Short Communication
Anodic abatement of organic pollutants in water in micro reactors

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Abstract

The electrochemical oxidation of oxalic acid (OA) was performed in a micro flow cell equipped with a boron doped diamond (BDD) anode. This preliminary study demonstrates that a flow cell with a micrometric distance between the cathode and the anode can be used to perform the electrochemical treatment of waters contaminated by organic pollutants in the absence of added supporting electrolytes with high abatements. The effect of the distance between the cathode and the anode, the flow rate and the current density on the abatement of oxalic acid and on the current efficiency was in particular studied.

Introduction

Microsystems technology, coming from information technology and miniaturization of data-processing devices, has entered many fields in our daily life. Process engineers are working in various areas ranging from the food industry through biotechnology to pharmaceutical products, from analytical and laboratory scale equipment through energy conversion to industrial chemistry applications for the production of millions of tons of chemicals [1]. In the context of chemical reactions, important potential benefits of microfluidic devices utilization come from enhanced heat and/or mass transfer that allow better thermal control of unit operations, and from the ability to directly scale down, scale up or modularize the processes. The achievement of higher product yield, selectivity and purity, improved safety and the access to new products have been furthermore claimed by researchers in the last years [1], [2].

Integration of electrodes in microfluidic devices has recently attracted a significant attention, particularly for analytical purposes [2], [3], [4], [5], [6], [7]. Electrochemical microfluidic devices were also used for preparative purposes [3], [5], [8], [9], [10], for monitoring flow velocities in microfluidic channels [11], [12], [13], [14] or evaluating electrochemical parameters such as diffusion coefficients, homogeneous and heterogeneous reaction kinetic constants, etc. [15], [16], [17].

To the best of our knowledge, electrochemical micro reactors have not been used for the treatment of wastewaters in spite of the great potential advantages offered by this approach as it will be evidenced here after. Electrochemical processes are considered as very promising for the abatement of organic pollutants toxic or refractory to biological degradation [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Indeed, this method uses a clean reagent, the electron, and very mild operative conditions (ambient temperature and atmospheric pressure) with limited operative costs [20]. However, electrochemical processes present two important disadvantages when performed in conventional reactors:

  • 1.

    To achieve reasonable cell voltages when the medium has not an adequate conductivity, one needs adding to the system a supporting electrolyte. This is certainly a main obstacle for a wide application of electrochemical abatement. Indeed, adding chemicals to a water stream is often a problematic issue, being opposed to general administrative regulations since this may lead to the formation of secondary pollutants during electrochemical incineration of organics, and anyway increases the operative costs.

  • 2.

    Low current efficiencies are usually achieved in direct oxidation processes when a high abatement of the organic pollutants is required, mostly due to the fact that mass transfer rates towards electrodes are extremely reduced at low pollutant concentrations.

Microcells (e.g. cells with a distance between the cathode and the anode of tens or hundreds of micrometers) lead to a drastic reduction of the ohmic resistances, thus allowing electrochemical operation without supporting electrolyte [3], [8], [9], [10]. Similarly, very small distances between electrodes spontaneously intensifies mass transport of the pollutants towards electrodes surfaces. Finally micro devices may simplify the scale up procedure, since this only requires a simple parallelization of many small units [5], [8], [9].

We wish to report the first time use of a micro electrochemical cell for the abatement of organic pollutants. Oxalic acid was chosen as model substrate owing to its resistance towards anodic oxidation, leading generally to an incomplete mineralization [18], [19], [21], [24], [26]. Boron doped diamond (BDD) electrodes were used as anodic materials since they are among the more promising ones for the incineration of organic compounds [21], [22], [25].

Section snippets

Experimental

Two different undivided flow cells were used. System I consisted of a micro-gap flow cell built upon inserting one or more polytetrafluoroethylene (PTFE) micrometric spacers in a commercial undivided filter press flow cell from ElectroCell AB (Fig. 1). The cell was equipped with two plate electrodes (9 cm2) of BDD/Nb (Condias, Germany) and nickel, used as anode and cathode, respectively. Working and counter electrodes were separated by one or more PTFE spacers (Bohlender GmbH, Germany) with

Results and discussion

As shown in Table 1, for a same constant current forced through the cell, the cell voltage ΔV across cathode and anode depended significantly on the thickness of the spacers. In the micro-gap flow cell, cell voltage decreased upon reducing the spacers thicknesses. In particular, in the experiments performed with a single spacer of 50 μm, a value of ΔV significantly lower than that observed in the conventional system was observed even in the absence of the supporting electrolyte.

A wide set of

Conclusions

The galvanostatic oxidation of oxalic acid was successfully performed with high abatement yields after a single passage of the solution inside a micro flow cell equipped with a boron doped diamond (BDD) anode. Even in the absence of added supporting electrolytes low cell voltages were observed. The abatement of oxalic acid and the current efficiency strongly depended on the inter-electrode gap. Lower micrometric gaps resulted in higher abatements of oxalic acid. Minor effects of flow rates and

Acknowledgments

In Palermo, this work was supported by Università di Palermo and Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR). In Paris, the work was partially supported by the CNRS (UMR 8640 PASTEUR), Ecole Normale Superieure (ENS), University Pierre and Marie Curie (UPMC), and the French Ministry of Research.

References (30)

  • D. Erickson et al.

    Anal. Chim. Acta

    (2004)
  • P. He et al.

    Electrochem. Commun.

    (2005)
  • D. Horii et al.

    Electrochem. Commun.

    (2005)
  • C. Amatore et al.

    J. Electroanal. Chem.

    (2004)
  • M. Thompson et al.

    J. Electroanal. Chem.

    (2005)
  • C. Amatore et al.

    J. Electroanal. Chem.

    (2006)
  • P. Canizares et al.

    J. Environ. Manage.

    (2009)
  • C.A. Martinez-Huitle et al.

    Electrochim. Acta

    (2004)
  • M. Panizza et al.

    Electrochim. Acta

    (2005)
  • O. Scialdone et al.

    Eletrochim. Acta

    (2008)
  • O. Scialdone et al.

    Electrochim. Acta

    (2008)
  • O. Scialdone et al.

    Electrochim. Acta

    (2009)
  • O. Scialdone et al.

    Wat. Res.

    (2009)
  • O. Scialdone

    Electrochim. Acta

    (2009)
  • S.G. Weber et al.

    Anal. Chim. Acta

    (1978)
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