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Surface chemical and physical behavior of chrysotile asbestos in natural waters and water treatment

Bales, Roger C. (1984) Surface chemical and physical behavior of chrysotile asbestos in natural waters and water treatment. California Institute of Technology , Pasadena, CA. (Unpublished)

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Chrysotile asbestos fibers enter California waters from physical weathering of magnesium-silicate, serpentine rocks in mountains of the northern and central portions of the state. Chrysotile particles, initially positively charged below pH 8.9 because of their magnesium-hydroxide surface, become negatively charged due to dissolution and adsorption of organic matter. Chrysotile suspended in 0.1 M inorganic electrolyte at pH 7-10 for up to five days dissolves with magnesium being released in excess of the 3:2 Mg:Si to silica molar ratio in the solid. The rate of magnesium release exhibits a fractional dependence on hydrogen-ion concentration: r = k_1'[H^+]^(0.24) The observed rate constant, k_l', depends on dissolution mechanism, specific surface area of the solid and charge-potential relation at the surface. Interpreted in terms of a site-binding model for adsorption and desorption of protons on the surface, the fractional dependence implies that dissolution is limited by a chemical reaction involving an average of less than one adsorbed proton per magnesium ion released into solution. Silica release from chrysotile shows no clear pH dependence. The rate of magnesium release is independent of the anions NO^(3-), Cl^- , SO_4^(2-), HCO_3^-, oxalate or catechol. Oxalate inhibited and catechol slightly enhanced silica release over the pH range 7.5-8.5; other anions had no systematic effect. Chrysotile's dissolution rate (10^(-15.7) mol/cm^2·s at pH 8) is consistent with observations on other magnesium silicates and brucite. Catechol adsorption onto chrysotile or aluminum oxide (pH 7.5-8.5) does not reach equilibrium but increases over five days. After one day the maximum adsorption density (Langmuir adsorption equation) on chrysotile is 0.7 x 10^(-9) mol/cm^2 (50 x 10^(-6) mg C/cm^2), approximately one-third of the estimated number of surface sites available for proton exchange. The maximum adsorption density for natural organic matter was near 30 x 10^(-6) mg C/cm^2 on both chrysotile and aluminum oxide. Chrysotile adsorbs sufficient catechol, oxalate, phthalate or natural organic matter within one day to reverse its surface charge. The extent of reversal is larger than observed for adsorption of the same organics on aluminum oxide, because of selective dissolution of chrysoti1e's outer magnesium-hydroxide layer. In reservoirs, submicron-sized chrysoti1e particles coagulate with larger (>2 μm), negatively-charged particles that subsequently settle out. The rate at which freshly-suspended, positively-charged chrysotile fibers coagulate is at least ten-fold greater than the rate for aged, negatively-charged fibers coagulate. Removal of chrysotile particles in water treatment occurs by deposition of fibers onto sand grains in filtration. Capture efficiency for single fibers is low; removal is enhanced 10-fold or more by incorporating fibers into larger flocs. Removal of chrysotile fibers in water filtration to levels near detection limits (typically 10^5-10^6 fibers/L) is possible; consistent achievement of this level will require a higher removal efficiency than is routinely achieved in treatment plants receiving water from the California aqueduct.

Item Type:Report or Paper (Technical Report)
Additional Information:© 1984 Roger Curtis Bales. All Rights Reserved. The work described in this report is intended to provide insight into the identity and rates of important surface-chemical reactions occurring on chrysotile asbestos under conditions encountered in surface waters of California. Experimental studies of dissolution and adsorption reactions comprise the majority of the material presented here. The intent of this work was to apply experimental and conceptual methods used in surface-chemical and geochemical studies on model systems to a potential drinking water quality problem. The results described here should provide both a better understanding of the environmental fate of chrysotile asbestos praticles in lakes and rivers and improved insight into fiber removal in water treatment. With the exception of Appendix VII, the material in this report was submitted by the author as a Ph.D. thesis at the California Institute of Technology in June, 1984. The water-filtration pilot experiments, described in Appendix VII, were carried out in July, 1984 as part of a continuing contract with The Metropolitan Water District of Southern California to study factors important in removal of chrysotile asbestos fibers in water treatment. I wish to express many thanks to my thesis advisor, James Morgan, for his support throughout this research. The counsel provided by Norman Brooks, Michael Hoffmann, Richard Flagan and George Rossman, who also served on my thesis examining committees, is greatly appreciated. Other graduate students, particularly Alan Stone and Simon Davies, provided a ready forum for discussion of key points. Gerald Zeininger spent a summer as an undergraduate assistant working on this research. Elton Daley, Leonard Montenegro, Joe Fontana and Rich Eastvedt provided technical support. Elaine Granger and Sandy Brooks typed the manuscript. Jean-Paul Revel made transmission electron microscope facilities available for my use; Pat Koen provided frequent technical assistance. Steven Hayward, various individuals at the Metropolitan Water District of Southern California, including Michael McGuire and Dale Newkirk, and persons at James M. Montgomery freely shared their knowledge and resources. My wife, Rebecca Bales, arranged her life to provide me with the maximum freedom to pursue my studies. With her devotion, pursuing a thesis program while maintaining spiritual and social health were possible. Financial support was provided by the Andrew W. Mellon Foundation through a grant to Caltech's Environmental Quality Laboratory, and by The Metropolitan Water District of Southern California. A portion of the Metropolitan grant was given through the American Water Works Association Research Foundation.
Group:W. M. Keck Laboratory of Hydraulics and Water Resources
Record Number:CaltechKHR:AC-8-84
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ID Code:26020
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Deposited On:07 Jan 2010
Last Modified:26 Dec 2012 13:51

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