Low Temperature Catalytic Oxidation of Hydrogen Sulfide and

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Environ. Sci. Technol. 2003, 37, 2568-2574

Low Temperature Catalytic Oxidation of Hydrogen Sulfide and Methanethiol Using Wood and Coal Fly Ash JAMES R. KASTNER,* K. C. DAS, QUENTIN BUQUOI, AND NATHAN D. MELEAR Department of Biological and Agricultural Engineering, Driftmier Engineering Center, The University of Georgia, Athens, Georgia 30602

The feasibility of reusing waste material as an inexpensive catalyst to remove sulfur compounds from gaseous waste streams has been demonstrated. Wood and coal fly ash were demonstrated to catalytically oxidize H2S and methanethiol (CH3SH) at low temperatures (23-25 °C). Wood ash had a significantly higher surface area compared to coal ash (44.9 vs 7.7 m2/g), resulting in a higher initial H2S removal rate (0.16 vs 0.018 mg/g/min) under similar conditions. Elemental sulfur was determined to be the end product of H2S oxidation, since X-ray diffraction analysis indicated the presence of crystalline sulfur. Catalytic decay occurred apparently due to surface deposition of sulfur and a subsequent decline in surface area (44.9-1.4 m2/g) during the reaction of H2S with the ash. Methanethiol was stoichiometrically converted to dimethyl disulfide ((CH3)2S2) without significant catalytic decay. Catalytic decay was reduced and H2S conversion increased (10% at 1.8 days vs 94% at 4.2 days) when H2S loading was decreased to levels typical of many environmental applications (500 ppmv inlet and 1.43 mg/min vs 60 ppmv, 0.09 mg/ min). Catalyst regeneration using hot water (85 °C) washing was possible, but only increased fractional conversion from 0.2 to 0.6 and the initial reaction rate to 50% of the original H2S oxidation activity.

Introduction High volume low concentration emissions (HVLC) from many industries (e.g., pulp and paper and wastewater treatment facilities) contain a range of reduced sulfur compounds, such as H2S, methanethiol, and dimethyl disulfide, which are odorous and toxic at high concentrations (1). Regenerative thermal oxidation (RTO) and wet scrubbers are two of the primary air pollution control technologies used to treat the reduced sulfur compounds (2, 3). RTOs have high operating costs, since oxidation occurs at high temperatures (760-870 °C), produce a greenhouse gas (CO2) due to combustion of an external carbon source, and require SO2 scrubbing (if sulfur is present). Wet scrubbers require costly oxidizing chemicals, such as ClO2 or NaOCl, and large amounts of water and can produce chlorinated hydrocarbons if not properly controlled. A cost-effective air pollution control technology for treatment of reduced sulfur compounds that reduces energy costs, water * Corresponding author phone: (706)583-0155; fax: (706)542-8806; e-mail: [email protected]. 2568

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consumption, and greenhouse gas production is required for many industries. Activated carbon (AC) is used in many cases to remove a wide range of compounds from gaseous emissions via adsorption and is reported to oxidize H2S over temperature ranges from ambient to 250 °C (4-6) with metal oxides suggested to be the active centers (7). It is theorized that chemisorption of oxygen and H2S is required and that elemental sulfur is the primary product at temperatures below 200 °C, resulting in surface deposition and catalytic decay (5, 7). The mechanism appears to be a function of temperature and the presence of water. At temperatures below 100 °C, condensed water and water vapor (i.e., humidity) is reported to enhance H2S oxidation (8-10). However, the presence of water reduces H2S conversion at temperatures above 125 °C (6). For example, reactors packed with activated carbon fiber (ACF) reduced inlet H2S concentrations from 200 ppmv to 0-2 ppmv (90% relative humidity, 80% moisture content), at 20 °C for 45 days, with periodic washing required to maintain activity (11). Dry ACF resulted in only a 12.5% H2S conversion over the same period. In addition, if H2S was present, both methanethiol (MT) and dimethyl sulfide (DMS) were oxidized to methane sulfonic acid and dimethyl sulfoxide or dimethyl sulfone respectively, using ACF (12). Finally, activated carbon oxidizes H2S in wastewater at temperatures ranging from 8 to 24 °C with conversions ranging from 60 to 100% for residence times of 8-11 min (13). Although activated carbon has both a high adsorption capacity and potential catalytic activity toward reduced sulfur compounds, high cost limits the use of activated carbon. Coal and wood ash are produced in large volumes in the United States (75 and 5.5 million tons/yr, respectively) with a majority landfilled (14, 15). It has been suggested that coal ash can act as an adsorbent for H2S (16), but little research has been performed to determine if ash can be reused as a catalyst. Recent research has demonstrated that coal fly ash can catalytically oxidize H2S and ethanethiol; however, limited information was obtained on the potential mechanism and kinetics of reaction (17). It is theorized that ash can act as a low temperature adsorbent/catalytic oxidizer of reduced sulfur compounds, given the presence of carbon, high surface area, and metal oxides in its matrix. Waste ash could be pelletized or incorporated into different matrices and used in different reactor designs for reduced sulfur compound removal in a wide range of industries. The objectives of this research were to compare the catalytic oxidation potential of coal and wood fly ash, determine the kinetics of H2S and methanethiol oxidation and decay, determine a potential mechanism for the catalysis, and study a method to regenerate the catalyst.

Experimental Methods Ash Characterization. Coal ash from a process utilizing selective catalytic reduction (SCR) via NH3 injection for NOx removal and wood ash from a pulp mill was used in this study. Activated charcoal (Sigma-Aldrich) was also tested for direct comparison to the fly ash (pH 9.26 ( 0.02, 700 ( 100 m2/g - Sigma Technical Services). The physical and chemical characteristics of the fly ash were determined and included pH (17), surface area (BET using N2 - Nova 3000 Quantachrome, Boynton Beach FL), bulk density, and the elemental composition. Surface area was calculated from N2 adsorption isotherms at -196 °C using the six point BET method. Original samples (0.18-0.26 g) were heated to 200 °C and degassed under vacuum (10-5 Torr) to constant pressure (12 h) before surface 10.1021/es0259988 CCC: $25.00

 2003 American Chemical Society Published on Web 04/25/2003

FIGURE 1. X-ray diffraction patterns of coal and wood fly ash, and wood ash reacted with H2S, identifying crystalline phases: M mullite (Al2O3); Mg - magnetite (Fe3O4); Ht - hematite (Fe2O3); B - bassanite (CaSO4‚1/2H2O); Hb - hannebachite (CaSO3‚H2O); Q - quartz (SiO2); C - calcite (CaCO3); S, orthorhombic sulfur.

FIGURE 2. The reaction of methanethiol (inlet b, outlet O), subsequent formation of dimethyl disulfide (9) with wood ash (2 g, 25% H2O), and decline in fractional conversion of methanethiol (3) at a 1.36 s residence time (9.1 cm packing height). A trace amount of dimethyl disulfide (0) was observed in the methanethiol source. area analysis. Treated samples were degassed at 100 °C to avoid sulfur loss (Figure 3, Supporting Information). Metal content was analyzed via ICP-MS (Perkin-Elmer Elan 6000) using EPA method 3051 for digestion, and the carbon and nitrogen contents were measured using combustion and infrared detection (Leco 2000). XRD Methods. X-ray diffraction (XRD) analyses were performed with a Scintag XDS 2000 diffractometer equipped with a cobalt X-ray tube. Powdered ash samples were mounted on quartz plates and stepped scanned over the angular range 15-50° 2θ. The step size for the analysis was 0.01° 2θ with a count time of 10 s/step. In one experiment, wood ash (25% moisture, 10 g) was reacted with an inlet stream of H2S (235 ppmv) for 48 h, and the ash was analyzed via XRD to detect elemental sulfur. All crystalline phases were identified based on comparison of the observed data with a reference database of crystalline materials produced by the International Center for Diffraction Data (ICDD). Continuous Flow Studies. The extent of H2S, MT, and DMDS conversion was measured in a continuous flow packed bed reactor (Figure 1, Supporting Information). Compressed air or N2 was first passed through a bubble column (5.0 cm i.d., 30.0 cm length) to humidify the gas, mixed with H2S (5% in N2), MT (0.5% in N2), or DMDS (0.5% in N2) using a mass

FIGURE 3. Continuous conversion of gaseous H2S stream (500 ppmv for wood ash and 343 ppmv for coal ash) and the overall reaction rate (insert) using a reactor packed with 10 g of wood fly ash (b) or coal fly ash (0) at 23 °C and a 4.6 s residence time. flow controller (Aalborg AFC 2600 D with Kalrez O-rings), passed through a static mixer (2.5 cm i.d., 30.0 cm length, 50 mm diameter glass beads), and then transported downward through the reactive column (Figure 1, Supporting Information). The compressed air or N2 rate was also controlled using a mass flow controller (UNIT UFC-8100). When the kinetics of reactant mixtures were studied, the second gas was connected via a tee into the main airflow in front of the mixer (Figure 1, Supporting Information). The mass flow controllers were factory calibrated and verified in our lab using rotameters specific for the gas of interest (Glimont Inc.). The air was prehumidified in a bubble column before being mixed with the reactants and entering the reactor. Coal fly ash, wood fly ash, or activated charcoal (210 g ash, 25% H2O (g/g), wet basis) was distributed in glass wool and packed over a defined height in the reactor. Humidity levels in the outlet of the bubble column were measured using a probe (Omega Engineering, Inc., Model RH30-3) and reached 100% relative humidity within 20 min after start-up. Tees (stainless steel, Swage-Lock) with septum were installed at the inlet and outlet of the column for sampling. All tubing was 6.35 mm (i.d.) Teflon, fittings were constructed of stainless steel (Swage-Lock), and the end of each column contained a threaded Teflon plug with an O-ring for an airtight fit. Residence times based on gas flow rate and packing height of the ash, ranged from 0.01 to 9.2 seconds, and inlet reactant concentrations ranged from 35 to 650 ppmv (0.0035-0.065%(v/v)). All kinetic studies were carried out at 23-25 °C and atmospheric pressure. The reactions were considered to be isothermal and constant pressure, since the temperature increase and pressure drop were minimal (0.2 °C and pressure drop of 0.4 kPa). The absolute pressure at the reactor inlet was 102.4 kPa (1.01 atm). Analytical Methods. Gas Analysis. Portable GC/MS units were used to measure H2S, MT, and DMDS concentrations at the inlet and outlet of the packed-bed reactor (HAPSITE Inficon, East Syracuse,, NY (17)). Gas samples were analyzed under isothermal conditions (60 °C or 80 °C) using a selective ion monitoring (SIM) method. In the SIM mode mass/charge ratios (m/z) of 34 (H2S), 47 (methanethiol), 79 (dimethyl disulfide), and 69 (internal standard) were selectively scanned. In addition to these reactants, SO2 (full scan), CS2 (full scan), and DMDS (SIM at 79) were monitored using SIM as potential reaction products of methanethiol and H2S oxidation. Standard curves and continuous flow studies were based on the peak area ratio of the component peak (e.g., 34 for H2S) to the internal standard. VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Comparison of the Physical and Chemical Characteristics of Coal Fly Ash (CFA) and Wood Fly Ash (WA) Used in Continuous Adsorption/Oxidation Studies properties m2/g

surface area, pH bulk density, g/cm3 carbon, % (dry basis) selected elements (ppm or mg/kg) Co Cu Mn Mo Ni V Fe a

CFA (mean ( SD)

WA (mean ( SD)

7.67 ( 0.96 11.88 ( 0.06 0.72 7.6 ( 0.14

44.89 ( 8.34 12.13 ( 0.17 0.54 18.75 (1.87

range

mean

range

mean

6.5-34.0 10.0-181.0 7.0-41.0 3.4-8.0 13.0-37.0 20.0-80.0 2200-19 656

13.38 54.52 70.92 5.26 21.75 47.66 7865.84

4.5-5.2 32.0-39.0 500.0-584 2.2-2.7 18-19 NDa 6600-8300

4.94 34.57 542.74 2.43 18.27 NDa 7470.54

ND: not determined.

Kinetics. The overall rate of oxidation was calculated from the measured fractional conversion, mass of the ash, volumetric gas flow rate, inlet mole fraction, pressure, and temperature using the following equation

-rH2S ) Q

X P y MW RT HSS W

(1)

where -rH2S is the reaction rate (mg/g/min), Q is the flow rate (L/min), y is the mole fraction in the inlet, X is the fractional conversion, W is the mass of ash (g), P is pressure (atm), R is the ideal gas constant, T is the temperature (K), and MW is the molecular weight of the reactant (e.g., H2S). Fractional conversion (X) was determined as the difference between the inlet and outlet concentration divided by the inlet concentration. Inlet and outlet concentrations at each time point were measured in triplicate (the coefficient of variation ranged from 0.3 to 8.0%). Reaction/Adsorption Capacity. The H2S reaction/adsorption capacity of the ash was calculated from the inlet and outlet concentration versus time curves. The adsorption capacity of the ash was calculated from the integrated area above the outlet concentration versus time curve (area ) ∫t0BCg,indt - ∫t0BCg,outdt), the flow rate, and mass of the ash. The reaction/adsorption capacity was a function of the integration limits defined by the breakthrough time, tB, at a defined outlet concentration. To compare reaction/adsorption capacities of the ash to activated carbon (18), breakthrough times occurring at 1/4th to 1/8th of the inlet concentration were also calculated. Regeneration Methods. In these experiments 10 g of wood ash (25% H2O) was reacted with 650 ppmv H2S until catalytic decay resulted in a fractional conversion of 0.20 (i.e., 20%). The ash was subsequently washed with 200 mL of deionized water and heated to 85 °C for 45 min. The solution was then filtered and dried at 105 °C for 3 h. Deionized water was added to the ash to reach 25% H2O before kinetic analysis in the continuous flow apparatus. Subsequent regeneration steps followed the same procedure.

Results and Discussion The physical and chemical properties of the ash indicated catalytic potential given the high surface area and pH and the presence of metals typically used in catalytic oxidation (Table 1). The surface area and carbon content for wood ash was significantly higher than coal ash (t-test, R ) 0.05), both suggesting better catalytic properties (Table 1). X-ray diffraction analysis also indicated the presence of crystalline phases typically associated with catalytic activity (Figure 1). 2570

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Mullite (3Al2O3•2SiO2), magnetite (Fe3O4), and hematite (Fe2O3 possibly with small amounts of other metals such as V, Mn, Cu, and Co substituting for Fe) were identified in wood ash and have been reported to catalyze the oxidation of reduced sulfur compounds or other VOCs (19, 20). In mullite, an alumina silicate, Fe3+ and Ti can replace Al in the structure. The crystalline phases identified in the coal ash were mullite, bassanite (CaSO4‚1/2H2O), and hannebachite (CaSO3‚H2O); bassanite and hannebachite have not been reported to act as catalysts. A catalytic mechanism for removal of H2S and MT (CH3SH) using the ash was clearly established due to the fact that without O2, H2S was not removed from the air stream (17) and that MT was converted to DMDS ((CH3)2S2)). DMDS was the primary product identified in the catalytic oxidation of MT, and the formation of CS2 or (CH3)2S was not detected (Figure 2). The fractional conversion of MT at an inlet concentration of 90 ppmv declined from 0.8 to approximately 0.4 after 25 h resulting in a decrease in the formation of DMDS (Figure 2). Previous research demonstrated the formation of diethyl disulfide from ethanethiol using coal fly ash in batch reactors (17) and activated carbon has been shown to catalytically oxidize MT to DMDS (21, 22), suggesting a similar catalytic mechanism between the materials. Contrary to activated carbon (22), wood ash (2 g) did not show a significant decline in catalytic activity toward MT oxidation. In our research, fractional MT conversion equilibrated at 0.38 after 25 h compared to activated carbon (1 g) in which conversion declined to 0.20 after 3.0 h (22). This was probably due to differences in the inlet MT concentration treated, 0.10% (v/v) with activated carbon and 0.01% (v/v) in our work. The lower concentration treated in our work is typical of expected environmental applications. Similar results were obtained for H2S in the presence of 21% O2, except that reaction products could not be measured in the outlet (e.g., SO2), and catalytic decay was typically faster (Figure 2, Supporting Information). Fractional conversion of H2S at an inlet concentration of 500 ppmv declined from 1.0 to 0.3 after 25 h. The decline in fractional conversion or catalytic decay was potentially due to deposition of sulfur on the surface of the wood ash. The surface area of wood ash used to treat 500 ppmv H2S declined from 44.9 to 1.4 m2/g (t-test, R ) 0.05) after the fractional conversion of H2S had significantly declined (Table 2). Moreover, if the degassing temperature in the surface area measurement method was not reduced below the boiling point of sulfur a yellow powder, apparently sulfur, was formed on the tubes above the sample (Figure 3, Supporting Information). Finally, XRD analysis of wood ash reacted with H2S indicated the formation of

TABLE 2. Comparison of the Physical and Chemical Characteristics of Wood Fly Ash after Catalytic Oxidation of H2S and Comparison to Activated Carbon wood ash properties m2/g

surface area, pH H2S reaction/adsorption capacity, (mg/g) CA,out/CA,in ) 0.06 (60 ppmv) CA,out/CA,in ) 0.13 (678 ppmv) CA,out/CA,in ) 0.16 (3000 ppmv) CA,out/CA,in ) 0.238 (500 ppmv) reaction rate, mg H2S/g-h initial, 500 ppmv inlet, 23-25 °C, 25% moisture, 95-100% RH initial, 40-60 ppmv H2S, 23-25 °C, 25% moisture, 95% RH, 60-80 ppmv MT initial, 1000 ppmv H2S, 20 °C, 2% moisture, 35% RH steady state (200 min), initial, 1000 ppmv H2S, 20 °C, 2% moisture, 35% RH 1-3 h online, 0.0625% H2S, 95-97 °C, dry gas 259 min, 3000 ppmv H2S, 200 °C, dry gas a

Activated carbon: 1050-1150 m2/g.

b

activated carbon

original

treated

44.89 ( 8.34 12.13 ( 0.17

1.39 ( 0.68 7.78 ( 0.10

original 500

treated 50 (5)

53.0 16.1 3-125 (18) 31.8 9.0 3.15

4.0 (activated charcoal) 18.4 (8)a 2.2 (8) 85.0 (5) 132 (6)b

Activated carbon: 487-1250 m2/g.

FIGURE 4. Effect of sulfur loading (500 ppmv inlet and 4.6 s residence time b, 61 ppmv inlet and 9.2 s O) on catalyst decay (10.04 g each) as indicated by the decline in fractional conversion of H2S. orthorhombic sulfur, indicating that catalytic decay was due to formation of elemental sulfur (Figure 1). Formation of sulfur deposits during the catalytic oxidation of H2S by activated carbon has been shown to significantly reduce surface area, decrease H2S conversion, and subsequently reduce catalytic activity (5, 8, 9, 10, 22, 23). For example, Meeyoo and Trimm (9) report a reduction in surface area from 1000 to 100 m2/g after 72 h when treating a 500 ppmv H2S stream at 25 °C. Similar to our results, fractional H2S conversion decreased from 0.76 to 0.05 over the 72- period (9). Slower catalytic decay in the activated carbon compared to the wood ash is probably due to the significantly higher surface area of the activated carbon and was observed in one experiment in which activated charcoal was directly compared to wood ash (Figure 5, Table 2). Unlike H2S and methanethiol, dimethyl disulfide (DMDS) did not react with the wood ash (2 g, 25% H2O), and no measurable conversion of DMDS was observed at an inlet concentration of 40 ppmv and a 1.36 s residence time (data not shown). We have previously demonstrated that coal fly

ash was not capable of oxidizing DMDS (17). These results imply that under the applied conditions ash is not capable of oxidizing the disulfide bond in DMDS or the C-S bond in dimethyl sulfide (DMS). Catalyst Decay. Given the significantly higher surface area of wood ash it was theorized that the overall reaction rate in the wood ash would be significantly higher than in the coal fly ash. Under similar conditions, the initial fractional conversion and reaction rate of H2S using wood fly ash was 2 and 8 times higher than that using coal fly ash (Figure 3). Although the overall reaction rate was higher for the wood ash, catalyst decay was still evident, as indicated by the decline in the fractional conversion and overall reaction rate (Figure 3). The higher surface area of the wood ash also appeared to reduce catalytic decay (relative to the coal ash), since the reaction rate for the wood ash was approximately 2 times that of the coal ash after 25 h (Figure 3). Similarly, catalytic decay toward H2S oxidation (in the presence of MT) was much slower for activated charcoal probably due to the significantly higher surface area (700 m2/g versus 44 m2/g for the wood ash - Figure 5); for example, the hydrogen sulfide oxidation rate declined from 3 to 1 mg/g-h (39 h) for wood ash compared to 4 to 2.3 mg/g-h for activated charcoal (53 h). Catalytic decay was also affected by the sulfur loading rate (CAo*Q). Hydrogen sulfide conversion remained higher than 90% after 100 h at a sulfur loading rate of 0.0897 mg H2S/min compared to only a 10% conversion after 48 h at a loading rate of 1.43 mg/min (Figure 4). Mixtures of Sulfur Compounds. Previous research indicated that DMS, DMDS, or MT could only be oxidized by activated carbon in the presence of H2S with methane sulfonic acid formation from MT and DMSO (dimethyl sulfoxide) and DMSO2 (dimethyl sulfone) from DMS and DMDS (11, 12). Contrary to Katoh et al. and similar to our results with MT only, MT was stoichiometrically converted to DMDS in the presence of H2S using wood ash (Figure 5). In addition, the presence of H2S did not result in DMDS oxidation, since DMDS was measured in the outlet (Figure 5). Thus, contrary to previous work with activated carbon fiber (12), the addition of H2S to DMDS did not result in the oxidation of this compound; H2S was removed but not DMDS. It is noteworthy that the fractional conversion of MT declined to a much VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Catalytic conversion of a H2S (inlet 9, outlet 0) and methanethiol (inlet b, outlet O) gas mixture, the formation of dimethyl disulfide (inlet 1, outlet 3) from methanethiol, and the fractional conversion of H2S ()) and methanethiol (() using 2 g each of wood ash and activated charcoal. A trace amount of dimethyl disulfide was observed in the methanethiol source for both experiments. lower level in the presence of H2S (compare Figures 5 and 2), suggesting that the deposition of sulfur also effected MT oxidation. Direct comparison with activated charcoal gave similar results to wood ash (MT was converted to DMDS in the presence of H2S); however, catalytic activity toward H2S oxidation showed reduced catalytic decay, and MT oxidation did not show signs of catalytic decay at the conditions tested (Figure 5). The difference between our work and that reported by Kato et al. (e.g., complete removal of MT without the formation of DMDS and subsequent formation of methane sulfonic acid in the presence of H2S) may be due to a different catalytic mechanism between activated carbon fiber and ash or activated charcoal, longer residence times (36 s compared to 0.01 to 9 s in our work), and a higher ratio of H2S to MT (10:1 versus 0.7:1). Katoh et al. (12) propose that H2S oxidation leads to polysulfide radical formation and the subsequent formation of oxygen and hydroxyl radicals which oxidize MT and DMS to methane sulfonic acid or dimethyl sulfoxide and dimethyl sulfone. Bashkova et al. (21) proposed a similar mechanism, except that H2S is not required since the activated carbon surface is able to generate oxygen and hydroxyl radicals, for oxidation of DMDS (formed from MT) to methane sulfonic acid. It is possible that the formation of the free radicals and subsequent oxidation of DMDS (formed from MT) could be rate limiting and thus would not have been measured at our residence times. However, we cannot discount the formation of methane sulfonic acid, since we did not analyze the ash or activated charcoal at the end of these experiments. Catalyst Regeneration. Methods to regenerate the catalyst were studied, given the fact that catalytic decay occurred during H2S oxidation. Catalyst regeneration would extend the use of the ash, minimize maintenance cost, and reduce the cost of spent catalyst treatment. A variety of methods has been used to regenerate activated carbon treating H2S, including hot and cold water washing (18), hydrogen gas treatment (24), and heating the carbon at high temperatures (e.g., 120-325 °C) in the presence of nitrogen (5, 13, 25). Given its simplicity, we studied the effect of washing the catalyst with water. Regeneration using hot water resulted in an increase in fractional conversion from 0.2 to 0.5 and an increase in the initial overall H2S reaction rate from 0.03 to 0.09 mg/g/min or 50% of the original wood ash reactivity (Figure 6). However, a second regeneration attempt did not 2572

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FIGURE 6. Effect of hot water washing on the overall reaction rate (A) and catalytic conversion (B) of H2S. Catalytic conversion of H2S (inlet b, outlet O) using fresh wood ash (10.04 g), after the first regeneration (inlet 9, outlet 0), second regeneration (inlet 1, outlet 3), and the subsequent effect on fractional conversion ((). Arrows indicate regeneration points. increase catalytic activity (Figure 6). These results are similar to Bagreev et al. (18) who observed a 60% reduction in adsorption capacity after catalytic decay and subsequent reaction after water regeneration. However, contrary to our results, the adsorption capacity, although lower than the original activated carbon, remained relatively constant after multiple regeneration steps (i.e., 60% of the original activated carbon). These differences may have been due to different washing methods. Bagreev et al. first washed the activated carbon five times via a Soxhlet apparatus (120 °C), which was then followed by cold water washing five times. The more extensive washing procedure potentially liberated more surface active sites by removing a larger fraction of elemental sulfur and H2SO4. High temperature heating under a nitrogen atmosphere has been demonstrated to regenerate activated carbon by removing sulfur (13, 25). Yang et al. (25) demonstrated that H2S conversion could be maintained above 55% (treating a 1.2% H2S stream) for ten 1000 min cycles, if

heated to 175 °C. Conversion could be increased to greater than 80% if the regeneration temperature was raised to 205 °C after 12 cycles (25). Given these results for activated carbon and our results demonstrating the release of sulfur from treated wood ash it is likely that heating at temperatures above 175 °C in a N2 environment would regenerate the ash for reuse and allow sulfur recovery. Potential Mechanism(s). Our results indicate that H2S and MT were catalytically oxidized in the presence of coal or wood fly ash and oxygen, with wood ash having a much higher catalytic activity, potentially due to higher surface area and the presence of catalytic crystalline phases. In experiments that received H2S or MT without ash (i.e., glass wool only), removal was not measured. DMDS did not react with the ash, since there were no measurable differences between inlet and outlet concentrations when treating DMDS. Moreover, MT oxidation resulted in the formation of DMDS and followed the stoichiometry of the proposed reaction.

2CH3SH + 1/2O2 f CH3SSCH3 + H2O

(2)

The fact that H2S and MT were oxidized, when DMDS was not, indicates that the sulfur atom is the site of the reaction and must be free for interaction with an active site on the ash (i.e. under the conditions studied). Multiple mechanisms for H2S and MT oxidation via activated carbon have been proposed with water affecting the proposed mechanisms at low temperatures. Below 100 °C, water vapor and a condensed water phase significantly enhance the rate of H2S oxidation (8-10, 17). The fact that water addition increased the H2S and thiol oxidation rate (17, 21) suggests that H2O along with base in the ash (pH ∼ 12) promotes partitioning into the liquid phase due to the rapid dissociation reaction that occurs (our reactions were performed with ash at 25% moisture and a 95-100% relative humidity gas stream). Once the reactants have partitioned into the water phase, H2S or CH3SH can undergo reaction with dissolved metal oxides (equations 3-7) or diffuse to the surface of the ash and undergo a surface based reaction with adsorbed oxygen (eqs 8 and 9 and Figure 4, Supporting Information).

H-SH + OH- f H-S- + H2O

(3)

H-S- + OH- f S2- + H2O

(4)

S2- + M+ f M2+S2(metal sulfides, both soluble and insoluble) (5) or

R-SH + OH- f R-S- + H2O

(6)

R-S- + M+ f M+RS(metal organic thiols, mostly insoluble) (7) The surface based reactions account for the increased reaction rate with higher surface area, catalytic decay due to deposition of elemental sulfur, and the need for oxygen. First, O2 is dissociatively adsorbed onto the ash matrix, which forms an oxygen free radical and is stabilized by surface active sites or X (e.g., mullite, hematite, magnetite) in the ash. The organic thiol is molecularly adsorbed to other active sites. The oxygen free radical then attacks the hydrogen bonded to the sulfur atom to form a hydroxide and organic thiol free radical. The free radicals are attracted to each other and result in the formation of a sulfur-hydroxide bond. Finally, water is removed and a disulfide bond is formed. If the “R” group is

replaced by hydrogen and the same mechanism is followed, HSSSSSSSSOH is formed. This molecule is large enough to allow a wrap-around reaction to occur causing a ring structure to form (Figure 4, Supporting Information for detailed mechanism).

NET RXN: 2R-SH + 1/2O2 f R-S-S-R + H2O (8) and

NET RXN: 8H2S + 4O2 + X f S8 + 8H2O

(9)

The following experimental results support the proposed mechanism. Decline in pH of the wood ash from 12.1 to 7.8 after reaction with H2S indicates the consumption of a base associated with the ash and the need for the presence of H2O. In addition, the fact that the overall reaction rate increased with surface area suggests a surface catalyzed reaction. Moreover, X-ray diffraction analysis of the wood ash indicated the presence of metal oxide phases capable of catalytic oxidation. The subsequent decay in catalytic activity, observation of a yellow powder during surface area measurements, and identification of orthorhombic sulfur suggest that sulfur was deposited on the ash, contrary to the production and release of SO2 often observed using activated carbon at high temperatures (25). Sulfur dioxide was not detected in the reactions of H2S with the ash, again suggesting the primary product was sulfur. Catalytic oxidation of H2S using activated carbon has not been reported to produce SO2 at low temperatures (12, 25). The formation of an insoluble sulfur product led to catalytic decay, potentially by clogging pores and covering surface active sites. Experimental results suggest that coal fly ash and to a much greater extent wood ash has the potential to replace activated carbon as a low cost catalyst for removal of H2S from high volume, low concentration streams. The adsorption capacity and reaction rate of wood ash for H2S, although lower than activated carbon (Table 2), is within an acceptable range for low concentration applications ( 94% for 4 days using wood ash at inlet concentrations typical of many environmental applications and if combined with a continuous regeneration process may lead to a low cost reaction/ adsorption process. Future research will focus on (1) methods to enhance the oxidation of other reduced sulfur compounds (e.g., methanethiol and dimethyl disulfide), (2) development of catalyst pellets to reduce pressure drop, and (3) methods to better understand the mechanism and regenerate the catalyst.

Acknowledgments This research was supported by the State of Georgia through the Traditional Industries Program and a grant (DE-FC2698FT40028) from the Combustion Byproducts Recycling Consortium (Program of the National Mine Land Reclamation Center, WVU, in cooperation with DOE National Energy Lab).

Supporting Information Available Figures describing the apparatus, elemental sulfur formation during H2S oxidation, the catalytic decay during oxidation of H2S at 500 ppmv using wood ash, and proposed surface reactions of H2S and CH3SH. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review July 25, 2002. Revised manuscript received February 18, 2003. Accepted March 19, 2003. ES0259988