<p>This work shows that infrared spectroscopy is an easy and fast analytical technique for monitoring the behavior of the humic acids Fluka and Leonardite in contact with binary solutions of mercury and lead. Given the complexity of the structure of humic acid, the experiments were carried out on simple model molecules.<strong></strong></p><p>The spectra showed a shift (30 cm<sup>-1</sup>) of the antisymmetric stretching vibration band from the carboxylate function -COO<sup>-</sup> from 1590 to 1560 cm<sup>-1</sup> for the humic acids Fluka-binary solution interaction. In the case of mercury alone, the shift was of 40 cm<sup>-1</sup>, it can be seen that this shift has dropped by 10cm<sup>-1</sup>, which proves that the cationic exchange becomes more difficult.<strong></strong></p><p>We have confirmed that the affinity is due to the ionic radius, the reduction potential and the ionic potential. We reinforced this by a fourth parameter which is the frequency separations between the-COO<sup>-</sup> antisymmetric and symmetric stretches (Dn<sub>as-s</sub>). We have found thatDn<sub>as-s </sub>is strongly related to the type of fixed metal. The spectra also showed the variation of the intensity of the C=O elongation vibration band of the carboxylic function -COOH up to 1610 cm<sup>-1</sup> for the Leonardite humic acid-binary interaction. The high concentration of the binary solution did not result in the total disappearance of the C=O band as in the Leonardite humic acid-mercury alone interaction study. This proves that some carboxylic sites are not accessible to Hg<sup>2+</sup> ions.</p>
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