Catalytic Removal of Alizarin Red Using Chromium Manganese Oxide Nanorods: Degradation and Kinetic Studies

: Dye removal through photocatalytic degradation employing nanomaterials as catalysts is a growing research area. In current studies, photocatalytic alizarin red (AR) dye degradation has been investigated by designing a series of Cr based manganese oxide nanomaterials (MH1–MH5). Synthesized nanomaterials were characterized by powder X-ray di ﬀ raction, scanning electron microscopy / energy dispersive x-ray, Brunauer–Emmett–Teller, and photoluminescence techniques and were utilized for photocatalytic AR dye degradation under UV light. AR dye degradation was monitored by UV–visible spectroscopy and percent degradation was studied for the e ﬀ ect of time, catalyst dose, di ﬀ erent dye concentrations, and di ﬀ erent pH values of dye solution. All the catalysts have shown more than 80% dye degradation exhibiting good catalytic e ﬃ ciencies for dye removal. The catalytic pathway was analyzed by applying the kinetic model. A pseudo second-order model was found the best ﬁtted kinetic model indicating a chemically-rate controlled mechanism. Values of constant R 2 for all the factors studied were close to unity depicting a good correlation between experimental data.


Introduction
Textile dyes produce large amounts of effluents having contaminants that are often discharged to surface water bodies and groundwater aquifers [1]. These discharges are a major source of concern as they have considerable potential to disturb the local and downstream ecosystems. Printing, textile, papermaking, pharmaceutical, food processing, and cosmetics industries can all be sources of effluent containing significant concentrations of dyes and pigments [2][3][4][5]. Most of the dyes which are being used in the textile industries are usually stable and nonbiodegradable [6]. These dyes belong to the class of synthetic organic compounds and are considered as major environmental pollutants due to their large-scale production, high aromaticity, chemical stability and toxicity [7][8][9]. To reduce the risk of environmental pollution, it is necessary to treat wastewater reserves before discharging them into the environment [10]. Various physical and chemical processes have been employed for dye degradation in wastewater treatment. However, these processes cannot be effectively employed to treat the wide range of wastewater effluents for dye degradation due to their high operation costs, energy consumption, reduced efficiency due to the chemical stability of dyes and/or the complexity of their degradation [11].
However, the recent trend is to combine techniques for sustainability, as an oxidation/catalysis or adsorption/catalysis to overcome the drawbacks of conventional techniques [12]. The photocatalytic degradation of dyes by nanoparticles as catalysts is a growing research area due to its quick oxidation, no formation of polycyclic products, and oxidation of pollutants [13]. Particles in the nanometer size range have a very large surface area, thereby offering many binding sites to the reacting species. A huge number of transition metal oxides catalysts were synthesized and employed for dye degradation purposes employing direct sunlight or UV light [14]. Metal oxide-based nanomaterials are widely employed due to their flexibility to modulate several characteristics-i.e., structure, composition, morphology, and surface area enhancements [15][16][17]. Oxides like TiO 2 , ZnO, and CdS are effective with UV light, easily available, relatively inexpensive and chemically stable photocatalysts [18][19][20][21]. Doped/undoped manganese oxides nanomaterials are promising materials due to their low cost, high stability, and good environmental compatibility [22].
Alizarin red (AR) has been widely employed in the textile industry [23]. It is a major textile effluent and has been used for the photocatalytic degradation of alizarin red in an aqueous medium [24]. Various studies were performed for photodegradation of AR dye using various photocatalysts. Joshi and Shrivastava used ZnO and TiO 2 as photocatalysts for AR dye degradation under UV and attained maximum removal in 200 min with very low initial dye concentration (20-60 ppm) [25]. Kansal et al. [26] used ZnO nanoparticles for AR dye degradation and achieved 77% degradation in 90 min by using a UV chamber.
In the present study, chromium-doped manganese oxide nanomaterials (MH1-MH5) were synthesized by the hydrothermal method. The effect of the time of contact between the synthesized nanomaterials and the AR dye, different catalyst doses, various dye concentrations in the aqueous solutions, and variating pH of the dye solutions on nanomaterials were investigated. The kinetic parameters have been evaluated to study the reaction mechanisms.

Elemental Analysis by EDX
The elemental analysis of all the synthesized materials (MH1-MH5) was carried out by energy dispersive X-ray (EDX) analysis technique coupled with scanning electron microscopy (SEM). The spectrographs obtained in this case are presented in supplemental data (Figure S1). The elemental composition found in this respect is reported in Table 1 along with other parameters. The results in Table 1 for nanomaterials (MH1-MH5) indicate that the analyzed elemental compositions are in close agreement with the expected elemental percentages in accordance with the reactants used for synthesis. The high-intensity peaks of Mn for MH1-MH5 indicate the presence of Mn with a major elemental composition, with percentage ranging from 48.05 to 54.88%. For the percentage composition ranges for Cr (5.06-6.26%) and O (37.90-43.68%), a minor presence was observed. EDX patterns are shown for MH1-MH5 in supplemental data ( Figure S1).

Powder X-Ray Diffraction (PXRD) Analysis
The phase composition and particle size of the synthesized materials were characterized by PXRD and the data obtained are reported in Table 2. The PXRD patterns of nanomaterials (MH1-MH5) are presented in Figure 1. Powder patterns for samples were matched to standard patterns of nanomaterials. Specific hkl values were evaluated corresponding to 2 theta values. These hkl values were matched with standard PXRD patterns for α-MnO 2 (JCPD No.44-0141), (JCPD No. 01-1127), CrO 3 (JCPD No. 74-0326, 70-3766). The PXRD of (MH1-MH5) depicted the presence of α-MnO 2 as a major phase with slight incorporation of CrO 3 and MnCrO 4 . It was indicated by the powder patterns that the peak for α-MnO 2 showed a gradual decrease in intensity as the concentration of Cr increased from MH1 to MH5 indicated by the peaks for CrO 3 . The major phase α-MnO 2 is tetragonal in structure having P4_2/mnm space group. These peak assignments in PXRD jointly with EDX results indicate that the MH1-MH5 samples are monocrystalline materials with the elemental composition given in Table 2. After the PXRD phase assignments, other parameters-i.e., the crystallite size, dislocation densities, crystallite volume, and microstrain, were calculated for all the synthesized materials and are reported in Table 2. The particle size calculation was performed using the Debye Sherrer formula [27] as follows:  After the PXRD phase assignments, other parameters-i.e., the crystallite size, dislocation densities, crystallite volume, and microstrain, were calculated for all the synthesized materials and are reported in Table 2. The particle size calculation was performed using the Debye Sherrer formula [27] as follows: where D is the particle size of the sample and λ is the wavelength of the X-ray diffraction beam and has a value of 1.5405980. The angle at which diffraction occurs is cos θ, and β is the FWHM (full width at half maximum) value [28]. The volume (V) of crystalline material was calculated using the relation (V= D 3 ) [29]. The dislocation density (δ), which represents the extent of defects in the sample, is defined as the length of the dislocation lines per unit volume of the crystal and is calculated using Equation (2) [30].
The strain-induced broadening in powders due to crystal imperfection and distortion is calculated as a microstrain (ε) [31]. Positive values of ε represent a tensile strain while negative values represent a compressive strain. Strain-induced broadening (microstrain) in nanomaterials was calculated by Equation (3) [31] as follows: The average crystallite sizes for (MH1-MH5) at their corresponding intensities were 44.02 for MH1, 38.90 for MH2, 49.60 for MH3, 50.22 for MH4 and 36.82 for MH5. It was noted that the crystallite size is increased by the inclusion of chromium which causes the crystal stress as depicted in Table 2. The values of dislocation densities show the extent of defects, which are fewer in nanomaterials (MH1-MH5). Nanomaterials with smaller crystallite sizes showing greater stabilities. Values for crystalline volume indicates that there is proper incorporation of chromium ions into regular lattice sites of O − in the samples. The inclusion of Cr ions in the α-MnO 2 structure causes an increase in the strain and increases crystal imperfections. Positive values depicted the tensile microstrain in MH1-MH5 nanomaterials which can be due to the lattice shrinkage.

Scanning Electron Microscopy (SEM) Analysis
Surface morphological characteristics were analyzed for all the synthesized series of nanomaterials by SEM and are reported in Table 3. SEM images of representative material MH5 are shown in Figure 2 while other nanomaterials (MH1-MH4) are enlisted in the supplemental data  and the surface morphological characteristics are reported in Table 3. SEM images indicated that synthesized nanomaterials (MH1-MH5) are crystalline and monodisperse. The surface morphological characteristics indicated crystalline rod-like structures for all the synthesized nanomaterials.  Table 3. SEM images of representative material MH5 are shown in Figure 2 while other nanomaterials (MH1-MH4) are enlisted in the supplemental data ( Figure S2a-d) and the surface morphological characteristics are reported in Table 3. SEM images indicated that synthesized nanomaterials (MH1-MH5) are crystalline and monodisperse. The surface morphological characteristics indicated crystalline rod-like structures for all the synthesized nanomaterials.

Brunauer-Emmett-Teller (BET) Analysis
The specific surface area of nanomaterials (MH1-MH5), calculated by multipoint BET, and pore structure parameters were calculated from the DFT method (based on molecular modelling and takes into account direct interaction of adsorbate with the adsorbent surface) and are enlisted in Table 4. The specific surface area of nanomaterials was in the range of 20-46 m 2 /g. MH3 has the maximum surface area of 46.32 m 2 /g, whereas MH5 has a surface area of 20.45 m 2 /g but smaller pore volumes-

Brunauer-Emmett-Teller (BET) Analysis
The specific surface area of nanomaterials (MH1-MH5), calculated by multipoint BET, and pore structure parameters were calculated from the DFT method (based on molecular modelling and takes into account direct interaction of adsorbate with the adsorbent surface) and are enlisted in Table 4. The specific surface area of nanomaterials was in the range of 20-46 m 2 /g. MH3 has the maximum surface area of 46.32 m 2 /g, whereas MH5 has a surface area of 20.45 m 2 /g but smaller pore volumes-0.038 and 0.032, respectively-indicating materials were microporous. Values of the constant (C) for MH2, MH3, and MH4 were <100, which indicates a strong adsorbate-adsorbent interaction. For MH1 and MH5, the values are high due to the high porosity of the material. Comparison of the experimental data with the BET results showed that varying the Cr concentration during the synthesis of nanomaterials (Section 2.2), significantly affected the specific surface area and pore size distribution of nanomaterials. The PL emission spectra of nanomaterials (MH1-MH5) have been obtained through a PL spectrophotometer with an exciton wavelength of 420 nm. The Cr-doped manganese oxide nanomaterials exhibited some sharp and broad PL peaks in the visible region from 420 to 700 nm. Figure 3 shows the PL spectra of nanomaterials (MH1-MH5) giving prominent peaks at 435, 452, 488, 588 and 630 nm corresponding to 2.85, 2.74, 2.54, 2.11 and 1.97 eV energy range. In this region, the emission results from the radiative recombination of the photogenerated hole with an electron occupying oxygen vacancy [32]. An increase in the emitted intensity in the visible region was observed in PL peaks with increasing Cr-doping [33] which most probably originated from the near band edge (NBE) due to the recombination of free exciton through exciton-exciton collision processes [32]. The narrower PL peaks also designate a lower trap density [34] in the synthesized materials which is advantageous for them to be applied as efficient photocatalysts [35]. Hence, synthesized nanomaterials may show prominent catalytic efficiencies.
The PL emission spectra of nanomaterials (MH1-MH5) have been obtained through a PL spectrophotometer with an exciton wavelength of 420nm. The Cr-doped manganese oxide nanomaterials exhibited some sharp and broad PL peaks in the visible region from 420 to 700nm. Figure 3 shows the PL spectra of nanomaterials (MH1-MH5) giving prominent peaks at 435, 452, 488, 588 and 630 nm corresponding to 2.85, 2.74, 2.54, 2.11 and 1.97 eV energy range. In this region, the emission results from the radiative recombination of the photogenerated hole with an electron occupying oxygen vacancy [32]. An increase in the emitted intensity in the visible region was observed in PL peaks with increasing Cr-doping [33] which most probably originated from the near band edge (NBE) due to the recombination of free exciton through exciton-exciton collision processes [32]. The narrower PL peaks also designate a lower trap density [34] in the synthesized materials which is advantageous for them to be applied as efficient photocatalysts [35]. Hence, synthesized nanomaterials may show prominent catalytic efficiencies.

Photocatalytic Degradation of AR by Nanocatalysts (MH1-MH5)
Photocatalytic AR dye degradation activity (see Section 3.4) carried out under a UV light and sample aliquots were taken at different time intervals. Absorbance at 505 nm was measured by the UV spectrophotometer for all the aliquots taken during the photocatalytic AR dye degradation under the UV light at different time intervals. For the absorbance, the percentage of dye degradation was calculated as described in Section 3.4. The peaks at 505 nm tend to decrease with the increase in the photocatalytic reaction time indicating the decomposition of the aromatic ring. Similarly, different factors like the effect of time, catalytic dose, initial dye concentrations, and pH were investigated by calculating the percentage of dye degradation.
The absorption spectrum of the dye solution (without catalyst) before and after keeping it without UV light irradiation for 180 min is given in Figure S3a (supplemental data) whereas the UV spectra of AR solution at start and after 180 min of UV light irradiation is given in Figure S3b (supplemental data). No change was observed in the AR dye solution after keeping it 180 min without UV light irradiation whereas a very minute change was observed for the dye solution irradiated with UV light for 180 min. The photocatalytic AR dye degradation with all synthesized nanomaterials (MH1-MH5) at various time intervals was calculated and showed more than 80% degradation of dye as shown in Figure S4 (supplemental data). Hence, the effect of different factors on the percent dye degradation is elaborated for most active catalysts in discussion below.

Effect of Time
Photocatalytic dye degradation under UV light was investigated at different time intervals-i.e., 30, 60, 90, 120, 150, and 180 min and the percentage of dye degradation values calculated are shown in Table 5. Nanomaterials (MH1-MH5) have shown a degradation of more than 40% dye after 30 min of photocatalytic dye degradation under UV light. An increase in the percentage of dye degradation was observed with every increasing time interval. A dye degradation of more than 80% was calculated at the end of photocatalytic activity by nanomaterials. Among all the nanomaterials MH1 and MH5 showed maximum photocatalytic efficiencies with 88.99 and 88.89% dye degradation, respectively (Figure 4). The correlation between the percentage of dye degradation and photocatalytic activity time was found to be significant as indicated by R 2 values ( Figure 5). The correlation between the percentage of dye degradation and photocatalytic activity time for (MH2-MH4) are presented in Figure S5 (supplemental data).

Effect of Photocatalyst Dosage
The effect of the catalyst dose on the dye degradation was investigated by changing the catalysts (MH1-MH5) dosage-i.e., 0.020, 0.025, 0.030, 0.035, and 0.040 g/L for 60 min irradiation of UV light as shown in Figure 6. The percent degradation of the AR dye (200 ppm) was calculated for (MH1-

Effect of Photocatalyst Dosage
The effect of the catalyst dose on the dye degradation was investigated by changing the catalysts (MH1-MH5) dosage-i.e., 0.020, 0.025, 0.030, 0.035, and 0.040 g/L for 60 min irradiation of UV light as shown in Figure 6. The percent degradation of the AR dye (200 ppm) was calculated for (MH1-MH5) nanomaterials and is given in Table 6. The percentage of dye degradation was found increase with increasing catalyst dose from 0.020 g/L to 0.040 g/L for nanomaterials. An increase in the percentage of dye degradation was observed by increasing the dose of the catalysts. A dye degradation of more than 38% was observed for 0.020 g of catalysts (MH1-MH5) dose which got increased to more than 80% for all the nanomaterials by increasing dose to 0.040 g. The highest dye degradations (83.78 and 84.34%) were achieved after 60 min irradiation of UV light for dosage 0.040 g/L of MH4 and MH5. The maximum increase in the dosage of nanomaterials (MH1-MH5) (0.040 g/L) results in a maximum dye degradation, possibly contributed by an increased surface provided by catalyst resulting in more photons AR arriving at the surface of catalysts. The correlation between the percentage of dye degradation and catalysts doses were found to be significant as the R 2 values calculated were close to 1 (Figure 7). The correlations between the percentage of dye degradation and catalyst dose for (MH1-MH3) are presented in Figure S6 (supplementary data).
Catalysts 2020, 10, x FOR PEER REVIEW 9 of 18 calculated were close to 1 (Figure 7). The correlations between the percentage of dye degradation and catalyst dose for (MH1-MH3) are presented in Figure S6 (supplementary data).    Catalysts 2020, 10, x FOR PEER REVIEW 9 of 18 calculated were close to 1 (Figure 7). The correlations between the percentage of dye degradation and catalyst dose for (MH1-MH3) are presented in Figure S6 (supplementary data).

Effect of Dye Concentration
The effect of AR dye solution concentration on its degradation percentage was investigated by changing the solution concentrations-i.e., 50, 100, 150, 200, and 250 ppm-for photocatalytic activity with 60 min UV light irradiation (shown in Figure 8). The percentage of dye degradation of different

Effect of Dye Concentration
The effect of AR dye solution concentration on its degradation percentage was investigated by changing the solution concentrations-i.e., 50, 100, 150, 200, and 250 ppm-for photocatalytic activity with 60 min UV light irradiation (shown in Figure 8). The percentage of dye degradation of different AR dye solution concentrations was calculated for (MH1-MH5) nanomaterials and is given in Table 7. The percentage of dye degradation of AR was found to be decreased with increasing concentrations of dye solution from 50 ppm to 250 ppm for nanomaterials. More than 80% dye degradation was observed for when 50 ppm solution of AR was employed for catalysts (MH1-MH5), which decreased to 60% for all the nanomaterials with increasing solution concentrations up to 250 ppm. of dye solution from 50 ppm to 250 ppm for nanomaterials. More than 80% dye degradation was observed for when 50 ppm solution of AR was employed for catalysts (MH1-MH5), which decreased to 60% for all the nanomaterials with increasing solution concentrations up to 250 ppm.
The highest dye degradations (83.87% and 83.54%) were achieved after 60 min irradiation of UV light for 50 ppm of dye solution by catalysts MH3 and MH5. In contrast, increasing dye concentrations gradually from 50 ppm to 250 ppm tended to decrease the percentage of dye degradation to 61.11% for MH3 and 63.21% for MH5. The correlation between the percentage of dye degradation and dye concentrations was found to be significant as the R 2 value calculated was close to 1 (Figure 9). The correlation between the percentage of dye degradation and different dye concentrations for (MH1-MH3) are presented in Figure S7 (supplemental data).
The decreased degradation at higher AR dye concentrations is ascribed to the adsorption of more dye molecules on the surface of the catalyst. This results in a decrease in the number of active sites on the catalyst surface. Additionally, an increase in dye concentration causes a decrease in the number of photons reaching the surface of catalysts, resulting in less UV light available for the exciton of catalyst particles due to more possible absorption by dye molecules.    The highest dye degradations (83.87% and 83.54%) were achieved after 60 min irradiation of UV light for 50 ppm of dye solution by catalysts MH3 and MH5. In contrast, increasing dye concentrations gradually from 50 ppm to 250 ppm tended to decrease the percentage of dye degradation to 61.11% for MH3 and 63.21% for MH5. The correlation between the percentage of dye degradation and dye concentrations was found to be significant as the R 2 value calculated was close to 1 (Figure 9). The correlation between the percentage of dye degradation and different dye concentrations for (MH1-MH3) are presented in Figure S7 (supplemental data).
Catalysts 2020, 10, x FOR PEER REVIEW 11 of 18 Figure 9. Correlation between different dye concentrations (50-250 ppm) and percentage degradation of alizarin red (AR) dye by 0.025 g of nanomaterials MH1 and MH5 after 60 min of photocatalytic activity.

Effect of pH of the Medium
The effect of pH of AR dye solution (200 ppm) on its percent degradation was investigated by changing the solution pH from acidic (pH = 3, 5), neutral (7) and basic (9,11) for photocatalytic activity with 60 min UV light irradiation (shown in Figure 10). The percentage degradations of different AR dye solutions with different pH were calculated for (MH1-MH5) nanomaterials and are given in Table 8. The percentages of dye degradation were found to be increased with increasing solution pH of AR from acidic to basic medium. Almost 25-29% dye degradation was observed at acidic pH (3 and 5). The percentage degradation of AR was increased up to 48-50% with a neutral medium (pH 7) of dye solution. In total, 64-79% dye degradation was observed at basic pH (9, 11) of dye solution. The highest dye degradation (76.97% and 78.51%) was achieved after 60 min irradiation of UV light for pH 11 of dye solution by catalysts MH4 and MH5. The correlation between the percentage of dye degradation and pH of AR dye solution was found to be significant as the R 2 value calculated was close to 1 ( Figure 11). The correlation between the percentage of dye degradation and different pH of dye solution for MH1-MH3 are presented in Figure S8 (supplemental data). MH5 was an efficient catalyst, among all showing more than 80% dye degradation for all the factors studied, indicating the positive influence on degradation by increasing Cr doping.  The decreased degradation at higher AR dye concentrations is ascribed to the adsorption of more dye molecules on the surface of the catalyst. This results in a decrease in the number of active sites on the catalyst surface. Additionally, an increase in dye concentration causes a decrease in the number of photons reaching the surface of catalysts, resulting in less UV light available for the exciton of catalyst particles due to more possible absorption by dye molecules.

Effect of pH of the Medium
The effect of pH of AR dye solution (200 ppm) on its percent degradation was investigated by changing the solution pH from acidic (pH = 3, 5), neutral (7) and basic (9,11) for photocatalytic activity with 60 min UV light irradiation (shown in Figure 10). The percentage degradations of different AR dye solutions with different pH were calculated for (MH1-MH5) nanomaterials and are given in Table 8. The percentages of dye degradation were found to be increased with increasing solution pH of AR from acidic to basic medium. Almost 25-29% dye degradation was observed at acidic pH (3 and 5). The percentage degradation of AR was increased up to 48-50% with a neutral medium (pH 7) of dye solution. In total, 64-79% dye degradation was observed at basic pH (9, 11) of dye solution. The highest dye degradation (76.97% and 78.51%) was achieved after 60 min irradiation of UV light for pH 11 of dye solution by catalysts MH4 and MH5. The correlation between the percentage of dye degradation and pH of AR dye solution was found to be significant as the R 2 value calculated was close to 1 ( Figure 11). The correlation between the percentage of dye degradation and different pH of dye solution for MH1-MH3 are presented in Figure S8 (supplemental data). MH5 was an efficient catalyst, among all showing more than 80% dye degradation for all the factors studied, indicating the positive influence on degradation by increasing Cr doping.

Kinetic Study
Pseudo kinetic models, such as pseudo-first-order kinetics and pseudo-second-order, are frequently used to explain the adsorption kinetics of solid/liquid systems [36][37][38]. The kinetics of dye degradation on the surface of nanomaterials (MH1-MH5) was illustrated best by the pseudo-secondorder kinetic model. The equation of the pseudo-second-order reaction is as follows:

Kinetic Study
Pseudo kinetic models, such as pseudo-first-order kinetics and pseudo-second-order, are frequently used to explain the adsorption kinetics of solid/liquid systems [36][37][38]. The kinetics of dye degradation on the surface of nanomaterials (MH1-MH5) was illustrated best by the pseudo-secondorder kinetic model. The equation of the pseudo-second-order reaction is as follows: Pseudo-second order chemical reaction kinetics of dye degradation at different time intervals provides the best correlation of the experimental data, which are close to the unity depicted in Figure   Figure 11. Correlation between different pH and percentage degradation of alizarin red (AR) dye (200 ppm) by 0.02 g of nanomaterials MH1 and MH5 after 60 min of photocatalytic activity.

Kinetic Study
Pseudo kinetic models, such as pseudo-first-order kinetics and pseudo-second-order, are frequently used to explain the adsorption kinetics of solid/liquid systems [36][37][38]. The kinetics of dye degradation on the surface of nanomaterials (MH1-MH5) was illustrated best by the pseudo-second-order kinetic model. The equation of the pseudo-second-order reaction is as follows: Pseudo-second order chemical reaction kinetics of dye degradation at different time intervals provides the best correlation of the experimental data, which are close to the unity depicted in Figure 12, and the reaction mechanism is found to be chemically rate-controlling [36]. As UV-light falls upon the metal oxide catalyst, electrons (e-) of the nanoparticles are excited from the valence band (VB) to the conduction band (CB), which results in generating the positive hole (h+) in the valence band. The electrons in the CB or stored in nanorods are trapped by the O 2 molecule to form a reactive superoxide anion radical (O 2− ). In contrast, the positive holes present in the VB react with water molecule and form hydroxyl radicals (OH). These created radicals are highly reactive toward the photo degradation of AR dye [8]. A proposed mechanism of AR dye degradation is illustrated in Figure 13.
Catalysts 2020, 10, x FOR PEER REVIEW 13 of 18 anion radical (O 2− ). In contrast, the positive holes present in the VB react with water molecule and form hydroxyl radicals (OH). These created radicals are highly reactive toward the photo degradation of AR dye [8]. A proposed mechanism of AR dye degradation is illustrated in Figure 13.  For all the factors, the correlation was found for dye concentration after 60 min or photocatalytic activity under UV light with the factor studied enlisted in Table 9. Values of correlation constant R 2 at different time intervals of photocatalytic activity were close to unity, as shown in Figure S9 of the supplementary data. Values of correlation constant R 2 for different catalyst doses, dye concentrations, and pH values were close to unity, as shown in Figures S10-S12 (supplemental data). anion radical (O 2− ). In contrast, the positive holes present in the VB react with water molecule and form hydroxyl radicals (OH). These created radicals are highly reactive toward the photo degradation of AR dye [8]. A proposed mechanism of AR dye degradation is illustrated in Figure 13.  For all the factors, the correlation was found for dye concentration after 60 min or photocatalytic activity under UV light with the factor studied enlisted in Table 9. Values of correlation constant R 2 at different time intervals of photocatalytic activity were close to unity, as shown in Figure S9 of the supplementary data. Values of correlation constant R 2 for different catalyst doses, dye concentrations, and pH values were close to unity, as shown in Figures S10-S12 (supplemental data). For all the factors, the correlation was found for dye concentration after 60 min or photocatalytic activity under UV light with the factor studied enlisted in Table 9. Values of correlation constant R 2 at different time intervals of photocatalytic activity were close to unity, as shown in Figure S9 of the supplementary data. Values of correlation constant R 2 for different catalyst doses, dye concentrations, and pH values were close to unity, as shown in Figures S10-S12 (supplemental data).

Chemicals and Reagents
KMnO 4 , K 2 Cr 2 O 7, HCl and AR dye were purchased from Sigma Aldrich (Saint Louis, MO, USA). All the chemicals were of analytical grade and used without further purification.

Synthesis of Chromium Manganese Oxide Nanomaterials (MH1-MH5)
The MH1 sample was synthesized as 0.948 g of KMnO 4 and 0.24 g of K 2 Cr 2 O 7 were dissolved in 50 mL of water and shifted to a Teflon line parr reactor. In total, 20 mL of HCl was added subsequently to the mixture. The reaction mixture was stirred for 30 min and the parr reactor was sealed at heated at 180 • C for 24 h. After, the heat treatment reactor was cooled down to the room temperature and opened. A dark brown powdered solid was obtained at the base of reactor. The powdered solid was filtered off and washed with adequate water. The MH2-MH5 samples were prepared by the same method used for MH1 by using 0.41, 0.35, 0.18 and 0.30 g of K 2 Cr 2 O 7 for synthesis, respectively. The synthesized MH1-MH5 were obtained as a dark brown powder and were characterized and utilized for catalytic studies. Their found unit formula, powder X-ray diffraction (PXRD) peaks and composition (by energy dispersive X-ray (EDX)), along with the synthetic conditions, are summarized in Table 1.

Characterization Techniques
Powder X-ray diffraction (PXRD) measurements were performed in an X-ray diffractometer (Bruker, AXS D8; Yokohama-shi, Japan) with Cu-Kα radiation (1.5406 Å) at a scanning rate of 10 • per min in the 2θ range from 10 • to 90 • . The morphological features of catalysts were characterized by a scanning electron microscope (SEM) (JEOL, JSM-6360 EO; Tokyo, Japan), and the elemental composition was estimated by an energy dispersive X-ray (EDX) diffractometer (JEOL JSM-6360 LV; Tokyo, Japan). The surface area and pore structure parameters were calculated by a Brunauer-Emmett-Teller (BET) analysis performed through Quantachrome Nova 2200e (Tokyo, Japan). A sonicator (Bransonic 2510EMT bath; Darmstadt, Germany) was used for the surface area deployment of nanomaterials. A PTFE(polytetrafluoroethylene)-lined, Stainless Steel, Parr 5521 high-pressure compact Autoclave was used for the temperature and pressure treatments. A UV lamp (length: 288 mm, pipe diameter: 16 mm, voltage: 220 V, power; 8 W, wavelength range: 240-285 nm) was used as a light source for photocatalytic alizarin red (AR) dye degradation activity. The catalytic dye degradation was studied by an ultraviolet (UV) spectrophotometer (Shimadzu; Osaka, Japan) in the frequency range of 250-800 nm.

Photocatalytic Activity
Photocatalytic alizarin red (AR) dye degradation was conducted according to previous reports [19,39,40] with some modifications. A 200 ppm solution of AR dye was prepared in water. Blank experiments were conducted for the AR dye solution without the presence of catalyst. One of the blank experiments was conducted by keeping the sample of dye solution for 180 min without UV light irradiation and the absorption spectra were taken at different time intervals. Another experiment was done by irradiating the dye solution for 180 min and the UV spectra were measured at various time intervals by taking sample aliquots.
To measure the photocatalytic activity in the presence of a catalyst, 50 mL of 200 ppm AR dye solution was subjected to stirring in beaker under UV light source in the presence of 0.02 g catalysts (MH1). Aliquots of 5 mL from reaction mixture were taken at different time intervals of 0, 30, 60, 90, 120, 150 and 180 min. is Catalysts in sample aliquots were spun down by a centrifuge for 1 min and were separated from reaction mixture. The separated catalyst was not added back to the reaction mixture. After the separation of the catalyst, the UV spectrum was measured from the UV spectrophotometer for all the taken aliquots at different time intervals. From the spectrum, absorbance at 501 nm was noted and the percentage degradation of AR was calculated for all the sample aliquots by following Equation (5) [39].
% Degradation = C o − C t /C o × 100 (5) where C o is the initial concentration and C t is the concentration at different sampling times. The concentration of AR after the reaction time was measured by the calibration curve between the absorbance and known AR concentration. The unknown AR concentration for the measured absorbance at different time intervals was calculated by the linear equation of known data. The same method was adopted to check the catalytic activity for all other catalysts (MH2-MH5). The percentage degradation of the AR dye was calculated by variating the initial dye concentration (50, 100, 150, 200 and 250 ppm), the change in catalyst dose (0.015, 0.02, 0.025, 0.03 g, 0.035 g 0.040 g) and the pH of the dye solution (3,5,7,9,11) after 60 min of photocatalytic activity under UV light irradiation by following the same procedure as described above. To study the effect of different pHs of the AR dye solution, the pH was adjusted by employing HCl for an acidic pH and NaOH for a basic medium.

Conclusions
The presented work describes photocatalytic AR dye degradation by a series of Cr-based manganese oxide nanomaterials (MH1-MH5). All the catalysts have shown more than 70% AR dye degradation, exhibiting the good catalytic efficiencies for dye removal. MH1 and MH5 showed more than 85% dye degradation for the effect of time intervals, whereas MH4 and MH5 showed more than 80% dye degradation for different catalyst doses. MH3 and MH5 showed more than 60% dye degradation for different dye concentrations employed. MH4 and MH5 showed more than 70% dye degradation for different pH values of dye solution. The catalytic pathways were analyzed by applying the kinetic model. The pseudo second order was found to be the best fitted kinetic model, indicating a chemically rate controlled mechanism. Values of constant R 2 for all the factors studied were close to unity, depicting good correlation between the factors studied. MH5 was an efficient catalyst among all, showing more than 80% dye degradation for all the factors studied.