This study evaluated the unmodified Norway Spruce Wood Residue (NSWR), an abundant lignocellulosic biomass, for the simultaneous removal of Pb²⁺, Cd²⁺, Zn²⁺, and Cu²⁺ from a quaternary aqueous system. A series of batch adsorption experiments were performed to assess the influence of key operational parameters (pH, contact time, adsorbent dose, temperature, particle size, initial concentration), with equilibrium data subsequently fitted to Langmuir and Freundlich isotherm models, and the NSWR characterized using FTIR and EDS analyses. The results demonstrated maximum Langmuir adsorption capacities following the order Pb²⁺ (10.3 mg/g) > Cu²⁺ (7.9 mg/g) > Cd²⁺ (6.3 mg/g) > Zn²⁺ (6.0 mg/g), corresponding to high removal efficiencies (up to 99% for Pb²⁺). Adsorption was rapid initially (~60% removal within 20 min) and favored slightly acidic conditions (pH 5–6) and moderately elevated temperatures (45°C). The Langmuir model provided an excellent fit to the data (R² ≈ 0.99), indicating favorable monolayer chemisorption, likely driven by a combination of cation exchange supported by EDS and interactions with surface functional groups. Conclusively, unmodified NSWR shows significant promise as an effective, inexpensive, and eco-friendly biosorbent for treating water contaminated with multiple heavy metals, presenting a viable waste valorization strategy for sustainable water management.

The discharge of synthetic dyes into aquatic systems remains a significant environmental concern due to their persistence, toxicity, and resistance to conventional treatment processes. This review critically examines recent advances (2020-2025) in glutaraldehyde-crosslinked chitosan (CS-Glu) adsorbents for dye removal from aqueous solutions. The scope covers major material families, including pristine CS-Glu hydrogels, magnetic CS-Glu composites, carbon-based hybrids, and inorganic-filler nanocomposites, as well as the adsorption of representative dye classes, such as cationic dyes (e.g., methylene blue), anionic dyes (e.g., Congo red), and reactive dyes commonly found in textile effluents. Reported adsorption capacities typically range from ∼50 to mor than 800 mg/g, depending on crosslinking density, composite structure, and operating conditions, with optimal performance generally observed within pH 4-9 and moderate ionic strength. The review discusses the crosslinking chemistry of CS-Glu systems, structure-property relationships, adsorption mechanisms (electrostatic interaction, hydrogen bonding, and π-π interactions), and the influence of physicochemical parameters on adsorption efficiency. Comparative analysis indicates that nanocomposite CS-Glu systems, particularly magnetic and carbon-based hybrids, provide improved adsorption capacity, stability, and recyclability compared with conventional chitosan adsorbents. Overall, the literature suggests that controlled crosslinking can significantly enhance chitosan stability while maintaining high adsorption performance. However, future research should prioritize testing under realistic wastewater matrices, long-term regeneration performance, and scalable synthesis strategies to facilitate practical applications.