Lignin-Based Magnesium Hydroxide Nanocomposite. Synthesis and Application for the Removal of Potentially Toxic Metals from Aqueous Solution

The pollution of water by potentially toxic metals or so-called heavy metals is the most severe form of environmental impact. Nanocomposites are considered promising materials for the removal of potentially toxic metals from aqueous solution through adsorption or ion exchange. To produce high-performance adsorbent for the removal of Ni²⁺, Cd²⁺, and Pb²⁺, lignin–Mg­(OH)₂ nanocomposite (LH-MH) was developed utilizing hydrolytic lignin waste with possible regeneration after metal uptake. The LH-MH was prepared using hydrolytic lignin, magnesium chloride, and NaOH solution by a thermally assisted method. The structure was studied using X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, while chemical composition was evaluated by Fourier transform infrared spectroscopy, thermogravimetric analysis/differential thermogravimetry, and energy-dispersive X-ray spectroscopy mapping. Adsorption was described by the Langmuir and Sips models, whereas kinetics was elucidated by pseudo-first-order and pseudo-second-order models. The obtained material demonstrated a nanocomposite structure indicating well-distributed nano-Mg­(OH)₂ onto the lignin polymer matrix. The nanocomposite demonstrated superior removal of Ni²⁺, Cd²⁺, and Pb²⁺. The mechanism of adsorption was investigated indicating ion exchange between toxic metals and Mg²⁺. The obtained adsorbent was successfully regenerated using combined treatment by HCl–MgCl₂–NaOH.