Predicting Abiotic Reduction Rates Using Cryogenically Collected Soil Cores and Mediated Reduction Potential Measurements

Abstract

Abiotic reduction of contaminants in iron-based laboratory model systems has been studied extensively, but the resulting understanding is not sufficient to allow prediction of reaction rates under field conditions. This study demonstrates that rates of contaminant reduction by aquifer sediments can be predicted by combining several recent methodological innovations into an integrated protocol that is amenable to use on field samples. The protocol involves (i) cryogenic collection of core samples to preserve redox conditions, (ii) contaminant reduction rate measurements, (iii) oxidation–reduction potential measurements of sediments using electron shuttle-mediated potentiometry, and (iv) regression analysis of mass-normalized rate constants for contaminant reduction and reduction potential. The protocol was demonstrated using carbon tetrachloride as the model contaminant and 43 aquifer sediment samples from nine locations at four sites. The abiotic rate of degradation covers 5 orders of magnitude and a 450 mV potential range. The 95% confidence interval was used to represent the minimum reactivity that would be observed at each of the field sites under the measured reduction potentials. This provides a novel and effective protocol for screening abiotic reduction capabilities or establishing site specific rates.

Reduction of 3-Nitro-1,2,4-Triazol-5-One (NTO) by the Hematite–Aqueous Fe(II) Redox Couple

“…On the one hand, the k SA for the reduction of these compounds is controlled by pH and pe, and pe in turn depends on the concentrations and strength of Fe 2+ -complexing ligands such as dissolved organic matter (DOM), sulfite, and carbonate. ,− On the other hand, k SA can be affected by pe-independent factors such as solutes (e.g., DOM) that alter particle charge through adsorption and/or reactive surface area through aggregation. For example, when dithionite is used for ISCR, ,,, its oxidation by organic and ferric constituents would yield predominantly sulfite, which can decrease the reactivity of iron oxide–Fe 2+ redox couples through multiple mechanisms noted above. Hence, our results highlight the importance of assessing the in situ {Fe 2+ (aq) } and the effects of solutes when using the LFERs to predict NTO and NB reduction rates.…”

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2+

“…Coupling this analysis with E H measurements could, for example, allow one to discriminate among possible reasons that humic acids alter nitroaromatic reduction rates by oxide-bound Fe 2+ , which has proven to be a difficult task. − Humic acid may alter pollutant reduction rates by aggregating particles, , complexing aqueous Fe 2+ , , changing Fe 2+ oxidation products, and/or by serving as electron shuttles . Coupling E H measurements with conventionally used approaches may allow researchers to discriminate among these possibilities for other classes of contaminants as well, as was recently shown for chlorinated solvents reacted with reduced natural sediments …”

“…[1][2][3][4][5][6][7][8][9][10][11][12] Consequently, Fe 2+ is a critical reductant to account for in naturally attenuated and engineered remediation systems. [13][14][15][16][17][18] Prior work has established that aqueous Fe 2+ in the presence of an iron (oxyhydr)oxide (i.e., an "iron oxide") reduces pollutants far more quickly than aqueous Fe 2+ alone, 2,9,14,[19][20][21][22][23][24][25][26][27] with reduction rates depending on the redox properties of the iron oxide present. 2, 4-9, 14, 16, 21, 28-41 This effect is a result of the iron oxide influencing what Fe 3+ oxidation product forms.…”

“…Ferrous iron (Fe 2+ ) can reduce and simultaneously alter the toxicities and/or solubilities of several classes of environmental pollutants found in groundwater. − Consequently, Fe 2+ is a critical reductant to account for in naturally attenuated and engineered remediation systems. − Prior work has established that aqueous Fe 2+ in the presence of an iron (oxyhydr)­oxide (i.e., an “iron oxide”) reduces pollutants far more quickly than aqueous Fe 2+ alone, ,,,− with reduction rates depending on the redox properties of the iron oxide present. ,− ,,,,− This effect is a result of the iron oxide influencing what Fe 3+ oxidation product forms. ,,, When aqueous Fe 2+ is oxidized in the absence of an iron oxide, it tends to form an aqueous Fe 3+ complex or ferrihydrite . When Fe 2+ is oxidized in the presence of a crystalline iron oxide, however, it tends to form a more thermodynamically stable iron oxide phase, typically via homoepitaxial growth. ,, Thus, the presence of an iron oxide alters the reduction potential ( E H ) value of the Fe 3+ /Fe 2+ redox couple by changing the Fe 3+ speciation (i.e., the iron oxide that forms).…”

“…51 Coupling E H measurements with conventionally used approaches may allow researchers to discriminate among these possibilities for other classes of contaminants as well, as was recently shown for chlorinated solvents reacted with reduced natural sediments. 16 ■ ASSOCIATED CONTENT * sı Supporting Information…”