Highly efficient metal-free nitrate reduction enabled by electrified membrane filtration

Abstract

Current methods for electrocatalytic destruction of nitrate in drinking water require metal catalysts to achieve sufficient nitrate removal. However, metal-based catalysts involve complicated synthesis, increase treatment costs and can lead to leaching of metals into treated water. Here we achieved nitrate reduction performance comparable to that of metal-based catalysts via electrofiltration through a metal-free nanoporous electrified membrane (EM) containing unmodified pristine carbon nanotubes (CNTs). Experimental results coupled with computational fluid dynamics simulations elucidated how the decreased diffusion boundary layer in the flow-through CNT-EM mitigates diffusion limitations to enhance overall nitrate reaction activity. Furthermore, defects in CNTs were identified as the catalytic active sites by comparing the activity of EMs containing acid-treated metal-free CNTs and CNTs with added defects. Through density functional theory and molecular dynamics calculations, we demonstrated enhanced *NO2 and *NO adsorption energies at intrinsic defect sites, which are present in most commercial CNTs and become more accessible to nitrate ions under flow-through operation. Finally, the long-term stability, tolerance of environmental interferences, and sufficient nitrate removal and scalability to meet drinking water standards were demonstrated in real surface water, exhibiting the outstanding performance of the metal-free CNT-EM for practical applications. By elucidating how nanoporous electrofiltration enables dynamic matching of reaction and transport rates, this study demonstrates a new strategy to drastically improve electrocatalytic reaction performance without complex catalyst materials innovation, bridging existing gaps for nitrate removal in drinking water treatment related to the use of metal-based catalysts. Current methods for electrocatalytic destruction of nitrate in drinking water require metal catalysts to achieve sufficient nitrate removal. Electrified membranes containing pristine carbon nanotubes operated under flow-through mode provide an alternative approach for efficient nitrate reduction without the use of metals.