A role of sodium hydroxide in the process of hydrogen sulfide adsorption/oxidation on caustic-impregnated activated carbons

Four activated carbons of various origins were impregnated with different concentrations of sodium hydroxide and used as hydrogen sulfide adsorbents in an accelerated test. The materials were characterized using nitrogen sorption, thermal analysis, and standard ASTM methods. The results showed that, with increasing loading of NaOH, the H2S breakthrough capacity increases 4-5 times until maximum capacity is reached at about 10% NaOH. This capacity per unit volume of the carbon bed is the same for all carbons and insensitive to their pore structures and surface areas. The specific capacity per unit surface area is also the same for all materials studied, including activated alumina. This indicates that the amount of NaOH present on the surface is a limiting factor for the capacity. By increasing the pH value of the carbon, sodium hydroxide causes an increase in the HS- ion concentration. These ions can be further oxidized to elemental sulfur or sulfuric acid, as suggested by changes in the surface pH values. The reactions proceed until all of the NaOH reacts with H2SO4 or CO2 (a product of surface reactions or gas present in the atmosphere) and the basic environment is no longer maintained.