Bioavailability Of Ionizable Pesticides Nicosulfuron And Carbendazim In Soil

Bioavailability is an important factor in assessing the risk of pesticide in the environment. Bioavailability of pesticide in soil is affected by physicochemical property and aging of soils, so the total concentration can not reflect actual bioavailability. Therefore, it is important to study the influence factors and evaluation method of bioavailability for the ecological risk assessment of the pesticide in soil.With the development of new pesticide, the ionizable pesticides which contain acid or alkaline functional groups can be applied more widely. Both sorption by soil and uptake by organism for ionizable organic pollutants depend on their speciation (i.e., neutral and ionized forms); thus, the bioavailability of ionizable organic pollutants is more complicated than that of neutral organic pollutants in soil. The weak acid herbicide nicosulfuron and weak base fungicide carbendazim which have been applied in our country were chosen for the target compounds in this paper, the influence of soil physical and chemical properties on their sorption and bioavailability were observed, and then various chemical methods were used to estimate the bioavailability of ionizable pesticides. The main results are as follows:The sorption of nicosulfuron and carbendazim in different soils were studied with the batch equilibrium method. The adsorption of nicosulfuron depended on soil type, and the Freundlich affinity coefficient (Kfads) ranged from0.42to6.58mg1-1/n kg-1L1/n. The organic matter content is the key factor governing the adsorption of nicosulfuron.The adsorption of carbendazim was stronger than nicosulfuron, which depended on soil type, and the Kfads ranged from1.02to12.69mg1-1/n kg-1L1/n. The adsorption increased with an increase in the organic matter and low pH values in the soils.The phytotoxicity of nicosulfuron to corn (Zea mays L.) in different soil was studied according to OECD guideline. The results indicated that the phytotoxicity of nicosulfuron in soils to corn depended on the soil type, and the concentration that inhibits growth by50%(IC50) ranged from0.77to9.77mg/kg among the five tested soils. Soil aging significantly reduced phytotoxicity of nicosulfuron to corn. The adsorption is the important factor governing phytotoxicity of nicosulfuron. To obtain a comparable value of IC50,0.01M CaCl2extracted concentrations (CCaC12), ex situ pore water concentrations (CEPW), and in situ pore water concentrations (CIPW) were measured and used to estimate the nicosulfuron phytotoxicity to corn. The range of the variation in the IC50values of nicosulfuron was smaller when the CCaC12and CEPW were used instead of the amended concentration. There was no significant difference among the values of IC50calculated from the in situ pore water concentrations of nicosulfuron in the five tested soils, suggesting that the concentration of nicosulfuron in the in situ pore water could be used to estimate the phytotoxicity of residual nicosulfuron in soils.The effect of soil type on toxicity of the weak base carbendazim to earthworms (Eisenia fetida) was observed according to OECD guideline. The results indicated toxicity of the carbendazim to earthworms was also dependent on soil type, and the median lethal concentrations (LC50) values ranged greatly from3.00to35.17mg/kg. The coefficient of variation (CV) of LC50S was61.79%. The soil pH and organic matter contents were the main factors governing the toxicity of carbendazim.The toxicity of the weak base carbendazim to earthworms was estimated using Soxhlet extracted concentrations (Cs), an excess of water extracted concentrations (CEEW), ex situ pore water concentrations (CEPW) and in situ pore water concentrations (CIPW) under various soil conditions. The results indicated that the median lethal concentrations (LC50) calculated from Cs ranged from2.32to33.95mg/kg in the five tested soils and the CV of LC50S was69.82%.When the LC50was calculated from the CEEW, CEPW and CIPW, the variability of the LC50gradually became smaller in these soils (where carbendazim exists as neutral form), with the CVs of LC50s being43.49%,29.72%and11.38%(for CEEW, CEPW and CIPW, respectively). The values of the LC50calculated from the in situ pore water concentrations were approximately equal.However, the mortality-concentration-response curves and LC50based on CIPW in strongly acidic soil (where carbendazim partially exists as ionized form) was significantly lower than other soils. The results indicated the in situ pore water concentration could be used to estimate the toxicity of carbendazim under various soil conditions where carbendazim exists in the neutral form. The in situ pore water concentration underestimated the toxicity of carbendazim to earthworms in low pH soil.The follow factors could be used to elaborate the reason the in situ pore water concentrations underestimated the toxicity of carbendazim in low pH soil. The earthworm activity increased the pH of the soil, especially in the soils where the earthworms were active. In low pH soils, increasing the pH causes the ionized carbendazim to change into neutral carbendazim, thereby decreasing the adsorption and causing the adsorbed carbendazim desorbed from soil; these changes increase the toxicity of carbendazim to the earthworms. For high pH soils, carbendazim exists in a unionized form; therefore, increasing the pH has little influence on sorption. Thus, the in situ pore water in the bulk soil is below the real pore water where the earthworm resides in the low pH soil. So the in situ pore water concentration underestimated the toxicity of carbendazim to earthworms in low pH soil.