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This study attempts to provide a theoretical evaluation of coprecipitation and fundamental data of binary mixing properties in the barite isostructural family. Mixing properties of binary solid solutions in the barite isostructural family were derived from evaluation of coprecipitation experiments and partitioning coefficients reported in the literature. The Margules parameters, W, for these binary systems correlate well through the relationship, where Δ&Gmacr;n° denotes the non-solvation contribution to the standard partial molal Gibbs free energy of formation for the major (M2+) and trace (Tr2+) aqueous ions, and ΔV is the molar volume mismatch between the two substituting end-members , and V° is the molar volume of end-member components. From this correlation, Margules parameters for the BaSO4-TrSO4 and SrSO4-TrSO4 binary systems, for which no experimental data are available, were estimated. Tr here stands for trace divalent metals and includes Cu2+, Co2+, Zn2+, Fe2+, Mn2+, Eu2+, Cd2+, Ca2+, Sr2+, Pb2+, and Ra2+. These estimated mixing properties allow predictions of miscibility gaps for the BaSO4-TrSO4 and SrSO4-TrSO4 binaries, and partitioning coefficients for Tr2+ between barite or celestine and aqueous solutions. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

We measured barium attachment and detachment fluxes from and to barite crystal surfaces in solutions with [Ba]/[SO 4 ] ratios from 0.06 to 52. The doping of 137Ba in the initial experimental solutions enabled us to measure unidirectional fluxes near solubility equilibrium (Q / K ≈ 1.0) in batch reactors. Both attachment and detachment fluxes increase with [Ba]/[SO 4 ] ratios. As expected, since the solutions were near solubility equilibrium ( S I ≈ 0.0 n), the attachment and detachment fluxes were nearly equal. The experimental data can be fit accurately using the Zhang and Nancollas (1998) (ZN98) process-based AB crystal growth model, which describes crystal growth and dissolution through nucleation and propagation of kink sites. Simultaneous fitting step velocity data of Kowacz et al. (2007) significantly reduced the number of non-uniqueness solutions. The remarkable fit implies that barite recrystallization at room temperature near equilibrium and moderate supersaturation occurs via ion-by-ion addition and removal, rather than the formation of amorphous precursors or aggregation of particles. The model is best fit using different detachment frequencies for Ba and SO 4 , suggesting the ZN98 assumption that detachment frequencies should be the same for the constituent ions does not apply to barite. Based on experimental data and model fitting, Ba attachment and detachment fluxes at a fixed saturation state increase with [Ba]/[SO 4 ] ratios monotonically. This has potentially significant implications for interpreting isotope and trace element cycling in natural environments. [ABSTRACT FROM AUTHOR]