| Absorption characteristics and passivation mechanism of red mud to heavy metals in contaminated paddy soil were studied through batch, pot and field experiments. I explored the characteristics of migration, input and output of heavy metals in soil-rice plant system, and revealed the behavior and efficiency of the red mud in passivation of the potential toxic elements in the soil. I also clarified the fertilizer efficiency of red mud and its impacts on the rice plant and soil environment. I also recommended the optimum amount of red mud application to provide a theoretic basis for the safe and effective application of red mud in agricultural production. The main results were as the follows:Application of red mud reduced soil exchangeable Pb, Zn and Cd content significantly. In comparison with the control, exchangeable Pb content was decreased by 39.25%, 41.38% and 50.19%, exchangeable Zn content was decreased by 49.26%,57.32% and 47.16%, and exchangeable Cd content was decreased by 19.53%,24.06% and 25.70% respectively after 30 d,60 d and 90 d of application of 4%(W/W) red mud. The remediation mechanism is that red mud has good fixing capacity on heavy metal ions such as Pb2+, Zn2+and Cd2+, which make them transformed from exchangeable state into a bonding oxide state. Consequently, mobility and reactivity of heavy metal ions in the soil were decreased. Application of red mud reduced the proportion of five forms of Pb, Zn and Cd, and reduced the proportion of soil exchangeable Pb, Zn and Cd to total Pb, Zn and Cd. Furthermore, the proportion of exchangeable Pb, Zn and Cd to total Pb, Zn and Cd decreased with the increasing amount of red mud application.Application of red mud, lime and sepiolite effectively promoted the stabilization of soil exchangeable Cd. Among them, the effect of red mud was the best. Compared with lime and sepiolite treatment, soil exchangeable Cd in the red mud treatment was decreased by 1.81% and 3.55% respectively, and Cd content in brown rice was decreased by 17.39% and 26.32% respectively. Among the three soil amendments, application of red mud can increase production dramatically, while lime and sepiolite application have no apparent effects on production. Application of soil amendments increased rice stomatal conductance (Gs), the net photosynthetic rate (Pn), intercellular CO2 concentration (Ci) and transpiration rate (Tr). Among them, the effect of application of red mud disposal is obvious. Each soil amendment increased the flag leaf area variously. Compared with lime and sepiolite, red mud application could alleviate the effect of Cd on rice growth and decrease Cd damage on rice plant. In Pb-, Zn- and Cd-polluted neutral mine soils, application of 0.25%(W/W) of red mud can promote the growth of rice plants and increase rice yield. Grain yield increased with increasing red mud application and then decreased. Compared with treatment without red mud application, grain yield increased by 12.04%,6.25% and 4.40% in treatments applied 0.25% (W/W),0.5% (W/W) and 0.75%(W/W) red mud respectively; decreased by 0.23% and 8.10% in treatments with 1% (W/W) and 1.25% (W/W) red mud application respectively. Application of red mud could significantly reduce Pb, Zn and Cd contents in rice plant, and the treatment with 1.25% (W/W) red mud application performed best, in which Pb, Zn and Cd contents reduced by 33.77%,9.84% and 15.40% respectively. Zinc content was below 50 mg/kg, which reached food hygiene standards (Zn 50 mg/kg, GB13106-1991). However, the Pb and Cd content in brown rice did not meet the food hygiene standards (Pb 0.4 mg/kg, GB14935-1994; Cd 0.2mg/kg, GB15201-95). Enrichment coefficients of Zn and Cd in rice plant were:root>stem and leaf>brown>shell, while Pb was root>stem and leaf>shell>brown. The enrichment capacity of rice plant on Pb, Zn and Cd reduced with the increasing application of red mud. Migration capacities of three heavy metals were:Cd>Zn>Pb.The appropriate application of red mud can apparently promote rice growth and increase rice yield. With increasing amount of red mud application, soil pH value and cation exchange capacity (CEC) increased. It can be concluded that the appropriate application of red mud can improve soil microbial survival environment, promote the reproduction of soil bacteria, fungi and actinomycetes, increase the soil microbial biomass carbon (SMBC) and nitrogen (SMBN) content, and improve soil urease, acid phosphatase and catalase activity at the tillering stage, whereas the influence of red bud application on the organic content is not obvious. For the yellow clayey paddy soil,0.5% (W/W) red mud application is more beneficial to improve the soil biological characteristics and soil fertility. For alluvial loamy paddy soil,0.75%(W/W) red mud application is optimum. The application of red mud can increased soil pH, reduce the content of soil exchangeable Cd, transform exchangeable Cd into iron manganese bound and carbonate bound, and reduce the cumulative amount of Cd in brown rice. When the amount of red mud application reached or was higher than 0.5%(W/W), Cd content in brown rice in two kinds of soil meet the national food hygiene standards. Considering the type of rice yield, the effect soil remediation and the quality of brown rice, the recommended amount of red mud application on the yellow clayey paddy soil and alluvial loamy paddy soil 1 were 0.5%(W/W) and 0.75%(W/W), respectively. In the Cd moderately contaminated acidity alluvial loamy paddy soil, grain yield increased with increasing red mud application and then decreased. In early rice, the yield in treatment with 4500 kg/hm2 red mud is highest. In late rice, the yield in treatment with 3000 kg/hm2 of red mud is highest. The grain yield in early rice and late rice was increased by 11.36% and 8.30%, respectively. Plant height, effective panicle number and 1000-grain weight in early rice and late rice were increased first and then decreased with increasing red mud application. Total grains in a particle decreased first and then increased with increasing red mud application. Soil pH value increased variably with increasing application of red mud during the stage of rice growth. Reduction of soil available Cd content was the combined results of increased soil pH value and soil adsorption capacity. The accumulation of Cd in rice decreased with increasing amount of red mud application. The primary reason is that soil available Cd content reduced after application of red mud.Grain yield increased within a range of red mud and pig manure ratio, but the yield decreased when beyond the upper limit amount of red mud. The changing trend of plant height, effective panicle, filled grain percent and 1000-grain weight is almost the same as the trend of grain yield in early rice and late rice. With red mud and pig manure ratio increases, soil available Cd content decreased gradually in early rice and late rice. The red mud-pig manure mixer can maintain higher physiological activity of superoxide dismutase (SOD) and peroxidase (POD) in blades, decreased the accumulation malondialdehyde (MDA) in leaves, and improved the balance relationship between generation and the clearance of intracellular reactive oxygen. With the red mud to pig manure ratio increases, the contents of cadmium in brown rice decreased gradually. The effect of red mud and pig manure on improvement of Cd-contaminated soil exists an optimal ratio (early rice,1.125; late rice,0.625), rather than the more the better. |
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