<p>In this work, the surfaces of cellulose nitrate (CN) membranes were modified through interfacial polymerization, employing halloysite nanotubes (HNTs) as additives to form a thin-film composite (TFC). The formation of CN-polyamide-HNT composite membranes (CN/HNTs) was confirmed by analyzing their structural and surface properties using various methods, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). The morphology and performance of the fabricated CN/HNT membranes exhibited a clear improvement, including alterations in surface roughness, charge, and hydrophilicity. The water contact angle and porosity of the membranes were also measured in relation to the HNT content (0–0.1 wt.%). As the HNT content increased, the contact angle, mean roughness, and zeta potential decreased from 56° to 26°, 65.2 to 26.5 nm, and − 13.5 to − 25.9, respectively. Although the pure water flux decreased from 61 to 39 L/m² h with the addition of HNTs, salt rejection increased with higher HNT content. Salt rejection values of approximately 96, 97, 92, and 95% were achieved for NaCl, Na₂SO₄, MgCl₂, and MgSO₄, respectively, reflecting improvements of 71, 59, 51, and 51%, with 0.1 wt.% being the optimal concentration for HNTs. The membranes were evaluated using samples of real seawater and groundwater. They demonstrated effective salt rejection and significantly reduced the total dissolved solids (TDS) in seawater. For the groundwater sample, the total hardness decreased markedly from 680 to 87 mg/L, corresponding to an 87% reduction.</p>