pH-switchable kraft lignin/silica gel sorbent for tunable removal of organic and inorganic water contaminants

The increasing threat of anthropogenic water contamination necessitates the development of sustainable, fossil-free materials for effective water remediation. This study presents an eco-friendly and scalable approach for fabricating lignin/silica gel composites, produced via the adsorption of kraft hardwood lignin (aKH) onto silica gel pre-modified with a quaternary ammonium salt (QAS). The resulting QAS-modified lignin–silica composite exhibited pH-dependent adsorption behavior, strongly influenced by the nature of the adsorbate. Optimal adsorption of a cationic dye was observed in the pH range of 5.7–6.5 (R ≥ 0.96), while anionic dye uptake was most effective between pH 4.2–5.2. For metal ions, the most efficient removal occurred at pH 3.2–4.5 for Fe(III) and 3.7–5.3 for Cu(II). In all cases, rapid adsorption kinetics were observed, with equilibrium reached within 20 min. The maximum sorption capacities, determined by Langmuir modeling, were 1.55 ± 0.05 mmol/g for Fe(III) and 1.21 ± 0.04 mmol/g for Cu(II). Isotherm analysis further revealed that the immobilization of lignin on QAS-modified silica gel significantly enhanced surface heterogeneity, promoting more favorable adsorption interactions. The composite’s high capacity, fast kinetics, and environmentally benign composition highlight its strong potential as an efficient and tunable adsorbent for water purification systems. Its pH-responsive behavior enables the selective removal of cationic and anionic pollutants, offering promising avenues for both effective decontamination and resource recovery.