Role of humic substances in the formation of nanosized particles of iron corrosion products. Pankratov D.A., Anuchina M.M. //Russian Journal of Physical Chemistry A. 2017. V.91. №2. P.233-239.
The corrosion of metallic iron in aqueous solutions of humic substances (HS) with limited access to air is studied. The HS are found to exhibit multiple functions. Acid–base, redox, and surfactant properties, along with the ability to form complexes with iron in solution, are displayed in the corrosion process. Partial reduction of the HS during the corrosion reaction and their adsorption onto the main corrosion product (Fe3O4 nanoparticles) are observed.
Humic substances (HS) are naturally occurring polyelectrolytes that play an important part in nature by influencing the formation of different iron-containing nanoparticles, their geological and biochemical migration, and the formation of minerals and ores. Making up the organic matrix of soils and natural waters, humic substances are major participants in the iron cycle. Natural compounds of iron with humic substances carry this transition element through soils and surface waters, contribute to the formation of iron-containing minerals, serve as sources of bioavailable iron for plants, and are promising for biomedical applications. However, the nature of iron–HS compounds is not yet fully understood. On the one hand, humic substances can act as chelating agents that bind iron in its different redox states. On the other hand, they can function as surfactants that keep iron-containing nanoparticles in a colloidal form in aqueous media . These mechanisms most likely operate concomitantly, but certain aspects of the formation of iron–HS compounds could cause the process to follow mostly one of the paths. The conventional approach to the synthesis of iron–HS compounds is based on the reaction between inorganic salts of iron (II) or (III) and humic substances in solution . However, this approach imposes limitations on a researcher by demanding a specific content, composition, and chemical environment of iron for the final product. In contrast, we propose a means of synthesis that closely mimics the formation of iron–HS compounds in nature: a complex and multicomponent (physicochemical) system that attains a thermodynamic equilibrium between different iron compounds on its own accord.
The corrosion of metallic iron in the presence of humic substances would seem to be a viable way of achieving this goal. Similar systems are promising as reactive barriers to diverse contaminants in groundwaters. Nanosized zero-valent iron (nZVI) is thought to be a principal reactive component in such barrier systems. This substance has reportedly been used to purify water of inorganic contaminants, chlorinated hydrocarbons, and such pathogenic flora as Salmonella. Since humic substances occur naturally in soils, they are considered promising environmentally-safe surfactants for stabilizing nZVI in aqueous environments. However, the possibility of stabilizing nZVI with humic substances is thought to be unlikely, due to the energetic chemical interaction between the two. The products formed during a long-term experiment in which metallic iron interacted with humic substances solutions for 90 days were analyzed via X-ray diffraction (XRD) and scanning electron microscopy (SEM) and were found to contain finely dispersed magnetite particles.
In this work, we investigated the products of (and processes accompanying) the corrosion of metallic iron in aqueous solutions of coal-derived humic substances. Our aim was to determine the role humic substances play in the corrosion of iron, the possible reaction pathways, and the form of the corrosion products.
Our data suggest that humic substances can oxidize metallic iron under the conditions explored and described in this work. It was shown that the reaction between iron and aqueous humic substances solutions is a multistage process that develops simultaneously through several possible pathways and gives rise to colloidal solutions of nanosized Fe3O4 particles stabilized by an humic substances. When this happens, the humic substances acts as a multifunctional reagent that participates in the corrosion process as an oxidizing and acidic reagent that enables iron to transfer into a solution; as a ligand for iron ions in solution; as a surfactant that stabilizes nanoparticles in solution; and as an adsorbate that affects the morphological properties of iron-containing reaction products. Our assumptions about the models of the reactions that occur between humic substances and metallic iron are obviously not exhaustive and require further confirmation and study.