The widespread use of pharmaceuticals and personal care products (PPCPs) has raised environmental concerns due to their presence in aquatic environments, unknown chronic low-dose exposure to humans, and recalcitrance to conventional water treatment technologies. In this dissertation, ultrasound, especially in pulsed wave (PW) mode has been explored to remove PPCPs. The focus was to fundamentally and mechanistically understand how ultrasound degrades PPCPs and what controls degradation kinetics.
First, ultrasound was employed to degrade the pharmaceuticals, ciprofloxacin (CIPRO) and ibuprofen (IBU), in the presence of Suwannee River fulvic acid (SRFA) and terephthalic acid (TA) to gain an understanding of the effect of environmentally relevant matrix organics on degradation kinetics. The matrix organics inhibited the sonolysis of CIPRO and IBU to different extents. Based on the results, SRFA stays in bulk solution, either quenching •OH and/or associating with the target compounds. Similar to SRFA, TA, a commonly used •OH scavenger, reacts with •OH in the bulk region but we also suspect it accumulates on or interacts with cavitation bubbles. The indication has caused us to reexamine the validity that TA can be used as a bulk •OH scavenger, because a flawed bulk •OH scavenger not only misestimates the contribution of •OH in bulk solution to the overall contaminant degradation, but also misrepresents the nature of the reaction in the aqueous cavitational systems.
By using PW ultrasound we evaluated the performance of different •OH scavengers (i.e., formic acid (FA), carbonic acid (CA), terephthalic acid (TA)/terephthalate (TPA), potassium iodide (KI), methanesulfonate (MS), benzenesulfonate (BS), and acetic acid (AA)/acetate) to determine the performance of •OH scavengers. The degradation of carbamazepine (CBZ), a probe compound serving as the reference compound occurred primarily at bubble-water interface in the study. Based on the pulsed enhancement (PE) of CBZ, acetic acid/acetate appears to scavenge •OH in bulk solution, and not interact with cavitation bubbles. For formic acid, carbonic acid, terephthalic acid/terephthalate, benzenesulfonate, and iodide, the PE was significantly decreased compared to in the absence of the scavenger. These scavengers not only quench •OH in bulk solution but also affect the cavity interface.
To apply the knowledge of AA as a bulk •OH scavenger, seven PPCPs, namely CBZ, IBU, CIPRO, acetaminophen (ATP), sulfamethoxazole (SFT), propyl gallate (PG), and diethyl phthalate (DP) were degraded by ultrasound. Degradation rates by PW ultrasound were compound dependent with degradation either faster for smaller compounds or slower for larger compounds than that under CW ultrasound. To investigate the discrepancy of degradation rate of the PPCPs between CW and PW ultrasound, AA was added and irradiated with each compound to differentiate the contribution of bulk •OH oxidation in its overall degradation. The results showed that the fraction of degradation occurring in bulk solution is positively correlated with the molar volume of the compound. Smaller PPCP compounds are able to more readily diffuse to bubble interfaces and are impacted most by pulsing ultrasound. Our results suggest PW ultrasound improves the energy efficiency of ultrasound as a treatment technology for small size PPCPs.