Performance evaluation of in situ Fenton-mediated photocatalysis of industrial dye effluent with enhanced TiO₂ nanoparticle

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, Fe₂O_3, 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.

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.

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)_.

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/P_o) 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).

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/N .	Samples .	BET surface area (m2 g−1) .	Pore volume (cm3/g) .	Pore size (nm) .
1		47.9654	0.17	14.22
2	3% AC-T	91.8267	0.42	18.33
3	5% AC-T	60.5754	0.32	21.28
4	3% AK-T	64.7434	0.30	18.37
5	5% AK-T	50.3858	0.21	16.64

S/N .	Samples .	BET surface area (m2 g−1) .	Pore volume (cm3/g) .	Pore size (nm) .
1		47.9654	0.17	14.22
2	3% AC-T	91.8267	0.42	18.33
3	5% AC-T	60.5754	0.32	21.28
4	3% AK-T	64.7434	0.30	18.37
5	5% AK-T	50.3858	0.21	16.64

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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/N .	Samples .	Percentage elemental composition (%) .
		Ti .	O .	Na .	Si .	Cl .	S .	Al .
1		29.91	47.74	–	22.35	–	–	–
2	3% AC-T	54.46	43.24	–	1.86	–	0.44	–
3	5% AC-T	59.63	37.69	–	2.22	–	0.47	–
4	3% AK-T	58.96	38.98	0.77	0.44	0.64	–	0.22
5	5% Ak-T	59.26	39.01	0.79	0.20	0.74	–	–

S/N .	Samples .	Percentage elemental composition (%) .
		Ti .	O .	Na .	Si .	Cl .	S .	Al .
1		29.91	47.74	–	22.35	–	–	–
2	3% AC-T	54.46	43.24	–	1.86	–	0.44	–
3	5% AC-T	59.63	37.69	–	2.22	–	0.47	–
4	3% AK-T	58.96	38.98	0.77	0.44	0.64	–	0.22
5	5% Ak-T	59.26	39.01	0.79	0.20	0.74	–	–

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• a.

  Performance test of enhanced TiO₂ nanocatalyst

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 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

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).

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)

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.

• 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.