Reactions of Acid Orange 7 with Iron Nanoparticles in Aqueous Solutions

The physicochemical properties of two commercial dispersions of iron nanoparticles were studied, together with their behavior in the room-temperature degradation in basic solutions of Acid Orange 7 (AO7), studied by UV-vis spectroscopy. In one dispersion (bare-RNIP), water was the solvent, and in the other (M-RNIP) a biopolymer (sodium aspartate) was added (RNIP standing for reactive nanoscale iron particles and M for modified). The features of iron nanoparticles (size, morphology, presence of oxidized phases) were studied both in the dispersions as such and in the corresponding dried powders. A protecting role of the biopolymer was observed, as well as changes in the properties with time (aging). With bare-RNIP, the fraction of Fe3O4 (magnetite) steadily increased with time at the expense of Fe-0, eventually reaching 99%, and with M-RNIP, the Fe-0 content was higher at any time than with bare-RNIP: aging, however, brought about the formation of the Fe3+ compound FeOOH besides magnetite. As for AO7 degradation, a similar behavior was observed with the two fresh dispersions: with M-RNIP, degradation was complete in a few minutes, and with fresh bare-RNIP, the same process was basically observed, though at a lower rate. In both cases, successive reactions were observed as a minor feature, for which an interpretation is advanced. Aging of M-RNIP does not prevent the degradation reaction: aging in the absence of the polymer, instead, leads, after 6 months, to an entirely different process, consisting in the mere adsorption through the phenol group of AO7 onto the magnetite external layer of bare-RNIP particles. Further aging of bare-RNIP prevents also this phenomenon. The different behavior of the two dispersions relates to the composition of iron nanoparticles. Reaction with water converts Fe-0 into magnetite. When Fe-0 is present and the thickness of the outer magnetite layer is moderate, AO7 degradation occurs. With a thick outer layer, only adsorption is possible, which does not take place on a fully oxidized surface.