Lead and Chromium Adsorption from Water using L-Cysteine Functionalized Magnetite (Fe₃O₄) Nanoparticles

L-Cysteine functionalized magnetite nanoparticles (L-Cyst-Fe₃O₄ NPs) were synthesized by chemical co-precipitation using Fe²⁺ and Fe³⁺ as iron precursors, sodium hydroxide as a base and L-Cysteine as functionalized agent. The structural and morphological studies were carried out using X-ray powder diffraction, transmission electron microscopy, dynamic light scattering, scanning electron microscopy and energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and UV-Vis spectrophotometric techniques. The zeta potential of bare Fe₃O₄ and functionalized L-Cyst-Fe₃O₄ NPs were +28 mV and -30.2 mV (pH 7.0), respectively. The positive surface charge changes to negative imply the presence of L-Cyst monolayer at particle interface. Band gap energy of 2.12 eV [bare Fe₃O₄NPs] and 1.4 eV [L-Cyst-Fe₃O₄ NPs] were obtained. Lead and chromium removal were investigated at different initial pHs, contact time, temperatures and adsorbate-adsorbent concentrations. Maximum Cr⁶⁺ and Pb²⁺ removal occurred at pH 2.0 and 6.0, respectively. Sorption dynamics data were best described by pseudo-second order rate equation. Pb²⁺ and Cr⁶⁺ sorption equilibrium data were best fitted to Langmuir equation. Langmuir adsorption capacities of 18.8 mg/g (Pb²⁺) and 34.5 mg/g (Cr⁶⁺) at 45 °C were obtained. Regeneration of exhausted L-Cyst-Fe₃O₄ NPs and recovery of Pb²⁺/Cr⁶⁺ were demonstrated using 0.01 M HNO₃ and NaOH. L-Cyst-Fe₃O₄ NPs stability and reusability were also demonstrated.