Fly ash and TiO2 modified fly ash as adsorbing materials for effective removal of methylene blue and malachite green from aqueous solutions

A cost-effective modified composite was prepared from fly ash (FA) and then it was coated with TiO2 by a process of refluxing in NaOH solution at 100 °C, resulting in modified fly ash (MFA). The adsorbent FA and MFA were analyzed by using pXRD, FT-IR, SEM, and BET techniques for the effective expulsion of malachite green (MG) and methylene blue (MB) from their aqueous solutions. The effect of adsorbent dose, concentration, pH, and the temperature was also studied. The MFA was found more efficient than FA for the adsorption of dyes MB and MG because crystallinity and surface area were increased after modification from 49.36% (FA) to 79.70% (MFA) and SBET = 10.795 m2g-1 (FA) to SBET = 30.597 m2g-1 (MFA) respectively. The maximum adsorption capacity (at 303 K) of MB-FA, MB-MFA, MG-FA, and MG-MFA was 78.4%, 95.1%, 78.7%, and 93.7%, respectively. We studied the different adsorption isotherms, but the Langmuir adsorption isotherm fits the best, and the maximum Langmuir adsorption capacity at 288K, 298K, and 308K for MB-FA is 96.1, 105.3, and 111.6 mg/g, for MFA-MB, is 116.9, 131.6 and 141.2 mg/g, for FA-MG is 94.8, 105 and 110.7 mg/g and for MFA-MG was 102.1, 112.2 and 120.5 mg/g respectively. A kinetics study of the adsorption of MB/MG onto FA/MFA shows that dyes were adsorbed according to a pseudo-second-order mechanism. Thermodynamics data (ΔGo,ΔHo,ΔSo) were also calculated at different temperatures and the value of ΔGo and ΔHo where negative and positive, respectively.