| In order to comprehensively understand the residue, existence form of quinclorac on soil after it was applied to paddy field, and its conversion condition, relevant reactivation mechanism, to fully explain and actively prevent its subsequent hazards to following crop and eco-environment, its adsorption, desorption, kinetics, thermodynamics and hysteresis effect on some soils developed from four parent materials in Hunan and eight kinds of mineral were systematically probed in the laboratory simulation by batch equilibrium technique. The relationship between the mineral structure and adsorption/desorption of quinclorac on mineral was analyzed by Fourier transform infrared spectra(FTIR), X-ray diffraction (XRD) and Scanning electron microscopy (SEM) method. At the same time, the effect of pH value, temperature and ionic strength on the adsorption/desorption of quinclorac was also explored. The effect of low molecular weight organic acid(LMWOA) on adsorption/desorption of quinclorac on the soil and mineral was researched to predict the adsorption/desorption behavior of quinclorac in plant rhizosphere environment, and the effect reason of LMWOA to the adsorption of quinclorac was analyzed with FTIR method. The main results showed as follows:1. The adsorption/desorption properties of quinclorac on the soil and mineral(1) The adsorption isotherm of quinclorac on the soil and mineral was well described by Linear and Freundlich equation when pH value was6.0, the adsorption capability of quinclorac on the mineral was higher than that on the soil. The adsorption quantity of quinclorac on the mineral decreased with temperature increasing when the initial concentration of quinclorac changed from2mg/L to12mg/L, The adsorption free energy(△G) was negative, and it indicated that this adsorption were spontaneous, the absolute value of△G were all less than40kJ/mol on the soil and mineral, this showed that the adsorption on the soil and mineral was mainly physical adsorption. Its adsorption enthalpy changes value (△H) on kaolinite, montmorillonite, zeolite and goethite were positive, which indicated that the adsorption was endothermic reaction, while its△H on diatomite and attapulgite were negative, indicated this adsorption was exothermic reaction, its△H on sepiolite and hematite could be positive or negative, indicated this adsorption was endothermic reaction or exothermic reaction, it decided to the initial concentration of quinclorac.(2) The adsorption of quinclorac on the soil and mineral included both rapid adsorption stage and slow adsorption equilibrium stage. Its adsorption kinetics could be described by the particle diffusion equation, and the contant C in this equation was not zero, which showed that the speed of particle diffusion was not the only factor. In addition, the adsorption kinetics of quinclorac corresponded with the Pseudo-second-order and Elovich equation, while the Pseudo-second-order equation was better than Elovich equation.(3) The organic carbon adsorption constant (Koc) of quinclorac on the soil changed from233.16to1238.10, which showed that quinclorac was easy to be adsorbed on this soil, its mobility was very weak.(4) The desorption isotherm of quinclorac on the soil and mineral could be described by Freundlich and Linear equation too. The desorption of quinclorac existed certain hysteresis on the soil and mineral, it indicated that quinclorac had the potential risk of long-term retention on the mineral and soil.(5) The desorption of quinclorac on the soil and mineral included fast, slow and equilibrium stage. The desorption kinetics of quinclorac conformed to the Pseudo-second-order equation, its desorption quantity depended on the physico-chemical properties of the mineral and soil, as well as the adsorption mechanism of quinclorac.(6) The adsorption acting force of quinclorac on the mineral was multiple. Quinclorac was adsorbed mainly with charge transfer and complexation on montmorillonite and diatomite, with H-bond, charge-dipole, complexation, charge transfer on kaolinite, with H-bond, charge-dipole and complexation on zeolite, attapulgite and sepiolite, with H-bond, complexation and charge transfer on goethite and hematite. The adsorption of quinclorac only occurred on the surface of sepiolite, hematite and diatomite, not reached their interlayer, while its adsorption on kaolinite, montmorillonite, zeolite, goethite and attapulgite not only occurred on the surface but also reached their interlayer.2. Effect of environmental factors on quinclorac adsorption/desorption properties on the soil and mineral(1) The adsorption quantity of quinclorac on kaolinite, montmorillonite, attapulgite, zeolite, diatomite and the soil decreased with the pH value increasing, while on sepiolite, goethite and hematite it firstly decreased, later increased slightly. When the pH value was4.0or5.0, the desorption quantity of quinclorac reached maximum, later decreased with the pH value increasing, The overall change trend of desorption quantity of quinclorac on the soil showed that firstly decreased, later increased slightly with the pH value increasing, The effect of pH value on the adsorption/desorption on the mineral and soil was relation to the dissociation constant of quinclorac, and the zero point charge (ZPC) of the mineral and soil.(2) The maximum adsorption quantity of quinclorac presented at15℃on the soil, decreased when the temperature ranged from15℃to55℃, while the desorption rate of quinclorac on the soil increased with the increase of temperature. However, the effect of temperature on the desorption quantity of quinclorac on the mineral was relatively small.(3) The effect of CaCl2concentration on adsorption quantity of quinclorac on the soil and mineral was relatively complicated. When the CaCl2concentration increased, the adsorption quantity of quinclorac on kaolinite, montmorillonite, attapulgite, the fourth red clay soil, alluvial sandy soil and shale red soil decreased, while increased significantly on purple paddy field soil, the adsorption quantity on sepiolite, zeolite, diatomite, yellow-clay paddy field soil, purple soil, granite red soil and reddish clay soil firstly increased, and later decreased, while firstly decreased, and later increased on goethite, on hematite it changed very small.3. Effect of LMWOA on the adsorption/desorption of quinclorac on the soil and mineral(1) The adsorption quantity of quinclorac on the soil and mineral increased sharply(Within2h), then became steady in the presence of LMWOA. The effect of LMWOA on adsorption kinetics of quinclorac mainly depended on its kind and concentration, as well as the physico-chemical properties of the soil and mineral. The adsorption kinetics of quinclorac on the soil and mineral could be well described by the Pseudo-second-order kinetic equation too in the presence of LMWOA.(2) The adsorption isotherm of quinclorac on the soil and mineral could be well described by the Linear or Freundlich equation in the presence of10.0mmol/L LMWOA. The adsorption capacity of quinclorac on the soil and mineral changed largely with the different types of soil, mineral and LMWOA, it was mainly due to the competition function both LMWOA and quinclorac, and LMWOA could change obviously the pH value of environmental medium, and then changed the surface chemical properties of the soil and mineral.(3) LMWOA concentration had largely influence on the adsorption amount of quinclorac on the soil and mineral, while this effect changed with the different types of soil, mineral, and LMWOA. Because of the bigger complex ability, oxalate had larger enhance effect than the rest of LMWOA (among the six acids), while acetic acid had the smallest effect.(4) The LMWOA’s influences on desorption capacity of quinclorac on the soil and mineral largely changed with its kind and concentration, and the types of soil and mineral. In general, because of the smaller molecular weight and simple chemical structure of LMWOA, acetic acid, succinic acid and malic acid had greater promoting desorption effect than the rest three kinds of LMWOA, which had bigger molecular weight and complex chemical structure. When LMWOA’s concentration was10mmol/L, the acetic acid, succinic acid and malic acid had greater promoting desorption effect than the rest three kinds of LMWOA on kaolinite, sepiolite, goethite, diatomite and zeolite, but the promoting desorption effect were very small on hematite, montmorillonite and attapulgite.(5) Results of FTIR showed that acetic acid, oxalic acid and citric acid could strongly combine on the Fe-O(Si-O) bond, formed the Fe(Si)-O-quinclorac-acetic acid (oxalic acid and citric acid) complexes on the surface of kaolinite, sepiolite, diatomite and hematite.(6) The adsorption quantity of quinclorac on sepiolite, attapulgite, goethite, reddish clay soil and shale red soil was lower than control bank when the mixed LMWOA’s concentration was2mmol/L on the mineral system (5mmol/L on the soil system), it showed inhibition, this inhibition gradually changed into activation by the mixed LMWOA’s concentration increasing, it was to say that the mixed LMWOA’s concentration could increase the adsorption amount of quinclorac. The adsorption amount of quinclorac on the rest soil and mineral gradually increased with the mixed LMWOA’s concentration increasing. The main reason was that the mixed LMWOA could significantly reduce the pH value of soil and mineral, which caused protonation, and could increase the quantity of positive charge on the surface of soil or mineral.(7) When the aqueous solution system contained six LMWOA, and its concentration changed from5mmol/L to80mmol/L, the desorption quantity of quinclorac on alluvial sandy soil, yellow-clay paddy field soil, granite red soil, shale red soil, reddish clayey soil, purple soil, purple paddy field soil, sepiolite, diatomite, zeolite, montmorillonite, hematite and goethite decreased with the mixed LMWOA’s concentration increasing, and it was lower than control bank. The mixed LMWOA could reduce the desorption quantity on the fourth red clay soil at lower concentration, while could promote the desorption quantity at higher concentration, this desorption quantity on kaolinite and attapulgite firstly increased, and then decreased with mixed LMWOA concentration increasing. |
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