Eggshell membrane derived nitrogen rich porous carbon for selective electrosorption of nitrate from water

Nitrate (NO3-) is a ubiquitous contaminant in water and wastewater. Conventional treatment processes such as adsorption and membrane separation suffer from low selectivity for NO3- removal, causing high energy consumption and adsorbents usage. In this study, we demonstrate selective removal of NO3- in an electrosorption process by a thin, porous carbonized eggshell membrane (CESM) derived from eggshell bio-waste. The CESM possesses an interconnected hierarchical pore structure with pore size ranging from a few nanometers to tens of micrometers. When utilized as the anode in an electrosorption process, the CESM exhibited strong selectivity for NO3- over Cl-, SO42-, and H2PO4-. Adsorption of NO3- by the CESM reached 2.4 × 10-3 mmol/m2, almost two orders of magnitude higher than that by activated carbon (AC). More importantly, the CESM achieved NO3-/Cl- selectivity of 7.79 at an applied voltage of 1.2 V, the highest NO3-/Cl- selectivity reported to date. The high selectivity led to a five-fold reduction in energy consumption for NO3- removal compared to electrosorption using conventional AC electrodes. Density function theory calculation suggests that the high NO3- selectivity of CESM is attributed to its rich nitrogen-containing functional groups, which possess higher binding energy with NO3- compared to Cl-, SO42-, and H2PO4-. These results suggest that nitrogen-rich biomaterials are good precursors for NO3- selective electrodes; similar chemistry can also be used in other materials to achieve NO3- selectivity.