Aquatic humic substances isolation

An extraction method for isolating humic substances from water by using XAD-8 has been proposed by Thurman and Malcolm (9) (see box). Humic substances in natural waters represent almost the entire hydrophobic acid fraction. This method has been used to isolate 4.25 g of humic substances from 24,500 L of ground water from the Fox-hills-Laramie aquifer and to obtain 500 g of humic material from 10,400 L of the Suwannee River (Table II). The sample from the Suwannee River was collected as a reference sample of aquatic humic substances by the International Humic Substances Society. In both of the examples cited, a fc cutoff of 100 was used. 

Aiken, G. R. 1985. Isolation and concentration techniques for aquatic humic substances. In Humic Substances in Soil, Sediment and Water Geochemistry, Isolation, and Characterization (G. R. Aiken, D. M. McKnight, R. L. Wershaw, and P. MacCarthy, Eds.), pp. 363—385. 

Thurman, E. M., and R. I. Malcolm. 1981. Preparative isolation of aquatic humic substances. Environmental Science Technology 15 463-466. 

Aiken, G. R. (1988). Critical evaluation of the use of macroporous resins for the isolation of aquatic humic substances. In Humic Substances and Their Role in the Environment, Frimmel, F. H., and Christman, R. F., eds., John Wiley Sons, Chichester, pp. 15-28. 

Standley, L.J. and Kaplan, L.A. (1998) Isolation and analysis of lignin-derived phenols in aquatic humic substances improvements on the procedures. Org. Geochem., 28, 689-697. 

Aiken, G. R. (Ed.). (1985). Isolation and Concentration Techniques for Aquatic Humic Substances. Humic Substances in Soil Sediment and Water. Wiley, New York, pp. 363-385. 

Thurman and Malcolm (1981) proposed a large-scale preparative method for isolation of aquatic humic substances, in which XAD-8 resin is used to adsorb humic substances from acidified water samples. The aquatic humic substances are then back-eluted with NaOH, and fractionated into aquatic HAs (insoluble at pH 1) and aquatic FAs (soluble at pH 1). The two fractions are further purified to remove inorganic solutes and freeze-dried separately. 

Little work has been done to compare the nature of ligands in riverine, estuarine, and coastal waters. Preston (1979) found similar selectivity coefficients for copper with humic compounds isolated from different salinity regimes of the Tamar estuary. His results are made uncertain by lack of knowledge of the molecular weights of the compounds, but it appeared that the selectivity for copper decreased with increasing salinity. The stability constant data of Mantoura et al. (1978) also show similar selectivities for copper by aquatic humic substances from river, lake, and marine waters, which would imply that little variation in selectivities should be found along an estuarine salinity gradient. 

A critical review is presented of the methods commonly used to isolate and concentrate aquatic humic substances, important steps in any extraction scheme are filtration, concentration, isolation, and preservation. Various filtration options are outlined and compared filtration is important for obtaining low-ash humic substances. Advantages and disadvantages of commonly used concentration and isolation techniques are discussed, and sorption on the synthetic macroporous resins XAD-8, a nonionic resin, and Duolite A-7, a weak anion-exchange resin, are recommended as the most efficient methods. Two established extraction schemes utilizing these resins are discussed. 

The final product should be free from chemical impurities, which hinder characterization of the isolated material, and should be in a form that can resist biological and chemical degradation. Many options are available for isolating and concentrating these compounds. Each method has advantages and disadvantage, and best results can be obtained by adopting a combination of techniques It is the purpose of this chapter to critically discuss and evaluate various ethods commonly used to isolate and concentrate aquatic humic substances. 

TABLE 2. Advantages and Disadvantages of Matrix Types Suitable for Filtration of Water for Isolation of Aquatic Humic Substances... 

Silver-membrane and glass-fiber filters are recommended for studying particulate organic carbon, because organic contamination of the sample is avoided (Cranston and Buckley, 1972). These filters are also recommended for the isolation of aquatic humic substances, because interactions with the filter by dissolved organic compounds are minimized, and the potential for organic contamination of the sample is slight. 

Freeze-drying is commonly used in conjunction with other concentration methods as the final step in isolating humic substances from water (Beck et al., 1974 Deinzer et al., 1975 Aldridge et al., 1976 Jolley et al., 1979 Thurman and Malcolm, 1981), and it is here that freeze-drying is most efficient. Samples of aquatic humic substances should be considerably concentrated and desalted prior to freeze-drying. The solid product obtained by freeze-drying can be easily handled and stored without fear of chemical degradation. 

Cheng (1977) found XAD-12, a very hydrophilic XAD resin with weak-base functional groups, to be the best sorbent for humic acid. Because of precipitation of humic acid at low pH, pH 5 was found best for sorption. Fulvic acid, however, adsorbs more strongly at lower pH (Fig. 3), and pH 2 is recommended. Humic acid constitutes only about 5% of dissolved humic substances in water for this reason, solution pH should be adjusted to pH 2 when isolating aquatic humic substances on XAD resins (Aiken et al., 1979). 

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