<p>The rapid expansion of the agriculture industry has led to the production of large quantities of biological wastes; however, innovating their potential applications still needs to be investigated. In this research, walnut hard waste has been valorized as a tannin-rich finishing material for the multi-functional finishing of wool yarns under the ultrasonic environment. The potential alternative of rice straw for the metal mordant was performed and it was used as a bio-mordant to improve the multi-functional properties of wool yarns toward the walnut hard waste without the risk of pollution. An investigation has been made to evaluate the main finishing process variables, optimize the process, develop a predictive model based on Box–Behnken Design, and evaluate the physio-chemical and multi-functional properties of the bio-yarns produced. The results of assayable tannin and flavonoid contents confirmed the worthwhile selection of the walnut hard waste as a tannin-rich finishing material. Statistical analysis of finishing conditions revealed that all the linear terms, the interaction terms of walnut concentration-finishing bath pH, rice concentration-finishing bath pH, time-finishing bath pH, time-rice concentration, the quadratic effects of finishing bath pH, walnut concentration, and rice concentration had significant influences on the response variable. The bio-yarns produced under optimized dyeing conditions showed excellent fastness qualities and bio-functional behaviors due to the formation of strong complexes between walnut molecules and peptide chains. The findings of this research address the urgent need for the bio-functional modification of textile materials via a facile impregnation strategy, providing a new starting point for the worthwhile application of agricultural waste in advanced bio-textiles.</p> Graphical Abstract <p></p>