Antibacterial properties of functionalized cellulose extracted from deproteinized soybean hulls

Soybean hulls (SBHs) are one of the main by-products of soybean crushing, usually destined for animal feeding or to become a putrescible waste. In this work, we upgraded the SBHs to materials with antimicrobial properties. After the extraction of soybean peroxidase from SBHs, an enzyme applicable in different technological sectors and naturally present in soybean hulls, the exhausted biomass was subjected to an acid-base treatment to isolate cellulose. The obtained material was, in turn, functionalized with 3-aminopropyl triethoxysilane (APTES) to achieve new hybrids with antimicrobial properties. The synthetic procedure was optimized by varying the solvent type (ethanol or toluene) and APTES amount. Overall, the amino-functionalization process was effective and the activity was outstanding against both gram-positive and gram-negative bacteria, reaching complete disinfection practically in all cases. The samples were studied by means of several characterization techniques, demonstrating that the solvent and cellulose types had a significant influence on the physical-chemical features, together with the eco-sustainability of the process. In particular, the use of greener ethanol and waste cellulose (with respect to a commercial one) resulted in a higher APTES immobilization efficiency and superior thermal stability of the final materials. Interestingly, the presence of various unremoved compounds from the lignocellulosic SBH matrix, although in small quantities, emerged as a crucial factor, also in terms of antibacterial activity, hypothesizing a role of residual phytochemicals.