Reducing sulfides

Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

At least one member of an active pair should probably exist to a greater extent on sulfided than on unsulflded catalysts. Exposed reduced metal sites, either slightly electropositive or uncharged, shown by Infrared to exist appreciably only on reduced sulfided catalysts, thus appear to be likely members of active pair sites. 

Leek and Bagander [221] determined reduced sulfide compounds (hydrogen sulfide, methyl mercaptan, carbon disulfide, dimethyl sulfide, and dimeth-yldisulfide) in water by gas chromatography using flame detection. Detection limits ranged from 0.2 ng/1 for carbon disulfide to 0.6 ng/1 for methyl mercaptan. Hydrogen sulfide was determined at the 1 ng/1 level. 

Leek and Baagander [311] determined reduced sulfide compounds in seawater by gas chromatography using a flame ionisation detector. Substances determined include methyl mercaptan, dimethyl sulfide, hydrogen sulfide and carbon disulfide. Detection limits range from 0.2ng/l (carbon disulfide) to 0.6 ng/1 (methyl mercapton). 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

The thickness of the sewer biofilm affects sulfide formation. Reduction of the biofilm thickness by increasing the wastewater velocity may lead to reduced sulfide problems. At very low velocities in an arerobic gravity sewer, a biofilm thickness may be more than 50 mm however, it may be substantially reduced to typically 1-5 mm when the velocity is increased. The thickness of... 

Chlorine the use of chlorine (Cl2) as a sulfide-controlling agent is based on its poisoning effect on the biological system and its oxidizing effect on sulfide. Chlorine is nonspecific, and a great number of side reactions with wastewater components may occur. Even concentrations of about 50 g m-3 have not efficiently been able to reduce sulfide concentrations of about 5-10 gS m-3. Chlorine is an environmentally problematic chemical that generally cannot be recommended for use in sewer systems. 

Removal of sewer biofilm and deposits by flushing and use of a cleaning ball for detachment of biofilm and resuspension of sewer sediments are examples of mechanical methods for reducing sulfide occurrence. 

Figure 16. X-ray powder diffraction of WS2 (a) bulk (2H) phase prepared by reducing-sulfidization of W03 in H2S (b) IF nanoparticles prepared by reducing-sulfidization of W03 in H2S and H2 gas (31b).

Swern oxidations have been performed using the PEG2000 bound sulfoxide 34 as a dimethylsulfoxide (DMSO) substitute (reaction 13).49-50 Several alcohols were efficiently oxidized to their aldehydes or ketones using this reagent, oxalyl chloride, and triethylamine. Precipitation of the polymer with cold diethyl ether and filtration through a pad of silica afforded the desired oxidized products in very good yields and purities. The reduced sulfide polymer could be reoxidized to sulfoxide 34 with sodium metaperiodate and used again in reactions with no appreciable loss in oxidation capacity. 

Moore, J. N. (1994). Contaminant mobilization resulting from redox pumping in a metal-contaminated river-reservoir system. In Environmental Chemistry of Lakes and Reservoirs, ed. L. A. Baker, pp. 451-71. Washington, D.C. American Chemical Society. Moore, J. N., Ficklin, W. H. Johns, C. (1988). Partitioning of arsenic and metals in reducing sulfidic sediments. Environmental Science and Technology, 22, 432-7. Morrison, G. M., Batley, G. E. Florence, T. M. (1989). Metal speciation and toxicity. Chemistry in Britain, 8, 791-5. 

Interpretation of these data suggests a question What is the oxidation process leading to the formation of pyrite (analytically determined as chromium-reducible sulfide, CRS) instead of simple precipitation of FeS (analytically determined as acid-volatile sulfide, AVS) AVS constituted less than 10% in the study of White et al. (35). 

Moore, J.N., Ficklin, W.H. and Johns, C. (1988) Partitioning of arsenic and metals in reducing sulfidic sediments. Environmental Science and Technology, 22(4), 432-37. 

The Mo hydroxylases are difficult to model because of the general redox incompatibility of the reducing sulfide and oxidizing Mo(VI) moieties found in the enzymes. The outcomes and directions of modeling endeavors up to 1997 have been reviewed by Young. ... 

A variety of other Lewis acids have been used in conjunction with LAH, including nickel, copper and titanium salts, the resulting reagents being used to reduce sulfides and benzyl ethers to hydrocarbons (equations 46-48). Addition of nickelocene to LAH produces a specific desulfurization reagent which does not reduce ketones and esters. ... 

For gold transported at temperatures of less than 400 °C in reduced sulfide-bearing solutions, gold solubility is maximized near the H2S-HS -S04 equal predominance point at any given temperature, pressure, and activity of water (see Figure 3). As conditions become more oxidized, the sulfide complexes that stabilize gold in aqueous solution break down, and gold metal precipitates. As pH increases (in the HS predominance field), the redox reaction... 

Reduced Sulfides Contributing to the Odor Control Problems in... 

TABLE 15.12 Total Reduced Sulfide Emissions from the Chemical Recovery Operations of a Kraft Pulp Mill, in kg S per Air Dry Tonne of Pulp"... 

Source Total reduced sulfide emissions (kg S/tonne) With No controls controls ... 

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