Formation,Migration And Transformation Mechanism Of CHCl₃ Under The Influence Of Colloid Effect In Managed Aquifer Recharge

Groundwater is an important part of water resources.More and more groundwater resources are used by human beings,and the excessive exploitation of groundwater by human can easily cause a series of environmental geological problems.The managed aquifer recharge is to recharge various water sources such as the surface water into the target aquifer,which can effectively alleviate the environmental geological problems caused by the unreasonable development and utilization of groundwater.However,the application of managed aquifer recharge involves not only the water quality conditions of external water sources,but also the disturbance of groundwater hydrodynamic and hydrochemical fields.Therefore,ensuring the safety and stability of the groundwater environment is a prerequisite for the effective application of managed aquifer recharge.Based on the diversity of recharge water,in order to prevent potential pollutants from entering the groundwater environment,disinfection and pretreatment of the recharge water can help control the groundwater from pathogenic microorganisms.This process effectively guarantees the water quality conditions of recharge water.However,the presence of disinfectants causes secondary pollution of groundwater by the formation of disinfection by-products?DBPs?.In addition,the disturbance of managed aquifer recharge process on groundwater makes the colloid effect obvious.The problem of colloid effect during managed aquifer recharge process is another key factor that affects the formation and migration of DBPs in groundwater.Although research has been conducted on the formation of DBPs and the environmental behavior in artificial recharge systems during the disinfection process,what are the formation laws of DBPs coupled with colloidal effects during this process?The mechanism of colloid effect on the migration and transformation of DBPs in the process of managed aquifer recharge needs to be further investigated.Therefore,the further study of the formation,migration and transformation mechanism under the influence of the colloid effect of DBPs during the managed aquifer recharge was carried out.DBPs pollution of groundwater is becoming increasingly prominent,and organic pollution of groundwater has become an important environmental problem that endangers human health.Chloromethane?CHCl₃?as an emerging pollutant of DBPs,has attracted attention because of its highest detection rate.After the formation of CHCl₃ in groundwater,it undergoes various physical,chemical and biological effects to redistribute it in groundwater and media.The colloid has good adsorption capacity due to its large specific surface area and rich pore structure,which affects the environmental behavior of CHCl₃ in groundwater systems.The study of environmental behavior characteristics of CHCl₃ under the influence of colloid during the managed aquifer recharge has become an important research content in DBPs pollution,but comprehensive understanding has yet been obtained.Based on this,this research relies on the National Natural Science Foundation of China"The formation mechanism of DBPs under managed aquifer recharge and its migration and transformation under the influence of coupled colloid effect"?41877175?and the Graduate Innovation Fund of Jilin University"Study on the formation mechanism and influencing factors of DBPs under the influence of coupling colloid effect during managed aquifer recharge"?101832018C051?.A recharge test site in Rushan River underground reservoir in Weihai City was taken as the research area?area of about 225 m²?.For the DBPs formed by recharged with chlorinated water,CHCl₃with the highest detection rate was selected as the research object.Based on the existing survey data and research results in the comprehensive analysis and research area,through supplementary geological,hydrogeological surveys and groundwater soil sample collection tests,the water and soil physical and chemical characteristics of the recharge site are identified.Subsequently,formation experiments of CHCl₃ and SiO₂synergistic studies were carried out under different artificial recharge conditions,and a formation prediction model of DBPs-SiO₂ was established.Carried out field artificial recharge experiments to analyze the changes of groundwater environmental elements,typical chemical components and microbial community characteristics during managed aquifer recharge,and influence of CHCl₃ coupled colloid effects in groundwater.Based on this,relevant parameters were obtained from field recharge experiments and formation experiments,a migration and transformation model under the influence of CHCl₃ coupled colloidal effects during managed aquifer recharge was established,to predict the migration and transformation process of CHCl₃ and spatiotemporal distribution characteristics.Determining the threshold value of disinfectant of recharge water suitable for site hydrogeological conditions will provide some guidance for the implementation and promotion of managed aquifer recharge.Through this study,the following understandings are mainly obtained:1.During the managed aquifer recharge,the secondary formation of CHCl₃ is accompanied,and its formation is affected by factors such as reaction time,colloidal concentration,chloride ratio,pH,ionic strength and type.?1?The formation of CHCl₃gradually increased with the extension of the reaction time,and then stabilized,and the formation rate increased first and then decreased to no longer form CHCl₃.The adsorption of colloidal SiO₂ on the reaction precursors inhibited the formation of CHCl₃at all times.Colloidal SiO₂ is easy to adsorb fast-reactive groups,so that the rate of formation in the rapid reaction stage at the beginning of the reaction is more affected by the colloid;?2?Colloidal SiO₂ is easy to adsorb aromatic organic compounds,and aliphatic precursors play a leading role at low chlorine rates.The formation of CHCl₃is suppressed.As the chlorine rate increases,chlorine is in an excess state relative to the precursors.The hydrolysate of NaClO occupies the adsorption site of colloid SiO₂,so that the influence of colloid effect on the formation is weakened.?3?With the increase of pH?5⁹?under non-colloidal condition,the formation of CHCl₃ gradually increased,but decreased in the presence of colloidal SiO₂.The pH of CHCl₃ production decreased when pH>7.This is because the colloidal SiO₂ potential is negative,and H⁺competes with the precursors under acidic conditions.In addition,humic acid has a linear structure under high pH conditions.The structural changes affect the adsorption of humic acid by colloidal SiO₂.?4?Colloidal SiO₂ enhances the ability of metal cations to inhibit the formation of CHCl₃.This is due to the higher surface charge of metal ions,which enhances the adsorption of aromatic organics under the influence of colloidal SiO₂,and reduces the formation of CHCl₃.Ca²⁺has complexing ability,the bridging effect of humic acid through Ca²⁺causes more humic acid to be adsorbed by colloid,resulting in a decrease in the formation.2.The tangential flow ultrafiltration device is used to classify the particle size of the experimental solution that has reached a stable formation with and without colloid.The concentration of CHCl₃ in the filtrate without colloid>100 nm,10¹00 nm and<10 nm after the fractionation accounted for 9.72%,9.08%and 81.20%of total,respectively.CHCl₃ mainly exists in the dissolved form.In the presence of 10 mg/L colloidal SiO₂,the concentration of CHCl₃ in the filtrates of each particle size grade accounted for 37.44%,7.63%,54.93%,and the colloidal CHCl₃ concentration of>100nm is 53.535?g/L.The colloidal SiO₂ has a strong adsorption effect on CHCl₃ in water,and its proportion in the total CHCl₃ concentration increases by 27.72%.3.The formation concentration of CHCl₃ was selected as the model dependent variable,and six indexes of reaction time,chloride ratio,pH,ion strength of Na⁺and Ca²⁺,and colloid concentration were used as independent variables.A multiple regression analysis and an artificial neural network analysis were used to establish CHCl₃ coupling the colloid effect formation prediction model,respectively.The artificial neural network model showed a higher adaptability with a correlation coefficient of 0.9753,which provided a scientific basis for the formation prediction.4.Field artificial recharge experiments show that,due to the effects of adsorption and biodegradation,the migration rate of CHCl₃ in groundwater is lower than the tracers,and the colloidal SiO₂ inhibits the migration of CHCl₃.In the presence of colloidal SiO₂,both the retardation coefficient R_f and natural decay rate constant k of CHCl₃ in each monitoring well increased.On the one hand,considering the formation effect,the reduction of the initial concentration of CHCl₃ in the presence of colloidal SiO₂ reduces its migration rate.On the other hand,aqueous media that adsorb colloidal SiO₂ provide more effective adsorption sites and inhibit the t migration of CHCl₃.The changes in the characteristics of groundwater environmental elements and water chemical components show that,under the influence of cation exchange and water-rock interaction,the increase of Ca²⁺concentration in groundwater makes the formation of CHCl₃suppressed.In the presence of colloids,the porous medium-CHCl₃ adsorption system was changed to the porous medium-colloidal SiO₂-CHCl₃ adsorption system,and the adsorption effect was enhanced.In addition,the biodegradation rate of CHCl₃ also increased,but by comparing the biodegradation rate constant,it was found that the influence level of the biodegradation decreased and the influence degree of the adsorption action is increased.5.The results of microbial community structure and diversity analysis during managed aquifer recharge indicate that Proteobacteria and Actinobacteria are the main sources of groundwater,accounting for 53.01%and 41.76%,respectively.Proteobacteria is the main source of recharge water,accounting for 84.62%.At the genus level,the dominat bacterial species of native groundwater and recharge water are Pseudobacterium?29.87%?and Acinetobacter?31.53%?.When there is colloidal SiO₂in the recharge water,Acinetobacter?46.81%?in aquifer enters the groundwater system and becomes the dominant bacteria.It play an important role in degradation of alkane,aromatic hydrocarbons and other components.Therefore,this process also has an impact on the degradation of CHCl₃.In addition to the above abundant microbial communities,a small amount of Nitrosomonas,Nitrospira,Flavobacterium,Methylophage,etc.bacteria that may degrade CHCl₃ through co-metabolism.Such microorganisms were detected in groundwater samples when the recharge water was injected for 170 min under colloid-free conditions,and the bacterial abundance increased in the later stage of the experiment,indicating that there is a co-metabolic process of CHCl₃,which makes it enriched in groundwater.However,in the presence of colloids,the above-metioned microorganisms were not detected at 170 min.on the one hand,the presence of colloids inhibited the migration of bacteria in groundwater.On the other hand,our research team's previous research results showed that colloidal SiO₂ in groundwater has a certain adsorption capacity for electron acceptors such as ammonia nitrogen,which inhibits the growth of microorganisms.6.Numerical and analytical model predictions under the influence of the CHCl₃coupling colloid effect during the managed aquifer recharge indicate that the presence of colloidal SiO₂ inhibits the secondary formation of CHCl₃ in groundwater,on the other hand,the natural decay rate increases,inhibiting the migration of CHCl₃ in groundwater.Based on the current CHCl₃ concentration limit of 0.06 mg/L stipulated in China's"Sanitary Standard for Drinking Water"?GB 5749-2006?,an effective chlorine concentration threshold for recharge water was proposed under the hydrogeological characteristics of the study area.That is,under the condition that the migration distance of CHCl₃ in groundwater is greater than 10 m and does not exceed the concentration limit,the effective chlorine concentration threshold of recharge water is 1.25 mg/L,and 1.93 mg/L in the presence of 10 mg/L colloidal SiO₂.When the migration distance is greater than 30 m and does not exceed the concentration limit,the effective chlorine concentration threshold of recharge water is 5.35 mg/L,and 7.05mg/L in the presence of 10 mg/L colloidal SiO₂.