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Environmental fate & pathways

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In case of NaOH and Na2CO3, hydrolysis is not relevant to be studied. Instead, for more environmentally hazardous sulfide compounds, hydrolysis is examined.

In aqueous environments, Na2S and NaHS are immediately hydrolyzed and, as a function of pH, an equilibrium is established between S2-, HS-, and H2S, with increasing H2S formation with decreasing pH. Temperature and salinity also affect this equilibrium but to a much lesser extent than pH. H2S is an extremely toxic gas that can stay present in water for several hours, depending on the oxygen level of the environment under consideration. In oxic systems, oxidation to polysulfides, elemental sulfur, thiosulfate, sulfite and - eventually - sulfate will occur. Half-lives of 0.4 to 65 h have been reported for sulfide oxidation in the aquatic environment (Bagarinao, 1992). In most aerobic systems however, half-lives are less than 1 hour. Sulfide oxidation is mediated via both biotic (sulfur oxidizing microorganisms) and abiotic processes and the half-lives reported do not distinguish between these two types of oxidation. In hypoxic or anoxic environments on the other hand (e.g., static environments with high concentrations of organic matter), H2S generation often occurs under the influence of sulfur reducing microorganisms.

Reducing conditions often occur in organic-rich sediments. However, depending on the sediment under consideration, the present dissolved sulfides will precipitate under the form of metal sulfides. In iron-rich environments, FeS and - eventually - FeS2will be the most abundant reduced sulfur compounds. H2S concentrations will be negligible or fluctuate depending on the importance of the other reactions but in some cases H2S concentrations may be relatively high during extented time periods. In view of the hazard assessment however, one must keep in mind that in sediments where these conditions occur naturally, living organisms are typically adapted to temporary fluctuations of H2S concentrations.

In well drained and oxic soils, released sulfides will be oxidized very fast to - eventually - sulfate and no H2S will be present. Here too, in soils with reducing conditions, such as in waterlogged soils or soils with excess organic matter, H2S formation will occur naturally. As in sediments, the presence of H2S will be driven by the importance of other reactions such as metal sulfide precipitation and formation of iron sulfides. Metal sulfide precipitation also occurs in deeper soil layers of well drained soils.

Due to low vapour pressure, Na2S and NaHS are not expected to be released to the air as such, however, when H2S is formed, H2S may be released. Atmospheric sulfur compounds undergo complex chemical and photochemical reactions, the overall effect being the oxidation of reduced compounds to sulfate (Brown, 1982). First, H2S is oxidized to SO2. Rate determining step is the first step where a hydroxyl radical abstracts hydrogen to form a hydrosulfide radical and water. Second step is reaction of the hydrosulfide radical with oxygen to form SO and then sulfur dioxide. Residence time of hydrogen sulfide in the troposphere has been calculated to be 18 h (Beauchamp et al., 1984). Sulfur dioxide then reacts with water to form sulphurous acid, which is rapidly oxidize to sulphuric acid (Brown, 1982).