Sulfides in freshwater and marine water

​Toxicant default guideline values for protecting aquatic ecosystems

October 2000

Description of chemical

Hydrogen sulfide (CAS 7783-06-4)is a poisonous gas with a characteristic odour of rotten eggs and is water soluble to 4 g/L at 20°C. It is commonly found as an anaerobic degradation product of chemicals containing sulfur, such as in natural sediments, and is found in industrial wastes and landfill leachates. Hydrogen sulfide is a by-product rapidly produced by organisms but it is non-cumulative. Hence it is unlikely that tissue concentrations in aquatic organisms reach levels affecting health if consumed by humans. Reduced oxygen availability in natural aquatic and sedimentary environments which leads to anoxia, is often correlated to increased sulfide production. Some anthropogenic activities possibly leading to anoxia in natural environments are direct discharge of organic-rich effluents (e.g. sewage) and enrichment of nutrients (N and/or P from fertiliser, detergent run-off) leading to eutrophication and eventually to organic enrichment from decomposition of algal matter.

Hydrogen sulfide is a diprotic acid that dissociates in aqueous solution to form an equilibrium between un-ionised H₂S, bisulfide ions HS¯ and sulfide ions S^2-.

The following equations illustrate the equilibria:

H₂S ≡ HS¯ + H⁺   K₁
HS¯ ≡ S^2- + H⁺    K₂

At environmental conditions (pH < 10), only the first dissociation is significant hence the concentration of S^2- is negligible compared to the concentrations of H₂S and HS¯. For instance at pH 9, around 99% is in the form of HS¯ and at pH 5 about 99% is at H₂S (USEPA 1986). The concentrations of sulfide are usually expressed either as total sulfides (the sum of concentrations of H₂S, HS¯ and acid-soluble metallic sulfides present in solution) or in terms of un-ionised hydrogen sulfide H₂S. Either expression for concentration may take into account the amount of sulfide as H₂S or simply S.

The dissociation of hydrogen sulfide is dependent on temperature (t in °C) and solution ionic strength (I in M²). The following formulas are used (by substituting known quantities to the formulas consecutively) for calculating the proportion as or concentration of un-ionised H₂S in freshwaters (Clesceri et al. 1998) when temperature, pH and solution conductivity C (in µS/cm) are known:

where T = 273.16 + t°C,

[H₂S] is concentration of un-ionised hydrogen sulfide and

[H₂S + HS¯] is total sulfide concentration.

Table 8.3.9 gives the percentages of un-ionised hydrogen sulfide in aqueous total sulfide solutions (for typical freshwater: low ionic strength 0.0032 M or conductivity ~ 200 µS/cm) for pH values between 6.5 and 8.5 and temperatures between 10 and 30°C. To determine sulfide concentration, the total sulfide is commonly measured and the un-ionised H₂S is calculated using the formulas above. Typical methods for measuring sulfide, such as colorimetric and iodometric methods and by ion-selective electrode, measure the total sulfide in solution.

Furthermore, the value for pK₁ is significantly affected by solution salinity or ionic strength > 0.1 M. Ionic strength may not be important in freshwaters, but it is in marine and estuarine waters. Goldhaber and Kaplan (1975) derived the following relationship between pK₁, temperature and solution salinity expressed as sulfide (in ‰):

In estuarine or marine waters, an increasing concentration of chloride at a given pH and temperature, decreases the proportion of un-ionised hydrogen sulfide to the total sulfides. Using the formulas given above for pK₁-T-salinity and for %H₂S-pH-pK₁, the percentage of un-ionised H₂S in total sulfide solutions can be calculated. Table 8.3.10 gives the % H₂S values as a function of temperature, pH and salinity. Water managers need to refer to Tables 8.3.9 and 8.3.10 or the formulas given above when considering sulfide toxicity.

Aquatic toxicology

Most of the studies done on the effects of sulfide to aquatic organisms have utilised hydrogen sulfide, or sodium sulfide being added with the total sulfides measured. When sodium sulfide had been used without measuring the concentrations, the concentrations based on amounts of Na₂S weighed would very likely overestimate the actual sulfides in solution because of the volatile nature of un-ionised hydrogen sulfide. The toxicity of sulfides is due mainly to the un-ionised hydrogen sulfide H₂S rather than HS¯ or S^2- (USEPA 1986). Studies with various fish species (obtained overseas) have shown a very narrow range of toxicity values obtained, indicating similar sensitivities among the different species tested. Twelve species gave a 96-hour LC50 range (geometric mean) for un-ionised H₂S of 7 (Salmo trutta) to 41 µg S/L (Carassius auratus), ten of which had LC50 values of ≥ 18 µg S/L. This was despite of different pH and test temperatures reported. Chronic concentrations were only as low as 1 µg S/L for exposures to 97 days (Lepomis macrochirus, Smith et al. 1976a), indicating that the toxicity of H₂S is not much greater for long exposures. This indicates that hydrogen sulfide is a non-cumulative toxicant and may be detoxified from the body (Torrans & Clemens 1982).

Furthermore, fish generally showed greater sensitivity than most invertebrates tested. With the exception of Baetis vagans (Smith et al. 1976b) and Gammarus pseudolimnaeus, the geometric mean LC50 values for eight other invertebrate species studied were approximately one order of magnitude higher (range 111 to 840 µg S/L un-ionised hydrogen sulfide). The very narrow range of acute toxicity values expressed as un-ionised H₂S (over a range of pHs and temperatures) for fish as well as for invertebrates, is not inconsistent with the un-ionised H₂S being the toxic form. In general, studies reported show the observed effect concentrations consistent with the un-ionised H2S form.

Table 8.3.9 Calculated percentages of un-ionised hydrogen sulfide in total aqueous sulfide solutions

Table 8.3.10 Calculated percentage of hydrogen sulfide in total aqueous sulfide solutions at different pH, temperature and salinity values

Factors that affect toxicity

There is limited information available to indicate any definite effects of pH and temperature on the toxicity of sulfides. As already mentioned, the un-ionised H₂S is accepted to be responsible for sulfide toxicity, and that pH, temperature and ionic strength are important in the determination of its concentration in solution.

Limited information indicates that temperature may increase the sensitivity of fish to hydrogen sulfide in the reported test temperatures of 10 to 20°C (Mance et al. 1988c). The concentration of hydrogen sulfide dissolved in solution is also inversely correlated to the concentration of dissolved oxygen, that is, greater H₂S is available to aquatic organisms when less oxygen is present.

Fish exhibit a strong avoidance reaction to hydrogen sulfide, assuming they can escape (USEPA 1986).

The US Environmental Protection Agency (EPA) (1986) considered that the aquatic hazard from hydrogen sulfide was often transient and localised and that ‘concentrations in excess of 2.0 µg/L would constitute a long-term hazard’, but this was based on earlier data. Neither the reviews by the US EPA (1986) nor the United Kingdom (Mance et al. 1988c) have specifically addressed the significance, or otherwise, of the low figures for lake whitefish.

Aquatic toxicology

The values given below are geometric means for species taken from all screened data that measured pH and temperature as well as dissolved oxygen in most cases. All concentrations are expressed as un-ionised hydrogen sulfide as µg S/L.

Freshwater fish: Acute data: 12 species, 48 to 96-hour LC50, 2 to 41 µg/L. Most sensitive were lake whitefish Coregonus clupeaformis (2 µg/L), Carassius auratus (4 µg/L) and Salmo trutta (7 µg/L). Least sensitive was fathead minnow Pimephales promelas (710 µg/L). Chronic data: C. auratus 294 to 430-day NOEC reduced growth and reproduction 6.6 to 13 µg/L; Lepomis macrochirus 7 to 8 days survival, 8 to 32 µg/L and 97 to 826-day growth and spawning NOEC 1 to 3 µg/L; Oncorhynchus mykiss 5 to 29-day LC50, 6 to 22 µg/L and 50 to 145-day NOEC survival and growth, 3 to 5 µg/L; P. promelas 84 to 354-day growth, survival, fecundity, 5 to 11 µg/L; Salvelinus fontinalis and Stizostedion vitreum 26 to 234-day reduced breeding, 4 to 12 µg/L.

Freshwater crustaceans: nine species, 96-h LC50, 32 to 2220 µg/L; Branchiura sowerbyi 96-hour LC50 19,500 µg/L. Gammarus pseudolimnaeus were most sensitive, Cyclops viridis, least. Chronic data: Gammarus pseudolimnaeus 10 to 105-day survival, 2 to 49 µg/L; Procambarus clarki 196 to 447-day survival 6 to 13 µg/L.

Freshwater insects: three species, 96-hour LC50, 26 to 404 µg/L; one species Chironomus spp. 96-hour LC50 23,000 to 33,400 µg/L. Chronic data: Hexagenia limbata 138-day survival 21 µg/L.

Freshwater mollusc: one species Lymnaea lutepla 96-hour LC50 6000 µg/L.

Marine mollusc: one species Mytilus edulis 48-hour EC50 development 1.5 µg/L.

Marine echinoderm: one species Strongylocentratus purpuratus 48-hour EC50 development 3 µg/L.

Marine crustacean: three species, Palaemonetes pugio, Rhepoxynius abronius, Eohaustorius estuarius 48 to 96-hour LC50, 24 to 112 µg/L.

Guideline

The trigger values were derived from screened acute data conducted at different pHs (6.4 to 8.7) and temperatures (6 to 30°C). All values were first converted to concentration as un-ionised H₂S using the formulas given above using the reported pH and temperature. Water managers need to distinguish between un-ionised H₂S from total sulfide, as it is the un-ionised H₂S concentration that is compared to the guideline trigger value. However, if the total sulfide concentration is below the guideline value, so is the un-ionised H₂S concentration.

Ammonium sulfide (CAS 12135-76-1) ionises readily in water to ammonium and sulfide ions. This chemical would be treated as forming two-way equilibrium with un-ionised ammonia NH₃₋ammonium NH₄⁺ (see first part of 8.3.7.2 in the ANZECC & ARMCANZ 2000 guidelines) and hydrogen sulfide H₂S-bisulfide HS¯. Similarly, sodium sulfide ionises to sulfide ions in water then hydrolyses to establish equilibrium with H₂S-HS¯. There are no separate guideline values for ammonium sulfide and sodium sulfide.

References

ANZECC & ARMCANZ 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, Canberra.

Clesceri LS, Greenberg AE & Eaton AD (eds) 1998. Standard methods for the examination of water and wastewater 1998, 20 edn, American Public Health Association, USA.

Goldhaber MB & Kaplan IR 1975. Apparent dissociation constants of hydrogen sulfide in chloride solutions. Marine Chemistry 3, 83–104.

Mance G, O'Donnell AR & Campbell JA 1988c. Proposed environmental quality standards for list II substances in water: Sulphide. Environmental Strategy Standards and Legislation Unit, Water Research Centre, Medmenham, UK.

Smith LL Jr, Oseid DM, Adelman IR & Broderius SJ 1976b. Effect of hydrogen sulfide on fish and invertebrates. Part I. Acute and chronic toxicity studies. US EPA Ecological Research Series Report. EPA-600/3-76-062a. 286 pp.

Smith LL Jr, Oseid DM, Kimball GL & El-Kandelgy SM 1976a. Toxicity of hydrogen sulfide to various life history stages of bluegill (Lepomis macrochirus). Transactions of the American Fisheries Society 105, 442–449.

Torrans EL & Clemens HP 1982. Physiological and biochemical effects of acute exposure of fish to hydrogen sulfide. Comparative Biochemistry and Physiology 71C, 183–190.

USEPA 1986. Quality criteria for water. US Department of Commerce, National Technical Information Service, US Environmental Protection Agency, Springfield, Virginia. PB87-226759, EPA 440/5 86-001.