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Immobilization and detoxification of chromium in the vadose zone is made possible by the existence of an effective reductant, SO2, that exists in a gaseous form at room temperature. Experimental studies were designed to characterize stoichiometry and kinetics of chromium reduction both in aqueous solutions at pH values near neutrality and in soil. First, batch experiments and elemental analyses were conducted to characterize the stoichiometry and kinetics of Cr(VI) reduction in water. The stoichiometric ratio of S(IV) removed to Cr(VI) removed ranged between 1.6 and 1.8. The overall reaction is believed to be the result of a linear combination of two reactions in which dithionate is an intermediate and sulfate is the stable oxidized product. The reaction was also rapid, with the half-time of about 45 minutes at pH 6 and about 16 hours at pH 7. A two-step kinetic model was developed to describe changes in concentrations of Cr(VI), S(IV), and S(V). Nonlinear regression was applied to obtain the kinetic parameters. The rate of reaction was assumed to be second-order with respect to [Cr(VI)] and first-order with respect to [S(IV)], and [S(V)]. The values for the rate coefficient for the first reaction (k1) were found to be 4.5 ("10%), 0.25 ("9.4%) (mM-2h-1) at pH 6 and 7, respectively. The values of the rate coefficient for the second reaction (k2) were 25 ("29%), 1.1 ("30%) (mM-2h-1) at pH 6 and 7, respectively. The reaction rate decreased as pH increased. Experiments showed that the rate at pH 7 was lower than that at pH 6 by one order of magnitude. Second, batch experiments and elemental analyses were conducted to characterize the stoichiometry and kinetics of Cr(VI) reduction in soil. The stoichiometric ratio of S(IV) removed to Cr(VI) removed was almost 2, which is slightly higher than that for the reaction in water. This higher value may be due to S(IV) oxidation by soil-derived Fe(III). The reaction was rapid, with the half-time less than 2 minutes, which is faster than in water. The rate coefficients, k1 and k2, were 22 ("41%) and 13 ("77%) (M-2h-1), respectively.
Chromium is a primary inorganic contaminant of concern at the Pantex Plant. Chromium concentrations have been found to be two orders of magnitude higher than the drinking water standards, particularly in certain wells in the perched aquifer below Zone 12. In situ reduction of a mobile form of chromium, Cr(VI) to an immobile form, Cr(III), was examined as a viable option to active soil restoration. Successfully immobilizing chromium in the vadose zone as Cr(III) will reduce the amount of chromium that reaches the groundwater table. The results from the solution experiments indicated that chromium was rapidly and stoichiometrically reduced by Fe(II) in solution. Also, the slurry experiments showed that the aquifer solids removed Fe(II) from solution, but a portion of the iron removed remained available for reaction with Cr(VI), but at a slower rate. A model to predict different amounts of iron pseudo-components was developed, which allowed prediction of iron amounts required to reduce chromium under in situ conditions.
Put together by a team of scientists, engineers, regulators, and lawyers, the Chromium(VI) Handbook consolidates the latest literature on this topic. The broad scope of this book fills the need for a comprehensive resource on chromium(VI), improving the knowledge of this contaminant at a time when the extent and degree of the problem is still being
This book discusses recent developments in the study of chemical processes and equilibria in the marine environment and in the air/water and water/sediment interfaces. The chemical cycle of carbon as well as the effect of organic substances on the speciation and distribution of inorganic and organometallic substances are extensively discussed. Much of the recent progress in the area is the direct result of advanced analytical technologies and chemometric applications which are highlighted in the book.
Chromium and its compounds are widely used by modern industries, resulting in large quantities of this element being discharged into the environment. To remove chromium from contaminated soils and ground water, it is necessary to predict chemical and physical processes that control the rate of reactions and transport of chromium in soils and aquifers. The goals of this experimental study were to determine (i) kinetics and equilibrium adsorption of chromium(VI) in a natural soil, (ii) reduction of Cr(VI) to Cr(III) in the soil, and (iii) the effect of competing oxyanions on Cr(VI) adsorption in the soil. The TLM was used to interpret surface complexation reactions of the chromate ions in the soil. A laboratory investigation of reactions between hexavalent chromium, Cr(VI), and a natural soil was conducted to evaluate factors that influence sorption and reduction of Cr(VI) in natural soils. Both batch and soil column experiments were conducted to study the chemical behavior and transport of Cr(VI) in the soil. Results indicated that adsorption and reduction of Cr(VI) are the major processes that control the rate of transport and mobility of chromium in natural soils. Cr(VI) removal from solution increased with increasing solute concentration and with decreasing solution pH. This experimental study provides insight on how the residual amount of ferrous ions in minerals such as magnetite can effect the redox speciation of chromium in natural soils. Experimental results indicated that the small amounts of magnetite Fe3O4 contained in the soil caused reduction of Cr(VI) to Cr(III) even at pH above 8. The ferrous iron contained in magnetite provides a source of electrons for the reduction of Cr(VI) to Cr(III). Competing oxyanions, phosphate (H2PO4−/HPO42−) and sulfate (SO42−), increased Cr(VI) desorption by direct competition for adsorption sites. The equilibrium adsorption capacity of the soil was described with the Langmuir model, while a triple layer model (TLM) was employed to describe the surface complexation reactions. Outer-sphere surface complexation reactions and two-site (FeOH and AIOH) modeling were used to simulate adsorption of the chromate (CrO42−) and bichromate (HCrO4−) ions.
Isotope Dilution Mass Spectrometry (IDMS) has become an essential tool in research laboratories and is increasingly used in routine analysis labs (including environmental, food safety and clinical applications). This is the first textbook to present a comprehensive and instructive view of the theory and applications of this growing technique. The main objective of this book is to cover the theory and applications of Isotope Dilution in Analytical Chemistry. The scope is comprehensive to include elemental analysis, speciation analysis, organic analysis and biochemical and clinical analysis together with applications in metabolism studies and traceability of goods. Until now there have been no books published with the same general scope (only book chapters on particular applications). This is a textbook focused at post-graduate level covering the basic knowledge required for doctoral studies in this field. Isotope Dilution Mass Spectrometry will also outline practical applications of interest for routine testing laboratories where isotope dilution procedures are implemented or can be implemented in the future. This unique book covers all the theoretical and practical aspects of Isotope Dilution Mass Spectrometry (IDMS). Due to the increasing application of IDMS in many research laboratories and the increasing implementation of IDMS methodologies in routine testing laboratories, scientists in industry and working in or affiliated to this area will this an invaluable source of information. Concerning the theoretical aspects, the authors present a uniform theoretical background which grows from previous developments in Organic, Speciation and Elemental analysis both in their own laboratory and in other laboratories around the world. This general approach will be simpler and will also include new emerging fields such as quantitative proteomics and metabolism studies.
The importance of understanding complex toxicological and chemical properties of hexavalent and trivalent chromium has increased rapidly over the last few years as state and federal regulators reevaluate environmental standards. The risk management of chromium-contaminated soils continues to be a very dynamic process that presents interesting challenges. Chromium in Soil discusses the challenges faced by those investigating and remediating chromium-impacted soils and groundwater. The chapters address numerous ground-breaking developments in various fields of environmental chromium research, including toxicity, chemistry, environmental fate and transport, remediation technology, and health-based cleanup standards.