Sorption of ionic dyes onto untreated low-rank coal – oxihumolite: A kinetic study

The kinetics of the sorption of basic dyes – Methylene Blue (C.I. Basic Blue 9), Malachite Green (C.I. Basic Green 4), Rhodamine B (C.I. Basic Violet 3), Crystal Violet (C.I. Basic Violet 10) – as well as acid dyes – Egacid Orange (C.I. Acid Orange 7), Egacid Yellow (C.I. Acid Orange 11), Midlon Black (C.I. Acid Black 26) – onto a special kind of naturally weathered and oxidised young brown coal called oxihumolite was investigated in a batch arrangement. Both pseudo-first-order and pseudo-second-order kinetic models were applied successfully to evaluate the kinetic dependencies. The intraparticle diffusion model was used to examine diffusion processes taking place during the dye sorption. It was found that the diffusion rate parameters decreased with increasing dose of the sorbent and with increasing particle sizes of the sorbent, whereas an effect of an agitation mode and intensity was much less pronounced. The diffusion processes were identified as the main mechanisms controlling the rate of the dye sorption onto oxihumolite. Malachite Green exhibited faster kinetics of the sorption in comparison with other dyes. However, this different behaviour may not be explained from differences in such parameters as a molecular weight or a dye structure.