Optimization study of caffeine adsorption onto large surface area wood activated carbon through central composite design approach

Abstract

Activated carbon from Acacia mangium wood was prepared by chemical activation using H3PO4 under optimum conditions (OAMW-AC). In this study, a rotatable central composite design of response surface methodology was used to optimize the adsorption capacity of OAMW-AC against caffeine molecules. The maximum removal capacity of the OAMW-AC was found to be 29.2 mg/g under optimized conditions. The experimental results established optimized conditions for maximum caffeine removal were; 61 min of contact time, 3.0 g/L of adsorbent dosage, 100 mg/L of initial caffeine concentration, and 7.60 solution pH. The surface morphology, surface elemental composition, and surface functional group changes on the OAMW-AC were monitored by field emission scanning electron microscopy images, energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy, respectively. The characterization data showed OAMW-AC had a Brunauer-Emmet-Teller (BET) surface area of 1767 m2/g with dominated mesopore area (94.8 %). The pH at the point of zero charges (pHzpc) was 2.25, and the negative value of proton binding capacity (Q = − 0.23 mmol/g) showing proton dissociation from the carbon surface at pH above pHzpc. A desorption study of the caffeine through 95 % ethanol solution was also carried out. It was observed that 37.2 % of the adsorbed caffeine could be reclaimed from activated carbon.