Enhancing the removal efficiency of methylene blue in water by fly ash <i>via</i> a modified adsorbent with alkaline thermal hydrolysis treatment

Abstract

An effective adsorbent of methylene blue was synthesized from coal fly ash (FA; waste material from a coal power plant) by a denaturing process with an alkaline solution at 90 °C. The denatured fly ash (D-FA) has a surface area and pore volume of 66.39 m² g^-1 and 15.33 cm³ g^-1, respectively, whereas the values of the original FA are negligible, <i>i.e.</i>, 3.55 m² g^-1 and 0.02 cm³ g^-1. The removal of methylene blue (MB) in aqueous solution by D-FA was increased in the range of initial MB concentration (10-20 mg L^-1); contact time (0-120 min); pH (2-8); D-FA dosage (1-4 g L^-1). However, a larger value of those operational parameters would not improve the removal activity. Furthermore, the methylene blue adsorption on the denatured FA was fitted with the Langmuir model with <i>R</i> ² = 0.9991; the maximum adsorption capacity was determined as 28.65 mg g^-1 from the model. Overall, the highest removal efficiency of MB using D-FA with the dosage of 4 g L^-1 was 97.1% in 30 mg L^-1 solution of methylene blue at pH = 7. The alkaline hydrothermal denaturation of waste FA is a promising approach to produce an adsorbent with beneficial environmental engineering applications.