Anthraquinone-2,6-disulfonate

An example of a process using O2 to oxidize HiS is the Stretford process, which is licensed by the British Gas Corporation. In this process the gas stream is washed with an aqueous solution of sodium carbonate, sodium vanadate, and anthraquinone disulfonic acid. Figure 7-9 shows a simplified process diagram of the process. 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

Stretford A process for removing hydrogen sulfide and organic sulfur compounds from coal gas and general refinery streams by air oxidation to elementary sulfur, using a cyclic process involving an aqueous solution of a vanadium catalyst and anthraquinone disulfonic acid. Developed in the late 1950s by the North West Gas Board (later British Gas) and the Clayton Aniline Company, in Stretford, near Manchester. It is the principle process used today, with over 150 plants licensed in Western countries and at least 100 in China. 

Takahax A variation of the Stretford process for removing hydrogen sulfide from gas streams, in which naphthaquinone sulfonic acid is used in place of anthraquinone disulfonic acid. Four variants have been devised types A and B use ammonia as the alkali, types C and D use sodium hydroxide or carbonate. Developed by the Tokyo Gas Company and licensed in the United States by Ford Baken and Davis, Dallas, TX. Many plants are operating in Japan. 

Absorption-oxidation processes oxidize absorbed H2S directly to elemental sulfur in solution (1). The principal example in current industrial use is the Stretford process (3). The chemistry of the process can be represented by the following idealized equations (ADA represents anthraquinone disulfonic acid) ... 

Stretford plants have been in operation for 30 years. There are hundreds of such plants worldwide, used in a variety of sulfur removal operations (Dalrymple 1989). In a Stretford process, the hydrogen sulfide in the feed gas stream is absorbed and oxidized to elemental sulfur in aqueous phase, using pentavalent vanadium which is subsequently reduced from a pentavalent form to a tetravalent form. Later in the process, the vanadium is re-oxidized back again, using anthraquinone disulfonic acid (ADA) as a catalyst, and the elemental sulfur is floated to the surface of the solution and removed. 

The boundary between all oxidized forms and all reduced forms of a substance can be drawn from Equation (18) by expanding Q (Equation (17)) to include acid/base speciation. Figure 16.1 shows this for five substances that exhibit moderately complex, but well characterized, speciation as a function of pH (uncomplexed Fe(II)/Fe(III), iron porphyrin, juglone, lawsone, and anthraquinone disulfonate). The resulting Eh-pH diagram shows, for example, that the hydroquinone of lawsone is a reductant relative to anthraquinone disulfonate, below pH 7.5, but the relationship is inverted at higher pH. A similar crossing... 

Figure 27. Schematic representation of the reconstituted Mb electron acceptor composite via electrostatic interaction. The photoinduced electron transfer occurs from the reconstituted zinc Mb to electron acceptor such as methyl viologen, anthraquinone disulfonate, cytochrome c within the stable complex.

Photosensitized vectorial electron transfer from the (morpholine)ethenesulfonate anion (MES ) to 1,5-anthraquinone disulfonate (AQDS ) mediated by dibutyl (BTDB) or diethyl (BTDE) esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid solubilized in CTAB micelles has also been reported [73], The proposed mechanism of BTD-mediated electron transfer is shown in Figure 7, where reductive quenching of photoexcited BTD by interfacially adsorbed MES is followed by electron transfer from reduced BTD to the AQDS , recycling the sensitizer. 

Electron transport across organised bilayers is an integral part of biological energy storage systems such as photosynthesis and provides a means of controlling back electron transfer and of separation of the products of redox reactions. Esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid (481) have been used to study the transfer of electrons from 2-(morpholino)ethanesulfonic acid (MES) to 1,5-anthraquinone disulfonate in micelles or across vesicle bilayers. The esters absorb the light, accept an electron from MES and transfer it to the... 

AQ, short for anthraquinone disulfonic acids, see text for details. 

The catalytic effect functions for disulfonation, as well as for mono-sulfonation, thereby allowing preparation of the various ot-fi, a-a, or fi-fi anthraquinone disulfonates at will, depending on whether the mercury was present during the mono- or the disulfonation step or during both or during neither. 

Fig.6. Cyclic voltammetry of 2,6-anthraquinone disulfonic acid at mercury ultramicroelectrodes. Scan rate = 10240 mV/s. The top voltam-mogram was obtained with a platinum electrode overcoated with mercury. The bottom voltammogram was obtained using a platinum electrode that was etched and silanized before coating with mercury as shown in Fig.5.

The first stage of the Stretford process absorbs H2S over sodium carbonate contained in the wash liquor together with sodium ammonium vanadate, anhydrous citric acid and anthraquinone disulfonic acid. The reaction... 

The V ions are re-oxidized to vanadium pentoxide by adding air in the oxidation section, the anthraquinone disulfonic acid acting as oxygen carrier and accelerator. 

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