Leon Tsao

Kinetics of silica polymerization

The polymerization of silicic acid in dilute aqueous solutions was studied experimentally and theoretically. Two basic processes were studied: the formation of colloidal particles by homogeneous nucleation, and their further growth by deposition of dissolved silica upon them. It was found that the state of ionization of the silica surface controls the rate of polymerization. The rate of deposition of dissolved silica on the surface of amorphous silica is proportional to the surface density of ionized silanol groups. The extent of surface ionization also determines the value of the surface tension, and thus also the rate of homogeneous nucleation. Added salts accelerate both molecular deposition and homogeneous nucleation by increasing the extent of surface ionization and decreasing the solubility of silica. Except for fluoride, salt ions were found not to have any specific catalytic effect. Fluoride was confirmed to be a powerful catalyst. Aluminum and boron were found to inhibit the reaction at pH 8. A successful quantitative theory of the homogeneous nucleation of colloidal silica particles was developed. The Lothe-Pound factor was shown to be about 3.34 × 1025 (kg H2O)−1. The nucleation model and results of our data analysis are incorporated in a computer code which models the homogeneous nucleation and growth of colloidal silica particles. This code is able to reproduce much of our data to within experimental error, and may be used to generate useful predictions for conditions that are typical of geothermal brines. The concentration of “adsorbed silica” on the surface of amorphous silica was determined, and was found to increase with the concentration of dissolved silica in the ambient solution. It is probably a reaction intermediate between dissolved silica and solid amorphous silica.

Surface study of HF- and HFH2SO4-treated feldspar using Auger electron spectroscopy

Auger electron spectroscopy has been used to study K-feldspar that has been reacted with both aqueous 10% HF and a 50% mixture of a 10% HF/0.1 N H/sub 2/SO/sub 4/ solution. In the feldspar/HF system, the resulting feldspar surface was shown to have been fluorinated; depth profiling, using argon ion sputtering showed the fluorination to have occurred substantially into the mineral bulk. In the feldspar/HF/H/sub 2/SO/sub 4/ system, the resulting surface contained both fluorine and sulfur. The fluorination had again penetrated into the bulk, but the sulfur could be removed with mild argon ion sputtering. The Al/F signal ratio was much lower on the feldspar surface treated with the 10% HF/0.1 N H/sub 2/SO/sub 4/ solution than the feldspar surface treated with the weaker 10% HF acid solution.

Catalytic Oxidation of Aqueous Hydrogen Sulfide in the Presence of Sulfite

Abstract Nickel sulfate catalyzes the reaction of hydrogen sulfide with oxygen in aqueous solution. This reaction was studied, and an empirical rate expression and a reaction mechanism were deduced. The rate of oxidation is independent of oxygen concentration and pH over the range investigated. The reaction rate is one half order in nickel, and it changes from second to first order in sulfide with increasing concentration. The oxidation reaction is an autocatalytic, free radical chain reaction. Nickel catalyzes the chain initiation step, and polysulfido radical ions propagate the chains. Colloidal sulfur is a major, frequently undesirable reaction product. Sodium sulfite suppresses formation of colloidal sulfur by converting it to thiosulfate. Cobalt is an equally potent catalyst, but a colloidal dispersion of cobalt oxysulfide is produced. Iron compounds are much weaker catalysts; iron citrate and iron HEDTA (N-hydroxyethylenediaminetriacetic acid) were the best among those tested.

Photophysical studies of uranyl complexes: VIII. Luminescence spectra of UO2SO4 · 312H2O and two polymorphs of bis(urea) uranyl sulfate

Abstract The photoluminescence spectra of hydrated and anhydrous uranyl sulfates have been studied under conditions of high resolution at cryogenic temperatures. All uranyl sulfate systems were found to yield nonequivalent spectra: the energies for the electronic and vibronic origins were found to vary with the system, and certain uranyl vibrational frequencies exhibited a dependence on environment. These differences must reflect the various ways in which the uranyl centers are linked by the bridging sulfate groups, as this linking is the main difference between the various structures.

The galena/dichromate solution interaction and the nature of the resulting chromium(III) species

Etude par spectrometrie photoelectronique de la reaction: il y a formation despeces Cr(III) et Cr(VI). Le Cr(III) serait sous la forme Cr 2 O 3 •4H 2 O avec une couche de CO 2 adsorbe sur la surface

Photophysical studies of uranyl complexes 5. Luminescence spectrum of K4UO2(CO3)3

The photoluminescence of K4UO2(CO3)3 has been studied under conditions of high resolution at cryogenic temperatures. The origin corresponding to the pure electronic transition was located at 4774 A (20945 cm−1), and it was found that the totally symmetric uranyl stretching mode was coupled to this transition. A progression of four band systems thus resulted, and from an examination of the energies of corresponding peaks in each system, a value of 813 cm−1 for the U-O stretching mode was determined. Two lattice modes (34 and 80 cm−1) and two molecular vibrational modes (205 and 276 cm−1) were also found to couple with the pure electronic transition, thus yielding approximately 15 major peaks in each band system. The 205 cm−1 vibration corresponded to a CO32− vibration, while the 276 cm−1 vibration was a UO22+ deformation. The low values obtained for the force constant and totally symmetric stretching frequency of the U-O bond suggested that in UO2(CO)34−, the uranium atom is bound in a complex species that may be considered as an intermediate between that of a uranyl (UO22+) and a uranate (UO810−) ion.

Luminescence of the hydrolysis products of Th(IV)

Abstract Raising the pH of an aqueous solution of thorium nitrate results in the precipitation of a hydrated thorium oxide. At room temperature this material does not exhibit any emissive properties, but upon cooling to 77 K a strong green luminescence can be observed. The emission spectrum is fairly broad, and is characterized by an emission lifetime of 125 μs. Calcination of the hydrolysis product produces an anhydrous thorium oxide which is not luminescent at any temperature. Through the production of a series of mixed lanthanide/thorium hydrolysis products, it was established that the luminescent species was a clustered thorium compound. Sensitization of Eu(III) emission by the thorium hydrolysis product was observed.

Photophysical studies of uranyl complexes—VI. Luminescence spectra of bis(imidazolium) tetrachlorodioxouranate(VI) and bis(2-methylimidazolium) tetrachlorodioxouranate(VI)

Abstract The photoluminescence spectra of the title compounds have been studied under conditions of high resolution at cryogenic temperatures. The luminescence obtained from the 2-methyl imidazolium salt was found to be well behaved, in that all emission originated from intrinsic uranyl centers and all luminescence decay curves consisted of single exponentials. In contrast, the emission associated with the imidazolium salt was found to originate both from intrinsic uranyl groups and from luminescence traps. The analysis of the luminescence decay curves obtained at cryogenic temperatures revealed that the lifetimes characteristic of the trap emission were significantly shorter than those associated with the intrinsic luminescence. The magnetic dipole allowed origins were found to be split by the rhombic crystal field in each system, but this splitting was observed to be 11 cm −1 or less. All other features present in either spectrum were assigned as vibronic in nature and they represent coupling of the various O-U-O or U-Cl vibrational modes with the pure electronic origins.

Reaction of hydrogen sulfide with oxygen in the presence of sulfite

Commonly, abatement of hydrogen sulfide emission from a geothermal powerplant requires that hydrogen sulfide dissolved in the cooling water be eliminated by chemical reaction. Oxidation by atmospheric oxygen is the preferred reaction, but requires a suitable catalyst. Nickel is the most potent and thereby cheapest catalyst for this purpose. One mg/L nickel in the cooling water would allow 99% removal of hydrogen sulfide to be attained. A major drawback of catalytic air oxidation is that colloidal sulfur is a major reaction product; this causes rapid sludge accumulation and deposition of sulfur scale. We studied the kinetics and product distribution of the reaction of hydrogen sulfide with oxygen, catalyzed by nickel. Adding sodium sulfite to the solution completely suppresses formation of colloidal sulfur by converting it to thiosulfate. The oxidation reaction is an autocatalytic, free radical chain reaction. A rate expression for this reaction and a detailed reaction mechanism were developed. Nickel catalyzes the chain initiation step, and polysulfidoradical ions propagate the chains. Several complexes of iron and cobalt were also studied. Iron citrate and iron N-hydroxyEDTA are the most effective iron based catalysts. Uncomplexed cobalt is as effective as nickel, but forms a precipitate of cobalt oxysulfide and is toomorexa0» expensive for practical use. 33 figures, 9 tables.«xa0less