Joseph P. Gould

The kinetics of hexavalent chromium reduction by metallic iron

Abstract The rate reduction of hexavalent chromium (Cr(VI)) by metallic iron under a range of conditions was studied in batch systems. The chemical variables studied were the Cr(VI) concentration, hydrogen ion concentration and surface area of iron. The influence of ionic strength and mixing rate was also examined. The reaction kinetics were found to be dependent on hydrogen ion concentration, hexavalent chromium concentration and iron surface area and to adhere to the following kinetic expression. d [ Cr(VI) ] d t = −k[ Cr(VI) ] 0.5 [ H + ] 0.5 ( Area ) . The rate constant was evaluated and found to have a value of 5.45 × 10−5 1 cm−2 min−1 over a wide range of conditions. The rate constant was found to increase as mixing rate increased up to a maximum value beyond which the rate was essentially independent of mixing. Increases in ionic strength were found to result in a rapid decrease in the rate constant at ionic strengths below 0.1 M. Further increases in ionic strength had no detectable impact on the rate constant. All rate determination studies were run in the mixing and ionic strength independent regions of these systems. Reaction stoichiometry was found to be, with one exception, independent of environmental conditions. In general, 1.33 mol of iron dissolved for each mol of Cr(VI) reduced. This highly efficient utilization of iron in the reduction suggests that hydrogen generated during iron dissolution may be acting as a reductant for the Cr(VI). The single parameter which influenced the reaction stoichiometry was the initial Cr(VI) concentration. The ratio of Cr(VI) reduced to iron dissolved increased rapidly as the Cr(VI) concentration increased. This observation was taken as being consistant with a surface interaction between the hexavalent chromium and some metastable hydrogen species at the iron surface.

The effect of solute competition on ozonolysis of industrial dyes

Abstract Ozonation of three industrial dyes was examined in batch studies with single- and multi-solute dye solutions. The kinetics of dye oxidation were controlled by gas transfer limitations and oxidation took place at the gas-liquid interface. Competition by dye molecules for the gas-liquid interface and relative chemical reactivity resulted in the preferential oxidation of selected dyes in mixtures of three dyes and auxilliary compounds typically found in dye-bath wastewaters.

Uptake, transformation and fixation of Se(VI) by a mixed selenium-tolerant ecosystem

The uptake, transformation and fate of Se, added as selenate to a mixed microbial ecosystem containing Se-tolerant bacteria and cyanobacteria, has been studied in simulated laboratory ponds. There was evidence of concerted activities of the microbial community in the system. Motile bacteria were responsible for transport into a surface mat and the combined actions of the microbial consortium provide chemical conditions under which the Se could be reduced to the elemental form and physically entrained in the mat. Strong qualitative indications of the formation of volatile alkylselenium compounds were also observed. The removal of Se from the water column was rapid and essentially quantitative. The advantages of such biological ecosystems for remediation of Se-contaminated aquatic ecosystems are discussed.

The kinetics and primary products of uracil chlorination

Abstract The reaction between free available chlorine (FAC) and the pyrinidine base uracil, was studied over a pH range of 5–9 and at a range of FAC to uracil ratios between 0.5 and 5 mol mol −1 . Rate constants for the reaction were obtained and the primary uracil chlorination products identified. The reaction was found to adhere very well to mixed second order kinetics at FAC to uracil molar ratios ⩾ ∼ 1.0. At a ratio of 5 mol mol −1 , the reaction demonstrated a reasonable empirical fit to the same kinetic model. The rate constant was markedly pH dependent increasing from approx. 0.3 M −1 s −1 at pH = 5 to 5 M −1 s −1 above pH = 7 at low molar ratios while the respective values at high molar ratios were approx. 7 and 90 M −1 s −1 . At low FAC uracil ratios the chlorination reaction yielded exclusively 5-chlorouracil while at the higher ratios, a dichlorouracil was isolated in a pure form and characterized spectrometrically. This material was the only organic chloramine detected in any of the systems. While evidence was obtained for breakdown of the uracil ring, little evidence for formation of secondary chloramines was found. The characteristic odor of nitrogen trichloride was noted at the higher FAC/uracil ratios. FAC consumption increased from 1 mol of FAC per mol of uracil consumed at the low molar ratios to 3 mol mol −1 at initial ratios of ≈ 5 mol FAC per mol uracil.

The formation of stable organic chloramines during the aqueous chlorination of cytosine and 5-methylcytosine

Abstract The interaction of aqueous hypochlorous acid with the aminopyrimidines, cytosine and 5-methylcytosine has been investigated. Analysis of the reaction mixtures indicated substantial formation of chlorine substitution and addition products with little reduction of chlorine to chloride. Very high levels of combined chlorine were observed. Ultraviolet spectra showed complex patterns of behaviour with peak shifts and intensity changes occurring in some cases and full loss of aromaticity in others. The latter process was suggestive of addition of HOCl to double bonds with formation of chlorohydrins. The behaviour was found to depend on reaction pH and chlorine to base ratios. Thin layer chromatography revealed complex patterns of stable chloramine formation with the reaction mixture complexity tending to increase with increasing pH and FAC/base ratios. Chemical ionization mass spectra disclosed the existence of a number of organic chloramines containing up to three chlorine atoms and, in the case of 5-methylcytosine a major ring cleavage and rearrangement product of as yet undefined structure. The half-life for the decay of the combined chlorine was found to be approx. 3.9 days and to be independent of precursor compound and pH.

The kinetics and products of the chlorination of caffeine in aqueous solution

Abstract The kinetics and products of the reaction between aqueous hypochlorous acid and caffeine have been evaluated. The reaction was found to be dependent on pH only to the extent that it influences the fraction of FAC which is in the form of HOCl. The kinetics were found to be well represented by the expression: d[Caff.] d t = −k [Caff.][HOCl] 2 The rate constant had a value of 162 ± 32 M−2 s−1. For each mol of caffeine consumed, 6.3 mol of FAC were consumed. The chlorination reaction proceeded primarily by ring cleavage and rearrangement forming nonoaromatic nitrogen heterocycles. 8-Chlorocaffeine was unequivocally identified as an aqueous chlorination product but was found to be formed only to a small extent and under rather limited conditions. No stable organic chloramines were formed.

Correlations Between Chemical Structure and Ozonation Kinetics: Preliminary Observations

Abstract The kinetics of the heterogeneous ozonation of o, p-activated aromatic organic compounds have been correlated with structural parameters defined by Linear Free Energy Relationships. The gross first order rate constants for these reactions were found to correlate excellently with the Hammett Polar Constant for a series of substituted phenols and pyrimidines. Strong correlation also was obtained for the kinetics of ozonation of an homologous series of 2-alkyl-4,6-dinitrophenols and the Steric Parameter. The reaction rate was observed to increase as the electron density in the ring increased and decreased with increasing size of the substituent alkyl group. A preliminary model structured around the kinetics of a gas diffusion controlled reaction with infinitely fast chemical kinetics has been proposed along with a program of research to evaluate such a model.

Multistage Decomposition Kinetics of Ozone In Dilute Aqueous Solutions

Abstract AbstractDecomposition of ozone in dilute aqueous solutions was found to be a complex process kinetically. Initial ozone dose-time had a significant impact on reaction order. Solutions dosed initially for less than one minute displayed uniform second-order kinetics. For doses applied over 5 and 15 min periods at a pH -2, reaction order changed from 2 to 1 to 0 as ozone decomposition progressed. At a pH of 6.65, the transition was from a reaction order of 2.5-3 to 2. This behavior has been ascribed to the effect of accumulated ozone decomposition products on the decomposition process.

Use of a Mixed Microbial System in the Removal of Lead from Contaminated Water

The potential of various microbes to concentrate elevated levels of heavy metals has long been observed in the natural environment (Stokes et al, 1975; Trollope and Evans, 1976). Metal tolerant species, apparently generated in polluted environments, may display an increased capacity for metal sorption and thus provide important biological material for both water purification and metal recovery (Stokes et al, 1975).