Mechanism Of The Effect Of DOM Collected From Crop Straw In The Decomposing Process On Adsorption Of Sulfamethoxazole In The West Sichuan Plain

Sulfonamides antibiotics are widely used in the treatment of human and animal diseases due to their stable chemical structure,easy to produce and broadspectrum antimicrobial effects.They can be accumulated in the natural environment easily and cause potential hazards to environment.Dissolved organic matter(DOM),which is a very active organic component in the environment,can control the environmental geochemical behavior of organic pollutants(OCs)in agricultural soils to a certain extent.Sulfamethoxazole(SMX)was used as the research object.This paper studied the adsorption process of SMX in the rice soil of the West Sichuan Plain.The characteristics of DOM released from rape and rice straws during the decomposition process were analyzed.The influence mechanism of the adsorption behavior of DOM-SMX in soil was discussed.It has great significance to accurately assess the environmental risk of SMX in the West Sichuan plain.The main conclusions are as follows:(1)The adsorption kinetic characteristics of SMX in rice soils collected from West Sichuan Plain suited the double-chamber first-order kinetic model.The thermodynamic process not only conformed to the Langmuir model but also to the Freundlish model,and showed certain nonlinear characteristics,but the distribution effect played the main role in this process.With the temperature changed,the adsorption capacity of Jinjiang rice soils on SMX changed irregularly,while the adsorption of Longxing rice soil was a spontaneous and endothermic process.(2)The aromaticity of DOM released from the rice straws in the initial period was higher than the rape straw.The aromaticity of two crop straws was both increased with the process of decomposition.The results of infrared spectrum analysis showed that the DOM released from two crop straws contains amino acids,lipids,sugars and cellulose,which transformed into humic substances,inorganic carbonates or ammonium salts after decomposition.It could also conclude that the contents of the fluorescent components were significantly different in the DOM released from two crop straws by using three-dimensional fluorescence spectrum.But the differences were weakened after a long term of humification.(3)The DOM derived from straws promoted the adsorption of SMX in soil in a certain concentration range,and the concentration was positively correlated with the degree of promotion.It was due to the adsorption capacity of the DOM in the soils.On one hand,the addition of DOM might increase the amount of soil organic matter,thus enhanced the distribution effect.On the other hand,it might be the effect of cumulative adsorption and co-adsorption.(4)Regardless decomposed or not,the DOM derived from straw showed a significant promoting effect on the adsorption of SMX in soils.With the prolongation of the decay time,the promoting effect gradually weakened.And the effect of DOM became stable after 90 days of decomposition.The increase of the adsorption of SMX DOM derived from decomposed straws was lesser than nondecomposed DOM,and it was because the proportion of hydrophobic components.(5)Whether the DOM derived from decomposed straws or not,it's effect on the adsorption of SMX was similar.In the initial stage,the adsorption of SMX in soils was increased rapidly,with time prolonged,the total amount of adsorption was promoted.The DOM decreased the proportion of the contribution of fast adsorption in the initial stage and slow adsorption in the subsequent stage.Meanwhile,it weakened the characteristic of the three-parameter biphasic firstorder adsorption model.At the initial stage of adsorption,the adsorption rate of SMX in the soil increase rapidly,which is related to the co-adsorption of DOMSMX.The adsorption equilibrium time was prolonged,which might due to the cumulative adsorption phenomenon.The increase of the total amount of equilibrium adsorption was related to the increase of the total organic content and the adsorption site,and both of them were provided by the external DOM derived from straws.(6)Whether the DOM derived from decomposed straw or not,the effect on the thermodynamics of the adsorption of SMX was similar.The adsorption process suited the Freundlish model rather than the Langmuir model.The adsorption process was still dominated by the partition-like interaction of OCs and DOM,and it was still a spontaneous and endothermic process.The effects of the DOM of decomposed and unhydrolyzed straws on the Gibbs free energy of the adsorption process were not significant different.After the addition of the DOM derived from decomposed straws,the entropy change and the enthalpy change of the soil adsorption process of SMX were different from the untreated rapeseed straw,which might occurre at the initial stage of adsorption,and it was because the components and functional groups of the DOM derived from decomposed straws have changed.