Environmenal Fates Of Five Sulfonylurea Herbicides In Soils

Sulfonylurea herbicides are an unique group of herbicides used to control a range of weeds and some grasses in a variety of crops and vegetables. Because of their low application rate, low mammalian toxicity, and unprecedented herbicidal activity, sulfonylurea herbicides have become very popular worldwide. On the other hand, the wide application would bring serious pollution to environments, ecological toxicity and injury to following crops even at very low level of the residue in soil. It is important to understand the environmental fates of sulfonylurea herbicides in soil. Therefore, this study was initiated to investigate the environmental fates of five sulfonylurea herbicides (nicosulfuron, bensulfuron-methyl, chlorimuron-ethyl, monosulfuron and metsulfuron-methyl) in eight type soils to provide some basic informations for safety use of these herbicides in field. The main results were summarized as follows:1. Adsorption-desorption characters of 5 sulfonylurea herbicides in 8 type of soilsThe adsorption-desorption experiments of the herbicides in soils were conducted by the batch equilibration and HPLC techniques. The results showed that the low adsorption ability of 5 sulfonylurea herbicides was found in 8 tested soils with different physical and chemical properties, and the free energy of adsorptions indicated the spontaneity of the given adsorption process of the herbicides in soils and unreversed adsorption existed during the process. The adsorption-desorption isotherms of the herbicides in the tested soils was nonlinear and fitted with Frendlich equation well. Furthermore, the difference of adsorption capability occurred in different soils to the herbicides. Among all tested soils, the red soil and black soil had strongest affinity with herbicides, and the sand soil had weakest affinity with herbicides, which led to higher adorptions to the herbicides for red soil and black soil and lowest adorption for sand soil. Further linear regression analysis showed that the product of Frendlich constants of adsoprion, K_ads-f(1/n_ads), had a positive correlation with organic matter content (OM %) and clay content (Clay %), but negative correlation with pH value of soils. The results of desorption indicated that the hystersis phenomena appeared during the desorption process for the herbicides in tested soils mostly.It also was conducted that analysis on correlation between hystersis phenomena and soil properties. The hystersis coefficients (H) decreased with OM % and Clay % increasing and increased with increasing of soils pH. Further calculation and analysis about K_oc, the mean partial free energy and the free energy of adsorption of the herbicides in soils performed middle or higher translocation in soils, which indicated that sulfonylurea herbicides had potential risk to groundwater or surface water.2. Adsorption characters of 5 sulfonylurea herbicides in sorbebtsThe batch equilibration and HPLC techniques were applied to investigate the adsorption characters of the herbicides in 4 typical sorbents including humic acid (HA), montorillonite, kaolin and diatomite. The results showed that the tested sorbents had strong adsorption capability to 5 sulfonylurea herbicides and the adsorption isotherms obeyed the Frendlich model. In all cases the isotherms for the herbicides in sorbents were nonlinear with slopes (1/n_ads) of <1, resembling the L-type curve. This shape suggested a relatively high affinity of the herbicides for the adsorbing sites in sorbents and the affinity decreased with the concentration of herbicides increasing. The results also showed that diatomite had lowest affinity for tested herbicides, however, kaolin had highest affinity for 3 herbicides including bensulfuron-methyl, chlorimuron-ethy(?) and nicosulfuron, montorillonite for monosulfuron and HA for metsulfuron-methyl3 Effects of fertilization on adsorption of 5 sulfonylurea(?)rbicides in soilsThe effects of the fertilizations using on 5 sulfonylurea herbicides adsorption by mixtures of soil and fertilizations through adding fertilizations to soil at different rates. The adsorotion isotherms of the herbicides in the mixtures of soil and the fertilizations were nonlinear and fitted Frendlich equation well. The isotherms for the herbicides in mixtures were nonlinear with stapes (1/n_ads) of <1, resembling the L-type curve, which suggesteds a relatively high affinity of the herbicides for the adsorbing sites in mixtures of soil and fertilizations, and the affinity decreased with the concentration of herbicides increasing. Among the fertilizers, the compost and sewage sludge addition could enhance the soil capacity of the herbicides adsorption obviously, and the positive correlation between rate of fertilizations in mixtures and adsorption coefficient was approved by linear regressions. Negative effect of plant ash on the herbicides adsorption in soil was found during the experiment, and correlation between rate of fertilizations in mixtures and adsorption coefficient was also approved by linear regressions. The phenomena were governed by amending the carbon content (OC %) of soil. The positive effect was found in soils amended with compost and sewage sludge because the fertilizations had higher OC % , which proved to hold more adsorption sites for organic matter. But there was a very low amount of OC in plant burned ash, which resulted in negative correlation to soils amended by plant burned ash. Furthermore, the addition of fertilizations would influence the distribution coefficient (K_d) mean partial molar free energy change (-F) of herbicides in amended soils, but no correlations were found.4. Effects of simulated acid rain on leaching of 5 sulfonylurea herbicides in soilsThe effects of simulated acid rain on leaching of 5 sulfonylurea herbicides in soils were investigated under laboratory condition. The herbicides were mobile on soils when tested using soil columns and leached by simulated acid rain with varied pH values. It was found that the herbicides were more mobile in soils with lower content of OM% and Clay% than in soil with the higher and there was a close relationship between the leaching of the herbicides and the properties of soil. The results also showed that pH values of simulated acid rain played an important action on the leaching. The lower pH values of simulated acid rain had greater contribution on the retentions capability of soils to herbicides, and the leaching amounts of the herbicides increased with increasing acid rain amount. Moreover, it was found primary that the herbicides with lower solubility had stronger retention in soil than the herbicides with higher solubility.5. Hydrolysis of 5 sulfonylurea herbicidesThe hydrolysis mechanism of 5 sulfonylurea herbicides at 4 various pH and 3 various temperature conditions was investigated. The primary studies showed that the hydrolysis rates of the herbicides were influenced by the chemical structure of compound, temperature and pH values, especially by pH values. The mechanism of hydrolysis was studied by identifying hydrolysis products with LC/MS technology under acidic and basic conditions. The results indicated that the broken of sulfonylurea bridge was the main pathway of hydrolysis both under acidic and basic conditions. But greater difference was proved during the pathway of hydrolysis under acidic and basic conditions. The two primary hydrolytic mechanisms are acid-catalyzed cleavage and base-catalyzed contraction/rearrangement of the sulfonylurea linkage. This bridge or the linkage is susceptible to attack by water on the carbonyl carbon of the bridge, and thus produces CO₂ and the corresponding aryl sulfonamide and amino-heterocyclic portions of the molecule. The rate of this reaction can often be hundreds of times faster under acidic conditions than neutral or slight base condition.