A set of bimetallic Pd−Cu/PVP (PVP = poly(N-vinylpyrrolidone)) colloids, with copper proportions ranging from 0 to 50 atom %, has been examined as catalysts in a batch reactor with flowing hydrogen for the reduction of aqueous nitrate and/or nitrite. The encapsulated Pd−Cu nanoparticles were characterized by powder XRD, TEM, EDX, and IR of adsorbed CO. A significant decrease in average particle diameter and changes in the Pd−Cu crystallinity occurred above ca. 30% copper content, and this transition corresponded with a significant increase in observed nitrate reduction rates. The strong dependence on composition suggests that specific Cun ensembles on the surface of the Pd−Cu nanoparticles are needed for effective nitrate-to-nitrite conversion. In contrast, nitrite reduction rates were only minimally enhanced by the presence of copper. Increasing pH had little effect on the nitrate reduction rates, but it strongly inhibited the rate of nitrite reduction. The requisite protonation of a palladium−nitrite surface intermediate is proposed.
This study tested the selectivity and sustainability of an alumina-supported Pd–In bimetallic catalyst for nitrate reduction with H2 in a continuous-flow packed-bed reactor in the presence of: (i) dissolved oxygen (DO), an alternative electron acceptor to nitrate, (ii) variable NO3 −:H2 influent loadings, and (iii) the presence of a known foulant, sulfide. The sustainability of the catalyst was promising, as the catalyst was found to be stable under all conditions tested with respect to metal leaching. The presence of DO at concentrations typical of treatment conditions will increase H2 demand for NO3 − reduction, but has no negative impact on the selectivity of the catalyst. Under optimal conditions, i.e., a pH of 5.0 and a high NO3 −:H2 influent loading, low NH3 selectivity (5%) was achieved for extended periods (36 days), resulting in sustained levels of NH3 that approached the European legal limit. The biggest challenge to the sustainability of the catalyst was the addition of sulfide, that initially increased NH3 selectivity and ultimately resulted in complete deactivation of the catalyst. Further work is required to identify regeneration methods to restore sulfide-fouled catalyst activity and selectivity; however, the most effective use would be to remove sulfide prior to catalytic treatment.
This study tested the stability, activity, and selectivity of an alumina-supported Pd–In bimetallic catalyst during repetitive sulfide fouling and oxidative regeneration conditions. Nitrate reduction with hydrogen was used as the probe reaction in a continuous-flow packed-bed reactor to assess changes in the catalyst structure as a result of the fouling and regeneration processes. Partial regeneration of a severely sulfide-fouled Pd–In catalyst was achieved with a NaOCl/NaHCO3 solution. However, the regenerated catalyst had a reduced activity for NO3 − reduction and increased selectivity towards NH3. Analysis of the catalyst bed after regeneration experiments using XPS, ICP-MS, and BET surface area revealed that bulk structural transformations of the Pd–In bimetallic catalyst occurred, as a result of preferential Pd dissolution near the column influent. The dissolved Pd showed limited mobility in the column, and was re-deposited on the catalyst, resulting in Pd enrichment on the catalyst surface and redistribution of Pd towards the end of the column. These changes along with residual sulfur content on the catalyst surface were likely responsible for the increased selectivity towards NH3. These results indicate the importance of limiting the exposure of reduced sulfur species to Pd-based catalysts, especially when treating contaminants like NO3 −, where product selectivity is a priority.
Knowledge of IFT values for chemical mixtures helps guide the design and analysis of various processes, including NAPL remediation with surfactants or alcohol flushing, enhanced oil recovery, and chemical separation technologies, yet available literature values are sparse. A comprehensive comparison of thermodynamic and empirical models for estimating interfacial tension (IFT) of organic chemical mixtures with water is conducted, mainly focusing on chlorinated organic compounds for 14 ternary, three quaternary, and one quinary systems. Emphasis is placed on novel results for systems with three and four organic chemical compounds, and for systems with composite organic compounds like lard oil and mineral oil. Seven models are evaluated: the ideal and nonideal monolayer models (MLID and MLNID), the ideal and nonideal mutual solubility models (MSID and MSNID), an empirical model for ternary systems (EM), a linear mixing model based on mole fractions (LMMM), and a newly developed linear mixing model based on volume fractions of organic mixtures (LMMV) for higher order systems. The two ideal models (MLID and MSID) fit ternary systems of chlorinated organic compounds without surface active compounds relatively well. However, both ideal models did not perform well for the mixtures containing a surface active compound. However, for these systems, both the MLNID and MSNID models matched the IFT data well. It is shown that the MLNID model with a surface coverage value (0.00341 mmol/m2) obtained in this study can practically be used for chlorinated organic compounds. The LMMM results in poorer estimates of the IFT as the difference in IFT values of individual organic compounds in a mixture increases. The EM, with two fitting parameters, provided accurate results for all 14 ternary systems including composite organic compounds. The new LMMV method for quaternary and higher component systems was successfully tested. This study shows that the LMMV may be able to be used for higher component systems and it can be easily incorporated into compositional multiphase flow models using only parameters from ternary systems.
H. Yoon, Werth, C. J., Barkan, C. P. L., Schaeffer, D. J., and Anand, P., “
North American railroads transport a wide variety of chemicals, chemical mixtures and solutions in railroad tank cars. In the event of an accident, these materials may be spilled and impact the environment. Among the chemicals commonly transported are a number of light non-aqueous phase liquids (LNAPLs). If these are spilled they can contaminate soil and groundwater and result in costly cleanups. Railroads need a means of objectively assessing the relative risk to the environment due to spills of these different materials. Environmental models are often used to determine the extent of contamination, and the associated environmental risks. For LNAPL spills, these models must account for NAPL infiltration and redistribution, NAPL dissolution and volatilization, and remediation systems such as pump and treat. This study presents the development and application of an environmental screening model to assess NAPL infiltration and redistribution in soils and groundwater, and to assess groundwater cleanup time using a pumping system. Model simulations use parameters and conditions representing LNAPL releases from railroad tank cars. To take into account unique features of railroad-tank-car spill sites, the hydrocarbon spill screening model (HSSM), which assumes a circular surface spill area and a circular NAPL lens, was modified to account for a rectangular spill area and corresponding lens shape at the groundwater table, as well as the effects of excavation and NAPL evaporation to the atmosphere. The modified HSSM was first used to simulate NAPL infiltration and redistribution. A NAPL dissolution and groundwater transport module, and a pumping system module were then implemented and used to simulate the effects of chemical properties, excavation, and free NAPL removal on NAPL redistribution and cleanup time. The amount of NAPL that reached the groundwater table was greater in coarse sand with high permeability than in fine sand or silt with lower permeabilities. Excavation can reduce the amount of NAPL that reaches the groundwater more effectively in lower permeability soils. The effect of chemical properties including vapor pressure and the ratio of density to viscosity become more important in fine sand and silt soil due to slow NAPL movement in the vadose zone. As expected, a pumping system was effective for high solubility chemicals, but it was not effective for low solubility chemicals due to rate-limited mass transfer by transverse dispersion and flow bypassing. Free NAPL removal can improve the removal efficiency for moderately low solubility chemicals like benzene, but cleanup times even after free NAPL removal can be prolonged for very low solubility chemicals like cyclohexane and styrene.