SOME INSIGHTS INTO PARTICLE SURFACE ION-EXCHANGE REACTIONS WITH FALLOUT RADIONUCLIDES

It is well-known that surfaces of fine sediment particles are capable of exchanging cations and anions. We are investigating how fallout radionuclides (FRN) ⁷Be, ¹³⁷Cs, and ²¹⁰Pb behave on particle surfaces, and how these isotopes sorb and desorb under experimental and natural conditions. Our first experiment involved comparing the ion exchange capacity of two commonly used ion-exchange resins with that of typical stream bank sediments found in southeastern Pennsylvania (USA). We collected precipitation from a single storm in three, 40 L, acid-washed (70% HN0₃) plastic containers. Ten grams each of 300 micron Amberlite® (MB-20), 40-75 micron Dowex® (50W X8), and stream bank sediment (80% silt, 10% clay, and 10% very fine sand) were added to the rain water and mixed for 15 min using an acid-washed steel paint mixer attached to a hand-held drill. Gamma spectra analyses were done using Mirion Broad-Energy Germanium (BEGe) detectors, and Mirion Technologies, Inc. Apex-Gamma® LabSOCS^TMsourceless calibration software. Results show that Dowex is 4.3 times more efficient in sorbing ⁷Be than Amberlite, and that stream bank sediment is 1.7 times more efficient than Dowex. Similarly, Dowex is 3.7 time more efficient than Amberlite in sorbing ²¹⁰Pb, while stream bank sediment is 5.5 times more efficient than Dowex.

The second experiment compared the efficiency of varying masses of stream bank sediment on the sorption capacity of FRN. Following the same procedures, we collected precipitation from a single storm in four, 40 L plastic containers, adding 10g, 25g, 50g, and 100g of sediment in turn to the four containers. Our results show that the 10g aliquot had the largest sorption yield for ⁷Be and ²¹⁰Pb, followed by the 25g, 50g, and 100 g aliquots in order of descending yield. These data form a linear relationship in ²¹⁰Pb and ⁷Be space, with an R²of 0.99 and a Y-intercept (²¹⁰Pb) of 5.3 x 10^-2Bqg^-1, which is equivalent to the activity of the sample prior to the experiment (5.7 x 10^-2+/- 7.1 x 10^-3 Bqg^-1). Together, these experiments show that small aliquots of stream bank sediments are exceedingly efficient in scrubbing FRN from precipitation, but also indicate dilution of ⁷Be and ²¹⁰Pb with increasing sediment concentration. The next phase of our research explores how ¹³⁷Cs might desorb in the presence of ²¹⁰Pb-enriched precipitation.