Adsorption-Desorption And Biodegradation Characteristics Of Pesticides On Soil Clay Minerals

Sorption and degradation are the two important processes governing the fate of pesticides in soil system.Sorption on minerals is shown to control the transport, transformation and biological processes of pesticides in the subsurface.Biodegradation is a fundamental attenuation process for pesticides and an important performance of soil purification.It is meaningful to investigate the sorption-desorption and biodegradation of pesticides for evaluating the potential effects on soil and groundwater and developing the soil chemical remediation and bioremediation.In this study,montmorillonite,kaolinite and goethite were selected as model minerals.The adsorption and desorption characteristics of methyl parathion(MP),carbaryl and their degrading cells on clay minerals were obtained.Batch experiments were applied to determine the biodegradation kinetics of the immobilized pesticides.Microbial activities in the absence and presence of montmorillonite,kaolinite or goethite were monitored by microcalorimetry.The main contents were outlined as follow:1.Chemical adsorption-desorption and Fourier transform infrared spectra(FTIR) were employed to investigate the adsorption characteristics of MP and carbaryl on soil clay minerals.The results showed that the adsorption of MP on clay minerals fitted Freundlich equation very well,and the adsorption of carbaryl more fitted Langmuir equation.Montmorillonite had a greater adsorption capacity for the two pesticides than goethite and kaolinite.Elovich equation was the optimum model for their adsorption kenitics,of which the coefficients(R~2) ranged between 0.93 and 0.98,indicating that the adsorption was a non-homogeneous diffusion process.In addition,the adsorption amounts of pesticides first decreased then increased with the increasing of the pH value. The binding strength of pesticides on minerals followed the sequence:montmorillonite＞kaolinite＞goethite.The FTIR spectra demonstrated that hydrogen bond and electrostatic bond could be formed in the adsorption process.2.The biodegradation kinetics of mineral-sorbed MP was determined.The metabolic activities of MP-degrading cells in the presence of various clay minerals or using free/immobilized MP were monitored by microcalorimetry.The biodegradation of MP was significantly impacted by the concentration of pesticide and the nature of minerals.For the free MP,the pesticide with a low concentration was degraded more rapidly than that with a high concentration.The degradation of sorbed MP proceeded at a rate faster than free pesticide over the initial 7 h of incubation.However,the percent degradation of free MP was obviously higher than that of immobilized pesticides afterwards.The differences of biodegradation were also observed when MP was sorbed on various minerals.The percent degradation of sorbed MP followed the sequence: montmorillonite＞kaolinite＞goethite,which was in accordance with the binding strength of MP on minerals.The presence of montmorillonite or kaolinite depressed the first exponential growth of MP-degrading cells but stimulated their second growth. However,goethite displayed inhibitory effect on bacterial activity at all times.The biodegradation of sorbed MP may be dominantly controlled by the activities of the MP-degrading microorganisms.The sorption characteristics of methyl parathion on minerals could be the secondary factors.3.Chemical adsorption-desorption and FTIR were employed to investigate the adsorption characteristics and mechanisms of MP-degrading cells on soil clay minerals. The kinetics of MP biodegradation by the immobilized cells were investigated.The metabolism curves of free/immobilized cells using MP were monitored by microcalorimetry.The adsorption amounts and the binding strength of MP-degrading cells on minerals examined followed the sequence:montmorillonite＞kaolinite＞goethite.The maximum adsorption of cells adsorption on montmorillonite and kaolinite occurred at pHs equivalent or close to the zero point charge of the two clay minerels. There were a variety of forces such as Van der Waals forces,electrostatic interactions, ligand exchange and hydrogen bonding in the process of MP-degrading cells adsorbed on minerals.The Van der Waals forces could dominated cells adsorption on montmorillonite, and the cells were adsorbed on kaolinite predominantly by non-electrostatic forces(e.g. hydrogen bond).The ligand exchange and electrostatic force were considered to be the main forces controlling the sorption of cells on goethite.Compared to free cells,the metabolism abilities of immobilized cells significantly weaken in the low MP concentration system.The presence of minerals enhanced the MP degradation in the high pesticide concentration system at the first 12h.Afterwards,the percent degradation of MP by immobilized cells was lower than that by free cells.The percent degradation of MP were different when cells sorbed on various minerals,which followed the sequence of montmorillonite＞kaolinite＞goethite.Montmorillonite presented a lower affinity for cells as compared to goethite and kaolinite,but it could enhance the activity of MP-degrading cells and further stimulated the bioavailability of pesticide.However, goethite displayed an inhibitory effect on the bacterial activity because of the tight attachment of bacterial cells and hindered the substrate biodegradation.4.The biodegradation kinetics of mineral-sorbed carbaryl was determined and the metabolic activities of carbaryl-degrading cells in the presence of various clay minerals were assessed by microcalorimetric technique for the first time.The impact on sorbed carbaryl bioavailability were illustrated by microbial activity and previous sorption experiments.The results showed that both degradation efficiency and rate of sorbed carbaryl were significantly impacted by the presence of minerals.The degradation of mineral-sorbed carbaryl proceeded at a rate faster than free pesticide over the initial 20 h of incubation.However,the percent degradation of aqueous carbaryl was obviously higher than that of immobilized pesticides afterwards.The differences of biodegradation were also observed when carbaryl was sorbed on various minerals.The percent degradation of sorbed carbaryl followed the sequence:montmorillonite＞kaolinite＞goethite,which is in accordance with the binding strength of carbaryl on minerals.The presence of montmorillonite or kaolinite enhanced the bacterial activity and further stimulated the bioavailability of carbaryl.However,goethite displayed inhibitory effect on bacterial activity and reduced the substrate degradation.The biodegradation of mineral-sorbed carbaryl may be dominantly controlled by the activities of the carbaryl-degrading cells.The sorption characteristics of carbaryl on minerals were considered to be the secondary factors.