The aim of this study was to develop a comprehensive strategy for the degradation of a pharmaceutical compound, i.e., diclofenac (DCF), in which the solubility is strongly related to the solution pH. Therefore, in this study, alternative treatments according to DCF concentration ranges were developed. Working at the concentration of 7.5 mg/L and pH below 4, the considered treatments by sonolysis, anodic oxidation and electro-Fenton resulted in a good performance with high abatement levels. At high concentrations (140 mg/L), it was necessary to operate at near neutral pH, and the three proposed treatments (sonolysis, anodic oxidation and electro-Fenton using iron-alginate spheres as the heterogeneous catalyst) were performed. Heterogeneous electro-Fenton (HEF) resulted in the best option for the pollutant removal, and three new chitosan-based heterogeneous catalysts were synthesized and evaluated. The operational conditions (intensity and dosage of catalyst) were optimized resulting in an intensity of 300 mA and catalyst dose of 20 g/L. Operating at these conditions, high levels of DCF degradation and mineralization were achieved. Several treatment cycles, using the same catalyst, were successfully carried out, and the catalysts demonstrated good stability and the potential to operate in a continuous way. Finally, the evolution of carboxylic acids and intermediate generation were analysed leading to the proposal of a plausible DCF degradation pathway for HEF.