Effective Adsorption of Sulfur Dioxide by Activated Carbons

Sep 11, 2014 - Superior de Ingeniería y Diseño Industrial, Universidad Politécnica de Madrid, Ronda de Valencia 3, 28012, Madrid, Spain. •S Suppo...
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Effective Adsorption of Sulfur Dioxide by Activated Carbons Containing Calcium Carbonate from Deinking Paper Sludge Antonio Nieto-Márquez,* Evangelina Atanes, Alberto Fernández, Sergio López-Villa, Alberto Cambra, and Francisco Fernández-Martínez Análisis y Caracterización Ó ptica de Materiales (ACOM), Departamento de Química Industrial y Polímeros, Escuela Técnica Superior de Ingeniería y Diseño Industrial, Universidad Politécnica de Madrid, Ronda de Valencia 3, 28012, Madrid, Spain S Supporting Information *

ABSTRACT: Two activated carbons have been prepared by chemical activation of deinking paper sludge with K2CO3, with further carbonization in N2 (AC-N2) or CO2 (AC-CO2) and characterized in terms of proximate analyses, adsorption/desorption of N2, skeletal and bulk pycnometry, acid/base and Boehm’s titrations, X-ray diffraction and scanning electron microscopy− energy dispersive spectroscopy (SEM-EDS) analyses. A commercial activated carbon (AC-COM) was subjected to the same characterization program for comparison purposes. Surface area/porosity followed the trend: AC-CO2 < AC-N2 < AC-COM, revealing a good development of porosity in samples prepared in the laboratory but not approaching that of the commercial sample. Surface basicity, in turn, followed the trend: AC-CO2 > AC-N2 ≫ AC-COM, due to the presence of CaCO3, which is a basic salt, in the carbons prepared. The three adsorbents were tested in the adsorption of SO2, where a proper selection of carbon particle size that maximized pore diffusion control led to a higher SO2 uptake. Adsorption equilibrium was successfully fitted to Langmuir and Freundlich isotherms. Final adsorption capacity was governed by textural properties, while adsorption affinity was influenced by both surface basicity and porosity.

1. INTRODUCTION Combustion of any material containing sulfuressentially fossil fuelsreleases sulfur oxides at different amounts depending on reaction conditions. Additionally, these oxides are emitted from mineral treatments or from the fabrication processes that employ sulfuric acid. Effects associated with SOx pollution depend on pollutant concentration, exposure time, receptor sensitivity, and synergies with other pollutants. Different effects on human health can occur, such as bronchitis and cardiac or respiratory problems. It can also lead to necrosis in plants and chemical damage to materials.1 Gas stream sulfur removal processes can be classified according to two different criteria: wet versus dry processes or recoverable versus nonrecoverable depolluting agent. Processes using calcium oxide or carbonate (promoted with sodium or magnesium) lead to calcium sulfates that are usually discarded. Processes with regeneration include Mg(OH)2, sodium citrate, or ammonia, which upon regeneration lead to S or SO2. Wet processes include absorption with amines, and dry processes refer to those including an adsorption or catalytic step.1,2 Adsorption of SO2 on activated carbon appears as a cheaper technique compared to wet processes, since no sludge is generated, that can remove simultaneously other air pollutants.3 Different works have reported the removal of SO2 by activated carbons.4−17 However, activation processes, either physical or chemical, generally lead to carbons with acid surfaces. Since SO2 is an acid gas, basic carbons are preferred. Surface basicity enhancement has been reported in the literature via the incorporation of N heteroatoms in the carbon.18Another way to introduce surface basicity is the incorporation of calcium compounds, such as CaO or CaCO3, in line with the processes © 2014 American Chemical Society

using lime reported above. Different works have reported the incorporation of calcium compounds to a carbon structure via impregnation or mixing processes.19,20 These Ca-activated carbon materials present a good adsorption response, but the main drawback is the multistep preparation procedure, that is, activation of carbon followed by introduction of calcium compounds. In this line, we report the preparation of Cacontaining activated carbons from the activations of a Cacontaining residue (i.e., deinking paper sludge (DPS)), which contains high amounts of CaCO3. DPS is largely produced in paper recycling industry, and it is composed of cellulose fibers, removed inks, clay filters, and other chemical additives and has been mainly used for agricultural or forestry applications.21 A few works have reported the use of DPS derived activated carbons in liquid-phase adsorption processes,22,23 but to the best of authors knowledge, no adsorption application has been performed in the gas phase. Herein, we report the adsorption of SO2 on Ca-containing activated carbons prepared from the activation of DPS with different activation strategies.

2. EXPERIMENTAL SECTION 2.1. Activated Carbon Preparation and Characterization. Deinking paper sludge was supplied by the company Holmen Paper Ibérica SL, presenting a fibrous gray aspect. In order to guarantee a homogeneous activation process, it was ball-milled to obtain a fine powder. Chemical activation employed K2CO3 (Scharlau) as activating agent. Typically, Received: Revised: Accepted: Published: 15620

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(60 cm3 min−1), (f) 15 min isothermal hold. Real (skeletal) densities of carbons were measured with in a Quantachrome SPY-3 stereopycnometer, by measuring the volume of He evacuated by a known mass of carbon, and the apparent (particle) density was accordingly calculated from real density and pore volume according to

DPS powder (ca. 50 g) was mixed with K2CO3 (1:1 w/w), diluted in deionized water (5 cm3 per gram of K2CO3), continuously stirred at 85 °C for 4 h, and dried overnight at 110 °C in static air. The resulting material was subjected to a thermal treatment in two different atmospheres: N2 and CO2. Thermal treatments were conducted in a ceramic tube (5 cm inner diameter (id)) located in a horizontal furnace (ENERGON HLT 18-50-14) electrically heated. Approximately 25 g of the chemically activated DPS powder filled a ceramic capsule located in the tube, was heated from room temperature to 800 °C at 5 °C min−1 in a flow of N2 or CO2 (100 cm3 min−1, mass flow controlled) with no further maintenance at that temperature, and allowed to return to room temperature under the same flow, in order to preserve the carbon from oxidation. The so-prepared materials were washed with HCl (1 M) to remove the activating agent and further washed with deionized water until the wash water approached neutral pH. The carbons were finally dried overnight at 110 °C in static air. At this point, two activated carbons have been prepared: one subjected to chemical activation and heated in N2 (AC-N2) and one subjected to chemical activation and to a further physical activation by heating in CO2 (AC-CO2). Additionally, a commercial activated carbon (PANREAC, product code: 211238) was used for comparison purposes (AC-COM), as well as a commercial CaCO3 (Scharlau, precipitated extra pure), for a physical mixture with AC-COM. Surface area/porosity measurements were conducted using a Quantasorb Sorption System (Quantachrome Instruments) with N2 (at −196 °C) as sorbate. Prior to analysis, the samples were outgassed overnight at 180 °C. Total specific surface areas were determined by the multipoint Brunauer−Emmett−Teller (BET) method at P/P0 ≤ 0.3, total specific pore volumes were evaluated from N2 uptake at P/P0 = 0.99, total specific micropore volumes and characteristic energies were determined using the standard Dubinin−Radushkevich treatment. Average micropore widths were calculated according to the Dubinin− Stoeckli equation, and pore size distributions were calculated from the adsorption branch of the isotherm using the Kelvin equation. The adsorption branch is preferred to the desorption one, in order to avoid tensile strength effects.24 Acid/base titrations were performed by immersing 25 mg of carbon in a 50 cm3 solution of NaCl (0.1 M), basified with 0.5 cm3 of NaOH (0.1 M) with constant stirring under inert atmosphere. To facilitate carbon dispersion, the solution was sonicated for 15 min. A 0.1 M HCl solution was used as titrant, added dropwise, and the pH was monitored using a Crison GLP21 pH-meter. The starting NaCl (plus NaOH) solution served as a blank. Boehm titrations were conducted by immersing four 0.25 g fractions of carbon in 50 cm3 0.05 M solutions of NaOH, Na2CO3, NaHCO3, and HCl, respectively, under continuous stirring for 24 h. Next, the solutions were filtered, and 5 cm3 of the liquids filtered were titrated either with HCl (0.1 M) or NaOH (0.1 M) to evaluate the base or acid excess, respectively.25 Proximate analysis was conducted in order to determine moisture, volatile matter, fixed carbon, and ash in the raw DPS and the carbons studied, under ASTM specified conditions. These experiments were carried out in the thermogravimetric SDT Q600 unit (TA Instruments), according to the following steps (TA Instrument Technical Note-129): (a) 10 mg of sample were ramped from room temperature to 200 °C at 25 °C min−1 under a flow of N2 (60 cm3 min−1), (b) 5 min isothermal hold, (c) ramp to 900 °C at 50 °C min−1, (d) 10 min isothermal hold, (e) switch gas to O2

ρp =

1 1/ρr + Vp

(1) −3

with ρr and Vp respectively being the real density (g cm ) and total pore volume (cm3 g−1) of the carbons. Bulk density was calculated by measuring the volume that a known mass of carbon occupies in a graduated cylinder. X-ray diffraction patterns were obtained in a Siemens Krystalloflex D5000 unit using a graphite monochromator with Cu Kα (1,2). The samples were scanned over the range 12° ≤ 2θ ≤ 90° every 0.02°, 1 s per step. SEM-EDS analyses were performed in a JEOL JSM-7400F scanning electron microscope with field emission gun and a PGT energy dispersive X-ray spectroscopy. 2.2. Adsorption of SO2. Adsorption experiments were conducted in the thermogravimetric analyzer described above, at 45 °C. In each experiment, a flow of SO2/N2 contacted the sample. Carbon samples were sieved and collected in the following batches: < 170, 170−300, 300−500, and 500−800 μm. The total flow was kept constant at 120 cm3 min−1, while the SO2 concentration spanned the range 500−11500 ppm, contacting ca. 10 mg of adsorbent. The weight gain was continuously monitored by the analyzer. These experiments presented an excellent reproducibility; that is, three repetitions of an uptake experiment under identical conditions resulted in the standard deviation of 0.79 mg g−1. Adsorption data (q) are given as the SO2 uptake in milligrams per gram of activated carbon and were calculated as the difference between the final and initial mass (mf − m0) recorded by the analyzer, divided by the initial mass, and multiplied by 1000. The error associated with these measurements (Δq) can be calculated according to the following standard logarithmic approach: (i) take logarithms at both sides of the equation, (ii) differentiate the equation, (iii) replace the differential by incremental (error), (iv) switch(−) to (+) sign. The error in mass measurements (Δm) is taken as 1 μg. The following error equation is obtained: ⎛ 2Δm Δm ⎞ Δq = q⎜ + ⎟ m0 ⎠ ⎝ m f − m0


In order to evaluate the adsorption strength, desorption experiments were done. In these experiments, previous adsorption (500, 2500, and 10000 ppm of SO2) was conducted, followed by an isothermal hold in N2 (120 cm3 min−1) at 45 °C.

3. RESULTS AND DISCUSSION 3.1. Characterization of Activated Carbons. Proximate analyses of activated carbons prepared, together with raw material and commercial sample are given in Table 1. Raw DPS was characterized by low contents of moisture and fixed carbon and high values of volatiles and ash. Comparable ash contents have been reported in the literature, mainly attributed to the presence of CaCO3.22 Upon activation, activated carbons noticeably increased the fixed carbon amount, due to the carbonization of the organic matter, mainly cellulose. The ash content was, as expected, comparable in the raw matter and 15621

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which results in the mixed oxides under the thermal treatment in CO2. This phenomenon was not observed when the thermal treatment was conducted in N2. In this case, the ions coming from the activating agent do not further react and are eliminated upon washing. This is in line with the ash content reported before; that is, the higher amount measured in ACCO2 respect to the raw matter must be linked to the formation of new carbonates upon treatment in CO2. Diffraction peaks appearing in DPS at 2θ ≈ 12.3 and 24.8° correspond to kaolinite, Al2Si2O5(OH)4 (ICDD-JCPDS: 00-006-0221), and were not present upon activation. The commercial sample, as expected, did not show any diffraction peak corresponding to Ca species. CaCO3 average crystal sizes, derived from Scherrer’s equation are given in Table 1. The activation process led to a decrease in average CaCO3 crystal size, compared with the raw DPS, obtaining the lowest for AC-CO2. Further information on surface composition and morphology of AC-N2 and AC-CO2 was obtained by SEM-EDS analyses. Figure 2 (1a and 1b) shows low resolution micrographs corresponding to AC-N2 and AC-CO2, respectively. The overall morphology of these samples consisted on irregular and conglomerated particles, where two different surface textures can be observed. These two textures were better appreciated at higher resolutions (Figure 2, 2a and 2b). Regions marked with I presented an irregular pseudogranular aspect, which can be attributed to CaCO3 particles. However, regions marked with II were smooth surfaces, which must correspond to the activated carbon obtained from the carbonization of cellulose fibers present in the raw matter.22 This hypothesis was confirmed by EDS analyses performed in each region, as shown in Figure 2 (3a and 3b). Plots corresponding to regions marked with I presented three main peaks, corresponding to carbon, oxygen, and calcium. The presence of these three species and the intensity proportion between them confirm the presence of CaCO3. However, for the regions marked with II, no signal corresponding to Ca was detected, and the signal corresponding to C was higher than the corresponding to O, confirming the carbonaceous nature of these smooth structures. The presence of Mg, Si, and Al come from the kaolinite and other inorganic species present in the original DPS. Note that the

Table 1. Physicochemical Properties of Activated Carbons and Raw Material







moisture (% w/w) volatiles (% w/w) fixed carbon (% w/w) ash content (% w/w) basic groups (meq g−1) CaCO3 crystal size (nm) skeletal density (g cm3) apparent density (g cm3) bulk density (g cm3)

10.8 18.7 38.5 32.0 3.3 23.7 0.34 0.30 0.08

7.5 5.5 40.6 46.4 5.0 18.3 0.53 0.47 0.19

6.9 4.7 86.9 1.5 0.8 0.59 0.45 0.25

2.5 57.3 9.3 30.9 n.m. 53.0 0.96 0.18a

As received.

AC-N2 but increased in the case of AC-CO2. Such increase can be attributed to the presence of a new phase; the mixed carbonate K2Ca(CO3)2, as will be explained next on the basis of XRD results. These results, as expected, are quite different from those corresponding to the commercial sample, with very high fixed carbon percentage and negligible ash content. The nature of the ashes present in the activated carbons prepared from DPS was evaluated by XRD. Figure 1 shows the XRD patterns corresponding to these carbons, together with the raw DPS. DPS and DPS-derived samples presented diffraction peaks at 2θ ≈ 23.0, 29.4, 35.9, 39.4, 43.1, 47.5, and 48.5°, characteristics of (012), (104), (110), (113), (202), (018), and (116) of rhombohedral calcite, CaCO3 (ICDD-JCPDS: 00-055-0586), with no contribution from calcium oxide, CaO (ICDD-JCPDS: 00-004-0777). A stronger, well-defined main diffraction peak (29.4°) was observed for AC-N2, contrary to a smaller one in the case of AC-CO2, accompanied by secondary peaks not present in AC-N2. Peaks at 28.5 and 34.0° correspond to fairchildite, K2CO3·CaCO3 (ICDD-JCPDS: 00-021-1287), hexagonal, and peaks at 29.5 and 31.2° correspond to buetschillite, 3K2CO3·2CaCO3 (ICDD-JCPDS: 00-025-0626), rhombohedral. Both are mixed carbonates, synthetic, with general formula K2Ca(CO3)2, and only appear in the carbon thermally treated under a CO2 atmosphere. This observation must be attributed to the use of K2CO3 as activating agent,

Figure 1. XRD patterns associated with activated carbons and DPS: (*) calcite, (●) kaolinite, (◆) fairchildite, and (■) buetschillite. 15622

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Figure 2. (1) Low and (2) high magnification SEM micrographs corresponding to (a) AC-N2 and (b) AC-CO2. (3) EDS analyses corresponding to areas I and II.

Figure 3. Acid−base titration curves associated with AC-N2 (▲), AC-CO2 (●), and AC-COM (■). Dashed line corresponds to blank solution. Inset: derivative curves.

signal corresponding to Al can also correspond to the sample holder. Potassium was also detected in elemental analyses, indicating that it was not completely eliminated upon the washing. It should be reminded that in the AC-CO2, potassium is part of the mixed carbonates K2Ca(CO3)2. Figure 3 shows the acid/base titration curves corresponding to the three carbons under study, where HCl was employed as titrant. Corresponding derivative curves of AC-N2 and AC-CO2 are given in the inset. The three curves corresponding to activated carbons were shifted right from the blank, indicating a higher HCl consumption than the bare NaCl solution, and therefore a basic character. However, a quite different behavior

between them was noticed. The curve corresponding to ACCOM presented a sigmoidal shape, similar to the corresponding to the blank, with a little associated consumption. This indicates the presence of some surface functionality that is providing a not very strong basic character to this activated carbon. However, in the case of those carbons prepared from DPS, the curves presented a totally different behavior. In both cases, two pH decays were obtained, being the HCl volume needed for the second decay approximately twice the needed for the first. This is better observed in the derivative curves, which is in good agreement with a two-step neutralization reaction: CaCO3, which is a basic salt, initially reacts with 15623

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Figure 4. (a) N2 adsorption/desorption isotherms and (b) pore size distributions associated with (I) AC-N2, (II) AC-CO2, and (III) AC-COM.

Table 2. Textural Properties of Activated Carbon Samples sample

ABET (m2 g−1)

Vp (cm3 g−1)

Vmeso (cm3 g−1)

VDR (cm3 g−1)

EDR (kJ mol−1)

L0 (nm)

micropore vol. percent (%)


395 176 752

0.41 0.22 0.53

0.22 0.13 0.18

0.19 0.08 0.35

15 15 16

0.86 0.86 0.80

46 38 66

loop, diagnostic of mesoporosity. N2 uptake at P/P0 ≤ 0.3 was fitted to BET equation, obtaining the specific surface areas. ACCOM presented the highest surface area, characteristic of a commercial activated carbon; however, AC-N2 and AC-CO2, while presenting an appreciable surface area, could not approach that of the commercial sample. Within the two carbons prepared from DPS, AC-N2 outperformed AC-CO2. Pore size distribution corresponding to AC-N2 presented two maxima at radii of ca. 1.0 and 1.5 nm, in the limit of micropores, and a secondary maximum at ca. 4.6 nm, associated with mesoporosity. AC-CO2 presented a main maxima at ca. 0.9 and a secondary at ca. 2.0 nm, and a similar shape was observed for AC-COM. Total and micropore volumes increased in the sequence AC-CO2 < AC-N2 < AC-COM and average micropore widths corresponding to AC-N2 and AC-CO2 were higher than the corresponding to the commercial sample (with subsequent lower characteristic energies), consistent with the lower surface areas. Taking into account that both samples were chemically activated with K2CO3, the thermal treatment in N2 developed a higher porosity than in CO2. During the chemical activation, the activating agent acts as a dehydrating compound, hydrolyzing and swelling the raw DPS and preventing from contraction during the thermal treatment, which after washing generates a high porosity. This was the case of thermal treatment in N2, as is usually done upon chemical activation. The aim of using CO2 (i.e., further physical activation) during the thermal treatment was taking advantage of its oxidizing capacity in order to selectively burn carbon atoms and further develop the porosity. However, no better results were obtained, probably attributed to the generation of CO and H2 that form very stable surface complexes that work counter activation.

protons from HCl to form the corresponding bicarbonate, which further reacts with protons to form H2CO3 in a second stage. The shift of these decays to higher HCl consumption values and the appearance of a third decay in the case of ACCO2 are attributed to the presence of the mixed carbonates explained above. Boehm’s titrations let us quantify the number and type of acidic and basic groups, under the assumption that NaOH neutralizes carboxylic, lactonic, and phenolic groups, Na2CO3 neutralizes carboxylic and lactonic groups and NaHCO3 neutralizes carboxylic groups. HCl, in turn, neutralizes basic groups.25 No contribution from acidic groups was measured in AC-N2 and AC-CO2; therefore, only the basic branch of Boehm’s analyses is given in Table 1. As commented above, the main contribution to basic groups in DPS-derived carbons come from Ca (Ca+K) carbonates. Therefore, it has been considered for Boehm’s calculations that 2 mol of HCl are needed to neutralize 1 mol of basic groups. AC-N2 and ACCO2 presented an order of magnitude higher loading of basic groups than AC-COM. Within these two carbons, the higher basicity in AC-CO2 must be linked to the higher ash content reported above, associated with the formation of the mixed carbonate. Skeletal density of carbons prepared was lower than the corresponding to the raw DPS and comparable to the corresponding to the commercial sample. N2 adsorption/ desorption isotherms and derived textural properties are given in Figure 4 and Table 2. The three carbons presented comparable isotherms, with a Type I behavior at low partial pressures, corresponding to the presence of microporosity, and a Type IV behavior at higher partial pressures, with hysteresis 15624

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uniform adsorbent particles. It is assumed that adsorbent particles are subjected to a step-change in the fluid phase concentration. For external mass transfer control, the following equations are applied:

Thus, far, we have presented the preparation and characterization of two activated carbon samples (together with a commercial one), where the thermal treatment upon chemical activation strongly influenced both textural and chemical properties. 3.2. Adsorption of SO2. In order to arrive at a suitable particle size that optimizes the adsorption of SO2, different experiments at varying particle sizes were conducted, from a SO2 inlet of 2500 ppm. Activated carbons were sieved and four fractions were obtained, collected between 0 and 170 μm, 170 and 300 μm, 300 and 500 μm, and 500 and 800 μm, namely, by the average size, 85, 235, 400, and 650 μm. Figure 5 shows the

⎛ n 0′ ⎞ 3k t ci0 (1 − R ) ⎜⎜1 − i 0 ⎟⎟F − R ln(1 − F ) = f rp ρp ni0 ni ⎠ ⎝


1 1 + K ici0


(5) −1

where kf is the mass transfer coefficient (cm min ), t is time (min), rp is the particle radius (cm), ci0 is the concentration of the fluid phase (mol cm−3), ni0 is the adsorption capacity in equilibrium (mol g−1), ρp is the apparent density of solid (g cm−3) and Ki is the Langmuir affinity constant, which provides a measurement of the strength with which the adsorbate is attached to the surface (cm3 mol−1). R is a dimensionless constant called separation factor. Regarding this factor, the adsorption is considered irreversible for R = 0, favorable for 0 < R < 1, linear for R = 1 and unfavorable when R > 1. For pore diffusion control, the following equations are applied: 6 π2


∑ n=1

⎛ n2π 2D t ⎞ 1 e ⎟ ⎜ exp , n2 ⎜⎝ rp2 ⎟⎠

F > 0.8,

n=1 (6)

Figure 5. Adsorption capacity (solid) and Bi numbers (open) associated with AC-N2 (squares) and AC-CO2 (circles).


equilibrium SO2 uptake values for the carbons prepared in the laboratory. In both cases, a maximum adsorption capacity value was obtained for an average particle size of 400 μm. Under identical conditions of total inlet flow and adsorbate concentration, such a trend must be associated with mass transfer limitations, which may control, to a different extent, the adsorption of SO2. The adsorption step itself is unlikely to affect the overall rate; however, there are two main preceding steps that may hinder the movement of a molecule of adsorbate from the bulk fluid outside a particle/pellet to an adsorption site on its internal surface. On the one hand, the adsorbate must move from the turbulent bulk fluid through a laminar (stagnant) boundary layer around the adsorbent particles to approach the external surface; this is known as external mass transfer or boundary film resistance. On the other hand, once the adsorbate has approached the external surface of the adsorbent, it must diffuse through the pores or lattice vacancies to the adsorption sites, known as internal mass transfer or pore diffusion.26 Both external and internal mass transfer constants were calculated by fitting the experimental kinetic data to the corresponding models (further kinetic study in Supporting Information, in terms of fractional approach to equilibrium (F), as described by Perry,27 which can be calculated as follows: F = (ni − ni0 ′)/(ni∞ − ni0 ′)

6 ⎛ Det ⎞ −3Det ⎜ ⎟ , rp ⎝ π ⎠ rp2 0.5

F < 0.8 (7)

where De is the effective diffusion coefficient in the pores of the solid (cm2 min−1). External mass transfer coefficients and effective diffusivities are given in Table 3. In order to analyze these parameters, Biot Table 3. Mass Transfer Parameters Associated with Activated Carbons for the Adsorption of SO2 at 45° C, 2500 ppm sample

avg. particle size (μm)

kf (cm min−1)

De × 105 (cm2 min−1)


85 235 400 650 85 235 400 650

0.45 1.55 2.43 2.13 0.33 0.95 2.02 2.62

1.1 3.5 8.5 20 0.5 1.2 4.0 7.5

173 520 607 346 280 930 1154 1064



(Bi) numbers, obtained by dimensional analysis, were calculated. Bi is a dimensionless group that compares the relative transport resistances, external and internal, and was calculated according to


Bi =

where ni, n0i ′, and n∞ represent the amount of adsorbate i adsorbed (mol g−1), respectively, at any time, at the beginning (zero in this study), and at the end of the process. A batch adsorption model has been employed, assuming a constant concentration of SO2 in the fluid phase with spherical and

k f rp De


Bi numbers are given in Figure 5 together with equilibrium uptake values as a function of particle size. High Bi numbers are diagnostic of an internal mass transfer control (high external mass transfer coefficient and low effective diffusivity), while low 15625

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Bi numbers (low external mass transfer coefficient and high effective diffusivity) are representative of external mass transfer control. Bi calculated were always higher than 170, indicating preferential control of pore diffusion, and increased up to average particle size of 400 μm, the same size that optimized SO2 uptake. It is well documented that adsorption is generally controlled by the transport within the pore network;28 in this line, the results obtained indicate a favored adsorption under those conditions maximizing the internal diffusion control (increasing Bi), which starts to decay when boundary film control gains importance (decreasing Bi). According to these results, 400 μm average was selected as a suitable size for further adsorption analyses. The adsorption equilibrium values, presented as the adsorption capacities of the solid as a function of the fluid phase concentration (i.e., adsorption isotherms) are presented in Figure 6. AC-CO2 presented an isotherm with a sharp

outperformed the others. This carbon presented the lowest surface area and pore volume but the highest HCl consumption in acid/base titrations and basic group determined by Boehm. This result indicates that at low partial pressures, SO2, as an acidic gas, is further adsorbed in basic surfaces. Comparable results have been reported elsewhere.30 Given the nonlinear behavior of the adsorption isotherms obtained in this work, it is necessary to fit the experimental data to mathematical models. Langmuir and Freundlich equations were employed. Langmuir isotherm considers a homogeneous surface with equivalent adsorption sites and only one molecule adsorbed per adsorption site. Interactions between adsorbed molecules are neglected. Therefore, Langmuir considers a saturated single adsorption layer, working in the low partial pressure range. In this line, experimental data were fitted to the low adsorption interval (up to 5700 ppm of adsorbate). In this equation, Cμs stands for the maximum concentration adsorbed per unit mass of adsorbent on a single layer and Ki is the Langmuir affinity constant, which is proportional to the heat of adsorption, so higher Ki values correspond to a more energetic uptake. The Freundlich model is an empirical equation based on the adsorption on heterogeneous surfaces with different adsorption sites, where sites with higher affinity for the adsorbate are occupied first, and the adsorption strength decreases as the surface coverage increases. The Freundlich constant KF measures the adsorption capacity of the solid. The parameter n takes values higher than unity and provides information about the affinity between adsorbate and adsorbent; higher n values correspond to a type I shaped isotherm, and subsequently, to a stronger interaction.31 Experimental data were fitted to Freundlich model over the entire range of concentrations studied. Table 4 shows the parameters associated with each model, together with the fitting goodness, in terms of R2, χ2, and p-value. The standard deviation associated with each parameter is also given. The two models studied provided a good fitting to experimental data, with R2 values close to unity, relatively low χ2 values and p-values close to zero. Derived parameters can be interpreted in the same manner. Affinity constant Ki from Langmuir model and the corresponding n derived from Freundlich followed the sequence AC-COM ≈ AC-N2 ≪ AC-CO2. The most basic AC-CO2 presented, by far, the highest affinity for the adsorbate. However, the similar affinity of AC-N2 and AC-COM suggests a combined effect, where the lower basicity of AC-COM is compensated by its superior surface area/porosity. In addition, the different origin of carbon from deinking paper sludge and the commercial sample introduces further differences, that is, elemental composition, ash content, or fixed carbon, that can also impact in adsorptive properties. The maximum adsorption capacity in

Figure 6. Adsorption isotherms associated with AC-N2 (○), AC-CO2 (□), and AC-COM (▲). (▼) corresponds to the physical mixture. Error bars given.

adsorption increase at low SO2 concentrations and that started to stabilize at ca. 3000 ppm of SO2 (0.003 atm), typical of a Type I isotherm, according to IUPAC, and indicative of a high affinity between the adsorbate and the adsorbent.29 This behavior was not as well-defined for AC-N2, and in the case of AC-COM, a more linear isotherm, closer to a Type II, indicated a poorer interaction. At saturation (>8000 ppm of SO2), the trend of increasing SO2 uptake clearly matched that of increasing surface area/porosity, that is, more sites available for adsorption. However, at low concentrations, AC-CO2

Table 4. Fitting Parameters to the Langmuir and Freundlich Isotherms for the Adsorption of SO2 at 45 °C Langmuir nonlinear

Cμ = Cμs


sample AC-N2 AC-CO2 AC-COM


K iP 1 + K iP

Cμ = KF(P)1/ n

P 1 1 = + P Cμ K iCμs Cμs Ki (atm−1) 455 ± 70 1951 ± 161 449 ± 59

Cμs (mg 53.6 ± 42.5 ± 68.1 ±

g−1) 0.1 0.0 0.1

R2 0.95 0.99 0.96

χ2 (mg g−1) 1.80 0.17 1.23

ln(Cμ) = ln(KF) + p-value 9.09 × 10−4 2.48 × 10−7 5.22 × 10−4 15626

n 2.17 ± 0.21 5.19 ± 0.51 1.98 ± 0.05

KF (mg g−1 atm−1/n) 403.06 ± 0.04 104.90 ± 0.02 705.27 ± 0.01

⎛1⎞ ⎜ ⎟ln(P ) ⎝n⎠

R2 0.92 0.92 0.99

χ2 (mg g−1) 4.58 2.67 0.71

p-value 3.01 × 10−6 7.67 × 10−6 1.08 × 10−10

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Industrial & Engineering Chemistry Research


the single layer, Cμs, derived from Langmuir and the corresponding KF from Freundlich followed the trend ACCO2 < AC-N2 < AC-COM, matching the experimental sequence of SO2 uptake at saturation and in line with an increasing surface area/porosity. Values obtained in this work from Langmuir and Freundlich models compare well with those reported in literature for comparable adsorbate−adsorbent systems30,32,33 and confirm how both the surface chemistry and textural properties of the adsorbent play a major role on adsorption strength/affinity, while the final adsorption capacity is rather governed by the textural properties of the solid. In order to assess the impact of the CaCO3 dispersed particles on the carbonaceous structure, an additional experiment with a physical mixture of commercial activated carbon (AC-COM) and a commercial CaCO3 powder was conducted. The AC/ CaCO3 proportion was kept at 60:40 w/w, in order to approximate the 32−46% ash content in the DPS derived carbons. Adsorption capacity for this mixture at 2500 ppm is given in Figure 6 by the down pointing triangle. It can be observed how the equilibrium adsorption of SO2 at such concentration expressed per gram of total adsorbent (AC +CaCO3) falls below the corresponding to AC-N2 or AC-CO2. This result shows the superior adsorptive behavior of CaCO3 particles when dispersed on the carbon structure. Additionally, with the aim of gaining further understanding of the adsorption strength between adsorbate and adsorbent, desorption experiments were conducted. The percentage of the SO2 desorbed during an isothermal hold under N2 atmosphere at 45 °C after adsorption at three different SO2 concentrations are given in Figure 7. It can be observed how the lower

adsorbent−adsorbate complex is lower than the needed to go back to the fluid phase, and therefore, desorption does not occur completely. These results confirm that the adsorbate− adsorbent interaction was dependent on both surface area and surface basicity.

4. CONCLUSIONS In this work, activated carbon was successfully prepared from deinking paper sludge via different activating procedures and employed as adsorbent of SO2 as a model environmental pollutant. Results presented support the following conclusions: (i) Deinking paper sludge is a suitable precursor for the preparation of activated carbons. Chemical activation with K2CO3 followed by carbonization in N2 or CO2 leads to a carbonaceous material with well-developed porosity, but still low when compared with a commercial sample. The presence of CaCO3 in the carbons prepared was responsible for a high basicity in these materials, making them suitable candidates for the adsorption of an acidic gas as sulfur dioxide. (ii) Adsorbent particle size played an important role in the final adsorption capacity, obtaining an optimum response for 400 μm average. At this sample size, the Bi number presented maximum values, indicating a favored adsorption under those conditions where pore diffusion was the main controlling mechanism. (iii) SO2 adsorption equilibria were successfully reproduced by Langmuir and Freundlich models. Final adsorption capacity was correlated to carbon surface area/porosity, while higher surface area and basicity led to a stronger interaction between adsorbate and adsorbent.


S Supporting Information *

Figure S1: representative example of fitting to the three models proposed (2500 ppm of SO2). Tables S1−S3: summary of data derived from fitting the experimental data to the models in eqs S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org/.


Corresponding Author

*Email: [email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Figure 7. Percentage of SO2 desorbed at 45 °C in N2 atmosphere (120 cm3 min−1).


The authors declare no competing financial interest.

desorption was obtained for AC-CO2, in line with its higher basicity and consistent with the highest Ki and n values derived from Langmuir and Freundlich (i.e., stronger interaction). A totally different behavior was observed for AC-N2, which desorbed practically ca. 100% of the SO2 adsorbed. AC-COM, with lower basicity but much higher surface area, desorbed ca. 70−90%. Physical adsorption occurs through van der Waals forces, which are typically weak and reversible. Therefore, when the concentration of adsorbate in the fluid phase decreases, the equilibrium displaces backward and desorption occurs. In the case of very narrow pores, or a stronger interaction between adsorbate and adsorbent (i.e., chemisorption), the energy of the

ACKNOWLEDGMENTS Holmen Paper Ibérica SL is gratefully acknowledged for providing the deinking paper sludge used in this work. Dr. A. J. dos Santos is greatly acknowledged for helpful discussion with XRD analyses.

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NOMENCLATURE ρb = bulk (package) density (g cm−3) ρp = particle (apparent) density (g cm−3) ρr = real (skeletal) density (g cm−3) m0 = initial mass measured (mg) dx.doi.org/10.1021/ie502955p | Ind. Eng. Chem. Res. 2014, 53, 15620−15628

Industrial & Engineering Chemistry Research


mf = final mass measured (mg) Δm = error in mass measurement (mg) ABET = specific surface area, as calculated by BET (m2 g−1) Vp = total pore volume (cm3 g−1) Vmeso = mesopore volume (cm3 g−1) VDR = micropore volume, as calculated by Dubinin− Radushkevich (cm3 g−1) EDR = characteristic energy (kJ mol−1) L0 = micropore width (nm) F = fractional approach to equilibrium ni = amount of adsorbate adsorbed at any time (mol g−1) n0i ′ = amount of adsorbate adsorbed prior to experiment (mol g−1) n∞ i = amount of adsorbate adsorbed at the end of experiment (mol g−1) kf = external mass transfer coefficient (cm min−1) rp = particle radius (cm) ci0 = concentration of the fluid phase (mol cm−3) ni0 = adsorption capacity in equilibrium (mol g−1) R = separation factor De = effective diffusion coefficient (cm2 min−1) Bi = Biot number Cμ = maximum concentration adsorbed per unit mass of adsorbent (mg g−1) Cμs = maximum concentration adsorbed per unit mass of adsorbent on a single layer (mg g−1) Ki = Langmuir affinity constant (atm−1) KF = Freundlich adsorption capacity (mgg−1atm−1/n) n = Freundlich affinity constant qe = mass adsorbed in equilibrium per gram of adsorbent (mg g−1)

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