There is growing
concern that rare earth elements (REEs) will become
emerging soil–water contaminants because of their increased
use in new technologies and products, which may lead to unavoidable
release to the environment. To better understand the environmental
behavior of REEs, a comprehensive set of adsorption and column transport
experiments was conducted in quartz sand media. The retention and
mobility of three representative REEs (La, Gd, and Er) were studied
in the presence and absence of humic acid (HA; 5, 20, and 50 mg L
–1
) and under a range of pH conditions (5–8).
Results show that REE mobility and retention are controlled by the
amount of REE–HA complexes formed in a solution, which increases
with increasing HA concentrations and solution pH. Gadolinium is the
most mobile among the representative REEs, followed by Er and La,
corresponding to the amount of (calculated) REE–HA complexes.
Increasing HA concentrations in the REE solution inhibits REE retention
in both the batch adsorption and column experiments. The same retardation
trend was observed for lower HA concentrations (Gd > Er > La).
In
a fixed HA concentration, HA and REE adsorption decrease simultaneously
as the solution pH increases, indicating the co-adsorption of REEs
and HA on the sand. Scanning electron microscopy detection of elongated
regions attached to the sand, where high REE and carbon (HA) concentrations
were measured, further suggests the co-adsorption of REE–HA
complexes. Modeling the column experiments shows that the time-dependent
attachment is dominant at high HA concentrations, while at lower HA
concentrations, both the time-dependent and spontaneous attachments
play equal roles. These results provide a quantitative characterization
of REE retention and mobility in sand media.