Dispersive clay flotation for the simultaneous removal of methylene blue and methyl orange from water

Synthetic dyes discharged from textile and industrial activities are persistent, toxic, and difficult to degrade, posing serious risks to aquatic ecosystems and human health. Therefore, developing rapid and effective technologies for removing dye pollutants from water is of significant environmental importance. A flotation method using dispersive clay (DC) was designed for the rapid and simultaneous removal of a basic dye, methylene blue (MB), and an acidic dye, methyl orange (MO), from water. The DC was successfully prepared by immersing montmorillonite K-30 (MT) in sodium fluoride solution, followed by sonicating in sodium hexametaphosphate solution. It lost the layered structure but possessed almost the same surface area (278 ± 8 m² g^-1) as the original MT (253 ± 7 m² g^-1), meeting the requirement of a large-capacity adsorbent. However, the DC particles were difficult to separate from water because of their high dispersibility in the aqueous solution. When flotation was carried out in the presence of a cationic surfactant, cetyltrimethylammonium chloride (CTAC), the DC particles rapidly coagulated to rise to the surface of water and were readily separated. MB in water was > 98% adsorbed on 100 mg L^-1 of DC because of its electrostatic interaction with the negatively charged DC surfaces. The removal ratio remained greater than 95% after adding up to 7.5 mg L^-1 of CTAC, after which the zeta potential of DC was nearly zero. Although very little MO adsorbed onto unmodified DC, the adsorption rate increased as added CTAC increased and exceeded 96% in the presence of 100 mg L^-1 DC and 7.5 mg L^-1 CTAC. The proposed method successfully removed MB and MO simultaneously from water within 5 min.