Sorption And Photolysis Of Antibiotics By Goethite/Humic Acid

Antibiotics were widely used in humans and animals to treat microbial infections, andalso as feed additive to promote growth of livestock animals. After administration of anantibiotic, a significant fraction is excreted in the parent form or its metabolite forms alongwith urine and feces, and then directly or indirectly reaches the aquatic and terrestrialenvironmental, which would pose a negative effect on aquatic and terrestrial organism andhuman health. Recently, many studies have been done about the environmental chemicalbehavior of antibiotics in the environments. However, few studies have been done about thetransport and transforation of antibioltics in the livestock farms, which are the major sourcesof antibiotics in the environments. Meanwhile the sorption mechanism of antibiotics in theenvironmental medium and the abiotic transformation in the natural environment is stillinsufficient. So tylosin (TYL) and sulfamethazine (SMT), the widely used veterinaryantibiotics were chosen as the sorbates. The sorption properties and sorption mechanism ofantibiotics on goethite and humic acid were investigated. The influences of environmentalfactors on the photodegradation of tylosin by goethite and the influence of the microcosmicinterfacial interaction between goethite and humic acid on the and transform of antibioltics inthe aqueous phase were also investigated. The main experiments and conclusions are asfollows:1．Sorption of TYL and SMT on goethite was systemaly measured using a batchtechnique. The results showed that the sorption of TYL and SMT could attain apparentequilibrium within5hrs and12hrs. The sorption rates for TYL and SMT decreased as theinitial concentration increased. The sorption process might be constituted with the initialboundary layer diffusion or external surface, then the intraparticle diffusion or pore diffusionstage and finally equilibrium stage related with the sorption on the interior surface of sorbent.Thermodynamics calculations revealed that the sorption of TYL on goethite was exothermaland spontaneous and SMT was endothermic and spontaneous. Sorption of TYL and SMTwere nonlinear and the sorption capacities were influenced by the pH and ionic strength. Thesorption mechanism of TYL might be related with surface complexation, electrostatic repulsion, and H-bonding on goethite. But for SMT the sorption mechanisms might involveVan der Waalsforce, electrostatic interactions, H-bonding, outer and inner-sphere complexesand π-π EDA interactions.2．The apparent equilibration times of sorption of TYL and SMT on humic acid were24hrs and the sorption rates for TYL and SMT decreased as the initial concentrationincreased. The sorption of TYL and SMT on humic acid was fitted by Linear and Freundlichmodels and the sorption was exothermal and spontaneous. Sorption of TYL and SMT wereobvious influenced by the pH and ionic strength and sorption were decreased with theincrease of pH and ionic strength. The main factors for sorption of TYL on humic acid werehydrophobic interaction, H-bonding and ion exchange but ionic exchange might dominate thesorption. The sorption mechanism of SMT on humic acid might be related to hydrophobicinteractions, surface complexation, electrostatic repulsion, and H-bonding.3．TYL and SMT could be degradation under the action of visible light in the natureenvironment, but when goethite existed the rate of catalytic degradation increasedsignificantly. The photodegradation rate of TYL and SMT decreased with the increase ofinitial concentrations and ionic strength, but the photodegradation rate of TYL and SMTincreased with the increase of concentrations of DOM. For TYL the photodegradation ratewas primarily decreased and then increased with the the increase of pH, but for SMT it wasdecreased all the time with the increase of pH. The photodegradation of TYL and SMT ongoethite might be related to the sorption of antibiotics on goethite and the soluble Fe3+concentration in the solution and the degradation process included the homogeneous reactionin the solution and the heterogeneous reaction at goethite surface. The DOM in the solutioncould promote the photodegradation of TYL and SMT by goethite. It should be noted toassess the influence of environmental factors on the transforation of TYL and SMT in theenvironment.4．The microcosmic interfacial interactions mechanism between goethite and humicacid were investigated the characterization of SEM, EDX, FTIR and Raman spectroscopy. Itcould be found that goethite could combined with humic acid through the surface active sites and active group and the combination is priority with chemical methods. The influence ofmicrocosmic interfacial interaction between goethite and humic acid on the transport andtransforation of antibioltics in the aqueous phase were also investigated the sorption andphotocatalysis of TYL and SMT by the complex of goethite and humic acid. It could be foundthat sorption rate and capacity of TYL and SMT on goethite increased significantly in thepresence of humic acid and the sorption rate and capacity increased with the increasedconcentrations of humic acid in the complexs. The sorption mechanism of TYL and SMT onthe complex might be related to hydrophobic interactions, H-bonding, ion exchange andsurface complexation. The presence of humic acids was conducive to the photolysis of TYLand SMT by goethite and the catalytic degradation efficiency of TYL and SMT increased withthe increase of concentration of humic acid in the complex. The ligand of goethite and humicacid could generate Fe2+into the liquid phase when the catalytic degradation of TYL andSMT by the complex of goethite and humic acid. The Fe2+could combined with humic acidand the oxidation for its ligands, which could improve the efficiency of catalytic degradationform high iron complex compound through the cycling of iron ions.