Leachability of Neutron Irradiated Fly Ash W. D. James,* Morteza Janghorbani,' and Terry Baxter Environmental Trace Substances Research Center, University of Missouri, Columbia, Missouri 6520 7
Leachablllty of neutron irradiated fly ash Is investlgated as a method for studying element leaching properties of this materlal wtth varlous leaching medla. Quantttatlve aspects of radlatlon damage are shown to be minor. Therefore, the technlque should be appllcable for this purpose. I t Is further shown that this technlque affords slgnlflcant advantages over the conventlonal post-leachlng analysis wlth respect to utlllzatlon of natural solutions for the lnvestlgatlon of fly ash leachablllty. However, for a few elements such as Pb, BI, V, and AI, the varlous nuclear parameters Involved lnvalldate appllcablllty of the present technlque.
With the dwindling supplies of petroleum and natural gas as basic raw materials for energy generation, coal is most likely t o be in increasing demand as a principal resource in the immediate future. Although estimates vary somewhat, Brackett ( 1 ) has predicted an annual consumption rate of 350 million tons by 1980 for conventional generation of electricity alone. This consumption will result in production of 40 miUion tons of fly ash per year which will pose a major disposal problem. To date, the only viable disposal methods are land fill and mine refill (2). The current practice of land fill and mine refill as primary disposal methods, together with an increasing rate of production, pose a potentially serious problem with respect to its leachability under natural conditions. Theis ( 3 ) and Holland e t al. ( 4 ) have studied the problem by equilibrating fly ash with either distilled water or millimolar solutions of EDTA on a mechanical shaker, filtering the aqueous phase, and analyzing it for a selected number of trace elements utilizing atomic absorption spectrometry as well as other methods. Conventional trace analysis of the leachates as applied to date t o this problem suffers from several major limitations. First, use of leaching media is limited to solutions not posing significant background problems. Therefore, acid mine drainage water or soil extract solutions cannot be employed as leaching media. Second, detailed evaluation of various parameters affecting the extent of leaching, such as nature of fly ash and its lime content, time-dependence of leaching, etc., usually generates an inordinately large number of samples t o be analyzed. And finally, the use of most trace analytical techniques imposes the major constraint of the necessity of determining likely constituents of the matrix before analysis, resulting in the possibility of overlooking important unsuspected elements. Leaching experiments performed on fly ash samples irradiated under appropriate fluence conditions inside a nuclear reactor permit not only utilization of natural leaching media without regard to contamination or background problems, but also significantly reduce the amount of effort required for leachate quantification since the only post-leaching work required is high-resolution gamma-spectrometry. Furthermore, where applicable, the technique is truly multielement Present address, M.I.T. Nuclear Reactor Laboratory, 138 Albany Street, Cambridge, Mass. 02139. 1994
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
in nature eliminating the need for preconception of element(s) to be investigated. Also, continuous monitoring of the leachate in an automated system becomes a practical possibility. The major question with respect to applicability of such a technique to leaching experiments is the extent of radiation damage affecting leaching properties of the material. If this is shown to be minor within the acceptable accuracy limits required, the technique can make a significant contribution to our understanding of leaching properties of this material under conditions similar to those encountered in nature. It will naturally not be applicable to elements such as P b and Bi, whose nuclear properties are not favorable for production of gamma-emitting radionuclides with sufficient sensitivity by thermal neutron capture or to those whose activation products possess half-lives which are very short relative to the time scale of the experiment (e.g., A1 and V). Nevertheless, the majority of elements can potentially be studied with this technique. In this communication, we report results of feasibility studies with emphasis on investigation of the extent of radiation damage affecting leaching properties of fly ash.
BACKGROUND If fly ash is irradiated with thermal neutrons, many of its constituent elements are transformed into radioactive isotopes some of which are gamma-ray emitters possessing sufficiently long half-lives to be useful for post-leaching spectrometry. Of the elements most commonly studied in environmental research, all but P b fulfill the requirements for this work, as seen from data of Table I. In addition to these elements, there are many others with favorable nuclear properties for this purpose. There are generally two types of radiation damage which might affect leaching properties of such materials. (a) General gamma and other radiation damage, altering structural properties of the material. For instance, it is likely that some compound within the matrix is sufficiently labile to radiation flux or ensuing temperature rise during irradiation so that a quantitatively significant chemical transformation takes place. A somewhat related effect that might affect leaching behavior of some elements is the extent of chemical reduction to lower oxidation state(s) due to the reducing atmosphere of the nuclear reactor. For instance, Figure 1 presents data on the observed extraction behavior of neutron compared with the expected behavior of As3+ irradiated h205 and As5+. The expected behavior curves were generated by post-extraction neutron activation analysis. Separate extractions were performed with varying shake times with aliquots of the organic phases being analyzed. These data clearly show that under these particular experimental conditions, a significant fraction of induced activity was present as As". (b) Specific recoil effects. The process of thermal neutron capture leading to production of radionuclides of interest is usually followed by emission of high-energy gamma-radiation (prompt gamma-rays). These photons generally possess energies of a few MeV. Emission of such energetic gamma-radiation leads to recoil of the activated radionuclide with recoil energies (>10 eV) exceeding those of chemical bonds (1-5 eV). This may result in the radionuclide recoiling out of its original lattice structure and, depending on subsequent
Nuclear Properties of Some Elements Pertinent to Leaching Experimentation Detection EPAb r-ray limita in water Radionuclide energy (KeV) leachate, standards, Ele men t of interest and half life Pg PPm 0.001 889, 84d sc 0.05 0.005 320, 28d cr 846, 2.6h Mn 1.0 1099, 45d Fe 0.00003 1173, 5 . 3 ~ co 0.00001 1.0 511, 12.8h cu 5.0 1115, 244d Zn 0.008 440, 14h 0.00002 0.1 559, 26h As 0.004 0.01 265, 120d se 777, 35h Br 0.0005 140, 67h Mo 0.01 0.0004 530, 54h Cd 0.0007 603, 60d Sb 159, 117d Te 0.003 364, 8d 0.002 0.002 279, 47d Hg 312, 27d Th 106, 2.4d V Data from Data from H. P. Yule, Anal. Chem., 37, 129 (1965). a 10-h irradiation at 4.5 X 1013n cm-2 s - l , t d = 0 . Table 11-1 (Ref. 4). -
Table I.
1
neutron-annealing), as well as the oxidation state of parent atoms in the matrix (5). The net effects associated with the above general mechanisms cannot be predicted reliably for a complex matrix and may not necessarily be quantitatively significant. They must be determined for each element of interest in the specific matrix involved experimentally. Two general methods can be applied t o quantitative evaluation of these irradiation effects. First, leaching properties of a preirradiated column may be quantitatively compared with those of an unirradiated matrix by postleaching analysis of leachates using any number of trace analysis methods. These types of experiments yield information only with respect t o the gross alterations of leaching properties, but do not provide any insight into the recoil-effect which may be potentially the most significant. Second, comparison of induced activities present in a leachate from an irradiated column before and after a similar re-irradiation of the leachate itself will yield quantitative data on the extent of the recoil effect.
-9
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EXPERIMENTAL
*
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Apparatus. Columns were prepared from vertically held disposable pipets containing a small glass wool plug and by lightly packing the irradiated material. The high resolution y-spectrometry system used included a detector (ORTEC VIP, resolution 2.2 keV, efficiency 10%) and the multichannel analyzer (ND 4410, 4096 channels). Materials. As fly ash representing the broad spectrum of the industry waste product, we have evaluated the NBS SRM 1633 sample which is a blend of coal fly ashes supplied by five electric power plants (TVA, Stevenson, Ala.; Commonwealth Edison, Chicago, Ill.; Baltimore Gas and Electric Co., Baltimore, Md.; Caroline Light and Power Co., Boxboro, N.C.; and Potomac Electric Power Co., Washington, D.C ). This material has a particle size range below 88 gm (170 mesh). Elemental analyses of this material are reported elsewhere ( 6 ) . Each column was eluted with one of the following solutions: deionized water (pH 5.6),1 X M EDTA solution (pH 4.8), acid-mine drainage water (pH 3.1), or soil extract (pH 7.7). The acid-mine drainage water was obtained from a nearby abandoned coal pit and was filtered prior to use. The soil extract was prepared by shaking a 5:l water-soil weight ratio of the top soil from a wooded area near Columbia, Mo., intermittently for 24 h and filtering the resulting solution through Whatman 40 frlter medium. Procedure. Seven hundred milligram portions of fly ash were weighed in precleaned Suprasil quartz irradiation vials. The ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
1995
Table 11. Comparing Induced Activities in the Same Leachate before and after the Second Irradiation u e m e nt and 7-ray energy, keV -1
IS1Ba, 216 496 7sSe, 265 279 400 51Cr, 320 134cs, 795 8SSr, 514 '24Sb, 603 1692
Decay corrected counts/500 s Beforea Afterb,c 1 0 333 7 981 1 9 576 8 189 5 298 144770 685 36681 16423 2 427
8 755 6 308 20360 8 209 5 872 152801 682 42662 46830 6 960
Table IV. Element Concentrations in Aliquot Number One of Deionized Water Leaching Medium
-
70
Differenced - 15 - 21 f 4
t0.2 +11
+6 0
+ 16 + 185 + 187
$ , t , = 1.76 x 1 O I 9 n cm-2. & t 2 = 2.07 x l o L 9 These data have been corrected for residual % difference = activity from the first irradiation. (counts after - counts beforelicounts before. a
n cm-'.
Element As Ba Br
Concn, ppm
