Adsorption And Desorption Characteristics Of Dimethyl Disulfide In Soil And Its Application In Soil Vapor Extraction

In facility agriculture,soil-borne diseases are more and more serious due to continuous stubble planting,and soil fumigation is the most direct and effective way to control soil-borne diseases.Dimethyl disulfide(DMDS)is a new fumigant,which has good control effect on root-knot nematodes,soil-borne pathogens and weeds.Previous studies found that DMDS diffused and distributed in tidal soil and red loam had significant differences in concentration,resulting in unstable control effect or phytotoxicity.In order to solve this problem,it is necessary to clarify the mechanism of diffusing ability difference of DMDS in different soils.Adsorption is a key factor affecting the diffusion and distribution of volatile organic compounds(VOCs)in soil,but the adsorption of soil to DMDS and its role in the diffusion remain unclear.Based on this,we designed a series of experiments to clarify the adsorption/desorption characteristics of soil to DMDS,and analyzed the relationship between the adsorption/desorption characteristics and diffusion capacity.Finally,soil vapor extraction(SVE)technology was combined with soil fumigation.The aim is to provide theoretical guidance and technical support for the safe and sustainable use of soil fumigants.Specific research results are as follows:(1)Adsorption characteristics of DMDS in different soilsThe adsorption kinetics of soil on vapor-phase DMDS was measured.The results showed that the first-order kinetic model could describe the adsorption process well.The maximum adsorption capacity of different soils on vapor-phase DMDS was different.With the increase of soil moisture content,the adsorption capacity of soil to DMDS decreased.The adsorption isotherms of soil to vapor-phase DMDS in gas-solid system,Henry’s law constant(K_H)in gas-liquid system and the adsorption isotherms of soil to DMDS in liquid-solid system were measured by batch equilibrium method in headspace vial.The results show that the linear Henry model and the nonlinear Freundlich model can describe the adsorption process of DMDS in gas-solid and liquid-solid systems well.In the gas-solid system,the Henry adsorption constant(K_d)of different soils was significantly different,indicating that the adsorption capacity of different soils to DMDS gas was different.And K_d decreased significantly with the increase of soil moisture content,that is,soil moisture significantly reduced the adsorption capacity of soil to vapor-phase DMDS.In the gas-liquid system,K_H increased from 0.0332(15℃)to 0.0827(25℃)with the increase of temperature,indicating that more DMDS could be separated from the water phase into the gas phase with the increase of temperature.The Freundlich adsorption constant K’_f was significantly different among different soils,indicating that the adsorption capacity of different soils for DMDS in the solid-liquid system was different.The higher the soil organic matter content was,the stronger the adsorption capacity for DMDS was.(2)Correlation and linear regression analysis between the adsorption capacity of soil to vapor-phase DMDS and soil physicochemical propertiesSpearman correlation coefficient method was used to analyze the correlation between vapor-phase adsorption constant(K_d)and soil physicochemical properties.It was found that the correlation between K_d and soil physicochemical properties changed with soil moisture content.When soil moisture content was 12%,K_d showed a significant positive correlation with soil specific surface area(SSA)(r=0.9527);the K_d in dry soil and soil pore size(<2 nm)showed a significant positive correlation;the correlation between K_d and SSA decreased when soil moisture content reached18%.According to the results of linear regression analysis of K_d and SSA when soil moisture content is 12%,there is a significant positive correlation between them(R~2=0.9385),and a model for predicting K_d or soil adsorption capacity C_s is established:K_d=0.4719×SSA-4.217 R~2=0.9385C_s=(0.4719×SSA-4.217)×C_a(3)Analysis of the relationship between soil adsorption characteristics and diffusion capacity of DMDS in different soilsA vapor-phase diffusion device was designed to minimize the influence of volatilization and degradation of DMDS on diffusion,and the vapor-phase diffusion rate(R_t),which can represent the vapor-phase diffusion capacity of DMDS,was measured.The linear regression analysis results of R_t and Henry adsorption constant K_d showed that there was a significant negative correlation between them(R~2=0.9734).Therefore,a model to predict R_t was established using K_d or SSA as parameters,as follows:R_t=-0.001040×K_d+0.023 R~2=0.9734R_t=-0.0004908×SSA+0.027With the increase of temperature,the gas-liquid distribution coefficient K_H of DMDS increases,and more DMDS are separated from the water phase into the soil air,thus improving the vapor-phase diffusion ability of DMDS in the soil.The linear regression analysis of R_t and Freundlich adsorption constant K’_f showed that the linear relationship between R_t and Freundlich adsorption constant K’_fwas poor,indicating that liquid-solid adsorption was not the main factor causing the difference of vapor diffusion of DMDS in different soils.(4)Desorption characteristics and influencing factors of DMDS in unsaturated soilsA ventilation desorption device was designed to monitor the concentration of DMDS vapor escaping from soil and residual concentration in soil.The results showed that DMDS adsorbed in soil could be quickly released as gas,especially in the initial stage of ventilation,and desorption could be more than 90%in the first 60 min.The desorption behavior of DMDS was affected by soil type,soil moisture content and soil bulk density.In the early ventilation stage,the desorption rate of DMDS from Wenshan soil and low moisture content soil was faster,but it took longer time to complete the desorption from the whole desorption cycle due to the high initial adsorption capacity and difficult separation of DMDS under the above soil conditions.The desorption rate of DMDS decreased with the increase of soil bulk density.Degradation determines the initial desorption amount of DMDS during field use,that is,the slower the degradation of DMDS in soil,the more DMDS would desorb and emit into the atmosphere.(5)Application of SVE technology in soil fumigationIn order to reduce the residue amount of DMDS in soil after film removal and reduce the risk of phytotoxicity to the next crop and the risk of DMDS emission to the environment,an indoor SVE device was designed to remove DMDS from soil.The results showed that the DMDS removal rate was different in different soils,and the DMDS removal rate in Wenshan soil was significantly higher than that in Shunyi soil and Suihua soil.DMDS removal rate increased with the increase of extraction airflow,but it did not increase significantly after the extraction airflow reached a certain peak.Sand culture method was established to determine the residue threshold of DMDS harmful to crops to evaluate the effect of SVE technology.The results showed that DMDS significantly inhibited the growth of cucumber when the concentration of DMDS in sand was up to 4μg/g,and the concentration of DMDS in the soil decreased to less than 1μg/g after 24 h extraction.This concentration proved to be theoretically safe for crops.The results showed that the SVE treated soil had no effect on crop growth index,and the soil was safe for cucumber crop.