Calcium sulphite

Bioaccumulation: terrestrial

Description of key information

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

Additional information

(1) Environmental fate and stability in soil:Calcium sulfite dissociates into sulfite anions and calcium cations in environmental solutions, including soil porewater.

(a) Calcium generally has a high mobility and, except under strongly alkaline conditions, occurs in solution as dissociated Ca2+ ions. Concentrations generally increase with stream order as a result of increasing contact time between water and soil or rock.The most common Ca-bearing minerals in sedimentary rocks, calcite and dolomite, weather on contact with acid solutions, typically carbonic acid (H2CO) derived from the dissolution of atmospheric CO in rain, releasing Ca or Ca and Mg, respectively. Solutions of most soil types contain an excess of Ca, which constitutes more than 90% of the total cation concentration; Ca, is, therefore, the most important cation in governing the solubility of trace elements in soil (Salminen et al. 2005).Conclusively, calcium cations become part of the global calcium cycle.

(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) Calcium is the fifth most abundant of all the elements, constituting approximately 3% by weight (equivalent to 4.2% CaO) of the upper continental crust and 5.29% (7.41% CaO) for the bulk continental crust (Salminen et al. 2005 and references therein).Monitoring data for elemental calcium background concentrations in soil are provided by the FOREGS Geochemical Baseline Mapping Programme (Salminen et al. 2005).The FOREGS dataset reports calcium/calcium 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. Calcium oxide data were converted into calcium concentrations. Based on 833 paired samples for EU-27, UK and Norway, baseline calcium levels in topsoilfrom 186 mg/kg to 340693 mg/kg calcium with 5^th and 95^th percentiles of 892 mg/kg and 161392 mg/kg calcium, respectively, and a median concentration of 6518 mg/kg calcium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of161392mg/kg can be regarded as representative background concentration for calcium in European topsoils. Additionally, calcium 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 calcium. Calcium concentrations of respective aqua regia extracts were measured by ICP-MS (LOQ: 100 mg/kg).Calcium levels of agricultural soil range from < 100.00 mg/kg to 347,847.34 mg/kg calcium with 5^th and 95^th percentiles of 558.73 mg/kg and 136111 mg/kg calcium, respectively, and a median of2865mg/kg calcium. In grazing land, soil concentrations of calciumrange from < 100.00 mg/kg to 328124.30 mg/kg with 5^th and 95^th percentiles of 555.09 mg/kg and 137576 mg/kg calcium, respectively, and a median of3019.67mg/kg calcium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 136111 mg/kg can be regarded as representative background concentration for calcium in European agricultural soils and the 95^thpercentile of 137576mg/kg can be regarded as representative background concentration for calcium in European grazing land soils. 

(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)The macro element calcium (Ca) is essential to living organisms since it plays an important role in muscle contraction, is a major component of bones (including crustacean exoskeletons) and cartilage and essential for the normal clotting of blood and milk in lactating animals. Calcium as an activator for several key enzymes (e.g. pancreatic lipase, acid phosphatase, cholinesterase, ATPases and succinic dehydrogenase) stimulates muscle contraction (muscle tone, heart beat) and regulates the transmission of nerve impulses. It regulates the permeability of cell membranes and, as such, is believed to be essential for the absorption of vitamin B12 from the gastro-intestinal tract. Finally, it is involved in the acid-base balance, i.e. pH regulation, and plays an important role in carbohydrate, protein and lipid metabolism (Tacon, 1987). Calcium deficiency therefore causes characteristic syndromes, reflecting its specific functions in the metabolism of the animal or plant (Adebayo & Omitoyin, 2013, Soetan et al. 2010). In general, calcium is readily absorbed through the gastro-intestinal tract (also gills and skin of aquatic organisms). In terrestrial animals, Vitamin D3 plays an essential role in calcium absorption, however, a similar role for fish has not yet been established. The dietary uptake of calcium is facilitated in presence of lactose (soluble sugar-Ca complex) and by high gastric acidities (Tacon, 1987). Calcium is then rapidly deposited in the skeleton (up to 99% of total calcium in the fish's body are present in bones, teeth and scales; ADCP, 1978) and body tissues. Moreover, it has been shown that the greater the need, the more efficient the calcium absorption (Soetan et al. 2010). The process of calcium homeostasis prevents bioaccumulation above concentrations which may harm the organism. Moreover, excess calcium may be excreted via kidneys and urine, but also via feces. Further routes of excretion encompass breast feeding (mammals) and gills (fish) (ADCP 1978).

(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) Calcium 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 (2002) on “calcium chloride (CAS# 10043-52-4)”, a potential for bioaccumulation/bioconcentration cannot be identified. Thus, calcium 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 calcium sulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural calcium and sulfur cycle, (ii) ubiquitousness of calcium and inorganic sulfur substances in soil and (iii) essentiality of calcium and sulfur in terrestrial organisms,calcium sulfite is expected to have a low potential for bioaccumulation in terrestrial organisms.

 

References:

OECD HPV Chemical Programme (2002): Calcium Chloride SIDS Initial Assessment Profile, UNEP Publications, SIAM 15, Boston, p. 3