The competitive binding of rare earth elements (REE) to humic acid (HA) and carbonates was studied experimentally at various pH and alkalinity values by combining ultrafiltration and inductively ...coupled plasma mass spectrometry techniques. The results show that the REE species occur as binary humate or carbonate complexes but not as ternary REE–carbonate–humate as previously proposed. The results also reveal the strong pH and alkalinity dependence of the competition as well as the existence of a systematic fractionation across the REE series. Specifically, carbonate complexation is at a maximum at pH 10 and increase with increasing alkalinity and with the atomic number of the REE (LuCO
3
≫
LaCO
3). Modeling of the data using Model VI and recently published stability constants for complexation of REE by humic acid well reproduced the experimental data, confirming the ability of Model VI to accurately determine REE speciation in natural waters. This modeling also confirms the reliability of recently published stability constants. This work shed more light not only on the competition between carbonates and HA for REE complexation but also on the reliability of WHAM 6 and Model VI for calculating the speciation of REE with organic matter in alkaline organic-rich water.
Competition between humic acid (HA) and carbonates (Carb) for rare earth elements (REE) complexation.
Arsenic (As) is a toxic and ubiquitous element which can be responsible for severe health problems. Recently, Nano-scale Secondary Ions Mass Spectrometry (nanoSIMS) analysis has been used to map ...organomineral assemblages. Here, we present a method adapted from Belzile et al. (1989) to collect freshly precipitated compounds of the re-oxidation period in a natural wetland environment using a polytetrafluoroethylene (PTFE) sheet scavenger. This method provides information on the bulk samples and on the specific interactions between metals (i.e. As) and the natural organic matter (NOM). Our method allows producing nanoSIMS imaging on natural colloid precipitates, including 75As−, 56Fe16O−, sulfur (32S−) and organic matter (12C14N) and to measure X-ray adsorption of sulfur (S) K-edge. A first statistical treatment on the nanoSIMS images highlights two main colocalizations: (1) 12C14N−, 32S−, 56Fe16O− and 75As−, and (2) 12C14N−, 32S− and 75As−. Principal component analyses (PCAs) support the importance of sulfur in the two main colocalizations firstly evidenced. The first component explains 70% of the variance in the distribution of the elements and is highly correlated with the presence of 32S−. The second component explains 20% of the variance and is highly correlated with the presence of 12C14N−. The X-ray adsorption near edge spectroscopy (XANES) on sulfur speciation provides a quantification of the organic (55%) and inorganic (45%) sulfur compositions. The co-existence of reduced and oxidized S forms might be attributed to a slow NOM kinetic oxidation process. Thus, a direct interaction between As and NOM through sulfur groups might be possible.
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•Distribution of arsenic in re-oxidation compounds within riparian wetland.•Statistical tools for a semi-quantification of colocalizations in nanoSIMS analyses.•PTFE sheets are a suitable tool for environmental sampling.•Speciation of sulfur within re-oxidation compounds for riparian wetlands.
Cerium anomaly development in natural waters is commonly related to the mechanism of oxidative scavenging of tetravalent cerium by iron and/or manganese oxides. In this study, a new mechanism for the ...development of Ce anomalies is described, which combines the oxidation of Ce at high pH by carbonate and the preferential adsorption of Ce(IV) to humic acids. This new mechanism was experimentally elucidated by studying the competition between carbonate and humic acids for complexing rare earth elements (REE). These experiments showed that above pH 8.2, 8.6 or 8.7 (with decreasing alkalinity from 10
−
2
to 10
−
3
mol L
−
1
), Ce(III) is readily oxidized into Ce(IV), which is then preferentially adsorbed onto humic acids. This preferential uptake of Ce results in the development of a negative Ce anomaly (as low as 0.05) in the “truly” dissolved part of the solution (i.e., <
5 kDa), and a complementary positive anomaly (up to 1.22) occurs in the organic colloidal fraction. The positive and negative Ce anomalies remained hidden until the organic and inorganic fractions of the solution were separated. Therefore, Ce anomalies became apparent only after ultrafiltration of the waters and the subsequent isolation of the two fractions. The Ce anomaly is thus more likely to be a proxy of redox conditions in ultrafiltered waters than in unfiltered waters or in waters filtered to <
0.2 µm. The removal (e.g., by coagulation and/or flocculation) of organic molecules in organic-rich alkaline waters might lead to the development of a negative Ce anomaly in the resulting organic-poor waters. In contrast, some organic-poor alkaline waters may develop positive Ce anomalies due to preferential complexation of Ce(IV) by dissolved carbonate.
The competitive binding of rare earth elements (REE) to purified humic acid (HA) and MnO
2 was studied experimentally using various HA/MnO
2 ratios over a range of pH (3 to 8). MnO
2, humic acid and ...REE solutions were simultaneously mixed to investigate the kinetics of the competitive reactions. Aqueous REE–HA complex is the dominant species whatever the experiment time, pH and HA/MnO
2 ratio. The value of the distribution coefficients between MnO
2 and solution (log
K
d
Ree/Mno
2
) increases with the HA/MnO
2 ratio, indicating that part of the REE–HA complexes are adsorbed onto MnO
2. The development of a Ce anomaly appears strongly limited in comparison with inorganic experimental conditions. Throughout the experimental run time, for HA/MnO
2 ratios of less than 0.4, MnO
2 acts as a competitor leading to a partial dissociation of the REE–HA complex. The majority of the dissociated REE is readsorbed onto the MnO
2 surface. The readsorption of REE is expressed by an increased Ce anomaly on the log
K
d
Ree/Mno
2
pattern as well as a change in shape of the coefficient distribution of REE between soluble HA and solution pattern (log
K
d
Ree/HA decrease for the heavy rare earth elements — HREE). Thus, REE are not only bound to MnO
2 as a REE–HA complex, but also as REE(III). Moreover, the competition between HA and MnO
2 for REE binding is shown to be higher at low pH (<
6) and low DOC/Mn ratio. This study partially confirms previous work that demonstrated the control of REE adsorption by organic matter, while shedding more light on the impact of pH as well as complexation reaction competition on long-term REE partitioning between solid surface and organic solutions. The latter point is important as regards to REE speciation under conditions typical of rock and/or mineral alteration.
Shallow groundwater samples (filtered at 0.2μm) collected from a catchment in Western France (Petit Hermitage catchment) were analyzed for their major- and trace-element concentrations (Fe, Mn, V, Th ...and U) as well as their dissolved organic carbon (DOC) concentrations, with the aim to investigate the controlling factors of vanadium (V) distribution. Two spatially distinct water types were previously recognized in this catchment based on variations of the rare earth element (REE) concentrations. These include: (i) DOC-poor groundwater flowing below the hillslope domains; this type has low V contents; and (ii) DOC-rich groundwater originating from wetlands, close to the river network; the latter water type displays much higher V concentrations. The temporal variation of the V concentration was also assessed in the wetland waters; the results show a marked increase in the V content at the winter–spring transition, along with variations in the redox potential, and DOC, Fe and Mn contents.
In order to allow the study of organo-colloidal control on V partitioning in water samples, ultrafiltration experiments were performed at different pore size cut-offs (30kDa, 10kDa and 5kDa). Two shallow, circumneutral waters were sampled: one was both DOC- and Fe-rich and the other was DOC-rich and Fe-poor. In terms of major- and trace-cations and DOC concentrations, the data were processed using an ascendant hierarchical classification method. This revealed the presence of two main groups: (i) a “truly” dissolved group (Na, K, Rb, Ca, Mg, Ba, Sr, Si, Mn, Co, Ni, Cr, Zn and Ni), and (ii) a colloidal group carrying DOC, Fe, Al, Pb, Cu, REE, U, Th and V. Vanadium has an unpredictable behavior; it can be either in the organic pool or in the inorganic pool, depending on the sample.
Moreover, V speciation calculations—using Model VI and SCAMP—were performed on both samples. Speciation modeling showed approximately the same partitioning feature of these elements as compared to ultrafiltration data, namely: a slight change of the V speciation in groundwaters along the studied topographic sequence.
This implies that vanadium in hillslope groundwater wells occurs as a mixing of organic and inorganic complexes, whereas V in wetland groundwater wells comprises mainly organic species. Using the dataset described above, factors such as aquifer–rock composition or anthropogenic input were demonstrated to probably play a minor role in determining the V distribution in shallow groundwaters. Although an anthropogenic impact can be ruled out at this local scale, we cannot preclude a perturbation in the global V cycle. Most likely, the two dominant factors involved are the organic matter content and the redox state either promoting competition with Fe-, Mn-oxides as V carriers in groundwater or not. In this context, it appears challenging to determine whether organic matter or redox-sensitive phases are the major V carriers involved, and a further study should be dedicated to clarify this partition, notably to address the processes affecting large-scale V transport.
► Increase of V content at the winter–spring transition. ► Organic matter and redox of waters are major factors controlling V distribution. ► Vanadium in hillslope groundwaters occurs as a mix of organic and inorganic complexes. ► Vanadium in wetland groundwaters comprises mainly organic species. ► Speciation modeling and ultrafiltration experiments showed the same partitioning.
Iron (Fe) reactivity and arsenic (As) reactivity in wetland soils were studied by applying a generalized dissolution rate law to data recovered from reductive dissolution experiments using ...As-bearing-Fe(III)-oxyhydroxides (ferrihydrite and lepidocrocite). Although As does not correspond to a separate mineral, the kinetic law can be successfully used to investigate the dynamics of As. This was possible as As was coprecipitated in all the tested Fe(III)-oxyhydroxides. The generalized rate law was also applied to available published and here produced datasets of reduction experiments of Fe(III)-oxyhydroxides (with reducing agent: ascorbate, Shewanella putrefaciens, purified soil Fe(III)-reducing bacteria and no purified autochthonous wetland soil bacteria). A comparison of the calculated kinetic parameters and modeling demonstrates that Fe reactivity is strongly increased in the wetland soil as compared to simple bacterial reduction experiments. Dissolved organic matter appears to be a key factor in the control of the Fe(III)-oxyhydroxide dissolution rate. More specifically, organic matter by strongly binding Fe(II) prevents Fe(II) readsorption and subsequent Fe secondary mineral formation, both of which are known to strongly decrease Fe(III)-oxyhydroxide dissolution rates. Arsenic solubilization is driven by Fe dissolution with the extent of the reduction pathway and therefore indirectly by the occurrence of dissolved organic matter. In this type of organic environment, where the formation of Fe secondary minerals is reduced or inhibited, As is not taken up and is thereby strongly solubilized. Therefore, wetlands appear to be favorable areas for the active transfer of As from the soil to both surface- and ground-waters.
► An experimental study of the organic matter impact on Fe(III) and As(V) bioreduction is proposed. ► Fe-oxide dissolution and As solubilization rates were studied using a kinetic modeling approach. ► As solubilization is driven by Fe dissolution and Fe binding by dissolved organic matter. ► Wetlands are privileged zones for active transfer of As from soil to water.
Competitive mechanisms between rare earth elements (REE) and aluminium for humic acid (HA) binding were investigated by combining laboratory experiments and modeling to evaluate the effect of Al on ...REE–HA complexation. Results indicates that Al3+ competes more efficiently with heavy REE (HREE) than with light REE (LREE) in acidic (pH=3) and low REE/HA concentration ratio conditions providing evidence for the Al high affinity for the few HA multidentate sites. Under higher pH – 5 to 6 – and high REE/HA conditions, Al is more competitive for LREE suggesting that Al is bound to HA carboxylic rather than phenolic sites. PHREEQC/Model VI Al–HA binding parameters were optimized to simulate precisely both Al binding to HA and Al competitive effect on REE binding to HA. REE–HA binding pattern is satisfactorily simulated for the whole experimental conditions by the ΔLK1A optimization (i.e. ΔLK1A controls the distribution width of logK around logKMA). The present study provides fundamental knowledge on Al binding mechanisms to HA. Aluminium competitive effect on other cations binding to HA depends clearly on its affinity for carboxylic, phenolic or chelate ligands, which is pH dependent. Under circumneutral pH such as in natural waters, Al should lead to LREE-depleted patterns since Al is expected to be bound to weak HA carboxylic groups. As deduced from the behavior of Al species, other potential competitor cations are expected to have their own competitive effect on REE–HA binding. Therefore, in order to reliably understand and model REE–HA patterns in natural waters, a precise knowledge of the exact behavior of the different REE competitor cations is required. Finally, this study highlights the ability of the REE to be used as a “speciation probe” to precisely describe cation interactions with HA as here evidenced for Al.
Wetlands are specific areas able to regulate metals mobility in the environment. Among metals, rare earth elements (REE) appear to be particularly interesting because of the information that could be ...provided by the REE patterns. Moreover, as REE are becoming a matter of great economic interest, their significant release into the environment may be expected over the next few decades. Wetlands would then play a key role in the regulation of their concentration in the environment. This review demonstrated that REE are released in wetland bound to colloidal organic matter. During the flood season, the released REE concentrations are largely higher than those released during the wet period. This solubilization is related to the organic matter desorption caused by the pH rise imposed by the reducing reactions. The resulting REE patterns depend on the heterogeneity of the humic acid (HA) binding sites and the presence of potential competitive cations, such as Fe(III) and Al(III). At high REE loading, REE are bound to HA carboxylic groups and the pattern exhibit a MREE downward concavity. At low loading, REE are bound to phenolic and chelate groups and the pattern exhibits a lanthanide contraction. At low loading, REE seem to act as cationic bridges between two organic molecules, whereas at high loading they seem to be engaged in strong multidentate bonding. Moreover, the REE patterns can be modified with the competitive cations amount and speciation. The prime factor governing all these processes is pH, which drives the organic colloid production, REE loading and solubility of competitive cations.
To obtain better constraints on the control of seasonal hydrological variations on dissolved organic carbon (DOC) dynamics in headwater catchments, we combined hydrometric monitoring with ...high‐frequency analyses of DOC concentration and DOC chemical composition (specific UV adsorption, δ13C) in soil and stream waters during one complete hydrological cycle in a small lowland catchment of western France. We observed a succession of four hydrological periods, each corresponding to specific DOC signatures. In particular, the rise of the upland water table at the end of the rewetting period yielded to a strong increase of the specific UV absorbance (from 2.5 to 4.0 L mg C−1 m−1) and of the δ13C values (from −29 to −27‰) of the soil DOC. Another striking feature was the release of large amounts of DOC during reduction of soil Fe‐oxyhydroxides at the end of the high‐flow period. Comparison of hydrometric data with DOC composition metrics showed that soils from the upland domains were rapidly DOC depleted after the rise of the water table in these domains, whereas wetland soils acted as quasi‐infinite DOC sources. Results from this study showed that the composition and ultimate source of the DOC exported to the stream will depend on the period within the annual hydrological cycle. However, we found that the aromatic DOC component identified during the high‐flow period will likely represent the dominant DOC component in stream waters on an annual basis, because most of the annual stream DOC flux is exported during such periods.
Key Points
DOC composition in soil and stream waters exhibit strong seasonal variations
Seasonal changes in DOC source pools in wetland soils driven by hydrology
Upland soils quickly depleted during the wet season contrary to wetland soils
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•Comparative study of DOM isolated using two filter types and two pore size filters.•No pore size or filter type effect was observed on bulk scale descriptors.•No filter effect was ...observed on the distribution of the 43 target molecules.•Possibility to compare molecular results for different filter type and porosity.•Breakup of colloids due to forces applied during filtration at 0.2µm.
Biogeochemistry of dissolved organic matter (DOM) is important to ecology, ecotoxicology and the carbon cycle. DOM is operationally defined as the OM fraction that passes through filters. Since different filter pore-sizes, ranging from 0.2 to 0.7μm, are commonly used, it is necessary to test if this choice has an effect on the concentration and composition of DOM in order to ensure the comparison between studies using different filter pore sizes and filter types (cellulose acetate vs glass fibre). The concentration and composition of DOM was investigated along a soil-river continuum in a lowland headwater catchment using two filter pore sizes (0.2 and 0.7μm). Dissolved organic carbon (DOC) was quantified, and the DOM composition was investigated using spectroscopic (specific UV absorbance) and isotopic (δ13C) bulk-scale descriptors and semi-quantitative molecular analysis by thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide coupled to a gas chromatography and mass spectrometry (THMGC-MS). No significant differences were detected between DOM<0.2μm and DOM<0.7μm for bulk scale descriptors. Moreover, at the molecular scale, the distribution of the 43 targeted compounds was not impacted. However their concentrations were slightly higher in the <0.2μm fraction. This could be due to a shield effect of mineral phases in the <0.7μm fraction implying a decrease in their recovery. Consequently, the DOM studies using 0.7 and/or 0.2μm filters can be compared. This similarity between those two pore-sizes is suggested to be due to the breakup of colloids by shear forces applying locally during filtration performed at 0.2μm.
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