Environ. Sci. Techno/. 1995, 29, 1504-1510
Introduction
~-MROZ:Oxidation Product and Factors Affecting ORidatien Rate BRONWEN WANGt AND RICHARD G. BURAU* Department of Land, Air, and Water Resources, University of California, Davis, California 95616
Volatile dimethylsele nide ( DMSe) was transformed to a nonvolatile Se compound in a 6-Mn02 suspension. The nonvolatile product was a single compound identified as dimethylselenoxide based on its mass spectra pattern. After 24 h, 100% of the DMSe added to a 6-Mn02 suspension was converted to nonpurgable Se as opposed to 20%, 18%, and 4% conversion for chromate, permanganate, and the filtrate from the suspension, respectively. Manganese was found in solution after reaction. These results imply that the reaction between manganese oxide and DMSe was a heterogeneous redox reaction involving solid phase 6-Mn02 and solution phase DMSe. Oxidation of DMSe to dimethylselenoxide [OSe(CH&] by a 6-Mn02 suspension appears to be first order with respect to 6-Mn02, t o DMSe, and to hydrogen ion with an overall rate law of d[OSe(CH& l/dt= 95 M-2 min-' [Mn02]1[DMSe]1[H+]1 for the MnOz - 2.46 x concentration range of 0.89 x M, the DMSe concentration range of 3.9 x - 15.5 x M Se, and a hydrogen ion concentation range of 7.4 x - 9.5 x lo-* M. A general surface site adsorption model is consistent with this rate equation if the uncharged IOMnOH is the surface adsorption site. DMSe acts as a Lewis base, and the manganese oxide surface acts as a Lewis acid. DMSe adsorption to IOMnOH can be viewed as a Lewis acid/ base complex between the largely p orbitals of the DMSe lone pair and the unoccupied e, orbitals on manganese oxide. For such a complex, frontier molecular orbital theory predicts electron transfer to occur via an inner-sphere complex between the DMSe and the manganese oxide.
Selenium volatilization is an important component of the global selenium cycle, contributing about 30 x lo* g of Se yr-' to the atmosphere (I). Volatile selenium compounds form as a result of biological activity in seleniferous terrestrial and aquatic environments. Microorganisms, higher plants, and animals all produce volatile selenium compounds when exposed to inorganic and organic forms of selenium, though microorganisms are considered to be the largest contributors to the volatile selenium pool (24). Since selenium volatilization is primarily a microbially mediated process, factors that affect the microbial population can affect the selenium volatilization rate. Volatilization is affected by selenium species (41,soil texture (3,soil water content (6,2),soil organic matter and carbon source (5, 7),temperature (8, 7),and trace metals (7). The major component of the volatile selenium emitted from soils is dimethylselenide [(CH3)2Sel(9, 10) with dimethyldiselenide [DMDSe, (CH3)2Se21 as a minor component. DMSe and DMDSe have also been identified over aqueous systems (3). In contrast to the numerous studies of Se volatilization, little work has been done on the environmental fate of DMSe. Using mass balance, the atmospheric selenium residence time has been calculated to be about 7 days (1). Atkinson (11) found that DMSe reacted rapidly with atmospheric hydroxyl (OH) and nitrate (NO3) radicals as well as with ozone (03).The lifetime of DMSe due to reaction with these species was calculated to be as follows: OH radical, 2.7 h; NO3 radical, 5 min; and 03,5.8 h. The difference in these lifetimes and that calculated by Mackenzie (1)using mass balance was attributed to the adsorption of the resulting products onto submicrometer particles in the atmosphere (11). Sorption by soil of DMSe from an airstream was found by Zieve and Peterson (12) for both nonsterilized and sterilized soils. Both soils sorbed volatile selenium to the same extent, and they concluded that the sorption was a physical or chemical process and not microbial uptake. Microbial activity may be important in the anoxic selenium cycle however. Oremland and Zehr (13) found that dimethylselenide was rapidly metabolized to both CH4and COz within anaerobic sediments and suggested that the rapid demethylation of DMSe by methanogenic and sulfaterespiring bacteria may be responsible for the lack of DMSe in anoxic waters. DMSe sorption by individual soil constituents followed the order organic matter > clay minerals > manganese oxides > iron oxides > acid-washed sand (12). Little of the sorbed selenium was extractable with 0.4 N K2S04, indicating that the sorbed selenium was not readily exchangeable (12, 14).
DMSe contains selenium in its reduced oxidation state of -11. In general, selenium in the -11 oxidation state is unstable within the oxidative environments where DMSe is found. Consequently, DMSe produced within the soil * E-mail address:
[email protected]. t Present address: U.S. Geological Survey,Water Resource Division, 2800 Cottage Way, Room 2233, Sacramento, CA,95825.
1904
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6, 1995
0013-936X/95/0929-1504$09.00/0
0 1995 American Chemical Society
may be susceptible to oxidation by soil constituents. Oxidation of dimethyselenide could produce an inorganic or organic selenium-containingoxidation product. Oxidation of dimethylselenide to inorganic elemental selenium, selenite, or selenate would require cleavage of the Se-C bond, while oxidation to oxidized organic selenium compounds such as dimethylselenone [(CH&Se021 or dimethylselenoxide [ (CH3)zSeOlwould not. Dimethylselenone has been found in the aquatic system (3, 10); dimethylselenoxide has not been reported, but dimethylsulfoxide has been found in oceanic environments (15). Given the similarities between selenium and sulfur, oxidation to dimethylselenoxide seems plausible. One class of potential oxidizing agents found in many soils is manganese oxide. Manganese oxides participate in a number of oxidation reactions involving both organic and inorganic compounds, and redox reactions occurring at the manganese oxide-solution interface have received considerable attention for a number of years (16-21). The objectives of this studywere (1)to establish whether DMSe can be abiotically oxidized by manganese oxide, (2) to measure the effect of experimental parameters on the oxidation rate, and (3) to identify the reaction products.
Materials and Methods Manganese Dioxide Preparation. Manganese dioxide was prepared by reacting excess potassium permanganate with hydrochloric acid (22). KMn04 (63 g) was dissolved in 1 L of distilled deionized water and heated to 90 "C. A 100-mL aliquot was transferred to a 3-L beaker, stirredwith a Tefloncoated magnetic stirring bar, and heated to 90 "C. The remaining permanganate solution and 66 mL of concentrated HC1 were alternately added with vigorous stirring to the 3-L beaker over a 1-min interval. Heating to 90 "C and stirringwere continued for 10min, at which time the heating was discontinued. The reaction was allowed to proceed for 30 min, after which time the suspension was vacuumfiltered on a fritted glass filter and washed with approximately 6 L of distilled deionized water. The oxide retained by the filter was resuspended in 4 L of water and allowed to sit overnight. The suspension was filtered and resuspended in 4 L of 0.5 M HC104. Alternate washing with distilled deionized water and perchloric acid was continued until the residual K content was 0.5%by weight. The resulting oxide was then dried and passed through an 80 mesh stainless steel sieve. This batch of manganese oxide was used throughout the study. Manganese Oxide Characterization. The oxide was characterized by X-ray diffraction (23). The surface area of the oxide was determined using the ethylene glycol monoethyl ether (EGME) vapor adsorption technique (24); stoichiometry was measured by iodometric titration (25); and the zero point of charge was measured by potentiometric titration (26). OxidationStudies. In all experiments,DMSe was added to the suspension as an aqueous solution. The DMSe aqueous solution was prepared by adding 100 pL of pure DMSe (Strem Chemicals) to 50 mL of distilled deionized water. A total of 2.5 mL of this solution was further diluted to 25 mL with distilled deionized water. A 100-pL aliquot of this dilute DMSe solution was added to the samples using a 1-mL Hamilton gas-tight syringe, giving a solution concentration of approximately 1 x M Se. The exact selenium concentration of the added DMSe solution was determined by oxidation with 2% ammonium persulfate
followed by reduction with 6 M HC1 for 30 min at 100 "C. The solution was then analyzed for selenium by hydride generation atomic absorption spectrophotometry (27).The DMSe solution was freshly prepared for each experiment, and each batch was analyzed for Se concentration. Batch studies of DMSe oxidation were conducted to compare the effect of different oxidizing agents. A MnOz suspension, a 1 x M KMn04solution, the filtrate from a Mn02 suspension, and a 1 x M KZCr207 solution were compared. The effects of manganese oxide concentration, pH, and DMSe concentration on the oxidation rate were determined in Mn02 suspensions containing a background electrolyte solution of 1 x M Na2S04. The pH was adjusted by adding 0.1 M NaOH to the suspension. The experimental setup was that of Zasoski and Burau (28) without the NZ purge. A 3-mL sample was withdrawn at timed intervals and filtered through a 0.45-pm syringe-end filter. The filtrate was sparged with Nz for 20 min to remove any unreacted DMSe and then digested using the persulfate method. DMSe oxidation was followed by monitoring the increase in the nonpurgable selenium with time. Nonpurgable selenium was chosen as the reaction parameter to follow because DMSe is easily removed by purging the solution with N2 (29). The reaction rate was studied to determine the reaction dependence on the molar concentrations of MnOZ,initial DMSe, and hydrogen ion. Activity corrections were ignored since the ionic streangth was low (approximately 2 mM). Rate dependence was evaluated using the initial rate method (30). A second-order polynomial was used to extrapolate experimental nonpurgable selenium concentration vs time data to t = 0. The polynomial was differentiated and evaluated at t = 0 to obtain the initial rate where the concentration of reactants was known. The reaction order was then determined from plots of log initial rate vs log initial concentration. Once the reaction order with respect to the various components was determined, the rate constant k was evaluated using the experimental conditions and solving the rate expression for k. Mass Spectra Mass spectral analysis required a higher concentration of reaction product. Therefore, 250 mg of MnOzwas hydrated in 50 mL of M NaZSO4,and DMSe was added to give a Se concentration of about 1 x M Se. After 24 h, the suspension was filtered and purged of any remaining volatile selenium. The mass spectra was acquired using direct insertion, and the sample was ionized with 70 eV. CryogenicTrapping. The cryogenic setup was modified by Gao (31) from the techniques of Cutter (32) and Masscheleyn (33). The sample was introduced into a bubbling tower containing 55 mL of 6 M HC1. Using a peristaltic pump, 4 mL of a 4% NaBH4-0.08 M NaOH solution was added to an aqueous sample over a 2-min period. The solution in the bubbling tower reaction vessel was continuously purged with NZat a rate of 70 mL min-'. Purging of the volatile Se species produced was continued for 6 min after the termination of the NaBH4 addition. The liquid nitrogen was then removed from around the collection tube, and the tube was heated bypassinglow voltage AC power through the Ni-Cr wire. Selenium in the gas stream passing through a heated quartz tube was measured by atomic absorption spectrophotometry at 196 nm. Identification of Reaction Products. The nonvolatile selenium species in the aqueous phase of the experiments VOL. 29, NO. 6,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1505
were identified by mass spectrometry and by the cryogenic trapping technique.
Results Manganese Oxide Characterization. X-ray diffraction analysis of the manganese oxide produced a spectrum with three broad peaks. The strongest peak was at a d-spacing of 0.708 nm, a broad weak peak was found at 0.353 nm followed by a weak peak at 0.243 nm. These d-spacings are consistent with those reported for 6-Mn02 (synthetic birnessite) (34, 35). Surface area measurements by the ethylene glycol monoethyl ether method (EGME) gave a value of 248 f 2 m2 g-l. The surface area compares reasonably well with the 223 m2 g-' reported for 6-Mn02 by Fendorf (35) but is considerably smaller than the 500 and 380 m2 g-' reported by Zasoski (34)and Loganathan (361, respectively. The average molar ratio of OlMn determined by iodometry was 1.91f 0.08. The zero point of charge was found to occur at pH 2.4; therefore, the net surface charge of the oxide was negative over the range of pH values used in this study. All these observations are consistent with the oxide being 6-MnC2 (21). DMSe Oxidation. Reaction of DMSe with 1.2 x M manganese oxide produced significantly more nonpurgable selenium in 24 h than 1 x M permanganate, the filtrate K2Cr207. of the manganese oxide suspension, or 1 x After 24 h, 100% of the selenium added as DMSe was converted to nonpurgable Se in the manganese oxide system compared to 20%, 18%, and 4% from chromate, permanganate, and the filtrate, respectively. Permanganate was tested to determine if residual permanganate from the manganese oxide synthesis could be responsible for the reaction. Permanganate produced significantly less nonpurgable selenium than the 6-Mn02 suspension, even though the permanganate concentration used was approximately 10 times that which would be present if all the K+ in the 6-Mn02were present as KMn04. In addition, the 6-MnO2filtrate produced less nonpurgable selenium than permanganate. These observations strongly suggest that the reaction was not due to residual soluble permanganate in the 6-Mn02 suspension. Selenium analysis of filtered and unfiltered samples showed that neither the reaction product nor DMSe adsorbed to the MnO2. Manganese oxide is reduced in hot 6 M HCl (37) in the reaction MnO,
+ 4HC1-
MnC1,
+ C1, + 2H20
(1)
Consequently, the manganese oxide in the unfiltered suspension will dissolve when the solution is heated in the reduction step of the persulfate/HCl method. If irreversible adsorption of the reaction product or DMSe had occurred, the concentration of the selenium would be greater in the unfiltered sample than in the filtered sample. The selenium concentrations were not significantly different, indicating that irreversible adsorption of the Se compounds did not occur (Table 1). Therefore, solution non-purgabale selenium could be used to follow the oxidation. Redox reactions involving a solid and a liquid phase often show an increase in reaction rate with increasing solid concentration. This increase is generally attributed to an increase in concentration of reactive surface sites. Increasing the dioxide concentration in the suspension from 0.89 x to 2.46 x M increased the proportion of nonpurgable selenium recovered in 10 min from 15% to 1506 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6,1995
-.
5
0
20
15
10 time (min)
FIGURE 1. Nonpurgable Se concentration with time resulting from the oxidation of DMSe by various concentrations of 6-Mn02.[DMSe] = 6.8 x M, [H+] = IOT7, T = 21.5 "C. TABLE 1
Effect of Filtering Reacted Suspensions" nonpurgable Se (x10-7 M) filtered unfiltered
6.8f 0.4 7.1 f 0.1
M; [DMSe] = 6.5 x IO-' M. n = 4; a pH = 5; [ M n 0 2 1 = 12 x ANOVA comparison between means ( F = 0.65, p = 0.466).
TABLE 2
Effect of Initial 6-MnOz Concentration on DMSe Oxidation Initial Ratesa [MnOzl initial rate ( x 10-3 M) ( x lo-' M Se rnin-')
2.46 1.78 1.06 0.89 a
0.21 0.13 0.09 0.07
[DMSel = 6.8
x
log MnOz
log rate
-2.61 -2.75 -2.97 -3.05
-0.68 -0.89 -1.05 -1.15
order
1.050.1
IO-' M, [H+l = lo-', T = 21.5 "C.
TABLE 3
Effect of Initial DMSe Concentration on DMSe Oxidation Initial Ratesa [DMSe] ( x lo-' M)
(x
15.57 1 1 -65 7.22 3.92 a
[6-Mn021=
initial rate M min-')
log Se
log rate
0.32 0.23 0.15 0.06
0.19 0.07 -0.11 -0.41
-7.49 -7.65 -7.82 -8.24
1.76 x
order
1.2iO.1
M, [H+l = IO-', T = 25 "C.
46% (Figure 1) and increased the initial rate from 0.07 to 0.21 M Se min-l (Table 2). The log initial rate vs log MnOz concentration was a linear relationship with a slope of 1.O, a standard error of 0.1, and an 6 of 0.97 (Table 2). Increasing the initial concentration of DMSe from 3.92 x lon7to 15.6 x M Se increased the reaction rate from 0.06 to 0.32 M Se min-' (Figure 2 and Table 3). A plot of the log initial rate vs the log DMSe concentration was linear with a slope of 1.24 a standard error of 0.07 (Table 3). The 95% confidence interval was calculated from the standard error (38)of the slope resulting in a range of 1.0- 1.4 for the slope. Because the 95% confidence interval encompassed 1,the reaction order was not considered to be significantly
- -1 I
,.--
0 = 3.92 x lo-’ M DMSe A = 7.72x 10-7
0 = normalized from pH data
- 1.90 .. -2.40
0 = normalized from DMSe data A = normalized from oxide data
.-
-2.90--
I.7”
-00
5
10
15
20
30
25
35
I
-7.10
-6.60
log
time (min)
FIGURE 2. Nonpurgable Se concentration with time resulting from the oxidation of DMSe by 6-MnO2 at various initial DMSe concentrations. [S-MnOJ = 1.8 x M, [H+] = 1 x lo-’, T = 21.5 “C.
A =pH5.48 A =pH6.00 0 =pH6.60 v
fi
a 3
5n I
0
time (min)
FIGURE 3. Nonpurgable Se concentration with time resulting from the oxidation of DMSe by 6-MnO2 at various hydrogen ion concentrations. [DMSel = 6.3 x lo-’ M,[ 6-MnOd = 0.094 x M, T=21.5 “C. TABLE 4
Effect of Initial Hydrogen Ion Concentration on Initial DMSe Oxidation Ratesa [H+l (mol 1-l) 10-5.13 10-5.48 10-6.00 10-6.60 a
(xIO-’
-6.10
rate M min-l)
log [H+]
log rate
2.81 1.51 0.41 0.14
-5.13 -5.46 -5.98 -6.58
-6.55 -6.82 -7.39 -7.86
[DMSel = 6.33 x IO-’ M, [6-Mn021= 0.092 x
order 0.92 f 0.05
M.
different than unity, and the reaction with respect to DMSe was considered to be first order. Hydrogen ion concentration also affected the oxidation of DMSe by the Mn02 suspension, with DMSe oxidation rate increasing as hydrogen ion concentration increased (29). Rapid oxidation of DMSe at the higher hydrogen ion concentrations necessitated a decrease in oxide concentration from that previously used. The oxide concentration in these experiments at lower pH was reduced to 0.092 x M, and the DMSe concentration was 6.3 x M Se. Complete recovery of the added selenium as nonpurgable selenium occurred within 5 min at [H+]= 7.4 x M, as opposed to 40% recovery after 30 min at [H+]= 2.5 x M (Figure 3). This corresponded to a rate increase from 0.01 M Se min-’ at [H+]= 2.5 x to 2.8 M Se min-l at [H+]= 7.4 x M (Table 4). Plotting the log rate vs log
-5.60
-5.10
Wfl
FIGUREQ. log initial rate vs log initial [H+I including data normalized from 6-Mn02 and DMSe initial rate data.
hydrogen ion concentration yielded a straight line with a slope of 0.92 and a standard error of 0.05 (Table 4). Again, as with the oxide concentration and initial Se concentration, the 95% confidence interval encompassed unity (0.771-07),and the reaction order of the oxidation of DMSe by Mn02 with respect to hydrogen ion is considered to be first-order. Because the oxide concentration used in the hydrogen ion experiment was lower than that used in the previous experiments, the rate data were corrected for MnOz concentration and DMSe concentration to determine if the first-order hydrogen dependence was applicable to the manganese oxide and DMSe concentrations previously used. All rate data were normalized with respect to Mn02 concentration and DMSe by
rate(exp)
[MnO,ll [DMSel
= rate(corr) = k[H]”
(2)
If the hydrogen ion dependence changed over either the oxide concentration or the DMSe concentration over the ranges used previously, the log rate(corr) for different oxide surfaces or DMSe concentrations vs log hydrogen ion would diverge from the trend established using the hydrogen ion data at lower oxide concentration. The log rate(corr) for the different experimental conditions are plotted together with hydrogen ion data obtained with a lower oxide concentration (Figure 4). The plotted line represents the least squares regression line derived by the hydrogen ion data with the lower oxide concentration. If all the data are used, the regression is y = 0 . 9 0 ~ 3.06 ($ =0.996). This slope is not different from that previously determined (0.92). Therefore, it appears that the reaction is first order with respect to hydrogen ion over an oxide concentration range from Mn02 concentration 0.89 x to 2.46 x loW3M, a DMSe concentration from 3.9 x to 15.5 x lo-’ M Se, and a hydrogen ion concentration to 9.5 x lo-* M. range from 7.4 x Rate Constant Determination. The rate expression for the oxidation of DMSe by 6-Mn02 can be summarized as
+
d[OSe(CH,),Ildt = k[MnO~l’[DMSel’[H+l’
(3)
The rate constant (k)was calculated using eq 3, the initial rate and the initial Mn02, DMSe, and hydrogen ion molar concentrations (Table 5). The average kis 95 f2 M-2 min-l, and the complete rate expression for the oxidation of DMSe VOL. 29, NO. 6,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1
1507
TABLE 5
Rate Constant Determination for DMSe Oxidation by 6=Mn02 initial rate initial [DMSe] initial S-Mn02 concn k (xlO-’ M min-l) (xlO-’M) [H+] (MI ( x ~ O - M) ~ (W2min-’) 10-5.13 0.93 2.81 6.33 64 10-5.48 0.93 1.51 6.33 76 10-6.00 0.93 78 0.41 6.33 10-6.60 0.93 95 0.01 6.33 10-6.92 1.78 96 0.32 15.57 10-6.98 1.78 100 0.23 11.65 10-6.32 1.78 7.72 91 0.15 10-7.02 1.78 85 0.06 3.92 10-7.00 2.46 0.21 130 6.83 10-7.01 1.78 110 0.13 6.83 10-7.00 1.31 0.09 6.83 100 mean= 95i2
. a) Y
i -
E
p4
1
10
20
L 1.0 2.0
min
Il 10
10
20
20
FIGURE 5. Cryogenic trapping: (a) separation of eH2 produced from 0.5 ng of Se as selenite and 0.04 ng of Se as DMSe, (b) 1 h reaction solution before NaBH, addition, (c) 1 h reaction solution after NaBH, addition, (d) 48 h reaction solution after N a B Y addition.
by 6-Mn02 is given by d[OSe(CH,),]/dt = 95 M-’ min-’[MnO,ll [DMSeI’[H’I
’ (4)
Product Identification. Potential oxidation products for DMSe oxidation by manganese oxide include elemental selenium, selenite, selenate, an oxidized organic selenium compound, or a mixture of compounds. A combination of hydride generationlcryogenic trapping and ion chromatography was used to determine whether an inorganic or an organic selenium compound was the reaction product. Hydride generationlcryogenic trapping was used successfullyto differentiate between selenite and organic selenides in natural waters (32,33). The method separates the species based on the boiling points of the hydrides formed on reduction and hydrogenation by NaBH4. Dihydrogen selenide formed from selenite and DMSe formed from organic selenides have different boiling points and can be separated from one another on this basis (32). Ion chromatographywas used to detect selenate since selenate concentrations of 5 pM could be easily detected by ion chromatography (29). Hydride generation/ cryogenic trapping achieved good peak separation between the two hydrides with elution times of 1.1 and 1.9 min for SeHz (dihydrogen selenide) and Se(CH3I2(DMSe),respectively (Figure 5a). The non1508
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6,1995
purgable reaction products for both the 1 and 48 h DMSe/ MnOz reaction periods produced only one hydride with the DMSe retention time of 1.9 min (Figure 5c,d). No hydride was detected prior to the addition of NaBH4(Figure 5b), so the detected DMSe must have resulted from reduction of the product by NaBH4 and was not residual unreacted DMSe. The longer reaction time of 48 h did not show any alteration in the product (Figure5d). In addition, selenate was not detected by ion chromatography. Detection of only DMSe after reaction ofthe nonpurgable Se product with NaBH4indicated that an organic selenium compound was present. Inorganic selenium oxides were ruled out as oxidation products since (1) no SeH2 was detected, indicating selenite was not present, and (2) selenate was not detected by ion chromatography. The possibility of elemental selenium being formed cannot be entirely ruled out since neither hydride generation/ cryogenic trapping nor ion chromatography will detect elemental selenium. However, there was no visual evidence for elemental selenium. The organic selenium compounds most likely to have formed are dimethylselenoxide [(CH3)2SeOlor dimethylselenone [CH&SeOz], representing a two- and a fourelectron oxidation, respectively. Unlike dimethylselenone, dimethylselenoxide has not been found in natural waters (10,3). Both however have been produced from DMSe by reaction with ozone (39, 40). Dimethylselenone and dimethylselenoxide can be distinguished via mass spectrometry. Both give strong molecular ion peaks corresponding to 126 atomic mass units (amu)for the selenoxide and 142 amu for the selenone (39). Mass spectrographs of the products from the reaction of DMSe with 6-Mn02 were acquired for the reaction at pH 5 and pH 7. The products of both the pH 5 and the pH 7 reactions produced similar spectra. The spectra for pH 7 is given in Figure 6. The molecular ion peak occurred at mass 126 with the characteristic six-peak selenium cluster occurring at 128,126,124,123,and 122amu corresponding to the natural abundances of the selenium isotopes of 82, 80, 78, 77, and 75 amu. The mass spectrum indicates the sample contains dimethylselenoxide. A peakwas not found at 142 amu, indicating that dimethylselenone was not at detectable levels in the samples (Figure 6). To determine if manganese was reduced during the reaction, reaction solutions were analyzed for manganese by ICP-MS for 55Mn. Manganese concentrations were higher for the reaction at pH 5 (1.7 x 10-6-2.0 x M Mn) than at pH 7 (0.2 x 10-6-0.3 x M Mn). The differencebetween the soluble manganese produced at the two pH values is most likely due to adsorption of manganese(I1) at higher pH. Manganese(W forms an insoluble oxide; manganese(I1) however is soluble, and the appearance of soluble manganese is an indication that some of the manganese oxide was reduced to Mn(I1).
Discussion DMSe was rapidly oxidized by a 6-Mn02 suspension to dimethylselenoxide (DMSeO). No other organic or inorganic Se-containingspecies was found in the aqueous phase of the suspension. The oxidation of DMSe to DMSeO is a two-electron oxidation of the Se(-11) in DMSe to Se(0) in DMSeO. A reasonable redox reaction between DMSe and 6-Mn02 would be
concentration of the surface complex such that
111
d(OSe(CH,),) l d t = k,[10Mn0,,a,,*Se(CH3~zl
90.
(9)
and
80.
70.
93
60.
I
d(~OMnOsurfac,*Se(CH,),)/dt = kl[lOMnO,~acel [Se(CH,),I {k-, + ~,~[IOMnO,urfac,*Se(CH,),l(10) Because the rate-limiting step has not been determined, the entire rate equation must be retained. If, however, the adsorption complex is assumed to be in steady state [i.e., d(~OMnOsu~ace*Se(CH~)~)Idt = 01 then =
~IOMnO,,,,,*Se(CH,),I
k,~10Mn0,~,,,1~Se~CH3~,1~~k-, + k,I (11) and d(OSe(CH,),)/dt = k~~~[lOMnOsurfacel [Se(CH,),ll{k-,
40
60
100
80
120
140
(OMnO-
FIGURE 6. Mass spectra of reaction product. Reaction time 24 h, M,57 x M MnOz. pH = 7, initial DMSe concentration 1 x
+ DMSe + 2H'
Mn2+
+ DMSeO + H,O
(5)
Oxidation of DMSe by the manganese oxide suspension could occur by either a homogeneous reaction involving soluble manganese in equilibriumwith the solid manganese oxide and solution DMSe or by a heterogeneous reaction involving the solid manganese oxide and solution phase DMSe. Reaction between the manganese oxide filtrate and DMSe was negligible, implying that a soluble species was not responsible for the rapid conversion of the DMSe to DMSeO. Therefore, the reaction between the manganese oxide and DMSe appears to be a heterogeneous redox reaction involving solid phase 6-Mn02and solution phase DMSe. Heterogenous reactions can be broadly described as diffusion to the oxide interface to form an adsorption precursor complex between the reactants, followed by electron transfer and product release (41). Applying this general surface reaction model to the DMSe-Mn02system yields 10MnOsurface + Se(CH,), IOMn0,,a,,*Se(CH3), lOMn
.-.
(12)
The pH-dependent charge exhibited by manganese oxide must be accounted for when applying a surface site binding model. Protonation of the oxide surface results in surface site changes (reactions 13 and 14):
amu
d-MnO,(s)
+ k21
(OMnO-
-
+ H+
+ 2H+
lOMnOH
(13)
&l
IOMn(OH,)+
Kb2
(14)
If protonation of the oxide surface is fast relative to DMSe adsorption and electron transfer, these reactions can be viewed as preequilibrium steps with equilibrium constants of Kbl and Kbz, respectively (16'). The total manganese surface must equal the sum of the species IOMnO-, IOMnOH, and IOMn(OHz)+(eq 15): [IOMnO,urfac,l= [IOMnO-I
+ [IOMnOHl + [I OMn(OH,)+l (15)
or [IOMnOsulfacel = [IOMnO-I
+ Kbl[H+][IOMnO-I + &[H+I2[loMno-] (16)
Substituting eq 16 into eq 12 gives the rate of formation of DMSeO as
ki
IOMnO,,,,,*Se(CH,),
k- 1
- lOMn + OSe(CH,), k2
5 + 2H+ Mn2+ + H,O
(6) (7)
(8)
where 10MnOsurfaceis the overall MnOz surface and IOMn0,hc,*Se(CH3)2 is asurface complexbetweend-MnO2 and DMSe. To preserve mass balance, the dimethylselenoxide formation rate must be directly proportional to the
The surface site binding model would predict a first-order dependence in DMSe and manganese oxide and a hydrogen ion dependence of 0,1, or 2 depending on whether IOMnO-, IOMnOH ,or IOMn(OH2)+is the favored solid phase reaction site. Experimentally, the rate law was determined to be first order in DMSe, manganese oxide, and hydrogen ion. The surface site binding model agrees with the experimentally determined rate law if IOMnOH was the primary reactive site. Implict in the above discusion is the following (1) the surface sites are not limiting and (2) the surface complex VOL. 29, NO. 6, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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reaches a steadystate. These assumptions appear resonable over the range 0.89 x 10-3-2.46 x M Mn02, DMSe concentration range of 3.9 x 10-'-15.5 x lo-' M Se, and a hydrogen ion concentation range of 7.4 x 10-6-9.5 x lo-* M, because this generalized surface reaction model is consistent with the observed rate expression. Complex Formation. DMSe behaves as a Lewis base (391, and the manganese oxide surface behavior as a Lewis acid (17). If the DMSe-6-Mn02 adsorption is viewed as a Lewis acidlbase complex,where DMSe donates one of its two lone electron pairs to the manganese oxide unoccupied d orbital, electron transfer could occur between the nonbonding lone electron pair on DMSe and the unoccupied manganese orbitals. Electron transfer between these two orbitals is consistent with the frontier molecular orbital theory for redox processes. Luther (42) used frontier molecular orbital theory to explain the rapid oxidation of hydrogen sulfide by manganese oxide versus the slower oxidation involving iron oxide or molecular oxygen. Frontier molecular orbital theory requires electron transfer to occur between the highest occupied molecular orbital of the reductant and the lowest unoccupied orbital of the oxidant (42). The type of bonding between the reductant and the oxidantis dependent on the hybridization of these orbitals. The nonbonding electron pairs on DMSe (the reductant) represent its highest occupied orbital. The highest occupied orbital hybridization for DMSe was calculated from a C-Se-C bond angle of 98" (39) to be 62% p-character and 38% s-character (43). The manganese center of manganese oxide is in octahedral coordination and is a low spin d3 coordination complex. The crystal field splits the d orbitals into three degenerate tZg orbitals containing one electron each and two empty degenerate eg orbitals (37). Manganese oxide acts as a Lewis acid by accepting two electrons into the unoccupied egorbitals, its lowest unoccupied orbital. Orbital overlap between the largely p nonbonding and the egorbitals results in a o-aelectron transfer and requires an inner-sphere mechanism (42). Therefore, frontier molecular orbital theory predicts an inner-sphere complex between the DMSe and the manganese oxide from which electron transfer occurs. The rate of oxidation of DMSe by manganese oxide observed in this study raises the possibility that volatile selenium can be rapidly converted to a nonvolatile compound in soil systems. Dimethylselenoxide has not been identified in soils or in water systems although dimethylselenone has. One possible explanation is adsorption of dimethylselenoxide to soil cation exchange sites. Selenoxides exhibit acid/ base characteristics and form positively charged acid ions under acidic conditions (39). Complete adsorption of dimethylselenoxide by a cation exchange column was noted at a pH of 4 (29), with recovery of Se only upon passing a basic solution through the column. Consequently, a positively charged cation of dimethylselenoxide may adsorb to the cation exchange complex in the soil under acidic conditions. The strong adsorption of the dimethylselenoxide to the cation exchange resin is consistent with the findings of Zieve and Peterson (12, 14) of the strong adsorption of DMSe from air passing through soils.
Received f o r review July 22, 1994. Revised manuscript re-
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Abstract published in Advance ACS Abstracts, April 15, 1995.
