Effect of Plant Uptake on Perchlorate Isotopic Composition

The occurrence of perchlorate (ClO₄^-) in the environment is attributed to both synthetic and natural sources. Unlike anthropogenic ClO₄^-, natural ClO₄^- exhibits a wide range in isotopic compositions, suggesting that natural ClO₄^- is formed through more than one pathway and/or undergoes post-depositional isotopic fractionation processes. One of these processes could be plant uptake and metabolism. Plants are known to reversibly accumulate ClO₄^-. However, there is little information available regarding the ability for plants to isotopically fractionate ClO₄^-. Plants could alter ClO₄^-isotopic composition either by mass dependent fractionation via transport carriers in the root, diffusion limitations through the root, translocation within the plant, reduction of ClO₄^- by plant enzymes, or non-specific exchange of oxygen in ClO₄^- catalyzed by plant compounds/processes. We examined the potential for plants to alter the isotopic composition of ClO₄^- (δ³⁷Cl, δ¹⁸O, and Δ¹⁷O) in both hydroponic and field scale experiments. Hydroponically grown snap bean plants were exposed to variable ClO₄^-concentrations (2mg/L and 10mg/L) in solutions prepared from ClO₄^- with both normal and anomalous O isotopic abundances. At maturity, we evaluated the uptake of ClO₄^-relative to other anions and the isotopic compositions of ClO₄^- in both plants and growth solutions. Additional experiments involved field scale exposures of snap beans to irrigation water containing low levels (< 10 ug/L) of ClO₄^-. The majority of the initial mass of ClO₄^- for both the low and high exposure hydroponic treatments was recovered in the growth solutions (20-40%) or plant compartments (40-60%), while some mass was not recovered (~20%). ClO₄^- isotopic compositions were essentially identical between recovered ClO₄^- in the plant tissues and hydroponic solutions. Anion ratios indicate that ClO₄^-was accumulated similarly to NO₃^- but preferentially to Cl^- (~4X). In field experiments, the isotopic composition of ClO₄^- in snap bean plants (stems and leaves) also was similar to that of ClO₄^- in the irrigation water, which resembled ClO₄^-imported from the Atacama Desert. Minor differences between plant and irrigation-water values in the field experiments may indicate additional ClO₄^- sources, such as indigenous ClO₄^- supplied by rain or present in the soil. Based on these data, snap bean plants do not alter ClO₄^-isotopic composition substantially either by uptake or during storage within the plant.