Showing papers by "Paul G. Tratnyek published in 2014"

Coupled effects of aging and weak magnetic fields on sequestration of selenite by zero-valent iron.

Abstract: The sequestration of Se(IV) by zero-valent iron (ZVI) is strongly influenced by the coupled effects of aging ZVI and the presence of a weak magnetic field (WMF). ZVI aged at pH 6.0 with MES as buffer between 6 and 60 h gave nearly constant rates of Se(IV) removal with WMF but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying WMF changes the mechanism of Se(IV) removal by ZVI aged for 6–60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se(IV) removal and Fe2+ release suggests direct reduction of Se(IV) to Se(0) by Fe0, in agreement with the XANES analysis. The numerical simulation of ZVI magnetization revealed that the WMF influence on Se(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe2+. In the presence of the WMF, the Lorentz force gives rise to convection in the solution, which narrows the diffusion layer, and the field gradient force, which tends to move pa...

Effects of metal ions on the reactivity and corrosion electrochemistry of Fe/FeS nanoparticles

Abstract: Nano-zerovalent iron (nZVI) formed under sulfidic conditions results in a biphasic material (Fe/FeS) that reduces trichloroethene (TCE) more rapidly than nZVI associated only with iron oxides (Fe/FeO) Exposing Fe/FeS to dissolved metals (Pd(2+), Cu(2+), Ni(2+), Co(2+), and Mn(2+)) results in their sequestration by coprecipitation as dopants into FeS and FeO and/or by electroless precipitation as zerovalent metals that are hydrogenation catalysts Using TCE reduction rates to probe the effect of metal amendments on the reactivity of Fe/FeS, it was found that Mn(2+) and Cu(2+) decreased TCE reduction rates, while Pd(2+), Co(2+), and Ni(2+) increased them Electrochemical characterization of metal-amended Fe/FeS showed that aging caused passivation by growth of FeO and FeS phases and poisoning of catalytic metal deposits by sulfide Correlation of rate constants for TCE reduction (kobs) with electrochemical parameters (corrosion potentials and currents, Tafel slopes, and polarization resistance) and descriptors of hydrogen activation by metals (exchange current density for hydrogen reduction and enthalpy of solution into metals) showed the controlling process changed with aging For fresh Fe/FeS, kobs was best described by the exchange current density for activation of hydrogen, whereas kobs for aged Fe/FeS correlated with electrochemical descriptors of electron transfer

Oxidative remobilization of technetium sequestered by sulfide-transformed nano zerovalent iron

Abstract: 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 Mossbauer 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.

IN SITU Chemical Reduction For Source Remediation

Abstract: In situ chemical reduction (ISCR) is a general term for a suite of in situ groundwater remediation technologies that rely primarily on chemical reduction of contaminants. ISCR has been used for over 15 years for plume treatment, but its use for source treatment is more recent. This chapter provides the first integrated assessment of the entire suite of ISCR technologies, and describes the technical basis, engineering aspects, past experiences and future prospects for using ISCR to treat chlorinated solvent source zones. In situ chemical reduction of contaminants can occur through natural intrinsic biogeochemical processes, as a result of stimulating in situ microbial activity to form reducing minerals, or after direct injection of chemical reductants. The chapter includes case studies of several ISCR technologies and summarizes the lessons learned to date from research and field experience.

Novel Contaminant Transformation Pathways by Abiotic Reductants

Abstract: Environmentally relevant abiotic reductants, such as zerovalent iron (ZVI) and minerals containing divalent iron (DVI), react predominantly by electron-transfer mechanisms with a variety of contaminant and probe compounds. Other reduction mechanisms involving activated forms of hydrogen (Hads or H–) have been suggested, but most evidence for these is only from systems containing noble metals that catalyze hydrogen activation (e.g., Pd). Here, 2-chlorophenylethanol and relatives of this aromatic halohydrin are used as probe compounds to show that ZVI can affect reduction by several novel pathways that are not observed with DVI minerals. These pathways include dechlorination by intramolecular nucleophilic substitution and epoxide ring opening by reduction. The former appears to be catalyzed by hydroxyl groups associated with oxides on actively corroding ZVI, and the latter can arise from hydride transfer (from NaBH4) or electron transfer (from ZVI).