Sorption of dimethylselenide by soils

Soil Eiol. Biochem.Vol. 17, No. I, pp. 105-107, 1985 Printed in Great Britain. All rights reserved

SORPTION

0038s0717/85 $3.00 + 0.00 Copyright 0 1985 Pergamon Press Ltd

OF DIMETHYLSELENIDE

BY SOILS

R. ZIEVE and P. J. PETERSON Department

of Biological

Sciences, Chelsea College, University of London, Hortensia Road, London SW10 OQR, U.K. (Accepted 20 June 1984)

Summary-Air-dry and moist soils were shown to possess the capacity to sorb substantial amounts of (‘?3e)dimethylselenide produced by the yeast Candida humicola in culture, or by soil supplied with (“Se)selenite, depending largely upon the organic matter content and selenium concentration of the soils. The sorption capacities of individual soil constituents followed the order; organic matter > clay minerals > manganese oxides z iron oxides > acid-washed sand. A chemical fractionation procedure applied to soils fumigated with (‘SSe)dimethylselenide revealed that the majority of the selenium sorbed was converted after 1 month to other forms, extractable mainly with strong acid solutions. Experiments with sterilized (autoclaved and y-irradiated) soils indicated that soil microorganisms played little, if any, part in the sorption process. The work reported here indicates that soil is an important natural “sink” for atmospheric dimethylselenide.

Our aim was to assess the capacity of wet and air-dry soils to sorb DMSe.

INTRODUCTION

The emission of volatile selenium compounds into the atmosphere from natural and anthropogenic sources has been established, but the environmental fate of these compounds is largely unkown (Mackenzie et al., 1979). The volatile compounds released by microorganisms (Francis et al., 1974) and non-accumulator selenium plants were demonstrated to be predominantly dimethylselenide (DMSe). The extent of volatilization in both laboratory-based and glasshouse experiments has been shown to be dependent upon microbiological activity, temperature, moisture, time, concentration of water-soluble selenium and season of the year (Zieve and Peterson, 1981, 1984). Recent direct measurements of selenium compounds in the atmosphere confirm the presence of DMSe at concentrations of up to 2.4ngm-3 (Jaing et al., 1983). Soil is an important natural “sink” for gaseous atmospheric pollutants, as has been established for nitrogen dioxide (Bremner and Nelson, 1967; Abeles et al., 1971) ethylene (Abeles et al., 1971) carbon monoxide (Inman and Ingersoll, 1971) and phosphine (Burford and Bremner, 1972) as well as several sulphur gases (Bremner and Banwart, 1976). Doran (1982) isolated four soil bacteria which apparently could utilize DMSe as their sole source of carbon. Although the selenium-containing products generated from DMSe and DMDSe (dimethyldiselenide) were not identified, it is feasible to suppose that some of the selenium would be retained in cellular material and eventually recycled.

MATERIALS AND METHODS

Soils

The soils used (Table 1) were surface samples (0-1Ocm) which differed markedly with respect to their organic matter content and total selenium. The low-selenium Compton soil and the high-selenium County Meath soil have been reported to give rise to selenium-deficiency and selenium-toxicity symptoms in grazing ruminants respectively (J. Allen, personal communication; Fleming and Walsh, 1957). Soil from the London site provides adequate selenium for animal health. Each sample was initially air-dried and crushed (< 2 mm). The pH was determined using a glass electrode (soil:water ratio, 1:2.5) and particle size distribution was determined by using the pipette method (Smith and Atkinson, 1975). Total soil selenium was measured using the 2,3-diaminonaphthalene fluorimetric procedure of Hall and Gupta (1969). Soils were sterilized by two techniques: (a) autoclaving carried out three times on three successive days at 120°C for 15 min; and (b) radiation using a %o source at 1 Mrad h-‘. The total radiation dose the soil received was 2.5 Mrad during which time the temperature of the soil was raised to approximately 35°C. Soil sterility was checked simultaneously with the setting up of each experiment. A soil slurry was

Table 1. Some ohvsical and chemical orooerties of the exoerimental soils Mechanical analysis Site of sampling Westfield College, London Compton, Berkshire County Meath, Ireland

PH 7.4 7.2 7.3

Organic matter %

Sand (Z-O.02 mm)

9 13 29

56 41 33 105

Silt (0.02-0.002 mm) 17.5 28 24

Clay ( < 0.002 mm) 17.5 18 14

Total Se (pgg-’ dry Wt) 5.0 0.3 91.0

R. ZIEVE and

106

J. PETERSON

extraction times are shown in Table 4. A 2g subsample was placed in a centrifuge tube that fitted a well-type crystal scintillation counter and the total radioactivity measured. Then 20 ml of the first extractant were added and equilibrated with the soil with end-over-end shaking. Samples were then centrifuged at 700 g for 10 min, the supernatant removed, and the remaining radioactivity was re-measured. The next solvent was added and the extraction procedure repeated.

Valve

DMSe

Fig.

P.

1.

generator

Se collector

Apparatus used in the study (‘%e)DMSe by soils.

HNO,

of sorption of

prepared by adding sterile water to the sterilized soil and, following inoculation of nutrient agar, was incubated at 25 and 37°C for 30 days. No colony formation was observed. Soil constituents

Sorption of (75Se)DMSe by the following major soil components was investigated. (a) organic matter-Fen peat was used in preference to material extracted from soil using conventional reagents so as to avoid artifacts produced during extraction. minerals-Fuller’s earth (Montclay (b) morillonite) powder. (c) metal oxides precipitated as a surface layer on acid washed sand; (i) iron oxide-by addition of 5% NaOH to 5% ferric chloride solution. (ii) manganese oxide-by slow addition of H,02 (20% w/v) to 5% solution of potassium permanganate. Experimental

procedures

The apparatus used to study DMSe sorption by soils is shown in Fig. 1. The soil selenium collector and the nitric acid trap were designed to fit a welltype crystal scintillation counter (Nuclear Enterprise Model 663C). Air, supplemented with (“Se)DMSe, was passed through 5 g soil contained in the soil selenium collector and then through a 10 ml concentrated nitric acid trap twice daily for 1 h at the rate of lOmlmin_’ for 6-7 days. To avoid diffusion of gases to, or from the experimental flasks, selenium collectors or nitric acid traps, solenoid valves and glass taps were used. (75Se)DMSe was obtained as the volatile product released either from Candida humicola (Zieve and Peterson, 1981) growing in the presence of sodium (“Se)selenite (The Radiochemical Centre, Amersham) or from 75Se-spiked London loam soil. C. humicola was grown in 100 ml nutrient broth (Oxoid) containing 0.25% sodium selenite and 25 PCi Se. This procedure gave rise to concentration of DMSe of 18-370 ng Se mm3 and 60&2000 p g Se mm3 for soil and C. humicola generated material, respectively. Solvent extractions

A sequential solvent-extraction procedure applied to the soils. The order, concentration

was and

RESULTS AND DISCUSSION

Adsorption of 75Se from (75Se)DMSe in different soils was established. Moist London loam soil (0.7 ml water added to 5 g air dried soil) sorbed larger amounts of soil-generated 7SSe(0.499 ng g-’ dry wt) than did air-dry soil (0.236ngg-’ dry wt). More selenium was sorbed by air-dried London loam from air containing a higher concentration of (75Se)DMSe generated by C. humicofa. Only 0.236 ng Se g-’ dry wt was sorbed from air containing 1.38 pg Se mm3 while up to 5 pg Se g-’ dry wt could be sorbed by soils from air containing 2000 pg Se me3. The County Meath soil, high in organic matter and selenium content, sorbed more selenium than the two loams (Table 2). The Compton soil, although containing a somewhat higher organic matter content than the London soil adsorbed slightly less selenium. It is interesting to note that the Compton soil contained very little total selenium (Table 1). The sorption of selenium by individual soil constituents was also examined (Table 3). Organic matter and clay minerals sorbed far greater quantities of selenium than manganese oxide and iron oxide. Very little selenium was sorbed by sand and surprisingly also by iron oxides, but in soils where iron oxides are abundant, its gross contribution may exceed that of free manganese oxides. The ability of sorbed 75Se to desorb into solution was also studied (Table 4) as an indication of its availability to plants. The readily-available selenium which would consist of selenate, selenite and organic selenium compounds of relatively low molecular weight was estimated by extraction with 0.4 N K,SO, (Cary et al., 1967). The amounts of 75Sewhich could be readily extracted were relatively small and deTable 2. The amount of “Se sorbed after 6 days fumigation with (“Se)DMSe (concentration 2000 pg Se II-“) generated by C. humicola Site of sampling

FgSeg-’

dry wt

1.28 kO.18 0.96 k 0.002 4.56 ir 0.88

Westfield College, London Compton, Berkshire County Meath, Ireland

Table 3. The amount of “Se sorbed by soil constituents after 7 days fumigation with (“Se)DMSe generated by C. humicofa (expressed as a percent of the concentration sorbed by the London loam soil) Soil constituents London loam Fen peat Clay Mn oxide on acid washed sand Fe oxide on acid washed sand Acid washed sand

Amount

of sorbed “Se 100 170 77 17 0.9 0.5

Sorption of DMSe by soils Table 4. Fractionation

of %Ienium

107

sorbed one month earlier by soils from soil-generated (‘%e)DMSe (values are expressed as percent of total)

Readily “available” 0.4 N

Potentially biologically available

Available with difficulty

Site of sampling

(72 h) soluble

0.05 N Na,SeO, (72hj ’ isotopically exchangeable

0.05 N NH,OH (96 h) soluble

6~ HCI (48 h) soluble

9~ HNO, (6 h) soluble

Insoluble residue

Westfield College, London County Meath, Ireland Fen peat

27.5 13 5.3

2.5 ND ND

7.9 ND ND

12.4 ND ND

20.6 ND ND

29.1 ND ND

KPO,

ND = not determined

creased with increasing organic matter (Table 4). This result agrees with those of Hamdy and Gissel-Nielsen (1976) which demonstrated that addition of organic matter reduces selenium availability. Similarly, in plant uptake studies, organic matter has been shown to reduce the availability of selenium (Bisbjerg, 1972). A separate series of experiments with London loam soil indicate the sorption of (‘%e)DMSe generated by C. humicolu (cultured separately from that used in Table 2) to be largely or entirely chemical. Soils sterilized by irradiation sorbed 0.82 f 0.07 pg Se g-’ dry wt which is a similar amount to unsterilized soil (0.76 + 0.02 pg Se g-’ dry wt), while autoclaved soils sorbed about half the amount (0.32 f 0.01 pg Se g-’ dry wt). The chemical composition of sterilized soil is undoubtedly changed by both irradiation and autoclaving, although autoclaving causes by far the greatest changes, especially in the amount of extractable inorganic ions (Cawse, 1975). Autoclaving has also been reported to solubilize more organic matter than irradiation (Powlson and Jenkinson, 1976). The experimental results are therefore consistent with the view that microorganisms do not play a role in the initial sorption of (75Se)DMSe. However, Bremner and Banwart (1976) concluded that soil microorganisms are partly responsible for the sorption of dimethylsulphide and dimethyldisulphide by moist soil, although their conclusion was based only on experiments with soil sterilized by autoclaving. There seems little doubt that soils represent an important natural “sink” for DMSe in the atmosphere. The data obtained show that the amount of selenium sorbed by dry loam soil is high and can account for approximately 50% of soil-generated DMSe although with the high concentration of DMSe generated by C. humicoIa, the sorption was only 10%. Thus, it can be expected that under natural conditions, most of the volatile selenium compounds released from the deeper soil layers would be trapped within the soil and the “net” release would be mainly from the surface layer. Acknowledgements-We thank the Agricultural Research Council for financial assistance with this work.

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