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DOI: 10.1016/j.jmrt.2019.08.038
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Available online 17 October 2019
Highly efficient removal of crystal violet dye from water by MnO2 based nanofibrous mesh/photocatalytic process
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Muniba Rahmata, Asma Rehmanb,
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asmanano@gmail.com

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, Sufyan Rahmatc, Haq Nawaz Bhattia, Munawar Iqbald, Waheed S. Khanb, Sadia Zafar Bajwab, Rubina Rahmate, Arif Nazird
a Department of Chemistry, University of Agriculture, Faisalabad, Pakistan
b Nanobiotechnology Group, National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
c Department of Molecular Biology and Genetics, Eastern Mediterranean University, Cyprus
d Department of Chemistry, The University of Lahore, Lahore, Pakistan
e Department of Physics, University of Punjab, Lahore, Pakistan
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Table 1. Comparison between the photo catalytic performances of MnO2 NFs mesh and other catalysts in the degradation of CV dye under sun light.
Abstract

The present study describes the potential use of 3D MnO2 nanofibrous mesh as photocatalyst for removal of organic dye. The MnO2 nanofibrous mesh has synthesized through facile, one-pot and cost-effective hydrothermal approach without using any template or other structure directing compounds. 3D MnO2 patterns clearly depict the well-dispersed nanofibers having diameter in the range of 10–25nm, and several microns in length; the unique mesh morphology offers a large surface area to promote enhanced photocatalytic activity, and also provided a macro porous network that supported efficient oxidation. Moreover, the strong electronic cloud and high aspect ratio (HAR) MnO2 nanofibers facilitated the fast oxidation of dyes molecule in a very short period of time. The designed nanostructured material was found to possess heterogeneous photocatalytic potential against the oxidative degradation of Crystal Violet (CV) dye taken as model pollutant. The photocatalytic performance of these materials was evaluated under ultra-violet as well as visible light. Best photocatalytic performance was achieved under visible light with almost complete degradation (97%) exhibited within 90min of irradiation time. Furthermore, the degradation efficiency of prepared photocatalyst was determined in presence and absence of initiator, hydrogen peroxide, and under different pH conditions. Finally, this technique can be easily scaled-up for removal of various polycyclic organic wastes that might be of potential industrial and environmental interests.

Keywords:
Manganese oxide
Nanofibrous mesh
Hydrothermal
Photocatalysis
Organic dyes
Visible light
Ultra violet light
Full Text
1Introduction

Water is the most important part of our life, including drinking, agriculture, public hygiene, industry and energy. Unfortunately, increase in world population rate and overharvesting have enhanced the threats for eradication of existing freshwater resources [1–3]. Though, urbanization as well as industrialization is becoming challenging for rapid decline in the water quality. Consequently, it imparts adverse impact on all living creatures [4–9]. Almost, 10,000 different pigments and dyes are produced per annum all around the world. These dyes are excessively used in tannery, textile, cosmetic and paint industries and are non-biocompatible, chemically resistant and carcinogenic due to presence of aromatic ring structures [10–13]. According to Ecological and Toxicological Association of the Dyestuffs (ETAD) Manufacturing Industry survey, a series of tests were conducted on almost 4000 dyes and found LD50 value greater than 2×103mg/kg for more than 90% of dyes [14–18]. Due to toxicological, environmental and chemical aspects, researchers are now focusing in finding of most appropriate methods for treatment of textile industry effluents [19]. Crystal violet (CV) is an industrial synthetic cationic dye with applications in biological staining, dermatological agent, veterinary medicine and dye processing. It is a member of the triphenylmethane family [20]. The CV dye easily interacts with the negatively charged cell membrane surfaces in mammals and enters into cells [21]. Excess inhalation of CV dye causes irritation of the respiratory tracts, vomiting, diarrhea, headache, dizziness and its long-term exposure might damage the mucous membrane. Previously these organic dyes were treated by some conventional protocols adsorption, flocculation, coagulation and reverse osmosis. Variety of adsorbents have been explored so far for wastewater treatment [6,22–26]. Although, this technique was found effective for removal of organic/inorganic pollutants but it is tedious, expensive, need high disposal cost and often ineffective in removing recalcitrant compounds [27].

To overcome these important issues, recently, photocatalytic degradation has got world attention as green technology for degradation of organic as well as inorganic pollutant from wastewater. Previous studies reports variety of adsorbants for removal of CV dye [28–31]. The excellent advantage of this technique is its ability to degrade the toxic dyes into harmless products. Semiconductor photocatalysts have been found highly applicable for effective wastewater treatment as well as for environmental remediation of organic pollutants, such as photocatalytic degradation of organic dyes under UV light and visible light have been achieved through ZnO, TiO2 and SnO [32]. TiO2 NPs have been extensively studied for degradation of organic pollutants due to its chemical stability, non-toxicity and high photocatalytic activity [33]. Additionally, transition metals like, copper, nickel, vanadium, tungsten, manganese and their oxides have used as advanced, economical, green and sustainable photocatalyst for effective oxidation of organic effluents/dyes under mild conditions. Among them manganese oxides have got an extensive attention as photocatalyst because it has more than five easily exchangeable transition states and possession of many structural forms over wide temperatures ranges (up 1200°C). Manganese oxide are also considered as auspicious oxidants for a variety of organic compounds including endocrine disruptors, antibacterial agents and pharmaceuticals. Additionally, the sorption and redox properties of MnO2 NPs (birnessite) have been explored in technical aspect for prospective use in remediation strategies or as water oxidation catalysis [34].

Table 1 shows the effectiveness of different catalyst for degradation of pollutants from industrial effluents.

Table 1.

Comparison between the photo catalytic performances of MnO2 NFs mesh and other catalysts in the degradation of CV dye under sun light.

Sr.no  Catalyst  Light source  Catalyst conc. (mg)  Dye conc. (ml)  Degradation time (min)  Degradation (%)  Ref. 
MoS2NFs  Sun light  20  100  40  99.3  [35] 
ZnO flowers  UV xenon arc lamp (300W)  100  100  80  96  [36] 
CaFe2O3  Microwave irradiation  50  50  10  90  [37] 
Sn@C-dots/TiO2  Sun light  60  100  210  60  [38] 
TiO2  UV-lamp (16W)  200  200  35  100  [20] 
rGo/CuS  Tungsten lamp (200W)  34  100  120  90  [39] 
MnO2NFs mesh  UV mercury lamp (160W)/sunlight  40  40  90  97  This Work 

Similarly, MnO2 nanorods were synthesized using mechanical processing with subsequent heat treatment and their photo catalytic activity was on decolorization of aqueous Rhodamine B at different pH levels. Irrespective of low price and high abundance, manganese oxides have a number of beneficial prospects involving tunable crystal structure and a scalable manufacturing process [40]. Additionally, it has been explored that reduced manganese oxide have ability to be re-oxidized by exposure to dioxygen, it means that manganese oxide can also act as an electron-transfer mediator for effective generation of a quick electron-transfer path during oxidation reactions. Tehseen and co-workers have revealed, that manganese oxides (II, III, IV, and VII) have noteworthy preference over other oxidants in potential remediation of organic contaminants. We have reported the synthesis of MnO2 nanofibrous (MnO2 NFs) mesh by using a one-pot, hydrothermal approach without using any template [41].

Herein we have investigated its potential for photocatalytic degradation of organic dye. In addition, the crystallite and particle size estimates were obtained from X-ray diffraction (XRD) and dynamic light scattering (DLS). Effect of different parameters like pH, addition of initiator on degradation of dye using the synthesized MnO2 nanofibrous mesh has been investigated. This mesh structure ensures high connectivity among fibers; that results in an efficient plasmonic photocatalysts for visible light driven mineralization of aqueous solution of CV dye with degradation rate of 99% in 90min. Furthermore, these lab scale tests can easily scaled-up to commercial scale with small modification and can resolve large industrial and environmental concerns.

2Materials and methods2.1Materials

All the chemicals and reagents used in this study were of analytical grades and purchased from Sigma Aldrich. The solutions were prepared using ultrapure distilled, deionized water (ρ=18MΩ cm) from a Millipore Milli-Q system.

2.2Preparation of MnO2 NFs

The MnO2 NFs mesh was synthesized using simple hydrothermal method as reported earlier [41]. Briefly, an aqueous solution of manganese acetate tetrahydrate (1mM) was prepared and stirred for about 10min at ambient conditions, to ensure homogeneity. In resultant solution, sodium hydroxide (1M; aqueous solution) was added drop wise with stirring, for another 15min. As the result, brown color precipitates were formed instantly. Next, the solution was transferred to a Teflon lined stainless-steel autoclave and placed in an oven at 180°C for 24h (optimized conditions), to complete the hydrothermal reaction. After stipulated time, the autoclave was allowed to cool down at room temperature. Finally, it was centrifuged, rinsed with deionized water dried at 60°C overnight in an oven, and stored in a desiccator at room temperature for further use.

2.3Photocatalytic degradation of crystal violet

Photocatalytic activity of as synthesized MnO2 NFs mesh was investigated for degradation of CV dye as a test contaminant. In a typical batch test, 15mg MnO2 NFs mesh was added as a photocatalyst to 15ml aqueous solution of CV (40ppm) and kept in dark for 30min to obtain the maximum adsorption/desorption of dyes on the catalyst surface). The photocatalytic degradation was carried out under mercury lamp (GYZ 220–230V 160W Philips electronics). The distance among source and reaction vials was approximately 25cm. The mixture of photocatalyst and dye was continuously stirred in air. After every 10min 2ml of final solution was withdrawn, centrifuged to remove the catalysts and analyze using UV–vis spectrophotometer. For kinetic study 40mg of MnO2 NFs mesh were added in the 40mg/100ml dye solution and illuminated under UV radiation and visible irradiation for different time intervals. Effect of initiator on photocatalytic degradation was monitored in the presence of oxidant (H2O2). The degradation efficiency of photocatalyst was evaluated by measuring the decline in maximum absorbance (λmax) of CV at 583nm. To ensure proper measurement, the collected samples were centrifuged at 400rpm to remove any suspended particles.

3Results and discussion3.1Photocatalytic degradation of organic dyes

In the presence of air or oxygen, the light illuminated semiconductor photocatalysts are capable of destroying many organics. The activation of semiconducting materials by light energy (hv) generates electron (e–) and hole (h+) pairs which are reductant and oxidant, respectively. In the degradation of organics, the hydroxyl radial (·OH) which comes from the oxidation adsorbed water or adsorbed hydroxyl (−OH), is the primary oxidant; and the presence of oxygen could prevent the recombination of electron-hole pairs [42]. When the photocatalyst is illuminated, a photon of energy higher than or equal to the band gap causes excitation of electrons into the conduction band (CB) of the photocatalyst. Simultaneously, an equal number of holes are generated in the valence band (VB). The high oxidative potential of the hole in the CB of catalyst permits the direct oxidation of the dye to reactive intermediates followed by degradation [43]. Based on above results and referred to previous reports [44] a possible mechanism for the degradation of crystal violet catalyzed by MnO2 NFs mesh is supposed:

In present study, MnO2 NFs mesh may effectively catalyze O2 and water molecules into OH radicals under the light irradiation. As earlier reported that the strong O2 activating ability of MnO2 nanowires have showed promising applications in the oxidative degradation of dye, like alizarin yellow R [45]. Kong et al. have suggested a photocatalytic mechanism and pointed out the oxidization of H2O and de-oxidization of O2 by photo-generated holes and photo-generated electrons respectively, occurred under UV-irradiation [46]. Wang et al. has reported that presence of H2O molecules in the tunnels of MnO2 nanorods as well as in the reaction mixture are responsible for the generation of OH, attack and/or destabilization of azo linkage (−N=N−) of the dyes and therefore photocatalytic efficiency is enhanced [47]. Previously reported by Lachheb et al. that the ultra-long morphology and mesoporous structure of a-MnO2 nanorods provide more active sites for the reactant molecules to reach the active sites. Finally, photocatalytic degradation rate of Congo red is higher in comparison to the methyl orange, which might be attributed due to the presence of two reaction sites, i.e., azo groups (−N=N−) in Congo red. In respect to that, OH radical adds onto the (−N=N−) double bond in very fast reaction and the conjugated system of Congo red molecule gets broken easily. Moreover, the OH addition to the (−N=N−) bonds produce hydrazyl radical. This reaction probably led to the destruction of the intensive color of the dye [48]. Our results are in good agreement to the results of Sadollahkhani et al regarding the better nanorods efficiency towards Congo red in comparison to nanoleave and nanosheets [49].

3.2Photocatalytic study of MnO2 NFs mesh

To evaluate the effectiveness of MnO2 NFs mesh as photocatalyst, CV solution was used as a model compound. The mixture of CV solution with MnO2 particles with specific morphology was exposed to UV light (254nm). It is clearly demonstrated from Fig. 1 that the crystal violet dye undergoes degradation in the presence of MnO2 NFs mesh as photocatalyst. The color of CV solution slowly faded when exposed to UV radiation for different time intervals (10–90min). The absorbance peak of CV significantly decreased from 1.384 (initial absorbance) to 0.039 in 90min when exposed to UV irritation.

Fig. 1.

Effect of UV on degradation of dye a) without initiator and b) with initiator.

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The high rate constant of MnO2 NFs mesh could be due to the high surface area to volume ratio. A larger surface area can provide more photocatalytic reaction centers for the absorption of reactant molecules. In addition, a larger surface area is also eff ;ective for UV light adsorption and thus generates more electrons and holes. The higher the number of carriers, the better the photocatalyst is. Thus, a better photocatalytic activity for MnO2 NFs mesh is achieved as a result of enhanced absorption capacity. The removal efficiency of the dye was estimated according to the following equation;

where A0 is the initial absorbance of the dye before mixing with the catalyst and At is the absorbance of the dye at a given reaction time t (min). It was 97% within 90min (Fig. 2). When MnO2 NFs particles were illuminated by UV light with energy greater than the band gap energy, the conduction-band electrons (e CB) and valence-band holes (h+VB) were generated on the surfaces of MnO2 NFs mesh as shown in Fig. 3. Holes could react with water adhering to the surfaces of MnO2 NFs mesh to form highly reactive hydroxyl radicals (OH). Meanwhile, oxygen acted as an electron acceptor by forming a superoxide radical anion (O2•−) CV was believed to be destroyed through direct oxidation by the (OH) radicals.

Fig. 2.

Effect of time on percentage degradation of dye.

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Fig. 3.

Generation of conduction-band electrons and valence-band holes on the surfaces of MnO2 NFs mesh.

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3.3Effect of initiator

The photocatalytic response of as-synthesized MnO2 NFs mesh was investigated at model pollutant CV under visible-light irradiation. Photocatalytic experiment was performed using hydrogen peroxide (H2O2) as initiator for photocatalytic degradation process. The absorption spectra of CV sample measured at different time intervals from 0 to 1.5h initiated by using optimized 0.04ml amount of H2O2. Lowering of the intensity peak in the absorption spectra indicates the decomposition or complex structure of CV into simpler ones in the presence of light. Fading color of CV in aqueous media is indicative of photocatalytic degradation of sample dye. It was observed that without initiator nanophotocatalyst takes 280min time to degradation and exhibits slow process and takes much more time for complete degradation but H2O2 reduced time for efficient degradation to just 60min as shown in Fig. 1. Quantity of initiator has definite effect on the degradation activity of nano-photocatalyst. Addition of oxidizing species to suspension is a usual practice to initiate the degradation rate. H2O2 being electron capturer, generates OH radical under visible light irradiation [50]. The highly oxidant (OH) radical initiates the chain photocatalytic reaction, whereas, at higher H2O2 concentration it facilitates the OH radical and hole recombination. In an aqueous solution, the holes at the surface are scavenged by surface hydroxyl groups and water molecules to generate OH radicals. The resulting OH radical, being a very strong oxidizing agent would oxidize the dye molecules to the mineral final products, i.e. CO2, H2O [51]. The effect of reaction time on photocatalytic performance of MnO2 NF mesh was estimated by degrading the CV aqueous solution.

3.4Comparative study of sunlight driven photodegradation of crystal violet dye with and without initiator H2O2

The hydroxyl radical plays a critical role in the degradation process of organics: it can be involved in hydroxyl substitution reaction, dehydrogenation reaction or electron transfer reaction, leading to sensitization and degradation of organics. In the photocatalytic reaction system, the addition of H2O2 can increase the rate of hydroxyl radical formation (Fig. 4). At the same time, H2O2 is also an electron capture agent that can inhibit the complex effects of photo- generated electron–hole pairs. So, the addition of H2O2 could accelerate the photocatalytic degradation efficiency. The reaction is described as follows [52].

MnO2 NFs mesh efficiently degraded 97% CV dye being initiated by H2O2 within 90min as well as degradation process was prolonged to 280min without initiator (Fig. 4).

Fig. 4.

Effect of Sunlight on degradation of dye a) without initiator and b) with initiator.

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3.5Effect of pH

Experiments were performed at different pH levels i.e. 3, 7 and 10. Acidic medium (pH 3) was found to be best for effective dye degradation under UV as well as sunlight irradiation as shown in Figs. 5 and 6. When the pH value is between 7 and 10 the degradation of dye is inhibited, due the hydroxyl ions competes with dye molecules in adsorption on the surface of silver nanoparticles. For the multi-phase photocatalytic reaction of semiconductors, the pH value of the solution is an important factor affecting the kinetics of catalytic reaction [52]. The photocatalytic degradation of dye is mainly dependent on the pH value due to its effect on the catalyst charge, aggregates size, and valance and conductance bonds position [4]. This work is in line with previous cited literature as Ahmed et al. has studied the effect of initial pH on the degradation efficiency for Alizarin yellow R by MnOx nanoparticles were also examined in the pH range 3–7at 323.15K as the degradation efficiency remarkably increased with the pH decreases [36]. Rashad et al. studied 100% degradation of same dye at pH 3 [20].

Fig. 5.

Effect of pH on dye degradation under the influence of UV light.

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Fig. 6.

Effect of pH on dye degradation under the influence of Visible light.

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3.6First order reaction kinetics for degradation of CV dye by MnO2 NFs mesh

The photo-degradation efficiency of MnO2 NFs mesh can be deduced from Figs. 7 and 8. When the absorbance data are plotted as in ln (C/C0) of the time-dependent normalized dye concentrations (which is the ratio between the initial concentration and the concentration upon reaction) and linear plots are obtained. This indicates that the decomposition of CV follows a first order kinetics regardless the morphology of MnO2 NFs mesh. The relationships between ln(C/C0) and irradiation time, indicating pseudo-first order kinetic models for the degradation process. This is a typical characteristic of photocatalytic reactions [53]. Under the dark condition, the dye removal proceeded quickly in the short time span after the mixing of MnO2 NFs mesh into the dye solution and then afterwards followed pseudo first order kinetics throughout the degradation process. Similar behaviors in the initial stage of mixing MnO2 NFs mesh with dye solutions were reported previously [54]. One possible explanation for this behaviors is the high rate of adsorption of dye molecules on the surface of MnO2 nanoparticles after mixing [55].

Fig. 7.

Degradation of crystal violet dye by MnO2 NFs a) 1st order reaction kinetics and b) 2nd order reaction kinetics.

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Fig. 8.

BMG kinetics for degradation of crystal violet dye by MnO2 NFs.

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4Conclusions

Removal of hazardous organic compounds from wastewater is quite challenging. These toxic organic compounds are adversely affecting the environment and are the major cause of pollution. To protect our environment their detoxification is very important to maintain the ecological balance. Among various proposed techniques for wastewater treatment, photocatalysis is most effective and sustainable approach for removal of organic wastes from water. The potential use of 3D MnO2 nanofibrous mesh as photocatalyst for removal of organic dye was monitored. The unique mesh morphology offers a large surface area to promote enhanced photocatalytic activity. The potential of MnO2 nanofibers facilitated the oxidative degradation of CV dye. Best photocatalytic performance was achieved under visible light with almost complete degradation (97%) exhibited within 90min of irradiation time. The degradation efficiency of prepared photocatalyst was optimum at pH 3 and in presence hydrogen peroxide. Finally, this technique can be utilized for depletion of organic pollutants that are potential industrial and environmental hazard.

Conflicts of interest

The authors declare no conflicts of interest.

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