Micropollutant removal by solar photo-Fenton at natural pH: Iron chelate selection, matrix effects, and costs

This study evaluates the efficiency and economic feasibility of solar photo-Fenton treatment at near-neutral pH using iron chelates (Fe-EDTA, Fe-DTPA, and Fe-EDDS (Fe:ligand molar ratios 1:1 and 1:10) for the removal of 21 structurally different micropollutants (MPs) from three wastewater conditions comprising two independent secondary effluents and one diluted condition derived from one of them, each spiked at environmentally relevant concentrations (∼1 μg L −1 ). Experiments were performed under simulated solar irradiation conditions. To assess reactivity-driven removal trends, micropollutants were classified into four reactivity groups based on their reported second-order rate constants for reactions with hydroxyl radicals, while TCEP was analyzed separately as a low-reactivity case relative to the other MPs investigated. Under the tested lower-DOC and lower-alkalinity condition, both Fe-EDTA and Fe-DTPA achieved high removal efficiencies (>90%) at the end of the treatment (6000 J L −1 ) for most MPs. Minor differences among the iron chelates were observed for highly reactive compounds. Under the tested higher-DOC and higher-alkalinity conditions, Fe-EDDS (1:10) outperformed the other iron chelate systems, achieving up to approximately ∼20 percentage points higher removal than Fe-EDTA (from 60 to 70% to 80–90%) for moderately reactive MPs. This improved performance was observed despite the higher estimated radical-scavenging contribution of excess EDDS and may be related to differences in complex persistence, redox lability, and secondary reactive pathways. The preliminary economic assessment indicated that reagent costs dominated the estimated treatment costs, accounting for up to 96% of the total cost for Fe-EDDS (1:10). Under the tested reagent doses and economic assumptions, Fe-EDTA yielded the lowest estimated treatment cost (0.21–0.22 € m −3 ), whereas Fe-EDDS (1:10) achieved higher removal under the wastewater condition with the highest DOC and alkalinity, albeit at a substantially higher estimated cost. These findings highlight the interplay between ligand stability, radical scavenging, and wastewater composition, and provide preliminary criteria for chelating agent selection in solar photo-Fenton applications.