Studies On The Molecular Speciation Of Heavy Metals In Contaminated Soils And The Mechanism On Their Transformation In The Rhizosphere

The bioavailability and environmental risk of heavy metals in soils depends greatly on their speciation, not their total amounts. Heavy metals, due to their high affinity with organic matter, were widely distributed in soils as organo-metallic complexes. However, few studies have been conducted to investigate the bioavailability of different organic-bound fractions of heavy metals in soils. The transformation of heavy metals in the rhizosphere is the critical process for the regulation of their transfer in the soil-plant systems. The rhizosphere is generally enriched with soil organic matter due to root exudates and microbial acitivies, thus influencing the spciation and bioavailability of heavy metals in the rhizosphere. However, the transformation of different organic-bound fractions of heavy metals in the rhizosphere has not been fully understood. Compared to chemical extraction methods, synchrotron based X-ray absorption fine structure (XAFS) spectroscopy gains more and more attentions because this technique can probe the molecular speciation of heavy metals in soils. However, synchrotron X-ray radiation may damage the targeted samples, which results in the inaccurate characterization of metal speciation. Therefore, the methodology of XAFS deserves further studies. Consequently, we selected two typical smelter contaminated soil and mining soil, and used chemical extraction methods and synchrotron based XAFS spectroscopy to systematically study the above scientific questions with focus on copper (Cu) and lead (Pb) in this study. The results of this study were concluded as followings.In this study, Cu-fulvic complexes and Pb-fulvic complexes were demonstrated to be with bioavailability in the smelter contaminated soil. Cu-fulvic complexes mainly contributed the bioavailable Cu in the soil. The bioavailable Pb significantly correlated with exchangeable Pb, adsorbed Pb and Pb-fulvic complexes. This study further verified the bioavailability and environmental risk of Cu/Pb in soils was determined by their speciation, not their total amounts. The total amounts of Cu/Pb in the smelter contaminated soil were lower than that in the mining soil. However, Cu/Pb in the smelter contaminated soil, dominate in adsorbed and mental-fulvic complexes, had higher bioavailability than that in the mining soil. Therefore, the smelter contaminated agricultural soil had higher environmental risk than the mining soil.This study firstly exhibited synchrotron soft X-ray beam could significantly induce the photoreduction of organic Cu(Ⅱ) compounds in a third generated synchrotron radiation facility on the time scale of XANES measurements. However, hard X-ray beam has insignificant impact on these samples during routine XANES and EXAFS experiments. High photo flux was the major reason for the above photoreduction induced by soft X-ray. Photoreduction on organic Cu(Ⅱ) compounds (Cu acetate) could be prevented when keeping the radiation dose at about 0.1 MGy. Furthermore, multiple synchrotron based XAFS spectroscopy (Cu L-edge, O and C K-edge XANES) and the Scanning Transmission X-ray Microscopy (STXM) were used to reveal the mechanism of photoreduction and to probe the specific radiation damage process for Cu acetate. Our study built the theoretical foundation in the methodology of soft XANES spectroscopy for its application in the probe of Cu speciation in soils.In this study, soil Cu was mainly associated with oxygen atom in the first shell for both the smelter contaminated soil and mining soil. Cu in the smelter contaminated soil was coordinated with 6 oxygen atoms and form distorted octahedron structure. The second shell Cu-C parameters of two Cu-C bonds equal to 3.23 A. Therefore, Cu in this soil mainly bounded to organic matter bridged by functional groups with oxygen binding sites and formed bidentate inner-sphere complexes with a six-membered ring. These results improve the further understanding of the molecular speciation of Cu in soils. For the mining soil with low organic matter content, each Cu atom mainly coordinated 6 oxygens at the first shell with octahedron structure. Moreover, soil Cu was mainly associated with one copper atom in the second shell with Cu-Cu distance equal to 2.75 A, and with two iron atoms in the third shell with Cu-Fe distance equal to 3.03 A. Therefore, Cu in the mining soil mainly associated with iron minerals, probably fixed in the crystal lattices. In conclusion, the molecular speciation of Cu in soils depends greatly on their origins and soil properties. Soil organic matter should be the primary factor controlling the speciation and bioavailability of Cu in soils. This study illustrated the significant root induced differences in the fractions of Cu/Pb, especially different organic-bound fractions, between the rhizosphere and non-rhizosphere of Elsholtzia splendens. Compared to the non-rhizosphere, Cu/Pb-fulvic complexes significantly increased in the rhizosphere for both of the smelter contaminated soil and the mining soil. Therefore, rhizosphere effect of E. splendens may enhance the bioavailability of soil Cu/Pb by the formation of Cu/Pb-fulvic complexes in the rhizosphere. The transformation of heavy metals in the rhizosphere also depended greatly on the soil properties and the species of heavy metals, and different soil properties. The transformation of Cu/Pb in the rhizosphere of E. splendens was mainly attributed to the root exudates and microbial activities in the smelter contaminated soil, but much more related to rhizosphere microorganisms than root exudates in the mining soil. In addition, bulk Cu K-edge XANES and EXAFS spectroscopy failed to probe the transformation of Cu in the rhizosphere of E. splendens at the molecular level. Consequently, selective sequential extraction methods still play an important part in the investigation of the rhizospheric transformation of heavy metals.