Over the last decade, considerable attention has been directed towards the electrochemical advanced oxidation processes (eAOPs), due to their ability to remove a large range of organic pollutants from water even at very low concentrations [1]. eAOPs rely on the in-situ generation of powerful oxidants, such as HO• by electron transfer at the anode surface. However, these processes are limited by mass transfer and require high energy consumption.
In this study, combined adsorption on granular activated carbon (GAC) and electro Fenton process (EF) is applied. The purpose of this coupled process is to overcome the mass transfer issue by pre-concentrating the compounds on AC and proposing a new AC regeneration method. During the EF the AC can directly be used as a cathode. Due to the electrostatic repulsions and oxidation process, the compounds are desorbed from the AC and oxidized by the HO• generated in the bulk.
Material and Methods
The adsorption cycles and EF experiments were performed in batch mode using deionized water (DW) and simulated wastewater (SWW) with caffeine as a model compound. Similar experimental steps and conditions as described in [2] are followed, using 1g of GAC, with pH=3, Fe2+= 0.15 mM, Na2SO4= 50mM, and a current intensity of 200 mA. To assess the degradation and mineralization of the caffeine, HPLC and TOC measurements were used, respectively. The regeneration efficiency was calculated by comparing the adsorption capacity of the AC after each regeneration cycle with its initial adsorption capacity.
Results and Conclusions
The efficiency of the process was discussed as regards (i) desorption of organic compounds and regeneration efficiency in DW and SWW, (ii) degradation and mineralization of organic compounds, and (iii) stability of GAC for several successive regeneration cycles. During the regeneration, over 80% of desorbed caffeine was degraded and 75% was mineralized due to the formed HO• by Fenton reaction and at the BDD anode surface (Fig.1). Furthermore, a recovery of the AC adsorption capacity by up to 50 % in PW and 37% in SWW was observed. These regeneration percentages decreased to 25% during the second cycle and then remained stable during the third and the fourth cycle. The incomplete regeneration is most likely due to remaining caffeine molecules at the AC [3], or a blockage of the carbon adsorption sites by formed degradation molecules. Hence, a longer treatment time might lead to higher efficiency. These hypotheses will be taken into consideration in future work to draw a clear conclusion on the regeneration phenomenon. The similar regeneration efficiencies and mineralization rates obtained during the 2nd,3rd, and 4th cycles indicate the stability of the GAC electrode.
In conclusion, the results from this study illustrate that combined adsorption and EF is a suitable process for the removal of caffeine and regeneration of AC. The advantage of such a process is that part of the AC can be regenerated while over 75% of the desorbed compounds from the AC are mineralized, thus avoiding toxic effluents. This process is currently implemented as a continuous e-cell for sequential adsorption and EF regeneration.
References
[1] Oturan,N.(2015),Occurrence and Removal of Organic Micropollutants in Landfill Leachates Treated by Electrochemical Advanced Oxidation Processes
[2] Trellu,C. (2021), Clofibric acid removal at carbon fibers by adsorption and electro-Fenton regeneration- Modeling and limiting phenomena. Electrochimica Acta, 138-283.
[3] Xiao,Y. (2018), Benefit of hydrophilicity for adsorption of Methyl Orange and polytetrafluoroethylene electrodes. Environ.Sci.Technol. 11761-11768