Molecular Speciation Of Heavy Metals Transportation In Soil

Heavy metals show significant influence on quality and safety of soil environment or agricultural products.At the mineral and biological interface,the characters of transportation behavior and speciation play an important role on heavy metal eco-toxicity and their end-result in pedosphere.With the developing of molecular environmental theory and the evolution of new technologies in recent years, a late hotspot in the environmental science is that how to understanding the reaction mechanisms of heavy metals in environment medium at molecular scale.Based on the traditional analysis methods,the synchrotron radiation X-ray absorption spectroscopy (SRXAS) were used in present study.The aim of our study was to clarify the changes of heavy metals molecular speciation and their transportation mechanism at the surface of clay minerals,soil particles and root-soil interface.The main results were as follows:The results showed that at the surface of montmorillonite and hematite,the first shell of Cu was bonded to four equatorial O atoms.At the surface of montmorillonite, Cu is adsorbed by edge-sharing O atoms in mineral slice,or adsorbs as binuclear complexes.At the surface of hematite,Cu is adsorbed by O atoms and forms as inner-sphere complexes or binuclear complexes at the mineral surface.The influence of oxalate,root exudates from Elsholtzia splendens and bacteria(Pseudomonas putida CZ1) on molecular speciation of Cu at the surface of minerals were studied.Results showed that at the montmorillonite and hematite surface,the Cu molecular speciation was changed by oxalate and Elsholtzia splendens root exudates,the binuclear complexes were dissolved and complex with organic acid.Studies clarified that oxalate and Elsholtzia splendens root exudates can activate metals on mineral surface. Our studies showed that Pseudomonas putida CZ1 promotes Cu adsorption at the montmorillonite and hematite surface,and more stable deposits were formed at the mineral surface.Studies clarified that Pseudomonas putida CZ1 can stabilize metals on mineral surface by organic acid and amino acid.Using Cu sulfide,mineral adsorbed Cu and humus adsorbed Cu as model references,the character of Cu XAFS showed that Cu were mainly bonded by humus, hematite,ferrihydrite,silicate minerals and sulfide in sampling soil collected from Zhuji mine site.In soil solution,Cu is mainly complex with oxalate and root exudates under the influence of Elsholtzia splendens root exudates.The hydrate Cu concentration significantly decreased in soil solution after Elsholtzia splendens root exudates were added into soil,this revealed that the Elsholtzia splendens root exudates can stabilize Cu in soil solution.The soil pH was significantly decreased and dissolved Cu increased after sulfur treatment.In soil solution,85.5%Cu were complex with organic acid.Column experiments studies showed that particle-facilitated transport of heavy metals is likely to represent the dominant transport pathway in soil influenced by acid rain.Silicate minerals could be a carrier of heavy metals to facilitate transport in soil, Fe oxide could partially dissolve to increase heavy metals release in soil at more acid pH,or could act as a vector of heavy metals transport in soil.Results from SEM-EDX analysis demonstrated that some nano-particles such as sulfide or Fe oxide are absorbed onto mobile soil particles.We quantitatively analyzed the heavy metal absorbance in mobile soil particles and results from SRXAS analysis demonstrated that the dominant heavy metal vectors are mainly composed with organic matters and Fe oxides.Results from SRXAS analysis showed that a competitive absorption occurred between the Elsholtzia splendens root exudates and soil particles,the hematite adsorbed Cu were dissolved by root exudates and complex with carboxyl.The present studies clarified the mechanisms that how sulfur influence the Cu molecular speciation at root-soil interface.Results showed that sulfur promotes the desorption of Cu from silicate and ferrihydrite,the released Cu were re-adsorbed by Elsholtzia splendens root exudates.It was illustrated that sulfur can promote the uptake of Cu and Fe in Elsholtzia splendens,and Cu is mainly precipitated on cell walls in Elsholtzia splendens root.