The aim of this book was to evaluate the efficiency of the complex formed between citric acid and ferric ions (ferricitrate) as an inducer of the photo-Fenton reaction for the treatment of landfill leachate. The leachate samples were collected at the Regional Urban Solid Waste Treatment Centre, located in the municipality of Giruá (RS). This leachate is characterised by high colour and turbidity, as well as high organic content expressed in high values of chemical and biochemical oxygen demand, total organic carbon, nitrogenous forms and phosphorus, presenting high polluting potential. The use of the organic ligand aims to improve the efficiency of the photo-Fenton process, since it forms stable and soluble complexes with Fe(III), as well as enabling reactions at a higher pH than traditionally possible.