Characterisation and bioremediation of hydrocarbon contaminated soils: a case of Murowa Diamonds mine

Abstract

Microbial biodegradation methods of hydrocarbon contaminated soils that can occur through biodegradation, bio augmentation, bio stimulation, and phytoremediation, have gained significant interest in recent years when compared to the conventional methods. The study was designed to explore the influence of petroleum hydrocarbon contamination on physicochemical and microbial characteristics of soils as well as determining the possibility of hydrocarbon biodegradation using biostimulation. The process involved soil characterisation and modification of nitrogen and phosphorus content to stimulate naturally adapting microorganisms. Characterisation process determined that hydrocarbon contamination of Murowa soils introduced hydrophobicity to the naturally wettable sandy loam soils. Naturally adapting microbial species capable of degrading hydrocarbons identified using Direct microscopy, Gram and Melzer’s iodine staining included Pseudomonas, Bacillus, Serratia marcescens, Flavobacterium, Micrococcus, Streptomyces Staphylococcus, Penicillium and yeasts. The N: P nutrient ratio and moisture levels were identified as potential limiting factors and hence experiments focused on manipulation of N: P nutrients to stimulate the identified hydrocarbon degrading organisms (bio stimulation). Hydrocarbons were identified by solvent extraction using hexane and gas chromatography. These included decane, undecane, hexadecanal, 2-ethylcridine, octadecane and 1-iodo. Soils weighing 10kgs with hydrocarbons levels of about 265mg/kg were subjected to eight (8) treatments with seven (7) different combinations of N (6000-12000mgN): P (600- 3000mgP) concentration ranges including the control. Nitrogen The moisture was adjusted and tilling for aeration was done on a weekly basis. Changes in Total Petroleum Hydrocarbon (TPH), C: N: P ratio, microbial mass and pH were evaluated over 111 consecutive days. The optimum N:P ratio was the determined to 2:1 molar ratio in form of 6000mgN:3000mgP. TPH concentration was reduced by 73% from the initial concentration within the first 74 days. Beyond 74 days there were no significant changes in the TPH concentration and this was attributed to the presence of more complex insoluble hydrocarbons which needed more time and an additional bio surfactant to complete mineralization. The conclusion was that a combination of natural attenuation and biostimulation methods can be used to bioremediate Murowa hydrocarbon contaminated soils using the 2:1 molar ratio of what