Layered double hydroxides in nanofiltration as a new approach to nitrate removal

High nitrate concentrations in water present serious risks to human health. This study evaluates two removal strategies: layered double hydroxide (LDH) nanoparticles (NPs) and LDH-incorporated thin-film composite nanofiltration (TFC-NF) membranes to reduce nitrate concentration. First, Mg-Al, Ni-Fe, and Mn-impregnated Zn-Al LDH NPs were co-precipitated, characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) analysis, field emission scanning electron microscopy (FESEM), and zeta potential measurement, and tested in batch adsorption experiments (20 mg/L nitrate solution, 1 g/L adsorbent dosage). Among the LDH NPs tested, Ni-Fe LDH demonstrated the highest nitrate rejection, achieving 13-14% at this concentration, with no significant change with further calcination. Next, the TFC-NF membranes were fabricated by embedding LDH NPs into the support layer; one variant received additional layer-by-layer (LBL) surface modification. The membranes were also characterized using FESEM and then evaluated using a 50 mg/L nitrate solution at 6 bar pressure and 25 °C. The TFC-NF membrane containing 0.25 wt% Mn-impregnated Zn-Al LDH achieved nitrate rejection of 43% with pure water flux (PWF) of 4.5 L/m² h bar^-1. Under the same conditions, the LBL-TFC NF membrane showed lower nitrate rejection of 17% but higher PWF of 7.7 L/m² h bar^-1.