Dual-frequency ultrasound (DFUS) coupled with sonocatalysts has emerged to be an advanced tool for antimicrobial applications in medicine but remains scarcely studied for water disinfection. In the present work, we first integrated high-frequency DFUS (120 + 1700 kHz), persulfate (S₂O₈²⁻) and ZnO nano- (50 nm) and microparticles (1 μm) for eradicating Escherichia coli and Enterococcus faecalis in synthetic water. For E. coli, the efficiency of DFUS-based processes can be ranked as follows: DFUS < DFUS/ZnO < DFUS/S₂O₈²⁻ < DFUS/ZnO/S₂O₈²⁻. A similar efficiency of the DFUS/S₂O₈²⁻ and DFUS/ZnO/S₂O₈²⁻ processes was found for more resistant E. faecalis. In the absence of persulfate, the performance of 1 μm ZnO was higher than that observed with 50 nm for inactivating E. coli via the DFUS/ZnO and 1700 kHz/ZnO processes. A synergy of DFUS in terms of 5-log (total) reduction was found in the S₂O₈²⁻/ZnO-based systems, being higher for E. faecalis (synergistic coefficient = 1.8–3.0). The synergistic effect was proposed to be driven by the boosted generation of reactive oxygen species and sonoporation. This study opens prospects for the development of novel DFUS-based piezo-catalytic systems for efficient water disinfection.

Dual-frequency ultrasound (DFUS) has received considerable attention for enhanced inactivation of microbial pathogens for medical treatment, but remains little investigated for water disinfection. This study is focused on inactivation of E. coli and E. faecalis in aqueous solution under dual-frequency ultrasonication at 120 + 1700 kHz using persulfate. Single-frequency ultrasonic inactivation showed the higher efficiency of 1700 kHz, compared to 120 kHz. Under the experimental conditions used, no measurable synergy between two frequencies was observed in the absence of persulfate. A high time-based synergistic effect in terms of total inactivation (5-log) of both bacterial species was achieved by DFUS-activated persulfate with synergistic indices of 1.8–5.0. We assume that this is attributed to increased generation of reactive oxygen species (primarily, sulfate anion (SO₄•⁻) and hydroxyl (•OH) radicals) as a result of enhanced acoustic cavitation. Radical probing and scavenging tests confirmed the generation of radicals and showed a nearly equal contribution of •OH and SO₄•⁻. This method could be an attractive alternative to ultraviolet technology for fast and effective water disinfection.

This study is the first to report the high performance of calcined natural sphalerite as a heterogeneous catalyst (Catalyst) in the piezo- and photocatalytic degradation of pharmaceuticals (bezafibrate and ceftriaxone) using high-frequency ultrasound (US, 1.7 MHz) and ultraviolet-light-emitting diodes (LED, 365 nm). The kinetic comparison showed that piezo-photocatalysis (LED + US + Catalyst) was more efficient than photocatalysis (LED + Catalyst) for degrading both contaminants in deionized water as well as in surface river water at natural pH (7.9). Despite reducing degradation rates (~1.7 times) in river water due to the scavenging effect of its constituents, ceftriaxone and bezafibrate were degraded by 77% and 48% after 1 h of exposure, respectively. Adding H₂O₂ increased the corresponding pseudo-first-order rate constants, and the complete degradation of ceftriaxone was achieved. However, the contribution of ultrasound at a given intensity was hidden, which resulted in a similar performance of piezo-photocatalysis and photocatalysis for treating river water. No pronounced synergy between the piezo- and photocatalytic processes was observed in the experimental conditions used. Nevertheless, the H₂O₂-assisted piezo-photocatalysis using high-frequency US, LED, and natural catalysts can be considered a novel and effective strategy for eliminating pharmaceuticals from real water without pH adjustment.

In present work, we have studied the kinetic fundamentals of sonophotochemical oxidation of emerging organic contaminants, atrazine and bisphenol A, in model aqueous solutions, simultaneously exposed to high-frequency ultrasound (US, 1.7 MHz) and ultraviolet light-emitting diodes (UV LEDs, 365 nm) in the absence and presence of persulfate (S2O82-) oxidant. Synergistic indices were calculated to assess a synergistic effect in the hybrid oxidation systems. It was found that the hybrid system {UV/US/S2O82-} exhibited the synergistic effect and was the most efficient for degrading bisphenol A in a raw: UV/US/S2O82- > UV/S2O82- > US/UV > US/S2O82- > > UV > US. In case of atrazine, no ultrasound effect was observed and the efficiencies of {UV/US/S2O82-} and {UV S2O82-} systems in terms of degradation rates were similar. In these oxidation systems, more than 90% of a contaminant was removed after 30 min treatment. Meanwhile, degradation rates for atrazine were higher than those wich were found for bisphenol A. This indicates a predomination of sulfate anion radicals, which react with bisphenol A rather slowly compared to atrazine. A synergism was also found under sonophotolysis of both contaminants without persulfate; however, this process requires a more prolonged irradiation time (similar to 20% degraded in 40 min), hence, it is less energy-effective. The obtained results are promising for application of UV LEDs and high-frequency ultrasound in persulfate-based advanced oxidation processes to degrade organic contaminants in natural water and wastewater.

Naturally occurring sono- and photoactive minerals, which are abundant on Earth, represent an attractive alternative to the synthesized sonophotocatalysts as cost-effective materials for water and wastewater treatment. This study focuses on characterizing and evaluating the sonophotocatalytic activity of natural sphalerite (NatS) from Dovatka deposit (Siberia) under high-frequency ultrasonic (US, 1.7 MHz) and ultraviolet light-emitting diodes (UVA LED, 365 nm) irradiation towards degradation of 4-chlorophenol as a model organic pollutant. Since raw natural sphalerite did not exhibit a measurable photocatalytic activity, it was calcined at 500, 900 and 1200 °C. The natural sphalerite after calcination at 900 °C (NatS*) was found to be the most effective for sonophotocatalytic degradation of 4-chlorophenol, attaining the highest efficiency (55%, 1 h exposure) in the following row: UV < US ≈ UV/US ≈ US/NatS* < UV/NatS* < UV/US/NatS*. Addition of 1 mM H₂O₂ increased the removal to 74% by UV/US/NatS*/H₂O₂ process. An additive effect between UV/NatS* and US/NatS* processes was observed in the sonophotocatalytic system as well as in the H₂O₂-assisted system. We assume that the sonophotocatalytic hybrid process, which is based on the simultaneous use of high-frequency ultrasound, UVA light, calcined natural sphalerite and H₂O₂, could provide a basis of an environmentally safe and cost-effective method of elimination of organic pollutants from aqueous media.

The efficiency of solar-induced destruction of herbicide atrazine in natural water and aqueous extract from agrogenic and unbroken soil by using photo-Fenton system was studied in this work. 80-90 % atrazine was removed in all water matrices. Using xenon lamp as a solar simulator, the rate of atrazine destruction was found to be higher in aqueous soil extracts than that observed in natural water. However, no significant difference was found between the destruction dynamics in natural water and soil extracts, exposed to natural solar light. This could be attributed to the higher hydroxyl radical exposure under the higher UV irradiance of the Sun.