The role of intra-NAPL diffusion on mass transfer from MGP residuals

Effects of size and composition of bitumen drops on intra-oil diffusion and dissolution of hydrocarbon solvents in froth treatment tailings ponds

The rate of mass transfer of lower molecular weight hydrocarbons (naphtha) from bitumen drops in mature fine tailings of oil sand tailings ponds (OSTPs) may control their bioavailability and the associated rate of GHG production. Experiments were conducted using bitumen drops spiked with o-xylene and 1-methylnaphthalene to determine the mass transfer rate of these naphtha components from bitumen drops. The results were compared to simulations using a multi-component numerical model that accounted for transport in the drop and across the oil-water interface. The results demonstrate rate-limited mass transfer, with aqueous concentrations after 60 days of dissolution that were different than those in equilibrium with the initial drop composition (less for o-xylene and greater for 1-methylnaphthalene). The simulations suggest that mole fractions were unchanged at the center of the drop, resulting in concentration gradients out to the oil-water interface. Numerical simulations conducted using different drop sizes and bitumen viscosities also suggest the potential for persistent naphtha dissolution, where the time required to deplete 80% of the o-xylene and 1-methylnaphthalene mass from an oil drop was estimated to be on the order of months to years for mm-sized drops, and years to decades for cm-sized drops assuming instantaneous biodegradation in the aqueous phase surrounding the bitumen.

Effects of source materials on desorption kinetics of carcinogenic PAHs from contaminated soils

Research investigating the desorptive behaviour of PAHs from contaminated soils often overlooked the effects of source materials, especially coal tar and coal tar pitch and materials alike. In this study, a refined experimental approach was adopted to establish a simple-to-complex continuum of systems that allow the investigation of desorption kinetics of benzo(a)pyrene (BaP) and 3 other carcinogenic PAHs (cPAHs) over an incubation period of 48 d. By comparing the modelled desorption parameters, elucidation of the effects of PAH source materials on their desorptive behaviour was achieved. Desorption of cPAHs from coal tar and pitch was enhanced when they were added to soils, with rapidly desorbing fraction (F_rap) of BaP increased from 0.68% for pitch to 1.10% and 2.66% for pitch treated soils, and from 2.57% for coal tar to 6.24% for coal tar treated soil G and 8.76% for coal tar treated sand (1 d). At 1 d, desorption of target cPAHs from solvent and source material spiked soils generally followed the order of solvent > coal tar > pitch. Increases in F_rap of cPAHs were observed in coal tar-treated soils after 48 d soil incubation (0.33%-1.16% for soil M, p ≥ 0.05, 6.24%-9.21% for soil G, p < 0.05) and was attributed to the continuous migration of coal tar as a non-aqueous phase liquid (NAPL) into soil pore structures. Slow desorption was dominated by source materials, whereas the extents and rates of rapid desorption (F_rap and k_rap) were more controlled by the quantity of soil organic matter (SOM), rather than quality of SOM (as in solvent-spiked soils). The results of this study challenged the role of PAH source materials as 'sinks' and led to the proposed roles of coal tar and pitch and source materials alike as 'reservoirs' with a risk-driven perspective.

Intra-NAPL diffusion and dissolution of a MGP NAPL exposed to persulfate in a flow-through system

Intra-NAPL diffusion is a critical process that can influence NAPL/water mass transfer. A series of physical model experiments was performed to investigate the role of intra-NAPL diffusion on the transient dissolution of a complex multicomponent NAPL subjected to persulfate treatment. To support these observations, a diffusion-based model was developed and calibrated using the experimental data. The experimental results indicated that while persulfate was able to completely degrade dissolved phase components, mass loss after ∼410 pore volumes of persulfate flushing was less than the no-treatment system. Intra-NAPL diffusion limitations were not observed in the physical model experiments. A comparison of experimental and simulated results indicated that processes related to persulfate/NAPL interactions restricted mass transfer, and yielded multicomponent mass transfer rate coefficients that were ∼30% of those estimated from an equivalent water-flushing experiment. Simulation results showed that a combination of NAPL composition and geometry, and interphase mass transfer rate can yield intra-NAPL diffusion limitations. Remedial technologies that rely on the aggressive flushing of reagents into NAPL zones may give rise to intra-NAPL diffusion limitations, which will directly affect treatment efficiency.

An empirical model for the evaluation of the dissolution rate from a DNAPL-contaminated area

This paper investigates dynamic variation in the morphologic distribution of dense non-aqueous phase liquids (DNAPLs), which take into account the coupled mass transfer. Experiments were carried out in a 2D tank representing a reconstructed aquifer model. DNAPL dissolution rates were investigated over a wide range of DNAPL saturations, several source configurations, and different hydraulic conditions. Morphometric indexes are presented that take into consideration further factors affecting the dissolution process. Local information regarding transport parameters related to the characteristics of the medium was obtained through a neural network and an optimization algorithm applied to experimental tracer tests. The history of DNAPL source architecture, in terms of saturation, indentation grade, and orientation, was determined by image analysis. Dissolved concentrations were registered and mass transfer rate coefficients were obtained for a wide range of source-zone configurations. A statistical analysis was performed to develop a constitutive equation that is descriptive of the mass transfer rate as a function of source-zone metric characteristics. A new empirical dissolution model using the proposed morphometric parameters is presented and compared with other models. The mass transfer correlation reported incorporates morphometric parameters and considers the complex and variable architecture of non-miscible contaminants. The proposed correlation can be used for an initial assessment of non-aqueous phase liquid (NAPL) dissolution rates over a wide range of saturation (residual and non-residual) conditions and different aqueous phase velocities within the NAPL source zone.