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Dynamics of arsenic at hydrothermal spring outlets: role of Fe oxyhydroxides and carbonates
1BRGM, 1, rue des Saumonières, 44000 Nantes, France
2BRGM, 3, avenue Claude Guillemin, BP 6009, 45060 Orléans cedex 2, France
Available online 1 November 2007.



The present chapter deals with the dynamics of arsenic (As) in a relative simple natural system consisting of hydrothermal spring outlets showing precipitation of iron (Fe) oxyhydroxides and carbonates. It was determined through a complete study comprising: (a) observation, (b) laboratory and field experimental partitioning studies and (c) geochemical modelling. The focus was on inorganically mediated geochemical processes, especially on the determination of the precipitation of mineral phases. Iron oxyhydroxides clearly show to control the fate of As through adsorption processes. Carbonates do not trap As, but play nevertheless an indirect role in the system. Their precipitation leads to the increase of pH causing desorption of As, which has been observed in the laboratory experiment. Field observations indicate, however, a stability of the As–Fe oxyhydroxide association. Arsenic remains linked to Fe oxyhydroxides in the travertine deposits associated to the springs as well as in the surrounding soils. The adsorption process appears very rapid, with around 90% of As being trapped within several minutes.

19.1. Introduction

The dynamics of As in soil and groundwater environment are linked to its partitioning between aqueous and solid phases. The partitioning is controlled by several processes including mineral precipitation/dissolution, adsorption/desorption, oxido-reduction and biological transformation. Adsorption is often the main process controlling the fate of As. It is known to occur on metal oxides and oxyhydroxides (iron (Fe), aluminium (Al) and manganese (Mn)) as well as on silica, clay minerals, carbonates and humic acids (Stollenwerk, 2003). The adsorption of As depends on the properties of the solid surface, pH, the concentration of arsenic and competing ions (phosphate, sulphate, silica, organic ligands, calcium (Ca) and magnesium (Mg)), as well as on As speciation (ibid.). Iron oxyhydroxides are well known to have a great adsorption potential for As. Due to their common presence in groundwater substrate and in soils, they are expected to play a major role in the fate of As in the environment. Due to complexity of natural systems (e.g. coating, mixture of minerals, heterogeneity, large number of trace elements in the liquid phase, etc.), it is rather difficult to verify the dominated process.
The saline and carbo-gaseous hydrothermal spring waters from the Cézallier area, French Massif Central ( [Criaud and Fouillac, 1986] and [Beaucaire et al., 1987] ), offer an exceptional opportunity to study the dynamics of As in a natural system. The CO2 degassing and the oxidation of the waters lead indeed to the precipitation of Fe oxyhydroxides and carbonates, which commonly form travertine deposits. Arsenic, that is present in significant concentrations in the spring waters, is partitioned in the solid phases.
More particularly, the partition of As during the simultaneous precipitation of Fe oxyhydroxides and carbonates was determined at the outlet of springs through a complete study that comprised: (a) observation of the travertine deposits and soils, (b) laboratory and field experimental partitioning studies and (c) geochemical modelling. In this context, the focus is on inorganically mediated processes.

19.2. The study site

19.2.1. Geography and geological setting

The studied thermo-mineral spring is 1 of the 50 cold mineral springs identified over the last 20 years in the Cézallier area (Fouillac, 1983). The Cézallier area belongs to an active natural hydrothermal system located in the north of the French Massif Central (Fig. 19-1). The geological setting of the Cézallier area is primarily made of a Variscan metamorphic basement, partially covered by Quaternary alkaline basalt bedrock (Feuga, 1987). The springs usually rise through fractures of the metamorphic basement ( [Criaud and Fouillac, 1989] and [Négrel et al., 2000] ). This is the case of the studied spring, named Le Bard, which lies along a fault and rises through heterogeneous anatexite ( [008] and [Cornu et al., 2001] ).

Fig. 19-1.
Location of (a) the Massif Central in France, (b) the Cézallier region in the Massif Central (France), (c) the studied spring in Cézallier

19.2.2. Spring waters

The spring waters, of neutral pH (6–7 dominated by Cl, Na and HCO3), are commonly rich in Ca, Fe, Al, phosphate and trace elements including As ( [008] , [Fouillac, 1983] , [Criaud and Fouillac, 1986] and [Beaucaire et al., 1987] ). They are supposed to result from the mixture of meteoric water with deep water during a convective cycle ( [Criaud and Fouillac, 1989] and [Négrel et al., 2000] ). A geochemical monitoring of some springs over several years indicated a chemical stability over time (Vuataz et al., 1987).
The spring waters are in a state of disequilibrium with respect to atmospheric conditions upon discharge: CO2 oversaturation [partial pressure ranging between 0.1 and 1 bar (Cornu et al., 2001)] and O2 undersaturation. The oxygenation and CO2 degassing (bubbling) occurring at the spring outlet are accompanied by the precipitation of Fe oxyhydroxides and carbonates, which commonly form travertine deposits.
As shown by Fouillac (1983), View the MathML source is the only important acid/base couple in CO2-rich waters. The inorganic aqueous speciation for these waters was calculated to assess the relative importance of the carbonate species (View the MathML source, FeCO3(aq)) and saturation indices of solid phases. The speciation calculations by Casanova et al. (1999) indicate that the View the MathML source complex dominates the Fe concentrations (from 83% to 91% of total Fe) in solution. After the bicarbonate complex, the free metal ion (Fe2+) is typically the next most abundant form of dissolved Fe, accounting for up to 11%. The percentage of the FeCO3(aq) ion pair is minor at 0–4%. The free ions, Ca2+ and Mg2+, represent more than 75% of total element concentrations.
The studies by Fouillac (1985) and Michard et al. (1987) indicate four groups of springs (according to their Cl/B molar ratio):
  • – a northern group (St Hérend) to which belongs Le Bard spring,
  • – a central group (Chassolle-Zagat),
  • – a southern group (Chantejail)
  • – and a south-western group (Pyronnée-Conche).

19.2.3. The origin of As

Arsenic content in the thermo-mineral springs of the Cézallier area ranges from 0 to 2 mg/l (0–27 μmol/l). In the central group of springs, As is mainly present as As(V) (over 90%). Two springs of this group show, nevertheless, as high as 30% of As(III), which is suspected to be linked to biological processes occurring due to the mixing with superficial waters (Baranger et al., 2001). It is interesting to note that As(III) dominates in the southern group of springs showing higher As concentrations (Criaud and Fouillac, 1989).
The isotopic study by Vuataz et al. (1987) does not allow to determine if the speciation of As is linked to deep thermalism or rather to the dissolution of sulphur minerals (Baranger et al., 2001). The concentration of As in the solution nevertheless appears associated with that of sulphates, indicating a probable common origin linked to the oxidation of sulphur minerals (ibid.).
Saturation indexes calculated by Criaud and Fouillac (1989) suggest that all spring waters are undersaturated for As-bearing minerals (native As, realgar, orpiment, arsenopyrite, claudetite).

19.3. Observation of the fate of As

Arsenic, like most of the chemical elements present in trace amounts in the aqueous phase, is partitioned during equilibration between the spring water and the precipitated phases ( [Beaucaire et al., 1987] , [Criaud and Fouillac, 1989] and [Casanova et al., 1999] ).
The fate of As in the travertine deposits and in the soils located in the vicinity of the northern and central groups of springs (including Le Bard spring) was studied mainly by Cornu et al. (2001) and to a lesser extent by Casanova et al. (1999). We present here the summary of the results.
The travertine is composed mainly of poorly and well crystalline compounds: Fe oxyhydroxides (two-line ferrihydrite) and well-crystallised calcite. Two mechanisms are expected to explain the spatial sequence observed in the layered carbonate deposit: chemical precipitation at the spring outlet and biologically controlled process at some distance from the spring (Cornu et al., 2001). Hydrous Fe oxide particles are the first to form upon discharge (Casanova et al., 1999). Their formation is linked at the emergence of the solution to the oxygenation of the solution, which leads to a change in the redox potential of the mineral water. Trace elements, and especially As, are clearly associated with Fe oxyhydroxides. The Fe oxyhydroxide-to-water ratio for As concentration ranges from 1 × 104 to more than 1 × 105 (ibid.).
The soils show traces of travertine decomposition and spring water influence. Highest Fe enrichment is observed at a few metres from the spring outlet, while Ca enrichment remains over a large distance (Cornu et al., 2001). Arsenic is clearly associated to Fe oxides. So does P, which is known to have a geochemical behaviour similar to that of As. Arsenic enrichment and the carbonate formation appear to be independent. Arsenic-bearing Fe oxides thus seem to have been preserved.

19.4. Dynamics of As: laboratory and field experiments

The complementary sets of experiments conducted in order to identify the mechanisms controlling As behaviour in the Cézallier spring waters are detailed by Le Guern et al. (2003). We remind here about the principle of the experiments and a part of the results.

19.4.1. Materials and methods Experimental systems and protocols
In addition to classical batch tests in the laboratory, an original dynamic experimental approach was developed to study the sequence of mineral precipitation and the associated As trapping. Because the Cézallier spring waters are in disequilibrium with the atmosphere, it is difficult to avoid changes in alkalinity, dissolved oxygen content and Fe precipitation during their sampling and transportation. The dynamic experiments, which require large amounts of fluids, were hence set up in the field to avoid sample preservation difficulties.
The laboratory experiments were carried out in 2-l “batch” reactors, each with a double glass envelope and a polyethylene lining (Le Guern et al., 2003). The solution, agitated with a Teflon-covered bar magnet, is brought into direct contact with the atmosphere for a period of 120 h. The temperature is maintained at 25 ± 2°C. The water used for the laboratory experiments was collected in in situ hermetically sealed polypropylene flasks (reflux sampling) or glass ampoules (vacuum sampling). Despite the precautions taken during water sampling, slight Fe precipitation nevertheless occurred. In order to compensate for this Fe loss, FeCl3 was added to the solution at the beginning of the experiments.
The dynamic field experiments, using a chemical engineering approach, consisted of a cascade of polypropylene reactors (Fig. 19-2) through which the Cézallier waters were circulated. Two systems were set up to study As trapping (see Fig. 19-2): a “Fast” (F) system, comprising five reactors (F1–F5) with a total solution residence time of 2 h, and a “Slow” (S) system, comprising four reactors (S1–S4) with a total solution residence time of 24 h. For each cascade, the solution residence time in the downstream reactors increases (using water level control) in order to gain a wider distribution of trace-element trapping by sharing out the precipitates among the reactors. To ensure precipitate retention in the reactors, the solution flow rate was calculated so that particles larger than 2 μm in diameter remained in the reactor in which they were formed. Calculations based on reactor diameter, water volume and particle density using Stokes’ law postulate that the precipitates are retained in the reactors by sedimentation and that the flow in the reactors is laminar. Both static and dynamic field experiments were conducted without agitation and the systems were fed directly with the discharging spring waters.

Fig. 19-2.
Principle of the reactor system set up for the dynamic field experiments. Reprinted from Le Guern et al. (2003) with permission from Elsevier Science
The waters were spiked at the beginning of both the laboratory and the field experiments with the elements to be studied, i.e. As, Se, U, Th and the rare earth elements. For As, added as As(V), the initial concentration was set at 0.5 μmol/l. No pH adjustment was made. Solution characterisation
The non-conservative parameters of the solutions (pH, Eh, temperature, dissolved oxygen, alkalinity) were measured before sampling and throughout the duration of each experiment. After sampling, the solutions were filtered through 0.22-μm Teflon filters under a N2 atmosphere (for preservation purposes) prior to chemical analysis, including determination of inorganic carbon (IC) (Shimadzu carbon analyser), ion chromatographic analysis of anions (Dionex AI-450 software), capillary electrophoresis analysis of cations (Millenium 2010 – Waters software), Fe and Si determination by colorimetry and As determination by atomic absorption spectrometry (Varian Z300 Zeeman effect spectrometer). Precipitate characterisation
Precipitate mineralogy was characterised using Fourier transform infra-red spectroscopy (FTIR) and X-ray diffraction (XRD) (Siemens D5000 automatic diffractometer and DIFFRACT AT software). Observations were made using a scanning electron microscope (SEM; JEOL JSM6100). The precipitates were also chemically analysed for trace and major elements, after acid dissolution in concentrated HCl, using the same analytical techniques as for the solutions.

19.4.2. Results and discussion Cézallier water characterisation
The physico-chemical composition of the Cézallier Le Bard spring waters used in the experiments is given in Table 19-1.
Table 19-1. Physico-chemical composition of the Le Bard spring waters (concentrations in mol/l)
Major cations
Major anions
Other elements
Other parameters
Ca2+6.8 × 10−3Cl16.9 × 10−3Fea0.16 × 10−3Temperature (°C)14
Na+44.1 × 10−3Alk.b56 × 10−3Si1.55 × 10−3pH6.47
K+5.3 × 10−3View the MathML source0.4 × 10−3As7 × 10−8Eh (mV)197
Mg2+6.6 × 10−3

Dissolved O2 (mg/l)2.39
a Total Fe.
b Alkalinity. Major element evolution
Figure 19-3 shows the results obtained from the laboratory experiments. A sharp drop is observed in the Fe concentration in solution during the first few hours (from 0.32 to 0.04 mg/l), and a more gradual drop in Ca concentration that tends towards a minimum after some 20 h (from 285 to <10 mg/l). Dissolved IC also tends towards a minimum after some 20 h (from 875 to 500 mg/l). In parallel, pH increases from 6 to 9.5 during the 120 h of the experiment. These evolutions are linked to the CO2 degassing (accompanied by oxygenation) of the Cézallier waters, and an associated rapid precipitation of Fe oxyhydroxides and a slower precipitation of calcite.

Fig. 19-3.
Evolution with time of As concentrations in solution (calculated from molarities) during the laboratory experiments conducted on Fe-enriched Cézallier waters in contact with the atmosphere. Comparison with Fe, Ca, inorganic carbon (C. inorg) and pH
The absence of significant variations in the concentration of other major cations (Na: 1050 ± 50 mg/l, K: 223 ± 17 mg/l, Mg: 160 ± 10 mg/l) and anions (Cl: 640 ± 35 mg/l, SO4: 45 ± 2 mg/l) with time suggests that no other major mineralogical phase is precipitated. The mineralogical characterisation confirms this interpretation: XRD detects calcite as the only crystalline phase, and FTIR and SEM show the presence of Fe oxyhydroxides and calcite alone (Le Guern et al., 2003).
The evolution of the major ions during the field experiments is in good accordance with that of the laboratory experiments, i.e. a relatively good correspondence between Fe and Ca (and pH) evolution vs time, and no significant variations in the concentrations of the other major elements. The results of the mineralogical characterisation also show the presence of calcite and Fe oxyhydroxides alone. Dynamics of As
During the laboratory experiments (Fig. 19-3), As was at first rapidly removed from the solution with as much as a 95% decrease in concentration during the first 15 h, related to the rapid precipitation of Fe oxyhydroxides. Arsenic is then released more gradually back into the solution (from 15 to 120 h), a process that appears to be pH-dependent; the increased pH is being related to CO2 degassing of the waters. The mineralogical characterisation indicates no ageing of Fe oxyhydroxides during the experiments.
The behaviour of As during the field experiments is in good agreement with the first few hours of the laboratory experiments (Fig. 19-4), i.e. rapid As removal from the solution (decreased in concentration of 80–98% in the field and 95% in the laboratory) correlated with the rapid precipitation of Fe oxyhydroxides. This was confirmed by the analysis of the mineral precipitates, which shows As trapping in large amounts by Fe oxyhydroxides (e.g. reactor F2: 500 mg/l) and in small amounts by calcite (calcite sampled in reactor S4: 11 mg/l).

Fig. 19-4.
Evolution of As trapping (calculated from molarities) with time during the “Fast” (above) and the “Slow” (under) system field experiments. Comparison with Fe, Ca and pH evolutions. Reprinted from Le Guern et al. (2003) with permission from Elsevier Science Mechanisms controlling As behaviour
The high As trapping by Fe oxyhydroxide precipitates, as observed in the studied system (Fig. 19-3 and Fig. 19-4), is in good agreement with literature data on As that underlines the importance of Fe in the cycling and regulation of As in surface water systems ( [Aggett and O’Brian, 1985] , [Belzile and Tessier, 1990] , [De Vitre et al., 1991] and [Fuller et al., 1993] ). The lack of correlation between the kinetics of Ca and As evolution in the system (Fig. 19-3 and Fig. 19-4) and the low concentration of As in calcite (11 mg/kg) indicates that calcite precipitation appears here to play a minor role in As trapping.
The correlation between As concentration and pH observed during the laboratory experiments, in agreement with a sorption process onto the Fe oxyhydroxide precipitates, is in good agreement with literature data: As(V) is known to strongly sorb onto Fe oxyhydroxides, and like the adsorption of other oxyanions by oxides, the sorbed amount increases with decreasing pH ( [Pierce and Moore, 1982] , [Goldberg, 1986] , [Fuller et al., 1993] and [Lumsdon et al., 2001] ). The absence of As release during the field experiments could be due to the pH (6.5–7.5) remaining below the point of zero charge (PZC) of the Fe oxyhydroxides. Schwertmann and Fetcher (1982) recorded a PZC between 5.3 and 7.5 for natural Si-bearing ferrihydrite and a PZC of around 8 for synthetic ferrihydrite.
The laboratory experiments show that the Fe oxyhydroxides are stable over time, which is in good accordance with field observations, e.g. Fe as oxyhydroxides in recent and older travertine deposits. This stability may be explained by the high elemental enrichment (Al, Si, Ca and As). Some foreign species indeed modify the surface structure of the ferrihydrite and limit the crystal growth. This was shown for Si (Zhao et al., 1994), As (Manceau, 1995) and PO4 ( [He et al., 1996] and [Rose et al., 1996] ), for instance. The total molar Si/Fe ratio of the Fe samples ranged between 0.07 and 0.51. This consequently pushes down the 0.36 limit required by Mayer and Jarell (1996) to obtain ferrihydrite rather than lepidocrocite when oxidising Fe(II) solutions at pH 7 in the presence of silica. Aluminium, Ca, As may combine with silica to stabilise the local structure of ferrihydrite (Casanova et al., 1999).

19.5. Mechanistic approach: geochemical modelling

A geochemical model was developed to test whether the identified mechanisms and the use of thermodynamic data from the literature are able to reproduce the studied system. In particular, the role of adsorption by Fe oxyhydroxides was investigated as the main mechanism for As trapping. We used the Phreeqc geochemical code (Parkhurst and Appelo, 1999), which has the capacity to calculate aqueous speciation and water–rock interactions, including sorption reactions such as surface complexation and/or ion-exchange-equilibria.

19.5.1. Model Conceptual model
Bearing in mind that the precipitation of ferrihydrite is associated with oxygenation of the solution, and that of calcite with CO2 degassing of the solution, then the key parameters of the system are the CO2 and O2 partial pressures (pCO2, pO2). Moreover, the trapping of As appears mainly controlled by sorption process onto Fe oxyhydroxides. Consequently, we took into account in the model four types of chemical reactions: complexation in the aqueous phase (speciation), precipitation/dissolution of the solid phases calcite and ferrihydrite, sorption on ferrihydrite and gas exchange (O2 dissolution, CO2 degassing).
Due to the very low As content of calcite compared to Fe oxyhydroxides revealed by the experiments (see chapter 19.4.2 section “Dynamics of As”), no co-precipitation or sorption reactions with calcite are considered. Although As probably also co-precipitates with Fe oxyhydroxides in addition to adsorption (cf. simultaneous precipitation and trapping), we only consider adsorption mechanisms. Aqueous speciation of the spring waters enriched in trace elements indicates that As is present mainly as the arsenate oxyanions View the MathML source and View the MathML source.
The input parameters are the initial physico-chemical composition of the solution. Final equilibrium conditions are constrained by equilibrium with atmospheric CO2 and O2 partial pressures. The kinetic aspects of gas exchange and mineral precipitation are taken into account to simulate the evolution of the system vs time. The target pCO2 and pO2 are set at –3.5 and –0.67, respectively, to simulate the evolution to equilibrium with the atmosphere (Table 19-2).
Table 19-2. Kinetic constants for dissolution/precipitation and gaseous exchange reactions
KineticsReactionKinetic constantPartial pressure
Fe oxyhydroxidesFe(OH)3 + 3H+ = Fe3+ + 3H2Okall = 1 × 10−9
CalciteView the MathML sourceparm1 = 0.6
parm2 = 0.6
CO2CO2(g) = CO2kall = 2 × 10−9−3.5
O2O2(g) = O2kall = 1 × 10−8−0.67
kall = one-way reaction; parm1 and parm2 as defined in Parkhurst and Appelo (1999). Thermodynamic and kinetic data
All calculations were made using a data set based on the WATEQ4F thermodynamic database (Ball and Nordstrom, 1991) complemented with sorption reactions and kinetics for CO2 degassing, O2 dissolution, calcite and Fe oxyhydroxide precipitation.
Selected sorption reactions and corresponding thermodynamic equilibrium constants are given in Table 19-3. Their description is based on the diffuse double-layer surface complexation model (Dzombak and Morel, 1990). The number of reactive sorption sites on Fe oxyhydroxides, linked to the amount of precipitated Fe oxyhydroxides, was calculated on the basis of (a) the site density for strong and weak binding sites and (b) the specific surface area recommended by Dzombak and Morel (1990) for a hydrous Fe oxide (600 m2/g).
Table 19-3. Selected sorption reactions and corresponding thermodynamic equilibrium constants
ReactionsLog KeqReference
View the MathML source7.29Dzombak and Morel (1990)
Ss,w−OH = Ss,w−O + H+−8.93Dzombak and Morel (1990)
View the MathML source9.3Waite et al. (1994)
Ss−OH + Ca2+ = Ss−OHCa2+4.97Dzombak and Morel (1990)
Sw−OH + Ca2+ = Sw−OCa2+ + H+−5.85Dzombak and Morel (1990)
Sw−OH + Mg2+ = Sw−OMg+ + H+−4.6Dzombak and Morel (1990)
View the MathML source7.78Wilkie and Hering (1996)
View the MathML source0.79Wilkie and Hering (1996)
View the MathML source29.31Dzombak and Morel (1990) and Wilkie and Hering (1996)
View the MathML source23.51Dzombak and Morel (1990) and Wilkie and Hering (1996)
View the MathML source−4.75Manning and Goldberg (1996)
Sw = weak sites; Ss = strong sites; Ss,w = strong and weak sites.
To simulate oxygenation and CO2 degassing of the water, we considered kinetically controlled exchange reactions between the aqueous phase and the atmospheric carbon dioxide and oxygen. Precipitation reactions of the Fe oxyhydroxides and calcite were also kinetically controlled. The data of Plummer et al. (1978), in Parkhurst and Appelo (1999), as given in the PhreeqC examples, were used for calcite. For the other introduced kinetic reactions, the following simple law was used:(19-1)V=kall(1−IAP/K)where V represents the reaction rate, IAP is the ion activity product, kall is the kinetic constant of dissolution and K is the thermodynamic constant of the reaction.
The IAP is deduced from the mass action law of the chemical reactions. Transport modelling
For the dynamic field experiments, consisting of circulating trace-element-enriched spring waters through a cascade of five reactors (Fig. 19-2), we simplified the transport as much as possible by using a discontinuous transport pattern; i.e. the calculated outlet solution from reactor no. 1 feeds reactor no. 2, from which the calculated outlet solution feeds reactor no. 3, etc., with each reactor being considered as a perfect reactor.

19.5.2. Modelling results Simulation of the laboratory experiments (with adjustment of kinetic constants)
The laboratory experiments were simulated adjusting the kinetic constants of precipitation reactions and gaseous exchanges according to the evolution of pH, dissolved Fe and calcium concentration with time (Table 19-2).
As far as the trends of the major elements and pH are concerned, adjustment against the values obtained during the laboratory experiments appears to be generally satisfactory, although the Fe and calcium concentrations are slightly underestimated by one order of magnitude (see Fig. 19-5).

Fig. 19-5.
Modelling Fe, Ca, pH and As evolution with time during laboratory experiments conducted on Fe-enriched Cézallier waters (Le Bard spring) brought into contact with the atmosphere. Comparison with experimental results. Reprinted from Le Guern et al. (2003) with permission from Elsevier Science Simulation of laboratory experiments (without adjustment of kinetic constants)
The previous adjustment was validated by testing the resultant model against another set of laboratory experiments that used another spring water of different composition but similar operating conditions. The spring water composition was replaced in the model. No adjustment of the thermodynamic or kinetic constants was made in this model.
The simulation provides a satisfactory prediction of the pH as with the Le Bard spring water simulation; the Fe concentrations are slightly underestimated by one order of magnitude. The slow calcium depletion calculated by the model indicates a slower calcite precipitation that was observed during the experiment.
The model provides a good reproduction of the evolution of As concentrations. However, as with the Le Bard spring water simulation, it underestimates the absolute value of As concentration, which may possibly be correlated with the overestimation of the amount of precipitated Fe (see Le Guern et al., 2003). Modelling of the “Fast” dynamic field experiments
The model was run with the following adjustments of the kinetic constants (kall): 1 × 10−9, 2 × 10−8 and 1 × 10−10, for ferrihydrite, CO2 and O2, respectively. These adjustments are introduced to reflect the fact that no agitation was applied to the field systems, so that oxygenation is accordingly slower than during the laboratory experiments.
The results of the “Fast” dynamic test are shown in Fig. 19-6. A good reproduction is obtained for the trend and absolute values of Fe in solution and pH. Conversely, the Ca concentration is slightly overestimated, which may be due to an underestimation of the calcite precipitation kinetics. The model provides a good reproduction of the evolution of As concentrations, which is slightly overestimated (by one order of magnitude).

Fig. 19-6.
Modelling Fe, Ca, pH and As evolution with time during dynamic field experiments conducted with the “Fast” system and comparison with experimental results. Reprinted from Le Guern et al. (2003) with permission from Elsevier Science

19.5.3. Discussion on the modelling results Laboratory experiment simulation
Although the observed general trends are well reproduced, adjustment of the kinetic parameters does not enable a simultaneous reproduction of the absolute Fe, Ca values and pH. The kinetic laws used would thus need some refinement. This has obvious repercussions when modelling the behaviour of As in the system. Nevertheless, the simulation results for As trapping appear generally highly satisfactory, despite a slight overestimation of the trapping, which correlates with the overestimation of Fe oxyhydroxide precipitation.
It must be underlined, however, that preliminary tests including the following sorption reaction (Dzombak and Morel, 1990) highly overestimate As trapping:(19-2)View the MathML sourceArsenic concentration is being calculated at 1 × 10−12 mol/l. The best approach of As behaviour was reached using the following equation (Manning and Goldberg, 1996):(19-3)View the MathML sourceAlthough this equation was initially developed for kaolinite, we tested its extrapolation to Fe oxyhydroxides. The tests encourage further to consider this reaction which may complete the set of equations and thermodynamic data available for As sorption onto Fe oxyhydroxides. Field experiment simulation
The simulations of the dynamic field experiments show that the model is capable of accurately estimating the overall trend of As concentration with time. However, the absolute values of trapped As are underestimated by one order of magnitude. In view of the conclusions drawn from the laboratory experiment simulation, this discrepancy may be linked to the lack of thermodynamic data available on As sorption onto Fe oxyhydroxides.
The influence of processes not taken into account by the model may also be considered, in particular the co-precipitation of a portion of the As with the Fe oxyhydroxides. Waychunas et al. (1993), however, observed no evidence for co-precipitation in a relatively similar system. They demonstrated that arsenate was removed from solution by adsorption onto ferrihydrite, during the formation of ferrihydrite, with no ferric arsenate or any As-bearing surface precipitate or solid–solution formation. These authors measured higher total site densities for ferrihydrite than those adopted by Dzombak and Morel (1990), which may constitute a factor of discrepancy between our model and the experimental results.
It is also interesting to mention that the Fe oxyhydroxides precipitated during our experiments contained traces of major and other trace elements in the spring waters, which may influence the properties of the precipitates. Arsenic reported by many authors ( [Anderson et al., 1985] , [Ainsworth et al., 1989] , [Anderson and Benjamin, 1990] and [Stollenwerk, 2003] ), showed that incorporation of Si, and to a lesser extent Al into the Fe oxyhydroxides, decreased their adsorption capacity.
Some other processes may occur, which are not considered by our model. Silicic acid, for instance, has been shown to compete with As for adsorption onto Fe oxyhydroxides (Swedlund and Webster, 1999). Taking into account this reaction, it would rather increase the discrepancy between model and experiments. On the contrary to silicic acid, bivalent cations, such as calcium are known to enhance adsorption of arsenic on ferrihydrite (Wilkie and Herring, 1996) in the pH range where cations show two positive charges. They may indeed act as an electrostatic bridge between the solid surface and the anions.
The simplification of the transport modelling, and in particular the adopted hypothesis whereby the reactors are comparable to perfect reactors, can also in part explain the discrepancies observed with respect to the dynamic field experiments.

19.6. Conclusion and perspectives

A good knowledge and understanding of the chemical mechanisms governing the transfer of As in the environment are of great importance in order to predict the impact of pollution and to find remediation solutions.
The dynamics of As were studied in a relative simple natural system showing precipitation of Fe oxyhydroxides and carbonates through a complete approach including a complete study that comprised: (a) observation of the solid and liquid phases, (b) laboratory and field experimental partitioning studies and (c) geochemical modelling. In this context, the focus was on inorganically mediated processes, and especially on the role of Fe oxyhydroxides and carbonates.
A new experimental set-up was developed to allow dynamic experiments. It offers the advantage of allowing segregation of the early and late precipitates: the Fe oxyhydroxides and calcite are distributed among the reactor series, as is the degree of As trapping. It may be applied to study the behaviour/sequestration of trace elements in any natural system, such as former mining sites, where there is continuous generation of a substrate (precipitation of solid phases).
The geochemical modelling using the PHREEQC code, included aqueous speciation, dissolution/precipitation of calcite and Fe oxyhydroxide, adsorption and gaseous exchanges. Thermodynamic and kinetic data from the literature were used, only for adjustment of the kinetics of the gaseous exchanges.
Iron oxyhydroxides clearly show to control the fate of As through adsorption processes. Carbonates do not trap As, but play nevertheless an indirect role in the system. Their precipitation leads indeed to the increase of the pH, which may lead to the desorption of As as observed at the laboratory (batch experiments). Field observations indicate, however, that As remains linked to Fe oxyhydroxides in the travertine deposits associated to the springs as well as in the surrounding soils. This may be due to the fact that the field is an open system, where the water-to-solid ratio becomes very low. The adsorption process appears very rapid, with around 90% of As being trapped within several minutes; thereby confirming previous observations ( [Xu et al., 1988] , [Fuller et al., 1993] and [Darlan and Inskeep, 1997] ).
The geochemical model is capable of accurately estimating the main As-trapping trends observed experimentally by using thermodynamic data from the literature. The sorption data set was completed by a sorption reaction extrapolated from Manning and Goldberg (1996) [see (Eq. 19-3)]. It may be interesting to further consider this reaction by determining the probability of its occurrence and its thermodynamic constant for Fe oxyhydroxides.
The model approaches the absolute As concentration values within approximately one order of magnitude. The most likely factors of discrepancy between the experimental and modelling results include the uncertainty on the thermodynamic constant of the added reaction describing arsenate adsorption onto Fe oxyhydroxides and the hypotheses made on specific area or surface site densities on Fe oxyhydroxides. The determination of specific data on Fe oxyhydroxides should allow us to reduce the discrepancy. Difficulties in simultaneously reproducing the key parameters of Fe and Ca concentrations and pH values on the basis of a calibration carried out on the O2 and CO2 degassing kinetics and on the Fe oxyhydroxide and calcite dissolution/precipitation may also be partly responsible for the observed discrepancy: the model is highly sensitive to variations in gas exchange kinetics.


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