Microwave-assisted hydrothermal carbonization of microalgae grown in swine wastewater: Process optimization and application of hydrochar for dye adsorption

Abstract

This study presents an eco-friendly approach for converting microalgae cultivated in swine wastewater into value-added hydrochar via microwave-assisted hydrothermal carbonization (MHTC). Process parameters, including temperature, residence time, and biomass-to-water ratio, were optimized using response surface methodology (RSM) based on a Box–Behnken design. The optimal conditions—190 °C, 10 min, and a 7.5 % biomass-to-water ratio—resulted in a maximum hydrochar yield of 58.8 wt%. The resulting hydrochar exhibited a high carbon content (52.83 %) and a higher heating value (23.56 MJ/kg), with well-developed surface functionality and microporous structure, as confirmed by FTIR, SEM, and BET analyses. Its applicability as an adsorbent was demonstrated through batch adsorption experiments targeting methylene blue dye. The hydrochar achieved a maximum adsorption capacity of 107.5 mg/g with a removal efficiency of 87 %. Adsorption data fitted best with the Freundlich isotherm and pseudo-second-order kinetic models, indicating multilayer sorption on a heterogeneous surface. These findings highlight the dual environmental benefit of using swine wastewater as a nutrient source for microalgal cultivation and the subsequent valorization of biomass into functional hydrochar. This work contributes to advancing circular bioeconomy strategies by integrating waste treatment, carbon recovery, and low-cost dye remediation solutions.