Dimin Fan

Sulfidation of Nano Zerovalent Iron (nZVI) for Improved Selectivity During In-Situ Chemical Reduction (ISCR)

The high reactivity of nano zerovalent iron (nZVI) leads to inefficient treatment due to competition with various natural reductant demand (NRD) processes, especially the reduction of water to hydrogen. Here we show that this limitation can be alleviated by sulfidation (i.e., modification by reducing sulfur compounds). nZVI synthesized on carboxylmethylcelluose (CMC-nZVI) was sulfidated with either sulfide or dithionite. The reactivity of the resulting materials was examined with three complementary assays: (i) direct measurement of hydrogen production, (ii) reduction of a colorimetric redox probe (indigo disulfonate, I2S), and (iii) dechlorination of trichloroethylene (TCE). The results indicate that sulfidation at S/Fe molar ratios of ≥0.3, effectively eliminates reaction with water, but retains significant reactivity with TCE. However, sulfidation with sulfide leaves most of the nZVI as Fe(0), whereas dithionite converts a majority of the nZVI to FeS (thus consuming much of the reducing capacity originally provided by the Fe(0)). Simplified numerical models show that the reduction kinetics of I2S and TCE are mainly dependent on the initial reducing equivalents and that the TCE reduction rate is affected by the aging of FeS. Overall, the results suggest that pretreatment of nZVI with reducing sulfur compounds could result in substantial improvement in nZVI selectivity.

Methods for characterizing the fate and effects of nano zerovalent iron during groundwater remediation.

The emplacement of nano zerovalent iron (nZVI) for groundwater remediation is usually monitored by common measurements such as pH, total iron content, and oxidation-reduction potential (ORP) by potentiometry. However, the interpretation of such measurements can be misleading because of the complex interactions between the target materials (e.g., suspensions of highly reactive and variably aggregated nanoparticles) and aquifer materials (sediments and groundwater), and multiple complications related to sampling and detection methods. This paper reviews current practice for both direct and indirect characterizations of nZVI during groundwater remediation and explores prospects for improving these methods and/or refining the interpretation of these measurements. To support our recommendations, results are presented based on laboratory batch and column studies of nZVI detection using chemical, electrochemical, and geophysical methods. Chemical redox probes appear to be a promising new method for specifically detecting nZVI, based on laboratory tests. The potentiometric and voltammetric detections of iron nanoparticles, using traditional stationary disc electrodes, rotating disc electrodes, and flow-through cell disc electrodes, provide insight for interpreting ORP measurements, which are affected by solution chemistry conditions and the interactions between iron nanoparticles and the electrode surface. The geophysical methods used for characterizing ZVI during groundwater remediation are reviewed and its application for nZVI detection is assessed with results of laboratory column experiments.

Oxidative Remobilization of Technetium Sequestered by Sulfide-Transformed Nano Zerovalent Iron

Our previous study showed that formation of TcS2-like phases is favored over TcO2 under sulfidic conditions stimulated by nano zerovalent iron. This study further investigates the stability of Tc(IV) sulfide upon reoxidation by solution chemistry, solid phase characterization, and X-ray absorption spectroscopy. Tc dissolution data showed that Tc(VII) reduced by sulfide-transformed nZVI has substantially slower reoxidation kinetics than Tc(VII) reduced by nZVI only. The initial inhibition of Tc(IV) dissolution at S/Fe = 0.112 is due to the redox buffer capacity of FeS, which is evidenced by the parallel trends in oxidation-reduction potentials (ORP) and Tc dissolution kinetics. The role of FeS in inhibiting Tc oxidation is further supported by the Mössbauer spectroscopy and micro X-ray diffraction data at S/Fe = 0.112, showing persistence of FeS after 24-h oxidation but complete oxidation after 120-h oxidation. X-ray absorption spectroscopy data for S/Fe = 0.011 showed significantly increasing percentages of TcS2 in the solid phase after 24-h oxidation, indicating stronger resistance of TcS2 to oxidation. At S/Fe = 0.112, the XAS results revealed significant transformation of Tc speciation from TcS2 to TcO2 after 120-h oxidation. Given that no apparent Tc dissolution occurred during this period, the speciation transformation might play a secondary role in hindering Tc oxidation. Collectively, the results indicate that sequestrating Tc as TcS2 under stimulated sulfate reduction is a promising strategy to improve the long-term stability of reduced Tc in subsurface remediation.

Reactivity of Fe/FeS nanoparticles: electrolyte composition effects on corrosion electrochemistry.

Zerovalent iron nanoparticles (Fe(0) NPs or nZVI) synthesized by reductive precipitation in aqueous solution (Fe/FeO) differ in composition and reactivity from the NPs obtained by reductive precipitation in the presence of a S-source such as dithionite (Fe/FeS). To compare the redox properties of these types of NPs under a range of environmentally relevant solution conditions, stationary powder disk electrodes (PDEs) made from Fe/FeO and Fe/FeS were characterized using a series of complementary electrochemical techniques: open-circuit chronopotentiometry (CP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). The passive films on these materials equilibrate within minutes of first immersion and do not show further breakdown until >1 day of exposure. During this period, the potentials and currents measured by LPR and LSV suggest that Fe/FeS undergoes more rapid corrosion and is more strongly influence by solution chemical conditions than Fe/FeO. Chloride containing media were strongly activating and natural organic matter (NOM) was mildly passivating for both materials. These effects were also seen in the impedance data obtained by EIS, and equivalent circuit modeling of the electrodes composed of these powders suggested that the higher reactivity of Fe/FeS is due to greater abundance of defects in its passive film.

Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals

Increasing recognition that abiotic natural attenuation (NA) of chlorinated solvents can be important has created demand for improved methods to characterize the redox properties of the aquifer materials that are responsible for abiotic NA. This study explores one promising approach: using chemical reactivity probes (CRPs) to characterize the thermodynamic and kinetic aspects of contaminant reduction by reducing iron minerals. Assays of thermodynamic CRPs were developed to determine the reduction potentials (ECRP) of suspended minerals by spectrophotometric determination of equilibrium CRP speciation and calculations using the Nernst equation. ECRP varied as expected with mineral type, mineral loading, and Fe(II) concentration. Comparison of ECRP with reduction potentials measured potentiometrically using a Pt electrode (EPt) showed that ECRP was 100-150 mV more negative than EPt. When EPt was measured with small additions of CRPs, the systematic difference between EPt and ECRP was eliminated, suggesting that these CRPs are effective mediators of electron transfer between mineral and electrode surfaces. Model contaminants (4-chloronitrobenzene, 2-chloroacetophenone, and carbon tetrachloride) were used as kinetic CRPs. The reduction rate constants of kinetic CRPs correlated well with the ECRP for mineral suspensions. Using the rate constants compiled from literature for contaminants and relative mineral reduction potentials based on ECRP measurements, qualitatively consistent trends were obtained, suggesting that CRP-based assays may be useful for estimating abiotic NA rates of contaminants in groundwater.

Dynamic interactions between sulfidated zerovalent iron and dissolved oxygen: Mechanistic insights for enhanced chromate removal

Recent research on contaminant removal by zerovalent iron (ZVI) has evolved from investigating simple model systems to systems that encompass increased dimensions of complexity. Sulfidation and aerobic conditions are two of the most broadly relevant complications. Combining these two, this study investigated the dynamic interactions between sulfidated microscale ZVI and dissolved O2, for removal of Cr(VI), a model contaminant for metals and metalloids. The results show that the coupling of sulfidation and oxygenation significantly improves Cr removal, which is attributed to enhanced Fe(II) production that resulted from accelerated corrosion of Fe(0). The Cr(VI) removal rate increased with increasing O2 saturation from 0% to 100% but showed a bimodal dependence on the S/Fe ratio. At the optimal S/Fe ratio, the ZVI exhibits a highly porous surface morphology, which, according to prior literature on sulfur induced corrosion, promotes corrosion. In addition, a novel time series correlation was developed between aqueous Fe(II) and Cr(VI) based on data collected in the presence and absence of 1,10-phenanthroline, to probe for changes of reductants during the reaction time course. The analysis indicated that Fe(0) was responsible for the initial small amount of Cr(VI) removal, which then transitioned to a phase controlled by surface Fe(II). The slopes of the time series correlations during the latter phase of the reaction vary with experimental conditions but are mostly much higher than the theoretical stoichiometric ratio between Cr(VI) and Fe(II) (i.e., 0.33), indicating that Fe(II) regeneration contributes significantly to Cr removal.