Inhibition of Biogenic Sulphide Production and Biocorrosion of Carbon Steel by Sulphate-reducing Bacteria Using Ocimum gratissimum Essential Oil

The inhibition effects of Ocimum gratissimum essential oil (OGEO) on biogenic sulphide production and biocorrosion of carbon steel by Sulphate-reducing bacteria (SRB) were investigated in the laboratory. SRB were isolated from injection and produced water from Bonga oilfield in Nigeria. Biogenic sulphide production experiment was setup to monitor sulphide production by SRB. The effects of temperature, pH and salinity on biogenic sulphide production were determined. High sulphide production was observed at temperatures ranging between 20°C – 40°C and pH 7 - 9. Sulphide production decreased as salinity increased. Significant production of sulphide (59-168 mg/l) was detected after 72 hours of incubation of >105 cells/ml SRB consortium under study conditions and the maximum of 293 mg/l was observed after 144 hours at 30°C when pH was 7 and salinity 25 g/l. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of OGEO were determined to be the same at 0.52 mg/ml. Corrosion inhibition efficiency (IE) increased with increase in concentration of extract and decreased with increase in temperature. OGEO gave the highest IE of 69.61% at 20°C, 61.050% at 30°C and 58.01% at 40°C for the 14 days test. For the 28 days test, OGEO gave the highest IE of 71.03% at 20°C, 63.61% at 30°C and 59.41% at 40°C. OGEO+50 ppm D-tyrosine blend gave IE ranging from 89.22 - 92.82% and 84.90 – 92.08% for 14 and 28 days test respectively. Adsorption mechanism of OGEO on carbon steel obeyed the Langmuir isotherm model. Thermodynamic parameters evaluated gave negative values for ΔHads and ΔSads, suggesting that the adsorption reaction is spontaneous at low temperature. OGEO showed activities against SRB isolated from oilfield environment and could be useful for the prevention of biocorrosion of carbon steel structures in this environment.