Selective adsorption and mechanistic insights into divalent cadmium, copper, and lead recovery using silica–magnetite modified Nitzschia sp. biomaterial

This study presents the synthesis and evaluation of a novel bio-based nanocomposite, Nitzs–Si@nMs, derived from Nitzschia sp. diatom biomass functionalized with a silica matrix and magnetite (Fe3O4) nanoparticles. The composite was fabricated through a facile room-temperature method and comprehensively characterized using FTIR, XRD, SEM–EDX, XRF, and particle size analysis. Nitzs–Si@nMs demonstrated outstanding adsorption performance for Cd(II), Cu(II), and Pb(II) ions in single, binary, and multicomponent systems. Under optimal conditions (pH 5.0, 120 min contact time, and 300 mg L−1 initial concentration), removal efficiencies exceeded 99%. Adsorption followed pseudo-second-order kinetics and conformed to the Langmuir isotherm, indicating monolayer chemisorption. Maximum adsorption capacities were 1.0204 mmol g−1 for Cu(II), 0.7299 mmol g−1 for Cd(II), and 0.5587 mmol g−1 for Pb(II). Competitive studies revealed a strong selectivity for Cu(II), attributed to its smaller hydrated radius and high affinity for Fe–O coordination sites introduced by magnetite. The composite exhibited robust chemical stability in acidic environments (84% Si retention after 96 h at pH 1.35) and retained over 80% of its adsorption capacity across four regeneration cycles using 0.1 M HCl. Compared to conventional bio-adsorbents, Nitzs–Si@nMs achieved superior adsorption capacities and operational advantages, including facile magnetic separation and reusability.