Metal-complexing ability of fulvic acids in various soil types

We investigated visible-uv absorption spectra, chemical composition, IR absorption spectra, carboxyl and the phenolic hydroxyl groups of fulvic acids extracted from various soil types containing a variety of humic acids in order to elucidate the metal-complexing ability of fulvic acids in soils. Furthermore, we proposed a simplified method of the ion exchange equilibrium method for the determination of the stability constant of metal-soil organic matter complexes. This method enables determination of soluble complexants including fulvic acids. Precisely, there have been difficulties in measuring the maximum complexing ability (MCA) and the molecular weight, which are required to calculate the stability constant. In this study, we adopted the chloranilic acid-Cu complex (insoluble) method to measure the MCA of fulvic acids. According to the visible-uv absorption spectra (delta log k) and the content of carbon (RF), the humic acids extracted from prepared soil samples were classified as A, B, P and Rp types (two each). In contrast, the fulvic acids showed less pronounced differences and could not be as easily categorized due to their lower molecular weight compared to the humic acids as indicated by the values of delta log k, RF and differences in the IR absorption spectra. The IR absorption spectra suggested that the humic acids were more aromatic than the fulvic acids and the major functional groups in both humic and fulvic acids were carboxyl and phenolic hydroxyl groups. Total carboxyl groups and phenolic hydroxyl groups in the fulvic acids were analyzed by a nonaqueous titration method. The COOH and phi-OH content was higher than the previously reported content in humic acid. The molar fulvic acid/Cu ratio, taken as the slope of log (lambda0/lambda-1) vs relative concentration of fulvic acid at pH 5.0 and 7.0 average 1.25 and 1.3, respectively. This ratio generally increased with increase in pH. However, our results showed little influence of pH on this ratio. In the case of Cu-fulvic acid complexes, approximately 5 moles of fulvic acid combined with 4 moles of Cu at both pH levels. The stability constant (log K) values were calculated using the MCA value determined by the chloranilic acid-Cu complex method. The average stability constant at pH 5.0 was 8.29 and rose to 8.69 at pH 7.0, only a 5 % increased in increase pH. In contrast, the average MCA value at pH 5.0 was 0.29 muM/mg F. A and rose to 1.04 muM/mg F. A at pH 7.0, a 360 % increased. This suggests that the stability constant (log K) values for Cu-fulvic acid complex is less affected by pH compared with the MCA. Moreover, MCA and stability constant (log K) showed some degree of correlation with the carboxyl group content at pH 5.0. However, no apparent relationship could be seen at pH 7.0. In contrast, some correlation was apparent between MCA and the stability constant, and the phenolic hydroxyl group content at pH 7.0. Thus, the fulvic acid-metal complex is formed by the carboxyl groups at low pH, and that part of the phenolic hydroxyl groups are involved in complex formation above pH 7.0