Synergistic mechanochemical phosphorylation and thermal curing of kraft fibers for high-capacity water remediation.

Abstract

Water scarcity and industrial effluent discharge demand sustainable and efficient materials for water treatment applications. Cellulose-based adsorbents are attractive due to their renewability and low environmental impact, yet their practical use is often constrained by limited surface charge density and adsorption capacity. The aim of this study was to develop a green and easy-to-implement modification strategy to convert industrial Kraft fibers into highly anionic, reusable adsorbents for effective removal of cationic dyes from aqueous systems. To achieve this aim, a solvent-free mechanochemical phosphorylation approach was employed using ball milling, with condensed phosphoric acid and urea as phosphorylating agents, followed by an optional thermal curing step. Two processing routes were compared to elucidate the individual and synergistic effects of mechanochemical activation and post-curing on charge development and adsorption performance. Methylene blue was used as a model cationic dye. Mechanochemically phosphorylated Kraft fibers exhibited exceptionally high anionic charge densities, reaching up to 6845 ± 147 mmol kg^-1. Compared to unmodified fibers (∼20 mg g^-1), the modified materials achieved a maximum methylene blue adsorption capacity of up to 1800 mg g^-1 after curing, corresponding to a 90-fold enhancement. The adsorbents showed good operational stability, retaining approximately 90% dye recovery after three adsorption-desorption cycles. The adsorption isotherm data were best described by Langmuir/Sips-type behavior, and kinetics were consistent with pseudo-second-order models, while density functional theory calculations confirmed the key role of phosphate groups in dye binding. Overall, this study establishes a simple, solvent-free, and environmentally benign strategy for converting industrial Kraft fibers into high-performance, regenerable adsorbents. The approach not only enhances the anionic functionality and adsorption efficiency of cellulose but also supports the broader valorization of Kraft fibers as low-cost, sustainable materials for real-world water remediation applications.