Interpreting nanoscale size-effects in aggregated Fe-oxide suspensions: Reaction of Fe(II) with Goethite

The Fe(II)/Fe(III) redox couple plays an important role in both the subsurface fate and transport of groundwater pollutants and the global cycling of carbon and nitrogen in iron-limited marine environments. Iron oxide particles involved in these redox processes exhibit broad size distributions, and the recent demonstrations of dramatic nanoscale size-effects with various metal oxides has compelled us, as well as many others, to consider whether the rate and extent of Fe(II)/Fe(III) cycling depends upon oxide particle size in natural systems. Here, we investigated the reaction of Fe(II) with three different goethite particle sizes in pH 7.5 suspensions. Acicular goethite rods with primary particle dimensions ranging from 7 by 80 nm to 25 by 670 nm were studied. Similar behavior with respect to Fe(II) sorption, electron transfer and nitrobenzene reduction was observed on a mass-normalized basis despite almost a threefold difference in goethite specific surface areas. Scanning electron microscopy (SEM) images, dynamic light scattering (DLS) and sedimentation measurements all indicated that, at pH 7.5, significant aggregation occurred with all three sizes of goethite particles. SEM images further revealed that nanoscale particles formed dense aggregates on the order of several microns in diameter. The clear formation of particle aggregates in solution raises questions regarding the use of primary particle surface area as a basis for assessing nanoscale size-effects in iron oxide suspensions at circum-neutral pH values. In our case, normalizing the Fe(II) sorption densities and rate constants for nitrobenzene reduction by BET surface area implies that goethite nanoparticles are less reactive than larger particles. We suspect, however, that aggregation is responsible for this observed size-dependence, and argue that BET values should not be used to assess differences in surface site density or intrinsic surface reactivity in aggregated particle suspensions. In order to realistically assess nanoscale size-effects in environmentally relevant systems that are likely to aggregate, new methods are needed to quantify the amount of surface area accessible for sorption and reaction in wet nanoparticle suspensions, rather than assuming that this value is equivalent to the surface area determined from the characterization of dry nanoparticles.