Time behaviour of metals in the leachate of a pretreated solid waste landfill

In the landfills of municipal solid wastes there are a number of constituents which will leach under the influence of infiltrating water. Among the leached constituents there are also trace inorganics which are potentially hazardous to the environment, the most important being the heavy metals. It follows that the waste landfills have a high contamination potential in terms of heavy metals, because leachate from landfills contains such substances. The amount and concentration of leachate that reaches the aquifer defines the potential for ground water contamination, and thus pollution of the drinking water supply. Data from literature indicates that very little of the original heavy metal content of a landfill is leached within the first decade after disposal. In a long term prospective, the remaining metals will either leach out, or be immobilized in the landfill. The degradation processes in landfills take place over a very long period of time; these degradation processes consist of a short initial aerobic phase and an early anaerobic phase. While most of the processes during these phases can be described qualitatively, the subsequent humic phase is poorly known. This phase is reached after perhaps one century and could possibly last thousand of years [1,2,3]. The leaching of individual trace inorganic pollutants is influenced strongly by the pH and the redox potential of the leachant as well as complexing agents carried by the water up stream. Until the humic phase, only a very small portion of the toxic metals deposited in a landfill are removed by the leachate. The mobility of heavy metals would increase during the humic phase, if oxygen penetrate into the landfill, leading to aerobic condition. As the oxygen intrudes into the landfill, the organic matter and the sulphides will degrade and release bound metals. If the reducing and the pH buffering capacity are depleted, the metals could be dissolved in the acid oxidising pore waters and can follow the water flowing through the deposit and, also by diffusion, can reach the ground water. In recent years, significant efforts have been made towards the characterization of the chemical properties of solid wastes and also towards the modelling of the hydraulic processes which occur in waste landfills. Material balance studies were performed and some of the retaining processes taking places in the humic phase were quantified. A recent material balance has shown that the abundance of some studied toxic metals (Cd, Cr, Zn, Hg and Pb) in the landfill varied from 5 to 127 times the concentrations of these in natural soils [4,5,6]. It is obviously not possible to conduct experiments over sufficiently long time scales in order to observe the long term leaching behaviour of wastes, so that information on the fate of contaminants (heavy metals) in landfills and its relation to waste processes requests additional research. Nevertheless, the general trend of pollutant release may be extrapolated also from experimental results from analysis of leachate produced in different phases and collected from suitable sites of landfills under investigation.