Evaluation of the Adsorption of Aquatic Humic Substances in Batch

Ind. Eng. Chem. Res. 2009, 48, 5445–5453

5445

Evaluation of the Adsorption of Aquatic Humic Substances in Batch and Column Experiments by Thermally Modified Activated Carbons ´ lvarez-Uriarte, Noemı´ Chimeno-Alanı´s, and Unai Iriarte-Velasco, Jon I. A Juan R. Gonza´lez-Velasco* Department of Chemical Engineering, Faculty of Science and Technology, UniVersity of The Basque Country, P.O. Box 644, E-48080, Bilbao, Spain

Two commercially available granular activated carbons (GACs) were thermally modified and evaluated for their adsorption properties in batch and column mode experiments. The widely used and well characterized aquatic humic substances were used as adsorbate. The aim was to relate the adsorption properties of GAC with their physicochemical properties and also evaluate the impact of heat treatment under nitrogen atmosphere on the performance of the adsorbent in both batch and column operation modes. Heat treatment had a qualitatively similar effect on surface basicity and pore volume of both adsorbents. However, in quantitative terms, significant differences were reported. Heat-treatment enhanced batch adsorption capacity only for GACA. Surface available in pores within 12-15 Å width was strongly correlated to the NOM adsorption capacity. Benefits of thermal treatment were more evident at high initial solute concentrations. Results from batch adsorption did not have a straightforward relationship with column adsorption performance. Column adsorption capacities at complete breakthrough reached 26% and 79% of batch mode capacities. The highest difference was observed for carbon GAC-A, which corresponds to the more mass transfer limited system. During column operation smaller fractions of NOM were more readily adsorbed compared to batch mode. 1. Introduction Aquatic natural organic matter is composed of a huge variety of compounds, such as humic acids, fulvic acids, proteins, carbohydrates, and carboxylic acids. These can cause color, odor, and heavy metal complexation and redisolution. However, as far as drinking water is concerned, it favors formation of disinfection byproducts (DBPs) and biological regrowth in the distribution system.1 Removal of natural organic matter (NOM) prior to disinfection was proven to be an effective method for reducing chlorinated DBPs.2 In this sense, the new European Directive stresses the need to ensure low DOC levels in potable water.3 Some fractions of NOM are barely removed from water using conventional coagulation-flocculation processes.4 These difficulties in removing sufficient NOM make it difficult to meet trihalomethane (THM) standards, and hence better alternative treatments are required. Activated carbons have been traditionally manufactured for the removal of small molecular weight hydrophobic synthetic organic contaminants. Adsorption by activated carbon is one of the best treatment technologies for the removal of NOM from water.5 It can be used either in powdered (PAC) or in granular (GAC) form, depending on whether it is used as a coagulation coadyuvant or in adsorption beds, respectively. From the ease of application point of view, the column adsorption procedure is preferred. However, low equilibrium uptake and slow adsorption kinetics of NOM by GAC have usually been a drawback. In this way, adequate chemical and physical properties of the adsorbent play major roles. Certain parameters that will affect the efficiency of an adsorption process include the particle size, surface area, and surface chemistry of the adsorbent, adsorbate concentration, pH, redox conditions, and other water quality parameters.6,7 While the effectiveness of activated carbon to act as adsorbent for a wide range of contaminants is well noted,8,9 more and * To whom correspondence should be addressed.

more research is gaining prominence due to the need to enable affinity for certain adsorbates. One of the most feasible ways to modify carbon properties is heat treatment under a given atmosphere. However, heat treatment is not always a welldefined effect on activated carbons. Adsorption of NOM strongly depends on the experimental setup, including carbon type, dosage, and operational conditions. Therefore, the impact of thermal treatment under nitrogen atmosphere was studied for both batch and column adsorption. Two commonly used activated carbons in the water treatment industry in Spain were studied. These have different origin, but similar characteristics, about 800 m2 g-1 specific surface area, and a priori, adequate values of pHPZC between 9.5-10.5. The main objective of this work is to present and discuss characterization and adsorption properties of these commercially widespread activated carbons in the virgin form and thermally modified. The first objective was to study in the batch mode the impact of heat treatment on the uptake of NOM and identify simple descriptors that facilitate the selection of adequate GAC. The second objective was to compare data obtained from batch and column experiments since parameters measured in batch experiments do not necessarily have to translate well into column adsorption experiments. 2. Experimental Section 2.1. Materials. Synthetic water was prepared with the characteristics shown in Table 1. Alkalinity and hardness of water were modified by addition of CaCl2 and NaHCO3 to ultrapure water. A mixture of a well-characterized, commercially available aquatic humic acid (Fluka Chemicals) and Fulvic acid (IHSS Suwannee river) was used as NOM. Two commercially available GACs were selected for this study: coconut shell-based GAC-A (Auxicarb) and coal-based GAC-F (Filtrasorb-400, Chemviron carbon). Samples were treated for 8 h at 800 °C under nitrogen flow (25 mL min-1) in

10.1021/ie900053p CCC: $40.75  2009 American Chemical Society Published on Web 05/08/2009

5446

Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009

Table 1. Physicochemical Properties of Synthetic Water parameter

value

pH DOC, mg L-1 UV254, cm-1 SUVA, L (mg m)-1 THMFPF, µgTHM mgDOC-1 SPTHMFP, µgTHM mgDOC-1 Ca2+, mg L-1 conductivity, µS cm-1 alkalinity, mgHCO-3 L-1

8.0 1; 5; 10; 20 0.067; 0.333; 0.691; 1.361a 6.74 110.1 36.7 41.1 472 125.2

a

1:2 dilution required for UV measurement.

a tubular quartz reactor within a furnace. These modified samples are identified as GAC-AT and GAC-FT. 2.2. Analytical Methodology. On the basis of the excellent correlation between UV254 of solution and DOC,10,11 the uptake of NOM was quantified by measurement of UV254. For the determination of the calibration curve, DOC was measured as Non Purgable Organic Carbon in a TOC analyzer with nonDispersive Infrared detector (Shimadzu TOC 5050A). UV absorbance measurements were carried out in a Helios-γ (TermoSpectronic) spectrophotometer with one centimeter optical path lengths. The pH measurements were carried out with a Crison (GLP-22) pH-meter. Molecular weight distributions of NOM were determined by HPLC-UV/RI. The experimental setup consists of an HPLC system with two detectors in series. Calibration standards were polyethylene glycol (PEG) at a concentration of 1 g L-1. The calibration curve was semilog linear over the range between 1000 and 41 000 Da. UV absorbance detector is NOM sensitive while RI detector is PEG sensitive (i.e., retention time of standard solutions was determined using refractive index detector, detector temperature 40 °C, whereas NOM solutions were analyzed using UV absorbance detector, 254 nm). Analytical conditions were as follows: column TSK R-3000, column temp. 40 °C, injection volume 150 µL, mobile phase phosphate buffer 0.004 M (pH 6.8), Na2SO4 0.0025 M, ionic strength 0.1 M, flow rate 1.0 mL min-1. 2.3. Characterization of Activated Carbons. A porosimeter system (ASAP 2010, Micromeritics) was used to determine the textural properties. The total pore volume was calculated from the adsorbed volume of gas near the saturation point (P/Po ) 0.98). Surface area was calculated from BET method. The pore size distributions, Smicro, Ssupermicro, and Smeso were determined using density functional theory (DFT) by assuming the graphite model with slit shape geometry. Thermogravimetric analysis (TGA) was performed with a SETSYS Evolution thermal analyzer. Samples were heated under synthetic air and from room temperature up to 900 °C by using a heating rate of 5 °C min-1. The contents of C, H, N, and S of the selected samples were measured using a CHNS-O Euro EA3000 Elemental Analyzer. The oxygen contents of samples were calculated by difference. The surface chemical properties of the carbons were characterized by different means. Carbon-KBr mixtures (1:100) were ground and pressed to obtain pellets, after which absorbance FTIR spectra of the carbon samples were recorded using an FTIR Spectrum Nicolet Protege´ spectrometer. Acidity and basicity of the carbon surface was measured from titration techniques12 65 mL vials were filled with 0.1 g of carbon and 20 mL of 0.05 N NaOH or 0.05 N of HCl. Vials without carbon and containing either acid or base were prepared as blanks. Samples and blanks were shaken at 200 rpm for 48 h at room temperature. Finally, samples were filtered (0.45 µm)

and 15 mL of the solution was titrated with 0.05 N of either NaOH or HCl solution. For the determination of the pHPZC, 20 mL of a NaNO3 0.1 M solution were put into several closed amber glass vials. The pH within each flask was adjusted to a value between 2 and 12 by adding HNO3 0.1 M or NaOH 0.1 M. Then, 0.1 g of carbon was added to each flask, and the final pH was measured after 24 h under agitation at room temperature. The pHPZC is defined as the point where the curve pHfinal vs pHinitial shows a plateau or crosses the line pHfinal ) pHinitial. The procedure was repeated for all of the activated carbon samples. Blank tests without carbon were also made in order to eliminate the influence of CO2 from air on pH. 2.4. Adsorption Equilibrium and Kinetics. Data were fitted by non-linear regression to the Freundlich (eq 1) and Langmuir equations (eq 2). Where qe (mgDOC gGAC-1) is the mass of solute adsorbed by dry weight of solid, Ce (mgDOC L-1) is the concentration of solute in equilibrium with concentration of solute adsorbed on solid. KL (L gGAC-1) and aL (L mgDOC-1) are Langmuir isotherm constants. This way, KL/aL (mgDOC gGAC-1) comes to represent the Langmuir monolayer adsorption capacity of the solid. KF represents the milligrams of solute adsorbed when Ce is unity, nF is Freundlich isotherm constant and is associated to adsorption strength. A modified isotherm model (eq 3) developed for isotherms of multicomponent organics was also used. It was originally developed to describe the adsorption of nonionic polymers by nonporous adsorbents and has been used for humic acids adsorption.13,14 The main benefit of eq 3 is that it eliminates the influence of the initial concentrations thus the adsorption capacity of NOM can be evaluated and compared correctly. qe ) KFC ne F qe )

kLCe 1 + aLCe

qe ) K F′ (Ce/mGAC)nF′

(1) (2) (3)

An adsorption mechanism involves several steps in series. Film diffusion, pore diffusion, surface diffusion, and adsorption contribute to overall uptake rate. The number of models developed to describe or predict the adsorption kinetics is high. Some of them provide detailed description of the system,15,16 however, extensive numerical calculations and computational effort are required. One approach to describe system dynamics is the use of empirical models. A lumped analysis of the adsorption rate is sufficient for comparison among adsorbents, especially in the case that experiments are to be performed in plant by operators. Among the commonly used models, the pseudo second order kinetic model (PSOM) and intraparticle diffusion control model (DCM) have been shown to adequately describe adsorption onto activated carbons.17,18 The integrated form of these models is given by eqs 4 and 5, respectively. All model parameters are defined in the nomenclature section. 1 1 1 + ) 2 qt q k2·qe ·t e

(4)

Dt ) kd·t0.5 (5) R2π 2.5. Batch Studies. Amber glass vials (65 mL) were placed in a constant temperature water bath with orbital shaking motion. All experiments were conducted at pH 8.0 with 30 mL of sample and 30 mg of GAC, at a stirring speed of 200 rpm, 30 °C and qt ) 6qe

Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009 a

Table 2. Some Physicochemical Properties of GACs GAC-A supplier

GAC-AT

chemviron carbon origin coconut coconut coal density, g cm-3 0.50 n.a. 0.425 Vtotal, cm3 g-1 0.341 0.381 (+12%) 0.483 SBET, m2 g-1 808 825 (+2%) 852 Smicro (0.7-1 nm) 702.9 668.2 639 Ssupermicro (1-2 nm) 105.1 140.2 (+30%) 170.4 Smeso (2-50 nm) 0 16.5 42.6 avg. pore size, Å 9.4 9.5 (+1%) 10.1 pHPZC 9.5 10.5 10.5 acidity (meq g-1) 0.13 0.03 0.10 basicity (meq g-1) 0.17 0.28 0.29 a

auxicarb auxicarb

GAC-F

GAC-FT chemviron carbon coal n.a. 0.502 (+4%) 963 (+13%) 674.1 250.4 (+30%) 38.5 10.8 (+6.9%) 10.6 0.06 0.25

n.a. not analyzed.

an average particle size of 0.25-0.30 mm. Samples were previously tempered for 1 h. For the kinetic study, at each contact time one vial was removed from the thermostatic bath and DOC remaining in solution was measured. The amount of adsorbed DOC onto GAC was calculated from the mass balance given by eq 6. All model parameters are defined in the nomenclature section. Varying initial DOC concentration of samples allowed establishing different DOC/GAC ratios. V (6) m 2.6. Column Studies. A glass column of length 21 cm and internal diameter 15 mm was packed with about 2.5 g of granular activated carbon having a particle size of 0.25-0.30 mm. Solution was pumped downward by a peristaltic pump and samples of effluent were collected at definite intervals of time. The experimental setup was designed to get a fast breakthrough by choosing relatively high flow velocities and low amounts of carbon. qt ) (Co - Ct)

3. Results and Discussion 3.1. Physical and Chemical Characteristics of GACs. Physical and chemical properties of activated carbons are summarized in Table 2. BET surface areas were similar for both virgin carbons. Micropore volume distributions (Figure 1) showed that pores were concentrated in the 6-10 Å diameter range. Some differences existed in the PSD being the average pore diameter 9.4 Å and 10.1 Å for GAC-A and GAC-F, respectively. Results from the elemental analysis can only be vaguely correlated. For example, the more acidic character of GAC-A corresponds to the higher oxygen percentage (Table 3). The main difference was in the surface chemistry. From FTIR spectra, the coal based carbon GAC-F exhibited more surface functional groups. Acid base titration results indicated that the content of acidic functionalities was similar for both virgin adsorbents, whereas the surface of GAC-F was richer in basic sites. This gave a pHPZC difference of one pH unit. In any case, both virgin carbons exhibited basic character with the point of zero charge occurring above 9.5. These values can be deemed adequate for adsorption of NOM from surface water (typical pH range 5.5-9.0). If adsorption occurs above the pHPZC, then the net charge of the adsorbent surface will be negative and therefore electrostatic repulsion between NOM molecule and surface could be expected.7 The most evident effect of heat treatment is the increase of the pHPZC of GAC-A from 9.5 to 10.5. For the modified carbons, a BET surface was increased by about 2% and 13% for GAC-A and GAC-F, respectively. Total pore volume showed a similar

5447

trend and increased by about 12% and 4% for GAC-A and GAC-F, respectively. Figure 2 shows FTIR spectra of the activated carbons. Spectra exhibit low OH stretching vibrations band (3600-3300 cm-1), which indicates the presence of strong hydrogen bonds. These hydrogen bonds are commonly the result of interactions between N-, H-, and O-containing moieties on the surface or the result of strongly adsorbed water on hydrophilic functional groups.19 The major difference in shapes of spectra shown in Figure 2 is in the region 1100-1800 cm-1, where GAC-F displays strong absorbance. Bands between 1300-1000 cm-1 can be assigned to CsO stretching and OsH bending modes of alcoholic, phenolic, and carboxylic groups existing in different structural environments.20,21 Absorbance in that region was very low for virgin and modified GAC-A. Reduction of the above-mentioned bands in sample GAC-FT was observed. This is because oxygen functionalities on carbon surface decompose below 900 °C.22,23 However, surface basicity was unchanged. Menendez et al.24 reported that heat treatment can also produce highly reactive sites such as free radical edge sites and dangling carbon atoms. Re-exposure of such carbons to the atmosphere can result in oxygen adsorption and the consequent formation of such functionalities in the coal-based carbon. This could explain the appearance of some bands in the 1300-1800 cm-1 region for GAC-FT. These bands can be attributed to CdO moieties in the carboxylic acids esters, and lactones (1650-1750 cm-1), quinine structures (1650 cm-1), and conjugated systems such as diketone, ketoester, and ketoenol structures (1570 cm-1).25,26 3.2. Batch Studies. Batch adsorption experiments were undertaken within the range of carbon to NOM ratios from 50 to 1000 mgGAC/mgDOC obtained at a fixed amount of GAC and varying NOM concentration, from 1 to 20 mg L-1. These ratios were representative of those applied in water treatment utilities, however, higher concentrations of NOM were used in order to favor adsorption differences among the evaluated adsorbents and ensure analytical precision. 3.2.1. Adsorption Equilibrium. Figure 3 summarizes isotherm data along with model fits for virgin and heat treated GACs. Table 4 displays best fitting parameters for eqs 1-3. The goodness of the fit was assessed by the correlation coefficient R2 and normalized deviation (ND).27 The percentage deviation between experimental and predicted values for each model was calculated by the following equation: normalized deviation )

100 N

∑|

(qe-exp - qe-pred) qe-exp

|

(7)

Although three models gave acceptable fits, the Freundlichtype models gave the highest R2 and lowest normalized deviation values. The minimum impact, in relative terms, of the heat treatment was in parameters from the modified Freundlich model what could be an effect of the carbon mass based normalization. With respect to virgin carbons, uptake capacity is significantly higher for GAC-F, which indicates that BET surface area was not a suitable adsorbent selection criterion (e.g., adsorption capacity of GAC-F is 300% higher, while BET surface area was only 5% higher). On the basis of model parameters shown in Table 4, it was observed that the impact of heat treatment on the adsorption properties was dissimilar for each carbon. For the coal based GAC-F equilibrium adsorption properties were unchanged. For adsorbent GAC-A, heat treatment increased NOM uptake on a mass basis. The stronger effect was observed in the adsorption capacity (KF) which increased 56%. Values of nF serve as an indicator of isotherm rise in the region of

5448


Figure 1. Comparison of fractional pore volume distributions. Table 3. Elemental Analysis of Virgin Carbonsa,b

GAC-A GAC-F a

C

H

N

S

Ash

O

90.24 87.43

0.47 0.08

0.26 0.55

n.d. 0.12

0.60 5.60

8.43 6.22

Percent w/w. b n.d. not detected.

Figure 2. FTIR spectra for different activated carbons.

Figure 3. Modified Freundlich adsorption isotherm for virgin and modified carbons. 200 rpm, dp 0.25-0.30 nm, pH 8.0.

lower solute concentrations which reflects the strength and affinity of the adsorbent for the solute.28 Heat treatment reduced surface acid sites as inferred from acid-base titration and pHPZC data. However, measured nF values for GAC-A were unchanged after heat treatment what suggests that surface chemistry was not the most significant factor in this adsorption system. Although total pore volume was vaguely increased after heat treatment, PSD was more readily modified. A trade-off between

micro and supermicroporosity occurred as observed in Figure 1. In general, primary micropore volume was reduced, whereas pore volume in the supermicropore size range was increased. Adsorption capacities at an initial concentration of 20 mgDOC L-1 were correlated with pore volumes at different pore size ranges. Following trial and error approach the results showed that NOM adsorption capacity was controlled by pores in the 12-15 Å width range (Figure 4). Characterization results indicate that heat treatment did not have any impact in the 12-15 Å width pore volume of GAC-F. This would explain the low or even null effect of treatment in the isotherm data. It is worth noting that the pHPZC and surface basicity of GAC-AT and GAC-F were very similar but equilibrium carbon load was not. Thus, results suggest that pore structure effects dominated over surface chemistry effects in the batch adsorption mode. The average molecular size of the fulvic and humic acids used in this study were reported to be about 7 Å and 11 Å, respectively.29 Considering the aforementioned optimum pore size range, our results indicate that in aqueous systems pore diameter to avoid size exclusion should be in the order of 1.5× the average size of the adsorbate. These values coincide with others,30 where it should be 1.7× the second largest dimension of the adsorbate. Pores with larger size (i.e., above 30 Å) can, of course, easily adsorb macromolecular organic components. However, their capability is lower since, given the same pore volume, the surface areas provided by such pores are smaller as compared to smaller pores. Obtained results suggest that available surface in pores within 12-15 Å width is strongly correlated to the NOM adsorption capacity of GAC. It should be noted that these results are specific to the type of NOM used in this study and the established treatment conditions. However, results suggest that based on this information a preliminary selection among a large number of commercial adsorbents could be feasible. Figure 5A shows HPLC-SEC data from batch adsorption experiments. Chromatograms correspond to the untreated and GAC treated NOM solutions. Untreated sample shows a main peak (P1 7.05 min, Mw 7180 Da) and two additional minor shoulders (P2 7.89 min, Mw 2485 Da; P3 8.20 min, Mw 1701 Da). In general terms, no evident differences exist in the Mw distribution of NOM contacted (in batch mode) by all four adsorbents. 3.2.2. Kinetics of Adsorption. Figure 6 depicts dissolved organic carbon removed as a function of contact time and adsorbent dose (mgGAC/mgDOC) where contact time to reach equilibrium varied from 50 to 300 h increasing with the initial


5449

a

Table 4. Isotherm Model Parameter Values Freundlich

Langmuir

modified Freundlich

adsorbent

KF

nF

R2

ND

KL

aL

KL/aL

R2

ND

K F′

nF′

R2

ND

GAC-A GAC-AT GAC-F GAC-FT

1.67 2.61 5.79 5.85

0.42 0.47 0.63 0.61

0.9999 0.9967 0.9996 0.9992

0.25 0.19 0.23 0.25

1.17 9.01 7.34 7.19

0.53 0.52 0.27 0.26

2.21 17.41 27.60 27.62

0.9804 0.9834 0.9984 0.9972

0.44 0.47 0.39 0.43

0.383 0.496 0.615 0.661

0.42 0.47 0.64 0.62

0.9999 0.9967 0.9998 0.9986

0.25 0.20 0.23 0.27

a

ND Normalized deviation measured by eq 7.

Figure 4. Effect of pore volume in NOM adsorption. Co 20 mgDOC L-1.

concentration of NOM. Best fitting parameters to eqs 4 and 5 are presented in Table 5. As occurred for the equilibrium data, heat treatment did not affect either the adsorption kinetics of GAC-F, thus model fitting was only applied to the virgin carbon. Although all of the correlation coefficient values were high, the PSOM model gave the best fit to experimental data. An overall trend of decreasing k2 was observed that coincides with other adsorption systems.31,32 This is a consequence of measuring an overall apparent kinetic constant rather than intrinsic kinetics. It reveals the fact that it is faster for an adsorption system with a lower initial concentration to reach a specific fractional uptake. From the intraparticle diffusion model, it was

observed that diffusivity (D/R2) decreased as the initial NOM concentration was increased. Contrary to our findings, previous works carried out with dyestuff16 and toluene33 reported that the intraparticle diffusivity increases with Co. This discrepancy could be explained by the higher aromaticity of the humic solutes used in this study compared to dyes. As liquid phase concentration increases more humic molecules would build up within adsorbent pores leading to a more hydrophobic environment what would hinder the diffusion of new humic molecules within the adsorbent pores. Thermally modified carbon GAC-AT showed reduced values of k2 kinetic constants as compared to the virgin mode. Thus, although higher equilibrium uptake values were achieved fractional uptake was lower for a given contact time. From eq 5, it can be easily deduced that the plot of adsorbed solute against t0.5 would yield a straight line passing through the origin if the adsorption process obeyed the intraparticle diffusion model. Figure 7 depicts that intraparticle diffusion contributes to the rate determining step where the slope of the curve is related to the diffusivity. The Kd parameter in intraparticle diffusion model represents an straightforward way to assess uptake rate when mass transfer control exist. At low solute concentrations, measured values of Kd were similar for virgin and modified samples. As solute concentration increased, the following trend was observed; GAC-FT ≈ GAC-F . GACAT > GAC-A. Thus, benefits of thermal treatment are more evident at high initial solute concentrations. For the modified carbon GAC-AT, the curves slightly move further from the origin indicating that other processes (i.e., adsorption) also contribute to overall NOM uptake rate. This suggests that thermal treatment could be a convenient pretreatment before

Figure 5. HPLC-SEC chromatogram of NOM in solution after treatment with GAC. DOCo ) 20 mg L-1. (A) Batch adsorption, contact time 95 h. (B) Column adsorption, for 0.2Co e Cefluent e 0.4Co.

5450


Figure 6. Uptake of NOM by virgin (solid) and modified (hollow) carbons. (A) GAC-F and (B) GAC-A. Table 5. Kinetic and Equilibrium Data diffusion controla (DCM)

pseudo second order (PSOM) -1

r, mgDOC gGAC

qe-exp, mg g

-1

qe-est, mg g

-1

k2 g mg

-1

-1

h

t50, h 1/k2qe

2

R

-1

kd mg g

h-0.5

D/R2 × 107 s-1

R2

GAC-A 1 5 10 20

0.81 2.46 3.64 5.16

0.80 2.48 3.86 5.76

0.1128 0.0221 0.0136 0.0037

1 5 10 20

0.80 3.40 4.43 8.73

0.85 3.71 4.19 8.43

0.0665 0.0071 0.0168 0.0030

1 5 10 20

0.93 4.26 8.36 15.39

0.97 4.44 8.97 17.97

0.0970 0.0101 0.0037 0.0010

10.9 18.3 19.0 47.1

0.9952 0.9941 0.9933 0.9911

0.1215 0.2648 0.4104 0.4100

5.55 2.97 3.01 1.54

0.9488 0.9972 0.9854 0.8996

17.7 37.9 14.2 39.4

0.9987 0.9998 0.9949 0.9992

0.0894 0.2743 0.4870 0.6333

3.05 1.58 2.93 1.28

0.9085 0.9383 0.9176 0.9348

10.5 22.2 29.9 55.1

0.9822 0.9925 0.9938 0.9912

0.1641 0.4357 0.7610 1.1625

7.54 2.54 2.01 1.19

0.9646 0.9787 0.9976 0.9738

GAC-AT

GAC-F

a

Data used correspond to a fractional uptake below that of 70% of equilibrium.

Figure 7. Intraparticle mass transfer curve for adsorption of NOM on virgin (solid) and modified (hollow) GAC-A.

any surface modification of adsorbent (i.e., nitration, metal impregnation) is faced. 3.3. Column Adsorption. Batch mode adsorption isotherms do not always give accurate scale up data in fixed bed process. Therefore, the effect of heat treatment was also studied in the column operation. The characteristic shape and general position of the breakthrough curve along the volume axis depends on inlet flow rates, concentration, and other properties, such as

column diameter and bed.34,35 Some of these effects can be observed in Figure 8A. Regarding feed characteristics, an increase in solute concentration speeds up the breakthrough of NOM (curve displaced to the left). Regarding operational conditions, the main manipulated variable in column adsorption systems is usually the flow rate. Most studies report a reduction in breakthrough time as flow increases, mainly due to reduction in EBCT. However, results from Figure 8A indicate that, at constant EBCT, higher flow rates can increase the time required for breakthrough. This could be explained in terms of the rate of solute diffusion to the surface of the activated carbon which is increased at higher flow rates. Experimental runs to compare influence of heat treatment were performed at a flow rate of 2.5 mL min-1 with C0 of 20 mg L-1 and 2,5 g GAC what corresponds to an EBCT of 2 min and fluid linear velocity of 0.9 m h-1. Obtained results are shown in Figure 8B. An immediate breakthrough was observed for the established conditions. This corresponds to a height of the mass transfer zone (HMTZ) that exceeds the GAC bed length (Z), indicating that DOC was not completely adsorbed by the carbons.36,37 After heat treatment the operation time for 50% breakthrough was unchanged for both adsorbents (Figure 8B). However, time for complete breakthrough was increased for GAC-A, whereas it was reduced for GAC-F. This is an interesting result since it implies a shorter operation time for GAC-F after heat treatment. Also, the result contradicts earlier batch experiments (Figure 6) where no differences were observed on the adsorption profile.


5451

Figure 8. Breakthrough curve for NOM adsorption. (+) mGAC-A ) 2.5 g, Co ) 1.5 mg L-1, 2.5 mL min-1; (4) mGAC-A ) 7.0 g, Co ) 1.5 mg L-1, 6.6 mL min-1; (3) mGAC-F ) 2.5 g, Co ) 1.5 mg L-1, 2.5 mL min-1; (9) mGAC-F ) 2.5 g, Co ) 20 mg L-1, 2.5 mL min-1; (b) mGAC-A ) 2.5 g, Co ) 20 mg L-1, 2.5 mL min-1 (0) mGAC-FT ) 2.5 g, Co ) 20 mg L-1, 2.5 mL min-1; (O) mGAC-AT ) 2.5 g, Co ) 20 mg L-1, 2.5 mL min-1.

The negative effect of heat treatment on column adsorption was attributed to the similar effective pore volume between virgin and modified GAC-F, and the fact that part of the basic surface functional groups were removed, as deduced from FTIR and acid-base titration results (Table 2). Adsorption capacities were calculated from the area under breakthrough curve. Measured values were 14.2 mg g-1 and 1.50 mg g-1 for virgin GAC-F and GAC-A, respectively. These are lower than adsorption capacities measured from batch mode runs (17.97 mg gGAC-F-1, 5.76 mg gGAC-A-1), especially for the coconut-based GAC-A. One potential reason for this discrepancy is nonequilibrium (i.e., rate-limited) adsorption, which could be manifested due to the differences in hydraulic residence time between the batch (250 h) and column (0.03 h) experiments. Evidence of rate-limited adsorption can be seen in the obtained breakthrough curves. Adsorption capacities for heat treated carbons were 11.9 mg g-1 and 2.1 mg g-1 for GACFT and GAC-AT, respectively. These indicate that the column adsorption capacity of GAC-A was enhanced which coincides with the previous batch runs. However, the overall uptake capacity was 17% reduced after thermal treatment of GAC-F. In the initial portion of breakthrough (inset graph Figure 8B) adsorption is favored, whereas the main difference was observed in the last portion of breakthrough. The decline in the last portion of the curve can be due to the bidisperse pore size distribution of the adsorbents where adsorption within the micropores is retarded due to steric interactions.33 Thus, the observed effect of heat treatment on the breakthrough properties of GAC-F could be explained through the displacement of average pore size in the micropore region (0.7-1 nm) to smaller values as shown in Figure 1. HPLC-SEC analyses of the GAC column effluent are presented in Figure 5B. Two are the main outcomes. First, the molecular weight distribution of NOM in the column effluent of the coal-based adsorbent (both as-received GAC-F and thermally treated GAC-FT) is shifted toward the larger molecular weight side. This indicates that during column adsorption, smaller organic constituents were adsorbed preferentially compared to batch operation, since in the batch mode such a shift was much less obvious. Second, the effect of thermal treatment not only depends on raw material properties, but also on the operation mode. For example, for the modified wood base carbon (GAC-AT) adsorption capacity was significantly increased (reduction of the area under chromatogram in Figure 5) in the batch adsorption experiments. On the contrary, for

the coal-based carbon, the effect of thermal treatment was much more marked if adsorption was performed in column mode. 4. Conclusions Two commercially available granular activated carbons were subjected to thermal treatment in an inert atmosphere of nitrogen. Thermal treatment reduced the surface acidity of GAC-A which significantly increased its pHpzc value from 9.5 to 10.5. It slightly increased, with a maximum increase of 13%, the specific surface area and total pore volume of both adsorbents. However, heat treatment does not always have a well-defined effect on activated carbon, and these later alterations did not have a straightforward correlation with the adsorption properties of the adsorbents. For carbon GAC-F (Filtrasorb-400), heat treatment had no effect on batch adsorption kinetics and equilibrium uptake capacity, whereas for GAC-A (Auxicarb) both increased. This improvement on adsorption performance was attributed to pore volume increase in the 12-15 Å width range. These results suggest that a preliminary selection among a large number of commercial adsorbents could be done based on this specific property of the adsorbents. Of course, further verification would be required for NOM of different sources and different treatment schemes (i.e., preoxidation of NOM). Although adsorption of NOM is a mass transfer limited process, an apparent second order kinetic model has shown to accurately reflect batch mode adsorption kinetics. Column adsorption runs showed that the 50% breakthrough time was unchanged by heat treatment. However, complete breakthrough time was increased for GAC-A, whereas it was reduced for GAC-F. This loss of performance of GAC-F was attributed to loss of surface functional groups and further reduction of pore size in the micropore region after heat treatment. Measured column adsorption capacities were lower than batch adsorption capacities. The difference was more pronounced for GAC-A adsorbent, which corresponds to the more mass transfer limited system. Obtained results indicate that equilibrium isotherm data are not sufficient to evaluate goodness of adsorbent. Furthermore, results obtained from isotherm data can somewhat contradict those obtained during column operation. Size exclusion chromatography data revealed that during column adsorption, smaller organic constituents were adsorbed preferentially compared to batch operation.

5452


qt ) NOM uptake per adsorbent gram, mg g-1 R ) average radius of spherical adsorbent particle, cm t50 ) adsorption time to reach 50% of equilibrium uptake, h V ) solution volume, L

In general terms, special care should be taken when batch adsorption data are used to assess the performance of adsorbents to be used in packed beds, especially in the case of highly microporous materials. It appeared that the GAC must present a particular association of surface basicity (in order to avoid electrostatic repulsions) and porous volume distributions to be efficient for NOM elimination. It would be interesting to assess and compare the impact of other carbon modification techniques in both batch and column adsorption performance. Such experiments are currently underway in our laboratories for iron-impregnated and ammonia-treated activated carbons.

Acknowledgment

5. Abbreviations

Literature Cited

DBPs ) disinfection byproduct DCM ) intraparticle diffusion control model DFT ) density functional theory DOC ) dissolved organic carbon EBCT ) empty bed contact time, min GAC ) granular activated carbon HMTZ ) height of the mass transfer zone, m ND ) normalized deviation NOM ) natural organic matter PEG ) polyethylene glycol pHPZC ) pH of point of zero charge PSD ) pore size distribution PSOM ) pseudo second order kinetic model RI ) refractive index SBET ) BET surface area, m2 g-1 SPTHMFP ) simulated distribution system THMFP, µgTHM

L-1 SUVA ) ultraviolet absorbance at 254 nm per DOC unit, L mg-1 m-1 TGA ) termogravimetric analysis THMFPF ) THMs formation potential for 7 days chlorination time, µgTHM L-1 TOC ) total organic carbon, mg L-1 UV254 ) ultraviolet absorbance at 254 nm 6. Parameters aL ) Langmuir equation parameter, L mgDOC-1 Ce ) equilibrium liquid phase concentration of NOM, mg

L-1 Co ) initial NOM concentration in liquid phase, mg L-1 Ct ) liquid phase concentration of NOM after a t period of time, mg L-1 D ) diffusivity coefficient of sorbate in the adsorbent particle, cm2 s-1 dp ) average diameter of GAC particle, mm k2 ) rate constant in pseudo second order model, g mg-1 s-1 kd ) intraparticle diffusion rate constant, mg g-1 s-1/2 KF ) Freundlich equation parameter, LnF mgDOC(1-nF) gGAC-1 KL ) Langmuir equation parameter, L gGAC-1 m ) mass of GAC mass, g M ) mass of adsorbent, g Mw ) weight average molecular weight, Da nF ) Freundlich equation parameter qe ) NOM uptake in equilibrium with the aqueous solution per adsorbent gram, mg g-1 qe-est, ) model predicted NOM uptake in equilibrium, mg g-1 qe-exp ) experimentally measured NOM uptake in equilibrium, mg g-1

The authors wish to thank to the Universidad del Paı´s Vasco, the Ministerio de Educacioń Cultura y Deporte (Grant No. AP 99 30687378), the Departamento de Industria, Comercio y Turismo (Project No. OD03UN57) and the water engineering firm Pridesa for their technical and economical support.

(1) Yamada, E.; Ozaki, T.; Kimura, M. Determination and behavior of humic substances as precursors of trihalomethane in environmental water. Anal. Sci. 1998, 14, 327. (2) Sadiq, R.; Rodriguez, M. J. Disinfection by-products (DBPs) in drinking water and the predictive models for their occurrence: a review. Sci. Total EnViron. 2004, 321, 21. (3) Directive 2000/60/CE of the European Parliament and of the Council, of 23 October 2000, establishing a framework for Community action in the field of water policy. (4) Iriarte-Velasco, U.; Alvarez-Uriarte, J. I.; Gonza´lez-Velasco, J. R. Removal and structural changes in natural organic matter in a Spanish water treatment plant using nascent chlorine. Sep. Purif. Technol. 2007, 57, 152. (5) Faust, S. D.; Aly, O. M. Chemistry of Water Treatment, 2nd ed.; Lewis Publishers: Boca Ratoń, FL, 1999. (6) Starek, J.; Zukal, A.; Rathousky, J. Comparison of the adsorption of humic acids from aqueous solutions on active carbon and activated charcoal cloths. Carbon 1994, 32, 207. (7) Fairey, J. L. Impact of natural organic matter on monochloramine reduction by granular activated carbon: the role of porosity and electrostatic surface properties. EnViron. Sci. Technol. 2006, 40, 4268. (8) Yang, R. T. Adsorption; John Wiley & Sons Inc.: Hoboken, NJ, 2003. (9) Cheremisinoff, P. N.; Ellerbusch, F. Carbon Adsorption Handbook; Ann Arbor Science: Ann Arbor, MI, 1980. (10) Korshin, G. V.; Li, C. W.; Benjamin, M. M. Use of differential spectroscopy to evaluate the structure and reactivity of humics. Water Sci. Technol. 1999, 40, 9. (11) Iriarte-Velasco, U.; Alvarez-Uriarte, J. I.; Gonza´lez-Velasco, J. R. Monitoring trihalomethanes in water by differential ultraviolet spectroscopy. EnViron. Chem. Lett. 2006, 4, 243. (12) Boehm, H. P. Some aspects of the surface chemistry of carbon blacks and other carbons. Carbon 1994, 32, 759. (13) Li, F.; Yuasab, A.; Ebie, K.; Azuma, Y.; Hagishita, T.; Matsui, Y. Factors affecting the adsorption capacity of dissolved organic matter onto activated carbon: modified isotherm analysis. Water Res. 2002, 36, 4592. (14) Kilduff, J. E.; Karanfil, T.; Chin, Y.; Weber, W. J. Adsorption of natural organic polyelectrolytes by activated carbon: a size-exclusion chromatography study. EnViron. Sci. Technol. 1996, 30, 1336. (15) Selene, M. A.; de Souza, G. U.; Peruzzo, L. C.; de Souza, A. A. U. Numerical study of the adsorption of dyes from textile effluents. Appl. Math. Model. 2008, 32, 1711. (16) Yang, X.; Otto, S. R.; Al-Duri, B. Concentration-dependent surface diffusivity model (CDSDM):numerical development and application. Chem. Eng. J. 2003, 94, 199. (17) Li, Z.; Yang, R. T. Concentration profile for linear driving force model for diffusion in a particle. AIChE J. 1999, 45, 196. (18) Iriarte-Velasco, U.; Alvarez-Uriarte, J. I.; Chimeno-Alanı´s, N.; Gonza´lez-Velasco, J. R. Natural organic matter adsorption onto granular activated carbons: implications in the molecular weight and disinfection by products formation. Ind. Chem. Eng. Res. 2008, 47, 7868. (19) Biniak, S.; Szymansky, G.; Siedlewski, J.; Swiatkowski, A. The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon 1997, 35, 1799. (20) Pradhan, B. K.; Sandle, N. K. Effect of different oxidizing agent treatments on the surface properties of activated carbons. Carbon 1999, 37, 1323. (21) Boonamnuayvitaya, V.; Sae-Ung, S.; Tanthapanichakoon, W. Preparation of activated carbons from coffee residue for the adsorption of formaldehyde. Sep. Purif. Technol. 2005, 42, 159. (22) Stohr, B.; Boehm, H. P.; Schlogl, R. Enhancement of the catalytic activity of activated carbons in oxidation reactions by thermal treatmentwith

Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009 ammonia or hydrogen cyanide and observation of superoxides species as a possible intermediate. Carbon 1991, 29, 707. (23) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M. Modification of the surface chemistry of activated carbons. Carbon 1999, 37, 1379. (24) Menendez, J. A.; Philips, J.; Xia, B.; Radovic, L. R. On the modification and characterisation of chemical surface properties of activated carboń. In the search of carbońs with stable basic properties. Langmuir 1996, 12, 4404. (25) Meldrum, B. J.; Rochester, C. H. In situ infrared study of the surface oxidation of activated carbon dispersed in potassium bromide. J. Chem. Soc. Faraday Trans. 1990, 89, 2997. (26) Nakahara, M.; Sanada, Y. FT-IR ATR Spectroscopy of the edge surface of pyrolitic graphite and its surface /PVC interface. J. Mater. Sci. 1995, 30, 4363. (27) Singh, R. K.; Kumar, S.; Kumar, S.; Kumar, S. Development of parthenium based activated carbon and its utilization for adsorptive removal of p-cresol from aqueous solution. J. Hazard. Mater. 2008, 155, 523. (28) Rengaraj, S.; Moon, S. H.; Sivabalan, R.; Arabindoo, B.; Murugesan, V. Agricultural solid waste for the removal of organics: adsorption of phenol from water and wastewater by palm seed coat activated carboń. Waste Manage. 2002, 22, 543. (29) Aiken, G. R.; Browm, P. A.; Noyes, T. I.; Pinckney, D. J. Molecular Size and Weight of Fulvic and Humic Acids from the Suwannee River. In Humic Substances in the Suwannee RiVer, Georgia: Interactions and Properties; Averett, R. C. Leenheer, J. A. McKinight, D. M. Thorn K. A. editors. United States Geological Survey Water Supply: Denver, CO Paper 2373. 1994.

5453

(30) Kasaoka, S.; Sakata, Y.; Tanaka, E.; Naitoh, R. Design of molecular sieve carboń: Studies on the adsorption of various dyes in the liquid phase. Int. Chem. Eng. 1989, 29, 734. (31) Rengaraj, S.; Kim, Y.; Joo, C. K.; Yi, J. Removal of copper from aqueous solution by aminated and protonated mesoporous aluminas: kinetics and equilibrium. J. Colloid Interface Sci. 2004, 273, 14. (32) Hameeda, B. H.; China, L. H.; Rengarajb, S. Adsorption of 4-chlorophenol onto activated carbon prepared from rattan sawdust. Desalination 2008, 225, 185. (33) Chatzopoulos, D.; Varma, A.; Irvine, R. L. Activated carbon adsorption and desorption of toluene in the aqueous phase. AIChE J. 1993, 39, 2027. (34) Honglay Chen, P.; Hui Jenq, C.; Mei Chsn, K. Evaluation of granular activated carbon for removal of trace organic compounds in drinking wa´ter. EnViron. Int. 1996, 22, 343. (35) Othman, M. Z.; Roddick, F. A.; Snow, R. Removal of dissolved organic compounds in fixed-bed columns: evaluation of low-rank coal adsorbents. Water Res. 2001, 35, 2943. (36) Weber, W. J. J. Physicochemical Processes for Water Quality Control; Wiley-Interscience: New York, 1972. (37) Michaels, A. S. Simplified method of interpreting kinetic data in fixed ion exchange. Ind. Eng. Chem. Res. 1952, 44, 1922.

ReceiVed for reView January 19, 2009 ReVised manuscript receiVed March 30, 2009 Accepted April 6, 2009 IE900053P

Evaluation of the Adsorption of Aquatic Humic Substances in Batch

Recommend Documents