Effects Of Dissolved Organic Matter(DOM) On The Fluorescence Properties And Membrane Filtration Of Difloxacin Hydrochloride(DFHC)

In recent years,the abuse of antibiotics in human and veterinary medicine caused a serious threat to the health of human and aquatic animals and plants.Dissolved organic matter(DOM)severs as a special carrier which was ubiquitously existing in sediment-water ecosystems and easy to bind with contaminants.DOM has significant effects on the mobility,transport and bioavailability of antibiotics.Thus,the study focused on the interaction between DOM and antibiotics could provide reliable theoretical support for the environmental behavior and risk assessment of antibiotics.In this study,Difloxacin hydrochloride(DFHC)was selected as a proxy of the fluoroquinolone antibiotics.The Suwannee River DOM(SRDOM),Aldrich humic acid(HA)and SRDOM+HA(with a ratio of SRDOM to HA of 1:1)were used as the proxy of DOM.The PES ultrafiltration(UF)membranes with molecular weight cut-offs of 20 kDa and 50 kDa were available for the ultrafiltration experiment.In addition,different molecular weight cut-offs(PVDF material,100 kDa,10 kDa,3 kDa)were used to understand the capacity of each fraction of SRDOM,HA and SRDOM+HA to bind DFHC.We applied excitation-emission matrix-parallel factor analysis(EEM-PARAFAC)to study the interaction between DFHC and DOM(SRDOM,HA and SRDOM+HA).The main findings in this work can be described as follows:1.Membranes with different molecular weight cut-offs of 20 kDa and 50 kDa were used in filtration experiments.The concentration of DFHC in filtration solution was detected by high-performance liquid chromatograph(HPLC).The membrane rejection degrees of DFHC by two membranes with pore sizes of 20 kDa and 50 kDa in the absence of SRDOM,HA and SRDOM+HA were determined to be 42%and 35%,respectively.The presence of SRDOM,HA and SRDOM+HA,particularly HA,greatly increased the membrane retention rate of DFHC.The HA(76%for 20 kDa and 69%for 50 kDa on average)exhibited a higher rejection degree than did SRDOM(65%for 20 kDa and 44%for 50 kDa on average).The macromolecules in SRDOM,HA and SRDOM+HA play a.leading role to bind DFHC.2.The EEM spectrum of the DFHC solution was characterized by two peaks at Ex/Em=280/450 nm(peak R)and Ex/Em=320/450 nm(peak S).The SRDOM,HA and SRDOM+HA exhibited an overlap phenomenon between the two main peaks,suggesting the occurrence of inter-component interactions within the DOM(SRDOM,HA and SRDOM+HA)components.The EEM spectrum of the SRDOM+HA mixture was largely different from those of their end-members,for instance,there was a relative blue shift(10 nm)in the excitation and a red shift(10 nm)in the emission of peak A occurred compared to that of the end member of SRDOM.Peak A of the EEM spectrum of the DOM-DFHC mixture almost completely overlapped with peak S,peak R elongated distinctly into peak C in the EEM spectra of the DOM-DFHC mixtures,which indicated the complex interactions between the DFHC and DOM(SRDOM,HA and SRDOM+HA).The presence of multiple fluorophores is a potential reason for the overlapping peaks.3.The PARAFAC modeling successfully decomposed the EEM spectra into four independent components(e.g.,Cl,C2,C3 and C4),which were attributed to antibiotic-like component(C1)and humic-like components(C2,C3 and C4).4.The HA showed the largest quenching effects on DFHC,followed by SRDOM+HA and SRDOM.The fluorescence intensity of the C1 component(300?g/L)decreased about 96.5%,95%and 75%when the concentrations of HA,SRDOM+HA and SRDOM reached approximately 12 mg/L,respectively.There was no effect of DFHC on the fluorescence of SRDOM,HA and SRDOM+HA.The antibiotic-like component and humic-like components exhibited unidirectional interactions.The qucnching mechanism between DFHC and DOM(SRDOM,HA and SRDOM+HA)was likely to be static quenching.5.The fitted stability constant of the antibiotic-like component for SRDOM,HA and SRDOM+HA was in the order of HA(4.07 L/mg)>SRDOM+HA(2.8 L/mg)>SRDOM(0.86 L/mg),which was consistent with the fluorescence-quenching efficiencies of DFHC.There was a significant positive correlation between the retention degree during UF and the fluorescence quenching efficiency of DFHC in the presence of SRDOM,HA and SRDOM+HA.The differences in type and amounts of functional groups in SRDOM,HA and SRDOM+HA may be the main reasons for the differences of DFHC quenching degree.