Physico-chemical characterization of colloidal iron suspensions for groundwater remediation

This work reports the physico-chemical and morphological characterization of colloidal nanoscale iron suspensions. Groundwater clean-up by nanoscale iron colloidal suspensions has received increasing interest in the last decade, due to the high reactivity of iron nanoparticles, their ability to treat a wide variety of the most common environmental contaminants and their great flexibility for in situ applications. In order to better understand these features, a physico-chemical characterization of the colloidal nanoscale iron dispersions is fundamental. Morphology, crystallinity, granulometric distribution, specific surface area and chemical composition have been investigated by optical microscopy; scanning and transmission electron microscopy (SEM-TEM); X-ray diffraction (XRD); Micro-Raman Spectroscopy and BET measurements of two nanoscale iron suspensions of Reactive Nanoscale Iron Particles (RNIP) from Toda Kogyo Corp: i) bare RNIP-10 and ii) RNIP-10 modified by a biopolymer. Chemical and physical properties have been studied of both the colloidal dispersion and its single phases (dispersed phase, i.e. nanoscale iron particles, and dispersing medium, constituted by a water solution with a biodegradable polymer). Nanoscale Iron Particles (NIP) show two kinds of irregular shapes, a spherical one and a crystalline one, according to SEM and TEM analysis. The NIP specific surface area, measured by means of N2 adsorption/desorption isotherms at 77 K and calculated according to the BET algorithm, is in the 8,5 - 30,6 m2 g-1 range, i.e. much higher than micrometric and millimetric iron. XRD diffractograms and Micro-Raman spectra show that NIP have a core-shell structure, with a core of iron covered by a shell of magnetite. Particles chemical composition and surface properties have been analyzed also by SEM-EDS microanalysis. These studies suggest that the colloidal suspensions are unstable and that particles form bigger microscale aggregates, as demonstrated by optical microscopy measurements too. Magnetite shell is liable for nanoparticles assembly. Therefore, these physico-chemical features may affect the ability of the colloidal suspensions to degrade groundwater contaminants and their delivery into porous medium.