Rare earth elements adsorption onto Carrizo sand: Influence of strong solution complexation

To better understand the mobility and transport of rare earth elements (REE) in groundwater flow systems, the influence of carbonate ions as well as humic substances on REE adsorption onto Carrizo Aquifer sand was investigated as a function of CO2 partial pressures (PCO2), pH, ionic strength, and initial REE concentrations. Batch adsorption experiments were carried out at room temperature (23° ± 1 °C) in a glove box under different PCO2 (i.e., 0 < PCO2 ≤ 10− 1.3 atm). Our experiments show that carbonate ions can either increase or decrease REE adsorption depending on the carbonate ion concentrations or the initial REE concentrations. More specifically, the degree that REEs are adsorbed onto the aquifer sand is inversely related to the amount of carbonate species of REEs. For example, an increase of PCO2 from 0 to 10− 3.5 atm, or the presence of carbonate ions in low initial REE concentrations (e.g.,10 μg/kg), generally result in enhanced REE adsorption, owing to the low amount of carbonate species of REEs in solution. With further increase of PCO2 at constant pH (7) and dissolved REE concentrations, solution complexation of REEs with carbonate ions increases and readily outcompetes mineral surface sites for REEs, leading to less REE adsorption. In addition, our experiments show that in the presence of carbonate ions, increasing pH from neutral pH to more alkaline pH leads to decreases in REE adsorption. The presence of humic substances can also either increase or decrease REE adsorption, depending on the pH range of the solution. Humic substances enhance REE adsorption at acidic pH range (i.e., pH < 4) and decrease REE adsorption when pH is above 4. Ionic strength has a negligible effect on REE adsorption in dilute solutions, and only when ionic strength was increased to 0.1 M was a decrease in REE adsorption noticeable. sorption onto Carrizo sand can be successfully described by a model that considers both surface complexation and surface exchange reactions. Specifically, the modeling studies point out the important role that surface exchange reactions play in REE adsorption onto Carrizo sand at high REE loading conditions (i.e., high initial REE concentration at low solid/solution ratio). Model calculations show that strong solution complexation of dissolved REEs with carbonate ions competes for uncomplexed REE ions, [Ln3+]F, where Ln is any of the lanthanides, and thus decreases REE adsorption at high PCO2 levels (> 10− 2.3 atm) or at high pH (> 8.0). Model calculations also indicate that competition between Na+ and uncomplexed REE ions, [Ln3+]F, for exchange sites results in less REE adsorption at high ionic strength (e.g., I = 0.1 M). Both experimental and modeling studies show that REE adsorption onto Carrizo sand is strongly influenced by REE solution complexation reactions, which are sensitive to PCO2 levels, pH, and the amount and type of solution complexing ligands.