Electrocatalytic reduction of nitrate using Pd-Cu modified carbon nanotube membranes

Excessive nitrate in water is harmful to the ecological environment and human health. Electrocatalytic reduction is a promising technology for nitrate removal. Herein, a Pd-Cu modified carbon nanotube membrane was fabricated with an electrodeposition method and used to reduce nitrate in a flow-through electrochemical reactor. The optimal potential and duration for codeposition of Pd and Cu were −0.7 V and 5 min, respectively, according to linear scan voltammetry results. The membrane obtained with a Pd:Cu ratio of 1:1 exhibited a relatively high nitrate removal efficiency and N2 selectivity. Nitrate was almost completely reduced (∼99 %) by the membrane at potentials lower than −1.2 V. However, −0.8 V was the optimal potential for nitrate reduction in terms of both nitrate removal efficiency and product selectivity. The nitrate removal efficiency was 56.2 %, and the N2 selectivity was 23.8 % for the Pd:Cu=1:1 membrane operated at −0.8 V. Nitrate removal was enhanced under acidic conditions, while N2 selectivity was decreased. The concentrations of Cl− ions and dissolved oxygen showed little effect on nitrate reduction. The mass transfer rate constant was greatly improved by 6.6 times from 1.14 × 10−3 m/h at a membrane flux of 1 L/(m2·h) to 8.71 × 10−3 m/h at a membrane flux of 15 L/(m2·h), which resulted in a significant increase in the nitrate removal rate from 13.6 to 133.5 mg/(m2·h). These findings show that the Pd-Cu modified CNT membrane is an efficient material for nitrate reduction.