Hydrogen sulfide (H2S), which can form in pond bottom sediment, is toxic to aquatic animals because it interferes with the cytochrome a3’s reabsorption during respiration. The main practices to minimize the risk of toxicity of hydrogen sulfide are careful feeding to avoid wasted wastewater at the bottom of ponds, aeration to avoid low dissolved oxygen levels and create a stream of water with additional oxygen. Across the interface between land and water, liming to prevent deposition and acidic water.
1. Hydrogen Sulfide (H2S) and Harm
A) Origin, concentration and types of hydrogen sulfide:
Sulfur is an essential element for plants, animals and bacteria, found in natural waters and water in aquaculture systems, mainly in the form of sulfate ions. In wet areas, the sulfate concentration in the water is usually 5 to 50 mg / L, but in the arid region the concentration usually exceeds 100 mg / L. Sea water contains an average of 2,700 mg / L sulfate. Although sulfate is rarely used for special aquaculture systems to increase ambient concentrations, it does exist in feed and some ways to improve water quality.
– Concentration of Hydrogen Sulfide:
+ The concentration of hydrogen sulfide must be estimated from the total sulfide concentration, because the methods for the determination of sulfide in water typically measure the total concentration of the three sulfides.
The H2S ratio at the different pH and temperature values given in Table 1 can be used to estimate the concentration of hydrogen sulfide. To illustrate, assume pH is 7.5 at 26 ° C in fresh water with a sulfur concentration of 0.5 mg / L. The coefficient for these conditions is 0.238. Multiply the coefficient with the sulfide concentration of 0.5 mg / L to the H2S concentration of 0.119 mg / L. In sea water the same temperature and pH, the concentration will be less and with a factor of 0.9.
– Sulfide in sediment:
+ The formation of hydrogen sulfide in sediment is mainly the result of sulfate reduction by microorganisms. The process of sulfate reduction occurs at lower redox potential than is required for the process of reducing iron and manganese by microorganisms.
Therefore, iron (chemo II) manganese (secondary chemistry) is usually found in places where hydrogen sulfide is produced.
+ Iron, manganese and other metals react rapidly with hydrogen sulfide to form insoluble metal sulfides that precipitate. This process usually reduces the concentration of hydrogen sulfide in sediment, but has reported concentrations above 100 mg / L of hydrogen sulfide in some sediment.
+ Hydrogen sulfide in sediment can diffuse into the upper surface water, can also be mixed into the water column by biological activity and sediment trapping by pulling nets and strong currents due to wind or ventilation. mechanics. If the rate of hydrogen sulfide diffuses into the water beyond its oxidation speed, it will detect the concentration of this potent toxin in the water column – especially in the water layer several centimeters above the boundary surface of the soil. Sediment and water.
B) Hydrogen sulfide and problems in aquaculture:
– The main problem associated with sulfur in aquaculture is the frequent presence of toxic levels of hydrogen sulfide. Sulfide can be found in water because it is the metabolite of Desulfovibrio and some other bacterial species found in anaerobic areas – usually in sediment.
These bacterial species use oxygen from sulfate as a molecular oxygen substitute in the respiratory tract. There are three types of sulfides (H2S, HS- and S2-) and they exist at a balanced level depending on temperature and pH. The influence of pH on the distribution of these three species at 25 ° C is shown in Figure (Effect of pH on the correlation coefficient of H2S, HS- and S2-). As pH increases, the proportion of hydrogen sulfide decreases and thus HS- increases until these two forms have approximately the same ratio at pH 7. At higher pH, HS is predominant and does not form S2- for Until pH is above 11.
Hydrogen sulfide generated in the sediment is mainly due to sulfate-reducing microorganisms, which can diffuse into the upper surface water and in the water column.
Hydrogen sulfide is toxic to aquatic animals because it inhibits the re-oxidation of cytochrome a3 during respiration. This effect is almost entirely caused by H2S, while the HS-nature is non-toxic. Even when toxic, S2- is not a problem, because it does not appear at the pH values found in aquaculture systems.
2. Toxicity of hydrogen sulfide
– 96-hour LC50 values (killing 50% of the test organisms in 96 hours) of hydrogen sulfide for freshwater fishes ranged from 20 to 50 μg / L, stress concentrations and fish mortality. The infection rate is much lower. A measure of toxicity – the LC50 reflects the concentration of a compound in water that killed 50% of the animals tested for a specified period of time, such as a 96-hour LC50.
– It’s ideally that freshwater fish should not be exposed to hydrogen sulfide concentrations above 2 μg / L for long periods. Shrimp and other seafood species tend to tolerate hydrogen sulfide more than freshwater species.
– The 96-hour LC50 of hydrogen sulfide for marine organisms ranges from 50 to 500 μg / L. However, the concentration of hydrogen sulfide should not exceed 5 μg / L in brackish water ponds with the highest seawater concentrations. As with freshwater fish, high concentrations of hydrogen sulfide make marine species more susceptible to disease – particularly Vibriosis in shrimp.
– Studies in soil and water systems in the laboratory conducted at Texas A & M University have shown that high levels of sulfide in water in sediment pore does not affect shrimp, as long as the faecal surface The soil and water balance is maintained in aerobic condition and the dissolved oxygen concentration in the water column is 70% saturated or higher oxygen. Studies have also shown an increased risk of hydrogen sulfide toxicity when sediment and water pH are lower.
3. How to determine hydrogen sulfide
– How to measure total sulfide concentrations is a complex task with standard laboratory methods, but aquaculture farmers can use hydrogen sulfide measurement kits to analyze total sulfide more easily. These kits give relatively reliable figures.
Of course, estimating the concentration of hydrogen sulfide from total sulfide concentrations requires data on water temperature and pH (factors to estimate hydrogen sulfide concentration). It is often possible to detect the presence of hydrogen sulfide due to the smell of rotten eggs. Hydrogen sulfide that can be measured in water usually means dissolved oxygen in water or at the interface between sediment and water at low levels, thus enhancing aeration.
He compared urgent due to h2s
4. How to manage hydrogen sulfide
– As mentioned above, streams created by aeration can disturb sedimentation, facilitating the mixing of hydrogen sulfide into water, but the positive benefits of adding oxygen by aeration Much more effective. However, aerators should be installed in a way that minimizes sediment disturbance.
– Key practices to minimize the risk of toxicity of hydrogen sulfide are careful feeding to avoid wasted wastewater at the bottom of ponds, aeration to avoid low dissolved oxygen levels, and an additional water runoff. Oxygen crosses the interface between land and water, liming to prevent deposition and acidic water.
– The bottom of pond pond should be exposed well. Sediment and sediment in areas that are too deep to be completely dry should be removed from the pond and the acidic pond bottom should be scattered.
– Some products are sometimes used for ponds because they are capable of reducing hydrogen sulfide. These products include potassium permanganate at concentrations up to six to eight times the hydrogen sulfide concentration – permanganates can oxidize sulfide. Iron compounds such as iron oxide were used for sedimentation at a rate of 1 kg / m 2 or more to induce hydrogen sulfide in the porous porous soil to precipitate iron sulfide. Sodium nitrate for water can help maintain additional oxygenation at the interface between water and soil, reducing the chance of hydrogen sulfide diffusing into the water.
– Bioproducts (probiotics) are commonly used in ponds with the belief that they reduce the risk of toxicity of hydrogen sulfide. Bacteria that oxidize sulfur are available in ponds and it is not certain that probiotics are effective in removing hydrogen sulfide. Zeolite is sometimes thought to absorb hydrogen sulfide, but the rate of treatment needed to be effective would be too much to be affordable.