Adsorption characteristics of polycyclic aromatic

Environ Sci Pollut Res DOI 10.1007/s11356-015-4936-2

PERSISTENT ORGANIC POLLUTANTS (POPS): A GLOBAL ISSUE, A GLOBAL CHALLENGE

Adsorption characteristics of polycyclic aromatic hydrocarbons from non-aqueous media using activated carbon derived from phenol formaldehyde resin: kinetics and thermodynamic study M. S. El-Shahawi 1,3 & A. S. Bashammakh 1 & H. Alwael 1 & A. A. Alsibaai 1 & A. M. Dowaidar 2

Received: 18 March 2015 / Accepted: 22 June 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Porous carbons were prepared by carbonization and activation of phenol formaldehyde resin by gasification with CO2 at 900 °C. Prepared activated carbon from phenol formaldehyde was characterized by measuring thermogravimetry (TG), differential thermal analysis (DTA), pH, surface area, porosity, and pore size distribution. The specific surface area (SSA) of these carbons ranges from 562 to 1904 m2/g, while their point of zero charge (pHPZC) varies from 2.6 to 8.8. The ability of the prepared activated carbon by gasification with CO2 at 900 °C from phenol formaldehyde resin (PFAC) to remove a series of polycyclic aromatic hydrocarbons (PAHs), e.g., naphthalene, fluorene, phenanthrene, pyrene, and fluoranthene, from mixtures of organic solvents with different polarities and chemical structures was tested. The adsorption capacity increases with the increasing the SSA and pHPZC of the carbons, confirming the roles of dispersive interactions. The kinetics and thermodynamics of the adsorption of phenanthrene as a model compound of PAH on PFAC in the organic solvent were studied. The adsorption capacity became notably greater with an increase in contact time and initial phenanthrene concentration. Responsible editor: Hongwen Sun * M. S. El-Shahawi [email protected]; [email protected] 1

Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia

2

Department of Chemical Engineering, Faculty of Engineering, UAE University, Al-Ain, United Arab Emirates

3

Department of Chemistry, Faculty of Science, Damiatta University, Damiatta, Egypt

Keywords Adsorption of PAHs . Phenol formaldehyde resin . Activated carbon . Non-aqueous media . Equilibrium isotherms . Kinetics and thermodynamics

Introduction Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds consisting of two or more fused aromatic rings (Boldrin et al. 1993; Arey and Atkinson 2003; Di-Toro et al. 2000). PAHs are chemically inert, soluble in many organic solvents, and are highly lipophilic. Thus, their behavior and the biological effects vary in the environment (Barr and Aust 1994). PAHs have received considerable attention due to their adverse effects on the health such as their association with skin, lungs, bladder, breast, and colon cancers in humans based on recent studies (Boffetta et al. 1997; Marti-Cid et al. 2008). Hence, PAHs are classified as hazardous materials by the US Environmental Protection Agency (EPA) and the World Health Organization (WHO) (Matina et al. 2014). Low-molecular weight PAHs, such as naphthalene, is highly mobile in an aquatic environment and present significant acute toxicity to many organisms. Many PAHs of higher molecular weights (e.g., benzo(a)pyrene) have been shown to be highly carcinogenic and have significant persistence up to many years in the environment (Chang et al. 2003; Sepic et al. 1997). This class of chemicals is formed in the environment as a result of incomplete combustion of organic material which is sourced either natural or anthropogenic (Menezes and de Lourdes Cardeal 2011). PAHs are found naturally in crude oil, creosote, coal tar, coal, olcanoes, and forest fires and have been also reported during production and processing of plasticizers, dyes, and pigments. Most PAHs enter the environment via the atmosphere from incomplete combustion

Environ Sci Pollut Res

processes, e.g., coal processing, crude oil refining, coal gasification, and coking (Blumer 1976). Based on the adverse effects of PAHs (Marti-Cid et al. 2008), a crucial need to develop highly precise, sensitive, and selective methods for the removal of traces of PAHs in complex matrices is of great importance. PAHs have been extracted utilizing different conventional and micro cleanup methods such as extraction using solid-phase extraction (SPE) on cartridges with alumina, florisil, silica, C18, PS-DVB (Tfouni et al. 2012; Lee and Shin 2010; Houessou et al. 2005; García-Falcón et al. 2005), cloud point extraction (CPE) (Shi et al. 2011), headspace solid-phase microextraction (HS-SPME) (Lei et al. 2011), stir-bar sorptive extraction (SBSE) (Zuin et al. 2005), liquid–liquid extraction (Tfouni et al. 2012; García-Falcón et al. 2005; Bishnoi et al. 2005), accelerated solvent extraction (Rey-Salgueiro et al. 2009), ultrasound-assisted extraction (Orecchio et al. 2009; Ramalhosa et al. 2009), solid-phase microextraction (SPME) (Rianawati and Balasubramanian 2009), soxhlet extraction (Chen and Lin 1997; Dujmov et al. 1994), sonication (Heemken et al. 1997; Jánská et al. 2004), supercritical fluid extraction (SFE) (Ali and Cole 2002; Yusty and Cortizo Daviña 2005), and pressurized liquid extraction (PLE) (Lund et al. 2009; Veyrand et al. 2007; Yusa et al. 2005; Shia et al. 2015) have utilized graphene oxide-bound silica as adsorbent for purification of 14 PAHs in mainstream cigarette smoke. Quinto et al. (2014) have developed two different microextraction packed sorbent (MEPS) methods for the analysis of 16 PAHs in real tap water and seawater samples, agricultural irrigation wells, and stream water samples. The first method consists of a sequence of cycles of aspirations and injections in the same vial and called Bdraw-eject;^ whereas, the second one consists of a similar cycle sequence, but the aspired sample in this case is discarded into waste and called Bextract-discard.^ The method was applied for the analysis of different PAHs in real samples from sea, agricultural irrigation wells, stream, and tap water. Many techniques, e.g., highperformance liquid chromatography (HPLC) with photometric detection, fluorimetric detection, or mass spectrometry detection (Bishnoi et al. 2005; Tfouni et al. 2012; Rey-Salgueiro et al. 2009) and gas chromatography coupled with mass spectrometry (GC-MS) (Plaza-Bolanos et al. 2010), have been used to measure PAHs in different matrices, e.g., in coffee and grilled meat (Ishizaki et al. 2010; Moazzen et al. 2013; Viegas et al. 2012). Phenol formaldehyde (PF) resin (Novolac) has been used as a starting carbonaceous material for preparation of activated carbon and chars (Youssef et al. 1994; Románmartínez et al. 1996; Kapteijn et al. 1999). The activated carbon obtained by carbonization of Novolac PF at 600 °C in nitrogen atmosphere and gasifying the carbonized product with carbon dioxide at 900 °C have shown excellent textural properties (surface area and porosity) toward adsorption of methylene blue and phenol

from their aqueous solutions. The results have revealed the existence of globular structures of different shapes and dimensions. The high-temperature chars (1373 K) exhibited Nfunctionalities attributed to pyridinic nitrogen, pyridones, and oxidic nitrogen species at the edges of the graphene structures and quaternary nitrogen incorporated in the graphene structure (Kapteijn et al. 1999). Based on the significant growth in Saudi Chemical industries, low cost-effective processes are required to phase out the problem of formation of small amounts of PAHs from hydrocracking reactions which can build up to concentrations that cause fouling of cooled heat exchanger surfaces, equipment, and fluid lines. This problem of fouling gradually reduces the heat transfer to some extent that reduces conversion and/or feed rate. In the light of the present trends toward heavier feedstocks, the adsorption, solubility, and precipitation of PAHs are of prime importance in petrochemical industry. Hence, a promising strategy is required to develop a low-cost method to remove and hamper these pollutants and to prevent their effects in hydrocracking. Thus, in continuation to our study (Dowaidar et al. 2007), this study is focused on (i) preparation and characterization of activated carbons derived from phenol formaldehyde resins for removal of PAHs in a batch mode; (ii) studying the kinetics and thermodynamic characteristics of the retention step of selected PAH; and finally, (iii) application of the developed SPE for removal of PAHs from the hydrocrackers. This study accelerates the development of SPE and broadens the application of SPE for removal of various organic and inorganic pollutant species in complex matrices. The study also possesses high academic value and application foreground in life science and analysis of PAHs in various matrices.

Experimental Apparatus A double beam UV-Visible spectrophotometer (SP400 PyeUnicam, UK), HPLC, (Waters, USA) with 600 controller, (UK6) injector, and photodiode array detector (PDA 996) were used for analysis of PAHs individually (at the optimum wavelength) and in their mixture before and after extraction on the solid phase extractor, respectively. Tube furnace, model F21130-33 (Barnstead/Thermolyne, USA) with temperature range (100–1200 °C) and water bath mechanical shaker (model SB-16, Techni Inc., UK), equipped with a temperature controller (Tempette Junior TE-8J)) were used. A Sartorius analytical balance (model A 200 S, Germany), an Orion pH meter (model 420, USA), a Shel Lab vacuum oven (model 1410, USA), an Alltech pump (model 526, USA), an IKA mechanical stirrer (model RW 10R, Germany), a cooling water bath (model MGW-Lauda K4 R, Germany), and an accelerated


surface area and porosity analyzer (model ASAP 2010 Micromeritics, USA) equipped with three different ranges (1000, 100, and 1 mm Hg) of transducers were used.

Recommended procedures Preparation of PFAC Preparation of the activated carbon from PF resin was performed by two consecutive steps namely carbonization and activation as follows:

Reagents and materials PF resin (Novolak) was purchased from Mansoura company for Resins and Chemicals Industries, Egypt. The resin was prepared by reacting phenol with formaldehyde in molar ratio 9:6.21 in the presence of oxalic acid as a catalyst. Melting point of PF was 75 °C, and the molecular weight was 677. Phenol formaldehyde activated carbon (PFAC) was prepared in the laboratory from PF resin. A commercial activated carbon (CAC) manufactured by BDH, chemical laboratory supplies (Poole, BH15 1TD, England), was dried in a vacuum oven at 473.15 K overnight. Activated carbon prepared from phenol formaldehyde resin was used as solid adsorbents in this study without further treatment. Naphthalene (NA), anthracene (AN), fluorene (FL), pyrene (PY), acenaphthene (AC), phenanthrene (PH), and fluoranthene (FLA) were obtained with high purity (98–99 %) from Merck. The solid adsorbates were dissolved in the required hydrocarbon solvents (hexane, n-heptane, cyclohexane, benzene, and methanol) to simulate the media of the hydrocracker output fluids. BDH solvents (hexane, nheptane, cyclohexane, benzene, and methanol) were used without further purification. Benzene was used as the base unit for the PAH compounds. Methanol was used to investigate the influence of solvent polarity on the adsorption of PAHs onto the CAC. The PAHs (their solubility in mole fraction) and solvents used in this study are listed in Tables 1 and 2 (David 2003–2004). A stock solution (103 mg/L) of each PAH was prepared by dissolving the exact weight of the appropriate compound in the selected solvent. More diluted solutions of PAHs of different concentrations were then prepared from their stock solutions. High purity (>99.99 %) nitrogen and carbon dioxide gases were used in the preparation of activated carbon.

Table 1

Carbonization of PF i. A known weight (50±0.01 g) of fine particles of phenol formaldehyde resin was placed in a stainless steel pyrolysis tube (5.0-cm diameter and 55-cm length). The pyrolysis tube was fitted with inlet and outlet connections to allow purging the tube from any atmospheric oxidation and also sweeping the carbonization gases and the tar from pyrolysis zone. The PF sample was positioned in the middle of combustion stainless steel tube using two beds of quartz wool before and after the sample. The position of the combustion tube inside the tube furnace was adjusted to put the sample in the homogenous heating zone of the furnace. Nitrogen gas was passed through the pyrolysis tube for 15 min to get an inert atmosphere over sample just to remove any traces of air or oxygen from the sample and heating zone. The sample was heated under N2 atmosphere at 100 cm3/min flow rate with 10 °C/min heating rate from 25 to 600 °C and left at this temperature for up to 3 h. The sample was allowed to cool under stream of nitrogen to prevent any adsorption or oxidation from air during cooling and finally. The carbonized sample (PFCo) was collected and crushed to the required particle size and used in the next activation process. Activation of PFCo Non-activated carbon (PFCo) prepared from phenol formaldehyde carbon was activated by physical

Physical properties of polycyclic aromatic hydrocarbons used in the present study (David 2003–2004)

Property

NA

AC

FL

AN

PH

FLA

PY

Molecular formula Molecular weight Melting point °C Boiling point °C Molecular volume (oA3) Molecular surface area (oA2) Solubility in water (mmol/L) Dielectric constant

C10H8 128.17 81.0 218.0 126.9 155.8 2.4*10−1 2.5

C12H10 154.21 96.2 279.0 148.4 180.8 2.9*10−2 3.0

C13H10 166.22 115.5 294.0 188.0 194.0 1.2*10−2 –

C14H10 178.23 216.4 340.0 170.3 202.2 3.7*10−4 –

C14H10 178.23 100.5 338.0 169.5 198.0 7.2*10−3 2.72

C16H10 202.26 108.8 383.0 187.7 218.6 1.3*10−3 –

C16H10 202.26 150.4 393.0 186.0 213.5 7.2*10−4 –

Environ Sci Pollut Res Table 2 Physical properties of the used solvents (David 2003– 2004)

Property/name

Methanol

Hexane

Heptane

Cyclohexane

Benzene

Purity % Boiling point °C Polarity Molecular weight Dielectric constant at 25 °C

99.5 64–65 Polar 32.01 32.63

99.5 68–69 Non-polar 86.18 1.80

99.5 98–99 Non-polar 100.21 1.90

≥99.7 80–81 Non-polar 84.16 2.015

≥99.9 80–81 Non-polar 78.12 2.3

activation process involving gasification with carbon dioxide at 900 °C as follows: i.

An accurate weight (25±0.01 g) of PFC was placed in a vertical silica tube inside the tube furnace and heated to 900 °C. Carbon dioxide was then allowed to pass up through the sample bed, supported upon a fine chrome screen under sufficient flow rate of nitrogen as a diluent through the sample to keep it Bjiggling^ during gasification with CO 2 at 900 °C. The samples of different burn-off percentages were then prepared by gasifying the PFC with carbon dioxide at 900 °C for different periods of time. The PF activated carbons obtained with different burn-off percentages (15.5, 22, 35, 58, and 71 %) were designated as PFAC16, PFAC22, PFAC35, PFAC58, and PFAC71, respectively (Table 3). Finally, the burn-off percentage was simply calculated employing the following equation:

Burn−off % ¼

ðW o −W Ac Þ 100 Wo

ð1Þ

where Wo is the original weight of PFC sample and WAc is the final weight obtained of PFAC sample and finally ii. The prepared samples were collected in tightly closed and labeled vials and stored in a desiccators containing P2O5.

Table 3

Surface characteristics of CAC and PFAC carbons

Sample

Burn-off, %

Vm, cm3/g

SBET, m2/g

CAC PFCo PFAC16 PFAC22 PFAC35 PFAC58 PFAC71

– 0 15.5 22.2 35.0 58.0 71.0

168.5545 66.518 87.448 123.809 161.959 254.878 383.038

733.75 286.72 380.68 538.97 705.04 1109.53 1556.91

Recommended batch experiments The effect of shaking time on the adsorption of the PAHs onto PFAC35 carbons was critically investigated. In a separate brown Erlenmeyer flasks (100 cm3 capacity), an accurate amount (0.2±0.001 g) of PFAC35 was equilibrated with 50.0 mL of phenanthrene (500.0 mg/L). The bottles were then tightly closed with bottle cap and parafilm tape and shaken for various time intervals up to 48 h at 25±0.1 °C. After reaching the equilibrium time, the liquid phase was separated and the amount of phenanthrene remaining in the organic solvent was separated out and the absorbance was measured at λmax against solvent blank. The amount of the PAH adsorbed on the activated carbon was finally calculated employing the following equation: qe ¼ ðC o −C e Þ

V W

ð2Þ

where qe is the amount adsorbed at equilibrium (mg/g), Co and Ce are the initial and equilibrium concentrations of PAH (mg/L), respectively, V is the volume of the sample in liter, and W is the weight of adsorbent in grams. Following these procedures, the influence various parameters, e.g., shaking time, pH, temperature, particle size, agitation rate, chemical structure of the PAH, and solvents were critically carried out. The values of %E and Kd are the average of three independent measurements, and the precision in most cases was ±3 %. Determination of adsorption capacity Semi-quantitative tests were performed separately for the adsorption of each solvent as a single component from a gaseous state. A known mass of activated carbon was placed in a reservoir (1), and a suitable volume of a solvent was then placed in the other reservoir (2). The apparatus was evacuated for several minutes while the valve on the solvent line was closed, and the evacuation continued in the activated carbon line. After 30 min, the valve from the evacuation line to the pump was closed and the activated carbon line went in a vacuum. The adsorption started slowly through the opened valve on the solvent line (4) and left for several days to reach equilibrium. After equilibrium, the activated carbon sample


was reweighed and the increase in the adsorbent weight was then calculated. The amount of solvent adsorbed at room temperature was then calculated. Kinetics of adsorption of PAH on activated carbon The kinetic study of adsorption of phenanthrene as model analyte of PAHs (200 mg/L) was critically investigated in methanol. The experiment was carried out at 15 and 35 °C, as follows: A 1700 mL of phenanthrene (200 mg/L) was placed in a vessel supported with baffles and placed in a water bath at the appropriate temperature. An accurate weight of activated carbon (1.0±0.01 g) was then added to phenanthrene solution. The solutions were stirred using a mechanical stirrer, and a time recording (stopwatch) started just after the addition of the adsorbent. A series of samples (2 mL each) was taken out at various time intervals (2–420 min), and the phenanthrene concentration remained in the methanol phase was then determined spectrophotometrically at λmax (292 nm).

Results and discussion Accumulation of PAHs in hydrocracking units in refinery plants represents a potentially serious operating problem. PAHs can build up to the limit of their solubility at temperatures commonly used for condensation and separation of reactor products. Hence, a detailed study on the characterization and adsorption characteristics of the prepared activated carbons from phenol formaldehyde are given below. Phenol formaldehyde resins have been used in many studies for the preparation of activated carbon with a controlled pore size distribution to be used as a molecular sieving and also for the preparation of highly porous carbons (Horikawa et al. 2002, 2003; Weng and Teng 2001). Characterization of PFAC Thermogravimetric analysis of PF resin Thermal properties of the polymer in different atmospheres under different experimental conditions are of prime importance for determining the stability of the polymer, the degradation products, and its chemical structure under some circumstances. Simultaneous mass spectrometry analysis employing TG-MS and pyrolysis-GC-MS are the most common techniques for detecting and identifying the thermal degradation products. TG and DTG analysis of PF resin under N2 at the same conditions during carbonization of PF sample are shown in Fig. 1. The sample was heated from room temperature up to 600 °C at 10 °C/min heating rate in nitrogen atmosphere. Three distinct regions in the TG were detected during the heating process (Fig. 1). The first region (35–120 °C)

corresponds to a weight loss of 1.25 %, indicating the loss of the absorbed water. The second region (120–320 °C) with a weight loss of 14.5 % represents the curing of the polymer (Kipling and Shooter 1965). The third region beyond 320 °C, thermal degradation of the polymer took place and continued throughout the heating process. At the end of the TG experiment, a fused black char of approximately 43.7 % of the original sample was remained. The sample was then left at 600 °C for 3 h to complete the carbonization process, and finally, a char of ~40 % of the original sample was left as solid residue. The water evolved at 210 °C was most likely produced from the condensation reactions among phenols and methylols leading to the formation of a three-dimensional macromolecule cross-linked via methylene and ether bridges. In the thermal degradation region of PF (>320 °C), the observed maximum weight loss occurred at 405 °C. The other weight loss observed at 560 °C is possibly attributed to the water as a major gas evolved and accompanied by a small amount of CO2 (Chang and Tackett 1991). DTA of phenol formaldehyde initially showed broad exothermic DTA peaks due to curing, i.e., further cross-linking. The existence of an extensively cross-linked aromatic structure in phenol formaldehyde during degradation is most likely proceeded to prevent rearrangement to full graphite structure on the subsequent heating. The chemical structure of phenol formaldehyde resin composed of carbon, hydrogen, and oxygen atoms and has the stoichiometric formula shown in Fig. 2. Phenol formaldehyde during pyrolysis generally produces a considerable amount of tar and water as pyrolysis products. Tar precipitation blocks partially or completely the developed porosity and retards the adsorption from gaseous phase as well as from solution. Thus, in the subsequent activation process, gasifying of PFC sample PFC at 900 °C with carbon dioxide was carried out. The non-activated carbon sample (PFC) was heated to 900 °C and maintained over a period of 30 to 180 min using a mixture of CO2 and N2 as fluidizing gases. Five activated carbons were obtained and designated as PFAC16, PFAC22, PFAC35, PFAC58, and PFAC71 with burn-offs equal 15.5, 22.2, 35, 58, and 71 %, respectively, as demonstrated in Table 3. The degree of carbon burn-off, θ (wt%), of the prepared five samples was calculated employing Eq. 1. pH measurements The pH for the aqueous suspensions of the prepared activated carbons were found in the pH range between 7.6 and 8.9 and slightly increased on increasing the burn-off percentage due to the formation of basic surfaces. The obtained char (nonactivated) surface has acidic nature of pH=5.4. The pH of commercial carbon is more basic than the prepared activated carbon (pH=9.4). The acidic nature of the non-activated

Environ Sci Pollut Res Fig. 1 TG and DTG curves for the phenol formaldehyde resin in nitrogen atmosphere

sample PFCo may be due to the presence of oxygen atoms in the original structure of polymer. The oxygen gas reacts with carbon atoms at the surface of these condensed structures to form complexes like phenols, carboxylic, and lactones. The basic nature of the activated carbons may be due to the heating of non-activated sample to high temperature (900 °C). Surface area and porosity of the activated carbons Low-pressure nitrogen adsorption measurements were carried out on volumetric adsorption analyzer at liquid nitrogen temperature (77 K) under a range of relative pressure (10−6 to 0.99). The adsorption of nitrogen was carried out on the non-activated samples as well as on its carbon dioxideactivated products. Preliminary experiments have shown that the adsorption process is reversible and desorption points lie on the same adsorption isotherms. Typical nitrogen adsorption–desorption isotherms at 77 K for PFC0, PFAC, and CAC are shown in Fig. 3. For PFCo, typical characteristics of a microporous material, corresponding to type I of the IUPAC classification, were noticed (Fig. 3). At moderate pressure >0.7, the isotherm showed a tendency to be type II isotherms. A significant amount of microporosity is most likely formed in the activated carbon samples. Adsorption of N2 by the prepared carbons increases with the increasing the burnoff percentage due to the developed porosity within the produced carbons. The activated carbons show microporous and mesoporous structures. On raising the burn-off percentage as a result of the penetration of carbon dioxide into the micropores, Fig. 2 Phenol formaldehyde resin chemical structure

the extent of mesoporosity increased (Fig. 3). At higher relative pressure, the isotherms are of type I and have a great tendency to be of type II as a result of capillary condensation in the mesopores. The plots of 1/[V (Po/P)−1] versus P/Po for the used adsorbents were linear over the range of a relative pressure from 0.05 to 0.35 (Fig. 4) where V is the quantity adsorbed by the solid, in cm/g, at the relative pressure of P/Po; P is an equilibrium pressure; and Po is the saturated vapor pressure. On extrapolation, the lines were passed close to the origin indicating a relatively large value of surface area, which is associated with the presence of a distinct knee in the adsorption isotherm. The specific surface area (SBET) was calculated using the equation: S BET ¼ V m N Am 10−20 =22414

ð3Þ

where Vm is the monolayer capacity in cm3 of adsorbate per gram of solid, N is the Avogadro’s number (N=6.023×1023 molecules/mole), and Am is the area in square angstrom (Ao 2) units occupied per one molecule of adsorbate in the complete monolayer. The calculated value of A m for nitrogen at 77.35 °C (liquid density ρ=0.808 gm/cm3) was found equal 16.2 Ao 2. The specific surface areas of samples were further calculated from the nitrogen isotherms adopting the value of 16.2 Ao 2 as the cross-sectional area of N2 molecule. Thus, the specific surface area (SBET) in square meters per gram of the adsorbent is given by the following equation: S BET ¼ 4:37 V m m2 =g

ð4Þ

The BET surface areas and monolayer capacities of PFC0, CAC, and PFAC carbons samples are summarized in Table 3. The data revealed that activation with CO2 is efficient in the

Environ Sci Pollut Res Fig. 3 The adsorption– desorption isotherms of nitrogen onto PFAC0, PFAC16, PFAC22 and PFAC35, CAC, PFAC58, and PFAC71 carbons at 77 K

preparation of activated carbon with porous structure and also better than the commercial one when applying activation to Fig. 4 BET linear plots for the adsorption of nitrogen on CAC and PFAC carbons at 77 K

high burn-off percentage. The carbonization of phenol formaldehyde resin produces a non-activated carbon with low

Environ Sci Pollut Res Fig. 5 Plot of burn-off percentage versus surface area, m2/g

surface area due to the deposition of less volatile combustion products on the micropores produced as a result of crosslinking and carbonization reactions. In the activation process, carbon dioxide reacts with the less volatile deposits producing a small change in the porosity by opening the closed micropores. On increasing the contact time between CO2 and surface of carbon, more loss of carbon and more micropores were created and also mesopores are formed. The plot of burn-off percentage versus the measured surface area (Fig. 5) showed a linear correlation confirming the increase of micropores and surface area on increasing the burn-off percentage. Porosity and pore size distribution determination Nitrogen adsorption measurements were used to evaluate the specific surface area, micropore surface area, micropore volume, and pore size distribution (PSD). The micropore volume and the external surface area were evaluated from the t-plot method (Gregg and Sing 1982). Practically, t-plot expresses the adsorption isotherm of a compared sample as a function of the amount adsorbed on the reference adsorbent. The micropore volume and the microporosity data are given in Table 4. Based with N2 only, the micropores might not be properly characterized. The

Table 4 Pore volumes of CAC and PFAC carbons

increase of burn-off percentage has positive effects on the total pores, micropore volumes, and consequently the total surface area. The influence of burn-off percentage on the porosity of the carbon dioxide-activated carbons was critically carried out. Representative results are shown in Fig. 6. The external surface area (surface area from mesopores and macropores), micropore surface area, and total surface area (BET) increased progressively on increasing the burn-off percentage (Fig. 6). The increase in the surface area of the prepared carbons with burn-off percentage up to 35 % is most likely due to opening of the closed micropores formed during carbonization process and also as a result of the creation of new micropores because of the gasification of the non-activated carbon with carbon dioxide. At burn-off percentage ≥58 %, an increase in the external surface area is observed. This behavior in most likely attributed to the widening of the micropores producing new mesopores with small diameter. The micropore volume increased linearly form low to high burn-off percentage (Fig. 6). At low burn-off percentage, the weight loss is mainly due to the possible removal of the deposit materials like tar and less volatile compounds which are responsible for closing the micropores during the carbonization process. An increase in the micropore volumes was

Sample

Total pore volume, cm3/g

Micropore area, m2/g

Total surface area, m2/g

Micropore volume, cm3/g

Mesopore volume, cm3/g

CAC PFC0 PFAC16 PFAC22 PFAC35 PFAC58 PFAC71

0.4335 0.1189 0.1773 0.2730 0.3705 0.7282 0.9779

516.55 277.34 366.91 513.09 667.80 1011.60 1435.16

733.75 286.72 380.68 538.97 705.04 1109.53 1556.91

0.3287 0.1091 0.1481 0.2090 0.2747 0.4337 0.6054

0.1049 0.0097 0.0306 0.0666 0.0958 0.2945 0.3725

Environ Sci Pollut Res Fig. 6 Influence of burn-off percentage on the total surface areas of PFAC carbons

observed on increasing the burn-off percentage. The reaction of the penetrated activating gas with carbon, through the narrow pores formed during carbonization process creating new micropores and widening the narrow ones, may account for the observed trend. The gasification of the non-activated carbon with carbon dioxide at 900 °C mainly causes a creation of microporosity, followed by a widening up the narrow pores. The pore diameter is slightly increased from 16.43 to 26.25 Ao as a result of increasing the burn-off percentage from 0.0 to 58 % and decreased at burn-off percentage of 71 % as a result of removal of wide pores from the carbon surface. During activation process, CO2 reacts with the less volatile deposits producing a small change in the porosity. Thus, increasing the contact time between CO2 and surface of carbon, more loss of carbon and more micropores and also mesopores were created. Plot of burn-off percentage versus the measured surface area confirmed the increase of micropores and surface area.

Fig. 7 The differential pore volume as function of pore width for PFC0, PFAC16, and PFAC22

Porous structure characteristics of the CAC, PFC0, and PFAC are summarized in Table 4. The data revealed that one third of the porosity of commercial carbon composed from mesopores and the average diameter for all pores is in the range of 23.6 Ao. The porosity of non-activated carbon (PFCo) mainly composed from micropores (~92 %) with small diameter. These data are in a good agreement with data obtained from nitrogen adsorption isotherm. On increasing burn-off percentage to 58 %, the mesopore percent increased up to ~40 % as a result of reaction of carbon dioxide with carbon surface inside the micropores. The increase in the burn-off percentage higher than 58 % initiates the reaction with the walls of outer mesopores and finally decreases its percentage. The nitrogen adsorption isotherms for the prepared carbons also revealed similar trend where the mesopore percent increased on raising the burn-off percentage. The increase of the amount of nitrogen adsorbed at higher relative pressure reflects the capillary condensations in the mesopores.

Determination of pore size distribution using DFT PSD of porous solids was determined via nitrogen adsorption. The number of molecules attached to a solid adsorbent at any instant increases on increasing the gas pressure until a point is reached where statistically it is reasonable to consider a monolayer to have formed. PSD curves show that the structure holds even for high burn-off carbons up to 71 % of the carbon oxidition. Gasification with oxidizing gases successively adds new pores of approximately the same size. The differential pore volume and differential surface area curves as a function of pore width calculated using DFT software. Representative results are shown in Fig. 7. The increase of pore volume and pore diameter on increasing the burn-off percentage is attributed to the reaction of CO2 with deposits closing the micropores formed during carbonization and penetration into narrow micropores to produce pores with larger size. The pore volume and pore diameter increase on increasing the burn-off percentage from 15.5 to 22 % due to the creation of new pores and widening the narrow pores. The maximum micropore volume was obtained with high burn-off percentage (Fig. 7). On increasing the burn-off percentage, a product has microporous and narrow mesoporous structure is obtained. The high surface areas of PFAC58 and PFAC71 are due to the microporous nature of carbons and narrow mesopores. Adsorption kinetics of PAHs onto PFAC carbons The influence of shaking time (0.0–50 h) on the uptake of phenanthrene (as a model compound for PAHs) onto CAC and PFAC35 in methanol at 25 °C is given in Fig. 8. The uptake increased on the increasing of contact time and attains a plateau after 12 h. The uptake was rapid in the initial stage (70 % within the first 2 h) and reached equilibrium after 12-h shaking time due to boundary layer diffusion. This conclusion was supported by calculating the half-life time (t1/2) of phenanthrene sorption from the methanol onto and CAC and PFAC sorbents. The values of t1/2 were found to be 1.2±0.04 and 1.05±0.008 h for the analyte onto CAC and PFAC sorbents,

Phenanthrene adsorbed (Q e), mg/g

Environ Sci Pollut Res 150.0

100.0

50.0 At 35 C At 15 C

0.0 0.0

5.0

10.0

15.0

20.0

Square root of time, min.

Fig. 9 The adsorbed concentration of phenanthrene onto CAC from methanol versus square root of shaking time at 15 and 35±0.1 °C

respectively. Thus, gel diffusion is not only the ratecontrolling step for CAC and PFAC sorbents as in the case of common solid phase extractor and the kinetic of analyte sorption onto the sorbent depends on film and intraparticle diffusion. The results were further subjected to WeberMorris 1963 model (Ho and McKay 1999). This model can be expressed by the following equation: qt ¼ Rd ðt Þ1=2 þ C

ð5Þ

where Rd is the rate constant of intraparticle transport (mg/g h1/2), qt is the sorbed phenol concentration (mg/g) at time t in hours, and C (mg/g) is the intercept. The plots of qt versus time were linear (R2 =0.9438–0.9652). The plots of the adsorbed phenanthrene as a representative example of PAHs onto activated carbon from methanol at 15 and 35±0.1 °C versus square root of time were linear (Fig. 9). The plot suggests a faster process at 15±0.1 °C (Fig. 9) in good agreement with the theory of diffusion control (Crank 1965). The two plots showed two distinct regions: an initial curved portion which is due to boundary layer diffusion effects (Crank 1965) and a final linear portion due to intraparticle diffusion effects (McKay et al. 1980). The intraparticle diffusion rate constant at 15 and 35 °C, evaluated from the plots, was found

2.5 at 35 C at 15 C

log (q1-qt)

2.0 1.5 1.0 0.5 0.0 0.0

100.0

200.0

300.0

400.0

Time, min.

Fig. 8 Influence of shaking time on the adsorption of phenanthrene onto CAC and PFAC35 samples from methanol at 25 °C

Fig. 10 Lagergren plots of phenanthrene adsorption onto CAC at 15 and 35±0.1 °C in methanol

Environ Sci Pollut Res 0.18

Fig. 11 Plots of 1/t versus 1/qt for the adsorption of phenanthrene onto CAC at 15 and 35±0.1 °C

0.16

1/qt (mg/g)-1

0.14 0.12 0.10 0.08 0.06 0.04

At 15 C At 35 C

0.02 0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

1/t, hr. -1

equal 5.3918 and 5.6792 mg/g/min1/2, respectively. The plot does not pass through the origin; thus, it can be assumed that a boundary layer effect occurs at a given degree, and intraparticle diffusion process is not the unique ratecontrolling step. The results were subjected to Lagergren model (Ho and McKay 1999) for pseudo-first order: log ðqe −qt Þ ¼ log qe − K Lager =2:303 t

ð6Þ

where qe is the amount of phenol sorbed at equilibrium per unit mass of sorbent (mg/g), KLager is the first-order overall rate constant of pseudo-first-order kinetic for the retention process per min, and t is the time in min. The plots of log (qe-qt) versus time (Fig. 10) were found linear and fit well with a first-order nature of the adsorption process involved particularly at 35±0.1 °C. The values of the first-order overall rate constant k1 for the PAH adsorption from methanol solution onto the solid adsorbent were found equal 0.00783 and 0.00967 (mg/g/min) at 15 and 35 °C, respectively with R2 in the range 0.97–0.98. Because of the large difference between the calculated and experimental values of qe, the order of the sorption process changed after elapsed time and the Table 5 Thermodynamic parameters for the adsorption of some PAHs onto CAC from different solvents

Thermodynamic parameters PAHs

Solvent

Naphthalene

Methanol Heptane Cyclohexane Cyclohexane Cyclohexane Methanol Heptane

Fluorene Anthracene Phenanthrene Pyrene

experimental data could not be adjusted to pseudo-first-order equation. This trend could indicate that the first stage of the retention process (external transport of analyte from the bulk solution onto sorbate surface) follows pseudo-first-order kinetics while the posterior processes (film diffusion and intraparticle diffusion) do not follow the same order. Thus, the results of PAH uptake were subjected to the pseudosecond-order kinetic model (Ho and McKay (1999). This model can be expressed by the following equation: t=qt ¼ 1= k 2 q2e þ ð1=qe Þt

ð7Þ

where k2, (g/mg/min) is the pseudo-second-order rate constant, The plots of t/qt versus t for phenanthrene were linear (Fig. 11). The pseudo-second-order constants (k2) for phenanthrene were 1.40 and 4.85 (g/mg/min), respectively. It can be concluded that the first step of the process (mass transport from solution until sorbent) followed pseudo-first-order kinetics and at the early stage of extraction, and the whole sorption process is governed by a pseudo-second-order kinetics. Thus, a chemisorption reaction is predominate in the rate-controlling step. The adsorption capacities for phenanthrene were 55.39 and 4.85 mg/g, respectively.

−ΔH°, K/Jmol

−ΔG°, K/Jmol

ΔS°, /Jmol

4.7767 1.2531 0.1710 0.8442 3.5071 0.4058 3.6758

2.5424 3.280 4.8262 6.507 6.4341 9.3975 6.1545

7.494 5.900 15.613 18.279 9.817 32.881 8.314


Thermodynamic characteristics of PAH adsorption The thermodynamic parameters (ΔH°, ΔG°, ΔS°) of naphthalene uptake onto activated carbon were evaluated. Plots of log Kc versus 1/T were linear over the range of temperature (283.15–328.15). The values of ΔH, ΔG, and ΔS are summarized in Table 5 for some PAHs in different solvents. The values of ΔG° for all PAH adsorption onto the activated carbons were found negative indicating that the adsorption processes are spontaneous. The negative values of ΔH° indicated that the adsorption processes are exothermic process and decreased on increasing of the temperature of adsorption. The positive values of ΔS° indicate that there is increased randomness at the solid interface during the adsorption of PAHs.

Conclusions Phenol formaldehyde activated carbon are highly porous with SSA values ranging from 562 to 1904 m2/g. The alkalinity of the carbons, measured by pHPZC, tends to be greater for the carbons that are more porous. The porosity of the carbons is dominated by micropores with a micropore volume fraction greater than 0.86. The adsorption of PAHs seems a two-stage process controlled by diffusive transport processes. The values of capacity parameter confirmed the importance of SSA and hydrophobicity of carbon surface in adsorbing PAHs. 4. The value of Kf follows the order of naphthalene > fluorene > phenanthrene > pyrene. This dependence of Kf on molecular size suggests that part of the internal surface may not be accessible to the large PAH molecules.

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