Download - [Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || The Nature of Commercial Humic Acids

4 The Nature of Commercial Humic Acids

Patrick MacCarthy

Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401

Ronald L. Malcolm

U.S. Geological Survey, Box 25046, Mail Stop 408, Denver Federal Center, Denver, CO 80225

In a follow-up to a study based primarily on solid-state 13C-NMR spectroscopy, commercial humic acids were investigated by infrared spectroscopy. Pronounced similarities were observed between the infrared spectra of all the commercial materials, and these "humic acids" appear to be partly in the salt form or complexed with metals. The commercial materials differ somewhat from each other in terms of their mineral compositions. The infrared spectra of the commercial materials are not remarkably different from humic acids isolated from peat, soils, and leonardite, but pronounced differences between these two classes of materials are revealed by 13C-NMR spectroscopy. The use of commercial humic acids as analogues of soil and water humic substances is criticized in this chapter. The adverse impact of high ash content on elemental analyses is illustrated, and the desirability of using direct methods for elemental determinations is discussed.

\J SE OF COMMERCIAL HUMIC ACIDS AS ANALOGUES of soil and water humic substances has been escalating over the past decade. The literature on this subject has been reviewed (I). The same article contains a detailed characterization of various commercial humic acids by elemental analysis and solid-state 1 3 C - N M R spectroscopy, and compares the commercial products with humic and fulvic acids that were isolated from soils and streams. A l though the commercial products bear some similarities to the soil and stream humic substances, there are, nevertheless, significant differences between

0065-2393/89/0219-0055$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

56 AQUATIC HUMIC SUBSTANCES

the two classes of materials. As a result, the use of commercial humic acids for simulating the properties and behavior of true soil and water humic substances needs to be reevaluated. Problems with the use of the commercial humic acids are further compounded by the lack of information on the source and methods of isolation and treatment, i f any, of these materials.

The objectives of this chapter are to provide further information on the commercial humic acids from five different suppliers, to discuss the nature of the commercial humic acids, and to illustrate the adverse impact of high ash content on the elemental analysis of humic substances.

Experimental Materials ami Methods The following commercial humic acids were used: No. 1: K&K humic acid (lot No. 215316, 1974) obtained from K&K Laboratories, Plainview, NY; No. 2: CPL humic acid (1974) obtained from Chemicals Procurement Laboratories, College Point, NY; No. 3: Fluka-Tridom humic acid Got No. 159128115, 1974) from Fluka Chemical Corp., Hauppauge, NY; No. 4: Pfaltz and Bauer humic acid (batch 1, 1974) and No. 5: Pfaltz and Bauer humic acid (batch 2, 1981) obtained from Pfaltz and Bauer, Stamford, CT; No. 7: Aldrich humic acid Got No. 113027, 1974); and No. 8: Aldrich humic acid, sodium salt (lot No. LE3601KE, 1981) from Aldrich Chemical Company, Milwaukee, WI.

Dopplerite (No. 10) was obtained from an outcrop at Stinking Springs, Fremont County, near Jackson, WY; this sample was used as dug from the outcrop and was not treated in any way. Peat for this study was obtained at a depth of about 1 m at the Peatland Experimental Station, Glenamoy, County Mayo, Ireland. Leonardite was taken from a homogenized 1000-kg sample obtained at the Biscoyne Mine, Bowman County, SD. This large sample of leonardite represents one of the bulk reference materials of the International Humic Substances Society, from which one of the standard humic acids (No. 12) has been extracted. The Irish peat humic acid (No. 11) (2) and the leonardite humic acid were obtained from the bulk materials by extraction with sodium hydroxide. The ash content of two of the commercial materials (Nos. 5 and 8) was diminished by subjecting the materials to liquid-liquid extraction with methyl isobutyl ketone (3), and the resulting materials are designated as sample Nos. 6 and 9, respectively.

Soil humic acid (No. 13) was extracted from the Sanhedron Al horizon near Petrolia, Mendocino County, in the Mattole River Valley of Northern California, by using a modified sodium hydroxide extraction procedure for soils (4). Dissolved stream humic acid (No. 14) was isolated from Ohio River water with a methyl meth-acrylate resin (XAD-8) extraction procedure (5).

Elemental and ash contents were determined by Huffman Laboratories, Golden, CO. All elemental contents were determined by direct methods. Ash content was determined by combustion to constant weight at 750 °C in an oxygen atmosphere. The elemental and ash contents are given in Table I. Infrared spectra were recorded using KBr disks (1% sample) on an IR spectrophotometer (Perkin Elmer Model 580). Solid-state 13C-NMR spectra were measured by G. Maciel, Colorado State University, Fort Collins, CO. All NMR spectra were run by the cross-polarization magic-angle spinning (CPMAS) technique.

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Tab

le I

. Ele

men

tal C

onte

nts

of C

omm

erci

al H

umic

Aci

ds a

nd S

elec

ted

Hum

ic A

cids

from

Dop

pler

ite, P

eat,

and

Leo

nard

ite

Sam

ple

C

H

0 Ν

S

F >

Tot

al

Ash

(%

) 1.

K&

K h

umic

aci

d 63

. 25

5. 17

32

. 22

0.68

4. 49

0.

15

106.

0

22. 4

8 2.

CPL

hum

ic a

cid

59. 8

9 5.

07

34. 8

1 0.8

1 0.

72

<0. 0

5 10

1. 3

9.

28

3. F

luka

-Tri

dom

hum

ic a

cid

65. 7

9 5.

51

37. 7

9 0.7

1 3.

16

<0. 0

5 11

3. 0

32

. 82

4. P

&B

hum

ic a

cid

(bat

ch 1

) 63

. 88

5. 69

35

. 23

0.80

3.

04

<0. 0

5 10

8. 6

24

. ,77

5. P

&B

hum

ic a

cid

(bat

ch 2

) 62

. 84

5. 38

34

. 99

0.80

3.

68

<0. 0

5 10

4. 9

15

. 01

6. P

&B

hum

ic a

cid

(pur

ified

bat

ch 2

) 61

. 87

4. 45

28

. 92

0.89

3.

35

<0. 0

5 99

. 5

4. 44

7.

Ald

rich

hum

ic a

cid

63. 1

5 5.

60

34. 9

8 0.

80

4. 58

<0

. 05

108.

9

22. 5

6 8.

Ald

rich

hum

ic a

cid,

sod

ium

salt

68. 9

8 5.

26

43. 4

5 0.7

4 4.

24

<0. 0

5 12

2. .7

31

. 21

9. A

ldri

ch h

umic

sal

t (pu

rified

to H

+ f

orm

) 65

. 31

5. 94

25

. 05

0.51

3. 36

<0

. 05

100.

2

4. 46

10

. W

yom

ing

dopp

leri

te

6L 16

4.

,39

43. 5

9 1.6

1 2.

36

<0. ,0

5 11

3. 1

31

. 02

11.

Irish

pea

t hum

ic a

cid

60. 7

0 5.

,22

32. 5

9 1.2

9 0.

90

<0. 0

5 10

0. ,7

1.

.05

12.

Leo

nard

ite h

umic

aci

d 63

. 25

3. ,64

31

. 05

1.17

0. ,84

<0

. ,05

100.

0

2. ,47

NO

TE: A

ll re

sults

are

exp

ress

ed o

n a

perc

ent-b

y-w

eigh

t, as

h-fr

ee,

and

moi

sture

-free

bas

is.

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Results and Discussion

Infrared Spectroscopy. The infrared spectra of all samples are shown in Figure 1. Overall, the infrared spectra of the commercial materials (No. 1-9) are generally similar to those of dopplerite (No. 10), peat (No. 11), and leonardite (No. 12) humic acids; however, some differences are discernible. Spectra of all the untreated commercial humic acids (samples

I 1 1 1 I I I ι ι ι ι ι 4000 3000 2500 2000180016001400 1000 800 600 400 200

Wavenumber (cm"1 )

Figure 1. Infrared spectra of commercial humic acids (Nos. 1-9) and other humic materials (Nos. 10-12): (1) Kà-K humic acid; (2) CPL humic acid; (3) Fluka-Tridom humic acid; (4) Pfaltz and Bauer humic acid (batch I); (5) Pfaltz and Bauer humic acid (batch 2); (6) "purified" Pfaltz and Bauer humic acid (batch 2); (7) Aldrich humic acid; (8) Aldrich humic acid, sodium salt; (9) "purified" Aldrich humic acid from No. 8; (10) Wyoming dopplerite; (11) Irish

peat humic acid; and (12) leonardite humic acid.

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4. MACCARTHY & MALCOLM The Nature of Commercial Humic Acids 59

No. 1-5 and 7) are very similar to each other, the major differences being in the —1750-cm 1 region of sample No. 3.

Because of the absence of a reasonably well-defined absorption band at —1750 cm" 1 in sample No. 3, the spectrum resembles that of the salt of humic acid more than humic acid itself. The resemblance becomes evident on comparison of this spectrum with that of sample No. 8, which is known to be in the salt form. The major difference between the spectrum of untreated (No. 5) and partially de-ashed (No. 6) Pfaltz and Bauer humic acid resides in the increased absorbanee of the 1750-cm"1 band in the latter. A similar, but more pronounced, change was observed on treating sample No. 8, which was known to be in the salt form, to produce sample No. 9.

The 1750-cm 1 band is due to the C = 0 stretching vibration of the unionized and uncomplexed carboxyl group. Evidently, the carboxyl group in commercial humic acids Nos. 1, 2, 4, 5, and 7 is partly in a salt or complexed form. The nominal "humic acid" No. 3 is evidently largely in the salt form, as is the Aldrich humic acid salt, sample No. 8. In the de-ashing procedure that was applied to samples No. 6 and 9, the humic acids are contacted with dilute H CI solution in the last step of the treatment, which ensures that all carboxyl groups are in the undissociated state. Wyoming dopplerite (No. 10) occurs naturally as a salt (6). This state is consistent with its infrared spectrum, which resembles that of the commercial humic acid salt (No. 8). The spectra of samples No. 11 and 12 show a pronounced absorption at —1750 c m - 1 because of the un-ionized carboxyl group. This characteristic is consistent with the method of preparing these samples.

The predominant ash components in many humic substances are ses-quioxides of iron, aluminum, and manganese, as well as amorphous silica and exchangeable metal ions such as sodium, calcium, magnesium, iron, and aluminum. Crystalline aluminosilicate clay minerals are also frequently associated with humic substances.

Sample No. 5 (Pfaltz and Bauer humic acid, purchased in 1981) had about 10% less ash than sample No. 4 (purchased in 1974) (Table I). The difference suggests either that two different source materials were used or that the 1981 sample had been subjected to some type of purification. This decrease in ash content apparently reflects a combination of fewer metal ions associated with carboxyl groups (increase in absorption because of unionized and uncomplexed carboxyl at —1750 cm 1 in the spectrum of sample No. 5 as compared to that of No. 4; see Figure 1) and the presence of less silica and aluminosilicate clay minerals (decrease in absorption in the 950-cm" 1 region of sample No. 5).

The decrease in ash content from 15% in sample No. 5 to 4.4% in sample No. 6, caused by liquid-liquid extraction, was largely caused by replacement of exchangeable metal ions with hydrogen ions at carboxylate sites. This replacement is shown by the increase in the intensity of absorption of the — 1750-cm 1 band because of the un-ionized carboxyl group and by

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the corresponding decrease in the absorption intensity in the 1650-1575-cm" 1 region, caused by the ionized carboxyl group, in the spectrum of sample No. 6.

The decrease in ash content after l iquid-liquid extraction is apparently not caused by a reduction in amorphous silica or aluminosilicate clay minerals, because the contribution to the S i - O stretching region near 1100 cm" 1

is essentially constant in the spectra of sample Nos. 5 and 6. Decrease in ash content because of a reduction in sesquioxides cannot be discerned from the infrared spectra.

The band at —1750 cm" 1 that indicates the un-ionized carboxyl group in sample No. 7 is absent from the spectrum of sample No. 8, and the band centered at —1650 cm" 1 has been broadened and intensified in sample No. 8. Sample No. 8 contains less amorphous silica and aluminosilicate clay minerals than sample No. 7, as indicated by the smaller absorption due to S i - O (—1100 cm"1) in the former. The large decrease in ash content from 31.21% in sample No. 8 to 4.46% in sample No. 9 is again caused, at least in part, by replacement of metal ions from carboxylate groups by hydrogen ions, as shown by the large increase in intensity of the —1750-cm"1 band and decrease in intensity of the carboxylate band at —1650 cm" 1 from sample No. 8 to sample No. 9.

Influence of Ash Content on Elemental Analysis. The elemental and ash contents for all samples are given in Table I. Ash interferes with elemental analyses (4), and some of the problems caused by high ash content in the study of soil organic matter have been discussed by Goh (7). As substantiated by the data in Table I, the elemental analysis (on an ash-free basis) of humic and fulvic acids with greater than 10% ash seldom yields a summation of elemental constituents (carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus) within 100 ± 5%.

The considerable improvement in the quality of elemental analyses with diminished ash content is apparent from a comparison of the values in the "total" column for the "purified" samples (Nos. 6 and 9) with the corresponding Table I values for the original samples (Nos. 5 and 8, respectively). The data also indicate that high ash content leads to anomalously high values for the organic oxygen content. Of the noncommercial samples, dopplerite (No. 10), with an ash content of 31.02%, gives a poor value for the summation of elemental content; the peat humic acid (No. 11) and leonardite humic acid (No. 12), both of which have low ash content, show good values for the summation of elemental content.

A l l six elements listed in Table I were determined by direct methods, and the discrepancies in the elemental analyses would not be evident if one of the elements had been determined by difference, as is frequently the case in humic-substances research.

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1 3 C-NMR Spectroscopy. The spectrum of sample No. 7 shown in Figure 2 is typical of the solid-state 1 3 C - N M R spectra of the commercial humic acids investigated in this research, all of which were found to be very similar to each other (I). The 1 3 C - N M R spectra of the commercial products (e.g., sample No. 7) were quite different from the spectra of humic acids isolated from peat (No. 11), soil (No. 13), and streams (No. 14), as illustrated in Figure 2. The remarkable similarity between the N M R spectra of the commercial humic acids and that of raw untreated dopplerite (sample No. 10) suggests a close similarity among these materials. A more detailed discussion of the 1 3 C - N M R spectra of the commercial humic acids and of humic acids isolated from peat, soils, and waters is provided in réf. 1.

Conclusions

Infrared spectra of commercial humic acids are similar to those of other humic acids. The commercial humic materials that are nominally in the acid form are, in fact, partly or largely in the salt or complexed form. However, these materials can be converted readily to the acid form. Infrared spectroscopy is not a sensitive technique for discriminating between commercial humic acids and humic acids isolated from soils and waters. However, distinct and significant differences between these two classes of materials are evident in the 1 3 C - N M R (I). There are also differences between these two classes of materials in elemental composition (J), but these differences are not as pronounced as those observed in the 1 3 C - N M R spectra.

Commercial humic acids have been used extensively by many researchers as analogues of soil and water humic substances. Many deductions concerning the environmental influence of humic substances have been based on studies involving such commercial samples. The inability of the more classical techniques, such as IR spectroscopy and elemental analysis, to clearly distinguish between the commercial materials and "true" humic substances isolated from soils and waters may have contributed to the hesitation of many environmental scientists to abandon the use of the commercial materials as analogues of soil and water humic substances, despite prior criticism (8) of the use of the commercial materials.

However, in view of the distinct differences between these classes of materials, as clearly revealed by 1 3 C - N M R spectroscopy, the commercial materials should not be used as analogues of soil and water humic substances in environmental studies. The problem involved in the use of the commercial materials is exacerbated by the almost complete lack of available information on the origin and pretreatment of these materials. A more recent quantitative study showed significant differences between the water solubility enhancement characteristics of commercial humic acids and those of stream-derived humic substances (9).

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Chemical shift in ppm, δ

Figure 2. CPMAS 13C-NMR spectra of Sample No. 7, Aldrich humic acid; No. 10, Wyoming dopplerite; No. 11, Irish peat humic acid; No. 13, Sanhedron

soil humic acid; and No. 14, Ohio River humic acid.

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In addition, high ash content leads to poor elemental analyses of humic substances. This results primarily from the anomalously high oxygen values and leads to a summation of elemental contents in excess of 100%. Reduction of the ash content results in considerably improved analyses. This points to a potentially serious problem in the common practice of determining the oxygen content of humic substances by difference.

References 1. Malcolm, R. L.; MacCarthy, P. Environ. Sci. Technol. 1986, 20, 904-911. 2. MacCarthy, P.; Mark, H. B., Jr. Soil Sci. Soc. Am. Proc. 1975, 39, 663-668. 3. Allen, R. S., III; MacCarthy, P. In Agronomy Abstracts; American Society of

Agronomy: Madison, 1982; p 180. 4. Malcolm, R. L. J. Res. U.S. Geol. Surv. 1976, 4, 27-40. 5. Thurman, Ε. M.; Malcolm, R. L. Environ. Sci. Technol. 1981, 15, 463-466. 6. Adam, Z. M.S. Thesis, Colorado School of Mines, 1985. 7. Goh, K. M. N. Z. J. Sci. 1979, 13, 669-686. 8. MacCarthy, P.; Malcolm, R. L. In Trace Organic Analysis: A New Frontier in

Analytical Chemistry; Chesler, S. N.; Hertz, H. S., Eds.; Proceedings of 9th Materials Research Symposium, National Bureau of Standards: Gaithersburg, MD, 1979; pp 793-796.

9. Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. Α.; MacCarthy, P. Environ. Sci. Technol. 1987, 21, 1231-1234.

RECEIVED for review March 28, 1988. ACCEPTED for publication June 27, 1988.

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