synthesis and characterization of goethite and

8/10/2019 Synthesis and Characterization of Goethite and

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Advances in Colloid and Interface Science

103 (2003) 5776

0001-8686/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0001-8686(02)00083-0

Synthesis and characterization of goethite andgoethitehematite composite: experimental study

and literature survey

Marek Kosmulski *, Edward Maczka , Elzbieta Jartych ,a,b, a c

Jarl. B. Rosenholmb

Department of Electrochemistry, Technical University of Lublin, Nadbystrzycka 38 A, 20618 Lublin,a

Poland

Department of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5, 20500 Abo, Finlandb

Department of Experimental Physics, Technical University of Lublin, Nadbystrzycka 38,c

20618 Lublin, Poland

Abstract

Aging of synthetic goethite at 140 8C overnight leads to a composite material in which

hematite is detectable by Mossbauer spectroscopy, but X-ray diffraction does not reveal anyhematite peaks. The pristine point of zero charge (PZC) of synthetic goethite was found atpH 9.4 as the common intersection point of potentiometric titration curves at different ionicstrengths and the isoelectric point (IEP). For the goethitehematite composite, the commonintersection point (pH 9.4), and the IEP (pH 8.8) do not match. The electrokinetic potentialof goethite at ionic strengths up to 1 mol dm was determined. Unlike metal oxides, fory3

which the electrokinetic potential is reversed to positive over the entire pH range atsufficiently high ionic strength, the IEP of goethite is rather insensitive to the ionic strength.A literature survey of published PZCyIEP values of iron oxides and hydroxides indicatedthat the average PZCyIEP does not depend on the degree of hydration (oxide or hydroxide).Our material showed a higher PZC and IEP than most published results. The present resultsconfirm the allegation that electroacoustic measurements produce a higher IEP than the

average IEP obtained by means of classical electrokinetic methods. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Point of zero charge; Adsorption; Isoelectric point; Goethite; Hematite; Mossbauer spec-

troscopy; X-Ray diffraction

*Corresponding author. Tel.: q48-81-5381355; fax: q48-81-5254601.E-mail address:[email protected] (M. Kosmulski).


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58 M. Kosmulski et al. / Advances in Colloid and Interface Science 103 (2003) 5776

Contents

1. Introduction ....... ........ ....... ........ ........ ........ ....... ........ ........ ........ ....... ........ ... 582. Experimental ........ ........ ........ ....... ........ ........ ....... ........ ........ ........ ....... ........ . 58

3. Results . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 60

4. Discussion.... ....... ........ ........ ........ ....... ........ ........ ........ ....... ........ ........ ........ . 65

References............ ........ ....... ........ ........ ....... ........ ........ ........ ....... ........ ........ ..... 68

1. Introduction

Iron (hydr)oxides substantially contribute to the uptake of toxic elements innatural mineral assemblies, even when the mass fraction of iron (hydr)oxides inthese assemblies is relatively low. The affinity of iron (hydr)oxides to anions andcations is pH-dependent, and this can be explained in terms of pH-dependent surfacecharging w14x. Namely, the surface carries a positive proton charge at pH belowthe point of zero charge (PZC) and a negative proton charge at pH above the PZC.This results in electrostatic attraction or repulsion of all ions other than protons, anda change in pH by one or two units can enhance or depress the uptake of ions byan order of magnitude. Thus, a serious study of the uptake of ions from solutioncan hardly be imagined without knowledge of the PZC. Iron (hydr)oxides havebeen used as model sorbents to examine the effects of different factors (pH,temperature, presence of other species in solution) on the sorption process. In thepresent paper, two model adsorbents are characterized and their PZC and IEP arecompared with the literature values.

The electrokinetic behavior of goethite at high ionic strengths was also studied.NaI was selected for this study as a salt that induced a more substantial shift in theIEP of metal oxides than any other salt.

2. Experimental

A portion of 400 cm of 2.5 M KOH solution was mixed with 1650 cm of pre-3 3

filtered 0.15 M Fe(NO ) solution in a plastic(Nalgene) container and the mixture3 3was stirred vigorously. The dispersion was aged for 48 h at 80 8C. The precipitatewas then centrifuged out and re-dispersed in 0.01 mol dm HNO solution,y3 3centrifuged again, and this procedure was repeated 10 times, and again three times,

with MilliQ water instead of HNO solution. Then a few portions of the yellow3brownish precipitate were mixed together and freeze-dried. The resulting material isreferred to as goethite 25. The other portion of goethite was dried in an oven at140 8C overnight. The resulting material is referred to as goethite 140 (although infact it is a goethitehematite composite).

Hyperfine interactions were determined by Mossbauer spectroscopy(MS). Meas-

urements were carried out at room temperature in the standard transmission geometryusing a source of Co in a rhodium matrix (activity of approx. 5 mCi). The isomer57

shifts given below are relative to a-iron. Numerical fitting of the experimental


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59M. Kosmulski et al. / Advances in Colloid and Interface Science 103 (2003) 5776

Fig. 1. X-Ray diffraction spectra of goethite 25 and 140.

spectra gives the hyperfine interaction parameters, e.g. isomer shift d, quadrupolesplitting D and hyperfine magnetic field B .hf

The potentiometric titrations of goethite dispersions (1 or 2 gy50 cm ; more3

concentrated dispersions become very viscous) were carried out in a thermostatedTeflon vessel under a nitrogen atmosphere. The NaNO concentration was 0.001,3

0.01 and 0.1 mol dm , and 0.1 or 0.2 mol dm NaOH was the titrant. They3 y3

titration was always started at acidic pH to facilitate removal of CO from the2system. The electrokinetic potential was determined with Acustosizer (Matec) andDT-1200 (Dispersion Technology) instruments at a 10% mass fraction of goethite.The results obtained at high ionic strengths were corrected for the electrolytebackground. The dispersions were not insulated from atmospheric CO during the2electrokinetic measurements. The electrokinetic and potentiometric titrations werecarried out at 25 8C. NaNO was used to establish a constant ionic strength unless3indicated otherwise.


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Fig. 2. Room-temperature Mossbauer spectra of goethite 25 (a) and 140 (b).

3. Results

Only the goethite structure was found using X-ray diffraction in goethite 25 andgoethite 140 (Fig. 1). The signalynoise ratio was rather poor. The mass loss oncalcination at 500 8C indicated nearly stoichiometric FeOOH in both samples.

Mossbauer spectra registered for goethite 25 and goethite 140 are presented in

Fig. 2, and the hyperfine interaction parameters calculated are summarized in Table


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Table 1The calculated hyperfine interactions parameters

Sample Bhf d D G k Phase Reference(T) (mm s )y1 (mm s )y1 (mm s )y1 (%)

Goethite 25 36.9"0.1 0.34"0.01 0.13"0.01 0.27"0.02 100 Goethite This workGoethite 140 38.3"0.1 0.35"0.01 0.13"0.01 0.23"0.03 65 Goethite This work

49.6"0.1 0.36"0.01 0.11"0.01 0.23"0.03 35 Hematite38.1 0.27 Goethite w5x36.0"1.5 0.45"0.01 0.24"0.05 Goethite w6x38.0 0.37 Goethite w7x51.9"0.1 0.38"0.01 0.09"0.01 Hematite w8x51.7"0.1 0.34"0.01 0.09"0.01 Hematite w9x

B , hyperfine field; d, isomer shift (relative to a-iron); D, quadrupole splitting; G, half-width of thehfspectral lines; k, relative contribution of the component.

Fig. 3. The surface charge density of goethite 25.

1. The results obtained for goethite 25 are typical for pure goethite. The resultsobtained for goethite 140 reveal two phases: goethite (dotted line in Fig. 2b) andhematite(solid line). The presence of a minor amount of hematite in well-crystallizednatural goethite has been reported w5x. The spectrum is a superposition of twocomponents, and their relative contributions were estimated from the area of eachsub-spectrum. The half-width of the spectral lines observed in the present study issubstantially greater than that of the calibration line (Gs0.12 mm s ). Our XRDy1

study also showed poorly developed Bragg peaks, especially for goethite 140. Bothresults are due to the poor crystallinity of our samples.


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Fig. 4. The surface charge density of goethite 140.

Fig. 5. The electrokinetic potential of goethite 140.


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Fig. 8. Histogram of the PZCyIEP values for iron (hydr)oxides reported in the literature. The black barsrepresent the matching CIP and IEP values.

The BET surface area of goethite 25 and 140 was 40.20"0.06 and 47.05"0.23m g , respectively (ASAP 2010).2 y1

The surface charging curves obtained at different ionic strengths intersect at pH9.4 in both samples (Figs. 3 and 4), and this value was interpreted as the pristinePZC values. Actually, the titration curves obtained at the ionic strengths of 0.001and 0.01 mol dm merge above pH 9.4 rather than intersect, but with 0.1 moly3

dm there is a distinct intersection point.y3

The IEP of the precipitate before freeze-drying (no salt added) was at pH 9.2(microelectrophoresis). The z potentials of goethite 140 in 0.01 mol dm NaNOy3 3

are shown in Fig. 5. The IEP obtained by two different instruments nearly match atpH 8.8, but they are substantially lower than the CIP (crossover point of titrationcurves obtained at different ionic strengths). The z potentials of goethite 25 areshown in Figs. 6 and 7. Fig. 6 indicates a perfect match between the commonintersection point of the charging curves (Fig. 3) and the IEP obtained by twodifferent instruments in 0.001 and 0.01 mol dm NaNO . The results obtainedy3 3with the Acustosizer indicate that the presence of NaI up to 0.5 mol dm (onlyy3

the results for 0.1 mol dm NaI are shown in Fig. 6) does not substantially shifty3

the IEP. The results obtained with the DT-1200 shown in Fig. 7 indicate a shift in


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Table 2The PZC of iron (hydr)oxides as the function of hydration and the crystalline structure

Entries Median Average Standarddeviation

Hematite, synthetic 62 8.1 7.82 1.17Hematite, natural 7 5.4 5.64 1.56Fe O other than hematite, mixtures of2 3 22 7.2 6.89 1.51

different forms or structure unknownGoethite 66 8.4 8.32 0.89FeOOH other than goethite, mixtures of 14 7.25 7.27 0.77

different forms or structure unknownFe (III) hydroxides and hydrous oxides 22 8 7.99 0.69Total 193 8 7.8 1.23

the IEP to pH 10.8 in 0.5 mol dm NaI, but a further increase in the NaIy3

concentration to 1 mol dm shifted the IEP back to its original value.y3

4. Discussion

Mossbauer spectroscopy is a very useful tool in studies of local interactions

between nuclear probes contained in the material investigated and their nearestenvironment. The sensitivity of Mossbauer spectroscopy is substantially higher than

XRD; it allows phases invisible to XRD to be revealed. Moreover, Mossbauer

spectroscopy can be used for quantitative analysis of mixtures of different phases.Goethite a-FeOOH contains the nuclear probe Fe. In Mossbauer spectroscopy57

experiments, hyperfine interactions may be determined, e.g. interactions of theelectromagnetic moments of the nuclear probe with the electric and magnetic fields.The present results indicate that XRD alone is not sufficient to exclude the presenceof hematite in aged goethite samples. Unfortunately, in many publications thestructure analysis of iron (hydr)oxides was carried out solely by means of XRD.

A literature search revealed a broad range of PZC and IEP values reported foriron (hydr)oxides w10177x. The numerical values of PZC from these publicationsare compiled elsewhere w2x, and here we only show a histogram. Fig. 8 indicatesthat the PZC values obtained in the present work are higher than most of the valuesreported in the literature. In order to explore the effect of the composition of the

material on the PZC, the iron (hydr)oxides were sorted into the following groups:

Synthetic hematite; Natural hematite; Fe O other than hematite, or a mixture of different forms or structure unknown;2 3 Goethite; FeOOH other than goethite, or a mixture of different forms or structure unknown;

and Iron (III)hydroxide or hydrous oxide.


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Table 3The PZC of iron (hydr)oxides as the function of the experimental method

CIPsIEP CIP IEP IEP Titration Total(classical) (electro- (one ionic

acoustic) strength)

Entries 21 79 69 1 24 193Median 8.2 8.1 7.9 9.4 7.85 8Average 8 7.98 7.58 9.4 7.63 7.8Standard deviation 1.05 0.9 1.51 1.34 1.23

The results of the literature survey sorted by composition are summarized in Table2.

Apparently the effect of the degree of hydration of the sample on the PZC is

rather insignificant, namely the average and median PZC for synthetic hematite andiron hydrous oxide and hydroxide are close to the average and median PZC in theentire data set, and the average and median PZC for goethite are only slightlyhigher. This coincidence can be explained in terms of a transformation of one ortwo external atomic layers of the solid into a somewhat different structure. Probablythis surface structure (and degree of hydration) is common for different crystallo-graphic forms representing the same chemical formula, and even for compounds ofdifferent degrees of hydration (oxide, oxohydroxide and hydroxide). The absenceof a significant difference in PZC (median and average based on over 100 entries)between a- and g-Al O , on the one hand, and between anatase and rutile on the2 3otherw2x, are in favor of this hypothesis. The PZC of natural hematite is significantly

lower than that of other types of materials, but this result is due to the substantialamount of silica that is found in natural hematite, as explicitly documented in manyoriginal publications, where specific numbers (e.g. mass fraction of silica and otherimpurities) are reported.

The PZC values for goethite reported in the literature are higher than those ofthe other crystallographic forms of FeOOH (akageneite, lepidocrocite), mixtures ofdifferent forms and FeOOH of unknown structure. The difference is of the order of1.5-fold the standard deviation in each population. Thus, in contrast with the degreeof hydration, for which the effect on the PZC of iron (hydr)oxides is ratherinsignificant, the structure within the same chemical formula(goethite vs. akageneiteand lepidocrocite) exercises a significant effect on the PZC. Yet, even when

compared with the data reported in the literature for goethite, our PZC and IEPvalues (Figs. 37) are rather high.The PZCyIEP of iron (hydr)oxides are sorted in Table 3 by method and the

following categories were distinguished:

Matching CIP and IEP (as in the present work for goethite 25); CIP; IEP (classical); IEP (electroacoustic); Titration (one ionic strength);


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A few less common methods (e.g. inflection point of titration curves, w22x) havebeen used to determine the PZC of iron (hydr)oxides, but the results obtained with

these methods are not analyzed here. In addition, PZC values reported withoutexplicitly stating the experimental method w24,110,177x or experiments that did notproduce any PZCyIEP w154x were purposely ignored.

Lyklema w178x recommended a combination of potentiometric titration andelectrokinetic methods to determine the pristine PZC of materials. Namely, the PZCand IEP shift in two opposite directions in the presence of specific adsorption ofcations or anions. Thus, matching CIP and IEP is very likely the pristine PZC ofthe adsorbent. On the other hand, a lack of coincidence indicates specific adsorption.Unfortunately, publications reporting CIP and IEP of the same sample are scarce.Publications reporting CIP only are more numerous, but the existence of a CIP doesnot prove the purity of the sample, i.e. a CIP also occurs in the presence of specific

adsorption w178x. Instead of titrations at different ionic strengths, some authors usedsalt titration (at PZC, addition of inert electrolyte does not induce a change in thepH of the dispersion), and such results are basically equivalent to the CIP and arelisted under the same category. Classical electrokinetic methods (electrophoresis,electroosmosis) make it possible to readily determine the IEP. User-friendly equip-ment is available nowadays, but it takes some effort and experience to avoiddifferent pitfalls, e.g. related to sample preparation. For example, storage of iron(hydr)oxide dispersion in glassware at neutral or basic pH for a few hours or daysresults in a shift of the IEP to lower pH than in the original sample. Some unusuallylow IEP values reported in the literature are probably due to this effect.

IEP values obtained using the electroacoustic method (as in the present study)

are listed under a separate category. On the other hand, nearly matching IEP fromelectroacoustic measurements and CIP w67x is listed under matching CIP and IEP,together with the results obtained by means of classical electrokinetic methods.

Mass titration has recently become popular as a method to determine the PZC.In this method, the natural pH of a concentrated dispersion is identified with thePZC. In this respect, mass titration is not very different from potentiometric titrationat one ionic strength (in which the PZC is also the natural pH of the dispersion)and the results obtained using these two methods are combined into one group inTable 3.

The choice of the method does not substantially affect the PZC (Table 3), i.e. acombination of titration (the common intersection point of titration curves for

different ionic strengths is identified with the PZC)and electrokinetic measurements,titration alone, classical electrokinetic measurements alone, and even potentiometrictitration at one ionic strength (the natural pH of a dispersion in inert electrolytesolution is identified with the PZC) produce practically the same average andmedian PZC as the entire data set. The IEP value from the electroacoustic methodis substantially higher than the IEP obtained by other methods. Interestingly, thishigh value exactly matches the CIP obtained in the present work.

As discussed above, the PZC value equal to matching IEP and CIP is morereliable than results obtained using a single method. The PZC obtained in this way


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for iron (hydr)oxides (black bars in Fig. 8)are distributed over a rather broad rangeof pH 5.59.4, but not evenly, i.e. five PZC values are concentrated in the relatively

narrow range pH 8.28.5, and six others in the range pH 99.4. These islands areto some degree associated with specific iron (hydr)oxide minerals. For example, allsix values in the range pH 99.4 represent synthetic hematite, but a value as lowas 6.7 was also reported for the same mineral. The black bars in Fig. 8 representinggoethite (pH 7.68.5) and FeOOH other than goethite (pH 7.17.6) can only befound over relatively narrow pH ranges.

Then again, our matching IEP and CIP for goethite 25 is substantially higherthan any other matching IEP and CIP for goethite reported in the literature. Therecent MUSIC model makes it possible to estimate the PZC of individual samplesfrom well-established physical quantities when the structure and contribution ofdifferent faces to the total surface area are known. For example, Hiemstra et al. w4x

found a PZC at pH 7.5 for the (100) face of goethite, and at pH 10.7 for the (001)and(010)faces. Then, our results may be interpreted as a relatively low contributionfrom the (100) face of goethite to its total surface area.

The absence of a substantial shift in the IEP of goethite 25 in 1 mol dm NaIy3

is rather surprising. With one exception of silica w179x, all other oxides for which zpotentials at high ionic strengths were studied using the electroacoustic methodshowed a reversal of sign to positive over the entire pH range at NaI concentrationsabove the critical value. The critical NaI concentrations are of the order of 0.5 moldm for metal oxides, and as low as 0.3 mol dm for hematite w180x, while fory3 y3

goethite such a critical concentration is either very high or does not exist at all. Inaddition, correlation of the critical concentration of NaI with the IEP (the critical

concentration is low for materials with a high pristine IEP) and the oddeven rule(oxides of metals showing odd valence have a low critical concentration) w180x

obviously do not apply to goethite. Perhaps there is a substantial difference betweenanhydrous oxides and metal oxohydroxides in the electrokinetic behavior at highionic strengths. A substantial difference in the mechanism of selenate binding tohematite on the one hand and to goethite on the other w181x is another example ofindividual sorption properties of these two minerals.

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