High basicity adsorbents from solid residue of cellulose and synthetic polymer co-pyrolysis for phenol removal: Kinetics and mechanism

Applied Surface Science 316 (2014) 435–442

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High basicity adsorbents from solid residue of cellulose and synthetic polymer co-pyrolysis for phenol removal: Kinetics and mechanism Ewa Lorenc-Grabowska ∗ , Piotr Rutkowski Wrocław University of Technology, Faculty of Chemistry, Department of Polymer and Carbonaceous Materials, Gda´ nska 7/9, 50-344 Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 17 June 2014 Received in revised form 5 August 2014 Accepted 5 August 2014 Available online 13 August 2014 Keywords: Biomass Activated carbon Phenol Adsorption mechanism Adsorption kinetics

a b s t r a c t The activated carbons (ACs) produced from solid residue of cellulose and synthetic polymer co-pyrolysis (CACs) and commercial activated carbon from coconut shell (GC) were used for phenol removal. The adsorption kinetics and mechanism were investigated. All studied activated carbons are predominantly microporous and are characterized by basic surface characteristics. Surface area SBET varies between 1235 and 1499 m2 /g, whereas the pHPZC changes from 7.70 to 10.63. The bath adsorption of phenol (P) was carried out at ambient temperature. The equilibrium time and equilibrium sorption capacity were determined. It was found that the boundary layer effect is bigger in AC with high basic characteristics of the surface. The rate controlling step is the intraparticle diffusion in CACs only, whereas in ACs with higher amount of acidic functionalities the adsorbate–surface interaction influences the rate of kinetic as well. The equilibrium isotherms are L2 type for commercial AC and L4 for CACs. The CACs are characterized by very high adsorption capacity that vary between 312 and 417 mg/g. The main mechanism of phenol adsorption is micropore filling within pores smaller than 1.4 nm. In the absence of solvent effect further adsorption of phenol on CACs takes place. The enhanced adsorption is due to dispersive/repulsive interaction induced by oxygen functionalities. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Human activity is reflected in a continuous wearing out of natural goods of the earth and increasing production of waste (industrial, urban and agriculture). That why, it becomes necessary to find a way to minimize the negative impacts of the waste on the environment and to reuse it. The used glass, metals, clothes can be easy recycled whereas solid organics waste including paper, plastic bottles, tires, agricultural or food waste are mainly considered as a source of the renewable energy. Energy cumulated in such waste can be recovered directly by burning or indirectly by pyrolysis and gasification processes. The pyrolysis of biomass leads to liquids, char and gaseous products. The yield of liquid product can be efficiently enhanced when process of fast pyrolysis is applied [1–3], whereas the quality and quantity of bio-oil can be satisfactorily improved by the addition of polymers including waste polymers [2]. The char obtained in co-pyrolysis of biomass and polymers is the by-product that can be utilized by burning. The char is mainly composed by a carbon-rich material [4,5], so it might be considered as a good precursor for other product manufacture.

∗ Corresponding author. Tel.: +48 713206431; fax: +48 713206203. E-mail address: [email protected] (E. Lorenc-Grabowska). http://dx.doi.org/10.1016/j.apsusc.2014.08.024 0169-4332/© 2014 Elsevier B.V. All rights reserved.

Due to the low surface area, biomass and the chars obtained from pyrolysis of biomass and waste are cheap but not very efficient adsorbents [4,6–10]. However, the solid residues become promising precursors for activated carbons (ACs) production by both physical and chemical activation [11–14]. Depending on the different texture properties and surface chemical characteristics, the ACs are used as adsorbents in air pollution purification or water and wastewater treatment. Recent years have shown remarkable increase in the level of different synthetic organic chemicals in water supplies. Hundreds of hazardous chemicals such as pesticides, herbicides, detergents, dyes, nitrosamines and phenolic compounds are found in water. Phenol and its derivatives are generally considered as a group of common environmental contaminants. Even low concentration of these compounds can be obstacle to use of water. Phenols not only cause unpleasant taste or odour but also exert negative effects on different biological processes. Moreover, these compounds are very often recognized/considered as muta- and/or carcinogenic. Phenol and phenolic compounds are present in wastewaters from coalrelated industries, i.e. coal gasification and coking plants as well as from pharmaceutical, plastics, rubber, disinfectant or agricultural run-off. The removal or destruction of phenolic compound is essential for purification of wastewater or raw water treatment. Various methods have been proposed for the treatment of wastewaters

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E. Lorenc-Grabowska, P. Rutkowski / Applied Surface Science 316 (2014) 435–442

like chemical oxidation, separation, filtration, etc. One of the most efficient ways of the water purification is adsorption on activated carbons (ACs) [12, 15]. Adsorption capacity of ACs depends on many factors, i.e. the porous structure of adsorbents, its surface characteristics or ash contamination. The characteristics of the adsorbates, i.e. the molecule size, molar mass, geometrical shape, solubility, pKa , pH of the solution, ionic strength and temperature influence the mechanism and kinetics of organic compounds adsorption. Phenol is relatively low-molecular-weight compound characterized by slightly acidic properties that why microporous ACs with basic surface properties is required for its efficient removal. There are many different methods proposed to prepare ACs with basic surface characteristics [16–21]. Among them, the high temperature treatment of AC in inert, hydrogen or ammonia atmosphere is believed to be the easiest way of producing porous carbons with basic surface properties [17–20]. Another way of producing basic porous carbons comprises carbonization and subsequent activation of the resultant chars from nitrogen containing polymers [20–25], carbon precursor like lignite and pitch with a nitrogen carrier, for example urea and melamine [26]. In present work microporous activated carbons with distinctively basic surface characteristic produced from solid residue of co-pyrolysis of cellulose and polymer have been used for adsorption tests from water. Taking into account the activated carbon characteristics, its adsorption ability towards phenol has been evaluated. Using the char of organic waste as an adsorbent precursor, brings a twofold solution to environmental problems. It reduces the volume of solid wastes and provides a low-cost adsorbent for the waste water treatment. Another advantage of using the waste as a precursor of AC is that due to low production cost, the regeneration of used AC is not required. This work has dual objective. The first one is to evaluate the utility of activated carbon from solid residue of fast co-pyrolysis of cellulose and biomass for organic compounds removal from water. The second is to gain an understanding of the mechanism and kinetics of the adsorption of phenol from aqueous solution on activated carbon with basic surface characteristics in terms of pore size distribution (PSD) influence. 2. Experimental 2.1. Activated carbon preparation The microporous activated carbons were produced from cellulose and synthetic polymer blends (CACs). The solid residues of a mixture of cellulose/polystyrene (3:1) (CPS), cellulose/ polypropylene (3:1) (CPP) and only cellulose (C) have been produced in two steps pyrolysis. In the first step the sample is slowly heated up to 400 ◦ C with heating rate 3 ◦ C/min and next the second step is the fast pyrolysis with heating rate 100 ◦ C/second up to 900 ◦ C. Finally, chars have been steam activated at 800 ◦ C to 50% burn off. For comparison commercial (Gryfscand) activated carbon (GC) produced by steam activation of coconut shells was used. The grain size of the studied activated carbons was below 0.2 mm. 2.2. Activated carbon characterization The elemental analysis of C, H, N and S was performed using a Vario III Elemental Analyzer. The oxygen content was calculated by difference. The porous texture was determined from nitrogen adsorption isotherms measured at 77 K with a NOVA 2200 (Quantachrome). The specific surface area was calculated using the BET method at p/p0 < 0.15. The amount of nitrogen adsorbed at relative pressure of p/p0 = 0.99 was employed to determine the total pore

Table 1 Physicochemical characteristic of phenol. Phenol (P) Molar mass (g/mol) Solubility (g/dm3 ) pKa  (nm)  (nm2 ) Molecular volume (nm3 )

Molecule dimension (nm)

94.11 80 9.98 210 0.437 0.162

volume (VT ). The micropore volume (VDR ) was calculated applying the Dubinin–Radushkevich equation up to p/p0 ≤ 0.05. The pore size distribution (PSD) was determined by means of the Density Functional Theory (DFT) method using Quantachrome software. The volume of pores smaller than 1.4 nm (V1.4 ) was calculated on the basis of DFT results. Base–acid titration based on Boehm’s method was performed to measure the total amount of basic and acidic surface groups of carbons using solutions of NaOH and HCl. The pHPZC (point of zero charge) of the ACs was determined according to the procedure described by Moreno-Castilla et al. [27]. 2.3. Phenol adsorption The bath adsorption of phenol (P), (POCH, Poland) from aqueous solutions was carried out at 24 ◦ C. For the adsorption, 0.01–0.2 g of AC was placed into Erlenmeyer flasks and 0.10 dm3 of adsorbate solution (150 mg/dm3 ) was added to each flask. The stoppered flasks were kept in a thermostat shaker bath and were agitated to reach equilibrium. Each set of flasks included two flasks containing blank solutions to check for sorbate volatilization and adsorption on the glass walls. The adsorption isotherms were determined without adding any buffer to control pH to avoid the presence of a new electrolyte in the system. The solution pH was measured by a digital pH-metre (Metler Tolledo) using a combined glass electrode. The concentration of each solute remaining in the water phase was determined using HITACHI U-2800A UV–vis spectrophotometer at the wavelength of 270 nm. The basic phenol characteristics are given in Table 1. 3. Results and discussion 3.1. ACs characterization The characteristic of series of three ACs produced from solid residue of co-pyrolysis of cellulose and polymer and one commercial AC used for phenol adsorption is given in Table 2. Activated carbons obtained by steam activation of solid residue of copyrolysis of cellulose and polymers are ash free and predominantly microporous. The micropores consist of 75–80% of the total pore volume of CACs. The SBET varies from 1235 to 1317 m2 /g, total pore volume is in the range of 0.616–0.570 cm3 /g whereas micropore volume (VDR ) changes from 0.500 to 0.530 cm3 /g. The commercial activated carbon is microporous as well, however is not ash free. The ash content is as high as 3.30 wt%. The GC is characterized by the highest SBET (1499 m2 /g), total pore volume (0.697 cm3 /g), micropore volume (VDR 0.540 cm3 /g) and mesopore volume (0.157 cm3 /g) of all tested adsorbents. As can be seen in Fig. 1 the pores with size 1.0–1.8 are prevalent for all studied carbons. The sharp and narrow maximum in pore volume at pore width 1.2–1.4 nm can be observed for the CACs. In the case of GC activated carbon, the maximum in PSD is assigned to pores with width 1.2–1.4 as well, however the volume of pore with this size is much smaller compared to CACs. The CACs are characterized by similar carbon content ranging from 94.9 to 95.1 wt% and no sulfur content. The GC has the lowest carbon content (92.2 wt%) and highest sulfur content (0.30 wt%).

E. Lorenc-Grabowska, P. Rutkowski / Applied Surface Science 316 (2014) 435–442 Table 2 Characteristics of studied activated carbons. C

CPP

Proximate analysis (wt%) 0.10 Moisturea a Ashd a 0.00

250 C

CPS

200 0.09 0.00

0.11 0.00

0.50 3.30

94.86 0.64 0.13 0.00 4.37 10.31

95.53 0.67 0.15 0.00 3.65 10.63

92.23 2.39 0.40 0.30 4.02 7.70

Functional groups (mmol/g) Basic 1.02 Acidic 0.40

0.98 0.39

1.10 0.41

0.63 0.87

Porous texture parameters 1317 SBET (m2 /g) 0.608 VT (cm3 /g) 3 0.530 VDR (cm /g) VDR /VT 0.80 1.29 L0 (nm)

1235 0.570 0.502 0.80 1.32

1240 0.616 0.500 0.75 1.15

1499 0.697 0.570 0.82 1.48

Ultimate analysis (wt%) Cdaf Hdaf Ndaf Sd Odiff b pHPZC

95.08 0.66 0.09 0.00 4.17 10.45

CPP

GC

CPS GC

qt (mg/g)

Parameter

437

150

100

50

c

a b c

d, moisture free content; a, sample in analytical state; daf, dry ash free content. diff, calculated content. Determined by Boehm method.

The content of heteroatoms O + N is similar for all ACs. The oxygen content is relatively low, ranging from 3.6 wt% for CPS to 4.4 wt% for CPP through 4.2 wt% for C and 4.0 wt% for GC. The nitrogen content is below 0.15 wt% in the case of CACs whereas for GC is 0.4 wt%. The CACs are characterized by basic surface characteristics. The pHPZC differs slightly between 10.31 and 10.63. The functional groups were determined by Boehm method. Results presented in Table 2 show that the amount of basic functional groups is twofold higher than the amount of acidic functional groups. The lack of ash and low heteroatom O + N content, lower than 4.5 wt%, allows to conclude that the basicity is governed mainly by the delocalized ␲ electrons of the graphene layers [28]. The commercial AC contains small amount of ash that influence the properties of activated carbon to some extent. The GC is characterized by slightly basic surface characteristic (pHPZC = 7.7) and comparable amounts of acidic and basic functional groups (Table 2). 3.2. Adsorption of phenol from aqueous solution 3.2.1. Kinetics of adsorption The adsorption of phenol on CACs and GC as a function of time is shown in Fig. 2. As can be seen, the adsorption on CACs is very fast.

0.14 C

Pore volume (cm3 /g)

0.12

CPP CPS

0.1

GC

0.08 0.06 0.04 0.02 0

Pore width (nm) Fig. 1. Pore size distribution of studied ACs determined by DFT method

0 0

50

100

150

200

time (h) Fig. 2. Phenol adsorption versus time on: C (♦); CPP (); CPS (); GC (); C0 = 150 mg/dm3 ; V = 0.10 dm3 ; mAC = 0.050 g.

Over 50% of equilibrium sorption capacity for CACs is obtained after only half an hour, whereas in the case of GC it is only 15%. The equilibrium time is increasing in following direction CPS (10 h) < CPP (12 h) < C (16 h) < GC (18 h). In systems with uniform experimental conditions, the equilibrium time is usually related with the volume of meso- and macropores that consists the transporting arteria. Such a relation is not observed in our case as mesopore volume increases in the direction CPP < C < CPS < GC. In the case of series of CACs the shortest equilibrium time is found for AC with the highest mesopore volume. Surprisingly, GC which is characterized by the highest mesopore volume among all studied ACs, has the longest equilibrium time. To get a better insight into kinetics, the model proposed by Weber and Morris [29] has been applied. In this model, described by Eq. (1), the intraparticle diffusion is considered as a rate-limiting step. qt =

kp t 1/2

,

(1)

where qt and kp are the amount adsorbed at time t (mg/g) and intraparticle rate constant (mg/g min1/2 ), respectively. The process of adsorption from solution can be divided into a few steps: molecules transportation from the bulk phase to the exterior surface of the adsorbent, next the diffusion in the film of boundary layer to the surface of adsorbate, the transport into the adsorbent by pores (intraparticle diffusion) and/or surface diffusion and the final adsorption onto active sites. Usually the first and last step are considered to be very fast, thereby cannot limit the rate of kinetics [30,31]. The plots of fractional uptake for P versus square root time on studied ACs are shown in Fig. 3. It can be observed that the plots are not linear over the whole time range, implying that more than one process affects the P adsorption. The occurrence of an external layer diffusion process can be deduced from the fact that the plots do not pass through the origin. Based on the thickness of the intercept of the plot, the boundary layer effect can be estimated. In our previous work [21] we observed that the decreasing order of the thickness of the layer was reflecting an enhanced affinity of phenol towards the ACs containing nitrogen functionalities. The higher the number of nitrogen groups contained in the ACs, the lower the boundary layer thickness was. In this work we observe that the activated carbons with higher basic surface characteristic (CACs) have stronger boundary layer effect compared to less basic carbon (GC). This can be explained by the difference in the movement of dissolved molecules together with the solvent in boundary of solution. In solution, the dissolved phenol

438

E. Lorenc-Grabowska, P. Rutkowski / Applied Surface Science 316 (2014) 435–442 Table 3 Langmuir and Freundlich parameters for phenol adsorption on ACs.

250 C CPP GC

qt (mg/g)

Adsorbent

C

CPP

CPS

Langmuir qmax (mg/g) b (dm3 /g)  R2

GC

417 0.025 0.885 0.993

407 0.033 0.922 0.964

312 0.070 0.704 0.994

145 0.160 0.207 0.998

51 0.391 0.935

49 0.238 0.968

69 0.149 0.822

CPS

200

150

Freundlich Kf (mg1−n dm3n /g) n R2

100

50

80 0.396 0.9282

pH of solution after adsorption 9.5–9.8

9.4–9.9

9.5–9.8

7.0–7.3

0 25

50

75

100

time 0.5 (min 0.5) Fig. 3. Intraparticle diffusion model on: C (♦); CPP (); CPS (); GC ().

molecules are surrounded by water molecules. The surface of ACs with insufficient amount of hydrophilic functional groups does not promote the water molecules to move towards the surface. The surface of CACs does not contain many hydrophilic groups (in contrast to mentioned N-based AC) hence is not very attractive to water molecule. A consequence of this is a large boundary layer effect. The GC with slightly basic surface characteristics, higher amount of acidic functionalities and hydrophilic ash contamination gives lower boundary layer effect as water molecules and phenol molecules surrounded by the water, are attracted by the surface of GC carbon. This leads to a faster molecules movement in the film of boundary layer. However, this should not be mistaken for the affinity of phenol molecules towards the active sites on surface of AC. The high affinity of phenol molecule to the AC surface is reflected in the shape of equilibrium isotherm, where even at low adsorbate concentration high adsorption capacity is observed. The diffusion in boundary layer performs when the concentration of adsorbate in solution is high. This is the first stage in the adsorption process that might be rate limiting. The second stage is the movement of molecules to the active sites through transporting pores and is represented by the first part of the plot. The final stage of intraparticle diffusion is the diffusion in micropores that is reflected in the last part of the plot. In the case of intraparticle diffusion the slope of the plot indicates the rate of adsorption. For all carbons studied in this work, the slope of the second part of plot is the smallest and nearly parallel to x-axis indicating that intraparticle diffusion into the micropores is the rate-controlling step. The slops are alike that allowed to conclude that the rate of phenol adsorption in these pores is very similar. Some differences between carbons in the slope of the first portion of plots are observed. Two couples of carbons C–CPP and CPS–GC are characterized by comparable slop of the first part of the plot (Fig. 3). The carbons with higher volume of mesopore are characterized by bigger slope of the first part of plot reflecting faster diffusion of molecule. The reason why the GC is characterized by the longest equilibrium adsorption time despite the fast diffusion in mesopores and low boundary layer might be explained by the mentioned earlier higher affinity of water to the surface of that carbon. The adsorbing phenol molecules have to displace the water molecules from active sites hence, despite the quick movement of phenol molecule towards active sites, the equilibrium time is not reached as fast as in the case of CACs as the water competition effect is observed. Nevskaia et al. [32] in his studies on kinetics of phenol also stated different rate controlling mechanisms for the non-modified and oxidized carbons. The diffusion in pores in the non-modified

carbon was postulated to be the rate controlling step whereas the adsorbate–surface interaction was the rate controlling in the case of oxidized carbons. However in that case shorter equilibrium time was observed for oxidized carbons.

3.2.2. Equilibrium adsorption of phenol The processes of phenol adsorption were carried out in unbuffered condition. Taking into account the pH of solution after the adsorption (Table 3) the electrostatic forces do not influence the adsorption processes on GC carbon. In the case of adsorption on CACs some phenol molecules might exist in ionic form that might insignificantly enhance the P adsorption. The adsorption isotherms for phenol on the ACs studied are shown in Fig. 4. According to Giles classification, the isotherms belong to the L type however, for the GC carbon it is L2 whereas for C, CPP, CPS carbons the type L4 is observed. The L2 class of isotherms is commonly reported for the adsorption of phenols from aqueous solution [16,21,28,33–36]. This is the case when no strong competition between the adsorbate and the solvent for occupying the adsorption sites is observed. The isotherm types other than L1 or L2 are rarely observed in the literature concerning phenolic compounds adsorption [20,37–39]. The appearance of this type of isotherm is attributed to an enhanced adsorption at higher concentration that might be explained by few factors, i.e. the multilayer adsorption, next the reorientation of P molecule from the flat position to vertical, and finally the adsorption on different type of adsorption sites. Despite the fact that in literature many different equations [33,37,40-44] are used to interpret the equilibrium adsorption

400

C CPP CPS GC

350 300

qe (mg/g)

0

250 200 150 100 50 0 0

50

100

150

200

ce (mg/dm3) Fig. 4. Equilibrium adsorption isotherm of P on: C (♦); CPP (); CPS (); GC ().

E. Lorenc-Grabowska, P. Rutkowski / Applied Surface Science 316 (2014) 435–442

439

Table 4 Phenol adsorption capacity on commercial AC and ACs from waste material. Adsorbent

SBET (m2 /g)

Vmic (cm3 /g)

pHPZC

qmax (mg/g)

Refs.

Vet-H20 Cane pitch/steam/810 CZ2.5–9 A-PAN A-PAN/CTP A-PET/CTP PET

1185 593 1339 730 740 760 1170

0.360 0.303 0.465 0.286 0.292 0.309 0.425

11.5

[39] [32] [33] [18]

AC-NA ACN650 CK10 Filtrasorb100

2340 1453 2451 937

1.018 0.370 1.048 0.400

145 301 250 167 161 160 (2.49 mmol/g) 234 238 243 369 206

8.82 7.83 7.36 6.20

9.40 – –

isotherms, in this work only the Langmuir (Eq. (2)) and Freundlich (Eq. (3)) models were used to interpret the data.

[26] [45] [34] [46]

(3)

It is observed that large surface area and a big micropore volume are necessary but not sufficient attributes of an optimized adsorbent for phenol removal. In this work we also observe that the phenol removal is not only dependent on the activated carbon porous surface area or micropore volume as GC with highest SBET and VDR shows the lowest adsorption capacity towards phenol.

where qe is the phenol amount adsorbed by carbon at equilibrium (mg/g), ce is the equilibrium phenol concentration in solution (mg/dm3 ), qmax is the monolayer capacity of the adsorbent (mg/g), b is the Langmuir adsorption constant (dm3 /mg), Kf is the Freundlich constant (mg1−n dm3n /g), and 1/n is the heterogeneity factor. These models are the simplest ones but directly give information on the adsorption capacity or heterogeneity factor that allow to study the mechanism of organic compounds adsorption. The use of other equations mostly refers to the search for the equation with best fit [33,42–45]. The calculated values of the Langmuir and Freundlich equation’s parameters are given in Table 3. The comparison of the R2 values of the linearized form of both equations indicates that the Langmuir model yields a better fit than the Freundlich model and can be applied to entire region of phenol concentration. The Langmuir monolayer capacity decreases as follows: C > CPP > CPS > GC and is between 417 and 145 mg/g. The adsorption capacity obtained for CACs Langmuir is very high. The amount of phenol adsorbed on different low-cost adsorbent usually does not exceed 100 mg/g [42,46], whereas the adsorption capacity of commercial ACs or ACs obtained from waste biomass and polymers usually varies between 100 and 300 mg/g [16,20,21,28,34–36,42–48]. The Langmuir adsorption capacity obtained for different activated carbons tested without any electrolyte addition to phenol solution is presented in Table 4.

3.2.3. On the mechanism of phenol adsorption Generally three mechanisms of phenolic compounds adsorption are considered: the ␲–␲ dispersion interaction, the electron–donor–acceptor complex formation and the hydrogenbond formation [47,49–54]. The first mechanism is based on the dispersion forces between the ␲ electrons of phenol ring and the ␲ electron of the graphene layer in the activated carbon. The ␲–␲ interactions are reinforced when adsorption takes place in smaller micropore as micropore filling. This is due to the enhanced adsorption potential occurring from the neighbouring graphene walls. The second mechanism proposes the formation of donor–acceptor complex between the surface electron donor groups (e.g. carbonyls) and the aromatic ring of phenol that acts as the electron acceptor. This type of interaction appears in wider micropores and mesopores. In the third, case the hydrogen of hydroxyl groups of phenol is involved in an intracomplex hydrogen bond with surface oxygen complex. In Fig. 5 the relationship between the Langmuir adsorption capacity (qmax ) and acidic (a, empty symbols), basic (a, filled symbols) functional groups, pHPZC (b, empty symbols) and the density of heteroatoms on the surface (b filled symbols) are given. It is generally accepted that with the increase in amount of acidic functionalities, the decrease in adsorption of acidic organic compounds

qe =

bqmax ce , 1 + bce 1/n

qe = Kf ce

(2)

,

O+N/SBET

basic functional groups (mmol/g) 0.0

0.2

0.4

0.6

0.8

1.0

0.002

a

0.004

0.005

b 400

qmax (mg/g)

400

qmax (mg/g)

0.003

300

200

100

300

200

100 0.4

0.6

0.8

acidic functional groups (mmol/g)

1.0

0

2

4

6

8

10

12

14

pHPZC

Fig. 5. Relationship between the Langmuir adsorption capacity (qmax ) and acidic (a, empty symbols), basic (a, filled symbols) functional groups, pHPZC (b, empty symbols) and the density of heteroatoms on the surface (b, filled symbols): C (♦); CPP (); CPS (); GC ().

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E. Lorenc-Grabowska, P. Rutkowski / Applied Surface Science 316 (2014) 435–442

0.5

V liq (cm3)

0.4

0.3

0.2

0.1

0

V 1.4 (cm3/g) Fig. 6. Correlation between the volume of pores smaller than 1.4 nm and volume of adsorbed phenol calculated on the basis of qmax (empty symbols) and amount adsorbed at first plateau (filled symbols): C (♦); CPP (); CPS (); GC ().

is observed. On contrary, high pHPZC and higher amount of basic functionalities favour the phenol adsorption [41]. Similar tendency is found in this work however the lack of linearity in these tendencies should be emphasized. Contrary to the literature [55] and our earlier reports [21], the increase in heteroatoms density is followed by the increase in adsorption capacity. Reported decreased the micropollutant adsorption from water with increasing the (O + N) content was explained by enhanced water adsorption on polar surface functional groups. Such a relationship is not observed in our work as the CACs do not contain hydrophilic groups. What more, the density of heteroatoms on the surface of the studied ACs is very similar while the difference in adsorption capacity for CACs and GC is huge. In our earlier studies [21] we have found that the volume of pores with a width smaller than 1.4 nm (V1.4 ) was similar to the volume of phenol calculated from qmax for the PET based ACs. In the case of nitrogen based ACs, the volume of the adsorbed phenol was slightly higher than V1.4 . In present work we observe that the volume of phenol calculated from qmax is insignificantly lower than the volume of pores smaller than 1.4 nm in the case of adsorption on GC, whereas is remarkably higher in the case of adsorption on CACs (Fig. 6; empty symbols). However, taking into account the shape of CACs isotherms (L4 types) and the calculated volume of adsorbed phenol from the amount of P adsorbed at first plateau a perfect correlation with the volume of micropore smaller than 1.4 nm (Fig. 6; filled symbols) is obtained. Based on that, it can be concluded that the phenol adsorption is governed mainly by micropore filling through the ␲–␲ dispersion interaction in small micropores. The close neighbourhood of walls enhances the dispersion interaction. This is in accordance with the statement given for multistep isotherms from gaseous phase. In the range of very low pressure/concentration adsorption takes place on the most active sites on the surface or within very narrow pores. At higher pressure/concentration the less active sites are occupied or the wider micropore are filled [56] that depends on particular surface and/or pore structure characteristics. In the case of CACs after filling the volume of small micropore, the phenol molecules start to adsorb until the second plateau is reached. If the multilayer adsorption or reorientation of phenol molecules were involved in the multi-step isotherm formation, the biggest volume of wider micropore and mesopore of the GC should have promote the stepped isotherm formation on this carbon. Hereby it is not observed. That is why at higher phenol concentration, the adsorption on less active sites should be considered.

In the case of GC that is characterized by higher hydrophilicity caused by higher amount of acidic functional groups and ash content, the “solvent effect” prevents further adsorption on less active sites. To study the mechanism of the second plateau formation, the polymerization effect should also be taken into account. When the adsorption is carried out in oxic condition, the oxidative coupling effect has been reported to influence the adsorption capacity towards phenol [57,58]. The elimination of acidic functional groups and introducing basic functional groups promotes the adsorption via phenol polymerization. However, in the case of adsorption on CACs we observe that the CPS with the highest basicity and highest amount of basic functionalities (Table 2) is characterized by lower adsorption capacity compared to C and CPP. Concluding, the oxidative coupling effect is not the force that determines the increase in adsorption on CACs. According to Hsieh and Teng [59], the increase in mesopore volume increases the adsorption capacity of activated carbon with similar surface area and micropore volume. Also in the case of pchlorophenol adsorption, it was observed that bigger volume of mesopore enhances the adsorption on ACs [20]. Contrary results are obtained in this work, as the CPS that is characterized by highest mesopore volume among CACs has the lowest qmax . Finally, the impact of the oxygen groups on adsorption capacity should be discussed. It is generally proofed that high quantity of oxygen in acidic functional group decreases the adsorption capacity due to the water adsorption [32,38,51,55]. Generally, the oxygen contribution to phenol adsorption is believed to influence the adsorption in two ways by hydrogen bond formation or through the donor–acceptor mechanism [60]. There is also third proposition given by Franz et al. [60] that the oxygen influences the adsorption of organic compound by dispersive/repulsive interaction. The heterogeneous oxygen groups attract and localize the electrons of the basal planes creating partially positive “islands” in the basal planes [61]. The hydroxyl group in phenol is an activating group that induces a partial negative charge of the phenol’s aromatic ring. What more, taking into account the adsorption condition in the case of phenol adsorption on CACs, some phenol will exist as dissociated molecule. Hence, the third proposition seems to explain the behaviour of P on CACs quite well. The attraction forces between negatively charged phenol molecule and “positive islands” in basal planes enhance the adsorption. The lower oxygen content in CPS (3.6 wt%) creates less “positive islands” in basal planes compared to other two cellulose based AC characterized by higher oxygen amount (4.2 wt% and 4.4 wt%) that explains the lower adsorption capacity (qmax ) found in this carbon despite its bigger basicity and similar porous characteristic.

4. Conclusions Activated carbons from solid residue from pyrolysis of cellulose and polymers are very efficient in the phenol removal. The obtained Langmuir adsorption capacity varied from 312 to 417 mg/g. The intraparticle diffusion is the rate limiting in the case of CACs. The kinetic in the case of commercial activated carbon (GC) is additionally limited by surface interaction. The equilibrium isotherms are L2 type for GC and L4 type in case of CACs. The main mechanism that rules the phenol adsorption is the micropore filling in pore with size smaller than 1.4 nm. The presence of hydrophilic functionalities and mineral mater lead to the “solvent effect” that result in decrease in the availability of less active sites on the basal planes to the phenol molecules. Further increment of adsorbed phenol molecule on cellulose base ACs is due to attraction forces created between “positive islands” in basal planes and phenol molecules.

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