Nanoscale iron characterization and mobility enhancements by means of biodegradable hydrocolloids

In the field of pollution remediation, and in particular of aquifer systems decontamination, the use of nanoscale zerovalent iron (NZVI) particles is one of the most interesting and promising technology. These particles can be suspended into a slurry, injected into the subsurface where they exploit the groundwater flow to embrace the source of contamination, thus treating a wide variety of contaminants and bypassing most of the limitation related to PRBs. However, very limited or no mobility of the nanoparticles has been shown in porous media. In order to enhance the mobility of iron nanoparticles in the subsurface, it is important to (i) prevent the formation of large aggregates that tend to be easily filtered and (ii) reduce the attachment of nanoparticles to the soil grains. Surface modifiers can increase the surface charge of the nanoparticles thereby providing electrostatic stabilization, and also create a surface brush layer which engenders long-range strong steric repulsion forces between particles and between particles and collectors. In the present study, after a description of the characterization of two commercial NZVI particles, the stabilizing effects and mobility enhancements deriving from dosing of biodegradable hydrocolloids on nanoscale iron suspensions were evaluated. Two commercial NZVI colloidal dispersions were employed: bare RNIP-DS particles and RNIP-10 AP SDS particles, modified by the supplier with addition of a biopolymer. Based on T.E.M. micrographs, the mean diameter of both suspensions is nanoscale (50÷80 nm), but particle size distributions exhibit a strong positive skewness. Bare RNIP-DS particles are generally larger than RNIP-10 AP SDS ones, due to the presence of larger magnetite particles. For both suspensions, theoretical values of specific surface area are smaller than experimental BET areas, especially for RNIP-10 AP SDS: it is likely that the particles have a surface roughness, which improves their surface area. Four biopolymers were added to RNIP-DS to study their effect on suspension stability. Guar gum was chosen as the best biopolymer between the four tested. Sedimentation profiles showed the ability of guar gum to effectively keep the iron nanoparticles suspended. Column experiments, run at conditions similar to those of natural groundwaters, confirmed the effectiveness of guar gum to enhance the mobility of NZVI particles in saturated porous media.