Effect of OH and silanol groups in the removal of dyes from aqueous solution using diatomite

ARTICLE IN PRESS

Water Research 39 (2005) 922–932 www.elsevier.com/locate/watres

Effect of OH and silanol groups in the removal of dyes from aqueous solution using diatomite M.A.M. Khraisheha,, M.A. Al-Ghoutib, S.J. Allenb, M.N. Ahmadb a

Department of Civil and Environmental Engineering, University College London, Gower Street, London WC1E 6BT, UK b School of Chemical Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK Received 29 April 2004; received in revised form 15 July 2004; accepted 21 December 2004

Abstract The removal of methylene blue, reactive black (C-NN), and reactive yellow (MI-2RN) from aqueous solution by calcined and raw diatomite at 980 1C was studied. These studies demonstrated the importance of the various functional groups on the mechanism of adsorption. The role of pore size distribution in the dye adsorption studies was also investigated. The adsorption isotherms were pH dependent. Henry and Freundlich adsorption isotherms were used to model the adsorption behavior and experimental results for all dyes used exhibited heterogeneous surface binding. The removal of the ionisable functional groups increased the pHZPC value from 5.4 to 7.7, while FTIR, SEM and XRD analysis showed a remarkable decrease of the characteristic Si–OH peaks after calcinations at 980 1C. The removal of hydroxyl groups from the surface of diatomite lead to a decrease in the adsorption. It was evident from pH and infrared spectra results that mechanisms of methylene blue and reactive yellow adsorption differed from that of reactive black. Accordingly, adsorption on the external surface by n–p interaction between the p system of the RB and the electron lone pairs of the oxygen atoms of siloxane group and columbic attraction between the dye and the surface of calcined diatomite was proposed as a possible adsorption mechanism. r 2005 Elsevier Ltd. All rights reserved. Keywords: Diatomite; Calcined diatomite; Adsorption; Reactive and basic dyes; Methylene blue; Textile wastewater

1. Introduction Colour removal from textile effluents has been the subject of great attention in the last few years. The presence of low concentrations of dyes in effluent streams is highly visible and undesirable, reducing light penetration and potentially inhibiting photosynthesis. Colour removal due to the water-soluble reactive dyes is Corresponding author. Tel.: +44 20 7679 7994;

fax: +44 20 7380 0986. E-mail address: [email protected] (M.A.M. Khraisheh).

problematic: current methods relying on activated sludge systems are not adequate, neither on site nor after dilution with domestic wastewater at sewage works. They are a very important class of textile dyes, whose losses through processing are particularly significant due to their high hydrolysing tendency. Thus, in the case of cellulose fibers dyed with these dyes, nearly 50% may be lost to the effluent (Laszlo, 1996). The adsorption process is widely used in wastewater treatments. Adsorption of dye from textile effluents has been investigated using a variety of materials such as peat, activated carbon, natural zeolite, and fly ash (AlQodah, 2000; Meshko et al., 2001). Diatomite is

0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.12.008

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

regarded as a mineral of organic origin, where the silica of fossilised diatom skeleton resembles opal or hydrous silica in composition (SiO2
H2O). The silica surface contains silanol groups that spread over the matrix of silica (Al-Ghouti et al., 2003). The silanol group is an active one which tends to react with many polar organic compounds and various functional groups. Diatomite has shown its effectiveness for the removal of heavy metals from wastewater (Al-Degs et al., 2001), but its use for the removal of reactive dyes has not been attempted and no reference is mentioned in literature. A preliminary study by the authors established the feasibility of using diatomite as a low-cost material for treating textile wastewater containing reactive dyes (AlGhouti et al., 2003). The studies also demonstrated the importance of the various functional groups, available on the surface of diatomite, on the mechanism of adsorption. Diatomite surface is terminated by OH groups and oxygen bridges (Table 1), which act as adsorption sites. In adsorption processes, it is important to know the characteristics of these different adsorption sites. Hence, in an effort to establish the importance of these groups, the diatomite was calcined to impair the surface of such groups and gain a better understanding of their role in the adsorption process. OH groups, Table 1, act as centres for adsorption through forming hydrogen bonds with the adsorbate (Zhuravlev, 2000) and could be divided into: (i) isolated free silanol (–SiOH), (ii) geminal free silanol (–Si(OH)2), and (iii) vicinal or bridged or OH groups bound through the hydrogen bond (Muster et al., 2001). In addition, diatomite consists of siloxane groups or –Si–O–Si– bridges with oxygen atoms on the surface. Zhuravlev (2000) showed that the hydrogen bonded water molecules were removed at room temperature in vacuum or

923

at 150 1C in the atmosphere in an amorphous silica sample. In a study of the surface concentration of different types of OH groups, it was found that the surface concentration of these groups on the amorphous silica surface decreased at elevation of temperature (between 200 and 1200 1C) to a zero at 1200 1C. In this study, thermal treatment of diatomite was attempted and the effect on dye removal from solution was investigated. Factors such as pH and initial dye concentration were tested. The adsorption capacity and adsorption mechanisms of basic and reactive dyes were investigated and compared with raw diatomite. Moreover, the effect of silanol groups on the adsorption process was evaluated. Different analytical methods such as Fourier transform infrared were used to identify the structure of calcined diatomite and to elucidate the proper mechanism of dye adsorption. pHZPC, acidity, and basicity were also carried out.

2. Materials and methods Diatomite samples were obtained from borehole BT36, depth 34–36 m in Al-Azraq region in East Jordan. Thermal treatment of diatomite was carried out by placing the diatomite in a crucible in the furnace at 980 1C for 4 h. The sample was then allowed to cool in a desiccator. The product was sieved to different particle sizes and o106–500 mm was used. The samples were kept in air tight plastic bags for further use. Methylene blue (MB), a basic dye, Cibacron reactive black (RB) and reactive yellow (RY) dyes were used; a summary of the main characteristics of these dyes is given in Table 2. The standard stock solutions of the dyes were prepared

Table 1 Infrared absorbance peak assignment for O–H stretching at the silica surface Peak (cm1)

Assignment

Silanol groups

3745710

Isolated Geminal

Si–OH Si-OH

OH Si OH 371575

366075 35207200

Weakly interacting vicinal

Internal silanol Strongly interacting vicinal

H

H O

O

Si

Si

—

H

H

H O

O

O

Si

Si

Si

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

924

Table 2 Main characteristics of the dyes used in this study Dye

MB

RB

RY

Type

Basic dye C.I.52015

Phase lmax, nm e, dm3 g1 cm1 Chemical structure

Solid 663 170.1

Cibacron reactive black C–NN Liquid, 33%wt 597 22.6 Unkonwn

Cibacron reactive golden yellow MI-2RN Liquid, 33%wt 430 23.0 Unknown

C16H18N3S+Cl

by appropriate dilution with deionised water to a final concentration of 1000 mg/dm3. To represent real textile effluent conditions, the reactive dyes were hydrolysed using the method described by Laszlo (Laszlo, 1996). The adsorption isotherm experiments were carried out in 60 cm3 glass bottle, where 0.05 g of calcined diatomite and 50 cm3 of the appropriate concentration of the test dye solution were added. Concentration between 100 and 1000 mg/dm3, pH 2–11, and a particle range o106–500 mm were employed. Preliminary investigations showed that equilibrium was attained in 48 h. After this period, samples were filtered through a 0.45 mm cellulose nitrate membrane filter (Swinnex-25 Millipore). In the case of MB, the samples were allowed to stand for several hours to settle the calcined diatomite particles since MB can easily be absorbed on the membrane. All experiments were replicated and blanks were used. The final dye concentrations were determined using a Perkin-Elmer UV–Vis spectrophotometer corresponding to lmax of each dye as shown in Table 2. Surface charge density, acidity, and basicity, pHZPC and Fourier transform infrared (FTIR-Perkin-Elmer Spectrophotometer RX I) of calcined diatomite were studied. Adsorbent samples were removed from the dye solution after equilibration and freed from the water by drying at 65 1C in preparation for the FTIR analysis. The spectra of the solid were then recorded. FTIRPerkin-Elmer Spectrophotometer RXI was used in all investigations. Scanning electron microscope of calcined diatomite were carried out using JEOL-JSM 6400 scanning microscope. In order to identify the structure and determine the compositions of adsorbents, XRD analysis was carried out using Siemens Difraktometer D ( 40 kV, 40 mA) 5000, Cu Ka1 radiation (l ¼ 1:5406 A; and the sample was scanned from 31 to 551 (2y) in step sizes of 0.041. 2.1. Surface area and pore size analysis The surface area of the samples was obtained by using the Brunauer, Emmett and Teller (BET) method by

assuming the section area of nitrogen molecule to be 0.162 nm2. Nitrogen adsorption at 77 K is a standard and widely used method for determining surface area, pore volume and pore size distribution the adsorbent (Altin et al., 1999). Nitrogen adsorption isotherms of the adsorbents were determined at 77 K using nitrogen adsorption apparatus, quantachrome instruments NOVA e-series. The weight of the samples was around 0.4 g. All the samples were outgassed under vacuum at 100 1C for 24 h before nitrogen adsorption in a vacuum system at about 1 atm. The aims of outgassing are: (i) to reach a well-defined intermediate state by the removal of physisorbed molecules and (ii) to avoid any drastic changes as a result of ageing or surface modification (Sing, 2001).

3. Results and discussion 3.1. Surface characteristics When diatomite is heated at high temperature, the active groups on the diatomite surface (–OH groups) are removed and the surface acquires stronger hydrophobic properties as suggested in Scheme 1. In order to investigate the surface characteristics of raw and calcined diatomite, FTIR analyses were carried out in the range 400–4000 cm1. The infrared spectra, shown in Fig. 1, indicated that the OH groups were removed from the surface after calcinations as the intensity of their absorption, in the high-frequency region, was decreased. The main absorption bands for raw diatomite, as depicted in Fig. 1, were found at 3690, 3614, 1088, 1026, 910, 786, and 714 cm1. The bands at 3690 and 3614 are due to the free silanol group (SiO–H) and the band at 1620 cm1 represents H–O–H bending vibration of water. The bands at 1088 and 1026 reflect the siloxane (–Si–O–Si–) group stretching and the 910 cm1 band corresponds to Si–O stretching of silanol group. Seven hundred and eighty-six and 714 cm1 bands represent SiO–H vibration. The absorption peaks around 532 and

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932 OH

O OH

Si O O Diatomite surface

925

1000oC -H2O

Si O

Si O

Si O

O O

O

Scheme 1. Representation of the hydrophobic characteristics of calcined diatomite at 980 1C.

100 90 a 80 70 910 3690 1620 60 3614 50 40 b 30 1088 1026 20 10 0 4000 3600 3200 2800 2400 2000 1600 1200 800 400

Transmittance

714 786

Wavelength (cm-1)

Fig. 1. Infrared spectra of (a) raw and (b) calcined diatomite at 980 1C.

466 cm1 are attributed to the Si–O–Si bending vibration (Rytwo et al., 2002; Hayakawa and Hench, 2000). All Si–OH peaks originally present on the surface of diatomite remarkably decreased in intensity upon calcination (as can be seen in Fig. 1(b)). Hydroxyl groups are either isolated or H-bounded on the surface of diatomite. Consequently, the surface is also predominantly covered by weakly adsorbed water in the cavity and water bounded to the surface hydroxyl groups via H-bonds. They appear as a broad band with middle wavelength at 3400–3500 cm1. By driving off adsorbed water, upon heating the diatomite at temperatures of 980 1C, the broad band faded gradually and all other significant peaks disappeared. The disappearance of the OH groups, which was originally found on the diatomite surface will, no doubt, have a detrimental effect on the adsorption from solution. Garcia-Santamaria et al. (2001) studied the properties of calcined colloidal amorphous silica as a function of calcination temperatures. This study confirmed that the most ionisable hydroxyl groups were removed during calcination. As a result, it is expected that these changes may have a significant effect on dye removal, which is dependent on not only the pore structure of diatomite but also on the surface functional groups. The effect of the removal of these groups on the dye adsorption will be considered in this investigation. The alkalimetric titration method was used to estimate the surface charge densities for the calcined diatomite (Stumm and Morgan, 1995; Chen and Lin, 2001). The results are depicted in Fig. 2. The

Fig. 2. pHZPC and surface charge density of raw and calcined diatomite at different pHs. Experimental conditions: particle size ¼ 106–250 mm, mass of adsorbent ¼ 0.1000 g, volume of solution ¼ 25.0 cm3, equilibrium time ¼ 24 h and shaking speed ¼ 125 rpm.

intersection of the curve with x-axis, at s equals zero, gives the zero point of charge (pHZPC), where the total charge from the cations and anions at the adsorbent surface equals zero. Close inspection of the results show that pHZPC values of raw and calcined diatomite were 5.4 and 7.7, respectively. It is obvious that the value of pHZPC of calcined diatomite was higher than that of the raw sample, which resulted from the removal of the ionisable functional groups from the surface of diatomite upon treatment. A constant value of surface charge density was observed between pH 6–10 and 6–8 for raw and calcined diatomite, respectively. Furthermore, the maximum positive surface charge (s+) of raw and calcined diatomite was obtained at pHo4 and the maximum negative charge (s) was at pH49. 3.1.1. XRD and SEM analysis XRD patterns of the raw and calcined diatomite are shown in Fig. 3. The figure shows that the main peaks in the diatomite sample correspond to quartz, smectite, illite, kalonite, halite, and small amounts of cristobalite. It is clear that the X-ray pattern of the raw diatomite is different from the pattern of the calcined diatomite. It is also noted that the amount of hematite and cristobalite was remarkably increased, while smectite–kalonite, halite, and illite completely disappeared as the diatomite was calcined at 980 1C. Therefore, the X-ray patterns show that the amorphous structure of the diatomite was changed drastically. Indeed, some peaks in the diatomite disappeared and some peaks were created upon calcinations. Similar behaviour was noted by Arik (2003). Scanning electron microscopy (SEM) has been a primary tool for characterising the fundamental physical properties of the adsorbent. It is useful for determining the particle shape and appropriate size distribution of the adsorbent. Scanning electron micrographs of

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

counts

926 800 700 600 500 400 300 200 100 0

(A)

Q

S-K Cr

S-K

I

0

10

SQ Cr I

H H

20

S He

30

H&I

40

Q

50

60

350 Q

300 counts

250 He

200 Cr

150

Cr

Fig. 4. SEM of calcined diatomite.

He &Cr

100

Q

He

50

He

0 10

20

30 2θ

40

50

60

Fig. 3. X-ray patterns of: (A) raw and (B) calcined diatomite (Q: quartz, S: smectite, Cr: a-cristobalite, I: illite, S–K: Smectite–kalonite, H: NaCl halite, He: hematite Fe2O3).

calcined diatomite is shown in Fig. 4. It can be seen that the solid structure of diatom becomes more noticeable after calcined at 980 1C. Diatomite frustules are divided into two main categories: centric (discoid) and pinnate (elongated) (Al-Degs et al., 2001). From Fig. 4, calcination diatomite has notable pores as discs or as cylindrical shapes. Thus, there is a good possibility for dyes to be adsorbed into these pores. Garcia-Santamaria et al. (2001) performed the SEM analysis of colloidal amorphous silica at 750 and 950 1C as calcinations temperatures. It was shown that the sphere size of the colloidal amorphous silica shrunk due to the internal chemical changes and volume reduction as demonstrated by SEM observation.

Dye removal (%)

0

100 90 80 70 60 50 40 30 20 10 0

MB+ diatomite

0

2

4

(a)

Dye removal (%)

(B)

100 90 80 70 60 50 40 30 20 10 0

RB+diatomite

0

2

4

(b)

The percentage removal of MB, RB, and RY from solution using raw and calcined diatomite was studied in the pH range 2–12. As shown in Fig. 5(a), the percentage MB removal by calcined diatomite dropped from 33% to 2% with decreasing pH of dye solution from 12.0 to 2.0. The maximum removal occurred at basic pH (10–12). Furthermore, the removal rate decreased with the decrease in pH which may be due to excess H+ ions competing with the dye cation for the adsorption sites. In addition, as the positive surface charge density decreases with an increase in the pH (pH4pHZPC) of the solution, the electrostatic repulsion between positively charged dye (MB+) and the surface of the calcined diatomite was reduced resulting in more adsorption. The

Dye removal (%)

3.2. Effect of calcinations on the dye removal ability of diatomite

100 90 80 70 60 50 40 30 20 10 0

(c)

RY+diatomite

0

2

4

MB+calcined diatomite

6 pH

8

10

12

RB+calcined diatomite

6 pH

8

10

12

RY+calcined diatomite

6 pH

8

10

12

Fig. 5. pH dependency of (a) MB, (b) RB, and (c) RY dyes onto raw and calcined diatomite. Experimental conditions: mass of adsorbent ¼ 0.05 g, volume of dye solution ¼ 50 dm3, equilibrium time ¼ 48 h, shaking speed ¼ 125 rpm, particle size o106 mm for calcined diatomite and 106–250 mm for diatomite, temperature ¼ 22 1C, initial MB concentration ¼ 200 mg/dm3, and initial RB and RY dye concentrations ¼ 100 mg/dm3.

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

percentage removal of RY by calcined diatomite (Fig. 5(c)), decreased with further increase in pH above pH 4 and the maximum percentage of removal was in acidic media (pH 2–3), while the percentage of RB (Fig. 5(b)), removal was nearly constant over the pH range. In general, the percentage RB and RY removal was decreased from 24% to 14% and 22% to 1%, respectively with increasing pH of dye solution from 2.0 to 11.0 when calcined diatomite was used as the adsorption media. The percentage of MB, RB, and RY removal from solution remained constant by raw and calcined diatomite in the pH range from 4 to 10 with better removal by diatomite. Fig. 6 shows that the behaviour of adsorption processes of MB, RB and RY onto the calcined diatomite is linear and consequently, the Langmuir adsorption isotherm was not successfully applied. Langmuir isotherm was not utilized to evaluate the results, since the obtained adsorption isotherms did not present the typical Langmuirian form. The experimental evidence indicates that an isotherm plateau was not reached. The isotherms exhibited the Freundlich behaviour, R240.97, which indicates a heterogeneous surface binding (Robinson et al., 2002; Kibe et al., 2000). The experimental data was fitted to the Freundlich and Henry’s models, Eqs. (1) and (2), respectively. Langmuir isotherm failed to describe the data and will not be discussed in this case (Al-Ghouti et al., 2003). qe ¼ K F C 1=n e ;

(1)

qe ¼ kH C e ;

(2) 3

where kH is the Henry constant (dm /g), KF is the Freundlich isotherm constant (mg/g(mg/dm3)n) and it is roughly an indicator of the adsorption capacity and n refers to adsorption tendency. Linerised plot of log qe vs. log Ce is obtained from the model and KF and 1/n can be determined from the slope and intercept (Chiou and Li, 2002), and the results were not a typical Langmuirian type (Fig. 6). The experimental results of Freundlich and Henry constants are shown in Table 3. Freundlich adsorption isotherm equation predicts that the dye concentrations on the adsorbent will increase so long as there is an increase in the dye concentration in the aqueous solution. More-

927

over, from Table 3, the value of n is nearly equal 1, in which case the Freundlich equation reduces to the Henry equation (Eq. (2)). The Freundlich model does not predict the adsorption capacity of adsorbent surface for specific adsorbate, but the KF value can be taken as a relative indicator of the adsorption capacity for a narrow sub-region having equally distributed energy sites toward specific adsorbate (Gemeay, 2002). However, the values of qe were predicted by introducing corresponding values of KF and 1/n as well as the initial dye concentration (Co ¼ 100 mg/dm3). Therefore, it is clear from the experimental results that the adsorption capacity corresponding to the initial equilibrium concentration was increased in order: RB4MB4RY. The (1/n) value from Freundlich equation indicates that the relative distribution of energy sites and depends on the nature and strength of the adsorption process. For example, the value of (1/n) of adsorption MB onto diatomite surface is 0.75, in fact this value refers to 75% of the active sites that have equal energy where adsorption take place. Moreover, the closer of the n value to 1, indicate homogenous surface. In the case of RB and RY, the adsorption capacity of RB was higher than RY. This is mainly due to functional groups and polarity of the dye. The results reported in Table 4 indicate that the dye adsorption on the silanol sites does not exceed 75% of the total MB uptake. As a result, the silanol groups play are an important factor in the MB adsorption, and to lesser extend in reactive dye adsorption. Surface functional groups in addition to the pore structure of the adsorbent play together to achieve a higher adsorption capacity. The percentage of micropores and mesopores in the calcined diatomite are 0% and 100%, respectively. This indicates a total absence of micropores in the calcined diatomite structure, which is supported by the results given in Table 4. The differences in the adsorption capacities reflect the large microporous surface area of the diatomite. Ko et al. (2003) studied the adsorption of basic (Basic Blue 69) and acid dye (Acid Red 114) onto activated carbon, FS-400, and bagasse pith. It was noticed that the differences in the adsorption capacities of these dyes onto activated carbon and bagasse pith reflected the large microporous surface area of activated carbon (Fig. 6).

Table 3 Henry and Freundlich parameters of adsorption isotherms of MB, RB, and RY onto calcined diatomite Dye

KF (mg/g)

1/n

Equation

R2

Capacitya (mg/g)

kH (dm3/g)

MB RB RY

0.20 0.41 0.40

1.042 0.980 0.855

log qe ¼ 0.700+1.042 log Ce log qe ¼ 0.387+0.980 log Ce log qe ¼ 0.398+0.855 log Ce

0.97 0.97 0.98

19.42 28.35 16.99

0.26 0.36 0.17

a

Adsorption capacities correspond to the initial equilibrium concentration at Co ¼ 100 mg/dm3.

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

928

Table 4 Adsorption capacities of MB, RB, and RY onto raw and calcined diatomite correspond to the initial equilibrium concentration at Co ¼ 100 mg/dm3 from the Freundlich equation Dye

Adsorption capacity (mg/g)

MB RB RY

Raw

Calcined

81.09 25.78 20.12

19.42 28.35 16.99

300

qe (mg/g)

P 1 c1 P ¼ þ ; V n ðPo  PÞ V m c V m c Po

MB RB RY

250 200 150 100 50 0 0

100

200

300

400

500

600

700

800

900

Ce (dm3/g)

Volume (cm3/g)

Fig. 6. Adsorption isotherms of MB, RB, and RY onto calcined diatomite. Experimental variables: mass of adsorbent ¼ 0.0500 g, volume of solution ¼ 50 cm3, equilibrium time ¼ 48 h, shaking speed ¼ 125 rpm, pH of MB, RB, RY dyes solution ¼ 11, 3, 3, respectively.

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.00

hysteresis loop of the desorption branch runs parallel to the adsorption curve as shown in the figure. This could be explained in terms of the swelling of the particles, which accompanies adsorption. The swelling distorts the structure, for example by prising apart weak junctions between primary particles and opens up cavities which were previously inaccessible to adsorbate molecules. Since the distortion is not perfectly elastic, some molecules become trapped and can escape only very slowly, or possibly not at all, during the desorption run (Sing, 2001). To estimate the surface area of the adsorbents, the BET equation was applied to P/Po ranges of the N2 isotherms:

Calcined diatomite Adsorption Desorption

(3)

where Vn is the volume of nitrogen adsorbed at pressure P, Po is the standard vapour pressure of the liquid at the temperature of experiment, Vm is the volume equivalent to an adsorbed monolayer and c is the BET constant which is related to molar energy of adsorption in the first monolayer. For the BET equation, the range of linearity is usually restricted to the P/Po values between 0.05 and 0.35 (Gomez-Serrano et al., 2001). For the adsorbents, however, these P/Po ranges are narrower (0.05–0.25). A plot of P=V n ðPo  PÞ vs. P/Po over the approximate range 0.05pP/Pop0.35 yields a straight line from which the monolayer capacity Vm can be evaluated. From the obtained Vm value, the specific surface area SBET can be calculated by using the cross-sectional area of nitrogen of 0.162 nm2. Surface area was calculated from the slope and y-intercept of the linear region of the BET transformation vs. relative pressure plot. The specific surface area, SBET, is then calculated from Vm by SBET ¼ V m na am =mV L ;

(4) 23

0.20

0.40 0.60 0.80 Relative pressure (P/Po)

1.00

Fig. 7. Nitrogen adsorption isotherms at 77 K of calcined diatomite.

3.3. Surface area and pore size analysis Nitrogen adsorption–desorption isotherms were recorded for calcined diatomite and the graphs are shown in Fig. 7. The uncommon Type III isotherm, which is usually associated with water vapour adsorption was observed for calcined diatomite as well as low-pressure hysteresis. The adsorbent–adsorbate interactions, in this type, are weak as compared with the adsorbate–adsorbate interactions (Allen et al., 1988). The shoulder of the

where na is Avogadro number (6.023  10 molecule/ mol), am is the cross section area occupied by each nitrogen molecule (0.162 nm2), m is weight of the sample and VL is the molar volume of nitrogen gas (22,414 cm3) (Sing et al., 1985; Conner et al., 1986). The results of surface area and monolayer capacity of the adsorbents estimated from BET method are summarised in Table 5. The c value of the BET equation is related to the enthalpy of adsorption in the first adsorbed layer, i.e., an indication of the magnitude of the adsorbent–adsorbate interaction energy (Chiang et al., 2001). A high value of the parameter c is an indication of strong adsorbent–adsorbate interaction. The higher c value was obtained for diatomite may be related to its microporosity. Table 5 illustrates that the adsorption capacity of the calcined diatomite is expected to be weak which is anticipated from the lower N2 monolayer adsorption capacity values, Vm, compared to

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

929

Table 5 Surface area and monolayer capacity raw and calcined diatomite Adsorbent

P/Po range of linearity

Surface areaa (SBET) (m2/g)

Vmb (cm3/g)

cb

Diatomite Calcined diatomite

0.056–0.25 0.05–0.021

54.38 0.35

0.0156 1.0  104

159.5 95.1

a

Calculated from Eq. (4). Calculated from Eq. (3).

b

Table 6 Micropore volume, micropore area, mesopore volume, mesopore area, total pore volume, average pore radius, and percentage of micropores and mesopores of raw and calcined diatomite Adsorbent

Diatomite Calcined diatomite

Micropore volume (cm3/g)

Micropore Mesopore area (m2/g) volume (cm3/g)

Mesopore area Total pore (external volume surface area) (cm3/g) 2 (m /g)

Average pore radius (nm)

0.022 0

33.161 0

21.22 0.35

2.290 7.824

0.0403 0.0014

raw diatomite. Once the values of monolayer adsorption capacity are calculated from the BET equation, the BET surface area was estimated by multiplying the value of Vm by the molecular surface area of N2 (0.162 nm2 or 16.2  1020 m2) (Altin et al., 1999). The BET surface areas of the adsorbents are given in Table 5. The BET plots are linear for the P/Po region taken for calculation of the surface area and Vm. The structure of the nitrogen monolayer is sensitive to any change in the chemical nature of the surface (ElAkkad, 1979). It is clear that the surface area of the diatomite was remarkably decreased as the samples were calcined at 980 1C. It indicates that a significant structure changes took place. Table 6 also indicates that the average pore volume of the calcined diatomite is 3.5fold higher than that for the raw material. It is clear that the adsorbents not only showed marked differences in their porous structure but also in the extent of their surface area. Based on the N2 adsorption experimental data, a correlation may be expected between the surface area, porosity and performance in solution. Calcined diatomite samples were found to exhibit an upward deviation in the region of high relative pressure, which is interpreted as capillary condensation in super micropores and small mesopores or adsorption on the external surface (Dabrowski, 1999). It is clear, that mesopores are dominant of the total pore volume for diatomite and calcined diatomite. The contribution of macropores in adsorption is less than 1% of the total surface area. The mesopores have the most influence in the adsorption of organic solutes,

0.0623 0.0014

Content of micro–mesopores Micropores (%)

Mesopores (%)

35.3 0

64.7 100

which enables their surfaces to be accessible to solute molecules (D’Silva, 1998). Surface functional groups added to the surface of the diatomite, as a result of modification, changed the surface character of the adsorbent. As expected, a significant change in the surface properties and micropores of the diatomite was observed after thermal treatment at 980 1C. The micropore volume of the diatomite changed from 0.022 to 0 cm3/g. In the case of calcined diatomite, raising the temperature to 980 1C leads to loss in the total surface area. A decrease in the total pore volume was also observed following the calcination of the diatomite. This means the pore widening takes place as a result of wall burning between micropores, which leads to an increase in internal porosity and reductions in micropores associated with high surface area. The same behaviour was observed by Girgis et al. (2002).

3.4. Proposed adsorption mechanisms pH analysis showed a clear difference in adsorption behaviour of RB and RY onto calcined diatomite The differences in solubility, molecular size, number of clusters of (–SO 4 ), and reactivity of aromatic ring may be the main contributory factors. The difference could also be attributed to the n–p interaction between the p system of the RB and the electron lone pairs of the oxygen atoms of siloxane group on the surface of calcined diatomite. The n–p interaction is enhanced by parallel adsorption of molecular structure of RB to

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

investigated. Fig. 8 shows the infrared spectra of raw and spent calcined diatomite after adsorption isotherms of these dyes. The removal of hydroxyl groups from the surface of diatomite lead to a decrease in the adsorption and the surface acquires more and more hydrophobic properties. Two small intensity bands at 2871 and 2938 cm1 are observed. It is assigned to symmetric and asymmetric C–H stretching vibrations. Calcination process lead to the formation of surface siloxane groups or –Si–O–Si– bridges, which probably act as an active site for dye adsorption. The intensity of siloxane peak (–Si–O–Si–) at 1099 cm1 decreased when the dye was adsorbed onto calcined diatomite. Moreover, the intensity of siloxane peak of RY and MB is lower than that after RB adsorption. It could be concluded that the mechanism of adsorption of RY and MB onto calcined diatomite was slightly different from RB adsorption. As a result two mechanisms are proposed: (a) adsorption on the external surface by n–p interaction, and (b) adsorption by an electrostatic attraction between the dye and the surface of calcined diatomite which is in agreement with work reported by O¨zacar and S- engil (2003). Moreover, it is clear from Fig. 5 that the intensity of hydroxyl group at 3438 cm1 decreases after

oxygen atoms of calcined diatomite (Este¯ vez et al., 1995). In the case of MB adsorption at high pH, the siloxane bond in calcined diatomite is cleaved by nucleophilic reagents such as sodium hydroxide. In aqueous solution, cleavage becomes noticeable at pH49.0 (Unger, 1979). Consequently, a careful examination of the structure of calcined diatomite after interacting by (–OH), silanol groups would be formed. Scheme 2 shows the electrostatic attraction between the surface of calcined diatomite and the cationic and anionic dye. Furthermore, a hydrogen bond interaction should be taken into account as it may play a dominant role in the adsorption process. In MB adsorption onto calcined diatomite, a hydrogen bond interaction between the nitrogen atoms in MB molecules and the silanol groups on the calcined surface at basic solution may take place. As a result, the silanol groups might play a very significant role in methylene blue adsorption in basic solution. Furthermore, the acidity and basicity of raw and calcined diatomite were studied by applying Boehm’s method (Otowa et al., 1997). The acidity and basicity of diatomite were 0.48 and 0.42 mmol/g, respectively. As the diatomite sample were heated to 980 1C, the acidity and basicity of calcined sample were reduced to 0.23 and 0.035 mmol/g, respectively. These results could also explain the reduction of adsorption capacity of calcined diatomite. It is noticed that the basicity of calcined diatomite is low, indicating that the possibility of calcined diatomite to react with hydrogen ion to form product (a) (see Scheme 2) is low. As a result, a weak electrostatic attraction between positively charged calcined diatomite and the reactive dyes, in particular RY, were formed. FTIR technique is an interesting application for studying the interaction between an adsorbate and the active groups on the surface of adsorbent. Consequently, in order to get some insight into the nature of the mechanism of dye adsorption onto calcined diatomite, FTIR of raw and spent calcined diatomite was

80 70

H+ O

3900

1099

3400

2900

2400 1900 1400 Wavelength (cm-1)

Si

DyeSi

O

Si O

O

O

O

O

OH

OH

O

Si

OH

Dye+

O O

O

Dye- + O

-

O

Si O

O O

a

Si

0 400

+

O

-

900

10

DyeOH

OH

O

Si

30

Fig. 8. Infrared spectra of: (a) calcined diatomite and spent calcined diatomite with (b) MB, (c) RB, and (d) RY.

O

O

40

3438

Si

Si

50

20

+

Si

60

b c d

O

O

a

Transmittance (a.u.)

930

Si O

Si

O

O

O

b

O O

Scheme 2. Electrostatic attraction of adsorption of cationic (MB) and anionic dye (RB or RY) onto calcined diatomite.

ARTICLE IN PRESS M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

adsorption of MB and RY onto calcined diatomite while it remains unaffected after RB adsorption.

4. Conclusion Understanding the adsorption parameters such as pH and initial dye concentration are of importance in adsorption processes. Calcined diatomite gave inadequate adsorbing capacity for of basic and reactive dyes from aqueous solution. This is because most of hydroxyl groups found originally in raw diatomite surface were removed. Silanol groups were found to be the main functional group responsible for most of dye adsorption, in particular MB molecules. However, pores of the calcined diatomite also play an important role in dye adsorption. It was evident that pH and FTIR methods are good tools in studying the dye adsorption behaviour onto calcined diatomite. As a result two mechanisms might be proposed: (a) adsorption by n–p interaction, and (b) adsorption by columbic attraction between the dye and the surface of calcined diatomite. Furthermore, hydrogen bond may also responsible for MB adsorption. The modification processes of the diatomite changed the surface area, porosity, diffusion properties and accessibility to internal sites. As a result, it is affected their adsorption performance. A significant change in the surface properties and micropores of the diatomite was observed after thermal treatment at 980 1C. The micropore volume of the diatomite changed from 0.022 to 0 cm3/g. Raising the temperature to 980 1C leads to loss in the total surface area.

References Al-Degs, Y., Khraisheh, M.A.M., Tutunji, M.F., 2001. Sorption of lead ions on diatomite and manganese oxides modified diatomite. Water Res. 35 (15), 3724–3728. Al-Ghouti, M.A., Khraisheh, M.A.M., Allen, S.J., Ahmad, M.N., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. J. Environ. Manage. 69 (3), 229–238. Allen, S.J., Whitten, L.J., McKay, G., 1988. Characteristic of activated carbon: a review. Dew. Chem. Eng. Mineral Process 615, 231–261. Al-Qodah, Z., 2000. Adsorption of dyes using shale oil ash. Water Res. 34 (17), 4295–4303. Altin, O., O¨zbelge, H.O¨., Dogu, T., 1999. Effect of pH in an aqueous medium on the surface area, pore size distribution, density, and porosity of montmorillonite. J. Colloid Interface Sci. 217, 19–27. Arik, H., 2003. Synthesis of Si3N4 by the carbo-thermal reduction and nitridation of diatomite. J. Eur. Ceram. Soc. 23, 2005–2014.

931

Chen, J.P., Lin, M., 2001. Equilibrium and kinetic of metal ion adsorption onto a commercial H-type granular activated carbon: experimental and modelling studies. Water Res. 35 (10), 2385–2394. Chiang, Y.-C., Chiang, P.-C., Huang, C.-P., 2001. Effects of pore structure and temperature on VOC adsorption on activated carbon. Carbon 39, 523–534. Chiou, M.S., Li, H.Y., 2002. Equilibrium and kinetic modelling of adsorption of reactive dye on cross-linked chitosan beads. J. Hazardous Mater. 2852, 1–16. Conner, W.C., Cevallas-Candau, J.F., Weist, E.L., Pajares, J., Mendioroz, S., Corte´s, A., 1986. Characterization of pore structure: Porosimetry and sorption. Langmuir 2, 151–154. Dabrowski, A. (Ed.) 1999. Adsorption and its applications in industry and environmental protection. Applications in Industry, vol. II, first ed. Elsevier Science B.V., Amsterdam, p. 8. D’Silva, A.P., 1998. Adsorption of antioxidants by carbon blacks. Carbon 36 (9), 1317–1325. El-Akkad, T.M., 1979. Analysis of microporosity and specific vapour sorption by zirconia gel. J. Chem. Technol. Biotechnol. 29, 413–418. Este¯ vez, M.J.T., Arbeloa, F.L., Arbeloa, T.L., Arbeloa, I.L., 1995. Characterization of rhodamine 6G adsorbed onto hectorite by electronic spectroscopy. J. Colloid Interf. Sci. 171, 439–445. Garcia-Santamaria, F., Palacios, E., Migues, H., Ibisate, M., Meseguer, F., Lopez, C., 2001. Reactive index properties of calcined silica spheres. Instituto de Ciencia de Materiales de Madrid (CSIC), Spain, September 3–7. Gemeay, A.H., 2002. Adsorption characteristics and the kinetics of the cation exchange of Rhodamine-6G with Na+-Montmorillonite. J. Colloid Interface Sci. 251, 235–241. Girgis, B., Yunis, S.S., Soliman, A.M., 2002. Characteristics of activated carbon from peanut hulls in relation to conditions of preparation. Mater. Lett. 57, 164–172. Gomez-Serrano, V., Gonzalez-Garcıa, C.M., Gonzalez-Martın, M.L., 2001. Nitrogen adsorption isotherms on carbonaceous materials. Comparison of BET and Langmuir surface areas. Powder Technol. 116, 103–108. Hayakawa, S., Hench, L.L., 2000. AM1 study on infra-red spectra of silica clusters modified by fluorine. J. NonCrystalline Solids 262, 264–270. Kibe, K., Takahashi, M., Kameya, T., Urano, K., 2000. Adsorption equilibriums of principal herbicides on paddy soils in Japan. Sci. Total Environ. 263, 115–125. Ko, D.C.K., Tsang, D.H.K., Porter, J.F., McKay, G., 2003. Application of multipore for the mechanism identification during the adsorption of dye on activated carbon and bagasse pith. Langmuir 19, 722–730. Laszlo, J.A., 1996. Electrolyte effects on hydrolysed reactive dye binding to quateruize cellulose. Textile Chemist Colourist 27 (4), 25–27. Meshko, V., Markoska, L., Mincheva, M., Rodrigues, A.E., 2001. Adsorption of basic dyes on granular activated carbon and natural zeolite. Water Res. 35 (14), 3357–3366. Muster, T.H., Prestige, C.A., Hayes, R.A., 2001. Water adsorption kinetic and contact angles of silica particles. Colloids Surfaces A: Physicochem. Eng. Aspects 176, 253–266.

ARTICLE IN PRESS 932

M.A.M. Khraisheh et al. / Water Research 39 (2005) 922–932

Otowa, T., Nojima, Y., Miyazaki, T., 1997. Development of KOH activated high carbon and its application to water purification. Carbon 35 (9), 1315–1319. O¨zacar, M., S- engil, I.A., 2003. Adsorption of reactive dyes on calcined alunite from aqueous solutions. J. Hazardous Mater. 4004, 1–14. Robinson, T., Chandran, B., Nigam, P., 2002. Removal of dyes from a synthetic textile dye effluent by biosorption on apple pomace and wheat straw. Water Res. 36, 2824–2830. Rytwo, G., Tropp, D., Serban, C., 2002. Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculation. Appl. Clay Sci. 20, 273–282. Sing, K., 2001. The use of nitrogen adsorption for the characterisation of porous materials Review.

Colloids Surfaces A: Physicochem. Eng. Aspects 187–188, 3–9. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouque´rol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 57, 603–619. Stumm, W., Morgan, J.J., 1995. Aquatic Chemistry, third ed. Wiley, New York, NY, pp. 518–557. Unger, K.K., 1979. Porous Silica: its Properties and Use as Support in Column Liquid Chromatography. Elsevier Scientific Publishing Company, New York, p. 86. Zhuravlev, L.T., 2000. The surface chemistry of amorphous silica. Zhuravlev model. Colloids Surfaces A: Physicochem. Eng. Aspects 173, 1–38.