Chemical Evaluation and Remediation of Petroleum Sludge Impacted Soils from Itsekiri Communities Around Warri Refinery in Delta State, Nigeria

ABSTRACT

Petroleum sludge samples from Warri Refinery and Petrochemical Company (WRPC), Delta State, Nigeria and soil samples from five selected oil-impacted communities (Ubeji- 500 m, Ekpan- 1 .5 km, Aja-Etan - 2.5 km, Ifie-Kporo - 3.0 km, ljala-Ikenren - 3.8 km from WRPC) around WRPC were collected and analyzed to appraise the levels of total petroleum hydrocarbons (TPH), Polycyclic aromatic hydrocarbons (PAH), and heavy metals (Cr, Cd, Pb, Ni, Cu, Co, Mn, Fe and Zn) caused by the petroleum sludge, using gas chromatography-flame ionization detector (GC-FID), gas chromatography-mass spectrometry (GC-MS) and Atomic Absorption Spectrophotometer (AAS) respectively. The soil samples coded A, 8, C, D and E respectively were subjected to bioremediation using Moringa Oleifera seed cake (MOSC) as organic fertilizer in the ratio of 3: I and monitored for contaminants reduction for 90 days at 30 days interval. Results obtained were compared with that of the control soil sample collected 8.5 km away from WRPC. This was further confirmed using Soil Quality Standards (SQSs) criteria for TPH and PAH indices while Enrichment Factor (EF) and Geoaccumulation Index (lgeo) were employed for heavy metals indicators. The risk assessment of the soil quality showed that the level of contamination was a function of proximity of each site to the refinery. The TPH and PAH contents from sites A to E for day I were (293846.00, 4255.87), ( 1 23874.00, 2942.04), (9729 1 .00, 181 8.38), (87561.23, 928.22), (48063.61, 1 89.93) (mg/kg) as against the control (651. I 8, 68.06) with the sludge sample having (68661 5.6, 1 3648.33) (mg/kg) respectively. After treatment of soils with the agro-waste from Moringa seed for 90 days, the TPH and PAH contents from site A to E reduced to (652.58, 5.66), (520.67, 3.09), (254.32, 4.81 ), (68.80, 0.18) and (6 1 .29, 1 .64) (mg/kg) while the control site exhibited (50.62, 0.46) (mg/kg) respectively. The SQSs conducted at day I indicated that all the sites were seriously contaminated with TPH and PAH while the control site also showed a slight contamination. SQSs conducted at day 30 revealed that only the control site was free from the traces of contamination recorded at day I. SQSs after 60 days indicated that sites D and E were no more contaminated but the SQSs conducted at 90 days revealed that all the sites were decontaminated of petroleum hydrocarbon. The results of the heavy metals obtained at day I showed that with the exception of Fe, the sludge sample had the highest concentrations of metals while Fe was highest in site A, and a stepwise decrease in metal contents was observed from A to E except for sites D and E in the case of Cr and Co. The computed EF at day 1 disclosed that most of the values exceeded I indicating a significant anthropogenic effect. The Igeo confirmed that sediment quality varied from moderately to strongly contaminated except Co and Fe in sites D to E. At the end of the study, EF revealed no to minor enrichment except for Cr and Pb. This was further confirmed by the lgeo which prompted further research on varying the proportion of soil to MOSC in the ratio of 3:2 and monitored it for another 30 days. Addition of higher mix ratio did not produce a corresponding increase in the removal of residual metals as expected. Further, the physicochemical properties (pH, conductivity, moisture content, organic carbon, organic matter, available phosphorous, total nitrogen, oil and grease, etc) of the samples were also determined accordingly. The initial pH values revealed acidic soil environment (5.31 - 5.54) unlike that of the control (7.81) which changed at 90 days of treatment to (6.50 - 6.63) with that of the control (7.95). In order to quantitatively analyze and confirm the relationship among soil physicochemical parameters and heavy metals content of day I, Pearson Correlation Analysis was applied to the dataset. A strong positive significant relationship (r =or> 0.9, p < 0.0 1) and a strong negative correlation (r = -0.9, p < 0.01) were seen among different parameters. However, most of the heavy metals were negatively correlated with pH which shows that increasing pH could reduce the metal availability. More so, moderately positive correlation was seen among most of the variables in the correlation matrix (r =or> 0.8, p < 0.05). A positive correlation among the metals in soil samples could indicate common sources of the metals which could be related to known geochemical associations among them. From the results, biostimulation through the application of organic amendments like MOSC into the contaminated soil has not only proven effective to accelerate bioremediation of petroleum sludge impacted soils, but also seen to increase the soil fertility. This confirms that biostimulation is a promising tool that can salvage petroleum sludge contaminated sites.