Study On Transport Mechanism And In-situ Bioremediation Of Aniline-degrading Bacteria In Heterogeneous Aquifer

Aniline is an essential chemical raw material in industrial production and poses carcinogenic,teratogenic and mutagenic effects,which causes serious environmental pollution.In-situ bioremediation has good application and development prospect because of the advantages of remarkable efficacy,low energy consumption and environmental friendliness.However,the aquifer often presents heterogeneous characteristics of cross-distribution of different permeability layers,which affects the migration of the functional microorganisms and the remediation effect of groundwater pollution.At present,there is a lack of systematic research on the mechanism of the influence of the heterogeneous layer,the interface of the low-permeability lens,the environmental conditions and co-existing pollutants on the bacterial transport.Therefore,it is of great significance to study the migration mechanism of aniline-degrading bacteria in heterogeneous aquifers and the effect of in-situ bioremediation for aniline-contaminated groundwater.The study on the migration mechanism of aniline-degrading bacteria in heterogeneous aquifers is also of significance to other similar bacteria.Aniline-degrading bacteria Pseudomonas migulae AN-1 was selected as the target microorganism,which is a kind of microorganisms widely existing in nature.In this paper,the migration mechanism of AN-1 was systematically studied under the influence of different environmental factors.Firstly,the chemotaxis of AN-1 to aniline during the bacterial transport in aquifer was studied.The adsorption and desorption characteristics of AN-1 with different media particle sizes were fitted with different types of isothermal adsorption models.The influence range and mechanism of environmental factors on bacterial transport were analyzed when aniline co-existed.The migration rule and influencing factors of AN-1 in heterogeneous aquifer containing low-permeability lens were studied;using green fluorescent protein labeling combined with light transmission technology to monitor the transport and distribution of AN-1 in real time.Finally,the feasibility of using AN-1 to simulate aniline-contaminated heterogeneous aquifers was evaluated.The main results obtained by the paper are as follows:1.The chemotaxis of AN-1 to aniline during the bacterial transport in aquifer(1)AN-1 can effectively degrade aniline and was chemotactic to aniline.Chemotaxis was related to aniline concentration.Under the condition of low aniline concentration(≤100 mg/L),AN-1 showed a positive chemotaxis towards aniline;when the concentration of aniline exceeded the range of 250 mg/L,the active migration rate of AN-1 decreased.(2)The chemotaxis of AN-1 to aniline was affected by temperature and p H.The chemotaxis of AN-1 was inhibited at 6℃;when the temperature increased,AN-1 can actively move towards aniline.The chemotaxis of AN-1 to aniline had a certain p H tolerance(6-8 section)and it was inhibited at p H value of 5 and 9 respectively.2.Transport capacity and influencing factors of AN-1 in different media(1)There was a hysteresis phenomenon in the desorption of AN-1 on adsorbed particles.The adsorption and desorption process of AN-1 on media with different particle sizes can be described by a linear equation,and the hysteresis coefficient(HI)values were all greater than 0.HI increased with decreasing media particle size,there was irreversible adsorption in the adsorption process.(2)The coexistence of aniline reduced the transport capacity of AN-1.When aniline coexisted,the transport rate and the breakthrough rate of AN-1 decreased,the retention of AN-1 and deposition rate coefficient(k_d)increased,which was caused by the chemotaxis of AN-1 to aniline near the surface of the media.The degree of influence of aniline increased with the decrease of the flow rate and the increase of ionic strength.3.Migration mechanism of AN-1 in heterogeneous aquifers with low-permeability lens(1)When a low-permeability lens was present,the degrading bacteria appeared to flow around,and some bacteria gathered on the surface of the low-permeability lens.The increase of flow rate,leading to the increase of fluid shear force,reduced the accumulation of the bacteria on the surface of the low-permeability lens and promoted AN-1 to flow around.The higher the flow rate,the smaller the maximum biomass at the interface of the low-permeability lens.(2)When the permeability ratio(R)was the same order of magnitude,AN-1 can enter the lens.When the range of R was larger by an order of magnitude,the flow-around and the adsorption and the filtration of the surface of the low-permeability lens played an important role and the biomass of bacteria entering the lens decreased.When R was greater than two orders of magnitude,the bacteria could not enter the low-permeability lens.(3)When aniline co-existed,the biomass of the adherent AN-1 increased.In the presence of aniline,the growth of AN-1 was promoted and the biofilm was formed,resulting in the relative amount of adherent AN-1 increased by 35 times.4.Remediation effect of AN-1 on aniline-contaminated heterogeneous aquifer(1)In the case of continuous pollution,AN-1 can effectively remediate aniline contamination in the background media and lens L1(R=10).The recovery rate of 150mg/L aniline was 95.16%.(2)When the source was exhausted,low-permeability lens L2(R=250)exhibited back diffusion.AN-1 aggregated around the low-permeability lens L2 under the blocking effect of low-permeability lens and the chemotaxis of AN-1 to aniline,and aniline can be degraded continuously.The main innovations of this paper are as follows:(1)Using green fluorescent protein labeling and light transmission technology,the transport and retention rules of degrading bacteria in heterogeneous media with low-permeability lens were revealed and the transport path of degrading bacteria was visually displayed.In the high-permeability region,the main mechanism of bacterial transport was convection-dispersion.In the low-permeability lens,in addition to the hydrodynamic effect,the bacterial transport included the retarding effect inside the low-permeability media and the adsorption and filtration of the surface of the lens.When R increased to 350,the bacteria accumulated on the surface of the low-permeability lens and could not enter the lens.These findings provide a theoretical basis for predicting the migration of functional microorganisms in heterogeneous aquifers.(2)The mechanism of coexisting with aniline on the transport of degrading bacteria in porous media was revealed.Under the coexistence of aniline,aniline adsorbed on the surface of the media and formed a concentration gradient from solid phase to liquid phase.AN-1 migrated to the surface of the media because of the chemotaxis,extended the residence time,and increased the retention.Previous studies on the migration mechanism of microorganisms in aquifers were mostly focused on convection-diffusion,adsorption/desorption and filtration,but few studies on the effect of chemotaxis on bacterial transport in aquifer.This study has improved the theoretical study on the migration mechanism of degrading bacteria,and has practical significance for clarifying the migration mechanism of degrading bacteria in aquifer.(3)A new method for in-situ bioremediation of aniline in heterogeneous aquifers has been proposed.After the remediation was completed in the high-permeability zone,utilizing the blocking effect of the low-permeability lens and the chemotaxis of the bacteria,the bacteria gathered on the surface of the low-permeability lens,and the back diffusion of aniline from the low-permeability lens was degraded,which overcame the tailing and rebound effects caused by back diffusion.This study is of great practical significance to improve the efficiency of in-situ bioremediation of heterogeneous aquifer.