a22 i t 's cracked up to be? What could be a sirnpler solutioii to the disposal of hazardous ( :hemids and waste radioactive materials than t o imcasc them i n wmcnt? l ' ~ t the practice is far from simple. "Arc hazardous constituents immobi- lized?" asks Frank Curtledge of Lou- isiana State Universitv. "And how permanmt arc! the matrix strii(:tiirv and its propertics under cnviron- mental stress?" . i t the August ACS meeting in New Ynrk Citv, scicn- tists discussed these questions a i d how to probe the changes that o(:(.ur within rrments.

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 ex- amine the degree ( 1 1 internal p d v - nierization of sili(:~rn-~ixvgen ti(inds in cemvnts mixcd with various chemicals. For example, they find that adding Pblll) slows the setting of the concrete initially but with time prumotes ~ io lymer iza t io t~ , whereas Cr(1ll) retards polym(~riza- tion. pnssiblv bv suhstituting for sil- icon 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 hnnd- ing to those elrmrnts. For instance. the chemical shifts of peaks arising from "'Si in silicatcs reflects thc dc- grce of cross-linking (Le.. the num- ber of Si-C-Si boiids). Thus NMR differentiates tietween the un(:r(isi- linked silicate ion Si(0l4'- and the series (O),Si-O-Si(O):,, . . Si(0- Si),-ahhreviated as Q", Q'

Ac(:ording to Cartledge. cement begins as priniarilv Q". but after 28 days forms a mixture of Q". Q', and Q'. 'l'lie presenw of Q' can tic em.

By Alan Newmon

pirically correlated to the cement's compression strength.

On the other hand , chemical shifts for "A1 NMR reflect tetrahe- dral versus octahedral geometry. NMR studies of cement show that octahedral aluminum begins form- ing one day after the material is mixed.

"A key goal of this research is to model the long4erm effects

of encapsulating hazardous wastes in

cement"

In another experiment, Cart- ledge's group added phenol to ordi- nary Portland cement (OPC) to ex- amine the chemical's leachability. Using 13C NMR, they observed that after 28 days all the phenol had ion- ized to calcium phenoxide. They also found evidence that phenol was dissolved in water trapped in- side cement pores.

Cartledge's group has even looked at cement entrapment of an experi- mental "sludge" containing ions

such as mercury, nickel, and cadmi- um combined with lime. In contact with air, a complex precipitate of hydroxide and carbonate salts forms, and this appears to be mi- croencapsulated within the cement matrix.

Electron microscopy A more complex assortment of

tools for probing cements was pre- sented by Maria Neuwirth from Canada's Alberta Environmental Centre. She compared data from scanning electron microscopy (SEMI with information available from transmission electron micros- copy (TEM) or scanning transmis- sion electron microscopy (STEM] (3). In general, SEM allows mapping of morphological changes in the ce- ment, with an image resolution on the order of 5 nanometers, whereas TEM/STEM provides localized, crystallographic, and chemical in- formation at image resolutions of less than 0.5 nanometer.

These microscopy techniques can be expanded to include other ana- lytical techniques. For instance, the addition of a backscattered electron detector on a scanning electron mi- croscope allows detection of re- gions 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 concen- tration profiles that provide some insight into mechanisms of leach- ing.

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

42 Environ. Sci. Technol., Voi. 26, No. 1, 1992 0013-936W92/0926-42$02.50/0 0 1991 American Chemical Society

thin, foil-like samples are required, the data can be quantified. Light el- ements can also be detected by mea- suring the energy loss of transmit- ted electrons (electron energy loss spectroscopy). Finally, crystallo- graphic lattice parameters and ori- entation for small grains or precipi- tates within the cement can be determined by electron diffraction methods (analogous to X-ray dif- fraction).

To prepare samples for SEM stud- ies, the cement is fractured and coated wi th 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 trans- parent . Neuwir th and her col- leagues have developed what they describe as a simple and reproduc- ible method to prepare cement sam- ples 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 chro- mium 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 en- capsulating hazardous wastes in ce- ment. Toward that end, researchers at Brookhaven National Laboratory have developed an accelerated leach test that, in combination with a computer model, could predict leach rates over time (4-6).

“Most of the work focuses on low- level radioactive waste disposal,” explains geochemist Mark Fuhr- mann. Experiments have followed the loss of sodium, potassium, and calcium ions, as well as radiochem- ical ions such as 137Cs, 57C0, and

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 absorp- tion spectroscopy or colorimetric tests. In addition, solid samples were analyzed with a SEM and an energy-dispersive X-ray spectrome- ter, which provided a picture of physical changes and chemical analyses of leachates, respectively, within cement samples,

The experimental procedure re- quires that the leachate solution be regularly changed. To accelerate the leaching process and simulate long time periods, parameters such as leachant volume, temperature, and

85

specimen size are varied. According to Fuhrmann, for Portland cement- based samples, the temperature can be raised to around 55 O C without changing the mechanism of leach- ing, (Arrhenius plots of data remain linear over this temperature range.)

From measurements of the frac- tion of material leached in these ex- periments, it is possible to estimate an effective diffusion constant. The estimate is tested against a comput- er 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. “Dif- fusion is the main mechanism,” ex- plains Fuhrmann, “but often there are several diffusion coefficients for the same elements. As a result there are several rates-one on top of an- other.”

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 ab- sorbed into the cement sample, pos- sibly within CaCO, regions that form on the cement sample’s sur- face. Thus, the mechanism for Cs combines diffusion and absorption.

The computer model allows re- searchers to project behavior at long times. “You have to be careful here. Each type of behavior will be ele- ment and material specific,” says Fuhrmann. [Copies of the program and a users manual are available from Fuhrmann, Nuclear Waste Re- search 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 Envi-

ronmental Aspects of Stabilization and Solidification of Hazardous and Radioactive Wastes; CBt6, P.; Gilliam, T. M., Eds.; ASTM: Philadelphia, PA, 1989. Fuhrrnann, M. et al. Materials Re- search Society Symposium Proceed- ings, 1990, 176, 75. Fuhrmann, M. et al. Optimization of the Factors That Accelerate Leaching; National Technical Information Ser- vice: Springfield, VA, 1984; BNL- 52204.

(5)

(6)

Alan Newman is an associate editor on the Washington editorial staff of ES&T.

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