HYDROGEN SULPHIDE H2S
MYTH OR MONSTER
DUNCAN GRIFFITHS
Duncan Griffiths Copyright © FEBRUARY, 2003, D, GRIFFITHS, ALL RIGHTS RESERVED
Hydrogen Sulphide or H2S seems to strike fear in the hearts of most if not all fish keeping hobbyists, none more so than koi keepers, the author has often heard reports on the internet of H2S being responsible for mortalities in koi carp, however, although the author has seen its existence on many occasion in ponds all over the UK, he has NEVER seen any mortalities attributed to this strange gas, from that stand point H2S it appears it can be a problem but it seems to be a small one.
It must be noted the occurrence of hydrogen sulphide is not that unusual in lakes inhabited with carp and other species with or without extensive vegetation. H2S’s very existence is a fact of life, yet wild carp and indeed other species of fishes do not readily die because of it’s is presence.
Wherever raw sewerage is processed H2S is always present; although H2S is a sanitizing killer, bacterium essential for processing raw sewerage flourishes and the purified water is released into lakes and reservoirs, fish also abound even though it is present in some form or another.
Hydrogen Sulphide
Molecular formula H2S
Physical appearance colourless gas
Chemical family inorganic gas
Melting point -850c
Boiling point – 600c
Water solubility 4370 ml/litre @ 00c; 1860 ml/litre @400c
Physical hazards flammable gas, may cause flash fire
Include irritation, lack of sense of smell, sensitivity to light, vomiting, breathing difficulties, lung congestion, nerve damage, heart damage, internal bleeding, brain-damage, coma and eventually death.
Major uses for H2S include dye manufacturing, tanning, cosmetics and wood pulp production; also therapeutic medicinal baths from natural spring waters containing H2S.
Hydrogen sulphide is quite common in the environment in low concentrations. E.g. mineral springs, bodies of water, waste water, and natural gases.
Hydrogen sulphide is manufactured during the decomposition sulphurous protein amino acids due to the influence of putrefactive sulphur anaerobic bacteria. E.g. standing water, marshes lakes and sewerage plants. Or, in the manufacture of chemical fibres, coking plants or refineries.
Hydrogen sulphide has an offending smell of "rotten eggs" which I am sure we have all smelt form time to time in our ponds especially if you have a water garden containing plants, the odour is detectable at very minute concentrations in the air. At higher concentrations (50-150 mg/m3it has a sweet sickly odour and at concentrations above this it can quickly deaden a humans sense of smell, leaving the subject unaware of it’s presence, therefore H2S in factories must be electronically monitored.
The threshold for smell and taste in water is estimated to be 0.05 – 0.1 mg/litre, the taste and odour threshold for sulphides is 0.2mg/litre
Hydrogen sulphide is formed when sulphides hydrolyse in water. Once H2S is a solute in water it quickly dissociates to form mono-hydrogen sulphide (1-)(HS-) and sulphide(S2-) ions. Methylene blue, colour-metric test kits are Ready available and quite cheap and will identify hydrogen sulphide at concentrations in the 0.1 and 20 mg/litre range
The concentrations of these two are dependent on pH of the body of water containing the hydrogen sulphide. The higher the pH the lower the concentrations of hydrogen sulphide and the higher the concentrations of dissociated mono-hydrogen sulphide. The lower the pH the higher the un-dissociated hydrogen sulphide.
At a pH of 7, 50.4% of the amount of sulphides will be hydrogen sulphide; the toxic form and the remaining 49.4% will be mono-hydrogen sulphide and sulphides.
At a pH of 5, 99% of sulphides present will be the toxic form of Hydrogen sulphide, at a pH of 9, less than 1.0% will be the toxic form of hydrogen sulphide.
Conversions
1mg/m3 = 0.706 PPM
1 PPM = 1.416 mg/m3.
|
PH |
%H2S |
|
5.0 |
99% |
|
5.5 |
97% |
|
6.0 |
91.1% |
|
6.5 |
76.4% |
|
7.0 |
50.6% |
|
7.5 |
24.4% |
|
8.0 |
9.3% |
|
8.5 |
3.1% |
|
9.0 |
1.0% |
|
Concentrations |
Effect |
|
0.02-0.4 mg /litre |
Sub-lethal effects, gill damage, (dependent on species) |
|
0.02-5.3 mg/litre |
Lethal for some species |
|
Generally the lower the organic load of the fishes natural habitat the more sensitive a species is, I.E. salmonoids are more susceptible than carp |
|
Hydrogen sulphide is present in the atmosphere by natural emissions from concentrations of 0.1 –1.0mg/m3, (mg=micrograms) although samples taken from near by factory emissions can elevate the concentrations locally, typically above 100 .mg/m3.
The estimated daily intake for a human averages10-20 .mg calculated on the assumption the 20 m3 of air containing hydrogen sulphide as calculated above, is inhaled.
Drinks and food may contain H2S, however the formation of sulphides while the cooking process takes place complicates available figures,
skimmed milk, less than 0.1% fat typically contains 0.8mg/litre while cream containing 30% fat and above can contain as much as 1.84mg/litre.
Cooked beef can contain 0.3mg/litre while lamb can contain 0.4mg/litre with levels being higher in anaerobically packed meats, with the average daily intake from food and drink per human being around 1.7mg/litre daily.
Water of course is a natural place to find H2S but they are most commonly derived from natural sources, I.E. underground sources such as well water from bore holes where water seeps up through anaerobic micro-organisms found in minerals rock formations.
In the USA, 500.mg has been found in fresh water supplies.
Hydrogen sulphide is easily absorbed in humans following ingestion; it rapidly spreads to the brain kidney, pancreas, liver and intestine.
Most consumed hydrogen sulphide is metabolised to form Methanethiol and Dimethyl sulphide most sulphides are quickly excreted via the kidney. Although a small portion of sulphide may be expelled by the lungs and some metallic forms in the faeces.
Exposure
The LD50 for sodium sulphide given orally in rats was reported as 250mg/kg of body weight.
Water containing 4-12 mg /litre of hydrogen sulphide was
found to be Embryo toxic in rats but water containing 2-3mg/litre had no
effects at all, also no birth and growth defects were recorded either.
Human exposure to low concentrations of Hydrogen sulphide has caused corneal
damage, headache, sleep disturbances, nausea, weight loss, and symptoms
suggestive of brain damage.
Higher concentrations can cause irritation of the lungs even
pulmonary oedema. Exposure around the 200 PPM has caused arm cramps, and low
blood pressure, unconsciousness, after 20 minutes. Coma may occur within
seconds inhaled at high concentrations and be followed rapidly by death. For example,
workers exposed to 930PPM hydrogen sulphide for less than 1 minute died.
Hydrogen sulphide is moderately toxic to animals via the inhalation route: LC50
(mice, 1 hour) = 673 PPM; LC50 (mice, 7.5 hours) = 140 PPM. Exposure to 10-13
PPM for 4-7 hours has caused eye irritation.
Skin absorption of Hydrogen sulphide is slight and not considered significant.
However, prolonged or repeated skin contact might cause mild irritation.
Treatment and control –
To
neutralise H2S
is with chlorine to be fed in sufficient quantities to eliminate it, while
leaving a residual amount in the water to prevent its reformation, (3 PPM
of chlorine is required for each PPM of hydrogen sulphide).
So to recap!
There is probably
more information than was required for this topic but fore-warned is fore-armed
From the koi
hobby point of view it seems it can be both myth and monster, but a reality
check is called for.
First of all
hydrogen sulphide when compared to other koi disease and problems, is small
indeed.
In a clean system H2S will not be a problem and further to this as the graph shows even if it was if you normally run high pH levels in the 8's if H2S was evident it would be at very low levels and would not pose much of a threat, further there are treatments available to the hobbyist to mop up this small proportion of hydrogen sulphide, namely H2O2 or KMnO4, in normal therapeutic doses
References:
Aquatex.com
Hydrogen sulphide, Geneva, world health organization.
H2S control municipal water-water reference library
Material safety data sheets