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Visible Light Assisted Photocatalytic Degradation of Chromium (VI) by Using Nanoporous [Fe.sub.2][O.sub.3].

1. Introduction

The pollution of unsafe metals is rising with widespread of industrial expansion [1]. Living point of view the metal chromium arises from its outstanding role in pollution in the form of industry and its toxicity to plants, animals, and microbes [2]. The vast amount of chromium is introduced into the surroundings throughout various sources like dyeing in fabric industries, compound manufacture, leather built-up, and metal plating which affect living organisms and cells [3, 4]. Chromium is in two oxidation states, Cr (III) and Cr (VI) hexa-valent, which is 600 times more toxic than the trivalent form in systematic manner [5, 6]. Several investigations have revealed that bulky sum of chromium (VI) is considered to be necessary for usual metabolic process [7]. The release into the sewage system causes a solemn environmental collision and chromium occurs mainly in Cr(III) form, which is oxidized into chromium (VI) due to the occurrence of organic compounds [8]. The ceiling stage of Cr (VI) tolerable in wastewater is 0.05 mg [L.sup.-1] [9]. However, the superior levels of chromium metals have been found to be toxic to the internal organs [10,11]. World health organization (WHO) has evidently stated that chromium (VI) is carcinogenic [9]. The human toxicity includes lung cancer and liver and gastric spoil [12 ]. Nowadays, the majority of the industries are facing the tricky problem of discarding of chromium (VI) in wastewater produced in huge quantity. Hexavalent chromium form chromate (Cr[O.sub.4.sup.-2]) is considerably more soluble in water than trivalent chromium Cr(III) [13]. The chromium elimination treatment includes adsorption process, ion exchange, precipitation, reverse osmosis, and photocatalysis [14]. Most of these methods require high capital and recuring expenses and, therefore, they are not suitable for small-scale industries [15, 16]. Photocatalysis has been attracting growing interest because it provides a new, promising way to meet the environmental challenges of energy and sustainability; semiconductor photocatalysis has conventional much attention because of its high efficiency without inferior pollution [17]. In recent advances in efficient photocatalytic systems, the synthesis of high-performance photocatalysts with good recyclability is the one of the most important purposes; deplorably, the broad band gap of metal oxides confines the reasonable maintenance under visible light [18]. Moreover, the recovery of photocatalysts requires tedious steps during recycling. Therefore, recent research works have focused on developing magnetically separable visible light responsive photocatalysts to carefully use sunlight [19, 20]. Most of the researchers used [Fe.sub.2][O.sub.3] as a magnetic core of the nanoparticle for effective efficiency and utilization [21, 22]. Hydrothermal methods were usually used to prepare nanoderivatives so as to effectively be utilized for visible light-driven activity [23]. The systematic photocatalytic removal of Cr(VI) from wastewater is by using emerging materials in the batch progression experiments (Figure 19). The chromium was removed using photocatalyst with UV light and in the gloomy at diverse pH range. The utmost removal of Cr(VI) was pragmatic in pH 2 and Ti[O.sub.2] showed peak ability for Cr(VI) removal than Ti[O.sub.2] thin film [24]. To the best of our knowledge, there are no reports on the structured nanoporous facile [Fe.sub.2][O.sub.3] and its impact on the photocatalytic degradation of chromium (VI) under visible light. In the present study, we have effectively synthesized magnetic [Fe.sub.2][O.sub.3] from hydrothermal method in the deficiency of any capping agents. The physical properties such as morphology, element analysis, and surface area of the nanoporous have been studied in detail, and toxicity of the samples was measured using Mus musculus skin melanoma cells to clear determination of toxicity of the synthesized nanomaterial with an appraisal for the degradation of Cr(VI) in systematic manner under visible light irradiation to be compared with different emerging materials in this paper (Scheme 1). Therefore, this organized approach provides a better option upconversion material for fundamental photoabsorption. Moreover, the stability and durability of the [Fe.sub.2][O.sub.3] have also been studied via recycling experiments.

[formula not reproducible]

2. Material and Methods

2.1. Chemicals. Ferrous sulfate heptahydrate (99+%, Sigma Aldrich), ferric sulfate hydrate (97%, Fe 21.6%, Sigma Aldrich), ammonium hydroxide solution (28-30%, Sigma Aldrich), hydrochloric acid (37%, Fisher Scientific), and methanol (ACS reagent, Fisher Scientific) were used without any further purification. Ferrous chloride tetrahydrate (Fe[Cl.sub.2] x 4[H.sub.2]O Merck>99%), ferric chloride hexahydrate (Fe[Cl.sub.3] x 6[H.sub.2]O Merck>99%), salts, and ammonium hydroxide (N[H.sub.4]OH Merck, 25% of ammonia) were used for the synthesis of iron oxide nanoparticles. Chemicals were of reagent grade and used without further purification; deionized water was purified by a Barnstead/Synbron Nanopure-II purification system.

2.2. Hydrothermal Preparation of Nanoporous [Fe.sub.2][O.sub.3]. The nanoporous [Fe.sub.2][O.sub.3] were synthesized using hydrothermal method. Fe[Cl.sub.3] and NaOH, ammonia (Aldrich, India), 10.14 g (37.5mol) Fe[Cl.sub.3] x 6[H.sub.2]O, and 7.45g (37.5mmol) Fe[Cl.sub.2] x 4[H.sub.2]O were dissolved into 25 ml of distilled water. Twenty-five milliliters of twenty-five percent ammonia was added to the salt solution under stirring condition at 700 rpm for 2 min right after 15 ml of mixture was put into a Teflon-lined stainless Morey autoclave, and the autoclave is heated to 180[degrees]C in an oven and maintained at 12 h reaction time. Temperature plays a vital role in the formation of well-defined spherical product. Autoclave was naturally cooled to room temperature, and the precipitates were washed with distilled water and isolated under magnet. The final products were dried at 60[degrees] C and were ready for characterization.

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2.3. Characterizations and Measurements. Magnetic measurement was accomplished using a vibration sample magnetometer (VSM, ADE EV9 Model). Brunauer-Emmett-Teller (BET) precise exterior area, aperture volume, and aperture size allocation of the samples determined by [N.sub.2] adsorption at 77 K by a Micromeritics ASAP, 2020, physico-adsorption analyzer. XRF band was recorded by Minipal- 4 benchtop model, with fine focus X-ray tube, MO target of multilayer monochromator of 17.5KeV for efficient elemental study. Ultima III Series, RIGAKU, TSX System, Japan, with Cu radiation (wavelength 1.54[degrees]A) at room temperature was used for wide-angle X-ray diffraction (XRD) patterns of the [Fe.sub.2][O.sub.3] recyclability studies. Scanning electron microscopic (SEM) images of the [Fe.sub.2][O.sub.3] was captured by HITACHI (S-3400 N, Japan) with 10 kV acceleration voltages. The [Fe.sub.2][O.sub.3] was measured using UV-Vis Spectrophotometer. The spectral analysis of [Fe.sub.2][O.sub.3] was carried out by measuring the optical density (OD) using Beckman Coulter, (DU739, Germany) scanning UV-Vis Spectrophotometer operated at a resolution of 1 nm between 280 and 800 nm. The Energy Dispersive Spectroscopy (EDS) analysis was carried out using HITACHI (Noran System-7, USA) system close to SEM for the finding of composite nanoparticles. The particle size distribution (PDS) and zeta capacity of iron oxide nanoparticle were observed with Microtrac (USA) element size monitor. The analyzer gives the size dimension and verification of particle size distribution. The concentration of profound metals in waste water is calculated with Inductively Coupled Plasma Atomic Emission Spectroscopy techniques (ICP-AES) using the Perkin-Elmer Optima 8000, ICP-OES; photocatalytic study was performed on a Heber-Immersion type photoreactor (HIPR-Compact, p-8/125/250/400) in an efficient manner and, finally, pH was observed by with an EU-TECH instrument pH meter. The electron-pore division development was confirmed by photoluminescence (PL) spectroscopy under an excitation wavelength of 325 nm.

2.4. Photocatalytic Study. The stock solution of Cr(VI) 1,000 ppm was equipped by dissolving [K.sub.2]Cr[O.sub.4] (A.R. grade); in the double-distilled water, batch studies were carried out using chromate stock solution to 1 L. The absorbance of chromate solution was calculated. The calibration curve of Cr(VI) was obtained at [lambda]max = 540 nm. An identical particle size of the photocatalysts was among 120 to 500 [mu] size for photocatalytic dreadful conditions. In every study, an accurately weighted amount of [Fe.sub.2][O.sub.3] was used in 100 ml solution by maintaining pH ofhexavalent chromium solution by means of adding of necessary amount of A.R. grade NaOH and HCl (E. Merck India) in the different flask at pH 2-7; the flask was sighted in Heber-Immersion type photoreactor, by steady shaking by magnetic stirrer. The photocatalyst is alienated from the solution by means of centrifugation. The removal efficiency of Cr(VI) was amplified through decline of pH from f 7 to 2. A highest exclusion was established to be high at pH 2, so pH was finalized at pH 2.

2.5. Photocatalysis of Industrial Effluent. The effluent containing Cr(VI) ions was taken away with diphenylcarbazide as masking agent creates the reddish purple color in the solution. The spectral study of [Fe.sub.2][O.sub.3] was performed by measuring the optical density (OD) by UV-Vis spectrophotometer operated at a resolution of 1 nm among 280 and 800 nm. The wastewater sample containing Cr(VI) was introduced with Heber-Immersion type photoreactor. In the test a precisely weighted quantity of photocatalysts was added in the flask, adjusting pH with NaOH and HCl in the different flask at pH 2 to 7. The flask was sited under Heber-Immersion type photoreactor, by steady shaking by magnetic stirrer. The photodegradation percentage was calculated by the following expression:

% Photo-degradation efficiency = [C.sub.0] - C/[C.sub.0] x 100 (2)

To authenticate the sorption of Cr(VI) over the [Fe.sub.2][O.sub.3] nanoporous material in the dark, the tests were carried out under identical conditions without visible light irradiation. The chemical oxygen demand (COD) was deliberate via dichromate oxidation method, subsequent to completion of photodegradation [25].

2.6. Cell Culture and Treatment. Cell culture and treatment of (B16-F10) cells were placed in tissue culture flasks and grown in Dulbecco's modified Eagle's medium (DMEM; Gibco, Thermo Fisher, USA), supplemented through 5% fetal bovine serum (FBS; Gibco, Thermo Fisher) and 1% antibiotic penicillin/streptomycin mixture (Santa Cruz Biotechnology, USA). The culture was maintained in a humidified atmosphere with 5% C[O.sub.2] at 37[degrees]C (Sanyo C[O.sub.2] Incubator; Sanyo, Japan). In the route of choosing the concentration of [Fe.sub.2][O.sub.3], a dose-response curve with different concentrations (0.01, 0.05, 0.1, 0.5, 1.0, and 2.5[micro]g/l) was plotted with explosion and MTT (2-(3,5-diphenyltetrazol-2-ium-2-yl)-4,5-dimethyl1-1,3-thiazole bromide) test, subsequent to 72 h of exposure. The concentrations of nanoporous [Fe.sub.2][O.sub.3] worn in the other experiments were 5, 50, 100, 200, 300, 400, and 500 [micro]g/l for a time tie of 72 h; the studies were carried out in triplicate and three self-determining repetitions [26].

2.7. Statistical Analysis. Three replicates were analyzed for each experiment and by analysis of variance (ANOVA) using SPSS Inc. 16.0. Significant things of treatments were determined by F values (p [less than or equal to] 0.05) and Tukey's HSD test.

3. Results and Discussion

The crystallinity and morphology of the synthesized nonporous materials were characterized by using XRF, SEM and DLS, and PL spectra analysis. The elements in the nanoporous were verified using EDX and elemental mapping analysis. The magnetic properties (coercivity (Hc), saturation magnetization (Ms), and remanent magnetization (Mr)) were studied via VSM analysis. The surface area and pore volume were calculated from the BET analysis. The photocatalytic activity was evaluated for Cr(VI) degradation under visible light irradiation. The electron-hole severance process in [Fe.sub.2][O.sub.3] was significantly supported by the PL analysis. The photochemical stability and reusability of [Fe.sub.2][O.sub.3] by using XRD was also discussed in detail in this article.

3.1. Characterization. X-ray fluorescence is recorded with fine focus X-ray tube, MO target of multilayer monochromator of 17.5KeV. The XRF spectra of [Fe.sub.2][O.sub.3] clearly indicate that the presence of [Fe.sup.+3] XRF patterns of [Fe.sub.2][O.sub.3] nanoporous materials is shown in Figure 1.

The diffraction peaks in 6.2 to 7.2 keV and the concentration of the compound Fe-KA being 153256.1 (cps) are perfectly aligned to the cubic phase [Fe.sub.2][O.sub.3] and are shown in Table 1.

The diffraction peaks related to other inferior phases of iron oxide are not detected. The magnetic parameters such as coercivity, saturation magnetization, and remanent magnetization are shown in Table 2. The Ms value of the nanoporous material is much lower than the other photocatalytic materials. Material is supposed to be renowned that the magnetic behavior (Mr) of [Fe.sub.2][O.sub.3] is still retained after photocatalytic procedure suggesting the suitability of the nanomaterials for magnetic separation and recovery [27, 28]. The decline in Mr for the nanoporous material not only reduces the aggregation of the catalyst but also enhances the reusability.

The nanoporous iron oxide control sample shows only an insignificant uptake at the squat relative pressure end (P/P0< 0.05) and a quick uptake with an H-3 type hysteresis loop at high relative pressure end (P/P0> 0.9), representing that the sample contains predominantly huge mesopores and/or macropores (average size: 28 nm). In this sample, the mesopores/macropores should be the inter-nanoparticle pores resulting from the aggregation of the crystalline [Fe.sub.2][O.sub.3] nanoparticles. The crystalline [Fe.sub.2][O.sub.3] nanoparticles in the iron oxide sample have an estimated average diameter of 16 nm, which is nearly identical to the average size estimated above significantly shows enhanced surface area (354 [m.sup.2] [g.sup.-1]) and pore volume (0.29 [cm.sup.3] [g.sup.-1]). Meanwhile, both surface area and pore volume show slight increases with the increase of the iron oxide pore. Surface area is an important parameter to determine the photocatalytic activity of nanoparticles and is performed by BET surface area analysis, shown in Table 3. A photocatalyst with a high surface area is likely to absorb more Cr(VI) and react more rapidly; [N.sub.2] adsorption-desorption isotherms of [Fe.sub.2][O.sub.3] are depicted. The isotherms of [Fe.sub.2][O.sub.3] are recognized as type-IV as per IUPAC classification, indicating the presence of mesopores [29]. Moreover, the isotherm profiles have [H.sub.2] (P/P0 from 0.4 to 0.9) and H-3 type (P/P0 from 0.9 to 1.0) hysteresis loops, signifying that there are slit-like pores [30]. The surface area and pore volume of the nanoparticle are lower than those of other photocatalytic metals and, in general, surface area is not the crucial factor that determines the photocatalytic activity of silver based catalysts [31, 32].

In the present research, [Fe.sub.2][O.sub.3] nanoporous material clutches the structural and functional groups on the surface, potentially fastening the metals. A piece of iron oxide has a specific number of active coordination sites, depending explicitly on the surface area. Surface of the iron oxides can be very influential for the chance of the allied metal ions to bind to the surface via specific or nonspecific adsorption. Adsorption to iron oxide surface is pH-reliant and slight fluctuations in the pH may engage in recreation in controlling the adsorption-desorption reactions. The morphology of [Fe.sub.2][O.sub.3] was carried out by Scanning Electron Microscopy; SEM images were displayed in Figure 2. [Fe.sub.2][O.sub.3] is mainly composed of spongy like squared nanoporous and the particle length and breadth are 73.92[micro]m and 31.90[micro]m, respectively. However, all the nanoporous materials are closely packed and aggregated. EDX was carried out to examine the elemental composition of the synthesized photocatalysts and the results are shown in Figure 3. The peaks corresponding to Fe and O are clearly observed at their normal energy levels, showing the elemental composition, atomic percentage, and weight percentage of the photocatalysts. Consistent with the XRF results, EDX spectra confirm the purity of the prepared samples.

The histogram of dynamic light scattering analysis for particle size distribution of controllable composites is depicted in Figure 4. DLS analysis is regarded as one of the reliable techniques for evaluating particle size, distribution, and zeta potential of nanoparticles in solution. The present scrutiny revealed the presence of particles with an average diameter of 33.70 nm in the aqueous colloidal solution. The surface charge (zeta potential) of the [Fe.sub.2] [O.sub.3] nanoporous plays a vital role during the interaction with other environmental degradation systems. The particles zeta potential value is 13.9 mV that was entirely determined in the present study. Nanoparticles with a zeta potential of -10 and + 10 mV have a neutral charge; when it is greater than +30 mV or less than -30 mV, it is considered to be strongly cationic and anionic, respectively. Based on the results, the growth mechanisms of the nanoporous [Fe.sub.2][O.sub.3] can be predicted.

In Figure 5, the peaks at 619 [cm.sup.-1] were attributed to the Fe-O bond vibration of the [Fe.sub.2][O.sub.3]. The spectrum showed the bands at 1390 [cm.sup.-1] and 1130 [cm.sup.-1] correspond to the out-of-level surface C-H vibration caused by the relic of triethylamine on the surface of particles and the peaks lying on around 1628.59 [cm.sup.-1] were tentatively assigned to the vibration of C-N bond [33]. The peaks at 3149 [cm.sup.-1] are assigned to the v(N [+ or -] H) vibrations [34], and peaks at 3448 [cm.sup.-1] were assigned to the O-H stretching vibration of absorbed water [35].

A predictable loom is working to estimate the band gap energy (Eg) values [36]. The band gap energy values of [Fe.sub.2][O.sub.3] nanoporous material were calculated to be 2.03 eV correspondingly.

3.2. Photocatalytic Activity. Sequentially to assess the photocatalytic activity of the geared-up samples under the visible light irradiation, the trials were conducted with an initial Cr(VI) concentration of 9 mM, catalyst concentration of 0.75 g/[L.sup.-1], and pH 5. The photocatalytic routine of [Fe.sub.2][O.sub.3] was shown in Figure 6.

pH is the imperative parameter that influences the photocatalytic activity [37]. Trials were performed at pH values of 3.0 (acidic), 5.0 (basel pH of Cr(Vl)), and the 7.0 (neutral) and 9.0 (alkaline). The deliberation of heavy metal and catalyst dosage were fixed at 9mM Cr(VI) and 0.75 g [L.sup.-1] ([Fe.sub.2][O.sub.3]), respectively. The outcome of pH on the photodegradation of Cr(VI) is revealed in Figure 6. The photodegradation percentage of Cr(VI) was found to be 60.24%, 82.12%, 65.17%, and 51.27% for pH 03, 05, 07, and 09 in that order. It is illustrious that the photocatalytic activity is decreased by the increase or decrease of Cr(VI) solution pH.

The photocatalytic activity of ZnO (viable, 99% purity by particle size 0.1-4.0 mM) and Ti[O.sub.2] (marketable, Ti[O.sub.2]-[P.sub.25]) was also willful for reference. The study confirmed that 81.11% of Cr(VI) was degraded by the [Fe.sub.2][O.sub.3] nanomaterials at 120 min of irradiation time. The degradation percentages of Ti[O.sub.2], ZnO, and Cu[O.sub.2] are 56.31%, 69.92%, and 71.32%, respectively. The activity of [Fe.sub.2][O.sub.3] (96.11%) is superior when compared to the standards in Figure 7. This may be credited toward its photosensitizing ability.

As per zero point charge ([P.sub.zc]), the external property of [Fe.sub.2][O.sub.3] is pretentious by the logical change in pH. The values of [Fe.sub.2][O.sub.3] were found to be 7.3, respectively. It is appealing to mention that Cr(VI) is a toxic heavy metal. An acidic environment is helpful to increase the electrostatic lure between the proton from the catalyst and the toxic heavy metal and thus the photodegradation is high. At low pH (below 5), the odds for agglomeration are high, which will decrease the active surface area available for heavy metal adsorption and photon absorption. At finest pH, the predominant iron, namely, Fe [(OH).sup.2+], not only forms Fe (II), the most important catalytic candidates in the photodegradation reactions, but also produces the further [sup.-]OH dependable for heavy metal [38]. pH was greater than [p.sub.zc]; the exterior of [Fe.sub.2][O.sub.3] becomes negatively charged. Consequently, the negatively charged Cr(VI) molecules are repelled by the catalyst surface, and this leads to a decrease in the photocatalytic activity. The extremely alkaline conditions are favorable for the cohort of more number of less reactive high-valence iron species [33].

3.3. Photodegradation Kinetics of Cr(VI). The Langmuir-Hinshelwood model was effectively used to investigate the kinetics of Cr(VI) photodegradation. The photocatalytic experiments were carried out under optimum reaction conditions [[Fe.sub.2][O.sub.3]= 0.75 g/[L.sup.-1], Cr(VI) = 9 mM and pH 5]. Figure 8 shows the logarithmic plot of Cr(VI) concentration as a function of irradiation time. The pseudo-first-order kinetics experiential rate constant for [Fe.sub.2][O.sub.3] is 1.21 x [10.sup.-2] [s.sup.-1] is appreciably higher than of Ti[O.sub.2] (4.36 x[10.sup.-3] [s.sup.-1] ) and Cu[O.sub.2] (6.45 x [10.sup.-3] [s.sup.-1]). Hence, the activity of the [Fe.sub.2] [O.sub.3] nanoporous material is about 2.4 times higher than that of other verified material in systematic manner. Negligible photocatalytic activity was shown by ZnO (2.11 x[10.sup.-3] [S.sup.-1]) in this research.

It is necessary to determine the degree of mineralization of Cr(VI) during photodegradation. Chemical oxygen demand (COD) is a fundamental investigation for assessing the superiority of effluents and wastewaters before release.

It predicts the oxygen obligation of the effluent and is used to monitor and control discharges and to assess the treatment plant routine. The fraction change in COD during photodegradation was measured under optimum reaction conditions [Cr(VI) concentration 9 mM, catalyst concentration 0.75g [L.sup.-1], pH 5, and irradiation time 180 min]. The solutions obtained after 180 min of photodegradation showed an important decrease in COD (59.52%); i.e., after 180 min COD was decreased from 89 mg [L.sup.-1] to 36.14 mg [L.sup.-1]. It has been observed that Cr(VI) molecules were to some extent degraded to intermediates, and small fraction was subjected to complete mineralization, persuading radical scavengers. To further gain insight into the degradation mechanism, the responsibility of the reactive species on the photocatalytic activity was determined using a series of scavengers. The experiments were carried out under optimum reaction conditions (Cr(VI) = 9mM, [Fe.sub.2][O.sub.3] = 0.75 g [L.sup.-1], and irradiation time = 120 min) in the company of scavengers (2 mM for 200 mL heavy metal solution) such as t-BuOH for *OH [39], benzoquinone (BQ) for [O*.sup.2] [40], and potassium iodide (KI) for holes and *OH [41]. The effect of t-BuOH, BQ, and KI on the photodegradation proportion of Cr(VI) is shown in Figure 9. It is perceptibly observed that the photodegradation percentage of Cr(VI) is reduced to 36.79%, 39.75%, 49.69, and 74.11% after the addition of KI, t-BuOH BQ, and, Blank respectively. The photocatalytic activity of the nanoporous was surprisingly concealed in the presence of scavengers, indicating that both [O.sup.2*] and the *OH are enthusiastically occupied in the photodegradation process.

3.4. Photocatalytic Mechanism. The valence band (VB) and the conduction band (CB) positions of [Fe.sub.2][O.sub.3] are expected by using the Butler and Ginley equation [42, 43].

[E.sub.VB] = EVB = x-[E.sup.e] + 0.5 Eg (3)

where x is the fixed electronegativity of the semiconductor, [E.sup.e] is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), [E.sub.VB] is the VB edge potential, and Eg is the band gap energy of the semiconductor. Te CB position can be deduced using the following equation:

[E.sub.CB] = [E.sub.VB] - Eg (4)

The x values of nanoporous [Fe.sub.2][O.sub.3] 3.16, accordingly, the VB and CB of [Fe.sub.2][O.sub.3], were estimated to be 2.11and 0.23 eV, respectively. Through light irradiation, electrons and holes are produced in the CB and VB of [Fe.sub.2][O.sub.3]. The photogenerated electrons can simply drift from the CB of nanoporous [Fe.sub.2][O.sub.3] under the action of a built-in electric field [44]. The holes will stay in the VB of [Fe.sub.2][O.sub.3]. These progression forces successfully extend the duration of charge carriers at the [Fe.sub.2][O.sub.3] nanoporous face. The electrons accumulate on the CB of nanoporous [Fe.sub.2][O.sub.3] corresponding with dissolved oxygen to form a super oxide radical ([O.sup.2*]), and they additionally react with [H.sup.+] to acquiesce to the hydroxyl radical (*OH). The photogenerated holes on the VB of iron oxide can unswervingly oxidize the Cr(VI) or react with [H.sub.2]O to produce hydroxyl radicals (*OH). The metal frame of the Cr(VI) compound is hurriedly tainted by the reactive species ([O.sup.2*], *OH and [h.sup.+]) generated in this photocatalytic process.

3.5. Tauc Relation for Finding Out the Band Gap. Tauc relation is the suitable tool and the appropriate technique to determine the optical absorption spectrum of a nanomaterial; the absorption coefficient a for material is given by

[alpha]h[gamma] = A [(h[gamma] - Eg).sup.n] (5)

The optical band gap was expected by plotting the straight line of [([alpha]h[gamma]).sup.2] v/s photon energy as shown in Figure 10. Bringing to a halt the tangent to the plot provides a good estimate of the band gap energy of nanoporous [Fe.sub.2] [O.sub.3]. The band gap of the hydrothermally synthesized nanoporous [Fe.sub.2][O.sub.3] was investigated that 3.16 eV. nanoporous [Fe.sub.2][O.sub.3] nanocomposite results in a remarkable increase in the photocatalytic activity of Cr(VI).

The electron-hole division method in [Fe.sub.2][O.sub.3] was additionally incorrigible by PL analysis. PL spectra of [Fe.sub.2][O.sub.3] are evidently shown in Figure 11. The PL emission peaks are observed at 416 nm and 440 nm [45, 46]. This is mostly accredited to the effectual separation of charge carriers on the marginal photocatalyst.

3.6. Reusability of Photocatalysts. In order to investigate the stability and durability of the [Fe.sub.2][O.sub.3] nanomaterial recycling experiments were conducted for the photodegradation of Cr(VI), after the successful recycling results of X-ray fluorescence were shown in Table 4. After the completion of each cycle, the photocatalyst was collected using an external magnet, washed with double-distilled water, dried overnight, and utilized. As depicted in Figures 12 and 13, the photodegradation percentages of Cr(VI) for five successive cycles were found to be 78.15%, 74.65%, 72.45%, 71.98%, and 70.24%, respectively; a photocatalyst is one of the most important supplies for unbeaten industrial applications. The [Fe.sub.2][O.sub.3] nonporous materials exhibit 76.24% of photocatalytic activity after five successive cycles in Figure 16 and, in Figures 14 and 15, the decrease in activity following two cycles is credited to the loss of catalyst throughout the easy washing process [47, 48]. In addition, there is no obvious change observed in the XRD pattern of [Fe.sub.2][O.sub.3] in Figure 17, after five cycles. These results indicate that the [Fe.sub.2][O.sub.3] nanoporous materials could be used as a stable photocatalyst for the degradation of heavy metals in industrial wastewater under sunlight as clearly indicated in the XRF pattern, Figure 18.

3.7. Toxicity Test. The exceptional upconversion materials for emitting visible light are the suitable candidates for both in vivo and in vitro bioimaging as well as in photodynamic therapy. In addition to photocatalytic activity, since the upconversion particles need to be used in living cells, the toxicity study is also very important. The outcome showed that the assorted sample decreases the viability of B16-F10 cells in a dose-dependent manner as in Figure 20. As shown in 5 g/mL concentration, the lowest concentration of sample used in the assay did not reduce the cell viability noticeably as compared to other increased concentrations. Among [Fe.sub.2][O.sub.3] found to be low in systematic manner, nanoporous [Fe.sub.2][O.sub.3] is the safest and the superlative alternative in terms of toxicity.

4. Conclusion

In summary, we have demonstrated a low-cost hydrothermal method for the preparation of a visible light responsive and magnetically discrete [Fe.sub.2][O.sub.3]. In such an easy process, the preparation procedure is simplified effectively, and the preparation time has been shortened which is beneficial for industrial production. The [Fe.sub.2][O.sub.3] nanocomposite exhibits the highest photocatalytic activity within 120 min of Cr(VI) and is degraded using 0.75 g [L.sup.-1] of [Fe.sub.2][O.sub.3], The electron-hole separation process in the nanocomposite has also been explained schematically. The high recyclability of [Fe.sub.2][O.sub.3] nanoporous material is expected to have excellent application prospects as a good photocatalyst in wastewater treatment, which needs further investigation. Moreover, the synthesized samples are highly stable in the photocatalytic process and nontoxic to the living cells. Hence, the combination of high photocatalytic stability, nontoxicity, and efficient upconversion properties in the synthesized material paves the mode for its application in photocatalysis, energy renovation, and the biological area.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding this manuscript.


The first author Abhilash M. R. (IF160104) is thankful to the Department of Science and Technology, Govt. of India, New Delhi, for awarding the financial assistance through DST-INSPIRE Junior Research Fellowship to carry-out the research in the University of Mysore, Mysuru 570 006, Karnataka, India.

Supplementary Materials

Overall process involved in visible light assisted photocatalytic degradation of chromium (VI) by using nanoporous [Fe.sub.2][O.sub.3]. (Supplementary Materials)


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Abhilash Mavinakere Ramesh (iD) (1) and Srikantaswamy Shivanna (iD) (1,2)

(1) Department of Studies in Environmental Science, University of Mysore, Manasagangotri, Mysore 570006, India

(2) Centre for Materials Science and Technology, Vijnana Bhavan, University of Mysore, Manasagangotri, Mysore 570006, India

Correspondence should be addressed to Srikantaswamy Shivanna;

Received 14 February 2018; Accepted 19 June 2018; Published 2 September 2018

Academic Editor: Sun-Jae Kim

Caption: Figure 1: XRF spectra of [Fe.sub.2][O.sub.3] nanoporous material.

Caption: Figure 2: SEM images of Cr(VI) adsorbed on [Fe.sub.2][O.sub.3] surface.

Caption: Figure 3: Energy dispersive X-ray analyzer result of [Fe.sub.2][O.sub.3].

Caption: Figure 4: Dynamic light scattering/particle size distribution result of [Fe.sub.2][O.sub.3].

Caption: Figure 5: FT-IR spectrum of nanoporous [Fe.sub.2][O.sub.3].

Caption: Figure 6: Role of pH on the photodegradation of Cr(VI) by [Fe.sub.2][O.sub.3] nanomaterials.

Caption: Figure 7: Photodegradation of Cr(VI) using various photocatalysts.

Caption: Figure 8: Kinetics of Cr(VI) photodegradation using different catalysts.

Caption: Figure 9: Role of different radical scavengers on the photodegradation of Cr(VI).

Caption: Figure 10: Tauc plot for the nanoporous [Fe.sub.2][O.sub.3].

Caption: Figure 10: Tauc plot for the nanoporous [Fe.sub.2][O.sub.3].

Caption: Figure 11: PL spectra of [Fe.sub.2][O.sub.3].

Caption: Figure 12: SEM (500 nm) scale image of Cr(VI) absorbed on the surface on nanoporous [Fe.sub.2][O.sub.3].

Caption: Figure 13: SEM (1 [micro]m) scale image of Cr(VI) absorbed on the surface on nanoporous [Fe.sub.2][O.sub.3].

Caption: Figure 14: SEM (1 [micro]m) scale image of nanoporous [Fe.sub.2][O.sub.3] after the recycle performance (c) recovered Cr(VI) [Fe.sub.2][O.sub.3] form for safe disposal.

Caption: Figure 15: SEM (500 nm) scale image of recovered Cr(VI) [Fe.sub.2][O.sub.3] form for safe disposal.

Caption: Figure 16: Recycling performance of [Fe.sub.2][O.sub.3] after the photodegradation of Cr(VI).

Caption: Figure 17: XRD patterns of [Fe.sub.2][O.sub.3] before and after successful cycles recycling.

Caption: Figure 18: XRF spectra of [Fe.sub.2][O.sub.3] nanoporous material after successful recycling.

Caption: Figure 19: Overall process in removal of Cr(VI) effectively by using [Fe.sub.2][O.sub.3] nanoporous material.

Caption: Figure 20: Toxicity testing of the [Fe.sub.2][O.sub.3] nanoporous material using Mus musculus skin melanoma cells using various concentrations.
Table 1: X-ray fluorescence spectrum compound concentration.

Compound              Al-KA    Cl-KA      Cr-KA    Fe-KA

Concentration (cps)   2.575   5163.511   439.862   19.643

Compound              Cu-KA    Hg-LA    Pb-LA     Fr-MAl

Concentration (cps)   45.822   45.763   17.247   3201.609

Table 2: Room temperature magnetic parameters of [Fe.sub.2][O.sub.3]
nano-porous material.

Catalyst              Coercivity        Remanent
                        Hc (G)       magnetization
                                   Mr (emu [g.sup.-1]

[Fe.sub.2][Q.sub.3]     45.36            0.0489

Catalyst                  Saturation
                      Ms (emu [g.sup.-1])

[Fe.sub.2][Q.sub.3]         0.5169

Table 3: Results from [N.sub.2] sorption characterization.

                      Surface area ([m.sup.2][g.sup.-1]) (a)

Sample                [S.sub.BET]   [S.sub.d<20 [Angstrom]]

porous                    354                 31

                            Surface area
                      ([m.sup.2][g.sup.-1]) (a)

Sample                 %[S.sub.d<20 [Angstrom]]

porous                           11%

                            Surface area
                      ([m.sup.2][g.sup.-1]) (a)

Sample                 [S.sub.d<20 [Angstrom]]

porous                           301

                         Surface area ([m.sup.2][g.sup.-1]) (a)

Sample                %[S.sub.d<20 [Angstrom]]   []

porous                          89%                  0.29

                      Pore volume ([cm.sup.3]
                          [g.sup.-1]) (b)

Sample                [V.sub.d<20 [Angstrom]]

porous                          0.02

                      Pore volume ([cm.sup.3]
                          [g.sup.-1]) (b)

Sample                %[V.sub.d<20 [Angstrom]]

porous                           7%

                         Pore volume ([cm.sup.3]
                             [g.sup.-1]) (b)

Sample                [V.sub.d<20 [Angstrom] (des)]

porous                             0.27

                      Pore volume ([cm.sup.3]         Average
                          [g.sup.-1]) (b)         meso-/macropore
                                                   size (nm) (c)
Sample                %[V.sub.d<20 [Angstrom]]   [D.sub.meso/macro]

porous                          93%                      7

(a) BET surface area ([S.sub.BET]), surface area of micropores
([S.sub.d<20[Angstrom]]) and surface area of meso-/macropores
([S.sub.d<20[Angstrom]]) determined with the t-plot method. The
percentage data denotes the percentage of surface area of micropores
or meso- and macropores relative to the total surface area.

(b) Total pores volume ([]), micro volume
([V.sub.d<20[Angstrom]]) determined with the t-plot method. The
percentage data denote the percentage of pore volume of micropores
relative to the total pore volume.

(c) Average meso-/macropores size ([D.sub.meso/macro]) determined
from the [N.sub.2] desorption data with NLDET model.

Table 4: X-ray fluorescence spectrum compound concentration after
successful recycling cycles.

Compound              Al-KA      Cl-KA          Cr-KA       Fe-KA

Concentration (cps)   1.025   No-intensity   No-intensity   19.643

Compound              Cu-KA   Hg-LA   Pb-LA   Fr-MAl

Concentration (cps)   2.367   6.324   2.145   3.265
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Title Annotation:Research Article
Author:Ramesh, Abhilash Mavinakere; Shivanna, Srikantaswamy
Publication:Journal of Materials
Geographic Code:9INDI
Date:Jan 1, 2018
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