Study of Selectivity Anionic Dye Removal and Sustainable Regeneration of Hydrochar from Spirogyra sp. Algae Biomass

This study investigates the synthesis of hydrochar from Spirogyra sp. algae biomass and its selective adsorption performance for anionic dyes. The hydrochar (SpiHC) was prepared via hydrothermal carbonization and characterized using XRD, FTIR, and BET analysis, revealing a significant increase in surface area (5.369 m²/g) and the development of a more ordered carbon structure. Selectivity studies showed that direct yellow (DY) exhibited the highest adsorption efficiency, with 62.07 % on SpiHC compared to 54.02 % on unmodified Spirogyra sp. (Spi). Adsorption experiments demonstrated optimal conditions at pH 6 and fit better with the pseudo-second-order kinetic model. Freundlich isotherm analysis indicated improved adsorption capacity and surface affinity of SpiHC. Thermodynamic parameters confirmed the endothermic and spontaneous nature of the adsorption process, with SpiHC showing higher adsorption affinity than Spi. Regeneration studies revealed that SpiHC retained 67.11 % adsorption efficiency after 4 cycles, highlighting its superior stability and reusability compared to Spi. These findings demonstrate the potential of Spirogyra-derived hydrochar as a sustainable adsorbent for water treatment applications. HIGHLIGHTS Hydrochar from Spirogyra sp was successfully synthesized by the hydrothermal method Selective adsorption on direct yellow is better than other anionic dyes The maximum adsorption capacity for direct yellow was 95.24 mg/g in hydrochar Regeneration studies revealed that SpiHC retained 67.11 % adsorption efficiency after four cycles. GRAPHICAL ABSTRACT