Sewage Sludge-derived Materials as Adsorbents of So2

Adsorption of sulfur dioxide on carbonaceous materials has been extensively studied (1-18). Activated carbon and activated carbon fibers have been found to be efficient adsorbents for the low temperature removal of SO2. It is believed that sulfur dioxide is adsorbed with two adsorption energies on activated carbons (3-12). The low energy, about 50 kJ/mol, corresponds to weak physical adsorption, and the second, about 80 kJ/mol, to chemisorption (11). The former process is likely to occur in small pores and is governed by microporosity and the pore size distribution of the activated carbon (12, 13). The strong adsorption is related to the presence of oxygenated groups on the carbon surface (3, 6, 8, 10, 11, 13), which are believed to be catalytic centers for sulfur dioxide oxidation to SO3 (14). Since usually the process is carried out in the presence of moisture and oxygen, sulfur dioxide is oxidized to sulfuric acid as the final product of the reaction. It was also found that three forms of adsorbed sulfur oxides could be present in such a situation. They are as follows: weakly adsorbed SO2, physically adsorbed SO3 (after oxidation of SO2), and strongly adsorbed H2SO4 (513). Recently, we showed that adsorbents obtained from sewage-sludge derived fertilizer can work efficiently as hydrogen sulfide adsorbents (19, 20). Their origin is in biosolids, which are mixtures of exhausted biomass generated in the aerobic and anaerobic digestion of the organic constituents of municipal sewage along with inorganic materials such as sand and metal oxides. Since 6.9 million tons of biosolids (dry basis) were generated in 1998 and only 60% were used beneficially (21), their conversion into useful materials is desirable. The problem of biosolid disposal is of major concern to environmentalists, especially in Europe where the standards for the heavy metal content of sewage sludgederived fertilizers are 100 times tighter than that in the U.S.A.(21). The materials obtained from carbonization of dewatered sewage sludge, with or without addition of various activating agents, usually have surface areas between 100 and 500 m/g. Reports on their performance as adsorbents of acidic gases differ (22-25). For example, the capacity for the adsorption of SO2 reported by Lu was less than 10% of the capacity of Ajax activated carbon (31). When the performance for H2S adsorption was compared, the capacity of the sludge-derived adsorbents was only 25% of the capacity of Calgon carbon, IVP 4x6 (30). Our recent studies (20) showed that sludge-derived adsorbents have a higher capacity for hydrogen sulfide removal than coconut shell-based carbon, which is considered an alternative to the caustic-impregnated carbons currently used in water treatment plants. The objective of this study is to determine the sulfur dioxide sorption capacity of sludge-derived adsorbents in comparison with the performance of coconut shell-based activated carbons. Differences in capacities result from differences in porosity, surface chemistry and inorganic constituents of the adsorbents. All of these have an effect not only on the sorption capacity but also on the chemical nature of the oxidation products.