Synergistic Effect of Nanocarbons and their Supported Single/Binary Transition Metal Oxides on Dye Removal

A significant volume of printing and dyeing effluent has been generating worldwide. The amount of discharged textile dye wastewater accounts for high percentage of total wastewater discharge, posing a serious danger to environmental security [1,2]. Additionally, the dyes are non-biodegradable, mutagenic and carcinogenic effects to humans. As a result, establishing modern and cost-effective techniques to meet stated criteria for remediating water and wastewater, as well as population needs, is now a requirement. Among developed methods, many researchers focused on the adsorption using metal oxide supported in single or binary form on different carbon materials as a promising method to overcome the previous shortcomings of adsorption by activated carbon (AC) alone, for example [3,4]. Accordingly, the combination of metal oxides with carbon materials could be enhanced the adsorption of dyes with further degradation operations according to type of these oxides including advanced oxidation processes (e.g., Fenton-oxidation, photo-oxidation as exposed to light source and wet oxidation with H2O2) [3-7]. Thus, a synergistic effect is obtained due to the presence of single/binary transition metal oxides (S/BTMOs) supported on carbon materials. Among them, binary transition metal oxides with spinel structures such as CuMn₂O₄, MnCo₂O₄, ZnMn₂O₄, NiMn₂O₄, CuCo₂O₄, MnFe₂O₄, and Mn₂CuO₄ have recently become an importance in industrial, environmental and energy applications [8-13].

Activated carbons and their supported single/binary transition metal oxides (S/BTMOs) for enhanced dye removal

In recent years, the loaded magnetic metal oxides on the AC gained much interest in order to improve the adsorption capacity and make the possible recovery of powdered adsorbents [3,4]. For example, some studies have prepared magnetic oxides in single/binary states using especially iron oxide for developing a magnetized activated carbon by simple economic separation. The adsorption and recovery by magnetic separation of AC loaded by F₂O₃ [14], Au-Fe₃O₄ [15], and NiFe₂O₄ [16] have greatly improved. Other oxides such as cerium dioxide (CeO₂), zinc oxide (ZnO) [17], manganese dioxide (MnO₂) [18] titanium oxide (TiO₂) [19] and Ce-TiO₂ [20] have been supported on AC to increase the catalytic degradation of dyes. These oxides loaded on AC can increase the convenience of operation, recovery, and efficiency of adsorption processes as a result of a synergistic effect.

Carbon nanotubes and their supported S/BTMOs for enhanced dye removal

Carbonaceous nanostructures are a deterministic support in water treatment because of their functional properties such as large surface area, high mechanical strength, high porosity, a significant aspect ratio, strong thermal and electrical properties, good chemical stability, impressive hydrophobicity, simplicity of use, and separation characteristics [21,22]. In particular, magnetic oxides loaded on carbon nanotubes (CNTs) hold remarkable properties such as surface-volume ratio, higher surface area and convenient separation methods to be effective adsorbents in removal of heavy metals and dye [23]. Recently, the synthesis of manganese dioxide (MnO₂) loaded on multi-walled carbon nanotubes (MWCNTs) has attracted tremendous increasing research interest in dye removal via Fenton-reaction degradation [24,25]. The combination of both adsorption process and catalytic oxidation processes through MnO2/MWCNTs nanocomposite system could be provided a simple, efficiently and environmentally friendly water treatment. Photodegradation of methylene blue dye enhanced significantly over fabricated CNTs-supported Mn-TiO₂ [5], CNTs/TiO₂/AgNPs/ surfactant nanocomposites [7], as well as Fenton-catalyst based Cu_Ssub>-TiO₂/MWCNTs nanocomposites [6]. NiO and Co₂O₃, NiCo₂O₄, and NiCo2O₄/MWCNTs nanocomposites have been synthesized and studied for photodegradation of Reactive Red 120 dye [26], where NiCo2O4/MWCNTs enhanced largely the photodegradation.

Graphenes and their supported S/BTMOs for enhanced dye removal

Graphene has emerged as a useful nano-adsorbent for environmental applications because of its high theoretical specific surface area (~2630m²g^-1) [27]. Moreover, abundant oxygencontaining functional groups have been incorporated in graphene to get graphene oxide (GO) and reduced graphene oxide (RGO) to enhance their adsorption capability [28,29]. Maximum adsorption capacities of methylene blue dye on the MFe₂O₄@GO (M=Cu, Co or Ni) were 25.81, 50.15 and 76.34mg g−1, respectively [30]. Hsieh et al. [31] reported degradation of acid orange 7 (AO7) by Pt-TiO2/G nanocomposites [31]. Pt-TiO2 can be served as a chargegenerating centre while graphene acted as an electron acceptor and transporter in the composite [31]. Ultra large surface area and strong π–π interaction on the surface of graphene - based metal oxides are responsible for adsorption [28].

Nanocarbons such as nanopore-structured carbon, carbon nanotubes (CNTs) and graphene can be used as substrates for the loading of single/binary transition metal oxides to make nanocompostes. The combination of metal oxides with nanocarbon materials gives synergistic effect on dye removal. The nanocomposites showed enhanced adsorption of dyes with further degradation processes according to type of these oxides, including advanced oxidation processes as reported in literature as Fenton-oxidation, photo-oxidation as exposed to light source and wet oxidation with H₂O₂. Among them, binary transition metal oxides with spinel structures such as CuMn₂O₄, MnCo₂O₄, ZnMn₂O₄, NiMn₂O₄, CuCo₂O₄, MnFe₂O₄, and Mn₂CuO₄ loaded on nanocarbons have recently become an importance in industrial, environmental and energy applications.

This work was supported by the Fundamental Research Funds for the Central Universities, China (20822041E4065,20826041F4087), Sichuan University-Yibin Municipal Government Cooperation Project (2020CDYB-33), and Sichuan Provincial Government in China (0082204151556).

The authors declare no conflict of interest.

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