double activation analysis. the simultaneous use of radiotracers and activable tracers in a stream...

Double Activation Analysis. The Simultaneous Use of Radiotracers and Activable Tracers in a Stream Microcosm

Ronald M. Knaus Nuclear Science Center, Louisiana State University, Baton Rouge, Louisiana 70803

Short-lived radiotracers produced in a research nuclear reactor were used in elemental distribution studies in an outdoor stream microcosm with black willow (Salix nigra Marsh.) roots serving as a bioaccumulator. Obvious limitations of the short-lived radiotracer were overcome by introducing trace amounts of the stable, activable element into the stream along with the radioactive tracer, thereby creating the dual tracer system. After the decay of the radiotracers, sampling from the microcosm was continued. The remaining amount and distribution of the stable, activable tracer was determined by instrumental neutron activation analysis. This combination of techniques supplies immediate short-term information from the radiotracer plus long-term information from the activable tracer, without introducing long-lived radioisotopes into the environment. In two separate experiments using the stable/radioactive pairs, Meg9Mo and La-140La, it was found that tracer experiments could be extended for weeks rather than hours.

Introduction Short-lived radiotracers, whose half-lives are measured in

terms of hours or a few days, used in large-scale field studies have advantages and a marked disadvantage. An advantage is that they yield accurate, short-term information on elemental movements in an ecosystem in less than an hour, if desired. Additionally, because they die out quickly, they are environmentally acceptable in these days of concern over radioactive contamination of the environment. However, most ecological studies follow elements far longer than a short-lived radiotracer will permit. To simply use more radiotracer to make up for its short half-life would call into question its environmental acceptability.

The disadvantage of the short-lived radiotracer can be overcome by the use of activable stable tracers. Stable tracers possess physical properties that make them radioactive upon neutron bombardment in a research nuclear reactor. These stable elements, called activable tracers, can be reduced in concentration only by dilution. Consequently, in studies using properly chosen activable tracers, sampling from the study area and subsequent neutron activation analysis of the samples could conceivably go on for months, years, or even longer. The disadvantage of using activable tracers in the field is the time it takes for a sample to be processed and the data from the sample to be reduced to give the investigator useful information. If the study site is located close to a research reactor, a concentrated effort by at least two personnel plus the reactor personnel are needed to get a series of samples com- pletely analyzed in one working day. In my experience it would be unrealistic for an investigator to expect to have the results from field samples analyzed the same day that they were collected.

The present study demonstrates that the combination of a short-lived radioisotope together with stable isotopes of the same element forms an environmental tracer technique called double activation analysis (2) . In double activation a small quantity of the element being studied is activated by neutrons in a research reactor. The tracer element, now with a small fraction of its atoms radioactive, is introduced into the study

site. The portion of the tracer which is radioactive gives immediate short-term data. The stable remainder of the elements remains in the study area and can be sampled over a long time. The samples can be activated later at the conve- nience of the investigator. If a study is concerned with both the short-term and the long-term dynamics of an activable tracer element in an ecosystem, and the use of long-lived tracers is forbidden, there is no alternative to double activation analysis.

To demonstrate the feasibility of the double activation analysis technique, an artificial stream microcosm was set up simulating natural aquatic conditions to study the sorption by black willow (Salix nigra Marsh.) roots of elements low in abundance in the earth’s crust. The willow roots had been shown previously to sorb high amounts of manganese in the stream microcosm ( I , 2). Additionally, the roots of red alder ( A h u s rubra Bong.) of the Pacific Northwest have been shown as bioaccumulators of indium and dysprosium (3). Characteristically, when growing near a stream, willow and alder plants have exposed roots and rootlets growing directly in the water, presenting a large surface area to the water. Such forms of root growth may be natural, as in the alder, or may result from soil erosion. These plants have been defined as rheophytes, meaning water-current plants (3 ) . The sorptive properties of the tracers by roots and root periphyton were investigated during this study and not the nonconservative qualities of the tracer as emphasized in hydrologic studies reported in IAEA proceedings ( 4 , 5 ) and by Hanson (6).

Materials and Methods Microcosm. The microcosm is an artificial stream bed,

which will be referred to as the stream (Figure 1). It is a 1.3 X 8 m wooden trough covered with resin-treated fiberglass. The walls of the trough are vertical 5 X 20 cm boards. The trough is elevated on a series of sawhorse benches which can be varied in height to regulate stream velocity. Water travels -25 m around 16 baffles (each measuring 90 cm long X 15 cm high) imposed alternately and regularly along the 8-m length of the trough. Three dams a t uniform intervals along the flow path elevate water depth to a maximum of 13 cm. Water is pumped into the stream from Corporation Canal, a natural urban drainage ditch on the Louisiana State University campus in Baton Rouge, LA. Water enters a reservoir a t the top of the stream (Figure 1). The reservoir is made from a sawed-off 55-gal steel drum with a hole drilled in its side near the bot- tom. Excess water and surges from the pump spill over the top of the reservoir and return to Corporation Canal. A constant hydrostatic head is thereby maintained. For this study a flow rate of 5.4 Llmin was maintained by a hydrostatic head of 47 cm above the hole (diameter of 0.9 cm). Other flow rates can be estimated from combinations of hydrostatic heads and hole diameters from tables found in King and Brater (7). The open mode was used in the present study which allowed stream water to spill off the end of the stream and seep into dense vegetation downstream from the pumping station.

Ca. 5 kg of untreated Mississippi River mud was placed behind each of the three dams in the artificial stream. Black willow shoots (40 cm long X 1-20 cm in diameter) cut from tree branches were placed upright in the mud and allowed to grow. After 50 days. willow roots grew out of the mud and into the

0013-936X/81/0915-0809$01.25/0 @ 1981 American Chemical Society Volume 15, Number 7, July 1981 809

TOP VIEW

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Figure 1. Schematic side and top views of the stream microcosm.

flowing water in a manner similar to that observed along the Mississippi River batture. These willow rootlets thus grown and exposed to the stream waters were used in the present study.

Selection and Production of Tracers. Molybdenum (Mol was chosen as a test element because of its ease of neutron activation and detection. Stable 9 8 M ~ has an isotopic abundance of 24.1% and is converted to radioactive 99Mo by the following reaction: 98Mo(n,y)99Mo. Molybdenum-99 (66.0-h half-life) decays to radiotechnetium (99mTc) daughter (6.01-h half-life). The 0.140 MeV y-ray of the decay of 99mTc daughter to 99Tc was counted to give quantitative results for Mo (8). (The 99Tc product has a half-life of 2.13 X 105 yr and decays by the emission of a 0.292-MeV (Emax) p particle, which was below the limits of detection using a Lowbeta I1 (Beckman) counting system.) From a biological viewpoint, Mo is of in- terest because it is the only representative of the 4d transi- tion-metal series that has been shown to be an essential element in the nutrition of cattle (9). MolYbdenum-99 is also a fission product. Information in Table I presents the procedure for radiomolybdenum production.

Stable lanthanum (La), 99.9% 139La, was chosen because of its ease of neutron activation (Table I) and ease of detection of the radioactive 140La produced by the (n,y) reaction. Lanthanum-140 (40.3-h half-life) emits a 0.487-MeV y-ray. Lanthanum was also chosen as a representative of the lan- thanide series of elements (rare earths) because many of the lanthanides are fission products associated with nuclear re- actors and nuclear fuel reprocessing ( I O ) .

Tracer Introduction into the Microcosm. In both experiments, the combined radioactive and stable tracers were dissolved in concentrated nitric acid and made up to a volume of 100 mL. These solutions were introduced into the stream a t station 12 (Figure 1) by a Holter peristaltic pump at a rate of 0.9 mL/min for 1.83 h.

Sampling of Roots and Water. Root and water samples were collected at station 2 (Figure 1) -18 m downstream from

station 12, the point of tracer input. Root samples of similar size and age were collected for the tracer studies. Ca. 0.6-g (wet weight) root samples were placed into 2-dram (1.6 X 5.5 cm) plastic polyethylene vials for direct y-ray counting. After counting, the root samples were dried under heat lamps, weighed, and transferred to 2/5-dram (1.2 X 2.5 cm) polyvials for instrumental neutron activation analysis (INAA). Water samples of 3 mL each were collected simultaneously with the root samples and placed in 2-dram polyvials for y-ray counting. For INAA, 0.5 mL of water was transferred to 2/5- dram polyvials. Background samples of willow roots, stream water, mud, and detritus were collected before tracer introduction and handled in the same manner as described above.

Instrumental Neutron Activation Analysis (INAA). INAA was carried out by using the TRIGA-type reactor located at the Radiation Center, Oregon State University in Corvallis. Standards, background samples, root samples, and water samples were placed in the rotating rack facility for 2 h at 500 kW, which provides a thermal-neutron flux of 1.2 X 10l2 n cm-2 s-l. After 4-h decay was allowed, the standards and the samples were counted on a 13% Ge(Li) detector with 1.9-keV resolution. Counts were accumulated until the standard counting error was less than 5% after background cor- rection. The techniques of neutron activation analysis in the biological sciences are reviewed in an up-to-date paper by Guinn and Hoste ( 1 1 ) .

Results The concentration of Mo in the roots increased from below

the limits of detection (-1 pg of Mo/g) to a maximum value of over 100 pg of Mo/g (dry weight) during the 1.83-h period of Mo introduction into stream waters (Figure 2). Molybde- num in the roots leveled off to a value of -25 pg of Mo/g of root by 23 h for radiotracer determinations but increased to values of near 40 and 60 pg of Mo/g of root for INAA at ap- proximately the same times. When radioactive determinations

x 2 100 I Z 0 [

0

A,O -RADIOTRACER DETERMINATIONS X X - INAA DETERMINATIONS 0

TIME, h

Figure 2. Micrograms of Mo/g of willow root on a dry weight basis (left axis) and micrograms of Mo/mL of stream water simultaneously sampled with a root sample (right axis) vs. log of time in h. At time = 0, Mo concentrations in roots and water were below the limits of detection.

Table 1. Production Parameters of Tracers Used in Stream Microcosm amount of neutron thermal

e I em en t stable radioisotope irradiation neutron irradi- usable -r-ray amount of

facility flu atlon used, MeV

99M0-99mTC TRlGA I1 2 x 101’ 5.5 443 0.140

(half-life, n cm-”s-l tlme, h lrCl

eie- activated, tracer, ment g g h) (location)

Mo 0.1 2.0

La 0.085 (66) (TAMU)a

0.085 ’“La (40.3) GTRR (GIT) 7 x 10” 0.5 582 0.487

* TAMU, Texas A & M University, Nuclear Science Center. GIT, Georgia Institute of Technology, The Neely Nuclear Research Center. Usable yield refers to

activity introduced into stream waters at time = zero, not activity produced at reactor.

810 Environmental Science & Technology

were no longer detected above background counts, an INAA value of 75 pg of Mo/g was recorded but then dropped to less than 10 pg of Mo/g at 96 and 188 h after the termination of Mo input.

Values for Mo in water sampled simultaneously with roots increase from below the limits of detection (0.01 pg of Mo/mL) to a concentration of 0.32 pg of Mo/mL a t the cessation of Mo input. Mo concentrations then fell below the limits of detection by 7 h after the cessation of Mo administration.

Near the end of M o - ~ ~ M o input, Mo was concentrated in the roots over 350 times the level found in the water (119 pg of Mo/g of root divided by 0.3 pg of Mo/mL of water). Before Mo water values decreased and became indistinguishable from background, the concentration in the roots was 3000 times that found in the water (30 pg of Mo/g of root divided by 0.01 wg of Mo/mL of water) 1.7 h after cessation of Mo input.

When one considers that the dried willow root samples ranged from 0.05 to 0.03 g and INAA detection was ca. 1 pg of Mo/g of sample, the data from INAA and radiotracer in roots are in reasonable agreement up to 24 h. Values of 9.4 and 4 pg of Mo/g of root at 98 and 190 h, respectively, were beyond the time for radioactive determinations due to decay of radiomolybdenum.

Lanthanum in roots (Figure 3) increased steadily from below the limits of detection (-1 pg of La/g) at time = 0 to a maximum value of -100 pg of La/g 1 h after cessation of La input. Values for La in roots did not fall below 20 pg/g for radiotracer determinations to 148 h, a t which point radioactive data could no longer be distinguished from background. INAA data are in good agreement with the radiotracer data. Infor- mation obtained from stable La tracer extends the duration of I4OLa radiotracer over $fold to 482 h (20 days).

A La content of 0.18 pg of La/mL in the water was recorded 25 min before the end of La input into stream waters. One hundred thirty minutes after the cessation of La addition, 140La was no longer detectable in the water. At the time of maximum La concentration in the water, 1.3 h of tracer administration, the La was concentrated in the roots 300 times the concentration found in the water (concentration of La in roots (dry weight) divided by La concentration in water). At 2.2 h after cessation of La input, roots contained 15 000 times the La found in the water.

Discussion Double activation analysis provided immediate data by

direct y-ray counting of root and water samples exposed to the radiotracers. Often these early radiotracer results, relayed from the laboratory to the stream by citizen band radio, fig- ured importantly in dictating the size and frequency of future

A.0 -RADIOTRACER DETERMINATIONS X - INAA DETERMINATIONS

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000 g 100 1000

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TIME, h

Flgure 3. Micrograms of La/g of willow root on a dry weight basis (left axis) and micrograms of La/mL of stream water simultaneously sampled with a root sample (right axis) vs. log of time in h. At time = 0, La concentrations in roots and water were below the limits of detection.

samples. These same water and root samples, plus additional samples collected after the decay of the short-lived radiotracer, were activated by neutrons a t a later date. The results from the activated samples were compared with radiotracer findings and found to be in agreement; the samples collected after the decay of the short-lived tracer yielded information beyond the time when the radiotracers had become too diluted and decayed to be useful. It became apparent only upon completion of INAA that additional results could have been obtained had samples been taken over even a longer time period.

Twenty-four times less La tracer was used than Mo because La has activation qualities superior to those of Mo. High isotopic abundance, large neutron cross section, high y-ray in- tensity and detection efficiency, and low spectral interferences favor La over Mo. The highest concentrations of the tracers in the waters of the two experiments showed Mo to be 17 times more concentrated than La. Yet, during and immediately after tracer inputs, the data in Figures 2 and 3 show that both the concentrations of La and Mo in the roots, and the shapes of the elemental adcumulation curves, are similar. This finding indicates that willow roots have an affinity for sorbing La at a rate -20 times higher than that for Mo. At -100 h after introducing the two tracers, Mo had washed out of roots 4 times faster than La. In comparing the two tracers, lanthanum was detected in the roots by INAA 2.5 times longer than Mo. At a flow rate of 5.4 L/min, Mo was Concentrated 350 times over water sampled simultaneously following addition of Mo, and 3000 times after the water had passed over the roots for an additional 1.7 h. Lanthanum in the roots was concentrated 440 times over water sampled simultaneously at the end of La input and over 15 000 times after the water had passed over the roots an additional 2.2 h. Knaus and Curry (I) have shown the potential use of Mn as a dual stable tracer and radiotracer under similar conditions, using the same microcosm as described in this work. In their work, at a flow rate of 10 L/min in which the effluent is reintroduced to the top of the stream (Figure 1, recycle mode), Mn was found to be concentrated in willow roots up to 500 times higher than water sampled simultaneously a t the end of Mn introduction and 105 times higher after the water had recycled 180 h.

A pattern similar to the Mo-La sorption rates was observed in other studies (31, which compared the sorptive rates of indium (an element in the same series as Mo) and dysprosium (a rare-earth element) in roots of wild-growing red alder (Alnus rubra Bong.). The Dy was preferentially sorbed to alder roots by a factor of 15 as compared with In.

The rise in the INAA values for Mo after 15.6-100 h (Figure 2) is not readily explained. The low values for samples a t 98 and 190 h may be partially attributed to dilution of root tissue by rootlet growth during the spring growing season, at which time the present work was carried out.

Data presented in Figure 3 were taken from rootlets collected from station 2 (Figure 1). Additional data, not presented here (12), were taken in duplicate during the La ex- periment from a station only 1.5 m below input station 12. As expected, results showed essentially no lag time for isotope sorbed, but the data also showed less than a 10% increase in maximum and plateau values of La in roots, as reported in Figure 3 for the lower station 2. These unpublished data cor- roborate the La radiotracer findings and indicate that, after stream water travels 18 m farther downstream in the microcosm, the reduction of La concentrations in water was small and sorption to roots was essentially the same a t the two sta- tions.

Future work involving aquatic systems hopefully will yield information to support predictive models for the sorption of technologically enhanced trace heavy elements in natural aquatic systems. The double activation technique used in this

Volume 15, Number 7, July 1981 811

work has been shown to overcome limitations of the use of short-lived radiotracers alone and of activable tracers alone in environmental studies. The use of double activation should lead to better understanding of the movements of heavy, nonconservative trace elements in hydrological and limnolo- gical tracer studies.

Acknowledgment I gratefully acknowledge support and help from R. Kirkland

and M. Davis of the Georgia Institute of Technology, J. Randall of Texas A & M University, C. H. Wang and W. Loveland of Oregon State University, A. H. Fawaris of the LSU Nuclear Science Center, and Nancy Knaus.

Literature Cited (1) Knaus, R. M.; Curry, L. R. Bull. Enuiron. Contam. Toxicol. 1979,

21 I 388.

(2) Curry, L. R. MS Thesis, Louisiana State University, Baton Rouge,

(3) Knaus, R. M.; El-Fawaris, A. H. Enuironm. Exp. Bot., in press. (4) IAEAIsot. Hydrol., Proc. Symp., 1962 1963. (5) IAEAIsot. Hydrol., Proc. Symp., 1966 1967. (6) Hanson, P. J. Doctoral Thesis, Oregon State University, Corvallis,

OR, 1970. (7) King, H. W.; Brater, E. F. “Handbook of Hydraulics; For the

Solution of Hydrostatic and Fluid-Flow Problems,” 5th ed.; McGraw-Hill: New York, 1963; pp 4-1-4-4,4-29.

(8) Arena, V. “Ionizing Radiation and Life”; C. V. Mosby Co.: St. Louis, MO, 1971; p 287.

(9) Underwood, E. J. “Trace Elements in Human and Animal Nu- trition”, 3rd ed.; Academic Press: New York, 1971; pp 116-27.

(10) Nelson, D. J.; Kaye, S. V.; Booth, R. S. River Ecol. Man, Proc. Int. Symp., 1971 1972,367-87.

(11) Guinn, V. P.;Hoste, J. Tech. Rep. 197,Ser.-I.A.E.A. 1980, No. 197,105-40.

(12) Knaus, R. M., Louisiana State University, unpublished data.

Received for review June 10,1980. Accepted March 11,1981.

LA, 1976.

Estimating Equilibrium Adsorption of Organic Compounds on Activated Carbon from Aqueous Solution

Wm. Brian Arbuckle Department of Environmental Engineering, University of Florida, Gainesville, Florida 3261 1

The solvophobic, Polanyi adsorption potential, and net adsorption energy theories have been proposed to correlate a calculated parameter with the equilibrium loading of an organic compound on activated carbon from aqueous solution. In this study, Polanyi and net adsorption energy parameters are calculated and correlated with the Freundlich K constants for nine alcohols, with the resultant correlation line used to predict K values for 13 additional compounds; the solvophobic theory is considered impractical in its present form for this purpose and was not tested. The net adsorption energy con- cept predicts K values better than the Polanyi theory, with both methods performing poorly for compounds having loadings greater than 1 mmol/g. Neither method predicts the proper branched-to-linear alcohol relationship nor predicts isotherms.

Background Activated carbon’s use has increased dramatically during

the past 20 yr, and, with the present concern for synthetic organic chemicals in our drinking water and for toxic and hazardous compounds in industrial wastes, it will increase substantially in the future. These newer applications involve activated carbon’s effectiveness for specific compound re- moval rather than reducing a gross parameter such as COD (chemical oxygen demand) or TOC (total organic carbon); therefore, a specific compound’s absorbability and its inter- action with competing organic compounds are now a major concern. Before competitive adsorption can be evaluated and predicted without performing numerous experiments, a technique is needed to estimate a compound’s ability to adsorb without competing organics present; this would enable a better understanding of the adsorption process and would provide a technique to optimistically estimate carbon usage and process costs before laboratory experiments are performed. Even though adsorption has been used for many years, no theory exists to predict adequately relative adsorbabilities of specific compounds on activated carbon.

Three methods have been proposed to evaluate relative adsorbability: solvophobic theory ( I ) , Polanyi theory (2), and

net adsorption energy (3). This paper uses isotherm data to evaluate whether the methods could be used by engineers to estimate relative adsorbability for specific compounds.

Experimental Section Adsorption isotherms were obtained by adding varied

quantities of pulverized Filtrasorb 400 (Calgon Corp.) to each of 6-10 Erlenmeyer flasks used per compound tested. Several studies with similarly sized compounds have shown little or no effect of carbon particle size upon equilibrium capacities (4-6); a slight effect has been observed with a larger compound (molecular weight of 349, ref 7). Two hundred milliliters of solution containing 500 mg/L of solute, 0.025 M KHzP04, and 0.025 M K2HP04 was added to each flask, stoppers were added, and the flasks were shaken for 2 h at 160 oscillations per minute on a gyratory shaker. For the small compounds and the small ads’orbent particle size used, this should have been sufficient to attain equilibrium. The samples were filtered through 0.45-pm filter paper and analyzed for total organic carbon, with solute concentration calculated from the TOC responses of standard solute solutions. (These data were collected by R. J. Romagnoli at Union Carbide’s South Charleston Research and Development Center, and most were reported in ref 8.)

Results and Discussion . -

The Freundlich isotherm represents the data:

X = KClIn (1)

with K and n being empirical constants; X is the solute loading on the adsorbent in mmol/g and C is the equilibrium solution concentration in mmol/L. A linear regression of

In X = In K + ( U n ) In C (2)

was used to obtain the constants, and a correlation coefficient calculated for the concentration range evaluated (Table I). The Freundlich K constants increase within each family of compounds with increasing molecular weight, as expected, as is the lower loading of the branched isomers compared to the linear ones. The ethanol and acetaldehyde K values may in-

812 Environmental Science & Technology 00 13-936X/8 1 /09 15-08 12$0 1.25/0 @ 198 1 American Chemical Society

double activation analysis. the simultaneous use of radiotracers and activable tracers in a stream...

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