Cyclic enrichment of chromium based on valence state transformation in metal-free photocatalytic reductive imprinted composite hydrogel

Abstract

Cr (VI) exists in anion form and can be reduced to positive charged Cr (III) under certain conditions. Can positive charged Cr (III) be continually used for absorbing Cr (VI) to achieve cyclic accumulation of chromium? In this paper, an ion imprinting material for adsorption of Cr (VI) was prepared by dispersing polypyrrole (PPy) in a gelatin/chitosan (Gel/CS) hydrogel network, named Gel/CS/PPy. Based on the conversion of Cr (VI) to Cr (III), a cyclic enrichment process including adsorption-photoreduction-fixation-readsorption of Cr (VI) was established in Gel/CS/PPy hydrogel. The composition and structure of the Gel/CS/PPy were analyzed by scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric (TGA), texture analyzer (Universal TA), zeta potential and ultraviolet-visible-near infrared spectra (UV–vis-NIR). The conversion of Cr (VI) and Cr (III) and its promoting effect on readsorption were verified by XPS. The results showed that Gel/CS/PPy has good adsorption capacity for Cr (VI) and excellent photocatalytic ability to reduce Cr (VI) to Cr (III). Cr (III)-loaded Gel/CS/PPy can be further used to adsorb Cr (VI) and showed good adsorption efficiency even after four cycles. The optimal operating condition for Cr (VI) adsorption is pH = 3; 2 g/L dose of Gel/CS/PPy; and the adsorption capacity of Cr (VI) was about 106.8 mg/g after six adsorption cycles. Since Gel/CS/PPy is composed of organic components, high purity chromium can be recovered by simple calcination method later. Therefore, the synthesized Gel/CS/PPy has great potential in the practical application of low concentration Cr (VI) treatment in water.