Bioaugmentation Of Crude-oil Contaminated Soil By Immobilized Microorganisms

Large volume of petroleum hydrocarbons coming from a variety of sources,e.g.human activities in the oil exploration,production,transportation and storage of petroleum,have been emitted into the environment,.These contaminants mainly accumulate in soil and pose serious risks to ecological environment and human health due to their toxic properties.As an economical and eco-friendly approach,there is growing public concern on bioremediation.However,bioremediation of petroleum-contaminated soil often encounter various problems,such as poor survival rates of inoculant,serious cell leakages of immobilized bacteria,and low efficiency of removal.Therefore,this study prepared a biomimetic hybrid material(Silica-IC).The characteristics,viability and activity of Silica-IC were then studied.Moreover,we used a constructed bacterial consortium immobilized on sawdust through a series of microcosm experiments to identify the main factors affecting decontamination and the optimal conditions for the bioremediation of heavily petroleum-contaminated acid soil.We expect this study will provide theoretical references and technical supports for bioremediation of petroleum-contaminated soil.The major accomplishments are as follows:(1)Silica-IC was prepared through pre-immobilization of bacterial cells on sawdust following the silica coating by vapor deposition.The silica layer covering on the bacterial agent could significantly reduce cell leakage from sawdust without losing reproductive capacity.Moreover,the retention rate of Silica-IC was higher than that of Phy-IC(simple immobilization by adsorption).When Silica-IC was stored at 4 °C and 30 °C for 28 days,the number of viable cells did not significantly decrease,even at 30 °C.This suggested that Silica-IC had a good ability to keep cells and could maintain long-term stability.(2)Degradation mechanism of Phenanthrene(Phe)by immobilized cells was studied.Results showed that degradation of Phe by Silica-IC was mainly controlled by an adsorption-degradation approach.Phe was employed as a model compound for their prevailing presence in the petroleum-contaminated environment,and Sphingomonas sp.GY2 B as the degrading bacterium.Phe degradation kinetics of immobilized cells in liquid medium was well fitted an adsorption-degradation model,while the Phe remove process by free cells was more consistent with first-order kinetic model.When immobilized cells were inoculated into slurry or soil matrix,a variety of pathways were existed,an adsorption-degradation approach may be dominant.Mass transfer of Phe from soil to carriers with degraders was accelerated,and with direct biodegradation of the carrier-associated Phe by immobilized cells,there would be more sites for Phe adsorption,and then successive biodegradation.Therefore,the distance between Phe and degraders was reduced and the mass-transfer limitation was further overcome,with enhancing the removal of Phe from soil.(3)The growth curves and degradation characteristics of Silica-IC in slurry and in soil were studied.Results showed that the degradation of Silica-IC toward Phe was efficient both in liquid and soil.After co-culturing for 48 h,Phe removal percentage of Silica-IC in soil was up to 93.4%.It was observed that after a short adjustment period,the immobilized cells were rapidly multiplied and maintained high levels of CFU.Therefore,the Silica-IC in soil showed benefit of survival and growth.(4)Bioremediation of heavily petroleum-contaminated acid soil by immobilized bacterial consortium was investigated.Results showed that repeated applications of incremental doses in a good microenvironment(proper pH,salinity and available substrates)were efficient to bioremediate the contaminated sites.An orthogonal experimental design(OED,L9(34))was employed to evaluate the effects of the main factors on the soil bioremediation.Results indicated the most significant negative factor was the inorganic fertilizer.The size and addition protocol of inoculant were positive factors,while the amounts of surfactants had a minimal impact.The addition of inorganic fertilizer could have caused excessive soil salinity and reduced the bioremediation effectiveness.The removal of total petroleum hydrocarbons(TPH)reached the highest level(60%)after 60 days under the optimal conditions of 2%(w/w)of the immobilized consortium,delivered in three increments and 6%(v/w)of SDS/TritonX-100(a molar ratio of 2:1).