Solubilities of Synthetic Schwertmannite and Ferrihydrite

Geochemical Journal, Vol. 36, pp. 119 to 132, 2002

Solubilities of synthetic schwertmannite and ferrihydrite

JAE-YOUNG YU,* MISUN PARK and JINHWAN KIM

Department of Geology, Kangwon National University, Chuncheon, Kangwon-Do 200-701, The Republic of Korea

(Received August 6, 2001; Accepted December 27, 2001)

Schwertmannite, 2-line ferrihydrite, and 6-line ferrihydrite were synthesized at 25°C in the laboratory to determine their solubilities. Chemical and thermal analyses of the synthesized minerals show that ≤ ≤ ≤ ≤ schwertmannite has the chemical formula Fe2O3Ðx(SO4)xánH2O (0.41 x 0.49, 1.51 n 2.81) and ≤ ≤ ≤ ≤ ferrihydrite Fe2O3Ð0.5y(OH)yánH2O (0 y 1.96, 0.82 n 1.14). The solubility products (K) of the minerals were estimated from the activities of the corresponding species calculated with the computer program PHREEQC. The estimated logK values are 2.01 ± 0.30 for schwertmannite, 8.46 ± 1.40 for 2-line ferrihydrite, and 10.12 ± 0.74 for 6-line ferrihydrite. The solubility of schwertmannite seems to vary depending on the sulfate content, but more investigation is needed to quantify the relationship. The solubility of ferrihydrite does not show any significant correlation with the water content. The stability boundary between schwertmannite and ferrihydrite predicted in this study is biased to higher pH than that observed in nature, which also need further investigation.

Schwertmannite is known to have variable INTRODUCTION chemical compositions. Bigham et al. (1994) sug- Schwertmannite and ferrihydrite are probably gested the chemical formula as ≤ ≤ the two most frequently observed minerals pre- Fe8O8(OH)8Ð2x(SO4)xánH2O (1 x 1.75). Later, cipitating from ferriferous aqueous systems, such Yu et al. (1999) reported the occurrence of as mine drainages, soil solutions and lake waters. schwertmannite having x ≥ 1.75. The relation be- Precipitation of these minerals not only controls tween the sulfate content and the solubility is not the chemical conditions, but also removes many known. Bigham et al. (1996) pointed out that part dissolved metals from the aqueous solution of the analyzed sulfate in schwertmannite may be through coprecipitation and adsorption. Thus, the adsorbed sulfate, but it has not been distinguished solubilities of schwertmannite and ferrihydrite from the structural sulfate yet. The solubility of should be known to describe the transport and fate schwertmannite has been reported only twice on of the metals and other chemicals in the systems. the literature. For the following dissolution reac- The solubilities of schwertmannite and tion ferrihydrite need to be better defined. Both min- + erals are poorly crystalline and metastable with Fe8O8(OH)8Ð2x(SO4)x + (24 Ð 2x)H 3+ 2Ð respect to goethite (Bigham et al., 1990, 1996; Yu = 8Fe + xSO4 + (16 Ð 2x)H2O, (1) et al., 1999), which might cause some difficulties in the solubility determination. The uncertainty in Bigham et al. (1996) and Yu et al. (1999) sug- solubilities of the minerals also arises due to a few gested 1018 and 1010 as the solubility product, re- other reasons. spectively.

*Corresponding author (e-mail: [email protected])

119 120 J.-Y. Yu et al.

Ferrihydrite has not only variable chemical Fe(NO3)3á9H2O, 0.01 M Na2SO4 + 0.01 M compositions but also variable crystallinity. The Fe(NO3)3á9H2O, and 0.005 M Na2SO4 + 0.01 M chemical formula of ferrihydrite has been reported Fe(NO3)3á9H2O. These solutions were hydrolyzed significantly diverse (Jambor and Dutrizac, 1998), for 12 min. at 60°C right after the mixing, and and generally represented by a bulk chemical for- then placed in a water bath at 25°C. We prepared mula like Fe2O3ánH2O (Schwertmann and Tayor, two sets of the first mixture. The samples of one 1989). Ferrihydrite may show up to 9 X-ray dif- of the first mixture were collected after 1, 4, 8, fraction lines depending on its crystallinity, but 12, 18, 24, 30, 48, 60, 72, 84, 96, 108, 120, 132, 2- and 6-line ferrihydrite are most common (Towe 144, 168 hours passed since the experiment and Bradley, 1967; Eggleton and Fitzpatrick, started. The samples of other first mixture and 1988; Martinez and McBride, 1998). Many inves- second one were collected after 1, 3, 7, and 14 tigators have tried to determine the solubility of days passed. The rest of the mixtures were sam- ferrihydrite. However, significant discrepancies pled after 1, 4, 7, 14, 25 days passed. are still present among the reported solubilities 2-line ferrihydrite was synthesized from and the relation among the solubility, chemical Fe(NO3)3á9H2O solutions of various concentra- composition and crystallinity is yet to be more tions titrated with 1N NaOH to various pH. The thoroughly investigated. For the following disso- concentrations of Fe(NO3)3á9H2O were 0.025, lution reaction 0.0165 and 0.005 M. The initial pH of 0.005

Fe(NO3)3á9H2O M solution was adjusted to ap- + 3+ 0.5Fe2O3 + 3H = Fe + 1.5H2O, (2) proximately 5, 6 and 9. We prepared two 0.025 M solutions; one is used for sampling at a shorter Nordstrom et al. (1990) summarized the reported time interval (1, 4, 7, 12, 17, 24, 31, 39, 48 hrs). values and gave the range of 103 to 105 for the The other 0.025 M and 0.0165 M solutions were solubility product. sampled after 0, 3, 7, and 14 days passed. Sam- The purpose of this study is to obtain more ples of 0.005 M solutions were collected after 2, reliable solubilities of schwertmannite and 4, 7, 12, 20, and 28 days passed. ferrihydrite and examine the relationship among 6-line ferrihydrite was also synthesized from the solubilities, chemical compositions, and 0.025, 0.0125, and 0.005 M Fe(NO3)3á9H2O solu- crystallinities of the minerals. Many of the re- tions prepared at 75°C. The solutions were allowed ported solubilities have been determined from the to be cooled for 13 min. at room temperature and natural waters or the solutions probably in a dis- then quenched with ice/water mixture. After the equilibrium state. This study synthesized temperature of the solution recovers to 25°C, the schwertmannite, 2-line ferrihydrite and 6-line solutions were titrated with 1 N NaOH. The sam- ferrihydrite under various conditions and collected pling intervals of the solutions were the same as data from the analyses of the synthesized miner- those for 2-line ferrihydrite. als and synthesis solutions. The samples were always taken from the mixtures of solutions and precipitates homogenized by shaking and then filtered with 0.1 µm METHODS micropore membrane to separate the aqueous Mineral syntheses phase from the precipitates. Each filtrate were Schwertmannite was synthesized from 1000 ml transferred to a presoaked 50 ml polyethylene

Na2SO4 + 1000 ml FeCl3á6H2O or Fe(NO3)3á9H2O bottle and acidified with c-HNO3. The precipitates solution of various concentrations. The concen- were washed three times with deionized water, trations were 0.02 M Na2SO4 + 0.02 M centrifuged 10,000 rpm for 5 min. and then dried ° FeCl3á6H2O, 0.0026 M Na2SO4 + 0.02 M at 50 C in an oven. FeCl3á6H2O, 0.02 M Na2SO4 + 0.01 M Solubilities of schwertmannite 121

Laboratory measurements and analyses Al2O3 used as a reference material. The samples Temperature of the synthesis solutions was were dried at 60°C for 2 hours prior to the ther- checked all the times and maintained at 25 ± 1°C. mal analysis and then analyzed from room tem- The pH of each solution was measured right after perature to 1,000°C with temperature increase rate the sampling and filtration with SUNTEX SP-701 of 10°C/min. pH/mV/ORP meter. The alkalinity measurement The amount of adsorbed sulfate on was performed only for the solutions having pH > schwertmannite was attempted to be determined 4.5 with Gran titration (Wetzel and Likens, 1991). using the method of Rose and Ghazi (1997). The Carbonate concentrations in aqueous phases were method is as follows; 0.75 g of schwertmannite estimated from the alkalinities (Stumm and was reacted with 7.5 ml (0.25M sodium oxalate : Morgan, 1996). 1M sodium hydroxide = 1:1) of solution. The so- The concentrations of Si, Al, Fe, Ca, Mg and lutions were gently machine shaken in a 25°C K in the aqueous solutions were determined with water bath for 24 hours and were also vigorously Perkin Elmer OPTIMA 3200XL ICP-AES at the hand shaken from time to time during this period. Seoul Branch of Korea Basic Science Institute The supernatant was centrifuged and then filtered (SB-KBSI). Na was analyzed with flame AAS with through a 0.1 µm micropore membrane. The filter Perkin Elmer Spectra AA-20 at the Department cake was analyzed with XRD and the filtrate was of Geosystem Engineering, Kangwon National analyzed with the methods described earlier for University (KNU). When the solution has very low the aqueous solutions. Fe and high Na contents, Na significantly inter- feres with the analysis of Fe. In this case, Fe was Chemical equilibrium calculation analyzed with standard addition method using the The speciation of the dissolved component and graphite furnace AES. The concentrations of ani- the activities of the species were calculated from onic components, including Cl, NO3, and SO4 in the chemical compositions of the aqueous solu- the aqueous phases were determined with Dionex tions using the computer program PHREEQC 4500i IC at SB-KBSI and at the Department of (Parkhurst, 1995). The database used in the cal- Chemistry, KNU. culation was that of MINTEQA2 (Allison et al., 100 mg of each precipitate sample was dried 1991). No solid or gas was allowed to be in con- at 120°C for 2 hrs in an oven and dissolved with tact with the solution during the calculation. 20 ml 5N HCl. This precipitate solution was di- luted to 500 ml and then analyzed by the same RESULTS AND DISCUSSION method as in the aqueous solution analyses. X- ray diffraction (XRD) and thermal analyses of the Chemical and mineralogical compositions precipitate samples were also conducted at the Tables 1, 2, and 3 list the chemical composi- Industrial Mineral Bank, KNU. XRD profiles were tions of the aqueous solution and the mineralogi- obtained with Rigaku Umax 2200V X-Ray cal compositions of the precipitate samples col- diffractometer, using a Co source with divergent- lected during the syntheses of schwertmannite, 2- scattering-receiving slits of 1°-1°-0.15 mm. Sam- line ferrihydrite, and 6-line ferrihydrite, respec- ples were continuously scanned from 5 to 100° tively. As expected, the aqueous phases mainly ° ° with scan rate of 1 /min and scan step of 0.02 in consist of Fe, Na, Cl, NO3, and SO4. The minera- 2θ. Thermal analyses included differential ther- logical compositions of the precipitates were de- mal analysis (DTA), thermogravimetric analysis termined from the XRD patterns. Figure 1 shows (TGA) and differential thermogravimetric analy- some typical XRD patterns of the precipitates. sis (DTGA), being performed with Rigaku TAS Many precipitates consists of schwertmannite, 2- 100. These techniques were used to analyze 10 line ferrihydrite, or 6-line ferrihydrite only. Some mg samples of precipitates in parallel with 10 mg other precipitates, however, also contain goethite, 122 J.-Y. Yu et al.

Table 1. The chemical compositions of the aqueous solutions and mineralogical compositions of the precipitates collected during the schwertmannite synthesis experiments

*Time passed since the synthesis started when the sample was taken. h = hours, d = days. **Mineral compositions of the precipitates. S = schwertmannite, G = goethite. #n.d. = not detected.

indicating that schwertmannite and ferrihydrite which might be directly precipitated from the so- were transformed to more stable goethite during lution. the experiments. It has been well known that Tables 4 and 5 summarize the chemical com- metastable schwertmannite and ferrihydrite pre- positions of some representative synthesized precipitate instead of stable goethite if pH is not too cipitates consisting of schwertmannite and high, which would be gradually transformed to ferrihydrite only, respectively. The thermal curves goethite (Chukrov et al., 1974; Bigham et al., were used to determine the amount of H2O(+) and 1996). There are many factors controlling this H2O(Ð) and to estimate the amount of the struc- transformation, but this study focuses only on the tural OH out of H2O(+). Figure 2 shows typical solubilities of schwertmannite and ferrihydrite. thermal curves of the precipitates consisting only Sample FII3 and FII6 consist only of goethite of schwertmannite (S2-3d), 2-line ferrihydrite Solubilities of schwertmannite 123

ecipitates collected

Table 2. The chemical compositions of the aqueous solutions and mineralogical compositions of the pr Table during the 2-line ferrihydrite synthesis experiments

*2F = 2-line ferrihydrite, G = goethite. 124 J.-Y. Yu et al.

ecipitates collected

Table 3. The chemical compositions of the aqueous solutions and mineralogical compositions of the pr Table during the 6-line ferrihydrite synthesis experiments

*6F = 6-line ferrihydrite, G = goethite. Solubilities of schwertmannite 125

Fig. 1. X-ray diffraction profiles of some representative precipitate samples collected during the syntheses. Abbreviations; S = schwertmannite, F = ferrihydrite, G = goethite.

(FII2-3d), and 6-line ferrihydrite (FVI2-1d). The amount of H2O(Ð) was calculated from the weight loss on TGA curve before temperature (T) reaches ° 120 C. The amount of H2O(+) was estimated from the total weight loss except that by SO3 gas re- ° lease after T passes 120 C. The release of SO3 is due to the SO4 in schwertmannite and occurs around 600°C. The amount of the structural OH was determined from the weight loss corresponding to the second endothermic peak around 200Ð 300°C. The amount of structural OH is denoted Fig. 2. Typical thermal curves of the precipitate consisting purely of schwertmannite (sample S2-3d), 2- as H O(str) and the rest of H O(+) as H O(cry) in 2 2 2 line ferrihydrite (FII2-3d), and 6-line ferrihydrite Table 5. (sample FVI2-1d). Schwertmannite was first named by Bigham et al. (1994), who gave the chemical formula of 126 J.-Y. Yu et al.

Table 4. Chemical compositions of the precipitates consisting of schwertmannite only and the chemical formulae of schwertmannite calculated with the chemical compositions

Table 5. Chemical compositions of the precipitates consisting of ferrihydrite only and the chemical formulae of ferrihydrite calculated with the chemical compositions

Ð *H2O(+) Ð H2O of structural OH . **Structural OHÐ.

≤ Fe16O16(OH)y(SO4)zánH2O (16 Ð y = 2z and 2 and Yu et al. (1999) but also from this study indi- z ≤ 2.5) with other physicochemical parameters cate that schwertmannite has little structural OH. of the mineral. Later, Bigham et al. (1996) and Yu The shapes of the DTA and TG curves due to wa- et al. (1999) investigated the stability of ter loss from schwertmannite (sample S2-3d in schwertmannite in an aqueous solution using the Fig. 2) rather resemble that of so-called “zeolitic same form of the chemical formula. However, the or colloidal water” (Todor, 1976). Bigham et al. thermal curves not only from Bigham et al. (1996), (1990) already recognized it and noted “similar Solubilities of schwertmannite 127 results were achieved by Margulis et al. (1975) The solubilities of schwertmannite and ferrihydrite from a synthetic gel with composition Figure 3 shows the variation of the Fe and SO4 2Fe2O3.SO3ámH 2O”. Thus, we think that concentration in the aqueous solutions as a func- schwertmannite has little or no structural OH and tion of time. The concentrations of the constitu- propose a new chemical formula for ent components of schwertmannite and schwertmannite as below: ferrihydrite in the solutions become more or less steady after at least 60 and 20 hours, respectively.

Fe2O3Ðx(SO4)xánH2O. Thus, the solubility of schwertmannite was calculated only with the data obtained from the sam- Table 4 compares the calculated x and n values ples collected after 60 hours passed among those according to the formula suggested by Bigham et consisting of schwertmannite only, while the solu- al. (1994) with that by this study. bility of ferrihydrite being estimated from the sam- The sulfate contents in the synthesized ples collected after 20 hours among those consist- schwertmannite in Table 4 are considerably higher ing of ferrihydrite only. than those originally reported by Bigham et al. The solubility of each mineral usually ex- (1994). Bigham et al. (1996) pointed out that some pressed with the solubility product for the corre- of the sulfates analyzed may be present not as the sponding dissolution reaction. If the dissolution structural but as the adsorbed form. We tried to reaction achieves an equilibrium state, the solu- determine the amount of the adsorbed sulfate on bility product equals to the equilibrium constant schwertmannite using the method of Rose and for the reaction. The calculated solubility prod- Ghazi (1997), but failed because most of ucts of schwertmannite and ferrihydrite in this schwertmannite was transformed to goethite dur- study, however, are highly improbable to be the ing the desorption experiment. equilibrium constants, because the dissolution re- Many investigators have proposed the chemi- actions of the minerals seem irreversible. This cal formula of ferrihydrite, but no single formula study actually examines the precipitation reactions is widely accepted (Jambor and Dutrizac, 1998). of the minerals and the dissolution experiments The disagreement among the reported formulae of the minerals have not been attempted. The re- lies in the amount of structural OH and crystalli- sults from work of Kim (2001) and the SO4 zation water. For example, Towe and Bradley desorption experiment of this study indicate that

(1967) gave Fe5HO8á4H2O, but Chukrov et al. schwertmannite and ferrihydrite show incongru- (1974) gave 5Fe2O 3á9H2O. Eggleton and ent dissolution to goethite in alkaline or dilute Fitzpatrick (1988) reported generalized bulk solutions. chemical formulae of Fe4(O,OH,H2O)12 for syn- Although the minerals are both metastable with thetic 2-line ferrihydrite and Fe4.6(O,OH,H2O)12 respect to goethite, they may experience instanta- for synthetic 6-line ferrihydrite. It is not very clear, neous dissolution and precipitation back and forth however, that how the electrical neutrality is sat- and achieve a kind of “pseudo-equilibrium” for a isfied between Fe, O, and OH. This study proposes finite amount of time. The XRD patterns (Fig. 1) a revised general chemical formula of ferrihydrite of the precipitates indicating only schwertmannite as below: or ferrihydrite suggest that this may actually hap- pen, but again we really do not know if these XRD

Fe2O3Ð0.5y(OH)yánH2O, patterns resulted from the true absence of goethite or from the presence of insufficient amount of which may be easy to formulate and balance the goethite due to very slow transformation of the electrical charges of Fe with O and OH. Table 5 minerals to goethite. lists the calculated y and n values in the above In any case, it is obvious that the precipita- formula for some 2- and 6-line ferrihydrites. tion-dissolution reactions of schwertmannite and 128 J.-Y. Yu et al.

Fig. 3. Concentration variation of the dissolved Fe and SO4 in the aqueous solutions as a function of time passed since the experiment started.

+ ferrihydrite are irreversible and the calculated Fe2O3Ðx(SO4)x + (6 Ð 2x)H 3+ 2Ð solubility products in this study should not be con- = 2Fe + xSO4 + (3 Ð x)H2O. (3) sidered as the equilibrium constants. In this sense, using the term “apparent solubility product” in- As expressed as reaction (3), however, the varia- stead of “solubility product” would be more ap- tion in sulfate concentration among the samples propriate. In the followings, however, we simply causes difficulties in representing the solubility use of the term “solubility product” for conven- product Ks on a solubility diagram. Slight modifi- ience. The concentrations of Fe and SO4 in Figs. cation of reaction (3) as below can circumvent this 3a and 3b show a little fluctuation, which may also problem: indicate the disequilibrium between the mineral and solution. Fe2O3Ðx(SO4)x + (3 + x)H2O 3+ Ð 2Ð + The dissolution reaction of schwertmannite = 2Fe + 6OH + xSO4 + 2xH . (4) may be represented as Solubilities of schwertmannite 129

Fig. 5. Plot of (pFe + 3pOH) versus (pSO4 + 2pH) for Fig. 4. Variation of the equilibrium constant of the aqueous phases of the samples precipitating pure schwertmannite dissolution reaction as a function of schwertmannite. The solid line represents the solubil- the sulfate content (x) in the mineral. The solid line ity line with pKs = Ð2.01 and the broken lines repre- ± represents the linear regression line for the whole data. sent the confidence limit defined by the 0.30. The broken ellipses represent separate solution batches.

(solid line in Fig. 4). However, examining the data

′ of FeCl3 solution batch separately from those of If the solubility product for reaction (4) is Ks , then Fe(NO3)3 solution batch (Table 1) reveals that pKs may be independent of x (broken ellipses in Fig.

logKa′=23() log + log a− s Fe3+ OH 4). For the time being, it is difficult to cleary understand the relationship between pK and x with ++ s xa()log2−+25 log a () SO4 H the given data. More data and through investigation on them is required for and the quantification or of the relation. Figure 5 plots (pSO4 + 2pH) versus (pFe + 3pOH) of the samples precipitating pure schwertmannite. pK of schwertmannite, calcu- pK′= pK + pK s ssw lated with Eq. (6), is Ð2.01 ± 0.30 on 95% confi- =+ ++ () 23()ppFe OH xpp() SO4 2 H , 6dence level. The solid line in Fig. 5 represent the solubility lines defined by Eq. (6) when pKs = Ð2.01 and x = 0.42. The broken lines show the where Kw is the dissociation constant of water and confidence limit. The calculated value is fairly p means Ðlog10. Equation (6) represents a line on close to pK = Ð2.68 which is the value recalcu- a (pSO4 + 2pH) versus (pFe + 3pOH) diagram s whose slope depends on the sulfate content in lated from pKs given by Yu et al. (1999) for reac- ° schwertmannite. tion (1) between 10 and 15 C. The solubility of schwertmannite may depend The dissolution reaction of ferrihydrite may be on x, the sulfate content in schwertmannite. Fig- expressed as ure 4 shows pKs against x. Taking account of the Fe O (OH) + 6H+ = 2Fe3+ + (3 + 0.5y)H O.(7) whole data, pKs seems increase as x increases 2 3Ð0.5y y 2 130 J.-Y. Yu et al.

Fig. 7. The solubility versus the water content of ferrihydrite.

Fig. 6. Plot of pFe versus pH for the aqueous solu- line 2-line ferrihydrite has a lower solubility than tions of the samples precipitating pure ferrihydrite. The solid lines are the solubility lines of 2- and 6-line 6-line ferrihydrite of better crystallinity. We cur- ferrihydrite. rently do not understand why the ferrihydrite having poorer crystallinity has lower solubility, and it may deserve further investigation. The solubility of ferrihydrite may depend on the content of

If the solubility product for reaction (7) is Kf, H2O(+) or structural OH as well as crystallinity. Figure 7, however, shows that the solubility and

logKa=−26 log log a+ () 8water content does not have any correlation. f Fe3+ H The stability boundary between or schwertmannite and ferrihydrite can be expressed in terms of the equilibrium constant, Kr, of the following transformation reaction: pFe = 3pH + logKf/2. (9)

Equation (9) represents a line on a pFe-pH dia- Fe2O3Ðx(SO4)x + (x + 0.5y)H2O 2Ð + gram. = Fe2O3Ð0.5y(OH)y + xSO4 + 2xH . (10) Figure 6 plots pFe versus pH of the aqueous solutions in equilibrium with the precipitates of The relative stabilities between schwertmannite and ferrihydrite may be determined with the ap- pure ferrihydrite. pKf of 2- and 6-line ferrihydrite, calculated from these data with Eq. (9), were parent solubility products as accurate as with true Ð8.46 ± 1.40 and Ð10.12 ± 0.74, respectively. thermodynamic equilibrium constants. If this is These values are within the range, 6 to 10, sug- the case, the stability boundary is given by the gested by Nordstrom et al. (1990), but a little equation higher than the value estimated by Yu et al. (1999) based on the stability relation between pK pK− pK ppSO=−22 H +r =−p H + sf. () 11 schwertmannite and ferrihydrite. Yu et al. (1999) 4 x x ± suggested pKf = Ð8.6 0.5 for 6-line ferrihydrite. It may be worth noting that more poorly crystal- Figure 8 shows the stability boundaries of differ- Solubilities of schwertmannite 131

≤ ≤ ≤ ≤ Fe2O3Ð0.5y(OH)yánH2O (0 y 1.96, 0.82 n 1.14). The aqueous solutions for schwertmannite and ferrihydrite synthesis reach steady states after 60 and 20 hours passed since the synthesis starts, respectively. The measured solubility of schwertmannite is in good agreement with the value reported by Yu et al. (1999), but that of ferrihydrite is a little higher than the value suggested by Yu et al. (1999). The solubility products for the dissolution reactions of schwertmannite, 2-line ferrihydrite, and 6-line ferrihydrite are 102.01±0.30, 108.46±1.40, and 1010.12±0.74, respectively. The synthesis experiments of this study reveal a few important problems which should be solved Fig. 8. Stability boundaries between schwertmannite to accurately estimate the solubilities of and ferrihydrite at different x values. The solid and schwertmannite and ferrihydrite and understand broken lines are for 2- and 6-line ferrihydrite, respec- the system including these minerals. The solubil- tively. ity of schwertmannite seems to be a function of sulfate contents, but we need more data to quantify the relation. The stability boundary between ent x values on pH-pSO4 diagram. The boundary between schwertmannite and 2-line ferrihydrite schwertmannite and ferrihydrite predicted with the when x = 0.42 (thick solid line in Fig. 8) is perti- above solubility products is in disagreement with nent to what we observe in nature. However, the that observed in natural system when the solubil- natural ferrihydrite is mostly 6-line ferrihydrite. ity of schwertmannite is assumed constant. The The boundary between schwertmannite and 6-line explanation of the disagreement may be possible ferrihydrite when x = 0.42 (thick broken line in only after the solubilities of schwertmannite and Fig. 8) is considerably biased to the higher pH than ferrihydrite are fully understood. The solubility the boundary observed in nature. This may be due of 2-line ferrihydrite is turned out to be lower than to determining the stability boundary with appar- 6-line ferrihydrite of better crystallinity. In most ent solubility products, due to the solubility change cases, polymorphs of better crystallinity have of schwertmannite as a function of sulfate con- lower solubilities. These problems may be of great tent, or due to other reasons that we have not rec- importance in dealing with ferriferous systems in ognized. If the solubility of schwertmannite in- near surface environments and expected to be creases as the chemical condition approaches to solved near future. the stability boundary, the boundary can be shifted much to the lower pH than that predicted with Acknowledgments—This study was supported by Korea Research Foundation Grant KRF-99-015- constant solubility. It also requires further inves- DP0432. The authors greatly thank Dr. H. Hwang in tigation. the Department of Chemistry, KNU and Dr. K. Min in the Department of Geosystem Engineering, KNU for allowing to use IC and AAS in their laboratories, re- SUMMARY spectively. The authors also appreciate the chemical and XRD data of excellent quality provided by the analyti- Synthesized schwertmannite and ferrihydrite cal staffs in SB-KBSI and IMB of KNU. The manu- have the chemical formula of Fe2O3Ðx(SO4)xánH2O script is greately improved only after the comments (0.41 ≤ x ≤ 0.49, 1.51 ≤ n ≤ 2.81) and given by Dr. Iwamori and an anonymous reviewer. 132 J.-Y. Yu et al.

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