Colloidal and chemical stability of iron oxides nanoparticles in aqueous solution
Résumé
Either used as nano-carriers in blood, depolluting agents in groundwaters or in soils, engineered iron nanoparticles are prone to a growing interest that explains their multiple uses as well as their increasing industrial production. The very small size of iron oxide nanoparticles (IONPs) having at least one space dimension <100nm gives rise to some exceptional physicochemical properties that ensue from their high reactivity. In environmental and agricultural fields, where IONPs could be particularly used, this reactivity is directly related to their colloidal stability which is of prime interest regarding groundwater- or soil-remediation, allowing IONPs to reach their target. NPs stability in aqueous environments depends on many parameters including the environmental conditions (pH, temperature, soil solution chemistry and ionic strength), NPs concentration, as well as NPs intrinsic and surficial characteristics. In this context, experimentations were investigated to assess the effects of pH, surface modifications and NPs intrinsic physicochemical properties (size, morphology and surface area (SA)) on their colloidal and chemical stability.
In a primary step, nm-sized Fe3O4-NPs and ɣ-Fe2O3-NPs were synthesized and their surface were then coated with HA and PC (Phosphatidylcholines) to model natural surface modification. In a second step, these IONPS were characterized with TEM, XRD, FT-IR and BET. Fe(II)/Fe(III) ratio measurements have been conducted on Fe3O4-NPs in aerobic conditions to assess the oxidation kinetics of magnetite to maghemite. Then, the effect of pH on the colloidal stability of these bare-IONPs and surface modified Fe3O4-NPs was studied in a pH range from 3 to 7.5. The results evidenced that pH played a key role in driving IONPs colloidal stability as pH changes affected the size distribution (SD) of all the IONPs investigated, leading at least to two SD configurations: IONPs were either stable or aggregated depending of the closeness of the pH regards to their respective pHzpc. Surface coatings with HA and PC induced surface chemical modifications, which shifted the pHzpc of bare magnetite and modified the ensuing pH-dependent SD through electrosteric interactions. HA turned out to be much more effective than PC in enhancing IONPs colloidal stability as it promoted smaller sized aggregates and widened magnetite pH-stability range (pH=4 to 7.5). The rapid transformation of magnetite into maghemite (five days) resulted in the loss of Fe(II) from its chemical structure and increased magnetite-NPs SA. Maghemite also displayed a higher sensitivity to pH than magnetite and the oxidized NPs formed almost exclusively μm-scaled aggregates in acidic medium (pH=3, 4 and 5). The colloidal stability of magnetite at acidic pH would thus likely be hindered in aerobic environments because of its rapid oxidation into maghemite which lead to higher aggregation at most pH values.