The Science of the Total Environment, 19 (1981) 277--284 277 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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FACTORS INFLUENCING THE VOLATILIZATION OF SELENIUM FROM SOIL

R. ZIEVE and P. J. PETERSON

Department of Botany and Biochemistry, Westfield College, University of London, Kidderpore A venue, London NW3 7ST (United Kingdom)

(Received March 4th, 1981; accepted March 13th, 1981)

ABSTRACT

The volatilization of selenium-75 from a soil amended with sodium selenite was inves- tigated under laboratory conditions and shown to be dependent upon microbiological activity, temperature, moisture, time, concentration of water~oluble selenium and season of the year when the soil sample was collected. Soil collected in the Spring and assayed under standard conditions, evolved substantially more selenium than that collected in the Summer, Autumn or Winter. Selenium volatilization decreased substantially within 10 days of isotope application and was correlated with the decrease in the water-soluble fraction.

Estimates of the low-temperature volatilization of selenium from soil indicate, that this source could approach the calculated value for anthropogenic release which emphasises the importance of microbiological transformations of selenium in the cycling of this ele- ment.

INTRODUCTION

Experiments with some plants, fungi, bacteria and rats have demonstrated their ability to synthesize volatile selenium compounds from inorganic sele- nium salts, or from several organo-selenium compounds (Lewis et al., 1966; Fleming and Alexander, 1972; Cox and Alexander, 1974). The volatilization of selenium from soils amended with organic or inorganic selenium has also been established (Abu-Erreish et al., 1968; Francis et al., 1974; Doran and Alexander, 1977; Reamer and Zoller, 1980). The volatile compounds were demonstrated to be predominantly dimethylselenide (DMSe) (Francis et al., 1974) although dimethyldiselenide (DMDSe) and dimethylselenone or methylmethylselenite have been reported (Reamer and Zoller, 1980}. It has been suggested that the volatilization of selenium from soil is largely or entirely a microbiological process (Abu-Erreish et al., 1968). Indeed, fungi

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belonging to several genera and at least one bacterium grown in culture are able to synthesize DMSe from inorganic selenium compounds (Challenger, 1945; Challenger and Charlton, 1947; Fleming and Alexander, 1972; Cox and Alexander, 1974; Barkes and Fleming, 1974).

Since selenium is highly susceptible to biomethylation, this reaction may be an important step in its transformation and ultimate transport in the environment. ' Our investigations were carried out to ascertain the quanti ty that could be released to the atmosphere from a soil and to determine some of the factors affecting it. The results are discussed in terms of the calculated anthropogenic input of selenium into the atmosphere and the significance of low-temperature volatilization in the global cycling of this element.

MATERIALS AND METHODS

A loam soil, whose relevant physical and chemical properties are listed in Table 1, was used throughout. Soil from the upper 10 cm was collected 10 days before each experiment, air dried and sieved through a 2-ram mesh.

Selenium-75 as sodium selenite was used to amend the soil; 5 pg of sel- enium and 5 pCi of radioactivity in 7 ml water (unless otherwise stated) were added to each replicate of 50 g air dried soil contained in 250-ml Q-fit conical flasks. The bottles were sealed with glass stoppers, combined with a vertical tubular inlet, while the side arm was used as the outlet. Air flushed through the bottles twice daily for 1 h at the rate of 5 ml min-I was passed through a 10-ml concentrated nitric acid trap. These traps, designed to fit a well-crystal scintillation counter, were assayed daily for radioactivity.

The flasks with their contents were weighed daily and the amount of water lost through evaporation was replaced as necessary. All experiments were conducted at 22°C unless otherwise stated and samples were triplicated throughout.

Moisture analysis was made by drying 2-g soil samples at 105°C for 16 h. Determinations of the water saturation percentage were carried ou t by the procedure as described by the U.S. Dept. Agric. (1954).

Total soil selenium was measured using the 2,3-diaminonaphthalene fluori- metric procedure of Hall and Gupta (1969). To measure water-soluble sel-

TABLE 1 SOME PHYSICAL AND CHEMICAL PROPERTIES OF THE EXPERIMENTAL LOAM SOIL

Mechanical separation

PH Organic Sand (%) Clay (%) Silt (%) Total matter (%) Se

(ppm)

7.4 9 56 17.5 17.5 5

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enium, 100 ml water was added to the flasks containing the 50 g soil and shaken for 2 h. The slurry was filtered through a Whatman 42 filter paper and the radioactivity measured in aliquots.

Autoclaving, where used, was carried out at 151b pressure, 121°C for 15 min.

RESULTS AND DISCUSSION

The data from this s tudy shows that selenium was volatilized from soil amended with sodium selenite and that the amount was dependent on several factors. Experiments with autoclaved soil conclusively show that no volatilization takes place thus indicating the essential role of microorganisms in this process.

The effect o f soil moisture on the volatilization of selenium-75 is shown in Fig. 1. No volatile selenium was determined above the air-dried soil again indicating the microbiological nature of the process. The rate of evolution of volatile selenium was highest at the 28% level, with the 16 and 40% levels evolving substantially less selenium. The highest moisture level used (40%) was below saturation but was approximately 75% of field-capacity. Similar results have been obtained by Abu-Erreish et al. (1968) using a heavy clay soil from S. Dakota.

The graphs in Fig. 1 show that irrespective of the degree of saturation, the rate of volatilization is highest at the commencement of the experiment and slows down with time. To examine whether the decreasing rates of evolution could be correlated with the water-soluble selenium, i.e. selenium available

0.30

2 8 % H ~

(D •

~ 010

D r y so i l 0 . 0 0 . ; " -- -" • • '

I I l I 5 10 15 2 0

T i m e ( d a y s )

Fig. 1. Effect of soil moisture o n t h e e v o l u t i o n of Se-75 from soi l

280

to the microorganism, as it comes to equilibrium with the "natural" soil sel- enium, an experiment was set up with replicate flasks for analysis. Flasks were taken at 0, 1, 7, and 17 days to determine the soluble selenium levels for comparison with the rates of selenium-75 evolution. The results (Fig. 2) show that the evolution of selenium and the decrease in water-soluble sel- enium are correlated but with a lag of several days between the two processes. This could be due to the prior incorporation of soluble selenium into the microorganisms indicating that they take several days to metabolize this ele- ment. Changes in the rate of volatilization with time have previously been noted by other workers for selenium (Hamdy and Gissel-Nielsen, 1976; Doran and Alexander, 1977) and for mercury (Rogers and McFarlane, 1979).

Under aerobic conditions many fungal species in pure culture are capable of biosynthesizing DMSe (Challenger and North, 1934; Challenger and Charlton, 1947; Challenger; 1945), and some authors have stated that they could not detect bacterial methylat ion (Barkes and Fleming, 1974). Never- theless, several bacterial species have now been shown to methylate selenium in culture (Doran and Alenxander, 1977). To test whether bacteria and/or fungi are responsible for the volatilization of selenium in soft, an experiment was set up under constant conditions but with chloramphenicol at two con- centrations being watered into the soil along with the radioactive selenium. The results shown in Table 2 indicate that the rate of volatilization with chloramphenicol present was approximately one half the normal rate which suggests that bacteria play an important role in this process in soil.

In a further experiment to examine the effects of microorganisms on volatilization, Candida humicola was grown in culture and aliquots containing 2.8 x 107 cells were watered into the soil. The organism was isolated in this

(b) 0-30 •

\ / o~ ° 1 5 i ~

0'10

(a}

o o i/ I I i 5 lO 15

Time (days)

100

90

so ~,

-40 ~

10

Fig. 2. The r e l a t i o n s h i p b e t w e e n t h e w a t e r - s o l u b l e s e l e n i u m a n d t h e e v o l u t i o n of Se-75 from so i l .

281

TABLE 2 THE EFFECT OF CHLORAMPHENICOL ADDITION ON Se-75 EVOLUTION FROM SOIL OVER 4-DAY PERIOD UNDER STANDARD CONDITIONS

Chloramphenicol addition Percent Selenium-75 evolved

None 0.032 100mg1-1 0.019 200 mg 1 -I 0.015

laboratory (Ramadan, 1980) and shown by g.l.c./m.s, to synthesize DMSe only (Zieve and Peterson, unpublished). The rate of selenium evolution approximately doubled with addition of the yeast {Fig. 3) but the rate of volatilization again decreased with time as for the control.

As a biological process, the volatilization of selenium is temperature dependent {Table 3). The total amount of selenium evolved after four days at 20°C was more than three times that at 10°C and more than fifteen times that at 4°C. Transferring soils from 20 to 4°C slows down the process markedly, while transferring from 4 to 20°C enhances the volatilization im- mediately.

The season of the year has an effect on the capability of the soil micro- organisms to volatilize selenium. As can be seen from Fig. 4, more selenium could be volatilized from soil collected in March, than from soil collected in June or September. Results obtained in March 1978 were similar to those of March 1979. As the soil was incubated under constant conditions the differ- ences in rate are probably due to differences in populations of the various microorganisms during the year which are involved in the methylat ion pro-

0 30 Soil + C. humicola

o. o

i o.Io ' " -

i I [ [ 5 10 15 20

Time (days)

Fig. 3. Effect of Candida Humicola addition to soil, o n e v o l u t i o n of Se-75 from soil.

2 8 2

TABLE 3 THE EFFECT OF TEMPERATURE ON Se-75 EVOLUTION FROM SOIL

Treatment First period Second period Third period 4 days 3 days 3 days

Temp. (°C) % evolved Temp. (°C) % evolved Temp. (°C) % evolved

1 4 0.004 20 0.021 2 10 0.018 20 0.024 3 20 0.062 20 0.014

_ i

20 0.010 4 0.002

21,3.79

"~ 0.20

e~ 21.6-79 29.12.79

0.10

219,80

I I I [ 5 10 15 20

Time (days)

Fig. 4. Effect of sampling during the year on evolution of 8e-75 from soil.

cess. These results could explain the findings of McDonald and Duncan {1979) when they recorded higher levels of atmospheric selenium in March when compared with the rest of the year. This contrasts with other elements where the atmospheric levels were generally higher in the Winter.

Hamdy and Gissel-Nielsen (1976) were able to show that the rate of evol- ut ion of selenium changes with the drying and re-wetting of the soil. This, along with the factors reported in the present study, demonstrates that the amount of selenium which is released into the atmosphere depends on many factors such as time of the year, temperature, rainfall, type of soil and avil- ability of the elements.

In estimating the output of selenium into the atmosphere from soil on a global scale, the following values were calculated from the data. If the maxi- mum Spring rate of selenium volatilization of 0.35 x 10 -a pg day -1 cm: was maintained throughout the year, then 172 x 10Sg selenium year -1 would have been released to the atmosphere taking the surface area of land less ice to be 1.33 x 101~ cm: (Garrels et al., 1975). The lower rate of volatilization during the autumn would result in an annual input into the atmosphere of

283

24 x 10 s g se len ium y e a r -1 . Mackenz ie e t al. (1979) have ca lcu la ted t h a t the a n t h r o p o g e n i c e m i s s i o n o f se len ium f r o m fossil fuel c o m b u s t i o n and the roas t ing o f su lphide ores, c o n t r i b u t e s 119 .8 x l 0 s g se len ium yea r -1 to the a t m o s p h e r e . Our da t a indica tes t h a t the a m o u n t o f mic rob io log ica l volat i l iz- a t ion o f se len ium cou ld a p p r o a c h the a n t h r o p o g e n i c f lux data .

D a t a s u p p o r t i n g the i m p o r t a n c e o f biological m e t h y l a t i o n of se len ium in the cyc l ing o f this e l e m e n t c o m e s f r o m the w o r k o f L~g and Ste innes (1978) . Dur ing a regional geochemica l survey o f t race e l emen t s in N o r w a y , t h e y d i scovered t h a t the se len ium c o n c e n t r a t i o n in the h u m u s layers o f fo res t softs dec reased wi th increas ing d is tance f r o m the coas t and was shown to be highly s igni f icant ly co r r e l a t ed wi th the annual rainfall data. Thus m u c h o f the e l e m e n t was supp l ied to the soils t h rough p rec ip i t a t ion . T h e y c o n c l u d e d t h a t a n t h r o p o g e n i c se len ium was n o t the m a j o r source o f the a t m o s p h e r i c in- p u t and t h a t na tu ra l m e c h a n i s m s m u s t be respons ib le fo r this process .

ACKNOWLEDGEMENT

We wish to a c k n o w l e d g e the s u p p o r t o f the Agr icul tura l Research Counci l fo r f inancia l ass is tance wi th this work .

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