John Bergendahl

Prediction of colloid detachment in a model porous media: hydrodynamics

Understanding the mobility of colloids through porous media is important in both engineered and natural applications. This study employed a model system to explore physically based colloid detachment. Polystyrene colloids were attached in the primary minimum to glass beads in a packed column, and a residually attached fraction was subsequently detached via hydrodynamic shear. Colloid fractions released from the surfaces in the porous media were experimentally quantified. A flowrate of 75 ml/min detached 50% of the residual colloid fraction from the surface. The residual colloid fraction released was predicted with a model incorporating an interaction energy distribution and system physics. In this model, detachment is realized when the applied rolling moment from hydrodynamic shear overcomes the resistance associated with rolling. With the exception of one parameter, the hysteresis loss factor, system characteristics were described a priori. Using the hysteresis loss factor and interaction energy distribution developed from extended-DLVO theory, the detachment of the residual colloid fraction from the packed bed was well predicted. This work decomposes colloid detachment into the constitutive mechanisms dependent upon thermodynamics and hydrodynamics.

Colloid generation during batch leaching tests: mechanics of disaggregation

Abstract Batch leaching tests are commonly used to assess the leaching potential of various organic and inorganic contaminants from soil. The toxicity characteristic leaching procedure (TCLP), a batch leaching test developed by the U.S. Environmental Protection Agency, employs an aggressive mixing technique that may allow colloidal fractions to appear in the filtrate. This study quantified the generation of colloid fractions during TCLP testing of a coal-tar contaminated soil, and explored the mechanics of disaggregation. Particle count data indicated that the concentration of 0.72 and 0.83 μm diameter colloids in the filtrate increased with agitation. The shear rate in the agitation vessel was determined, as well as the hydrodynamic forces acting on the 0.72 and 0.83 μm colloids attached to the soil grains. Through use of force and moment balances, and the Johnson-Kendall-Roberts and Derjaguin-Muller-Toporov adhesion models, it was determined that the operative detachment mechanism is most likely rolling or sliding, depending on the contact radius and the coefficient of static friction. Colloid generation during the TCLP test results in an increase in total colloidal surface area in the filtrate, and may concomitantly result in an overprediction of the aqueous phase concentration of hydrophobic contaminants.

Adsorption of methyl tertiary butyl ether on granular zeolites: Batch and column studies.

Methyl tertiary butyl ether (MTBE) has been shown to be readily removed from water with powdered zeolites, but the passage of water through fixed-beds of very small powdered zeolites produces high friction losses not encountered in flow through larger sized granular materials. In this study, equilibrium and kinetic adsorption of MTBE onto granular zeolites, a coconut shell granular activated carbon (CS-1240), and a commercial carbon adsorbent (CCA) sample was evaluated. In addition, the effect of natural organic matter (NOM) on MTBE adsorption was evaluated. Batch adsorption experiments determined that ZSM-5 was the most effective granular zeolite for MTBE adsorption. Further equilibrium and kinetic experiments verified that granular ZSM-5 is superior to CS-1240 and CCA in removing MTBE from water. No competitive adsorption effects between NOM and MTBE were observed for adsorption to granular ZSM-5 or CS-1240, however there was competition between NOM and MTBE for adsorption onto the CCA granules. Fixed-bed adsorption experiments for longer run times were performed using granular ZSM-5. The bed depth service time model (BDST) was used to analyze the breakthrough data.

Pilot-scale Fenton's oxidation of organic contaminants in groundwater using autochthonous iron.

A pilot-scale study was conducted to evaluate Fentons oxidation with autochthonous iron for treating extracted groundwater contaminated with organic solvents. Based on a previous bench-scale treatability study, a batch reactor pilot-plant system was designed and operated to evaluate the effects of various parameters including pH, iron concentration, hydrogen peroxide dose, and reaction time. Effective system conditions were found to be pH of 3.5, hydrogen peroxide to iron molar ratio of 75/1, and autochthonous iron at an average concentration of 10mg/l. The data collected demonstrate the effectiveness of Fentons oxidation using autochthonous iron for treating this contaminated water, with reductions to below method detection limits for many contaminants. This pilot-scale study provided kinetic rate constants for predicting contaminant disappearance, information necessary for designing a full-scale Fentons oxidation system.

Batch leaching tests: Colloid release and PAH leachability

The Toxicity Characteristic Leaching Procedure (TCLP) was developed by the U.S. Environmental Protection Agency to assess leaching potential of contaminants from waste, and to provide a test to classify hazardous waste. It is a batch leaching test where a waste (such as contaminated soil) and an extraction fluid are agitated for a predetermined time. Since TCLP employs an aggressive mixing technique, it is possible that hydrophobic contaminant-laden colloidal fractions may appear as “dissolved” constituents. In this study, TCLP was employed to determine the leachability of PAH contamination from a coal tar contaminated site. Generated colloids and the apparent aqueous concentrations of naphthalene and phenanthrene were measured at various mixing times in the extraction fluid. A mathematical model was developed that predicted the apparent aqueous contaminant concentration in the filtrate. This model accounted for the presence of colloids in the filtrate, and quantified contaminant desorption from colloids. The fraction of colloid-bound contaminant was predicted to be negligible for naphthalene. However, phenanthrene was predicted to have a significant fraction of the total contaminant in the colloidal phase, while naphthalene was primarily dissolved. The desorption model and PAH desorption data are presented here to determine the extent of colloid-facilitated desorption during leaching tests.

Environmental Issues of Gasoline Additives – Aqueous Solubility and Spills

Publisher Summary This chapter focuses on various environmental issues related to gasoline additives such as ethers and alcohols. Gasoline additives are soluble in water and may be quite mobile in the environment during gasoline spills. In the event of substantial spills of gasoline to the environment, the gasoline phase may move because of gravity until a barrier is encountered. The gasoline light non-aqueous phase liquid (LNAPL) constituents may have a tendency of getting transferred to the gas phase in the unsaturated zone above the water table and the LNAPL, but dissolution of the constituents from the LNAPL into flowing groundwater is typically an environmental concern because the groundwater plume created by dissolution can travel significant distances. Gasoline may also be trapped as a residue in soil pores, thereby providing a continuous source for dissolution. Organic compounds with high aqueous solubilities, such as ethanol and methanol, may be present in water at high concentrations and may function as co-solvents with the ability to solubilize other organic gasoline constituents that are more harmful to human health and the environment. At high co-solvent concentrations, the surface of the solubilized organic molecules in the water is surrounded by the organic co-solvent as well as water, thereby affecting the net molecular interactions.

Kinetic Model for the Degradation of MTBE by Fenton's Oxidation

Environmental Context.Since the early 1990s, methyl tert-butyl ether (MTBE), a possible human carcinogen, has been used as a gasoline oxygenate at concentrations of up to 15% by volume; however, a fraction of the MTBE produced has inevitably been released to the environment. And, spills at gasoline service stations have resulted in local groundwater contamination levels of MTBE over 100 mg L−1, because of its very high water solubility. Advanced oxidation is a common technique for mineralizing organic contaminants, but the reaction chemistry needs to be better understood to facilitate design of remediation systems. Abstract.A kinetic model for the degradation of methyl tert-butyl ether (MTBE) in batch reactors with Fenton’s oxidation (Fe2+/ H2O2) in aqueous solutions was developed. This kinetic model consists of equations accounting for (1) hydrogen peroxide chemistry in aqueous solution, (2) iron speciation, and (3) MTBE oxidation. The mechanisms of MTBE degradation, and the resultant pathways for the formation and degradation of the byproducts, were proposed on the basis of previous studies. A set of stiff nonlinear ordinary differential equations that describe the rate of formation of each species in this batch system was solved using Matlab (R13) software. The kinetic model was validated with published experimental data. The degradation of MTBE by Fenton’s oxidation is predicted well by the model, as are the formation and degradation of byproducts, especially methyl acetate (MA) and tert-butyl alcohol (TBA). Finally, a sensitivity analysis based on calculating the sum of the squares of the residuals (SSR) after making a perturbation of one rate constant at a time was applied to discern the effect of each reaction on MTBE disappearance.