Many, but not all, other graduate students in the department provided a ready forum for discussion of key points. Magnesium urn re1 facilitated dissolution of fran chrysotile in 0.1 M inorganic electrolyte at pH 7-10 for up to five days is greater than the 3:2 molar ratio of Mg:Si to silica in solid id.
MOTIVATION TO STUDY CHRYSOTILE 1. Asbestos in Drinking Water
Serpentine minerals are phyllosilicates (layer silicates); crystal is formed by repeated alternating stacking of the infinite magnesium hydroxide and silica sheets. Nevada and the mountains of the Pacific coast, but can also be found associated with ultramafics.
SCOPE AND OBJECTIVES
Experimentally verify the effectiveness of chrysotile coagulation with larger silica particles under a limited set of well-defined conditions. The last level, field investigations, is aimed at better defining the actual behavior of chrysotile particles in the systems of interest.
CHAPTER 2 BACKGROUND
MOLECULAR MODEL OF SURFACE
- Chrysotile Surface Structure
Predicted Chrysoti1e Surface Speciation
The estimated pH zpc of magnesium hydroxide is <12.5 (Parks, 1967), suggesting that >Mg-OH 2 + should be the predominant surface species in the pH range of interest in Cal ifornia surface waters, 7.5-8.5.
Magnesium-Anion Complexes
The predicted behavior for a magnesium silicate surface is shown in Figure 2.3, using these reported constants and with pK s. When the first and second acidity constants for H2A can be used and with the plexing equilibrium constant M90H+, this takes the form:
DISSOLUTION MODEL
- Dissolutlon Process
- Reaction Kinetics 1. Theory
Electron micrographs of the fresh and magnesium leached material showed a slight change in fiber morphology, indicating that both are present. Chowdhury (1975) observed release rates for magnesium from chrysotile on the order of 10-15 mol/cm 2 s at pH 7 and 37°C, which is considerably lower than the rate in strong acid. The surface area of the material was not reported in his experiments, so a value of 50 m2/g was assumed.
Ak is the affinity of step k in the forward direction, equal to the negative of. Writing (2.20) in terms of the activated complex precursors, the species that react to form the activated complex in step j, and far from.
Kinetic Model for Chrysotile Dissolution Possible activated-complex precursors for
In the present case, the probability that any surface >Mg-OH site is occupied by a proton is {>M9-OH. This is equal to the square of the fraction of sites that are protonated, {>~g-OH 2.
Model Appl ied to Fiber Ranoval
The effect of fibers aligned in a flow will be to reduce the collision radius and therefore the rate constant. The magnitude of this uncertainty in rate constant is insignificant (small) in the present context (southern Cal ifornia surface waters) for two reasons.
Deposition in Sand Fil ters 1. Model Predictions
Chrysotile fibers, which fall in the size (length) range of ~0.1-1, must be removed primarily by Brownian diffusion; settlement only becomes important above 1 y.I/T1. The contact efficiency is in the range of 10 -3 for 0.5 µm fibers and decreases at sizes of 1 ager.
Particle Size, }J-m
PARTICLES
CHAPTER 3
MATERIALS AND METHODS
Silica Silica (Si0
OTHER MATERIALS AND CHEMICALS
EXPERIMENTAL DESIGN
0 TEM
Potentiometric Experiments
The amount of catechol or oxalate adsorbed to the solid surface was determined throughout the experiment by measuring changes in solution concentration. To verify that particle concentration did not affect results, experiments were performed at pH 8 with 0.18, 1.0 and 2.0 grams of. To verify that the glass components did not contribute significant concentrations of silica, parallel experiments were performed using brucite (magnesium . hydroxide) as the starting material.
The experimental conditions were designed such that both magnesium and silica were far from solubility equilibrium (Figure 3.6).
Hours
- Electrophoretic Experiments
- Adsorption Experiments
- APPARATUS
- El ectrophoresis
- NATURAL ORGANIC MATTER
- CHEMICAL AND ANALYTICAL TECHNIQUES
The mobility of chrysotile was determined in the presence of three low molecular weight organic anions - phthalate, oxalate and catechol. Organic matter concentrations were higher in the adsorption experiments to maintain the same ratio of solid to organic matter as in the mobility experiments. The increased illumination provided by the laser made it possible to observe chrysotile particles in the 0.11m size range; many of these are not visible with white 1 rank II light.
Total organic carbon (TOC) and UV. UV) absorbance in the range 240-300 nm in the column effluent was monitored periodically.
Wavelength, nm
COAGULATION
The number concentration of silica particles was determined as 109/L by Coulter counter and 2x108/L by counting on the TEM (a). The lower TEM number and the TEM size distribution used in subsequent calculations because the Coulter counter showed a high background. a) The average count was 18 particles on a section of each of six squares of a 200-mesh copper grid; standard deviation was two per square. Electrophoretic mobility of the silica in the stock suspension was -1.8 Ilm·s-1V·cm- 1• M initial mobility measurement performed on the.
On the four days prior to the coagulation experiment, 50 ml samples of the 49 µg/L chrysotile suspension were analyzed by TEM for total fiber count.
CHAPTER 4
RESULTS AND INTERPRETATION OF EXPERIMENTAL WORK
WEATHERING BY DISSOLUTION
- Surface Coordination of Inorganic Ions
- Development of Surface Charge on Chrysotile Results of a typical constant-pH dissolution,
- Estimation of Surface Equil ibrium Constants From a least-squares fit to the data and for the
- Dissolution Stoichiometry
- Comparison with Brucite Dissolution
In the absence of adsorption of ions other than Hand + OH-, the species contributing to the surface charge are given by (2.4) and the pH elm is equivalent to the pHzpc. The different amounts of magnesium and silica released into solution in the presence of the four inorganic anions studied are shown in Figure 4.5. The gradual decrease in the rate could partly reflect a decrease in the total number of areas.
Following (2.28), the resulting rate law for the release of magnesium in solution is: 4.1) Kll constant depends on the specific digestion mechanism.
4.1) The constant kll depends on the dissolution mechanism, the specific
- pH Dependence of Initial Magnesium Release A second observed pH effect was on the initial
- Effect of Inorganic Ions
- Effect of Low Molecul ar Weight Organic Ions As the data on Figure 4.6 suggest, catechol and
- RELATION BETWEEN ADSORPTION OF ORGANIC MATTER, DISSOLUTION AND SURFACE CHARGE
- Adsorption
At lower pH (7.0–8.5) in the chrysotile system, the release of silica more closely follows the rate of release of magnesium. The presence of the effect only early in the dissolution suggests that 1 igand attack is more important at higher energy sites on the surface. Similarly, phthalate did not affect the dissolution rate of enstatite, diopside, or tremolite in the pH range 1-6 (Schott et al., 1981).
Degree of dissolution of chrysotile in the presence of organic anions; D symbols are for experiments in the absence of organics; conditions are stated on fig.
Solution Concentrationr t (mmoyL)
Adsorption of Oxidized Polymers of Catechol
The initially clear catechol-chrysotile suspensions darkened in color over time and became very black after a few days.
Time, hours
Consistency of Surface Charge and Dissolution Results
Using the surface equilibrium constants derived in section 4.1.1, the surface charge can then be calculated as a function of time (Figure 4.23). This calculation suggests that the surface charge at pH 8 should be zero after about ten weeks, which is about five times longer than the two week observed time for mobility reversal (Figure 4.21). This lack of close agreement between the two sets of data reflects the oversimplified nature of the surface chemistry model used.
The addition of organic anions makes the surface charge and mobility of chrysotile negative in two ways -- z.
Effect of Natural Organic Matter
The pH-iep of Alox in the absence of organic matter is seen from Figure 4.28 to be around 9.4. Concentrations of the order of 10-4 mg C/cm2 at pH 8 cause a charge reversal on chrysotile but not on Alox. Figure 4.14 shows that catechol adsorbs more strongly on Alox than on chrysotile.
The greater mobility of chrysotile in the presence of catechol (Figure 4.27) is then due to 1 argued that the dissolution rather than greater adsorption.
Appendices II and III
COAGULATION
The experimental blank -- a 4.9 giL chrysotile suspension with no silica present -- showed a relatively constant fiber concentration in the ion. There is a qualitative agreement between the departures from coagulation predicted in ion theory and observed in experiment. The lower rate or efficiency in natural waters is apparently due to the more uniform negative surface charge on all particles, caused by
Tipping and Higgins (1982) observed that adsorbed humics increase the stability of hematite particles; the mechanism is believed to be steric stabilization.
CHAPTER 5 CONCLUSIONS
- CHRYSOTILE DISSOLUTION
- pH Dependence of Chrysotile Dissolution
Silica release from chrysotile shows a weaker pH dependence and the rate cannot be characterized by a simple equation like (5.1). The rate at which chrysotile dissolves (10-15•7 mo1/cm2·s at pH 8) is consistent with the rates observed for other magnesium silicate minerals and for magnesium hydroxide; the magnitude of the rate further confirms that a chemical reaction rather than diffusion of reactants (protons) to the surface is rate-limiting. Chrysotile suspended in inorganic electrolytes exhibits a rapid but decreasing rate of release of Mg2+ into solution for 12-24 hours, followed by a slower, steady release rate for the following four to five days.
Up to 15 percent of the outer magnesium hydroxide surface is released into solution (based initially on a 100 percent magnesium surface) during the initial 12-24 hour period.
10 (Wirth & Gieskes, 1979)
ADSORPTION AND SURFACE CHARGE
Particles that age in natural water acquire a negative surface charge due to rapid (within hours) adsorption of natural organic matter and slower (within days) dissolution of the outer particles. At pH values below pHzpc (8.9), the surface charge of chrysotile is initially positive, but becomes negative through both. Chrysotile absorbs enough catechol, oxalate, phthalate or natural organic matter within one day to reverse the surface charge.
Surface charge then changes more slowly, reflecting the relative rates of magnesium and silica release.
The 1 arger charge reversal on chrysotil e refl ects an enhanced renoval of the surface brucite layer accompanying the rapid (order of hours)
- Surface Chemical t-bdel
Aluminum oxide has an iep pH of 9.4, close to that of chrysotile aged for 1 ess than a week. Removal of chrysotile particles in water treatment occurs by deposition of coagulated fibers in sand grains in filtration.
CHAPTER 6 RECOMMENDATIONS
- CHEMICAL WEATHERING
- PARTICLE AGGREGATION AND DEPOSITION
Experiments with well-characterized magnesium silicates are recommended as a first step in investigating these dependencies. It is recommended that one or more long-term dissolution experiments be performed on a well-characterized crysotile to confirm this projection. It is recommended that the pH dependence and transport of brllcite dissolution be determined in a system with.
It is recommended as a first step to determine the rate and extent of coagulant deposition on chrysotile.
Pre-Cenozoic geology of the southern half of the IS-minute Auburn quadrangle, California. An XPS study of the dissolution kinetics of chrysotile in 0.1 N oxalic acid at different temperatures. Considerations on the kinetics and mechanism of dissolution of metal oxides in acidic solutions.
A study of the adsorption of Ni(II) on an amorphous silica surface by chemical and tf.1R methods.
Before addition of organic substances After addition of organic substances Organic load Mobility Duration, Mobi 1ity, Duration.
