Abstract:
The brewing industry produces large amounts of solid organic wastes, of which spent grains, surplus yeast and spent hops represent the majority. For years, despite the low-cost and great availability, their reuse has been scarce and limited to animal feed. More recently, alternative valorisation strategies to transform the brewing residues into value-added products have been investigated. Following this approach, in this study the wastes of two high-fermentation beers, i.e. IPA and stout from local breweries, were collected and used to generate N-doped carbon dots (CDs) by one-step hydrothermal carbonization (HTC), without the employment of any oxidising agents, surface passivation or external doping molecules. HTC has been chosen being a facile, eco-friendly and cost-effective method. Initially, CDs from the sole spent grains and surplus yeasts were produced to evaluate the contribution of each residue separately. Afterwards, in view of a potential technology scale-up, all wastes were blended together and residual beer was used instead of water for the HTC treatment.
Optical and structural proprieties of CDs were studied by various techniques including UV-visible and fluorescence spectroscopy, transmission and scanning electron microscopy, dynamic light scattering and nuclear magnetic resonance. The blended CDs exhibit high aqueous solubility with a bright blue emission (λex=420 nm) under UV light and a quantum yield of up to 11.3%. The presence of nitrogen-containing functional groups in the CDs was confirmed by elemental analysis. Lastly, envisaging potential photocatalytic applications, the CDs were successfully dispersed in a polyvinyl alcohol-based matrix, leading to flexible and luminescent hydrogels.
