Adsorption of an acid dye onto coal fly ash

In this study, we found the raw coal fly ash (CFA) that had not been subjected to any pretreatment process had superior adsorbing ability for the anionic dye Acid Red 1 (AR1) than did two modified coal fly ashes (CFA-600 and CFA-NaOH). The adsorption capacities followed the order CFA > CFA-600 > CFA-NaOH, and they each increased upon increasing the temperature (60 °C > 45 °C > 30 °C). The adsorptions of AR1 onto CFA, CFA-600, and CFA-NaOH all followed pseudo-second-order kinetics. The isotherms for the adsorption of AR1 onto the raw and modified coal fly ashes fit the Langmuir isotherm quite well; the adsorption capacities of CFA, CFA-600, and CFA-NaOH for AR1 were 92.59–103.09, 32.79–52.63, and 12.66–25.12 mg g−1, respectively. According to the positive values of ΔH° and ΔS°, these adsorptions were endothermic processes. The ARE and EABS error function methods provided the best parameters for the Langmuir isotherms and pseudo-second-order equations, respectively, in the AR1–CFA adsorption system.