Electro-Fenton Regeneration of Activated Carbon Fibers for the Removal of Pharmaceutical Residues

Abstract

Introduction The electro-Fenton process is based on in-situ production of the Fenton’s reagent in order to continuously generate hydroxyl radicals in the bulk solution [1]. It is also possible to promote the generation of additional powerful oxidant species by using anode material with high oxygen evolution overpotential. Therefore, such electrochemical advanced oxidation process (EAOP) is able to degrade a large range of organic pollutants [1]. However, energy efficiency of EAOPs is strongly affected by mass transport limitations during the treatment of low concentrations of pollutants such as pharmaceutical residues. In fact, a large amount of hydroxyl radicals are wasted in parasitic reactions such as hydroxyl radical dimerization. The combination of adsorption and electro-Fenton processes aims at overcoming this drawback. Adsorption on activated carbon fibers (ACFs) is used for pre-concentration of organic pollutants. Then, ACFs can be directly used as cathode during the electro-Fenton process for desorption, degradation and mineralization of organic compounds as well as for regeneration of ACFs for additional adsorption steps [2]. The possibility to easily regenerate ACFs improve the cost-effectiveness of this material, which presents more suitable characterisitcs than grain or powder activated carbon (e.g. low intra-particle diffusion limitation leading to fast adsorption kinetics, narrow pore size distribution leading to selective adsorption of micropollutants). This innovative strategy was applied for the removal of pharmaceutical residues. Scale-up from batch to continuous filtration reactor was investigated. Material and methods A first set of experiments was performed in batch reactor for both adsorption and electro-Fenton processes, by using clofibric acid and ofloxacin as model pharmaceutical residues. Design parameters (e.g. anode material: boron-doped diamond (BDD) or TiOx) and operational parameters (current density, treatment time) were optimized for improving regeneration efficiency and mineralization of pharmaceutical residues. Regeneration efficiency was assessed by comparing the adsorption capacity of regenerated ACFs and pristine ACFs. Mineralization rate of desorbed organic compounds was measured by total organic carbon analysis. Biodegradability and acute toxicity (Microtox®) of the solution was also assessed. ACFs were characterized by scanning electron microscopy, Hg porosimetry, and Raman spectroscopy. Then, the study was focused on the design, implementation and optimization of this combined process in a single continuous filtration reactor and using a real effluent. Results and discussion Batch experiments using clofibric acid as model pollutant were performed in order to better understand the influence of critical design and operational parameters. The efficiency of the process was discussed as regards to (i) desorption of organic compounds and regeneration efficiency, (ii) degradation and mineralization of organic compounds and (iii) stability of ACFs for several successive adsorption/regeneration cycles. By using BDD as anode and a current density of 21 mA cm-2, it was observed that increasing the treatment time from 3 to 6 h strongly improved the regeneration effectiveness from 45 to 75%. Further increasing the treatment time to 9 h did not show significant positive effect because of the lower availability of a portion of adsorbed compounds within the ACF porosity. Besides, dividing the current density by a factor of 2.5 (8 mA cm-2) only reduced the regeneration efficiency from 75 to 64%, indicating that low current density might be more cost-effective. Using TiOx instead of BDD anode also only reduced the regeneration efficiency from 64 to 60%. TiOx is a new promising electrode material that is much less expensive than BDD. Therefore, using TiOx anode was considered as a more cost-effective configuration. A great advantage of this process is also that >80% of organic compounds desorbed from ACF were mineralized, thus avoiding the production of a toxic effluent. Regeneration efficiency was then assessed during 10 cycles of adsorption/regeneration without significant decrease due to the protection of the ACF surface by the cathodic polarization effect. Finally, this treatment strategy was implemented in a continuous filtration reactor where both adsorption and electro-Fenton regeneration could take place sequentially. Process efficiency for treating a real effluent was assessed by comparing breakthrough curves of pristine and regenerated filters. The development of this technology is currently focused on point-of-use filters for domestic applications (about 5 liters per day) as well as on industrial wastewater treatment (pilot-scale unit able to treat 10-20 liters per hour).