Adsorption of Phosphates on Alumina Irradiated by Neutrons and γ-Rays

Abstract

The purpose of the present work is to study the influence on the surface of aluminium oxide of reactor irradiations. The method used for the purpose is the adsorption of phosphate ions. Earlier results have been published in ref.[1]. ExperimentalThe alumina was a Brockmann-type Merck product, particle size 100-200 mesh. It was dehydrated before irradiations 24 hours at 140 °C. After degassing its nitrogen surface area was 24 m 2 /g. The X-ray analysis indicated an amorphous structure. The aluminium oxide was irradiated in central channels of the Vinßa heavy water reactor. Samples were divided in two groups, one group was irradiated in sealed quartz ampoules, containing air, nitrogen and oxygen or evacuated at 10~2 mm Hg. The second group of samples was irradiated either in unsealed quartz ampoules, or in aluminium cans with screw caps. The latter containers were filled with ordinary air and were not gastight. Neutron fluences were between 10 10 and 10 18 n/cm 2 , the flux density of fast neutrons was 1,5 X 10 12 n/cm 2 s. The absorbed gamma-dose varied from 10 7 -10 9 rads. During activation the temperature rose to 150 °C. (In the graphite channel temperatures were appreciably higher. This irradiation is therefore not comparable to the other ones). Samples were left to cool during one week before measurements were made.Adsorption experiments were carried out in aqueous solutions of Na 2 HP0 4 labelled with 32 P. The pn was kept at 9,5 and the temperature at 22 °C. The equilibration time was two hours, adsorption was determined by the difference in radioactivity of the solution before and after adsorption. A special series of measurements was performed by means of spectrophotometric phosphorus determinations. The results were in satisfactory agreement with the radioactivity measurements. Adsorption measurements of methylene blue were done in aqueous solutions containing from 1 X 10~4 to 1 χ 10~3 mol/1 of that compound. The pn was kept at 6 and the temperature at 22 °C. The equilibration time was 20 hrs. Results and DiscussionAdsorptions isotherms on non-irradiated and irradiated oxides are shown in Fig. 1 for phosphates. For methylene blue similar curves were obtained, with a pseudo-4 Kadiochimica Acta, Band 12, Heft 1 plateau above concentrations of about 0.3 millimolar. The shape of the isotherms is quite similar from different samples. The adsorption capacity is appreciably changed, however, for samples irradiated in the aluminium cans. Fig. 1. Isotherms of phosphate adsorption on alumina. At p H = 9.5, and at 22 + 0,5 °C. a) Non-irradiated; b) irradiated in sealed ampoule (air present); c) irradiated in aluminium can Adsorption capacities of a number of samples irradiated under different circumstances are compared in Table 1.It is seen that all samples which are irradiated with free access to the atmosphere show abnormally large adsorption of phosphates. Adsorption of methylene blue was, however, reduced by irradiation. Post-irradiation annealing of samples was found to decrease adsorption capacity of the oxid...