Desarrollo de nuevas tecnologías de separación basadas en líquidos iónicos para la recuperación de metales pesados de efluentes acuosos

Abstract

The economic system based on maximum output, consumption, the unlimited exploitation of resources and profit as the sole criterion of the good economic performance is unsustainable. For this reason, the idea of moving towards real development has been imposed, allowing the improvement of living conditions, but that is consistent with a rational exploitation of the planet. It is called sustainable development. The concept of sustainability or sustainable development can be promoted by a wide variety of disciplines. In this framework, the "Green Chemical Engineering" has arisen, whose mission is to promote innovations in chemical technologies in order to reduce or eliminate the generation and/or use of toxic or hazardous in the design, manufacture and use of processes and chemicals substances. One of the areas where more effort has been made by researchers in the implementation of the green chemistry engineering principles is the removal/recovery of heavy metals from industrial effluents. These efforts are motivated by the high toxicity of heavy metals and their every decreasing abundance in nature reserves. Some of the customary purification methods have serious environmental problems and/or high energy consumption. In this thesis the development of new separation technologies based on ionic liquids for the recovery of heavy metals from aqueous effluents is discussed. This study is part of a recent line of research group Chemical and Process Engineering (INQUIPRO), Department of Chemical and Environmental Engineering at the Technical University of Cartagena, on the recovery of metal ions in aqueous effluents by "green separation technologies" based on ionic liquids. This work aims to contribute to the design of separation, concentration and/or recovery processes, of heavy metals normally found in industrial effluents by using ionic liquids as extraction agents. As model metals, iron (III), zinc (II), cadmium (II) and copper (II) have been used, either by conventional processes or through immobilized liquid membranes. In relation to the use of ionic liquids as extracting agents for removing heavy metals from liquid streams by the conventional process of liquid-liquid extraction, a study was performed to determine the ability of various commercial ionic liquids of different nature such as sole extraction agents for the recovery of the four metal ions studied. Ionic liquids based on cations methyltrioctylammonium (MTOA+) and dialkylimidazolium (omim+ and bmim+) and the anions chloride (Cl-), hexafluorophosphate (PF6-), bis(trifluoromethylsulfonyl)imida (NTF2-) and tetrafluoroborate (BF4-) were used. It has been found that for these ionic liquids, the composition of cation and anion significantly influences the efficiency of the extraction process. In this context, with the ionic liquid [MTOA+][Cl-] an almost total removal of Fe (III), Zn (II) and Cd (II) and very high for Cu (II) was been achieved. With [omim+][BF4-] almost complete extraction of Zn (II) and Cd (II) is reached as well, while extraction of Fe (III) and Cu (II) were almost negligible. Thus, the use of this ionic liquid allows the separation of Zn (II) and Cd (II) of Fe (III) and Cu (II). Moreover, [omim+][PF6-], [bmim+][PF6-], [bmim+][NTf2-] and [omim+][NTf2-] made possible the selective separation of Zn (II)/Cd (II) and Zn (II)/ Fe (III) due to the low percentage of extraction of Zn (II) reached using these ionic liquids. The influence of the initial concentration of the metal ion was also analyzed. It resulted to affect inversely the stripping rates. Also, the HCl concentration plays an important role in the extraction process, increasing significantly the percentage of extraction of Fe(III), Zn(II), Cd(II) and Cu(II). This demonstrates the great potential of ionic liquids for its use as single agents in the extraction removal of metal ions, since changes the composition of the cation and anion and the extraction conditions, allow the design of a specific extraction process for each metal ion. However, in practical use of these new extraction agents, not only the extraction, but also the metal ions stripping processes are required to be studied. Such processes can be carried out simultaneously using supported liquid membranes, in this case based on ionic liquids (SILM). Based on the results obtained in the study of liquid-liquid extraction with ionic liquid, membranes are prepared by immobilizing the ionic liquids [omim+][BF4-], [omim+][PF6-] and [MTOA+][Cl-] on Nylon® porous substrates. Assays with individual solutions of metal ions and their mixture were conducted, varying the composition of the receptor phase. The supported liquid membranes prepared from ionic liquids based on dialkylimidazolium presented stability problems, and no satisfactory results are achieved with regard to the separation of metal ions. With [MTOA+][Cl-], however, the percentage of ionic liquid retained in the membrane after pertraction tests was considerably high. With this ionic liquid, three different composition receiving phases (Milli-Q water, sodium carbonate 0.1 M and ammonia 6M) were tested. It was observed that modifications in the composition of the receptor phase, the selective separation of the ions studied could be achieved. In order to improve the stability of SILM, casting methods were used to obtain polymer inclusion membranes (PIM). For this, various plasticizers and solvents of different polarity were studied in order to find the best combination and ratio with the ionic liquids [MTOA+][Cl-] and [omim+][PF6 -] since these ionic liquids showed good transport properties for the studied metal ions, and because of their low solubility in water. With the ionic liquid [omim+][PF6 -] stable membranes were obtained, but nevertheless failed to separate the mixture of metals. The use of PIM based on [MTOA+][Cl-], conducted to the selective separation of a mixture of the studied metal ions. However, flow passage resistance was significantly reduced. With the obtained results in studies of liquid-liquid extraction and pertraction with supported liquid membranes and polymer inclusion membranes based ionic liquids, a general process scheme for the selective separation of iron (III), zinc (II), cadmium (II) and copper (II) from hydrochlorinated effluents is proposed. This process combines the two technologies studied: liquid-liquid extraction and pertraction by liquid membranes based on ionic liquids.