Dilute Pb(II) aqueous solutions were nanofiltered through a tubular membrane with good rejections. Retention was modeled using the Modified Spiegler–Kedem theory. The true retention, evaluated from concentration-polarization measurements, was similar to the observed value. The three characteristic parameters of the model: reflection coefficient $$\sigma$$ , solute permeability $$P$$ , and mass transfer coefficient $$K_{\text{m}}$$ were evaluated simultaneously. The reflection coefficient decreased with an increase in concentration until a plateau was reached at a concentration of 30 ppm. At low concentrations, the solute permeability increased with an increase in concentration, reaching a maximum at a concentration of 30 ppm. Subsequently, the permeability decreased with further increase in concentration, until at concentrations ≥ 100 ppm, it reached values close to those observed for very dilute solutions (< 10 ppm). Industrial scale nanofiltration of dilute solutions of Pb(II) is viable with high retentions. High pressures and tangential speeds and low temperatures increase retention. Moreover, moderately high concentrations of aqueous Pb(II) solutions can be reduced to totally sure levels in less than four nanofiltration steps. This makes nanofiltration a suitable tool to decrease Pb(II) levels below those recommended by the world health organization.