Comparative Assessment of Pretreatment Strategies for Integrated Phenolic Recovery and Bioenergy Production from Sunflower Hulls

Sunflower hulls are an abundant lignocellulosic waste with significant but underexploited potential for the recovery of high-value compounds and production of renewable energy. In this study, an integrated biorefinery approach was developed for the sequential recovery of waxes, free and bound phenolic compounds, and biomethane. Three pretreatment strategies (acid-thermal, mechanically assisted alkaline, and ultrasound-assisted ethanolic) were comparatively evaluated to determine their effects on bound phenolics extraction and biomethane production. Sequential extraction recovered 0.94 g of waxes 100 g⁻¹ d.b. and 145.52 mg GAE 100 g⁻¹ d.b. of free phenolics, with chlorogenic acid as the predominant compound. The different pretreatments exhibited contrasting performances depending on the valorisation criterion considered. Although acid–thermal pretreatment achieved the highest recovery of bound phenolic compounds (99.3 mg GAE 100 g⁻¹ d.b.), ultrasound-assisted ethanolic pretreatment preserved the largest proportion of the original biomass and resulted in the highest overall methane recovery (150% relative to the untreated biomass). Each pretreatment modified the lignocellulosic matrix through hemicellulose solubilization, lignin disruption, and fibre reorganisation, thus explaining the observed differences in phenolic release and biomethane production. The results demonstrate that each pretreatment favours a specific product stream, and that the selection of the most suitable pretreatment depends on the biorefinery’s objective.