Iron oxides and (hydroxy)oxides are abundant in aquatic systems and, when associated with ferrous iron (Fe(II)), are capable of reducing organic contaminants such as nitroaromatics (NACs) through surface-mediated redox reactions. Hence, the surface chemistry of these particles plays an important role in determining the extent of these redox reactions. Natural organic matter (NOM) is ubiquitous in groundwater and interacts significantly with mineral surfaces. Due to its complicated and heterogeneous chemical structure and functionalities, however, these interactions may alter the mineral reactivity. Understanding iron oxide interactions with NOM in groundwater, therefore, is crucial for predicting the reactivity of particles and designing efficient remediation processes. In this study the interaction of three well-defined NOM samples: Suwannee River NOM (SRNOM), Suwannee River fulvic acid (SRFA), and Elliot soil humic acid (ESHA) with goethite nanoparticles is evaluated. NOM samples were contacted with goethite particles and adsorbed fractions on the goethite surface and dissolved fraction in the aqueous phase were characterized. The physical, chemical, and optical properties of the parent material and aqueous fraction after sorption of each NOM were analyzed by size exclusion chromatography, total organic carbon analysis, and UV-visible and fluorescence spectroscopy. The impact of each fraction on the reactivity of goethite/Fe(II) toward 4-chloronitrobenzene (4-ClNB) was studied. Results showed that the adsorbed fractions of all NOM isolates hinder the reactivity of goethite and reduced the rate constant from 1.9 ± 0.15 h-1 for fresh particles to 1.5 ± 0.14 h-1 for SRNOM, 1.1 ± 0.07 h-1 for ESHA, and 1.0 h-1 for SRFA exposed materials. [C1] [AS2] [b3] Results indicate that adsorbed fractions contain more fluorophores and have higher molecular weight. By observing desorption behavior of adsorbed NOM from the goethite particles, it was found that for SRNOM and SRFA the fluorophores partially desorbed back to aqueous phase, for ESHA however, the fluorophores remained adsorbed to the mineral surfaces.

Goethite-induced NOM fractionation and its impact on goethite/Fe(II) reactivity toward 4-chloronitrobenzene (4-ClNB)

ACS Spring 2022

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