Sulphate, to sulphide

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

The sodio sulphate (technically temed tdU take) ia next heated with calcic carbonate and small coal. The carbon reduces the sodio sulphate to sulphide, and the calojo carbonate transforms the sodio sulphide into sodio carbonate, insoluble calcic osysulpbide being simultaneously produced —... 

If the drift-weed were to be burnt to a loose ash, it would furnish 25 to 30 lbs. of iodine per ton in practice, it rarely contains more than 12 lbs. per ton. The low yield is due to faulty treatment in calcination-—e.g. (i) burning at too high a temp, which causes the volatilization of part of the iodine, and the fusion or fluxing of the ash with sand and pebbles and (ii) imperfect protection of the kelp-ash from the weather whereby some of the soluble iodides are washed out by rain. High temp, burning also reduces some of the sulphates to sulphides, which later causes a high consumption of acid per unit of iodine. 

Le Blanks Process.—This process involves three stages the conversion of sodium chloride into sodium sulphate, or salt-cake process the reduction of the sulphate to sulphide by means of carbon, and the conversion of the sulphide into sodium carbonate by the action of calcium carbonate, or black-ash process and the extraction of the sodium carbonate with water, or lixiviation process ... 

Some isotope fractionations, notably those in biological systems, are primarily controlled by kinetic effects. For example, the bacterial reduction of seawater sulphate to sulphide proceeds 2.2 % faster for the light isotope than for S. -/For the reactions... 

If there is no access to air, there is usually no significant corrosion on ferrous materials when pH > 5. However, in some environments, heavy localized corrosion may occur due to bacteria that thrive in near neutral environment (pH = 5-8) at temperatures of 10-40°C in the absence of air (anaerobic bacteria). These bacteria promote reduction of present sulphate to sulphide. Hence, they are usually called sulphate-reducing bacteria (SRB). The classical mechanism comprises the following reactions at/on the cathodes [6.16] ... 

In the absence of oxygen, they reduce sulphate to sulphide. As a result of... 

One example of such reduced metabolic products that readily react with electrodes is Fl2S, which is produced by sulphate reducers in MFCs [19]. The primary oxidation product in this process is insoluble sulphur (S°). Reduction of sulphate to sulphide requires eight electrons, but oxidation of sulphide to S° releases only two of fhese eight electrons at the electrode surface. For example, when pyruvafe was used as the substrate, the above electrochemical reactions might be expressed as... 

The reduction of sulphate to sulphide involves a change from... 

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