Ultrasound pretreatment of third-generation biomass (invasive macroalga Rugulopteryx okamurae) to obtain platform biocommodities

Abstract

Volatile fatty acids (VFA) and reducing sugars (RS) are widely used as platform molecules in biorefineries, facilitating the production of valuable biofuels and chemicals. From an environmental, economic and social perspective, third generation biomass, including macroalgae beach-cast, represents an innovative and optimal solution for the production of these commodities. This study explores the impact of ultrasound pretreatment on the invasive macroalga Rugulopteryx okamurae, aiming to produce RS and VFA through enzymatic hydrolysis and dark fermentation. Several ultrasound conditions were tested: amplitudes (0, 70-100 %), suspension volumes (300, 600 mL), and algal concentrations (4-8 %). Optimal results emerged with 100 % amplitude, 300 mL volume, and 4 % (w/v) algal concentration, leading to the maximum COD solubilization of 61.5 mg COD g-biomass^-1. For enzymatic hydrolysis, the pretreated sample achieved maximum RS concentrations (0.124 g-RS g-biomass^-1) with half the enzyme dosage required by the non-pretreated alga (25 vs 50 FPU g-biomass^-1), implying significant economic benefits for large-scale processes. The kinetic model proposed by Romero-Vargas et al. aligned perfectly with the experimental data, obtaining higher values of all the kinetic parameters for the pretreated sample. Dark fermentation showed substantial increases in organic matter solubilization and VFA production (10.36 mg-HAc g-biomass^-1) post ultrasound pretreatment: 21.1 % higher solubilization and 9.4 % increased VFA compared to non-pretreated biomass. The resulting VFA composition comprised 73 % acetic acid, 13 % propionic acid, and 8 % butyric acid. Utilization of chemical agents during sonication may further enhance overall processing yields.