Factors Affecting Mass Transfer by Ultrasound
Ultrasonic, solution, and media conditions influence the efficacy of the ultrasonic mass transfer mechanism, and ultimately aqueous phase degradation efficiency. Frequency effects on contaminant desorption are predicated on cavitational intensity. Longer bubble growth periods and increased energy absorption associated with low-frequency cavitation cause more intense physical effects.
Lower ultrasonic frequencies also have higher desorption rates of phenol from activated carbon than higher frequencies. At the same frequency, increased acoustic power intensities enhance degradation and desorption rates. Higher power settings increase the acoustic amplitude, producing more intense shockwaves and microjets from stronger cavitational collapses.
Solution conditions, such as bulk temperature and pH, also influence contaminant sorption processes. Since sorption is an exothermic process (∆H < 0), increases in bulk temperature typically decrease equilibrium Kd values. Kd values are inversely related to aqueous solubility. Since aqueous solubility typically increases at higher temperatures, decreased compound association with the soil/sediment is expected.
The influence of temperature on sorption processes is determined by the strength of interactions between contaminant and soil/sediment. Weaker interactions have lower magnitude AH values, and therefore are not as influenced by temperature changes. Bulk temperature increases also increase surface diffusivity for desorption. pH effects, however, are complex and specific to the pollutants and media. Pollutant-soil/sediment interactions depend on physicochemical properties of the pollutant (e.g. log KOW and pKa values for organic pollutants), as well as media properties such as mineral phase, type of SOM, and surface charge (see Further Reading section).
Little work investigates pH effects on ultrasound enhanced desorption of organic compounds. However, soil pH remains constant under the presence of ultrasound. For ultrasonic treatment of metal-contaminated media, low pH values enhance desorption.
Particle size and type are critical to mass transfer processes by ultrasound. Removal efficiencies are higher for coarse sediments than fine sediments, attributed to lower specific surface area of coarse solids and smaller path lengths. Physical effects, such as shockwaves and microjets, reduce particle size. Larger particles have larger size reduction rates than smaller particles in the presence of ultrasound.
Consequently, larger particles such as sands experience more desorption than smaller particles, such as clays. Smaller particles, however, have more cavitation nuclei and greater •OH production due to fewer asymmetric bubble collapses. Likewise, water-soil ratios increase with smaller particle sizes. A higher ratio creates more cavitation nuclei, increasing degradation. Increasing the water-soil ratio for the same sized particle can increase desorption, as well.
Date added: 2025-01-04; views: 28;