Sunlight-Induced Molecular Progression of Oil into Oxidized Oil Soluble Species, Interfacial Material, and Dissolved Organic Matter

Abstract

Spilled oil is highly susceptible to sunlight-induced transformations, both as films on the surface of water and material dissolved or dispersed in the water column. We utilized ultrahigh-resolution mass spectrometry and optical spectroscopy to understand shifts in oil photoproduct distributions as a function of photo-oxygenation. Oxygenation of oil produces compounds that have increased polarity, resulting in greater partitioning to the oil−water interface and eventually greater partitioning into the aqueous phase. Such partitioning was shown to be dependent on the carbon number and oxygen content of the photoproducts, providing an empirical basis for predicting the partitioning of oil photodegradation products between the oil phase, the interfacial region, and into the aqueous phase to form petroleum-derived dissolved organic matter. While such photochemical transformations have been predicted for many years, there has not been direct evidence previously for the photodissolution process. Furthermore, the relationship of carbon number and oxygen content with progression from the oil phase to the interfacial phase to the aqueous phase has not been demonstrated. This paper details this progression and observable properties that can be used to understand oil behavior after a spill during sunlight exposure, thus providing greater predictability of oil fate, transport, impact, and effective remediation strategies.

Sunlight-driven dissolution is a major fate of oil at sea

“…Photochemistry matters for oil spill modeling and response efforts because sunlight-generated species have distinct physical and chemical properties from crude oil (1,2), thus affecting oil transport, environmental partitioning, and the effectiveness of response tools such as chemical dispersants (6). Furthermore, photochemistry may have opposing effects on different pools of oil compounds depending on the relative importance of different reaction pathways (for example, oxidation, fragmentation, and polymerization), resulting in observations of both persistent, presumably high-molecular weight polymerized species [for example, tar balls on beaches (5,(7)(8)(9)] and lower-molecular weight volatile and water-soluble species (1,3,10). Because oxygenation is a ubiquitous feature of both the high-and low-molecular weight materials produced by sunlight (5,10), "photo-oxidation" is commonly used to refer to the suite of photochemical processes influencing oil and will be used throughout this article.…”

Molecular Characterization of Photochemically Produced Asphaltenes via Photooxidation of Deasphalted Crude Oils

“…Other reports by Ruddy, Niles, and Tarr indicate that photoproducts from petroleum in the environment comprise acidic and ketone/aldehyde functionalities, which are readily accessible by negative- and positive-ion electrospray ionization (ESI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). ,− Furthermore, oil- and water-soluble photooxidation products have been shown to contain abundant oxygenated S- and N-containing compounds (S x O y and N x O y ). ,,,, Many of these weathering products may exhibit solubility behavior consistent with asphaltenes, i.e., remain oil-soluble but alkane-insoluble. In addition, a higher content of alkyl-side chains (higher carbon number) decreases water solubility. − Thus, several works suggest that water-soluble photoproducts have carbon numbers between ∼10–30, whereas nonweathered petroleum and oil-soluble photoproducts typically have a carbon number above 30 …”

“…Recent works demonstrate that upon sunlight irradiation, asphaltenes can produce oil- and water-soluble compounds with up to 18 oxygen atoms per molecule. − Chacón-Patiño et al demonstrated that water solubility increases as a function of increasing oxygen content and decreasing molecular weight, which is sample- and structure-dependent.…”

What Was Released? Assessing the Physical Properties and Chemical Composition of Petroleum and Products of Burned Oil

“…As determined through ultrahigh resolution mass spectrometry (FT-ICR MS), elemental assignments for tens of thousands of molecules in the native (unaltered) and weathered Macondo oil revealed the chemical changes induced by weathering (Figure 5d; Ruddy et al, 2014). Subsequent analysis of different crude oil fractions (oil-soluble non-interfacially active, oil-soluble interfacially active, and water soluble) indicated that decreasing carbon and increasing oxygen numbers determined the progression of molecules from oil-soluble to water-soluble (Zito et al, 2020). Thus, weathering-induced chemical changes (see arrows in Figure 5) were linked to changes in oil solubility.…”