Ba attachment and detachment fluxes to and from barite surfaces in 137Ba-enriched solutions with variable [Ba2+]/[SO42−] ratios near solubility equilibrium

Abstract

We measured barium attachment and detachment fluxes from and to barite crystal surfaces in solutions with [Ba]/[SO4] 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]/[SO4] ratios. As expected, since the solutions were near solubility equilibrium (SI≈0.0n), 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 SO4, 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]/[SO4] ratios monotonically. This has potentially significant implications for interpreting isotope and trace element cycling in natural environments.