Kinetics of the nucleophilic dissolution of hydrophobic and hydrophilic elemental sulfur sols by sulfide

Abstract

The reaction most directly responsible for polysulfide formation in aquatic settings, as a key part of overall biotic and abiotic sulfur cycling, is the nucleophilic reaction between elemental sulfur and sulfide. The reactivity of elemental sulfur, and therefore the production of polysulfide, is fundamentally tied to a suite of poorly understood surface properties of elemental sulfur, that in natural environments is thought to primarily exist in the most stable state (α-S8), or as biologically produced elemental sulfur (S8bio). Sulfur sols, a group of particles from nanoparticle to colloidal size, can be used as model systems to isolate key characteristics responsible for poorly understood reactivity associated with sulfur cycling. Specifically, Weimarn sols (S8weimarn), as hydrophobic particles, and Raffo sols (S8Raffo) as hydrophilic particles, represent ideal end members for biologically produced elemental sulfur.The reaction most directly responsible for polysulfide formation in aquatic settings, as a key part of overall biotic and abiotic sulfur cycling, is the nucleophilic reaction between elemental sulfur and sulfide. A series of pseudo-first order kinetic batch reactions were performed after development of a technique to measure sulfur sol concentrations with sufficient quality at second time scales on a 96-well microplate UV–Vis spectrophotometer. The experiments facilitated determination of the kinetics of the reaction between elemental sulfur sols and sulfide as a function of specific surface area, sulfide concentration, pH, ionic strength, and temperature for both Raffo and Weimarn sols, and determination of the effect of organic surfactants (using sodium dodecylsulfate, SDS, as an analogue). A broad governing rate law for either sol proved statistically undefinable, likely a result of key back reactions, surface reaction details, and undefined side reactions. However, a series of rate laws for specific conditional variables and defined rates for an array of conditions set a kinetic framework to define these rates. Activation energy for sols are relatively close to that determined for S8bio and significantly lower than those measured for α-S8. Reaction rates as a function of surface character (hydrophilic v hydrophobic) are significantly different, and rates as a function of surfactant concentration suggest enhancement of reaction rates as a surface process, not linked to dissolution of the elemental sulfur by surfactants. An accurate description of kinetic pathways governing intermediate sulfur species thus requires specific definition of the form of elemental sulfur.