Sorption/Desorption and Photodegradation of Dispersed Oil Components in Marine Ecosystem

Abstract

This effects of a prototype oil dispersant on solubilization, sorption, and desorption of three model Polycyclic aromatic hydrocarbons (PAHs) in sediment-seawater systems were investigated. Increasing dispersant dosage linearly enhanced solubility for all PAHs. Conversely, the dispersant enhanced the sediment uptake of the PAHs, and induced significant desorption hysteresis. Such contrasting effects (adsolubilization vs. solubilization) of dispersant were found dependent of the dispersant concentration and PAH hydrophobicity. The dual-mode models adequately simulated the sorption kinetics and isotherms, and quantified dispersant-enhanced PAH uptake. Sorption of naphthalene and 1-methylnaphthalen by sediment positively correlated with uptake of the dispersant, while sorption of pyrene dropped sharply when the dispersant exceeded critical micelle concentration (CMC). The deepwater conditions diminished the dispersant effects on solubilization, but enhanced uptake of the PAHs, albeit sorption of the dispersant was lowered. The information may aid in understanding roles of dispersants on distribution, fate and transport of petroleum PAHs in marine systems. Fate of dispersed petroleum hydrocarbons in marine systems were investigated and the effects of typical weathering process including dispersion, sorption and photodegradation on distribution of representative petroleum hydrocarbons were examined in this work. Corexit 9500A was used to prepare dispersed water accommodated oil (DWAO) under varied dispersant-oil-ratios (DORs). Higher doses of dispersant can disperse more n-alkanes and PAHs into the water column. Some petroleum with lower molecular weight was dispersed more favorably, while the solubilization of longer chained alkanes (C30-C40) and PAHs with 5-6 rings were not enhanced even with a high DOR of 1:2. Three dispersants (Corexit 9500A, Corexit 9527A and SPC 1000) were compared for dispersing the petroleum hydrocarbons. For n-alkanes, the Corexit 9500A disperse both C11-C18 and C19-C28 effectively, and Corexit 9527A favored to disperse short chained alkanes (C10-C16). SPC 1000 was the most effective dispersant which can disperse a broader range of alkanes (C12-C28). Further addition of 180 mg/L of dispersant into the DWAO can enhance the solubilization effect thereby lowering the uptake of both PAHs and n-alkanes onto a loamy sand. In the DWAO, both n-alkanes and PAHs were readily photo-degraded under typical marine surface sunlight. Intensive alkylation of PAHs was observed in Day 3 and the concentrations of alkylated PAHs reached the maximum values in Day 3, subsequently followed by a rapid decay in Day 4- continuing until Day 14. We prepared and characterized a new type of photocatalyst, referred to as cobalt-doped titanate nanotubes (Co-TNTs), using TiO2 (P25) as the precursor through a two-step process (hydrothermal reaction at 150 ºC followed by calcination at 600 ºC). The optimal catalyst (Co-TNTs-600) was obtained at a Co loading of 2.26 wt.% and calcination temperature of 600 ºC. The catalyst effectively catalyzes photodegradation of phenanthrene (a model polycyclic aromatic hydrocarbon) under simulated solar light. The pseudo first-order rate constant was determined to be 0.39 h-1, about 10 times that of the conventional photocatalyst TiO2. TEM, XRD and XPS analyses indicate that Co-TNTs-600 is a composite nanomaterial containing titanate, anatase and CoO crystals. The hydrothermal treatment converts TiO2 into tubular, multilayered titanate nanotubes, allowing for incorporation Co(II) ions on the matrices. The subsequent calcination partially transforms titanate into anatase and the adsorbed Co2+ ions into CoO. UV-vis DRS spectra suggest that the absorption edge of Co-TNTs-600 shifts to the visible light region compared to P25 and un-calcined titanate nanotubes (TNTs), and the new catalyst displays a narrower optical energy band of 2.8 eV compared to 3.2 eV for P25 and 3.4 eV for TNTs. The incorporated CoO acts as an electron transfer mediator, which prevents the recombination of hole-electron pairs created mainly by anatase under solar irradiation. In addition, the Co-TNTs-600 exhibits good reusability and can be gravity-separated and reused in multiple cycles of operations for phenanthrene photodegradation. This new catalyst appears promising for catalyzing photodegradation of persistent PAH.