Resource Disposal and Products of Fly Ash from Domestic Waste Incineration in Zhejiang Province, China: Migration and Change of Hazardous Heavy Metals

Abstract

At present, domestic waste incineration fly ash is classified as hazardous waste. The technical hurdle of fly ash detoxification and resource usage must be overcome in order to effectively utilize fly ash resources. In this study, we investigate the migration and transformation of heavy metal contaminants in the course of utilizing domestic waste incineration fly ash resources through the technology pathway of low-temperature pyrolysis, cyclic rinsing, and evaporation crystallization. Firstly, a comparative analysis was conducted on the fly ash (FA), pyrolysis ash (PA), and water-washing ash (WA) resulting from domestic waste incineration, revealing 24 types of metals, 3 types of non-metals, and 8 types of oxides. We observed the variations in heavy metal concentrations as well as the acidity and alkalinity in three types of ash resulting from the regenerated salt (RS) and incineration processes. Moreover, we analyzed the changes in heavy metal levels and acidity and alkalinity of treated saline water (TSW) and industrial brine (IB), which originate from the fly ash treatment process. The study’s results have confirmed that the heavy metal content in RS was below the detection limit following resource treatment. In addition, the regenerated salt product is determined to be a CaO-SiO₂-Al₂O₃-Fe₂O₃ system, which enables the utilization of fly ash as a valuable resource. Notably, there were significant changes observed in heavy metal content in TSW and IB. Continued attention needs to be paid to the potential risk of environmental contamination from heavy metals and dioxins in FA infiltration. This research will prove beneficial in assessing resource utilization potential of products subjected to environmentally sound incineration fly ash treatment.

1. Introduction

With the rapid development of urbanization, the generation of domestic waste has been increasing annually, and according to the data of National Statistical Yearbook, only in China, the amount of urban domestic waste removal has reached 242 million tons in 2019. Although domestic waste incineration is an effective waste minimization solution, it also generates acid gas, heavy metals, and other toxic and hazardous substances such as dioxins and fly ash during actual operation, creating secondary pollution in the ecological environment. This secondary pollution includes dioxins, fly ash, and other toxic and harmful substances [1,2,3]. The amount of fly ash generated nationwide in 2020 has exceeded 10 million tons, and the resulting problem of harmless and safe disposal of fly ash has also become one of the main challenges for the sustainable development of the domestic waste incineration power generation industry [4,5,6]. Potential emissions can only be mitigated through increased reuse of waste incineration byproducts, including ashes, residues, and fly ash.

Based on the analysis of fly ash disposal and recycling technology at home and abroad, solidification/stabilization and sending to landfill are still the most important fly ash disposal methods. However, there are still potential environmental risks in landfills [7,8,9]. Fly ash contains a variety of heavy metals, and is therefore considered a hazardous waste, with greater environmental hazards [10,11,12]. The heavy metals found in fly ash mainly include Zn, Pb, Cu, Cd, Cr, As, Hg, and Ni [13,14]. Previous studies have shown that the content of Zn in fly ash is the highest, which can reach 2088-65,850 mg/kg, followed by Pb, with a content of about 793–12,113 mg/kg, and Cu with a content of 113–8822 mg/kg, and the rest of the heavy metal elements have different contents [13,14,15]. The dioxin content of raw fly ash can be much more than 3 ng TEQ/g, which are important problems to be faced in fly ash disposal. Quina et al. found that cement curing does not stabilize Hg, Pb, and Cr, and that there is a long-term, potential threat to the environment [16,17], so the use of cement curing technology for fly ash treatment must be assessed for risk before further resource utilization [18]. Heavy metals not only seriously pollute soil and water sources, but also have great toxicity to human body [19]. The main hazards to the human body are chronic poisoning, brain lesions, anemia, chronic renal failure and carcinogenesis [16]. Therefore, the use of cement curing technology for fly ash treatment must be evaluated for risk assessment before further resource utilization [17,20]. At present, there is a lack of systematic and in-depth research on the migration of heavy metals from the solid phase to the liquid phase in the process of water dichlorination, the removal of heavy metals in the fly ash eluent, and the resource utilization of the by-product chlorine salts, etc.

Zhejiang Province is one of the earliest provinces in China to adopt domestic garbage incineration power generation technology [21]. Up to now, there are 75 domestic garbage incineration power generation enterprises in Zhejiang Province that have been put into operation, with a total of 166 treatment units, 85,000 tons of garbage per day, and 838,000 tons of fly ash per year. By the end of 2030, the province’s domestic waste incineration power generation capacity will reach more than 90,000 tons/day, the province’s domestic waste all realize incineration treatment, the amount of cured fly ash will be more and more, even if 5% of the amount of generation, the daily will produce 0.45 million tons/day fly ash, much higher than the aforementioned 10% of the domestic garbage landfill capacity to deal with the ability to handle (0.241 million tons/day), a large number of incineration of cured fly ash will be Faced with the predicament of nowhere to landfill [2]. More serious is the cured fly ash landfill still exists a lot of fly ash heavy metals and dioxin leaching potential environmental pollution risk [20,21,22], is the province’s ecological civilization construction and sustainable development of environmental security hidden danger one of the problems.

On 11 May 2021, the State Council’s General Office issued the Reform and Promotion Plan for Strengthening Supervision, Utilization, and Disposal Capacity of Hazardous Waste. The plan explicitly promotes advancing the high-quality development of the industry for hazardous waste utilization and disposal. The project concentrates on researching the appropriate application and disposal methods for hazardous waste like domestic waste incineration fly ash. The implementation of environmental pollution prevention and control technologies will be demonstrated, along with providing efficient policy support for the comprehensive utilization of domestic waste incineration fly ash and its by-products. To address these needs, this study examines the low-temperature pyrolysis of fly ash, water washing, evaporation, and wastewater crystallization in Zhejiang Province. It explores how heavy metals and dioxins migrate and transform during domestic waste incineration fly ash resource utilization. The study determines if the resulting products meet industry performance standards.

2. Materials and Methods

2.1. Fly Ash Source

Fly ash sampling came from Huzhou City, Zhejiang Province. The specific parameters of the fly ash treatment system process are shown in

Table 1. Parameters related to fly ash treatment system.

	Technology	Input
Fly ash treatment system	Solid Phase Catalytic Pyrolysis Furnace Solid Phase Thermal Dioxin Removal System	Natural gas: 20 m3/ton Electricity: 43 kWh
	Fly ash rinsing system	Industrial water: 180 kg
Exhaust gas treatment process	Water washing system workshop	Electricity: 32 kWh
	Evaporation crystallization drying waste gas	Industrial water: 0.96 kg
Wastewater treatment process	Heavy metal removal system	Electricity: 12 kWh Hydrochloric acid: 25.8 kg PAM: 0.028 kg Sodium sulfide: 0.112 kg
	Primary softening system	Electricity: 17 kWh Sodium sulfate: 170 kg
	Secondary softening system	Electricity: 10.5 kWh Sodium carbonate: kg Hydrochloric acid: kg
	Separation membrane system	Electricity: 31 kWh
	Medium water tank	Industrial water: 0 kg
	MVR evaporation and crystallization system	Electricity: 140 kWh
Total process energy consumption		Electricity: 285.5 kWh Natural gas: 20 m3 Industrial water: 180.96 kg

. The specific process flow is described in the Section 3.1. Fly Ash Resource Utilization Processes.

2.2. Determination of Metallic Elements in Solid Waste

Weigh 0.1~0.5 g (accurate to 0.1 mg) of air-dried, ground sieve (100 mesh) samples, add a small amount of pure water to wet, then add 10 mL of concentrated nitric acid, 6 mL of hydrofluoric acid and 2 mL of perchloric acid, placed in half an hour of cold ablation, in the fully automated graphite ablator (DEENA II, Thomas Cain, MA, USA) on the 160 °C open heating for 2 h. Continue heating to the end of the white smoke until the ablution liquid is less than 0.5 mL, observe the color of the residue colorless or light yellow gelatinous to determine the completion of the ablation. The color of the residue is colorless or light-yellow jelly like that is to judge the completion of digestion. After reduction to room temperature, the volume of pure water is fixed at 50 mL. After sample pretreatment, an inductively coupled plasma mass spectrometer (PE NexION 2000G, Thomas Cain, MA, USA) and an inductively coupled plasma emission spectrometer (PE Optima 8300, Thomas Cain, MA, USA) were used for heavy metal detection.

2.3. Determination of Hexavalent Chromium in Solid Waste

The detection method was alkaline digestion/flame atomic absorption spectrophotometry (HJ 687-2014). First, the sample was pre-treated, 2.5 g of sample was weighed into a beaker, 50 mL of alkaline extract, 0.4 g of MgCl₂ and 0.5 mL of K2HPO4-KH2PO4 buffer solution was added, put into the stirring magnet, polyethylene film sealing, stirring at room temperature for 5 min, heating and stirring to 90–95 °C, and kept for 60 min, removed and cooled to room temperature. 0.45 μm filtration was performed to adjust the pH to 9.0 ± 0.2, transferred to the laboratory and transferred to the laboratory for analysis. The pH was adjusted to 9.0 ± 0.2, transferred to a 100 mL volumetric flask, the volume was adjusted, shaken well and waited for measurement. An atomic absorption spectrophotometer (PE AAnalyst 800, Thomas Cain, MA, USA) was used for analysis.

2.4. Determination of Heavy Metal Elements in Water Quality

Inductively coupled plasma emission spectrometry HJ776-2015 was used to detect and analyze the heavy metal content in water quality. First take 100 mL of water samples to add 5 mL of nitric acid solution, add plugs to mix well, placed on the hot plate heating in the case of non-boiling slowly heated to almost dry, cooling, repeated until the sample solution becomes lighter in color. After cooling, add a few milliliters of nitric acid, then add a small amount of water, placed on the hot plate to continue to heat the residue to dissolve. After cooling, the solution was fixed with experimental water to the original sampling volume, so that the solution to maintain 1% (v/v) of nitric acid acidity. Inductively coupled plasma mass spectrometer (PE NexION 2000G, Thomas Cain, MA, USA) and inductively coupled plasma emission spectrometer (PE Optima 8300, Thomas Cain, MA, USA) were used for detection.

2.5. Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony in Water Quality

Take 25 mL of water samples, add 5 mL of hydrochloric acid—nitric acid solution, mixed with a stopper, placed in a boiling water bath heating digestion 1 h, during the period of shaking 1 to 2 times and open the lid deflated, cooled, fixed volume of water to the standard line, mixing, to be measured. After that, it was determined by atomic fluorescence method. The analyzing instrument was atomic fluorescence photometer (BAF-3000, Beijing, China).

2.6. Determination of Hexavalent Chromium in Solid Waste

Hexavalent chromium in solid waste was detected by alkaline digestion/flame atomic absorption spectrophotometry (HJ 687-2014). First, the sample was pre-treated, 2.5 g of sample was weighed into a beaker, 50 mL of alkaline extract, 0.4 g of MgCl₂ and 0.5 mL of K₂HPO₄-KH₂PO₄ buffer solution were added, put into the stirring magnet, sealed by polyethylene film, stirred for 5 min at room temperature, heated and stirred to 90–95 °C, and kept for 60 min, then taken off and cooled to room temperature. A 0.45 μm filter was used, and the pH was adjusted to 9.0, transferred to a 100 mL volumetric flask, the volume was adjusted, shaken well, and waited for measurement. The analytical instrument was an atomic absorption spectrophotometer (PE AAnalyst 800, Thomas Cain, MA, USA).

2.7. Determination of pH Value in Ash and Water Samples

For the determination of the pH value of the ash in the municipal solid waste incineration process, refer to the standard “Solid Waste Value Determination Method”. Take dry fly ash samples placed in a plastic bottle, add distilled water so that the solid–liquid ratio of 1:5, sealed with a lid, continuous shaking at room temperature, followed by rest, using a pH meter to determine the pH of the top layer of clear liquid and water samples.

2.8. Statistical Analysis

Data were analyzed using OriginPro 2019b software (OriginLab, Northampton, MA, USA). Comparisons were made using one-way analysis of variance followed by multiple comparison of means (Tukey’s test). Differences were considered statistically significant when the p value was less than 0.05. All experiments were performed with three replicates.

3. Results and Discussion

3.1. Fly Ash Resource Utilization Processes

Waste incineration fly ash into the plant first into the low-temperature pyrolysis equipment, filled with nitrogen to keep inert, heating to 300–400 °C, keep 60–90 min; catalytic induction of dioxin dichlorination/condensation reaction: The C-Cl bond is broken and decomposed by the dichlorination/condensation reaction of the phenyl parent body, and the polymerized product generated by the degradation of dioxin and other POPs (finally amorphous carbon) is non-toxic, harmless and safe, which solves the impact of POPs on the environment. Meanwhile, the alkaline atmosphere in the low-temperature thermal decomposition equipment has a strong neutralizing effect on the volatile acid gases such as HF, HCl and SO_x generated by the decomposition. The chlorinated salt produced by the reaction is more easily separated by leaching. After the waste incineration fly ash dichlorination reaction is completed, it is cooled to below 150 °C in an adiabatic and inert atmosphere, which effectively inhibits the resynthesis of dioxin. The pyrolysis exhaust gas passes through the exhaust gas treatment system, completes the treatment of dust, NO_x, SO₂, dioxin and other substances, and is discharged in accordance with the standard, and the pollutant emission meets the standard of “Hazardous Waste Landfill Pollution Control Standard” (GB 18598) [23], and is discharged in accordance with the standard.

The fly ash after pyrolysis enters the three-stage countercurrent water washing system, and in the process of circulation washing, the soluble substances in the fly ash are transferred to the solid–liquid phase, and the soluble salts and heavy metals in the fly ash are eluted from the fly ash into the water washing wastewater. After washing, the fly ash treatment product is CaO-SiO₂-Al₂O₃-Fe₂O₃ system, which can be used as raw material for building materials, with wide application scenarios [24]. At present, the downstream building materialization enterprises have already adopted the fly ash treatment product as raw material for building materials to produce water stabilizing materials and free burning bricks [25,26,27], which has achieved a better raw material substitution economic value.

The washing wastewater generated from fly ash enters the pretreatment process of evaporation crystallization system, removing heavy metals, calcium and magnesium ions in the wastewater using chemicals, forming a higher concentration of salt water, which enters the evaporation crystallization system as raw material liquid. Evaporation crystallization system for sodium chloride, potassium chloride solution at different temperatures in different saturated concentrations, using high-temperature evaporation crystallization of sodium salt, cooling crystallization of potassium salt pre-cipitation way, the raw material liquid in the water will be separated into sodium salt, potassium salt and condensate. Sodium salt, potassium salt as industrial salt products for sale and recycling, condensate into the water washing system recycling, industrial waste water zero discharge. The specific process flow diagram is shown in Figure 1.

3.2. Compositional Analysis of Different Ash Fractions

We first analyzed the composition of FA, PA, and WA in the fly ash revitalization process. The results of elemental analysis showed that the different ash fractions contained the non-metallic elements P, S and CI (Figure 2A). Among them, there was no significant difference in the contents of P and S in the different ash fractions, and only the concentration of CI in WA was significantly lower than the concentration of CI in FA and PA (Figure 2A). In addition, we detected 23 metallic elements in different ash fractions, namely Ti, V, Cr, Mn, Cu, Zn, Rb, Sr, Ba, Pb, Sc, Co, Ni, Ga, As, Y, Zr, Nb, La, Hf, Bi, Ce, and Th (a radioactive element). Among them, the first 11 metallic elements have high contents in the ash fractions, especially Zn, Ti, Cu, and Pb (Figure 2B). It is noteworthy that the Zn concentration in WA is significantly higher than that in PA and FA, with concentrations approaching 8000 mg/kg, while the Ti content in WA is the lowest in comparison. As shown in Figure 2C, Ni, As and Zr were the most abundant metal elements in different ash fractions with concentrations below 200 mg/kg, and these three were also the most abundant in FA. In contrast, Bi was significantly higher in WA than in PA and FA. Our research indicates that during the low temperature pyrolysis and circulating water scrubbing processes, different heavy metals accumulate in different ashes. Further treatment is required to remove these heavy metals.

The primary chemical components present in varying ash fractions were evaluated using X-ray fluorescence spectrometry. The results showed that the different ash fractions contained a variety of metal oxides, including MgO, Na₂O, AI₂O₃, SiO₂, K₂O, CaO, and Fe₂O₃ (Figure 3). These results confirm that the fly ash treatment product after washing is a CaO-SiO₂-Al₂O₃-Fe₂O₃ system. Among them, the highest detected content was CaO, which was the highest in the WA, and the content of PA was significantly higher than that of the FA. In addition, Na₂O content was detected in the WA, but Na₂O in the FA was below the detection limit. In particular, the content of K₂O was also significantly different among the three ash fractions, with the content of K₂O in the WA being significantly lower than that in the FA and PA. In addition, we also detected the content of non-metallic oxides CO₂, which was found to be the highest in FA, followed by PA, and the lowest in WA at less than 2%. Fly ash that has been disposed of in a safe manner and further processed can be utilized as a resource under certain conditions. The prevalent approach for utilizing fly ash resources involves using it as a building material, where it can be added directly to cement and concrete to construct road bases, dams, and other infrastructure projects [28,29,30,31]. Additionally, fly ash that has underwent melt vitrification can be utilized for the production of glass and ceramics [32].

3.3. Analysis of Heavy Metals in Different Ash and Regenerated Salts

As shown in Figure 4, we compared the concentrations of 12 heavy metals (Cd, Cu, Fe, Pb, Mn, Ni, Zn, AI, Ba, Sr, Ca, and Mg) in FA, PA, WA, and RS, respectively. Based on the results, the top four heavy metals with the highest content in fly ash were Ca, AI, Fe, and Mg. In addition, Fe, Mg and AI showed similar trends in the three ash fractions, with significantly lower concentrations of Fe, Mg and AI in PA and WA compared to FA. And the concentrations of Fe, Mg and AI were essentially the same in PA and WA. In contrast, the concentrations of Cd and Zn were significantly higher in PA and WA compared to FA. While the concentrations of Cu and Ba in different ash fractions were not significantly different. There was no significant difference in the content of Cu in different ash fractions. There was no significant difference in the concentrations of Mn and Sr in both FA and PA ashes, but the concentrations of these two heavy metals were significantly lower in WA compared to FA and PA. Overall, the concentrations of Cd, Zn, and Ca were significantly higher in PA, but the concentrations of Fe, Pb, Ni, AI, and Mg were significantly lower compared to FA. In addition, the concentrations of Cd and Zn in WA were significantly higher, but the concentrations of Fe, Pb, Mn, Ni, AI, Sr, and Mg were significantly lower compared to FA. It is worth noting that all heavy metal levels in RS were found to be below detection limit, indicating successful removal of heavy metals from RS to a significant extent. Efficient drenching agents were used to enhance the leaching of heavy metals, including Zn, Pb, and Cu, which have high concentrations in fly ash, transferring the metals from the solid phase to the liquid phase. Subsequently, electrochemical methods were employed for hierarchical reduction to recover the heavy metals, leading to an effective removal of heavy metals from RS.

3.4. Total Heavy Metals of Recycled Salt Water (TSW) and Industrial Brine (IB)

In order to clarify whether there is a significant difference between the two types of brines, the TSW and the IB were collected separately for compositional analysis. The comparative analysis of the concentrations of inorganic salt ions in TSW and IB showed that the concentrations of CI⁻ in IB was about 14.13% higher than that in TSW, and the concentrations of SO₄²⁻ and NO₃⁻ were about 38.37 and 31.64 times higher than those in TSW, respectively. On the other hand, both NO₂⁻ and S²⁻ concentrations were reduced by more than 100-fold in IB compared to TSW (

Table 2. The heavy metal content in TSW and IB. Different lowercase letters indicate statistically significant differences (p < 0.05).

	Detection Limit	TSW	IB
Cl−/mg·L−1	/	184,000	210,000
SO42−/mg·L−1	/	281.5	10,800
NO2−/mg·L−1	/	0.52	<0.003
S2−/mg·L−1	/	0.7135	0.007
NO3−/mg·L−1	/	4.045	128
Cd/mg·L−1	0.05	<0.05	<0.05
Cr/mg·L−1	0.03	<0.03	<0.03
Cu/mg·L−1	0.04	0.398 ± 0.007	<0.04
Fe/mg·L−1	0.01	0.164 ± 0.010 a	0.179 ± 0.007 b
Pb/mg·L−1	0.1	410.373 ± 8.707 a	0.209 ± 0.022 b
Mn/mg·L−1	0.01	<0.01	<0.01
Ni/mg·L−1	0.007	<0.007	<0.007
Zn/mg·L−1	0.009	2.502 ± 0.037	<0.009
Al/mg·L−1	0.009	0.445 ± 0.007	<0.009
Ba/mg·L−1	0.01	5.665 ± 0.099 a	0.030 ± 0.003 b
Sr/mg·L−1	0.01	76.415 ± 1.507 a	0.099 ± 0.002 b
Ca/mg·L−1	0.02	47,125 ± 970 a	37.950 ± 1.372 b
Mg/mg·L−1	0.003	0.853 ± 0.348 a	0.122 ± 0.001 b
Hg/μg·L−1	0.04	4.607 ± 0.061 a	0.060 ± 0.028 b

Note: Some of the results were lower than the detection limit.

).

On the other hand, it can be concluded from Table 2 that the total heavy metal concentrations in IB are significantly lower than in TSW. In TSW and IB, the concentrations of Cd, Cr, Mn and Ni were below the detection limits. In addition, the concentrations of Zn and AI in IB were also very low. It is noteworthy that the concentrations of Pb, Ca, and Sr in these two are extremely different, with the concentrations of Pb, Ca, and Sr in TSW being 2051.85, 1241.77, and 771.87 times higher than those in IB, respectively. This confirms that soluble salts and heavy metals in the fly ash are efficiently removed from the fly ash and transferred into the wastewater of the water washing process during the cyclic rinsing process.

3.5. pH Analysis

As shown in Figure 3A, the pH of the FA, PA, and WA samples ranged from 11.5 to 13.1, which is alkaline, and there was no significant difference in the pH of the three ash fractions. This result indicates that the process of pyrolysis and water washing experienced by the ash did not significantly change the pH in the ash. In particular, the pH of the regenerated salt samples underwent a significant decrease in pH compared to the other samples, with the RS samples having pH values between 8.8 and 9.8 (Figure 5A). In addition, we also compared the variation of pH in treated saline water (TSW) and industrial brine (IB), and the results showed that the pH of TSW was about 11.347 ± 0.06, which was similar to that of different ash fractions. And the pH of IB was about 8.925 ± 0.079 (Figure 5B), which was almost the same as that of RS. The results provide evidence that non-hazardous treatment effectively removes excess impurities from the RS.

4. Conclusions and Implications for Sustainability

In this study, we conducted qualitative and quantitative analyses of pollutants such as chlorine and heavy metals in fly ash, which confirmed the effectiveness of our approach. Our results indicate that in the entire process of treating fly ash resources through waste incineration, all wastewater is reused in the recycling system, thus achieving complete zero discharge. Moreover, the by-product salt has a heavy metal content below the detectable limit, and the pollutants in it can be discharged to fulfill the “Hazardous Waste Landfill Pollution Control Standards” (GB 18598) [23] and emission standards. Furthermore, the salt product regenerated after water washing was determined to be a CaO-SiO₂-Al₂O₃-Fe₂O₃ system. Currently, building material companies are utilizing fly ash treatment products to create water-stable materials and baking-free bricks, resulting in better economic value compared to using raw materials [33]. This provides a viable solution for the proper, efficient and cost-effective disposal and utilization of fly ash from municipal solid waste incineration.

In summary, this study will contribute to (1) rational planning of the industrial layout for washed fly ash; (2) development of fly ash washing industry standards that specify requirements for controlling toxic and hazardous substances content, which would support subsequent management and resource utilization; and (3) development of accompanying management standards for utilizing by-products of waste salt, stipulating regulations and quality testing requirements for legal sales of by-products.