Ultrasonic-assisted hydrogen peroxide leaching of cadmium-containing flue dust under room temperature and neutral conditions: Efficient recovery and mechanism

Graphical abstract

Highlights

• •Cadmium recovery of 96.76 % was achieved at room temperature, driven by cavitation-enhanced ·OH radicals.
• •Ultrasound inhibits particle agglomeration, opens packages, and promotes H₂O₂ to produce more ·OH.
• •Activation energy drops from 16.198 to 3.389 kJ·mol⁻¹ under ultrasonic synergy.
• •Neutral conditions eliminate acid wastewater, reducing environmental impact.

Abstract

A novel ultrasonic-enhanced hydrogen peroxide leaching method is proposed for efficient cadmium recovery from flue dust under room temperature and neutral conditions, addressing the limitations of conventional methods such as low efficiency and environmental pollution. Under optimal conditions (room temperature, 5 mL H₂O₂, 2:1 liquid–solid ratio, 20 min, and 360 W ultrasonic power), the cadmium leaching efficiency reaches 96.76 %, significantly higher than the conventional oxidative leaching efficiency of 88.57 %. Mechanistic studies indicate that ultrasound inhibits particle agglomeration, disrupts encapsulated structures, and generates hydroxyl radicals, enhancing oxidation of cadmium and lead-containing phases for efficient separation. Kinetic studies reveal that the introduction of ultrasound reduces the activation energy from 16.198 to 3.389 kJ·mol⁻¹, changing the rate-determining step from a mixed control to diffusion control, thereby accelerating leaching kinetics. This method not only enhances cadmium recovery but also reduces environmental impact, offering a greener and more efficient solution for the lead smelting industry.

1. Introduction

During the lead smelting process, a large amount of cadmium-containing flue dust is generated from the bottom-blown furnace, with cadmium content reaching about 15 % to 30 % [1]. Taking a Chinese lead smelting enterprise as an example, its annual lead production is 300,000 tons, while the bottom-blown furnace dust generated amounts to 5,500 tons per year. Globally, the lead smelting industry produces about 11–12 million tons of lead annually, leading to a continuous accumulation of cadmium-rich dust. Cadmium is a highly toxic heavy metal that can contaminate soil, water, and air, leading to serious ecological degradation and adverse health effects [2,3]. On the other hand, due to its unique physicochemical properties, cadmium is widely used in rechargeable batteries, coatings, and plastic stabilizers, making its recovery economically valuable [4,5]. Recovering cadmium from flue dust not only mitigates environmental pollution but also enables the dust to be returned to the smelting system for lead extraction, thereby achieving synergistic resource utilization of lead and cadmium [6]. The dual benefits of environmental protection and economic value make the development of efficient and eco-friendly technologies for metal extraction from flue dust a key research focus.

The treatment methods for cadmium-containing flue dust can be categorized into pyrometallurgy and hydrometallurgy [7]. Pyrometallurgical methods exploit the differential volatilization characteristics of heavy metals, where cadmium and its compounds initiate volatilization at 700–800 °C with complete volatilization achieved by 1000 °C, while lead and zinc compounds require higher temperature ranges of 1000–1200 °C and 900–1000 °C respectively. This thermal gradient enables selective cadmium recovery at lower temperatures prior to other metal constituents. However, due to their high energy consumption and lengthy process, these methods are gradually being phased out [8]. In contrast, hydrometallurgy methods enable efficient metal recovery under relatively mild conditions, with significant clean production advantages [9]. In industrial applications, the hydrometallurgical treatment of cadmium-containing dust typically involves acid leaching or water leaching to transfer cadmium compounds into solution, followed by enrichment and recovery of cadmium through methods such as zinc powder replacement, electrowinning, or solvent extraction [10]. Among these steps, leaching is one of the key steps to determine the effect of cadmium enrichment and ensures the resource utilization of flue dust, which directly determines the efficiency and economy of cadmium recovery [11]. While acid leaching efficiently extracts cadmium, it also dissolves a large number of impurities (e.g., arsenic, lead, etc.), leading to complex leaching solution compositions and increased difficulty in subsequent separation and purification [12]. Additionally, the generation of acidic wastewater raises environmental treatment costs. For instance, Wang et al. [13] compared the leaching effects of dilute acid and water on cadmium-containing flue dust from a lead smelting bottom-blown furnace. They found that although dilute acid leaching increased the cadmium leaching rate by about 3 %, it also caused the arsenic impurity leaching rate to increase by 14 %. In comparison, water leaching is widely used in lead smelting companies in Henan, China, due to its operational simplicity and environmental friendliness. However, under practical operating conditions, the cadmium leaching efficiency rarely exceeds 60 % [14]. Even with enhanced stirring, the addition of oxidants, or raising the temperature above 80 °C, the leaching rate has never been able to exceed 90 %, still falling short of production requirements [15].

Research has confirmed that mechanical activation (such as high-energy ball milling) can enhance the metal leaching efficiency by disrupting particle agglomeration and increasing the specific surface area [16]. However, this approach fails to directly break the chemical bonds of chemically stable refractory phases due to the absence of effective chemical activation pathways, resulting in limited leaching effectiveness. Ultrasound technology, with its unique cavitation and mechanical effects, has significantly enhances mineral leaching during hydrometallurgy [17]. Ultrasound, typically above 20 kHz, creates cavitation in liquids, forming bubbles that grow, contract, and burst. The collapse of these bubbles releases energy, generating temperatures over 5000 K and pressures around 50 MPa locally, accompanied by shock waves and microjets [18]. These effects increase the contact area between liquids and solids, promoting mass transfer and chemical reactions. Additionally, ultrasound stirs liquids intensely, reduces diffusion resistance, and accelerates reactions [19]. It can also disrupt particle surfaces, open inclusions, and create new reaction interfaces [20]. For instance, Gui et al. [21] demonstrated that ultrasound can improve gold mineral leaching efficiency by about 20 %, while Sari [22] reported that ultrasonication enhances metal recovery rates from complex matrices (e.g., smelting residues), highlighting its broad applicability in hydrometallurgical processes. When combined with oxidants like hydrogen peroxide or ozone, ultrasound generates stronger oxidizing free radicals, enhancing oxidative leaching by promoting phase transformations [23]. However, the effects of ultrasound and oxidants on cadmium-containing flue dust leaching, their impact on phase transformation, and their influence on the leaching kinetics remain unexplored.

To address the key technical challenges associated conventional water leaching processes for cadmium-containing dust, such as high energy consumption, long duration, and low leaching efficiency in conventional water leaching, an ultrasonic-enhanced hydrogen peroxide leaching method is developed for efficient cadmium recovery from flue dust under room temperature and neutral conditions in this work. The content, phase composition, and occurrence forms of cadmium, lead, and other elements in the flue dust are analyzed. The impacts of various reaction factors on cadmium leaching efficiency are investigated, and the process conditions are optimized. Through multiscale characterization and analysis, the role of oxidants in complex mineral systems and the enhancement mechanism of ultrasound in oxidative leaching for cadmium separation are elucidated. The kinetic characteristics of cadmium leaching under the coupled effects of ultrasonic fields and oxidants are investigated. Ultimately, an efficient and eco-friendly ultrasonic-enhanced oxidative leaching process is established. This process achieves selective valence conversion-based efficient recovery of cadmium and lead, reduces cadmium accumulation and hazards in lead–zinc smelting systems, enhances cadmium resource utilization, and promotes sustainable development in the lead–zinc smelting industry.

2. Experimental

2.1. Materials

The flue dust used in this study was collected from bottom-blown furnace of a lead smelting enterprise in Henan Province, China, and the hydrogen peroxide (H₂O₂, pure grade) was supplied by Sinopharm Chemical Reagent Co., Ltd., China. All solutions were prepared using deionized water.

2.2. Experimental procedure

The experimental flow chart is illustrated in Fig. 1. The ultrasonic-enhanced water leaching with oxidation (ULO) experiments were conducted as follows: 50 g of flue dust sample was placed in a 200 ml beaker, followed by the addition of a magnetic rotor and deionized water. The beaker was then sealed with plastic wrap to prevent water evaporation. An ultrasonic probe (20 kHz, maximum power 1800 W) was placed vertically in the center of the beaker, about 1 cm from the bottom. The beaker was placed in a water bath with magnetic stirring (200 rpm). During the leaching process, H₂O₂ solution was added dropwise at a flow rate of 0.5 g/min and ultrasound was turned on. The conditioned experiments were carried out by adjusting various related parameters such as leaching temperature (20, 30, 40, 50, 60 °C), time (10, 20, 30, 40, 50 min), liquid–solid ratio (1; 1, 1.5: 1, 2: 1, 2.5: 1, 3: 1), and amount of oxidizer (1, 2, 3, 4, 5, 6 mL). After each experiment, the mixture was transferred to a filtration flask for vacuum filtration to separate the leachate and leach residue, which were subsequently characterized using various analytical techniques. For comparison, conventional water leaching (CL), ultrasonic-enhanced water leaching (UL), conventional water leaching with oxidation (CLO) experiments were conducted under identical conditions, with the only variations being the absence of oxidant, ultrasound assistance, or both. The ultrasonic reaction system was independently developed by the Key Laboratory of Unconventional Metallurgy, Ministry of Education at Kunming University of Science and Technology. It operates at an ultrasonic frequency of 20 kHz, with the core components of the ultrasonic generator made from titanium alloy. The ultrasonic power is controlled by adjusting the output percentage, with a maximum output power of 1800 W. During the experiments, a thermostatic circulating water bath system (accuracy: ±0.5 °C) was used to maintain temperature stability under high-power ultrasonic conditions. The reaction solution temperature was monitored in real time with an immersion-type precision thermometer, and the cooling water flow rate was dynamically adjusted to maintain a constant reaction system temperature.

Fig. 1.

Experimental setup and flow chart.

2.3. Analytical methods

The phase composition was performed using powder X-ray diffractometer (XRD, Rigaku dX 2000, Japan), with data analyzed via MDI Jade software. Bulk elemental analysis of samples was carried out using an X-ray fluorescence spectrometer (XRF, Rigaku ZSX Primus, Japan). Chemical state analysis was conducted by X-ray photoelectron spectroscopy (XPS, PHI PHI-5300, USA), where the binding energies of elements were calibrated against the C1s peak at 284.8 eV and processed using Avantage software. Micromorphology of the sample were performed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS, Zeiss Sigma 300, Germany). Particle size distribution was measured with a laser particle size analyzer (Malvern Mastersizer 3000+ Ultra, UK). The elemental content of the filtrate samples was quantified by inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110, USA). The specific surface area of filter cakes under different conditions was measured using a fully automatic specific surface area and porosity analyzer (Micromeritics ASAP 2460, USA). All samples were degassed at 120 °C for 8 h prior to measurement. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Ion chromatography (IC) analysis was performed using a chromatograph (HIC-20A SUPER, Shimadzu, Japan) equipped with an appropriate anion separation column and suppressor to detect sulfite (SO₃^2-) and sulfate (SO₄^2-) ions. The eluent was a potassium hydroxide (KOH) solution in gradient mode. Electron paramagnetic resonance (EPR) analysis was performed using a spectrometer (JES-FA200, JEOL, Japan) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent.

The leaching degree of cadmium is calculated by Eq. (1) [24].

$$\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{C}\mathrm{d}(\%)={(C}_t\times V_t)/(m_0\times {\omega }_0)\times 100\%$$ (1)

Where $C_t$ represents the concentration of Cd in the filtrate (g/L) and $V_t$ represents the volume of filtrate (L). Meanwhile, $m_0$ represents the mass of raw flue dust (g), ${\omega }_0$ represents the Cd content of Cd in raw flue dust (%).

3. Results and discussion

3.1. Analysis of raw flue dust

The cadmium-containing dust used in this study was obtained from the flue dust generated by a bottom-blown furnace at a lead smelter in Henan Province, China. The average particle size of the dust was measured as 3.61 μm, with a particle size range of 0.405 to 51.8 μm. As shown in Table 1, XRF analysis revealed that lead (Pb) was the predominant metallic component (41.51 wt%), followed by cadmium (Cd, 14.33 wt%) and sulfur (S, 11.81 wt%). Other significant constituents included potassium (K, 1.75 wt%), tellurium (Te, 0.99 wt%), and bismuth (Bi, 0.97 wt%).

Table 1.

The chemical composition of raw flue dust (wt.%).

Pb	Cd	S	K	Te	Bi
41.51	14.33	11.81	1.75	0.99	0.97
Na	Tl	Zn	Cl	Ag	Si
0.64	0.47	0.33	0.25	0.10	0.07

The XRD analysis of raw flue dust (as shown in Fig. 2) reveals the presence of multiple cadmium and lead phases, which exhibit distinct leaching behaviors that critically impact the leaching efficiency and subsequent separation of lead from the solution. Cadmium is primarily present as cadmium sulfate (CdSO₄, PDF#97-010-0317), cadmium sulfite (CdSO₃, PDF#97-006-2641) and cadmium sulfide (CdS, PDF#97-060-0773). Among these, CdSO₄ is highly soluble in water (77.2 g/100 mL at 20 °C), making it easier to leach under conventional conditions. However, CdSO₃ and CdS exhibit lower solubilities (0.001 g/100 mL and <0.0001 g/100 mL, respectively), with CdS being particularly challenging to dissolve due to its stable crystal structure and low reactivity. The coexistence of these less soluble cadmium phases substantially limits the overall cadmium leaching efficiency, especially when they are encapsulated or aggregated within the flue dust particles, which further hinders their contact with the leaching solution. Regarding lead speciation, lead sulfate (PbSO₄, PDF#97-009-2609) and lead sulfite (PbSO₃, PDF#97-003-5360) are identified. PbSO₄ is typically insoluble in water (0.003 g/100 mL) and thus remains in the solid residue, ensuring it does not contaminate the leaching solution [25]. However, PbSO₃ has a slight solubility (0.004 g/100 mL) that increases at elevated temperature, allowing a portion of it to dissolve into the solution. This temperature-dependent solubility of PbSO₃ leads to the introduction of lead impurities into the solution at higher leaching temperatures, complicating the separation process and reducing the purity of the final product [26].

Fig. 2.

X-ray diffraction pattern of the raw flue dust.

The XPS survey spectrum (Fig. 3a) detects the presence of multiple elements, including Cd, Pb, S, and O. The Pb 4f spectrum (Fig. 3b) shows peaks attributed to PbSO₄ (139.0 ± 0.1 eV for 4f7/2 and 144.2 ± 0.1 eV for 4f5/2) and PbSO₃ (138.2 ± 0.1 eV for 4f7/2 and 143.3 ± 0.1 eV for 4f5/2), confirming the coexistence of these lead phases. As shown in the Cd 3d spectrum (Fig. 3c), the observed peaks are deconvoluted into two main components corresponding to Cd 3d5/2 (405.4 ± 0.2 eV) and Cd 3d3/2 (412.0 ± 0.2 eV). Moreover, loss peak features are observed on the high binding energy side of Cd 3d3/2 [27,28]. This stems from the intrinsic electronic structure rather than spectral fitting artifacts and does not interfere with the phase transition analysis of cadmium-containing species. In cadmium compounds such as cadmium sulfide (CdS), cadmium sulfate (CdSO₄), and cadmium sulfite (CdSO₃), cadmium always exists in the +2 valence state. Since XPS primarily probes the chemical states of elements, the nearly identical Cd 3d binding energies in these compounds make them difficult to distinguish at standard XPS resolution. However, the XRD results (Fig. 2) provide complementary structural information, confirming the presence of these phases. The overlap in binding energies suggests that cadmium exists in multiple phases within the flue dust, which can complicate the leaching process as different phases have varying solubilities and reactivities. The S 2p spectrum (Fig. 3d) reveals three distinct components corresponding to metal sulfate (S 2p3/2, 169.0 ± 0.1 eV), metal sulfite (S 2p3/2, 165.9 ± 0.2 eV), and metal sulfide (S 2p3/2, 161.5 ± 0.1 eV). This multistate sulfur distribution confirms that the coexistence of sulfate, sulfite, and sulfide phases, which is in full agreement with XRD results.

Fig. 3.

High-resolution XPS spectra of raw flue dust: (a) XPS full spectrum, (b) Pb 4f, (c) Cd 3d, (d) S 2p.

The complex phases of cadmium and lead in raw flue dust, including soluble, sparingly soluble and insoluble phases, which complicates subsequent Cd-Pb separation. Ultrasound can facilitate phase transformations, breakdown aggregates, and increase the effective surface area for leaching, thereby improving the overall efficiency and purity of the process [29]. This approach is crucial for overcoming the barriers posed by raw flue dust characteristics and achieving cleaner, more efficient metal extraction.

3.2. Performance comparison of various leaching methods

To preliminarily demonstrate the effectiveness of the introduction of ultrasound and H₂O₂ in enhancing the leaching degree of Cd from cadmium-containing flue dust, the leaching degree of Cd under four different conditions-conventional water leaching (CL), conventional water leaching with oxidation (CLO), ultrasonic-enhanced water leaching (UL), and ultrasonic-enhanced water leaching with oxidation (ULO) were systematically evaluated. The leaching conditions were uniformly set as follows: temperature of 20 °C, time of 10 min, liquid–solid ratio of 2:1, ultrasonic power of 360 W (only UL and ULO), and H₂O₂ addition of 0.5 mL (only CLO and ULO). As shown in Fig. 4, the leaching degree of Cd increased sequentially from 84.17 % (CL) to 87.40 % (CLO), 91.26 % (UL) and 93.47 % (ULO), which indicates that both ultrasound and H₂O₂ facilitated the water leaching process.

Fig. 4.

Comparative Cd leaching degree under four conditions: conventional water leaching (CL), conventional water leaching with oxidation (CLO), ultrasonic-enhanced water leaching (UL), ultrasonic-enhanced water leaching with oxidation (ULO).

The comparative effects of air and hydrogen peroxide on cadmium leaching with and without ultrasound were also investigated in our work. Under identical control parameters (air flow rate 5 L/h, duration 10 min, liquid-to-solid ratio 2:1, temperature 20 °C), the leaching degree of Cd reached 83.14 % with conventional air-assisted leaching and increased to 89.68 % with ultrasound-assisted air leaching (360 W). The comparative study demonstrated that regardless of ultrasound application, the air-assisted leaching efficiency was significantly lower than that of the hydrogen peroxide system (87.4–93.47 % vs. 83.14–89.68 %), which is attributed to the lower oxidation potential (+1.23 V) and solubility (9.1 mg/L at 20 °C) of oxygen in air. Notably, air bubbling even showed slightly inferior performance compared to pure water leaching in certain cases, likely due to fine particle entrainment by air flow.

Compared with that of CL, the increase in Cd leaching degree by CLO is small (Δ ∼ 3.2 %). In contrast, UL (Δ ∼ 7.1 % over CL) can significantly improve the leaching degree of Cd. Meanwhile, the superior performance of ULO (Δ ∼ 9.3 % over CL) indicates that ultrasound not only disperses particles, increasing solid–liquid contact and enhancing mass transfer, but also promotes H₂O₂ decomposition into ·OH [30]. These radicals, with strong oxidizing power, can accelerate oxidation, transforming CdSO₃ to CdSO₄, CdS to CdSO₄ and PbSO₃ to PbSO₄ (Eqs. (2), (3), (4)) [17]. Simultaneously, ultrasonic waves generate cavitation bubbles whose collapse creates a momentary high-temperature and high-pressure environment. This expedites oxidation and enhances leaching efficiency [31]. In addition, the Gibbs free energy values of reactions (2–4) confirm the thermodynamic spontaneity of the reactions. Simulation experiments in solution demonstrated that H₂O₂ does not undergo rapid decomposition in the presence of CdS, highlighting the validity of selecting hydrogen peroxide as the oxidant in this work (see Supporting Information Text S1 and Fig. S1 for details). The relevant reactions and their thermodynamic data are as follows:

CdSO₃ + H₂O₂ → CdSO₄ + H₂O ΔG⁰ =-374.194 kJ/mol (2)

CdS + 4H₂O₂ → CdSO₄ + 4H₂O ΔG⁰ =-1144.306 kJ/mol (3)

PbSO₃ + H₂O₂ → PbSO₄ + H₂O ΔG⁰ =-417.973 kJ/mol (4)

The above experiments show the feasibility of ultrasound and H₂O₂ in the water leaching of cadmium-bearing flue dust. Therefore, it is necessary to further explore the optimal process conditions and the mechanism of ultrasonic enhancement.

3.3. Optimization of ultrasonic-enhanced H₂O₂ leaching conditions

The effect of temperature on Cd leaching from cadmium-bearing flue dust was investigated under the conditions of 10 min leaching time, a liquid-to-solid ratio of 2:1, 5 mL H₂O₂ dosage, and 360 W ultrasonic power, with the results shown in Fig. 5a. For CLO, the leaching degree of Cd increased from around 87.56 % at 20 °C to 92.25 % at 60 °C. In ULO, the leaching degree of Cd reached 94.53 % at 20 °C and achieved a maximum value of 96.06 % at 40 °C. A higher temperature intensifies the cavitation effect of ultrasound. On one hand, the increased vapor pressure inside cavitation bubbles promotes their formation and violent collapse, generating strong micro-jets that disperse the flue dust particles. The high-temperature, high-pressure microenvironment created when the bubble collapses reduces the viscosity of the liquid and enhances the mass-transfer process [32]. On the other hand, under the effect of ultrasound, the increase in temperature accelerated the decomposition of H₂O₂ into ·OH, which has stronger oxidizing ability (Eq. (5)). The symbol “)))” in Eq. (5) indicates the application of ultrasound. The mechanism of ·OH generation will be discussed in detail in subsequent chapters. This process not only facilitated the conversion of insoluble CdSO₃ and CdS into soluble CdSO₄ but also promoted the oxidation of PbSO₃ to PbSO₄, which helps fix lead in the solid phase and prevent it from entering the solution [33]. However, the effect of temperature exhibited duality. As the temperature rose from 40 to 60 °C, the Cd leaching degree in CLO showed a slight increase, while that in ULO decreased from 96.06 % to 89.75 %. At 60 °C, the ULO efficiency is even lower than that of CLO. Although the thermostatic water bath ensured macroscale temperature stability throughout the reaction system, ultrasonic cavitation simultaneously generated transient high temperature and high-pressure micro-regions. These localized high temperatures frequently exceeded the 65 °C decomposition threshold of hydrogen peroxide, inducing its decomposition as described in Eq. (6). This inherent sonochemical effect of localized overheating accelerated both the volatilization of active oxygen species and their subsequent escape from the leaching system, thereby reducing the overall leaching efficiency.

Fig. 5.

Comparative the leaching degree of Cd under five conditions. (a) temperature, (b) H₂O₂ dosage, (c) liquid–solid ratio, (d) time, (e) ultrasonic power.

Considering that the Cd leaching degree in ULO at room temperature already reached 94.53 % (20 °C). From an industrial application perspective, room-temperature operation not only avoids heating costs but also offers greater economic benefits; therefore, the optimized temperature condition is room temperature (20 °C).

H₂O₂ + ))) → 2·OH (5)

H₂O₂ → H₂O + 1/2 O₂ ΔG⁰ =-116.409 kJ/mol (6)

The effect of H₂O₂ dosage on Cd leaching from cadmium-bearing flue dust was examined at room temperature, with a 10 min leaching time, a 2:1 liquid-to-solid ratio, and 360 W ultrasonic power, as shown in Fig. 5b. The addition of H₂O₂ had a notably positive effect on the leaching process. In the CLO method, the leaching degree of cadmium increased from 76.62 % with 1 mL of H₂O₂ to 88.51 % with 6 mL. This can be attributed to H₂O₂ ability to transform CdSO₃ and CdS into more soluble CdSO₄. When ultrasound was incorporated (ULO), the leaching degree surpassed that of CLO at every H₂O₂ dosage level, increasing from 82.51 % with 1 mL to 94.34 % with 6 mL. This underscores the synergistic interplay between ultrasound and H₂O₂. The cavitation effect of ultrasound accelerates the decomposition of H₂O₂ into ·OH, as shown in Eq. (5) [34], which exhibits superior oxidative capability compared to H₂O₂. This is quantitatively demonstrated by their standard reduction potentials under identical acidic conditions: +2.80 V for ·OH versus +1.78 V for H₂O₂ as an oxidant. Although system pH may affect absolute potential values, this direct comparison confirms ·OH's significantly stronger oxidative power. Additionally, mechanical action of ultrasound improves the dispersion of flue dust particles, exposing more active sites to oxidants [35]. However, using too much H₂O₂ has drawbacks. Excessive H₂O₂ can decompose and be wasted. More importantly, an overabundance of H₂O₂ can generate an excess of ·OH, which may recombine (as shown in Eq. (7)), thereby reducing their oxidizing capacity [36]. Considering these factors, the optimal H₂O₂ dosage was determined to be 5 mL. At this level, the ULO achieved a Cd leaching degree of 94.53 %, whereas the CLO reached 87.56 %.

$$2\bullet \mathrm{O}\mathrm{H}\to {\mathrm{H}}_2\mathrm{O}+1/2{\mathrm{O}}_2$$ (7)

The influence of different liquid–solid ratios on Cd leaching from cadmium-bearing flue dust was assessed at room temperature, with a 10 min leaching time, 5 mL H₂O₂ dosage, and 360 W ultrasonic power, as shown in Fig. 5c. For CLO, the Cd leaching degree increased from approximately 34.95 % at a liquid-to-solid ratio of 1:1 to about 89.73 % at a ratio of 3:1. In ULO, the leaching degree climbed from around 41.5 % at a ratio of 1:1 to nearly 95.66 % at a ratio of 3:1. For CLO, the increase in leaching indicates that the additional liquid helps to overcome the mass transfer limitations by providing more solvent for the dissolution process, which facilitates contact between the reactants and improves the diffusion of H₂O₂ into the solid phase allowing more cadmium to be dissolve [37,38]. When ultrasound is introduced (ULO), the effect of liquid-to-solid ratio becomes even more pronounced. A higher liquid–solid ratio not only improves the dispersion of flue dust particles and reduces the propagation resistance of ultrasonic waves, but also provides more space for the formation and collapse of cavitation bubbles. As a result, the cadmium-containing material phase has more chances to dissolve into the liquid phase, thus increasing the leaching degree of Cd. However, beyond a certain liquid–solid ratio, the rate of increase in leaching degree may slow down. This is because excessive liquid can dilute the concentration of H₂O₂ and other reactants, potentially reducing the reaction rate. Moreover, higher liquid–solid ratios may increase the energy consumption and operational costs due to the need to handle larger volumes of solution. For ULO, the leaching degree reaches 94.53 % at a liquid-to-solid ratio of 2:1, and further increasing the ratio has limited effect. Thus, the optimal liquid-to-solid ratio is 2:1, where CLO's leaching degree is 86.65 % and ULO's is 94.53 %.

The effect of leaching time on cadmium leaching from cadmium-bearing flue dust was assessed at room temperature, with a liquid-to-solid ratio of 2:1, 5 mL H₂O₂ dosage, and 300 W ultrasonic power, as shown in Fig. 5d. For CLO, the leaching degree of Cd increased from about 87.07 % at 10 min to roughly 95.47 % at 50 min. In ULO, the leaching degree climbed from around 93.28 % at 10 min to nearly 98.19 % at 50 min. This indicates that longer leaching time provides more opportunity for the oxidative and dissolution reactions to proceed. The leaching degree improvement can be attributed to the extended contact time between the leaching solution and the solid phase. For CLO, the increase in leaching degree suggests that additional time allows more cadmium to be dissolved as the reaction progresses. In ULO, prolonged leaching time works with ultrasonic cavitation and mechanical effects to disperse the flue dust, increasing the contact of cadmium-bearing phases (especially CdSO₃ and CdS) with the solution [39]. This enhances dissolution and other reactions, improving the leaching degree. However, beyond a certain leaching time, the rate of increase in leaching degree slow down. This is because the most readily leachable cadmium species (CdSO₄) are dissolved first, and additional time does not significantly increase recovery. Moreover, longer leaching times may increase energy consumption and operational costs. In summary, the optimal leaching time is 20 min, at which point the leaching degree for CLO is 88.57 %, and for ULO it is 96.91 %. Within this range, the process achieves a favorable balance between efficient mass transfer and cost-effectiveness, maximizing cadmium recovery while maintaining operational feasibility.

Under the optimized conditions, the impact of ultrasonic power was investigated, as illustrated in Fig. 5e. For ULO, the Cd leaching degree increased from 89.73 % at 120 W to a peak of 96.76 % at 360 W, followed by a slight decrease to 91.94 % at 600 W. This non-linear trend underscores the dual role of ultrasonic power in governing cavitation intensity and energy utilization. At lower power levels (≤360 W), the enhanced leaching degree correlates with intensified cavitation effects, characterized by increased bubble density and collapse energy [40]. The transient high-pressure microjets (>1000 MPa) generated during bubble implosion effectively fracture particle agglomerates, exposing fresh CdSO₃ and CdS surfaces to H₂O₂, while the localized high-temperature (∼5000 K) microenvironments promote the decomposition of H₂O₂ into ·OH, which oxidizes refractory CdSO₃ and CdS into soluble CdSO₄ [41]. However, excessive power (>360 W) induces acoustic decoupling and bubble shielding, where dense bubble clouds attenuate ultrasonic energy transmission, reducing effective cavitation activity [42]. Additionally, prolonged exposure to high power accelerates H₂O₂ self-decomposition, diminishing oxidant availability [43]. Moreover, excessive power causes extra electricity use. Therefore, the optimal ultrasonic power is 360 W.

In summary, compared with conventional water leaching with oxidative (CLO), ultrasonic-enhanced water leaching with oxidative (ULO) significantly enhances Cd leaching. Under optimized conditions: room temperature, 5 mL H₂O₂, a 2:1 liquid-to-solid ratio, 20 min, and 360 W ultrasonic power, the cadmium leaching efficiency reaches 96.76 %, an 8.19 % improvement over that of CLO (88.57 %). For the recovery of Cd²⁺ from high-purity cadmium-rich leachate, industrial practice typically employs either zinc powder replacement (Cd²⁺ + Zn → Cd↓ + Zn²⁺) or aluminum powder replacement (3Cd²⁺ + 2Al → 3Cd↓ + 2Al³⁺), while the PbSO₄-containing leaching residue is valorized by direct recycling to the lead smelting furnace as feedstock, thereby achieving complete solid waste elimination and demonstrating an effective implementation of circular economy principles through comprehensive resource recovery.

To further illustrate the advantages and innovations of our study on cadmium recovery from flue dust, a comprehensive comparison with previous research is presented in Table 2. Existing methods, such as oxidative pressure leaching, water leaching, acid leaching, and bioleaching, still suffer from challenges such as low cadmium recovery efficiency, high energy consumption, or the generation of acidic wastewater. In contrast, our approach enables more efficient cadmium recovery under milder conditions.

Table 2.

Comparison of Cd recovery efficiency in different leaching environments.

Leaching environment	Conditions	Cd Recovery (%)	Reference
Pressure oxidation leaching	100 g/L acid concentration, 2.0 MPa oxygen pressure, 170 °C, liquid–solid ratio of 10 mL/g, 2.5 h, 500 r/min.	98.96 %	Li et.al [26]
Acid leaching(H2SO4)	20 g/L acid concentration, 70 °C, liquid–solid ratio of 4:1, 3 h.	85.70 %	Chen et.al [44]
Water leaching	room temperature, liquid–solid ratio of 4:1, 2 h.	87.18 %	Ji et.al [45]
Bioleaching	SOB (sulfate-oxidizing bacteria) and IOB (iron-oxidizing bacteria) inoculation dose 5 % (V/V), 30 °C, 120 rpm.	86 %	Wang et.al [46]
Ultrasound-enhanced H2O2 leaching	room temperature, liquid–solid ratio of 2:1, 20 min, 5 mL H2O2, 360 W.	96.76 %	This work

3.4. Effects of ultrasound on particle agglomeration and encapsulation in cadmium leaching

Under the optimal conditions obtained in the previous section, the leach residue of CLO and ULO along with the raw flue dust were subjected to SEM-EDS and particle size analyses, and the results obtained are shown in Fig. 6 and Fig. 7. Different points in the image were analyzed for elemental content during SEM inspection, and the results obtained are shown in Table 3. The BET analysis results of raw flue dust, CLO residue, and ULO residue are listed in Table 4.

Fig. 6.

SEM-EDS image under different conditions and magnifications and element mapping images of Cd, Pb, S and O: (a) raw flue dust, (b) conventional water leaching with oxidative (CLO), (c) ultrasonic-enhanced water leaching with oxidative (ULO).

Fig. 7.

Particle size distribution of different samples: (a) raw flue dust, (b) conventional water leaching with oxidative (CLO), (c) ultrasonic-enhanced water leaching with oxidative (ULO).

Table 3.

Elemental mass fractions at different points in SEM-EDS analysis (wt.%).

No.	Pb	Cd	S	O
1#	11.98	53.87	11.26	22.89
2#	47.46	15.52	11.38	25.64
3#	37.57	20.15	10.93	31.35
4#	16.98	48.85	7.26	26.91
5#	59.70	14.36	10.51	15.44
6#	63.17	7.51	9.27	20.04
7#	76.02	0.92	9.44	13.63
8#	77.24	1.03	9.19	12.55

Table 4.

BET surface area analysis results under different conditions.

Sample	surface area (m2/g)	Pore Size (nm)	Pore Volume (cm3/g)
raw flue dust	2.545	54.308	0.011
CLO	2.096	24.809	0.008
ULO	3.371	59.018	0.014

3.4.1. Inhibition of particle agglomeration by ultrasound

The SEM images (Fig. 6) and particle size distributions (Fig. 7) and BET analysis results (Table 4) show that ultrasonic effectively inhibits particle agglomeration during leaching process. The raw flue dust exhibits agglomeration (Fig. 6a''), which reduces the effective solid–liquid contact area and affects the interaction between the leaching solution and active sites on the particles. After CLO, the particle size increases dramatically, with the D₉₀ rising from 8.08 µm (Fig. 7a) in raw flue dust to 105 µm (Fig. 7b). These findings are further corroborated by the data in Table 4, which demonstrates concurrent reductions in specific surface area, pore size and pore volume of CLO residue compared to raw flue dust, consistent with the results shown in Fig. 7. The SEM images of the CLO residues (Fig. 6b'') also reveal larger particle clusters, indicating that conventional leaching doesn't prevent agglomeration and may even promote it due to insufficient mechanical energy to disrupt particle interactions.

In contrast, ULO treatment significantly reduces particle size and agglomeration. The particle size distribution results show a D₉₀ of 6.70 µm and a D₅₀ of 2.50 µm (Fig. 7c), demonstrating the formation of smaller and more uniformly dispersed particles. The BET analysis confirms that compared with CLO treatment, the ULO treatment increases the specific surface area from 2.096 to 3.371 m²/g, expands the pore size from 24.809 to 59.018 nm, and enhances the pore volume from 0.008 to 0.014 cm³/g. These parameter changes are consistent with the variations observed in particle size distribution results (Fig. 7). The SEM images of the ULO residue (Fig. 6c'') further verify these findings, demonstrating that ultrasound effectively inhibits particle agglomeration during leaching. The inhibition of agglomeration by ultrasound can be attributed to its unique cavitation mechanism and mechanical stirring. During ultrasonic irradiation, cavitation bubble formation and collapse generate intense local turbulence and shock waves. The mechanical stirring produces vibrations at frequencies as high as 20 kHz. These mechanical forces act on the particle surfaces, breaking up existing agglomerates and preventing new ones from forming [47,48]. These actions of ultrasound ensure that particles remain dispersed throughout the leaching process.

3.4.2. Opening of encapsulated structures by ultrasound

In addition to inhibiting agglomeration, ultrasound significantly aids in disrupting the encapsulation of cadmium-containing phases within flue dust particles. SEM-EDS analysis indicates that cadmium − bearing phases are often encapsulated inside particles, limiting their contact with the leaching solution. For instance, EDS signals for cadmium are concentrated in specific regions (as shown in the arrow-marked areas of Fig. 6a-6a', Fig. 6b-6b'), suggesting these phases are embedded within the particle structure. The encapsulation of cadmium-containing phases severely hinders efficient leaching by restricting the interaction between reactants and target metals. However, ultrasound effectively breaks down these encapsulated structures. The combined effects of cavitation and mechanical action penetrate the particles, shattering the encapsulating matrices [43]. SEM results after ULO show no obvious cadmium-containing phases and smaller particles compared to the raw flue dust and CLO residues. Particle size analysis also confirms this, indicating that ultrasound breaks down larger particles enclosing cadmium phases. In contrast, CLO residues still exhibit many encapsulated structures due to insufficient mechanical energy to disrupt them.

The elemental mass fractions in Table 3 further support ultrasonic disruption of encapsulation. In the raw flue dust and CLO residues, the cadmium content at points 1# and 4# is 53.87 % and 48.85 %, respectively, indicating large, encapsulated cadmium. At points 2#, 3#, 5#, and 6#, the cadmium content is 15.52 %, 20.15 %, 14.36 %, and 7.51 %, respectively, showing uneven cadmium distribution, possibly due to encapsulation or agglomeration. Conventional methods fail to leach cadmium efficiently from particles. After ULO, cadmium content is more uniform and lower (the cadmium contents of positions 7# and 8# are 0.92 %, 1.03 % respectively, suggesting successful disruption of encapsulation and more thorough leaching of cadmium across the entire particle.

In summary, ultrasonic dual effects of inhibiting particle agglomeration and disrupting encapsulation significantly enhance cadmium leaching from raw flue dust. By preventing agglomeration, ultrasound increases the effective surface area for leaching, enabling reactants to access more sites on the particle surface. Moreover, the disruption of encapsulated structures exposes previously hidden cadmium-containing phases to the leaching solution, making them available for reaction. These mechanisms collectively increase the effective surface area, improve mass transfer, and expose previously inaccessible cadmium phases, ultimately leading to a significant improvement in leaching efficiency.

3.5. Effects of ultrasound on phase transition

3.5.1. XRD and XPS analysis

A comparison of the XRD patterns between CLO residue and ULO residue reveals the impact of ultrasound on phase transformations during the leaching process, as shown in Fig. 8. The raw flue dust exhibits multiple phases, including CdSO₄, CdSO₃, CdS, PbSO₄, and PbSO₃ (Fig. 2). After CLO treatment, the peak intensity of CdS only slightly decreased, and the CdSO₃ peak remained significant, indicating incomplete conversion of CdS and CdSO₃ into CdSO₄, which directly results in the low cadmium leaching efficiency of the CLO process. Additionally, the persistent presence of the PbSO₃ peak in the CLO residue suggests that lead may enter the solution in a slightly soluble form. This speculation was confirmed by lead content tests in the leaching solution. As shown in Table 5, the lead concentration in the CLO leaching solution increased significantly over time, reaching as high as 2307 mg·L⁻¹ at 20 min.

Fig. 8.

XRD patterns of CLO residue (red) and ULO residue (blue).

Table 5.

Lead content in leach solution of conventional water leaching with oxidative (CLO) and ultrasonic-enhanced water leaching with oxidative (ULO) under different leaching time (mg·L^-1).

Leaching time	CLO	ULO
5 min	103	19
10 min	576	56
20 min	2307	78

In stark contrast, the XRD pattern of the ULO-treated residue showed complete disappearance of the characteristic peaks of CdS and CdSO₃, confirming that ultrasonic action facilitated their full conversion into soluble CdSO₄. Meanwhile, the disappearance of the PbSO₃ peak and the enhancement of the PbSO₄ peak indicate that the ultrasonic environment promoted the conversion of PbSO₃ into insoluble PbSO₄. This transformation mechanism is further supported by ULO leaching solution data, where the lead concentration remained consistently low (19–78 mg·L⁻¹) across different time intervals (5–20 min).

To further investigate the phase transformation under various conditions, sampled XPS analysis was conducted on the CLO and ULO residues at 5 and 10 min (with the optimized reaction time being 20 min). The results are shown in Fig. 9.

Fig. 9.

Comparison of Pb 4f, Cd 3d, and S 2p of XPS in different conditions: (a1-a2, b1-b2, c1-c2) conventional water leaching with oxidative (CLO), (a3-a4, b3-b4, c3-c4) ultrasonic-enhanced water leaching with oxidative (ULO).

The Pb 4f spectrum shows that lead is present as both PbSO₄ and PbSO₃ in the raw flue dust (Fig. 3b). After 5 min of CLO, the PbSO₃ peak intensity decreases slightly, yet it remains notably high even after 10 min. In contrast, ULO exhibits a more pronounced decrease in the PbSO₃ peak intensity after just 5 min, with a corresponding increase in the PbSO₄ peak, a trend that persists after 10 min. The decline in the PbSO₃ peak could be due to both phase transformation and dissolution. These findings reveal that the weak oxidizing ability of CLO causes slow conversion of PbSO₃ to PbSO₄, allowing micro-soluble PbSO₃ to release Pb²⁺ into the solution. Conversely, ultrasonic treatment promotes rapid oxidation of PbSO₃ to insoluble PbSO₄, as conclusively evidenced by the consistently low lead concentration measured in the ULO leachate (see Table 5).

The Cd 3d spectrum shows that cadmium in the raw flue dust mainly exists in the forms of CdSO₄, CdSO₃, and CdS (Fig. 3a). The binding energy shifts of different cadmium compounds in the XPS spectra are minimal, making it challenging to distinguish between them. However, the overall peak changes can be used to assess the leaching efficiency of cadmium. After 5 min of CLO, there is almost no change in the Cd 3d peak. After 10 min, the peak intensity decreases slightly. In contrast, ULO shows a rapid decline in the Cd 3d peak intensity after just 5 min, indicating a higher cadmium leaching rate. After 10 min, the Cd 3d peak intensity is even lower, indicating that leaching is still proceeding effectively.

The S 2p spectrum indicates the presence of metal sulfate, metal sulfite, and metal sulfide in the raw flue dust (Fig. 3c). After 5 min of CLO, the metal sulfide and sulfite peaks remain prominent, suggesting limited oxidation. After 10 min, there is a slight decrease in the sulfite peak, but the sulfide peak persists. ULO shows a more significant decrease in the sulfide and sulfite peaks after 5 min, with an increase in the sulfate peak. This trend is even more pronounced after 10 min, indicating that ULO effectively promotes the oxidation of sulfide and sulfite to sulfate.

To validate the reaction pathway of CdS under the action of H₂O₂ and ultrasound, we performed systematic experiments. Under optimized conditions, a 3 g/L analytical grade CdS solution (100 mL) was oxidized. Color changes progressed from dark yellow (CdS) to turbid light yellow, finally becoming clear (see Fig. 10a), demonstrating stepwise oxidation. At 15 min, the turbid light-yellow solution suggested intermediates like elemental sulfur (S⁰). XRD of the 15-min residue confirmed S⁰ and CdSO₃, indicating intermediate sulfur species (see Fig. 10b). Solution turbidity supported colloidal S⁰ formation. IC analysis of the final solution (20 min) showed >99 % conversion to sulfate (29.94 mg/L SO₄²⁻) with minimal sulfite (214.3 µg/L SO₃²⁻). The oxidation pathway of CdS under ULO synergy involves: CdS → S⁰ → CdSO₃ → CdSO₄, the following valence transition: S^-2 → S⁰ → S⁺⁴ → S⁺⁶.

Fig. 10.

Changes and analysis of CdS solution under the action of ultrasound-enhanced hydrogen peroxide: (a) 100 ml of 3 g/L CdS solution, under conditions of 5 ml H₂O₂, 20 °C, and 360 W power, changes in the color and transparency of the reaction solution at different time points (0, 5, 15, 20 min), (b) XRD analysis of residue at 15 min.

3.5.2. EPR analysis

Electron paramagnetic resonance (EPR) is a powerful technique for detecting short-lived, reactive species such as ·OH radicals. To further investigate the role of ultrasound in enhancing phase transformation during leaching, EPR detection was employed to analyze the generation of ·OH radicals in the leaching solution. The equipment used was a JEOL JES-FA200 EPR spectrometer with the following parameters: operating frequency 9182 MHz, central magnetic field 327.0 mT, power 0.998mW, scan width ±5 mT, modulation amplitude 1.0 G and sweep time 30 s.

Samples from distinct reaction systems (CL, UL, CLO, and ULO) were immediately mixed with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and analyzed within 2 min after mixing to trap transient ·OH radicals, forming stable DMPO-OH adducts that exhibited the signature 1:2:2:1 quartet EPR spectrum, and the results are shown in Fig. 11. The results showed no detectable peaks in CL and UL due to the absence of conditions for ·OH generation. During the CLO process, a weak peak is observed, which is attributed to the production of trace amounts of OH from hydrogen peroxide at room temperature due to its own homolytic reaction. In contrast, ULO exhibited strong peaks characteristic of ·OH radicals (1:2:2:1), demonstrating the synergistic effect of ultrasound and hydrogen peroxide, which promotes hydrogen peroxide decomposition into ·OH radicals. As confirmed by Sari and Tura [49], this ·OH radical-generation pathway significantly enhances the oxidative capacity of the leaching system.

Fig. 11.

Comparison of EPR results under different conditions: conventional water leaching (CL), conventional water leaching with oxidation (CLO), ultrasonic-enhanced water leaching (UL), ultrasonic-enhanced water leaching with oxidation (ULO).

Combined with XRD, XPS, and EPR analyses, the results collectively confirm the superiority of the ULO process in phase transformation. Ultrasound promotes the generation of highly oxidative ·OH radicals from hydrogen peroxide. The combined effects of hydrogen peroxide and ·OH radicals accelerate the conversion of CdSO₃ and CdS to CdSO₄, and PbSO₃ to PbSO₄.

3.6. Effects of ultrasound on leaching kinetics characteristics

Kinetic analysis is crucial for understanding the mechanisms of cadmium leaching from raw flue dust, as it helps identify the rate-controlling steps and the factors influencing the leaching process. In this study, kinetic analyses were conducted for both conventional water leaching with oxidation (CLO) and ultrasonic-enhanced water leaching with oxidation (ULO) methods to compare their rate-controlling mechanisms and energy requirements. The kinetic experiments were conducted under the following conditions: temperature (20, 30 and 40 °C), liquid-to-solid ratio (2:1), ultrasonic power (360 W), and H₂O₂ dosage (5 mL). The kinetic analysis commenced at time zero, with samples collected at regular intervals (0, 2, 4, 6, 8 and 10 min) to monitor the leaching efficiency over time. The average particle size of the raw flue dust used for kinetic assessment was 3.61 μm, with a particle size range of 0.405 to 51.8 μm.

The leaching of cadmium from cadmium dust is fundamentally a liquid–solid reaction involving porous solid particles. The shrinking core model is widely employed in the kinetic study of such reactions because it effectively quantifies the internal chemical reactions within particles, the mass transfer of ions in pore solutions, and the diffusion process through product layers, while also distinguishing the rate-controlling steps (e.g., surface chemical reaction control, product layer diffusion control, or mixed control). Based on the shrinking core model framework, this study adopts its two classical kinetic models, which are the chemical reaction-controlled model and the diffusion-controlled model, to investigate the leaching behavior of the dust. Both models have been extensively validated in similar leaching studies [50,51]. The kinetic equations for the surface chemical reaction-controlled model and the diffusion-controlled model are given by Eqs. (8), (9), respectively.

The experiments were conducted at various temperatures, with samples taken at different time intervals to measure the leaching efficiency of cadmium (Fig. 12a and 12b). The kinetic parameters derived from different kinetic models for CLO and ULO treatments at various leaching temperatures are summarized in Table 6. The data obtained from these experiments were then fitted to the shrinking core model equations to determine the rate constants and identify the controlling mechanism. Considering the influence of non-ideal factors such as incomplete mixing and heat transfer delays during the initial reaction phase, the kinetic fitting procedure was initiated only after the experimental data demonstrated a stable linear trend (excluding the transient zero-time region), thereby ensuring that the kinetic analysis accurately characterizes the steady-state behavior of the dominant reaction stage and provides reliable rate constants—an established methodology in hydrometallurgical kinetics research [17,52,53]. Meanwhile, the apparent activation energy, a key parameter, was calculated using the Arrhenius equation (Eq. (10)) to identify the controlling step.

$$1-{(1-x)}^{1/3}=k_{1}t$$ (8)

$$1-2/3x-{(1-x)}^{2/3}=k_{2}t$$ (9)

Where $x$ represents leaching degree of cadmium (%); $k_1$ and $k_2$ denote the apparent rate constant (min⁻¹); $t$ is leaching time (min).

$$\ln k=\ln A-E_a/RT$$ (10)

Where $k$ represents the apparent rate constant; $A$ is the frequency factor, $E_a$ denotes the activation energy (J·mol⁻¹); $R$ represents the universal gas constant (J·mol⁻¹·K⁻¹); $T$ is leaching temperature (K).

Fig. 12.

Kinetic analysis of conventional water leaching with oxidation (CLO) and ultrasonic-enhanced water leaching with oxidation (ULO): (a, b) leaching degree of cadmium at different temperatures and time conditions, (a1, a2) the chemical reaction-controlled model of ULO, (b1, b2) the diffusion-controlled model of ULO, (a3, b3) arrhenius plot of reaction rate against reciprocal of temperature.

Table 6.

Kinetic parameters derived from different kinetic models for CLO and ULO treatments at various leaching temperatures.

	1-(1-x)1/3			1-2/3x-(1-x)2/3
	T (℃)	Rate constant k	R2	T (℃)	Rate constant k	R2
CLO	20	0.04751	0.9929	20	0.01818	0.9794
	30	0.04903	0.9761	30	0.01969	0.9604
	40	0.05104	0.9888	40	0.0218	0.9693
ULO	20	0.04999	0.9578	20	0.02299	0.9657
	30	0.05213	0.9799	30	0.0244	0.9882
	40	0.05269	0.9554	40	0.02512	0.9699

At activation energies between 4.18 and 12.55 kJ·mol⁻¹, the reaction is controlled by diffusion; above 40 kJ·mol⁻¹, the reaction is controlled by surface chemistry [17,54,55]. For CLO, the leaching rates at different times and temperatures were fitted to two models, and the chemical reaction-controlled model showed better results (Fig. 12a1-12a2). However, the activation energy calculated via the Arrhenius equation was 16.198 kJ·mol⁻¹, which falls between 13 and 40 kJ·mol⁻¹ (Fig. 12a3), indicating that CLO is influenced by both diffusion and chemical reaction controls. This aligns with the earlier analysis suggesting that CLO struggles to effectively disrupt encapsulation and inhibit aggregation, thereby highlighting mass transfer barriers. Moreover, it suggests that CLO has insufficient reactivity towards CdSO₃ and CdS. The analysis of ULO shows that the diffusion-controlled model better fits the results (Fig. 12b1-12b2). The calculated activation energy of ULO is 3.389 kJ·mol⁻¹ (Fig. 12b3), confirming diffusion control as the rate-determining step. This finding provides critical process optimization guidance, indicating that priority should be given to enhancing ultrasonic power density and optimizing hydrodynamic conditions (e.g., adjusting solid–liquid ratio and agitation intensity) to disrupt boundary layers and improve mass transfer. Alternatively, reducing particle size to increase specific surface area can significantly enhance cadmium leaching efficiency. These results further demonstrate that increasing oxidant concentration or raising temperature would have limited effects on reaction rate enhancement, thereby preventing unnecessary energy and reagent consumption.

The significant difference in activation energies between CLO and ULO highlights the impact of ultrasound on the leaching mechanism. In CLO, the higher activation energy suggests that the process is limited by both mass transfer and chemical reactions. The conventional method lacks the mechanical energy to effectively breakdown particle structures, leading to incomplete exposure of reactive sites and a dependence on slower diffusion processes. And hydrogen peroxide such a case cannot carry out an effective oxidation reaction. This dual control mechanism results in lower leaching efficiencies and longer reaction times. On the other hand, the addition of ultrasound alters the reaction's controlling step and effectively reduces the activation energy. This is because ultrasound can separate aggregated particles and disrupt encapsulations, increasing the contact between the leaching solution and active sites on the particles. Additionally, under ultrasound, hydrogen peroxide generates more reactive ·OH, which enhances the oxidation process and improves the oxidation reaction.

4. Conclusion

This study demonstrated the effectiveness and innovation of ultrasonic-enhanced hydrogen peroxide leaching for recovering cadmium from flue dust under room temperature and neutral conditions. The key findings and conclusions were as follows:

The ultrasonic-enhanced leaching method significantly improved cadmium recovery efficiency. Under optimized conditions (room temperature, 5 mL H₂O₂, 2:1 liquid-to-solid ratio, 20 min, and 360 W ultrasonic power), the cadmium leaching efficiency reached 96.76 %, representing a substantial improvement over the 88.57 % achieved with conventional oxidative leaching.

The enhanced mechanism can be attributed to three main factors:

• 1.Inhibition of particle agglomeration: Ultrasound prevented particle aggregation, thereby increasing the effective surface area for leaching.
• 2.Generation of hydroxyl radicals: Ultrasound promoted the decomposition of hydrogen peroxide into highly oxidative hydroxyl radicals, which enhanced the oxidation of cadmium-and lead-containing phases for efficient separation.
• 3.Reduction of activation energy: The activation energy for the leaching process decreased from 16.198 to 3.389 kJ·mol⁻¹, changing the rate-determining step from dual control (diffusion and chemical reactions) to diffusion control. This significantly accelerated the leaching process.

CRediT authorship contribution statement

Yuxi Xie: Writing – original draft, Formal analysis. Tian Wang: Writing – review & editing. Yuanru Wang: Writing – review & editing. Yan Gao: Writing – review & editing. Shihao Liu: Writing – review & editing. Fei Niu: Writing – review & editing. Phan Duc Lenh: Writing – review & editing. Thiquynhxuan Le: Writing – review & editing, Project administration, Conceptualization. Libo Zhang: Supervision, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article: