Improved Carboxylate Density in Functionalized Biochar for Methylene Blue Adsorption

Carboxylate appended biochar BCKA was synthesized by a two-step functionalization of leached rice straw biochar using ethyl 2-bromoacetate/K₂CO₃/DMF and hydrolysis to furnish the modfied biochar. The modified biochar was characterized using proximate analysis, CHNS, SS CPMAS ¹³C NMR, FESEM, FTIR, XPS, BET, and zeta potential. Biochar BCKA was optimized for pH and dose, respectively, as 7 and 0.2 g/L for methylene blue (MB) adsorption with a contact time of 6 h at 298 K. MB removal up to 98% from 20 mg/L dye solution, using BCKA as adsorbent pH 7, 298 K. The nonlinear fit to Langmuir, Freundlich, Temkin, SIPS, and D-R adsorption isotherm showed a good fit with correlation coefficients (R²) 0.970, 0.969, 0.988, 0.984, and 0.983, respectively, and maximum MB adsorption (q_m) of 261 mg/g. Thermodynamic parameters for the adsorption of MB using BCKA were 38.72 kJ/mol (∆H°), 225.42 J/mole K (∆S°), and negative ∆G° confirming the spontaneity driven by entropy. Kinetic studies showed good nonlinear fit to pseudo-second order (R² 0.967), intraparticle diffusion (R² 0.977), and Elovich equation (R² 0.958) with chemisorption-based interaction between BCKA and MB. Chemisorption, particularly ion exchange on the surface of BCKA with MB, is inferred from the D-R isotherm and kinetic analysis. Column breakthrough studies showed Thomas model fit with q_t 267.09 mg/g, and efficiency >80% was retained for two cycles of MB adsorption.