Effects Of Freezing-thawing On Dissolved Organic Matters In Soils And Their Binding With Phenanthrene

Dissolved organic matters（DOMs）were ubiquitous in the environment,with heterogeneity and photochemical activity.They got a wide range of molecular weights and contained complex and diverse functional groups,which played an important role in the dissolution,migration,transformation,biodegradability and biotoxicity of hydrophobic organic contaminants（HOCs）and heavy metals.Polycyclic aromatic hydrocarbons（PAHs）were widely present in the global environment,it was a kind of persistent organic pollutants（POPs）.PAHs were not only carcinogenic,teratogenic and mutagenic to animals and humanbins,but also bioaccumulative and genetically toxic.DOMs could combined with PAHs so that they could control the behavior of PAHs in the environment.Because of the heterogeneous characteristics of DOMs,DOMs got lots of influences from natural and human factors.The author extracted two kinds of DOMs from urban park soil（PS）and roadside soil（RS）in Shenyang city,respectively.The basic structure and the changes of their structure after different freezing-thawing treatment were characterized by infrared spectroscopy,ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscopy.Fluorescence quenching method was used to determine the binding constant（K）of DOM with phenanthrene.The change of K values under different freezing-thwaing treatments was also investigated.Besides,the different freezing-thawing treatmens contains different freezing temperature,different thawing temperature,different freezing time and different freezing-thawing times.The difference of K values were also analyzed by correlation matrix method.The reasons of the difference of K and other optical properties between before and after freezing–thawing treatment were also analyzed in this paper.The content of dissolved organic carbon（DOC）of PS and RS were 3.7 mgC·L^-11 and 3.0mgC·L^-1 respectively.PS and RS got similar infrared spectra,ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscopy,which showed that PS and RS had the similar structure and composition.There were benzene ring,phenolic hydroxyl and alcohol haydroxyl functional groups,both of PS and RS contained aliphatic hydrocarbon chain structure and aromatic ring structure.The unit DOC concentration of the degree of aromaticity at 254 nm for PS and RS were comparable.And the main fluorescen components were fulvic-like acids and humic-like acids in both PS and RS.The DOC of PS and RS were decreased by different degree,after different freezing and thawing treatments,and the former one was influenced more deeply than the other one.The DOC of PS ranged from 2.1 mgC·L^-11 to 3.6 mgC·L^-1,while the DOC of RS ranged from 1.6mgC·L^-11 to 2.9 mgC·L^-1.They were all decreased as the freezing temperature increased.Freeze-thaw treatment could significantly enhance the degree of aromatic and humification of DOM extracted from soils.Freeze-thaw treatment also could change the molecular weights of DOM,and produced more chromophoric dissolved organic matter（CDOM）。The result showed that seasonal freezing and thawing processes would deepen the differences between different types of soil DOM on both the stuctures and components.Freezing and thawing treatment resulted in significan changes in three-dimensional fluorescence spectra of PS and RS.The peak of aromatic protein-like were became more obvious when the freezing temperature was higher,or the melting temperature was appropriate,and its content became more as the freezing time was longer,or the freezing-thawing cycle times were more.The content of humic-like acids decreased becaused of longer freezing time of more times of freezing-thawing cycles.The content of fulvic-like acids would decreased as the freezing time longer,while it would decreased first then increased as the freezing-thawing times were more.Different freezing and thawing conditions significantly changed the contribution of each fluorescent components to the total amout of fluorescent substances in DOM,and the fluorescent components were able to convert to each other.The fulvic-like acids became the most abundant fluorescent substance in DOM because of freezing and thawing treatment.The macromolecular fused ring structure was destroyed by freezing-thawing processes,which produced more phenolic compounds such as fulvic acids,phenols and terpenoids.The humin-like substance would decrease,however the tyrosine-like substance would increased as the freezing time became longer or the times of freezing–thawing cycles became more.All the Stem-Volmer plots showed obvious linear characteristic in this paper,introduced that the main mechanism for the combination of all the DOM tested in this paper with phenanthrene was hydrophobic distribution.The K values of both PS and RS were 3.5×10⁴ L·Kg^-1.All the different freezing-thawing conditions had extremely significant effects on the K values at 0.01level,including different freezing temperature,different thawing temperature,different freezing time and different freezing-thawing times.Besides there existed significant difference between PS and RS.The K values of PS ranged from1.6×10⁵ L·Kg^-11 to 8.4×10⁵ L·Kg^-1,while the K values of RS ranged from 1.5×10⁵ L·Kg^-11 to 3.7×10⁵ L·Kg^-1,both of them decreased with increasing freezing temperature,moreover,they were linearly related to the freezing temperature.The K values of PS and RS exhibited quadratic polynomials as a function of freezing time and freezing-thawing cycle times,they were all became larger than before the freezing-thawing treatment one order of magnitude.The increase in the binding constant between DOM and PAHs would improve the mobility of PAHs existed in the environment on the one hand,it would also reduce the bioavability of PAHs on the other hand.The correlation analysis showed that the aromatic rings were the core functional group of DOM binded with phenanthrene.The aromatic ring content was significantly correlated with fulvic-like acids,humic-like acids and aromatic-like proteinⅠ.The aromatic of DOM after freezing-thawing treatment had a great relationship with tyrosine-like,humic-like and the total amount of fluorescent substances,but there were obvious differences among different DOM with different kinds of sources.