Fourier transform infrared study of mercury interaction with carboxyl groups in humic acids

An interaction between humic acid, an organic part of soil and mercury was studied by Fourier transform infrared spectroscopy (FTIR) and by ICP-AES analysis under given pH and concentration conditions. First the spectroscopic model was validated on the interaction of simple molecules representing the structural components of humic acid such as benzoic acid, catechol and salicylic acid with mercury. The interaction of carboxylic parts of humic acid with mercury is very interesting and easily characterised by infrared spectroscopy, an ideal mean for molecular study. Under the salt form (commercial humic acid Fluka TM: FHA), humic acid reacts with mercury in a different way from its acid form (FHA purified noted PFHA) and the Leonardite (LHA). Because of the straightforward exchange between Na+, Ca2+ and Hg2+, fixation of the latter is much more important with the salt form (FHA). However, this reaction is reduced under the acid form (PFHA, LHA) because the exchange with protons is difficult. The effect of this exchange was studied by FTIR showing the intensity decrease of νCO (COOH), the carboxylic functional group band of the acid, and the shifting of νas (COO−), the carboxylate functional group band under given pH and mercury conditions. For the FHA salt form, the characteristic band νCO (COOH) represented by a shoulder did not evolute, whereas the corresponding band to νas (COO−) strongly shifted (40 cm−1) for a maximum Hg2+ concentration (1 g l−1). On the other hand, for the acid form (PFHA, LHA), the intense band of νCO (COOH) disappeared proportionally to the increase of Hg2+concentration and the νas (COO−) band moved for about 20 cm−1. The same results were reached with pH variations. Our results were confirmed by ICP-AES mercury analysis. This study shows that humic acids react differently according to their chemical and structural state.