Reduction of nitroaromatic compounds (NACs), an important class of groundwater pollutants, by Fe(II) associated with iron oxides has been studied widely. However, the significant differences between the well-controlled, batch reactor conditions of the laboratory and the complicated conditions encountered in the field are not well understood. Continuous flow column reactors offer an experimental scenario that mimics an environmental system more authentically than do batch reactors.  Here, we present results tracking the reductive degradation of the model pollutant 4-chloronitrobenzene (4-ClNB) by Fe(II) in flow-through column reactors packed with quartz sand coated with iron oxide minerals.  Important experimental variables include the degree of saturation; exposure to oxygen; pH; concentrations of organic matter, Fe(II), and 4-ClNB; and phase composition, elemental composition, and abundance of the iron oxide minerals on the sand. During reaction, the mobile phase is collected and analyzed for concentrations of 4-ClNB, 4-chloroaniline, and Fe(II). After reaction, particles from different sections of each column are collected, and the iron oxide minerals are detached from the sand grains for characterization using scanning and transmission electron microscopy, X-ray diffraction, elemental analysis, and more. In experiments with organic matter, its concentration and composition are also monitored. Results are compared to those from batch reactors in order to better understand how results from batch reactors can be used to better understand field environments.  Results demonstrate that the oxidative mineral growth is qualitatively similar in batch and flow-through reactors but that the extent of growth depends on the proximity to the inlet for the column.  The findings provide insight into evolving particle reactivity in contaminated groundwater systems.

Reductive degradation of a model polluant with concurrent oxidative mineral growth of iron oxide-coated sand in column reactors

ACS Spring 2022

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