Main Factors Affecting The Forms And Bioavailability Of Cd And Its Key Regulation Technologies In Paddy Soils

With the increasing cadmium?Cd?pollution in farmland soil and the particularity of Cd enrichment in rice paddy system,the Cd over-standard rate of rice has been increasing in recent years.Cd pollution has become one of the major limiting factors affecting rice production and improving rice quality in China.The content of Cd absorbed by rice from the soil is not necessarily related to the total amount of Cd in the soil,but is often significantly related to the content of available Cd.The bioavailability of heavy metals?Cd?in soil depends mainly on their chemical form.Cd in the soil exists in different forms in the soil media after adsorption-desorption,complexation-precipitation,oxidation-reduction reaction and the action of soil microorganisms.Therefore,factors affecting the distribution and availability of Cd in soil mainly include soil pH,Eh,organic matter content and colloidal types and quantities.Rice paddy soil is affected by man-made factors under the disturbance of FDD alternately formed by the special soil type,due to the frequent flooding?reduction?and dry?oxide?alternating wet and dry processes,such as rice roots secrete oxygen and fertilizer production activities,the effects of paddy soil pH and Eh value is in unstable state,unsteady pH and Eh and drive a series of physical/chemical role in paddy soil.For example,the precipitation/reduction hydrolysis of iron?manganese?oxides,the synthesis/decomposition of organic matters,the leaching and oxidation/reduction reactions of both haline-base and haline-base and oxidation/reduction reactions resulted in the complex transformation of Cd morphology and availability in paddy soils.At present,there are many studies on the changes of the morphology and availability of heavy metals in paddy fields under the condition of single change of Eh or pH.However,under the condition of pe+pH,the mechanism and influence factors of S morphological transformation and its influence on Cd morphology and effectiveness are not systematic and in-depth.Around this study unsteady Cd pollution in paddy soil under the condition of pe+pH,from the soil acidification and driving mechanism of soil S morphological transformation of available forms and rice paddy soil Cd to absorb impact Angle,and combining with XRD,SEM-EDS and NMT in situ detection technology,such as in paddy soil Cd form transformation and influence factor and so on has carried on the deeper research.At the same time,the effect of soil remediation in paddy fields polluted by Cd was evaluated in combination with Cd drop rate of rice seeds and changes of soil Cd morphology,so as to provide scientific reference for the prevention and control of Cd pollution in paddy fields in China.The main conclusions are as follows:?1?Pot experiments were conducted to simulate three different types of soil acidification:industrial wastewater?sulphuric acid acidification,AP1?,artificial acid rain?AP2?and long-term fertilization?AP3?,and the change mechanism of Cd morphology and availability in rice soil by different acidification methods were studied.The results showed that soil acidification could directly or indirectly improve the availability and extractability of Cd in paddy soil,and increase the absorption of Cd by rice.And different acidification methods have different effects on Cd bioavailability in different paddy soils.In the two soils,the increase of Cd content in rice was the largest under AP3 treatment.The content of Cd in stem,leaf and root of AP3 rice in S1 increased to 6.04 mg·kg^-1 and 12.85 mg·kg^-1,and Cd content in stem,leaf and root of AP3 rice in S2 increased to 7.93 mg·kg^-1 and 18.86 mg·kg^-1.The effects of three kinds of soil acidification on soil available Cd and rice Cd absorption increase were all simulated long-term fertilization acidification>simulated artificial acid rain acidification>simulated wastewater irrigation soil acidification?sulfuric acid acidification?.?2?Three types of soil acidification were simulated:industrial wastewater?sulfuric acid acidification,AP1?,artificial acid rain?AP2?and long-term fertilization?AP3?.The effects of different acidification methods on Cd uptake and transport in rice and Cd availability in soil were studied by hydroponics and NMT in situ detection.The results showed that Cd accumulation in rice tissue and average velocity of Cd²⁺in rice root were negatively correlated with soil pH.When the pH value of soil S1 was reduced by 0.20 and 0.45 units,the Cd²⁺flow sequence of rice roots at 200 m and 500 m from the root tip was both simulated long-term fertilization acidification>simulated artificial acid rain acidification>simulated wastewater irrigation soil acidification?sulfuric acid acidification?.?3?Eexogenous addition of 0,150 mg·kg^-1,300 mg·kg^-1 and 600 mg·kg^-1 S the effects of different S contents on the soil Cd availability and Cd accumulation of rice at different growth stages were studied by pot experiments during the whole growth period of rice.The results showed that the percentage of DTPA-Cd in total Cd decreased by 2.6%,6.9%and 4.1%respectively in 150 mg·kg^-1,300mg·kg^-1 and 600 mg·kg^-1 S treatments compared with the treatment without S addition.The order of reduction of Cd accumulation in rice treated with different concentration S was exogenous addition of300 mg·kg^-1>exogenous addition of 600 mg·kg^-1>exogenous addition of 150 mg·kg^-1>without exogenous addition of S.The decrease of soil available Cd and rice Cd accumulation in different S concentrations was firstly strengthened and then decreased with the increase of S concentration.Furthermore,the order of contribution rate of rice Cd accumulation at different growth stages>heading stage at tillering stage>maturation stage.Therefore,it is of great significance for reducing Cd accumulation in rice to control the availability and activity of heavy metal Cd by S application at tillering stage.?4?Under the conditions of exogenous addition of 300 mg·kg^-1 S and exogenous addition of 0mg·kg^-1 and 2.4 mg·kg^-1 Cd,soil 70%MWHC treatment,flooding treatment?the water surface is 1-2cm above the soil surface?and anaerobic flooding treatment were carried out.The change of pe+pH value in soil under different water management and oxygen supply conditions was studied by soil culture experiment.The coupling relationship between S and Cd morphologic transformation in rice soil under the condition of pe+pH was discussed.The results showed that the soil pe+pH value under different treatments was anaerobic waterlogging?AF?>waterlogging?WF?>70%MWHC?DT?.Compared with DT treatment,soil available Cd decreased by 9.34%and 14.20%respectively in WF and AF treatment.Available Cd concentration in soil decreased with the decrease of pe+pH.Compared with DT-Cd treatment,the proportion of soil available S increased by 12.23%and 31.46%under WF-Cd and AF-Cd treatment respectively,and the soil available S increased with the decrease of pe+pH value.In addition,available Cd was negatively correlated with the available S content in soil.Therefore,reducing soil pe+pH value can effectively reduce the effectiveness of Cd in soil.?5?The effects of different soil pe+pH and S concentration on sulfhydryl biosynthesis and Cd accumulation in rice were studied by pot experiment in the whole growth period of rice,using two kinds of water treatment methods of S(0 and 300 mg·kg^-1),continuous drying?CD?,alternate dry wet cycle?DW?and continuous flooding?CF?.The results showed that flooding treatment was more effective than aerobic treatment in reducing pe+pH,Cd availability and Cd accumulation in rice tissue.The decrease of pe+pH value in soil and the enhancement of the biosynthesis of plant chelating peptide?PCs?and glutathione?GSH?in rice root were the main reasons for the decrease of Cd absorption and transport in rice induced by S.SEM-EDS was used to observe the root growth of rice treated with S soil.TEM-EDS analysis showed that the addition of S and waterlogging could promote the formation of iron film on the surface of rice root system and increase the Fe concentration in rice tissue.However,in the drying treatment without S,part of root epidermis disintegrated.?6?Based on in-situ passivation technology,the effects of several passivation agents on the remediation effect of Cd contaminated paddy field soil were studied,and the remediation evaluation of contaminated farmland with remediation marginal efficiency was studied.The results showed that the reduction rate of Cd in two kinds of Cd sensitive rice grains was 41.6%?65.3%,that in XS09 was slightly higher than that in JH212,and that in acid red soil was higher than that in paddy soil.The marginal efficiency of Cd contaminated red soil and paddy soil was 9.10%?15.4%with different passivators.Considering the two indexes of Cd reduction rate and marginal efficiency,the order of the effects of different passivators on Cd contaminated soil was clay mineral passivator?AT?>humus passivator?WG?>rock based passivator?FS?.In addition,the effect of different passivation agents in pot experiment was slightly higher than that in field experiment,but the difference was not significant?P<0.05?.