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The retention of transition metals

October 20, 2003

Transition metals are essential for life but, depending on their concentration in the environment, they can prove to be toxic and provoke serious environmental impact. The aim of this PhD is to study the retention of transition metals by humic substances.

Humic substances are, on the other hand, macromolecules arising from the physical, chemical and microbiological transformation (humification) of biomolecules, explains '�lvarez Puebla researcher of the Public University of Navarre. Their importance is fundamental as they make up the most widespread an ubiquitous source of non-living organic material that nature knows. Approximately 80% of the total carbon in terrestrial media and 60% of the carbon dissolved in aquatic media are made up of humic substances.

Beyond their relevance for life, these substances have industrial applications for making absorbents of metal-poisoned sources. Being natural substances, their purification process is cheaper than the synthesis of any other absorbent and, moreover, due to their high operability, they absorb more than the absorbents used to date such as active charcoals or clays.

Mechanisms of retention

Humic substances have a great capacity for retaining transition metals, forming metalorganic complexes which cause these metals to be more or less available for plants and the plants pas to animals and these to humans.

Thus, '�lvarez Puebla proposed a study of how the retention mechanisms operate in order to find out both the capacity and strength of retention a specific humic substance might have; i.e. how this metal-humic substance complex behaves after being subjected to different conditions.

To this end, copper, nickel and cobalt were chosen as all three are in the First Transition Series, are consecutive, are necessary for life, are quite common in industrial effluents, are very used and are very well parameterised for study by means of computer techniques.

Mathematical model for analysis

'�lvarez Puebla drew up a mathematical model which enabled an interpretation of the interaction between metals and humic substances. This model considers the global process retention as a series of simple processes and, according to the author, provides good results and reveals a considerable selectivity of humic substances in order to retain the various metallic species as a function of pH and, to a lesser extent, of the concentration of the metal species in the solution.

Also, the theses held by other researchers, which have even been used as a basis for a United States legislative bill, argue that the transition metals may be liberated in alkaline soil, given that an insoluble hydroxide is formed which stays retained in the soil.

However, '�lvarez Puebla has shown that humic substances, present in all soils, can mobilise the metal in question because they co-ordinate with it and make it soluble. On the metal mobilising with rain or river water, it can reach subterranean waters and return to the food chain, contaminating the environment.

All this research has been carried out both at a macroscopic level and a microscopic one, by means of various analysis techniques and mathematical and molecular modelling. In this way, the mechanisms of interaction of Co(II), Ni(II) and Cu(II) have been determined in a humic substance complex and its fractions, as a function of the metal concentration and pH of the medium.

Elhuyar Fundazioa