Comparative assessment of the characteristics and Cr(VI) removal activity of the bimetallic Fe/Cu nanoparticles pre- and post-coated with carboxymethyl cellulose

High redox activity and superior magnetic property of nano-scale zero valent iron (nZVI) leads to the formation of surface oxide layers and large agglomerates, limiting its application in remediation activities. To inhibit surface oxidation and to overcome the aggregation challenge, copper (Cu) doped bimetallic iron/copper (Fe/Cu) nanoparticles with/without polymeric stabilization (carboxymethyl cellulose (CMC)) were chemically synthesized and compared with bare iron (Fe) nanoparticles for hexavalent chromium (Cr(VI)) removal. CMC stabilization of Fe/Cu nanoparticles was carried out in both post and pre-grafting pathways to evaluate the role of stabilization method on nanoparticle characteristics and reactivity. Detailed material characterization and CMC bonding mechanisms onto nanoparticle surface were explored by performing TEM, FTIR, XRD, XPS, and TGA analysis. TEM results showed spherical morphology of nanoparticles with better stability and smaller particle size for CMCpre-Fe/Cu (~25 nm). Batch experiment results exhibited lower Cr(VI) removal efficiency for stabilized nanoparticles in contrast with bare nanoparticles (CMCpost-Fe/Cu (20.68%) < CMCpre-Fe/Cu (41.79%) < Fe/Cu (58.19%) < Fe (62.09%)) - attributable to the electrostatic repulsion between stabilized nanoparticles and Cr(VI) species. Contrary to other materials, despite the low Cr(VI) removal efficiency of stabilized nanoparticles, CMCpre-Fe/Cu showed a faster initial adsorption rate (22.23 mg.g(-1).min(-1)). Acidic pH and particle dispersion were found to impact the removal process positively. XPS analysis of the materials before and after the reaction described the removal mechanism as a synergistic effect of adsorption, reduction, and precipitation. In conclusion, Cu doping can boost the removal process by facilitating reductive pathways, and polymer coating may result in reduced Cr(VI) removal from wastewater due to surface charge properties.