A physical-based interpretation of mechanism and kinetics of Cr(VI) reduction in aqueous solution by zero-valent iron nanoparticles.

The aim of this paper is to show the results obtained by investigating the reduction of hexavalent Chromium [Cr(VI)] by iron nano-particles in aqueous solution, interpreted in light of the particle-grain model. The diffusional and geometric parameters that govern and describe the reacting system were estimated from the evidences deriving from the characterization and the experiments conducted, allowing assumptions based on physical principles. Such procedure rendered the particle-grain model a valid choice for the interpretation of the results obtained. The model, used in its dimensionless form, was tested according to a preliminary procedure aimed at analyzing the sensitivity of the system, by varying within wide ranges the ratio between the reaction rate, the diffusive mass transfer rate, and the particle-grain radius, to show how reliable its potential application may be. Subsequently, a non-linear regression procedure was used to estimate the two main parameters of the model that affect the reduction process: (i) the diffusion coefficient within the solid layer produced along with the reaction, Dpc (6.02 E-13 m2 s-1), and (ii) the kinetic constant of the surface reaction, kc (0.21 m s-1). The values found for the parameters were perfectly in line with theoretical considerations and experimental evidences.