1944
Anal. Chem. 1985, 57, 1944-1946
Artifacts in the Use of Selective Chemical Extraction To Determine Distributions of Metals between Oxides of Manganese and Iron Edward Tipping,* Nigel B. Hetherington, and John Hilton Freshwater Biological Association, The Ferry House, Ambleside, Cumbria LA22 OLP, United Kingdom
Dudley W. Thompson School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 ITS, United Kingdom
Emma Bowles and John Hamilton-Taylor Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, United Kingdom
Treatment of a naturally occurrlng mlxture of Mn and Fe oxIdes with aaidifled hydroxylamlne released Into solution almost all the Mn, Ca, Zn, and Ba present, together with about onethird of the Pb. Treatment of the residue with oxalate released the Fe and the other two-thirds of the Pb. Parallel examlnatlon of the solids by electron mlcroscopy coupled with electron probe mlcroanalysls showed that the dlssolutlon behavlors of Ca and Pb dld not reflect thelr dlstrlbutlons In the original mlxture, where Ca had been associated with both the Mn and Fe oxldes, but Pb largely with the Mn oxlde alone. The dlscrepancles occurred because the conditlons of the hydroxylamlne treatment were sufficiently acidlc for the Fe oxlde to release Ca but sufflclently basic for It to take up a substantlal amount of the Pb that had been released by dissolution of the Mn oxlde.
Selective chemical extraction schemes are widely employed to obtain information about the “solid speciation” of metals in soils, sediments, and other natural particulate phases (1). Major components of such materials are the oxides of Mn and Fe. These can be selectively extracted by treatment with acidified hydroxylamine to dissolve the Mn, followed by oxalate to dissolve the Fe (2-4). It is usually assumed that metals and other species brought into solution by hydroxylamine and oxalate were associated with Mn oxides and Fe oxides, respectively, in the original solid. We attempted to apply this procedure to determine the partitioning of associated metals between manganese and iron oxides that had been precipitated together on the walls of a disused lead mine. However, although good separation of the Mn and Fe phases was achieved, electron probe microanalysis (EPMA) of the solid material before and after hydroxylamine treatment showed that the release of associated metals was not necessarily related to their original distributions.
EXPERIMENTAL SECTION The sample studied was a dark-brown,earthy material. It was removed from the wall of the lead mine with an acid-washed polyethylene spatula and transferred to an acid-washed polyethylene container for transport. Note that this was a different sample to that used for the study of pH-dependent metal desorption (5). The extraction procedure was essentially that of Shuman (4). The sample was treated first with 0.1 M hydroxylamine/O.OlM HNO, at room temperature, at a concentration of 4 g/L, the reaction being allowed to continue until effervescence had ceased and the sample had become distinctly lighter brown. This cor-
responded to a reaction time of 20 min rather than the 30 min used by Shuman. The final pH of the suspension was 5.2. After centrifugationand removal of the supernatant for metal analysis, the solid residue was treated with 0.2 M ammonium oxalate/0.2 M oxalic acid (pH 3) for 4 h at room temperature in the dark. Again the supernatant was taken for metal analysis. A parallel pair of extractions was performed to obtain solid materials for electron microscopy and X-ray diffractometry. Concentrations of Mn, Fe, Ca, Zn,and Pb were determined by flame atomic absorption spectrophotometry (Perkin-Elmer 2380). Barium was measured in the emission mode. Electron microscopic examinations were made with a JEOL lOOCX Temscan instrument equipped with a Link Systems 290 energy dispersive electron probe microanalyzer. Specimens for electron microscopy were prepared by transferring small volumes of suspension (briefly sonicated) to nylon or copper grids and airdrying. Elemental analyses were obtained from specimen areas within the range 0.002-0.2 pm2. It should be noted that (a) the Cu peaks in the EPMA spectra are due almost entirely to the electron microscope, not the sample, and (b) the peaks at -4.5 keV in Figure 1 are due to any Ba in the samples, together with Ti in the nylon specimen grids; Ba contents (Table 11) were obtained by EPMA of samples deposited on copper grids. The carbon content of the dried solid was measured with a Carlo Erba 1106 instrument. Oxidizing equivalentsof manganese were determined by reaction with excess Fe2+in acid solution followed by back titration with KMn04 (6). X-ray diffraction measurements were carried out with a Philip8 PW1730 instrument.
RESULTS AND DISCUSSION The lead mine at Wanlockhead was opened early in the 18th century. The manganese and iron oxides have therefore been precipitating over a period of at the most 250 years. We suppose that they are formed when anoxic water, rich in dissolved metals, percolates into the mine and is exposed to oxygen. The damp atmosphere in the tunnel from where the sample was taken has probably prevented any drying out of the oxides, As collected, the material was approximately 10% by weight water. The material was low in carbon (0.9% by weight). Its manganese had an oxidation state of 4.0, i.e., the Mn oxide had the stoichiometry Mn02. Of the diffraction peaks given by the untreated material only one, a broad reflection a t d = 7.3 A, corresponded to known d spacings of Mn and Fe oxides, being ascribable to birnessite-type Mn oxide (7). As expected, this peak was not given by the residue from the hydroxylamine extraction step. A number of other, sharper reflections were noted, but these increased in intensity after leaching and can be attributed to small amounts of aluminosilicates and quartz. Electron microscopy showed the material to contain a number of phases (Figure 1). Before any chemical treatment
0003-2700/85/0357-1944$01.50/0 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57. NO. 9. AVGVST 1985
I
I
cs after
NH~OH
I
CShforrNH20H
1945
7keV
keV
F before N%OH
F after NHzOH
C"
si
m
I
u
2
4
6
8
1 0 1 2 1 4
XeV
2
4
L L e
b
8
1 0 1 2
keV
Flpure 1. Electron micmgaph of the cfiginel. untreated sample (a). and representative EPMA spectra before (b, c) and after (d. e) treatment wlth hydroxylarnhe. Scale bar represents 200 nm. CS is the crurnpledaheet phase and F Is the iron ox& phase.
all the Mn waa in a phase consisting of thin crumpled sheets, similar in appearance to Mn oxide formed hy the oxidation of Mn(I1) in soils (7)and lake water (8). This phase also contained Fe, but most of the Fe was present in roughly spherical particles, or in twisted cylinder8 reminiscent of the "iron bacterium^ Siderococcus (9). Minor amounts of more crystalline iron oxides (goethite and possibly feroxyhite, d'FeOOH) were also found. The solid residue after hydroxylamine treatment contained the w e phases as the untreated
material except that the crumpled-sheet phase now contained very little Mn and was present in much smaller amounts (Figure 1). After oxalate treatment there were no disordered Fe phases remaining, the residue consisting of only crystdine iron oxides together with detrital silicates. Many metals in addition to Mn and Fe were present in the disordered oxides. Four (Ca, Zn, Ba, and Pb) occurred in sufficiently high concentrations to be detected by EPMA (cf. Figure l),thus allowing the effecta of chemical leaching to be monitored by
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table I. R e l e a s e of M e t a l s i n t o S o l u t i o n b y Treatment w i t h H y d r o x y l a m i n e , F o l l o w e d by O x a l a t e "
metal released, mg/g dry wt
metal
hydroxylamine
oxalate
Mn
300.3 19.3 26.3 21.9 9.4 2.1
6.3 177.3 0.08 1.17 0.29 4.39
Fe Ca Zn
Ba Pb
% released hydroxylamine oxalate
97.4 9.8 99.7 94.9 97.0 32.5
2.1 90.2 0.3 5.1 3.0 67.5
" See text for experimental details. T a b l e 11. R e l a t i v e C o n t e n t s
of
S o l i d P h a s e s D e t e r m i n e d by
EPMA"
metal
before hydroxylamine treatment crumpledFe oxide sheet phase phases
Mn
100
Fe Ca Zn Ba Pb
48
6.1 3.6 1.0 1.1
0 100 4.1 0.7 0 0
after hydroxylamine treatment crumpledFe oxide sheet phase phases 4.2 100 0.3 0.5 0 1.6
1.0 100 0.6 0.3
0 1.0
"Results are based on peak heights and are not corrected for variations in sensitivity with atomic no. The major element in each phase (Mn or Fe) is assigned a value of 100. Each value shown is a mean from 4 to 10 individual spectra. The crumpledsheet and Fe oxide phases are identified in Figure 1. examination of the solid phases. Table I shows the results of the chemical extraction experiments. It is clear that, as indicated by electron micrmmpy, the hydroxylamine reagent solubilized nearly all the Mn in the sample, but only about 10% of the Fe. The behaviors of Ca, Zn, and Ba during the chemical extractions suggest that in the original material these metals were predominantly associated with the Mn oxide, since, together with Mn, they were each solubilized almost completely by treatment with hydroxylamine. In the case of Pb it appears that about two-thirds was originally associated with the iron oxide phases. However, the results from EMPA show that for Ca and Pb these conclusions are not justified. Thus EPMA shows that in the original material Ca was rather evenly divided between the crumpled-sheet phase (mainly Mn oxide) and the iron oxides, while Zn, Ba, and P b were predominantly in the Mn phase (Figure 1and Table 11). After hydroxylamine treatment the iron oxides contain little Ca, Zn, or Ba but they do contain considerable amounts of Pb, of which they were devoid in the original solid. The transfer of a large amount of Pb from manganese to iron oxide occurred because during hydroxylaminetreatment the pH of the suspension increased, reaching a final value of 5.2. Under these conditions considerable adsorption of Pb (but not Ca, Zn, or Ba) is expected (1,10).The fiial pH value in hydroxylamine treatment reflects the consumption of protons during Mn(IV) reduction together with buffering by the remaining iron oxide and detrital phases, and it will vary among samples, depending on their chemical nature and the solid/solution ratio employed. (Note that in our experiments
we worked with a concentrationof 4 g of solids/L, at the lower end of the range used by Chao (2)in the original development of the method.) Thus the extent of Pb readsorption is difficult to predict. Readsorption similar to that found for Pb would be expected to occur with other strongly adsorbed metals and may account for the release of only 0.036 mg of Cu/g from the present sample by hydroxylamine treatment, compared to 0.237 mg of Cu/g by oxalate. The behavior of Ca during treatment with hydroxylamine (Tables I and 11) shows that a metal initially associated with iron oxide can be released into solution as a result of a decrease in pH (from ca. 7 in the original sample) without dissolution of the oxide (cf. ref 5). By analogy, this could apply to other relatively weakly adsorbed metals having potentially greater environmental impact, e.g., Cd. Again, the extent of release will be pH dependent and therefore will vary from sample to sample. Such release on lowering the pH means that the readsorption problems found here for Pb cannot be overcome satisfactorily by maintaining a low pH during dissolution of Mn oxides, since that would greatly encourage desorption from nondissolving Fe oxide. The results for P b illustrate the readsorption problems predicted by Rendell et al. (11) on the basis of experiments with spiked sediment samples, while those for Ca show that weakly adsorbed metals can be released from phases other than those being dissolved. Taken together, these findings mean that considerable care must be taken when attempting to employ selective chemical extraction to evaluate metaloxide interactions in the environment.
ACKNOWLEDGMENT We thank J. P. Lishman (Freshwater Biological Association) and J. Bowman (University of Lancaster) for technical assistance and E. M. Evans for typing the manuscript. We are indebted to G. Downs-Rose for granting us access to the lead mine (now part of the Museum of Scottish Lead Mining, Wanlockhead, Dumfriesshire, Scotland). R e g i s t r y No. ",OH, 7803-49-8; Mn, 7439-96-5; Ca, 7440-70-2; Zn, 7440-66-6; Ba, 7440-39-3; Pb, 7439-92-1; Fe, 7439-89-6; ammonium oxalate, 1113-38-8; manganese oxide, 11129-60-5; iron oxide, 1332-37-2.
LITERATURE CITED Salomons, W.; Forstner. U. "Metals in the Hydrocycle"; Springer-Verlag: Berlin, 1983. Chao, L. L. SollScl. SOC.Am. Proc. 1972, 3 6 , 764-768. Schwertmann, U. Z . Pflanzenernaehr., Dung., Bodenkd. 1984, 105, 194-202. Shuman, L. M.; SoilSci. SOC.Am. J . 1982, 4 6 , 1099-1102. Tlpping, E.; Thompson, D. W.; Ohnstad, M.; Hetherlngton, N. B., submltted to Environ Scl. Technol. Kolthoff. I . M.; Eelcher, R. "Volumetrlc Analysls"; Wiley-Interscience: New York, 1957; Vol. 111. Chukhrov, F. V.; Gorshkov, A. I.; Rudnitskaya, E. S.; Eeresovskaya, V. V.; Sivtsov, A. V. C/ays C/ay Mlner. 1980, 28, 346-354. Tipping, E.; Thompson, D. W.; Davison, W. Chem. Geol. 1984, 4 4 ,
.
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Chukhrov, F. V.; Gorshkov, A. I.; Zvyagln, E. B.; Ermliova, L. P. In "Geology and Geochemistry of Manganese"; Varentsov, I. M., Grasselly, G., Eds.; Schweizerbart'sche: Stuttgart, 1980; Vol. I . Klnnlburgh. D. 0.;Jackson, M. L. I n "Adsorptlon of Inorganics at Solid-Liauid Interfaces"; Anderson, M. A,, Rubln, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981. Rendell, P. S.; Eatley, G. E.; Cameron, A. J. Environ. Sci. Technol. 1980, 14, 314-318.
RECEIVED for review February 25,1985. Accepted April 26, 1985. This work was partly supported by the Natural Environment Research Council.
