Removal of humic acids from water by hybrid titanium-based electrocoagulation with ultrafiltration membrane processes

Desalination 300 (2012) 51–57

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Removal of humic acids from water by hybrid titanium-based electrocoagulation with ultrafiltration membrane processes Xin Chen ⁎, Huiping Deng Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, Shanghai 200092, China

a r t i c l e

i n f o

Article history: Received 1 March 2012 Received in revised form 17 May 2012 Accepted 5 June 2012 Available online 23 June 2012 Keywords: Electrocoagulation Ultrafiltration Membrane fouling Titanium salt coagulation

a b s t r a c t Coagulation imposes major impact on the removal of humic acids (HAs) and the reduction of ultrafiltration (UF) membrane fouling. In this study, titanium was used instead of iron and aluminum as a novel alternative sacrificial anode, and the removal of HAs from water by titanium-based electrocoagulation with submerged flat sheet ultrafiltration membrane was investigated. Results clearly indicate that the current intensity did have an apparent effect on the size and fractal structure of flocs. A combination of electrocoagulation with ultrafiltration can increase HAs rejection, reduce membrane fouling and decrease transmembrane pressure. Membrane permeability and the specific resistance of cake layer were controlled directly by coagulated floc structure, which was affected strongly by the applied current intensity. Hydrous titanium dioxide (TiO2) flocs were formed by anodizing of titanium and one-step hydrolysis process with titanium salt solution. TiO2 nanoparticles with large specific surface area produced by the processes showed high photocatalytic activity. Thus, the processes are not only efficient in the removal of HAs from water but also economical in sludge recycling and reduction. It may offer a novel solution to many economic and environmental problems associated with handling iron and aluminum salt coagulation. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Membrane technology has provided an important solution in water pollution treatment recently. However, the accumulation of colloids on the surface or inside the pores of the membrane, i.e. the colloidal fouling, is leading to severe energy loss during filtration, and consequently to high operational costs that limit the application of the membrane technology. The addition of a coagulant prior to membrane filtration has been adopted to improve product water quality and reduce membrane fouling [1–3]. At the same time, electrocoagulation (EC) is an alternative method. EC has been widely used in oily, dye and textile wastewater, potable water, dairy effluents and landfill leachate, to study the removal of organic matter, heavy metals, fluoride, etc. [4–7]. Electrocoagulation removes the small colloidal particles efficiently in comparison with the conventional techniques, because the small charged particles have greater probability of being coagulated and destabilized by the electric field that forces them in motion. Simultaneously, gas bubbles are produced due to electrolysis, which can enhance flotation [8]. Floc formed by EC is similar to the floc formed by chemical coagulant, but the EC floc contains less bound water and tends to be much larger, more stable, and acid-resistant. Therefore, it can be separated faster by filtration [9]. A few studies have investigated ⁎ Corresponding author at: Room 215, Building Mingjing, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China. Tel.: + 86 13601968692; fax: + 86 021 65986313. E-mail addresses: [email protected], [email protected] (X. Chen). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.06.004

hybrid electrocoagulation membrane processes [10], which have shown to be highly efficient in removing a large range of pollutants from water, such as virus [11], nickel [12], selenium [13], and silica [14]. The sacrificial anodic electrodes, commonly consisting of iron and aluminum, are used to continuously supply metallic ions as the source of coagulants, which can hydrolyze near the anode to form a series of metallic hydroxides capable of destabilizing dispersed particles. The EC process using these sacrificial anodes produces large quantities of iron and aluminum salt coagulated sludge, which inhibits efficient water treatment. Most component of the sludge is solid waste from which nothing may be recovered or reused, and requires further incineration and landfill treatment. However, reports on novel electrodes materials remain very scarce in the literature. Furthermore, it should be noted that the residual iron can cause esthetic problems. In general, the appearance of dissolved iron in aquatic suspensions can lead to visual, odor and taste problems resulting from later growth of iron bacteria [15]. Even aluminum salts are suspected to be harmful to human and living things [16]. Seventy-two percent of the commonly used coagulants in chemical flocculation are aluminum sulfate, 23% are iron salts, and 5% are poly-aluminum chlorides nowadays [17]. Consequently, a coagulant that is safer and produces more reusable coagulated sludge could offer a novel solution to many environmental and economic problems associated with sludge handling. Humic acids account for about 50–90% of the total freshwater organic matter and present a yellowish or brown color in water. It has been noted that their functional groups such as carboxylic and phenolic acids can adsorb metal ions and influence their fate. The

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high affinity of HAs for complexation with various pollutants including heavy metals and pesticides causes contamination of surface water [18]. For the given optimum coagulation dosage, the flocs formed by titanium tetrachloride (TiCl4) showed larger floc strength than those by polyaluminum chloride (PACl) and with larger average size during growth period [19]. Removal of organic matter and different molecular sizes by titanium salt flocculation was similar to that of the most widely used iron and aluminum salt flocculation. The results suggest that the concentration of titanium salt remained in the supernatant after the TiCl4 flocculation is in accordance with the requirement of the Ti concentration in drinking water supplies according to World Health Organization's (WHO) environmental health guidelines. And long-term toxicological studies have not found titanium salt in water to have any adverse effects. All the above factors suggest that the titanium salt can be used as an alternative coagulant [17]. Humic substances represent the common agents contributing to flux decline during membrane filtration of natural water, and modifying the operation of UF presents a promising alternative in order to minimize the fouling. In this paper, titanium was used instead of iron and aluminum as a novel alternative sacrificial anode, and the removal of HAs from water by titanium-based electrocoagulation with submerged flat sheet ultrafiltration membrane (Ti-EC-UF) was investigated. This study aimed to solve three major problems: (1) the relationship between the size and fractal structure of flocs and the current intensity; (2) the improvement of removal performance and the mitigation of fouling mechanisms; and (3) characteristics of the flocs generated during the titanium -based electrocoagulation. 2. Materials and methods 2.1. Chemicals Humic acids were purchased from Sigma-Aldrich (St. Louis, USA). Titanium dioxide P-25 (specific surface area=50 m2/g, particle size= 27 nm, and anatase:rutile=80:20) was from Degussa Corporation, Germany. Ultra pure water was prepared by a Milli-Q purification water unit (Millipore, Watford, UK). Hydrochloric acid, sodium hydroxide and sodium chloride, all of analytically pure grade, were supplied from Sinopharm Chemical Reagent, Shanghai, China. 2.2. Feed water Humic acid stock solution was prepared by dissolving 2 g of HAs powder in 10 mL of sodium hydroxide solution (2 mol/L) and then diluting in 2 L of ultra pure water. The stock solution was stirred by a magnetic agitator for 24 h. Then the stock solution was filtered through a 0.45um filter to remove solid residues and stored at 4 °C until used. The feed water containing HAs solution was prepared by diluting 400 mL stock solution in 10 L of deionized water for each experiment. Concentrations of HAs solution were evaluated with the ultraviolet absorbance measured at 254 nm(UV254) by UV–VIS Spectrophotometer (UV-2550, Shimadzu, Japan), which was 1.12 cm− 1 on average of the feed water. The conversion of UV254 data to an equivalent humic acid concentration is based on the linear relationship between UV254 absorbance and humic acid concentration [20]. The removal of UV254 absorbance was used to evaluate the HAs removal efficiency. Multi-parameters of water quality analyzer (PH/cond 340i, WTW, Germany) was used to measure the pH of the feed water. The original pH of the humic acid solution in the feed tank was adjusted to be approximately 7, which was close to the natural water pH in most of drinking water source.

plates. Polyvinylidene fluoride (PVDF) UF membrane (Jiangsu Lantian Peier Membrane CO., Ltd., China) was used. The size of the membrane module was 22 cm ×16.5 cm×0.6 cm, with the effective membrane area 193.7 cm2. Mean pore size of the membrane was 0.08 μm. Both titanium anode and cathode were made from plates with the dimensions of 20 cm×16 cm×0.1 cm. The distance separating the electrodes was fixed to 3 cm. The anode was placed on the influent side of the flat membrane, and the cathode was placed at the other side to create direct current electric field. The membrane module was located 1 cm from the anode and 1 cm from the cathode. 2.4. Filtration experiments The feed water was pumped into the reactor using an electromagnetic feed pump under the control of electric liquid level meter to keep the steady liquid level in the reactor. A peristaltic pump (BT100-2 J, Longer Precision Pump Co., Ltd., China) was used to convey the solution to the filtration module, while the rotational speed was controlled at specific levels for the applied flux. The electric field was supplied by a power supply (Shanghai Huayang Electronic Instrument Plant, China), of which the voltage can be regulated from 0 V to 60 V and the current from 0 A to 3 A. Filtration experiments were carried out without recirculating the permeate in the feed tank. Permeate collected in a container was recorded by an electrical balance to calculate the flux (UW-6200 H, Shimadzu, Japan). Transmembrane pressure (TMP) was recorded by the vacuum pressure gauge (Shanghai Automation Instrumentation Co., Ltd., China). The filtration reactor was placed in a low temperature water bath (DKB-1615, Shanghai Jing Hong Laboratory Instrument Co., Ltd., China) in order to keep the water temperature close to room temperature most of the time except several occasions when it increased owing to joule heating at high current. With the apparatus above, the precise control of pressure, flow rate and temperature was allowed. Fig. 1 shows the schematic diagram of the bench-scale Ti-EC-UF system. The membrane was filtered with ultra pure water before experiments in order to remove preservatives. The pure water permeability measurements were performed, and TMP seemed to be a direct indication of the membrane surface state. Ultrapure water was forced to permeate through the membrane at a constant flux of about 23 mL/min. During the 20‐min operation period, results indicate that TMP of the UF module is about 0.018 MPa with minor fluctuation for different original membranes due to the relatively uniform pore size. After experiments, a back-flushing method was used to clean the membrane and followed by an alkaline bath (pH=2, NaOH solution) for 15 min, an acid bath (pH =12, HCl solution) for 15 min, and rinsing with deionized water for 5 min. The membrane foulants can be effectively removed by dilute HCl and NaOH solution combined with hydraulic flushing, and the membrane filtration flux was recovered well.

2.3. Ti-EC-UF module A bench-scale reactor with the dimensions of 32 cm ×18 cm×4.5 cm was installed with the submerged UF flat sheet membrane and electrode

Fig. 1. Schematic diagram of the bench-scale Ti-EC-UF system. 1-Power supply,2-Feed tank,3-Cathode plate,4-Submerged UF flat sheet membrane,5-Titanium anode,6-Vacuum pressure gauge,7-Peristaltic pump,8-Electrical balance,9-Computer.

X. Chen, H. Deng / Desalination 300 (2012) 51–57

2.5. Analytical methods The photographic image analysis method was used to study the fractal structure of flocs formed during electrocoagulation. The relationship between projected area and projection perimeter is given as follows [21]: A ¼ αP

D2

ð1Þ

where A is the projected area; P is the projection perimeter; and D2 is the fractal dimension in two-dimension space. Performing the natural logarithm on Eq. (1), it transforms into the following form: LnA ¼ D2 LnP þ Lnα

ð2Þ

In Eq. (2), plotting Ln(A) against Ln(P) gives a straight-line with slope D2; and Ln(α) is the Ln(A)-intercept. The area and perimeter were measured using an optical microscope (Nikon eclipse80i, Japan) together with the Simple PCI software (Compix Inc., USA). Measurements were conducted on five different samples to ensure reproducibility. The zeta-potential curve of the flocs that were dispersed in water at various pH values was measured, using a Zetasizer Nano Z Potential Analyzer (Malvern Instruments Ltd., UK). The TiO2 nanoparticles produced from the flocs were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Brunauer, Emmett, and Teller (BET) surface area and photocatalytic activity to evaluate their quality. 3. Results and discussion Current density was identified as the key operational parameter, because it directly determined coagulant dosage and bubble generation rates, and strongly influenced both solution mixing and mass transfer at the electrodes [22]. The influence of current intensities on the performance of the Ti-EC-UF processes was studied at various voltages. Sodium chloride was used as supporting electrolyte to provide ionic strength and maintain the conductivity of the feed solution. The experiment was designed at three current intensities of 0.1 A (equivalent to a current density of 3.125 A/m2), 1A (31.25 A/m 2) and 3 A (93.75 A/m2). The coagulation suspension samples were collected from the reactor after the end of experiment to measure the zeta-potential and particle size of the flocs, and obtain the data for fractal dimension calculation. 3.1. Relationship between flocs and current intensity 3.1.1. Fractal structure of flocs The characteristic of the flocs formed in the processes is a key factor in understanding the mechanisms of Ti-EC-UF processes. Fractal mathematics enables the representation of the apparently wild and complex structures of aggregates by simple parameters known as fractal dimensions [23]. Aggregates formed as a result of a coagulation–flocculation processes using hydrolyzing metal salts as coagulant have a four‐level structure. Primary particles form dense flocculi. Flocculi which are formed during the flocculation group themselves to flocs. Flocs together form weak aggregates. The particles in each level of organization are linked with elastic bonds, as are the different levels in the structure [24]. Due to the low current intensity, hydrolyzing titanium salts which were formed during the electrolysis of titanium anode were not enough to group themselves to flocs at 0.1 A. Dense flocculi can be only observed, as shown in the photographic image of 0.1 A. The results show that the average values of fractal dimension for 1 A and 3 A were 1.87 and 1.56, respectively. In general, the high D2 values indicate a compact structure while low ones indicate a loose structure [25,26].

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3.1.2. Zeta-potential of flocs at various pH values The pH of the post-electroflocculated suspension containing flocs was adjusted to be between 2 and 12 using dilute NaOH and HCl solutions. The point of zero charge pH (pHpzc) was derived by linear interpolation of the two points above and below the x-axis [27]. The pHpzc of the flocs was 4.75, where the net surface charge was zero (Fig. 2). The final pH was always higher than the initial pH 7, and the higher current intensity produced a higher final pH due to more hydroxyl formation at the cathode. The final pH value of 7.10, 8.86 and 10.36 were produced at 0.1 A, 1 A and 3 A, respectively. The zeta potential is an indirect measurement of the charge on particles, and its value determines the extent of the electrostatic forces of repulsion among charged particles, which indicates the extent of stability of the suspension [23]. The zeta-potential of flocs at 1 A was lower than that at 3 A, which was consistent with the electrostatic and configuration characteristics of flocs that were dependent on pH. The effect of higher intramolecular electrostatic repulsion and stretched configuration at the current intensity of 3 A, eventually led to larger and looser floc structure. 3.2. Effect of current intensity on HAs removal The current intensity not only determines the coagulant dosage and bubble production rate but also the size and growth of the flocs, which can influence the treatment efficiency of the EC. Therefore, the effect of current intensity on the HAs removal in the Ti-EC-UF processes was investigated. Fig. 3 shows the removal efficiency of HAs in terms of UV254 rejection during 140‐min operation period. The rejection of HAs was less than 10% in the absence of electric field. It was not efficient to remove HAs by UF alone. Membrane sieving occurred, when HAs were transported to the membrane surface. Meanwhile, membrane would adsorb HAs either on surface or in pores. When the current intensity was varied from 0.1 A, 1 A to 3 A, for a given 140‐min operation period, the removal efficiency increased significantly with increase of current intensity. At the applied current intensity of 3 A, it was able to reduce UV254 by over 60% in the permeate. Several mechanisms are possible for the removal of HAs from the water by Ti-EC-UF processes [22,28,29]. As the titanium sacrificial anode corroded due to an applied current, the titanium salt coagulant was formed, and electrocoagulation occurred subsequently. Indeed, more quantities of the complex titanium oxides–HAs organic matters produced, namely called flocs at higher current density. With EC treatment, a relatively looser layer formed on the membrane due to solid titanium salt flocs deposition. The cake layer may also reduce fouling by keeping the smaller particles from reaching the surface and the pores of the membrane. The simultaneous electrophoretic migration of the negatively charged HAs colloids toward the anodic surfaces forced chemical coagulation between particles and metallic hydroxides in the

Fig. 2. Zeta potential of the flocs as a function of pH.

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Fig. 3. Changes of removal efficiency at different current intensities.

Fig. 5. Changes of total resistances at different current intensities.

vicinity of the anode. Thus, less foulants accumulated or deposited on the membrane surface. At the same time, electrolytic hydrogen gas bubbles were generated at the cathode which may float some portion of the coagulated HAs to the surface. Additionally, other electrochemical reactions including electrolysis, oxidation and reduction reactions could occur also near the electrodes which benefited the UF process as HAs may be mineralized during these processes. 3.3. Effect of current intensity on filtration resistance For the membrane filtration of coagulated suspension, the permeation flux for UF is usually written in terms of transmembrane pressure difference (TMP) and a total resistance by Darcy's law [3]: J¼

ΔP ΔP ¼ ηRt ηðRc þ Rm Þ

ð3Þ

where J = permeation flux of solution (m/s), ΔP = transmembrane pressure (Pa), Rt = total resistance (m − 1), Rc = the resistance of the cake layer (m− 1), Rm = membrane resistance (m− 1) and η = viscosity of solution (Pa·s). The feed water containing HAs solution was filtered through the membrane at applied current intensity of 0 A, 0.1 A, 1 A and 3 A, respectively. During the 140‐min operation period, TMP was plotted in a TMP-time graph, and the total resistance was calculated by the Darcy equation. Results indicate that TMP and the total resistance of the UF module decreased dramatically with the EC applied as the current increased from 0 A to 3 A (Figs. 4 and 5). The obvious increase of

Fig. 4. Changes of transmembrane pressure at different current intensities.

the TMP during the operation period at 0 A without electric field may be due to adsorption or clogging of HAs on the surface or in the pore of the membrane of UF. At this situation, the membrane surface was covered with a dense layer (cake) and had only small number of open pores, indicating severe fouling occurrence in agreement with the dramatic increase of the TMP–time curve at 0A. With EC treatment, a relatively looser layer formed on the membrane due to solid titanium salt flocs deposition. This layer protected the membrane pores from plugging, keeping them clean and open to flow. At the same time, electroosmosis was an important process in electrofiltration, which was the movement of liquid under the influence of the electric field [28]. It may occur within the cake layer and medium, and generally would allow more fluid to flow across the membrane. It is well accepted that two main types of colloidal fouling mechanisms exist simultaneously: internal fouling mechanism that acts to reduce the membrane permeability and external fouling which results from colloidal accumulation that forms a cake layer on the membrane surface. The cake layer may also reduce fouling by preventing the smaller particles from reaching the surface and the pores of the membrane. The increase in particle size as a result of the flocculation process is a common explanation given for fouling mitigation. The larger flocs cannot penetrate the pores of the membrane, thus preventing the internal fouling from taking place. The flocs also produce a cake with hydraulic resistance lower than the cake produced by untreated particles [15]. Floc cake resistance is lower than resistance due to the unsettled floc and the uncoagulated organics [30]. Compact cakes are formed in nonaggregating systems where the particles would pack uniformly on the membrane surface. More porous cakes are commonly formed when particles aggregate prior to deposition on the membrane surface. Indeed, the fractal properties of aggregates may influence membrane filtration behavior [3,31]. Membrane permeability and the specific resistance of cake layer are influenced significantly by coagulated floc structure. The more compact flocs produce the higher specific resistance, whereas the looser flocs result in the lower specific resistance. In this experiment, the structure of the flocs was affected strongly by the applied current intensity. At 0.1 A, due to the low current intensity, hydrolyzing titanium salts which were formed during the electrolysis of titanium anode were not enough to group themselves to flocs, so that only dense flocculi could be observed. It caused incomplete aggregation of colloidal particles, which had a relatively slight effect on the TMP of UF operation. At higher current intensity, a large proportion of the formed titanium salt flocs sank in the chamber of the UF module, never reaching the membrane surface. Therefore, higher operation current intensity did not lead to higher accumulation rates on the membrane surface in this experiment. The more open and looser structure with the lower average fractal dimension D2 characterized the flocs at 3 A, as opposed to the flocs at 1 A. The specific cake resistance decreases

X. Chen, H. Deng / Desalination 300 (2012) 51–57

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upon increasing the floc size and decreasing the fractal dimension [32]. Thus, the cake layer resistance of 3 A was lower than that of 1 A. And the total resistance was lower at 3 A, as shown in Fig. 5. Additionally, the electrokinetic potential of the flocs and membrane surface was negative under the experiment condition. The zeta potential of flocs tended to increase with the increase of current intensity. This may lead to an increase in the repulsive potential between the membrane surface and the flocs, and more beneficial conditions for cake removal by weakening the physicochemical attraction between the cake and membrane components [33]. The fouling was more reversible. Furthermore, easier cleaning may reduce the chemical or physical cleaning steps for extending the life expectancy of the membrane. 3.4. Characteristics of TiO2 from the EC flocs To further study the characteristics of Ti-EC flocs, using deionized water as medium without HAs, and NaCl as supporting electrolyte, after EC with titanium anode at 3 A for 140 min, the settled flocs were collected and dried for analysis of characterization. 3.4.1. XRF X-ray fluorescence spectroscopy is a common measurement method of multi-elements simultaneous determination in various materials [34–36]. XRF technology illuminates a sample with high-energy photons generated by a low-power X-ray source. By measuring the scattered X-rays, the XRF can estimate density and calculate a concentration. The X-ray fluorescence analysis was performed using WDXRF spectrometer (S4 Explorer, Bruker-Axs, Germany), equipped with Rh target and X-ray tube (50 kV, 40 mA). The floc samples dried in a drying chamber at 100 °C were ground in the agate mortar. As shown in Table 1, 90% of the floc samples by weight were TiO2. Sodium chloride was the electrolyte, and calcium and barium elements came from the impurity in the titanium electrode. 3.4.2. XRD The crystal structure of TiO2 nanoparticles was analyzed by X-ray powder diffractometer (D8 Advance, BRUKER-AXS, Germany) operating with CuKα radiation source filtered with a graphite monochromator. The XRD pattern was analyzed with MDI Jade 5.0 (Materials Data Inc., USA). The crystallite size D was determined from the broadening of corresponding strongest X-ray diffraction peaks by using Scherrer's formula [37]: D ¼ 0:9λ=βcosθ

ð4Þ

where λ is the average wavelength of the X-ray radiation (λ=1.5418 Å), β is the line-width at half-maximum peak position, and θ is the diffracting angle (2θ=25.4°). Fig. 6 shows the X-ray diffraction pattern of TiO2 nanoparticles obtained by the Ti-EC-UF processes. It demonstrates that most diffraction peaks belong to the anatase phase. Applying a constant voltage to the titanium effectively dissolved Ti according to Ti → Ti + 2 → TiO + 2. TiO + 2 combined easily with OH − to form TiO2·H2O or Ti(OH)4. Ti(OH)4 is an unstable substance, which changes gradually into TiO2·H2O by dehydration. The reaction equations were, þ2

TiO

−

−

−2

þ 6OH þ 2e →TiðOHÞ4 þ H2 þ 3O

ð5Þ

Table 1 Metal contents determined by XRF. Component

NaCl

CaO

TiO2

BaO

Intensity (KCPSa) Mass percent (wt%)

1.6 9.91

0.3 0.0255

445.1 90.0

1.8 0.0226

a

Kilocounts per second.

Fig. 6. X-ray diffraction pattern of TiO2 nanoparticles from titanium -based electrocoagulation process.

and, þ2

TiO

−

þ 2OH →TiO2  H2 O

ð6Þ

[38]. Hydrous titanium dioxide (TiO2·xH2O) flocs were formed by anodizing of titanium and a one-step hydrolysis process with titanium salt solution(mainly TiCl4) in the experiment performed herein. The TiO2 produced in this method is referred to hereafter as photocatalyst from titanium -based electrocoagulation process (EC TiO2). EC TiO2 nanoparticle, with the average size of ~7 nm, was smaller than the P-25 TiO2, the most widely used photocatalyst, resulting in a large specific surface area and high removal capability as the following experiment data proved. TiO2 is the widely used metal oxide in the manufacture of cosmetics, paints, electronic paper, solar cells, and other environmental applications. Thus, the processes are not only efficient in the removal of HAs from water but also economical in sludge recycling and reduction. 3.4.3. BET surface area The surface area and pore size distribution of the EC TiO2 nanoparticles were measured by specific surface area and porosity analyzer (ASAP 2000, Micromeritics, USA). The nitrogen adsorption– desorption isotherms of nanoparticles were used to analyze their microstructure characteristics. The BET‐specific surface area of these TiO2 nanoparticles was found to be 169.75 m 2/g, which was higher than that of the P-25 TiO2. The average pore diameter was determined to be 6.8 nm, which indicates that most pores were mesopores and reflects the inter-particle porosity in the TiO2 nanoparticles. Thus, these EC TiO2 nanoparticles have a relatively large surface area, which is beneficial for the adsorption. 3.4.4. Photolysis and photocatalytic experiment The photocatalytic activity test of EC TiO2 was investigated under irradiation of ultraviolet using the method of photodecomposition of HAs and was compared to that of the P-25 TiO2. All the experiments were carried out under irradiation of two UV lamps (16 W × 2, 254 nm, Light Sources Co., Ltd, America) in the same open glass dish reactor shielded from out light. The irradiation intensity impinging on the surface was 620 μW/cm2 on average measured by the UV radiometer (UV-B, Photoelectric Instrument Factory of Beijing Normal University, China). The HAs solution of 300 mL was adjusted to pH 7. The solution was stirred by a magnetic agitator at 200 rpm with 50 mg photocatalyst (EC TiO2 and P-25 TiO2, respectively) for 30 min. The removal of HAs was the combined effect of adsorption and photocatalysis degradation.

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After 30 min of photocatalytic reaction, the HAs removal efficiency in terms of UV254 removal with the EC TiO2, and the those of P-25 TiO2 were 32.91% and 26.86%, respectively. According to this result, EC TiO2 with UV light showed high photocatalytic activity, mainly due to a relatively large surface area of the EC TiO2, which was preferable for the adsorption and degradation [37,39]. The results for EC TiO2 were consistent with the recent reports. TiO2 nanoparticles were synthesized by the one-step hydrolysis process with aqueous TiCl4 solution. These TiO2 nanoparticles ranged from 3 to 8 nm and formed aggregates with a highly porous structure, resulting in a large specific surface area and higher adsorption capacity on As(III) than that of commercial P-25 TiO2 nanoparticles [37]. Similarly, the titanium salt flocculated sludge was recycled to produce valuable by-product namely TiO2. The removal of acetaldehyde with these TiO2 was higher than that with the P-25 TiO2 under UV irradiation [17]. 4. Conclusions (1) The application of current intensity did have an apparent effect on the size and fractal structure of flocs. Hydrolyzing titanium salts, which were formed during the electrolysis of titanium anode, were not enough to group themselves to flocs at 0.1 A. The more open and looser structure with the lower average D2 characterized the flocs at 3 A, as opposed to the flocs at 1A. (2) The removal efficiency increased significantly with increase of current intensity. Several mechanisms are possible for the removal of HAs from water by Ti-EC-UF processes: simultaneous electrocoagulation, electrophoretic migration, electroosmosis, electroflotation and mineralization. (3) In addition, TMP and hydraulic resistances also reduced substantially with EC process. Membrane permeability and cake layer‐specific resistance were controlled directly by coagulated floc structure, which was affected strongly by the applied current intensity. At 0.1 A, dense flocculi caused incomplete aggregation of colloidal particles so that it had a relatively slight effect on the TMP of UF operation. For the high current intensity, the more compact flocs produced the higher specific cake resistance at 1 A, whereas the looser flocs resulted in the lower specific cake resistance at 3 A. Correspondingly, the hydraulic resistance was lower at 3 A. (4) Hydrous titanium dioxide flocs were formed by anodizing of titanium and a one-step hydrolysis process with titanium salt solution. EC TiO2 was found to be superior in terms of specific surface area and photocatalytic activity. The proposed novel hybrid processes of Ti-EC-UF show very promising results for the improvement of effluent quality, reduction of membrane fouling and production of economically useful TiO2 byproducts. Thus, the processes are not only efficient in terms of removal of HAs from water but also economical in sludge reduction and recycling. It may offer a novel solution to many economic and environmental problems associated with handling of iron and aluminum salt coagulation. Acknowledgments This study was financially supported by the Technology Research and Demonstration of Typical Rural Drinking Water Safety Safeguard Project (No. 2008ZX07425-007) within the National Key Program of Water Pollution Control and Reclamation in China. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.desal.2012.06.004.

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