| Cadmium(Cd)is a wide spread contaminant in the environment.It is highly toxic to all organisms and poses a serious threat to human health.In recent years,human activities such as fuel combustion,mining and smelting of nonferrous metals,industrial manufacture and sewage irrigation have resulted in Cd contamination in soils,especially in some paddy soils of Southern China.With 7%of the soil samples exceeding the soil environmental quality limits,Cd is the most prevalent and crucial contaminant in Chinese soils according to a recent report.Because Cd is retained in human body with a long half-life,even a slight Cd exposure daily could lead to chronic toxic effects.Cadmium can cause renal damage and osteoporosis in human body.Environmental factors including soil pH,redox conditions,organic matter content and soil texture,can impact soil Cd bioavailability greatly,among which soil pH and redox conditions are the most important.However,how pH and redox potential affect soil Cd speciation is still not fully understood.Cadmium in cereals is a major source of Cd exposure for humans.Cadmium can enter the food cycle through plant absorption.As a stable food,rice is a major source of Cd intake for a large numbers of people in China.There is a substantial variation among different rice cultivars in Cd accumulation.Understanding rice Cd absorption,translocation and accumulation mechanisms are crucial for controlling Cd accumulation in rice.Immobilization is an effective soil Cd contamination remediation strategy.Common soil amendments include lime materials,biochar and phosphate fertilizers.The chemical mechanisms underpinning Cd immobilization by amendments remain to be investigated.Phytoextration is a promising soil bioremediation method,which also has a number of limiting factors in practical application.This thesis presents three aspects of investigation on soil Cd environmental behaviors,rice Cd translocation and related transporters,and soil Cd remediation strategies.The main results are summarized as follows:(1)To understand the impacts of redox conditions on soil Cd availability,a series soil incubation experiments were performed.A soil sample was collected from a mining contaminated paddy site in Guizhou province(total Cd concentration 145 mg kg-1,pH 7.1).Paddy soil redox conditions changing processes were simulated,with the soil samples flooded for 1,7,30 or 120 days in reaction columns,and then drained to field moisture capacity and immediately collected or cultured for another 5 days and prepared for further analysis.A stirred-flow kinetic method was used to determine the desorption of Cd from different soil samples.The desorption agent was DTPA(5 mM)-TEA(100 mM)-CaCl2(10 mM),pH 7.2.The results show that,after reaction with the desorption agent,soil Cd maximum desorption ranges from 19 to 49 mg kg-1,accounting 13%to 32%of the total soil Cd content.Meanwhile,increasing flooding duration decreased both the maximum amount of Cd desorption and the rate coefficient significantly,which then increased rapidly after drainage of water for 5 days.These findings suggest that increasing flooding duration is an effective way to reduce paddy soil Cd availability.(2)To understand the impacts of redox conditions and soil amendments on Cd speciation and release kinetics in paddy soils,a series of soil incubation experiments were performed.The soil samples used in this study were collected from Xiangtan,Hunan Province(total Cd concentration 2 mg kg-1,pH 5.8).All soil samples were spiked with CdCl2 to achieve a final concentration of 165 mg kg-1 to ensure the collection of high quality Cd extended X-ray absorption fine structure(EXAFS)data.Three g CaCO3(enhance the soil pH by 1 unit)and 1 g CaSO4 kg-1 soil(enhance the total S concentration by 235 mg kg-1)were added individually or in combination.The soil samples were aged for 4 weeks,and then cultured under moisture conditions for another 2 weeks before use.Soils were flooded for 1,7,30 or 120 days,drained to field moisture capacity and then immediately collected or cultured for another 5 days and prepared for further analysis.A stirred-flow kinetic method was conducted to determine the desorption of Cd from different soil samples.Synchrotron-based techniques and linear combination fitting(LCF)were employed to identify and characterize Cd speciation in soil samples.The results show that increasing flooding duration and using amendments both decreased the maximum amount of Cd desorption and the rate coefficient greatly.LCF of bulk EXAFS spectra of different samples indicate that most of the Cd was sorbed on iron minerals(50%-60%),while a small amount of CdS(?20%)was detected as the flooding duration increased.CdS disappeared after drainage for 5 days.Furthermore,a high Cd and a low Cd rice cultivars were grown in a pot experiment with the two amendments and the same soil used in the soil incubation experiment.The results show that both CaCO3 and CaSO4 treatments lowered the Cd concentration in rice shoot and grain,with the effect of CaCO3 being greater than that of CaSO4.(3)A Japonica rice cultivar with consistently high Cd accumulation in both shoots and grain was identified in several field and greenhouse experiments.This cultivar possesses an OsHMA3 allele with a predicted amino acid mutation from Ser to Arg at the 380th position.When expressed in yeast,the gene of this haplotype showed no Cd transport activity,and the allele did not complement a reported nonfunctional allele of OsHMA3 in F1 test.Among diversity panels of 1483 rice cultivars,this allele is only present in temperate Japonica cultivars.Different cultivars possessing this allele all showed significantly increased root-to-shoot Cd translocation and a shift of Cd speciation from Cd-S to Cd-O bonding in root determined by synchrotron X-ray absorption spectroscopy.These findings indicate that 380th amino acid mutation of OsHMA3 is a new loss-of-function allele,which can lead to high Cd accumulation in shoots and grain of Japonica rice cultivars.(4)Twenty-nine different lines of Cd and Zn hyperaccumulator Noccaea caerulescens were grown in a pot experiment to test the Cd bioaccumulation ability for phytoextraction.The soil used was collected from Xiangtan,Hunan Province(total Cd concentration 2.1 mg kg-1,pH 6.4).The Cd concentration in the N.caerulescens leaves ranged from 50 to 327 mg kg-l,and the bioaccumulation factor ranged from 40 to 95,with the highest biomass reached 5.0±3.3 g pot-1,leading to a 0.64 mg pot-1 Cd maximum removal,and lowered the total soil Cd concentration by 30%.The majority Cd in the leaves were bound to thiol groups(61%-69%)in all N.caerulescens samples determined by extended X-ray absorption fine structure.Meanwhile,Cd-carboxyl was detected in samples with a high Cd bioaccumulation ability,Cd-phytate was detected in those with a low Cd bioaccumulation ability.The difference in leaf Cd speciation may be associated with the Cd bioaccumulation ability.Noccaea caerulescens can accumulate a large amount of Cd even from low Cd content soils,making it an ideal material for phytoextraction.In conclusion,soil Cd bioavailability and rice grain Cd accumulation are controlled by many factors,which must be considered in order to develop strategies to minimize rice Cd grain accumulation.The results presented in this thesis will provide a scientific basis for developing practical strategies to mitigate soil Cd contamination and minimize the translocation of Cd from paddy soils to rice grain. |
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