Evaluating the stability and efficiency of Fe(III)-DTPA complex for prednisolone degradation by solar photoelectro-Fenton process at neutral pH

Abstract

The stability of the Fe(III)-DTPA complex for its use as a promising homogeneous catalyst to solve the pH bottleneck in solar photoelectro-Fenton (SPEF) process was evaluated for the first time. Viability depends on its resistance to photodegradation and attack by reactive oxygen species; ideally, it should allow sufficient release of free iron catalyst while preserving ligand stability to prevent iron precipitation. Here, Fe(III)-to-ligand molar ratio was first optimized to ensure complete chelation within a wide pH range, as well as high photostability against UVA and sunlight and resistance to •OH attack. Results showed that Fe(III)-DTPA complex at ratios of 1:1 and 1:2 remained stable at pH 3–9 for at least 180 min. Notably, the 1:2 complex exhibited superior photostability and oxidative resistance during SPEF treatment, minimizing the iron precipitation. As a proof of concept, removal of the glucocorticoid prednisolone (PREDN) by SPEF at pH 7 was investigated. Complete degradation of 0.055 mM PREDN in 50 mM Na2SO4 medium containing 0.05 mM Fe(III)-DTPA (1:2) was achieved after 90 min at 30 mA cm−2. Although 95% of the complex was also degraded, the organic species derived from iron complex maintained a significant percentage of soluble iron (> 40%). Additionally, phytotoxicity and QSAR tests revealed that an initially pronounced toxicity due to by-products generated from DTPA and PREDN resolved into a non-toxic mixture with enhanced biodegradability. Therefore, Fe(III)-DTPA complex (1:2) can be considered a sustainable catalyst for SPEF treatment of drugs at circumneutral pH, combining acceptable operational stability and environmental safety.