Electrochemical oxidation of contaminants of emerging concerns (CECs) using CNT hollow fiber membrane

Abstract

Recently, contaminants of emerging concern (CECs) such as pharmaceuticals and personal care products (PPCPs) have been growing concern during water and wastewater treatment processes because they cannot be easily removed by current wastewater treatment technologies. Among advanced technologies, electrochemical oxidation (EO) processes have been considered as one of cost-effective and versatile CEC removal technologies, while the conventional EO system composed with stacked flat-sheet structure has critical limitations such as low diffusional rate of contaminants onto electrode surfaces. In this study, we suggested convective flow system of contaminated water containing CECs into the electrode to increase the mass transfer and adsorption rate. To achieve this, all CNT based hollow fiber membrane (CHM) was fabricated to be used as an electrode in EO system. We found that prepared CHM had a thickness of 1000 $\mu$m with an average pore size of 43 nm, and exhibited 95 $LMHbar^{-1}$, which is similar to permeability of ultrafiltration (UF) membrane. CHM also experienced electrical conductivity of 57.5 S $cm^{-1}$ and OER (oxygen evolution reaction) potential of 2.1 V. Here, using the CHM as an anode, we develop the novel filtration-electrochemical degradation (F-ED) system that water containing CECs flows convectively into anodic CHM electrode. BPA (bisphenol A), SMX (sulfamethoxazole), DEET (N,N-diethyl-meta-toluamide) were chosen as model CECs and oxidized with an CHM anode at the current density of 4 $mAcm^{-2}$ for 30 minutes. The oxidation performances of BPA, SMX, and DEET with F-ED (filtration-electrochemical degradation) system were significantly better than those with conventional A-ED (adsorption-electrochemical degradation) system due to the direct contact between CECs and CNTs during the filtration of contaminated water through electrically conductive CHF pores. In addition, the mixed solution of BPA, SMX and DEET were conducted by F-ED for 30 minutes and the removal efficiency of each substances was much higher than that of A-ED. To investigate the degree of mineralization of CECs, BPA degradation pathway was specifically proposed by analyzing BPA intermediates resulting from the degradation of BPA by HPLC-MS (high-performance liquid chromatography-mass spectroscopy). It is suggested that hydroxyl radicals generated by water decomposition reacted with BPA to form BPA hydroxylated derivatives and they transformed into one aromatic ring compound and aliphatic acid, finally mineralized as $CO_2$ and $H_2 O$. Overall, CHM provided excellent adsorptions and electro-chemical oxidations of CECs by novel CNT properties and convective transport of contaminants by filtration.