New Insights into the Role of Natural Organic Matter in Fe-Cr Coprecipitation: Importance of Molecular Selectivity

Coprecipitation of Fe/Cr hydroxides with natural organicmatter(NOM) is an important pathway for Cr immobilization. However, therole of NOM in coprecipitation is still controversial due to its molecularheterogeneity and diversity. This study focused on the molecular selectivityof NOM toward Fe/Cr coprecipitates to uncover the fate of Cr via Fouriertransform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). Theresults showed that the significant effects of Suwannee River NOM(SRNOM) on Cr immobilization and stability of the Fe/Cr coprecipitatesdid not merely depend on the adsorption of SRNOM on Fe/Cr hydroxides.FT-ICR-MS spectra suggested that two pathways of molecular selectivityof SRNOM in the coprecipitation affected Cr immobilization. Polycyclicaromatics and polyphenolic compounds in SRNOM preferentially adsorbedon the Fe/Cr hydroxide nanoparticles, which provided extra bindingsites and promoted the aggregation. Notably, some specific compounds(i.e., polyphenolic compounds and highly unsaturated phenolic compounds),less unsaturated and more oxygenated than those adsorbed on Fe/Crhydroxide nanoparticles, were preferentially incorporated into theinsoluble Cr-organic complexes in the coprecipitates. Kendrickmass defect analysis revealed that the insoluble Cr-organiccomplexes contained fewer carbonylated homologous compounds. Moreimportantly, the spatial distribution of insoluble Cr-organiccomplexes was strongly related to Cr immobilization and stabilityof the Fe/Cr-NOM coprecipitates. The molecular information of theFe/Cr-NOM coprecipitates would be beneficial for a better understandingof the transport and fate of Cr and exploration of the related remediationstrategy. The study analyzed the molecularselectivity of naturalorganic matter, promoting Fe/Cr coprecipitation using FT-ICR-MS anddemonstrated the importance of insoluble Cr-organic complexesin Cr immobilization and stability of coprecipitates.