Sawdust hydrochar derived using microwave-assisted hydrothermal carbonization for solar vapor generation

In response to the global water scarcity crisis, this study explores the use of waste sawdust (WS), which is an underutilized lignocellulosic byproduct of the timber industry, as a sustainable feedstock for photothermal conversion materials (PCM) in interfacial solar vapor generation (ISVG) systems. Carbonized sawdust hydrochars (CSHs) were produced from WS via eco-friendly and efficient microwave-assisted hydrothermal carbonization (MHC) at 230 °C with residence times of 30, 60, and 90 min, yielding samples denoted as CSH30, CSH60, and CSH90, respectively. All resulting CSHs exhibited a rougher surface morphology, a BET surface area of approximately 5.7 m2 g− 1, enhanced carbon content (~ 80%), and broad-spectrum light absorption. Hydrochar-based solar absorbers (HSAs) were fabricated and tested in an outdoor ISVG setup using seawater under direct solar irradiation (1.18 kW m− 2). Among them, HSA30 (based on CSH30) demonstrated the most optimal performance, achieving a photothermal efficiency of 66.45% and an evaporation rate of 1.15 kg m− 2 h− 1. Extending the residence time beyond 30 min yielded negligible improvement in performance. Heat loss analysis of HSA30 showed a low total heat loss of 13.8%. HSA30 also successfully rejected salt by dissolving and absorbing accumulated salt. The condensed vapor yielded clean water with a salinity of 220 ppm and a pH of around 6.85, meeting the World Health Organization (WHO) drinking water standards. These findings demonstrated that CSH derived via MHC is an efficient material for solar desalination.