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Differential thermal analysis of metal-fulvic acid salts and complexes
Geoderma Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
DIFFERENTIAL THERMAL A N A L Y S I S OF METAL-FULVIC ACID SALTS AND COMPLEXES 1 M. SCHN1TZER and H. KODAMA
Soil Research Institute, Canada Department of Agriculture, Ottawa, Ont. (Canada) (Received April 14, 1971) ABSTRACT Schnitzer, M. and Kodama, H., 1972. Differential thermal analysis of metal-fulvic acid salts and complexes. Geoderma, 7: 93-103. Salts and complexes of fulvic acid, a water-soluble soil humic fraction, with 14 different mono-, diand tri-valent metal ions, were prepared and characterized by differential thermal analysis (DTA). The major exotherm for untreated fulvic acid, signalizing the decomposition of the fulvic acid "nucleus", occurred at 450°C. For salts of fulvic acid with monovalent ions, major exotherms occurred between 430 ° and 450°C, but for most polyvalent metal-fulvic acid complexes the major exotherms appeared at significantly lower temperatures. Ferric iron was especially effective in lowering the temperature of the major fulvic acid exotherm. This effect was not catalytic but due to the formation of a chemical complex between iron and fulvic acid. The thermal stabilities of the salts and complexes were related to the nature of the metal-fulvic acid bonding. For metal-fulvic acid salts the main exotherm temperatures tended to decrease as the size of the metal ion increased. For transition metal-fulvic acid complexes the major exotherm temperatures were inversely proportional to the ionization potential o f the metal ion. The DTA method permits differentiation between metal chemically complexed by fulvic acid and metal physically mixed with fulvic acid. At the present time this cannot be done by any other method in such a relatively simple manner. Also, the DTA method may serve as a "fingerprint" for the rapid identification of metal-fulvic acid complexes.
INTRODUCTION Fulvic acid ( F A ) is a water-soluble, relatively low-molecular weight h u m i c fraction that occurs in soils, lakes (Ishiwatari, 1969), rivers (Lamar, 1968) and in the sea (Rashid and King, 1969). Fulvic acid can form stable water-soluble salts and water-soluble and waterinsoluble c o m p l e x e s with di- and tri-valent metal ions (Schnitzer and H o f f m a n , 1967; Schnit. zer and Skinner, 1964) and so influence the physical, chemical and biological properties o f soils and waters. Products resulting f r o m interactions o f F A with m e t a l ions m a y also play a significant role in e n v i r o n m e n t a l pollution. There is, therefore, a need for the developm e n t o f specific b u t relatively simple m e t h o d s for the analysis o f m e t a l - F A complexes. In view o f the chemical complexities of such systems, the use o f physical rather than o f chemical m e t h o d s appeared attractive. Schnitzer and H o f f m a n (1967) p r o p o s e d a differential t h e r m o g r a v i m e t r i c ( D T G ) m e t h o d for this purpose. The thermal stabilities o f the metalF A c o m p l e x e s were f o u n d to depend on the nature o f the cation, b u t the m a j o r d e c o m p o 1 Contribution No. 371.
94
M. SCHNITZERAND H. KODAMA
sition peaks were often not sufficiently separated to permit differentiation between the various complexes. In the present investigation we have attempted to characterize metal-FA complexes by differential thermal analysis (DTA) in the hope that this approach would be more specific than DTG. EXPERIMENTAL
FA The FA originated from the Bh horizon of the Armadale soil, an imperfectly drained podzol developed on sandy loam in Prince Edward Island, Canada. Methods of extraction and purification of the FA were exactly as described previously (Schnitzer and Hoffman, 1967). The extracted and purified FA contained 1.0% ash; its elementary composition on a dry, ash-free basis was 50.90% C, 3.35% H, 0.75% N, 0.25% S and 44.75% O. One gram of FA contained 9.1 mequiv. COOH, 3.3 mequiv, phenolic OH, 3.6 mequiv, alcoholic OH and 3.1 mequiv. C=O groups. The number-average molecular weight (~ln) of the FA, as measured by vapor-pressure osmometry and corrected for dissociation of COOH groups, was 951 (Hansen and Schnitzer, 1969). From the molecular weight, ultimate and functional group analyses, the following molecular formula was calculated: C2aH16(COOH)9(OH)7
(co)3. Preparation of salts and complexes For the sake of clarity some experimental details in this section, although described earlier (Schnitzer and Hoffman, 1967), will be summarized briefly. To solutions of 250 mg (0.263 mmoles) of FA, dissolved in 25 ml of double-distilled water, aqueous solutions (2.5 mmoles in 10 ml) of the following salts were added: NaC1, KC1, LiC1, NH4 C1, CuC12.6H2 O, PbC12, NiC12.6H2 O, MnC12.6H: O, ZnC12, CaC12.2H2 O, MgC12.6H2O, SrCI: .6H2 O, FeCla .6H20 and A1C13.6H20.The pH of each mono- and di-valent cation system was adjusted to 5.0 with dilute NaOH solution. Each solution was stirred for one hour at room temperature, transferred to a seamless cellulose dialysis tubing and dialyzed against distilled water until the excess salt was removed (usually 72 h) as shown by a negative chloride test in the water outside the tubing. The dialyzed solution was dried first in a rotary evaporator, and then in a vacuum desiccator over P20s. In order to minimize formation of hydroxides, the pH of the solutions used for the formation of the Fe- and Al-complexes was adjusted to 2.5 and 4.0 respectively. The "low" and "medium" Fe- and A1-FA complexes were prepared according to Schnitzer and Skinner (1964).
Metal analysis All salts and complexes were digested with 5:1 HNO3 :HC1 solution and then dissolved in O.1 N HC1. Na, K, Li, Cu, Pb, Ni, Mn, Zn, Ca and Mg were determined by atomic ab-
DTA OF METAL-FULVICACID SALTS AND COMPLEXES
95
sorption spectroscopy; Fe and A1 were measured colorimetrically (Shapiro and Bran nock, 1957) and Sr by titration with EDTA.
Differential Thermal Analysis (D TA ) DTA curves were recorded on a Robert L. Stone DTA apparatus. 10 mg of sample, diluted with 20 mg of calcined alumina, was sandwiched between layers of calcined alumina. The apparatus was run at a constant rate of heating of 10°C/min under a flow of air of 12.5 ml/min. These experimental conditions have previously been found to be optimum for investigating the thermal decomposition of FA (Kodama and Schnitzer, 1970). RESULTS AND DISCUSSION
Chemical composition of salts and complexes Since the original FA contained approximately 50% C, the FA content of the salts and complexes was estimated by multiplying the carbon percentage value by two (Table I). It is generally assumed that monovalent cations form salts rather than complexes with polyelectrolytes such as FA. On the other hand, the formation of stable complexes of FA with di- and trivalent metal ions has been reported by several workers (Schnitzer and TABLE I Chemical composition of salts and complexes (air-dry basis) Salt or complex
FA FA (C%×2) (mmoles)
Li-FA Na-FA K-FA NH,-FA
89.7 91.0 80.1 90.0
0.094 0.096 0.084 0.095
Mg-FA Ca-FA Mn-FA Ni-FA Cu-FA Zn-FA Sr-FA Pb-FA
81.8 82.5 81.6 80.0 67.9 77.4 78.9 50.7
"Low" A1-FA "Medium" A1-FA "High" A1-FA "Low" Fe-FA '"Medium" Fe-FA "High" Fe-FA
80.2 66.8 50.6 81.5 67.8 51.4
Metal (%)
Metal (mmoles)
mmoles metal mmoles FA
1.40 3.58 6.10 3.61
0.200 0.156 0.156 0.201
2.13 1.63 1.86 2.12
0.086 0.087 0.086 0.084 0.071 0.082 0.083 0.053
4.18 6.38 10.00 11.00 21.75 10.80 11.67 47.35
0.172 0.160 0.181 0.188 0.343 0.165 0.133 0.229
2.00 t.84 2.10 2.24 4.83 2.01 1.60 4.32
0.084 0.700 0.053 0.086 0.071 0.054
5.62 10.16 13.14 6.72 16.79 27.59
0.208 0.376 0.487 0.120 0.300 0.493
2.48 5.37 9.19 1.40 4.23 9.13
96
M. SCHNITZER AND H. KODAMA
Hoffman, 1967). The molar metal/FA ratios in Table I are about 33% higher than those reported previously (Schnitzer and Hoffman, 1967). This is so because we have in this investigation re-determined the l~ln of the FA by vapor pressure osmometry, probably the most reliable method presently available for measuring number-average molecular weights of water-soluble polyelectrolytes, and corrected the Mn for the dissociation of COOH groups (Hansen and Schnitzer, 1969). Thus, we believe that the ratios shown in Table I are more reliable than those published earlier. The ratios for salts of FA with monovalent cations range from 1.63 to 2.13. Complexes of FA with divalent cations have molar metal/FA ratios varying from 1.60 for Sr to 4.83 for Cu, whereas the ratios for the "high" Fe- and A1-FA complexes are approximately 9.0. Thus, one atom of Fe and A1 reacts with one, COOH group of the FA. By comparison with untreated FA, IR spectra of the salts and complexes showed strong bands near 1610 and 1400 cm "1 , indicating conversion of COOH groups of the FA to COO" groups, to which metal ions were bonded by electrovalent linkages (Schnitzer and Hoffman, 1967).
DTA of FA The DTA curve of untreated FA (Fig.l) shows a broad but shallow endotherm near 100°C, a shoulder-like exotherm at about 330°C and a prominent exotherm at 450°C.
i
I
200
4OO
6OO
~
1OO0
TEMPERATURE*C Fig. 1. DTA curves of untreated FA and M+-FA salts. According to earlier thermal studies (Schnitzer and Hoffman, 1964; Kodama and Schnitzer, 1970), the endotherm and the two exotherms arise from dehydration, decarboxylation and decomposition, respectively, of the FA "nucleus".
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
. 97
DTA of metal (M+)-FA salts DTA patterns of the Li-, Na-, K-, and NH4-FA salts (Fig. 1) show shallow endotherms between 66 ° and 80°C, due to dehydration. The Li-salt exhibits a small exotherm at 275°C, followed by a very prominent exotherm at 468°C and three smaller ones at 558 °, 588 ° and 658°C ~. The DTA curve of the Na-FA is quite different; it shows smaller and broader exotherms at 268 °, 390 °, 464 °, 725 ° and 771°C and a high-temperature endotherm near 850°C The curve for the K-FA is it_,many respects similar to that of the Na-FA, except that the exotherms occur at lower temperatures, that is, at 258 ° , 330 ° , 434 ° , 455 ° (shoulder) and 660°C. In addition a well-defined endotherm appears at 858°C. The pattern o f the NH4-FA is quite different from that of the other salts. A shallow exotherm occurs at 327°C, followed by a very strong exotherm at 387°C and a smaller one at 435°C.
DTA of divalent metal-FA complexes DTA curves o f divalent metal-FA complexes are shown in Fig.2. All complexes show shallow endotherms, due to dehydration, at temperatures ranging from 6 0 ° - 9 0 ° C . The Mg-FA complex is characterized by a medium-sized exotherm at 312°C, followed by astrong
\
r TEMPERATURE °C
Fig.2. DTA curves of M2+-FA complexes. t Peak temperatures are accurate within _+3°C and are reproducible in replicate runs.
98
M. SCHNITZERAND H. KODAMA
exotherm at 382°C. The Ca-complex exhibits an exothermic doublet at 300 ° and 372°C, and then a less-pronounced exotherm at 462°C, accompanied by a shoulder around 500°C. The Mn-FA complex shows an exotherm at 297°C, followed by a prominent exotherm at 383°C. The DTA curve of the Ni-FA complex is similar to that of the Mg-FA complex, except that the peak temperature of the second exotherm is 37 I°C, that is, about 10°C lower than that of the Mg-FA complex. The Cu-FA complex is characterized by a triplet of welldefined exotherms at 2310,260 ° and 314°C. The DTA of the Zn-FA complex shows an exotherm at 319°C followed by a slightly broader exotherm at 420°C. The Sr-FA complex gives a pattern similar to that of the Ca-FA complex, that is, the pattern consists of an exothermic doublet, followed by a small exotherm. Peak temperatures of the three exotherms are at 297 ° , 359 ° and 444°C, all of which are lower than the corresponding temperatures for the Ca-FA complex. The curve of the Pb-FA complex exhibits a slightly broad exotherm at 275°C, which is followed by smaller and multi-complex exotherms at 279 °, 410 ° with shoulders around 400 ° and 434°C.
DTA of trivalent metal-FA complexes The A1-FA complexes (Fig.3) exhibit broad and shallow endotherms between 90°C and 120°C, due to dehydration. The "low" A1-FA complex shows a very strong exotherm at 452°C with shoulders near 350°C and 490°C. The "medium" AI-FA complex gives a large
0
2oo
4oo
6oo
800
lOOO
TEMPERATURE °C
Fig.3. DTA curves of M3+-FA complexes.
DTA OF METAL-FULVICACID SALTS AND COMPLEXES
99
and broad exotherm at 452°C with a shoulder around 385°C. The curve of the "high" A1-FA complex is very similar to that of the "medium" A1-FA complex, except that the main exotherm occurs at higher temperature (476°C). By contrast, the DTA curves of Fe-FA complexes show major exotherms at much lower temperatures than those of the A1-FA complexes. The "low" Fe-FA complex (fig.3) shows a minor exotherm at 272°C, followed by a strong exotherm at 362°C and a very small exothermic hump at 442°C. In case of the "medium" Fe-FA complex the major exotherm occurs at 341°C, while the minor exotherm changes to a shoulder at about 278°C. In the "high" Fe-FA complex the peak temperature of the major but less pronounced exotherm is further lowered to 290°C. Thus, as the molar ratio of Fe to FA increases, the peak temperature of the major exotherm decreases. This situation is similar to that observed when the three Fe-FA complexes were investigated by DTG (Schnitzer and Skinner, 1964). The DTA data are summarized in Table II. Some observations on the thermal decomposition reactions
As shown in Fig.l, the main decomposition of untreated FA, that is, the oxidation of the FA "nucleus", occurs at 450°C under the experimental conditions employed in this investigation. For all metal-FA salts, the major exotherms occur between 430 ° and 470°C. In addition, a number of complex exotherms appear at higher temperatures. By contrast, the major exotherms of the polyvalent metal-FA complexes, except those of the A1-FA complexes, occur at temperatures that are significantly lower than that of the main exotherm for untreated FA. These differences in thermal stability are most likely related to differences in the types of bonds that mono- and polyvalent metal ions form with FA. By selectively blocking COOH and phenolic OH groups in FA and reacting the resulting preparations with metal ions, it was found in an earlier study (Schnitzer and Skinner, 1965) that two types of reactions could occur: (1) those involving simultaneously both COOH and phenolic OH groups; and (2) those in which only COOH participated. Thus, if a di- or trivalent metal ion reacts simultaneously with two groups (see Fig.4) a severe strain may be put on the polyelectrolyte structure, so that the latter may become susceptible to thermal decomposition at relatively low temperatures. On the other hand, each monovalent ion reacts with one active site only (Fig.4), so that less strain is exerted on the FA structure, and this may account for the relatively high thermal stabilities of the M+-FA salts. For monovalent metal-FA salts, the main exotherm temperatures tend to decrease as the size of the ion with which the FA combines increases. This is exemplified by the Ca- and Sr-FA complexes. For transition metal-FA complexes, the main exotherm temperatures trend to be inversely proportional to the ionization potentials of the associated metal ions. As can be judged from the DTA curves, individual metal ions certainly play important roles in the process of thermal decomposition. To obtain a better understanding of the specific mechanisms of each decomposition reaction it will be necessary to identify the reaction products. i
M. SCHNITZER AND H. KODAMA
100
TABLE II Summary of DTA data
FA
T(-)
S
100(b)
vw
Li-FA Na-FA K-FA NH 4-FA
80(b) 75(b) 66(b) 78(b)
vw vw vw vw
Mg-FA Ca-FA Mn-FA Ni-FA Cu-FA Zn-FA Sr-FA Pb-FA
86(b) 78(b) 60(b) 88(b) 83(b) 78(b) 82(b) 80(b)
vw vw vw vw vw vw vw vw
90(b) 107(b) l19(b) 86(b) 78(b) 80(b)
vw w vw vw vw vw
Low A1-FA Med. A1-FA High A1-FA Low Fe-FA Medium Fe-FA High Fe-FA
T(-) T(+) S
T(+)
231
OH
275 268 258
w w w
ms
260
ms
297 275
ms m
272 mw (278) 290(b)
T(+)
S
T(+)
330 w (327) 312 300 297 315 314 319
ms ms mw s ms ms
390
w
387
vs
382 372 383(s) 371(s)
vs ms s vs
359 379
s mw
362 vs 341 vs (330)
H
t o~ ~-° COO\ 0/Cu
[
COO\ 0/Fe-OH
F CO0\ i o/Fe-O
OH
S
H
COO-Fe (OH)2
COOH
COO\
COOH
I Coo/C°
COO-Fe(OH)2
COOH
! co0\ i jco
COO-Ee(Ol~) 2
CO0
COO-Fe(OH)2
COOH
i COOH
COO-Fe(OH)7
COOH Li-FA Salt
t COOH Cu-FA Complex
COO-Fe(OH)7 "High' Fe-FA Complex
T(+)
S
450
(b) = exceptionally broad (s) = exceptionally sharp
coo..
COOH
COOH
S
(350) (385) (385)
COO-Li
COO-Li
T(+)
(320)
= endothermic peak = exothermic peak = size of peak
OH
S
Fig.4. Simplified structures for the Li-FA salt, Cu-FA and "high" Fe-FA complexes.
468 464 434 435(s)
vs mw m vs
462
mw
420 444 (400)
ms mw
452 452
s s
(442)
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
T(+)
T(+) S
T(+) S
T(+) S T(+)
558
588
658
w
660
w
mw
mw
(720) (455)
101
S T(+) S T(-)
770
m 850 858
S
vw mw
(5OO)
410
w
(536) 434
mw
(490) 476(b)
ms
455
vw
vs = very strong; s = strong; ms = medium strong m = medium; mw= medium weak; w = weak vw = very weak
( ) indicates shoulder-like peak
Practical applications of the proposed DTA method Iron-pans occur in soils of many regions having cool climates but little is known about their analytical characteristics. The sample that we examined was a black, shiny, dense and amorphous material which cemented gravel fragments. It occurred as a sharp band 22.0 to 22.3 cm below the soil surface ofa humic podzol in Newfoundland (McKeague et al., 1967). After air-drying, the sample was crushed carefully to avoid excessive breakdown and passed through a 2-mm sieve. Gravel fragments were removed as carefully as possible before grinding. The upper curve in Fig.5 shows the DTA pattern of the ironpan. Aside from a shallow en~ dotherm near 90°C, one major exotherm occurs at 306°C. The general shape of this curve is very similar to that of the "high" Fe-FA complex (Fig.3), indicating that the iron in the pan is complexed by FA, the major organic component of the pan (McKeague et al., 1967). Chemical analysis confirmed the information provided by the DTA curve.
102
M. SCHNITZER AND H. KODAMA
,,°=/\ J
Mb d A-Fe- A
0
I
200
400
600
BOO
TEMPERATURE °C
Fig.5. DTA curves of ironpan and of mixed Fe-AI-FAcomplex extracted from the soil. In another investigation we attempted to extract a metal-FA complex from a soil (Schnitzer and Skinner, 1964). 10 g of soil, taken from the Bh horizon o f a podzol, was shaken for 1 h with 100 ml of O. 1 N HC1. The dark brown supernatant solution was separated from the residue by filtration, dialyzed against distilled water until free of chloride, and dried under reduced pressure at room temperature. Chemical analyses of the extract indicated a molar FA/A1/Fe ratio of 7/24/4. The DTA curve of the mixed A1-Fe-FA complex (Fig.5) is characterized by an endotherm at IO0°C, and then a large and broad exotherm at 370°C with a shoulder around 320°C. The shape of the curve is reminiscent of the curves for the "high" and "medium" A1-FA complexes (Fig.3), although the peak temperature of the major exotherm is about 100°C lower than that of the A1-FA complexes. A probable explanation for the low temperature of the exotherm lies in the presence of Fe in the complex. It is noteworthy that previous DTG analysis (Schnitzer and Skinner, 1964) showed that the major decomposition temperature of FA was substantially lowered when Fe plus A1 were complexed simultaneously. The effect of Fe on lowering the major peak temperature of FA is quite remarkable. Suspecting that this effect was due to catalysis by hydrous oxides, we prepared mechanical mixtures of FA with iron and aluminum monohydroxides in the same proportions in which they occurred in the "medium" complexes. The resulting DTA patterns were identical to that of untreated FA (Fig.l), indicating that catalytic effects due to hydrous oxides were not associated with the drastic lowering of the main decomposition temperature of the Fe-FA complexes but that chemical complex formation was involved here. It is possible that the thermal decomposition of FA is to a minor extent affected by silicates which were present in the soil extract. We have found earlier (Kodama and Schnitzer, 1970) that mixing 24.5 mg of FA with 40.0 mg of Na-montmorillonite lowered the temperature of the main FA-exotherm from 450°C to 400°C, but that the shape of the exotherm remained unchanged. The results presented herein show that the DTA method permits differentiation between metal chemically complexed by FA and metal physically mixed with FA. At the present
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
103
time this t y p e o f i n f o r m a t i o n c a n n o t be o b t a i n e d in a relatively simple m a n n e r b y a n y o t h e r m e t h o d . T h e D T A m e t h o d m a y also serve as a " f i n g e r p r i n t " for the rapid i d e n t i f i c a t i o n o f different metal-FA complexes. ACKNOWLEDGEMENTS The a u t h o r s a c k n o w l e d g e the t e c h n i c a l assistance o f G. S c o t t a n d J.G. Desjardins. REFERENCES Hansen, E.H. and Schnitzer, M., 1969. Molecular weight measurements of polycarboxylic acids in water by vapor pressure osmometry. A hal. Chim. A cta, 46: 247-254. Ishiwatari, R., 1969. Aft estimation of the aromaticity of a lake sediment humic acid by air oxidation and evaluation of it. Soil Sci., 1.07: 5 3 - 5 7 . Kodama, H. and Schnitzer, M., 1970. Thermal analysis of a fulvic acid-montmorillonite complex. Proc. Int. Clay Conf., Tokyo, 1: 765-774. Lamar, W.L., 1968. Evaluation of organic color and iron in natural surface waters. U.S. Geol. Surv.. Prof. Pap., 600-D, 24-29. McKeague, J.A., Schnitzer, M. and Heringa, P.K., 1967. Properties of an ironpan humic podzol from Newfoundland. Can. J. Soil Sci., 47: 2 3 - 3 2 . Rashid, M.A. and King, L.H., 1969. Molecular weight distribution measurements on humic and fulvic acid fractions from marine clays on the Scotian Shelf. Geochim. Cosmochirn. A c ta, 3 3 : 1 4 7 - 1 5 1 . Shapiro, L. and Brannock, W.W., 1957. Rapid analysis of silicate rocks. U.S. Geol. Surv., Bull., 1036-C., pp.34 36. Schnitzer, M. and Hoffman, I., 1964. Pyrolysis of soil organic matter. Soil Sci. Soc. Am. Proc., 28: 5 2 0 525. Schnitzer, M. and Hoffman, I., 1967. Thermogravimetric analysis of the salts and metal complexes of a soil fulvic acid. Geochim. Cosmochirn. Acta, 31 : 7 - 1 5 . Schnitzer, M. and Skinner, S.I.M., 1964. Organo-metallic interactions in soils, 3. Soil ScL, 98: 197203. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Soil Sci., 99: 2 7 8 284.
DIFFERENTIAL THERMAL A N A L Y S I S OF METAL-FULVIC ACID SALTS AND COMPLEXES 1 M. SCHN1TZER and H. KODAMA
Soil Research Institute, Canada Department of Agriculture, Ottawa, Ont. (Canada) (Received April 14, 1971) ABSTRACT Schnitzer, M. and Kodama, H., 1972. Differential thermal analysis of metal-fulvic acid salts and complexes. Geoderma, 7: 93-103. Salts and complexes of fulvic acid, a water-soluble soil humic fraction, with 14 different mono-, diand tri-valent metal ions, were prepared and characterized by differential thermal analysis (DTA). The major exotherm for untreated fulvic acid, signalizing the decomposition of the fulvic acid "nucleus", occurred at 450°C. For salts of fulvic acid with monovalent ions, major exotherms occurred between 430 ° and 450°C, but for most polyvalent metal-fulvic acid complexes the major exotherms appeared at significantly lower temperatures. Ferric iron was especially effective in lowering the temperature of the major fulvic acid exotherm. This effect was not catalytic but due to the formation of a chemical complex between iron and fulvic acid. The thermal stabilities of the salts and complexes were related to the nature of the metal-fulvic acid bonding. For metal-fulvic acid salts the main exotherm temperatures tended to decrease as the size of the metal ion increased. For transition metal-fulvic acid complexes the major exotherm temperatures were inversely proportional to the ionization potential o f the metal ion. The DTA method permits differentiation between metal chemically complexed by fulvic acid and metal physically mixed with fulvic acid. At the present time this cannot be done by any other method in such a relatively simple manner. Also, the DTA method may serve as a "fingerprint" for the rapid identification of metal-fulvic acid complexes.
INTRODUCTION Fulvic acid ( F A ) is a water-soluble, relatively low-molecular weight h u m i c fraction that occurs in soils, lakes (Ishiwatari, 1969), rivers (Lamar, 1968) and in the sea (Rashid and King, 1969). Fulvic acid can form stable water-soluble salts and water-soluble and waterinsoluble c o m p l e x e s with di- and tri-valent metal ions (Schnitzer and H o f f m a n , 1967; Schnit. zer and Skinner, 1964) and so influence the physical, chemical and biological properties o f soils and waters. Products resulting f r o m interactions o f F A with m e t a l ions m a y also play a significant role in e n v i r o n m e n t a l pollution. There is, therefore, a need for the developm e n t o f specific b u t relatively simple m e t h o d s for the analysis o f m e t a l - F A complexes. In view o f the chemical complexities of such systems, the use o f physical rather than o f chemical m e t h o d s appeared attractive. Schnitzer and H o f f m a n (1967) p r o p o s e d a differential t h e r m o g r a v i m e t r i c ( D T G ) m e t h o d for this purpose. The thermal stabilities o f the metalF A c o m p l e x e s were f o u n d to depend on the nature o f the cation, b u t the m a j o r d e c o m p o 1 Contribution No. 371.
94
M. SCHNITZERAND H. KODAMA
sition peaks were often not sufficiently separated to permit differentiation between the various complexes. In the present investigation we have attempted to characterize metal-FA complexes by differential thermal analysis (DTA) in the hope that this approach would be more specific than DTG. EXPERIMENTAL
FA The FA originated from the Bh horizon of the Armadale soil, an imperfectly drained podzol developed on sandy loam in Prince Edward Island, Canada. Methods of extraction and purification of the FA were exactly as described previously (Schnitzer and Hoffman, 1967). The extracted and purified FA contained 1.0% ash; its elementary composition on a dry, ash-free basis was 50.90% C, 3.35% H, 0.75% N, 0.25% S and 44.75% O. One gram of FA contained 9.1 mequiv. COOH, 3.3 mequiv, phenolic OH, 3.6 mequiv, alcoholic OH and 3.1 mequiv. C=O groups. The number-average molecular weight (~ln) of the FA, as measured by vapor-pressure osmometry and corrected for dissociation of COOH groups, was 951 (Hansen and Schnitzer, 1969). From the molecular weight, ultimate and functional group analyses, the following molecular formula was calculated: C2aH16(COOH)9(OH)7
(co)3. Preparation of salts and complexes For the sake of clarity some experimental details in this section, although described earlier (Schnitzer and Hoffman, 1967), will be summarized briefly. To solutions of 250 mg (0.263 mmoles) of FA, dissolved in 25 ml of double-distilled water, aqueous solutions (2.5 mmoles in 10 ml) of the following salts were added: NaC1, KC1, LiC1, NH4 C1, CuC12.6H2 O, PbC12, NiC12.6H2 O, MnC12.6H: O, ZnC12, CaC12.2H2 O, MgC12.6H2O, SrCI: .6H2 O, FeCla .6H20 and A1C13.6H20.The pH of each mono- and di-valent cation system was adjusted to 5.0 with dilute NaOH solution. Each solution was stirred for one hour at room temperature, transferred to a seamless cellulose dialysis tubing and dialyzed against distilled water until the excess salt was removed (usually 72 h) as shown by a negative chloride test in the water outside the tubing. The dialyzed solution was dried first in a rotary evaporator, and then in a vacuum desiccator over P20s. In order to minimize formation of hydroxides, the pH of the solutions used for the formation of the Fe- and Al-complexes was adjusted to 2.5 and 4.0 respectively. The "low" and "medium" Fe- and A1-FA complexes were prepared according to Schnitzer and Skinner (1964).
Metal analysis All salts and complexes were digested with 5:1 HNO3 :HC1 solution and then dissolved in O.1 N HC1. Na, K, Li, Cu, Pb, Ni, Mn, Zn, Ca and Mg were determined by atomic ab-
DTA OF METAL-FULVICACID SALTS AND COMPLEXES
95
sorption spectroscopy; Fe and A1 were measured colorimetrically (Shapiro and Bran nock, 1957) and Sr by titration with EDTA.
Differential Thermal Analysis (D TA ) DTA curves were recorded on a Robert L. Stone DTA apparatus. 10 mg of sample, diluted with 20 mg of calcined alumina, was sandwiched between layers of calcined alumina. The apparatus was run at a constant rate of heating of 10°C/min under a flow of air of 12.5 ml/min. These experimental conditions have previously been found to be optimum for investigating the thermal decomposition of FA (Kodama and Schnitzer, 1970). RESULTS AND DISCUSSION
Chemical composition of salts and complexes Since the original FA contained approximately 50% C, the FA content of the salts and complexes was estimated by multiplying the carbon percentage value by two (Table I). It is generally assumed that monovalent cations form salts rather than complexes with polyelectrolytes such as FA. On the other hand, the formation of stable complexes of FA with di- and trivalent metal ions has been reported by several workers (Schnitzer and TABLE I Chemical composition of salts and complexes (air-dry basis) Salt or complex
FA FA (C%×2) (mmoles)
Li-FA Na-FA K-FA NH,-FA
89.7 91.0 80.1 90.0
0.094 0.096 0.084 0.095
Mg-FA Ca-FA Mn-FA Ni-FA Cu-FA Zn-FA Sr-FA Pb-FA
81.8 82.5 81.6 80.0 67.9 77.4 78.9 50.7
"Low" A1-FA "Medium" A1-FA "High" A1-FA "Low" Fe-FA '"Medium" Fe-FA "High" Fe-FA
80.2 66.8 50.6 81.5 67.8 51.4
Metal (%)
Metal (mmoles)
mmoles metal mmoles FA
1.40 3.58 6.10 3.61
0.200 0.156 0.156 0.201
2.13 1.63 1.86 2.12
0.086 0.087 0.086 0.084 0.071 0.082 0.083 0.053
4.18 6.38 10.00 11.00 21.75 10.80 11.67 47.35
0.172 0.160 0.181 0.188 0.343 0.165 0.133 0.229
2.00 t.84 2.10 2.24 4.83 2.01 1.60 4.32
0.084 0.700 0.053 0.086 0.071 0.054
5.62 10.16 13.14 6.72 16.79 27.59
0.208 0.376 0.487 0.120 0.300 0.493
2.48 5.37 9.19 1.40 4.23 9.13
96
M. SCHNITZER AND H. KODAMA
Hoffman, 1967). The molar metal/FA ratios in Table I are about 33% higher than those reported previously (Schnitzer and Hoffman, 1967). This is so because we have in this investigation re-determined the l~ln of the FA by vapor pressure osmometry, probably the most reliable method presently available for measuring number-average molecular weights of water-soluble polyelectrolytes, and corrected the Mn for the dissociation of COOH groups (Hansen and Schnitzer, 1969). Thus, we believe that the ratios shown in Table I are more reliable than those published earlier. The ratios for salts of FA with monovalent cations range from 1.63 to 2.13. Complexes of FA with divalent cations have molar metal/FA ratios varying from 1.60 for Sr to 4.83 for Cu, whereas the ratios for the "high" Fe- and A1-FA complexes are approximately 9.0. Thus, one atom of Fe and A1 reacts with one, COOH group of the FA. By comparison with untreated FA, IR spectra of the salts and complexes showed strong bands near 1610 and 1400 cm "1 , indicating conversion of COOH groups of the FA to COO" groups, to which metal ions were bonded by electrovalent linkages (Schnitzer and Hoffman, 1967).
DTA of FA The DTA curve of untreated FA (Fig.l) shows a broad but shallow endotherm near 100°C, a shoulder-like exotherm at about 330°C and a prominent exotherm at 450°C.
i
I
200
4OO
6OO
~
1OO0
TEMPERATURE*C Fig. 1. DTA curves of untreated FA and M+-FA salts. According to earlier thermal studies (Schnitzer and Hoffman, 1964; Kodama and Schnitzer, 1970), the endotherm and the two exotherms arise from dehydration, decarboxylation and decomposition, respectively, of the FA "nucleus".
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
. 97
DTA of metal (M+)-FA salts DTA patterns of the Li-, Na-, K-, and NH4-FA salts (Fig. 1) show shallow endotherms between 66 ° and 80°C, due to dehydration. The Li-salt exhibits a small exotherm at 275°C, followed by a very prominent exotherm at 468°C and three smaller ones at 558 °, 588 ° and 658°C ~. The DTA curve of the Na-FA is quite different; it shows smaller and broader exotherms at 268 °, 390 °, 464 °, 725 ° and 771°C and a high-temperature endotherm near 850°C The curve for the K-FA is it_,many respects similar to that of the Na-FA, except that the exotherms occur at lower temperatures, that is, at 258 ° , 330 ° , 434 ° , 455 ° (shoulder) and 660°C. In addition a well-defined endotherm appears at 858°C. The pattern o f the NH4-FA is quite different from that of the other salts. A shallow exotherm occurs at 327°C, followed by a very strong exotherm at 387°C and a smaller one at 435°C.
DTA of divalent metal-FA complexes DTA curves o f divalent metal-FA complexes are shown in Fig.2. All complexes show shallow endotherms, due to dehydration, at temperatures ranging from 6 0 ° - 9 0 ° C . The Mg-FA complex is characterized by a medium-sized exotherm at 312°C, followed by astrong
\
r TEMPERATURE °C
Fig.2. DTA curves of M2+-FA complexes. t Peak temperatures are accurate within _+3°C and are reproducible in replicate runs.
98
M. SCHNITZERAND H. KODAMA
exotherm at 382°C. The Ca-complex exhibits an exothermic doublet at 300 ° and 372°C, and then a less-pronounced exotherm at 462°C, accompanied by a shoulder around 500°C. The Mn-FA complex shows an exotherm at 297°C, followed by a prominent exotherm at 383°C. The DTA curve of the Ni-FA complex is similar to that of the Mg-FA complex, except that the peak temperature of the second exotherm is 37 I°C, that is, about 10°C lower than that of the Mg-FA complex. The Cu-FA complex is characterized by a triplet of welldefined exotherms at 2310,260 ° and 314°C. The DTA of the Zn-FA complex shows an exotherm at 319°C followed by a slightly broader exotherm at 420°C. The Sr-FA complex gives a pattern similar to that of the Ca-FA complex, that is, the pattern consists of an exothermic doublet, followed by a small exotherm. Peak temperatures of the three exotherms are at 297 ° , 359 ° and 444°C, all of which are lower than the corresponding temperatures for the Ca-FA complex. The curve of the Pb-FA complex exhibits a slightly broad exotherm at 275°C, which is followed by smaller and multi-complex exotherms at 279 °, 410 ° with shoulders around 400 ° and 434°C.
DTA of trivalent metal-FA complexes The A1-FA complexes (Fig.3) exhibit broad and shallow endotherms between 90°C and 120°C, due to dehydration. The "low" A1-FA complex shows a very strong exotherm at 452°C with shoulders near 350°C and 490°C. The "medium" AI-FA complex gives a large
0
2oo
4oo
6oo
800
lOOO
TEMPERATURE °C
Fig.3. DTA curves of M3+-FA complexes.
DTA OF METAL-FULVICACID SALTS AND COMPLEXES
99
and broad exotherm at 452°C with a shoulder around 385°C. The curve of the "high" A1-FA complex is very similar to that of the "medium" A1-FA complex, except that the main exotherm occurs at higher temperature (476°C). By contrast, the DTA curves of Fe-FA complexes show major exotherms at much lower temperatures than those of the A1-FA complexes. The "low" Fe-FA complex (fig.3) shows a minor exotherm at 272°C, followed by a strong exotherm at 362°C and a very small exothermic hump at 442°C. In case of the "medium" Fe-FA complex the major exotherm occurs at 341°C, while the minor exotherm changes to a shoulder at about 278°C. In the "high" Fe-FA complex the peak temperature of the major but less pronounced exotherm is further lowered to 290°C. Thus, as the molar ratio of Fe to FA increases, the peak temperature of the major exotherm decreases. This situation is similar to that observed when the three Fe-FA complexes were investigated by DTG (Schnitzer and Skinner, 1964). The DTA data are summarized in Table II. Some observations on the thermal decomposition reactions
As shown in Fig.l, the main decomposition of untreated FA, that is, the oxidation of the FA "nucleus", occurs at 450°C under the experimental conditions employed in this investigation. For all metal-FA salts, the major exotherms occur between 430 ° and 470°C. In addition, a number of complex exotherms appear at higher temperatures. By contrast, the major exotherms of the polyvalent metal-FA complexes, except those of the A1-FA complexes, occur at temperatures that are significantly lower than that of the main exotherm for untreated FA. These differences in thermal stability are most likely related to differences in the types of bonds that mono- and polyvalent metal ions form with FA. By selectively blocking COOH and phenolic OH groups in FA and reacting the resulting preparations with metal ions, it was found in an earlier study (Schnitzer and Skinner, 1965) that two types of reactions could occur: (1) those involving simultaneously both COOH and phenolic OH groups; and (2) those in which only COOH participated. Thus, if a di- or trivalent metal ion reacts simultaneously with two groups (see Fig.4) a severe strain may be put on the polyelectrolyte structure, so that the latter may become susceptible to thermal decomposition at relatively low temperatures. On the other hand, each monovalent ion reacts with one active site only (Fig.4), so that less strain is exerted on the FA structure, and this may account for the relatively high thermal stabilities of the M+-FA salts. For monovalent metal-FA salts, the main exotherm temperatures tend to decrease as the size of the ion with which the FA combines increases. This is exemplified by the Ca- and Sr-FA complexes. For transition metal-FA complexes, the main exotherm temperatures trend to be inversely proportional to the ionization potentials of the associated metal ions. As can be judged from the DTA curves, individual metal ions certainly play important roles in the process of thermal decomposition. To obtain a better understanding of the specific mechanisms of each decomposition reaction it will be necessary to identify the reaction products. i
M. SCHNITZER AND H. KODAMA
100
TABLE II Summary of DTA data
FA
T(-)
S
100(b)
vw
Li-FA Na-FA K-FA NH 4-FA
80(b) 75(b) 66(b) 78(b)
vw vw vw vw
Mg-FA Ca-FA Mn-FA Ni-FA Cu-FA Zn-FA Sr-FA Pb-FA
86(b) 78(b) 60(b) 88(b) 83(b) 78(b) 82(b) 80(b)
vw vw vw vw vw vw vw vw
90(b) 107(b) l19(b) 86(b) 78(b) 80(b)
vw w vw vw vw vw
Low A1-FA Med. A1-FA High A1-FA Low Fe-FA Medium Fe-FA High Fe-FA
T(-) T(+) S
T(+)
231
OH
275 268 258
w w w
ms
260
ms
297 275
ms m
272 mw (278) 290(b)
T(+)
S
T(+)
330 w (327) 312 300 297 315 314 319
ms ms mw s ms ms
390
w
387
vs
382 372 383(s) 371(s)
vs ms s vs
359 379
s mw
362 vs 341 vs (330)
H
t o~ ~-° COO\ 0/Cu
[
COO\ 0/Fe-OH
F CO0\ i o/Fe-O
OH
S
H
COO-Fe (OH)2
COOH
COO\
COOH
I Coo/C°
COO-Fe(OH)2
COOH
! co0\ i jco
COO-Ee(Ol~) 2
CO0
COO-Fe(OH)2
COOH
i COOH
COO-Fe(OH)7
COOH Li-FA Salt
t COOH Cu-FA Complex
COO-Fe(OH)7 "High' Fe-FA Complex
T(+)
S
450
(b) = exceptionally broad (s) = exceptionally sharp
coo..
COOH
COOH
S
(350) (385) (385)
COO-Li
COO-Li
T(+)
(320)
= endothermic peak = exothermic peak = size of peak
OH
S
Fig.4. Simplified structures for the Li-FA salt, Cu-FA and "high" Fe-FA complexes.
468 464 434 435(s)
vs mw m vs
462
mw
420 444 (400)
ms mw
452 452
s s
(442)
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
T(+)
T(+) S
T(+) S
T(+) S T(+)
558
588
658
w
660
w
mw
mw
(720) (455)
101
S T(+) S T(-)
770
m 850 858
S
vw mw
(5OO)
410
w
(536) 434
mw
(490) 476(b)
ms
455
vw
vs = very strong; s = strong; ms = medium strong m = medium; mw= medium weak; w = weak vw = very weak
( ) indicates shoulder-like peak
Practical applications of the proposed DTA method Iron-pans occur in soils of many regions having cool climates but little is known about their analytical characteristics. The sample that we examined was a black, shiny, dense and amorphous material which cemented gravel fragments. It occurred as a sharp band 22.0 to 22.3 cm below the soil surface ofa humic podzol in Newfoundland (McKeague et al., 1967). After air-drying, the sample was crushed carefully to avoid excessive breakdown and passed through a 2-mm sieve. Gravel fragments were removed as carefully as possible before grinding. The upper curve in Fig.5 shows the DTA pattern of the ironpan. Aside from a shallow en~ dotherm near 90°C, one major exotherm occurs at 306°C. The general shape of this curve is very similar to that of the "high" Fe-FA complex (Fig.3), indicating that the iron in the pan is complexed by FA, the major organic component of the pan (McKeague et al., 1967). Chemical analysis confirmed the information provided by the DTA curve.
102
M. SCHNITZER AND H. KODAMA
,,°=/\ J
Mb d A-Fe- A
0
I
200
400
600
BOO
TEMPERATURE °C
Fig.5. DTA curves of ironpan and of mixed Fe-AI-FAcomplex extracted from the soil. In another investigation we attempted to extract a metal-FA complex from a soil (Schnitzer and Skinner, 1964). 10 g of soil, taken from the Bh horizon o f a podzol, was shaken for 1 h with 100 ml of O. 1 N HC1. The dark brown supernatant solution was separated from the residue by filtration, dialyzed against distilled water until free of chloride, and dried under reduced pressure at room temperature. Chemical analyses of the extract indicated a molar FA/A1/Fe ratio of 7/24/4. The DTA curve of the mixed A1-Fe-FA complex (Fig.5) is characterized by an endotherm at IO0°C, and then a large and broad exotherm at 370°C with a shoulder around 320°C. The shape of the curve is reminiscent of the curves for the "high" and "medium" A1-FA complexes (Fig.3), although the peak temperature of the major exotherm is about 100°C lower than that of the A1-FA complexes. A probable explanation for the low temperature of the exotherm lies in the presence of Fe in the complex. It is noteworthy that previous DTG analysis (Schnitzer and Skinner, 1964) showed that the major decomposition temperature of FA was substantially lowered when Fe plus A1 were complexed simultaneously. The effect of Fe on lowering the major peak temperature of FA is quite remarkable. Suspecting that this effect was due to catalysis by hydrous oxides, we prepared mechanical mixtures of FA with iron and aluminum monohydroxides in the same proportions in which they occurred in the "medium" complexes. The resulting DTA patterns were identical to that of untreated FA (Fig.l), indicating that catalytic effects due to hydrous oxides were not associated with the drastic lowering of the main decomposition temperature of the Fe-FA complexes but that chemical complex formation was involved here. It is possible that the thermal decomposition of FA is to a minor extent affected by silicates which were present in the soil extract. We have found earlier (Kodama and Schnitzer, 1970) that mixing 24.5 mg of FA with 40.0 mg of Na-montmorillonite lowered the temperature of the main FA-exotherm from 450°C to 400°C, but that the shape of the exotherm remained unchanged. The results presented herein show that the DTA method permits differentiation between metal chemically complexed by FA and metal physically mixed with FA. At the present
DTA OF METAL-FULVIC ACID SALTS AND COMPLEXES
103
time this t y p e o f i n f o r m a t i o n c a n n o t be o b t a i n e d in a relatively simple m a n n e r b y a n y o t h e r m e t h o d . T h e D T A m e t h o d m a y also serve as a " f i n g e r p r i n t " for the rapid i d e n t i f i c a t i o n o f different metal-FA complexes. ACKNOWLEDGEMENTS The a u t h o r s a c k n o w l e d g e the t e c h n i c a l assistance o f G. S c o t t a n d J.G. Desjardins. REFERENCES Hansen, E.H. and Schnitzer, M., 1969. Molecular weight measurements of polycarboxylic acids in water by vapor pressure osmometry. A hal. Chim. A cta, 46: 247-254. Ishiwatari, R., 1969. Aft estimation of the aromaticity of a lake sediment humic acid by air oxidation and evaluation of it. Soil Sci., 1.07: 5 3 - 5 7 . Kodama, H. and Schnitzer, M., 1970. Thermal analysis of a fulvic acid-montmorillonite complex. Proc. Int. Clay Conf., Tokyo, 1: 765-774. Lamar, W.L., 1968. Evaluation of organic color and iron in natural surface waters. U.S. Geol. Surv.. Prof. Pap., 600-D, 24-29. McKeague, J.A., Schnitzer, M. and Heringa, P.K., 1967. Properties of an ironpan humic podzol from Newfoundland. Can. J. Soil Sci., 47: 2 3 - 3 2 . Rashid, M.A. and King, L.H., 1969. Molecular weight distribution measurements on humic and fulvic acid fractions from marine clays on the Scotian Shelf. Geochim. Cosmochirn. A c ta, 3 3 : 1 4 7 - 1 5 1 . Shapiro, L. and Brannock, W.W., 1957. Rapid analysis of silicate rocks. U.S. Geol. Surv., Bull., 1036-C., pp.34 36. Schnitzer, M. and Hoffman, I., 1964. Pyrolysis of soil organic matter. Soil Sci. Soc. Am. Proc., 28: 5 2 0 525. Schnitzer, M. and Hoffman, I., 1967. Thermogravimetric analysis of the salts and metal complexes of a soil fulvic acid. Geochim. Cosmochirn. Acta, 31 : 7 - 1 5 . Schnitzer, M. and Skinner, S.I.M., 1964. Organo-metallic interactions in soils, 3. Soil ScL, 98: 197203. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Soil Sci., 99: 2 7 8 284.