Sonication Improves Fenton Reactions

Fenton reactions are based on the generation of free radicals such as hydroxyl •OH radical and hydrogen peroxide (H₂O₂). The Fenton reaction can be significantly intensified when combined with ultrasonication. The simple, but highly efficacious combination of Fenton reaction with power ultrasound has been shown to drastically improve the desired radical formation and thereby process intensifying effects.

How Does Power Ultrasound Improve Fenton Reactions?

When high-power / high-performance ultrasonication is coupled into liquids such as water, the phenomenon of acoustic cavitation can be observed. In the cavitational hot-spot, minute vacuum bubbles arise, and grow over several high-pressure / low-pressure cycles caused by the power ultrasound waves. At the point, when the vacuum bubble cannot absorb more energy, the void collapses violently during a high-pressure (compression) cycle. This bubble implosion generates extraordinarily extreme conditions where temperatures as high as 5000 K, pressures as high as 100 MPa, and very high temperature and pressure differentials occur. The bursting cavitation bubbles also generate high-speed liquid microjets with very intense shear forces (sonomechanical effects) as well as free radical species such as OH radicals due to hydrolysis of water (sonochemical effect). The sonochemical effect of free radical formation are the major contributor for ultrasonically intensified Fenton reactions, whilst the sonomechanical effects of agitation improve mass transfer, which improves chemical conversion rates.
(The picture left shows acoustic cavitation generated at the sonotrode of the ultrasonicator UIP1000hd. Red light from the bottom is used for improved visibility)

Exemplary Case Studies for Sonchemically Enhanced Fenton Reactions

The positive effects of power ultrasound on Fenton reactions has been widely studied in research, pilot and industrial settings for various applications such as chemical degradation, decontamination and decomposition. The Fenton and sono-Fenton reaction is based on the hydrogen peroxide decomposition using an iron catalyst, which results in the formation of highly reactive hydroxyl radicals.
Free radicals such as hydroxyl (•OH) radicals are often purposely generated in processes to intensify oxidation reactions, e.g., to degrade pollutants such as organic compounds in wastewater. Since power ultrasound is an auxiliary source of free radical formation in Fenton type reactions, sonication in combination with Fenton reactions enhanced pollutant degradation rates in order to degrade pollutants, hazardous compounds as well as cellulose materials. This means that an ultrasonically intensified Fenton reaction, the so-called sono-Fenton reaction, can improve the hydroxyl radical production making the Fenton reaction significantly more efficient.

Sonocatalytic–Fenton Reaction for Enhances OH Radical Generation

Ninomiya et al. (2013) demonstrate successfully that a sonocatalytically enhanced Fenton reaction – using ultrasonication in combination with titanium dioxide (TiO2) as catalyst – exhibits a significantly enhanced hydroxyl (•OH) radical generation. The application of high-performance ultrasound allowed to initiate an advanced oxidation process (AOP). Whilst sonocatalytic reaction using TiO2 particles have been applied to the degradation of various chemicals, the research team of Ninomiya used the efficiently generated •OH radicals to degrade lignin (a complex organic polymer in cell walls of plant) as a pretreatment of lignocellulosic material for a facilitated subsequent enzymatic hydrolysis.
The results show that a sonocatalytic Fenton reaction using TiO2 as sonocatalyst,enhances not only the degradation of lignin but also is an efficient pretreatment of lignocellulosic biomass in order to enhance the subsequent enzymatic saccharification.
Procedure: For the sonocatalytic–Fenton reaction, both TiO2 particles (2 g/L) and Fenton reagent (i.e., H2O2 (100 mM) and FeSO4·7H2O (1 mM)) were added to the sample solution or suspension. For the sonocatalytic–Fenton reaction, the sample suspension in the reaction vessel was sonicated for 180 min with the probe-type ultrasonic processor UP200S (200W, 24kHz) with sonotrode S14 at an ultrasound power of 35 W. The reaction vessel was placed in a water bath maintaining a temperature 25°C using a cooling circulator. The ultrasonication was performed in the dark in order to avoid any light-induced effects.
Effect: This synergistic enhancement of OH radical generation during the sonocatalytic Fenton reaction is attributed to the Fe3+ formed by Fenton reaction being regenerated to Fe2+ induced by the reaction coupling with the sonocatalytic reaction.
Results: For the sono-catalytic Fenton reaction, the DHBA concentration was enhanced synergistically to 378 μM, whilst the Fenton reaction without ultrasound and TiO2 only achieved DHBA concentration of 115 μM. The lignin degradation of kenaf biomass under Fenton reaction achieved only a lignin degradation ratio, which increased linearly up to 120 min with kD = 0.26 min−1, reaching 49.9% at 180 min.; whilst with sonocatalytic–Fenton reaction, the lignin degradation ratio increased linearly up to 60 min with kD = 0.57 min−1, reaching 60.0% at 180 min.

Scanning electron micrographs (SEM) of kenaf biomass (A) untreated control, pretreated with (B) sonocatalytic (US/TiO2), (C) Fenton (H2O2/Fe2+), and (D) sonocatalytic–Fenton (US/TiO2 + H2O2/Fe2+) reactions. Pretreatment time was 360 min. Bars represent 10 μm.
(Picture and study: ©Ninomiya et al., 2013)

Naphtalene Degradation via Sonochemical Fenton

the highest percentage of naphthalene degradation was achieved at the intersection of the highest (600 mg L-1 hydrogen peroxide concentration) and the lowest (200 mg kg1 naphthalene concentration) levels of both factors for all ultrasound irradiation intensities applied. It resulted in 78%, 94%, and 97% of naphthalene degradation efficiency when sonication at 100, 200, and 400 W, respectively, was applied. In their comparative study, the researchers used the Hielscher ultrasonicators UP100H, UP200St, and UP400St. The significant increase in the degradation efficiency was attributed to the synergism of both oxidizing sources (ultrasonication and hydrogen peroxide) which translated into the increased surface area of Fe oxides by applied ultrasound and the more efficient production of radicals. The optimum values (600 mg L−1 of hydrogen peroxide and 200 mg kg1 of naphthalene concentrations at 200 and 400 W) indicated up to a maximum 97% reduction in naphthalene concentration in soil after 2 h of treatment.
(cf. Virkutyte et al., 2009)

SEM–EDS microgram of a) elemental mapping and b) soil prior and c) after ultrasound irradiation treatment
(Picture and study: ©Virkutyte et al., 2009)

Sonochemical Carbon Disulfide Degradation

Adewuyi and Appaw demonstrated the successful oxidation of carbon disulfide (CS2) of in a sonochemical batch reactor under sonication at a frequency of 20 kHz and 20°C. The removal of CS2 from the aqueous solution significantly increased with an increase in ultrasound intensity. Higher intensity resulted in an increase in the acoustic amplitude, which results in an intenser cavitation. The sonochemical oxidation of CS2 to sulfate proceeds mainly through oxidation by the •OH radical and H2O2 produced from its recombination reactions. In addition, the low EA values (lower than 42 kJ/mol) in both the low- and high-temperature range in this study suggest that diffusion-controlled transport processes dictate the overall reaction. During ultrasonic cavitation, the decomposition of water vapour present in the cavities to produce H• and •OH radicals during the compression phase has been already well studied. The •OH radical is a powerful and efficient chemical oxidant in both the gas and liquid phase, and its reactions with inorganic and organic substrates are often near the diffusion-controlled rate. The sonolysis of water to produce H2O2 and hydrogen gas via hydroxyl radicals and hydrogen atoms is well-known and occurs in the presence of any gas, O2, or pure gases (e.g., Ar). The results suggest that the availability and the relative rates of diffusion of free radicals (e.g., •OH) to the interfacial reaction zone determine the rate-limiting step and the overall order of the reaction. Overall, sonochemical enhanced oxidative degradation is an effective method for carbon disulfide removal.
(Adewuyi and Appaw, 2002)

Ultrasonic Fenton-like Dye Degradation

The effluents from industries that use dyes in their production are an environmental problem, which is requires an efficient process in order to remediate the waste water. Oxidative Fenton reactions are widely used for treating dye effluents, whilst improved Sono-Fenton processes are getting increasingly attention due to its enhanced efficiency and its environmental-friendliness.

Sono-Fenton Reaction for Degradation of Reactive Red 120 Dye

The degradation of Reactive Red 120 dye (RR-120) in synthetic waters was studied. Two processes were considered: homogeneous Sono-Fenton with iron (II) sulfate and heterogeneous Sono-Fenton with synthetic goethite and goethite deposited onto silica and calcite sand (modified catalysts GS (goethite deposited onto silica sand) and GC (goethite deposited onto calcite sand), respectively). In 60 min of reaction, the homogeneous Sono-Fenton process allowed a degradation of 98.10 %, in contrast with 96.07 % for the heterogeneous Sono-Fenton process with goethite at pH 3.0. The removal of RR-120 increased when the modified catalysts were used instead of bare goethite. Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) measurements showed that the highest TOC and COD removals were achieved with the homogeneous Sono-Fenton process. Biochemical Oxygen Demand (BOD) measurements allowed to find that the highest value of BOD/COD was achieved with a heterogeneous Sono-Fenton process (0.88±0.04 with the modified catalyst GC), demonstrating that the biodegradability of the residual organic compounds was remarkably improved.
(cf. Garófalo-Villalta et al. 2020)
The picture left shows the ultrasonicator UP100H used in the experiments for red dye degradation via sono-Fenton reaction.(Study and picture: ©Garófalo-Villalta et al., 2020.)

Heterogeneous Sono-Fenton degradation of azo dye RO107

Jaafarzadeh et al. (2018) demonstrated the successful removal of azo dye Reactive Orange 107 (RO107) via sono-Fenton like degradation process using magnetite (Fe3O4) nanoparticles (MNP) as catalyst. In their study, they used the Hielscher UP400S ultrasonicator equipped with 7mm sonotrode at 50% duty cycle (1 s on/1 s off) to generate acoustic cavitation in order to obtain the desired radical formation. The magnetite nanoparticles function as peroxidase-like catalyst, therefore an increase in the catalyst dosage provides more active iron sites, which in turn accelerates the decomposition of H2O2 leading to the production of reactive OH•.
Results: Complete removal of azo dye was obtained at 0.8 g/L MPNs, pH = 5, 10 mM H2O2 concentration, 300 W/L ultrasonic power and 25 min reaction time. This ultrasonic Sono-Fenton like reaction system was also evaluated for real textile wastewater. The results showed that chemical oxygen demand (COD) was reduced from 2360 mg/L to 489.5 mg/L during a 180 min reaction time. Moreover, cost analysis was also conducted on the US/Fe3O4/H2O2. Finally, ultrasonic/Fe3O4/H2O2 showed high efficiency in decolorization and treatment of coloured wastewater.
An increase in ultrasonic power led to an enhancement in reactivity and surface area of magnetite nanoparticles, which facilitated the transformation rate of `Fe3+ to `Fe2+. The as-generated `Fe2+ catalyzed a H2O2 reaction in order to produce hydroxyl radicals. As a result, the increase of ultrasonic power was shown to enhance the performance of US/MNPs/H2O2 process by accelerating the decolorization rate within a short period of contact time.
The authors of the study note that ultrasonic power is one of the most essential factors influencing on the degradation rate of RO107 dye in the heterogeneous Fenton-like system.
Learn more about highly efficient magnetite synthesis using sonication!
(cf. Jaafarzadeh et al., 2018)

RO107 degradation in different combinations at pH of 5, MNPs dosage of 0.8 g/L, H2O2 concentration of 10 mM, RO107 concentration of 50 mg/L, ultrasonic power of 300 W and reaction time of 30 min.
Study and picture: ©Jaafarzadeh et al., 2018.

HEAVY-DUTY ULTRASONICATORS

Hielscher Ultrasonics designs, manufactures and distributes high-performance ultrasonic processors and reactors for heavy-duty applications such as advanced oxidative processes (AOP), Fenton reaction, as well as other sonochemical, sono-photo-chemical, and sono-electro-chemical reactions. Ultrasonicators, ultrasonic probes (sonotrodes), flow cells and reactors are available at any size – from compact laboratory test equipment to large-scale sonochemical reactors. Hielscher ultrasonicators are available a numerous power classes from laboratory and bench-top devices to industrial systems capable to process several tons per hour.

Precise Amplitude Control

The amplitude is one of the most important process parameter influencing the results of any ultrasonic process. Precise adjustment of the ultrasonic amplitude allows to operate Hielscher ultrasonicators at low to very high amplitudes and to fine-tune the amplitude exactly to the required ultrasonic process conditions of applications such as dispersion, extraction and sonochemistry.
Choosing the right sonotrode size and using optionally a booster horn for and additional increase or decrease of the amplitude allows to setup an ideal ultrasonic system for a specific application. Using a probe / sonotrode with a larger front surface area will dissipate the ultrasonic energy over a large area and a lower amplitude, whilst a sonotrode with smaller front surface area can create higher amplitudes creating a more focused cavitational hot spot.

Hielscher Ultrasonics manufactures high-performance ultrasonic systems of very high robustness and capable to deliver intense ultrasound waves in heavy-duty applications under demanding conditions. All ultrasonic processors are built to deliver full power in 24/7 operation. Special sonotrodes allow for sonication processes in high-temperature environments.

The table below gives you an indication of the approximate processing capacity of our ultrasonicators: