NOTE THE POINT OF ZERO CHARGE OF NATURAL AND SYNTHETIC FERRIHYDRITES AND ITS RELATION TO ADSORBED SILICATE

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1 Clay Minerals (1982) 17, NOTE THE POINT OF ZERO CHARGE OF NATURAL AND SYNTHETIC FERRIHYDRITES AND ITS RELATION TO ADSORBED SILICATE The point of zero charge (pzc) of synthetic Fe-oxides is well documented and usually ranges between ph 7 and 9 (Parks, 1965; Schwertmann & Taylor, 1977). In contrast, the pzc of natural Fe-oxides has only rarely been determined. Using electrophoretic mobility, Van Schuylenborgh & Arens (195) found that a natural goethite had a much lower pzc (~3) than synthetic goethites. They attributed this to better crystallinity of the natural goethite caused by slower crystallization. Soils dominated by Fe- (or A1-) oxides rarely have pzc values as high as those of pure oxides. This is usually attributed to the presence of negatively charged impurities such as clay silicates and organic matter (Parfitt, 1981). Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe23.9H2, occurs in hydromorphic soils (Schwertmann et al., 1982) and is the main component in ochrous precipitates formed when Fe-bearing fresh waters come in contact with air (Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). Under these conditions the ferrihydrite is reasonably free of other charge-active minerals. The aim of this study was to find out if the pzc of these natural ferrihydrites differed from those of synthetic samples. Materials and methods The natural ferrihydrites were collected from ochrous fresh-water deposits in Finland and used without further pretreatment. They were of different crystallinity as judged from X-ray diffractograms which varied between 2- and 6-line material (Table 1). Besides X-ray diffraction, Feo/Fe d ratios (Fe o = oxalate-extractable Fe; Fe d = dithionite-extractable Fe) close to unity showed ferrihydrite to be the only Fe-oxide mineral in most of these samples, except for two where small amounts of goethite were detected. The samples had high surface areas, and contained % organic C and % oxalate-extractable Si (Sio) (Table 1). On the basis of a strong IR adsorption band at cm -1 and results from selective extraction methods, Carlson & Schwertmann TABLE 1. Properties of natural ferrihydrites. Sample Nature of Fea~ Feot C t Sixo Sio? SiNaon Sio/Si o + Fe o IR adsorption Surface no, sample % % Feo/d % % % % mole ratio band cm 1 area m2/g pzc 31 Fh(5)(Gt)* A Fh(2) $4 Fh(2) S6A Fh(2) S7B Fh(4)(Gt) $8 Fh(2) * Fh - ferrihydrite; number in parentheses indicate number of XRD lines; (Gt) - trace of goethite. t dithionite- (d) and oxalate (o) extractable Fe and Si, respectively. (~ 1982 The Mineralogical Society

2 472 Note (1981) concluded that this Si existed in close association with the ferrihydrite. Small differences between Si o and HCl-extractable Si indicated that the samples contained hardly any quartz or crystalline silicates. Two synthetic ferrihydrites were used for comparison. One was prepared by adding KOH to an Fe(NO3) 3 solution immediately before pzc determination. The second was synthesized by a 12-rain hydrolysis of a -2 M Fe(NO3) 3 solution at 65~ followed by dialysis and freeze-drying of the dialyzed sol (Towe & Bradley, 1967). The first sample was a very poorly crystalline, 2-line material, whereas the second was well crystalline and showed a full 6-line X-ray diffraction trace. The pzc was determined by potentiometric titration. A known amount of the oxide (usually between 1 and 2 mg) was added to a temperature-controlled (25 ~ 1 ml titration vessel filled with N2-saturated water and equipped with a glass electrode, N 2 inlet and outlet, base or acid inlet and a thermometer. The suspension was adjusted to a given ph and, when equilibrium was attained, solid KNO 3 was added to achieve KNO 3 concentrations of 1,.1, or.1 M. The suspension was stirred magnetically and flushed with N 2 while being titrated automatically with.1 g KOH or.1 M HNO 3 using a Metrohm titrator E526 with an accuracy of.1 ml. Titration was only continued after the ph had remained constant for at least 2 s. The cross-over point of the three curves was taken as the pzc and the charge per mole of Fe was calculated from the base or acid consumption, after correction for the blank titration if necessary. Fe and Si were extracted by acid oxalate in the dark (Schwertmann, 1964). Fe was also extracted using the dithionite/citrate/bicarbonate method (Mehra & Jackson, 196). For additional Si fractionation, an HC1 extract (2 mg sample + 4 ml 5 M HC1, ambient temperature, 1 min extraction, immediate dilution) and an NaOH extract (2 mg sample + 1 ml M NaOH, 2 h, 8~ were produced. Fe and Si were measured photometrically in the extracts with a sulpho-salicylic acid (Koutler-Anderson, 1953) and a molybdenum blue method (Boltz & Mellow, 1947), respectively. Results and discussion Typical titration curves for natural ferrihydrites are shown in Fig. 1. In each case definite crossing points were obtained. The pzc values of the six natural ferrihydrites were between 5.3 and 7.5 (Table 1). The values are lower than those usually obtained for synthetic ferrihydrites (Gast, 1977; Davis & Leckie, 1978) and this may be partly due to the presence of Si in the samples. In fact, the pzc decreased with an increasing mole ratio Sio/(Si o + Fe o) (Fig. 2). Fe o and Si o correspond essentially to the Si and Fe of the ferrihydrite in the samples. Further support for the influence of Si comes from the pzc values of pretreated samples. A mild NaOH treatment which extracts considerable amounts of Si (Table 1) led to a significant increase of the pzc (Table 2). A smaller increase was obtained with sample $4 after an H22-treatment to remove organic matter, whereas with sample 4A the pzc dropped to 4-7 and rose again to 6.8 after a subsequent NaOH treatment. A possible explanation for the drop of pzc after H22 treatment of sample 4A may lie in the release of Si on peroxidation which is then adsorbed by the ferrihydrite. An effect of Si on the pzc was also obtained with synthetic ferrihydrite. The pure ferrihydrite had a pzc of 8-. If solid, non-crystalline Si-oxide was added, the pzc was lowered only slightly to 7.9 for 17.5 mole % Si added and to 7-3 following 28 mole % Si

3 Note 473 Sio =.1] Sio*Feo , i +2/31-2 ~ 5 ~ =.11 oj E -1 E :~, ~, ~A -'1 ~.3 Sio ~= $4. 2 //pzc : 5.3 ph FIG. 1. Potentiometric titration curves of 3 natural ferrihydrites in 1,.1 and.! M KNO 3 solutions. addition. This is in agreement with results recently obtained by Pyman et al, (1981) for the analogous Al,Si-system. A much stronger effect was obtained if soluble Si was added either before, or immediately after, ferrihydrite precipitation (Fig. 3). The slightly stronger effect of Si if added after Fe precipitation seems to indicate that more of the Si is located at the surface of the ferrihydrite. TABLE 2. Point of zero charge of two natural ferrihydrites after various pretreatments. Treatment NaOH H22 Sample None NaOH H22 then H22 then NaOH $ n.d. 4A 6,

4 474 Note ~ o 9 synthetic nafulol pzc..1 i.2!.3 Sio/Sio + Feo FIG, 2. Point of zero charge of natural and synthetic ferrihydrites in relation to their oxalate-soluble Si/(Si + Fe) mole ratio. The more than additive effect of Si points to a strong interaction between Si and the ferrihydrite surface as already suggested from the IR spectrum (Carlson & Schwertmann, 1981). Conclusions Si-containing natural ferrihydrites have a much lower pzc than pure ones. Because equal quantities of non-crystalline Si-oxides mixed mechanically with ferrihydrite reduce the pzc to much less than in coprecipitates, it must be assumed that the soluble Si modifies the charge characteristic of ferrihydrite by converting proton acceptor FeOH groups into proton donor SiOH groups, as shown previously by Herbillon & Tran Vinh An (1969)for coprecipitates of FeC13 and NazSiO 3. The association between silicate and surface FeOH groups is also indicated by a shift of the Si-O vibration from 18 cm -1 for non-crystalline Si-oxide to between 97 and 937 cm -1 depending on the amount of Si present (Herbillon &

5 Note 475 ~ ' (~ _ 1 84 E O.1M ] liv.4,, %sil -, ~u +1 LL- Oa "~ +2 E - ] ~" - 2 j6.8 ~pzc: " + 2 ~pzc : ph I [oprecipitated FegSi I [ after Si~ Fe precipitqt~ FIG. 3. Potentiometric titration curves of synthetic, Si-containing ferrihydrites in 1,.1, -1 M KNO 3 solutions. Tran Vinh An, 1969; Schwertmann & Thalmann, 1976; Henmi et al., 198; Carlson & Schwertmann, 198 I). A decrease in pzc of synthetic goethite from 7.7 to 6.5 following addition of Si was noted by Hingston et al. (1967). The natural ferrihydrites used for this study were precipitated from Fe-bearing waters at their appearance at the surface and it is likely that silicate and other anions were coprecipitated in a similar manner to our synthetic preparations. Mobile silicate is a compound present in almost every soil. It can be assumed, therefore, that the common Fe-oxides in soils, such as goethite and hematite, whose pzc is usually around 8, will, if pure, also interact with silicate and other anions and have lower pzc values. Preliminary results with goethites from hydromorphic soils seem to support this.

6 476 Note ACKNOWLEDGMENT The authors gratefully acknowledge the help of Dr. D.G. Schulze in revising the text. Technische Universitiit Mfinchen, Institut fiir Bodenkunde, 85 Freising-Weihenstephan, FRG 3 March U. SCHWERTMANN H. FECHTER REFERENCES BOLTZ D.F & MELLOW M.G. (1947) Determination of P, Ge, Si and As by the heteropoly blue method. Anal. Chem. 19, CARLSON L. & SCHWERTMANN U. (1981) Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Aeta 45, DAVIS J. & LECKIE J.O. (1978) Surface ionization and complexation at the oxide/water interface. J. Colloid Interface Sei. 67, GAST R.G. (1977) Surface and Colloid Chemistry. Pp in: Minerals in Soil Environments. (J.B. Dixon & S.B. Weed, editors). Soil Sci. Soc. Amer. HENMI T., WELLS N., CHILDS C.W. & PARFlTT R.L. (198) Poorly-ordered iron-rich precipitates from springs and streams on andesitic volcanoes. Geochim. Cosmochim. A cta 44, HERmLLON A.J. & TRAr~ VINH AN J. (1969) Heterogeneity in silicon-iron mixed hydroxides. J. Soil Sci. 2, HINGSTON F.J., ATKINSON R.J., POSNER A.M. & QUIRK J.P. (1967) Specific adsorption of anions. Nature 215, KOUTLER-ANDERSON E. (1953) The sulfosalicylic method for iron determination and its use in certain soil analysis. Ann. Roy. Agric. Coll. Sweden 2, MEHRA O.P. & JACKSON N.L. (196) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, PARFITT R.L. (1981) Chemical properties of variable charge soils. Pp in: Soils with Variable Charge. (B.K.G. Theng, editor). N.Z. Soc. Soil Sci. PARKS G.A. (1965) The isoelectric points of solid oxides, solid hydroxides and aqueous hydroxy complex systems. Chem. Rev. 65, PYMAN M.A.F., BOWDEN J.W. & POSNER A.M. (1979) The point of zero charge of amorphous coprecipitates of silica with hydrous aluminium or ferric hydroxide. Clay Miner. 14, SCHUYLENaORGH J. & ARENS P.L. (195) The electrokinetic behaviour of freshly prepared y- and cf-feooh. Recueil des Travaux Chimiques des Pays-Bas 69, SCHWERTMANN U. (1964) Differenzierung der Eisenoxide des Bodens durch Extraktion mit Ammonlumoxaiat- L6sung. Z. Pflanzenernfihr., Dfing. Bodenkunde 15, SCHWERTMANN U. & FISCHER W.R. (1973) Natural 'amorphous' ferric hydroxide. Geoderma, 1, SCHWERTMANN U. & THALMANN H. (1976) The influence of Fe(II), Si and ph on the formation oflepidocrocite and ferrihydrite during oxidation of aqueous FeCl2-solutions. Clay Miner. 11, SCHWERTMANN U. & TAYLOR R.M. (1977) Iron oxides. Pp in: Minerals in SoilEnvironments. (J.B. Dixon & S.B. Weed, editors). Soil Sci. Soc. Amer. SCHWERTMANN U., SCHULZE D.G. & MURAD E. (1982) Identification of ferrihydrite in soils by dissolution kinetics, differential X-ray diffraction and M6ssbauer spectroscopy. Soil Sci. Soc. Am. J. 46, (in press). TowE K.W. & BRADLEY W.F. (1967) Mineralogical constitution of colloidal hydrous ferric oxides. J. Colloid Interface Sci. 24,