The Dynamic Change Of Dissolved Organic Matter And Its Effects On The Sorption-desorption Of Organic Pollutants

Dissolved organic matter(DOM),also called water suluable organic matter,is defined as it can be extracted by water,acid and alkali,like organic matter in water,soil solution and organic materials.The operation definiation of DOM is defined through filtration,the size limit,which is used to differentiate DOM from particulate organic matter,is somewhat arbitrary,but there is an almost universal consensus that it is around 0.45μm.In the past 20 years,much progress has been made in the understanding of DOM functions in terrestrial and aquatic ecosystems.In the presence of DOM,weathering rates can be accelerated,and DOM plays a central role during podsolisation.Furthermore,DOM contains organically bound nutrients such as N,P and S,and DOM dynamics will therefore also affect their mobility and availability.Today,it is commonly acknowledged that DOM can affect the solubility,biodegradation and mobility of organic compounds and thus contributes to pollutant transport or to micronutrient availability.For more attention was payed to organic pollutants,studying the relationship between DOM and organic pollutants will be the frontier of soil science,ecology and environment science.The dynamics of DOM derived from various stages of crop straw decomposition have been studied in this research,its effects on the sorption-desorption behavior of two probe organic pollutants in soil had also been studied,one is herbicide(Bensulfuron-methyl,BSM)and the other is polycyclic aromatic hydrocarbons(Pyrene,PYR).The main results as follows:(1)The dynamic changes of DOM derived from crop straw decomposition was studied by using chemical analysis and XAD-8 resin.The results showed that the content and constituents of DOM was in dynamic changes at all the incubation stages.At the initial decomposition stages,the DOM orignated from water-dissolved matter derived from organic materials,temperately,the primary composition of DOM was carbonhydrates and micromolecular organic acid,its hydrophile was a little strong.With crop straw incubation time prolonged,the DOM was origanted from the product of cellulose and hemicellulose decomposition.Temperately,the DOM was macromolecular matter,its aromaticity and hydrophobic nature increased.The DOC,dissolved nitrogen,dissolved sugar and phenol acid increased gradually until constant after 91 days' incubation,but the compositions were still changing.The correlation annlysis among the different index was carried out. The results showed that:DOC was positive correlated to DPC and HA,and negative correlated to FA significantly at 0.05 level respectively.Dissolved sugar was positive correlated to hydrophilic fraction.DPC was positive correlated to HA significantly at 0.01 level.HA was positive correlated to hydrophobic fraction,and FA was negative correlated to hydrophilic fraction significantly at 0.01 level respectively.(2)The dynamical changes of optical property and structure characteristics of DOM by using wave scanning and Frourier Transform Infra Spectroscopy(FTIR).During the crop straw decomposition,theâ–³logK value increased at first,then decreased,while A₂₈₀,A₂₅₄and E₄/E₆ increased,which indicated that the aromaticity and oxidation degree of DOM increased,their molecular structure became less aliphatic and more complex.The content of carboxyl,carbonnyl phenol hydroxyl increased,methoxy and methanol hydroxyl decreased.The propotion of HA in DOM(PQ value)gradually increased, indicating that FA took up formation faster than HA in the initial stage of incubation.With prolonged incubation,FA transformed into HA.Frourier Transform Infra Spectroscopy indicated that the DOM derived from crop straw decomposition had siminar structure characteristics,such as methyl,methylene,methine,carboxyl,hydroxyl et al.The compounds in DOM were saccharide,aliphatic and aromatic organic acid,hydroxybenzene et al..(4)Sorption of model pollutants in soil was biphasic,a fast sorption phase followed by a slow sorption phase.The equilibrium reached in 8 hours.Likewise,desorption were biphasic,too,containing a fast phase no more than 8 hours,in which a small portion model pollution adsorbed in soil desorbed into aqueous phase quikly,however,the other portion seemed to be resistant to desorption.It was found that the sorption-desorption hystersis of soil was statistically significant.(5)The Freundlich and linear model can comletely explained the mechanism of BSM and PYR sorption and desorption.Within the selected concentration range of the BSM, PYR and DOM in this study,the sorption capacity of soil BSM and PYR reduced significantly after applying DOM into soil,which derived from different decomposing stages.Fuetheremore,there is significant difference(p＜0.01)comparing to control treatment(i.e.,no DOM in sorption-desorption course).The effects of DOM on the sorption capacity of soil BSM were weakened with longer DOM compositon time,but the sorption capacity of soil PYR was reverse to BSM.The hysteresis index(HI)value was much lower and was not statistically significant comparing with control treatment after adding DOM. The DOM accelerated desorption of soil BSM.The restraint and accelaration effects were caused by comprehensive factors.The hydrophilic fraction and fulvic acid palyed the primary role on the BSM sorption-desorption,while hydrophobic fraction and humic acid palyed the primary role on the PYR sorption-desorption.This research was important to direct the reasonable application of organic fertilizer.