PHYTOREMEDIATION OF CRUDE OIL-CONTAMINATED SOIL IN EKPAN, DELTA STATE, NIGERIA

Abstract

Crude oil spillage poses a threat to public health in the Niger Delta as a result of gross pollution of the environment. Physical and chemical remediation methods are expensive for addressing the situation. Several plants have been used in previous studies for phytoremediation of crude oil-contaminated soils. This study was therefore designed to identify native plants with phytoremediation potential for the degradation of Total Petroleum Hydrocarbon (TPH) in crude oil-contaminated soils in Ekpan, Delta State. Composite soil samples collected from an uncontaminated area were deliberately contaminated with Bonny light crude oil to a level of 5 g/kg (w/w) resulting in 8.510.0±50.5 mg TPH/kg soil. Four native plants (Brachiaria ruziziensis, Cyperus esculentus, Kyllinga bulbosa and Telfairia occidentalis) were purposively affected and screened for their tolerance to crude oil-contamination for three weeks. Three of the selected plants (excluding B. ruziziensis) which showed tolerance to crude oil were grown for eight weeks on the contaminated soil amended with six levels (0, 1.5, 2.5, 5.0, 10.10 and 20.0 tons/hectare) of organic manure (OM), which was added to improve soil fertility under greenhouse conditions (temperature 26.7±1.2⁰ humidity, 74.1±2.3%). Plant height (cm) was monitored weekly using a metre rule. Plant weight (g) was determined monthly using a mettler balance. Soil samples collected monthly from the pots were analysed for TPH using gas chromatography (Varian 3400). Field experiment was done in a randomised block design with for treatment groups: (A) contaminated soil amended with 10 tons/ha OM; (B) contaminated soil without OM; (C) uncontaminated soil without OM (control) and (D) an unplanted contaminated soil without OM (control). Kyllinga bulbosa, the plant with the best phytommediation potential in the greenhouse experiment was grown on a plot contaminated with 0.8 liter Bonny light crude oil/m² (8,546.0±20.0 mg TPH/kg soil). All experiments were conducted in triplicates and randomised with controls. Total heterotrophic soil bacteria were counted at eight weeks using standard bacteriological methods. Data were analysed using descriptive statistics and ANOVA at 0.05 level of significance. The three crude oil tolerant plants used in the greenhouse experimenti at eight weeks and at 10 tons/ha reduced TPH in the contaminated soil: Kyllinga 97.0%; Cyperus 92.8% and Telfaira 91.0%. Plant dry weight progessively increased (Kyllinga: 2.4±0.1 to 7.9±1.3g: Cyperus 2.1±0.1 to 7.4 ±1.6g: Telfairia 20.2±0.5 to 48.1±2.3 g) with increasing rates of OM application. In the field experiment, after eight weeks, significant reductions in soil TPH were recorded in two treatment groups: A (93.6%) and B (88.8%) compared with the controls, C (0.0%) and D (35.8%). The composition of the composite soil showed 84.3±0.6% and 9.3±0.3% Clay and 6.4±0.2% Silt. Total heterotrophic bacterial counts (cfu/g) were significantly higher in the planted OM amended soil (1.92±4.25x10¹¹) compared to the unplanted OM untreated soil (1.70 ±0.01 x 10⁵). The highest concentration of total petroleum hydrocarbon (97%) removal from the contaminated soil was obtained with Kyllinga bulbosa plants. This result was enhanced by the application of organic manure and the presence of soil bacteria. Kyllinga is therefore recommended for phytoremediation of crude oil-contaminated soils.