CHARACTERISTICS AND REACTIVITY OF HUMIC ACIDS EXTRACTED FROM FOREST AND SAVANNA SOILS OF NIGERIA
CHARACTERISTICS AND REACTIVITY OF HUMIC ACIDS EXTRACTED FROM FOREST AND SAVANNA SOILS OF NIGERIA
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Date
2012-02
Authors
OLUSEGUN, AYORINDE MICHAEL
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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.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF
POSTGRADUATE STUDIES, AHMADU BELLO UNIVERSITY,
ZARIA, NIGERIA IN PARTIAL FULFILMENT FOR THE
AWARD OF DOCTOR OF PHILOSOPHY IN SOIL SCIENCE
DEPARTMENT OF SOIL SCIENCE
FACULTY OF AGRICULTURE
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA
FEBRUARY, 2012
Keywords
CHARACTERISTICS,, REACTIVITY,, HUMIC,, ACIDS,, EXTRACTED,, FROM,, FOREST,, SAVANNA,, SOILS,, NIGERIAN,