Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2012
Publication Date: 3/18/2012
Publication URL: http://dx.doi.org/10.1021/es204007t
Citation: Khaokaew, S., Chaney, R.L., Landrot, G., Pandya, K., Sparks, D.L. 2012. Speciation and release kinetics of zinc in contaminated paddy soils. Environmental Science and Technology. 46:3957-3963. Interpretive Summary: Zinc contamination is the most frequent metal contamination of soils. Excessive zinc contamination may be phytotoxic to plants at acidic pH. Zinc mine wastes cause important contamination of soils in many countries where co-contamination with cadmium causes adverse health effects in humans. However, if soil pH is high, excessive zinc is transformed to other minerals in soil, reducing phytoavailability and potential phytotoxicity. In the present study, X-ray Spectroscopy was used in conjunction with mild acid extraction in a stirred-flow apparatus to evaluate chemical speciation of the soil zinc, and kinetics of release of soil Zn from a contaminated paddy soil from Thailand. X-ray Spectroscopy identified the zinc species present in the soil as Zn-Layered Double Hydroxides (LDH). When the soil was flooded, or when it was acidified, zinc remained mostly as the Zn-LDH compounds which have low extractability with mild acid, indicating that most of the high soil zinc has been inactivated into new solid phases with very limited solubility or release with mild acid. These findings show that keeping Zn contaminated soils calcareous can transform zinc to forms with low phytoavailability which protects soil fertility.
Technical Abstract: Zinc is an important nutrient for plants, but it can be toxic at high concentrations. The solubility and speciation of Zn is controlled by many factors, especially soil pH and Eh, which can vary in lowland rice culture. This study determined Zn speciation and release kinetics in Cd-Zn co-contaminated alkaline and acidified paddy soils, under various flooding periods and draining conditions, by employing synchrotron-based techniques, and a stirred-flow kinetic method. Results showed almost no change in Zn speciation and release kinetics in the two soils, although the soils were subjected to different flooding periods, and draining conditions. The mineral phases in which Zn is immobilized in the soil samples were constrained by Linear Least Squares Fitting (LLSF) analyses of bulk X-ray Absorption Fine Structure (XAFS) spectra of soil samples. Only two main phases were identified by LLSF, i.e., Zn-layered double hydroxides (Zn/Mg-hydrotalcite-like, and ZnAl-LDH) and Zn-phyllosilicates (Zn-kerolite). Under both soil pHs, under all flooding and draining conditions, less than 25% of Zn was desorbed from the soil after a two-hour desorption experiment. The information obtained in this study will be useful in finding the best strategy to control Cd and Zn bioavailability in Cd-Zn co-contaminated paddy soils.