Influence of surface charge of an Fe-oxide and an organic matter dominated soil on iodide and pertechnetate sorption

Abstract

Summary Iodine-129 and technecium-99 are commonly the largest contributors to the calculated health risk associated with long-term nuclear-waste burial. The high proportion of risk from these radionuclides is due to their large inventories in many types of waste, long half-lives, and the perception that they are highly mobile in sediments. In most aquifer systems, these radionuclides exist as anions, iodide (I-) and pertechnetate (TcO4 -), and sorb poorly to soils that possess a net negative charge. A series of iodide and pertechnetate sorption experiments were conducted over a pH range of 3 to 9. The two soils used in this study possessed a pH-dependent charge; one soil was collected from a wetland and derived most of its charge from organic matter, whereas the second soil was collected from an upland site and derived most of its charge from Fe/Al-oxide coatings. Although both soils had nearly identical particle size distributions, pH values, and mineral compositions, they had dissimilar surface charge and I- and TcO4 - sorption behavior. The pH where the wetland soil did not have any net charge (more specifically, the Point-of-Zero-Salt Effect) was 4.4. The pH where the upland soil did not have any net charge was 4.1. Under ambient conditions, the wetland soil had a pH of 4.2 and a slight positive net charge of +0.1meq/100g. The upland soil had a natural pH of 5.0 and a net charge of -0.25meq/100g. Both iodide and pertechnetate sorbed appreciably more to the wetland soil than to the upland soil, likely the result of more anion sorption sites derived from the organic matter in the wetland soil. In both soils, iodide sorption was greater and exhibited a greater pH-dependency than pertechnetate sorption. Pertechnetate exhibited anion exclusion (negative K d values) or no sorption at pH values above the Point-of-Zero-Salt Effect. Iodide sorption decreased markedly as the pH increased to the zero-point-of-charge, and remained largely unchanged at pH values above the zero-point-of-charge.