Adsorption mechanism of different dyes on chemical activated carbon as quantitative assessment for wastewater treatment: Comparative study between ZnCl2 and its eutectic

Abstract

A recent development in the field of activated carbon is chemical activation via physical mixing using molten salts. Previous research discussed the differences in characteristics of activated carbons from pinewood created by pure ZnCl2 and its eutectic mixture of ZnCl2-KCl-NaCl (60:20:20 mol %). Herein a contrast in mesoporous pore size distribution and its corresponding surface area was discovered. ZnCl2 generates a higher surface area over a broad pore range whereas the eutectic creates lower surface area but with a distinctive narrow peak in the larger mesopores. The question remains whether this difference in pore distribution and surface area has an impact on the adsorption mechanism. For this instance, the effect of varying pH on adsorption was tested for methylene blue (cationic), neutral red (neutral) and methyl orange (anionic). Next to that, the adsorption kinetics and equilibrium were investigated using methyl orange and brilliant blue FCF. The latter is also anionic but larger in size and molecular weight compared to the former. A variety of adsorption models were compared to the obtained adsorption data. The pseudo-second-order model provided the best description of adsorption kinetics with both dyes. Highest adsorption in this case was noted for carbons activated with the eutectic mixture, amounting to 229.6 mg/g for methyl orange and 121.8 mg/g for brilliant blue FCF. Regarding the adsorption equilibrium, adsorption of methyl orange obtained the best fit with the Langmuir model. Adsorption of brilliant blue on the other hand had the best correlation with the Fowler-Guggenheim model, which accounts for lateral interactions between adsorbate molecules. Again, the eutectic treated carbons showed the most promising results, amounting to 447.5 mg/g and 146.4 mg/g for methyl orange and brilliant blue FCF, respectively. Furthermore, removal efficiency was determined and mass transfer resistance that might have occurred during adsorption, were investigated for both anionic dyes. Finally, regeneration of the carbons was examined. After 3 cycles, the carbons treated with the eutectic mixture maintained 60.7 and 49.1 % of their original adsorption capacity of methyl orange and brilliant blue, respectively, while this amounted to 65.9 and 46.3 % for the ZnCl2 activated carbons. This research shows that adsorption capacity not solely relies on the adsorbents surface area but is also dependent on the pore distribution. Bearing this in mind, it can be stated that eutectic salt mixtures can serve as useful activating agents for tailor-made activated carbons suitable for wastewater treatment.