Silk/Polyamidoamine Membranes for Removing Chromium VI from Water

The polyamidoamine hydrogels prepared by the radical post-polymerization of alfa-omega-bisacrylamido-terminated M-AGM oligomers, in turn obtained by the polyaddition of 4-aminobutylguanidine with N,N’-methylenebisacrylamide, were reinforced with raw silk fibers, which can establish covalent bonds with the polyamidoamine matrix via the reaction of the amine groups in the lysine residues with the acrylamidic terminals of the M-AGM oligomer. Silk/M-AGM membranes were prepared by impregnating silk mats with M-AGM aqueous solu-tions and subsequently crosslinking by UV-irradiation. The guanidine pendants of the M-AGM units imparted the ability to form strong, but reversible interactions with oxyanions, including the highly toxic chromate ions. The potential of the silk/M-AGM membranes to purify Cr(VI)-contaminated water down to the drinkability level, that is, below 50 ppb, was tested by performing sorption experiments both in static (Cr(VI) concentration 20 - 2.5 ppm) and flow con-ditions (Cr(VI) concentration 10 - 1 ppm). After static sorption experiments, the Cr(VI)-loaded silk/M-AGM membranes could easily be regenerated by treating with a 1 M sodium hydroxide solution. Dynamic tests performed using two stacked membranes and a 1 ppm Cr(VI) aqueous solution reduced Cr(VI) concentration down to 4 ppb. Remarkably, the use of renewable sources, the environmental-friendly preparation process and the goal achieved meet the eco-design requirements.