Structural characteristics of modified activated carbons and adsorption of explosives

Structural characteristics of modified activated carbons and adsorption of explosives

Journal of Colloid and Interface Science 266 (2003) 388–402 www.elsevier.com/locate/jcis Structural characteristics of modified activated carbons and...

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Journal of Colloid and Interface Science 266 (2003) 388–402 www.elsevier.com/locate/jcis

Structural characteristics of modified activated carbons and adsorption of explosives W. Tomaszewski,a V.M. Gun’ko,b J. Skubiszewska-Zi˛eba,a and R. Leboda a,∗ a Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland b Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine

Received 15 August 2002; accepted 13 June 2003

Abstract Several series of activated carbons prepared by catalytic and noncatalytic gasification and subsequent deposition of pyrocarbon by pyrolysis of methylene chloride or n-amyl alcohol were studied by FTIR, chromatography, and adsorption methods using nitrogen and probe organics (explosives). The relationships between the textural characteristics of carbon samples and the recovery rates (η) of explosives on solidphase extraction (SPE) using different solvents for their elution after adsorption were analyzed using experimental and quantum chemical calculation results. The η values for nitrate esters, cyclic nitroamines, and nitroaromatics only partially correlate with different adsorbent parameters (characterizing microporosity, mesoporosity, pore size distributions, etc.), polarity of eluting solvents, or characteristics of probe molecules, since there are many factors strongly affecting the recovery rates. Some of the synthesized carbons provide higher η values than those for such commercial adsorbents as Hypercarb and Envicarb.  2003 Elsevier Inc. All rights reserved. Keywords: Activated carbon; FTIR; Nitrogen adsorption; Nitrate esters; Cyclic nitroamines; Nitroaromatics adsorption; SPE method; Microporosity; Mesoporosity; Surface properties

1. Introduction Activated carbons utilized as adsorbents for different purposes [1–5] can be modified to change the texture and surface composition enhancing their efficiency for such specific applications as solid-phase extraction (SPE) of trace amounts of organic or inorganic compounds from different media. Clearly the adsorptive properties depending on the specific surface area, pore size distribution, type and concentration of active surface sites, etc. [1,2] strongly affect the recovery rates (η) on SPE [3]. For clean-up and enrichment of traces of nitro compounds and their degradation products, different adsorbents can be applied, e.g., silica [6], modified silica [7,8], resins [9–11], or carbonaceous materials [8,12–14]. For charcoal, recovery rates close to 70% were obtained for ethylene glycol dinitrate, trinitroglycerin and pentaerythritol tetranitrate on elution with * Corresponding author.

E-mail address: [email protected] (R. Leboda). 0021-9797/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00633-7

methanol [8,9,12], isopropyl alcohol [15] or methylene chloride [16]; however, low η values were found for nitroaromatics [9]. Previously it was shown [14] that oxidation, reduction, catalytic activation, and graphitization of microporous carbons affect the recovery rates on SPE of nitrate esters, cyclic nitroamines, and nitroaromatics differently. For instance, carbon graphitization enhances the η values for some nitroaromatics by several times but reduces η for nonaromatic compounds. For the latter, mild oxidation or reduction of carbons changes the η values by only 10–20%. For nitrate esters and cyclic nitroamines, η increases with increasing microporosity of carbons, but for nitroaromatics, there is the opposite tendency [14]. Despite published investigations, the influence of many of the mentioned factors on the effectiveness of adsorbents in SPE is inadequately elucidated in the literature. Therefore the aim of this work was to study the influence of different activations of carbons on changes in their structural characteristics and efficiency in concentration and SPE recovery rates of a variety of explosives depending on used solvents and eluents.

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2. Experimental 2.1. Materials Commercial activated carbon A2PS (HPSD, Hajnówka, Poland) prepared from plum stones carbonized at 823– 873 K, activated at 1173–1273 K, and demineralized (deashed) with a mixture of HCl and methanol [17,18] was utilized as the initial material (granule fraction of 0.1– 0.2 mm), which was also modified by catalytic gasification and deposition of pyrocarbon. The catalytic activation of A2PS was carried out as described previously [19] utilizing impregnation of the carbon by solution of 0.25% Ca(II) and subsequent activation at 1073 K (sample labeled A2PSM). Then pyrocarbon was deposited on A2PS and A2PSM by carbonization of methylene chloride and n-amyl alcohol (pentanol-1). Methylene chloride was pyrolyzed in a rotary reactor with a nitrogen flow at 773 K [20] for 15 min (A2PS→A2PSC at the pyrocarbon amount CC = 22 wt%) and 30 min (A2PSM→A2PSMC at CC = 40 wt%). Additional pyrocarbon was deposited on A2PSC (A2PSCA) and A2PSMC (A2PSMCA) by pyrolysis of n-amyl alcohol (5 cm3 per 1.5 g of a carbon adsorbent) in an autoclave at 773 K for 6 h. Samples cooled to room temperature were washed with dimethylformamide, then with acetone and finally were dried at 423 K. A2PSCA and A2PSMCA contained an additional 11 and 21 wt% of pyrocarbon deposits, respectively; i.e., total CC was 33 (A2PSCA) and 61 wt% (A2PSMCA). Coke (irregular-shaped granules of 0.20–0.32 mm) (WDDW, Hajnówka, Poland) of natural origin washed with HCl and methanol was used as the second initial material. The first coke series was prepared by water steam gasification for 2, 3, 6, and 8 h (burnoff was 9, 14, 32, and 38% respectively) labeled coke-2h, -3h, -6h, and -8h. The second series with deposited Ca(II) (3%) on the initial coke was activated by water vapor for 1, 4, and 8 h (burnoff was 53, 63, and 74% respectively), labeled coke-1h/Ca, -4h/Ca, and -8h/Ca. The third coke series was prepared by modification of the latter samples by pyrocarbon (pyrolysis of CH2 Cl2 for 15 min) at CC = 16, 17, and 29 wt% (labeled coke-1h/Ca/C, -4h/Ca/C, and -8h/Ca/C). Additionally, two commercial SPE adsorbents, porous graphitic carbon Hypercarb (ThermoHypersil, UK, particle diameter of 0.03–0.04 mm, SBET = 94 m2 /g, Vp = 0.565 cm3 /g) and graphitized carbon black Envicarb (Supelco, USA, particles of 0.04–0.06 mm, SBET = 98 m2 /g, Vp = 0.447 cm3 /g) were used to compare their adsorptive properties in SPE with those of synthesized carbons. Three groups of explosives were chosen as probe compounds to study the efficiency of the concentration and recovery process using prepared carbons: (i) nitrate esters: nitroglycol (ethylene glycol dinitrate, EGDN), nitroglycerine (glycerol trinitrate, NG), and pentaerythritol tetranitrate (PETN); (ii) cyclic nitroamines: octogen (1,3,5,7tetranitro-1,3,5,7-tetrazacyclooctane, HMX) and hexogen

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(1,3,5-trinitro-1,3,5-triazacyclohexane, RDX); and (iii) nitroaromatics: 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), tetryl (2,4,6,N-tetranitro-N-methylaniline, TNMA), hexyl (2,2 ,4,4 ,6,6 -hexanitrodiphenylamine, HNDPA), and 2,2 ,4,4 ,6,6 -hexanitrodibenzyl (HNDB). All applied explosives were obtained as certified standards from Promochem and Institute of Organic Chemistry (Warsaw). Some properties of these organics with respect to SPE by activated carbons were described previously [13,14]. The HPLC grade solvents water, dimethylformamide (DMFA), tert-butyl methyl ether, dimethyl sulfoxide (LabScan, Ireland), acetonitrile, isopropyl alcohol, tetrahydrofuran, methanol, ethyl acetate, and chloroform (Merck) were used in SPE experiments. 2.2. SPE procedure Standard stock solutions of explosives were prepared in acetonitrile at a concentration of 1 mg/10 ml, except nitrate esters at a concentration of 5 mg/10 ml. The sample solutions were stored at 4 ◦ C to examine their stability (interesting for practical investigations of trace amounts of explosives), which was appropriate at least for a month. For recovery studies, aqueous samples were prepared daily and spiked with a described standard solution to get the concentration of 25 µg/100 ml for nitroamines and nitroaromatics but 125 µg/100 ml for nitrate esters. Acetonitrile of 5 vol% (0.48 M) was added to the aqueous samples for complete dilution of explosives. SPE cartridges contained the same amounts (100 mg) of adsorbents packed in 3-ml polyethylene tubes with porous polyethylene frits (J.T. Baker, Philipsburg). Prepared cartridges were washed out and conditioned before experiments. Carbon beds were rinsed with 6 ml of DMFA to wash out impurities and then with 20 ml of water. The SPE experiments with the aqueous solutions of explosives using modified carbons and HPLC quantification were described in detail elsewhere [14]. The 100-ml samples were percolated through the cartridges at a flow rate of 10 ml/min using a vacuum manifold. The residual water was removed by nitrogen aspiration for 10 min and elution was performed with 6 ml of DMFA in a 10-ml graduated flask. Water was added to obtain a final eluate volume of 10 ml. When elution with another solvent (tert-butyl methyl ether, dimethylsulfoxide, acetonitrile, tetrahydrofuran, methanol, ethyl acetate, or chloroform) was performed adequately modified procedure of sample preparation was applied. All obtained 6-ml eluates were evaporated to dryness under nitrogen at 45 ◦ C. The residues were finally reconstituted to a volume of 10 ml with 6 ml of DMFA and the necessary portion of water. 2.3. Chromatography HPLC analysis was performed using a LC-51 (Bruker) ternary gradient system with UV detection at 220 nm. The separation of analytes was carried out using an analytical

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BDS-Elite 5-µm 150 × 4 mm column (ThermoHypersil, UK) and a 10 × 4.6 mm precolumn slurry packed with LiChrospher RP-select B (Merck), all thermostated at 50 ◦ C. The injection volumes for eluates and standard solutions were 50 µl. A linear binary gradient elution with isopropyl alcohol containing 5% (v/v) of acetonitryle and water was used for the separation of the studied explosives. The elution started from 5% (v/v) of isopropyl alcohol with a flow rate of 1.5 ml/min and reached 50% (v/v) of organic solvent in 8 min. The SPE recovery rates η were calculated by an external standard method from the peak heights. The calibration curves were obtained by linear regression in the range 0.25–30 µg/10 ml for nitroamines and nitroaromatics but 1.25–150 µg/10 ml for nitrate esters. The correlation coefficients for studied explosives obtained from five points were from 0.97 to 0.99. The mean values of the recovery rates were evaluated from six SPE measurements and mean errors were less than ±5%. Isosteric heat of adsorption (qis0 ) of benzene, n-hexane, and chloroform as testing adsorbates was calculated from temperature dependence of their zero retention volume. The chromatographic measurements were carried out by means of a GC-6000 Vega 2 (Carlo Erba Instr.) gas chromatograph (with 70 cm × 2.5 mm glass columns) equipped with a thermoconductivity detector (TCD) at 383–563 K. He purified by molecular sieves was used as a carrier gas (30 cm3 /min). 2.4. Nitrogen adsorption–desorption and structural parameters The nitrogen adsorption–desorption isotherms (shown as standard in Fig. 1 and in the form of αS plots in Fig. 2) were recorded at 77.4 K using a Micromeritics ASAP 2405N analyzer. The specific surface area SBET [21–23], pore volume Vp (estimated at p/p0 ≈ 0.98, where p and p0 denote the equilibrium and saturation pressures of nitrogen, respectively), average pore half-width Xp and other parameters listed in Table 1 were determined on the basis of the nitrogen adsorption–desorption data. The micropore parameters (Table 1, VDS , SDS , micropore half-width xDS , and distribution dispersion δDS ) were estimated using the modified (introducing an additional variable parameter n) Dubinin–Stoeckli (DS) equation [24],     2 2 n  µxDS A xDS VDS 1 + erf , exp − a= √ 2DV0 D2 δDS D 2 (1) where V0 is the molar volume; xDS is the pore half-width at a maximum of the distribution; δDS is the distribution dispersion; A = RT ln(p0 /p) is the differential molar work equal, with inverse sign, to changes in the Gibbs free energy; D =  z −t 2 2 A2 )0.5 ; µ = (βk)−2 ; erf(z) = √2 e dt ; k = (1 + 2µδDS π 0 10–12 kJ nm/mol; and βN2 = 0.33. The VDS and SDS values were determined over the range of the pore half-width 0.2–1.0 nm. The Dubinin–Astakhov equation [25–28] was

Fig. 1. Nitrogen adsorption–desorption isotherms for (a) A2PS-x, (b) noncatalytically activated coke, and (c) catalytically activated coke and covered by pyrocarbon.

also used to calculate the specific surface area SDA of micropores. Notice that the value of the specific surface area of mesopores (SK ) calculated using the Kiselev method [29] was utilized to calculate corrected parameters (DS and DA) of micropores. The specific surface area of mesopores (Table 1, Smes ) was also calculated using the αS plot method with Carbopack F (graphitized carbon black) as a reference adsorbent [30]. It should be noted that the application of the BET equation to microporous active carbons characterized by the first type of nitrogen adsorption isotherm is questionable; however, application of this equation is acceptable if the constant cBET < 450 [23]. All the studied carbons are characterized by a significant (major for some samples) contribution of mesopores (Table 1, Smes ). Therefore, the cBET value criterion is satisfied (<450) for series with A2PS-x (excepting the initial sample A2PS at cBET = 798, as its isotherm is close to type I (Fig. 1a)) and for many coke-x samples. Therefore, not only SBET but also S! = Smes + SDA values are shown in Table 1 to characterize the total specific sur-

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Fig. 2. The αS plots with reduced adsorption divided by (a, c, e) V0.4 (i.e., pore volume filled at p/p0 = 0.4) and (b, d, f) SBET for (a, b) A2PS-x, samples of (c, d) noncatalytically activated coke, and (e, f) catalytically activated coke and covered by pyrocarbon.

face area of carbons. As a whole, the S! and SBET values are closely related and their changes on carbon modifications are concordant. Consequently, one can assume that SBET (as a conventional parameter) can be used for characterization of structural features of all the studied carbons. The pore size distributions f (x) were calculated using the overall isotherm equation [31,32] described in detail elsewhere [13,14,33–37]. Desorption data (overall isotherms) were utilized to compute the f (x) distributions

using regularization procedure [38] under a nonnegativity condition (f (x)  0 at any x) with a fixed regularization parameter α = 0.01 using a model of slit-like pores and renormalized (for transition to the pore volume increment versus pore half-width) in comparison with the PSD computed according to the literature [33,34], fnew (x) = 

xf (x)Vp . xf (x) dx

(2)

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Table 1 Structural characteristics of synthesized carbon adsorbents No

Adsorbent

SBET

S!

Smes

SDS

Vp

VDS

Xp

xDS

δDS

CC,pyrocarb

(m2 /g)

(m2 /g)

(m2 /g)

(m2 /g)

(cm3 /g)

(cm3 /g)

(nm)

(nm)

(nm)

(wt%)

1 2 3

A2PS A2PSC A2PSCA

1250 49 3

1216 63 3

698 55 3

746 2 0

0.713 0.073 0.007

0.548 0.014 0.001

0.57 1.48 2.90

0.69 1.94 2.17

0.34 0.78 1.85

0 22 33

4 5 6

A2PSM A2PSMC A2PSMCA

1501 429 47

1582 431 59

1139 320 55

625 137 1

1.318 0.493 0.127

0.716 0.173 0.007

0.88 1.15 2.72

0.92 0.98 2.78

0.45 0.39 1.31

0 40 61

7 8 9 10 11

Coke Coke-2h Coke-3h Coke-6h Coke-8h

707 767 939 1140 1234

622 678 828 1032 1174

225 244 348 551 713

574 564 770 756 687

0.394 0.418 0.509 0.617 0.677

0.319 0.333 0.413 0.525 0.593

0.44 0.44 0.43 0.43 0.55

0.45 0.56 0.57 0.70 0.80

0.27 0.28 0.18 0.26 0.32

0 0 0 0 0

12 13 14

Coke-1h/Ca Coke-4h/Ca Coke-8h/Ca

609 669 923

526 584 855

291 306 567

284 331 432

0.584 0.638 0.975

0.267 0.273 0.386

0.76 0.76 1.06

0.55 0.63 0.81

0.56 0.45 0.36

0 0 0

15 16 17

Coke-1h/Ca/C Coke-4h/Ca/C Coke-8h/Ca/C

278 370 451

264 421 333

179 279 210

90 139 260

0.307 0.423 0.500

0.111 0.145 0.188

1.10 1.14 1.11

0.82 0.72 0.73

0.67 0.58 0.26

16 17 29

The renormalized PSDs are akin to those computed using the DFT Micromeritics software (this was tested for several microporous and microporous/mesoporous carbons). Additionally, the PSDs of mesopores were calculated using the Micromeritics software with the Barrett–Joyner–Halenda (BJH) method [40] utilizing desorption data. 2.5. FTIR spectroscopy The IR spectra of synthesized carbons were recorded using a FTIR1725× (Perkin–Elmer) spectrophotometer. The measurements were carried out over the range 4000– 400 cm−1 . Carbon samples (0.33 wt%) were stirred with dry KBr (Merck, spectroscopy grade) and then pressed to form appropriate tablets. This technique was used since diffuse reflection FTIR spectra of pure carbon samples yielded little information. 2.6. Quantum chemical calculations The solvation free energy (#Gs ) and geometrical and electronic parameters of explosive molecules were calculated using a solvation model, SM5.42/HF/6-31G(d) and SM5.42R/DFT/6-31G(d)//SM5.42/HF/6-31G(d) (program package GAMESOL, Version 3.1, and its nonstandard modification to use DFT with the combined B3LYP exchange and correlation functional) assuming infinite dilution [41,42]. In the case of the DFT calculations, the geometry was optimized by SM5.42/HF/6-31G(d) and the free energy of solvation was calculated by SM5.42R/B3LYP/6-31G(d). The #Gs values were calculated at the gas-phase geometry (SM5.42R) or with consideration for the geometry relax-

ation in the liquid phase (SM5.42). In the case of SM5.42R #Gs = #GEP + GCDS

(3)

where #GEP = #EE + GP ,  Ak σk , GCDS =

(4) (5)

k

#GEP (determined by self-consistent reaction field calculation) is the bulk electrostatic component of the solvation free energy, GP is the polarization energy, #EE is the distortion energy, Ak is the exposed surface area of atom k, and σk is the atomic surface tension of atom k (function of the solute’s 3D geometry and a set of solvent descriptors). The solvation free energy in the case of SM5.42 can be determined as #Gs = EE,l − EE,g + GP,l + GCDS,l ,

(6)

where the subscripts l and g correspond to liquid and gas phases, respectively. These computation methods were described in detail elsewhere [41,42].

3. Results and discussion Different types of modification of carbons result in significant changes in their structural characteristics (Table 1 and Figs. 1–4). In the case of the A2PS-x series, only A2PSM (activated with Ca(II)) possesses more developed porosity (Vp and VDS ) and specific surface area (SBET , S! , and SDS ) than initial A2PS. However, A2PS (nitrogen adsorption isotherm is close to type I (Fig. 1a)) has narrower pores (Xp and xDS ) at a minimal dispersion (δDS ) than other A2PS-x samples; and contribution of micropores is maximal

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Fig. 3. Ratios between (a) micropore (VDS ), mesopore (Vp − VDS ), and total pore volumes and between (b) micropore (SDS ), mesopore (SK ), and total specific surface areas for A2PS-x.

for A2PS (Fig. 3). Samples A2PSC, A2PSCA, A2PSMC, and A2PSMCA modified by pyrocarbon deposits (CVD-C) are characterized by significantly diminished porosity and accessible surface area. However, catalytic modification of A2PS (enhanced mesoporosity) before pyrocarbon deposition leads to diminution of pore blocking by CVD-C in A2PSMC and A2PSMCA in comparison with that for A2PSC and A2PSCA, respectively, despite twice as much CC (Table 1, Figs. 1–3). Modified coke-x samples, with one exception (coke1h/Ca/C), possess larger pore volume Vp , and for half of them, SBET is larger than that of the initial coke (Table 1). Noncatalytically-activated samples are characterized by significant contribution of micropores (Table 1, SDS , VDS , Xp , and xDS ). The nitrogen adsorption–desorption isotherms (with the same shape) rise higher and higher with increasing activation time (Fig. 1b). Increase in burnoff with activation time leads to nearly linear enhancement of the specific surface area, the total porosity (Fig. 4a), the volume of micropores VDS (Fig. 4b), and the specific surface area of mesopores SK (Fig. 4c), SBET − SDS (Fig. 4d), and Smes (Table 1). However, the specific surface area of micropores SDS (Fig. 4b) and the volume of mesopores (Vp − VDS ) (Fig. 4d) change nonlinearly. Catalytic activation of the initial coke

393

enhances SBET only for a sample activated for 8 h, since the contribution of micropores decreases for coke-nh/Ca, but the total pore volumes Vp and Smes increase for this series. After pyrocarbon deposition on the latter, the porosity and the specific surface area, especially contribution of micropores, decrease, but Xp increases, as well as xDS and δDS (excepting coke-8h/Ca/C); i.e., narrow pores become more nonuniform. Notice that the porosity of coke-nh/Ca/C samples is greater than that of A2PS-x after pyrocarbon grafting, as for the latter the CC values were greater (Table 1). On the whole, activation of samples using the calcium catalyst enhances the contribution of mesopores, and the shape of the nitrogen adsorption–desorption isotherms (possessing a larger hysteresis loop, isotherm slope, and adsorption at p/p0 > 0.4) gives a pictorial view (Figs. 1 and 2) of this effect. Deposition of pyrocarbon on the carbon surfaces leads to the opposite effect, reducing the porosity and the specific surface area, due to filling and blocking of matrix pores [43]. Therefore the nitrogen adsorption– desorption isotherms drop and the hysteresis loops are unclosed (Fig. 1); i.e., pores have complicated structures, e.g., with narrow entrances (bottlelike pores). Similarity in the isotherm shape for modified coke samples (Fig. 1b or 1c) corresponds to similarity of their reduced (V /V0.4) αS plots (Fig. 2c or 2e). In the case of A2PS-x samples, the reduced (V /V0.4) αS plots (Fig. 2a) differ due to reduction of the total porosity and diminution of contribution of the microporosity, and these plots demonstrate a significant increase in relative adsorption (i.e., V /V0.4 1) at p/p0 > 0.4. Notice that the αS plots reduced by dividing by SBET (Fig. 2b) are akin to those shown in Fig. 2a (due to enhancement of contribution of broad pores for samples with smaller SBET ). For the first series of modified coke samples, the differences between the αS plots reduced by dividing by SBET (Fig. 2d) are greater (excepting the initial coke sample) than those for the αS plots reduced by dividing by V0.4 (Fig. 2c), and the curve order differs. However, these differences are smaller than those for A2PS-x. For other modified coke samples (Figs. 2e and 2f), two types of the αS plots give close results, since relative contributions of micro- and mesopores to the total porosity and the specific surface area change concordantly (as the shape of the nitrogen isotherms is nearly the same (Figs. 1b and 1c)). Changes in the texture of carbon samples are clearly seen as changes in contribution of mesopores and micropores (Figs. 3–6). For A2PS-x samples, modification leads to enhancement of mesopore contribution, as well as catalytic activation of the coke (Table 1), which results in growth of the pore (Xp ) or micropore (xDS) half-width. Changes in the structural parameters (Table 1) correlate with the PSDs calculated using different methods (Figs. 5–7). As a whole, the PSDs calculated using an integral equation [33–37] with the regularization procedure [38,39] are akin to those calculated by the BJH method over the mesopore range. Modified A2PS-x samples depict significant contribution of mesopores at x > 1 nm; however, A2PSM, as well as initial

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Fig. 4. Pore parameters as functions of burnoff for noncatalytically activated coke for (a) overall characteristics SBET and Vp , (b) micropores SDS and VDS , and mesopores (c) SK , and (d) SBET –SDS and Vp –VDS .

A2PS, possesses significant microporosity (Fig. 5). The f(x) curves for A2PS-x have a complicated shape that corresponds to relatively irregular pore structure for samples with developed (Figs. 5a and 5b) or low (Figs. 5c and 5d) porosity. Notice that for the latter, contribution of large pores increases due to pyrocarbon grafting. Consequently, pyrocarbon particles have relatively large sizes, which are typical for hybrid pyrocarbon–mineral adsorbents [43–46]. Noncatalytically modified coke samples (in contrast to other studied carbons) demonstrate PSDs similar in their shapes (Figs. 6a and 6b) (as well as the isotherm shape in Fig. 1b) but differing in the intensity (akin to nitrogen adsorption), since the micropore and mesopore parameters depend on the activation time (Table 1) or burnoff (Fig. 4). A more complex picture is observed for catalytically activated or pyrocarbon-grafted coke samples (Figs. 6c, 6d, 6e, and 6f) because of changes in the pore structure with increasing contribution of both mesopores and micropores separately for these two series (Table 1). The summary effect of changes in meso- and micropore contributions is nonlinear and the structural parameters are nonlinear functions of the amount CC of grafted pyrocarbon. Notice that commercial Envicarb and Hypercarb, whose structural characteristics with respect to the SPE effectiveness were described in detail elsewhere [14], possess well-defined mesoporosity

(Fig. 7). Therefore, one can assume that activated carbons with developed mesoporosity and a marked contribution of the microporosity can be effective adsorbents for SPE of explosives; however, the chemical composition of the adsorbent surfaces can also play an important role due to specific interaction of adsorbed molecules with active (polar) surface sites. According to the FTIR spectra of the studied carbons (Figs. 8–11) and the literature [47–52], one can assume that they have surface functionalities with C=O (carboxylic, anhydride, lactone, and ketene groups having IR bands at 1750–1630 cm−1 ), C–O (lactonic, ether, phenol, etc., with a very intensive band at 1300–1000 cm−1 ), CH (3100–2800 cm−1 ), CC (1600–1450 cm−1 ), and CH (3070– 3030 cm−1 ) in aromatic groups. A low-intensity band at 2070–2040 cm−1 observed in the FTIR spectra of some samples (e.g., Figs. 9 and 10) can be assigned to twinned bonds (C=C=C, C=C=O, C=C=N). Aliphatic groups on all samples characterized by valence vibrations νCH at 2930 and 2850 cm−1 can be linked to both organic molecules adsorbed on samples and to characteristic functional groups on the active carbon surface. One can assume that there are only very low amounts of N-, P-, and S-containing functionalities [47–52] on the surfaces of prepared materials.

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395

Fig. 5. Pore size distributions for A2PS-x samples calculated using (a, c) regularization procedure applied to overall adsorption equation and (b, d) the BJH method (only for mesopores).

An intensive band at 3700–3200 cm−1 of adsorbed water, which is observed for all samples, can mask a band ν OH in different groups CO–H. Notice that the IR bands between 2400 and 2300 cm−1 (observed for many samples) can be assigned to adsorbed CO2 molecules. Notice that the diffuse reflection FTIR spectra of these samples (not shown here) depict only low-intensity bands over the ranges 1000–660 and 1650–1100 cm−1 against an intensive background absorbance. Thus, the chemical structure of the surface of the studied carbons is relatively complex with different CO, COC, CC, and C–H bonds. The FTIR spectra of Hypercarb and Envicarb (Fig. 12) differ significantly from the spectra of synthesized carbons (Figs. 8–11),

especially over the range 1000–1200 cm−1 related to different oxygen–containing functionalities; e.g., for Envicarb, the absorbance is very low there. However, the intensity of a band at 1650 cm−1 for this carbon is high (Fig. 12, curve 2). Consequently, one can assume that the surfaces of Hypercarb and Envicarb are more hydrophobic than those of other studied carbons. Clearly, polar functional groups can play a significant role (other positive or negative depending on the characteristics of adsorbates, solvents, and eluents) in SPE and elution of adsorbed organics from pores in liquid media. The textural properties of adsorbents (as well as the characteristics of adsorbates and liquid media) impact the concentration and SPE recovery rates for explosives [14].

Table 2 Recovery rates η (%) for the first series of A2PS-x carbons Adsorbent

EGDN

NG

PETN

TNB

HNDPA

TNMA

TNT

HNDB

ηaver,ad

HMX

RDX

A2PS A2PSC A2PSCA

99 20 10

94 13 6

97 29 17

70 53 40

10 79 47

75 30 12

2 45 23

32 69 48

99 30 11

99 21 9

67.7 38.9 22.3

A2PSM A2PSMC A2PSMCA

90 81 14

72 62 10

88 83 23

56 76 47

27 98 45

73 36 17

28 57 37

64 95 71

93 78 19

95 77 12

68.6 74.3 29.5

ηaver,ex

52.3

42.8

56.2

57.0

51.0

40.5

32.0

63.2

55.0

52.2

Note. ηaver,ad and ηaver,ex correspond to average recovery rate for a given adsorbent and explosive, respectively.

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Fig. 6. Pore size distributions for samples prepared from coke calculated using (a, c, e) regularization procedure applied to overall equation and (b, d, f) the BJH method (only for mesopores); (a, b) noncatalytic activation, (c, d) catalytic (label Ca) activation; and (e, f) catalytic activation and pyrocarbon deposition (label Ca/C).

SPE investigations of three classes of explosives adsorbed by A2PS-x (Tables 2 and 3) show minimal irreversible adsorption and maximal η values for nitrate esters and cyclic nitroamines for the initial A2PS with a large contribution of micropores (Fig. 5), but significant amounts of nitroaromatics are irreversibly adsorbed on A2PS (see A in Table 3), i.e., poorly desorbed on elution, that gives low η values for these compounds. A2PSMC is more effective for nitroaromatics,

possibly because of the smaller contribution of narrow micropores (Fig. 5), in which adsorption of nitroaromatics can be stronger (e.g., because of strong dispersive interaction with both pore walls simultaneously) than that in mesopores (interaction with only one pore wall). Additionally, elution of explosive molecules adsorbed in narrow micropores is more difficult than that from mesopores. Thus, low recovery of nitroaromatics is caused by their irreversible adsorption

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Table 3 Recovery rates ηw (%) for residual solution after adsorption of explosives on carbons, total η! = η + ηw , and relative amounts (A, %) of irreversible adsorbed explosives Adsorbent

Parameter

EGDN

NG

PETN

TNB

HNDPA

TNMA

TNT

HNDB

HMX

RDX

A2PS

ηw η! A

0 99 1

0 94 6

0 97 3

0 70 30

0 10 90

0 75 25

0 2 98

0 32 68

0 99 1

0 99 1

A2PSCA

ηw η! A

65 75 25

60 66 34

45 62 38

30 70 30

35 82 18

55 67 33

45 68 32

20 68 32

70 81 19

80 89 11

A2PSMC

ηw η! A

0 80 20

15 77 23

0 83 17

0 76 24

0 98 2

0 36 64

0 57 43

0 95 5

5 83 17

5 82 18

Initial coke

ηw η! A

0 75 25

0 63 37

0 70 30

0 79 21

0 40 60

0 71 29

0 0 100

0 34 66

0 60 40

0 65 35

Coke-8h/Ca/C

ηw η! A

0 100 0

0 80 20

0 100 0

0 100 0

0 100 0

0 100 0

0 57 43

0 95 5

0 100 0

0 95 5

Evnicarb

ηw η! A

30 82 18

55 70 30

0 85 15

0 75 25

0 88 12

0 95 5

0 40 60

0 83 17

0 90 10

30 82 18

Hypercarb

ηw η! A

35 84 16

60 75 25

0 84 16

0 77 23

0 88 12

0 96 4

0 40 60

0 82 18

0 89 11

30 77 23

16

22

17

22

28

23

62

30

14

16

Aaverage

Table 4 Recovery rates (%) for several activated carbons in dimethyloformamide Substance

ηaver,ex (%)

Recovery rates η (%) Coke initial

Coke 1h/Ca/C

Coke 4h/Ca/C

Coke 8h/Ca/C

Envicarb

Hypercarb

EGDN NG PETN

63.8 42.3 76.8

75 63 70

47 35 54

60 46 68

100 80 100

52 15 85

49 15 84

TNB TNT TNMA HNDPA HNDB

83.8 30.5 86.3 84.0 78.2

79 0 71 40 34

84 23 77 92 88

88 23 79 96 87

100 57 100 100 95

75 40 95 88 83

77 40 96 88 82

HMX RDX ηaver,ad (%)

87.2 68.7

60 65 55.7

84 64 64.8

100 95 92.7

90 52 67.5

89 47 66.7

100 89 73.6

Note. ηaver,ad and ηaver,ex correspond to average recovery rate for a given adsorbent and explosive, respectively.

(Table 3, A) in micropores, especially for TNT adsorbed in narrow micropores of A2PS. Notice that a maximal average η = 78.5% for the same explosives was found previously for activated carbon A2PS oxidized under mild conditions [14]. Average SPE rates (ηaver,ad ) increase with increasing specific surface area and total pore volume (however, SBET and Vp may be not too great, e.g., A2PSMC, Hypercarb, or Envicarb) but decrease with pore width (Fig. 13). Analysis of the SPE recovery rates for the same substances adsorbed by modified coke samples and Hypercarb or Envicarb (Tables 3

and 4, Fig. 14) confirms these relationships. Coke-8h/Ca/C demonstrates a very high average η value, significantly larger than those for other samples (Tables 2–4) synthesized in this work or previously [14] or commercial Hypercarb and Envicarb adsorbents. This can be caused by (i) a significant contribution of broad micropores at x → 1 nm and narrow mesopores at x between 1 and 3 nm (Fig. 6) (easier occurrence of adsorption and desorption than on microporous carbon), and (ii) lower amounts of surface functionalities (Fig. 11, ranges 1400–1700 and 500–800 cm−1 ) than for

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Fig. 7. Pore size distribution for Envicarb and Hypercarb.

Fig. 10. FTIR spectra of the second coke series: (1) coke-1h/Ca, (2) coke-4h/Ca, and (3) coke-8h/Ca.

Fig. 8. FTIR spectra of A2PS-x series (curve numbers in figure correspond to sample numbers in Table 1).

Fig. 11. FTIR spectra of the third Coke series: (1) coke-1h/Ca/C, (2) coke-4h/Ca/C, and (3) coke-8h/Ca/C.

Fig. 9. FTIR spectra of coke-x series: (1) initial coke, (2) coke-2h, (3) coke-3h, (4) coke-6h, and (5) coke-8h.

some other carbons, i.e., low contribution of specific interaction (e.g., hydrogen bonds) between nitro groups of adsorbates and surface hydroxyls, which can play a negative role in elution of explosives. It should be noted that simple linear correlation between the solvent polarity and the recovery rates is absent (that

Fig. 12. FTIR spectra of (1) Hypercarb and (2) Envicarb.

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399

Fig. 14. Average SPE recovery rates η for 10 explosives adsorbed onto 6 carbons (initial coke, coke-1h/Ca/C, -4h/Ca/C, -8h/Ca/C, Envicarb, Hypercarb) and eluted with 8 solvents, as a function of (a) SBET and (b) average pore half-width Rp and micropore half-width xDS .

Fig. 13. Average SPE recovery rates η as functions of (a) SBET , (b) Vp , and (c) average pore half-width Rp for different organics adsorbed by A2PS-x samples.

should be expected). However, the tendency to enhancement of the average η values with increasing medium polarity is observed (Fig. 15) because of enhancement of elution of polar adsorbate molecules by polar solvent. For example, a maximal average η value was found for DMFA and minimal for nonpolar tert-butyl methyl ether with a low dielectric constant and methanol (however, its ε value is close to that of DMFA). It should be noted that the hydrogen bonds between nitro groups and surface OH groups are weaker than those for the C=O group of DMFA (according to quantum chemical calculations). Therefore, the maximal η values (see η and ηaver,ad ) are observed for coke-8h/Ca/C (Tables 3 and 4), which is characterized by not too large amounts of surface functional groups containing oxygen and a significant contribution of mesopores (Fig. 6), since elution of adsorbates

from larger pores is more effective than from narrow micropores. The heat of adsorption (qis0 ) plays an important role (as a stabilizing contribution to the free energy of adsorption) in adsorption–elution of organics. The qis0 values depend on the nature of an adsorbate and the electronic (e.g., availability of polar surface sites) and structural (pore size distribution) characteristics of an adsorbent, as it can increase with SBET and Vp (Fig. 16). Besides the chemical nature of the adsorbent surfaces affects the qis0 values; therefore, it can depend on structural characteristics nonlinearly. Adsorption– desorption of organics in carbon pores from the solution depends also on the characteristics of solvents, which affect the electronic structure of the solute molecules depending, of course, on their chemical composition. For example, according to quantum chemical calculations, the dipole moment µ of TNMA is 3.83 and 4.92 D, and µ = 4.18 and 5.64 D for NG in the gas (calculations for vacuum) and DMFA (Table 5), respectively. However, for molecules with a low dipole moment, µ is near the same in the gas and liquid media, e.g., µ is the same (0.22 D) for HMX in the gas and DMFA media, and a similar picture is observed for EGDN, TNB, and HNDB. The solvation free energy of explosives depends on their spatial characteristics (affecting the structure of a cavity in

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Table 5 Free energy of solvation (#Gs ) and dipole moment of explosives in dimethyloformamide or water Solute

Fig. 15. Average SPE recovery rates η for 10 substances adsorbed onto 6 carbons (initial coke, coke-1h/Ca/C, -4h/Ca/C, -8h/Ca/C, Envicarb, Hypercarb) and eluted with 8 solvents, as a function of the dielectric constant of solvents.

EGDN NG PETN TNB TNT TNMA HNDPA HNDB HMX RDX

−#Gs (in DMFA) (kJ/mol)

µ (in DMFA) (D)

−#Gs (in water) (kJ/mol)

25.6 46.1 53.5 31.4 28.5 42.3 47.9 54.8 56.9 57.0

0.02 5.64 6.15 0.02 2.07 4.92 1.50 0.03 0.22 9.63

20.8 41.1 37.5 27.1 22.0 38.7 43.8 40.0 71.0 78.4

Fig. 17. Models of (a) PETN, (b) HNDPA, (c) RDX, and (d) TNB in micropores at x = 0.5 nm (c, d) and 0.75 nm (a, b) with consideration for the external surface of molecules for solvation.

Fig. 16. Isosteric heat of adsorption qis0 of benzene, hexane, and chloroform as a function of (a) the specific surface area and (2) the pore volume for samples 3, 5, and 6 (Table 1) and Envicarb.

solvent) and electronic structure (contributing electrostatic and dispersive interaction, formation of the hydrogen bonds, etc.) and the solvent type (Table 5, #Gs ) because of different number of nitro groups and availability of benzene ring in solutes, different polarity and polarizability of solute and solvent molecules and other factors contributing different components in #Gs (see Eqs. (3)–(6)). These factors can affect not only adsorption of explosives but also their elution from carbon pores. Adsorption of two-dimensional aromatic molecules, e.g., TNB, in narrow micropores (Fig. 17d) should be irreversible because of not only their strong dis-

persive interaction but also the difficulties of penetration of solvent molecules between an organic molecule and the pore walls in the confined space of micropores. Additionally, simple “ejection” of an adsorbed molecule from narrow pores by solvent molecules is difficult in consequence of the mentioned dispersive interaction with two walls simultaneously. The availability of even a small “anchor” such as the CH3 group in TNT (i.e., this molecule is nearly twodimensional) inhibits its motion from narrow pores, which can explain low recovery rates (η, ηaver,ex ) for TNT adsorbed onto different carbons, especially microporous (Tables 2–4, especially Aaverage in Table 3, and Refs. [13,14]). Additionally, the −#Gs value is lower for TNT in both DMFA and water than that for other nitroaromatic and other explosives, except EGDN (Table 5); i.e., elution of TNT does not lead to strong lowering of the free energy of the system. Clearly, elution of organics from carbon surfaces is easier for threedimensional molecules (with lower dispersive interaction with carbon pore walls than that for two-dimensional aromatics) adsorbed in mesopores or even in micropores but at

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tendencies (correlation coefficient about 0.5 or lower) are seen (Fig. 18). For instance, lowering of the solvation free energy for a free explosive molecule leads to increase in η (Fig. 18a) because of enhancement of elution efficiency. Increase in the volume of an adsorbate molecule and the surface area of the corresponding solvent cavity (Sc ) causes its adsorption in larger pores. Since explosive elution from larger pores is easier than that from narrow ones, there is tendency of increase in η with Sc (Fig. 18c). Notice that for separated narrower series of explosives, e.g., for nitroaromatics, there is a nearly linear relationship between ηaver,ex for A2PS-x samples (Table 2) and −#Gs (for DMFA as a solvent), except for TNB. Additionally, ηaver,ex of a given explosive compound is relatively high if its −#Gs is significantly higher than −#Gs for solvent molecules per se (e.g., for a molecule of DMFA in DMFA or a water molecule in water, #Gs ≈ −25 kJ/mol according to calculations for the saluted molecule (i.e., infinite dilution) in the corresponding solvent by SM5.42/HF/6-31G(d)), since this causes enhancement of elution efficiency of explosives.

4. Conclusion

Fig. 18. Recovery rate versus (a) free energy of solvation, (b) dipole moment of molecules, and (c) cavity surface area of explosives in solution with (a–c) DMFA and (a) water.

x → 1 nm (Fig. 17 and A for A2PS and Aaverage in Table 3) accompanied by greater solvation effects for these molecules (Fig. 18, Table 5); as the free energy in the adsorbed state is higher than that in the bulk solution, the equilibrium shifts towards dissolved explosives in comparison with adsorbed ones. Therefore, the recovery rates of explosives with threedimensional molecules are typically greater (and irreversible adsorption is lower) than those for two-dimensional aromatics (Tables 2–4). Since there are many factors affecting recovery rates, it is difficult to expect simple linear relationships between the η values and different (structural and electronic) characteristics of adsorbates and adsorbents. However, certain

Analysis of the relationships between the SPE recovery rates and the structural characteristics of both adsorbents and adsorbates should be accompanied by analysis of their chemical compositions and electronic structure parameters affected by solvents, i.e., by consideration of the free energy of solvation, adsorption, and elution. Catalytic or noncatalytic activation of carbons of different origin allows us to prepare adsorbents (e.g., coke-4h/Ca/C and coke-8h/Ca/C, A2PS, A2PSM, and A2PSMC) possessing appropriate properties for effective adsorption, SPE, and recovery of the studied explosives, both aromatic and nonaromatic. The maximum effective multistep modified coke-8h/Ca/C adsorbent possesses not too high structural characteristics related to micro- and mesopores, whose contributions are close to one another. Its pore size distribution with respect to mesopores is akin to those of commercial Hypercarb and Envicarb carbons; however, coke-8h/Ca/C has a larger contribution of narrow pores at x < 2 nm and large pores at x > 30 nm. The effectiveness of coke-8h/Ca/C in SPE is significantly higher (average value for ten probe compounds of approximately 93% using DMFA as an eluent) than that of the mentioned commercial adsorbents characterized by average recovery values of 67–68% (DMFA as an eluent). Acknowledgments This research was supported by NATO (Grant EST.CLG. 976890) and the Polish State Committee for Scientific Research (KBN, Warsaw, Grant OTOOC 02422 (W.T.)). One of the authors (R. Leboda) is very grateful to the Foundation for Polish Science for financial support.

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