The Aging Kinetic Model Of Exogenous Cr(?) In Soils And Its Toxic Effect On Wheat

Due to the gradual acceleration of industrialization and urbanization,pollution caused by heavy metals has gradually become a major environmental problem in China.The migration and transformation of heavy metal chromium not only leads to environmental pollution of water,soil,air,etc.,but also can enter the human body through food chains such as crops,resulting in an increased rate of human teratogenicity and cancer.Therefore,the study on the migration and conversion of cadmium and its bioavailability have important scientific and environmental significance for the prevention and treatment of cadmium pollution.Chromium in the soil mainly exists in form of Cr???or Cr???,and Cr???is much more toxic than Cr???.The aging process of chromium in soils will affect the content of available Cr???and Cr???,resulting in a large difference in toxicity.However,the aging mechanism of cadmium is still unclear,and its effects on plant availability are also lacking in current.In response to these scientific issues,this paper expands the following research content:?1?The aging mechanism model of soil chromium.Six typical farmland soils of wheat planting provinces and cities were selected for the model study.In this study,The mass fraction of chromium in soil was adjusted to 175 mg·kg^-1 by adding Cr???solution externally.The aging experiment in the soil was carried out to obtain the content of different extracted Cr in 105 days.Three classic aging models were used to analyze the aging mechanism of chromium,the results show that the adsorption?R²>0.821—0.985,P<0.01?works more important than the diffusion?R²>0.690,P<0.01?in the aging process of soil Cr???.However,these three models are more suitable for fitting a specific reaction process,and cannot be based on the mechanism that multiple reactions occur together during the aging process.Therefore,a kinetic model of with three elementary reactions was build,including the effective state Cr???into the aging state,the effective state Cr???reduces to the effective state Cr???,and the generated effective state Cr???continues to age to produce the aging state.The results show that the new model can better reflect the different aging mechanisms of chromium and calculate its reaction rate.The Cr???content?11.78?32.54%?in the soil is higher than the Cr???content?11.03?24.87%?during the soil aging balance process in 105 days,and most of the available chromium in the soil will exist in the form of Cr???.?2?Bioavailability of chromium in soil.In this study,wheat was grown in soils with exogenous Cr???solution to investigate the bioaccumulation and bioavailability of chromium.The results showed that chromium inhibited root elongation when the chromium content in wheat reached about 200 mg·kg^-1 in 5 days of seedling stage,and the bioaccumulation factor of wheat decreased with chromium concentration,probably because in the low concentration(5 mg·kg^-1),root elongation of wheat is less inhibited,while chromium bioaccumulation factor were relatively high.The growth of wheat root in soil is affected by the increase of chromium treatment(5?175 mg·kg^-1),and the bioaccumulation capacity is gradually reduced.Meantime,the bioavailability of chromium in soil was studied using wheat root elongation as the end point of test.The results showed that extracted chromium?by ammonium acetate?in soil was significantly correlated with the relative root elongation of wheat?R²=0.741,P<0.01?.Compared with total chromium content in soil,effective chromium is more suitable as a predictor of the bioavailability model.A bioavailability prediction model was established by regression analysis to predict the relative root elongation based on the extracted chromium content and the ratio of Cr???or Cr???to the total chromium?R²>0.75,P<0.001?.Due to the comprehensive impact of soil physical and chemical properties such as organic matter content and pH value during the aging process,the available chromium content of Chongqing purple soil(pH=6.40,OM=17.94 g·kg^-1)was less in the six soils after 30 days of aging equilibrium.The extracted chromium content and Cr???proportion of the soil in Chongqing was relative low,and the chromium toxicity was the least among six soils.While the effect of physical and chemical properties of soil on the available Cr???and Cr???content cannot be ignored,the bioavailability model could be easily conducted by extracted chromium.?3?The key aging process and soil properties on the bioavailability of Cr.A regression analysis of the chromium toxicity threshold of wheat on the typical soil of18 provinces and cities by the response rate of the aging process and the soil physical and chemical properties were made to study key aging process and soil properties effect on Cr bioavailability.The results showed that the organic matter content had the greatest influence on the effectiveness of chromium in soil.Based on the parameters of elementary reaction kinetic model,a toxicity threshold model was established to evaluate the effectiveness of soil chromium.The rate constants of different aging reactions were taken as the influencing factors.By analyzing the weight of standardized influencing factors,it is concluded that the reduction process of Cr???to Cr???has the greatest impact on the available state of Cr in soil compared with other adsorption,precipitation,diffusion and other effects in aging reaction.