Effect of different ratio KOH treatment on neem bark in the formation of activated carbon: Adsorption characteristics study

Abstract

In this study, mature neem tree bark (NB) was chemically transformed into an activated carbon (NBAC) through different ratios (1:1, 1:2, 2:1, 1:3, 3:1) of KOH and NB biomass. The obtained Neem bark-activated carbons (NBAC-1, NBAC-2, NBAC-3, NBAC-4, and NBAC-5) were tested for methylene blue (MB) dye removal efficiency. The NBAC-1 was found to be highly effective against MB dye. The NBACs have a low Brunauer-Emmet-Teller (BET) surface area (10.8 m2/g) with surface functional groups dominated by -OH and -CO. It is thermally stable up to 700°C, and its surface elemental composition is mainly carbon, oxygen, and potassium. The adsorption characteristics against MB dye show that 99.15% of dye from 100 mg/L initial concentration solution was removed with 4 g/L NBAC-1 dosage in 200 min of contact time at 50°C solution temperature, and pH of 10 (pHzpc = 7.85), resulting in an adsorption capacity of 24.79 mg/g. Kinetic studies indicated that the adsorption process of MB dye utilizing NBAC-1 adhered to the linear pseudo-second-order (R2 = 0.9726) and nonlinear Elovich model (R2 = 0.9419), signifying chemisorption, alongside the nonlinear Weber-Morris model (R2 = 0.9231), highlighting the significance of mass diffusion of MB dye onto the NBAC-1 surface in multilayers. Isotherm investigations favored better conformity with the Freundlich model, exhibiting a higher R2 value of 0.999, suggesting a heterogeneous multilayer adsorption mechanism. Adsorption thermodynamics analysis inferred that the adsorption was feasible (∆G = -4.342 KJ/mol), endothermic (∆H = +50.0203 KJ/mol), and demonstrated a high affinity of NBAC-1 towards MB dye (∆S = +168.2255 J/K/mol). Recyclability of the NBAC-1 adsorbent was also conducted to check for its sustainable usage.