Date of Award

Spring 1980

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

Abstract

This study describes complexation of metal ions by fulvic acid, the acid soluble fraction of organic matter found in soils (SFA) and water bodies (WFA). Such complexation affects the toxicity and geochemical mobility of heavy-metal ions. All the work was performed with two previously isolated and characterized fulvic acids: one from a podzol soil and the other from a freshwater river. The work is divided into three major sections.

I. Complexation of Cd('2+) by fulvic acid. The conditional stability constants for complexes between aqueous Cd('2+) and the two fulvic acids were studied by Cd('2+) ion-selective electrode potentiometry. A temperature of 25(DEGREES)C and a medium of 0.1 M KNO(,3) was used throughout. The best results arose when Cd('2+) (rather than fulvic acid) was the titrant. The experiments show that (1) stability constants increased with increasing pH, and (2) stability constants decreased as the fulvic acid concentration rose toward 70 mg/L. The second effect does not occur for the Cu('2+)-fulvate system. In relatively concentrated solutions, interactions among fulvic acid molecules apparently block sites otherwise available to Cd('2+). From pH 4.0 to 8.0, the logarithm of the 1:1 conditional stability constant increases from 3.15 to 4.08 for WFA and from 3.23 to 4.63 for SFA. Fewer oxygen-containing complexation sites per mole of WFA is a reason for the lower WFA constants found during this study and in the next one with Pb('2+).

II. Complexation of Pb('2+) by fulvic acid. Pb('2+) forms much stronger complexes with SFA and WFA than does Cd('2+), as measured by Pb('2+) ion-selective electrode potentiometry at 25(DEGREES)C in 0.1 M KNO(,3). From pH 4.0 to 6.0, the logarithm of 1:1 Pb('2+)-SFA conditional stability constants increases from 4.0 to 6.3. The corresponding constants for Pb('2+)-WFA in the range pH 4.5 to 6.0 increase from 3.7 to 5.1.

Pb('2+)-fulvate precipitates at very low mole ratios of metal ion to fulvic acid. This precipitation limits the range of reagent concentrations that can be used for solution-phase studies of Pb('2+)-fulvate complexes. Pb('2+) and Cu('2+) form similar strength complexes with fulvic acid until Pb('2+)-fulvate precipitation begins. At that point, removal of hydrated Pb('2+) is more complete than is the removal of hydrated Cu('2+) as demonstrated in parallel experiments. The extra Pb('2+) removal from solution appears to occur through physical association with Pb('2+)-fulvate solids as well as by chelation by sites still available in the precipitates. Only at much higher mole ratios of metal ion to fulvic acid does Cu('2+)-fulvate precipitate.

III. Fluorescence studies. These fulvic acids have a broad, featureless fluorescence peak at 445-450 nm upon excitation at 350 nm. The work shows that (1) fluorecence intensity varies with pH, being the greatest near pH 5.0 and dropping off most notably below pH 4. (2) Metal ions at concentrations of approximately 10('-3) M or less that are not complexed by fulvic acid do not quench the fluorescence. (3) paramagnetic ions such as Cu('2+), Co('2+), and Ni('2+), when complexed by fulvic acid quench the fluorescence. A comparison of ion-selective electrode and fluorescence studies for Cu('2+)-SFA complexes shows a direct proportionality between the amount of complex and the percentage of fluorescence quenched from pH 3.0 to 6.0. With further development, fluorescence offers promise as a means of measuring stability constants for complexes between fulvic acid and paramagnetic metal ions.

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