Comprehensive comparative evaluation of methylene blue adsorption on pristine and ZnCl₂ modified montmorillonite K10

The main objective of the present research is the removal of methylene blue (MB) dye using both montmorillonite K10 clay (MT) and the same clay modified with ZnCl₂ (Zn-MT). The adsorption behaviour of both the adsorbents towards MB are compared. Both the adsorbents are properly characterized through XRD, FTIR, SEM–EDS, pzc, and specific surface area etc. The EDS spectra showed clear indication of the presence of Zn in Zn-MT. The surface area improved from 67.3 to 79.1 m²/g after modification. Pseudo second order kinetics and Langmuir isotherm were obeyed well for both the materials. Langmuir monolayer adsorption capacities of unmodified and modified MT are 12.2 and 12.66 mg/g respectively. In both the cases, optimum contact time and pH are 60 min and 6 respectively. At optimum contact time, pH, temperature—30 °C, adsorbent dose—5 g/L, 74.23 and 98.42% of MB can be removed from a 50 mg/L solution using MT and Zn-MT respectively. Although adsorption capacity increased only slightly, substantial improvement in MB % removal on Zn-MT indicates that modification may increase adsorption affinity with access to active sites rather than increasing the total number of adsorption sites. This type of improvement in adsorption efficiency is highly relevant for real water treatment applications. The increase of ionic strength decrease MB adsorption % for both the adsorbents. Using 0.1 M HCl as desorbing agent, both MT and Zn-MT can be reused and recycled for fifth adsorption–desorption cycles.