EVALUATION OF SELECTED REMEDIATION METHODS FOR THE TREATMENT OF HYDROCARBON CONTAMINATED SOIL IN OLOGBO, NIGERIA

Abstract

In this study, oil exploitation field at Ologbo, Edo state was assessed in order to determine chemicals of concern at the site. The research work was carried out by first determining the extent of pollution in the oil field; and then treating hydrocarbon contaminated soil collected from the site in laboratory and field pilot study. The three methods of treatment applied are; phytoremediation, land farming and chemico-biological stabilization. This was to ascertain the best remediation method for total petroleum hydrocarbon (TPH) treatment at the oil field. The site assessment was carried out by measuring 1 km2 of the land and gridded at 100 m interval using ARCGIS 9.8 and Google Earth Imagery. The exact locations at the field were established using GARMIN handheld GPS receiver while hand auger was used to obtain soil samples at the grid intersections at depths of 15 cm, 30 cm and 60 cm respectively in accordance with United State Environmental Protection Agency new test methods. Using EPA/530/UST-90/010 procedure, crude oil detection test was carried out. Samples were taken to Laboratory for TPH quantification. The results obtained were exported into ARCGIS 9.8 software where it was used to generate TPH pollution assessment map of the study area. The screen house study was carried out at the department of crop science, university of Benin. Phytoremediation treatment involved 10 kg of contaminated soil samples (in 2%, 4% and 6% concentrations) + 5 kg of organic amendment (cow dung) + 3No Panicum Maximum (guinea grass). Land farming treatment was carried out using 10 kg of contaminated soil (in 2%, 4% and 6% concentrations) + 2.5 kg NPK fertilizer + 2.5 kg organic amendment (cow dung) while the chemico-biological stabilization involved 10 kg of spiked soil samples (in 2%, 4% and 6% concentrations) + 2.5 kg of CaO + 2.5 kg organic amendment (palm kernel husk) + 3No humidicola grass (Brachiaria Humidicola). For each treatment procedure and concentration, there was control samples (contaminated without treatment) and a total of 36 experimental setups were used since each concentration level was replicated. The field pilot study was carried out in the same garden with same procedure but with larger samples in twelve plots with sub-plot attached. The pollution assessment and interpolated maps revealed higher concentration of TPH at the North east location of the study area at 15 cm depth but reduced with increase in depth. The screen house and pilot study results revealed that after 150 days of treatment, TPH had loss more than 80% of its initial concentration while the total heterotrophic bacteria count increased from 7.06E-01 to 9.88E+13. Mean plot performance revealed that land farming treatment was the best of the three treatment methods (except at 6% concentration where phytoremediation was the best) and pseudo first order was the most suitable kinetic model for land farming kinetic reactions. The significant contributions of each input variables associated in the land farming treatment process was analysed using Design Expert 7 software; the result revealed that turning rate with 82.79% had the highest contribution. pH had 4.36%, moisture content had 0.48% while mass of substrate had 0.046% contribution respectively. The numerical optimization performed to ascertain the desirability of the overall model revealed that with initial contamination concentration of 5,000 mg/kg; turning rate of 5.0, pH of 6.01, moisture content of 10% and substrate mass of 1.00 kg; TPH value reduction of 635.907 mg/kg with 98.60% reliability will be achieved. Land farming procedure should be practiced in TPH contaminated sites so as to promote clean environment since there is increasing awareness of monitoring and remediation of subsoil in exploration fields in Nigeria.