Ligand-Enhanced Electron Utilization for Trichloroethylene Degradation by ^·OH during Sediment Oxygenation

Abstract

The potential of oxygenating Fe(II)-bearing sediments for hydroxyl radical ( • OH) production and contaminant degradation has been proposed recently. Here, we further show that specific ligands can largely enhance contaminant degradation during sediment oxygenation due to increased utilization efficiency of sediment electrons. With the addition of 0− 2 mM sodium ethylene diamine tetraacetate (EDTA) or sodium tripolyphosphate (TPP) in sediment suspension (50 g/L, pH 7.0), trichloroethylene (TCE, 15 μM) degradation increased from 13% without ligand to a maximum of 80% with 2 mM TPP and was much higher with TPP than EDTA because EDTA competes for • OH. Electron utilization efficiency for • OH production increased with increased ligand concentration and was enhanced by up to 6−7 times with 2 mM EDTA or TPP. Electron transfer from sediment to dissolved Fe(III)-ligand is mainly accountable for the enhanced electron utilization efficiency by the ligands with low adsorption affinity (i.e., EDTA), and additional variation of sediment surface Fe(II) coordination is mainly responsible for the enhancement by the ligands with high adsorption affinity (i.e., TPP). Output of this study provides guidance and optional strategies for enhancing contaminant degradation during sediment oxygenation.

Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

“…A possible reason is that at early time, when Fe­(II) concentrations are the greatest, more •OH is needed for TCE oxidation because of competition with Fe­(II) scavenging, and this is considered in the model. Another reason not considered in the model is that other reactive species besides •OH are oxidizing TCE, but there is little support for this in the literature. ,, The best fit rate constant values for k sol , k fre , and k oe are 0.14 M –1 s –1 , 2.52 M –1 s –1 , and 4.10 × 10 –4 s –1 . respectively.…”

“…As illustrated, both free ( k fre ) and solid ( k sol ) Fe­(II) react with O 2 to produce iron (oxyhydr)­oxides and •OH, with free Fe­(II) reacting at a higher rate than solid Fe­(II), and solid Fe­(II) in biogenic RIM reacting at a higher rate than in abiotic RIM. Higher rates of reaction with O 2 for free versus solid Fe­(II) may be due to faster mass transfer of the former which is a homogeneous reaction, , and thermodynamic changes via the complexation of free Fe­(II) with inorganic ions (i.e., carbonate), , organic ligands (from biomass), , and mineral surface association. , It should be noted that the mechanism for •OH production with free or solid Fe­(II) in two-phase (i.e., liquid and solid) systems is well documented and yields O 2 •– , H 2 O 2 , and finally •OH. ,, …”

“…The environmental significance of ROS generated from oxygenated RIM has been demonstrated in many laboratory studies showing pollutant destruction. For example, TCE, ,, phenol, , p-chloroaniline, and As­(III) can all be oxidized by •OH generated during oxygenation of RIM. The impact of these ROS on natural attenuation of TCE may be especially important when biological reduction is not active , because half-lives of •OH-promoted TCE oxidation are much smaller than those for abiotic reductive dechlorination with similar RIM (∼0.25 vs 60 years) .…”

“…Hydroxyl radicals produced during mineral oxygenation react with TCE ,,, and other organic contaminants (such as p-chloroaniline and phenol , ) in pure and sediment-containing reduced iron systems amended with oxygen. A primary factor affecting contaminant reaction with •OH is competition for •OH or •OH precursors; these can include the destructive oxidation of dissolved organic matter and microbial biomass with •OH, , the oxidation of reduced sulfur species, as well as the oxidation of excess Fe­(II) in aqueous, adsorbed, and solid phases with •OH to form iron (oxyhydr)­oxide minerals. , The relative rates among these reactions likely affect the extent of contaminant degradation.…”

Hydroxylamine promoted Fe(III) reduction in H2O2/soil systems for phenol degradation

“…However, the e ciencies of •OH production and contaminant degradation were relatively low (Tong et Based on our early recognition, the capacity of •OH production depends on Fe(II) content and speciation in soils and sediments (Xie et al 2020). The addition of ligands to change Fe(II) speciation during sediment oxygenation enhanced TCE degradation (Xie et al 2021). The e ciency of TCE degradation increased by up to 6 times by ligand addition.…”