ANALYSIS OF METAL-HUMIC ACID INTERACTION BY PAPER CHROMATOGRAPHY USING HUMIC ACID-IMPREGNATED FILTER PAPER

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Abstract
We propose and describe a new paper chromatographic method for evaulating metal-humic acid interactions. In this method, a mixed standard solution of metal ions was spotted on the starting line of humic acid-impregnated filter paper and then developed with a 0.1 M acetate buffer solution (pH 5, 80 percent ethanol) containing 0.05 M Cd2+ or Mn2+. The Rf values of the metal ions were calculated on the basis of the spot front of the same ion, obtained on plain filter paper not impregnated with humic acid. We examined how the Rf value varies with species of metals and varieties of humic acids. The results obtained are as follows1. The Rf values of the metal ions seem to decrease as the humification of the humic acids proceeds. An exceptionally strong combination appeared, however, between Cu2+ and the humic acid of humified rice straw or peaty soil.2. The intensity order of the metal-humic acid interaction was observed in many of the samples analyzed as follows: Cu2+ > Hg2+ > Pb2+ > Ni2+ > Zn2+ > Cd2+ = Co2+ > Mn2+. In nature, however, it is considered that Hg2+ combines more strongly with humic acid than does Cu2+.3. Part of the Hg2+ was converted into unknown mobile compounds by several humic acids with advanced humification during the development of the chromatography.4. The results of a multiple regression analysis of the Rf values and the functional groups of the humic acids revealed that the carboxyl group plays an important role in the combination of humic acids with metals and that the carbonyl group and nitrogen-containing functional groups also seem to combine with metals by coordinate bonding. However, the phenolic hydroxyl group does not seem to take part in the combination in 80 percent ethanol solution. The results also showed that the affinity of metal ions for nitrogen-containing functional groups is in the order Cu2+ > Zn2+ > Ni2+ > Co2+ > Mn2+ > Pb2+, and that the affinity of metal ions toward oxygen-containing functional groups is almost the exact opposite.5. The average Rf values of metal ions are in good correlation with the overall stability constants (K0) quoted from the literature (log K0 = −6.34 Rf + 9.53, r = −0.992); therefore, the Rf values may be able to replace the stability constants of metal-humic acid complexes. We propose and describe a new paper chromatographic method for evaulating metal-humic acid interactions. In this method, a mixed standard solution of metal ions was spotted on the starting line of humic acid-impregnated filter paper and then developed with a 0.1 M acetate buffer solution (pH 5, 80 percent ethanol) containing 0.05 M Cd2+ or Mn2+. The Rf values of the metal ions were calculated on the basis of the spot front of the same ion, obtained on plain filter paper not impregnated with humic acid. We examined how the Rf value varies with species of metals and varieties of humic acids. The results obtained are as follows 1. The Rf values of the metal ions seem to decrease as the humification of the humic acids proceeds. An exceptionally strong combination appeared, however, between Cu2+ and the humic acid of humified rice straw or peaty soil. 2. The intensity order of the metal-humic acid interaction was observed in many of the samples analyzed as follows: Cu2+ > Hg2+ > Pb2+ > Ni2+ > Zn2+ > Cd2+ = Co2+ > Mn2+. In nature, however, it is considered that Hg2+ combines more strongly with humic acid than does Cu2+. 3. Part of the Hg2+ was converted into unknown mobile compounds by several humic acids with advanced humification during the development of the chromatography. 4. The results of a multiple regression analysis of the Rf values and the functional groups of the humic acids revealed that the carboxyl group plays an important role in the combination of humic acids with metals and that the carbonyl group and nitrogen-containing functional groups also seem to combine with metals by coordinate bonding. However, the phenolic hydroxyl group does not seem to take part in the combination in 80 percent ethanol solution. The results also showed that the affinity of metal ions for nitrogen-containing functional groups is in the order Cu2+ > Zn2+ > Ni2+ > Co2+ > Mn2+ > Pb2+, and that the affinity of metal ions toward oxygen-containing functional groups is almost the exact opposite. 5. The average Rf values of metal ions are in good correlation with the overall stability constants (K0) quoted from the literature (log K0 = −6.34 Rf + 9.53, r = −0.992); therefore, the Rf values may be able to replace the stability constants of metal-humic acid complexes. © Williams & Wilkins 1983. All Rights Reserved.