Geology topics

Balaji Rao

Publications and source records attributed to Balaji Rao.

5 recordsLinked to original sources

USGS2021

Origin of the isotopic composition of natural perchlorate: Experimental results for the impact of reaction pathway and initial ClOx reactant

Natural perchlorate (ClO 4 − ) exists in many places on Earth, in lunar regolith, meteorites, and on the surface of Mars. Terrestrial natural ClO 4 − has widely variable Cl and O stable isotopic compositions (δ 37 Cl, δ 18 O, Δ 17 O). The δ 18 O and Δ 17 O values of ClO 4 − from the most hyper-arid locations co-vary. ClO 4 − from less arid areas has relatively little 17 O excess and poor Δ 17 O-δ 18 O correlation. ClO 4 − from the Atacama Desert has unusually low δ 37 Cl (<−10‰) and exhibits a positive correlation between δ 37 Cl and δ 18 O, while the δ 37 Cl of ClO 4 − from all other locations varies between −5 and +7‰ with no δ 37 Cl-δ 18 O covariation. To evaluate the impact of different precursors (ClO x ) and reaction pathways on the isotopic composition of ClO 4 − , we measured the isotopic composition of ClO 4 − produced in the laboratory by UV or O 3 mediated aqueous oxidation of Cl − , OCl − , ClO 2 − , and ClO 2 ° as well as O 3 mediated oxidation of dry NaCl. ClO x oxidation in aqueous or dry systems enriched in O 3 produced ClO 4 − with Δ 17 O values that generally increased with the number of O atoms required and included evidence that the site-specific 17 O anomaly in O 3 was preferentially transferred to ClO 4 − . Based on the inferred number of O atoms sourced from O 3 , and known Cl and O reaction pathways, it appears that ClO 2 ° and ClO 3 * were required intermediates in the production of ClO 4 − in the O 3 experiments. ClO x aqueous oxidation by UV irradiation produced ClO 4 − with a large range of δ 18 O values and little or no 17 O anomaly. ClO 3 − was produced to a much greater extent than ClO 4 − in all experiments except dry oxidation of NaCl by O 3 . The isotopic composition of ClO 3 − was distinct from that of ClO 4 − produced from the same initial reactants. Combined results of O 3 and UV mediated reactions largely bracketed the range of natural ClO 4 − δ 18 O and Δ 17 O values as well as δ 37 Cl values of non-Atacama natural samples, but no conditions produced the low δ 37 Cl values of Atacama ClO 4 − . Our results indicate that variation in production mechanisms, possibly combined with isotopically variable precursors, could be responsible for much of the observed isotopic variation in natural ClO 4 − and ClO 3 − .

Geochimica et Cosmochimica Acta↗

USGS2017

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

Natural perchlorate (ClO 4 − ) in soil and groundwater exhibits a wide range in stable isotopic compositions (δ 37 Cl, δ 18 O, and Δ 17 O), indicating that ClO 4 − may be formed through more than one pathway and/or undergoes post-depositional isotopic alteration. Plants are known to accumulate ClO 4 − , but little is known about their ability to alter its isotopic composition. We examined the potential for plants to alter the isotopic composition of ClO 4 − in hydroponic and field experiments conducted with snap beans ( Phaseolus vulgaris L.). In hydroponic studies, anion ratios indicated that ClO 4 − was transported from solutions into plants similarly to NO 3 − but preferentially to Cl − (4-fold). The ClO 4 − isotopic compositions of initial ClO 4 − reagents, final growth solutions, and aqueous extracts from plant tissues were essentially indistinguishable, indicating no significant isotope effects during ClO 4 − uptake or accumulation. The ClO 4 − isotopic composition of field-grown snap beans was also consistent with that of ClO 4 − in varying proportions from irrigation water and precipitation. NO 3 − uptake had little or no effect on NO 3 − isotopic compositions in hydroponic solutions. However, a large fractionation effect with an apparent ε ( 15 N/ 18 O) ratio of 1.05 was observed between NO 3 − in hydroponic solutions and leaf extracts, consistent with partial NO 3 − reduction during assimilation within plant tissue. We also explored the feasibility of evaluating sources of ClO 4 − in commercial produce, as illustrated by spinach, for which the ClO 4 − isotopic composition was similar to that of indigenous natural ClO 4 − . Our results indicate that some types of plants can accumulate and (presumably) release ClO 4 − to soil and groundwater without altering its isotopic characteristics. Concentrations and isotopic compositions of ClO 4 − and NO 3 − in plants may be useful for determining sources of fertilizers and sources of ClO 4 − in their growth environments and consequently in food supplies.

Science of the Total Environment↗

USGS2015

Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments

Natural perchlorate (ClO 4 − ) is of increasing interest due to its wide-spread occurrence on Earth and Mars, yet little information exists on the relative abundance of ClO 4 − compared to other major anions, its stability, or long-term variations in production that may impact the observed distributions. Our objectives were to evaluate the occurrence and fate of ClO 4 − in groundwater and soils/caliche in arid and semi-arid environments (southwestern United States, southern Africa, United Arab Emirates, China, Antarctica, and Chile) and the relationship of ClO 4 − to the more well-studied atmospherically deposited anions NO 3 − and Cl − as a means to understand the prevalent processes that affect the accumulation of these species over various time scales. ClO 4 − is globally distributed in soil and groundwater in arid and semi-arid regions on Earth at concentrations ranging from 10 −1 to 10 6 μg/kg. Generally, the ClO 4 − concentration in these regions increases with aridity index, but also depends on the duration of arid conditions. In many arid and semi-arid areas, NO 3 − and ClO 4 − co-occur at molar ratios (NO 3 − /ClO 4 − ) that vary between ∼10 4 and 10 5 . We hypothesize that atmospheric deposition 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 including N 2 fixation, N mineralization, nitrification, denitrification, and microbial ClO 4 − reduction, as indicated in part by NO 3 − isotope data. In contrast, much larger ranges of Cl − /ClO 4 − and Cl − /NO 3 − ratios indicate Cl − varies independently from both ClO 4 − and NO 3 − . The general lack of correlation between Cl − and ClO 4 − or NO 3 − implies that Cl − is not a good indicator of co-deposition and should be used with care when interpreting oxyanion cycling in arid systems. The Atacama Desert appears to be unique compared to all other terrestrial locations having a NO 3 − /ClO 4 − molar ratio ∼10 3 . The relative enrichment in ClO 4 − compared to Cl − or NO 3 − and unique isotopic composition of Atacama ClO 4 − may reflect either additional in-situ production mechanism(s) or higher relative atmospheric production rates in that specific region or in the geological past. Elevated concentrations of ClO 4 − reported on the surface of Mars, and its enrichment with respect to Cl − and NO 3 − , could reveal important clues regarding the climatic, hydrologic, and potentially biologic evolution of that planet. Given the highly conserved ratio of NO 3 − /ClO 4 − in non-biologically active areas on Earth, it may be possible to use alterations of this ratio as a biomarker on Mars and for interpreting major anion cycles and processes on both Mars and Earth, particularly with respect to the less-conserved NO 3 − pool terrestrially.

Geochimica et Cosmochimica Acta↗

USGS2010

Natural chlorate in the environment: Application of a new IC-ESI/MS/MS method with a Cl18O3- internal standard

A new ion chromatography electrospray tandem mass spectrometry (IC-ESI/MS/MS) method has been developed for quantification and confirmation of chlorate (ClO 3 − ) in environmental samples. The method involves the electro-chemical generation of isotopically labeled chlorate internal standard (Cl 18 O 3 − ) using 18 O water (H 2 18 O). The standard was added to all samples prior to analysis thereby minimizing the matrix effects that are associated with common ions without the need for expensive sample pretreatments. The method detection limit (MDL) for ClO 3 − was 2 ng L −1 for a 1 mL volume sample injection. The proposed method was successfully applied to analyze ClO 3 − in difficult environmental samples including soil and plant leachates. The IC-ESI/MS/MS method described here was also compared to established EPA method 317.0 for ClO 3 − analysis. Samples collected from a variety of environments previously shown to contain natural perchlorate (ClO 4 − ) occurrence were analyzed using the proposed method and ClO 3 − was found to co-occur with ClO 4 − at concentrations ranging from <2 ng L −1 in precipitation from Texas and Puerto Rico to >500 mg kg −1 in caliche salt deposits from the Atacama Desert in Chile. Relatively low concentrations of ClO 3 − in some natural groundwater samples (<0.1 μg L −1 ) analyzed in this work may indicate lower stability when compared to ClO 4 − in the subsurface. The high concentrations of ClO 3 − in caliches and soils (3−6 orders of magnitude greater) as compared to precipitation samples indicate that ClO 3 − , like ClO 4 − , may be atmospherically produced and deposited, then concentrated in dry soils, and is possibly a minor component in the biogeochemical cycle of chlorine.

Environmental Science & Technology↗

USGS2007

Widespread natural perchlorate in unsaturated zones of the southwest United States

A substantial reservoir (up to 1 kg ha -1 ) of natural perchlorate is present in diverse unsaturated zones of the arid and semi-arid southwestern United States. The perchlorate co-occurs with meteoric chloride that has accumulated in these soils throughout the Holocene [0 to 10−15 ka (thousand years ago)] and possibly longer periods. Previously, natural perchlorate widely believed to be limited to the Atacama Desert, now appears widespread in steppe-to-desert ecoregions. The perchlorate reservoir becomes sufficiently large to affect groundwater when recharge from irrigation or climate change flushes accumulated salts from the unsaturated zone. This new source may help explain increasing reports of perchlorate in dry region agricultural products and should be considered when evaluating overall source contributions.

Environmental Science & Technology↗