Metal distribution in a lysimeter : experimental methods

North Americans have traditionally used landfills as a cheap method of refuse disposal. Over the past few years governments have started forming legislation to prevent further landfilling of some industrial wastes. This reflects the realization that metals and organics are not always retained by landfills. The lack of information with respect to metal mobility has caused many metal wastes to be barred from landfills. This has created a disposal problem for commercial and industrial refuse producers. In 1978 a co-disposal project was initiated at the University of British Columbia to determine the potential for enhanced retention of metals in a landfill. Electroplating sludge and septic tank pumpings were added to six airtight lysimeters and the liquid effluent was monitored for Cd, Cr, Fe, Ni, Pb and Zn releases. The study did not determine specific metal retention mechanisms. Therefore, the study report herein was initiated to establish experimental procedures to determine: relative metal mobility, the role of bacteria as metal binding agents and the importance of natural ligands as metal binding agents. Three separate experiments were conducted using samples which had been removed from one of the UBC co-disposal study lysimeters. A technique for removing the samples in a nitrogen atmosphere was developed to keep the samples anaerobic. By using anaerobic samples any changes caused by exposure to air were avoided. The first experiment used a progressive extraction technique to determine the relative mobility of Cd, Cr, Cu, Fe, Ni, Pb, and Zn. Three samples were tested using this technique. The samples were suspensions which had been made by adding deionized distilled water to lysimeter material. An untreated sample suspension was used as a control. Another was sonicated to rupture cells and fragment particles. The third sample was aerated to determine the effects of exposure to air. By comparing the sonicated sample data with the control sample data it may be possible to estimate the mass of metal bound by bacteria. A comparison of the aerated sample and control sample data should give an upper bound to the proportion of the metal mass that will remain fixed in a lysimeter. The extraction procedure separated the metals into five distinct groups: mobile, easily complexed, organically bound, strongly bound and stable metals. The second experiment was developed to enumerate the bacteria in a sample. A quick and accurate enumeration technique was required so an estimate of the metals associated with bacteria could be made. For this purpose the fluorescent dye 4,6-diamidino-2-phenylindole was selected to enhance visual detection of bacteria. Enumeration was unreliable because of the relatively small quantity of bacteria that were detected in the complicated mixture of organic materials. The third experiment compared the complexing strength of natural ligands with that of EDTA, ethanoic acid, glycine, histidine, 8-hydroxyquinoline (oxine), NTA and oxalic acid. Stability constants for the expected metal-chelate complexes were used to form a gradient of complexing agents. It was not possible to determine the presence or absence of natural ligands from the data collected. The technique may provide a method for determining metal species in complex samples. It was theoretically possible to determine the mass of each metal species by using chelating agents which vary in their ability to complex each metal species. The major limitation of the theory is the lack of an accurate method for determining the stability constants of complexes which are expected to form. With further work the three experimental methods tested could be used to determine the metal mobility, the mass of metal associated with bacteria, the strength of natural ligands and the metal species in an anaerobic landfill sample. The progressive extraction technique should be modified to use dry oxidation of organics to ensure complete oxidation. The chelation experiment requires further testing before a chelate gradient can be used to determine metal speciation and ligand strength. The fluorescent dye technique was the least successful of the three experiments tested. It may be possible to improve the technique by using mithramycin instead of 4,6-diamidino-2-phenylindole, by experimenting with the concentration of ethanol required to fix bacteria or by using a chemical other than sodium pyrophosphate to break down the cellulose fibers in the sample. Once the three techniques have been successfully used to test landfill samples it will be possible to develop more informed landfill management policies.Applied Science, Faculty ofCivil Engineering, Department ofGraduat