Production of Activated Bio-chars for Wastewater Treatment: Characterization, Activation and Evaluation of the Adsorption Capacity

Abstract

Bio-char prepared from lignocellulosic wastes is a sustainable and low-cost material with many interesting applications, such as the adsorption of contaminants from wastewaters, among others. It is obtained from the pyrolysis of biomass through a thermochemical process in the absence of oxygen. In this process, biomass hydrocarbons, volatile matter, oxygen and hydrogen are converted into a carbon-enriched material, known as Bio-char (BC). Several parameters will define the structural properties and characteristics of the BC, some of which are related to the used raw material or biomass type and others to the process conditions. In order to evaluate the potential of the BC as adsorbent of toxic compounds from wastewaters, the most relevant properties are related to the active surface area, pore size and distribution, and stability. For the use of BC as adsorbent, in order to increase the adsorption capacity, activation of the BC is typically done. In this work, cork residues from wine bottles were thermo-chemically treated under a pyrolysis process and subsequently activated with KOH. The obtained Activated Bio-Chars (ABC) were characterized in terms of proximate analysis, active surface area and pore size distribution. Furthermore, BC and ABC adsorption capacity was tested. In particular, the adsorption of the dye methylene blue, MB; was studied. The obtained results indicated that the activation process was necessary in order to reach high MB adsorption capacities, over 90%. Because of its characteristics, it is proposed that BC without activation could be used to remove lower molecular weight components, such as gaseous ones. Finally, focusing on the design of a low-impact process and following green chemistry and sustainable practices, the recycling potential of the ABC was studied in terms of number of cycles, used solvent, dye recovery and adsorption performance. Reused ABC presented good performance in terms of adsorption capacity during several batches, similar to the one of raw ABC. The performance of several solvents was studied and acetone was selected as the better one due to good results in terms of ABC regeneration capacity and low boiling point for its subsequent recuperation.