Temperature dependence of the point of zero charge of oxidized and non-oxidized activated carbons

CARBON

4 6 ( 2 0 0 8 ) 7 7 8 –7 8 7

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journal homepage: www.elsevier.com/locate/carbon

Temperature dependence of the point of zero charge of oxidized and non-oxidized activated carbons ´ lvarez-Merinoa, M.A. Fontecha-Ca´maraa, M.V. Lo´pez-Ramo´na, M.A. A C. Moreno-Castillab,* a

Departamento de Quı´mica Inorga´nica y Orga´nica, Facultad de Ciencias Experimentales, Universidad de Jae´n, 23071 Jae´n, Spain Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain

b

A R T I C L E I N F O

A B S T R A C T

Article history:

Three activated carbons in the form of grains, fibers and pellets were oxidized with ammo-

Received 17 December 2007

nium peroxidisulfate. The oxidized and non-oxidized activated carbons were characterized

Accepted 1 February 2008

by: N2 adsorption at 77 K, temperature programmed desorption up to 1273 K, and potenti-

Available online 9 February 2008

ometric titrations in the temperature range between 288 and 318 K. The point of zero charge (PZC) of the oxidized activated carbons decreased with increasing temperature and this decrease was in the same direction as the change in 1/2pKw, the neutral point of water. The non-oxidized activated carbons behaved in a similar manner, but the PZC decreased by more than the corresponding change in 1/2pKw. Variation of surface charge of the three oxidized activated carbons was fitted to a single second-order function with respect to pH and temperature. However, a single equation could not be found for the three non-oxidized activated carbons. Standard thermodynamic functions at the PZC were obtained from potentiometric curves. Acidity constant distributions were obtained and showed an increase in the number of acidic groups when the temperature increased. The number of carboxylic and phenolic groups obtained from these distributions was compared with that obtained from the deconvolution of temperature programmed desorption spectra. Ó 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

The PZC is an important property of carbon–water interfaces in many technological applications. The PZC is defined as the pH at which the carbon surface has no charge in the absence of specific adsorption. The carbon surface is positively charged at pH values below the pHPZC and negatively charged at pH values above the pHPZC [1,2]. The negative surface charge is produced by dissociation of surface oxygen complexes of acid character, e.g., carboxyl and phenolic groups. The origin of the positive charge is more uncertain. In carbons without nitrogen functionalities it can be due to surface oxygen complexes of basic character, e.g.,

pyrones or chromenes, or to the presence of electron-rich regions within the graphene layers that accept protons from the aqueous solution. Because small changes in the acid and base concentration generally change the magnitude of the potential of the carbon–water interface, H+ and OH ions are considered the potential-determining ions. Knowledge of the variation of carbon surface charge with solution pH, and hence the pHPZC, is of great importance because it allows the Coulomb interactions between the carbon surface and the adsorbate dissolved in water to be controlled [3,4]. In this case, the adsorbate must be an organic or inorganic electrolyte that is dissociated or protonated in aqueous solution. The pHPZC also controls the stability of

* Corresponding author: Fax: +34 958 248 526. E-mail address: [email protected] (C. Moreno-Castilla). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.02.002

CARBON

779

4 6 (2 0 0 8) 7 7 8–78 7

colloidal suspensions. Thus, close to the pHPZC, the Coulomb repulsion of colloid particles is too weak to prevent flocculation [4]. Adsorption studies on carbon materials from aqueous solutions have largely focused on environmental applications, and most were conducted at 25 °C. However, the effect of temperature on adsorption is of great interest, since the temperature of water in nature ranges from 30 °C in the tropics to almost 0 °C in cold regions, and the temperature range of industrial wastewaters is even wider. Temperature is therefore a further variable that can have a major influence on the adsorption of organic and inorganic adsorptives on carbon materials from aqueous solution, see, i.e., Refs. [5–16]. In the same way, adsorption temperature might influence the carbon surface chemistry. However, so far there are no available data on the effect of temperature on the surface charge and PZC of carbon materials. By contrast there have been several investigations into the effect of temperature on the PZC of metal oxides/hydroxides [3,17–25] and phosphates [26]. These studies generally show that the pHPZC decreases with higher temperature. In many cases, this decrease follows that of the water dissociation constant (pKw). With this background, the objective of this investigation was to study variations in PZC and surface charge as a function of the temperature of original and oxidized activated carbons in the form of grains, fibers, and cloth.

equation were obtained from the adsorption isotherms. The external surface area (Sext) in pores with a diameter >3.7 nm was obtained from mercury porosimetry. Temperature programmed desorption (TPD) experiments were carried out by heating samples up to 1273 K in He flow at a rate of 5 K/min and recording amounts of CO, CO2 using a quadrupole mass spectrometer from Balzers (model MSC 200 Thermocube), as a function of temperature, as described elsewhere [27]. The oxygen content (OTPD)was determined from the amount of CO and CO2 evolved. Surface charge in the temperature range 288–318 K was determined by potentiometric titration as described elsewhere [28], using an automatic titrator (Metrohm 702 Titrino) for the measurements. The equilibration and titrations were performed under a nitrogen atmosphere cover to eliminate the influence of atmospheric CO2. Different carbon suspensions were prepared with 0.10 g of carbon and 40 mL of electrolyte solution 0.01 N in NaCl, and allowed to equilibrate for 48 h at the chosen temperature. Carbon suspensions were then titrated with either 0.1 N HCl or 0.1 N NaOH, adding 0.01 mL of titrating agent, with 180 s of equilibration time between additions up to pH 3 or pH 11, respectively. A blank titration was also performed under the same conditions. The proton balance was obtained from the following equation:

2.

where m is the mass of sample, V0 is the initial volume of solution, Vt is the volume of added titrating agent, Nt is the normality of the titrating agent and the i and f subscripts indicate the initial and final concentrations, respectively, of H+ and OH ions. A negative Q value indicates the presence of surface acid groups whereas a positive Q value indicates the presence of surface basic groups.

Experimental

Three commercial carbon materials were used: a granular activated carbon (GAC) from Sutcliffe Carbons, and an activated carbon fiber (ACF) and activated carbon cloth (ACC) from Kynol Europe. Samples were oxidized with a saturated solution of (NH4)2S2O8 in 1 M H2SO4 as described elsewhere [27], and are referred to in the text by the name of the carbon material followed by ox. Surface area and porosity of samples were obtained from N2 adsorption isotherms at 77 K and mercury porosimetry up to 4200 kg/cm2. Adsorption isotherms were measured in a volumetric glass system free of grease and mercury and capable of reaching a dynamic vacuum of 106 Torr. The pressure was measured with a Baratron transducer from MKS. Before measurement of adsorption isotherms, samples were outgassed at 383 K overnight under a dynamic vacuum of 106 Torr. The apparent BET surface area (SBET) as well as the micropore volume (W0) and its mean width (L0) from DR

Q¼

3.

1  ½V0 f½Hþ i  ½OH i g þ Vt Nt mSBET  ðV0 þ Vt Þf½Hþ f  ½OH f g

ð1Þ

Results and discussion

Surface characteristics of carbons used are compiled in Table 1. The apparent BET surface area and micropore volume diminish whereas external surface area increases after oxidation of the original activated carbons, which may largely result from destruction of micropore walls during the oxidation process [29]. CO2- and CO-evolution profiles of oxidized samples are depicted in Fig. 1. The amounts of desorbed CO and CO2, and the OTPD obtained from them, are shown in Table 1.

Table 1 – Surface characteristics, amounts of CO and CO2 evolved up to 1273 K and oxygen content of original and oxidized activated carbons Sample

SBET (m2/g)

GAC ACF ACC GAC-ox ACF-ox ACC-ox

1191 1709 2128 1050 1315 1636

Sext (m2/g) 35 35 40 49 85 94

W0 (cm3/g)

L0 (nm)

0.49 0.73 0.91 0.44 0.57 0.71

1.41 1.69 1.69 1.53 1.71 1.80

CO (mmol/g) 1.73 1.57 1.10 3.78 5.65 5.83

CO2 (mmol/g) 0.55 0.26 0.06 1.90 3.18 2.70

CO/CO2 3.1 6.0 18.3 2.0 1.8 2.2

OTPD (%) 4.5 3.3 1.9 12.1 19.2 18.0

780

CARBON

4 6 ( 2 0 0 8 ) 7 7 8 –7 8 7

0.9

0.6

0.3

GAC-ox

CO(µmol/g.s)

CO2(µmol/g.s)

GAC-ox

#2 #3

0.6 #3 #4 0.3

#2 #1

#4 0.0 273

0.0 473

673

873

1073

273

1273

473

673

1.5

0.8

CO(µmol/g.s)

CO2(µmol/g.s)

ACF-ox #2 #3 0.4

#4 0.0 273

473

673

873

#3 0.5

#2

1073

0.0 273

1273

473

#4 673

873

#5

1073

1273

T(K) 1.2

CO(µmol/g.s)

ACC-ox

CO2(µmol/g.s)

1273

1.0

#1

0.8

#1 #2

1073

ACF-ox

T(K)

0.4

873

T(K)

T(K)

#3

ACC-ox #3

0.6 #2 #1

#4

#5

#4 0.0 273

473

673

873

1073

0.0 273

1273

473

673

T(K)

873

1073

1273

T(K)

Fig. 1 – CO2- and CO-evolution profiles of the oxidized activated carbons and their deconvolution using a multiple Gaussian function.

Oxidation of the original carbons greatly increased their oxygen content, especially in ACF and ACC samples, due to their more developed porosity and surface area. Oxidation also brings about a decrease in the CO/CO2 ratio, which is very marked in the case of ACC-ox. This is due to the large increase in CO2-evolving groups. TPD spectra were deconvoluted following the method described by Figueiredo et al. [30–32] to determine the amount

of surface groups. A multiple Gaussian function was used to fit each profile, taking the position of the peak center as the initial estimate. Results obtained from the CO2- and CO-evolution profiles are compiled in Tables 2 and 3, respectively, which include the temperature of the maxima, T, their width at half height, W, and the area of the peaks, A. The CO2-evolution profile can be deconvoluted into three or four peaks depending on the sample. Peaks #1 and #2

Table 2 – Results of the deconvolution of the CO2-evolution profiles of oxidized samples Peak #1 Tm (K) GAC-ox ACF-ox ACC-ox

– – 497

W (K) – – 24

Peak #2

A (mmol/g) – – 0.32

Tm (K) 533 562 547

W (K) 45 44 42

Peak #3

A (mmol/g)

Tm (K)

W (K)

0.48 0.67 0.46

683 698 667

100 96 100

Peak #4

A (mmol/g) 1.02 1.82 1.53

Tm (K)

W (K)

906 900 872

100 96 100

A (mmol/g) 0.29 0.71 0.39

CARBON

781

4 6 (2 0 0 8) 7 7 8–78 7

Table 3 – Results of the deconvolution of the CO-evolution profiles of oxidized samples

GAC-ox ACF-ox ACC-ox

W (K)

465 515 502

36 24 44

Peak #2

A Tm(K) (mmol/g) 0.03 0.03 0.18

783 798 748

W (K) 100 96 100

A (mmol/g) 1.02 1.82 1.53

Peak #3 Tm W (K) (K) 942 941 948

correspond to carboxyl groups. The first peak, at a lower temperature, is due to strong acidic carboxyl groups and the second, at a higher temperature, to weaker acidic carboxyl groups. Carboxylic anhydrides decompose by releasing one CO2 and one CO molecule. The CO2- and CO-deconvolution peaks corresponding to anhydrides, #3 in Table 2 and #2 in Table 3, respectively, occur at different temperatures, T. This was previously observed [31] in oxidized samples with a high concentration of carboxyl groups and was attributed to either the readsorption of CO on carbon sites vacated after decomposition at lower temperatures of carboxyl groups or the stronger adsorption of CO on the carbon surface. In this case, the temperature difference between CO2 and CO can be 50–100 K [31,33]. Therefore, in the deconvolution of the CO2 and CO peaks corresponding to anhydrides, the same W and A values were maintained for both peaks while a variation in T was allowed. Peak # 4 (Table 2) is due to lactones. The low temperature peak in the CO-evolution profile (#1) can be attributed to the decomposition of carbonyl groups from a-substituted ketones and aldehydes [27,32]. In the same profile, peak #3 correspond to phenols and/or hydroquinones and #4 to carbonyls and/or quinones [30–32]. Finally, peak # 5 might correspond also to the same groups than those of peak # 4 but placed on energetically different sites [30,34,35] or to other basic groups. Results obtained show that the carboxyl, anhydride and phenol group predominate in all oxidized samples. Potentiometric titration curves obtained at temperatures between 288 and 318 K are depicted for sample ACC-ox, as an example, in Fig. 2. These curves show that the negative

2

Surface charge (μmol/m )

1.0

0.0 0.4

Peak #4

A (mmol/g)

78 80 101

1.48 2.66 3.10

Peak #5

Tm (K)

W (K)

A (mmol/g)

1100 1040 1076

73 33 36

1.27 0.59 0.54

Tm (K)

W (K)

– 1149 1191

– 46 57

A (mmol/g) – 0.56 0.48

10.0

pHPZC or 1/2pKw

Peak #1 Tm (K)

6.5

3.0 3.1

3.2

3.3

3.4

3.5

1000/T (K) Fig. 3 – Relationship of pHPZC and 1/2pKw with temperature: h ACC-ox; s ACC and n 1/2pKw.

surface charge at a given pH increases with higher temperature due to an increase in proton release from surface oxygen complexes of acid character. Fig. 2 also shows that the pHPZC value decreased with higher temperature, and Fig. 3 depicts the linear decrease in pHPZC with decreasing 1/T. The slope of this decrease is the same as that of the neutral point of water, 1/2pKw. Thus, for each oxidized activated carbon, the (1/2pKw  pHPZC) value remained constant in the temperature range studied (Table 4). Hence, the change in pHPZC is due to the change in the ionization constant of water with temperature and may be based on similarities in the ionization behavior of surface oxygen complexes and water [20]. Similar results have been found with other systems, e.g., NiO, Co3O4 [18], and rutile [20]. The constancy in the (1/2pKw  pHPZC) value is very useful for predictive purposes, since the pHPZC value can be

0.2

-1.0 0.0

Table 4 – Effect of temperature on the PZC of the oxidized activated carbons

-0.2

-2.0 -0.4 3.0

3.2

3.4

3.6

3.8

Sample

4.0

-3.0 2

4

6

8

Fig. 2 – Variation of the surface charge of ACC-ox with the solution pH at different temperatures — 318 K; – – 308 K; - - 298 K; – - – 288 K.

pHPZC

1/2pKw  pHPZC

GAC-ox

288 318

3.91 3.44

3.26 3.26

ACF-ox

288 318

3.65 3.20

3.52 3.50

ACC-ox

288 318

3.66 3.18

3.51 3.52

10

pH

Temperature (K)

CARBON

4 6 ( 2 0 0 8 ) 7 7 8 –7 8 7

predicted from its measurement at room temperature, at least in the temperature range studied here. Results found indicate that the relative affinity of H+ and OH ions for the oxidized carbon surface is independent of temperature, and that the concentration ratio of these ions in solution is also independent of temperature at the PZC. The thermodynamic relation describing the temperature dependence of PZC was derived by Be´rube´ and De Bruyn [17] and is given by the following equation: 4:6Rð1=2pKw  pHPZC Þ ¼ DH =T  DS

40

-a

782

20

ð2Þ +

Q ¼ aT þ b

DH* (kJ/mol)

Sample GAC-ox ACF-ox ACC-ox GAC ACF ACC

DS* (J/K mol)

0 0 0 65 67 106

135 123 138 195 255 296

2

Q (μmol/m )

4

5

6

7

8

9

10

7

8

9

10

pH 24

16

8

0 3

4

5

6

pH Fig. 5 – Variation of the a and b parameters with the solution pH. h ACC-ox; n ACF-ox; s GAC-ox.

Furthermore, the a and b parameters of this equation can be related to the pH by second-order functions such as those given in Fig. 5. An interesting feature of this figure is that the a parameter for the three oxidized activated carbons can be fitted by a single second-order function, with a correlation coefficient of 0.927. The same is observed for the b parameter, with a correlation coefficient of 0.919. Therefore, variations in surface charge of the three oxidized activated carbons can be expressed as a function of pH and temperature (in the 288–318 K range) by the following equation:

0.5

Q ¼ ½ð0:91pH2  6:95pH þ 17:70Þ=500  T þ ½0:52pH2

-0.5

 4:04pH þ 10:40

pH=3.5 pH=5 pH=6 pH=7 pH=8 pH=9 -2.5 283

3

ð3Þ

Table 5 – Derived standard thermodynamic functions at the PZC by Eq. (2)

-1.5

0

b

where DH* is the standard enthalpy of transferring H and OH ions from the bulk solution to the interfacial region at the PZC, DS* is the standard entropy of hydration of H+ and OH ions at the PZC, and R is the gas constant. According to the above authors, this equation is applicable to all oxide systems and may also be extended to other systems. Results obtained by applying Eq. (2) to the above (1/2pKw  pHPZC) values in the temperature range studied are compiled in Table 5. Surface charge depended on temperature and solution pH, as shown in Fig. 2. To quantify this influence, the surface charge was plotted as a function of temperature for a given pH. This relationship is shown in Fig. 4 for ACC-ox as an example. At a given pH, the surface charge, Q, linearly decreases as temperature increases, in the following equation:

293

303

313

323

T (K) Fig. 4 – Variation of the surface charge of ACC-ox versus temperature at various pH values.

ð4Þ

Potentiometric titration curves obtained at different temperatures with ACC are shown in Fig. 6 as an example of those obtained with the original, non-oxidized, activated carbons. In this case, the negative surface charge also increases with higher temperature, whereas there is practically no change in positive surface charge with temperature variations. The pHPZC (see inset of Fig. 6) also decreases with higher temperature. The linear relationship between the pHPZC and 1/T is depicted in Fig. 3. However, with the non-oxidized activated carbons, the decrease in pHPZC is greater than the corresponding change in 1/2pKw, hence the (1/2pKw  pHPZC) value does

CARBON

0.6

-0.5

0.3

0.0

1/2pKw - pHPZC

2

Surface charge (μmol/m )

783

4 6 (2 0 0 8) 7 7 8–78 7

0.02

0.00

-0.3

-1.5

-0.02

-0.6 -0.04 7

8

9

10

-0.9

-2.5 3

5

7

9

11

3.1

3.2

pH Fig. 6 – Variation of the surface charge of ACC with the solution pH at different temperatures — 318 K; – – 308 K; - - 298 K; – - – 288 K.

Table 6 – Effect of temperature on the PZC of original, non-oxidized, activated carbons Sample

Temperature (K)

pHPZC

3.3

3.4

3.5

1000/T (K)

1/2pKw  pHPZC

GAC

288 318

8.01 6.97

0.84 0.27

ACF

288 318

6.60 5.57

0.57 1.13

ACC

288 318

9.10 7.70

1.93 1.00

Fig. 7 – (1/2pKw  pHPZC) versus 1/T for ACC.

and OH ions to the adsorbed state will require a large degree of desolvation in non-oxidized activated carbons, due to their higher hydrophobic character, giving rise to greater disorder in the system.

-a

7.0

0.0 6

7

8

9

10

11

9

10

11

pH 4.0

b

not remain constant in the temperature range studied, as shown in Table 6. This behavior has been found with some oxides and hydroxides, e.g., rutile, magnetite, hematite, cAl2O3, Ni(OH)2 [3,17], and a-Al2O3 [22]. The difference in behavior between non-oxidized and oxidized activated carbons may be due to their distinct surface structures. Thus, the surface of oxidized activated carbon is more hydrophilic because it has a higher concentration of surface oxygen complexes compared with non-oxidized carbon. Fig. 7 depicts the application of Eq. (2) to non-oxidized activated carbon, taking ACC as an example. Results obtained are compiled in Table 5 and show that DS* is larger for non-oxidized than oxidized reactivated carbons. Again, this may be due to their distinct surface structure, which produces differences in their double layer systems. Thus, the water molecules will be bound to the surface oxygen complexes of oxidized activated carbon by H bonds due the higher hydrophilicity of its surface. This will give rise to a surface water layer with a very similar physical state to that of the bulk aqueous phase. The adsorption process that results in ionic double layer requires penetration of the H+ and OH ions into the inner region of this relatively ordered but liquid interface. The standard entropy of H+ and OH ions in this surface water layer would not be very different from that in the bulk aqueous phase. However, the transfer of H+

3.5

2.0

0.0

6

7

8

pH Fig. 8 – Variation of the a and b parameters with the solution pH. h ACC; n ACF; s GAC.

784

4 6 ( 2 0 0 8 ) 7 7 8 –7 8 7

CARBON

4

The a and b parameters of non-oxidized activated carbons were obtained by applying Eq. (3) to the potentiometric titration curves and were related to the pH by second-order functions, as shown in Fig. 8. However, results obtained for the three non-oxidized activated carbons could not be fitted by a single equation, unlike the case of the oxidized activated carbons. Thus, the variation of Q as a function of pH and T is given by the following equations:

2

Q GAC ¼ ½ð1:23pH2  18:75pH þ 71:74Þ=500  T

12

GAC-ox

f(pKa) (mmol/g)

10 8 6

þ ½0:72pH2  11:03pH þ 42:31

ð5Þ

0 2

3

4

5

6

7

8

9

10

11

pK a

Q ACF ¼ ½ð1:01pH2  14:36pH þ 51:38Þ=500  T þ ½0:77pH2  11:58pH þ 43:76

5

Q ACC ¼ ½ð0:72pH2  12:00pH þ 49:95Þ=500  T þ ½0:38pH2  6:15pH þ 25:27

ACF-ox

f(pKa) (mmol/g)

4 3 2 1 0 2

3

4

5

6

7

8

9

10

11

7

8

9

10

11

pK a 10

ACC-ox 8

f(pKa) (mmol/g)

ð6Þ

6 4 2 0

2

3

4

5

6

pKa Fig. 9 – pKa distributions for oxidized samples at different temperatures. — 288 K; – 318 K.

ð7Þ

The numerical SAIEUS procedure (solution of adsorption integral equation using splines) [28,36,37] was applied to the potentiometric titration curves to determine the acidity constant distribution, f(pKa), of species present on the surface of the samples. Acidity constant distributions for the oxidized activated carbons obtained at 288 and 318 K, as examples, are depicted in Fig. 9 and the amount of the different acidic groups, according to their pKa, and peak position are compiled in Table 7. Results obtained in samples with high oxygen content, ACFox and ACC-ox, reveal a highly heterogeneous distribution of surface acidic groups, with pKa values ranging from 3 to 10. This distribution was less heterogeneous in sample GAC-ox. The surface of the oxidized carbons can be seen as a complex mixture of acidic functionalities with a distribution of different pKa values. In complex mixtures of organic acids containing only C, H, and O [38], the frequency of occurrence of carboxyl groups is a Gaussian distribution extending up to a pKa value of 8, with the maximum at 4.5. The phenolic groups show a similar distribution for pKa values, extending from 8 to 12 with the maximum at 10. Therefore, carboxyl groups will be considered here to have pKa values < 8 and phenolic groups values of 8–10, although basic species can appear at pKa values >9 [38]. Results in Fig. 9 and Table 7 show that the number of acidic groups increases with higher temperature. In the case of ACC-

Table 7 – Results of potentiometric titration: number of groups in mmol/g and peak position in parenthesis Sample

GAC-ox

ACF-ox

ACC-ox

T (K)

pKa 2–4

pKa 4–6

pKa 6–7

288 318

0.73 (2.98) 1.22 (2.98)

288 318

1.03 (2.94) 1.52 (2.98)

0.10 (5.68) 0.52 (5.13)

0.70 (6.83)

288 318

1.11 (2.91) 1.86 (2.94)

0.03 (5.77) 0.32 (4.76)

0.49 (6.25)

pKa 7–8

pKa 8–9

TPDa

pKa 9–10 Carboxylic Phenolic

Carboxylic Phenolic 0.36 (7.83) 0.16 (7.47)

0.06 (7.30) 0.81 (7.48)

0.04 (8.59) 0.90 (8.47)

0.30 (9.98) 4.43 (9.98)

0.73 1.58

0.30 4.43

2.52

1.48

0.62 (9.94) 3.12 (9.82)

1.29 2.74

0.66 4.02

4.31

2.66

0.31 (9.29) 3.22 (9.36)

1.20 3.48

0.31 3.22

3.84

3.10

Number of carboxylic and phenolic groups obtained by TPD. a The number of carboxylic groups from TPD was obtained from the addition of the number of carboxylic groups and twice the number of anhydrides.

CARBON

10

GAC

f(pKa) (mmol/g)

8 6 4 2 0 2

3

4

5

6

7

8

9

10

11

pKa 8

f(pKa) (mmol/g)

ACF 6

4

2

3

4

5

6

7

8

9

10

11

7

8

9

10

11

pKa 3

ACC

f(pK a) (mmol/g)

ox, the number of carboxylic and phenolic groups obtained from TPD was similar to that obtained from potentiometric titration at 318 K. However, for GAC-ox and ACF-ox, the number of carboxylic groups from TPD was greater than that from potentiometric titration at 318 K. This suggests that not all surface acid groups are dissociated at 318 K. In contrast, the number of phenolic groups obtained from TPD was smaller than that from potentiometric titration at 318 K, which might be due to the inclusion of some basic groups in the peak at pKa  10. Acidity constant distributions for non-oxidized activated carbons are depicted in Fig. 10, which shows a practically monomodal distribution at pKa  10. Results obtained are compiled in Table 8 and indicate that the number of groups increases with higher temperature. In this case, a comparison with TPD data was not possible because due to the low oxygen content of these samples their TPD spectra could not be deconvoluted. Finally, the effect of temperature on the surface charge and PZC of oxidized and non-oxidized activated carbons might explain the increase in metal ion uptake from aqueous solution, observed in some systems [13–15,39–41] when temperature rises.

4.

0 2

2

1

0

2

3

4

5

6

785

4 6 (2 0 0 8) 7 7 8–78 7

pK a Fig. 10 – pKa distributions for original samples at different temperatures. — 288 K; – 318 K.

Conclusions

At a given pH, the negative surface charge of oxidized and non-oxidized activated carbons increases and the pHPZC decreases with higher temperature. In oxidized activated carbon, the change in pHPZC is due to the change in the ionization constant of water with temperature variations, whereas in non-oxidized activated carbons, the decrease in pHPZC is greater than the change in the neutral point of water (1/2pKw). These differences found may be due to the distinct surface structure of the two types of carbon. The variation in the standard entropy of hydration of H+ and OH ions at the PZC is greater in non-oxidized than in oxidized activated carbons. This may be due to their distinct surface structures, which produce differences in their double layer systems. The variation in surface charge of the three oxidized activated carbons can be fitted by a single second-order function with respect to pH and temperature. However, the variation in surface charge of the three non-oxidized activated carbons with changing pH and temperature cannot be fitted by a single equation.

Table 8 – Results of potentiometric titration: number of groups in mmol/g and peak position in parenthesis Sample

T (K)

GAC

288 318

ACF

288 318

ACC

288 318

pKa 5–6

pKa 7–8

pKa 9–10

Carboxylic

0.15 (10.10) 3.14 (10.02) 0.08 (5.74) 0.07 (5.66)

0.07 (7.19)

0.14 (10.16) 3.18 (10.03) 0.14 (10.02) 0.88 (10.08)

Phenolic 0.15 3.14

0.08 0.14

0.14 3.18 0.14 0.88

786

CARBON

4 6 ( 2 0 0 8 ) 7 7 8 –7 8 7

Acidity constant distributions were obtained for the oxidized and non-oxidized activated carbons and show that the number of acidic groups increases with higher temperature. The numbers of carboxylic and phenolic groups obtained from these distributions at the highest temperature used (318 K) was compared with those obtained from the deconvolution of TPD spectra. Acidity constant distributions for non-oxidized activated carbons show a monomodal distribution at pKa  10 and the number of groups also increases with higher temperature. The effect of temperature on the surface charge and PZC of oxidized and non-oxidized activated carbons might explain the observation in some systems of an increase in metal ion uptake from aqueous solution at higher temperatures.

[12]

[13]

[14]

[15]

[16]

Acknowledgements Authors are grateful to MEC and FEDER project CTQ200767792-C02-02 and Junta de Andalucı´a project RNM 547 for financial support.

[17]

[18]

R E F E R E N C E S

[19]

[1] Leo´n y Leo´n CA, Radovic LR. Interfacial chemistry and electrochemistry of carbon surfaces. In: Thrower PA, editor. Chemistry and physics of carbon, vol. 24. New York: Dekker; 1994. p. 213–310. [2] Radovic LR, Moreno-Castilla C, Rivera-Utrilla J. Carbon materials as adsorbents in aqueous solutions. In: Radovic LR, editor. Chemistry and physics of carbon, vol. 27. New York: Dekker; 2001. p. 227–406. [3] Schoonen MAA. Calculation of the point of zero charge of metal oxides between 0 and 350 °C. Geochim Cosmochim Acta 1994;58(13):2845–51. [4] Lyklema J. Fundamentals of interface and colloid science. Fundamentals, vol. 1. New York: Academic Press; 1993. ´ lvarez P, Masa FJ. Activated [5] Garcı´a-Araya JF, Beltra´n FJ, A carbon adsorption of some phenolic compounds present in agroindustrial wastewater. Adsorption 2003;9(2):107–15. [6] Srivastava VC, Swamy MM, Mall ID, Prasad B, Mishra IM. Adsorptive removal of phenol by bagasse fly ash and activated carbon: equilibrium, kinetics and thermodynamics. Colloid Surf A 2006;272(1):89–104. [7] Mishra SK, Kanungo SB, Rajeev. Adsorption of sodium dodecyl benzenesulfonate onto coal. J Colloid Interf Sci 2003;267(1):42–8. [8] Terzyk AP, Rychlicki G. The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: the temperature dependence of adsorption at the neutral pH. Colloid Surf A 2000;163(2):135–50. [9] Terzyk AP, Rychlicki G, Biniak S, Lukaszewicz JP. New correlations between the composition of the surface layer of carbon and its physicochemical properties exposed while paracetamol is adsorbed at different temperatures and pH. J Colloid Interf Sci 2003;257(1):13–30. [10] Terzyk AP. Molecular properties and intermolecular forcesfactors balancing the effect of carbon surface chemistry in adsorption of organics from dilute aqueous solutions. J Colloid Interf Sci 2004;275(1):9–29. ´ lvarez-Merino [11] Fontecha-Ca´mara MA, Lo´pez-Ramo´n MV, A MA, Moreno-Castilla C. About the endothermic nature of the

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

adsorption of the herbicide diuron from aqueous solutions on activated carbon fiber. Carbon 2006;44(11):2335–8. ´ lvarez-Merino Fontecha-Ca´mara MA, Lo´pez-Ramo´n MV, A MA, Moreno-Castilla C. Temperature-dependence of herbicide adsorption from aqueous solutions on activated carbon fiber and cloth. Langmuir 2006;22(23):9586–90. Youssef AM, El-Nabarawy T, Samra SE. Sorption properties of chemically activated carbons. 1. Sorption of cadmium(II) ions. Colloid Surf A 2004;235(1):153–63. Li YH, Di Z, Ding J, Wu D, Luan A, Zhu Y. Adsorption thermodynamics, kinetic and desorption studies of Pb2+ on carbon nanotubes. Water Res 2005;39(4):605–9. Meena AK, Mishra GK, Rai PK, Rajagopal C, Nagar PN. Removal of heavy metal ions from aqueous solutions using carbon aerogel as an adsorbent. J Hazard Mat 2005;122(1):161–70. ´ az-Flores PE, Arago´n-Pin˜a A, MendozaLeyva-Ramos R, Dı Barron J, Guerrero-Coronado RM. Adsorption of cadmium(II) from an aqueous solution onto activated carbon cloth. Sep Sci Technol 2005;40(10):2079–94. Berube YG, de Bruyn PL. Adsorption at the rutile-solution interface: I. Thermodynamic and experimental study. J Colloid Interf Sci 1968;27(2):305–18. Tewari PH, Campbell AB. Temperature dependence of point of zero charge of cobalt and nickel oxides and hydroxides. J Colloid Interf Sci 1976;55(3):531–9. Fokkink LGJ, De Keizer A, Lyklema J. Temperature dependence of the electric double layer on oxides: rutile and hematite. J Colloid Interf Sci 1989;127(1):116–31. Machesky ML, Wesolowski DJ, Palmer DA, Ichiro-Hayashi K. Potentiometric titrations of rutile suspensions to 250 °C. J Colloid Interf Sci 1998;200(2):298–309. Halter WE. Surface acidity constants of a-Al2O3 between 25 and 70 °C. Geochim Cosmochim Acta 1999;63(19): 3077–3085. Morel JP, Marmier N, Hurel C, Morel-Desrosiers N. Effect of temperature on the acid-base properties of the alumina surface: microcalorimetry and acid-base titration experiments. J Colloid Interf Sci 2006;298(2):773–9. Angove MJ, Wells JD, Johnson BB. The influence of temperature on the adsorption of mellitic acid onto goethite. J Colloid Interf Sci 2006;296(1):30–40. Angove MJ, Wells J, Johnson BB. Influence of temperature on the adsorption of mellitic acid onto kaolinite. Langmuir 2006;22(9):4208–14. Blesa MA, Figliolia NM, Maroto AJG, Regazzoni AE. The influence of temperature on the interface magnetite-aqueous electrolyte solution. J Colloid Interf Sci 2006;101(2): 410–418. Finck N, Drot R, Mercier-Bion F, Simoni E, Catalette H. Temperature effects on the surface acidity properties of zirconium diphosphate. J Colloid Interf Sci 2007;312(2): 230–236. Moreno-Castilla C, Carrasco-Marı´n F, Mueden A. The creation of acid carbon surfaces by treatment with (NH4)2S2O8. Carbon 1997;35:1619–26. Benaddi H, Bandosz TJ, Jagiello J, Schwarz JA, Rouzaud JN, Legras D, et al. Surface functionality and porosity of activated carbons obtained from chemical activation of wood. Carbon 2000;38(5):669–74. Moreno-Castilla C, Alvarez-Merino MA, Lopez-Ramon MV, Rivera-Utrilla J. Cadmium ion adsorption on different carbon adsorbents from aqueous solutions. Effect of surface chemistry, pore texture, ionic strength, and dissolved natural organic matter. Langmuir 2004;20(19):8142–8. Figueiredo JL, Pereira MFR, Freitas MMA, Orfao JJM. Modification of the surface chemistry of activated carbons. Carbon 1999;37(9):1379–89.

CARBON

4 6 (2 0 0 8) 7 7 8–78 7

[31] Aksoylu AE, Madalena M, Freitas A, Pereira MF, Figueiredo JL. The effects of different activated carbon supports and support modifications on the properties of Pt/AC catalysts. Carbon 2001;39(2):175–85. [32] Figueiredo JL, Pereira MFR, Freitas MMA, Orfao JJM. Characterization of active sites on carbon catalysts. Ind Eng Chem Res 2007;46(12):4110–5. [33] Puri BR. Surface complexes on carbons. In: Walker PL, editor. Chemistry and physics of carbon, vol. 6. New York: Dekker; 1970. p. 191–281. [34] Zielke U, Hu¨ttinger KJ, Hoffman WP. Surface-oxidized carbon fibers: I. Surface structure and chemistry. Carbon 1996;34(8):983–98. [35] Aguilar C, Garcı´a R, Soto-Garrido G, Arriagada R. Catalytic wet air oxidation of aqueous ammonia with activated carbon. Appl Catal B 2003;46:229–37. [36] Jagiello J. Stable numerical solution of the adsorption integral equation using splines. Langmuir 1994;10(8):2778–85.

787

[37] El Sayed Y, Bandosz TJ. Adsorption of valeric acid from aqueous solution onto activated carbons: role of surface basic sites. J Colloid Interf Sci 2004;273(1):64–72. [38] Perdue EM, Reuter JH, Parrish RS. A statistical model of proton binding by humus. Geochim Cosmochim Acta 1984;48(6):1257–63. [39] Mohan D, Singh KP. Single- and multi-component adsorption of cadmium and zinc using activated carbons derived from bagasse-an agricultural waste. Water Res 2002;36(9): 2304–18. [40] Guo Y, Qi J, Yang S, Yu K, Wang Z, Xu H. Adsorption of Cr(VI) on micro and mesoporous rice husk-based active carbon. Mat Chem Phys 2002;78(1):132–7. [41] Lyubchik SL, Lyubchik AI, Galusko OL, Tikhonova LP, Vital J, Fonseca IM, et al. Kinetics and thermodynamics of the Cr(III) adsorption on the activated carbon from co-mingled wastes. Colloid Surf A 2004;242(1):151–8.