3.1 Properties of Commercial ACs
The textural characteristics SBET, VP, dP, Vmic, L0, and Smic of all ACs are registered in Table 2. The SBET of the ACs decreased from 1357 to 510 m2/g, and the decreasing order is as follows: Wood > Merck > Coconut > Norit > Darco. Furthermore, the total Vp was evaluated at (P/Ps) = 0.99 and varied from 0.44 to 1.18 cm3/g, showing that the porosity of the ACs changed broadly.
Table 2
Textural properties of the commercial ACs.
AC
|
SBET
(m2/g)
|
VPa
(cm3/g)
|
dPb
(nm)
|
Vmicc
(cm3/g)
|
L0d
(nm)
|
Smic
(m2/g)
|
Wood
|
1357
|
1.18
|
3.51
|
0.40
|
1.41
|
746
|
Merck
|
1074
|
0.57
|
2.13
|
0.33
|
1.27
|
554
|
Coconut
|
960
|
0.44
|
1.81
|
0.38
|
0.97
|
800
|
Norit
|
646
|
0.55
|
3.43
|
0.28
|
0.93
|
433
|
Darco
|
510
|
0.58
|
5.34
|
0.21
|
1.22
|
328
|
a Total pore volumen |
b Average pore diameter |
c Micropore volume |
d Micropore average width |
The adsorption isotherms of N2 on the ACs are plotted in Fig. S.1. According to the classification recommended by IUPAC (Rouquerol et al. 2014), the isotherm shapes of Coconut and Merck ACs are type Ia (Fig. S.1 a) and Ib (Fig. S.1 b), respectively. These isotherms are reversible and have a high opening in the adsorption shoulder, which is characteristic of microporous materials. For the isotherm Ia, the type of microporosity is narrower if compared to that of isotherm Ib, where the diameters of the micropores are wider according to the opening of the adsorption shoulder (see isotherm Ib). The adsorption isotherms of the Darco, Norit, and Wood ACs have shapes of type IIb (Fig. S.1 c, d, and e), distinctive of mesoporous materials, showing the hysteresis loops H3 and H4 type. The hysteresis loop H3 (Fig. S.1 c) often occurs in materials formed by aggregates of particles with sheet morphology, while the H4 (Fig. S.1 d and e) is typical of activated carbons and other adsorbents, which have slit shape pores and high distribution of micropores (Boehm 1966).
The Vmic of the Coconut, Merck, Norit, Darco and Wood ACs represented 86, 58, 51, 36 and 34 % of the total Vp, respectively, confirming that the Coconut and Merck ACs consisted mainly of micropores.
Figure S.2 displays the cumulative pore volume and distribution of pore size for all ACs. The accumulated pore volume distribution of the Coconut AC (see Figure S.2 a) shows that the volume of micropores is 94.3 % of the entire pore volume, although the remaining 5.7 % is mesoporous. Furthermore, the pore size distribution is almost unimodal, and the approximate pore diameter was about 0.65 nm. Likewise, in Figure S.2 e, the cumulative volume distribution of the Wood AC revealed that the micropores and mesopores represented 49 and 51 % of the total pore volume, correspondingly. On the other hand, it can be corroborated that most of the micropore sizes are between 0.5 and 0.75 nm for Coconut AC and varying from 0.65 to 0.75 nm for the Wood AC.
Table 3 shows that the concentrations of the total basic and acid sites for the ACs varied from 0.093 to 4.995 meq/g and 0.093 to 1.874 meq/g, respectively. As can be seen, the total acidic and basic sites concentrations ranged widely. The Wood AC surface exhibited a more acidic character (pHPZC = 3.64), considering that the concentration of acidic sites was 7.7 times larger than that of the basic ones. Otherwise, the concentration of basic sites of Coconut AC was 7-fold larger than those of acid sites (pHPZC = 10.85). In general, the acid sites concentrations of the ACs decreased as follows: Wood > Coconut > Norit > Darco > Merck, whereas the basic sites diminished in the subsequent order: Coconut > Norit > Wood > Merck ≈ Darco.
Table 3
The concentration of acidic and basic sites of the commercial ACs.
Activated Carbon
|
Total acid sites (meq/g)
|
Total basic sites (meq/g)
|
Coconut
|
0.711
|
4.995
|
Merck
|
0.093
|
0.093
|
Darco
|
0.141
|
0.093
|
Norit
|
0.313
|
1.366
|
Wood
|
1.874
|
0.243
|
3.2 Modeling the adsorption data of RNZ and DCF
The adsorption isotherm of Radke-Prausnitz (R-P) interpreted the data for the adsorption equilibrium of both pharmaceuticals. This isotherm model is mathematically expressed as follows (Leyva-Ramos 2007):
where the R-P isotherm parameters are β, a (L/g) and b (Lβ/mgβ).
The parameters for adsorption isotherm were calculated by matching the adsorption model to the data using the Rosenbrock-Newton optimization algorithm. Besides, the average percent deviation for each adsorption model, %D, was appraised using the succeeding mathematical relationship:
The data were also described by the Langmuir and Freundlich isotherms (Moral-Rodríguez, 2019); however, the %D values for the R-P isotherm model were shorter than the %D values of the Freundlich and Langmuir adsorption models in 22 out of the 28 experimental conditions registered in Tables 4 and 5. Therefore, the R-P model better interpreted the experimental data since it is a three-parameter isotherm, while the Langmuir and Freundlich isotherms are two-parameter adsorption models. Tables 4 and 5 list the parameters and % D for the R-P isotherm. The R-P adsorption model adequately represented the experimental data since the % D varied from 0.9 to 21.0 %.
Table 4
Parameter values of the Radke-Prausnitz adsorption isotherms for the adsorption of RNZ and DCF in aqueous solution on ACs at T = 25 ºC and pH = 7.
Compound
|
AC
|
a
(L/g)
|
b
(Lβ/mgβ)
|
β
|
%D
|
RNZ
|
Wood
|
50.3
|
0.46
|
0.81
|
3.7
|
Merck
|
46.5
|
0.51
|
0.83
|
0.9
|
Coconut
|
55.6
|
0.42
|
0.80
|
8.0
|
Norit
|
165.3
|
1.26
|
0.89
|
4.4
|
Darco
|
188.1
|
3.01
|
0.82
|
4.6
|
DCF
|
Wood
|
29.2
|
0.10
|
0.92
|
9.9
|
Merck
|
103.9
|
1.30
|
0.82
|
10.5
|
Coconut
|
709.3
|
6.70
|
0.88
|
14.1
|
Norit
|
103.9
|
1.30
|
0.82
|
4.9
|
Darco
|
5.4
|
0.05
|
0.90
|
1.8
|
Table 5
Parameters of the Radke-Prausnitz adsorption isotherms for the adsorption of RNZ on Coconut and DCF on Wood from aqueous solution at different operating conditions and I = 0.01 N.
Operating
Conditions
|
RNZ on Coconut
|
DCF on Wood
|
T
(°C)
|
pH
|
a
(L/g)
|
b
(Lβ/mgβ)
|
β
|
%D
|
a
(L/g)
|
b
(Lβ/mgβ)
|
β
|
%D
|
25
|
6
|
|
|
|
|
114.0
|
0.26
|
0.95
|
15.0
|
25
|
7
|
55.6
|
0.42
|
0.80
|
8.0
|
29.2
|
0.10
|
0.92
|
9.9
|
25
|
9
|
312.6
|
2.55
|
0.78
|
21.0
|
36.3
|
0.17
|
0.94
|
1.4
|
25
|
11
|
27.9
|
0.20
|
0.75
|
2.5
|
118.9
|
1.15
|
0.84
|
3.1
|
15
|
7
|
84.2
|
0.63
|
0.84
|
9.0
|
22.7
|
0.23
|
0.78
|
18.6
|
25
|
7
|
55.6
|
0.42
|
0.80
|
8.0
|
29.2
|
0.10
|
0.92
|
9.9
|
35
|
7
|
167.2
|
1.34
|
0.80
|
1.0
|
23.7
|
0.06
|
0.97
|
2.0
|
3.3 Adsorption of RNZ and DCF on ACs
At T = 25 ºC and pH = 7, the isotherms of RNZ and DCF adsorbed on ACs are shown in Fig. 2a and 2b. As depicted in Fig. 2a, the capacities of ACs for adsorbing RNZ in water solution diminished as follows: Coconut > Wood > Norit > Merck > Darco. At the RNZ equilibrium concentration of 500 mg/L, the uptake of RNZ adsorbed (Q500) upon the Coconut, Wood, Norit, Merck and Darco is 434, 350, 283, 261 and 188 mg/g, respectively. It can be noted that Coconut and Darco presented the largest and lowest adsorption capacity towards RNZ. In Fig. 2b, it is observed that Wood had the highest adsorption capacity towards DCF. The Q500 for DCF (Q500) on the Wood, Merck, Coconut, Norit and Darco is 396, 248, 222, 182 and 166 mg/g, correspondingly, so that the AC capacities for adsorbing DCF decreased in the subsequent series: Wood > Merck > Coconut > Norit > Darco.
In this work, the maximum uptake of RNZ adsorbed on Coconut AC was 444 mg/g at pH of 7 and T of 25°C and was slightly larger than those presented in previous studies (Méndez-Díaz et al. 2010; Moral-Rodríguez et al. 2016). The maximum adsorption capacities of three commercial carbons towards RNZ ranged from 376 to 394 mg/g (Méndez-Díaz et al. 2010; Moral-Rodríguez et al. 2016). While the Wood AC presented the maximum uptake of DCF adsorbed of 441 mg/g, which is within the range (47.12–1033 mg/g) found for the adsorption capacities of pristine and modified ACs (Moral-Rodriguez et al. 2019; Viotti et al. 2019).
Figure 3 depicts the molar uptake of DCF and RNZ adsorbed for the concentration at equilibrium of 500 mg/g, Q500 (mmol/g), graphed vs. the BET surface area of the AC. It can be noticed that the capacity of ACs for DCF incremented approximately linearly by augmenting SBET. In the case of RNZ, the capacity of ACs for adsorbing RNZ increased somehow linearly with surface area, except for the Coconut AC. The finding that the surface area affected the adsorption capacity corroborated that the π-π dispersive interactions were the predominant adsorption mechanism. These interactions are related to the π electrons of the aromatic ring of RNZ or DCF and the π electrons existing in the AC graphene planes. The Coconut AC had the greatest adsorption capacity towards RNZ, but the Coconut AC did not have the highest SBET and was the AC having the largest concentration of basic sites. This result demonstrated that the surface chemistry of ACs could also affect their adsorption capacity.
The molar Q500 of RNZ was always higher than that of DCF independently of the AC. This result can be ascribed to the molecular dimensions of RNZ (Table 1), which are shorter than those of DCF so that the RNZ molecules can access more micropores than the DCF molecules, and more adsorption sites are available for adsorbing RNZ.
The molar Q500 for RNZ and DFC vs. the concentration of acidic sites per unit surface area of AC (acidic sites/SBET) are plotted in Fig. 4. Overall, the acidic sites concentration promoted the adsorption capacity of ACs. Except for Merck AC, the capacities of the ACs for adsorbing DCF were raised linearly by increasing the concentration of acidic sites. Again, the Coconut AC exhibited the highest Q500 for RNZ, but this carbon did not have the largest concentration of acidic sites. The acidic site concentration favored the AC adsorption capacity because some of the acidic sites can activate the π-π dispersion interactions (Carrales-Alvarado et al. 2014). The results indicated that the textural and chemical characteristics of ACs significantly influenced the ACs adsorption capacities towards RNZ and DCF from water solutions.
3.4 Influence of pH on the capacity of Coconut AC for adsorbing RNZ
The dependence of the Coconut AC capacity for RNZ on the pH is exhibited in Fig. 5, and the adsorption capacity rises marginally by augmenting the pH from 3 to 7; however, at pH = 11, the adsorption of RNZ on Coconut AC was significantly enhanced when the concentrations of RNZ at equilibrium were higher than 200 mg/L. For the RNZ concentration of 300 mg/L, the RNZ uptakes at pH 3, 7, and 11 were 364, 389, and 548 mg/g, respectively. Therefore, the mass of RNZ adsorbed at pH 11 was 1.5 and 1.4-fold higher than those at pH 3 and 7, respectively.
The above results can be rationalized according to the speciation diagram of RNZ (Fig. 1), which indicates that in the pH range of 3–9, the RNZ molecule exists as the undissociated species, and the surface of the Coconut AC is positively charged (pHPZC = 10.85). Hence, in this pH range, the electrostatic interactions did not influence the adsorption of RNZ, confirming that the RNZ is adsorbed on Coconut AC by π-π interactions mainly. Although the surface of the Coconut AC is now slightly negatively charged at pH = 11, and the RNZ is still present as the neutral species, substantiating that the RNZ adsorption was not changed by electrostatic interactions. However, at pH = 11, the adsorption capacity increased for concentrations higher than 100 mg/L. The increase was due to the reduction of the RNZ solubility since the concentration of Na+ ions is augmented by incrementing the solution pH. Consequently, the hydrophobic interactions between the surface of the Coconut AC and the RNZ were favored. Thus, the adsorption of RNZ en Coconut AC at pH = 11 is related to the π-π dispersive interactions and hydrophobic effect.
3.5 Influence of pH on the capacity of Wood AC for adsorbing DCF
Figure 6 illustrates the pH influence on the capacity of Wood AC for adsorbing DCF. The adsorption capacity diminished considerably and moderately by incrementing the pH from 6 to 9 and 9 to 11, correspondingly. None adsorption runs were performed at pH < 6 because the DCF solubility in water is low (Llinàs et al. 2007). For a DCF equilibrium concentration of 300 mg/L, the uptake of DCF adsorbed was 568, 353, 278 and 244 mg/g for the pH values of 6, 7, 9 and 11, correspondingly. The capacity of Wood AC at pH = 6 was 1.6, 2.0 and 2.3 times higher than those at pH of 7, 9 and 11.
The above behavior can be ascribed to the fact that the DCF molecules in water are the anionic species (DCF−) in the pH span from 6 to 11, while the surface of Wood AC is negatively charged (pHPZC = 3.64). Thus, the lessening of the adsorption capacity was associated with the increment of the electrostatic repellence existing between the surface of the Wood AC and DCF− because the negative charge of AC surface augments by raising the pH.
The influence of the electrostatic interactions in the DCF adsorption mechanism on Wood AC was further analyzed by carrying out adsorption runs at the solution ionic strengths of 0.01, 0.1 and 1.0 N (Moral-Rodriguez, 2019). The results (not shown in this work) demonstrated that the capacity of Wood AC for adsorbing DCF increased while raising the ionic strength. The ionic strength was varied by changing the NaCl concentration in the solution, so the Na+ ions adsorb on the AC negative surface, balancing the negative charge of the AC and decreasing the repulsion between DCF− and the Wood AC surface, enhancing the adsorption of DCF. This effect is known as the screening effect (Moreno-Castilla 2004).
3.6 Effect of temperature on the capacity of Coconut and Wood ACs for adsorbing RNZ and DCF
The influence of temperature on the uptake of RNZ and DCF adsorbed on Coconut and Wood ACs at pH = 7 is depicted in Fig. 7. For an RNZ equilibrium concentration of 400 mg/L (See Fig. 7a), the uptake of RNZ adsorbed on Coconut AC was promoted and non-influenced by incrementing the temperature from 15 to 25°C and 25 to 35°C, respectively. A comparable tendency was also noted for the adsorption of RNZ on an AC commercially known as Filtrasorb 400 when the temperature varied from 10 to 40°C (Moral-Rodríguez et al. 2016). The mass of RNZ adsorbed at 35, 25 and 15°C was 408, 403 and 352 mg/g, correspondingly indicating that the capacity of Coconut AC increased 14.5 % when the temperature rose from 15 to 25°C. Likewise, Fig. 7b shows that the adsorption of DCF was significantly influenced by increasing the temperature from 15 to 25 and slightly augmented from 25 to 35°C. For a DCF concentration of 400 mg/g, the uptakes of DCF adsorbed were 363, 413 and 427 mg/g at 15, 25 and 35°C. These outcomes verified that the adsorption capacity of Wood AC towards DCF was raised 14 % and 3.4 % while incrementing the temperature from 15 to 25°C and 25 to 35°C.
The isosteric adsorption heat, (ΔHads)q, for RNZ and DCF, was estimated using the experimental data at 15 and 25°C since the adsorption capacity varied in this temperature range. The (ΔHads)q was estimated employing the following equation (Leyva-Ramos 1989):
where (ΔHads)q is the isosteric adsorption enthalpy, J/mol; R is the gas law constant, 8.314 J/K mol; C2 and C1 are the equilibrium concentrations of the pharmaceutical at temperatures T2 and T1, correspondingly, and at the same mass of the pharmaceutical adsorbed at equilibrium (q), mg/L; T2 and T1 are the temperatures at the conditions 2 and 1, respectively, K.
The ΔHads was estimated to be 56.5 and 56.3 kJ/mol for the adsorption of RNZ on Coconut AC at q = 358 mg/g, and DCF on Wood AC at q = 372 mg/g, correspondingly. Thus, the adsorption of both pharmaceuticals was endothermic. It is worthwhile to mention that the ΔHads decreased as the q was reduced because the experimental adsorption equilibrium data were overlapped for q less than 270 mg/g.