Highly efficient simultaneous ultrasonic-assisted adsorption of methylene blue and rhodamine B onto metal organic framework MIL-68(Al): central composite design optimization

In the present work, metal organic framework MIL-68 (MIL = Material of Institute Lavoisier), with metal sites of aluminum ions (namely MIL-68(Al)), was synthesized by a simple, fast and low-cost process for simultaneous removal of Methylene Blue (MB) and Rhodamine B (RhB), regarded to be toxic and even carcinogenic, from aqueous solution. The adsorbent was characterized by FTIR, SEM, XRD, BET and TG analysis. Optimization of significant variables such as pH, adsorbent dosage, initial MB concentration, initial RhB concentration and adsorption time affecting both single and binary dye removal was developed by Central Composite Design (CCD) under response surface methodology (RSM), and the optimized values for these parameters in binary solutions were found to be 6.45, 0.014 g, 60 mg L-1, 15 mg L-1 and 9.9 min, respectively. The kinetics and isotherm of the adsorption were investigated in detail. Maximum sorption capacities of 1666, 1111, 227 and 29 mg g(-1) were obtained from the Langmuir isotherms for MB and RhB in single and binary solutions, respectively. The kinetics study results suggested that the sorption of the studied dyes onto MIL-68(Al) follows the pseudo-second order model. According to the reusability test of the adsorbent, dye uploaded MIL-68(Al) can be regenerated using methanol. The regenerated sorbent was used for three cycles use with high performance. The effect of ionic strength on the removal efficiency of the sorbent was also tested with different kinds of salts. The applicability of the sorbent was examined for real samples spiked with specific quantities of both dyes in water from tap water, sea water, spring water and laundry wastewater. The short time required for dye uptake makes this MOF a promising sorbent for simultaneous and rapid removal of dyes even in the real polluted environment.