Nanocellulose-sewage sludge ash composite beads as sustainable sorbents for advanced wastewater treatment

Herein, we report the formulations of composite beads to be used as adsorbent materials for water treatment, based on acid-treated sewage sludge ash (ASSA), obtained from mono-combustion, combined with polysaccharide-based biopolymers (chitosan, carboxymethylcellulose and TEMPO-oxidized cellulose nanofibers (TOCNF)), with glutaraldehyde (GA) or sodium alginate (SA) as cross-linkers. The resulting materials were characterized by SEM-EDS, both on the surface and in the inner section, and by tomography, highlighting the homogeneous distribution of ashes in the polysaccharide matrix and a highly porous network. The composites were tested as adsorbents for water treatment via batch adsorption experiments to evaluate their removal capacity for textile dyes and heavy metals. Comparative analyses revealed that beads reinforced with polymeric networks exhibited markedly higher adsorption efficiencies than corresponding blank formulations and raw ash-based sorbents. Notably, composite beads fabricated with TOCNF, SA, and ASSA displayed enhanced adsorption performance and mechanical integrity. Adsorption performances were described by the Langmuir isotherm model, indicating monolayer adsorption on uniform sites, whereas pseudo-second-order kinetic fits suggested chemisorption-dominated processes. Overall, the TOCNF-SA-ASSA composite beads demonstrate superior adsorption efficiency and operational robustness across multiple pollutant classes, underscoring their potential for advanced water treatment.