Hydrothermal valorization of biomass waste into hydrochar towards circular economy and sustainable adsorptive dye contaminants clean-up: A review

Abstract

Circular economy including hydrothermal conversion of biomass waste into valuable products like hydrochar for environmental application has attracted researchers' attention in the 21st century because of its importance on environmental sustainability and responsibility. The goal of this study was to examine the adsorptive clean-up of dye contaminants from aqueous media using biomass waste-derived hydrochar (BWDHC). The study evaluated the performance of different BWDHCs, scrutinized the adsorption mechanism from density functional theory and spectroscopic viewpoint, and analyzed the kinetics, isotherm, and thermodynamics. It was found that the highest reported adsorptive dye clean-up (2468 mg/g) was observed for malachite green using litchi peel-derived hydrochar. The chief mechanisms of dye sorption were π-π stacking/interaction, H-bonding, and electrostatic interaction through the BWDHC oxygenated functional groups and dyes nitrogen and sulfonate groups. Also, dye sorption was largely best fitted to the pseudo-second-order kinetics and Langmuir isotherm models suggesting that the BWDHC active sites' numerical strength affects the rate of adsorption. It was observed that various spent BWDHCs can be easily regenerated and the majority of these maintained over 75 % of the original adsorption capacity after 4–6 rounds. Competitive adsorption findings also revealed that BWDHC holds great potential in real-world applications. Furthermore, problematic issues and future research directions were underscored. This study leads to the conclusion that adsorption methods using BWDHC can eco-effectively clean up dye in industrial runoffs and wastewater. Moreover, this review not only contributes to water pollution remediation advancement but also the advancement of circular economy, responsible consumption, and sustainable waste management practices.