PNNL

Abstract

Although “intragranular” pore space within grain aggregates, grain fractures, and mineral 24 surface coatings may contain a relatively small fraction of the total porosity within a porous 25 medium, it often contains a significant fraction of the reactive surface area, and can thus strongly 26 affect the transport of sorbing solutes. In this work, we demonstrate a batch experiment 27 procedure using tritium and bromide as high-resolution diffusive tracers to characterize the 28 intragranular pore space. The method was tested using uranium-contaminated sediments from 29 the vadose and capillary fringe zones beneath the former 300A process ponds at the Hanford site 30 (Washington State, USA). Sediments were contacted with tracers in artificial groundwater, 31 followed by replacement of bulk solution with tracer-free groundwater and monitoring of tracer 32 release. From these data, intragranular pore volumes were calculated and mass transfer rates 33 were quantified using a multirate first-order mass transfer model. Tritium-hydrogen exchange 34 on surface hydroxyls was accounted for by conducting additional tracer experiments on sediment 35 that was vacuum dried after reaction. The complementary (“wet” and “dry”) techniques allowed 36 for the simultaneous determination of intragranular porosity and surface area using tritium. The 37 Hanford 300A samples exhibited intragranular pore volumes of ~1% of the solid volume and 38 intragranular surface areas of ~20-30% of the total surface area. Comparison with N2 gas 39 adsorption suggests that this pore space includes both “micropores” ( 2 nm). Intragranular porosity estimates obtained using bromide were 41 significantly smaller, likely due to anion exclusion of Br- from pores with negatively charged 42 surfaces.