Biomass-Derived Sustainable Adsorbents for the Removal of Crystal Violet from Water

The increasing discharge of synthetic dyes into aquatic environments by the dyeing and textile industries has become a major environmental concern due to their persistence, toxicity, and resistance to biodegradation. Among the available treatment technologies, adsorption is highlighted, particularly when sustainable adsorbents are employed [1–3]. This study evaluated the potential of lignocellulosic biomass residues—cork sawdust, coconut shells, and stubble—and their corresponding activated carbons (ACs) for the removal of crystal violet (CV) from aqueous solutions. The activated carbons were prepared by chemical activation with KOH at a precursor-to-activating-agent ratio of 1:2 and carbonized at 873 and 973 K. The adsorbents were characterized by elemental analysis, FTIR-ATR spectroscopy, nitrogen adsorption at 77 K, and determination of the point of zero charge (pHpzc). Batch adsorption experiments were conducted to investigate the effects of adsorbent particle size, solution pH, contact time, temperature, and initial CV concentration. The raw biomasses exhibited predominantly acidic pHpzc values, whereas the ACs showed neutral to basic pHpzc values. The highest CV removal efficiencies were achieved at alkaline pH (≈9), with adsorption equilibrium being reached after approximately 6 h. The raw biomasses exhibited maximum adsorption capacities of 34.0, 44.5, and 60.2 mg g⁻¹ for cork, stubble, and coconut shells, respectively, corresponding to efficiencies of 90% for dilute solutions. The ACs, prepared at 973 K, significantly enhanced the adsorption performance, with ACs derived from stubble and coconut shells reaching maximum adsorption capacities of 134 and 181 mg g⁻¹, respectively. Temperature had little influence on CV adsorption by the raw biomasses, whereas adsorption onto the ACs was predominantly endothermic. Overall, the results demonstrate that raw biomasses and biomass-derived ACs are promising, low-cost, and sustainable adsorbents for the efficient removal of CV from water, while simultaneously promoting biomass valorization and producing treated water of sufficient quality for potential reuse.