The Mechanism Of Butachlor Adsorption On Soils And Soil Components

Soil is an important natural resource and a basic component of ecological environment. With the rapid economic development, a sharp rise in population and the acceleration of industrialization process, more and more toxic substances enter into the environment, and soil pollution is becoming a serious problem. The organic pollution is much greater as compared to the heavy metal contaminations. Herbicide is one of the important soil organic contaminants. Butachlor is a pre-emergence herbicide belonging to chloroacetanilide group which is used widely in oriental countries for the control of annual grasses, and is one of the three most used herbicides in China. Butachlor is toxic to aquatic organisms. Especially, it has genotoxicity to the amphibian animals. Moreover it could induce apoptosis in mammalian cells. Application of butachlor in soil can cause toxicity to earthworms, change the microbial populations and enzyme activities, and sometimes adversely affects the growth and activities of beneficial microorganisms in soils. A commonly used herbicide in tropical rice soils, even at field-application level, would influence the biogeochemical cycling of CH4 by inhibiting methanogenic bacterial populations and their activities. The sorption and desorption are the most important processes which control other processes such as bioavailobility, biodegradation, persistence, and leachability in soil. Investigation of the sorption/desorption of butachlor in soil is essential to understand the forementioned processes.This dissertation is aimed to reveal the distribution pattern of butachlor in the soil/mineral/organic matter-water system, to understand the contribution of soil components to butachlor adsorption as well as to explore the mechanisms of butachlor adsorption on different soil components. In our study, the thermodynamics and kinetics of butachlor sorption in 13 soil samples with different physical and chemical properties was investigated. Soil samples were collected from eleven provinces from south (19°N) to north (47°N) in eastern China. Furthermore, on the basis of above study, the different particle-size fractions of organo-mineral complexes, montmorillonite, kaolinite, amorphous oxides and humic acids (HAs) were used to investigate the butachlor sorption behavior and mechanisms. The main results are summarized as follows:(1)Butachlor sorption by 13 soils was well described by Freundlich equation. The quantity and quality of organic matter were generally the preponderant factors in influencing the butachlor sorption on the soils, but was affected by inorganic fractions, such as clay and amorphous sesquioxides. Combination of the data obtained from the 13 soils in the present study with other 23 soil samples reported by other researchers in the literature showed that K_oc would be a poor predictive parameter for butachlor adsorption on soils with tatal oranic carbon (TOC) content higher than 4.0% and lower than 0.2%.The relative importance of organic matter and clay in butachlor adsorption will depend on the ratio of clay to organic carbon content (RCO), which is a useful parameter to predict the butachlor behavior in different soils. The soils with the ratio of clay content to TOC content (RCO) values less than 60 adsorbed butachlor mainly by the partition into soil organic matter matrix. The soils with RCO values higher than 60 apparently adsorbed butachlor by the combination of the partition into soil organic matter matrix and adsorption on clay surface.(2)Butachlor sorption by soils apparently equilibrated in 24h, but the soil with special low total organic carbon (TOC) content (<0.5%) needed longer time (60h). The processes of butachlor sorption could be divided into fast and slow reaction stages. Total organic carbon was the major factor in influencing the rate of butachlor sorption; clay and CEC also affected to a certain degree. Kinetics of butachlor sorption could be fitted well by the Elovich equation and the dual constant equation, which could be better understood when both of the equations were used together.(3)The results on sorption/desorption of butachlor by different size organo-mineral complexes isolated from the black, yellow-brown, yellow soils and latosols indicated that about 53%-70% of butachlor was adsorbed on clay fraction; 47%-31% on silt fraction; and less than 4% on fine sand fraction. Hysteresis occurred in all systems, and the hysteresis decrease in the order: clay>silt>fine sand. For many fractions, the K_d values are concentration dependent at low equilibrium conc entration (<1.5 mg L^-1), but tends to be constant at high concentration. Accordingly, the nonlinearity adsorption was significant at low equilibrium concentrations.(4)Sorption of butachlor by pure minerals (Montmorillonite, Montmorillonite-Ca, Kaolinite, Kaolinite-Ca, Fe hydrous oxides, Al hydrous oxides) and humic acids extracted from four soils were measured to obtain additional perspective on the potential contribution of both clay minerals and soil organic matter to contaminant retention in soils. The sorption affinity for butachlor was in the order: HAs >> Montmorillonite > Montmorillonite-Ca >> Kaolinite > Kaolinite-Ca > Al hydrous oxides > Fe hydrous oxides. The K_d values were concentration (C_e) dependent except the HAs-water system. The adsorption of butachlor by HAs was a partition mechanism between water phase and organic phase. The K_d values were nearly constant with increasing C_e. Therefore, the adsorption isotherm was linear type. For the montmorillonite/Ca-montmorillonite-water system, the K_d values increased with anincrease in C_e, approaching a constant value at high C_e values. The isotherm was S-type which results from the competitive adsorption of H₂O molecule on mineral hydrophilic surface at low butachlor concentration. However, for the hydrous oxides-water system, the K_d values decreased with increase in C_e, also approaching a constant value at high C_e values, and the isotherm was L-type. Chemical bonding occurred between the hydroxyl on oxides surface and the butachlor molecule resulting in strong hysteresis.(5)The sorption capacity of black soil and latosol treated by Fe hydrous oxides is much larger than before treatment, and the degree of nonlinearity for isotherm at low concentration is also larger than before treatment. An explanation is that the hydroxyl oxides adsorbed butachlor. Another interpretation is that the characteresties of SOM in soil might change, resulting in an increased adsorption of butachlor.(6)The K_d values for black, yellow-brown, yellow soils and latosol treated by H₂O₂ is significantly smaller than before treatment, The K_d values increased with increase in C_e, which is the inverse of untreated soils. The K_oc values of soils treated with H₂O₂ are 2.4-3.7 folds greater than untreated soils, and larger than the K_oc values of HAs, which result from the adsorption contribution of inorganic components. For untreated soils, some of the SOM associated with clay, and even wrapped by clay, and their adsorption function was prevented by clay, which account for lower K_oc value than HAs.(7)The reasons for variations in the K_oc values are as follows: a) The chemical characteristics/nature of the organic matter varied with soil/aggregate size, which led to difference in polarity, and resulted in differences in sorption capacities. b) The pattern and extent of closeness of SOM associated to the inorganic components varied with soils/aggregate size, which resulted in differenced in sorption affinities. C) The adsorption attribution of inorganic components originating from soils samples with low TOC may have contributed to larges values of the estimated K_oc than the true values.(8)The formula K_d=K_dmin+f_oc×K_oc, is we developed on the assumption of onlyorganic carbon and inorganic components play the adsorption role, can easily identifythe inorganic component attribution to overall adsorption and estimate the thresholdf_oc at which inorganic contributions to overall adsorption will be measurable(approximately 10% of overall adsorption).