Beach-cast brown seaweed (Ascophyllum nodosum) for the extraction of bioactive compounds and production of hydrochar using subcritical water as a green solvent

Beach-cast brown seaweeds are a rich source of potential bioproducts but are currently either left to degrade on coastlines or harvested and discarded in landfills, becoming an environmental burden and resulting in underutilized biomass. This thesis explores the valorization of beach-cast seaweed using hydrothermal processing. A common brown seaweed, Ascophyllum nodosum, abundant along the North Atlantic seashores, is used to investigate lower temperature hydrothermal processing, subcritical water (SCW) extraction as an effective process to produce bioactive compounds (i.e., alginate, fucoidan, phenolics, and carotenoids) in liquid product while simultaneously producing a valuable solid by-product (hydrochar). A comprehensive literature review identified knowledge gaps in bioactive compound extraction and challenges in scaling SCW processing, guiding the subsequent experimental investigations. The beach-cast A. nodosum was extracted using an Accelerated Solvent Extractor (ASE) under high-pressure SCW conditions (100–200 °C and 100 bar) to evaluate its valorization potential. Experimental design, kinetic studies, and rate modeling identified temperature as the most critical parameter impacting the ASE process. The key bioactive compounds (crude alginate, crude fucoidan, and antioxidant-rich crude extract) and nutrients (minerals and vitamins) exhibited comparable yields and qualities to those reported from fresh seaweeds, demonstrating the feasibility of using beach-cast seaweed as a sustainable feedstock. The scalability of SCW was studied using a 600 mL bench-scale pressurized reactor under low-pressure SCW conditions (100–240 °C, 1–33 bar). A screening factorial design (SFD) and central composite design (CCD) were used to study the effects of final temperature, hold time at that final temperature, water-to-biomass ratio, water pH, and agitation speed. Final temperature and hold time were identified as the most significant parameters impacting yields and qualities of bioactive compounds and hydrochar. A key challenge identified in the SCW scale-up is the variability in heating times across reactors, which complicates process optimization and control. To address this, the severity factor (log Ro) was evaluated as a single parameter (0.59–5.10) to integrate final temperature (100–240 °C), heating time (31–99 min), and hold time (0–24 min) for beach-cast A. nodosum valorization. The results demonstrated that the SCW process could recover crude alginate (15.75 dry wt% at log Rₒ = 1.99), crude fucoidan (39.94 dry wt% at log Rₒ = 2.39), antioxidant-rich crude extract (53.84–55.07 dry wt% at log Rₒ = 3.18–3.22), and hydrochar (29.53 dry wt% at log Rₒ = 3.79) at the maximum yields and/or qualities. Characterization of solid and liquid products revealed potential applications across biofuels, animal feeds, diet supplements, cosmetics, and natural antioxidant ingredients. This thesis establishes a holistic framework for the sustainable valorization of beach-cast seaweeds. By demonstrating the conversion of “waste” A. nodosum into multiple high-value products, the research contributes to regional sustainability, supports zero-waste concepts, and provides practical design tools (i.e., severity factor) for guiding future SCW scale-up and industrial application.

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