John G. Darab

Bio: John G. Darab is an academic researcher from Pacific Northwest National Laboratory. The author has contributed to research in topics: Catalysis & Borosilicate glass. The author has an hindex of 16, co-authored 47 publications receiving 2280 citations. Previous affiliations of John G. Darab include Delphi Automotive & United States Department of Energy.

Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron

Abstract: Borohydride reduction of an aqueous iron salt in the presence of a support material gives supported zero-valent iron nanoparticles that are 10−30 nm in diameter. The material is stable in air once it has dried and contains 22.6% iron by weight. The supported zero-valent iron nanoparticles (“Ferragels”) rapidly separate and immobilize Cr(VI) and Pb(II) from aqueous solution, reducing the chromium to Cr(III) and the Pb to Pb(0) while oxidizing the Fe to goethite (α-FeOOH). The kinetics of the reduction reactions are complex and include an adsorption phase. About 10% of the iron in the material appears to be located at active surface sites. Once these sites have been saturated, the reduction process continues but at a much lower rate, which is likely limited by mass transfer. Rates of remediation of Cr(VI) and Pb(II) are up to 30 times higher for Ferragels than for iron filings or iron powder on a (Fe) molar basis. Over 2 months, reduction of Cr(VI) was 4.8 times greater for Ferragels than for an equal weigh...

Surface chemistry and electrochemistry of supported zerovalent iron nanoparticles in the remediation of aqueous metal contaminants

Abstract: The microstructure, physical characteristics, corrosion behavior, and reactivity of zerovalent iron nanoparticles synthesized on a support (primarily a nonporous, hydrophobic polymer resin) were studied. The remediation of groundwater by zerovalent iron in in situ permeable reactive barriers relies on the redox reaction between metallic iron and a reducible contaminant. Decreasing the size of the iron particles and dispersing them on a support increases the specific surface area of the iron, as well as the ratio of surface to bulk iron atoms, and should thereby increase both the reaction rate and the fraction of iron atoms available for the reaction. Borohydride reduction of aqueous ferrous sulfate gives supported iron nanoparticles, 10−30 nm in diameter, which consist of 85% zerovalent iron by weight. These materials (“ferragels”) are stable in air and have corrosion behavior comparable to iron filings. Interestingly, the presence or absence of a support, as well as the boron remaining from the borohydri...

Chemistry of Technetium and Rhenium Species during Low-Level Radioactive Waste Vitrification

Abstract: We reviewed the chemistry of radioactive technetium species and their nonradioactive rhenium surrogates in the context of Hanford Site low-level radioactive waste processing and vitrification. Information concerning the hydrolysis, precipitation, phase transformation, volatilization, and redox chemistries of these species during the drying, calcining, and vitrification of aqueous waste slurries is condensed and extrapolated from the literature. The similarities between the chemistry of technetium and rhenium species were highlighted to evaluate the performance of rhenium as a surrogate for technetium in laboratory and engineering-scale low-level radioactive waste vitrification experiments.

The structure of Na2O–Al2O3–SiO2 glass: impact on sodium ion exchange in H2O and D2O

Abstract: The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10–50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D 2 O compared to rates in H 2 O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the OH bond as the rate-limiting reaction in Na + –H + exchange whereas matrix dissolution is controlled by OH − or H 2 O catalyzed hydrolysis of SiOSi and SiOAl bonds. Changes in Na exchange rate with increasing Al 2 O 3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using NaO bond length and co-ordination as measured by Na K-edge X-ray absorption spectroscopy, and binding energy shifts for SiONa sites measured by X-ray photoelectron spectroscopy (XPS). The energy barrier was calculated to increase from 34 kJ mol −1 for Na 2 O·2SiO 2 glass to 49 kJ mol −1 for a glass containing 15 mol% Al 2 O 3 , consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion-exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.

An xafs study of strontium ions and krypton in supercritical water

Abstract: We present the first direct measurement of ion hydration in supercritical water using X-ray absorption fine structure (XAFS). Radial structure functions were determined for strontium ions in supercritical water at 385[degree]C and 269-339 bar at a concentration of strontium of 0.2 M. For supercritical water, at a temperature of 385[degree]C and density of 0.54 g/cm[sup 3], the number of waters of hydration was a factor of 0.52 of the number in liquid water under ambient conditions. The radius of the first solvation shell changes very little at these elevated temperatures. This large local depletion of water around the ion would affect the short-range interactions with counterions and may increase the ion reactivity. We also report XAFS results for krypton in supercritical water and show that, in contrast to strontium ions, the local solvent environment is more gaslike in the first few solvation shells of the krypton atom. 36 refs., 5 figs., 1 tab.

Nanoscale Iron Particles for Environmental Remediation: An Overview

Abstract: Nanoscale iron particles represent a new generation of environmental remediation technologies that could provide cost-effective solutions to some of the most challenging environmental cleanup problems. Nanoscale iron particles have large surface areas and high surface reactivity. Equally important, they provide enormous flexibility for in situ applications. Research has shown that nanoscale iron particles are very effective for the transformation and detoxification of a wide variety of common environmental contaminants, such as chlorinated organic solvents, organochlorine pesticides, and PCBs. Modified iron nanoparticles, such as catalyzed and supported nanoparticles have been synthesized to further enhance the speed and efficiency of remediation. In this paper, recent developments in both laboratory and pilot studies are assessed, including: (1) synthesis of nanoscale iron particles (10–100nm, >99.5% Fe) from common precursors such as Fe(II) and Fe(III); (2) reactivity of the nanoparticles towards contaminants in soil and water over extended periods of time (e.g., weeks); (3) field tests validating the injection of nanoparticles into aquifer, and (4) in situ reactions of the nanoparticles in the subsurface.

A review of water treatment membrane nanotechnologies

Abstract: Nanotechnology is being used to enhance conventional ceramic and polymeric water treatment membrane materials through various avenues. Among the numerous concepts proposed, the most promising to date include zeolitic and catalytic nanoparticle coated ceramic membranes, hybrid inorganic–organic nanocomposite membranes, and bio-inspired membranes such as hybrid protein–polymer biomimetic membranes, aligned nanotube membranes, and isoporous block copolymer membranes. A semi-quantitative ranking system was proposed considering projected performance enhancement (over state-of-the-art analogs) and state of commercial readiness. Performance enhancement was based on water permeability, solute selectivity, and operational robustness, while commercial readiness was based on known or anticipated material costs, scalability (for large scale water treatment applications), and compatibility with existing manufacturing infrastructure. Overall, bio-inspired membranes are farthest from commercial reality, but offer the most promise for performance enhancements; however, nanocomposite membranes offering significant performance enhancements are already commercially available. Zeolitic and catalytic membranes appear reasonably far from commercial reality and offer small to moderate performance enhancements. The ranking of each membrane nanotechnology is discussed along with the key commercialization hurdles for each membrane nanotechnology.