neutral pH. This is probably because hydroxide, carbonate, and phosphate phases control heavy metal solubility at higher pHs, and, at acidic pHs, free metal ions are predominant. Metal-ligand complex stability generally decreases with a reduction in pH, reflecting the role of R-COO- in metal complexation [13].

However, not only do the organic phases bear different binding abilities for metals, but also different metals have different affinities for humic substances. Metals like Cd, Ni, and Zn are generally reported to be more mobile in the soil environment than Pb, Cu, and Co. Table 3.1 shows the relative affinities of different metals onto humic and fulvic acids at various pH values, as reported by various researchers [14-17].

In addition to this, various researchers have differentiated humic behavior in accordance with the hydrophobic (Ho) or hydrophilic (Hi) nature of humic substances. According to Han and Thompson [18], in the DOC of molecular weight (MW) < 3500 g mol-1, 56% is Hi and 44% is Ho; the total concentration is 860 mg DOC kg-1 organic matter. A MW > 14,000 g mol-1 83% is Hi and only 17% is Ho (with a total concentration of 580 mg DOC kg-1 organic matter).

Explaining the binding of metals by organic matter has two main approaches. The first one is the discrete ligand model (DLM) and the second is the continuous distribution model (CDM). The DLM approach suggests that only a few ligands on the humic substance are required to fit experimental data. Given the complexity of humic phases, these ligands (or ligand classes) must be less than the actual total number presented on these molecules. For an individual adsorption site, v, the following formula is valid:

where Ki is the stability constant of the binding of metal M onto the given ligand class vi and in brackets are the metal concentrations.

Although the DLM represents metal binding quite successfully, the CDM approach allows a large number of sites to be involved in binding metals, and thus it represents real conditions more closely. In this model, no discrete Ki defined, but rather a continuum of K values. This model assumes an irregular frequency distribution of the functional groups. The formula describing the macroscopic free ligand concentration, L/, is:

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