Environmentally-friendly alkaline ionized water pretreatment and hydrolysis of macroalga via microwave-assisted heating to improve monosaccharide yield for bioethanol production

This study synthesizes three sequential steps: pretreatment, acid hydrolysis, and enzyme hydrolysis for bioethanol and hydrochar production from macroalga Gracilaria. The Taguchi method is employed separately in each step to optimize relevant operating factors, aiming to maximize the monosaccharide yield. During pretreatment, environmentally friendly alkaline ionized water is utilized as it does not contain chemical additives. Afterward, microwave-assisted hydrolysis is adopted to enhance the monosaccharide yield of the macroalga for bioenergy production. The results show that the total sugar of Gracilaria undergoing acid hydrolysis alone is 28.72 g‧L⁻¹. The total sugar content after alkaline pretreatment followed by acid hydrolysis is 32.64 g‧L⁻¹, and it increases to 34.76 g‧L⁻¹ after adding enzymes. Meanwhile, the higher heating value of Gracilaria increases from 10.884 MJ‧kg⁻¹ to 12.620 MJ‧kg⁻¹ after undergoing alkaline pretreatment. After the acid and enzyme hydrolysis processes, it increases to 15.164 MJ‧kg⁻¹. The solid biofuel's calorific value increases by 39 % from the three-stage processes. The liquid product combined with Saccharomyces cerevisiae can produce bioethanol, while the produced hydrochar can be used as a solid fuel. This research promotes the development of macroalgal biomass for energy and environmental applications, thereby advancing the circular bioeconomy.