Seawater humic substances

The most striking characteristic of the dissolved humic substances is their chromophoric nature. As part of the DOM, they impart a yellow-brown cast to marine and freshwaters and, hence, are part of the CDOM pool. Terrestrial hiunic substances compose a significant fraction of the riverine DOM entering the ocean. In seawater, humic substances compose 5 to 15% of the HMW DOM. Differences exist in the bulk properties of marine and terrestrial humic substances. These are summarized in Table 23.6. They have been used to trace the fate of terrestrial organic matter in the ocean. 

Figure 9.10 (a) Schematic soil humic acid structure proposed by Schulten and Schnitzer (1997). Note that the symbols stand for a linkages in the macromolecules to more of the same types of structure. (b) Schematic seawater humic substances structure proposed by Zafiriou et al. (1984). (c) Schematic black carbon structure proposed by Sergides et al. (1987). 

Further work on differentiating marine and coastal runoff humic substances was hindered by the lack of suitable isolation techniques for the marine material since concentrations in the open sea rarely exceed 0.25 mg/ L. Sorption of marine humic substances from seawater onto solid phases is now a standard technique and can be used to extract gram quantities of marine humic substances for chemical and physical studies (see Aiken, Chapter 14). Sieburth and Jensen (1968) first used rolled nylon stockings as an adsorbant but the method suffered from contamination. Kerr and Quinn (1975) used a specially treated charcoal and obtained quantitative recovery of the dissolved colored substances in seawater. Riley and Taylor (1969) introduced the use of cross-linked polystyrene resins, specifically Amberlite XAD-2. This polymer is now the most widely used for open-ocean work (Stuermer and Harvey, 1974 Bada et al., 1982 Harvey et al., 1983) and in estuaries (Mantoura and Riley, 1975). These isolation methods have made available sufficient quantities of seawater humic substances for detailed chemical studies. 

Differences in the distribution of Fe zones in the 10% seawater-humic substances systems, when the same concentrations of different humic substance samples are used, are presented in Figure 6. There is no doubt that the FAC sample produces the highest amount of the anionic Fe zone but all samples of the humic substances significantly influence the distribution of the electrophoretic... 

Mn in estuarine or seawater-humic substance systems might be partly attributed to the formation of the Mn-humic substance complex (besides positively charged hydrolytic products of Mn(III) which are present to some extent in the estuarine or diluted seawater without the addition of humic substances). 

A study of the bulk properties of seawater humic substances was carried out by Kerr and Quinn (1975), while a detailed structural analysis was undertaken by Stuermer (1975) and Stuermer jmd Harvey (1978). Stuermer discussed the structural features in terms of origin, chemical and physical properties, interaction in the sea and eventual fate. As an example of the formation of a humic substance in seawater, we will discuss Stuermer s proposed structure of seawater fulvic material (Gagosian and Stuermer, 1977), the precursor compounds to its formation, and the condensation and polymerization reactions responsible for its synthesis. Although the material isolated by Stuermer represents only a small portion of the total hiunic material, it serves as an example of a possible condensation product. 

Stuermer, D. H. and Payne, J. R. (1976). Investigation of seawater and terrestrial humic substances with carbon-13 and proton nuclear magnetic resonance. Geochim. Cosmochim. Acta 40,1109-1114. 

Kogut, M. B. and Yoelker, B. M. (2001). Strong copper-binding behaviour of terrestrial humic substances in seawater, Environ. Sci. TechnoL, 35, 1149-1156. 

The basic structure of humic substances involves a backbone composed of alkyl or aromatic units crosslinked mainly by oxygen and nitrogen groups. Major functional groups attached to the backbone are carboxylic acids, phenolic hydroxyls, alcoholic hydroxyls, ketones, and quinones. The molecular structure is variable as it is dependent on the collection of DOM available in seawater to undergo the various polymerization, condensation, and oxidation reactions and reaction conditions involved in humification, as well as the ambient physicochemical reaction conditions, such as temperature and light availability. 

Recall from Chapter 23.2.4 that humic substances are isolated from seawater by adsorption on a hydrophobic resin followed by elution using solvents of varying pH. The desorbed compounds are fractionated into two classes, humic acids fulvic acids based on their solubility behavior. A model structure for a humic acid is illustrated in Figure 23.10a in which fragments of biomolecules, such as sugars, oligosaccharides. 

Since most of the riverine DOM is comprised of humic substances, considerable attention has been fiacused on its fete in seawater. Little terrestrial DOM is detectable in seawater, suggesting the existence of an efficient removal process. This is surprising given the traditional view that humic substances are relatively refractory. Marine chemists are currently investigating the redox and photochemistry of humic substances to better understand its chemical fete in the oceans. 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Calculated equilibrium speciation of (a) mercury and (b) copper during estuarine mixing of hypothetical river water with seawater. Hum, humic substance. Note logarithmic scale on y-axis. Source. From Mantoura, R. F. C., et al. (1978). Estuarine and Coastal Marine Science 6, 387 08. 

Humic substances High-molecular-weight organic compounds that are variable in composiUon, have complex structures, and are relaUvely inert. They comprise a large fracUon of the DOM. Found in sods, sediment, fresh, and seawater. 

Raspor, B., Nurnberg, H.W., Valenta, P. and Branica, M., 1984. Studies in seawater and lakewater on interactions of trace metals with humic substances isolated from marine and estuarine sediments. 11. Voltammetric investigations on trace metal complex formation in the dissolved phase. Mar. Chem., 15 231-249. 

D.H. Stuermer and G.R. Harvey, Humic substances from seawater, Nature 250 (1974) 480-481. 

Hedges, J. I., P. G. Hatcher, J. R. Ertel, and K. J. Meyers-Sculte. 1992. A comparison of dissolved humic substances from seawater with Amazon River counterparts by 13C-NMR spectrometry. Geochimica et Cosmochimica Acta 56 1753—1757. 

Stuermer, D. H., and Harvey, G. R. (1977). The isolation of humic substances and alcohol-soluble organic matter from seawater. Deep Sea Res. 24, 303-309. 

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

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