Mechanochemical synthesis and interfacial binding analysis of chitosan/alum sludge composite particles

Solvent-based approaches have advanced the synthesis of nanocomposites, though opportunities remain to improve their environmental footprint and scalability. This study presents a green, organic solvent-free mechanochemical approach for synthesizing chitosan/sludge composite particles, aiming to convert low-reactivity alum sludge waste into high-value materials. Through high-energy ball milling, chitosan serves as a natural dispersant, significantly enhancing the refinement and dispersion of sludge particles to the nanoscale (about 100 nm) and forming a uniform encapsulating layer. FTIR, TGA, and XPS analyses revealed three distinct binding interactions between chitosan and sludge, i.e. electrostatic adsorption (+31.4 vs −15.2), hydrogen bonding and chelation with metal ions (Al 3+ and Fe 3+ ). TGA distinguished the decomposition ranges of these interactions and quantified their relative proportions before and after acid washing, with chelation demonstrating the highest stability. Chitosan-assisted mechanochemical synthesis enabled the formation of nanocomposite particles in which highly reactive sludge nanoparticles are stably and uniformly dispersed within a chitosan matrix. The interfacial structure between the two phases is composition-dependent and tunable. Due to the pH-responsive nature of chitosan, the materials show potential for future development as controlled-release self-healing agents in acidic sewer environments. • A green mechanochemical method was developed to synthesize chitosan/sludge nanocomposites. • Chitosan significantly reduced sludge particle size and formed a uniform encapsulating layer. • FTIR, TGA, and XPS analyses revealed electrostatic, hydrogen bonding, and chelation interactions. • TGA distinguished thermal ranges of different binding types and quantified their relative proportions.