Abstract

Lan Y.; Huang X.; Deng, B. (2002) "Suppression of Pyrite Oxidation by Iron 8-Hydroxyquinoline", Archives of Environmental Contamination and Toxicology, 43:168-174.

Abstract:One of the important approaches to prevent pyrite (FeS2) oxidation and subsequent formation of acid mine drainage (AMD) is to create a surface coating on pyrite. In this study, a coating of iron 8-hydroxyquinoline was formed by leaching pyrite with a 0.10 M H2O2 / 0.0034 M 8-hydroxyquinoline solution, and stability of the coated pyrite was tested under various pH and temperature conditions. The results showed that iron 8-hydroxyquinoline coating could significantly suppress further pyrite oxidation by both chemical (H2O2) and biological (e.g. thiobacillus ferrooxidans) processes. At pH from 3.0 to 5.0 and temperature from 10 - 40 oC, the amount of SO42- leached out by 0.10 M H2O2 from the coated pyrite samples was 54.8 - 70.1 % less than that from the uncoated controls. The oxidation of pyrite followed a pseudo-zero-order kinetics under the constant concentration of H2O2. In the presence of micro-organisms, sulfate leached out of the uncoated pyrite in one year accounted for 5.32% of the total pyrite in the system, with a concurrent pH drop to 2.35 under the ambient room temperatures. In contrast, the amount leached out from the coated samples was only 0.15% of the total pyrite and the final pH was 5.48. Thus, the coating decreased the leachability of pyrite by 97% in the inoculated systems. In comparison to the more widely studied iron phosphate coating,the advantage of iron 8-hydroxyquinoline coating was that it inhibited both chemical and biological pyrite oxidation, while iron phosphate coating could only inhibit chemical pyrite oxidation.

Deng, B.; Hu, S.; Whitworth, T. M.; Lee, R. (2002) "Trichloroethylene reduction on zerovalent iron: Probing reactive versus nonreactive sites", Chapter 13, in: Innovative Strategies for the Remediation of Chlorinated Solvents and DNAPLs in the Subsurfaces, Susan Henry, Ed., ACS book series, pp 181 - 205.

Abstract: Sorption and reduction of trichloroethylene (TCE) on zero-valent iron (ZVI) were examined using batch experiments in the presence and absence of cysteine and potassium sulfate. Cysteine, an amino acid containing a -SH functional group, did not significantly affect TCE adsorption at concentrations lower than 1.0 mM. However, the rates of TCE reduction and iron corrosion were dramatically decreased at comparable or even much lower concentrations. The results suggested that there were two types of surface sites on the iron: the reactive sites responsible for TCE reduction and the non-reactive sites for the bulk of TCE adsorption. The reactive site density was estimated to be only 2% of the total surface sites. A two-site analytical model was proposed to explain the experimental results, in which adsorption took place on both reactive and non-reactive sites, but dechlorination reaction occurred only on reactive sites. Additionally, it was observed that the rate of TCE reduction was correlated to the rate of iron corrosion. This suggested that the same type of surface sites was responsible for both dechlorination of TCE and iron corrosion, and/or the hydrogen producing corrosion process took place prior to the dechlorination reaction.

Kim, C.; Zhou, Q.; Deng, B.; Thornton, E. C.; Xu, H. (2001) "Chromium (VI) Reduction by Hydrogen Sulfide in Aqueous Media: Stoichiometry and Kinetics ", Environ. Sci. Technol, vol. 35, 2219-2225.

Abstract: The objective of this work was to investigate the reaction stoichiometry, kinetics, and mechanism for Cr(VI) reduction by hydrogen sulfide in the aqueous phase. Batch experiments with excess [Cr(VI)] over [H2S]T indicated that the molar amount of sulfide required for the reduction of one molar of Cr(VI) was 1.5, suggesting the following stoichiometry:

2CrO42- + 3H2S + 4H+ ® 2Cr(OH)3(S) + 3S(S) + 2H2O

Further study with Transmission Electron Microscopy (TEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) confirmed that chromium hydroxide and elemental sulfur were the stable products. The kinetics of Cr(VI) reduction by hydrogen sulfide was measured under various initial concentrations of Cr(VI) and sulfide, as well as pHs controlled by HEPES, phosphate, and borate buffers. Results showed that the overall reaction was second order, i.e., first order with respect to Cr(VI) and first order to sulfide. The reaction rate increased as pH was decreased, and the pH dependence correlated well with the fraction of fully protonated sulfide (H2S) in the pH range of 6.5 to 10. The nature of buffers didn't influence the reaction rate significantly in the homogeneous system. The reaction kinetics could be interpreted by a three-step mechanism: formation of an inner-sphere chromate-sulfide intermediate complex ({H2O4CrVIS}2- ), intramolecular electron transfer to form Cr(IV) species, and subsequent fast reactions leading to Cr(III).

Deng, B.; Hu, S. (2001) "Reductive Dechlorination Of Chlorinated Solvents On Zerovalent Iron Surfaces" In: S. Burns and J. Smith, Eds. Physical and Chemical Remediation of Contaminated Aquifers, Kluwer Academic Publishers, Chapter 7, pp. 139 - 159.

Abstract: Optimal design of zerovalent iron-based permeable reactive barriers requires a complete understanding of dechlorination kinetics and mechanism. The effect of other ambient constituents, which may retard or enhance the dechlorination processes, should be considered. Literature has revealed that reduction of chlorinated compounds occurs on the iron surface and the reaction rate is limited by surface processes, rather than transport processes. Adsorption onto the surface can take place on both reactive sites that are responsible for the reductive dechlorination, and nonreactive sites that only sequester the contaminants. This chapter explores a model based on the assumptions that adsorption equilibrium on the two types of surface sites is always maintained, but the reduction rate is directly proportional to the amount sorbed onto reactive sites only. Numerical solutions are obtained to illustrate the effect of coadsorbates on the adsorption and reduction of chlorinated compounds under this mechanistic framework.

Deng, B.; Campbell, T.; Burris, D. (1999) "Reduction of Vinyl Chloride in Metallic Iron-Water Systems", Environ. Sci. Technol. Vol 33, pp. 2651-2656.

Abstract: Vinyl chloride (VC) is a highly toxic groundwater contaminant. The objective of this study is to examine the kinetics and mechanism of VC reduction by metallic iron, a process used for contaminated site remediation. A series of batch experiments were performed under various iron loadings, VC concentrations, pH conditions, temperatures, and in the presence of 1,10-phenanthroline, 2,2-dipyridyl and nitrilotriacetic acid. Results indicated that ethylene was the major product of VC reduction under all conditions examined. The reduction rate increased with increasing VC concentration, iron loading, and temperature. The amount of VC adsorption on iron surfaces was estimated. A simple kinetic model was developed to interpret the observed reaction kinetics, in which the adsorption of VC on the iron surface sites was considered to be in equilibrium with its aqueous concentration, and the reaction rate was directly proportional to the amount of VC adsorbed. The activation energy of the reaction was measured to be 41.6+2.0 kJ/mole, indicating that the chemical reaction at the surface, rather than aqueous phase diffusion to the surface, was controlling the overall rate of the reaction. Experiments with Fe(II)/Fe(III) chelating agents suggest that the effect of Fe(II) on VC reduction is not significant.

Burris, D.; Allen-King, R.; Manoranjan, V.; Compbell, T.; Loraine, G.; Deng, B. (1998) "Chlorinated Ethene Reduction by Cast Iron: Sorption and Mass Transfer", J. Environ. Eng. Vol. 124(10), pp. 1012-1019.

Abstract: Tetrachloroethylene (PCE) and trichloroethylene (TCE) exhibited significant nonlinear sorption to nonreactive sites when exposed to four cast irons. Cast iron is a reactive material that promotes reductive dechlorination and has recently been used for in-situ remediation of chlorinated solvent contaminated ground water. Comparisons between PCE sorption to cast iron, graphite, and iron-containing minerals indicate that nonreactive sorption is due to exposed graphite inclusions in the cast iron. Sorption of the homologous series of chloroethenes to a cast iron adheres to Traube's rule; thus, the extent of sorption is related primarily to compound hydrophobicity. An analytical model incorporating rate-limited sorption/desorption to nonreactive sites was used to assess sorption nonequilibrium. Effective sorption and desorption rate coefficients determined how significant mass transfer limitations to nonreactive sorption sites exist for PCE and not for TCE. The nonreactive sorption observed indicates that flow-through cast iron treatment systems will exhibit significant delayed attainment of steady-state conditions for chlorinated ethenes, particularly PCE and TCE.

Burris, D.; Delcomyn, C.; Deng, B.; Buck, L.; Hatfield, K. (1998) "Kinetics of Tetrachloroethylene Reductive Dechlorination Catalyzed by Vitamin B12", Environ. Toxicol. Chem. Vol. 17, pp. 1681-1688).

Abstract: Reductive dechlorination kinetics of tetrachloroethylene (PCE) to ethylene catalyzed by vitamin B12 using Ti[III] citrate as the bulk reductant was examined in a vapor/water batch system. A kinetic model incorporating substrate-B12 electron-transfer complex formation and subsequent product release was developed. The model also accounted for the primary reductive dechlorination pathways (hydrogenolysis and reductive b elimination) and vapor/water-phase partitioning. Reaction rate constants were sequentially determined by fitting the model to experimental kinetic data while moving upward through consecutive reaction pathways. The release of product from the complex was found to be second order with respect to substrate concentration for both PCE and acetylene; all other substrates appeared to release by first order. Reductive b elimination was found to be a significant reaction pathway for trichloroethylene (TCE), and chloroacetylene was observed as a reactive intermediate. Acetylene production appears to be primarily due to the reduction of chloroacetylene derived from TCE. The reduction of cis-dichloroethylene (cis-DCE), the primary DCE isomer formed, was extremely slow, leading to a significant buildup of cis-DCE. The kinetics of acetylene and vinyl chloride reduction appeared to be limited by the formation of relatively stable substrate-B12 complexes. The relatively simple model examined appears to adequately represent the main features of the experimental data.

Deng, B.; Stone, A. T. (1996) "Surface-Catalyzed Chromium(VI) Reduction: TiO2-CrVI- Mandelic Acid System". Environ. Sci. Technol. Vol. 30, pp. 463-472.

Abstract: As part of a study exploring the pathways and rates of CrVI reduction under environmentally relevant conditions, this paper examines the kinetics and mechanism of CrVI reduction by mandelic acid in the presence of 1.0 g/L TiO2. The redox reaction is dramatically catalyzed by the oxide surface, and benzoylformic acid and benzaldehyde are the major products of mandelic acid oxidation. Comparisons of adsorption and reaction among mandelic acid, methyl mandelate, and atrolactic acid suggest that CrVI adsorption is required for the reaction to take place while the adsorption of organic reductant may be unnecessary. The reaction, to our surprise, is zero order with respect to mandelic acid, but fractional orderwith respect to the ester methyl mandelate. This change in reaction order indicates that CrVI reduction by mandelic acid and methyl mandelate occurs via different rate-limiting steps.

Deng, B.; Stone, A. T. (1996) "Surface-Catalyzed Chromium(VI) Reduction: Reactivity Comparisons among Different Organic Reductants and Different Catalytic Surfaces". Environ. Sci. Technol. Vol. 30, pp. 2484-2496.

Abstract: CrVI reduction by low molecular weight organic compounds with various functional groups was examined in the presence and absence of oxide surfaces. Surface-catalyzed CrVI reduction has been demonstrated with a-hydroxyl carboxylic acids (glycolic acid, lactic acid, mandelic acid, and tartaric acid) and their esters (methyl glycolate, methyl lactate, and methyl mandelate), with a-carbonyl carboxylic acids (glyoxylic acid, pyruvic acid, and benzoylformic acid), with oxalic acid, and with substituted phenols (salicylic acid, 4-methoxy phenol, and resorcinol). Both goethite (a FeOOH) and aluminum oxide (g-Al2O3) exert an appreciable catalytic effect, although somewhat less than that observed with titanium dioxide. This study indicates that surface-catalyzed CrVI reduction may occur in soils, sediments, aquifers and other aquatic environments rich in mineral surfaces.

Allen, H.; Fu, G.; Deng, B. (1993), "Analysis of Acid Volatile Sulfide (AVS) and Simultaneously Extracted Metals (SEM) for the Estimation of Potential Toxicity in Aquatic Sediments", Environ.Toxicol. Chem. 12: 1441-1453.

Abstract: Acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) are operationally defined methods for the analysis of sulfide and associated metals in aquatic sediments. The SEM-to-AVS ratio has been useful in explaining the results of bioassay tests of metal toxicants. This paper describes apparatus that can be used in the evolution of sulfide from sediments and a method for the analysis of the evolved sulfide and the liberated metal. The method was studied with respect to gas flow rate, digestion time, and acid concentration. Liberated and trapped sulfide was determined by a colorimetric method of analysis. Using the apparatus and conditions described in this paper, the colorimetric method of analysis is capable of detecting AVS at concentrations normally encountered with a recovery of sulfide of at least 90%. High precision is possible if this apparatus is used. The limit of detection of the method is approximately 0.01 mmol/g dry sediment. We added 6 M HCl to produce a final concentration of approximately 1 M for the release of the AVS and SEM from unheated samples. We found that sulfide was not released from pyrite (FeS2) or copper sulfide (CuS) under these conditions. The liberation of copper from the two studied sediments indicates that copper was probably associated with another phase in these sediments. AVS is stable for several weeks in refrigerated or frozen samples.