Gamma (60Co) irradiation and thermal effect on redox behavior of interlayer iron in montmorillonite

Abstract

Fe2+-montmorillonite (Mt) is expected to be one of the major altered clay minerals formed in the engineered barrier system proposed for deep geological disposal of high-level waste (HLW). The high energy radiations imparted by the radionuclides present in HLW as well as the temperature can impact the redox behavior of interlayer iron in clay. Therefore, we have studied the effect of γ- radiation (60Co) on the stability of Fe2+-Mt dispersed in water. Irradiation at a dose rate of 0.6 kGy/h for 18 h resulted in the oxidation of about 14% interlayer Fe2+ as indicated by Fe2+/Fetotal ratio. On further irradiation, these oxidized Fe3+ are reduced back to Fe2+. Such behavior was observed in inert, aerated, and N2O-saturated conditions.The effect of temperature was found to be similar to that of γ-irradiation, particularly on the reduction of interlayer Fe3+ from Fe3+-Mt. When the temperature was increased from 50 to 300 °C, > 90% reduction (Fe3+ → Fe2+) was observed, which is ascribed to the self-reduction mechanism. This sample is similar to that of Fe2+-Mt as evident from XRD, FTIR, TG and magnetic data anaysis. The increase in Fe2+/Fetotal ratio of the heat-treated samples enabled the solid-state reaction with o-phenanthroline (L) by in-situ complexation to form organo-modified Mt. (d001 = 18.5 Å) at room temperature (RT, 27 °C). Furthermore, the presence of Fe2+ in this heat-treated Mt. is evident from their reduction of aqueous CrVI. For instance, 55% of CrVI reduction occurred with Fe3+-Mt@200. On the other hand, the gradual oxidation of interlayer Fe2+ occurred when Fe2+-Mt was heated up to 300 °C. Only about 12% oxidation was observed when heated to 200 °C for 4 h. This is probably due to an equilibrium between oxidation and their self-reduction. In this heat treated sample, about 73% of CrVI reduction occurred, which is comparable to 80% reduction observed with freshly prepared Fe2+-Mt@RT. Thus, the redox behavior of interlayer iron was significantly influenced by γ-irradiation as well as thermal treatment. We believe the observations made here have significant implications on the plausible existence of clay minerals in the geothermal and extraterrestrial environment.