Valorization of Brewery Wastes for the Synthesis of Silver Nanocomposites Containing Orthophosphate

Brewery wastes from stage 5 (Wort precipitate: BW5) and stage 7 (Brewer's spent yeast: BW7) were valorized for the synthesis of silver phosphate nanocomposites. Nanoparticles were synthesized by converting silver salt in the presence of brewery wastes at different temperatures (25, 50, and 80 °C) and times (10, 30, and 120 min). Unexpectedly, BW7 yielded Ag₃PO₄ nanoparticles with minor contents of AgCl and Ag metal (Ag_met). Contrastingly, BW5 produced AgCl nanoparticles with minor amounts of Ag₃PO₄ and Ag_met. Nanocomposites with different component ratios were obtained by simply varying the synthesis temperature and time. The morphology of the nanocomposites contained ball-like structures representative of Ag₃PO₄ and stacked layers and fused particles representing AgCl and Ag_met. The capping on the nanoparticles contained organic groups from the brewery by-products, and the surface overlayer had a rich chemical composition. The organic overlayers on BW7 nanocomposites were thinner than those on BW5 nanocomposites. Notably, the nanocomposites exhibited high antibacterial activity against Escherichia coli ATCC 25922. The antibacterial activity was higher for BW7 nanocomposites due to a larger silver phosphate content in the composition and a thin organic overlayer. The growth of Ag_met in the structure adversely affected the antimicrobial property of the nanocomposites.