Dispersive liquid-liquid microextraction as a novel enrichment approach for compound-specific carbon isotope analysis of chlorinated phenols.

Compound-specific isotope analysis (CSIA) via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is a potent tool to elucidate the fate of (semi-)volatile organic contaminants in technical and environmental systems. Yet, due to the comparatively low sensitivity of IRMS, an enrichment step prior to analysis often is inevitable. A promising approach for fast as well as economic analyte extraction and preconcentration prior to CSIA is dispersive liquid-liquid microextraction (DLLME) - a well-established technique in concentration analysis of contaminants from aqueous samples. Here, we present and evaluate the first DLLME method for GC-IRMS exemplified by the analysis of chlorinated phenols (4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol) as model compounds. The analytes were simultaneously acetylated with acetic anhydride and extracted from the aqueous phase using a binary solvent mixture of acetone and tetrachloroethylene. With this method, reproducible δ13C values were achieved with errors ≤ 0.6‰ (n = 3) for aqueous concentrations down to 100 µg L-1. With preconcentration factors between 130 and 220, the method outperformed conventional liquid-liquid extraction in terms of sample preparation time and resource consumption with comparable reproducibility. Furthermore, we have demonstrated the suitability of the method (i) for the extraction of the analytes from a spiked river water sample and (ii) to quantify kinetic carbon isotope effect for 2,4,6-trichlorophenol during reduction with zero-valent zinc in a laboratory batch experiment. The presented work shows for the first time the potential of DLLME for analyte enrichment prior to CSIA and paves the way for further developments, such as the extraction of other compounds or scaling up to larger sample volumes.

Transformation of Iminodi(methylene phosphonate) on Manganese Dioxides - Passivation of the Mineral Surface by (Formed) Mn2.

Aminopolyphosphonates (APPs) are strong chelating agents with growing use in industrial and household applications. In this study, we investigated the oxidation of the bisphosphonate iminodi(methylene phosphonate) (IDMP) - a major transformation product (TP) of numerous commercially used APPs and a potential precursor for aminomethylphosphonate (AMPA) - on manganese dioxide (MnO2). Transformation batch experiments at pH 6 revealed AMPA and phosphate as main TPs, with a phosphorus mass balance of 80 to 92% throughout all experiments. Our results suggest initial cleavage of the C-P bond and formation of the stable intermediate N-formyl-AMPA. Next, C-N bond cleavage leads to the formation of AMPA, which exhibits lower reactivity than IDMP. Reaction rates together with IDMP and Mn2+ sorption data indicate formation of IDMP-Mn2+ surface bridging complexes with progressing MnO2 reduction, leading to the passivation of the mineral surface regarding IDMP oxidation. Compound-specific stable carbon isotope analysis of IDMP in both sorbed and aqueous fractions further supported this hypothesis. Depending on the extent of Mn2+ surface concentration, the isotope data indicated either sorption of IDMP to the mineral surface or electron transfer from IDMP to MnIV to be the rate-limiting step of the overall reaction. Our study sheds further light on the complex surface processes during MnO2 redox reactions and reveals abiotic oxidative transformation of APPs by MnO2 as a potential process contributing to widespread elevated AMPA concentrations in the environment.

Expanding the calibration range of compound-specific chlorine isotope analysis by the preparation of a 37 Cl-enriched tetrachloroethylene.

RATIONALE: The recent development of reliable GC/qMS methods for δ37 Cl compound-specific stable isotope analysis (CSIA) paves the way for dual carbon-chlorine isotope analysis of chlorinated ethenes and thus allows deeper insights into underlying transformation processes/mechanisms. A two-point calibration is indispensable for the precise and correct conversion of raw data to the international δ37 ClSMOC scale. The currently available calibration standards for tetrachloroethylene (PCE) span only a very narrow range from -2.52‰ (EIL2) to +0.29‰ (EIL1), which is considerably smaller than observed δ37 Cl isotope enrichment in (bio-)transformation studies (up to 12‰). METHODS: We describe the preparation and evaluation of a new 37 Cl-enriched PCE standard to avoid bias in δ37 Cl CSIA arising from extrapolation beyond the calibration range. The preparation comprised: (i) partial PCE reduction by zero-valent zinc in a system of PCE, ethanol (initial volume ratio 3/5) and trace amounts of water followed by (ii) liquid-liquid extraction and (iii) a subsequent fractional distillation to purify the 37 Cl-enriched PCE. RESULTS: The obtained PCE (PCEenriched ) showed a purity of 98.8% (mole fraction) and a δ37 ClSMOC value of +10.8 ± 0.5‰. The evaluation of an experimental dataset with and without extrapolation showed no significant variation. CONCLUSIONS: The new PCE standard (PCEenriched ) expands the calibration range to 13.3‰ (previously 2.8‰) and thus prevents potential bias introduced by extrapolation beyond the calibration range.

Two Pathways Compete in the Mn(II)-Catalyzed Oxidation of Aminotrismethylene Phosphonate (ATMP).

Mn(II)-catalyzed oxidation by molecular oxygen is considered a relevant process for the environmental fate of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP). However, the potential roles of Mn(III)ATMP-species in the underlying transformation mechanisms are not fully understood. We combined kinetic studies, compound-specific stable carbon isotope analysis, and equilibrium speciation modeling to shed light on the significance of such Mn-ATMP species for the overall ATMP oxidation by molecular oxygen. The fraction of ATMP complexed with Mn(II) inversely correlated with both (i) the Mn(II)-normalized transformation rate constants of ATMP and (ii) the observed carbon isotope enrichment factors (εc-values). These findings provide evidence for two parallel ATMP transformation pathways exhibiting distinctly different reaction kinetics and carbon isotope fractionation: (i) oxidation of ATMP present in Mn(III)ATMP complexes (εc ≈ -10 ‰) and (ii) oxidation of free ATMP by such Mn(III)ATMP species (εc ≈ -1 ‰) in a catalytic cycle. The higher reaction rate of the latter pathway implies that aminopolyphosphonates can be trapped in catalytic Mn-complexes before being transformed and suggests that Mn(III)ATMP might be a potent oxidant also for other reducible solutes in aqueous environments.

Stable carbon isotope analysis of polyphosphonate complexing agents by anion chromatography coupled to isotope ratio mass spectrometry: method development and application.

Compound-specific carbon isotope analysis (carbon CSIA) by liquid chromatography/isotope ratio mass spectrometry (LC-IRMS) is a novel and promising tool to elucidate the environmental fate of polar organic compounds such as polyphosphonates, strong complexing agents for di- and trivalent cations with growing commercial importance over the last decades. Here, we present a LC-IRMS method for the three widely used polyphosphonates 1-hydroxyethane 1,1-diphosphonate (HEDP), amino tris(methylenephosphonate) (ATMP), and ethylenediamine tetra(methylenephosphonate) (EDTMP). Separation of the analytes, as well as ATMP and its degradation products, was carried out on an anion exchange column under acidic conditions. Quantitative wet chemical oxidation inside the LC-IRMS interface to CO2 was achieved for all three investigated polyphosphonates at a comparatively low sodium persulfate concentration despite the described resilience of HEDP towards oxidative breakdown. The developed method has proven to be suitable for the determination of carbon isotope fractionation of ATMP transformation due to manganese-catalyzed reaction with molecular oxygen, as well as for equilibrium sorption of ATMP to goethite. A kinetic isotope effect was associated with the investigated reaction pathway, whereas no detectable isotope fractionation could be observed during sorption. Thus, CSIA is an appropriate technique to distinguish between sorption and degradation processes that contribute to a concentration decrease of ATMP in laboratory batch experiments. Our study highlights the potential of carbon CSIA by LC-IRMS to gain a process-based understanding of the fate of polyphosphonate complexing agents in environmental as well as technical systems.