Carboxymethyl cellulose stabilized and sulfidated nanoscale zero-valent iron: Characterization and trichloroethene dechlorination

Polymer stabilization can enhance subsurface transport of nanoscale zero-valent iron (nZVI) while sulfidation can reduce its hydrogen evolution reaction (HER) and enhance electron utilization efficiency (εe). However, the combined impacts of stabilization and sulfidation on particle characteristics, reactivity and efficiency, and therefore longevity for contaminant remediation/capacity, has not been adequately investigated. Carboxymethyl cellulose stabilized and sulfidated nZVI (CMC-S-nZVI) was synthesized by both one-pot and two-step approaches and characterized with SEM, TEM-SAED, XRD, XPS and FTIR techniques. Dithionite was used as the sulfur source for the one-pot particles and sulfide was used for the two-step particles. We found that CMC stabilization did not affect the degree of sulfidation. Both stabilization of S-nZVI and sulfidation of CMC-nZVI enhanced particle reactivity by one order of magnitude. The two-step CMC-S-nZVI generally exhibited much higher reactivity than the one-pot counterparts, mainly because dithionite consumes Fe0. CMC stabilization significantly decreased εe, an effect not quantified before, while sulfidation compensated for the decrease. Almost full εe compensation against stabilization was obtained at S/Fe 0.5, where the εe was ∼14% and 13%, for the non-stabilized and the two-step CMC-S-nZVI, respectively. Sulfidation at S/Fe > 0.2 completely eliminated the HER in the two-step CMC-S-nZVI after 2d with > 73% of Fe0 preserved, and had a perfect εe of 100% after 8d of ageing (i.e., the longevity of CMC-nZVI). Our results suggest that through stabilization and sulfidation, a balance between particle stability and longevity/capacity of nZVI could be achieved.