Comparative study of wastewater treatment efficiency: ozone vs. atmospheric oxygen with a manganese dioxide catalyst

In the modern world, with the growth of industrial production, people are universally confronted with the challenge of purifying polluted natural waters from industrial effluents (Hosseinzadeh et al. 2020). There is a trend toward not only increasing emissions but also an increased diversity of toxic compounds entering the waters (Sun et al. 2020). Traditional standard purification methods (chemical treatment, filtration, sedimentation, and biological treatment) are showing decreasing effectiveness, particularly in areas such as water treatment and the disposal of sludge contaminated with toxic organic substances (Wang & Chen 2020). Industries that discharge hard-to-treat substances into water include the wood processing industry, pulp and paper manufacturing, as well as metallurgy and the chemical industry (Song et al. 2021).

In recent years, there has been a trend toward the increasingly widespread use of oxidative technologies for the removal of harmful organic impurities from water. As oxidants, various compounds are employed, including ozone, chlorine, and chlorine dioxide (Flores-Chaparro et al. 2020; Tian et al. 2020). More complex oxidative systems are also utilized, such as ozone in combination with ultraviolet irradiation, hydrogen peroxide, and ferrous ions, as well as hydrogen peroxide and ultraviolet irradiation in tandem (Mohammadi et al. 2020a; Foroughi et al. 2022). The essence of this method lies in the fact that under the influence of oxidation, organic compounds are broken down into inorganic compounds with lower molecular weights, rendering them non-toxic (Malik et al. 2020; Lim et al. 2022). Ozone is considered one of the most promising oxidants since it can oxidize even refractory organic compounds into carbon dioxide and water, provided that specific conditions are met (Nurkenov et al. 2008; Gkorezis et al. 2016).

Furthermore, oxidative methods are actively applied in the purification of wastewater from hydrocarbon impurities. Ozone treatment is not only used for industrial effluent treatment but also for meeting international drinking water standards and water conditioning (Xie et al. 2017). However, practical ozone application has its drawbacks, primarily being energy-intensive due to significant electricity consumption (Ji et al. 2018; Akhmetbaev & Dzhandigulov 2019). Wastewater can also be treated by introducing atmospheric oxygen, which is an energy-efficient, environmentally friendly, and technologically safe method (Mecha & Chollom 2020). However, it is worth noting that automated water purification processes utilizing atmospheric air may not be as effective, especially when dealing with wastewater containing a high concentration of organic substances, as the oxidation process becomes less efficient under such conditions (Itzel et al. 2020). Therefore, the search for new methods to enhance wastewater treatment performance through oxidation remains relevant today.

Despite the numerous methods developed for the purification of wastewater and natural waters at present, the challenge lies in the fact that each specific contaminant requires a unique technology for removal (Ahmed & Fakhruddin 2018).

Therefore, there is no single purification method that possesses high efficiency in all cases. In this study, the authors propose a method for wastewater purification using an oxidative approach with the incorporation of additional catalysts. The authors hypothesize that manganese dioxide may serve as an effective catalyst in this process.

The objective of this research is to develop a methodology for wastewater treatment using ozone and atmospheric oxygen, aiming to achieve higher efficiency compared to standard methods. The research tasks include (a) assessing the effectiveness of catalysts in the oxidation process, resulting in the generation of active hydroxyl and peroxyl radicals at the end of the process, and (b) comparing the efficiency of this approach with standard purification methods.

The research was conducted between 2019 and 2020 in Moscow, Russian Federation, at the Institute of Organic Chemistry named after N.D. Zelinsky, Russian Academy of Sciences. A specialized experimental setup was employed for the investigation. The setup comprised the following components: a primary unit (compressor), as well as a block for purifying and drying the incoming air stream, an O3 generator, a rotameter, a reactor in which the oxidation processes occurred, a gas analyzer to analyze gases produced during chemical reactions, and a degasser for decomposing any remaining O3. When atmospheric oxygen was supplied, no current was provided to the O3 generator. In total, 540 repetitions of each experiment were conducted, corresponding to various concentrations of manganese dioxide when oxygen and ozone were introduced (270 repetitions for each delivery method). The number of repetitions (540) was chosen to ensure the statistical significance of the results, to account for the variability in the composition of wastewater, and to verify the reproducibility of the experiment under various catalyst concentration conditions. This facilitated the acquisition of more reliable and generalized data regarding the efficacy of the wastewater treatment methodology employing ozone and catalyst.

Aromatic hydrocarbons such as benzene, toluene, xylene, and others are formed as a result of organic synthesis. These compounds are characterized by high levels of toxicity and chemical oxygen demand.

Aliphatic hydrocarbons, simple hydrocarbons, or their derivatives, such as methane, ethane, and propane, also exhibit toxicity and oxygen consumption capacity.

Sulfates are formed as a result of neutralizing the alkaline components of wastewater with sulfuric acid.

Emulsions from concentrations of hydrocarbon resin are formed as a result of mixing organic hydrocarbons with water and other components of wastewater. These components are characterized by high levels of toxicity and chemical oxygen demand, making them key research targets in the context of wastewater treatment.

For a more detailed analysis of the composition of organic compounds in wastewater and to identify details of degradation pathways during treatment, gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–mass spectrometry (LC–MS) methods were employed. These methods enable the identification of types of organic compounds and their changes during wastewater treatment processes, which are important for understanding purification mechanisms and process optimization.

The following ozone dosages were used during the study: 3 g/l of wastewater for each experiment. The efficiency of the purification process was assessed based on changes in the chemical oxygen demand (COD) level over various time intervals. The maximum efficiency of oxidative processes was observed within the first 15–30 min after the start of treatment both when using ozone and atmospheric oxygen. Subsequently, practically identical COD level change curves were observed, regardless of the oxidizer used. However, after 1 h of treatment and in the presence of a catalyst (manganese dioxide), a significantly lower COD value was observed when using ozone compared to atmospheric oxygen as the oxidizer.

The object of our research is wastewater originating from an organic synthesis plant, specifically selected due to their significant contamination with aromatic and aliphatic hydrocarbons, which are considered toxicants. Additionally, these compounds exhibit specific oxygen consumption (or COD). They are characterized by elevated alkalinity levels, necessitating neutralization with sulfuric acid. As a result of this neutralization, sulfates and emulsions from hydrocarbon resin concentrations significantly exceed permissible limits. The presence of such a high quantity of toxic substances in the plant's wastewater served as the basis for our investigation.

In each experiment, 1 L of wastewater was carefully introduced into the reactor. Subsequently, various concentrations of manganese dioxide were added: 0.01 g/l (concentration 1), 0.05 (concentration 2), 0.1 (concentration 3), 0.2 (concentration 4), 0.5 (concentration 5), 1.0 (concentration 6), 2.0 (concentration 7), 4.0 (concentration 8), and 8.0 (concentration 9). Each experiment was repeated 30 times. Simultaneously, a mixture of air and ozone was supplied, with an ozone concentration of 3.0 g/l. The flow rate of the mixture was 80 l/h. Similarly, oxygen from atmospheric air was supplied at the same flow rates. The experiments were conducted at a temperature corresponding to normal room temperature (20–22 °C). Thus, the experiments corresponded to the kinetic region of the process. The specific oxygen consumption was determined using the dichromate method, which involves the oxidation of organic substances using a potassium dichromate solution (18 N) in a 1:1 ratio with sulfuric acid. We selected this method because it provides the highest accuracy in calculating COD. Additionally, the light transmittance coefficient was taken into account using a 5 cm cuvette and a wavelength of 400 nm.

The LabAnalyt SP-V1000 spectrophotometer was utilized in this study. The molar ratio of oxidant to pollutant was 2:1. Quantitative analysis was conducted using calibration curves. The detection limit was 0.01 mg/L, and the limit of quantification was 0.05 mg/L.

All experiments were conducted in strict accordance with quality assurance protocols and standards.

The manganese dioxide used in the study had particle sizes ranging from 50 to 100 μm. It originated from the laboratory of chemical reagents ‘Alpha,’ exhibited a surface porosity of 0.3 cm³/g, and was synthesized via the high-temperature treatment of manganese ore.

The obtained data were recorded in the Microsoft Excel 2016 (Microsoft Corp., USA) database. Subsequently, statistical analysis methods were applied. The arithmetic mean and the standard error of the mean were calculated for each of the variables. To determine the significant difference between the results of different experiments, a Student's t-test was employed, with a minimum significance level of p ≤ 0.05. Parameters such as light transmittance coefficient and the degree of wastewater purification during ozonation are presented in the text of the article as percentages.

The effect of anions and water matrix on the degradation process should be discussed. Variations in water compositions, such as the presence of carbonates or other ions, may influence the efficiency of the oxidation process.

It is noted that the presence of carbonates in the water matrix can influence the degradation efficiency. Further investigation into the interaction between carbonates and the oxidation process is warranted.

The degradation process may occur through the direct reaction with ozone or via radical pathways. The identification of radical species present in the system requires scavenging tests.

When the catalyst concentration is increased from 0.1–0.5 g to 1 and 2 g/l, effective water purification is observed with both oxidants. The method's efficiency increases to 53% after half an hour of treatment with ozone and 41% for atmospheric oxygen. In other words, ozone proves to be a more effective oxidant when a catalyst is present (p ≤ 0.05). The enhanced efficiency of water purification in the presence of manganese dioxide as a catalyst can be explained by the fact that its introduction leads to a 2-electron oxidation process, resulting in the formation of manganates, which themselves act as efficient oxidants in an alkaline environment. When they react with hydrocarbons present in the water (in an alkaline medium), manganese dioxide is regenerated, forming a stable compound.

In light of the emerging trends in using ultraviolet irradiation as an alternative to chlorination for water purification, we also considered the water transparency indicator. It is well known that ultraviolet irradiation is judiciously applied only when high transparency levels are achieved. Figure 1(c) and 1(d) presents the results of changes in the light transmittance coefficient during the treatment of water with atmospheric oxygen and ozone, depending on the catalyst concentration. From Figure 1(c) and 1(d), it can be observed that the maximum increase in the light transmittance level occurs within the first quarter-hour to half an hour after the initiation of water treatment in the purification system.

The maximum values of the light transmittance coefficient in the presence of the catalyst were achieved with ozone treatment lasting for 1 h, reaching 83% at a concentration value of 1 (or 0.01 g of the catalyst per liter). For atmospheric oxygen, these values were significantly lower (52%, p ≤ 0.05 compared to ozone), with a higher concentration of the catalyst (1 g/l or concentration 5). This indicates that even in the presence of the catalyst, a higher concentration of the catalyst is required for higher light transmittance values, which ultimately still leads to lower results compared to the ozone mixture. Furthermore, we attribute the higher results for ozone to the fact that it undergoes addition reactions to multiple chemical bonds. In the case of atmospheric oxygen treatment, these bonds are broken as they are disrupted by hydroxyl and peroxide radicals.

We did not observe significant changes in the light transmittance coefficient through water at catalyst concentrations 1 and 2 (0.01 and 0.05, Table 1) both for ozonation and atmospheric oxygen treatment. However, after 1 h of treatment, a significant decrease in the pH level occurred (Table 1), nearly by one unit, for both ozone (p ≤ 0.01) and atmospheric oxygen (p ≤ 0.05). The largest decrease in pH was observed within the first 15–30 min of treatment, after which pH values decreased substantially slower (p ≤ 0.02 between 15–30 min and 1 h of treatment).

Moreover, when the catalyst concentration exceeded 0.5 g per 1 L following a 30-min treatment, hydrocarbons were not detected in the water, irrespective of the oxidizing agent employed. This suggests that the most effective method for purifying water from hydrocarbons when using atmospheric oxygen is the introduction of a catalyst at a concentration of 2 g, and for ozone, it is 1 g per 1 L. In this case, the following water purification efficiency indicators were recorded: 53% for ozone treatment and 42% when atmospheric oxygen was supplied. Consequently, the values of light transmittance also vary, with 62% for ozone and 43% for atmospheric oxygen.

In our study, an examination was conducted on water samples containing high concentrations of hydrocarbons from the same plant. It was noted that when a catalyst is present, hydrocarbon oxidation reaches its peak within 30 min following its introduction. The type of oxidant used does not significantly impact this process. Significantly increased wastewater treatment efficiency was noted when introducing the catalyst at concentrations up to 2 g per 1 L when atmospheric oxygen was supplied (p ≤ 0.05 compared to initial treatment values) during the first half an hour after its introduction. For ozone, these indicators were half as much (p ≤ 0.05 compared to oxygen, Table 2) at 1 g per 1 L.

In the case where the turbidity values reached 35% within half an hour after the start of the water purification procedure, this was achieved with ozone in the presence of a catalyst at concentrations of 2, 4, and 8 g per 1 L. These values were 26% for atmospheric oxygen under the same catalyst concentrations (p ≤ 0.05 compared to ozone). Thus, ozone demonstrated greater efficiency compared to atmospheric oxygen in this context.

Significantly higher values were achieved with ozone when the alkalinity was altered (Tables 2 and 3). Alkalinity decreased most rapidly within the first half an hour of the treatment at catalyst concentrations ranging from 1 to 8 g per 1 L. In contrast, for atmospheric oxygen, these values remained within the range of 4–8 g per 1 L (p ≤ 0.05 compared to ozone). Further analysis through chromatography–mass spectrometry revealed that the reduction in alkalinity was associated with the formation of organic acids during the purification process.

To assess the reusability of the catalyst, additional experiments should be conducted to evaluate its performance after multiple cycles of use.

The toxicity of intermediates can be evaluated using the ECOSAR program to gain insights into their environmental impact.

The effectiveness of degradation should be compared with other commonly used processes for pollutant removal to assess its practicality and efficiency.

It is well known that the oil and gas industry represents a major source of widespread toxic emissions into the environment. This is primarily because the majority of extracted oil consists of hydrocarbons (Logeshwaran et al. 2018). These hydrocarbons, in turn, serve as the primary raw materials in the production of various goods and products in industries such as organic chemistry and energy-saving technologies (Phan et al. 2022). Currently, annual oil production continues to rise and has already reached a staggering 3.5 billion tons (Mainardis et al. 2020). According to international agreements, no more than one percent of extracted oil should enter the environment, yet even this figure amounts to a significant 35 million tons (Szabová et al. 2020). When considering the annual losses of hydrocarbons resulting from accidents involving oil pipelines, tankers, fires at oil rigs, and oil refineries, the detrimental impact on the environment increases exponentially (Heebner & Abbassi 2022). Currently, a variety of physical, chemical, and biological methods are used to clean the environment from oil and its byproducts. In the following sections, we will discuss the most prevalent ones and evaluate their effectiveness in comparison to our approach.

The coagulation method involves the use of aluminum and iron hydroxides as coagulants (Kienle et al. 2022). Additionally, secondary products from oil refining processes can also be utilized as coagulants (Pang et al. 2023). Compositions of coagulants and flocculants are employed to enhance coagulation efficiency (Campo & Di Bella 2019). These compositions separate soluble and insoluble forms of hydrocarbons and other organic compounds from wastewater. However, a drawback of this method is the substantial amount of insoluble precipitate generated, which was not observed in our approach. It is known that the resulting precipitate from this method can only be used in the subsequent purification stage after extended drying, a process taking 1.5–2 h (de Abreu Domingos & da Fonseca 2018).

The flotation method yields the following results for the separation of organic products: 45–55% insoluble forms and 35–45% soluble forms (Esmaeili & Saremnia 2018). In our case, the purification efficiency consistently exceeded 50%. Additionally, during flotation, there is a notable 49% reduction in the level of COD (Esmaeili & Saremnia 2018). We observed a reduction in this parameter of approximately 40–53%, bringing it to a comparable level. Furthermore, a modified flotation method, known as pressure flotation, is utilized (Filatova & Soboleva 2019). All these methods are relatively energy-intensive, with comparable energy costs to ozone generation. However, an advantage of the pressure flotation method is the reduced amount of precipitate generated and a shorter purification time, typically around half an hour.

The electrochemical method's primary objective is to separate the hydrocarbon component from wastewater. In this method, oxidation occurs at the anode, while reduction takes place at the cathode. An alloy of aluminum and iron serves as the electrode material. However, a drawback of this method is its substantial energy consumption (Mohammadi et al. 2020b).

Lastly, the adsorption method is highly effective, with removal rates reaching up to 55%. However, it is crucial to take into account that individual pollutants necessitate specific combinations of adsorbents, each characterized by its distinct physicochemical properties (Sadare et al. 2018; Nazifa et al. 2019). In our method, employing manganese dioxide as a catalyst has demonstrated significant purification results, comparable to other methods. Moreover, our technique enables the repeated use of the catalyst at least five times without a significant loss of its properties.

This study presents an innovative approach to wastewater treatment from high concentrations of hydrocarbons using ozone and manganese dioxide catalysts. The key findings demonstrate that ozone exhibits more reliable purification properties compared to atmospheric oxygen. Additionally, it is shown that the application of the catalyst enhances the efficiency of water purification from pollutants.

The highest clarity in reducing overall water pollution is achieved when using ozone due to accelerated oxidative reaction. The efficiency of this process increases during the first 15–30 min of purification.

Changes in the levels of COD and total organic carbon (TOC) during the treatment process confirm the effectiveness of using ozone and catalysts in reducing pollution levels in water.

The water's biotoxicity decreases as a result of purification, as evidenced by changes in the concentration of harmful organic compounds and a decrease in pollutant content.

To analyze economic efficiency, it is necessary to consider investment costs, operating expenses, and overall operational costs for both water purification methods. It is important to determine operating costs for the best purification process, which provides maximum efficiency at minimal costs.

The research findings affirm the potential of employing ozone treatment with a manganese catalyst in the process of wastewater purification from organic contaminants. This method can be widely applicable for treating wastewater with high hydrocarbon content, particularly pertinent to industrial enterprises dealing with organic compounds.

The study unveils new prospects in wastewater treatment, demonstrating the advantages of ozone utilization compared to traditional methods such as atmospheric oxygen. The application of manganese catalyst further enhances process efficiency and opens avenues for developing more effective water purification technologies. This research represents a significant contribution to the environmental domain and may constitute a crucial step toward improving the ecological sustainability of industrial processes.

The work was carried out within the framework of program-targeted funding for scientific and technical programs for 2023–2025 Ministry of Science and Higher Education of the Republic of Kazakhstan BR21882415 “Development of technology for the safe disposal of wastewater for irrigation of fodder crops and tree plantations in conditions of water shortage in the Kyzylorda region”.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

S.U., G.A., and D.S. conceptualized the study and wrote, reviewed, and edited the article. S.U. wrote, reviewed, and edited the article.

The research was conducted ethically in accordance with the World Medical Association Declaration of Helsinki. The research was approved by the local ethics committees of Kyzylorda University named after Korkyt Ata (Protocol no. 4993 dated from 02/02/2019). Informed consent was signed by participants.

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

The authors declare there is no conflict.