Removal of methylene blue from water by ultrasound-assisted adsorption using low-cost bentonites

Abstract

This study focused on the comparison of ultrasound-assisted (UA) and conventional (shaking or stirring) adsorption techniques, taking into account the time-dependent adsorption capacities. Six different bentonite samples were used as adsorbents, and methylene blue (MB) was used as adsorbate. Effects of pH, contact time, MB initial concentration, and temperature on the UA-adsorption were also investigated. XRD analyzes showed that all the bentonite samples were mostly composed of montmorillonite clay minerals, while XRF analyzes showed that the samples contained high percentage of silicon and aluminum oxides (≥70 wt%). Maximum zeta potential values (37.1–42.1 mV) were measured at pH 12 and, to confirm this, maximum adsorption capacities were also determined at this point. Ultrasound irradiation significantly increased the adsorption capacities of all bentonite samples in the early stages. For example, the adsorption capacity of the B-1 bentonite sample was measured as 388 mg/g by UA-adsorption at the first 5th minute, while it was 286 mg/g by conventional (C) adsorption technique at the same time. All bentonite samples showed increasing adsorption capacity with increased temperature confirming endothermic reaction. Maximum adsorption capacities of B-1, B-2, B-3, B-4, B-5, and B-6 bentonite samples with the 96–100 % MB removal percentages at selected conditions (pH: 11–12, temperature: 25 °C, contact time: 60 min, and MB initial concentrations for B-1: 800, B-2: 700, B-3,4: 650, B-5: 600, and B-6: 500 mg/L) were measured as 398, 342, 320, 296, 288, and 243 mg/g, respectively. Pseudo-second-order kinetic model and Langmuir isotherm model with higher correlation coefficients were found to be more suitable to explain the adsorption of MB cations onto bentonites. In addition, maximum adsorption capacities of B-1, B-2, B-3, B-4, B-5, and B-6 samples at 25 °C calculated from Langmuir isotherm data determined as 500, 454.5, 400, 384.6, 370, and 357.1 mg/g, respectively.