Seasonal fluctuations in zinc speciation within a contaminated wetland

The cycling of common sorbents such as metal (hydr)oxides, carbonates, and sulfides in redox-active environments influences the partitioning of associ ated trace elements such as zinc. Consequently, fluctuations in redox statu s may in part determine the availability and mobility of Zn and other trace elements. This research examines changes in Zn speciation in a contaminate d wetland soil that undergoes seasonal flooding. X-ray absorption spectrosc opy (XAS) was employed to identify and quantify Zn species from soil cores collected over a 1-year cycle as a function of water depth, location, and s oil depth. Zinc associated with (hydr)oxide phases in dry, oxidized soils a nd with sulfides and carbonates in flooded systems. An increase in water le vel was accompanied by a reversible change in Zn fractionation toward ZnS a nd ZnCO3. However, a small, recalcitrant fraction of Zn associated with (hy dr)oxides remained even when the soils were exposed to highly reducing cond itions. Water depth and redox potential were the most important factors in determining Zn speciation, although spatial variation was also important. T hese data indicate that zinc sorption is a dynamic process influenced by en vironmental changes.