X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface

X-Ray Absorption Spectroscopic Study of the Sorption of Cr(lll) at the Oxide-Water Interface II. Adsorption, Coprecipitation, and Surface Precipitation on Hydrous Ferric Oxide LAURENT

CHARLET*

AND A L A I N A. M A N C E A U ' ~

* Institutfiir anorganische, analytische, und physikalische Chemie, Freiestrasse 3, 3000 Bern, Switzerland, and t Laboratoire de Mindralogie-Cristallographie, CNRS UA09, Tour I6, 4 place Jussieu, Paris 75252, France

Received October 30, 1990; accepted June 25, 1991 The sorption of Cr(III) by hydrous Fe oxides involves adsorption, surface precipitation, and coprecipitation phenomena. These phenomena lead to different phases which are here compared with regard to both their Cr solubility and their local structure. The former is simulated by the "surface precipitation model" and the latter is derived from X-ray absorption fine-structure (EXAFS) spectroscopic data interpreted by the "polyhedral approach method." The adsorption of a Cr(IlI) atom onto goethite or hydrous ferric oxide (HFO) occurs via the formation of inner-sphere surface complexes. In such complexes, Cr atoms are never isolated, but present as small surface hydroxy polymers. This polymerization has been catalyzed by the surface, and it occurs when Cr(III) bonds only 10% of the surface "active" sites (i.e., when it covers 1% of the BET surface area). In these surface polymers, Cr(IIl) atoms are surrounded by three metal (Fe or Cr) shells at 3.00-3.05, 3.40-3.46, and 3.94-4.03 A, as in a mixed a- and 3'MeOOH local structure (Me = Fe or Cr). These polymers act as nuclei for the precipitation of a surface hydrous Cr oxide, a precipitation that starts under conditions undersaturated with respect to the homogeneous precipitation of the same oxide, a phenomenon well described by the surface precipitation model. This surface precipitate and the pure hydrous chromium oxide have identical solubility products and the same local structure (3,-CrOOH). On the contrary, when Cr(IIl) is coprecipitated with Fe(III), only two cation shells are detected around Cr(llI) at 2.99 and 3.40 A, indicating that Cr substitutes for Fe in an a-(Fe, Cr)OOH framework. A difference in local structure (a-CrOOH versus 3"-CrOOH) therefore accounts for the difference in solubility products of the surface-precipitated and coprecipitated Cr hydroxides. © 1992 Academic Press, Inc. INTRODUCTION The transition from metal ion adsorption to either p r e c i p i t a t i o n o n a m i n e r a l surface or c o p r e c i p i t a t i o n with o t h e r m e t a l ions is o f prim a r y i m p o r t a n c e in a wide variety o f contexts, such as m a t e r i a l science ( d o p i n g o f r e c o r d i n g m a g n e t i c particles a n d c o r r o s i o n c o n t r o l ) , catalysis ( d o p i n g o f s u p p o r t surfaces), e n v i r o n m e n t a l engineering ( i n d u s t r i a l w a s t e w a t e r t r e a t m e n t a n d recycling o f m e t a l s ) , oceanogr a p h y (scavenging o f trace e l e m e n t s ) , a n d geochemistry ( c o n t r o l o f g r o u n d w a t e r pollution a n d ore d e p o s i t i o n ) . T h i s t r a n s i t i o n has been previously investigated a l o n g two different lines. O n o n e h a n d , v a r i o u s spectroscopic m e t h o d s have b e e n used in catalysis research

to s h o w t h a t m e t a l ions are in m a n y cases n o t i n d i v i d u a l l y s o r b e d o n a surface, b u t i n s t e a d are p r e s e n t as surface clusters. O n t h e o t h e r h a n d , g e o c h e m i s t s have d e s c r i b e d w i t h i n t h e r m o d y n a m i c f r a m e w o r k s such as t h e " s u r face p r e c i p i t a t i o n m o d e l " ( 1 ) a n a s s u m e d c o n t i n u u m b e t w e e n the f o r m a t i o n o f i s o l a t e d surface c o m p l e x e s ( 2 ) a n d surface p r e c i p i t a tion. Efforts to relate m o l e c u l a r s t r u c t u r e s to surface c o o r d i n a t i o n m o d e l s are few ( 3 - 5 ) . T h e p h e n o m e n o n investigated in t h e prese n t s t u d y is t h e s o r p t i o n o f C r ( I I I ) o n t o h y d r a t e d F e o x i d e s ( H F O ) . This s o r p t i o n o c c u r s in soils a n d rivers (6, 7 ). It is also w i d e l y u s e d in w a t e r t r e a t m e n t p l a n t s a n d in a n a l y t i c a l c h e m i s t r y to r e m o v e c h r o m i u m f r o m d i l u t e s o l u t i o n s ( 8 ) . T h r e e distinct s o r p t i o n p h e 443 0021-9797/92 $3.00

,lournal of Colloid and Interface Science, Vol. 148, No, 2, February 1992

Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

444

CHARLET AND MANCEAU

The hydrous chromium oxide (HCO) sample is synthesized by coulometrie titration (see below) of a mixed CrCI3/HC1 solution up to neutral pH. X-Ray diffraction analysis indicates.that the HCO material is "amorphous." The local structure of HCO has been investigated in Part I ( 19): two metal atomic shells are present about each average Cr atom, one with 1.9Cr at 2.9 A and one with 1.5Cr at 3.98 A. In contrast to HFO, HCO possesses a 3'CrOOH local structure, i.e., is isostructural to lepidocrocite (see Fig. 1 ). The number of Cr neighbors in HCO is particularly low for a solid constituted by a 3D framework ofpolymerized octahedra, a n d may have been underestiMATERIALS A N D METHODS mated, due to an important structural disorder at the local level, which is consistent with the Material disorder already observed on a larger scale by Two adsorbents have been used: a well- X-ray diffraction. This disorder decreases the crystallized goethite (a-FeOOH) a n d a hy- amplitude, of the signal and, hence, leads to drous Fe oxide (HFO). The classical method underestimated values ofthe number of metal for synthesizing goethite has been slightly atoms present in each shell. modified (use of sulfate-iron instead of niThe coprecipitated Fe/Cr hydroxide (HFO / trate-iron salts) to produce a sample with no HCO) is synthesized in the same manner as microporosity (14) and a N2 BET specific sur- HFO, but it is allowed to age afterward for a face area of 19,1 m z g-1. The precipitation of week as a suspension. X-Ray diffraction analH F O involves the dissolution of Fe(1",103)3 salt ysis indicates that it has a poorly crystallized in a 0.01 M HCI solution a n d the rapid titra- goethite structure. Its chemical composition is tion of this solution up to pH 7.0 under nitro- feo.994Cr0.0o6OOH. gen with a 0.1 M NaOH solution free of COa (13). The product is then filtrated, rinsed with Experimental deionized water, and freeze-dried. Its specific surface area is equal to 256 m 2 g-1. This prodSample preparation methods. In method 1, uct is considered "amorphous" through X-ray an HFO (or goethite) sample is suspended in diffraction ( X R D ) a s its pattern displays two 75 ml of a C r / N a chloride solution, pH 2.5 broad hk bands at about 2.5 and l.5 A which ([CI-] = 0.6 M). This suspension is couloare characteristic of what is often referred to metrically titrated (see below) to pH 4.00, eias a "two-line ferrihydrite" ( 15, 16 ). The local ther directly or step by step. Base is generated structure of this product has been investigated at a slow rate (1.8 gmole min-1) at the surface in a previous work by EXAFS spectroscopy of a large Pt electrode to avoid the presence (17), and has been found to be characterized of any local excess of base in solution. An enby the presence of two Fe shells about each tire titration to pH 4.00 takes at the most 24 Fe atom, at 3,05 and 3.43/~. HFO is thus well h, an interval during which about 1% of the ordered at the local scale, and its short-range Cr(III) initially present in solution could have order is similar to those of o~-FeOOH and/3- been, in the absence of adsorption, dimerized FeOOH ( 18 ) (Fig. 1 ). Since aging of HFO (20). During this time however, 95% of the leads to goethite, its local structure is most Cr(III) present in solution is adsorbed. Thereprobably close to that of goethite (c~.FeOOH). fore, at the time when adsorption occurs, nomena (9) have been described in the cr(IIl) literature ( 10): the adsorption of Cr( III ) onto HFO ( 11 ), the surface precipitation of Cr(III) hydroxide onto hematite (12), and the coprecipitation of Fe(III) and Cr(III) (13) '. In this paper we report (i) on the description at a macroscopic level of the transition from ad' sorption to surface precipitation (or to eoprecipitation), (ii) on the local structures found in these different phases by extended X-ray absorption fine-structure spectroscopy (EXAFS), and (iii) on an attempt to bridge the macroscopic (thermodynamic) and the microscopic (structural) information.

Journal of Colloid and Intoface Science, Vol. 148; No, 2, February 1992

Cr(III)

SORPTION

ON

HYDROUS

445

Fe OXIDES

iepidocroci~(te~. ~~" 'FeOOH) ~~,. :'~~ 3.87]k

akaganei(l~ teFeOOH) goethite

0

1

t

~

0

2

3

4 R (A)

/..1

l

2

3

4 R (A)

l

2

3

4 R (A)

(eFeOOH)

hematite

(c~Fe203)

~ ~

u

~t ,~

v

2'89/i"

~,

\~

'~

r

I

I

0

1

2

I ¥

~

3

4 R (]~)

V

I

FIG. 1. Polyhedral approach to the structure of trivalent Fe oxides. In the 3' structure, where octahedra share single corners (SC), thethird RDF peak is shifted to larger distances compared with the one in a and structures (DC linkage). Despite their identical number of nearest and next-nearest Fe neighbors, aFeOOH and/3-FeOOH structures yield different RDFs, as a result of the difference in coherency of the FeFe distances along and between octahedral chains. A similar situation has been observed for Mn oxides (Part I). Fe-Fe distances have been calculated from crystallographic data (46-49).

chromium atoms are present in solution cally titrated to pH 4.00. In this Cr solution, mainly as free ions. about 20% of the total Cr is polymerized (20). In method 2, to study the effect of equiliCoulometric titration. The titration of a I0 bration time and sample water content o n g liter -1 HFO (or 100 g liter -I goethite) susEXAFS results, some experiments had to be pension is done under a presaturated N2 atperformed at the synchrotron facility during mosphere in a reactor immersed in a therthe synchrotron beam session. For these ex- mostated water bath ( T = 25,0 +_0.1 ° C ). The periments, a simplified method, which is sim- proton concentration, [ H +], is measured with ilar to that described in Part I, was followed a Keithley 196 voltmeter and a Metrohm (19). An aliquot of HFO suspension, pre- combined glass electrode, previously calibrated equilibrated at pH 6.00, is mixed with an aged by Gran titration. For the sake of simplicity, Cr mother solution, previously coulometri- we use the term pH to refer not to an activity Journal of Colloid and InterJdce Science, VoL 148, No. 2, February 1992

446

CHARLET

AND

scale, but to a concentration scale, i.e., to - l o g [ H +]. Base is generated at the surface of a Pt electrode. This electrode is connected to a salt bridge Ag/AgC1 electrode by a Keithley 224 programmable current source. All samples of the "sorption isotherm" (see below)were titrated directly at p H 4.00, and then allowed to equilibrate for 5 days. On the contrary, the titration curves of H F O in the absence or presence of 3 m M C r ( l l I ) were done step by step, with "steps" roughly 0.2 pH unit apart one from another. At each step, the suspension was allowed to equilibrate for 3 h. This equilibration time had previously been shown to be long enough for Cr sorption onto HFO (or goethite) to reach equilibrium (11, 21 ). At each step, the apparent net proton surface excess, 6nn, is calculated as (22)

MANCEAU

excess in chromium, I~cr, is defined as the unitless moles of chromium sorbed per mole of Fe present in the solid phase ( 1 ). The log Fc~ vs log [ Cr 3+] curve is called the "sorption isotherm." Macroscopic M o d e l

The sorption isotherm is interpreted by the "surface precipitation model," as the result of different sorption reactions. A single-site adsorption of free Cr(III) metal ions is assumed to occur at low total Cr (III) ion concentration in a Langmuir-type fashion. Once the active surface sites are saturated, a (Fe, Cr)(OH)3(s) solid solution is assumed to precipitate (1). In fact, three reactions occur simultaneously TABLE 1

6nil = (CA -- CB + [ H +1 - [ O H - I ) V

Reactions and Basic Equations of the Surface Precipitation Model

where cA is the initial total proton concentration ( 10 -2 M ) , cB is the total concentration of hydroxyl ions generated since the start of the titration, and V is the suspension volume.

Reactions Cr 3+ + ~ F e O H = Fe(OH)3(s) + ~ C r O H + 3H +

Concentration m e a s u r e m e n t s a n d data analysis. At the end o f the equilibration time,

the suspension is filtered under nitrogen through a 0.0l-#m filter paper and repeatedly washed with a 0.6 M NaC1 solution, p H 4.00. The filter pad is either air-dried (method 1 ) or directly analyzed as a paste (method 2). Metal (Cr or Fe) total concentrations are measured on the filtered solution as well as on a concentrated nitric acid solution in which a sample of the adsorbent has been dissolved. These chemical analyses are done by GTAAAS on a Varian AA 875 spectrometer or, when the metal concentration is lower than 0.2 # M , by absorption polarography. Speciation of the solution is performed by the computer program M I C R O Q L (23) on the basis of available hydrolysis constants (24). Conventional free ion activity coefficients are computed according to the specific interaction theory (25). Total and free metal ion concentrations are given in moles per liter (or molar) and indicated by brackets. Subscript TS refers to total concentration in solution. The surface Journal of Colloid and Interface Science, Vol. 148, No. 2, February 1992

Fe 3÷ + 3 H 2 0

=

Fe(OH)3(s) + 3H +

Cr 3+ + 3 H 2 0

=

Cr(OH)3(s) + 3H ÷

D] [2] [3]

Basic equations ST = [~FeOH] + [~CrOH]

[41

[Cr(OH)3 (s)] ._ . Xe,~onl~ = [Cr(OH)3(s)] + [Fe(OH)~(s)] (= zj K~a~

[~CrOH]XF~°m~ • Kmco (E B. Kmco) ~FeOH]Xc~om3

K~ro =

XF~on)~l [H*] 3 [Fe3+]

Kmc°

Xc~on)~) [H+] 3 [Cp+]

[5] [6]

[71

[83

T O T F e = [Fe(OH)3(s)] + [EFeOH] + [Fe]rs [91 Sorpfion isotherm equation [10]

Pc, = [ECrOH] q [Cr(OH)3(s)] TOTFe TOTFe with [~CrOH] = TOTFe [Cr(OH)3(s) ] = TOTFe

Sr - - (1 + / F l ( z -j -- 1))-I ( ~ y) TOTFe 1

_ Y _~ z([H+] 3 + Q,,[H+] 2 + Q,3 [H+])

(z-' - 1)

and z=

[CP+]K, nco [H*] 3

B

KmvoTOTFe

Cr(III) SORPTION

ON HYDROUS

(Eqs. [1 ] to [ 3 ], Table I): the adsorption of Cr(III) onto a surface site - F e O H and the precipitation of both Fe(OH)3(s) and Cr(OH)3(s). On adsorption, the metal is hydroxylated and becomes a surface site itself (-=-CrOH) while the site on which it has been adsorbed (----FeOH) is integrated into the solid phase (Eq. [ 1], Table I). The total surface site concentration therefore remains constant (Eq. [4], Table I). Thermodynamic computations on this three-equilibrium system can be carried out in terms of conditional equilibrium constants (Eqs. [6] to [8], Table I), i.e., in terms of mole fraction of the solid species (Eq. [ 5 ], Table I) and of concentration of the species present in solution or at the surface, under the following conditions (5). The two surface species must have equal activity coefficients and, within the range of isomorphic substitution considered (here, 0-20%), the activity coefficients of the two solid species must be constant. The first assumption is valid because the surface charge remains constant at pH 4.00 whatever the Cr loading, as will be shown later. The validity of the second assumption for Fe(OH)3(s) is a result of its constant ion activity product throughout the isotherm (Table II). The Cr(OH)3(s) case is also discussed later. Farley et al. (1) have solved the system formed by Eqs. [4] to [9] (Table I) at constant proton concentration, and have shown that the metal surface excess, l~cr, is a function of a single variable: the free metal concentration in solution (Eq. [ I 0 ], Table I). This function contains a few parameters the values of which are chosen as follows. The first two iron hydrolysis constants, Q , and QJ2, are taken from the literature (18). The HFO solubility product, KsnFo, is computed from the [Fe]T~ experimental values. The conditional equilibrium constants, Kads and KsHco (for reactions [1] and [ 3 ], Table I), are obtained from an analysis of the sorption isotherm described in detail elsewhere ( 1, 5). The total concentration in surface sites, ST, is set equal to the concentration of sorbed chromium at the inflection

Fe O X I D E S

447

point which is commonly observed in such isotherms (1). The sorption phenomenon is therefore dominated below this inflection point by a Langmuir-type adsorption, and above it, by a coprecipitation phenomenon.

EXAFS Data Collection EXAFS data were collected at the L U R E synchrotron facility (Orsay, France) on the EXAFS IV spectrometer with running conditions of 1.85 GeV and 300-260 mA beam current. The methods followed in data collection and data reduction have been described in Part I (19). We hereafter refer to the radial distribution function obtained from the Fourier transform of the EXAFS spectrum as RDF. All RDFs presented are uncorrected for phase shifts. In a preliminary study, the effect of sample wetness was investigated. Comparison of spectra recorded on wet paste at room temperature and on dried material at 77 K indicated (see Results) no change in structural information, while the signal-to-noise ratio was maximized by lowering the temperature. Thus, all RDFs reported here are derived from EXAFS spectra recorded at 77 K on air-dried material.

Polyhedral Approach The polyhedral approach is a method of interpretation of the metal-metal distances obtained from the RDF, based on the recognition of a tight correspondence between these distances and octahedra linkages in oxides (26). Four octahedron linkages are relevant to Fe and Cr (hydr)oxides (Fig. 1 ) and they correspond to the sharing of the following 1. A single corner (SC), as in a monodentate mononuclear complex. A given octahedton shares in this case one oxygen with another octahedron. 2. A double corner (DC), as in a bidentate binuclear complex. A given octahedron shares in this case two nearest oxygens with two different octahedra. Journal of Colloid and Interface Science, Vol. 148, No. 2, February 1992

448

CHARLET AND MANCEAU 10

charge [ P Z N P C (22)] equal to 7.8. This value is very close to the absolute P Z N P C of HFO, E 7.96, the end p H value in a titration of a stan5 ¸ dardized Fe(III) solution by a base solution • AA ".r containing an equal total a m o u n t of equiva,g • AA . A ,J.jL lent charge (L. Spadini, personal communication). Therefore, 6nn is, within the accuracy •e of such experiments, a measure of the absolute oo °o ° .5 ¸ net proton H F O surface charge (22). At low • . . , . . . , • . . , p H ( p H ~< 3.5) this surface charge reaches a 2 4 6 8 m a x i m u m value, equal in terms of density to pH 1.97 protonated sites/nm z. This density, the FIG. 2. Titration curvesof HFO obtained in the absence PZNPC, and the intrinsic acidity and basicity (triangles) or presence (circles) of a total concentration constant values of H F O (data not shown) are (solid + solution) of 3 rnM Cr(III). in very close agreement with those measured for well-crystallized goethite samples (30, 31 ). The two solids, H F O and goethite, can indeed 3. An edge (E), as in a bidentate m o n o be considered to differ only by their degree of nuclear complex. A given octahedron shares crystallinity (and thus by specific surface area) in this case two nearest oxygens with another since they have a similar local structure. octahedron. 4. A face (F), as in a tridentate m o n o n u Sorption of Cr(III) onto HFO clear complex. An octahedron shares three The titration of H F O in the presence of 3 nearest neighbors with another octahedron. m M C r ( I I I ) is presented in Fig. 2 (circles). 3,-MeOOH structures (e.g., the H C O local As reported in studies performed on similar structure) are characterized by 2SC + 6E linkmaterial ( 1 1, 21 ), c h r o m i u m ( I I I ) is adsorbed ages per octahedron; ~ - M e O O H (e.g., H F O in the low p H range. The adsorption is comlocal structure) by 4DC + 4E linkages (Fig. pleted at 99.7% at p H 4.00 (Table II), and 1 ). In Fe(III) oxyhydroxides the F e - F e distances across SC, DC, and E are, respectively (26, 27) (Fig. l ) , 3.87, 3.46-3.7, and 3.01TABLE It 3.07 A (with an additional distance of 3.28

eAA •

~A

-

in a - F e O O H ) . In C r O O H , C r - C r distances across D C and E are, respectively (28, 29), 3.56 and 2.96-2.98 A. Therefore the C r - M e distances measured by EXAFS spectroscopy will permit the way c h r o m i u m octahedra are bridged to other Me octahedra in the bulk or at the surface o f solids investigated herein to be deciphered. RESULTS

Properties of the Adsorbent Phases (HFO and Goethite) T h e H F O proton titration curve is given in Fig. 2 (triangles). It indicates, for an H F O surface, an apparent point o f zero net proton Journal of Colloid and Interface Science,

Vol.

148, No.

2, February

1992

Adsorption/Surface Precipitation Experimental Data after 5 Days of Equilibration at pH 4.00, I = 0.6 MNaCI, and Log TOTFE = -0.9487 LoglCr]rs

Log[C?

+]

-6.938 -6.938 -6.762 -6.675 -5.540 -5.354 -4.968 -3.813 -3.374 -3.017

-7.105 -7.105 -6.929 -5.842 -5.707 -5.521 -5.135 -3.980 -3.541 -3.184

Log

I¥~

-3.263 -3.090 -2.794 -2.238 -2.059 -1.694 -1.563 -1.320 -0.973 -0.693

Log[Fe],rs

-6.633 -6.543 -6.492 -6.405 -6.706 -6.571 -6.405 -6.435 -6.601 -6.522

RDF

---#1 #2 #3 #4 #5 #6

a Subscript TS refers to total concentration in solution. The Cr surface excess, rc~, is defined in the text.

Cr(III) SORPTION ON HYDROUS Fe OXIDES

beyond, the titration curves run parallel (Fig. 2). This adsorption is completed 3.8 pH units below PZNPC. It therefore occurs against a positive surface charge which indicates a strong inner-sphere bonding between the surface =-FeO groups and the chromium ion. This sorption results in a net release of protons, which can be estimated from the comparison of the HFO titration curves performed in the absence (Fig. 2, triangles) and in the presence (Fig. 2, circles)of 3 m M Cr(III). At pH 4.00 the vertical distance between the two lines indicates a release of about three protons per Cr 3+ ion adsorbed, as assumed in Eq. [1] (Table I). This adsorption therefore does not induce any change in the surface charge, a result which contrasts that obtained in studies of bivalent heavy metal ion adsorption on goethite and hematite, where adsorption was never electrically balanced by an equal release of protons (32, 33). The sorption isotherm of Cr(III) onto HFO at pH 4.00 + 0.10 is given in Fig. 3 (solid squares). This isotherm can be subdivided into

"[.0

•

i

'

~c3 -1.o o.0

--o

-2.0

i

•

i'

.

,

.

i

,

co-precipitation ~ ~

/

three regions. In the low concentration region the amount of metal sorbed is proportional to the free metal ion concentration in solution, and represents at least 99% of the total Cr(III) (Table II). At larger concentration (4.0 ~< - l o g [ C r 3+] ~< 5.2), the adsorption curves present an inflection point and a "plateau" around this point. The metal surface excess at this inflection point is equal to - l o g rcr = 1.4 _ 0.1. According to the surface precipitation model, this value is a measure of the number of surface sites. In terms of site density it is equal to 1.3 sites n m -2, a value remarkably close to that obtained on the basis of proton maximum adsorption (1.97 sites nm-2). At larger total Cr (III) concentrations, the amount of metal sorbed increases again, proportionally to the concentration of free chromium ion, [Cr3+]. The conditional equilibrium constants obtained from the analysis of this isotherm ( 1, 5) are, for Eqs. [ 1] to [ 3 ] (Table I), Kads = 10 -6"66, KsHFO = 10 -3'82, and KsHco = 10 -9.68. These solubility products are very close to the values reported previously in the literature: pKsnco = 9.8 in 0.01 M NaC104 (34) and PKsnFO = 3.96 in 3 M NaC104 respectively (35). These constants allow an adequate description of the data by the surface precipitation model (Fig. 3, lower solid line). Local Order about Cr(IlI) Atom Sorbed on Goethite

precipitation

~

449

-3.0

-4.0 -8.0

a,ds°rpti°n, -7.0

-6.0

,

, -5.0

,

, -4.0

, -3.0

, -2.0

log [Cr 3+] FIG. 3. Sorption isotherm of Cr(IlI) either adsorbed and surface precipitated on HFO (solid squares) or coprecipitated with Fe(III) [empty squares; data from Fig. 1 and Table 1 of Ref. ( 13)]. rcr is unitless and [Cr 3+] is in M. Experimental and model parameters: T = 25°C, pH = 4.00. In the surface precipitation case, equilibration time = 5 days, I = 0.6 M NaC1, TOT Fe = 0.1125 M, ST = 4.48 × 10 -3 M, log K, ds = --6.66, log Ksr~vo = -3.82, log Ksnco = -9.68. In the coprecipitation case, equilibration time = 1 week, Sr = 4.48 × 10-2 M, log Ka~ = -6.66, log KsHvo= --3.82, log KsHco = --8.68.

The RDF of Cr adsorbed at 13.4% surface coverage on a well-crystallized goethite [assumed to have a total of 2.31 sites n m -2 (31 ) ] is given in Fig. 4. It clearly shows three weak but well-defined Cr-Me peaks, between 2 and 4 ~, i.e., beyond the intense Cr-O contribution. The Fourier backtransform in the range 2.2-4.0 ~, results in a typical wave beating between 4 and 5 ~-1 (Fig. 5). This beat pattern can be well reproduced by fitting the experimental filtered EXAFS contribution, assuming three (Fe, Cr) atomic shells at 3.01, 3.45, and 3.95-3.99 A, with 1.1-1.6, 0.5-0.8, and 1.2-1.9 Me atoms, respectively (Fig. 5 and Journal of Colloid and Interface Science,

Vol. 148,No. 2, February 1992

450

CHARLET AND MANCEAU

coprecipitation A coprecipitation ~v~. I

I

/

I --

oc FeOOH

CrK-edge

1:""I ........:oK-edge

I: il

~"

~~

0

F e 3+ "~ 3.28A - '

Cr(lll) for Fe(lll) isomorphic substitution 1

2

3

4

R (A)

adsorption

Fe OOH :Cr

-

adsorption

-

~'~"

? CrOOH

y CrOOH

surface precipitation

........ p,¢

LL

~, CrOOH I

2

3

4

R (A)

~.~,A

0

F e 3+

l'lullinuclear 'YCrOOH-like surface complexes

FIG. 4. Lower left: CrK-RDF of Cr(III) sorbed on goethite (-log I'cr = 2.86). Upper left: RDF of coprecipitated (Fe, Cr)OOH(s), recorded at the FeK absorption edge (dotted line) and CrK absorption edge (solid line). Right: structural interpretation of these RDFs.

Table III). The second and third C r - M e distances are very close to the second Me shell distance in H F O ( d [ F e - F e ] = 3.43 A) and in H C O ( d [ C r - C r ] = 3.98 A), respectively. A straightforward application of the polyhedral approach tells us therefore that, on average, each Cr octahedron sorbed onto goethite is associated with other Cr a n d / o r Fe octahedra by 1.1 to 1.6 edges, 0.5 to 0.8 double corners, and 1.2 to 1.9 single corners. These results contradict a model in which some Cr octahedra would share only one edge and others only one or two comers (e.g., within several types of isolated surface mononuclear Cr surface complexes), since such a model would Journal of Colloid and Interface Science, Vol. 148, No. 2, February 1992

result in a m u c h lower average n u m b e r of Me neighbors in each metal shell. Instead, they are fully consistent with the formation ofmultinuclear (Fe, Cr) complexes. The existence of such complexes is further suggested by the fact that the n u m b e r of atomic neighbors herein determined does not take into account the possibility of a structural disorder. Because o f this local disorder the n u m b e r of Cr neighbors measured by EXAFS is underestimated

(NExA~S < NREAL)Three structural models can be further hypothesized for the sorption mechanism of Cr onto goethite: (i) diffusion of Cr into the lattice; (ii) formation o f a discrete, separate Cr

Cr(III) SORPTION ON HYDROUS Fe OXIDES

.0~

.00~ 11 v 4

6 8 WAVEVECTOR k(,~-1)

10

FIG. 5. Fourier-filtered EXAFS of the three Cr-Me RDF peaks of Cr (III) sorbed on goethite (solid line) compared with a least-squares-fit theoretical EXAFS function (dotted line) assuming 1.1 (Fe, Cr) at 3,01 A, 0.8(Fe, Cr) at 3.45 /~ and 2.2(Fe, Cr) at 3.95 A.

oxyhydroxide phase not bounded to the surface functional groups (homogeneous precipitation); and (iii) formation of multinuclear Cr complexes at the surface of goethite. Possibilities (i) and (ii) cannot be retained because the present RDF does not look like that of either Feo.994Cro.o06OOH or HCO (Fig. 4). Likewise, there are not enough nearest (Fe, Cr) atoms at 3.01 A (1.1 ~< N~< 1.6) to be consistent with a combination of both mechanisms, (i) and (ii), as both H F O / H C O and HCO have 1.7 to 2.4 nearest neighbors across edges (Table III and Table II of Part I). Thus, Cr sorbed onto goethite is present as small surface polymers or "clusters" (3) in which the chromium atoms are in both an aMeOOH and a y-MeOOH local environment. Local Order about Cr(III) Atom Sorbed on HFO

The RDFs obtained for samples taken along the sorption isotherm (Table II and Fig. 3) are presented in Fig. 6 ( full lines). The lowest surface coverage which can be reached with the present X-ray flux corresponds to Cr(III) occupying 10% of the "total reactive surface site density," as defined by the chromium coverage at the sorption isotherm inflection point (Ns). Samples from the three regions of the sorption

451

isotherm ("adsorption," "surface saturation," "surface precipitation") are found to be labeled by distinct shapes in the portion of their RDF between 3 and 4 ~., which corresponds to the second- and third-neighbor metal shell (Fig. 6). This evolution of the local structure is also visualized on the Fourier-filtered CrMe EXAFS contributions (Fig. 7). With increasing Cr loading the first metal shell peak (Fig. 6) and, hence, the number of neighbors within this shell (Table III) decreases. This indicates a reduction of the average number of neighbors across edges. But the most striking feature of this series of RDFs is the progressive diminution of the number of next-nearest Me neighbors (DC linkage) as surface loading increases (Table II). This contribution is a "signature" of the a-MeOOH structure surrounding the Cr atom. As long as it exists, the Cr atom is in part in the same structural environment as it is in the coprecipitated sample, i.e., in a goethite-like local structure. The second RDF peak fully disappears only in the true "surface precipitation" part of the isotherm, i.e., when Fcr exceeds the "plateau" value. The RDF of the sorbed Cr atoms (#6, Fig. 6) is then identical to that of the HCO phase (dotted line, Fig. 6). The third Me RDF peak (indicative of SC linkage) is present even in the very first spectrum. This peak indicates the presence of multinuclear chromium complexes even at 10% monolayer coverage. These complexes have a y-type structure identical to that of HCO. Thus a heterogeneous nucleation is already occurring at Cr concentrations lower than the saturation of the surface sites and under conditions where, for kinetic reasons, polymerization in solution is negligible. Of particular interest is the constant intensity of this third Me contribution (Table III and Fig. 6) whatever the chromium content may be. As this contribution is attributed to Cr-Cr SC linkages, one expects the size of the Cr polymers not to increase with progressive coverage of the surface. EXAFS spectroscopy, however, fails to differentiate small ordered chromium clusters from large disordered 3D Journal of Colloid and Interface Science,

Vol. 148,No. 2, February 1992

CHARLET AND MANCEAU

452

TABLE III EXAFS Parameters Sample

Reference phyllomanganate

Atomic pair

Fe0.994Cr0.0o60OH

One-shell fit Two-shell fit

FeOOH:Cr

Three-shell fit

Three-shell fit

#1

Three-shell fit

#2

Three-shell fit

Three-shell fit

#3

Three-shell fit

#4

Three-shell fit

#5

Three-shell fit

#6

Two-shell fit

Mn-Mn Cr-Fe Cr-Fe Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe) Cr-(Cr, Fe)

~ (A)a

2.90 2.99 3.40 3.01 3.45 3.95 3.01 3.45 3.99 3.04 3.42 4.03 3.05 3.40 4.03 3.04 3.43 4.03 3.03 3.41 3.98 3.03 3.46 3.95 3.00 3.45 3.95 3.00 3.94

N~

a (A)"

&E.a-

Qe

6 2.4 1.0 1.1 0.8 1.2 1.6 0.5 1.9 3.0 0.8 1.5 2.6 0.8 1.5 2.7 0.4 2.2 2.9 0.8 1.5 2.5 0.6 1.5 2.1 0.4 1.7 1.5 1.5

0.11 0.11f 0.11f 0.11f 0.11f 0.11f 0.13 0.05 0. l 3 0.11: 0.11: 0.11: 0.11f 0.11: 0.11f 0.11 0.09 0.13 0.11f 0.11: 0.11f 0.11: 0.11f 0.11f 0.11: 0.11i 0.11f 0.11f 0.11f

0.0

--

3.8

0.025

3.1

0.013

2.4

0.007

0.5

0.020

-0.8

0.006

-0.7

0.004

1.1

0.021

0.9

0.027

2.0

0.010

0.8

0.007

Cr-Me distance. b Number of atomic neighbors. c Debye-Waller factor. a Difference of energy threshold between spectra of the sample and of the reference. e Q = ( Xexp- xth) 2Ik ~X~xp. 2 : P a r a m e t e r h e l d fixed d u r i n g t h e fitting p r o c e d u r e ,

f r a m e w o r k s o f c h r o m i u m atoms, as b o t h w o u l d lead to small average n u m b e r s o f neighbors, T h u s we c a n n o t d e t e r m i n e with accuracy the size o f these surface clusters, since the n u m b e r o f Cr neighbors m e a s u r e d b y EXA F S is n o t sensitive to this size. T h e c o n s t a n t a p p a r e n t n u m b e r o f C r a t o m s at 3.9-4.0 f o u n d i n # 1 a n d #6 ( T a b l e II a n d Fig. 6) indicates that m u l t i n u c l e a r C r complexes have a certain extent even at low C r surface coverage. Journal ~?fColloid and lnterJbce Science, Vol. 148, No. 2, February 1992

G o i n g further into this spectral analysis, one m a y also a s s u m e that the c o n t r i b u t i o n of cluster C r - C r pairs to the R D F peak at 3.05-3.00 is also i n d e p e n d e n t of the Cr content, a n d thus equal to the one detected in the last spect r u m o f Fig. 6 where the sorbent c o n t r i b u t i o n is n o longer detectable. According to this scheme, c o m p a r i s o n of the first metal peak height i n the first a n d last R D F s of Fig. 6 indicates that a m o n g the three nearest (Fe, Cr) a t o m s detected i n the first spectrum, about half

Cr(III) SORPTION ON HYDROUS Fe OXIDES

453

creases (Fig. 3), the numbers of nearest and next-nearest Fe neighbors decrease (Table III). These next-nearest neighbors, indicative of a DC linkage (a contribution of the adsorbent), can still be detected up to the end of

.o2

~

h

a

-.02

6 8 10 WAVEVECTOR k(A-1 )

4

-'01

V

4

6 8 10 WAVEVECTOR k(,~,-1)

12

12

,02

.01 1

2

3

4

R(A)

FIG. 6. CrK-RDF ofCr(III) sorbed on HFO at different surface coverages. From top downward: -log I'cr = 2.24 (#1), 2.06 (#2), 1.69 (#3), 1.56 (#4), 1.32 (#5), and 0.69 (#6). The dotted line represents the RDF of HCO.

~.01 4

of them are Fe and the other half are Cr atoms. This possible progressive diminution in the number of Fe atomic neighbors across edges in going from #1 to #6 is also substantiated by the regular decrease in the average Cr-Me distance from 3.05 to 3.00 A (Table III). Thus as the surface excess of chromium in-

6

8

10

12

WAVEVECTOR k(.&-1) PIG. 7. Fourier-filtered Cr-Me contributions to EXAFS of HFO. (a) Comparison of Cr-Me contributions at different surface coverages. Dotted line: #2, solid line: #4, dashed fine: #6. (b) Fit of #2 (dotted line) assuming 2.61 (Cr, Fe) at 3.05/~, 0.8(Cr, Fe) at 3.40/~, and 1.5(Cr, Fe) at 4.03 A. (c) Fit of#6 (dotted line) assuming 1.5(Cr, Fe) at 3.00/k and 1.5(Cr, Fe) at 3.94 A. Journal of Colloid and Interface Science.

Vol 148,No. 2, February1992

454

CHARLET AND MANCEAU

,~

t - 40rnin

.2=:

I-LL

t-

60rain

between 3 and 4 A, which is consistent with the presence of Fe neighbors at 3.4-3.5 A and Cr neighbors at 3.9-4.0 A. Keeping in mind that this kinetic series was realized with a Cr mother solution in which about 20% of Cr was polymerized, whereas all previous RDFs were obtained from samples in which sorbed Cr has been taken up from solutions containing at the most 1% polymers, it appears that the formation of these surface polymers is independent of the amount of polymers present in solution. The similarity of the RDFs (Fig. 8) obtained at room temperature with that of #3 (Fig. 6), obtained at 77 K with a sample containing an equivalent amount of chromium, demonstrates also that the local structure of the surface is in no case perturbed by cooling the samples to 77K.

Coprecipitation Data The RDF of the (Feo.994Cr0.006)OOH(i.e., H F O / H C O ) solid phase (log Pcr = - 2 . 2 2 ) recorded at the C r K absorption edge (Fig. 4) ., looks like that recorded at the FeKabsorption edge. The strong enhancement of the Cr-O first-shell peak, compared with that of the Fe..,, /i O peak, indicates that the coherency of the Cr-O distances, known to be higher in Cr than 1 3 4 R(A) in Fe oxyhydroxides, has been preserved durFIG. 8. CrK-RDF ofCr(Ill) sorbedon HFO at different ing the substitution. Evidence for such a local equilibration times. From top downward: t = 20 min, 40 min, 60 min, 1 h 30 min, and 20 h. The dotted line rep- relaxation of the Cr crystallographic site in host lattices is widely documented in the literature resents the RDF of HCO (log Pcr= - 1.69). [see, e.g., (36) and references therein]. This local relaxation extends up to the second cothe "plateau." Then, when the a m o u n t o f ordination shell inasmuch as the Cr-Fe dissorbed Cr exceeds the number of surface "ac- tance is lower than the Fe-Fe distance, and is tive" sites, the R D F obtained is quite identical similar to the Cr-Cr distance encountered in to that of a pure 3,-CrOOH solid phase (Fig. HCO (Table III). The important result re6: compare #6 and the dotted-line RDFs); i.e., mains, whatever the above details, that Cr surface precipitation occurs. atom substitutes for Fe in the H F O / H C O coThe shape of the RDF is roughly indepen- precipitated lattice. It is therefore present in a dent of the equilibration time, as indicated in local a- (Cr, Fe) O O H environment. Fig. 8. The similarity of these RDFs indicates The solubility of (CrxFel_x)OOH solid sothat the formation of surface multinuclear Cr lutions has been studied by Sass and Rai as a structures is completed in less than 20 rain. function of X, the Cr mole fraction in the solid, All the RDFs show an enlarged double peak with X being varied from 0.01 to 0.69 (13). ..._

t- lh30

J o u r n a l o[Colloid a n d lntetJhce Science, Vol. 148, No. 2, February 1992

Cr(III) SORPTION ON HYDROUS Fe OXIDES Since Pcr is related to X by I'cr = X~ ( 1 - X ) , the results of this study could be replotted in a sorption isotherm format (empty squares, Fig. 3). These data are adequately described by the "surface precipitation model" (Fig. 3, top solid line), using the same parameters used to compute the sorption isotherm model curve, except the surface site density (ST = 0.0448 M) and the solubility product of the CrOOH (s) end-member phase of the solid solution (pKsc~ooH = 8.68). This pKsc~oon value is, as reported by the authors of the original study, an order of magnitude lower than the values obtained for the hydrous chromium oxide synthesized by homogeneous precipitation (13). DISCUSSION

Precipitation and Coprecipitation The H F O / H C O coprecipitate has a unique a-MeOOH local structure about both chromium and iron atoms. No trace of other local structures (such as ~ - M e O O H ) could be detected. Fe is therefore isomorphicaUy substituted by Cr. The two octahedra are of similar size and are both found in a - M e O O H (Me = Cr, Fe) mineral structures (37). This isomorphic substitution accounts for the infrared stretching frequencies (38) and Cr solubility ( 13 ) of (Crx Fe l-x )OOH (s), which have been found to vary continuously with X, the chromium mole fraction. Once aged and crystallized, these solid solutions also dissolve congruently (39). Solubility data of H F O / H C O can be described by the "surface precipitation model," which assumes that, at low Cr content, Cr atoms are present not within but at the surface of hypothetical Fe(OH)3(s) particles. These particles are very small, since they present four hydroxyl reactive surface sites for every ten Fe atoms they may contain (log S T / T O T F e = - 0 . 4 ) . Such small particles have been observed to be formed in the course of Fe polymerization: electron microscopy, photon correlation spectroscopy, and small-angle Xray scattering have indicated that this poly-

455

merization starts with the formation of 15-~. spherules containing 10-15 Fe atoms (40, 41 ). The H F O / H C O solubility data indicate therefore that Cr adsorption on these spherules occurs at the very beginning of the "coprecipitation" process, at low p H range, i.e., under conditions where no 3,-CrOOH polymers are formed in solution. This results in the single a-(CrxFel_x)OOH local structure, as observed in the final precipitate. If the ( C r x F e l - x ) ( O H ) 3 • n H 2 0 ( s ) precipitates were ideal solid solutions, their solubility should reach that o f C r ( O H ) 3 , n H 2 0 ( s ) as X tends toward 1. This is not observed. Instead a jump from 1.0 (this study) to 1.62 (13) log units separates the saturation Cr concentration of the Cr-rich H F O / H C O from that of riCO. Thermodynamically, it indicates high exothermic mixing enthalpies, that is, attractive forces which are greater (bonds are stronger) between unlike component molecules than between like components (13). These components are present in different local structures, so that this solubility j u m p can be interpreted as a shift from one structure (aCrOOH ) to its polymorph (3~-CrOOH). Such differences in solubility among polymorphs has also been reported in the case of Zn hydroxides (42).

Heterogeneous Nucleation At low chromium total concentration, the Cr sorption onto H F O can be macroscopically described by a Langmuir-type adsorption, i.e., by a two-dimensional process in which one site is occupied per chromium ion adsorbed (Eq. [1], Table I). In this model, adsorption would lead to progressive coverage of the surface by chromium ions until a " m o n o l a y e r " formed, i.e., until one atom was sorbed per "active" surface site. This simple picture does not occur. Instead, once adsorbed, the Cr atoms have too many neighbors to be present as isolated adsorbed Cr atoms. These sorbed atoms are found simultaneously in two different environments. They are first in an a-type environment (as indicated by the two nearest metal Journal of Colloid and &l~er.&ceScie,.ce, V ol. 148, No~ 2, February 1992

456

CHARLET

AND

shells, at ~3.0 and 3.4 A), most probably with respect to the adsorbent Fe atoms. But they are also present in a second type of environment, where they are found associated together in small hydroxide clusters of ~,-type structure, as indicated by the ~4.0-A Me-Me distance (SC bridging). These clusters, whose structure is similar to that found in the Te isomer of Cr tetramer (42), could not, for kinetic reasons, have been formed in solution before being adsorbed. Therefore, the surface catalyzes the polymerization, which occurs when Cr(III) bonds only 10% of the "reactive" surface sites (or 1% of the BET surface area). Thus, heterogeneous nucleation occurs in the so-called adsorption domain. The average number of DC linkages per Cr octahedron (diagnostic of an a-type structure) decreases from 0.8 to 0.4 in going from sample #1 to #5 (Table III), while in the meantime the quantity of sorbed Cr increases 16 times. Thus when Cr concentration increases, the newly formed Cr surface clusters bridge the sorbent surface octahedra instead of the already formed surface clusters. At the end of the "plateau" (Fig. 3) and under conditions undersaturated with respect to the homogeneous precipitation of Cr hydroxide (HCO), a new phase, whose local structure is identical to that found in HCO and in the surface clusters, starts to grow. The adsorbent structure is then no longer detected in the RDF (RDF #5, Fig. 6). The solubility of this surface precipitate is equal to that of HCO. No solubility gap and no structural discontinuity could therefore be observed between this heterogeneously nucleated hydrous Cr oxide and the homogeneously precipitated HCO.

Effect of Surface Crystallinity on the Nucleation Process HFO and goethite have the same local structure but they differ in their crystallinity. At comparable Fcr (-log I~cr = 2.82), chromium atoms sorbed on goethite (Fig. 4 and Table III) have fewer--about half--metal neighbors than chromium atoms sorbed on HFO (Fig. 6, RDF #1, and Table III). FurJournal of Colloid and Interface Science, Vol. 148, No, 2, February 1992

MANCEAU

thermore, the weak amplitude of the first Me peak and the short Cr-Me distance (3.01 A, Table III) corresponding to this peak strongly suggest that very few Cr atoms are bound to Fe goethite atoms via edge sharing of their octahedra. At the same time, Cr-Fe DC linkages are relatively numerous. Thus, on well-crystallized goethite, multinuclear Cr species tend to bridge ferric octahedra only, or mainly, by sharing double corners, whereas on HFO Cr octahedra also share edges with octahedral Fe neighbors. This main difference in~sorption mechanism between goethite and HFO is at first sight surprising as these two sorbents possess the same bulk structure and differ only in their crystallinity. This contrasting behavior likely originates from a difference in the surface structures of goethite and HFO. Given their good crystallinity and needle shape, goethite crystallites consist of infinite double chains (see Fig. 1). Sorbate ions tend to model the substrate and to behave like the Fe octahedron of an additional double chain. They are linked mainly by DC to the surface octahedra as in binuclear bidentate surface complexes. In contrast, given their high density of defects, HFO chains are short and some of them are likely to be single chains. The density of edge surface sites is then expected to be much higher in HFO than in goethite, and this would account for the increased possibility of E linkage. As the aging of HFO leads to goethite, crystal growth can be viewed as a change in the relative proportion of surface sites, and thus as a change in the possible modes of metal adsorption, and hence in sorbate/sorbent affinity. For example the enrichment factor (44) of uranyl is about 500 times greater on HFO than on fine-grained natural goethite (45). The reason for such a dramatic enhancement of the adsorption capacity has recently been elucidated at the molecular level (45). Uranyl ions bond HFO via E linkage. The observed reduction in U affinity, in going from HFO to goethite, arises therefore from the reduction in the density of available edges and, thus, in active surface sites during crystal growth.

Cr(llI) SORPTION ON HYDROUS Fe OXIDES SUMMARY

This EXAFS study demonstrates that mixed (Fe, Cr)(OH)3(s) metal hydrous oxides have different local structures, depending on their history, i.e., on whether the two metal ions were coprecipitated or whether one ion was sorbed on an hydrous oxide previously formed with the other metal ion. In a coprecipitation process, the chromium atoms are integrated in the HFO matrix and do not significantly change its structure, so that they are found in an a-type structure (i.e., with two nearestneighbor metal shells at 3.00-3.05 and 3.403.45 A). This phase is in structural discontinuity with the homogeneously precipitated Cr oxide ( H C O ) , whose local structure is "~C r O O H (characterized by two atomic shells at 3.00-3.05 and 3.94-4.03 A). This structural discontinuity accounts for the difference in Cr solubility observed between pure Cr(OH)3 (s) and (CrxFet-x)(OH)3(s) solid solutions even for high X values (2). Sorption of Cr(III) on goethite or HFO occurs via formation of inner-sphere surface complexes. Cr(III) is present in two local environments: an a - M e O O H structure (similar to that of the adsorbent) and a 3,-MeOOH structure (Me = Cr, Fe). It indicates that surface clusters are formed, even at intermediate surface coverage (10% of the active surface sites or 1% of the BET surface area). As Cr coverage increases beyond full surface coverage, a new phase starts growing, which is characterized by a single local structure, namely y-CrOOH. This growth starts under conditions largely undersaturated with respect to the homogeneous precipitation of HCO, a phenomenon well described by the "surface precipitation model" ( 1 ). The solubility product of the two phases (surface precipitated and homogeneously precipitated hydrous Cr oxide) is the same and the two products have identical local structure ( 7 - C r O O H ) . The macroscopic part of this study quantifies the much greater efficiency of the coprecipitation phenomenon, with respect to the surface precipitation, in removing chromium(IlI ) from solution. This efficiency is due

457

in part to the size of the sorbent particle, which is much smaller in a coprecipitation process, and in part to the Cr solubility, which is m u c h smaller for Cr present in an a-type than in an 7-type local environment. This coprecipitation occurs widely in natural systems (e.g., soils) and is used in water treatment plants to remove Cr from industrial effluent waters. ACKNOWLEDGMENTS We thank B. Trusch for performing chemical analysis; F. Thomas for the BET measurements; Dr. Bottero, Dr. Buxbaum, Dr. Nordstrom, and Dr. Wersin for stimulating discussions; and Dr. Sposito and two anonymous persons for their constructive reviews. L.C. acknowledges the support from the Swiss National Science Foundation (Project 20-28270.90), and A.M. acknowledges CNRS/INSU Grant 5.06 (program "Equipement mi-lourd 1990") and Grant 91 DBT 4.01 (program "Fluides Minrraux et Cinrtique," contribution 317). REFERENCES 1. Farley, K. J., Dzomback, D. A., and Morel, F. M. M., J. Colloid Interface Sei. 106, 226 (1985). 2. Sehindler, P. W., and Stumm, W., in "Aquatic Surface Chemistry" (W. Stumm, Ed.). Interscience, New York, 1987. 3. Bleam,W. F., and McBride,M. B., J. Colloidlnterface Sei. 103, 124 (1985). 4. Motschi, H., Colloids Surf. 9, 333 (1984). 5. Wersin, P,, Charlet, L., Karthein, R., and Stumm, W., Geochim. Cosmochim. Acta 53, 2787 (1989). 6. Schwertmann, U., and Latham, M., Geoderma 38, 105 (1986). 7. Gibbs, M. M., WaterRes. 13, 295 (1979). 8. Cranston, R. E., and Murray, J. W, AnaL Chim. Acta 92, 275 (1978). 9. Sposito, G., in "Geochemical Processes at Mineral Surfaces" (J. A. Davis and K. F. Hayes Eds.), ACS Syrup. Ser. No. 323. Am. Chem. Soc., Washington, DC, 1986. 10. Richard, F. C., and Bourg, A., Water Res. 25, 807 (1991). 11. Leckie, J. O., Appleton, A. R., Ball, N. B., Hayes, K. F., and Honeyman, B. D., EPRI RP-910-1, Electric Power Research Institute, Palo Alto, CA, 1984. 12. Garg, A., and Matijevic, E., Langmuir4, 38 (1988). 13. Sass, B. M., and Rai, D., Inorg. Chem. 26, 2228 (1987). 14. Okuda, Y., and Harada, T., U.S. Patent 4495164 (1985). 15. Chukhrov, F. V., Zvyagin, B. B., Gorshkov, A. I., Yermilova, L. P., and Balashova, V. V., Int. GeoL Rev. 16, 1131 (1973), Journal of Colloid and lntecface Science, Vol. 148, No. 2, February 1992

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Journal of Colloid and Interface Science, Vol. ~14-8~,No. 2, February 1992

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