Tunicate-Inspired Gallic Acid-Conjugated Chitin Nanofibrous Matrix for Metal Sequestration and Water Treatment Applications

Abstract

Tunicate, marine filter feeder, exhibit an outstanding metal sequestration ability on their blood cells through a series of complexation reaction with oligopeptide containing 3,4,5-trihydroxyphenilalanin (TOPA) moieties called tunichrome. Inspired by this naturally occurred strategy, we design a nano-bio filtration matrix mimicking tunichrome using two cheap and abundant precursor; pyrogallol-5-carboxylic acid (gallic acid) and chitin nanofiber, which is the chitin nanofibrous matrix tailored with gallic acids (CGa). The tunicate mimetic matrix (CGa) demonstrated the versatility of strategy in the generation of water filtration material, especially for 1) gold recovery and 2) carcinogenic hexavalent chromium removal. CGA showed unprecedented fine and clean gold recovery (>99%, 533 mg gold per gram weight), which exceeds that of the present most popular materials. Extended X-ray absorption fine structure (EXAFS) spectroscopy and data-fitting elucidated the decreased coordination numbers for Au-Au nearest neighbors, demonstrating gold coordinated to pyrogallol units, followed by an intramolecular association of Au0. A catalytic reduction of 4-nitrophenol mediated by the CGa with harvested gold revealed an excellent recyclability up to 30 cycles (~95% reduction), together with methylene blue reduction and antimicrobial performances, indicating a versatile characteristic of sustainable processes by the CGa. In addition, the CGa also exhibits a superior performance towards carcinogenic hexavalent chromium removal (nearly 119 mg chromium per g weight of CGa). Cr K-edge extended X-ray absorption fine structure (EXAFS) corresponds well to nontoxic CrIII (average four Cr-O bond lengths of 1.91 Å) and a small fraction of CrVI (average two Cr-O bond lengths of 1.62 Å). A coupled reductive complexation mechanism was identified by combining near edge X-ray absorption fine structure (NEXAFS) and linear combination fits. This strategy opens the door for fast-developing new biomimetic alternatives for metal sequestration which is not restricted to gold and chromium and can offer a tool for multifaceted soft/hard nanomaterials.