Acetic acid interactions with NH4+ - smectite : prediction of the behaviour of polluants in waste landfill
Résumé
According to legislation, clays are used as barriers in bottom of waste landfill in order to prevent pollution by leachates. Among clays, smectites are generally chosen because of their low permeability after compaction, expansion ability and high Cation Exchange Capacity (CEC). Landfill leachates are characterized by large amount of ammonium. Therefore a natural cation exchange can be predicted in clay barriers. Moreover, waste decomposition leads to produce high amount of acetic acid in landfill leachates. For these reasons we focused our study on acetic acid (pKa=4.74) in interaction with ammonium montmorillonite. Experiments were performed with the reference SWy-2 montmorillonite totally exchanged with NH4+ cations and acetic acid at higher concentrations than in landfill. NH4-smectite samples were stirred during 7 days in acetic acid solutions, then rinsed and dried at 40°C. The pH was controlled during experiments. Analyses performed by FTIR spectrometry indicate no adsorption of acetic acid onto montmorillonite. The main observed modification is a variation of the H-N-H-bending after acid treatment, which tends to decrease with the increase of acid concentration. Carbon, nitrogen and sulphur in the solid phase were quantified with a LECO CNS-2000 analyser. Nitrogen quantitative analyses indicate that ammonium cations are partially exchanged during the interaction with acetic acid. For example, 44% NH4+ cations were exchanged with 0.1M acetic acid (pH=3.2), although no carbon was fixed. Ammonium concentrations in solution after interaction were quantified with ionic chromatography, and confirm the partial cationic exchange of NH4+ by H+. Moreover, d001 reflection values have been measured by x-ray powder diffraction. A shift from 1.19 nm for the sample before treatment to 1.25 nm (cation exchange proportion=53%) was observed, according to a NH4+-to-H+ exchange. This study with acetic acid at several concentrations indicates that cation exchange is directly dependent on pH value, but also limited at highest concentrations. Interactions between formic acid (pKa=3.75) and ammonium smectite samples have also been investigated. These experiments show that the pKa value of carboxylic acid is an important factor in exchange control. For a same acid concentration, the NH4+-to-H+ exchange rate is higher with formic acid due to its lower pKa. For example it is 61 % for 0.1M formic acid (pH=2.3). Besides, the same general phenomenon is confirmed with other studied montmorillonites with various Cation Exchange Capacity. In waste landfill, carboxylic acids and ammonium smectites interactions seem to be especially characterized by the NH4+-to-H+ exchange.