Perchlorate (ClO4-) has both synthetic and natural sources. Stable isotope analysis of O (δ18O, δ17O) and stable (δ37Cl) and radioactive (36Cl) isotope analysis of Cl in ClO4- allows natural and synthetic ClO4- to be distinguished from each other and provides information on the potential origin(s) of natural ClO4-. The goal of this research effort was to develop an improved understanding of (1) the distribution and isotopic characteristics of natural ClO4- worldwide, (2) the mechanisms of natural ClO4- production, and (3) the contributing processes resulting in the ubiquitous distribution of this anion and its stable isotope characteristics in soils, groundwater, and vegetation.
In order to achieve these goals, several different laboratory and field research tasks were undertaken. These objectives and tasks were designed to fill existing data gaps on the production, distribution and isotopic characteristics of natural ClO4-. The first tasks were designed to improve our understanding of ClO4- distribution, significant environmental ClO4- production mechanisms and the stable isotopic signature of the produced ClO4-. These tasks included the following: (1) determination of the isotopic signature of ClO4- produced by O3 and ultra violet (UV) oxidation of Cl- and relevant ClOx species; (2) development of an improved understanding of the occurrence and isotopic signature of natural ClO4- and its relationship with NO3- and other anions in arid and semi-arid environments worldwide; and (3) evaluation of the potential for microbial production of ClO4. A second set of tasks were designed to better understand plant accumulation of environmental ClO4-, whether its isotopic signature is altered during uptake and accumulation, and/or whether plants can generate ClO4- via an O3-mediated mechanism.
The results from a range of different laboratory studies evaluating ClO4- formation mechanisms confirm that there are multiple potential pathways of ClO4- generation from both UV-photolysis and O3-mediated oxidation of Cl- and other ClOx precursors. Laboratory studies were successful in producing Cl and O isotopic variations in ClO4- that incorporate much of the reported stable isotope variation in natural ClO4-. Data indicate that final ClO4- isotopic composition is dependent on the precursor species oxidized. The reaction rates and intermediate species proposed to be involved in ClO4- formation require further study and additional experiments are required to resolve the reason for the enigmatic low δ37Cl values of Atacama ClO4- (Chile), but significant progress was made in constraining pathways of ClO4- production in nature by application of stable isotope analysis.
Soil and groundwater sampling was conducted worldwide (including the continental US, South Africa, South America, China, Antarctica, and the Middle East) to evaluate concentrations of ClO4- and its relationship with common co-occurring anions such as Cl- and NO3- among others. The data indicate that ClO4- is globally distributed in soil and groundwater in arid and semi-arid regions on Earth at concentrations ranging from 10-1 to 106 µg/kg. Generally, the ClO4- concentration in these regions increases with aridity index, but this also depends on the duration of arid conditions. In many arid and semi-arid areas, NO3- and ClO4- co-occur at consistent ratios (NO3-/ClO4-) that vary between ~104 and ~105. This is not the case for Cl-/ClO4- ratios, which vary widely among locations. The NO3-/ClO4- ratios are largely preserved in hyper-arid areas that support little or no biological activity (e.g. plants or bacteria), but can be altered in areas with more active biological processes.
Stable isotope analysis of Cl and O and radioactive isotope analysis of 36Cl in natural ClO4- confirmed and extended initial data suggesting that indigenous ClO4- sources in the southwestern U.S. show some isotopic variation by location and environment. ClO4- concentration and isotope analysis was conducted in all five of the North American Great Lakes. The data showed average ClO4- concentrations ranging from 0.05 to 0.13 µg/L (varying by lake) with concentrations being nearly constant with depth. Interestingly, the overall ranges of stable isotopic compositions of Great Lakes ClO4- resemble those of indigenous natural ClO4- measured in groundwater of the western USA indicating a predominantly natural atmospheric source of ClO4- in all of the lakes. Bomb-pulse 36Cl is largely retained in Lake Superior because of the 191-yr water residence time in the lake.
A variety of plant species were also evaluated for their potential to accumulate and even generate ClO4- via O3-mediated processes. Plants, particularly in arid environments, may contain abundant Cl- in their tissues; display a vast array of hydrated internal and external reaction surfaces; and catalyze a multitude of redox reactions that could be involved in biosynthesis of ClO4-. These factors, the ubiquitous distribution of plants, and the post-industrial increase in O3 exposure are consistent with the possibility that tropospheric O3 may induce biosynthesis of ClO4- from Cl- in plants. This was evaluated as a potentially novel source of ClO4- in the environment.
The impact of plant accumulation of ClO4- on Cl and O stable isotope values was also evaluated in both hydroponic laboratory studies and field crops grown in different parts of the U.S. In hydroponic studies with snap beans, no substantial differences were observed in the δ 37Cl, δ18O, or Δ17O values of ClO4- between the growth solutions and leaf extracts. In contrast to ClO4-, δ15N of NO3- in plant tissue was fractionated substantially (~10-20‰). The ε15N/ε18O ratios of 1.04 ‒ 1.07 support previous experimental studies showing similar ratios via assimilatory nitrate reductase. The data indicate that plants do not metabolize and assimilate ClO4- similarly to NO3-. Similar to hydroponically grown plants, field grown plants exposed to environmentally relevant ClO4- concentrations also did not appear to affect foliar ClO4- isotopic composition.
Overall, the results of this project have provided important new information on natural ClO4- in the environment. Significant progress was made concerning potential mechanisms of its formation, isotopic characterization of natural ClO4- sources in groundwater, lakes, soils and plants, and its worldwide occurrence and accumulation in arid and semi-arid environments. The data support previous studies showing that natural and synthetic ClO4- can be differentiated by stable isotope methods, and suggest for the first time that the source(s) of ClO4- in food crops may be determined by isotopic analysis of ClO4- in plant tissue.