Application of activated carbon produced from phosphoric acid-based chemical activation of oil fly ash for the removal of some charged aqueous phase dyes: role of surface charge, adsorption kinetics, and modeling

Oil fly ash (OFA) waste from electric power plants poses a serious management challenge. To address this problem, we studied the conversion of OFA into activated carbon (AC) and investigated the application of the produced AC for the removal of several aqueous phase dyes. The latter investigation included a study of the effects of various process parameters, such as pH, AC amount, and initial dye concentration, on the efficiency of AC adsorption. The highest specific surface area (SSABET) value of 63 m2/g was achieved for the produced AC sample using a phosphoric acid strength of 40% (w/w %), an impregnation ratio R of 0.8 (mL-acid/gm-fly ash), and a furnace temperature of 500°C (for 2 h). Increased methylene blue (MB) adsorption onto AC was observed when the pH was above 5. At higher pH, the presence of positive amino groups on MB was indicated to favor its adsorption onto AC, which was found to have a dominant negative surface charge at pH values above 5.6 (i.e. pHzpc). Furthermore, speciation of the AC surface functional groups, including O–H groups, was also invoked to explain the pH-dependent adsorption of MB onto OFA-based AC. In addition, methyl orange (MO) dye showed higher adsorption at acidic pH values, and an increase in pH from 2 to 8 caused a decrease in its adsorption, which was also attributed to its AC's of 5.6. The kinetics studies showed that the adsorption rate values are higher at lower dye concentrations. A better fit was observed for the second-order kinetics compared to the first-order kinetics. The Freundlich isotherm model provided a better fit for the adsorption of MB and MO, whereas the Langmuir model provided a better fit for rhodamine B (RB) adsorption. Furthermore, the response surface methodology (RSM)-based models also showed that the RSM approach can be used to predict the removal of RB, MB, and MO dyes from the aqueous phase using OFA-based AC under a varying set of operational conditions. In summary, the results from the present investigation indicate that OFA-based AC can be successfully used for the removal of several dye contaminants from wastewater streams by carefully optimizing the process conditions described in this work.