A green oxidation route to prepare polysaccharides-based wood adhesives from sewage sludge

Bio-adhesives are increasingly needed due to notorious formaldehyde emissions from conventional wood adhesives. Sewage sludge (SS) is rich in proteins and polysaccharides, which are potential feedstocks for bio-adhesives, while the lack of aldehyde groups impedes its efficacy as bio-adhesives. Herein, NaIO₄ oxidation was proposed as a green route to enrich aldehyde groups on polysaccharides for the synthesis of bio-adhesives from SS. From in-situ Fourier transform infrared spectroscopy, it confirmed that aldehyde groups were formed after oxidation treatment which is beneficial for the Schiff-base reaction. The maximum wet shear strength (WSS) of the bio-adhesives was increased to 1.46 MPa after the oxidation treatment of SS, which exceeded the requirement of China national standard GB/T 9846-2015 (≥ 0.7 MPa). Schiff-base reaction was beneficial for the formation of macromolecular networks to increase the WSS. Furthermore, the addition of ammonium polyphosphate endowed the bio-adhesive with high flame retardancy and further enhanced the WSS of the resultant bio-adhesive. Subsequent mechanistic analyses elucidated that the presence of active functional groups and the establishment of covalent bonds were pivotal determinants in the functionality of bio-adhesives. Finally, this study established a novel avenue for the upgrading of SS, offering an environmentally friendly and sustainable alternative for wood adhesives.