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Effects of environmental weathering on the properties of ASC-whetlerite
~-~223/91 $300+ ,011 Copyright 8 1991Pergamon Presspie
Vol. 29. No. 2. pp. 197-205. 1991 Printedin GreatBritain.
Carbon
EFFECTS OF ENVIRONMENTAL WEATHERING THE PROPERTIES OF ASC-WHETLERITE
ON
JOSEPH ROSSIN, ERICA PETERSEN, AND DAVID TEVAULT Air Purification Branch, U.S. Army CRDEC, Aberdeen Proving Ground, MD 21010
ROBERT LAMONTAGNE Naval Research Laboratory, Washington DC 20375
and LOUIS ISAACSON Geo-Centers,
Inc.. Ft. Washington, MD 20744
(Received 26 March 1990; accepted in revised form 15 May 1990)
Abst~et-Activated, impregnated carbon from a radial Row air purification filter was separated into fractions after the filter had been exposed to shipboard environmental conditions for 21 months. Carbon samples from the discrete filter locations were analyzed using surface analysis methods. Breakthrough times of the segregated carbon fractions were recorded for a cyanogen chloride challenge. The inlet portion of the filter was most severely affected by the weathering. For carbon removed from the filter inlet. the weathering process caused impregnants to leach from the granules and form metal sulfates, presumably by reaction with atmospheric SO,. No sulfur was detected on carbon samples removed from the center and outlet portions of the filter; however, a significant fraction of the chromium(VI) impregnant had been reduced to chromium(II1). In all cases, breakthrough times recorded for a cyanogen chloride challenge were significantly less than that recorded for the unweathered carbon sample. Key Words-ASC.
EDS. weathering, whetlerite. XPS.
1. INTRODUCTION
ASC whetlerite is an activated carbon impregnated with salts of copper, chromium, and silver. This material is used by the military to provide protection against chemical warfare agents by either adsorption of the agent or by adsorption followed by chemical reaction with the impregnants. The carbon filter employed in the modern-day gas mask is sealed under vacuum. It is unsealed and installed only in the event of a chemical threat. In this manner, the filter is not exposed to the environment prior to use, and loss of performance due to atmospheric exposure need not be taken into consideration. Uniike the gas mask filter, filters employed in collective protection applications (fixed installations. armored vehicles, naval vessels. etc.) may be required to function continuously, whether or not a chemical threat exists. This results in the collective protection filter being continuously subjected to environmental contaminants which may hinder the filter’s performance against a chemical attack. Deitz et a[.[11 performed studies involving the effects of weathering on the performance of ASC whetlerite. In these experiments, ambient air was drawn through impregnated, activated carbon filters for periods varying from one month to one year. Results of this study showed the carbon located at the inlet 25% of the filter provided minimal protection against cyanogen chloride. Carbon located further into the filter bed possessed greater cyanogen chloride reac-
tivity than that of the inlet, based on cyanogen chloride breakthrough times. In all cases, however, the cyanogen chloride reactivity was less than that of the unweathered carbon sample. The loss of reactivity from whetlerite located at the inlet portion of the filter was attributed to the presence of airborne contaminants (ozone, sulfur dioxide, and nitric oxide), while the decreased activity of carbon located futher into the filter was attributed to deactivation by water of the vapor. No spectroscopic characterization weathered carbons was reported. The effects of aging activated carbon (impregnated with copper and chromium) in a humid environment were investigated by Brown et al.[2]. Their studies indicated that aging the impregnated carbon in a humid environment did not alter the oxidation state of copper, based on XPS analysis. The aging did. however, result in the reduction in a portion of the chromium(V1) to chromium(II1). The decrease in the HCN and (CN)? breakthrough times foilowin~ aging in a humid environment was attributed to the reduction in the fraction of chromium(V1). The objective of the present study is to identify and characterize the effects of environmental weathering on collective protection filter containing impregnated, activated carbon. The work reported here focuses on characterizing the changes in metal speciation and distribution within carbon granules removed from discrete locations within a radial flow carbon filter following 21 months of weathering in 197
J. ROSIN et al.
198
a marine environment. Surface analysis methods (X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy) and cyanogen chloride breakthrough times have been employed in characterizing the carbon from the collective protection filter.
2.1 Materials ASC whetlerite (Lot 103) was obtained from Calgon Corporation as 12-30 mesh (U.S. Standard Sieve) granules. This sample contained 8% copper, 3% chromium, 0.05% silver, and 2% triethylenediamine (TEDA) by weight, based on manufacturer specifications. This sample will be referred to as ASCTEDA. BPL carbon (Lot 7502) was obtained for comparison studies from Calgon Corporation as 1230 mesh granules. BPL has a pore structure similar to that of ASC whetlerite but does not contain the metal impregnants. High purity copper(I1) sulfate and chromium(II1) sulfate were obtained from Aesar Chemical.
the particle filter is to prevent aerosols (including ocean spray) from contacting the carbon bed. Following weathering, carbon was removed from the filter and separated into four layers perpendicular to the direction of air flow (radial direction). The four layers consisted of the inlet-most portion of the filter (referred to as inlet throughout this article); a mixture of carbon from the filter inlet to a point extending one-third of the way into the bed (inlet l/3); a mixture of carbon from a point onethird of the way into the bed to a point extending two-thirds of the way into the bed (center 113); and a mixture of carbon from a point two-thirds of the way into the bed extending to the filter outlet (outlet l/3). The inlet-most portion of the filter was approximately 15% to 20% of the total filter thickness. Carbon particles removed from the inlet region of the filter were clumped together, while samples removed from the other portions of the bed displayed little evidence of clumping. The samples which were clumped together were easily separated using very light pressure.
2.2 Weathering of ASC-TEDA carbon The weathered samples of ASC-TEDA carbon were obtained from a radial-flow filter which had been exposed to a marine environment in a prototype collective protection system. This collective protection filter assembly was exposed to normal operating conditions on an ocean-going vessel for 21 months. A schematic depiction of the collective protection filter is shown in Fig. 1. Note that the air flow is in the radial direction. A particle filter is present in front of the carbon bed. The purpose of
2.3 Preparation of impregnated BPL carbon Approximately 3.0 g dried BPL were impregnated to incipient wetness with 2.0 mL of 1 mol/L copper sulfate solution. Following impregnation, the carbon was again dried at llo”C, and the impregnation procedure was repeated a second time in order to produce a sample containing approximately 8% by weight copper. An additional 3.0 g of BPL were dried and impregnated to incipient wetness with 2.0 mL of 0.3 mol/L solution of chromium sulfate in order to produce a sample containing approximately
2. PROCEDURE
aw FILT ER
Fig. 1. Cut-away diagram of radial flow carbon filter
199
Effects of weathering on ASC-whetlerite 2% by weight chromium. Both the copper and chromium sulfate impregnated samples were dried overnight at 110°C following impregnation. These samples are intended to serve as references for surface analysis studies.
at all analysis regions within the bisected granule. A manufacturer-supplied quantification routine was employed in calculating atomic concentrations. 2.5 Cyanogen chloride breakthrough times The reactivities of the unweathered and weathered samples of ASC-TEDA were determined by a cyanogen chloride breakthrough test. Approximately 18 g of carbon were placed into a tubular glass reactor and equilibrated with 80% RH air at 300 K flowing at a linear velocity of 9.6 cm/s overnight. This correponds to ca. 50,000 column volumes and is expected to approach equilibrium loading according to previous experience. The cyanogen chloride test was performed at 80% RH, 300 K with a cyanogen chloride feed concentration 4 mg/L and a linear velocity of 9.6 cm/s. The breakthrough time is defined as the time at which the effluent cyanogen chloride concentration exceeds 8 kg/L.
2.4 Surface analysis X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Perkin-Elmer Phi 570 ESCAi SAM system employing MgK, X-rays. Samples were analyzed for carbon, oxygen, copper, chromium, and sulfur. No additional elements were detected. No silver could be detected due to its low impregnation level (0.05 wt%). XPS analyses are reported for samples prepared by crushing the granules to a fine powder and sieving to below 325 mesh. All binding energies reported are referenced to the carbon 1s photoelectron peak at 284.6 eV. The fraction of chromium(V1) was determined by using a standard peak subtraction routine. Atomic ratios pertaining to elements of interest were determined by employing sensitivity factors supplied by the manufacturer. Energy dispersive X-ray spectroscopy (EDS) was performed using a Tracer Northern 5700 EDS/WDS automation system interfaced with a JEOL 35CF scanning electron microscope. All spectra were recorded using 20 KeV electron accelerating voltage and count rates of 1,500 to 2,000 counts per second. Samples were analyzed as 12-30 mesh granules attached to the sample mount with colloidal carbon adhesive paint. Cross-sectional analysis was performed on single carbon granules bisected with a diamond knife. Analyses of the bisected carbon granules were performed by collecting spectra from a 100 km-by-100 IJ-msquare area stepped across the largest sample dimension (see Fig. 2). The 100 km square area was chosen over spot analysis in an attempt to account for statistical variations between the individual spots. The radial distribution of copper and chromium is reported relative to the atomic concentration of silicon plus aluminum. This method was selected because the absolute intensities of silicon plus aluminum were found to be nearly constant
Figure 3a shows a backscattered electron image (BEI) of a representative inlet carbon granule. A BE1 image of an unweathered carbon granule is shown in Fig. 3b for comparison. In comparing these figures, note the difference in the surface morphology. Figure 3a shows that large regions of the surface of the weathered granule appear to be covered with a metallic overlayer. Some inlet granules were found to be entirely covered with this overlayer. A metallic overlayer was also observed upon analysis of inlet 113 granules; however, it was not nearly as severe as that of the inlet carbon sample. BEI recorded for carbon samples from the center 113 and outlet 113 were similar to the unweathered sample. XPS spectra of the weathered carbon samples from the discrete locations within the filter were compared to spectra corresponding to the unweathered carbon sample. A significant fraction of sulfur was detected on the samples from the filter inlet and inlet 113. Surprisingly, no sodium was detected. as one might expect from the salt water environment. This indi-
Fig. 2. Scanning electron micrograph of bisected TEDA carbon granule showing 100 )*m by 100km beam analysis regions.
Fig. 3a. Backscattered electron image (BEI) ot an ASCTEDA carbon granule from the filter inlet following weathering.
ASCmicro-
3. RESULTS
J. ROSIN et al.
200
Fig. 3b. Backscattered electron image (BEI) of an unweathered AX-TEDA carbon granule.
cated
that ocean
spray did not contact
the carbon
bed. Figure 4 compares the XPS spectra of the copper 2p photoelectron region of the weathered carbon samples from the discrete locations within the filter. The spectrum corresponding to the unweathered sample is provided as a reference. Figure 4 shows the position of the copper 2p peak to be shifted to a higher binding energy for carbons from the inlet and inlet l/3 region of the filter relative to that of the unweathered sample. These peaks also appear broader than the peak corresponding to the unweathered sample. Note also the changes that occur
in the satellite shake-up region (about 939 to 949 eV) of the spectrum corresponding to the inlet sample as compared with that of the unweathered carbon. Some similar (but more subtle) changes in the shake-up region are detected in the spectrum of the inlet l/3 sample. The copper 2p spectra of the weathered samples from the center l/3 and outlet l/3 appear identical to that of the unweathered sample. Figure 5 compares the XPS spectra of the chromium 2p photoelectron region of the weathered carbon samples from the discrete filter locations. The spectrum corresponding to the unweathered carbon sample is shown for comparison. Note from this figure that the chromium peaks in all the weathered samples are shifted to a lower binding energy compared to those of the unweathered sample. However, spectra recorded for the center l/3 and outlet l/3 samples display strong shoulders at approximately 578.5 eV, consistent with the position of the 2p3,*peak of the unweathered sample. Figure 6 compares the copper 2p photoelectron region of the inlet sample to that of BPL carbon impregnated with copper sulfate. As the figure indicates, the positions and shapes of all peaks associated with the spectrum of the inlet sample are in excellent agreement with those of the impregnated BPL sample. Figure 7 compares the chromium 2p photoelectron region of the inlet sample to that of BPL impregnated with chromium sulfate. The figure indicates the positions of the chromium 2p photo-
2p3/2
w OUTLET
l/3
UNWEATHERED
I \ 974
964
954
BINDING
944 ENERGY
934
924
(eV)
spectra of the copper 2p photelectron region for unweathered and weathered carbon samples.
Fig. 4. XPS
595
590
565
BINDING
580
575
ENERGY
Unweathered
570
565
(eV)
Fig. 5. XPS spectra of the chromium 2p photoelectron ^ region tor unweathered
and weathered carbon samples.
Effects of weathering on AK-whetlerite
201
2% Cr,lS0,~,~12H*0
974
954
944
BINDING
ENERGY
964
934
on
BPL
924
(eV)
176
Fig. 6. XPS spectra of the copper 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 8% copper sulfate.
electron peaks of the inlet sample
to be consistent with those of the reference sample (BPL impregnated with chromium sulfate). The sulfur 2p pho-
toelectron region comparing the inlet cabon sample to BPL impregnated with copper sulfate and BPL impregnated with chromium sulfate is illustrated in Fig. 8. From the spectrum corresponding to the inlet sample, note that only one sulfur peak is present. The position of this peak is in agreement with that of the sulfur associated with the impregnated BPL carbons. Figure 9 reports the ratio of copper to silicon-plus-
174
170
BINDING
166
ENERGY
162
156
(eV)
Fig. 8. XPS spectra of the sulfur 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 8% copper sulfate and BPL impregnated with 2% chromium sulfate.
(Cu/[Si + Al]) at the different analysis areas (see Fig. 2) within the bisected carbon granules from the discrete locations within the filter. Data corresponding to the unweathered carbon granule are reported for comparison. Data for the inlet sample are not presented in Fig. 9, as virtually no copper was detected within the granule. Figure 10 reports the ratio of chromium to silicon-plus-aluminum at the different analysis areas within the bisected carbon granules from the discrete locations within the filter. Data corresponding to the unweathered carbon granule are shown for comparison. Data for the inlet sample are not reported, as virtually no chromium were detected within the granule. The cyanogen chloride breakthrough times for the unweathered and weathered carbon samples are reported in Table 1. No cyanogen chloride data were obtained from the inlet sample, as there was not enough sample available to perform the test. aluminum
4. DISCUSSlON
I
595
I
I
I
I
590
565
560
575
BINDING
ENERGY
I
570
I
565
(eV)
Fig. 7. XPS spectra of the chromium 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 2% chromium sulfate.
Freshly prepared ASC whetlerite is believed to contain copper-ammonium-chromium complexes[3]. These complexes possess chromium in the + 6 oxidation state and are postulated to be responsible for the destruction of cyanogen chloride and hydrogen cyanide(2-51. Changes in the impregnant speciation are expected to reduce the reactive properties of the whetlerite. It is evident from the XPS spectra presented in Figs. 4 and 5 that significant changes in the copper and chromium speciation have occurred as a result of marine weathering for samples
J. ROSSINet al.
202
8
\-
2 1 O0
I
1
Fig. 9. Relative copper ~ncentrations
I
I
I
I
I
2
3
4
5
6
POSITION
as a function of position within the bisected carbon granules corresponding weathered and unweathered samples.
removed from the inlet region of the filter. Figure 4 compares the XPS spectra of the copper 2p photoelectron peaks of the weathered carbon samples to that of the unweathered carbon sample. The position of the copper 2p photoelectron peaks corresponding to the unweathered carbon sample is indicative of copper in the +2 oxidation state and is
q
7 to
similar to what has been reported elsewhere[4,6,7]. Note from Fig. 4 that the binding energy of the copper 2p3,2 photoelectron peak corresponding to the inlet sample has increased about 1.3 eV to 935.1 eV. Also note the change in the satellite shake-up peaks. The changes in the shake-up region are important, as these peaks provide information regard-
0.9
+
z
yO.6 s
1
2
3 4 POSITION
5
6
7
Fig. 10. Relative chromium concentration as a function of position within the bisected carbon granules correspanding to weathered and unweathered samples.
Effects of weathering on ASC-whetlerite Table 1. Cyanogen chloride breakthrough time for ASC-TEDA samples from discrete filter locations Cyanogen chloride breakthrough time (min)
Sample
c5.0 24.2 24.6 37.8
Inlet Inlet 113 Center l/3 Outlet 113 Unweathered ing the chemical
states
and speciation
of paramag-
netic compounds[8]. The changes in the copper XPS spectra suggest that the majority of the copper has been transformed to copper sulfate. A comparison is made between the copper 2p spectra of weathered ASC-TEDA and BPL impregnated with copper sulfate (see Fig. 6). Note that the two spectra are quite similar. suggesting that most, if not all, the copper associated with the inlet sample was transformed to copper sulfate during the weathering process. Further evidence for the formation of copper sulfate is reported in Fig. 8. Data presented in this figure show that the position of the sulfur peak is consistent with that of copper sulfate. The sulfur 2p photoelectron spectrum of the inlet sample is interesting in that only one sulfur peak is present. This suggests that additional sulfur complexes (such as metal sulfides and adsorbed sulfuric acid) are not present in any significant quantity, The copper 2p photoelectron spectrum of the inlet 113 sample is different from that of the unweathered sample. Note that the position of the 2p,.? peak is shifted to a slightly higher binding energy, and that the satellite shake-up region displays a different shape. A small fraction of sulfur was detected during the analysis of this sample. It is likely that the changes in the shape and positions of the peaks are due to the transformation of a portion of the original copper(H) species to copper sulfate. The spectra corresponding to the center 113 and outlet 113 samples are very similar to that of the unweathered sample. For these samples. copper has remained in the +2 oxidation state. This is important, as copper(U) is reported to have a role in the removal of hydrogen cyanidej21. Virtually no sulfur or any other foreign elements were detected upon analysis of the center 113 and outlet l/3 samples. Evidently, the environmental exposure did not alter the oxidation state of copper past the inlet 113 of the filter. Analysis of the chromium XPS data presented in Fig. 5 reveals that changes in the chromium oxidation state(s) have occurred and that these changes are dependent on the carbon location within the filter. For the unweathered sample, the position of the chromium 2pv, photoelectron peak at 578.5 eV is due to the presence of chromium in the + 6 oxidation state[4,6,7]. The shoulder present at about 577 eV is indicative of chromium(II1) complexes. For this sample, approximately 60% of the chromium was determined to be in the +6 oxidation state. The
203
fraction of chromium(V1) associated with a whetlerite sample is important, as chromium(V1) complexes are believed to be responsible for the destruction of cyanogen chloride[3-51 and hydrogen cyanide[2]. Figure 5 indicates that only a small amount of chromium, if any, is present in the +6 oxidation state for the inlet carbon sample. The position of this peak is indicative of chromium in the +3 oxidation state and is consistent with that of chromium sulfate, as shown in Fig. 7. These data suggest that the weathering process has changed the majority of the chromium (whether in the + 6 or + 3 oxidation state) associated with the inlet carbon sample to chromium sulfate. The chromium spectrum corresponding to the inlet 113 sample is difficult to interpret. A small amount of sulfur was detected upon analysis of this sample. The position of the chromium 2pi.? peak suggests that both chromium(II1) complexes, chromium sulfate, and chromium(VI) complexes may be associated with the sample. However. the fraction of chromium(V1) associated with this carbon sample appears to be small. based on the intensity of the signal present at about 578.5 eV. The chromium spectra corresponding to the center 113 and outlet 113 samples appear similar. For both samples, the position of the 2p?.? peak has shifted approximately 1.2 eV down field; however. a very strong shoulder exists at 578.5 eV. This shoulder is attributed to chromium in the + 6 oxidation state, and the shifting of the chromium peak is due to the reduction of a portion of the chromium(V1) to chromium(II1). For both the center l/3 and outlet 113 carbon samples. approximately 40% of the chrdhum was determined to be in the + 6 oxidation state. This value represents a 33% reduction in the fraction of chromium(V1) compared to the unweathered sample. Brown et a/.[21 have shown that exposure of impregnated carbons to water vapor results in a partial reduction of chromium(V1). The partial reduction of chromium(V1) as observed for the center li3 and outlet 113 samples may be due to exposure to humidity in the shipboard environment. The atomic ratio of sulfur to copper-plus-1.5-timeschromium (S/[Cu + 1.5 Cr]), as determined by XPS. provides an indication of the extent of metal sulfate formation. For the inlet sample, this ratio was determined to be 1.03, suggesting that virtually all the copper and chromium associated with the inlet sample had been transformed to metal sulfates. For the inlet li3 sample, the atomic ratio of sulfur to copperplus-1.5-times-chromium was determined to be 0.20. This indicates that approximately 20% of the impregnants had been transformed to metal sulfates. Virtually no sulfur was detected upon analysis of the center 113 and outlet 113 samples. For the inlet and inlet 113 samples. the formation of copper and chromium sulfate was likely the result of contact with airborne sulfates. The source of the airborne sulfates has not been determined, but acid rain and/or SO, from diesel exhaust gases are likely sources. The cyanogen chloride reactivity of impregnated
204
J. ROSIN et al.
carbons is reported to be proportional to the chromium(V1) fraction of the sample[3-51. XPS analyses suggested only a small quantitiy of chromium(V1) was associated with the inlet l/3 sample. It was therefore expected that this sample would provide little cyanogen chloride protection. These results were indeed observed and are reported in Table 1. The center l/3 and outlet l/3 samples possessed similar fractions of chromium(VI), and, as expected, similar cyanogen chloride breakthrough times were recorded. The fraction of chromium(V1) associated with these samples was approximately two-thirds that of the unweathered sample (40% versus 60%). It is interesting to note that the cyanogen chloride breakthrough times recorded for these samples were also approximately two-thirds less than that recorded for the unweathered sample (24.6 minutes versus 36.8 minutes). These data appear to confirm the role of the chromium(V1) species in the destruction of cyanogen chloride and support previous literature observations. Based on the cyanogen chloride reactivity of the discrete sections of this carbon filter, the reactive properties of this weathered carbon filter are expected to be less than 50% of the original design. Figure 3a shows the inlet carbon granule to be encrusted with a metallic overlayer. This overlayer was found to consist primarily of copper sulfate, based on EDS microbeam analysis. This overlayer was not observed on the unweathered carbon granule (Fig. 3b) nor on the carbon granules located past the inlet l/3 of the filter. EDS analysis of a bisected carbon granule (inlet sample) revealed virtually no copper or chromium present within the granule. Wet chemical analyses were performed on 8-10 g portions of each carbon sample to determine whether impregnants were lost during the weathering process. To within experimental error, no loss of copper, chromium, or silver occurred during the weathering process. It is likely that the metallic overlayer was formed by the leaching of impregnants to the external surface of the granule. Migration of impregnants to the external surface will result in a loss of dispersion, thereby reducing the reactive properties. EDS analyses of the weathered and unweathered bisected carbon granules were performed for the purpose of establishing the effects of weathering on impregnant migration. Comparing the radial impregnant distribution of the weathered granules to that of the unweathered granules will allow one to determine whether the impregnants have migrated to the external surface of the granule. In an effort to normalize and better present the data, the ratios of copper and chromium to silicon-plusaluminum are reported. It should also be noted that the numerical value of these ratios varies from sample to sample. This may be the result of the silicon and aluminum fraction being different in each granule analyzed and/or the fraction of copper and chromium varying from granule to granule. Therefore, the discussion is focused on trends in the distribution of impregnants within the individual granules.
Figure 9 illustrates the copper distribution as a function of position within the different bisected carbon granules. For the unweathered carbon, the concentration of copper at the outer analysis area (position 1) is greater than that found further into the granule and is consistent with previous reports[6]. Further into the granule, the copper distribution appears uniform. For the inlet l/3 sample, the relative concentration at the outer regions of the granule is greater than that of the unweathered granule. These data suggest that a portion of the copper has begun to migrate towards the external surface of the granule. The copper concentration at the outer analysis areas for the center l/3 and outlet l/3 samples was less than that observed for the unweathered sample. However, just as in the case of the unweathered sample, the copper concentration is nearly uniform at all internal analyses areas. These data suggest that migration of the impregnants to the external surface of the center l/3 and outlet l/3 samples did not occur. The reason for the decrease in the concentration gradient near the external surface of the granule is not clear. The chromium distribution within the weathered and unweathered carbon granules is illustrated in Fig. 10. For the unweathered sample, the concentration of chromium is greatest at the external surface of the granule (position 1) and declines to a near constant value further into the granule. The gradient is similar to that observed for copper, but not quite as steep. Virtually no chromium was detected at the internal analysis areas for the inlet sample. Therefore these data are not reported in Fig. 10. The chromium distribution for the inlet l/3 and center l/3 granules are similar to that of the unweathered sample; neither sample shows evidence of chromium migration. The fact that the inlet l/3 sample showed evidence of copper migration suggests that copper is more readily leached from the sample than chromium. The outlet l/3 sample shows an increase in the concentration of chromium near the external surface of the granule (moving towards the center of the granule). This is in contrast to what was observed for the unweathered sample but consistent with the copper distribution reported in Fig. 9. Migration of the impregnants was accompanied by the formation of copper and chromium sulfates. For the inlet sample, virtually all the impregnants were present as metal sulfates and were found to be located almost exclusively at the external surface of the granule. For the inlet l/3 sample, only a portion of the impregnants had been transformed to metal sulfates. EDS analyses confirmed the migration of copper, but chromium apparently remained in place. For the center l/3 and outlet l/3 samples, no metal sulfates were detected, and migration of copper and chromium to the external surface of the granule did not occur. A possible explanation for the migration of impregnants to the external surface of the granule as observed for the inlet sample follows. Copper and chromium sulfates may be formed by treating
205
Effects of weathering on AX-whetlerite ASC-TEDA with dilute suluric acid. For the case at hand, the environmental sulfuric acid may be supplied by either acid rain or SO, from diesel exhaust. When the acid contacts the impregnants, copper and chromium sulfates are formed. This is consistent with the observed speciation. Copper and chromium sulfates are soluble in water, and their solubility increases as the solution pH is decreased. It is likely that the filter cycles through periods of changing humidity. As the humidity is decreased, a portion of the water and the soluble compounds migrates through the pores to the external surface of the granule, where the water evaporates. This results in the soluble sulfates accumulating at the external surface of the granule. Increasing the humidity will result in refilling the pores with water, which brings in more acids from which the metal sulfates are formed. Repeating this cycle a number of times over the age of the filter would likely remove a large fraction of the impregnants located within the granule and deposit them onto its external surface.
5. CONCLUSIONS
The inlet portion of the carbon filter was severely affected by prolonged environmental exposure. For the shipboard environment encountered, sulfates appear to be the only contaminant detected which affected the reactive properties of the carbon. Airborne sulfates lead to the formation of metal sulfates, which ultimately leach from within the carbon granules to the external surface. Past the inlet l/3 of the bed, virtually no trace of sulfur was present; how-
ever, a significant fraction of chromium(V1) was reduced to chromium(II1) species (about one-third). Past the inlet third of the bed, the degree of chromium reduction was independent of the position within the filter. Standardized cyanogen chloride breakthrough times of the different filter fractions suggest that the environmental exposure reduced the filter performance to 50% of its initial value. Acknowledgemenls-One of us (J. A. R.) wishes to thank the National Research Council for financial support on this project. R. A. L and L. I. thank NAVSEA for financial support. The authors extend thanks to Mr. R. Grue for providing the CK data and to Mr. R. Herd for performing the wet chemical analyses. REFERENCES
1. V. R. Deitz, R. J. Puhala, D. B. Stroup, and G. F. Dickey, Influence of Atmospheric Weathering on the Performance of Whetlerite, Naval Research Laboratory Memo. Report No. 4752, Washington DC (1982). 2. P. N. Brown, G. G. Jayson, G. Thompson, and M. C. Wilkinson, Carbon 27, 821 (1989). 3. L. L. Pytlewski, Studies of ASC Whetlerite Reactivity, Report ARCSL-CR-79008, U.S. Army Chemical Systen& Laboratory, Aberdeen Proving Grbund. MD. NOV. 1979. UNCLASSIFIED Reoort. 4. M. M. Ross, R. J. Colton. ahd V. R. Deitz, Carbon 27. 426 (1989).
5. P. N. Krishnan, S. A. Katz, A. Birenzvige, and H. Salem. Carbon 26. 914 (1988). 6. J. A. Rossin, Carbon 2;. 61i (1989). 7. J. L. Hammarstrom and A. Sacco, J. Catal. 112, 267 (1988). 8. C. D. Wagner, W. M. Riggs, L. E. Davis. J. F. Moulder, and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corp., Edin Prairie. MN (1979).
Vol. 29. No. 2. pp. 197-205. 1991 Printedin GreatBritain.
Carbon
EFFECTS OF ENVIRONMENTAL WEATHERING THE PROPERTIES OF ASC-WHETLERITE
ON
JOSEPH ROSSIN, ERICA PETERSEN, AND DAVID TEVAULT Air Purification Branch, U.S. Army CRDEC, Aberdeen Proving Ground, MD 21010
ROBERT LAMONTAGNE Naval Research Laboratory, Washington DC 20375
and LOUIS ISAACSON Geo-Centers,
Inc.. Ft. Washington, MD 20744
(Received 26 March 1990; accepted in revised form 15 May 1990)
Abst~et-Activated, impregnated carbon from a radial Row air purification filter was separated into fractions after the filter had been exposed to shipboard environmental conditions for 21 months. Carbon samples from the discrete filter locations were analyzed using surface analysis methods. Breakthrough times of the segregated carbon fractions were recorded for a cyanogen chloride challenge. The inlet portion of the filter was most severely affected by the weathering. For carbon removed from the filter inlet. the weathering process caused impregnants to leach from the granules and form metal sulfates, presumably by reaction with atmospheric SO,. No sulfur was detected on carbon samples removed from the center and outlet portions of the filter; however, a significant fraction of the chromium(VI) impregnant had been reduced to chromium(II1). In all cases, breakthrough times recorded for a cyanogen chloride challenge were significantly less than that recorded for the unweathered carbon sample. Key Words-ASC.
EDS. weathering, whetlerite. XPS.
1. INTRODUCTION
ASC whetlerite is an activated carbon impregnated with salts of copper, chromium, and silver. This material is used by the military to provide protection against chemical warfare agents by either adsorption of the agent or by adsorption followed by chemical reaction with the impregnants. The carbon filter employed in the modern-day gas mask is sealed under vacuum. It is unsealed and installed only in the event of a chemical threat. In this manner, the filter is not exposed to the environment prior to use, and loss of performance due to atmospheric exposure need not be taken into consideration. Uniike the gas mask filter, filters employed in collective protection applications (fixed installations. armored vehicles, naval vessels. etc.) may be required to function continuously, whether or not a chemical threat exists. This results in the collective protection filter being continuously subjected to environmental contaminants which may hinder the filter’s performance against a chemical attack. Deitz et a[.[11 performed studies involving the effects of weathering on the performance of ASC whetlerite. In these experiments, ambient air was drawn through impregnated, activated carbon filters for periods varying from one month to one year. Results of this study showed the carbon located at the inlet 25% of the filter provided minimal protection against cyanogen chloride. Carbon located further into the filter bed possessed greater cyanogen chloride reac-
tivity than that of the inlet, based on cyanogen chloride breakthrough times. In all cases, however, the cyanogen chloride reactivity was less than that of the unweathered carbon sample. The loss of reactivity from whetlerite located at the inlet portion of the filter was attributed to the presence of airborne contaminants (ozone, sulfur dioxide, and nitric oxide), while the decreased activity of carbon located futher into the filter was attributed to deactivation by water of the vapor. No spectroscopic characterization weathered carbons was reported. The effects of aging activated carbon (impregnated with copper and chromium) in a humid environment were investigated by Brown et al.[2]. Their studies indicated that aging the impregnated carbon in a humid environment did not alter the oxidation state of copper, based on XPS analysis. The aging did. however, result in the reduction in a portion of the chromium(V1) to chromium(II1). The decrease in the HCN and (CN)? breakthrough times foilowin~ aging in a humid environment was attributed to the reduction in the fraction of chromium(V1). The objective of the present study is to identify and characterize the effects of environmental weathering on collective protection filter containing impregnated, activated carbon. The work reported here focuses on characterizing the changes in metal speciation and distribution within carbon granules removed from discrete locations within a radial flow carbon filter following 21 months of weathering in 197
J. ROSIN et al.
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a marine environment. Surface analysis methods (X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy) and cyanogen chloride breakthrough times have been employed in characterizing the carbon from the collective protection filter.
2.1 Materials ASC whetlerite (Lot 103) was obtained from Calgon Corporation as 12-30 mesh (U.S. Standard Sieve) granules. This sample contained 8% copper, 3% chromium, 0.05% silver, and 2% triethylenediamine (TEDA) by weight, based on manufacturer specifications. This sample will be referred to as ASCTEDA. BPL carbon (Lot 7502) was obtained for comparison studies from Calgon Corporation as 1230 mesh granules. BPL has a pore structure similar to that of ASC whetlerite but does not contain the metal impregnants. High purity copper(I1) sulfate and chromium(II1) sulfate were obtained from Aesar Chemical.
the particle filter is to prevent aerosols (including ocean spray) from contacting the carbon bed. Following weathering, carbon was removed from the filter and separated into four layers perpendicular to the direction of air flow (radial direction). The four layers consisted of the inlet-most portion of the filter (referred to as inlet throughout this article); a mixture of carbon from the filter inlet to a point extending one-third of the way into the bed (inlet l/3); a mixture of carbon from a point onethird of the way into the bed to a point extending two-thirds of the way into the bed (center 113); and a mixture of carbon from a point two-thirds of the way into the bed extending to the filter outlet (outlet l/3). The inlet-most portion of the filter was approximately 15% to 20% of the total filter thickness. Carbon particles removed from the inlet region of the filter were clumped together, while samples removed from the other portions of the bed displayed little evidence of clumping. The samples which were clumped together were easily separated using very light pressure.
2.2 Weathering of ASC-TEDA carbon The weathered samples of ASC-TEDA carbon were obtained from a radial-flow filter which had been exposed to a marine environment in a prototype collective protection system. This collective protection filter assembly was exposed to normal operating conditions on an ocean-going vessel for 21 months. A schematic depiction of the collective protection filter is shown in Fig. 1. Note that the air flow is in the radial direction. A particle filter is present in front of the carbon bed. The purpose of
2.3 Preparation of impregnated BPL carbon Approximately 3.0 g dried BPL were impregnated to incipient wetness with 2.0 mL of 1 mol/L copper sulfate solution. Following impregnation, the carbon was again dried at llo”C, and the impregnation procedure was repeated a second time in order to produce a sample containing approximately 8% by weight copper. An additional 3.0 g of BPL were dried and impregnated to incipient wetness with 2.0 mL of 0.3 mol/L solution of chromium sulfate in order to produce a sample containing approximately
2. PROCEDURE
aw FILT ER
Fig. 1. Cut-away diagram of radial flow carbon filter
199
Effects of weathering on ASC-whetlerite 2% by weight chromium. Both the copper and chromium sulfate impregnated samples were dried overnight at 110°C following impregnation. These samples are intended to serve as references for surface analysis studies.
at all analysis regions within the bisected granule. A manufacturer-supplied quantification routine was employed in calculating atomic concentrations. 2.5 Cyanogen chloride breakthrough times The reactivities of the unweathered and weathered samples of ASC-TEDA were determined by a cyanogen chloride breakthrough test. Approximately 18 g of carbon were placed into a tubular glass reactor and equilibrated with 80% RH air at 300 K flowing at a linear velocity of 9.6 cm/s overnight. This correponds to ca. 50,000 column volumes and is expected to approach equilibrium loading according to previous experience. The cyanogen chloride test was performed at 80% RH, 300 K with a cyanogen chloride feed concentration 4 mg/L and a linear velocity of 9.6 cm/s. The breakthrough time is defined as the time at which the effluent cyanogen chloride concentration exceeds 8 kg/L.
2.4 Surface analysis X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Perkin-Elmer Phi 570 ESCAi SAM system employing MgK, X-rays. Samples were analyzed for carbon, oxygen, copper, chromium, and sulfur. No additional elements were detected. No silver could be detected due to its low impregnation level (0.05 wt%). XPS analyses are reported for samples prepared by crushing the granules to a fine powder and sieving to below 325 mesh. All binding energies reported are referenced to the carbon 1s photoelectron peak at 284.6 eV. The fraction of chromium(V1) was determined by using a standard peak subtraction routine. Atomic ratios pertaining to elements of interest were determined by employing sensitivity factors supplied by the manufacturer. Energy dispersive X-ray spectroscopy (EDS) was performed using a Tracer Northern 5700 EDS/WDS automation system interfaced with a JEOL 35CF scanning electron microscope. All spectra were recorded using 20 KeV electron accelerating voltage and count rates of 1,500 to 2,000 counts per second. Samples were analyzed as 12-30 mesh granules attached to the sample mount with colloidal carbon adhesive paint. Cross-sectional analysis was performed on single carbon granules bisected with a diamond knife. Analyses of the bisected carbon granules were performed by collecting spectra from a 100 km-by-100 IJ-msquare area stepped across the largest sample dimension (see Fig. 2). The 100 km square area was chosen over spot analysis in an attempt to account for statistical variations between the individual spots. The radial distribution of copper and chromium is reported relative to the atomic concentration of silicon plus aluminum. This method was selected because the absolute intensities of silicon plus aluminum were found to be nearly constant
Figure 3a shows a backscattered electron image (BEI) of a representative inlet carbon granule. A BE1 image of an unweathered carbon granule is shown in Fig. 3b for comparison. In comparing these figures, note the difference in the surface morphology. Figure 3a shows that large regions of the surface of the weathered granule appear to be covered with a metallic overlayer. Some inlet granules were found to be entirely covered with this overlayer. A metallic overlayer was also observed upon analysis of inlet 113 granules; however, it was not nearly as severe as that of the inlet carbon sample. BEI recorded for carbon samples from the center 113 and outlet 113 were similar to the unweathered sample. XPS spectra of the weathered carbon samples from the discrete locations within the filter were compared to spectra corresponding to the unweathered carbon sample. A significant fraction of sulfur was detected on the samples from the filter inlet and inlet 113. Surprisingly, no sodium was detected. as one might expect from the salt water environment. This indi-
Fig. 2. Scanning electron micrograph of bisected TEDA carbon granule showing 100 )*m by 100km beam analysis regions.
Fig. 3a. Backscattered electron image (BEI) ot an ASCTEDA carbon granule from the filter inlet following weathering.
ASCmicro-
3. RESULTS
J. ROSIN et al.
200
Fig. 3b. Backscattered electron image (BEI) of an unweathered AX-TEDA carbon granule.
cated
that ocean
spray did not contact
the carbon
bed. Figure 4 compares the XPS spectra of the copper 2p photoelectron region of the weathered carbon samples from the discrete locations within the filter. The spectrum corresponding to the unweathered sample is provided as a reference. Figure 4 shows the position of the copper 2p peak to be shifted to a higher binding energy for carbons from the inlet and inlet l/3 region of the filter relative to that of the unweathered sample. These peaks also appear broader than the peak corresponding to the unweathered sample. Note also the changes that occur
in the satellite shake-up region (about 939 to 949 eV) of the spectrum corresponding to the inlet sample as compared with that of the unweathered carbon. Some similar (but more subtle) changes in the shake-up region are detected in the spectrum of the inlet l/3 sample. The copper 2p spectra of the weathered samples from the center l/3 and outlet l/3 appear identical to that of the unweathered sample. Figure 5 compares the XPS spectra of the chromium 2p photoelectron region of the weathered carbon samples from the discrete filter locations. The spectrum corresponding to the unweathered carbon sample is shown for comparison. Note from this figure that the chromium peaks in all the weathered samples are shifted to a lower binding energy compared to those of the unweathered sample. However, spectra recorded for the center l/3 and outlet l/3 samples display strong shoulders at approximately 578.5 eV, consistent with the position of the 2p3,*peak of the unweathered sample. Figure 6 compares the copper 2p photoelectron region of the inlet sample to that of BPL carbon impregnated with copper sulfate. As the figure indicates, the positions and shapes of all peaks associated with the spectrum of the inlet sample are in excellent agreement with those of the impregnated BPL sample. Figure 7 compares the chromium 2p photoelectron region of the inlet sample to that of BPL impregnated with chromium sulfate. The figure indicates the positions of the chromium 2p photo-
2p3/2
w OUTLET
l/3
UNWEATHERED
I \ 974
964
954
BINDING
944 ENERGY
934
924
(eV)
spectra of the copper 2p photelectron region for unweathered and weathered carbon samples.
Fig. 4. XPS
595
590
565
BINDING
580
575
ENERGY
Unweathered
570
565
(eV)
Fig. 5. XPS spectra of the chromium 2p photoelectron ^ region tor unweathered
and weathered carbon samples.
Effects of weathering on AK-whetlerite
201
2% Cr,lS0,~,~12H*0
974
954
944
BINDING
ENERGY
964
934
on
BPL
924
(eV)
176
Fig. 6. XPS spectra of the copper 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 8% copper sulfate.
electron peaks of the inlet sample
to be consistent with those of the reference sample (BPL impregnated with chromium sulfate). The sulfur 2p pho-
toelectron region comparing the inlet cabon sample to BPL impregnated with copper sulfate and BPL impregnated with chromium sulfate is illustrated in Fig. 8. From the spectrum corresponding to the inlet sample, note that only one sulfur peak is present. The position of this peak is in agreement with that of the sulfur associated with the impregnated BPL carbons. Figure 9 reports the ratio of copper to silicon-plus-
174
170
BINDING
166
ENERGY
162
156
(eV)
Fig. 8. XPS spectra of the sulfur 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 8% copper sulfate and BPL impregnated with 2% chromium sulfate.
(Cu/[Si + Al]) at the different analysis areas (see Fig. 2) within the bisected carbon granules from the discrete locations within the filter. Data corresponding to the unweathered carbon granule are reported for comparison. Data for the inlet sample are not presented in Fig. 9, as virtually no copper was detected within the granule. Figure 10 reports the ratio of chromium to silicon-plus-aluminum at the different analysis areas within the bisected carbon granules from the discrete locations within the filter. Data corresponding to the unweathered carbon granule are shown for comparison. Data for the inlet sample are not reported, as virtually no chromium were detected within the granule. The cyanogen chloride breakthrough times for the unweathered and weathered carbon samples are reported in Table 1. No cyanogen chloride data were obtained from the inlet sample, as there was not enough sample available to perform the test. aluminum
4. DISCUSSlON
I
595
I
I
I
I
590
565
560
575
BINDING
ENERGY
I
570
I
565
(eV)
Fig. 7. XPS spectra of the chromium 2p photoelectron region comparing weathered carbon from the filter inlet to BPL impregnated with 2% chromium sulfate.
Freshly prepared ASC whetlerite is believed to contain copper-ammonium-chromium complexes[3]. These complexes possess chromium in the + 6 oxidation state and are postulated to be responsible for the destruction of cyanogen chloride and hydrogen cyanide(2-51. Changes in the impregnant speciation are expected to reduce the reactive properties of the whetlerite. It is evident from the XPS spectra presented in Figs. 4 and 5 that significant changes in the copper and chromium speciation have occurred as a result of marine weathering for samples
J. ROSSINet al.
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8
\-
2 1 O0
I
1
Fig. 9. Relative copper ~ncentrations
I
I
I
I
I
2
3
4
5
6
POSITION
as a function of position within the bisected carbon granules corresponding weathered and unweathered samples.
removed from the inlet region of the filter. Figure 4 compares the XPS spectra of the copper 2p photoelectron peaks of the weathered carbon samples to that of the unweathered carbon sample. The position of the copper 2p photoelectron peaks corresponding to the unweathered carbon sample is indicative of copper in the +2 oxidation state and is
q
7 to
similar to what has been reported elsewhere[4,6,7]. Note from Fig. 4 that the binding energy of the copper 2p3,2 photoelectron peak corresponding to the inlet sample has increased about 1.3 eV to 935.1 eV. Also note the change in the satellite shake-up peaks. The changes in the shake-up region are important, as these peaks provide information regard-
0.9
+
z
yO.6 s
1
2
3 4 POSITION
5
6
7
Fig. 10. Relative chromium concentration as a function of position within the bisected carbon granules correspanding to weathered and unweathered samples.
Effects of weathering on ASC-whetlerite Table 1. Cyanogen chloride breakthrough time for ASC-TEDA samples from discrete filter locations Cyanogen chloride breakthrough time (min)
Sample
c5.0 24.2 24.6 37.8
Inlet Inlet 113 Center l/3 Outlet 113 Unweathered ing the chemical
states
and speciation
of paramag-
netic compounds[8]. The changes in the copper XPS spectra suggest that the majority of the copper has been transformed to copper sulfate. A comparison is made between the copper 2p spectra of weathered ASC-TEDA and BPL impregnated with copper sulfate (see Fig. 6). Note that the two spectra are quite similar. suggesting that most, if not all, the copper associated with the inlet sample was transformed to copper sulfate during the weathering process. Further evidence for the formation of copper sulfate is reported in Fig. 8. Data presented in this figure show that the position of the sulfur peak is consistent with that of copper sulfate. The sulfur 2p photoelectron spectrum of the inlet sample is interesting in that only one sulfur peak is present. This suggests that additional sulfur complexes (such as metal sulfides and adsorbed sulfuric acid) are not present in any significant quantity, The copper 2p photoelectron spectrum of the inlet 113 sample is different from that of the unweathered sample. Note that the position of the 2p,.? peak is shifted to a slightly higher binding energy, and that the satellite shake-up region displays a different shape. A small fraction of sulfur was detected during the analysis of this sample. It is likely that the changes in the shape and positions of the peaks are due to the transformation of a portion of the original copper(H) species to copper sulfate. The spectra corresponding to the center 113 and outlet 113 samples are very similar to that of the unweathered sample. For these samples. copper has remained in the +2 oxidation state. This is important, as copper(U) is reported to have a role in the removal of hydrogen cyanidej21. Virtually no sulfur or any other foreign elements were detected upon analysis of the center 113 and outlet l/3 samples. Evidently, the environmental exposure did not alter the oxidation state of copper past the inlet 113 of the filter. Analysis of the chromium XPS data presented in Fig. 5 reveals that changes in the chromium oxidation state(s) have occurred and that these changes are dependent on the carbon location within the filter. For the unweathered sample, the position of the chromium 2pv, photoelectron peak at 578.5 eV is due to the presence of chromium in the + 6 oxidation state[4,6,7]. The shoulder present at about 577 eV is indicative of chromium(II1) complexes. For this sample, approximately 60% of the chromium was determined to be in the +6 oxidation state. The
203
fraction of chromium(V1) associated with a whetlerite sample is important, as chromium(V1) complexes are believed to be responsible for the destruction of cyanogen chloride[3-51 and hydrogen cyanide[2]. Figure 5 indicates that only a small amount of chromium, if any, is present in the +6 oxidation state for the inlet carbon sample. The position of this peak is indicative of chromium in the +3 oxidation state and is consistent with that of chromium sulfate, as shown in Fig. 7. These data suggest that the weathering process has changed the majority of the chromium (whether in the + 6 or + 3 oxidation state) associated with the inlet carbon sample to chromium sulfate. The chromium spectrum corresponding to the inlet 113 sample is difficult to interpret. A small amount of sulfur was detected upon analysis of this sample. The position of the chromium 2pi.? peak suggests that both chromium(II1) complexes, chromium sulfate, and chromium(VI) complexes may be associated with the sample. However. the fraction of chromium(V1) associated with this carbon sample appears to be small. based on the intensity of the signal present at about 578.5 eV. The chromium spectra corresponding to the center 113 and outlet 113 samples appear similar. For both samples, the position of the 2p?.? peak has shifted approximately 1.2 eV down field; however. a very strong shoulder exists at 578.5 eV. This shoulder is attributed to chromium in the + 6 oxidation state, and the shifting of the chromium peak is due to the reduction of a portion of the chromium(V1) to chromium(II1). For both the center l/3 and outlet 113 carbon samples. approximately 40% of the chrdhum was determined to be in the + 6 oxidation state. This value represents a 33% reduction in the fraction of chromium(V1) compared to the unweathered sample. Brown et a/.[21 have shown that exposure of impregnated carbons to water vapor results in a partial reduction of chromium(V1). The partial reduction of chromium(V1) as observed for the center li3 and outlet 113 samples may be due to exposure to humidity in the shipboard environment. The atomic ratio of sulfur to copper-plus-1.5-timeschromium (S/[Cu + 1.5 Cr]), as determined by XPS. provides an indication of the extent of metal sulfate formation. For the inlet sample, this ratio was determined to be 1.03, suggesting that virtually all the copper and chromium associated with the inlet sample had been transformed to metal sulfates. For the inlet li3 sample, the atomic ratio of sulfur to copperplus-1.5-times-chromium was determined to be 0.20. This indicates that approximately 20% of the impregnants had been transformed to metal sulfates. Virtually no sulfur was detected upon analysis of the center 113 and outlet 113 samples. For the inlet and inlet 113 samples. the formation of copper and chromium sulfate was likely the result of contact with airborne sulfates. The source of the airborne sulfates has not been determined, but acid rain and/or SO, from diesel exhaust gases are likely sources. The cyanogen chloride reactivity of impregnated
204
J. ROSIN et al.
carbons is reported to be proportional to the chromium(V1) fraction of the sample[3-51. XPS analyses suggested only a small quantitiy of chromium(V1) was associated with the inlet l/3 sample. It was therefore expected that this sample would provide little cyanogen chloride protection. These results were indeed observed and are reported in Table 1. The center l/3 and outlet l/3 samples possessed similar fractions of chromium(VI), and, as expected, similar cyanogen chloride breakthrough times were recorded. The fraction of chromium(V1) associated with these samples was approximately two-thirds that of the unweathered sample (40% versus 60%). It is interesting to note that the cyanogen chloride breakthrough times recorded for these samples were also approximately two-thirds less than that recorded for the unweathered sample (24.6 minutes versus 36.8 minutes). These data appear to confirm the role of the chromium(V1) species in the destruction of cyanogen chloride and support previous literature observations. Based on the cyanogen chloride reactivity of the discrete sections of this carbon filter, the reactive properties of this weathered carbon filter are expected to be less than 50% of the original design. Figure 3a shows the inlet carbon granule to be encrusted with a metallic overlayer. This overlayer was found to consist primarily of copper sulfate, based on EDS microbeam analysis. This overlayer was not observed on the unweathered carbon granule (Fig. 3b) nor on the carbon granules located past the inlet l/3 of the filter. EDS analysis of a bisected carbon granule (inlet sample) revealed virtually no copper or chromium present within the granule. Wet chemical analyses were performed on 8-10 g portions of each carbon sample to determine whether impregnants were lost during the weathering process. To within experimental error, no loss of copper, chromium, or silver occurred during the weathering process. It is likely that the metallic overlayer was formed by the leaching of impregnants to the external surface of the granule. Migration of impregnants to the external surface will result in a loss of dispersion, thereby reducing the reactive properties. EDS analyses of the weathered and unweathered bisected carbon granules were performed for the purpose of establishing the effects of weathering on impregnant migration. Comparing the radial impregnant distribution of the weathered granules to that of the unweathered granules will allow one to determine whether the impregnants have migrated to the external surface of the granule. In an effort to normalize and better present the data, the ratios of copper and chromium to silicon-plusaluminum are reported. It should also be noted that the numerical value of these ratios varies from sample to sample. This may be the result of the silicon and aluminum fraction being different in each granule analyzed and/or the fraction of copper and chromium varying from granule to granule. Therefore, the discussion is focused on trends in the distribution of impregnants within the individual granules.
Figure 9 illustrates the copper distribution as a function of position within the different bisected carbon granules. For the unweathered carbon, the concentration of copper at the outer analysis area (position 1) is greater than that found further into the granule and is consistent with previous reports[6]. Further into the granule, the copper distribution appears uniform. For the inlet l/3 sample, the relative concentration at the outer regions of the granule is greater than that of the unweathered granule. These data suggest that a portion of the copper has begun to migrate towards the external surface of the granule. The copper concentration at the outer analysis areas for the center l/3 and outlet l/3 samples was less than that observed for the unweathered sample. However, just as in the case of the unweathered sample, the copper concentration is nearly uniform at all internal analyses areas. These data suggest that migration of the impregnants to the external surface of the center l/3 and outlet l/3 samples did not occur. The reason for the decrease in the concentration gradient near the external surface of the granule is not clear. The chromium distribution within the weathered and unweathered carbon granules is illustrated in Fig. 10. For the unweathered sample, the concentration of chromium is greatest at the external surface of the granule (position 1) and declines to a near constant value further into the granule. The gradient is similar to that observed for copper, but not quite as steep. Virtually no chromium was detected at the internal analysis areas for the inlet sample. Therefore these data are not reported in Fig. 10. The chromium distribution for the inlet l/3 and center l/3 granules are similar to that of the unweathered sample; neither sample shows evidence of chromium migration. The fact that the inlet l/3 sample showed evidence of copper migration suggests that copper is more readily leached from the sample than chromium. The outlet l/3 sample shows an increase in the concentration of chromium near the external surface of the granule (moving towards the center of the granule). This is in contrast to what was observed for the unweathered sample but consistent with the copper distribution reported in Fig. 9. Migration of the impregnants was accompanied by the formation of copper and chromium sulfates. For the inlet sample, virtually all the impregnants were present as metal sulfates and were found to be located almost exclusively at the external surface of the granule. For the inlet l/3 sample, only a portion of the impregnants had been transformed to metal sulfates. EDS analyses confirmed the migration of copper, but chromium apparently remained in place. For the center l/3 and outlet l/3 samples, no metal sulfates were detected, and migration of copper and chromium to the external surface of the granule did not occur. A possible explanation for the migration of impregnants to the external surface of the granule as observed for the inlet sample follows. Copper and chromium sulfates may be formed by treating
205
Effects of weathering on AX-whetlerite ASC-TEDA with dilute suluric acid. For the case at hand, the environmental sulfuric acid may be supplied by either acid rain or SO, from diesel exhaust. When the acid contacts the impregnants, copper and chromium sulfates are formed. This is consistent with the observed speciation. Copper and chromium sulfates are soluble in water, and their solubility increases as the solution pH is decreased. It is likely that the filter cycles through periods of changing humidity. As the humidity is decreased, a portion of the water and the soluble compounds migrates through the pores to the external surface of the granule, where the water evaporates. This results in the soluble sulfates accumulating at the external surface of the granule. Increasing the humidity will result in refilling the pores with water, which brings in more acids from which the metal sulfates are formed. Repeating this cycle a number of times over the age of the filter would likely remove a large fraction of the impregnants located within the granule and deposit them onto its external surface.
5. CONCLUSIONS
The inlet portion of the carbon filter was severely affected by prolonged environmental exposure. For the shipboard environment encountered, sulfates appear to be the only contaminant detected which affected the reactive properties of the carbon. Airborne sulfates lead to the formation of metal sulfates, which ultimately leach from within the carbon granules to the external surface. Past the inlet l/3 of the bed, virtually no trace of sulfur was present; how-
ever, a significant fraction of chromium(V1) was reduced to chromium(II1) species (about one-third). Past the inlet third of the bed, the degree of chromium reduction was independent of the position within the filter. Standardized cyanogen chloride breakthrough times of the different filter fractions suggest that the environmental exposure reduced the filter performance to 50% of its initial value. Acknowledgemenls-One of us (J. A. R.) wishes to thank the National Research Council for financial support on this project. R. A. L and L. I. thank NAVSEA for financial support. The authors extend thanks to Mr. R. Grue for providing the CK data and to Mr. R. Herd for performing the wet chemical analyses. REFERENCES
1. V. R. Deitz, R. J. Puhala, D. B. Stroup, and G. F. Dickey, Influence of Atmospheric Weathering on the Performance of Whetlerite, Naval Research Laboratory Memo. Report No. 4752, Washington DC (1982). 2. P. N. Brown, G. G. Jayson, G. Thompson, and M. C. Wilkinson, Carbon 27, 821 (1989). 3. L. L. Pytlewski, Studies of ASC Whetlerite Reactivity, Report ARCSL-CR-79008, U.S. Army Chemical Systen& Laboratory, Aberdeen Proving Grbund. MD. NOV. 1979. UNCLASSIFIED Reoort. 4. M. M. Ross, R. J. Colton. ahd V. R. Deitz, Carbon 27. 426 (1989).
5. P. N. Krishnan, S. A. Katz, A. Birenzvige, and H. Salem. Carbon 26. 914 (1988). 6. J. A. Rossin, Carbon 2;. 61i (1989). 7. J. L. Hammarstrom and A. Sacco, J. Catal. 112, 267 (1988). 8. C. D. Wagner, W. M. Riggs, L. E. Davis. J. F. Moulder, and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corp., Edin Prairie. MN (1979).