Simultaneous removal of single and mixed cationic/anionic dyes from aqueous solutions using flotation by colloidal gas aphrons

The treatment of dye-contaminated wastewater poses a challenge to industries due to stringent effluent discharge regulations. In this study, the removal of individual and mixed dyes from aqueous solutions using flotation by colloidal gas aphrons (CGAs) was investigated. The natural surfactant extracted from the leaves of the Ziziphus spina-christi (ZSC) tree, available abundantly in the Gulf region, was used to prepare CGAs to remove cationic and anionic dyes from aqueous media in a flotation column. Freshly prepared CGAs were used to remove cationic dye (methylene blue, MB) and/or anionic dye (Congo red, CR), resulting in concentrate and tailings. The concentrate solution was then collected and reused to generate new CGAs to remove oppositely charged dyes from the aqueous media. The maximum MB dye removal with the fresh CGAs was 79% at an initial concentration of 50 mg/L. Regenerated and reused CGAs achieved an additional 37% removal of CR. The synthetic anionic surfactant, sodium dodecyl benzenesulfonate (SDBS), was also used to prepare CGAs for the removal of mixed dyes (CR and MB). At pH 11, the dye removal was about 75% for MB and 88% for CR. Therefore, a one-step or two-step CGAs flotation process can potentially be used for the removal of single and mixed dyes.