A Comparative Study on the StructureâPerformance Relationships of

Article pubs.acs.org/IECR

A Comparative Study on the Structure−Performance Relationships of Chemically and Electrochemically Coagulated Al(OH)3 Flocs† Orhan Taner Can*,‡ and Mahmut Bayramoglu§ ‡

Bitlis Eren University, Department of Environmental Engineering, 13000 Bitlis, Turkey Gebze Institute of Technology, Department of Chemical Engineering, Cayirova, 41400 Gebze, Turkey

§

S Supporting Information *

ABSTRACT: In this paper, various physical and chemical characteristics of electro-coagulated (ECC) and chemical-coagulated (CC) aluminum hydroxide flocs were explored comparatively in the context of their impacts on the pollutant removal performance of the processes. First, the sedimentation rate, floc size and zeta potential of the flocs at different pH values were measured in solution. Furthermore, flocs dried at 105 °C were characterized by various techniques such as TGA,FT-IR, SEM, XRD, BET and XPS as well as by chemical analysis. It was found that ECC flocs were bigger in size, faster in sedimentation and better pressed, while they had higher BET area, less water and higher isoelectric pH than CC flocs. Furthermore, pollutant removal tests using various chemicals such as benzoquinone, hydroquinone and a reactive dye showed that ECC flocs outperformed CC flocs, due to superior physical-chemical characteristics.

1. INTRODUCTION Conventional coagulation/flocculation processes usually involve three stages namely, the addition of chemical reagents to destabilize the pollutants by the formation of particles with reduced solubility from the pollutants, the formation of larger sized flocs attained by a soft mix that allows the collision between particles and their aggregation, and finally the separation of the solids by settling or by dissolved air flotation. As an alternative to conventional chemical coagulation (CC), electrocoagulation (ECC) consists of in situ generation of coagulants by the electrodissolution of a sacrificial anode, usually of aluminum or iron.1,2 The main reactions occurring at the electrodes during ECC are as follows: Anode: Al(s) → Al3 +(aq) + 3e−

protonation/deprotonation reaction of surface sites, is mainly a function of pH and the ionic strength of the aqueous solution. On the other hand, the fundamental differences in the surface morphology of typical aluminum hydroxides can be attributed to the type and combination of singly or doubly bonded hydroxyl with aluminum ions. More aluminum-bonded hydroxyls means stronger surface acidity, and more oxygen involved in the surface bonding means stronger surface alkalinity. Potentiometric titration techniques are commonly used to evaluate the surface acid−base properties.8−12 ECC removes soluble or colloidal pollutants by means of various mechanisms such as ionic complexation or ion exchange on the floc surface active sites and the enmeshment of the colloidal pollutants into the sweep flocs which are eventually removed by sedimentation or by flotation by means of hydrogen bubbles generated at cathodes.13,14 ECC as an eco-friendly and cost-effective process,15−19 has been successfully applied to treat a broad range of wastewaters of various industries such as chemical mechanical polishing,20,21 phosphate,22,23 surfactant,24 food process,25 semiconductor,26 olive mill,27,28 restaurant,29 metal plating,30 tannery,31 chromium(VI),32 potato chip manufacturing,33 dairy,34 poultry slaughterhouses,35,36 pulp and paper mills37 and textile wastewaters.38−50 CC and ECC methods have been compared both technically and economically in a study which showed the beneficial advantages of ECC such as lower operating cost,51,52 less material consumption, and less sludge production than CC for similar COD and turbidity removal levels.52 Furthermore, a literature survey has shown that ECC and CC processes were not assessed comparatively in the light of physical and chemical characteristics of coagulated flocs. Thus, the one

(1)

Cathode: 3H 2O(l) + 3e− →

3 H 2(g) + 3OH−(aq) 2

(2)

Al3+ and OH− ions generated by electrode reactions 1 and 2 react to form various monomeric and polymeric species Aln(OH)3n−m m which transform finally into solid Al(OH)3 according to complex precipitation kinetics.3−5 Al3 +(aq) + 3H 2O(l) → Al(OH)3(s) + 3H+(aq)

(3)

The physicochemical reactions on the surface of aluminum hydroxides flocs are complicated largely due to various physical− chemical processes. The correlation between the crystal structure, morphology, and surface chemistry is complicated. The surface hydroxyl groups ionize as Brønsted acid or base sites in appropriate circumstances. The nature of the surface sites determines the ability to bind protons and then the concentration of these sites macroscopically presents acid− base properties.6,7 The surface charge, which is due to the © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3528

August 24, 2013 February 6, 2014 February 11, 2014 February 11, 2014 dx.doi.org/10.1021/ie402789w | Ind. Eng. Chem. Res. 2014, 53, 3528−3538

Industrial & Engineering Chemistry Research

Article

Figure 1. The experimental setup.

aim of this paper is to investigate various floc properties such as sedimentation rate, floc size, and zeta potential relevant to the performance of the processes. For this purpose, both types of flocs were characterized by various techniques such as thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS) surface charge analysis as well as by chemical analysis (CA). Another important aim of this study is to put forth the effect of pollutant chemical structure on the performances of the flocs. Thus, various chemicals with different structures and polarities, namely, a polar chemical pbenzoquinone (BQ), a nonpolar chemical hydroquinone (HQ), and a textile dye with a complex structure containing strong ionic groups Remazol Red RB 133, were selected as pollutant prototypes. These chemicals are used widely in various industries, but they cause serious health and environmental problems; acute exposure to high levels of BQ via inhalation in humans is highly irritating to the eyes, resulting in discoloration of the conjunctiva and cornea, while dermal exposure causes dermatitis with skin discoloration and erythema.53−55 On the other hand, HQ is an important phenolic compound used in a wide number of biological and industrial processes,56,57 and in the aquatic environment it is considered an important xenobiotic micropollutant. Exposure to HQ causes health hazard effects to humans and animals.58,59 Finally, Remazol Red RB 133 is a widely used reactive textile dye that contains two of the most commonly used anchors, monochlorotriazine and vinyl sulfone groups. The pollution induced by dyestuff losses and discharge during dyeing and finishing processes has been a serious environmental problem; dyes in wastewater undergo chemical as well as biological changes, consume dissolved oxygen from the streams, and destroy aquatic life by forming toxic and

carcinogenic aromatic amines under uncontrolled anaerobic conditions.60

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. The experimental setup is shown in Figure 1. The electrocoagulation cell was made of plexiglass with the dimensions of 70 mm × 70 mm × 250 mm and a 650 cm3 working capacity. Seven electrodes with the dimensions of 200 mm × 60 mm × 3 mm and a total effective area of 684 cm2 consisting of four aluminum anodes of 99.53% purity and three stainless steel cathodes were placed at equal distance of 7 mm in the electrocoagulation cell. The electrodes were connected in monopolar parallel mode to a digital DC power supply (Agilent 6674A system; 0-60 V/0-35A) equipped with potentiostatic or galvanostatic operational options. During the runs, the pH of the solution was controlled at the desired value within 0.1 pH unit accuracy by means of a process control system consisting of a PID microcontroller, acid (A) and base (B) dosing pumps, a recirculation pump (P), and a pH sensor with a response time of 3 s. 2.2. Experimental Procedure and Chemicals. Procedure for ECC. The runs were conducted at a constant temperature of 25 °C. On the basis of preliminary experiments, the liquid circulation rate in the EC reactor solution was chosen as 1200 cm3 min−1 to control the pH efficiently and to homogenize the composition of the solution. Before each run, aluminum electrodes were washed with acetone to remove surface grease, then surfaces impurities were removed by dipping for 5 min in a solution prepared by mixing 100 mL of HCl solution (35%) and 200 mL of hexamethylenetetramine aqueous solution (2.80%).61 For each run, 0.65 dm3 of pure water was placed into the electrolytic cell whose conductivity was adjusted by NaNO3 solution, the current density was adjusted to a desired value, and the run was started. At the end of the run, the solution was 3529

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scanning speed of 5° min−1. Sample surfaces gold coated with a sputter in the vacuum were examined using the Philips XL30 S FEI model SEM. The BET surface area of the dried aluminum hydroxides were determined by nitrogen physisorption using the Quantachrome Autosorb1. Before each measurement, the sample was degassed at 300 °C for 3 h. For XPS, the structure and morphology of the aluminum hydroxides floc surfaces were studied using the Phobus 150 Specs electron analyzer with conventional X-ray source (Al Kα).

filtered, the filtrate was centrifuged at 2000 rpm, and the electrodes were washed thoroughly with water to remove any solid residues on the surfaces, dried, and reweighed. In each run, 0.65 dm3 of distilled water of which the conductivity was adjusted by NaNO3 1000 μS cm−1 solution was placed into the electrolytic cell. The current density was adjusted to 1.37 A and the run was started. At the end of the run, the solution was filtered, the filtrate was centrifuged at 2000 rpm, and the electrodes were washed thoroughly with water to remove any solid residues on the surfaces, dried, and reweighed. The experiment time was adjusted to 20 min. Procedure for CC. A known quantity of Al2(SO4)3·18H2O (equivalent to the amount of aluminum anode dissolved during the corresponding ECC run) was dissolved in 0.65 dm3 of distilled water at 25 °C. While the mixture was stirred at 500 rpm, 1 M NaOH was added to adjust the pH to the desired value and finally the slurry was mixed at 200 rpm for 20 min. Merck quality Al(OH)3·xH2O (hydrargillite) was used as a reference in various floc characterization tests. Furthermore, Merck quality BQ, HQ, and a reactive textile dye containing strong ionic groups (Remazol Red RB 133 provided by DyStar Company, defined as Reactive Red 198 in the Color Index) were used for testing the pollutant removal performances of CC and ECC. Spectrophotometric analysis of the chemical solutions was performed using the Perkin-Elmer UV−vis spectrophotometer (model Lambda 35) at 246 nm, 288 nm, and 518 nm for BQ, HQ and Remazol Red RB 133 respectively. 2.3. Floc Characterization Procedures and Equipment. 2.3.1. Analyses Conducted with Floc Suspensions. Surface charge analysis, floc size analysis and sedimentation tests were performed in floc suspensions to calculate the point of zero charge (pHpzc), single particle and floc (aggregate) sizes, the floc density, and the sedimentation rate. The equipment and analysis procedures are summarized as follows. The zeta potential measurements of the surface charge analysis were obtained using the Malvern Zetasizer Nano ZS instrument. Floc size analysis was performed using the Malvern Mastersizer 2000 Particle Size Analysis device. For the sedimentation test, 650 cm3 of floc solutions obtained from ECC or CC were slowly drained without disturbing the flocs into a transparent plexiglas sedimentation column with 32 mm internal diameter and 670 mm length. The column was brought to an upright position after shaking gently to ensure homogeneity. The stopwatch was started and the heights of sedimented flocs were recorded at predetermined time intervals. 2.3.2. Analyses Conducted with Dried Flocs. Aluminum hydroxide suspensions were filtered and dried in the oven at 105 °C until constant weight. TGA/DTA, CA, FTIR, XRD, SEM, BET, and XPS analysis were conducted on the dried flocs. The equipment and analysis procedures are summarized as follows: TG/DTA was carried out on a Mettler Toledo, TGA/SDTA 851 thermal analyzer, with 5 °C min−1 heating rate up to 1000 °C. For chemical analysis, the Al contents of dried aluminum hydroxide flocs were determined by the complexometric titration method using xylenol orange indicator. FTIR adsorption spectra were obtained in the frequency range 500−4000 cm−1 using the BioRad Tropical Option for FTS 175 C FT-IR spectrometer. For the analyses a semitransparent disk of 13 mm diameter containing 1 mg of mixture of dried ECC or CC floc and 200 mg of KBr was prepared by pressing under 10 MPa. The crystal structure of the flocs was determined using a Rigaku Dmax 2200 XRD diffractometer with Cu Kα radiation at 40 kV/40 mA. Each sample was scanned from 5° to 75° with 0.020° resolution and a

3. RESULTS AND DISCUSSION 3.1. Surface Charge Analysis. The zeta potential value allows the understanding of the relevant characteristics of colloidal systems such as the nature and density of electrical charges on particle surfaces. A high zeta potential value stabilizes colloidal suspensions by preventing the formation of aggregates. Furthermore, zeta potential and the thickness of the diffuse layer decrease with increasing ionic strength of the solution, causing an increase in aggregate sizes.62 The relationship between the zeta potential and the degree of coagulation63 is given in Supporting Information Table 1. The zeta potentials of ECC, CC, and standard aluminum hydroxide floc solutions dosed with equal amounts of Al at different pH values are presented in Figure 2. The points of zero

Figure 2. Zeta potential as function of pH for ECC, CC, and Al(OH)3 suspensions at equivalent Al dose (0.331 g L−1).

charge (pHpzc) where zeta potential is zero were detected as 8.5 for ECC floc, 5.5 for the CC floc, and 5.0 for merck Al(OH)3. In terms of the surface charge; it is negative above pH 8.5 and 5.5 in the cases of ECC and CC, respectively. The occurrence of zeta potential within the range of +15 and −15 mV is due to the high ionic forces. As seen from Figure 2, ECC floc has a high positive surface charge within a wider pH range while CC floc has a negative surface charge. Consequently, the surface charge/pH relations of ECC and CC are quite different than each other. Therefore, the chemical−electrochemical coagulation processes, where mechanisms such as absorption−ionic interaction are important, may exhibit different removal performances at the same pH, depending on the type of the charged groups carried by the pollutant (cationic−anionic) and the polarity of the pollutant (polar−apolar). 3.2. Sedimentation Analysis. In this study, a generalized semiempirical form of the flux density function was used.64−69 The sedimentation rate of the flocs is defined as

vS = − 3530

dz dt

(4) dx.doi.org/10.1021/ie402789w | Ind. Eng. Chem. Res. 2014, 53, 3528−3538


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The solid concentration at a height z is calculated as z φ(z) = φ0 0 zc ≤ z ≤ z 0 z T (t )

obtained νs−φ data set, linear regression was implemented between ν1/n s and φ(z) according to eq 6; u∞ the floc settling rate at infinite dilution and k the solid volume fraction the flocs (the ratio of the floc total volume to the solid volume in the floc) were calculated.

(5)

where, z0 is the initial height of the suspension (0.64 m), z is the amount of sedimentation (m), νs is the speed of the flocs (m s−1), φ is the volumetric concentration of solid in suspension, φ0 is the initial volumetric concentration of solid in suspension and amount of floc filtered and dried after sedimentation, and zC is the height of the critical point of suspension. From the sedimentation data of ECC and CC flocs’ initial sedimentation velocities, single particle sedimentation velocity, floc densities, and diameters were calculated. The following method was used for the calculations: First, φ(z) values were calculated according to eq 5. Afterward, νs sedimentation velocity was calculated on a time-dependent basis from the sedimentation velocity curves (Figures 3 and 4); for this purpose, the derivative

1/ n 1/ n vs1/ n = u∞ − ku∞ φ (z )

ρF =

zc ≤ z ≤ z 0

(6)

ρS + (k − 1)ρL (7)

k −3

ρF, the floc density (kg m ) was calculated by eq 7. Here ρS is the density of the solid particles (density of the aluminum hydroxide 2420 kg m−3) and ρL is the density of the suspension liquid. Finally, the floc diameter D was calculated by eq 8: ⎛ 18μu ⎞1/2 ∞ ⎟⎟ D = ⎜⎜ ⎝ g (ρF − ρL ) ⎠

(8)

Here μ is the viscosity of the suspension liquid (0.89 × 10−4 kg m−1 s at 25 °C) and g is the gravity acceleration (9.81 m s−2). Figures 3 and 4 show the sedimentation velocity curves of ECC and CC flocs. ECC flocs are sedimented faster than CC flocs and ECC flocs are more compressible than CC flocs. ECC and CC sedimentation parameters, initial sedimentation rate u0, k, and ρF are listed in Tables 1 and 2. As seen in Figure 5, both ECC and CC flocs have the biggest floc diameters and highest sedimentation velocities at acidic and basic ends; at the acidic pH 4.0, ECC floc diameter reached to 386 μm, while it reaches to the minimum 215 μm at the neutral pH 6.5 and to the maximum 523 μm at the basic pH 9.5. Similarly, CC floc diameter reach to 254 μm at the acidic pH 4.5, to the minimum 182 μm at the neutral pH 6.5 and to the maximum 340 μm at the basic pH 9.5. As seen in Figure 6, ECC flocs have bigger diameter values than CC flocs. In addition, their sedimentation velocities are nearly double those of CC flocs. Floc diameters were also measured using particle sizer equipment. As seen in Figure 7, the measured values are nearly 10 times smaller than the floc diameter values calculated in Figure 6. The reason why the diameter values measured with the particle sizer were much smaller than the values calculated from the sedimentation data is due to the high mixture rate and ultrasound implemented during the measurement by the particle sizer. In this way, floc aggregates are desaggregated and the microflocs, most probably created in the beginning stage of the sedimentation, emerge. Nevertheless, it is seen from Figure 7 that the pH-dependent variation of the microfloc diameter obtained from the device exhibits the same trend as the variation obtained from the sedimentation data. An important point is the fact that the difference between ECC and CC seen in Figure 6 no longer exists here. However, despite having the same diameter as

Figure 3. Sedimentation velocity curves of ECC flocs.

Figure 4. Sedimentation velocity curves of CC flocs.

of the second rank polynomial (4) obtained through regression from the height−time curves was taken. Then, by using the

Table 1. ECC Sedimentation Parameters: Initial Sedimentation Rate u0, k, ρF pH

a

b

R2

u0 (10−4 m s−1)

k (m3floc/m3solids)

ρF (kg m‑3)

4.0 5.5 6.5 7.5 8.5 9.5

0.185 0.174 0.164 0.172 0.181 0.197

61.39 47.15 29.49 53.90 68.96 89.49

0.992 0.976 0.993 0.996 0.989 0.995

2.88 1.85 1.68 1.77 2.22 2.96

332 271 180 313 381 454

1004.7 1005.6 1008.3 1004.9 1004.1 1003.5

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Table 2. CC Sedimentation Parameters: Initial Sedimentation Rate u0, k, ρF pH

a

b

R2

u0 (10−4 m s−1)

k (m3floc/m3solids)

ρF (kg m‑3)

4.5 5.5 6.5 7.5 8.5 9.5

0.160 0.145 0.143 0.145 0.155 0.169

45.19 34.60 34.79 40.88 41.28 65.90

0.997 0.993 0.997 0.990 0.984 0.979

1.08 0.76 0.70 0.71 1.09 1.40

282 239 243 282 266 390

1006.7 1007.6 1007.5 1006.7 1007.0 1005.3

are written as Al2O3·xH2O. The three surface states of alumina, that is, physically adsorbed water, chemically adsorbed water, and oxygen bridges on the surface of aluminum hydroxide, are depicted in Figure 8. By losing the water in its structure, Al2O3· xH2O first turns into AlO(OH) and eventually to Al2O3. The thermal behavior of the ECC and CC dried flocs was studied using TG/DTA investigations. Analysis started at 20 °C, with 0.5 °C increments and finished at 1000 °C. TG and DTA curves are give in the Supporting Information, Figures 1 and 2. Weight loss (water loss) of ECC and CC dried flocs was calculated from the TG/DTA data (at 950 °C all water is accepted to be lost). Table 3 shows the results of these calculations. From the TG and DTA peaks, it is observed that the weight loss of ECC and CC flocs generally occurs within the 20−550 °C and 550−950 °C temperature bands and in 2 main phases. In the first phase, which is accepted as the phase where the bound (crystal) water is lost, weight losses for ECC and CC are observed to be independent from pH and around 35%. However, in the second phase weight losses vary according to pH and process type (ECC−CC). According to this, the structural H2O or OH amount in CC is higher than that in ECC. Furthermore, Al contents of flocs were also determined by wet chemical analysis. Table 4 shows the percentage of Al obtained by both methods. As it is observed in Figures 9 and 10, the Al percentage of ECC flocs is higher than that of CC flocs, while bound-water contents are lower. Also, it is observed that as the pH of the medium increases, the water contents of the flocs decrease and consequently, aluminum content increases, and the increase rate is higher for ECC flocs. 3.4. FT-IR Analysis. The bond structures of the flocs were examined by obtaining the absorption spectra at the frequency range of 500−4000 cm−1 from the transparent discs prepared by mixing the samples taken from ECC and CC floc samples dried at 105 °C (potassium bromide) with KBr. The FT-IR spectrum of ECC flocs, CC, and Merck Al(OH)3 depending on pH are given in Supporting Information Figure 3. General FT-IR vibrations for Al(OH)3 and the corresponding wave numbers are given in Supporting Information Table 2. From the FT-IR spectrum of ECC the O−H stress band within 2600−3800 vibration range and at a 3453 peak and the H−O−H inflection band within 1600−1700 vibration range and at a 1640 peak are observed. While both the absorbed water in aluminum hydroxide and structural OH groups contribute to the formation of the O−H stress band within the range of 2600−3800 vibrations, only the H−O−H absorbed water contributed to the formation of the 1640.71 The 615 wavenumber shows the Al−O vibration in the gamma aluminum phase72 (the 1384 wavenumber belongs to the N−O vibration caused by KNO3 electrolyte used in ECC solutions for conductivity setting). Comparing FT-IR spectra of ECC flocs and CC flocs in terms of

Figure 5. Single particle sedimentation rates of ECC and CC flocs as function of pH.

Figure 6. ECC and CC flocs (aggregate) sizes as function of pH (measured by sedimentation test).

Figure 7. ECC and CC flocs (single particle) sizes as function of pH (measured with Particle Sizer).

CC microflocs, ECC microflocs turn into macroflocs and aggregate faster. 3.3. Thermogravimetric Analysis. Different forms of aluminum hydroxide that contain some bound (crystal) water 3532



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Figure 8. Hydration-dehydration steps of aluminum hydroxide.70

Table 3. Weight Loss of ECC and CC Flocs weight loss (%) temp (°C)

temp range (°C)

550

20−550

950

550−950

pH

ECC

CC

33 36 35 36 34 34 25 12 11 9 7 4

33 34 36 38 35 34 26 19 17 13 9 7

4−4.5 5.5 6.5 7.5 8.5 9.5 4−4.5 5.5 6.5 7.5 8.5 9.5

Figure 10. Al content of ECC and CC flocs as a function of pH.

generated in acidic conditions have a more amorphous structure than those generated in basic conditions and that as the pH increases, the crystallization level of the floc structure increases slightly. From the TGA results it was observed that the crystal water decreases with increasing pH. Thus, it can be asserted that with the increase in crystallization, the crystal water contents of the flocs decrease. 3.6. SEM Analysis. Figure 11 depicts SEM images of the surfaces of ECC, CC flocs, and Merck Al(OH)3, respectively, with a 50000 magnification. It is seen that ECC and CC flocs have a more clustered amorphous structure while the standard Al(OH)3 had a more crystalline flake-like structure. Both ECC and CC SEM images show that the particles are spherical in shape; therefore, they fit well into the hindered settling theory. 3.7. XPS Analysis. XPS analysis provides information on the physical−chemical structure of a solid surface (up to a depth of 10 mm). The determination of the differences in the surface and mass properties of the flocs is important. To calculate the chemical composition and elemental composition of the floc surface from XPS spectra, the n (atom g cm−3) values for each of the elements are calculated by eq 9:

Table 4. Percentage Al of ECC and CC Flocs Obtained in TGA and Al Chemical Analysis (CA) at Different pH Values ECC CC

CA TGA CA TGA

pH =

4

%Al %Al %Al %Al

25 22

4.5

5.5

6.5

7.5

8.5

9.5

26 22

29 28 25 25

31 29 27 25

32 29 25 26

32 31 29 30

28 33 31 31

n = I /S Figure 9. H2O content of ECC and CC flocs vs pH.

(9)

Here, I is the electron number (intensity) and S is the atomic sensitivity coefficient. When working with Al Kα rays and for a 54.7 degree X-ray source, for an Al 2p peak S = 0.234 and for an O 1s peak S = 0.711. By using these values, the atomal O/Al ratio for Merck Al(OH)3 was calculated as 2.85 which indicates some amount of crystal water bound on the hydroxide, by considering that the stoichiometric ratio is 3. XPS spectra showing Al and O intensities of Merck Al(OH)3, ECC, and CC flocs are given in the Supporting Information Figures 6, 7, and 8. Here, it is noted that hydrogen could not be determined with XPS. Consequently, it is not possible to

absorbance values shows semiquantitatively that the OH group is more common in CC flocs. 3.5. XRD Analysis. XRD spectra of ECC and CC flocs are given in Supporting Information Figures 4 and 5, as function of pH. Due to their quite amorphous or weak crystal structures, the XRD peaks of the ECC and CC flocs have very low intensities and exhibit very wide and shallow distributions. While the crystal structures of the flocs created in both conditions are usually in boehmite form, bayerite form is also observed for the flocs produced with ECC. For both ECC and CC cases, the flocs 3533



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Figure 11. SEM images of (a) ECC floc for pH 4, (b) CC floc for pH 4.5, (c) ECC floc for pH 9.5, (d) CC floc at pH 9.5, (e) Merck Al(OH)3.

Figures 13 and 14, it is observed that the surface areas and pore volumes of flocs are directly related with pH and that they increase in line with the increasing pH. The BET surface area of ECC flocs is bigger than that of CC floc rates ranging from 30% to 85%, as shown in the Figure 13. It is not clear whether these rates are current for a dispersed floc in water. BET surface area and total pore volume of ECC and CC flocs as a function of pH are give in Supporting Information Tables 3 and 4. 3.9. Pollutant Removal Performance Tests. The goals of this section are to compare the performances of ECC and CC flocs and to investigate the effect of pollutant chemical structure on the performances of the flocs as well.

determine exactly the surface composition consisting basically of oxygen, hydrogen, and aluminum atoms. The changes in the O/Al ratio on the surfaces of ECC and CC with the change in pH are shown in Figure 12; this ratio, higher in the case of ECC, decreases with increasing pH and stabilizes at pH 9.5. These results are in accordance with the results obtained via TGA and chemical analysis methods shown in Tables 3 and 4 and in Figures 9 and 10. Consequently, it may be concluded that the surface and mass structures of both floc samples are similar to each other and also change similarly as a function of pH. 3.8. BET Analysis. BET analysis was determined for ECC and CC flocs degassed for 3 h at 300 °C. The BET surface area of the Merck Al(OH)3 sample was measured to be 17 m2 g−1. From 3534



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HQ by the ECC process. The removal performance is inversely proportional to concentration for both pollutants. At high BQ concentrations, electrocoagulant flocs are more effectively used in terms of adsorption capacity, despite lower removal efficiency. On the other hand, with increasing HQ concentration, the adsorption capacity reaches a maximum and then decreases. Furthermore, it can be said that ECC flocs adsorb BQ more effectively than HQ: By considering the dipole moments of the two compounds (0.67 D for BQ and 0 D for HQ), it can be safely concluded that the removal capacity is strongly dependent on the polarity of the adsorbed molecule. 3.9.2. Removal Performance of the Dye by ECC and CC Flocs. Domestic and industrial wastewaters mostly contain anionic surfactant and particulate matters. The zeta potential of these particulates is usually between −10 and −40 mV. In the event of using aluminum salts in purification, aluminum ions form the polymeric aluminum hydroxide gel network by becoming hydrolyzed and aerial particulates become confined in this gel. To obtain a weak zeta potential, the pH is adjusted somewhere close to pH 6 (for instance +5 mV). Electrostatic stability is trivial in this zeta potential but by producing positively charged flocs, an important removal is acquired by the anionic pollutant adsorption on these flocs.74 Dye removal efficiencies by the ECC and CC methods are shown in Figure 15 as function of pH. Removal efficiencies above

Figure 12. Oxygen/Al atom ratio of flocs as a function of pH.

Figure 13. BET surface area of ECC and CC flocs as a function of pH.

Figure 15. Dye removal efficiencies of ECC and CC flocs as function of pH.

99% are obtained with ECC flocs in the pH range of 5.5−7.5, while the removal with CC decreases steadily upon the increase in pH. Furthermore, Figure 16 reveals that the adsorption capacity of ECC flocs is approximately two times higher than that of CC flocs. On the other hand, the removal efficiency plots (Figure 16) and zeta potential plots (see Figure 2) of ECC and CC flocs bear a great resemblance to each other, which indicates that the sulfonic groups help the dye to be removed more successfully by the ECC flocs that have higher positive zeta potential. Furthermore, the BET surface area of ECC floc is higher than CC floc (see Figure 13). From these considerations it can be concluded that zeta potential is as effective as BET surface area in terms of adsorption capacity. Finally, the removal efficiency of ECC is shown comparatively for polar BQ and ionic dye as function of pH in Figure 17.

Figure 14. Total pore volume of ECC and CC flocs as a function of pH.

The performance of the flocs was assessed using the removal efficiency, E (%), and the adsorption capacity, Q (kg pollutant/kg floc), according to eqs 10 and 11, respectively, E=

C i − Cf 100 Ci

(10)

V (11) m −1 where Ci is the initial pollutant concentration (mg L ), Cf is the final concentration (g L−1), V is the volume of solution (L), and m is the amount of dried floc mass (g). 3.9.1. Removal Performances of BQ and HQ by ECC Flocs. Removal efficiency and adsorption capacity of ECC flocs with respect to BQ and HQ were explored in detail in a previous study.73 A brief summary of the experimental results are given here. First, it can be said that BQ is more effectively removed than Q = (C i − Cf )

4. CONCLUSION The study demonstrated that the success of the ECC process in wastewater treatment was mainly due to the outstanding characteristics of the ECC flocs outperforming CC flocs in 3535



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In conclusion, the zeta potential and BET surface area of the flocs are effective structural properties of the adsorption capacity.

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ASSOCIATED CONTENT

* Supporting Information S

Tables listing the following: zeta potential as a function of degree and type of coagulation; wavenumbers and range of FT-IR vibrations; BET surface area of ECC and CC flocs as a function of pH; total pore volume of ECC and CC flocs as a function of pH. Also, TG and DTA spectra of ECC flocs and CC flocs as function of pH; FT-IR spectra of ECC flocs; CC flocs and Merck Al(OH)3 depending on pH; XRD spectra of ECC flocs and CC flocs as function of pH, XPS spectra of Merck Al(OH)3, ECC flocs and CC flocs depending on pH. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 16. Dye adsorption capacities of ECC and CC flocs as function of pH.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 434 2283127/560. Fax: +90 (434) 2285171. E-mail address: [email protected]; [email protected]; otanercan@ yahoo.com. Notes

† This paper is a part of O.T. Can’s doctoral thesis. The authors declare no competing financial interest.

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REFERENCES

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Figure 17. Comparison of the removal efficiencies of ECC flocs for BQ and the dye.

term of removal perfromance and adsorption capacity for all three types of pollutant as well as the sedimentation rate. The experimental results supporting this conclusion may be summarized as follows: • ECC flocs have higher diameter values than CC flocs are better pressed and their sedimentation velocities are nearly twice those of CC flocs. • BET surface areas of ECC flocs are 30%−85% greater than CC flocs, the surface area increasing with increasing pH values. From the chemical composition point, the crystal water content is less while Al content is higher in the case of ECC flocs. On the other hand, the isoelectric pH of ECC flocs is higher than that of the CC flocs. Furthermore, the results showed that the adsorption efficiency of the flocs depends on the polarity−ionicity of the adsorbed molecule. The existence of ionic groups or higher dipole moments favors a high adsorption capacity. BQ with a high dipole moment is more effectively adsorbed than HQ with zero dipole moment, although both bear hydroxy or oxo-polar groups. On the other hand, the sulfonic groups help the dye to be removed more successfully by the ECC flocs that have higher positive zeta potential than CC flocs. The pH of the solution affecting the zeta potential is also crucial for an efficient removal; for BQ the removal efficiency increases in neutral pH between 5.5 and 7.5, showing a maximum (77%) at pH 7.5, and decreases in acidic and basic pH values. Ionic adsorption is dominant in the adsorption mechanism while the interaction between polar oxohydroxyl groups on Al(OH)3 surface and polar pollutant molecules also plays an important role in physical adsorption. 3536



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