Oxidation is one of the two ...

Oxidation is one of the two major forms of chemical reaction in a landfill. Obviously, the extent of the oxidation reactions is rather limited, in as much as the reactions depend upon the presence of oxygen trapped in the landfill when the landfill is made. Ferrous metals are the components likely to be affected.

The second major form of chemical reaction includes the reactions that are due to the presence of organic acids and carbondioxide (CO2) synthesized in the biological processes and dissolved in water (H2O). Reactions involving organic acids and dissolved CO2 are typical acid-metal reactions . Products of these reactions are largely the metallic ions and salts in the liquid contents of the fill. The acids lead to the solubilization and, hence, mobilisation of materials that otherwise would not be sources of pollution . The dissolution of CO2 in water deteriorates the quality of the water, especially in the presence of calcium and magnesium.

The importance of biological reactions in a landfill is due to the following two results of the reactions:

1. The organic fraction is rendered biologically stable and, as such, no longer constitutes a potential source of nuisances.
2.  The conversion of a sizeable portion of the carbonaceous and proteinaceous materials into gas substantially reduces the mass and volume of the organic fraction.

Since there is variation in the composition of leachate due to the factors like waste composition, waste age, landfilling technology, leachate sample collection techniques may also influence the measured leachate quality . For example, colloids have a high affinity for heavy metals thus the concentration of heavy metals measured in a leachate sample may depend strongly on the amount of colloidal matter present in the sample and the handling of the sample. No standard protocols for sampling, filtration, and storage of leachate samples exist. The content of colloidal matter in a sample depends to a large extent on the sampling technique used [20] where samples are obtained from groundwater monitoring wells. A high pumping rate will increase the colloid content of the sample significantly , and the heavy metal concentration may also be increased. Thus leachate samples should be filtered in the field before analysis of heavy metals, especially when the sampling is done quickly. Alternatively, samples could be withdrawn under very low pumping rates and after sufficient removal of the well. Leachate samples should be maintained under anaerobic condition until they are preserved because metal solubility varies according with their oxidation stage