CHARACTERISTICS AND REACTIVITY OF HUMIC ACIDS EXTRACTED FROM FOREST AND SAVANNA SOILS OF NIGERIA

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CHARACTERISTICS AND REACTIVITY OF HUMIC ACIDS EXTRACTED FROM FOREST AND SAVANNA SOILS OF NIGERIA

ABSTRACT

The study was carried out to determine the characteristics and reactivity of humic acids extracted from soils sampled from three ecological zones of Nigeria: Forest, Southern Guinea savanna and Northern Guinea savanna. Surface soil samples were obtained from undisturbed (virgin) plots and from fallowed plots for not less than twenty five years. Soil samples were analyzed for selected physico-chemical properties. Humic acid was extracted from each soil sample under atmospheric N2 atmosphere. Chemical properties and reactivity of the extracted humic acid were investigated by chemical analysis and infra-red spectroscopic methods. Attempts were also made to examine the effect of environmental gradients on the characteristics and reactivity of humic substances extracted from the soil samples. Soil acidity increased from forest to Northern Guinea savanna. Metal bound and water-soluble carbon, nitrogen content and available P all decreased from Forest to Northern Guinea savanna. Humification ratio (HR) and humification degree (HD) of the extracted humic substances increased from Forest to Northern Guinea savanna, while colour quotient (E4/E6 ratio) decreased in the same direction. Analysis of some functional groups in the humic acid indicated that phenolic group was significantly higher than carboxylic group. Total acidity of the humic substances was significantly lower for soils from the Northern Guinea savanna than the other ecological zones. The same trend was observed for the concentration of phenolic and carboxylic groups in the humic substances (HS). Reactivity of the HS was investigated by adsorption of Cu, Zn, Pb and Cd added to HS at the pH range of 4-6. In almost all cases, metal adsorption increased with increasing pH and increasing concentration of the metal adsorptive. Except for Cu adsorption at pH 6 which exhibited the H-isotherm curve, the adsorption isotherm curves of the other metals were either the S-type or L-type. Using the Langmuir equation to model the adsorption isotherm of the metal was generally unsuccessful probably because of the heterogeneity of the adsorption sites in contrast to the homogeneity of adsorption sites assumed in the derivation of the Langmuir equation. However, the Freundlich isotherm of the form q = KC n where ‘q’ was the metal sorbed, ‘k’ and ‘n’ adjustable coefficient and C, the concentration of the added metal in solution successfully described the adsorption pattern of the metal with coefficient of determination (R2) ranging from 0.59 to 0.99 or 56% to 99%. Variations in the adsorption of Cu, Zn, Pb and Cd across the three ecological zones were accounted for by the ratio of humic acid (HA) to fulvic acid (FA), humification ratio and the concentration of phenolic groups and total acidity of the humic substances. Indeed, the ratio of HA:FA accounted for between 90 and 94% of the variation of the Freundlich ‘k’ and ‘n’ value for Zn adsorption. Based on the analysis of ‘n’ value of the Freundlich equation, interpreted to represent the heterogeneity of the adsorption sites in the HS, the result indicated that the sites for the adsorption of Cu and Zn were more variable than those of Pb and Cd. In other words, Cu and Zn could be adsorbed unto a wide range of sites (heterogeneous) in contrast to a narrow range of sites (localized) for Cd and Pb. Spectroscopic studies, using infra-red spectroscopy to examine functional groups most active in the retention of metals or ions in HS in soils showed that the carboxylic (-COOH) and the carbonyl (=C=O) groups were most active in metal complexation, This was deduced from the changes in the stretching and bending vibrations of the functional groups in the HS as inferred from infra-red absorption bands. In general, HS from the soils showed strong absorption bands at the regions of 2990 to 2860cm-1at pH range of 4 to 6, indicating the C-H vibration of aliphatic compounds, at between 1460 to 1330 cm-1 for OH and C-O stretching vibrations of phenolic group, at 1630 to 1650 cm-1due to C=C vibration of aromatic compounds most likely conjugated to C=O stretching vibrations of groups like amide, ketones, quinines and COO- symmetric stretching. Differences across zones in the concentration of functional groups of the humic acids were critical enough to accept that there will be differences in their reactivity across zones.

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