Ozonation and Photodegradation of Oil Compounds by Surface Level Ozone and Engineered Catalysts in Marine Ecosystem
Type of DegreePhD Dissertation
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Marine oil spill has been an environmental challenge for many decades. As the use of oil dispersants has become one of the main approaches to clean up spilled oil, the environmental behavior of dispersed oil remains poorly understood. Specifically, Oil degradation by surface-level atmospheric ozone has been largely ignored in the field. To address this knowledge gap, this study in Chapter 2 investigated the ozonation rate and extent of typical petroleum compounds by simulated surface-level ozone, including total petroleum hydrocarbons (TPHs), n-alkanes, and polycyclic aromatic hydrocarbons (PAHs). Moreover, the work explored the effect of a prototype oil dispersant, Corexit EC9500A, on the ozonation rate. Rapid oxidation of TPHs, n-alkanes and PAHs was observed at various gaseous ozone concentrations (i.e. 86, 200 and 300 ppbv). Generally, the presence of the oil dispersant enhanced ozonation of the oil compounds. The addition of humic acid inhibited the reaction, while increasing salinity accelerated the degradation. Both direct ozonation by molecular ozone and indirect oxidation by ozone-induced radicals play important roles in the degradation process. The findings indicate that ozonation should be taken into account in assessing environmental fate and weathering of spilled oil. Meanwhile, little is known on the removal of dispersed oil from marine water using conventional adsorbents. This study in Chapter 3 investigated the effectiveness of three model activated charcoals (ACs) of different particle sizes (4-12, 12-20 and 100 mesh, i.e., GAC4×12, GAC12×20, PAC100) for removal of oil dispersed in marine water by two prototype oil dispersants (Corexit EC9500A and Corexit EC9527A). Sorption kinetic data revealed that all three ACs can efficiently adsorb dispersant Corexit EC9500A and two dispersed oil (DWAO-I and DWAO-II) following the same trend: PAC100 > GAC12×20 > GAC4×12. PAC100 showed much faster and more effective adsorption rate than the GACs, with a pseudo second-order rate constant k2 (g/(mg∙h)) value of 8.94, 7.62 and 0.51 for Corexit EC9500A, DWAO-I, and DWAO-II, respectively, and equilibrium uptake 5.23, 31.65, and 29.52 mg/g. Sorption isotherms confirmed PAC100 showed the highest adsorption capacity for dispersed oil in DWAO-I with a Freundlich KF value of 10.90 mg/g∙(L/mg)1/n (n = 1.38). In addition, PAC100 also showed the highest sorption of n-alkanes and PAHs. The various characterization results showed PAC100 have the higher measured BET surface area as 889 m2/g and total pore volume as 0.95 cm3/g (pHPZC = 6.1). Furthermore, the presence of additional dispersant Corexit EC9500A showed two contrasting effects on the oil sorption, i.e., adsolubilization effect and solubilization effect. Meanwhile, the solution pH only modestly affected the sorption of dispersed oil, i.e. the oil uptake decreased by <8% when increasing the solution pH from 6.0 to 9.0. However, salinity presents the inhibitive effect for the sorption uptake of oil hydrocarbons. This information is important for understanding roles of oil dispersant on the sorption of dispersed oil by activated charcoals from the dispersant-seawater-oil system. The results also indicate that the sorption of dispersed oil by activated charcoal is a low-cost and efficient way for the oil and dispersed oil cleanup process. In addition, we prepared various silica aerogel supported TiO2 (TiO2/SiO2) composite materials through initial sol-gel method and subsequent calcination, and tested the materials for removal of a model of PAHs, i.e., phenanthrene, through adsorption and photocatalysis as shown in Chapter 4. Anatase was formed at calcination temperature of 400 and 600 C, while mixed crystal phases of anatase and rutile were found at 800 C. All the TiO2/SiO2 composite materials were able to rapidly adsorb phenanthrene, with equilibrium being reached within 180 min. Higher calcination temperature resulted in better crystallinity of TiO2, higher photocatalytic activity, and reduced the adsorption affinity of the material toward phenanthrene. TiO2/SiO2-800 showed minimal phenanthrene uptake (only 5.2% of total phenanthrene) but the strongest photocatalytic activity, and it was able to completely degrade phenanthrene within 3 h without any residual in the solid phase. For TiO2/SiO2-800, TiO2 acts as the primary photocatalyst and silica serves as the support for nano-TiO2, which can facilitate accumulation and subsequent photodegradation of phenanthrene at the surface of the material by acting as a weak adsorbent as well as a medium. The material can be repeatedly used in multiple cycles of operations without significant loss in its photocatalytic activity. Meanwhile, we proposed and tested a composite material, activated charcoal supported titanate nanotubes (TNTs@AC), for efficient adsorption and photodegradation of oil components from (dispersed water accommodated oil) DWAO. TNTs@AC was prepared through a one-step hydrothermal method, and is composed of an activated charcoal core and a shell of carbon-coated titanate nanotubes. TNTs@AC held great potential to substantially advance current practices in treating spilled oil from water/seawater. >99.5% of TPHs, n-alkanes and PAHs could be adsorbed at 1.25 g/L TNTs@AC within 24 h, and the adsorption mainly occurred in the first 4 h. In the following photocatalysis process, 98.0%, 94.8% and 98.4% of the pre-concentrated (thourgh adsorption) TPHs, n-alkanes and PAHs could be degraded within 4 h. >97% of TPHs could be further removed by photo-regenerated TNTs@AC.