Catalytic ozonation of N-methyldiethanolamine over mixed oxides derived from Mg/Fe-LDH

The MDEA (N-methyldiethanolamine) is widely employed in natural gas processing plants to remove acid gases (CO₂ and H₂S) from raw natural gas (Laila et al. 2018). However, a difficulty develops during the regeneration process in the gas purification, when a small amount of non-biodegradable alkanolamine is discharged in the wastewater. The COD of the released wastewater containing amine was high. The purpose of this research was to assess the efficiency of catalytic ozonation reaction, which improves the removal of aqueous methyldiethanolamine and can be used as a pre-treatment before a biological process using the system (catalyst + O₃). Currently, there is a great deal of research on the treatment of alkanolamines contained in wastewater for their toxicity, the use of adsorbents as (activated carbon, chitosan, alumina, zeolite) such as the benefit of using this method to reuse these alkanolamines in the trial (Razali et al. 2010). Furthermore, there are many techniques in the field of wastewater treatment for the removal of solids and oil, grease, biodegradable or non-biodegradable organic compounds, and toxic molecules (Haithem et al. 2019) Such these techniques applied alone or in cascade improve the level of treatment. Advanced oxidation processes (AOPs) occupy a very important place among the different treatment technologies; have been considered as popular techniques to treat the high concentration of organic contaminants in wastewater. However, we report the treatment methods such as the application of AOP (UV/Fenton, O₃/Ultraviolet, O₃/H₂O₂, UV/H₂O₂, UV/TiO₂) are of the most alternative techniques for the destruction of many other organic matters in wastewater and effluents. Several AOPs have been observed in the literature such as the heterogeneous Fenton degradation of persistent organic pollutants using natural chalcopyrite: effect of water matrix and catalytic mechanism (Jiapeng et al. 2022), additionally, these AOPs are very interesting alternatives for the degradation of non-biodegradable organic pollutants by the biological treatment process, such these techniques are based on the generation and use of a powerful oxidant especially the hydroxyl radical (OH^•) which can be produced by the photochemical and non-photochemical process, while attack the pollutant and form the degraded product (Che et al. 2014). In addition, the catalytic ozonation is one of these processes; such that the catalyst provides better performance both in terms of reaction rate and energy consumption (Legube & Karpel 1999). Overall, the efficiency of the oxidation and the fact that it does not form solid residues, such as sewage sludge, are the main advantages of AOP. In the other hand, the major disadvantage generally lies in the high cost of operation due to energy needs. This hinders industrial development a lot in technology according to a recent study (Oller et al. 2011). Currently, ozone with a redox potential of (2.07 V), as a strong oxidant, has drawn increasing attention in various environmental systems for potential oxidant capacity. Ozone reacts rapidly with organic substances in aqueous solution in two ways: direct reaction with molecular O₃ or indirect reaction with the active radicals, such hydroxyl radicals (OH^•) (redox potential of 2.33 V), react with most of the pollutants no selectively, destroying them and converting into harmless organic compound's such as CO₂ and H₂O (Hrvoje et al. 2006; Violante et al. 2010). O₃ molecules react selectively with the electron-rich compounds, so the reaction rate is quite slow for some other organic compounds such as pesticides and increases energy consumption for ozone production. These are examples of ozone application limitations for the direct removal of pollutants. As a consequence, the application of some combined oxidation technologies, such as catalytic ozonation facilitates the depletion of ozone, and the formation of hydroxyl radicals (OH^•). Recently, the process of catalytic ozonation has received more attention due to its effectiveness in treating wastewater (Lim et al. 2020). Moreover, the reaction can be run at ambient conditions of temperature and pressure with complete mineralization of the organic matter into carbon dioxide and water.

In this context, several materials have been used as catalysts in the catalytic ozonation, such as metal oxides (MnO₂, FeOOH, Fe₂O₃, MgO, TiO₂, etc.) (Bing et al. 2019a, 2019b), support metals (Cu, Ru, Pt, Co), zeolites modified with metals, activated carbon, and mesoporous structures (Hui et al. 2020). The mixed metal oxides have also been used in these reactions, which are promising catalysts because of their higher reactivity (Mahesh et al. 2014). Hence, various research papers dealing with LDH materials have been published (Nazrizawati et al. 2022). However, the synthesis of highly dispersed mixed oxides can be obtained from lamellar double hydroxides (LDHs) structures by calcination of parent LDH (Mir Saeed et al. 2021). These structures are considered to be a class of anionic clays and have received much attention in the field of catalysis because of their interesting physicochemical properties (Vaccari 1999). Moreover, they are used in hydrogen generation reactions (Bert et al. 2001), 5-hydroxymethyl furfural formation (Xiangbo et al. 2021), photocatalysis (Haithem et al. 2019), oxidative dehydrogenation of ethanol to acetaldehyde (Haolan et al. 2022), and as a potential adsorbent on the removal of pollutants from the environment due to their high surface area, high anion exchange capacity, and good thermal stability (Amy-Louise et al. 2021). Suggesting, the degradation in tetracycline for copper-based catalysts from electroplating sludge by ultrasound treatment and their antibiotic degradation was performed and the results showed that the product had very good performance over a wide pH range (2–11). At an initial pH = 2, the copper-based catalysts could degrade 91.9% of 50 mg/L tetracycline aqueous solution within 30 min (Zhenxing et al. 2023). So far, various catalysts have been investigated for AOP such as LDHs (layered double hydroxides), Haung et al. (2020) elaborated the Ni₂Fe-LDHs and used them as a catalyst in the heterogeneous catalytic ozonation of Bisphenol A, they found that a Ni:Fe ratio of 3:1 was an excellent catalyst for complete removal of Bisphenol A (Yuanxing et al. 2019). Hence, the degradation of aniline by the ozonation process using Co₂Fe-LDH and Co₂Al-LDH layered double oxides was studied (Yuanfeng et al. 2020). Furthermore, they used the Mg₂Al-LDH as a catalyst for the decomposition of p-nitroaniline from aqueous solutions by ozonation (Mohammad et al. 2020). Recently, several synthesis methods can be used to prepare LDH, among them; the co-precipitation reaction under mild conditions is relatively a simple and economical method (Ali et al. 2020a, 2020b). The LDH structure is defined as [M_(1–x)²⁺ M_x³⁺ (OH)₂]^x+(Aⁿ⁻) x/n·mH₂O, where (M²⁺ = Mg²⁺, Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺) and (M³⁺ = Al³⁺, Fe³⁺, Cr³⁺) metal cations, Aⁿ⁻ is interlayer exchangeable anions such as ⁠, ⁠, Cl⁻, and I⁻ and m is the number of water molecules (Ali et al. 2020a, 2020b).

The chemical reagents used were: sodium hydroxide pellets (Fisher Scientifique, 99%), hydrochloric acid (Fisher Scientifique, 36%), iron (III) chloride hexahydrate FeCl₃·6H₂O (Prolabo, 99.8%), magnesium chloride hexahydrate MgCl₂·6H₂O (prolabo, 99.9%), and deionized water purified by the Water Purification System Milli-Q (MERCK).

For the synthesis of LDHs phases, we follow the experimental protocol based on the co-precipitation method (Aaron & Dahn 2019; Ali et al. 2020a, 2020b). In a typical synthesis, magnesium chloride MgCl₂·6H₂O (0.2 mol for ratio 2 and 0.3 mol for ratio 3) and iron chloride FeCl₃·6H₂O (0.1 mol) were continuously stirred into NaOH solutions at pH 10 and room temperature of 25 °C. The solution while stirring at 700 rpm for 24 h. The synthesis was carried out under an inert atmosphere (N₂ gas bubbling). Then, the suspensions obtained were hydrothermally treated at a temperature of 80 °C for 24 h. To collect the products, the suspensions were centrifuged and rinsed five times with deionized water. Ultimately, the products were obtained by drying the powders in an oven at a temperature of 65 °C for 24 h to achieve the Mg₂Fe-LDH and Mg₃Fe-LDH. Activation of the powders was conducted via calcination in a muffle furnace at 400 °C in an air atmosphere for 6 h. The Mg₂Fe-LDH after calcination gives a product denoted as HTcMg₂Fe and the second Mg₃Fe-LDH gives a product designated as HTcMg₃Fe.

XRD patterns were recorded of 2⊖ = (5–70°) using a Bruker-AXS Advance diffractometer (40 kV, 30 mA, radiation λ = 0.15406 nm). FT-IR using a tensor 27 spectrometer (Bruker Optik GmbH, Germany). TG was recorded by thermal analysis using the TA Micrometrics 2050 TGA in the range of 30–800 °C with a heating rate of 5 °C/min. The surface areas were determined at 77.35 K by application of the N₂ adsorption–desorption isotherms (BET) method in a Micromeritics Tristar 3000 after degassing the sample in vacuo under nitrogen flow overnight at 150 °C L (Laila et al. 2018). The pH value and electrical conductivity were measured by a pH meter inolab pH 730 (WTW) and an electrical conductivity meter inolab cond 730 (WTW), respectively.

The chemical oxygen demand (COD) analysis using the HACH model DR/2800 spectrophotometer (HACH Company, USA) equipped with the HACH test tube.

The MDEA content was also analyzed using an automatic titrator (T50 METTLER TOLIDO serial number SNR: B134208930 with a reference electrode DX 200, DGI 111-SG (0–14) pH, (0–80 °C), KCl 3 mol/L AgCL sat).

As shown in Figure 3(a), three basal reflections typical of an LDH structure were observed: at 2θ of about 10° for (003), 23° for (006), and 35° for (009) (Ligita et al. 2020). Furthermore, the calculated d-spacing and the cell parameters (a and c) are given in Table 1. From this table, the calculated values of parameter a (lattice parameter) with the molar ratio for Mg/Fe = 2 are a little small compared with the lattice parameter for Mg/Fe = 3M. However, a small increase for lattice parameter a with increasing for the molar ratio R = 3 Mg/Fe. Overall, the calcined LDH (HTcMg₂Fe and HTcMg₃Fe) are shown in Figure 3(b). It can be seen that the lamellar structure collapsed completely toward the formation of mixed oxides during the calcination process (Esthela et al. 2019). The peaks at 2θ = 43.05° and 62.55° which corresponds to MgO (LCPDS 78-0430) (Dhal et al. 2015), and at 2θ = 30.1°, 35.5°, 43.06°, 57.0°, and 62.55°, that of the spinel structure of MgFe₂O₄ (magnesioferrite) (JCPDS 17-0465) (Yi et al. 2021; Lingyu et al. 2022).

In addition, the ferrite spinel with the MgFe₂O₄ structure has two types of hydroxyl groups on the surface Fe–OH bond attributed 547 cm⁻¹, which are the main active centers for binding various cationic and anionic compounds. In aqueous solutions, an outer layer of hydroxyl groups (surface) is formed on the surface of ferrite spinel (Ligita et al. 2020). The surface charge of a metal ferrite depends on the solution pH and can be described using the zero charge point (pH_pzc). In the case of pH < pH_pzc, the surface is dominated by an excess positive charge (–OH²⁺) due to an increase in the number of H⁺ ions, as a result, the adsorbent behaves like a Brensted acid. At pH > pH_pzc, the surface of the adsorbent acquires a negative charge as a result of the deprotonation of hydroxyl groups, and the adsorbent will behave as a Brensted base. Thus, denoting that anions are commonly adsorbed at pH < pH_pzc and cations at pH > pH_pzc (Ligita et al. 2020).

In the case of HTc, the FT-IR spectrum shows complete disappearance of the characteristic bands of water molecules (at about 3,386 cm⁻¹) and the carbonate group (at around 1,360 cm⁻¹). Meanwhile, an increase in the intensity of the bands corresponds to the characteristic vibrations of the M–O bonds (M = Fe or Mg) in the range of 405 cm⁻¹ (Shutang et al. 2021).

The dosage of adsorbent mass is one of the most important influencing parameters in the mass transfer process during the adsorption reaction. This provides the number of active sites capable of interacting with the molecule in solution, which we initially studied the effect of catalyst mass. The adsorption performance of activated materials HTc was investigated for the removal of MDEA-wastewater conditions: (100 ml of a solution containing MDEA (4 Wt%), catalysts dose (10–60 mg), time of 30 min, and pH 10). A 5 ml of each solution was taken and filtered before analysis.

The kinetics of ozonation of aqueous MDEA solution in the presence of 30 mg of catalysts was studied at pH 10 and a temperature of 30 °C, the experiments were carried out using a fixed initial concentration of aqueous MDEA solution (4 Wt%), and the ozone gas was introduced through a porous diffuser at the bottom of the reactor at a flow rate of 5 mg/min under continuous steering. A 5 ml of sample was taken every 10 min to separate the catalyst particles by filtration.

The removal efficiency decreased with the increasing initial concentration of MDEA. This could be attributed to the insufficiency of systems (catalysts + O₃) to remove high MDEA molecules in the aqueous solution. To investigate the performance of this process, the COD analysis was performed under the same conditions (Figure 10(a)). The highest value of the COD removal was (80.64%) obtained when using (4 Wt%) of MDEA, then the COD removal decreased with increasing of MDEA content in the aqueous solution. Overall, these results indicate that both catalysts with ozone could cause a low degree of degradation and mineralization of the highly concentrated MDEA (Gholamreza et al. 2009).

The results showed that the highest COD removals were observed over catalysts HTcMg₂Fe up to 80.76% at pH 7 and HTcMg₃Fe at 80.36% at pH 8, due to the reactivity and stability of catalysts in this pH range. In addition, the high removal of MDEA in alkaline pH can be related to pH_pzc of catalysts (Andrei et al. 2021). The pH_pzc of HTc was determined by the solid addition method to be 7.7 for HtcMg₂Fe and 8.2 for HtcMg₂Fe. Therefore, the pH_pzc is the pH at which the catalyst's surface is electrically neutral can be explained the best removal efficiency was obtained while the high stability of the catalyst. Many studies have reported that in alkaline conditions the performance catalytic ozonation efficiency increases, because the decomposition of ozone generating free active radicals especially OH^• radicals), which are extremely oxidizing species and selectively react with organic compounds in aqueous solutions. Additionally, in our study, the destruction performance of MDEA was started in an acidic pH 3 environment and was 77.6% for the process (HTcMg₂Fe + O₃) and 77.8% for the process (HTcMg₃Fe + O₃). Also in an alkaline solution, the MDEA removal efficiency in the catalytic ozonation reaction was 80.61% in the process (HTcMg₂Fe + O₃) and 80.28% in the process (HTcMg₃Fe + O₃) at pH 11, under these conditions, it becomes ozone combined with the catalyst increasing the production of hydroxyl radicals OH^• can be very reactivity with MDEA solution in the presence of a catalyst, which increases the treatment efficiency of MDEA-wastewater.

Both calcined LDH (HTc) have almost the same catalytic activity toward the degradation of MDEA with a mean removal COD of 80.76% in the pH 7 by HTcMg₂Fe, this value remained significant when compared to the work of Samira Molareza et al., who used ultraviolet UV light and peroxy disulfate for the photochemical oxidation of MDEA (Samira et al. 2016).

The present study for catalytic ozonation of MDEA has been compared with previous research work to understand the novelty of this work (Table 4). From this table, all applied materials (HtcMg₂Fe and HtcMg₃Fe) were better catalysts for the removal efficiency of MDEA. Moreover, this study was evaluating the efficiency of catalytic ozonation reaction, which increased the removal of aqueous methyl diethanolamine. Indicating possibilities of applications in industrial as a pre-treatment before a biological process using the system (catalysts + O₃).

In summary, Mg₂Fe-LDH and Mg₃Fe-LDH are successfully synthesized via the co-precipitation method. A portion of the LDH was calcined at 400 °C for 6 h to obtain two calcined powders designated respectively as HTcMg₂Fe and HTcMg₃Fe. The LDHs powder exhibited rhombohedral symmetry and after activation by calcination, the structure of both LDHs completely collapse to give a mixture of oxides (MgO + MgFe₂O₄) with a spinel structure. The study of the catalytic ozonation process for MDEA degradation revealed higher MDEA removal efficiency compared to the direct ozonation process. Overall, both catalysts with ratio R = 2.3 Mg/Fe exhibited the best degradation performance. After 60 min reaction, 30 mg catalysts, (4 Wt%) MDEA, an average value of COD removal was 80.76% obtained by HTcMg₂Fe pH 7 and 80.36% by HTcMg₃Fe at pH 8. Both catalysts showed almost higher COD removal versus MDEA degradation in aqueous solutions with a small improvement for HTcMg₂Fe compared to HTcMg₃Fe. Ultimately, after calcination the Mg₂Fe-LDH structure can yield a mixed oxide (HTcMg₂Fe), which can be considered as an excellent practical alternative catalyst with high performance in the removal of organic pollutants such as MDEA from aqueous solutions.

We thank the Algerian University for financial support, for providing various facilities, and for necessary approval.

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

The authors declare there is no conflict.