| Cadmium contamination has a huge potential threat to the ecosystem, food safety and human health, which has become a major environmental problem widely concerned. There are abundant straw resources in China. By using straw returning to reduce mobility of cadmium not only solves environmental problems caused by agricultural waste, but also realizes resource utilization of straw, which accords with the requirements of sustainable agriculture. Based on the studies on geological background and geochemical characteristics of Tongling mine area, this study selected farmland soil from Tongling nonferrous metal mining area, carried out static adsorption, adsorption kinetics, soil incubation and pot experiments, in order to determine the impact mechanism of straws returning on Cadmium speciation and bioavailability in polluted soils. All the results of the study could not only seek effective countermeasures to control soil Cd environmental behavior, but also have practical application value to the realization of straw recycle and agricultural production safety.The dynamic adsorption experiment and static adsorption experiment were carried out to study the adsorption characteristics of straw power on cadmium. The results showed that the dynamics adsorption process conformed to the pseudo first order kinetics equation. The isotherms adsorption of cadmium was described well by the Langmuir model, chemisorption maybe dominated the adsorption process. With increased dosage of straw, the removal rate of Cd2+increased gradually, and unit mass adsorbent adsorption amount decreased. The best dosage of the straw in the study is lOg/L; In the initial pH range of2.0-7.0, the adsorption capacity increased with the pH value increasing; at pH value6, rice and rape straw reached highest adsorption capacity,13.30mg/g and13.64mg/g, respectively. In the research, the best oscillation rate was150rmp. Therefore, the straw used as adsorption material in wastewater treatment of heavy metal Cd2+could keep the wastewater pH value at6, the oscillation velocity at150rmp, straw dosage at10g/L, which are the best treatment effects. Rape straw on the removal effect of Cd2+is better than that of rice straw.Adsorption and desorption experiments were conducted to study the effect of straw returning on the adsorption and desorption characteristics of cadmium in soil. It was found that the isotherms adsorption of soil were described well by Langmuir model, while on paddy soil with straw returning Temkin model fitted the best according to the statistical analysis of the data. Straw returning promoted soil absorption of Cd2+, also reduced the amount of soil Cd2+desorption. In general, rice straw treatment on soil desorption of Cd2+was greater than that by rape straw treatment. In soil seriously contaminated by cadmium, straw returning can effectively reduce the ecological risk of cadmium.A soil incubation experiment was conducted to study the effect of non-decomposed straw returning on soil properties and dynamic changes of cadmium fractions in soils. The results showed that, in submerged incubation mode and dry cultivation mode, straw returning significantly increased soil pH in both red soil and paddy soil, the Eh value of soil was reduced, but soil DOC concentration increased. In native cadmium contaminated soil, soil cadmium existed in residual cadmium form; while in added cadmium contaminated soil, most of the added cadmium existed in exchangeable form, and residual cadmium ratio decreased significantly with added cadmium. Compared with the control, added straw treatment significantly increased soil water soluble cadmium, EDTA bound cadmium and residual cadmium content, while reduced the exchangeable cadmium content, and the changes were more violent within the former30days. The DOM extracted from straw is rich in carboxyl, aldehyde and ketone groups, which are high unsaturation material. Through the decomposition of soil, DOM’s aliphatic properties decreased, and aromatic properties rose, which should be the cause of the increase of soil EDTA bound cadmium and residual cadmium content. In the simulated contaminated soil, different forms of cadmium correlated with soil pH, Eh and DOC in straw returning treatments, and all reached a significant level.Rape and rice pot experiments were conducted to study the effect of non-decomposed rape straw power or rice straw power returning on bioavailability of cadmium in cadmium contaminated soils and the mechanisms. Results showed that, with the increase of cadmium concentration, rape and rice growth inhibition increased (P<0.05); straw returning alleviated the toxicity of cadmium on plant growth in a certain extent, but did not reach the significant difference level in biomass, economic characters, photosynthesis, and physiological traits. Cadmium concentration in different parts of rape and rice increased with the degree of cadmium pollution in soil (P<0.05), straw returning significantly reduced cadmium concentration and BCF, the effect was more significant in high concentration of cadmium contaminated soil, and the full amount of straw returning treatment had better effect to reduce the absorption of cadmium. Straw returning influenced plant cadmium uptaking mainly through changing soil cadmium fractions. Under the condition of added cadmium, cadmium content in plants was mainly related to the exchangeable cadmium contents (P<0.05), straw returning promoted soil cadmium transformation from active exchangeable cadmium to low activity state of EDTA combination state and residual cadmium, which reduced the bioavailability of cadmium.In summary, straw returning effectively promoted polluted soil absorption of Cd2+, decreased the amount of soil Cd2+desorption. And through reduced the exchangeable cadmium content, significantly reduced cadmium concentration and BCF in each tissues of crops. That reduced cadmium migration to the environment and avoided transfer in the food chain. In the red soil region of paddy-upland rotation system, recommended the rape straw total returning, can better decreased soil Cd uptake of rice, avoided cadmium rice production. |
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