Fundamentals of Ultrasound. Factors Affecting Degradation
Several solution and compound parameters can also impact contaminant location in the cavitating system and contaminant degradation rates. These include pH, bulk temperature, initial concentration, other constituents in solution, and physicochemical properties of the target compound. Some factors, though discussed previously regarding cavitation, may affect compound degradation differently.
pH effects on sonochemical reactions are dependent on compound properties. Effects of solution pH on ionizable organics depend on the pKa of the compound (Section 1.3). Ionized compounds, whether negative or positive, remain in bulk solution during sonolysis. Sonolytic rate constants of ionized compounds are lower than rate constants for neutral compounds. Having relatively lower solubility compared to its ionized form, neutral compounds accumulate at the bubble-water interfacial region or partition to the gaseous region where •OH concentrations are higher than bulk solution.
Studies show different pH effects on neutral, nonionizable organics. Park et al. found decreasing degradation efficiency of PAH sonolysis with increasing pH. They attributed this behavior to higher one-electron oxidation potential of •OH at lower pHs. Drijvers et al. studied TCE degradation by sonolysis. Lower rate constants at acidic pH values were credited to more dissolved CO2 gas formation through TCE degradation. CO2 has a lower polytropic index than air, lowering bubble collapse temperature, pressure, and •OH production. Cavitation also generates electrical charge. The bubble-liquid interface is negatively charged, but changes to electrostatic charge with varying pH and implications on sonochemical reactions are poorly understood.
Bulk temperature affects volatile and nonvolatile organic compounds differently due to changes in compound partitioning in cavitation bubbles. As temperature increases, volatile organic compound (e.g. benzene, toluene, ethylbenzene, and xylenes (BTEX) and chlorinated solvents) degradation rate constants increase because compounds diffuse faster into the gaseous region whereby they undergo thermolysis, whereas for hydrophobic nonvolatile organics, degradation rates increase and then decrease as bulk temperature increases. This changing rate with temperature is attributed to less-favorable compound accumulation at the bubble-water interface and increased diffusivity at higher bulk temperatures. These effects have opposite effects on degradation rates resulting in the observed trend.
Initial concentration of the target compound affects sonolysis, too, depending on compound type. Sonolytic rate constants for volatile compounds decrease with increasing concentration. Increased vapor concentrations in the cavitation bubble lower the peak collapse temperature, decreasing sonolytic rate constants. Degradation of hydrophobic, nonvolatile compounds at the bubble-water interface generally follows the same trend as volatile compounds. However, at low concentrations, preferential accumulation of hydrophobic compounds to the bubble-water interface reduces surface tension, promoting bubble growth and temporarily increasing degradation rate constants.
Additional bulk solution constituents compete with target compounds for reaction with •OH. Three such constituents studied are dissolved organic matter (DOM), salts, and organic solvents. DOM reacts with •OH and competes with neutral hydrophobic organics at the bubble-water interface. Neutral, hydrophobic organics may bind to DOM, limiting their availability to reach cavitation bubble surfaces and/or react with •OH. Higher DOM concentrations increasingly inhibit degradation of neutral hydrophobic organics. However, volatile organic compounds, such as MTBE, in the presence of humic acids, do not exhibit inhibited degradation. Volatile organics primarily degrade in the gaseous bubble, and are not as reliant on •OH availability in the interfacial region. Salt presence increases the hydrophilicity of the bulk solution, thereby “salting out" organic compounds and enhancing diffusion to the hydrophobic, bubble-water interface.
Seymour et al. observed enhanced degradation rates of organic contaminants with increased salt concentrations, attributing this behavior to increased reaction with •OH at the interfacial region. Meanwhile, Psillakis et al. observed the opposite trend, stating increased salt concentrations decreased degradation rates of organic contaminants because of vapor pressure decreases and surface tension increases, thus reducing the number of cavitation bubbles formed. Lastly, dissolved organic solvents compete with target compounds for •OH.
In some cases, the presence of organic solvent inhibits target compound degradation by preferentially reacting with •OH. Organic solvents decrease the polytropic index of the gas in the cavitation bubble, taking energy from the bubble due to energy for reaction. Thus, the bubble collapse temperature decreases, lowering degradation rates. In other cases, volatile organic solvents can form organic radicals in the cavitation bubble interior and react with volatile organic compounds to enhance degradation. For example, the presence of CCl4 enhanced the sonolytic rate constant of the p-nitrophenol.
Physicochemical characteristics of the target compound, such as log KOW, Henry’s constant, and aqueous solubility, affect the location of contaminant degradation. Hydrophobicity, as expressed by log KOW, influences compound accumulation at the bubble-water interface. More hydrophobic compounds with higher log KOW values exhibit more accumulation at the interfacial region and increase the likelihood for reactions between the compound and radical species.
The Henry’s constant (KH) is the air-water partitioning coefficient. Compounds with higher KH are more likely to partition and degrade in the gaseous region, while compounds with lower KH are more likely to degrade by radical attack at the bubble-water interface or bulk solution. Lastly, compounds with high aqueous solubilities remain in solution and have slower rate constants than compounds with low solubility which partition to the bubble-water interface. Empirical results show that sonolytic rate constants depend more on aqueous solubility than log KOW because water solubility values are more precise.
Date added: 2025-01-04; views: 22;