Hydrochar-Based Wood Bio-Adhesives from Sewage Sludge: The Role of Sodium Periodate Oxidation

The development of technologies for replacing formaldehyde-based adhesives and disposing of sewage sludge (SS) are two standing issues of high interest to the academy and industry. This study reports the production of formaldehyde-free wood bio-adhesive using hydrochar from hydrothermal carbonization of SS aided by sodium periodate (NaIO4) oxidation. The performance of the bio-adhesives can meet the requirements of the Chinese National Standard GB/T 9846-2015 (≥0.7 MPa). The aldehyde group forms after NaIO4 treatment of sewage sludge hydrochar, which should benefit adhesion performance. However, the oxidation process also breaks some polysaccharides' carbon chains, disrupting the extracellular polymeric substances (EPS) structure in hydrochar and forming small molecular compounds. As a result, the wet shear strength value of bio-adhesive decreases, but the water resistance increases. Nevertheless, the interaction between active functional groups (e.g., between the aldehyde and amine groups) in bio-adhesives during hot-pressing treatment promotes adhesion performance. In addition, after doping polyethyleneimine, the cross-linked network in bio-adhesives can be further enhanced, improving the wet shear strength and water resistance. Hence, introducing active functional groups is the key step to bio-adhesives, proving NaIO4 oxidation as a novel and practical approach to valorize sewage sludge for producing formaldehyde-free adhesives.