| The yield of rice,especially the yield per unit area,has greatly increased,and agrochemicals,especially fertilizers,have made a significant contribution.The increase in the application amount of agrochemicals has resulted in a large amount of companion ions remaining in the soil,such as a large amount of Cl-brought in with the application of potassium fertilizer.Due to Cl-has a strong ability to coordinate and complex,it can form a series of complexes with the general formula CdCln2-n with soil Cd(such as:CdCl+,CdCl20,CdCl3-and CdCl42-),thereby affecting the existing form,environmental behavior and biological effectiveness of Cd in rice fields system.At present,in the research on the influence of companion ions on the environmental behavior of Cd,there are many discussions about companion cations,and the effect of Cl-,which has the widest source and the largest input,has not been paid due attention to its effects after entering the environment.In this paper,soil culture experiments,rice sand culture experiments and rice pot experiments were conducted to explore the effects of exogenous Cl-addition on the adsorption characteristics of Cd in paddy soils of different parent materials,the absorption and accumulation of Cd by rice,and the migration and transformation of Cd in the soil-rice system.The research aims to reveal the influence of Cl-on the environmental chemical behavior of Cd in the solid-liquid phase of the soil,and to clarify the correlation between the accumulation of Cd in rice and Cl-,with a view to the rational application of Cl-containing agrochemicals and heavy metal pollution.It can provide suggestions on the safe use of cultivated land.The main findings are as follows:(1)Through soil culture experiments,the effect of external addition of Cl-on the Cd adsorption characteristics of different parent material soils was studied.The results show that there are significant differences in the adsorption capacity of different parent soils to Cd.In the early stage of the adsorption process,with the increase of Cl-addition,the adsorption amount of Cd by the three parent soils was significantly reduced and all were less than the blank control,which decreased by 1.3-22.5%(purple clayey soil),6.95-23.34%(red and yellow soil)and 5.89-23.80%(granitic sandy soil),which shows that adding Cl-can reduce the adsorption capacity of soil to Cd and has the most significant effect on the red and yellow mud with the highest organic matter content.By fitting the quasi-first-order kinetics and quasi-second-order kinetic equations,it can be found that the fitting effect of the quasi-second-order kinetic equations is better,and the coefficient of determination is high(R2>0.99),the obtained equilibrium adsorption capacity is not much different from the actual test results.The relationship between the adsorption process rate of Cd in the three soils is:red and yellow soil>purple clayey soil>granitic sandy soil.By fitting Langmuri and Freundlich type adsorption isotherms,it can be obtained that the Langmuri equation has a better fitting effect on the adsorption of Cd on red yellow mud and burlap mud(R2>0.90),and the Freundlich equation has a better fitting effect on the adsorption of Cd in the purple mud field(R2>0.95).Comprehensive analysis shows that when Cl-is added externally,the adsorption effect of the heavy metal Cd in the purple mud field is relatively strong.(2)Through the rice sand culture experiment,the effect of exogenous addition of Cl-on the absorption and accumulation of Cd by rice was studied.The results have shown that in the treatment without Cd,compared with the Ca(NO3)2 treatment group,the rice plant height of the T1-T4 treatment with Cl-increased by 1.94-25.0%,indicating that within a certain concentration range,Cl-can promote the growth of rice;In the treatment with Cd,the plant height of rice treated with T1-T4 was reduced by 10.0-11.29%compared with the group treated with Ca(NO3)2,and Cl-enhanced the toxicity of Cd.The Cd in the root of rice was significantly higher than that in the shoot,and there was no significant difference in the Cd of the shoot part of rice in each treatment.With the increase of Cl-addition,the Cd in the rice underground also increased,and was significantly lower than the corresponding Ca(NO3)2 treatment group(CK1-CK4),which decreased by 81.48%,70.30%,45.51%and 1.70%,respectively.The Cl-and Cd in the shoot of rice showed a very significant positive correlation(R2=0.980).(3)Through the rice pot experiment,the effect of exogenous addition of Cl-on the migration and transformation of Cd in the soil-rice system was studied.The results had shown that the addition of external sources of Cl-will significantly increase the fresh weight of rice and the acid-extractable Cd(E1)in the soil.During the whole growth period of rice,the Cd of the soil solution of T1-T4 treatment was 0.068-2.994 times lower than that of Ca(NO3)2 treatment group(CK1-CK4).At the heading stage and the filling stage,the Cd of each part of rice treated with T1-T4 was lower than the corresponding Ca(NO3)2 treatment group(CK1-CK4),which decreased by 3.45-32.17%,1.52-26.17%,0.43-25.67%and 1.14-32.03%.However,with the increase of Cl-addition,the accumulation of Cd in rice gradually decreased,and the downward trend was most significant during the filling stage.Using the Debye-Huckel limit equation to calculate the concentration of each component of CdCln2-n in the soil solution,it can be seen that when the Cd/Cl ratio in the soil-water medium is between 0.625-2.5,Cl-is in the soil solution and the free state of Cd2+mailly forms CdCl+ complexes.In summary,Cl-reduces the adsorption capacity of soil to Cd and increases the Cd content of the soil solution;The Cd and Cl-content in the above-ground part of rice showed a very significant positive correlation;Exogenous addition of Cl-will significantly increase the fresh weight of rice and the acid-extractable Cd(El)in the soil,indicating that Cl-enhances the migration ability and bioavailability of Cd in the soil-rice system.Therefore,the rational application of chlorine-containing agricultural chemicals is closely related to the safe use of heavy metal polluted farmland,and the impact of Cl-on the environmental behavior of Cd should be paid attention to. |
