3D macroporous boron-doped diamond electrode with interconnected liquid flow channels: A high-efficiency electrochemical degradation of RB-19 dye wastewater under low current

https://doi.org/10.1016/j.apcatb.2018.12.074Get rights and content

Highlights

  • A novel three dimensional BDD foam electrode was prepared for the first time.

  • A simple and reproducible preparation method of 3D-BDD was proposed.

  • The electro-active surface area of 3D-BDD electrode is 20 times larger than that of 2D-BDD electrode.

  • The highest oxidation reaction rate of 0.09 min−1 so far was obtained for the degradation with BDD electrode.

Abstract

Boron-doped diamond (BDD) has proved to be an ideal anode material for the electrolysis of organic sewage. However, the existing two dimensional BDD electrodes with small active area and low mass transfer rates, limit their further improvement in degradation efficiency. In this paper, a novel three dimensional macroporous BDD (3D-BDD) foam electrode with a structure of evenly distributed pores and interconnected networks in which wastewater can flow freely was prepared using a simple and reproducible method. Compared to two dimensional BDD electrodes of the same geometry, the electro-active surface area of 3D-BDD electrode increased by ˜20 times, and the electrochemical oxidation reaction rate constant of RB-19 increased by ˜350 times. Under optimized conditions, the energy consumption reduced to a minimum of 0.03 kWh(gTOC) −1, and the MCE reach a maximum of 325.86%.

Introduction

Since the last century, the treatment of wastewater produced by printing and dyeing, textile, pesticide, pharmaceutical, petrochemical and other industries has attracted much attention [1,2]. These industrial wastewater normally contain complex, chemically stable, and bio-refractory organic pollutants [1,3]. Due to their toxicity, carcinogenicity and mutagenicity, these organic pollutants are likely to cause some unimaginably disastrous consequences on the environment and human health even with low concentrations in water [4]. As a typical active anthraquinone dye, RB-19 was widely used in the printing and dyeing industry due to its strong chroma stability and good reproducibility. On the other hand, it has strong chemical stability (half-life cycle = ˜46 years, pH = 7, T = 25 °C), non-biodegradability (50% < EC20 < 75%), toxicity and good water solubility [5], therefore RB-19 would remain in water for a very long time without an effective way of treatment. Unfortunately, traditional treatments like biochemical methods were proven to be ineffective [6].

Electrochemical Advanced Oxidation Processes (EAOPs), as a clean, effective and environmentally friendly electro-catalytic technology, is a research focus in the field of organic wastewater treatment in recent years [7,8]. Boron-doped diamond (BDD) is an ideal anode material for electrochemical oxidation processes due to its excellent mechanical properties, high oxidation potential, wide potential window, extremely low background current and good corrosion resistance [9]. Many studies have reported that RB-19 can be effectively removed by electro-catalytic oxidation with BDD electrodes [6,10,11].

The electro-catalytic activity of BDD is greatly affected by the characteristics of the electrode (including boron concentration, sp3/sp2 ratio, film thickness, surface termination, etc.), particularly by the surface roughness [12]. Because the rougher the surface, the larger effective electro-active surface area (EASA) will be, which not only increases the contact area between the sewage and the electrode, but also increases the active reaction sites for the generation of hydroxyl radicals (∙OH) during electrolysis, thereby enhancing the oxidation efficiency of BDD electrode. Based on the above analysis, considerable research efforts have been devoted to the study of BDD electrodes with large EASA and their applications in the fields of electrochemical oxidation [9,13], supercapacitors [14], biosensors [15], solar cells [16] and so on. The preparation methods of BDD with high specific surface area are mainly divided into two types in principle: (i) etching the surface of BDD [13,[17], [18], [19], [20]], and (ii) deposition on a porous template [14,[21], [22], [23]]. Method of etching BDD usually first deposits a planar BDD film and then etches it to form a porous structure, including mask etching [17], high temperature activation [18,19], metal-assisted etching [13,20], etc. For example, Honda et al. produced BDD with honeycomb structures by oxygen plasma etching using porous alumina masks [17]. Kondo et al. obtained a structure with dense pores on BDD surface using a two-step method, graphitizing it at 1000 ℃ in argon atmosphere, then removing the graphite phase in air at 425 ℃ [19]. Li et al. obtained porous BDD films with nanopores of different shapes (including square and triangular) by Ni-nanoparticle-assisted plasma etching, and the EASA was 2.43 times larger than its geometric area [9]. However, there are some shortcomings in the method of BDD etching: one is the size of nanopores are difficult to control accurately, and it is a complex preparation process which is far from energy efficient, also the BDD could be easily contaminated by unnecessary masking metal impurities during the etching process [21]. Compared to etching process, depositing BDD films by chemical vapor deposition (CVD) on porous templates with large specific surface area is a better method. A number of works have been done on BDD to increase EASA by depositing diamond on various substrates such as carbon nanotubes [14], glass fiber [21,22] and porous silicon [23], etc. For example, Zanin et al. grown nanostructured BDD films on vertically aligned carbon nanotubes, as a result, the electrochemical active area and double-layer capacitance of the “teepee” structured BDD electrode are ˜450 times greater than those of the planar BDD electrode [14]. Petrák et al. deposited BDD films on SiO2 fibers by spin coating or doctor blade techniques on a planar BDD film, and then removed SiO2 fibers by pickling to obtain a porous film [21]. However, so far as we know, the reported preparation methods of substrates with large specific area are complicated to operate. And since the size and depth of the pores are mostly in the nanometer order, namely they still belong to two-dimensional porous electrodes, not allowing organic wastewater to readily flow in and out through the channels of material. As a result, the electrochemical oxidation efficiency will be hindered by the limited diffusion. He et al. obtained a BDD foam electrode by depositing on a porous titanium substrate prepared by powder metallurgy, expanding the preparation of porous BDD from two dimensions to three dimensions, and endowed it three-dimensional pores and adjustable porosity [24]. However, the irregular shape, uneven distribution of the pores and the inhomogeneous small pore size (1–2 μm) make it difficult to obtain a continuous and uniform diamond film inside the substrate and cannot form through-type channels for organic wastewater to flow freely inside. So it still belongs to the plate electrode from the macroscopic, not helpful for the improvement of liquid mass transfer, resulting in the current efficiency is less than 30%, thus limits the further improvement of electrochemical oxidation efficiency [24].

Based on the above discussion, this paper aims to increase the EASA of BDD films and to improve the liquid phase mass transfer in the degradation process by a new method which is easy to operate, reproducible, and does not require complicated substrate preparation process. A novel three-dimensional BDD fully coated Ni foam electrode with adjustable pore size and interconnected channels in which wastewater can flow freely was prepared. In addition, the electrocatalytic performance and degradation mechanism of 3D-BDD electrode were studied by the degradation of refractory dye RB-19. This work has expanded the possibility of 3D design of high-quality three dimensional BDD electrodes and its application in actual wastewater treatment.

Section snippets

preparation of BDD

Planar BDD (referred to be 2D-BDD) films were deposited on commercial titanium sheets of 20 mm × 30 mm × 1.5 mm using hot filament CVD technique in the mixed gas atmosphere for 24 h. The mixture of gas was in the ratio of B2H6(diluted by 95% of hydrogen) : CH4 : H2 = 0.2 sccm : 2 sccm : 98 sccm. B2H6 was used as the source of boron and the ratio of [B]/[C] was 10,000 ppm. The deposition system’s temperature was kept at ˜850 ℃ and the pressure was maintained at 3 kPa.

A batch of high-quality

Morphology of BDD electrodes

The morphologies of 2D-BDD, the inside holes and surface of 3D-BDD films were observed through SEM images in Fig. 1. Holes of 3D-BDD with a porosity of 50 ppi were evenly distributed and in regular shape. Diamond grown on surface and inside frame was continuous and uniform, fully coated by the three-dimensional foam skeletons without cracks or holes. The pore diameter was around 1 mm and the width of skeletons was around 200 μm (Fig. 1a). Due to the effect of hot filament radiation distance and

Discussion

Macroporous 3D-BDD foam electrodes with three-dimensional interconnected (mm) network structure were constructed in this paper. It can be seen from SEM and Raman characterization that the quality of BDD was not affected by the three-dimensional structure substrate. The EASA of 3D-BDD was increased by ˜20 times comparing to the planar BDD of the same volume, which will greatly improve the space utilization when applied to practical wastewater treatment device. 3D-BDD electrode exhibits excellent

Conclusions

In this paper, high-quality 3D-BDD electrodes with uniformly distributed pores were obtained by CVD deposition on Ni foams. The diameters of pores were 1 mm and the skeleton width was 200 μm, forming interconnected channels through which the liquid phase can flow freely. The EASA of 3D-BDD electrode reaches 20.96 cm2/cm2, which was 20 times of that of 2D-BDD electrode. The peak current of the 3D-BDD electrode at the same sweep rate was much higher (200 times) and the electron transfer

References (45)

  • C. Salazar et al.

    J. Hazard. Mater.

    (2016)
  • E.C. Paz et al.

    Appl. Catal. B Environ.

    (2018)
  • M. Schriks et al.

    Water Res.

    (2010)
  • A.C. Johnson et al.

    J. Hydrol.

    (2008)
  • E.J. Weber et al.

    Water Res.

    (1993)
  • E. Petrucci et al.

    Chem. Eng. J.

    (2011)
  • X. Li et al.

    Carbon

    (2018)
  • D. Montanaro et al.

    Chem. Eng. J.

    (2009)
  • T. Ohashi et al.

    Electrochim. Acta

    (2011)
  • T. Kondo et al.

    Carbon

    (2014)
  • H.A. Mehedi et al.

    Carbon

    (2013)
  • V. Petrák et al.

    Carbon

    (2017)
  • S. Ruffinatto et al.

    Diam. Relat. Mater.

    (2015)
  • Y. He et al.

    Sep. Purif. Technol.

    (2015)
  • A. Thiam et al.

    Sep. Purif. Technol.

    (2015)
  • S. Garcia-Segura et al.

    Water Res.

    (2011)
  • J.R. Steter et al.

    Electrochim. Acta

    (2016)
  • R. Ramamurti et al.

    Diam. Relat. Mater.

    (2008)
  • Z. Frontistis et al.

    J. Environ. Manage.

    (2018)
  • N. Barhoumi et al.

    Appl. Catal. B Environ.

    (2017)
  • M. Ghazouani et al.

    J. Electroanal. Chem.

    (2016)
  • E.M. Siedlecka et al.

    Water Res.

    (2018)
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