Bioremediation of a Crude Oil Polluted Tropical Mangrove Environment

Abstract

A combination of bioaugumentation with hydrocarbon utilizing indigenous bacteria, biostimulation with agricultural fertilizer (NPK 15:15:15) and tilling were employed as remedial options for 20 weeks in a crude oil polluted tidal plain dominated by mangrove ( Rhizophora, Laguncularia and Avicenia) vegetation. Soil moisture rose from 48.5% prior to remediation to 60.48% four weeks after treatment and dropped to 48.22% after remediation. Other physicochemical parameters of soil such as total hydrocarbon (1.22 to 0.20ppm), organic carbon (18.14 to 8.85%) and carbon/nitrogen ratio (91 to 25) dropped during the remediation period. Total Nitrogen (0.20 to 0.36%) increased during the remediation period. The total heterotrophic bacteria (THB) count increased in treatment options A (biostimulation, bioaugumentation and tilling), B (bioaugumentation and tilling) and D (biostimulation and tilling). The THB count of option C1 (double control), C2 (control) and E (tilling alone) were fairly constant throughout the remediation period. There was however an increase in the hydrocarbon utilizing bacterial counts for all treatment options throughout the remediation period. This increase was greater in option A, B and D. Option A (84%) and option D (82%) recorded higher levels of hydrocarbon loss (P> 0.01) than the other four options B (76%), C1 (36%), C2 (65%) and E (63%). Option B also recorded a significantly higher level of hydrocarbon loss (P>0.01) than C1 , C2 and E while that of C1 was significantly lower than C2 and E. The addition of limiting nutrients (biostimulation) with tilling (option D) is a preferred remedial option for a crude oil polluted soil in a tropical mangrove environment @ JASEM In the Niger Delta, increasing petroleum exploration has led to wide spread contamination of most of its creeks, swamps, soils and groundwater (Okpokwasili and Odokuma, 1990, Odokuma and Ibor, 2002). These contaminants present several environmental problems (Vandermeulen and Lee, 1986). They may affect physiological processes, population and behavioral profile of organisms (Teh and Lee, 1974). Studies have shown that the environmental transformations of the organic chemicals in terrestrial and aquatic systems are induced by physical, chemical and microbial factors within the system (Plimmer, 1978). Environmental factors such as increased salinity and water logging, slow down considerably the recovery rates of oil contaminated salt water ecosystems. Biodegradation rates have been observed to decrease with an increase in salinity (Okpokwasili and Odokuma, 1990). Mangroves possess air pores also called lenticels situated in aerial roots, which at low tide permit diffusion of oxygen. Crude oil from a spill could blanket the pores directly or float freely in water during high tide until it encounters a root and sticks to it. Microbial degradation (biodegradation) is a complete process facilitated by appropriate ambient and seasonal conditions, the presence of the optimum concentration of the right type of nutrients and the composition of the indigenous microbial community (Leahy and Colwell, 1990). Bioremediation involves the use of microorganisms to remove pollutants (Lee et. al. , 1993). Bioremediation of crude oil polluted soil is becoming an increasingly important remedial option. Bioremediation has a great potential for destroying environmental pollutants (Song et. al ., 1990). The use of inexpensive equipment, environmentally friendly nature and simplicity of the process are some of its advantages over other remedial alternatives such as physical and chemical treatments. Biostimulation and bioaugumentation either in-situ or ex-situ are various modifications of bioremediation (Lee et al ., 1993). Bioaugumentation involves the addition of external (indigenous or exogenous) microbial populations to the waste. Biostimulation involves the addition of appropriate microbial nutrients to a waste stream (Lee et. al., 1993). The objective is to stimulate the indigenous microbial flora of the waste to bring about its degradation. The biostimulation (nutrient enhancement of microbial organic waste break down) option, is in it's infancy in Nigeria . The objective of this study was to evaluate a combination of options bioaugumentation with indigenous hydrocarbon utilizing bacteria, biostimulation, with an agricultural fertilizer (NPK 15:15:15 ) and tilling for the remediation of a crude oil polluted mangrove swamp