Reactive transport of Rare Earth Elements: surface complexation with quartz surfaces and modeling investigations
Résumé
Rare Earth Elements (REE) are not only considered as high-
technology components but also as contaminants of emerging
concern creating long-term ecological effects when released into
ecosystems. However, there is a lack of comprehensive
investigations on their mobility, environmental fate and transport
in geochemical settings. In addition, most of the previous studies
reported adsorption of one or a few REE onto minerals, and
knowledge on dynamic changes of the entire REE patterns in
natural porous media is scarce. Here, we have thoroughly
examined the interactions of the entire REE group with 100-300
μm quartz sand particles under water-saturated flow conditions.
Both batch and dynamic column experiments are conducted at
environmentally relevant geochemical conditions.
Adsorption of REE on the quartz surface was investigated over
a wide range of pH and varying REE concentrations. As typically
encountered for cations, REE binding increased with increasing
pH values. Results from batch experiments showed that there is a
stronger adsorption for middle REE as compared to light REE or
heavy REE. A surface Complexation Model (SCM) based on
inner-sphere coordination with silanol groups has been
developed for quartz sand, which successfully predicted both
sorption edges and sorption isotherms.
We also observed the REE competitive binding in the column
experiments where the middle REE are preferentially adsorbed
compared to the light and heavy REE. Tracer experiments
showed that a homogeneous flow predominates in the sand-
packed column. We then developed a transport model using
batch-derived surface complexation constants and flow
parameters, which account for physical and chemical processes
in the column system. This combined model can well describe
the breakthrough curves of REE in sand columns under various
hydrochemical conditions. These results have strong implications
for an accurate assessment of the transport behavior of REEs in
natural systems.