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Bifunctional Redox Iron/Cerium (Hydr)oxide Process for H2S Removal from Pulp and Paper Emissions Catalin F. Petre and Faı1c¸ al Larachi*,† Department of Chemical Engineering, Laval University, Que´ bec, Canada G1K 7P4
Hydrogen sulfide is the most abundant contaminant among the total reduced sulfur (TRS) quartet in the kraft mill atmospheric emissions. An advantage of the association of oxygen with TRS in the pulp and paper effluents is exploited in a new bifunctional redox scrubbing process based on iron chemistry for the removal of hydrogen sulfide. As synthesized iron/cerium (hydr)oxide composite materials were tested at ambient and alkaline conditions in an agitated batch reactor for oxidizing dissolved bisulfide (HS-) in both aerobic and anoxic environments. Polysulfides and thiosulfate were the main detected reaction products. The presence of dissolved oxygen contributed to the sustained active surface iron sites regeneration thereby improving significantly HS- removal. Testing the material in three-cycle bisulfide oxidation without reactivation and in the presence of oxygen demonstrated its long-term efficiency. Introduction The control and abatement of the sulfurous odorss referred to as the total reduced sulfur gases (or TRS gases)sare difficult tasks to achieve in the air pollution problems afflicting the kraft pulp mills. This is due to the very low human olfactory levels of TRS and also because odors, per se, are difficult to quantify in terms of tolerable or acceptable levels. TRS gases in the pulp and paper industry refer to the following low molecular mass sulfur-bearing components: hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3), and dimethyl disulfide (CH3S2CH3). Two main TRS sources are recognized to occur in the kraft mill emissions: (i) the LVHC (low volume high concentration) gases, usually dealt with by incineration, emerge from brownstock washers, digester, and evaporator systems, etc.; (ii) the HVLC (high volume low concentration) gases, dealt with by means of alkaline/ amine wet scrubbing, are released by recovery furnaces and lime kilns.1 Typically, HVLC point sources release on average 200 ppmv of TRS at high discharge rates (8000-25 000 Nm3/ h),2 which is nearly 40 times greater than the enacted concentration levels (5 ppmv in both Canada and the United States).3 LVHC point sources exhibit, on the contrary, large TRS variability even though average concentrations above 500 ppmv4 are not atypical. This turns the LVHC regulation equivalently steeper (>102 times the regulated level). In addition, the olfactory threshold of TRS for human beings is as low as 5 ppb, ca. 103 times current regulation. Considering the progressive nature of legislation in response to increased public awareness, stricter regulations combined with highly efficient abatement technologies would eventually be required to achieve ultimately odor-free pulp and paper atmospheric effluents in the foreseeable future. In recent years, gas desulfurization processes based on iron chemistry have received increasing attention in * To whom correspondence should be addressed. E-mail:
[email protected]. † Present address: ARKEMA/TOTALsCentre Technique de Lyon Chemin de la Loˆne, BP32 69492 Pierre-Be´nite, Ce´dex, France.
the natural gas and oil refining industry.5,6 A distinctive feature between this area and the pulp and paper industry is the association of oxygen with TRS pollutants in the latter. This feature might imply a potential scrubbing technology, tailored for TRS removal, where the simultaneous presence of dissolved molecular oxygen is able to regenerate in situ iron from Fe(II) to active Fe(III). Such a bifunctional redox system enables conducting within the same vessel and simultaneously the ferric-mediated oxidation of the sulfur-containing pollutant (i.e., reduction of ferric site) on one hand and the oxygen-mediated ferrous site reoxidation on the other hand.7 This redox process requires lenient operating conditions (20-60 °C, atmospheric pressure) and would consist of three steps: (i) pollutant absorption in soft basic absorbing water (pH ≈ 9-10); (ii) redox reaction between TRS and Fe(III); and (iii) reoxidation of iron from ferrous to ferric by means of dissolved oxygen. Usually, for preventing its precipitation in alkaline media during desulfurization, homogeneous iron is stabilized as an organometallic chelate, such as ferric ethylenediaminotetraacetate (Fe3+EDTA4-) or ferric trans-1,2-diaminocyclohexanetetraacetate (Fe3+CDTA4-). One way to avoid use of expensive iron chelates while preserving oxidative performance is to use heterogeneous iron in the form of iron (hydr)oxides, which can be suitable for the oxidation of hydrogen sulfide. The reaction between dissolved sulfide and iron (hydr)oxides occurs in a variety of natural anaerobic environments and is well-documented in the literature.8-12 A complex reaction scheme yields metastable monosulfide11 that converts into stable elemental sulfur in the presence of iron. Over the past few years, cerium oxide and CeO2containing materials have come under scrutiny as catalysts, structural and electronic promoters in heterogeneous catalysis.13,14 When associated with transition metal oxides, cerium oxide has been shown to promote oxygen storage and release, to enhance oxygen mobility, to form surface and bulk vacancies, and to improve catalyst redox properties of the composite oxide. Most previous studies dealing with the use of iron for the removal of hydrogen sulfide are related to homogeneous iron (hydr)oxides reacting with hydrogen sulfide
10.1021/ie050194x CCC: $30.25 © 2005 American Chemical Society Published on Web 04/29/2005
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Table 1. Symbols, BET Specific Surface Area, and Iron Content (AAS) for Materials Used in Anoxic Reactions with Bisulfide
material
symbol
BET specific surface area (m2/g)
iron (hydr)oxide, conventional synthesis iron-cerium (hydr)oxide, conventional synthesis iron (hydr)oxide, modified synthesis iron-cerium (hydr)oxide, modified synthesis cerium (hydr)oxide, modified synthesis
FeOx(c) Fe/CeOx(c) FeOx(m) Fe/CeOx(m) CeOx(m)
22 127 48 125 71
dissolved in natural anoxic waters8-12 at slightly acidic or neutral pH. The focus of the present study is on the reaction between iron/cerium (hydr)oxides and dissolved bisulfide at moderately basic pH with and without the mediation of molecular oxygen. First, it will be shown that iron/cerium (hydr)oxide, owing to enhanced ceria mediation, performs better than iron (hydr)oxide in oxidizing the bisulfides. Second, in anoxic conditions, the main reaction products are polysulfides and thiosulfate and not sulfate as suggested previously.8,9 Third, the presence of dissolved oxygen re-oxidizes the iron surface sites involved in the reaction, thus improving significantly bisulfide conversion. Fourth, testing the material in three-cycle bisulfide oxidation without reactivation and in the presence of oxygen demonstrates the material’s long-term efficiency. Experimental Section Materials Preparation and Characterization. A modified coprecipitation protocol was set to synthesize the iron/cerium composite (hydr)oxides. In conventional coprecipitation, the (hydr)oxides composite forms when a solution containing the two metal cation precursors is directly added to a precipitant. Due to rapid changes in solution concentration, this method yields poor control on precipitation, grain size, and morphology.14 In addition, the pH at which precipitation initiates differs for each metal cation, thus implying significant compositional discrepancies of the hydroxides composite mixture from the beginning until the end of precipitation. The coprecipitation method of Imamura and Doi15 was therefore modified to elude these problems through the (i) use of metal-chelate complexes as metal precursors and (ii) dropwise addition of the precipitating solution under vigorous mixing. (Hydr)oxides and hydrous oxides precipitation of a number of metal cations can be shifted to appreciably higher pH levels by adding chelating agents.16 Chelation of dissolved metal ions indeed forms organometallic complexes that preserve the metal ion homogeneous character up until high pH values. This delayed precipitation enables more homogeneous coprecipitation of the metals to take place. In addition, the controlled (dropwise and fast-mixing) release of the reactionparticipating ligands ensures the formation of more homogeneous (hydr)oxides with finer particles size. All chemicals were ACS grade (Sigma-Aldrich Canada) and were used without further purification. The composite oxide (9/1 Fe/Ce bulk atom ratio basis) was prepared as follows: (i) dissolving 6.55 g (for Fe complex) and 0.75 g (for Ce complex) of EDTA (ethylenediaminetetraacetic acid), respectively, in 85 mL and 10 mL of NaOH 1M; (ii) after complete dissolution of EDTA at pH ≈ 7, 9.05 g of Fe(NO3)3‚9H2O and 0.95 g of CeCl3‚7H2O were added to each chelate solution resulting in 1:1 molar-basis metal ions and EDTA ratio
Fe (at. %) 71 56 71 56 0
at pH ≈ 4; (iii) the two solutions were mixed together, and 150 mL of a 3 M NaOH solution was added dropwise under vigorous mixing; (iv) the resulting precipitate was washed with 500 mL of deionized water, air-dried at 60 °C for 5 h, and calcined in air at 350 °C for 3 h. Single metal (iron and cerium) (hydr)oxides were also prepared using the above-described method (see also Table 1 for this modified synthesis). In addition, a second composite (hydr)oxide (with 9/1 Fe/Ce bulk atom ratio) and single metal (iron) (hydr)oxides were prepared using the coprecipitation method of Imamura and Doi15 (see also Table 1 for this conventional synthesis). The resulting powders were characterized by means of the following analytical techniques: (i) atomic absorption spectroscopy (AAS) was performed on a PerkinElmer AAnalyst 800. The samples were first dissolved in an aqueous HCl-HF digestion mixture. (ii) X-ray powder diffraction patterns (XRD) were recorded on a Siemens D5000 diffractometer using the Cu KR radiation at 40 kV and 30 mA (λ ) 1.54184 Å), at 1°/min [2θ]. (iii) The specific BET surface areas was determined using N2 adsorption and BET model. Nitrogen adsorption isotherms at 77 K were measured on a Micrometrics Tristar 3000 at residual pressures of ca. 10-5 Torr. (iv) Texture and morphology of the composite materials were examined using secondary and/or backscattered electron images on a JEOL 840-A scanning electron microscope (SEM) at different scales/magnifications. (v) X-ray microanalysis was done on a fully automated CAMECA SX-100 electron microprobe equipped with five vertical wavelength-dispersive spectrometers. (vi) Temperature-programmed reduction (TPR) was carried out using an automated Altamira AMI-1 instrument where about 10 mg of the sample was loaded in a U-shaped quartz microreactor and heated from room temperature to 800 °C (heating rate 3 K/min) in a flowing hydrogen mixture (50 mL/min, 10% H2 in Ar). Hydrogen consumption was monitored using a thermal conductivity detector. Reaction Setup. Temperature-controlled reactions between iron/cerium (hydr)oxide and bisulfide (HS-) were carried out at pH ) 9.5 in a 1.5 L batch, sealed, baffled, double-jacketed, and magnetically stirred glass reactor under anoxic (nitrogen) and active (air) headspace. Dissolved oxygen was measured using a dissolved oxygen probe (DOB-930 model from Omega), and pH was held constant using a 0.1 M borate (Sigma-Aldrich) buffer, adjusted to the desired pH with NaOH (Fisher Scientific). During the anoxic runs, N2 was bubbled through the aqueous solution prior to sodium sulfide addition to eliminate residual oxygen. The sulfur precursor was Na2S‚9H2O salt (Sigma-Aldrich). A prescribed amount of sodium sulfide was added to the buffer solution. The reactional liquid volume was adjusted to 300 mL turning all the sulfide ions into hydrosulfide anions (HS-) at the prevailing pH conditions. The amount of powdered metal (hydr)oxide in-
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9393 Table 2. AAS, X-ray Microanalysis, and XPS Bulk and Surface Elemental Compositions in Fresh and Spent Iron/Cerium (hydr)Oxide Materials X-ray microanal.
XPS
Fe/(Fe + Ce) (at. %) fresh iron/cerium hydr(oxide) 89.7 spent iron/cerium hydr(oxide)
91.0
85.6 85.2
S/(Fe + Ce) (at. %) spent iron/cerium hydr(oxide) -
4.5
0.0
material
AAS
troduced into the slurry reacting medium corresponded to stoichiometric proportions between iron and hydrosulfide (Fe:HS- molar ratio 2:1). The iron-based material was abruptly injected into the reactor to trigger reaction. For the runs in the presence of O2, air was supplied in the reactor headspace (1.2 L) prior to introducing the powdered material. During the course of reaction, aliquots were periodically withdrawn from the reactor and filtered prior to analyses on a 0.2 µm Millipore membrane filter to remove solid particles. Sample Analysis. Bisulfide consumption and products (thiosulfate, sulfate, sulfite, and polysulfides) formation were monitored by means of a capillary electrophoresis (CE) system (Agilent Technologies) used to separate, identify, and quantify the ionic species being formed or consumed during reaction. The capillary electrophoresis was equipped with a pre-aligned deuterium lamp and a UV-visible absorbance diode array detector (DAD). Separation was achieved using indirect UV detection at two different wavelengths: 230 nm with reference at 372 and 214 nm with reference at 372 nm. A negative voltage bias of 30 kV was applied. The analytes were separated in fused silica capillaries with 50 µm i.d. and 48 cm of effective length. The detector window was formed by burning off a 1 cm section of the outer polyimide coating. The temperature of the capillary cassette was 25 °C, and the sample injection was performed in the hydrodynamic mode by applying a 50 mbar pressure for 10 s. The Agilent ChemStation software was used for data acquisition and interpretation.
Figure 1. Effect of various powdered Fe/Ce-containing materials on HS- conversion (pH ) 9.5, 22 °C, 1 atm, Vliq ) 0.3 L, 600 rpm): (*) CeO2 (363 mg) and HS- (3.5 × 10-3 mol/L); (4) FeOx(c) (166 mg) and HS- (3.5 × 10-3 mol/L); (2) FeOx(m) (166 mg) and HS(3.5 × 10-3 mol/L); (0) Fe/CeOx(c) (210 mg) and HS- (3.5 × 10-3 mol/L); (9) Fe/CeOx(m) (210 mg) and HS- (3.5 × 10-3 mol/L).
Results and Discussion Calcination at relatively low temperature (350 °C) resulted in amorphous powdered materials as confirmed through XRD analysis. Atomic absorption spectroscopy indicated that the as-synthesized composite material had a bulk atomic Fe/(Fe + Ce) ratio of 89.7%, while XPS analysis indicated a surface ratio of 85.6% (Table 2). Inspection of the powdered material through SEM showed more or less narrow grain size distribution centered around 5 µm. The presence of Ce in the iron/ cerium (hydr)oxide had a noticeable effect on the BET specific surface area as shown in Table 1. Both Fe/Ce composites (conventional and modified synthesis) demonstrated significantly high surface areas (factor 6 for Fe/CeOx(c) and factor 3 for Fe/CeOx(m)) as compared to the corresponding Ce-free iron (hydr)oxide. Also, the modified chelate synthesis yielded an iron (hydr)oxide material BET specific surface area double that with respect to the conventional synthesis. However, the Fe/ Ce composites synthesized according to the two ways yielded almost the same BET surface area values. The presence of Ce in the composite oxide seems to be more important for the BET surface area masking the positive effect of the chelate synthesis method.
Figure 2. Typical electrophoregrams of detected species for the reaction between HS- and Fe/CeOx(m): (a) beginning of reaction; (b) anoxic conditions; (c) in the presence of oxygen. Reaction conditions as in Figure 3a for (0) and (b). Peaks: 1, polysulfides; 2, thiosulfate; 3, sulfate; 4, bisulfide; 5, sulfite.
Bisulfide Oxidation. To evaluate the efficacy of iron/ cerium (hydr)oxide in reacting with hydrogen sulfide, the effect of various powdered Fe and/or Ce containing materials on HS- conversion under anoxic conditions was compared in Figure 1. Note that, since pH was maintained at 9.5, H2S virtually converts almost totally into bisulfide through hydrogen sulfide first acidity equilibrium. Similarly, most sulfides shift to bisulfides through second acidity equilibrium of H2S.18 Hence, under such alkalinity conditions, Na2S was used for commodity as a sulfide source to generate bisulfide instead of sparging gaseous H2S in the reacting slurry. The tested materials are summarized in Table 1. Regardless of tested materials and whether dissolved oxygen was available, polysulfides and thiosulfate were
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Figure 4. Polysulfides oxidation to thiosulfate for the reaction between HS- and O2 (2) and HS- Fe/CeOx(m) and O2 (b). Reaction conditions as in Figure 3a for (2) and (b). Yield S2O32- ) S2O32produced/HS- initial. Yield Sx2- ) Sx2- produced/HS- initial.
Figure 3. (a) Direct and indirect mediation of oxygen in the reaction between iron/cerium (hydr)oxide and HS- (pH ) 9.5, 22 °C, 1 atm, Vliq ) 0.3 L, 600 rpm): (2) O2 gas (1.12 × 10-1 mol/L) and HS- (6.55 × 10-3 mol/L), without iron/cerium (hydr)oxide; (0) Fe/CeOx(m) (210 mg) and HS- (3.5 × 10-3 mol/L) without O2; (b) Fe/CeOx(m) (210 mg), O2 gas (1.12 × 10-1 mol/L) and HS- (3.5 × 10-3 mol/L). (b) Dissolved O2 profile for the last case.
the main reaction products whereas the SO42- and SO32- peaks remained relatively marginal as shown in the electrophoregrams of Figure 2. As seen from Figure 1, the Fe/CeOx(m) and Fe/CeOx(c) materials exhibited the highest HS- conversion, followed in order of decreasing activity by FeOx(m), FeOx(c), and CeOx(m). Recall that all these profiles were obtained when no oxygen is available in the system. The results portrayed in Figure 1 indicate, first, that controlling iron (hydr)oxide precipitation allows more efficient exploitation of the role of iron in the redox reactions. Second, the presence of Ce in the iron/cerium (hydr)oxide affected positively HS- oxidation (Figure 1), which can be speculatively ascribed to a modified material’s structure caused by Ce. Association of Ce to Fe in iron/cerium (hydr)oxides creates more accessible (or effective) reaction sites for bisulfide conversion. Another interesting result highlighting the viability of a self-sustained bifunctional redox process using molecular oxygen is illustrated in Figure 3. The role of molecular oxygen in the reaction is double. It participates directly to homogeneously oxidize the aqueous bisulfide without mediation of any transition metal
(2, Figure 3a).17,19-21 It also allows, through an indirect heterogeneous pathway (b, Figure 3a), fast regeneration of trivalent iron from divalent iron yielded through bisulfide oxidation (Figure 1). Both direct and indirect mediations of oxygen in the reaction between Fe/CeOx(m) and HS- are illustrated in Figure 3a. Via the direct mode, oxygen slowly oxidizes bisulfide to elemental sulfur that combines with residual bisulfide to form polysulfides, which can ultimately be oxidized to thiosulfate.17,22 Bisulfide oxidation undergoes a sulfide-sulfur-polysulfide cycle with HS- oxidation to sulfur being the rate-limiting step.17 Figure 3a shows that, when only O2 is present in the system (i.e., direct mode ON), a relatively low bisulfide conversion (ca. 25%) is achieved after 180 min. This is almost 2.5 times less than the bisulfide conversion (ca. 80%) given by iron/ cerium (hydr)oxide in anoxic conditions (i.e., direct mode OFF) over a comparable time interval. Nevertheless, the most effective conversion was attained when O2 is present along with iron/cerium (hydr)oxide proving that oxygen accelerates bisulfide conversion via the indirect mode. As a matter of fact, the reaction attained completion (i.e., total bisulfide conversion) in less than 60 min. Oxygen is believed to reoxidize primarily iron from ferrous to ferric (indirect mode ON) and, second, to promote polysulfide oxidation into higher oxidation state sulfur-bearing anions such as thiosulfate (Figure 4). The main reaction products observed are polysulfides (mainly S32- and/or S42-)22,23 and thiosulfate. No sulfate was produced (Figure 2a,c). Note in this latter case the nondepleted dissolved oxygen level (around 9 mg/L), which indicates no oxygen gas-liquid mass transfer limitation (Figure 3b). Figure 4 shows the influence of iron/cerium (hydr)oxide in the oxidation reaction of polysulfides to thiosulfate. When only oxygen is present in the system a very slow oxidation of polysulfides to thiosulfate is observed, which is 4-6 times lower than when both material and oxygen are present in the system. This suggests that iron/cerium (hydr)oxide could exhibit a catalytic role in the oxidation of polysulfides to thiosulfate.
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Figure 5. (a) Performance of spent and fresh iron/cerium (hydr)oxide for HS- conversion (pH ) 9.5, 22 °C, 1 atm, Vliq ) 0.2 L, 600 rpm): (2) Fe/CeOx(m)-fresh (100 mg) and HS- (2.50 × 10-3 mol/L); ([) Fe/CeOx(m)-spent (100 mg) and HS- (2.50 × 10-3 mol/L). (b) Proof of concept of the bifunctional redox process (pH ) 9.5, 22°C, 1 atm, Vliq ) 0.2 L, 600 rpm) for HS- (4.34 × 10-3 mol/L) removal by the iron/cerium (hydr)oxide (170 mg) in the presence of O2 gas (1.12 × 10-1 mol/L). Lines represent tendencies.
The performance of spent and fresh iron/cerium (hydr)oxide for HS- conversion is illustrated in Figure 5a. The spent material is collected after exposing it once to a bisulfide-containing solution in a previous reaction run under the same anoxic conditions as in Figure 1. After recovery, the spent sample endured neither thermal regeneration nor calcination, except air exposure overnight for drying purposes. As seen in Figure 5a, this appears to be sufficient to restore the material’s activity as in its pristine state. Figure 5b shows the proof of concept of the bifunctional redox process. The iron/cerium (hydr)oxide long-term efficiency in HSremoval was tested in a three-cycle reaction, in the presence of oxygen. Each 90 min, a cycle starts with the same amount of bisulfide to be oxidized and the same quantity of oxygen in the reactor headspace. As shown in Figure 5b, HS- was totally converted in 20 min (first
cycle) to 40 min (last cycle). Two plausible reasons could explain such a drop off in efficiency between last and first cycle: (i) reduction in available oxygen for iron reoxidation because part of the oxygen becomes increasingly rerouted for polysulfide conversion into thiosulfate due to the buildup of polysulfides; (ii) as speculated earlier, the iron/cerium (hydr)oxide could take part in polysulfide oxidation as a catalyst, thus becoming less available for HS- oxidation to polysulfides. Nevertheless, when large excess of oxygen is permanently ensured, uninterrupted removal of dissolved hydrogen sulfide by the iron/cerium (hydr)oxide could be achieved in a sustainable manner as shown in Figure 5b. Characterization of Spent and Fresh Iron/ Cerium (Hydr)oxides. To unveil some aspects on the modes of action of iron/cerium (hydr)oxides in oxidizing bisulfides with and without oxygen, a number of char-
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surface (Table 2). This apparent contradiction suggests that the probed depths, typically between 20 and 100 Å, from where photoelectrons are ejected and used for detection in XPS are not sufficient to sense the presence of sulfur. On the contrary, the penetrative capability of X-ray microanalysis allowed positive detection. Such observation could be an indirect evidence for valence (III) iron re-precipitation over the iron/cerium (hydr)oxide surface burying the formed sulfurs beneath. The fact that the Fe/(Fe + Ce) atomic ratio at the materials surface remains virtually cerium rich and unchanged for both fresh and spent materials might point to the fact that sulfur does not build up on top of cerium. Conclusion
Figure 6. TPR analysis for fresh and spent chelate-route synthesized iron/cerium (hydr)oxide: (a) Fe2O3; (b) Fe/CeOx(m)fresh; (c) Fe/CeOx(m)-spent.
acterizations were completed on both fresh and spent materials. The surface reaction between iron (hydr)oxide and bisulfide is very complex. Previous studies8-12 suggested that chemical reaction between surface iron(III) and bisulfide causes the leaching-off of a reduced ferrous iron into the solution thus leaving a new trivalent iron surface site. The leached divalent iron cation, in the presence of an oxidant (e.g., dissolved oxygen) quickly oxidizes to Fe(III),24 which in turn precipitates at the current working pH values and rebuilds afresh a trivalent iron (hydr)oxide surface site. BET analysis of spent FeOx (conventional and modified) materials revealed that the difference in surface area between the two oxides almost disappeared after reaction. The specific surface area of spent FeOx(c) increased to 41 m2/g from its orignal 22 m2/g whereas that of FeOx(m) remained unchanged near 47 m2/g. Change in specific surface area for FeOx(c) is likely to be due to surface reordering giving rise to micropores that are responsible for surface increase. Such surface reordering is likely linked with iron leaching off and/or re-adsorption on the surface to create, after re-oxidation (and precipitation), newly distributed iron sites. The TPR profiles, shown in Figure 6, reveal that the fresh material TPR profile is reminiscent of an Fe3O4 bulk structure (curve b),25 whereas that of the spent one (aging conditions as in Figure 5a) is typical of Fe2O3 bulk structure (curve c).25 In this latter case, the TPR profile featured a prominent peak near 400 °C similar to that of curve a for Fe2O3 commercial powder. Inception nearby 290 °C of a very pronounced low-temperature peak on curve c coincides correspondingly with the shoulder position on curve a for Fe2O3 commercial powder and curve b for fresh iron/cerium hydr(oxide). Exacerbation of such a low-temperature peak clearly indicates an improvement in the distribution of trivalent iron surface sites for the spent iron/cerium hydr(oxide). However, though TPR profiles for fresh and spent materials exhibit noticeable differences, this did not seem to affect dramatically the materials’ reactivity toward bisulfide oxidation as exemplified by the conversion profiles discussed above (Figure 5a). The X-ray microanalysis revealed the existence of sulfur for the spent material, when, on the contrary, XPS analysis could not detect sulfur deposits on its
This study highlighted the proof of concept of a new bifunctional redox process for the abatement of hydrogen sulfide in the context of pulp and paper atmospheric emissions using heterogeneous iron/cerium composite (hydr)oxides in alkaline conditions. The presence of cerium in the (hydr)oxide composite gave rise to more accessible and effective reaction sites and improved the redox properties of the composite oxide. Availability of oxygen (present in association with TRS gases in the pulp and paper industry emissions) improved significantly bisulfides conversion and helped regenerating active Fe(III) surface sites of iron/cerium (hydr)oxide. The material, tested in a three-cycle bisulfide oxidation reaction in the presence of oxygen, proved its long-term efficiency. Obviously, pH, temperature, and initial bisulfides-oxygen and bisulfides-(hydr)oxide ratios play a very important role and affect products’ distribution. This will be analyzed in future work along with extension of this regenerative oxidative removal of bisulfides to other sulfur-bearing TRS. Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada Strategic Grant Program Environment and Sustainable Development is gratefully acknowledged. Literature Cited (1) Smook, G. A. Handbook for Pulp & Paper Technologist, 2nd ed.; Angus Wilde Publications: Vancouver, BC, Canada, 1992. (2) Normandin, A. Private communication, 2001. (3) Pinkerton, J. E. Trends in U.S. kraft mill TRS emissions. TAPPI J. 1999, 82, 166. (4) Sittig, M. Pulp and Paper Manufacture: Energy Conservation and Pollution Prevention; Noyes Data Corporation: Park Ridge, NJ, 1977. (5) Holbrook, D. L. Handbook of Petroleum Refining Processes, 2nd ed.; Meyers, R. A., Ed.; McGraw-Hill: New York, 1996. (6) Kohl, A.; Nielsen, R. Gas Purification, 5th ed.; Gulf Publishing Co.: TX, 1997. (7) Iliuta, I.; Larachi, F. Concept of bifunctional redox ironchelate process for H2S removal in pulp and paper atmospheric emissions. Chem. Eng. Sci. 2003, 58, 5305. (8) Stumm, W.; Sulzberger, B. The cycling of iron in natural environmentssconsiderations based on laboratory studies of heterogeneous redox processes. Geochim. Cosmochim. Acta 1992, 56, 3233. (9) dos Santos Afonso, M.; Stumm, W. Reductive dissolution of iron(III) (hydr)oxides by hydrogen-sulfide. Langmuir 1992, 8, 1671. (10) Peiffer S.; dos Santos Afonso M.; Wehrll B.; Gachter R. Kinetics and mechanism of the reaction of H2S with lepidocrocite. Environ. Sci. Technol. 1992, 26, 2408.
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9397 (11) Poulton, S. W.; Krom, M. D.; van Rijn, J.; Raiswell, R. The use of hydrous iron(III) oxides for the removal of hydrogen sulphide in aqueous systems. Water Res. 2002, 36, 825. (12) Poulton, S. W. Sulfide oxidation and iron dissolution kinetics during the reaction of dissolved sulfide with ferrihydrite. Chem. Geol. 2003, 202, 79. (13) Hamoudi, S.; Larachi. F.; Sayari A. Wet oxidation of phenolic solutions over heterogeneous catalysts: Degradation profile and catalyst behavior. J. Catal. 1998, 177, 247. (14) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: London, UK, 2002. (15) Imamura, S.; Doi, A. Wet oxidation of amonia catalyzed by cerium-based composite oxides. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 75. (16) van Dillen A. J.; Terode, R. J. A. M.; Lensveld, D. J.; Geus J. W.; de Jong, K. P. Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes. J. Catal. 2003, 216, 257. (17) Chen, K. Y.; Morris, J. C. Kinetics of oxidation of aqueous sulfide by O2. Environ. Sci. Technol. 1972, 6, 529. (18) Dore´, M. Chimie des oxydants et traitement des eaux; Technique & Documentation: Paris, France, 1989. (19) O’Brien, D. J.; A.; Birkner, F. B. Kinetics of oxygenation of reduced sulfur species in aqueous solution, Environ. Sci. Technol. 1977, 11, 1114.
(20) Hoffman, M. R.; Lim, B. C. Kinetics and mechanism of the oxidation of sulfide by oxygen: catalysis by homogeneous metalphthalocyanine complexes. Environ. Sci. Technol. 1979, 13, 1406. (21) Fischer, H.; Schulz-Ekloff, G.; Wohrle, D. Oxidation of aqueous sulfide solutions by dioxygensPart II: Catalysis by soluble and immobilized cobalt(II) phthalocyanines. Chem. Eng. Technol. 1997, 20, 462. (22) Steudel, R. Mechanism for the formation of elemental sulfur from aqueous sulfide in chemical and microbiological desulfurization processes. Ind. Eng. Chem. Res. 1996, 35, 1417. (23) Giggenbach, W. Optical spectra and equilibrium distribution of polysulfide ions in aqueous solution at 20 °C. Inorg. Chem. 1972, 11, 1201. (24) Stumm, W.; Lee, G. F. Oxygenation of ferrous iron. Ind. Eng. Chem. 1961, 53, 143. (25) Jung, K.-D.; Joo, O.-S.; Cho, S.-H.; Han, S.-H. Catalytic wet oxidation of H2S to sulfur on Fe/MgO catalyst. Appl. Catal. A 2003, 240, 235.
Received for review February 18, 2005 Revised manuscript received April 5, 2005 Accepted April 5, 2005 IE050194X
