A Simulation Study Of Effect Of Dissolved Black Carbon On Photodegradation Of Typical Antibiotics In Water

Dissolved black carbon（DBC）is a water-soluble and highly mobile component of black carbon.It is an important constituent of dissolved organic matter（DOM）pool and a key form in the carbon cycle.Compared with the well-studied DOM,DBC has a relatively homogeneous structure,and shows higher quantum yields of reactive species and stronger photosensitization for organic pollutants.However,there are still few studies on the influence of DBC on the photodegradation behavior of antibiotics in water.The paper focus on the influence of DBC on the photodegradation of typical antibiotics in water.DBC derived from potato straw biochar was selected.In the study,ultraviolet visible absorption spectroscopy（UV-vis absorption spectroscopy）,Fourier transform infrared spectroscopy（FT-IR）,three-dimensional fluorescence spectroscopy（3D-EEM）,ultra-high performance liquid chromatography（HPLC）determination and Folin-Ciocalteu method and other techniques and methods are mainly used.This paper explored the following two parts:（1）The influence of selective adsorption on ferrihydrite on the properties of DBC,and the influence and mechanism of DBC on the photodegradation of SD before and after adsorption;（2）The effects of DBC with different preparation temperatures on the photodegradation of ofloxacin（OFL）were studied.The major research contents and results were as follows:（1）The present study investigated the effects of the selective adsorption of DBC on ferrihydrite on its chemical and optical properties,as well as on the photodegradation of SD induced by DBC,under different Fe/C molar ratios(i.e.0,7.50 and 11.25,and denoting as DBC₀,DBC_7.50and DBC_11.25,respectively).The results showed that the UV absorbance,aromaticity,molecular weight and phenolic antioxidant contents of DBC were significantly reduced after adsorption on ferrihydrite,with a greater reduction as the increase of Fe/C ratio.Photodegradation kinetics experiments showed that the selective adsorption of DBC on ferrihydrite significantly changed the effect of DBC on photodegradation behavior of SD,and observed photodegradation rate constant of SD(k_obs)increased from 4.14×10^-5s^-1in DBC₀solution to 5.75×10^-5s^-1in DBC_7.50solution while decreased to 3.40×10^-5s^-1in DBC_11.25solution.The photochemical steady-state experiments showed that adsorption of DBC on ferrihydrite reduced the steady-state concentration and quantum yield of ³DBC*and ¹O₂photo-induced by DBC,which was attributed to the reduction of quinones/aromatic ketones.At the same time,the co-reduction of phenol and quinones/aromatic ketones made the second-order reaction rate constant between³DBC*and SD(k _SD,3DBC*)increase from 0.87×10⁸M^-1s^-1for DBC₀to 2.54×10⁸M^-1s^-1for DBC_7.50while decreased to 0.82×10⁸M^-1s^-1for DBC_11.25.Quantitative calculation of the contribution of reactive species to the photodegradation of SD showed that ³DBC*played a major role in the indirect photodegradation of SD（41.18-66.43%）,and ¹O₂played a minor role（2.50-4.29%）,and·OH did not participate in the reaction.In addition,the preferential adsorption of phenol in DBC on ferrihydrite weakened the back-reduction process of DBC on the oxidized intermediates of SD,that was,weakening the inhibition of DBC on SD photodegradation.The above results suggested that the selective adsorption of DBC on ferrihydrite changed the reactivity between DBC photo-induced ³DBC*and SD and the steady-state concentration of reactive species,further affecting the effect of DBC on photodegradation behavior of SD.（2）The chemical composition and optical properties of DBC at different pyrolysis temperatures（pyrolysis temperatures were 300℃and 500℃,and DBC were denoted as 300℃and 500℃,respectively）were characterized by spectral techniques including UV-vis absorption spectrum,3D-EEM,and FT-IR,and the photodegradation behavior and mechanism of DBC with different pyrolysis temperature on OFL were explored.The results showed that higher pyrolysis temperature reduced the UV absorbance,molecular weight and aromaticity of DBC,and reduced the content of phenol while increased the content of quinone/aromatic ketone substances,and increased the percentage content of humic substances.The photodegradation kinetics experiments showed that DBC with different pyrolysis temperature have significantly different effects on the photodegradation of OFL.The photodegradation rate of OFL in pure water was（6.04±0.06）×10^-4s^-1.DBC₃₀₀significantly inhibited the photodegradation rate of OFL and the photodegradation rate decreased to（3.40±0.10）×10^-4s^-1,while DBC₅₀₀slightly promoted the photodegradation of OFL and the photodegradation rate increased to（6.21±0.16）×10^-4s^-1.The quenching experiments showed that all of ³DBC*,¹O₂and·OH promoted the photodegradation of OFL.At the same time,the lower light screening factor of DBC₃₀₀（0.796）and k _SQ/k _DBC=1.41>1 in DBC₃₀₀system indicated that DBC₃₀₀inhibited the photodegradation of OFL mainly through light screening effect and dynamic quenching effect,and the light screening factor of DBC₅₀₀（0.813）indicated that DBC₅₀₀inhibited the photodegradation of OFL mainly through the light screening effect.The static quenching effect of DBC₃₀₀and DBC₅₀₀had no significant effect on the photodegradation of OFL.The steady-state concentration and quantum yield of photo-generated reactive species of DBC₅₀₀were 2.83-4.15 times and 1.30-1.74times higher than DBC₃₀₀,respectively,which mainly might be due to the higher contents of quinone/aromatic ketone substances in DBC₅₀₀.At the same time,due to the lower content of phenol in DBC₅₀₀,the second-order reaction rate constant of³DBC*and OFL(k _OFL,3_DBC*)of DBC₅₀₀was higher than that of DBC₃₀₀,and the higher pyrolysis temperature enhanced the reactivity of ³DBC*and OFL.DBC₅₀₀had the higher k _OFL,3_DBC*and steady state concentration of reactive species,making the photosensitized degradation of OFL stronger than DBC₃₀₀.