Insights of mechanism into enhanced removal of Cr(VI) by Ginkgo biloba leaves synthesized bimetallic nano-zero-valent iron/copper

Abstract

For purpose of green preparation of nano-zero-valent iron and copper (GB-nZVI/Cu), the use of Ginkgo biloba extract as a reducing agent is reported herein. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) analyzed the properties of nanoparticles. GB-nZVI/Cu exhibited a better Cr(VI) removal performance than nZVI/Cu materials synthesized by apricot leaves (Al-nZVI/Cu) and hawthorn leaves (Hl-nZVI/Cu). GB-nZVI/Cu could remove 97.2% Cr(VI) in 30 min. Low pH and 313 K promoted the reaction while humic acid (HA) had inhibitory effect. The promoting effects of coexisting anions were observed following the order: SO42->HCO3->Cl-. The rate constant K2 increased from 0.0303 to 4.8865 g·mg-1·min-1 as SO42− concentration increased. Na+ and low Mg2+ concentration promoted the removal of Cr(VI) while NO3−, K+, Ca2+ and high Mg2+ concentration had the negative effects. The removal reaction followed the pseudo-second-order kinetic model, with a chemical adsorption as a rate-controlling step. Visual MINTEQ models and mechanism studies showed that the removal of Cr(VI) by GB-nZVI/Cu was a combination of adsorption, reduction and precipitation. The Fe-Cu galvanic cell can accelerate the corrosion of Fe0, enhancing the reduction of Cr(VI). Cu0/Cu(I) could be oxidation into Cu(II) by receiving electrons from Fe3+, resulting in the generation of Fe2+. Cr(III) finally precipitated in the form of FeXCr1−X(OH)3 and Cr(OH)3. GB-nZVI/Cu had superior sedimentation behavior and more Fe2+ production to promote Cr(VI) reduction. GB-nZVI/Cu maintained a better removal performance of Cr(VI) than Al-nZVI/Cu and Hl-nZVI/Cu after five reuses.