Sewage Sludge Protein-based Fully Bio-Adhesive for Plywood

To advance sewage sludge (SS) valorization, this study proposed a novel approach that integrated protein recovery from SS with its conversion into a fully bio-based adhesive for plywood production. A thermal-alkaline pretreatment (pH = 12, 90 ℃) followed by acidic precipitation enabled efficient recovery of sewage sludge protein (SSP). Compared to the commonly used sulfuric acid, citric acid partially co-precipitated with SSP, potentially promoting amidation reactions between amino and carboxyl groups during the curing process. This interaction contributed to the enhanced adhesive strength of SSP. Inspired by the catechol-mediated adhesion mechanism of mussel foot proteins, tannic acid (Tan) and Zn²⁺ ions were incorporated into the alkali-modified SSP. Covalent and hydrogen bonding occurred through cross-linking between the polyphenolic moieties of Tan and the amide/carbonyl groups of SSP, while Zn²⁺ ions served as coordination centers, further strengthening interfacial cohesion. These procedures reassembled the fragmented peptide chains of SSP, resulting in enhanced hydrophobicity, thermal stability, and mold resistance. Under optimized curing conditions (140 ℃, 8 min), the resulting adhesive achieved a wet shear strength of (1.09 ± 0.08) MPa, surpassing the Chinese National Standard (≥ 0.7 MPa) by 55.7%. X-ray micro-computed tomography revealed that 24.1% of the adhesive penetrated the wood micropores during curing, with mechanical interlocking complementing bulk adhesion to improve shear performance. Heat-induced structural reorganization further promoted β-sheet formation and esterification between SSP peptides and Tan, thereby reinforcing the adhesive’s cross-linked architecture. Finally, a preliminary assessment of the process’s economic viability and carbon neutrality potential highlighted its promise for sustainable, engineering-scale implementation.