Sorption Equilibrium Research Of Tylosin On Clay Mineral Surface

Antibiotics are widely used in medicine, livestock industries and aquiculture industries.As the increasing demand for meat products, livestock industries and aquiculture industriesdevelop quickly, the farms begin to largely use antibiotic as a growth promoter to shortenanimal's growth period for the maximum profit. Growth promoter is a major role ofantibiotics used in modern agriculture and livestock industries. High to30%-90%veterinaryantibiotics are excreted as the parent compounds through urine and feces and they are stillactivity. When the urine and feces are used as manure to applicate on soil, the antibioticsmetabolize or their paret compound could reach in environment to form hard controllablewildspreas pollution. It is believed that antibiotics in environment could induce resistance ofmicroorganism and lead to potential health problems for animals and even human beings.Antibiotic pollution in environment is a hot research in recent yeas, but it does not get enoughconcentration in Chian especially in Pearl River Delta. In this area, there are a lot of culturefarms. Developed livestock industries would be sure to cause antibiotics pollution in soiland water. In this paper, tylosin (TYL), a widely used veterinary antibiotic was chose as thesorbate. Around the sorption characters of on soil, the paper discussed deeply the sorptionmechanism of TYL behaviour on the interface of soil-water. The main experiments andconclusions are as follows:1. Sorption of TYL was measured for four agricultural soils using a batch technique. Theexperimental data showed that the sorption could attain apparent equilibrium within24hrsand little or insignificantly desorption hysteresis was observed. The Freundlich isothermequation could fit data well, with the nonlinearity parameter n ranging from0.83to1.03.The single point distribution coefficients (KD) were in a narrow range from2.01to12.4L/kg.The Kocvalue at different concentration ranged from118to15850L/kg-OC. The KDvaluescorrelated well with soil pH and surface area, but poorly with the soil organic carbon content.The clay in the soil also influenced the sorption especially montmorillonite and kaolinite.Because of large molecular weight and ionization character of tylosin, the sorption of tylosinon soil was nearly linear. The little or no desorption hysteresis was observed. The majorinteraction of sorption-desorption may be ionic exchange and Van der Waals'force. Thestudy indicated that the mobility of tylosin is less than those of smaller and polar antibioticsand because of the reversible sorption on soils, the soil-bound TYL could transport readily togroundwater.2. From the previous experiments, minerals in soil were considered as the most important factor to influence the TYL sorption. Thereby, TYL sorptions on several pure minerals weremeasured. The minerals included kaolinite, montmorillonite, quartz, goethite and Al2O3.The completely mixed batch reactor (CMBR) systems were used in the experiment. Theresults showed that sorption of TYL on minerals decreased in the order of montmorillonite>goethite> kaolinite> quartz> Al2O3. All of the sorptions were abviously nonlinear with then value from0.22to0.48. TYL sorptions on minerals were correlation with surface area ofmineral, point of zera charge (pzc) and solution pH value. Under acid solution condition,strong electrostatic attraction between the negative charges of mineral surface and positivecharges of TYL determined the sorption of TYL on minerals. However, under netural andalkaline solution, distribution interaction of no charge tylosin on mineral surface dominated.Among the five representive minerals in soils, montmorillonite, kaolinite and goethite werethe most important minerals to affect TYL sorption. Further, analysis of the thermodynamicparameters for these three minerals shown that tylosin sorption on them were spontaneous,thermodynamically favorable, and exothermic. The site energy distribution indicated thatthe sorption sites with lower site energies were more heterogeneous. Temperature mayaffect the sorption site heterogeneity because of the different structure of minerals.3. Tylosin has an ionizable functional group. It is partly positive charge form atenvironmentally relevant pH values. So it complicates to predict its sorption, availabilityand transport. To futher discusse the sorption mechanism of tylosin on minerals, the study oftylosin sorption on montmorillonite and kaolinite were contrasted. The pH effect onsorption was first discussed. The results showed as followe: The sorption isotherms for twoclays were nonlinear and sorption capacity on motmorillonite was two orders of magnitudehigher than kaolinite at specific aqueous concentration. The sorption decreased as pH andthe overall uptake might be dominated by different sorption process in the varied pHcondition. The pH experiments indicated cation exchange and surface sorption were themain sorption mechanism of TYL on minerals. Further XRD analyses revealed thatinterlayer of montmorillonite could expand due to the uptake of tylaosin. The sorption oftylosin on montmorillonite was likely limited to monolayer coverage. A sorption model inwhich species-specific sorption coefficients were weighted by the pH dependent fraction ofeach species fit the data well. DMM sorption model was used to evaluate the contribution ofcation exchange and surface sorption for whole sorption process. The results showed thatcation exchange controlled the sorption. Meanwhile, surface sorption is more and moreimportant as the concentration of TYL increased. Hydrophobic interactions betweenadsorbed tylosin may exist. The tylosin adsorbed via ion exchange may act as an organic phase enhancing the hydrophobic interactions of pollutant with the mineral surfaces.4. In the natural environment, ionic condition of tylosin solution was always not stable. Sothe sorption of tylosin by pure clay minerals varying in ionic strength, ionic type in solutionand preliminarily studied interlayer interaction of tylosin on different type montmorilloniteswere investigated. The sorption capacity of tylosin at different ionic strengths decreasedwith the order of0.008M>0.051M>0.108M>0.508M. In different metal ions solution,the sorption of tylosin decreased in the following order: K⁺> Na⁺> Ca²⁺≧Mg²⁺.However, for montmorillonite the sorption displayed an opposite tendency at low initialconcentration of tylosin. While the initial solution was low, sorption capacities at the ionicstrength of0.051M and0.108M were nearly the same, however were larger than the sorptioncapacity at the ionic strength of0.008M. Sorption of tylosin at different ionic type solutionat pH6.5decreased in the order of Mg²⁺≧Ca²⁺> K⁺> Na⁺at low solution concentration.These results indicated that the sorption sites on minerals were heterogeneous. When theinitial concentration of tylosin was low, the molecular first occupied high affinity sites likehydrogen bond. Not only tylosin can interact with the mineral surface group, but alsointeract with the water molecular around metal ions in solution. Thereby, proper ionicstrength and high valence state metal ions in soultion could increase tylosin sorption. Whenthe tylosin concentration was high, the sorption was major on the low energy sorption site.Cation exchange was the major interaction. Competitive sorption between tylosin and cationin solution resulted in decrease sorption. The sorption of tylosin on differentmontmorillonites decreased in the order of Ca-montmorillonite> Na-montmorillonite>K-montmorillonite. It indicated that interlayer hydration influenced the sorption rather thaninterlayer cation exchange.5. To understand tylosin effect on the sorption of other organic pollution on minerals,phenanthrene was chosen as the typical pollution. Phenanthrene sorptions pure mineral andmineral with tylosin were investigetd. The results showed that montmorillonite andkaolinite with no tylosin can weakly sorb the phenanthrene and the sorption capacity onmontmorillonie was higher than that on kaolinite. Linear sorption model and Freundlichmodel can fit the sorption isotherms well. The R²values were more than0.98. Thesorption coefficients K_D on montmorillonite and kaolinite were0.012and0.0035L/g,respectively. However, after tylosin was sorbed on calys, the sorptions of phenanthrene onclays were increased obviously. The sorption capacities of phenanthrene on clays increasedwith the initial concentration of tylosin, while the nolinearity decreased with the initialconcentration of tylosin. On montmorillonite, the sorption capacity logKFincreased from -4.047to0.591, and the n values decreased from1.692to0.596, corresponding to initialconcentration of tylosin from0mg/L to100mg/L. After tylosin was sorbed on clays, thetylosin played as the role of organic matters of clays. The sorption coefficients ofphenanthrene on clays were positive relation with the content of organic matter (f_oc). The nvalues were negative relation with the f_oc. Hydrophobic distribution was the majorinteraction of phenanthrene sorption on organominerals. And as the f_oc of clay increased, thesorption nonlinearity of phenanthrene increased. This indicated tylosin could obviouslypromote phenanthrene sorption on minerals. As antibiotic and phenanthrene coexisted, thepresence of antibiotic could increase the mobility and bioavailability of phenanthrene in theenvironment.