SORPTION OF PHENANTHRENE BY NATURAL ORGANIC MATTER FRACTIONS IN SELECTED ROCKS AND SOILS FROM SOUTHWESTERN NIGERIA

Abstract

Natural Organic Matter (NOM) fractions in rocks, sediments and soils are known to have good sorptive potentials for frequently occurring Hydrophobic Organic Contaminants (HOCs) such as phenanthrene. Despite widespread occurrence of organic matter-rich rocks, sediments and soils in Nigeria, there is limited information on the sorptive potentials of their NOM fractions for HOCs which may be useful in remediation and clean-up design. This study is aimed at investigating the sorption of phenanthrene by NOM fractions in selected rocks and soils from southwestern Nigeria. Coal and mudstone, carbonaceous shale and sandy soils (500 g each) were purposively collected from Ute, Auchi and Lagos respectively. The rocks and soils (250 g each) were demineralised with 1M HCl and mixture of 1M HCl/10%HF (1:3) successively. Lipid and bound-lipid were extracted via solvent extraction and saponification methods respectively. Lignin was extracted from the soils with dioxane-acid hydrolysis method. Organic carbon, hydrogen and oxygen contents of demineralised, lipid free, bound-lipid free and lignin free (NOM fractions) were determined using bulk method. Organic matter type in the rocks and soils were determined from atomic ratios of the elements. Sorption parameters (nonlinearity and organic carbon normalised sorption coefficients, KOC) of the bulk samples (control) and NOM fractions were determined from sorption experiments using phenanthrene concentrations ranging from 10 to 1250 ppb. The skeletal carbons of NOM fractions were determined using 13C Cross Polarisation Magic Angle SpinningNuclear Magnetic Resonance. The ranges of organic carbon and oxygen contents of NOM fractions in the rocks were respectively: 53.0-70.8% and 11.6-23.1% (demineralised), 54.4-70.2% and 12.8-26.1% (lipid free), 33.4-51.6% and 28.3-38.6% (bound-lipid free). Those of NOM fractions in the soils were correspondingly: 27.8-63.1% and 7.0-21.4% (demineralised), 27.2-65.2% and 16.4-22.1% (lipid free), 9.2- 54.7% and 14.7-29.3% (bound-lipid free), 15.2-65.3% and 11.2-17.4% (lignin free). The bound-lipid free has the lowest organic carbon and highest oxygen. The H/C and O/C ratios in the NOM fractions were: 0.6-1.7, 0.6-1.3, 0.7-2.3, 0.5-1.3 and 0.1-0.5, 0.1-0.6, 0.3-1.2, 0.2-0.6 respectively, indicating increasing degree of maturity in NOM fractions with progressive extraction except in bound-lipid free. Higher O/C ratio in bound-lipid free indicates high UNIVERSITY OF IBADAN LIBRARY iii polarity. The plot of H/C and O/C ratios indicated that the rocks contained type III kerogen while the soils contained type II/III kerogen. Phenanthrene sorption isotherms for bulk samples and NOM fractions were nonlinear (n<1). The nonlinearity increases with degree of maturity. The KOC range in bulk rocks, soils, demineralised, lipid free, bound-lipid free and lignin free were: 3305-21241, 751-3133, 570-2883, 2663-14084, 651-28751, 10459-63217 mL/g respectively, indicating higher sorption capacity in rocks than soils, and increasing sorption capacity in NOM fractions except in bound-lipid free. Single point KOC decreased with increasing concentration of phenanthrene in bulk samples and NOM fractions. The skeletal carbons in the NOM fractions showed that aliphatic carbon peak (0-93 ppm) decreased while aromatic carbon peak (93-165 ppm) increased, which further confirmed the degree of maturity. The sorption of phenanthrene by natural organic matter fractions in the rocks and soils depended strongly on the degree of maturity and polarity. Keywords: Natural phenanthrene, Sorption capacity, Natural organic matter, Degree of maturity. Word count: 497