Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2011
Publication Date: 4/22/2011
Citation: Khaokaew, S., Chaney, R.L., Landrot, G., Ginder-Voget, M., Sparks, D.L. 2011. Speciation and release kinetics of cadmium in an alkaline paddy soil under various flooding periods and draining conditions. Journal of Environmental Science and Technology. 45:4249-4255. Interpretive Summary: Discharge of mine wastes to paddy rice fields in northern Thailand cause extensive soil contamination with cadmium and zinc, raised soil pH, and caused rice and soybean to accumulate excessive levels of cadmium which have caused adverse health effects in the local population. The present research was conducted to characterize the chemical forms of cadmium in contaminated soils when they were aerobic or flooded. Flooding normally raises soil pH and lowers redox potential such that cadmium solubility is reduced. It has seemed likely that cadmium sulfide was formed when sulfate was converted to sulfide in the flooded soil. By using synchrotron radiation to examine the chemical forms of cadmium present in the soil during different soil redox treatments, we showed that little or no CdS was present in the aerobic soil, but upon flooding for 7 days, significant amounts of CdS were formed. In the aerobic soil, Cd bound to organic matter predominated along with Cd bound to or incorporated within CaCO3. Further, in the paddy soil, most of the cadmium was associated with calcium rather than associated with zinc; in non-paddy mine waste contaminated soils, cadmium and zinc are commonly associated in sulfides or carbonates. Thus this work confirms that flooding of zinc-mine waste contaminated paddy soil causes the formation of cadmium sulfide which has very low phytoavailability to rice plants.
Technical Abstract: Understanding the chemical forms in which Cd is present in paddy soils is needed to develop efficient and cost-effective strategies to clean up the soils, and/or minimize Cd uptake by rice. This study aims to determine Cd speciation and release kinetics in an alkaline paddy soil, at various flooding periods and draining conditions, by employing synchrotron-based techniques, and a stirred-flow kinetic method. Results revealed that varying flooding periods and draining conditions affected Cd speciation and release kinetics. Linear Least-Squares Fitting (LLSF) of bulk X-ray absorption fine structure (XAFS) spectra of the air-dried soil sample, and the 1 day-flooded soil sample, showed that the XAFS spectrum of Cd bound to humic acid contributed to at least 50% of the fits. However, this standard was not dominant in the fit for the soil flooded for 7 days and longer periods. The XAFS spectrum of cadmium carbonates (either Cd-CaCO3, or/and CdCO3) significantly contributed to the LLSF fits for all soil samples. At all experimental conditions, less than 25% of Cd was released after a two-hour desorption experiment. Results obtained by micro x-ray fluorescence (µ-XRF) spectroscopy showed that Cd was less associated with Zn than Ca, in most soil samples.