This study aims to investigate the treatability of electrocoagulated biodiesel wastewater (ECBD) by KMnO4 and KMnO4/O3 processes. The ECBD removal efficiencies of both KMnO4/O3 and KMnO4 methods were compared, and the COD and MeOH removal efficiencies with the KMnO4/O3 process gave better results than the KMnO4 process. For the ECBD removal efficiencies, the experimental parameters including pH, KMnO4 dose, O3 dose and reaction time were optimized by changing one parameter at a time. As a result of 6 h of KMnO4 oxidation, 91.74% of COD and 95.93% of MeOH removal was achieved under the optimum conditions. However, the COD and MeOH removal efficiencies have been obtained under optimum conditions for KMnO4/O3 as 97.79% and 98.30%, respectively. The second-order kinetic model has been found to be the most suitable model for both processes and the R2 has been found as 0.999 and 0.999 for KMnO4 and KMnO4/O3, respectively. The reaction k has also been calculated as 6 × 10−5 L·mg−1·min−1 and 1.63 × 10−4 L·mg−1·min−1 for COD and MeOH in KMnO4 oxidation, respectively. Furthermore, the k has also been calculated as 6 × 10−5 L·mg−1·min−1 and 1.6 × 10−4 L·mg−1·min−1 for COD and MeOH in KMnO4/O3 oxidation, respectively.

• Biodiesel wastewater is difficult to treat due to the impurities it contains.

• I used KMnO4 oxidation and KMnO4/O3 oxidation.

• For the ECBD removal efficiencies, the experimental parameters were optimized.

• The results show that the receiving environment discharge standard was reached by removing pollutants.

• The second-order kinetic model has been found to be the most suitable model for both processes.

Increase in oil prices, fossil fuel depletion and environmental problems encourage the development of renewable fuels (Patiño et al. 2018). Biodiesel has a great interest in the world as renewable and environmentally friendly fuel energy derived from biomass (Chi et al. 2018).

Generally, the biodiesel production process is the transformation of triglycerides into the fatty acid methyl ester by using the transesterification process in the presence of alcohol and a catalyst (Veljković et al. 2014). The impurities of produced biodiesel are removed with a large amount of water (Mohana et al. 2011). As a result of the washing process, large quantities of wastewater containing soap, catalyst, alcohol, free glycerol and free fatty acids (FFA) occurs (Berrios & Skelton 2008; Atadashi et al. 2011; Rattanapan et al. 2011). Generated wastewater has a high organic load, pungent odour and milky colour (Veljković et al. 2014). Due to the high levels of chemical oxygen demand (COD), biological oxygen demand (BOD), oil, suspended solid matters and dissolved solids, biodiesel wastewater treatment is too hard.

In the literature for biodiesel wastewater treatment, various physical and chemical treatment methods are studied, such as membrane filtration (Mozaffarikhah et al. 2017), adsorption (Pitakpoolsil & Hunsom 2014), acidification-coagulation (Rattanapan et al. 2011), coagulation-flocculation (Daud et al. 2015, 2018), electrocoagulation (Srirangsan et al. 2009; Ngamlerdpokin et al. 2011; Ahmadi et al. 2013; Tanattı et al. 2018a, 2018b), electroflotation-electrooxidation (Jaruwat et al. 2010; Romero et al. 2013), ozonation (Tanatti et al. 2019) and solvent extraction (Tanattı et al. 2018a, 2018b). Although the biological treatment of biodiesel wastewater is quite difficult due to its high pollution load, aerobic (Suehara et al. 2005) and anaerobic treatment (Nishio & Nakashimada 2007; Siles et al. 2010) have also been studied.

When the literature on biodiesel wastewater treatment is examined, permanganate and permanganate/ozone methods are unavailable. Therefore, these methods are an innovative approach for biodiesel wastewater treatment. Permanganate used as a strong oxidant (E0 = 1.51 V) through the high reduction potential (Zhong & Zhang 2019). In neutral, alkaline and acidic environments permanganate ion is an important oxidizing agent. Furthermore, especially in an acidic medium, widely used for oxidation of organic and inorganic compounds (Althagafi & Fawzy 2016). In addition, permanganate is an inexpensive, relatively stable, easy to use, and highly effective oxidizing agent over a wide pH range (Song et al. 2019). Also, as a green oxidant, permanganate is widely used in wastewater treatment, reduction of micro-pollutants and drinking water treatment (Rodriguez et al. 2007; Zhang et al. 2014).

Permanganate is an effective oxidant for some chemicals and drugs existing in wastewaters such as triclosan (Jiang et al. 2009; Chen et al. 2016), sulfamethoxazole (Gong & Chu 2018), bisphenol A (Zhang et al. 2013), sulfadiazine (Yang et al. 2018), chlorophene (Xu et al. 2018) and tetrabromobisphenol A (Pang et al. 2014).

Another strong oxidant used in this study is ozone (E0 = 2.07 V) (Yang et al. 2017). In the oxidation of organic and inorganic compounds with ozone, reactions occur either directly as molecular ozone or indirectly with hydroxy radicals formed by ozone decomposition (Nawrocki & Kasprzyk-Hordern 2010; Li et al. 2018). Ozone oxidation mechanism takes place depending on pH. While ozone reacts directly with organic substances under acidic conditions, at high pH (pH> 7) performs indirect oxidation reactions and rapidly generates hydroxyl radicals and other species (Pazdzior et al. 2017).

The study's main aim is to ensure the discharge standards of biodiesel wastewater, which is treated in two stages in this way. In this project, the treatment of pre-treated raw biodiesel wastewater (electrocoagulated) by oxidation of potassium permanganate (KMnO4) and potassium permanganate/ozone (KMnO4/O3) was investigated. In the KMnO4 oxidation process to investigate the removal efficiencies of COD and methanol (MeOH) and in electrocoagulated biodiesel wastewater (ECBD) pH, KMnO4 dose and reaction time parameters were examined. At KMnO4/O3 oxidation process pH, KMnO4 dose, O3 dose and reaction time parameters were studied. Depending on time studies and obtained optimum conditions, KMnO4 and KMnO4/O3 oxidation processes were examined in terms of kinetics for ECBD wastewater.

### Characterization of biodiesel wastewater

Biodiesel wastewater was produced in the Environmental Engineering Laboratory of Sakarya University with the method given at Tanatti et al. (2018a, 2018b). Biodiesel wastewater was produced with 100 L reactors in the laboratory. Biodiesel wastewater comes with a very high pollution load and it varies in content and pollution values depending on the type of oil produced. In order to eliminate these negative effects, raw biodiesel wastewater was produced from sunflower oil by transesterification method and purified by wet washing. In this study, the COD value of the raw biodiesel wastewater is approximately 400,000 mg·L−1, the BOD value is 206,000 mg/L and the oil-grease value is around 25,000 mg·L−1. Thus, biodiesel wastewater cannot be treated with a single-stage process or conventional treatment method. This is because the COD value is not below 1,000 mg·L−1 and the oil-grease value is very high, and these parameter values make biological treatment very difficult. The produced raw biodiesel wastewater was pre-treated with electrocoagulation (EC) (Tanattı et al. 2018a, 2018b) and got ready for oxidation studies. The EC process was operated at an initial pH 6, conductivity (NaCl) 1 g·L−1, current density 0.3226 mA·cm−2 and optimum electrolysis time of 1 minute. Although all fatty acid methyl esters are removed in the EC method, COD and MeOH values are not appropriate for discharge to the receiving environment. Therefore, after EC, ECBD needs an advanced treatment method such as KMnO4 and KMnO4/O3 oxidation. Characterization of ECBD wastewater is shown in Table 1.

### Chemical analysis

COD analyses were measured according to the Standard Methods for Control of Water and Wastewater, method 5220 D, 23rd edition (APHA 2017). pH adjustments were made with 0.5 M NaOH and 0.5 M H2SO4 solutions.

MeOH concentrations were measured spectrometrically with SHIMADZU UV/Vis 1,700 model spectrophotometer device. 10% (m/v) sodium nitroprussite, 10% (m/v) potassium ferricyanate and 5% (m/v) sodium hydroxide solutions were used as reagents in the analysis (Zhan et al. 2010). For the preparation of the chromogenic reagent (SNP), first the reagent solutions are combined and then doubled with pure water at volumetric flask. For methanol analysis, 6 mL of SNP is added to 1 mL of sample and shaken for 1 min. After waiting 45 minutes at room temperature, the absorbance value is measured in the spectrophotometer at 481 nm wavelength.

### Reactor design

As shown in Figure 1, the KMnO4/O3 process has consisted of an ozone generator with a capacity of 15 g·L−1·h−1, a magnetic stirrer and a 360 ml glass reactor. The ozone generator (SABO ELEKTRONİK BRAND SL-10 model) used in the experiments produces ozone with a high frequency corona discharge. In addition, the ozone generator has a current of 4 A and a power consumption of 1,000 Wh. The remaining ozone in each experiment was removed from the system by applying a mixing process for 5 minutes.

### Effect of pH in KMnO4 and KMnO4/O3 processes

In KMnO4 oxidation, pH is considered one of the most important parameters to remove organic pollutants (Yang et al. 2018). Many studies have proved permanganate to perform an exchange of five electrons under acidic conditions, three electrons under neutral mild alkaline conditions and one electron under alkaline conditions. The reactions of permanganate due to pH changes are as in Equations (1)–(3) (Xu et al. 2016, 2017; Yin et al. 2017).
(1)
(2)
(3)

Ozone is a very strong oxidant (redox potential of 2.07 V). Through the special dipole structure, it reacts rapidly with organic and inorganic materials in aqueous solutions (Li et al. 2018). Depend on pH changes two types of reactions occur as direct and indirect in the oxidation of wastewater with ozone (Waldemer et al. 2010).

The direct and indirect reactions of ozone are as follows in Equations (4) and (5):

Direct reactions:
(4)
Indirect reactions: (total reaction)
(5)

In this study, the effects of pH on the treatment efficiency of ECBD wastewater were investigated by using advanced oxidation processes of KMnO4 and KMnO4/O3. Experiments were conducted at a dose of 1 g·L−1 KMnO4 and 0.06 g·L−1·h−1 O3, reaction time of 1 hour at chosen pH values (between 2 and 13). After the implementation of both methods, removal efficiencies were determined by measuring COD and MeOH parameters in ECBD wastewater. Figure 2 shows the COD and MeOH removal efficiencies for both processes.

In the KMnO4 process, while the maximum COD removal efficiency was observed at pH 2, a decreasing trend was observed with the pH increase. If numerically expressed, the COD removal efficiency was 83.99% at pH 2 and 78.59% at pH 13. As can be seen from Figure 2, the MeOH removal decrease was observed with the pH increase. The highest MeOH removal efficiency was obtained at pH 2 as 90.78%. Under acidic conditions, MnO4− gets the highest E0 (electrode potential) value. Due to the pH increment, E0 decreases caused removal efficiencies to fall away. In the research conducted with p-arsanilic acid and roxarsone, researchers achieved high efficiency in reduction of these substances with permanganate under acidic (pH 2–7) conditions (Xie et al. 2019).

In the oxidation of sulfadiazine with permanganate, close to 100% at pH 3.76 was removed in 10 minutes (Yang et al. 2018) has been reported that the chemical properties of organic compounds may play a role in the reactions with permanganate. For example, the highest removal efficiency of trichloracan was determined as pH 8. (Jiang et al. 2009).

The COD removal efficiency in the KMnO4/O3 process shows a gentle increasing trend depending on pH. The COD removal efficiency was 81.95% at pH 2 and rises up to 85.50% at pH 13. Also, in the same process, determined that the MeOH removal was not changed significantly depending on the pH as clearly seen in Figure 2. While at pH 2 the MeOH removal efficiency was 89.60%, at pH 13 the highest removal performance was observed as 92.32%. Ozone oxidation performs with indirect reactions by generating OH· radicals at high pHs. Since the E0 value of the generated OH· radicals were higher, it has been observed that the removal efficiency increases at high pH. Although there are many studies about biodiesel wastewater treatment, none uses KMnO4/O3 and KMnO4 processes. In the study of oxidation of ECBD wastewater with ozone, COD removal was investigated and the highest removal efficiency was found as pH 13.9 (Tanatti et al. 2019). In the treatment of wastewater, OH· radicals are formed due to the indirect reaction of ozone at high pH and high removal efficiencies are achieved in wastewater treatment. (Zhan et al. 2019).

### Effect of KMnO4 dose in KMnO4 and KMnO4/O3 processes

Oxidant dose has a significant effect on the breakdown of organic compounds (Xie et al. 2019). In order to determine the effect of KMnO4 dose on both processes, studies have been carried out between 0.5 g·L−1 and 7.5 g·L−1 doses. In the previous pH determination studies conducted, the optimum pH was 2 for the KMnO4 process and 13 for the KMnO4/O3 process. For both processes, studies were maintained with 0.06 g·L−1·h−1 as ozone dose and 1 hour as the reaction time. The results obtained are shown in Figure 3. COD and MeOH removal efficiencies increase with increasing KMnO4 dose, but after a certain KMnO4 dose, the increase in removal is very small. COD and MeOH removal efficiencies increase up to 5 g·L−1 KMnO4 dose, and there is an increase of 0.2% in removal efficiency after 5 g·L−1 KMnO4 dose. As can be seen from Figure 3, the COD and MeOH removal efficiencies of 81.82% and 89.11, respectively, at 0.5 g·L−1 KMnO4 dose, obtained as 86.87% and 92.92% at the 5 g·L−1 KMnO4 dose. On the other hand, at a dose of 7.5 g·L−1 KMnO4, the COD and MeOH removal efficiencies were found to be 87.02% and 92.97%.

According to the experimental results in the KMnO4/O3 process, COD removal efficiency was reached the maximum level at a dose of 2 g·L−1 KMnO4. Over this dose, KMnO4 did not have an extra effect on COD removal and created a stable trend. Up to a dose of 2 g·L−1 KMnO4 has shown a rapid increase regarding the MeOH removal. Above this dose the MeOH removal yields were determined as about average 0.15%. At the dose of 2 g·L−1 KMnO4, COD and MeOH removal efficiencies were obtained as 88.47% and 94.58%, respectively.

In a study conducted with pulp and paper wastewater, it was determined that removal efficiencies increased with the permanganate dose increment; a significant change was not observed after 1.5 mM dose (Jaafarzadeh et al. 2017). In another study, it was founded that with a rise in permanganate dose the treatment of triclosan advanced up to 100% (Chen et al. 2016).

### Effect of O3 dose in KMnO4 and KMnO4/O3 processes

Ozone dose is one of the most important parameters affecting the treatment efficiency of the KMnO4/O3 process. With the increase of the ozone dose, the formation of OH radicals in the process also rises. The study was carried out with varying ozone doses as 0.03, 0.06, 0.1, 0.15, 0.3 and 0.9 g·L−1·h−1, keeping pH 13, KMnO4 dose 2 g·L−1 and 1 hour reaction time constant. The Figure 4 shows the effect of ozone dose on COD and MeOH removal efficiencies. COD removal efficiencies at these ozone doses were found to be 80.31, 82.01, 82.57, 83.65, 85.14 and 85.35%, respectively. MeOH removal efficiencies were obtained as 90.14, 90.92, 91.12, 91.58, 93.38 and 93.18%, respectively. Up to 0.3 g·L−1·h−1, ozone dose removal efficiencies rises in conjunction with the ozone flow, after this point the increase is negligible. The solubility of ozone in wastewater is limited, so after the saturation with ozone, the increased ozone dose has no extra effect on the removal efficiency (Deng 2020). O3 doses between 0.075 g·L−1·h−1 and 2 g·L−1·h−1 were studied in the treatment of biodiesel with the O3 process. It has been reported that the highest removal efficiency of biodiesel wastewater was achieved at a dose of 1.5 g·L−1·h−1 O3. Moreover, the COD removal efficiency was found to be 62.89% and the MeOH removal efficiency was 87.64% at this O3 dose in 2 h reaction time (Tanatti et al. 2019).

### Effect of reaction time in KMnO4 and KMnO4/O3 processes

The experiments conducted in both processes within the study's scope were carried out between 0.25 hours and 6 hours to determine the reaction time. The effects of the reaction time on the COD and MeOH removal efficiency were investigated by using the optimum parameters (pH 2, 5 g·L−1 KMnO4) in the KMnO4 process. According to Figure 5, in the KMnO4 process, COD and MeOH removal efficiencies increased with longer reaction times. However, the increase in removal efficiencies after 3 hours is an insignificant level. When the experimental results were examined, the COD removal efficiency was 79.91% in the 0.25 hour reaction time, while it was 91.74% in the 6 hour reaction time. At the reaction time of 0.25 hours, 85.02% of the existing MeOH in the biodiesel wastewater was removed, at the end of 6 hours, 95.83% removal was achieved.

Studies to determine the effect of reaction time in the KMnO4/O3 process were carried out with pH 13, 2 g·L−1 KMnO4 and 0.3 g·L−1·h−1 O3 dose. In the KMnO4/O3 process, COD and MeOH removal efficiencies increase with the extension of the reaction time as in the KMnO4 process. When Figure 5 is examined, COD and MeOH removal yields were obtained as 85.97% and 87.33, respectively, at a reaction time of 0.25 hours. However, in the reaction time of 6 hours, 97.79% of COD is removed and the MeOH removal efficiency has observed as 98.30%. The effect of reaction time on the treatment of biodiesel wastewater with the O3 process, pH 13.9, 1.5 g·L−1·h−1 O3 dose and 25 °C temperature was investigated. In the study, 0.5 and 9 h reaction times were studied, but the highest COD and MeOH removal efficiencies were achieved at the end of 9 h (Tanatti et al. 2019). As seen in our study and other studies, biodiesel wastewater is treated in a shorter time with the KMnO4 and KMnO4/O3 processes.

### Kinetic analysis of COD and MeOH removal efficiencies in KMnO4 and KMnO4/O3 processes

Kinetic parameters are obtained using experimental data. Reaction rate constants and regression coefficients can be achieved by using kinetic modeling (Kaur et al. 2020).

In the study, first-order, second-order and pseudo-first-order, pseudo-second-order kinetic models, which are widely used in the literature, were examined for the KMnO4 and KMnO4/O3 processes. The equations of the used kinetic models are given below (Simonin 2016; Bener et al. 2019; Samal & Trivedi 2020; Titchou et al. 2020).
(6)
(7)
(8)
(9)
where Co: Inital concentration (mg·L−1), C: Final concentration (mg·L−1), t: Reaction time (min), Ce: Concentration coefficient (mg·L−1), k2: Mean mass transport coefficient (min−1).

The reaction rate constants (k) and regression coefficient (R2) values for the COD and MeOH parameters obtained with the kinetic examination performed for the KMnO4 process are given in the Table 2. Four models were examined separately and the highest R2 values were reached with the pseudo-second-order model. While the R2 value obtained as 0.9999 for COD with the pseudo-second-order model for the MeOH removal was achieved as 1 with the same model. k values were found as 0.00006 L·mg−1·min−1 and 0.000163 L·mg−1·min−1 for COD and MeOH, respectively.

The reaction rate constant (k) and the regression coefficient (R2) in the kinetic analysis for the COD and MeOH parameters in the KMnO4/O3 process are given in the Table 3. The highest R2 values among the examined kinetic models were obtained in the pseudo-second-order model. The determined R2 values by the kinetic examination for the COD and MeOH parameters are 0.9994 and 0.9999, respectively as shown in Table 3. k values were specified for COD and MeOH removal as 0.00006 L·mg−1·min−1 and 0.00016 L·mg−1·min−1.

When the regression coefficients in Tables 2 and 3 are examined, the most significant kinetic model is found to be the pseudo-second-order kinetic model for both processes. However, the second-most significant model in the COD and MeOH parameters for the KMnO4 process is the pseudo-first-order kinetic model. However, when R2 is evaluated here, it is seen as significant around 90%. In addition, the most significant model for COD removal in the KMnO4/O3 process is the first order, while the most significant model for MeOH removal is the second order. The R2 values are evaluated within 5% significance in the kinetic models found suitable for this process.

Theoretical and experimental removal efficiencies obtained by the pseudo-second-order models for KMnO4 and KMnO4/O3 processes and results are given in Figures 6 and 7. As seen in Figure 6, the COD removal efficiency rapidly increases in the first 90 minutes, and then it slows down. In the comparison of the experimental and theoretical results, both results overlap in the longer time periods. However, as seen in Figure 7, the MeOH removal efficiencies given theoretically and experimentally are in agreement. As with COD, MeOH removal efficiencies increase slowly depending on time.

### Cost analysis

The cost of KMnO4 and KMnO4/O3 processes are affected by electrical energy consumption (ozone production and stirrer) and chemical reagents. The operational cost of KMnO4 and KMnO4/O3 processes of treated effluent has been estimated in US$.m−3 by considering several parameters including electrical energy, amount of KMnO4, NaOH and HCl. The amount of electricity consumed for ozone used in the system has been calculated as 20 kWh·m−3 and the electrical energy price was calculated as US$1.81. However, the amount of electricity for both processes consumed for mixing the system is 1.9 kWh·m−3 and the price of electricity is US$0.182. The NaOH and HCl consumtions are 0.2 kgm−3 and 0.5 Lm−3 and their costs are US$ 1.12 kg−1 and US$3.33 L−1, respectively. The operating cost for the treatment per m3 of the KMnO4 and KMnO4/O3 processes is calculated in USD. The cost of the KMnO4 and KMnO4/O3 processes are US$3.512 and US\$3.112, respectively. The results showed that the KMnO4 and KMnO4/O3 processes showed similar cost. Therefore, the KMnO4/O3 process provides superior removal efficiency and cost effectiveness, resulting in excellent performance for the decontamination of biodiesel wastewater.

The treatment of biodiesel wastewater is difficult due to the high pollution loads and it is not possible to meet the discharge standards by most methods. The treatment to be made by using these methods can be carried out with the processes used instead of providing the discharge standard by passing through many treatment systems and cost-effective maintenance. The KMnO4/O3 process especially emerges as a very important process in terms of both cost and removal efficiency.

Two different advanced oxidation processes have been used on ECBD wastewater treatment. The optimum conditions for KMnO4 process were determined as pH 2, 5 g·L−1 KMnO4 dose and 6 hour reaction time. Moreover, the optimum conditions for KMnO4/O3 process were determined as pH 13, 2 g·L−1 KMnO4 dose 0.3 g·L−1·h−1 O3 dose and 6 hour reaction time. Therefore, under the optimum conditions, the COD removal efficiencies for the KMnO4 and KMnO4/O3 processes was obtained as 91.74% and 97.79%, respectively. Furthermore, the MeOH removal efficiencies for KMnO4 and KMnO4/O3 processes was obtained as 95.93% and 98.30%, respectively. The result of this study was verified that the COD and MeOH removals were reached the highest efficiency for both processes, whereas compared to the KMnO4 process, it was seen that the KMnO4/O3 process removal efficiency is higher than the KMnO4 process.

The observation of the the KMnO4 and the KMnO4/O3 processes show that with a 6 hour reaction time obtained COD values 1,421.36 mg·L−1 and 380.1 mg·L−1 respectively. According to Turkey's water pollution and control regulation, the COD limit value is 400 mg·L−1 for biodiesel wastewater to discharge to the receiving environment. With this scope, the comparison of the KMnO4 and KMnO4/O3 processes for COD parameter shows that with the KMnO4 longer reaction time is necessary to achieve regulation standards for biodiesel wastewater treatment. However, in the KMnO4/O3 process, COD can be reduced to the limit value with shorter reaction time.

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

The authors declare there is no conflict.

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