Insights into estrogenic activity removal using carbon nanotube electrochemical filter

This study reports the performance of a carbon nanotube (CNT) electrochemical filter applied to 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) degradation and their estrogenic activity removal (calculated in terms of E2 equivalent, EQ-E2). The performance of CNT electrochemical filter was assessed at different applied voltages (0-2.5 V) and aqueous matrices (ultrapure water and urban wastewater), using 37 μM of E2 and EE2, a flow rate of 1.5 mL min^-1 and 10 mM of Na₂SO₄, used as supporting electrolyte. Surface characterization of CNT anodic filters was completed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was used to investigate electron transfer mechanisms. The CNT electrochemical filter was successfully applied to E2 and EE2 degradation and removals higher than 95.3% (oxidative fluxes >2.94 ± 0.05 mmol h^-1 m^-2) were achieved when 2.5 V was applied for both ultrapure water and urban wastewater. CV results indicate that the oxidation in the CNT electrochemical filter is an irreversible process. SEM and XPS results showed evidence of the polymer formation on the CNT surface after 300 min of reaction, which probably reduced the efficiency of the process under low applied voltages. Estrogenic activity was considerably reduced and minimal EQ-E2 levels were observed when 2.5 V was applied. A residual EQ-E2 was observed, likely due to the presence of estrogens, which suggests the non-formation of estrogenic intermediates. At 2.5 V total cell potential, the energy required to remove estrogenic activity was 0.014 ± 0.001 kWh m^-3 for ultrapure water and 0.021 ± 0.001 kWh m^-3 for post-secondary wastewater. These results suggest a CNT electrochemical filter may have potential to effectively and efficiently remove estrogenic activity and may be a feasible process for wastewater polishing treatment.