Synergistic Catalysis for Algae Control: Integrating Sonocavitation and Chemical Catalysis

Abstract

This review systematically summarizes recent advances in ultrasound–chemical catalytic synergistic technology for controlling harmful algae blooms, focusing on the multi-mechanism cooperation of catalysts, oxidants, and nanomaterials within sonocavitation systems. The technology enhances coupling efficiency between cavitation effects and radical oxidation while leveraging interfacial regulation capabilities of catalysts (e.g., charge adsorption, carrier migration) to selectively disrupt algae cell structures and efficiently degrade extracellular organic matter. Three key innovations are highlighted: (1) development of a multi-mechanism synergistic system that overcomes traditional technical limitations through moderate pre-oxidation strategies for precise algae control; (2) first systematic elucidation of the bridging role of sonoporation in ultrasound–chemical synergy; (3) decipherment of interface-targeted regulation mechanisms that enhance oxidation efficiency. Collectively, these advances establish an engineerable new paradigm characterized by high efficiency, operational stability, and minimized ecological risks.

1. Introduction

In recent years, the safety of drinking water has become an increasingly severe global issue. With rapid economic development, eutrophication in freshwater bodies such as lakes and reservoirs has intensified, leading to frequent cyanobacterial blooms [1,2]. These algae blooms not only disrupt aquatic ecosystems but also pose direct threats to drinking water safety. Secondary metabolites produced by cyanobacteria, including microcystin-LR (MC-LR), anatoxin-a, and cylindrospermopsin, have been confirmed to exhibit hepatotoxicity, neurotoxicity, and genotoxicity [3]. The World Health Organization (WHO) has established a stringent safety limit of 1 μg/L for MC-LR in drinking water. However, during algae bloom outbreaks, actual toxin concentrations frequently exceed this threshold, posing significant health risks to humans [4].

Current mainstream cyanobacteria removal technologies predominantly rely on chemical pre-oxidation coupled with coagulation sedimentation (or flotation) processes [5]. The pre-oxidation can effectively disrupt organic coatings on particle surfaces and alter zeta potential to enhance coagulation efficiency [6]. For instance, ozone (O₃) or chlorine pre-oxidation reduces head loss development rates during rapid sand filtration and prevents turbidity breakthrough—this technology presents significant limitations. First, the pre-oxidation process may degrade high-molecular-weight organic matter into low-molecular-weight fractions while compromising algae cell membrane integrity, leading to the release of both extracellular and intracellular algae organic matter (AOM) along with toxins [7,8]. Second, inefficient oxidant mass transfer results in delayed contact with target pollutants, causing oxidant self-quenching or unproductive reactions with water molecules. These factors not only reduce treatment efficacy but also increase the risk of disinfection by-product (DBP) formation, increasing the risk of drinking water safety [6].

To overcome the aforementioned limitations, the synergistic effect between catalysts and ultrasonic cavitation technology has been proven to significantly enhance algae removal efficiency [9,10,11]. Ultrasonic cavitation refers to the dynamic process in which ultrasonic waves propagate through a liquid, creating alternating high- and low-pressure cycles that induce the formation of microscopic gas or vapor bubbles. These bubbles grow during the rarefaction phase and collapse violently during the compression phase. The implosion produces extreme localized conditions, temperatures exceeding 5000 K and pressures above 500 MPa, accompanied by intense shock waves and microjets [12]. The high temperature also promotes the thermal dissociation of water molecules, generating highly reactive species such as hydroxyl radicals (·OH) and hydrogen radicals (·H) [13,14]. These physical and chemical processes constitute the two principal effects of cavitation: mechanical effects, which physically disrupt particulate matter or algae cell structures, and sonochemical effects, in which radicals drive potent oxidative degradation of organic pollutants. The synergistic system of catalyst/oxidant and ultrasonic cavitation technology system achieves enhanced performance through the collaborative action of multiple mechanisms, such as physical fragmentation and free radical oxidation, which are strengthened by the ultrasonic cavitation effect: (1) selectively damaging cyanobacterial cell structures without triggering massive intracellular release, (2) efficiently degrading extracellular organic matter (EOM), and (3) enabling dynamic process control via precise frequency/power modulation. Notably, ultrasound synergizes with chemical agents or catalytic materials to significantly enhance radical mass transfer efficiency at cyanobacteria/catalyst interfaces, achieving “moderate pre-oxidation”—effectively balancing algae removal performance with minimized adverse effects from over-oxidation [15,16,17,18,19,20,21,22]. This self-terminating treatment characteristic provides innovative solutions to critical bottlenecks in conventional pre-oxidation technologies.

This review aims to systematically expound on the recent research progress of ultrasonic algae removal technology, with a focus on the following aspects: (1) this review employs bibliometric methods to systematically quantify the research status in the field of ultrasonic algae removal, covering dimensions such as publication volume, co-occurrence of keywords, and temporal sequence of literature, thereby revealing the research hotspots and development trends in this field. (2) Based on the research progress of ultrasonic bioeffects, the review explores the biological effect mechanisms of the interaction between ultrasonic cavitation and algae cells. (3) The review analyzes the influence of different operational parameters (frequency, power, and time) on the treatment efficacy. (4) The synergistic enhancement principles of ultrasound combined with other technologies such as flocculants, oxidants, and nanosonosensitizers are investigated. (5) The technical and economic feasibility of actual engineering applications is assessed. By clarifying the key scientific issues and technological bottlenecks in current research, this review provides theoretical support for the development of new strategies for efficient and safe ultrasound treatment, ultimately facilitating the transformation of this technology from laboratory research to large-scale applications.

2. Research Progress in Ultrasonic Bioeffects

2.1. Physical Damage

Ultrasonic cavitation has emerged as a highly promising strategy for microbial inactivation and has garnered significant attention in recent years [16]. Simultaneously, ultrasonic cleaning technology leverages the powerful mechanical effects of cavitation bubbles in liquids to disrupt cellular membrane structures without the risk of secondary pollution, while maintaining continuous system operation, thus significantly improving treatment efficiency and cost-effectiveness [23]. Consequently, the inactivation technology based on cavitation offers safety, effectiveness, and economic advantages, with high practical value with strong potential. However, when addressing cyanobacterial blooms, ultrasonic treatment still faces severe challenges. Particularly, ensuring water quality while preventing excessive cell damage under high-intensity cavitation mechanical forces has become a core problem in this field [24]. Specifically, the primary challenge lies in the precise control of cavitation bubble positioning. The generation and movement of cavitation bubbles are influenced by a complex interplay of factors such as sound wave intensity, frequency, and propagation medium, among others. The uncertainty of these factors makes it difficult to predict the distribution and movement trajectory of cavitation bubbles in liquids, which in turn affects the effective contact between cavitation bubbles and the biofilm formed by microorganisms [24]. Furthermore, the biofilm, composed of algae cells, extracellular polymers, proteins, etc., is an elastic biological material that poses a challenge in its resistance to cavitation bubbles. The biofilm not only has elasticity and integrity but also possesses a certain repair capacity to resist external pressure and impact [25,26]. This resistance characteristic often prevents cavitation bubbles from achieving the desired cleaning effect on cells. Moreover, the interaction mechanism between cavitation bubbles and cells involves multi-physics field coupling problems in complex media, such as sound propagation and the flow of sound currents. This high complexity makes it extremely difficult to thoroughly analyze the interaction between cavitation bubbles and cells. Therefore, precise control of the spatial and temporal distribution and action mechanisms of cavitation bubbles to enhance their mechanical effects on algae inhibition is an urgent issue to be addressed in the current application of cavitation technology for cyanobacteria control.

2.2. Free Radical Oxidation

Free radical oxidation plays a key role in ultrasonically mediated microbial inactivation and the regulation of microbial life cycles. The extreme high-temperature and high-pressure environment generated by ultrasonic cavitation can cause thermal decomposition reactions in water molecules, breaking them down into highly reactive ·OH and ·H radicals [27]. These radicals attack the phospholipid bilayer of the cell membrane, causing lipid peroxidation, disrupting membrane integrity, and inhibiting the photosynthetic system [28]. Despite the significant potential of ultrasonically mediated free radical oxidation in the field of algae removal, current research still faces many challenges. Firstly, the generation and action mechanisms of radicals are extremely complex, influenced by a combination of factors such as ultrasonic parameters, medium properties, and reaction systems, making it difficult to precisely control the generation process of radicals [29,30]. Moreover, the random distribution of cavitation bubbles leads to fluctuations in radical concentration, with local concentration changes potentially leading to differences in local oxidative damage and affecting reaction efficiency [31]. Secondly, the three-dimensional network structure formed by extracellular polymeric substances (EPSs) in the cell membrane can trap polar molecules with diameters greater than 2 nm (such as ·OH radicals), significantly reducing the concentration of radicals reaching the inner layer of the biofilm, and functional groups such as carboxyl groups (-COOH) in EPSs can interact with ·OH radicals, directly consuming them and reducing their oxidation efficiency [32,33]

2.3. Sonoporation

The sonoporation effect induced by ultrasonic cavitation offers a new perspective for ultrasonically mediated algae removal strategies. Studies have shown that transient channels produced by bubble cavitation under ultrasonic action can significantly enhance cell membrane permeability, which may promote the targeted delivery of oxidants and algicides into cyanobacterial cells [34,35]. This drug delivery mechanism based on inertial cavitation has been confirmed by multiple studies to significantly improve the intracellular transport efficiency of bactericides [36,37]. Among these, Wu et al. [38] achieved efficient intracellular delivery while maintaining the viability of microalgae cells through the optimization of ultrasonic parameters. In the process of cyanobacteria inactivation, the sonoporation effect operates through a dual mechanism: on one hand, the mechanical shearing force and microjets generated by cavitation create reversible channels in the cell membrane, providing a penetration path for oxidants and algicides [21]; on the other hand, the radicals produced along with cavitation can synergistically enhance the bactericidal effects of oxidants [39]. This physical–chemical synergistic effect not only disrupts cell membrane integrity [18] but also interferes with energy metabolism gene expression [40] and induces apoptosis pathways [18], achieving multi-targeted intervention in the life cycle of cyanobacteria. Notably, Wu et al. [28] found that, although cyanobacteria show a certain self-repair ability after ultrasonic treatment, the regeneration can be effectively suppressed by optimizing treatment parameters, such as using intermittent ultrasound. Current research challenges include the significant differences in sensitivity to the sonoporation effect among different species of cyanobacteria, as well as the potential impact of complex components in natural water bodies on cavitation efficiency and oxidant stability.

This review summarizes three primary bioeffect mechanisms of ultrasonic technology applied to cyanobacterial bloom control: physical damage, free radical oxidation, and sonoporation (Figure 1). Ultrasonic frequency is a key determinant of the dominant bioeffect mechanism during cavitation. Precise regulation of frequency enables targeted utilization of algae cell characteristics, directing the physical and chemical processes of cavitation toward specific modes of biological damage. Low-frequency ultrasound (20–40 kHz) predominantly induces vigorous inertial cavitation, generating intense shear forces, shock waves, and microjets that cause direct structural disruption of algae cells [41]. Intermediate frequencies (40–200 kHz) can provide a balanced interplay between physical disruption and radical-mediated oxidation, enabling tunable intervention strategies depending on treatment goals. In contrast, high-frequency ultrasound (>200 kHz) favors stable cavitation and promotes sonochemical pathways, particularly water sonolysis to produce reactive oxygen species (ROS) such as ·OH, which oxidatively damage cell membranes, proteins, and pigments essential for photosynthetic metabolism [18,42]. Understanding this frequency dependence is critical for optimizing sonocatalytic processes, enabling the selection of operating conditions that maximize desired effects while minimizing excessive cell lysis or toxin release.

This frequency-dependent control of physical and chemical effects highlights the importance of optimizing ultrasonic parameters to achieve the desired bioeffects while minimizing unintended damage. However, avoiding excessive cell damage under high-intensity cavitation mechanical forces while ensuring water quality remains a core challenge in ultrasonic treatment of cyanobacterial blooms. Based on the reviewed literature, ultrasonic technology should emphasize challenges such as precise control of cavitation bubbles, the elasticity and repair capacity of biofilms, and multi-physics field coupling. Addressing these challenges through precise control of the spatial and temporal distribution and action mechanisms of cavitation bubbles is crucial. In terms of free radical oxidation, the highly reactive radicals produced by ultrasonic cavitation can attack the phospholipid bilayer of the cell membrane, causing lipid peroxidation, disrupting membrane integrity, and inhibiting the photosynthetic system. However, the generation and action mechanisms of radicals are complex and influenced by various factors, making precise control difficult. The impact of EPS in the cell membrane on the trapping and consumption of radicals and the influence of these interactions on radical oxidation efficiency should be a focal point of research. Additionally, sonoporation, as a new strategy for ultrasonically mediated algae removal, enhances cell membrane permeability to promote the intracellular delivery of oxidants and algicides. Research has shown that sonoporation can operate through a dual mechanism of mechanical shearing force and radical generation to achieve multi-targeted intervention in the life cycle of cyanobacteria. However, the self-repair ability of cyanobacteria after ultrasonic treatment necessitates the optimization of treatment parameters, such as intermittent ultrasound, to effectively inhibit regeneration. The sensitivity differences of different cyanobacterial species to the sonoporation effect and the potential impact of complex components in natural water bodies on cavitation efficiency and oxidant stability are identified as key research areas for the future, which are expected to further enhance the practical application of ultrasonic technology in the management of cyanobacterial blooms.

3. Algae Removal by Ultrasound Synergized with Catalysts

Accurate evaluation of “algae removal” is critical for comparing the performance of different ultrasonic–chemical catalysis systems. In the literature, this term is quantified using a variety of indicators, each reflecting different aspects of algae biomass reduction and physiological damage. Commonly employed metrics include: (i) chlorophyll a concentration, serving as a proxy for photosynthetic capacity, typically determined by spectrophotometry; (ii) turbidity, representing the reduction of suspended algae particles; (iii) cell density or cell count, obtained via optical microscopy, flow cytometry, or hemocytometry; and (iv) cell viability or membrane integrity, assessed through assays such as live/dead staining, ATP quantification, or metabolic activity measurements. These indicators capture distinct removal mechanisms—such as pigment degradation, coagulation and sedimentation, physical disruption, and loss of metabolic function—which are particularly relevant in ultrasound–catalyst synergistic systems where multiple bioeffects occur simultaneously. For robust cross-study comparison, it is essential to clearly specify the chosen indicator(s), analytical method, and calculation approach when reporting removal rates.

Based on these metrics as comparative benchmarks, recent studies have demonstrated that ultrasound-coupled catalyst technology significantly enhances the removal efficiency of harmful algae by modulating sonochemical effects and interfacial reaction kinetics. This approach provides a novel strategy to overcome the limitations of conventional algae control methods, such as chemical residue accumulation and toxin release resulting from cell rupture. The core breakthrough lies in that catalysts enhance free radicals such as ·OH generated by cavitation effects through interfacial interactions and promote the separation of electron–hole pairs, thereby improving the production and utilization efficiency of ROS [43,44,45] and achieving low energy consumption and high-efficiency algae inactivation and organic matter degradation. In the ultrasound-synergized catalytic algae removal system, carbon-based, iron-based, titanium-based, and special polymer-based catalysts have shown excellent performance due to their unique structures and properties (

Table 1. The performance of different types of catalysts for algae removal under ultrasound.

Catalysts	Dosage	Algae	Density	Frequency	Time	Removal Rate (%)	Reference
FeSO4	1 mg/L	Microcystis aeruginosa	4.19 × 106 cells/mL	20 kHz,	5 min	89.26%	[21]
Fe3O4/CNTs	20 mg/L	Microcystis aeruginosa	1.8 × 106 cells/mL	40 kHz	20 s	94.40%	[46]
Co-MOF/PVA	0.5 mg	Navicula	Using appropriate growth concentration	20 kHz	137.2 s	99.96%	[47]
Nitrogen-doped nanodiamonds (N-NDs)	20 mg/L	Microcystis aeruginosa	1.5 × 106 cells/L	800 kHz	5 min	Over 90%	[48]
Polytetrafluoroethylene (PTFE) film	5 × 5 cm PTFE film	Microcystis aeruginosa	1.76 × 106 cells/mL	40 kHz	5 h	60.19%	[43]
Fe3O4/multi-walled carbon nanotubes (Fe3O4/MWCNTs)	5 mg/L	Microcystis aeruginosa	2.0 × 106 cells/mL	20 kHz (probe), 40 kHz (bath, for catalyst preparation), 600 kHz (synergistic treatment)	30 min	90.58%	[21]
TiO2	0.5 g/mL	Microcystis aeruginosa	5 × 106 cells/mL	36 kHz	15 min	87%	[44]
TiO2/biochar (TiO2/BC)	50 mg/L	Microcystis aeruginosa	1.3 × 107 cells/mL	600 kHz	90 s	92%	[49]
T-BaTiO3/Ag3PO4 (T-BTO/AP-50)	50 mg/L	Microcystis aeruginosa	-	28 kHz	4 h	96.10%	[45]

).

3.1. Carbon-Based Catalyst Systems

Carbon-based materials have demonstrated great potential in water treatment due to their excellent electrical conductivity, large specific surface area, and surface modifiability [50,51,52]. However, pure carbon-based materials suffer from limitations such as limited free radical generation efficiency and rapid recombination of electron–hole pairs. Their catalytic activity is generally improved through doping, composite modification, and design of special structures to enhance interfacial reactions [53,54]. For example, N-NDs, with their large specific surface area and surface structures rich in active sites such as pyridinic nitrogen and graphitic nitrogen, significantly enhance cavitation effects under high-frequency ultrasonic irradiation (e.g., 800 kHz), promoting the cleavage of water molecules to generate ROS (e.g., ·OH, ¹O₂). Their positively charged surfaces further shorten the mass transfer distance between free radicals and negatively charged algae cells, enhancing oxidation targeting, thereby achieving efficient algae removal while improving the removal efficiency of AOM and microcystins [48]. In addition, polymer carbon-based materials such as PTFE membranes induce a liquid–solid contact electrification effect under low-frequency ultrasound (e.g., 40 kHz), generating a strong electric field at the interface that promotes the cleavage of water molecules into reactive species such as ·O₂⁻ and H₂O₂. The algae removal rate reaches 60.19% within 5 h, which is 5.1 times that of ultrasound alone. Their excellent surface chemical stability ensures no significant loss of activity after 10 cycles of reuse, providing a new idea for low-cost and pollution-free algae removal technologies [43]. However, this synergistic system still faces issues such as relatively long treatment duration and high difficulty in regenerating some catalysts in practical applications. Future efforts can be directed towards further optimizing the preparation process of composite catalysts to enhance their synergistic efficiency with ultrasonic cavitation, as well as exploring efficient recovery and recycling technologies for catalysts.

3.2. Iron-Based Catalyst Systems

In ultrasound-assisted algae removal systems, iron-based catalysts exhibit significant advantages due to their low cost, recyclability, and high safety [55,56]. They mainly include three typical categories: (1) single-component iron oxides (e.g., magnetite Fe₃O₄); (2) iron-based composite catalysts (e.g., Fe₃O₄/MWCNTs); and (3) soluble Fenton reagents (represented by FeSO₄). In algae removal applications, pure iron-based catalyst particles are prone to agglomeration due to magnetic interactions, resulting in reduced active surface area and catalytic efficiency, which limits their practical application effects [9,57]. In contrast, iron-based composites and Fenton systems are more widely used in this field. To address the aforementioned issues of pure iron-based catalysts in algae removal, first, additives such as transition metals (manganese, copper) can be introduced to modify the electronic structure of the catalyst, promote Fe²⁺/Fe³⁺ cycling, and enhance catalytic efficiency [58,59]. Second, support materials with a large specific surface area and good electrical conductivity, such as carbon nanotubes and graphene, can be incorporated to inhibit catalyst particle agglomeration, thereby improving active surface area and electron transfer efficiency [55,60].

Wu et al. [61] synthesized Fe₃O₄/MWCNT composites via a sonochemical method (20 kHz probe or 40 kHz bath ultrasound) and elucidated the mechanism underlying their synergistic effect with ultrasound in the removal of Microcystis aeruginosa. Firstly, the mechanical vibration of 40 kHz ultrasound effectively depolymerizes Fe₃O₄ agglomerates, facilitating their uniform dispersion and anchoring on the surface of MWCNTs. Meanwhile, the high electrical conductivity of MWCNTs, in synergy with the ultrasonic cavitation effect, accelerates electron transfer, significantly enhancing the kinetic efficiency of the Fe²⁺/Fe³⁺ cycle and driving the ·OH radicals generated by cavitation to attack algae cells. On the basis of slightly increasing cell membrane permeability, this process remarkably strengthens coagulation and sedimentation by inducing moderate surface oxidation; a 94.4% algae cell removal rate can be achieved with merely 20 mg/L of catalyst combined with 20 s of ultrasonic irradiation. Further studies revealed that, in a 600 kHz high-frequency ultrasound system, 20 mg/L of Fe₃O₄/MWCNTs can synergize with the activation of 20 mg/L persulfate (PS), reducing the residual rate of algae cells to 9.4% within 30 min, which highlights the enhancement effect of high-frequency cavitation on oxidant activation efficiency. In addition, the catalyst retains over 90% of its activity after five cycles, and, compared with the sole ultrasound or pure Fe₃O₄ system, the combined strategy significantly reduces energy consumption and coagulant dosage, thus providing an efficient, stable, and economical solution for the control of harmful algae blooms.

In Fenton systems, Fe²⁺ (e.g., FeSO₄) as a classic catalyst can further enhance algae removal efficiency when synergized with ultrasound [9]. The local high-temperature and high-pressure environment generated by ultrasonic cavitation accelerates the reaction between Fe²⁺ and H₂O₂, promoting ·OH generation. Meanwhile, the mechanical effect of ultrasound creates pores on the algae cell surface, facilitating the penetration of free radicals into cells to cause oxidative damage. Notably, FeSO₄ assisted by ultrasound can also effectively control the release of microcystins. Unlike standalone Fenton treatment, which leads to increased toxin concentrations, in 20 kHz and 800 kHz sono-Fenton systems, free radicals can simultaneously degrade the released microcystins [21]. This indicates that the Fe²⁺-mediated sono-Fenton system not only achieves efficient algae removal but also reduces the risk of secondary pollution, offering a more comprehensive solution for eutrophic water treatment. However, this technology is plagued by issues such as high energy consumption, complex interference in actual water bodies, and high costs in large-scale production. Future efforts should focus on optimizing parameters and dosages and improving processes to enhance stability.

3.3. TiO₂-Based Catalytic Systems

TiO₂ catalysts can effectively reduce the dosage of subsequent algicide agents through free-radical-mediated pre-oxidation reactions [62,63] and thus have been widely applied in the fields of photocatalytic and sonocatalytic algae removal. However, visible-light-driven (VLD) photocatalysis relies on the band gap of semiconductors, and there are significant limitations in its visible light absorption efficiency and the light transmittance of algae solutions (with a penetration depth typically < 10 cm) [64,65,66]. In contrast, sonocatalysis utilizes ultrasonic cavitation effects, which enables it to operate efficiently in dark or highly turbid environments. With a sound wave penetration depth > 1 m, sonocatalysis is more suitable for practical water bodies [67,68]. Sonocatalysis combines the synergistic effects of physical fragmentation (shock waves destroying cell structures) and chemical oxidation (·OH attack), whereas photocatalysis is dominated by chemical oxidation [64]. For instance, the T-BaTiO₃/Ag₃PO₄ material constructed by [69], when synergized with 28 kHz ultrasound, achieved a chlorophyll a removal rate as high as 96.1% within 4 h, with excellent cyclic stability—the degradation rate remained > 90% after five uses. It is worth noting that single photocatalytic components often require carrier floating designs due to insufficient penetration [69], which further highlights the penetration advantage of sonocatalysis in engineering applications.

Ultrasound enhances TiO₂ catalysis via three pathways: (1) surface cavitation erodes passivation layers, exposing fresh active sites; (2) shockwaves promote mass transfer of reactants to catalyst interfaces; (3) localized heating accelerates charge separation, suppressing electron–hole recombination [37,54]. Studies have shown that 0.5 g/mL pure TiO₂ combined with 15 min of ultrasonic irradiation can reduce the survival rate of Microcystis aeruginosa to 0.13, much lower than 0.87 with ultrasound alone, and can inhibit its regrowth within 10 days [44]. However, excessive residual TiO₂ nanoparticles in water may pose potential ecological and health risks, making catalyst modification research increasingly important. Wang et al. [9] synthesized TiO₂-loaded BC nanocomposites and found that BC not only improved the dispersibility and stability of TiO₂ but also effectively enriched algae cells through its adsorption performance. This material achieved a 92% cell removal rate under 600 kHz ultrasonic irradiation for 90 s, with low dosage (5–50 mg/L) and high safety. Fan et al. [45] constructed a T-BTO/AP heterojunction catalyst, which combines piezoelectric effect and photocatalysis, showing stronger algae removal ability when synergized with ultrasound. The ultrasonic mechanical force induces a built-in electric field in T-BaTiO₃, converting the carrier transfer path from a traditional Type II to a Z-scheme heterojunction, thereby inhibiting electron–hole pair recombination. Under the synergy of 28 kHz ultrasound and visible light, the chlorophyll a removal rate reached 96.1% within 4 h, with excellent cycling stability—retaining over 90% degradation efficiency after five uses. Future research should focus on improving the practicality, economy, and environmental safety of this technology. Key directions include: immobilization and efficient recovery of catalysts, evaluation of adaptability and efficiency in complex real water bodies, and promotion of pilot-scale verification and engineering application research. However, this technology still faces challenges including difficulties in catalyst separation and recovery, as well as complexity in scalable system design. Future research should focus on achieving efficient catalyst immobilization and recovery, along with evaluating its adaptability and performance in complex real water matrices.

4. Ultrasound-Assisted Oxidant Catalysis for Algae Removal

In ensuring the safety of drinking water, the mechanical force and free radical oxidation effects produced by ultrasonic cavitation can influence the structure of EPS and cell activity of blue-green algae, thus achieving the removal of algae blooms. In addition, oxidants have also been introduced into the ultrasonic algae removal process. For instance, peracetic acid, Fenton’s reagent, hydrogen peroxide, etc. (

Table 2. The performances of ultrasound–oxidant collaboration for microalgae treatment.

Oxidant	Concentration	Free Radicals	Species	Density	Time (min)	Frequency	Intensity	Device	Efficiency (%)	Reference
PS	20 mg/L	SO4−·, ·OH	Microcystis aeruginosa	2.0 × 106 cells/mL	30		0.3 W/mL	20 kHz probe, 40 kHz cleaning bath, 600 kHz device	90.58	[63]
Peracetic acid	5–20 mg/L	·OH, ·CH3CO2, ·CH3CO3	Microcystis aeruginosa	2.2 × 106 cells/mL	20	20 kHz		A 20 kHz ultrasonic generator	Reduction of membrane fouling resistance by 76.26	[22]
Potassium permanganate (KMnO4)	5–30 mg/L	·OH, O2−	Microcystis aeruginosa	2.0 × 106 cells/mL	10	1000 kHz	0.12 W/mL, 0.39 W/mL	1000 kHz ultrasonic generator		[70]
O3	0.27 g O3/g	·OH	Cenedesmus sp., Chloroccum sp.	0.5 g/L		30 kHz	50 W	Ultrasonic processor equipped with an 8 cm long and 7 mm diameter ultrasonic probe		[71]
O3	27 mg O3/L	·OH	Scenedesmus obliquus			42 kHz	100 W	2.81 L ultrasonic bath		[72]
H2O2, Fe2+	H2O2: 1 mg/L; Fe2+: 1 mg/L	·OH	Microcystis aeruginosa	4.19 × 106 cells/mL	5	20 kHz	0.42 W/mL	Probe	89.26	[21]

) have been used in conjunction with ultrasound for algae inactivation, all yielding favorable results (Figure 2).

4.1. Potassium Permanganate

KMnO₄ is a relatively mild oxidant. Research has found that the combination of 1000 kHz moderate-intensity ultrasound with KMnO₄ can efficiently inactivate Microcystis aeruginosa, with cell damage but no increase in extracellular organics and toxins, making the filtrate safe for use [63]. It is not only easy to use but also produces fewer by-products compared to other oxidants, and its application in the pre-oxidation process of drinking water plants in China has become increasingly widespread. The MnO₂ produced during the KMnO₄ oxidation process has adsorption capabilities, which help in the coagulation and precipitation of macromolecular substances in the water. Moreover, KMnO₄ can inhibit the activity of algae cells, which is beneficial for subsequent process treatment. However, excessive dosing of KMnO₄ can damage algae cells, releasing a large amount of intracellular organics, affecting the coagulation process and increasing the potential for the formation of DBP. Therefore, when enhancing the pre-oxidation with KMnO₄ to improve algae removal, sufficient attention should also be given to the removal of algae-origin organic matters [73,74].

4.2. Persulfate

PS activation technology has become a research hotspot in the field of water treatment due to its ability to generate highly reactive radicals (SO₄⁻·, ·OH) and, especially when combined with ultrasound, it demonstrates significant synergistic potential. Previous studies have shown that the combination of ultrasound and PS has achieved good results in controlling various water pollutants. For example, Pandya et al. [75] achieved a COD removal rate of 39.5% within 60 min using ultrasound combined with PS, verifying the feasibility of this technology in complex industrial wastewater treatment; Tsenter et al. [76] studied the inactivation of bacteria using high-frequency dual-frequency ultrasound combined with ZnO and PS and found that the system required only 25 min to achieve 5-log inactivation of Escherichia coli; Yang et al. [77] treated methyl orange with ultrasound and PS combined, achieving a removal rate of 87.38% within 60 min, which is significantly higher than that of either ultrasound (16.7%) or PS (52.22%) alone. These studies provide new ideas for the treatment of algae water using ultrasound coupled with PS. The local high-temperature and high-pressure environment generated by ultrasound can accelerate the decomposition of PS and enhance the exposure of active sites on the catalyst surface, promoting radical generation. Wu et al. [63] synthesized Fe₃O₄/MWCNTs, which, under the assistance of 20 kHz ultrasound, synergistically acted with PS to reduce the remaining algae cell count to 9.4% within 30 min, with its high efficiency attributed to the synergistic effect of ultrasound cavitation and free radical oxidation. In addition, ultrasound can also damage the algae cell membrane, exposing internal organic matter for radical attack, and promote the dispersion of catalysts, forming a synergistic system of “oxidation–coagulation–filtration”. Despite the significant advantages of ultrasound/PS technology, its practical application still faces challenges. For example, during the activation of PS by pyrite, H₂PO₄⁻ may inhibit radical generation due to chelation of iron species; while the PMS/Fe²⁺ system can effectively control pollution in membrane treatment, the dosage of Fe²⁺ needs to be balanced to avoid secondary pollution [78]. Future research needs to further optimize the matching of ultrasound energy input and PS dosage and combine catalyst regeneration technology with the assessment of the impact of complex matrices in actual water bodies, which can promote the transformation of this technology from laboratory to engineering applications.

4.3. Ozone

The combined technology of ultrasound and O₃ has emerged as a critical approach to enhance the efficiency of algae removal in drinking water treatment due to the synergistic effects of physical disruption and chemical oxidation. The synergistic interaction is primarily manifested in the following ways: ultrasound cavitation generates high-energy shock waves that disrupt the cell walls and gas vesicles of cyanobacteria, enhancing the efficiency of O₃ contacting intracellular carbohydrates, phosphorus, and other substances. For instance, González-Balderas et al. [72] found that ultrasound pre-treatment led to a 99±3% release rate of intracellular proteins in microalgae cells and an increase in lipid recovery from 11% to 90 ± 2%, creating conditions for subsequent O₃ degradation. Additionally, ultrasound cavitation promotes the decomposition of O₃ to generate ·OH radicals with stronger oxidizing capabilities, accelerating the degradation of adhesive metabolites on the surface of algae cells and reducing the electrostatic repulsion between particles. Shokoohi et al. [79] confirmed through Taguchi experiments that the yield of ·OH in this system is 40% higher than that of single O₃ treatment, and under optimal conditions, the removal rate of chlorophyll a can reach 100%. Furthermore, O₃ first oxidizes to weaken the stability of the algae cell wall, and combined with the mechanical shearing force of ultrasound, it can further promote the release and degradation of IOM. Keris-Sen et al. [71] found that this combined treatment increased the recovery rates of microalgae lipids and carbohydrates to 59% and 81%, respectively, which is 2.7 times and 1.7 times higher than those achieved by single ozone treatment. In terms of process optimization, Wu et al. [80] increased the O₃ utilization rate from 32% to 61% through suction cavitation, achieving a 91% removal rate of cyanobacteria within 5 min, which is significantly better than the 35% achieved by single O₃ treatment; González-Balderas et al. [72] also confirmed that this combined process can reduce solvent usage by 95% and extraction time by 90%, demonstrating advantages in reducing energy consumption and costs. However, this technology still faces challenges such as the risk of intracellular algae toxin release (requiring control of ultrasound energy ≤ 200 kW h/kg), interference from the competitive reaction of natural organic matter, and optimization of the cavitation flow field in large-scale equipment. Its engineering application needs to focus on release kinetics and water quality adaptability, providing new pathways for ensuring the safety of drinking water.

4.4. Peroxyacetic Acid

Ultrasound can produce ROS (such as ·OH) through cavitation effects, while PAA decomposes to generate ·CH₃CO₂ radicals and theoretically form a synergistic oxidation effect. Previous studies have shown that ultrasound-assisted MnO₂ can efficiently catalyze the homolytic cleavage of PAA, producing a large number of active radicals within 60 min, accelerating the oxidation process [81]. Additionally, some studies have combined PAA with coagulation or ultraviolet (UV) light for algae removal, achieving good results, and PAA can avoid excessive cell lysis and reduce harmful by-products under appropriate conditions [82,83,84]. However, only one study on the use of ultrasound coupled with PAA for algae removal has been retrieved. Zhu et al. [22] conducted algae removal experiments under three systems: PAA, UV/PAA, and US/PAA. The experimental results showed that, compared to US/10 mg/L PAA treatment, the proportion of cell structure fragments after UV/10 mg/L PAA treatment increased significantly (90.8% vs. 24.3%), indicating that ultrasound is generally less effective than UV in activating reactive oxygen radicals. Moreover, the water quality after US/PAA treatment was significantly deteriorated, and when the PAA dose exceeded 10 mg/L, the intracellular organic matter (IOM) released by US/PAA caused irreversible fouling of ultrafiltration membranes. This suggests that ultrasound may stimulate the stress metabolism of algae cells, leading to the secretion of a large amount of organic matter [22]. From this perspective, the effect of ultrasound coupled with PAA for algae removal is suboptimal, and more research is needed to reveal the mechanism of the combined use of ultrasound and PAA for algae removal.

4.5. Hydrogen Peroxide and Fenton’s Reagent

In the control of algae blooms, the combined use of ultrasound and Fenton systems, leveraging the dual-frequency mechanism of low-frequency extracellular damage and high-frequency intracellular oxidation, can achieve efficient algae removal. Studies have shown that low-frequency high-intensity ultrasound can form pores in the cell walls of cyanobacteria through cavitation effects, causing extracellular oxidative damage, while high-frequency low-intensity ultrasound promotes the entry of Fenton reagents (H₂O₂ and Fe²⁺) into cells through endocytosis, triggering intracellular oxidative reactions. This dual-frequency mechanism significantly enhances the efficiency of algae removal. Low-frequency sono-Fenton treatment for 5 min can reduce the density of Microcystis aeruginosa from 4.19 × 10⁶ cells/mL to 0.45 × 10⁶ cells/mL (removal rate 89.26%), while the high-frequency system has a slightly lower removal rate (44.39%) but an energy efficiency (634.16) nearly twice that of the low-frequency system (212.52) [21]. Furthermore, the precise addition of Fenton reagents is key to optimizing this system. It has been found that, when the concentrations of H₂O₂ and Fe²⁺ are both 1 mg/L, low-frequency ultrasound pre-oxidation for 2 min can render blue-green algae cells almost invisible and simultaneously degrade MC-LR to below the WHO standard (1.0 µg/L), avoiding the risk of toxin leakage associated with traditional methods [21]. Compared to the selective algae removal technique combining hydrodynamic cavitation with trace amounts of H₂O₂, the sono-Fenton system has advantages in rapid inactivation (5 min) and toxin control, making it particularly suitable for emergency treatment scenarios. In addition, high-frequency ultrasound can achieve efficient inactivation with low concentrations of Fenton reagents by enhancing reagent delivery and intracellular oxidation, reducing chemical residues and the risk of secondary pollution, demonstrating the potential of green technology. Although significant results have been achieved with this system, its synergistic mechanism requires further investigation. For example, how does ultrasound cavitation regulate the generation rate of ·OH radicals in Fenton reactions? Does the endocytosis triggered by high-frequency ultrasound depend on specific cell structures? The answers to these scientific questions will provide theoretical support for the optimization of process parameters.

5. Ultrasound-Assisted Nanomaterial Catalysis for Algae Removal

In the treatment of eutrophic water bodies, the synergistic application of nanomaterials and physical technology offers a solution for algae pollution control that is both highly effective and environmentally friendly. In fact, the mechanical effects of acoustic cavitation include acoustic radiation force, shear force, microjets, and shock waves, among others, which can alter the structure of biological materials. However, the related contributions of different mechanical forces and their synergistic effects in practical applications are still in the research stage [85]. For example, the authors’ team previously conducted research on ultrasonic biological effects using low-power ultrasound combined with functional nanomaterials. The results showed that carbon-based materials (nanodiamonds) can significantly reduce the acoustic pressure threshold and induce the generation and collapse of acoustic cavitation bubbles, and the mechanical effects produced during this process are the main causes of changes in biological structures [48,86].

To further enhance the effects of acoustic cavitation, researchers have used heterogeneous materials to promote bubble nucleation, increasing the likelihood of nucleation and positioning the cavitation bubbles ideally to boost the cavitation effect. Currently, widely reported heterogeneous nucleation materials include lipid vesicles prepared by various strategies [87], phase-change materials [88], microfabricated materials with nanoscale patterns [89], inorganic materials (such as gold nanoparticles, carbon materials, etc.) [48,90], or colloidal nanoparticles [91,92]. Among these, inorganic nanoparticles have become the ideal nucleation materials in biological and environmental fields due to their ease of preparation and control and their ability to continuously and stably mediate cavitation under ultrasound [48,80,87,91].

The relative distance between the cavitation bubble and the elastic cell membrane has a significant impact on the physical damage to the biological membrane. Studies have shown that, under elastic boundary conditions, the intensity of cavitation bubble collapse is significantly weakened. It is confirmed that the distance between the cavitation bubble and the elastic material significantly affects the kinetic behavior of bubble collapse, the response characteristics of elastic boundaries, and the degree of material damage [93,94]. Other studies have shown that the distance between the bubble and the wall directly affects the jet strength [95]. Ding et al. [96] pointed out that shock wave pressure decreases rapidly with increasing propagation distance, and a moderate distance is more conducive to using axial jets to damage elastic boundaries. Studies have shown that the collapse behavior of cavitation bubbles near the interface of elastic materials is an important direction for promoting related research, but current studies have not yet thoroughly explored the specific mechanisms of the interaction between cavitation bubbles and microbial biofilms. In addition, the combination of the lattice Boltzmann method and numerical modeling methods has developed into an efficient tool for simulating the growth [96] and collapse [97] of cavitation bubbles, aiding in the in-depth study of the interaction mechanisms between cavitation bubbles and elastic interfaces.

For algae removal, ultrasound cavitation damages the cell walls of algae, prompting the release of intracellular substances, and the rich nanoscale pores and large specific surface area of the nanomaterials provide an ideal carrier for the conduction of ultrasound energy and the adsorption of pollutants (Figure 3). Furthermore, the porous structure can physically adsorb AOM released by cells after ultrasound treatment, causing the UV254 value (indicating the content of aromatic organic matter) of the water treated by ultrasound to decrease by 15% compared to that treated by ultrasound alone, thus inhibiting the accumulation of precursor substances for DBP [98,99,100].

6. Conclusions

Ultrasound-assisted algae removal technology has emerged as an environmentally benign water treatment strategy, demonstrating significant research advances and application potential in environmental catalysis. This review systematically synthesizes recent progress in synergistic systems integrating ultrasonic cavitation with chemical catalysts (catalysts, oxidants, nanomaterials) for controlling harmful algae blooms. We comprehensively investigate molecular mechanisms underlying ultrasound–algae cell interactions and critically analyze cooperative efficiency, advantages, and limitations of ultrasound coupled with carbon-based, iron-based, and TiO₂-based catalysts; oxidants including KMnO₄, PS, and O₃; as well as functional nanomaterials.

The current research landscape and future perspectives are summarized as follows:

(1) Technological Mechanism Innovation: The ultrasound–catalysis synergy leverages a multi-level physico-chemical cooperative mechanism driven by cavitation effects, integrating cavitation-bubble-induced physical disruption and radical oxidation to efficiently inactivate algae cells while synchronously degrading extracellular organic matter. Sonoporation-enhanced membrane permeability facilitates targeted oxidant/catalyst delivery, endowing the technology with dual high-efficiency and controllability. This approach selectively damages algae structures while suppressing massive release of intracellular toxins, overcoming core limitations of conventional chemical pre-oxidation—notably high by-product risks and low radical utilization efficiency due to “over-oxidation”—thus demonstrating significant potential for algae bloom control.

(2) Interfacial Synergy in Sonocatalysis: Ultrasound–catalysis systems break traditional technical barriers through interfacial regulation and cooperative enhancement:

Carbon-based catalysts enhance radical targeting via surface charge modulation;

Iron composites utilize carriers to inhibit aggregation and improve electron transfer efficiency;

TiO₂ heterojunctions optimize carrier migration paths through piezoelectric effects;

Oxidants (e.g., KMnO₄/PS) coupled with ultrasound achieve adsorption–oxidation coupling and radical chain reactions;

Nanomaterials synergize cavitation nucleation with porous adsorption to form a closed “rupture–oxidation–adsorption” loop, reducing DBP precursors.

(3) These mechanisms establish a theoretical foundation for efficient, controllable, low-risk algae control technologies, although catalyst recyclability and scalable recovery remain pivotal engineering challenges.

Future Perspectives:

(1) Mechanistic Deepening: Future studies should elucidate species-specific algae responses to ultrasound and associated biological mechanisms. Concurrently, reactive species generation/transformation pathways in ultrasound–oxidant systems should be investigated to guide technical optimization.

(2) Advanced Material Development: Prioritize multi-functional hybrid catalysts (C/Fe/Ti-based) with synergistic gain and anti-aggregation stability. Develop biodegradable nanocarrier-loaded eco-friendly oxidants for targeted delivery to minimize secondary contamination. Establish structure–activity relationships between nanomaterial surface properties and cavitation efficiency, unveil electron-transfer pathways at ultrasound–catalyst interfaces, and construct acoustic/optical/electrical multi-field coupling models for precision-controlled moderate oxidation.

(3) Environmental Impact and Safety Assessment: Enhance evaluation of ecological impacts, focusing on transformation by-products generated during ultrasound treatment and their ecosystem effects. Optimize operational parameters and process design to prevent water quality deterioration while ensuring high algae removal efficiency.

(4) Engineering Application Advancement: Accelerate real-water validation and economic feasibility studies. Foster cross-sector collaboration to promote global implementation of ultrasound-based algae control, enabling sustainable water resource management.