Cadmium Translocation In Paddy Soil-Rice System And Its Passivation By Iron And Biochar Composites

The development of coal-fired power plants and other industries not only provide energies for economic development,but can also cause serious environmental contaminations,especially heavy metals contamination such as cadmium?Cd?in the surrounding soil.Cd is easy to accumulate in rice grain.It is important and urgent to ensure the safety production of rice because it is the major foodstuff in China.A thermal power plant in northern Guangdong Province,China,annually generates 50 TWh,meeting 80%of northern Guangdong electricity demand.An environmental impact assessment of the power plant is needed to identify the potential health risks from the soil-rice system in the surrounding area.The aims of present study are to,?i?delineate the extent of heavy metal contamination in surface soil and rice grains;?ii?identify main soil properties that affect Cd availability;?iii?select low-Cd cultivars by growing 101 different cultivars of rice?Oyza sativa L.?in field;?iv?inhibit Cd translocation from soil to rice by passivants composed of biochar and iron by pot experioments;?v?apply and verify the passivants in field scale.The obtained results are summarized as follows:?1?High concentrations of As,Cd,Cu,Pb and Zn in the predominant wind direction of the power plant were observed and there were significant negative correlations between the metals concentrations and the distances from the power plant,suggesting that the power plant represents a important source of topsoil heavy metals in this region.Based on the threshold values for heavy metal pollution,the paddy soils near the power plant showed serious Cd contamination.As?Cr?Cu?Hg?Ni?Pb and Zn were also higher than the background values.Cd and Pb contents in rice grain exceeded the food standard,this presents a serous risk to humain health for the surrounding people.distributions of Cr and Ni;The significant correlations between Hg and N,P,K,OM indicated that it was dominated by agricultural activities.Soil properties including pH,CEC,OM,Fe oxide,exhibited noticeable effect on the availability of most heavy metals in the paddy field.The rice grown in the study area showed potential Cd health risk.?2?The contents of the amorphous Fe and DCB-extractable Fe oxides were high in the studied soils,they are significantly and negatively correlated with the Cd contents in the rice grain,hull and straw.These results suggest that soil active Fe could affect the availability of Cd in rice plant.Besides,Si fractionation and pH in soil also should be important factors affecting Cd content in rice plant.?3?The amount of Fe plaque differed among different rice cultivars.Cd immobilized on the root surface increased with the increase of Fe in the Fe plaque and Cd translocated into the inner rice decreased.Under the experiment conditions,the conventional rice cultivars accumulated higher Cd than the hybrid cultivars.Cd in Fe plaque,rice straw and hull of hybrid rices are higher than those of conventional rice cultivars.The hybrid cultivars TU122 and JCU308 had lowest Cd accumulation in rice grain among the tested cultivars,contained?0.13±0.01?and?0.14±0.01?mg·kg^-1,respectively were lower than the permissible limit(0.2 mg·kg^-1)of the food standards in China.?4?The applications of Fe?NO₃?₃ or Fe based biochar decreased significantly Cd accumulation in rice.These applications increased significantly the concentrations of the ammonium oxalate extractable Fe as well as its combined Cd in soil;and also decrease exchangeable Cd and carbonate bound Cd in soil.In addition,the applications enhanced Fe plaque on the rice root surface and increased the Cd fixed in Fe plaque.Therefore,the Cd translocation from soil to rice grain is decreased and the health risk is attenuated..?5?The field experiments in 2014 with the application of biochar,zero-valent Fe and Fe based biochar showed that these aditions could increase the output of rice grain,decreased significantly Cd content in rice grain.Fe based biochar was better than biochar and zero-valent Fe in enhancing Fe-plaque formation and its Cd fixation on the rice root surface.Considering both the passivation efficiency and the cost,Fe based biochar with3%?w/w?Fe content is recommended with an application rate of 1500 kg/hm².