The nanoscale size of plastic debris makes them potential efficient vectors of many pollutants and more especially of metals. In order to evaluate this ability, nanoplastics were produced from ...microplastics collected on a beach exposed to the North Atlantic Gyre. The nanoplastics were characterized using multi-dimensional methods: asymmetrical flow field flow fractionation and dynamic light scattering coupled to several detectors. Lead (II) adsorption kinetics, isotherm and pH-edge were then carried out. The sorption reached a steady state after around 200 min. The maximum sorption capacity varied between 97% and 78.5% for both tested Pb concentrations. Lead (II) adsorption kinetics is controlled by chemical reactions with the nanoplastics surface and to a lesser extent by intraparticle diffusion. Adsorption isotherm modeling using Freundlich model demonstrated that NPG are strong adsorbents equivalent to hydrous ferric oxides such as ferrihydrite (log Kadsfreundlich=8.36 against 11.76 for NPG and ferrihydrite, respectively). The adsorption is dependent upon pH, in response to the Pb(II) adsorption by the oxygenated binding sites developed on account of the surface UV oxidation under environmental conditions. They could be able to compete with Fe or humic colloids for Pb binding regards to their amount and specific areas. Nanoplastics could therefore be efficient vectors of Pb and probably of many other metals as well in the environment.
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•Pb(II) binding experiments onto nanoplastics extracted from environmental micro-plastics were performed.•Pb(II) was largely bound onto nanoplastics by specific adsorption and intraparticle diffusion.•Nanoplastics could be significant vectors of metals in the environment.
The complexation of Fe(II) with organic matter (OM) and especially with humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range ...varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HA depending on the pH. Experimental data were used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA=2.19±0.16, log KMB=4.46±0.47 and ΔLK2=3.90±1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose and Waite, 2003). They were then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe2+; Fe(OH)+ appears at basic pH (from pH 8.13 to 9.9). The occurrence of OM and the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As.
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•We measured Fe(II)-humic acid adsorption isotherm and pH sorption edge.•Results were modeled using a coupling of PHREEPLOT–PHREEQC-Model VI.•We determined the binding parameters for Fe(II)-humic acid complex.•We validated the binding parameters using LFERs and published datasets.•We determined the speciation of Fe(II)-humic acid complexes using PHREEQC-Model VI.
Transmission electron microscopy micrographs of Fe nanoparticles: (a) and (b) correspond to pure Fe system; whereas (c) and (d) correspond to Fe–HA system. Display omitted
► New data demonstrating ...the impact of HS on the formation and reactivity of Fe oxides. ► Humic substances reduce the extent of the Fe oxidation-hydrolysis reaction. ► Humic substance directly impact the size and the nature of formed Fe oxides. ► A fraction of Fe does not contribute to the genesis of nanoparticles. ► Mixed Fe nanoparticles-organic colloids are much more bioavailable than Fe oxides.
Few studies have so far examined the kinetics and extent of the formation of Fe-colloids in the presence of natural organic ligands. The present study used an experimental approach to investigate the rate and amount of colloidal Fe formed in presence of humic substances, by gradually oxidizing Fe(II) at pH 6.5 with or without humic substances (HS) (in this case, humic acid – HA and fulvic acid – FA). Without HS, micronic aggregates (0.1–1μm diameter) of nano-lepidocrocite is obtained, whereas, in a humic-rich medium (HA and FA suspensions at 60 and 55ppm of DOC respectively), nanometer-sized Fe particles are formed trapped in an organic matrix. A proportion of iron is not found to contribute to the formation of nanoparticles since iron is complexed to HS as Fe(II) or Fe(III). Humic substances tend to (i) decrease the Fe oxidation and hydrolysis, and (ii) promote nanometer-sized Fe oxide formation by both inhibiting the development of hydroxide nuclei and reducing the aggregation of Fe nanoparticles.
Bioreduction experiments demonstrate that bacteria (Shewanella putrefaciens CIP 80.40T) are able to use Fe nanoparticles associated with organic matter about eight times faster than in the case of nano-lepidocrocite. This increase in bioreduction rate appears to be related to the presence of humic acids that (i) indirectly control the size, shape and density of oxyhydroxides and (ii) directly enhance biological reduction of nanoparticles by electron shuttling and Fe complexation. These results suggest that, in wetlands but also elsewhere where mixed organic matter-Fe colloids occur, Fe nanoparticles closely associated with organic matter represent a bioavailable Fe source much more accessible for microfauna than do crystallized Fe oxyhydroxides.
Colloids have been recognized as key vectors of pollutants in aqueous environment. Amongst them, those formed by iron (Fe) and organic matter (OM) are of major importance due to their ubiquity in the ...surface environment and strong affinity for metals. In the recent years, Fe stable isotopes have been increasingly used to elucidate the sources and biogeochemical cycling of Fe in Earth's surface environments. In this study, we aim to elucidate (i) the possible Fe isotopic signature resulting from the Fe/OM colloid formation and (ii) the mechanisms involved in the development of such isotopic signature. For this purpose, Fe-OM associations were synthesized through binding and titration experiments. Various pH levels were used in order to study the isotope behavior of Fe occurring as free species at pH 1, as Fe-OM complexes at pH 2 and as mixed Fe-oxyhydroxide/OM nanoaggregates or particles at pH 6.5. Organic matter-free, Fe-free and OM membrane-deposition experiments were also performed. These suspensions were (ultra)filtered at 0.2 µm, 30 kDa and 5 kDa to evidence the possible Fe isotope fractionation between fractions. This protocol allowed also testing the potential of (ultra)filtration techniques to generate isotope fractionation. The results provided evidence that abiotic Fe precipitation, (ultra)filtration techniques and OM deposition were not able to produce significant Fe isotope fractionation under the experimental conditions. However, at circum-neutral pH, the Fe-OM binding and titration experiments displayed a significant enrichment of heavy Fe isotopes in the <30 kDa fractions relative to the total Fe pool δ56Fe = 0.35 ± 0.05‰ and 0.26 ± 0.05‰ (95% confidence interval, 2σ and relative to international standard IRMM-14), respectively. Mass balance and error propagation calculation showed Fe isotope fractionation in binding and titration experiments between the >30 kDa and <30 kDa fractions for −0.35 ± 0.05‰ and −0.27 ± 0.05‰, respectively. This Fe isotope fractionation could be due to the complexation of Fe by OM in the <30 kDa fractions. At pH 2, the OM-free experiment, the <30 kDa fraction showed Fe isotope ratio δ56Fe = 0.75 ± 0.03‰ with an enrichment in heavy Fe isotopes of δ56Fe’ = 0.14 ± 0.04‰ relative the total Fe pool (δ56Fe’ is δ56Fe value which was corrected by δ56Fe of total fraction). This enrichment in heavy Fe isotopes induced an isotopic fractionation factor of −0.87 ± 0.26‰ between the >30 kDa and <30 kDa fractions produced by the complexation between the heavy Fe isotopes and OH− ligands in the <30 kDa fraction. The natural Fe-OM associations were further investigated through oxidation experiments of a reduced wetland soil solution. The oxidized soil solution was (ultra)filtered at 5 µm, 3 µm, 0.2 µm, 30 kDa and 5 kDa. The highest δ56Fe was obtained in the smallest size fraction, i.e. <5 kDa fraction, yielding a negative isotopic fractionation Δ56Fe >5kDa − <5kDa = −0.23 ± 0.08‰ suggesting that Fe heavy isotopes are preferentially bound to small humic OM molecules in the form of Fe monomers or small clusters. This study highlights the importance of organic matter for metals’ isotopic systems.
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.
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.
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.
The Humic Ion Binding Model VI (Model VI) – previously used to model the equilibrium binding of rare earth elements (REE) by humic acid (HA) – was modified to account for differences in the REE ...constant patterns of the HA carboxylic and phenolic groups, and introduced into PHREEQC to calculate the REE speciation on the HA binding sites. The modifications were shown to greatly improve the modeling. They allow for the first time to both satisfactorily and simultaneously model a large set of multi-REE experimental data with the same set of equations and parameters. The use of PHREEQC shows that the light rare earth elements (LREE) and heavy rare earth elements (HREE) do not bind to HA by the same functional groups. The LREE are preferentially bound to carboxylic groups, whereas the HREE are preferentially bound to carboxy–phenolic and phenolic groups. This binding differentiation might lead to a fractionation of REE–HA patterns when competition between REE and other metals occur during complexation. A survey of the available data shows that competition with Al
3+ could lead to the development of HREE-depleted HA patterns. This new model should improve the hydrochemical modeling of the REE since PHREEQC takes into account chemical reactions such as mineral dissolution/precipitation equilibrium and redox reactions, but also models kinetically controlled reactions and one-dimensional transport.
Dynamic structure of humic substances according to pH.
Whereas humic substances are known to play a key role in controlling metal speciation and trace element mobility within soils and waters, the ...understanding of their structure is still unclear and remains a matter of debate. Several models of humic substance structure have been proposed, where humic substances were composed of either: (i) macromolecular polyelectrolytes that can form molecular aggregates or (ii) supramolecular assemblies (molecular aggregates) of small molecules without macromolecular character, joined together by weak attraction forces. This experimental study was designed and dedicated: (i) to follow the size of organic molecules versus ionic strength or pH by the combined means of ultrafiltration and aromaticity data and rare earth element (REE) fingerprinting, and (ii) to investigate the pH and ionic strength effect on the distribution of associated rare earth elements in soil solution. This study supports the presence of supramolecular associations of small molecules and probably the presence of macromolecules in the bulk dissolved organic matter. By contrast to ionic strength, pH appeared to be the major parameter playing on the stability of the humic substance structure. Humic substances displayed dynamic structures, which evolved with regard to pH. Low pH led to a destabilization of the humic substance conformation. This destabilization had an impact on the trace element distribution in soil solution, as assessed by REE data, and conversely, the destabilization degree of humic substances seemed to be influenced by the metal ion charge.
Adsorption experiments of rare-earth elements (REE) onto hydrous ferric oxide (HFO) were performed to evaluate the impact of organic complexation on both REE(III) adsorption and the Ce(III) oxidation ...rate. Scavenging experiments were performed at pH 5.2 with NaCl and NaNO
3 solutions containing either free REE (III) or REE(III)–humate complexes. The log
K
d
REE patterns obtained from HFO suspensions exhibit a slight positive Ce anomaly and an M-type lanthanide tetrad effect, in contrast with the partitioning between REE(III)–humate complexes and HFO, which yields completely flat distribution patterns. The “organic” partitioning runs yield log
K
d
REE
organic
/log
K
d
DOC ratios (DOC=dissolved organic carbon) close to 1.0, implying that the REE(III) and humate remain bound to each other during the adsorption experiment. The lack of any positive Ce anomaly or M-type lanthanide tetrad effect in the organic experiments seems to reflect an anionic adsorption of the REE–humate complex. Adsorption onto HFO takes place via the humate side of the REE(III)–humate complexes. The oxidation of Ce(III) by Fe(III) and the proportion of surface hydroxyl groups coordinated to REE(III) at the HFO surface are the two most commonly invoked processes for explaining the development of positive Ce anomalies and the M-type tetrad lanthanide effect. However, such processes cannot proceed since the REE are not in direct contact with the HFO suspensions, the latter being shielded by PHA. The present results further complicate the use of Ce anomalies as reliable paleoredox proxies in natural precipitates. They are also further demonstration that organic matter may inhibit the lanthanide tetrad effect in geological samples.
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