Characteristics Of Dissolved Organic Matter In Aquatic Environment And Interaction Mechanisms Between Dissolved Organic Matter And Typical Antibiotics

Dissolved organic matter(DOM),which is regarded as a“microreactor”in aqueous solution,can react with antibiotics to impact their environmental processes such as migration,transformation,distribution,bioavailability,etc.Therefore,it is of great significance to explore the composition and distribution characteristics of DOM and the interaction mechanisms between typical antibiotics and DOM.In this paper,UV-vis and three-dimensional excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis(3DEEM-PARAFAC)were used to study the composition,spatial distribution and source characteristics of DOM in Lanzhou reach of the Yellow River.Meanwhile,two kinds of commercial DOM(humic acid(HA)and natural organic matter(NOM))were selected to study the complexation mechanisms of DOM with typical antibiotics in aqueous solution.The interactions between HA and fluoroquinolone antibiotics(FQs),NOM and sulfathiazole antibiotics(STZ),NOM and ceftazidine antibiotics(CAZ)were examined by Nuclear magnetic resonance(~1H NMR),fluorescence,UV-vis and Fourier transform infrared(FT-IR)spectroscopic techniques.The main research results are as follows:(1)DOM in Lanzhou reach of the Yellow River was possibly comprised of low molecular weight organic substances with aromatic structure.It belonged to the complex"protein-like-humic-like"substances,dominated by protein-like substances.The protein-like components and the humic-like components had different sources.The spatial distribution trend of DOM underwent low-high-low along the upstream to downstream continuum,which was mainly affected by the changes of the protein-like substances content.Anthropogenic activities of high-intensity interference strongly influenced variations of protein-like materials dynamics,and this phenomenon was very obvious in densely populated tourist attractions.The input of contaminations generated by human activities was closely related to DOM concentrations and composition of the Lanzhou reach of Yellow River,including increased residential/commercial wastewater,catering,water recreation facilities pollution,shipping and a small amount of industrial discharge,etc.In addition,it was concluded that the endogenesis pollution may become main internal loading in the Lanzhou reach of Yellow River,which implied that stronger support for endogenesis pollution control is needed to alleviate DOM pollution and improve water quality.(2)The micro-environmental changes and the quenching effect of fluorophores in FQs occurred when HA interacted with four FQs(ciprofloxacin(CIP),enrofloxacin(ENR),ofloxacin(OFL)and norfloxacin(NOR)).The location adjacent oxygen right of OFL and the aromatic ring(the adjacency replaced by two nitrogen-containing groups)of CIP,ENR,NOR might be highly affected by HA molecule.The binding processes were exothermic and the formation of HA-FQs complexes would be powered chiefly by the?S~0.FQs could bind to one class of sites on HA and the quenching processes were static.H-bonding,electrostatic effect,van der Waals force were the acting force in the binding reactions and the?-?stacking effect was the major binding force under alkaline conditions.Moreover,the configuration,the charge and the size/type of charged groups of FQs and HA molecules would change under influence of different pH,inducing different binding capability between them.As ionic strength increased,the binding reactions for FQs-HA system were suppressed.Changes in UV-vis,~1H NMR and FT-IR spectra further demonstrated that the spatial conformation and micro-environment of FQs had altered in the presence of DOM(3)The main components of NOM were fulvic acid(FA)and humic acid(HA).Addition of STZ and CAZ could quench the fluorescence intensity of both FA and HA.Comparing with STZ molecule,the binding degree of CAZ molecule to FA/HA was stronger.And the binding extent of FA to STZ/CAZ molecule was greater than that of HA to STZ/CAZ molecule.The binding processes of FA/HA-STZ and FA/HA-CAZ were static and dynamic,respectively.Specifically,the double bond adjacent to N in STZ molecule may be affected greatly by the FA/HA components and the methylene group attached to N and the double bond adjacent to N and S in CAZ molecule may be affected highly by the FA/HA.The main binding forces of FA/HA-STZ were hydrogen bonding and electrostatic interaction,while the main binding force of FA/HA-CAZ was hydrophobic effect.The binding capability of FA/HA-STZ system was stronger under low pH conditions and the complexation constants of FA/HA-CAZ system were higher under neutral pH conditions.Increasing ionic strength promoted the binding capability of the FA-STZ and FA/HA-CAZ systems,while inhibited the complexation capability of the HA-STZ system.The spectral intensity and shift alterations of ~1H NMR,FT-IR and UV-vis demonstrated that the spatial structure and polarity of NOM molecule changed after interacting with STZ/CAZ.