Cheap adsorbent. Part 1: Active cokes from lignites and improvement of their adsorptive properties by mild oxidation

Abstract

In this article, cheap adsorbents were produced starting from two different lignites. About 500 kg of each coal was pyrolyzed in a rotary kiln at semi-pilot scale. Characterization of the obtained chars are made in terms of porosity development, surface functional groups and adsorptive properties determined for both 4-nitrophenol and lead. A post-oxidative treatment at low temperature is proposed in order to improve the adsorptive capacities of the initial chars by the introduction of oxygen containing functional groups, which are widely involved in chemisorption. The conditions of the post-treatment were chosen to be easily exported in an industrial process, for example during the cooling step. The importance of both lignite characteristics (ashes content, water content) and pyrolysis conditions is demonstrated. The positive effect of a simple post-oxidative treatment is shown by a strong increase in the adsorptive capacities, and correlated to the evolution of the porosities and surface oxygen functionalities. Considering unit area, the properties of our active cokes are similar to those of commercial active carbon, but their specific areas are less developed (about 400 m 2 g −1).