Recovery of alginate, fucoidan, phenolics, carotenoids, and hydrochar from beach-cast Ascophyllum nodosum using subcritical water extraction

Beach-cast brown seaweeds are an underutilized biomass with high potential for sustainable valorization within a circular economy framework. This study focuses on subcritical water (SCW) as a green, chemical-free solvent to recover high-value bioactive compounds (crude alginate, crude fucoidan, phenolics, carotenoids) and hydrochar from beach-cast Ascophyllum nodosum . Despite increasing interest in seaweed valorization, limited research has optimized SCW processing for beach-cast seaweeds, and the effects of non-isothermal heating time (critical factor for scale-up) remain largely unexplored. To address this, factor screening followed by a central composite design was used to evaluate the effects of final temperature, heating time, hold time, water-to-biomass ratio, pH, and agitation speed on yields and antioxidant activities. Final temperature and hold time significantly affected all responses. Maximum crude alginate (15.80 dry wt%) and crude fucoidan (38.48 dry wt%) yields were obtained at 120 °C, 20 min and 138 °C, 12 min hold times, respectively. Comparable yields were also achieved under non-isothermal heating conditions (no hold time) at 140 °C (crude alginate) and 160 °C (crude fucoidan). At 160 °C and 20 min hold time (total time of 76 min), the crude extract was richest in phenolics and carotenoids and concomitant antioxidant activities. Hydrochar was also recovered 31.23–61.59 dry wt% over 112–168 °C and 0–24 min hold time (total 38–76 min). This work provides not only a framework to develop SCW processes for other types of seaweeds but also demonstrates the value in beach-cast seaweeds compared to their cultivated counterparts. • SCW was used to extract bioactive compounds and hydrochar from waste seaweed. • Final temperature, heat time, and hold time significantly influenced product yields. • Maximum crude alginate and fucoidan yields were 15.80 % and 38.48 %, respectively. • Crude extract at 160 °C and 20 min had the “best” phenolic and carotenoid contents. • Hydrochar recovered from 31.23 to 61.59 dry wt% among the conditions studied.