Transport Of Glyphosate In Saturated Raw And Manganese Oxide-Coated Quartz Sand

Glyphosate is a widely used herbicide due to its fast degradation and low toxicity.In recent years,however,the toxic effects of glyphosate on human health have been controversial.The detention of glyphosate in soil directly affects whether it is decomposed and transported to the surrounding area.Due to its chemical characteristics,glyphosate is very easy to combine with mineral and organic compounds in soil.If this combination is very close,it is generally impossible to separate glyphosate from the soil through water leaching;on the contrary,it might be transported to the ground water by the rainfall.As glyphosate is mainly degraded by microorganisms,the lack of microbe in groundwater may lead to the detention and accumulation of glyphosate in the groundwater for a long time and bring environmental risk.Current studies mainly focus on the adsorption of glyphosate on soil,and fewer reports on its transport in saturated porous media are obtained.In this paper,the detention/transport of glyphosate in two saturated porous media,quartz sand(Quartz sand,QS)and manganese loaded quartz sand(Manganese oxide coated quartz sand,MOCS),was studied.Batch adsorption experiment was used to determine the sorption capacity of glyphosate on QS and MOCS and the transport of glyphosate in the sand column was investigated by detecting the breaking curve.At the same time,the effects of pH,ionic strength and surface active agent(SAA)on the transport of glyphosate were also investigated.The experimental data obtained were finally simulated by Hydrus-1D software,and the solute transport model of glyphosate in saturated sand column was established.The present study provides theoretical reference for pollution control and risk management of glyphosate and other herbicides in the environment.The main results are as follows:The main results are as follows:(1)The adsorption of glyphosate on QS is weak,while MOCS can adsorb glyphosate.pH showed a negative correlation with glyphosate adsorption on MOCS,and low pH increased the adsorption capacity.The glyphosate adsorption on MOCS increased with the increase of adsorbent dosage.Isotherm data obtained for glyphosate adsorption on MOCS were best fitted to Langmuir model giving the maximal adsorbed amount within monolayer and the adsorption process can be described by pseudo-second order kinetics equation.(2)The mobility of glyphosate in the saturated QS column is stronger than that in the saturated MOCS column.The detention of glyphosate in saturated MOCS column was promoted by lower pH in the range of 3 to 8,and the stronger the acidity was,the more glyphosate was blocked.Ionic strength has little effect on glyphosate transport in MOCS column,but the rise of ionic strength promotes glyphosate detention.The effects of surfactants on glyphosate transpor varied according to their chemical charicteristics.Alkyl polyglycosides have a slight effect on the migration of glyphosate,while organosilicon surfactants have little effect on the transport of glyphosate.The two surfactants on the glyphosate transport in MOCS column showed the same rule,that is,the blocking coefficients of glyphosate reached a peak when the concentration of surfactant is 0.2 g/L,and the block coefficients decreased when the concentration increased afterwards.(3)The experimental data were simulated by Hydrus-1D.The transport of glyphosate in saturated QS and MOCS column can be described by Van Genuchten one-dimensional convection-dispersion model.The correlation coefficients R2 were alarger than 0.98.Among the parameters,the transport of glyphosate was mainly affected by the retention coefficient Kd.Asymmetric breakthrough curves may be the result of non-equilibrium sorption.Changes in different environmental factors let to changes in Kd,resulting in different degrees of trapping in the breakthrough curve.In summary,glyphosate is ease to transport in QS porous media.It might because the lack of available adsorptive site on QS surface.Retention rate of glyphosate in MOCS is higher.This finding would help prevent glyphosate pollution in underground water.Further study of glyphosate transport in other porous media,is needed in the future.