Neil C. Sturchio

Per- and polyfluoroalkyl substances (PFAS) are globally distributed environmental contaminants whose persistence and toxicity necessitate advanced source identification strategies. Stable isotope analysis serves as a powerful forensic tool, providing intrinsic signatures to trace contaminant origins and transformation pathways. However, multi-element isotope characterization of PFAS remains underdeveloped, with most studies limited to carbon. Here, we present a robust method for multi-element (δ34S, δ15N, and δ18O) compound-specific isotope analysis of PFAS using Orbitrap mass spectrometry (MS). Using perfluorobutanesulfonamide (FBSA) as a model compound, we evaluated the isotopic characteristics of its NSO2– fragment generated via higher-energy collisional dissociation (HCD) and in-source fragmentation. Orbitrap δ34S measurements across diverse FBSA samples showed high precision and good alignment with elemental analyzer–isotope ratio mass spectrometry (EA–IRMS), effectively resolving isotopic variations among samples (ranging from −4.3‰ to +4.4‰). Simultaneous δ15N measurements and optimized δ18O measurements exhibited good agreement with EA–IRMS values, demonstrating robust multi-element capability. Across 3 orders of magnitude in concentrations of FBSA (0.01 to 10 μM), Orbitrap δ34S values measured using HCD remained stable (2 SD = 0.96‰), whereas δ34S measured using in-source fragmentation at 0.01 μM deviated from the mean δ34S values of higher concentrations by 2.9‰. In binary mixtures with perfluorohexanesulfonamide (FHxSA), HCD-based analysis of FBSA yielded consistent isotope values, with deviations observed only under equimolar, high PFAS loading conditions (7 μM each of FBSA and FHxSA). These results demonstrate that the developed framework provides a high-resolution platform for PFAS isotope analysis, enabling new opportunities in environmental forensics.

The Arabian Peninsula encompasses one of the world's most extensive aeolian systems, yet the provenance and transport history of its dune sediments are not well understood. Using hyperspectral data from NASA's Earth Surface Mineral Dust Source Investigation (EMIT), we mapped mineral compositions and abundance patterns across dune fields and adjacent source regions by analyzing diagnostic visible to shortwave infrared absorption features. Six diagnostic minerals (Calcite, Kaolinite, Muscovite–illite, Chlorite, Hematite, and Goethite) exhibit spatially coherent and elevated (>0.002 < 0.16) spectral mineral abundances (SMA: fractional area occupied by a mineral in a pixel) that distinguish 10 dune fields. Mineralogical concordance, SMA attenuation along transport pathways, prevailing wind regimes, and relict fluvial pathways reveal three provenance domains: (a) intracontinental inputs from the Arabian Shield and overlying Paleozoic siliciclastics; (b) marginal contributions from carbonate and ophiolitic terrains; and (c) extra‐basinal inputs from marine carbonate aeolianites and Bitlis–Zagros orogenic detritus.

Krypton-81 (⁸¹Kr) and chlorine-36 (³⁶Cl) are among the few isotopic tracers capable of constraining groundwater residence times on 10⁵–10⁶ year timescales. In sedimentary aquifer systems bounded by low-permeability units, however, diffusive solute exchange can strongly modify tracer distributions and bias apparent ages derived from concentration ratios. In the transboundary Milk River Aquifer (MRA), progressive chloride enrichment caused by diffusion across shale aquitards complicates the interpretation of ³⁶Cl/Cl as a chronometer. Here, we combine new measurements of ⁸¹Kr, ³⁶Cl, stable chlorine isotopes (³⁷Cl/³⁵Cl), and ¹⁴C with advection–diffusion transport modeling to quantify the importance of matrix diffusion on tracer systematics and inferred groundwater ages. The simulations reproduce the observed decrease in ³⁶Cl/Cl and concomitant increase in δ³⁷Cl along regional flow paths, demonstrating that diffusive influx of Cl-rich aquitard water dominates the evolution of the chlorine isotope system. In contrast, modeled and observed ⁸¹Kr activities show substantially lower sensitivity to diffusive exchange over the timescales considered. A comparison of simulated and measured tracer relationships indicates that, in the MRA, apparent ages derived from ³⁶Cl primarily reflect chloride addition rather than radioactive decay, whereas ⁸¹Kr provides a more robust and conservative chronometer for fossil groundwater. These results highlight the value of integrating stable and radioactive chlorine isotopes with noble gas dating and explicit transport modeling to disentangle decay from transport effects. The approach developed here provides a quantitative framework for interpreting multitracer data sets in regional aquifers affected by long-term diffusive exchange and has broader implications for assessing fossil groundwater resources in similar hydrogeological settings.

The reaction of dissolved Pb²⁺ with calcite surfaces at near-equilibrium conditions involves adsorption of Pb²⁺ and precipitation of secondary heteroepitaxial Pb-carbonate minerals. A more complex behavior is observed under far-from-equilibrium conditions, including strong inhibition of calcite dissolution, development of microtopography, and near-surface incorporation of multiple monolayers (ML) of Pb²⁺ without precipitation of secondary phases [where 1 ML ≡ 1 Ca/20.2 Ų, the crystallographic site density of the calcite (104) lattice plane]. However, the mechanistic controls governing far-from-equilibrium reactivity are not well understood. Here, we observe the interfacial incorporation of dissolved Pb²⁺ during the dissolution of calcite (104) surfaces at pH ∼ 3.7 in a flow-through reaction cell, revealing the formation of a ∼1 nm thick Pb-rich calcite layer with a total Pb coverage of ∼1.4 ML. These observations of the sorbed Pb distribution used resonant anomalous X-ray reflectivity, X-ray fluorescence, and nanoinfrared atomic force microscopy. We propose that this altered surface layer represents a novel sorption mode that is stabilized by conditions of sustained disequilibrium. This behavior may significantly impact the transport of dissolved metals during disequilibrium processes occurring in acid mine drainage and subsurface CO₂ injection and, if appropriately accounted for, could improve the predictive capability of geochemical reactive-transport models.

Reactions of mineral surfaces with dissolved metal ions at far-from-equilibrium conditions can deviate significantly from those in near-equilibrium systems due to steep concentration gradients, ion-surface interactions, and reactant transport effects that can lead to emergent behavior. We explored the effect of dissolved Pb2+ on the dissolution rate and topographic evolution of calcite (104) surfaces under far-from-equilibrium acidic conditions (pH 3.7) in a confined single-pass laminar-flow geometry. Operando measurements by digital holographic microscopy were conducted over a range of Pb2+ concentrations ([Pb2+] = 0 to 5 x 10(-2) M) and flow velocities (v = 1.67-53.3 mm s(-1)). Calcite (104) surface dissolution rates decreased with increasing [Pb2+]. The inhibition of dissolution and the emergence of unique topographic features, including micropyramids, variable etch pit shapes, and larger scale topographic patterns, became increasingly apparent at [Pb2+] >= 5 x 10(-3) M. A better understanding of such dynamic reactivity could be crucial for constructing accurate models of geochemical transport in aqueous carbonate systems.

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (Λ^N/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from Λ^N/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe²⁺-bearing minerals and iron-oxide-bound Fe²⁺, and UV-driven phototransformations. The Λ^N/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe²⁺-bearing minerals and iron-oxide-bound Fe²⁺, albeit the mean Λ^N/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.