iron speciation in natural organic matter colloids

E L S E V I E R Colloids and Surfaces

A: Physicochemical and Engineering Aspects 136 (1998) 1 I- 19

COLLOIDS AND A SURFACES

Iron speciation in natural organic matter colloids

J6r6me Rose ~, Astride Vilge a, Gwenaelle Olivie-Lauquet b, Armand Masion a,

Carole Frechou ~, Jean-Yves Bottero a . ,

a Laboratoire des Geosciences tie i'Environnement, URA 132 CNRS, CEREGE, Europole Mediterraneen de i'Arbois, BP 80, 13545 Aix-en-Provence, Cedex 04, France

b Laboratoire de Mineralogie Cristalloch#nie, Universitb Pierre et Marie Cur.e, 5 place Jussieu, 75242 Paris, Cedex 05, France

Accepted 10 April 1997

Abstract

Natural colloids sampled in rivers in the Nsimi region (south Cameroon) were studied by extended X-ray absorption fine structure ( EXAFS ) spectroscopy and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) to determine the speciation of iron within these colloids. The analysis and modeling of the EXAFS spectra revealed that iron is poorly polymerized due to the complexation of Fe by the organic matter, since monodentate and bidentate complexes were detected. Py-GC-MS gave a closer insight into the chemical nature of the organic matter: the colloidal fraction, which corresponds to the EXAFS sample, consists mainly of carboxylic acids, thus explaining the high level of complexation of Fe. © 1998 Elsevier Science B.V.

1. Introduction

Iron plays a key role as regards the retention and transportation of natural organic matter (NOM) in aquatic media [1,2]. A better under- standing of the natural cycles of Fe and NOM requires a precise knowledge of the Fe -NOM interactions. The nature of NOM plays an important role in these interactions, since their physicochemical properties can lead to the solubilization as well as the precipitation of metal ions. During the last years, a large number of studies have been devoted to adsorption phenomena of NOM onto iron oxyhydroxide particles [3-6]. Among all the mechanisms of adsorption, the ligand exchange between surface-coordinated OH and H20 from

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iron oxides and NOM was favored by the investi- gations [3-6]. Moreover, these NOM adsorption phenomena can lead to the dissolution of the adsorbent [7] and thus to th,~: soiubilization of Fe. This solubilization of iron by NOM in natural aquatic systems or in soils is often considered to result from a reduction of Fe(III ) to Fe(II) [8]. The chemistry of the organic molecules is essential in the interactions between Fe and NOM. For instance, the carboxyl/hydroxyl functional groups from humic and fulvic acids are well known to form stable complexes with Fe 3 ÷ [9]. The chemical characterization of NOM is very complex. Never- theless, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) is an analytical technique suitable for a rapid characterization of the chemical nature of organic matter in soils and aquatic environments [ 10-14].

The Fe -NOM interactions in natural colloidal

12 J. Rose et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 11-19

particles have received only limited attention. This is mainly due to the relatively high dilution of metal ions in such colloidal particles and the amorphous nature of samples, which make the direct determination of the chemical form of metal species a difficult task. Only one recent study has successfully determined the speciation of a metal ion (lead) in contaminated soils using EXAFS spectroscopy [ 15].

The aim of this paper is to examine the potential of X-ray absorption spectroscopy (XAS) and Py-GC-MS to characterize the Fe speciation and Fe-NOM interactions in natural amorphous colloidal particles from natural water at pH 6.

2. Materials and methods

Natural colloidal particles were extracted from rivers in the Nsimi region (south Cameroon). The site is representative of conditions encountered in central Africa, i.e. tropical humid climate under forests. The samples were collected during the rain season. The concentration of carbon (total organic carbon (TOC)) in the water is 12.5 mg/l and the minor and trace elements have a concentration lower than 10 lag/l (Table 1 ). The iron concentration in the sample is 478 lag/l. ESR analyses showed that the iron in the Nsimi sample is in the Fe(llI) state [ 16]. Different colloidal fractions were obtained using tangential ultrafiltration. The sample examined by EXAFS at the Fe-K edge corresponds to the 20-300kD fraction. Py-GC-MS analyses were carried out on the

Table I Characteristics of the Nsimi water (20-300 kD fraction)

Nsimi

pH at 20:C 5.7 Total organic carbon (TOC) (mg I ) 12.5 [Fe] (lag I) 478 [All (lag I) 192 [Si] (lag !) 7100 Traces (lag I ) < 10 Carbonates ND

ND: not detected. C content determined with a standard carbon analyser, all other concentrations were obtained by ICP-AES.

20-300kD and >0.45 pm fractions. They are representative of the colloidal fraction and the particulate fraction respectively [ 16].

A solution obtained by mixing FeCI36H20 (5 x 10 -2 M) with oxalic acid (Ox) at constant pH 6 and [Ox]/[Fe] molar ratio 5 was used as reference for the EXAFS experiments. In these conditions 2-1 and 3-1 complexes are formed [17,18].

2.1. EXAFS exper#nents

The atomic environment of iron was determined using Fe-K edge EXAFS spectroscopy. XAS spectra were recorded at room temperature in the fluorescence mode at the DCI synchrotron (Orsay, France), on the D42 EXAFS beamline. The posit- ron storage ring was running at 1.85 GeV and 280-320 mA.

2.2. Data reduction

The EXAFS spectra were analysed in the classi- cal way [19]: the oscillatory part was extracted using a "spline" function fit and normalized using the Heitler approximation [20]. A Kaiser window (r=2.5) was then applied to the k"'/,(k) weighted data before Fourier transform from k=2.5 to 14 A-~; k stands for the modulus of the wave vector associated with the electronic wave. A value of n = 1 was used in order to maximize the weight of the contribution of a light element whose maxi- mum amplitude is at low k value. The Fourier transform yields radial distribution functions (RDFs) consisting of n peaks representing n coordination spheres at distances R, from the central atom. Distances are uncorrected for phase shift and have to be displaced toward long distances by 0.3-0.4,A, from crystallographic positions. The contributions of the various shells were singled out by a back Fourier transform (including ;' 'emoval of the Kaiser window contribution) from real to k space. Th,:se partial EXAFS functions were then least-squares fitted by a theoretical function in order to determine the structural and chemical parameters: R~, N~ (number of neighbors) and nature of atomic neighbors in the jth shell around Fe. For the different atomic pairs used in the

J. Rose et al. / Colloids Stofaces A: Physicochem. Eng. Aspects 136 (1998) 11-19 13

calculations of EXAFS spectra, the amplitude functions F~Fe-O/C;Fe~ and the phase shift functions q~tF~-O/C/F~ were determined using theoretical functions [21 ]. They were validated with pure and well-crystallized compounds such as 7FeOOH (Lepidocrocite) for the Fe-Fe and Fe-O pairs, and Fe acetate for the Fe-C pair. The uncertainty in R and N is 0.06 A, and 10% respectively.

atom, 0.5 Fe atoms are present at a distance of o

3.20 A, corresponding to an edge sharing between iron octahedra, and 1.5 Fe at a distance of 3.85 A, corresponding to a single corner bond. These EXAFS data indicate that 2:1 oxalate-Fe bidentate complexes are formed and the low coordination numbers N~el and Nre3 (Table 3) are characteristic of poorly polymerized iron [23-26].

2.3. Analysis of organic matter

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was performed on the freeze- dried water using a pyroprobe 100 (CDS, Oxford) filament pyrolyzer connected with the spit/spitless injection port of a Fisons GC/MD 800. About 1 mg of freeze-dried sample was directly put into a quartz tube and submitted to a flash pyrolysis. The platinum filament was programmed to a final temperature of 640~C at a heating rate of 20°C/ms and held at this temperature for 20 s. The pyrolysis products were separated on a DB-WAX column programmed from 30 to 220°C at a rate of 3~C/min and held at this temperature for 20 min. Then they were detected by the mass spectrometer operated at 70 eV and scanned frona 30 to 350 amu at 1 scan/s.

Pyrolysis products were identified by comparison with standard spectra in the literature or in the NIST spectrum library. Characteristic compounds were attributed to one of the following categories (Table 2): polysaccharides (PS), proteins (PR), aminosugars (AS), polyhydroxyaromatics (PHA) and miscellaneous, i.e. compounds not identified or not attributed. The proportion of each category was calculated following the method developed by Bruchet et al. [22].

3.1.2. Nsimi sample Significant differences appear between the radial

distribution functions (RDFs) (uncorrected for phase shift) of natural samples from Nsimi compared with a Goethite reference phase (Bayer A.C:., specific surface area 66 rn2/g) (Fig. 3). For each =arve the first peak corresponds to back- scattering atoms of the first coordination sphere of iron, i.e. O (O, OH, OH2) in our case. The other peaks at larger distances indicate the presence of atoms in the next nearest shell of the central iron atom. The RDF of the Nsimi sample displays a weak and relat!vely short contribution at a distance of 2.7-2.8 A. This distance is too short to correspond to Fe-Fe contributions. The shortest Fe-Fe interatomic distance, corresponding to face sharing octahedra, is 2.85 ,A, [27]. Knowing that organic molecules are at large concentration and that Fe -O-C or Fe-O-N bonds are in a 2.7-3.1 A range [28-30], such an interatomic distance could correspond to the existence of Fe-organic complexes. Nevertheless, it is not possible to attribute unequivocally this short distance without a modeling step. Fe-Fe contributions at approximately 3.35 A and 3.85 A are attributed respectively to edge sharing and single corner sharing linkages between iron octahedra [27,31-33].

3. Results

3.1. EXAFS results

3.1.1. Fe-oxalate The analysis of the second and third peaks of

RDF (Figs. 1 and 2 and Table 3) of the Fe-oxalate solution indicates that four carbon atoms are present at a distance of 2.85 A, from the central

3.2. Pv-GC-MS results

The Py-GC-MS experiments were performed on two fractions (20-300 kD and >0.45 lam) of the samples collected in the catchment area of Nsimi. The position of the peaks of the pyrochromatograms allows us to identify the pyrolysis by-products which are then attributed to one of the four categories mentioned earlier (Table 2). The first interpretation of the pyrochromatograms

14 J. Rose et al. / Colloids Sterfaces A: Physicochenl. Eng. Aspects 136 (1998) 11-19

Table 2 Identification of pyrolysis by-products, their origin (a blank entry denotes unknown or multiple origins) and their attribution according

to Ref. [22]

Compound Possible origin Attribution

CO_, Carbonate, carboxylic acid Benzene PHA Acetonitrile Nitriles PR Toluene Lg PR Pyridine PR Methylpyridine PR Styrene PR Hydroxypropanone PS Methylstyrene PR 2-Methylfuran Furan PS 2- Methyl2cyclopenten- 1 -one C)'clopentenones PS Furfural Furan PS Acetic acid Carboxylic acid Misc. Furaldehyde Furan PS Indene PS Benzofuran Furan PS 2-Acetylfu, an Furan PS 3-Methy12cyclopenten -l-one Cyclopentenones PS Pyrrol,. Pyrrole PR Pro,,tonic acid f arboxylic acid Misc. M,~thylpyrrole Pyrrole PR M ethylfurfural Furan PS Furfuryl alcohol Bacteria Misc. E enzonitrile Nitriles PR Acetophenone Lg Misc. i'~aphthalene Furan PS ,~,cetamide AS 1~ lethylacetamide AS Butenoic acid Carboxylic acid. bacteria Misc. M :thylnaphthalene Furan PS Pheaylacetonitrile Nitriles PR Phenol Lg PHA. PR p-Cresol Lg PHA m-Cresol Lg PHA Ethyiphenol Lg PHA Indole Pyrrole PR Methylindole Pyrrole PR Ethylbenzene Lg Misc. Benzaldehyde Lg Misc. Methoxyphenol Lg Misc. Furanone Furan PS Furan derivative Furan PS Pyrrole derivative Pyrrole PR

Lg: lignine; PHA: polyhydroxyaromatics: PR: proteins: PS: polysaccharides: AS: aminosugars. Misc.: miscellaneous.

was carried out based on the method described by Bruchet et al. [22]. The distribution of Ol~i is listed in Table 4. The proportions of each category between the colloidal and the particulate fraction

are quite comparable: the PS and PHA are the predominant compounds in these samples, with a slightly higher proportion of PHA in the 20-300 kD fraction than in the particulate frac-

J. Rose et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 136 (1998) 11-19 15

i + + , , ,

0 1 2 3 4 5 6 R(A)

Fig. l. Fe-radial distribution functions for the Fe-oxalate solution.

v

a

1 . ..°" '°"" o t o . . . . . . i . . . . . : . . . . . . . . . . . /; ~

. , i o

-1

4 6 8 10 12 14

klA")

Fig. 2. Partial EXAFS functions generated by+ Fourier back- transforming the (a) 2.3-2.7 ,h, and (b) 2.7-3.8 A regions of the RDF ofoxalate-Fe solution L/M = 5 (solid linej compared with least-squares fits (dashed-line).

l--'--Ns+m' 1

0 ! 2 3 4 5 6 R(A)

Fig. 3. Radial distribution functions lbr the Nsimi sample compared with the RDF of Goethite.

tion. A more detailed analysis of the two pyrochromatograms provides supplementary information: in both fractions, the lignin and carboxylic acid by-products are present in large proportions,

whereas the nitriles, pyrroles and furans are less abundant (Fig. 4). The proportions of carboxylic acids and CO2 are higher in the colloidal (20-300 kD) than in the particulate fraction. In our case, the area of the CO2 peak is directly proportional only to carboxylic groups, since carbonates were not detected (Table 1 ). On the other hand, the proportion in lignin compounds is higher in suspended matter than in the colloidal fraction.

4. Discussion

The major problem encountered in the fitting of partial EXAFS curves is to determine the nature of the backscattered atom. For example, at a given distance it is not possible to differentiate atoms with similar atomic number (AI, Si, and P for example) [25]. Moreover, the presence of low atomic number elements in the vicinity of a heavy element such as Fe is not easily detectable because the amplitude of Fe-Fe contribution to the EXAFS signal is larger than the Fe-O-(light element) contribution [25,26 ].

For the Nsimi sample, the analysis of the first peak indicates that approximately six oxygen atoms (__+ 10%) are present: 3.4 oxygen atoms at 1.97 A and 2.3 oxygen atoms at 2.09 ,A.

The calculation of partial EXAFS curves corresponding to the 2.2-2.8 A and 2.8-3.6 A, ranges was carried out using two carbon shells and two iron shells respectively (Table 5, Fig. 5). For the first region ( Fe-O-C contributions, Fig. 5 (a)), the fit is satisfactory. For the second region (Fe-Fe contributions, Fig. 5(b)), the calculated curve does not fit well the experimental spectrum for k > 8 ,A-1. This problem is due to the important chemical and structural heterogeneity of the natural sample and the possible presence of multiple scattering phenomena.

In order to validate the presence of carbon atoms in the environment of iron in this sample, and to quantify the multiple scattering effect, ab initio calculations in a multiple scattering approach were carried out using the FEVF601 code [34]. This ab initio code allows the recalculation of the general shape of z(k) by determining separately the contribution of each multiple scattering path


Table 3 Structural parameters extracted from the partial Fe EXAFS curve fitting for the Fe-oxalate solution

R window (A) for the Fourier back-transform Rt, (A) Nc, R~e, (A,) NFe, Rye3 (A) Nve, Q

2.3-2.7 2.85 4.3 0.08 2.7-3.8 2.85 4.2 3.20 0.5 3.85 1.5 0.04

The electron mean free path 2=2k/L, where L= 1.8 A, -2 (determined from the Lepidocrocite) was chosen for the Fe-Fe pairs and for the Fe-C atomic pair L = 2.8 A, -2 (determined from the Fe-acetate). Fe-C~ corresponds to Fe-O-C linkages. Fe-Fet corresponds to iron octahedra sharing one edge, Fe-Fe3 corresponds to iron octahedra sharing one corner. R = distance between the two atoms of each atomic pair (A); N= number of atoms in the different coordination spheres of iron; Q=

residual of fit.

Table 4 Distribution of the main categories of organic compounds in the two fractions of the Nsimi water

> 0.45 pm 20-300 kD fraction fraction

Polysaccharides 31% 27% Proteins 12% 12% Aminosugars 5% 7% Polyhydroxyaromatics 32% 37% Miscellaneous 20% 18%

40 .

30

2(1

Z E ~-

20-300kD

.45 micron

Fig. 4. Distribution of the main organic compounds in the >0.45 pm and the 20-300 kD fractions of the Nsimi water.

and keeping those which exceed a given threshold. This code can only be used when the exact atomic coordinates of a given cluster are known. This cluster must take into account the experimental EXAFS data in terms of interatomic distances and number of neighboring atoms around the central atom. The 3D model developed in the case of the

Nsimi sample (Table 6, Fig. 6) represents only one structural solution among all, in agreement with the experimental EXAFS data (Table 5). This model corresponds to two clusters. In this model, bidentate complexes correspond to Fe-C distances of 2.82 A as in oxalate-Fe 2:1 complexes, whereas monodentate complexes correspond to Fe-C distances of 2.98 A.

There is a good agreement between the experimental RDF and the ab initio calculation in the 2.0-2.7 A range (Fig. 7). The back Fourier trans- forms of this region show an even better fit between the Nsimi sample and the model (Fig. 8). This suggests that, as far as Fe-C contributions are concerned, the model developed on the basis of the first EXAFS analysis is realistic, i.e. the iron in the natural Nsimi sample is bound to organic molecules forming monodentate and bidentate type complexes since both Fe-C distances are accurately accounted for by the model. In the 2.7-4.0 A range of the RDF, the fit between the model and the experimental curve is less satisfactory ( Fig. 7). This is due to the presence of multiple scattering paths which cannot be defined without a precise knowledge of the 3D geometry of the clusters. However, modeled Fe-Fe distances and the number of Fe neighbors (Table6) are not expected, to differ significantly from the actual structural parameters in the Nsimi sample. Thus it can be inferred from the, ab initio calculation that iron has a low level ,,," polymerization in the natural sample and : ~ edge and single corner sharing are :~. :Jredominant types of linkage between iron octahedra.

It is now well known that the hydrolysis of

J. Rose et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 11-19

Table 5 Structural parameters extracted from the partial Fe EXAFS curve fitting for the Nsimi sample

R window (A) for the Fourier back-transform Rc, (A) Nc, Rc, (A) Arc " Rre_, (A) N~e., Rre3 (~,) N~o3

17

2.2-2.8 2.82 1.9 2.98 1.5 2.8-3.6 3.35 0.4 3.86

0.08 0.7 0.12

Ct and C2 corresponds to mono- and bidentate F e - O - C linkages respectively. Fez corresponds to edge sharing iron octahedra.

O.l

0.05

0 O3

-0.05

-0.1

~ ~ ~ ] a . . . . . . . . . Calculated

. ; ~ , .,

..... ii

I I I I

4 6 8 I0 12 14 k(A" )

Table 6 Coordination numbers for the different Fe atoms of the 3D model

N F e - 7 Ct N F e - 7 C2 N F e - - Fe 2 N F e - ~- Fe a

2.82 A 2.98 A 3.35 ~, 3.86 A

Fel 2 2 0 1 Fe 2 2 2 0 2 Fe3 2 2 0 1 Fe4 3 0 I 0 Fes 1 2 1 0

Model ( 2 + 2 + 2 + 1.6 0.4 0.8 3 + I )/5 = 2

0,8

0,6

0,4

0,2 2~ -~ o

co

X.O 2

-0,4

-0,6

~8

Experimental ! b . . . . . . . . . Cala~ed

4 6 8 I0 12 k(A" )

Fig. 5. Partial EXAFS functions generated by Fourier back- transforming the (a) 2.2-2.8 ,~ and (b) 2.8-3.6 ,~ regions of the RDF of the Nsimi sample (solid line) compared with least-squares fits (dashed lines).

Fe 3+ leads to the formation of double corner sharing linkages between iron octahedra which are the premise of the nucleation and growth processes of polycations and infinite FeOOH phases [23, 31- 33]. The absence of such double corner bonds in the Nsimi sample clearly indicates a strong hindrance of the Fe hydrolysis. This hindrance is due to the blocking of the Fe growth sites by complex-

ation. Complexation of Fe by Si is highly unlikely in our case, because (i) the concentration of Si (~ 7 mg/l ) determined by chemical analysis of the Nsimi sample (Table 1 ) represents the soluble Si in equilibrium with silica [35]. Lower concentrations of Si would have been measured in the case of significant complexation between Fe and Si; (ii) the experimental EXAFS data could not be fitted using Fe-Si contributions. Therefore, it can be assumed that complexation by organic ligands plays the major role in the hydrolysis

n atom

Fe:P-~2/Fe2-Fe3 = 3.85/k

Fe4-Fe5= 3.35 ,g,

3D model

Fig. 6. 3D model developed in order to fit the experimental EXAFS spectrum of the Nsimi sample using a multiple scattering approach.


..". " Experimental , ,A" ] . . . . . . . . . Calculated I

1 2 3(,~) 4 5 6

Fig. 7. Comparison of the RDF curves for the Nsimi sample and the model. Solid line: experimental: dashed line: calculated.

0,03

0,02

0,01

o -0,01

-0,02

-0,03

[ ~ Exoeriment,I ] . . . . . . . . . . . t . . . . . . . . . c'r°ul=e t . . . . . . . . . . . . .

4 6 8 10 12 klA")

Fig. 8. Partial EXAFS spectra of the 2.0-2.7 ,~, region of RDF. for the Nsimi sample (sohd line) and for the model (dashed line).

Similar to the PHA, the proportion of lignin compounds is very high. Thus the major information encountered in the nature of NOM in Nsimi's catchment area is that the organic matter consists predominantly of terrestrial matter. Thus, it can be assumed that, in Nsimi's catchment area, the terrestrial matter migrated to the aquatic environment. This assessment is confirmed by a C/N ratio between 20 and 25, which is indicative of terrestrial organic matter [ 16]. The comparison of the two samples shows that there are more lignin pyrolysis by-products in the particulate fraction than in the colloidal fraction, and on the other hand there are fewer carboxylic acids in the >0.45 pm fraction than in the 20-300 kD fraction. Since the carboxylic acids mainly originate from the degradation of molecules or from bacterial and fungal activity, the colloidal fraction appears more degraded than the other one. The Nsimi sample previously studied by XAS corresponds to the colloidal fraction enriched in carboxylic acids which are very reactive, thus explaining the high level of complexation of iron. Moreover, the presence of carboxylic acid is strongly correlated with PHA and related molecules [39,40].

hindrance. Recent results on coagulation of river water NOM by Fe a+ salts at pH ~7, leading to the formation of Fe-organic complex aggregates in which the hydrolysis of Fe is very limited [36], support this hypothesis.

The analysis of the NOM of the Nsimi sample can tentatively help to bring some explanations of the complex formation potential. The interpretation of Py-GC-MS chromatograms proves that the overall compositions of colloidal and particulate fractions are similar. The PHA and PS are the predominant compounds (Table 4). The sources of PS can be aquagenic (structural PS from cell walls or algae) or pedogenic (cellulose types resis- tant to degradation), while the PHA originate from extensive humification of plants and degradation products [37,38]. It appears that the aquagenic materials are less present in our samples: the proportion of PR mainly originating from algual or phytoplanctonic sources is about 12% of the NOM, and the proportion of AS derived from cell membranes of microorganisms is very low (5%).

5. Conc lus ion

In this study we showed that XAS is a powerful technique to determine the speciation of iron in natural colloids in which NOM are the major complexing ligands. The NOM, similarly to the small organic acids (such as oxalic acid), hinder the hydrolysis of Fe even at pH 6. Infinite phases such as 0cFeOOH, which are formed in the absence of complexing ligands, are not detected in the Nsimi sample.

We showed that EXAFS spectroscopy allows us to determine the structure of complexes formed by heavy and light elements. As an example, bidentate and monodentate organo--Fe complexes are detected in our natural Nsimi sample.

A c k n o w l e d g m e n t

The authors wish to thank Dr. Alain Manceau for his help in obtaining the EXAFS data and the

J. Rose et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 136 (1998) 11-19 19

staff of the D42 X-ray beamline at the LURE synchrotron (Universit6 Paris-Sud, Orsay, France).

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iron speciation in natural organic matter colloids

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