spectra in Figure 5d revealed a significant reduction in the presence of the four elements—Zn, Pb, Cd, and Ni—which leached out of HFA on the surface of the sample following treatment with the combined stabilizing agent. This observation aligns with the heavy metal data, indicating the successful precipitation or chelation of most of the excess heavy metals, resulting in effective solidification.

4. Conclusions

This study explored the physicochemical attributes, heavy metal content, leaching concentration of heavy metals, and dioxin content in mixed hazardous waste incineration fly ash, which arises from a novel hazardous waste co-disposal method developed by a hazardous waste disposal enterprise. Additionally, the stability features of silicate cement, sodium dihydrogen phosphate, SDD, and three agent combinations of these three agents were compared. Moreover, the stability features of these agent combinations with differing additive ratios were explored to ascertain the optimal combination ratio for stabilizers. Our findings are summarized as follows:

(1)

The hazardous waste incineration fly ash produced through the new co-disposal process exhibits differences in appearance, chemical composition, and dioxin content compared to traditional fly ash, primarily due to distinct generation conditions. Notably, the heavy metal leachate from HFA surpasses the pollution control standards established for municipal landfills in China concerning the content of zinc, lead, cadmium, and nickel. Therefore, effective stabilization is imperative before landfill disposal.

(2)

Cement was found to exert a commendable stabilizing influence on Pb and Cd, achieving compliance with pollution control requirements at a cement addition rate of 150%. Nevertheless, it should be noted that this approach is associated with a relatively high volume change ratio, amounting to 2.7%.

(3)

Sodium dihydrogen phosphate can complement the deficiency of SDD in stabilizing Pb and reduce the quantity of SDD required. Simultaneously, cement can enhance the solidifying strength to a certain extent. When employing a combined stabilizing agent consisting of 5% cement, 15% SDD, and 10% NaH₂PO₄, the heavy metal concentration in the leachate from HFA falls below the Chinese municipal waste landfill pollution control standard. Furthermore, this combination ensures compliance with dioxin content disposal requirements. Importantly, it leads to a lower volume change ratio of approximately 1.31, rendering it a cost-effective and space-saving solution for landfill disposal.

(4)

The combined stabilizing agent demonstrates a strong binding capacity and stability when interacting with the target heavy metals. Analysis of XRD, FTIR, and SEM data reveals that the combined stabilizing agent can effectuate the transformation of heavy metals from unstable to stable states through precipitation, adsorption, and chelation reactions with HFA. Additionally, it partially adheres to the surface of HFA particles, thereby densifying the surface structure of the particles and substantially decreasing the potential for heavy metal leaching from HFA.

(5)

The findings of this study demonstrate a significant step forward in the sustainable management of hazardous waste. The proposed combined S/S system offers a multi-faceted sustainability benefit: (1) Environmental Sustainability: It ensures the long-term immobilization of heavy metals, preventing ecosystem pollution and protecting human health. (2) Technical and Economic Sustainability: By significantly reducing the dosage of cement and expensive chelators, it lowers the treatment cost and energy consumption associated with material production. More importantly, the controlled volume expansion (~1.31 times) directly translates to a substantial reduction in the demand for precious landfill space, which is a critical economic and environmental factor in urban areas.