Electrochemistry has an important role to play in increasing the sustainability and energy efficiency of water treatment and reuse systems. In Singapore for example, the water demand has been projected to more than double by 2060, with non-domestic water demand expected to account for 70% (560 millions of gallons per day). In this context, industrial water reuse is becoming a necessity and the electronics sector has been identified as one of the priority targets due to the extra-pure water requirements. The main issue with industrial water reuse is the removal of “hard-to-treat” contaminants and current interest is focusing on advanced oxidation processes (AOPs), because they produce highly oxidizing species like ^•OH radicals that can non-selectively mineralize all sorts of organic pollutants. Among them, electro-Fenton has emerged because of its many advantages, including ease of operation and high degradation rates of numerous organic pollutants, thanks to the recent development of improved electrode materials (e.g., carbon nanotubes, graphene, etc.) (1-3). In this work, we showcase the performance at lab-scale and pre-pilot scale of an electro-Fenton prototype to treat a real waste stream from a semi-conductor factory using a novel carbon-fiber brush cathode.

First, the brush was fully characterized in terms of its electrical conductivity and surface area. Its interfacial charge-transfer resistance (R_ct) – as determined by electrochemical impedance spectroscopy – equaled 1.40 Ω. Cyclic voltammetry was used to determine the electroactive surface area of the electrode, which was found equal to 416 cm², by using the Randles-Sevcik equation (Eq. 1):

Ip = 2.69 × 10⁵ × AD^1/2n^1/2γ^1/2C (Eq. 1)

where Ip is the peak current (A), n is the number of electrons participating in the redox reaction (n = 1), A is the area of the electrode (cm²), D is the diffusion coefficient of the molecule in solution (7.60 × 10^-6 cm² s^-1), C is the concentration of the probe molecule in the bulk solution (1 × 10^-5 mol cm^-3), and γ is the scan rate of the potential perturbation (0.01 V s^-1).

The catalytic activity of the novel electrode was then assessed by its capacity to electrogenerate H₂O₂. Since H₂O₂ is an intermediate required to produce ^•OH in the electro-Fenton reaction (Eq. 2) and is easier to detect, due to its longer half-life, its production arguably constitutes a better indicator of the cathode efficiency than the generation of ^•OH (4, 5):

H₂O₂ + Fe²⁺ → Fe³⁺ + ^•OH + HO^- (Eq. 2)

The influence of the current density (normalized to the cathode projected surface area) on H₂O₂ production through cathodic reduction of O₂ was thus evaluated and the best results were obtained at 1.25 mA cm^-2, yielding a specific maximum H₂O₂ concentration of 3.00 ± 0.12 mg-H₂O₂ L^-1 cm^-2. Upon addition of Fe²⁺ to trigger the electro-Fenton reaction following Eq. 2, a mineralization efficiency of 80% was attained within 3 hours using real wastewater from the microelectronics industry in a lab-scale batch setup (Fig. 1) and the energy requirement was as low as 0.14 kWh g^-1of total organic carbon (TOC).

Following these promising results, a continuous-flow pre-pilot scale prototype was designed and a computational flow simulation allowed optimizing various parameters, from electrode spacing to flowrate and mass transfer rates towards the electrodes (Fig. 2). The performance of this pre-pilot prototype was evaluated in the same conditions as the lab-scale reactor, confirming and even improving the results obtained at lab-scale with over 90% TOC removal reached within 3 hours (Fig. 1). However, the energy consumption increased with the intensity of the treatment from 0.8 KWh g^-1 of TOC in the first hour to 1.34 kWh/g^-1of TOC, after 3 hours, showing that electro-Fenton would be better used as a pre-treatment technology, over a limited period of time. Overall, these results demonstrate the potential of electro-Fenton as a water treatment and reuse technology and constitute an important step towards full-scale applications. A detailed technical economic assessment of the technology for industrial water reuse will be presented at the conference.

References
1. E. Mousset, Z. T. Ko, M. Syafiq, Z. Wang and O. Lefebvre, Electrochim. Acta (in press).
2. E. Mousset, Z. Wang, J. Hammaker and O. Lefebvre, Electrochim. Acta, 214, 217 (2016).
3. E. Mousset, Z. Wang and O. Lefebvre, Water Sci. Technol., 74, 2068 (2016).
4. L. Zhou, M. Zhou, Z. Hu, Z. Bi and K. G. Serrano, Electrochim. Acta, 140, 376 (2014).
5. F. Yu, M. Zhou and X. Yu, Electrochim. Acta, 163, 182 (2015).