Enhanced mercury adsorption in activated carbons from biomass materials and waste tires

Fuel Processing Technology 88 (2007) 749–758www.elsevier.com/locate/fuproc

Review

Enhanced mercury adsorption in activated carbons from biomassmaterials and waste tires

G. Skodras a,b,c,⁎, Ir. Diamantopoulou a, A. Zabaniotou d,G. Stavropoulos a, G.P. Sakellaropoulos a,b

a Chemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greeceb Laboratory of Energy and Environmental Processes, Chemical Process Engineering Research Institute, Thessaloniki, Greece

c Institute for Solid Fuels Technology and Applications, Ptolemais, Greeced Chemical Process and Plant Design Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece

Received 18 January 2007; received in revised form 8 February 2007; accepted 29 March 2007

Abstract

Agricultural residues and waste tires constitute an important source of precursors for activated carbon production. Activated carbons offer apotential tool for mercury emissions control. In this work, pine and oak wood, olive seed and tire wastes have been used for the preparation ofactivated carbons, in order to be examined for their mercury removal capacity. In the case of activated carbons produced from pine/oak woods andtire wastes, a two stage physical activation procedure was applied. Activated carbons derived from olive seeds were prepared by chemicalactivation using KOH. Pore structure of the samples was characterized by N2 and CO2 adsorption, while TPD-IR experiments were performed inorder to determine surface oxygen groups. Hg° adsorption experiments were realized in a bench-scale adsorption unit consisting of a fixed-bedreactor. The influence of activation technique and conditions on the resulted activated carbon properties was examined. The effects of porestructure and surface chemistry of activated carbons were also investigated. Activated carbons produced from olive seeds with chemical activationpossessed the highest BET surface area with well-developed micropore structure, and the highest Hg° adsorptive capacity. Oxygen surfacefunctional groups (mainly lactones) seem to be involved in Hg° adsorption mechanism.© 2007 Elsevier B.V. All rights reserved.

Keywords: Ativated carbon; Biomass materials; Waste tires; Surface chemistry; Hg adsorption

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7502. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 750

2.1. Sample preparation and characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7502.2. Hg° bench-scale adsorption tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751

3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7513.1. Pore structure characterization of the samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7513.2. Surface chemistry characterization of the samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7523.3. Mercury adsorption results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 757References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758

⁎ Corresponding author. P.O. Box 1520, Thessaloniki 54006, Greece. Tel.: +30 2310 996260; fax: +30 2310 996168.E-mail address: [email protected] (I. Diamantopoulou).

0378-3820/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2007.03.008

mailto:[email protected]

http://dx.doi.org/10.1016/j.fuproc.2007.03.008

750 G. Skodras et al. / Fuel Processing Technology 88 (2007) 749–758

1. Introduction

Activated carbons become more and more interesting onaccount of their excellent properties as adsorbents, renderingpossible their use in the removal of heavy metals, such asmercury, from gaseous media. The problem associated with theuse of activated carbons for mercury flue gas control is the highcost, leading to the investigation of alternative low costsorbents. Agricultural residues, wood and waste tires constitutean important source of precursors for the production of activatedcarbons [1,2].

The characteristics of activated carbon depend on the physicaland chemical properties of the precursor as well as on theactivation method. There are two methods of preparing activatedcarbons: physical and chemical activation. John et al. haveemployed the physical activation procedure for the production ofactivated carbons by using steam and carbon dioxide. Theynoticed that microporosities were similar between steam andcarbon dioxide activations, but mesoporosity was favored bysteam activation [3]. In addition to the starting material and theoxidizing agent, activation time and temperature affect thestructural properties of the resulting activated carbon. Manyresearchers observed that BET surface area and pore volumeincreased with activation time and temperature [4–6]. Forinstance, Vitidsant et al. observed that the optimum conditionsfor pyrolysis and steam activation were 750 °C for 2 h [7]. Theadvantage of chemical activation over physical activation is that itis performed in one step and at relatively low temperatures. Themost important and commonly used activating agents arephosphoric acid, zinc chloride and alkaline metal compounds,such as KOH [8,9]. It is noted that activated carbons with well-developed porosity and highlymicroporous structure with narrowpore size distribution could be prepared from KOH–impregnatedprecursors [10,11]. It can be seen that surface area and microporevolume reach a maximum for the KOH/precursor ratio of 4:1,using a high temperature (800 °C) and a relatively long activationtime [12,13].

A large surface area, micropore and total pore volumesenhance activated carbons mercury adsorptive capacity [14,15].Usually micropores posses the majority of the active sites formercury adsorption while mesopores act as transportation routes[16,17]. Additional to pore structure characteristics, activationprocedure affects the nature of carbon complexes formed oncarbon surface. Generally, at modest activation temperaturesalmost all kinds of oxygen functionalities are detected in carbonsurface. As the activation temperature increases up to 900 °C,lower concentrations of surface organic components are observedand only quinones remain [18]. Similar results are obtained duringthe KOH activation and carbonyl, carboxylic acid, lactones, ether,quinone, and hydroxyl groups are formed. In the activation ofKOH-impregnated samples for 2 h at 700 °C, the large amount ofunstable oxygen groups decomposes and only hydroxyl andcarbonyl groups remain [19]. TPD can assess all functionalgroups qualitatively, and combined with infrared analysis (IR),quantitative information for the functional groups that decomposebelow 1250 °C can be obtained. Carboxylic acids and lactonegroups evolve CO2 upon heating, while carboxylic anhydride

produces both CO and CO2. CO is derived from phenols, ethersand carbonyls/quinone groups [20,21]. Among the functionalgroups detected, both lactone and carbonyl groups are active sitesthat favor mercury adsorption [22].

In this work, the efficiency of seven activated carbonsproduced from pine and oak woods, waste tires and olive seeds,by physical and chemical activation, in removing Hg° from thegas phase was evaluated. The correlation between preparationconditions and resulting physical and chemical properties hasbeen determined. The effect of the pore structure and surfacechemistry on Hg° adsorption was also investigated. Aninnovative aspect of the present study is that pyrolysis andactivation for pine/oak wood and tires were performed at pilotscale and therefore the results give more realistic approach ofprocess feasibility. Since tires are made mostly from rubber andhave negative environmental impacts, their re-use for theproduction of activated carbons can be very rewarding. Some ofthe obtained activated carbons, such as those derived from pinewood and tires activated for 2 h at 900 °C, demonstrated similarBET surface areas and Hg° adsorptive capacity to commercialproducts, such as F400 (Calgon Corporation) and RWE(Rheinbraun Company). Thus, the prepared activated carbonscan be considered as a potential solution in Hg° abatementtechnologies [23]. Additionally to the production of low costsorbents from by-products or waste materials, the target of thisresearch is the correlation of their physical and chemicalproperties with the mechanisms that govern Hg° capture.

2. Experimental

2.1. Sample preparation and characterization

Mercury adsorption experiments were performed using activated carbonsproduced from pine and oak wood (samples A and B respectively) and tirewastes (samples C1, C2, C3) by physical activation. Chemical activationprocedure was employed in the case of activated carbons derived from oliveseeds (samples D1, D2, D3). Pyrolysis of used pine/oak woods and tires hasbeen performed at the Compact Power pyrolysis plant (Avonmouth, UK), at800 °C in an inert atmosphere for 45 min, on a scale of 1 t/h. About 5 kg of thechar has been activated in the pilot station of the NESA at Louvain-la-Neuve at970 °C in the presence of steam between 0.5 and 2 h. Pyrolysis and activationprocedure is described in detail elsewhere, and the results of preparationconditions are summarized in Table 1 [24,25]. About 30 g of dry olive kernelshave been pyrolyzed in a bench-scale fixed-bed system consisted of a verticalstainless steel reactor, for 1 h at 800 °C, under N2 flow adjusted at 100 cm3/min.Chemical activation process was initiated by introducing ∼8 g of KOH and∼2 g of char sample (KOH–precursor ratio: 4:1) in the same reactor. Mixing hasbeen lasted for 10 min, and activation has been realized at 800 °C, less than100 cm3/min N2 flow for 1 and 3 h [26,27]. Physical adsorption methods (N2

adsorption at 77 K and CO2 adsorption at 298 K) were used to characterize thepore structure of the examined activated carbons. A conventional volumetricapparatus (Quantasorb Co.) was used for the N2 adsorption experiments andCO2 adsorption isotherms were obtained by using the same laboratoryvolumetric apparatus. The adsorbed volumes of N2 and CO2 are expressed asScm3 (referred to Standard conditions). BET and Dubinin–Radushkevichequations were used to calculate surface areas and micropore volumes from N2

(V μpN

2) and CO2 adsorption (V μpCO

2).Temperature programmed desorption (TPD) analyses were conducted by

using a ThermoFinnigan TPD/R/O 1100 equipped with a TCD and coupled witha Multi-channel Gas Analyzer using the infrared technique (Bernt Messtechnik).0.1 g of sample was placed in a quartz tube reactor and heated, under He flow(55 cm3/min) and 10 °C/min heating rate. A multiple Gaussian function was

Table 1Activated carbons produced under different conditions

Raw material Pine wood Oak wood Waste tires Olive seed waste

Activated carbon A B C1 C2 C3 D1 D2Pyrolysis conditions (°C, h) 800/0.75 800/0.75 800/1Chemical treatment – – KOHActivating gas/flow rate (cm3/min) H2O–CO2/758·10

3 H2O–CO2/758 ·103 N2/100

Activating conditions (°C, h) 900/2.5 900/0.5 900/1 900/2 800/1 800/3

751G. Skodras et al. / Fuel Processing Technology 88 (2007) 749–758

used for fitting the composite TPD spectra and CO/CO2 peaks produced by GasAnalyzer results, using the PeakFit Deconvolution procedure (SeaSolveSoftware Inc.).

2.2. Hg° bench-scale adsorption tests

Mercury adsorption tests were conducted with activated carbon mass 20 mg,mixed with 1 g sand, in a differential fixed-bed reactor (0.635 cm inner diameterstainless steel column), enclosed in a temperature-controlled oven, Fig. 1, asdescribed in more detail elsewhere [23]. The mass of adsorbent was selected inorder not to extend the experiment time unnecessarily, and 1 g of sand wasmixed with activated carbon as inert material to provide enough bed length(4 cm). The particle size of activated carbon was ranged between 150 and250 μm, since a larger particle would result to increased internal mass transferlimitations inside the particle and high external film diffusion coefficient. Thiscould produce a reduced adsorption rate. On the other hand, a smaller particlesize could increase the pressure drop along the fixed-bed column and promote anundesirable fluidization. A mercury permeation device was used as a source ofelemental mercury Hg° (VICI Metronics Inc, Santa Clara, CA). The device,designed to produce constant release of mercury vapor per unit time at thespecified temperatures, was secured in a temperature-controlled stainless steelU-tube holder, and nitrogen at preadjusted constant flow was fed through it. Amass flow controller kept the nitrogen flow constant, at 200 cm3/min.The Hg°inlet concentration was 0.35 ng/cm3 using 200 cm3/min of nitrogen, while thesorption temperature was 50 °C. For the continuous measurement of the outletelemental mercury concentration, a Mercury Instruments Analyzer (Combina-tion model VM-3000-LabAnalyzer 254) was used, based on Cold Vapor

Fig. 1. Schematic diagram of

Absorption Spectrometry. Before starting each experiment, 200 cm3/min ofclean nitrogen was directed through the empty reactor and Mercury Analyzer(U-tube bypassed), in order to equilibrate the carrier gas flow rate in the system.Then the mercury permeation device was placed on line and the temperature ofthe oven was adjusted to generate the desired mercury concentration. The flowrate of the mercury laden carrier gas was maintained at 200 cm3/min, and thevapor phase mercury concentration was allowed to equilibrate. At the same time,the bypassed reactor was charged with the adsorbent and heated at the desiredadsorption temperature. When this was achieved, the mercury gas stream waspassed through the fixed-bed reactor and the adsorption time was started.

3. Results and discussion

3.1. Pore structure characterization of the samples

The nitrogen adsorption isotherms of the resulting activatedcarbons, used for calculating their BET surface area and totalpore volume, Table 2, are presented in Fig. 2, corresponding to amixed type H3 according to IUPAC classification, based only onthe adsorption branch. A steep increase of the adsorbed volumesis observed at low pressures due to the enhanced potential ofmicropores, and a less sudden increase at high pressures showsthe presence of mesopores in the samples [28]. Pore structurecharacteristics vary in dependence of rawmaterial and activation

mercury adsorption unit.

Table 2Pore structure characteristics and Hg° adsorptive capacity of activated carbons

Sample BET (m2/g) V μpCO

2 (cm3/g) Vtot (cm

3/g) V μpCO

2 (%)Vtot V μpN

2 (cm3/g) V μp

N2 (%)Vtot Hg° (ng/mg AC) a

A 896.7 0.27 0.61 44 0.34 56 421B 684.4 0.26 0.45 58 0.22 49 379C1 132.9 0.12 0.25 48 0.06 23 188C2 251.6 0.19 0.26 73 0.09 36 331C3 358.5 0.19 0.31 61 0.15 49 342D1 1058.2 0.27 0.66 41 0.49 74 795D2 1690 0.35 0.9 39 0.7 78 869a 360 min adsorption experiment, 50 °C.


conditions. Activated carbon produced from pine wood disposeshigh BET surface area, total pore volume and microporecontribution to the total pore volume, compared with the oneresulted from oak wood (SBET are 896 and 684 m2/g re-spectively). In the case of activated carbons derived from wastetires, the poorest surface development occurred when activationtime was 30 min, while activation time increase resulted to anincrease of pore structure values (SBET=358 m

2/g), Table 2. Thesurface areas of the activated tire char are lower than thosereported in literature, where many researchers produced sampleswith surface areas between 389 and 530 m2/g. This could beattributed to the different activation conditions and high ashcontent that is about 20% in the starting material [24,29,30]. Ascan be seen in Figs. 2 and 3 and Table 2, N2 adsorbed amountsfor the activated carbons prepared from olive seeds by chemicalactivation increased with activation time. KOH activatedsamples posses a well-developed micropore structure andconsequently high micro- and total pore volume as well asBET surface area, the highest compared to the other samples(1690 m2/g). This is in agreement with the previous experimen-tal results indicating that high activation time and KOH/charratio 4:1 favor microporosity development [10,19]. Thecontribution of N2 micropore volume (V μp

N2) to the total pore

volume of samples is relatively low for tire carbons, increases upto 50% for the wood based samples and is high (about 75 %) forthe olive seed samples that can be characterized as mainlymicroporous carbons. Micropore volumes calculated from CO2

Fig. 2. N2 adsorption isotherms of the activated carbons produced from variousstarting materials.

adsorption isotherms (V μpCO

2) show a lower contribution to totalporosity than those estimated from N2 isotherms for olivecarbons but higher for used tire ones. That kind of discrepancy isjustified because each of the N2 and CO2 gases has access topores of different sizes.

3.2. Surface chemistry characterization of the samples

Figs. 4–6 show a comparison of the TPD plots obtainedduring the thermal treatment of the activated carbons tested.TPD plots of the activated carbons produced from the samestarting material are quite similar, but significant differences areobserved for TPD plots of the samples derived from differentraw materials. Activated carbons produced from pine and oakwoods present more intense peaks in the temperature range600–800 °C, Fig. 4, compared to the ones derived from tires andolive seeds. Peaks centered at 1100 °C are sharper in the case ofsamples produced from waste tires, Fig. 5. TPD can assess thebehavior of the thermal treated samples qualitatively, but thecomposite TPD spectra cannot give information for the specificfunctional groups. Thus, TPD results have been coupled withinfrared quantitative information in order to separate the CO andCO2 spectra. The composite CO and CO2 spectra obtained fromthe infrared spectra during the TPD tests should be de-convoluted to estimate the surface composition. For this reasona multiple Gaussian function was used and the de-convolutedCO, CO2 profiles are given in Figs. 7–20. The CO and CO2

Fig. 3. CO2 adsorption isotherms of the activated carbons produced from variousstarting materials.

Fig. 4. TPD profiles of activated carbons produced from pine and oak woods.

Fig. 5. TPD profiles of activated carbons produced from waste tires.

Fig. 7. De-convolution of the CO2 evolution curve of pine wood activated carbon.

Fig. 8. De-convolution of the CO evolution curve of pine wood activated carbon.


evolved amounts during the thermal treatment of the samples,Table 3, were calculated by employing the integration methodof the areas under CO and CO2 curves constructed from TPD-IRresults. Generally, CO2 evolution at low temperatures isattributed to acidic groups, while CO produced at higher

Fig. 6. TPD profiles of activated carbons produced from olive seeds.

temperatures is derived from weak acidic and basic groups. CO2

groups centered at temperatures up to 400 °C come fromcarboxyls, or lactones at temperatures up to 600 °C. Phenols,ethers, carbonyls and quinones evolve CO between 700 and

Fig. 9. De-convolution of the CO2 evolution curve of oak wood activated carbon.

Fig. 10. De-convolution of the CO evolution curve of oak wood activated carbon.

Fig. 11. De-convolution of the CO2 evolution curve of activated carbonproduced from waste tires (C1).


Fig. 14. De-convolution of the CO evolution curve of activated carbon producedfrom waste tires (C2).


1000 °C, and carboxylic anhydrides produce both CO and CO2

in the temperature range of 350–600 °C [31,32].CO2 release for both pine and oak wood activated carbons,

Figs. 7 and 9, begins at low temperature, around 200 °C, and


continues up to 900 °C. The peaks that are centered at 235 and336 °C for pine wood activated carbon and 248 at 341 °C foroak wood activated carbon can be attributed to carboxyl groupdecomposition.



Fig. 18. De-convolution of the CO evolution curve of activated carbon producedfrom olive seeds (D1).


In the case of pine wood activated carbon, CO2 peak found at474 °C is higher than the one formed at 494 °C in oak woodactivated carbon de-convoluted profile. These two peaksrepresent lactone decomposition. The opposite is observed forCO2 peaks centered at 636, 697, 776 °C (Fig. 7), as well as at642, 675 °C, Fig. 9, for pine and oak wood activated carbonsrespectively. These five peaks are considered to result fromcarboxylic anhydrides desorption. The de-convoluted COcurves of pine and oak wood activated carbons, Figs. 8 and10, are quite similar. CO peaks centered at low temperatures upto 792 °C, Fig. 8, are produced from the decomposition ofphenols and ethers. Both CO de-convoluted curves exhibit apeak at 1055 and 1058 °C respectively, that represents carbonyland quinone decomposition and it is more distinguishable in thecase of oak wood activated carbon. CO yield for pine woodactivated carbon is about 13% smaller than that of oak woodsample, while CO2 evolved amount is 10% higher, Table 3.

CO2 evolution curves of the activated carbon produced fromwaste tires (C1), Fig. 11, increase smoothly from 200 °C up to978 °C, with two sharp peaks at 624 and 908 °C that can be

Fig. 17. De-convolution of the CO2 evolution curve of activated carbonproduced from olive seeds (D1).

attributed to carboxylic anhydride decomposition. In the case ofsamples C2 and C3, Figs. 13 and 15, CO2 curves consist of onlyone distinguishable peak centered at 929 and 675 °C respec-tively that are referred to carboxylic anhydride degradation.This is in agreement with the decrease of CO2 produced withactivation time, Table 3. CO release occurred from 700 °C up to1100 °C during the thermal treatment of activated carbonsproduced from waste tires, Figs. 12, 14, and 16. Thistemperature range reveals the absence of thermal weak groupsthat evolve CO during their decomposition, such as phenols andethers.

In the CO2 de-convoluted profile of olive seed activatedcarbons, Figs. 17, 19, peaks centered at 450 and 621 °C forsample D2 are higher compared to those at 481 and 629 °C forsample D1. These peaks represent carboxylic anhydridedecomposition. This is confirmed by the higher CO2 releaseduring TPD experiment of D2 sample, Table 3. From the CO de-convoluted profile of D1 and D2 samples, Figs. 18 and 20, COpeaks formed during D2 decomposition are higher than thosereleased from D1 sample, except the peaks that are centered at

Fig. 19. De-convolution of the CO2 evolution curve of activated carbonproduced from olive seeds (D2).

Fig. 21. Hg° breakthrough curves for activated carbons produced from biomasssamples.Fig. 20. De-convolution of the CO evolution curve of activated carbon produced

from olive seeds (D2).


high temperatures around 1000 and 1100 °C. For both oliveseed activated carbons, CO peaks centered at high temperatures(above 1000 °C) represent quinone group decomposition andare higher compared to the ones presented in the CO de-convoluted profile of pine and oak wood activated carbons,Figs. 8 and 10. Their formation was favored by the high KOH-to-precursor ratio [19].

Finally, it can be said that in activated carbon produced frompinewood (A), the functionalities consist largely of lactones andcarbonyls. The contribution of lactones to the total oxygengroup number is higher than in the sample derived from oakwood (B). The opposite is observed for carbonyl groups that areincreased in samples produced from oak wood. In the case ofactivated carbons prepared from waste tires, their surfaceoxygen functionalities consist of carboxylic anhydrides and ofthe most thermal stable groups, such as carbonyls/quinones.The absence of carboxyls, phenols and ethers is noticeable.Samples resulted from olive seeds possess all the types ofoxygen groups. Their thermal stable groups, such as quinones,are higher, compared to pine and oak wood samples, while theyseem to be smaller than carbonyls/quinones presented in wastetire samples.

3.3. Mercury adsorption results

Hg° breakthrough curves of the activated carbons are givenin Figs. 21–23. The minimum values of Cout/Cin reached at the

Table 3CO and CO2 evolution during TPD experiment

Sample CO (mmol/100 g) CO2 (mmol/100 g)

A 130.9 104B 150.1 94C1 153.7 31C2 132.6 15.5C3 153 8.1D1 172 34.4D2 106 49

initial times, are different in the various breakthrough curvesdue to the different properties (pore structure, surfacechemistry) of the examined samples. Hg° adsorptive capacitiescalculated by integrating the breakthrough curves obtained at50 °C, during 360 min adsorption experiment, are presented inTable 2. As it is observed, the lower Hg° removal ability wasdemonstrated by samples originated from waste tires (C1–C3),while the higher from olive seed waste activated carbons (D1,D2). Activated carbons produced from oak/pine wood chars,presented Hg° adsorptive capacities comparable to the retentionability of some commercial activated carbons, such as RWE(Rheinbraun Company, 394 ng/mg Hg° adsorptive capacity)and BPL (Calgon Corporation, 380 ng/mg Hg° adsorptivecapacity) [23,28,33]. Sorbents prepared from olive seeds,possessed Hg° adsorption ability higher than all the examinedsamples, commercial F400 (Filtrasorb 400, Calgon Company)and Norit FGD included, which dispose 680 and 450–618 ng/mg Hg° adsorptive capacity, respectively [23,34].

As far as it concerns the influence of pore structure char-acteristics on Hg° adsorption, Table 2, it is observed that anincrease in BET surface area, total pore volume and microporevolume is accompanied by an almost proportional increase inHg° adsorptive capacity. The above suggests that the available

Fig. 22. Hg° breakthrough curves for activated carbons produced from wastetires.

Fig. 23. Hg° breakthrough curves for activated carbons produced from oliveseeds.


BET surface area, the pore volume and the contribution ofmicropores affect Hg° removal [35,36]. Particularly, thepresence of micropores in KOH-activated carbons improvestheir adsorptive capacity, since inside the micropores, theinteraction potential between the solid and the Hg° moleculesis significantly higher than in wider pores [37]. It has beenobserved that activated carbons produced from pine wood (A)and tires activated for 2 h at 900 °C (C3), dispose similar BETsurface areas to F400 and RWE commercial activated carbons(816 and 343 m2/g respectively) [23]. However, sample Apresents smaller Hg° removal capacity compared to F400,because of its reduced micropore volume. On the contrary, C3sample disposes similar Hg° adsorptive capacity to RWE com-mercial activated carbon, due to similar percentages of micro-pores contributing to their pore structure (49% for C3 and 46%of total pore volume for RWE [23]. The used-tire based activatedcarbons show lower adsorption capacities compared to the A, Band D samples but comparable to the RWE. The production ofactivated carbons by employing the chemical activationprocedure leads to the formation of microporous samples withincreased surface area and total pore volume. These samplesseem to present superior pore structure characteristics comparedto the commercial activated carbons, and enhanced Hg° ad-sorption capacity, (868 ng/mg), which is the higher between thesamples prepared here and one of the highest found in literature,Table 2 [23,34].

Additional to the effect of high BET surface area andmicroporosity, the enhanced Hg° adsorptive capacity of sampleA compared to the one of sample B, could be attributed to thetype and number of surface oxygen groups. The evolved amountof CO2 is higher in the case of sample A, Table 3, probably due tothe higher amount of lactone decomposition. This is betterconfirmed by CO2 peaks centered at 336 and 474 °C, Fig. 7. Inthe CO de-convoluted profile of sample A, Fig. 8, the sharperpeak at 893 °C that represents carbonyl decomposition is higherthan the one at 848 °C in the case of sample B, Fig. 10. So, thepresence of lactones and carbonyls is more noticeable in the caseof activated carbon produced from pine wood resulting inenhanced Hg° removal ability, since these groups are consideredto be possible active sites that favor Hg° adsorption [22].

Elemental mercury posses two electrons in the 6s outer shell inits electronic configuration and its adsorption involves anoxidation mechanism that is favored by the reactions whichaccepts Hg° electrons, such as the following reaction: quinines+2H++2e−→phenols i.e. C6H4O2+2H++2e−=C6H4 (OH)2.In the case of lactones and carbonyls or quinones, carbon atompresented in the _C_O bond acts as an electron acceptor, so,these oxygen functionalities facilitate the electron transferprocess (mercury oxidation) that is taking place on activatedcarbon surface acts as an electrode [22].

As it is observed in CO2 de-convoluted profiles of activatedcarbons produced from waste tires, Figs. 11, 13 and 15, CO2

release starts only above 500 °C, as a result of carboxyls andlactones absence, that affects their limited adsorptive capacity,Table 2, additional to their poor developed pore structure.

The high participation of carbonyls in total oxygen content ofD1 and D2 samples, Figs. 18 and 20, in comparison to carbonylpresence in pine/oak wood samples surface, Figs. 8 and 10,enforces their increased Hg° removal ability. Although CO andCO2 emitted amounts during D1 and D2 temperatureprogrammed desorption are comparable to the correspondingamounts of the other samples, the pore structure of the chemicallyactivated samples dominates over the other examined samples'physical properties and seems to be the crucial factor that affectstheir Hg° adsorption capacity.

4. Conclusions

A number of activated carbons have been produced from lowcost–highly available materials by various activation methods(physical–chemical), and they have been examined for theirHg° adsorptive ability. Produced carbons demonstrated BETsurface areas, total pore volumes and micropore volumes,ranging from lower to higher than commercial products.

In a similar fashion, Hg° adsorptive capacities ranged betweenmuch higher than smaller to the same commercial activatedcarbons, rendering some of their potential candidates for future Hg°entrapment application. Particularly, the best behaviour is demon-strated from chemically activated samples produced from KOH-activated olive seeds, while the other activated carbons follow withthis order: TiresbOak WoodbPine WoodbOlive Seed Wastes.Despite the high adsorption potential of KOH-activated carbons, anexcess amount of KOH is required (C: KOH=1:4) for theirpreparation. This may increase their production cost in acommercial unit. The recovery and recycle of the KOH maysolve at least partially that problem.By correlating the experimentalresults, we have shown that an increase in BET surface area, totalpore volume and micropore percentage is accompanied by analmost proportional increase in Hg° adsorptive capacity.

Apart from pore structure, the surface oxygen functionalgroups, and especially lactone and carbonyl groups, seem to affectthe Hg° adsorptive capacity of the activated carbons, asexemplified by the TPD-IR results. These oxygen functionalitiesfacilitate the electron transfer process and mercury oxidation onactivated carbon surface and act as possible chemisorption centersfor Hg°. The absence of lactones from the surface chemistry ofactivated carbons derived from waste tires contributes to their


limited mercury adsorptive capacity. On the contrary, the highparticipation of carbonyls in the total number of oxygen groups ofKOH-activated samples promotes their Hg° retention ability.

References

[1] V. Serrano-Gomez, M.E. Correa-Cuerda, M.C. Gonzalez-Fernandez, F.M.Franco-Alexandre, A. Garcia-Macias, Preparation of activated carbonsfrom walnut wood: a study of microporosity and fractal dimension, SmartMaterials and Structures 14 (2005) 363–368.

[2] A. Rubel, R. Andrews, R. Gonzalez, J. Groppo, T. Robl, Adsorption of Hgand NOx on coal by-products, Fuel 84 (2005) 911–916.

[3] M.M. Johns, E.W. Marshall, A.C. Toles, The effect of activation methodon the properties of pecan shell-activated carbons, Journal of ChemicalTechnology and Biotechnology 74 (1999) 1037–1044.

[4] J. Guo, A. Lua Chong, Characterization of adsorbent prepared from oil-palm shell by CO2 activation for removal of gaseous pollutants, MaterialsLetters 55 (2002) 334–339.

[5] J. Villegas-Pastor, J.C. Valle-Duran, Pore structure of chars and activatedcarbons prepared using carbon dioxide at different temperatures from extractedrockrose, Journal of Analytical and Applied Pyrolysis 57 (2001) 1–13.

[6] T. Yang, A. Lua Chong, Characteristics of activated carbons prepared frompistachio-nut shells by physical activation, Journal of Colloid and InterfaceScience 267 (2003) 408–417.

[7] T. Vitidsant, T. Suravattanasakul, S. Damronglerd, Production of activatedcarbon from palm-oil shell by pyrolysis and steam activation in a fixed bedreactor, Science Asia 25 (1999) 211–222.

[8] V. Serrano-Gomez, M.E. Correa-Cuerda, C.M. Gonzalez-Fernanadez, F.M.Franco-Alexandre, A. Garcia-Macias, Preparation of activated carbons fromchestnut wood by phosphoric acid-chemical activation. Study of micropo-rosity and fractal dimension, Materials Letters 59 (2005) 846–853.

[9] C. Srinivasakannan, M. Bakar Abu Zailani, Production of activated carbonfrom rubber wood sawdust, Biomass and Bioenergy 27 (2004) 89–96.

[10] Jin-Soo Park, Young-Woo Jung, Influence of activation temperature onmicroandmesoporosity of synthetic activated carbons, Carbon Science 2 (2) (2001)105–108.

[11] Yeh-Li Hsu, H. Teng, Influence of different chemical reagents on thepreparation of activated carbons from bituminous coal, Fuel ProcessingTechnology 64 (2000) 155–166.

[12] H. Teng, Chuan-Yu Lin, Y.L. Hsu, Production of activated carbons frompyrolysis of waste tires impregnated with potassium hydroxide, Air &Waste Management Association 50 (2000) 1940–1946.

[13] D. Castello-Lozano, A.M. Rodenas-Lillo, D. Amoros-Cazorla, A.Solano-Linares, Preparation of activated carbons from Spanish anthraciteI. Activation by KOH, Carbon 39 (2001) 741–749.

[14] S.D. Serre, G.D. Silcox, Adsorption of elemental mercury on the residualcarbon in coal fly ash, Industrial & Engineering Chemistry Research 39(2000) 1723–1730.

[15] R.T. Carey, W.O. Hargrove, F.C. Richardson, R. Chang, B. Meserole, J. Frank,Factors affecting mercury control in utility flue gas using activated carbon, Air& Waste Management Association 48 (1998) 1166–1174.

[16] M. Valer-Maroto Mercedes, Y. Zhang, J.E. Granite, Z. Tang, W.H. Pennline,Effect of porous structure and surface functionality on themercury capacity ofa fly ash carbon and its activated sample, Fuel 84 (2005) 105–108.

[17] Z. Hu, H. Guo, P.M. Srinivasan, N. Yaming, A simple method fordeveloping mesoporosity in activated carbon, Separation and PurificationTechnology 31 (2003) 47–52.

[18] A. Lua Chong, J. Guo, Activated carbon prepared from oil palm stone byone-step CO2 activation for gaseous pollutant removal, Carbon 38 (2000)1089–1097.

[19] Jin-Soo Park, Yung-Woo Jung, Effect of KOH activation on the formationof oxygen structure in activated carbons synthesized from polymericprecursor, Journal of Colloid and Interface Science 250 (2002) 93–98.

[20] L.J. Figueiredo, R.F.M. Pereira, A.M.M. Freitas, M.M.J.J. Orfao,Modification of the surface chemistry of activated carbons, Carbon 37(1999) 1379–1389.

[21] A.Montoya, F. Mondragon, N.T. Tuong, Formation of CO precursors duringchar gasification with O2, CO2 and H2O, Fuel Processing Technology 77–78(2002) 125–130.

[22] H.Y. Li, W.C. Lee, K.B. Gullett, Importance of activated carbon's oxygensurface functional groups on elemental mercury adsorption, Fuel 82 (2003)451–457.

[23] G. Skodras, Ir. Diamantopoulou, P. Natas, A. Palladas, G.P. Sakellaropoulos,Postcombustion measures for cleaner solid fuels combustion: activatedcarbons for toxic pollutants removal from flue gases, Energy and Fuels 19(2005) 2317–2327.

[24] A. Zabaniotou, P. Madau, D.P. Oudenne, G.C. Jung, P.-M. Delplancke,A. Fontana, J. Active carbon production from used tire in two-stageprocedure: industrial pyrolysis and bench scale activation with H2O–CO2 mixture, Journal of Analytical and Applied Pyrolysis 72 (2004)289–297.

[25] A. Zabaniotou, D.P. Oudenne, G.C. Jung, A. Fontana, Active carbonproduction char issued from used tires pyrolysis: industrial improvement,ERDOL, ERDGAS, KOHLE 121 (2004) 160–162.

[26] G.G. Stavropoulos, Precursor materials suitability for super activated carbonsproduction, Fuel Processing Technology 86 (11) (2005) 1165–1173.

[27] G.G. Stavropoulos, A. Zabaniotou, Production and characterisation ofactivated carbons from olive-seed waste residue, Journal of Microporousand Mesoporous materials 82 (2005) 79–85.

[28] G.G. Stavropoulos, I.R. Diamantopoulou, G. Skodras, G.P. Sakellaropoulos,High activity carbon sorbents for mercury capture, Proceedings of theInternational Symposium onMoving Towards Zero Emission Plants, Greece,June 20th–22th, 2005.

[29] R. Helleur, N. Popovic, M. Ikura, M. Stanciulescu, D. Liu, Characterisationand potential applications of pyrolytic char from ablative pyrolysis of usedtires, Journal of Analytical and Applied Pyrolysis 58–59 (2001) 813–824.

[30] A. Zabaniotou, G. Stavropoulos, Pyrolysis of used automobile tires andresidual char utilisation, Journal of Analytical and Applied Pyrolysis 70(2003) 711–722.

[31] L.J. Figueiredo, R.F.M. Pereira, A.M.M. Freitas, M.J.J. Orfao, Modificationof the surface chemistry of activated carbons, Carbon 37 (1999) 1379–1389.

[32] G. de la Puente, J.J. Pis, A.J. Menendez, P. Grange, Thermal stability ofoxygenated functions in activated carbons, Journal of Analytical andApplied Pyrolysis 43 (1997) 125–138.

[33] Z. Sun, X. Li, J. Luo, J.Y. Hwang, Unburned carbon from fly ash for Hg°adsorption. II. Adsorption isotherms and mechanisms, Journal of Mineralsand Materials Characterization and Engineering 1 (2) (2002) 79–96.

[34] S. Sjostrom, Assessment of low cost novel sorbents for coal-fired powerplant mercury control, DOE/NETL's Programme, DOE Number: DE-FC26-01NT41180, 2002, pp. 1–7.

[35] M.A. Mastral, T. Garcia, R. Murillo, S.M. Callen, M.J. Lopez, V.M.Navarro, Moisture effects on the phenanthrene adsorption capacity bycarbonaceous materials, Energy and Fuels 16 (1) (2002) 205–210.

[36] M.A. Mastral, T. Garcia, R. Murillo, S.M. Callen, M.J. Lopez, V.M.Navarro, PAH mixture removal from hot gas by porous carbons. Frommodel compounds to real conditions, Industrial & Engineering ChemistryResearch 42 (21) (2003) 5280–5286.

[37] R. Yan, T.D. Liang, L. Tsen, P.Y. Wong, K.Y. Lee, Bench-scale experimentalevaluation of carbon performance on mercury vapor adsorption, Fuel 83(2004) 2401–2409.

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Enhanced mercury adsorption in activated carbons from biomass materials and waste tires