Susan E. Burns

Microbubble generation for environmental and industrial separations

Small gas bubbles are used in many environmental and industrial processes for solid-liquid separations or to facilitate heat and mass transfer between phases. Typically, smaller bubbles are preferred in treatment techniques due to both their high surface area-to-volume ratio and their increased bubble density at a fixed flow rate. This study examines some of the factors that affect the size of bubbles produced in the processes of electroflotation, dissolved air flotation, and a relatively new method known as electrostatic spraying. The effect of voltage, current and ionic strength was studied in electroflotation, the effect of pressure was studied in dissolved air flotation and the effect of voltage, capillary dimensions and flow rate was studied in electrostatic spraying. In electroflotation, the flow rate of gas produced increased as a function of voltage and current. Flow rate also increased as the ionic strength of the aqueous medium was increased. However, no clear trends in bubble size as a function of these parameters were evident. The bubbles produced in dissolved air flotation showed a decrease in size as saturation pressure was increased; however, the differences were insignificant at high pressures. Bubble size in electrostatic spraying decreased as voltage was increased. Finally, this study compares the three methods of bubble production in terms of average bubble diameter, bubble size distribution and power consumed during production. Dissolved air flotation produced the largest average bubble diameters, while electroflotation produced the smallest average bubble diameters. In terms of bubble size distribution, dissolved air flotation produced the most narrow distribution, electrostatic spraying produced the widest distribution, and electroflotation produced an intermediate distribution. In terms of power consumption, the pilot-scale dissolved air flotation system maximized surface area production, electroflotation produced an intermediate value, and electrostatic spraying of air produced the least surface area as a function of power consumed.

Nonionic organic solute sorption onto two organobentonites as a function of organic-carbon content

Sorption of three nonionic organic solutes (benzene, trichloroethene, and 1,2-dichlorobenzene) to hexadecyltrimethylammonium bentonite (HDTMA bentonite) and benzyltriethylammonium bentonite (BTEA bentonite) was measured as a function of total organic-carbon content at quaternary ammonium cation loadings ranging from 30 to 100% of the clays cation-exchange capacity. Sorption of all three solutes to HDTMA bentonite was linear and sorption of all three solutes by the HDTMA bentonite increased as the organic-carbon content of the clay increased. 1,2-Dichlorobenzene sorbed most strongly to HDTMA bentonite, followed by benzene and TCE. The stronger sorption of benzene to HDTMA bentonite compared to TCE was unexpected based on a partition mechanism of sorption and consideration of solute solubility. LogK(oc) values for all three solutes increased with organic-carbon content. This suggests that the increased organic-carbon content alone may not explain the observed increase in sorption capacity. Sorption of the three solutes to BTEA bentonite was nonlinear and solute sorption increased with decreasing organic-carbon content, with a peak in the magnitude of solute sorption occurring at an organic-carbon content corresponding to 50% of CEC. Below 50% of CEC, sorption of all three solutes to BTEA bentonite decreased with decreasing organic-carbon content. Surface area measurements indicate that the surface area of both organobentonites generally decreased with increasing organic-carbon content. Since nonionic organic solute sorption to BTEA bentonite occurs by adsorption, the reduced sorption is likely caused by the reduction in surface area corresponding to increased organic-cation loading.

Sorption and permeability of gasoline hydrocarbons in organobentonite porous media

We investigate the use of organobentonites as liners for underground gasoline storage tanks to reduce the risk of subsurface contamination. A series of permeability measurements were conducted on two types of organobentonites: benzyltriethylammonium-bentonite (BTEA-bentonite) and hexadecyltrimethylammonium-bentonite (HDTMA-bentonite). Both water and commercial unleaded gasoline were used as the permeant liquids. Results of these measurements indicate that the intrinsic permeability of the organobentonite decreases by one to two orders of magnitude when the permeant liquid is changed from water to gasoline. Results of batch sorption measurements reveal that benzene sorption to both organobentonites from water is greater than benzene sorption to conventional bentonite. The magnitude of benzene sorption is related to the loading of the organic quaternary ammonium cation on the clay. As the HDTMA cation loading increases from 25% of cation exchange capacity (CEC) to 120% of CEC, benzene sorption increases. However, as the BTEA cation loading increases from 40 to 120% of CEC, benzene sorption decreases. Collectively, these results suggest that organobentonites can be used effectively to reduce hydrocarbon migration rates beneath leaking underground gasoline storage tanks, and that the optimal organic cation loading with respect to pollutant sorption may be less than 50% of cation exchange capacity for some organobentonite-solute combinations.

Utilization of Savannah Harbor river sediment as the primary raw material in production of fired brick

A laboratory-scale study was conducted to assess the feasibility of the production of fired bricks from sediments dredged from the Savannah Harbor (Savannah, GA, USA). The dredged sediment was used as the sole raw material, or as a 50% replacement for natural brick-making clay. Sediment bricks were prepared using the stiff mud extrusion process from raw mixes consisted of 100% dredged sediment, or 50% dredged sediment and 50% brick clay. The bricks were fired at temperatures between 900 and 1000 °C. Physical and mechanical properties of the dredged sediment brick were found to generally comply with ASTM criteria for building brick. Water absorption of the dredged sediment bricks was in compliance with the criteria for brick graded for severe (SW) or moderate (MW) weathering. Compressive strength of 100% dredged sediment bricks ranged from 8.3 to 11.7 MPa; the bricks sintered at 1000 °C met the requirements for negligible weathering (NW) building brick. Mixing the dredged sediment with natural clay resulted in an increase of the compressive strength. The compressive strength of the sediment-clay bricks fired at 1000 °C was 29.4 MPa, thus meeting the ASTM requirements for the SW grade building brick. Results of this study demonstrate that production of fired bricks is a promising and achievable productive reuse alternative for Savannah Harbor dredged sediments.

Molecular dynamics simulation of secondary sorption behavior of montmorillonite modified by single chain quaternary ammonium cations.

Organoclays synthesized from single chain quaternary ammonium cations (QAC) ((CH(3))(3)NR(+)) exhibit different mechanisms for the sorption of nonpolar organic compounds as the length of the carbon chain is increased. The interaction between a nonpolar sorbate and an organoclay intercalated with small QACs has been demonstrated to be surface adsorption, while partitioning is the dominant mechanism in clays intercalated with long chain surfactants. This study presents the results of a molecular dynamics (MD) simulation performed to examine the sorption mechanisms of benzene in the interlayer of three organoclays with chain lengths ranging from 1 to 16 carbons: tetramethylammonium (TMA) clay; decyltrimethylammonium (DTMA) clay; and hexadecyltrimethylammonium (HDTMA) clay. The basis of the overall simulation was a combined force field of ClayFF and CVFF. In the simulations, organic cations were intercalated and benzene molecules were introduced to the interlayer, followed by whole system NPT and NVT time integration. Trajectories of all the species were recorded after the system reached equilibrium and subsequently analyzed. Simulation results confirmed that the arrangement of the surfactants controlled the sorption mechanism of organoclays. Benzene molecules were observed to interact directly with the clay surface in the presence of TMA cations, but tended to interact with the aliphatic chain of the HDTMA cation in the interlayer. The simulation provided insight into the nature of the adsorption/partitioning mechanisms in organoclays, and explained experimental observations of decreased versus increased uptake capacities as a function of increasing total organic carbon (TOC) for TMA clay and HDTMA clay, respectively. The transition of sorption mechanisms was also quantified with simulation of DTMA clay, with a chain length between that of TMA and HDTMA. Furthermore, this study suggested that at the molecular level, the controlling factor for the ultimate sorption capacity is available surface sites in the case of TMA clay, and density of aliphatic chains within the interlayer space for HDTMA clay.

Effect of total organic carbon content and structure on the electrokinetic behavior of organoclay suspensions.

This experimental investigation measured the zeta potential of the clay mineral, montmorillonite, which was modified with six different quaternary ammonium cations. The organic cations were chosen to quantify the effect of cation functional groups, including chain length and cation size, on the resulting zeta potential; each of the six cations were exchanged onto the clay surface at three levels of total organic carbon. The zeta potential of the unmodified and the organically modified clays was measured as a function of pH, and in all cases, became less negative as the total organic carbon was increased and as the length of the attached carbon chain was increased, indicating that the organic cations were more strongly bound within the particles shear plane as total organic carbon content was increased. Measured zeta potential was also less negative for all clays tested (including unmodified montmorillonite) as pH was decreased. When compared on the basis of total organic carbon content, increasing the length of one carbon chain in the quaternary positions was a more effective method of neutralizing surface charge than was increasing the overall size of the cation (i.e., increasing the chain length in all quaternary positions).

Modeling Sorption and Diffusion of Organic Sorbate in Hexadecyltrimethylammonium-Modified Clay Nanopores – A Molecular Dynamics Simulation Study

Organoclays are highly sorptive engineered materials that can be used as amendments in barrier systems or geosynthetic liners. The performance of confining and isolating the nonpolar organic contaminants by those barrier/lining systems is essentially controlled by the process of organic contaminant mass transport in nanopores of organoclays. In this article, we use molecular dynamics (MD) simulations to study the sorption and diffusion of organic sorbates in interlayers of sodium montmorillonite and hexadecyltrimethylammonium (HDTMA(+))-modified montmorillonite clays. Simulated system consisted of the clay framework, interlayer organic cation, water, and organic sorbate. Their interactions were addressed by the combined force field of ClayFF, constant-valence force field, and SPC water model. Simulation results indicated that in HDTMA coated clay nanopores, diffusion of nonpolar species benzene was slowed because they were subjected to influence of both the pore wall and the HDTMA surfactant. This suggested the nonpolar organic compound diffusion in organophilic clays can be affected by molecular size of diffusive species, clay pore size, and organic surfactant loading. Additionally, a model that connected the diffusion rate of organic compounds in the bulk organoclay matrix with macropores and nanopores was established. The impact of intercalated organic cations on the diffusion dominated mass transport of organic compounds yielded insight into the prediction of the apparent diffusion behavior of organic compounds in organic-modified clays.

Impact of Organic Coatings on Frictional Strength of Organically Modified Clay

AbstractOrganic matter is frequently encountered in both naturally occurring and engineered particulate media. Charged functional groups in the organic matter can lead to cation exchange within the clay interlayer, which results in the formation of an organic coating on the clay surfaces and alters the interfacial frictional regime in the soil mass. This study investigated the triaxial shear frictional behavior of montmorillonite particles coated with a controlled organic phase composed of quaternary ammonium cations. Through cation exchange, organic cations were loaded onto the clay’s interlayer exchange sites, with control on the density of organic coverage and structure of the organic cation. Results demonstrated that increasing the total organic carbon content of the clay resulted in increasing frictional resistance regardless of whether the increase in carbon content was attributable to increased density of organic loading, increased cation size, or increased cation tail length. Concentrating organic...

MIXTURES OF FINE-GRAINED MINERALS KAOLINITE AND CARBONATE GRAINS

The behavior of mineral mixtures can be significantly different from the behavior of the individual components of the mixture due to differences between the mechanical and chemical properties of the individual minerals, and their ensuing effects on interparticle interactions and fabric formation. This study examines mixtures of kaolinite and calcium carbonate at different mass fractions using sedimentation, viscosity, and liquid-limit tests. These macroscale tests represent a wide range of solid-volume fractions and strain levels, with emphasis on high water-content conditions to magnify the effects of electrical forces. The results demonstrate that interparticle interactions depend on mineral surface-fluid effects, particle geometry, relative particle size, and solids content. With small solids contents, the kaolinite/calcium carbonate mixture behavior is a function of electrostatic interactions between oppositely charged mineral particles that promote flocculation; however, with large solids contents, the specific surface area of the minerals is the controlling factor. These results are relevant to many natural soil environments and to the possible development of engineered mineral mixtures for industrial applications.

Small- and High-Strain Measurements of In Situ Soil Properties Using the Seismic Cone Penetrometer

Seismic cone penetration tests provide an economical and expedient means of assessing small-strain properties (low-amplitude shear modulus, GMAX) and large-strain behavior (shear strength, τmax) of soil deposits from a single sounding. That measurements are taken at complete opposite ends of the strain spectrum permits development of the entire stress-strain-strength representation of soil layers with depth. A modified hyperbola is shown to be appropriate in a modulus degradation scheme for application to static monotonic loading of soils. Also, a global correlation of mass density (ρ) with shear wave velocity (Vs) for all types of geomaterials is presented, which facilitates the calculation of GMAX = ρVs2.