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Effect of one-step and two-step H3PO4 activation on activated carbon characteristics
Bioresource Technology Reports 8 (2019) 100307
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Effect of one-step and two-step H3PO4 activation on activated carbon characteristics
T
Oluwatosin Oginnia, , Kaushlendra Singha, Gloria Oportoa, Benjamin Dawson-Andoha, Louis McDonaldb, Edward Sabolskyc ⁎
a
School of Natural Resources, West Virginia University, Morgantown, WV 26506, United States of America Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV 26506, United States of America c Department of Mechanical Engineering, West Virginia University, Morgantown, WV 26506, United States of America b
ARTICLE INFO
ABSTRACT
Keywords: Activated carbon Activation H3PO4 Adsorption Biochar Biomass
The objective of this study was to investigate the effect of one-step and two-step H3PO4 activation on activated carbon characteristics. Kanlow Switchgrass and Public Miscanthus and their biochars obtained via pyrolysis were used as precursors. The precursors were impregnated with H3PO4 and activated at a temperature of 900 °C to produce biomass-derived and biochar-derived activated carbons. The activated carbons were characterized for textural characteristics, surface chemistry and microstructure. The activated carbons were also tested for their adsorption capacities for caffeine and acetaminophen. The biomass-derived activated carbons (KSBM and PMBM) had the highest BET surface areas of 1373 and 999 m2/g, respectively. The total pore volumes of the biomass-derived activated carbons were about two-fold higher than the biochar-derived activated carbons. In addition, the monolayer adsorption capacities of the biomass-derived activated carbons were about five folds higher than that of biochar-derived activated carbons for the adsorption of caffeine and acetaminophen.
1. Introduction Chemical synthesis of activated carbon involves the impregnation of the precursor with activating agent prior to activation/carbonization. The chemical activating agent plays a significant role in the characteristics of the resulting activated carbon. For example, the use of zinc chloride as an activating agent, produces highly microporous activated carbon. However, its use is associated with the challenges such as equipment corrosion, inefficient chemical recovery and possibility of product contamination when the activated carbons are used in pharmaceutical and food industries (Oginni, 2018; Prahas et al., 2008). Significant preference is given to the use of phosphoric acid because of the non-polluting nature of the resulting activated carbon, the development of both micropores and mesopores of the activated carbon, and the ease of chemical recovery (Kumar and Jena, 2016). Synthesis of activated carbon via the chemical pathway can be divided into a one-step or two-step activation. In the one-step activation, the biomass precursor is impregnated with the chemical activating agent and the impregnated precursor is activated/carbonized at an elevated temperature to produce a biomass-derived activated carbon. While the two-step activation involves the thermal conversion of the biomass to biochar and subsequent impregnation of the biochar precursor, followed ⁎
by the activation of the impregnated biochar precursor to produce a biochar-derived activated carbon (Oginni et al., 2019). In a one-step activation using phosphoric acid, the activating agent interacts with the biomass precursor during the impregnation procedure by cleaving the arylether bonds in the lignin and hydrolyzing the glycosidic linkages in the hemicellulose and cellulose through dehydration and condensation reactions (Marsh and Rodríguez-Reinoso, 2006; Villota et al., 2019). Also, the phosphoric acid interacts with the organic species in the biomass precursor to form phosphate and polyphosphate groups, which promote a dilation process that leaves an accessible porous structure after elimination of the acid (Gueye et al., 2014; Villota et al., 2019). Yakout and Sharaf El-Deen (2016) prepared a biomass-derived activated carbon from olive stones using a one-step H3PO4 activation and reported a BET surface area of 1218 m2/g and total pore volume of 0.6 cm3/g. The N2 adsorption isotherm of the activated carbon was classified as type I according to International Union of Pure and Applied Chemistry (IUPAC) classification, which is an indication that the activated carbon is a microporous material. The development and widening of activated carbon porosity was associated with the amount of H3PO4 intercalated in the internal structure of the biomass precursor. Villota et al. (2019) investigated the microwave assisted H3PO4 activation of waste cocoa pod husk and reported a BET
Corresponding author at: 322 Percival Hall, School of Natural Resources, West Virginia University, Morgantown, WV 26506, United States of America. E-mail address: [email protected] (O. Oginni).
https://doi.org/10.1016/j.biteb.2019.100307 Received 6 July 2019; Received in revised form 9 August 2019; Accepted 10 August 2019 Available online 14 August 2019 2589-014X/ Published by Elsevier Ltd.
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surface area of 1139.17 m2/g and a total pore volume of 1.062 cm3/g. The distribution of the surface area and pore volume showed that a larger proportion of the surface area and the pore volume were mesopores, which indicates that the activated carbon is mesoporous in nature. Valero-Romero et al. (2019) characterized a biomass-derived activated carbon produced from a one-step H3PO4 activation of olive stones at a temperature of 800 °C for 2 h. The activated carbon had a BET surface area of 1380 m2/g and mesopore volume of 0.654 cm3/g. The high surface area is an indication of large number of active sites and the high mesopore volume enhances mass transfer rate, making the activated carbon suitable for catalytic applications. Also, the surface chemistry of the activated carbon showed a high surface oxygen content, which was associated with the presence of C–O–P surface functional groups formed via the oxidation of CeP bonds. The presence of the C–O–P surface functional groups on the activated carbon surface is responsible for high chemical and thermal stability, as well as surface acidity of the activated carbon. However, in a two-step H3PO4 activation, the phosphoric acid facilitates the removal of tarry substances in the pores of the biochar and act as a template for the creation of microporosity during the activation stage (Marsh and Rodríguez-Reinoso, 2006). Shamsuddin et al. (2016) reported an increase in the BET surface area of a biochar-derived activated carbon obtained from the H3PO4 activation of biochar made from kenaf core fiber. The biochar had a BET surface area of 13.68 m2/ g and a micropore surface area of 7.16 m2/g, which increased to 299.02 m2/g and 229.20 m2/g, respectively due to activation. The pore development of the biochar-derived activated carbon was enhanced by the diffusion of H3PO4 molecules into the pores of the biochar precursor and thereby increasing the H3PO4-carbon reaction via acid hydrolysis process which resulted in the creation of more pores. Nitnithiphrut et al. (2017) prepared a biochar-derived activated carbon from H3PO4 activation of biochar made from para wood and reported a BET surface area of 153.73 m2/g, which was higher than the BET surface area of 86.91 m2/g obtained for the para wood biochar. The SEM morphology of the activated carbon showed a well-developed porous structure, which was attributed to chemical etching on the biochar surface and deep penetration of the H3PO4 molecules into the biochar pores. While independent studies were conducted on either one-step or two-step activation using phosphoric acid as an agent, it is hard to establish which pathway is better because different biomass were tested for different pathway. Recently, Oginni et al. (2019) compared one-step and two-step activation on activated carbon characteristics using KOH as an activating agent. The KOH acts like a catalyst and reacts with the biomass/biochar carbon as opposed to phosphoric acid, which act like a cleaning agent and cleans up the deposited tarry products in biochar's pores. Therefore, the objective of this study was to investigate the effect of one-step and two-step H3PO4 activation on characteristics of biomass-derived and biochar-derived activated carbons and test their aqueous adsorption characteristics with respect to their potential use for water treatment.
2019; Oginni et al., 2017). The pyrolysis vapor generated during the carbonization process was swept into a series of ice-bath condensers. The biochars were removed from the reactor after it was cooled down to room temperature under a constant flow of nitrogen gas. The impregnation procedure was carried out by soaking about 50 g of the precursors in 200 ml solution of phosphoric acid (85% wt.) and continuously stirred inside a pressure reactor (Model: 4500, Parr Instrument Company, Moline, IL) at a temperature of 85 °C for 24 h. After the impregnation process was completed, the impregnated precursors were oven dried at a temperature of 103 °C for 24 h. The oven-dried impregnated precursors were thereafter subjected to activation at a temperature of 900 °C for 1 h under continuous nitrogen flow using a thermogravimetric analyzer (Model: TGA 701, LECO Corporation, St. Joseph, MI). At the end of the activation procedure, the activated carbons were allowed to cool down to room temperature under the continuous flow of nitrogen. Thereafter, the activated carbons were washed with boiling deionized water in a lab-line multiunit extraction equipment until the pH of the activated carbons became neutral. The washed activated carbons were oven dried at a temperature of 103 °C for 24 h. The biomass-derived activated carbons were labelled as: KSBM (Activated Carbon from Kanlow Switchgrass Biomass), PMBM (Activated Carbon from Public Miscanthus Biomass); while the biocharderived activated carbons were labelled as KSBC (Activated Carbon from Kanlow Switchgrass Biochar), PMBC (Activated Carbon from Public Miscanthus Biochar). 2.2. Activated carbon characterization 2.2.1. Morphology Scanning electron microscope (Model: Hitachi-S4700, Hitachi High Technologies America, Schaumburg, IL) at the West Virginia University Shared Research Facilities was used in examining the morphology of the activated carbons. A double sided adhesive tape was used in fixing the activated carbon sample to the specimen stub. The specimen stub was placed on the specimen holder and transferred into the vacuum chamber where the microscope scanned the sample with a beam of electrons. The SEM images were collected using the equipment software. 2.2.2. Surface area and pore characteristics Nitrogen adsorption/desorption isotherms at 77 K of the activated carbons were determined using Micromeritics Accelerated Surface Area and Porosimetry System (Model: ASAP 2020, Norcross, GA, USA). The samples were degassed at a temperature of 105 °C for 24 h and cooled down to 30 °C prior to the analysis. The surface areas were calculated using the BET equation. The t-plot model was used in calculating the micropore volume. The Non-Local Density Function Theory (NLDFT) model in the SAIEUS software (Kupgan et al., 2017) was used in obtaining the pore size distributions of the activated carbons. 2.2.3. XPS and Raman analysis The surface functional groups were determined using X-ray photoelectron spectroscopy (Model: PHI 5000 VersaProbe XPS/UPS, ULVACPHI Inc., Kanagawa, Japan) in a spectral range of 0 to 1400 eV binding energy and energy resolution of 0.50 eV. The sample was mounted on a steel specimen disk using an Ultra High Vacuum approved spectral grade double-sided carbon tape and loaded into the introduction chamber. Thereafter, the sample was transferred into the analysis chamber where the photoelectron spectra were acquired. The element identification and the peak fitting were carried out using the PHI MultiPak software. Raman microscope (Model: Renishaw InVia) equipped with an excitation laser of 532 nm wavelength was used in examining the microstructure of the activated carbons. The laser beam was operated on a 1% laser power input and at a 50× optical magnification. The scanning of the sample was within a spectrum range of 1000–2000 cm−1.
2. Materials and methods 2.1. Material preparation Biomass and biochar precursors were used in synthesizing the activated carbons in this study. The biomass precursors were Kanlow Switchgrass and Public Miscanthus, which were oven dried at 103 °C for 24 h and thereafter ground to < 1 mm particle size using a Retsch Grindomix (Model: GM 200). The biochar precursors were produced by carbonizing the ground biomass samples in a fixed-bed batch reactor having a diameter of 22.86 cm and a height of 25.40 cm. The reactor was filled with oven-dried biomass and the sample-filled reactor was placed in a furnace (Model: BF51728C, Thermo Scientific, NC). The furnace was heated from room temperature to 500 °C under a nitrogen gas flow of 2 L/min and kept at that temperature for 30 min (Oginni and Singh, 2
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2.3. Adsorption of caffeine and acetaminophen
main conduit to the micropores on the inner surface of the activated carbons (Xu et al., 2014; Yorgun and Yıldız, 2015).
Caffeine (purchased from Fischer Science Education, Nazareth, PA) and acetaminophen (purchased from Acros Organics, Morris, NJ) were used as adsorbates for the adsorption experiment. Standard solutions having concentration of 200 ppm were produced by dissolving 10 mg of the adsorbate in 50 ml distilled deionized water. From this standard solution, samples were taken and adjusted with distilled deionized water to give the desired concentration. The solution concentration was measured with UV spectrophotometer (Model: Varian Cary 50) at wavelength of 242 nm and 274 nm for acetaminophen and caffeine respectively. The selection of the wavelength was based on the peak absorbance recorded for both adsorbates when their solutions were scanned in the spectrophotometer between 200 nm and 400 nm on the spectrum mode, using distilled deionized water as a blank. The experiments conducted in investigating the adsorption kinetics involved the addition of 10 mg of the activated carbon to 40 ml of adsorbate solution in glass vials with an initial concentration of 40 ppm. The mixture was agitated in a multipoint agitation plate at a temperature of 25 °C. The time recording started when the agitation began and several samples were collected between 15 min and 9 h. The collected samples were filtered through a filter paper (Fisher Scientific, Qualitative P4) to separate activated carbon and adsorbate solution. The quantity of adsorbate uptake by activated carbon was calculated using Eq. (1);
qt =
Co
Ct W
V
3.2. BET surface area The BET surface areas of the activated carbons were assessed by the N2 adsorption/desorption isotherms shown in Fig. 2. The isotherm for the biochar-derived activated carbon (PMBC) can be classified as type I, as the isotherm is horizontal and parallel over the relative pressure range. Microporous materials are known to exhibit this type of adsorption isotherm, which implies that PMBC is microporous in nature. However, the adsorption/desorption isotherms for the other biocharderived activated carbon (KSBC) and the biomass-derived activated carbons (KSBM and PMBM) was more S-shaped and it can be classified as type IV isotherm. It is visually clear that the biomass-derived activated carbons adsorbed more nitrogen over the relative pressure range (0.20 to 0.80) as opposed to the biochar-derived activated carbons where no significant nitrogen was absorbed in the same range. It essentially indicates that the use of phosphoric acid directly on the biomass precursor did not allow depositing of tarry material in the mesopores and preserved the pores. The type IV isotherm is characterized by its hysteresis loop, which is associated with capillary condensation taking place in the mesopore (Sing et al., 1985). The initial part of the isotherm (prior to the hysteresis loop), also known as the inflection point of the knee of the isotherm is attributed to monolayer-multilayer adsorption, which is similar to the middle section of type II isotherm and it indicates the stage at which monolayer coverage is complete and multilayer adsorption begins (Lowell et al., 2006; Sing et al., 1985). The textural characteristics of the biomass-derived and biochar-derived activated carbons are presented in Table 1. The biomass-derived activated carbons (KSBM and PMBM) had higher BET surface areas in comparison to the biochar-derived activated carbons. However, the biochar-derived activated carbon (PMBC) had the lowest BET surface area of 161.97 m2/g. This may be attributed to a high degree of burn off from the activated carbon during the activation process. The high BET surface areas of the biomass-derived activated carbons may be attributed to chemical reaction between the chemical components of the biomass precursors and the activating agent during the carbonization/activation stage (Oginni et al., 2019). The acidic impregnation of the biomass precursor led to the hydrolysis of cellulose and hemicellulose present in it (Sun and Cheng, 2002). The phosphoric acid impregnation has been reported to have the following two important functions: it promotes the pyrolytic decomposition of the biomass precursor and the formation of cross-linked structure (Budinova et al., 2006; Yorgun and Yıldız, 2015). The cross-linking is due to interactions between the acid and the organic material in the precursor leading to formation of phosphate linkages between the fragments in the biopolymer (Budinova et al., 2006). The BET surface area, which simply represents the internal surface area, is generally divided into micropore and mesopore. The micropore surface area represents the fraction of the activated carbon that are categorized as microporous. Bansal and Goyal (2005) explained that due to the large proportion of the surface area and pore volume that the micropore constitutes, the micropore plays a significant role in the adsorption behavior of an activated carbon, given that the molecular dimensions of the adsorbate are not too large to enter the micropores. The micropore surface areas (Smicro) of the activated carbons ranged between 137 and 285 m2/g (Table 1). The percentage of the micropore surface area to the total BET surface area showed that the biochar-derived activated carbons (KSBC and PMBC) had a higher proportion of their BET surface areas as micropores. The Smicro/SBET percentage for the biocharderived activated carbons (KSBC and PMBC) was 39.14 and 86.06% respectively while the biomass-derived activated carbons (KSBM and PMBM) had a Smicro/SBET percentage of 9.98 and 28.48% respectively. The higher Smicro/SBET percentage for the biochar-derived activated carbons may be attributed to the fact that the activating agent helped in removing deposited tarry substances that blocked the pore network of
(1)
where qt is the quantity (mg/g) of adsorbate adsorbed at time t, Co is the initial concentration (ppm), Ct is the concentration at time t (ppm), V is the volume (ml) of the adsorbate solution and W is the weight (mg) of the activated carbon used. The pseudo-second order kinetic model (Mestre et al., 2007) was used in estimating the adsorption kinetics and it is expressed as;
t 1 1 = + t qt qe k2 qe2
(2)
where qe is the maximum adsorption capacity for the pseudo-second order adsorption, k2 is the equilibrium rate constant for the pseudosecond order adsorption. Values of k2 and qe were estimated from the intercept and slope, respectively, of the plot of t/qt versus t. The Langmuir and Freundlich isotherm equations were used in modelling the adsorption equilibrium. In conducting the adsorption equilibrium experiment, 10 mg of the activated carbon was mixed with 40 ml solution of the adsorbates (initial concentration of 10–40 ppm) and agitated for 5 h. After the agitation, the concentration of the adsorbate in the solution at equilibrium was determined. The choice of 5 h as agitation time was based on the result of the adsorption kinetics, where there was no difference in the uptake of the adsorbates by the activated carbons between 5 and 9 h. 3. Results and discussion 3.1. Solid morphology Fig. 1 shows the scanning electron micrographs of the biomass-derived and biochar-derived activated carbons. All the activated carbons showed visible cavities and pores, which is an indication that original cell wall structures of the precursors were retained. Phosphoric acid act as a cleaning and dehydrating agent, which removes tarry products in the pores of the biochar precursor and promotes bond cleavage reactions in the biomass precursor and facilitate crosslinking, condensation and formation of linkage such as polyphosphosate esters, which can protect the internal pore structure of the resulting activated carbon (Xu et al., 2014). The pores showed in the SEM images are tunnel shaped with varying sizes and these pores are exterior pores which act as the 3
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A
C
B
D
Fig. 1. Scanning electron micrographs of the biochar-derived activated carbons [(A) KSBC, (B) PMBC] and biomass-derived activated carbons [(C) KSBM, (D) PMBM].
Fig. 2. N2 adsorption/desorption isotherms for biochar-derived and biomass-derived activated carbons.
The mesopore size distribution for the biomass-derived activated carbons can be seen to be higher than the biochar-derived activated carbons. This corroborates the values reported in Table 1 for the mesopore volume of the activated carbons. The biomass-derived activated carbons (KSBM and PMBM) showed a higher mesopore volume of 1.35 and 0.92 cm3/g respectively. However, the micropore volumes for all the activated carbons were similar. The micropores are characterized by adsorption through volume filling and there is no capillary filling taking place in these pores whereas the mesopores are characterized by capillary condensation with formation of a meniscus of the liquified adsorbate (Bansal and Goyal, 2005). The average pore diameters for all the activated carbons were between 2.01 and 4.15 nm (Table 1). It is also noteworthy that the biomassderived activated carbons possessed higher average pore diameters. This difference is attributed to the fact that the biochar precursors had an ordered porous structure, such that the activating agent had a minimal impact on improving the porous structure of the resulting biochar-derived activated carbons (Bazan-Wozniak et al., 2017).
Table 1 Textural characteristics of biomass-derived and biochar-derived activated carbons. Sample
SBET (m2/g)
Smicro (m2/g)
Smicro/SBET (%)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
dp (nm)
KSBC PMBC KSBM PMBM
697.96 161.97 1372.93 999.06
273.20 139.39 137.05 284.51
39.14 86.06 9.98 28.48
0.55 0.08 1.45 1.04
0.12 0.06 0.10 0.12
0.43 0.02 1.35 0.92
3.13 2.01 3.42 4.15
*dp: average pore diameter (4V/A by BET); V: pore volume; S: surface area.
the biochars, hence leading to the higher Smicro/SBET percentage. 3.3. Pore size distribution Pore size distribution is an intrinsic property of the activated carbon, which influences its adsorption performance. Fig. 3 shows the pore size distributions of the biomass-derived and biochar-derived activated carbons. All the activated carbons except PMBC showed multiple modal distributions across the microporous and mesoporous range. The biochar-derived activated carbon (PMBC) showed a narrow monomodal distribution which is centered in the microporous range.
3.4. Raman analysis The Raman spectra of the biomass-derived and biochar-derived activated carbons showed that all the activated carbons exhibited the 4
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Pore Size Distribution (cc/g/nm)
0.35 0.3
KSBC
PMBC
KSBM
PMBM
Microporosity
0.25
Mesoporosity 0.2 0.15 0.1 0.05 0 0
2
4
6
8
10
Pore Width (nm) Fig. 3. Pore size distribution of the biomass-derived and biochar-derived activated carbons.
two-signature D (disorderness) and G (graphitization) bands. Table 2 shows the band positions and intensity ratio (ID/IG) for all the activated carbons. The presence of both the D and G bands in the Raman spectra of the activated carbons is a representation of their heterogenous carbon microstructure (Yumak et al., 2018). Also, there was an overlapping of the D and G bands in the Raman spectra of the activated carbons, which is a depiction of the presence of high proportion of amorphous carbon structures (Guizani et al., 2017). The D-band positions for all the activated carbons were between 1347 and 1386 cm−1. This band represents structural defect, which is absent in a defect-free graphite and when present in microstructure, it appears at a Raman active mode of about 1350 cm−1 (Ciftyurek et al., 2018). This is an indication that the activated carbons are not defect-free. The ID/IG ratio of the biochar-derived activated carbons (KSBC and PMBC) was lower than their respective biomass-derived activated carbons (KSBM and PMBM). This ID/IG ratio describes the ratio of disordered structure of turbostratic carbon to ordered graphite crystallites with lower ID/IG ratio being a representation of more aromatic ring structures and less carbon-containing defects that enhance the formation of surface oxygen functional groups (Peng et al., 2016; Zhao et al., 2013). This implies that the lower ID/IG ratio of the biochar-derived activated carbon is an indication of their higher extent of graphitization, which is not conducive to adsorption applications.
a sample is irradiated with x-ray and the kinetic energy of electrons emitted from atoms is measured by the spectrometer. The binding energy of the electron depends on the chemical environment of the atom(s) from which the electron has been emitted and by quantifying the kinetic energy of the emitted electron, the proportion of the binding energy of the electron with the environment of the atom gives rise to different peaks in the XPS spectrum (Bansal and Goyal, 2005). The C1s, O1s and P2p spectra for the activated carbons were deconvolved and the relative content of the functional groups are presented in Table 3. The C1s spectra were deconvolved into the following peaks based on the chemical shifts showing the aliphatic/aromatic carbon groups: Peak 1 (284.6 eV) – graphitic carbon C-C/C=C, Peak 2 (285.7–286.3 eV) – phenolic, alcohol, ether or C]N groups, Peak 3 (288.4–289.2 eV) – carboxyl or ester groups (Yue et al., 1999; Zhou et al., 2007). The nonfunctionalized/graphitized carbon (Peak 1) was higher than the functionalized carbon (Peak 2 and Peak 3) for both the biochar-derived and biomass-derived activated carbons. A ratio of the functional C to graphitized C (FC/GC) showed that all the activated carbons had similar ratio except PMBC, which has a FC/GC ratio of 0.51. The non-functionalized/graphitized carbon is hydrophobic and not useful for adsorption whereas the functional carbon is hydrophilic (Xi et al., 2019). If the FC/GC ratio are similar for activated carbons, then their adsorption capacities would largely depend on disorderness and availability of internal surface area for molecules to be absorbed. The O1s deconvolved peaks were assigned to the following known chemical shifts: Peak I (531.0–531.9 eV) – carbonyl oxygen of quinines, Peak II (532.2–532.8 eV) – carbonyl oxygen atoms in esters, anhydrides and oxygen atoms in hydroxyl groups, Peak III (533.1–534.6 eV) – non‑carbonyl oxygen single bond in esters and carboxyl group (Zhou et al., 2007). For all the activated carbons, the proportions of the hydroxyl and carboxyl groups were higher than the proportion of the quinines. The presence of the carboxyl and hydroxyl groups are known to make the surface of the activated carbon to be acidic in nature (Li et al., 2002) and this also influence the adsorption characteristics of the activated carbon. The location and number of oxygen functional groups on the pore structure of activated carbon plays a significant role in its adsorption capacity when employed for aqueous adsorption (Franz et al., 2000). For example, water molecules are adsorbed to the
3.5. Surface functional groups The X-ray photoelectron spectroscopy (XPS) was used in analyzing the surface functional groups of the activated carbons. In this technique, Table 2 Raman Spectra D and G band positions and the corresponding intensity ratios for the biochar-derived and biomass-derived activated carbons. Sample
D peak (cm−1)
G peak (cm−1)
ID/IG
KSBC PMBC KSBM PMBM
1386.16 1359.73 1352.08 1347.61
1598.76 1593.49 1588.97 1594.75
0.73 0.89 0.78 0.98
5
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Table 3 Relative content of C1s, O1s and P2p functional groups of the biochar-derived and biomass-derived activated carbons from XPS spectra. Activated carbon
KSBC PMBC KSBM PMBM
C1s (% wt.)
O1s (% wt.)
P2p (% wt.)
Peak 1
Peak 2
Peak 3
FC/GC ratio
Peak I
Peak II
Peak III
Peak A
Peak B
C1s
O1s
Si2p
P2p
44.98 66.43 47.21 46.03
39.11 14.81 30.82 32.09
15.91 18.75 21.97 21.88
1.22 0.51 1.12 1.17
14.72 33.53 12.30 13.27
69.55 39.37 20.52 18.17
15.73 27.10 67.18 68.56
84.13 10.60 62.37 39.96
15.87 89.40 37.63 60.04
74.14 56.14 79.68 80.30
18.75 35.01 15.03 14.61
1.26 – 0.61 0.74
4.03 8.84 3.36 3.57
hydrophilic polar oxygen functional groups on the surface of the activated carbon forming three-dimensional clusters which can block the pore entrance (Franz et al., 2000; Mestre et al., 2007). The P2p spectra was deconvolved into two peaks, namely; Peak A (132.1–133.5) – phosphate and pyrophosphate and Peak B (134.1–134.9) - metaphosphate and phenyl-phosphate (Moulder and Chastain, 1992). The presence of the phosphate functional groups on the activated carbons was due to the use of phosphoric acid in impregnating the precursors and this functional group is acidic in nature, hence making the surfaces of the activated carbons to be acidic as well. A similar result was reported by Xu et al. (2014) for activated carbon produced from chemical activation of reedy grass using phosphoric acid.
From the kinetics parameters presented in Table 4 (pseudo-second order rate constant, k, initial adsorption rate, h, and half-life time, t1/2), it is evident that the adsorption processes of both adsorbates are controlled by the textural and surface chemical characteristics of the activated carbons. For example, PMBC was reported earlier to have the lowest BET surface area of 161.97 m2/g, total pore volume of 0.08 cm3/ g and a FC/GC ratio of 0.51 (which is an indication of its hydrophobicity). This led to its poor adsorption characteristics for both caffeine and acetaminophen. Meanwhile, the biomass-derived activated carbons (KSBM and PMBM) showed a good adsorption performance for both adsorbates due to their high surface area, mesopore volume and FC/GC ratio. This observation is in accordance with the assertion that the overall rate of adsorption process in a pseudo-second order kinetic model is controlled by chemisorption which involves valency forces through electron sharing between the adsorbent and the adsorbate (Foo and Hameed, 2012).
3.6. Adsorption kinetics The adsorption kinetics describes the controlling mechanism of adsorption process, which in turns governs the mass transfer and equilibrium time (Foo and Hameed, 2012; Mestre et al., 2011). The pseudo-second order kinetic model parameters for both acetaminophen and caffeine using the biomass-derived and biochar-derived activated carbons are presented in Table 4. The pseudo-second order kinetic model is based on the sorption capacity on solid phase and it has been shown to predict adsorption behavior of the adsorbent when the initial concentration of solute is low (Ho, 2006). The correlation coefficients for acetaminophen adsorption were higher than for the caffeine adsorption except for caffeine adsorption on PMBM which had the highest correlation coefficient (R2) of 0.99. There is a slight disparity between the calculated adsorption capacity (qe calc) and the experimental adsorption capacity (qe exp) values for caffeine. However, the experimental and the calculated adsorption capacity values for acetaminophen were very similar. The experimental adsorption uptake (qe exp) values for the biomass-derived activated carbons were higher than the biochar-derived activated carbons for both adsorbates. The high adsorption uptake presented by the biomass-derived activated carbons may be attributed to their high mesopore volume. This observation was also reported by Mestre et al. (2011) for the adsorption of paracetamol and ibuprofen with activated carbons having higher mesopore volumes in comparison to activated carbons having smaller mesopore volumes.
3.7. Equilibrium adsorption isotherms Fig. 4a & b illustrate the adsorption isotherms of caffeine and acetaminophen, respectively using the biochar-derived and biomassderived activated carbons. The isotherms displayed by the activated carbons for both acetaminophen and caffeine can be classified as the L type, based on the Giles classification, showing an initial concave curvature at low equilibrium concentrations followed by a plateau or saturation limit (Giles et al., 1960; Mestre et al., 2009). For this type of adsorption isotherm, the adsorbate has a strong affinity to the adsorbent, which is an indication that there is no strong competition by the solvent for the active sites (Mestre et al., 2009). Also, an important characteristic of this adsorption isotherm is its initial curvature, which shows the initial rate of adsorption onto the vacant sites in the activated carbon and as the sites are filled up with the adsorbates, it becomes difficult for the remaining adsorbate molecules to find a vacant site (Giles et al., 1960). It is noteworthy that all the isotherms except KSBC isotherm for acetaminophen did not fully attain the saturation limit. To obtain the maximum adsorption capacity at equilibrium, the experimental isotherms were fitted to the Langmuir and Freundlich models. Table 5 shows the Langmuir and Freundlich model parameters alongside the coefficients of determination (R2). The experimental data fit better to the Langmuir model as its coefficients of determination (R2) was higher than the Freundlich model. The Langmuir model describes monolayer adsorption of adsorbate onto a homogenous adsorbent surface (Mestre et al., 2009; Rey-Mafull et al., 2014). This model accounts for surface coverage by balancing the relative rates of adsorption and desorption with the former being proportional to the fraction of the available active sites on the surface and the latter is proportional to the fraction that is covered (Rey-Mafull et al., 2014). The monolayer adsorption capacities (qm) for the biomass-derived activated carbons were higher than the biochar-derived activated carbons for both caffeine and acetaminophen. KSBM had monolayer adsorption capacities of 102.04 mg/g and 29.87 mg/g while PMBM had monolayer adsorption capacities of 101.01 mg/g and 156.25 mg/g for both caffeine and acetaminophen respectively. A similar result was reported in previous work of the authors for adsorption of caffeine and acetaminophen using KOH biomass-derived and biochar-derived
Table 4 Pseudo-second order kinetic model parameters for the adsorption of caffeine and acetaminophen.
Caffeine
Acetaminophen
Atomic concentration (%)
Activated carbon
qe, exp (mg/g)
qe, calc (mg/g)
kx 10−3 (g/ mg/ min)
t1/2 (min)
h (mg/g/ min)
KSBC PMBC KSBM PMBM KSBC PMBC KSBM PMBM
21.33 3.89 93.73 120.74 43.37 12.04 85.41 90.80
21.65 5.37 107.53 129.87 43.10 13.41 86.21 90.09
7.78 2.28 0.96 0.46 0.49 2.37 1.29 0.58
5.94 81.79 9.68 16.83 47 31.48 8.99 19.28
3.65 8.03 11.11 7.72 0.92 0.43 9.59 4.67
R2
0.83 0.75 0.93 0.99 0.94 0.96 0.96 0.96
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Fig. 4. a: Adsorption isotherms for caffeine with the Langmuir and Freundlich fitting the experimental data for all the activated carbons. b: Adsorption isotherms for acetaminophen with the Langmuir and Freundlich fitting the experimental data for all the activated carbons. Table 5 Langmuir and Freundlich isotherm parameters for the adsorption of caffeine and acetaminophen. Adsorbate
Caffeine
Acetaminophen
Activated carbon
KSBC PMBC KSBM PMBM KSBC PMBC KSBM PMBM
Langmuir equation
Freundlich equation
qm (mg/g)
b (l/mg)
20.16 12.60 102.04 101.01 49.02 13.99 129.87 156.25
0.53 0.10 0.63 0.41 1.71 0.50 0.37 0.25
2
R
0.89 0.83 0.99 0.95 0.99 0.95 0.99 0.95
1/n 0.26 0.44 0.22 0.22 0.15 0.19 0.37 0.44
KF 8.10 2.16 49.77 45.48 14.19 6.99 40.36 38.04
R2 0.28 0.66 0.83 0.85 0.90 0.59 0.94 0.96
qm – maximum monolayer adsorption capacity; b - Langmuir constant. n – Freundlich exponent; KF – Freundlich constant; R2 – linear regression coefficient of determination. Ce – solution concentration at equilibrium (mg/l); qe - uptake at equilibrium (mg/g).
activated carbons. The biomass-derived activated carbon (KOH-KSBM) had monolayer adsorption capacities of 173.42 mg/g and 187.52 mg/g, which were higher than the monolayer adsorption capacities of 11.13 mg/g and 26.36 mg/g presented by the biochar-derived activated carbon (KOH-KSBC) for both caffeine and acetaminophen respectively (Oginni et al., 2019). Meanwhile, Beltrame et al. (2018) reported a higher monolayer adsorption capacity (155.5 mg/g) than our results for caffeine using a biomass-derived activated carbon prepared from pineapple leaves and using phosphoric acid as an activating agent, which could be very well due to different biomass. The high monolayer adsorption capacities of the biomass-derived activated carbons (KSBM and PMBM) in this present study, can be attributed to the porous nature and surface chemical characteristics of the activated carbons. The biomass-derived activated carbons had higher surface areas and total pore volume in comparison to the biochar-derived activated carbons. Also, the mesopore volumes of the biomassderived activated carbons were higher than the biochar-derived activated carbons. With respect to the surface chemistry of the activated carbons, the surfaces of the activated carbons except PMBC were characterized to be hydrophilic in nature. The hydrophilicity of the activated carbon surface enables its wettability, hence enhancing its adsorption behavior. The biomass-derived activated carbons were also shown to have a higher proportion of their oxygen functional groups to be carboxyl and hydroxyl groups, hence making their surface acidic in
nature and enhancing their adsorption behaviors. 4. Conclusion Biomass-derived activated carbons presented higher surface areas alongside total pore volumes and average pore diameters in comparison to the biochar-derived activated carbons. The biomass-derived activated carbons showed more disordered sp2 carbon cluster, which is associated with the presence of more amorphous region. The monolayer adsorption capacities of the biomass-derived activated carbons (KSBM and PMBM) for caffeine and acetaminophen were about 3–5 folds higher than the biochar-derived activated carbons. Similarly, the biomass-derived activated carbons had higher experimental adsorption uptakes for both caffeine and acetaminophen. The better adsorption characteristics of the biomass-derived activated carbons were attributed to their suitable textural and surface chemical characteristics. Declaration of competing interest The authors declare no conflict of interest including financial, personal or other relationships with anybody or organizations. 7
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Acknowledgement
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The authors wish to acknowledge the financial supports from the USDA National Institute of Food and Agriculture, McIntire Stennis project (accession number 1007044) and Hatch Project (accession number 1019107) towards this research work. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biteb.2019.100307. References Bansal, R.C., Goyal, M., 2005. Activated Carbon Adsorption CRC Press. Boca Raton, FL, USA. Bazan-Wozniak, A., Nowicki, P., Pietrzak, R., 2017. The influence of activation procedure on the physicochemical and sorption properties of activated carbons prepared from pistachio nutshells for removal of NO2/H2S gases and dyes. J. Clean. Prod. 152, 211–222. Beltrame, K.K., Cazetta, A.L., de Souza, P.S.C., Spessato, L., Silva, T.L., Almeida, V.C., 2018. Adsorption of caffeine on mesoporous activated carbon fibers prepared from pineapple plant leaves. Ecotoxicol. Environ. Saf. 147, 64–71. Budinova, T., Ekinci, E., Yardim, F., Grimm, A., Björnbom, E., Minkova, V., Goranova, M., 2006. Characterization and application of activated carbon produced by H3PO4 and water vapor activation. Fuel Process. Technol. 87 (10), 899–905. Ciftyurek, E., Bragg, D., Oginni, O., Levelle, R., Singh, K., Sivanandan, L., Sabolsky, E.M., 2018. Performance of Activated Carbons Synthesized from Fruit Dehydration Biowastes for Supercapacitor applications. Environ. Prog. Sustain. Energy 38 (3), 1–14. https://doi.org/10.1002/ep.13030. Foo, K.Y., Hameed, B.H., 2012. Preparation, characterization and evaluation of adsorptive properties of orange peel based activated carbon via microwave induced K2CO3 activation. Bioresour. Technol. 104, 679–686. Franz, M., Arafat, H.A., Pinto, N.G., 2000. Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon. Carbon 38 (13), 1807–1819. Giles, C.H., MacEwan, T.H., Nakhwa, S.N., Smith, D., 1960. 786. Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J. Chem. Soc. (Resumed) (0), 3973–3993. Gueye, M., Richardson, Y., Kafack, F.T., Blin, J., 2014. High efficiency activated carbons from African biomass residues for the removal of chromium(VI) from wastewater. J. Environ. Chem. Eng. 2 (1), 273–281. Guizani, C., Haddad, K., Limousy, L., Jeguirim, M., 2017. New insights on the structural evolution of biomass char upon pyrolysis as revealed by the Raman spectroscopy and elemental analysis. Carbon 119, 519–521. Ho, Y.S., 2006. Review of second-order models for adsorption systems. J. Hazard. Mater. 136 (3), 681–689. Kumar, A., Jena, H.M., 2016. Preparation and characterization of high surface area activated carbon from Fox nut (Euryale ferox) shell by chemical activation with H3PO4. Results Phys. 6, 651–658. Kupgan, G., Liyana-Arachchi, T.P., Colina, C.M., 2017. NLDFT pore size distribution in amorphous microporous materials. Langmuir 33 (42), 11138–11145. Li, L., Quinlivan, P.A., Knappe, D.R.U., 2002. Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon 40 (12), 2085–2100. Lowell, S., Shields, J.E., Thomas, M.A., Thommes, M., 2006. Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density. Springer, Netherlands, pp. 350. https://doi.org/10.1007/978-1-4020-2303-3. Marsh, H., Rodríguez-Reinoso, F., 2006. Chapter 6 - Activation Processes (Chemical). in. Elsevier Science Ltd. Oxford, Activated Carbon, pp. 322–365. Mestre, A.S., Pires, J., Nogueira, J.M.F., Carvalho, A.P., 2007. Activated carbons for the adsorption of ibuprofen. Carbon 45 (10), 1979–1988. Mestre, A.S., Pires, J., Nogueira, J.M.F., Parra, J.B., Carvalho, A.P., Ania, C.O., 2009. Waste-derived activated carbons for removal of ibuprofen from solution: Role of
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Effect of one-step and two-step H3PO4 activation on activated carbon characteristics
T
Oluwatosin Oginnia, , Kaushlendra Singha, Gloria Oportoa, Benjamin Dawson-Andoha, Louis McDonaldb, Edward Sabolskyc ⁎
a
School of Natural Resources, West Virginia University, Morgantown, WV 26506, United States of America Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV 26506, United States of America c Department of Mechanical Engineering, West Virginia University, Morgantown, WV 26506, United States of America b
ARTICLE INFO
ABSTRACT
Keywords: Activated carbon Activation H3PO4 Adsorption Biochar Biomass
The objective of this study was to investigate the effect of one-step and two-step H3PO4 activation on activated carbon characteristics. Kanlow Switchgrass and Public Miscanthus and their biochars obtained via pyrolysis were used as precursors. The precursors were impregnated with H3PO4 and activated at a temperature of 900 °C to produce biomass-derived and biochar-derived activated carbons. The activated carbons were characterized for textural characteristics, surface chemistry and microstructure. The activated carbons were also tested for their adsorption capacities for caffeine and acetaminophen. The biomass-derived activated carbons (KSBM and PMBM) had the highest BET surface areas of 1373 and 999 m2/g, respectively. The total pore volumes of the biomass-derived activated carbons were about two-fold higher than the biochar-derived activated carbons. In addition, the monolayer adsorption capacities of the biomass-derived activated carbons were about five folds higher than that of biochar-derived activated carbons for the adsorption of caffeine and acetaminophen.
1. Introduction Chemical synthesis of activated carbon involves the impregnation of the precursor with activating agent prior to activation/carbonization. The chemical activating agent plays a significant role in the characteristics of the resulting activated carbon. For example, the use of zinc chloride as an activating agent, produces highly microporous activated carbon. However, its use is associated with the challenges such as equipment corrosion, inefficient chemical recovery and possibility of product contamination when the activated carbons are used in pharmaceutical and food industries (Oginni, 2018; Prahas et al., 2008). Significant preference is given to the use of phosphoric acid because of the non-polluting nature of the resulting activated carbon, the development of both micropores and mesopores of the activated carbon, and the ease of chemical recovery (Kumar and Jena, 2016). Synthesis of activated carbon via the chemical pathway can be divided into a one-step or two-step activation. In the one-step activation, the biomass precursor is impregnated with the chemical activating agent and the impregnated precursor is activated/carbonized at an elevated temperature to produce a biomass-derived activated carbon. While the two-step activation involves the thermal conversion of the biomass to biochar and subsequent impregnation of the biochar precursor, followed ⁎
by the activation of the impregnated biochar precursor to produce a biochar-derived activated carbon (Oginni et al., 2019). In a one-step activation using phosphoric acid, the activating agent interacts with the biomass precursor during the impregnation procedure by cleaving the arylether bonds in the lignin and hydrolyzing the glycosidic linkages in the hemicellulose and cellulose through dehydration and condensation reactions (Marsh and Rodríguez-Reinoso, 2006; Villota et al., 2019). Also, the phosphoric acid interacts with the organic species in the biomass precursor to form phosphate and polyphosphate groups, which promote a dilation process that leaves an accessible porous structure after elimination of the acid (Gueye et al., 2014; Villota et al., 2019). Yakout and Sharaf El-Deen (2016) prepared a biomass-derived activated carbon from olive stones using a one-step H3PO4 activation and reported a BET surface area of 1218 m2/g and total pore volume of 0.6 cm3/g. The N2 adsorption isotherm of the activated carbon was classified as type I according to International Union of Pure and Applied Chemistry (IUPAC) classification, which is an indication that the activated carbon is a microporous material. The development and widening of activated carbon porosity was associated with the amount of H3PO4 intercalated in the internal structure of the biomass precursor. Villota et al. (2019) investigated the microwave assisted H3PO4 activation of waste cocoa pod husk and reported a BET
Corresponding author at: 322 Percival Hall, School of Natural Resources, West Virginia University, Morgantown, WV 26506, United States of America. E-mail address: [email protected] (O. Oginni).
https://doi.org/10.1016/j.biteb.2019.100307 Received 6 July 2019; Received in revised form 9 August 2019; Accepted 10 August 2019 Available online 14 August 2019 2589-014X/ Published by Elsevier Ltd.
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surface area of 1139.17 m2/g and a total pore volume of 1.062 cm3/g. The distribution of the surface area and pore volume showed that a larger proportion of the surface area and the pore volume were mesopores, which indicates that the activated carbon is mesoporous in nature. Valero-Romero et al. (2019) characterized a biomass-derived activated carbon produced from a one-step H3PO4 activation of olive stones at a temperature of 800 °C for 2 h. The activated carbon had a BET surface area of 1380 m2/g and mesopore volume of 0.654 cm3/g. The high surface area is an indication of large number of active sites and the high mesopore volume enhances mass transfer rate, making the activated carbon suitable for catalytic applications. Also, the surface chemistry of the activated carbon showed a high surface oxygen content, which was associated with the presence of C–O–P surface functional groups formed via the oxidation of CeP bonds. The presence of the C–O–P surface functional groups on the activated carbon surface is responsible for high chemical and thermal stability, as well as surface acidity of the activated carbon. However, in a two-step H3PO4 activation, the phosphoric acid facilitates the removal of tarry substances in the pores of the biochar and act as a template for the creation of microporosity during the activation stage (Marsh and Rodríguez-Reinoso, 2006). Shamsuddin et al. (2016) reported an increase in the BET surface area of a biochar-derived activated carbon obtained from the H3PO4 activation of biochar made from kenaf core fiber. The biochar had a BET surface area of 13.68 m2/ g and a micropore surface area of 7.16 m2/g, which increased to 299.02 m2/g and 229.20 m2/g, respectively due to activation. The pore development of the biochar-derived activated carbon was enhanced by the diffusion of H3PO4 molecules into the pores of the biochar precursor and thereby increasing the H3PO4-carbon reaction via acid hydrolysis process which resulted in the creation of more pores. Nitnithiphrut et al. (2017) prepared a biochar-derived activated carbon from H3PO4 activation of biochar made from para wood and reported a BET surface area of 153.73 m2/g, which was higher than the BET surface area of 86.91 m2/g obtained for the para wood biochar. The SEM morphology of the activated carbon showed a well-developed porous structure, which was attributed to chemical etching on the biochar surface and deep penetration of the H3PO4 molecules into the biochar pores. While independent studies were conducted on either one-step or two-step activation using phosphoric acid as an agent, it is hard to establish which pathway is better because different biomass were tested for different pathway. Recently, Oginni et al. (2019) compared one-step and two-step activation on activated carbon characteristics using KOH as an activating agent. The KOH acts like a catalyst and reacts with the biomass/biochar carbon as opposed to phosphoric acid, which act like a cleaning agent and cleans up the deposited tarry products in biochar's pores. Therefore, the objective of this study was to investigate the effect of one-step and two-step H3PO4 activation on characteristics of biomass-derived and biochar-derived activated carbons and test their aqueous adsorption characteristics with respect to their potential use for water treatment.
2019; Oginni et al., 2017). The pyrolysis vapor generated during the carbonization process was swept into a series of ice-bath condensers. The biochars were removed from the reactor after it was cooled down to room temperature under a constant flow of nitrogen gas. The impregnation procedure was carried out by soaking about 50 g of the precursors in 200 ml solution of phosphoric acid (85% wt.) and continuously stirred inside a pressure reactor (Model: 4500, Parr Instrument Company, Moline, IL) at a temperature of 85 °C for 24 h. After the impregnation process was completed, the impregnated precursors were oven dried at a temperature of 103 °C for 24 h. The oven-dried impregnated precursors were thereafter subjected to activation at a temperature of 900 °C for 1 h under continuous nitrogen flow using a thermogravimetric analyzer (Model: TGA 701, LECO Corporation, St. Joseph, MI). At the end of the activation procedure, the activated carbons were allowed to cool down to room temperature under the continuous flow of nitrogen. Thereafter, the activated carbons were washed with boiling deionized water in a lab-line multiunit extraction equipment until the pH of the activated carbons became neutral. The washed activated carbons were oven dried at a temperature of 103 °C for 24 h. The biomass-derived activated carbons were labelled as: KSBM (Activated Carbon from Kanlow Switchgrass Biomass), PMBM (Activated Carbon from Public Miscanthus Biomass); while the biocharderived activated carbons were labelled as KSBC (Activated Carbon from Kanlow Switchgrass Biochar), PMBC (Activated Carbon from Public Miscanthus Biochar). 2.2. Activated carbon characterization 2.2.1. Morphology Scanning electron microscope (Model: Hitachi-S4700, Hitachi High Technologies America, Schaumburg, IL) at the West Virginia University Shared Research Facilities was used in examining the morphology of the activated carbons. A double sided adhesive tape was used in fixing the activated carbon sample to the specimen stub. The specimen stub was placed on the specimen holder and transferred into the vacuum chamber where the microscope scanned the sample with a beam of electrons. The SEM images were collected using the equipment software. 2.2.2. Surface area and pore characteristics Nitrogen adsorption/desorption isotherms at 77 K of the activated carbons were determined using Micromeritics Accelerated Surface Area and Porosimetry System (Model: ASAP 2020, Norcross, GA, USA). The samples were degassed at a temperature of 105 °C for 24 h and cooled down to 30 °C prior to the analysis. The surface areas were calculated using the BET equation. The t-plot model was used in calculating the micropore volume. The Non-Local Density Function Theory (NLDFT) model in the SAIEUS software (Kupgan et al., 2017) was used in obtaining the pore size distributions of the activated carbons. 2.2.3. XPS and Raman analysis The surface functional groups were determined using X-ray photoelectron spectroscopy (Model: PHI 5000 VersaProbe XPS/UPS, ULVACPHI Inc., Kanagawa, Japan) in a spectral range of 0 to 1400 eV binding energy and energy resolution of 0.50 eV. The sample was mounted on a steel specimen disk using an Ultra High Vacuum approved spectral grade double-sided carbon tape and loaded into the introduction chamber. Thereafter, the sample was transferred into the analysis chamber where the photoelectron spectra were acquired. The element identification and the peak fitting were carried out using the PHI MultiPak software. Raman microscope (Model: Renishaw InVia) equipped with an excitation laser of 532 nm wavelength was used in examining the microstructure of the activated carbons. The laser beam was operated on a 1% laser power input and at a 50× optical magnification. The scanning of the sample was within a spectrum range of 1000–2000 cm−1.
2. Materials and methods 2.1. Material preparation Biomass and biochar precursors were used in synthesizing the activated carbons in this study. The biomass precursors were Kanlow Switchgrass and Public Miscanthus, which were oven dried at 103 °C for 24 h and thereafter ground to < 1 mm particle size using a Retsch Grindomix (Model: GM 200). The biochar precursors were produced by carbonizing the ground biomass samples in a fixed-bed batch reactor having a diameter of 22.86 cm and a height of 25.40 cm. The reactor was filled with oven-dried biomass and the sample-filled reactor was placed in a furnace (Model: BF51728C, Thermo Scientific, NC). The furnace was heated from room temperature to 500 °C under a nitrogen gas flow of 2 L/min and kept at that temperature for 30 min (Oginni and Singh, 2
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2.3. Adsorption of caffeine and acetaminophen
main conduit to the micropores on the inner surface of the activated carbons (Xu et al., 2014; Yorgun and Yıldız, 2015).
Caffeine (purchased from Fischer Science Education, Nazareth, PA) and acetaminophen (purchased from Acros Organics, Morris, NJ) were used as adsorbates for the adsorption experiment. Standard solutions having concentration of 200 ppm were produced by dissolving 10 mg of the adsorbate in 50 ml distilled deionized water. From this standard solution, samples were taken and adjusted with distilled deionized water to give the desired concentration. The solution concentration was measured with UV spectrophotometer (Model: Varian Cary 50) at wavelength of 242 nm and 274 nm for acetaminophen and caffeine respectively. The selection of the wavelength was based on the peak absorbance recorded for both adsorbates when their solutions were scanned in the spectrophotometer between 200 nm and 400 nm on the spectrum mode, using distilled deionized water as a blank. The experiments conducted in investigating the adsorption kinetics involved the addition of 10 mg of the activated carbon to 40 ml of adsorbate solution in glass vials with an initial concentration of 40 ppm. The mixture was agitated in a multipoint agitation plate at a temperature of 25 °C. The time recording started when the agitation began and several samples were collected between 15 min and 9 h. The collected samples were filtered through a filter paper (Fisher Scientific, Qualitative P4) to separate activated carbon and adsorbate solution. The quantity of adsorbate uptake by activated carbon was calculated using Eq. (1);
qt =
Co
Ct W
V
3.2. BET surface area The BET surface areas of the activated carbons were assessed by the N2 adsorption/desorption isotherms shown in Fig. 2. The isotherm for the biochar-derived activated carbon (PMBC) can be classified as type I, as the isotherm is horizontal and parallel over the relative pressure range. Microporous materials are known to exhibit this type of adsorption isotherm, which implies that PMBC is microporous in nature. However, the adsorption/desorption isotherms for the other biocharderived activated carbon (KSBC) and the biomass-derived activated carbons (KSBM and PMBM) was more S-shaped and it can be classified as type IV isotherm. It is visually clear that the biomass-derived activated carbons adsorbed more nitrogen over the relative pressure range (0.20 to 0.80) as opposed to the biochar-derived activated carbons where no significant nitrogen was absorbed in the same range. It essentially indicates that the use of phosphoric acid directly on the biomass precursor did not allow depositing of tarry material in the mesopores and preserved the pores. The type IV isotherm is characterized by its hysteresis loop, which is associated with capillary condensation taking place in the mesopore (Sing et al., 1985). The initial part of the isotherm (prior to the hysteresis loop), also known as the inflection point of the knee of the isotherm is attributed to monolayer-multilayer adsorption, which is similar to the middle section of type II isotherm and it indicates the stage at which monolayer coverage is complete and multilayer adsorption begins (Lowell et al., 2006; Sing et al., 1985). The textural characteristics of the biomass-derived and biochar-derived activated carbons are presented in Table 1. The biomass-derived activated carbons (KSBM and PMBM) had higher BET surface areas in comparison to the biochar-derived activated carbons. However, the biochar-derived activated carbon (PMBC) had the lowest BET surface area of 161.97 m2/g. This may be attributed to a high degree of burn off from the activated carbon during the activation process. The high BET surface areas of the biomass-derived activated carbons may be attributed to chemical reaction between the chemical components of the biomass precursors and the activating agent during the carbonization/activation stage (Oginni et al., 2019). The acidic impregnation of the biomass precursor led to the hydrolysis of cellulose and hemicellulose present in it (Sun and Cheng, 2002). The phosphoric acid impregnation has been reported to have the following two important functions: it promotes the pyrolytic decomposition of the biomass precursor and the formation of cross-linked structure (Budinova et al., 2006; Yorgun and Yıldız, 2015). The cross-linking is due to interactions between the acid and the organic material in the precursor leading to formation of phosphate linkages between the fragments in the biopolymer (Budinova et al., 2006). The BET surface area, which simply represents the internal surface area, is generally divided into micropore and mesopore. The micropore surface area represents the fraction of the activated carbon that are categorized as microporous. Bansal and Goyal (2005) explained that due to the large proportion of the surface area and pore volume that the micropore constitutes, the micropore plays a significant role in the adsorption behavior of an activated carbon, given that the molecular dimensions of the adsorbate are not too large to enter the micropores. The micropore surface areas (Smicro) of the activated carbons ranged between 137 and 285 m2/g (Table 1). The percentage of the micropore surface area to the total BET surface area showed that the biochar-derived activated carbons (KSBC and PMBC) had a higher proportion of their BET surface areas as micropores. The Smicro/SBET percentage for the biocharderived activated carbons (KSBC and PMBC) was 39.14 and 86.06% respectively while the biomass-derived activated carbons (KSBM and PMBM) had a Smicro/SBET percentage of 9.98 and 28.48% respectively. The higher Smicro/SBET percentage for the biochar-derived activated carbons may be attributed to the fact that the activating agent helped in removing deposited tarry substances that blocked the pore network of
(1)
where qt is the quantity (mg/g) of adsorbate adsorbed at time t, Co is the initial concentration (ppm), Ct is the concentration at time t (ppm), V is the volume (ml) of the adsorbate solution and W is the weight (mg) of the activated carbon used. The pseudo-second order kinetic model (Mestre et al., 2007) was used in estimating the adsorption kinetics and it is expressed as;
t 1 1 = + t qt qe k2 qe2
(2)
where qe is the maximum adsorption capacity for the pseudo-second order adsorption, k2 is the equilibrium rate constant for the pseudosecond order adsorption. Values of k2 and qe were estimated from the intercept and slope, respectively, of the plot of t/qt versus t. The Langmuir and Freundlich isotherm equations were used in modelling the adsorption equilibrium. In conducting the adsorption equilibrium experiment, 10 mg of the activated carbon was mixed with 40 ml solution of the adsorbates (initial concentration of 10–40 ppm) and agitated for 5 h. After the agitation, the concentration of the adsorbate in the solution at equilibrium was determined. The choice of 5 h as agitation time was based on the result of the adsorption kinetics, where there was no difference in the uptake of the adsorbates by the activated carbons between 5 and 9 h. 3. Results and discussion 3.1. Solid morphology Fig. 1 shows the scanning electron micrographs of the biomass-derived and biochar-derived activated carbons. All the activated carbons showed visible cavities and pores, which is an indication that original cell wall structures of the precursors were retained. Phosphoric acid act as a cleaning and dehydrating agent, which removes tarry products in the pores of the biochar precursor and promotes bond cleavage reactions in the biomass precursor and facilitate crosslinking, condensation and formation of linkage such as polyphosphosate esters, which can protect the internal pore structure of the resulting activated carbon (Xu et al., 2014). The pores showed in the SEM images are tunnel shaped with varying sizes and these pores are exterior pores which act as the 3
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A
C
B
D
Fig. 1. Scanning electron micrographs of the biochar-derived activated carbons [(A) KSBC, (B) PMBC] and biomass-derived activated carbons [(C) KSBM, (D) PMBM].
Fig. 2. N2 adsorption/desorption isotherms for biochar-derived and biomass-derived activated carbons.
The mesopore size distribution for the biomass-derived activated carbons can be seen to be higher than the biochar-derived activated carbons. This corroborates the values reported in Table 1 for the mesopore volume of the activated carbons. The biomass-derived activated carbons (KSBM and PMBM) showed a higher mesopore volume of 1.35 and 0.92 cm3/g respectively. However, the micropore volumes for all the activated carbons were similar. The micropores are characterized by adsorption through volume filling and there is no capillary filling taking place in these pores whereas the mesopores are characterized by capillary condensation with formation of a meniscus of the liquified adsorbate (Bansal and Goyal, 2005). The average pore diameters for all the activated carbons were between 2.01 and 4.15 nm (Table 1). It is also noteworthy that the biomassderived activated carbons possessed higher average pore diameters. This difference is attributed to the fact that the biochar precursors had an ordered porous structure, such that the activating agent had a minimal impact on improving the porous structure of the resulting biochar-derived activated carbons (Bazan-Wozniak et al., 2017).
Table 1 Textural characteristics of biomass-derived and biochar-derived activated carbons. Sample
SBET (m2/g)
Smicro (m2/g)
Smicro/SBET (%)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
dp (nm)
KSBC PMBC KSBM PMBM
697.96 161.97 1372.93 999.06
273.20 139.39 137.05 284.51
39.14 86.06 9.98 28.48
0.55 0.08 1.45 1.04
0.12 0.06 0.10 0.12
0.43 0.02 1.35 0.92
3.13 2.01 3.42 4.15
*dp: average pore diameter (4V/A by BET); V: pore volume; S: surface area.
the biochars, hence leading to the higher Smicro/SBET percentage. 3.3. Pore size distribution Pore size distribution is an intrinsic property of the activated carbon, which influences its adsorption performance. Fig. 3 shows the pore size distributions of the biomass-derived and biochar-derived activated carbons. All the activated carbons except PMBC showed multiple modal distributions across the microporous and mesoporous range. The biochar-derived activated carbon (PMBC) showed a narrow monomodal distribution which is centered in the microporous range.
3.4. Raman analysis The Raman spectra of the biomass-derived and biochar-derived activated carbons showed that all the activated carbons exhibited the 4
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Pore Size Distribution (cc/g/nm)
0.35 0.3
KSBC
PMBC
KSBM
PMBM
Microporosity
0.25
Mesoporosity 0.2 0.15 0.1 0.05 0 0
2
4
6
8
10
Pore Width (nm) Fig. 3. Pore size distribution of the biomass-derived and biochar-derived activated carbons.
two-signature D (disorderness) and G (graphitization) bands. Table 2 shows the band positions and intensity ratio (ID/IG) for all the activated carbons. The presence of both the D and G bands in the Raman spectra of the activated carbons is a representation of their heterogenous carbon microstructure (Yumak et al., 2018). Also, there was an overlapping of the D and G bands in the Raman spectra of the activated carbons, which is a depiction of the presence of high proportion of amorphous carbon structures (Guizani et al., 2017). The D-band positions for all the activated carbons were between 1347 and 1386 cm−1. This band represents structural defect, which is absent in a defect-free graphite and when present in microstructure, it appears at a Raman active mode of about 1350 cm−1 (Ciftyurek et al., 2018). This is an indication that the activated carbons are not defect-free. The ID/IG ratio of the biochar-derived activated carbons (KSBC and PMBC) was lower than their respective biomass-derived activated carbons (KSBM and PMBM). This ID/IG ratio describes the ratio of disordered structure of turbostratic carbon to ordered graphite crystallites with lower ID/IG ratio being a representation of more aromatic ring structures and less carbon-containing defects that enhance the formation of surface oxygen functional groups (Peng et al., 2016; Zhao et al., 2013). This implies that the lower ID/IG ratio of the biochar-derived activated carbon is an indication of their higher extent of graphitization, which is not conducive to adsorption applications.
a sample is irradiated with x-ray and the kinetic energy of electrons emitted from atoms is measured by the spectrometer. The binding energy of the electron depends on the chemical environment of the atom(s) from which the electron has been emitted and by quantifying the kinetic energy of the emitted electron, the proportion of the binding energy of the electron with the environment of the atom gives rise to different peaks in the XPS spectrum (Bansal and Goyal, 2005). The C1s, O1s and P2p spectra for the activated carbons were deconvolved and the relative content of the functional groups are presented in Table 3. The C1s spectra were deconvolved into the following peaks based on the chemical shifts showing the aliphatic/aromatic carbon groups: Peak 1 (284.6 eV) – graphitic carbon C-C/C=C, Peak 2 (285.7–286.3 eV) – phenolic, alcohol, ether or C]N groups, Peak 3 (288.4–289.2 eV) – carboxyl or ester groups (Yue et al., 1999; Zhou et al., 2007). The nonfunctionalized/graphitized carbon (Peak 1) was higher than the functionalized carbon (Peak 2 and Peak 3) for both the biochar-derived and biomass-derived activated carbons. A ratio of the functional C to graphitized C (FC/GC) showed that all the activated carbons had similar ratio except PMBC, which has a FC/GC ratio of 0.51. The non-functionalized/graphitized carbon is hydrophobic and not useful for adsorption whereas the functional carbon is hydrophilic (Xi et al., 2019). If the FC/GC ratio are similar for activated carbons, then their adsorption capacities would largely depend on disorderness and availability of internal surface area for molecules to be absorbed. The O1s deconvolved peaks were assigned to the following known chemical shifts: Peak I (531.0–531.9 eV) – carbonyl oxygen of quinines, Peak II (532.2–532.8 eV) – carbonyl oxygen atoms in esters, anhydrides and oxygen atoms in hydroxyl groups, Peak III (533.1–534.6 eV) – non‑carbonyl oxygen single bond in esters and carboxyl group (Zhou et al., 2007). For all the activated carbons, the proportions of the hydroxyl and carboxyl groups were higher than the proportion of the quinines. The presence of the carboxyl and hydroxyl groups are known to make the surface of the activated carbon to be acidic in nature (Li et al., 2002) and this also influence the adsorption characteristics of the activated carbon. The location and number of oxygen functional groups on the pore structure of activated carbon plays a significant role in its adsorption capacity when employed for aqueous adsorption (Franz et al., 2000). For example, water molecules are adsorbed to the
3.5. Surface functional groups The X-ray photoelectron spectroscopy (XPS) was used in analyzing the surface functional groups of the activated carbons. In this technique, Table 2 Raman Spectra D and G band positions and the corresponding intensity ratios for the biochar-derived and biomass-derived activated carbons. Sample
D peak (cm−1)
G peak (cm−1)
ID/IG
KSBC PMBC KSBM PMBM
1386.16 1359.73 1352.08 1347.61
1598.76 1593.49 1588.97 1594.75
0.73 0.89 0.78 0.98
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Table 3 Relative content of C1s, O1s and P2p functional groups of the biochar-derived and biomass-derived activated carbons from XPS spectra. Activated carbon
KSBC PMBC KSBM PMBM
C1s (% wt.)
O1s (% wt.)
P2p (% wt.)
Peak 1
Peak 2
Peak 3
FC/GC ratio
Peak I
Peak II
Peak III
Peak A
Peak B
C1s
O1s
Si2p
P2p
44.98 66.43 47.21 46.03
39.11 14.81 30.82 32.09
15.91 18.75 21.97 21.88
1.22 0.51 1.12 1.17
14.72 33.53 12.30 13.27
69.55 39.37 20.52 18.17
15.73 27.10 67.18 68.56
84.13 10.60 62.37 39.96
15.87 89.40 37.63 60.04
74.14 56.14 79.68 80.30
18.75 35.01 15.03 14.61
1.26 – 0.61 0.74
4.03 8.84 3.36 3.57
hydrophilic polar oxygen functional groups on the surface of the activated carbon forming three-dimensional clusters which can block the pore entrance (Franz et al., 2000; Mestre et al., 2007). The P2p spectra was deconvolved into two peaks, namely; Peak A (132.1–133.5) – phosphate and pyrophosphate and Peak B (134.1–134.9) - metaphosphate and phenyl-phosphate (Moulder and Chastain, 1992). The presence of the phosphate functional groups on the activated carbons was due to the use of phosphoric acid in impregnating the precursors and this functional group is acidic in nature, hence making the surfaces of the activated carbons to be acidic as well. A similar result was reported by Xu et al. (2014) for activated carbon produced from chemical activation of reedy grass using phosphoric acid.
From the kinetics parameters presented in Table 4 (pseudo-second order rate constant, k, initial adsorption rate, h, and half-life time, t1/2), it is evident that the adsorption processes of both adsorbates are controlled by the textural and surface chemical characteristics of the activated carbons. For example, PMBC was reported earlier to have the lowest BET surface area of 161.97 m2/g, total pore volume of 0.08 cm3/ g and a FC/GC ratio of 0.51 (which is an indication of its hydrophobicity). This led to its poor adsorption characteristics for both caffeine and acetaminophen. Meanwhile, the biomass-derived activated carbons (KSBM and PMBM) showed a good adsorption performance for both adsorbates due to their high surface area, mesopore volume and FC/GC ratio. This observation is in accordance with the assertion that the overall rate of adsorption process in a pseudo-second order kinetic model is controlled by chemisorption which involves valency forces through electron sharing between the adsorbent and the adsorbate (Foo and Hameed, 2012).
3.6. Adsorption kinetics The adsorption kinetics describes the controlling mechanism of adsorption process, which in turns governs the mass transfer and equilibrium time (Foo and Hameed, 2012; Mestre et al., 2011). The pseudo-second order kinetic model parameters for both acetaminophen and caffeine using the biomass-derived and biochar-derived activated carbons are presented in Table 4. The pseudo-second order kinetic model is based on the sorption capacity on solid phase and it has been shown to predict adsorption behavior of the adsorbent when the initial concentration of solute is low (Ho, 2006). The correlation coefficients for acetaminophen adsorption were higher than for the caffeine adsorption except for caffeine adsorption on PMBM which had the highest correlation coefficient (R2) of 0.99. There is a slight disparity between the calculated adsorption capacity (qe calc) and the experimental adsorption capacity (qe exp) values for caffeine. However, the experimental and the calculated adsorption capacity values for acetaminophen were very similar. The experimental adsorption uptake (qe exp) values for the biomass-derived activated carbons were higher than the biochar-derived activated carbons for both adsorbates. The high adsorption uptake presented by the biomass-derived activated carbons may be attributed to their high mesopore volume. This observation was also reported by Mestre et al. (2011) for the adsorption of paracetamol and ibuprofen with activated carbons having higher mesopore volumes in comparison to activated carbons having smaller mesopore volumes.
3.7. Equilibrium adsorption isotherms Fig. 4a & b illustrate the adsorption isotherms of caffeine and acetaminophen, respectively using the biochar-derived and biomassderived activated carbons. The isotherms displayed by the activated carbons for both acetaminophen and caffeine can be classified as the L type, based on the Giles classification, showing an initial concave curvature at low equilibrium concentrations followed by a plateau or saturation limit (Giles et al., 1960; Mestre et al., 2009). For this type of adsorption isotherm, the adsorbate has a strong affinity to the adsorbent, which is an indication that there is no strong competition by the solvent for the active sites (Mestre et al., 2009). Also, an important characteristic of this adsorption isotherm is its initial curvature, which shows the initial rate of adsorption onto the vacant sites in the activated carbon and as the sites are filled up with the adsorbates, it becomes difficult for the remaining adsorbate molecules to find a vacant site (Giles et al., 1960). It is noteworthy that all the isotherms except KSBC isotherm for acetaminophen did not fully attain the saturation limit. To obtain the maximum adsorption capacity at equilibrium, the experimental isotherms were fitted to the Langmuir and Freundlich models. Table 5 shows the Langmuir and Freundlich model parameters alongside the coefficients of determination (R2). The experimental data fit better to the Langmuir model as its coefficients of determination (R2) was higher than the Freundlich model. The Langmuir model describes monolayer adsorption of adsorbate onto a homogenous adsorbent surface (Mestre et al., 2009; Rey-Mafull et al., 2014). This model accounts for surface coverage by balancing the relative rates of adsorption and desorption with the former being proportional to the fraction of the available active sites on the surface and the latter is proportional to the fraction that is covered (Rey-Mafull et al., 2014). The monolayer adsorption capacities (qm) for the biomass-derived activated carbons were higher than the biochar-derived activated carbons for both caffeine and acetaminophen. KSBM had monolayer adsorption capacities of 102.04 mg/g and 29.87 mg/g while PMBM had monolayer adsorption capacities of 101.01 mg/g and 156.25 mg/g for both caffeine and acetaminophen respectively. A similar result was reported in previous work of the authors for adsorption of caffeine and acetaminophen using KOH biomass-derived and biochar-derived
Table 4 Pseudo-second order kinetic model parameters for the adsorption of caffeine and acetaminophen.
Caffeine
Acetaminophen
Atomic concentration (%)
Activated carbon
qe, exp (mg/g)
qe, calc (mg/g)
kx 10−3 (g/ mg/ min)
t1/2 (min)
h (mg/g/ min)
KSBC PMBC KSBM PMBM KSBC PMBC KSBM PMBM
21.33 3.89 93.73 120.74 43.37 12.04 85.41 90.80
21.65 5.37 107.53 129.87 43.10 13.41 86.21 90.09
7.78 2.28 0.96 0.46 0.49 2.37 1.29 0.58
5.94 81.79 9.68 16.83 47 31.48 8.99 19.28
3.65 8.03 11.11 7.72 0.92 0.43 9.59 4.67
R2
0.83 0.75 0.93 0.99 0.94 0.96 0.96 0.96
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Fig. 4. a: Adsorption isotherms for caffeine with the Langmuir and Freundlich fitting the experimental data for all the activated carbons. b: Adsorption isotherms for acetaminophen with the Langmuir and Freundlich fitting the experimental data for all the activated carbons. Table 5 Langmuir and Freundlich isotherm parameters for the adsorption of caffeine and acetaminophen. Adsorbate
Caffeine
Acetaminophen
Activated carbon
KSBC PMBC KSBM PMBM KSBC PMBC KSBM PMBM
Langmuir equation
Freundlich equation
qm (mg/g)
b (l/mg)
20.16 12.60 102.04 101.01 49.02 13.99 129.87 156.25
0.53 0.10 0.63 0.41 1.71 0.50 0.37 0.25
2
R
0.89 0.83 0.99 0.95 0.99 0.95 0.99 0.95
1/n 0.26 0.44 0.22 0.22 0.15 0.19 0.37 0.44
KF 8.10 2.16 49.77 45.48 14.19 6.99 40.36 38.04
R2 0.28 0.66 0.83 0.85 0.90 0.59 0.94 0.96
qm – maximum monolayer adsorption capacity; b - Langmuir constant. n – Freundlich exponent; KF – Freundlich constant; R2 – linear regression coefficient of determination. Ce – solution concentration at equilibrium (mg/l); qe - uptake at equilibrium (mg/g).
activated carbons. The biomass-derived activated carbon (KOH-KSBM) had monolayer adsorption capacities of 173.42 mg/g and 187.52 mg/g, which were higher than the monolayer adsorption capacities of 11.13 mg/g and 26.36 mg/g presented by the biochar-derived activated carbon (KOH-KSBC) for both caffeine and acetaminophen respectively (Oginni et al., 2019). Meanwhile, Beltrame et al. (2018) reported a higher monolayer adsorption capacity (155.5 mg/g) than our results for caffeine using a biomass-derived activated carbon prepared from pineapple leaves and using phosphoric acid as an activating agent, which could be very well due to different biomass. The high monolayer adsorption capacities of the biomass-derived activated carbons (KSBM and PMBM) in this present study, can be attributed to the porous nature and surface chemical characteristics of the activated carbons. The biomass-derived activated carbons had higher surface areas and total pore volume in comparison to the biochar-derived activated carbons. Also, the mesopore volumes of the biomassderived activated carbons were higher than the biochar-derived activated carbons. With respect to the surface chemistry of the activated carbons, the surfaces of the activated carbons except PMBC were characterized to be hydrophilic in nature. The hydrophilicity of the activated carbon surface enables its wettability, hence enhancing its adsorption behavior. The biomass-derived activated carbons were also shown to have a higher proportion of their oxygen functional groups to be carboxyl and hydroxyl groups, hence making their surface acidic in
nature and enhancing their adsorption behaviors. 4. Conclusion Biomass-derived activated carbons presented higher surface areas alongside total pore volumes and average pore diameters in comparison to the biochar-derived activated carbons. The biomass-derived activated carbons showed more disordered sp2 carbon cluster, which is associated with the presence of more amorphous region. The monolayer adsorption capacities of the biomass-derived activated carbons (KSBM and PMBM) for caffeine and acetaminophen were about 3–5 folds higher than the biochar-derived activated carbons. Similarly, the biomass-derived activated carbons had higher experimental adsorption uptakes for both caffeine and acetaminophen. The better adsorption characteristics of the biomass-derived activated carbons were attributed to their suitable textural and surface chemical characteristics. Declaration of competing interest The authors declare no conflict of interest including financial, personal or other relationships with anybody or organizations. 7
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