Cd(II) Sorption by Novel Polymer Inclusion Membranes Based on L-Glutamic N,N-Diacetic Acid or Citric Acid from Aqueous Solution

Despite the availability of many chelating agents able to efficiently bind metal cations in solution, only few are actually viable as a large-scale countermeasure to the persistent problem of metal-polluted wastes. For instance, among them, the non-biodegradable ethylenediaminetetraacetic acid (EDTA) ended up being overused, thus becoming a polluting agent itself, pointing out the necessity to search for new strategies for efficient cations removal and environmental remediation. In this scenario, this contribution reports some preliminary results on a research focused on the development of novel ecofriendly efficient and “practically usable” materials. Although this goal is still far from being achieved, the fundamental idea of the material biocompatibility led to the synthesis of a series of novel and ecofriendly polymer inclusion membranes (PIMs), whose formulation is based on a biopolymer matrix, namely polylactic acid (PLA) including Aliquat336 (Ali, an ionic liquid) and L-glutamic N,N-diacetic acid (GLDA), the latter being a readily biodegradable chelating agent, or citric acid (Cit). Such material has already shown a remarkable chelating capability toward Cd2+ along with high tolerance to extreme conditions such as low pH and high electrolyte concentration. In this regard the chemometrics approach during the experimental design was of pivotal importance. This versatile method greatly simplified the assessment of the chemical behavior of these materials in heterogeneous solutions. In particular, the acid-base equilibria of L-glutamic N,N-diacetic acid and citric acid entrapped inside the polymeric membrane were investigated by potentiometric titrations in a pH range ranging from 2 to 11 and ionic strength between 0.1 and 3 mol dm-3. Moreover, kinetics experiments for the sorption of the Cd2+ cation were performed by means of differential pulse anodic stripping voltammetry (DP-ASV) to assess the time required to reach equilibrium between the PIMs and a solution containing Cd(II), at different pH and chloride concentration values. This information was then taken into account while performing isotherm experiments by DP-ASV. Last, the surface structural characterization was performed by means of attenuated total reflectance infrared spectroscopy (ATR). The preliminary data obtained are encouraging and may suggest the use of these polymer inclusion membranes for remediation purposes.