Abstract
The textile industry consumes a large volume of organic dyes and water. These organic dyes, which remained in the effluents, are usually persistent and difficult to degrade by conventional wastewater treatment techniques. If the wastewater is not treated properly and is discharged into water system, it will cause environmental pollution and risk to living organisms. To mitigate these impacts, the photo-driven catalysis process using semiconductor materials emerges as a promising approach. The semiconductor photocatalysts are able to remove the organic effluent through their mineralization and decolorization abilities. Besides the commonly used titanium dioxide (TiO2), manganese dioxide (MnO2) is a potential photocatalyst for wastewater treatment. MnO2 has a narrow bandgap energy of 1~2 eV. Thus, it possesses high possibility to be driven by visible light and infrared light for dye degradation. This paper reviews the MnO2-based photocatalysts in various aspects, including its fundamental and photocatalytic mechanisms, recent progress in the synthesis of MnO2 nanostructures in particle forms and on supporting systems, and regeneration of photocatalysts for repeated use. In addition, the effect of various factors that could affect the photocatalytic performance of MnO2 nanostructures are discussed, followed by the future prospects of the development of this semiconductor photocatalysts towards commercialization.
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Introduction
Intensified development of various dye-related industries such as textile, food, and furniture manufacturing throughout the years without appropriate wastewater management has resulted in serious environmental pollution, aggravated by nonbiodegradability, high toxicity, and carcinogenic nature of dyes. It has been reported that around 20% of the world produced dyes are lost during the dyeing process and being expelled as colored effluent (Konstantinou and Albanis 2004; Langhals 2004; Yahya et al. 2018). Aside from affecting the aesthetic value of water sources, the presence of organic dyes increases the chemical oxygen demand (COD) in wastewater. The generation of by-products through chemical reaction in the wastewater such as oxidation and hydrolysis, resulted in potential hazards to living organisms (Akpan and Hameed 2009; Konstantinou and Albanis 2004; Xiong et al. 2001; Yu et al. 2014).
In response to this concern, advance water treatment technologies are crucial to remediate and reclaim wastewater to ensure a sustainable management and development of clean water. Various conventional water treatment technologies, such as coagulation, adsorption, and membrane separation, have been used to eliminate organic dye from effluents (Konstantinou and Albanis 2004; Ong et al. 2018; Tang and An 1995). Nonetheless, these methods only focus on the reclamation of organic dyes from the wastewater liquid phase to solid phase, which in turn creating secondary pollutants to the environment (Guo et al. 2012). Additional cost is needed in treating these solid wastes and the recovery of the adsorbent. Although biological treatment involving aerobic and anaerobic process could be used to treat the organic dyes, most of the synthetic dyes are persistent to the bacteria’s degradation (Kristensen et al. 1995; Prahl et al. 1997). It is also reported that azo dye may reduce to potentially hazardous aromatic amines under anaerobic condition (Chan et al. 2011; Han et al. 2009; Rai and Dos-Santos 2017; Sharma et al. 2011).
For this reason, oxide semiconductor photocatalysis has been developed and extensive research have been performed to explore its full potential in the degradation of organic dyes particularly for those with low biodegradability (Das et al. 2017; Elbasuney et al. 2019; Kim et al. 2017). Photocatalysis is a nonselective and low-temperature route approach, capable to remove organic dyes through degradation process and to transform them into non-harmful particles (Barreca et al. 2019; Hao et al. 2013). Moreover, its employment of solar energy in the activation of the chemical redox reaction is a merit towards green and sustainable technology for pollution management. TiO2 is the most explored semiconductor photocatalysts owing to its chemical inertness, photocatalytic stability, ease of production and utilization, as well as not harmful to environment and living organism (Akpan and Hameed 2009; Konstantinou and Albanis 2004). TiO2 has been an active candidate in photocatalyst for the degradation of organic compounds such as nonbiodegradable azo dyes, pesticides, saturated compound (alkane), aromatic compound, and nitrite compound under the exposure of ultraviolet (UV) light (Fujishima et al. 2008; Nakata and Fujishima 2012). However, TiO2 has the weakness of not be able to be activated by visible light as a result of its wide bandgap energy (3.2 eV). This has highly restricted its practical application using solar energy as in fact there is only about 3% of UV in the sunlight (Dong et al. 2015). Various approaches such as doping, coupling with other semiconductor material (nanocomposite), and surface modification have been employed to improve the photocatalytic activity of TiO2 in visible and infrared region of solar light (Huang et al. 2015; Kwon et al. 2017; Li et al. 2015a, b; Low et al. 2017; Xiao et al. 2015; Xu et al. 2013). Those modification processes require extra and complicated procedures, which in turn increase the cost of production, confining the application of TiO2 as photocatalyst.
Manganese dioxide (MnO2) with narrow bandgap energy (1–2 eV), which is able to function in the visible region of solar energy, is a promising candidate for photocatalyst applications (Das and Bhattacharyya 2014). Its photocatalytic activity was first validated two decades ago by Cao and Steven through the oxidation of 2-propanol (Cao and Suib 1994). The conversion of as high as 100% to acetone have been realized by the amorphous MnO2 photocatalyst system. Cao and Steven proposed that the photocatalytic capability of manganese is due its ability to rotate among the different oxidation states (4+, 3+, 2+), the produced oxygen to move to the surface of the amorphous catalyst, and the regeneration by atmospheric oxygen during photocatalysis to produce more radicals for the destruction of toxic hydrocarbon species. More in-depth studies have been carried out after that, further proving the feasibility of MnO2 in photocatalysis (Ferreira et al. 2018; Lekshmi et al. 2018). The promising catalytic properties coupled with naturally abundance, acid resistance, and nontoxic has marked MnO2 as a compelling candidate in the degradation of organic dyes. Furthermore, MnO2 could present in various crystal phases with diverse morphologies. This has opened up various photocatalytic possibilities for MnO2-based compounds, awaiting to be fully explored and optimized.
Recently, lots of research works on the synthesis of MnO2-based compounds and their employment in the photocatalytic degradation of organic dyes have been reported. To the best of authors’ knowledge, there are reviews on MnO2-based compound in energy storage (Liu et al. 2013), in catalytic reduction of nitrogen-based compound (Liu et al. 2016) and in adsorption (Nitta 1984). However, the review on MnO2-based photocatalysts to remove organic dyes is yet to be reported. Hence, along with the rising amount of harmful wastewater progressively with the remarkable growth of textile industry, a comprehensive summary in promoting this sustainable technology is in need to ease the application of this technology in a larger scale. The key purpose of this paper is to assess the use of MnO2-based photocatalysts, the synthesis methods of MnO2 nanostructures, as well as the influence of various operating parameters on the photocatalytic performance. A brief review on the future challenges and prospects of MnO2-based nanostructures in photocatalysis are also be presented in this paper to provide perspectives and directions for future development of this photocatalyst.
Fundamental and mechanism of MnO2 photocatalyst
Manganese dioxide (MnO2) is an n-type semiconductor, possessing band gap energy in the range of 1–2 eV, depending on its polymorphic forms (Chan et al. 2016). Its narrow band gap energy makes it a visible light-responsive semiconductor which is of great advantage over other wide bandgap semiconductors, i.e., TiO2 and ZnO in photocatalysis applications. In addition, it is an excellent semiconductor that possesses various redox activities, excellent flexibility, and good electrical properties (Gagrani et al. 2018). Thus, it is commonly applied in the areas of ion exchange (Recepoğlu et al. 2018), biosensors (Chen et al. 2018), molecular adsorption (Dey et al. 2018), energy storage (Zhu et al. 2018) and also photocatalysis (Lai et al. 2018). Furthermore, MnO2 particles are approximately 75% cheaper as compared to TiO2 particles (www.sigmaaldrich.com). Due to these merits, MnO2 nanostructure is suggested as a promising semiconductor photocatalyst in the removal of organic dyes.
MnO2 is a heterogeneous photocatalyst. The photodegradation of organic compounds by MnO2 involves of (a) organic molecules diffuse from the wastewater liquid phase to the surface of the MnO2 catalyst, (b) adsorption of the organic dyes on the surface of MnO2 crystallites, (c) a series of redox reactions occurs in the adsorbed phase, and (d) the desorption of the degraded products of organic dyes from the surface of MnO2 (Oh et al. 2016).
Generally, the redox reaction occurs in the adsorbed phase can be illustrated in Fig. 1. When solar light with energy larger than the bandgap energy of MnO2 is irradiated on its surface, the photocatalysis reaction is being induced. The electrons from the filled valence band are promoted to the empty conduction band, forming electron-hole pairs. The electron-hole pairs then migrate to the surface of the MnO2 catalyst for a series of redox reactions. To favor a photocatalyst process, the recombination of the electron hole pair must be prevented to the greatest extent; as the most critical part of this photo-induced reaction is to have reactions between the generated holes and the reductant; and between the active electrons with the oxidant. The photogenerated holes react with H2O and OH− and oxidize them into hydroxyl radical (OH•). Meanwhile, the photogenerated electrons react with O2 and reduce them into superoxide radical anions (O2−•). The O2−• may then be protonated by H+ in water (depending on the reaction), forming hydroperoxyl radical (HO2•) which subsequently converted to H2O2. The H2O2 formed then dissociates into more reactive OH• for the degradation of organic compound. Both OH• and HO2• radicals are the active species in the photocatalytic activities of MnO2. Particularly, OH• is a very strong oxidizing species and is able to degrade the organic dyes to their end products non-selectively (Shayegan et al. 2018; Yahya et al. 2018). The related equations involved in the MnO2 photocatalysis are shown in Eqs. (1) to (9):
Schematic diagram showing the degradation process of organic dye by MnO2 with the aid of solar energy: (i) photocatalytic reaction induced by solar light, (ii) photoexcitation of electrons from valance band to conduction band, and (iii) formation of free radicals (OH• and O2−•) by photoexcited electron and holes through a series of redox reaction for degradation of dye
The nature as well as the number of reactive species generated in a photocatalytic activity may be differ according to the photocatalyst system. They are strongly interrelated to (a) the presence of surface defects; (b) morphological, structural, and compositional properties of the photocatalysts; (c) nature of the dye; and (d) the optical excited sources.
Crystal structure of MnO2
MnO2 exists in several crystallographic forms, i.e., α-, β-, γ-, δ-, and λ- (Thackeray 1997). The ways of MnO6 octahedral interlink in the MnO2 nanostructures causing the variation in the crystallographic forms. Each crystallographic form possesses its unique tunnels’ structure or interlayers. The vertices and edges in MnO6 octahedral sharing in several different directions lead to the formation of tunnel structures with different dimension. 1D tunnel structure can be found in α-, β-, and γ- MnO2 while the δ- MnO2 possess 2D layered in its structure and the λ-MnO2 is a 3D spinel structures (Feng et al. 1998). They are characterized by the tunnel’s size with the octahedral subunits’ number (n × m). Figure 2 shows the schematic diagrams of their structures, and their basic crystallographic data are presented in Table 1. α-MnO2 (Fig. 2a) comprises of double chains of edge-sharing MnO6 octahedral. The MnO6 octahedral in α-MnO2 link at the corners to form 1D (2 × 2) and) and (1 × 1) tunnels. β-MnO2 (Fig. 2b) is composed of single strand of edge-sharing MnO6 octahedral to form a 1D (1 × 1) tunnel. γ-MnO2 (Fig. 2c) is random intergrowth of ramsdellite (1 × 2) and pyrolusite (1 × 1) domains (De Wolff, 1959). δ-MnO2 (Fig. 2d) with an interlayer separation of ~ 7 Å is a 2D layered structure. The structure of λ-MnO2 (Fig. 2e) is a 3D spinel (Devaraj and Munichandraiah 2008).
Crystal structures of α-, β-, γ-, δ-, λ- MnO2 (Devaraj and Munichandraiah 2008)
The nature of crystallographic structures such as geometry, lattice parameter, and tunnel’s size greatly influence the physical and chemical properties of MnO2 nanostructures (Lin et al. 2017). For instance, band gap energy and surface area are two of the dominant factors that drive the photocatalytic activity. The band gap energy is inversely proportional to the lattice constant due to the binding force between valance electron and parent atom (Kwon et al. 2008). Different MnO2 crystal structures possess different lattice constants owing to their unique arrangement of the MnO6 octahedral, resulting in the variation of photocatalytic performance. Meanwhile, the surface area of nanostructures which are affected by its morphology is correlated to the variation in the interfacial strain of crystal structure during the phase transition (Li et al. 2016); causing the variation in the photocatalytic behaviour.
Crystal structure is one of the most important and primitive aspects in the synthesis of materials as many properties of materials depend on the crystal structure. Therefore, understanding the crystal structure of MnO2 is of importance to help in revealing the photocatalytic behavior and mechanism of the photocatalyst system. The nature of crystal structure can be altered easily by the synthesis conditions such as temperature of reaction, concentration of precursor, presence of impurities as well as dopant, or through coupling. Tracking of the changes in crystal structure along the synthesis condition could help in producing a suitable MnO2 nanostructure fordel "D:/Programs/ProductionJournal/Temp/ccc.bat" photocatalysis.
MnO2 nanostructures
Metal oxide nanostructures play a crucial role in determining their application in various fields. Different nanostructures display distinguished physical properties. In photocatalysis reaction, a suitable MnO2 nanostructure helps in boosting the degradation efficiency and in enhancing the recovery and reuse of photocatalyst after the water treatment process. Various MnO2 nanostructure-based photocatalyst have been synthesized and reported to possess promising degradation efficiency.
Figure 3 shows the MnO2 nanostructures with various morphologies. Nanoparticles are nanostructure with dimension not larger than 100 nm (Shinde and More 2019). Nanoparticles can easily be suspended in solution. Their large specific surface area has made them a popular choice in catalysis. However, nanoparticles have the tendency to agglomerate in the solution which greatly reduced their available active sites for photocatalytic reaction (Pacheco-Torgal and Jalali 2011). In addition, complete recovery of nanoparticles from reacting solution is difficult and is a drawback for the photocatalytic reaction. Surface modifications have been carried out to help in improving the uniform dispersion of nanoparticles in the solution and prevent agglomeration (Cabello et al. 2018; Zhu et al. 2005; Zhu et al. 2006).
1D MnO2 include nanorods, nanofibers, nanoneedles, nanotubes, and nanowires. The attractive properties of 1D MnO2 are efficient electron transportation and excitation along the longitudinal pathway with outstanding mechanical strength. Hence, 1D nanostructure has been widely used in the field of electrical, optical, sensor, and photocatalysis (Wang et al. 2009). In photocatalysis, 1D nanostructures with high aspect ratio are able to increase the effective surface area for reaction. Besides, the longitudinal charge separation of 1D nanostructure (caused by high aspect ratio of 1D nanorods) resulting in the low photoluminescence intensity. The low photoluminescence intensity reduces the emission of absorbed photon. Consecutively, it helps in better preservation of the photoexcited electrons in the series of redox reactions. This has been a great advantage in photocatalysis as light source is the main element to initiate the reactions (Baral et al. 2016).
Nanosheets and nanoflowers MnO2 are classified as the 2D and 3D nanostructure arrays, respectively. Sun et al. demonstrated that the ultrathin lamellar structure of 2D MnO2 nanosheet with high BET surface area enables sufficient organic pollutant to absorb on its surface, in turn more organic dyes could be attacked by the reactive hydroxyl radicals, improving the photocatalytic efficiency (Sun et al. 2015). Ye et al. revealed that 3D MnO2 nanoflowers possessed a larger surface to volume ratio compared to other dimension nanostructures (Ye et al. 2017). In photocatalysis, the photo-induced activity is greatly governed by the effective surface area. Hence, the fabrication of 2D and 3D nanostructure has also caught the attention of researchers.
Crystallinity of the MnO2 nanostructures has also been found to affect the photocatalytic reaction. Amorphous MnO2 nanorods have been reported to perform better than its crystalline structure in photocatalytic reaction. This could be due to the presence of defects, for example oxygen or surface defects, in the amorphous structure have promoted the trapping of excited electron. Thus, it prolonged the separation of charge carriers, allowing more holes to diffuse to the surface of particles for subsequent degradation of organic compounds (Gagrani et al. 2018; Zhang et al. 2014c).
Synthesis of MnO2 nanostructures
Synthesis of MnO2 particles
Several studies reported that the band gap energy of semiconductor oxides are size and shape dependent (Ekimov et al. 1985; Li and Li 2006; Smith and Nie 2009). For example, Alexander et al. found that the band gap energies of MnO2 increased as the MnO2 particles became smaller (Soldatova et al. 2019); Gao et al. observed a band gap of 1.32 eV in α-MnO2 nanofibers with typical diameters of 20–60 nm and lengths of 1–6 mm (Gao et al. 2008); Sakai et al. also reported that MnO2 nanosheets with a very small thickness of about 0.5 nm had bandgap energy of about 2.23 eV (Sakai et al. 2005). The shift in the band gap to higher energies was attributed to the carrier confinement in the small semiconductor particles (Li et al. 2015a, b). The results suggest that it is possible to tune the response of MnO2 photocatalyst from infrared to visible light through controlled synthesis.
Solution-based approaches are the most commonly reported in the synthesis of MnO2 nanostructures, i.e., hydrothermal (Liang et al. 2008; Wang and Li 2003; Xiao et al. 2018; Zhang et al. 2014b), sol-gel (Chan et al. 2016), wet chemical (Dang et al. 2016; Xia et al. 2017), precipitation (Chen et al. 2009), microwave (Ai et al. 2008), ice-templating (Sun et al. 2017), and reflux (Cui et al. 2015) method. Solution-based approaches are preferred owing to their flexibility, controllability, and the least consumption of energy. By proper control on the experimental variables such as type of solvents, type of precursors, and temperature of reaction, the size as well as morphology of the desired MnO2 nanostructures could be obtained.
Among these methods, hydrothermal is the most preferred approach to synthesize MnO2 nanostructures due to the simplicity of the process, good repeatability, high reliability, and easy tailored over the size and morphology of the nanostructure. Nonetheless, hydrothermal method is sometimes controversial due to the use of large amount of solvents such as NaOH and HCl as medium which is a considerable drawbacks to environment (Li et al. 2019; Zhang et al. 2014b). In respect to this, some of the ongoing researches have modified the synthesis methods by using water as medium. Kim et al. synthesized MnO2 nanorods via mild hydrothermal route with water as solvent. The MnO2 nanorods exhibited outstanding result where they were able to fully degrade the methylene blue (MB) organic dye in 20 min and were able to reuse for five times without affecting its degradation efficiency (Kim et al. 2017). This suggested that a good quality catalyst could be produced through an environmental friendly approach with a lower cost of production.
Over the years, researchers prone to use environmental friendly approaches or natural precursors for the synthesis process. A lot of efforts are seen in correspond to the green and sustainable technology to cope with various globally environmental issue nowadays. Green synthesis of MnO2 nanoparticles using natural products such as Y. gloriosa leaf extract and curcumin has been reported recently (Hoseinpour et al. 2018). The antioxidant properties of Y. gloriosa leaf extract acted as a promising alternative in the synthesis of metal based oxide materials. The reaction was carried out at ambient temperature and pressure by a simple co-precipitation method. By showing positive response in the photodegradation of Acid Orange dye, the study has shown the possibility of using natural products that are affordable and easily available for the mild synthesis of MnO2 based photocatalyst.
Alternatively, mechanochemical dry route approaches have also been described in the synthesis of MnO2 nanostructures. It is a solvent-free synthesis method, dealing with the chemical transformation induced by mechanical energy (Achar et al. 2017). However, the common drawback of this process was the agglomeration of end products that tended to affect the available effective surface area for photocatalytic reaction to occur. In respect to this, Gagrani et al. reported an improved version of mechanochemical process on the synthesis of MnO2 nanorods to be used as photocatalyst (Gagrani et al. 2018). The mechanochemical process was carried out without using hazardous chemical such as sulphuric acid. The end products possessed high surface area without agglomeration (Liu et al. 2017; Yang et al. 2015). This work has given an insight on a green, fast, and economic method in the synthesizing of MnO2-based photocatalyst.
In addition, rapid synthesis method under mild temperature has also been introduced by researcher to reduce the consumption of electrical power and cut down the cost of production. For instance, sonochemical method by the aid of ultrasonicator was able to synthesize MnO2 photocatalyst in 15 min at 60 °C (Rajrana et al. 2019). The rate of chemical reaction was greatly enhanced by the additional vibration provided by the ultrasonicator; hence, rapid nucleation of fine particles was achieved in short period of time. This facile rapid one step method has provided researcher another idea of green synthesis approach to be explored. For ease of reference, various synthesis methods of MnO2 nanostructure and their effect on the end-products have been summarized in Table 2.
D diameter; L length, T thickness
Synthesis of MnO2 photocatalyst on supporting system
Using of MnO2 in particles form for wastewater treatment has many limitations. This includes deterioration of removal efficiency overtime as the MnO2 particles could easily drain away by running water. The washed away MnO2 particles itself become secondary pollutants in the water system. Thus, extra processes are needed to remove these secondary pollutants from the water and slurry. This incurs additional cost and time, which are not economical feasible.
Development of supporting system for MnO2 photocatalysts has been observed in recent years to mitigate the drawbacks of photocatalyst in particles form. For instance, MnO2-based photocatalyst was deposited on TiO2 sheet using electrodeposition technique by Xu et al. (Fig. 4a) (Xu et al. 2014). This supporting system not only reduced the loss of MnO2 photocatalyst during reaction but also prolonged the separation of electron hole pairs thus improved the overall photocatalytic performance in visible light (Moulai et al. 2018). Besides electrodeposition, MnO2 nanotubes were successfully grown on the polyethylene terephthalate (PET) fiber using sol-gel method by Chan et al. (Fig. 4b) (Chan et al. 2016). Meanwhile, pulsed laser deposition (PLD) method has been applied in the deposition of MnO2-based photocatalyst on the FTO film (Fig. 4c). Seventy-six percent of MB’s degradation was observed for the Ag/BiVO4/MnO2 thin films after an hour of visible light irradiation (Trzciński et al. 2016).
Nevertheless, the peeling off of the photocatalysts from the supporting system after long hour of wastewater treatment should be taken extra consideration. This phenomenon is mainly due to the poor adhesion between semiconductor photocatalyst and supporting system. The semiconductor photocatalysts were intended to grow rather than to deposit on the supporting system. Deposited photocatalyst would not adhere well and tended to fall off from the supporting system. Besides, poor adhesion of semiconductor photocatalysts on supporting system could be also due to (a) improper surface activation of supporting system before synthesis process, (b) internal stress attributed to lattice mismatch between photocatalyst and supporting system, and (c) contamination of the precursors solution (Arai et al. 1987)
In general, it can be perceived that the performance of the photocatalyst which deposited on the supporting system is inferior to the photocatalyst in its particle form. This could be due to the reduction of accessible surface area as well as the incompatibility of the supported system with the photocatalyst. Nevertheless, growth of photocatalyst on the supporting system is a necessity for practical application.
Parameters affecting the degradation efficiency of MnO2-based photocatalysts
pH of the solution
The pH of reacting medium influences the overall photocatalytic performance of MnO2 photocatalysts. The pH dictates the surface charge properties of the photocatalyst and influences the oxidation potential of the reaction. Thus, it is crucial that the photocatalysts work in their optimum and stable pH range for fully boosting of its inherent ability.
The surface of photocatalyst is positively charged when the reacting solution is below its point of zero charge (pzc) value and is negatively charge when is above its pzc. This changes altered by the pH need to be highlighted as it will influence the attraction and repulsion relation between the catalyst and organic dye compound. Thus, this is an influential factor in a photocatalytic reaction. The effect of pH on the ionization state of photocatalyst’s surface can be explained by Eqs. (10) to (12) (Akpan and Hameed 2009):
For instance, according to Kim et al. and Zhang et al., the degradation of MB by MnO2 catalyst was the best in near neutral pH range of 6.0–7.0 (Kim et al. 2017; Zhang et al. 2014b). In acidic medium, the production of OH• could be speed up by the positively charged photocatalyst, which benefit the photocatalytic reaction. However, both the positively charged MnO2 catalyst and the MB organic dye tended to repel away, making the adsorption reaction difficult and hence delaying the degradation of MB (Zhao et al. 2013). In contrary, the negatively charge of MnO2 catalyst in alkaline medium favoured the adsorption of positively charged MB molecule. The competition between the dye and the excess OH− for OH• tended to occur (OH• + OH− → O− + H2O) (Hayon 1965). This had decreased the availability of OH• for the photocatalytic reaction. Thus, the balance between the electrostatic attraction and the diffusion rate of surface generated OH• towards the organic dye must be achieved in order to achieve optimum photocatalytic performance (Ai et al. 2008).
Some MnO2 photocatalyst systems are reported to perform better in alkaline medium due to the preferable electrostatic attraction (David and Vedhi 2017; Mittal et al. 2009; Nanda et al. 2016). In alkaline medium, the negatively charged MnO2 favoured the adsorption of cationic dyes. Besides, the generation of active OH• radicals increased as a result of the increase reaction between the photogenerated holes and the negatively charged hydroxyls OH− (MnO2 (hvB+) + OH− → MnO2 + OH•). For instance, the study from Naveen et al. reported that the photodegradation of Bengal red dye using commercially available MnO2 powder increased with pH value until pH = 8. Further increase in the pH deteriorated the reaction’s rate as excess of OH− may change the ionic form of dye (Mittal et al. 2009).
Nevertheless, some studies also reported that MnO2-based photocatalysts perform well in low pH range (acidic medium) (Cui et al. 2015; Dang et al. 2016; Gagrani et al. 2018; Sun et al. 2017; Sun et al. 2015). Sun et al. reported that MnO2 aerogel showed excellent degradation of Rhodamine B (RhB) in low pH of 2.5 compared to that at pH 3.5 and pH 5.9. They proposed the change in the pH condition altered the reduction potential of the reaction. In fact, the redox reaction of MnO2/Mn2+ in the presence of H+ could be expressed as MnO2 (s) + 4H+ + 2e− → Mn2+(aq) + 2H2O, with standard reducing potential of 1.29 V (Stumm et al. 2012). From Nernst equation, the reduction potential could be improved by low pH value (excess H+), favoring the reduction process. The decolorization of the dyes were due to the reduction of Mn4+ to Mn2+, favoring the redox process and hence the oxidation of RhB. In addition, Mn2+ ions were able to generate more oxidizing hydroxyl radical (•OH) under acidic condition. These radicals subsequently reacted and mineralized the organic dyes (Stone and Morgan 1984). The presence of Mn2+ ion in the reacting medium was indeed needed to be taken into consideration. Excess Mn2+ in the aqueous system for a certain period of time could lead to several impacts such as biofouling, odor, water turbidity, and corrosion (Güneş Durak et al. 2013) .
The understanding and elucidation of pH on the photodegradation process by MnO2-based photocatalysts is complicated. The photocatalytic behavior is greatly affected by the nature of MnO2-based photocatalyst system itself and the organic dyes. Some photocatalyst system performed better in low pH while others at neutral or higher pH (Yao et al. 2013). Appropriate pH should be carefully picked to ensure achievement of optimum photocatalytic efficiency. Table 3 summarizes the effect of pH on the photodegradation of various reported MnO2 photocatalysts system and various organic dyes.
Loading of catalyst
The loading of catalyst is often in association with the cost of operation. This is particularly important in commercial application. Thus, it is essential in evaluating the feasibility and efficiency of the MnO2 photocatalysts under different loading. An optimum amount of catalyst is required to ensure the optimum performance of catalyst as well as avoiding the wastage of materials. Many studies have reported on the optimum loading of MnO2 catalyst in the photocatalytic process involving organic dyes (Dang et al. 2016; Das and Bhattacharyya 2014; Hao et al. 2013; Kim et al. 2017; Sun et al. 2017; Sun et al. 2015; Yu et al. 2014)
In general, the degradation rate increases with the amount of catalysts until it reaches the optimum level. Then, the degradation rate steadily decreases after the optimum concentration. With the increase amount of catalyst, the total effective surface area as well as the reaction sites of the catalyst were increased (Bond 1987). Then, a higher amount of hydroxyl or other related radicals were produced, enhancing the photocatalytic performance of MnO2. This phenomenon would quench once the optimum loading was achieved and followed by the deterioration of degradation efficiency (Yu et al. 2014). This was attributed to the rate of production of free radical species was far quicker compared to their consumption by the organic dyes for degradation. These excess free radicals tended to react with each other and vanish rather than degrading the organic dyes. Hence, the reduction in the rate of photocatalytic reaction was observed (Huang et al. 2008). In addition, high catalyst loading also promoted agglomeration between MnO2 nanostructures, causing severely decrease in the active surface area for reaction (Kim et al. 2017; Zhang et al. 2014a). Besides, the solution turbidity increased with the catalyst loading. The penetration of the light was reduced. This caused poor photocatalytic performance of MnO2 (Chakrabarti and Dutta 2004; Huang et al. 2008). In short, the optimum loading of MnO2 catalyst to be used should be examined to ensure the effectiveness of the photocatalytic reactions.
Temperature of reaction
The photocatalytic studies were normally performed under ambient temperature and atmospheric pressure. However, some studies reported that a slight rise in the operating temperature could result in the improvement of degradation rate (Kim et al. 2017; Liang et al. 2008; Wang et al. 2018; Yao et al. 2013). For example, Wang et al. reported that a 2.5-fold increase in the degradation rate was achieved with the increased operating temperature from 25 to 40 °C (Wang et al. 2018). This was mainly contributed by the higher diffusion rate of the reacting free radical towards the organic pollutants, resulting in higher degradation efficiency. Yet, the reaction temperature should not go beyond certain range. Too high temperature may alter the photocatalyst’s surface and hence affect the adsorption capacities on organic pollutants. The ideal temperature for a catalytic process is in the range of 20 to 80 °C, involving few kJ/mol of activation energy (Malato et al. 2003).
Intensity of light
Photocatalytic activity of MnO2 particles is a light-dependent reaction. Its efficiency in removal of organic pollutants could be improved by exposing with more radiation. The generation of electron-hole pairs is controlled by the light intensity. Hence, more radicals could be formed with a higher light intensity and thus increased the photocatalytic activity. According to the study of Kormann et al., the rate of photocatalytic activity was proportional with light intensity at light intensity of 0–20 mW/cm2 (Kormann et al. 1991). If the light intensity was higher than 25–30 mW/cm2, the rate of photocatalytic degradation was proportional to the square root of the light intensity. The reaction rate always increased with the light intensity until the mass transfer limit was encountered. At much higher light intensity (˃ 35 mW/cm2), the rate of the photocatalytic reaction was independent on the light intensity. The generation rate of electron hole pairs would be faster than the rate of photocatalytic activities. Excess amount of electron hole pairs tended to combine with each other. In addition, the surface of the catalyst was occupied by a large amount of charges. This limited the mass transfer for both adsorption and desorption. Hence, further increase in the light intensity would not enhance the reaction rate (Malato et al. 2009). The light intensity used in the photocatalytic studies was commonly in the range of 1–5 mW/cm2 (Horie et al. 1996; Luan et al. 2016).
Oxidizing agent
The electron hole recombination process have been pointed out to be one of the major factor that leading to poor photocatalytic activity of MnO2 photocatalyst. As an alternative, oxidizing agents have been employed to reduce the recombination and to ensure the effectiveness of photocatalytic reaction. The oxidizing agents could address the electron hole recombination issue by (1) increasing the number of trapped electrons and (2) producing more active species for degradation of organic pollutants (Selvam et al. 2007; Singh et al. 2007; Wei et al. 2009).
Several oxidizing agent for example hydrogen peroxide (H2O2) and sulfate radical anions (SO4−•) have been studied for their influence on the photocatalytic performance of MnO2-based catalyst. H2O2 is a powerful oxidizing agent and electron acceptor. It is a competent candidate in enhancing the photodegradation efficiency of various organic compounds (Kim et al. 2017; Lu et al. 2019; Nanda et al. 2016; Qu et al. 2014; Trzciński et al. 2016; Yu et al. 2014). Electron-hole pairs would be generated on the surface of MnO2 photocatalyst upon photoexcitation. Ideally, a series of redox reactions would occur by these photogenerated holes and electrons to produce strong oxidizing radicals for the degradation of organic pollutants. Nonetheless, the recombination of electron hole pairs tended to occur before the redox reactions, leading to the deterioration of photocatalytic activity. By adding H2O2 in the photocatalytic system, it enabled the acceptance of photoexcited electrons from the conduction band of MnO2 to form the hydroxyl radicals via the redox reaction (Eq. (13)). In addition, H2O2 could also split directly into hydroxyl radicals photocatalytically (Eq. (14)) (Nanda et al. 2016).
The hydroxyl radicals that formed by H2O2 on the surface of MnO2 enhanced the degradation of organic pollutants into less harmful molecules. Nevertheless, excess dosage of H2O2 has opposite effect due to the scavenging of OH• by excess H2O2 to form perhydroxyl radicals (HO2•). Perhydroxyl radical is a much weaker oxidant as compared to OH• radicals (Eq. (15)). Therefore, an appropriate amount of H2O2 is crucial to enhance the photodegradation efficiency (Molina et al. 2006).
Recently, peroxymonosulfate (Oxone, PMS), a type of sulfate radical anions (SO4−•) has been studied (Liang et al. 2012; Liu et al. 2015; Saputra et al. 2012; Yao et al. 2013). It possess a higher oxidizing potential (1.82 V) than H2O2 (1.76 V). It is an affordable and environmental friendly oxidant and has showed strong oxidizing power in the mineralization of various organic pollutants. Besides high reactivity, its key advantages over H2O2 are easy to be handled as it is in solid form. PMS (HSO5−) possess similar role as H2O2 in photocatalysis, as an electron scavengers and strong oxidizing agent. PMS reacts with the photoexcited electrons to form two powerful oxidizing agents, i.e., sulfate radicals (SO4−•) and hydroxyl radicals (OH•) (Eqs. (16) and (17)). The sulfate radicals could easily react with the hydroxyl species from water and thus improve the degradation efficiency (Lee et al. 2016):
In brief, studies showed that oxidizing agents have an influential role on the photocatalytic degradation of organic dyes. Hence, there is a need to consider their effect in the water treatment process that involving photocatalysts. Recently, a group of researchers has presented a model that was able to produce H2O2 itself using a UV assisted cavity bubble oxidation reactor (Mohod et al. 2018). The reactor used a combination of high flow rate of water, temperature, and pressure exerting on the surface of glass balls. This was to created sufficient shear effect for the formation of oxidizing radicals (Mahale et al. 2016). The reactor system is shown schematically in Fig. 5, and the related equations are presented in Eqs. (18) to (20):
Schematic representation of UV-cavity-bubble oxidation reactor (Mohod et al. 2018)
Coupling effect
Various MnO2 nanostructures have been reported to be promising photocatalysts. However, to further improve its photocatalytic efficiency, coupling of MnO2 with other semiconductor or nanocarbon material and forming nanocomposite has becoming another important field to be explored. Studies have proven that the enhancement of photocatalytic performance could be achieved through coupling of nanostructure compounds. The favourable coupling effect between the nanocomposites promoted charge separation, offered better suppression of electron-hole pair recombination and provided a larger total effective surface area for better organic compounds removal (Serpone et al. 1995).
MnO2 coupled with titanium dioxide (TiO2) has been extensively investigated in photocatalysis application. TiO2 is a well-known semiconductor in the photocatalyst’s field owing to its outstanding photocatalytic properties. Coupling of TiO2 into the MnO2 photocatalyst system created a win-win situation. The slow charge transfer rate and fast recombination of photogenerated electron-hole pairs of MnO2 could be improved; while the enhancement of TiO2 in the visible light could be achieved (Lekshmi et al. 2017; Wang et al. 2018; Xue et al. 2008; Zhang et al. 2009a). For instance, the study from Xue et al. demonstrated that the synergistic effect between MnO2 and TiO2 has produced a photocatalytic system with higher surface area and larger pore size compare to their individually element. In addition, remarkable performance of MnO2/TiO2 under the irradiation of visible light was achieved. There was 10-fold increase in the MB degradation as compared to that of TiO2 alone (Xue et al. 2008).
Besides, addition of specific properties such as magnetic property to MnO2 nanostructures could be achieved by coupling with other metal oxides. For instance, the recovery and the durability of the MnO2 nanostructures have been an important part in photocatalyst. The production of magnetic composites that enable the utilization of magnetic separation technology appears to be a very efficient method to remove the MnO2 nanostructures. Thus, Zhang et al. have reported the coupling of MnO2/ Fe2O3 to produce magnetic nanocomposite through mild hydrothermal method. This nanocomposite demonstrated high photodegradation of MB under UV-vis light, superior to that of MnO2 or Fe3O4 component. Moreover, coupling with ferromagnetic Fe3O4 allowed the easy recovery and reusability of photocatalyst using external magnetic force, enhancing their potential industrial application in wastewater treatment (Zhang et al. 2014b).
Alternatively, coupling with nanocarbon based materials have been extensively study by the researchers. Carbon-based materials are known to have excellent electrical properties, high accessible surface area, and superior electronic properties (Ong et al. 2016). Particularly, graphene-based materials have caught great interest due to its promising outcome in photocatalysis. For instance, Qu et al. reported that the hybrid graphene/MnO2 displayed superior catalytic activities than the bare MnO2 nanoparticles. With the in cooperation of graphene, the problems of MnO2 nanostructure such as aggregation, poor conductivity and stability, are well addressed. Furthermore, the large surface area of graphene based materials enhanced the photocatalysis process (Qu et al. 2014).
Miyazaki et al. synthesized ternary composite that composed of MnO2-loaded Nb2O5 carbon cluster (coupled semiconductor/nanocarbon) for MB degradation (Miyazaki et al. 2009). The smooth pathway for transfer of electrons from MnO2 to the carbon cluster and subsequently to the Nb2O5, leading to the efficient separation of photo-induced electron-hole pairs. This improved the degradation of MB by this MnO2 composite under visible light irradiation (Wang et al. 2016).
Various possible MnO2 composites had been synthesized to enhance the properties of MnO2 for the development of promising photocatalytic systems. Table 4 summarizes the effect of various MnO2 coupling systems for photocatalyst applications. Generally, the photocatalytic activities were enhanced by the increase amount of semiconductor oxide or nanocarbon until a certain concentration. This was because the excess semiconductor oxide or nanocarbon tended to agglomerate on the surface of MnO2, blocking the accessible surface area for the adsorption of organic dyes, hence reducing the photocatalytic activity (Siddiqui et al. 2019). Excess coupled materials also shaded the photocatalyst from the light source, resulting in poor photocatalytic activity (Wang et al. 2019).
Recovery and reuse of photocatalyst
The recovery and reuse of photocatalyst after a reaction is an important aspect in development of MnO2 photocatalyst. In fact, for a photocatalyst to be industrial viable, it must be able to withstand the reaction condition repeatedly and able to fully recover from the treated system to prevent secondary pollution.
Magnetic separation technology is one of the most commonly studied methods in the recovery of the photocatalysts. The implementation of magnetic separation technique (Sirofloc process) in the water treatment system has been found to offer several benefits over sedimentation and flotation, especially with both the reduction in capital and operational costs (Herrmann et al. 1998). With the development of a magnetic photocatalyst (with inclusion of paramagnetic materials such as iron or tungsten),various problems arise from the solid-liquid separation of the photocatalyst particles from reacting system could be well addressed (Beydoun et al. 2001; Kurinobu et al. 2007; Zhang et al. 2009b). By application of the external magnetic force, the magnetic photocatalyst could be easy and fully recovery from the reacting solution while the preservation of large surface area of submicron/nano-particles could be achieved. The photocatalyst could be ready to reuse once separated. Figure 6 demonstrates a laboratory scale unit of photoreactor with magnetic separation unit for reference.
Schematic diagram of photoreactor and magnetic separation unit (Beydoun et al. 2001)
Meanwhile, ceramic membrane microfiltration with the aid of ultrasonic has been reported for the recovery of photocatalyst from the photoreactor (Cui et al. 2011). It is known that the compaction of the filtration cake tended to occur after lengthen reaction’s time in conventional microfiltration. Hence, the ultrasonic-enhanced membrane microfiltration process was proposed. This ultrasound irradiation could bring four specific effects towards the microfiltration process. Firstly, sonication with its vibration ability reduced the blockage of pore and cake coagulation on the filtration membrane as well as helped in preventing the agglomeration of particles in solution. Secondly, the mechanical vibration energy from sonication enabled the particles to partly suspend in the aqueous media, hence provided more free passages for the elution of solvent. Thirdly, the cavitation gas bubbles that produced during ultrasonication was able to reach and clean the crevices in the membrane that were difficult to be accessed through conventional cleaning methods. In short, the ultrasonic-enhanced membrane filtration improved the conventional microfiltration method by promoting the recovery of photocatalyst through the conservation of a stable and clean membrane surface while preventing the suspension of photocatalyst in the photoreactor. Figure 7 displays the schematic diagram of the photocatalyst recovery process through ceramic membrane microfiltration and ultrasonication.
Schematic diagram of membrane separation module: (12) inlet, (13) ceramic membrane, (14) membrane module, (15) ultrasound generator and transducer, (16) permeate outlet, (17) flange, and (18) retentate outlet (Cui et al. 2011)
Future challenges and Prospect
The semiconductor-based photocatalysts are able to address the organic dyes issue effectively and sustainably as compared to conventional methods such as reverse osmosis and adsorption. The overall benefits of this technology may include saving a huge amount of water and minimizing environment pollution. The treated water could be recycled in the same factory or reused in other applications that have less stringent water quality. This technology gives a good impact especially in this decade where clean water crisis is alarming. Therefore, there is a need to bring MnO2-based photocatalysis to real-life application. In response to this, there are a few challenges that need to be addressed.
Firstly, the photocatalytic degradation process should be carry out using the organic wastewater effluent from dye-related industry rather than a proxy. Most of the studies in literature were targeted on one specific type of dye. Thus, the dye was relatively easier to remove hence high degradation efficiency could be easily obtained. One should be aware that the composition of wastewater from the industrial is complex and contained thousands of impurities. Thus, the interactions between those impurities with MnO2 photocatalyst should be taken into consideration during the degradation process. In-depth studies on their impacts towards the efficiency of MnO2 photocatalyst are needed to facilitate its real life application in wastewater treatment.
Secondly, most of the reported dyes such as rhodamine B, methylene blue, and congo red consist of conjugated double bonds. These conjugated double bonds are known to be vulnerable and easily attacked oxidatively. Yet, there are many organic dyes do not contain such delocalised π system. Future study may need to explore and improve the effectiveness of MnO2-based photocatalysis on these types of persistent organic pollutants in order to broaden the application of this technology in wastewater treatment.
Thirdly, more research are still needed in the development of MnO2 nanostructures. Mostly, the effectiveness of a photocatalytic reaction is greatly affected by the photocatalyst itself. Although with today technology, a good MnO2 nanostructure photocatalyst could be easily synthesized. Efforts are always needed in this area to come out with an even better MnO2 photocatalyst with suitable nanostructure in order to deal with the increase amount of pollutants and the ever changing environment.
Fourthly, complete reclamation of the MnO2 photocatalyst from the treated wastewater is remained as a big challenge. More efficient and promising engineering design of the reactor system or an appropriate nanostructure model should be developed and explored to compete with the rapid development of various dye related industries. The proposed measure should be cost effective and able to provide complete prevention of catalyst from remaining in the treated wastewater, avoiding contamination that may bring negative impacts on living organism.
Last but not least, a modelling work for MnO2 photocatalytic system is required for a better and solid understanding on the mechanism involved during the reaction. This helps in the prediction on the optimum reaction conditions, degradation efficiency and kinetic of reactions.
Conclusions
MnO2 exists in various polymorph forms, and thus, its bandgap varies between 1 and 2 eV. This unique property allows MnO2 becomes an attractive photocatalyst that responsive to visible light or even infrared. The degradation of organic pollutants by MnO2 photocatalyst involves a series of redox reactions, producing less harmless by products. MnO2 nanostructures were mainly synthesized by solution route such as hydrothermal, sol-gel, solution precipitation, and reflux methods owing to their flexibility and controllability. The physical properties of MnO2 nanostructures such as dimension, morphology, particle size and pore size are greatly affected by these synthesis methods. Therefore, optimization of MnO2 nanostructures is needed as their photocatalytic performances are affected by factors such as pH, amount of photocatalyst, temperature, intensity of light, oxidizing agent, and coupling.
It is worth mentioning that synthesis of MnO2 nanostructures on supporting systems, such as PET fiber, TiO2 sheet, and FTO substrate, is getting attention by researchers. The supporting system helps to minimize the loss of MnO2 nanostructures during photocatalytic process. In addition, the current development trend in photocatalysts field is to establish techniques such as magnetic separation and membrane microfiltration for recovery and reuse the photocatalysts. Although tremendous efforts and achievements have been done by researchers, many challenges still need to be addressed to increase its industry viability. These include the development of MnO2 photocatalyst that can treat effluent from dye-related industry which contain more complex dyes and impurities, removal of persistent organic dyes that do not contain conjugated double bonds, more effective MnO2 photocatalyst with suitable nanostructures, reclamation of photocatalysts from treated effluent, and modelling of photocatalytic mechanism of MnO2.
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This study was funded by Universiti Sains Malaysia, Research University Grant (1001.PBAHAN.8014095). The authors also acknowledge the support from USM Fellowship (RU 1001/CIPS/AUPE001).
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Chiam, SL., Pung, SY. & Yeoh, FY. Recent developments in MnO2-based photocatalysts for organic dye removal: a review. Environ Sci Pollut Res 27, 5759–5778 (2020). https://doi.org/10.1007/s11356-019-07568-8
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DOI: https://doi.org/10.1007/s11356-019-07568-8