removal of methyl mercaptide by iron/cerium oxide−hydroxide in anoxic and oxic alkaline media

Removal of Methyl Mercaptide by Iron/Cerium Oxide-Hydroxide in Anoxic andOxic Alkaline Media

Catalin F. Petre† and Faı1cal 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 dealwith the methyl mercaptan contamination from total reduced sulfur (TRS) emissions in the pulp and paperindustry. 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, whereasin oxic conditions it was noticeably higher but tributary of dissolved oxygen (DO2). Interference with bisulfidecomixed with mercaptide caused an inhibition in the conversion for both pollutants. Such inhibition was dueto the incipient polysulfides formed via bisulfide oxidation. On the contrary, the mercaptide conversion wasfound to improve at the expense of bisulfide when large quantities of exogenous polysulfides were presentinitially in the reaction medium. The conversion of mercaptide was unaffected with the comixing of dimethylsulfide. The oxidation of methyl mercaptide by the O2/Fe/Ce oxide-hydroxide system did not affect thereoxidative regeneration of surface Fe(III) by DO2. This feature pinpoints the sine qua none condition for aredox process based on the O2/Fe/Ce oxide-hydroxide system to remove methyl mercaptan in pulp andpaper emissions.

Introduction

Unlike mechanical pulping, and despite the fact that nearlyhalf of the wood content is being degraded by chemical pulping,Kraft chemical pulping still pervades the pulp and paper industryin North America owing, principally, to the properties it confersto the freed (hemi)cellulose rich fibers.1 The wood cooking,the black liquor evaporation and incineration, and the chemicalrecovery systems from Kraft pulping are responsible for theemission of the so-called total reduced sulfurs (TRS) which areknown for their nauseous character. TRS gases refer specificallyto the quartet: hydrogen sulfide (H2S), methyl mercaptan (CH3-SH), dimethyl sulfide (CH3SCH3, also known as DMS), anddimethyl disulfide (CH3S2CH3, also known as DMDS). Al-though current North American regulation limits TRS concen-tration levels in atmospheric effluents to 5 ppmv, this is stillwell above the human olfactory threshold (ca. 1-10 ppbv)2

explaining hence the public resentment and the bad presssurrounding the emissions of TRS. A great deal of efforts todevelop efficient abatement approaches have been deployedrecently with the goal of turning the TRS-containing effluentsinto odor-attenuated and less objectionable streams.3

Several TRS abatement processes have been advocated suchas alkaline/amine scrubbing and gas incineration, chemicaloxidation by NaClO, ClO2, H2O2, and KMnO4 and gas phaseoxidation by ClO2, wet oxidation, aerobic biofiltration, activatedcarbon, and green liquor dregs adsorption (see for example refs2 and 3). However, only incineration and alkaline scrubbingare currently massively implemented industrially.

Because of their lower boiling points, hydrogen sulfide andmethyl mercaptan (MM) contribute for the largest compositionsin the TRS mix. In a recent work,4 a simple redox approach forscrubbing H2S-contaminated air effluents was evaluated inwhich Fe/Ce oxide-hydroxide (FeCeOx) was used for oxidizingthe absorbed pollutant within an alkaline solution at 298 K and

0.1 MPa. The approach was motivated by the association inthe Kraft mill atmospheric effluents of the TRS gases withsufficient molecular oxygen, by the lenient conditions character-izing the TRS streams (20-60 °C, 0.1 MPa) as well as by theprevalence of numerous alkaline liquid streams that could betaken advantage of in the mill and could serve for scrubbingpurposes. Dissolution of oxygen in the alkaline solution/FeCeOxslurry was shown to accelerate the conversion of bisulfide4

(HS-, i.e., the H2S alter egoat basic pH). The system consistsof two consecutive reactions with a reactant in the one being aproduct in the other being run within the same vessel at thesame time. The oxidation of bisulfide by ferric iron and, inreturn, the O2 reoxidation of ferrous into ferric iron perpetuatethe pollutant conversion while requiring little amounts of cheap,easily regenerable and recyclable oxidant.

Methyl mercaptan is formed in the Kraft cooking liquor dueto the reactions between bisulfide and the methoxy moieties inthe lignin:5

In addition, MM is the precursor of DMS from lignin duringthe cooking phase and of DMDS via the reaction with dissolvedoxygen:5

Unlike hydrogen sulfide, odor removal due to methyl mer-captan received much less literature coverage. Wallace6 studiedthe oxidation of thiols by transition metal oxides in xylenesolvent at 55°C in anoxia (absence of dissolved oxygen). Thereaction was found to yield disulfides with manganese(IV) andiron(III) being the most active metal oxides in thiols oxidation.Thiols oxidation by metal oxides was proposed to obey a radicalmechanism with thiols adsorption on the surface of the metaloxide as the rate-determining step.

* To whom correspondence should be addressed. E-mail:[email protected].

† E-mail: [email protected].

HS- + R-OCH3 + OH- f CH3S- + RO- + H2O (1)

CH3S- + R-OCH3 f CH3-S-CH3 + RO- (2)

2CH3S- + 1/2O2 + H2O f CH3-SS-CH3 + 2OH- (3)

1990 Ind. Eng. Chem. Res.2007,46, 1990-1999

10.1021/ie061586m CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 02/24/2007

Bentvelzen et al.7 studied the methyl mercaptan oxidationby dissolved oxygen and the DMDS hydrolysis in alkalinesolutions (pH) 11-12). MM oxidation was found to be veryrapid and to follow a two-step mechanism: the first step beingthe MM conversion to DMDS (eq 3), while, in the second, theDMDS slowly hydrolyzes to nonvolatile methane sulfinic acidreleasing some regenerated MM lying predominantly asmethyl mercaptide in the pH conditions prevailing in themedium (eq 4):7

Kask and Teder5 studied the absorption of methyl mercaptanfrom the gas phase into aqueous alkaline polysulfides andpolythionates solutions. The latter solutions were prepared bymixing sulfide and sulfite solutions. MM absorption wasobserved to improve with increasing pH in polythionatessolutions, whereas MM absorption in polysulfides solutions wasreported to increase while increasing their concentration.However, the concentrated polysulfides solutions were foundto outperform the most efficient polythionates solutions testedfor MM removal.5

In a recent study, Kastner et al.8 showed that hydrogen sulfideand methyl mercaptan can be catalytically oxidized by woodand coal fly at low temperatures (23-25 °C) in oxic conditions,i.e., with dissolved oxygen. Elemental sulfur was identified asthe end-product of H2S oxidation, while MM conversion wasreported to be stoichiometric with the formed DMDS withinsignificant catalytic decay.

A series of similar but more detailed studies was conductedby Bashkova et al.9-11 who studied the influence of differentproperties of activated carbon for adsorption/oxidation of CH3-SH. Methyl mercaptan adsorption at ambient conditions wasfound to be sensitive to the size and volume of pores, surfaceoxygen groups (particularly basic oxygen), pH, and the presenceof water on the activated carbon surface. The presence of iron(most probably in the form of iron oxide) on the activated carbonsurface was reported to improve the removal capacity of thematerial. The reaction involves the reduction of iron sites byaccepting electrons from the mercaptide ions during theiroxidation to DMDS. Further re-exposure to oxygen of themoisturized activated carbon was found to allow regenerationof the iron sites. The presence of moisture was also establishedto facilitate the dissociation of methyl mercaptan to its methylmercaptide conjugate ion.

As a continuation of our previous work on hydrogen sulfide,4

we propose to test in the present study possible extension ofthe FeCeOx/O2 bifunctional reaction system for the removal ofMM at basic pH (10.5-12) with the following two objectives:

(1) Assess the effect of pH, methyl mercaptide and dissolvedO2 concentrations (oxic and anoxic conditions, DO2 ) 0), andFeCeOx areal concentration on the conversion of methylmercaptide.

(2) Assess the influence of other TRS components (H2S,DMDS, DMS) and polysulfides interfering with methyl mer-captide oxidation by the FeCeOx/O2 bifunctional reactionsystem.

Experimental Section

Materials Preparation and Characterization. All materialswere ACS grade, and all solutions were prepared with distilledwater. A modified coprecipitation protocol was used to syn-thesize the composite Fe/Ce oxide-hydroxide (FeCeOx) with

an Fe:Ce bulk mole ratio of 9:1 (atomic absorption analysisperformed on a Perkin-Elmer AAnalyst 800).4 The powderedmaterial, calcined in air at 295°C for 3 h, yielded a specificsurface area of 110 m2/g which was determined by themultipoint BET method (Micrometrics TRISTAR3000 ana-lyzer). The powder had an ensemble-average diameter of 35µm as measured by means of an optical microscope. Thesynthesized material was amorphous as verified by X-raydiffractometry performed on a Siemens D5000 diffractometerusing Cu KR radiation at 40 kV and 30 mA (λ ) 1.54184 Å)at 1 °/min [2θ].4

Reaction Setup.Temperature-controlled (22°C) reactionsbetween 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) placedin sealed, double-jacketed, and magnetically stirred (700 rpm)glass reactors having headspace capacities varying betweenVG

) 50 and 1100 mL (Table 1). The borate buffer system wasshown to ensure constant pH (variation within(0.1 pH units)over the reaction time,4,12and thus, only initial pH was adaptedwithout any further on-line control. The system was runbatchwise for both the slurry and gas phases.

In the anoxic reaction tests, nitrogen (very high purity, PraxairCanada) was bubbled through a fritted distributor for 15 minin the solutions to get rid of residual oxygen prior to mercaptideaddition. Deoxygenation of the solutions was verified by meansof a dissolved oxygen (DO2) probe (DOB-930 model fromOmega) by always maintaining the DO2 level below the sensordetection limit (<0.1 ppm). For the oxic runs, the liquid wassaturated with oxygen (∼8.6 mg/L at 22°C) from the reactorheadspace air (Praxair Canada) prior to reaction. DO2 consump-

2CH3-SS-CH3 + 2OH- f

CH3SO2- + 3CH3S

- + 2H+ (4)

Table 1. Initial Conditions for Mercaptide Oxidation Experiments

runno. pH

init FeCeOxsurf area(m2/L)

init CH3S-

conc(mmol/L)

Vgas

(mL)DO2

(mg/L) observations

1 12 120 1 150 02 12 120 1 150 0 no PDTA3 12 120 0.5 150 04 12 120 1.8 150 05 12 120 0.7 150 06 12 100 1 150 07 12 120 1 150 08 12 140 1 150 09 10.5 120 1 150 0

10 11 120 1 150 011 11.5 120 1 150 012 10.5 120 1 150 013 11 120 1 150 014 12 120 1 150 015 12 0 1 1100 8.6 oxygen only16 12 50 1 1100 8.617 12 50 1.5 1100 8.618 12 50 2 1100 8.619 12 50 0.5 1100 8.620 10.5 50 1 1100 8.621 11.5 50 1 1100 8.622 11 50 1 1100 8.623 12 50 1 1100 8.624 12 50 1 50 8.6 initial gas vol.25 12 50 1 250 8.6 initial gas vol.26 12 120 1 250 8.627 12 50 1 1100 8.6 test DMS 0.5 mM28 12 50 1 1100 8.6 test DMS 1 mM29 12 50 1 1100 8.6 test HS- 2 mM30 12 50 1 1100 8.6 test HS- 0.5 mM31 12 0 1 1100 8.6 test HS- 0.5 mM32 10.5 120 1 1100 8.6 test HS- 0.5 mM33 10.5 50 1 1100 8.6 test Sx

2-

34 10.5 120 1100 test HS- 1 mM0.5 8.6 2 cycles

Ind. Eng. Chem. Res., Vol. 46, No. 7, 20071991

tion during the course of oxidation was monitored with thedissolved oxygen probe.

Stock solutions were prepared weekly by dissolving 300 mgof sodium methyl mercaptide (CH3SNa, Sigma-Aldrich Canada)in 50 mL of distillated water, adjusted with NaOH (FisherScientific) at pH ) 12. The desired initial organic sulfideconcentration,C0, was obtained by injecting a prescribed volumefrom the stock solution in the borate-buffered liquid.

A suspension of the iron-based material in 5 mL deionizedwater was briefly added to the reactor to trigger the heteroge-neous reactions. To prevent precipitation of the leached iron inthe course of the anoxic tests, a 0.5 mmol/L solution of apolydentate chelate was initially introduced in the borate bufferfor iron complexation. The chelate used was 1,3-diaminopro-pane-N,N,N′,N′-tetraacetic acid (PDTA, Sigma-Aldrich Canada).As also verified previously for HS- oxidation,12 addition of 0.5mmol/L PDTA to prevent precipitation of leached Fe2+ had noeffect on the mercaptide conversion (Table 1: run no. 1 withPDTA and run no. 2 without PDTA, results not shown).

Small aliquots (1 mL) were periodically withdrawn duringthe course of reaction from the slurry phase and filtered througha 0.2µm Millipore membrane filter. The samples were analyzedby capillary electrophoresis (CE) to monitor the time evolutionof methyl mercaptide, bisulfide, and thiosulfate. For analyzingthe dissolved iron, 0.3 mL aliquots were diluted in 10 mL ofwater and analyzed by atomic absorption. Prior to CE analysisof the sulfur-bearing species, the samples were preventively keptin an oxygen-free atmosphere to quench any adventitious DO2-mediated reaction.

Table 1 summarizes the initial conditions for the reaction runscarried out in the present study. Out of 34 reaction runs, 23were conducted at pH) 12 for which virtually completedissociation of MM to its methyl mercaptide anion conjugateis achieved (98.5% CH3S-, 1.5% CH3SH):13

We did not attempt to analyze the methyl mercaptan(molecular form) lying in the dissolved state and coexisting withthe mercaptide in the solution nor that fraction of methylmercaptan that might strip toward the reactor headspace. Thelatter would resorb back, via gas-to-liquid mass transfer, whilemercaptide is being oxidized. We positively identified DMDSas a reaction product of mercaptide oxidation by FeCeOx. Beinguncharged, DMDS could not be resolved via our capillaryelectrophoresis protocol. Although not quantified, it was identi-fied in this study by analyzing the composition of the reactorheadspace by gas chromatography using a Perkin-Elmer GCequipped with a flame ionization detector (FID) and a capillarycolumn of 0.32 mm i.d. and 30 m length with a 5.0µm filmthickness of DB-1 (J&W Scientific).

Samples Analysis.Consumption of methyl mercaptide andbisulfide anions, and formation of thiosulfate were monitoredby means of a capillary electrophoresis (CE) system (AgilentTechnologies) to separate, identify, and quantify the ionicspecies formed or consumed during the oxidation of methylmercaptide. The capillary electrophoresis system was equippedwith a prealigned deuterium lamp and a UV-visible absorbancediode array detector (DAD). Separation was achieved usingindirect UV detection at three different wavelengths: 214 nmwith reference at 372 nm (thiosulfate), 230 nm with referenceat 372 nm (bisulfide), and 240 nm with reference at 372 nm(methyl mercaptide). The ions migration and separation wasachieved by applying simultaneously a negative voltage of 10kV and an inlet pressure of 25 mbar. The analytes were

separated in fused silica capillaries with 50µm i.d. and 37 cmeffective length. The detector window was formed by burningoff a 1 cm section of the outer polyimide coating. Thetemperature of the capillary cassette was 25°C, and the sampleinjection was performed in the hydrodynamic mode by applyinga 50 mbar pressure for 10 s. Agilent ChemStation software wasused for data acquisition and quantification.

The background electrolyte (BGE) was prepared weekly bymixing in 50 mL of distilled water, 142 mg of Na2HPO4 (ACS,Fisher Scientific), and 4.2 mL of a 6 mmol/L hexamethoniumbromide solution (98%, Sigma-Aldrich Canada), i.e., the electro-osmotic flow (EOF) modifier. The pH of BGE was adjusted to12 with 3 mol/L NaOH. The resulting carrier electrolyte,containing 20 mmol/L phosphate and 0.5 mmol/L EOF modifier,was filtered and degassed for 15 min in an ultrasonic bath priorto use.

Figure 1 shows typical electrophoregrams as a function oftime of mercaptide, bisulfide, and thiosulfate peaks for the HS-/CH3S-/FeCeOx/O2 system at pH) 10.5. The CE detectionlimits for both methyl mercaptide and hydrosulfide weredetermined to be ca. 1 ppm.

Results and Discussion

Anoxic Reaction between Methyl Mercaptide and Fe-CeOx. On the basis of the few heterogeneous reaction mech-anisms available in the literature for bisulfide and MMoxidations,6,12 it is proposed to analyze the anoxic reactionbetween methyl mercaptide and FeCeOx according to thefollowing simplified pathway:

Formation of methyl mercaptide radical

Formation of DMDS

Leaching of incipient surface Fe(II)

CH3SH a CH3S- + H+ pKa ) 10.3 at 25°C (5)

Figure 1. Typical electrophoregram of detected species for the reactionbetween CH3S-, HS-, and FeCeOx in the presence of oxygen. The reactionconditions are like those in Figure 8b (2).

[FeIII (OH)] + CH3S- + H+ a CH3S

• + [FeII(OH2)+ ]

(6)

2CH3S• a CH3SSCH3 (7)

[FeII(OH2)+ ] + H+ f Fe2+ + new site (8)

1992 Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007

Figure 2 illustrates the influence of the initial concentration,C0, of organic sulfide on the time evolution of the normalizedmethyl mercaptide concentration (C/C0) in the anoxic oxidationwith FeCeOx (Table 1, runs 3-5 and 7). For a given FeCeOxareal concentration and pH,C0 is barely affected the mercaptideultimate conversion. For instance, foraFeCeOx) 120 m2/L andpH ) 12, the mercaptide conversion stagnated around 40% in1 h despite a change by a factor 3.6 onC0. Note that themercaptide conversion shown in Figure 2 is virtually indepen-dent of the initial concentration,C0, as the available Fe(III)surface sites of the FeCeOx material are likely to be the limitingreactant even at a relatively low mercaptide initial concentration(0.5 mmol/L).

The influence of the FeCeOx initial areal concentration onthe normalized mercaptide concentration is shown in Figure 3.Mercaptide conversion expectably increases with an increasingFeCeOx initial areal concentration, e.g., a 40% rise of initialoxide area improved mercaptide conversion by 50% after 2 h.

Figure 4 shows the influence of pH on the anoxic oxidationof methyl mercaptide by FeCeOx for an initial organic sulfideconcentrationC0 ) 1 mmol/L andaFeCeOx ) 120 m2/L. Theleaching off of iron during the reaction exhibited virtually nodependence with respect to pH (Figure 4). At first sight, thetrends of the iso-pH mercaptide concentration profiles seemerratic with the highest contrasts in reactivity at pH) 11(CH3S- conversion) 55% at 2 h) and pH) 11.5 (CH3S-

conversion) 27% at 2 h). Mercaptide conversion was slightlygreater at pH) 11 than at pH) 10.5; a similar pattern persistsfor pH ) 12 and 11.5. These trends were confirmed by triplicatetests at pH) 12 (Table 1: runs 1, 7, and 14) and by duplicatetests at pH) 10.5 (Table 1: runs 9 and 12) and 11 (Table 1:runs 10 and 13). Reproducibility of the same trends rules outtherefore any influence of uncontrollable error sources and isindicative of the complex dependence on pH of the manyreaction steps involved in the mercaptide oxidation as shall bediscussed later.

The limited reactivity of the material and the lack ofmonotonous trends in the way mercaptide conversion dependson pH in Figure 4 are analyzed in the light of the likelihood of

the following four factors: (1) pH dependence of MM speciationaffecting theactual concentration of mercaptide vis-a`-vis thetotal organic sulfide; (2) extent of Fe2+ dissolution thus exposingthe virgin underneath Fe(III) sites to the surface; (3) sensitivityto pH of the reducibility of Fe(III) surface sites; (4) possiblecatalysis by FeCeOx of mercaptide oxidation product competingwith mercaptide oxidation for available surface sites.

Ascribing the limited reactivity of the material and the lackof monotony of C/C0 versus pH to the effect of pH on sulfidespeciation, i.e., first factor above, seems rather unlikely as theinitial mercaptide concentration was shown in Figure 2 to havea minor effect on the mercaptide conversion.

Figure 2. Anoxic reaction between FeCeOx and methyl mercaptide atconstant surface area and pH (100 mmol/L borate, 120 m2/L FeCeOx, pH) 12); effect of MM initial concentration (C0) on CH3S- conversion: (O)C0 ) 0.5 mmol/L, (b) C0 ) 0.7 mmol/L, (∆) C0 ) 1 mmol/L, and (2) C0

) 1.8 mmol/L.

Figure 3. Anoxic reaction between FeCeOx and methyl mercaptide, atconstant MM initial concentration and pH (100 mmol/L borate,C0 ) 1mmol/L, pH ) 12); effect of oxide surface area on CH3S- conversion:(b) 100, (9) 120, and (2)140 m2/L.

Figure 4. Anoxic reaction between FeCeOx and methyl mercaptide atconstant surface area and MM initial concentration (100 mmol/L borate,120 m2/L FeCeOx,C0 ) 1 mmol/L); effect of pH on CH3S- oxidation(filled symbols) and Fe2+ dissolution (empty symbols): (b) pH ) 10.5,(9) pH ) 11, ([) pH ) 11.5, and (2) pH ) 12. Data point marked withthe * symbol are taken from ref 12 and represent the dissolved iron duringthe reaction between FeCeOx and HS- under similar conditions. The linesshow trends.


Figure 4 reveals that the concentration of leached Fe2+ soaredduring the anoxic oxidation of bisulfide by FeCeOx.12 This isshown for pH) 11 where the oxidation of bisulfide releasedalmost nine times more Fe2+ than the mercaptide oxidation fornearly similar initial areal concentrations of FeCeOx. At pH)11, the leached Fe2+ fraction is estimated not to exceed 1% ofthe total divalent iron being formed after 2 h assuming a 2:1mole ratio between the reduced iron and the CH3S- oxidizedto DMDS (eqs 6 and 7). Hence to be in line with stoichiometricexpectation, it appears that the amount of leachable Fe2+ (eq8) was negligibly small compared to the amount of Fe(II) thatremained attached to the FeCeOx surface. The Fe(II) speciessitting on the surface prevent new Fe(III) surface sites fromemerging from beneath to pursue reaction. Therefore, the secondfactor evoked regarding the influence of Fe2+ dissolutionexplains in part why mercaptide conversion halted after sometime (Figures 2-4).

The reducibility of Fe(III) surface sites also depends on pH.There is a tendency of Fe(III) reactivity to decline withincreasingly pH.12,14,15This is globally coherent with the saggingin mercaptide conversion at pH) 11.5-12 as compared to thatat pH ) 10.5-11, as shown in Figure 4.

DMDS alkaline hydrolysis is known to release back someMM (eq 4), though the uncatalyzed hydrolysis of DMDS wasreported to be relatively slow at ambient temperature.7 Theemergence of local shoulders for some iso-pH mercaptideconcentration profiles in Figure 4 suggests that eq 4 is coherentwith the reappearance of MM through DMDS hydrolysis andthat furthermore a catalytic effect via FeCeOx must be presentalready at 22°C to facilitate DMDS hydrolysis. This catalyticeffect would explain why mercaptide asymptotic concentrationsleveled off relatively rapidly even at ambient temperature (Figure4). In addition, a catalyzed hydrolysis suggests a competitionbetween DMDS and methyl mercaptide for accessing theavailable unreacted Fe(III) surface sites. This solicits the materialsurface by an overaccumulation of adsorbed mercaptidesdueto DMDS subsequent hydrolytic decompositionsthus accelerat-ing reduction of the Fe(III) surface sites whereupon deactivationof the FeCeOx material ensues. Unlike bisulfide, mercaptidecannot be totally converted thus giving rise to the plateaus inthe C/C0 profiles of Figure 4.

Oxic Reaction between Methyl Mercaptide and FeCeOx.Bentvelzen et al.7 showed that MM reacts rapidly with dissolvedoxygen to yield DMDS at pH) 12 and temperatures in the20-25 °C range. Figure 5a compares normalized methylmercaptide concentration profiles (C/C0) in oxic conditions withand without FeCeOx (Table 1, runs 15 and 16) along with thecorresponding DO2 profiles. In agreement with Bentvelzen etal.,7 the homogeneous oxidation of mercaptide by dissolvedoxygen is relatively fast (95% conversion after 30 min).Moreover, the monotonic “shoulderless” falloff of theC/C0

profile (filled triangle) suggests no evidence of mercaptidereformation via DMDS hydrolysis as should be expected forthe homogeneous route at such low temperatures (no catalysisFeCeOx).7 Introduction of FeCeOx at 120 m2/L areal concentra-tion, while everything else being identical to the homogeneousreaction conditions, occasioned a slight slow down in mercaptideconsumption in the long run. Appearance of a shoulder (filledsquare in Figure 5a) near 20 min of reaction is interpreted asinception of MM through FeCeOx-catalyzed DMDS hydrolysis.High conversion levels of mercaptide are also achieved hetero-geneously as seen in Figure 5b for variousC0 values. Persistence

of the shoulders on theC/C0 profiles is also noticeable in Figure5b which lean towards a FeCeOx-catalyzed hydrolysis ofDMDS.

When FeCeOx was added, DO2 dropped quickly during thefirst 15 min before nearly leveling off when virtually nomercaptide was left (Figure 5a). Comparatively, the DO2 declinefor the homogeneous oxidation of mercaptide was barelyimperceptible dropping by just 0.2 mg/L in 120 min. It is worthyof notice that the DO2 depletion as revealed in the measuredprofiles does not fairly reflect the actual consumption ofdissolved oxygen because the gas-liquid mass transfer fromthe headspace air replenishes, more or less rapidly, the dissolvedoxygen (Table 1); besides, the total O2 available outweighs thestoichiometric requirement for mercaptide removal by a factor24.2 (see below) even via the homogeneous oxidation, eqs 3and 4. It appears that mercaptide oxidative conversion is affecteddifferently by the homogeneous and heterogeneous routes. The

Figure 5. (a) Effect of FeCeOx on methyl mercaptide oxidation (filledsymbols) by dissolved oxygen (empty symbols) (100 mmol/L borate, 120m2/L FeCeOx, pH) 12,C0 ) 1 mmol/L): (2) DO2 only and (9) FeCeOx+ DO2. (b) Oxic reaction between FeCeOx and methyl mercaptide atconstant surface area and pH (120 m2/L FeCeOx, pH) 12); effect of MMinitial concentration on CH3S- conversion: (9) C0 ) 0.5 mmol/L, (0) C0

) 1.0 mmol/L, (2) C0 ) 1.5 mmol/L, and (∆) C0 ) 2.0 mmol/L. The linesshow trends.


strong falloff in DO2 in the early instances of heterogeneousreaction is in all likelihood attributable to the consumption ofsupplementary DO2 in the reoxidative regeneration of surfaceFe(II) into Fe(III). Recall that the split between the Fe(II) lefton the material’s surface and the Fe2+ that leached off was infavor of an overwhelming fraction of surface Fe(II) indicatingthat DO2 mostly reoxidized thenonleachediron.

The influence of pH on mercaptide oxidation by FeCeOx andDO2 is shown in Figure 6. Regardless of the prevailing pH,high conversion levels of mercaptide (85%-98%) were obtainedafter 40 min. Note also that the iso-pH mercaptide profiles inoxic conditions (Figure 6) arranged in exactly the same mannerwith respect to pH as in the anoxic tests (Figure 4).

Figure 6 also shows that the lower the pH the higher the DO2

consumption, in line with the facilitated reducibility of Fe(III)into Fe(II) outlined earlier in the analysis of the anoxic kinetics.This reaction, in return, requires much more oxygen to reoxidizeFe(II) back into Fe(III). A 15% deviation between DO2 at pH) 12 and 10.5 occurred after 15 min resulting in only a 4%deviation on mercaptide concentrations. This suggests that, asidefrom mercaptide homogeneous oxidation (eq 3) and DO2-mediated Fe(II) reoxidation, DO2 might have been consumedby other reactions between oxygen and presumably mercaptidebyproducts. DO2 ultimately plateaued after thermodynamicequilibrium was attained between DO2 and O2 left in theheadspace after the reactions reached termination.

As seen in Figures 5 and 6, total mercaptide conversion isachievable by the FeCeOx/DO2 system. This means that in acontinuous industrial process relying on the same reactingsystem, the gas residence time (or flow rate) can be controlledin order to absorb methyl mercaptan, to convert it according toeq 5 into mercaptide, and finally to deeply oxidize it thusyielding a nearly MM-free air stream leaving the FeCeOx/DO2

based scrubber.Figure 7 shows the effects on mercaptide oxidation kinetics

of the initial areal concentration of FeCeOx, on the one hand,and of the initial headspace volume of the reactor on the otherhand (Table 1, runs 23-26). When the reactor headspace waslimited to VG ) 50 mL for aFeCeOxas in Figure 5, a maximum

of 40% mercaptide conversion by FeCeOx was achieved (Figure7). ExpandingVG to 250 mL, leaving unchanged the otherconditions with respect to run 23, improved the ultimatemercaptide oxidation by 25%. A further expansion ofVG to 1100mL, everything else being constant, shifted the final mercaptideconversion to 95%. In view of reactions eqs 3 and 4, the netstoichiometric excess ratiosε ) n0(O2):n0(MM) were 1.1 (VG

) 50 mL), 5.8 (VG ) 250 mL), and 24.2 (VG ) 1100 mL)indicating that the reaction could be oxygen-limited forε )1.1 due to Fe(II) concurrent reoxidation and oxidation of otherorganic sulfur byproducts. This explains the lack of performancein mercaptide conversion. For the remaining two otherε cases,the reactions were conducted under a stoichiometric shortageof mercaptide. Despite the fact that there was plenty of oxygenfor converting all the mercaptide via eqs 3 and 4, full mercaptideconversion was completed only at the highestε ratio. Theseresults are ex-post-facto confirmation that a great deal of DO2

could be diverted and consumed by reactions other thanhomogeneous mercaptide oxidation. More than doubling theareal FeCeOx concentration for the run atε ) 5.8 did not appearto further the mercaptide conversion (Figure 7). An increase inareal concentration of FeCeOx promotes the catalytic hydrolysisof DMDS, yielding alas more reformed mercaptide thusdiverting more oxygen from homogeneous mercaptide oxidationto the reoxidation of surface Fe(II) into Fe(III).

As suggested from the above analysis, the reaction betweenmercaptide, FeCeOx, and DO2 displays a much more complexpattern than the anoxic reaction. In oxic conditions, besides thehomogeneous mercaptide oxidation with DO2 (eq 3), mercaptidealso reacts with FeCeOx (eqs 6 and 7) resulting in its rapidconsumption in the early stages of the reaction (Figures 4-6).Moreover, a plausible route for DO2 overconsumption by sulfur-bearing byproducts is believed to be the oxidation of methanesulfinic acid to form the stable and nonvolatile methane sulfonicacid (MSA):16

Figure 6. Oxic reaction between FeCeOx and methyl mercaptide (100mmol/L borate, 120 m2/L, C0 ) 1 mmol/L); effect of pH on CH3S-

oxidation (filled symbols) and DO2 profile (empty symbols): (b) pH )10.5, (9) pH ) 11, ([) pH ) 11.5, and (2) pH ) 12. The lines showtrends.

Figure 7. Oxic reaction between FeCeOx and methyl mercaptide (100mmol/L borate,C0 ) 1 mmol/L, pH ) 12); effect of net stoichiometricexcess ratioε ) n0(O2):n0(MM) and aFeCeOxon CH3S- conversion: ([) ε

) 1.1 andaFeCeOx) 50 m2/L, (2) ε ) 5.8 andaFeCeOx) 50 m2/L, (∆) ε )5.8 andaFeCeOx) 120 m2/L, and (9) ε ) 24.2 andaFeCeOx) 50 m2/L. Thelines show trends.

CH3SO2- + 1/2O2 + H+ f CH3SO3H (9)


Obviously, in addition to this reaction, DO2 will be consumedto regenerate the Fe(III) being reduced in reaction eq 6:

The above results and observations highlight a practical valuefor these reactions. Methyl mercaptide can be rapidly destroyedin alkaline conditions by DO2 via eq 3 (Figure 5a), while theDMDS, regardless of its source (incipient via eq 4 or exog-enous), is also destroyed through FeCeOx-catalyzed hydrolysisto yield nonvolatile methane sulfinic acid (via a heterogeneousversion of the reaction, eq 4) or sulfonic acid (eq 9). Therefore,one can conclude that the FeCeOx/DO2 system, in addition toits potency vis-a`-vis hydrogen sulfide oxidation,4 is also ableto mitigate the odor disadvantages associated with two volatilemalodorous TRS compounds, i.e., methyl mercaptan (via eqs3, 6, and 7) and DMDS (via eqs 4 and 9).

Interference of Other TRS Components in the OxicMercaptide-FeCeOx Reaction.Bentvelzen et al.16 showed thatthe presence of bisulfide inhibits the reaction between MM andoxygen. The explanation put forward was that the longer-livedbisulfide radical (HS•) allows more encounters with oxygen thandoes the mercaptide radical (CH3S•) and hinders therefore MMoxidation.

Our results on the homogeneous (i.e., without FeCeOx)oxidation of mercaptide with DO2 confirm the inhibition ofmercaptide uptake in the presence of HS-. This is shown inFigure 8a which illustrates the time evolution of bisulfide (emptysquares) and mercaptide (filled squares) normalized concentra-tions in comparison to mercaptide oxidation in bisulfide-freesolutions (Figure 5a, filled triangles) under otherwise identicalconditions. When carried out separately, the reaction betweenbisulfide and oxygen is very slow compared with the mercaptideoxidation.4,16,17Mixing and oxidizing (along with O2) mercaptideand bisulfide together at pH) 12 exhibited the unexpectedthree-regime patterns in theC/C0 profiles shown in Figure 8a.The first regime, lasting ca. 30 min, is characterized by bothbisulfide and mercaptide being consumed. The second one,elapsing in ca. 30 min, suggests a surge in HS- and CH3S-

concentrations before a third regime where both theC/C0

profiles plateaued. Interestingly, the bisulfide final concentrationnearly gained back its initial value meaning that, after all, theHS- could be left unconverted.

Addition of a small quantity of FeCeOx (50 m2/L) to theformer CH3S-/HS-/O2 system attenuated the second regime inthe previous pattern for both S-bearing reactants (Figure 8a,filled triangles for mercaptide and empty triangles for bisulfide).This addition still did not achieve complete removal ofmercaptide unlike the tests without bisulfide (Figure 5a).Increasing FeCeOx areal concentrations from 50 to 120 m2/Lfurthered the bisulfide and mercaptide conversions (Figure 8a,empty and filled circles). As will be discussed later and unlikethe explanation of Bentvelzen et al.,16 the observed inhibitionis believed to be related to the formation of polysulfides fromHS- oxidation17 which affect methyl mercaptide oxidation viaa number of other reactions to be discussed below.

The time evolution of mercaptide concentration in presenceof DO2 was unaffected by the pH changes as shown in Figure8b. However, bisulfide consumption was improved by 25%when the pH dropped from 12 to 10.5. The better conversionobtained for bisulfide is thought to be caused, as discussedearlier, by the enhanced reactivity of the Fe(III) surface siteson FeCeOx with decreasing pH.

Let us now rationalize the observations from Figures 8a andb. For the case without FeCeOx, the oxidation of mercaptideyields DMDS via eq 3, while the oxidation of bisulfide yields

disulfide (the polysulfides precursor) and polysulfides:17

A rapid equilibrium is established among the distributionof the resulting polysulfides which exhibit different chainlengths:18,19

Figure 8. (a) Effect of FeCeOx on methyl mercaptide (filled symbols)and bisulfide (empty symbols) oxidation by DO2 (100 mmol/L borate, pH) 12, C0 (MM) ) 1 mmol/L, C0 (HS-) ) 0.5 mmol/L): (9) DO2 only,(2) 50 m2/L FeCeOx, and (b) 120 m2/L FeCeOx. (b) Effect of pH on methylmercaptide (filled symbols) and bisulfide (empty symbols) oxidation byFeCeOx and DO2 (100 mmol/L borate, 120 m2/L FeCeOx,C0 (MM) ) 1mmol/L, C0 (HS-) ) 0.5 mmol/L): (2) pH 10.5 and (b) pH 12. The linesshow trends.

2HS- + 1/2O2 f SS2- + 2H2O (11)

SnS2- + HS- + OH- a Sc-1S

2- + Sd-1S2- + H2O

n + 1 ) c + d andn ) 4-8 (12)

(n - 1)SnS2- + HS- + OH- a nSn-1S

2- + H2On ) 2-8 (13)

2[FeII(OH2)+] + 1/2O2 f 2[FeIII (OH)] + H2O (10)


Teder and Tormund20 found that MM in aqueous solutionsreacts with polysulfur compounds (polysulfides and polythion-ates) to form nonvolatile products. More precisely, mercaptidereacts with polysulfides whereas no reaction takes place betweenmethyl mercaptan and polysulfides.20 Nilvebrant et al.,21 using1H NMR, identified most of the products in the reactiveabsorption of methyl mercaptan within polysulfides solutionsat pH ) 11.5 and 60°C. Under these conditions, the mainidentified methylpolysulfides were DMDS, dimethyl trisulfide(CH3SSSCH3), and dimethyl tetrasulfide (CH3SSSSCH3).21 Thereaction products distribution of methylpolysulfides was shownto be strongly dependent on pH presumably because of thedependence of the polysulfides chain length on pH.18-21 Asimilar mechanism is assumed to hold in our study consideringthe compatibility in our pH ranges. Incipient DMDS (viareaction eq 3) is likely to react with incipient polysulfides (viaeqs 11-13) as follows:

Therefore, the buildup of polysulfides due to the oxidationof bisulfide with oxygen (eqs 11-13) can be held responsiblefor the reformation of mercaptide via eq 14.

Furthermore, Nilvebrant et al.21 explained that one of thecharacteristics of the sulfur-sulfur bond is its ability to undergoionic (heterocyclic) scission in reactions with agents. The surgeof HS- in the absence of FeCeOx from Figure 8a suggests thatthe nascent methylpolysulfides will undergo a nucleophilicattack by the hydroxide to yield bisulfide and methyl polysul-foxo ions:

Hence, the three-regime patterns observed in Figure 8a inthe absence of FeCeOx can be interpreted as resulting fromrelatively fast reactions: eq 14 for methyl mercaptide and eq15 for bisulfide that are able to counterbalance the oxidation ofmethyl mercaptide (eq 3) and bisulfide (eq 11) by DO2.

Addition of a relatively large quantity (120 m2/L) of FeCeOxto the reaction medium affects both mercaptide and bisulfidedynamic profiles. In fact, bisulfide is consumed by a furthersurface redox reaction involving the FeCeOx material toheterogeneously generate the disulfide:12

The heterogeneously mediated polysulfides route (eqs 16 and17) appears to be more rapid than the homogeneous route (eqs11-13). The disulfide, either formed from eqs 11 or 16, reactswith FeCeOx to yield higher polysulfides (eq 17), thiosulfate(eq 18),12 and plausibly elemental sulfur18 (eq 19) according tothe following set of reactions:

Thiosulfate was indeed detected in the present study as anoxidation product of bisulfide in the presence of FeCeOx (seeFigure 1). Since the reformation of bisulfide is no more observed

with FeCeOx present, it is suggested that reactions eqs 16-19,aided by DO2 sites regeneration (eq 10), are faster then reaction15. Furthermore, the loss of disulfide in the form of thiosulfate(eq 18) possibly quenches its propagation toward the morereactive polysulfides needed in eq 14. A consequence of this isthat less mercaptide is reformed by eq 14 in coherence withFigure 8a.

The curbing effect of bisulfide on mercaptide consumptionhas been speculated to be caused indirectly via endogenouspolysulfides. Let us turn our attention to the behavior of thereaction between mercaptide and FeCeOx/O2 when plenty ofexogenous polysulfides are available from start. Exogenouspolysulfides, consisting actually of a polysulfides-bisulfidemixture solution, have been synthesized using a protocolpresented elsewhere.22 The mole average chain length ofpolysulfides was computed to be 4.7 for pH) 10.5.23 Figure 9compares the oxidation of mercaptide with FeCeOx/O2 interfer-ing either with endogenous or exogenous polysulfides. A similar,though slightly faster and deeper, mercaptide conversion wasobserved with the exogenous polysulfides. Surprisingly, thebisulfide time profiles were very dissimilar for the two casesdespite very close final conversions (ca. 63%). Sulfur-richpolysulfides are known to break out into smaller polysulfidesunits via their reaction with mercaptide according to thefollowing:21

The (exogenous) polysulfides-mediated consumption of mer-captide (eq 20) could mean that less polysulfides are beingutilized by reaction eq 17 resulting in a lesser bisulfideconsumption which is coherent with the HS- profile in Figure9. When nearly no mercaptide was left, some 40 min later,reaction 20 loses impetus so that reaction 17 regains strengthand, while consuming polysulfides, also converts bisulfide asobserved experimentally (Figure 9).

Addition of 0.5-1 mmol/L of DMS brought no changes tothe mercaptide response (data not shown). Note that DMS wasnot monitored in our study. DMS is known to react24 very

CH3 - SS- CH3 + SnS2- f CH3Sn+1S

- + CH3S- (14)

CH3Sn+1S- + OH- f CH3SnSO- + HS- (15)

2[FeIII (OH)] + 2HS- a SS2- + 2[FeII(OH2)+] (16)

2[FeIII (OH)] + Sn-1S2- + HS- + H2O a

SnS2- + 2[FeII(OH)+] + OH- n ) 2-8 (17)

7[FeIII (OH)] + SS2- + H+ + 3H2O f

Fe2+ + S2O32- + OH- + 6[FeII(OH)+] + [*] (18)

S8S2- + H+ a S8 + HS- (19)

Figure 9. Effect of polysulfides on methyl mercaptide (filled symbols)and bisulfide (empty symbols) oxidation by FeCeOx and DO2 (100 mmol/Lborate, 120 m2/L FeCeOx,C0 (MM) ) 1 mmol/L, C0 (HS-) ) 0.5 mmol/L, pH ) 10.5): (9) bisulfide only and (2) polysulfides-bisulfide solution.The lines show trends.

CH3S- + SnS

2- f CH3Sn-xS- + SxS

2- (20)


rapidly with OH• radicals in aqueous phase via a complexreaction mechanism to form mainly nonvolatile dimethylsulfoxide (DMSO). DMS was also found to be oxidized toDMSO by O3 and H2O2 in distilled water or in seawater.24 Thesereactions are known to be relatively slow at pHg 7 in theabsence of a catalyst. Therefore, the indifference of themercaptide oxidation profiles to DMS could mean either thatDMS might be rapidly and completely converted into DMSOowing to potential effects of iron catalysis and OH• radicals orsimply that no interference of DMS occurs with the system.No firm proof can be advanced at this level of the study toexplain these results.

A key question that remains to be addressed concerns thebehavior of the in situ oxidative regeneration of iron by DO2

(eq 10) of the FeCeOx/O2 system and whether or not it isinfluenced by methyl mercaptide and its daughter oxidationproducts. This aspect is crucial for an industrial process relyingon FeCeOx for TRS abatement.4 In the case of bisulfideoxidation by the FeCeOx, it was demonstrated that the materialis able to keep its reactivity toward HS- during a certain numberof oxidation cycles via reoxidation of Fe(II) surface sites byDO2. This performance was achieved despite the fact that a smallfraction of reduced divalent iron was found to leach during HS-

oxidation from the material’s surface.4,12 In the case of MMoxidation by FeCeOx, Fe2+ dissolution was found to beinsignificant as observed above (Figure 4). Therefore, it isexpected that the oxidation of mercaptide by FeCeOx shouldaffect even less the material’s regenerability from the leachingstandpoint.

A two-cycle oxidation test was performed in order to analyzethe potential of in situ oxidative regenerability of Fe(II) by DO2

(Figure 10). In the first cycle, 0.5 mmol/L of methyl mercaptidewas oxidized at pH) 10.5 for 90 min by the FeCeOx/O2system. Both the DO2 and mercaptide concentrations weremonitored all throughout the 90 min. The second cycle wasresumed immediately afterward, after adding 1.0 mmol/L of HS-

to the “ripened” reaction mixture. The reaction continued foranother 90 min during which DO2, bisulfide, and mercaptideconcentrations were monitored (Figure 10). Note that noexogenous mercaptide was added to the system in the secondcycle.

Mercaptide was fully oxidized in 40 min in the first cycle(filled squares, Figure 10). The DO2 profile decreased duringthe first 30 min and then started to slowly increase, as a resultof starved mercaptide, via gas-liquid mass transfer replenish-ment. Addition of HS- after 90 min to the ripened reactionmixture was accompanied by a rapid consumption of bisulfide(Figure 10, filled triangles). The HS- concentration exhibitedca. 45% conversion after 30 min. Declining afterward at asensibly slower rate, bisulfide conversion reached ca. 60% in90 min, i.e., the second cycle duration.

For comparison, the time evolution of bisulfide oxidation withthe FeCeOx/O2 system in the absence of mercaptide is shownin Figure 10 as empty triangles. The reaction was conducted ina pristine solution under initial conditions regarding the FeCeOxareal concentration and DO2 nearly similar to those used in thefirst cycle. Over the first 30 min, the conversion of HS-

appeared to be quite comparable for the pristine mercaptide-free solution and the ripened reaction mixture in use in thesecond cycle. In comparison with the mercaptide-free solution,accumulation of mercaptide oxidation products in the lattersolution appeared to slow down the bisulfide conversion forthe next 60 min. In the authors’ opinion, the slowdown after30 min of bisulfide conversion cannot be ascribed to an activityloss of FeCeOx caused by the mercaptide oxidation in the firstcycle but instead is due to secondary reactions. For example,the polysulfides, formed either heterogeneously (eq 16) orhomogeneously (eq 11), preferentially react via eq 14 with theDMDS that built up after the first cycle rather than with FeCeOx(eqs 17 and 18). As a result, a drop in bisulfide consumptionshould be expected as depicted in Figure 10. The consumptionof DMDS via eq 14 could also explain the mercaptide peakaround 40 min in the second cycle. Note that the origin of thispeak is believed to be different from the one caused by theDMDS FeCeOx-catalyzed hydrolysis as seen in the first cyclearound 10 min (Figure 10). The above analysis indicates thatthe oxidation of methyl mercaptide by the FeCeOx/O2 systemdoes not affect the Fe(II) reoxidation capabilities of the materialby DO2. This feature is crucial for any redox process based onthe FeCeOx/O2 system for TRS abatement.

Conclusion

The oxidation of methyl mercaptide by FeCeOx was studiedin aqueous solutions at different pH values (10.5-12) in a stirredbatch reactor at 22°C in anoxic and oxic conditions. A capillaryelectrophoretic protocol was developed to separate and quantifymethyl mercaptide, bisulfide, and thiosulfate ions. The mainoxidation product of methyl mercaptide in anoxia was thenonionic dimethyl disulfide (DMDS) detected via gas chroma-tography in the reactor headspace. The pH appeared to be acritical parameter in methyl mercaptide conversion in anoxia.The highest and lowest conversions were encompassed at thepH values of 11 and 11.5, respectively.

In oxic conditions, the mercaptide oxidation by FeCeOx wassensibly improved attaining very high conversion levels up to100%. Dissolved oxygen (DO2) was found to react rapidly withCH3S- at pH ) 12. DO2 was the principal oxidizing agentresponsible for the observed high mercaptide conversions. Theinitial amount of oxygen (dissolved and gas-phase) was foundto be critical for obtaining high conversions of mercaptide.

Figure 10. Two-cycle reaction between FeCeOx, methyl mercaptide (firstcycle, C0 (MM) ) 0.5 mmol/L and bisulfide; second cycle,C0 (HS-) )1.0 mmol/L) in the presence of DO2 (100 mmol/L borate, 120 m2/L FeCeOx,pH ) 10.5): (9) mercaptide oxidation, (2) HS- oxidation, and (0) DO2

profile. The data points marked with the∆ symbol represent the oxidationof HS- by FeCeOx and DO2 under similar conditions but without MM.The lines show trends.


When CH3S- oxidation by the FeCeOx/O2 system was carriedout simultaneously with HS- oxidation, an inhibition in conver-sion was observed for both pollutants. This inhibition onmercaptide oxidation is thought to be related to the in situaccumulation of polysulfides via HS- oxidation. On thecontrary, if a large quantity of polysulfides is present initiallyin the reaction medium, the mercaptide conversion was foundto improve.

The addition of DMS initially to the reaction betweenFeCeOx/O2 and mercaptide had no effect on the mercaptideconversion. Finally, it was concluded that the oxidation ofmethyl mercaptide by FeCeOx did not affect the material’sreoxidation capabilities by DO2. This feature is crucial for anyredox process based on the FeCeOx/O2 system for TRSabatement.

Acknowledgment

Financial support from the Natural Sciences and EngineeringResearch Council of Canada Strategic Grant Program Environ-ment and Sustainable Development is gratefully acknowledged.The help of Laurent Gras, a visiting student from I.U.T. St.Jerome Marseille-France, in performing the experiments isacknowledged with gratitude.

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ReceiVed for reView December 10, 2006ReVised manuscript receiVed January 22, 2007

AcceptedJanuary 26, 2007

IE061586M


removal of methyl mercaptide by iron/cerium oxide−hydroxide in anoxic and oxic alkaline media

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