| Sulfonylurea herbicides are the novel herbicides and exhibit highly selective herbicidal activity, as well as low mammalian toxicity. This type of herbicides is widely used in many crops to control annual broadleaf and gramineal weed at ultra-low application dosage. However, the very low level residues of sulfonylurea herbicides in soils and water would bring very serious pollution to environment, ecological toxicity and phytotoxicity to crops. Sulfonylurea herbicides are highly phytotoxic to some current or subsequent crops at only 1% or even less of the originally applied amount, resulting in growth inhibitions of certain subsequent crops. Methiopyrsulfuron and monosulfuron-ester are the relatively new selective sulfonylurea herbicides developed by Hunan Research Institute of Chemical Industry and Pesticide National Engineering Research Center (Tianjin), Nankai University. To date, little data are available on their environmental fates, especially adsorption, desorption, and leaching. The objectives of this study were ascertaining the influences of soil physical and chemical properties, experiment conditions, surfactants, sorbents, and organic amendments on adorption, desorption, and leaching of methiopyrsulfuron and monosulfuron in soils for environmental risk assessments after their application. The main results were summarized as follows:1 Adsorption-desorption characteristics of methiopyrsulfuron and monosulfuron-ester in seven type of soilsMethiopyrsulfuron and monosulfuron-ester adsorption and desorption were measured in seven type of soils sampled from Heilongjiang, Shandong, Jiangxi, Sichuan, Anhui, and Chongqing provinces of China using a batch equilibrium method. The results showed that their adsorption and desorption were well-described by the Freundlich equation, which showed their adsorption isotherms were nonlinear with 1/nf-ads less than 1.0, resembling the L-type curve. Methiopyrsulfuron and monosulfuron-ester adsorption abilities were relatively low in seven tested soils with different physicochemical properties, and free energy change of adsorptions (ΔG) indicated that physical adsorption was dominant in soils and irreversible adsorption existed during the process. The results of the linear correlation analyese between Kf-ads and the soil physicochemical properties showed that soil pH and organic matter content (OM) were the main factors influencing methiopyrsulfuron adsorption; methiopyrsulfuron adsorption was negatively correlated with pH and positively correlated with OM. Methiopyrsulfuron desorption was hysteretic on soils with higher OM content and lower pH. Soil pH, OM content, and clay content were the main factors influencing monosulfuron-ester adsorption and desorption. Monosulfuron-ester adsorption and desorption were negatively correlated with pH while positively correlated with OM and clay content.Solution pH, temperature, background electrolyte, soil/water ratio, and part removal of OM could affect methiopyrsulfuron and monosulfuron-ester adsorption in soils. Methiopyrsulfuron or monosulfuron-ester adsorption decreased as solution pH increased, and kept constant as solution pH rose to a certain value. Methiopyrsulfuron or monosulfuron-ester adsorption capacity slightly increased when the temperature increased from 15 to 35℃or soil/solution ratio increased from 1:25 to 1:2.5 in soils. Methiopyrsulfuron or monosulfuron-ester adsorption all increased with increasing CaCl2 concentration, whereas its adsorption decreased significantly when soil organic matter was removed partially.2 Effects of surfactants on methiopyrsulfuron and monosulfuron-ester adsorption in soilsThe effects of anionic surfactant sodium dodecyl sulfate (SDS), cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB), and nonionic surfactant Tween-80 on methiopyrsulfuron and monosulfuron-ester adsorption in soils were investigated in laboratory studies. The results showed that the type or concentration of surfactant could influence methiopyrsulfuron and monosulfuron-ester adsorption in soils. Below the critical micelle concentration (CMC) of SDS, methiopyrsulfuron or monosulfuron-ester adsorption in S1 and S7 was increased as SDS concentration increased; around the value of CMC, methiopyrsulfuron or monosulfuron-ester adsorption in S1 and S7 reached the greatest adsorption abilities; while above the CMC, methiopyrsulfuron or monosulfuron-ester adsorption in SI and S7 decreased slightly. CTAB could increase methiopyrsulfuron or monosulfuron-ester adsorption in S1 and S7. The Kf-ads values of methiopyrsulfuron or monosulfuron-ester in S1 and S7 increased with increasing concentration of CTAB. Nonionic surfactant Tween-80 could reduce methiopyrsulfuron or monosulfuron-ester adsorption in S1 and S7. Within the range of five tested concentrations, the Kf-ads values of methiopyrsulfuron or monosulfuron-ester in S1 and S7 were less than the corresponding controls.3 Adsorption characteristics of methiopyrsulfuron and monosulfuron-ester in five sorbebts Methiopyrsulfuron and monosulfuron-ester adsorption in five typical sorbents including humic acid (HA), montmorillonite, kaoline, Na-bentnite, and diatomite could be well described by the Freundlich equation, which suggested their adsorption isotherms in five sorbents were nonlinear with 1/nf-ads less than 1.0, resembling the L-type curve. The five tested sorbents had strong adsorption abilities to methiopyrsulfuron and monosulfuron-ester. The order of Kf-ads value was:HA>kaoline > montmorillonite> Na-bentnite> diatomite. Their adsorption abilities were negatively correlated with pHs. The Freundlich equation described methiopyrsulfuron and monosulfuron-ester adsorption in CTAB modified organic clays well. The adsorption isotherms of CATB-kaoline, CTAB-montmorillonite, and CTAB-Na-bentnite were S-type curve, while the adsorption isotherm of CTAB-diatomite was L-type curve. The adsorption abilities of CTAB modified organic clays were stronger than those of raw clays. The Kf-ads values of methiopyrsulfuron on CTAB-montmorillonite, CTAB-kaoline, CTAB-Na-bentnite, and CTAB-diatomite increased 119.15,163.38,72.85, and 81.15 times respectively compared to the raw clays, and increased 114.64,140.77,90.18, and 99.68 times espectively for monosulfuron-ester. The order of Kf-ads values for methiopyrsulfuron or monosulfuron-ester on CTAB modified organic clays was: CTAB-kaoline> CTAB-montmorillonite> CTAB-Na-bentnite> CTAB-diatomite.4 Effects of organic amendments on methiopyrsulfuron and monosulfuron-ester adsorption-desorption in soilsThe effects of organic amendments on methiopyrsulfuron and monosulfuron-ester adsorption-desorption in soils were determined in laboratory studies using a batch equilibrium method. Organic amendments used in the experiments were peat, sewage sludge, and HA. The results showed that methiopyrsulfuron or monosulfuron-ester adsorption-desorption in organic amendments and organic amended soils fitted the Freundlich model well. Methiopyrsulfuron or monosulfuron-ester adsorption isotherms were nonlinear with 1/lf-ads value less than 1. Methiopyrsulfuron or monosulfuron-ester adsorption isotherms in organic amended soils belonged to L-type curve, except that methiopyrsulfuron adsorption in 2% HA amend soil was S type curve. Organic amendments resulted in an enhancement of adsorption capacities of soils. The Kf-ads values of methiopyrsulfuron or monosulfuron-ester in organic amended soils increased with increasing organic amendment concentrations in soils. Organic amendments resulted in an decrease of desorption capacities of soils. The Kf-des values of methiopyrsulfuron or monosulfuron-ester in organic amended soils increased with increasing organic amendment concentrations. The Kf-des values of methiopyrsulfuron or monosulfuron-ester in organic amended soils were obviously greater than those in natural soils. The 1/nf-des values of methiopyrsulfuron or monosulfuron-ester in organic amended soils were obviously smaller than those in natural soils, which implied that organic amendments could improve methiopyrsulfuron and monosulfuron-ester desorption hystersis and prevent their leaching in soils.5 Leaching of methiopyrsulfuron and monosulfuron-ester in soilsLeaching of methiopyrsulfuron and monosulfuron-ester in three soils was investigated using soil column leaching method under laboratory conditions. The results showed that methiopyrsulfuron or monosulfuron-ester was easily mobile in three soils. The correlation between methiopyrsulfuron leaching ratio from soil column and pH was significant, while there was no significant correlation between methiopyrsulfuron leaching ratio and the other soil physicochemical properties. There was also no significant correlation between monosulfuron-ester leaching ratio and soil physicochemical properties, suggesting that monosulfuron-ester leaching in soils was related with all of soil physicochemical properties. Leaching water volume, leaching speed, herbicide application dosage, simulated acid rain, surfactants, and organic amendments could influence methiopyrsulfuron or monosulfuron-ester leaching in soils. Methiopyrsulfuron or monosulfuron-ester leaching ratio from three soils increased with leaching water volume and herbicide application dosage, but decreased with leaching speed at the same leaching water volume. pH values of simulated acid rain played an important action on methiopyrsulfuron or monosulfuron-ester leaching. The lower pH values of simulated acid rain had greater contribution to the retentions capability of soils to methiopyrsulfuron or monosulfuron-ester. CTAB and SDS could prevent methiopyrsulfuron or monosulfuron-ester leaching in three soils, while Tween-80 could promote methiopyrsulfuron or monosulfuron-ester leaching in three soil. Organic amendments could decrease methiopyrsulfuron or monosulfuron-ester leaching ratio from three soil. Leaching of methiopyrsulfuron or monosulfuron-ester in three soils was negatively correlated with organic amendment concentrations in soils.In conlusions, this article studied adsorption, desorption, and leaching characteristics of methiopyrsulfuron and monosulfuron-ester in soils, and the effects of soil physicochemical properties, experiment conditions, surfactants, sorbents, and organic amendments on their environmental behaviors in soils were also investigated systematically. The results showed that the adsorption abilities of methiopyrsulfuron and monosulfuron-ester were relatively low in seven tested soils, and their desorption for all tested soils was hysteretic. Soil pH and organic matter content were the main facters influencing methiopyrsulfuron and monosulfuron-ester adsorption-desorption in soils. The herbicides were mobile in soils when tested using soil columns leaching method. Adsorption, desorption, and leaching characteristics of methiopyrsulfuron and monosulfuron-ester indicated that they had potential risk to environment and crop. Five tested sorbebts and three organic amendments had strong adsorption capability to methiopyrsulfuron and monosulfuron. Organic amendments could significantly improve herbicide adsorption and decrease leaching behavior in soils. The study on methiopyrsulfuron and monosulfuron-ester could offer scientific basis for how to effectively use the herbicides and reduce potential pollution to environment. At the same time, it would correctly assess their mobility, transformation, and would take effective and scientific measures to monitor and manage the contamination, to remediate the polluted soil. |
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