Preparation of sludge-derived KOH-activated carbon for crude glycerol purification

Abstract

Activated carbon (AC) was prepared from wastewater treatment sludge by KOH activation and then used for the adsorptive purification of crude glycerol (CG). The effect of different KOH: char (w/w) ratios (1–6), KOH soaking times (5–25 h), and activation temperatures (500–900 °C) on the surface properties of the obtained sludge-derived ACs and their adsorption capacity for impurities in H3PO4 pretreated CG was explored. The preparation conditions affected the textural properties and surface chemistry of the sludge-derived AC and its efficiency in the adsorptive purification of CG. Oxygen-containing surface functional groups, particularly carbonyl groups, as well as the textural properties, played a more important role in the adsorptive purification of CG. Among the sludge-derived ACs, the one impregnated at a KOH: char (w/w) ratio of 5.0 for 25 h and then activated at 800 °C exhibited the most suitable surface properties for the purification of pretreated CG, attaining a 93.0 % (w/w) glycerol level at an AC dose of 67 g/L for 2 h with agitation at 250 rpm. The AC adsorption of impurities from the pretreated CG followed the Langmuir isotherm. However, the reusability of the used AC by hexane or diethyl ether extraction was not suitable.