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Review

Emerging Catalytic Strategies Driven by External Field for Heavy Metal Remediation

by
Xinyue Zhang
1,2,†,
Shanliang Chen
2,†,
Attiq Ur Rehman
1,
Suwei Zhang
2,3,
Qingzhe Zhang
4,*,
Yong Liu
2,* and
Shun Li
1,*
1
Institute of Quantum and Sustainable Technology (IQST), School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
2
Foshan (Southern China) Institute for New Materials, Foshan 528231, China
3
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
4
Shandong Key Laboratory of Environmental Processes and Health, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(9), 602; https://doi.org/10.3390/catal14090602
Submission received: 8 August 2024 / Revised: 28 August 2024 / Accepted: 3 September 2024 / Published: 7 September 2024
(This article belongs to the Section Nanostructured Catalysts)
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Abstract

:
Heavy metal pollution presents significant environmental and public health risks due to its widespread occurrence and resistance to degradation. There is a pressing need for innovative solutions to address the challenge of heavy metal ion removal from water resources. In this review, we highlight recent advancements in emerging catalytic strategies for efficient heavy metal remediation, leveraging various external fields such as electric, mechanical, magnetic, and thermoelectric fields, as well as their synergetic coupling with photocatalysis technology. These novel approaches offer promising avenues for enhancing heavy metal removal efficacy and environmental sustainability. In particular, this review focuses on recent breakthroughs in new materials systems capable of functioning under diverse external fields, heralding future advancements in heavy metal remediation. Finally, we discuss the current challenges and future perspectives in this emerging research area.

1. Introduction

The escalating pace of industrial and societal development has heightened the urgency to address the heavy metal pollution issue, presenting a pivotal challenge that demands innovative and effective solutions [1,2]. As a result, there is an increasing need to develop efficient, low-cost, and environmentally friendly strategies for removing heavy metal contaminants from water sources [3]. Heavy metals such as cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), zinc (Zn), and uranium (U) are particularly harmful in their ionic forms due to their toxicity and persistence in the environment [4]. These metals cause toxicity through similar pathways, including the generation of reactive oxygen species, enzyme inhibition, and oxidative stress [5]. Taking Hg as an example, the World Health Organization (WHO) has set stringent regulations to mitigate its harmful effects, with a maximum allowable concentration of 30 nM in water, emphasizing the critical importance of controlling Hg contamination to protect human health and environmental well-being [6,7,8]. Moreover, the interaction of Cu ions with proteins and enzymes in the body can lead to gastrointestinal issues and osteoporosis, while exposure to Pb ions is associated with an increased risk of cardiovascular diseases. The WHO has also reported close connections between heavy metal exposure and serious health problems, including cirrhosis, arthritis, hypertension, and kidney failure [9]. This is because heavy metals not only contaminate soils but also accumulate in the food chain, particularly in vegetables grown in contaminated soils [10]. Consequently, it is essential to develop advanced and effective technologies to address the challenges posed by heavy metal contamination.
In the past few decades, numerous techniques have been developed to remove heavy metals in wastewater, including coagulation [11], chemical precipitation [12], ion exchange [13], membrane filtration [14,15], adsorption [16,17,18], photocatalysis [19,20], advanced oxidation [21], and bioremediation [22]. These traditional methods indeed have their advantages, such as simple process design and high heavy metal removal capacity [23]. However, these methods often lack high selectivity, leading to the removal of non-target ions and requiring additional treatment steps. Moreover, they also tend to be less effective at treating low concentrations of heavy metals effectively and can be costly due to high energy consumption, chemical usage, and disposal costs [24]. For instance, coagulation and chemical precipitation methods generally require pH adjustment and generate significant amounts of sludge, which presents challenges for proper handling and disposal [25,26]. While adsorption processes are versatile, low-cost, and capable of achieving high removal efficiencies, they are limited by factors such as adsorbent capacity, slow kinetics, and the need for regeneration or disposal of spent adsorbents. Furthermore, the chemicals used in these processes can cause secondary pollution to environmental implications if not managed properly. These shortcomings underline the urgent need for more advanced and sustainable technologies to improve the efficiency, cost-effectiveness, and environmental sustainability of heavy metal treatment.
In recent years, innovative approaches that utilize various external fields, such as electric [27,28], mechanical [29,30], magnetic [31,32,33], and thermoelectric fields [34,35], have demonstrated significant potential in driving catalytic processes with enhanced performance. The use of different fields leads to several novel catalytic effects termed as electro-catalysis [36], mechano-catalysis [37,38,39], magneto-catalysis [40,41], and thermoelectro-catalysis [42,43]. These emerging techniques offer new opportunities for heavy metal removal, as illustrated and summarized in Table 1 and Figure 1. The key of these approaches relies on the design and fabrication of functional nanomaterials [44,45], offering rapid and effective solutions to reduce toxicity while enabling the selective removal and recovery of specific contaminants [46,47,48]. Furthermore, these external fields can be integrated with extensively investigated photocatalysis technology [49,50,51], including photo-electrocatalysis [52], piezo-photocatalysis [53,54], magneto-photocatalysis [55,56], and thermoelectric-photocatalysis [57,58]. Such coupling systems have the potential to maximize removal efficiencies and expand the scope of applications, aligning closely with sustainable development goals. Recent studies demonstrate that utilizing external fields is a highly promising avenue in heavy metal remediation. Therefore, summarizing these critical advances in this emerging research area is both timely and essential.
In this review, we provide a comprehensive examination of advanced methodologies for heavy metal removal, with a focus on the utilization of various external fields including electric, mechanical, magnetic, and thermoelectric fields, as well as their integration with photocatalysis through synergistic coupling effects. Each section delves into the specific influences of these external fields on catalytic processes, elucidating the unique mechanisms of field-assisted catalysis. Importantly, the key advances in materials design that address the complex challenge of heavy metal pollution are also highlighted. These strategies offer distinct advantages in improving the selectivity, efficiency, and sustainability of heavy metal removal. Finally, we summarize and propose challenges and perspectives in this promising research field.

2. External Field-Driven Heavy Metal Removal

2.1. Electrocatalysis

The application of electric fields in driving or modulating various catalytic reaction processes has been well-established [59,60,61]. Recently, electric field-assisted removal of heavy metals from wastewater has emerged as a promising route due to its high efficiency and selectivity. Significant advancements in electrode material design and electrochemical devices have bolstered this approach, providing a robust strategy for the reduction and recovery of metals from polluted water [62,63]. Integrating well-designed electrode materials is crucial, as it can greatly enhance treatment efficiency by optimizing the electrochemical interactions required for metal ion deposition [64,65].
In electrocatalysis, the constancy of the current direction plays a pivotal role, with two distinct modes being explored: steady-state direct current (DC) and pulsed current (PC) electrocatalysis. The DC mode involves a continuous, unidirectional current, while the PC mode utilizes an electrical signal composed of multiple pulses, resulting in a dynamically changing current waveform [66]. Each mode presents unique advantages and challenges, depending on the specific application and desired outcomes. This section delves into the mechanisms of electrocatalysis, recent material innovations, and the practical applications of electric field-assisted technologies in the treatment of heavy metal-contaminated wastewater. Through these discussions, the potential of this emerging technology to provide efficient and sustainable solutions for environmental remediation is highlighted.

2.1.1. Direct Current (DC) Electrocatalysis Mode

The DC electrocatalytic process operates under a constant voltage, allowing for precise control of the chemical reaction process by adjusting the current or voltage. Notably, Li et al. [67] demonstrated the efficacy of DC potential in reducing the bioavailability of heavy metals in soil. Their study showed significant reductions in concentrations of Mn, Zn, Cu, Ni, and Cd, with decreases of 61.7%, 63.8%, 64.9%, 83.7%, and 63.8%, respectively. In a study by Hemmatifar et al. [68], an electrochemical flow cell with redox-active electrodes was proposed for the selective removal and recovery of vanadium (V) oxyanions from aqueous streams. This system leveraged the intrinsic affinity of the redox-active metallocene polymer poly(vinyl)ferrocene (PVFc) for oxyanion species on the anode, paired with a matched-capacity cathode featuring conductive polymer polypyrrole (PPy) doped with sodium dodecyl benzene sulfonate (Figure 2a,b). This setup illustrates how electrode material design can be tailored for specific metal recovery. Cathodic reduction, in particular, offers unique advantages for the treatment of Cu-based complexes. This method directly precipitates Cu through a two-electron process involving the central Cu2+ ion, without requiring ligand degradation. For instance, the heterojunction Ni-Sb-SnO2 anode was investigated for electrocatalytic treatment of Cu-TEPA, a typical complex in industrial wastewater, achieving 94.36% decomplexation and 86.52% Cu recovery [69]. O and OH produced at the anode first attack Cu-TEPA to generate Cu-organic nitrogen intermediates, which further catalyze O to generate OH, thereby catalyzing the decomposition process. The released Cu is gradually reduced to Cu and finally deposited on the stainless steel cathode in the form of CuO and Cu. Qin et al. [70] reported the use of a MoS2 nanosheet/graphite felt (GF) cathode, achieving an average Faraday efficiency of 29.6% and a specific removal rate (SRR) of 0.042 mol cm−2 h−1 for Cu-EDTA at –0.65 V vs. SCE (saturated calomel electrode), outperforming many commonly reported electro-oxidation systems (Figure 2c).
In practical applications, high concentrations of heavy metal ions often coexist with organic contaminants, leading to challenges such as electrode fouling and degradation. To address these issues, capacitive deionization (CDI) technology, which utilizes an electric field to drive the directional movement of charged ions, has gained extensive attentions in the removal of heavy metal ions. When the polarity of the applied voltage is reversed, the adsorbed heavy metal ions can be desorbed and recovered. Chen et al. [71] proposed a capacitive deionization-electro-oxidation (CDI-EO) system that employs activated carbon layer coated graphite paper as the cathode and RuO2-IrO2/Ti as the anode, enabling simultaneous removal of heavy metal ions and organic contaminants. During this unique process, Cu2+ ions were removed through cathodic electro-sorption and electro-deposition, while AO7 dye was degraded via anodic oxidation. This dual-function approach not only addresses the issue of electrode fouling but also enhances the overall efficiency of the remediation process.

2.1.2. Pulsed Current (PC) Electrocatalysis Mode

Recently, there has been increasing research attention on the irreversible changes in electrocatalyst structure or active sites, such as dissolution and passivation, that occur during catalytic reactions. Beyond the regulation of electrocatalysts and active sites, the influence of mass transfer processes and the interfacial microenvironment on the reaction is critical for maintaining the high activity and stability of the active sites [72]. To address these challenges, the emerging pulsed current (PC) method has been developed. This technique alternates between two potentials at specific time intervals, creating continuously looping cycles that can optimize reaction conditions. Wang et al. [73] designed a Fe2+-doped NiFe LDH/NF porous electrode that utilized rapid switching between pulsed anodic and cathodic voltages. This pulsed current method intermittently accelerated the migration of Cr(VI) ions to the electrode surface, where they were efficiently reduced to Cr(III). This approach significantly enhanced the reduction kinetics of Cr(VI), achieving a reduction efficiency three times higher than that of traditional constant voltage methods.
Additionally, Guo et al. [74] elucidated the mechanism of pulsed and direct current electrochemical methods for the removal of representative heavy metals (e.g., Pb2+, Cd2+, Mn2+). As depicted in Figure 2d, the periodic variation between low and high voltages plays a crucial role in modulating the concentration of heavy metal ions on the electrode surface. This novel approach is particularly beneficial for overcoming kinetic limitations caused by mass transport, ultimately improving the recovery efficiency of heavy metals.
In summary, the DC electrocatalysis mode provides a stable and continuous current flow, ensuring consistent reaction, which is advantageous for processes requiring uniformity over time. Nevertheless, it may lead to increased electrode degradation due to the constant current flow. In contrast, PC electrocatalysis operates by intermittently applying current in controlled pulses, offering advantages in enhancing reaction selectivity and minimizing unwanted side reactions. This technique is particularly valuable for achieving high selectivity in complex reactions and preserving electrode integrity, making it suitable for prolonged operation while also reducing overall energy consumption.
Catalysts 14 00602 g002
Figure 2. (a) Photograph of a continuous-flow system comprising an electrochemical cell, influent reservoir, peristaltic pump, automated fraction collector, and a flow-through conductivity sensor. (b) Effluent concentration profiles for vanadium during adsorption at 1.1 V and desorption at –0.5 V [68]. Copyright 2020, Wiley. (c) Schematic diagram of the electrocatalytic reduction of Cu-EDTA at a MoS2 cathode, along with a comparison of Faraday efficiency (FE) and specific removal rate (SRR) between the MoS2/GF system and other reported electrooxidation-based Cu-EDTA removal systems [70]. Copyright 2023, Springer. (d) Conversion process and mechanisms of heavy metal ions during electrodeposition [74]. Copyright 2024, ACS Publications.
Figure 2. (a) Photograph of a continuous-flow system comprising an electrochemical cell, influent reservoir, peristaltic pump, automated fraction collector, and a flow-through conductivity sensor. (b) Effluent concentration profiles for vanadium during adsorption at 1.1 V and desorption at –0.5 V [68]. Copyright 2020, Wiley. (c) Schematic diagram of the electrocatalytic reduction of Cu-EDTA at a MoS2 cathode, along with a comparison of Faraday efficiency (FE) and specific removal rate (SRR) between the MoS2/GF system and other reported electrooxidation-based Cu-EDTA removal systems [70]. Copyright 2023, Springer. (d) Conversion process and mechanisms of heavy metal ions during electrodeposition [74]. Copyright 2024, ACS Publications.
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2.2. Mechano-Catalysis

Mechanochemistry, which utilizes mechanical stimuli to initiate chemical reactions, represents a promising route for various catalytic applications [37,38,39]. Among these, piezocatalysis (PZC) over piezoelectric materials is emerging as a promising catalytic approach driven by external mechanical forces in the surrounding environment [75,76,77,78,79,80]. Additionally, a novel catalytic effect termed contact-electro-catalysis (CEC) has been discovered [81]. Under mechanical stress, CEC utilizes electron transfer that occurs at the liquid-solid interface or even at the liquid-liquid interface, driven by contact electrification (CE) effects, to stimulate redox reactions [82]. This section summarizes the recent progress in PZC and CEC processes for heavy metal removal.

2.2.1. Piezocatalysis

The piezoelectric effect, characterized by materials with non-centrosymmetric structures, induces an asymmetric distribution of surface charges when subjected to mechanical force. These surface-enriched non-equilibrium charges can initiate electrochemical reactions [83,84,85], leading to the generation of reactive oxygen species (ROS) and thereby facilitating heavy metal remediation.
Barium titanate (BaTiO3), a typical ABO3-type perovskite, exhibits excellent piezoelectric performance due to its non-centrosymmetric lattice [86,87]. Recently, BaCO3@BaTiO3 composite microspheres were synthesized by a hydrothermal-calcination method, as reported by Xie et al. [88] Under mechanical excitation, these composite microspheres generate active ROS including hydroxyl radicals (•OH) and superoxide radicals (•O2−), which can effectively reduce toxic Cr(VI) to Cr(III). Additionally, Pan et al. [89] demonstrated a multifunctional composite composed of piezoelectric BaTiO3 nanowires and graphene (BaTiO3@graphene), and then assembled into a 3D millimeter-sphere for Cu(II) recovery. The incorporation of graphene drastically increased the surface potential from ~19.8 to 96.8 mV, which effectively promoted the transfer of mechanical-excited carriers and thus the mechano-catalytic performance.
In addition, Wei et al. [90] designed and fabricated a novel bifunctional piezocatalyst, Au/BiVO4 nano-hybrid, for the simultaneous removal of 4-chlorophenol and Cr(VI) upon mechanical forces provided ultrasonic vibration (Figure 3a–c). This system achieved a high removal efficiency of ~83% in 2 h, with the piezocatalytic activity increasing alongside the ultrasonic power. In 2021, Tian et al. [91] reported a membrane catalyst composed of carbon nanofibers (CNFs) and few-layer piezoelectric SnS2 nanosheets for Cr(VI) treatment. The optimal SnS2/CNFs nanocomposite demonstrated a high Cr(VI) reduction efficiency, with a rate of ~0.132 min−1 in 40 min.
Piezocatalysis technology has also been applied to the removal of U(VI) [92,93,94]. For example, Cai et al. [93] reported a SnO2/ZnSnO4 piezocatalytic composite capable of converting U(VI) into UO2O2 precipitate. Additionally, a low-cost calcium phosphate biological piezocatalyst was developed to extract U(VI) [94]. A proposed reaction mechanism is presented in Equations (1)–(4). Initially, U(VI) is adsorbed onto the catalysis to form a compound meta-autunite. Subsequently, piezo-excited electrons react with the adsorbed U(VI) to form solid UO2 and U3O8. Alternatively, the generated electrons can react with soluble oxygen to form H2O2, which may then react with UO2 to produce uranium peroxide (UO2)O2•2H2O. This process offers new pathways for the treatment of heavy metals through mechanical forces-assisted elimination.
2UO22+ + Ca2+ + 2PO43− + 6H2O → Ca(UO2)2(PO4)2(H2O)6
UO22+ + 2e− → UO
2UO22+ + UO2 + 4•OH + 4e− → U3O8 + 2H2O
UO2 + 2H2O2 + 2H2O → UO4•4H2O

2.2.2. Contact-Electro-Catalysis

In 2022, the contact-electro-catalysis (CEC) effect was first proposed [82], leveraging contact-induced charge carrier transfer at interfaces between liquids and solids, triggering redox reactions. The energy source of CEC is external mechanical stimuli, including stirring, ball milling, and ultrasound. Generally, the solid to be used is general organic and inorganic materials, despite they are chemically inert [95,96]. The polytetrafluoroethylene (PTFE) onto ZSM-5 was constructed by Li et al. [95] Introducing the Fe(III)-initiated self-cycling Fenton system features synergistic activation of O2 and Fe(III)-activated H2O2, leading to increased ROS generation and showing potential in catalytic metal recovery. Su et al. [96] demonstrated that the CEC effect can effectively promote the reduction of a series of heavy metal ions, including Rh3+, [PtCl4]2−, Ag+, Hg2+, Pd2+, [AuCl4]−, and Ir3+, in the presence of fluorinated ethylene propylene (FEP) microparticles. As displayed in Figure 3d–j, the CEC performance of catalysts is closely related to their ability to undergo water-solid contact, electrically conduct, and release electrons.
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Figure 3. (a) Removal of Cr(VI) using Au/BiVO4 nanocomposites under different ultrasonic powers. (b,c) Proposed piezocatalysis mechanism for the removal of Cr(VI) [90]. Copyright 2019, Elsevier. (d–j) Reduction of metal ions in solution by ultrasonication in the presence of fluorinated ethylene propylene (FEP) microparticles [96]. (Denoted: FEP* in (f) represents charged FEP) Copyright 2024, Springer Nature.
Figure 3. (a) Removal of Cr(VI) using Au/BiVO4 nanocomposites under different ultrasonic powers. (b,c) Proposed piezocatalysis mechanism for the removal of Cr(VI) [90]. Copyright 2019, Elsevier. (d–j) Reduction of metal ions in solution by ultrasonication in the presence of fluorinated ethylene propylene (FEP) microparticles [96]. (Denoted: FEP* in (f) represents charged FEP) Copyright 2024, Springer Nature.
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Building on this, researchers have proposed a novel electrochemical approach that combines CEC with capacitive deionization (CDI). This integrated method effectively degrades metal-organic complexes while simultaneously recovering metals. Shen et al. [97] employed dielectric FEP powder as catalysts within a electrochemical system that integrates CEC and CDI technologies. This innovative approach enabled efficient degradation of metal-EDTA complexes (M-EDTA) and concurrent recovery of metal ions, presenting a viable and sustainable solution to the pressing issue of heavy metal pollution in aquatic environments.
Recent advancements have shown that the efficiency of mechanically driven PZC and CEC processes can be optimized through the design of advanced piezoelectric and dielectric materials. Unlike the PZC effect over piezoelectric nanomaterials, CEC allows for a broader range of dielectric materials to act as catalysts. Since CEC can occur during the contact process without the need for intense friction, it may provide milder conditions for accelerating catalytic reaction rates, offering significant advantages for heavy metal remediation.

2.3. Magneto-Catalysis

In the past few decades, researchers have been exploring the incorporation of magnetic fields into chemical reactions [31,32,33,98,99]. Studies have shown that the Lorentz force generated by magnetic fields can influence various aspects of free radical reactions, including the lifespan of free radicals and reaction intermediates, leading to improved catalytic activity. In addition, recent studies have uncovered a fascinating phenomenon in multiferroic nanocomposites, which exhibit a direct strain-mediated magnetoelectric-catalytic effect [33,100,101,102]. This effect allows for the manipulation of chemical reactions on the polarized surface by applying an external magnetic field, which offers an entirely new catalytic mechanism. The magneto-catalytic and magnetoelectric-catalysis effects have spurred growing interest in activating diverse catalytic reactions and provided a promising approach for the treatment of heavy metal contaminants under a magnetic field.
Ramadan et al. [103] prepared a ferromagnetic BiFeO3(BFO)/Ni0.1Fe2.9O4 nanocomposite by incorporating Ni-doped magnetite into antiferromagnetic BFO, which effectively enhanced its magnetic properties. The enhancement could be attributed to the dipole interaction at the interface boundary between the ferromagnetic Ni0.1Fe2.9O4 and the antiferromagnetic BFO phase. The composite demonstrated remarkable efficiency in removing heavy metals, especially Cr(VI) from wastewater, achieving a removal rate of 75% within 40 min at a pH value of 6. Moreover, Gonzalez-Vazquez et al. [104] investigated the effect of varying external magnetic field intensities on the adsorption of Cd2+, Pb2+, and Zn2+ ions. It was found that magnetic fields can greatly affect factors such as the hydrating ion radius and magnetic susceptibility, resulting in enhanced heavy metal remediation.
When an alternating current (AC) magnetic field is applied, the magnetostrictive phase in multiferroic composite materials induces strain that is transmitted to the ferroelectric (piezoelectric) phase, altering the electrical polarization. Consequently, multiferroic magnetoelectric composites can initiate catalytic reactions under a magnetic field and generate ROS [101], as depicted in Figure 4a,b. Mushtaq et al. [105] synthesized a core-shell structured magnetoelectric catalyst composed of CoFe2O4@BiFeO3 (CFO@BFO) and utilized it for catalyzing the reduction of Cr(VI) via the magnetoelectric-catalytic effect. Driven by the AC magnetic field, a great number of electrons and holes are excited on the surface of the CFO@BFO catalyst. These magneto-excited electrons directly facilitate the reduction of Cr(VI) to Cr(III). In addition, the oxidation of Cr(III) is driven by the free radicals of •O2− and •OH, formed by these holes and electrons, thus completing the catalytic cycle (Figure 4c,d).
The catalytic heavy metal remediation through the application of magnetic fields holds significant promise. Magnetic fields directly influence surface charge distribution and reaction kinetics of catalysts, thereby boosting catalytic activity. Unlike photocatalysis, which can be hindered by variations in light intensity and wastewater turbidity, magnetic fields ensure stable and reliable performance in treating complex wastewater.

2.4. Thermoelectrocatalysis

Thermoelectric materials function as energy suppliers by converting temperature gradients into electricity via the Seebeck effect [106,107,108]. In addition to the main focus on thermal to electricity energy conversion, thermoelectrocatalysis (TECatal) has emerged as a promising catalytic approach aimed at directly converting waste heat into chemical energy [109,110]. Over the past decade, researchers have explored intriguing temperature gradient-induced catalytic reactions in thermoelectric materials, which can drive a wide range of reactions in the fields of clean energy, chemical production, and environmental remediation [35,43,111,112,113,114,115].
Free electrons/holes and ROS (e.g., •O2− and •OH) are crucial for catalyzing heavy metal remediation, as discussed in piezocatalysis and electrocatalysis. Thermoelectric nanomaterials, such as Bi2Te3, Sb2Te3, BiCuSeO, and PbTe [43,112,116,117,118], can generate ROS under modest environmental temperature differences. When a temperature gradient is applied, negative charges in n-type thermoelectric material migrate from the hot end to the cold end, generating a potential difference across the material. The potential drives the separation and migration of carriers in opposite directions and induces ROS production on the surface of catalysts. Thus, thermoelectric-catalysis technology, by integrating thermoelectric effects with catalytic reactions, holds the promise of achieving efficient removal of heavy metals at relatively low temperature gradients, utilizing waste heat energy from both environmental and industrial sources.

2.5. Integrated Coupling Catalysis

In addition to the individual field-driven catalysis, the coupling of these fields with the widely investigated photocatalysis technology that suffered from low efficiency in separating photogenerated electron-hole pairs opens new avenues for the efficient and selective removal of heavy metals [110,119,120,121]. By leveraging the synergistic effects of various physical fields, this approach promises to overcome the limitations of single-field systems, providing excellent complementary capabilities. Analyzing the mechanisms and existing examples of multi-field coupled photocatalysis will deepen our understanding of the effects by external field, leading to more comprehensive synergistic strategies. Recently, multi-field coupled catalysis, including photo-electrocatalysis, piezo-photocatalysis, magneto-photocatalysis, and thermoelectric-photocatalysis, has emerged as a forefront area of research (Figure 5). These approaches revolutionize traditional photocatalytic processes by synergistically combining multiple driving forces and sustainable energy inputs, thereby enhancing kinetics, efficiency, and versatility.

2.5.1. Photo-Electrocatalysis

Photoelectrocatalytic (PEC) technology utilizes semiconductor materials to generate electron-hole pairs under light illumination assisted by an external electric field. This process enables simple, efficient, and harmless treatment of heavy metal ions, holding significant practical application potential. In a PEC system, the photocatalyst is typically positioned on either the anode or cathode. Upon applying a current or voltage, photogenerated electrons (e−) move to the cathode, while holes (h+) remain at the anode, thereby extending the lifetimes of these charge carriers.
The effectiveness of combining light irradiation and electrical circuit in a bio-electrochemical setup was presented by Sun et al. [122] As illustrated in Figure 6a–c, this system can simultaneously remove recalcitrant organic pollutants and recover valuable heavy metals such as Cu, Ni, and Zn. This dual-functionality is attributed to the active species of •O2−, •OH, and H2O2 generated during the PEC processes. In addition, a photocatalytic flow-through system with a biomimetic C3N4-Cu-TCPP photoanode and CeO2-BiOCl photocathode was developed [123]. Additionally, a membrane catalyst with synergistically interacting electronic properties was engineered using a highly active CdS/TiO2 heterojunction and ferroelectric polyvinylidene fluoride. This catalyst achieved a highly efficient reduction of Cr(VI) to Cr(III) at a rate of 1.6 × 10−2 min−1 [124]. These studies present an environmentally friendly alternative for the complete recovery and separation of various heavy metals in complex environments, enabling the simultaneous recovery of valuable resources.

2.5.2. Piezo-Photocatalysis

The piezo-photocatalytic effect is another emerging coupling strategy that harnesses light and external mechanical fields to achieve enhanced performance by introducing piezoelectric semiconductor materials [125,126,127,128]. Upon subjected to external stress, piezoelectric materials produce a dipole moment, resulting in the formation of an internal polarization field [129]. If the potential created within the piezoelectric material surpasses the reaction threshold, piezo-photocatalytic reactions can take place. Moreover, the piezoelectric effect can effectively separate photogenerated electrons and holes, thereby improving photocatalytic activity.
Recently, several ferro-/piezoelectric materials (e.g., PbTiO3, Na0.5Bi0.5TiO3, and Bi3TiNbO9) received considerable attention in the field of piezo-photocatalysis for heavy metal removal. For example, a ferroelectric material of SrBi2Nb2O9 with defect-engineered Na-Sm bimetal-regulated layered structure was developed by Guo et al. [130], achieving an enhanced removal rate of U(VI) by coupling light and the piezoelectric field (Figure 6d,e). A similar strategy was employed by Dong et al. [131] using piezoelectric CdS, with an asymmetric hexagonal phase structure, for the piezo-photocatalytic removal of U(VI). The combination of piezoelectric effects and photocatalytic activity enabled CdS nanowire to achieve the highest photocatalytic rate under ultrasonic vibration and 5 W LED illumination. The rate constant (0.042 min−1) was 12 times and 53.8 times higher than those under LED and ultrasonic conditions, respectively. The catalytic mechanism involves a two-step reduction reaction of O2 to produce H2O2, followed by interactions between H2O2 and UO2+ or the oxidation of UO2 to form (UO2)O2H2O.

2.5.3. Magneto-Photocatalysis

Magnetic fields exert profound impact on photocatalysis, offering unique effects that are not achievable by other external fields. Utilizing permanent magnets to generate a magnetic field requires no additional energy, making it advantageous for industrial applications. When an external magnetic field is applied to photocatalysts, it can induce effects such as negative magnetoresistance (MR), Lorentz forces, and spin polarization. These effects effectively prolong the lifetime of photo-generated electrons, reduce the activation energy of reactions, and enhance reaction selectivity [132].
A composite catalyst comprising BiFeO3/CoFe2O4/Co3O4(BFO/CFO/CO) was reported to exhibit remarkable photocatalytic performance under an external magnetic field [133]. As shown in Figure 6f,g, when a magnetic field of 0.4 T was applied, the catalyst achieved a significantly higher reduction efficiency for Cr(VI), reaching 99.3% within 35 min. Moreover, the reduction efficiency increased with increasing magnetic field intensity. The photoreduction kinetic rate under the magnetic field was 0.132 min−1, which is significantly higher compared to that without a magnetic field (0.026 min−1). This outstanding photoreduction performance is attributed to the magnetic field-induced separation of photogenerated electron/hole pairs. Upon the applied magnetic field, a perpendicular force acts on the movement direction of the photogenerated electrons and holes, which results in the enhanced catalytic removal rate of the heavy metal Cr(VI). Furthermore, Lan et al. [134] developed La-doped BiFeO3 multiferroic materials (La-BFO) that couple magnetic, piezoelectric, and photoexcited properties. The internal piezoelectric field, generated by the synergistic effects of multiple fields, adjusts the band structure and facilitates the separation of photogenerated charge carriers, leading to the production of more reactive species.

2.5.4. Thermoelectric-Photocatalysis

Thermoelectric materials have been extensively explored for potential applications in thermocouples, electricity generators, and refrigerators. Besides their traditional use in thermal-electrical power generation, there is a growing interest in directly integrating the thermoelectric effect with catalytic processes [109]. By coupling thermoelectric materials with semiconductor photocatalysts, a Schottky heterojunction with an ohmic contact could be constructed. He et al. [135] developed a TiO2−x/1T-MoS2 photocatalyst that is capable of utilizing both light and thermal energy, achieving a high removal rate of U(VI) of 98.2% within 60 min (Figure 6h). The mechanism of photocatalysis assisted by the thermoelectric field is illustrated in Figure 6i. In a liquid phase environment, a temperature gradient forms near the heterojunction interface, generating a thermoelectric potential difference via the Seebeck effect. This effect facilitates the transfer of photogenerated electrons to the substrate surface and participate in the capture and photoreduction of U(VI) ions. Besides, bioelectrocatalysis combined with the thermoelectric effect has been explored. Ai et al. [136] demonstrated a pioneering bioelectrochemical system integrated with thermoelectric generators to sequentially recover heavy metals, including Cu2+, Cd2+, and Co2+, from smelting wastewater. In summary, thermoelectric effect-assisted photocatalytic processes show significant potential for applications under thermal environment and promote the resource utilization of waste heat.
Thoroughly, external field catalysis systems such as electrocatalysis, piezoelectric, and multi-field coupling for heavy metal remediation have been demonstrated superior performance (Table 2). The outstanding performance has allowed for more breakthroughs in this field while inspiring more insightful opinions.
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Figure 6. (a,b) Possible photoelectrochemical pathways of heavy metals recovery and recalcitrant organic mineralization, and (c) heavy metal and COD removal [122]. Copyright 2022, Elsevier. (d) Potential mechanisms of piezocatalytic, photocatalytic, and piezo-photocatalytic removal of U(VI), and (e) crystal structure of SrNb2Bi2O9 [130]. Copyright 2024, Wiley. (f) Recycling Cr(VI) reduction efficiencies (0.0 T and 0.4 T of magnetic field), and (g) schematic diagram of Cr(VI) decontamination over BFO/CFO/CO nanocomposites [133]. Copyright 2022, Elsevier. (h) Comparison of reaction kinetics with/without external thermoelectric field, and (i) mechanism of photogenerated electron transfer driven by the thermoelectric field [135]. Copyright 2023, Elsevier.
Figure 6. (a,b) Possible photoelectrochemical pathways of heavy metals recovery and recalcitrant organic mineralization, and (c) heavy metal and COD removal [122]. Copyright 2022, Elsevier. (d) Potential mechanisms of piezocatalytic, photocatalytic, and piezo-photocatalytic removal of U(VI), and (e) crystal structure of SrNb2Bi2O9 [130]. Copyright 2024, Wiley. (f) Recycling Cr(VI) reduction efficiencies (0.0 T and 0.4 T of magnetic field), and (g) schematic diagram of Cr(VI) decontamination over BFO/CFO/CO nanocomposites [133]. Copyright 2022, Elsevier. (h) Comparison of reaction kinetics with/without external thermoelectric field, and (i) mechanism of photogenerated electron transfer driven by the thermoelectric field [135]. Copyright 2023, Elsevier.
Catalysts 14 00602 g006

3. Conclusions and Perspectives

3.1. Conclusions

This review provides an overview of emerging catalytic strategies driven by external fields for heavy metal remediation, focusing on electro-catalysis, mechano-catalysis (including piezocatalysis and contact-electro-catalysis), magneto-catalysis, and thermoelectro-catalysis, as well as coupling of these fields with photocatalytic processes. These techniques hold significant promise for enhancing the efficiency and effectiveness of heavy metal removal, presenting valuable solutions for environmental cleanup and resource recovery. They offer several distinct advantages, including selective targeting of heavy metals, improved energy efficiency, and reduced environmental impact.

3.2. Future Outlook

To fully realize their potential, future research should aim at optimizing these external field-driven strategies to enhance their practical applicability and effectiveness.
First, developing catalytic materials is crucial for advancing these innovative approaches. It is essential to construct well-designed materials and devices with tailored properties to enhance their responsiveness to external fields. This includes improving the stability, activity, and recyclability of catalysts under varying field conditions. Advanced synthetic methods will be necessary for the rational construction of new materials with well-engineered electronic structures, surfaces, and interfaces. By fine-tuning these material properties, researchers can maximize the efficiency and durability of catalytic systems, making them more suitable for real-world applications in environmental remediation.
Second, a comprehensive understanding of the underlying mechanisms driving these novel external field-driven catalytic processes is still lacking. The critical factors that determine catalytic performance are not fully understood, particularly when multiple external fields, such as electric, mechanical, magnetic, and thermoelectric fields, are combined. To address this, theoretical simulations and in situ experiments are necessary to elucidate the coupling mechanisms of different fields with catalytic reactions, as well as the interactions at the atomic or molecular scale, including adsorption, carrier transport, active sites, radicals, intermediates, and surface catalytic reaction kinetics. Developing more diverse external field coupling strategies, such as magneto-thermoelectric, piezo-electrochemistry, and piezoelectric-thermoelectric, will expand the frontier of catalysis research and open new avenues for advanced remediation techniques.
Third, translating laboratory-scale successes to industrial-scale applications remains a significant challenge. To bridge this gap, research must address critical issues related to scalability, cost-effectiveness, and the seamless integration of these advanced catalytic systems into existing remediation technologies. Compared to traditional treatment methods such as chemical precipitation or ion exchange, these emerging catalytic methods driven by external fields are often more selective and efficient, which can result in significant cost savings for industrial facilities by reducing chemical and energy consumption, as well as lowering waste treatment costs. However, there are still many challenges in promoting their practical applications, facing the problems of the cost-effectiveness of the reactors. With ongoing technological advancements and economies of scale, these methods may become more cost-effective and are anticipated to be widely adopted in heavy metal treatment applications. For practical adoption, these systems must be compatible with current remediation infrastructure, requiring catalysts that can be easily integrated without extensive modifications or additional equipment. Developing modular and adaptable systems that can be tailored to different environmental conditions and remediation needs is essential. These approaches offer tailored solutions to address the challenges posed by heavy metal contamination, providing scalable, cost-effective, and efficient technologies for environmental cleanup and resource recovery.
In summary, the integration of external fields into catalytic processes offers transformative potential for heavy metal remediation. Continued advancements in material science, mechanistic understanding, and practical applications will be crucial for harnessing the full benefits of these emerging technologies.

Author Contributions

Conceptualization, X.Z., S.C. and A.U.R.; writing—original draft preparation, X.Z. and S.L.; writing—review and editing, supervision, S.Z., Y.L., Q.Z. and S.L.; funding acquisition, S.L., Q.Z. and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by financial aid from the National Natural Science Foundation of China (Grant No. 22075126 and 52172187), the Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515111137 and 2021A1515110185), and the Guangdong-Macao joint funding program for science and technology innovation (2022A0505020025).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 14 00602 g001
Figure 1. Schematic of emerging catalytic strategies for heavy metal removal driven by external fields.
Figure 1. Schematic of emerging catalytic strategies for heavy metal removal driven by external fields.
Catalysts 14 00602 g001
Catalysts 14 00602 g004
Figure 4. (a) Schematic diagram of magnetostrictive-piezoelectric-catalytic effect over CoFe2O4@BiFeO3 nanocomposite under a magnetic field. (b) EPR spectra of •O2− and •OH trapped by DMPO under a magnetic field [101]. Copyright 2021, ACS Publications. (c) Reduction curves of Cr(VI) to Cr(III) using CFO@BFO under a magnetic field (15 mT and frequency of 1.1 kHz). (d) Scheme of magnetoelectric-catalytic effect induced Cr(VI) reduction [105]. Copyright 2022, Elsevier.
Figure 4. (a) Schematic diagram of magnetostrictive-piezoelectric-catalytic effect over CoFe2O4@BiFeO3 nanocomposite under a magnetic field. (b) EPR spectra of •O2− and •OH trapped by DMPO under a magnetic field [101]. Copyright 2021, ACS Publications. (c) Reduction curves of Cr(VI) to Cr(III) using CFO@BFO under a magnetic field (15 mT and frequency of 1.1 kHz). (d) Scheme of magnetoelectric-catalytic effect induced Cr(VI) reduction [105]. Copyright 2022, Elsevier.
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Figure 5. Integrated coupling photocatalytic system with multiple external fields for heavy metal remediation.
Figure 5. Integrated coupling photocatalytic system with multiple external fields for heavy metal remediation.
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Table 1. The key pros and cons of various catalytic methods driven by different external fields.
Table 1. The key pros and cons of various catalytic methods driven by different external fields.
Catalytic MethodsProsCons
Electro-catalysisFine-tuning of reaction kinetics and selectivity; Potential for real-time monitoring and control.The electricity input increases operational costs; Limited to systems with accessible electrical connections.
Mechano-catalysisUtilizing waste mechanical energy; Able to integrate into continuous processes.Mechanical wear and tear on treatment equipment; Potential for uneven reaction conditions.
Magneto-catalysisImproves reaction selectivity; Enables non-contact remote control of catalytic process.Effectiveness depends on the strength of the magnetic field; May require specialized magnetic equipment.
Thermoelectro-catalysisOperating under low temperature differences; Easy to scale up and achieve industrial-scale applications.Limited by kinetic or thermodynamic equilibrium; Lack of high-frequency thermal cycling.
Table 2. Performance comparison for removing target heavy metals by different catalytic methods.
Table 2. Performance comparison for removing target heavy metals by different catalytic methods.
MethodsCatalystsTarget Heavy MetalsRemoval
Efficiency 		Ref.
ElectrocatalysisCarbon feltsPb(II)
Cd(II)
Mn(II)		100%
100%
>98%		[74]
Mechano-catalysisAu/BiVO4Cr(VI)83%[90]
Magneto-catalysisBiFeO3/Ni0.1Fe2.9O4Cr(VI)75%[103]
Photo-electrocatalysisWO3/MoO3/g-C3N4Cu(II)
Ni(II)
Zn(II)		85.8%
71.6%
67.7%		[122]
Piezo-photocatalysisCdSU(VI)98.32%[131]
Magneto-photocatalysisBiFeO3/CoFe2O4
/Co3O4		Cr(VI)99.3%[133]
Thermoelectric-photocatalysisTiO2−x/1T-MoS2U(VI)98.2%[135]
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Zhang, X.; Chen, S.; Rehman, A.U.; Zhang, S.; Zhang, Q.; Liu, Y.; Li, S. Emerging Catalytic Strategies Driven by External Field for Heavy Metal Remediation. Catalysts 2024, 14, 602. https://doi.org/10.3390/catal14090602

AMA Style

Zhang X, Chen S, Rehman AU, Zhang S, Zhang Q, Liu Y, Li S. Emerging Catalytic Strategies Driven by External Field for Heavy Metal Remediation. Catalysts. 2024; 14(9):602. https://doi.org/10.3390/catal14090602

Chicago/Turabian Style

Zhang, Xinyue, Shanliang Chen, Attiq Ur Rehman, Suwei Zhang, Qingzhe Zhang, Yong Liu, and Shun Li. 2024. "Emerging Catalytic Strategies Driven by External Field for Heavy Metal Remediation" Catalysts 14, no. 9: 602. https://doi.org/10.3390/catal14090602

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