Environ. Sci. Technol. 1997, 31, 2345-2349
Role of Carbonation in Transient Leaching of Cementitious Wasteforms J O H N . C . W A L T O N , * ,† SAZZAD BIN-SHAFIQUE,† ROBERT W. SMITH,‡ NEYDA GUTIERREZ,† AND ANTHONY TARQUIN† Department of Civil Engineering, University of Texas at El Paso, El Paso, Texas 79968 and Biotechnologies Department, Idaho National Engineering Laboratory, Idaho Falls, Idaho
A combined experimental and modeling program approach was applied to evaluate the role of the carbon dioxide reactive component of soil gas on the long-term performance of cementitious wasteforms. Small wasteforms were cast with Portland cement and synthetic wastewater containing metals and nitrate as a tracer. A series of wasteforms was exposed to an accelerated environment for carbonation and then subjected to transient leaching tests in deionized water. A second set of control wasteforms was not carbonated but was otherwise treated identically. Results are analyzed by comparison of experimental data with theoretical models of the leaching process. The results indicate that carbonation increases the apparent diffusion coefficient for unreactive species while resulting in chemical binding of metals through solid solution in calcite. For strontium, carbonation resulted in lower net leaching while leaching of calcium and nitrate was increased by carbonation.
Introduction Solidification and stabilization with cementitious materials is a widely accepted and economically attractive waste management option for disposal of low-level radioactive and hazardous wastes. Advantages of cementitious wasteforms include (a) availability of materials locally on a worldwide basis, (b) low cost, (c) ability to tailor the mixture for different wastes, and (d) high physical strength. The solidified wasteform can be developed in place (grouting of wastes), poured into steel drums or concrete canisters for later disposal, or formed directly into large monoliths. In most applications, cementitious materials are used primarily for their structural and physical properties. However, the effect of cementitious materials on the geochemical environment in the immediate vicinity of the concrete is pronounced. The geochemistry of the pore fluid in contact with hydrated cementitious materials is characterized by persistent alkaline pH values buffered by the presence of hydrated calcium silicates (C-S-H) and portlandite [Ca(OH)2]. The high pH and large internal surface area for sorption make cementitious wasteforms ideal for sequestering metals and radionuclides. Wasteforms to which blast furnace slag (BFS) has been added are additionally characterized by the presence of high * Corresponding author fax: 915-747-8037; e-mail: jwalton@cs. utep.edu. † University of Texas at El Paso. ‡ Idaho National Engineering Laboratory.
S0013-936X(96)00964-9 CCC: $14.00
1997 American Chemical Society
concentrations of reduced sulfur species in the pore fluid and by strongly reducing redox potential (Eh values) (1), both controlled by reduced sulfur in the BFS. These highly reducing conditions are effective in sequestering multivalent radionuclides such as Tc-99 in insoluble reduced forms. Because the geochemical environment within the wasteform pores is profoundly different from the ambient environment within the vadose zone (i.e., the region above the water table where water saturation is variable), the wasteform reacts irreversibly with and is altered by reactive agents within the soil. Among the reactive agents are oxygen and carbon dioxide, both components of soil gases. Although soil gas composition can be highly variable, increased levels of CO2 and reduced levels of O2 (relative to the atmosphere) are expected because of microbial activities. The fugacity of CO2 for equilibrium among water, portlandite, and calcite is so low (10-13 bar compared to the atmospheric value of ∼10-3 bar) that the introduction of CO2 can have a significant effect on cementitious materials as initial mineral phases are converted to carbonates. Limitations of Current Test Methods for Cementitious Wasteforms. Although significant research has been expended upon design of cementitious wasteforms, relatively little effort has been put into evaluation of how the wasteforms will perform within the context of the overall disposal system. Standardized leach tests provide only a limited amount of information relevant to estimation of long-term performance (risk assessment). The standard test for radioactive wastes is the ANSI/ANS-16.1 leach test (2). ANSI/ANS-16.1 generates a single parameter called the leachability index. The leachability index is derived by fitting the diffusion equation to short-term leaching results. The index equals the negative logarithm of the average diffusion coefficient (in units of cm2/ s). The diffusion equation is generally successful in fitting the results of short-term tests because several processes, in addition to simple diffusional release, lead to a square root of time dependence for total contaminant release. For example, adsorption-controlled release (3), solubility-controlled release, and oxidation-controlled release (4) all lead to a square root of time dependence for release in short-term tests. Although a variety of phenomena lead to similar shortterm leaching behavior and can thus be described by a leachability index, over longer time periods, performance diverges depending upon the processes controlling release rate. For this reason, simple leach tests combined with “black box” analyses are inadequate to predict absolute or relative long-term performance of wasteforms. The Toxicity Characteristic Leaching Procedure (TCLP) (5), promoted by the U.S. Environmental Protection Agency (EPA), is based on overnight extraction in acetic acid solution and a crushed wasteform. Quantitative interpretation of these tests are hampered because of the lack of correspondence between test conditions and conditions encountered in the disposal environment. Three broad areas have been identified (6) where further research is needed in cementitious wasteforms: (a) correlation of physical properties to performance, (b) fundamental chemistry and microstructure, and (c) performance tests. One of the limitations of current tests for wasteform leaching is that water-saturated conditions are specified for testing. The stipulation of water-saturated conditions for leach tests contrasts with current regulatory requirements (in the United States) and engineering practice of placing waste disposal facilities in the vadose zone, above the water table. In short-term diffusion-controlled leaching tests, saturated conditions may represent a worst case and therefore provide a conservative estimate of in situ performance.
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wasteform. Another change in wasteform behavior caused by carbonation is the potential for solid solution of some ions in the calcium carbonate present in the carbonated zone (10). Initial calculations of the importance of calcite solid solution on contaminant release, from cementitious wasteforms, suggest that the process is potentially important in limiting the release of some ions (8). The carbonated shell is likely to be important in long-term performance, but is not addressed in current leach tests.
Experimental Method
FIGURE 1. Schematic of carbonation process. However, long-term consideration of geochemical processes involved in in situ leaching suggests that the vadose or unsaturated zone, in general, provides a much more aggressive long-term environment for cementitious materials. The aggressive environment is caused by increased contact with carbon dioxide and oxygen in the soil air and variable contact with water. More generally, enhanced design of disposal facilities requires improved fundamental understanding of wasteform leaching in likely service environments, and this information cannot be produced from standardized leach tests. Reaction with Carbon Dioxide. Carbon dioxide interacts with cementitious wasteforms in a process known as carbonation. Carbonation occurs when Portlandite and other calcium-bearing phases in the wasteforms react with CO2 to form calcite (CaCO3). Major mineralogical changes associated with carbonation include conversion of Portlandite (7)
Ca(OH)2(s) + CO2(g) f CaCO3(s) + H2O(l)
(1)
and calcium silicate hydrate gel
C-S-H(s) + CO2(g) f CaCO3(s) + SiO2‚nH2O(s) + H2O(l) (2) The conceptual model for the reaction of soil gas with a wasteform is given in Figure 1. Reactive components present in soil gas diffuse into the wasteform, resulting in the alteration. A growing rind of carbonated material surrounds an inner zone of intact material. Because the geochemical and physical environment in the altered rind is significantly different from the environment of the intact core, the behavior of radionuclides and toxic metals is expected to be different. The rate of penetration of the carbonated reaction front can be approximated with a shrinking core model (8). Experimental investigation and modeling (9) have demonstrated that the migration of the front is most rapid at a relative humidity of 50-65% and declines at greater and lower relative humidity. This is because the increasing relative humidity increases the fraction of pores filled with water and thus hinders diffusion of CO2. At the same time, the carbonation reaction is aqueous and slows as water activity decreases. The two conflicting processes (diffusion of CO2 and reaction kinetics) lead to most rapid carbonation at intermediate relative humidity. Carbonation has several influences on the wasteform, both physical and chemical. Porosity tends to drop as previously open, large pores fill with calcium carbonate (7). With progressive carbonation, the pH of the system gradually drops from around 13 to 8.4 (9), leading to altered solubility (sometimes higher, sometimes lower) of waste metals previously immobilized by the high pH of the
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Small wasteforms (diameter 3.1 cm, height 6.7 cm) were cast with Portland cement and synthetic wastewater containing known amounts of dissolved metal ions. Synthetic wastewater was prepared by dissolving nitrate, chloride, or oxide salts of cadmium, cobalt, lead, strontium, zinc, and antimony in deionized water to yield concentrations of 3000 mg/L for Cd, Co, Pb, and Zn; 1000 mg/L for Sr;
