J. Samuel Arey

Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill

Quantitative information regarding the endmember composition of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for determining the oil flow rate, total oil volume released, and trajectories and fates of hydrocarbon components in the marine environment. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1,600 standard cubic feet per petroleum barrel. Based on the measured endmember gas-to-oil ratio and the Federally estimated net liquid oil release of 4.1 million barrels, the total amount of C1-C5 hydrocarbons released to the water column was 1.7 × 1011 g. The endmember gas and oil compositions then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1,100 m. The most abundant petroleum hydrocarbons larger than C1-C5 were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L-1. Comparison of the endmember gas and oil composition with the composition of water column samples showed that the plume was preferentially enriched with water-soluble components, indicating that aqueous dissolution played a major role in plume formation, whereas the fates of relatively insoluble petroleum components were initially controlled by other processes.

Stacking and spreading interaction in N-heteroaromatic systems

π-π interactions in heteroaromatic systems are ubiquitous in biological systems. In the present study, stabilization energies of stacked and hydrogen-bonded dimers of N-heteroaromatic systems (pyridine, pyrazine, sym-triazine, and sym-tetrazine) have been computed using a benchmark quality coupled cluster through the perturbative triples (CCSD(T)) method at the estimated complete basis set (CBS) limit. In the case of stacking, monomer units are found to be stacked in parallel planes with displaced geometries. The stabilization energies for the most stable stacked geometry of pyridine, pyrazine, sym-triazine, and sym-tetrazine dimers are found to be -3.39, -4.14, -4.02, and -3.90 kcal/mol, respectively at the est. CCSD(T)/CBS level of theory, which is clearly larger than the stabilization energy for the most stable geometry of the benzene dimer. In the case of spreading, hydrogen bonded dimers and trimers are stabilized by weak C-H···N interactions. The stabilization energies for the stacked and the spread out complexes are found to be comparable. The stabilization energy for the trimers is computed using the MP2, MP3, and B3LYP-D methods. The present study is aimed at unraveling the basis of preferred conformations of N-heteroaromatic dimers. These model systems explain partly the stability of double helical DNA and RNA structures that are formed by stacking and hydrogen bonding between nucleic acid bases.

Critical Evaluation of Implicit Solvent Models for Predicting Aqueous Oxidation Potentials of Neutral Organic Compounds

Quantum chemical implicit solvent models are used widely to estimate aqueous redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aqueous single electron oxidation potentials of a diverse test set of neutral organic compounds for which accurate experimental oxidation potential and gas-phase ionization energy data are available. Using a thermodynamic cycle, we decomposed the free energy of oxidation into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aqueous oxidation potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidation potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted experimental data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aqueous oxidation potentials and related properties.

Resolving Biodegradation Patterns of Persistent Saturated Hydrocarbons in Weathered Oil Samples from the Deepwater Horizon Disaster

Biodegradation plays a major role in the natural attenuation of oil spills. However, limited information is available about biodegradation of different saturated hydrocarbon classes in surface environments, despite that oils are composed mostly of saturates, due to the limited ability of conventional gas chromatography (GC) to resolve this compound group. We studied eight weathered oil samples collected from four Gulf of Mexico beaches 12-19 months after the Deepwater Horizon disaster. Using comprehensive two-dimensional gas chromatography (GC × GC), we successfully separated, identified, and quantified several distinct saturates classes in these samples. We find that saturated hydrocarbons eluting after n-C22 dominate the GC-amenable fraction of these weathered samples. This compound group represented 8-10%, or 38-68 thousand metric tons, of the oil originally released from Macondo well. Saturates in the n-C22 to n-C29 elution range were found to be partly biodegraded, but to different relative extents, with ease of biodegradation decreasing in the following order: n-alkanes > methylalkanes and alkylcyclopentanes+alkylcyclohexanes > cyclic and acyclic isoprenoids. We developed a new quantitative index designed to characterize biodegradation of >n-C22 saturates. These results shed new light onto the environmental fate of these persistent, hydrophobic, and mostly overlooked compounds in the unresolved complex mixtures (UCM) of weathered oils.

Hydrogen Bonding Described Using Dispersion-Corrected Density Functional Theory

In recent works, dispersion-corrected atom-centered potentials (DCACPs) were developed as a method to account for long-range dispersion forces between molecules in density functional theory calculations within the generalized gradient approximation (GGA). Here, we test the ability of DCACPs to improve the GGA treatment of hydrogen-bonded systems. We assessed both BLYP and dispersion-corrected BLYP with respect to benchmark calculations for the hydrogen bond lengths and binding energies of 20 complexes containing the elements C, H, N, O, and S. Benchmark data included geometries calculated using MP2 and CCSD(T) and binding energies using W2, W1, CBS-QB3, and other CCSD(T) extrapolation schemes. With respect to benchmark methods, dispersion-corrected BLYP exhibited a mean signed error of 0.010 A in the hydrogen bond length and a mean relative error of 5.1% in the hydrogen bond binding energy. By comparison, uncorrected BLYP exhibited error statistics of 0.036 A and 15.9%, respectively. We conclude that DCACPs robustly improve the BLYP description of hydrogen-bonded systems at small additional computational cost. New benchmark geometries (MP2/aug-cc-pVTZ) and new benchmark binding energies (W1) are presented for seven complexes, and the remaining benchmark data were taken from previous literature.

First day of an oil spill on the open sea: Early mass transfers of hydrocarbons to air and water

During the first hours after release of petroleum at sea, crude oil hydrocarbons partition rapidly into air and water. However, limited information is available about very early evaporation and dissolution processes. We report on the composition of the oil slick during the first day after a permitted, unrestrained 4.3 m(3) oil release conducted on the North Sea. Rapid mass transfers of volatile and soluble hydrocarbons were observed, with >50% of ≤C17 hydrocarbons disappearing within 25 h from this oil slick of <10 km(2) area and <10 μm thickness. For oil sheen, >50% losses of ≤C16 hydrocarbons were observed after 1 h. We developed a mass transfer model to describe the evolution of oil slick chemical composition and water column hydrocarbon concentrations. The model was parametrized based on environmental conditions and hydrocarbon partitioning properties estimated from comprehensive two-dimensional gas chromatography (GC×GC) retention data. The model correctly predicted the observed fractionation of petroleum hydrocarbons in the oil slick resulting from evaporation and dissolution. This is the first report on the broad-spectrum compositional changes in oil during the first day of a spill at the sea surface. Expected outcomes under other environmental conditions are discussed, as well as comparisons to other models.

Reactivity of BrCl, Br2, BrOCl, Br2O, and HOBr Toward Dimethenamid in Solutions of Bromide + Aqueous Free Chlorine

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.

Floating oil-covered debris from Deepwater Horizon: identification and application

The discovery of oiled and non-oiled honeycomb material in the Gulf of Mexico surface waters and along coastal beaches shortly after the explosion of Deepwater Horizon sparked debate about its origin and the oil covering it. We show that the unknown pieces of oiled and non-oiled honeycomb material collected in the Gulf of Mexico were pieces of the riser pipe buoyancy module of Deepwater Horizon. Biomarker ratios confirmed that the oil had originated from the Macondo oil well and had undergone significant weathering. Using the National Oceanic and Atmospheric Administrations records of the oil spill trajectory at the sea surface, we show that the honeycomb material preceded the front edge of the uncertainty of the oil slick trajectory by several kilometers. We conclude that the observation of debris fields deriving from damaged marine materials may be incorporated into emergency response efforts and forecasting of coastal impacts during future offshore oil spills, and ground truthing predicative models.

Aqueous oxidation of sulfonamide antibiotics: Aromatic nucleophilic substitution of an aniline radical cation

Sulfonamide antibiotics are an important class of organic micropollutants in the aquatic environment. For several, sulfur dioxide extrusion products have been previously reported upon photochemical or dark oxidation. Using quantum chemical modeling calculations and transient absorption spectroscopy, it is shown that single-electron oxidation from sulfadiazine produces the corresponding aniline radical cation. Density functional theory calculations indicate that this intermediate can exist in four protonation states. One species exhibits a low barrier for an intramolecular nucleophilic attack at the para position of the oxidized aniline ring, in which a pyrimidine nitrogen acts as a nucleophile. This attack can lead to a rearranged structure, which exhibits the same connectivity as the SO2 -extruded oxidation product that was previously observed in the aquatic environment and characterized by NMR spectroscopy. We report a detailed reaction mechanism for this intramolecular aromatic nucleophilic substitution, and we discuss the possibility of this reaction pathway for other sulfonamide drugs.

Accurate DFT Descriptions for Weak Interactions of Molecules Containing Sulfur

Dispersion corrected atom centered potentials (DCACPs) have been shown to significantly improve the density functional theory (DFT) description of weak interactions. In this work, we have calibrated a DCACP for sulfur in combination with the widely used Generalized Gradient Approximation (GGA) BLYP, thereby augmenting the existing library of DCACPs for the first- and second-row elements H, C, N, O, and rare gases. Three weakly bound complexes as well as elemental (orthorhombic) sulfur are used as test cases to evaluate the transferability of the DCACP to different chemical environments. It is found that the sulfur DCACP systematically improves the agreement of DFT-calculated weak interactions with respect to MP2 and CCSD(T) level results.