Metal-Organic Interaction in Landfill-Polluted Environments

In spite of such general knowledge about metal-organic complexation stability, it must be stressed that a great deal of experimental studies do not reflect the environment of a landfill-polluted water.

Thus, the expected behavior of metals may differ substantially from what may be previously known, and several factors may induce such a view: first, most experimental work deals with manipulated and treated organic compounds; second, very few reported investigations actually dealt with metal-organic complexation in landfill leachate or leachate-polluted groundwater systems; and third, the influence that a high ionic strength environment and the presence of multiple competing ligands has on the ability to form metal-organic complexes is not known.

In these environments, metals may exist as free metal ions, inorganic/organic complexes, colloids, and associated to nanoparticles. The speciation of metals, which determines its relative mobility, is thus dependent on chemical parameters such as pH, redox, concentration of primary ions, type and amount of organic compounds, and also colloids and other nanophases. Specific studies on metal-organic complexation associated to landfill leachate waters or leachate-polluted groundwaters are restricted to a few metal cations, most notably Pb, Cd, Ni, Cu, Cr, and Zn.

Despite how scarce and, ultimately, complex these studies may be, it has been possible to gather important knowledge on metal-organic compound interaction. However, more recent studies have been not as much concerned with metal speciation, but rather on landfill leachate degradation mechanisms including the effects transition metals have in organic compound degradation.

These mechanisms, whose knowledge is important for the implementation of cost-effective solutions for waste degradation and water treatment, aim at producing simpler and safer organic compounds. But metals remain a concern and the efficiency of adsorbent substrates that scavenge these metals is likely to differ under aqueous media with competing com- plexing agents, whose behavior is crucial to know.

The spilling of leachates to the environment may substantially modify metal leaching and transport. Organics complex and transport metals and waste waters usually have a low redox potential that in some circumstances can promote the reductive dissolution of metal-bearing minerals under particular conditions.

Speciation studies on a landfill leachate based on an exchange procedure with Chelex 100 resin demonstrates that the free or labile complexed fraction is the most abundant for all the metals studied by Majone at al. [16], increasing from 50% to 80% in the order Cd < Pb < Ni < Cu («80%). The slowly exchangeable fraction represents 14% for Cu, 28% for Pb, and 47% for Cd. Ni has the higher relative percentage in the inert or stable complexed fraction (17%).

In the case of Cu, this metal becomes complexed either with low MW compounds (<1000 Da, around 47%) or with high MW compounds (>12 000 Da, around 45%), these latter corresponding probably to humic acids. Overall, these authors concluded that the high MW organic acids have an important influence on metal speciation, and bonding to these substances increases in the rank Ni < Cd < Cu.

The work of Jensen and Christensen and Jensen et al. showed that a significant but highly variable fraction of heavy metals have affinity for dissolved or colloidal organic matter. It must also be stressed that the distribution of metals among the different groups considered (dissolved and colloidal forms, organic- or inorganic-associated) varied between samples and between metals within the same sample, which demonstrates the heterogeneity of these systems and difficulty in achieving general conclusions.

However, in leachates, it has been possible to show that heavy metals, such as Cu, Pb, and Cd, are predominantly associated with organic matter, either in colloidal form or dissolved. A substantial fraction of dissolved metals, such as Zn and Ni, is mostly associated with dissolved inorganic complexes, the remaining are associated with colloidal fractions (either organic or inorganic).

The importance that colloidal organic matter has on the speciation and binding of Cu and Pb seems to be the most plausible explanation for the differential partitioning of metals to sediments near the source of spilled landfill leachate to the surface environment. Besides, it has also been verified that metals such as Zn and Ni had much less affinity to organic compounds, preferentially forming insoluble sulfide phases, as in the case of Zn. Christensen et al. also verified for Zn and Ni in leachate-contaminated groundwater that at the low concentrations usually observed in the natural system, complex stability decreases significantly.