1. Introduction
The “reduced pollution, low carbon, and elegant ecological” concept calls for new routes and processes for advancing the water ecological environment toward more sustainable development. Currently, the application of green electricity has proven to be an effective approach to improving traditional water treatment technology while compensating for its shortcomings
[1],
[2],
[3]. For example, electricity can effectively and tunably regulate the surface charge state of materials to manipulate pollutant migration, achieving membrane antifouling
[4],
[5],
[6],
[7]. Moreover, the electroporation and electroosmotic dehydration of microbial cells contribute to sterilization and sludge dewatering
[8],
[9],
[10],
[11]. Additionally, the carrier concentration, Fermi level position, density of state distribution, and so forth, of catalysts can also be regulated by the electric field applied to improve the catalytic reaction
[12],
[13],
[14],
[15]. However, the application of external electricity in the water treatment process faces three main critical challenges: ① inconvenient operation and external equipment costs; ② high energy consumption for realizing a high electric field, especially the 10
6–10
7 V·m
−1 needed for cell electroporation
[11],
[16]; and ③ undesirable faradic reactions that occur in aqueous systems resulting from high electric voltages
[17],
[18].
Hydraulic energy, including kinetic and potential energy, is inherent in the water treatment process, such as the mechanical stirring of industrial processes, pipeline flow, general flow, and fluctuations in rivers and lakes
[19],
[20],
[21]. Despite the characteristics of this energy, including their weak strength, low frequency, and high dispersion, they are ubiquitous and result in a large total energy. Typically, piezoelectricity is a common force–electricity conversion mechanism based on the asymmetric shift of charges or ions in piezoelectric materials when exposed to mechanical strain, which depends on the intrinsic polarization of materials
[19],
[20], such as mechanical deformation-driven electricity. Owing to its high electromechanical coupling capacity (piezoelectric coefficient
d33 represents the electrical charge generated in the three-direction (polarization direction) when mechanical stress is applied along the same three-direction, reaching hundreds of pC·N
−1)
[4],
[13],
[22],
[23], high power density (hundreds of mC·m
−2)
[24],
[25] and capacity to function in broad frequency applications, piezoelectricity is regarded as one of the most promising strategies for converting hydraulic pressure to electricity to address common challenges in water treatment. The fundamental properties of representative piezoelectric materials, including both inorganic and organic catalogs, and their fundamental properties are summarized in
Table 1. Typically, inorganic piezoelectric materials generally possess strong piezoelectric properties (piezoelectric coefficient
d33) and a high temperature tolerance (Curie temperature) but are stiff and brittle (a large Young’s modulus), whereas organic materials are often flexible and lightweight but do not exhibit prominent piezoelectric performance.
Compared with directly applying external electricity, the
in situ conversion of hydraulic energy to electricity does not require complex operations or external infrastructure. Notably, the local piezoelectric field can reach up to 10
7 V·m
−1 [12],
[27], which is sufficient for enhancing conventional water treatment processes. Moreover, piezoelectric materials possess high electrical resistance that restricts charge transfer, preventing faradic reactions from generating high piezoelectricity. Overall,
in situ utilization of hydraulic piezoelectricity in the water treatment process is simple and efficient and can adapt to harsh and complex working environments, which is also environmentally friendly. Notably, such piezoelectric coupling is a regulated and improved strategy for innovating traditional water treatment technologies rather than a revolution.
With this in mind, we discuss the piezoelectric concept of the
in situ utilization of hydraulic pressure–piezoelectricity conversion to address common challenges in water treatment processes (
Fig. 1 [28]). On the basis of the different working mechanisms of piezoelectric integration systems, several representative water treatment technologies, including piezo-membranes, catalytic reactions, and sludge piezodewatering applications, are regulated to compensate for their shortcomings. While past efforts have laid a solid foundation, we expect further breakthroughs in practical verification and investigations of integration with other emerging hydraulic-based technologies or energy sources, aiming to provide more inspiration and scenarios of
in situ electricity that advance water treatment processes.
2. Hydraulic piezoelectricity conversion and promising applications
2.1. Self-cleaning piezomembrane
Pressure-driven membrane technology is a widely applied matter separation method in water treatment processes
[4],
[29],
[30]. Although it possesses multiple advantages, including selective permeation and low energy consumption, inevitable membrane fouling is a critical challenge in practical applications
[31],
[32]. Generally, fouling decreases membrane performance and lifespan, which leads to increased energy consumption and operating costs. Traditional static membrane modifications can only alleviate the fouling process and are unable to prevent fouling; thus, the resulting membranes are ultimately contaminated
[30]. Additionally, dynamically responsive membranes require an external stimulus source that intimately combines with membrane processes, which greatly increases the complexity and cost of the operating system
[33].
The adoption of pressure-responsive membranes containing piezoelectric materials is a proposed alternative for realizing electroactive antifouling behavior. Owing to hydraulic pressure (> 1 bar; 1 bar = 10
5 Pa) being the intrinsic driving force for membrane filtration, pressure pulses can be naturally transformed into electroactive responses to achieve
in situ antifouling via a piezoelectric membrane (PiezoMem)
[4],
[6],
[34],
[35] without additional energy requirements. Fundamentally, piezoelectric current pulses and rapid voltage (interface electric field) oscillations converted by transient hydraulic pressure fluctuations can generate abundant near-surface reactive oxygen species (ROS) and dielectrophoretic forces (
Fig. 2), contributing to the degradation and repulsion of pollutants without the need for additional chemical cleaning agents
[27], secondary waste treatment or further external stimuli. Notably, PiezoMem shows a universal
in situ antifouling action for a wide range of foulants, including organic molecules, colloids, oil droplets, and microorganisms. Furthermore, the findings and experience in PiezoMem antifouling can also be extended to enhanced membrane permeability, rejection, and demulsification under weak hydraulic energy. Pioneering work has preliminarily reported that membrane permeability and rejection can be slightly improved owing to the superhydrophilicity and high surface potential caused by piezocatalytic polarization
[25]. Overall, coupling the piezoelectric concept is a regulated and improved strategy for membrane technologies. Piezomembranes are still suitable for various membrane application scenarios, including municipal/industrial wastewater treatment, drinking water treatment, and so forth. However, this proven concept requires further practical validation, especially regarding long-term durability. Furthermore, the underlying piezoelectric transduction mechanism needs to be explored in detail to strengthen aspects of membrane performance.
2.2. Catalytic reaction regulated by piezoelectricity
In heterogeneous catalysis, the catalytic performance of a material essentially depends on its electronic structure. Modulating the electronic structure and properties of catalysts through tuning physical fields is important for achieving precise catalytic regulation. Among them, applying an electric field is a simple and effective way to regulate catalytic processes
[36],
[15]. First, the carrier concentration, Fermi level position, and density of state distribution of catalysts can be changed by an electric field
[37],
[38], promoting charge transfer/separation, optimizing the redox potential, and creating effective active/adsorption sites
[39],
[40]. Second, an electric field can activate reactant molecules and lower the energy of their transition states by increasing the bond ionicity and mixing of charge-transfer states
[41],
[42]. Finally, the electric field can adjust the adsorption energy/strength between catalysts and reactant molecules and ultimately affect the activation energy and reaction rate according to the volcano curve and Sabatier principle
[43],
[44],
[45]. Overall, applying an external electric field alters heterogeneous catalytic processes/chemical reactions, which further controls the reactivity, selectivity, and reaction mechanism. However, the undesired faradic reaction or limited field strength of the external electric field limits its further development
[46].
On the basis of ubiquitous hydraulic energy, a prospective strategy is to construct an
in situ electric field using piezoelectricity to regulate catalytic processes/chemical reactions (
Fig. 3 [28]). Under mechanical force or strain, the center of positive and negative charges in the unit cell of piezoelectric materials, such as non-centrosymmetric crystal structures, is separated and a net dipole moment is generated, forming a built-in electric field
[47],
[48]. This built-in electric field can effectively modulate the activation and conversion of molecules (e.g., CO, O
2, N
2, and NO)
[49], as well as their activity and selectivity in photocatalytic reactions (e.g., water splitting
[50],
[51] and reduction of Ag
+ to Ag
[52]) and electrocatalysis (e.g., oxygen evolution reaction
[53], hydrogen evolution reaction
[54], and CO
2 reduction reaction
[55]). Additionally, the switchable piezoelectric field can capture oppositely charged reactants or intermediates, causing differences in adsorption strengths
[40],
[52],
[56], which further regulates the absorption/desorption behavior between reactive molecules and catalytic surfaces
[57],
[58]. Notably, the local piezoelectric field can reach up to 10
7 V·m
−1 [12],
[27],
[59], which could effectively separate/migrate carriers and promote the regeneration of active sites in catalysts
[47],
[60],
[61]. Notably, hydraulic piezoelectricity is used to regulate the catalytic reaction by constructing an
in situ electric field rather than as a piezoelectric catalyst.
Pioneering work has introduced this strategy into water treatment applications. In natural water bodies, harmful and toxic organic pollutants seriously threaten ecosystems and human health. A floatable piezo-photocatalytic platform, which utilizes natural water waves and solar light for piezo-photocatalytic water decontamination, was constructed to achieve the goals of environmental protection and sustainable development
[62]. Piezoelectric polymeric/inorganic materials coupled with photocatalysts to form floatable membranes in the form of “nanoalgae” or core–shell nanocomposites can achieve impressive self-purification of polluted rivers
[62],
[63],
[64]. The piezo-photocatalytic performance is greatly improved, such as reaching 400% enhancement, which is attributed to the efficient separation and reduced recombination of electrons and holes in the catalysts and the manipulation of pollutants by the
in situ piezoelectric field
[28],
[65]. Moreover, in advanced oxidation processes of water treatment, inherent mechanical energy (e.g., ultrasound, agitation, and friction) can also be converted into piezoelectricity to regulate catalytic reactions. For example, a piezocatalytic self-Fenton system has emerged as a promising technique for wastewater treatment. The generated piezoelectric potential could offer a strong electrochemical driving force for facilitating the reaction kinetics of water-oxidative H
2O
2 production and drive the sustained reduction of Fe
3+ [66]. Similarly, for peroxymonosulfate activation, a hydraulic-driven piezoelectric field can not only promote the regeneration of Fe
2+ [67] but also effectively separate the charges of catalysts, which accelerates the generation of ROS for catalytic degradation
[68]. Therefore, hydraulic piezoelectricity can be effectively introduced into the catalytic reactions of current municipal/industrial wastewater treatment and drinking water treatment. Additionally, coupling the piezoelectric field to high-value catalytic reactions, such as green chemical/fuel feedstock production (e.g., hydrogen/oxygen evolution, oxygen reduction to H
2O
2, CO
2 reduction to chemicals/fuels, and nitrogen reduction to ammonia), is highly desirable for achieving greater efficiency. The realization of efficient catalytic regulation under weak hydraulic energy requires more elaborate designs of piezoelectric materials and composite structures in catalytic systems. There is still a long way to go for practical applications, and intensive studies are needed.
2.3. Sludge piezo-dewatering/sterilization
Treatment of municipal wastewater daily produces substantial amounts of sewage sludge as a byproduct, which contains massive amounts of water and harmful pathogens
[69],
[70]. Sludge dewatering plays an important role in reducing sludge volume, facilitating transport, and preventing leachate pollution. The current technologies for sludge dewatering are mainly categorized into physical and chemical methods, but these methods have expensive running costs and require large energy inputs resulting in severe secondary chemical pollution
[9],
[71]. The application of an external electric field during the dewatering of a sludge segment, so-called electro-dewatering, is considered to accelerate liquid–solid separation and sterilization, resulting in low water content and fewer viruses in the sludge cakes
[10]. However, electro-dewatering is limited by the strength of the external electric field, poor operability, and the high cost of additional high-voltage equipment. Exploring a more convenient, affordable, and efficient technique for sludge electro-dewatering is crucial.
During the dewatering process, mechanical pressure is inevitably applied to squeeze water out of the sludge. In this regard, coupling with piezoelectric processes naturally occurs during dewatering, where piezo-dewatering may be more effective and economical (
Fig. 4). Notably, sludge often contains intracellular water and pathogenic bacteria
[9], so sterilization is a necessary step in sludge dewatering. One advantage is that under filtration pressure, the induced strong piezoelectric field (8.1 × 10
7 V·m
−1) can result in electroporation of the microbial cell membrane followed by the penetration of generated ROS, releasing intracellular water and thus alleviating the environmental risk of subsequent sludge treatment
[72],
[73],
[74],
[75]. Moreover, the piezoelectric field disrupts the sludge structure and microbial organization; hence, the present organic substances flow out along with the water flow
[8]. Another advantage is that the piezoelectric field can cause electro-osmosis and electromigration
[71],
[76], which may improve the sludge dewatering capacity and promote the separation of mud and water. Additionally, the remaining piezofields of piezomaterials are electrically neutralized with sludge particles, reducing the zeta potential of the sludge and enhancing its dewaterability
[8]. From the perspective of practical application, the costs of equipment and operation remain essentially unchanged because the piezoelectric effect is driven naturally by the pressure of the filtration process, which is based on existing devices without any additional energy input or new equipment. However, piezo-dewatering technology is still in its infancy, and certain scientific and technological issues, such as the operational stability and recyclability of piezoelectric materials, the universality of various types of sludge, and the scale-up of the process, require further in-depth investigation. In addition, for high-concentration organic sludge, such as industrial sludge, the piezo-dewatering strategy also needs further validation.
Notably, piezoelectricity not only is suitable for piezosterilization in sludge but can also be extended to sterilization in aqueous systems. Recent studies have reported that piezosterilization shows distinct universal water bacterial disinfection performance among different microbes and is about 1000 times more efficient than an equivalent amount of preformed hydrogen peroxide, greatly reducing chemical usage and the generation of disinfection byproducts
[27],
[77]. Thus, designing more efficient piezoelectric materials and flexible systems to optimize force–electricity conversion with high-strength and fast-response piezoelectric fields under weak hydraulic energy systems is needed.
3. Conclusions and outlook
Hydraulic energy, which is naturally accompanied by diverse water treatment processes, generally has weak strength, low frequency, and high dispersion; however, it is ubiquitous and results in a large amount of total energy.
In situ hydraulic pressure–electricity conversion and utilization are convenient, efficient, and practical and are promising strategies for addressing common challenges in water treatment. Piezoelectricity is promising for regulating water treatment technologies and has sparked extensive study interest, resulting in pioneering advancements in piezomembrane technology, regulated catalysis, and sludge piezo-dewatering applications. Such
in situ piezoelectricity coupling can strengthen traditional water treatment processes while overcoming their limitations. The findings and attempts in piezoelectric water treatment can also be extended to other processes, such as flocculation, precipitation, sterilization, and demulsification. To further promote the piezoelectric regulation of universal water treatment technologies, elucidating the fundamental physicochemical transduction mechanism, such as the microscopic charged state of materials and carrier transfer, is important. In addition, the failure risks from depolarization, fatigue, and dissolving loss of the piezoelectric material over long-term operation in an aqueous environment constitute a primary challenge for advancing such piezoelectric coupling technology. With successful piezoelectric applications, various hydraulic-based technologies could also be used to advance water treatment. For example, emerging hydrovoltaics have greatly expanded the technical capability of harvesting the mechanical and latent energies of hydraulic energy, allowing for advancements in generating electricity from water waves/flows, raindrops, natural evaporation, and moisture
[78],
[79],
[80],
[81],
[82]. Additionally, there are ubiquitous solid–liquid contact processes in environments, which have been demonstrated to generate so-called solid–liquid triboelectricity
[21],
[83],
[84],
[85]. These new transduction principles show promise in water treatment but are in their infancy, and further exploration is needed for their
in situ utilization similar to that of piezoelectricity. In addition to hydraulic kinetic energy, various energy sources, such as available thermal energy, also have potential application value and development prospects, providing more inspiration and scenarios for advanced water treatment. For example, industrial waste heat can be utilized
in situ for pipeline anticorrosion and membrane distillation processes.
Using the inherent energy in water treatment processes to generate in situ electroactivity is not only a proven sustainable concept but also has practical application potential. Such in situ electricity coupling is a regulated and improved strategy for innovating traditional water treatment rather than a revolution, which shows the feasibility of developing low-carbon, efficient, and advanced water treatment technologies. Thus, we expect that breakthroughs at a fundamental level and investigations into its integration with other hydraulic-based technologies or energy sources will offer the community powerful tools to address common challenges in water treatment and enable the achievement of the outlined milestones.
CRediT authorship contribution statement
Qiancheng Xia: Writing – review & editing, Writing – original draft, Investigation, Conceptualization. Xinrong Fang: Writing – original draft. Jiaming Yao: Writing – original draft. Xiaohan Yang: Writing – original draft. Yongguang Bu: Writing – original draft. Wenkai Zhang: Writing – original draft. Guandao Gao: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
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.
Compliance with ethics guidelines
Qiancheng Xia, Xinrong Fang, Jiaming Yao, Xiaohan Yang, Yongguang Bu, Wenkai Zhang, and Guandao Gao declare that they have no conflict of interest or financial conflicts to disclose.
Acknowledgments
We thank C. Wang for the detailed discussion. This work was supported by the National Natural Science Foundation of China (22276092), the Fundamental Research Funds for the Central Universities (2022300304), and the National Innovation Center par Excellence Joint Graduate Program.
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