Behaviour of carboxylic acids on NH4-smectites : pollution prediction in waste landfill
Résumé
Waste storage induces environmental problems like soil and water pollutions by landfill leachates. Smectites are usually used as pollutant barriers at bottom of waste landfill. For this application, most interesting properties of smectites are their low permeability after compaction, expansion ability and high Cation Exchange Capacity (CEC). Waste decomposition leads to produce high amount of pollutants and it is necessary to predict the long-term behaviour of the clay barrier. Landfill leachates are first characterized by large amount of ammonium. Therefore a natural cation exchange can be predicted in clay barriers. Moreover, high amount of acetic acid are observed in landfill leachates. For these reasons we focused our study on acetic acid (pKa=4.74) in interaction with ammonium montmorillonite, then extended to others carboxylic acids to better understand carboxylic acids and ammonium smectites interaction mechanisms. Experiments were performed with the reference SWy-2 montmorillonite totally exchanged with NH4+ cations and different carboxylic acids at several concentrations but higher than found 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 decrease of the H-N-H-bending intensity 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 0.1 mol/L 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. In room relative humidity and temperature a shift from 1.19 nm for the sample before treatment to 1.25 nm (cation exchange proportion=53%) was observed, corresponding to a NH4+-to-H+ exchange. These results are in accordance with the d001 reflection value of SWy-2 montmorillonite saturated by protons: 1.26 nm at 40% relative humidity (FERRAGE et al, 2005). Besides, the same general phenomenon was confirmed with the other montmorillonites of various CEC we studied. Interactions between ammonium SWy-2 montmorillonite and other short-chain carboxylic acids (formic acid, pKa=3.75; chloroacetic acid, pKa=2.86; oxalic acid, pKa=1.25) have also been investigated with the same protocol. At equal acid concentration, lower is the pKa of acid, higher is the NH4+-to-H+ exchange rate. Furthermore, this study indicates that cation exchange is directly dependent on pH value for acid concentrations lower than 1 mol/L. Results showed that the behaviour of formic acid on ammonium smectite is very close to the one of acetic acid. The interaction is characterized by the absence of adsorption and only by the NH4+-to-H+ exchange. This exchange was also found when chloroacetic acid and oxalic acid were interacted with ammonium smectite. But in addition, we observed fixation on the clay with these two last acids, clearly characterized by an increase of the carbon concentration in the solid phase. These observations were confirmed by FTIR thanks to a stretching band in the C=O vibration range. Besides, XRD patterns showed a significant increase of d001 reflections from 0.5 to 2.5 nm which may be attributed to a fixation in interlayer space. Additionally, a decrease of the width at mid-height of the 00l reflections is observed for clay treated with oxalic acid, indicating a better stacking of the layers which can be attributed to a structural modification of the interlayer space. Reference: FERRAGE, E., TOURNASSAT, C., RINNERT E. & LANSON B. (2005): Influence of pH on the interlayer cationic composition and hydration state of Ca-montmorillonite: Analytical chemistry, chemical modeling and XRD profile modeling study.- Geochimica et Cosmochimica Acta, Vol. 69, No. 11, 2797-2812.