Aggregate formation of humic acids from marine sediments

Marine Chemistry, 33 ( 1991 ) 229-241

229

Elsevier Science Publishers B.V., Amsterdam

Aggregate formation of humic acids from marine sediments Noriko Shinozuka and Chang Lee Institute of Industrial Science, Universityof Tokyo, 22-I, Roppongi 7-chome, Minatoku, Tokyo 106, Japan (Received November 13, 1989; revision accepted October 15, 1990 )

ABSTRACT Shinozuka, N. and Lee, C., 1991. Aggregate formation of humic acids from marine sediments. Mar. Chem., 33: 229-241. Humic acids were extracted from marine sediments and the aggregate-forming properties were studied by surface tension measurements and the soluhilization of hydrocarbons. The surface tension of aqueous humic acid solutions decreases with the humic acid concentration and becomes constant; the relationship between the surface tension and the logarithm of the concentration of humic acid is represented by two straight lines. The break point is considered to he due to aggregate formation. The solubility of benzo (a)pyrene and icosane in humic acid solution increases with humic acid concentration. A marked solubility increase observed at a humic acid concentration near 0. 1% indicates that the hydrocarbon is solubilized in humic acid aggregates. The concentration of aggregation can be determined from the point of intersection of two straight lines representing the relationship between solubility of hydrocarbons and log concentration of humic acid. Concentrations of aggregate formation for various humic acids determined by surface tension measurements and by the solubilization of hydrocarbons are in good agreement.

INTRODUCTION

Humic substances are the most widely distributed natural organic matter on the surface of the Earth (Schnitzer and Kahn, 1972); they are thought to play an important role in the environment. The reactions, persistence and behavior of many chemicals in seawater and sediments are largely dependent on the nature and concentrations of humic substances (Rashid, 1985a). Soil humic and fulvic acids have been shown to be surface active (Visser, 1964; Tschapek and Wasowski, 1976; Chen and Schnitzer, 1978). We reported that the surface activity of solutions of humic acids extracted from marine sediments was higher than that of commercially available humic acids (Hayano et al., 1982). Structural studies of marine humus suggest a considerably more aliphatic nature than for the terrestrial counterparts (Rashid, 1985b), which may result in high surface activity of solutions of marine humic 0304-4203/9 i/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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N. SHINOZUKA AND C. LEE

acids. It has already been pointed out that humic acids from marine sediments form aggregates at a certain concentration, according to surface tension measurements (Hayano et al., 1982; Hayase and Tsubota, 1983). Boehm and Quinn (1973 ) reported that dissolved organic matter can solubilize alkanes and other organic compounds. Interactions of soil humic substances with pesticides have been studied and reviewed (Khan, 1978 ). Humic substances adsorb pesticides and increase their solubility compared with that in water. Wershaw et al. ( 1969 ) reported that the solubility of DDT is at least 20 times higher in a 0.5% sodium humate solution than in water. Recently, Chiou et al. (1986) reported the solubility enhancement of PCB and other pollutants in the presence of aquatic humus. We found that the water solubilities of aliphatic and aromatic hydrocarbons increase on addition of humic acids extracted from marine sediments. A marked increase of solubility is observed for humic acid concentration high enough for aggregate formation to occur, as determined by surface tension measurements (Shinozuka et al., 1987 ). This solution behavior of water insoluble substances indicates the solubilization of hydrocarbons in micelle-like aggregates, inside of humic acid. In this paper, we will focus on the aggregate-forming properties of humic acids as determined from the results of surface tension measurements and the enhancement of hydrocarbon solubility in humic acid solutions. The relation of surface tension to humic acid concentration indicates aggregate formation, but does not necessarily suggest a micelle-like aggregate. Sometimes dimers and trimers give a similar relation, so other evidence must be used for confirmation (Attwood and Agaewal, 1980). We used solubilization measurements to determine the concentration for micelle-like aggregate formation. Commercially available humic acids were used to compare the aggregate-forming property with humic acids from marine sediments. MATERIALS AND METHODS

Humic acid preparation Humic acids were extracted from marine sediments sampled near Japan and Indonesia. Sampling sites are given in Table 1. The sediments were collected as a 50-cm box core with an Ocean-50 sampler. The extraction procedures were similar to those used for the International Humic Substances Society's reference samples (1982) with a little variation. In brief, the frozen sediment was thawed and dried. The powdered sample was passed through an 80-mesh sieve, and suspended in water acidified with HC1 to remove carbonates. After washing with water, 0.2 M NaOH was added with stirring for 24 h at 30 ° C under nitrogen to extract humic acid. This extraction procedure was repeated three times with separate alkaline solutions. The solution was

231

AGGREGATE FORMATION OF HUMIC ACIDS FROM MARINE SEDIMENTS TABLE 1 Sampling sites of marine sediments Name

Sampling sites

Depth

Data

(m) BS-12 BS-14 SJ-I FS-E

34°57'12"N, 34°59'54"N, 38°17'12"N, 07°22'42"S,

139°15'36"E 139°20'48"E 135°29'30"E 119°36'12"E

1333 1488 2980 3785

Dec. Dec. Sept. Feb.

1978 1978 1984 1985

centrifuged to eliminate suspended mud and then filtered through a 0.45-/~m Millipore filter. The filtrate was acidified to below pH 2 with 6 M HC1 and the resulting humic acid precipitate was separated by centrifugation. Dissolution into alkaline solution and precipitation by acid was repeated three times. The final alkaline solution was filtered through a 0.22-/~m Millipore filter, and then passed through an Amberlite IR- 120 cation exchange resin column to obtain the acid form of humic acid. The effluent was acidified and the precipitate was freeze-dried. Commercially available humic salts (Aldrich Chemical Co. and Fluka AG) were also purified by repeated dissolution with alkali and precipitation with acid. All chemicals used were of guaranteed reagent grade. Total acidity of the humic acids was determined by the method reported by Gillam and Riley (1982).

Surface tension measurement A Wilhelmy type surface tensiometer (ST-1, Shimadzu Seisakusho Co.) was used for surface tension measurements with a glass plate. The surface tension value was read when it was constant within 0.1 mN m-~ h -~. The temperature was kept at 25 °C. The pH of humic acid solution was adjusted to nine with HCI or NaOH.

Solubilizing procedure Solubility of hydrocarbon was measured by the so-called shake flask method. An aliquot of acetone solution of benzo (a)pyrene (BaP) was put into a Teflon flask. After evaporation of the solvent, 20 ml of humic acid solution was added, and the flask was shaken in a shaker with horizontal movement (Tokyo Rikakikai Co. Ltd., Model SS-80) for 4 h at 25°C (shaking for 3 h was needed to obtain constant solubility values). After standing for 30 m the solution was filtered through a glass fiber filter (Whatman G F / C ) to separate undissolved BaP. To the filtrate, transferred to a separatory funnel, 20 ml of dichloromethane was added. The solution was shaken for about 2 min and

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N. SHINOZUKA AND C. LEE

the organic solvent was transferred to a volumetric flask. No detectable BaP was extracted f r o m the residual humic acid solution. The absorption spectrum was measured in the visible wavelength range to determine the concentration ofBaP solubilized in humic acid solution. BaP is known to be a strong carcinogen; care must be taken in handling the reagent. Solubilization experiments with icosane have already been reported (Shinozuka et al., 1987).

RESULTS A N D D I S C U S S I O N

Elemental composition of humic acids The elemental composition and total acidity of humic acids examined are summarized in Table 2. The samples may contain small amounts of P and S but these elements were not determined. Humic acids from marine sediments show a relatively high H / C ratio and a high nitrogen content compared with the commercial humic acids, which were said to be terrestrial in origin. The elemental composition of sedimental humic acids was characteristic of the marine humic acid (Rashid, 1985b). The total acidity of the samples varied in the range 4-10, but it seems to have little effect on the surface activity at pH 9. The molecular size of the humic acid B S-14 was determined by fractionation with ultrafilter membranes in four fractions. The weight per cent of each fraction was as follows: >300 000, 49%; > 100 000, 15%; > 10 000, 16%; < 10 000, 20% (Shinozuka et al., 1984). Molecular weight of Fluka humic acid was estimated to be 700-800 by mass spectrometry (Y. Miyashita, personal communication, 1986). TABLE 2 Elemental composition and total acidity of humic acids Humic acid

C (%)

H (%)

N (%)

(H/C)atom

Total acidity (mequiv g-J )

BS-12 SJ- 1 FS-E BS-14 Aldrich Fluka

54 54 51 50 56 52

6.7 5.0 6.3 5.4 4.3 4.7

5.3 4.4 5.4 4.2 0.9 1.0

1.49 1.11 1.48 1.29 0.92 1.07

10.3 7.8 4.2 8.0 7.8 7.0

C,H,N,: ash-flee bases.

AGGREGATE FORMATION OF HUMIC ACIDS FROM MARINE SEDIMENTS

233

Surface tension measurement The surface tension of humic acid solutions changes with humic acid concentration as shown in Fig. 1. Two straight lines are obtained for the relationship between the concentration (on a log scale) and the surface tension, except in the case of SJ-1 and Fluka, in the concentration range below 0.2%. The fact that the slopes of the lines in the lower concentration range are nearly equal in the case of BS-12, FS-E and SJ-1 indicates that these humic acids have similar surface excess values. Aldrich and Fluka humic acids exhibit less steep slopes than those of humic acids from sediments. The surface excess calculated using the Gibbs equation was 1.85 × 10-1o mol cm -2 for BS-12, FS-E and SJ- l, 1.43 × 10- l o mol c m - 2 for Fluka, and 1.15 X 10- ~o mol c m - 2 for Aldrich. These values are comparable with that of soil fulvic acid and smaller than that of soil humic acid reported by Chen and Schnitzer ( 1978 ). Because the molecular weight and the shape of humic acids are unknown, surface excess does not directly reflect the molecular size. It is interesting that humic acids from marine sediments show larger values than commercial ones. The size of the latter is considered to be smaller than for humic acids from marine sediments. The surface tension of the BS-12 solution was about 41 (raN/m)

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mN m-~ at its constant value, whereas the other humic acids from marine sediments showed higher values. From these values, the humic acids examined are thought to have moderate surface activity compared with surfactants. The inflection observed in the relation of surface tension to humic acid concentration indicates the formation of a micelle-like aggregate of humic acid, similar to that of surfactant systems. However, as was pointed out by Attwood and Agaewal (1980), other evidence is needed to confirm this.

Solubilization of hydrocarbons The relationship between the solubility of BaP in humic acid solution and the log concentration of humic acid is shown in Fig. 2a-f. The solubility increases with the log humic acid concentration linearly in the lower concentration range. A marked increase is observed at around 0.1% in the case ofhumic acids from marine sediments. The enhancement of solubility compared with the water solubility of hydrocarbons (Whitehouse, 1984) in the lower humic acid concentration range can be attributed to an interaction such as adsorption of pesticides on soil humic substances (Pierce et al., 1974 ) or a partition mechanism. The details of this interaction will be the subject of further investigation. Plots of the solubility of BaP against the humic acid concentration on a log scale gave in some cases two straight lines (Fig. 2a, b, c and e). It is clear from these figures that two processes are involved in the phenomenon of solubility enhancement. At the lower humic acid concentration, hydrocarbon molecules may interact with the humic acid, which may be assumed to provide a sufficiently large intramolecular nonpolar organic environment (Chiou et al., 1986 ). Above the concentration of the crossing point the mechanism of solubility enhancement may change. The concentration at the crossing point shows the same value at which aggregation is suggested from surface tension measurements. By analogy with surfactant systems, the increase of solubility at higher humic acid concentration is thought to be due to the solubilization of hydrocarbon in aggregates of humic acid, like the solubilization of waterinsoluble substances in surfactant micelles, in which an organic nonpolar phase is formed. Aldrich humic acid also increases the water solubility of BaP but a linear relation between solubility and log concentration was not obtained, whereas in the solubilization of icosane in Aldrich humic acid solution, plots of solubility of icosane against log concentration of humic acid gave two straight lines (Hayano et al., 1984); the solubility was enhanced at a humic acid concentration of 0.14 g ( 100 ml ) - i. As for SJ- l, although aggregate formation was not indicated from the surface tension measurement, aggregation may be suggested by the solubility-concentration relationship. The solubility of BaP in Fluka humic acid solution increases linearly against log humic acid concentration; the solubility is high compared with the corresponding con-

AGGREGATE FORMATION OF HUMIC ACIDS FROM MARINE SEDIMENTS

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centration of other humic acids but no aggregation is indicated from the surface tension and solubility measurements in the concentration range examined. In the case of BS-14, solubilization of BaP is represented as two straight lines (Fig. 2e), which is similar to the solubilization oficosane (Fig. 3 ). The aggregate-forming concentration from BaP solubilization is somewhat smaller than those from the other two methods.

Aggregateformation of humic acid From the behavior of surface tension of humic acid solution and the solubility change of hydrocarbons with humic acid concentration, we conclude that the humic acid forms aggregates and solubilizes hydrocarbons in these aggregates. This is the reason for the solubility enhancement of hydrocarbons in humic acid solutions. The concentrations at the inflection on the surface tension-log concentration curve and the crossing point of the solubility-log concentration curve are listed in Table 3. The concentrations for aggregate formation of BS-14, FS-E and SJ-1 are around 0.1%. A low value for BS-12 may be attributed to its high surface activity, as indicated earlier. Aldrich humic acid shows a higher concentration for aggregate formation compared (6g/ml)

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AGGREGATE FORMATION OF HUMIC ACIDS FROM MARINE SEDIMENTS

239

TABLE 3 Concentration of aggregate formation Humic acid

Method Surface tension ( g ( l O O m l ) -~)

Solubilization (g(lOOml) ')

BS-12 FS-E BS-14

0.054 0.10 0.10

SJ-I Aldrich

0.15

0.058 0.099 0.11 0,085 0,078 0,14

(BaP) (BaP) (icosane) (BaP) (BaP) (icosane)

with those for humic acids from marine sediments. The values from the two measurements are in good agreement except for sample SJ- 1. The solubilizing behavior of humic acids for BaP may be affected by structural factors such as aromaticity, functional group contents, hydrophobicity and molecular size. Malcolm and MacCarthy (1986) pointed out that the structure of commercial humic acids is distinctly different from that of natural humic acids. Comparison of water solubility enhancements of organic solutes by aquatic humic materials and commercial humic acids indicated stronger interaction of commercial humic acids (Chiou et al., 1987 ). The interaction of Aldrich humic acid with BaP seems to be affected by a structural feature of the commercial humic acids, such as the high degree of aromaticity and the high degree of condensation, which are cited from the elemental analysis. Generally, in the critical miceUe concentration (CMC) determination by the solubilization method, care must be given to a specific interaction between solubilizate and surfactant. The reason for the discrepancy in the case of SJ-1 remained obscure, and the details of the interaction of hydrocarbons and humic acids are under investigation. The concentration for aggregate formation is around 0.1%, except for BS12. This value is similar to that reported by Hayase and Tsubota ( 1983 ) for the higher molecular weight fraction; they studied the surface tension behavior of sedimentary humic and fulvic acids. Rochus and Sipos ( 1978 ) reported the CMC values of alkali metal salts of soil humic acid. Their values were around 1% for lithium, potassium and sodium humates, and decreased to 0.3% in the presence of 2% KC1. Their high CMC values compared with our results will be the result of structural differences between soil humic acids and humic acids from marine sediments. Although our CMC values are lower than those reported, the concentration of humic acid for aggregation is by two orders of magnitude higher than that in the environment. From the results described above, we confirm that humic acids form micelle-like aggregates and solubil-

240

N. SHINOZUKA AND C. LEE

ize hydrocarbons into their hydrophobic interior. Because the C M C values o f humic acids we determined are more than a few h u n d r e d s milligrams per liter, the solubility e n h a n c e m e n t that occurs in natural environments cannot be attributed to the solubilization in a micelle-like interior. The shape, size and nature of the aggregate are being studied at our laboratory. CONCLUSIONS (1) H u m i c acids extracted from marine sediments decrease the surface tension of water and show moderate surface-active properties. (2) Plots of surface tension against log concentration o f humic acid give two straight lines with a break point which indicates the formation o f aggregates. (3) H u m i c acids enhance the water solubility o f hydrocarbons. The solubilities of icosane and benzo ( a ) p y r e n e increase markedly at higher concentration o f h u m i c acid; the p h e n o m e n o n can be attributed to the solubilization of hydrocarbons in h u m i c acid aggregates. (4) The concentrations o f aggregate formation determined by surface tension measurements and by the solubilization o f hydrocarbons are in good agreement. H u m i c acids from marine sediments form aggregates at concentrations around 0.1%. ACKNOWLEDGEMENT Financial support by the Tokyo Ohka F o u n d a t i o n for the P r o m o t i o n of Science and Technology is acknowledged.

REFERENCES Attwood, D. and Agaewal, S.P., 1980. Investigation of the self-associationof an amphiphilic drug in aqueous solution. J. Chem. Soc. Faraday Trans., I, 76: 570-577. Boehm, P.D. and Quinn, J.G., 1973. Solubilization of hydrocarbons by the dissolved organic matter in sea water. Geochim. Cosmochim. Acta, 37: 2459-2477. Chen, Y. and Schnitzer, 1978. The surface tension of aqueous solutions of soil humic substances. Soil Sci., 125: 7-15. Chiou, C.T., Malcolm, R.L., Brinton, T,L. and Kile, D.E., 1986. Water solubility enhancement of some organic pollutants by dissolved humic and fulvic acids. Environ. Sci. Technol., 20: 502-508. Chiou, C.T., Kile, D.E., Brinton, T.I., Malcolm, R.L., Leenheer,J.A. and MacCarthy, P., 1987. A comparison of water solubility enhancements of organic solutes by aquatic humic materials and commercial humic acids. Environ. Sci. Technol., 21: 1231-1234. Gillam, A.H. and Riley, J.P., 1982. Microscale functional group analysis of marine and sedimentary humic substances. Anal. Chim. Acta, 141: 287-299. Hayano, S., Shinozuka, N. and Htakutake, M., 1982. The surface active properties of marine humic acid. J. Japan Oil Chem. Soc. (Yukagaku), 31: 357-362.

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Hayano, S., Shinozuka, N. and Lee, C., 1984. Solubilizing action of marine humic acid. Proc. World Surfactants Congress, Vol. 1, Kurle Druck Verlag, Gelnhausen, pp. 244-248. Hayase, K. and Tsubota, H., 1983. Sedimentary humic acid and fulvic acid as surface active substances. Geochim. Cosmochim. Acta, 47: 947-952. International Humic Substances Society, 1982. Outline of Extraction Procedures. Khan, S.U., 1978. The interaction of organic matter with pesticides. In: M. Schnitzer and S.U. Khan (Editors), Soil Organic Matter. Elsevier, Amsterdam, pp. 137-171. Malcolm, R.L. and MacCarthy, P., 1986. Limitations in the use of commercial humic acids in water and soil research. Environ. Sci. Technol., 20:904-911. Pierce, R.H., Jr., Olney, C.E. and Felbeck, G.T., Jr., 1974. pp'-DDT adsorption to suspended particulate matter in sea water. Geochim. Cosmochim. Acta, 38:1061-1073. Rashid, M.A., 1985a. In: Geochemistry of Marine Humic Compounds. Springer-Verlag, New York, Chapter 3, p. 74. Rashid, M.A., 1985b. In: Geochemistry of Marine Humic Compounds. Springer-Verlag, New York, Chapter 3, p. 105. Rochus, W. and Sipos, S., 1978. Die Micellbilding bei Huminstoffen. Agrochimica, XX 11 : 446454. Schnitzer, M. and Khan, S.U., 1972. In: Humic Substances in the Environment. Marcel Dekker, New York, Chapter 1, p. 1. Shinozuka, N., Shinji, O. and Hayano, S., 1984. Solution properties of marine humic acid II. J. Japan Oil Chem. Soc. (Yukagaku), 33: 88-94. Shinozuka, N., Lee, C. and Hayano, S., 1987. Solubilizing action of humic acid from marine sediment. Sci. Total Environ., 62:311-314. Tschapek, M. and Wasowski, C., 1976. The surface activity of humic acids. Geochim. Cosmochim. Acta, 40: 1343-1345. Visser, W.A., 1964. Oxidation reduction potentials and capillary activities of humic acid. Nature, 204:581. Wershaw, R.L., Burcar, P.J. and Goldberg, M.C., 1969. Interaction of pesticides with natural organic material. Environ. Sci. Technol., 3:271-273. Whitehouse, B.G., 1984. The effects of temperature and salinity on the aqueous solubility of polynuclear aromatic hydrocarbons. Mar. Chem., 14:319-332.