COLLOIDALLY MEDIATED ARSENIC TRANSPORT EXPERIMENTS ANALYZED WITH SYNCHROTRON X-RAY TECHNIQUES

We investigated the mobility of As-laden colloids through sequentially encountered porous matrices with contrasting isoelectric points, quartz and calcite sands. In one experiment, colloid suspensions of ripidolite, gibbsite, natural soil colloids, and synthetic goethite + hematite, prepared in 100 µM sodium arsenate solutions at pH 5, were successively pumped through the matrices. Sampling ports tapped each matrix separately. Using synchrotron X-ray diffraction and X-ray absorption spectroscopy together with ICP-MS analysis of influents and effluents, we observed preferential trapping of ripidolite in the calcite matrix, and gibbsite in the sand matrix, in the initial steps. Soil colloids passed through both matrices. Goethite was retained in both matrices. Arsenic tended to remain affiliated with ripidolite and goethite; less was retained in the quartz matrix than in the calcite matrix.

In another experiment, real-time X-ray diffraction was performed on “mini-columns” mounted on an electronically controlled mobile sample stage. A colloidal iron oxide suspension was pumped by a motorized syringe through polycrit tubes, encountering quartz and marble sand in succession. Two-dimensional XRD patterns were collected at five points along the column, including at the boundary between quartz and marble, every 10 minutes during the course of the experiments. Colloid suspensions and the coarse matrix materials produce different characteristic diffraction patterns. The small size and numerous crystallites of colloids leads to the production of uniform Bragg cones, recorded on the CCD detector plate as rings, whereas the larger matrix grains produce oriented Bragg reflections, recorded as spots. Consequently, the appearance and disappearance of rings in the otherwise spotted diffraction pattern provides time-resolved visual evidence for accumulation and dissipation of colloids through the matrix. Iron oxides accumulated temporarily in both matrices and remained at the quartz-calcite interface.