Hydrodynamic cavitation-enhanced activation of sodium percarbonate for estrogen removal

Estrogens such as naturally occurring estrone (E1), 17β-estradiol (E2), and estriol (E3), as well as the synthetic 17α-ethinylestradiol (EE2), are contaminants that are causing particular environmental concern (Kuster et al. 2004, 2008; Meffe & de Bustamante 2014; Barreiros et al. 2016). They represent one group of so-called endocrine-disrupting chemicals, compounds altering the function of the endocrine system, and consequently have adverse health effects on intact organisms even at ng/L levels (Lafont & Mathieu 2007; Pessoa et al. 2014; Adeel et al. 2017; Vilela et al. 2018). Therefore, the European Decision E.U. 2015/495 included E1, E2, and EE2 in the ‘watch-list’ of the Water Framework Directive (E.U.). Estrogens enter the environment through insufficient wastewater treatment and animal waste disposal (Sarmah et al. 2006; Atkinson et al. 2012; Tiedeken et al. 2017). The typical concentrations of these substances in wastewater treatment plant effluents and surface water can reach up to tens ng/L (Meffe & de Bustamante 2014; Adeel et al. 2017), but occasionally they can exceed 500 ng/L (Tiedeken et al. 2017). As the conventional methods of estrogen removal fail, other processes, including sorption, membrane filtration, biodegradation, photolysis, and advanced oxidation processes (AOPs), have been investigated (Rosenfeldt et al. 2007; Pereira et al. 2012; Silva et al. 2012).

Recently, the AOPs have been used in advanced water treatment processes as an efficient, rapid, and simple technology. During AOPs, oxygen radicals such as hydroxyl radicals (^·OH) (Miklos et al. 2018) or sulfate radicals (Wen et al. 2022) are formed, as highly reactive species that interact usually unselectively with target molecules, in quantity sufficient to oxidize the majority of organic pollution present. These technologies involve different activation methods and can potentially utilize several diverse mechanisms. The AOPs could be broadly classified into chemical (e.g. O₃, H₂O₂ and persulfate), photolytic (e.g. ultra violet (UV)/oxidant or UV/photocatalyst), electrochemical (e.g. TiO₂ electrodes), catalytic (e.g. Fenton process), and physical (e.g. ultrasound, plasma discharge, microwave) or combinations of them. Each of these particular techniques has its benefits and drawbacks, including the formation of toxic transformation products, which can have similar or increased toxicity relative to the parent compounds (Tufail et al. 2021). Thus, to overcome the drawbacks or increase the efficiency of pollution degradation, the use of combinations of different mechanisms is advantageous (Silva et al. 2012; Tufail et al. 2020). In addition to effectiveness, the operational costs, sustainability, and general feasibility are definitely of great importance.

Sodium percarbonate (SPC; Na₂CO₃·1.5 H₂O₂), as a solid carrier of H₂O₂, acts similarly and has several advantages compared to the liquid phase. The main benefits are its stability and safe handling, its environmentally nature with no toxic residues, effectiveness across a wide pH range, and its relatively low cost (Ma et al. 2020; Zhang et al. 2021). In addition, SPC decomposition products (⁠ and ⁠) can act as a buffer for an undesired pH decrease. Although SPC can directly decompose various pollutants, catalyst or energy activation enhances the degradation efficiency. In an activated SPC system, hydroxyl, superoxide, and carbonate radicals are generated (Zhang et al. 2021). In order to activate the SPC, various chemicals such as transition metal ions, ozone, and FeOCl (Miao et al. 2015; Fu et al. 2016, 2017; Guo et al. 2021) and physical approaches such as discharge plasma (Wang et al. 2018; Tang et al. 2019), UV light (Sindelar et al. 2014), and microwaves (Zuo et al. 2020) were tried. The combination of chemical and physical methods seems to be a reasonably promising and effective method of degrading pollutants.

Hydrodynamic cavitation (HC) belongs to the up-and-coming physical methods of AOPs (Zupanc et al. 2013; Dular et al. 2016; Marsalek et al. 2020; Panda et al. 2020). HC occurs when water is forced through a tapering device (Venturi tube, orifice plate, or convergent-divergent nozzle). The gaseous microbubbles are formed after liquid passing through the constriction because of a drop in local pressure. Subsequently, these cavities collapse violently downstream due to the pressure recovery. This phenomenon is accompanied by an increase in local temperature, causing water molecules to dissociate and form reactive oxygen species (Zheng et al. 2022). Combining HC with other AOPs is quite a novel approach and offers several advantages, such as higher degradation efficiencies, cost effectiveness, and a reduction in chemicals utilized which means lower environmental impact (Wang et al. 2021). The combination of HC and SPC has been proven to degrade lignocellulose in corn stovers (Nakashima et al. 2016). Most recently, the HC was used as an activator in a hybrid system of SPC and ozone to remove 1,4-dioxane (Fedorov et al. 2023).

This paper studies the degradation of estrogens using SPC activated by HC. We hypothesized that HC could promote the dissolution rate and activate the SPC without any further activation by heating or adding chemicals. Such a process should be more economically convenient and environmentally acceptable. The work focuses on optimizing the new treatment technology for removing organic micropollutants in treated wastewater, especially on the effects of SPC dosage, the number of cavitation cycles (time), and the temperature.

Standard compounds of estrone (≥99%), 17β-estradiol (≥98%), estriol (≥98%), and 17α-ethinylestradiol (≥98%), as well as all solvents and reagents, were purchased from Sigma-Aldrich (St. Louis, MO, USA). Internal standards 17β-estradiol-d₄ and 17α-ethinylestradiol-d₄ were acquired from C/D/N Isotopes Inc. (Pointe-Claire, Quebec, Canada). All standards were prepared in HPLC-grade methanol. Methanol, acetonitrile, and acetone (HPLC grade), formic acid, hydrochloric acid, SPC (sesquihydrate, available in H₂O₂ 20–30%), titanium (IV) oxysulfate in a sulfuric acid solution, dansyl chloride, and sodium bicarbonate were used.

The tank was filled with 2 L of the aqueous solution containing 300 ng/L of each estrogen. The SPC in concentrations of 80 and 200 mg/L was added. Neither the ionic strength nor the pH of the solutions was adjusted to minimize operating steps, so that the procedure remained economical and suitable for practical implementation. The pH value, temperature, and conductivity were measured by the Combo pH/EC meter (Hanna, HI 98129). After collection, the samples were left to stand for 3 and 24 h before the LC-MS/MS analysis.

Prior to analysis, the estrogens were extracted from water samples using SPE Oasis^® HLB (500 mg, 6 mL) cartridges (Waters, Milford, MA, USA) according to Sadilek et al. (2016) with slight changes (for more details, see Supplementary Material, p. S2).

In this study, the Agilent 1260 Infinity high-performance liquid chromatography system (Agilent Technologies, CA, USA) combined with the Agilent 6460 TripleQuad mass spectrometer (Agilent Technologies, CA, USA) equipped with electrospray ionization interface were used for estrogen quantification. The separation was achieved using Poroshell 120 EC-C18 (2.1 × 100 mm; 2.7 μm) fitted with a security guard column of the same packing material (Agilent Technologies, CA, USA). The mobile phases A and B consisted of formic acid (7 mM) and acetonitrile, respectively. A gradient raising the acetonitrile from 50 to 100% in 10 min, then held for 1 min, was used. The flow rate was 350 μL/min, and the injection volume was 10 μL. Mass data were acquired using the MassHunter Workstation software (Agilent Technologies, CA, USA) with multiple reaction monitoring (MRM) in the positive mode. The detector settings were capillary voltage 3,500 V, nozzle voltage 2,000 V, gas temperature (N₂) 200°C, gas flow 10 mL/min, nebulizer 50 psi, sheath gas temperature (N₂) 350°C, and sheath gas flow 10 mL/min. MRM transitions of protonated dansyl-derivatives to 171 and 156 were used for the quantification and qualification of the estrogens.

The colorimetric method previously described was used to determine the content of hydrogen peroxide (Eisenberg 1943). This method is based on a specific reaction between hydrogen peroxide and titanyl ions, resulting in a yellow complex of pertitanic acid with absorption maxima at 407 nm. The sample was mixed with the titanium reagent in a ratio of 2:1 (sample:reagent, v/v), and the absorbance was measured using a Spark™ multimode microplate reader (Tecan, Austria).

Statistical analyses were performed using GraphPad Prism 7.04 (GraphPad Software Inc., CA, USA). A t-test was used to determine the differences between individual groups of samples. A probability level of p< 0.05 was considered statistically significant. All data are represented as mean ± SD of relative concentrations (related to the initial concentration).

It is evident from Figure 2 that a higher SPC concentration increased the estrogen degradation rate, especially after a longer period of cavitation. With increasing cavitation time from 4 to 20 s, the estrogen degradation went from 31 to 62% once 80 mg/L of SPC was added. Furthermore, the degradation increased from 47 to 90% when 200 mg/L of SPC was used. The observation was supported by comparing degradation rate constants (see Table 1 and Figure S1). The increase in the amount of SPC more than doubled the corresponding kinetic constants.

As SPC was used for estrogen removal for the first time in this study, we compared our results with the corresponding dose of liquid H₂O₂ used by other authors. For example, Perondi et al. (2020) employed a combination of H₂O₂ (15 mg/L) and UV to remove E2 from surface water, and they obtained constants that were 100 times lower than those reached in our experiments. In that case, 60 min of treatment was needed to remove 89% of E2. In another study, Sun et al. (2019) combined H₂O₂ (0.7 mg/L) with 2.5 mg of O₃ to remove E1, E2, and EE2. They generated ozone from high-purity oxygen and reached the estrogen removal efficiency of 89.7, 84.5 and 90.5% for E1, E2, and EE2 after 10 min treatment with reaction rate constants of 0.18, 0.22, and 0.24 min⁻¹, respectively. Our experiments achieved 90% removal using 200 mg/L of SPC (equal to 50 mg/L of H₂O₂) in 20 s treatment without any ozone needed. Other studies focusing on eliminating estrogens using H₂O₂-based methods are given in Table 2.

The oxidation process of estrogens involves the orbitally controlled reactions that are initiated by an attack on the aromatic ring (C3, C4, and C5 positions). The OH functional group of estrogens can also be oxidized (to the carbonyl group by deprotonation) (Rokhina & Suri 2012). Due to incomplete mineralization, the transformation products can be formed during the AOP processes. An overview of possible transformation products is summarized in the literature (Pereira et al. 2011; Perondi et al. 2020). Cedat et al. (2016) tested the removal of E1, E2, and EE2 in the mixture using the UV/H₂O₂ process. Alongside the degradation rate constants of hormones, they also measured the estrogenic activity removal rate constants. They found that they are in the same range, suggesting that no high estrogenic or toxic by-products (tested on Vibrio fischeri) were formed during the oxidation process. Contrarily, Silveira et al. (2022) used Ceriodaphnia silvestrii as the test organism for the toxicity assay after UV-assisted peroxidation of EE2 and demonstrated the likelihood of an acute effect occurring in the aquatic environment. Therefore, before considering a full-scale pilot, the ecotoxicological evaluation should also be a part of the process.

Our previous study (Pribilova et al. 2022) investigated the synergy of HC and sodium persulfate (0.1 mM). We obtained similar results regarding the efficiency of estrogen degradation (Table S1) and electric energy per order (EEO) under the same operating conditions (see the SPC activation section for details), but the rate constants were 2–3 times lower. In that case, sulfur-based radicals were also involved in estrogen degradation.

The pH and conductivity of the solutions were also monitored throughout the process (Figure S2). After the addition of SPC at a dose of 80 mg/L, the pH of the solution averaged 9.5 and dropped to 9.1 after a 20-s treatment. A slight decrease was also observed when a dose of 200 mg/L was used; in this case, the initial pH value was 10.2 and dropped to 10.0. Such high values could pose a potential risk to the environment, but it should be noted that the experiments were conducted in distilled water. Moreover, in real wastewater, the buffering capacity of SPC could be an advantage.

Liu et al. stated in their review (Liu et al. 2021) that temperature is a key parameter affecting the efficiency of the SPC-activated system. Therefore, we examined the effect of temperature on SPC activation. Our results showed no significant impact on estrogen removal when the solution of SPC was heated to 60°C and combined with HC (compare Figure 2(a) and 2(b)). Similarly, the observation was confirmed by the values of the rate constants, which increased moderately with increased temperature (see Table 1). However, the differences between the individual rate constants ranged from 0.45 to 1.72 min⁻¹. In contrast, Ma et al. (2018) found that sole SPC is highly affected by increased temperature. They observed that as the temperature increased from 4 to 30°C, the peroxide rate constants increased by two orders of magnitude (from 8.26 × 10⁻³ to 7.69 × 10⁻¹ h⁻¹). The difference we observed can be most probably explained by the fact that during the process of HC, high local temperatures (up to 5,000 K; Zupanc et al. 2013) occur; thus, the influence of solution temperature is negligible. This could be advantageous since water treatment operates under different weather and temperature conditions (depending on the season). Additionally, the duration of our whole process is so short, as it took place in units of tens of seconds, and thus, it was not affected by the temperature of the solution.

SPC, calcium percarbonate, or peroxymonocarbonate are per-oxo substances, which are used in chemical engineering or water treatment as oxidizing substances. Most commonly, the SPC is used as a solid carrier of hydrogen peroxide because it is considered safer to transport and store, has lower explosive potential than liquid hydrogen peroxide, is suitable for longer storage, and is an environmentally friendly oxidant. It is particularly important for one practical reason: SPC can react in a wide range of pH compared to a broad spectrum of different Fenton-like reactions. The practical use of SPC is supported by the fact that carbonate radicals boost a multiple reactive oxygen species (multi-ROS) process (Equation (5)–(10)), where reactions can generate superoxide radicals, singlet oxide, or hydroxyl radicals (Li et al. 2021). This process was elucidated by Li et al. (2021) as the mechanism of SPC activation by CuFeS₂. The resulting OH^· via the traditional heterogeneous Fenton system reacts with carbonate ions to form additional ROS.

Even though SPC can react with pollutants directly, dominantly due to hydrogen peroxide, to reach the maximum effect of SPC oxidation potential, activated SPC enhances decomposing potential, which is why appropriate SPC activation is recommended. Two types of SPC activation can be differentiated: (i) homogenous and (ii) heterogeneous. Homogenous activation is based on direct contact with reactants, which is typically activated by increased temperature, UV, ozone, microwave, ultrasound, or discharge plasma activation (Liu et al. 2021). Heterogenous activation requires the addition of other chemicals, which is dominantly based on transition metal ions, especially iron. The forms of iron ions depend on the technology used and vary from ferrate (Fe⁶⁺), through Fe^2+/3+ used in modified Fenton reactions up to nano zero-valent iron (Fu et al. 2017; Liu et al. 2021). Recently, the degradation of sulfamethazine via the activation of 157 mg/L SPC by 500 mg/L chalcopyrite (CuFeS₂) was published (Li et al. 2021). The concentration of SPC used in their study was between the concentrations used in our research. Moreover, the chalcopyrite used in the concentration of 500 mg/L can contaminate water with copper. HC is quite a novel way of homogenous activation of SPC, which does not need any chemicals or expensive activation such as ozone, UV, or heating. HC provides the opportunity to treat realistic volumes of water (the outlet of wastewater treatment plants in tens m³/h). HC, in combination with hydrogen peroxide, provides synergic effects (Marsalek et al. 2020) and represents a promising tool for the intensification of AOP reactions, particularly for the cheap and environmentally responsible activation of SPC for organic pollutant removal.

Other effects of HC also have to be mentioned because they significantly contribute to SPC activation. First, these are high-shear stresses that enhance the disintegration of SPC. The second effect is connected with the multiphase bubbly flow with important velocity gradients, which help in the mixing process. It is interesting and important to note that while medium velocity in the Venturi tube throat is around 20 m/s, the bubble wall, during the collapse, moves with velocities higher by two orders of magnitude, and also the velocity of the microjet can reach up to the velocity of sound in liquid medium.

In the case of heat-combined SPC/HC, energy consumption would be higher because the energy of external heating, depending on the heating source, must be considered. According to the calculated values, the SPC/HC system belongs to group 2 (1–100 kWh/m³/order); thus, the methods are likely quite energy-intensive for most practical applications, but they might still provide attractive solutions for specific challenges, and full-scale applicability of these processes should be further investigated.

Our experiments showed that activation of SPC via HC is potent and green technology. Although the energy consumption belongs to group 2, the process does not require any other heating, UV radiation, or adding additional chemicals that could burden the environment. However, further experiments evaluating different water matrices should be continued as the experiments were performed in deionized water. The toxicity and estrogenicity tests should also be considered when considering a full-scale pilot.

We investigated the possibility of SPC activation via HC and demonstrated its usefulness for the simultaneous degradation of four estrogens: E1, E2, E3, and EE2 in an aqueous solution. By the synergistic action of these two AOP methods, more than 97% of the investigated pollutants were removed. Post-treatment processes played an important role. The experimental results indicated that the proposed treatment method was independent of the solution temperature. All tested estrogens compete to react with the generated radicals because the individual reaction constants were similar. The proposed process is a promising technique for the efficient removal of estrogens from aqueous solution. The advantage of our proposed activation is its low operating costs compared to other combinations, such as the heating of the solution. In addition, this method does not require any additional chemicals, e.g. to adjust pH or increase efficiency, so it does not pose an additional burden on the environment. HC can treat large volumes of water, which is important for any practical applications; thus, we expect that this practical and environmentally safe technology can help clean waters, for example, as the tertial treatment.

This work was supported by the Czech Science Foundation, project number 19-10660S.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.