Disseny d'un sistema de membrana líquida suportada per la determinació de Zn lliure en medi aquós

Abstract

Zinc is an essential trace element for organisms, but an excessive concentration can cause serious toxicity problems affecting their metabolism. It is one of the most widely-spread metals in aquatic ecosystems, and often its concentration exceeds the established thresholds for European and American legislation (59-210 ppb / 30-500 ppb respectively, in function of water hardness). The total metal concentration present in the aquatic environment doesn’t provide information about its distribution, mobility, biological availability and toxicity. Therefore, the bioavailable metal amount for an organism is proportional to the free or labile metal fraction present in the environment. Nowadays different analytical methods are being developed related to the trace elements speciation, applicable to the free metal quantification, such as supported liquid membranes. In this study, it has been used a system based on a supported liquid membrane of tubular configuration, hollow fiber (HF) for free zinc determination in aquatic environment. HF device consists in a three-phase system composed of two separate aqueous phases which are in contact with each other through a microporous polypropylene hollow fiber, with a solvent immobilized in its pores. The organic solution containins an extractant which facilitates metal transport through the membrane. Various studies are performed to evaluate Zn2+ transport efficiency of the device with ZnSO4 aqueous solutions. On one hand, it is studied the best composition of the organic and stripping phase, making trials with different concentrations of the extractant D2EHPA 0,5M / 0,25M / 0,1M membrane, and changing stripping phase composition using acidified aqueous solutions HNO310-1M / HNO310-2M + Ca(NO3)2 3,6 ·10-2M / HNO310-4M + Ca(NO3)2 4·10-2M. The best Zn transport efficiencies are obtained with a chemical system composition of 0,1M D2EHPA in the membrane and HNO310-2M + Ca(NO3)2 3,6 ·10-2M in the stripping phase. On the other hand, it is studied the effect of the presence of different ligands (EDTA and citric acid) in the donor phase 30,60 μM Zn, 15 μM EDTA and citric acid 8 μM, where observed that the EDTA influence about the amount free zinc, is more important than citric acid. Finally, it is studied the chemical system selectivity with respect to Zn2+ in the presence of other divalent ions, such as Cu2+. The results show that the system is selective for Zn extraction under the tested conditions. The device is applied in river water samples, getting low rates of Zn2+ transport due to the presence of humic acids, chlorides, sulfates that can form zinc complexes decreasing its free labile fraction and other metals such as Fe3+ that can also bind to the extractant