An Effective Strategy for the Photocatalytic Elimination of Industrial Wastewaters via MgFe-LDH/TiO 2 Heterojunction

Reactive dyes are presently the most common type of dye utilised by the garment sector to colour wool, cotton, and viscose-based clothing. By using the typical biological degradation & sediment absorb approaches, it is quite impossible to get rid of them. Thus reactive dye colors in the effluent are major environmental issues in recent times. In this work Reactive dye series has been choosen for the photocatalytic degradation studies over calcined form of TiO 2 and MgFe-LDH cal nanocomposites (TiO 2 /MgFe-LDH cal ). This work assesses, a detailed investigation of the photocatalytic degradation of the reactive class dyes Ca . Reactive green 19, Reactive red 120, Reactive blue 4 and Eosin Yellowish dye by layered double hydroxide (LDH) and TiO 2 heterojunction (MgFe-LDH cal /TiO 2 ). Here for the first time, photocatalytic application of the said heterojunction was investigated toward the degradation of the reactive dyes. As these dyes are cytotoxic, mutagenic, and inhibit certain mitochondrial functions and often are carcinogens. This heterojunction system degrades the reactive dyes in 1 hour of visible light irradiation. The said LDH heterojunction was characterised by powder XRD to confirm the phase and composite formation, Uv-Visible and PL spectra to know the optical activity of the material in presence of light. The size, interplanar spacing and morphology of the nanoparticles were inspected by HR-TEM analysis. All the batch experiments were carried out at room temperature, without any adjustment of pH, at the dye concentration varying from 20 to 50 mg/L. Furthermore, according to the rejuvenation study, the prepared LDH heterojunction system could potentially be put to use for a number of cycles.


Introduction
Water is used extensively throughout the dyeing and finishing processes in textile manufacturing.Due to the widespread usage of reactive dyes, wastewaters released into waterways or wastewater treatment plants are heavily polluted.Because of this reactive class dyes utilization in huge quantities, a sizable portion of these dyes enter to the environment through wastewater, hence threatens the environment.Pollution of the environment and lately have increased consciousness about the possibility of worldwide catastrophe.The creation of environmentally friendly, low-energy cleanup technology is a critical undertaking for the sustainable growth of human society.Even extremely small levels of colors in water from factories could be easily observed through naked eye, and the breakdown byproducts from these reactive dyes can often be hazardous. [1]The textile manufacturing effluents tend to be brightly coloured and dumping into open seas provides an environmental issue since dye concentration typically ranges from 10 to 200 ppm. [2]Approximately 7x10 5 tonnes of dye are manufactured annually, and in total there are more than 10,000 commonly accessible dyes . [3]With the increased usage of a wide range of reactive dyes, damage by these wastewater is becoming more concerning.The majority of dyes utilised by clothing industries are not biodegradable and are photo-stable. [4]Such effluent must be treated before being released into the environment in order to mitigate the danger of environmental degradation. [5]The most crucial oxidation process uses heterogeneous photocatalysts, a mix of TiO2 and light, to destroy water soluble organic contaminants in water and wastewater.Many methodologies, such as chemical or physical procedures, have been suggested or are being explored in this regard. [6]Due to its resistance to corrosion in water and UV light reacivity, TiO2 is a frequently used photo-catalyst.TiO2 has the ability to act as an oxidative and a reductive catalyst.Due to its powerful oxidising properties, non-toxicity, and long-term photostability, TiO2 is regarded as being very near to the perfect semiconductor for photocatalysis. [7]Although there are some lacuna with TiO2 photocatalysis, such as lower catalytic active sites, lack of convenient structural properties, and a wide band gap energy (3.2 eV), these issues could be resolved by using conventional synthesis methods with other semiconductor photocatalyst combination.Due to their special characteristics, such as tiny particle sizes, high surface to volume ratios, and easy synthesis process, with which they could be fixed onto solid matrices for improved wastewater treatment, nanoscale particles seem promising in this field. [8]Commercial dye degradation by TiO2-mediated photocatalytic oxidation looks to be a promising method.Organic contaminants found in waste streams can be effectively mineralized into CO2 and inorganic ions by heterogeneous advanced oxidation processes that utilise hydroxyl radicals (•OH). [9,10].TiO2 has a significant potential for industrial applications in the photocatalytic remediation of water contaminants due to its low cost and ease of manufacture.A cost-effective technique is required to remove colours from municipal and industrial water effluents due to increased environmental concerns and awareness.A potential technique to induce degradation of dyes includes the oxidation process through TiO2.By using heterogeneous advanced oxidation techniques that make use of hydroxyl radicals (•OH), organic pollutants present in waste streams may be effectively broken down into CO2 and other inorganic ions. [9,10]Due to its low cost and simplicity in manufacturing, TiO2 has enormous possibilities for usage including photocatalytic treatment of water-based pollutants.Owing to the increasing environmental consciousness and concerns, a low-cost approach is needed for eliminating hazardous water contaminants from urban and industrial wastewater discharges.Water soluble, brightly colored dyes provide the greatest removal challenges, including reactive and acid dyes.Both types of dyes have been shown to be ineffective when treated by traditional and urban oxygenation treatment methods. [11,12]Currently, there is a lot of interest in the growth of visible light reactive catalysts for the efficient and sustainable harvesting of the 43% visible light present in solar radiation.Numerous studies have been conducted on titanium dioxide (TiO2) as a photocatalyst for the conversion of solar energy and the degradation of pollutants. [13,14]The introduction of advanced oxidation processes for the treatment of waste fluids is the subject of several studies. [15,16][19][20] For the development of LDH materials with effective photocatalytic capabilities, many approaches have been taken.LDH hybrids were recently created using carbon-based components and semiconductors to improve photocatalytic activity. [17]lternately, incorporating cations in the layers that showed enhance photocatalytic activity due to the flexible composition of LDHs were shown by many researchers.In fact, binary and ternary LDHs and materials generated from LDH have demonstrated promising photocatalytic properties when replaced with transition metals were well documented by the researchers recently. [18]For instance, the n-type semiconductor NiFe LDH [19] that absorbs visible light up to 800 nm and have extraordinary photoactivity as compared to other LDH materials was reported.Recently published article [20] also depicts the visible light activity in the remediation of water pollutants by ternary ZnCoFe LDH.But no investigation has been done by using Mg-Fe-LDH based TiO2 composites till date despite of its narrow band gap, high visible light absorptive capability, stability and economic features.Apart from doping and composite synthesis by using carbon based materials, LDH material, which consists of special 2D morphology along with typically positively charged brucite-like host layers, inspires us for the preparation of heterojunction hybrid with TiO2 photocatalyst.In this article, first ever attempt has been made to synthesize visible light active heterojunction MgFe-LDHcal/TiO2 for photocatalytic degradation of VAT class dyes present in waste waters collected directly from local textile industries (Sambalpuri Vastralaya, Odisha, India) as well as laboratory prepared sample waste water (for reactive class dyes degradation) in very high concentration.

Experimental 2.1. Apparatus and reagents
XRD, diffuse reflectance UV (DRUV)-Vis, photoluminescence, TEM, and energy dispersive X-ray (EDX) investigations and analysis were used to characterize all of the as prepared composites.CuKα radiation with 2θ ranging from 10 o to 80 o at a scanning rate of 5 o per minute was used in a Rigaku Miniflex powder diffractometer (operated at 30 kV and 15 mA) to acquire Xray diffraction patterns.With Barium Sulfate used as a reference, the DRUV-Vis spectra of the catalyst samples were collected using a Varian Cary 100 spectrophotometer outfitted with a diffuse reflectance accessory in the wavelength range of 200-800 nm.PL spectra were captured using a Perkin-Elmer LS 55 fluorescence spectrometer with 380 nm excitation at room temperature.An FEI TECNAI G2 model running at 200 kV was used to capture the TEM pictures.the study's sampling.For preparing materials for the experiment, the as-prepared catalytic composites were sonicated in 2-propanol for 3 min.The samples were then drop-dried on a copper grid coated with carbon film.Titanium isopropoxide (purity >97%, Sigma Aldrich), Isopropanol (purity >99%, Sigma Aldrich), Mg (NO3)2.6H2Oand Fe(NO3)3.9H2O(Alfa Aesar).No additional purification was performed on any of the compounds utilised.

Synthesis of catalysts 2.2.1. Synthesis of TiO2
Initially, anatase TiO2 was synthesized by the following method.10 ml titanium isopropaxide was mixed with 60 ml of Isopropanol.Then deionised water in the amount of 3 ml was then added to the solution.The mixture was vigorously stirred until a cloudy solution was obtained.Then the solution was centrifuged by laboratory centrifuge equipment at 5000 rpm for 30 min.Then the precipitate was dried at 80 o C for overnight.The dried precipitate grinded with a mortar until fine white powder was obtained.The powder was then calcined in a muffle furnace at 500 o C.

Synthesis of MgFe-LDHcal
Then MgFe-LDH was synthesized by taking an aqueous solution of Mg(NO3)2.6H2O(0.22 mol) and Fe(NO3)3.9H2O(0.11 mol) in 200 ml of distilled water.The NaOH (0.72 mol) solution was added drop wise in the above mixture and then undergone with vigorous stirring at room temperature for 2 h.Then, the mixture was aged at 60 °C for 18 h.The precipitate formed was filtered and washed with hot distilled water until there is a neutral pH of the filtrate was obtained.Finally, the powder was dried in an oven at 80 °C overnight.Then, it is calcined at 800 °C for 4 h.After the cacination process at high temperature, the LDH is now named as MgFe-LDHcal.

Synthesis of MgFe-LDHcal/TiO2 heterojunction
To form the heterojunction, different wt.% of calcined MgFe-LDH powders (2, 5, and 10) were loaded onto TiO2 by the in-situ growth of TiO2 nanoparticles on its layers.Then, it is calcined at 500 °C for 4 h and these were designated as xMgFe-LDHcal/TiO2 heterojunction.These products were applied for the photocatalytic degradation experiments.

2.3.
Photocatalytic Experiment A stoppered Pyrex conical flask that contained the catalyst (20 mg) and 20-50 mg/L of dye concentration was exposed to visible light in an illumination UV-Vis.chamber (BS 02, Germany) for 1 h.The photodegradation of laboratory prepared organic textile dyes namely Reactive green 19, Reactive red 120, Eosin Yellowish and Reactive blue 4 were observed.Additionaly, direct industrial waste water treatment was carried out with dye concentrations (209 mg/L for BLUE VAT DYE and 1657 mg/L for VIOLET VAT DYE collected from local textile industry) with 1 h of visible light irradiation.All the experiments were performed at room temperature without adjusting pH of the experiments.After reaction completion, the results were analysed in Uv-Visible spectrophotometer.

2.4.
Photocatalytic Experiment A stoppered Pyrex conical flask that contained the catalyst (20 mg) and 20-50 mg/L of dye concentration was exposed to visible light in an illumination UV-Vis.chamber (BS 02, Germany) for 1 h.The photodegradation of laboratory prepared organic textile dyes namely Reactive green 19, Reactive red 120, Eosin Yellowish and Reactive blue 4 were observed.

After 1h illumination and separation of catalyst
Scheme 1. Photoreactor with highly concentrated indstiral waste collectced from sambalpuri bastryalaya industry.The direct industrial waste water treatment was carried out with dye concentrations (209 mg/L for BLUE VAT DYE and 1657 mg/L for VIOLET VAT DYE) with 1 h of visible light irradiation through a Lelesil photoreactor with MPMVL Lamp.All the experiments were performed at room temperature without adjusting pH of the experiments.After reaction completion, the results were analysed in Uv-Visible spectrophotometer.

XRD patterns of the samples
Representative XRD patterns of the samples exhibit the characteristic reflections of the LDH materials. [21,22]XRD spectra (Figure 1

3.2.
Uv-Vis DRS Analysis The optical absorption behaviour of the TiO2, MgFe-LDHcal and its heterojunction was conducted by UV-vis absorption spectroscopy and is depicted in Figure 2, the absorption edge shifted towards longer wavelength, indicated enhanced photocatalytic activity of the heterojunction upon illumination of visible light.UV/Vis diffuse reflectance spectra of xMgFe-LDHcal/TiO2 heterojunction absorbs the broad range of radiation unlike pure TiO2.This consequently shows that these heterojunction should have good photo activity in presence of visible spectrum of solar radiation.

3.3.
Photoluminescence Study PL spectroscopy of TiO2, MgFe-LDHcal and MgFe-LDHcal/TiO2 shows emission peaks at 540 nm upon excitation at 350 nm.In the above spectra pure TiO2 shows the highest charge carrier recombination thus having the highest intensity.Pure MgFe-LDHcal also shows high intensity, with elevated charge carrier recombination property.But at the same time as the heterojunction designed with the weight percentage of 5MgFe-LDHcal/TiO2 has the lowest intensity as compared to the other synthesized composites and so it is showing best activity towards photocatalytic reaction.The spectrum region or comparative intensity of peaks are unaffected by the formation of heterojunction, but the overall emission intensity is gradually decreased.Our model predicts the decrease in PL intensity following semiconductor insertion with the LDH heterojunction formation, charge separation occurs which diminishes the recombination of photogenerated charge carriers and hence the lowest intensity.

Transmission electron microscopy Study
From the Morphological study of pure MgFe-LDHcal (Figure 4A), an equi-axed layered morphology with layers having thickness range of 1-2 nm can be observed.Energy dispersive X-ray (EDX) spectroscopy investigation further supports the material's composition with strong Mg, Fe, and O signals may be seen in EDX point spectra that were collected at the layer scan's central spot (Figure 4B).This result implies the absence of impurities during the synthesis of photocatalysts.The elemental mapping images (figure 4C-G) clearly indicate that the MgFe-LDHcal nanolayers consist of Mg, Fe, and O atoms.Interestingly, thin layers consist of O-Mg-Fe trimetal and are covered homogeneously with other atoms.From the Morphological study of pure MgFe-LDHcal (Figure 4A), an equi-axed layered morphology with layers having thickness range of 1-2 nm can be observed.With the use of energy dispersive X-ray (EDX) spectroscopy investigation, the elemental content is further verified.To gain a better insight into the chemical nature of the deposits, TEM-guided Energy-dispersive X-ray spectroscopy (EDX) analysis was performed on selected sample 5MgFe-LDHcal/TiO2 after calcinations (Figure 5) and EDX chemical analysis made it possible to determine their elemental composition.The TEM image of 5MgFe-LDHcal/TiO2 heterojunction shows beautiful nanoparticles with pseudo spherical shapes and average diameter of 7-12 nm (Figure 5A) on the thin layers of MgFe-LDH.This again evidenced the formation of heterojunstion as the nanopartcles are evenly distributed on the surface of layered structure.The structures have a strong elemental signal (Figure 5C-H) of Mg, Fe, O and Ti which confirms the successful synthesis of the heterojunction without any impurity.The lattice images of LDH layers and nanoparticles were clearly observed, which indicated that these have high degrees of crystallinity and phase purity.From the distance between the adjacent lattice fringes, we can assign the lattice plane on the LDH layer and TiO2 nanoparticles.The pure MgFe-LDHcal layer shows the interplaner spacing of d = 0.45 nm for the (311) plane, which is consistent with the XRD value (figure 6a).Interestingly, in the HRTEM images of the heterojunction of 5MgFe-LDHcal/TiO2 the TiO2 nanoparticles showed very clear lattice spacing of d = 0.33 nm for the (101) plane of the anatase phase on the layer of MgFe-LDHcal layer having the lattice fringes with interplanar spacing of d = 0.45 nm as shown in figure 6B.The distance between the lattice fringes (d = 0.33 nm) in the pseudo sphere can be assigned to the interplanar distance of anatase phase (101) plane, which is well consistent with XRD results as well.Further observation by SAED (inset image in Fig. 6A-B) confirmed that the TiO2 had a polycrystalline anatase structure, and the layer of MgFe-LDHcal were single crystalline when heterojunction with TiO2 was not made.Further HRTEM, TEM and SAED analysis, it can be concluded that the formation of heterojunction affect not only the crystalline phase and crystallinity but also the morphologies of TiO2 nanostructures.Band gap is the predominant characteristics of any photocatalytic semiconductor which determines the light harvesting efficacy of it.Using the reflectance data, the Kubelka-Munk technique was used to measure the band gap (Eg), which is crucial to test the light-harvesting effectiveness, for the 5MgFe-LDHcal and 5MgFe-LDHcal/TiO2 samples (Figure 7A and B). [23]Extrapolating the linear component of the (Ahv) 1/2 curve vs photon energy hυ yielded the value of Eg.A Tauc plot was used to assess the band gap's nature (direct or indirect).Equation ( 1) provides the Tauc relationship as given below: [1]   in which a is the absorption coefficient, hν is the energy of the incident radiation, A is the constant based on effective masses of electrons and holes, and n can take the values of 1/2 and 2 for a direct or indirect band gap transition, respectively.In accordance to the equation [1], the band gap of pristine MgFe-LDHcal and 5MgFe-LDHcal/TiO2 heterojunction were calculated where pristine MgFe-LDHcal shows the indirect band gap transition values of the order of 1.81 eV, which could be suitable for visible-light sensitivity.The 5MgFe-LDHcal/TiO2 heterojunction displayed two Eg values of 1.8 and 3.1 eV, which are as predicted and is may be correspond to 5MgFe-LDHcal and 5MgFe-LDHcal/TiO2, respectively.The photodegradation of any organic/reactive is initiated by de-ethylation and ring-opening pathways.
With the purpose to distinguish between these pathways, UV-vis spectra of EY, RB4, RG19, RR 120 were experimented during the degradation reaction over 5MgFe-LDHcal/TiO2 sample were showed in figure 9A-D.It is pretty obvious that the degradation response of the dyes over these materials occurs via ring-opening mechanism based on the dwindling in intensity of the primary absorption peaks of all dyes as the irradiation period increases with no variations in peak positions.Additionally, nearly every colours tend to disintegrate within 90 minutes.From this observation we can conclude that this catalyst is very effective in degrading industrial effluents with very high concentrations.Furthermore, the total carbon (TC) analyses in figure 10b also corroborate the removal of carbon contents after the reaction under visible light in presence of the catalyst from the industrial effluent.

Mechanism
Apart from the band gap calculation of both the semiconductors, the conduction band edge potential (CB) and the valence band edge potential (VB) were also calculated of the calcined MgFe-LDHcal to propose a plausible mechanism to explain the enhanced photocatalytic activity of the 5MgFe-LDHcal/TiO2 heterojunction as illustrated in figure 11.With the intention of clarifying the separation of photogenerated electron-hole pairs over the said heterojunction, it is necessary to find out the CB and VB potentials of the components.These energy levels were calculated using the following empirical equations [2]: In which a, b, and c are the number of atoms in the compounds.The values of Eg and χ (Mullikens electronegativity of atoms) for MgFe-LDHcal are 1.8 and 5.72 eV, respectively.Therefore, the ECB and EVB of MgFe-LDHcal were calculated to be -0.32 and +1.48 eV versus normal hydrogen electrode (NHE).In addition, Eg and χ values for TiO2 are 3.2 and 5.81 eV, respectively.Furthermore, ECB and EVB for TiO2 were calculated to be -0.29 and +2.91 eV/NHE, respectively.When the MgFe-LDHcal/TiO2 heterojunction were irradiated by the visible light, the photons attaining to the photocatalyst would be absorbed by MgFe-LDHcal due to its narrow band gap leading to generation of some electron-hole pairs.As can be seen in Figure 10, the CB of MgFe-LDHcal is higher than that of TiO2.Hence, the photogenerated electrons in the CB of MgFe-LDHcal can be transferred to TiO2 CB.But due to wider band gap of TiO2, visible light cannot excite the VB electrons to the CB as it is active only under UV • Email: editor@ijfmr.comIJFMR240112289 Volume 6, Issue 1, January-February 2024 13 light.These interfacial charge transfers prolong lifetime of the charge carrier by suppressing the recombination of electron-hole pairs, which is more favorable to increase the photocatalytic activity.Some of the photogenerated electrons on the CB of MgFe-LDHcal react with adsorbed O2 to form o O2 − radicals which may be responsible for the degradation of the dyes.And the holes available in the VB of MgFe-LDHcal could directly oxidize the dyes to get the unpolluted water.So finally, we can predict that the CB electron of TiO2 and the VB holes are responsible for the overall degradation of the organic/reactive/industrial dyes.

Conclusion
TiO2-incorporated MgFe-LDHcal nanocomposites were synthesised in this study utilising a costeffective co-precipitation approach with varied doses to improve textile dye degradation.XRD confirmed the presence of calcined MgFe-LDH and anatase TiO2 phases without any impurities.The morphology of composites was recorded using TEM, and HR-TEM computed interplanar d-spacing (0.45 and 0.33 nm) that was well matched with XRD.In comparison to doped free samples, TiO2incorporated MgFe-LDHcal nanocomposites demonstrated superior catalytic activity of up to 99.9%.This study evaluates a thorough analysis of the photocatalytic degradation of the reactive class dyes Ca.By using layered double hydroxide (LDH) and TiO2 heterojunction (MgFe-LDHcal/TiO2), the hazardous category dye i.e. reactive green 19, reactive red 120, reactive blue 4, and Eosin Yellowish dye were degraded.For the first time, the photocatalytic use of the aforementioned heterojunction was examined.
The most important future development in this area will be the development of a large scale synthesis reactor that can use direct industrial effluents for treat the hazardous materials present in waste water.
) show significant formation of semiconductor TiO2 and MgFe-LDH in the synthesis method.The TiO2 nanoparticles exhibited pure anatase phase in case of pristine as well as in the synthesized composite.This reflects the transition of the TiO2 nanoparticles structure at a calcination temperature of 500 o C. The high intensity located 2θ = 35.54,43.21, 62.75 represents the peak 511, 400, 440 show the formation of heterojunction.The XRD patterns of the as-synthesized xMgFe-LDH/TiO2 solids indicate the formation of a well crystallized layered structure.The calculated basal spacing values corresponding to the (220) and (511) planes indicates at 2θ= 30 o , 57 o were easily recognized in all the patterns for MgFe-LDH in the calcined form.The XRD pattern indicates planes (101), (004), (200) refer to 2θ= 25 o , 37 o and 37 o point out the TiO2 is in anatase phase.Thus heterojunction formation was confirmed from the XRD pattern with having both the diffraction pattern.

Figure 4 .
Figure 4. Shows the (A) TEM image of MgFe-LDH pure after calcination.(B) EDX spectra of MgFe-LDH showing the presence of Mg,Fe and O elements in the photocatalyst.(C-G) Elemental mapping analysis in the same area of the MgFe-LDH, showing the presence of O, Mg, Fe elements without any other impurities.

Figure 8 .
Figure 8. (A-B) Degradation efficiency of 5MgFe-LDHcal/TiO2 for reactive dyes, (B) Rate of change of concentration of RR120, RB4, EY, RG19 with varying initial concentration over 5MgFe-LDHcal/TiO2 at different times.(C) Tabular image showing the degradation of highly concentrated industrial dye effluents collected directly from textile industry.