Natural Organic Matter Adsorption onto Granular Activated Carbons


Sep 23, 2008 - Unai Iriarte-Velasco, Jon I. Álvarez-Uriarte, Noemí Chimeno-Alanís and Juan R. González-Velasco*. Department of Chemical Engineering, ...
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7868

Ind. Eng. Chem. Res. 2008, 47, 7868–7876

Natural Organic Matter Adsorption onto Granular Activated Carbons: Implications in the Molecular Weight and Disinfection Byproducts Formation ´ lvarez-Uriarte, Noemı´ Chimeno-Alanı´s, and Juan R. Unai Iriarte-Velasco, Jon I. A 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

Adsorption of natural organic matter (NOM) by granular activated carbon (GAC) was studied. Three different carbons of different origin were initially used. The physical properties were studied by means of N2 adsorption. Chemical properties were studied by means of thermogravimetric analysis (TGA), acid-base titrations, and FTIR. Only one of the carbons showed a broad adsorption band in the 1300-1000 cm-1 region in FTIR spectra, which can be assigned to C-O stretching and O-H bending modes of alcoholic, phenolic, and carboxylic groups. Adsorption of NOM was studied by batch adsorption experiments. Uptake of NOM by the different carbons was evaluated by UV absorbance, disinfection byproduct formation potential tests, and HPLC-SEC chromatography. Freundlich equation was used to fit equilibrium data. pHPZC and overall surface basicity were shown to improve the removal of THM precursors. Differences in the molecular weight distribution of the adsorbed material by different carbons were reported. A clear correlation was found between the reduction in the THM formation capacity of sample and the reduction in intensity of a specific peak in SEC chromatograms. Furthermore, THMFP tests showed the existence of some fractions of NOM not adsorbable with activated carbons and undetected by measurement of DOC. 1. Introduction The concern over the need to protect drinking water quality is growing in developed countries and has driven the formulation of new quality standards.1 Control over the formation of disinfection byproducts (DBPs) is one of the main treatment goals. Natural organic matter (NOM) can bind or complex synthetic organic chemicals and/or toxic metals; it also can serve as a substrate for bacterial growth in distribution systems and is considered one of the main precursors of disinfection byproduct formed during water treatment.2-34 Organic matter in natural waters cannot be completely removed from water using conventional treatment processes.5 Furthermore, some treatment technologies such as ozone can partially oxidize NOM and make it more refractory to coagulation-flocculation-settling processes.6 Activated carbon has been widely used to remove a broad spectrum of organic compounds from water.7,8 The use of activated carbon adsorption includes applications in taste and odor control, the removal of specific organics, such as aliphatic and aromatic hydrocarbons, and the removal of DOC for the reduction of DBPs precursors,suchashumicsubstances;andalsoDBPsthemselves.9-11 Granular activated carbon (GAC) can be used as part of a multimedia filter to remove particulates or as a postfilter to remove specific contaminants. The adsorption capacity of activated carbons is typically assessed by the iodine number or the BET surface area. However, molecular size and chemical properties of iodine and nitrogen are not representative of aquatic NOM. These parameters do not account for effects such as the molecular sieving or electrostatic interactions that can occur during adsorption of NOM. Consequently, actual carbon usage rate in adsorption processes is frequently unrelated to these parameters.12-14 Another characteristic property of activated carbons is the point * To whom correspondence should be addressed. Tel.: +034946012681. Fax: +34-946015963. E-mail: [email protected] ehu.es.

of zero charge (pHPZC). Above the pHPZC, the net charge of the adsorbent surface will be negative, and therefore electrostatic repulsion between humic molecules and surface could be expected.15 This way, higher pHPZC can markedly improve the adsorption of NOM, particularly when NOM has an acidic character and salt concentration is low.10 However, Weng et al.11 observed that the adsorption is still positive at pH values 1.5 units above the pHPZC of the adsorbent. This behavior was attributed to the existence, besides the electrostatic interactions, of other mechanisms such as specific adsorption and hydrophobic interactions. Previous research has also revealed the importance of size exclusion effects and textural characteristics of the activated carbon.16-19 Selecting an effective activated carbon is not a straightforward task especially for water treatment because the nature of NOM strongly affects the adsorption process. Furthermore, NOM chemical properties and also its content can vary considerably between drinking water production facilities and even in a relatively short period of time for the same production facility. Consequently, adsorption rate and capacity of GAC can be severely affected.20 The capacity of activated carbon to reduce formation of DBPs during drinking water production depends on its ability to remove NOM over a wide molecular weight spectrum. The main objective of this work was to investigate the role of carbon surface chemistry and pore structure on the uptake of NOM from the DBPs formation potential and molecular weight distribution (MWD) point of view. A good understanding of the impact of activated carbon on the MWD of organic matter and its relationship with its disinfection byproduct formation potential is required as a basis for selecting the best adsorbent material for drinking water production. 2. Experimental Section 2.1. Materials. The adsorbents selected for this study were one coconut shell-based GAC-A (Auxicarb), one bituminous

10.1021/ie800912y CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7869 Table 1. Physicochemical Properties of Synthetic Waters parameter

value

pH DOC, mg L-1 UV254, cm-1 SUVA, L (mg m)-1 THMFPF, µg THM mg DOC-1 SPTHMFP, µg THM mg DOC-1 Ca2+, mg L-1 conductivity, µS cm-1 alkalinity, mg HCO3- 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.

coal-based carbon GAC-F (F-400, Chemviron carbon), and a wood-based GAC-M (Merck) activated carbon. All three carbons were produced by steam activation. Synthetic water was prepared for which alkalinity, hardness, and pH were modified to the typical values found in surface waters from the Basque Country. 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, ref 53680) and fulvic acid (IHSS Suwannee river) was used as NOM. Working solutions were prepared at four initial DOC concentrations, 1.0, 5.0, 10.0, and 20.0 mg L-1. Physicochemical properties of the prepared synthetic waters are shown in Table 1. Adsorption and characterization experiments (i.e., acidity-basicity, pHPZC) were conducted in borosilicated amber glass vials (65 mL) with Teflon joint and screw cap. THMFPF tests were carried out to measure DBPs precursor content.21 THMFPF refers to the final or maximum THMs that can be formed and thus denotes total precursor content. High chlorine residuals were used to evaluate an extreme condition to better understand THM formation. Chlorination conditions for THMFPF correspond to 22 °C, initial free chlorine residual (FCR) 78 mg L-1, pH 7.0, and chlorination time 168 h. Simulated plant THM formation potential (SPTHMFP) refers to the amount of THM that could form under typical water treatment plant (WTP) operational conditions. Chlorination conditions for SPTHMFP are less extreme and denote those applied at WTP 10 °C, initial FCR 15 mg L-1, pH 8.0, and chlorination time 15 h. Initial FCR was selected to ensure a residual value between 1 and 3 mg L-1 for a solution with 10 mg DOC L-1. 2.2. Analytical Methodology. DOC was measured as nonpurgeable organic carbon in a TOC analyzer with non-dispersive infrared detector (Shimadzu TOC 5050A). Method minimum reporting level (MRL) and minimum quantification level (MQL) have been calculated as 3 and 10 times the standard deviation of a low concentration standard. By using 2.0 mg DOC L-1 potassium hydrogen phthalate standard solution, MRL and MQL correspond to 0.3 and 1.0 mg L-1, respectively. The four THMs species, chloroform, bromoform, bromodichloromethane, and dibromochloromethane, were determined by gas chromatography/ECD (HP 5890 Series II, column Hp-1). Method detection and quantification levels for each THM species (defined as peak height to noise ratio of 3:1 and 10:1, respectively) correspond to 0.8 and 2.8 µg L-1, respectively. pH measurements were carried out with a Crison (GLP-22) pH meter. UV absorbance measurements were carried out in a Helios-γ (TermoSpectronic) spectrophotometer with 1 cm optical path lengths. 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 to 900 °C by using a heating rate of 5 °C min-1. The surface chemical properties of the carbons were characterized by different means. FTIR spectra was recorded for which 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. FTIR spectra of modified GAC-F were also measured. Activated carbon was treated under N2 atmosphere for 8 h at 800 °C (GAC-FT) and oxidized with a concentrated nitric acid solution (15.7 N) for 3 h at 150 °C (GAC-FOx). Acidity and basicity of carbon surface were measured from titration techniques.22 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. Batch equilibrium method for determination of the point of zero charge (pHPZC) was used as described elsewhere.23 Accordingly, GAC samples were put into several closed amber glass vials containing 20 mL of a NaNO3 0.1 M. The initial pH within each flask was adjusted to a value between 2 and 12 by adding HNO3 0.1 M or NaOH 0.1 M. Next, 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 versus 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 to eliminate the influence of CO2 from air on pH. 2.4. Experimental Setup. NOM was adsorbed from aqueous solutions. Adsorption of DOC was studied by the bottle-point method (i.e., different initial DOC concentrations). pH and electrical conductivity were maintained constant to control the possible effect of inorganic content. On the basis of the excellent correlation between UV254 of solution and DOC,24,25 the uptake of NOM was quantified by measurement of UV absorbance at 254 nm (UV254). Vials were introduced in a thermostatic water bath with orbital shaking. Experiments were conducted at pH 8.0 with 30 mL of sample and 30 mg of GAC. To avoid undesirable temperature fluctuations among different adsorption experiments, they were carried out in a thermostatic bath at 30 °C. The selection of the temperature was limited by the absence of a refrigeration system (only heating) of the bath; therefore, to ensure the control capacity, the temperature was set slightly above room temperature. In any case, the established temperature (30 °C) should not alter the drawn conclusions. Although results are not presented, the existence of void volume in vials significantly enhanced interphase mass transfer. This effect was positive to minimize to the maximum the external mass transfer control. 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 as follows:

7870 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

qt ) (Co - Ct)

V m

(1) -1

where qt is the amount of DOC adsorbed onto GAC (mg g ), Co and Ct are the DOC concentrations at the beginning and after a certain period of time (mg L-1), V is the solution volume (L), and m is the GAC mass (g). 2.5. Adsorption Isotherms. Adsorption of humic acid was evaluated and compared to popular two-parameter single-solute isotherm models: the Langmuir and the Freundlich equations, which are described by eqs 2 and 3, respectively. qe )

KLCe 1 + RLCe

(2)

Table 2. Some Physicochemical Properties of Adsorbentsa

supplier origin Vtotal, cm3 g-1 SBET, m2 g-1 % Smicro (0.7-1 nm) % Ssupermicro (1-2 nm) % Smeso (2-50 nm) av. pore size, Å pHPZC iodine number, mg g-1 acidity (mequiv g-1) basicity (mequiv g-1) ash (% w/w) a

qe ) KFCne F

(3)

where qe (mg DOC g GAC-1) is the mass of solute adsorbed by dry weight of solid, Ce (mg DOC L-1) is the concentration of solute in equilibrium with the concentration of solute sorbed -1 on solid, KL (L g GAC-1), RnFL (L mg DOC ) are Langmuir (1-nF) isotherm constants, and KF L mg DOC g GAC-1 and nF are Freundlich isotherm constants. This way, KL/µL (mg DOC g GAC-1) comes to represent the Langmuir monolayer adsorption capacity of the solid, while KF represents the milligrams of solute adsorbed when Ce equals 1.0 mg L-1. 2.6. HPLC-SEC Chromatography. Molecular weight distributions of NOM were determined by HPLC-UV/RI. The experimental setup consists of an HPLC system with two detectors in series. UV absorbance detector is NOM sensitive, while IR detector is PEG and PEO 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). Calibration standards were polyethylene glycol (PEG) and polyethylene oxide (PEO) at a concentration of 1 g L-1. The calibration curve was semilog linear over the range of 1000-41 000 Da. 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. 3. Results and Discussion 3.1. Physical and Chemical Characteristics of Carbons. The data of specific surface area (SBET), total pore volume (Vtotal), micropore surface (Smicro), supermicropore surface (Ssupermicro), mesopore surface (Smeso), inorganic content, surface aciditybasicity, as well as the usually used pHPZC are summarized in Table 2. Among these activated carbons, GAC-F is by far the most studied one. Physicochemical properties for GAC-F vary significantly in the literature (SBET 866.4-1215 m2 g-1, pHPZC 8.9-10.4, VT 0.553-0.600 cm3 g-1).16,23,26 The values measured in this study fall within the reported range. It can be seen that all of the carbons are essentially microporous and exhibit similar BET surface. GAC-F and GAC-M show a slightly higher mesopore volume, which resulted in about 1 Å higher average pore size. Most of the available pore volume is homogeneously distributed in the micropore region (dp < 10 Å); thus the used carbons can be regarded as porous solids with dispersed microporosity. All three adsorbents have more basic sites than acidic sites and a pHPZC value above 9.5 units, which implies that surface maintains basic characteristic within the typical pH range (from 5.5 to 9.5 pH units) of surface waters. The TGA values for the 50% (T50) weight loss were 645, 645, and 620 °C for GAC-F, GAC-M, and GAC-A, respectively. These results

GAC-A

GAC-F

Auxicarb coconut 0.341 808 87 13 0 9.4 9.5 900 0.13 0.17 0.6

Chemviron Carbon coal 0.483 852 75 20 5 10.1 10.5 1050 0.10 0.29 5.6

GAC-M Merck wood 0.505 840 71 22 7 10.1 10.0 N/A 0.09 0.22 4.2

N/A: not analyzed.

Figure 1. FTIR spectra for different activated carbons.

indicate a similar thermal stability of all adsorbents, especially for GAC-F and GAC-M. These have slightly higher T50 values and also a higher inorganic content. FTIR spectra of activated carbons are shown in Figure 1. All carbons exhibit OH stretching vibrations band (3600-3100 cm-1) characteristic of surface hydroxylic groups and chemisorbed water.27 The asymmetry of this band at lower wavenumbers indicates the presence of strong hydrogen interactions between surface functional groups and adsorbed water molecules. The spectra for GAC-A and GAC-M showed fewer peaks and weaker intensity, which is indicative of a less amount of surface functional groups. This is probably because of differences in the activation procedure and precursor material of each carbon. The bands in the (i) 1730-1705 cm-1 and (ii) 1570-1550 cm-1 wavenumber ranges attributed to the stretching vibrations of CdO moieties in the (i) carboxilic, quinonic, ester lactonic, or anhydre groups and (ii) conjugated systems like diketone, keto-esters, and keto-enol structure are absent in the GAC-F sample. A weak band near 1570-1550 cm-1 is observed for GAC-A. This coincides with the slightly higher acidity measured for GAC-A. This somewhat confirms the overall low surface acidity of these activated carbons measured by acid-base titration (Table 2). Thermal treatment followed by oxidation with concentrated nitric acid (150 °C, 3 h) of virgin GAC-F generates an increase of these bands, which confirms that these bands correspond to acidic groups.28 Below 2000 cm-1, the FTIR spectra of the carbons display typical absorption of surface functional groups and structural oxygen species. The main difference between unmodified carbons is the presence of overlapping bands between 1300 and 1000 cm-1, which can be assigned to C-O stretching and O-H bending modes of alcoholic, phenolic, and carboxylic groups existing in different structural environments.29,30 Thermal treat-

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7871

Figure 2. Percent removal of UV absorbance at 254 nm for (A) different mixing conditions and (B) different particle sizes.

ment reduces oxygen content and produces more basic carbon.31 Reduction of the above-mentioned bands in sample GAC-FOx suggests abundant oxygen-containing structures on the GAC-F surface. 3.2. Adsorption Equilibrium. Figure 2 shows UV absorbance removal from solution under different mixing conditions and for different carbon particle size. From the analysis of these preliminary experiments and data summarized in Table 2, it was concluded that GAC-M and GAC-F had similar physicochemical and adsorption properties. For this reason, the study was continued with the two most different adsorbents, GAC-F and GAC-A. Experimental conditions for the rest of the experiments were set as follows: stirring intensity of 200 rpm, pH 8.0 units, 30 °C, and an average particle size of 0.25-0.30 mm. Adsorption equilibrium data were fit to Langmuir and Freundlich equations by nonlinear regression. To evaluate the goodness of fit, the sum of squares due to the errors (SSE), determination coefficient (R2), and standard deviation (SD) associated with the output model results were calculated. Results summarized in Table 3 indicate that the quality of fit is excellent for both isotherms. However, the Freundlich equation was used to describe adsorption of NOM on the activated carbon as it gave the maximum R2 and the lowest SSE values. Isotherm data and the Freundlich model curve are plotted in Figure 3. Theoretical background of Langmuir isotherm makes the quantity KL/RL quite useful. It is found that adsorption capacity (KL/RL) is 1 order of magnitude higher for GAC-F as compared to GAC-A, being 27.6 and 2.21 mg DOC g GAC-1, respectively. Values of nF were 0.42 and 1.59 for GAC-A and GACF, respectively. nF can serve as an indicator of isotherm rise in the region of lower solute concentrations, which reflects the strength and affinity of the adsorbent for the solute.32 Again, higher values of GAC-F reflect higher affinity of natural organic matter toward this carbon surface. Adsorption values measured for GAC-F are comparable to those reported for adsorption of humic acid onto activated natural clays.33 3.3. Adsorption Dynamics. Adsorption kinetics, demonstrating the rate of solute uptake, is one of the most important characteristics representing the adsorption efficiency of activated carbon. Figure 4 shows the uptake of NOM as a function of time. The effect of the initial solute to adsorbent concentration ratio given by parameter r (mg NOM g GAC-1) is shown. Adsorption rate progressively decreases as the system approaches equilibrium. In agreement with previous results shown in Figure 3, GAC-F reaches higher equilibrium uptake values. Normalized adsorption curve (Figure 4c,d) was independent from r values for GAC-A, which indicates a strong mass transfer control for that adsorbent. The contact time to reach equilibrium

varied from 75 to 300 h, which is far from typical hydraulic residence times applied during water treatment processes. Adsorption of phenolic compounds by granular activated carbon is known to be extremely rapid, and within the first hour of contact 60-80% of equilibrium uptake is reached.34 However, the higher molecular radius of humic acid, about 1.72 nm,35,36 which is well above the average pore size of the used adsorbents, would explain such a slow adsorption rate. Several diffusion models can be used to interpret the adsorption kinetics; however, it is very difficult to make reliable parameter estimations for intraparticle diffusion in activated carbon due to the system complexity.37As a consequence, mathematically simple kinetic models may be adequate for practical operation. To analyze the adsorption rate of NOM, the pseudo first order (PFO) (eq 4), the pseudo second order (PSO) (eq 5), the modified pseudo first order (MPFO) (eq 6), and the intraparticle diffusion control (DC) (eq 7) model were evaluated on the basis of the experimental data.38,39 Parameter D (cm2 s-1) is intraparticle diffusivity, R (cm) is the average radius of GAC particle, qt (mg g-1) is the average solid phase concentration of solute at contact time t (min), qe (mg g-1) is the solid phase concentration of solute at equilibrium, and k1 (h-1), k2 (g mg-1 h-1), k1m (h-1), and kd (mg g-1 min-1) are rate constants. ln(qe - qt) ) ln qe - k1 · t

(4)

1 1 1 + ) qt k · q2 · t qe 2 e

(5)

qt + ln(qe - qt) ) ln qe - k1m · t qe

(6)



qt )6 qe

Dt ) kd · t0.5 R2π

(7)

Nonlinear regression was applied, and obtained values are summarized in Table 4. The first order model gave, in general, a very poor fit. Pseudo second order and the modified pseudo first order models can be seen to describe adequately the kinetics of adsorption of NOM with the highest correlation coefficient. Apparent kinetic constants, k1m and k2, decrease with increasing solute to adsorbent mass ratio (r). Exception occurs for k1m values measured for GAC-A system where no clear trend was observed and a constant value of k1m could be assumed. Measured diffusivity values (D/R2) were in the range of 0.28 × 10-6 to 0.55 × 10-6 s-1 and 0.20 × 10-6 to 0.75 × 10-6 s-1 for GAC-A and GAC-F, respectively. Increase in solute concentration led to a decrease in diffusivity for GAC-F. This trend may be due to solute agglomeration at high bulk liquid concentrations, which would further increase the diffusional resistance. The existing differences in the textural properties (i.e., SBET and pore size distribution) of the carbons are not very marked and do not seem to be responsible for the observed differences in adsorption rate and adsorption capacity. Previous works suggest that effective adsorbent should exhibit a large volume of micropores with widths that are about 1.3-1.8 times larger than the kinetic diameter of the target adsorbate.40 However, average pore size of GAC used in the present study is well below those values, whereas NOM adsorption onto GAC-F was high. The observed adsorption pattern suggests that the effects of pore surface chemistry on adsorbate/adsorbent interactions can overcome molecular exclusion effects. The high surface basicity measured for GAC-F with a pHPZC value of 10.5 favors the

7872 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 3. Parameter Values from Fitting the Adsorption Equilibrium Data GAC-A isotherm model

parameter values

GAC-F R2

SSE

SD

parameter values

R2

SSE

SD

7.34 0.27 27.60

0.9983

0.2668

0.3190

5.85 1.59

0.9996

0.0505

0.1590

Langmuir KL RL KL/RL

1.17 0.53 2.21

0.9803

0.2631

0.3627

KF nF

1.67 0.42

0.9999

0.0001

Freundlich 0.0058

electrostatic attraction within the surface and the negatively charged organic molecule. 3.4. THMFP Tests. Figure 5 illustrates the reduction of precursors of THM after treatment with activated carbon. Contrary to the observed trend for the removal of DOC, the reduction of THM precursors is more affected by solute to adsorbent mass ratio (r parameter) for GAC-A. When equilibrium was reached, GAC-A removed between 40-68% and 55-75% of THMFPF and SPTHMFP, respectively, while GAC-F removed for all cases above 85%. In general terms, removal efficiencies attained for SPTHMFP were higher than those observed for the THMFPF. It is noteworthy the fact that even at the longest contact times and for the lowest NOM

Figure 3. Equilibrium isotherms for GAC-A and GAC-F.

concentration (Figure 5a), some residual THMFPF exists, whereas DOC was reduced below detection limits (Figure 4b). This indicates the existence of some fractions of NOM that do not adsorb onto the activated carbons. The contribution of these organic molecules to total DOC is very low; however, they could lead to significant concentrations of DBPs during the water disinfection step and also within the distribution system, especially when rechlorination stations exist. 3.5. Effect on Molecular Weight Distribution. Figure 6A,C shows the HPLC-SEC chromatograms for NOM solution before and after treatment with GAC. At low concentrations, NOM elutes as a main peak (P1 7.05 min, Mw 7180 Da) with a welldefined shoulder (P2 7.63 min, Mw 3451 Da). The existence of two additional minor shoulders was also observed (P3 7.89 min, Mw 2485 Da, P4 8.19 min, Mw 1701 Da), indicative of the presence of a small quantity of lower Mw molecules. At the highest concentration of 10 mg DOC L-1 (results not shown), there is a loss of resolution, and a unique band with the two minor shoulders (P3, P4) was observed. In this study, weight-average (Mw) and number-average (Mn) molecular weight of the NOM were 6512 and 3725 Da, respectively (polydispersivity 1.74). Because of the large heterogeneity of NOM, its molecular weight has been reported to be highly variable. Measured values in the present work coincide with those reported by Li et al.41 for commercial humic substances, whereas they are above the average Mw values (i.e., from 1000 to 3000 Da) reported by others for natural dissolved organic matter (DOM) in surface waters.42,43 Our SUVA values are also higher, confirming that humic fraction contains the larger DOM components with a higher degree of aromaticity.

Figure 4. Batch adsorption curve for GAC-A and GAC-F for different solute to adsorbent mass ratios (r, mg DOC g GAC-1): pH 8.0, T 30 °C, 200 rpm. (A and C) GAC-A, (B and D) GAC-F.

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7873 Table 4. Best Fit Parameters by Least-Square Nonlinear Regression for GAC-A and GAC-F GAC-A r mg DOC g qe,exp GAC-1 0.81 2.46 3.64

1 5 10

qe,exp

r mg DOC g GAC-1 1 5 10

0.81 2.46 3.64 GAC-F qe,exp 0.931 4.257 8.365 qe,exp 0.931 4.257 8.365

r mg DOC g GAC-1 1 5 10 r mg DOC g GAC-1 1 5 10

first order (PFO)

first order modified (PFOM)

k1 h-1 0.0706 0.0398 0.0408

qe

R2

K1m h-1

SSE

0.7276 0.9508 0.042 (1.20%) 0.0134 2.2613 0.9731 0.1812 (1.78%) 0.0099 3.4441 0.9805 0.307 (1.98%) 0.0151

second order (PSO) k2 g mg-1 h-1 qe

R2

SSE

diffusion controla (DC) kd mg g-1 h-0.5

0.1166 0.0180 0.0122

0.7948 0.9818 0.0140 (0.40%) 0.1215 2.5645 0.9902 0.0662 (0.65%) 0.2648 3.8952 0.9928 0.1131 (0.73%) 0.4104

first order (PFO) k1 h-1

qe

0.0808 0.0366 0.0267

0.8396 0.9462 0.0597 (1.44%) 0.0227 3.9221 0.9707 0.787 (2.76%) 0.0087 7.9765 0.9871 1.830 (3.37%) 0.0081

second order (PSO) k2 g mg-1 h-1 qe 0.0970 0.0101 0.0037

R2

R2

SSE

SSE

first order modified (PFOM) K1m h-1

diffusion controla (DC) kd mg g-1 h-0.5

0.9796 0.9822 0.0850 (0.55%) 0.1641 4.4425 0.9925 0.2319 (0.81%) 0.4357 8.9662 0.9938 0.7154 (1.33%) 0.7610

qe 0.8110 2.5260 4.0630

R2

SSEb

0.9942 0.0046 (0.13%) 0.9972 0.0528 (0.52%) 0.9942 0.1363 (0.88%)

D/R2 ×106 s-1 R2

SSE

0.546 0.281 0.308

0.9488 0.0171 (1.37%) 0.9972 0.0033 (0.10%) 0.9854 0.0095 (0.18%)

qe

R2

1.0100 4.3160 8.0567

0.9902 0.0120 (0.48%) 0.9984 0.0588 (0.31%) 0.9941 1.3436 (2.47%)

D/R2 ×106 s-1 R2 0.754 0.254 0.201

SSEb

SSE

0.9646 0.215 (5.87%) 0.9787 0.1320 (1.50%) 0.9976 0.0492 (0.32%)

a Diffusion control model was fit only to data corresponding to a fractional uptake below that of 75% of equilibrium. This corresponds to the first 23 and 47 h of adsorption for GAC-A and GAC-F, respectively. b Within brackets SSE to ∑qt,exp ratio, which represents the percentage of qt not explained by the model.

Figure 5. THMFP of solutions versus contact time with GAC. Solid line THMFPF, dashed line SPTHMFP. (A) r ) 1 mg DOC g GAC-1, (B) r ) 5 mg DOC g GAC-1, (C) r ) 10 mg DOC g GAC-1.

To evaluate MWD of the adsorbed DOM, each of the chromatograms shown in Figure 6A,C was subtracted from the chromatogram of the original untreated solution. Obtained differential chromatograms are shown in Figure 6B,D and illustrate the difference between DOM in the original untreated solution and that remaining after GAC treatment, thus allowing the determination of the MWD of the adsorbed fractions. It was observed that at low NOM concentrations, as occurs in natural waters, both systems present comparable initial uptake rates for high Mw solutes (peak P1 in Figure 6B,D). Coal-based GAC-F more effectively removed large Mw compounds. Other works42 reported that even for highly microporous adsorbents DOM molecules with Mw values up to approximately 2700 Da were able to access the carbon pores. Smaller molecules are likely to be removed more efficiently, which converts activated carbon in one of the most efficient means to remove DOC and THMFP in water treatment processes.44 The main difference between the studied adsorbents occurs on the molecular size distribution of the adsorbed molecules. During the first stages of adsorption, the total amount of adsorbed solute (total area under curve) is 25% lower for carbon GAC-A. However, for this activated

carbon, the height ratio P2/P1 is much higher, which indicates a preferential uptake of low Mw molecules. For example, for GAC-A and GAC-F, values of P2/P1 after 2 h of contact time were 1.25 and 1.01, and after 24 h of contact time, 1.03 and 0.82, respectively. Furthermore, MWD of adsorbed fractions of NOM was more homogeneous for GAC-F, whereas GAC-A adsorbs two clearly differentiable lumps of organic matter. Neither average pore size distribution nor specific surface area values allow explaining these differences, which are likely to arise from differences in the surface chemistry or chemical composition. Regarding the latter, Olson et al.45 found that large concentrations of divalent cations in activated carbon could promote binding to the negatively charged NOM molecules in the water. This way, electrostatic interactions would overcome limitations due to size exclusion effects, and DOM would adsorb over a wider range of Mw as observed for GAC-F. However, this would require further investigation by other analytical techniques. In the literature, it can be found that characteristic HPLCSEC chromatograms of humic substances can consist of different key peaks and shoulders46,47 or a unique band48 depending on

7874 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

Figure 6. SEC chromatograms for GAC-treated solutions. Initial DOC ) 1 mg L-1, r ) 1 mg DOC g GAC-1.

Figure 7. Relationship between THMFP (solid symbols) and SPTHMFP (hollow symbols) and the reduction of selected peak height in HPLC-SEC chromatograms.

analytical conditions. Commonly, these data are used to evaluate the average MW of DOM and/or the process efficiency related to adsorbate size. In the present study, we have tried to analyze these fractions of DOM based on their ability to form THM upon chlorination. Figure 7 shows the relationship between the values of THMFP and SPTHMFP of solutions treated with activated carbon at different contact times and the corresponding removal of peak 1 and peak 2 in HPLC-SEC chromatograms in Figure 6B,D. Two approaches can be considered when analyzing these results: first, to identify which of the HPLCSEC bands is more efficient for the characterization of GAC treated sample; and second, to identify which of the chlorination

tests used to quantify DBPs better correlates with chromatographic properties. Regarding the first approach, all linear regression R2 values and slope values are higher for P2. The calculated slope values are from 70% to 200% higher for P2 as compared to P1. These results indicate that the removal of DBP precursors better correlates and also is more sensitive to the removal of organic molecules comprised in the fraction defined as P2. Based on calibration carried out using PEG and PEO standards, P2 corresponds to molecules with a molecular weight of about 3451 Da. With respect to the second approach, the slope values are significantly higher for THMFP; however, this is due to the

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7875

also higher absolute values measured for this variable. For GACA, no remarkable differences are observed between R2 values measured for THMFPF and those for SPTHMFP. All curves gave values above R2 ) 0.943. However, for activated carbon GAC-F, it is observed that THMFP data better correlate with HPLC-SEC data as inferred from the higher R2 obtained. This can be related to the fact that this adsorbent adsorbed a more heterogeneous mixture of NOM. In general terms, it can be concluded that the relationship between removal of P2 (3451 Da) and THMFP of solution was the strongest and the most significant relationship. Considering that uptake of NOM is higher and less selective onto GACF, these results gain importance because they allow the identification and also quantification of NOM fractions, which are especially problematic from the DBPs formation point of view. The preferential removal of molecules comprised in this specific fraction could allow enhanced control over DBPs. 4. Conclusion Adsorption of a NOM, a commercially available humic acid in this study, onto two commercial activated carbons was studied at relatively extreme conditions. Both Langmuir and Freundlich equations gave an acceptable fit of the experimental data. Although there were similarities in pore size distribution, the adsorption performance significantly differed between adsorbents. On a mass basis, GAC-F carbon showed the best kinetic and thermodynamic properties for NOM adsorption as compared to GAC-A. The higher NOM uptake was attributed mainly to its higher surface basicity. Results from size exclusion chromatography indicate that coconut shell made adsorbent, GACA, showed more selective adsorption properties. Nonequilibrium data revealed that adsorption was favored for two clearly differentiable lumps of organic matter. Molecules in the molecular weight range of 3400-7200 Da were hardly removed from solution by GAC-A. SBET values and pore size distribution could not explain this difference in the adsorption pattern. The surface chemistry or the significantly higher inorganic content of GAC-F could be responsible. At a GAC to NOM ratio of 5 mg DOC g GAC-1, GAC-F removed above 85% of the THMFP and SPTHMFP. In relative terms, the removal of SPTHMFP was higher than removal of THMFPF, which indicates that adsorption onto activated carbon could be effective in reducing THM formation under less severe chlorination conditions (i.e., full-scale operation), although confirmation from experiments with real samples would be required. Reduction of THMFP and DOC followed a similar trend. However, THMFP data showed the existence of some refractory fractions of NOM, undetected through measurement of UV absorbance. Even at the longest contact times and for the lowest NOM concentration, some residual THMFPF was measured. Our overall results show that an accurate and effective removal of precursor should focus on the selective removal of certain fractions of NOM. Size exclusion chromatography data indicate that the removal of low Mw NOM fractions (peak P2) strongly correlates with the reduction in the THM formation capability of sample. This relationship should be further studied also for other other DBPs (i.e., HAA, POX, AOX) and for other activated carbon contact systems (i.e., column adsorption). In this sense, the development of comparable calibration procedures for SEC is of the first necessity for obtaining comparable data from one laboratory to another. An effective adsorbent should possess an appropriate micropore size distribution, surface basicity, and also inorganic content depending on target adsorbate.

Acknowledgment We wish to thank the anonymous IECR reviewers for their thorough review of this paper, the Universidad del Paı´s Vasco, the Ministerio de Educacio´n Cultura y Deporte (grant AP 99 30687378), the Departamento de Industria, Comercio y Turismo (project OD03UN57), and the water engineering firm Pridesa for their technical and economical support. Abbreviations DBPs ) disinfection byproduct DFT ) density functional theory DOC ) dissolved organic carbon DOM ) dissolved organic matter ECD ) electron capture detector FCR ) free chlorine residual GAC ) granular activated carbon MQL ) minimum quantification level MRL ) minimum reporting level MWD ) molecular weight distribution NOM ) natural organic matter PEG ) polyethylene glycol PEO ) polyethylene oxide pHPZC ) pH of zero charge RE ) percent removal efficiency RI ) refractive index SEC ) size exclusion chromatography SD ) standard deviation SPTHMFP ) simulated plant THMFP TGA ) thermogravimetric analysis THM ) trihalomethanes THMFP ) THM formation potential THMFPF ) final THMFP TOC ) total organic carbon UV254 ) UV absorbance at 254 nm Parameters Ce ) equilibrium liquid phase concentration of NOM, mg L-1 Co ) initial liquid phase concentration of NOM, mg L-1 Ct ) liquid phase concentration of NOM after a t period of time, mg L-1 D ) intraparticle diffusivity, m2 s-1 dp ) average diameter of GAC particle, mm k1 ) first-order model kinetic constant, h-1 k1m ) modified first-order model kinetic constant, h-1 k2 ) second-order model kinetic constant, g mg-1 h-1 nF (1-nF) KF ) Freundlich equation parameter, L mg DOC g GAC-1 -1 KL ) Langmuir equation parameter, L g GAC kp ) intraparticle diffusion rate constant, mg g-1 min-1 m ) mass of GAC, g Mn ) number average molecular weight Mw ) weight average molecular weight nF ) Freundlich equation parameter qe ) NOM uptake in equilibrium with the aqueous solution per adsorbent gram, mg DOC g GAC-1 qt ) NOM uptake per adsorbent gram, mg DOC g GAC-1 R ) average radius of GAC particle, mm T50 ) temperature for 50% loss in TGA analysis, K V ) liquid volume, L RL ) Langmuir equation parameter

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ReceiVed for reView April 7, 2008 ReVised manuscript receiVed July 30, 2008 Accepted August 12, 2008 IE800912Y