EFFECT OF SPENT ENGINE OIL AND ITS CONSTITUENT HEAVY METAL UPTAKE USING COWPEA AS A MODEL

Abstract

Disposal of Spent Engine Oil (SEO) on farmlands is one of the environmental problems creating unsatisfactory conditions for plant growth and human health. Studies have shown that SEO contain contaminants which can be absorbed by plants thus, constituting danger to the health of consumers of such plant products. This study was conducted to assess the effect of SEO on cowpea growth and toxicity. Topsoil (0-15 cm depth) was obtained from teaching and research farm, University of Ibadan. A representative grab sample of the soil and that of SEO were taken for baseline heavy metal analysis. The soil was put into perforated polythene bag to a weight of 350 g. The experiment was laid out as completely randomised design in 5 replicates. Seven treatments of SEO (A: 5 ml, B: 10 ml, C: 15 ml, D: 20 ml, E: 25 ml, F: 50 ml, and G: 75 ml) were applied to the soil while the 8th group devoid of SEO served as the control. Three cowpea seeds were dipped into each bag and watered daily. Growth parameters including plant height and leaf number were recorded weekly. After harvesting, seed weight and total plant weight were obtained. The seeds and roots were also subjected to heavy metal analysis through Atomic Absorption Spectrophotometry assessment (AAS). The grease and oil content in soil samples were determined by obtaining the absorbance on AAS at 450 nm. The data were analysed using ANOVA, while Duncan Multiple Range test was used for post-hoc comparisons. Baseline heavy metal analysis showed that only lead and cadmium were present in the SEO at 0.003 ppm and 0.462 ppm but absent in the soil. There was significant reduction in plant height (control: 50.72 cm, A: 41.79 cm, B: 39.39 cm, C: 37.36 cm, D: 33.27 cm, E: 32.13 cm, F: 26.85 cm, G: 24.33 cm) and number of leaves (control: 25.91, A: 22.54, B: 22.38, C: 21.57, D: 19.06, E: 20.65, F: 17.61, G: 17.67) in contaminated soil with increasing SEO concentration (p<0.05). There was also significant decrease (p<0.05) in the seed weight of plant grown on contaminated soil (control: 24.40 g, A: 24.00 g, B: 22.94 g, C: 16.38 g, D: 15.95 g, E: 9.66 g, F: 5.44 g, G: 5.03 g). Similar pattern was observed in the total plant weight (control: 171.60 g, A: 170.25 g, B: 167.60 g, C: 136.60 g, D: 135.00 g, E: 124.20 g, F: 62.60 g, G: 54.70 g). Plants grown on contaminated soil contained more cadmium with increasing oil concentration as manifested in its higher values of cadmium in seeds (control: 0.000 ppm, A: 0.046 ppm, B: 0.056 ppm, C: 0.057 ppm, D: 0.078 ppm, E: 0.081 ppm, F: 0.102 ppm, G: 0.119 ppm) and roots (control: 0.000 ppm, A: 0.063 ppm, B: 0.070 ppm, C: 0.079 ppm, D: 0.091 ppm, E: 0.113 ppm, F: 0.163 ppm, G: 0.263 ppm) from contaminated soil. Also, the concentration of grease and oil in the polluted soil increased with increasing SEO (control: 1.81 mg/l, A: 8.13 mg/l, B: 8.92 mg/l, C: 10.30 mg/l, D: 11.36 mg/l, E: 11.69 mg/l, F: 14.88 mg/l, G: 17.47 mg/l). Cowpea grown on polluted soil with spent oil had stunted growth, higher concentration of cadmium as well as reduced crop yield and plant weight. Indiscriminate disposal of spent oil should be controlled.