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Fig. 1.
A schematic diagram illustrating the overall geochemical processes controlling REEs in groundwater.
Fig. 2.
A simplified diagram showing redox processes, complexation, and desorption control on REE in groundwater. The subscript
“
Nor
”
means normalization of
REE using standard reference.
①
REE complexation with anions, which typically generates HREE-enriched pattern due to REE-carbonate complexes (
Luo and Byrne,
2004
);
②
Oxidative scavenging, where Ce and LREE are preferentially adsorbed over HREE leading to negative Ce anomaly and HREE enrichment in groundwater (
De
Carlo et al., 1998; Bau and Koschinsky, 2009
);
③
Preferential desorption of Ce(IV) over Ce(III) from surfaces upon Ce oxidation (
Bau, 1999
);
④
Microbially-mediated
oxidation of Ce(III) to Ce(IV) (
Moffett, 1990
);
⑤
Desorption of REEs from Fe/Mn oxide surfaces along with Ce desorption (
Bau et al., 1998
);
⑥
Reduction of Fe(III) to
Fe(II) forming Fe-S minerals in the presence of sulfide (Luther III et al., 1992;
Roy et al., 2011
);
⑦
Reduction of Ce(IV) to Ce(III) (
Tang and Johannesson, 2006
). REE
mobilization, fractionation, and re-distribution in groundwater serve as a powerful tool to reconstruct the effects of redox processes, complexation, adsorption
and desorption.
Fig. 3.
A diagram illustrating the impact of adsorption onto metal (i.e., Fe, Mn, and Al) oxyhydroxides and organic substances on groundwater REE (Note that
property of metal and organic colloids or particles changes from upstream to downstream. This will result in different REE concentrations and normalized patterns for
groundwater and colloids or particles. Upstream, the oxidative groundwater favors preferential scavenging of LREEs and Ce with metal oxyhydroxides, and thus
LREEs depletion and negative Ce anomaly occur in the groundwater herein. Downstream, the remaining REEs sorbed onto metal colloids or particles are depleted in
LREEs, and negative Ce anomaly decreases. Under the conditions of high particle load downstream, REEs would shift from colloidal facies to particle facies).
Fig. 4.
A diagram illustrating the impact of chemical weathering on groundwater REE (The evolution trend of REE patterns was adopted from
Li et al. (2020)
. The
HREE-enriched patterns observed in groundwater of upper slope primarily result from preferential dissolution of HREE-bearing minerals during weathering under
mildly acidic conditions, while carbonate complexes and their preferential sorption onto mineral surfaces play a critical role in formation of HREE-enriched patterns
in groundwater of lower slope. During migration, groundwater pH value increases, and carbonate concentration increases due to mixing with the alkaline and
carbonate-rich groundwater. This will facilitate the transport of HREE in groundwater).
Fig. 5.
A simplified diagram showing the REE patterns and Gd anomaly in tracing migration of AMD and WWTP effluent in groundwater systems (Note that the
presence of anthropogenic Gd is shown by positive Gd anomaly in normalized REE patterns (REE
NOR
). The appreciable positive Gd in tap water signals that
groundwater should be monitored with continuous efforts, especially in the areas where groundwater is artificially recharged by surface water, and is used for
drinking water supply).
Fig. 6.
Distribution of REE risk expressed as hazard quotient (HQ) in groundwater and tap water (REE data used for risk calculation were taken from
Krohn et al.
(2024), Knappe et al. (2005), Johannesson et al. (2017), Wysocka et al. (2023)
, and Kulaksız and Bau (2011a). The normalized REE pattern in panel (a) is shown with
average REE concentrations, and risk distribution in panel (b) is displayed with average values as well. The specific method for REE risk calculation has been
published in one of our recent studies (
Liu et al., 2024
)).
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