The heterogeneous Fenton process has been widely applied though some aspects of this process are still poorly understood. In this study, we simultaneously quantify the adsorption and decomposition of formate and H 2 O 2 at pH 4.0 in the presence of freshly formed ferrihydrite and provide new insights into the ferrihydrite-induced heterogeneous Fenton mechanism with the aid of kinetic and reactive-transport modeling. Our results show that the decomposition of H 2 O 2 and formate is controlled by surface-initiated reactions. Adsorbed formate occupies the surface sites otherwise available for reaction with H 2 O 2 and therefore hampers the surface Fenton reactions despite the negligible accumulation of H 2 O 2 on the surface. The minimal impact of methanol (an effective HO • scavenger) on formate oxidation as well as the poor oxidation of fully adsorbed oxalate compared with the ready oxidation of partially adsorbed formate demonstrates that oxidation mainly occurs in the solid−liquid boundary layer, rather than in bulk or on the surface. This is suggested to be due to the diffusion of surface-generated HO • , rather than surface Fe(II), to the boundary layer with the results of kinetic and reactive-transport modeling supporting this conclusion. The new findings are critical to our understanding of the removal behavior of more complex organic target species and to the design of more effective heterogeneous Fenton technologies.
The
heterogeneous Fenton process in the presence of Fe-containing
minerals is ubiquitous in nature and widely deployed in wastewater
treatment. While there have been extensive relevant studies, the dependence
on pH of the nature and extent of oxidant generation and key reaction
pathways remain unclear. Herein, the adsorption and decomposition
of formate and H2O2 were quantified in the presence
of ferrihydrite within the pH range of 3.0–6.0, and experiments
with methyl phenyl sulfoxide were conducted to distinguish between
HO• and weaker oxidant(s) which react via oxygen
atom transfer including ferryl ion ([FeIVO]2+) and/or ferric hydroperoxo intermediates (FeIII(O2H)). Both HO• and [FeIVO]2+/FeIII(O2H) are concurrently
produced on the surface over the acidic to near-neutral pH range.
Despite the simultaneous formation of both oxidants, HO• is the major oxidant responsible for substrate oxidation in the
interfacial boundary layer with [FeIVO]2+/FeIII(O2H) exhibiting limited exposure to substrates.
With an increase of pH, the yield of both oxidants is inhibited by
the decreasing availability of surface sites due to ferrihydrite particle
aggregation. Increasing pH also favors the nonradical decay of H2O2 as evident from the consistent oxidant production
rate relative to the surface area (SSA) despite an accelerated H2O2 decay rate relative to SSA with pH increase.
Iron oxychloride (FeOCl) has been reported to be a highly efficient heterogeneous Fenton catalyst over a wide pH range. In order to determine the true catalytic performance of FeOCl, we simultaneously quantified the adsorptive and oxidative removal of formate, oxalate, and rhodamine-B (RhB) and the formation of RhB oxidation products at both pH 4.0 and 7.0. FeOCl was found to be a poor Fenton catalyst at either pH, as gauged by the oxidation of formate, oxalate, and rhodamine B and the decomposition of H 2 O 2 , in comparison with ferrihydrite (Fhy), one of the most common Fe-containing Fenton catalysts. The adsorption of target contaminants to FeOCl and homogeneous Fenton processes, induced by dissolved iron, resulted in overevaluation of the catalytic performance of FeOCl, especially for (i) the use of strongly adsorbing target compounds, without consideration of the role of adsorption in their removal and (ii) exceedingly high concentrations of H 2 O 2 to remove trace quantities of target contaminants. Overall, this study highlights that the systematic quantification of H 2 O 2 decomposition, target compound adsorption, and oxidation as well as the concentrations of oxidized products formed are prerequisites for unequivocal elucidation of the catalytic nature and reaction mechanism of solid Fenton catalysts.
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