Application of fractal flocculation and vertical transport model to aquatic sol-sediment systems.

In estuarine and coastal environments, flocculation occurs between particles of different fractal dimensions and of different densities. Questions remain concerning the level of detail required to model particle flocculation and settling in these heterogeneous systems. This paper compares the goodness of fit between two flocculation models, using measured time series particle size distribution data collected from clay, colloidal silica, emulsified crude oil, clay-crude oil, and silica-crude oil systems. The coalesced sphere (CS) flocculation model includes the effects of heterogeneous particle size and density; the modified coalesced fractal sphere (mCFS) model adds effects due to heterogeneous fractal dimension. Goodness of fit was quantified using values of a minimized objective function, the mean of the sum of the square of the relative residuals (MSSRR). For nearly all tested experimental conditions, MSSRR values varied less than 5% between the CS and mCFS flocculation models. Additionally, collision efficiency values for single-particle-type (alpha(HOMOO)) and dual-particle-type (alpha(HETT)) systems were obtained through parameter regression using the CS and mCFS models. Using the mCFS model, estimated fractal dimension (D) values obtained for clay and clay-oil systems were between 2.6 and 3.0, lower than that postulated by the CS model but higher than that estimated experimentally by the particle concentration technique. The Stokes settling velocity of a clay aggregate of a given mass is reduced with decreased fractal dimension. This results in clay-oil flocculation occurring faster than floc sedimentation in the studied hydrodynamic range. Thus, the mCFS model provides insights to the fate of spilled oil in inland and coastal waters.

Modeling crude oil droplet-sediment aggregation in nearshore waters.

This paper describes a modeling approach that simulates changes in particle size distribution and density due to aggregation by extending the Smoluchowski aggregation kinetic model to particles of different density. Batch flocculation studies were conducted for clay, colloidal silica, crude oil, clay-crude oil, and silica-crude oil systems. A parameter estimation algorithm was used to estimate homogeneous collision efficiencies (alphaHOMO) for single-particle-type systems and heterogeneous collision efficiencies (alphaHET) for two-particle-type systems. Homogeneous collision efficiency values (alphaHOMO) were greater for clay (0.7) and for crude oil (0.3) than for silica (0.01). Thus, clay and crude oil were classified as cohesive particles while silica was classified as noncohesive. Heterogeneous collision efficiencies were similar for oil-clay (0.4) and oil-silica (0.3) systems. Thus, crude oil increases the aggregation of noncohesive particles. Data from the calibrated aggregation model were used to estimate apparent first-order flocculation rates (K') for oil, clay, and silica and apparent second-order flocculation rates (K'') for oil and clay in oil-clay systems and for oil and silica in oil-silica systems. For oil or clay systems, aggregation Damköhler numbers ranged from 0.1 to 1.0, suggesting that droplet coalescence and clay aggregation can occur on the same time scales as oil resurfacing and clay settling, respectively. For mixed oil-clay systems, the relative time scales of clay settling and clay-oil aggregation were also within an order of magnitude. Thus, oil-clay aggregation should be considered when modeling crude oil transport in nearshore waters.

Chemical dispersant effectiveness testing: influence of droplet coalescence.

Thermodynamic and kinetic investigations were performed to determine the influence of coalescence of chemically dispersed crude oil droplets in saline waters. For the range of pH (4-10) and salinity (10 per thousand, 30 per thousand, 50 per thousand ) values studied, zeta-potential values ranged from -3 to -10 mV. As the interaction potential values calculated using Derjaguin-Landau-Verway-Overbeek (DLVO) theory were negative, the electrostatic barrier did not produce significant resistance to droplet coalescence. Coalescence kinetics of premixed crude oil and chemical dispersant were determined within a range of mean shear rates (Gm = 5, 10, 15, 20 s(-1)) and salinity (10 per thousand, 30 per thousand ) values. Coalescence reaction rates were modeled using Smoluchowski reaction kinetics. Measured collision efficiency values (alpha = 0.25) suggest insignificant resistance to coalescence in shear systems. Experimentally determined dispersant efficiencies (alpha = 0.35) were 10-50% lower than that predicted using a non-interacting droplet model (alpha = 0.0). Unlike other protocols in which the crude oil and dispersant are not premixed, salinity effects were not significant in this protocol. This approach allowed the effects of dispersant-oil contact efficiency eta(contact) to be separated from those of water column transport efficiency (eta(transport)) and coalescence efficiency (eta(coalescence)).

Characterizing aquatic sediment-oil aggregates using in situ instruments.

The effects of emulsified crude oil and salinity (15, 30 per thousand ) on the steady state aggregate volume distributions and fractal dimensions were determined for a range of mean velocity gradients, (G(m) =5-50 s(-1)). Aggregation was performed in a 40-L cylindrical tank with a 4-blade paddle mixer. Three-dimensional fractal dimensions (D3) and volume distributions were determined using a procedure integrating data from an electrozone and an in situ light scattering instrument. Two-dimensional fractal dimensions (D2) and derived volume distributions were determined using a recently developed submersible flow cytometer equipped with a digital camera and image analysis software. For latex beads or emulsified crude oil systems, the above listed instruments yielded consistent size distributions and fractal dimensions (D2=1.92 +/- 0.16, D3=2.94 +/- 0.12). Mean volume aggregate diameters determined using the FlowCAM were consistently larger that those determined using the LISST-100 or Coulter Multisizer due to aggregate orientations during measurements. With increasing G(m) values, all colloidal aggregates showed increasing D3 values due to reduced aggregate length. Because of the compactness of all the aggregates (D3 >2), D2 values remained constant at 2. Neither salinity nor sediment type significantly affected D2 values calculated for sediment-crude oil aggregates. However, clay-oil aggregates showed higher D3 values than clay aggregates. This suggests that colloidal oil and mixing shear are the more dominant factors influencing aggregate morphology in nearshore waters. Overall, the data suggests that the analysis methods provide consistent size distribution results. However, because of the shear and salinity of coastal waters, resulting aggregates are too compact to estimate their D3 values using image analysis alone.

Partitioning of crude oil polycyclic aromatic hydrocarbons in aquatic systems.

This paper investigates the hypothesis that observed polycyclic aromatic hydrocarbon (PAH) concentrations in an aqueous system are equal to the sum of the organic phase and soluble phase molar concentrations. While the organic phase concentrations are proportional to the PAH mole fraction in the oil, the soluble phase molar concentrations are estimated using Raoult's law. A batch laboratory mixing vessel with a scalable mixing energy was loaded initially at various oil layer thicknesses (0.4-3.2 mm) which correspond to oil surface loadings (40-310 mg/cm2). The vessel was agitated at constant mean shear rates (Gm = 5, 20 s(-1)). Total petroleum hydrocarbon (TPH) samples were taken periodically to estimate the entrainment rate as a function of initial oil layer thickness. TPH concentrations were measured in-situ using a laser scattering instrument (LISST-100) and ex-situ using gravimetric analysis. At a steady-state TPH concentration (>72 h), additional samples were analyzed for PAH concentration using GC/MS analysis. TPH concentrations increased over time according to a first-order kinetic model. Generally, the first-order rate constant and steady-state concentration both increased with increased oil loading and with increased Gm. In addition, measured PAH concentrations correlated well (r2 > 0.96) with those predicted by a partitioning model. These results are useful for assessing the effects of mixing and oil loading conditions on crude oil entrainment and PAH partitioning.