Removal of Cationic and Ionic Dyes on IndustrialMunicipal Sludge

smithsonite (ZnCO3) are revealed. Indeed, new components formed have their origin in the addition of silica (coming from sewage sludge) and iron and z...
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Ind. Eng. Chem. Res. 2007, 46, 1786-1793

Removal of Cationic and Ionic Dyes on Industrial-Municipal Sludge Based Composite Adsorbents Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 138th Street and ConVent AVe, New York, New York 10031

The new class of reactive adsorbents obtained by pyrolysis of individual and mixed industrial sludges were used as media for the removal of cationic (Basic Fuchsin) and anionic (Acid Red 1) dyes. Materials were characterized using adsorption of nitrogen, thermal analysis (TA), potentiometric titration, X-ray diffraction (XRD), and scanning eletron microscopy (SEM). Kinetic measurements showed a fast decolorization process. Dye adsorption isotherms were fitted to the Langmuir-Freundlich model isotherm. The limiting capacities for Acid Red were between 35 and 73 mg/g, whereas the capacities for Basic Fuchsin removal ranged between 70 and 127 mg/g. While the Acid Red removal capacity was comparable to that on commercial activated carbon, the sludge-derived materials adsorb more Basic Fuchsin than carbon. The high efficiency of adsorption for both cationic and ionic dyes was linked to surface chemical heterogeneity and a high volume of mesopores. The diversity in surface chemistry, which leads to ion exchange processes, is a result of the presence of minerals, which are formed during pyrolysis. Introduction Dye wastewaters discharged from textile and dyestuff industries have to be treated due to their negative environmental impact.1 To remove dyes from aqueous solutions, various materials have been used including clays,2 activated carbons,3,4 chitosan and chitin,5,6 and cellulose.7 Recently, growing interest has been shown in application of adsorbents derived from waste products. Examples are sewage sludge based adsorbents8,9 or metal hydroxide sludge.10 Carbonaceous adsorbents recently tested to remove dyes are those obtained by pyrolysis of sewage sludge.8,9 Besides a carbon phase,11,12 they contain inorganic oxides/hydroxides,13,14 which might be important for a specific adsorption process. Such adsorbents contain a high volume of mesopores which was indicated as a factor governing the adsorption capacity for either cationic or anionic dyes. Those dyes were found to be adsorbed on the surface of sewage sludge based adsorbents in large quantity in spite of the opposite charge of the surface as a function of pH.9 On the other hand, the high adsorption capacity of metal hydroxide sludge for dye removal was attributed to either formation of metal dye complexes on the surface or precipitation of those complexes, depending on the system pH.10 The objective of this paper is to evaluate the applicability of new composite adsorbents derived from industrial sludges. The materials obtained besides high volume of large mesopores also have appealing surface chemistry, which was expected to play a role in dye adsorption via formation of dye-surface complexes and ion exchange processes. The performance of new materials is linked to their surface features, which are the results of complex solid-state reactions occurring during pyrolysis. This leads to the formation of unique minerals dispersed on the surface in the large pores.

Island Water Pollution Control Plant, (SS) with a 50:50 ratio based on the wet mass, homogenized, dried at 120 °C for 48 h, and then carbonized at 950 °C in a nitrogen atmosphere in a fixed bed (horizontal furnace). The heating rate was 10 °C/min with holding time of 1/2 h. The same treatment was applied for single components of the mixture, sewage sludge and waste oil sludge, separately. To compare the changes in the sample chemistry and porosity imposed by temperature, the waste oil sludge and sewage sludge mixture was also carbonized at 650 °C maintaining the same carbonization conditions as those at 950 °C. The materials after pyrolysis at 950 °C are referred to as SS, WO, and SSWO. The SSWO sample pyrolyzed at 650 °C is referred to as SSWO650. To properly evaluate the performance of the materials obtained, the adsorption experiments addressed below were also carried out on wood based carbon, WVA-1100, manufactured by Westvaco (WVA). The prepared materials were studied as adsorbents of dyes form aqueous solution. After adsorption of Acid Red 1 and Basic Fuchsin (Pararosaniline), they are designated by adding additional letters, -AR or -BF, to their names, respectively. Moreover, the samples exposed only to water adsorption have a -H2O added to their names. Acid Red 1 (AR) and Basic Fuchsin (BF) were selected as adsorbates. Since they differ either in molecular size (roughly estimated sizes based on the size of the bezene molecule and assuming a flat arrangement of atoms are 30 Å × 15 Å × 4 Å and 20 Å × 20 Å × 4 Å for AR and BF, respectively) or chemistry, the adsorption selectivity of sludge based composite adsorbents in terms of porous structure and surface chemistry can be evaluated.

Experimental Materials. Industrial oil sludge (WO) from Newport News Shipyard was mixed with dewatered sewage sludge from Wards * To whom correspondence should be addressed. Tel.: (212) 6506017. Fax: (212) 650-6107. E-mail: [email protected]. 10.1021/ie0610997 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007

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Adsorption of Dyes from Aqueous Solution. Kinetic studies were conducted in a temperature controlled bath shaker using 30 mL of dyes solution (concentration 500 mg/L) and a fixed adsorbent dosage of 0.1 g. Samples at different time intervals (0-25 h) were taken and filtrated. The concentration of the samples was analyzed using a spectrophotometer (512 nm for AR and 550 nm for BF). Equilibrium studies were conducted in a series of 100 mL Erlenmeyer flasks at 293 K. Each flask containing between 0.050 and 0.100 g was filled with 10 mL of dye solution with concentrations between 10-1000 mg/L. After equilibration for 72 h (based on the kinetic curves, it was judged that 72 h was a sufficiently long time to reach equilibrium for all samples), the samples were filtrated and analyzed for their dyes content and the equilibrium adsorption capacity was calculated. To check if the basic pH of the samples does not have an effect on the precipitation of dyes in the solution and thus on concentration measurements, the solutions of AR and BF of 20 mg/g were prepared. Their pH was adjusted to 10.7 by adding NaOH. Then, the concentration of the dyes was measured again using spectrophotometer. Less than 5% error on the dye concentration indicated that this method could be used to evaluate the amount adsorbed needed to plot and fit adsorption isotherms. The experiments were done without pH control, but for all experimental points, the resulting pH was between 9 and 11 at which we consider each dye to be at the same ionization form. The pKa of R-NH3+ groups of Basic Fuchsin based on the aniline molecule should be lass than 7. On the other hand, phenolic and >NH2+ are expected to dissociate under our experimental conditions. Although the ionization forms are important for the adsorption process, we expect cations or anions of chosen dyes to play a crucial role in the adsorption process. The equilibrium data was fitted to the so-called LangmuirFreundlich single solute isotherm:15

qe (KCe)n ϑt ) ) qo 1 + (KC )n

(1)

e

where ϑt(ce) is the fractional coverage of the adsorbent surface, qe is the absolute adsorbed amount of solute per unit gram of adsorbent, qo is the monolayer-adsorption capacity, which we consider as a limiting adsorption capacity, ce is the solute equilibrium concentration, K is the Langmuir-type equilibrium constant, and the exponential term n is the heterogeneity parameter of the site energies. The fitting range was for the equilibrium concentration from 0 to 400 mg/L for AR and from 0 to 13 mg/L for BF. The Langmuir-Freundlich (LF) equation was chosen as appropriate to estimate the values of the monolayer capacity. Textural Characterization. Nitrogen adsorption-desorption isotherms were measured using an ASAP 2010 analyzer (Micromeritics) at -196 °C. Before the experiments, the samples were degassed at 120 °C to a constant pressure of 10-5 torr under a vacuum. The isotherms were used to calculate the specific surface areas (SBET), micropore volumes (Vmic), mesopores volume (Vmes), total pore volumes (Vt), and pore size distributions (PSD). The latter was determined using density functional theory (DFT).16,17 For determination of the volume of the micropores, the Dubinin-Radushkevich method was applied.18 The relative microporosity was calculated as a ratio of the micropore volume to the total pore volume. Thermal Analysis. Thermal analysis was carried out using a TA Instruments thermal analyzer. The instrument settings were

as follows: heating rate 10 °C/min in a nitrogen atmosphere at a 100 mL/min flow rate. An average sample size was about 30 mg. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator. The instrument was set in the equilibrium mode when the pH was collected. Approximately 0.1000 g samples were placed in a container thermostated at 25 °C with 50 mL of 0.01 M NaNO3 and equilibrated overnight. To eliminate any interference by dissolved CO2, the suspension was continuously saturated with N2. The adsorbent suspension was stirred throughout the measurement. Each sample was titrated with 0.1 M NaOH titrant for wood based carbon (WVA-1100) and 0.1 M HCl titrant for sludge-derived materials using 0.001 mL increments. Experiments were carried out in the pH range 3-10.19,20 The experimental data was transformed into a proton binding isotherm, Q, representing the total amount of protonated sites. pH of Water Suspension. A 0.4 g sample of dry adsorbent powder was added to 20 mL of distilled water or dye solution (c ) 20 mg/g), and the suspension was stirred overnight to reach equilibrium. Then, the pH of the suspension was measured using a Accumet Basic pH meter. Fourier Transform Infrared (FTIR). Infrared spectroscopic measurements were done on a Varian 7000 FT-IR spectrophotometer using the attenuated total reflectance method (ATR). The spectrum was collected 35 times and corrected for the background noise. The experiments were done on the powdered samples, without KBr addition. SEM. Scanning electron microscopy images were obtained at Zeiss-LEO using a LEO 1550 FESEM. The as-received samples were mounted using silver support. Results and Discussion The kinetics of dye adsorption are presented in Figure 1. For both adsorbates and the adsorbents chosen in this study, the adsorption process is very fast, and after 24 h, equilibrium is practically reached for both dyes. The kinetic studies also indicate the differences in the performance of adsorbents. Based on the kinetic experiments, the isotherms on the sludge derived materials and WVA carbon were measured, and they are shown in Figure 2. The differences in adsorption uptakes of both dyes studied result from complex interactions of various factors such as dye solubility, surface chemistry, molecular weight, ion exchange capacity, the specific interactions with active sites on adsorbents, and the possible sieving effects. Although the shapes of the isotherms may be related to the mechanism of adsorption via Giles classification,21 we do not analyze them since the significant differences in the concentration range for AR and BF exist as a result of the differences in the adsorption capacity for both dyes. Based on the experimental points, the limiting capacities for AR and BF on sewage-derived materials are between 45 and 71 and 70 and 123 mg/g, respectively. The fitting parameters for the Langmuir-Freundlich model (Table 1) indicate the complexity of the adsorption process. The adsorption capacity of BF at equilibrium is about twice that of AR. For BF also, the heterogeneity parameter is much smaller than that for AR, which suggests that adsorption sites of broad energy range take part in the removal process. Comparing the capacity of adsorbents for both dyes, it is clearly seen that mixing waste oil and sewage sludge results in a significant enhancement in the capacity compared to the individual components. The high temperature of pyrolysis leads

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Figure 2. Adsorption isotherms for Acid Red (A) and Basic Fuchsin (B). The solid lines indicate the goodness of the fit to the LF equation. Figure 1. Kinetic curves for Acid Red (A) and Basic Fuchsin (B) adsorption on the surface of sludge-derived materials.

to an increase in the amount adsorbed for both dyes; however, the effect on n, representing the energetic heterogeneity of adsorption sites, is negligible. With an increase in the amount adsorbed, an increase in the n parameter is noticed, which indicates that even though the surface becomes energetically more uniform more centers, on which the adsorption is enhanced, are formed. All sludge-derived adsorbents have higher adsorption capacity than commercial, wood based, activated carbon. The differences in the amount adsorbed described above must be related to differences in the porosity and surface chemistry of adsorbents. Shapes of the nitrogen adsorption isotherms indicate a predominantly mesoporous structure with some contribution of micropores. Hysteresis loops show the complexity of pore shapes. On the basis of the nitrogen uptake, mixing two sludges results in a significant increase in the amount adsorbed, suggesting the synergetic effect during pyrolysis. The high temperature of treatment seems to be beneficial for porosity development since the amount adsorbed on SSWO is much higher than that on SSWO650. The structural parameters presented in Table 2 show that sludge-derived materials have surface areas 5-10 times smaller that that for WVA activated carbon. Although the volumes of micropores of the sludge-derived adsorbent are about 10 times smaller than that for the carbon, only a 2-fold difference in the volume of mesopores exists. As expected based on the shapes

Table 1. Fitting Parameters of Dyes’ Adsorption Isotherms to the Langmuir-Freundlich Equation n

R2

Acid Red 1 0.02 0.08 0.28 0.08 0.01

0.51 0.78 0.99 0.74 0.76

0.9879 0.9970 0.9866 0.9698 0.9967

Basic Fuchsin 0.28 0.33 0.48 0.33 0.29

2.76 1.55 1.69 1.76 2.34

0.9984 0.9925 0.9964 0.9901 0.9964

sample

qo [mg dye/g]

SS WO SSWO SSWO650 WVA

34.7 56.1 72.8 68.4 71.4

SS WO SSWO SSWO650 WVA

70.4 94.2 126.9 105.9 78.40

K [l/mg]

of isotherms, mixing sludges and heating at a high temperature results in a 100% increase in the volume of micropores, compared to the adsorbents derived from individual sludges. The details about the porous structure are seen in pore size distributions (PSDs) presented in Figure 3. WVA carbon was chosen owing to its broad pore size distribution and the high volume of mesopores. Nevertheless, in comparison with our sludge-derived adsorbents, it has a smaller volume of pores larger than 500 Å (macropores). Mixing sludges results in an increase in the volume of both micropores (less than 20 Å in diameter) and mesopores larger than 100 Å. This development of porosity can be important for the adsorption of large dye molecules. The complexity of the porous structure is seen on the examples of SEM images for the SS sample (Figure 4).

Ind. Eng. Chem. Res., Vol. 46, No. 6, 2007 1789 Table 2. Structural Parameters sample

SBET (m2/g)

Vmic (cm3/g)

Vmes (cm3/g)

Vt (cm3/g)

Vmic/Vt

SS SS-H2O SS-AR SS-BF WO WO-H2O WO-AR WO-BF SSWO SSWO-H2O SSWO-AR SSWO-BF SSWO650 SSWO650-H2O SSWO650-AR SSWO650-BF WVA WVA-H2O WVA-AR WVA-BF

103 100 48 57 128 109 97 68 192 174 128 93 108 199 195 135 1843 1822 1707 1558

0.043 0.041 0.021 0.022 0.047 0.040 0.036 0.027 0.077 0.068 0.048 0.035 0.043 0.077 0.072 0.052 0.674 0.662 0.624 0.561

0.100 0.095 0.086 0.087 0.363 0.390 0.364 0.303 0.288 0.301 0.270 0.280 0.317 0.253 0.228 0.220 0.570 0.570 0.530 0.510

0.143 0.136 0.107 0.109 0.414 0.431 0.407 0.326 0.356 0.369 0.318 0.315 0.356 0.332 0.299 0.265 1.243 1.225 1.154 1.065

0.301 0.302 0.196 0.202 0.114 0.093 0.089 0.083 0.216 0.184 0.151 0.111 0.121 0.232 0.241 0.196 0.542 0.540 0.541 0.527

Since the volume of pores in the sludge-derived materials is small in comparison to that for the WVA activated carbon, their high dye adsorption capacities should be linked to surface chemistry effects. It has to be mentioned here that, due to the complexity of the chemical composition of sludges, the effects of surface chemistry cannot be presented without any ambiguity and the mechanisms proposed here are judged as the most plausible at this stage of our study. All sludge-derived materials are basic with pH values between 9 and 11. In the case of sewage sludge based materials, the source of their basicity is calcium and magnesium.14 Table 3 presents the high amounts of metals in the adsorbents studied. It is interesting that exposing those samples to water causes only marginal (less than 100 ppm) leaching of metals, mainly calcium and magnesium. A decrease in the pH as a result of leaching of some hydroxides/salts is less than 1 pH unit. The pH of WVA-1100 carbon is slightly acidic (pH ) 5.5). This stability in the chemistry of the sludge-derived adsorbents is the result of high temperature pyrolysis, which leads to the formation of new mineral-like structures, which might be active in dye complexation and anion exchange processes. These mineral-like structures also contribute to “encapsulation” of metals which was proposed previously as a method of toxic waste neutralization.22,23 The X-ray diffraction results presented in detail elsewhere24,25 showed that in SS wurtzite (ZnS), ferroan (Ca2(Mg, Fe)5(SiAl)8O22(OH)2), chalcocite (Cu1.96S), spinel (MgAl2O4), and feroxyhite (FeO(OH)) are present, while in WO, besides metallic iron, bornite (Cu5FeS4), hibonite (CaAl12O19), zincite (ZnO), and ankerite (Ca(Fe, Mg)(CO3)2) were identified. In the case of SSWO, besides bornite, wurtzite, and spinel common for both of the single component adsorbents, new minerals such as sapphirine (Mg3.5Al9Si1.5O20), maghemite (Fe2O3), cohenite (Fe3C), lawsonite (CaAl2Si2O7(OH)2H2O), and smithsonite (ZnCO3) are revealed. Indeed, new components formed have their origin in the addition of silica (coming from sewage sludge) and iron and zinc from waste oil sludge. Heating the sewage-waste oil sludge mixture at 650 °C results in a formation of different chemistry than that at 950 °C. The SSWO650 sample is not highly mineralized (only a few peaks were revealed), and only anorthite (CaAl2Si2O8), diaspore (AlO(OH)), and metallic iron are detected. The presence of carbonates after high temperature heating is the result of carbon phase gasification and reaction of freshly formed CO2 with metal oxides.

Figure 3. Pore size distributions for the sludge-derived adsorbents (A) and WVA (B).

To “localize” the dye adsorption in the pore system, the nitrogen adsorption isotherms were measured on the exhausted samples on which high amounts of Acid Red and Basic Fuchsin were adsorbed (the amounts are listed in Table 2). As a result, an interesting phenomenon of an increase in the surface area after dye adsorption was noticed for the SSWO650 sample (Table 2). This was related to a significant increase in the volume of micropores. Even though the formation of new micropores as a result of leaching can contribute to that increase for the sample obtained at 650 °C, the more plausible explanation, which might work for all samples, seems to be hydration of the surface and formation of hydrated salts or hydroxides. To check this hypothesis, the dehydroxylated adsorbents were exposed to water adsorption under the same conditions as adsorption of dyes occurred. Then, after drying to a constant mass at 120 °C, the nitrogen adsorption isotherms were measured and structural parameters and PSDs were calculated. As seen from Table 2 for the materials heated at 950 °C, a very slight decrease in the volume of micropores is noticed after exposure to water. This likely happens due to blocking pore entrances by small amounts of water molecules bonded to the surface after the formation of hydroxides. On the other hand, for SSWO650, the surface area and volume of micropores increased almost 100%. This suggests the formation of new small pores in larger pore spaces where reactive oxides such as calcium or magnesium can form hydroxides or some metals can leach. Due to the low pyrolysis temperature, this material is not stable in water and the simultaneous processes mentioned above along with dye adsorption can occur. Moreover, new

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Figure 4. SEM images of the SS sample. Table 3. Metal Contents and the Results of pH Measurements sample SS WO WOSS650 WOSS WVA

Fe [%]

Ca [%]

Mg [%]

Cu [%]

Zn [%]

6.1 3.7 4.05 4.9

5.1 5.1 4.4 5.1

1.1 8.4 6.15 4.75

0.17 0.25 0.16 0.21

0.09 0.51 0.36 0.3

pHa

pHb

pHc

pHd

10.9 10.0 10.4 9.4 8.5

9.2 9.2 9.0 9.4 8.4

10.6 10.7 10.7 9.6 8.3

10.3 9.8 10.1 9.4 7.3

a Initial. b After exposure to water followed by drying. c After AR adsorption (initial pH of AR solution 7.9). d After BF adsorption (initial pH of BF solution 6.1)

hydrated species formed can enhance the complexation/ion exchange processes. The formation of new species/dissolution upon contact with water can be seen on the proton uptake curves obtained from potentiometric titration experiments (Figure 5). Here, the titration was done on the initial samples and those exposed to water adsorption and dried at 120 °C. On the basis of the proton uptake, the mixtures of two sludges are the most basic materials and the trend in the amount of dyes adsorbed follows the differences in the amount of basic sites. The majority of the surface species has conjugate acids with pKa in a very acidic range. While the sewage sludge derived adsorbent is not sensitive to any acid-base reactions (smooth curve), in the case of waste oil sludge containing materials, and especially for its mixture with sewage sludge, the reaction with acid occurs in the range between 4 and 5 where salts are formed and released to the solution (dissolution). For the waste oil sludge based adsorbent, WO, a rapid increase in the proton uptake between pH 5-6 must be related to chemical reaction with the oxides

present in the system. That step in the uptake is much less pronounced for SSWO, indicating that new chemistry, much resistant to reaction with acid, is formed. In the case of SSWO650, a continuous uptake of protons occurs, which is related to chemical instability of this material/the surface affinity toward reaction with acid. When the adsorbents are exposed to water adsorption, as expected, the chemistry of the surface changes. It become more basic (with higher proton uptake) and the reaction with acid occurs even in the basic range at pH 7-8 for SSWO-H2O. Generally speaking, on the basis of the shapes of the proton uptake curves, the materials obtained should be chemically stable in the pH range where dyes adsorption occurs under our experimental conditions, which was determined to be between 9 and 11. As seen from Table 2 adsorption of both dyes decreases the surface area and the volume of micropores. The effect is much more pronounced for BF than for AR, which is in agreement with the differences in their amounts adsorbed. Since the dye molecules are too big to enter the micropores, the observed decrease is caused by blocking pore entrances for nitrogen by the adsorbed dye molecules. In Table 4, the specific amounts of dyes adsorbed on the samples used for porosity studies are listed in miligram per gram and in milimole per gram. The latter values were used to calculate the numbers of molecules adsorbed. Having this quantity, and the estimated surface area occupied by each dye molecule and assuming either perpendicular or parallel location on the surface the hypothetical areas covered by adsorbed molecules were calculated. As seen from Table 3 and 4, the areas obtained assuming parallel orientation on the surface are much greater than those calculated from nitrogen adsorption, which indicates that this mechanism of

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Figure 5. Proton uptake curves for materials as-received and exposed to water adsorption followed by drying. Q > 10 represents proton uptake; Q < 10 represents proton release. Table 4. Amount of Dyes Adsorbed on the Specific Adsorbent Samples Used for Porosity Study (Table 2) and the Hypothetical Surface Area Occupied by Dye Molecules Assuming Their Parallel and Perpendicular Orientation on the Surface hypothetical surface [m2/g] sample

ql [mg dye/g]

SS WO SSWO SSWO650 WVA

26.5 51.0 72.9 65.0 46.4

SS WO SSWO SSWO650 WVA

70.0 84.4 120.0 96.0 75.6

ql [mmol dye/g]

parallel

perpendicular

Acid Red 0.052 0.100 0.143 0.128 0.091

141 271 387 345 246

19 36 52 46 33

Basic Fuchsin 0.216 0.261 0.371 0.297 0.234

520 629 893 715 563

104 126 179 143 113

adsorption is not possible. When the perpendicular orientation is chosen, the hypothetical surface occupied by dye molecules is much smaller especially for Acid Red where numbers close to or smaller than the decrease in the surface area measured after dye adsorption are found. In the case of BF, the decreases in the hypothetical area are still about twice as big as those measured. The reason for this may be in overestimation of the surface occupied by the dye molecule (estimated size) and/or

in formation of aggregates. The definitely smaller than observed decreases in the surface areas in the case of Acid Red may indicate some contribution of parallel orientation of the molecules. This orientation can lead to micropore blocking. In fact, one molecule of Acid Red can interact with two active sites, when geometry allows. This may happen when a molecule is located flat on the surface. AR is an anionic dye, which has a tendency for dimerization.4,26 Those dimers can be strongly adsorbed even in relatively small mesopores due to the linear shape of the molecules. Moreover, the dye dissociates in water and its anions can be adsorbed via anion exchange processes. All our materials are basic and release OH- in aqueous solution, which makes chemisorption of AR anions favorable. Since the differences in the amount adsorbed does not directly follow the differences in samples’ pH values (small variations), the adsorption process must be complex depending on the texture of adsorbents. Low adsorption on SS can be easily explained by its lowest volume of mesopores. This happens in spite of the fact that this material contains ferroan (Ca2(Mg, Fe)5(SiAl)8O22(OH)2) and feroxyhite (FeO(OH)) which might contribute to anion exchange. On the other hand, the low porosity argument does not apply for the WO sample. A plausible explanation for its low capacity is the absence of minerals capable of exchanging anions. Mixing the two sludges results in the formation of two new minerals, sapphirine (Mg3.5Al9Si1.5O20) and lawsonite (CaAl2Si2O7(OH)2H2O),24,25 and the latter might contribute to an increase in the anion exchange capacity. Although SSWO650 does not have a pore volume higher than that in WO, on its surface diaspore (AlO(OH)) is formed, and it likely contributes to an increased adsorption. In this sample, we have also a high content of oxides (calcium and magnesium), which are converted to hydroxides when in contact with water. Since the amount adsorbed is high compared to the available surface area where anions exchange can occur, the dispersive interactions of dyes with the adsorbent surfaces must also occur, and here, the high volume of pores becomes crucial. The mechanism of BF adsorption must be different than that of AR owing to the difference in the chemical nature of the dyes. BF is a cationic dye, and its large cation must find the energetically favorable sites on the surface or precipitate with an anion. If the surface chemistry were active in the process, the perpendicular orientation suggested from the hypothetical surface occupied by adsorbed molecules (Table 4) would be the most efficient one from the point of view of the removal capacity. This is important since, opposite to AR, in BF the charge of the ion is only +1. Thus, adsorption likely occurs via cation exchange with the minerals, mainly aluminosilicates present in our materials. In SS, ferroan (Ca2(Mg, Fe)5(SiAl)8O22(OH)2) and spinel (MgAl2O4) are important while in WO hibonite (CaAl12O19) and ankerite (Ca(Fe, Mg)(CO3)2) might be active. After mixing and carbonization at 950 °C, sapphirine (Mg3.5Al9Si1.5O20) and lawsonite (CaAl2Si2O7(OH)2H2O) appear. On the other hand, in WO650, anorthite (CaAl2Si2O8) is present. Generally speaking, the main mechanism of chemisorption can be summarized in the form of following equations: (a) for AR

MnXm / nMm+(solid) + mXn-(aq) Dye - SO3Na / Dye - SO3-(aq) + Na+(aq)

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nMm +(solid) + mDye - SO3-(aq) / Mn - (O3S - Dye(s))m or

M(OH)n / M(OH)n-1+(solid) + OH-(aq) Dye - SO3Na / Dye - SO3-(aq) + Na+(aq) M(OH)n-1+(solid) + Dye - SO3-(aq) / M - (OH)n-1O3S - Dye(s) (b) for BF

XS(OH)n / Xn+(aq) + S(OH)n-(solid) Dye - NH2Cl / Dye - NH2+(aq) + Cl-(aq) S(OH)-(solid) + Dye - NH2+(aq) / Dye - NH2OH - S Recalculation of the capacity evaluated from the LF equation for the number of moles adsorbed per gram and the unit surface area of the materials reveals interesting results (Table 4). The amount of moles of BF adsorbed on the surface exceeds the amount of AR by 2-3 times (around 0.1 mmol/g compared to 0.3 mmol/g), which is related to the sizes of molecules, their charge, and orientation on the surface. When the amount adsorbed per unit surface area is compared, all sludge-derived adsorbents obtained at the same condition (950 °C) exhibit similarity in the amount adsorbed in each dye category (about 0.8 × 10-3 mmol/m2 for AR and about 2 × 10-3 mmol/m2 for BF). This suggests that besides the surface chemistry, which causes the differences in the amounts adsorbed between the two dyes, the textural features’ effect is important. Normalized adsorption of both dyes is much higher on SSWO650 than that on the sample pyrolyzed at 950 °C, likely as a result of chemical instability/reactivity of the surface of the former sample. The presence of dyes adsorbed and the differences in the chemistries of the materials are seen on DTG curves (Figure 6). For comparison, the DTG curves for pure dyes are also

Figure 6. Comparison of DTG curves for as-received adsorbents, exposed to water and dye adsorption.

Figure 7. FTIR spectra for composite samples.

included. The peaks represent the weight loss as a result of evaporation of compounds such as water or thermal decomposition of adsorbents or dyes during pyrolysis. The samples were dried under the same conditions to avoid differences in the water content. The dyes decompose in a broad temperature range up to 800 °C. The first peak seen on all samples at temperatures less than 200 °C must represent the dehydration of mineral structures present in the sludge-derived adsorbents. It is especially well pronounced for SSWO650, which is the most reactive toward water. This reactivity is seen as a broad peak for the water-exposed sample between 200 and 600 °C, which is linked to the dehydroxylation of hydroxides formed after reactions with water. After adsorption of dyes, the thermal behavior of this sample is similar to that one after only water adsorption, which is likely the result of its chemical heterogeneity and competition between water and dyes for adsorption sites. As from potentiometric titrations, the effect of water on surface chemistry is also seen on the WO sample. For the SS series of samples, the effects of BF present on the surface are only seen as small decomposition peaks. The effects of AR adsorption are not distinguishable due to the limitations in the sensitivity of the thermal analysis (TA) method. A large peak representing BF decomposition on the WO sample indicates that water does not compete for adsorption sites but creates new sites for BF removal. On the other hand, that competition is seen for AR adsorption on this sample (less intense peaks than these after only adsorption of water). AR and BF peaks seem to be comparable on the SSWO sample; however, the amounts adsorbed differ.

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Support for the changes in the chemistry and the mechanism described above are the FTIR studies of the surface of samples before and after adsorption of dyes presented in Figure 7. As examples, the composite samples, SSWO, obtained at 650 and 950 °C were chosen. The peaks between 800 and 1200 cm-1 represent Si-O(Si) and Si-O(Al) vibration form tetrahedral or alumino- and silico-oxygen bridges in aluminosilicates.27 The presence of OH- is represented by a broad peak between 3200 and 3600 cm-1. Its intensity increases when samples are exposed to water adsorption. These vibrations are also more intense for the sample exposed to 650 °C than for its 950 °C-treated counterpart. High reactivity of the former sample toward water is also seen as an increase in the intensity of the vibrations between 1300 and 1700 cm-1. Adsorption of Acid Red results in new peaks at about 1210, 1250, 1470, 1520, 1570, and 1610 cm-1. The peaks at the region 1570-1600 and 1400-1525 cm-1 are characteristic of CdC aromatic skeletal vibrations.28 A band at 1508 cm-1 represents azo bond vibrations in the AR molecule.29 The peaks at 1210 and 1470 cm-1 represent SO3groups and aromatic rings of the dye, respectively.28 Compared to Acid Red, Basic Fuchsin adsorption results in much more intense peaks, which can be related to the differences in the amounts adsorbed between AR and BF. The peaks at about 1170, 1620, and between 3380 and 3420 cm-1 are related to the presence of nitrogen as sCsN, sNH2, amide, and NH vibrations, respectively.28 In the region between 1260 and 1340 cm-1, there are the peaks of strong C-N stretching vibrations of aromatic amines. The N-H stretching vibration of primary amines is observed at 1580-1650 cm-1.30 Conclusions The results presented in this paper show the excellent dye removal capacity of industrial sludge based adsorbents. The measured values are higher than those obtained on commercial micro/mesoporous activated carbons. The new adsorbents can effectively remove both cationic and anionic dyes. This happens owing to their surface chemical heterogeneity and the presence of minerals with the ability to ion exchange. Moreover, the sludge-derived adsorbents have a very high volume of mesopores (and macropores). Those mesopores are the ideal sites for adsorption of large molecules such as AR or BF. Acknowledgment We are grateful to Dr. David Frey of Zeiss for SEM images. The experimental help of Ms. Anna Kleyman is appreciated. The authors are grateful to Prof. Daniel Akins, Dionne Miller, and Shiunchin (Chris) Wang for their help in receiving FTIR spectra. Literature Cited (1) Wong, Y.; Yu, J. Laccase-catalyzed decoloration of synthetic dyes. Water Res. 1999, 33, 3512-3520. (2) Alkan. M.; Celikcapa, S.; Demirbas, O.; Dogan, M. Removal of reactive blue 221 and acid blue 62 anionic dyes from aqueous solutions by sepiolite. Dyes Pigm. 2005, 65, 251-259. (3) McKay, G.; Al-Duri, B.; Allen, S. J.; Thomson, A. Adsorption for liquid process effluents. Ich. E. Semp. Pap. 1998, 17, 1-17. (4) Walker, G. M.; Weatherley, L. R. Adsorption of dyes from aqueous solution-the effect of a adsorbents pore size distribution and dye aggregation. Chem. Eng. J. 2001, 83, 201-206. (5) Juang, R. S.; Tseng, R. L.; Wy, F. C.; Lee, S. H. Adsorption behavior of reactive dyes from aqueous solutions on chitosan. J. Chem. Technol. Biotechnol. 1997, 70, 1169-80. (6) Longhinotti, E.; Pozza, F.; Furlan, L.; de Nazzare de, M.; Sanches, M.; Klung, M.; Laranjeira, M. C. M.; Favere V. T. Adsorption on anionic dyes on the biopolymer chitin. J. Braz. Chem. Soc. 1998, 9, 435-440.

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ReceiVed for reView August 19, 2006 ReVised manuscript receiVed January 13, 2007 Accepted January 22, 2007 IE0610997