- Email: [email protected]
Polypyrrole-Coated Granules for Humic Acid Removal
Journal of Colloid and Interface Science 243, 52–60 (2001) doi:10.1006/jcis.2001.7843, available online at http://www.idealibrary.com on
Polypyrrole-Coated Granules for Humic Acid Removal Renbi Bai1 and X. Zhang Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received March 22, 2001; accepted July 14, 2001; published online September 24, 2001
substances in drinking water and other process waters. Processes such as chemical coagulation, adsorption, and membrane separation have been developed for removing humic substances (1, 6, 7). Although coagulation using alum has widely been used, it incurs a high operational cost and generates high volumes of extra sludge that is difficult to handle. In addition, it is suspected that aluminum exposure aids in the development or acceleration of the onset of Alzheimer’s diseases (8). Humic substances also tend to foul membranes seriously and thus limit membrane application in this field (9). Removing humic substances by conventional adsorbents often leads to difficulties because of the water-soluble formations and their wide ranges of molecular weight and size distribution (2, 10). Thus, searching for new adsorbents for effective removal of humic substances has attracted great interest (10, 11). Granular media are widely used in filtration and adsorption processes in many industrial applications, especially in water and wastewater treatment. The two processes often have a similar design involving passing the liquid to be treated through a bed of granular media. Purification takes place when particles or other substances in the fluid attach to the surface of the granules. Filtration is often used for the removal of larger particles, whereas adsorption is used for smaller ones or dissolved substances. The effectiveness of both processes, however, relies on the surface interactions between the media grains and the particles or substances to be removed. For example, sand and anthracite, due to cost-effective considerations, have been commonly used as filter media. These materials, however, are not particularly efficient in removing suspended/colloidal particles, nor are they effective in removing any dissolved inorganic and organic substances, such as humics. The difficulty arises from the fact that sand, anthracite, and the suspended particles or dissolved organic matter to be removed often carry the same kind of surface charges (negative), leading to unfavorable grain–particle surface interactions (12, 13). A simple remedy to the unfavorable surface interactions is to employ granules possessing positive surface charges. Toward this end, attempts were made to coat materials (such as sand) with metallic oxides, hydroxides, or peroxides in order to develop positively charged surfaces (14–19). Although these modified materials were shown to give improved performance, the coated layers, particularly metallic hydroxide, were vulnerable to dissolution when the modified granules were placed in service (20).
Granules with positive surface charges were prepared by coating glass beads with polypyrrole (PPy). The coated glass beads were found to possess high positive zeta potentials over a wide range of pH values. Batch and fixed bed humic acid adsorption experiments using the coated glass beads as adsorbents were conducted. Scanning electron microscope (SEM) was used to examine the surface morphology of the coated glass beads before and after humic acid adsorption. X-ray photoelectron spectroscopy (XPS) was applied to assess the protonation and charge transfer of the PPy coating with and without adsorbed humic acid. SEM images showed that the PPy coating considerably increased the surface roughness of the granules and a significant amount of humic acid was adsorbed on the PPy coating. From XPS analysis, it was found that 28% of the nitrogen atoms in the PPy coating were protonated, leading to a highly positively charged surface at pH < 10.5. The results also showed that the amount of protonated nitrogen atoms decreased by up to 25% due to humic acid adsorption, suggesting that humic acid uptake by the PPy-coated glass beads was affected at least partly by charge neutralization. Humic acid adsorption also resulted in a reverse of the positive zeta potential of the PPy coating, indicating the importance of macromolecular adsorption in the process. The adsorption equilibrium data can be fitted with either the Langmuir or the Freundlich expression. Both pH and ionic concentration were found to affect the extent of humic acid adsorption by the PPycoated granules. °C 2001 Academic Press Key Words: positive surface charges; polypyrrole coating; surfacemodified granule; humic acid removal; adsorption; surface interactions.
INTRODUCTION
Humic substances, which constitute a major class of organic matter present in natural waters (such as lakes, groundwater, and rivers), affect water quality adversely in several ways: causing undesirable color and taste, serving as food for bacterial growth in water distribution systems (1), binding with heavy metals and biocides (pesticides and herbicides) to yield high concentrations of these substances and enhance their transportation in water (2), and reacting with chlorine in water treatment to produce trihalomethanes, which are known human carcinogens (3–5). Hence it is desirable to minimize the presence of humic 1 To whom correspondence should be addressed. Fax: (65) 779 1936. E-mail: [email protected].
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
52
COATED GRANULES FOR HUMIC ACID REMOVAL
In recent years, polypyrrole (PPy) has found increasing applications because of its stability to oxygen and water and its straightforward synthesis (21). PPy is usually made from converting pyrrole into a polymer with a “backbone” of anions incorporated along the chain. The three methods of making PPy include electrochemical polymerization, chemical oxidation– polymerization, and chemical vapor deposition. PPy belongs to a class of intrinsically conducting polymers and displays several interesting properties: redox activity, high surface free energy and polarity, Br¨onsted acid–base chemistry, ion-exchange capacity, strong adsorptive capabilities of gases and macromolecules, and room temperature conductivity (22). Industrial applications of PPy include the preparation of high performance materials in actuators, chemical and biosensors, modified electrodes and electronic devices, etc. (23–26). Its possible application in water and wastewater treatment, however, has not been considered. A new type of granular media that possess positive surface charges has been prepared in our laboratory by coating glass beads with PPy. In this study, we investigated the adsorption of humic acid by surface-modified glass beads. The mechanism of adsorption was studied by X-ray photoelectron spectroscopy (XPS) analysis and zeta potential measurements. The effects of pH and ionic concentration on humic acid removal were also examined. These results are presented below. EXPERIMENTAL
Pyrrole (99%), FeCl3 · 6H2 O (97%), and humic acid (defined as sodium salt in particulate form) were purchased from the Aldrich Chemical Co. Ballotini glass beads, with specific weight of 2.55, were obtained from Jencons (Scientific) Ltd., UK. All the reagents used were of analytical reagent grade, and deionized (D.I.) water was used to prepare the test solutions. Obtaining a PPy coating on the glass beads involved chemical oxidation–polymerization of pyrrole, with Fe3+ as oxidant and water as solvent. FeCl3 · 6H2 O was dissolved in D.I. water with stirring. Glass beads were then added into the solution. After 5 min, pyrrole solution diluted by D.I. water was added in droplets into the mixture with vigorous stirring. The color of the solution rapidly became black. The system was continuously mixed for 3 h to allow the growth of PPy coating on the glass beads. Then the coated glass beads were washed with D.I. water and alcohol, dried in air for 24 h, and stored in a desiccator prior to use. The sizes of the coated and uncoated glass beads were measured using an optical microscope (Olympus BX60, Olympus Optical Co. Ltd., Japan) to determine the coating thickness. The surface morphologies of the coated and uncoated glass beads were examined using a scanning electron microscope (SEM, JEOL JSM-6400) at 10–20 kV. Batch and fixed bed humic acid adsorption experiments were conducted. Before a test, humic acid stock solution was prepared
53
by dissolving a certain amount of humic acid in a known volume of D.I. water. The solution was then mixed with a magnetic stirrer for 1 h and filtered through a Whatman membrane filter (0.45 µm). Batch tests were made by placing 15 g of the PPy-coated glass beads in 250 ml humic acid solution of an initial concentration of 15 mg/L. The solution was shaken slowly in an orbit shaker for 24 h. The history of the humic acid concentrations of the solution was determined through taking and analyzing samples periodically. Fixed bed adsorption tests were conducted in a filter column of 25 mm internal diameter, packed with varying depths of the PPy-coated glass beads. Different influent concentrations and flow rates through the column were used and the effluent humic acid concentrations were monitored against time. For comparison purposes, the use of uncoated glass beads for humic acid adsorption was also included. For the PPy-coated glass beads, batch adsorption equilibrium tests were also made. PPy-coated glass beads (9 g) were placed in a tube of 75 ml humic acid solution with an initial concentration in the range of 10 to 60 mg/L, respectively. Equilibrium was established when three consecutive samples taken at 3-h intervals did not show any change in humic acid concentration. To study the effects of initial pH and ionic concentration on the adsorption of humic acid, HCl and NaOH were added as necessary to adjust the pH values, and NaCl was used to vary the ion concentrations of the test solutions. In these experiments, 3 g of the PPy-coated glass beads was placed into 25 ml of humic acid solution with a known initial concentration until the adsorption equilibrium was established. An ultraviolet–visible spectrometer (Hitachi UV-2000) was used to determine the humic acid concentrations of the samples in both batch and fixed bed experiments. A standard concentration–light intensity curve was first prepared from solutions of known humic acid concentrations (those particles retained on the membrane filter during humic acid solution preparation were dried and weighed to eliminate their effects on the preparation of the standard curve). XPS analyses of the coated glass beads before and after humic acid adsorption were made on a VG ESCALAB MKII spectrometer with an Al K α X-ray source (1486.6 eV of photons) to determine the N atoms present in the PPy coating and their oxidation states. To compensate for the surface charging effects, all binding energies were referenced to the C1’s neutral carbon peak at 284.6 eV (27). Surface elemental stoichiometries were determined from the sensitivity-factor corrected peak area ratios. The software package XPSpeak 4.1 was used to fit the XPS spectra peaks. The zeta potentials of the PPy-coated glass beads were determined by the method described by Bai and Tien (13). The coated glass beads were placed in a 100-ml vial with 50 ml D.I. water and the vial was vibrated in a sonic bath for 24 h. The liquid in the vial with small fragments from the PPy coating in it was then decanted for measurements. The zeta potentials so
54
BAI AND ZHANG
determined were found to be similar to those determined using PPy particles from the polymerization reaction, indicating that the fragments in the decanted liquid were essentially from the PPy coating and were completely made of PPy. A Zeta-Plus4 Instrument (Brookhaven Instruments Corp.) was used to measure the zeta potentials of the PPy fragments, as well as that of humic acid macromolecules. RESULTS AND DISCUSSION
The average diameters of the coated and uncoated glass beads were determined through measuring 50 individual grains using the optical microscope. The uncoated glass beads had an average diameter of 549.96 ± 24.38 µm, and the glass beads after one-time coating and twice coatings had average diameters of 562.33 ± 22.83 µm and 590.36 ± 52.31 µm, respectively. The thickness of the PPy coating was therefore estimated to be 6.2 µm for one-time coating and 20.2 µm for twice coatings. The results reported below used glass beads from one-time coating of PPy. Figure 1 shows the zeta potential values of the coated glass beads as a function of the solution pH. For pH below 10.5, the zeta potentials were positive, and, over the range of pH from 4 to 10, it remained relatively constant (approximately 37.4 mV). In contrast, the uncoated glass beads were found to have negative zeta potentials for pH > 3 (approximately −25 mV at pH around 7) (13). Because of the positive zeta potential, the coated glass beads can be expected to enhance the removal of all negatively charged substances from liquid solutions. XPS analysis was used to characterize the PPy coating on the glass beads. XPS is a relatively new technology used to identify the elemental composition and oxidation states of a solid material surface. For clarity of the presentations to be followed later, the principle of XPS analysis is briefly described here.
FIG. 1. Zeta potentials of the PPy coating on the coated glass beads before and after humic acid adsorption.
FIG. 2. N 1s XPS spectrum for the PPy coating on the coated glass beads.
An atom is made of a nucleus and a number of electrons that orbit the nucleus with defined orbits. The analysis of XPS involves catching the electrons from the matter being studied in order to find out from which atom they are coming. To be able to free the electrons from the nucleus attractive force, the sample to be analyzed is exposed to an X-ray source. This particular type of source brings the electrons enough energy to free them from the nucleus. Once they have been freed up, some of them carrying enough energy may leave the solid matter and are collected by an electron analyzer. The most important information in the analysis is the so-called binding energy (BE) which an electron had before leaving the atom. By counting the electrons for various BEs, a corresponding spectrum is obtained. Since each value of the BE is characteristic for a given element, and each peak area is proportional to the number of atoms being present in the solid matter studied (23, 28), the chemical elemental composition of the sample surface is therefore determined by calculating the respective contribution of each peak area. Appropriate data processing then leads to the determination of the chemical-bound nature that exists between these elements. In addition, the BE of an element increases with increasing its oxidation state and the energy shifts are typically in the range of 0–3 eV. For the PPy coating in this study, our interest has focused on the oxidation state of nitrogen. The N 1s core-level spectrum from XPS analysis of the coated glass beads (dry PPy fragments) is presented in Fig. 2. The BE of the main pyrrolylium nitrogen component (–NH–) was about 399.4 eV. The higher BE tail in the N 1s spectrum at 401.7 eV was attributed to the positively charged nitrogen atoms (N+ ). The proportion of positively charged nitrogen atoms in the PPy coating layer was found to be 28% (in terms of N+ /N). It is these protonated N+ atoms that contribute to the positive surface charges of the PPy-coated glass beads. PPy has a neutral polymer backbone in its reduced state (pyrrole). Applying an oxidant in the chemical polymerization results in the backbone placed in its oxidized state, with
COATED GRANULES FOR HUMIC ACID REMOVAL
55
positive charges accompanying the incorporation of electrolyte anions (29). The oxidation–polymerization reaction stoichiometries have been proposed, respectively, as in Eq. [1] (30) and Eq. [2] (31), 4(C4 H5 N) + 9FeCl3 → (C4 H3 N)4 + Cl− + 8HCl + 9FeCl2 , [1] 3(C4 H5 N) + 7FeCl3 → (C4 H3 N)3 + Cl− + 6HCl + 7FeCl2 , [2] where Cl− is the dopant anion. Equation [1] indicates a pyrrole/ Cl− ratio of 4 : 1 (25% N+ /N) while Eq. [2] a ratio of 3 : 1 (33% N+ /N) in the PPy complex. The PPy synthesized by oxidative polymerization in other studies was reported to have a proportion of positively charged nitrogen atoms in the range of 25 to 30% (23), which was comparable to the 28% determined in this study. The 25–30% of N+ /N ratio suggests that in the polymerization of pyrrole into PPy complex, both Eq. [1] and Eq. [2] reactions are likely to be coexisting, and, on an average, at least one pyrrole monomer among every four pyrrole monomers in the PPy complex is protonated. The N atoms in the PPy complex may exist as imine (==N–), amine (–NH–), and/or electron deficient nitrogen (N+ ). The results in Fig. 2, however, show that under the present experimental condition, imine nitrogen was absent in the PPy coating because a peak at a BE lower than 399.4 eV in the spectrum was not observed. PPy may be partially deprotonated by a base or the deprotonated polymer may in turn be reprotonated by an acid through a process as shown in Scheme 1. Furthermore, the insertion or aggregation of OH− or H+ on the surface of PPy in strong basic or strong acidic conditions may occur (32). These properties of PPy are responsible for the observed changes of PPy’s zeta potentials with solution pH, as shown in Fig. 1. The morphologies of the uncoated and coated glass beads were examined under a scanning electron microscope. The images are shown in Fig. 3. The surface of glass beads was relatively smooth (Fig. 3a). Polymerization of pyrrole generated a large number of fine PPy particles (ranging from a few tens to a few hundred nanometers) which deposited over the surfaces
SCHEME 1
FIG. 3. SEM images showing the surface morphology of the uncoated and coated glass beads: (a) surface of glass bead, (b) surface of the PPy-coated glass bead, and (c) surface of the PPy-coated glass bead (enlarged).
of the glass beads. The coated surface was rough with a considerable number of ringlike structures (Figs. 3b and 3c). These structures may be attributed to the adsorption and release of gas bubbles (i.e., O2 ) during the oxidation–polymerization reaction of the pyrrole (33). The rough surface due to the formation of the ringlike structures has been considered as surface defects in other applications, but it is actually beneficial for particle removal through adsorption and filtration in water and wastewater treatment.
56
BAI AND ZHANG
SCHEME 2
FIG. 4. Typical results of humic acid adsorption by the coated and uncoated glass beads (initial solution pH of 6.7; other conditions as specified in the experimental section in the text).
Typical batch adsorption results of humic acid are shown in Fig. 4. For the uncoated glass beads, the adsorption of humic acid was very limited, probably due to the repulsive interaction between the negatively charged glass beads and the humic acid molecules. The PPy-coated glass beads, however, had considerably higher adsorptive capacity for humic acid. After 10 h, adsorption equilibrium appeared to be established. The specific amount of humic acid adsorption was calculated from mass balance to be 0.17 mg (humic acid)/g (coated glass beads) in this case. Typical XPS results for the N 1s spectrum of the Ppy-coated glass beads with humic acid adsorption are shown in Fig. 5.
FIG. 5. Typical N 1s XPS spectrum for the PPy coating adsorbed with humic acid.
In contrast with the results of Fig. 2, the N+ /N ratio reduced to about 21%. Since no imine nitrogen was found in the spectrum, the results therefore indicate that a portion of the positively charged nitrogen atoms in the PPy coating layer was neutralized by humic acid. The reaction may proceed as shown in Scheme 2, where R–COO− stands for a humic acid macromolecule. The percentage of the N+ atoms reacted with humic acid may be calculated, as in this case, to be only 25% of the total positively charged N+ atoms presented before humic acid adsorption. Compared with the size of a pyrrole monomer in the PPy polymer molecule, humic acid is believed to have a much larger molecular size. Therefore, only some of the protonated pyrrole monomers in the PPy molecules may contact the reactive groups (i.e., COOH, etc.) of a humic acid molecule and thus result in reaction. In the N 1s XPS spectrum in Fig. 5, the BE of the main pyrrolylium nitrogen component (–NH–) is found to be 399.7 eV. In comparison with 399.4 eV for the main pyrrolylium nitrogen in the PPy layer absent of humic acid (Fig. 2), the increase of 0.3 eV in BE may indicate a partial “protonation” of the main pyrrolylium nitrogens in the PPy layer adsorbed with humic acid. This phenomenon may be explained by “charge transfer” because PPy is electrically conductive. When some of the N+ atoms in the PPy molecules reacted with humic acid, partial positive “charge transfer” may occur from those unreacted N+ atoms to the reacted N atoms through conjugated bonds in the PPy backbone. As a consequence, the neutral nitrogen components (–NH–) may become partially or slightly positively charged. This could greatly enhance the adsorption of humic acid as –NH– constitutes more than 70% of the nitrogen atoms in the PPy coating. Special attention was paid in obtaining the results of Fig. 5. Humic acid obtained from Aldrich was specified at containing a trace amount of nitrogen (the supplier specified 0.68% by weight). To make sure that the N 1s spectrum in Fig. 5 was representative of the nitrogen in the PPy layer but not that of humic acid, XPS analyses were performed for the uncoated glass beads
57
COATED GRANULES FOR HUMIC ACID REMOVAL
(without nitrogen component) with deposited humic acid and for the Aldrich humic acid dry particles, respectively. The results confirmed that the nitrogen component in the humic acid was indeed very small and that no protonated nitrogen atoms (N+ ) were detected. In addition, since the humic acid had a wide range of sizes (from subnanometer up to 84 nm with an average size around 5 nm), and since the sampling depth of the XPS analysis was at about 5–10 nm (28), the angle of the sample in the XPS analysis was adjusted to obtain a N 1s spectrum of significance (if the analysis were located at a place on the PPy layer where a large humic acid macromolecule was adsorbed, the XPS analysis may not give a noticeable N 1s spectrum because the sampling depth of the XPS analysis did not reach the PPy layer of the coated glass beads). Zeta potential measurements were performed to study the effect of humic acid adsorption on the electrical properties of the PPy coating. These results under various solution pH values are also given in Fig. 1. The most noticeable difference is that the zeta potentials of the PPy coating (PPy fragments from the coated glass beads) adsorbed with humic acid were mostly negative. Adsorption of humic acid therefore not only neutralized the positive charges but also reversed the surface charges of the PPy coating. The mechanisms of humic acid removal by the PPy-coated glass beads therefore include at least chemical reaction between the reactive groups, which is responsible for charge neutralization, and macromolecular adsorption, which is responsible for charge reversal. The first mechanism may act quickly but the second mechanism can be a slow process. The images in Fig. 6 clearly show that a layer of humic acid deposited on the surface of the Ppy-coated glass beads. The adsorption equilibrium data were modeled using the Langmuir equation given as
FIG. 6. SEM images showing (a) the PPy coating of the glass bead and (b) the PPy coating with humic acid deposition.
bC X = , X max 1 + bC
Alternatively, the Freundlich equation is often used in describing the adsorption isotherm of a single substance, i.e.,
[3]
where X is the amount of humic acid adsorbed per unit weight of coated glass beads at equilibrium concentration (mg/g), C is the equilibrium concentration of the humic acid solution (mg/L), X max is the maximum amount of adsorption at monolayer coverage (mg/g), and b is the adsorption equilibrium constant (L/mg). Equation [3] can be rewritten to give 1 1 1 1 + = . X X max X max bC
[4]
A plot of 1/ X versus 1/C would generate a straight line. The intercept gives access to the estimation of Xmax and the slope to b. Figure 7a shows the fitted results of Eq. [4] to the experimental points from the adsorption equilibrium tests at neutral pH and without addition of any salt (indifferent electrolyte). The results give 1/ X = 2.5254 + 214.9986/C (R = 0.999) with X max = 0.395974 mg/g and b = 0.011746 L/mg.
1
X = PC n
[5]
where X and C have the same definitions as in Eq. [3], P is a constant representing the adsorption capacity (mg/g)(L/mg)n , and n is a constant depicting the adsorption intensity (dimensionless). Equation [5] can be transformed to the log-linearized form log X =
1 log C + log P. n
[6]
The plot of log X versus log C based on Eq. [6] to the same experimental results in Fig. 7a is shown in Fig. 7b, with a regression result of log X = 0.79993 log C − 2.179 (R = 0.994). The values of P and b are therefore determined to be P = 0.006622 and n = 1.25011 (A value of n larger than one suggests that the amount of adsorption will approach a limit). Figure 7 indicates that either the Langmuir or the Freundlich equation is able to fit the adsorption equilibrium data adequately.
58
BAI AND ZHANG
FIG. 7. Adsorption equilibrium data of humic acid on the PPy-coated glass beads, fitted with the Langmuir and Freundlich equations respectively (pH 7, without addition of any indifferent electrolyte). (a) Langmuir model fitted to the experimental results. (b) Log-linearized Freundlich model fitted to the experimental results.
that with the PPy coating, the available surface area for humic acid adsorption was significantly increased. The rough surfaces of the coated glass beads shown in Figs. 3 and 6 are in line with this speculation. In fact, BET surface area analyses for the coated and uncoated glass beads indicated that the surface area of the glass beads increased by approximately 347 times per bead after coating with PPy. Because of the macromolecular feature of humic acid, however, not all of the surface area was available for humic acid adsorption (especially those in the small pores). Solution pH and ionic concentration are often shown to be the important parameters determining adsorption performance because of their influence on the nature of surface interactions between media grains and the substances to be removed. The results on the effect of the initial solution pH are shown in Fig. 8. It is clear that the extent of adsorption generally improved with a decrease of the initial pH, and there was hardly any adsorption at pH > 12. It is also interesting to note that the amount of humic acid adsorbed by the PPy-coated glass beads decreased slightly from pH 3.1 to pH 1.1 although the zeta potential of the PPy coating was even more positive at pH 1 than at pH 3 (see Fig. 1). The results can be explained by the interactions between the PPy coating and the humic acid to be adsorbed. The pK a of the–COOH groups in humic acid molecules was at around pH 3 (34). For pH > 10.5, both the PPy coating and the humic acid had negative zeta potentials. Owing to the strong electrostatic repulsion, no humic acid adsorption therefore took place at pH > 12. For pH < 3, however, the PPy coating had strong positive zeta potential and the humic acid became positively charged with decreasing pH. Again, the electrostatic repulsion developed between the PPy molecules and the humic acid limited the adsorption of humic acid on the coated glass beads, leading to the observed decrease of humic acid adsorption at pH 1.1, as compared to that at pH 3.1. The significant amount of humic acid adsorption at pH 3.1 (the point of zero charge of humic acid) also indicates the importance of molecular adsorption in the process.
Assuming that the average diameter and specific weight of the coated glass beads are 562 µm and 2.55 g/cm3 (see the earlier sections) and that the sizes of humic acid molecules are 2–80 nm with an average size of 5 nm and a specific weight of 1.587 g/cm3 (2), one can calculate, based on the maximum monolayer adsorption quantity of X max = 0.395974 mg/g, the surface areas occupied by the adsorbed humic acid molecules to be 5.64688 × 10−5 –1.41172 × 10−6 m2 per bead, corresponding to humic acid sizes of 2–80 nm, and 2.25875 × 10−5 m2 per bead if the average humic acid size is 5 nm (assuming humic acid is spherical and its deposition has a minimum excluded surface area factor of 1.273). The occupied surface areas will therefore be 56.9–1.42 times that of the surface area of a smooth media grain (9.923 × 10−7 m2 ) for humic acid sizes of 2–80 nm, or 22.76 times that of the media grain for an average humic acid size of 5 nm. In other words, the calculation suggests that humic acid adsorption may occur in multiple layers. However, if a monolayer adsorption did occur, the calculation indicates
FIG. 8. Effect of initial solution pH on the adsorption of humic acid by the PPy-coated glass beads (3 g grains in 25 ml solution).
COATED GRANULES FOR HUMIC ACID REMOVAL
FIG. 9. Effect of ionic concentration on the adsorption of humic acid by the PPy-coated glass beads at pH 6.5 (3 g grains in 25 ml solution).
From pH 10 to 3.1, the adsorption of humic acid showed a gradual increase with decreasing pH. Since the negative zeta potentials of the humic acid reduced with decresing pH and the positive zeta potentials of the PPy coating were relatively constant, humic acid adsorption on the PPy surface was therefore in a more compact pattern in the lateral direction due to reduced repulsion between the adsorbed humic acid molecules, which would result in more humic acid molecules being adsorbed on the surface at a lower pH (35). The effect of ionic concentration on humic acid removal at near neutral pH is shown in Fig. 9. Improved removal with increasing ionic concentration (NaCl) was observed, but the effect was not significant. It is important again to consider the electrostatic interactions between the humic acid molecules themselves as well as between the humic acid molecules and the PPy coating, as adsorption performance depends on both of these interactions. Generally speaking, an increase in the ionic concentration would reduce the lateral electrostatic repulsion between the adsorbed humic acid molecules, thus leading to increased adsorption. Increasing the ionic concentration is also believed to reduce the size of humic acid molecules and hence to increase the amount of humic acid adsorption (35). On the other hand, when the ionic concentration is increased, some of the charges are screened and the attractive force between the humic acid molecules and the PPy molecules is reduced. The positive ions in the solution will also compete with the PPy coating for the negative sites in the humic acid molecules. These last two effects will contribute to reduced adsorption of humic acid. Fixed bed adsorption tests for humic acid removal were also conducted. The results are given in Fig. 10. The major difference between the fixed bed and the batch adsorption tests is that in the former the solution flowed through the media bed at a certain velocity and the humic acid in the solution had a much shorter contact time with the media grains (only about 1–3 min). Similar to the batch experiments, the uncoated glass beads did not show much capacity for humic acid adsorption
59
in fixed bed adsorption tests, but the PPy-coated glass beads again showed considerably higher adsorption capacity. The history of humic acid removal was found to consist of two distinct phases: an initial high but rapid decreasing removal phase, followed by a low but relatively constant removal phase. The initial phase may be attributed to the charge neutralization, while the later phase to a molecular adsorption mechanism. Charge neutralization is a faster process and depends on the number of protonated N+ - atoms (which decreased with time), but molecular adsorption is generally slower and depends on the surface areas of the media grains only. Two-phase breakthrough curves were also reported for humic acid removal with activated carbon (6). In the initial phase, the adsorption is found to have a stronger dependence on the amount of the media grains in the bed than on the contact time. For example, the experiment with bed depth L = 11 cm and flow rate q = 24 ml/min had only slightly lower removal rate than the one with L = 11cm and q = 12 ml/min, even though the contact time in the first one was only half of that in the second one. Calculations were made for the amount of humic acid adsorption in the experiments shown in Fig. 10. The amounts of humic acid adsorbed in the three experiments with the coated glass beads at a time up to 420 min are in the range of 0.17–0.30 mg (humic acid)/g (coated glass beads). These quantities are still considerably lower than that of X max = 0.395974 determined in the batch isotherm adsorption equilibrium tests. It therefore explains the observed relatively constant removal of humic acid in the second phase shown in Fig. 10 because the media grains were still adsorptive. PPy has been reported to be very stable under normal temperature and pH conditions in many other applications (23–26). A preliminary test was conducted in this work to examine whether there would be any dissolution of the PPy coating on the glass beads in normal water pH condition. Coated and
FIG. 10. Removal of humic acid in fixed bed tests with the coated and uncoated glass beads as media grains (L is media depth, q is flow rate through the bed, influent humic acid concentration Co = 15 mg/L, solution pH is 6.9).
60
BAI AND ZHANG
ACKNOWLEDGMENTS The financial support of the Academic Research Funds, National University of Singapore, is acknowledged.
REFERENCES
FIG. 11. Changes of conductivities in solutions immersed with the coated and uncoated glass beads, respectively.
uncoated glass beads (10 g) were placed respectively into two of three flasks (one served as blank) containing 250 ml of D.I. water with pH 6.7. The conductivities of the solutions in the three flasks were monitored at room temperature (22–24◦ C) for over 40 days. As can be seen from the results in Fig. 11, although the conductivities of the solutions with the coated or uncoated glass beads exhibited a certain degree of increase over time, the changes were essentially very small (possibly due to the impurities on the grain surface). The coated glass beads in fact appeared to be even more stable than the uncoated glass beads (lower increase of conductivity). In a subsequent batch adsorption test, the performances of the freshly coated glass beads and the coated glass beads immersed in D.I. water after 40 days were compared and no significant difference was observed (results not shown). These results confirmed the stability of the PPy coating. CONCLUSIONS
Granules with positive surface charges are desirable for the removal of most suspended, colloidal, and dissolved substances in water and wastewater as those substances are usually negatively charged. Glass beads coated with PPy were found to have high positive zeta potentials over a wide range of pH values and to be very stable in normal water pH conditions. The new material gives considerably better performance in humic acid removal than the uncoated glass beads. The mechanisms of humic acid removal by the PPy-coated glass beads are found to include both charge neutralization as well as molecular adsorption.
1. Jacangelo, J. G., DeMarco, J., Owen, D. M., and Randtke, S. J., J. Am. Water Works ASS. 87, 64 (1995). 2. Jone, M. N., and Bryan, N. D., Adv. Colloid Interface Sci. 78, 1 (1997). 3. Bellar, T. A., Lichtenberg, J. J., and Kroner, R. C., J. Am. Water Works ASS. 66, 703 (1974). 4. World Health Organization, in “Health Criteria and Other Supporting Information,” Vol. 2. WHO, Geneva, 1996. 5. Singer, P. C., Water Sci. Technol. 40, 25 (1999). 6. Fettig, J., Water Sci. Technol. 40, 173 (1999). 7. Alborzfar, M., Jonsson, G., and Gron, Ch., Water Res. 32, 2983 (1998). 8. World Health Organization, in “Addendum to Vol. 1, Recommendations.” WHO, Geneva, 1998. 9. Nystrom, M., Ruohomaki, K., and Kaipia, L., Desalination 106, 79 (1996). 10. Seida, Y., and Nakano, Y., Water Res. 34, 1487 (2000). 11. Teermann, I. P., and Jekel, M. R., Water Sci. Technol. 40, 199 (1999). 12. Lukasik, J., Cheng, Y. F., Lu, F., Tamplin, M., and Farrah, S. R., Water Res. 33, 769 (1999). 13. Bai, R., and Tien, C., J. Colloid Interface Sci. 218, 488 (1999). 14. Zerda, K. S., Gerba, C. P., Houand, K. C., and Goyal, S. M., Appl. Environ. Microbiol. 49, 91 (1985). 15. Farrah, S. R., and Preston, D. R., Appl. Environ. Microbiol. 50, 1502 (1985). 16. Lukasik, J., Truesdail, S., Shah, D., and Farrah, S. R., Kona 14, 87 (1996). 17. Chaudhuri, M., and Sattar, S. A., Water Sci. Technol. 10, 77 (1986). 18. Gerba, C. P., Houand, K., and Sobsey, M. D., J. Environ. Sci. Health 23, 41 (1988). 19. Edwards, E., and Benjamin, M. M., J. Water Pollut. Fed. 9, 1523 (1989). 20. Chen, J., Truesdail, S., Lu, F., Zhan, G., Belvin, C., Koopman, B., Farrah, S., and Shah, D., Water Res. 32, 2171 (1998). 21. Skotheim, T. A., Elsenbaumer, R., and Reynolds, J., (Eds.), “Handbook of Conducting Polymers.” Marcel Dekker, New York, 1998. 22. Saoudi, B., Despas, C., Chehimi, M. M., Jammul, N., Delamar, M., Bessi`ere, J., and Walcarius, A., Sens. Actuators B 62, 35 (2000). 23. Kang, E. T., Neoh, K. G., and Tan, K. L., Adv. Polym. Sci. 106, 135 (1993). 24. Meagher, L., Klauber, C., and Pashley, R. M., Colloids Surf. A 106, 63 (1996). 25. Lee, H. S., and Hong, J., Synth. Met. 113, 115 (2000). 26. Thi´eblemont, J. C., Gabelle, J. L., and Planche, M. F., Synth. Met. 66, 243 (1994). 27. Kim, D. Y., Lee, J. Y., Kim, C. Y., Kang, E. T., and Tan, K. L., Synth. Met. 72, 243 (1995). 28. Cairns, D. B., Armes, S. P., Chehimi, M. M., Perruchot, C., and Delamar, M., Langmuir 15, 8059 (1999). 29. Deronzier, A., and Moutet, J. C., Coord. Chem. Rev. 147, 339 (1996). 30. Pron, A., Kucharski, Z., Budrowski, C., Zagorska, M., Krichene, S., Suwalski, J., Dehe, G., and Lefrant, S., J. Chem. Phys. 83, 5923 (1985). 31. Machida, S., Miyata, S., and Techagumpuch, T., Synth. Met. 31, 311 (1989). 32. Michalska, A., and Maksymiuk, K., Electrochim. Acta 44, 2125 (1999). 33. Unsworth, J., Innis, P. C., Lunn, B. A., Jin, Z., and Norton, G. P., Synth. Met. 53, 59 (1992). 34. Thurman, E. M., Wershaw, R. L., and Malcolm, D. J., Org. Geochem. 4, 27 (1982). 35. Avena, M. J., and Koopal, L. K., Environ. Sci. Technol. 33, 2739 (1999).
Polypyrrole-Coated Granules for Humic Acid Removal Renbi Bai1 and X. Zhang Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received March 22, 2001; accepted July 14, 2001; published online September 24, 2001
substances in drinking water and other process waters. Processes such as chemical coagulation, adsorption, and membrane separation have been developed for removing humic substances (1, 6, 7). Although coagulation using alum has widely been used, it incurs a high operational cost and generates high volumes of extra sludge that is difficult to handle. In addition, it is suspected that aluminum exposure aids in the development or acceleration of the onset of Alzheimer’s diseases (8). Humic substances also tend to foul membranes seriously and thus limit membrane application in this field (9). Removing humic substances by conventional adsorbents often leads to difficulties because of the water-soluble formations and their wide ranges of molecular weight and size distribution (2, 10). Thus, searching for new adsorbents for effective removal of humic substances has attracted great interest (10, 11). Granular media are widely used in filtration and adsorption processes in many industrial applications, especially in water and wastewater treatment. The two processes often have a similar design involving passing the liquid to be treated through a bed of granular media. Purification takes place when particles or other substances in the fluid attach to the surface of the granules. Filtration is often used for the removal of larger particles, whereas adsorption is used for smaller ones or dissolved substances. The effectiveness of both processes, however, relies on the surface interactions between the media grains and the particles or substances to be removed. For example, sand and anthracite, due to cost-effective considerations, have been commonly used as filter media. These materials, however, are not particularly efficient in removing suspended/colloidal particles, nor are they effective in removing any dissolved inorganic and organic substances, such as humics. The difficulty arises from the fact that sand, anthracite, and the suspended particles or dissolved organic matter to be removed often carry the same kind of surface charges (negative), leading to unfavorable grain–particle surface interactions (12, 13). A simple remedy to the unfavorable surface interactions is to employ granules possessing positive surface charges. Toward this end, attempts were made to coat materials (such as sand) with metallic oxides, hydroxides, or peroxides in order to develop positively charged surfaces (14–19). Although these modified materials were shown to give improved performance, the coated layers, particularly metallic hydroxide, were vulnerable to dissolution when the modified granules were placed in service (20).
Granules with positive surface charges were prepared by coating glass beads with polypyrrole (PPy). The coated glass beads were found to possess high positive zeta potentials over a wide range of pH values. Batch and fixed bed humic acid adsorption experiments using the coated glass beads as adsorbents were conducted. Scanning electron microscope (SEM) was used to examine the surface morphology of the coated glass beads before and after humic acid adsorption. X-ray photoelectron spectroscopy (XPS) was applied to assess the protonation and charge transfer of the PPy coating with and without adsorbed humic acid. SEM images showed that the PPy coating considerably increased the surface roughness of the granules and a significant amount of humic acid was adsorbed on the PPy coating. From XPS analysis, it was found that 28% of the nitrogen atoms in the PPy coating were protonated, leading to a highly positively charged surface at pH < 10.5. The results also showed that the amount of protonated nitrogen atoms decreased by up to 25% due to humic acid adsorption, suggesting that humic acid uptake by the PPy-coated glass beads was affected at least partly by charge neutralization. Humic acid adsorption also resulted in a reverse of the positive zeta potential of the PPy coating, indicating the importance of macromolecular adsorption in the process. The adsorption equilibrium data can be fitted with either the Langmuir or the Freundlich expression. Both pH and ionic concentration were found to affect the extent of humic acid adsorption by the PPycoated granules. °C 2001 Academic Press Key Words: positive surface charges; polypyrrole coating; surfacemodified granule; humic acid removal; adsorption; surface interactions.
INTRODUCTION
Humic substances, which constitute a major class of organic matter present in natural waters (such as lakes, groundwater, and rivers), affect water quality adversely in several ways: causing undesirable color and taste, serving as food for bacterial growth in water distribution systems (1), binding with heavy metals and biocides (pesticides and herbicides) to yield high concentrations of these substances and enhance their transportation in water (2), and reacting with chlorine in water treatment to produce trihalomethanes, which are known human carcinogens (3–5). Hence it is desirable to minimize the presence of humic 1 To whom correspondence should be addressed. Fax: (65) 779 1936. E-mail: [email protected].
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
52
COATED GRANULES FOR HUMIC ACID REMOVAL
In recent years, polypyrrole (PPy) has found increasing applications because of its stability to oxygen and water and its straightforward synthesis (21). PPy is usually made from converting pyrrole into a polymer with a “backbone” of anions incorporated along the chain. The three methods of making PPy include electrochemical polymerization, chemical oxidation– polymerization, and chemical vapor deposition. PPy belongs to a class of intrinsically conducting polymers and displays several interesting properties: redox activity, high surface free energy and polarity, Br¨onsted acid–base chemistry, ion-exchange capacity, strong adsorptive capabilities of gases and macromolecules, and room temperature conductivity (22). Industrial applications of PPy include the preparation of high performance materials in actuators, chemical and biosensors, modified electrodes and electronic devices, etc. (23–26). Its possible application in water and wastewater treatment, however, has not been considered. A new type of granular media that possess positive surface charges has been prepared in our laboratory by coating glass beads with PPy. In this study, we investigated the adsorption of humic acid by surface-modified glass beads. The mechanism of adsorption was studied by X-ray photoelectron spectroscopy (XPS) analysis and zeta potential measurements. The effects of pH and ionic concentration on humic acid removal were also examined. These results are presented below. EXPERIMENTAL
Pyrrole (99%), FeCl3 · 6H2 O (97%), and humic acid (defined as sodium salt in particulate form) were purchased from the Aldrich Chemical Co. Ballotini glass beads, with specific weight of 2.55, were obtained from Jencons (Scientific) Ltd., UK. All the reagents used were of analytical reagent grade, and deionized (D.I.) water was used to prepare the test solutions. Obtaining a PPy coating on the glass beads involved chemical oxidation–polymerization of pyrrole, with Fe3+ as oxidant and water as solvent. FeCl3 · 6H2 O was dissolved in D.I. water with stirring. Glass beads were then added into the solution. After 5 min, pyrrole solution diluted by D.I. water was added in droplets into the mixture with vigorous stirring. The color of the solution rapidly became black. The system was continuously mixed for 3 h to allow the growth of PPy coating on the glass beads. Then the coated glass beads were washed with D.I. water and alcohol, dried in air for 24 h, and stored in a desiccator prior to use. The sizes of the coated and uncoated glass beads were measured using an optical microscope (Olympus BX60, Olympus Optical Co. Ltd., Japan) to determine the coating thickness. The surface morphologies of the coated and uncoated glass beads were examined using a scanning electron microscope (SEM, JEOL JSM-6400) at 10–20 kV. Batch and fixed bed humic acid adsorption experiments were conducted. Before a test, humic acid stock solution was prepared
53
by dissolving a certain amount of humic acid in a known volume of D.I. water. The solution was then mixed with a magnetic stirrer for 1 h and filtered through a Whatman membrane filter (0.45 µm). Batch tests were made by placing 15 g of the PPy-coated glass beads in 250 ml humic acid solution of an initial concentration of 15 mg/L. The solution was shaken slowly in an orbit shaker for 24 h. The history of the humic acid concentrations of the solution was determined through taking and analyzing samples periodically. Fixed bed adsorption tests were conducted in a filter column of 25 mm internal diameter, packed with varying depths of the PPy-coated glass beads. Different influent concentrations and flow rates through the column were used and the effluent humic acid concentrations were monitored against time. For comparison purposes, the use of uncoated glass beads for humic acid adsorption was also included. For the PPy-coated glass beads, batch adsorption equilibrium tests were also made. PPy-coated glass beads (9 g) were placed in a tube of 75 ml humic acid solution with an initial concentration in the range of 10 to 60 mg/L, respectively. Equilibrium was established when three consecutive samples taken at 3-h intervals did not show any change in humic acid concentration. To study the effects of initial pH and ionic concentration on the adsorption of humic acid, HCl and NaOH were added as necessary to adjust the pH values, and NaCl was used to vary the ion concentrations of the test solutions. In these experiments, 3 g of the PPy-coated glass beads was placed into 25 ml of humic acid solution with a known initial concentration until the adsorption equilibrium was established. An ultraviolet–visible spectrometer (Hitachi UV-2000) was used to determine the humic acid concentrations of the samples in both batch and fixed bed experiments. A standard concentration–light intensity curve was first prepared from solutions of known humic acid concentrations (those particles retained on the membrane filter during humic acid solution preparation were dried and weighed to eliminate their effects on the preparation of the standard curve). XPS analyses of the coated glass beads before and after humic acid adsorption were made on a VG ESCALAB MKII spectrometer with an Al K α X-ray source (1486.6 eV of photons) to determine the N atoms present in the PPy coating and their oxidation states. To compensate for the surface charging effects, all binding energies were referenced to the C1’s neutral carbon peak at 284.6 eV (27). Surface elemental stoichiometries were determined from the sensitivity-factor corrected peak area ratios. The software package XPSpeak 4.1 was used to fit the XPS spectra peaks. The zeta potentials of the PPy-coated glass beads were determined by the method described by Bai and Tien (13). The coated glass beads were placed in a 100-ml vial with 50 ml D.I. water and the vial was vibrated in a sonic bath for 24 h. The liquid in the vial with small fragments from the PPy coating in it was then decanted for measurements. The zeta potentials so
54
BAI AND ZHANG
determined were found to be similar to those determined using PPy particles from the polymerization reaction, indicating that the fragments in the decanted liquid were essentially from the PPy coating and were completely made of PPy. A Zeta-Plus4 Instrument (Brookhaven Instruments Corp.) was used to measure the zeta potentials of the PPy fragments, as well as that of humic acid macromolecules. RESULTS AND DISCUSSION
The average diameters of the coated and uncoated glass beads were determined through measuring 50 individual grains using the optical microscope. The uncoated glass beads had an average diameter of 549.96 ± 24.38 µm, and the glass beads after one-time coating and twice coatings had average diameters of 562.33 ± 22.83 µm and 590.36 ± 52.31 µm, respectively. The thickness of the PPy coating was therefore estimated to be 6.2 µm for one-time coating and 20.2 µm for twice coatings. The results reported below used glass beads from one-time coating of PPy. Figure 1 shows the zeta potential values of the coated glass beads as a function of the solution pH. For pH below 10.5, the zeta potentials were positive, and, over the range of pH from 4 to 10, it remained relatively constant (approximately 37.4 mV). In contrast, the uncoated glass beads were found to have negative zeta potentials for pH > 3 (approximately −25 mV at pH around 7) (13). Because of the positive zeta potential, the coated glass beads can be expected to enhance the removal of all negatively charged substances from liquid solutions. XPS analysis was used to characterize the PPy coating on the glass beads. XPS is a relatively new technology used to identify the elemental composition and oxidation states of a solid material surface. For clarity of the presentations to be followed later, the principle of XPS analysis is briefly described here.
FIG. 1. Zeta potentials of the PPy coating on the coated glass beads before and after humic acid adsorption.
FIG. 2. N 1s XPS spectrum for the PPy coating on the coated glass beads.
An atom is made of a nucleus and a number of electrons that orbit the nucleus with defined orbits. The analysis of XPS involves catching the electrons from the matter being studied in order to find out from which atom they are coming. To be able to free the electrons from the nucleus attractive force, the sample to be analyzed is exposed to an X-ray source. This particular type of source brings the electrons enough energy to free them from the nucleus. Once they have been freed up, some of them carrying enough energy may leave the solid matter and are collected by an electron analyzer. The most important information in the analysis is the so-called binding energy (BE) which an electron had before leaving the atom. By counting the electrons for various BEs, a corresponding spectrum is obtained. Since each value of the BE is characteristic for a given element, and each peak area is proportional to the number of atoms being present in the solid matter studied (23, 28), the chemical elemental composition of the sample surface is therefore determined by calculating the respective contribution of each peak area. Appropriate data processing then leads to the determination of the chemical-bound nature that exists between these elements. In addition, the BE of an element increases with increasing its oxidation state and the energy shifts are typically in the range of 0–3 eV. For the PPy coating in this study, our interest has focused on the oxidation state of nitrogen. The N 1s core-level spectrum from XPS analysis of the coated glass beads (dry PPy fragments) is presented in Fig. 2. The BE of the main pyrrolylium nitrogen component (–NH–) was about 399.4 eV. The higher BE tail in the N 1s spectrum at 401.7 eV was attributed to the positively charged nitrogen atoms (N+ ). The proportion of positively charged nitrogen atoms in the PPy coating layer was found to be 28% (in terms of N+ /N). It is these protonated N+ atoms that contribute to the positive surface charges of the PPy-coated glass beads. PPy has a neutral polymer backbone in its reduced state (pyrrole). Applying an oxidant in the chemical polymerization results in the backbone placed in its oxidized state, with
COATED GRANULES FOR HUMIC ACID REMOVAL
55
positive charges accompanying the incorporation of electrolyte anions (29). The oxidation–polymerization reaction stoichiometries have been proposed, respectively, as in Eq. [1] (30) and Eq. [2] (31), 4(C4 H5 N) + 9FeCl3 → (C4 H3 N)4 + Cl− + 8HCl + 9FeCl2 , [1] 3(C4 H5 N) + 7FeCl3 → (C4 H3 N)3 + Cl− + 6HCl + 7FeCl2 , [2] where Cl− is the dopant anion. Equation [1] indicates a pyrrole/ Cl− ratio of 4 : 1 (25% N+ /N) while Eq. [2] a ratio of 3 : 1 (33% N+ /N) in the PPy complex. The PPy synthesized by oxidative polymerization in other studies was reported to have a proportion of positively charged nitrogen atoms in the range of 25 to 30% (23), which was comparable to the 28% determined in this study. The 25–30% of N+ /N ratio suggests that in the polymerization of pyrrole into PPy complex, both Eq. [1] and Eq. [2] reactions are likely to be coexisting, and, on an average, at least one pyrrole monomer among every four pyrrole monomers in the PPy complex is protonated. The N atoms in the PPy complex may exist as imine (==N–), amine (–NH–), and/or electron deficient nitrogen (N+ ). The results in Fig. 2, however, show that under the present experimental condition, imine nitrogen was absent in the PPy coating because a peak at a BE lower than 399.4 eV in the spectrum was not observed. PPy may be partially deprotonated by a base or the deprotonated polymer may in turn be reprotonated by an acid through a process as shown in Scheme 1. Furthermore, the insertion or aggregation of OH− or H+ on the surface of PPy in strong basic or strong acidic conditions may occur (32). These properties of PPy are responsible for the observed changes of PPy’s zeta potentials with solution pH, as shown in Fig. 1. The morphologies of the uncoated and coated glass beads were examined under a scanning electron microscope. The images are shown in Fig. 3. The surface of glass beads was relatively smooth (Fig. 3a). Polymerization of pyrrole generated a large number of fine PPy particles (ranging from a few tens to a few hundred nanometers) which deposited over the surfaces
SCHEME 1
FIG. 3. SEM images showing the surface morphology of the uncoated and coated glass beads: (a) surface of glass bead, (b) surface of the PPy-coated glass bead, and (c) surface of the PPy-coated glass bead (enlarged).
of the glass beads. The coated surface was rough with a considerable number of ringlike structures (Figs. 3b and 3c). These structures may be attributed to the adsorption and release of gas bubbles (i.e., O2 ) during the oxidation–polymerization reaction of the pyrrole (33). The rough surface due to the formation of the ringlike structures has been considered as surface defects in other applications, but it is actually beneficial for particle removal through adsorption and filtration in water and wastewater treatment.
56
BAI AND ZHANG
SCHEME 2
FIG. 4. Typical results of humic acid adsorption by the coated and uncoated glass beads (initial solution pH of 6.7; other conditions as specified in the experimental section in the text).
Typical batch adsorption results of humic acid are shown in Fig. 4. For the uncoated glass beads, the adsorption of humic acid was very limited, probably due to the repulsive interaction between the negatively charged glass beads and the humic acid molecules. The PPy-coated glass beads, however, had considerably higher adsorptive capacity for humic acid. After 10 h, adsorption equilibrium appeared to be established. The specific amount of humic acid adsorption was calculated from mass balance to be 0.17 mg (humic acid)/g (coated glass beads) in this case. Typical XPS results for the N 1s spectrum of the Ppy-coated glass beads with humic acid adsorption are shown in Fig. 5.
FIG. 5. Typical N 1s XPS spectrum for the PPy coating adsorbed with humic acid.
In contrast with the results of Fig. 2, the N+ /N ratio reduced to about 21%. Since no imine nitrogen was found in the spectrum, the results therefore indicate that a portion of the positively charged nitrogen atoms in the PPy coating layer was neutralized by humic acid. The reaction may proceed as shown in Scheme 2, where R–COO− stands for a humic acid macromolecule. The percentage of the N+ atoms reacted with humic acid may be calculated, as in this case, to be only 25% of the total positively charged N+ atoms presented before humic acid adsorption. Compared with the size of a pyrrole monomer in the PPy polymer molecule, humic acid is believed to have a much larger molecular size. Therefore, only some of the protonated pyrrole monomers in the PPy molecules may contact the reactive groups (i.e., COOH, etc.) of a humic acid molecule and thus result in reaction. In the N 1s XPS spectrum in Fig. 5, the BE of the main pyrrolylium nitrogen component (–NH–) is found to be 399.7 eV. In comparison with 399.4 eV for the main pyrrolylium nitrogen in the PPy layer absent of humic acid (Fig. 2), the increase of 0.3 eV in BE may indicate a partial “protonation” of the main pyrrolylium nitrogens in the PPy layer adsorbed with humic acid. This phenomenon may be explained by “charge transfer” because PPy is electrically conductive. When some of the N+ atoms in the PPy molecules reacted with humic acid, partial positive “charge transfer” may occur from those unreacted N+ atoms to the reacted N atoms through conjugated bonds in the PPy backbone. As a consequence, the neutral nitrogen components (–NH–) may become partially or slightly positively charged. This could greatly enhance the adsorption of humic acid as –NH– constitutes more than 70% of the nitrogen atoms in the PPy coating. Special attention was paid in obtaining the results of Fig. 5. Humic acid obtained from Aldrich was specified at containing a trace amount of nitrogen (the supplier specified 0.68% by weight). To make sure that the N 1s spectrum in Fig. 5 was representative of the nitrogen in the PPy layer but not that of humic acid, XPS analyses were performed for the uncoated glass beads
57
COATED GRANULES FOR HUMIC ACID REMOVAL
(without nitrogen component) with deposited humic acid and for the Aldrich humic acid dry particles, respectively. The results confirmed that the nitrogen component in the humic acid was indeed very small and that no protonated nitrogen atoms (N+ ) were detected. In addition, since the humic acid had a wide range of sizes (from subnanometer up to 84 nm with an average size around 5 nm), and since the sampling depth of the XPS analysis was at about 5–10 nm (28), the angle of the sample in the XPS analysis was adjusted to obtain a N 1s spectrum of significance (if the analysis were located at a place on the PPy layer where a large humic acid macromolecule was adsorbed, the XPS analysis may not give a noticeable N 1s spectrum because the sampling depth of the XPS analysis did not reach the PPy layer of the coated glass beads). Zeta potential measurements were performed to study the effect of humic acid adsorption on the electrical properties of the PPy coating. These results under various solution pH values are also given in Fig. 1. The most noticeable difference is that the zeta potentials of the PPy coating (PPy fragments from the coated glass beads) adsorbed with humic acid were mostly negative. Adsorption of humic acid therefore not only neutralized the positive charges but also reversed the surface charges of the PPy coating. The mechanisms of humic acid removal by the PPy-coated glass beads therefore include at least chemical reaction between the reactive groups, which is responsible for charge neutralization, and macromolecular adsorption, which is responsible for charge reversal. The first mechanism may act quickly but the second mechanism can be a slow process. The images in Fig. 6 clearly show that a layer of humic acid deposited on the surface of the Ppy-coated glass beads. The adsorption equilibrium data were modeled using the Langmuir equation given as
FIG. 6. SEM images showing (a) the PPy coating of the glass bead and (b) the PPy coating with humic acid deposition.
bC X = , X max 1 + bC
Alternatively, the Freundlich equation is often used in describing the adsorption isotherm of a single substance, i.e.,
[3]
where X is the amount of humic acid adsorbed per unit weight of coated glass beads at equilibrium concentration (mg/g), C is the equilibrium concentration of the humic acid solution (mg/L), X max is the maximum amount of adsorption at monolayer coverage (mg/g), and b is the adsorption equilibrium constant (L/mg). Equation [3] can be rewritten to give 1 1 1 1 + = . X X max X max bC
[4]
A plot of 1/ X versus 1/C would generate a straight line. The intercept gives access to the estimation of Xmax and the slope to b. Figure 7a shows the fitted results of Eq. [4] to the experimental points from the adsorption equilibrium tests at neutral pH and without addition of any salt (indifferent electrolyte). The results give 1/ X = 2.5254 + 214.9986/C (R = 0.999) with X max = 0.395974 mg/g and b = 0.011746 L/mg.
1
X = PC n
[5]
where X and C have the same definitions as in Eq. [3], P is a constant representing the adsorption capacity (mg/g)(L/mg)n , and n is a constant depicting the adsorption intensity (dimensionless). Equation [5] can be transformed to the log-linearized form log X =
1 log C + log P. n
[6]
The plot of log X versus log C based on Eq. [6] to the same experimental results in Fig. 7a is shown in Fig. 7b, with a regression result of log X = 0.79993 log C − 2.179 (R = 0.994). The values of P and b are therefore determined to be P = 0.006622 and n = 1.25011 (A value of n larger than one suggests that the amount of adsorption will approach a limit). Figure 7 indicates that either the Langmuir or the Freundlich equation is able to fit the adsorption equilibrium data adequately.
58
BAI AND ZHANG
FIG. 7. Adsorption equilibrium data of humic acid on the PPy-coated glass beads, fitted with the Langmuir and Freundlich equations respectively (pH 7, without addition of any indifferent electrolyte). (a) Langmuir model fitted to the experimental results. (b) Log-linearized Freundlich model fitted to the experimental results.
that with the PPy coating, the available surface area for humic acid adsorption was significantly increased. The rough surfaces of the coated glass beads shown in Figs. 3 and 6 are in line with this speculation. In fact, BET surface area analyses for the coated and uncoated glass beads indicated that the surface area of the glass beads increased by approximately 347 times per bead after coating with PPy. Because of the macromolecular feature of humic acid, however, not all of the surface area was available for humic acid adsorption (especially those in the small pores). Solution pH and ionic concentration are often shown to be the important parameters determining adsorption performance because of their influence on the nature of surface interactions between media grains and the substances to be removed. The results on the effect of the initial solution pH are shown in Fig. 8. It is clear that the extent of adsorption generally improved with a decrease of the initial pH, and there was hardly any adsorption at pH > 12. It is also interesting to note that the amount of humic acid adsorbed by the PPy-coated glass beads decreased slightly from pH 3.1 to pH 1.1 although the zeta potential of the PPy coating was even more positive at pH 1 than at pH 3 (see Fig. 1). The results can be explained by the interactions between the PPy coating and the humic acid to be adsorbed. The pK a of the–COOH groups in humic acid molecules was at around pH 3 (34). For pH > 10.5, both the PPy coating and the humic acid had negative zeta potentials. Owing to the strong electrostatic repulsion, no humic acid adsorption therefore took place at pH > 12. For pH < 3, however, the PPy coating had strong positive zeta potential and the humic acid became positively charged with decreasing pH. Again, the electrostatic repulsion developed between the PPy molecules and the humic acid limited the adsorption of humic acid on the coated glass beads, leading to the observed decrease of humic acid adsorption at pH 1.1, as compared to that at pH 3.1. The significant amount of humic acid adsorption at pH 3.1 (the point of zero charge of humic acid) also indicates the importance of molecular adsorption in the process.
Assuming that the average diameter and specific weight of the coated glass beads are 562 µm and 2.55 g/cm3 (see the earlier sections) and that the sizes of humic acid molecules are 2–80 nm with an average size of 5 nm and a specific weight of 1.587 g/cm3 (2), one can calculate, based on the maximum monolayer adsorption quantity of X max = 0.395974 mg/g, the surface areas occupied by the adsorbed humic acid molecules to be 5.64688 × 10−5 –1.41172 × 10−6 m2 per bead, corresponding to humic acid sizes of 2–80 nm, and 2.25875 × 10−5 m2 per bead if the average humic acid size is 5 nm (assuming humic acid is spherical and its deposition has a minimum excluded surface area factor of 1.273). The occupied surface areas will therefore be 56.9–1.42 times that of the surface area of a smooth media grain (9.923 × 10−7 m2 ) for humic acid sizes of 2–80 nm, or 22.76 times that of the media grain for an average humic acid size of 5 nm. In other words, the calculation suggests that humic acid adsorption may occur in multiple layers. However, if a monolayer adsorption did occur, the calculation indicates
FIG. 8. Effect of initial solution pH on the adsorption of humic acid by the PPy-coated glass beads (3 g grains in 25 ml solution).
COATED GRANULES FOR HUMIC ACID REMOVAL
FIG. 9. Effect of ionic concentration on the adsorption of humic acid by the PPy-coated glass beads at pH 6.5 (3 g grains in 25 ml solution).
From pH 10 to 3.1, the adsorption of humic acid showed a gradual increase with decreasing pH. Since the negative zeta potentials of the humic acid reduced with decresing pH and the positive zeta potentials of the PPy coating were relatively constant, humic acid adsorption on the PPy surface was therefore in a more compact pattern in the lateral direction due to reduced repulsion between the adsorbed humic acid molecules, which would result in more humic acid molecules being adsorbed on the surface at a lower pH (35). The effect of ionic concentration on humic acid removal at near neutral pH is shown in Fig. 9. Improved removal with increasing ionic concentration (NaCl) was observed, but the effect was not significant. It is important again to consider the electrostatic interactions between the humic acid molecules themselves as well as between the humic acid molecules and the PPy coating, as adsorption performance depends on both of these interactions. Generally speaking, an increase in the ionic concentration would reduce the lateral electrostatic repulsion between the adsorbed humic acid molecules, thus leading to increased adsorption. Increasing the ionic concentration is also believed to reduce the size of humic acid molecules and hence to increase the amount of humic acid adsorption (35). On the other hand, when the ionic concentration is increased, some of the charges are screened and the attractive force between the humic acid molecules and the PPy molecules is reduced. The positive ions in the solution will also compete with the PPy coating for the negative sites in the humic acid molecules. These last two effects will contribute to reduced adsorption of humic acid. Fixed bed adsorption tests for humic acid removal were also conducted. The results are given in Fig. 10. The major difference between the fixed bed and the batch adsorption tests is that in the former the solution flowed through the media bed at a certain velocity and the humic acid in the solution had a much shorter contact time with the media grains (only about 1–3 min). Similar to the batch experiments, the uncoated glass beads did not show much capacity for humic acid adsorption
59
in fixed bed adsorption tests, but the PPy-coated glass beads again showed considerably higher adsorption capacity. The history of humic acid removal was found to consist of two distinct phases: an initial high but rapid decreasing removal phase, followed by a low but relatively constant removal phase. The initial phase may be attributed to the charge neutralization, while the later phase to a molecular adsorption mechanism. Charge neutralization is a faster process and depends on the number of protonated N+ - atoms (which decreased with time), but molecular adsorption is generally slower and depends on the surface areas of the media grains only. Two-phase breakthrough curves were also reported for humic acid removal with activated carbon (6). In the initial phase, the adsorption is found to have a stronger dependence on the amount of the media grains in the bed than on the contact time. For example, the experiment with bed depth L = 11 cm and flow rate q = 24 ml/min had only slightly lower removal rate than the one with L = 11cm and q = 12 ml/min, even though the contact time in the first one was only half of that in the second one. Calculations were made for the amount of humic acid adsorption in the experiments shown in Fig. 10. The amounts of humic acid adsorbed in the three experiments with the coated glass beads at a time up to 420 min are in the range of 0.17–0.30 mg (humic acid)/g (coated glass beads). These quantities are still considerably lower than that of X max = 0.395974 determined in the batch isotherm adsorption equilibrium tests. It therefore explains the observed relatively constant removal of humic acid in the second phase shown in Fig. 10 because the media grains were still adsorptive. PPy has been reported to be very stable under normal temperature and pH conditions in many other applications (23–26). A preliminary test was conducted in this work to examine whether there would be any dissolution of the PPy coating on the glass beads in normal water pH condition. Coated and
FIG. 10. Removal of humic acid in fixed bed tests with the coated and uncoated glass beads as media grains (L is media depth, q is flow rate through the bed, influent humic acid concentration Co = 15 mg/L, solution pH is 6.9).
60
BAI AND ZHANG
ACKNOWLEDGMENTS The financial support of the Academic Research Funds, National University of Singapore, is acknowledged.
REFERENCES
FIG. 11. Changes of conductivities in solutions immersed with the coated and uncoated glass beads, respectively.
uncoated glass beads (10 g) were placed respectively into two of three flasks (one served as blank) containing 250 ml of D.I. water with pH 6.7. The conductivities of the solutions in the three flasks were monitored at room temperature (22–24◦ C) for over 40 days. As can be seen from the results in Fig. 11, although the conductivities of the solutions with the coated or uncoated glass beads exhibited a certain degree of increase over time, the changes were essentially very small (possibly due to the impurities on the grain surface). The coated glass beads in fact appeared to be even more stable than the uncoated glass beads (lower increase of conductivity). In a subsequent batch adsorption test, the performances of the freshly coated glass beads and the coated glass beads immersed in D.I. water after 40 days were compared and no significant difference was observed (results not shown). These results confirmed the stability of the PPy coating. CONCLUSIONS
Granules with positive surface charges are desirable for the removal of most suspended, colloidal, and dissolved substances in water and wastewater as those substances are usually negatively charged. Glass beads coated with PPy were found to have high positive zeta potentials over a wide range of pH values and to be very stable in normal water pH conditions. The new material gives considerably better performance in humic acid removal than the uncoated glass beads. The mechanisms of humic acid removal by the PPy-coated glass beads are found to include both charge neutralization as well as molecular adsorption.
1. Jacangelo, J. G., DeMarco, J., Owen, D. M., and Randtke, S. J., J. Am. Water Works ASS. 87, 64 (1995). 2. Jone, M. N., and Bryan, N. D., Adv. Colloid Interface Sci. 78, 1 (1997). 3. Bellar, T. A., Lichtenberg, J. J., and Kroner, R. C., J. Am. Water Works ASS. 66, 703 (1974). 4. World Health Organization, in “Health Criteria and Other Supporting Information,” Vol. 2. WHO, Geneva, 1996. 5. Singer, P. C., Water Sci. Technol. 40, 25 (1999). 6. Fettig, J., Water Sci. Technol. 40, 173 (1999). 7. Alborzfar, M., Jonsson, G., and Gron, Ch., Water Res. 32, 2983 (1998). 8. World Health Organization, in “Addendum to Vol. 1, Recommendations.” WHO, Geneva, 1998. 9. Nystrom, M., Ruohomaki, K., and Kaipia, L., Desalination 106, 79 (1996). 10. Seida, Y., and Nakano, Y., Water Res. 34, 1487 (2000). 11. Teermann, I. P., and Jekel, M. R., Water Sci. Technol. 40, 199 (1999). 12. Lukasik, J., Cheng, Y. F., Lu, F., Tamplin, M., and Farrah, S. R., Water Res. 33, 769 (1999). 13. Bai, R., and Tien, C., J. Colloid Interface Sci. 218, 488 (1999). 14. Zerda, K. S., Gerba, C. P., Houand, K. C., and Goyal, S. M., Appl. Environ. Microbiol. 49, 91 (1985). 15. Farrah, S. R., and Preston, D. R., Appl. Environ. Microbiol. 50, 1502 (1985). 16. Lukasik, J., Truesdail, S., Shah, D., and Farrah, S. R., Kona 14, 87 (1996). 17. Chaudhuri, M., and Sattar, S. A., Water Sci. Technol. 10, 77 (1986). 18. Gerba, C. P., Houand, K., and Sobsey, M. D., J. Environ. Sci. Health 23, 41 (1988). 19. Edwards, E., and Benjamin, M. M., J. Water Pollut. Fed. 9, 1523 (1989). 20. Chen, J., Truesdail, S., Lu, F., Zhan, G., Belvin, C., Koopman, B., Farrah, S., and Shah, D., Water Res. 32, 2171 (1998). 21. Skotheim, T. A., Elsenbaumer, R., and Reynolds, J., (Eds.), “Handbook of Conducting Polymers.” Marcel Dekker, New York, 1998. 22. Saoudi, B., Despas, C., Chehimi, M. M., Jammul, N., Delamar, M., Bessi`ere, J., and Walcarius, A., Sens. Actuators B 62, 35 (2000). 23. Kang, E. T., Neoh, K. G., and Tan, K. L., Adv. Polym. Sci. 106, 135 (1993). 24. Meagher, L., Klauber, C., and Pashley, R. M., Colloids Surf. A 106, 63 (1996). 25. Lee, H. S., and Hong, J., Synth. Met. 113, 115 (2000). 26. Thi´eblemont, J. C., Gabelle, J. L., and Planche, M. F., Synth. Met. 66, 243 (1994). 27. Kim, D. Y., Lee, J. Y., Kim, C. Y., Kang, E. T., and Tan, K. L., Synth. Met. 72, 243 (1995). 28. Cairns, D. B., Armes, S. P., Chehimi, M. M., Perruchot, C., and Delamar, M., Langmuir 15, 8059 (1999). 29. Deronzier, A., and Moutet, J. C., Coord. Chem. Rev. 147, 339 (1996). 30. Pron, A., Kucharski, Z., Budrowski, C., Zagorska, M., Krichene, S., Suwalski, J., Dehe, G., and Lefrant, S., J. Chem. Phys. 83, 5923 (1985). 31. Machida, S., Miyata, S., and Techagumpuch, T., Synth. Met. 31, 311 (1989). 32. Michalska, A., and Maksymiuk, K., Electrochim. Acta 44, 2125 (1999). 33. Unsworth, J., Innis, P. C., Lunn, B. A., Jin, Z., and Norton, G. P., Synth. Met. 53, 59 (1992). 34. Thurman, E. M., Wershaw, R. L., and Malcolm, D. J., Org. Geochem. 4, 27 (1982). 35. Avena, M. J., and Koopal, L. K., Environ. Sci. Technol. 33, 2739 (1999).