234
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
products. Thiosulfate with its molecule has two sulfur atoms opposite by their
//0
valence: one S2+ and the second S6-: Na - S - S -O-Na. Thiosulfate is quite
\ \0
resistant to direct chemical oxidation, but when being produced in water
bodies, it is readily oxidized by thiobacilli to sulfate (Sorokin 1970b, 1972b).
Thus, we may conclude that the process of sulfide oxidation in the redox zones
of water basins actually proceeds via two steps. During the first (chemical) step
of sulfide oxidation, about 70% of its sulfur is oxidized to sulfate and about
30% is hidden within the thiosulfate molecules as sulfane sulfur, which has the
same valence as sulfide (S2-). It eludes analysis during colorimetric determination of H 2S and precipitation as ZnS. Therefore the measurement of the
sulfide oxidation rate by measuring only the decrease of H 2S concentration in
water overestimates it by 20-30%, just as the sequencing of only the sulfate
formation rate underestimates it. It is obvious that to evaluate the real rate of
H 2S oxidation it is necessary to measure the amount of sulfur of sulfate and
the sulfate contained in thiosulfate thus formed. Another way to account for
the sulfane sulfur hidden within the thiosulfate molecules is to hydrolyze the
thiosulfate with AgN03. In its presence the thiosulfate is split by silver sulfide
and sodium sulfate: Na2S203 + 2 AgN03 + H 20 Ag2S + 2HN03 + Na2S04. Silver
nitrate is added to the water sample, where remains of H 2S are present, and
the newly formed thiosulfate during its oxidation will precipitate as Ag2S both
H 2S and the sulfone sulfur of thiosulfate. So in this case we will have in this
precipitate the whole reduced remaining sulfur. At the same time, in the filtrate
we will have sulfur oxidized to sulfate and represented by the sulfate itself plus
the sulfate of thiosulfate. Thus the BaCtz added to the filtrate will precipitate
whole sulfur oxidized to sulfate. This enables measurement of the real rate of
H 2S oxidation in water samples (Sorokin 1970b). With the use of 35S-labeled
sulfied, the sensitivity of experimental estimations of H 2S oxidation rates in
the laboratory or in situ increases by several orders of values. When employing the radioisotope method for estimation of H 2S oxidation rates, both these
approaches can be used: measuring the rate of sulfide disappearance or of the
sulfate formation. In practice this can be achieved as follows.
5.5.2.1 Preparation of the Working Solution of Labeled Sulfide
Commercial batch preparations of labeled Na-sulfide (Na~5S9H20), most
often contain a significant share of its oxidation products (SO, S20~-, SOl-).
Therefore they should be cleaned if this share is over 10% of total 35S radioactivity in the batch. To estimate the share of these products first it is necessary
to adjust the appropriate specific radioactivity of sulfide sulfur contained in
the batch. The batch should be purchased containing 0.2 to 1 m Ci of 35S. When
the batch container is opened and Na2S9H20 solution in 1-2ml of water is
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
products. Thiosulfate with its molecule has two sulfur atoms opposite by their
//0
valence: one S2+ and the second S6-: Na - S - S -O-Na. Thiosulfate is quite
\ \0
resistant to direct chemical oxidation, but when being produced in water
bodies, it is readily oxidized by thiobacilli to sulfate (Sorokin 1970b, 1972b).
Thus, we may conclude that the process of sulfide oxidation in the redox zones
of water basins actually proceeds via two steps. During the first (chemical) step
of sulfide oxidation, about 70% of its sulfur is oxidized to sulfate and about
30% is hidden within the thiosulfate molecules as sulfane sulfur, which has the
same valence as sulfide (S2-). It eludes analysis during colorimetric determination of H 2S and precipitation as ZnS. Therefore the measurement of the
sulfide oxidation rate by measuring only the decrease of H 2S concentration in
water overestimates it by 20-30%, just as the sequencing of only the sulfate
formation rate underestimates it. It is obvious that to evaluate the real rate of
H 2S oxidation it is necessary to measure the amount of sulfur of sulfate and
the sulfate contained in thiosulfate thus formed. Another way to account for
the sulfane sulfur hidden within the thiosulfate molecules is to hydrolyze the
thiosulfate with AgN03. In its presence the thiosulfate is split by silver sulfide
and sodium sulfate: Na2S203 + 2 AgN03 + H 20 Ag2S + 2HN03 + Na2S04. Silver
nitrate is added to the water sample, where remains of H 2S are present, and
the newly formed thiosulfate during its oxidation will precipitate as Ag2S both
H 2S and the sulfone sulfur of thiosulfate. So in this case we will have in this
precipitate the whole reduced remaining sulfur. At the same time, in the filtrate
we will have sulfur oxidized to sulfate and represented by the sulfate itself plus
the sulfate of thiosulfate. Thus the BaCtz added to the filtrate will precipitate
whole sulfur oxidized to sulfate. This enables measurement of the real rate of
H 2S oxidation in water samples (Sorokin 1970b). With the use of 35S-labeled
sulfied, the sensitivity of experimental estimations of H 2S oxidation rates in
the laboratory or in situ increases by several orders of values. When employing the radioisotope method for estimation of H 2S oxidation rates, both these
approaches can be used: measuring the rate of sulfide disappearance or of the
sulfate formation. In practice this can be achieved as follows.
5.5.2.1 Preparation of the Working Solution of Labeled Sulfide
Commercial batch preparations of labeled Na-sulfide (Na~5S9H20), most
often contain a significant share of its oxidation products (SO, S20~-, SOl-).
Therefore they should be cleaned if this share is over 10% of total 35S radioactivity in the batch. To estimate the share of these products first it is necessary
to adjust the appropriate specific radioactivity of sulfide sulfur contained in
the batch. The batch should be purchased containing 0.2 to 1 m Ci of 35S. When
the batch container is opened and Na2S9H20 solution in 1-2ml of water is