COPRECIPITATION IN THE BARITE ISOSTRUCTURAL FAMILY: 2. NUMERICAL SIMULATIONS OF REACTIONS AND MASS TRANSPORT

Abstract

Coprecipitation of barite with trace constituents was simulated with consideration of aqueous speciation and complexation, mixing properties for the binary solid solutions (Zhu, this issue), precipitation and dissolution kinetics, and advective-dispersive transport. Speciation-solubility modeling was used to reproduce BaSO4-RaSO4 coprecipitation experimental results, and to calculate CrO42− aqueous concentrations in equilibrium with a Ba(SO4,CrO4) solid solution. Kinetic reaction path modeling was used to simulate the coprecipitation of barite with RaSO4 to form an onion-like chemically zoned solid upon the cooling of oil field brine.A one-dimensional coupled reactive mass transport model shows a strikingly different transport pattern for the tracer Ra2+, when the dominant attenuation reaction is with solid solution (Ba, Ra) SO4 as compared to the case when it is controlled by pure RaSO4 and barite solids under local equilibrium conditions. A self-enrichment of Ra2+ in the groundwater and aquifer solid matrix—higher concentrations of Ra2+ downstream from the reaction front—results from the coprecipitation reaction and advective-dispersive transport. This self-enrichment process generates a secondary tracer source, which has tracer concentrations higher than that of the original source. On the other hand, coprecipitation reactions can reduce Ra2+ concentrations in groundwater to a much lower level (below ppb) than that of pure RaSO4(c) solubility (near ppm), which has been used to establish the Ra2+ concentration limits in groundwater, soil, and nuclear waste repositories.