Fractionation and characterization of leachate from a municipal landfill: advantage of high performance size exclusion chromatography
Landfill leachates represent a source of organic pollution characterized by an important organic load, with high chemical oxygen demand in recent sanitary landfills and some organic compounds refractory to biodegradation. Several researchers have examined the organic matter in these landfill leachates. In addition to measuring parameters such as chemical and biological oxygen demands (COD and BOD) and UV-absorbance, different analytical techniques were applied: gas chromatography with flame ionization or mass detection; high performance liquid chromatography; infrared spectroscopy; nuclear magnetic resonance spectrometry; and elementary analysis. Raw leachates or samples after fractionation on Sephadex gel were characterized by ultrafiltration or adsorption on XAD resins. Conclusions from these earlier studies were as follows: - physico-chemical properties of leachates revealed not only a high organic pollution but also diversity and variability according to the age of the sanitary landfill and the climatic conditions; - gel permeation chromatography and ultrafiltration revealed two main fractions in the leachates: one with molecular weights below 1000 Daltons (Da), and another with molecular weights above 5000-10000 Da; - infrared spectroscopy and nuclear magnetic resonance spectrometry showed functional groups present in the humic and fulvic acid fractions of natural organic matter; - a varying number of peaks detected by gas chromatography with flame ionization or mass detection proved the complexity of the matrix. Few compounds were identified and quantified, with the exception of fatty acids. Moreover this technique was only applicable to molecules with low molecular weight. The purpose of the present work was to develop a new method of fractionation of organic matter in landfill leachates and to study their characterization and treatment biodegradation. Ultrafiltration, as a prefractionation step, divided the leachate into four fractions according to their molecular weight: above 10000 Da, from 10000 - 3000 Da, from 3000 - 1000 Da, and below 1000 Da. The second fractionation step was carried out using gel permeation chromatography. This technique has been was applied by earlier researchers for the characterization of landfill leachates, but at low pressure on Sephadex gels. In our study, we developed a high performance size-exclusion chromatography method using a polymer based TSK PW column, a hydrophilic cross-linked polyether. Three TSK G3000 PW columns and one G5000 PW column were tested with water at pH 4 with acetic acid and with a water/methanol mixture as mobile phases. This rapid method of separation, with short retention times, was coupled with on-line multidetection: UV-visible (254 nm - aromatic compounds), fluorescence spectroscopy (275/325 nm, protein-type molecules; 320/430 nm, humic-type molecules) and evaporative light scattering detection, ELSD. The ELSD allowed detection of all mineral and organic compounds that did not evaporate at the working temperature (45°C). The effect of the sodium chloride concentration on retention times was tested with eluants and columns. Secondary effects, often observed with size-exclusion chromatography, occurred with the gel chosen. The elution of sodium chloride solutions at different concentrations showed that the TSK PW gel bears electronegative charges, and that the density of these charges differs from one column to another. For the leachate we observed this influence: chromatograms obtained on two TSK G3000 columns were different for fractions with molecular weights below 1000 Da. The comparison of chromatograms obtained with the four detection methods provided information about the identity of the types of compounds present. For fractions with molecular weights below 1000 Da, separation was performed using a TSK G3000 PW column, with an eluant pH of 4 and a water-methanol mixture; three main families were detected. For fractions with molecular weights above 10000 Da, chromatographic separation was improved by elution with water/methanol (70/30) with TSK G5000 and G3000 columns in series; two main groups were identified, humic substances and protein-type compounds. The constituents of the two intermediate fractions with molecular weights between 10000 and 1000 Da were essentially humic substances, identified after separation on a TSK G3000 PW column with water-methanol (70/30) as the eluant.