Transport Mechanism And Adsorption Technology Study For Ciprofloxacin In Groundwater

Ciprofloxacin（CPX）,the third generation of fluoroquinolones,has been widely detected in the groundwater.CPX reached the groundwater by various pathways is toxic to microbial community and could lead to antibiotic-resistant bacteria,which finally result in a discounted efficacy of drugs and apparent increase of risks for human health.Understanding the transport mechanism of CPX in groundwater,such as advection,dispersion,adsorption and other environmental processes,can provide scientific basis for the remediation of CPX-contamianted groundwater.Therefore,this paper studies the migration mechanism of CPX in groundwater,develops a new adsorption material combining sodium alginate and grapheme,and establishes the adsorption technology for CPX-contaminated groundwater.The main results of this study were as follows:1.The adsorption-desorption properties of CPX in sandy silt soil were investigated by a series batch tests.Kinetics data followed the psedo-second-order model（R²>0.999）.Sorption isotherms were well fitted to the Langmuir model.The maximum adsorption capacity for CPX based on the Langmuir model was 5.50 mg g^-1.CPX adsorption decreased sharply by increasing the pH from 7.0 to 10.0 and the ionic strength from 0.01to 0.2 mol L^-1 CaCl₂.Comparatively,CPX was more readily desorbed from sandy silt at alkaline and high ionic strength conditions.Cation exchange interaction,electrostatic interaction,and hydrophobic interaction were proposed to be the important adsorption mechanisms.2.The transport mechanisms of CPX in aquifers were explored by studying the effect of aquifer heterogeneity on tracer transport,the applicability of multiple solute transport models,and the effect of adsorbed CPX on the transport of CPX through flow-through columns（A-F）tests.First,systematic research of aquifer heterogeneity on tracer transport was applied.Transport of tracer conforms to the advection-dispersion equation when the hydraulic conductivity was higher than 1.53 cm min^-1;conforms to the two-region model when lower than 1.53 cm min^-1.Second,to explore the transport of CPX in sandy silt soil,modeling results of the two-region model,the Thomas model,and the Yan model were evaluated.The breakthrough curve of CPX reached the breaking time（C/C₀=0.05）,median time（C/C₀=0.5）and exhaustion time（C/C₀=0.9）at 170 PV（pore volume of the column）,397PV,and 470 PV,respectively.The retention percentage was 99.7%,94.6%,and 83.2%,respectively.The correlation coefficients of these three models were 0.9859,0.9016,and0.7245,respectively.Of three models,the two-region model was most suitable to describe the transport of CPX.Last,the effects of adsorbed CPX on the transport of CPX in groundwater were conducted by the column F filled with medium sand.The results showed the adsorbed CPX affected the re-adsorption capacity of medium sand.The adsorbed CPX on medium sand undergone desorption under deionized water flushing and came into the groundwater.3.Novel sodium alginate/reduced graphene oxide hydrogel beads（GSA）were synthesized characterized,and their adsorption properties of CPX in the solution were tested.GSA was characterized to investigate the surface appearance,functional groups,crystallinity,and thermal stability.The morphological examination of GSA revealed wrinkles and pore space.There were many functional groups,such as C=C,C-OH,and O-C=O.GSA has a high thermal stability.The adsorption of CPX onto GSA followed the pseudo-second-order kinetic model.The isotherm data followed the Freundlich model.The maximum adsorption capacity was 100 mg g^-1 at pH 7.0.The adsorption process was sensitive to contact time,initial CPX concentration and ionic strength.However,it was not pH sensitive.Hydrophobic interaction,electrostatic interaction,ionic exchange,H-bonding,and pore filling were proposed to be the main adsorption mechanisms.4.The effects of flow rate,influent CPX concentration,and ionic strength on the performance of PRB were confirmed throught flow-through column experiments and by using a chemical non-equilibrium two-site model.The results showed that the median time of breakthrough curves increased from 7 min to 16 h as the flow rate decreased from5.8 to 0.29 mL min^-1;decreased from 14.28 to 2.73 h as the influent CPX concentration decreased from 20 to 2.5 mg L^-1;decreased from 14.28 to 2.37 h as the ionic strength increased from 0.01 to 0.1 mol L^-1.Accordingly,this study provides an example of how a PRB can be designed to respond to water matrix properties and hydrogeological conditions.The PRB lifetime obtained in this study is 288 d for step injection of a 20 mg L^-1 CPX solution with an ionic strength of 0.01 mol L^-1 at a flow rate of 0.29 mL min^-1.5.The long-term performance of PRB by a one-dimensional flow-through column H filled with GSA and coarse sand were investigated.The CPX solution of 50 mg L^-1(background electrolyte 0.01 mol L^-1 CaCl₂)was introduced at a flow rate of 13.9 mL min^-1,the breakthrough curve reached the break time,median time and exhaustion time at12 PV,18 PV,and 47 PV,respectively.The PRB system had an almost constant hydraulic conductivity and then could be effectively remediate the CPX-contaminated groundwater for a long time.