pH-Guided morphological transition of Bergenia ciliata -derived ZnO nanoparticles: from spheres to rods/sheets for enhanced Pb(II) adsorption

The presence of lead in water sources poses significant toxicological threats due to its persistence and bioaccumulation potential. Therefore, effective removal of lead from contaminated water is imperative to safeguard public health and ensure water quality. For this purpose, ZnO NPs were synthesized via green synthesis using Bergenia ciliata stem extract at five different pH levels (7–11) and applied for Pb2+ removal from water. For structural characterization, XRD confirmed the hexagonal wurtzite phase of ZnO, while FESEM revealed morphological transitions from spherical particles at pH 7 to rod-like and sheet-like structures at pH 11. EDS and color mapping verified the absence of impurities. FTIR and Raman spectra verified Zn-O bonding and structural evolution with pH. Moreover, TGA/DTG confirmed high thermal stability up to ∼800 °C with over 97% weight retention at pH 10–11. Adsorption performance was evaluated under varying pH, time, dose, and temperature. The maximum Pb2+ removal was achieved at pH 7 and 8. Kinetic data followed the pseudo-second-order model (R2 = 0.98–0.99 ± 0.01–0.02), and the Langmuir isotherm best fit the equilibrium data (R2 = 0.98–0.99 ± 0.01–0.02), indicating monolayer adsorption. The highest adsorption capacity (qmax) was 102.04 ±1.04 mg/g (pH7, 308 K). Freundlich and Temkin models also supported surface heterogeneity (R2=0.87-0.96 ± 0.01–0.02). Additionally, thermodynamic parameters confirmed the exothermic nature of adsorption (ΔH = −15.03 to −28.60 ± 0.21–0.56 kJ/mol) and decreasing spontaneity with temperature (ΔG= +5.11 to +7.84 ± 0.09-0.16 kJ/mol), indicating physisorption. Eventually, Post-adsorption FTIR and XPS analysis confirmed Pb-O bond formation without significant ZnO structural loss.