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Section Three WASTE TREATMENT PROCESS A. PHYSICAL/CHEMICAL PROCESS 47 MIXED SOLID PHASE EQUILIBRIUM James W. Van Nortwick, Jr., Research Assistant Pritzker Department of Environmental Engineering Illinois Institute of Technology, Chicago, Illinois 60614 Charles N. Haas, Betz Chair Professor Drexel University, Philadelphia, Pennsylvania 19104 INTRODUCTION In many industrial wastewater treatment systems soluble metal contaminants are often removed from the waste stream by alkaline precipitation. These processes result in the formation of mixed solid phases containing a number of metal hydroxides. In the past, it has been shown that (1) mixed solid phases generally have different solubilities than pure solid phases'"3 and (2) precipitation processes do not generally obey simple equilibrium relationships.3"5 Attempts to describe this deviation from ideality have included studies on kinetics and surface adsorption.5"7 However, no studies have been reported on solid phase non-idealities of environmentally important systems, which are known to exist.80 The recent emphasis on kinetic and surface adsorption studies has somewhat obscured the fact that a knowledge of thermodynamic properties is still of fundamental importance. This arises from the dependence of theoretical dissolution models on solubilities, which in turn depend on their thermodynamic properties. The practical significance of non-idealities in metal precipitation processes is that there is potential for a change in the point of minimum solubility of all the components in a solid mixture. The existence of such phenomena may make possible new separation processes based upon exploitation of differential non-ideal behavior among the components, however, no data on mixed solid phase non-idealities are available for the metal hydroxide solid phases encountered in wastewater treatment. The specific objectives of this research project were to investigate the existence of non-ideal behavior (with respect to ideal solution theory) in the hydroxide system composed of mixtures of cupric and cadmium hydroxide and to determine the dependency of solid phase activity corrections (non- idealities) on facets of the solution such as ionic strength, and total metal concentration. Cadmium and copper were chosen in light of their prevalence in industrial wastewaters, current environmental concerns regarding their disposal and their physical properties. BACKGROUND Traditionally, classical solubility products have been used extensively to describe the solid-solution phase equilibrium in single and mixed solid phases. This classical theory assumes that the activity of the solid phase is equal to unity, i.e., the solid mixture behaves as an ideal solid solution. Although this approach has become a versatile tool in rationalizing the dissolution and precipitation behavior of many systems, direct application of the solubility product principle and the assumption of solid phase ideality may not adequately describe mixed solid equilibrium relationships. The solid solution is in equilibrium with its ions but the equilibrium relationships are not always immediately obvious and quite often the assumption of unit activities is inappropriate. According to the Gibbs equilibrium condition,21 if a mixed solid and a solution are in equilibrium, the chemical potential of each component is the same in both phases. The solubility of the solid is therefore dependent on the activity of the solid phase. A number of researchers have shown that the mixed solid-solution equilibrium varies as a function of solid phase composition."16,19,20 These deviations from the ideal solid solution theory have often been attributed to solid phase non- 46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 455
Object Description
Purdue Identification Number | ETRIWC199147 |
Title | Mixed solid phase equilibrium |
Author |
Van Nortwick, James W. Haas, Charles N. |
Date of Original | 1991 |
Conference Title | Proceedings of the 46th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,42649 |
Extent of Original | p. 455-466 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital object copyright Purdue University. All rights reserved. |
Language | eng |
Type (DCMI) | text |
Format | JP2 |
Date Digitized | 2009-11-24 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Resolution | 300 ppi |
Color Depth | 8 bit |
Description
Title | page 455 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital copyright Purdue University. All rights reserved. |
Language | eng |
Type (DCMI) | text |
Format | JP2 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Transcript | Section Three WASTE TREATMENT PROCESS A. PHYSICAL/CHEMICAL PROCESS 47 MIXED SOLID PHASE EQUILIBRIUM James W. Van Nortwick, Jr., Research Assistant Pritzker Department of Environmental Engineering Illinois Institute of Technology, Chicago, Illinois 60614 Charles N. Haas, Betz Chair Professor Drexel University, Philadelphia, Pennsylvania 19104 INTRODUCTION In many industrial wastewater treatment systems soluble metal contaminants are often removed from the waste stream by alkaline precipitation. These processes result in the formation of mixed solid phases containing a number of metal hydroxides. In the past, it has been shown that (1) mixed solid phases generally have different solubilities than pure solid phases'"3 and (2) precipitation processes do not generally obey simple equilibrium relationships.3"5 Attempts to describe this deviation from ideality have included studies on kinetics and surface adsorption.5"7 However, no studies have been reported on solid phase non-idealities of environmentally important systems, which are known to exist.80 The recent emphasis on kinetic and surface adsorption studies has somewhat obscured the fact that a knowledge of thermodynamic properties is still of fundamental importance. This arises from the dependence of theoretical dissolution models on solubilities, which in turn depend on their thermodynamic properties. The practical significance of non-idealities in metal precipitation processes is that there is potential for a change in the point of minimum solubility of all the components in a solid mixture. The existence of such phenomena may make possible new separation processes based upon exploitation of differential non-ideal behavior among the components, however, no data on mixed solid phase non-idealities are available for the metal hydroxide solid phases encountered in wastewater treatment. The specific objectives of this research project were to investigate the existence of non-ideal behavior (with respect to ideal solution theory) in the hydroxide system composed of mixtures of cupric and cadmium hydroxide and to determine the dependency of solid phase activity corrections (non- idealities) on facets of the solution such as ionic strength, and total metal concentration. Cadmium and copper were chosen in light of their prevalence in industrial wastewaters, current environmental concerns regarding their disposal and their physical properties. BACKGROUND Traditionally, classical solubility products have been used extensively to describe the solid-solution phase equilibrium in single and mixed solid phases. This classical theory assumes that the activity of the solid phase is equal to unity, i.e., the solid mixture behaves as an ideal solid solution. Although this approach has become a versatile tool in rationalizing the dissolution and precipitation behavior of many systems, direct application of the solubility product principle and the assumption of solid phase ideality may not adequately describe mixed solid equilibrium relationships. The solid solution is in equilibrium with its ions but the equilibrium relationships are not always immediately obvious and quite often the assumption of unit activities is inappropriate. According to the Gibbs equilibrium condition,21 if a mixed solid and a solution are in equilibrium, the chemical potential of each component is the same in both phases. The solubility of the solid is therefore dependent on the activity of the solid phase. A number of researchers have shown that the mixed solid-solution equilibrium varies as a function of solid phase composition."16,19,20 These deviations from the ideal solid solution theory have often been attributed to solid phase non- 46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 455 |
Resolution | 300 ppi |
Color Depth | 8 bit |
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