1990
Ind. Eng. Chem. Res. 2007, 46, 1990-1999
Removal of Methyl Mercaptide by Iron/Cerium Oxide-Hydroxide in Anoxic and Oxic Alkaline Media Catalin F. Petre† and Faı1c¸ al Larachi* Department of Chemical Engineering, LaVal UniVersity, Que´ bec, Canada G1K 7P4
The oxidation of methyl mercaptide by Fe/Ce oxide-hydroxide was studied as a potential approach to deal with the methyl mercaptan contamination from total reduced sulfur (TRS) emissions in the pulp and paper industry. The reaction was studied in aqueous solutions at 22 °C and different alkaline pH values (10.5-12) both in anoxic and oxic conditions. Mercaptide reactivity was strongly dependent on pH in anoxia, whereas in oxic conditions it was noticeably higher but tributary of dissolved oxygen (DO2). Interference with bisulfide comixed with mercaptide caused an inhibition in the conversion for both pollutants. Such inhibition was due to the incipient polysulfides formed via bisulfide oxidation. On the contrary, the mercaptide conversion was found to improve at the expense of bisulfide when large quantities of exogenous polysulfides were present initially in the reaction medium. The conversion of mercaptide was unaffected with the comixing of dimethyl sulfide. The oxidation of methyl mercaptide by the O2/Fe/Ce oxide-hydroxide system did not affect the reoxidative regeneration of surface Fe(III) by DO2. This feature pinpoints the sine qua none condition for a redox process based on the O2/Fe/Ce oxide-hydroxide system to remove methyl mercaptan in pulp and paper emissions. Introduction Unlike mechanical pulping, and despite the fact that nearly half of the wood content is being degraded by chemical pulping, Kraft chemical pulping still pervades the pulp and paper industry in North America owing, principally, to the properties it confers to the freed (hemi)cellulose rich fibers.1 The wood cooking, the black liquor evaporation and incineration, and the chemical recovery systems from Kraft pulping are responsible for the emission of the so-called total reduced sulfurs (TRS) which are known for their nauseous character. TRS gases refer specifically to the quartet: hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3, also known as DMS), and dimethyl disulfide (CH3S2CH3, also known as DMDS). Although current North American regulation limits TRS concentration levels in atmospheric effluents to 5 ppmv, this is still well above the human olfactory threshold (ca. 1-10 ppbv)2 explaining hence the public resentment and the bad press surrounding the emissions of TRS. A great deal of efforts to develop efficient abatement approaches have been deployed recently with the goal of turning the TRS-containing effluents into odor-attenuated and less objectionable streams.3 Several TRS abatement processes have been advocated such as alkaline/amine scrubbing and gas incineration, chemical oxidation by NaClO, ClO2, H2O2, and KMnO4 and gas phase oxidation by ClO2, wet oxidation, aerobic biofiltration, activated carbon, and green liquor dregs adsorption (see for example refs 2 and 3). However, only incineration and alkaline scrubbing are currently massively implemented industrially. Because of their lower boiling points, hydrogen sulfide and methyl mercaptan (MM) contribute for the largest compositions in the TRS mix. In a recent work,4 a simple redox approach for scrubbing H2S-contaminated air effluents was evaluated in which Fe/Ce oxide-hydroxide (FeCeOx) was used for oxidizing the absorbed pollutant within an alkaline solution at 298 K and * To whom correspondence should be addressed. E-mail:
[email protected]. † E-mail:
[email protected].
0.1 MPa. The approach was motivated by the association in the Kraft mill atmospheric effluents of the TRS gases with sufficient molecular oxygen, by the lenient conditions characterizing the TRS streams (20-60 °C, 0.1 MPa) as well as by the prevalence of numerous alkaline liquid streams that could be taken advantage of in the mill and could serve for scrubbing purposes. Dissolution of oxygen in the alkaline solution/FeCeOx slurry was shown to accelerate the conversion of bisulfide4 (HS-, i.e., the H2S alter ego at basic pH). The system consists of two consecutive reactions with a reactant in the one being a product in the other being run within the same vessel at the same time. The oxidation of bisulfide by ferric iron and, in return, the O2 reoxidation of ferrous into ferric iron perpetuate the pollutant conversion while requiring little amounts of cheap, easily regenerable and recyclable oxidant. Methyl mercaptan is formed in the Kraft cooking liquor due to the reactions between bisulfide and the methoxy moieties in the lignin:5
HS- + R-OCH3 + OH- f CH3S- + RO- + H2O (1) In addition, MM is the precursor of DMS from lignin during the cooking phase and of DMDS via the reaction with dissolved oxygen:5
CH3S- + R-OCH3 f CH3-S-CH3 + RO-
(2)
2CH3S- + 1/2O2 + H2O f CH3-SS-CH3 + 2OH-
(3)
Unlike hydrogen sulfide, odor removal due to methyl mercaptan received much less literature coverage. Wallace6 studied the oxidation of thiols by transition metal oxides in xylene solvent at 55 °C in anoxia (absence of dissolved oxygen). The reaction was found to yield disulfides with manganese(IV) and iron(III) being the most active metal oxides in thiols oxidation. Thiols oxidation by metal oxides was proposed to obey a radical mechanism with thiols adsorption on the surface of the metal oxide as the rate-determining step.
10.1021/ie061586m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 1991
Bentvelzen et al.7 studied the methyl mercaptan oxidation by dissolved oxygen and the DMDS hydrolysis in alkaline solutions (pH ) 11-12). MM oxidation was found to be very rapid and to follow a two-step mechanism: the first step being the MM conversion to DMDS (eq 3), while, in the second, the DMDS slowly hydrolyzes to nonvolatile methane sulfinic acid releasing some regenerated MM lying predominantly as methyl mercaptide in the pH conditions prevailing in the medium (eq 4):7
2CH3-SS-CH3 + 2OH- f CH3SO2- + 3CH3S- + 2H+ (4) Kask and Teder5 studied the absorption of methyl mercaptan from the gas phase into aqueous alkaline polysulfides and polythionates solutions. The latter solutions were prepared by mixing sulfide and sulfite solutions. MM absorption was observed to improve with increasing pH in polythionates solutions, whereas MM absorption in polysulfides solutions was reported to increase while increasing their concentration. However, the concentrated polysulfides solutions were found to outperform the most efficient polythionates solutions tested for MM removal.5 In a recent study, Kastner et al.8 showed that hydrogen sulfide and methyl mercaptan can be catalytically oxidized by wood and coal fly at low temperatures (23-25 °C) in oxic conditions, i.e., with dissolved oxygen. Elemental sulfur was identified as the end-product of H2S oxidation, while MM conversion was reported to be stoichiometric with the formed DMDS with insignificant catalytic decay. A series of similar but more detailed studies was conducted by Bashkova et al.9-11 who studied the influence of different properties of activated carbon for adsorption/oxidation of CH3SH. Methyl mercaptan adsorption at ambient conditions was found to be sensitive to the size and volume of pores, surface oxygen groups (particularly basic oxygen), pH, and the presence of water on the activated carbon surface. The presence of iron (most probably in the form of iron oxide) on the activated carbon surface was reported to improve the removal capacity of the material. The reaction involves the reduction of iron sites by accepting electrons from the mercaptide ions during their oxidation to DMDS. Further re-exposure to oxygen of the moisturized activated carbon was found to allow regeneration of the iron sites. The presence of moisture was also established to facilitate the dissociation of methyl mercaptan to its methyl mercaptide conjugate ion. As a continuation of our previous work on hydrogen sulfide,4 we propose to test in the present study possible extension of the FeCeOx/O2 bifunctional reaction system for the removal of MM at basic pH (10.5-12) with the following two objectives: (1) Assess the effect of pH, methyl mercaptide and dissolved O2 concentrations (oxic and anoxic conditions, DO2 ) 0), and FeCeOx areal concentration on the conversion of methyl mercaptide. (2) Assess the influence of other TRS components (H2S, DMDS, DMS) and polysulfides interfering with methyl mercaptide oxidation by the FeCeOx/O2 bifunctional reaction system. Experimental Section Materials Preparation and Characterization. All materials were ACS grade, and all solutions were prepared with distilled water. A modified coprecipitation protocol was used to synthesize the composite Fe/Ce oxide-hydroxide (FeCeOx) with
Table 1. Initial Conditions for Mercaptide Oxidation Experiments run no.
pH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
12 12 12 12 12 12 12 12 10.5 11 11.5 10.5 11 12 12 12 12 12 12 10.5 11.5 11 12 12 12 12 12 12 12 12 12 10.5 10.5 10.5
init FeCeOx init CH3Ssurf area conc Vgas DO2 (m2/L) (mmol/L) (mL) (mg/L) 120 120 120 120 120 100 120 140 120 120 120 120 120 120 0 50 50 50 50 50 50 50 50 50 50 120 50 50 50 50 0 120 50 120
1 1 0.5 1.8 0.7 1 1 1 1 1 1 1 1 1 1 1 1.5 2 0.5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.5
150 150 150 150 150 150 150 150 150 150 150 150 150 150 1100 1100 1100 1100 1100 1100 1100 1100 1100 50 250 250 1100 1100 1100 1100 1100 1100 1100 1100
0 0 0 0 0 0 0 0 0 0 0 0 0 0 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6
observations no PDTA
oxygen only
initial gas vol. initial gas vol. test DMS 0.5 mM test DMS 1 mM test HS- 2 mM test HS- 0.5 mM test HS- 0.5 mM test HS- 0.5 mM test Sx2test HS- 1 mM 2 cycles
an Fe:Ce bulk mole ratio of 9:1 (atomic absorption analysis performed on a Perkin-Elmer AAnalyst 800).4 The powdered material, calcined in air at 295 °C for 3 h, yielded a specific surface area of 110 m2/g which was determined by the multipoint BET method (Micrometrics TRISTAR3000 analyzer). The powder had an ensemble-average diameter of 35 µm as measured by means of an optical microscope. The synthesized material was amorphous as verified by X-ray diffractometry performed on a Siemens D5000 diffractometer using Cu KR radiation at 40 kV and 30 mA (λ ) 1.54184 Å) at 1 °/min [2θ].4 Reaction Setup. Temperature-controlled (22 °C) reactions between Fe/Ce oxide-hydroxide and methyl mercaptide ion (CH3S-) were monitored in borate-buffered (100 mmol/L borate, Sigma-Aldrich) aqueous solution (VL ) 400-500 mL) placed in sealed, double-jacketed, and magnetically stirred (700 rpm) glass reactors having headspace capacities varying between VG ) 50 and 1100 mL (Table 1). The borate buffer system was shown to ensure constant pH (variation within (0.1 pH units) over the reaction time,4,12 and thus, only initial pH was adapted without any further on-line control. The system was run batchwise for both the slurry and gas phases. In the anoxic reaction tests, nitrogen (very high purity, Praxair Canada) was bubbled through a fritted distributor for 15 min in the solutions to get rid of residual oxygen prior to mercaptide addition. Deoxygenation of the solutions was verified by means of a dissolved oxygen (DO2) probe (DOB-930 model from Omega) by always maintaining the DO2 level below the sensor detection limit (
