Catalytic degradation of organic pollutants by manganese oxides: a comprehensive review

ABSTRACT Manganese dioxide (MnO2) is a very promising catalyst material due to its unique physicochemical properties and synergistic effects with other metals or metal oxides. Especially, MnO2 was widely used to remove organic pollutants. In this paper, we summarize the phase and morphology structures of MnO2. The effects of doping and composite on the structures and catalytic properties of MnO2 materials are also compared and described. The catalytic properties of MnO2-based materials on organic pollutants (phenolic compounds, antibiotics, dyes, and pesticides) are also analyzed. In addition, we summarize the degradation mechanisms, degradation pathways, and degradation efficiency of different MnO2-based materials on organic pollutants. The development status and shortcomings of MnO2 are discussed. Its development trend of catalytic degradation of organic pollutants by manganese oxides is provided.


Introduction
Plenty of organic pollutants were generated with the growing rate of urbanization, such as phenolic compounds, antibiotics, dyes, pesticides, etc [1,2]. Some of these pollutants are biodegraded into harmful compounds, while others are inherently difficult to degrade, leading to environmental accumulation [3][4][5]. There is an urgent need to find effective ways to eliminate toxic pollutants that strike a balance between economic feasibility and environmental friendliness to eliminate the threats to humans, animals, and plants [6].
In recent years, many researchers have reported that manganese oxides and their composites are effective adsorbents and catalysts for removing organic pollutants from wastewater compared with other metal oxides [7][8][9][10][11][12]. Manganese is a transition metal that occurs widely in nature (the tenth most abundant element in the earth's crust) [13,14]. Manganese exists in nature mainly in the form of manganese oxide and other compounds. The nano-sized manganese metal oxides have many advantages, such as large specific surface area, porous structure, many active sites, good thermal stability, easy recovery, high environmental compatibility, and low toxicity [2,[15][16][17]. The crystalline phase MnO 2 material consists of [MnO 6 ] octahedral units in various tunnel-like and laminar structures, which are one-dimensional tunnel structure, two-dimensional laminar structure, and three-dimensional mesh structure, respectively. Due to its special structure, single MnO 2 species can be modified by element doping, morphology control, facet engineering, structure construction. In addition, MnO 2 -based composites can be prepared using homojunction and heterojunction structures [2]. Due to their unique physicochemical properties and synergistic effects with other metals or metal oxides, they have received extensive attention as excellent adsorbents and catalysts for organic pollutants [18][19][20].
In this paper, different phase and morphological structures of MnO 2 are reported ( Figure 1). The application, degradation mechanism, and development status of MnO 2 -based materials on organic pollutants are described. The organic pollutants were divided into phenolic compounds, antibiotics, dyes, and pesticides. The review provides an overview of the research related to manganese oxides and their application, including future areas of research and limitations in the current body of research.

Phase structure
There are many kinds of MnO 2 in the environment, including α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , ε-MnO 2, and λ-MnO 2 [14,21]. The six main types of crystalline MnO 2 can be classified into three categories, namely, 1D tunnel structures, 2D layer structures, and 3D mesh structures, respectively [2,22]. α-MnO 2 belongs to the tetragonal crystal system, which is widely found in nature. It has (2 × 2) tunnels of large square vacancies, which can be partially occupied by K + , Na + , Ba + , Mg 2+ or Ca 2+ ions and water molecules [23]. This structure would increase the adsorption ability of α-MnO 2 . β-MnO 2 belongs to the tetragonal system and its structure is relatively stable. The narrow tunnel of β-MnO 2 (1 × 1) can only accommodate small ions such as H + or Li + , which is not conducive to ion diffusion [24]. γ-MnO 2 has a hexagonal dense row structure with alternating growth of (1 × 1) and (1 × 2) tunnels. The disorderly and irregular alternating growth of γ-MnO 2 tunnels leads to low crystallinity and the generation of defects and vacancies. This structural feature enhances its electron exchange capacity and thus improves the catalytic performance [25]. The two-dimensional layered δ-MnO 2 belongs to a typical monoclinic system with a large interlayer distance, which can accommodate many water molecules, metal cations, and other substances [26,27]. λ-MnO 2 is a typical spinel structure. Its 3D (1 × 1) tunnel structure is conducive to electron transfer [2]. ε-MnO 2 has a polycrystalline structure with hexagonal symmetry and many cationic vacancies [28]. Therefore, the difference in the phase structure of MnO 2 also determines the difference in its properties.

Morphological structure
In addition to the phase structure, the morphology structure also has a great influence on the properties of MnO 2 . The morphological structures of MnO 2 include nanorods, nanotubes, nanowires, nanofibers, nanoribbons, nanosheets, and nanoflowers. As shown in Figure 1.
The catalytic performance for toluene decreased in the order of rod-like α-MnO 2 , tube-like α-MnO 2 , flowerlike Mn 2 O 3 , wire-like α-MnO 2 , which was consistent with the oxygen specie concentration and lowtemperature reducibility [29]. The low-temperature reducibility for toluene decreased in the order of nanotube, nanowire, nanoflower, and nanocube. The nanotubes have the highest low-temperature reducibility because of their larger specific surface area and higher the Mn 4+ content [30]. ε-MnO 2 microcubes prepared for oxidation of toluene possessed extraordinary features including the high porosity, reducibility, lattice oxygen reactivity, and Mn 4+ fraction [31]. The crystalline structure of α-MnO 2 was more important than the porous structure for phenol degradation [16]. Layered MnO 2 nanosheets have a large specific surface area and are highly porous, which provides more active sites for methylene blue molecules [32]. Nanoflower ε-MnO 2 exhibited the best removal efficiency of triclosan because of its high oxygen vacancy and Mn 3+ content, easily released lattice oxygen, and unique tunnel structure [33,34]. The crystal defects in the amorphous structure in MnO 2 nanoflowers facilitated the absorption and oxidative degradation of Rhodamine [35]. The catalytic properties of MnO 2 nanotubes are like nanorods but have a larger surface area than nanorods for degradation of phenol with higher charge transfer rate [36]. Nanofibers had the higher activity and stable properties, which have good adsorption performance in addition to excellent catalytic oxidation performance for propane [37,38]. MnO 2 nanowires can effectively degrade methylene blue at low temperatures, with only a slight decrease in the generation of free radicals and degradation efficiency after recycling [39].
The morphological structure control can determine the specific surface area, low-temperature reducibility, oxygen vacancies, surface defects, mass transportation, and charge motion electron-hole pairs, which have a great influence on the adsorption and catalytic oxidation properties of MnO 2 -based materials.

Doping and composite of MnO 2 -based materials
With the application of MnO 2 in environmental protection, the demand for catalytic properties of MnO 2 is gradually increasing too. However, the specific surface area, crystal structure, oxygen-manganese bond strength, and other aspects of MnO 2 are not ideal for some substances that are difficult to degrade. To cope with different organic pollutants, MnO 2 has been doped and compounded to improve its catalytic capacity.

Doping of MnO 2 -based materials
Elemental ion doping can adjust or change the intrinsic properties of MnO 2 , including morphology, specific surface area, oxygen vacancy formation energy, and oxygen mobility [40]. Hence, many researchers have endeavored to dope alkali and alkaline earth metal ions, other metal ions, and nonmetal anions into MnO 2 , as shown in Figure 2.
Alkali metals and alkaline earth metal ions can change the morphology and lattice structure of MnO 2 , which in turn affect their adsorption and catalytic properties. It has been reported that K + and Tb 3+ doped MnO 2 materials have higher specific surface area, Mn 3+ content, surface oxygen vacancy, and lattice oxygen activity for methyl blue [41]. The addition of K + , Mg 2+ , Ca 2+ , and Na + can affect the specific surface area, the binding energy of lattice oxygen, the oxygen vacancy, and the interlayer space of MnO 2 materials [42][43][44].
In addition to metal anion doping, nonmetal anion (N [54], B [55]) doping has also been investigated. Non-metal anion doping can reduce the energy of oxygen vacancy formation and bandgap, promote the formation of oxygen vacancy, and electron transport thus improving the optical and electrical catalytic activity of the catalyst.

Fabrication of MnO 2 -based composites
Combining with other substances is a good way to improve the catalytic performance of MnO 2 . So far, MnO 2 composite catalysts can be divided into four types: MnO 2 /MnO 2 , metal/MnO 2 , metal oxide/MnO 2 , and carbon materials/MnO 2 . MnO 2 -based composites have a larger surface area, faster electron-hole dissociation efficiency, stronger light absorption, and charge separation efficiency compared to MnO 2 catalysts alone.
A variety of composites can be synthesized by combining metal oxides and MnO 2 . The addition of metal oxides and the generation of special structures can increase the concentration of oxygen vacancies on the surface of manganese oxide, which forms a low resistance electron-defective surface and promotes the rapid transfer of carriers, giving the catalyst a high oxidation potential and a high photoreaction current. Metal oxides/MnO 2 catalyst also has special structures, such as hollow sphere structures, intercalation structures, hollow tube structure, etc. Many  Carbon materials are widely applied to enhance the catalytic performance of MnO 2 due to their lightweight, high strength, high electrical conductivity, and chemically stable. In recent years, nanotube, nanosphere, nanofiber, and graphene have been used for enhancing the activity of MnO 2 . The combination of carbon material increased the surface area, the active site of manganese dioxide, more surface oxygencontaining functional groups, and the carrier mobility of MnO 2 [78][79][80][81].
The Layered Double Hydroxides (LDH) is so known as a hydrotalcite-like material. Recently, MnO 2 -based LDH materials have been widely used for the catalytic degradation of pollutants. Due to its unique structure, MnO 2 -based LDH materials have a large specific surface area and more exposed active groups. The characteristics of its adsorption and catalytic degradation of pollutants are improved. Chen et al. prepared FeMn-LDH by co-precipitation method and used it to activate the peroxymonosulfate (PMS) for octadecylamine degradation [82]. Kabel

Organic pollutant degradation
Organic pollutants widely exist in water and soil environment. Some of them are highly toxic, easily accumulative, and difficult to degrade. Organic pollutants can be divided into phenolic compounds, antibiotics, dyes, pesticides, etc.  [93]. Zhang et al. found that the •OH concentration was increased by a factor of two when using the prepared MnO 2 with mesoporous structure and high specific surface area to degrade phenolic acid [94]. MnO 2 can also generate •OH to participate in the degradation of phenolic compounds when used as electrode materials, such as MnO 2 electrodes, polypyrrole/β-MnO 2 modified graphite electrodes [95], and MnO 2 /Nano-G|Foam-Ni/Pd composite cathodes [67].

Sulfate radical oxidation methods
More recently, SO 4 •have been proposed as an alternative to •OH for organic oxidation. The SO 4 •- can be obtained by heating, light radiation, and metal activation from sulfate oxidizers such as persulfate and PMS [96].

In addition to •OH and SO 4 •-, superoxide radical (O 2
•− ) is also produced in some cases. Wang et al. found that δ-MnO 2 crystals with high exposure surfaces promoted the formation of O 2 •− and accelerated the degradation of phenol [102]. Another process is the mechanical ball milling of oxygen vacancy enriched manganese dioxide. It was found that the obtained BM20-MnO 2 produced O 2 •− and the degradation rate of tetrabromobisphenol was increased by 22 times [4]. Bisphenol A was oxidized by a non-radical mechanism using the formation of reactive complexes between amorphous MnO 2 and PMS. The amorphous MnO 2 could activate PMS, and the generated active MnO 2 /PMS system degraded about 94% of bisphenol A within 60 min [103].

Antibiotics
Antibiotics are widely used around the world to treat infectious diseases, and they enter soils and water systems through leachates of sewage, manure, and leachate from pharmaceutical waste. They are persistent in the environment and remain toxic to the hematopoietic system, posing a serious threat to the environment and humans. Therefore, it is urgent to develop effective and economical methods to eliminate antibiotics, considering their chemical and biological stability [75,104]. Antibiotics mainly include ciprofloxacin, tetracycline, sulfa antibiotics, and other antibiotics. Most of the degradation of antibiotics is accomplished by oxidative free radicals. •OH, SO 4 •-, and O 2 •− are the main active species.

Ciprofloxacin
Ciprofloxacin is a quinolone antibacterial drug with strong penetrating properties. Many researchers have explored the degradation of ciprofloxacin. 98.3% of ciprofloxacin was degraded within 30 min in a visible light/PMS system mediated by magnetic γ-Fe 2 O 3 -MnO 2 with many oxygen vacancies [105]. Wang et al. synthesized copper-based bimetallic oxides to decompose ciprofloxacin in wastewater by activating H 2 O 2 to produce •OH. The degradation efficiency was up to 100% [106]. The degradation efficiency of ciprofloxacin by α-MnO 2 combined with dielectric barrier discharge could reach 93.1% after 50 min [107]. In summary, MnO 2 has an important effect in the degradation of ciprofloxacin.

Tetracycline
Tetracycline is a very common antibiotic in polluted environment. And its contamination area is very large. Tetracyclines also induce resistance in microorganisms and their metabolic intermediates are more toxic [108]. The α-MnO 2 /ZnO-C z-type photocatalyst degraded 96.69% of tetracycline within 60 min with O 2 •− and •OH [109]. The adsorption capacity of Erdite/MnO 2 nanorods on tetracycline was 2613.3 mg/g, which was due to the coordination reaction between the -NH 2 group of tetracycline and the hydroxyl group [110]. Minale et al. degraded 91.46% of oxytetracycline at pH 5 by using a sodium polyacrylate hydrogel loaded with MnO 2 [111].

Sulfa antibiotics
Sulfa antibiotics are typical toxic antibiotics. Khan  the main active substances [114].

Other antibiotics
Many other types of antibiotics can also be harmful to the environment and humans, such as ceftiofur (CEF),

Dyes
Water pollution resulted from organic dyes and their residues with the rapid growth of textile and dye industries has become a major environmental problem [72,117,118]. In addition to traditional adsorption methods, advanced oxidation technologies (i.e. photocatalytic oxidation, Fenton, and Fenton-like reactions) have been widely applied to eliminate dyes from wastewater [119].

Adsorption and oxidation method
The conventional adsorption-oxidation method is a more economical, mature, and environment friendly technology compared to other technologies. It can be used for degrading most dyes, such as RhB, Reactive Blue 19 (RB19), and so on. Core-shell MnO 2 -SiO 2 nanorods were synthesized by Gong et al. It was found that the final decolorization rate of RhB was 98.7% under the action of the absorption and oxidization [120]. Fathy et al. synthesized a novel and efficient nanocomposite catalyst γ-MnO 2 /MWCNT by in situ co-precipitation method. The results showed that the degradation rate of RB19 dye was 100% under the combined effect of adsorption and catalytic oxidation [121]. Adsorption oxidation is widely used in the field of organic dye degradation and has a large potential for development.

Photocatalytic reaction
In recent years, more and more studies have been applied to the photocatalysis technology for the degradation of dyes. Chiam

Fenton reaction
The

Fenton-like reaction
With the development of the Fenton reaction, Fentonlike reaction has been gradually applied to the prevention and control of dye pollution. MnO 2 @ZIF-8 coreshell nanoparticles were synthesized by Cao et al. The nanoparticles were used as photocatalysts to degrade RhB in a Fenton-like process and the final degradation rate was greater than 96.0% [126]. Yu et al. prepared a series of β-MnO 2 macro catalysts for the degradation of some dyes (MB, methyl orange (MO), RhB, and acid orange II (AOII)) under hydrothermal conditions. After 40 min of reaction, the degradation rates of MB, MO, RhB, and AOII were 95%, 45%, 52%, and 63%, respectively [127].

Pesticides
As food production increases, the use of pesticides is also growing rapidly. Of the total pesticide use, about 0.1% meets the target, and the rest remains in the environment. This leaded to the deterioration of soil quality and following crop yield reduction with poor quality, and water pollution. Ultimately, it poses a threat to animals and humans [128][129][130]. Pesticides can generally be classified as phenols, organophosphates, and other types.

Organophosphorus pesticides
Among the common pesticide contaminants, organophosphorus pesticides are the most commonly used agricultural pesticides, most of which are highly or moderately toxic [131].

Other pesticides
In addition to organophosphorus compounds, some other pesticides are found in sewage and soil, such as toxaphene, DIN (difenoconazole), and DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane) and so on. 96.5% of toxaphene was removed by the MnO 2 /cellulose fiber nanocomposite [135]. Zhao [136]. New pesticide contaminants are being discovered one after another. This is an issue that needs to be addressed urgently.

Other organic pollutants
In addition to the above pollutants, perfluorooctanesulfonic acid (PFOS), cationic blue (X-GRL), carbamazepine (CBZ), methyl benzoate, methylparaben, paracetamol, and dimethylhydrazine were also found in wastewater and contaminated soil, which can be degraded by manganese oxides. The degradation mechanisms are mostly related to free radicals. The degradation rate can reach 80-100% in a few hours (Table 1). Although manganese oxide has a good effect on the degradation of most organic pollutants, new pollutants are being discovered one after another. Therefore, we still need to continuously develop organic degradation technologies.

Summary and outlook
Environmental pollution caused by rapid industrialization is one of the major challenges faced by human society. Manganese oxides are one of the most promising catalytic materials at this stage due to their unique properties. In the past few years, this material has made encouraging breakthroughs in catalytic degradation of organic pollution. This paper reviews the various phase and morphological structures of MnO 2 , three modification methods (self-mixing, doping, and composite) of MnO 2 . In addition, the catalytic degradation effect and mechanism of phenolic compounds, antibiotics, dyes, pesticides, and other organic pollutants are also reviewed. MnO 2 materials have been developed for the degradation of organic pollutants and their catalytic performance has been improved. However, it must be acknowledged that the application of MnO 2 materials in this field of catalytic organic degradation still faces various unresolved problems. The current problems we need to solve or the direction we need to study in the future are: 1) To enhance the mechanical and thermodynamic stability of MnO 2 materials to maintain their structures; 2) To ensure the activity of advanced oxidation free-radicals catalyzed by the catalyst; 3) To explore the synthesis of hetetoatom-doped MnO 2 and construction its composites with specific function in a controllable method; 4) To clarify the intermediates and reaction path in the catalytic process of MnO 2 catalyst; 5) To explore the recovery and regeneration of MnO 2 catalysts.

Disclosure statement
No potential conflict of interest was reported by the author(s).