EXCHANGEABILITY AND FORMATION CONSTANTS OF COPPER, ZINC AND CADMIUM WITH HUMIC SUBSTANCES AT INDIGENOUS CONCENTRATIONS

AUTOR(ES)

Pedro Alexandre Varella Escosteguy

DATA DE PUBLICAÇÃO

2001

RESUMO

A series of laboratory experiments and modeling investigations studied the exchangeability and the formation constants of trace metals with water-soluble humic substances at environmentally realistic concentrations. Copper (Cu2+), zinc (Zn2+), and cadmium (Cd2+) were the studied trace metals. These metals have different affinities for the binding sites of humic substances and have a range of functional group preferences when interacting with humic materials. Water saturation extracts, soil humic substances extracted from soils, and Suwannee River natural organic matter solutions (NOM) were used. The NOM studied systems were the same as those described in the previous research that determined the log K values for metal binding to humic substances used by the computer-implemented program MINTEQA2. Environmentally relevant concentrations of metal were added during the titrations rather than the millimolar concentrations used in the earlier research. A Donnan membrane equilibrium technique rather than the lanthanide spectrofluorometry technique used in the original research was employed to determine the activities of free cations. Free metal activities were modeled by using a composite ligand and the continuous Gaussian distribution sub-models. No significant competition for binding sites between the free Cd2+, Zn2+, Cu2+, and H+ was observed in response to changes in total dissolved and free Cd, H+, Ca, and Cu activities in solution. The mean log K value for Cu agrees with the value used in the MINTEQA2 database, while the values of log K for Cd (4.7) and Zn (4.2) are higher than those obtained in the published research that determined the log K values included in MINTEQA2 (log K = 3.3 and 3.5, respectively).

ASSUNTO(S)

substâncias húmicas metais poluição do solo especiação de metais quimica do solo concentrations of metal soil humic substances

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