Out of the blue: Liquid emulsion membranes remove methylene blue from water

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  • Published: Feb 13, 2012
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Out of the blue: Liquid emulsion membranes remove methylene blue from water

Liquid emulsion membranes

The industrial dye methylene blue, which is in widespread use for colouring fabrics, cotton and silk, is also employed in aquaculture for treating fungal infections in fish, as a clinical dye for staining tissues, and even as a drug. Its industrial applications invariably result in the escape of residual dye into wastewater, from which it should be removed before release into the environment.

Although several established procedures are in place for the wastewater treatment of methylene blue, such as oxidation or adsorption, a trio of Indian scientists has been examining another well-established separation process. Abhijit Ramchandra Chavan, Pradip Rajabhau Gadade and Mamta Devrao Sardare from the Dr. Babasaheb Ambedkar Technological University, Lonere, looked at the use of liquid emulsion membranes as an alternative.

Liquid emulsion membranes were first reported in 1968 and they can be used extensively to remove heavy metals, phenols and organic acids from solution. Rather than employing a piece of membrane material, the liquid membrane is constituted in solution by mixing a series of appropriate liquids.

The membrane phase is generally prepared by mixing the emulsifying agent with a suitable diluent to produce small droplets of the diluent as an inner phase within globules of emulsion. These globules are then dispersed within the sample solution so that the inner phase never comes into direct contact with the sample. The target compound diffuses from the sample solution through the membrane into the inner phase, sometimes with the aid of an added carrier.

Agitation of the emulsion produces many small droplets within the sample so that the contact surface area for the analyte is very large, providing efficient transfer to the inner phase. This whole process is energy efficient and takes place at room temperature.

Membrane removal for methylene blue

In the case of methylene blue, the researchers prepared the emulsion by mixing the non-ionic surfactant Span 80, which is an oleate ester of the carbohydrate sorbitan, with the diluent. The mixture was stirred for 20 minutes during the dropwise addition of a solution of sodium hydroxide. The purpose of this salt was to form sodium ion complexes with the methylene blue molecules to aid transfer from solution through the membrane into the inner phase.

The emulsion was added gently to the sample solution with stirring to aid contact between the emulsion droplets and the sample. After a suitable period, the solution was left to separate out and the concentration of methylene blue was measured by UV spectrophotometry.

The diluent forms the major proportion of the emulsion, so the performances of several solvents were compared. The main properties to consider are the density and viscosity, which determine the permeability and thickness of the membrane walls, so will affect stability of the emulsion and the extraction time and yield.

Hexane and heptane both performed well, giving recoveries of 89.5 and 97.9%, respectively, after 50 minutes of contact time. The yield with toluene maximised at 82.4% after 25 minutes but the best solvent tested was kerosene, which extracted 99.6% of methylene blue in just 12 minutes under optimum conditions.

The extraction performance was influenced by many factors, such as the concentrations of the surfactant and sodium hydroxide, the volume ratio of the kerosene phase to the sodium hydroxide phase, and the stirring speed.

The initial concentration of methylene blue in the aqueous solution also had an effect. For levels up to 30 ppm, maximum recovery was maintained but it began to fall away for more concentrated solutions, ranging from about 90-95% for 100-200 ppm methylene blue to about 67% for 1000 ppm.

So, for applications in the real world, it would be desirable to have an idea of the initial concentration of the dye and diluting the feed solution as necessary to 30 ppm or lower before beginning the extraction process.

The use of liquid emulsion membranes for removing methylene blue from aqueous solutions can provide almost complete recovery, although it should be pointed out that all of the experiments in this study were performed with standard solutions. Real wastewater samples are likely to contain other compounds which might interfere with the extraction process or affect emulsion stability, so further studies are required before the process can be rolled out.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The dye methylene blue has been removed from wastewater using liquid emulsion membranes, which offered almost complete recovery with kerosene as the extraction phase

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