| Rice is a kind of staple grain that is easy to enrich Cadmium(Cd),and Cd pollution in rice field could reduce rice yield and quality,thus endangering food supply and safety.Biochar is an excellent adsorbent and carrier material,which contains abundant pore structure and oxygen-containing functional groups,so it is widely used to remove heavy metals in environmental fields.Due to its high surface activity,large specific surface area,many adsorption sites and excellent mechanical properties,iron oxide nanoparticles can be applied to the modification of biochar and other materials.At present,nano-Fe3O4 modified biochar have showed a good adsorption property in Cd/Pb wastewater,but its effect and action mechanism on rice-soil system are rarely studied.Iron plaque is an iron oxide film deposited on the surface of rice roots,various elements on rice roots can be affected by its adsorption and co-precipitation,the application of biochar or iron-materials can affect its formation and rhizosphere Cd migration.Nano-Fe3O4-modified biochar(BC-Fe)was prepared by the coprecipitation of nano-Fe3O4on a rice husk biochar surface.The effects of BC-Fe on Cd bioavailability in soil and on Cd accumulation andtranslocation in rice(Oryza sativa L.)were investigated in a pot experiment with 7 application rates(0.05-1.6%,w/w).Furthermore,the effects of three types of iron sources(EDTA-Fe、BC-Fe、FeSO4)on the rice iron plaque formation were compared through hydroponic experiments,in order to explore the mechanism of rice on Cd absorption and transport in rice roots.It provides a new theoretical and data supports for the application of nano-Fe3O4 modified biochar on Cd-contaminated rice fields in China.The main results from this research are given as follows:(1)Loading of nano-Fe3O4 particles increased the surface roughness and pore structure of biochar,increased the specific surface area and total pore volume by 49.4 times and 60.5 times respectively,increased the number of functional groups(-OH,C=O,-COOH)on surface,but decreased its pH value by 1.10 units.(2)In heavily Cd containment soil(Cd=1.6 mg·kg-1),the application of 0.05%-1.6%BC-Fe reduced the soil pH by 0.04-0.39 units,increased soil cation exchangecapacity(CEC)by 9.44%-161%,and reduced the soil available Cd(HOAc-Cd)by 6.81-25.0%,but had no significant effect on soil organic matter.(3)The application of BC-Fe increased the aboveground biomass of rice plant,among which the yield of brown rice increased by 38.2%-72.5%under 0.8%-1.6%treatments.0.05%-0.8%BC-Fe treatments reduced the Cd concentration in brown rice by 10.4%-49.0%,but the concentration was increased by 55.1%in 1.6%BC-Fe treatment.0.8%-1.6%BC-Fe treatments promoted Cd transport to leaves,which could increase the risk of Cd accumulation in brown rice,thus the recommended ratio BC-Fe is 0.2%-0.4%.(4)Fe concentrations in iron plaque increased by 23.5%-79.0%with the application of 0.05%-1.6%BC-Fe,the concentrations of Cd in the root iron plaque increased first and then decreased.After DCB-Fe concentration in iron plaque induced by BC-Fe higher than 22.0-23.0 g·kg-1,the formation of iron plaque reduced Cd absorption of rice roots,but when it below the threshold interval,the formation of iron plaque promoted Cd migration and accumulation into the rice roots.(5)Under hydroponic experiment,EDTA-Fe、BC-Fe and FeSO4 treatments could induce the formation of yellowish-brown or dark-brown iron plaque,which based on 1-2 layers surface cells of rice roots.The main components of iron plaque were hydropyrite,fibronite,goethite and hematite,and the concentration of amorphous iron plaque was significantly higher than crystalline iron plaque,while the proportion of BC-Fe induced crystalline iron plaque was higher than orther treatments,accounting for 31.77%-35.91%.Under different types of Fe treatments,the Fe concentration of iron plaque was as follows:FeSO4>EDTA-Fe>BC-Fe;and the Cd adsorption capacity of iron plaque introduced by different Fe treatments was as follows:BC-Fe>EDTA-Fe>FeSO4,in which the Cd concentration of amorphous iron plaque was significantly higher than that of crystalline iron plaque.Under BC-Fe treatment(40,320 mg·L-1),the proportion of Cd in crystalline iron plaque increased with the increase of BC-Fe treatment concentration,from 15.76%to 16.41%.(6)Under hydroponic experiment,different types of Fe treatments(EDTA-Fe、BC-Fe and FeSO4)of 40 and 320 mg·L-1 increased the Cd/Fe concentrations in different rice parts(roots,stems and leaves).Where,in the case of BC-Fe,Cd/Fe concentrations in rice roots was increased with the increase of the treatment’s concentration,Cd/Fe concentrations in stems and leaves showed an opposite trend.The Cd and Fe concentration between stems and leaves of rice was significantly positively correlated.BC-Fe treatment could enhance the interception of Cd and Fe by rice roots,but promoted the migration and transport of Cd and Fe between stems and leaves.BC-Fe treatment reduced Cd accumulation in overground rice tissues mainly by inducing iron plaque formation and regulating Fe level in rice roots. |
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