| Sulfa antibiotics is one of the most prescribed antibiotics in livestock husbandryworldwide. Given that more than45-90%of the ingested sulfa antibiotics may beexcreted by animal without modification. The sulfa antibiotics are discharged into thesoil environment, leaching into the aquifer, threat the safety of groundwater. Sulfaantibiotics in groundwater, stress the generation of sulfa resistance of microorganisms.The resistance genes horizontally transfer to the harmful bacteria, through the foodchain threat to human health. Although sulfa antibiotics concentration in groundwaterhas no specific hazard standard, the harm of the long-term presence of trace amountsulfa antibiotic to human body is immeasurable.In recent years, the widespread detection of sulfa antibiotics in groundwater hasattracted the attention of domestic and foreign scholars. For a long time, due to lackunderstanding of the dangers of antibiotic pollution, researches on the sulfa antibioticspollution in environment in China are later. Researches on the antibiotics pollutionsituation and mechanism of migration and transformation in groundwater are almostunder unknown conditions. Therefore, it has an important significance to research themechanism of sulfa antibiotics migration and transformation in groundwater and theremediation technology.This paper selected a sulfa antibiotic contaminated site in northeast china as studyarea, an area of about1000m2. Due to the long-term discharge of swine wastewaterfrom an adjecent pig farm, a diameter of about4m sewage pit formed on the surfaceground, containing up to0.5mg/L of sulfa antibiotics mainly includesulfamethyldiazine (SM), sulfamethazine (SM2), sulfathiazole (ST) andsulfamethoxazole (SMX), leaching into aquifer and resulting in groundwater pollution.The investigation results show that0.5~3.5m lithology of shallow aquifer was yellowish silt,3.5~7.0m mealy sand and7.0~10.0m fine sand. In this study area,the groundwater recharge are mainly from precipitation and surface watergroundwater interaction, and its main discharge are mainly through evapotranspirationand artificial exploitation. The groundwater level is shallow, and the hydraulicgradient is about5‰. Based on comprehensive analysis of existing exploration dataand research results, through the geological, hydrogeological survey, soil andgroundwater sampling and testing, the sulfa antibiotics polluted characteristics ingroundwater and soil was detected. Through laboratory static experiments, as well aspercolation column simulation experiments, reveal the mechanism of migration andtransformation of sulfa antibiotics and resistance gene expression in groundwater.Screening suitable adsorbent material researches remediation technology of sulfaantibiotic contaminated groundwater.Groundwater and soil analysis results showed that there were amount of sulfaantibiotics detected in soil and groundwater in this site. Sulfa antibiotics in soil mainlyconcentrated in lithology medium of0.5~5.0m. The detection rate of SM was highest,and the concentration of SM up to3.5mg/kg. The detection rate of ST (0.5mg/kg) wassecond, and the concentration of SMX was lowest (0.07mg/kg). With the depthincrease, the sulfa antibiotic detection decrease. Sulfa antibiotic concentration ingroundwater was from0to98ng/L. In11groundwater samples, the detection rate ofSMX was the highest, concentration more than40ng/L, the detection rate of SM washigher and that of SM2was lowest.Adsorption is one of the main function affected the migration and transformationof the pollutants in vadose zone and phreatic water. Adsorption is not only affected byphysical and chemical properties of pollutants, but also affected by other factors, suchas the organic matter, iron and manganese oxide content of the lithology media.The results of indoor static adsorption experiments showed that the adsorptiondata fitted well with quasi-second-order kinetic model and Freundlichthermodynamics equation. The adsorption rate of silt> mealy sand> fine sand.Adsorption of sulfa antibiotics on silt was quickest and equilibrium time was about 10h. Equilibrium time of the silt was about16h and equilibrium time of fine sand was24h. Four sulfa antibiotic distribution coefficient Kdin different media were Kd silt> Kdmealy sand> Kd fine sand，which showed that sulfa antibiotics was easiest to be adsorbed bysilty, second was mealy sand and hardest to be adsorbed by fine sand. The reason wasthat the content of organic matter and iron and manganese oxides in silty was morethan other lithology mediums. The fitted results of intramolecular diffusion modelshowed that adsorption of sulfa antibiotics on lithology media could be divided intotwo processes: diffusion boundary layer and intramolecular proliferation, in whichmolecular diffusion was the rate-limiting step. Humus and iron and manganese oxideshad great adsorption capacity to sulfa antibiotics, and played a promotion role in theadsorption process. The TDS of groundwater could promote the adsorption of sulfaantibiotics onto lithology medium, which mainly because with the ionic strength ofthe solution reducing, the diffuse electric double layer of lithology medium surfacebecome thin, negative charge repulsion between sulfa antibiotics and lithologymedium became stronger. Iron and manganese ions in groundwater played a favorableeffect on adsorption of sulfa antibiotics on aquifer medium, the reason was that part ofthe iron and manganese ions could form a key bridge to promote the adsorption ofsulfa antibiotics on aquifer medium.Biodegradation is one of the important functions during the migration andtransformation process of natural attenuation. In this paper, the study has carried outfrom the microbial community structure analysis, the diversity index analysis, naturaldegradation kinetics, metabolite monitoring etc., to explain the natural biodegradationof sulfa antibiotics in groundwater. High-throughput sequencing technology wasemployed to research microbial community structure in groundwater and resultsshowed that the dominant microbial group was Proteobacteria, Firmicutes andBacteroides. There are a lot of sulfa antibiotics resistant microbial community andsulfa antibiotics degradation bacteria exist in groundwater. The analysis results of theAlpha diversity index, Beta diversity index, OTU cluster and PCA statistical analysisshowed that microbial community of groundwater in this site were poor diversity and uneven distribution in space. Sulfa antibiotic biodegradation accounted for a dominantposition in this site compared to other abiotic factors, such as hydrolytic degradation.The natural degradation process of sulfa antibiotics fitted well with first orderdegradation kinetics and the half-life of sulfa antibiotics in groundwater was42.52~115.52d. The sulfa antibiotic half-life in silt medium was shorter than that in mealysand and sand medium, which mainly because the organic matter of aquifer mediumpromoted the biodegradation of sulfa antibiotics in groundwater. With the sulfaantibiotics degradation, metabolic end products SO42-and NH4+concentration in thegroundwater showed increase trend, which also indicated that in the groundwater thedegradation of sulfa antibiotics went through hydrolysis and desulfurization metabolicprocesses.The results of vadose zone simulated column leaching experiments showed thatsulfa antibiotics are difficult to adsorption, easy migration. Four sulfa antibioticsdetected the maximum concentration in simulated column at192h (SMX),216h (SM),264h (SM2) and336h (ST), respectively. The retardant coefficient of vadose zonemedia to four sulfa antibiotics was ST> SM2> SM> SMX, which showed that themigration capacity of four sulfa antibiotics was SMX>SM>SM2>ST. pH of theeffluent fluctuated between7.2to7.8, the concentration of SO42-and NH4+showedincreasing trend.The aquifer simulated tank experiments results showed that with migration pathprolonging, the concentration of four sulfa antibiotics decreased due to adsorption anddegradation function. During the sulfa antibiotics migration process, quantitative PCRtechnology was employed to detect sulfa resistance gene expression. The resultsshowed that closer to the inlet (A8, B8, C8, D8), the sulfa resistance gene expressionwas more intensity, which indicated that the longer the exposure time to sulfaantibiotics, the higher the sulfa resistance genes expression.Based on the migration and transformation of sulfa antibiotic in groundwater andthe resistance genes expression, combining the characteristics of the groundwater, thispaper chose adsorption method to remove trace sulfa antibiotic from groundwater. Contrasting the adsorption capacity of spent mushroom substrates(SMS), scoria, bonecharcoal, the adsorption capacity of SMS was better than the other adsorptionmaterials. The results of the static adsorption experiments showed that adsorptioncurve of four sulfa antibiotics onto SMS fitted well with quasi-second-order kineticmodel and Langmuir thermodynamics model. The mechanisms of sulfa antibioticsadsorption onto SMS were mainly electrostatic attraction, hydrogen keys and otherphysical and chemical effects. The results of dynamic simulation experiments showedthat, after30days the sulfa antibiotic concentration in groundwater were basicallybelow the detection limit, indicating that the SMS was an effective material removingsulfa antibiotics from groundwater. |
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