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Article

A Sustainable Development Strategy for Municipal Solid Waste Incineration Bottom Ash: Adsorption Performance and Mechanism in Removing Heavy Metals from Water

by
Yao Zhao
*,
Wenqian Li
,
Jiaqing Wang
and
Zekunyun Hu
College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(8), 3466; https://doi.org/10.3390/su17083466
Submission received: 15 March 2025 / Revised: 1 April 2025 / Accepted: 10 April 2025 / Published: 13 April 2025
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Abstract

:
As urbanization progresses rapidly, the pollution of heavy metal wastewater and the disposal of municipal solid waste incineration bottom ash (MSWI-BA) have emerged as significant challenges. MSWI-BA is a porous material recognized as an environmentally friendly adsorbent. To prevent escalating costs in future practical engineering applications, this study employed unmodified, natural MSWI-BA. This research assessed the adsorption capabilities of MSWI-BA for Pb(II) and Zn(II) through static adsorption experiments, which included adsorption kinetics and isotherm studies. The influence of various factors on the adsorption performance of MSWI-BA was investigated through adjusting the solution pH and the amount of ash, competitive adsorption conditions, and regeneration experiments. Advanced techniques, including ESEM-EDS, XRD, and FTIR, were utilized to analyze the adsorption mechanisms. The results indicated that under the conditions of pH values of 4 and 5, a temperature of 318 K, and an ash dosage of 0.1 g/20 mL, the maximum adsorption capacities of MSWI-BA for Pb(II) and Zn(II) were 89.09 mg/g and 33.77 mg/g, respectively. MSWI-BA demonstrates robust regeneration potential over multiple cycles, validating its practical feasibility. The principal mechanisms for removal include chemical precipitation, ion exchange, and surface complexation. By repurposing it as an efficient and low-cost adsorbent, this represents a sustainable strategy.

1. Introduction

The rapid development of urbanization and a booming economy have led to significant environmental issues, particularly heavy metal pollution in wastewater and the excessive generation of municipal solid waste (MSW) [1]. Heavy metals are non-biodegradable and can be carcinogenic; therefore, their presence in water, even in small amounts, poses a serious health threat to living organisms [2]. Numerous studies have consistently highlighted the prevalent heavy metals in wastewater around the globe, including lead (Pb), mercury (Hg), zinc (Zn), cadmium (Cd), and arsenic (As) [3]. Additionally, industrial discharges and inadequate wastewater treatment facilities have greatly exacerbated the ongoing release of heavy metals into natural water sources [4,5].
Common methods for treating wastewater contaminated with heavy metals include biological methods, adsorption, membrane separation, chemical precipitation, and ion exchange [6]. Among these, adsorption is renowned for its low preparation cost, simple operation, and wide availability of adsorbents, along with its regeneration capability, and has been widely and effectively used for the removal of heavy metal ions from wastewater. Among various adsorbents, inorganic adsorbents have gradually become a research hotspot due to their remarkable physical and chemical properties, as well as their environmental friendliness. Inorganic adsorbents can generally be classified into traditional inorganic adsorbents, modified inorganic adsorbents, and novel adsorbents. Traditional inorganic adsorbents, such as zeolite, aluminum oxide, and diatomaceous earth, have constraints in application due to their low adsorption efficiency and poor regeneration performance [7,8,9]. For example, volcanic ash possesses volcanic ash reactivity, but its maximum adsorption capacity for Pb(II) is only 8.15 mg/g [10]. For enhanced adsorption performance and environmentally friendly purposes, traditional inorganic adsorbents need to be modified. The porous geopolymer (PGP) prepared from coal gangue and fly ash has an optimal adsorption capacity of 85.67 mg/g for Pb(II), but the cost reaches 2.07 RMB/kg [11]. However, the availability of materials is declining. This decrease leads to higher costs, making the economics of using these materials for heavy metal pollution treatment in wastewater unfavorable.
On the other hand, an increasing number of cities in both developing and developed countries are facing significant challenges regarding MSW. Currently, Chinese MSW production accounts for approximately 27% of the total generated globally [1]. In 2022, MSW output in China reached a concerning 244 million tons, reflecting an average annual increase of 7% over the past five years. If this trend persists, it is projected that the total will exceed 261 million tons by the end of the current year [12]. Currently, the primary methods for managing MSW are landfilling and incineration. However, landfilling is insufficient for achieving waste reduction and resource recovery objectives, as it requires substantial land area and fails to eliminate toxic components present in the waste [13].
The incineration method is actively promoted by the government due to its significant reductions in both the volume and weight of MSW. Two main residues are generated after the incineration process: fly ash (FA) and bottom ash (BA) [14]. There are numerous heavy metals and other harmful substances in FA, most of which exceed the standard values. Comparatively, only a few heavy metals are found in BA and they are lower than the standard values [15]. Hence, most scholars and engineers have utilized MSWI-BA in cement, soil, cement concrete, and asphalt mixtures due to its stable physicochemical properties and similar mechanical characteristics to natural aggregates. However, the addition of MSWI-BA has not significantly improved the properties of these materials; consequently, its actual utilization rate remains low (approximately 20%) in the construction field. It has been found that MSWI-BA is a porous material rich in SiO2 and CaCO3, which exhibits high volcanic ash activity [16]. Its mineral composition and high specific surface area contribute to a strong affinity for binding heavy metal ions, such as Pb, Zn, and Cu, demonstrating its potential for removing heavy metals from wastewater. Nevertheless, most studies have reported the properties of MSWI-BA and the performance of constructional materials containing MSWI-BA, with few studies focusing on the reuse of MSWI-BA as an adsorbent material based on its high specific surface area and volcanic ash activity [17]. There is a need to study the adsorption behavior and properties of reusing MSWI-BA for removing typical heavy metals from wastewater to evaluate the feasibility and eco-friendliness of using MSWI-BA as an adsorbent material.
Previous studies identified two key challenges: evaluating the feasibility of MSWI-BA as an eco-friendly adsorbent for heavy metal removal from wastewater and elucidating its adsorption mechanism. Pb(II) and Zn(II), prevalent pollutants in urban and industrial wastewater, were the focus of this study. Solid waste has been used as an adsorbent, but it often necessitates modification for improved adsorption efficiency [18]. Such modifications not only raise costs but can also result in environmental contamination. Consequently, using natural MSWI-BA as the research focus makes it easier to apply in future engineering practices (such as permeable asphalt pavement and permeable concrete [19]). This study evaluated the adsorption performance of MSWI-BA for Pb(II) and Zn(II) through static adsorption experiments, including adsorption kinetics and isotherm studies. The impact of various factors on the adsorption performance of MSWI-BA was explored through adjusting the solution pH and the amount of ash, competitive adsorption, and regeneration experiments. The physicochemical properties and porous characteristics of MSWI-BA were characterized using ESEM-EDS, XRD, FTIR, and low-temperature nitrogen adsorption tests. The findings highlight MSWI-BA’s potential as a sustainable adsorbent for heavy metal remediation and provide valuable insights into the mechanisms governing Pb(II) and Zn(II) adsorption.

2. Materials and Methods

The MSWI-BA used in this study was sourced from a municipal waste incineration power plant situated in Nanjing City, Jiangsu Province, China. Upon delivery to the laboratory, iron-containing and non-ferrous metal impurities were meticulously removed, followed by a natural drying process that lasted 90 days indoors. The dried MSWI-BA was then sieved according to the standard method (T0302-2005) [20]. Samples with particle sizes between 1.18 mm and 2.36 mm were selected and subsequently dried in a blast dryer at a temperature of 333 K until they reached a constant weight. After drying, all samples were cooled to room temperature (T = 298 K) and sealed in polypropylene (PP) bags.
Before conducting the adsorption experiments, the microscopic morphology of MSWI-BA, along with the composition and distribution of the major elements on its surface, was examined using Environmental Scanning Electron Microscopy (ESEM) (Prisma E, Thermo Fisher, Waltham, MA, USA). The structure of the organic functional groups and crystal composition in MSWI-BA were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) (VERTEX 80v, Bruker, Ettlingen, Germany) and X-ray Diffraction (XRD) (Ultima IV, Rigaku, Tokyo, Japan), respectively. Additionally, the pore distribution of the MSWI-BA particles was characterized through a low-temperature nitrogen adsorption test, employing the Brunauer–Emmett–Teller (BET) model and the t-plot method.
Dynamic experiments were first conducted to investigate the influence of contact time on the adsorption of heavy metals by using MSWI-BA as an adsorbent. Herein, the dosage of 1.0 g/20 mL was selected according to previous studies [21]. A 1 g MSWI-BA sample and a 20 mL solution of Pb(II) or Zn(II) with a concentration of 100 mg/L were mixed in a 50 mL centrifuge tube, and then shaken for 0–1440 min at a rotational speed of 180 r/min, under a room temperature of 298 K. The initial pH values of the Pb(II) solution and Zn(II) solution were 4 and 5, respectively.
To explore the impact of initial concentration on the adsorption performance of Pb(II) and Zn(II) by MSWI-BA under various temperatures, based on the results of the adsorption kinetics experimental, a 0.1 g/20 mL MSWI-BA sample was mixed with a 20 mL solution of Pb(II) or Zn(II) in a 50 mL centrifuge tube and then shaken at 180 r/min at 298 K, 308 K, and 318 K for 1440 min, respectively. The initial concentrations of Pb(II) or Zn(II) in the solutions were in the range (20–500 mg/L), respectively. Moreover, the initial pH values of the Pb(II) solution and Zn(II) solution were 4 and 5, respectively.
In addition to the above factors, the effects of MSWI-BA dosage and the starting pH of the solution on the uptake of heavy metals were also taken into consideration. The dosage of an adsorbent significantly influences its capacity to retain a specific concentration of adsorbate. To investigate the effect of MSWI-BA dosage on the adsorption of heavy metals, 0.02 g/20 mL–1.8 g/20 mL of sample was separately mixed with 20 mL of the Pb(II) or Zn(II) solution in a 50 mL centrifuge tube, and then shaken for 1440 min at a rotational speed of 180 r/min under a room temperature of 298 K. The initial concentrations for both heavy metals were 100 mg/L. Moreover, the initial pH of the Pb(II) solution and Zn(II) solution was 4 and 5, respectively. The pH value has a marked effect on the equilibrium capacity. To examine how the initial pH value affects the adsorption of heavy metals by MSWI-BA, an experiment was conducted where 0.1 g/20 mL of the adsorbent was combined with 20 mL of either Pb(II) or Zn(II) solution in a 50 mL centrifuge tube. The mixture was subsequently shaken for 1440 min at a rotational speed of 180 rpm under a room temperature of 298 K. The initial pH levels of the solutions were systematically adjusted to 2–6. Consequently, the competitive adsorption of Pb(II) and Zn(II) on MSWI-BA was thoroughly investigated.
Heavy metals coexist in wastewater, so the results of a single system cannot provide an effective theoretical basis for practical use [22]. Consequently, a study was conducted to explore the competitive adsorption phenomena of Pb(II) and Zn(II) on MSWI-BA. In this study, a 0.1 g/20 mL MSWI-BA sample was mixed with 10 mL of Pb(II) solution and 10 mL of Zn(II) solution in a 50 mL centrifuge tube and then shaken under a temperature of 298 K with a rotational speed of 180 rpm for 1440 min. The initial concentration for both solutions was 100 mg/L. The pH values of the Pb(II) solution and Zn(II) solution were 4 and 5, respectively.
Adsorption of Pb(II) and Zn(II) was accomplished using HCl as the elution agent. This process involved shaking the contaminants at room temperature for 8 h, which facilitated the complete desorption of Pb(II) and Zn(II) from the MSWI-BA. After filtration, the collected MSWI-BA was washed with deionized water until neutral and then dried in an oven for 8 h before the cyclic adsorption experiments were conducted. The cycling performance of the slag was subsequently evaluated to assess its regeneration capacity.
It is important to note that after each experiment, all solutions were passed through a 0.45 μm imported polyethersulfone (PES) filter membrane. The concentration of heavy metals in the filtrate was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES)(NEXION 300, PerkinElmer, Waltham, MA, USA).
The adsorption capacity (Qt) and the removal efficiency (R) of Pb(II) or Zn(II) at contact time t were separately calculated using Equations (1) and (2). The equilibrium adsorption capacity (Qe) was calculated using Equation (3).
Q t = ( C 0 − C t ) × V X  
R = C 0 − C t C 0 × 100 %
Q e = ( C 0 − C e ) × V X
where C0, Ct, and Ce are the heavy metal concentration of the solution at contact time 0, t, and equilibrium, respectively, mg·L−1; V is the volume of the solution, mL; and X is the weight of the adsorbent, g.
Kinetic models can characterize the changes during the adsorption process. The adsorption of heavy metal ions by adsorbents is commonly characterized through the utilization of primary and secondary kinetic models. The pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were employed to analyze the kinetic experimental data, thereby elucidating the adsorption behavior of MSWI-BA with respect to heavy metal ions. Kinetics analysis can reveal the mechanism and rate-limiting step of the adsorption process. If the adsorption process is primarily governed by physical adsorption, it may align better with the PFO model. Conversely, if it is dominated by chemical reactions, it is likely to correspond more closely with the PSO model. Additionally, two widely utilized kinetic models were selected to assess the adsorption performance of MSWI-BA and to elucidate the interactions between the adsorbent and the adsorbate [23].
In this study, adsorption data were analyzed by two kinetic models. The PFO model and PSO model are described by Equations (4) and (5), respectively [24,25].
Q t = Q e ( 1 − e − K 1 t )
Q t = K 2 Q e 2 t 1 + K 2 Q e t
where K1 and K2 are separately the rate constants for the PFO and PSO kinetic model, g·mg−1·min−1. Other symbols have the same meaning as above.
The Langmuir model, primarily indicative of the surface retention phenomenon of MSWI-BA, is characterized by the presence of uniform binding sites, as articulated in Equation (6). The affinity associated with the adsorption process can be computed using Equation (7) and is denoted by the parameter RL. When RL > 1, it suggests that the adsorption process is unfavorable; conversely, when RL < 1, it indicates that the adsorption occurs readily [26]. The Freundlich model describes multilayer adsorption characterized by increased surface coverage resulting from the heterogeneous nature of the solid surface. The associated empirical model is expressed in Equation (8).
Q e = Q m K L C e 1 + K L C e
R L = 1 1 + K L C 0
Q e = K F C e 1 / n
where Qm is the maximum adsorption capacity, mg·g−1; KL is the Langmuir model constant, L·mg−1; KF is the Freundlich model constant, (mg·g−1); and 1/n represents the intensity of the effect of equilibrium concentration on the adsorption amount.
Based on the results concerning the influence of the material on heavy metal adsorption performance, the two aforementioned isothermal adsorption models were employed. To analyze and fit the experimental data, a multivariate nonlinear fitting method was utilized, thereby elucidating the interaction mechanism between MSWI-BA and heavy metal ions.
Stock solutions containing Pb(II) and Zn(II) ions were formulated at a concentration of 1 g/L using Pb(NO3)2 and ZnSO4·7H2O, respectively. These concentrates were then appropriately diluted to prepare the necessary working solutions for the experiments. The concentrations of these solutions were adjusted as required, and the pH was modified using NaOH and HCl to achieve the desired experimental conditions.
All utilized chemicals and reagents were of analytical grade. Pb(NO3)2 was obtained from Xilong Chemical Company (Shantou, China), whereas ZnSO4·7H2O was purchased from Sinopharm Chemical Reagent Co. (Shanghai, China).

3. Results and Discussion

3.1. Characterization of MSWI-BA

The microscopic morphology and surface elemental composition of MSWI-BA are detailed in Figure 1 and Table 1. The results of crystal morphology and composition of MSWI-BA are shown in Figure 2. The results in Figure 1 and Table 1 illustrate that the appearance of MSWI-BA closely resembles that of natural aggregates commonly used in road construction, and it has a coral-like morphology with an uneven surface structure featuring abundant grooves and pores on the micro level. The surface of MSWI-BA particles is predominantly composed of various elements, including O, C, Si, and Al, with atomic percentages of 61.5%, 26.9%, 6.9%, and 2%, respectively.
It can be seen in Figure 2 that the presence of a broad characteristic absorption peak at 3431 cm dicates the existence of the hydroxyl group (-OH). Two characteristic adsorption peaks observed near 1428 cm−1 and 1630 cm−1 are attributed to the asymmetric and symmetric stretching vibrations of the carboxyl (-COO-) group [27]. Conversely, signal peaks around 1796 cm−1 and 1039 cm−1 correspond to carbonyl -C=O and Si-O/C-O bonds, respectively. The XRD patterns were analyzed, revealing that the MSWI-BA mainly consists of two crystal structures: CaCO3 and SiO2.
MSWI-BA samples were tested through a low-temperature nitrogen adsorption test, and the data were analyzed using the BET model and t-plot method. Additionally, the calculations regarding specific surface area, micropore volume, and total pore volume are consolidated in Table 2. As indicated in Table 2, the average specific surface area of MSWI-BA is 7.46 m2/g, with a total pore volume of 0.029 cm3/g, leading to an average pore size of 3.36 nm. During the adsorption process, only the pores penetrable by heavy metal ions are deemed effective, as they provide sufficient opportunities for heavy metal ions to enter [28].

3.2. Adsorption Behavior Pb(II) and Zn(II) onto MSWI-BA

3.2.1. Kinetic Adsorption Properties

The adsorption data were analyzed using two kinetic models, with the results presented in Figure 3. As shown in the figure, the adsorption of Pb(II) and Zn(II) onto MSWI-BA as the adsorbent increases with contact time (t). Specifically, Qt increases rapidly with t, reaching equilibrium at approximately 120 min. The adsorption process can be divided into three stages: 0–60 min, 60–120 min, and post-120 min. Initially, the high concentration of metal ions in the solution creates a noticeable concentration gradient on the surface of MSWI-BA, serving as a strong driving force that facilitates the dispersion and aggregation of heavy metal ions on the adsorbent’s surface through electrostatic interactions. Consequently, the adsorption capacity of MSWI-BA increases sharply during this period. From 60 to 120 min, the adsorption sites on the surface of MSWI-BA become progressively occupied by heavy metal ions, leading to a slower increase in adsorption capacity. After 120 min, all active sites on MSWI-BA are completely occupied, and the adsorption reaches equilibrium. The final adsorption quantities were measured at 1.922 mg/g for Pb(II) and 1.986 mg/g for Zn(II). These results suggest that MSWI-BA is effective in removing Pb(II) and Zn(II) from liquid solutions.
All the correlation coefficients (R2) values listed in Table 3 exceed 0.91, indicating that the adsorption of both Pb(II) and Zn(II) ions by MSWI-BA involves contributions from both chemisorption and physisorption [29]. The adsorption process for removing Pb(II) using MSWI-BA aligns closely with the PFO model, achieving a fitted correlation coefficient of R2 = 0.97. In contrast, the adsorption process for Zn(II) can be described by both models with R2 values of 0.99. These results mean the adsorption process for Pb(II) is primarily governed by physical adsorption, while the adsorption of Zn(II) is not only influenced by physical adsorption but also chemical adsorption [30]. During the adsorption process, electron transfer or sharing occurs between the heavy metal ions in the solution and the nitrogen and oxygen atoms present on the surface of MSWI-BA. Functional groups on the surface of the slag (-OH, C=O, and C–O–C) may participate in the adsorption process. Among these, the carboxyl group plays a key role in Pb adsorption [31].

3.2.2. Isothermal Adsorption Properties

The isothermal models and adsorption thermodynamics of MSWI-BA for Pb(II) and Zn(II) at various temperatures (298 K, 308 K, and 318 K) are illustrated in Figure 4 and Figure 5. According to Formulas (6)–(8), the parameters of the isothermal adsorption models for Pb and Zn onto MSWI-BA were obtained (Table 4). Additionally, thermodynamic parameters, including the standard enthalpy change (ΔH°), standard entropy change (ΔS°), and standard Gibbs free energy change (ΔG°), were calculated according to established formulas, with the results presented in Table 5. The effects of temperature and different heavy metal concentrations on the adsorption behavior of MSWI-BA, along with the corresponding thermodynamic changes, were investigated.
Overall, the adsorption of Pb(II) and Zn(II) by MSWI-BA exhibits a rapid initial increase, followed by a gradual leveling off as the initial heavy metal concentration in the aqueous solution rises. In the tested concentration range (0–500 mg/L), the active adsorption sites on the surface of MSWI-BA are sufficient to accommodate the heavy metal ions, facilitating the rapid binding of Pb(II) and Zn(II) ions to these sites. As the initial concentration increases, competitive adsorption occurs among the heavy metal ions due to the limited availability of active sites on the MSWI-BA surface, resulting in a slower increase in Qt until equilibrium is reached. At a temperature of 318 K, the maximum adsorption capacities were measured at 89.09 mg/g for Pb(II) and 33.77 mg/g for Zn(II), observed at an initial concentration of 500 mg/L.
The R2 listed in Table 4 indicates that the adsorption isotherm for Pb(II) conforms to both the nonlinear Langmuir and Freundlich models, but shows a better fit with the Langmuir model, which has an R2 of 0.99. This suggests that the adsorption of Pb(II) by MSWI-BA is dominated by monolayer adsorption [32]. The active adsorption sites on the surface of MSWI-BA exert a homogeneous and uniform force on Pb(II) ions. In contrast, the adsorption isotherm simulation for Zn(II) fits both the Langmuir and Freundlich models very well (R2 = 0.98) [33]. The adsorption of Zn(II) onto the surface of MSWI-BA is more intricate, possibly involving both monolayer and multilayer adsorption [34].
The theoretical maximum adsorption capacities (Qm) calculated using the Langmuir model are 107.88 mg/g for Pb(II) and 60.29 mg/g for Zn(II). Moreover, the adsorption equilibrium constant (KL) of the Langmuir model is less than 1, and the reciprocal of the Freundlich model exponent (1/n) is less than 0.5, indicating that the adsorption of both Pb(II) and Zn(II) by MSWI-BA in aqueous solutions occurs easily without the need for additional promotion.
The effect of solution or experimental temperature is a significant physicochemical factor, as it can influence the amount of adsorption [35]. As observed in Figure 5, better adsorption performance is exhibited with increasing temperature. This can be explained by the enhanced surface activity and kinetic energy of the MSWI-BA, which enhance adsorption. As the temperature rises, the speed of molecular motion increases, resulting in a higher binding rate of heavy metal ions to active sites.
According to Table 5 and Figure 5, ΔH° > 0 indicates that the adsorption of Pb(II) and Zn(II) onto MSWI-BA is an endothermic process. The adsorption capacity of the material for heavy metals effectively increases with rising temperature [36]. Furthermore, ΔS° > 0 reflects an increase in the disorder at the solid–liquid interface between MSWI-BA and the heavy metal solution [37]. Additionally, ΔG° < 0 suggest that the adsorption process of heavy metal ions by MSWI-BA is spontaneous [38]. Moreover, the increasing absolute values of Gibbs free energy further confirm that elevated temperatures are beneficial for enhancing the adsorption performance of MSWI-BA. Furthermore, the ionization of organic functional groups (-OH, -NH3⁺, etc.) on the surface of MSWI-BA is enhanced at elevated temperatures, leading to greater availability of active sites [39]. When combined with the previously discussed adsorption kinetic models, it can be analyzed that the adsorption process of MSWI-BA for Pb(II) and Zn(II) is primarily governed by physisorption.
A comparative analysis of the adsorption capacities of various modified low-cost adsorbents and MSWI-BA for Pb(II) and Zn(II) ions was conducted, with the results presented in Table 6. It is evident that the MSWI-BA utilized in this study, even without modification, exhibited superior adsorption performance for heavy metal ions while simultaneously providing environmental benefits.

3.3. Factors Affecting Pb(II) and Zn(II) Adsorption onto MSWI-BA

3.3.1. The Effect of Solution pH

Several studies have identified the initial pH value of the aqueous solution as a key parameter, as it significantly influences adsorption efficiency and adsorption capacity. The adsorption of MSWI-BA for Pb(II) and Zn(II) at various initial pH values is presented in Figure 6. According to Figure 6, the trends in Pb(II) and Zn(II) adsorption using MSWI-BA are similar. As the initial pH increases, the adsorption capacity also rises. Specifically, within the pH range of 2 to 6, the adsorption rate of Pb(II) increases from 69.18% to 94.6%, while that of Zn(II) rises from 19.55% to 99.69%. At an initial solution pH of 4 to 6, the MSWI-BA exhibits good adsorption effectiveness for Pb(II) and Zn(II). Within this range, the R values are all greater than or equal to 90%.
In this study, when HCl was added to the solution, the surface of MSWI-BA became protonated, and the rate of proton migration increased, competing for the limited adsorption sites. Additionally, proton adsorption leads to increased electrostatic repulsion between the MSWI-BA surface and the metal cations, resulting in poorer adsorption performance. Conversely, a higher pH value reduces proton adsorption, thereby enhancing adsorption.

3.3.2. The Effect of MSWI-BA Dosage

The amount of adsorbent is a crucial parameter that affects the quantitative ratio of adsorbent to adsorbate, thereby influencing the adsorption process. Generally, increasing the adsorbent dose is positively correlated with the efficiency and performance of heavy metal ion removal. The changes in Qt and R of Pb(II) and Zn(II) with various doses of MSWI-BA are depicted in Figure 7.
It can be observed that the efficiency of both Pb(II) and Zn(II) removal increases with the dosage of MSWI-BA, achieving high removal rates of 98.99% and 99.31% at an MSWI-BA dosage of 1.8 g/20 mL, respectively. In cases involving smaller amounts of MSWI-BA (0.02 g/20 mL to 0.1 g/20 mL), the R increases significantly. This can be attributed to the substantial increase in MSWI-BA dosage by five times, which provides a larger active surface area and more available adsorption sites for Pb(II) and Zn(II). Conversely, with higher amounts of MSWI-BA (0.1 g/20 mL to 1.8 g/20 mL), R ≥ 90% remains relatively stable. At these higher MSWI-BA contents, the adsorption of heavy metals gradually decreases. This phenomenon is primarily due to the substantial increase in the number of active sites that can accommodate metal ions, resulting in a gradual reduction in metal loading per unit mass of the adsorbent [44].

3.4. Competitive Adsorption Behavior in Mixed Systems

The competitive adsorption of Pb(II) and Zn(II) ions in a mixed system is presented in Figure 8. It can be observed that both the Qt and R in the mixed system are lower than those in the single system, indicating that competitive adsorption occurs between the two types of heavy metal ions. Related studies have confirmed significant differences in the surface affinity of the adsorbent for different heavy metal ions within the mixed system, primarily attributed to the ionic radius and electronegativity of the ions involved [45]. Generally, a larger ionic radius corresponds to lower restricted migration on the surface of the adsorbent [46]. Consequently, the Zn(II) ion demonstrates a better binding ability to the MSWI-BA surface due to its larger ionic radius of 0.74 Å compared to the Pb(II) ion, which has an ionic radius of 1.20 Å. However, Pb(II) possesses stronger electronegativity, making it more attractive to MSWI-BA and more likely to form stable complexes [47]. This suggests that MSWI-BA has almost the same affinity and adsorption capacity for both heavy metal ions in the mixed system. As a result, the removal rates of Pb(II) and Zn(II) ions in the mixed system reach as high as 97.66% and 99.84%, respectively.

3.5. Results and Discussion on the Regeneration Performance of MSWI-BA

Desorption and regeneration treatments were subsequently applied to the MSWI-BA following the adsorption of heavy metal ions, with the results of the cyclic adsorption experiments presented in Figure 9. The findings reveal that the removal efficiencies of Pb(II) and Zn(II) decline as the number of cycles increases, although this reduction remains below 15%. After four cycles, the removal rates for Pb(II) and Zn(II) were recorded at 74.8% and 83.5%, respectively. These results underscore the commendable regeneration performance of the MSWI-BA, affirming its efficacy as an exceptional adsorbent material for heavy metal ions. The observed decrease in adsorption efficiency can primarily be attributed to incomplete desorption during the elution process, as some heavy metal ions occupied the adsorption sites, ultimately diminishing overall adsorption efficacy with increasing cycle repetitions.

4. Adsorption Mechanism of MSWI-BA on Pb(II) and Zn(II)

As mentioned above, the adsorption of heavy metal ions by MSWI-BA is influenced not only by physical adsorption but also by chemical adsorption. The adsorption process depends on the surface characteristics of the adsorbent material, the adsorption conditions, and the properties of heavy metals. To verify the theoretical adsorption mechanism, MSWI-BA samples before and after adsorption were subjected to FTIR analysis, XRD tests, and ESEM analysis, with the results shown in Figure 10, Figure 11 and Figure 12.
The adsorption of MSWI-BA on heavy metal ions in aqueous solution is attributed to several processes. Firstly, physical adsorption occurs through van der Waals forces and pore filling within the MSWI-BA structure. Secondly, chemical adsorption plays a significant role, as functional groups on the surface of MSWI-BA, such as -OH and -NH2 groups, can form complexes with heavy metal ions through chelation or coordination bonds. Additionally, ion exchange processes may take place, where cations from the heavy metals replace other cations already present on the adsorbent surface. Furthermore, electrostatic attraction between the charged surface of MSWI-BA and the heavy metal ions of opposite charge contributes to the overall adsorption process.
After the adsorption of heavy metals, the FTIR spectra of MSWI-BA showed significant changes; the characteristic peaks corresponding to oxygen- and nitrogen-containing functional groups on their surfaces exhibited noticeable shifts. Specifically, the intensity of the Si-O absorption peak at 1046 cm−1 and the -OH absorption peak at 3435 cm−1 decreased and shifted after the adsorption. This can be attributed to the attraction of heavy metal ions to the hydroxyl groups during the adsorption process, leading to the aggregation of metal ions near the Si-O groups. Additionally, the characteristic absorption peaks around 1637 cm−1 and 1432 cm−1, which correspond to the asymmetric and symmetric stretching vibrations of the -COO- group, also shifted. This means that oxygen-containing functional groups are involved in the adsorption reactions of heavy metals. The results indicate that, serving as active adsorption sites for heavy metal ions, these groups may form new chemical bonds with Pb(II) and Zn(II), thereby exhibiting some degree of surface complexation.
From the XRD spectra, it is evident that there is no significant change in the composition of the material phases, indicating that no new material phases are generated during the adsorption process. Two sets of characteristic diffraction peaks related to the crystal structures of SiO2 (PDF #85-0865) and CaCO3 (PDF #88-1807) are observed in the XRD spectra of MSWI-BA, both before and after adsorption. The diffraction peaks near 2θ = 20.87°, 26.64°, 50.14°, 59.97°, and 68.15° are associated with the SiO2 crystal phase, while the characteristic peaks of the CaCO3 crystal structure appear near 2θ = 23.05°, 29.39°, 35.98°, 39.42°, 43.17°, 47.50°, 48.51°, and 59.97°. All of these crystalline materials originate from the inorganic fraction of MSWI-BA. Notably, the characteristic diffraction peaks near 2θ = 9.70° and 15.81°, observed in Figure 10, decrease after heavy metal adsorption. This indicates that a reaction occurs between the ettringite in the MSWI-BA and the heavy metal ions in the solution during the adsorption process. Existing studies have shown that there is a lattice substitution effect with Pb(II), while the interaction with Zn(II) is primarily through interstitial doping [48]. These results indicate that ion exchange is a significant adsorption mechanism for the removal of Pb(II) using MSWI-BA, while chemical precipitation plays a crucial role in the efficient removal of Zn(II).
The characterization of the MSWI-BA before and after heavy metal adsorption reveals differences in the surface microstructure and elemental composition, as illustrated in Figure 12. Specifically, grooves and clusters become densely aggregated and subsequently deposit within their pores. XRD results indicate that ettringite likely undergoes decomposition, leading to the continuous destruction of grooves and resulting in a relatively smoother surface. FTIR analyses suggest that heavy metals interact with the oxygen-containing functional groups in MSWI-BA, forming surface complexes. The differences observed between Figure 11 show that MSWI-BA exhibits fewer grooves and complexes after Pb(II) adsorption compared to Zn(II) adsorption; this finding supports conclusions drawn from both the adsorption kinetic and isotherm models. As shown in Figure 12d,e, the elemental composition of the MSWI-BA surface is predominantly oxygen (O), accounting for approximately 50% both before and after heavy metal adsorption. Following the adsorption of Pb(II), the Pb content on the MSWI-BA surface increased from 0% to 2.2%, while the Zn content rose from 0% to 1.9% after the adsorption of Zn(II). This further indicates the successful loading of Pb(II) and Zn(II) onto the surface of the MSWI-BA [49].
In summary, it is evident that the adsorption mechanism for removing Pb(II) using MSWI-BA is primarily governed by monomolecular layer adsorption. In contrast, Zn(II) removal is predominantly characterized by multimolecular layer adsorption. Throughout this process, the interaction between MSWI-BA and heavy metal ions may involve the complexation of functional groups, chemical precipitation, and ion exchange. The above results also demonstrate that MSWI-BA exhibits promising applicability in the treatment of heavy metals. As a resource-efficient adsorbent, MSWI-BA can reduce raw material costs, aligning with the principles of sustainable development and environmental protection.

5. Conclusions

After conducting an experimental investigation into the reuse of MSWI-BA as an eco-friendly adsorbent material for the removal of Pb(II) and Zn(II) from aqueous solution, the following conclusion can be drawn:
(1)
MSWI-BA exhibited excellent adsorption capacity for Pb(II) and Zn(II) removal from aqueous solutions. The adsorption of Pb(II) on MSWI-BA fits the PFO and Langmuir models better; at pH = 4 and T = 318 K, the maximum adsorption capacity is 89.09 mg/g. The adsorption of Zn(II) on MSWI-BA fits well with the PFO, PSO, Langmuir, and Freundlich models; at pH = 5 and T = 318 K, the maximum adsorption capacity is 33.77 mg/g.
(2)
The adsorption efficiency of MSWI-BA for Pb(II) and Zn(II) is influenced by the initial pH of the solution, the amount of ash, the competition between heavy metal ions, and the number of regeneration cycles. At pH = 2, the removal rates of Pb(II) and Zn(II) by MSWI-BA are only 69.18% and 19.55%, respectively. When the pH value is in the range of 4–6 and the amount of ash is between 0.1 g/20 mL and 1.8 g/20 mL, the R values for the ash are all greater than or equal to 90%. In mixed systems, competitive adsorption between Pb(II) and Zn(II) reduced overall adsorption compared to single systems. After four cycles, the removal rates for Pb(II) and Zn(II) were recorded at 74.8% and 83.5%, respectively.
(3)
Characterization via ESEM-EDS, FTIR, and XRD revealed structural and compositional changes in MSWI-BA after adsorption. Chemical precipitation, ion exchange, and surface complexation were identified as key mechanisms.
The above results indicate that MSWI-BA exhibits good adsorption properties for Pb(II) and Zn(II) in water without the need for modification. It is characterized by low cost and large storage capacity, making it suitable for direct use in treating heavy metal wastewater to utilize its adsorption properties. The resource recovery and reuse of MSWI-BA as an efficient and low-cost adsorbent constitutes a sustainable strategy.

Author Contributions

Conceptualization, Y.Z.; methodology, Y.Z.; writing—review and editing, Y.Z. and J.W.; data curation, W.L.; writing—original draft, W.L.; investigation, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Jiangsu Province “333 High-level Talent”, grant number non.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

This work was supported by Jiangsu Province “333 High-level Talent” project (Sixth round). The authors would like to appreciate the Advanced Analysis and Testing Center of Nanjing Forestry University for helping in FT-IR, XRD, and ESEM tests.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Images of pre-treated natural aggregate and MSWI-BA: (a) natural aggregate and MSWI-BA particles after pretreatment; (b) ESEM image of MSWI-BA at 3000× magnification.
Figure 1. Images of pre-treated natural aggregate and MSWI-BA: (a) natural aggregate and MSWI-BA particles after pretreatment; (b) ESEM image of MSWI-BA at 3000× magnification.
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Figure 2. Characterization results of MSWI-BA: (a) FTIR spectrum of MSWI-BA; (b) XRD pattern of MSWI-BA.
Figure 2. Characterization results of MSWI-BA: (a) FTIR spectrum of MSWI-BA; (b) XRD pattern of MSWI-BA.
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Figure 3. Fitting graph of adsorption kinetics data of MSWI-BA: (a) the adsorption of Pb(II); (b) the adsorption of Zn(II).
Figure 3. Fitting graph of adsorption kinetics data of MSWI-BA: (a) the adsorption of Pb(II); (b) the adsorption of Zn(II).
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Figure 4. Fitting graph of isothermal adsorption model of MSWI-BA: (a) fitting of isothermal adsorption data for Pb(II) at 298 K; (b) fitting of isothermal adsorption data for Zn(II) at 298 K; (c) fitting of isothermal adsorption data for Pb(II) at 308 K; (d) fitting of isothermal adsorption data for Zn(II) at 308 K; (e) fitting of isothermal adsorption data for Pb(II) at 318 K; and (f) fitting of isothermal adsorption data for Zn(II) at 318 K.
Figure 4. Fitting graph of isothermal adsorption model of MSWI-BA: (a) fitting of isothermal adsorption data for Pb(II) at 298 K; (b) fitting of isothermal adsorption data for Zn(II) at 298 K; (c) fitting of isothermal adsorption data for Pb(II) at 308 K; (d) fitting of isothermal adsorption data for Zn(II) at 308 K; (e) fitting of isothermal adsorption data for Pb(II) at 318 K; and (f) fitting of isothermal adsorption data for Zn(II) at 318 K.
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Figure 5. Isothermal adsorption capacity and thermodynamics: (a) absorption capacity of MSWI-BA for Pb(II) at different temperatures; (b) absorption capacity of MSWI-BA for Zn(II) at different temperatures.
Figure 5. Isothermal adsorption capacity and thermodynamics: (a) absorption capacity of MSWI-BA for Pb(II) at different temperatures; (b) absorption capacity of MSWI-BA for Zn(II) at different temperatures.
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Figure 6. Effect of solution pH value on the Pb(II) and Zn(II) adsorption using MSWI-BA.
Figure 6. Effect of solution pH value on the Pb(II) and Zn(II) adsorption using MSWI-BA.
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Figure 7. Effect of MSWI-BA dosage on the adsorption of heavy metals from aqueous solution.
Figure 7. Effect of MSWI-BA dosage on the adsorption of heavy metals from aqueous solution.
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Figure 8. Effect of co-existing substances on the adsorption amount and removal rate of heavy metals by MSWI-BA.
Figure 8. Effect of co-existing substances on the adsorption amount and removal rate of heavy metals by MSWI-BA.
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Figure 9. Removal rate of MSWI-BA at different cycle counts.
Figure 9. Removal rate of MSWI-BA at different cycle counts.
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Figure 10. FTIR spectra of MSWI-BA samples before and after heavy metal adsorption.
Figure 10. FTIR spectra of MSWI-BA samples before and after heavy metal adsorption.
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Figure 11. XRD spectra of MSWI-BA samples before and after heavy metal adsorption.
Figure 11. XRD spectra of MSWI-BA samples before and after heavy metal adsorption.
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Figure 12. ESEM-EDS analysis of MSWI-BA surface: (a) ESEM image before adsorption; (b) ESEM image after Pb(II) adsorption; (c) ESEM image after Zn(II) adsorption; (d) EDS spectra before and after adsorption; and (e) Atomic% before and after adsorption.
Figure 12. ESEM-EDS analysis of MSWI-BA surface: (a) ESEM image before adsorption; (b) ESEM image after Pb(II) adsorption; (c) ESEM image after Zn(II) adsorption; (d) EDS spectra before and after adsorption; and (e) Atomic% before and after adsorption.
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Table 1. Major elements on the surface of MSWI-BA according to the results of the ESEM-EDS test.
Table 1. Major elements on the surface of MSWI-BA according to the results of the ESEM-EDS test.
ElementONaMgAlSiKCa
Atomic/%61.50.90.82.06.90.826.9
Table 2. Pore structure parameters of MSWI-BA.
Table 2. Pore structure parameters of MSWI-BA.
MSWI-BA SampleBET-Specific Surface Area/m2·g−1Pore Volume
/10−3 cm3·g−1		Pore Diameter
/nm
(1)7.480.0273.25
(2)7.660.0313.45
(3)7.240.0293.42
Average value7.460.0293.36
Table 3. Fitted parameter values of PFO model and PSO model.
Table 3. Fitted parameter values of PFO model and PSO model.
IonsQe/mg·g−1PFO Model (R2)PSO Model (R2)
Pb(II)1.980.970.91
Zn(II)1.920.990.99
Table 4. Fitted parameter values for the Langmuir and Freundlich models.
Table 4. Fitted parameter values for the Langmuir and Freundlich models.
IonsLangmuirFreundlich
Qm/mg·g−1KL/L·mg−1R21/nKF/L·mg−1R2
Pb(II)107.883.17 × 10−40.990.400.810.97
Zn(II)60.293.78 × 10−40.980.460.530.98
Table 5. Thermodynamic parameters of MSWI-BA adsorption.
Table 5. Thermodynamic parameters of MSWI-BA adsorption.
IonsΔH°/K·mol−1ΔS°/J·mol−1·K−1ΔG°/KJ·mol−1
298 K308 K318 K
Pb(II)9.931336.7638−0.4134−0.54357−0.6655
Zn(II)23.465280.8864−0.25786−0.56536−0.8535
Table 6. Comparison of adsorption performance of MSWI-BA with other materials.
Table 6. Comparison of adsorption performance of MSWI-BA with other materials.
AdsorbentPb(II)/mg·g−1Zn(II)/mg·g−1References
MgAl-layered double hydroxide10.887.30[40]
Spirulina platensis 60026.927.8[41]
Cement-modified biochar composites38.7626.53[42]
High-efficiency hydroxyapatite–hydrochar composites--24.99[43]
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Zhao, Y.; Li, W.; Wang, J.; Hu, Z. A Sustainable Development Strategy for Municipal Solid Waste Incineration Bottom Ash: Adsorption Performance and Mechanism in Removing Heavy Metals from Water. Sustainability 2025, 17, 3466. https://doi.org/10.3390/su17083466

AMA Style

Zhao Y, Li W, Wang J, Hu Z. A Sustainable Development Strategy for Municipal Solid Waste Incineration Bottom Ash: Adsorption Performance and Mechanism in Removing Heavy Metals from Water. Sustainability. 2025; 17(8):3466. https://doi.org/10.3390/su17083466

Chicago/Turabian Style

Zhao, Yao, Wenqian Li, Jiaqing Wang, and Zekunyun Hu. 2025. "A Sustainable Development Strategy for Municipal Solid Waste Incineration Bottom Ash: Adsorption Performance and Mechanism in Removing Heavy Metals from Water" Sustainability 17, no. 8: 3466. https://doi.org/10.3390/su17083466

APA Style

Zhao, Y., Li, W., Wang, J., & Hu, Z. (2025). A Sustainable Development Strategy for Municipal Solid Waste Incineration Bottom Ash: Adsorption Performance and Mechanism in Removing Heavy Metals from Water. Sustainability, 17(8), 3466. https://doi.org/10.3390/su17083466

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