Effluents resulting from the frequent use of industrial azo dyes in textile operations have posed great toxicological impacts on man and the environment. The limitations of conventional treatment infrastructure necessitate the use of rapid Fenton-mediated catalytic systematic process to tackle the attendant treatment limitations. The study applied in situ Fenton-mediation process with constructed low power UV-LED reactor for rapid catalytic treatment of dye-laden effluent using enhanced acid and alkali TiO2-nanoparticles (Nps) (1–5%, i.e. 1–5 M) at definite experimental conditions, respectively. A comprehensive instrumental study was done to access the morphological, functional and elemental constituents of these nanocatalysts. The performance of the respective catalyst was evaluated using methylene blue (MB) dye at definite experimental conditions of pH, dosage, concentration and irradiation time. The results revealed a mesoporous structural nanocatalyst with increasing surface area after enhanced modification. The optimal experimental conditions of pH and concentration were recorded as 5 and 10 mg/L, respectively; while the most efficient nanocatalyst was 3 wt% alkali-modified TiO2 (3% Ak-TiO2) having a degradation efficiency of 89.15% at 90 min of irradiation using 50 mg dosage in contrast to higher irradiation time and catalyst dosage for other catalysts.

  • Development of rapid photocatalytic treatment process with built low-powered UV-LED reactor.

  • Use of unique Fenton reagents and in situ mediation for effective dye degradation.

  • Unique morphological enhancement of nanocatalyst for more effective degradation.

The evolution of textile industries and the increasing demands on the use of synthetic dyes corresponds to the generation of a large volume of textile effluent consisting of 10–30% of loosed dyes (Sarmah & Kumar 2011; Al-Rubaie & Mhessn 2012). Among the industrial dyes commonly used are the azo dyes (N = N). This category of dye takes a significant use of 70% among other dyes commercially (Oyetade et al. 2022). The industrial attention is based on their vast use in dyeing, printing, pigmentation and extensively in medicine and food colorants (Ameen et al. 2011; Kumar & Pandey 2018). Although they are less toxic and chemically stable in raw form, the indiscriminate discharge of their corresponding effluents is deleterious to human health and the environment at large (Kaur & Singhal 2014; Abayomi et al. 2020; Deriase et al. 2021; Oyetade et al. 2023). Treatment technologies categorized as physical, biological and chemical treatments have been applied for the remediation of recalcitrant dye pollutants (Xu et al. 2010; Kumar & Pandey 2018). However, the recalcitrant nature and chemo-transformative behavior of the dye molecules in textile wastewater remain challenging (Oyetade et al. 2022). In recent times, studies have focused on the use of photocatalysis as an efficient treatment technology to completely mineralize the selected recalcitrant dye pollutants (Xu et al. 2010; Zhu et al. 2010). Generally, the photocatalytic approach is a photon-induced process occurring which takes place at the surface of photon excited nanocatalysts, such as TiO2, ZnO h-BN, GO, SnO2 ZrO2, ZnO, CdS, Fe2O3, and so on (Hou et al. 2018). The mechanism involves the capturing of photon energy from light by the photocatalyst, excitation of the electrons from the valence band (VB) to the conduction band (CB) (electron–hole pair), formation of oxidizing and reducing sites, generation of radical species (•OH, and •OOH) and radical attack which mineralizes the dye molecules (Lopes Colpani et al. 2019). The radical attack leads to the degradation of dye molecules into CO2, H2O and other harmless substances (Sarmah & Kumar 2011; Hossain et al. 2020). Among the wide spectrum of photocatalysts semiconductors used, the selection of titanium dioxide (TiO2) is based on its cost-effectiveness, nontoxicity, biocompatibility, stability, possibility of recovery and reuse, corrosion resistance, flexibility in synthesis and surface modification with good adsorption–desorption rate (Haider et al. 2019). Furthermore, the semiconductor has high surface activity and excellent dye mineralization potential based on its unique oxidizing capacity (Jangid et al. 2021). This semiconductor generally exists as anatase, rutile and brookite, although other forms of TiO2 are hollandite, columbite and ramsdellite which are generally unstable (Haider et al. 2019). Among the existing forms, rutile and anatase have the most appreciable use with known synthetic pathways such as the sol–gel method, hydrothermal, microemulsion and micelles, pulse laser deposition, and so on (Mezni et al. 2017). The vast applicability of rutile and anatase is based on its high oxidative power, photochemical stability, excellent potential energy of photogenerated electrons and appreciable photocatalytic activity (Macwan et al. 2011; Xia et al. 2014). However, the challenges of agglomeration of the nanoparticles (Nps), low reaction kinetics, insensitivity to visible light, frequent electron-hole recombination and low sorption properties resulted in the need for simultaneous enhancement of the performance via modification of the surface morphology and application of in situ Fenton-mediation for the process (Hou et al. 2018; Qutub et al. 2022; Oyetade et al. 2022; Zhang et al. 2021a, 2021b). The incorporation of Fenton-mediation in photocatalysis is based on its ability to generate or increase the availability of more radicals at the catalyst surface via the combination reaction of Fe2+ and H2O2 under acidic conditions. This action incredibly enhances capturing of photons from UV light and solar light via its association with a modified photocatalyst and the generation of more HO• to initiate an effective dye-radical attack (Li et al. 2020; Zhang et al. 2021a, 2021b). Hence, this study comparatively investigates the performance of in situ mediated photocatalytic degradation of methylene blue (MB) dye with acid- and alkali-enhanced TiO2-Nps and statistical optimization of the most efficient catalyst for industrial scale-up.

Chemicals, instruments and glassware

High purity grade (99.5%) titanium dioxide (P25) powder consisting of mixed anatase and rutile phase was purchased at LOBA Chemicals in India. Other reagents such as HCl (37% v/v), H2SO4 (97% v/v), anhydrous FeCl3, H2O2 (30% V/V), NaOH and methylene blue dye MB (C16H18ClN3S) purchased from the same location were analytical grade. Furthermore, de-ionized water was used throughout the experiment, while other glassware and equipment such as a centrifuge machine, analytical weighing balance and oven were procured at the Nelson Mandela African Institution of Science and Technology (NM-AIST), Tanzania. A low power rating (18 W) UV-LED reactor was constructed at Arusha Technical College (ATC) in Tanzania with a power rating of 18 W.

Preparation of enhanced TiO2-Nps

Acid enhancement of the nanocatalyst (TiO2) was carried out according to the modified method reported by Park & Shin (2014) and Hou et al. (2018). This involved weighing 1.5 g of TiO2 into 100 mL of 0,1,2, 3, 4 and 5 mol dm−3 concentrations of H2SO4 (AC-TiO2), with stirring for 3 h. The solution was then centrifuged at 3,000 rpm, filtered and washed with ultrapure water three times before drying at 105 °C in the oven for 2 h. The resulting material was calcinated at 450 °C for 2 h to eliminate water trappings in the catalyst matrix and pulverized to obtain powdered material. Each sample was labeled appropriately 0% (untreated TiO2-Nps) and then 1–5% (i.e. 1–5 M), respectively. The procedure was repeated using NaOH to obtain alkali-modified TiO2-Nps (AK-TiO2).

Nanocatalyst characterization

The morphological properties of prepared samples were analyzed using a Zeiss Ultra Plus 55 Field Emission Scanning Electron Microscope (FE-SEM) at 2.0 KV acceleration potential with energy-dispersive X-ray spectroscopy (EDX). N2 adsorption–desorption isotherms of the samples were achieved through the aid of a micromeritics TriStar II 3020 device run at a relative pressure (P/Po) ranging from 0.01 to 1.0 with the samples being pre-degassed at a temperature of 120 °C for 16 h in a vacuum before the analysis was performed. A Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) model was adopted to estimate the samples' surface area and pore size distribution (PSD), respectively. X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The Fourier-transform infrared spectra (FT-IR) of the samples were recorded on a Vertex 70 spectrometer in a range from 4,000 to 400 cm−1. The initial and final concentrations of the treated effluent were quantified as a function of the absorbance using UV-Vis diffuse reflectance spectroscopy (UV-vis DRS; Shimadzu 2600, Kyoto, Japan).

Photocatalytic performance evaluation

Performance evaluation of the photodegradation of the dye molecules (MB) in aqueous solution via the simultaneous use of catalyst with in situ Fenton-mediation was studied under constructed cost-effective low-powered UV-LED reactor. Aqueous suspension of MB effluent (100 mL, 10 mg/L) was placed in a 250 mL beaker and a measured amount of the TiO2-Nps was added. Before irradiation, the suspensions were magnetically stirred in the dark for approximately 60 min. Constant stirring of the suspensions was maintained before and during the irradiation of UV light (Shahabuddin et al. 2018). A unique Fenton reagent involving the anhydrous FeCl3 and H2O2 was prepared via the modified method of Liu et al. (2019) and added in situ to the process before irradiation. At a definite time interval, 5 mL of the aliquot was taken and centrifuged at 3,000 rpm. The supernatant was quantified using the UV-Vis at a predetermined λmax of 666 for the MB. Equation (1) was used to determine the degradation efficiency. The same process was carried out for acid (AC-TiO2) and alkali-modified (AK-TiO2) nanocatalysts at 50, 70, 90 and 110 mg dosages, respectively. The procedure was repeated at the pH range of 3–11, a concentration of 10–40 and an irradiation time of 15–90 min at 50 mg/100 mL of the aqueous solution
formula
(1)
where Ct is the concentration at time t, C0 initial dye concentration

Morphological characterization of nanocatalysts

The FE-SEM image in Figure 1(a) and 1(b) describes the morphological arrangement of the untreated P25 TiO2–Nps catalyst, Figure 1(c) and 1(d) depicts the 3% acid and alkali-enhanced catalyst while Figure 1(g)–1(j) shows a 5% enhanced catalyst at different magnifications. From the imaging, a highly stacked and clustered morphological arrangement was observed for untreated TiO2-NPs in Figure 1(a) and 1(b) and for all the acid-enhanced catalysts. The clustered morphology from the imaging was equally reported by Kumar & Pandey (2018), which is connected with lower binding sites available and undesirable agglomeration. From Figure 1, the structural expression of the catalyst is a spherical cylindrical nanocatalyst with an average diameter of 2.8 mm. These structural features agree with the FE-SEM studied by Sarmah & Kumar (2011) and Zhou et al. (2022) on TiO2-Nps; however, Hou et al. (2018) added that the clustered morphological characteristics of the TiO2-Nps and the acid-enhanced catalyst are often responsible for the agglomeration phenomenon which lowers the photocatalytic performance. Also, the cylindrical morphological shape suggests its unique advantage and use as support in the formation of composite via in situ polymerization with macromolecules (Zhu et al. 2010; Wahyuni et al. 2018; Jangid et al. 2021). The modification of the surface morphology of synthesized catalysts may facilitate more interparticle spacing and surface growth, thus increasing the availability of active sites for catalyst-adsorbent binding (Zare et al. 2018; Lee & Patel 2022; Oyetade et al. 2022). The impacts of the treatment, respectively are revealed in terms of the displayed surface growth topology TiO2-Nps in Figure 2. It is necessary to add that the rutile and the anatase components of TiO2-Nps are structurally cylindrical as equally exhibited from the imaging in Figure 3 (Hou et al. 2018; Haider et al. 2019).
Figure 1

FE-SEM of (a,b) TiO2–Nps with 3% AC-TiO2 (c,d); 3% AK-TiO2 (e,f); 5% AC-TiO2 (g,h); 5% AK-TiO2 (i,j) with their structure.

Figure 1

FE-SEM of (a,b) TiO2–Nps with 3% AC-TiO2 (c,d); 3% AK-TiO2 (e,f); 5% AC-TiO2 (g,h); 5% AK-TiO2 (i,j) with their structure.

Close modal
Figure 2

3D Surface imaging of TiO2 (a); (b,c) 3% and (c–e) 5% AC-TiO2 and AK-TiO2.

Figure 2

3D Surface imaging of TiO2 (a); (b,c) 3% and (c–e) 5% AC-TiO2 and AK-TiO2.

Close modal
Figure 3

BET adsorption–desorption isotherm of 3% (a) and 5% (b) and pore size distribution of TiO2-Nps with enhanced nanocatalyst of 3% (e) and 5% (e) of TiO2-Nps with enhanced nanocatalyst.

Figure 3

BET adsorption–desorption isotherm of 3% (a) and 5% (b) and pore size distribution of TiO2-Nps with enhanced nanocatalyst of 3% (e) and 5% (e) of TiO2-Nps with enhanced nanocatalyst.

Close modal

BET analysis

The pattern of the BET adsorption–desorption isotherm in Figure 3(a) and 3(b) structurally describes the features of the pure and enhanced catalyst. The results showcase a typical type IV hysteresis loop of N2 adsorption–desorption isotherm for all samples, indicating well-developed mesopores (Carja et al. 2001; Xiang et al. 2010). This report agrees with a similar observation on the mesoporous structure of TiO2-Nps made by Kumar & Pandey (2018), Liu et al. (2019), and Wahyuni et al. (2018). From the results, the significant increase in the quantity adsorbed set in at P/P0 region > 0.8. At this point and beyond, more opening of the hysteresis loop was observed which accounts for stronger multilayer adsorption (Nie et al. 2013). Furthermore, the vital features of a specific area, pore diameter and pore volume are described in Table 1, while Figure 3(c) and 3(d) comparatively shows the pore size of 3 and 5% enhanced TiO2-Nps with its acid and alkali, respectively. From Table 1, the lowest pore size observed was 14.33 nm similar to the 14.73 nm size reported by Hou et al. (2018). However, the value increases significantly after respective acid and alkali treatment. This action was similar to the BET surface areas of 47.97 for untreated, while the highest was 91.82 m2g−1 for 3% acid modification followed by 64.74 m2g−1 for 3% alkali modification, respectively. The tunability of the morphological properties increased the surface area of the catalyst, facilitating the prompt electron transport dynamics and high ion diffusion, allowing improved photochemical performance (Carja et al. 2001; Mezni et al. 2017; Kumar & Pandey 2018).

Table 1

Physicochemical properties of the samples

S/NSamplesBET surface area (m2 g−1)Pore volume (cm3/g)Pore size (nm)
 47.9654 0.17 14.22 
3% AC-T 91.8267 0.42 18.33 
5% AC-T 60.5754 0.32 21.28 
3% AK-T 64.7434 0.30 18.37 
5% AK-T 50.3858 0.21 16.64 
S/NSamplesBET surface area (m2 g−1)Pore volume (cm3/g)Pore size (nm)
 47.9654 0.17 14.22 
3% AC-T 91.8267 0.42 18.33 
5% AC-T 60.5754 0.32 21.28 
3% AK-T 64.7434 0.30 18.37 
5% AK-T 50.3858 0.21 16.64 

XRD analysis

XRD patterns of pure TiO2-Nps, 3% (Figure 4(a)) and 5% (Figure 4(b)) acid and alkali-enhanced catalysts were, respectively, reported. Figure 4 shows the patterns for pure anatase TiO2-Nps, at 25°, 38°, 48°, 55.1° and 63.2° which correspond to (101), (103), (200), (211) and (204) planes of TiO2. However, the structural patterns at the plane (110) correspond to the rutile structure (JCPDS No. 21–1272) (Ananpattarachai et al. 2009; Moon et al. 2011). The observed features are in agreement with the high purity grade P25 TiO2 used by Plermjai et al. (2019); however, lower than those reported of TiO2 synthesized via the sol–gel method. From Figure 4(a) and 4(b), it was observed that the intensity of the peaks decreases significantly after the acid and alkali modification process. Furthermore, the higher the intensity of the peaks the more crystalline the arrangement of the Nps, while the decrease in peak intensity suggests the transformation of the surface morphology of the Nps to an amorphous state (Xia et al. 2014; Hou et al. 2018; López-López et al. 2022). This morphological transformation justifies the observed increasing BET surface area and pore volume in Table 1.
Figure 4

XRD pattern of TiO2-Nps with enhanced nanocatalyst of 3% (a) and 5% (b).

Figure 4

XRD pattern of TiO2-Nps with enhanced nanocatalyst of 3% (a) and 5% (b).

Close modal

FT-IR analysis

Figure 5 shows the FT-IR spectra of the nanocatalyst with 3% (Figure 8(a)) and 5% (Figure 8(b)) acid- and alkali-enhanced nanocatalyst, respectively. From the spectra, the peak intensity around 400–800 cm−1 reveals the functional characteristic of the metal oxide bond (Ti–O bond), while the peak at 1,400 cm−1 shows stretching vibrations of Ti–O–Ti (Wang et al. 2015; Hou et al. 2017). The functional features at this region may facilitate the dye-adsorbent coordination in adsorption which enhances the improved photocatalytic performance of the nanomaterial (Abebe et al. 2020). Also in Figure 5, the bend of O–H observed at 3,454.51 cm−1 is often assigned to the OH free radical associated with hydrogen bond (Ahmadizadegan 2017; Martakov et al. 2018). Additionally, an observed band shift in Figure 5(a) and 5(b) could significantly contribute to the dye coordination and radical formation at the dye-catalyst interphase which improves the catalyst efficiency (Lu & Astruc 2020). The peak at 1,417 cm−1 corresponds to the S = O linkage on the acid-enhanced catalyst with increasing intensity at 5%. This band shows the presence of sulfate ions which may act as radical scavengers to lower the generation of hydroxyl radicals during photocatalysis (Park & Shin 2014). Generally, the FT-IR spectra band of photocatalyst interprets the functionality of the material and the bond existing within the atoms, and these forecast the expected reaction mechanism and rate of reaction (Shahabuddin et al. 2018; Abebe et al. 2020; Oyetade et al. 2022).
Figure 5

FT-IR spectra of TiO2-Nps with enhanced nanocatalyst of 3% (a) and 5% (b).

Figure 5

FT-IR spectra of TiO2-Nps with enhanced nanocatalyst of 3% (a) and 5% (b).

Close modal

EDX analysis

The elemental composition and the percentage abundance are described in Table 2. Table 2 reveals that the predominant elemental composition of Ti (29.91%), O2 (47.74%) and Si (22.35%) were present in the pure TiO2-Nps which agrees with the observed functional features of the FT-IR spectra band in Figure 5. However, with alkaline modification the percentage abundance of Ti increases to 54.46 and 59.63% in 3 and 5% acid modification, respectively. On the other hand, the highest titanium composition quantified after alkali treatment was 58.96 and 59.26% for 3 and 5% alkali modification, respectively. The sulfur content of 1.86 and 2.22% in 3% AC-TiO2 and 5% AC-TiO2 can be attributed to the sulfuric acid treatment while the presence of Na can be attributed to the NaOH treatment of the Nps. It is necessary to add the increase in element composition of titanium to the treatment completely remediate the silicon content of the TiO2-Nps which could influence the band gap features and the performance of the semiconductor during adsorption and photocatalysis (Martakov et al. 2018; Lu & Astruc 2020). Hence, the higher the amount of titanium and oxygen elements present, the more the possibility of effective coordination and bond establishment needed to adsorb dye molecules before photocatalysis (Angkaew & Limsuwan 2012).

Table 2

EDX elemental composition

S/NSamplesPercentage elemental composition (%)
TiONaSiClSAl
 29.91 47.74 – 22.35 – – – 
3% AC-T 54.46 43.24 – 1.86 – 0.44 – 
5% AC-T 59.63 37.69 – 2.22 – 0.47 – 
3% AK-T 58.96 38.98 0.77 0.44 0.64 – 0.22 
5% Ak-T 59.26 39.01 0.79 0.20 0.74 – – 
S/NSamplesPercentage elemental composition (%)
TiONaSiClSAl
 29.91 47.74 – 22.35 – – – 
3% AC-T 54.46 43.24 – 1.86 – 0.44 – 
5% AC-T 59.63 37.69 – 2.22 – 0.47 – 
3% AK-T 58.96 38.98 0.77 0.44 0.64 – 0.22 
5% Ak-T 59.26 39.01 0.79 0.20 0.74 – – 

Photocatalytic activities and effects of independent parameters

  • a.

    Performance test of enhanced TiO2 nanocatalyst

The results in Figure 6(a) evaluated the preliminary performance of Fenton-mediated TiO2-Nps enhanced with 1–5 M concentration of H2SO4 and NaOH, respectively. The result reveals the minimal performance of 54.6 and 59.14% for acid- and alkali-enhanced nanocatalysts. However, the optimal performance recorded was at 3 M (64.78 and 74.15%) and (60.1 and 74.03%) 5 M, respectively, for acid- and alkali-enhanced nanocatalysts.
Figure 6

Preliminary performance test of enhanced TiO2-Nps at 10 mg/L (a); 1 g/L and 45 min; effect of pH on degradation efficiency of 3% (b) and 5% (c) ehanced TiO2-Nps.

Figure 6

Preliminary performance test of enhanced TiO2-Nps at 10 mg/L (a); 1 g/L and 45 min; effect of pH on degradation efficiency of 3% (b) and 5% (c) ehanced TiO2-Nps.

Close modal

A similar performance was recorded by Hou et al. (2018), however, at a considerably higher concentration of 10 M NaOH. Furthermore, Park & Shin (2014) added that although the acid treatments of TiO2 nanoparticle yield hydroxyl and increase the hydrophilicity of the catalyst, the presence of from the H2SO4-treated catalyst at some point acts as •OH scavenger which prevents the photocatalytic process. However, it is necessary to add that the in situ Fenton-mediation during the process may account for the appreciable dye photodegradation, especially at 3 and 5 M concentrations of acid. The use of Fenton-mediation facilitates the generation of a significant amount of desired .OH which promotes decolourization of dye molecules adsorbed onto the surface of the modified nanocatalyst (Yasar & Yousaf 2012; Zhang et al. 2021a, 2021b).

  • b.

    Effect of pH on photocatalysis

Figure 6(b) and 6(c) shows the effect of varying pH (3–11) for the Fenton-mediated photocatalytic process using the most efficient acid- and alkali-enhanced catalysts in Figure 6(a). From the results, the optimal pH value of 5 was recorded with degradation efficiency of 51, 59.67, 61.02, 63.67 and 66.02%, respectively, for pure, 3 and 5 M acid- and alkali-enhanced photocatalysts at 45 min. Abdellah et al. (2018) suggested that the basic nature of MB and its interaction with and amphoteric catalyst (TiO2-Nps) accounts for a desired slightly acidic pH (<5.8) which builds up a positively charged surface to establish electrostatic interaction with the dye molecules. Furthermore, the result of the optimal pH of 5 agrees with the study carried out by Jangid et al. (2021) and Shahabuddin et al. (2018) using MB as a model dye. The higher performance at this pH is because the positive surface of TiO2-Nps in an acidic medium facilitates the hydroxyl ion generation needed to form radicals (Jiménez et al. 2015). Additionally, the OH. concentration increases with the increasing presence of OH ions due to the H+ ions in the acidic medium (Ma et al. 2014). Also, the formation of electrostatic interaction between the catalyst and dyes in the effluent and radical generation depend on the pH of the medium (Dresselhaus et al. 2018). It is necessary to add that the natural and synthetic dyes are generally called either cationic, anionic or neutral. Thus, the nature of the dye constitutes to its reactivity in effluents and suggests optimal pH for its remediation (Joshi & Shrivastava 2012; Abdellah et al. 2018).

  • c.

    Effect of initial concentration on photocatalysis

The effect of initial concentration on the Fenton-mediated process was studied at different concentrations ranging from 10 to 40 mg/L with 50 mg catalyst dosage and 45 min light UV irradiation. Figure 7(a) and 7(b) shows significant reduction with increasing concentration up to 40 mg/L, although we noted optimal concentration of 10 mg/L for the process. Similar optimal concentration was reported by Marziyeh et al. (2012), Jangid et al. (2021) and Mpelane et al. (2020) for MB dye using the nanocatalyst and with other materials forming composites. The descending plot in Figure 7(a) and 7(b) showcases possible covering and oversaturation of the active sites of the catalyst which reduces dye–photocatalyst interaction. Hence, it lowers the generation of free radicals which enhances dye degradation due to the accessibility of photon energy to the active sites (Marziyeh et al. 2012; Dutta et al. 2021; Oyetade et al. 2022).
  • d.

    Effect of dosage on photocatalysis

Figure 7

Effect of initial concentration on degradation efficiency of for 3% (a) and 5% (b) ehanced TiO2-Nps.

Figure 7

Effect of initial concentration on degradation efficiency of for 3% (a) and 5% (b) ehanced TiO2-Nps.

Close modal

The effect of dosage generally accounts for the reactive performance of the catalyst and its economic implication. Figure 8 reveals the degradation efficiency at different catalyst dosage (30–110 mg) under 45 min UV irradiation, optimum pH (5) and initial concentration of 10 mg/L, respectively. From the results, the optimum dosage ascribed to the pure TiO2-Nps was 70 mg which slightly increases to 90 mg. However, 50 mg optimal dosage was recorded for 3 and 5 M alkali-enhanced TiO2-Nps in contrast to 70 mg optimal dosage for acid-enhanced TiO2-Nps. Beyond this optimal point, there is a level of constancy followed by a gradual decline in performance. From the result, the alkali-enhanced catalyst has an efficiency of 69.3% at 50 mg, compared to 52.65 and 66. 34% degradation at 70 and 90 mg for pure TiO2-Nps and acid-enhanced TiO2 (AC-TiO2-Nps).

The increase in photodegradation is due to the increasing amount of sites available for binding which results in the formation of more active radicals (hydroxyl and superoxide) that initiate the degradation reaction (Suttiponparnit et al. 2011; Abdellah et al. 2018). Although high catalyst loading of TiO2 particles impedes incident UV irradiation apart from the possibility of agglomeration owing to clustered surface area (Wang et al. 2010; Saleh & Gupta 2012; Abdellah et al. 2018). Therefore, beyond the optimal dosage, coupled with a low surface area of the catalyst, reduction in decolorization efficiency is set in, although a similar optimal dosage was recorded by Hou et al. (2018) for alkali-enhanced TiO2 (AK-TiO2-Nps). On the other hand, Park & Shin (2014) uses 500 mg of acid-enhanced TiO2 for optimal dye photocatalysis. The significant efficiency of the AK-TiO2-Nps catalyst is due to the increase in the surface area and lowered agglomeration as revealed by the BET, SEM and 3D surface image, while the saturation and eventual agglomeration with increasing dosages accounts for the gradual reduction of the process (Chatterjee et al. 2017; Dutta et al. 2021; Bayuo et al. 2023). Hence, the surface modification of the nanocatalyst often enhances bonding activities such as electrostatic interactions, Van der Waals forces, hydrogen bonding and π–π interactions which enhance the photocatalytic process, however, at optimal dosage to avoid agglomeration (Ma et al. 2014; Jiménez et al. 2015).

  • e.

    Effect of irradiation time on photocatalysis

The degradation efficiency of the Fenton-mediated photocatalysis of MB at varying irradiation times using pure and enhanced TiO2-Nps as described by Figure 9(a) and 9(b), while Figure 9(c) and 9(d) shows the degradation rate (Ct/C0) of MB dye over time. From the results, 71.48, 69.01 and 89.15% were recorded for TiO2, 3% AC-TiO2 and 3% AK-TiO2 (3 M), respectively. In contrast, 68.19 and 88.37% for 5% AC-TiO2, 5% AK-TiO2 (5 M) were, respectively, quantified. Although a similar amount of degradation was put forward by Hou et al. (2018) and Kumar & Pandey (2018) at a higher time range of 120 and 150 min. The lower time rate may be due to the novel application of in situ Fenton-mediation to improve the reaction rate of the process using an enhanced catalyst. This accounts for the high radical generation with lower recombination desirable to initiate the radical attack and eventual degradation (Zhang et al. 2021a, 2021b). Also, the adoption of in situ mediation improves the homogeneity of the solute–solvent interface while facilitating the efficient mass transfer and shorter reaction time (Tunc Dede et al. 2019). Although the Fenton-mediation is challenged with the generation of iron sludge, studies revealed that the use of the modified co-photocatalyst (TiO2-Nps) with the low concentration of iron in the Fenton reagent leads to the generation of little or no iron sludge (Chacó et al. 2006; Dong et al. 2020). The use of this multiple process is called photo-Fenton where the electron generated by the co-photocatalyst is captured by Fe3+ to produce Fe2+ which combines again with H2O2, thereby preventing covering the surface of the catalyst leading to its deactivation (Mokhbi et al. 2019; Dong et al. 2020).
Figure 8

Effect of catalyst dosage on degradation efficiency for 3% (a) and 5% (b) ehanced TiO2-Nps.

Figure 8

Effect of catalyst dosage on degradation efficiency for 3% (a) and 5% (b) ehanced TiO2-Nps.

Close modal
Figure 9

Effect of irradiation time on (a,b) degradation efficiency and (c,d) degradation rate of MB.

Figure 9

Effect of irradiation time on (a,b) degradation efficiency and (c,d) degradation rate of MB.

Close modal

Additionally, at an optimum time of 90 min, the lower degradation of acid-enhanced catalyst is due to the possibility of agglomeration as described from the SEM image and inhibition of radical attack as a result of sulfate ion acting as radical scavengers (Abdellah et al. 2018; Shahabuddin et al. 2018). Furthermore, after the darkroom adsorption–desorption process, at 15 min irradiation, the lower degradation efficiency for all catalysts is due to the generation of fewer electron–hole pairs before radical formation and degradation of adsorbed dye molecules (Jangid et al. 2021). However, as time increases, the degradation efficiency increases, respectively. It is worthy of note that the highest efficiency of 89.15 and 88.97% for 3 and 5 M alkali-enhanced TiO2 indicates the successful modification of the surface morphology as described by the SEM results which enrich the catalyst with significant binding sites and possibly influence its bandgap (Ahmadizadegan 2017; Debnath et al. 2021; Oyetade et al. 2022). The process of bandgap tunability and surface modification in this regard limits the possibility of agglomeration and recombination of electron–hole pair (Bingham & Daoud 2011; Eskizeybek et al. 2012; Jangid et al. 2021). Shahabuddin et al. (2018) added that the larger the surface areas the more active sites available for binding and the lower the agglomeration rate of the nanocomposite. However, the organic dye has light-absorbing potential and can undergo electronic transition via intersystem crossing leading to self-decomposition as the singlets and triplets species are formed (Subramanian et al. 2014; Shahabuddin et al. 2018). The formation of these species is enhanced by their interaction with photon-active and active site-enriched nanocatalyst which reacts with oxygen and water molecules to generate peroxide, superoxide and hydroxyl radicals. This process is facilitated at a lower time via Fenton-mediation (Shahabuddin et al. 2018; Oyetade et al. 2022).

Reaction mechanism

The surface modification via alkali modification is significant in charge separation during the photocatalytic process. The preliminary adsorption–desorption in a dark room establishes an electrostatic bond and enhances the adsorbent–adsorbate linkage between TiO2-Nps and MB dye (Figure 10). Irradiation of the system with UV-LED results in the excitation of electrons from the valance band to the CB with appreciable charge separation due to the preliminary alkali modification process. Radicals such as hydroxyl radicals (OH•), superoxide radical anions () and hydroperoxyl radicals (•OOH) are generated during this process (Zangeneh et al. 2015; Qamar et al. 2019). Also at the optimal pH of 5 (slightly acidic condition), the surface of the amphoteric photocatalyst (TiO2-Nps) becomes positively charged, thereby establishing electrostatic reaction with the negatively charged model dye (MB), this process facilitates effective adsorption of the dye molecules unto the surface of the catalyst before the radical attack on irradiation (Jiménez et al. 2015; Abdellah et al. 2018). During the irradiation process, a spontaneous and simultaneous redox reaction alongside a radical attack of the adsorbed dye molecules occurs, leading to the preliminary opening of the ring as described in Figure 10. However, the in situ mediation of the process by Fenton reagents accounts for the generation of a larger amount of radical which combines with the radical generated by the action of the nanocatalyst in Equations (2)–(6). The reaction eventually terminates at the point of mineralization of the dye molecules into CO2 and H2O which is reflected in decolorization of the effluent in Figure 10 (Asghar et al. 2015; Jangid et al. 2021).
formula
(2)
Figure 10

Reaction mechanism.

Figure 10

Reaction mechanism.

Close modal
formula
(3)
formula
(4)
formula
(5)
formula
(6)

Comparison of degradation efficiency

The performance of the most effective enhanced TiO2 (3% AK-TiO2) is compared with other reports at optimal experimental conditions for the degradation of MB in Table 3. From the comparative results, the use of in situ Fenton-mediated photocatalysis and enhanced catalyst (3% AK-TiO2) shows appreciable efficiency in contrast to reagents such as rihodizonic acid and salicylic acid used apart from their reported toxicity and cost intensiveness. Furthermore, a lower amount of NaOH concentration is required for the current study compared to the 10 M NaOH used by Hou et al. (2018). However, it is necessary to add that the variation in the light sources from Table 3 greatly influences the efficiency. Hence, at a shorter time and with cost-effective energy, the economically viable nanocatalyst can be applicable for optimal remediation of effluent laden with recalcitrant dyes thereby addressing environmental concerns and contributing to the sustainable development goal (6) clean water and sanitation, goal (7), affordable energy (14) life below water and goal (15) life on land. Furthermore, the modification creates the possibility of recycling and reuse of the photocatalyst without additional treatment due to lowered agglomeration from the alkali enhancement and the conversion of Fe3+ to Fe2+ via the capturing of the electron generated from the redox process of the catalyst (AK-TiO2-Nps) (Dong et al. 2020; Bilici et al. 2021). Also, when coupled without other photon-sensitive nanomaterials or immobilized on nanopolymeric support leaching is reduced and a higher recovery yield for reuse is obtainable (Oyetade et al. 2022)

Table 3

Comparison between the present study and reported TiO2-enhanced catalyst for photodegradation of MB dye

NanocatalystpHInitial conc (mg/L)SourceCatalyst dosage (mg)Irradiation time (min)Efficiency (%)References
10 M NaOH-treated TiO2 – 10 300 W Xenon lamp 20 150 – Hou et al. (2018)  
Fenton-mediated-3% AK-TiO2-Nps 10 18 W UV-LED 50 90 89.15 Present study 
TiO2 (rod) 10 100 W Xenon lamp 50 90 25.03 Wahyuni et al. (2018)  
TiO2-NPs 10 30 W UVC lamp 100 150 28 Koysuren & Koysuren (2019)  
TiO2 Nanopowder 11 10 24 W Hitachi black light lamp 30 60 85 Marziyeh et al. (2012)  
Commercial TiO2 particles. 10−5 UV lamp 70 60 99 Yuangpho et al. (2018)  
H2O2/TiO2 20 39 W/m2 UV lamp with light intensity 150 80 97.6 Farouq (2018)  
TiO2-Nps – 10 Visible 100 150 50 Melinte et al. (2019)  
TiO2−SF – 10 300 W Xenon lamp 200 120 54 Xu et al. (2020)  
Ti-SA 2 × 10−6 400 W UV light 200 180 93.3 Mohammadi & Aliakbarzadeh Karimi (2017)  
NF/TiO2−RA  10 150 W high-pressure xenon 10 90 60 Sun et al. (2020)  
NanocatalystpHInitial conc (mg/L)SourceCatalyst dosage (mg)Irradiation time (min)Efficiency (%)References
10 M NaOH-treated TiO2 – 10 300 W Xenon lamp 20 150 – Hou et al. (2018)  
Fenton-mediated-3% AK-TiO2-Nps 10 18 W UV-LED 50 90 89.15 Present study 
TiO2 (rod) 10 100 W Xenon lamp 50 90 25.03 Wahyuni et al. (2018)  
TiO2-NPs 10 30 W UVC lamp 100 150 28 Koysuren & Koysuren (2019)  
TiO2 Nanopowder 11 10 24 W Hitachi black light lamp 30 60 85 Marziyeh et al. (2012)  
Commercial TiO2 particles. 10−5 UV lamp 70 60 99 Yuangpho et al. (2018)  
H2O2/TiO2 20 39 W/m2 UV lamp with light intensity 150 80 97.6 Farouq (2018)  
TiO2-Nps – 10 Visible 100 150 50 Melinte et al. (2019)  
TiO2−SF – 10 300 W Xenon lamp 200 120 54 Xu et al. (2020)  
Ti-SA 2 × 10−6 400 W UV light 200 180 93.3 Mohammadi & Aliakbarzadeh Karimi (2017)  
NF/TiO2−RA  10 150 W high-pressure xenon 10 90 60 Sun et al. (2020)  

RA, rhodizonic acid; SS, salyscilic acid; Nf, nano fiber; Nr, nano rods; Nps, nanoparticles.

In summary, the Fenton-mediated photocatalytic process vis-à-vis the enhanced TiO2-Nps was comprehensively evaluated and statistically modeled by the optimization of the independent variable factors for degradation of industrially used azo dye (MB). The XRD result reveals the predominate peak of anatase with rutile, while the SEM imaging indicated modified surface morphology having a mesoporous structure from the BET isotherm. The availability of more pore size and active sites after alkali modification is in tandem with the higher degradation efficiency coupled with the in situ mediation of the process by the Fenton reagent to generate more radicals. Furthermore, the study reveals that the slightly acidic pH of the medium enhanced the presence of H+ ions which increase OH necessary for the generation of the desired hydroxyl radicals. Hence, at a slightly acidic medium, alkali-modified TiO2-Nps performs most effectively in the mineralization of organic dye pollutants.

Recommendations

  • 1.

    The use of Fenton-mediated process at measured effluent concentration influences the efficiency of the process.

  • 2.

    Development of nanocatalyst trapping system to recover and reuse catalyst for effluent decolorization.

  • 3.

    Reuse of treated water for basic industrial operations such as desizing, singeing, and cooling of boilers.

  • 4.

    Development of CO2 sequestration system to capture the gas resulting in photocatalysis in further industrial process.

This work was supported and funded by the Regional Scholarship for Innovation Fund (RSIF), a flagship program of the Partnership for Skills in Applied Sciences, Engineering and Technology (PASET).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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