Establishment Of Cr(Ⅵ) Aging Model In Typical Farmland Soils In China And Application

With the wide application in industry and agriculture,Cr has become one of the most important heavy metal pollutants in soil environment.Soil Cr pollution not only affects soil ecological security,but also affects human health through food chain enrichment.Soil Cr mainly exists in two forms:Cr（III）and Cr（Ⅵ）,and Cr（Ⅵ）has attracted widespread attention due to its high toxicity and mobility.The aging of Cr（Ⅵ）in different soils has an important effect on the toxicity and availability of Cr（Ⅵ）.Due to the diversity of soil types in China,the aging mechanism of Cr（Ⅵ）in different soils is still unclear,and the current soil environmental quality standard in China lacks the risk screening and control values of Cr（Ⅵ）for agricultural land.In view of these problems,the following aspects were conducted in this thesis:（1）In this study,a HPLC-ICP-MS method was established for the determination of Cr species in soil samples,and the influences of mobile phase concentration,flow rate and methanol addition on the separation and quantification of Cr species were explored.The results showed that the retention time of EDTA-Cr（III）and Cr（Ⅵ）increased with the increase concentration of TBAOH,and the retention time of Cr（Ⅵ）increased more than that of EDTA-Cr（III）.Moreover,with the increase of EDTA-Na₂concentration,the retention time of EDTA-Cr（III）and Cr（Ⅵ）decreased,the retention time of Cr（Ⅵ）decreased more compared to EDTA-Cr（III）.Both increasing the flow rate and addition of methanol could reduce the time required to analyze the entire sample,but addition of methanol significantly increased the strength of baseline and reduced ICP-MS sensitivity.Thus,based on the separation performance of Cr species as well as to minimize the time required to complete a sample analysis by HPLC-ICP-MS,the following conditions were determined:3 mmol·L^-1 TBAOH+0.4 mmol·L^-1EDTA-Na₂ as mobile phase,p H=6.9,and flow rate at 1.2 m L·min^-1.（2）A 360-day aging experiment was conducted with 50 mg·kg^-1 Cr（Ⅵ）exogenetically added to 12 farmland soils with different properties collected from around China.The dynamic change process of available Cr（Ⅵ）and Cr（III）and the aging mechanism of Cr（Ⅵ）were investigated by means of dynamic model fitting and multiple linear regression.The results showed that available Cr（Ⅵ）in the soils decreased rapidly from 6.71%-68.71%to 4.53%-45.17%at the initial time（t<60 d）,and then the trend slowed down and stabilized gradually（3.25%-34.35%）.During the aging process of Cr（Ⅵ）,the available Cr（III）produced by the reduction of Cr（Ⅵ）increased rapidly and peaked（0.27%–3.27%）in 2 to 30 days,then decreased rapidly,and finally stabilized（0.01%-2.19%）.The aging process of available Cr（Ⅵ）in all soils can be well described by the second-order dynamics model（R²>0.99）,based on the pseudo-second-order model,the approximate aging equilibrium time of spiked Cr（Ⅵ）was derived to be from 6 to 158 days.According to the aging model,Cr（Ⅵ）aging is affected by the combined effects of p H,organic matter,and soil clay content,among which,p H is the most important factor.（3）The short-term experimental ecotoxicological data of Cr（Ⅵ）in Chinese soil obtained from literature were normalized to long-term data using aging model.The Cr（Ⅵ）toxicity prediction model was established,and the EC₁₀ value prediction model suitable for interspecific extrapolation was determined by prediction performance and intraspecific variation analysis.Using the model,the ecological toxicological data of Cr（Ⅵ）from different soils were normalized to the typical soil scenarios in China,and the threshold of soil Cr（Ⅵ）ecological safety in China was derived.The results showed that Cr（Ⅵ）species sensitivity sequence is consistent in the four types of soil except for a few species.Ammonia oxidizing bacteria abundance and root elongation of spinach and celery were sensitive to Cr（Ⅵ）,which can be used as monitoring and early warning biological indicators for soil Cr（Ⅵ）pollution.The soil cellulose enzyme activity and soil catalase activity were at the top end of SSD curves,representing the least sensitive indicator of Cr（Ⅵ）pollution.Aging correction can eliminate the difference between the laboratory toxicology experiment results and the actual situation in the field to a certain extent,and avoid the excessively conservative soil threshold of Cr（Ⅵ）.The effect of aging correction is gradually weakened with the increase of p H.the HC₅values of Cr（Ⅵ）in acidic soil,neutral soil,alkaline soil,and calcareous soil were 8.7,7.1,6.0 and 5.3mg·kg^-1,respectively,which were far lower than the screening values of total Cr that specified in the current soil environmental quality standards for agricultural land in China.