Investigation of washing and storage strategy on aging of Mg-aminoclay (MgAC) coated nanoscale zero-valent iron (nZVI) particles

The tendency towards agglomeration and oxidation of nanoscale zero-valent iron (nZVI) particles limits its application for in situ groundwater and soil remediation. Although the effect of surface coatings on nanoparticle stabilization has been commonly practiced, the effect of preparation procedures on aging of stabilized nZVI needs to be investigated. The effect of washing and storage, up to seven days, on aging of Mg-aminoclay (MgAC) coated nZVI is evaluated, following three procedures: pre- and post-storage washing with a 1mM NaHCO₃ solution, and pre-storage washing with a stabilizer (MgAC) solution. Even though the initial particle size is identical, the observed size of pre-washed nZVI increases up to six times. This high aggregation tendency appears to be due to the desorption of MgAC during washing and storage, as verified by a decrease in the zeta potential, indicating a decrease of repulsion between nZVI particles. On the other hand, pre-storage washing is essential, in order to retain nZVI reactivity, by removal of residual reactants in the synthesis mixture. The reactivity of nZVI is examined with three parameters: optical density at 508nm as a measure of particle concentration, reactive iron content measured by H₂ generation with acid digestion, and nitrate reduction capacity. All three parameters decrease significantly for post-storage washed nZVI, which corresponds to XRD results that exhibit transformation of Fe(0) to iron oxides. The reactivity tests display high linear correlations (r²>0.95, p<0.05) with respect to one another. Pre-storage washing, followed by addition of MgAC, exhibits high stability as pre-storage washing, as well as high reactivity as post-storage washing. Here, it is found that the proper washing procedure is crucial in coated nZVI preparation, to provide long lasting stability and to maintain reaction capacity during its preparation and transport.