Hydrochar from Shorea spp.: a dual-purpose approach for sustainable biofuel and efficient methylene blue adsorbent

Abstract

This study delves into the multifaceted potential of hydrochar from a locally sourced wood species (Shorea spp.) as both an energy source and an adsorbent for dye removal. Hydrochars were produced under varied hydrothermal carbonisation (HTC) conditions, and the optimal variant, hydrochar-220, generated in a semi-flow hot compressed reactor at 220 °C, was selected for in-depth investigation. Hydrochar-220 exhibited the highest heating value at 22.81 MJ, representing a 33% improvement over the Shorea precursor, confirming its energy-positive nature. The CHN elemental analysis, TGA-PKM, XRD, DSC, SEM/EDX and BET results collectively highlight the valuable characteristics exhibited by the hydrochar. Furthermore, the adsorptive properties of hydrochar-220 were evaluated for dye removal, focusing on the cationic dye methylene blue (MB). The pseudo-second-order model best described the adsorption kinetic (R2 = 0.999), indicating the dominance of chemisorption. The Langmuir isotherm model best described the adsorption process, which predicted a theoretical maximum adsorption capacity of 37.8 mg.g−1 and suggested monolayer adsorption. Importantly, regeneration experiments demonstrated only minor reductions in adsorption capacity over successive wash cycles for both distilled water and NaOH washing mediums, highlighting the sustainability and cost-effectiveness of hydrochar for dye removal applications. In summary, the investigated hydrochar holds promise as both an energy source and a potent adsorbent for dye removal, offering multifaceted applications with economic and environmental benefits. Graphical Abstract: (Figure presented.)