Degradation activity of plasma catalysis between dielectric barrier discharge (DBD) and carbon nanotubes-graphene-nickel foam (CNTs-G-Nif) has been studied in treatment of dye wastewater. CNTs-G-Nif was prepared through a two-step chemical vapor deposition (CVD) approach. The composite has been characterized by different techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. SEM results showed that the Nif as the growth substrate was evenly wrapped by G and then CNTs were successfully grown on G as the support. The growth mechanism of composite was proposed. The possible coupled catalytic mechanism between DBD and CNTs-G-Nif were addressed. In addition, the modification on G-Nif was found by SEM during the discharge process in liquid phase. And the modification mechanism of DBD plasma (DBDP) acting on composites was discussed. Finally, by means of analyses of ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS), the general degradation pathway and stepwise degradation pathways of alizarin green (AG) were proposed in detail.

  • A novel degradation system DBD plasma/CNTs-G-Nif is proposed.

  • The new model of coupling factor is put forward to evaluate the degradation effect.

  • DBD plasma has modification on G-Nif in degradation of dye wastewater.

  • Comprehensive mechanisms of preparing,coupling,modifying and degrading are explored.

With high chroma, large displacement, high content of electrolyte and complex composition, the printing and dyeing wastewater was considered as the most pernicious industrial wastewater. Therefore, it must be thoroughly mineralized and degraded to eliminate environmental impacts (Wang et al. 2016; Ben et al. 2017). Alizarin green (AG), a kind of typical anthraquinone dyes was selected as a main research object in this study. The current treatment methods for wastewater pollution mainly included physical methods, biological methods (Farabegoli et al. 2010), physical and chemical methods (Hayeeye et al. 2016; Liu et al. 2017), and advanced oxidation processes (AOPs). AOPs are most suitable for removing persistent and recalcitrant organic compounds. AOPs do not form harmful byproducts and have a short reaction time because of the use of hydroxyl radicals formed by a combination of ozone (O3), hydrogen peroxide, and ultraviolet (UV) (Colindres et al. 2010), Fenton's reagent (Kumar et al. 2011), ultrasonic (Shen et al. 2017a, 2017b), electrochemical oxidation (Rubi-Juarez et al. 2016), photocatalytic oxidation (Liu et al. 2006; Hou et al. 2019), plasma process (Jiang et al. 2014; Magureanu et al. 2015), combined technologies, etc.

It has been proven that the low temperature plasma is capable of producing a variety of highly oxidizing substances such as hydroxyl radical (·OH), O3, hydrogen peroxide (H2O2) and superoxide radical (·O2) (Jiang et al. 2014). In addition, low temperature plasma also contained UV radiation, shock wave, strong electric field and pyrolysis. Therefore, low temperature plasma could be used for treating pollutants (Magureanu et al. 2015), especially for the refractory aromatic organics in printing and dyeing wastewater. The discharge forms of low temperature plasma mainly included pulse discharge, direct current corona discharge, glow discharge, sliding arc discharge and dielectric barrier discharge (DBD). Among these discharge forms above, low temperature plasma technology based on DBD was easy to operate and able to obtain high treatment effects. In recent years, it has received great attention in the area of water treatment (Wang et al. 2008; Wang et al. 2011; Lou et al. 2012; Jiang et al. 2016). Jiang et al. (Feng et al. 2014) reported that 250 mg·L−1 of Indigo Carmine solution was completely degraded in 10 minutes at the voltage of 25 kV and frequency of 200 Hz on a dielectric barrier discharge plasma (DBDP) reactor. Compared with other AOPs, plasma technology alone showed certain superiority in degradation. However, there were still some block blanks in this field, especially in the reduction of the energy consumption and further enhancing utilization effect of the active substances in plasma system (Chen et al. 2009). Therefore, the combined systems of plasma have been extensively studied to solve the problems above recently, mainly including the combination with inorganic ions and various kinds of catalytic materials.

In recent years, electrode catalytic materials of carbon class (Cc) composite have been studied. A wide range of C materials included activated carbon, carbon nanotubes (CNTs), carbon nanofibers, G and various composite materials. Mousset et al. (2016) used different forms of G as cathode material in electro-Fenton (EF), implying the possibility of using pristine G in EF for the treatment of wastewater. Ji et al. (2015) demonstrated that flow-through electrode of polyaniline supported on lamellar-structured G could be used for electrocatalytic reduction of Cr6+. Huang et al. (2013) provided a simple hydrothermal method to synthesize wall-like layered metal oxide MMoO4 (M = Ni, Co) nanosheet arrays, which are directly grown on flexible carbon cloth and used as a non-removal for advanced supercapacitors. Adhesive electrode. Zhang et al. (2016) have used a simple two-step method to fabricate Ni1/3Co2/3Se2 nanowires and nanosheets on carbon cloth. They are interconnected to form a layered three-dimensional network structure, showing high electrochemical HER active. Vecitis et al. (2011) reported that active MWCNT filter was effective toward the adsorptive removal and electrochemical oxidation of the aqueous dyes, Methylene Blue and Methyl Orange, and the oxidation of the aqueous anions, chloride and iodide. Shen et al. (2015) confirmed that G materials could enhance the removal effect of dye, oil, organic solvents, heavy metal ions and gas pollutants. Zhang et al. (2010) reported that the CNTs-pillared RGO composite materials could exhibit an excellent visible light photocatalytic performance in degrading dye Rhodamine B because of the unique porous structure and the exceptional electron transfer property of G. Based on the studies above, there were reasons to consider that Cc electrode catalytic materials could also be combined with DBDP to treat wastewater on account of its unique discharge structure. Recently, (Sun et al. 2010) used the solid carbon source at 1,073 K to produce large, high quality and controllable G and doped G. Latorre et al. (2010) used phenomenological model to describe the growth process of CNTs prepared in chemical vapor deposition (CVD) method. Lv et al. (2014) suggested that CNTs could be combined with G to form three-dimensional (3D) structure. However, the CNTs-G-Nif was prepared through a two-step CVD approach with methane (CH4) functioned as the carbon source which was distinguished from the ethanol functioned likewise (Zhu et al. 2014). 2D materials can be water treated through photothermal evaporation, antibacterial and photocatalysis. Therefore, research on this application is also explosive growth (Xie et al. 2020a, 2020b). Due to its large absorption surface, strong absorption in the broad solar spectrum and effective photothermal conversion, two-dimensional nanomaterials have attracted great attention in water production induced by photothermal evaporation. And two-dimensional photothermal materials surpassed graphene in terms of clean water production (Xie et al. 2020a, 2020b). However, since the band gap of the most commonly used 2D materials is not as good as 3D carriers in terms of photocatalysis (Zhang et al. 2019), there are certain limitations.

As a kind of fixed phase of 3D material, CNTs-G-Nif should be more suitable for the DBDP reaction system, compared with powder catalysts. Such CNTs-G-Nif was convenient for subsequent separation and cleaning. At the same time, it could be used as a pseudo electrode material due to its special electrochemical properties. Meanwhile, DBDP could solve the problem of activation on CNTs which should be activated before used. In addition, DBDP also had modification on G and CNTs in wastewater, which was interesting to study as well. In summary, a novel degradation system, where CNTs-G-Nif was combined with DBDP, would be first proposed and investigated in this study. The degradation effects AG by DBDP and DBD/CNTs-G-Nif were evaluated with kinetics model, respectively. The growth mechanism of prepared composites, coupled with the catalytic mechanism between DBD and CNTs-G-Nif, and degradation mechanism of AG were explored one by one.

Experimental materials

All of chemicals were of analytical grade. Polyethylene glycol (PEG, MW = 20,000), ethanol, acetone, hexamethylenetetramine, Ni(NO3)2·6H2O, ZnSO4·7H2O, NaOH and KCl were purchased from Xilong Scientific Co., Ltd (Guangdong, China). AG and H2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. The deionized water was used throughout this study.

Preparation

The Nif (2 cm × 2 cm, thickness 1.0 mm, 99.99% purity) was washed for several times by an ultrasonic cleaner using acetone, ethanol and distilled water, respectively. Then, the Nif was taken out and dried slowly. A Nif was placed in quartz boat (20 mm). The quartz boat was pushed into the center of a horizontal quartz tube (100 mm diameter) in a tube furnace (Anhui BEQ Equipment Technology Co., Ltd, China, Figure 1(a)), and heated to 1,273 K under mixed gas of 30 standard m2/min H2 and 200 standard m3/min (sccm) Argon (Ar). Then, reaction gas of CH4 was fed to the system at flow rates of 15 sccm, keeping 30 min at 1,273 K for G growth. Subsequently, the sample was rapidly cooled down to room temperature under H2 and Ar flow, resulting in high quality G layers grown on both sides of the Nif. The dip-drawing method was used to coat G with a catalyst solution. The G-Nif substrate was dipped into 10 mL ethanol/polyethylene glycol (PEG) (10%, w/w) solution containing 0.1 mol·L−1 Ni(NO3)2 for about 5 min, and then drawn out and dried in air slowly. Prior to the growth of CNTs, the G-Nif substrate was heated to 773 K for 10 min under Ar flow (170 sccm), followed by H2 annealing (50 sccm). Then, the furnace temperature was continued to increase to 1,173 K under Ar and H2 flow. The CH4 (30 sccm) mixed with Ar and H2 was added into the tube furnace at the reaction temperature of 1,173 K for 40 min. Subsequently, the sample was cooled down to room temperature under protection of Ar flow. And the CNTs-G-Nif could be obtained.

Figure 1

CVD reaction equipment (a) (1. quartz tube, 2. tube furnace, 3. gas control valves, 4. temperature indicator, 5. breather pipe) and DBDP generator (b) (1. power source, 2. cathode interface, 3. anode inteface, 4. circulating pump, 5. reaction kettle).

Figure 1

CVD reaction equipment (a) (1. quartz tube, 2. tube furnace, 3. gas control valves, 4. temperature indicator, 5. breather pipe) and DBDP generator (b) (1. power source, 2. cathode interface, 3. anode inteface, 4. circulating pump, 5. reaction kettle).

Characterization

The phase and composition of the as-prepared samples were detected by X-ray diffraction (XRD) studies using an X-ray diffractometer with Cu Kα radiation under 40 kV and 40 mA with ranging from 10° to 80° at a scanning rate of 10°·min−1 (D/MAX-IIIA, Japan). Raman spectroscopy studies were carried out on a Dispersive MicroRaman spectrometer (Invia, Renishaw) with an argon-ion laser at an excitation wavelength of 514 nm in the range 500–3,000 cm−1. The morphology of the prepared samples was examined using a scanning electron microscope (SEM) (Hitachi S-4700, Japan).

Degradation activities

Figure 1(b) illustrated the DBDP reactor (DBD-50, Nanjing Suman Plasma Scientific Co., Ltd). The H2SO4 (0.1 M) and NaOH (0.1 M) were applied to adjust the pH of solution (pH 2.0–11.0) determined by a pH meter (pHS-25C, Shanghai Yidian Scientific Instrument Co., Ltd, China). The conductivity of solution (105–2,000 μS·cm−1) was adjusted by KCl and determined using an electrical conductivity meter (DDS-307, Shanghai Yueping Scientific Instrument Co., Ltd, China). The corresponding concentrations of AG (50–200 mg·L−1) were configured and the corresponding input powers (56–112 W) were adjusted by controlling currents. Coupling system of DBDP/CNTs-G-Nif were operated with CNTs-G-Nif 2 cm × 2 cm. The absorbance of the sample was determined at regular intervals during the degradation process.

Analysis of samples

The samples were taken from the degradation process of AG under DBDP/CNTs-G-Nif. The UV-Vis spectrum was determined by using Shimadzu UV-1601PC UV-Vis Scanning Spectrophotometer with the wavelength ranging from 200 to 800 nm. FT-IR spectra of the samples were recorded by Tensor 27, Bruker spectrometer (Germany) over the wavelength range of 4,000–400 cm−1. The GC-MS analysis was performed to identify the intermediates produced through the AG degradation using a method described in our previous study. Meanwhile, the samples were also analyzed by LC-MS spectrometry (Water Alliance 2795 LC) to assist the identification of degradation products.

Characterization analysis

Macroscopic feature

It could be seen from Figure 2 that they were Nif, G-Nif and CNTs-G-Nif from left to right in order. The clean Nif presented silvery white and had metallic luster. After the growth of G on Nif, the color of composite was changed to be grey and the texture became hardened. After the growth of CNTs on G-Nif, the color of composite was jet black. Inverting the colors of the digital photo, some changes on the surface of Nif could be found after compositing G and CNTs. Black dots could be found on the surface of G-Nif. It could be explained that Ni may be separated out from sample during the cooling process. But few black dots could be found on the surface of CNTs-G-Nif, which illustrated that G-Nif was completely covered with CNTs.

Figure 2

The digital and colorinvert photos of Nif, G-Nif and CNTs-G-Nif.

Figure 2

The digital and colorinvert photos of Nif, G-Nif and CNTs-G-Nif.

Morphology and structure analysis

Figure 3 showed images of XRD and Raman in regard to CNTs-G-Nif. XRD spectrum depicted in Figure 3(a) showed a series of peaks. The peak (002) stood for the existence of element C while the peaks (111), (200) and (220) represented the existence of element Ni. Besides, there was no existence of other elements or oxides. This was consistent with the result that the material composition could be seen from SEM. From Figure 3(b), the existence of D-peak and 2D-peak indicated that the material had a typical graphite structure (He et al. 2013). 2D-peak was relatively weak, indicating the G was multilayer (Zhu et al. 2014). G-peak was mainly caused by the stretching vibration of C = C (sp2) on the sheet of G. D-peak was referred to as the defect peak, which was caused by the stretching vibration of disordered or sp3 hybridized C atoms in the carbon material, corresponding to the defect level of carbon materials (Georgakilas et al. 2016). The literature proved that the integrity of the composite's structure could be maintained during the growing process of G and CNTs on Nif (He et al. 2013).

Figure 3

The XRD (a) and Raman (b) patterns of samples.

Figure 3

The XRD (a) and Raman (b) patterns of samples.

Figure 4 showed the images of SEM in different growth stages of composite materials. As illustrated in Figure 4(a), the surface of Nif was slightly rough, but the whole was relatively smooth within 100 μm. It could be seen clearly that the surface of Nif substrate was encased by squamous G layers after first step CVD growth from Figure 4(b). The relative datum manifested that there were 2–5 layers G when it was grown based on Ni (Zhang et al. 2013). It was obvious that there were many wrinkles on G through the illustration of Figure 4(b). This was due to the inconsistent coefficient of thermal expansion between Nif and G (Dong et al. 2012a, 2012b). It could also be seen that there were some white matters on the surface of G, which may be the Ni separated out from the skeleton of Nif during the cooling process (Dong et al. 2012a, 2012b). The micro display was exactly echoed with the macro display of colorinvert photos in Figure 2. As shown in Figure 4(c), it could be found that there were thickly-dotted tiny crystals clustered on the G layer randomly. This was because that Ni salt on the surface of G layer was turned into Ni nanoparticles after high temperature with 773 K in the tube furnace, which could be the subsequent catalyst for growing CNTs (Dong et al. 2012a, 2012b). As shown in Figure 4(d), rough and curved CNTs were randomly grown on the surface of G after the second step of CVD growth. As can be seen from Figure 4, the whole composite maintained the honeycomb-shaped structure after repeated growth. The overall morphology of each stage was similar to that reported by the literatures (Dong et al. 2012a, 2012b; Zhu et al. 2014). This fully demonstrated that it was feasible to prepare similar products in different carbon source conditions by replacing ethanol with CH4.

Figure 4

The SEM photographs of samples (a) Nif; (b) G-Nif; (c) Ni crystaline-G-Nif; (d) CNTs-G-Nif.

Figure 4

The SEM photographs of samples (a) Nif; (b) G-Nif; (c) Ni crystaline-G-Nif; (d) CNTs-G-Nif.

Application of DBDP alone

Effect of operational parameters

In terms of wastewater treatment with DBDP, there were many influence factors. Precisely, four common influence factors would be studied in this study, including concentration of dye, pH of solution, conductivity of solution and input power of DBDP. Figure S1 (Supplementary material) shows the changes between decolorization rate of AG and values of four factors. The decolorization rate was decreased when values of concentration, pH and conductivity were gradually increased. But the decolorization rate increased when values of input power were gradually increased. The amount of active material and high energy electron produced in plasma discharge was constant at the same time as when the input power was unchanged. When the concentration of AG was higher, the relative decolorization rate was lower. It could be found that acidic condition was of benefit to degradation of AG. The data showed that the oxidation potential of ·OH was 2.74 eV when pH was 3 but the oxidation potential of ·OH was only 2.34 eV when pH was 9. It illustrated that the oxidation ability of ·OH was stronger under acidic condition (Dong et al. 2012a, 2012b). DBD was non-equilibrium discharge and the energy was transferred to other molecules through the collision so that the target could be excited or dissociated. With the increase in conductivity, the process of micro-discharge was hindered and the formation of strong reactive free radicals was slowed down, thus the degradation reaction was reduced. The increase of input power could promote the increase of energy in the reaction, which was conducive to produce more active substances and high energy electrons. Thus, the increase in input power was beneficial to the degradation of AG.

Establishment of the kinetics model equation

The reaction order showed the relationship between the concentration of the remaining reactants and the reaction rate. Through the analysis of the experimental data, the reaction order could be determined. It was demonstrated that the degradation of AG could be regarded as the pseudo-first-order reaction kinetics in DBDP alone by plotting of ln(c/c0)-t. It can also be obtained from Figure S2 that ln(c/c0)-t presented better linear fitting before 60 seconds. As can be seen in Table 1, the R2 values of ln(c/c0)-t that were greater than 0.99, 0.97 and 0.93 accounted for 45, 85 and 100% of the total R2 values, respectively, which illustrated the degradation process of AG could be described in a first-order kinetic model in 60 seconds. When the reaction time exceeded 60 seconds, the data points started to fluctuate, and then the degradation process was no longer accurate with the first-order reaction kinetics model. It also reflected that the degradation reaction process became complicated and the large number of intermediates would be produced. Thus, the relationship between concentration of residual AG and reaction rate was affected. The degradation process of AG was an imprecise first-order reaction kinetics model when the reaction time was more than 60 seconds.

Table 1

The value of R2 under different factors

Concentration (mg·L−150 75 100 150 200 Figure S2a 
R2 0.99204 0.99669 0.97806 0.99726 0.96614 
pH 11 6.5 Figure S2b 
R2 0.93729 0.97557 0.99726 0.99297 0.99651 
Conductivity (μS·cm−1105 105 105 105 105 Figure S2c 
R2 0.97323 0.98248 0.98284 0.98949 0.97974 
Input power(W) 56 70 84 98 112 Figure S2d 
R2 0.94054 0.99273 0.98284 0.99461 0.99356 
Concentration (mg·L−150 75 100 150 200 Figure S2a 
R2 0.99204 0.99669 0.97806 0.99726 0.96614 
pH 11 6.5 Figure S2b 
R2 0.93729 0.97557 0.99726 0.99297 0.99651 
Conductivity (μS·cm−1105 105 105 105 105 Figure S2c 
R2 0.97323 0.98248 0.98284 0.98949 0.97974 
Input power(W) 56 70 84 98 112 Figure S2d 
R2 0.94054 0.99273 0.98284 0.99461 0.99356 
The proportionality constant of the apparent chemical reaction rate equation could be called the apparent chemical reaction rate constant. In the DBDP reaction system, the apparent chemical equation was determined as follows:
formula
(1)
where c is the concentration of target pollutant, kDBDP is the kinetic constant of DBDP system, kobs is the apparent chemical reaction rate constant, and t is time.

There were many factors influencing the apparent reaction rate, such as concentration, temperature and pH (Beltran-Heredia et al. 2001; Gao et al. 2004; Cheng et al. 2013). Beltran-Heredia et al. (2001) used a modified Arrhenius expression to explain that the kinetic rate constants were affected by the temperature and pH. Cheng et al. (2013) showed that the kinetic constant was affected by the target concentration and ozone consumption when the light intensity was constant. The similar kinetics model equation was used to describe the degradation process by other AOPs except DBDP in our previous research (Shen et al. 2017a, 2017b).

For the study of dye wastewater treatment using plasma, concentration of AG, pH, conductivity, and the input power were selected as the factors. The following empirical model was used to describe the apparent reaction rate constant in this study:
formula
(2)
where kobs is the apparent reaction rate constant, A refers to the former factor, c0 is the concentration of AG (mg·L−1), [OH] is pH value, Kcon is the solution conductivity (μS·cm−1), P is input power (W), and α, β, γ and ɛ are corresponding mathematical constants.
Equation (3) could be got by logarithm of the above equation.
formula
(3)
Since the voltage in this study was set to 70 V, Equation (3) becomes:
formula
(4)

Then, linear fitting of the ln kobs served as function of ln c0, ln [OH], ln Kcon and ln P.

A multiple regression analysis of the kobs value vs. c0, [OH], Kcon and P was followed, leading to: α = -0.547, β = −0.0743, γ = −0.1036 and ɛ = 2.68 from Figure S3.

Then, the apparent reaction rate constant, kobs becomes:
formula
(5)
A = 0.681 by calculating, Equation (5) becomes as follows:
formula
(6)
When Equation (6) was substituted into Equation (1), Equation (7) could be obtained:
formula
(7)
where cu was the residual amount of AG.

Equation (7) was the apparent chemical equation established for treatment process of AG wastewater under DBDP.

Application of combined systems

As shown in Figure 5, the kinetic constant of DBDP alone was 0.0179. The kinetic constant of DBDP/CNTs-G-Nif was namely k(DBDP/CNTs-G-Nif) = 0.0257. The kinetic constant of CNTs-G-Nif alone k(CNTs-G-Nif) could be approximated as 0 in 150 s. Therefore, cofactor E, which was commonly used to evaluate synergistic effect of the combined system before, would be got through the Equation (8):
formula
(8)
Figure 5

The comparison analysis on combined systems: (a) decolorization rate; (b) kobs.

Figure 5

The comparison analysis on combined systems: (a) decolorization rate; (b) kobs.

Data presentation of synergistic effect was that 1 + 1 > 2. The cofactor E = 1.44> 1 illustrated that there was synergistic effect DBDP/CNTs-G-Nif. This result was consistent with coupling effect.

Exploration of reaction mechanisms

The preparation mechanism on composite

In order to understand the growth of composite at various stages better, a feasible preparation mechanism was proposed by probing into the process of preparation in this study. As Figure 6 illustrated, C atoms would infiltrate into metallic Ni and the whole temporarily formed a solid solution state influenced by high temperature when CH4 and foam nickel were used as the carbon source and the base of growth, respectively. Relevant data study indicated that the solubility of C in Ni was about 0.9% when the reaction temperature reached to 1,173 K (Li et al. 2009). Because of the cooling effect, the oversaturated C atoms were separated out of Ni surface to form grapheme (Kim et al. 2009; Guermoune et al. 2011), namely G-Nif. The G-Nif dipped in Ni salt/ethanol/PEG solution was slowly pulled out to form coating state on its surface (Kim et al. 2012). Then, Ni nanoparticles, the growth catalysts of CNTs, were randomly formed on the surface of G due to the thermal expansion and H2 etching during the process of heat treatment (Nessim et al. 2008). On account of subsequent high temperature, the rapid spread of C atoms piled and accumulated from the bottom of nano-Ni growth catalysts to form directional CNTs (Czekaj et al. 2007; Kim et al. 2012) which was confirmed as multi-walled CNTs (Dong et al. 2012a, 2012b), namely CNTs-G-Nif. In addition, to make a vision, the nanocatalysts with smaller size could be loaded on the surface of CNTs to form nanocatalysts-CNTs-G-Nif in order to further enhance the properties of composite materials.

Figure 6

The schematic illustration for preparation mechanism of the class of X-Nif.

Figure 6

The schematic illustration for preparation mechanism of the class of X-Nif.

The coupled catalytic mechanism between DBDP and CNTs-G-Nif

An explicably coupled catalytic mechanism was expounded in Figure 7 relying on the process of degradation treatment in this study. CNTs-G-Nif was located in the bottom of water phase and was separated by a layer of quartz medium with low voltage pole of DBDP. The current flowed from high voltage pole would firstly flow through CNTs-G-Nif then flowed to low voltage pole. Therefore, the CNTs-G-Nif could be regarded as the pseudo-cathode materials of DBDP, enhancing the stability of discharge and the performance of mass transfer, to improve the effect of plasma catalysis. In addition, the composite CNTs-G-Nif which presented the honeycomb structure on the whole could target contaminants in a relatively wide range. The CNTs grown under the support of G could lower the overpotential of the substrate in the reaction of DBDP. At the same time, CNTs as the channel of electron transport could promote electron transfer and enlarge current response (Sun et al. 2010). Thus, the special plasma catalysis effect could be produced for the mineralization of substances such as dye of AG and its degradation products.

Figure 7

The mechanism of composite materials coupled with DBDP.

Figure 7

The mechanism of composite materials coupled with DBDP.

Since CNTs had special one-dimensional hollow tubular structures and high specific surface area (Lv et al. 2014), the surface of inside and outside layer of CNTs had a lot of active points which could be regarded as host of active material generated by discharge (Sánchez-Polo et al. 2005; Valdman et al. 2018) and adsorption points which could be utilized to capture and adsorb organic contaminants in aqueous solution (Liu et al. 2013).

On account of the effect in the atmosphere of DBDP, dye molecules adsorbed on the surface of CNTs were in an excited state to originate photoinduced electron which would firstly transfer to carbon tube, and then transfer to Ni substrate step by step following energy level. O2 in the solution or on the surface of composite material would be captured by photo-induced electron, then reduced into ·O2 (Yim & Liu 2004). Thus, the efficiency of electron transfer was accelerated in reduction reaction of O2, which could improve the productivity of H2O2. The positive hole reacted with H2O or OH on the surface of the composite material, leading to generate a large amount of ·OH. Meanwhile, H2O around the active points on the CNTs would also be more easily influenced by shock waves or strong electric fields of plasma to form ·OH and ·H. The O2 in the water medium could have the reaction with ·H to form ·O2 which would be converted into ·OH and other strong oxidation free radicals through chemical reactions (Zhang et al. 2008; Gong et al. 2019). In a word, the cyclic degradation model ‘adsorption capture/catalytic oxidation’ was realized in DBDP/CNTs-G-Nif.

Degradation mechanism on intermediates and pathway of AG

Analysis of UV-Vis spectrum

Among the wave length of 200–800 nm, there were five main characteristic peaks of AG which were at 254 nm, 284 nm, 412, 606 and 644 nm, respectively. Peak at 254 nm corresponded to the absorption peak of benzene ring. Peaks at 284 and 412 nm corresponded to the absorption peaks of anthraquinone structure. Peaks at 606 and 644 nm meant that the absorption of the conjugate structure was in visible part (Xiong et al. 2013). It could be seen from Figure 8(a) that the structure of AG was very symmetrical while the conjugate effect made the color of AG green. With time, the absorption peaks gradually decreased in DBDP/CNTs-G-Nif system, indicating that the molecular structure was constantly opened. When the treatment time was 60 seconds, the absorption peaks of the visible part disappeared completely, indicating that large molecular structures of the solution were almost completely destroyed. When the treatment time reached 90 seconds, there were weak absorption peaks only in the ultraviolet region, indicating that organic matters with aromatic ring were almost completely oxidized. At 120 seconds, all characteristic peaks disappeared completely, indicating that the decolorization was complete without the presence of the chromophoric groups, and the macromolecule organics with the aromatic ring structures were mineralized into micro-molecule acids and other small molecule products.

Figure 8

The spectrum analysis of AG: (a) UV-Vis spectrum; (b) FT-IR spectroscopy.

Figure 8

The spectrum analysis of AG: (a) UV-Vis spectrum; (b) FT-IR spectroscopy.

Analysis of FT-IR spectrum

As could be seen from Figure 8(b), peaks near 1,500 cm−1 represented the C = C vibration absorption peak of the typical benzene ring skeleton when the sample was in absence of degradation. Usually, there were four peaks, but the four peaks sometimes did not appear at the same time. Peak at 1,029.89 cm−1 was the vibration absorption peak of -SO3Na. Peaks at 1,261.02 cm−1 and 1,177.95 cm−1 represented the stretching vibration peak of C = O in anthraquinone. Peak at 3,444.21 cm−1 represented the vibration absorption peak caused by -NH. The peak position of substituent group in benzene ring was at 617.14 cm−1. After 30 seconds of treatment, C = C vibration absorption peak of benzene ring skeleton turned into 1,633.49 cm−1, moving into takanami, which meant that original molecules with big conjugate system had become a relatively small system of benzene ring. In addition, the characteristic peak of -SO3Na disappeared. The peak of -NH was transferred to 3,440.54 cm−1. Although the peak type still existed, the vibration amplitude of the peak had decreased. The peak of C = O disappeared, indicating that C = O had been oxidized by active substances. In terms of the dye with anthraquinone structure, the adjacent benzene rings could be oxidized and broken easily once C = O was destroyed (Salazar et al. 2012). When the degradation time was 60 seconds, most peaks were disappeared and the remained peaks were weakened in some degree, indicating that the large molecular structure had disappeared, and the intermediate products were continuously mineralized to form small molecular products in the DBDP/CNTs-G-Nif system. This was consistent with the analysis of UV-Vis spectrum at 60 seconds.

Analysis of GC-MS and LC-MS spectrum

According to the analysis results of GC-MS, it could be doped out from Figure S4 that the main intermediate products were phthalic acid, hydroquinone, mesoxalicacid and various small molecular acids like acetic acid during the degradation process of AG by DBDP/CNTs-G-Nif. From the analysis results of LC-MS, it could be conjectured from Figure S5 that the relative molecular weights of the main intermediates existed in the degradation process of AG by DBDP/CNTs-G-Nif were 240, 124, 116 and 108, respectively. Through the analysis, possible intermediates were 1, 4-dihydroxyanthraquinone, 4-methyl catechol, butenedioic acid as well as p-benzoquinone, etc.

Analysis of degradation pathway

The analysis of the spectrums above, including UV-Vis, FT-IR, GC-MS and LC-MS allowed proposing a possible degradation pathway of Figure 9 for AG mineralization by DBDP/CNTs-G-Nif. Table 2 showed the major identified intermediates. Macromolecule of AG was broken at C-N and 1,4-dihydroxyanthraquinone (A1) and 4-methyl catechol (C2) were formed after oxidation and nucleophilic reactions. The ring of A1 would be opened through the bombardment of high-energy free radicals to form hydroquinone (A2) and phthalic acid (A3). The p-benzoquinone (A4) was turned into maleic acid (A5). Meanwhile, A1 could be also oxidized to 1,4,9,10-anthraquinone (B1) which was the transition state, and then A3 and A5 or catechol (B4) and B3 were formed by ring open. A3, B4, and C2 were translated into a long chain of acids such as hexadiendioic acid and keto acid by further oxidation. Long-chain acids continued to turn into small molecular carboxylic acids such as oxalic acid (C4), acetic acid and formic acid, which were generally reported as the end-products before totally mineralized into CO2 and H2O (He et al. 2008; Salazar et al. 2012).

Table 2

The determined main intermediates of AG

NumberNameCAS numberMolecular formulaStructural formulaGC-MS/LC-MS
1,4-dihydroxyanthraquinone 81-64-1 C14H8O4  LC-MS 
4-methyl catechol 452-86-8 C7H8O2  LC-MS 
phthalic acid 88-99-3 C8H6O4  GC-MS R.time = 7.215 min 
hydroquinone 123-31-9 C6H6O2  GC-MS R.time = 7.670 min 
pyrocatechol 120-80-9 C6H6O2  GC-MS R.time = 7.670 min 
mesoxalicacid 473-90-5 C3H2O5  GC-MS R.time = 3.705 min 
p-benzoquinone 106-51-4 C6H4O2  LC-MS 
butenedioic acid 110-16-7 C4H4O4  LC-MS,GC-MS R.time = 6.235 min 
acetic acid 64-19-7 C2H4O2  GC-MS R.time = 2.035 min 
NumberNameCAS numberMolecular formulaStructural formulaGC-MS/LC-MS
1,4-dihydroxyanthraquinone 81-64-1 C14H8O4  LC-MS 
4-methyl catechol 452-86-8 C7H8O2  LC-MS 
phthalic acid 88-99-3 C8H6O4  GC-MS R.time = 7.215 min 
hydroquinone 123-31-9 C6H6O2  GC-MS R.time = 7.670 min 
pyrocatechol 120-80-9 C6H6O2  GC-MS R.time = 7.670 min 
mesoxalicacid 473-90-5 C3H2O5  GC-MS R.time = 3.705 min 
p-benzoquinone 106-51-4 C6H4O2  LC-MS 
butenedioic acid 110-16-7 C4H4O4  LC-MS,GC-MS R.time = 6.235 min 
acetic acid 64-19-7 C2H4O2  GC-MS R.time = 2.035 min 
Figure 9

The general degradation pathway of AG.

Figure 9

The general degradation pathway of AG.

Figure 10 explained the general sub-step pathways of degradation. As shown in S1, the products of two different states would be formed during the first moments of oxidation when the bonds were broken at sites 1 or 2. But -NH2 could turn into -OH under the function of ·OH (He et al. 2008) so that the final transformations presented as A1 and C1. As shown in S2, the phenol on the benzene ring of C2 would be further oxidized into the ketone. Then, on the one hand, C3 and C4 could be formed through progressive oxidization of benzene ring. On the other hand, the methyl groups on the benzene ring could be replaced to form alcohols. The alcohols were gradually oxidized to form aldehydes and acids (Song et al. 2009), and then were finally converted to C3 and C4 by ring-open reaction.

Figure 10

The general substep degradation pathway of AG(S1-S3).

Figure 10

The general substep degradation pathway of AG(S1-S3).

As shown in S3, A1 was transformed into other intermediates by breaking bonds or further oxidation under the action of active substances. The specific transition processes of intermediate products were illustrated in Figure 11. From S4-S8, the oxidation transformation paths of the intermediate product in S3 were explained in detail. In S4, double bond was broken, ·H was transferred, and the structure was rearranged to form A5. In S5, 1,4-naphthoquinone through protonation and transferring of double bond was turned into 1,3-indenadione (Fanchiang & Tseng 2009; Yuan et al. 2011). In S6, the ketone was decomposed into A3 after cleavage of ring, and B3 was formed by the further oxidation until it was mineralized to CO2 and H2O completely. In S7, phenol on the benzene ring of B4 would be further oxidized to ketone and then oxidized to form B5 by opening ring (Mvula & Sonntag 2003; Shen et al. 2016, 2017a, 2017b). In S8, A2 could be oxidized to A4 (Khataee et al. 2016), and A4 was finally oxidized to form A5 due to the breakage of ring (Li et al. 2005).

Figure 11

The detailedly substep degradation pathway of AG(S4-S8).

Figure 11

The detailedly substep degradation pathway of AG(S4-S8).

Degradation process of AG could be described in a first-order kinetic model by DBDP alone within 60 seconds. It was feasible to prepare similar product CNTs-G-Nif using different carbon sources by replacing ethanol with methane through a two-step CVD approach. The cofactor E = 1.44 > 1 illustrated that there was synergistic effect DBDP/CNTs-G-Nif. The cyclic degradation model ‘adsorption capture/catalytic oxidation’ was realized in DBDP/CNTs-G-Nif. AG could be mineralized into CO2 and H2O by DBDP/CNTs-G-Nif. The results above illustrated that such CNTs-G-Nif composite represented the possibility of a new family of innovative carbon materials for plasma catalysis.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21246010).

All relevant data are included in the paper or its Supplementary Information.

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Author notes

These authors contributed equally to this work and should be considered co-first authors.

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Supplementary data