Phytoremediation of Some Tropical Soils Contaminated with Petroleum Crude Oil

Abstract

This study was undertaken in three phases to identify (phase 1), screen (phase 11) and evaluate (phase 111) plants for their phytoremediation potential. In Phase 1, 15 plant species made up of grasses and legumes namely: Paspalum. vaginatum, Cynodon.dactylon, Pueraria. phaseoloides, Centrosema. pubescens, Panicum. maximum, Schrankia. leptocarpa, Eclipta. alba (Linn.), Cyperus. haspen (Linn.), Melastromastrum. capitatum, Acreceras. zizanoides Dandy, Pteridum aquilinum (Linn), Ludwigia.decurrens Walt,Setaria longiseta P.Beauv., Physalis angulata (Linn.), and Desmodium scorpiurus Desv.were identified on sites previously polluted by crude oil spills in the Niger Delta Area of Nigeria. The first 6 species were used in phase 11 while the first four species were earmarked (rolled over) for phase 111. Responses to Questionnaire indicated that majority of residents in the selected sites/communities had lived in these areas for 10 or more years had mainly JHS/SHS education; were self employed – mainly farmers and fishers although most were unemployed in the public sector. Adverse effects of the operations of oil companies particularly oil spillage on the environment and local residents include: loss of vegetation and farmlands, soil and water body contamination, weak social and cultural institutions (disrespect by youth for elders and institutions), militancy and hostage taking among youth from the area. In phase 11, seeds of legumes among the six selected species were collected from Accra, Aburi environs and Kusi in the Eastern region of Ghana; they were scarified, cultured in growth medium and the seedlings which emerged from them were transplanted into experimental pots, each containing 2000g of either Alajo or Toje soil series. One week after transplanting, each pot was simulated with a corresponding serial crude oil concentration of 0% (control) 1 % (24ml), 3% (83ml), 5.5% (130ml) and 8% (189ml) or 10% (237ml) in three replicates. These concentrations were arrived at, using the fomular [(vol of crude = % X wt / 0.844) ml]. Similarly, three replicates of each selected grass species were planted vegetatively into 2000g of either Alajo or Toje soil series using known numbers of propagules (stolons). After one week of planting, pots were treated with the above serial crude oil concentrations using the appropriate volume indicated. Germination rates were variable and results of percentage plant Survival Difference (PSD) indicated that Paspalum vaginatum and Cynodon dactylon were highly tolerant of crude oil concentrations at 3%, 5.5% and 8% on the Toje soil series and 3%, 5.5% and 10% on the Alajo soil series as compared to Panicum maximum and where therefore selected as high contaminant tolerant species for phase 111; whereas Puraria phaseoloides and Centrosema pubescens which had - 45% & ±42% and -16% & -35%respectively, as PSD for Alajo and Toje soil series as compared to Schrankia leptocarpa, were also selected as low/poor contaminant tolerant species (at 1% & 2% - introduced in phase 111as upper contamination limit for the poor tolerant species) also, for the last stage (phase 111) of the study. For phase 111, C. dactylon, P .vaginatum, P. phaseoloides and C. pubescens were evaluated for phytoremediation potential at crude oil concentrations of 3%, 5.5% (added to determine mid-point tolerance for high contamination tolerant species) and 8% for Toje soil series, and going up to 10% for Alajo soil series for high contaminant tolerant species and 1% & 2% for low/poor tolerant species respectively, using the randomized complete block design. Results of plant biomass (fresh weight) decreased (between 0.5g/fw & 20g/fw) at low contaminant concentrations and increased (between 14g/fw & 122g/fw) at high contaminant concentrations.Values of plant enrichment were higher for Cynodon dactylon (0.9% & 4.49%) than for Paspalum vaginatum (1.2% & 3.12%) respectively for Toje and Alajo soil series. Therefore, both Cynodon dactylon and Paspalum vaginatum can be good plants species for phytoremediation having accumulated total hydrocarbons up to 33,270mg/kg at 5.5% on Toje soil series and 40,466mg/kg at 8% on Alajo soil series.