EDXRF analysis and risks assessment of potentially toxic elements in sand fraction (tailing) of Nigerian oil sands

Abstract

Elemental composition of sand fraction of Nigerian oil sand was determined with a view to assessing the human and ecological risks to the sand fractions. Soxhlet extraction of bitumen from the oil sand was done using toluene as the extracting solvent. Elemental analysis was done using energy-dispersive X-ray fluorescence spectrophotometry and risks calculated using elemental concentrations as variables. Fe had the highest mean concentration (9453 ± 1477.72 µg/g), while As had the least mean concentration (81.75 ± 17.91 µg/g) in the sand fractions. It was observed that the elemental concentrations in this study were comparatively higher than those of Nigerian oil sands and bitumen, indicating that the larger percentages of the elements were extracted into the sand fraction which is part of the tailings (including water) that will be discarded as wastes during the recovery of bitumen from the oil sands. Close inter-element clustering existed between the elements (Fe and V) and (Cu, As, Nb, Sc, Ga, Rb, Ca, Se, V, Mn, Zn, K, Ti, Ni, Sr and Zr), indicating chemical affinity and/or similar sources. Contamination factor results indicated varying degrees of contamination by the elements, while modified degree of contamination (mCd > 32) indicated an ultra-high degree of contamination of the sand fractions by the elements. Pollution load index (PLI > 1) indicated deterioration of site quality, while potential ecological risk index and modified ecological risk index results indicated very high ecological risks by the elements. Study concluded that the elemental levels were high enough to degrade the environment and cause ill-heath to humans upon long exposure if the sand fraction (tailing) is disposed indiscriminately. Therefore, regular environmental monitoring assessment should be carried out to screen uncritical disposal of tailings during the development of the natural resource.