Novel activated carbon prepared from an agricultural waste, Stipa tenacissima, based on ZnCl2 activation—characterization and application to the removal of methylene blue

Abstract

Activated carbon (AC) was prepared by means of a novel physiochemical activation method from low-cost biosorbent, agricultural waste (Stipa tenacissima fiber). A two-step pyrolysis was considered instead of a single-step pyrolysis, which involved zinc chloride for the first activation step and a steam mixture of water, CO2 and acetic acid for the second step. The obtained AC was tested as an adsorbent for the removal of a basic dye, Methylene blue (MB) from aqueous solutions. Batch experiments were conducted to examine the effect of the main parameters, such as the initial MB concentration, the pH, and the kinetic adsorption of this dye. Results showed that a pH value of 7 is favorable for the adsorption of MB. Rate constants of pseudo-first-order, pseudo-second-order, and intraparticle diffusion coefficient were calculated to analyze the dynamic of the adsorption process; they showed that adsorption kinetics followed a pseudo-second-order and an intraparticle diffusion model, while the two straight lines describing experimental data indicated that intraparticle diffusion was not the limiting mechanism for adsorption. Among the tested isotherm models, the Sips isotherm was found to be the most relevant to describe MB adsorption onto both activated and non-ACs with the best maximum adsorption capacity (Qm), 178.44 and 27.21 mg g−1, respectively. The negative values of ΔG° revealed that the adsorption process was spontaneous. The positive values of ΔH° and ΔS° showed the endothermic nature and an increase in disorder of MB molecules during the adsorption process, respectively.