Next Article in Journal
Channel Deformations and Hazardous Processes of the Left-Bank Tributaries of The Angara River (Eastern Siberia)
Next Article in Special Issue
Comparative Study on Photocatalytic Performance of TiO2 Doped with Different Amino Acids in Degradation of Antibiotics
Previous Article in Journal
Application of Water Quality Indices, Machine Learning Approaches, and GIS to Identify Groundwater Quality for Irrigation Purposes: A Case Study of Sahara Aquifer, Doucen Plain, Algeria
Previous Article in Special Issue
Bioremediation Treatment of Polyaromatic Hydrocarbons for Environmental Sustainability
 
 
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Comparative Study on Enhanced Photocatalytic Activity of Visible Light-Active Nanostructures for Degradation of Oxytetracycline and COD Removal of Licorice Extraction Plant Wastewater

1
Department of Environmental Health Engineering, School of Public Health, Research Center for Environmental Determinants of Health (RCEDH), Kermanshah University of Medical Sciences, Kermanshah 51351, Iran
2
Social Development and Health Promotion Research Center, Kermanshah University of Medical Sciences, Kermanshah 51351, Iran
3
School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
4
Department of Chemical Engineering, Sharif University of Technology, Tehran 11155, Iran
5
Department of Civil Engineering, Technical and Vocational University, Isfahan 73441, Iran
6
Fars Power Generation Management Company, Shiraz 45616, Iran
*
Author to whom correspondence should be addressed.
Water 2023, 15(2), 290; https://doi.org/10.3390/w15020290
Received: 11 December 2022 / Revised: 31 December 2022 / Accepted: 3 January 2023 / Published: 10 January 2023

Abstract

:
This study evaluates the effects of carbon, nitrogen, and sulfur dopants on the photocatalytic activity of TiO2 for degradation of oxytetracycline (OTC) and chemical oxygen demand (COD) removal from licorice extraction plant wastewater (LEPW). Three novel visible-light-responsive nanostructures, including L-Histidine-TiO2, L-Methionine-TiO2 and L-Asparagine-TiO2, were successfully synthesized. The results showed that the modification of TiO2 with these three amino acids made the catalyst active in the visible light region and reduced the recombination rate of e/h+ pairs according to PL analysis. The photodegradation efficiency of L-Histidine (2 wt.%)-TiO2 was 100% and 94% for OTC and COD, respectively. It showed the highest photocatalytic activity under illumination, compared to L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2. Synthesized composites were characterized with SEM, XRD, FTIR, DRS, and PL analyses. The biological oxygen demand to COD (BOD5/COD) ratio for treated LEPW was determined to be 0.5–0.6, confirming the enhanced biodegradability of the treated effluent. The effect of the independent variables, namely, initial concentration of OTC and COD, catalyst dosage, irradiation time, pH of solution, and light intensity, on the photocatalytic process was evaluated by Response Surface Methodology (RSM), and the optimum value of each independent parameter for maximum degradation of OTC and COD by L-Histidine (2 wt.%)-TiO2 was determined. The radical trapping experiment was performed with various scavengers in order to propose a photocatalytic mechanism, showing that hydroxyl radicals were the main active species. L-Histidine (2 wt.%)-TiO2 showed a stable and reusable structure even after four cycles of COD removal under the following optimal conditions of [COD]: 300 mg/L, [catalyst]: 1 g/L, light intensity: 25 W/cm2 at pH = 4 after 180 min irradiation.

1. Introduction

Contaminants discharged from industries and through anthropogenic activities into aqueous media are responsible for water pollution [1], water shortage crises [2], and contamination of the environment [3]. In wastewater, most contaminants are non-biodegradable and resistant to natural degradation, such as OTC [4] and licorice [5]. Tetracycline-based OTC antibiotics are used worldwide for human and veterinary medicine [6]; however, OTC is poorly absorbed in the digestive tract (30%) and most of it enters the environment through body excretions [7]. Ultimately, these residues can cause serious health problems and threaten the ecosystem. They can contribute significantly to the increase in antibiotic resistance genes [8,9]. As a result of their hydrophilic properties and stable naphthalene rings, OTC residues cannot be conventionally treated by biodegradation or chlorination [10].
The compound licorice is widely used in tobacco, dietary supplements, sweets, and medicines [11]. Due to this, large quantities end up in water, wastewater, and soil as residues. Environmental pollution with licorice causes serious problems for the ecosystem [11,12,13].
Photocatalysis is a useful and practical method for degrading non-biodegradable contaminants, such as OTC and licorice, among other treatment methods [14]. The ability of the photocatalysis method to convert contaminants to harmless compounds, its affordability, reusability, and sustainability make it an environmentally friendly and effective water treatment method [15]. As a result of its strong oxidizing power [15] and chemical and biological inertness [16], its wide band gap energy leads to poor performance under visible light irradiation, and its high recombination rate of electron/hole pairs negatively affects its photocatalytic activity [17,18]. To address these problems, non-metal doping is an appropriate method because it reduces band gap energy and makes TiO2 a visible light-driven photocatalyst [19,20]. In contrast to metal doping, non-metal doping enhances the photocatalytic properties of TiO2 because it creates new levels near the valence band [21]. Some studies have investigated the photocatalytic activity of C-N and S-doped TiO2, Sushma et al., investigated the effect of tri-doping C, N and S on the photocatalytic activity of TiO2 [22]. Wan et al. assessed the influence of tri-doping on the performance of TiO2 nanorods [23]. Their findings showed that non-metal modifications of TiO2 resulted in increased active sites, longer lifetimes of holes and electron pairs, and a red shift to the visible region, as well as a narrowing of the band gap [21,22]. L-amino acids are rich sources of N, C and S that can be used for co-doping and tri-doping TiO2. Zanganeh et al., studied the role of L-proline [24], L-Lysine [25], L-Histidine [26] in improvement of the photocatalytic performance of TiO2.
The purpose of this study was to assess the effect of co-doping and tri-doping TiO2 on photocatalytic degradation of OTC and COD in LEPW. L-Histidine, L-Methionine and L-Asparagine were used as sources of nonmetals to modify TiO2, and their positive effects were compared. Based on the photocatalytic performance of L-amino acid-TiO2 networks for removal of OTC with different contents of dopants, the best weight percent was selected. L-histidine-TiO2 which showed the highest OTC and COD degradations were selected for investigating the optimal conditions based on response surface methodology (RSM) as a statistical technique. The prepared photocatalysts were characterized with X-ray diffraction (XRD), Fourier transform infrared (FT-IR), diffuse reflectance spectra (DRS), field emission scanning electron microscopy (FESEM), energy dispersive analysis of X-ray (EDX) and photoluminescence analysis (PL). The novelty of this study was its fabrication of three reusable visible-light responsive photocatalysts, including L-Histidine (C, N) codoped-TiO2, L-Asparagine (C, N) codoped-TiO2, and L-Methionine (C, N, S) tri-doped-TiO2. The impact of variables, including OTC and COD concentration, irradiation time, light intensity, catalyst concentration, and pH, for the photodegradation process were evaluated, based on central composite design (CCD). The variables were optimized to obtain the maximum COD removal and OTC removal efficiency. The reusability of the prepared catalyst, the biodegradability of the treated LEPW and total organic carbon (TOC) for OTC were evaluated.

2. Experimental

2.1. Materials

Tetra-n-butylorthotitanate (TBOT (C16H36O4Ti, 99%)), ethanol (C2H5OH, 99.0%) and acetic acid (CH3COOH, 99.8%) were provided by Merck (Germany). OTC (C22H24N2O9, 99%), L-Methionine (C5H11NO2S, 99%), L-Asparagine (C4H8N2O3, 99%), L-Histidine (C6H9N3O2, 99%) were purchased from Sigma-Alderich (UK). Licorice (liquorice) extraction plant wastewater was taken from a Zagros liquorice company, Kermanshah, Iran. The characteristics of the licorice extraction plant wastewater samples are shown in Table 1. Since the BOD5/COD ratio of raw LEPW was 0.19 and the minimum BOD5/COD ratio, as an acceptable factor for the wastewater to be biodegradable, was 0.4 [12,13] raw LEPW was considered non-biodegradable.

2.2. Synthesis of L-Asparagine-TiO2, L-Histidine-TiO2 and L-Methionine-TiO2

All photocatalysts were prepared by the sol-gel method [27]. Initially, different amounts (1.0, 1.5, 2.0, 2.5 wt.%) of L-Asparagine, L-Methionine, L-Histidine were dissolved in mixtures of 15 mL ethanol, 0.2 mL deionized water, and 3.4 mL acetic acid and mixed for 1 h with a magnetic stirrer (solutions A1–A3). Then, 5 mL TBOT was added to 10 mL ethanol and dispersed for 20 min by an ultrasonic bath (solution B). The solutions A1–A3 were added dropwise to solution B and were sonicated for 1.5 h. The solutions were kept in darkness at room temperature for 24 h. Finally, the solutions were dried in an electric oven at 120 °C for 12 h and were calcined at 500 °C for 2 h inside a muffle furnace to obtain L-Asparagine-TiO2 (C,N co-doped TiO2), L-Histidine-TiO2 (C,N co-doped TiO2) and L-Methionine-TiO2 (C,N,S tri-doped TiO2) nanostructures.

2.3. Catalysts Characterization

The X-ray diffraction (XRD) patterns were obtained by using Rigaku D-max C III (Tokyo, Japan) with Ni-filtered Kα radiation (λ = 1.542 °A). The functional group of catalysts were determined by Fourier transform infrared (FT-IR) spectra (Shimadzu Varian 4300, Kyoto, Japan) with KBr pellets containing the powder samples. The UV–Vis diffuse reflectance spectra (DRS) were obtained using a UV–Vis spectrophotometer (Shimadzu 1800, Kyoto, Japan). The morphologies of synthesized composites were studied by field emission scanning electron microscopy energy dispersive analysis of X-rays (FESEM-EDX) (Philips XL 30 and S-4160, Eindhoven, Netherlands) and to make the samples conductive, they were covered with gold. The photoluminescence (PL) analysis was performed in a (fluorescence spectrophotometer Perkin Elmer/LS55, Waltham, Massachusetts, USA) at the excited wavelength of 410 nm.

2.4. Photocatalytic Activity Experiments

The photocatalytic experiments were conducted in a cylindrical Pyrex equipped with a cooling jacket to adjust the temperature of the solution to 25 °C. Irradiation was performed using LED lamps (50 W, emission: 405 nm and light intensity of 13 lm per m2). The specific amount of catalyst was suspended to the solution of OTC or LEPW, followed by adjusting the pH with NaOH and H2SO4 (1 M). The solution was stirred in dark conditions for 30 min to obtain the adsorption–desorption equilibrium. After which, the light sources were switched on and the photocatalytic experiments were conducted under visible light irradiation and the solution continuously stirred. At different time intervals 2 mL of the OTC solution or LEPW was sampled and centrifuged to remove the remaining nanoparticles. The COD concentration of LEPW samples was determined by a colorimetric method and COD calibration curve. The COD removal was measured by means of Equation (1):
Removal   % = 1 C t C 0 × 100
where C0 and Ct are the initial and the final COD concentrations of LEPW in the solution, respectively. OTC concentration was determined using the spectrophotometry method in a spectrophotometer device V-570 (Jasco, Japan) and the degradation efficiency was calculated according to Equation (1) (C0 and Ct are the initial and the final OTC concentration). Total organic carbon (TOC) of the OTC solutions was measured with a TOC analyzer (2200, Shimadzu, Kyoto, Japan).

2.5. Experimental Design Methodology

The CCD with five independent factors at three levels was applied to design the experiments in the present study. The number of experiments was determined according to Equation (2) [15]:
N = 2 k q + 2 × k + n c
where k stands for the number of independent factors and q and nc represent a fraction of the number of factor and the repetition number of the central point, respectively.
The independent variables for this study were OTC concentration, COD in LEPW, catalyst dosage, irradiation time, pH of solution, and light intensity, and OTC photodegradation and COD removal were chosen as the responses (Table 2). The factors and their levels were chosen based on the screening experiments and the literature. A total number of 50 experiments were designed for photodegradation of OTC (Table S1) and also COD removal from LEPW (Table S2), according to Equation (2) (q is zero for a full factorial design). The effect of independent variables, their interaction, the mathematical model and optimization of variables were determined by CCD.

3. Results and Discussion

3.1. Optimization of L-Asparagine, L-Histidine and L-Methionine Loading in TiO2 Network

In order to determine the optimum content of L-Asparagine, L-Histidine, and L-Methionine in composites, their ability to photodegrade OTC at an initial concentration of 50 mg/L, pH = 4.5, and 1 g/L of catalyst dosage was assessed (Figure 1). After 60 min illumination, the removal efficiency of L-Methionine-TiO2 increased from 69% to 90% after increasing the weight percentage of L-Methionine from 0.5 wt.% to 1.5 wt.% (Figure 1b). A further increase in L-Methionine loading (2.0 wt.%), however, resulted in a drop in OTC removal efficiency to 86.6% (Figure 1b). In the case of L-Asparagine and L-Histidine, increasing L-amino acids content improved the photocatalytic performance of TiO2, and the degradation efficiency of L-Histidine (2.0 wt.%)-TiO2 and L-Asparagine (2.0 wt.%) in removal of OTC reached 100% and 74% (Figure 1a,c). Nonetheless, more increment of L-Asparagine, as well as L-Histidine loading, to 2.5 wt.% had negative effects on the OTC removal efficiency and it declined to about 2–4%. The improvement could be attributed to reduction of the band gap because of formation of new levels between VB and CB due to the introduction of C, N and S to the oxygen sites of TiO2 [15,23]. However, the downward trends of removal efficiency with excessive doping could be explained by the reduction of the distance between trapping sites in photocatalysts, and increase in the recombination centers and the recombination rates of electron/hole pairs [28,29].The results showed that L-Histidine at 2 wt.%, L-Methionine at 1.5 wt.% and L-Asparagine at 2 wt.% were the optimal amounts of loading in the TiO2 network. Meanwhile, the role of the adsorption process on OTC removal was assessed and OTC removal efficiency after 30 min stirring in darkness was about 15–22% for L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2.
To further investigate the photocatalytic activities of L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2, the reaction kinetics of photodegradation of OTC were studied and are illustrated in Figure S1. The photocatalytic removal rate constant (K1 and k2 for Pseudo first and second order models, respectively, and determination of the coefficient (R2) are also represented in Table 3. The results exhibited that the degradation reaction for all the prepared nanocomposites followed pseudo first-order kinetics based on Equation (3) [30]:
Ln C 0 C t = K 1 t
t C t = 1 k 2 C 0 + t C 0
where C0, Ct, K1, k2 and t are concentrations at time zero and t, rate constant and irradiation time, respectively. The k1 values of pseudo first order kinetic models were achieved at about 0.0251, 0.0149 and 0.0111 min−1 for L-Histidine-TiO2, L-Methionine-TiO2 and L-Asparagine-TiO2, respectively. It was confirmed that there was more photocatalytic activity of L-Hisitindine-TiO2, as explained earlier. Therefore, L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2 showed the highest photocatalytic performances for OTC and COD photodegradation. Consequently, the characterization tests were carried out for these optimum loading catalysts
The photocatalytic ability of the three synthesized catalysts at optimum weight percentages for COD removal was also studied at an initial COD concentration of 500 mg/L, 2 g/L of catalysts and original pH (Figure 2a,c). The biodegradability of the treated LEPW was studied by classification of various possible intermediates, including COD mineralized (the complete conversion of a compounds to CO2 and H2O and other inorganic compound), bCOD (Biodegradable COD concentration), nbCOD (Non-biodegradable COD concentration), and CODe (COD contents remained as parent form) as defined in the following Equations:
Total COD = COD mineralized + bCOD + nbCOD + CODe
CODe = bCOD + nbCOD
bCOD = UBOD + 1.42fdYxCODe
BOD5 = 0.68UBOD
where UBOD, fd and Yx are, ultimate biochemical oxygen demand, fraction of cell mass remaining as cell debris (0.15 g/g) and biomass yield coefficient (0.2 gVSS/gCOD).
The results showed that L-Histidine (2.0 wt.%)-TiO2 was the most effective catalyst for the photocatalytic removal of COD (Figure 2a). As can seen in Figure 2 the mineralized COD was achieved at 470 mg/L by using L-Histidine (2 wt.%)-TiO2 nanoparticles after 120 min visible irradiation. It was obtained at 400 and 330 mg/L for L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2, respectively. The results showed that the nbCOD content of raw LEPW was much higher than the bCOD amount, and after photocatalytic removal of COD by all synthesized photocatalysts the nbCOD values reduced more than bCOD, which showed increasing biodegradability of the treated LEPW.
Moreover, the biodegradability of the produced unknown products was studied by determination of the BOD5/COD ratio for the treated wastewater. The COD removal and BOD5/COD ratio versus irradiation time are represented in Figure 3a,b, respectively. The maximum ratio of BOD5/COD was determined to be 0.6, 0.54 and 0.5 after 120 min visible irradiation for L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2 respectively (Figure 3b). This confirmed the effectiveness of the photocatalytic process in enhancing the biodegradability of the treated LEPW. The COD removal efficiencies were about 94, 80 and 74% for L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2 catalysts under 120 min visible light irradiation time, respectively. The contribution of the adsorption process for LEPW using synthesized nanoparticles was insignificant (5–8%).
The higher photocatalytic activity of L-Histidine (2 wt.%)-TiO2 in elimination of OTC and COD, in comparison with L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2 nanostructures, can be explained by PL analysis, which was used for evaluation of the recombination rate of the photo-induced electron/hole pairs. The weaker emission peak intensity of PL spectra signified lower recombination rate and higher lifetime of charge carriers [31]. The PL spectra for the three prepared nanoparticles is illustrated in Figure 4. The intensity of emission peak for L-Histidine (2 wt.%)-TiO2 was far lower in comparison with the two other modified catalysts (L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2). Consequently, the modification of TiO2 with L-Histidine (2 wt.%) had a more significant role in reduction of the e/h+ recombination rate than those of the two other nanoparticles.

3.2. Characterization of Catalysts

The XRD patterns of three catalysts, including L-Histidine-TiO2, L-Methionine-TiO2 and L-Asparagine-TiO2 are illustrated in Figure 5a. The peaks at 2θ = 25°, 38°, 48°, 54.5°, 55.3° and 63° that correspond with (101), (103), (200), (105), (204) and (213) diffraction planes, respectively, could be attributed to TiO2 [32]. There were no peaks related to L-Methionine, L-Asparagine and L-Histidine, because the amounts of loading on TiO2 were low.
FTIR analysis (Figure 5b) showed that the bands at 522 and 713 cm−1 (range 400 and 800 cm−1) were characteristic bands of TiO2 and related to the vibration band of Ti-O and Ti-O-Ti bonds [33].The absorption peaks around at 3420 cm−1 could be attributed to the hydroxyl groups (O-H) of water molecules absorbed on the surface of the catalysts [34]. The bands observed around 2355 cm−1 were related to the presence of dissolved, or atmospheric, CO2 in the samples [35].
The UV–Vis diffuse reflectance spectra (DRS) and Tauc plots of the as-synthesized TiO2 samples are illustrated in Figure 5c,d, respectively. The noticeable red shift with C–N-codoped-TiO2 and C–N–S-tridoped-TiO2 catalysts to the visible light region were observed (Figure 5c). The extension of the light absorption edge of catalysts into the UV-visible region could be a decisive factor in increasing the number of photogenerated electrons and holes to generate more reactive species which improved the photocatalytic activity of TiO2. The optical band gap energy of each of the three L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2 catalysts were obtained at 2.1, 2.3 and 2.17 eV (Figure 5d), which was lower than the band gap of pure TiO2 (3.1 eV) [30]. The reduction of the band gap was due to the introduction of doping species (C, N and S) into the TiO2 lattice. Similar results were obtained for co-doped TiO2, and the band gap energy of L-Arginine (1 wt.%)-TiO2, L-Lysine (C, N co-doped)-TiO2/WO3 and L-Proline (2 wt.%)-TiO2 were obtained as 1.7, 2.1, and 1.7, respectively [25,36].
The SEM images of the synthesized catalysts are illustrated in Figure 6. The SEM images of catalysts showed the formation of spherical-like particles and uniform distribution without agglomeration structures. The composition of elements in L-Histidine (2 wt.%)-TiO2 nano photocatalysts were estimated as shown in the EDX spectroscopy results (Figure 6d). Based on an EDX spectrum, L-Histidine (2 wt.%)-TiO2 mainly consisted of Ti and O, along with a small amount of C and N, indicating that nonmetals had been successfully incorporated into TiO2. The uniformity and size of nanoparticles for all the photocatalysts were also investigated by image processing software (ImageJ 1.44p software) and the result is given in Table S3.

3.3. Statistical Analysis and Modeling for Photodegradation of OTC and COD by L-Histidine (2 wt.%)-TiO2

The effects of the independent variables on the OTC and COD removal were statistically assessed by applying CCD. The analysis of variance (ANOVA) verified that the quadratic models of the photocatalytic process with p-value < 0.0001 and F-value of 129.72 for OTC removal and 49.06 for COD removal were significant (Table 4). The reduced quadratic models, based on the significant parameters, are depicted with the following Equations:
Y OTC % = + 74.19 8.82 A + 5.41 B 13.56 C + 5.38 D + 10.56 E 3.53 CE 7.01 B 2 28.51 C 2
Y COD % = + 68.92 10.56 A + 7.44 B 7.26 C + 6.15 D + 8.65 E 7.92 A 2 + 8.08 C 2 8.92 D 2
The p-values of independent variables indicated that all five factors, including COD or OTC concentration (A), catalyst concentration (B), pH (C), irradiation time (D), and light intensity (E), were significant.
According to their F-values (Table 4) and the regression coefficients of Equations (9) and (10) the degree of effect and significance of independent variables on OTC removal had the following orders: C > E > A > B > D. It should be noted, however, that the importance of independent parameters for the removal of COD was A > E > B > C > D.
The ANOVA results also confirmed that there were interactions between A and C for OTC removal, whereas no interaction between variables was found for COD removal. The quadratic terms, including B2 and C2 for OTC removal and A2, C2, and D2 for COD removal, were statistically significant. The coefficient of A and C were negative, implying that the increase of the OTC or COD concentration and pH could lead to the decline of OTC and COD removal efficiency (Table 4).
The coefficient of determination (R2) was 0.97 for OTC removal and 0.91 for COD removal, close to 1, confirming that the predicted data were matched with experimental values [37,38]. The lack of fit for both models was non-significant, indicating the suitability of the models. Adeq precision was obtained as 39.90 for OTC and 33.61 for COD removal, which represented a sufficient signal [39]. The adjusted R2 of 0.96 for OTC removal and 0.90 for COD removal were in good agreement with their R2, implying the adequacy of the proposed models. The normal probability plot of the residuals (Figure S2) further confirmed that the models fitted the experimental runs well, as the distribution of the experimental data was normal and the points were near the straight lines [38]. The value of coefficient of variation (CV) and standard deviation for both of the proposed models indicated the high reliability and precision of the models, respectively [39].

3.4. Photodegradation Modeling and Optimization of Independent Variables

As shown in Figure 7, Figure 8 and Figure 9, the counter plots illustrate the relationship between OTC and COD removal efficiency and the five independent variables. The effects of A (OTC or COD concentration) and B (L-Histidine (2 wt.%)-TiO2 catalyst concentration) on responses are illustrated in Figure 7a,b, respectively. By increasing the OTC concentration from 50 to 100 mg/L, the removal efficiency decreased from 83% to 55%. With an increase in COD from 300 to 700 mg/L, around 35% of COD removal efficiency was lost. This reduction could be explained by occupation of the active sites with pollutants at higher concentration levels. Furthermore, the presence of more pollutants inhibited photocatalytic activity by blocking visible light photons, preventing them from reaching the surface of the photocatalysts [40].
The effect of L-Histidine (2 wt.%)-TiO2 catalyst dosage on response was also assessed and higher removal efficiency for both OTC and COD removal were obtained at higher levels of catalyst dosage. The enhancement of activity could be attributed to the availability of more active sites for the adsorption of pollutants. In addition, these sites could also absorb more photons to produce active species [21,40].
The photodegradation of OTC and COD, as a function of pH and irradiation time, is shown in Figure 8a,b. The photodegradation efficiency of OTC increased significantly to around 80% by bringing the pH from acidic to neutral, however it decreased significantly to almost 35% when the pH was increased to alkaline (Figure 8a). The reason why the maximum removal efficiency of OTC was observed at pH = 7 could be attributed to the properties of both the catalyst and the OTC (pHpzc of the catalyst and surface charge of the OTC). The pHpzc of L-Histidine (2 wt.%)-TiO2 catalyst was determined as 6.9 (Figure S3), which meant that it was positively charged at a pH less than 6.9 and it had negative charge at a pH more than 6.9. Further, the OTC had pKa values of 3.22, 7.46, and 8.94 [41] and, therefore, under acidic conditions, it existed as both neutral (H2OTC) and positively charged (H3OTC+) forms. At pH levels greater than 9, the OTC had a negatively charged form [42]. Therefore, the repulsive force caused by equal charges of catalyst and OTC dissociations led to a decreased degradation yield, both in acidic and basic media. The COD removal of efficiency was also highly influenced by pH and Figure 8b shows that the acidic condition (pH = 4) was the most favorable to the COD degradation compared to neutral or alkaline pH, owing to the attraction forces between licorice (pKa = 2.6) [43] and L-Histidine (2 wt.%)-TiO2 at this pH. Moreover, Figure 8a,b also depicts the effect of irradiation time on the responses. The positive effects of longer irradiation time on the responses were observed for both OTC and COD removal. The photodegradation removal efficiency improved by almost 20–30% when the irradiation time went up from 15 to 60 min for OTC and 60 to 180 min for COD removal. The reason for this was that there was more time for radical hydroxyls to form and for reactions to occur between the generated species and the pollutant molecules, which could lead to enhanced photodegradation.
The effect of light intensity on the photodegradation efficiency are depicted in Figure 9. The upward trend of the OTC removal efficiency was observed by increasing the light intensity from 15 to 25 W/cm2 (Figure 9a). This upward trend was also observed for the COD removal from LEPW at higher light intensity and it reached around 90% removal at the light intensity of 25 W/cm2 (Figure 9b). This improvement in removal efficiency was because of increased photon absorption by L-histidine (2 wt.%)-TiO2 in higher light intensities [44]. The optimum conditions for OTC photodegradation and COD removal from LEPW were determined using CCD while considering the desirability of 1. The COD in LEPW was completely removed under the optimal conditions of initial COD of 300 mg/L, catalyst concentration of 2 g/L at pH = 4 and light intensity of 25 W/cm2 after 180 min visible light irradiation, which was close to the predicted removal efficiency. Further, the optimization results determined that, at an initial concentration of 50 mg/L of OTC, and at a light intensity of 25 W/cm2 and a L-Histidine (2 wt.%)-TiO2 concentration of 1 g/L, OTC was completely removed after 60 min of irradiation, which was quite close to the predicted removal efficiency (99.5%), confirming the suitability of the proposed models.

3.5. Photocatalytic Mechanism of L-Histidine (2 wt.%)-TiO2

The first active species that are produced in photocatalysis are e and h+, and then these photoinduced electrons and holes react with H2O and O2 on the conduction band (CB) and valance band (VB) of photocatalysts [45]. To suggest a photocatalytic mechanism, finding the main reactive species is necessary. Therefore, a radical trapping experiment, by introducing different scavengers to the reaction media, was performed. In this study 1,4-benzoquinone (BQ), sodium azide (SA), isopropanol alcohol (IPA), and ammonium oxalate (AO) were introduced as scavengers of superoxide radical (O2), singlet oxygen (1O2), hydroxyl radical (OH), and hole (h+), respectively [46,47,48]. The effect of scavengers on photocatalytic removal of OTC and COD were evaluated at conditions of initial concentrations of 50 and 300 mg/L, photocatalyst loadings of 1 and 2 g/L, pH of 7 and 4, and at 25 W/cm2 light intensity 25 W/cm2 for OTC and COD, respectively. Without any scavengers, OTC and COD were completely degraded after 60 and 120 min, respectively. Among different radical trappings, the introduction of SA had no effect on photodegradation efficiency, which showed that 1O2 did not have any role in removal of pollutants. However, the photocatalytic performance of L-Histidine (2 wt. %)-TiO2 was significantly reduced by adding IPA. Therefore, hydroxyl radicals (OH) were the main species in degradation of OTC and COD, as seen in Figure 10a,b. Similarly, introducing AO and BQ reduced removal efficiency of OTC and COD, which indicated that h+ and O2 played roles in the photodegradation mechanism. The results of the radical trapping experiments provided information which suggested a photocatalytic mechanism for removal of OTC and COD. The possible mechanism could explain the degradation pathway in two ways. At first, the reaction between dissolved oxygen (O2) and photogenerated electrons produced superoxide radical (O2), which later transformed to OH and H2O2, resulting in degradation of OTC molecules [49]. Then, the photoinduced holes and adsorbed OTC molecules on the photocatalyst surface could directly oxidize OTC radicals to OTC+. In addition, N-Ti-O bonds were formed when N was doped into the TiO2 lattice. In the meantime, the combination of the N2p and O2p orbitals created a new level above VB, resulting in a reduction in the band gap of TiO2 [50]. Additionally, doped C could serve as a photosensitizer for TiO2, by injecting electrons into its CB [51], as seen in Figure 11.

3.6. Reusability Performance

The reusability and photostability of L-Histidine (2 wt.%)-TiO2 for COD removal from LEPW under optimum conditions for four following cycles was studied to evaluate its potential for industrial usage. Figure 12a shows that the COD photodegradation efficiency reduced slightly (only 7–8%) after four cycles. This indicated that L-Histidine (2 wt.%)-TiO2 nanostructure could be used as an effective and potentially reusable catalyst for photocatalytic treatment of water and wastewater.
Further, the structural decomposition and mineralization effectiveness of L-Histidine (2 wt.%)-TiO2 catalyst was assessed for OTC removal under the optimum conditions through a TOC analyzer. The initial OTC solution (50 mg/L) had 22.4 mg/L of TOC and the complete removal of TOC was observed after 300 min irradiation (Figure 12b). The mineralization results verified that OTC could be effectively degraded to byproducts and then it could be completely converted to harmless compounds, which indicated the effectiveness of the L-Histidine (2 wt.%)-TiO2/UV–Vis degradation process. In order to distinguish the contribution of adsorption, photolysis and photocatalysis in the removal of the pollutants, experiments were done in darkness, without catalysts, under visible light illumination and in visible light/L-Histidine (2 wt.%)-TiO2 system, respectively. In the photolysis process, the COD and OTC removals from LEPW and OTC were achieved 3 and 8% after 180 and 60 min, respectively. The role of adsorption was more significant than photolysis, which could eliminate 47% OTC and 39% OTC after 60 and 180 min, respectively.

4. Conclusions

L-Histidine-TiO2, L-Methionine-TiO2 and L-Asparagine-TiO2 novel photocatalysts were successfully synthesized through the sol-gel method. The optimum weight ratio of amino acid to TiO2 was determined by assessing their photocatalytic performances in degradation of OTC and COD, which obtained L-Histidine (2 wt.%), L-Methionine (1.5 wt.%) and L-Asparagine (2 wt.%). The performance evaluation of the synthesized photocatalysts showed L-Histidine (2 wt.%)-TiO2 had the best photocatalytic activity and the lowest recombination rate. Characterization of as-prepared photocatalysts proved enhancement of TiO2 photocatalytic performance through reducing the band gap energy of TiO2 from 3.1 to 2.1, 2.3 and 2.17 eV for L-Histidine (2 wt.%)-TiO2, L-Methionine (1.5 wt.%)-TiO2 and L-Asparagine (2 wt.%)-TiO2. PL analysis showed reduction of the recombination rate of the co-doped TiO2 photocatalyst.
Analysis of variance (ANOVA) showed the accuracy of the proposed models for photodegradation of OTC and COD by L-Histidine (2 wt.%)-TiO2. The models showed that decreasing initial concentration and increasing catalyst dosage, irradiation time and light intensity positively influenced photocatalysis. The complete OTC removal was obtained at optimal conditions of 50mg/L OTC, 1g/L of L-Histidine (2 wt.%)-TiO2, at neutral pH and light intensity of 25 W/cm−1 after 60 min visible light irradiation and 100% reduction of TOC after 300 min irradiation time. The increase of BOD5/COD of treated LEPW from 0.19 to about 0.6 confirmed the improvement of the biodegradability. The scavenger study showed the role of radicals in the degradation process followed the order of (OH) > (O2) > (h+). The slight decline of COD removal efficiency of LEPW (about 7–8%) after four cycles verified the reusability and stability of the catalyst. Based on the results, L-Histidine (2 wt.%)-TiO2 could be considered a promising photocatalyst for industrial and practical wastewater and water treatments.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15020290/s1, Figure S1: Kinetic models of OTC photocatalytic removal for optimum L-Amino acid-TiO2 at catalyst loading of 1 g/L, OTC concentration of 50 mg/L and original pH (4.5); Figure S2: Normal Probability plot of the residuals (a) OTC removal and (b) COD removal from LEWP; Table S1: Experimental conditions and response data for OTC photocatalytic removal; Table S2: Experimental conditions and response data for COD removal from LEPW; Table S3: The particle size distribution of the photocatalysts.

Author Contributions

Conceptualization, H.Z., S.A.M., P.E. and E.A.; methodology, H.Z., P.E. and E.A.; software, H.Z., P.E. and E.A.; formal analysis, H.Z., P.E., E.A., J.F. and M.R.Z.; investigation, H.Z., J.F. and M.R.Z.; resources, S.A.M.; data curation, H.Z., S.A.M., P.E., E.A., J.F. and M.R.Z.; writing—original draft preparation, H.Z. and S.A.M.; writing—review and editing, E.A.; visualization, H.Z.; supervision, H.Z. and S.A.M.; project administration, H.Z., S.A.M., J.F. and M.R.Z.; funding acquisition, S.A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Kermanshah University of Medical Sciences with Grant number: 97649 and ethical code: IR.KUMS.REC.1397.587).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the Kermanshah University of Medical Sciences, Kermanshah, Iran, for the financial support provided for this research work (Grant number: 97649 and ethical code: IR.KUMS.REC.1397.587).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Ramanayaka, S.; Sarkar, B.; Cooray, A.T.; Ok, Y.S.; Vithanage, M. Halloysite nanoclay supported adsorptive removal of oxytetracycline antibiotic from aqueous media. J. Hazard. Mater. 2020, 384, 121301. [Google Scholar] [CrossRef] [PubMed]
  2. Sun, Y.; Chen, Z.; Wu, G.; Wu, Q.; Zhang, F.; Niu, Z.; Hu, H.-Y. Characteristics of water quality of municipal wastewater treatment plants in China: Implications for resources utilization and management. J. Clean. Prod. 2016, 131, 1–9. [Google Scholar] [CrossRef]
  3. Zangeneh, H.; Zinatizadeh, A.A.; Zinadini, S.; Feyzi, M.; Bahnemann, D.W. Preparation ultrafine L-Methionine (C, N, S triple doped)-TiO2-ZnO nanoparticles and their photocatalytic performance for fouling alleviation in PES nanocomposite membrane. Compos. Part B Eng. 2019, 176, 107158. [Google Scholar] [CrossRef]
  4. Li, J.; Tian, J.-J.; Wang, L.-G.; Chang, C.-T. Photocatalytic hydrogen production and photodegradation of oxytetracycline by Cu-CdS composites. Mater. Lett. 2018, 217, 243–246. [Google Scholar] [CrossRef]
  5. Mahmoodi, M.; Rafiee, E.; Eavani, S. Photocatalytic removal of toxic dyes, liquorice and tetracycline wastewaters by a mesoporous photocatalyst under irradiation of different lamps and sunlight. J. Environ. Manag. 2022, 313, 115023. [Google Scholar] [CrossRef]
  6. Liu, C.; Mao, S.; Shi, M.; Hong, X.; Wang, D.; Wang, F.; Xia, M.; Chen, Q. Enhanced photocatalytic degradation performance of BiVO4/BiOBr through combining Fermi level alteration and oxygen defect engineering. Chem. Eng. J. 2022, 449, 137757. [Google Scholar] [CrossRef]
  7. Xiao, Y.; Sun, X.; Chen, J.; Zhao, S.; Jiang, C.; Yang, L.; Cheng, L.; Cao, S. Simultaneous formation of a C/N-TiO2 hollow photocatalyst with efficient photocatalytic performance and recyclability. Chin. J. Catal. 2019, 40, 765–775. [Google Scholar] [CrossRef]
  8. Zhao, C.; Deng, H.; Li, Y.; Liu, Z. Photodegradation of oxytetracycline in aqueous by 5A and 13X loaded with TiO2 under UV irradiation. J. Hazard. Mater. 2010, 176, 884–892. [Google Scholar] [CrossRef]
  9. Zhao, C.; Pelaez, M.; Duan, X.; Deng, H.; O’Shea, K.; Fatta-Kassinos, D.; Dionysiou, D.D. Role of pH on photolytic and photocatalytic degradation of antibiotic oxytetracycline in aqueous solution under visible/solar light: Kinetics and mechanism studies. Appl. Catal. B Environ. 2013, 134, 83–92. [Google Scholar] [CrossRef]
  10. Vaiano, V.; Sacco, O.; Sannino, D.; Ciambelli, P. Photocatalytic removal of spiramycin from wastewater under visible light with N-doped TiO2 photocatalysts. Chem. Eng. J. 2015, 261, 3–8. [Google Scholar] [CrossRef]
  11. Kooravand, S.; Goshadrou, A.; Hatamipour, M.S. Enhanced ethanol production from Glycyrrhiza glabra residue by fungus Mucor hiemalis. Ind. Crops Prod. 2017, 108, 767–774. [Google Scholar] [CrossRef]
  12. Andrio, D.; Asmura, J.; Yenie, E.; Putri, K. Enhancing BOD5/COD ratio co-substrate tofu wastewater and cow dung during ozone pretreatment. In MATEC Web of Conferences; EDP Sciences: Les Ulis, France, 2019. [Google Scholar]
  13. Parsaa, Z.; Motevassel, M.; Khosravi-Nikou, M.R.; Jorfi, S.; Ghasemi, B. Biodegradability enhancement of azo dye Direct Orange-26 using UV/Fenton-like process: Optimization using response surface methodology. Desalination Water Treat. 2017, 81, 233–241. [Google Scholar] [CrossRef]
  14. Dai, Y.; Liu, Y.; Kong, J.; Yuan, J.; Sun, C.; Xian, Q.; Yang, S.; He, H. High photocatalytic degradation efficiency of oxytetracycline hydrochloride over Ag/AgCl/BiVO4 plasmonic photocatalyst. Solid State Sci. 2019, 96, 105946. [Google Scholar] [CrossRef]
  15. Zangeneh, H.; Zinatizadeh, A.A.; Zinadini, S.; Rafiee, E.; Feyzi, M.; Bahnemann, D.W. A novel L-Histidine (C, N) codoped-TiO2-CdS nanocomposite for efficient visible photo-degradation of recalcitrant compounds from wastewater. J. Hazard. Mater. 2019, 369, 384–397. [Google Scholar] [CrossRef]
  16. Liu, C.; Mao, S.; Shi, M.; Wang, F.; Xia, M.; Chen, Q.; Ju, X. Peroxymonosulfate activation through 2D/2D Z-scheme CoAl-LDH/BiOBr photocatalyst under visible light for ciprofloxacin degradation. J. Hazard. Mater. 2021, 420, 126613. [Google Scholar]
  17. Azzam, E.M.; Fathy, N.A.; El-Khouly, S.M.; Sami, M. Enhancement the photocatalytic degradation of methylene blue dye using fabricated CNTs/TiO2/AgNPs/Surfactant nanocomposites. J. Water Process Eng. 2019, 28, 311–321. [Google Scholar] [CrossRef]
  18. Wang, J.; Fan, C.; Ren, Z.; Fu, X.; Qian, G.; Wang, Z. N-doped TiO2/C nanocomposites and N-doped TiO2 synthesised at different thermal treatment temperatures with the same hydrothermal precursor. Dalton Trans. 2014, 43, 13783–13791. [Google Scholar] [CrossRef]
  19. Chen, W.; Liu, T.-Y.; Huang, T.; Liu, X.-H.; Duan, G.-R.; Yang, X.-J.; Chen, S.-M. A novel yet simple strategy to fabricate visible light responsive C, N-TiO2/gC3N4 heterostructures with significantly enhanced photocatalytic hydrogen generation. RSC Adv. 2015, 5, 101214–101220. [Google Scholar] [CrossRef]
  20. Cheng, X.; Yu, X.; Xing, Z. Synthesis and characterization of C–N–S-tridoped TiO2 nano-crystalline photocatalyst and its photocatalytic activity for degradation of rhodamine B. J. Phys. Chem. Solids 2013, 74, 684–690. [Google Scholar] [CrossRef]
  21. Wang, N.; Li, X.; Yang, Y.; Zhou, Z.; Shang, Y.; Zhuang, X.; Zhang, T. Two-stage calcination composite of Bi2O3-TiO2 supported on powdered activated carbon for enhanced degradation of sulfamethazine under solar irradiation. J. Water Process Eng. 2020, 35, 101220. [Google Scholar] [CrossRef]
  22. Sushma, C.; Kumar, S.G. C–N–S tridoping into TiO2 matrix for photocatalytic applications: Observations, speculations and contradictions in the codoping process. Inorg. Chem. Front. 2017, 4, 1250–1267. [Google Scholar] [CrossRef]
  23. Wan, N.; Huang, Y.; Ho, W.; Zhang, L.; Zou, Z.; Lee, S. Oxygen vacancy-mediated efficient electron-hole separation for CNS-tridoped single crystal black TiO2 (B) nanorods as visible-light-driven photocatalysts. Appl. Surf. Sci. 2018, 457, 287–294. [Google Scholar] [CrossRef]
  24. Eskandari, P.; Amarloo, E.; Zangeneh, H.; Rezakazemi, M.; Zamani, M.R.; Aminabhavi, T. Photocatalytic activity of visible-light-driven L-Proline-TiO2/BiOBr nanostructured materials for dyes degradation: The role of generated reactive species. J. Environ. Manag. 2023, 326, 116691. [Google Scholar] [CrossRef] [PubMed]
  25. Dolatshah, M.; Zinatizadeh, A.A.; Zinadini, S.; Zangeneh, H. Preparation, characterization and performance assessment of antifouling L-Lysine (C, N codoped)-TiO2/WO3-PES photocatalytic membranes: A comparative study on the effect of blended and UV-grafted nanophotocatalyst. J. Environ. Chem. Eng. 2022, 10, 108658. [Google Scholar] [CrossRef]
  26. Zangeneh, H.; Zinatizadeh, A.A.; Zinadini, S.; Nazari, S.; Sibali, L.; McKay, T. Highly efficient azo dye degradation in a photocatalytic rotating disc reactor with deposited l-histidine-TiO2-CdS. Mater. Sci. Semicond. Process. 2022, 152, 107071. [Google Scholar] [CrossRef]
  27. Amaro-Medina, B.M.; Martinez-Luevanos, A.; Soria-Aguilar, M.D.J.; Sanchez-Castillo, M.A.; Estrada-Flores, S.; Carrillo-Pedroza, F.R. Efficiency of Adsorption and Photodegradation of Composite TiO2/Fe2O3 and Industrial Wastes in Cyanide Removal. Water 2022, 14, 3502. [Google Scholar] [CrossRef]
  28. Wang, Y.; Huang, Y.; Ho, W.; Zhang, L.; Zou, Z.; Lee, S. Biomolecule-controlled hydrothermal synthesis of C–N–S-tridoped TiO2 nanocrystalline photocatalysts for NO removal under simulated solar light irradiation. J. Hazard. Mater. 2009, 169, 77–87. [Google Scholar] [CrossRef]
  29. Xue, X.; Wang, Y.; Yang, H. Preparation and characterization of boron-doped titania nano-materials with antibacterial activity. Appl. Surf. Sci. 2013, 264, 94–99. [Google Scholar] [CrossRef]
  30. Yang, B.; Park, H.-D.; Won Hong, S.; Lee, S.-H.; Parke, J.-A.; Choi, J.-W. Photocatalytic degradation of microcystin-LR and anatoxin-a with presence of natural organic matter using UV-light emitting diodes/TiO2 process. J. Water Process Eng. 2020, 34, 101163. [Google Scholar] [CrossRef]
  31. Liu, C.-F.; Perng, T.-P. Fabrication and band structure of Ag3PO4–TiO2 heterojunction with enhanced photocatalytic hydrogen evolution. Int. J. Hydrog. Energy 2020, 45, 149–159. [Google Scholar] [CrossRef]
  32. Mei, P. The enhanced photodegradation of bisphenol A by TiO2/C3N4 composites. Environ. Res. 2020, 182, 109090. [Google Scholar] [CrossRef]
  33. Farhadian, N. Chitosan modified N, S-doped TiO2 and N, S-doped ZnO for visible light photocatalytic degradation of tetracycline. Int. J. Biol. Macromol. 2019, 132, 360–373. [Google Scholar] [CrossRef]
  34. Shayegan, Z.; Haghighat, F.; Lee, C.-S. Carbon-doped TiO2 film to enhance visible and UV light photocatalytic degradation of indoor environment volatile organic compounds. J. Environ. Chem. Eng. 2020, 8, 104162. [Google Scholar] [CrossRef]
  35. Ba-Abbad, M.M. Synthesis and catalytic activity of TiO2 nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation. Int. J. Electrochem. Sci. 2012, 7, 4871–4888. [Google Scholar]
  36. Zangeneh, H.; Mousavi, S.A.; Eskandari, P. Comparison the visible photocatalytic activity and kinetic performance of amino acids (non-metal doped) TiO2 for degradation of colored wastewater effluent. Mater. Sci. Semicond. Process. 2022, 140, 106383. [Google Scholar] [CrossRef]
  37. Mousavi, S.A.; Farrokhi, F.; Kianirad, N.; Falahi, F. Degradation of aniline from aqueous solution by Fenton process: Modeling and optimization. Desalination Water Treat. 2018, 125, 68–74. [Google Scholar] [CrossRef]
  38. Mousavi, S.A.; Vasseghian, Y.; Bahadori, A. Evaluate the performance of Fenton process for the removal of methylene blue from aqueous solution: Experimental, neural network modeling and optimization. Environ. Prog. Sustain. Energy 2020, 39, e13126. [Google Scholar] [CrossRef]
  39. Mousavi, S.A.; Nazari, S. Applying Response Surface Methodology to Optimize the Fenton Oxidation Process in the Removal of Reactive Red 2. Pol. J. Environ. Stud. 2017, 26, 765–772. [Google Scholar] [CrossRef]
  40. Samy, M.; Ibrahim, M.G.; Gar Alalm, M.; Fujii, M.; Ookawara, S.; Ohno, T. Photocatalytic degradation of trimethoprim using S-TiO2 and Ru/WO3/ZrO2 immobilized on reusable fixed plates. J. Water Process Eng. 2020, 33, 101023. [Google Scholar] [CrossRef]
  41. Pereira, J.H.; Vilar, V.J.; Borges, M.T.; González, O.; Esplugas, S.; Boaventura, R.A. Photocatalytic degradation of oxytetracycline using TiO2 under natural and simulated solar radiation. Sol. Energy 2011, 85, 2732–2740. [Google Scholar] [CrossRef]
  42. Hassandoost, R. Hierarchically structured ternary heterojunctions based on Ce3+/Ce4+ modified Fe3O4 nanoparticles anchored onto graphene oxide sheets as magnetic visible-light-active photocatalysts for decontamination of oxytetracycline. J. Hazard. Mater. 2019, 376, 200–211. [Google Scholar] [CrossRef] [PubMed]
  43. Yu, W.; Jin, H.; Shen, A.; Deng, L.; Shi, J.; Xue, X.; Guo, Y.; Liu, Y.; Liang, X. Purification of high-purity glycyrrhizin from licorice using hydrophilic interaction solid phase extraction coupled with preparative reversed-phase liquid chromatography. J. Chromatogr. B 2017, 1040, 47–52. [Google Scholar] [CrossRef] [PubMed][Green Version]
  44. Surenjan, A. Synthesis, characterization and performance of visible light active C-TiO2 for pharmaceutical photodegradation. J. Environ. Chem. Eng. 2017, 5, 757–767. [Google Scholar] [CrossRef]
  45. Kumar, A.; Khan, M.; Zeng, X.; Lo, I. Development of g-C3N4/TiO2/Fe3O4@ SiO2 heterojunction via sol-gel route: A magnetically recyclable direct contact Z-scheme nanophotocatalyst for enhanced photocatalytic removal of ibuprofen from real sewage effluent under visible light. Chem. Eng. J. 2018, 353, 645–656. [Google Scholar] [CrossRef]
  46. Dugandžić, A.M. Effect of inorganic ions, photosensitisers and scavengers on the photocatalytic degradation of nicosulfuron. J. Photochem. Photobiol. A Chem. 2017, 336, 146–155. [Google Scholar] [CrossRef]
  47. Huangch, S.; Chen, C.; Tsai, H.; Shaya, J.; Lu, C. Photocatalytic degradation of thiobencarb by a visible light-driven MoS2 photocatalyst. Sep. Purif. Technol. 2018, 197, 147–155. [Google Scholar] [CrossRef]
  48. Liu, C.; Mao, S.; Wang, H. Peroxymonosulfate-assisted for facilitating photocatalytic degradation performance of 2D/2D WO3/BiOBr S-scheme heterojunction. Chem. Eng. J. 2022, 430, 132806. [Google Scholar] [CrossRef]
  49. Fakhravar, S.; Farhadian, M.; Tangestaninejad, S. Excellent performance of a novel dual Z-scheme Cu2S/Ag2S/BiVO4 heterostructure in metronidazole degradation in batch and continuous systems: Immobilization of catalytic particles on α-Al2O3 fiber. Appl. Surf. Sci. 2020, 505, 144599. [Google Scholar] [CrossRef]
  50. Li, W.; Xie, L.; Zhou, L.; Ochoa-Lozano, J.; Li, C.; Chai, X. A systemic study on Gd, Fe and N co-doped TiO2 nanomaterials for enhanced photocatalytic activity under visible light irradiation. Ceram. Int. 2020, 46, 24744–24752. [Google Scholar] [CrossRef]
  51. Chen, D.; Jiang, Z.; Geng, J.; Wang, Q.; Yang, D. Carbon and nitrogen co-doped TiO2 with enhanced visible-light photocatalytic activity. Ind. Eng. Chem. Res. 2007, 46, 2741–2746. [Google Scholar] [CrossRef]
Figure 1. Effect of the amount of dopant agents in L−Amino acid−TiO2 composites on OTC photocatalytic removal, (a) L−Histidine, (b) L−Methionine and (c) L−Asparagine at catalyst loading of 1 g/L, OTC concentration of 50 mg/L and original pH (4.5).
Figure 1. Effect of the amount of dopant agents in L−Amino acid−TiO2 composites on OTC photocatalytic removal, (a) L−Histidine, (b) L−Methionine and (c) L−Asparagine at catalyst loading of 1 g/L, OTC concentration of 50 mg/L and original pH (4.5).
Water 15 00290 g001aWater 15 00290 g001b
Figure 2. COD removal by (a) L−Histidine (2 wt.%) −TiO2 (b) L−Methionine (1.5 wt.%) −TiO2 (c) L−Asparagine (2 wt.%) −TiO2 at catalyst loading of 2 g/L, COD concentration of 500 mg/L and original pH = 5.7.
Figure 2. COD removal by (a) L−Histidine (2 wt.%) −TiO2 (b) L−Methionine (1.5 wt.%) −TiO2 (c) L−Asparagine (2 wt.%) −TiO2 at catalyst loading of 2 g/L, COD concentration of 500 mg/L and original pH = 5.7.
Water 15 00290 g002aWater 15 00290 g002b
Figure 3. Photocatalytic performance of optimum L-Amino acid-TiO2 in (a) COD removal and (b) BOD5/COD ratio versus irradiation time at catalyst loading of 2 g/L, COD concentration of 500 mg/L and original pH = 5.7.
Figure 3. Photocatalytic performance of optimum L-Amino acid-TiO2 in (a) COD removal and (b) BOD5/COD ratio versus irradiation time at catalyst loading of 2 g/L, COD concentration of 500 mg/L and original pH = 5.7.
Water 15 00290 g003aWater 15 00290 g003b
Figure 4. The PL analysis of the prepared catalysts.
Figure 4. The PL analysis of the prepared catalysts.
Water 15 00290 g004
Figure 5. (a) XRD patterns, (b) FTIR spectra, (c) DRS spectra and (d) Tauc plots of photocatalysts.
Figure 5. (a) XRD patterns, (b) FTIR spectra, (c) DRS spectra and (d) Tauc plots of photocatalysts.
Water 15 00290 g005aWater 15 00290 g005b
Figure 6. FESEM/EDX of the synthesized photocatalysts (a) L-Asparagine (2 wt.%)-TiO2 (b) L-Methionine (1.5 wt.%)-TiO2 (c) L-Histidine (2 wt.%)-TiO2 (d) EDX spectrum of L-Histidine (2 wt.%)-TiO2.
Figure 6. FESEM/EDX of the synthesized photocatalysts (a) L-Asparagine (2 wt.%)-TiO2 (b) L-Methionine (1.5 wt.%)-TiO2 (c) L-Histidine (2 wt.%)-TiO2 (d) EDX spectrum of L-Histidine (2 wt.%)-TiO2.
Water 15 00290 g006aWater 15 00290 g006b
Figure 7. Contour plot of (a) OTC and (b) COD removal efficiency as a function of pollutant concentration and catalyst dosage.
Figure 7. Contour plot of (a) OTC and (b) COD removal efficiency as a function of pollutant concentration and catalyst dosage.
Water 15 00290 g007
Figure 8. Contour plot of (a) OTC and (b) COD removal efficiency as a function of pH and irradiation time.
Figure 8. Contour plot of (a) OTC and (b) COD removal efficiency as a function of pH and irradiation time.
Water 15 00290 g008
Figure 9. Contour plot of (a) OTC and (b) COD removal efficiency as a function of PH and light intensity.
Figure 9. Contour plot of (a) OTC and (b) COD removal efficiency as a function of PH and light intensity.
Water 15 00290 g009
Figure 10. Photodegradation of (a) OTC and (b) COD with L-Histidine (2 wt.%)-TiO2 in presence of different scavengers.
Figure 10. Photodegradation of (a) OTC and (b) COD with L-Histidine (2 wt.%)-TiO2 in presence of different scavengers.
Water 15 00290 g010
Figure 11. OTC photodegradation mechanism for L-Hisitidine-TiO2.
Figure 11. OTC photodegradation mechanism for L-Hisitidine-TiO2.
Water 15 00290 g011
Figure 12. (a) Reusability of the L-Histidine (2 wt.%)-TiO2 catalyst for LEPW removal and (b) TOC removal of OTC photodegradation.
Figure 12. (a) Reusability of the L-Histidine (2 wt.%)-TiO2 catalyst for LEPW removal and (b) TOC removal of OTC photodegradation.
Water 15 00290 g012
Table 1. Characteristics of licorice extraction plant wastewater.
Table 1. Characteristics of licorice extraction plant wastewater.
Wastewater PropertiesInitial Conditions before Treatment
ColorBrown
COD (mg/L)700–800
BOD5/COD ratio (mg/L)0.17–0.19
pH5.7–6.4
TSS (mg/L)120–250
Table 2. Experimental ranges and levels of the processing factors for OTC and COD removal.
Table 2. Experimental ranges and levels of the processing factors for OTC and COD removal.
FactorsLevels
−101
OTC
A: [OTC] (mg/L)5075100
B: [Catalyst] (g/L)0.511.5
C: pH4710
D: Irradiation time (min)153045
E: Light Intensity (W/cm2)152025
LEPW
A: [COD] (mg/L)300500700
B: [Catalyst] (g/L)11.52
C: pH4710
D: Irradiation time (min)60120180
E: Light Intensity (W/cm2)152025
Table 3. Kinetic constant and determination of coefficient of pseudo first and second order models for photocatalytic removal of OTC.
Table 3. Kinetic constant and determination of coefficient of pseudo first and second order models for photocatalytic removal of OTC.
Pseudo First Order Model K1R2
L-Histidine-TiO20.02510.977
L-Methionine-TiO20.01490.983
L-Asparagine-TiO20.01110.977
Pseudo Second Order Modelk2R2
L-Histidine-TiO20.01080.824
L-Methionine-TiO20.00820.902
L-Asparagine-TiO20.00800.947
Table 4. ANOVA results for models of COD and OTC photocatalytic degradation.
Table 4. ANOVA results for models of COD and OTC photocatalytic degradation.
SourceSum of SquaresdfF-Valuep-Value
LEPWOTCLEPWOTCLEPWOTCLEPWOTC
Model12,391.427,990.108849.06129.72<0.0001<0.0001
A: [dye]3790.62647.0611120.0598.14<0.0001<0.0001
B: [catalyst]1882.6995.761159.6236.92<0.0001<0.0001
C: pH1794.46250.621156.83231.75<0.0001<0.0001
D: Irradiation time1284.7984.971140.9636.52<0.0001<0.0001
E: light Intensity2542.23790.621180.52140.54<0.0001<0.0001
CE-399.03-1-14.79-0.0004
A2181.8-1-5.76-0.0210-
B2-178.29-1-6.61-0.0139
C2189.22951.641-5.99109.440.0187<0.0001
D2230.7-117.319.990.0100-
Residual1294.51105.824142
Lack of Fit1127876.9434351.390.790.34570.7047
Pure Error167.5228.8877
LEPW: R2 = 0.91, adjusted R2 = 0.9, Std. Dev = 5.62, Adeq precision = 33.06, C.V.% = 8.92, Lack of fit = 0.34
OTC: R2 = 0.96, adjusted R2 = 0.95, Std. Dev = 5.19, Adeq precision = 39.67, C.V.% = 10.38, Lack of fit = 0.7
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zangeneh, H.; Mousavi, S.A.; Eskandari, P.; Amarloo, E.; Farghelitiyan, J.; Zamani, M.R. Comparative Study on Enhanced Photocatalytic Activity of Visible Light-Active Nanostructures for Degradation of Oxytetracycline and COD Removal of Licorice Extraction Plant Wastewater. Water 2023, 15, 290. https://doi.org/10.3390/w15020290

AMA Style

Zangeneh H, Mousavi SA, Eskandari P, Amarloo E, Farghelitiyan J, Zamani MR. Comparative Study on Enhanced Photocatalytic Activity of Visible Light-Active Nanostructures for Degradation of Oxytetracycline and COD Removal of Licorice Extraction Plant Wastewater. Water. 2023; 15(2):290. https://doi.org/10.3390/w15020290

Chicago/Turabian Style

Zangeneh, Hadis, Seyyed Alireza Mousavi, Parisa Eskandari, Ehsan Amarloo, Javad Farghelitiyan, and Mohammad Reza Zamani. 2023. "Comparative Study on Enhanced Photocatalytic Activity of Visible Light-Active Nanostructures for Degradation of Oxytetracycline and COD Removal of Licorice Extraction Plant Wastewater" Water 15, no. 2: 290. https://doi.org/10.3390/w15020290

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop