Preparation of High-Performance KOH-Activated Biochar from Agricultural Waste ( Sapindus mukorossi ) and Its Application in Organic Dye Removal

The separation of organic dyes from wastewater (WW) is a major challenge in water pollution control. The present research utilised agricultural residuals from Sapindus mukorossi were utilised to prepare high-performance biochar through carbonization and KOH activation, while its efficiency was evaluated in removing methylene blue (MB). The physicochemical characteristics of the unactivated Sapindus shell biochar (SH0) and activated Sapindus shell biochar (SH2) material were characterised via EA, FTIR, BET, and SEM analyses. The findings indicated that the KOH activated Sapindus shell biochar (SH2) exhibited higher adsorption efficiency in comparison to Sapindus shell biochar (SH0). In particular, the SH2 demonstrated an 11.2-fold higher adsorption capacity for MB (502.11 mg·g −1 ) compared to SH0, a performance enhancement driven by its remarkably porous structure, substantial total pore volume (0.56 cm 3 ·g −1 ), and high specific surface area (871.04 m 2 ·g −1 ). A high MB removal efficiency of 98.36% was achieved within 30 min under the following optimal conditions: a KOH/SH0 activation ratio of 2:1, pH 6.5, and a biochar dose of 0.2 g·L −1 . The MB adsorptive process was studied by applying the Langmuir isotherm and PSO kinetic models, suggesting physical and chemical interaction mechanisms between MB dyes and SH2. These findings provide a feasible strategy for the application of Sapindus shells and offer technical support for effectively removing dyes from wastewater by KOH-modified biochar.