Reversed Micellar Extraction of Methylene Blue by using Di(2-ethylhexyl) Phosphoric Acid

Abstract

The development of efficient methods for the removal of organic dyes, which are considered to be toxic to the aquatic biota and therefore destroy the ecosystem, is required. In this study, the extraction of methylene blue (MB) from aqueous solution into DEHPA/isooctane solution was investigated. MB was hardly extracted at all from the aqueous to the organic phase at pH values below 5, and the extraction ratio of MB greatly increased with increasing pH in the range of 5-6 by the electrostatic attraction between dissociated DEHPA and MB. MB was not extracted to the organic phase at pH values above 6, since DEHPA leaked from the organic to the aqueous phase. The addition of 2-ethyl-1-hexanol suppressed the DEHPA leakage, and then MB was successfully extracted to the organic phase at pH values above 6. It was found that MB was solubilized as an MB-DEHPA complex in the organic phase in the pH range of 5-5.5, whereas MB would be entrapped in the waterpool of the reversed micelle at pH values above 5.5. The back extraction ratio of MB from the organic to the aqueous phase was about 100 % at pH values below 5 because of the destruction of the reversed micelles and the disappearance of the electrostatic attraction between DEHPA and MB.

Extraction Kinetics of Methylene Blue by AOT and DEHPA Reversed Micellar Solution

“…Figure 4 shows the effect of AOT and KCl concentration on the K W value of the MB extraction from KCl solution to AOT/EHA solution (C EHA = 100%). It is known that EHA prevents the leakage of DEHPA from an organic phase to an aqueous phase [12]. Although the addition of EHA into an AOT/ isooctane system is not necessary, the effect of the organic solvent was examined in order to compare the AOT and the DEHPA systems.…”

“…Therefore, it was shown that the extraction rate of MB by AOT or DEHPA is governed by the electrostatic interaction between the MB and extractant, regardless of the size and shape of reverse micelles formed. In the previous paper [12], it was reported that MB was solubilized as a MB-DEHPA complex in the organic solution and/or entrapped in the waterpool of a reverse micelle at pH values above 5.5. From the kinetic study, it was suggested that the positively charged MB was bound to the negatively charged head group of the extractant and solubilized in the organic phase as the MB-extractant complex, rather than MB being taken up by the reverse micelles at the interface.…”

“…The extraction of MB by di(2-ethylhexyl)phosphate (DEHPA) reverse micelles was also discussed. The formation of DEHPA reverse micelles depends on pH, and it was shown that the extraction of MB under neutral conditions and back-extraction with weak acidity were possible [12].…”