The stability of emulsion liquid membranes (ELMs) and their ability to extract Cr(VI) were investigated. The objective of this study is to compare different ELM formulations using combinations of two hydrophilic (Tween 20 and Tween 80) and two lipophilic (polyglycerol polyricinoleate (PGPR) and Span 80) emulsifiers. TOPO (tri-n-octylphosphine oxide) as a carrier and a green solvent (sunflower oil) were used to provide high extraction efficiency of Cr(VI). All these double emulsions were characterized in droplet size distribution, zeta potential, visual inspection with a microscope, and stability. The best formulation was obtained with PGPR as the inner stabilizer and Tween 80 as the outer stabilizer, leading to Cr(VI) ion removal efficiency of up to 96%.

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The influence of several formulation and operating conditions on hexavalent chromium extraction by green emulsified liquid membranes (GELMs) was studied. An excellent removal efficiency was achieved under optimized conditions. The optimum GELM formulation in terms of stability was obtained with tri-n-octylphosphine oxide (TOPO) as extractant, polyglycerol polyricinoleate (PGPR) and polyoxyethylene sorbitan monooleate (Tween 80) as stabilizers in sunflower oil. Optimum GELMs showed a monomodal distribution of sizes around 1.29 mm. Results confimed that the use of a vegetable solvent and PGPR for GELM formulation is a promising alternative to petroleum organic solvents.

a b s t r a c tThe focus of this study is the extraction of hexavalent chromium (Cr(VI)) from wastewater using emulsion liquid membrane (ELM) in order to contribute to sustainable design by replacing the common synthetic organic solvents to a bio-based solvent. The membrane phase consisted of sunflower oil as solvent and two non-ionic surfactants (Span 80 and Tween 80) as emulsifiers. Tri-n-octylphosphine oxide (TOPO) was used as carrier and Na 2 CO 3 aqueous solution (0.5 mol/L) as stripping phase. The extraction of hexavalent chromium by using ELMs with sunflower oil as green solvent was modeled and optimized. The influence of several factors was studied: the volume fraction of the internal aqueous phase (X 1 ), initial concentration of hexavalent chromium [Cr(VI)] 0 (X 2 ), and the percentage of carrier concentration (X 3 ). A full 2 3 factorial design was performed to study the influence on the yield of the extraction process of Cr(VI); it provided important information regarding the optimum level of each variable along with its interactions with other variables and their effects on yield. Characterization of the primary water-in-oil (W 1 /O) and double emulsions (ELM) are also reported. Results showed that the use of a vegetable solvent in ELMs formulation is a good alternative to petroleum organic solvents reaching Cr(VI) removal efficiency values up to 87.43% using 4% of TOPO at an optimal pH of 1.

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