| Dimethyl Phthalate(DMP),which is a kind of endocrine disruptors widely found in the environment.It is a kind of phthalate acid esters(PAEs)which is mainly used as a plasticizer.In recent years,PAEs has been widely detected in the soil and water of Three Gorges Reservoir.The results of animal experiments show that PAEs has teratogenicity,mutagenicity and reproductive toxicity.Its homologue solubility in water is better than the others,and the concentration in water can reach a large value so that the water environment safety of the three Gorges Reservoir area will be exposed to greater risk.Soil/water are likely to become DMP source/sink in the conversion process because of the periodical flooding and exposing in soil due to its special dispatching mode during the transition between high and low water periods.During this period,the changes of soil and water properties may affect the migration and transformation of DMP at the water-soil interface and change the source/sink relationship.Typical soils in the fluctuating zone of Three Gorges Reservoir,purple soil is the tested soil,indoor dry-wet alternate culture experiment under controllable conditions is conducted using Flooding-Drying-Re-flooding-Re-drying mode to explore the ability of DMP migration and release between sediment,pore water and overlying water during the periodic fluctuation of the water level in fluctuating zone.In addition,this experiment also examined the effect on DMP with different soil DMP background concentrations,different water properties,and different environmental conditions.Then,the results will explain the relationship of DMP migration and transformation between sediment,pore water and overlying water.The results show that:1.The overall variation trend of DMP in sediment,pore water and overlying water is as follows: the content of DMP in sediment decreases significantly in the initial periods of two flooding(0~2d and 40~42d),and the content of DMP in sediment increases in different degrees at the initial periods of two drying(20 ~21d and 60~61d).During the first flooding,there is a two-phase repeated distribution phenomenon between DMP in the sediment and pore water: desorption(0~2d)-adsorption(2~4d)-re-desorption(4~8d)-re-adsorption(8-16d).There is also a weak distribution phenomenon between DMP in the sediment and overlying water: desorption(0~2d)-adsorption(2~4d)-re-desorption(4~8d)-re-absorption(8~20d)during the first flooding.During the first 0~4 days of the second flooding,the distribution phenomenon between the sediment and the pore water/overlying water is the same as that of the first flooding.However,in the later stage of the second flooding,the distribution phenomenon is not obvious due to the small change of the three-phase DMP concentration and the weak desorption/adsorption effect.2.Therefore,it is known that the source/sink of DMP is not always the same,but is constantly changing during the flooding: during the 0~2d and 4~8d of first flooding,the sediment is the source of DMP,while the pore water and the overlying water are the sink of DMP and on the 2~4d and 8~20d,the sediment is the sink of DMP,while the pore water and the overlying water are converted into the source of DMP.During the second flooding,the source and sink situation of 0~4 days is the same as that of the first flooding and on the 4~8 day,the pore water is still the source of DMP,while the overlying water is converted into the sink of DMP,and the three-phase reaches a stable state on 8~20day.During the first flooding,the migration process of the DMP content in pore water and overlying water reaches the maximum value and then shifts to sediment can be fitted by the pseudo-second-order kinetic equation.The results of correlation analysis show that,the correlation between overlying water and pore water is the highest in the first flooding,and that in the second flooding is similar to the first flooding.3.During the two flooding periods,when the initial DMP content in soil is 10、40、100 mg/kg,respectively,the variation trend of DMP concentration in sediment,pore water and overlying water is approximately the same,which is positively proportional to the initial DMP content in soil,and the the maximum release is reached at 2th day.However,when the initial DMP content of soil increased to 200 mg/kg,there is a different trend of DMP concentration in three phases,especially in the first flooding: the DMP content in the sediment and the pore water shows a different uptrend from the other three groups on the 4-16 d,and the DMP concentration in the overlying water shows a different downtrend from the other three groups in the same period.The initial DMP content of soil increases to 200 mg/kg will cause the change of the source and sink path of DMP in three phases.4.The water conditions of overlying water have effects on the migration and release of DMP at soil-water interface during the periodic dry-wet alternation.(1)With the increase of ionic strength in overlying water,the absorption of sediment on DMP decreases to varying degrees,but it contributes to the release of DMP in sediment turns to pore and overlying water.The DMP concentrations of pore and overlying water are proportional to the ionic strength of overlying water.And the change of the ionic strength in overlying water will cause the change of the source and sink path of DMP in three phases.(2)When the p H of overlying water fluctuates between weak acidic and weak alkaline,it has little effect on the DMP content in sediment.The experimental results show that the weak alkaline conditions is not conducive to the occurrence of DMP in the pore water and the overlying water,and it may be existed the hydrolysis of DMP in weak alkaline conditions.And the change of the p H in overlying water will cause the change of the source and sink path of DMP in three phases.After the maximum release of DMP was reached under four different overlying water p H treatments,the migration process of shifting to soil could be fitted by the pseudo-second-order kinetic equation,and with the increase of p H,R2 increased.5.Environmental conditions have effects on the migration and release of DMP at soil-water interface during the periodic dry-wet alternation.(1)On the whole,with the increase of organic matter content in the soil,the absorption of DMP in soil is decreased,and the content of DMP in sediment releasing into overlying water and pore water is increased,and the difference is significant.But during the second flooding period,the results show that the DMP concentration in pore water will be decreased when the organic matter content is increased to a certain extent.And the change of the organic matter content in the soil has little effect on the source and sink path of DMP in three phases.During the first flooding,the migration process of the DMP content in pore water reaches the maximum value and then shifts to sediment can be fitted by the pseudo-second-order kinetic equation under the treatment of different organic matter content in the soil,and the R2 values are all 0.9523 and above.(2)Under aerobic condition,the content of DMP in the sediment is the lowest,while the concentrations of DMP in the pore and overlying water do not increase under aerobic conditions.Through the study of DMP volatilization at the air-water interface,it is found that the concentration of DMP in the air phase that above the air-water interface under aerobic conditions is significantly higher than that in anoxic and natural condition.The result shows that air agitation is beneficial to the substance exchange between air-water phases of DMP under aerobic conditions,and there are part of DMP volatilize into the air phase through the air-water interface.The aerobic or anoxic condition will cause the change of the source and sink path of DMP in three phases.During the first flooding,the migration process of the DMP content in the pore water and overlying water reaches the maximum value and then shifts to the sediment can be fitted by the pseudo-second-order kinetic equation under the treatment of different oxygen levels in the overlying water,and the R2 values are all 0.8142 and above.(3)During the first flooding,with treatment with microbial inhibitors,the content of DMP in sediment is significantly higher than that of the control group,while the concentration of DMP in pore water was lower than that of the control group.At the same time,the addition of microbial activity inhibitors results in different trends of DMP in sediment,pore and overlying water during the two flooding periods,which will cause the change in the source and sink paths of DMP in three phases.During the first flooding,the migration process of the DMP content in pore water and overlying water reaches the maximum value and then shifts to the sediment can be fitted by the pseudo-second-order kinetic equation under different microbial activities,and the R2 values are all 0.8142 and above. |
PDF Full Text Request