Study On Environmental Behavior Characteristics Of Escherichia Coli O157:H7 In The Porous Media Under The Action Of Composite Colloid

In the biocolloid of groundwater,the particle size of bacteria is often between that of viruses and protozoa,and their migration and retention behavior in the vadose zone presents a more complicated mechanism.Some low-infectious doses and highly pathogenic bacteria may migrate long distances in groundwater and multiply in groundwater,leading to the spread of serious diseases and posing health risks to humans and animals.The vadose zone is the first line of defense for groundwater.It is necessary to understand the migration and retention behavior of pathogenic microorganisms in vadose zone.Driven by precipitation and irrigation,surface sewage seeps into the seepage area and eventually pollutes groundwater.As biocolloids,some bacteria have unique biological characteristics.While they are pathogenic,they can also interact with colloids in the underground environment to promote the migration of some pollutants.The target pathogenic Escherichia coli O157:H7 of this study is a pathogenic E.coli that produces Shiga toxin,which can coordinately migrate with natural colloids in the formation environment and produce complex behavior characteristics.The colloids in the vadose zone do not exist alone.They often interact in the underground environment and cement each other to form complexes.Considering the influence of the presence of composite colloids on the migration of E.coli can make the study more closely simulate the real groundwater environment,and further understand the migration characteristics of E.coli in the vadose zone.Therefore,in the presence of different types of composite colloids,explore the migration behavior of pathogenic bacteria in the vadose zone,which provides a basis for assessing groundwater health risks,predicting the behavior of groundwater environmental pollutants,and remediation of pathogenic bacteria contaminated groundwater.In this study,the column experiment was used,E.coli O157:H7 was selected as the target bacteria,and the representative of iron oxide colloid(Fe₂O₃)and inorganic colloid(montmorillonite)in the vadose zone was selected as the composite clay mineral colloid(Mt-Fe₂O₃).The representative of iron oxide and organic colloid(HA)is used as the composite colloid of organic clay mineral(HA-Fe₂O₃).Explore the effect of composite colloid on the migration of pathogenic bacteria in the vadose zone.To further supplement the cognition of the research on the cooperative migration behavior characteristics of pathogenic bacteria pollutants and composite colloids at this stage,and provide some references for the risk of pathogenic microorganisms in groundwater pollution.The main conclusions of this paper are as follows:1.The basic properties of the"E.coli O157:H7-composite colloid"systemThe results of exploring the effect of the composite colloid on the activity of E.coli during the time period of the migration experiment show that the effect of the activity of E.coli under the condition of the presence of the composite colloid during the experiment meets the experimental requirements.The E.coli in this study is a hydrophilic bacterium.It can be found from the scanning electron microscope that the shape of a single E.coli is rod-shaped.The size of E.coli at different growth stages is different.The results of the particle size analyzer showed that the average particle size of E.coli was 1088.3 nm.In Mt-Fe₂O₃ composite colloid,the montmorillonite will wrap the E.coli,and Fe₂O₃ may play a bonding role in it.It was observed that there were more E.coli on the surface of the local medium with more Fe₂O₃ colloids attached,indicating that Fe₂O₃ played a significant role in promoting the adhesion of E.coli on the medium.Compared with the condition of single humic acid and E.coli,when the humic acid and Fe₂O₃ co-exists,the colloid binds to E.coli more tightly,indicating that the Fe₂O₃ promotes the combination of composite colloid and E.coli.Therefore,the presence of the composite colloid and its different composition have a significant impact on the behavior of E.coli in groundwater.2.The influence of water content and media type on the migration of E.coliThe results of the migration experiment showed that as the water content decreased from 80%to 60%,the amount of E.coli deposited in the column increased from 67.14%to 71.36%,and the deposition rate coefficient kc increased from 3.77×10^-3 to 6.39×10^-3,indicating that the reduction of water content is not conducive to the migration of E.coli.When the humic acid-iron oxide composite colloid is present,the peak value of the penetration curve increases from 0.45 to 0.48 as the moisture content decreases,indicating that humic acid has good migration ability and is not easy to adhere to the air-water interface.Combine with E.coli and move quickly in the liquid phase pathway.The montmorillonite-iron oxide composite colloid will wrap the E.coli,making it easy to stay in the porous medium.The sedimentation amount in the sand column is more than 87%,and the retention phenomenon becomes more obvious as the water content decreases.In the natural environment,the moisture content of the vadose zone increases due to precipitation,artificial recharge,etc.,which may increase the migration ability of bacteria and re-migrate the originally retained bacteria,which requires attention.Comparing the migration of E.coli in different media,it can be found that the amount of E.coli deposited in natural sand is reduced from 67.14%to 65.05%in quartz sand.In HA-Fe₂O₃ composite colloid,the amount of E.coli deposited was reduced from 64.86%in natural sand to 60.11%in quartz sand.In Mt-Fe₂O₃ composite colloid,the amount of E.coli deposited was reduced from 87.45%in natural sand to 81.91%in quartz sand.All indicate that the reduction of the surface roughness of the medium facilitates the migration of E.coli in the porous medium.3.The effect of composite colloid on the migration of E.coliThe influence of the type and concentration of the composite colloid on the migration behavior of E.coli is the focus of this study.The study on the influence of the concentration of humic acid in the HA-Fe₂O₃ composite colloid on the migration of E.coli and found that under the condition of HA(0)Fe₂O₃(5),the migration ability of E.coli is extremely weak,and the deposition amount is as high as 95.11%.The reason is that there is electrostatic attraction between the Fe₂O₃ and E.coli,and the Fe₂O₃ is easy to adhere to the medium,providing extra adsorption sites for E.coli.Under the condition of HA(5)Fe₂O₃(5),the migration ability of E.coli was significantly improved,and the deposition amount was reduced from 95.11%to 64.85%.The reason is that the humic acid colloid has a strong migration ability.The promotion of Fe₂O₃ facilitates its attachment to E.coli and can carry E.coli to migrate faster in the liquid phase pathway.Under the condition of HA(10)Fe₂O₃(5),the migration ability of E.coli is further improved.At this time,the deposition amount of E.coli is 60.95%.This indicates that the combination of humic acid colloid and E.coli is beneficial to the migration of E.coli.In view of the influence of Fe₂O₃ concentration change in HA-Fe₂O₃ composite colloid on the migration of E.coli,it was found that the deposition of E.coli was 64.85%in HA(5)Fe₂O₃(5),and increased to 87.15%in HA(5)Fe₂O₃(10).The results show that a small amount of Fe₂O₃ is beneficial to the combination of humic acid colloid and E.coli and facilitates the migration of E.coli.Excessive Fe₂O₃ will make E.coli adhere to the medium and hinder the migration of E.coli.Under the condition of Mt-Fe₂O₃ composite colloid,the migration ability of E.coli is weak,and the deposition amount is more than 85%.The study on the influence of the change of the concentration of montmorillonite in the Mt-Fe₂O₃ composite colloid on the migration of E.coli found that the deposition of E.coli under the conditions of Mt(5)Fe₂O₃(5)and Mt(10)Fe₂O₃(5)increased from 87.45%to 90.80%.The results showed that the increase of montmorillonite colloid concentration increased straining phenomenon,increased the water stop zone,increased the deposition amount of E.coli,and reduced its migration ability.The study on the influence of Fe₂O₃ concentration change in Mt-Fe₂O₃ composite colloid on the migration of E.coli found that when the iron oxide concentration in Mt(5)Fe₂O₃(X)is 0,5,10 mg/L,the corresponding E.coli deposition amount is 85.29%,87.45%,95.22%,indicating that with the increase of Fe₂O₃ concentration,the amount of E.coli deposits gradually increased.The reason is that the migration ability of montmorillonite colloid itself is weak,and it is easy to adsorb E.coli and adhere to the medium.The addition of Fe₂O₃ promotes the combination of montmorillonite colloid and E.coli,and further inhibits the migration of E.coli.4.The simulation of bacteria migration experimentThe model results show that the influence of the change of moisture content on the deposition rate of E.coli in this study is far greater than the change of the medium type.In most cases,the presence of composite colloids promoted the increase in the deposition rate of E.coli.Comparing the colloidal deposition rate coefficient calculated by the model and the experiment,the difference of the colloidal deposition rate coefficient in the presence of HA-Fe₂O₃ composite colloid is larger than that of other groups,and the calculated value of the actual measurement is much smaller than the simulated value.In the future simulation and prediction of the migration of pathogenic microorganisms,attention should be paid to the presence of humic acid-iron oxide composite colloid to avoid underestimation of the migration of pathogenic bacteria.