Isotherm Behavior and Mass Transfer Kinetics of Curcumin Adsorption onto Granular Jengkol Peel Waste Biochar

The abundance of jengkol peel waste (JPW) poses environmental and health risks, necessitating proper management. Converting JPW into biochar for dye decolorization offers a promising solution for promoting sustainable waste utilization and environmental remediation. This study investigated the adsorption of curcumin onto granular jengkol peel waste biochar (GJPWBC), examining isotherm behaviors (using ten isotherm models) and elucidating adsorption mechanisms and mass transfer kinetics via Generalized Fulazzaky equations. According to the Fowler-Guggenheim model, increasing the GJPWBC grain size from 105 to 250 and 500 μm decreases the interaction energy (W) from -3.788 to -5.100 and -5.996 kJ mol^-1, respectively. The corresponding K_FG values also decrease from 0.005 to 0.002 and 0.001 L mg^-1. The Hill-DeBoer (K₂ < 0 kJ mol^-1) and Fowler-Guggenheim (W < 0 kJ mol^-1) models effectively described the isotherm behavior of curcumin adsorption, indicating an endothermic process. The decrease in external mass transfer rate was compensated by an increase in internal mass transfer (IMT) rate, suggesting multilayer adsorption on the GJPWBC surface, as evidenced by negative [k_La]_d values (<0 min^-1) and positive [k_La]_f values (>0 min^-1). The rate-limiting step of curcumin adsorption depended on the IMT rate. Analysis of the global mass transfer rate revealed that increasing GJPWBC grain size (from 105 to 250 and to 500 μm) enhanced multilayer adsorption, optimizing curcumin adsorption performance. The [k_La]_g values increased from 0.000414 to 0.001206 min^-1 (105 μm, 40 mg L^-1 curcumin), from 0.014223 to 0.018839 min^-1 (250 μm, 60 mg L^-1 curcumin), and from 0.024778 to 0.059279 min^-1 (500 μm, 60 mg L^-1 curcumin), indicating optimal performance of the conditioning formulations. Curcumin dye decolorization by GJPWBC was most favorable at 40 mg L^-1 for the 105 μm grain size and at 60 mg L^-1 for the 250 and 500 μm grain sizes. Using GJPWBC as an adsorbent for curcumin decolorization expands current knowledge on adsorption isotherms and mass transfer kinetics in dye effluent management, offering valuable insights into surface science, green remediation technologies, and sustainable environmental solutions.