Abstract's Details

Effects of Aggregation Conditions on the Uptake of Zn(II) and Cu(II) onto Iron Oxyhydroxides
Abstract ID	ENV-06
Presenter	James Dale
Presentation Type	Poster
Full Author List	J. G. Dale (1), C. S. Kim (1), J. P. Stegemeier (1)
Affiliations	(1) Chapman University
Category	Environmental Science
Abstract	Iron oxyhydroxides have a high capacity to sequester heavy metals in aqueous solutions through sorption onto their surfaces and incorporation into their structure. This property makes them potentially useful in the remediation of heavy metal contaminated sites. Additionally, aggregation of nanoscale iron oxyhydroxides may be induced by changes in the geochemical conditions of the aqueous environments through which they travel, impacting their ability to adsorb and retain heavy metals. In order to accurately assess the effects of these varying conditions, experiments were devised to simulate both varying pHs and ionic strengths of the geochemical environments that may be encountered in natural bodies of water. The sorption properties of iron oxyhydroxides for Zn(II) and Cu(II) under these various aggregation conditions were tested to determine which conditions resulted in the largest initial metal uptake and the greatest metal retention under desorption (i.e. low pH) conditions. Suspensions of nanoparticulate (~5 nm) iron oxyhydroxides were exposed to 0.5 mM Zn(II) or Cu(II) either before or after aggregation was induced. Three different aggregation conditions were tested, one at pH 10 and ionic strength 0.001M, another at ionic strength 1.00M and pH 7, and a final control condition, aggregated at pH 7 and 0.001M. After going through both aggregation and adsorption, the samples were split into two equal aliquots. One aliquot went through a desorption process by decreasing the pH to 5, followed by centrifugation and decanting. The supernatants were then analyzed via AA spectroscopy for metal concentrations. This data shows that both metals reach their own respective maximums after being desorbed under all aggregation conditions. Furthermore, copper exhibited very low desorptions, indicating a high percentage of incorporated metal, whereas zinc was exactly the opposite, with large desorptions, indicating more surface bound than incorporated zinc. EXAFS spectroscopy was conducted at beamline 11-2 on the wet pastes to identify the speciation of metals associated with the solid phase and determine which conditions yielded strongly bound, incorporated metal species. The EXAFS data shows that the mechanism of aggregation has a noticeable effect on the amount of metal incorporated into the structure of the iron oxyhydroxides, following the trend that pH has a greater effect than the control, which has a greater effect than ionic strength. Samples exposed to metals before aggregation were also found to incorporate metals more thoroughly than those that were aggregated before being exposed to metals. The AA data reinforces the trends seen in the EXAFS data, with uptake correlating with the extent of metal incorporation. This research could help to identify optimal conditions and mechanisms when using iron oxyhydroxides in the remediation of heavy metal contaminated sites.
Footnotes
Funding Acknowledgement	Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. This study was supported by a grant from the National Science Foundation, Division of Earth Sciences, Grant #0618217, and beamtime provided through SSRL beamtime proposal #2929.