Nano-TiO₂, ultrasound and sequential nano-TiO₂/ultrasonic degradation of N-acetyl-para-aminophenol from aqueous solution

Pharmaceuticals such as analgesics, antibiotics, antiepileptics, etc., are not completely removed during wastewater treatment. Pharmaceuticals are usually present in the treatment effluents, contaminating waterways and public water supplies all over the world; they are among important emerging water contaminants (Moussavi et al. 2016). The release of pharmaceuticals into water bodies has led to their accumulation which adversely affects the ecosystem as well as causing prolonged effects on human health. Pharmaceuticals are intended to preserve their chemical structure long enough to exert their therapeutic effect. This property, as well as the continuous use of these drugs, enables pharmaceutical products to persist in the environment for a long period of time (Isariebel et al. 2009). N-acetyl-para-aminophenol (AAP; acetaminophen or paracetamol) is a drug component that has been used to reduce inflammation and relieve fever and pain by blocking enzymes and proteins associated with pain. It is regarded as one of the most widely prescribed pharmaceuticals around the world (Galhetas et al. 2014; Zavala & Estrada 2016). It was estimated in 1998 that 3.2 × 10⁹ AAP tablets were consumed in the UK (Xagoraraki et al. 2008). Stackelberg et al. (2004), Gomez et al. (2007), Ternes (1998), and Gros et al. (2006) reported the occurrence of AAP in surface water, raw wastewater, treated wastewater, and treated drinking water, respectively. Zavala & Estrada (2016) identified the toxic effects of AAP on microorganisms in aquatic systems. Therefore, there is a need to treat water and wastewater containing pharmaceutical products in order to improve water quality and protect human health.

Moussavi et al. (2016) investigated the adsorption of AAP on double-oxidized graphene oxide. The authors reported that the maximum adsorption capacity of AAP (704 mg/g) was obtained at pH ≤ 8, the adsorption data was well fitted by the pseudo-second-order kinetic and Langmuir equilibrium model. Isariebel et al. (2009) evaluated the influence of operating conditions of the ultrasound process on the degradation of AAP and levodopa. The authors reported that the ultrasonic degradation of these drugs follows a pseudo-first-order reaction kinetics and the best results were obtained with 574 kHz ultrasonic frequency. The ultrasonic degradation of AAP and naproxen in the presence of powdered activated carbon and biochar adsorbents has been reported by Im et al. (2014). The authors described that the degradation of pollutants in aqueous phase by ultrasound occurs by the formation, growth, and implosive collapse of bubbles in a liquid; this involves some reaction pathways such as pyrolysis inside the bubble and hydroxyl radical-mediated reactions at the bubble–liquid interface and/or in the liquid bulk (Isariebel et al. 2009).

TiO₂ is a promising oxide that has been commonly used in photocatalytic applications due to its unique strong oxidizing power, high photostability, and redox selectivity. TiO₂ exists in rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic) forms. It can be synthesized by chemical impregnation, hydrothermal method, direct oxidation of TiCl₄, metal organic chemical vapor deposition method, physical vapor deposition, etc. (Haider et al. 2015). Well dispersed TiO₂ nanoparticles (nano-TiO₂) with fine sizes are promising in many applications (Vijayalakshmi & Rajendran 2012) such as in pigments, catalytic supports, photocatalysts, and adsorbents for the remediation of contaminated water and wastewater. Several investigations such as chemical (Hiremath et al. 2006), electrochemical (Arredondo Valdez et al. 2012), adsorption (Galhetas et al. 2014), Fenton (Basavaraju et al. 2011), photocatalysis (Yang et al. 2009; Aguilar et al. 2011), ozonation (Andreozzi et al. 2003), ultrasound (Isariebel et al. 2009), UV (Ratpukdi 2014) and the combined methods have been carried out on the removal of AAP in aqueous solutions. Nevertheless, partial degradation of AAP was achieved rather than total degradation; besides, many of these techniques require high voltage or incessant chemical feeds which increase the operational cost (Zavala & Estrada 2016). Previous research studies have shown that the integration of different treatment systems enhances treatment efficiency as well as reduces cost. An advanced oxidation process such as ultrasound is regarded as a pretreatment method in an integrated system. Therefore, the integration of ultrasound treatment process and adsorption will be useful to take advantage of the methods and to minimize the drawback of the two methods.

The objective of this study is to investigate the potential of nano-TiO₂, ultrasound and a combined/sequential nano-TiO₂/ultrasound for the remediation of a model aqueous solution of AAP. The influence of various operating conditions on the removal efficiency of AAP was investigated as well as the modeling of the kinetics, equilibrium, and thermodynamic studies. The importance of this treatment technique is its low cost and it could be employed in a scaled-up water treatment process for large volumes containing organic and inorganic pollutants.

Titanium(IV) oxide nanopowder, 21 nm primary particle size, and AAP (>99%) were supplied by Sigma-Aldrich. The nano-TiO₂ was chosen based on its unique chemical and biological inertness, high surface area, and low cost of production. Stock solution containing 1,000 mg/L AAP was prepared by dissolving 1.0 g of AAP in 1,000 mL of deionized water and the made up solution was stored in the dark at 4°C. Working solutions were prepared daily by serial dilution. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust the pH of AAP solutions during the study of the influence of pH on the degradation efficiency.

The mineral phase identification of the nano-TiO₂ was conducted by X-ray diffraction (Siemens D8 Advance Bruker XRD). The morphology of nano-TiO₂ was obtained by scanning electron microscopy (SEM; Nova Nano SEM 230) and transmission electron microscopy (TEM; FEI Tecnai G² 20). Energy dispersive spectroscopy (EDS) was used to study the elemental composition of nano-TiO₂ and the infrared spectrum was obtained by Perkin Elmer Universal attenuated total reflection–Fourier transform infrared (UATR-FTIR) Spectrum Two™ spectrometer.

The adsorption of AAP was achieved by mixing a range of nano-TiO₂ (0.01–0.05 g) at ambient temperature with 50 mL of 100 mg/L AAP aqueous solution (pH 5.66), which was stirred at 120 rpm for 10 min. After the optimization of the nano-TiO₂ dosage, a 0.03 g of nano-TiO₂ was mixed with 50 mL of 100 mg/L AAP aqueous solution and stirred at 120 rpm for different contact times ranging from 15 to 100 min. The results obtained on the influence of contact times were used for the kinetic investigation. The equilibrium study was examined at AAP concentration ranging from 6.25 to 100 mg/L. The pH (adjusted with 0.05 M HCl and NaOH) was investigated at pH 3–8.37, while the thermodynamic parameters were obtained by treating 100 mg/L AAP aqueous solution with nano-TiO₂ at 295 K to 343 K.

A 100 mL of 100 mg/L AAP solution was placed in a 250 mL beaker and sonicated (Misonix Ultrasonic Liquid Processors Sonicator S-4000) at 20 kHz ultrasonic frequency for 60 min. This followed the optimization of pH and time on the ultrasound degradation efficiency of AAP. After the optimization of pH and time, AAP aqueous solution was sonicated at 20, 40, 80, and 100 kHz ultrasonic frequencies with or without nano-TiO₂.

The morphological study of nano-TiO₂ by SEM (Figure 1(a)) and TEM (Figure 2(b)) indicated regular spherical shaped particles of nano-TiO₂. The EDS (Figure 1(b)) as an attachment of the scanning electron microscope showed that the nano-TiO₂ is made up of C (20.53 ± 4.39%), O (42.82 ± 5.06%), and Ti (36.65 ± 3.85%). From Figure 2(a), it was observed that the diffraction peaks of nano-TiO₂ that appeared in the pattern corresponded to the anatase and rutile TiO₂ phase with good crystallinity.

Anatase (JCPDS Card no. 21-1272) peaks are found at 2θ values of 25.3°, 36.9°, 37.8°, 48.1°, 53.9°, 55.06°, 62.7°, 68.8°, 70.3°, 75.0°, and 82.7°; these correspond to (101), (103), (004), (200), (105), (211), (204), (116), (220), (215), and (224) crystal planes. Rutile (JCPDS Card no. 21-1276) peaks are found at 2θ values of 27.4°, 41.2°, and 56.6°; these correspond to crystal planes of (110), (111), and (220). The anatase peaks were dominant; consequently, the nano-TiO₂ that was used in this study was mainly of anatase form.

The FTIR spectrum of nano-TiO₂ (Figure 3) showed very weak absorption in the frequency region of 1,638.46 cm⁻¹; the absorption at 1,638.46 cm⁻¹ corresponds to Ti-O-Ti vibrations. Adsorption bands which may correspond to Ti-O bonding were obtained below the 400 cm⁻¹ regions (400–350 cm⁻¹).

Figure 4 shows the results on the influence of nano-TiO₂ dosage on the treatment of 100 mg/L AAP. The results showed an initial rapid removal of AAP (46.09 mg/g) within 10 min with 0.01 g nano-TiO₂. The increase of the nano-TiO₂ dosage from 0.01 g to 0.03 g brought about equilibration after which the percentage of AAP removed decreases as the nano-TiO₂ dosage increased from 0.03 g to 0.05 g. The aggregation of nano-TiO₂ particles in AAP aqueous solution at these dosages (0.03–0.05 g) might possibly limit the available active site of nano-TiO₂ and consequently lead to the reduction of the removal of AAP. This is supported by Pettibone et al. (2008) who mentioned that the aggregation of nano-TiO₂ plays a significant role in the amount of surface area available for adsorption and that aggregation negatively impacts the activity of nanoparticles. A 0.03 g dosage of nano-TiO₂ was chosen and maintained for further experiments.

Likewise, a rapid adsorption of AAP (77.23 mg/g) within 15 min was observed with 0.03 g nano-TiO₂. The removal efficiency sharply increased to 112.81 mg/g in 60 min, which may be credited to a larger surface area of nano-TiO₂ being available with time for the AAP molecules. The percentage removal of AAP, however, slightly reduced from 67.69% to 60.67% as the contact time was further increased from 60 to 100 min (Figure 5). Thus, 60 min contact time was maintained for further experiments. The slight reduction in the amount of AAP adsorbed at higher contact times could be attributed to the saturation of the active sites of the nano-TiO₂ by the AAP.

The kinetic information obtained from fitting the pseudo-first and pseudo-second-order kinetic models with the experimental data are summarized in Table 2. As seen in Table 2, the experimental data had better fitness (higher R² value; R² = 0.9848) with the pseudo-second-order model than with the pseudo-first-order kinetic model. Moreover, the experimental adsorption capacity q_e (113.98 mg/g) was close to the calculated q_e (102.04 mg/g), confirming the high correlation of AAP adsorption by nano-TiO₂ to the pseudo-second-order model. This implies that chemisorption is the rate limiting step, and that the AAP and nano-TiO₂ were affecting the adsorption process under the investigated conditions. Chen et al. (2015) and Moussavi et al. (2016) likewise reported that the removal of pharmaceuticals (sulfamethoxazole, ciprofloxacin, AAP) from aqueous solutions by graphene oxide followed the pseudo-second-order kinetic model.

The results of the influence of the initial AAP concentration are depicted in Figure 8. Figure 8 shows that the adsorption capacity of AAP initially increased from 61.94% at AAP concentration of 6.25 mg/L to 74.38% at AAP concentration of 25 mg/L; it slightly reduced from 73.38% to 66.64% when the AAP concentration was increased to 40 mg/L after which the adsorption capacity tended towards equilibrium. On the contrary, the equilibrium adsorption capacity increased from 6.45 mg/g to 112 mg/g when the initial AAP concentration was increased from 6.25 mg/L to 100 mg/L. The increase could be as a result of the enhancement of mass transfer rates due to a higher AAP gradient concentration at a higher initial AAP concentration, leading to the uptake of more AAP molecules per nano-TiO₂ mass unit.

The equilibrium parameters obtained from fitting the Langmuir and Freundlich equilibrium models are also summarized in Table 2. Based on Table 2, the equilibrium data had better fitness (higher R² value; R² = 0.9545) with the Freundlich model than with the Langmuir equilibrium model, which implies that AAP uptake occurs as a multilayer onto a heterogeneous nano-TiO₂ surface.

The study of the influence of pH on the adsorption of AAP at pH range 3–8 (Figure 11) showed that the treatment efficiency initially increased from 66.12% at pH 2 to 77.33% at pH 4 after which the adsorption decreased to 63.22% at pH 8. The pH increase of the medium may have led to changes in the adsorbate structure and adsorbent properties thereby resulting in a decrease in the amount of AAP removed at pH 4–8. Bernal et al. (2017) reported that AAP (pKa = 9.5) coexist in both the ionized (a base) and non-ionised (an acid) forms. Thus, the distribution of acid and basic forms is strictly dependent on the solution pH and their interaction with solid may or may not favor the adsorption process if the forces of attraction or repulsion prevail, respectively.

The trend of Figure 12 indicates that the adsorption of AAP by the nano-TiO₂ decreased with the increase of temperature. This suggested that the nano-TiO₂ adsorption of AAP is exothermic. The adsorption of AAP by nano-TiO₂ at ambient temperature was 67.69% and decreased to about 43.89 at 343 K corresponding to 112.81 mg/g to 73.15 mg/g, respectively. The decrease of AAP adsorption capacity with the increase of AAP solution temperature might be due to desorption of the AAP molecules from the surface of nano-TiO₂ as the temperature increases. This is supported by Moussavi et al. (2016), who reported that the thermodynamic analysis of the adsorption of AAP onto a double-oxidized graphene oxide is exothermic.

The Van't Hoff plot (log K vs 1/T) and thermodynamic parameters of AAP adsorption by nano-TiO₂ are presented in Figure 13 and Table 3, respectively. The calculated ΔH^o and ΔS^o of the adsorption of AAP by the nano-TiO₂ are −16.88 kJ/mol and −52.69 J/mol/K, respectively. The negative value of ΔG^o at 295 K and 303 K implies that the adsorption of AAP by nano-TiO₂ is spontaneous at these temperatures, but non-spontaneous at higher temperatures. The value of ΔG^o increased with the increase of AAP solution temperature, ΔH^o is negative, and K value decreased with the increase of temperature (Table 3). All these confirmed that the adsorption of AAP by nano-TiO₂ is exothermic under the selected conditions. The negative value of ΔS^o showed the decreased randomness of solid–liquid interphase during the treatment processes of AAP by the nano-TiO₂.

The experiments on the treatment of AAP by nano-TiO₂ have proven to be efficient and promising for the removal of AAP from contaminated water and wastewater.

The ultrasound degradation of 100 mL of 100 mg/L AAP (without pH adjustment (pH 5.66)) led to 53.84% removal efficiency. This prompted the need to investigate whether the change of pH of the AAP aqueous solution will enhance the degradation efficiency. Figure 14 shows that the rate of AAP degradation is strongly pH-dependent; the ultrasound treatment of AAP decreased as the pH of the solution increased from pH 3 to 8. This is supported by Im et al. (2014), who reported that the most favorable degradation pH of AAP is acidic. The hydrophobicity of AAP is favored at acidic conditions, thereby resulting in the accumulation of AAP (molecular form) at the interface of the cavitation bubbles, where a relatively higher concentration of OḢ is present (Villaroel et al. 2014). Ultrasonic degradation efficiency of approximately 57.43% and 53.78% were achieved at a pH of 3 and 8, respectively. A pH of 3 was maintained for further experiments.

Although a pH of 3 improved the ultrasound degradation of AAP (57.43%), the ultrasound treatment of AAP under normal condition (pH 5.66) should be encouraged (53.84%), due to the fact that the adjustment of solution pH is an additional cost and the discharge of the acidic resultant solution is hazardous to the environment.

The results on the influence of ultrasound time showed a rapid increase of the degradation of AAP from 12.30% at 10 min to 57.43% at 60 min sonication time (Figure 15(a)).

A plot of ln AAP_o/AAP_t against t (Figure 15(b)) was linear and k was obtained from the slope of the graph. The value obtained for the pseudo-first-order rate constant was 1.55 × 10⁻²min⁻¹ and is similar to the study by Emery et al. (2005). The authors reported that the ultrasound degradation of pharmaceuticals obeys pseudo-first-order reaction kinetics.

The rate of ultrasound degradation of organic compounds usually depends on ultrasonic frequency. Moreover, there is the need to investigate the optimum frequency that will favor the degradation of pollutants. Thus, Figure 16 shows the percentage removal–frequency profiles of AAP during the ultrasound degradation of 100 mL of 100 mg/L AAP at a pH 3, contact time of 60 min, and a stirring speed of 180 rpm.

For the range of ultrasonic frequencies studied (20 kHz–100 kHz), the amount of AAP degradation decreased with the increase of the ultrasonic frequencies. This may be due to a reduced cavitational effect as ultrasonic frequency increased. The highest AAP removal was achieved with 20 kHz ultrasonic frequency. This finding is supported by Isariebel et al. (2009) and Im et al. (2014), who stated that the degradation of AAP decreased at higher ultrasonic frequencies. Im and co-workers attributed the lower degradation efficiency at a higher ultrasonic frequency to small bubble size and insufficient ultrasound reactions deriving from low collapse events.

In ultrasound degradation, AAP was degraded by reacting with OḢ which are generated from thermally dissociated water under ultrasound irradiation, while AAP molecules are adsorbed on the surface of nano-TiO₂ particles in the nano-TiO₂ treatment method. Researchers have suggested that the combination of adsorbents with ultrasound irradiation is suitable for the removal of persistent organic compounds and pharmaceuticals (Hamdaoui 2011; Zhao et al. 2011; Milenkovic et al. 2013). In our study, the addition of nano-TiO₂ into AAP aqueous solution improved the treatment efficiency for the nano-TiO₂/ultrasound treatment at 100 kHz but not for other ultrasound frequencies. At 100 kHz, 15.94% of AAP was degraded by ultrasound treatment and 40.01% of AAP was degraded by the combined nano-TiO₂/ultrasound treatment. However, approximately 52.61% of AAP was degraded by the use of ultrasound combined with nano-TiO₂, while 57.43% of AAP was degraded by the use of ultrasound without nano-TiO₂ when 100 mL of 100 mg/L AAP solution was sonicated for 60 min, at 20 kHz, and pH of 3. The application of all the optimum operating conditions to the remediation of AAP in a sequential approach showed that the degradation of AAP by ultrasound (20 kHz, pH 3, 60 min) followed by adsorption with nano-TiO₂ (0.03 g, 60 min) resulted in approximately 99.50% AAP removal efficiency.

This study has proven the effectiveness of ultrasound, nano-TiO₂, and the integrated treatment processes for the remediation of AAP or other emerging water and wastewater contaminants. Thus, we proposed a schematic diagram of a sequential nano oxides/ultrasound removal of pollutants from contaminated water or wastewater (Figure 17).

A comparison of the percentage removal of AAP using various techniques is listed in Table 4. The percentage removal of AAP by ultrasound, nano-TiO₂, and the integrated treatment processes in comparison with others' methods show satisfactory performance.

The extent of nano-TiO₂, ultrasound, and nano-TiO₂/ultrasound degradation of AAP was strongly influenced by the operating conditions, such as nano-TiO₂ dosage, contact time, initial AAP concentration, ultrasound frequency, pH, and temperature. The kinetics and equilibrium studies of AAP adsorption by the nano-TiO₂ followed the pseudo-second-order and Freundlich models, respectively. The thermodynamic parameters showed that the adsorption process is exothermic, spontaneous at 295 K and 303 K but non-spontaneous at higher temperatures. The optimum AAP removal efficiency achieved when 50 mL of 100 mg/L AAP solution was treated with 0.03 g nano-TiO₂ at 60 min contact time, 120 rpm, and pH 4 was 77.33%. Ultrasound treatment without the addition of nano-TiO₂ resulted in 57.43% degradation, whereas 52.61% degradation was achieved when nano-TiO₂ was mixed with AAP aqueous solution under ultrasound at a frequency of 20 kHz and pH 3. This investigation has shown that ultrasonic treatment alone or the addition of nano-TiO₂ into the AAP aqueous solution before sonolysis was not able to degrade AAP completely. However, the use of inline ultrasound degradation of AAP before nano-TiO₂ treatment effectively removed 99% AAP, showing that a sequential process had an additive effect whereas a simultaneous combined approach was not significantly effective.

The authors express thanks to the Vaal University of Technology, South Africa for granting a Postdoctoral Research Fund (PDRF) for this study.