Water purification using sponge like behaviour of poly (N-isopropylacrylamide) ferrogels. Studies on silver removal from water samples

A new method for metal removal from waste water samples using thermoresponsive ferrogel sorbents is presented. For this purpose a novel method of synthesising composite materials made from a polymeric hydrogel and sorbent materials was developed. The key aspect of synthetic work was the selection of a polymer matrix for homogeneous implementation of a sorbent material. Poly(N-isopropylacrylamide) (pNIPA) hydrogel, characterised by discontinuous volume phase transition induced by the temperature change, was selected as the polymeric matrix. The synthesised materials were applied for the removal of silver cations in aqueous media. The phenomenon of volume phase transition of pNIPA gels allowed one to absorb solution containing examined metal into the gel matrix with the sorbent material and effectively separate polymer phase from the solution depleted of metal after the phase transition was completed. Magnetite (Fe3O4 ) acted as a sorbent in designing of the composite material. Magnetite particles implemented in the pNIPA gel matrix form a ferrogel, which can be fully controlled using a magnetic field. The maximum adsorption capacity was estimated experimentally to be 13.6 mg Ag / g Fe3O4. The number was verified by the kinetic data. The proposed mechanism of silver sorption on magnetite postulates a chemical deposition of metallic silver followed by intensified diffusion of silver cations to magnetite grains. The ferrogel sorption system is stable from pH 5 to 8 and can be easily restored via the swelling - shrinking cycling. The sorption procedure was validated for the real samples containing excessive amount of copper ions.