Cascade Valorization of Brewer's Spent Grain Residues Through Green Solvent Extractions and Subsequent Thermochemical Conversion to Activated Carbon Adsorbers

Abstract

To maximize the recovery of multiple added-value products from biomass waste streams, optimal valorization should follow a cascade process integrating extraction and conversion steps. In this context, natural deep eutectic solvents (NADES) can be employed as green solvents to significantly expand the possible valorization routes. In this study, brewer's spent grain (BSG) was first used for antioxidant (polyphenolic compound) extraction with NADES, whereupon the remaining biomass was converted into activated carbons (ACs). These adsorbents were characterized in-depth, and their performance for subsequent NADES clean-up is described for the first time as a proof-of-concept. Extraction with NADES caused a significant reduction in AC yield, due to associated degradation effects on structural elements (lignin, cellulose, and hemicellulose) within the biomass. The largest effect (only 3% AC yield for choline chloride:malic acid) was correlated to the solvent acidity, and not inherent to the NADES hydrogen bond interactions. In addition, the acidic pretreatment caused significantly lower surface area and less micropore development, which negatively impacted the adsorption properties. Nonetheless, NADES pretreatments created ACs that outperform traditional woody-biomass-derived commercial AC reference materials (adsorption q e of 203.9 +/- 12.5, 257.3 +/- 6.8, and 204.9 +/- 2.8 mg/g, respectively, from choline chloride:glycerol NADES stock solution).