Optimization of the sequestering ability of modified polymer inclusion membranes (Pims) towards Sn(II) in aqueous solution

The employment of polymeric membranes to afford a wide range of energy- and environmental-related applications has attracted both research and industry. Among them, Polymer Inclusion Membranes (PIMs) have been investigated to produce materials with appealing properties in fields such as separation science, sensors, water treatment, while featuring cost competitiveness and ease of processability. PIMs are usually composed by a polymeric matrix, i.e. PVC or CTA (polyvinylchloride or cellulose triacetate), an extractant (a carrier) and a plasticizer and/or modifier. Such a simple chemistry may be finely tuned to gain membranes able to exert selective pollutants extractions with high efficiency, avoiding the environmental issues of organic solvents use. Bearing this in mind, this work has been focused on the synthesis of PIMs and surface modified PIMs able to exert sequestering properties towards Sn2+. For this purpose, several PIMs were prepared, containing different amounts of PVC and CTA as polymeric matrix, Aliquat 336 or Alicy as plasticizers, and Thiomalic acid (SMAL) or montmorillonite modified with a thiolic group as extractants. In this last case, the use of a compatibilizing agent 3-aminopropyltriethoxysilane (APTES) was necessary to anchor the inorganic montmorillonite with the organic matrix of the polymer. The aim of this double approach is to obtain membranes where the extraction of the metal can be connected either to its diffusion within the bulk of the membrane or to a specific interaction with the functionalized surface. The composition of PIMs and the solution in which test their sequestering ability were selected with a D-Optimal experimental design, with a total of 9 factors, 6 of which are 3-levels and 3 are 2-levels. The maximum number of experiments was kept to 30, while the optimal number resulted to be 16. The sequestering ability of the PIMs was evaluated in a aqueous solution of Sn(II) 1 ppm (also containing ionic medium and inorganic acid) by differential pulse – anodic stripping voltammetry. The PIMs was immersed directly into the electrochemical cell, thus allowing to collect a scan every two minutes and to profile the absorption kinetic in one hour. The kinetics were fitted to a first-order model, determining Qe (the amount of tin extracted at the equilibrium) and K1 (kinetic constant). All the tested membranes were characterized by means of static contact angle measurements, tensile module, thickness and Raman investigations. The surface functionalization of the PIM increased the Qe of the membrane, indicating that the interaction between the solution of the PIM occurs through a surface configuration rather than the bulk of the material. In this light, the use of more rigid CTA-based membranes is better than the use of more elastic PVC-based membranes. As regards the ionic liquid, Alicy seems to improve Qe and this can be attributed to: 1. better interaction with APTES in the functionalization step, 2. interaction with metal cation through the P-OH site. Alicy makes the membrane less hydrophilic than Aliquat, while remaining within the hydrophilic membranes. The average value of the contact angles of the membranes containing Alicy (74°) is almost exactly twice that of the membranes containing Aliquat (36°), as can be expected from the composition of these ionic liquids.