Improved Carboxylate Density in Functionalized Biochar for Methylene Blue Adsorption

Abstract Carboxylate appended biochar BCKA was synthesized by a two‐step functionalization of leached rice straw biochar using ethyl 2‐bromoacetate/K 2 CO 3 /DMF and hydrolysis to furnish the modfied biochar. The modified biochar was characterized using proximate analysis, CHNS, SS CPMAS 13 C 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 ( R 2 ) 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 ( R 2 0.967), intraparticle diffusion ( R 2 0.977), and Elovich equation ( R 2 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.