Separation and Recovery of Anionic Pollutants by the Emulsion Pertraction Technology. Remediation of Polluted Groundwaters with Cr(VI) Eugenio Bringas, M. Fresnedo San Roma ´ n, and Inmaculada Ortiz* Departamento Ingenierı ´a Quı ´mica y Quı ´mica Inorga ´ nica, ETSII y T, UniVersidad de Cantabria, 39005 Santander, Spain Polluted groundwaters with an average anionic composition of 9 mol Cr 2 O 4 2- /m 3 , 64 mol SO 4 2- /m 3 , and 9.2 mol Cl - /m 3 were treated by the emulsion pertraction technology using Alamine 336 as the extractant and NaOH as the stripping agent, to reduce the concentration of chromate compounds and recover them in a concentrated solution. A careful experimental design was carried out to analyze the equilibrium and kinetic behavior of the separation system. The equilibrium parameters of the chemical reactions involved, K i ex , were calculated from the experimental data. A mathematical model that described the kinetics of the separation and concentration processes was developed, and the unknown model parameters, that is, the membrane and the organic phase mass transport coefficients, K m ) 1.5 × 10 -3 m/h and K o A v ) 2.94 × 10 4 h -1 , were estimated. Within experimental system the kinetic control is shared between the internal (feed phase) and the membrane mass transport resistances. 1. Introduction The widespread use of chromium in industrial applications such as leather tanning, metallurgy, electroplating processes, and so forth has caused chromium contamination of surface and groundwaters. 1-3 Public concerns arise as a result of the sufficient evidence of chromium(VI) carcinogenicity in humans and the bio-accumulation into flora and fauna. 4,5 Conventional processes reduce chromium(VI) to chromium- (III), which is less toxic, less soluble, and less mobile than chromium(VI). Chromium(III) is then converted to the hydrox- ide form and removed after precipitation. 6 Other technologies have been developed to achieve the removal and recovery of hexavalent chromium from wastewaters: (i) adsorption; 6,7 (ii) ion exchange technology; 3,8 (iii) membrane processes, 9 espe- cially microfiltration, 10 ultrafiltration, 11 and nanofiltration; 12 and (iv) solvent extraction processes, especially emulsion liquid membranes (ELMs), 13,14 supported liquid membranes (SLMs), 15,16 and nondispersive solvent extraction (NDSX). 17-19 In this work a novel extraction technology, called emulsion pertraction (EPT), is used to carry out the simultaneous extraction and back-extraction of anionic pollutants. This technology combines the advantages of ELMs and NDSX. 20 In the EPT process the aqueous phase with the target species is kept apart from the emulsion phase by a hydrophobic mi- croporous membrane. The emulsion phase consists of an organic phase with a dissolved extractant where an aqueous strip solution is dispersed forming aqueous droplets. There is no need to make the dispersion very stable because the droplets of the strip solution do not penetrate into the membrane pores as a result of its hydrophobic character. 21,22 Although there are scarce references to this alternative, its viability to the recovery of different pollutants has been reported: (i) hexavalent chromium, copper, zinc, and nickel in wastewater, 23-28 (ii) zinc and iron from passivating bath liquids, 21,29 (iii) hydrocarbons, 30 (iv) radionuclides, 31 and (v) penicillin and organic acids. 32 The application of a new technology requires a reliable mathematical model and parameters that serve for design and optimization purposes. This work studies the separation of a multicomponent system, by means of the EPT technology. The particular problem under study is the selective removal and recovery of chromium(VI) from polluted groundwaters where other competitive anionic species are present, mainly sulfate and chloride anions. A careful experimental design was carried out to obtain equilibrium and kinetic data. Despite the high complexity of the system, the use of a methodology previously developed by the authors allowed the development of the mathematical model with determination of the unknown design parameters, namely, membrane mass transport coefficients and mass transport coefficients through the organic phase stagnant layer. 2. Experimental Setup 2.1. Kinetic Experiments. The case of study is the selective removal and recovery of chromium(VI) from polluted ground- waters where other competitive anions are present, mainly sulfate and chloride anions. The average composition of the polluted groundwaters is shown in Table 1. The presence of high concentrations of calcium required the elimination of the cation previous to the anionic separation to avoid precipitation of calcium hydroxide and fouling of the microporous hollow fiber (HF) membranes. The organic phase was prepared using 10% (v/v) of tri-octyl/decylamine (Alamine 336, COGNIS) as the anionic extractant and Isopar L Fluid, which is an isoparaffinic hydrocarbon, (ExxonMobile Chemical) as the solvent. The 3% (v/v) of Pluronic PE 3100 (block copolymer of ethylene oxide and propylene oxide; BASF) was added to enhance the phase separation of the aqueous stripping solution from the organic solution in the absence of agitation * To whom correspondence should be addressed. E-mail: ortizi@ unican.es. Table 1. Composition of the Polluted Groundwater component concentration component concentration Cr(VI) 26.1 mol/m 3 CO3 2- 1.6 mol/m 3 SO4 2- 33.5 mol/m 3 Si 0.5 mol/m 3 Cl - 29.1 mol/m 3 Al 2.1 mol/m 3 Ca 13.6 mol/m 3 TOC 4.3 mol/m 3 pH 7.3 conductivity 11.8 mS/cm 4295 Ind. Eng. Chem. Res. 2006, 45, 4295-4303 10.1021/ie051418e CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006