Influence of water on adsorption of organic compounds from its aqueous solutions on surface of synthetic active carbons

Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) 61–67

Influence of water on adsorption of organic compounds from its aqueous solutions on surface of synthetic active carbons M. Seredycha , A. Gierakb,∗ b

a European Collegium’s Polish and Ukrainian University, Lublin, Poland Faculty of Chemistry, Maria Curie-Skłodowska University, M.C. Skłodowska Sq. 3, 20-031 Lublin, Poland

Received 29 August 2003; accepted 20 July 2004 Available online 27 August 2004

Abstract The investigations of preparation of active carbons by carbonization of porous copolymer (4,4 -diphenyl sulfone dimethacrylate) have been described. The carbonization of polymers was carried out in inactive atmosphere (N2 ) under supervised conditions. A spherical shape of grains is a result of carbonization. The obtained carbon adsorbents were oxidized with nitric acid (6 M HNO3 ) and heated in hydrogen (H2 ) atmosphere. Adsorption from aqueous solutions of chosen organic compounds on the modified synthetic active carbons, adsorption of organic compounds in relation to water as well as structural characteristics of carbon adsorbents are discussed in this paper. The interaction of the organic substances and the synthetic active carbons in aqueous solutions depends on the structural properties of carbons (volume of microand mesopores, distribution of pore sizes, specific surface area) and the chemical properties (i.e. concentration of functional oxygen groups, presence of mineral admixtures). © 2004 Elsevier B.V. All rights reserved. Keywords: Synthetic carbon adsorbents; Water; Phenol; Naphthalene adsorption

1. Introduction Various organic substances (often of the concentrations below 10−5 g/l) contaminating the natural habitat occur in natural and wastewaters. They are determined by means of different separation and concentration methods which frequently employ adsorption on suitable adsorbents [1–5]. Active carbons are, among others, adsorbents used for this purpose. Carbon adsorbents are also used for air purification [6–9]. As active carbons contain various functional oxygen groups on their surface, large amounts of water can accumulate in their pores which leads to sorption capacity decrease [10,11]. Water adsorption (as well as of other polar molecules) takes place on active surface sites which are the functional polar groups like: phenolic, carboxylic, lactone, etc. ones as well as those containing other heteroatoms (N, S, P, etc.) [6–11]. Water adsorption causes formation of clusters ∗

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(of nanometer or larger size [12]) and appearance of droplets of nanometer size. Therefore heat of water adsorption can be relatively low in the range 15–30 kJ/mol [13]. This adsorption heat is much lower than the latent heat of water condensation (45 kJ/mol) which is due to hydrophobicity of part of carbon surface [6–11]. Carbon wetting heat (Him ) in water depends on both structure of complete micro-, meso- and transport pores and amount of functional oxygen groups on the adsorbent surface. As an example Him changes in the range from 40 to 540 mJ/m2 (or from 2 to 94 J/g) for carbons of various structural properties (mainly micropores) and chemical properties (depending on the concentration of functional oxygen groups and specific surface area). Water can fill in active carbon pores effectively but does not fill in mesopores. It can also fill in macropores (energy of water adsorption on such samples was close to enthalpy of pure water melting) [9,11]. Therefore, it is difficult to interpret adsorption of organic compounds on carbon adsorbents in the presence of adsorbed water or adsorption of water and organic substance mixture [11,14,15]. Reduction of organic compounds breakthrough

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Table 1 Characteristics of porous structure of synthetic carbon adsorbents Sample

Specific surface area SBET [m2 /g]

Specific surface area, St (m2 /g)

Surface of mesopores, Sme (m2 /g)

Pore volume, Vpor (cm3 /g)

Pores average dimension, ˚ Rpor (A)

Volume of micropores, Vmicr (cm3 /g)

Adsorption capacity ami (mmol/g)

Per cents of micro pores (%mic )

M6P M6P-1 M6P-2

438 297 542

550 373 679

34 31 36

0.27 0.20 0.33

20 21 19

0.18 0.12 0.22

5.2 3.5 6.4

67 60 67

volume was found to occur only in the presence of water in the system which can be explained by adsorption of small water clusters in micropores and partial masking of mesopores and pores transported by water droplets of nanosize [11]. Comparison of Him for microporous carbons (also for other adsorbents [16]) for water and organic substances shows that water molecules interact with hydrophobic surface in a weaker way than organic molecules [11–16]. Therefore adsorbing organic compounds can replace water in the pore system. Competitive adsorption of water and organic compounds on the adsorbent surface results from energetic interactions in the surface layer of the carbon adsorbent [11–16]. Adsorption of single organic substances from the diluted aqueous solutions on active carbons was taken as the function of their molecular mass, size and geometric shape, kind of functional groups, polarity and solubility. Papers [17–19] show that increase of adsorbate molecular mass causes increase of adsorption due to greater affinity of larger mass molecules for the carbon surface. However, increase of molecule volume, polarity and solubility decreases adsorption as it is a factor inhibiting effective use of carbon adsorption space. Puri et al. [5] showed that specific surface of active carbons plays the greatest role in physical adsorption. However, chemical nature of carbon surface is responsible for the differences in the shape of experimental isotherms. Glushenko et al. [20] show that carbon oxidation decreases adsorption of non-polar and weakly polar substances. Though many papers dealing with adsorption of organic substances from aqueous solutions have been published, detailed properties of water surface layer and organic compounds on active carbons are still unknown. The reasons for it are carbon porous structure and its surface chemistry. Therefore the aim of this paper is to study adsorption of some organic compounds from aqueous solutions on modified synthetic active carbons.

2. Experimental The initial active carbon (M6P) was prepared by carbonization of the polymers (4,4 -diphenyl sulfone dimethacrylate) [21]. Carbonization of polymers was carried out in the fluidal reactor at 600 ◦ C heating the oven with the rate 10 ◦ C/min in the inert atmosphere (in the nitrogen stream). The obtained carbon adsorbents were washed away from the inorganic catalysts and subjected to the additional modification: active carbon surface was oxidized using ni-

tric acid (initial active carbon M6P was flooded with 6 M HNO3 solution and boiled under the reflux condenser for 1 h – adsorbent M6P-1). Another portion of carbon (M6P) was heated in the hydrogen atmosphere (800 ◦ C for 3 h – M6P-2). Carbon was washed with distilled water till neutral reaction was reached and dried at 200 ◦ C. The structural properties of the obtained carbon adsorbents are presented in Table 1. Structure of the adsorbents was determined from the nitrogen adsorption–desorption isotherms at −196 ◦ C using the Brunauer–Emmett–Teller method in the relative pressure range from 0.05 to 0.2. The total pore volume (Vp , cm3 /g) was determined from the values of nitrogen adsorption for the relative pressure p/po equal to 0.975. The total specific surface area of active carbons (St ), volume of micropores (Vmi ) as well as the specific surface area of mesopores (Smezo ) were determined by means of ␣s ˚ was calculated method [22,23]. The average pore size (R, A) using the Langmuir method. The percentage contents of micropores (%mic ) was calculated as the ratio of micropores volume and total pores volume (%mic = (Vmic /Vp ) × 100%). The course of nitrogen adsorption–desorption isotherms and at the same time estimation of microporosity of the obtained synthetic active carbons by means of ␣s method is presented in paper [24]. 2.1. Studies of adsorption Adsorption of water, phenol (polar substance) and naphthalene (non-polar substance) on the prepared synthetic carbon adsorbents was studied. Adsorption measurements of phenol and naphthalene from aqueous solutions on carbon adsorbents were made at 20 ◦ C, measuring initial and equilibrium concentrations using the UV spectrometer produced by HELIOS-y (Thermo Spektronic) in the wave length range 200–300 nm. Phenol and naphthalene adsorption isotherms were measured as follows: 0.1–0.2 g carbon (weighed with the accuracy ± 0.01 g) was added into the flasks containing 10 ml phenol solution (of the concentration 0.025–0.25 mg/l) and naphthalene (of the concentration 0.00112–0.0112 mg/l) each. Tightly closed flaks were shaken at 20 ◦ C for 3 days and nights. The conditions of adsorption isotherms measurement were based on the analysis of kinetics of the adsorption process of these substances on carbon adsorbents (adsorption equilibrium time was about 48–50 h). After the equilibrium was reached, quantitative analysis of the studied solutions was made based on the calibration curves of standard solutions.

M. Seredych, A. Gierak / Colloids and Surfaces A: Physicochem. Eng. Aspects 245 (2004) 61–67

Fig. 1. Isotherms of adsorption of phenol on carbons: 1 – M6P; 2 – M6P-1; 3 – M6P-2 (Tables 1 and 2).

Fig. 2. Isotherms of adsorption of naphthalene on carbons: 1 – M6P; 2 – M6P-1; 3 – M6P-2 (Tables 1 and 2).

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Table 2 The Freundlich constant of adsorption isotherms of phenol and naphthalene on investigated carbons Adsorbent

Freundlich constants Phenol

M6P M6P-1 M6P-2

Naphthalene

k (mmol/g)

1/n

10−4 G

0.0074 0.0078 0.0045

0.74 0.54 0.77

0.2 0.4 0.17

(mmol/m2 )

2.2. Thermogravimetric measurements Water thermodesorption from the surface of studied samples was carried out under quasi-isothermal conditions using the Derivatograph PC (Paulik, Erdey, Budapest, Hungary). Water vapor was adsorbed on the carbon surfaces at room temperature (22 ◦ C). A portion of the earlier out-gassed adsorbent (1 g – 200 ◦ C, pressure 10−3 Torr) was put into the vessel containing saturated water vapor and left for 72 h to establish adsorption equilibrium. Then the weighed adsorbent samples (about 50 mg) were measured thermogravimetrically using a spiral platinum crucible at the temperature range 20–300 ◦ C. The sample mass loss (TG) was registered depending on temperature. From the obtained results, the total amount of adsorbed water was calculated per a gram of adsorbent and 1 m2 of its surface [25].

3. Results and discussion Figs. 1 and 2 present the course of phenol and naphthalene adsorption isotherms from the aqueous solutions on the studied synthetic active carbons. The measured adsorption isotherms were described using the Freundlich equation: X = kc1/n m

(1)

where X/m is the amount of phenol or naphthalene adsorbed per a unit of carbon mass in mmol/g, c the equilibrium concentration in mol/l, k and 1/n are the Freundlich constants. Physically k is the empirical constant corresponding to a segment cut off by the straight line with the axis of ordinates with the concentration C = 1 mmol/l in the diagram made in

k (mmol/g)

1/n

10−6 G (mmol/m2 )

0.0026 0.0030 0.0014

0.90 0.88 0.97

0.32 0.64 0.29

Table 3 The content of iron and sulfur in carbon adsorbents obtained by means of XRF method Sample

Content of Fe (%)

Content of S (%)

M6P M6P-1 M6P-2

2.21 ± 0.10 1.24 ± 0.05 3.75 ± 0.16

4.61 ± 0.35 2.73 ± 0.20 4.53 ± 0.35

the logarithmic scale (log(X/m) = f(log c)). 1/n is the slope of the straight line on the isotherm. The optimum values of the Freundlich parameter of equation 1/n (0.1–0.5) have been presented by the authors of the papers [2,3]. Table 2 presents the Freundlich constants for the studied systems. The studied adsorbents have differentiated values of the coefficient 1/n for phenol and naphthalene. This probably is connected with the reversible adsorption of the compounds on carbons. Higher values of parameter 1/n are presented by the authors of papers [26–29]. Active carbons have a complicated porous structure and complex chemical character of the surface. In the case of adsorption from liquid phase, the chemical structure of carbon surface affects its adsorption properties significantly. Functional oxygen groups of acidic character are the most important. Temperature as well as concentration of hydrogen ions (pH) and ionic strength of the solution also have influence on adsorption. Moreover, a competitive character of adsorbent surface filling by solution components mainly by other dissolved substances and water should be taken into consideration. Oda et al. [30] assume that the functional groups of acidic character are responsible for adsorption increase for more polar organic compounds. As follows from the studies by Asakawa et al. [2,3] on phenol adsorption from aqueous

Table 4 The characteristics of water layers adsorbed on the surface of the carbon adsorbents: initial (M6P), oxidized with nitric acid (M6P-1) and reduced with hydrogen (M6P-2) Sample

Specific surface, SBET (m2 /g)

max CH 2O (mg/g)

s CH (mg/g) 2O

Gs (kJ/mol)

Gb (kJ/mol)

Gb − G65 (kJ/mol)

∆GsΣ (mJ/m2 )

G (mJ/m2 )

M6P M6P-1 M6P-2

438 297 542

345 332.5 368.5

27 170 150

39.6 39.1 37.5

14.8 15.4 16.6

7.1 7.6 8.8

68 623 289

37240 74181 41071

max s CH – the total concentration of adsorbed water; CH – concentration of strongly bounded water; Gt – the maximum change of free energy of water (G300 2O 2O = 54 kJ/mol); Gb – the free energy of water in temperature of evaporation of the elemental part of water (temperature ∼100 ◦ C); Gs = Gt − Gb – the ◦ change of free energy of water strongly bounded; G65 Σ = 7.8 kJ/mol – the free energy of water in temperature 65 C, i.e. in moment of beginning of process of evaporation from full pores the water; Gb − G65 – the change of free energy of water in range temp. 65–100 ◦ C; GsΣ – the free superficial energy on border of structural phase adsorbent/water strongly bounded; G – the total free superficial energy on border of structural constituents adsorbent/water.

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Fig. 3. The thermodesorption of water from the surface of modified active carbons as a function of the temperature (curves Q – TG): (a) the quantity of grams of water per gram of the adsorbent; (b) the quantity of grams of water per 1 m2 of the adsorbent.

solutions on modified graphitized carbon blacks not only specific surface area and adsorbent pore volume affect adsorption value. This process is mainly determined by adsorbent surface polarity. Carbon adsorbents enriched with functional polar groups decrease phenol adsorption from water solution due to competitive adsorption of more polar water molecules. Slightly different results were observed studying the synthetic active carbons prepared by us. The greatest adsorption of both phenol and naphthalene was observed on the adsorbent oxidized with nitric acid (M6P-1 – Table 2), i.e. on the adsorbent which theoretically possesses the largest number of functional groups. It proves that the functional polar groups are responsible for adsorption of both polar phenol molecules and non-polar naphthalene molecules from the aqueous solution. However, it should be noted that the adsorbent M6P-1 possesses the smallest specific surface area SBET , pore volume Vpor and the smallest volume of micropores of the tested adsorbents. Organic substance adsorption from electrolyte solutions is also affected by occurrence of various heteroatoms in their structure (i.e. S, N, P, metals). As follows from the fluorescence studies of atomic spectroscopy (XRF), the adsorbent M6P-1 possessed the smallest number of heteroatoms (Fe and S) in its composition (Table 3). Phenol adsorption from aqueous solutions on carbon adsorbents depends also on the quantitative ratio of acidic and basic groups on its surface [4]. Table 4 presents the characteristics of water layers bonded with the surface of carbons determined from the data in Figs. 5 and 6. The numerical values of proper adsorption (G, mmol/m2 ) for phenol and naphthalene in the samples M6P and M6P-2 are similar in our studies. For the sample M6P-1, the numerical value of proper phenol adsorption increases four times but for naphthalene twice (Table 2). Figs. 3 and 4 present the derivatographic studies (Q-TG and Q-DTG curves) showing the dependences of water mass loss in the temperature function on the modified active carbons. Fig. 3a presents loss of water mass per gram of the adsorbent (∆mH2 O /gads ) and Fig. 3b presents the data per 1 m2 of the adsorbent surface (∆mH2 O /m2 ). In the course

Fig. 4. Speed of water thermodesorption process (Q – DTG) from the micro pores of the investigated carbons as a function of the process temperature.

of TG curves in Fig. 3a and b three segments can be distinguished the first of which (horizontal) in the temperature range 25–100 ◦ C refers to the part of thermodesorption when adsorbent pores are filled with adsorbed water. The second segment from 110 to 300 ◦ C is connected with thermodesorption of water from functional surface groups (denotes strongly bonded water). As follows from the comparison of TG curves course in Fig. 3a the amount of adsorbed water on the studied adsorbents is directly proportional to the pore volume Vp (Table 1). The carbon M6P-1 (modified with 6 M HNO3 ) is

Fig. 5. The changes of the free energy of water (G) as a function of its content on the surface of the carbon adsorbents.

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Fig. 6. Changes of the free energy of water (a) in the initial stage (almost all of the pores filled with the water) and (b) the final stage (the pores being almost empty) as a function of its superficial concentration on the synthetic carbon adsorbents.

an exception. For this adsorbent (M6P-1) of the smallest pore volume and the specific surface area, the amount of adsorbed water is the largest. Water adsorption is larger here than in the case of initial carbon and reduced in hydrogen atmosphere (M6P and M6P-2). This fact can be explained by the presence of various functional oxygen (phenolic, carboxylic and other) groups and micropores (being strong adsorption centers) on the surface of carbon (M6P-1) which take part in adsorption of various polar compounds. The differences in water mass loss shown in Fig. 3 can also be seen in Fig. 4 illustrating the rate of water thermodesorption processes (Q-DTG) in the temperature function. More significant changes on the TG curves are observed if the changes of desorbed water amounts are related to 1 m2 adsorbent surface (Fig. 3b). This results from the differences in adsorption properties of the specific surface area of the studied adsorbents. The quantity ∆mH2 O /m2 can be treated as a parameter describing water sorption on the carbon adsorbent surface at a given temperature. Figs. 3 and 4 indicate that the significant initial water mass losses from the carbon samples are probably associated with the moisture loss (of adsorbed water vapor) whose largest amount is found on the oxidized carbon (functional oxygen groups constitute hydrophilic adsorption centers). However, for the initial carbon (M6P) and that thermally reduced in H2 (M6P-2) water mass loss (second peak) is associated with the moisture loss from micropores. Distinct shift of the maximum towards higher temperature from 98 ◦ C (minimum of endothermal transformation on the DTG curve for the oxidized carbon Fig. 4) can be observed which may indicate strong interactions between the adsorbed water molecules and the functional surface groups. Fig. 5 presents the curve of dependence of water free energy change G on its content on the carbon adsorbent surfaces (∆G = f (CH2 O )) in the whole range of studied concentrations. Fig. 6 presents the initial and final segments of this dependence. With large values of water adsorption on the adsorbent surface all pores are filled with water which evaporates with the temperature increase. In this case water is

evaporated from the water–air interface and the water surface is separated from the adsorbent surface with a thick water layer filling the pores. The initial segment of the dependence ∆G = f (CH2 O ) corresponds to this process (Fig. 6a). On this segment water concentration changes are small not exceeding 10–15% of the initial concentration of the adsorbed water. In the studied series of adsorbents, the maximum value of surface free energy at the adsorbent/water interface (G ) could be expected for the oxidized adsorbent which exhibits the greatest adsorption affinity for water molecules. Water interaction results mainly from formation of hydrogen bonds between water molecules and functional groups. Carbon surface heating in hydrogen atmosphere decreases surface concentration of hydrophilic centers. Large values of total surface free energy at the adsorbent/water interface for all samples result from the structure of micropores present in carbon, constituting strong adsorption centers and from chemical properties (different functional oxygen groups – phenolic, carboxylic and others).

4. Conclusions Interaction of organic substances occurring in aqueous solutions with the surface of synthetic active carbon depend on structural properties (such as micro- and mesopore volume, pore size distribution, specific surface area) and chemical properties (concentration of functional oxygen groups, presence of mineral admixtures). Water remaining in pores of carbons which are applied in adsorption of organic compounds occupies part of total pore volume and probably adsorbs in narrow pores where cooperation with outer functional groups is possible. Probably more intensive “dehydration” of active carbons can be made by adsorption of organic compounds on carbon adsorbents of developed mesoporous structure for which a number of transport pores should be comparable with volume of micropores.

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