The Effect And Mechanism Of DOM Collected From Crop Straw On Adsorption Of Butachlor In The West Sichuan Plain

Amide herbicides are widely used in the weed control of grasses in paddy fields due to their high broad spectrum,low dosage and strong selectivity.However,it is easy to accumulate in the natural environment and poses a potential hazard to the ecological environment.Dissolved organic matter(DOM)derived from crop straws is an extremely active organic component in the environment,and controls the geochemical behavior of organic pollutants(OCs)in farmland soil to some extent.In this paper,butachlor was used as the research object to study its adsorption process in the paddy soil of the West Sichuan Plain.The characteristics of DOM released from the decomposition process of returned rape and rice straws were analyzed.The influence mechanism of DOM and its different molecular weight particle on the adsorption behavior of butachlor in soil was discussed.The research has important significance for accurately assessing the environmental risk of butachlor in the West Sichuan Plain.The main conclusions are as follows:(1)The adsorption kinetics of butachlor in paddy soil collected from the West Sichuan Plain met the two-compartment first-order kinetics model,while the thermodynamic process was consistent with both the Langmuir model and the Freundlich model.The adsorption process showed a certain degree of nonlinearity,but it was still dominated by the distribution.(2)The spectral characteristics of DOM during the decomposing process of returned straw were significantly different,with the dissolution and degradation of aromatic substances and the formation of new humic substances;the aromaticity and molecular weight of the dissolved substances in the rapid leaching stage of straw were relatively low.The dissolution of aromatic components is mainly in the early stage of decomposition(? 10 days);difficultto-decompose cellulose,hemicellulose and macromolecular proteins were mainly degraded in the later stage of decomposition.(3)The fraction of <1000 molecular weight in DOM derived from rapeseed and rice straws accounted for the highest,followed by 10,000,and the least among 1000-10000.The combination of butachlor and DOM was dominated by a single complete static quench.Hydrophobic interaction was one of the major 2 forces between DOM and butachlor.Within a certain range,DOM in high concentration promoted its binding to butachlor.Binding interaction of Butachlor and high molecular weight DOM was stronger than low molecular weight.(4)The adsorption amount of DOM in soil increased with the concentration of DOM.Soil particle-size,specific surface area and surface micropores were the main reasons for differences in the adsorption amount of DOM in soil.The hydrophilic DOM derived from rapeseed straws had stronger adsorption capacity in soil.The Freundlich model fitted best for The thermodynamic process of the adsorption of DOM in soil,exactly opposite to the Langmuir model.Adsorption of DOM in two paddy soils seemed to be nonlinear.(5)DOM with certain concentration could improve the adsorption of butachlor in soil,and its concentration was positively correlated with the degree of promotion.After adding DOM,the adsorption equilibrium time was significantly delayed,and the equilibrium adsorption capacity increased significantly.Before the adsorption equilibrium,the adsorption amount of DOM derived from rapeseed straws in soil was greater than rice straws.However,when the adsorption reached equilibrium,the difference between the two decreased gradually.(6)After the addition of DOM,the adsorption kinetics of butachlor in soil was still consistent with two-compartment first-order kinetics model.Although DOM had a competitive adsorption effect on the adsorption of butachlor in soil,it was more likely to enter the soil by co-adsorption or cumulative adsorption.After adding DOM,the Langmuir model was no longer suitable for explaining the thermodynamic process of adsorption of butachlor in soil.Instead,the Freundish equation as an empirical model behaved excellent correlation.Among the different molecular weight components of same-origin DOM,the high molecular weight DOM had the strongest enhancement effect on the adsorption of butachlor in soil,and the lowest molecular weight DOM had the weakest effect.