Potassium sulphite

Bioaccumulation: terrestrial

Description of key information

Taking into account (i) the rapid dissociation of potassium sulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural potassium and sulfur cycle, (ii) ubiquitousness of potassium and inorganic sulfur substances in soil and (iii) essentiality of potassium and sulfur in terrestrial organisms, potassium sulfite is expected to have a low potential for bioaccumulation in terrestrial organisms.

Additional information

(1) Environmental fate and stability in soil:

Potassium sulfite dissociates into sulfite anions and potassium cations in environmental solutions, including soil porewater.

(a)Potassium is very soluble and occurs as monovalent cation under environmental conditions. Although potassium is an abundant element, its mobility is limited by three processes: (a) it is readily incorporated into clay-mineral lattices because of its large size; (b) it is adsorbed more strongly than sodium on the surfaces of clay minerals and organic matter; and (c) it is an important element in the biosphere and is readily taken up by growing plants (Salminen et al. 2005).

(b)Sulfites are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. Under oxygen-rich conditions, sulfites are rapidly oxidized catalytically by (air) oxygen or by microbial action to sulfate. Microbial oxidation of reduced sulfur species including elemental sulfur (S), sulfide (HS^-), sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) is an energetically favorable reaction carried out by a wide range of organisms, i.e. sulfur oxidizing microorganisms (SOM) resulting in ultimate transformation into sulfate (SO₄^2-, Simon and Kroneck, 2013).

Observed SO₃^2-oxidation rates in soils are dependent on soil characteristics, i.e. are decreasing with increasing soil pH. In a study performed by Lee et al. (2007), soils collected from the surface horizon (0 to 20 cm) were amended with 0.3 % w/w CaSO₃. Based on the analysis of soils leachates initial SO₃^2-oxidation rates were dependent on soil pH – however final recovered sulfate concentrations were similar among all tested soils (pH range 4.0 - 7.8) irrespective of pH, yielding > 75% recovery of the total added sulfur.

In highly reduced (water-logged) soils, reduction to sulfides may take place with subsequent formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, including FeS, ZnS, PbS and CdS (Lindsay, 1979, Finster et al., 1998). Thus, under anoxic conditions, sulfate is readily reduced to sulfide by sulfate-reducing bacteria (SRM) that are common in anaerobic environments. Other sulfur-containing microbial substrates such as dithionite (S₂O₄^2-), thiosulfates (S₂O₃^2-) or sulfite (SO₃^2-) may also be anaerobically utilised, ultimately resulting in the reduction to sulfide (H₂S).

A significant set of microbial populations grows by disproportionation of sulfite, thiosulfate or elemental sulfur, ultimately yielding sulfate or sulfide (Simon and Kroneck 2013 and references therein; Janssen et al. 1996, Bak and Cypionka, 1987).

In sum, sulfites may reasonably be considered chemically unstable under most environmental conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle.

(2) Ubiquity and natural/ambient background:

(a)Potassium is the 8^thmost abundant element in the earth crust with an estimated concentration of 1.84%, is ubiquitous and a constituent of soil minerals (Salminen et al. 2005 and references therein).Monitoring data for elemental potassium background concentrations in soil are provided by the FOREGS Geochemical Baseline Mapping Programme (Salminen et al. 2005). The FOREGS dataset reports potassium/potassium oxide concentrations for 845 topsoil samples (dried, grinded, sieved to < 2 mm, pulverised to < 0.063 mm, and analysed by XRF, LOQ: 0.01 %) sampled on a grid across Europe. Potassium oxide data were converted into potassium concentrations. Based on 833 paired samples for EU-27, UK and Norway, baseline potassium levels in topsoil from 216 mg/kg to 50,855 mg/kg potassium with 5^thand 95^th percentiles of 5,595 mg/kg and 30,805 mg/kg potassium, respectively, and a median concentration of16,055mg/kg potassium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 30,805 mg/kg (based on XRF data) can be regarded as representative background concentration for potassium in European topsoils.

Additionally, potassium concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for potassium. Potassium concentrations of respective aqua regia extracts were measured by ICP-MS (LOQ: 20 mg/kg). Potassium levels of agricultural soil range from 40.37 mg/kg to 26,738.7 mg/kg potassium with 5^thand 95^th percentiles of 235.3 mg/kg and 3,895.5 mg/kg potassium, respectively, and a median of 1,198 mg/kg potassium. In grazing land, soil concentrations of potassium range from 43.2 mg/kg to 27,956.9 mg/kg with 5^thand 95^thpercentiles of 201.9 mg/kg and 3,993.0 mg/kg potassium, respectively, and a median of 1,086 mg/kg potassium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 3,895.5 mg/kg can be regarded as representative background concentration for potassium in European agricultural soils and the 95^thpercentile of 3,993.0mg/kg can be regarded as representative background concentration for potassium in European grazing land soils (based on aqua regia extracts).

(b) Sulfur is a ubiquitous natural component of soil. Most terrestrial environments have substantial sulfur levels whereas sulfur-deficient environments are rare. In soil, sulfur can be found as pure element, sulfide (salts containing S^2-) and sulfate (SO₄^2-) minerals and in various organic substances. In all but highly reduced soils, sulfate is the most stable species at environmentally relevant pH > 4. Other stable sulfur species such as SO(g), S^2-, S₂O₃^2-and S₂O₄^2-are, however, not prevalent in soils (Lindsay, 1979). Due to its ability to exist in a wide range of oxidation states, sulfur plays an important role in living organisms, both as a structural component and a redox-active element. Soluble states of sulfur such as sulfates and sulfites are common in their various elemental forms. The three most abundant forms of sulfur are elemental sulfur, sulfate (SO₄^2-) and sulfide (S^2-) and sulfur containing oxyanions, i.e. sulfite (SO₃^2-), dithionite (S₂O₄), thiosulfate (S₂O₃) and polythionates such as trithionate (S₃O₆^2-) and tetrathionate (S₄O₆^2-, Simon and Kroneck 2013). Due to its key importance for biological processes and unique metabolic versatility, i.e. its appearance in amino acids, iron-sulfur proteins, thioredoxins and sulfolipids, the major fraction of the sulfur in surface soil horizons is present in organic combinations, e.g. in plant litter, microbial biomass or stabilized in soil organic matter with the remainder occurring as inorganic sulfate (Maynard et al., 1998).

A total of 837 topsoil samples were processed in the FOREGS-program to determine sulfur background concentrations. Sulfur concentrations of respective aqua regia extracts were measured by ICP-AES (limit of quantification (LOQ): 50 mg/L). Based on 775 paired samples from the FOREGS dataset, the median sulfur content of European topsoil amounts to 222 mg/kg ranging from <50 to 6,518 mg/kg, and the 95^thpercentile of 645 mg/kg can be regarded as representative background in European topsoils (Salminen et al. 2005).

Additionally, sulfur concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for sulfur. Sulfur concentrations of respective aqua regia extracts were measured by ICP-OES and/or ICP-MS. Sulfur levels of agricultural soil range from < 5 to 68,226 mg/kg sulfur with a median of 209 mg/kg and a 95^thpercentile of 783.91 mg/kg. In grazing land, soil concentrations of sulfur range from < 5.00 to 98,189 mg/kg with a median of 310 mg/kg and a 95^thpercentile of 645 mg/kg (Reimann et al. 2014). Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 783.91 mg/kg can be regarded as representative background concentration for sulfur in European agricultural soils and the 95^thpercentile of 645 mg/kg can be regarded as representative background concentration for sulfur in European grazing land soils.

(3) Essentiality:

(a)Potassium is essential for the most basic cellular mechanisms in animal cells. In most animal cells, differences in K+ and Na+ concentrations between the interior and exterior of the cell exist. More than one third of the total ATP consumption in resting animal cells is used to maintain this gradient. This gradient is crucial in the control of cell volume, in rendering nerve and muscle cells electrically excitable, and is the driver of active sugar and amino-acid transport. In a reverse mechanism, it can be used to generate ATP (Stryer, 1988). Locations and functions of membrane potassium channels have been investigated in the nematodeCaenorhabditis elegans.Apparently, individuals with deficient potassium channels show severe malfunctions, including locomotive and motoric disorders, defects in defecation, inability to lay eggs, feeding behavior and sensory malfunctions (Salkoff et al. 2005). Deficiency symptoms, thus, clearly highlight the universal essentiality of potassium for maintenance of membrane potential and muscular and neural functions. Given its universal importance, potassium is an absolute essential element inC. elegansgrowth medium. Growth is not observed when this element is missing in the culture medium (Zečić et al. 2019 and references therein).

Effects on growth have also been observed in other invertebrate species: Low-Potassium diet reduces consumption rate and conversion of ingested food and thus results in an overall prolonged growth/feeding phase and overall reduced growth rate in third instar Tobacco Hornworms (Manduca Sexta) (Wulfson & Stamp, 1991). Chen et al. (2018) reported that development of soybean loopers was facilitated when feeding on soy plants fertilized with high amounts of potassium in comparison to loopers feeding on plants fertilized with lower amounts.

In the environment, potassium primarily leaches from plant litter (Anderson 1983). However, soil arthropods also act as detritivores in the remineralization of nutrients and enhance the amount of available potassium in soil (Anderson et al. 1983, Pramanik et al. 2001). Arthropods regulate absorption and secretion of potassium in osmoregulation via Malphigian Tubes, guts and recta (Beyenbach 2016). The average potassium content of 37 species of forest-floor arthropods was determined with 6.2 mg/g ash free dry weight (Reichle et al. 1969). In a study on the nutritional value of different insects for birds, Studier et al. (1992) measured the mineral content of different insect species and the potassium content ranged from 7.1 mg/g dry weight in Trichoptera to 14.9 mg/g dry weight in Phasmatodea.

(b)Sulfur is essential in two different ways: as a structural component and on a metabolic level. Sulfur forms a foundation of life itself: it is needed in the synthesis of membrane lipids, is part of the amino acids cysteine and methionine and consequently part of proteins and enzymes (Sekowska et al. 2000). It is found in iron-sulfur centers of Ferredoxin (which functions as electron carrier in anaerobic respiration) and of Glutathione (which plays an important role as antioxidant at the prevention of ROS-induced cell-damage). Further, it is present in the cofactors Thiamine and Lipoic acid (which is linked to many dehydrogenases), and biotin.

Sulfur is taken up by invertebrates via the diet and most insects seem to require to take up methionine with their diet to cover their sulfur demands. Lack of sulfur-containing amino acids or organic sulfur in the diet leads to detrimental effects, as observed in the aphidMyzus persicae.Omission of methionine or cysteine leads to a strong reduction of growth and a greatly reduced number of viable offspring, with effects more pronounced in the absence of methionine. However, the demand for cysteine can be met by provision of other sulfur substances so that cysteine may not be an essential amino acid but might serve as sulfur carrier instead (Dadd & Krieger, 1968). Retnakan and Beck (1967) reported that provision of inorganic sulfur mitigated negative effects of the lack of methionine or cysteine and concluded that aphids could synthesize either of these amino acids, possibly with the help of symbionts. Supply of inorganic sulfur mitigated the need for cysteine and methionine uptake in the aphidsAcyrthosiphon pisumandNeomyzus circumflexus.However, the synthesis of the amino acids is also attributed to symbionts(Dadd, 1973 and references therein). Thus, the lack of sulfur-containing amino acids in the diet affects aphids but symbionts seem to counteract and to contribute to the diet by synthesizing these amino acids from inorganic sulfur (Douglas, 1988). In food-choice experiments, it was demonstrated that methionine had the strongest positive effect on selection of diet from an array of served diets, underlining the demand for methionine (Dadd,1973 and references therein).

In spiders, the sulfur-containing amino acid taurine is furthermore a component of spider silk and venom (Wiesenborn 2012 and references therein). In earthworms, S-containing dialkylfuransulfonates allow consumption of leaf litter by preventing adverse effects of polyphenols to the earthworms. Dialkylfuransulfonates comprise up to 20% of the total sulfur found in earthworms and play a vital role for detritus consumption in ecosystems all over the world (Liebeke et al. 2015).

Conclusion:

(a) Potassium is ubiquitous in the environment, occurring in minerals, soil, sediments and natural waters, and is present as an essential and actively regulated element in biota. According to the OECD SIDS (2001) on “potassium chloride (CAS# 7447-40-7)”, a potential for bioaccumulation/bioconcentration cannot be identified. Thus, potassium as essential element is actively regulated and does not bioaccumulate.

(b) Sulfur is ubiquitous in the environment, is actively regulated and fulfils essential roles in all cells, determining the structure and activity of a number of molecules and modulating a myriad of metabolic and catalytic processes. Accordingly, the bioaccumulation potential of sulfite is expected to be low.

Taking into account (i) the rapid dissociation of potassium sulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural potassium and sulfur cycle, (ii) ubiquitousness of potassium and inorganic sulfur substances in soil, and (iii) essentiality of potassium and sulfur in terrestrial organisms,potassium sulfite is expected to have a low potential for bioaccumulation in terrestrial organisms.

 

References: OECD SIDS (2001). Potassium chloride. CAS# 7447-40-7.SIDS Initial Assessment Report for 13th SIAM (Bern, 6-9 November 2001).