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Description of key information

Sodium thiosulfate dissociates into thiosulfate anions and the respective sodium cations in aqueous solution or upon contact with soil moisture. Thiosulfate anions are unstable under environmentally relevant conditions and are readily oxidized to sulfate (Lindsay, 1979, Barbosa-Jefferson et al. 1998, Jørgensen et al. 1990). Any quantitatively relevant adsorption of sodium thiosulfate or its dissociation products onto soil, sediments or suspended matter is therefore not expected.

 

Due to rapid abiotic and biotic transformation processes, the thiosulfate ion is considered unstable under relevant environmental conditions rendering the assessment of representative, thiosulfate-specific global partition coefficients not feasible. However, data are available on sulfur partitioning in soils and sediments (see additional information below), yielding the following partition coefficients:

log Kp(solids-water in freshwater sediment):     2.02 L/kg (sulfur, n = 750)

log Kp(solids-water in marine sediment):          1.58 L/kg (sulfur, n = 2)

log Kp(solids-water in soil):                             1.64 L/kg (sulfur, n = 25))

 

The mobility of sulfur in soil and sediments is therefore considered to be low as adsorption and reduction processes constrain it. In poorly drained, peaty soil, sulfur is immobilised and enriched as sulfide. In agricultural soils with oxidative conditions, inorganic sulfur almost always occurs in the form of sulfate.

 

Sulfate, as negatively charged ion even at low pH, is expected to be adsorbed by non-specific anion exchange and most effectively at low pH. Soils with a pH > 6 are not expected to adsorb a significant amount of sulfate so that nearly all sulfate is present in soil solution and, as a consequence, highly susceptible to leaching. The main soil constituents responsible for sulfate adsorption are clays with positive edge charges and iron and aluminium oxides. Nevertheless, there does not seem to be a consensus in the literature on the role of nonspecific and specific adsorption of sulfates so that specific adsorption by ligand exchange has also been described (McBride, 1994, Environmental chemistry of soils. Oxford University press.; Sokolova and Alekseeva, 2008, Adsorption of sulfate ions by soils (a review). Eurasian Soil Science 41/ 2: 140–148.).

 

Regarding the partitioning of sodium in sediments, a European median log Kp value of 2.89 L/kg is derived for sediment-water partitioning of sodium.

Key value for chemical safety assessment

Additional information

Due to rapid abiotic and biotic transformation processes, the thiosulfate ion is considered unstable under relevant environmental conditions with the transformation processes and the chemical identity of the sulfur species resulting thereof being predominantly dependant on the respective environmental conditions. Consequently, the assessment of representative, thiosulfate-specific global partition coefficients is not feasible.

Since sulfur exists in streamwater predominantly as the free sulfate anion (Salminen et al. 2005), concentrations of sulfate in streamwater and sulfur in sediment concentrations are applied to examine the respective partitioning

Data on environmental sulfur and sulfate concentrations are available based on monitoring data for elemental sulfur concentrations in water and corresponding sediments provided by the FOREGS Geochemical Baseline Mapping Programme that aimed to provide high quality, multi-purpose homogeneous environmental geochemical baseline data for Europe. A total of 750 paired samples, i.e. samples with the same coordinates for the sampling location of stream water (filtered to < 0.45 µm) and sediment (wet sieved in the field to <0.15 mm) were processed (Salminen et al. 2005) and results correspond to steady-state conditions of S, independent of sulfur speciation. Sampled stream water and sediments cover a wide range of environmental conditions. Water parameters such as pH, hardness and organic carbon concentrations cover several magnitudes.

Based on the FOREGS dataset, the median sulfate concentration of 16.88 mg/L (n = 750) can be regarded as a typical background concentration in European surface waters, whereas the median sulfur concentration of 508 mg/kg can be considered a typical background concentration of sulfur in European stream sediments. Corresponding log sediment/water partition coefficients range from 0.11 to 4.20 with 5th and 95th percentiles of 0.99 and 3.07, respectively. Therefore, an European median sulfur log Kp(solids-water in sediment) of 2.02 can be derived based on European stream water sulfate concentrations as the dominant sulfur species. In addition, a marine sediment-water partition coefficient log Kp(solids-water in sediment) for sulfur of 1.58 L/kg is derived in the study. Results are however based on limited sample size (n=2) and data should therefore be treated with caution.

Regarding the partitioning of sulfur in European soils, data are available from a study by Sheppard et al. (2011) based on data from five different soil types, i.e. clay till, clay gyttia, glacial clay, cultivated peat and wetland peat (n=25), yielding a median logKp(solids-water in soil) of 1.64 L/kg.

 

Salminen, R. et al., 2005. Geochemical Atlas of Europe. Part 1: Background Information, Methodology and Maps. http://www.gtk.fi/publ/foregsatlas/.