Chromium(VI) and Oxyanion Remediation Of Vadose Zone Soils With Zero Valent Iron (ZVI) and Biological Reduction
Creators: name: Wong, Nicolas Kim
Publisher: name: University of Nevada, Las Vegas
Year: 2019

description: In areas with high industrial development, soil and groundwater are often heavily contaminated with hexavalent chromium [ Cr(VI) ], which commonly occurs as the oxyanions chromate ( CrO42- ) and dichromate ( Cr2O72- ). By itself, Cr(VI) is a common contaminant in various industrial wastes, but other oxyanions such as nitrate [ NO3- ], chlorate [ ClO3- ], and perchlorate [ ClO4- ] can appear with Cr(VI) as co-contaminants based on the type of industrial waste. Cr(VI) and ClO3- occur as co-contaminants in areas where sodium chlorate is manufactured as a bleaching agent for the pulp and paper industry (ERCO Worldwide, 2012). ClO4- and Cr(VI) are common co-contaminants due to their shared applications in electroplating and leather tanning (Sorensen et al., 2006). ClO4-, NO3- and Cr(VI) can occur simultaneously in areas associated with the manufacture, use and disposal of rocket fuel (Rong, 2018). ClO4- and NO3- are also noted to be common co-contaminants in soil and groundwater. (Logan and Lapoint, 2002; Ziv-El and Rittman, 2009; Rong, 2018) 
Prior to the implementation of RCRA regulations in 1986, wastes containing these contaminants were simply disposed of into the ground, resulting in the contamination of both vadose zone soils and groundwater. Technological options for remediation of vadose zone soils are limited in comparison to groundwater remediation due to lack of development and field testing, with very few options having been successfully implemented in vadose zone treatment (Dresel et al., 2011). This thesis focuses on bioremediation options for vadose zone soils, specifically on the remediation of Cr(VI), NO3-, and ClO3- using biological reduction. 
The research objective of this study was to assess the viability of bioremediation as an alternative for the removal of Cr(VI) from vadose zone soils using bioremediation methods. Specifically, autotrophic removal through biotic contaminant removal under maintained anaerobic conditions and bio-augmented remediation using zero-valent iron [ ZVI ] were compared to determine which method of treatment was more effective at reducing Cr(VI) and its co-contaminants from vadose zone soils. 
Microcosm experiments were performed using contaminated fine-grained soils taken from a site in the southwestern United States with high levels of Cr(VI), NO3-, and ClO3-. Biotic reduction tests comparing EOS-Pro and molasses as carbon sources were performed, where soil was divided, mixed with different carbon source and nutrients, prepared and placed in an anaerobic chamber to incubate. A second microcosm test was performed where contaminated soils were mixed with varying amounts of carbon source, nutrients, bacteria and stoichiometric ratios of ZVI to determine which combination of biological reduction and ZVI reduced the most contaminant in the least amount of time. Sample blanks were formed for both experiments to determine which soil amendment enhanced contaminant reduction, if any, and by how much. 
During the biotic reduction experiments, it was determined that while molasses was more effective in stimulating Cr(VI) removal, neither carbon source had any significant effect on NO3- or ClO3- removal due to incomplete Cr(VI) reduction. Low soil moisture in the samples also inhibited Cr(VI) reduction, which in turn also inhibited soil denitrification and ClO3- reduction. In comparison, the ZVI remediation experiments showed that significant reduction of all three contaminants took place within 50 days of regular treatment of the vadose zone soils, with Cr(VI) and ClO3- being almost completely removed from the soil. As the ZVI experiments involved regular soil wetting to prevent desiccation, it raises the implication that a combination of soil flushing techniques with biological reduction using ZVI could be employed to treat highly contaminated vadose zone soils. Considerations for the use of either ZVI or biological reduction techniques in vadose zone treatment include the costs of using high stoichiometric ratios of ZVI to contaminant, the removal of potential byproducts like iron [ Fe ] and ammonia [ NH3 ], and the ambient soil conditions at the time of treatment.