Highly efficient immobilization of NZVI onto bio-inspired reagents functionalized polyacrylonitrile membrane for Cr(VI) reduction

To provide superior substrates and determine the specific species of immobilized nano zero-valent iron (NZVI) system, polyacrylonitrile (PAN) membrane was functionalized by bio-inspired polydopamine (PDA) and poly(_L-DOPA) (PDOPA) for efficient immobilization of NZVI. The synthesized composites were denoted as PAN/PDA-NZVI (PPN) and PAN/PDOPA-NZVI (PON), respectively. Analyses of XRD, SEM/EDS and XPS show that the aggregation and release of iron nanoparticles had been successfully controlled by improving membrane hydrophilcity and iron-chelating capacity via the graft of functionalized groups (i.e. –OH and –COOH) of PDA and PDOPA on PAN membrane. Both PPN and PON composites exhibited superior reactivity for Cr(VI) removal (Cr(VI) removal efficiency and reaction rate were 2.21–2.22 and 9.90–10.14 times higher than that of bare NZVI, respectively). The stability and recyclability of PPN and PON composites could be maintained over repeated cycles. Further analyses indicate that PON is more capable for Cr(VI) elimination than PPN due to the proprietary carboxyl of _L-DOPA. With the addition of 1,10-phenanthroline, membrane-chelated Fe(II) was determined to be the major species in Cr(VI) removal system, accounting for 56.9% and 53.8% with regard to PPN and PON composites, and Fe⁰ was responsible for the reduction of residual Cr(VI). Analyse of reacted composites revealed that Cr(VI) was completely converted into Cr(III), followed by formation of dominant Cr(III)/Fe(III) (oxy)hydroxides and partial desorption from NZVI reactive sites. This study suggested that both synthesized PPN and PON composites have potentials for Cr(VI)-contaminated wastewater treatment.