On-Site Application of Solar-Activated Membrane (Cr–Mn-Doped TiO2@Graphene Oxide) for the Rapid Degradation of Toxic Textile Effluents

Solar-activated water treatment has become an emerging research field due to its eco-friendly nature and the economic feasibility of green photocatalysis. Herein, we synthesized promising, cost-effective, and ultralong-semiconductor TiO2 nanowires (NW), with the aim to degrade toxic azo dyes. The band gap of TiO2 NW was tuned through transition metals, i.e., chromium (Cr) and manganese (Mn), and narrowed by conjugation with high surface area graphene oxide (GO) sheets. Cr–Mn-doped TiO2 NWs were chemically grafted onto GO nanosheets and polymerized with sodium alginate to form a mesh network with an excellent band gap (2.6 eV), making it most suitable to act as a solar photocatalytic membrane. Cr–Mn-doped TiO2 NW @GO aerogels possess high purity and crystallinity confirmed by Energy Dispersive X-ray spectroscopy and X-ray diffraction pattern. A Cr–Mn-doped TiO2 NW @GO aerogels membrane was tested for the photodegradation of Acid Black 1 (AB 1) dye. The synthesized photocatalytic membrane in the solar photocatalytic reactor at conditions optimized by response surface methodology (statistical model) and upon exposure to solar radiation (within 180 min) degraded 100% (1.44 kg/m3/day) AB 1dye into simpler hydrocarbons, confirmed by the disappearance of dye color and Fourier transform infrared spectroscopy. An 80% reduction in water quality parameters defines Cr–Mn-doped TiO2 NW @GO aerogels as a potential photocatalytic membrane to degrade highly toxic pollutants.


Introduction
Water contamination is a major problem faced by the entire world, especially in developing countries. According to a rough estimation, around four billion people throughout the world have no or very little supply of clean water, and yearly, millions of people died due to drinking contaminated water [1][2][3]. Many industries, such as paper, pulp, dyestuff, pharmaceutical textiles, etc., are running throughout the world [4][5][6]. A huge number of pollutants are being discharged from these industrial processes that cause noticeable effects on the environment [7]. Around 20% of colored waste effluents are being discharged into water bodies from textile industries without treatment, which is badly polluting the environment. These colored effluents are rich in organic dye pollutants, and because of the non-biodegradable nature of these contaminants, they are a serious environmental risk more feasible redox photocatalytic degradation process of dyes [37]. After co-doping with Cr-Mn, TiO 2 was conjugated with GO to form a Cr-Mn-doped TiO 2 @GO composite that was polymerized into aerogel photocatalytic membrane for the solar-assisted photocatalytic degradation of Acid Black 1 dye. All operational parameters, i.e., oxidant concentration (H 2 O 2 ), pH and sunlight exposure time, and the size of the aerogel to study the photodegradation of Acid Black 1, were optimized by response surface methodology (RSM). Because of our synthesized Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane's large internal surface area, porosity, high optical properties, low density, and high adsorptive site due to GO functionalities, it favors the greater interaction of pollutants with the membrane leading to 100% degradation within 180 min of exposure to solar light. The employment of our designed photocatalytic membrane will be cost-effective because of the simultaneous adsorption and photodegradation of toxic organic pollutants [45][46][47].

Synthesis of Graphene Oxide (GO) and TiO 2 Nanowire (NW)
Graphene oxide (GO) was synthesized by a bottom-up approach using the method reported in the literature with slight modification [48]. In brief, 2 g of citric acid (CA) was heated at 200 • C in heating in the mantle. Within 5 min of heating, the CA was liquified and changed color from white to pale yellow and finally turned to dark brown after three hours, indicating the formation of GO sheets. Later, the formed dark brown was removed from the mantel, cooled, and neutralized to pH 7 with 50 mL NaOH (0.1 M) solution. The resulting dark brown GO solution was stored for further use.
Cr-Mn-doped TiO 2 nanowires (NW) were prepared according to the previously reported literature [49]. In brief, the molten salt flux method was used for the synthesis of the Cr-Mn-doped TiO 2 NW. A total of 0.50 g of TiO 2 powder was mixed with the 2.0 g of NaCl and 0.50 g of NaHPO 4 in a 1:4:1 ratio, and 2.5% atomic percentage of Mn(NO 3 ) 2 and Cr(NO 3 ) 3 dopants. The whole reaction mixture was finely ground in a pestle mortar to form a fine powder mixture and then calcined in a crucible at the temperature of 825 • C for eight hours in a muffle furnace (ramping rate of 2 • C/min). After calcination, the prepared mixture was allowed to naturally cool down at room temperature, and then it was thoroughly washed with water to remove unreacted salts and dried in an oven at 80 • C for eight hours.

Synthesis of Cr-Mn-Doped TiO 2 /Graphene Oxide Aerogels
Individually synthesized GO and Cr-Mn-doped TiO 2 NWs were used to prepare Cr-Mn-doped TiO 2 NW @GO aerogels using the polymerization method. In brief, 2.5 mL (0.2 g/100 mL) of GO was taken and diluted with 48.5 mL of distilled water followed by the addition of 0.5 g/50 mL of Na alginate (at 50 • C) and 0.16 g of Cr-Mn-doped TiO 2 NW; the whole reaction mixture was stirred for 20 min at room temperature to ensure polymerization. Later on, the gel was dropped into CaCl 2 solution (1.0 M) followed by freeze-drying for 4-5 h to obtain Cr-Mn-doped TiO 2 NW @GO aerogels.

Characterization of Cr-Mn-Doped TiO 2 /Graphene Oxide Aerogels
The synthesized Cr-Mn-doped TiO 2 NW @GO aerogel was characterized through X-ray diffraction (XRD, Jeol JDX-3532 diffractometer, Tokyo, Japan) to study the crystal structure of the aerogel. Scanning Electron Microscopy (SEM, Quanta 250, FEG (Waltham, MA, USA) analysis was conducted to determine the surface morphology. Energy Dispersive X-ray (EDX) analysis was performed to identify the elements present in the synthesized material. Fourier transform infrared spectroscopy (FTIR, Bruker IFS 125HR Japan, Tokyo, Japan) was conducted to identify the functional groups. The Brunauer-Emmett-Teller (BET) surface area and pore size analysis were performed by measuring the N 2 adsorptiondesorption isotherms.

Statistical Analysis
The optimization of the operational reaction parameters, i.e., the size of the aerogels, irradiation time, pH, and oxidant concentration, was performed using Design Expert 7 pro software under Response Surface Methodology (RSM). RSM includes different designs, these designs are three-level factorial, central composite (CCD), Box-Behnken (BBD), and D-optimal [50]. Here, we used RSM in combination with central composite design (CCD) to optimize four operational reaction parameters; these parameters were chosen as independent variables while the degradation rate of the dye was chosen as the output response variable. The list of variables, and their actual lower and higher levels for optimization are given in Table 1. CCD design (CCD) was adopted to evaluate the combined effect of the four independent variables by 30 sets of experiments. A general expression of the mathematical relationship describing the response of four independent variables (A, B, C, D) can be approximated by quadratic polynomial Equation (1): Y = β + β 1 X 1 + β 2 X 2 + β 3 X 3 + β 4 X 4 + β 12 X 12 + β 2 X 2 + β 33 X 32 + β 44 X 42 + β 12 X 1 X 2 + β 13 X 1 X 3 + β 14 X 1 X 4 + β 23 X 2 X 3 + β 24 X 2 X 4 + β 34 X 3 X 4 (1) where Y presented the predicted response and β is the coefficient constant. While β 1 , β 2 , β 3 , and β 4 are the linear effect coefficients, β 11 , β 22 , β 33 , and β 44 are the quadratic effect coefficients, and β 12 , β 13 , β 14 , β 23 , β 24 , and β 34 are the interaction effect coefficients. X1, X 2 , X 3 , and X 4 are the independent variables corresponding to A, B, C, and D, respectively.  10 15 The data were analyzed by analysis of variance (ANOVA), and the mean values were considered of significant difference when p < 0.0001. The optimal values of the operational parameters were estimated by the three-dimensional response surface analysis of the independent variables and the dependent variable.

Photocatalytic Degradation Potential of Acid Black Dye by Cr-Mn-Doped TiO 2 NW @GO Aerogels
Solar light-assisted photocatalytic degradation of Acid Black 1 dye (AB 1) (C 36 H 23 N 5 Na 2 O 6 S 2 ) by Cr-Mn-doped TiO 2 NW @GO aerogels photocatalyst was evaluated in borosilicate glass reactors (500 mL) filled with the Cr-Mn-doped TiO 2 NW @GO aerogel membrane (12.5 mm thickness). A reaction mixture containing a 500 ppm solution of AB 1 dye was exposed to solar radiation (one Sun illumination, wavelength = 530 nm) at conditions optimized by RSM in the solar photocatalytic reactor (Scheme 1). The rate of the degradation was measured with a UV-Vis spectrophotometer (Maximum wavelength = 620 nm) followed by the degradation % calculation using Equation 1 dye was exposed to solar radiation (one Sun illumination, wavelength = 530 nm) at conditions optimized by RSM in the solar photocatalytic reactor (Scheme 1). The rate of the degradation was measured with a UV-Vis spectrophotometer (Maximum wavelength = 620 nm) followed by the degradation % calculation using Equation (2): (Degradation (%) = (A0 − At)/(A0) * 100 (2) where Ao: Absorbance of Dye at Zero minutes, At: Absorbance of Dye at time t. The water quality parameters of the treated and untreated AB 1 dye-containing textile industrial effluents such as BOD, COD, and TOC were analyzed by using previously reported literature (Ahmad et al., 2019). Scheme 1. Schematics of the photocatalytic reactor design and experimental setup for the degradation of AB 1 dye.

Characterization of Cr-Mn-doped TiO2 NW @GO Aerogel
The crystal structure of the prepared Cr-Mn-doped TiO2@GO aerogel was studied through XRD analysis ( Figure 1a). The appearance of the XRD diffraction peaks at 2θ° values of 27.5°, (110), 36.04 (101), 41.31 (111), 54.28 (211), and 55.3° (102) confirmed the presence of the rutile phase of TiO2 [51], while the absence of diffraction peaks corresponding to the oxides or metallic phase of Cr-Mn dopants confirmed the successful substitution of Cr-Mn with the Ti in the TiO2 crystal structure. Furthermore, the poor crystalline nature of the dopant will favor enhanced oxygen vacancies and the catalytic nature of the Cr-Mn-doped TiO2@GO aerogel. Moreover, the appearance of a diffraction peak at 2θ = Scheme 1. Schematics of the photocatalytic reactor design and experimental setup for the degradation of AB 1 dye.

Characterization of Cr-Mn-Doped TiO 2 NW @GO Aerogel
The crystal structure of the prepared Cr-Mn-doped TiO 2 @GO aerogel was studied through XRD analysis ( Figure 1a). The appearance of the XRD diffraction peaks at 2θ • values of 27.5 • , (110), 36.04 (101), 41.31 (111), 54.28 (211), and 55.3 • (102) confirmed the presence of the rutile phase of TiO 2 [51], while the absence of diffraction peaks corresponding to the oxides or metallic phase of Cr-Mn dopants confirmed the successful substitution of Cr-Mn with the Ti in the TiO 2 crystal structure. Furthermore, the poor crystalline nature of the dopant will favor enhanced oxygen vacancies and the catalytic nature of the Cr-Mn-doped TiO 2 @GO aerogel. Moreover, the appearance of a diffraction peak at 2θ = 10.9 • corresponded to the (100) plane of GO [52], confirming the presence of GO and the successful formation of the Cr-Mn-doped TiO 2 NW @GO aerogel.  10.9° corresponded to the (100) plane of GO [52], confirming the presence of GO and the successful formation of the Cr-Mn-doped TiO2 NW @GO aerogel. In order to analyze the structure and morphology of synthesized Cr-Mn-doped TiO2@GO aerogel the SEM analysis was carried out (Figure 1b). A fine network of Cr-Mndoped TiO2 NWs surrounding the nanosheets of GO can be clearly seen in SEM images, which confirmed that Cr-Mn-doped TiO2 NW is successfully grafted over GO nanosheets. During the polymerization process for the formation of aerogel, Cr-Mn-doped TiO2 NWs completely attached to the surface of GO sheets which makes it difficult to differentiate between the Cr-Mn-doped TiO2 NWs and the GO sheets. Further, the elemental analysis and purity of the Cr-Mn-doped TiO2@GO aerogel were confirmed by EDX (Figure 1c-g). It is quite obvious from Figure 1c-f that the Ti, O, and C (a component of GO) in the prepared sample are present in higher concentrations, while Mn and Cr, being the dopants, are present in a lesser concentration in the EDX mapping. All these elements are the main components of the prepared Cr-Mn-doped TiO2@GO aerogel.
The specific surface area (SBET) and porosity of the prepared Cr-Mn-doped TiO2@GO aerogel were evaluated with the help of N2 adsorption-desorption isotherms through BET. From the results, it was obvious that the incorporation of Cr-Mn in TiO2 enhanced In order to analyze the structure and morphology of synthesized Cr-Mn-doped TiO 2 @GO aerogel the SEM analysis was carried out (Figure 1b). A fine network of Cr-Mndoped TiO 2 NWs surrounding the nanosheets of GO can be clearly seen in SEM images, which confirmed that Cr-Mn-doped TiO 2 NW is successfully grafted over GO nanosheets. During the polymerization process for the formation of aerogel, Cr-Mn-doped TiO 2 NWs completely attached to the surface of GO sheets which makes it difficult to differentiate between the Cr-Mn-doped TiO 2 NWs and the GO sheets. Further, the elemental analysis and purity of the Cr-Mn-doped TiO 2 @GO aerogel were confirmed by EDX (Figure 1c-g). It is quite obvious from Figure 1c-f that the Ti, O, and C (a component of GO) in the prepared sample are present in higher concentrations, while Mn and Cr, being the dopants, are present in a lesser concentration in the EDX mapping. All these elements are the main components of the prepared Cr-Mn-doped TiO 2 @GO aerogel.
The specific surface area (S BET ) and porosity of the prepared Cr-Mn-doped TiO 2 @GO aerogel were evaluated with the help of N 2 adsorption-desorption isotherms through BET. From the results, it was obvious that the incorporation of Cr-Mn in TiO 2 enhanced its S BET (79.3 m 2 /g) compared to the pure TiO 2 (46.2 m 2 /g). This enhancement is attributed to the control of the crystal grain size due to the incorporation of dopants. The surface area was further increased upon the addition of GO and the formation of an aerogel structure (280.2 m 2 /g). These results confirmed that the synthesized materials are highly porous in nature ( Table 2). The high porosity of aerogel provides a microchannel for the efficient interaction of pollutants with the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane. Moreover, the conjugation of Cr-Mn-doped TiO 2 NW with the GO nanosheets will not only enhance the surface area for pollutant interaction but also facilitate the photogenerated electron-hole separation and their transmission between the GO nanosheets and Cr-Mn-doped TiO 2 NW.  Figure 1d) the prominent broad peak from 500-1000 cm −1 corresponding to the Metal-O/Ti-O bonds in the crystal lattice and with the C of GO (Ti-O-C). A broad absorption peak at 3340 cm −1 was due to the stretching vibration of the O-H (hydroxyl) group of water molecules, which indicated the existence of water molecules. Additionally, the presence and conjugation of GO in the Cr-Mn-doped TiO 2 @GO aerogel is confirmed from the characteristic stretching vibration peaks at 1030 cm −1 (epoxy C-O), 1425 cm −1 (C-OH) and 1570 cm −1 (ketone or carboxylate C=O) [53]. These hydroxyl and other oxygenated functional groups present at the corners and on the basal planes of GO could form bonds with the Ti-O and with functional groups of alginate to form a robust Cr-Mn-doped TiO 2 @GO aerogel matrix.

Statistical Evaluation of Photo Catalytic Activity of Cr-Mn-Doped TiO 2 @GO Aerogel
The RSM was utilized to optimize the conditions for the rapid photodegradation of AB 1 dye. Three-dimensional response surfaces were generated by using the Design Expert 7 pro software, which presented the visual relationship between different variables [54,55]. The RSM results of 30 runs were obtained on the basis of polynomial Equations (1) and (3) and are provided in Table 3. According to the ANOVA, the p-value was less than 0.0001, which means that the model was significant; on the other hand, the p-value greater than 0.1000 showed that the model was not significant. Furthermore, the p-value of <0.0001 and the agreement of adj. R 2 (0.9148) and R 2 (0.9559) values with the reduced C.V. value of 8.34% showed that the model is highly significant, while the lack of a fit test showed good predictability. In the case of A, B, AB, AC, BC, and CD, A2, B2, C2, and D2 are significant model terms. The interactive effect of these variables in terms of response surfaces is shown in Figure 2. An ANOVA table was formed by applying design expert software for checking the AB 1 degradation rate from wastewater. If more values are insignificant, then the reduction in the model improves the model. The level of the f-value of the lack of fit applied to the model is not significant relative to the pure error. Hence, a non-significant lack of fit is good. Three-dimensional surfaces graphically represent the interaction between two variables while the other variables are kept constant (Figure 2). pH and oxidant concentration are very sensitive factors that influence the degradation of AB 1; their interactive effect in terms of the 3D response surface over the AB 1 degradation is given in Figure 2a. As the value of the pH increases from acidic to neutral, the degradation of the dye is increased and reaches a maximum value at neutral pH. Upon further increasing the pH of the AB 1 dye sample, the degradation rate decreased. Therefore, pH 7 is optimum for the degradation of AB 1 dye. The rate of AB 1 degradation was enhanced from 1-3 mmol of oxidant concentration, and upon the further addition of oxidant concentration, the degradation rate decreased.
dation but up to a certain limit, i.e., neutral pH and an irradiation time of 180 min ( Figure  2c), after which, more time did not affect the degradation rate. The size of the aerogel has a reasonable effect on the degradation rate of the dye (Figure 2d); upon an increment in the size of the aerogel, the % degradation of AB 1 dye is increased. However, upon a further increase in the aerogel size from 12.50 mm, no prominent effect in the rate of AB 1 degradation was observed. Hence, at the optimum aerogel size of 12.50 mm at neutral pH (7), maximum AB 1 degradation was achieved. Further increases in pH decreased the degradation rate to the extent to which the aerogel size was increased. Conclusively, according to the RSM response surfaces, the optimized parameters were pH = 7, solar irradiation time = 180 min, oxidant concentration = 1-3 mmol, and at an aerogel size of 12.50 mm, the maximum degradation of AB 1 dye was achieved. Higher or lower values than the optimized conditions led to decreases in the AB 1 degradation rate, either due to changes in the surface charge (pH effect), surface area change (size of aerogel), or the scavenging or self-decomposition of hydroxyl free radicals (effect of irradiation and oxidant) produced during photocatalysis that are responsible for AB 1 dye degradation.

Evaluation of Extent of AB 1 Dye Degradation by Cr-Mn-doped TiO2@GO Aerogel
The AB 1 degradation by a Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane in the photocatalytic reactor was evaluated by comparing the UV/Vis spectra of AB 1 before and after treatment with a Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane. The extent of the degradation of AB 1 was evaluated by UV/Vis spectroscopy at 620 nm, and the reduction in the peak corresponding to AB 1 dye after treatment with Cr- The interactive effect of irradiation time and pH also influences the rate of AB 1 degradation (Figure 2b). An increase in irradiation time along with pH increased AB 1 degradation but up to a certain limit, i.e., neutral pH and an irradiation time of 180 min (Figure 2c), after which, more time did not affect the degradation rate. The size of the aerogel has a reasonable effect on the degradation rate of the dye (Figure 2d); upon an increment in the size of the aerogel, the % degradation of AB 1 dye is increased. However, upon a further increase in the aerogel size from 12.50 mm, no prominent effect in the rate of AB 1 degradation was observed.
Hence, at the optimum aerogel size of 12.50 mm at neutral pH (7), maximum AB 1 degradation was achieved. Further increases in pH decreased the degradation rate to the extent to which the aerogel size was increased. Conclusively, according to the RSM response surfaces, the optimized parameters were pH = 7, solar irradiation time = 180 min, oxidant concentration = 1-3 mmol, and at an aerogel size of 12.50 mm, the maximum degradation of AB 1 dye was achieved. Higher or lower values than the optimized conditions led to decreases in the AB 1 degradation rate, either due to changes in the surface charge (pH effect), surface area change (size of aerogel), or the scavenging or self-decomposition of hydroxyl free radicals (effect of irradiation and oxidant) produced during photocatalysis that are responsible for AB 1 dye degradation.

Evaluation of Extent of AB 1 Dye Degradation by Cr-Mn-Doped TiO 2 @GO Aerogel
The AB 1 degradation by a Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane in the photocatalytic reactor was evaluated by comparing the UV/Vis spectra of AB 1 before and after treatment with a Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane. The extent of the degradation of AB 1 was evaluated by UV/Vis spectroscopy at 620 nm, and the reduction in the peak corresponding to AB 1 dye after treatment with Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane confirmed the efficient degradation of AB 1 dye (Figure 3a). Moreover, the AB 1 dye degradation was kinetically monitored (Figure 3b); it is quite obvious from the results that within 30 min of exposure to the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane, almost 50% of the dye was degraded, whereas in the case of blank (no membrane) and oxidant (no membrane), no remarkable AB 1 degradation was observed, confirming the efficient role of Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane in AB 1 degradation, with complete removal at 180 min.
Membranes 2022, 12, x FOR PEER REVIEW 10 Mn-doped TiO2@GO aerogel photocatalytic membrane confirmed the efficient degr tion of AB 1 dye (Figure 3a). Moreover, the AB 1 dye degradation was kinetically m tored ( Figure 3b); it is quite obvious from the results that within 30 min of exposure t Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane, almost 50% of the dye wa graded, whereas in the case of blank (no membrane) and oxidant (no membrane), n markable AB 1 degradation was observed, confirming the efficient role of Cr-Mn-d TiO2@GO aerogel photocatalytic membrane in AB 1 degradation, with complete rem at 180 min. Further, FTIR of treated and untreated AB 1 dye solution was compared. From F 4, it is clear that peaks corresponding to AB 1 dye clearly vanished, confirming the effi degradation of AB 1 dye in wastewater. Peaks corresponding to stretching vibration sociated with the N-H (3200-3400 cm −1 ), C-N (1000-1200 cm −1 ), and N=N azo bonds (1 1490 cm −1 ) completely disappeared after treatment with the Cr-Mn-doped TiO2@GO ogel photocatalytic membrane, confirming that dye is mineralized into hydrocarbon cause of the absence of other aromatics, specifically aryl amines peak (1600-1650 cm −1 as depicted in Figure 4. Further, FTIR of treated and untreated AB 1 dye solution was compared. From Figure 4, it is clear that peaks corresponding to AB 1 dye clearly vanished, confirming the efficient degradation of AB 1 dye in wastewater. Peaks corresponding to stretching vibrations associated with the N-H (3200-3400 cm −1 ), C-N (1000-1200 cm −1 ), and N=N azo bonds (1400-1490 cm −1 ) completely disappeared after treatment with the Cr-Mndoped TiO 2 @GO aerogel photocatalytic membrane, confirming that dye is mineralized into hydrocarbons because of the absence of other aromatics, specifically aryl amines peak (1600-1650 cm −1 ) [56] as depicted in Figure 4. Mn-doped TiO2@GO aerogel photocatalytic membrane confirmed the efficient degradation of AB 1 dye (Figure 3a). Moreover, the AB 1 dye degradation was kinetically monitored ( Figure 3b); it is quite obvious from the results that within 30 min of exposure to the Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane, almost 50% of the dye was degraded, whereas in the case of blank (no membrane) and oxidant (no membrane), no remarkable AB 1 degradation was observed, confirming the efficient role of Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane in AB 1 degradation, with complete removal at 180 min. Further, FTIR of treated and untreated AB 1 dye solution was compared. From Figure  4, it is clear that peaks corresponding to AB 1 dye clearly vanished, confirming the efficient degradation of AB 1 dye in wastewater. Peaks corresponding to stretching vibrations associated with the N-H (3200-3400 cm −1 ), C-N (1000-1200 cm −1 ), and N=N azo bonds (1400-1490 cm −1 ) completely disappeared after treatment with the Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane, confirming that dye is mineralized into hydrocarbons because of the absence of other aromatics, specifically aryl amines peak (1600-1650 cm −1 ) [56] as depicted in Figure 4. A quantitative assessment of the degradation of the pollutants from water quality parameters has been analyzed before and after photocatalytic treatment with a Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane. The % reduction in the COD, TOC, and BOD in wastewater has been measured. From Figure 5, it can be seen that COD is 71%, BOD~79%, and TOC~61% reduced, confirming the high efficiency of the prepared Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane. Figure 4. FTIR spectra of AB 1 dye wastewater before and after photocatalytic treatment with Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane.
A quantitative assessment of the degradation of the pollutants from water quality parameters has been analyzed before and after photocatalytic treatment with a Cr-Mndoped TiO2@GO aerogel photocatalytic membrane. The % reduction in the COD, TOC, and BOD in wastewater has been measured. From Figure 5, it can be seen that COD is ⁓71%, BOD ⁓79%, and TOC ⁓61% reduced, confirming the high efficiency of the prepared Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane.

Figure 5.
Water quality parameters evaluated before and after with Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane.

Reusability of Cr-Mn-doped TiO2@GO Aerogel Photocatalytic Membrane
The Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane can be facilely separated from the reaction system for recycling. The photocatalyst with the remarkable potential of AB 1 degradation can be easily separated from the solution and can be reused with unvaried performance. Herein, the results showed that as the number of cycles of reusability increased, initially, the efficiency of the catalyst was retained, and then it continuously decreased. However, even after eight cycles of reuse, the catalyst retained its efficiency of dye degradation of more than 50% in wastewater ( Figure 6). Here, the first cycle means that catalyst has already been used.

Reusability of Cr-Mn-Doped TiO 2 @GO Aerogel Photocatalytic Membrane
The Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane can be facilely separated from the reaction system for recycling. The photocatalyst with the remarkable potential of AB 1 degradation can be easily separated from the solution and can be reused with unvaried performance. Herein, the results showed that as the number of cycles of reusability increased, initially, the efficiency of the catalyst was retained, and then it continuously decreased. However, even after eight cycles of reuse, the catalyst retained its efficiency of dye degradation of more than 50% in wastewater ( Figure 6). Here, the first cycle means that catalyst has already been used.

Mechanism of Degradation of AB 1 Dye by Cr-Mn-doped TiO2@GO Aerogel
In the first step, (a) Cr-Mn-doped TiO2@GO aerogel photocatalytic membrane, upon exposure to solar light radiation, generates an electron-hole pair (e − h + ). h + is located in the valence band (VB) that is formed upon the excitation of efrom the VB to the conduction band (CB); the availability of both e − and h + causes the redox reaction to take place [35,57].

Mechanism of Degradation of AB 1 Dye by Cr-Mn-Doped TiO 2 @GO Aerogel
In the first step, (a) Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane, upon exposure to solar light radiation, generates an electron-hole pair (e − h + ). h + is located in the valence band (VB) that is formed upon the excitation of e − from the VB to the conduction band (CB); the availability of both e − and h + causes the redox reaction to take place [35,57]. In the second step (b), the Cr-Mn-doped TiO 2 @GO aerogel adsorbed the water molecules through GO functional groups. These water molecules become oxidized by the h + , resulting in the formation of hydroxyl ions (OH − ). In the third step, (c) OH − upon further oxidation formed a hydroxyl radical ( • OH), which attacks AB 1 dye, leading to its degradation into lower hydrocarbons through an oxidation reaction. Another possibility (d) of dye degradation is attributed to the presence of e − at the CB. This e − will reduce the O 2 molecule at the CB, resulting in the formation of • OH and superoxide anion free radicals (O 2 − ), leading to the reductive degradation products of AB 1 dye. (a) Redox reaction involved in photocatalytic degradation of AB 1 dye by Cr-Mndoped TiO 2 @GO aerogel photocatalytic membrane: Moreover, GO supports the Cr-Mn-doped TiO 2 and prevents charge recombination by enhancing the charge transfer to the AB 1 dye, leading to more efficient, rapid, and effective AB 1 dye degradation. Because rapid charge recombination slows down or even stops the photocatalytic activity, the presence of GO facilitates the charge separation along with the charge transfer at the surface, thus enhancing the reaction kinetics and rate of degradation of the dye. Because photocatalysis is a surface phenomenon, increased surface area leads to enhanced photocatalysis. The incorporation of GO enhanced the surface area of Cr-Mn-doped TiO 2 . Further, • OH radical generation and the availability of active sites are proportional to the enhanced surface area. All these factors helped the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane to degrade upon exposure to solar radiation.

Conclusions
TiO 2 doping is among the hot topics, due to its high catalytic performance toward the mitigation of environmental pollution; however, the band gap reduction and separation of charge holes require strategic modification by the incorporation of multiple metals. TiO 2 doping with Cr/Mn could reduce the band gap but has fewer adsorption capacities to selectively remove organic pollutants, especially when found in an aqueous form. GO has great potential to overcome this challenge due to its excellent absorption properties, and its conversion to an aerogel-based membrane could expose most of the reactive sites of all elements. The prepared Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane possesses the ability to directly absorb solar radiation to degrade dyes.
Herein, the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane was successfully synthesized by the polymerization method using sodium alginate as a linker. Characterizations of the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane proved it to be pure and highly crystalline, and it successfully grew over the GO nanosheets. The hydrophilic nature and higher photocatalytic properties of the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane successfully degraded AB 1 dye in textile industry wastewater upon exposure to solar radiation, as confirmed by FTIR analysis, a kinetics study of AB 1 dye degradation, and a UV-Visible spectrophotometer. Moreover, a clear~80% reduction in COD (reduction), BOD, and TOC were observed that defines the Cr-Mn-doped TiO 2 @GO aerogel photocatalytic membrane as a potential material for the economical and efficient remediation of toxic pollutants released in textile industry effluents to make that water reusable.