Optimization of ibuprofen degradation in water using high frequency ultrasound-assisted biological reactor

The rapid industrial investment and the growing water demand due to increasing population are leading to huge pressure on water resources. This fact is appearing more in water-scarce countries. Thus, adopting integrated water resources management has become indispensable. Wastewater reuse is a key pillar in the water cycle management that would help in avoiding the deterioration of the ecosystem and reduce the pressure on the available water resources. However, wastewater contains pollutants that are characterized by high chemical stability and by recalcitrance to mineralization. Consequently, the requirements for the treatment process of wastewater for reuse purposes has become increasingly stringent, especially with the strict directives related to the aforementioned challenging pollutants (Abdelhay et al. 2019). These pollutants are not completely eliminated from the wastewater treatment plants (Miège et al. 2008) because they are not amenable to conventional treatment (Wang et al. 2019a, 2019b). Therefore, they persist in the environment, which has triggered a search for innovative non-conventional treatment methods. Recently, advanced oxidation processes (AOPs) received considerable attention as potential methods to remove such non-biodegradable substances (Méndez-Arriagad et al. 2008; Xiang et al. 2016; Boczkaj & Fernandes 2017; Gągol et al. 2018; Wang et al. 2019a, 2019b). Sonication is coming up as an interesting AOP process due to its competitive capacity to destroy organic contaminants (Chakinala et al. 2008; Oztekin & Sponza 2013; Tran et al. 2015). Sonication relies on the acoustic cavitation phenomenon, which could be generated in two ways, first by passage of ultrasonic waves; in this case, it is called acoustic cavitation or ultrasonication. However, if it occurs due to pressure variation in liquid, it is called hydrodynamic cavitation. Due to cavitation, the bubbles in the liquid collapse violently releasing large amount of energy over a very small location according to the ‘hot spot’ theory (Doosti et al. 2012). As a result, water molecules undergo homolysis yielding free hydroxyl radicals (OḢ) (Mahvi 2009). Hydroxyl radicals are powerful oxidizing agents that have the capability to attack organic compounds (Adewuyi 2005).

Among the persistent organic contaminants that can be potentially eliminated using ultrasonication are pharmaceutical residuals. Those are considered as an emerging environmental problem as their presence in water poses a potential ecotoxicological risk (Odabasi & Buyukgungor 2017). Ibuprofen (IBP) is one of the most widely used non-steroidal anti-inflammatory drugs and it is most commonly found in water bodies (Daughton 2001; Daneshvar et al. 2010). In spite of the good oxidation rate of IBP obtained by several types of AOPs (Tran et al. 2015; Xiang et al. 2016; Wang et al. 2019a, 2019b), ultrasound (US) still offers some unique features such as not requiring adding chemicals, short treatment time, and insensitivity to electrode or piezoelectric disc coverage by the organic intermediates (Abdelhay et al. 2020). Little information is available in literature about a comprehensive optimization and of IBP degradation using US as pretreatment to conventional methods.

The present study aimed to: (i) investigate the effect of several experimental parameters on the degradation of IBP using a high frequency ultrasound, namely, the power density (480, 960) W/L, US frequency (1,142, 860, 578) kHz, working volume (500, 250) mL, pH (8.2, 4), and initial concentration (30, 60) mg/L; (ii) evaluate the degradation kinetics; (iii) evaluate the performance of a conventional aerobic biological reactor for IBP degradation after pretreating water with US under the optimal conditions.

Ibuprofen is (±)-2-(p-Isobutylphenyl) propionic acid, a non-steroidal anti-inflammatory drug. Its molecular formula is C₁₃H₁₈O₂. Figure 1 represents the chemical structure of IBP. The IBP used in this study was obtained from Jerash Pharma, from IOL Chemicals and Pharmaceuticals Limited, Industrial Area, Punjab, India. The physicochemical properties of IBP are listed in Table 1.

The water samples were prepared by dissolving IBP in deionized water using a magnetic stirrer at room temperature (25 °C). Two stock solutions were prepared with two different IBP concentrations (30 and 60 mg/L).

The batch assays of this study were performed in a 1 L cylindrical Plexiglas reactor (Meinhardt Ultrasonics) with an internal diameter of 6.925 cm (Figure 2). The temperature of the sonicated water samples was controlled by means of the water flow in a double-jacketed vessel. Ultrasound waves were produced by piezoelectric ceramic transducer (Meinhardt Ultrasonics) placed at the base of the reactor. The transducer (diameter: 75 mm, height: 75 mm) was adjusted to provide the system with three different working frequencies (578, 800 and 1,142) kHz and a power output (400 W/cm²) that could be controlled and varied using an ultrasonic multi-frequency generator with a maximum value of 240 Watt (Meinhardt Ultrasonics). The temperature of the transducer and the generator was monitored through sensors. The transducer temperature was kept at 40 °C.

Figure 3 shows the schematic diagram of the used aerobic biological reactor (Solteq TR 28). The bioreactor consists of a 10 L clear acrylic vessel mounted on a base with a pH meter, electrical heater, dissolved oxygen meter, and a temperature controller. The reactor was operated in batch mode for 4 days. The temperature of the reactor was maintained at 20 °C.

The ultrasonication experiments were used to carry out several tests to determine the effect of different operating conditions on the degradation of IBP. Three different working frequencies were studied, including 1,142, 860, and 578 kHz. Likewise, the effect of power density (960 and 480 w/L), IBP initial concentration (30 and 60 mg/L), working volume (250 and 500 mL), and pH (4 and 8.2) were studied as well. All water samples were sonicated and treated for 120 min. The degradation of IBP using sonication was followed up by withdrawing a 2 mL sample every 30 minutes from the reactor. All sonication experiments were conducted twice.

The potential of ultrasonication as a pretreatment system was investigated. The treated effluent from the high frequency ultrasound was used as feed to the biological reactor after adjusting the pH back to around 7 using NaOH. The biological treatment batch experiments were carried out at room temperature in the 10 L pre-described aerobic reactor with a working volume of 5 L filled with activated sludge and treated for 4 days at 25 °C and pH of 7 using an airflow rate of 4.5 L/min. The activated sludge was collected from the German Jordanian University local wastewater treatment plant (WWTP) (Table 2). The samples were taken from the supernatant of the aeration tank. The chemical oxygen demand (COD) was measured every 24 hours, whereas the IBP concentration was measured at the beginning and at the end of the experiment. The total nitrogen (TN) and COD values observed for the activated sludge collected from the university were higher than the reported values for activated sludge (Li et al. 2009). This is due to the fact that the WWTP receives sewage and wastewater from all chemistry laboratories at the university.

COD, TS, TN were measured according to APHA Standard Methods 1998. COD was measured using a Hach test kit. The samples were first digested by a Hach digester (DRB 200-Germany) then the COD was measured using a Hach spectrophotometer (DR/2010-Germany). The pH of the tested samples were measured using a digital calibrated Jenway 3,540 pH meter.

The kinetic study consists of following up the IBP removal percentage versus the sonication time at the determined optimal operating conditions (578 kHz and 480 W/L) for samples having different initial COD concentrations (30 and 60 mg/L). Then, the first order kinetic model was tested by nonlinear regression techniques to describe the kinetics of IBP removal during sonication.

The relevant equations are as follows.

One of the key parameters in the application of the ultrasonication process is the applied frequency. Therefore, the effect of frequency on IBP degradation has been investigated by testing three values: 578 kHz, 860 kHz, and 1,142 kHz. The frequency values were selected based on the operational settings of the piezoelectric ceramic disc as it is adjusted to provide only these values. Figure 4 shows the time course changes in IBP removal percentage over 120 min. The IBP concentration and power density imposed during this test were 30 mg/L and 480 W/L respectively. It can be noticed that the removal percentage of IBP decreased with increasing ultrasound frequency. About 96% of IBP was removed when a frequency of 578 kHz was applied while only 39% was removed at 1,142 kHz. This is consistent with the fact that low frequency of ultrasound has stronger sonophysical effects than high frequency, since the bubbles which experience low frequency ultrasonic field grow bigger and collapse more violently. Consequently, this would lead to higher localized temperatures and pressures at the cavitation site or higher release of OḢ radicals (Petrier & Francony 1997; Chen et al. 2012; Adityosulindro et al. 2017).

Another important parameter in terms of deciding the percentage of IBP removal is the power density. Thus, the experiments were carried out at two different power densities (480 w/L and 960 W/L). It follows from the data obtained in Figure 5 that the IBP removal percentages at a frequency of 578 kHz were 96%, and 99% at 480 W/L and 960 W/L respectively. Therefore, it can be concluded that the removal percentage increases as the power density increases. This could be justified by the logic that increasing the power dissipated per treated volume will increase the cavitation activity, which in turn increases the number of bubbles generated and the amount of (OḢ) free radicals released (Emery et al. 2003; Naddeo et al. 2010; Tran et al. 2013). However, the increment in IBP removal with increasing power density is more noticeable for the samples sonicated at 860 kHz than at 578 kHz. As is shown in Figure 5, the increase in IBP removal with power density was 3% and 33% for 578 kHz and 860 kHz respectively. This observation may be attributed to the fact that sonication at high frequency results in small diameter cavities with weak energetic collapse, which makes the power density a limiting factor in promoting IBP degradation.

To assess the effect of the sonicated sample volume, the sonication experiment was carried out for 120 min, using two sample volumes of 250 mL and 500 mL. Preliminary experiments were carried out (data not shown here) and the results revealed that minimal IBP removal was observed at volumes exceeding 700 mL. Therefore, Figure 6 shows the IBP removal percentage as a function of time at two different working volumes (250 mL and 500 mL). As clearly seen, the working volume with the investigated values has no significant effect on the IBP removal. This observation is valid at the two different power inputs (Figure 6(a) and 6(b)). For both volumes, the IBP removal obtained in the US reactor operated with 240 W was around 99%. However, it dropped to 40% at a power input of 120 W. The effect of the working volume was explained by Matouq & Al-Anber (2007), who mentioned that the energy transmitted to the liquid through the transducer would be higher when the height of the treated sample was lower. Consequently, the aforementioned results could be explained by sufficient energy being dissipated from the piezoelectric transducer through the height of the liquid at the two volumes tested in the experiment. Thus, the adverse effect of the working volume was not observed in the reported experiment.

Figure 7 shows the effect of IBP initial concentration on its degradation at a pH of 8.2, a frequency of 578 kHz, and a power density of 480 W/L. Two initial concentrations were selected for the experiments (30 mg/L and 60 mg/L) which are respectively below and equal to the IBP solubility in water (58 mg/L) (Dahane et al. 2013). The results revealed that the removal percentages of IBP at sonication time less than 120 min were found to decrease with increasing initial IBP concentration. For example, the IBP removal decreased from 87% to 57% after 60 min of sonication time when the IBP initial concentration increased from 30 to 60 mg/L respectively. However, after 120 min of irradiation time, there was no significant difference in the degradation of IBP between 30 and 60 mg/L. This is in good agreement with previous studies showing that the rate of IBP removal is inversely proportional with its initial concentration (Elsayed 2015; Wang et al. 2019a, 2019b). The increase in IBP concentration results in a weakening effect of cavitation reactions and consequently in insufficient generation of hydroxyl radicals to destroy the organic pollutants, and thus lower IBP removal rates (Wang et al. 1999; Stapleton et al. 2007). Nonetheless, it should be noted that the total amount of IBP degraded after 120 min at 60 mg/L was as much as two times larger than that degraded at 30 mg/L (Elsayed 2015).

The effect of IBP initial concentration on the ultimate IBP removal rate was also investigated at different power densities (Figure 8). It is obvious that the ultimate removal rate after 120 min sonication time was marginally higher at lower IBP initial concentration (30 mg/L). This was observed at the two tested power density values.

A kinetic study was performed to determine the rate at which IBP was removed during the sonication time. The first order model was tested in the analysis of the sonication data of IBP. The normalized form of the model at two different concentrations is depicted in Figure 9. The kinetic parameters, rate constants and regression coefficients are summarized in Table 3. Figure 9 and Table 3 show that the time profile of the IBP removal percentage was fitted appropriately by the first-order kinetics (R² > 0.93) at the two initial concentrations. This inverse relationship between the reaction rate constant and the initial concentration compared well with existing literature (Tran et al. 2015; Wang et al. 2019a, 2019b). The reaction rate constants presented in Table 3 are considerably higher than those reported by Le et al. (2019). This might be attributed to the higher power density applied in the current study.

It is well known that the pH of solutions markedly influences the ultrasonic degradation of organic pollutants. The effect of pH on IBP degradation by sonication is shown in Figure 10. The pH value was adjusted from 8.2 to 4 for an IBP initial concentration of 30 mg/L. The IBP removal percentage increased from 96% to 99.5% when the solution pH was decreased from 8.2 to 4. This confirms the results that the degradation rate of organics is enhanced in acidic media (Villaroela et al. 2014). The IBP at a pH of 4 exists in its molecular form since the pH value is lower than the pKa (4.9) of IBP. The molecular form of IBP has a higher hydrophobicity than its ionic form, which is produced at pH values exceeding the pKa. Under this condition, the molecular IBP accumulates in the vicinity and at the surface of the cavitation, which stimulates the degradation rate due to the high concentration of OḢ at the cavitation site [3, 5].

Under the predetermined optimal ultrasonication parameters (30 mg/L IBP, 578 kHz, 960 w/L, pH 4, 120 min) one experiment was carried out to investigate the efficiency of the high frequency ultrasound as an effective pretreatment method for biological degradation of IBP. Under these working conditions, 99.5% of the IBP was removed from the ultrasound-treated effluent (as previously mentioned). However, Figure 11 shows that the COD was not completely removed, which is due to the accumulation of degradation byproducts in the solution during ultrasonication. The percentage of COD removal in an IBP-containing solution reached only 37% of an initial COD of 54 mg/L. Therefore, the sonicated effluent was fed to an aerobic bioreactor and treated for 4 days. Figure 12 illustrates that the COD was further removed from the presonicated samples in the bioreactor reaching a percentage of 47% after 4 days. The enhancement of COD removal occurred most probably because more soluble byproducts were released under ultrasonication pretreatment in comparison with the case without pretreatment (Le et al. 2019). However, no effect was observed on COD removal in the samples directly fed to the biological reactor without pretreatment (Figure 12,). This could be a result of the weak capacity of the biological treatment to initiate a breaking down of the IBP molecule within the given operating time. In one study (Krishnan et al. 2016), it was reported that biological wastewater treatment methods are deemed less effective for complete elimination of highly persistent chemicals, thus necessitating research on AOPs. This conclusion is justified by the need for biological treatment of successive culture enrichments in order to get adapted bacteria to IBP.Langenhoff et al. (2013) tested the degradation of 50 mg/L of ibuprofen in aerobic bioreactor using unadapted sludge and found that the IBP was mineralized after 27 days in the aerobic microcosms. The fact that the IBP-containing wastewater was presonicated helped in the conversion of IBP to intermediates that can be degraded by the unadapted sludge used in this study. Furthermore, the IBP concentration in the samples sonically pretreated was measured before and after the biological treatment. The results (Figure 13) revealed that the IBP residues after ultrasonication remained untouched in the biological treatment, as justified by the constant peak area in Figure 13 detected by the HPLC. Hence, the biological treatment was efficient in completing the degradation of IBP byproducts generated during the ultrasonication pretreatment process but it failed in breaking down the IBP residues. This finding supports the aforementioned discussion about the weak capability of unadapted activated sludge to degrade IBP within 4 aeration days. These results suggest that ultrasonication is a promising essential technology to assist the conventional treatment when dealing with recalcitrant organics. Arnoud de (2018) reported enhanced biological removal of IBP from water pretreated using intensive photocatalysis where IBP was completely removed within 3 days. It is hypothesized that the enhanced biodegradation of ibuprofen was triggered by the presence of photocatalytic products.

The objective of this work was to optimize IBP removal by ultrasonication as a pretreatment step before conventional biological treatment. The results presented in this work depict the interesting potential of ultrasound to breakdown IBP into biodegradable byproducts that can be handled by conventional treatment. Thus, ultrasonication was effective as a pretreatment process in assisting the biological treatment. Interestingly, the highest ultrasonication efficiency (99.5%) was obtained at a power density, working frequency, IBP initial concentration, and pH of 960 W/L, 578 kHz, 30 mg/L, and 4 respectively. The IBP was remaining in samples treated in the biological reactor without prior sonication. In addition, the IBP residues in the samples pretreated by ultrasound was also not removed after the biological treatment. Thus, a standalone biological reactor was not efficient in cleaving the IBP ring and trigger the IBP degradation.