a22 i t ' s cracked up to be? What could be a sirnpler solutioii to the disposal of hazardous ( : h e m i d s and waste radioactive materials than t o imcasc them i n wmcnt? l ' ~ t the practice is far from simple. "Arc hazardous constituents immobilized?" asks Frank Curtledge of Louisiana State Universitv. "And how permanmt arc! the matrix strii(:tiirv and its propertics under cnvironmental stress?" . i t the August ACS meeting in New Ynrk Citv, scicntists discussed these questions a i d how to probe the changes that o(:(.ur within rrments.
By Alan Newmon
pirically correlated to the cement's compression strength. On the other h a n d , chemical shifts for "A1 NMR reflect tetrahedral versus octahedral geometry. NMR studies of cement show that octahedral aluminum begins forming one day after the material is mixed.
"A key goal of this
An NMR view of cement Cartlcdgc's group uses Fourier transform UMK ( w i t h magic angle spinning to offset problems arising from studying solid samples) to examine the degree (11 internal p d v nierization of sili(:~rn-~ixvgen ti(inds in cemvnts m i x c d with various chemicals. For example, they find that adding Pblll) slows the setting of the concrete initially but with time prumotes ~ i o l y m e r i z a t i o t ~ , whereas Cr(1ll) retards polym(~rization. pnssiblv bv suhstituting for silicon in the mixturp ( 1 . 2 ) . To gnin these insights. they rely on correlations between the NMR cheniicdl shifts rifrither *'Si or ",\I resonan(:es and thi! chemical hnnding to those elrmrnts. For instance. In another experiment, Cartthe chemical shifts of peaks arising ledge's group added phenol to ordifrom "'Si in silicatcs reflects thc dc- nary Portland cement (OPC) to exgrce of cross-linking (Le.. the num- amine the chemical's leachability. ber of Si-C-Si boiids). Thus NMR Using 13C NMR, they observed that differentiates tietween the un(:r(isi- after 28 days all the phenol had ionlinked silicate ion Si(0l4'- and the ized to calcium phenoxide. They series (O),Si-O-Si(O):,, . . Si(0- also found evidence that phenol was dissolved in water trapped inSi),-ahhreviated as Q",Q' Ac(:ording to Cartledge. cement side cement pores. begins as priniarilv Q".but after 28 Cartledge's group has even looked days forms a mixture of Q".Q',and at cement entrapment of an experiQ'. 'l'lie presenw of Q' can tic em. mental "sludge" containing ions
research is to model the long4erm effects of encapsulating hazardous wastes in cement"
42 Environ. Sci. Technol., Voi. 26,No. 1, 1992
such as mercury, nickel, and cadmium combined with lime. In contact with air, a complex precipitate of hydroxide a n d carbonate salts forms, and this appears to be microencapsulated within the cement matrix. Electron microscopy A more complex assortment of tools for probing cements was presented by Maria Neuwirth from Canada's Alberta Environmental Centre. She compared data from scanning electron microscopy (SEMI with information available from transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM] (3).In general, SEM allows mapping of morphological changes in the cement, with an image resolution on the order of 5 nanometers, whereas TEM/STEM provides localized, crystallographic, and chemical information at image resolutions of less than 0.5 nanometer. These microscopy techniques can be expanded to include other analytical techniques. For instance, the addition of a backscattered electron detector on a scanning electron microscope allows detection of regions containing heavy elements such as cadmium and chromium, whereas the detection of X-rays (arising from the interaction of the microscope's electron beam with an atom's core electrons) scanned in a line generates qualitative concentration profiles that provide some insight into mechanisms of leaching. TEM/STEM can also produce X-ray photons from a sample. With a suitable detector, the X-rays can identify light elements such as carbon and oxygen, and because
0013-936W92/0926-42$02.50/0 0 1991 American Chemical Society
thin, foil-like samples are required, the data can be quantified. Light elements can also be detected by measuring the energy loss of transmitted electrons (electron energy loss spectroscopy). Finally, crystallographic lattice parameters and orientation for small grains or precipitates within the cement can be determined by electron diffraction methods (analogous to X-ray diffraction). To prepare samples for SEM studies, the cement is fractured and coated w i t h carbon to prevent charging effects. On the other hand, TEM specimens require greater care because they must be only about 100 nm thick to be electron transparent. Neuwirth a n d her colleagues have developed what they describe as a simple and reproducible method to prepare cement samples with the correct dimensions. As part of their work, Neuwirth and co-workers have studied the fate of Cr(II1) in OPC using X-ray methods and electron diffraction. Like Cartledge, they find evidence for an unusual replacement of chromium ions for silicon ions within the cement. A model for leaching A key goal of this research is to model the long-term effects of encapsulating hazardous wastes in cement. Toward that end, researchers at Brookhaven National Laboratory have developed a n accelerated leach test that, in combination with a computer model, could predict leach rates over time (4-6). “Most of the work focuses on lowlevel radioactive waste disposal,” explains geochemist Mark Fuhrmann. Experiments have followed the loss of sodium, potassium, and calcium ions, as well as radiochemical ions such as 137Cs,57C0, and 85 Sr from waste forms made with Portland Type I cement. Gamma ray spectroscopy is used to detect the radioisotopes, whereas the stable ions are analyzed by atomic absorption spectroscopy or colorimetric tests. In addition, solid samples were analyzed with a SEM and an energy-dispersive X-ray spectrometer, which provided a picture of physical changes and chemical analyses of leachates, respectively, within cement samples, The experimental procedure requires that the leachate solution be regularly changed. To accelerate the leaching process and simulate long time periods, parameters such as leachant volume, temperature, and
specimen size are varied. According to Fuhrmann, for Portland cementbased samples, the temperature can be raised to around 55 O C without changing the mechanism of leaching, (Arrhenius plots of data remain linear over this temperature range.) From measurements of the fraction of material leached in these experiments, it is possible to estimate an effective diffusion constant. The estimate is tested against a computer model program developed by the Brookhaven researchers, and the value for the diffusion coefficient is refined. When data and model agree then some details of how ions escape from the cement are revealed. “Diffusion is the main mechanism,” explains Fuhrmann, “but often there are several diffusion coefficients for the same elements. As a result there are several rates-one on top of another.” Leaching mechanisms can be complex. For instance, in fitting the data on 137Cs with the computer’s prediction, it became apparent that some of the element was being absorbed into the cement sample, possibly within CaCO, regions that form on the cement sample’s surface. Thus, the mechanism for Cs combines diffusion and absorption. The computer model allows researchers to project behavior at long times. “You have to be careful here. Each type of behavior will be element and material specific,” says Fuhrmann. [Copies of the program and a users manual are available from Fuhrmann, Nuclear Waste Research Group, Brookhaven National Laboratory, Upton, NY 11973.1
References (1) Cartledge, F. K. et al. Environ. Sei. Technol. 1990, 24, 867. (2) Ortego, J. D. et al. Environ. Sei. Tech-
nol. 1991, 25,1171. (3) Ivey, D. G. et al. Lewis: Chelsea, MI, in press. (4) Fuhrmann, M.; Colombo, P. In Environmental Aspects of Stabilization and Solidification of Hazardous and Radioactive Wastes; CBt6, P.; Gilliam, T. M., Eds.; ASTM: Philadelphia, PA, 1989. (5) Fuhrrnann, M. et al. Materials Research Society Symposium Proceedings, 1990, 176, 75. (6) Fuhrmann, M. et al. Optimization of the Factors That Accelerate Leaching; National Technical Information Service: Springfield, VA, 1984; BNL52204.
Alan Newman is an associate editor on the Washington editorial staff of ES&T.
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