Dynamics of Clay Nanoparticle-Associated Trace Metals from Soil to Groundwater: Insights from Contrasting Geological Settings.

Fluid infiltration plays a crucial role in transporting dissolved elements and may serve as a pathway for nanoparticles from soil surface to the subsurface. Smectite-type nanoparticles, as a key soil mineral component, can act as efficient carriers of cations due to their negative surface charge and large specific surface area. This study aims to understand the dynamics of smectite-type nanoparticles-associated trace metal, focusing on rare earth elements (REEs), from soil to groundwater at two contrasting sites in Thuringia, Germany, namely the Hainich Critical Zone Exploratory (carbonate/siliciclastic bedrock) and Saale-Elster-Sandsteinplatte Observatory (siliciclastic bedrock). Engineered Ni-montmorillonite (Ni-mnt) nanoparticles, synthesized hydrothermally as described by (Reinholdt et al., 2013) were used as tracers.

Nanoparticle migration requires stability against aggregation, influenced by pH, ionic strength, and natural organic matter (NOM). The effect of above-mentioned parameters on stability of Ni-mnt was investigated under controlled conditions in synthetic waters simulating surface-to-subsurface transitions and natural waters from lysimeter and well samples at both sites. Stability was assessed using dynamic light scattering (DLS), while REE adsorption and dissolved organic carbon (DOC) were evaluated with Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Liquid Chromatography – Organic Carbon Detection – Organic Nitrogen Detection (LC-OCD-OND), respectively.

As expected, Ni-mnt stability decreases in Calcium-rich environments and increases in high pH and NOM-rich environments as indicated by the critical coagulation concentration (Ca-CCC). Without NOM, Ca-CCC values of Ni-mnt were in the range of 2.5 mM to 5 mM in the pH range 5 to 8. In contrast, in the presence of NOM, (3.3 mg/L of [DOC]), Ca-CCC values rose to 8 mM at pH 5 and 6, and 15 mM at pH 7 and 8. As revealed by LC-OCD-OND measurements Ni-mnt stabilization is likely due to an association of high molecular weight DOC such as biopolymers and humics.

REEs preferentially adsorb onto organics rather than Ni-mnt under the competitive conditions chosen. Desorption experiments show that light REEs are stronger bond by Ni-mnt (slower reversibility kinetics).

These results highlight the critical role of NOM, particularly biopolymers and humics, in stabilizing clay nanoparticles and influencing REE transport. While NOM reduces aggregation under low to moderate ionic strengths, high ionic strength induces aggregation through cation bridging.

 

Reference

Reinholdt, M. X., Brendle, J., Tuilier, M. H., Kaliaguine, S., & Ambroise, E. (2013). Hydrothermal Synthesis and Characterization of Ni-Al Montmorillonite-Like Phyllosilicates. Nanomaterials (Basel), 3(1), 48-69. https://doi.org/10.3390/nano3010048