Abstract

The application of nano-TiO2 as adsorbent combined with ultrasound for the degradation of N-acetyl-para-aminophenol (AAP) from aqueous solution was investigated. The nano-TiO2 was characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR). Experimental results revealed that the adsorption of AAP by nano-TiO2 fitted the pseudo-second-order kinetic model, the equilibrium could be explained by the Freundlich isotherm and the treatment process is exothermic. The optimum removal efficiency of AAP (128.89 mg/g (77.33%)) was achieved at pH 4 when 0.03 g of nano-TiO2 was mixed with 50 mL of 100 mg/L AAP aqueous solution at ambient temperature, 60 min contact time, and a stirring speed of 120 rpm. Ultrasound at 20 kHz and pH 3 was favorable and it resulted in 52.61% and 57.43% removal efficiency with and without the addition of nano-TiO2, respectively. The degradation of AAP by ultrasound followed by nano-TiO2 treatment resulted in approximately 99.50% removal efficiency. This study showed that a sequential ultrasound and nano-TiO2 treatment process could be employed for the removal of AAP or other emerging water and wastewater contaminants.

INTRODUCTION

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.

MATERIALS AND METHODS

Reagents

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.

Characterization of nano-TiO2

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.

Experimental design

Adsorption of N-acetyl-para-aminophenol with nano-TiO2

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.

Degradation of N-acetyl-para-aminophenol with ultrasound

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.

Samples were taken (from the ultrasonic reactor), filtered through 0.45 µm Biofil syringe filter (as the case when nano-TiO2 was involved) after the elapsed time and analyzed by a UV/visible spectrophotometer (model: Nicolet Evolution 100) at a wavelength of 245 nm (Isariebel et al. 2009). All experiments were conducted in triplicate. The percentage of AAP removal and the amount (mg) of AAP per g nano-TiO2 were calculated using Equations (1) and (2), respectively:  
formula
(1)
 
formula
(2)
where AAPo and AAPe (mg/L) are the initial and equilibrium concentration of the AAP aqueous solution, respectively, WnTiO2 (g) is the mass of nano-TiO2, and VAAP (mL) is the volume of the AAP solution. The experimental runs and operational conditions are given in Table 1.
Table 1

Experimental runs and operational conditions

Adsorption by Nano-TiO2
 
Experimental conditions
 
Run Aim Nano-TiO2 dosage (g) Concentration of AAP (mg/L) Volume of AAP (mL) Contact time (min) pH Temperature (K) Stirring speed (rpm) 
Influence of nano-TiO2 dosage 0.01–0.05 100 50 10 5.66 Ambient 120 
Influence of contact time 0.03 100 50 15–100 5.66 Ambient 120 
Influence of initial AAP concentration 0.03 6.25–100 50 60 5.66 Ambient 120 
Influence of pH 0.03 100 50 60 3–8.37 Ambient 120 
Influence of temperature 0.03 100 50 60 5.66 295–343 120 
Ultrasound degradation
 
Run Aim Ultrasonic frequency (kHz) Concentration of AAP (mg/L) Volume of AAP (mL) Time (min) pH Stirring speed (rpm) 
Influence of pH 20 100 100 60 3–8 180 
Influence of ultrasound time 20 100 100 10–60 180 
Influence of ultrasonic frequency 20–100 100 100 60 180 
Combined nano-TiO2/ultrasound degradation 
Run Aim Ultrasonic frequency (kHz) Concentration of AAP (mg/L) Volume of AAP (mL) Time (min) pH Stirring speed (rpm) Nano-TiO2 dosage (g) 
Influence of pH 20 100 100 60 3–8 180 0.03 
Influence of ultrasound time 20 100 100 10–60 180 0.03 
Influence of ultrasonic frequency 20–100 100 100 60 180 0.03 
Adsorption by Nano-TiO2
 
Experimental conditions
 
Run Aim Nano-TiO2 dosage (g) Concentration of AAP (mg/L) Volume of AAP (mL) Contact time (min) pH Temperature (K) Stirring speed (rpm) 
Influence of nano-TiO2 dosage 0.01–0.05 100 50 10 5.66 Ambient 120 
Influence of contact time 0.03 100 50 15–100 5.66 Ambient 120 
Influence of initial AAP concentration 0.03 6.25–100 50 60 5.66 Ambient 120 
Influence of pH 0.03 100 50 60 3–8.37 Ambient 120 
Influence of temperature 0.03 100 50 60 5.66 295–343 120 
Ultrasound degradation
 
Run Aim Ultrasonic frequency (kHz) Concentration of AAP (mg/L) Volume of AAP (mL) Time (min) pH Stirring speed (rpm) 
Influence of pH 20 100 100 60 3–8 180 
Influence of ultrasound time 20 100 100 10–60 180 
Influence of ultrasonic frequency 20–100 100 100 60 180 
Combined nano-TiO2/ultrasound degradation 
Run Aim Ultrasonic frequency (kHz) Concentration of AAP (mg/L) Volume of AAP (mL) Time (min) pH Stirring speed (rpm) Nano-TiO2 dosage (g) 
Influence of pH 20 100 100 60 3–8 180 0.03 
Influence of ultrasound time 20 100 100 10–60 180 0.03 
Influence of ultrasonic frequency 20–100 100 100 60 180 0.03 

RESULTS AND DISCUSSION

Nano-TiO2 characterization

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.

Figure 1

SEM image (a) and EDS (b) of nano-TiO2.

Figure 1

SEM image (a) and EDS (b) of nano-TiO2.

Figure 2

XRD (a) and TEM (b) of nano-TiO2. A, anatase TiO2; R, rutile TiO2.

Figure 2

XRD (a) and TEM (b) of nano-TiO2. A, anatase TiO2; R, rutile TiO2.

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 3

FTIR spectrum of nano-TiO2.

Figure 3

FTIR spectrum of nano-TiO2.

Adsorption capacity of AAP by nano-TiO2

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.

Figure 4

Influence of nano-TiO2 dosage. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; contact time: 10 min; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

Figure 4

Influence of nano-TiO2 dosage. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; contact time: 10 min; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

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.

Figure 5

Influence of contact time. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

Figure 5

Influence of contact time. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

The kinetics of the nano-TiO2 AAP treatment process were investigated by applying the pseudo-first and pseudo-second-order kinetic models as represented in Equations (3) and (4), respectively:  
formula
(3)
 
formula
(4)
where qe is the equilibrium amount of AAP adsorbed per unit mass of nano-TiO2 (mg/g), t is time (min), qt is the amount of AAP adsorbed per unit mass of nano-TiO2 at time t (mg/g), k1 and k2 obtained from Figures 6 and 7 are the pseudo-first and pseudo-second-order adsorption rate constants, respectively.
Figure 6

Pseudo-first-order kinetic plot.

Figure 6

Pseudo-first-order kinetic plot.

Figure 7

Pseudo-second-order kinetic plot.

Figure 7

Pseudo-second-order kinetic plot.

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 qe (113.98 mg/g) was close to the calculated qe (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.

Table 2

Equilibrium and kinetic parameters for the removal of AAP by nano-TiO2

Equilibrium models
 
Langmuir model
 
Freundlich model
 
(mg/g)  (L/mg) R2  (mg/g (L/mg)1/nR2 
1,250 2.76 × 10−3 0.028 3.5653 1.025 0.9545 
Kinetic models
 
Pseudo-first-order Pseudo-second-order 
      
2.99 × 10−3 4.982 0.0066 8.66 × 10−3 102.041 0.9848 
Equilibrium models
 
Langmuir model
 
Freundlich model
 
(mg/g)  (L/mg) R2  (mg/g (L/mg)1/nR2 
1,250 2.76 × 10−3 0.028 3.5653 1.025 0.9545 
Kinetic models
 
Pseudo-first-order Pseudo-second-order 
      
2.99 × 10−3 4.982 0.0066 8.66 × 10−3 102.041 0.9848 

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.

Figure 8

Influence of initial AAP concentration. Experimental conditions: vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

Figure 8

Influence of initial AAP concentration. Experimental conditions: vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; pH of solution 5.66; ambient temperature.

The equilibrium study of the nano-TiO2 treatment process was considered by applying the linearized form of the Langmuir (Equation (5)) and Freundlich (Equation (6)) models to the experimental data presented in Figure 8:  
formula
(5)
 
formula
(6)
where AAPe, AAPa, qm, and n are the equilibrium concentration of AAP solution (mg/L), the amount of AAP adsorbed per unit mass of nano-TiO2 (mg/g), Langmuir constant representing adsorption capacity (mg/g), and the number of layers, respectively. KL and KF from Figures 9 and 10 are the Langmuir (L/mg) and Freundlich (mg/g (L/mg)1/n) constants, respectively.
Figure 9

Langmuir equilibrium plot.

Figure 9

Langmuir equilibrium plot.

Figure 10

Freundlich equilibrium plot.

Figure 10

Freundlich equilibrium plot.

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.

Figure 11

Influence of pH. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; ambient temperature.

Figure 11

Influence of pH. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; ambient temperature.

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.

Figure 12

Influence of temperature. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; pH of solution 5.66.

Figure 12

Influence of temperature. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 50 mL; nano-TiO2 dosage: 0.03 g; contact time: 60 min; stirring speed: 120 rpm; pH of solution 5.66.

The standard free energy (ΔGo), enthalpy (ΔHo), and entropy (ΔSo) changes were calculated from Equations (7)–(9):  
formula
(7)
 
formula
(8)
 
formula
(9)
where AAPo − AAPe is the amount of the AAP adsorbed per liter, AAPe is the equilibrium concentration of the solution in mg/L, T is temperature (K), R is ideal gas constant (8.314 J/mol/K), and K is thermodynamic equilibrium constant.

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 ΔHo and ΔSo 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 ΔGo 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 ΔGo increased with the increase of AAP solution temperature, ΔHo 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 ΔSo showed the decreased randomness of solid–liquid interphase during the treatment processes of AAP by the nano-TiO2.

Table 3

Thermodynamic parameters

T (K) ΔHo (kJ/mol) ΔSo (J/mol/K) ΔGo (J/mol) K 
295 − 16.88 − 52.69 −1,813.31 2.0945 
303 −790.69 1.3687 
313 29.26 0.9888 
323 585.56 0.8041 
333 653.21 0.7898 
343 700.74 0.7821 
T (K) ΔHo (kJ/mol) ΔSo (J/mol/K) ΔGo (J/mol) K 
295 − 16.88 − 52.69 −1,813.31 2.0945 
303 −790.69 1.3687 
313 29.26 0.9888 
323 585.56 0.8041 
333 653.21 0.7898 
343 700.74 0.7821 
Figure 13

Van't Hoff plot.

Figure 13

Van't Hoff plot.

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.

Degradation capacity of AAP by ultrasound

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.

Figure 14

Influence of pH on ultrasonic degradation. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; ultrasonic frequency: 20 kHz; time: 60 min; stirring speed: 180 rpm.

Figure 14

Influence of pH on ultrasonic degradation. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; ultrasonic frequency: 20 kHz; time: 60 min; stirring speed: 180 rpm.

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)).

Figure 15

(a) Influence of time on ultrasonic degradation: Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; ultrasonic frequency: 20 kHz; pH: 3; stirring speed: 180 rpm. (b) Pseudo-first-order plot of AAP ultrasonic degradation.

Figure 15

(a) Influence of time on ultrasonic degradation: Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; ultrasonic frequency: 20 kHz; pH: 3; stirring speed: 180 rpm. (b) Pseudo-first-order plot of AAP ultrasonic degradation.

The degradation rate constant was determined by assuming a pseudo-first-order reaction kinetics (Equation (10)).  
formula
(10)
where AAPo and AAPt are AAP concentrations at time 0 and t, respectively, and k is the pseudo first-order rate constant.

A plot of ln AAPo/AAPt 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.

Figure 16

Degradation efficiency by ultrasound with or without nano-TiO2. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; pH: 3; time: 60 min; stirring speed: 180 rpm.

Figure 16

Degradation efficiency by ultrasound with or without nano-TiO2. Experimental conditions: concentration of AAP: 100 mg/L; vol. of AAP aqueous solution: 100 mL; pH: 3; time: 60 min; stirring speed: 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).

Figure 17

Proposed schematic diagram of a sequential nano oxides/ultrasound degradation of pollutants.

Figure 17

Proposed schematic diagram of a sequential nano oxides/ultrasound degradation of pollutants.

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.

Table 4

Comparison of AAP removal percentage with the literature

Techniques Percentage removal (%) Reference 
Ultrasound (100 mg/L AAP, 574 kHz) 70 (4 hours); 96 (8 hours) Isariebel et al. (2009)  
UV <10 Aguilar et al. (2011)  
UV + TiO2 as catalyst 97 
Electrolysis with reticulated vitreous carbon (RVC) electrodes 90 Arredondo Valdez et al. (2012)  
Electrolysis with modified TiO2/RVC electrode 90 
Electrolysis with modified CuO2/TiO2/Al2O3/RVC electrode 98 
Electrolysis with modified TiO2/RVC electrode + UV 95 
Electrolysis with modified CuO2/TiO2/Al2O3/RVC electrode + UV 99 
Chlorination (dose 49–54 mmol L−1, pH 5.5, 24 h) >95 Westerhoff et al. (2005
Electrochemical oxidation (5.7 mA/cm2, pH 3, 6 hr) 96 Zavala & Estrada (2016)  
Double-oxidized graphene oxide as adsorbent 83 Moussavi et al. (2016)  
Chemically treated chicken bone waste as adsorbent 93 Yusoff et al. (2017)  
Ultrasound (100 mg/L AAP, 20 kHz, 60 min) 57.43 Our study 
Nano-TiO2 as adsorbent 77.33 
Combined ultrasound (20 kHz) and nano-TiO2 52.61 
Sequential ultrasound (20 kHz) and nano-TiO2 99.50 
Techniques Percentage removal (%) Reference 
Ultrasound (100 mg/L AAP, 574 kHz) 70 (4 hours); 96 (8 hours) Isariebel et al. (2009)  
UV <10 Aguilar et al. (2011)  
UV + TiO2 as catalyst 97 
Electrolysis with reticulated vitreous carbon (RVC) electrodes 90 Arredondo Valdez et al. (2012)  
Electrolysis with modified TiO2/RVC electrode 90 
Electrolysis with modified CuO2/TiO2/Al2O3/RVC electrode 98 
Electrolysis with modified TiO2/RVC electrode + UV 95 
Electrolysis with modified CuO2/TiO2/Al2O3/RVC electrode + UV 99 
Chlorination (dose 49–54 mmol L−1, pH 5.5, 24 h) >95 Westerhoff et al. (2005
Electrochemical oxidation (5.7 mA/cm2, pH 3, 6 hr) 96 Zavala & Estrada (2016)  
Double-oxidized graphene oxide as adsorbent 83 Moussavi et al. (2016)  
Chemically treated chicken bone waste as adsorbent 93 Yusoff et al. (2017)  
Ultrasound (100 mg/L AAP, 20 kHz, 60 min) 57.43 Our study 
Nano-TiO2 as adsorbent 77.33 
Combined ultrasound (20 kHz) and nano-TiO2 52.61 
Sequential ultrasound (20 kHz) and nano-TiO2 99.50 

CONCLUSION

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.

ACKNOWLEDGEMENTS

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

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