Green synthesis of novel biochar from Abelmoschus esculentus seeds for direct blue 86 dye removal: Characterization, RSM optimization, isotherms, kinetics, and fixed bed column studies

Abstract

The presence of Direct blue 86 dye (DB86) in water can lead to various health hazards to the humans and animals. The study explored efficacy of biochar derived from Abelmoschus Esculentus seeds (AESB) to remove DB86 from an aqueous solution. BET analysis of AESB delineated H4 classification with the predominance of micropores and mesopores spread throughout the surface. FTIR study demonstrated the presence of the alkyl (C–H), Alkene (C]C), Carbonyl (C]O) and O–H bond of the sulphonic group which helped in adsorption of DB86 molecules through various mechanisms i.e., pore filling, π-π interactions, and hydrogen bonding interactions. Response surface methodology (RSM) was used for designing the adsorption experiment and analysing the optimum operating parameters. Batch experiments demonstrated excellent adsorption capacity (277.04 mg/g) of AESB and was efficient in 98.06% removal of DB86 at optimal conditions i.e., dye conc. = 300 mg/L, dose = 2.5 g/L, pH = 2, time of 120 min. Adsorption followed nonlinear Sips model (R2 = 0.999) with an error (X2 = 0.13, RMSE = 0.83, MAPE 0.56 and MSRE = 0.0006). The kinetic analysis revealed intra-particle diffusion being the rate-determining step and followed nonlinear pseudo-first-order kinetics (R2 = 0.997). Thermodynamic study revealed that the adsorption of DB-86 proceeded spontaneously and exhibited endothermic characteristics, with the enthalpy change primarily governed by the physisorption mechanism. Thomas model revealed inverse relation of breakthrough and exhaustion time with flow while it was proportional to bed height. The sorption capacity (N0) (2.2493 mg/l min) and rate constant (Ka) (0.028 L/min. mg) of BDST model can accurately be used for predicting the performance of AESB in full scale column.