Sericin-based resins from silk degumming wastewater for the removal of heavy metal ions from water

Chromium (VI) is a water pollutant categorized as ‘likely to be a carcinogen to humans’ compound when orally ingested with estimated cancer potency 0.5 mg/kg/day. The European Directive 2001/59/EC poses a 5 µg/L threshold concentration for Cr(VI) in groundwaters. In this work, a chemical process was devised to obtain heavy metal ion absorbing resins by the polyaddition of bisacrylamides and 1,2-diaminoethane with sericin using as reaction solvent raw waste-water from silk degumming processes. Silk sericin (SS) is a natural globural protein deriving from silk worm Bombyx mori with molecular weight ranging from 10000 to 300000. Following the alkaline degumming process, sericin is degraded to peptides with molecular weight 20000. These peptides contain lysine-deriving residues that participate in the polyaddition leaving to a resin. This resin is a hybrid one in which a substantial portion is constituted by sericin peptides. The rationale of this approach is that the guanidinum ion has the ability to strongly bind oxoanions, due to its geometrical Y-shaped, planar orientation, optimizing charge distribution and hydrogen bonds [1]. SS resins were evaluated for the removal of both positively charged (Cu2+, Co2+, Ni2+, Mn2+) and negatively charged heavy metals oxoanions (CrO42-) from water. Different resins were obtained containing different amounts of sericin. These resins were characterized by elemental analysis and their structure confirmed by FT-IR/ATR spectroscopy. The swelling capacity of the new absorbents in different media and their thermal stability by DSC and TGA techniques were evaluated. The removal properties of resins towards Cu2+, Co2+, Ni2+, Mn2+ and CrO42- ions in aqueous single metal dilute and concentrate solutions were performed in batch absorption experiments and evaluated by EDTA titration in the case of Cu2+, Co2+, Ni2+, Mn2+, and by the UV-VIS spectroscopy in the case of CrO42-. The products showed different absorption capacities depending on the SS content in the resin. Treatment with 0.1 M HCl showed excellent regeneration with maintenance of the resins absorption capacity for 20 regeneration cycles. In conclusion, sericin-based resins, besides being biocompatible, were endowed with environmental friendly preparation process; biodegradability; moderate cost; ability to fast and quantitatively absorb from aqueous solutions even at low pollutant concentration; full reversibility of the absorption process making it economically convenient both for regeneration and metal recovery.