Effects of Watering Regimes on the Intrinsic Qualities of Bioremediated Waste Engine Oil-Polluted Soil

Abstract

The present study investigated the effects of soil watering regimes on the intrinsic qualities of bioremediation of a waste engine oil-polluted soil. Five (5) kg of sun-dried top-soil was each placed into large perforated bowls and mixed thoroughly with waste engine oil (WEO) to obtain similar concentrations of 2.5% w/w oil in soil. The polluted soil was thereafter amended with 4g NPK (15:15:15) fertilizer to enhance microbial activity. Having previously determined the soil’s water holding capacity to be 215 ml/kg soil, the entire setup was divided into 6 sets according to watering regimes. One set was irrigated with 1000 ml distilled water only once a week (1PW); the other sets were irrigated twice in a week (2PW), once a month (1PM), and the other twice a month (2PM). One set was deprived of moisture throughout the duration of the study (NWT), while the control experiment was carefully irrigated daily to saturation (CTRL). The entire experimental set up was left for three months in a well ventilated screen house with inherent room temperature range (28 – 30ºC). Results of the present study showed reductions in heavy metal contents, but in differing degrees. One week after pollution (WAP) concentration of Fe was 1097.34 mg/kg, Mg was 18.4mg/kg, Cu was 5.63 mg/kg and Ni was 2.95 mg/kg. Total hydrocarbon content (THC) was 3425.63mg/kg. However, 3 months later when polluted soil was subjected to varying soil watering regimes, Fe in soil was 875.43mg/kg, Cu was 3.83 mg/kg and THC was 1095.54mg/kg respectively in the control, compared to values for Fe (687.3 mg/kg), Cu (3.83 mg/kg) and THC (445.45 mg/kg) respectively, when soil was wetted once a week. The contamination factor (CF) values presented for Fe, Mg, Cu, and Ni were all less than unity (CF < 1) an indication, that these heavy metals were remediated to levels below when soil was not yet exogenously polluted. There were reductions in polyaromatic hydrocarbon (PAH) contents of the soil. At one WAP total PAH was 923.90mg/kg as against 458.59 mg/kg 3 months later. When soil was wetted once a week, total PAH was 85.98mg/kg, 104.89 mg/kg when irrigated twice a week, 170.74 mg/kg when irrigated twice a month and 302.60mg/kg when soil receive no wetting at all. Comparatively total concentration of PAH was lowest at 1PW. Bacterial isolates of the oil polluted soil subjected to watering treatment once a week were Bacillus subtilis, Pseudomonas sp., and Serratia marcescens, whereas fungal species included Aspergillus niger, Penicillium sp. and Trichoderma sp.