Release And Loess Immobilization Of Heavy Metals In The Sediment From ADrainage Stream In A Mine-based Oasis

In this thesis,the sediments from a suburban drainage stream in an arid mine-based oasis were studied.The distribution characteristics and pollution status of heavy metals(Cu,Zn,Cd,Pb,Ni and Cr)in sediments of Dongdagou and Xidagou streams in Baiyin City were investigated through the field investigation experiment.On the basis of the field investigation experiment,the release experiment was conducted to study the environmental effects of heavy metals in sediments under different water environmental conditions.The immobilization of heavy metals(Cu,Zn,Cd and Pb)contaminated sediments from Dongdagou stream was carried out using Chinese loess.The fraction changes,longitudinal migration and bioavailability of heavy metals in loess-amended sediments were studied using Tessier sequential extraction procedure,desorption experiments,toxicity characteristic leaching procedure(TCLP),soil column leaching experiments and pot experiments.The main results of this thesis are summarized as follows:(1)The average concentrations of heavy metals(except for Cr)in sediments from the Dongdagou and Xidagou streams were significantly higher than the soil background values in Gansu Province,China.The average concentrations of Cu,Zn,Cd and Pb in sediments from Dongdagou stream and Cd in sediments from Xidagou stream were highly elevated compared to the category two standards limits of environmental quality standard for soils in China,and the Cd pollution was the most serious.Furthermore,the analysis of chemical fractionation indicated that Zn,Ni,and Cr in sediments from Dongdagou stream were dominated mainly by the fraction of residual,the existence of Cu and Cd was mainly in organic fractions,while Pb was composed mostly by its Fe-Mn oxides fraction.Pb in sediments from Xidagou stream existed by Fe-Mn oxides fraction,other metals appeared mainly associated with the residual fraction.The enrichment factor(EF)indicated that the enrichments of Cd in sediments from Dongdagou and Xidagou streams were very serious,which posed a strong pollution level.The risk assessment code(RAC)showed that the risk level of heavy metals in sediments from Dongdagou stream descended in the order: Ni>Cd>Pb>Cu>Zn>Cr,Ni posed a highest risk level.The risk level of heavy metals in sediments from Xidagou stream decreased in the order: Pb>Cd>Ni>Cu>Zn>Cr,Cd,Pb and Ni posed a highest risk level.(2)The results of this p H-dependent release experiment showed that the release patterns of Cu,Zn,Pb,and Cr were similar,while Cd and Ni release patterns were similar under different p H conditions.The changes in the release of heavy metals in sediments were related to their redistribution between chemical fractionations.After reaching the release equilibrium of heavy metals under different p H conditions,the percentages of organic Cu,Zn,Cd,and Fe-Mn oxyhydroxide Pb decreased,compared to their initial fractions.The residual fractions of Ni and Cr were dominant,regardless of p H.The release of Cu,Zn,Pb,Ni and Cr increased with the increasing ionic strength.However,the release of Cd increased first and then decreased with the increasing ionic strength.(3)The loess can effectively transform Cu from the carbonate fraction into the residual fraction when the loess dose in 10 kg wet sediment(the water content was 32.6%)was ≥ 5 kg.However,loess had little effect on Zn,Cd and Pb immobilization.Zn was mobilized when the loess dose was ≥ 2 kg.Cd and Pb were also mobilized when the loess dose was 20 kg.Correlation analysis showed that these four single extractants(Ca Cl2,HCl,EDTA-2Na and DTPA)can provide a good indication of the availability of Cu,Zn,Cd and Pb in the amended sediment.Results from TCLP confirmed loess reduced leaching rate of Cu and Zn achieving up to 42.43% and 17.57% reductions,respectively,when compared with untreated sediment.The loess could significantly immobilize Cu and Zn in sediment,and the optimum dose of loess in 10 kg wet sediment(the water content was 32.6%)was 5 kg.However,loess was inefficient for Cd and Pb immobilization,and they were also mobilized when the loess dose was 20 kg.(4)The concentrations of Zn,Cd,and Pb in the leachate of each soil column were lower than the detection limit after leaching of HNO3 solution with p H=3 for 30 d.The concentration of Cu in the leachate decreased with increasing leaching time,and the maximum concentration was lower than the quality third standard(1.0 mg L-1)for ground water in China(GB/T 14848-2017).Sediment,loess-amended sediment and sierozem have a strong buffering capacity.After leaching,the contents of Cu,Zn,Cd,and Pb in 0-10 cm layer sediment or loess-amended sediment have different degrees of leaching reduction,while the contents of Cu,Zn,Cd,and Pb in 10-35 cm layer sierozem samples have different degrees of leaching increase.By calculating the simulated precipitation,it can be obtained that the Cu concentration in the leachate of each soil column will not cause pollution to the groundwater in the period of 7.2-9.8 y.(5)Roots were the main organs to accumulate Cu,Zn,Cd and Pb at seedling stage,jointing stage and maturation stage.The concentration of Cu and Zn in the roots of corn at the seedling stage was the largest,the concentration of Cd in the roots of the jointing stage was the largest,and the concentration of Pb in the roots of corn at the maturation stage was the largest,while the concentrations of Cu,Zn,Cd,and Pb in the stems and leaves of corn at the seedling stage were the largest.The bioconcentration factor(BCF)indicated that the BCF of Cu,Zn,and Pb in loess-amended sediment in roots,stems,and leaves of corn at seedling stage,jointing stage,and maturation stage were all less than 1,while Cd was more easily accumulated in corn.The translocation factor(TF)showed that Cd and Zn were more likely to transfer from roots to the upper parts of the corn.In general,the release of heavy metals in the sediments under the conditions of different p H value and ion intensity would cause water pollution.The application of loess was effective in immobilizing Cu and Zn in contaminated sediment.However,the mobilization of Cd and Pb should be considered for sediment remediation using loess in practical use,and it should be carried out in a long-term experiment and fully assess the effects of metal immobilization in the loess-amended sediment.