Preconcentration of heavy metals by Donnan dialysis

Abstract

The aim of this study was to investigate the applicability of Donnan dialysis for the preconcentration of iron from synthetic wastewaters. Moreover, the competition between magnesium and calcium on the preconcentration of iron as well as the effect of adding a complexing agent (EDTA) on the enrichment factor of iron were also studied. Heavy metals are of great concern because of their extreme toxicity, even at low traces. Even with the sensitivity and selectivity of existing analytical techniques, direct determination of heavy metals is not always possible due to the low concentration of the ions. For this reason there is a great necessity for the preconcentration of heavy metals prior to their determination. Donnan dialysis experiments were performed in a laboratory scale with a dialysis cell, according to an experimental design created with the MODDE 8.0 software also used for modelling the process. Experiments were conducted with a full factorial design, which consisted in 11 experiments, three (3) of were central points. Donnan dialysis was proved to be a feasible technique to preconcentrate iron from synthetic wastewater samples. However, the metal competition decreased the enrichment factors of the main metal to be preconcentrated and the presence of the complexing agent in the receiver side did not affect the preconcentration process. Microscope images were taken to study the membrane structure. Membrane fouling caused by the absortion of iron explained the low enrichment factors and poor reproducibility of the process. Moreover, it was observed that the regeneration process was not good enough to regenerate properly the membrane. Different studies were done with another membrane and with another metal such as copper; nonetheless similar results were achieved.