Catalytic activation and application of micro-spherical carbon derived from hydrothermal carbonization of lignocellulosic biomass: statistical analysis using Box–Behnken design

Abstract

In this study, activated carbon was produced by physico-chemical activation of hydrothermally carbonized (HTC) material derived from the dried stem of Corchorus olitorius commonly known as jute (JS), using potassium hydroxide (KOH) as an activation agent. The activation process was optimized using Box–Behnken factorial design (BBD), with outcome of 29 different experiments under predefined conditions. Four different parameters, namely activation temperature (x₁), activation time (x₂), ratio of char to KOH (x₃) and CO₂ flow rate (x₄), were optimized with respect to their influence on maximum removal percentage for divalent cations of Cu(II) (Y₁) and carbon yield (Y₂). All the four process parameters had strong positive effect on adsorption capacity up to a certain limit; beyond which it started to decline. The specific surface area of the hydrochar (HTC) was enhanced substantially after the activation process. Scanning electron microscopy (SEM) revealed that the morphology of the JS based hydrochar (JSC) changed significantly after KOH impregnation and activation under the flow of CO₂ gas. The Langmuir maximum monolayer adsorption capacity for Cu(II) cations was 31.44 mg g⁻¹. Equilibrium isotherm data were well followed by Freundlich and Temkin models. Due to an increase in temperature, the Langmuir maximum monolayer adsorption capacity, q_m (mg g⁻¹) and Freundlich constant, K_F increased successively, representing an endothermic nature of the adsorption.