Cadmium speciation and release kinetics in a thai paddy Soil: an in situ molecularscale investigation using X-ray absorption spectroscopy

摘要

Understanding the factors controlling Cd speciation and bioavailability in flooded, drained, and alternate flooded/ drained paddy soil will be crucial to developing and implementing best management practices needed for productive agriculture in mine- waste impacted areas. Transformation of Cd species can occur in response to fluctuating soil moisture conditions. Redox potential is a master variable controlling Cd speciation and bioavailability in paddy soils, and soil pH influences the transport and fate of Cd (e. g., via sorption to organic matter or metal oxides and formation of sparingly-soluble species). Synchrotron X-ray absorption spectroscopy (XAS) was used to investigate Cd speciation in a contaminated paddy soil incubated at redox regimes representative of field conditions (e. g., flooded and drained). Cd K-edge XAS data were collected at bearnlines 10-ID-B and 13-BM-D of the Advanced Photon Source (APS), and S K-edge XANES spectra were collected at beamline X-15B of the National Synchrotron Light Source (NSLS). Preliminary μ-XAS data revealed CdS persists in microenvironments of dry soil, and S XANES data showed multiple valence states for soil S (e. g., S(-Ⅱ) and S(-Ⅵ)). Bulk XAS data of dry soil revealed CdCO <,3> was the most abundant species in limed Thai paddy soil (83%), and the soil also contained an appreciable amount of CdS (～15%). A stirredflow reaction chamber was used to investigate Cd and Zn release kinetics from paddy soil. Kinetic experiments revealed Cd and Zn release from soil was initially rapid followed by a gradual slow release of the metals at longer periods of time.