PB DESORPTION AT BARITE-WATER INTERFACES USING RESONANT ANOMALOUS X-RAY REFLECTIVITY

High concentrations of toxic metals, such as lead (Pb), in the environment can cause severe ecological and human health impacts. Adsorption and desorption processes at mineral-water interfaces control retention and release of ions at the surfaces, affecting the mobility of toxic metals in groundwater. Here, we measured desorption of lead (Pb) ions at the barite (001) - water interface using in situ x-ray reflectivity (XR) to understand the effects of Pb on surface structure and resonant anomalous x-ray reflectivity (RAXR) to resolve the amount of sorbed and desorbed Pb.

Barite (BaSO₄) is often used as a weighting agent in drilling pipes during oil recovery and is also utilized in industrial fracking which affects groundwater systems and aquifers. Based on previous studies on barite, Pb²⁺ and Sr²⁺ ions adsorb and incorporate into the uppermost layers of the barite (001) surface. However, little surface distortions were observed in the presence of strontium solution relative to high concentrations of Pb solution. This suggests barite is effective as a host for the sequestration of Pb even under conditions where the Pb-bearing phases remain undersaturated in the bulk solution.

While the sorption behavior of Pb at barite is known, it is essential to study the desorption mechanisms at barite crystal to better understand the potential for rerelease of toxic metals back into the environment. In situ XR and RAXR measurements were conducted using a transmission cell to control the solution chemistry. The results show that ~50% of the sorbed lead (Pb) species desorb after reacting the barite sample with Pb free solution (BSS) for 0.5 hr. Three additional measurements made in BSS indicate that amount of desorbed Pb increases over time Pb free barite saturated solution (BSS). However, even after reacting for 12 hours, 31% of the sorbed Pb still remains.

The desorption experiments suggest that at [Pb] ≥ 225µM incorporated species are resistant to desorption. While incorporation of Pb is a relatively fast process, the reverse process (exchange of Ba for incorporated Pb) is slow. The incorporation of Pb within the barite surface leads to stabilization of Pb, which is partially irreversible, further making barite an ideal host mineral for Pb sequestration. In conclusion, desorption results demonstrate the reproducibility and partial irreversibility of the Pb²⁺ ions at the barite (001) surface.