Environ. Sci. Technol. 1903, 17, 709-713
Surface Areas and Porosities of Fe( I I I)- and Fe( I I)-Derived Oxyhydroxides Stuart A. Crosby, Douglas R. Glasson,t Alan H. Cuttler,t Ian Butler,$Davld R. Turner,$ Mlchael Whitfield,' and Geoffrey E. Millward* Department of Marine Science, Plymouth Polytechnic, Plymouth, PL4 8AA Devon, U.K.
This paper reports the surface areas and pore characteristics of iron oxyhydroxide precipitates derived from ferric and ferrous iron by using conditions very similar to those encountered in natural waters. Precipitates derived from Fe(II1) had surface areas in the range 159-234 m2 g-' with hysteresis loops which suggested pores of the narrow-necked, wide-bodied type. The identification experiments involving X-ray diffraction, infrared spectroscopy, and Mossbauer spectroscopy showed that the precipitates were amorphous in the initial phases but on aging for 12 days they contained 10-20% a-FeOOH. Precipitates derived from Fe(I1) had surface areas in the range 97-120 m2 g-l, and they were identified as poorly crystalline yFeOOH. The natural samples from iron-rich sources showed surface areas in the range 6.4-164 m2 g-' with both porous and nonporous characteristics. The results of this study are of relevance to the adsorption of dissolved trace constituents by iron oxyhydroxides in natural waters.
Introduction Naturally occurring iron oxyhydroxides have sorption properties which are known to be important in the control of the solution chemistry of natural waters (1). Many studies have been caried out on the adsorption of both cations (2-6) and anions (7-12)onto iron oxyhydroxides. These have shown that the uptake of the dissolved components appears to be a function of the surface characteristics. As a consequence some recent studies have been directed toward the evaluation of the nature of both synthetic and natural oxyhydroxides (13-18). However, in many of these studies (and those involving the adsorption of trace constituents) a wide range of preparative techniques have been used, and it is difficult to see how definitive comparisons could be made in view of the contrasts in methodology. In particular the iron concentrations used are often much higher than those found in natural waters, the pH of the precipitation lacks control, and the aging times are ill-defined. All these factors could contribute to a variability in precipitate character and adsorption capacity. The situation is further complicated by the fact that most studies involve the precipitation of ferric salts whereas in the environment the precipitates are more commonly derived from the hydrolysis and oxidation of Fe(I1). This could occur where anoxic pore waters are injected into estuaries by tidal stirring, where mixing takes place across the oxic/anoxic boundary in stabilized lakes and fiords and where iron-rich mine streams drain into natural waters. Indeed, a recent study of the uptake of phosphate onto Fe(I1)-derived precipitates has shown dramatic differences when compared to adsorption onto Fe(II1)-derived material (12).There were also differences in the uptake kinetics depending on whether the precipitate was fresh or aged, and a pH hysteresis effect has been observed for phosphate adsorption onto Fe(II1)-derived material (11). Present address: Department of Environmental Science, Plymouth Polytechnic, Plymouth, Devon, U.K. Present address: Marine Biological Association, The Laboratory, The Hoe, Plymouth, Devon, U.K. 0013-936X/83/0917-0709$01.50/0
The surface area and porosity of these materials could contribute to some of the observed adsorption phenomena. The object of this study, therefore, was to examine the surface areas and porosities of iron oxyhydroxide precipitates prepared under carefully controlled conditions, which were close to those encountered in the environment.
Experimental Section The synthetic iron oxyhydroxides were prepared by adding 0.05 M FeC1, or FeClz in 0.01 M HC1 to 30 L of distilled water, to give a final iron concentration of 1X lo4 M. The distilled water contained a 2 mM NaHC03 buffer to stabilize the pH to within k0.5 unit. The precipitations were carried out in a constant temperature room held at 15 "C, and the solids were aged for 2 h, 48 h, and 12 days. To isolate the short-term aged material, the suspensions were centrifuged in 600-mL containers at 2000 rpm for 10 min. The precipitates were recovered by washing from the containers with acetone, which is miscible with water in all ita proportions and does not react with the iron matrix. The removal of water from the surface and pores arrests the aging process (19),and the solids were dried slowly under vacuum at room temperature. In the case of the precipitates aged for several days the materials were allowed to settle out, and about half the liquid was decanted off prior to centrifugation. The natural materials were obtained from two iron-rich sources located on the mineralized catchment area of Devon, South-West England (20). The sediment interstitial waters were collected from one site by using a technique involving dialysis bags (21). From another location, 30-L samples of water were returned to the laboratory and filtered by using a combination of GF/C and 0.45-pm Millipore filters. The dissolved iron was precipitated by adding NaHC03 solution and bubbling air through the solution to give a final pH of 8.0, and it was then aged for 48 h. The relative amounts of dissolved Fe(II1) and Fe(I1) were measured by using the Ferrozine complex (22),on a Pye Unican SP 500 spectrophotometer at 5620 A. The surface areas of the iron oxyhydroxides were measured by using a gravimetric BET Nz adsorption technique, after outgassing for 24 h at room temperature on a vacuum microbalance, C.I. Mark 2B. This instrument measured the adsorption of Nz at 77 K with a microgram to milligram sensitivity by using samples of 0.25 g or less of the solid. The adsorption isotherms also indicated any porosity present (from the hysteresis) and pore size ranges. Morphology and aggregate sizes of the materials were observed by transmission electron microscopy (TEM) by using a Philips 300 TEM. Powder X-ray diffraction patterns of the precipitates were obtained from a Hilger-Watts X-ray diffractometer with an Mo source. Infrared spectra of the samples were obtained by dispersing 1 or 2 mg of the solid in 400 mg of dry KBr and compressing them into a disk. The spectra were recorded on a Perkin-Elmer 257 double-beam spectrophotometer. Mossbauer spectra were obtained from 10-20-mg samples mounted between polyethylene disks in a perspex holder. The y-radiation source was a 25 mCi of 57C0in a rhodium matrix, and the mea-
0 1983 American Chemical Society
Environ. Sci. Technol., Vol. 17, No. 12, 1983 709
Table I. Comparison of Surface Areas of Iron Hydroxides iron oxyhydroxide
method of measurement
am-FeOOH adsorption of phosphate am-FeOOH negative adsorption of Na+ am-FeOOH negative adsorption Mgz+ am-FeOOH BET Ar am-FeOOH BET N, am-FeOOH BET N, am-FeOOH BET N, am-FeOOH BET N, am-FeOOH electron microscopy am-FeOOH BET N, a-FeOOH or-Fe00H or-FeOOH a-FeOOH 7-FeOOH 7-FeOOH y-FeOOH 7-FeOOH
electron microscopy BET N, BET N, BET N, BET& BET N, BET N, BET N,
' 1 year old.
surface area, mz g-'
ref
720
24
270-335
25
~700 215'-265b 159 215 182 320 250 159-234 11-18 89 48 71 1 1 4 k 3.6 100 171 97-121
26 27 26 28 29 30 31 this work 31 32 33 34 35 36 37 this work
1 month old.
surements were made over 24 h. Iron metal was used to provide the calibration and zero velocity reference. The spectra were taken at room temperature (293 K) and at liquid N2temperature (77 K)on a Mossbauer spectrometer of similar design to that of Clarke et al. (23).
Results and Discussion The previous studies reported in Table I show that there are discrepancies between the surface areas determined by using in situ methods and those where the precipitate is isolated and dried. Ideally surface characterization studies should be carried out in suspension with surface areas determined by the adsorption of an appropriate dissolved constituent. However, care must be taken when some of the in situ techniques are applied such as negative adsorption which applies only to smooth nonporous surfaces (38),an assumption which cannot be made for all iron oxyhydroxides. The major advantage of the gas adsorption/desorption experiment is that estimates of pore shape and size can be obtained from the hysteresis loops. Comparable hysteresis experiments using a dissolved constituent are difficult to perform in situ. Thus, the gas ad-
sorption method was preferred when precipitates were used that had been carefully prepared at room temperature, in such a way as to retain the reticular structure of the solid matrix (19). Fe(II1)-DerivedPrecipitates. Preliminary studies were carried out on the precipitation of the Fe(II1)-derived material in which the development of the colloid and appearance of the precipitate aggregates were followed by nephelometry (39). This showed rapid formation of the colloid,
