calcite is assumed. The equilibrium composition has been computed for two values of Pco,. The solution composition for Pco, = 10+ is very similar to that given for the median composition of terrestrial waters. Water of the type of Model 6 has a nearly constant [Si(OH)& The latter concentration varies with (Pco,)~/~. Such a hypothetical univariant system shows (as long as equilibrium between all phases is maintained and PcoZ kept constant) an infinite buffer capacity with respect to strong acid or strong base addition (Sillen, Morgan). If this idea is carried a little bit further one can appreciate the working hypothesis, proposed by Sillen, that the ocean represents a coexistence of a sufficient number of phases, so that for the given number of components only one degree of freedom ( F = C P 2 = 1) remains; then PCO,in the atmosphere of the model will be determined by the equilibria (especially equilibria among clays) and cannot be varied.
8 The Chemistry of Rivers and Lakes: The Nature and Properties of Natural Product Organics and Their Role in Metal Ion Transport R. F. CHRISTMAN University of Washington, Seattle, W a s h .
+
Comparison w i t h real systems
Our models predict that feldspars at the incipiently high COz partial pressures are unstable, with respect to kaolinite. With decreasing C 0 2 pressure and sufficient accumulation of and Na+ or Ca+2 other clays such as montmorillonite are being formed. If [Ca+*]and [HC03-] become sufficiently large, calcite precipitates and represents an upper limit for the accumulation of soluble ions. Reactions between aluminum silicates are pertinent in regulating the water composition. The predictions of the equilibrium models appear to be in qualitative agreement with many field observations. As demonstrated by Feth et al. [Si(OH)4] shows little variation with stream discharge. In Figure 1, a few experimental data of analysis of ground waters originating in igneous rocks are depicted. Interestingly, many points fall very close to the montmorillonite-kaolinite boundary. In presenting simplified equilibrium models, we have tried to abstract from the complexity of nature. For those who are familiar with the complexity of the weathering reactions, the simplification may have gone too far. But it was our intent to reach those who are not students of weathering and to stimulate them to read the excellent papers by Garrels and Mackenzie, Feth, Roberson and Poltzer, Sillen, Holland, Kramer, Hem, and Hemley.
It is generally recognized that the relatively common occurrence of a yellow color in lake water is due to the presence of complex organic matter of natural origin. A reasonably accurate understanding of the environmental factors involved in the production of color or of the significance of this material to the aqueous biological community has been hindered by a lack of significant structural information. In this article I want to question two general theories which have resulted from previous research on the complex natural color system: first, that the molecules producing color are aliphatic in nature; and, second, that their interaction with iron does not involve chelation as a primary mechanism. The presence of aromaticity is not strikingly apparent from an inspection of spectrophotometric data obtained either on the natural waters or on the organic solids isolated from them. However, we have recently subjected these organic materials to oxidative degradation employing an alkaline CuO system (Christman and Ghassemi, 1966). The degradation products (50 yield) isolated and identified using thin layer chromatography and ultraviolet spectrophotometry are shown in Table 1. Evaluation of the oxidative system using model compounds has shown that aromatic ring fissure does not occur and that alkyl side chain oxidation occurs only when the alkyl-aryl carbon-
302 Environmental Science and Technology
”...
to-carbon linkage is appropriately activated by oxygen-bearingsubstituents. If the ring is strongly activated, decarboxylation results. Thus it seems justifiable to consider 2,4- and 2,6-dihydroxybenzoic acids as intermediate oxidation products which would be converted to resorcinol during oxidation. These compounds can be regarded as structural nuclei in the parent macromolecule since it is doubtful they are produced during oxidation and they are definitely not extractable from the untreated colored water. The manner in which these phenolic moieties are bound in the macromolecular structure is unknown. The simplest model one could visualize consists of the aromatic groups bound through extensive alkyl side chain networks. The real structure must be more complicated, probably involving interaromatic linkages, since the foregoing model would demand a chemical reactivity and U.V. absorbance behavior that is not known to exist in real color-water systems. Considering the number and kind of ionogenic groups involved, the ultimate size of any given color-producing structure is probably also a function of pH and total iron concentration. All of the aromatic substitution patterns shown in Table 1 naturally occur in the extractive fraction of woody tissue and in soil organic matter. Thus, this result is more attractive than the “aliphatic” theory of organic color structure. One of the most widely recognized properties of organic color is its tendency to associate with iron. Recently, Shapiro (1964) studied this interaction quantitatively and concluded that the organic acids producing color peptize iron in aqueous solution and that this mechanism is more important than chelation. It is significant that Shapiro also observed the iron-holding capacity of organic color to increase markedly with pH. His theoretical argument, however, included a molecular weight for organic color of 322 and a requirement that aqueous iron be fully coordinated with the color molecule. The magnitude of calculated molar ratios of iron to color seemed to Shapiro to be inconsistent with a chelation model of interaction. If a molecular weight of 10,000 is assumed (this may be low), a recalculation of Shapiro’s data at pH 7.0 reveals
If it is now assumed that HA is a very weak acid [(A-)
log [Fe]
- [HA] + (A-)
=
-
[HA]/(HA).
The reason I have arranged this expression in the above manner is simply because the quantities [Fe], [HA], and pH are essentially the experimental quantities reported by Shapiro. Thus this chelation model predicts a straight line relationship between log l/([Fe] - [HA]) and pH, with an intercept of log K. When the reported quantitative interaction data are plotted with this relation a perfect straight line results and a K value of approximately lo5 is obtained. There are of course many questionable assumptions involved in this model, but the result indicates the point I wish to make, namely that the existing data on color-iron interaction are not inconsistent with a chelation mechanism.
iron-color chelation [HA] = (A-)
- [HA]
+
Christman, R. F., and Ghassemi, M., "Chemical Nature of Organic Color
+ (HA)
Rr, in solventa 1
2
i
'
Absorption maxima, mp 3
EtOH
EtOH-NaOH
0-Dihydroxybenzene
0.96
0.41 I 0.55
203, 219, 279
262, 290
Ivf-Dihydroxybenzene
0.98
0.31
0.48
205, 223, 277, 283
245, 295
3-Methoxy-4-hydroxybenzaldehyde
0.89
0.59
0.68
210, 255, 320
250, 350
3-Methoxy-Chydroxybenzoic acid
0.26
0.10
0.57
259, 293
300
3,5-Dimethoxy-4hydroxybenzoic acid
0.03
0.03
0.51
213, 272
303
3,4-Dihydroxybenzoic acid
0.37
0.05
0.29
208, 252, 292
277, 302
3,5-Dihydroxybenzoic acid
0.20
0.00
0.24
a
4
Chemistry of the Oceans: Some Trace MetalOrganic Associations and Chemical Parameter Differences in Top O n e Meter of Surface
References
Table 1. Degradation products of natural organic color
Compound
in Water," J. Amer. Wafer Works ASSOC., 58 (6), 723 (1966). Shapiro, J., "Effect of Yellow Organic Acids on Iron and Other Metals in Water," J . Amer. Water Works ASSOC., 58 (8), 1062 (1964).
Solvent 1 : ethyl ether. Solvent 2 : benzene and ethanol (9O:lO) v./v. Solvent 3 : benzene, methanol, and acetic acid (95:8:4) v./v.
The impact of the present surge of interest in oceanography on chemistry of the oceans will doubtless be far reaching. In the past few years, chemistry has grown from a one- or two-man operation, mostly of analytical type, at each oceanographic institute or department to a group of fundamentally trained scientists attacking broad chemical problems of the oceans. As is usually the case, the recent advances have been preceded by new chemical concepts and techniques. The long-awaited utilization of instrumental methods for a much refined analysis of the major component relationships, such as salinity, have greatly enlarged our ability to detect minute differences in water properties. Perhaps the greatest advances, however, have occurred in the minor component and trace element areas. Development of such sensitive procedures as neutron activation analysis, radioactive detectors, isotopic dilution, polarography, mass spectrometry, gas chromatography, atomic absorption spectrometry, and thin layer, paper, and column chromatography has Volume 1, Number 4. April 1967 303
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