Unmodified versus caustics-impregnated carbons for control of hydrogen sulfide emissions from sewage treatment plants

Unmodified and caustic-impregnated carbons were compared as adsorbents for hydrogen sulfide in the North River Water Pollution Control Plant in New York City over a period of 2 years. The carbons were characterized using accelerated H2S breakthrough capacity tests, sorption of nitrogen, potentiometric titration, and thermal analysis. The accelerated laboratory tests indicate that the initial capacity of caustic-impregnated carbons exceeds that of unmodified carbon, but the nature of real-life challenge streams, particularly their lower H2S concentrations, nullifies this advantage. As the caustic content of the impregnated carbon is consumed, the situation reverses, and the unmodified carbon becomes more effective. When the concentration of H2S is low, the developed surface area and pore volume along with the affinity to retain water create a favorable environment for dissociative adsorption of hydrogen sulfide and its oxidation to elemental sulfur, S4+, and S6+. In the case of the caustic carbon, the catalytic impact of the carbon surface is limited, and its good performance lasts only while active base is present. The results also show the significant differences in performance of unmodified carbons due to combined effects of their porosity and surface chemistry.