The co-precipitation of ions into solid solutions has long been studied for their applications in wastewater treatment, the storage of radioactive waste, and to understand the cycling of metals in the environment. Hazardous ion removal through co-precipitation into barite (BaSO4), for example, is one viable, cost-effective method of wastewater treatment. Here, we focus on modeling Sr co-precipitation into barite directly from an aqueous solution. A geochemical model was developed to understand the controls on Sr incorporation into barite particles, the distribution of particle sizes produced from the reaction, and the total Sr removal from solution. The model incorporates nucleation and growth of particles, along with Sr partitioning into the solid phase as barite particles form through the attachment of ions onto its surfaces. Simulations examined co-precipitation at a low barite saturation index (SI) of 2.34 in comparison to a high barite saturation index of 3.80. At high SI, barite particles are more uniform in size and contain low amounts of Sr. In contrast, simulations at low SI show zones of Sr-richness forming near the end of the precipitation reaction, leading to Sr-rich edges on barite particles. The number of particles formed and the periods of high Sr incorporation result in higher removal of Sr at low SI. These findings corroborate experimental results by Hunter et al. (2020), in which nano-X-ray fluorescence maps of co-precipitated (Sr,Ba)SO4 particles under the same solution conditions produced relatively uniform, ~6 µm-sized Sr-poor particles at high SI, while experiments at low SI produced relatively diverse particles ranging in the extent of Sr incorporation and in particle sizes of ~2-8 µm. The percentage of Sr removal from solution was also higher at low SI. Our model can improve predictions of metal removal from co-precipitation reactions, which can ultimately help determine optimal metal removal solution conditions for applications such as wastewater treatment systems.

Modeling the nucleation, growth, and co-precipitation of Sr into barite particles

ACS Spring 2022

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