Improved sono-assisted adsorption of a binary dye mixture using bis(2-ethylhexyl) phosphate modified Amberlite XAD-2 resin and response optimization

Abstract

Methylene blue (MB) and crystal violet (CV) was simultaneously adsorbed ultrasonically on bis (2-ethylhexyl) phosphate (D2EHPA) modified Amberlite XAD-2 resin. The prepared resin was characterized by FESEM, SEM with EDS, FTIR, and point of zero charge. Preliminary studies for the removal of MB and CV dye in the mixture was done by using different ratio of D2EHPA impregnated XAD-2 resin and at different pH. Central Composite Design (CCD) of response surface methodology (RSM) was used to design and optimize the removal process. The effective parameters chosen for this study were: ultrasonication time (tus: 3–15 min), adsorbent dosage (m: 0.3–1.5 g), and initial concentration of MB (Cmbin) and CV (Ccvin) dye in solution (5–25 mg L−1). At optimized condition (tus = 10.5 min, m = 1.16 g, and concentration of Cmbin = 16 mg.L−1and Ccvin = 16 mg L−1), the maximum percentage removal for MB and CV dye was 93.38% (Qt = 1.29 mg g−1) and 97.80% (Qt = 1.32 mg g−1), respectively. pH study was performed at optimized condition by varying the pH of the dye solution from 3 to 12. At optimized condition, kinetic and equilibrium studies were performed. For single dye adsorption study, Freundlich model and for binary dye adsorption study extended Freundlich model was best fitted. The pseudo-second-order model best fitted the kinetic data for both the dyes with R2 = 0.998 for MB dye and R2 = 0.999 for CV dye which indicated the chemisorption nature of adsorbent. The possible interaction mechanism between D2EHPA and MB/CV dye was proposed. The continuous study was performed by varying flow rates using a recirculating flow vessel and found to be 12.5% more energy efficient than the batch mode. Regeneration of the adsorbent was done successfully using ethanol-water solution up to five cycles.