Effect of solution pH and influence of water hardness on caffeine adsorption onto activated carbons

Abstract

There has been little research into the effects of the water hardness and pH of surface waters on the adsorption of caffeine on activated carbons. The aim of this study was to determine the influence of these water characteristics on different activated carbons. Caffeine adsorption from the aqueous phase was studied using biomass derived activated carbons (DD: dende coco and BB: babassu coco) and a commercially available activate carbon (NO: Norit® GAC 1240 plus). The functionalized carbons in an inert atmosphere was also studied and were denominated DI, BI, NI. Results highlight the importance of pH in caffeine adsorption: the highest removals were obtained for pH 3.0 and decrease for higher pH. The adsorption isotherms obtained were fitted to the Freundlich and Langmuir models. Calcium and magnesium ions were adsorbed to a varied extent on the activated carbons. The hardness in solution decreased their adsorption due to a competition effect. KF and qm from the Freundlich equation linearly decreased with water hardness due to salt-screened electrostatic repulsions between charged molecules. The amount adsorbed from deionized water was largest because there was no competition between inorganic ions and molecules.