Stability Constants of Metal–Humic Acid Complexes and Its Role in Environmental Detoxification

Humic acid with hydroxyl-, phenoxyl-, and carboxyl-reactive groups can form coordination compounds with metals. The ion-exchange equilibrium method using Dowex AG 50W-X8, 20-25 mesh Na+ form was used to determine stability constants of complexes formed between humic acid (isolated from the soil) 50–250 μg (≈3×10−5–15×10−5 mol/liter) and metal salts solution 200 μg at pH 3.5. The stability constant (log K) for different metal–humic acid complexes indicated the following order of the stabilities of complexes formed between humic acid and metal ions: Cu>Fe>Pb>Ni>Co>Ca>Cd>Zn>Mn>Mg. The data on stability constants demonstrated substantial deviation from Irwing–Williams series reported for divalent ions. The molar humic acid/metal ratios were also calculated. Some of the factors affecting the stability constants such as cation exchange capacity of humus soil, molecular radius, and molecular surface area of humic molecules were also estimated. The significance of the data to predict the behavior of these complexes in the environment is discussed.