Removal of Methylene Blue Dye from Aqueous Solutions by Sodium Carbonate Activated Bentonite

Abstract

The leather and textile industries are the leading dyestuff consumers and the main sources of pollutants in the water environment. There is a continuously increasing worldwide concern for the development of organic dye wastewater treatment technologies. The present study focused on the removal of methylene blue (MB) dye from an aqueous solution using sodium carbonate activated bentonite adsorbent and its ZnO/bentonite nanocomposites for photodegradation. To achieve the objective of the study, the bentonite sample was collected from Gewane, Afar regional state, purified, and activated by Na2CO3. Proximate analyses and characterizations of adsorbents and ZnO/bentonite nanocomposites were carried out by using XRD, FT-IR, SEM, and AAS. The XRD showed the bentonite adsorbent contains Montmorillonite, Quartz, Muscovite, Cristobalite, Hematite, and Feldspar as a major phase and ZnO/bentonite nanocomposites contain the ZnO with hexagonal Wurtzite structure and bentonite characteristics. The SEM study revealed the availability of porous morphology on the surface of the bentonite adsorbent before adsorption and coverage of the surface porous by methylene blue after adsorption. The AAS study showed that the local Gewane bentonite is Ca-bentonite but after activation by 5% sodium carbonate, it was changed to Na-bentonite. Optimization of adsorbent was carried out to identify the best adsorbent for the removal of MB dye from an aqueous solution. The result indicated 5% Na2CO3 activated bentonite (AB5) has (100%) removal efficiency of the targeted dye. Then, the adsorption experiment was conducted using activated bentonite (AB5) to establish the effect of different parameters such as adsorbent dosage, pH of the solution, initial concentration of MB, contact time, and temperature on the adsorption efficiency of MB dye. The adsorption isotherm was performed using Langiumer, Freundlich, and Temkin isotherm models. The result indicated Langiumer adsorption isotherm best fit the adsorption study for the present experimental data with R2=1. The kinetic adsorption of MB dye by activated bentonite (AB5) was conducted using pseudo-first-order and pseudo-second-order models. The result specified that the pseudo second-order fit experimental data well and the process was chemisorption. The endothermic and spontaneous nature of the adsorption process of MB dye by activated bentonite (AB5) was carried by studying the thermodynamic parameters of the system. The result indicated the positive value of the ΔS˚ and negative values ΔG˚ shows the adsorption capacity of MB dye molecules is on the adsorbent surface. Desorption of MB dye from the spent adsorbent and recyclability of adsorbent were studied by thermo-chemical methods by heating the spent adsorbent at 200°C for 45 min and dipped in ethanol, acetone, and distilled water heated at 25, 35, and 45 °C for 2hrs. The result showed the desorption capacity of MB dye in this study was low. But, the ZnO/bentonite nanocomposites have high degradation efficiency. The adsorbent in this study was recycled at least three times and its adsorption capacity decreases with adsorbent recycling times.