Role of extract purification in iron complexation and photo-Fenton reactivity under mild pH conditions

Photo-Fenton processes operated at circumneutral pH require iron-complexing agents able to maintain Fe(III) solubility while sustaining efficient Fe(III)/Fe(II) cycling. In this work, polyphenol-rich extracts obtained from grape-processing residues were evaluated as natural ligands for photo-Fenton degradation of a mixture of six emerging contaminants (30 mg L⁻¹ total) at pH 5–7 using 5 mg L⁻¹ Fe(III). Raw alkaline extracts and membrane-refined fractions (300 and 150 kDa) were comprehensively characterized (TOC, COD, AOS, DLS, TPC, flavonoids and EEM fluorescence) and correlated with iron stability and degradation performance through a Doehlert multivariate design. Membrane fractionation significantly modified extract functionality. The 300 kDa retentate exhibited the highest phenolic and flavonoid densities (0.181 and 0.175 mg mg C⁻¹, respectively) and maintained soluble iron at pH 7 with minimal losses over 60 min. Under optimized conditions (pH 6, 16 mg C L⁻¹, stoichiometric H₂O₂), this fraction achieved 80% pollutant removal in 60 min, approaching the performance of EDTA and NTA when compared at equivalent carbon loads. In contrast, the raw extract was penalized by non-functional organic matter, while the 150 kDa fraction showed reduced chelating efficiency due to lower coordination-site density. The novelty of this work lies in establishing a direct correlation between membrane-driven compositional tuning (AOS and fluorescence signatures), iron stability, and photo-Fenton efficiency, demonstrating that functional group density rather than total organic carbon governs performance. These results highlight the potential of agro-residue-derived polyphenolic extracts as sustainable iron-complexing agents for photo-Fenton processes at mild pH. • Membrane purification enriches functional groups relevant for Fe(III) complexation. • The 300 kDa fraction provides optimal iron stability and photoreactivity at pH 5–7. • Enhanced iron stability directly correlates with improved photo-Fenton reactivity. • Natural extracts approach synthetic chelators at equivalent carbon doses.