Submitted to: Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/15/2006
Publication Date: 7/16/2007
Citation: Khaokaew, S., Tappero, R., Jin, Y., Ravel, B., Chaney, R.L., Sparks, D.L. 2007. Cadmium Speciation and Release Kinetics in a Thai Paddy Soil: An In situ Molecular-Scale Investigation Using X-ray Absorption Spectroscopy (XAS) [abstract]. Meeting Proceedings.
Interpretive Summary: Our research has shown Cd in rice soils is readily transferred to rice grain, and the malnutrition of subsistence rice farmers promotes absorption and risk from soil Cd. This is true whether the Cd contamination comes from pure Cd sources (pigments), or geogenic Zn+Cd sources such as mine wastes. In many Asian nations, Zn-Pb mining has contaminated rice soils and caused human Cd disease. Following this logic, we examined a site in Thailand where Zn mine wastes contaminated rice land and confirmed high Cd transfer. These findings raise questions about the chemical form of Cd and Zn in such soils because Zn is not increased in rice grain even when soil Zn is highly contaminated. Thus, we used X-ray absorption spectroscopic methods to determine the chemical species of Cd and Zn present in flooded or aerobic rice soils, and examined the release of Zn and Cd from the anaerobic or aerobic soil in a stirred cell reactor. Using X-ray absorption spectroscopy at the Brookhaven National Laboratory, we were able to determine the chemical forms of Cd and S in the soil to date. In the flooded soil, nearly all Cd has been transformed to CdS, a very insoluble Cd species. Interestingly, even when the soil become aerobic, not all Cd is transformed to more soluble forms. The transformation of S is also evident in the spectroscopic results. Additional study of transformation of Cd and Zn as the soil is allowed to become aerobic similar to field rice soils will be undertaken.
Technical Abstract: 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 beamlines 10-ID-B and 13-BM-D of the Advanced Photon Source (APS) at Argonne National Lab, and S K-edge XANES spectra were collected at beamline X-15B of the National Synchrotron Light Source (NSLS) at Brookhaven National Lab. Preliminary -XAS data revealed CdS persists in microenvironments of dry soil, and S K-edge XAS data revealed multiple valence states for soil S (e.g. S(-II) and S(-VI)). Bulk XAS data of dry soil indicated CdCO3 was the most abundant species in the limed Thai paddy soil (83%), and the soil also contained an appreciable amount of CdS (~15%). Cadmium speciation in paddy soil will depend predominantly on soil Eh and pH conditions. A stirred-flow reaction chamber was used to investigate Cd and Zn release kinetics from paddy soil. Stirred-flow kinetic experiments revealed Cd and Zn release from soil was initially rapid. Additional work is underway to identify changes in Cd speciation under flooded, drained, and alternate flooded/drained soil moisture conditions. Understanding the factors controlling Cd speciation and bioavailability in flooded and drained paddy soil will be crucial to developing and implementing best management practices needed for productive agriculture in mine-waste impacted areas.