Metal–Metal Correlation of Biodegraded Crude Oil and Associated Economic Crops from the Eastern Dahomey Basin, Nigeria

Abstract

The presence of heavy metals in plants from oil sand deposits may reflect mineralization resulting from petroleum biodegradation. Petroleum composition and heavy metal analyses were performed using thermal desorption gas chromatography and atomic absorption spectrophotometry on oil sand and plant root samples from the same localities in the Dahomey Basin. The results from the oil sand showed mainly heavy-end hydrocarbon components, humps of unresolved complex mixtures (UCM), absences of C6-C12 hydrocarbon chains, pristane, and phytane, indicating severe biodegradation. In addition, they showed varying concentrations of vanadium (2.699–7.708 ppm), nickel (4.005–11.716 ppm), chromium (1.686–5.733 ppm), cobalt (0.953–3.223 ppm), lead (0.649–0.978 ppm), and cadmium (0.188–0.461 ppm). Furthermore, these heavy metals were present in Citrus, Theobroma Cacao, Elaeis guineensis, and Cola. The chromium, nickel, vanadium, lead, cobalt, and cadmium concentrations in the Citrus were 7.475, 4.981, 0.551, 0.001, 0.806, and 0.177 ppm, respectively. For the Theobroma Cacao, the concentrations of chromium, nickel, vanadium, lead, cobalt, and cadmium were 7.095, 16.697, 2.151, 0.023, 3.942, and 0.254 ppm. Elaeis guineensis also showed the presence of chromium (32.685 ppm), nickel (32.423 ppm), vanadium (11.983 ppm), lead (0.190 ppm), cobalt (4.425 ppm), and cadmium (0.262 ppm). The amounts of chromium, nickel, vanadium, lead, cobalt, and cadmium in the Cola were 9.687, 9.157, 0.779, 0.037, 0.695, and 0.023 ppm. The World Health Organization’s (WHO) safe and permissible limits for Cd (0.003 ppm), Cr (0.1 ppm), Ni (0.05 ppm), and Pb (0.1 ppm) in agricultural soils were all exceeded in the oil sand. The presence of these metals in the oil sands and their uptake by the plants could potentially be toxic, resulting in high mortality. The metal–metal correlation of the plant’s rootsto the oil sand indicates the nonanthropogenic origin of the heavy metals, which leads to the conclusion that their source is related to the hydrocarbon accumulation in the Afowo sand.