Environ. Sci. Technoi. 1988, 22, 463-467
Discrimination between Adsorption and Coprecipitation in Aquatic Particle Standards by Surf ace Analysis Techniques: Lead Distributions in Calcium Carbonates Julla E. Fulghum, Scott R. Bryant and Richard W. Linton" Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
Chrlstopher F. Bauer Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824
Dieter P. Grlffls Engineering Research Services Division, North Carolina State University, Box 7903,Raleigh, North Carolina 27695
rn The evaluation of new analytical approaches for aquatic sediment characterization requires standard particles with controlled spatial distributions of trace elements. For example, because of the influence on environmental availability, it is important to distinguish particles with trace elements uniformly distributed in the particle bulk from those with trace elements adsorbed to the surface. To this end, standard calcium carbonate (calcite) particles were generated to contain either coprecipitated or adsorbed lead. Conventional analytical techniques (atomic spectroscopy, X-ray diffraction, X-ray microanalysis, electron microscopy) fail to distinguish the two particle types. However, distinguishing between particle surface and interior compositions is possible with surface analytical techniques, notably secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). This permits the discrimination of surface adsorbed from coprecipitated lead in particles with similar average (bulk) concentrations. Trace metal distribution between the solution and sediment phases affects transport, bioavailability, and toxicity in natural water systems. Both adsorption and coprecipitation remove trace metals from solution, but changes in the chemical environment may reverse either process. The solution concentrations of toxic metals, such as Pb and Cd, and essential nutrients, like Zn and Fe, are influenced by these processes. Distinguishing adsorption from coprecipitation is important in evaluating the bioavailability of metals in sediments and suspended particulate matter. Adsorption is a two-dimensional metal binding on the particle surface, while coprecipitation is a three-dimensional process incorporating the metal into the bulk-particle structure. Dissolution and desorption kinetics differ; coprecipitated metals may be less readily released by changes in pH or ligand concentration. It is difficult, however, to distinguish between these two processes in the laboratory. A common type of sediment analysis sequentially extracts trace elements, categorizing them by the extraction conditions. For example, the method suggested by Tessier separates metals into fractions that are bound to carbonate, bound to metal oxides, and bound to organics (1). Although this may be empirically useful, a correlation between these operational classes and actual chemical speciation has not been demonstrated (2). A less empirical approach to this problem requires the synthesis of standard particles of known trace element Present address: Standard Oil R&D Laboratory, 4440 Warrensville Center Rd., Cleveland, OH 44128. 0013-936X/88/0922-0463$01.50/0
distribution and speciation. C.F.B. is developing procedures for making particles containing both adsorbed and coprecipitated trace elements at varying concentrations. These samples will be used as standards for the evaluation of various extraction techniques. However, before model particulates can be used to evaluate extraction methods, analytical methodologies must be developed to determine trace metal distributions within the standard particles. Many techniques of use for particulate characterization such as scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX), cannot distinguish very well between bulk and surface concentrations. Surface analytical spectroscopies can probe metal distributions within particulate samples, although these techniques have been applied only infrequently to environmental samples. Secondary ion mass spectrometry (SIMS) supplies data related to the three-dimensional spatial distribution of elements, including trace constituents ( 3 , 4 ) . Energetic ion bombardment is used to sputter and ionize the sample. Resultant secondary ion intensity profiles as a function of time reflect concentration variations of elements with depth. X-ray photoelectron spectroscopy (XPS) provides concentrations and chemical speciation information on major and minor surface elements ( 5 , 6 ) . R.W.L. has used XPS and SIMS extensively to analyze both airborne and solution particulates (3-6). General instrumental details are available elsewhere (7, 8). The unique focus of this study is the development of SIMS and XPS techniques to discriminate between the adsorption and coprecipitation of lead associated with model calcium carbonate sediment particles. Experimental Section
Sample Preparation. (A) Adsorbed Pb. Reagentgrade CaC03 (2 g) was mechanically sieved to obtain a particle fraction passing through a 45-pm mesh. This sample was suspended with continuous stirring in either 200 mL of 5 pM Pb(N03)2or 50 pM Pb(N03), for 20 min. Pb(N03)2solutions were not at saturation with respect to PbC0, before CaCO, addition. However, CaC03 dissolution could result in PbC03 formation. (B) Coprecipitated Pb. A stoichiometric amount of 0.1 M Ca(N03)2with Pb(N03)2at either 5 or 50 yM was added dropwise into 0.1 M Na2C03. The system was open to the atmosphere and thermostated in a water bath at 37 "C. No pH control was used. The solution was always supersaturated with respect to PbC03 during precipitate formation. The precipitate was stirred constantly. After the addition was complete (about 2.5 h), the suspension was allowed to age with constant stirring at 37 OC for 24 h. In one case, the suspension was split into two batches, one of which was aged for only 15 min.
0 1988 American Chemical Society
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Flgue 1. semndary elernon micrographs of calcium carbonate coprecipilaled with Pb. Magnification 01 9OOX. (A) Calcite coprectpbte c o n t a l n ~ 134 ppm Pb (sample 1: Table 11). (6) Vaterne coprecipitate containing 124 ppm Pb (sample 5 ; Table 11).
Table 1. SIMS Instrumental Parameters-Primary Ions composition current, nA impact energy on sample, keV sputtered ares analyzed area, diameter, pm
OZ+
11w
10.5 250 pm X 250 pm 150
Bulk and Microscopic Analysis. Adsorbed and coprecipitated solids were vacuum filtered on cellulose, washed with water, and freeze-dried for at least 15 h. A portion of each sample was dissolved in dilute HCI, and P b was determined by atomic absorption spectroscopy (AAS) (Instrumentation Labs Model 951) a t 217 nm with an air/acetylene flame with background correction for a Ca interference. For X-ray powder diffraction (XRD), samples were mounted on glass slides with double-face adhesive tape. Diffraction patterns were obtained with a General Electric XRD-3 diffractometer with a Cu tube and a Ni filter. For SEM (Model AMR 1000A) inspection, samples were attached to AI stubs with a press-apply adhesive (E. F. Fullam, Inc.) and were sputter-coated with a thin layer of Au/Pd. Energy dispersive X-ray analysis (EDAX Model 711) was performed at 20 kV. Secondary Ion Mass Spectrometry (SIMS). Particles were pressed into high-purity AI foil (Alfa Products, 99.9995% pure) for SIMS analysis to assure low-substrate background contributions to Ca and P b signals. Initial investigations revealed that the blank P b level in In foil (Alfa Products, 99.999% pure) was approaching the P b intensity observed in the lowest concentration samples. Substrate background was potentially significant since low particle coverages were necessary to minimize electrical charging artifacts during SIMS analysis. Each SIMS experiment represents the average for a number of particles present in the analyzed area. Data were acquired using a Cameca IMS 3f ion microscope with a duoplasmatron 0 ,' ion source and a primary beam magnetic mass filter (8). Typical primary beam parameters are given in Table I. Computer-controlled magnetic peak switching was used for multielement depth profiling to establish concentration gradients with depth into the particles. Sputtering rates are on the order of 1 nm/s. P b was determined by using the mass 206 isotope (natural abundance = 24.1%) due to a molecular interference (Ca,O,+) at the more abundant mass 208 (natural abundance = 52.4%). As a measure of the CaCO, matrix, T a Z *was used rather than %a+ to avoid saturation of the electron multiplier under the high-sensitivity conditions required for P b detection. The use of a chemically reactive ion beam (Oz+)generally results in an enhancement of positive secondary ion 464
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emission for many elements (9). However, artifactual variations in secondary ion yield (ion yield transient) may be observed in the depth profile until the primary ion implantation depth is reached corresponding to a steady. state concentration of implanted 0 (typically l(t30 nm of sputtering). Although the carbonate matrix contains 0 as a major component, a small surface transient in ion yield is apparent for Ca. Since slightly different ion-yield variations were observed for %a2+ and ,Oca+, it is inaccurate to assume that ratioing 206Pb+to %az+ will entirely correct for surface transient effects on P b intensities. However, it is still necessary to take into account differences in P b intensities due to variations in particle coverages for different samples. Therefore, the normalized SIMS depth profdes are presented as raw 206Pb+ intensities divided by the constant %a" signal after the steady-state condition was reached. X-ray Photoelectron Spectroscopy (XF'S). Particles were pressed into In foil (Alfa Products, 99.999% pure) for XPS analysis. High coverages could be utilized, as indicated hy the lack of an In photoelectronsignal, thereby eliminating any potential P b background from impurities in the foil. Data were acquired with a Physical Electroniol Perkin-Elmer Model 548 electron spectrometer equipped with a Mg anode X-ray source (40 mA, 10 keV) and double-pass cylindrical mirror analyzer operated in the retarding mode. High-resolution spectra were acquired by use of 50 eV pass energy and a 1 eV/s scan rate with multiple scans over the following energy ranges: 549-524 eV (0Is), 364-339 eV (Ca 2p), 305-280 eV (C ls), and 155-130 eV (Pb 4f). Quantitative elemental analysis results were calculated with photoelectronpeak integral-area intensities and elemental sensitivity factors (IO). All peak positions were charge-corrected relative to C at 284.6 eV. Results and Discussion Bulk a n d Microscopic Analysis. Six samples were analyzed for this study; descriptions of the samples and their bulk P b concentrations as determined by AAS are given in Table 11. The corresponding adsorbed and coprecipitated P b samples were prepared to have similar bulk P b concentrations (samples 1 and 3; samples 2 and 4) and to cover a range of ea. lo(t1000 ppm by weight. Additional preparations (samples 5 and 6) were used to study the effect of aging time in solution on coprecipitate formation. SEM showed rhombohedral calcite crystals (Figure 1A) for both the adsorbed samples and the coprecipitates, which were aged for 24 h (samples 1-4,6). Coprecipitate 5 (aged 15min) contained mainly globular vaterite crystals (Figure 1B). The EDX technique did not detect P b in any of the samples. XRD on samples 1-4 showed narrow-line
Table 11. Sample Descriptions-Lead in Calcium Carbonates
'bt/Catt
1E-
sample no.
sample type
bulk P b concn"
crystal structureb
1 2 3
coprecipitate coprecipitate adsorbed adsorbed coprecipitate coprecipitate
134 1210 73 1120 124 122
calcite calcite calcite calcite vaterite calcite
4 5 6
\ \
I E-
\
ppm by weight determined by atomic absorption spectroscopy. Determined by X-ray powder diffraction and scanning electron microscopy.
\
l E-
l Ef
I
Figure 3. Normalized SIMS depth profiles of 'OePb+ for samples 1-4. ,---.__ ----------___..____ Pb intensities are normalized to steady state 4oCa2fsignals (see Experimental Section). Maximum sputter time corresponds to an estimated depth of 1 pm.
I
8 TS/S
258
588 TIHUSI SFRE 3
758
188
I
I
ifil
-8EI
8
250
588
TIKkI
758
ME
Ffgure 2. SIMS depth profiles of 'OSPb+(dashed lines) and %a2+ (soli lines) for samples 1-4 (Table 11). Maximum sputter tlme corresponds to an estimated depth of 1 pm.
spectra characteristic of calcite with no evidence of P b incorporation in the structure. Therefore, adsorption and coprecipitation of Pb are indistinguishable by these analysis techniques. SIMS Depth Profiling Analysis. Depth profiles of Pb+ and Ca2+for samples 1-4 are shown in Figure 2. The time axis is proportional to depth; 1000 s corresponds to a depth of approximately 1pm. The adsorbed (samples 3 and 4) and coprecipitated (samples 1and 2) cases show significantly different Pb distributions. The coprecipitates have a nearly uniform P b distribution throughout the analysis depth, while the samples prepared to contain adsorbed P b show a significant surface enrichment. Depth profiles for Pb+ normalized to the steady-state Ca2+signal are compared in Figure 3. The samples containing adsorbed P b have higher normalized Pb+ intensities near the surface relative to the corresponding coprecipitates having similar bulk concentrations (Figure 3; sample 3 vs sample 1; sample 4 vs sample 2). The observed Pb signals for the adsorbed samples also decrease by about
an order of magnitude during the depth profiles, but they do not reach the P b detection limit with increasing sputtering time (Figures 2 and 3; samples 3 and 4). This is the consequence of several factors. The sputtering of particle agglomerates continually exposes new surfaces and diminishes depth resolution. Variations in sputter rate within single particles are expected, reflecting differences in the local angle of incidence of the primary beam on the nonplanar surface ( 4 ) . Adsorbed Pb within internal particle pores will also limit the dynamic range of intensities. Finally, the blank P b level in the foil substrate may contribute significantly to the signal observed after a long sputtering time. Thus, replicate profiles show considerable variability in the normalized Pb/Ca ion-intensity ratios. For example, a relative standard deviation of 35% was observed for four depth profiles of sample 1. This results from the above factors as well as compositional variations among individual particles and variations in the number of particles analyzed due to low coverage of the A1 foil substrate. The effect of solution aging time on coprecipitate formation is an additional consideration. Samples 5 and 6 differ only in the solution residence time and have similar bulk P b levels (Table 11). Sample 5 (aged 15 min) consists largely of vaterite, which is kinetically favored over calcite (11). However, as the coprecipitate ages, vaterite converts to calcite, the thermodynamically preferred form (sample 6) ( 1 2 ) . The normalized depth profiles of these samples are essentially identical with that of sample 1 in Figure 3. The results indicate that the SIMS profiles are not sensitive to the crystalline form of the carbonate and that the P b distribution indicated by SIMS is unaffected by particle aging times greater than 15 min. The SIMS data lack the precision necessary to distinguish small bulk concentration differences between samples 1, 5, and 6 under these analysis conditions. XPS Surface Analysis. Samples 1-6 were analyzed by XPS in order to further evaluate the extent of Pb surface enrichment. The XPS analysis depth is determined by the inelastic me,an-free path of the ejected photoelectrons, i.e., the average distance within a solid Envlron. Scl. Technol., Vol. 22, No. 4, 1988 485
Table 111. Surface Atomic Ratios Determined by XPS” sample no.b
C (carbonate)/Ca
Pb/Ca
1.2 1.2 1.2 1.2 1.3 1.3 1.3
NDe ND 0.01 0.04 ND ND ND
1
2 3 4 5 6
pure CaCO,
a Using tabulated elemental sensitivity factors. *See Table I1 for sample descriptions. “ND = P b not detected.
Table IV. Comparison of Quantitative AAS, SIMS, a n d XPS Results for Lead i n Calcium Carbonates
SIMS XPS, bulkb surfacee interiord surfacee
AAS, P b concn ratiosn
A sample 2/sample 1 9 B sample 4/sample 2 0.9 C sample B/sample 1 0.5 D sample 4/sample 3 15
8 10 10
8
6
0.6 0.4 9
4
See Table I1 for sample descriptions. b P b value for each sample is bulk concentration in ppm by weight. ‘Pb value for each sample is 2oePb+intensity from the initial data point in the depth profile (normalized to 40Ca2+intensity). d P b value for each sample is zoaPb+intensity after a constant P b signal was achieved in the depth profile (normalized to 40Ca2+intensity). ‘Pb value for each sample is the atomic percent of P b ratioed to the atomic percent of Ca.
sample from which a photoelectron can be ejected into the vacuum without energy loss. Mean-free paths for Pb, Ca, 0, and C are about 3 nm, reflecting the surface specificity of the XPS measurements (12). This determination of surface concentration complements the SIMS data; it is more difficult to estimate immediate surface concentrations through SIMS due to possible ion-yield transients. Table I11 summarizes some of the quantitative XPS results. Only the adsorbed samples (3 and 4) have P b levels that are detectable by XPS; relative detection limits for XPS are often several orders of magnitude higher than for SIMS. The XPS results do not provide useful information on P b speciation due to the narrow range of binding energy shifts observed for various oxides and carbonates (13). For example, both Pb304at 138.4 eV and 2PbC03.Pb(OH)2at 138.2 eV are possibilities since the P b 4f7/2peak was observed at 138.65 eV for sample 3 and 138.25 eV for sample 4. The Ca 2p and C 1s photoelectron peak positions are consistent with CaC03for all samples (346.8 eV for Ca 2p, 289.2 for C ls, carbonate component). All samples, when compared to a pure CaC03 standard, have carbonate to calcium ratios close to the stoichiometric value (Table 111). Quantitative Comparison of Techniques. Ratios of P b concentrations for various pairwise combinations of samples 1-4 resulting from AAS, XPS, and/or SIMS measurements are compared in Table IV. The XPS and SIMS results often differ from both the bulk AAS values and from each other. This is the expected consequence of the different sensitivities and analytical volumes sampled by each technique as reviewed below. The AAS and SIMS results for the P b ratio of the highvs low-concentration coprecipitates (Table IV, case A) are expected to be similar since fairly constant P b levels were observed throughout the SIMS depth-profile analysis. Indeed, the P b ratio by AAS is 9, while the SIMS-derived P b ratios are comparable, 8 at the particle surfaces and 6 in the particle interiors. Since the relative standard deviation of the SIMS measurements for a given sample 466
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is as high as 35%, differences in the surface vs interior P b ratios may not be significant. The unique advantage of SIMS is illustrated in the quantitative results shown for the adsorbed vs coprecipitated samples (Table IV, cases B and C). The bulk AAS results do not distinguish adsorption from coprecipitation since the average P b concentrations are similar for the two samples in case B or for the two samples in case C. However, the surface P b ratios in SIMS are elevated in the adsorbed samples by an order of magnitude over the coprecipitates. Thus, SIMS data clearly distinguish between the two sample types. The interior ratios derived by SIMS in case B or C also are slightly lower than the corresponding AAS values. Since the surface region of the adsorbed samples is enriched in P b relative to the bulk (AAS)concentration values, the particle interiors must be somewhat depleted in P b relative to bulk (AAS) concentrations. P b was detected by XPS only for the adsorbed samples. The P b ratio of the high- vs low-concentration adsorbed sample is somewhat lower for XPS compared to SIMS (Table IV, case D). This may reflect more extensive multilayer adsorption or surface precipitation of Pb species in sample 4, as well as greater penetration into internal pores, which could result in a greater enriched depth than is accessed by XPS analysis. Indeed, the SIMS depth profiles suggest P b enrichment well into the particle interiors for the adsorbed samples and a greater P b concentration as a function of depth for sample 4. Additionally, both the XPS and SIMS data for case D in Table IV show considerably lower P b ratios than the bulk AAS data. This further suggests that the higher concentration sample (sample 4) has more P b adsorbed in internal pores that is largely excluded from the analytical volume of the XPS surface analysis or is only partially included in the analytical volume comprising the SIMS depth profiles of the near surface region. A final comparison of the XPS and SIMS results suggests that SIMS, under these experimental conditions, may somewhat underestimate the immediate surface P b concentration for the adsorbed samples, especially for sample 3. For example, samples 2 and 3 have nearly the same initial P b secondary ion intensities in the SIMS depth profiles (Figure 2 and 3), but a surface P b signal was detected by XPS only for sample 3 (Table 111). The ion yield transient in SIMS in the initial portion of the depth profile can obscure changes in surface P b levels as previously discussed. Perhaps more importantly, the ion microscope acquires depth profiles by cycling through the acquisition of each element in order of increasing mass, Le., 40Ca2+is measured before z06Pb+for each data acquisition cycle. Under the required analysis conditions, several nanometers of material may be sputtered before the acquisition of the first P b data point is completed. Although a slower sputter rate results in the removal of less material per data acquisition cycle, the resulting decrease in signal intensity results in inadequate SIMS sensitivity for P b in some of the samples. Thus, the immediate surface peak for adsorbed P b (samples 3 and 4) may be underestimated, reflecting the limitations of SIMS data acquisition time relative to sputter rates. This limitation may be more severe for the lower concentration adsorbed sample (sample 3). Further illustration of the last point above is provided by the comparison of P b surface enrichments observed by XPS vs SIMS for the adsorbed samples (Table V). Although the two techniques are in excellent agreement for sample 4, it is apparent that SIMS data provide a low
Envlron. Sci. Technol. 1000, 22, 467-470
Table V. Comparison of Lead Surface Enrichments Observed by XPS vs SIMS for the Adsorbed Samples
sample no.' 3 4
Literature Cited (1) Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51, 844. (2) Kheboian, C.; Bauer, C. F. Anal. Chem. 1987, 59, 1417. (3) Linton, R. W.; Loh, A.; Natusch, D. F. S.;Evans, C. A., Jr. Science (Washington, D.C.) 1976, 191, 852. (4) Cox, X. B.; Bryan, S. R.; Linton, R. W.; Griffis, D. P. Anal. Chem. 1987,59, 2018. (5) Farmer, M. E.; Linton, R. W. Environ. Sci. Technol. 1984, 18, 319. (6) Harvey, D. T.; Linton, R. W. Colloids Surf. 1984, 11, 81. (7) Riggs, M. W.; Parker, M. J. in Methods of Surface Analysis; Czanderna, A. W., Ed.; Elsevier: Amsterdam, 1975; pp 103-158. (8) Lepareur, J. Rev. Tech. Thomson-CSF 1980,12, 225. (9) Blaise, G. Surf. Sci. 1976, 60, 65. (10) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Raymond, R. H.; Gale, L. H. SIA, Surf. Interface Anal. 1981, 3, 211. (11) Turnbull, A. G. Geochim. Cosmochim. Acta 1973,37,1593. (12) Seah, M. P.; Dench, W. A. SIA, Surf. Interface Anal. 1979, 1, 2. (13) Pederson, L. R. J. Electron Spectrosc. Relat. Phenom. 1982, 28, 203. (14) Davis, J. A.; Fuller, C. C.; Cook, A. D. Geochim. Cosmochim. Acta 1987,51, 1477.
extent of lead surface enrichment XPSb SIMSc 45 9
13 10
'See Table I1 for sample descriptions. Ratio of surface weight percent (XPS) to bulk weight percent (AAS). CRatioof surface and interior zoePb+intensities normalized to the steady state 40Ca2+ signals (SIMS). estimate of the immediate surface region P b concentration for sample 3. A much greater fraction of the totaladsorbed Pb apparently is in the outermost surface layers of sample 3 vs sample 4 on the basis of the XPS results shown in Table V. This again suggests that the higher concentration adsorbed sample (sample 4) has more extensive P b penetration and adsorption in internal pores of the carbonate particles. The higher concentration of Pb in sample 4 also may allow for the formation and precipitation of a Pb-Ca solid solution, as has recently been illustrated for Cd and calcite (14). No attempt was made to distinguish between surface precipitation and adsorption in this study. Acknowledgments
We thank Sabrina Hettinger and Michele Dube at the University of New Hampshire for their assistance in the preparation and bulk characterization of the calcite samples. Registry No. CaC03, 13397-26-7; Pb, 7439-92-1.
Received for review December 8,1986. Accepted October 26,1987. This work was supported in part by the National Science Foundation (XPS Instrumentation Grant). Support for the ion microscope was provided in part by a grant from the National Science Foundation (DMR-8107499)and by matching funds from the Microelectronics Center of North Carolina, North Carolina State University, and the University of North Carolina-Chapel Hill. Additional support was provided by the Ion Microscope Laboratory (NCSU) and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
NOTES Hydrogen Sulfide Removal by Supported Vanadium Oxide Miguel J. Bagajewicz, Satish S. Tamhankar,t Maria F. Stephanopoulos,t and George R. Gavaias" Department of Chemical Engineering and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 125
The retention of hydrogen sulfide by alumina-supported vanadium oxide at 650-700 "C is studied with flow reactor experiments. The effects of sorbent prereduction and gas-phase composition (H2and H20 content) are discussed. It is found that hydrogen sulfide is chemisorbed reversibly on a nonstoichiometric vanadium oxide. Bulk sulfide is not formed.
"C), various transition metal oxides have been considered as potential sorbents. Comparative evaluation of various oxide sorbents have been published in US.Department of Energy reports (1)and in journals (2, 3). In a fuel gas atmosphere most metal oxides are first reduced to lower oxides or even metals, which in turn react with H2S. Thus, in general, the reactions can be represented as
Introductibn
In efforts to develop a process for the removal of H2S from coal-derived fuel gas at high temperatures (500-800 'Present address: The BOC Group, Technical Center, Murray Hill, NJ 07974. t Present address: Department of Chemical Engineering, MIT, Cambridge, MA 02139. 0013-936X/88/0922-0467$01.50/0
MOy + Y H ~ + S MS,
+ yH2O
(2)
The level of H2Sin the purified gas and the sulfur loading of the sorbent at H2S breakthrough are governed by the kinetic as well as the thermodynamic parameters of these reactions. In the case of V205, thermodynamic data
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 4, 1988 467
