Utilization of raw and dried Victorian brown coal in the adsorption of model dyes from solution

Being plentiful and readily available in many parts of the world, brown coal is probably one of the most potential substitutes for activated carbon. Many reports include the brown coal drying stage prior to adsorption despite the fact that it may lead to coal shrinkage and reduction of its pore volume, thus diminishing its adsorption capacity. In this study, various types of raw (wet) brown coal were investigated as adsorbents for anionic and cationic dyes in aqueous solution. The adsorption capacities of these raw brown coals were compared with those of oven dried brown coals and also with activated carbon. It was found that raw brown coals exhibit high adsorption capacities for cationic dye, but very low capacities for anionic dyes, suggesting the involvement of a cation-exchange mechanism. Additionally, the cation exchange capacities correlate positively with its cation adsorption capacity highlighting the major role of strongly acidic groups in the adsorption process. Drying the brown coal significantly reduces its adsorption capacity for cationic dye, supporting the hypothesis of coal shrinkage and the consequent reduced pore volume upon drying. These findings suggest that raw (wet) brown coal may perform as well as activated carbon for the removal of positively charged species in aqueous solution. Regeneration of the used adsorbent may also not be necessary, since the used brown coal can be simply burnt as fuel or composted.