Environ. Sci. Technol. 1994, 28, 1791-1800
Determination of Metal Behavior during the Incineration of a Contaminated Montmorillonite Clay Eric G. Eddings,t JoAnn S. Llghty,' and Janusz A. Kozlnskl*
Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12 The goal of this study was to develop an understanding of metals behavior during thermal treatment. Clay samples, contaminated with metals to obtain a surrogate waste, were analyzed prior to and following thermal treatment using nitric acid and/or hydrogen fluoride digestion, followed by inductively coupled plasma emission spectrophotometry analysis. Techniques were used to examine particle surface and metal distribution within cross sections. Lead, cadmium, and chromium results are discussed. With hydrogen fluoride-digested samples, the results indicated that vaporization increased slightly with increasing temperature for cadmium and lead. Chromium did not show increased vaporization. At higher temperatures, the nitric acid digestions did not completely remove the metals. Scanning electron microscopepictures showed that, at higher temperatures, the particle structure became compact and glassy; the electron microprobe results indicated that lead and cadmium were located in regions with high silicon, suggesting reactions with the silicon. Chromium distribution remained uniform, suggesting that chromium was immobilized due to structural changes not reactions.
Introduction Thermal treatment is a proven, commercially available technology for the remediation of most organically contaminated solids and soils. If heavy metals are also present, there is a potential for these metals to vaporize and potentially escape air pollution control (APC) devices. Oppelt (1)has identified the emission of toxic heavy metals as one of the greatest health risks associated with incineration; however, there is currently a lack of fundamental knowledge regarding the mechanisms affecting the fate of heavy metals during thermal treatment (2, 3). There are several primary mechanisms that can occur to influence the fate of a metal contaminant: (1)the metal can remain in the solid; (2) the metal may be entrained with other particulate matter; (3) the metal can react and form another species that may remain in the ash or vaporize; or (4) the metal can vaporize. If the metal vaporizes, it can condense on fly ash, often becoming highly enriched in the fly ash particle. The metal vapor may also homogeneously condense (4).Particles formed by the latter, gas-phase mechanism are potentially dangerous due to the fact that they are often small in size, on the order of microns, and are difficult to capture in typical APC equipment. It is important, therefore, to understand the mechanisms which result in vaporization of metal contaminants from the solid substrate.
* Author to whom correspondence should be addressed; e-mail address:
[email protected]. t Present address: Reaction Engineering International, Salt Lake City, UT 84101. t Present address: McGill Metals Processing Centre, McGill University, Montreal, Canada H3A 2A7. 0013-938X194/0928-1791$04.50/0
0 1994 American Chemical Society
The objectives of this research were to determine the important factors in the vaporization of metals from solids. Due to the exponential temperature dependence of the vapor pressure of metal compounds, temperature is considered an important parameter. In addition, previous studies by this research group and others have shown the importance of the binding phenomena of the metal with the solid substrate (5-7). This binding can influence the effective volatility of the metal. Experiments have been performed in both bench-scale and pilot-scale facilities (5, 8) using a montmorillonite clay sorbent as the solid substrate. The clay sorbent was used because this material has been extensively studied in previous work of the investigators (9). Metals used included lead, chromium, cadmium, barium, arsenic, copper, strontium, and zinc. The first five metals are regulated under current EPA guidelines (IO),and the other three metals represent nonregulated metals that might be possible surrogates for volatility behavior. Lead, chromium, and cadmium were studied in detail since they represent different classes of volatility. Chromium is usually quite refractory (nonvolatile),lead is semivolatile, and cadmium is volatile. This paper reviews the experimental results and provides explanations for some of the physical mechanisms involved with lead, cadmium, and chromium behavior in solids. Comparisons with published field data are also shown. In addition, a brief review of the basic structure and properties of pertinent clay minerals is given.
Physical Description of Clay Substrates The clay material utilized in the bench-scale and pilotscale experiments discussed in this paper was montmorillonite, which is one of the more commonly encountered clay minerals. It is a three-layer smectite clay (Si:Al:Si) with approximately one-third of the octahedral A13+ ions exchanged with Mg2+ions, yielding a net negative charge. One of the unusual properties of montmorillonite, as well as many other three-layer clays, is the ability for the clay to swell upon hydration (11). Water is absorbed between the clay layers, and the interlayer distance subsequently increases to accommodate the water. The negative charge associated with the Mg-substituted layers is balanced by positive ions that can be found between the triple sheets (12-14). The most commonly encountered interlayer cations are Na+ and K+, and these free cations distributed over a very high surface area contribute to the high cation-exchange capacity of clays (15). The ability of transition metal cations to exchange with these alkali cations has been the subject of much research, and numerous investigators have successfully exchanged the naturally occurring ions in the clays with such ions as Cu2+,Ni2+, Cr3+,Fe3+,Co2+,Pd2+,Ru3+,Zn2+, Cd2+,and Pb2+(12,16-22). It should be noted that the exchange of cations in these referenced studies was Environ. Sci. Technol., Vol. 28, No. 11, 1994 1701
accomplished using aqueous solutions and procedures similar to those used with the pilot-scale and bench-scale tests discussed in this work. Detailed investigations regarding the nature of the adsorbed metals in fully hydrated clays indicated that the metal ions were present as hydrated ion complexes,similar to what would be found in aqueous solutions of the ions (23-25). In addition, it has been noted that the process of adsorption of heavy metal ions by clay minerals involves more than ion exchange and that in some cases the adsorption is irreversible (26). Experimental Facilities and Methods Bench-scale testing was completed in a differential-bed reactor (DBR) described in detail elsewhere (5). In this reactor, a small bed of contaminated particles is contained between two stainless-steel screens and placed within an electrically heated ceramic block. A preheated flow of gas is passed through the particles. The amount of metal vaporized is determined by the difference between the metal concentration in the particles prior to and following the experiment. The temperature limit of this facility is approximately 600 "C. A rotary-kiln simulator was used for the pilot-scale testing and for testing at higher temperatures (650-980 "C). The facility, described in detail by Owens (27),is a batch system where the main section is 0.6 m in diameter by 0.6 m long. The burner section and main section rotate, while the exhaust section is stationary. The bed temperature was measured by a thermocouple which extends into the middle of a kiln bed in the main section. A shielded suction pyrometer was used to determine the gas temperature. Waste was loaded through a blast-gate valve located in the stationary exhaust section, and solid samples were also takenvia this opening. The advantage of utilizing the batch system was that the removal of solid samples during an experiment simulated the collection of timeresolved samples of a charge as it would move through a full-scale facility. Therefore, additional information was obtained beyond sampling the bottom ash as it exits the kiln, which is the typical measurement used in full-scale facilities. The size of the charge was approximately 3500 g, and the solid samples removed were approximately 2 g. Aqueous solutions of metal salts were mixed with the montmorillonite clay, using enough water to completely saturate the material. Three source compounds of each metal were used: nitrate, chloride, and sulfate salts (see Table 1). The different metal salts were used to identify any effects due to the initial speciation of the metal. The aqueous metal solution was combined with the montmorillonite clay in a known mass ratio (Table l ) , and the mixture was tumbled for at least 1h. After being mixed, the particles were dried overnight at 105 "C. Water was then added back to the sorbent to a level of 10 wt 5%. Tetrachloroethylene (TCE) and toluene were also added to determine if organic contamination had an effect on metal volatilization. TCE was added as a source of chlorine for comparison with metal chloride tests. For this reason, TCE was not added in the tests involving metal chlorides. It should be noted that the clay sorbent particles were sieved prior to contamination to remove particles smaller than 1mm in diameter in an effort to minimize entrainment of contaminated particles. 1792 Envlron. Scl. Technol., Vol. 28, No. 11, 1994
Table 1. Initial Species of Metal Salts Utilized in Aqueous Contamination and Initial Solid Concentrations. of Metal Contaminants in Montmorillonite Clay Sorbent Used in Rotary Kiln Tests metal Cr Cd Ba cu Pb Sr
As Zn
nitrate tests
Cr(NOd~3H20 CrCl~6H20 Cd(N0&4HzO CdC12.2.5HzO Ba(N03h BaClr2HzO Cu(NOs)z.3HzO CuClrH20 P b (Nod2 PbClz Sr(NO3h SrClz.6H20 As203 As203
metal
nitrate tests
Cr Cd Ba cu
278 243 30'2 308 280 273
Pb Sr As Zn
metal salts utilized chloride tests
sulfate tests Crz(S0*)~15HzO CdS01.8Hz0 CuSOp5HzO Pb (CzHa0z)z AS208
ZnS01.7H20 av initial solid concn (pg/g) chloride testa sulfate tests untreated
12
56
293 232 336 309 287 302 14 54
140 718 97 324 172 76 7 317
42 5 74 19 10 64 14 63
Based on U.S.EPA Method 3050.
Solid samples were ground to a fine dust and digested with nitric acid according to EPA Method 3050 (28).The solutions were then analyzed by inductively coupled plasma emission spectrophotometry (ICP) using the procedures of EPA Method 6010 (28). Due to recent publications on the limitation of Method 3050 (29-31) to recover all of the metal, a limited number of samples were also analyzed using a hydrofluoric acid (HF)digestion technique. The HF method fully dissolves the solid sample, ensuring that the metals are released from the aluminosilicate matrix, which may not be the case with Method 3050. Finally, the samples were analyzed for the distribution of selected metals (Pb, Cd, and Cr) using a Cameca 50X electron probe microanalyzer (EMPA) (32). Pictures of particle surface structure were taken with a Cambridge 240 scanning electron microscope. Results and Discussion
General Results. The following discussion is most applicable to clay materials which have been contaminated with trace levels of metal Contaminants (
