Established water and wastewater treatment technologies inadequately address the degradation and removal of Emerging Contaminants, specifically pharmaceuticals, posing significant concerns for human health and the environment. Sonication has emerged as a potential additional treatment method, with studies suggesting a synergistic effect under multi-frequency irradiation, that enhances process efficiency in terms of energy consumption. However, the underlying mechanisms of this synergy remain unknown, necessitating further investigation. This thesis focused on the sonodegradation of paracetamol, a widely found pharmaceutical in the environment with potential harmful impact on kidney as well as proliferation of breast cancer cells. To investigate the synergy and identify the underlying mechanisms, single- and dual-frequency sonication systems were examined across various paracetamol concentrations (5 – 1000 mg.L^-1), frequencies (20 – 2000 kHz), and power settings. Results revealed that dual-frequency sonication (combination of 20 kHz with a frequency higher than 100 kHz), significantly enhanced pollutant degradation while minimally affecting the yield of oxidants including HO•. The novel method proposed in this study for determining bubble size distribution suggest that dual-frequency sonication extends the lifetime of cavitation bubbles, by making them smaller and more uniform, and thus more stable. Long-lasting oscillating bubbles are expected to enhance the mixing at macroscopic scale and provide more time for the pollutants to diffuse towards the bubbles. Considering the heterogeneous nature of sonodegradation, and the 2^nd-order kinetic of the sonodegradation of paracetamol, the findings suggest that the observed synergy under dual-frequency sonication is the result of the improvement in mass transfer in the reactor on both macroscopic and microscopic scales. The order of the sonodegradation kinetics were found to be notably sensitive to solution concentration, which further highlights the significant role of mass transfer in this process. The trend of degradation rates with the yield of HO• under the various studied sonication conditions as well as the investigation of the sonodegradation intermediates revealed that chemical oxidation by HO• is the dominant degradation mechanism of the pollutant, irrespective of the sonication systems and conditions. Additionally, the degradation of smaller and more hydrophilic intermediates than paracetamol was found to be improved under dual-frequency sonication, which shows the attractiveness of this system where the more hydrophilic intermediates are more toxic than the parent pollutant. In this study, no intermediate more toxic than paracetamol was found in the solution, indicating the safety of sonodegradation as a remediation method for this pollutant.