MECHANISM FOR THE FORMATION OF ELEMENTAL SULFUR FROM AQUEOUS SULFIDE IN CHEMICAL AND MICROBIOLOGICAL DESULFURIZATION PROCESSES

A detailed reaction mechanism is proposed for the formation of crystal line elemental sulfur from aqueous sulfide by oxidation with transitio n-metal ions like V-V, Fe-III, Cu-II, etc. The first step is the forma tion of HS. radicals by one-electron oxidation of HS- ions. These radi cals exist at pH values near 7 mainly as S-.-. Their spontaneous decay results in the formation of the disulfide ion S-2(2-). The further ox idation of disulfide either by S-.- radicals or by the transition-meta l ions yields higher polysulfide ions from which the homocyclic sulfur molecules S-6, S-7, and S-8 are formed. In water these hydrophobic mo lecules form clusters which grow to droplets of liquid sulfur (sulfur sol). Depending on the composition of the aqueous phase, crystallizati on of the liquid sulfur as either alpha- or beta-S-8 is rapid or delay ed. Surfactants delay this solidification, while certain cations promo te it. All these reactions are proposed to take place in desulfurizati on plants working by the Stretford, Sulfolin, Lo-Cat, SulFerox, or Bio -SR processes. In addition, the sulfur produced from sulfide by oxidiz ing sulfur bacteria is formed by the same mechanism, which now explain s many observations made previously (including the formation of the by products thiosulfate, polythionates, and sulfate).