Adsorption and Photocatalytic Mineralization of Bromophenol Blue Dye with TiO 2 Modiﬁed with Clinoptilolite/Activated Carbon

: This study presents a hybridized photocatalyst with adsorbate as a promising nanocomposite for photoremediation of wastewater. Photocatalytic degradation of bromophenol blue (BPB) in aqueous solution under UV-irradiation of wavelength 400 nm was carried out with TiO 2 doped with activated carbon (A) and clinoptilolite (Z) via the co-precipitation technique. The physiochemical properties of the nanocomposite (A–TiO 2 and Z–TiO 2 ) and TiO 2 were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared (FTIR) spectroscopy. Results of the nanocomposite (A–TiO 2 and Z–TiO 2 ) efﬁciency was compared to that with the TiO 2 , which demonstrated their adsorption and synergistic effect for the removal of chemical oxygen demand (COD) and color from the wastewater. At an optimal load of 4 g, the photocatalytic degradation activity (Z–TiO 2 > A–TiO 2 > TiO 2 ) was found favorably by the second-order kinetic model. Consequently, the Langmuir adsorption isotherms favored the nanocomposites (Z–TiO 2 > A–TiO 2 ), whereas that of the TiO 2 ﬁtted very well on the Freundlich isotherm approach. Z–TiO 2 evidently exhibited a high photocatalytic efﬁcacy of decomposition over 80% of BPB (COD) at reaction rate constant (k) and coefﬁcient of determination (R 2 ) values of 5.63 × 10 − 4 min − 1 and 0.989, respectively.


Introduction
The recent uprising of industrialization is being associated with the depletion of water resources owing to its consumption rate, making it essential to maintain healthy living [1,2]. Notwithstanding, the impact of anthropogenic activities and socio-economic development has resulted in poor sanitation and inadequate freshwater supply for drinking, even in countries or regions considered as water-rich [1]. Water shortages in many developing countries have been reported as one of the most significant political and societal problems confronting them [1,3].
South Africa, like many African countries, continues to suffer from the fresh water supply, which is declining in quality because of an increase in the pollution index and impeding of river catchments being caused by urbanization, deforestation, industrial and agricultural activities [4][5][6]. Furthermore, dyes and pigments produced by various industries such as cosmetics, textile, paper, leather, pharmaceutical, nutrition, and plastics are the foremost common contaminants in wastewater [7]. Distinctively, with a yearly estimation of over 70,000 tons of dyes produced, during the dyeing processes, 15% are being misapplied [8,9]. The release of these dyes and other organic pollutants into wastewater and the aquatic environment, even at low concentrations, are noticeable to be highly toxic with low transparency [9,10]. Additionally, emerging contaminants (antibiotics, pesticides, nanoplastics) complexity opposing water quality poses a great threat to aquatic life and The change in the crystallographic structure of TiO 2 after being modified into A-TiO 2 and Z-TiO 2 nanocomposites were determined by X-ray diffraction (XRD). As can be seen (Figure 1), the XRD results showed the presence of crystalline phases in phase mixtures, with most of the TiO 2 structures as the anatase phase. Comparatively, the change in the intensities of the TiO 2 peaks to that of A-TiO 2 and Z-TiO 2 signpost the effect that the isomorphic interaction may have occurred during the modification mechanism [32,33]. Notwithstanding, the dominating phase of the TiO 2 pattern in all the samples by the XRD peaks ( Figure 1) within the angle range of 2θ (5 • > 2θ > 70 • ) is anatase corresponding to the JCPDS Files No.21-1272 [33,34] (Figure 1) by the broad bump of A-TiO 2 around 2θ = 25.28 • . Adding to this, the reflection peak in the A-TiO 2 of the anatase around 2θ = 25.67 • decreased, whereas the rutile peak at 2θ = 27.45 • (002) became sharper and stronger. However, the anatase peak in the Z-TiO 2 remained unchanged, even though the rutile peak intensity heightened. The most intense reflection of clinoptilolite (Z) was found at 2θ = 9.88 • . Conversely to this result, Zhong et al. [33] suggested the steadiest rutile polymorph of TiO 2 observed at 2θ = 27.45 • of the Z-TiO 2 displayed the same signal characterized for natural zeolite (clinoptilolite). In terms of A-TiO 2 , there is no peak at the 2θ = 24.67 • position, which is generally characterized as a crystalline plane (002) of activated carbon (A), which seems to be isolated by the TiO 2 (anatase peak at 25.28 • ). This makes A-TiO 2 be in an amorphous state, by its carbon suppressing the TiO 2 (anatase peak at 2θ = 25.45 • ) transition to reveal the rutile peak at 2θ = 27.45 • . Additional, peaks at 2θ values of 52.06 • (004), 55.2 • (211) and 58.68 • (204) of A-TiO 2 and Z-TiO 2, aside it being rutile, might also be attributed to reflections of crystallographic phases of impurities as some are depicted in Table 1. Additionally, the anatase being the predominant phase, therefore, expedited the nanocomposite for the photocatalytic activity [35]. Furthermore, the dominant crystal size (22-34 nm) of TiO 2 present in the nanocomposites (A-TiO 2 and Z-TiO 2 ) as anatase was calculated by using the Scherer's Equation (1) and the diffraction peak at the 2θ = 25.28 • (101) plane [27], where δ is the wavelength of X-ray radiation (CuKα = 0.15406 nm), K is a constant taken as 0.98, β is the first line width at half maximum height (FWHM) of the peak (determined using Origin software, version 2018), and θ is the diffracting angle (2θ = 25.28 • for 101 planes).
where δ is the wavelength of X-ray radiation (CuKα = 0.15406 nm), K is a constant taken as 0.98, β is the first line width at half maximum height (FWHM) of the peak (determined using Origin software), and θ is the diffracting angle (2θ = 25.28 • for 101 planes).  (Figure 1) by the broad bump of A-TiO2 around = 25.28°. Adding to this, the reflection peak in the A-TiO2 of the anatase around 2θ = 25 decreased, whereas the rutile peak at 2θ = 27.45° (002) became sharper and stronger. H ever, the anatase peak in the Z-TiO2 remained unchanged, even though the rutile p intensity heightened. The most intense reflection of clinoptilolite (Z) was found at 2 9.88°. Conversely to this result, Zhong et al. [33] suggested the steadiest rutile polymo of TiO2 observed at 2θ = 27.45° of the Z-TiO2 displayed the same signal characterized natural zeolite (clinoptilolite). In terms of A-TiO2, there is no peak at the 2θ = 24.67° p tion, which is generally characterized as a crystalline plane (002) of activated carbon which seems to be isolated by the TiO2 (anatase peak at 25.28°). This makes A-TiO2 b an amorphous state, by its carbon suppressing the TiO2 (anatase peak at 2θ = 25.45°) tr sition to reveal the rutile peak at 2θ = 27.45°. Additional, peaks at 2θ values of 52.06° (0 55.2° (211) and 58.68° (204) of A-TiO2 and Z-TiO2, aside it being rutile, might also be tributed to reflections of crystallographic phases of impurities as some are depicted Table 1. Additionally, the anatase being the predominant phase, therefore, expedited nanocomposite for the photocatalytic activity [35]. Furthermore, the dominant crystal (22-34 nm) of TiO2 present in the nanocomposites (A-TiO2 and Z-TiO2) as anatase w calculated by using the Scherer's Equation (1) and the diffraction peak at the 2θ = 25 (101) plane [27], where is the wavelength of X-ray radiation (CuKα = 0.15406 nm), a constant taken as 0.98, β is the first line width at half maximum height (FWHM) of peak (determined using Origin software, version 2018), and θ is the diffracting angle = 25.28° for 101 planes).   Scanning electron microscopy (SEM) at the University of Cape Town, South Africa was employed to characterize the surface morphologies of the TiO 2 , A-TiO 2, and Z-TiO 2 nanocomposites, and the results obtained are presented in Figure 2. It is observed (Figure 2a) that the TiO 2 with no covering has very hexagonal and unformulated structures with abundant pores to accommodate other materials [31,33,35]. The average grain size (Figure 2a) of the pores was 4.15 µm under the scale of 1 µm with a magnification of 50 kx and a working distance (WD) of 6.36 mm. Conversely, for the A-TiO 2 and Z-TiO 2 representing a WD of 6.11 mm ( Figure 2b) and 6.14 mm (Figure 2c), respectively, a grain pore size of 20.8 µm under the same scale of 5 µm and magnification of 10 kx was obtained. where is the wavelength of X-ray radiation (CuKα = 0.15406 nm), K is a constant taken as 0.98, β is the first line width at half maximum height (FWHM) of the peak (determined using Origin software), and θ is the diffracting angle (2θ = 25.28° for 101 planes).

Morphological Analysis
Scanning electron microscopy (SEM) at the University of Cape Town, South Africa was employed to characterize the surface morphologies of the TiO2, A-TiO2, and Z-TiO2 nanocomposites, and the results obtained are presented in Figure 2. It is observed (Figure 2a) that the TiO2 with no covering has very hexagonal and unformulated structures with abundant pores to accommodate other materials [31,33,35]. The average grain size (Figure 2a  Subsequently, the EDX images at 100 µm for the TiO2 (Figure 3a), A-TiO2 (Figure 3b), and Z-TiO2 (Figure 3c) revealed the possible hybridization with well-dispersed TiO2 on the A-TiO2 and Z-TiO2 nanocomposites. Evidently, the hexagonal structure (Figure 3a) of TiO2 nanoparticles was well dispersed and intertwined on the surface of A-TiO2 or Z-TiO2 [33], which transformed their surface structures (Figure 3b,c). The porous and agglomerated structures are revealed in the A-TiO2 ( Figure 3b) and Z-TiO2 ( Figure 3c). However, the surface chemistry of A-TiO2 or Z-TiO2 with other elements present, as shown in Table  1, may have obstructed the TiO2 surface coating and photocatalytic activities. The elemental compositions of the TiO2, A-TiO2, and Z-TiO2 nanocomposites, is shown in Table  1. This revealed the elements in the TiO2 (O > Ti > C > Na) were distinctively present in the A-TiO2, and Z-TiO2 which proofed the success of hybridizing the nanocomposite. Considering the elemental structures, Z-TiO2 contained O > Ti > C > Si > K > Al > Na > Mg > Ca, where Si, Al, O and other cationic elements forms the internal framework of Z [34,35]. The unique pores of Z provided a large surface area and improved the homogeneous dispersion of TiO2 nanoparticles on the surface (Figure 3c) of A-TiO2 constituted O > K > C > Ti > Cl > Al > P > Si > Ca > Mg > S, whereby most of the cationic carriers sup- Subsequently, the EDX images at 100 µm for the TiO 2 (Figure 3a), A-TiO 2 (Figure 3b), and Z-TiO 2 (Figure 3c) revealed the possible hybridization with well-dispersed TiO 2 on the A-TiO 2 and Z-TiO 2 nanocomposites. Evidently, the hexagonal structure ( Figure 3a) of TiO 2 nanoparticles was well dispersed and intertwined on the surface of A-TiO 2 or Z-TiO 2 [33], which transformed their surface structures (Figure 3b,c). The porous and agglomerated structures are revealed in the A-TiO 2 ( Figure 3b) and Z-TiO 2 ( Figure 3c). However, the surface chemistry of A-TiO 2 or Z-TiO 2 with other elements present, as shown in Table 1, may have obstructed the TiO 2 surface coating and photocatalytic activities. The elemental compositions of the TiO 2 , A-TiO 2, and Z-TiO 2 nanocomposites, is shown in Table 1. This revealed the elements in the TiO 2 (O > Ti > C > Na) were distinctively present in the A-TiO 2, and Z-TiO 2 which proofed the success of hybridizing the nanocomposite. Considering the elemental structures, Z- Al, O and other cationic elements forms the internal framework of Z [34,35]. The unique pores of Z provided a large surface area and improved the homogeneous dispersion of TiO 2 nanoparticles on the surface (Figure 3c) of A-TiO 2 constituted O > K > C > Ti > Cl > Al > P > Si > Ca > Mg > S, whereby most of the cationic carriers suppressed the presence of the Ti composition ( Figure 3b). In addition, the strong synergy between Al and Si decreased the charge carriers of the Ti as there is a high possibility of forming composite oxides [33,35].

Molecular Structures and Functional Groups
The nanocomposites (A-TiO2 and Z-TiO2) and TiO2 were furthermore analyzed by Fourier-transform infrared (FTIR) spectroscopy. The chemical bonds and functional groups within a fingerprint region of wavenumber of 400-2000 cm −1 revealed by the FTIR spectra are presented in Figure 4. The spectrum also reveals the vibration bands between the reactive hydroxyl groups, as well as the hydrophilic nature of the TiO2 [28,36]. The peaks observed around 3250 cm −1 was attributed to the strong vibrations of OH groups bonded with water or TiO2, where the shift in the OH bonding or the overlayer might be due to the combined A or Z with the TiO2. The peaks detected around 1630-1820 cm −1 were ascribed to the OH and C=O vibration band. The single band around 1635 cm −1 associated with the vibration mode of the Ti-O bond and other oxygenated functional groups [30,33]. The weak absorption band around 1590 cm −1 and 1450 cm −1 for the A-TiO2 and Z-TiO2 were attributed to the carboxyl (C-OH/C-H) and hydroxyl (H-O-H) bending vibration mode, which might have occurred by the nanocomposite adsorbing moisture during the preparation [36]. Furthermore, the broad and weak peaks observed around 1070 cm −1 for the A-TiO2 and Z-TiO2 might also be due to the modification of the TiO2 spectrum being attributed to the C-O-C stretching vibrations [36,37]. Finally, the broad peaks within the range of 400 cm −1 to 1000 cm −1 may be instigated by the vibration stretch of Ti-O and -OH, as observed in all the samples [35].

Molecular Structures and Functional Groups
The nanocomposites (A-TiO 2 and Z-TiO 2 ) and TiO 2 were furthermore analyzed by Fourier-transform infrared (FTIR) spectroscopy. The chemical bonds and functional groups within a fingerprint region of wavenumber of 400-2000 cm −1 revealed by the FTIR spectra are presented in Figure 4. The spectrum also reveals the vibration bands between the reactive hydroxyl groups, as well as the hydrophilic nature of the TiO 2 [28,36]. The peaks observed around 3250 cm −1 was attributed to the strong vibrations of OH groups bonded with water or TiO 2 , where the shift in the OH bonding or the overlayer might be due to the combined A or Z with the TiO 2 . The peaks detected around 1630-1820 cm −1 were ascribed to the OH and C=O vibration band. The single band around 1635 cm −1 associated with the vibration mode of the Ti-O bond and other oxygenated functional groups [30,33]. The weak absorption band around 1590 cm −1 and 1450 cm −1 for the A-TiO 2 and Z-TiO 2 were attributed to the carboxyl (C-OH/C-H) and hydroxyl (H-O-H) bending vibration mode, which might have occurred by the nanocomposite adsorbing moisture during the preparation [36]. Furthermore, the broad and weak peaks observed around 1070 cm −1 for the A-TiO 2 and Z-TiO 2 might also be due to the modification of the TiO 2 spectrum being attributed to the C-O-C stretching vibrations [36,37]. Finally, the broad peaks within the range of 400 cm −1 to 1000 cm −1 may be instigated by the vibration stretch of Ti-O and -OH, as observed in all the samples [35].

Effect of pH on Photocatalytic Degradation
The pH of an effluent strongly plays an important role in photocatalytic degradation of organic compounds, such as dyes in an aqueous suspension with TiO 2 NPs. First and foremost, the pH of the BPB synthesized effluent was investigated to ascertain the TiO 2 , Z-TiO 2, and A-TiO 2 photocatalytic activities on COD removal. This was carried out by varying the pH of the effluent from 5-9 while keeping the catalyst load and reaction time constant at 2 g and 30 min, respectively. Figure 5 shows the degradation efficiency obtained by the TiO 2 , Z-TiO 2 and A-TiO 2 on the COD removal. Evidently, the pH affected the adsorption shift of the organic substrates onto the surface of the photocatalyst as well as their photodegradation efficiency from the neutral to the acidic and basic mediums. The degradation efficiencies recorded for the acidic medium (pH 5-6) were 75-79%, 75-77%, and 77-80% for TiO 2 , Z-TiO 2 and A-TiO 2, respectively. Under the neutral medium, almost the same degradation efficiency of about 63% was recorded for all the catalysts.
In the basic or alkaline medium (pH 8-9), the degradation efficiency of 66%, 64%, and 63% was, respectively recorded for the TiO 2 , Z-TiO 2, and A-TiO 2 . The results confirm the occurrence of photocatalysis on the surface of NPs are mostly influenced by their charged ions and the pH of the solutions [18,24]. Thus, the charged ions and change in pH promoted the pollutant's stability, neutralization, agglomeration, and adsorption onto the photocatalyst surface [24]. The high acidic medium-efficiency can be attributed to the presence of negatively charged contaminants in the effluent, which were attracted and adsorbed onto the surface of the positively charged photocatalysts (TiO 2 , Z-TiO 2 and A-TiO 2 ) [21,24]. In addition, the low-efficiency exhibited within the basic medium (pH 8-9) may be due to the electrostatic repulsion between the positively charged NPs and the cationic charges in the solution.

Effect of pH on Photocatalytic Degradation
The pH of an effluent strongly plays an important role in photocatalytic degradation of organic compounds, such as dyes in an aqueous suspension with TiO2 NPs. First and foremost, the pH of the BPB synthesized effluent was investigated to ascertain the TiO2, Z-TiO2, and A-TiO2 photocatalytic activities on COD removal. This was carried out by varying the pH of the effluent from 5-9 while keeping the catalyst load and reaction time constant at 2 g and 30 min, respectively. Figure 5 shows the degradation efficiency obtained by the TiO2, Z-TiO2 and A-TiO2 on the COD removal. Evidently, the pH affected the adsorption shift of the organic substrates onto the surface of the photocatalyst as well as their photodegradation efficiency from the neutral to the acidic and basic mediums. The degradation efficiencies recorded for the acidic medium (pH 5-6) were 75-79%, 75-77%, and 77-80% for TiO2, Z-TiO2 and A-TiO2, respectively. Under the neutral medium, almost the same degradation efficiency of about 63% was recorded for all the catalysts. In the basic or alkaline medium (pH 8-9), the degradation efficiency of 66%, 64%, and 63% was, respectively recorded for the TiO2, Z-TiO2, and A-TiO2. The results confirm the occurrence of photocatalysis on the surface of NPs are mostly influenced by their charged ions and the pH of the solutions [18,24]. Thus, the charged ions and change in pH promoted the pollutant's stability, neutralization, agglomeration, and adsorption onto the photocatalyst surface [24]. The high acidic medium-efficiency can be attributed to the presence of negatively charged contaminants in the effluent, which were attracted and adsorbed onto the surface of the positively charged photocatalysts (TiO2, Z-TiO2 and A-TiO2) [21,24]. In addition, the low-efficiency exhibited within the basic medium (pH 8-9) may be due to the electrostatic repulsion between the positively charged NPs and the cationic charges in the solution.

Effects of Photocatalyst Dosage
The photocatalytic performance of single (A, Z, TiO2) and binary nanocomposites (A-TiO2 and Z-TiO2) were investigated by measuring catalyst-load-dependent degradation of the COD (367.7 mg/L), and color (458 Pt. Co) initial concentrations from the BPD simulated effluent under a UV-light. The effluent pH was kept constant at an acidic medium of 6.5 to enhance the protonation of the active sites of the catalyst. Subsequently, to ascertain the optimal dosage (2-4 g) of the nanocomposites (A-TiO2 and Z-TiO2), first, the load (1-6 g) of a single catalyst (A, Z, and TiO2) was investigated at the reaction time of 30 min.

Effects of Photocatalyst Dosage
The photocatalytic performance of single (A, Z, TiO 2 ) and binary nanocomposites (A-TiO 2 and Z-TiO 2 ) were investigated by measuring catalyst-load-dependent degradation of the COD (367.7 mg/L), and color (458 Pt. Co) initial concentrations from the BPD simulated effluent under a UV-light. The effluent pH was kept constant at an acidic medium of 6.5 to enhance the protonation of the active sites of the catalyst. Subsequently, to ascertain the optimal dosage (2-4 g) of the nanocomposites (A-TiO 2 and Z-TiO 2 ), first, the load (1-6 g) of a single catalyst (A, Z, and TiO 2 ) was investigated at the reaction time of 30 min. Figure 6 shows that photocatalytic degradation efficiency increased with an increase in the catalyst load until an optimal dosage is attained. As seen (Figure 6a,b), an increase in the catalyst load (1-5 g) promoted the photodegradation rate with more active sites, whereby after the optimal load (5 g), there was a degeneration as compared to the A and Z loads at 6 g. Thus, an increase in the active sites of the catalyst surfaces was too dense to interact with the contaminant, thereby destabilizing and deactivating them as they agglomerate [20,30]. At a single catalyst load of 5 g, about 60-80% and 65-90% of the initial concentration of color ( Figure 6a) and COD (Figure 6b) was respectively removed. In addition, the increasing order of the single catalyst loads was observed for the color (Z > TiO 2 > A) and COD removal (TiO 2 > Z > A).
Evidently, the single catalyst load results favored the TiO 2 due to its high active sites and hydroxyl radicals, which entrapped the contaminants [31,32]. This affirms that the photocatalytic nature of TiO 2 , once activated by UV radiations, produces a very reactive species, which led to the degradation of the organic substance in the solutions [29,36]. However, in the subsequent investigation of the nanocomposites in 1:1 ratio (A-TiO 2 and Z-TiO 2 ) loads (2-4 g), there is a prompt increase in the contaminant removal efficiency (Figure 6c,d) as compared to the individual components (Figure 6a,b). Thus, the TiO 2 in the nanocomposite was photo-activated by producing a high amount of the hydroxyl radicals [30,36]. Thus, an increase in the nanocomposite load to a high level (4 g) produced more of the photogenerated electron holes [37]. This increased the removal efficiency of the A-TiO 2 by 20% as compared to the TiO 2 , for the removal of color and COD, as shown in Figure 6c,d, respectively. In addition, the increasing order of color (( Figure 6c) and COD (Figure 6d) removal by the catalyst was found as A-TiO 2 > Z-TiO 2 > TiO 2 . However, among the nanocomposites, activated carbon (A) is recounted as a good electron acceptor, whereas TiO 2 is also known to be an effective electron donor [33,34]. Consequently, the A-TiO 2 was found to be very effective in suppressing the photogenerated species from destabilization during overload as compared to that of the TiO 2 [35,37]. Therefore, the synergistic effect of hybridizing A-TiO 2 ( Figure 6) eventually reduced the destabilization of the hole-pair electrons at the high dosage which increased its efficiency [36]. Figure 7 presents the adsorption and photocatalytic degradation performance of the nanocomposites (A-TiO 2 and Z-TiO 2 ) and TiO 2 , which were simultaneously achieved with a catalyst load of 4 g by monitoring the removal of COD. Firstly, the dark adsorption (no UV-light) was carried out within a stipulated time interval of 60 minutes, after which the solution was illuminated for another 60 minutes to enhance the photodegradation. However, the adsorption and photocatalytic activities of the supported TiO 2 (A-TiO 2 and Z-TiO 2 ) under the irradiation time of 60 min led to higher degradation (>85% COD) as compared to the unsupported TiO 2 (80% COD). In the dark adsorption experiment, TiO 2 was seen to be less efficient because of its deactivated reactive sites, which makes it slow to react with other complex compounds. Even though the increasing order (Z-TiO 2 > A-TiO 2 > TiO 2 ) of the photocatalytic degradation efficiency favored Z-TiO 2 (Figure 7b), the hybridized A-TiO 2 also had effective adsorptive (Figure 7a) capacity owing to its low density and surface area [33,35]. Thus, the support on TiO 2 with A and Z increased the irradiated surface area and adsorption capacity of the TiO 2 . According to Zhong et al. [33], the high specific surface area of the nanocomposite creates an interface, which tolerates immobilization and destabilization of the photocatalyst to trap the organic pollutants. Results obtained have also confirmed the three steps of the decomposition mechanism of TiO 2, such as (i) movement of the excited radicals, (ii) adsorption and photodegradation of the organic pollutants from the bulk solution onto the photocatalyst surface [34]. among the nanocomposites, activated carbon (A) is recounted as a good electron acceptor, whereas TiO2 is also known to be an effective electron donor [33,34]. Consequently, the A-TiO2 was found to be very effective in suppressing the photogenerated species from destabilization during overload as compared to that of the TiO2 [35,37]. Therefore, the synergistic effect of hybridizing A-TiO2 ( Figure 6) eventually reduced the destabilization of the hole-pair electrons at the high dosage which increased its efficiency [36].  Figure 7 presents the adsorption and photocatalytic degradation performance of the nanocomposites (A-TiO2 and Z-TiO2) and TiO2, which were simultaneously achieved with a catalyst load of 4 g by monitoring the removal of COD. Firstly, the dark adsorption (no UV-light) was carried out within a stipulated time interval of 60 minutes, after which the solution was illuminated for another 60 minutes to enhance the photodegradation. However, the adsorption and photocatalytic activities of the supported TiO2 (A-TiO2 and Z-TiO2) under the irradiation time of 60 min led to higher degradation (>85% COD) as compared to the unsupported TiO2 (80% COD). In the dark adsorption experiment, TiO2 was seen to be less efficient because of its deactivated reactive sites, which makes it slow to react with other complex compounds. Even though the increasing order (Z-TiO2 > A-TiO2 > TiO2) of the photocatalytic degradation efficiency favored Z-TiO2 (Figure 7b), the hybridized A-TiO2 also had effective adsorptive (Figure 7a) capacity owing to its low density and surface area [33,35]. Thus, the support on TiO2 with A and Z increased the irradiated surface area and adsorption capacity of the TiO2. According to Zhong et al. [33], the high specific surface area of the nanocomposite creates an interface, which tolerates immobilization and destabilization of the photocatalyst to trap the organic pollutants. Results obtained have also confirmed the three steps of the decomposition mechanism of

Kinetic Models
The kinetics of heterogeneous photocatalyst reactions in the slurry is usually described by the Langmuir-Hinshelwood model expressed in (2). However, the degradation and adsorption processes can be monitored either by a first-order (3) or the second-order kinetic models (5) [38,39].
where , and represents the reaction rate, the rate constant (mg/min) and ad-

Kinetic Models
The kinetics of heterogeneous photocatalyst reactions in the slurry is usually described by the Langmuir-Hinshelwood model expressed in (2). However, the degradation and adsorption processes can be monitored either by a first-order (3) or the second-order kinetic models (5) [38,39].
where r, K r and K a C e represents the reaction rate, the rate constant (mg/min) and adsorption constant (mg −1 ), respectively. Assuming K a C e is negligible at a low concentration, the rate of degradation in (2) can then be expressed as a first-order model (3).
To estimate the rate of reaction constants for the first and second-order kinetic models, ln C o C vs. t (Figure 8a) and 1 C − 1 C o vs. t (Figure 8b), respectively, was plotted. Where, k 2 is the second-order rate constant (min −1 ), C 0 is the initial COD concentration, t is time, and C is the concentration at the specified time.  Table 2 presents the kinetics data obtained, indicating that the reactions do not fit well on the first-order kinetic model. However, all the reactions fitted well on the secondorder kinetic model as observed based on their rate constants (k) and coefficient of determination (R 2 ) values. A 95% confidence level with a difference of less than 0.2 between the and Adjusted , also suggests the significance and accuracy of the model predictability. Additionally, the probability functions (p < 0.05) were significantly different from the zero levels. The increasing order of the and values were seen as Z-TiO2 > A-TiO2 > TiO2, whereby Z-TiO2 had the highest and values of 5.63 × 10 −4 min −1 and 0.989, respectively. This validates the trend observed for the highest removal of the COD by photocatalytic degradation (Z-TiO2 > A-TiO2 > TiO2), as shown in Figure 7, the data obtained favored the second-order kinetic model. In addition, the adsorbates (A) presence in the nanocomposites (A-TiO2) suppressed the TiO2, which made it more favorable for adsorption [40,41].   Table 2 presents the kinetics data obtained, indicating that the reactions do not fit well on the first-order kinetic model. However, all the reactions fitted well on the second-order kinetic model as observed based on their rate constants (k) and coefficient of determination (R 2 ) values. A 95% confidence level with a difference of less than 0.2 between the R 2 and Adjusted R 2 , also suggests the significance and accuracy of the model predictability. Additionally, the probability functions (p < 0.05) were significantly different from the zero levels. The increasing order of the k and R 2 values were seen as Z-TiO 2 > A-TiO 2 > TiO 2 , whereby Z-TiO 2 had the highest k and R 2 values of 5.63 × 10 −4 min −1 and 0.989, respectively. This validates the trend observed for the highest removal of the COD by photocatalytic degradation (Z-TiO 2 > A-TiO 2 > TiO 2 ), as shown in Figure 7, the data obtained favored the second-order kinetic model. In addition, the adsorbates (A) presence in the nanocomposites (A-TiO 2 ) suppressed the TiO 2 , which made it more favorable for adsorption [40,41].  Figure 9 presents Freundlich and Langmuir isotherms best fits of the data obtained from the adsorption of COD. The adsorption was done in the dark until equilibrium was achieved, whereas the photocatalytic degradation was carried out under UV-irradiation for 60 min. The Langmuir isotherm defined by (6) is expressed in a linearized form as (7).
where, q e is the adsorbate at equilibrium (mg/g), q m is the theoretical maximum adsorption capacity (mg/g), K L is the Langmuir adsorption constant (L/mg) and C e is the supernatant concentration at equilibrium (mg/L). From (7), a plot of C e q e vs. C e gives a straight line (Figure 9b) with slope 1 q m and intercept 1 K L q m . The Freundlich isotherm described in (8) is expressed in a linear form as (9), where K f and n are the Freundlich isotherm constants representing the adsorbate capacity (mg/L) and intensity, respectively.
Catalysts 2021, 11, x FOR PEER REVIEW 11 of 15 where, is the adsorbate at equilibrium (mg/g), is the theoretical maximum adsorption capacity (mg/g), is the Langmuir adsorption constant (L/mg) and is the supernatant concentration at equilibrium (mg/L). From (7), a plot of .
gives a straight line (Figure 9b) with slope and intercept . The Freundlich isotherm described in (8) is expressed in a linear form as (9), where and are the Freundlich isotherm constants representing the adsorbate capacity (mg/L) and intensity, respectively.
gives a straight line (Figure 9a) with as the slope and as the intercept. The Freundlich isotherm (Figure 9a), which is used for non-ideal adsorption containing heterogeneous surfaces with different affinity, was preferred for the TiO2 adsorption. The order of increasing the deactivated sites towards adsorption in the Freundlich isotherm (Figure 9a) was TiO2 > Z-TiO2 > A-TiO2, whereby TiO2 fits well with the R 2 value of 0.984. This confirms that adsorption occurred at reactive sites with different strengths of affinity, whereby the adsorption efficiency reduced (Figure 7) as the reactive sites being occupied increased [38,39]. Conversely, the nanocomposites (Z-TiO2 > A-TiO2) were well fitted with the Langmuir isotherms (Figure 9b), which assumes unchanging monolayer adsorption with no interaction between the adsorbates as compared to TiO2. However, Z-TiO2 was found to fit on both Freundlich (R 2 = 0.929) and Langmuir (R 2 = 0.988) isotherms. This can be attributed to the heterogeneous surface of Z-TiO2 which, with the presence of the TiO2, makes it possible to exhibit different energy or affinity levels [36][37][38]. Wolf [39], therefore itemized that (i) monolayer adsorption occurs on the adsorbent surface, which has a finite number of similar active sites, (ii) the interaction does not take place between adjacent adsorbed molecules, (iii) the active sites have equal affinities towards the adsorbate molecules and finally (iv) the adsorption process is reversible.

Chemicals and Synthetic Effluent
A locally manufactured TiO2 used in this study was supplied by Huntsman Tioxide, South Africa (Pty). Its crystal structure composes of anatase (75%) and rutile (25%) with a mean particle size of 25 nm, specific surface area and density of 48 m 2 /g and 4.26 g/cm 3 , respectively. AC with a specific surface area, pore-volume and ash content of 1576 m 2 /g, 0.967, and 3%, respectively, was supplied by a local South Africa industry (RTB Chemicals). Natural zeolite powder (95-98% of clinoptilolite balanced with silica, 125-250) was A plot of lnq e vs. lnC e gives a straight line (Figure 9a) with 1 n as the slope and lnK f as the intercept. The Freundlich isotherm (Figure 9a), which is used for non-ideal adsorption containing heterogeneous surfaces with different affinity, was preferred for the TiO 2 adsorption. The order of increasing the deactivated sites towards adsorption in the Freundlich isotherm (Figure 9a) was TiO 2 > Z-TiO 2 > A-TiO 2 , whereby TiO 2 fits well with the R 2 value of 0.984. This confirms that adsorption occurred at reactive sites with different strengths of affinity, whereby the adsorption efficiency reduced (Figure 7) as the reactive sites being occupied increased [38,39]. Conversely, the nanocomposites (Z-TiO 2 > A-TiO 2 ) were well fitted with the Langmuir isotherms (Figure 9b), which assumes unchanging monolayer adsorption with no interaction between the adsorbates as compared to TiO 2 . However, Z-TiO 2 was found to fit on both Freundlich (R 2 = 0.929) and Langmuir (R 2 = 0.988) isotherms. This can be attributed to the heterogeneous surface of Z-TiO 2 which, with the presence of the TiO 2 , makes it possible to exhibit different energy or affinity levels [36][37][38]. Wolf [39], therefore itemized that (i) monolayer adsorption occurs on the adsorbent surface, which has a finite number of similar active sites, (ii) the interaction does not take place between adjacent adsorbed molecules, (iii) the active sites have equal affinities towards the adsorbate molecules and finally (iv) the adsorption process is reversible.

Chemicals and Synthetic Effluent
A locally manufactured TiO 2 used in this study was supplied by Huntsman Tioxide, South Africa (Pty). Its crystal structure composes of anatase (75%) and rutile (25%) with a mean particle size of 25 nm, specific surface area and density of 48 m 2 /g and 4.26 g/cm 3 , respectively. AC with a specific surface area, pore-volume and ash content of 1576 m 2 /g, 0.967, and 3%, respectively, was supplied by a local South Africa industry (RTB Chemicals). Natural zeolite powder (95-98% of clinoptilolite balanced with silica, 125-250) was supplied by Pratley Minerals, South Africa.

Nanocomposite Synthesis (A-TiO 2 and Z-TiO 2 )
The composites (A-TiO 2 , Z-TiO 2 ) were synthesized based on a modified low-temperature co-precipitation method [31,35]. First, 30 g of AC and 10 g of TiO 2 was added to 200 mL of deionized water and sonicated (45 kHz at 80 • C) for 1 h. After the sonication, the samples were oven-dried at 80 • C for 24 h, which resulted in A-TiO 2 . This was then washed with deionized water, filtered, and dried for 1 hour at 110 • C. Prior to usage, the A-TiO 2 was calcined at 600 • C for 1 h at a ramp speed of 15 s. The above procedure was also repeated for the Z-TiO 2 .

Characterization of Composite (A-TiO 2 and Z-TiO 2 )
3.2.1. X-Ray Diffraction (XRD) Analysis X-ray diffractometer equipment (Bruker AXS, D8 Advance) coupled with PANalytical software (version 6.01) (Empyrean, PRO MPD, Almelo, The Netherlands) operated at 40 kV with a target current of 40 mA was used for the analysis. The samples were closely packed in a rectangular glass cell and scanned with a J-J scan coupled with the copper anode (Cu-Kα radiation: λ = 1.5406 Å) and Bragg-Brentano configuration. This was carried out at a room temperature with a speed of 0.033 • min −1 over a region of 5 to 70 (2θ) [43].

Fourier Transform Infrared Spectroscopy (FTIR) Analysis
The FTIR spectra of the samples were obtained by using an FTIR-spectrometer (Shimadzu FTIR 8400) in the wavenumber range of 400-4000 cm −1 with a resolution of 4 cm −1 [43]. This method provided information about the molecular structure and functional groups of the composite.

Scanning Electronic Microscope (SEM) Analysis
A Scanning electronic microscope at the University of Cape Town, South Africa (Nova NanoSEM coupled with EDT and TLD detector) was used to analyze the morphological structure of the samples. This was operated at an acceleration voltage of 5 kV with a magnification in the range of 10-50 kx [43].

Experimental Procedure
The adsorption and photocatalytic experiments were carried out with a modified jar tester reactor, which consisted of six 1 L beakers, stirred at a speed of 150 rpm. At a distance less than 2 cm, the setup was exposed to a UV light of wavelength 400 nm and power rating of 18 W (T8 blacklight-blue bulb 400 nm, 18 W, Philips, and Netherland). Three sets of experiments, as presented in Table 3, were used to investigate the effects of single catalyst load (1-6 g), binary catalyst load (2-4 g), and reaction time (10-60 min). Furthermore, the adsorption capacity of the binary composites was investigated without the UV radiation for the 60 min reaction time, whereas samples were collected in 10 min intervals. The supernatant samples collected with a syringe were filtered through a Whatman filter (Grade 5; 2.5 µm). The effluent was initially characterized for COD (367.7 mg/L) and color (458 Pt. Co) using HACH DR 3900 within the wavelength of 455-635 nm (Hach Company, Loveland, CO, USA) as well as turbidity (14.48 NTU) using HI 98,703 portable turbidimeter (Hanna Instruments, Veneto, Italy). Likewise, the Hannah pH-meter (HI98130, Hanna Instruments, Woonsocket, RI, USA) was used to measure the pH (7.5) of the effluent. Based on primary studies, the pH (6.5) was kept constant by adjusting it with 1 M HCl and NaOH. Upon analyzing the treated samples, the contaminants removal efficiency was calculated using the following Equation (10).
Removal efficiency (%) where C 0 and C f are initial and final concentrations of contaminants before and after treatment. The removal rate of COD obtained by the Z-TiO 2 and A-TiO 2 composites contributed to the adsorption and photocatalysis process. All the experiments were carried out in triplicates, and the average values obtained were reported.

Conclusions
In this study, the treatability of bromophenol blue (BPB) dye in aqueous solution by adsorption and photocatalytic degradation with and without UV light of wavelength 400 nm and a power rating of 18 W, was investigated. This was carried out under different catalyst loads (1-6 g) and reaction times (10-60 min) for the removal of COD, color, and turbidity. Furthermore, nanocomposites (A-TiO 2 and Z-TiO 2 ) were synthesized, where their physicochemical analyses (SEM, FTIR, and XRD) demonstrated large pore size (20.8 µm) and crystalline size (22-34 nm) with high surface area and chemical stability improvement as compared to the uncoated TiO 2 . At the optimal single catalyst load of 5 g, above 70%