Abstract

Annals of Environmental Science
Volume 1, February 2007, Pages 23-34

Chemical Characteristics of Humic Acids in Relation to Lead, Copper and Cadmium Levels in Contaminated Soils from South West Nigeria

I.M. Adekunle, T.A. Arowolo, N.P. Ndahi, B. Bello and D.A. Owolabi, Department of Environmental Management and Toxicology, University of Agriculture Abeokuta, and Department of Chemistry, University of Maiduguri, Nigeria.

Received July 1, 2006; in final form December 27, 2006; Accepted January 2, 2007

Humic acids (HAs) extracted from topsoils obtained from the highways, motor parks and auto-repair work-shops in Lagos, Ibadan and Abeokuta, Nigeria were characterized by infrared spectroscopy, titration and UV-visible spectroscopy for reactive groups. The rel-ative abundance of metals as determined by atomic absorption spectrophotometry decreased in the order Pb > Cu > Cd. Concentrations of soil total Cd (1.7 ± 0.2 to 1.9 ± 0.2 mg/kg), Cu (184 ± 25 to 360 ± 31 mg/kg) and Pb (1039 ± 123 to 1524 ± 88 mg/kg) exceeded international reference values for uncontam-inated soils. Parametric and nonparametric correla-tions of the metal concentrations with humic acid chemical characteristics gave both negative and positive coefficients, some of which were significant at p ≤ 0.05. Competitive trace metal complexation reactions with an equimolar solution of the cations revealed that the HAs bound more Pb (0.911 mmol/g) than Cu (0.779 mmol/g) or Cd (0.237 mmol/g). The oxygen-containing groups (carboxyl and phenol), the chromophores that absorbed in the UV-visible region and the hydrophobic nature (E4/E6 ratios from 4.4 ± 0.2 to 5.7 ± 0.2) of the humic acids affect soil metal sorption.