Desalination 265 (2011) 169–176

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Desalination

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Potential of nano-carbon xerogels in the remediation of dye-contaminatedwater discharges

B.S. Girgis ⁎, A.A. Attia, N.A. FathySurface Chemistry and Catalysis Laboratory, Physical Chemistry Department, National Research Centre, 12622 Cairo, Egypt

⁎ Corresponding author. Tel.: +20 2 3371433; fax: +E-mail address: girgisbs@hotmail.com (B.S. Girgis).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.07.048

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 May 2010Received in revised form 19 July 2010Accepted 19 July 2010Available online 25 August 2010

Keywords:Carbon xerogelsMethylene blueRhodamine BBatch adsorption

A series of porous carbon xerogels were synthesized from resorcinol-formaldehyde resin (RF), usingresorcinol/catalyst (R/C), molar ratios (50, 200, 500, and 1000), and carbonization temperatures of 500, 600and 700 °C, under no external gas flow. Drying of the RF hydrogels was carried out by the conventionalevaporation technique in static air. The produced xerogels were characterized by N2/77 K adsorption, C,H,and O-content, TEM, and FTIR techniques. Adsorption of two probe dye molecules, methylene blue andRhodamine B, was determined to assess the removal capacity in relation to porosity characteristics andsynthesis conditions. Raising the R/C ratio was associated with considerable development of internal porosity(surface area and pore volume), mostly in the micropore size range, especially by carbonization of theprimary xerogels at 600 or 700 °C. Good adsorbing carbons were obtained with appreciable capacity foruptake of the two cationic dye molecules, which increased with extended internal porosity. The describedsimple conditions of preparation might enhance the pilot-scale production of carbon xerogels suitable forliquid phase adsorption treatment of various organics from the aqueous solutions.

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1. Introduction

Among the different pollutants of the aquatic ecosystem, dyes are alarge and important group of synthetic organic compounds (SOC).They are widely used in diverse industries such as textiles, paper,rubbers, plastics, foodstuffs, cosmetics, etc., to color their products.These dyes are invariably left in the industrial wastes and conse-quently discharged mostly in surface water resources. Dyes, even inlow concentrations, are visually detected and meanwhile affect theaquatic life and food web. These colored compounds are not onlyaesthetically displeasing but also inhibiting sunlight into the waterstreams and thus reduces the photosynthetic reactions [1]. Sincemany organic dyes are harmful to human beings, the color removalfrom process or water effluents environmentally demanding. Due tothe large degree of organics in these molecules and to the stability(heat and/or light) of modern dyes, conventional physicochemicaland biological treatment methods are insufficient for their removal[2]. Among the well-known treatment processes; coagulation [3],oxidation [4], ultrafiltration [5], electrochemical [6,7] and adsorption[8] techniques were employed for dye removal. Activated carbonadsorption proved to be one of the most effective and widely usedadsorbent; it has rapidly gained prominence as treatment processwith good quality effluents with low concentrations of dissolved dyes

from textile industries [9,10]. Meanwhile, it is a sludge-free treatmentscheme and thus considered as one of the best available technologies(BAT) [11]. A large number of different types of dyes were found tobe amenable to this process, e.g., reactive, direct, acid and basic dyes[9,12–15].

Carbon gels, a novel class of synthetic porous carbons wasdeveloped by Pekala et al. in 1989 [16], and received considerableattention in the literature over the past 10 years [17–23]. The firstbeing synthesized from the polycondensation of resorcinol (R) andformaldehyde (F) in aqueous solution [17]. The texture and densityproperties of these materials were established to be controlled eitherby using different synthesis protocols or drying and pyrolysisconditions. The porosity characteristics can be controlled over abroad range, and the pore and particle size can be tailored at thenanometer scale [24]. For these reasons, such type of carbons wassuggested to be suitable for a wide range of applications [25], rangingfrom energy storage devices for example, as intercalation anodes forrechargeable lithium ion cells [26], as electrodes for electric doublelayer capacitors (EDLCs) [21,27,28], in thermal and phonic insulators,chromatographic packing, adsorbents, and catalyst supports [29–32].

Three main stages can be distinguished in the preparation ofcarbon aerogels, cryogels and xerogels [33]. In the first stage, R and Fare mixed at the appropriate ratio, commonly 1:2, although a ratio of1:1.33 was reported by Teng and Wang [34], followed by gellingand curing steps. The most important factors that control theproperties of the organic gel in this stage are the concentration ofcatalyst (C) and reactants (R and F), and the initial pH of aqueoussolution [29]. In general, the surface area of RF-carbon xerogels has a

Table 1Conditions of preparation of the studied carbon xerogels and their slurry pH.

Notation R/C HTT °C % Yield Slurry pH

RFX-50 50 – – 4.0CX50-500 50 500 32.0 7.2CX200-500 200 500 22.5 6.8CX500-500 500 500 24.7 6.7CX1000-500 1000 500 15.7 6.2CX50-600 50 600 13.1 7.3CX200-600 200 600 9.5 7.2CX500-600 500 600 16.7 6.9CX1000-600 1000 600 10.1 6.7CX50-700 50 700 10.3 7.3CX200-700 200 700 12.3 5.2CX500-700 500 700 12.3 6.9CX1000-700 1000 700 6.0 6.5

170 B.S. Girgis et al. / Desalination 265 (2011) 169–176

weak dependence on the initial solution pH in the acidic range [24],but at a pH higher than 7.0, the surface area diminishes completely[16].

The second stage in the preparation of the organic gels is thedrying of the wet gel. Simple evaporation of the solvent by heating thehydrogel was believed to a collapse of the pore structure due tochanges in surface tension of the solvent upon the formation of thevapor-liquid interface. This collapse results in a large shrinkage of thegel, giving a dense polymer called a “xerogel” [29]. The wet gel may bedried with supercritical fluids to preserve the structure, and carbondioxide is widely used. Supercritically dried gels are called “aerogels”,and they have higher surface areas and pore volumes than thecorresponding xerogels. When the liquid solvent is removed byfreeze-drying method, the gel obtained is a “cryogel,” the solvent isfrozen and then removed by sublimation, thereby avoiding theformation a vapor-liquid interface. Therefore, the supercritical- orfreeze-drying methods retain the formal skeletal structure of the wetgel during the subsequent drying stage.

The third preparation stage of carbon xerogels, aerogels, andcryogels is the carbonization (or pyrolysis) of the dried R/F gels,carried out by heating the sample in flowing N2, He, or Ar at atemperature between 500 and 2500 °C [21]. During carbonization, thedried gel loses O and H functionalities with an enrichment in carbon,giving a high pure carbon structure. The surface area, pore volume,and pore size distribution of the RF-carbon gels are tunable surfaceproperties related to the synthesis and processing conditions whichcan produce a wide spectrum of materials with unique properties.Because of their structure and texture, carbon aerogels/xerogelscan be designed and controlled at the nanometer scale, accordingly,they have recently been classified as nanostructured carbons[21,24,29,34,35].

It is noticeable that most of the literature pertinent to carbon gels,xerogels or activated gels focused mostly on the optimization of theirsynthesis conditions and characterization. However, with respect toadsorption from solution onto such synthetic porous carbons, veryfew relevant studies are reported, e.g. adsorption of phenol andreactive dyes: Black 5 and Red E [36]. The adsorption capacity ofsimilar xerogels was noticed to be enhanced by both the texture andsurface chemical nature [37].

The present work focuses on the synthesis and characterization ofcarbon xerogels, produced from resorcinol-formaldehyde aquagels(resins), under standard conditions: type of precursors, catalyst,gelling–curing, and evaporative drying. Variations of resorcinol-catalyst (sodium carbonate) molar ratios (50, 200, 500, and 1000)and pyrolysis temperatures (500, 600, and 700 °C) were studied,under static conditions, in the absence of any external flow of gases.Development of porosity as function of the two factors wasdetermined, and adsorption capacity towards two cationic dyes,methylene blue and Rhodamine B, was evaluated.

2. Experimental

2.1. Synthesis of carbon xerogels

Four organic xerogels were prepared by the sol-gel polymerizationreaction of resorcinol (R) (99%, Panreac Quimica SA) and formalde-hyde (F) (37%, Aldrich), to which a basic salt was added followingPekala's method [16,20,21,24]. In all cases, the hydrogels wereprepared using the following amounts: 7.995 g of R, 11.0 mL of F(stabilized by 10–15 wt.% methanol) and 48 mL of water; molar ratioswere R/F=0.5. A solution of 20.0 w/v% solids was obtained. Theamounts of metallic precursor (basic catalyst), sodium carbonate,were 0.154, 0.038, 0.015, and 0.008 g, the corresponding molar ratiosof R/C became 50, 200, 500, and 1000, respectively. Formaldehydewasadded first to resorcinol, dissolved in prescribed amount of water,followed by drops of dilute (0.1 N) NaOH solutions to adjust pH to 7.4,

6.8, 6.0 and 5.6, respectively, and the resultant sol solution stirred for30 min at 80 °C. The formed hydrogel was kept in a stoppered glassflask, left at room temperature for 24 h, and then placed in an air ovenat 85 °C for 1 day to achieve the gelation and curing process. Thesolution progressively changed color from clear colorless to yellow,orange and dark brown at pH 7.4 or yellowish brown at pH 6.8–5.6;the initial sol solution pH was measured in all cases. After curing, thegel was removed from the oven and cooled to room temperature. Thehydrogel was then transferred to an open beaker, placed in an air drierand heated slowly to 60 °C and held for 2 h, then to 110 °C for another2 h to get the organic xerogels (RFX), or resins.

The carbon xerogels (CXs) are formed by pyrolysis of the driedorganic xerogels at 500, 600, or 700 °C for 2 h, in a temperature-controlled muffle furnace. Slow heating and cooling rates are set at10 °C/min. Then, the resulting carbons were thoroughly washed withhot water until the pH of the drained solution reached 6.0. Finally, theobtained carbon xerogels were filtered and dried in an air oven at110 °C overnight.

2.2. Sample nomenclature

Throughout the paper, the samples are denoted as follows; theletter C (for carbon), X (for xerogel) is followed by the R/C ratio andfinally followed by the carbonization temperature. For example,sample CX500-500 is a synthesized carbon xerogel with an R/C molarratio equal to 500 and the carbonization temperature is 500 °C(Table 1), while RFX is denoted as resorcinol-formaldehyde organicxerogel.

2.3. Physicochemical characteristics

Chemical analysis for carbon and hydrogen present in the carbonxerogels was determined by using an apparatus of the type ElementarVario EL (Made in Germany). The content of oxygen was estimated bydifference. The surface functional groups were qualitatively deter-mined by FTIR spectra of the organic xerogel (RFX at R/C=50) as wellas selected carbon xerogels CX1000-500, CX 1000–600, and CX 1000–700, recorded within the wavenumber range of 4000 to 400 cm−1.Analysis was achieved by a spectrophotometer Type FTIR-2000 PerkinElmer (USA), employing the KBr-pellet technique, was performedon those samples. Pressed KBr-pellets at a sample/KBr ratio of0.2% (w/w) were scanned after drying overnight at 100 °C. Thesurface morphology and texture properties were photographed bytransmission electron microscopy (TEM, Zeiss-EM10). Slurry pH wasdetermined by contacting 1 g of the finely powdered xerogels with100 mL of distilled water, shaken well for 5 min, left overnight inthe stoppered bottles, and then the pH of supernatant liquid wasmeasured.

Table 2Chemical analysis of C, H and O elements in the prepared samples.

Sample % C % H % Odiff Atomic H/C Atomic O/C

RFX-50 58.4 4.74 36.86 0.973 0.473CX50-500 76.9 2.91 20.19 0.454 0.197CX200-500 77.3 3.01 19.70 0.466 0.191CX500-500 78.5 3.22 18.32 0.489 0.175CX1000-500 79.5 3.31 17.23 0.498 0.162CX50-600 85.6 2.15 12.25 0.301 0.106CX200-600 88.2 2.75 9.05 0.373 0.077CX500-600 89.6 2.84 7.56 0.381 0.063CX1000-600 90.1 3.18 6.82 0.424 0.056CX50-700 83.3 1.73 14.97 0.249 0.135CX200-700 84.0 1.86 14.14 0.249 0.135CX500-700 85.1 2.12 12.78 0.299 0.112CX1000-700 89.5 2.16 8.34 0.289 0.070

171B.S. Girgis et al. / Desalination 265 (2011) 169–176

2.4. Porosity characterization

Texture characteristics of the prepared carbon xerogels weredetermined by the N2 adsorption at 77 K, using an Autosorb I(Quantachrome Micrometrics, USA), computer-controlled apparatus.Major texture parameters such as the total surface area which wasestimated by applying BET-equation, total pore volume Vp deter-mined from nitrogen held as liquid as P/Po=0.95, and the averagepore radius from rp=2 Vp/SBET, were evaluated. An estimate formicropore volume (Vo

0.1) was taken as volume of liquid nitrogen heldat P/Po=0.1 [38].

2.5. Adsorption of dyes from aqueous solution

Equilibrium adsorption experiments were carried out using twoprobe cationic dyes, methylene blue (MB) [1,39–42] and Rhodamine B(RB) [43–47], from an aqueous solution. A 50 mg of the powderedcarbon xerogels were added to 100 mL of dye solutions of variousinitial concentrations (10–100 mg/L), contained in 250 mL Erlen-meyer flasks. The stoppered flasks were kept at ambient conditions,in a flask shaker, for 72 h to attain equilibrium. The residual dyeconcentration was determined using a UV–Vis spectrophotometer(Shimadzu Model PC-2401) with 1.0 cm length-path cell. Absorbancemeasurements of the properly diluted filtrates were made at themaximum wavelengths of at 664 nm (MB) and 554 nm (RB). Theamount of dye uptaken by the carbon xerogels was calculated byapplying the equation [43,46]:

qe =Co−Ceð Þ

mV ð1Þ

where qe is the amount of dye uptaken by the adsorbent (mg/g), Coinitial dye concentration put in contact with the adsorbent (mg/L), Ceequilibrium dye concentration (mg/L) after the batch adsorptionprocedure, m mass of adsorbent in (g) and V is the volume of dye putin contact with the adsorbent (l). The isotherm model of Langmuirwas fitted to describe the equilibrium adsorption. The linear equationof Langmuir isotherm is given as [43,46]:

Ce

qe=

1QoKL

+1Qo

Ce ð2Þ

where qe and Ce are same as above, Qo is the maximum adsorptioncapacity (mg/g), and KL is the Langmuir affinity constant (L/mg). Theessential characteristics of the Langmuir isotherm can also beexpressed in terms of a dimensionless equilibrium parameter [1], RL,defined as;

RL = 1= 1 + KLCoð Þ ð3Þ

where, KL is the Langmuir constant; and Co is the highest initial dyeconcentration (mg/L). The free energy of adsorption, ΔGo can also beevaluated from the parameter KL (mol/L) according to the expression;ΔGo

ads= RT ln KL [1].

3. Results and discussion

3.1. Xerogels preparation and properties

Thus, twelve carbon xerogels are obtained, in addition to oneorganic xerogel included for comparison (Table 1).

Resorcinol [1,3 dihydroxy benzene, C6H4 (OH)2] is a phenoliccompound, which is capable of adding formaldehyde (HCHO) in the2-, 4-, and/or 6-positions in the aromatic ring [21,24]. Oxygen andhydrogen in the final R/F-resin are structurally original and inherentcomponents of the two precursors [H/C=6/8+4/2=10/8 (1.25) andO/C=2/6+2/2=4/8 (0.5), respectively]. The H/C (1.25) and O/C

(0.5) ratios come of the starting reactants; 1 mol resorcinol with2 mol formaldehyde. The addition reaction retains the parent O andH atoms, with slight reduction in their ratios, as only water isformed through the polycondensation and polymerization reactions.This takes place as a result of the extensive formation of methylene(−CH2−) and methylene ether (−CH2−O−CH2−) bridges [21,24].

The produced organic resin (RFX) exhibits the H/C and O/C ratiosof 0.997 and 0.473, respectively, proving that slight amounts of H andO are released through the H2O formation reaction (Table 2). The FTIRspectra of the selected samples (Fig. 1) exhibit several absorptionbands mostly attributed to C–O, C O, and C–OH groups and adsorbedwater [47–50]. An absorption band at 3480 cm−1, is assigned tostretching in hydroxyl and adsorbed water, at 1736 cm−1 due to C Ostretching, while at 1239, 1157, 1101, and 1034 cm−1 are assignedto stretching in aromatic and aliphatic esters, ethers, and alcohols[47–50]. Other bands of small intensities at 2924, 2842, 1511, and1439–1300 cm−1, are usually ascribed to asymmetric and symmetricstretching and bending modes of aliphatic and aromatic structures, inaddition to those below 800 cm−1 which are characteristic of out-of-plane vibration of C–H moieties of aromatic structures.

Thermal treatment at 500–700 °C, to get the carbon xerogels,reduces considerably the H/C and O/C ratios (Table 2). It is wellestablished that carbonization or pyrolysis, particularly decomposesthe heteroatoms containing groups, releasing them in the form ofsmall compounds (alcohols, ethers, ketones, acids and water) andthus enriching the carbon content (cf. Table 2, C from 58 to around90%). Hydrogen and oxygen are accordingly reduced from 4.74 downto 1.73% and from 36.8 down to 6.8%, respectively. The most drasticthermal effect is observed at 600 °C, and particularly at the highest R/Cratio of 1000 (alternatively, the lowest catalyst content) as shown inTable 2. It is of interest to note that the 700 °C-xerogels are apparentlyslightly re-enriched with O more than the 600 °C-products, despitethe anticipated extra-depletion with raising temperature (H isnormally reduced in same direction). Ambient oxygen inside anoven atmosphere, thus, seems to be captured at 700 °C forming newstable C–O groups. This trend is confirmed by the FTIR spectra (Fig. 1),in addition to the O-content (%O) and O/C ratio (Table 2). The TEMmicrographs in Fig. 2, confirm the fine and nanostructured nature ofthe resin or carbon xerogels under consideration. It was shown thatraising R/C at 500 °C results in widening of the pores and size ofinterconnected particles. It means that higher R/C values lead in fact tolarger polymer nodules as in case of CX500-500.

3.2. Development of porosity under varying processing conditions

From the N2-adsorption isotherms, Table 3 summarizes theestimated texture parameters for the xerogels synthesized undervarying R/C molar ratios and/or heat treatment temperatures (HTT).Both variables are known to have appreciable impact on porosity of

Fig. 1. FTIR spectra of the selected carbon xerogels and the parent RFX.

172 B.S. Girgis et al. / Desalination 265 (2011) 169–176

the developed carbon xerogels [20–23]. Inspection of the data inTable 3 points to the following observations:

1) Increasing the initial resorcinol-formaldehyde (R/C) ratio bringsabout a general development in the porosity (pore volume)with more than 50% in the wider micropore range, especially atR/C≥500. An associated considerable regular increase in the totalsurface area is observed (4–7 fold), where most of the developedporosity lies within the micropore range (up to more than 70%).Such type of porosity contributes to the appearance of high surfacearea (SBET=613–735 m2/g), however with less contribution to thepore volume.

2) On the other hand, raising the HTT modifies appreciably theinternal porosity, where at R/C=200 and 500, the total surfaceareas are 363, 439, and 516 m2/g and 441, 653, and 661 m2/g at500, 600, and 700 °C, respectively. At the highest R/C value of 1000with varying HTT, the corresponding values of SBET are 613, 687and 735 m2/g, respectively. Pyrolysis at 600 °C seems to be morefavorable than either 500 or 700 °C, as much higher pore volumeswith considerable mesoporosity are obtained (Table 3).

3) In general, good quality carbon adsorbents are obtained by heattreatment within 500–700 °C, for the RF-resins with initial molarratios of 50–1000. The best precursor being that with the highestR/C=1000 (i.e. lowest catalyst content), followed by pyrolysis at600 °C, whereas heating at 700 °C is not accompanied by furtherevolution of porosity. However, carbon xerogels obtained at 500 °C

Fig. 2. TEM micrographs of RFX-5

can produce adsorbents with good content of mesoporosity (Vmes/VP~70%) and wide average dimensions around 24 Å at all R/Cratios. Most of the other carbons exhibit narrow microporosity,rP=11–14 Å (except two carbon xerogels). This might recom-mend the production of mesoporous carbons suitable for a widevariety of treatment processes as will be assessed and demon-strated in the next section.

3.3. Adsorption of methylene blue and Rhodamine B from aqueoussolution

As the case of traditional activated carbons, color bodies are targetmaterials for removal from water courses generated in domestic,industrial and agricultural wastewaters. For this purpose, currentlydeveloped carbon adsorbents were usually tested to evaluate theircapacity towards probe molecules and to determine the adsorptionefficiency. Methylene blue (MB) is one of these conventional solutesextensively used since long time [39], and still in current use, in manyresearch reports to the present time [1,40–42], as well as in technicalbrochures of industrial carbons. It has formula weight (319), cross-sectional area (120 Å2) and molecular size (13–15 Å), which are well-recognized over the previous decades. An equally important andcurrently employed dye with higher molecular structure is Rhoda-mine B (RB) [43–46], with formula weight (479), cross-sectional area(160 Å2) and molecular size (18 Å).

0, CX50-500 and CX500-500.

Table 3Porous characteristics of the prepared carbon xerogels.

Samples SBET(m2/g)

VP

(cm3/g)rP(Å)

V0.1o

(cm3/g)Vmeso

(cm3/g)V0.1o /VP

(%)Vαmeso/VP

(%)

RFX-50 194 0.325 28.3 0.043 0.282 13.3 86.7CX50-500 87 0.108 24.9 0.032 0.076 29.6 70.4CX200-500 363 0.433 23.9 0.144 0.289 33.2 66.8CX500-500 441 0.538 24.4 0.173 0.365 33.1 66.9CX1000-500 613 0.742 24.2 0.230 0.512 31.0 69.0CX50-600 453 0.305 13.3 0.179 0.123 57.4 42.6CX200-600 439 0.312 14.2 0.198 0.114 63.4 36.6CX500-600 653 0.417 12.8 0.263 0.154 63.1 36.9CX1000-600 687 0.897 26.1 0.278 0.619 40.0 60.0CX50-700 174 0.093 10.7 0.065 0.028 69.9 30.1CX200-700 516 0.331 12.8 0.210 0.121 63.4 36.6CX500-700 661 0.832 25.2 0.271 0.561 50.1 49.9CX1000-700 735 0.451 12.3 0.308 0.143 47.1 52.9

173B.S. Girgis et al. / Desalination 265 (2011) 169–176

The adsorption capacities of carbon xerogels under investigationwere studied as adsorbents for these cationic dye molecules (MB andRB) and isotherms are illustrated in Figs. 3 and 4 and the derivedcharacteristics data are collected in Tables 4 and 5. Figs. 3 and 4 showan interesting trend, where the first points fall on, or approach the qe-axis indicating high affinity carbons (H-type isotherms). Suchphenomenon is confirmed by the very low RL values (b 1.0), and thehigh negative values of ΔG (see Tables 4 and 5).

Current knowledge about the fundamental factors that influencethe adsorption process from the aqueous phase were postulated to bethe characteristics of the adsorbent and the adsorptive, the solution

Fig. 3. Adsorption isotherms of MB dye molecules onto the prepared carbon xerogels.

Fig. 4. Adsorption isotherms of RB dye molecules onto the prepared carbon xerogels.

chemistry and the adsorption temperature. Keeping the last twovariables as constant (solution chemistry and the temperature) itremains that the properties of both the adsorbent and adsorptivebecome the major factors controlling the adsorption process. Thecharacteristics of carbon that affect the adsorption process are the poretexture, surface chemistry and mineral matter content. In the case ofcarbon xerogels, the slurry pH range between 6.2 and 7.3 (i.e., withinthe neutral range) and meanwhile they are free of any mineral matter(only insignificant amounts from the original basic catalyst). The major

Table 4Adsorption parameters of methylene blue onto carbon xerogels.

Sample Qo

(mg/g)KL

(L/g)RL ΔG

(kJ/mol)SMB

(m2/g)SMB/SBET(%)

SD(μmol/m2)

RFX-50 33 0.211 0.009 −28.0 75 39 0.531CX50-500 26 0.180 0.053 −27.6 59 68 0.937CX200-500 46 0.122 0.077 −26.6 104 29 0.397CX500-500 57 0.071 0.123 −25.2 129 29 0.405CX1000-500 80 0.073 0.121 −25.3 181 30 0.372CX50-600 55 0.156 0.156 −24.6 124 27 0.380CX200-600 65 0.127 0.127 −25.2 147 33 0.465CX500-600 114 0.076 0.076 −26.6 257 39 0.547CX1000-600 154 0.133 0.133 −25.0 347 50 0.703CX50-700 45 0.056 0.152 −24.6 102 59 0.810CX200-700 70 0.054 0.156 −24.5 159 31 0.424CX500-700 147 0.051 0.164 −24.4 332 50 0.697CX1000-700 222 0.033 0.233 −23.3 502 68 0.947

Table 5Adsorption parameters of Rhodamine B onto carbon xerogels.

Sample Qo

(mg/g)KL

(L/g)RL ΔG

(kJ/mol)SRB(m2/g)

SRB/SBET(%)

SD(μmol/m2)

RFX-50 42 0.077 0.025 −26.5 84 43 0.454CX50-500 30 0.030 0.250 −24.1 60 69 0.724CX200-500 84 0.045 0.182 −25.1 169 46 0.482CX500-500 100 0.047 0.175 −25.2 201 45 0.474CX1000-500 117 0.055 0.154 −25.6 235 38 0.398CX50-600 83 0.039 0.204 −24.7 166 36 0.382CX200-600 89 0.046 0.178 −25.2 178 40 0.424CX500-600 118 0.084 0.106 −26.7 263 40 0.377CX1000-600 128 0.058 0.147 −25.7 256 37 0.389CX50-700 69 0.042 0.192 −24.9 138 79 0.828CX200-700 111 0.051 0.146 −25.4 222 43 0.450CX500-700 120 0.076 0.116 −26.4 240 36 0.485CX1000-700 160 0.049 0.169 −25.3 320 43 0.602

Fig. 5. Correlation diagram between Qo of MB (mg/g): (a) BET-surface area (m2/g) and(b) micropore volume (cm3/g) of the obtained carbon xerogels.

174 B.S. Girgis et al. / Desalination 265 (2011) 169–176

factor, thus, would be the porous parameters (total surface area, totalpore volume, and mean pore dimensions). The adsorption capacity willthen depend on extent of internal porosity as well as pore entrancedimensions. The surface chemistry of activated carbons essentiallydepends on their heteroatom contents, mainly on their surface oxygencomplex content. These determine the surface charges (positive ornegative), which results from the dissociation of surface oxygencomplexes of acid or basic character. In addition, surface oxygencomplexes also affect the surface hydrophobicity, where in general anincrease in their oxygen content brings about a decrease in the carbonhydrophobicity. Now, solvent molecules (e.g., water) are preferablybound to the surface O-complexes thus reducing the accessibility of thehydrophobic chains of organic compounds.

Among the characteristics of the adsorptive that influence theadsorption process are themolecular size, solubility, pKa and nature ofthe substituent if they are aromatic. The molecular size controls theaccessibility to the pores of the carbon and the solubility determinesthe hydrophobic interactions. The pKa controls the dissociation of theadsorptive if it is an electrolyte. The substituent on the aromatic ringcan withdraw or release electrons from it, which would affect thedispersion interactions between the aromatic ring of the adsorbateand the surface of the adsorbent.

3.3.1. Adsorption of methylene blue from aqueous solutionThe RF-xerogels carbonized at 500, 600, or 700 °C show progres-

sive increase in adsorption capacity for MB as function of theprecursor R/C ratios. Increasing the R/C ratio promotes the uptake ofMB, alternatively, raising the pyrolysis temperature develops bettercarbon adsorbents for the specific R/C values (except for the CX50-700sample). It means that increasing either R/C or HTT enhances the dyeuptake capacity by the prepared carbon xerogels. Accordingly, thebest adsorbent appears to be carbon CX1000-700 with adsorptioncapacity of 222 mg/g, which is comparable to good adsorbingactivated carbons. It is noticeable that this adsorbent is derived atthe highest R/C (1000, which corresponds to lowest catalyst content)and highest HTT of 700 °C.

Taking into consideration the adsorbate (MB) cross-sectional areaof 120 Å2, surface areas covered by MB (SMB) were evaluated, as wellas their ratio to SBET surface area (SMB/SBET) are shown as well inTable 4. Irregular fractions of area available to the adsorption of MBare observed (e.g. 0.68–0.31 at 500 °C, 0.27–0.51 at 600 °C and 0.64–0.57 at 700 °C). Meanwhile, raising the R/C ratio from 50, 200, 500 and1000 is also associated by an irregular change in accessible fraction toMB, especially at R/C of 50 and 200. At decreased content of catalyst,R/C=500 and 1000, a regular increased capacity of adsorption isobserved: at R/C=500, it shows 29, 39 and 50% with HTT, and 30, 50and 68% also with HTT. Thus, raising either R/C or HTT enhances thecapacity or the accessibility of MB into the internal structure of thecarbon xerogels. Around 1/3 up to 2/3 of the total surface area is

available to the uptake of MB, the highest being obtained at thehighest synthesis parameters; R/C or HTT.

Upon plotting adsorbed amounts of MB (Qo, in mg/g) as function ofeither BET-surface area (SBET, m2/g) or micropore volume (Vo

0.1, cm3/g),exhibits an apparently good relationship (Fig. 5a and b). An increase ofeither parameter promotes the adsorption capacity, though not as alinear function. All points of the Qo–SBET relation appear to fall on asmooth asymptotic curve, whereas the Qo–Vo

0.1 exhibit apparently samecurve but is not similarly good as some values show up or downdeviation. Up to 600 m2/g, the uptake of MB appears a slow function ofsurface area and beyond this area the dye uptake is greatly acceleratedand enhanced. Porosity generated at 600 and 700 °C seems to becomposed of wide microporosity accessible to the dye molecules,particularly xerogels developed at R/C ratios of 500 and 1000. Suchspecific conditions appear the most appropriate for deriving ofgood adsorbents for treatment of MB-contaminated water streams(or molecularly similar other dyes).

3.3.2. Adsorption of Rhodamine B from aqueous solutionRaising the HTT is generally associated with promoted uptake of

the larger dye molecule RB to attain its optimum adsorption at thehighest R/C and HTT of 700 °C. However, the changes are mostlysmall at R/C ratios of 50 up to 500; the considerable increase appearsat the highest R/C of 1000. Thus, the maximum uptake is attainedat the highest conditions of xerogel synthesis, i.e. HTT=700 °C andR/C=1000, which amount to 160 mg/g of RB.

Converting the amounts uptaken into corresponding accessiblesurface area, the values in columns 5 and 6 in (Table 5) are obtained.This appears as 60 up to 320 mg/g, corresponding to fractions of 36 upto 79% of the total surface area (SRB/SBET) determined by the N2-

175B.S. Girgis et al. / Desalination 265 (2011) 169–176

adsorption. Nevertheless, excluding two high fractions for twosamples, CX50-500 and CX50-700, the rest vary between 36 and46% (average 41%) for the ten carbon xerogels. An interesting linearrelation between the uptake amounts (Qo of RB, mg/g) and either SBETor Vo

0.1, inferring that the dye uptake is a reasonable function ofavailable porosity as shown in Fig. 6 (a and b). Thus, although this dyeis larger than MB, its uptake is directly related to internal porosity.

3.3.3. Comparative uptake of MB and RB by carbon xerogelsFig. 7 (a and b) displays a comparative correlation between Qo

(inmg/g or inmmol/g) for the uptake ofMB and RB as function of SBET-areas. It appears that the uptake of RB (in mg/g) seems to beappreciably higher than MB, and tends to a limiting value beyond500 m2/g (Fig. 7a). On the contrary, MB uptake is much lower than RBup to 700 m2/g and tends to be asymptotic beyond this area (Fig. 7a).Since the texture characteristics of the adsorbents are the same forboth adsorptives, then the adsorbate properties would become thedetermining factor in this case. Upon normalizing the monolayercapacities of the two molecules (in mg/g) to their correspondingmolecular weights to become mmol/g, a new relation emerges(Fig. 7b) in which the uptake of MB and RB appears to be almostlinear and identical at and below 500 m2/g, whereas adsorption ofMB becomes appreciably higher beyond this limit. As the amount inmmol/g corresponds to the number of adsorbed molecules, it thusbecomes normal that the xerogel internal porositywould be accessibleto more molecules of the smaller size (MB) than the relatively bulkierones. The texture properties here seem to be of lower effect on the

Fig. 6. Correlation diagram betweenQo of RB (inmg/g) and (a) BET-surface areas (m2/g)and (b) micropore volume (cm3/g) of the obtained carbon xerogels.

Fig. 7. Correlation diagram between Qo of MB and RB (in mg/g or mmol/g) and BET-surface areas (m2/g) of the obtained carbon xerogels.

extent of uptake of both dyes as the pore size of the adsorbent seems tobe equally accessible to the dyemolecules. This could be observed fromTables 4 and 5 as the average value of accessible areas in comparisonto the BET-surface areas fall within 40 and 50%. Few lower and/orhigher values are out of range for both dyes, and the surface packing(μmol/m2) per unit of surface area of MB on the total carbon gels ishigher by around 18% than the corresponding RB molecules. This goeswell with the corresponding molecular dimensions or cross-sectionalarea (Fig. 8).

It is noticeable that the associated free energies of adsorption forMB are −23.3 to −28.0 kJ/mol and −24.1 to −26.7 kJ/mol for RB.These indicate that the adsorption of both dyes on the carbon xerogelsis a spontaneous (−ve) and of physical nature. The calculated valuesof Langmuir separation factors, RL, for the uptake of MB molecules onthe hereby developed carbon xerogels, are found to be in the range of0.033 to 0.233 and 0.025–0.250 for RB adsorption. These valuesconfirm that the adsorption of both dyes was highly favorable onto

Fig. 8. Correlation diagram between surface density of MB and RB (μmol/m2).

176 B.S. Girgis et al. / Desalination 265 (2011) 169–176

the prepared RF-porous carbons under the conditions prescribed inthis study.

4. Conclusions

The sol-gel synthesis process of RF-resins (aquagels), followed bysubcritical dehydration, resulted in the well-known organic xerogels.Subsequent pyrolysis at 500–700 °C, in absence of any flow of inertgases, developed satisfactory adsorbent carbon xerogels in good yield.Preparation of the primary sols of resorcinol-formaldehyde involveda change in the basic catalyst (Na2CO3) molar ratio in a decreasingorder in relation to R. Raising the R/C ratio (50, 200, 500, and 1000)resulted in considerable development of internal porosity (surfacearea and pore volume), mostly in the micropore size range. Highlydeveloped porous carbons are obtained by carbonization of theprimary xerogels at high temperatures of 600 or 700 °C. Good ad-sorbing carbon xerogelswere formedwith appreciable capacity for theuptake of two bulky cationic dyes, methylene blue and Rhodamine B,attaining values comparable to conventional activated carbons.Around 1/3 up to more than 1/2 of the total internal area is thusavailable to the uptake of MB, the highest being obtained at theirhighest values of R/C=1000 or highest HTT=700 °C. Therefore, theraising of either HTT, or R/C molar ratio, favors the pore developmentwith generation of relatively wide micropores suitable for uptake ofthe bulky dye molecules. On the other hand, a low to moderateadsorption capacity (b200 mg/g) is observed for the adsorption of RBdye molecules on the prepared carbon xerogels at different HTTs withvarying R/C (50 to 1000).

Overall, the produced carbon xerogels that was subjected to noactivation treatment, proved to be prospective good adsorbingcarbons in the remediation processes involving large solute organicspecies from the aqueous medium, especially in medical applicationsas well as in biological fluids separations.

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