The Science of Chemical. Presented by. SolidTek Systems, Inc. Cincinnati, Ohio EXECUTIVE SUMMARY

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Roderick Gray
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1 The Science of Chemical Presented by SolidTek Systems, Inc. Cincinnati, Ohio EXECUTIVE SUMMARY It is obvious that solidification and chemical fixation of waste materials provides the safest form of pretreatment prior to final disposal. The following text describes the chemical fixation of hazardous materials; the test results which prove the effectiveness of the process; and studies which show the long term stability of the solid product of such fixation. The process referred to as solidification may be one of adding bulking or absorptive agents such as "speedy dry" or other clay absorbant; or some other inert agent like kiln dust, flyash, Fullers Earth, dirt, sand or sawdust. Solidification may also be accomplished by adding reactive agents which combine chemically with the liquid or sludge to fo-a "stable" solid. Neither of these solidification mechanisms is concerned with the potential of the final product for leaching out its hazardous components. Chemical fixation, on the other hand, is a process in which reagents are added to chemically bind hazardous components within a sludge or liquid. Non reactive components are treated or removed prior to final chemical fixation. The reagents added are designed to prevent the specific contaminants within the waste from leaching when the final, solid, product is exposed to landfill conditions. In the specific case of 'ID" series materials, on-site chemical fixation can eliminate the characteristic component from the leachate thus rendering the waste stream nonhazardous. Chemical fixation is a relatively new science and has only been practiced for approximately 10 years. During this time the leaching characteristics of the solidified product have been subjected to numerous tests. These tests have been conducted on the solid waste product for a variety of purposes such as: a. To qualify for disposal in a sanitary landfill. b. To qualify for disposal in a secure landfill. c. To qualify for a reclassification based on a reduction of toxicity. d. To qualify a waste for delisting. The results of these tests are contained in this report, along with the results of research into the operative chemical reactions which occur in this process. Also included is a pilot study on the applicability of this process for the solidification and chemical fixation of sewer sludge. 99

2 The information presented herein shows that this process has been refined beyond an emperical problem solving technique, Rather, it is a true science in which the physical and chemical, characteristics of the final product can be predicted based on the chemistry of the waste stream to be treated. Even though we do not have 20 or 30 years of experience with this process and its resultant products, our research and testing program shows predictable, verifiable test results both on current and accelerated aging studies. These results show that this process can and will be a significant factor in the future treatment and detoxification of hazardous materials. The SolidTek process shows great promise for treating large quantities of hazardous wastes and allowing them to be either placed in-situ after treatment or taken to a nearby sanitary landfill. This will save millions of dollars in transportation and disposal costs while, at the same time providing increased pub1 ic safety and protection of the environment. SOLIOIFICATION AND CHEMICAL FIXATION SYSTEMS Solidification systems are chemically reactive formulations which, together with the water and other Components in sludges and other aqueous wastes, form stable solids. "Stable", in this sense, means that the solids are physically stable under normal or expected environmental conditions and will not revert to the original liquid or semi-liquid state. The compositions formed by the use of these solidification systems may or may not be "non-toxic" as defined by any applied leaching tests. Chemical Fixation/Solidification systems not only chemically solidify the waste, but also insolublize, immobilize, encapsulate, destroy, absorb, or otherwise interact with selected waste components which are toxic or hazardous. The goal of these systems is to produce a solid which is non-hazardous (or less hazardous) than the original waste. Again, the degree of hazard for these kinds of materials and systems is usually defined by 1 eac hi ng tests. SYSTEMS TECHNOLOGY Solidification systems are of two types - inorganic or organic - according to the nature of the solidification chemicals used. Organic systems have been little used for industrial wastes except in the area of radioactive waste solidification, where urea-formaldehyde and bitumen systems have been widely used and other polymerization processes have been marketed to some degree. These systems are sometimes hydrophobic in nature and difficulties are encountered when incorporating the water-based wastes which make up the primary disposal problem. In addition, organic systems suffer from the problem of instability with regard to ambient environmental factors such as micro-organisms and ultra-violet light. Inorganic systems are used by SolidTek for the chemical fixation and solidification of complex wastes and/or mixtures thereof, with the aim of producing a non-toxic, environmentally safe material which could be used as 100

3 landfill. The process uses a multi-part, inorganic chemical system which reacts with all polyvalent metal ions and with certain other waste components; it also reacts within itself to form a chemically and mechanically stable solid. This system is based on reactions between catalyzers and setting agents which occur in a controlled manner to produce a solid matrix. The matrix itself, as produced, is actually a pseudo-mineral. It is based on tetrahedrally coordinated silicon atoms a1 ternating with oxygen atoms along the backbone of a linear chain. The charged side groups - in this case oxygen - when reacted with polyvalent metal ions result in strong ionic bonding between adjacent chains to form a cross linked, three dimensional, polymer matrix which is very much like many of the natural puroxene minerals. This type of structure displays properties of high stability, high melting point, and a rigid, friable, structure similar to many soils. The primary questions asked about any solidification system are: "Is it stable? W i l l the waste revert under some environmental condition to the original sludge or liquid waste? How do we determine stability?" In fact, we have only three ways of determining the long term stability of solidified waste or, for that matter, of any other material. These are: - Accelerated testing - designed to speed up the natural processes which might be involved in degradation of the material so as to simulate the results at any point in time in the future. - Actual environmental testing - placing the solidified material into real and actual environmental conditions for the period of time constituting its lifetime. - Chemical modeling - determining the lifetime of the material by means of "paper" analysis of the chemistry of the system and its relationship to other systems of known stability. Since we expect solidified waste to be stable for an indefinite and, hopefully, infinite period of time, it is obviously not possible to run actual environmental tests before such systems are put into commercial use. We do have approximately 12 years of experience with actual solidified wastes which have been disposed of or stored under a variety of natural environmental conditions. There are many accelerated tests developed for other types of materials whlch could be applied to solidified wastes; however, no standards have yet been set in this area. While various physical and mechanical tests have been run on solidified wastes, these are primarily for the purpose of determining suitability of the waste for structural uses such as landfill or road base, or for permeability determination if the solidified waste is to be used in a landfill. Most accelerated testing has really been involved in determining leaching characteristics, which are primarily associated with toxicity rather than physical stability. Some of the long term leaching procedures such as laboratory column tests and field lysimeter tests are useful in evaluating structural stability over a period of time in contact with water and/or according to the E.P. extraction procedure under RCRA. If products formed from chemical reactions involved in the solidification process 101

are thermodynamically or kinetically stable under normally encountered ambient temperature conditions, then we need only concern ourselves about other environmental factors such as groundwater

4 are thermodynamically or kinetically stable under normally encountered ambient temperature conditions, then we need only concern ourselves about other environmental factors such as groundwater activity, wind and rain erasion, biological degradation, and the effects of ultraviolet radiation. If the solidified waste is to be buried or covered, as is normally the case, then erosion and the effects of ultraviolet radiation do not apply and the important factors are the effect of ambient groundwater and biological degradation. Now we can see why, in addition to low cost and availability, Solidl'ek's systems are scientifically acceptable. These systems are non-b and have a very high degree of resistance to most types of groundwater. Because of these characteristics, it is expected that properly formulated, solidification systems would change only very slowly in the ground - on the same time scale as natural rocks and minerals; the possibility of sudden and catostrophic changes in such materials is very, very small. Chemical Fixation/Solidification systems introduce an additional problem fn evaluation - that of toxicity. This characteristic, however, is measurable by leaching tests. While considerable controversy still rages over hew these leaching tests should be run and with what leachants, much of the basic protocol for running different kinds of tests has already been established. Whether running a flow-through, dynamic, column or lysimeter test, a statfc, equilibrium, shaker-type test or E.P.A. Extraction Procedure, it is possible to make measurements which allow extrapolation of the probable leaching characteristics of the material into the future with a good probability of accuracy in predicting what the results would be under a given environmental condition. In formulating chemical fixation/soli di f ication systems, it is absolutely necessary to know the detailed composition of the waste material so that the fixation part of the process can be designed to limit leaching of the hazardous components to the necessary level established by law or regulation. Because liquid waste residuals are often very complex materials, all kinds of interferences and synergisms occur between various ions and other components contained in the waste. For this reason, a great deal of knowledge and experience is necessary to formulate a system which will achieve the desired results. Chemical fixation can occur by one or more of a fairly large number of mechanisms: neutralization, insolubliration, encapsulation, sorption, ion exchange, chemical modification, oxidation, reduction, permeation limitation, and surface area reduction. In the case of inorganic solidification systems, the solidification chemistry itself normally accomplishes some of these fixation functions by formation of metal hydroxides, silicates, absorption and/or ion exchange, encapsulation, and limiting the permeability of the wa,ste material, Soli dtek Chemical Fixation/Soli dification/stabil ization Systems SolidTek's present inorganic chemical fixation/sol idification systems are based on various siliceous compounds and catalyzing agents to control the reaction rate. Unless otherwise noted, they are all designed to work on aqueous systems. In addition to the solidification agents, systems designed for chemical fixation also contain various types of fixatives, both organic and inorganic. 102

5 One of the primary considerations in formulating SolidTek's solidification and chemical fixation systems is that the chemicals used in these systems be basically non-toxic and safe to handle. Unless otherwise specified, this is the case for all SolidTek solidification chemicals. They are formulated from either natural or synthetic materials which have a long history of industrial use and are considered to be generally non-hazardous. Since many of the formulations are quite alkaline, and nearly all of them contain very fine powders which can be irritating to the mucous membranes, it is necessary to utilize normal caution in handling such material, such as.eye protection and, in closed environments, dust masks. Essentially the same precautions as one would take in handling lime will be adequate with SolidTek Solidification systems. BASIC MECHANISMS The primary reactions of importance in the SolidTek process are similar to the extensively studied hydraulic cement alumino-silicate reactions, some of which are : 2Ca3Si05 + 6H2O -- H6Ca3Si Ca(OH)2 2Ca2Si04 + 4H20 -- HgCa3Si Ca(OH)2 Ca3A H2O -- 3Ca(OH)2 + AlO6 Ca3A HqCaSi05 t 26H20 -- H64Ca6A12S13050 CaqAl~Fe HqCaSi H20 -- H64Ca6A12Si H6Fe206 Or H64CagFe2Si HgA1206 The first two reactions, hydration of the half-hydrates of tricalcium and dicalcium silicate, are the primary reactions in the portland cement process and very likely in the SolidTek process. Calcium silicate hydrate is not a well defined compound. The formula H Ca Si 0 (C-S-H) is but an approximate description, as the stoichiometry is quite variable. The Ca/Si ratio in C-S-H is not exactly 1.5 but varies from 1.5 to 2.0. The result is a poorly crystalized amorphous gel consisting of extremely small, irregular particles of indefinite morphology. The result of addition of calcium alumino silicates to sewage sludge is a solidified material with the following positive results to varying degrees:. Sequestration of metals and other inorganics Sequestration of organics Pathogen reductiion. Odor control 10 3

6 METALS BINDING In the solidtek alumino-silicate and related reactions, basically three mechanisms are available to sequester metals:. Precipitation of simple metal salts such as carbonates and hydroxides Ionic-binding, loosly, within micropores of the alumino-silicate structures (exchangeable via ion exchange). Mineral fixation into the alumino-silicate complex The long-term stability of these processes from most to least stable fs: I Least Stable Most Stable Precipitation > Ion Exchange > Mineral Pixation This same series indicates the general biological availability of the metals. such as for crop uptake. Simple precipitation due to ph adjustment would be similar to lime stabilization. The metals bound via this mechanism may be leachable after the alkalinity of the product has been satisfied. Ion exchange is a more permanent fixation mechanism than precipitation and results in metals being less biologically available than precipitation. where M+z + 2RB --> R2M + 2B+ M = metal R = ion exchange sites B = exchangeable cation ' Cation selectively most likely follows the lyotrophic series: Hg > Cu > Ni > Pb > Co > Zn > Cd > Fe > Mn > Mg > Ca Divalent cations are generally preferred to monovalent, trivale to divalent, etc. in an ion exchange scheme. Mineral fixation as a part of the alumino-silicate structure is a very complex reaction. The general formula for the alumino-silicate minerals is: where xmyn(zp0q)wr X = large, weakly charged cations in 8 or higher coordinations with oxygen Y = medium sized, two or four valent ions in 6 coordination with oxygen Z = small, highly charged ions in tetrahedral coordinatiion with oxygen 0 = oxygen W = additional anionic groups such as OH or anions such as C1- or F-. 104

7 The ratio p:q depends on the degree of polymerization of the silicate framework, and the other subscripts m, n, and r depend on the need for electroneutral ity. Naturally occurring alumino-silicate minerals were formed under long-term stable conditions, conducive to uniform crystaline growth; the same is not true of relatively fast setting hydraulic cements. Rapid alumino-silicate hydrolysis results in, rather than a well crystalized regular structure such as that which occurs in nature, a C-S-H "tobermorite" gel in which the nature of the alumino-silicate polymer is highly irregular. Since the SolidTek process is similar to cement hydration, a similar irregular tobermorite gel is most likely formed. This would result in a stable alumino-silicate polymer with the ability to bind some cationic metals and anionic compounds. The coordination nunber of an ion is of great importance to its potential role in the alumino-silicate complex. The following is the listing of cations of concern and their coordination nunbers: 2 Coordination Number 4 - Ion si+4 ~1+3 Ion Radii (A) Y Y or X x 6 6 or or It should be noted that not all of these ions have been identified in alumino-silicate structures. This listing is based on theoretical structures extrapolated from ionic radii. Although th ionic radii of Cd+2 is of the same order as Ca+2, which would suggest Cd+? could replace Ca+2 in an alumino-silicate complex, experience has not shown this to be the case. Cd2 may actually be found only in 12 coordination. 10 5

8 In general, the more highly structured highly polymerized alumino-silicate contain sites providing lower coordination binding. The nature of the hydraulic cement gel may therefore increase the available sites for binding of high coordination metals over naturally occurring minerals. Information developed in conjunction with delisting petitions suggests that the solid matrix retains the contaminants in such a form as to be resistant to normal exposures to rainfall and surface water. Thus, chemical fixation is a viable technique in the arsenal of major site remediation activities. 106

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