For the first time a method is presented to study the distribution of metals between humic and fulvic fractions in natural soils. Total organometallic complexes are separated by extraction with 0.1 M pyrophosphate. After acidification to pH 1.5 with H2SO4 the fraction associated with humic acids is separated by precipitation and the remaining solution contains the complexes associated with fulvic complexes. Concentrations of 11 elements (Al, Fe, Ca, Mg, K, Mn and the trace elements Cu, Pb, Zn, Cd, and Cr) were determined in the pyrophosphate and the fulvic acid extracts, and the fraction bound to humic acids was estimated by difference. Typical results from application of the method to andosols, cambisols, and podzols are presented. Fulvic acid is generally the main humic fraction reacting with metals, except for Ca where around 70% of the organic bound pool is found in the humic fraction. The corresponding figure is of the order of 30–40% for Al, Fe, Mn and Cr and even less for Pb, Cu, Zn, and Cd. For some elements the humic/fulvic distribution appears to be different between soil types. Some factors that may disturb the results obtained by the present method are discussed.
Humic substances have the capacity to interact with metal ions, which makes them important agents for speciation and mobility of metals in soils. In many cases they exert strong control on the behaviour of pollutant metals in the environment. A recent review on the interaction of metals with humic substances was published by Logan et al. (1997).
Characteristics of humus fractions have proven useful in interpretation of organic matter dynamics in soil. A rather extensive literature exists on characterization of humic matter (Schnitzer, 1967, 1976, 1978; Schnitzer et al., 1984; Schnitzer and Calderoni, 1985; Schnitzer and Preston, 1986) and on binding of metals to humic matter mainly concerning laboratory experiments with humic or fulvic fractions purified from water or soil (Schnitzer, 1966, 1969, 1971; Schnitzer and Skinner, 1963a,b, 1964, 1965, 1966, 1967; Schnitzer and Hansen 1970; Ogner and Schnitzer, 1971; Bloomfield et al., 1976; Cheshire et al., 1977; Schnitzer and Kendorff, 1981; Murray and Linder, 1984). In some cases sequential extraction schemes were used to distinguish between different binding forms of metal in soils and sediments (Riche and Weir, 1963; Tessier et al., 1979; Slavek et al., 1982; Schnitzer and Scuppli, 1989b).
The present investigation represents the first attempt to study the specific association of metals with the humic and fulvic acid fractions within natural soils, using a combination of well-known procedures. For the extraction of humic and fulvic acid fractions and the associated metals from the soils 0.1 M Na4P2O7, pH 10 was used. This extractant was chosen for its high extraction capacity and selective complexation(Schnitzer et al., 1958; Alexandrova, 1960; Bascomb, 1968; Schnitzer and Desjardins, 1966; Schnitzer and Skinner, 1968; Schuppli et al., 1983; Schnitzer and Schuppli, 1989a). The further separation of humic/fulvic fractions was performed according to the classical method (Alexandrova, 1960; Kononova and Belchikova, 1960) and atomic absorption spectrometry was used for determination of trace metals concentrations. The present paper describes the methodology used and shows some typical results from a comprehensive study on humic/fulvic distribution of 11 metals (Fe, Al, Ca, Mg, K, Mn, Cu, Pb, Zn, Cd and Cr) in three types of natural soils: andosols, podzols and cambisols.