Aluminum effect on dissolution and precipitation under hyperalkaline conditions: I. Liquid phase transformations

The Hanford Site has been contaminated by hyperalkaline, saline fluids containing high levels of aluminum, which can influence the chemical behavior of sediments. While prior research has shown that such fluids can cause mineral dissolution and release of elements like silicon and iron, the specific role of aluminum in these processes remains unclear. Existing studies have not fully addressed how aluminum affects the kinetics of mineral dissolution and precipitation under hyperalkaline conditions. This uncertainty limits the ability to predict how contaminants like cesium and uranium might move through the soil. The study of aluminum’s influence is critical for understanding long-term environmental impacts. No prior work has resolved the interaction between aluminum concentration and the dissolution of potassium-bearing minerals. This gap motivated the current investigation into the effects of aluminum on mineral transformations in hyperalkaline environments. The need to better understand these processes is driven by the potential for radionuclide mobility in contaminated soils.

This study aimed to evaluate how aluminum affects the dissolution and precipitation of minerals in hyperalkaline, saline fluids at the Hanford Site. The researchers sought to determine the role of aluminum in altering the rates of mineral dissolution and the formation of secondary phases. The specific problem addressed is the lack of understanding regarding how aqueous aluminum concentrations influence the release of elements like silicon, iron, and potassium from soil minerals. The motivation stems from the need to predict how contaminants might behave in such environments. The study focused on the first 48 hours of mineral-fluid interactions, as this period is critical for initial dissolution processes. The researchers also aimed to quantify how aluminum affects the kinetics of these transformations. By isolating the effects of aluminum in metal- and glass-free systems, the study sought to provide a clearer picture of its role in mineral transformations. The ultimate goal was to improve predictions of radionuclide mobility in contaminated sediments.

The researchers conducted batch experiments using metal- and glass-free systems to isolate the effects of aluminum. The experiments were carried out at 323 K under CO2- and O2-free conditions to simulate hyperalkaline environments. The study focused on the dissolution of soil minerals and the formation of secondary phases. The experimental setup allowed for the measurement of element release into the soil solution over time. The researchers monitored the release of silicon, iron, and potassium as indicators of mineral dissolution. They also tracked the formation of secondary phases, such as alumino-silicate compounds. The experiments were designed to determine how aqueous aluminum concentrations influence dissolution rates. The results were analyzed to calculate dissolution and precipitation rates based on element concentrations in the solution.

The study found that base-induced dissolution of soil minerals was rapid in the first 48 hours, with immediate release of silicon and iron into the soil solution. Potassium release lagged, with increased dissolution of K-bearing minerals after 2 to 3 days. Silicon and iron release were highly dependent on aqueous aluminum concentration, with rate orders less than -1. Initial potassium release showed lower dependence on aluminum, with fractional rate orders. Dissolution rates based on silicon release ranged from 29.47 to 4.35 × 10⁻¹² mol m⁻² s⁻¹ depending on aluminum concentration. Aluminum participated in the formation of secondary phases, with precipitation rates of 10⁻⁸ mol s⁻¹. The overall precipitation rate of alumino-silicate phases was likely controlled by aqueous silicon concentration, with rates of 10⁻⁹ and rate constants between 0.0054 and 0.0084 h⁻¹. These findings suggest that aluminum can significantly influence the chemical transformations in hyperalkaline sediments.

The authors concluded that aluminum plays a significant role in altering the dissolution and precipitation processes in hyperalkaline environments. The study demonstrated that aluminum can inhibit mineral dissolution by reducing free hydroxide concentration in the solution. The findings suggest that aluminum may slow the release of silicon and iron from soil minerals. The delayed release of potassium indicates that K-bearing minerals dissolve more slowly in the presence of aluminum. The study also showed that the formation of secondary phases is influenced by both aluminum and silicon concentrations. The researchers propose that the overall precipitation rate of alumino-silicate phases is controlled by aqueous silicon levels. These results may have implications for the mobility of radionuclides like cesium and uranium in contaminated sediments. The authors suggest that future work should focus on the long-term effects of aluminum on mineral transformations.