Uni-Bayreuth

Sprungmarken

 

Reactivity of nanoscale zero-valent iron particles used for in situ groundwater remediation

Doris Schmid1, S. Laumann1, S. Wagner1, T. Hofmann1
1 Department für Umweltgeowissenschaften, Universität Wien

P 12.11 in Grundwasserqualität

 

Nanoscale zero-valent iron particles have a high potential for transformation or degradation of common groundwater and wastewater contaminants such as chlorinated solvents, metals, and pharmaceuticals [1, 2, 3, 4]. Deploying nZVI for in situ groundwater remediation might offer an alternative to conventionally used ex situ techniques like, e.g., pump-and-treat. The effectiveness of the nZVI-based remediation technique strongly depends on the reactivity and longevity of nZVI particles. The aim of this study is to determine the effect of I) various hydrochemical conditions (e.g., high carbonate content and Ca-Mg rich water) and II) the type of porous media on the reactivity of three commercially available nZVI particles. Reactivity was determined in batch and column experiments using iopromide, an X-ray contrast agent, as a model compound to study the dehalogenation process with nZVI.

The investigated nZVI particles included non-coated nZVI (Nanofer 25), polyelectrolyte-coated nZVI (Nanofer 25S), both in aqueous suspension, and air-stable nZVI powder (Nanofer Star) (NANOIRON s.r.o., Czech Republic).

The reaction order and reaction rate constants were derived from batch and column experiments. Two different water types were deployed in batch and column tests (a) pH 7 buffered water with 50 mM HEPES and (b) Ca-Mg-HCO3-rich water at pH 8.6. Columns tests were used to simulate dehalogenation in porous quartz and carbonate sand. All experiments were carried out under anoxic conditions. Pore water velocities during column tests was adjusted to ~1.5 and 8.5 m d-1.

Batch tests showed that all types of nZVI tested were able to fully degrade iopromide under the applied conditions. The reaction kinetics can be described by a pseudo-first order model. The surface area normalized first order reaction rate constant (kSA) of the different nZVI particles ranged between 8.94∙10-1 and 1.14 L m-2 h-1 for the pH 7 buffered water. Reaction rate constants were highest for Nanofer 25 followed by Nanofer 25S and Nanofer Star. For the Ca-Mg-HCO3-rich water the values for kSA were 3 times lower, reflecting the pH dependency of the reaction.

The amount of zero valent iron and the residence time of iopromide in the column were the crucial factors for the dehalogenation of iopromide. With decreasing residence times of iopromide in the column the conversion of iopromide becomes incomplete. Iopromide was continuous introduced into the column, causing a lower Fe(0) to iopromide ratio and therefore a lower reaction rate compared to the batch tests In column experiments with quartz sand kSA was 2.6∙10-4 L m-2 h-1 for pH 7 buffered water and 5.7∙10-4 L m-2 h-1 for the Ca-Mg-HCO3-rich water. For carbonate sand and under similar hydrochemical conditions kSA was in the same range. Therefore, no effect of the porous media on the nZVI reactivity could be observed.

Further experiments will be conducted to elucidate the reaction rate limiting step during the dehalogenation of iopromide with nZVI.



 

[1] Tratnyek and Johnson (2006): Nano Today 1(2):44-48.

[2] Zhang, W. (2003): J. Nanopart. Res. 5, 323-332.

[3] Zboril et al. (2012): Hazard. Mater. (211-212), 126-130.

[4] Stieber et al. (2011): ES&T 45 (11), 4944-4950.

Letzte Änderung 23.12.2013