The present work describes in detail the chemical structure of the complex Fe(III)-EDDS and the predominance of different species with respect to pH. These results were obtained with ab initio calculations. From the photoredox process, the formation of hydroxyl radical was confirmed, and HO* is the main species responsible for the degradation of the organic compound present in aqueous solution. The degradation of 4-tert-butylphenol (4-t-BP), used as a model pollutant, was investigated in different conditions. For the first time, the second-order rate constant of the reaction between HO* and 4-t-BP and the formation rate of HO* (RHO*f) from the photochemical process were evaluated. Through the degradation of 4-t-BP, the effect of Fe(III)-EDDS concentration, oxygen, and pH was also investigated. The pH, which plays a role in the iron cycle and in the Fe(III)-EDDS speciation, was noticed as an important parameter for the efficiency of 4-t-BP degradation. Such a result could be explained by taking into account the complex speciation and presence of a predominant form (FeL-) up to pH 8. These results are very useful for the use and optimization of such iron complexes in water treatment processes.