Metal-Organic Interactions in Landfill Leachates and Associated Groundwaters

Landfills are necessary infrastructures for the safe disposal of a variety of wastes produced in our society. These infrastructures are currently regulated in such way as to define which types of wastes they may receive, and accordingly, which type of operation and monitoring procedures or construction regulations should be implemented. With respect to wastes, there are three classes of landfills according to waste type: hazardous, nonhazardous, and inert.

These landfills will develop leachates, as water percolates the waste piles while the landfill is being operated, so the type of leachate produced depends on the solubility of the waste constituents. These leachates are drained and collected into pools for water treatment and safe release to the environment. However, risk assessment also considers leachate leaking, especially to groundwaters, because even in the best-engineered sites leachate leaks to some extent.

This can be due to such causes as development of fissures in the impermeable barrier or wearing out of materials with time. Although metals are not leachate major constituents, as persistent potential hazardous elements, establishing their fate is of major importance when they are released into groundwater systems.

Organic compounds are the core constituents of landfill leachate derived waters. Their nature and relative amount are dependent on landfill age and evolutionary stage. Four pathways are generally acknowledged: initial transition stage, in which waste decomposes under aerobic conditions with production of heat; aci- dogenic phase, which occurs in anaerobic conditions developing high concentrations of soluble degradable organic compounds; methanogenic phase, in which the leachate becomes neutral to more alkaline; final maturation phase, in which aerobic conditions may return and leachate eventually ceases to be hazardous to the environment.

In the early stages of landfills, organic compounds present in the leachate are mostly species with molecular weight (MW) less than 1000 Da, although with a nonnegligible fraction of higher MW species; less than 18% have MW greater than 10 000 Da. The leachate pH is typically around 5 in these early stages because of the abundance of low MW organic acids. It is in this phase that metal concentrations usually increase.

This acidic stage can last for a few years or up to 10 years depending on landfills. Gradually, landfill leachates evolve toward the methanogenic stage, attaining pH values around 8, where higher MW organic acids develop, including fulvic- and humic-type compounds, sustaining the precipitation of sparingly soluble metal phases. Mature leachates have about 67% of organic molecules with a MW greater than 10 000 Da.

Organic compounds, with their abundant functional groups, are recognized important metal ligands in this system. Among the different types of compounds, carboxylic acids and phenols are always the most abundant ones.

Pollutants that may be present in landfill leachates are grouped into four major categories: dissolved organic matter, inorganic compounds and molecules, heavy metals, and xenobiotic organic compounds. Among the inorganic components commonly found in landfill leachates are ammonium, bicarbonate, Ca, K, Na, iodide, nitrate, and sulfate. Among the heavy metals are Cd, Cu, Zn, Ni, Cr, Co, Pb, Hg, and the metalloid As.