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Title: Stable isotopic composition of perchlorate and nitrate accumulated in plants: Hydroponic experiments and field data

Author
item ESTRADA, NUBIA - Texas Tech University
item BOHLKE, J - Us Geological Survey (USGS)
item STURCHIO, NEIL - University Of Delaware
item GU, BAOHUA - Oak Ridge National Laboratory
item HARVEY, GREG - United States Air Force
item Burkey, Kent
item GRANTZ, DAVID - University Of California
item MCGRATH, MARGARET - Cornell University
item ANDERSON, TODD - Texas Tech University
item RAO, BALAJI - Texas Tech University
item SEVANTHI, RITESH - Texas Tech University
item HATZINGER, PAUL - Cb&i
item JACKSON, ANDREW - Texas Tech University

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2017
Publication Date: 10/1/2017
Citation: Estrada, N., Bohlke, J.K., Sturchio, N., Gu, B., Harvey, G., Burkey, K.O., Grantz, D., McGrath, M., Anderson, T., Rao, B., Sevanthi, R., Hatzinger, P., Jackson, A. 2017. Stable isotopic composition of perchlorate and nitrate accumulated in plants: Hydroponic experiments and field data. Environmental Science and Technology. 595:556-566.

Interpretive Summary: Perchlorate salts are detected throughout the environment. High levels of perchlorate are considered toxic to mammals, including humans, because perchlorate disrupts thyroid metabolism by interfering with the uptake of iodide. Natural sources of perchlorate include atmospheric oxidation of chloride mediated by lightning, ultraviolet radiation, or tropospheric ozone. In addition to natural atmospheric sources, a number of anthropogenic point sources have been identified that include industrial and military applications, consumer products such as fireworks and highway flares, and in Chilean nitrate fertilizer which contains high natural concentrations of perchlorate. Differentiating between various sources of perchlorate is possible based on stable isotopic composition. Currently, it is unclear why natural perchlorate isotopic composition varies between locations. Processes that could alter perchlorate isotopic compositions in the environment include microbial transformation, diffusion, isotope exchange with water, and plant uptake. In collaborative research by a team of scientists from multiple organizations, we show that plant metabolism is not a significant factor contributing to the observed variation in perchlorate isotopic composition.

Technical Abstract: Natural perchlorate (ClO4-) in soil and groundwater exhibits a wide range in stable isotopic compositions (37Cl, 18O, and 17O), suggesting that ClO4- is formed through more than one pathway and/or undergoes post-depositional isotopic fractionation processes. One such fractionation process could be plant uptake and metabolism. Plants are known to accumulate ClO4-, but little is known about the ability of plants to isotopically alter ClO4-. We examined the potential for plants to alter the isotopic composition of ClO4- in hydroponic experiments conducted with snap beans (Phaseolus vulgaris L.), grown to full maturity in solutions of varying ClO4- concentrations with both normal and anomalous O and Cl isotopic abundances. The fate of ClO4- was compared with that of NO3- and Cl-. Consistent with past studies, the majority of the initial mass of ClO4- for both low and high concentration exposure treatments was recovered in growth solutions (20-40%) or plant compartments (40-60%), while some mass was not recovered (~20%). Anion ratios indicated ClO4- was transported from solutions into plants similarly to NO3- but preferentially to Cl- (~ 4X). ClO4- isotopic compositions were essentially identical between recovered ClO4- in plant tissue, hydroponic solutions, and initial ClO4- source values, indicating no significant isotope effects due to ClO4- uptake or accumulation in plants. NO3- uptake also had little or no effect on NO3- isotopic compositions in growth solutions, but a large fractionation effect (10-20 permil), with a delta(15N/18O) ratio of ~1, was observed between NO3- isotopic compositions of the growth solution and leaf extracts. This effect was attributed to partial NO3- reduction during assimilation within plant tissue. For comparison with experimental results, we also analyzed the isotopic composition of ClO4- leached from spinach obtained commercially. The ClO4- isotopic composition of commercial spinach was similar to that of indigenous natural ClO4- reported for the southwestern United States and the Great Lakes. Our results suggest that isotopic compositions of ClO4- in plants could be used to detect the source of ClO4- in the environment and/or food chain.