Kinetic study of mineral oil removal from wastewater by the sono-electrochemical process

Wastewater containing oil from industrial sources is particularly problematic, as mineral oils (total hydrocarbons) and other pollutants contained in this wastewater are treated as hazardous waste under the European Legislation. Researchers have explored and developed various approaches to separate oil components from water (Coca et al. 2011; Perez et al. 2016). However, the increasing number of studies on some newer technologies, such as electrocoagulation and advanced oxidation processes, has opened the door for their wider application (Asaithambi et al. 2017; Ozyonar et al. 2020; Moradi et al. 2021). The sono-electrochemical (sono-EC) process is based on the dissolution of metal ions at the electrodes, the neutralization of contaminants and the formation of flocs that can be easily removed by sedimentation (or flotation), while at the same time, reactive radicals are generated by ultrasonic cavitation, leading to additional degradation of contaminants (Moradi et al. 2021). Although electrochemical and sono-EC methods are widely known and have been shown to be very effective for oil removal (Tir & Moulai-Mostefa 2008; Safari et al. 2016; Posavcic et al. 2022; Moneer et al. 2023), there are still some unanswered questions, such as the reaction rate of this specific contaminant.

Chemical kinetics is a branch of physical chemistry that studies the rate and mechanism of chemical reactions. It is helpful in predicting how quickly a chemical reaction will occur. In the laboratory, chemical kinetics is usually monitored by changing the concentration, temperature, or pressure of the reactants (Atkins & de Paula 2010; García-Carrillo et al. 2019; Deokate et al. 2023). In electrochemistry, the reaction rate depends on the dissolution of metal ions and the elimination of hydrogen ions (H⁺) by H₂ evolution (García-Carrillo et al. 2019).

Determining the rate of chemical reactions is one of the main tasks in chemical research. Chemical kinetics can be a useful tool to determine the optimal operating time of EC and sono-EC processes. The lower the reaction rate constant (k), the slower the reaction proceeds and the longer it takes to remove the contaminant. Thus, the efficiency of the EC process increases over time, but only up to a certain point when saturation with metal hydroxides occurs (Khan et al. 2023). A review of the state of knowledge revealed that the rate of chemical reactions in the treatment of oily wastewater by sono-EC treatment has not yet been investigated. Therefore, the main objective of this study was to determine the mineral oil removal rate by sono-EC treatment.

There are some models that have been used to describe the kinetics of mineral oil removal by electrochemical treatment, such as Körbahti & Artut 2010; Rincon 2011; Sangal et al. 2013. Körbahti & Artut (2010) investigated the electrochemical oil/water demulsification and purification of bilge water using Pt/Ir electrodes, and kinetic studies revealed an overall first-order electrochemical conversion rate of pollutants. The specific reaction rate constant was calculated to be 6.775 × 10⁻² min⁻¹ at 32 °C. Rincon (2011) conducted a series of EC experiments with a synthetic emulsion containing hexane extractable materials (HEMs). It was confirmed that the EC of HEM follows first-order kinetics. Kinetic constants of 0.0441 and 0.0443 s⁻¹ were determined by applying both the dispersion and tanks-in-series models, respectively. R² was 0.97 in both cases. Sangal et al. (2013) observed the change in turbidity with oil removal. In their study, the kinetic constant was maximum at a current density of 138.8 A/m² and the process followed first-order kinetics. The kinetic constant was 0.105128 min⁻¹.

However, as far as the authors know, there is no study describing the kinetics of sono-EC process for mineral oil removal.

Therefore, zero-, first-, and second-order kinetic models were used in this study to determine the reaction rate of mineral oil removal with the sono-EC process, i.e., to describe the adsorption kinetics on aluminium hydroxides.

The oily wastewater was obtained from the cleaning of oil and grease separators on roads. The reaction rate experiments were carried out under the optimal conditions defined in the work of Posavcic et al. (2022). Regarding that research, a total of 34 experiments were conducted, each with 8 L of real oily wastewater. Four aluminium electrodes were placed in a container, while eight ultrasonic transducers were attached to the bottom of the reactor. The oily wastewater circulated through this experimental device, and a water sample was taken after each experiment. A reduced cubic regression model with estimated coefficients was developed to describe the mineral oil removal efficiency, and the optimal conditions for the sono-EC with circulating flow were 8 min of treatment time, a current density of 53.1 A/m², a flow rate of 0.23 L/s, and an ultrasonic intensity of 5.1 kW/m². It was concluded that US acted as an additional mixer and enhanced the flocculation phase of EC. This also accelerated the sedimentation process as the flocs separated faster from the aqueous phase (Posavcic et al. 2022).

The aim of this study is to determine the mineral oil removal rate by kinetic modelling, as an extension of the preliminary study. For this purpose, a wastewater sample with the same volume (8 L), an initial mineral oil concentration of 381.5 mg/L, a pH of 7.2, and a temperature of 20.4 °C was subjected to optimal conditions (8 min of treatment time, a current density of 53.1 A/m², a flow rate of 0.23 L/s, and an ultrasonic intensity of 5.1 kW/m²). The mineral oil concentration was analysed with the NEXIS GC-2030 (Shimadzu, Japan), and the temperature and the pH with the HI-98194 Multiparameter Waterproof Meter (Hanna Instruments, Romania). The temperature and the pH did not change during the 8 min of treatment time.

The experimental part was the same as in the preliminary study. EC was used to neutralize the charge of the oil droplets, and US cavitation increased the removal efficiency by exposing the oil to extreme conditions (high temperature and high pressure).

Regarding the contribution of ultrasound to the sono-EC process, the results of the first-order kinetics can be used for comparison with other studies. The kinetic model in the study by Sangal et al. (2013) followed first-order kinetics. The kinetic constant for the current density of 59.2 A/m², which is comparable to the current density of this study (53.1 A/m²), was 0.02923 min⁻¹. In another study, Rincon (2011) concluded that the electrocoagulation of HEM in the synthetic emulsion follows first-order reaction kinetics, and the kinetic constant is 0.0447 s^{− 1}. The higher the value of the constant, the faster the reaction. Since the slope k₁ for the first-order reaction in this study is 0.3359 min⁻¹ (Table 2), it can be concluded that ultrasound has a positive influence on the reaction rate.

There are also kinetic analyses for sono-EC, but none of them consider the removal of mineral oil. For example, He et al. (2016) concluded that a variable model describes the color removal better than a pseudo first-order kinetic model. On the other hand, Asaithambi et al. (2017) described COD (chemical oxygen demand) removal with first-order. It is emphasized that the mass of dissolved metal from the electrodes must be known for variable and pseudo models (He et al. 2016). Although Figure 3 indicates that mineral oil removal with sono-EC also follows first-order kinetics, which would be consistent with the previously mentioned research, it is concluded that the changes in mineral oil concentrations follow second-order kinetics due to the coefficient of determination being smaller (R² = 0.8837) than for second order.

Chemical kinetics can be a useful tool for determining the optimal operating time of EC processes. A review of the state of knowledge revealed that the rate of chemical reactions in the treatment of oily wastewater by sono-EC treatment has not yet been investigated. Therefore, the main objective of the study was to determine the mineral oil removal rate by sono-EC treatment. The kinetic study of mineral oil removal was performed at constant current density, flow rate, and ultrasonic intensity. Although the results indicate that the mineral oil removal with sono-EC follows first-order kinetics, it is concluded that the changes in mineral oil concentrations follow second-order kinetics, as the coefficient of determination (R²) is the highest (R² = 0.9732). The specific rate of chemical reactions for second order is 0.0061 [mg⁻¹ L min⁻¹]. The initial mineral oil concentration was 381.5 mg/L. After 8 min of treatment, the achieved mineral oil concentration was 21.4 mg/L.

This work is a first step towards improving the knowledge of the kinetics of mineral oil removal from oily wastewater by a sono-EC process. The results of this study indicate that the sono-EC process could be a promising technology for the removal of mineral oil from wastewater and that the mineral oil removal rate can be determined by chemical kinetics. The results considering kinetics could be useful for the optimization of the sono-EC process and reactor design. However, it is recommended to conduct a more comprehensive study on the kinetics of the sono-EC process for mineral oil removal, for example on large-scale devices.

This work has been supported by the Croatian Science Foundation under the project ‘IP-2019-04-1169 – Use of treated oily wastewater and sewage sludge in brick industry – production of innovative brick products in the scope of circular economy’.

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

The authors declare there is no conflict.