Bioremediation Potential of Microorganisms in selected Hydrocarbon-contaminated soils of the Niger-Delta, Nigeria
Niger Delta, being the centre of petroleum production and process activities in Nigeria, is directly exposed to large and repeated oil spills or leaks that result in hydrocarbon pollution. As a consequence, there is a strong need to give attention toremediationtechniques such assoil replacement, encapsulation, thermal desorption, and bioremediation. This study focuseson bioremediation, especially in-situ bioremediation. This requires the establishment of indigenous microbial potential and evaluation of the limiting factors such as physicochemical properties of the soil and the nature of substrate. This study, therefore seeks to evaluate the bioremediation potentials of the microbial population in selected petroleum hydrocarbon contaminated soils in the Niger Delta area of Nigeria. Soil samples were collected from two towns, Effurun and Jesse in Delta State, as well as three towns, Eleme, Bori and Pete in Rivers State. Conventional methods of isolation of microorganisms using serial dilution and plating on nutrient agar and potato dextrose agar were adopted to evaluate the population densities of the total heterotrophic bacteria and fungi as well as total hydrocarbon utilizing bacteria and fungi in the soil samples. The abilities of undefined consortium in the various sites were studied and the isolates were screened for selection using specific test on different hydrocarbon sources. The abilities of the selected isolates were tested on mineral salts medium (MSM) spiked with 1-2 % (V/V) hexadecane and 2- methylnaphthalene. In addition, the ability of a consortium of selected isolates to biodegrade mixture of polyaromantic compounds such as 1-methylnaphthalene (1MN), 2-methylnapthalene (2- MN) and 1, 3-dimethylnaphthalene (1, 3-DMN), and aliphatic compounds such as hexadecane (HX) and heneicosane (HN) was studied. The heterotrophic bacterial counts were found to range from 1.8 x 105- 2.85 x 106cfu g-1 while the total hydrocarbon utilizing bacterial counts ranged from 3.0 x 103 - 5.75 x 104 cfu g-1. The heterotrophic fungal counts ranged from 5.5 x 107 - 5.57 x 108cfu g-1 while the total hydrocarbon utilizing fungi ranged from 1.8 x 105- 2.18 x 106cfu g-1. The physicochemical properties such as the pH, organic matter, moisture content, water holding capacity and nutrient level were evaluated experimentally, and the result indicates that the total organic matter ranged from 1.80 - 21.30 %. The pH in CaCl2 (pH(C)) solutionof the soil samples was in the range of 4.28 - 6.28, while that of the pHin water (pH (W)) was within the range of 5.4 - 7.3. The concentration of nitrate ions (NO3 -1) in all the soil samples varied from 27.2– 218.0 mg kg-1 and that of sulphate ions (SO4 -2) varied from 9.71- 327.0 mg kg-1. The moisture content of contaminated soils ranged from 11.30- 39.30 %(w⁄w) while that of uncontaminated soils was in the range of 9.30- 26.30 % (w⁄w). The study showed that the obtained microbiological and physicochemical parameters of the soils are consistent with the conditions in an environment that encouraged microbial activity. The consortia of the microbial in the different soil samples studied showed different degrees of capabilities on the crude oil, as these removed high number of components of the crude oil and thus make the areas potentially suitable for in-situ bioremediation. The selected isolates degraded hexadecane and 2-methylnaphthalene respectively ranging from 89.98- 97.94 % and 84.27- 93.48 % at the end of 21 days. At the end of the 120 hours the extent of degradation of the various components of mixture, 2-MN, 1MN, 1, 3-DMN, HX and HN were 86.54, 81.85, 81.71, 59.65 and 73.61 % respectively by the formulated consortium. The kinetic parameters such as the associated saturation constant (43.8- 602.0) and maximum specific substrate consumption rate (0.564-3.674) indicated that the consortium grew on the constituent of the substrate. A suitable mathematical model was developed to predict the fate of multicomponent substrate in a porous media, especially soil that incorporated appropriate biodegradation kinetics that is adjudged to describe the consumption rate of the substrates. The Equation was solved using Finite Volume Method and simulated in a Matlab environment. The model was validated using a column experiment, and it was observed that the model appropriately described the degradation process and predicted extent of degradation in multicomponent substrate systems. Therefore, the formulated consortium has the ability to substantially degrade most of the components of the hydrocarbon mixture though at different rates.