Oxidation of a Dimethoxyhydroquinone by Ferrihydrite and Goethite Nanoparticles: Iron Reduction versus Surface Catalysis

Abstract

Hydroquinones are important mediators of electron transfer reactions in soils with a capability to reduce Fe(III) minerals and molecular oxygen, and thereby generating Fenton chemistry reagents. This study focused on 2,6-dimethoxy hydroquinone (2,6-DMHQ), an analogue to a common fungal metabolite, and its reaction with ferrihydrite and goethite under variable pH and oxygen concentrations. Combined wet-chemical and spectroscopic analyses showed that both minerals effectively oxidized 2,6-DMHQ in the presence of oxygen. Under anaerobic conditions the first-order oxidation rate constants decreased by one to several orders of magnitude depending on pH and mineral. Comparison between aerobic and anaerobic results showed that ferrihydrite promoted 2,6-DMHQ oxidation both via reductive dissolution and heterogeneous catalysis while goethite mainly caused catalytic oxidation. These results were in agreement with changes in the reduction potential (E) of the Fe(III) oxide/Fe(II) redox couple as a function of dissolved Fe(II) where E of goethite was lower than ferrihydrite at any given Fe(II) concentration, which makes ferrihydrite more prone to reductive dissolution by the 2,6-DMBQ/2,6-DMHQ redox couple. This study showed that reactions between hydroquinones and iron oxides could produce favorable conditions for formation of reactive oxygen species, which are required for nonenzymatic Fenton-based decomposition of soil organic matter.

Generation of hydroxyl radicals from reactions between a dimethoxyhydroquinone and iron oxide nanoparticles

“…The corresponding values in the ferrihydrite system under aerobic and anaerobic conditions were 100% and 90%, respectively. These results corroborate previous findings showing that goethite mainly mediated catalytic 2,6-DMHQ oxidation, whereas the reaction with ferrihydrite exhibited a large contribution from reductive dissolution, which was evidenced by the extensive 2,6-DMHQ oxidation observed under anaerobic conditions 26 .

Figure 3 Oxidation of 90 µM 2,6-DMHQ (0.44 µmol/m 2 ) at pH 4.5 (black: aerobic and green: anaerobic) and pH 7.0 (red: aerobic and blue: anaerobic) in the presence of ferrihydrite ( a , c ) and goethite ( b , d ).

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“…30 μM was dissolved from goethite. Re-adsorption of Fe 2+ under these conditions is very low, as shown by previous studies 15 , 26 , and therefore, these results were a direct reflection of the magnitude of reductive dissolution. In the presence of O 2 , the Fe 2+ concentrations decreased for both iron oxides (Fig.…”

“…4 , black lines), and relative to the anaerobic conditions, the effect was largest in the case of goethite. In a previous study, we showed that the re-oxidation of Fe 2+ in the presence of ferrihydrite was low at pH 4.5 26 , and therefore, the reduced Fe 2+ concentration indicated an increase in the contribution from the catalytic oxidation of 2,6-DMHQ at the expense of reductive dissolution. Some re-oxidation of Fe 2+ likely occurred in the presence of goethite, as indicated by the decreases in the concentration after ca.…”

“…Interestingly, substantial oxidation was also observed at pH 7.0 under anaerobic conditions. This oxidation is likely caused by reductive dissolution, which will be favoured if the Fe 2+ concentration is maintained at low concentrations through re-oxidation by the low O 2 concentrations that exists also under conditions where we tried to exclude oxygen 26 . In addition, these oxygen trace concentrations can also contribute to the catalytic oxidation of 2,6-DMHQ.…”

“…This feature makes the reactions between such nanoparticles and redox-active metabolites particularly interesting because of the possibility of forming Fenton reagents at the particle surfaces, which increases the likelihood that molecules accumulated at these interfaces will be oxidized. Recently, reactions between 2,6-dimethoxy-1,4-hydroquinone (2,6-DMHQ (1,4-(CH 3 O) 2 -2,6-C 6 H 2 (OH) 2 )) and iron oxides were shown to cause reductive dissolution as well as catalytic hydroquinone oxidation in the presence of O 2 26 , but whether these reactions also generate significant amounts of ·OH remains unknown.…”