Systematic comparison of the size, surface characteristics and colloidal stability of zero valent iron nanoparticles pre- and post-grafted with common polymers

Abstract

This study systematically examines the effect of four common polymers on the size, surface chemistry, colloidal stability, and sedimentation behaviour of nanoparticles of zero valent iron (NZVI). The mean diameters of NZVI ranged from 5 to 189 nm, depending on the synthesis conditions. The influence of the pre- or post-grafting of the polymers on the above parameters was also investigated. The four polymers are carboxymethyl cellulose (CMC), poly(styrenesulfonate) (PSS), poly(acrylic acid) (PAA) and polyacrylamide (PAM). The molecular weight of these polymers range over three orders of magnitude, from 1.8 × 103 to 5000 × 103 g mol −1 and they are all negatively charged with the exception of the non-ionic PAM. All NZVI samples were synthesized by sodium borohydride reduction of ferrous sulfate solutions. For pre-grafting, the NZVI was synthesized in the presence of the different polymers and identical synthesis conditions were maintained which allowed for an unbiased and direct comparison of the characteristics of the different particles. ATR-FTIR analyses revealed that both pre- and post-grafted polymers were bound to the iron nanoparticle surface with polar bonds between carboxylate groups of CMC and PAA, sulfonate groups of PSS, and with hydrogen bonds with the amine groups of the non-ionic PAM. TEM images and nanoparticle tracking analysis revealed that iron nanoparticles synthesised in the presence of the polymers were larger in diameter, with TEM mean diameters ranging from 84.5 to 189 nm, than the bare-NZVI (59.1 nm), when synthesized with the same initial Fe 2+ concentration. Altering the CMC or Fe 2+ concentration during synthesis allowed synthesis of relatively monodisperse, colloidally stabilized CMC-NZVI with defined average diameters ranging from 5 nm to 129 nm. XPS studies indicated that all pre-grafted polymer-stabilized NZVI had a polymer coating covering a core–shell structure with a shell of iron oxide–hydroxides and a core of Fe 0. Pre-grafted CMC was found to be the most efficient in colloidally stabilizing NZVI. Sedimentation tests demonstrated that after 60 min pre-grafted CMC-NZVI were slightly more stable than pre-grafted PAA-NZVI, but at least two-times more stable than pre-grafted PSS- or PAM-NZVI and 13 times more stable than bare-NZVI. The colloidal stability of all post-grafted NZVI was lower than the corresponding pre-grafted particles by 1.3 to 6 times, with the exception of PSS. The nature of polymer–Fe interactions were further explored by ATR-FTIR spectra of pre- and post-grafted particles. These results highlight the relevance of the extent and the nature of the polymer–Fe interactions during synthesis of polymer-stabilized NZVI.