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a b s t r a c tThe main objective of this work is the utilization of W/O emulsions prepared by low-frequency ultrasonic irradiation (22.5 kHz) for the removal and recovery of cationic dyes from aqueous solution by emulsion liquid membrane containing di(2-ethylhexyl) phosphoric acid (D2EHPA) as carrier. The important parameters governing the extraction behavior of dyes have been investigated. These parameters were contact time of the external phase with W/O emulsions, external phase concentration, feed phase pH, stirring speed, extractant concentration, presence of salts, and type of diluent. The extraction of a mixture of four cationic dyes (methylene blue, rhodamine B, malachite green, and basacryl red GL) is also investigated in the absence and presence of salt in the external phase. The possibility of stripping the dyes is evaluated and the influence of sulfuric acid concentration on the stripping efficiency is also examined. The obtained results show that, under most favorable conditions, practically all the dye cations present in the feed phase were extracted. The removal percentage of the mixed dye was found to be higher than 98% even in the presence of salt in the continuous phase. The transport mechanism of cationic dyes for the present system was determined and divided into six stages. The best sulfuric acid concentration in the internal phase that conducted to very good stripping efficiency (≥97%) and excellent emulsion stability was 0.5 M. In all cases, the recovery of the membrane phase was total and the extraction of dyes was not decreased up to seven runs.

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The stability of an emulsified liquid membrane composed of Span80 as a surfactant, D2EHPA as an extractant and sulfuric acid as an internal phase was first studied according to different diluents and many operating parameters using the Plackett–Burman design of experiments. Then the removal of methylene blue from an aqueous solution has been carried out using this emulsified liquid membrane at its stability conditions. The effects of operating parameters were analysed from the Box–Behnken design of experiments. The optimization of the extraction has been realized applying the response surface methodology and the results showed that the dye extraction yielding 98.72% was achieved at optimized conditions.