Evaluation of the Movement of Sodium Phosphate Through a Silicon Oxide Matrix Using Thin Layer Chromatography Coupled with Secondary Ion Mass Spectrometry

Potential migration of subsurface contaminants into the underlying aquifer is a concern at the Idaho National Engineering and Environmental Laboratory. One approach to attenuate migration would be to create a calcium phosphate barrier, which might be accomplished serially infusing the subsurface with soluble phosphate and then calcium to form a precipitate barrier. To do this, the migration of soluble phosphate and calcium through mineral oxide matrices must be understood. In the present study, Thin Layer Chromatography (TLC) and Secondary Ion Mass Spectrometry (SIMS) are used to investigate the mobility of soluble phosphate. TLC plates were used to measure mobility, and SIMS analysis of the developed plates tracked phosphate mobility. In SIMS, a sample removed from a developed TLC plate was bombarded by energetic ions that disrupted the surface and caused fragments of molecules on the surface to be sputtered into the gas phase, where they are sorted by mass and detected. The resulting mass spectra were correlated with the phosphate species under investigation. The extent of mobility was assessed, and rates of solvent flow as a function of the solute were measured. The results show that the solute slowed the rate of aqueous infiltration through cellulose and silica matrices. Rate of infiltration decreased in the order: water . calcium nitrate > sodium phosphate. in addition, three different sodium phosphate species were evaluated, and the rate of infiltration was found to decrease in the order: NaH2PO4 > Na2HPO4 > Na3HPO4. Phosphate displayed a broad, regular distribution with retention factors from about 0.3 to 0.8. In contrast, sodium displayed a bimodal distribution, with an abundant peak at Rf ∼ 0.3, and a faster-moving, broad distribution that ranged from 0.3 to 0.8. These observations show that phosphate and a faction of the sodium could be moved at a fast rate through the matrix, but a second fraction of the sodium was more strongly retained. SIMS detection limits of sodium on TLC plates were also evaluated. The abundance of Na+ increased nearly linearly with increasing concentration from 0.038 mM to 2.5 M Na3HPO4. Above 2.5 M, Na+ abundance did not increase, indicating surface saturation above this concentration. The results indicated that soluble phosphate could be readily moved through the silica matrix, which might enable barrier formation by subsequent infusion of calcium nitrate. This latter experiment is the next stage for this research