Al2O3-PVDF ultrafiltration

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Adsorption mechanism of O/W on TiO2/Al2O3-PVDF ultrafiltration membrane Xuesong Yi, Dexin Wang, Fei Yang, Yong Wang, Yuguang Zhu, and Wenxin Shi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01222 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Adsorption mechanism of O/W on TiO2/Al2O3-PVDF ultrafiltration membrane Xuesong Yi1, Yuguang Zhu 1, Dexin Wang1*, Fei Yang1, Yong Wang2, Wenxin Shi3 *Corresponding author: Dexin Wang. Tel.:+86-0898-66192915; Fax: +86-0898-66192915; E-mail address: [email protected]; Xuesong Yi, E-mail address: [email protected]; Fei Yang, E-mail address: [email protected]; Yong Wang, E-mail address: [email protected]; Yuguang Zhu, E-mail address: [email protected] Wenxin Shi, E-mail address: [email protected]. 1. School of Environmental science and engineering, Hainan University, Haikou 570028, China 2. State Key Laboratory Breeding Base of Marine Genetic Resources, Third Institute of Oceanography, Xiamen 361005, China 3. State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China

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ABSTRACT For the sake of gaining a clear idea of adsorption mechanism on oil emulsion-membrane system, Daqing crude oil emulsion and two kinds of PVDF ultrafiltration membranes made in our laboratory were used as the objects to pursue the adsorption characteristics in this system. Several isotherm and kinetics models used here for simulating adsorption process, effect of variables including: time, initial concentration, temperature; and SEM and ATR-FTIR spectrum was investigated to assist in understanding mechanism. The results show that the Redlich-Peterson model and pseudo-first-order kinetic model are the best fitting models with all of R2 values higher than 0.98, suggesting a endothermic, chemical and physical combined adsorption process. Moreover, the thermodynamic parameters, such as

∆ r Gmθ , ∆ r H mθ ,

and

∆ r S mθ

were also calculated

from the temperature dependence, indicating a non-spontaneous process, and increase of temperature counted against o/w adsorption. Further evidence is made from the analysis of microstructure and infrared spectrum, ultimately. Keywords: adsorption; o/w; thermodynamics; kinetics; UF membranes

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INTRODUCTION Once the oilfield produced water discharges into rivers, lakes, and sea water, a piece of 2.0×104 m2 water surface can be covered by a layer of oil thin film with its’ thickness of 2.8×10-4 mm only formed with 4.5 dm3 crude oil [1], which prevents oxygen from diffusing from the atmosphere to the water body and weakens solar energy casting into. Results in water self-purification capacity decreases due to the limited photosynthesis of algae for less of dissolved oxygen; at the same time, the normal growth of aquatic organisms, such as fish, shrimp, shellfish are affected, sometimes, damaging the practical function for the oil flavor in the organism, even, leading to asphyxia and death of the aquatic animals in the serious polluted oily water, directly [2~4]. According to some reports [5,6], over the past 50 years, more than 1000 kinds of marine organisms have been extinct because of oil pollution, and marine organisms have decreased by nearly 40%, while, the damages of reduced diversity of marine organisms and the accumulation of carcinogens in marine organisms cannot be estimated for environmental and human. These substances can be assimilated, absorbed, and concentrated by aquatic biological easily, leading to aquatic lives distortion, then entering into the human bodies through the food chain; especially, can promote pathological changes in organs such as intestines, stomach, liver and kidneys, even destroy the central nervous system and hematopoietic system by the action of accumulating toxicity [7,8] . The traditional technology for treating oilfield produced water can no longer adapt to the concept of oil field sustainable development. While, ultrafitration (UF) technology known for high efficiency and green characteristics can separate the pollutants from the water phase directly without demulsification and acidification process. In the recent years, many scholars [9~13] did a lot of research and practice in treating oilfield produced water by UF technology, and the research 3


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results show that UF technology had great potential for development and application in oilfield water treatment. However, the problem of membrane fouling control plays a key role in oilfield produced water treatment by membrane techonology[14,15]. The adsorption pollution of the ultrafiltration membrane by oil/water (o/w) emulsion was studied by taking the crude oil from Daqing oil field as the experimental object, which mainly contains hydrocarbons, gum, and other lipophilic organic matters, as well as lots of acidic oxygen compounds including: fatty acids, naphthenic acids, aromatic acids, and phenols, etc. contributes about 0.3~0.4% of the total; nitrogenous and sulfur compounds of about 0.05%~0.5% and 1%, respectively[16]. This composition shows that the emulsion contains generous hydrophilic, oil pro and two parent groups, which can form hydrogen bonds with the strong polar elements such as O, N, and F on membrane surface easily. In addition, on the basis of “similarity and mutual attraction”, the oil droplets in motion can interact strongly with the porous membrane under the action of Van Edward force. Therefore, exploring the adsorption characteristics of o/w on the membrane surface, especially, the adsorption isotherm, thermodynamics, and kinetics, play an important role in understanding adsorption mechanism, quantifying adsorption capacity and reducing membrane fouling.

MATERIALS AND METHODS Membranes PVDF plate membrane (OM) and its’ corresponding form modified by nano-particles of TiO2 and Al2O3 (MM) was prepared by phase inversion method in our laboratory[17], which has the surface Zeta potential of -13.21mv and -15.18mv, respectively, were used for static adsorption experiments. Table 1 shows characteristics of the two kinds membranes. The membrane were cut 4


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into square shape of 50 mm × 50 mm with a total area of 50 cm2 for both sides. All the new membranes were soaked in distilled water for 1 hour to remove the formaldehyde protective liquid before each adsorption experimental operating[18]. Each fresh membrane was used for the adsorption measurement with only one time, then discarded after determining of the adsorption capacity. All the membranes were stored in refrigerator with 4℃.

Solutions O/w is a kind of hydrophobic mixture with low solubility in water, which can adsorb on hydrophobic surface easily for its’ high octanol water partition coefficient, such as organic surface. Moreover, Zeta potential of the solution is always negative in the range of pH 2.0~13.0, which increases first and then decreases along with the increase of pH, and it has a relatively stable maximum value of about -107 mv in the range of PH 6-10 [19]. Thus, it can be inferred that some parameters, such as hydrocarbon and hydrogen bond should should have an impact on adsorption process, impelling this adsorption process on the hydrophobic surface. Other parameters, such as ion concentration, pH value, temperature, and time may also affect it[20]. 1000 mg/L o/w stock solution was prepared by adding 1.000 g oil solid (dry weight) with 1000 mL distilled water into emulsification mixer with 28000 rpm shear agitation, then the raw liquid can be used after diluted to the required concentration with o/w particle diameter in the range of 0.1-0.6µm, determined by Malvern Mastersizer Particle size analyzer, even after 7days, the oil droplets were still in the range of 0.2-0.8µm, which indicated the oil emulsion was stability [21].

Adsorption experiments The adsorption experiments were carried out in 500 mL glass erlenmeyer flasks in an air bath shaker with 200 rpm under constant temperature through batch method. In order to determine the 5


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kinetics and thermodynamics of adsorption, o/w solution with a volume of 300 mL has a specified concentration, and a piece of membrane with a surface area of 50 cm2 which was adequately sensitive for this adsorption process was added to the flasks[22], then, each test lasted 48 hours, until the concentration of the o/w solution determined by infrared spectrophotometry method[23] was no longer changed obviously. Kinetic curves can be achieved by monitoring o/w concentration of the liquid solution at regular time intervals until not changing more, also, every sample used only once[24, 25]. Moreover, the impact of influencing factors, such as pH value adjusted to certain value with 5 N NaOH and 5 N HCl, and temperature for evaluating in the tests were conducted at the fixed values. The samples without membranes were used as control groups in each test, and all the data were the average values obtained from two parallel experiments in this study.

Ultrafiltration experiments Fig.1 shows the schematic diagram of experimental set-up. In this system, a dead-end stirred ultrafiltration cell (XFUF 07601, Millipore Co., U.S.A.) with a volume capacity of 300 mL was used as the main ultrafiltration equipment, which has an inner diameter of 76 mm with an effective membrane area 40 cm2. Nitrogen gas was used as the extra driving force in ultrafiltration experiments under a constant room temperature of 25±1 ℃, the stirring speed of the rotor is set to 100 rpm. The membranes used for ultrafiltration were immersed into an o/w solution until adsorption saturation, then a gently washing step for membrane surface was made for 30 s by distilled water. However, in order to elimate the densification of organic membranes by compression, each membrane was compacted for 0.5 h at 0.2 MPa TMP before using, meanwhile, the pure water flux was determined as the initial flux J0. 6


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RESULTS AND DISCUSSION Adsorption equilibrium isotherm Five different types Isothermal Equations used commonly, eg. Freundlich (F), Langmuir (L), Temkin (T), Redlich-Peterson (R-P), and Langmuir-Freundlich (L-F) [22~24], have been adopted here to determine adsorption isotherm of o/w from solution onto the two kinds of membranes. O/w adsorption isotherm on the two membranes experiments are shown in Fig. 2. As can be seen from the diagram, the overall trend of the adsorption isotherm data points is: in the initial stage, the adsorption amount increases significantly with the increase of O/W equilibrium concentration in the solution; and then tends to be gentle gradually, which is in line with the typical R-P isotherm curve[26]. This result was consistent with Wibowo. et al. who investigated salinity adsorption at natural zeolite [27]. The adsorption amount of o/w on OM and MM increased considerably with the o/w equilibrium concentration in solution, nearly reached to as high as 24.7 µg/cm2 and 18.5 µg/cm2, respectively. After the point, adsorption amounts were no longer increased obviously with increase of o/w concentration in aqueous solution. Furthermore, MM had a smaller adsorption capacity than that of OM, for several reasons: Firstly, this may be attributed to the complexity of the chemical composition and structure of oil molecules, which has strong hydrophobicity. Secondly, it was believed that the improvement of hydrophilic property of MM played an important role[28], resulting in the amount of adsorption reduced greatly. This result is different from the adsorption isothermal line obtained by Tian et al.[29] who use novel reusable porous polyimide fibers for hot-oil adsorption. The main reasons for the difference between these two phenomena can be draw as the difference of water affinity of nano-particles modified materials and polyimide fibers. On the one hand, the membrane surface energy and 7


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hydrophilic property improved [30], leading to the decrease of the adsorption capacity of o/w, due to the addition of TiO2 and Al2O3 nano-particles; on the other hand, o/w is mainly composed of various hydrocarbons and carbon-chain polymer, which is adsorb on the hydrophobic membrane surface more easily, based on the "similarity attraction" principle. Generally, three-parameter equations eg. Redlich-Peterson and Langmuir–Freundlich are not use as common as two-parameter equations, eg. Langmuir, Freundlich and Temkin to describe adsorption isotherm parameters. Thus, Langmuir equation is the most popular in studying BSA adsorption on different membranes, such as Nakamura et al. [31] and Bowen et al. [32]. However, according to the research works in the recent years [33], three parameters equations is usually more suitable for the isotherm data simulation of more complex adsorption process than that of two parameters equations. The least squares method is used to analyze the adsorption data of o/w on two kinds of membranes to carry out the nonlinear regression simulation of the above five isotherm models, and parameters and correlation coefficients (R2) of each model were calculated use Origin software, the results are as follows in Table 2. It can be seen from this Table 2, apart from the Freundlich model, the other four models for the process of fitting had thermodynamic characteristics of two kinds of integrated membrane adsorption of o/w, with R2 values indicating applicability order: Redlich-Peterson = Langmuir-Freundlic > Temkin = Langmuir. Therefore, according to the significance of the fitting model, it can be preliminarily determined that the adsorption process is a physical-chemical adsorption.

Adsorption thermodynamics Endothermic or exothermic occurred in the o/w adsorption process had been revealed with 8


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experimental condition of o/w concentration 20 mg/L, pH value 6.8, contact time 24 h, and a broad variety of temperature 298 K, 293 K, 303 K, 308 K, 313 K. The result of the change of adsorption capacity with temperature, and Van’t Hoff plots of o/w adsorption onto OM and MM were shown as Fig. 3, meanwhile, thermodynamic parameters were also calculated, just show in Table 3. As can be seen from Fig. 3(a), the adsorption of o/w on the two kinds of membranes gradually decreases with the increase of temperature, indicating that the increase of temperature is not conducive to the absorption process. In general, the higher the temperature, the greater the entropy of the system, which means the efficiency of the collision is increased, leading to the adsorption capacity increasing. However, for the exothermic adsorption process, temperature increase has a negative effect on the adsorption process, and the mechanism is similar to the adsorption of Pb2+ on biological apatite aqueous solution stated by Shen et al[34]. In order to determine the mechanism of the enthalpy control process, the plot of lnK to 1/T and the formula (1) was used to calculate standard Gibbs free energy; formula (2) and formula (3) were used to calculate and estimate heat of adsorption and the change of entropy, then all the results are shown in Table 3. ∆ r G mθ = − RT ln K (1) ∆ r Gmθ = ∆ r H mθ − T∆ r S mθ

ln K =

∆ r S mθ ∆ r H mθ − R RT

(2) (3)

When 293 K, 298 K, 303 K, 308 K and 313 K were used, the standard Gibbs free energies of o/w adsorption on OM and MM were 0.81, 0.96, 2.00, 2.88 and 3.84 KJ/mol, 1.97, 2.39, 3.36, 3.76 and 4.80 KJ/mol, respectively. The lower of temperature the lower of standard Gibbs free 9


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energy obtained, indicating the two kinds of membranes have strong adsorption capacity for o/w, that is to say, the adsorption process tends to be spontaneous[35], and the rise of temperature makes the spontaneous reduction of the process, namely, the feasibility is non-spontaneous during this adsorption process. In addition, this value of MM is significantly higher than that of OM, indicating anti fouling ability of MM increased. It can be deduced from Table 3 that this adsorption is an endothermic process for the standard molar enthalpy change is negative. Obviously, the amount of adsorption gradually decreased with the increase of temperature, because of the solubility increases with the increase of temperature, which promotes the process of desorption; on the contrary, o/w particles agglomerate easily as temperature decreases, then easily deposited on the membrane surface. Carpintero-Tepole, et al. had also achieved this similar conclusion[36]. Moreover, the standard molar enthalpy changes values of o/w by OM and MM were 41.37 and 39.22 kJ/mol, separately, indicating it seems to be a chemical and physical combined adsorption process with neglecting the factors of systematic error, on the basis of the conclusion stated by Alkan [37] that chemisorption with the standard enthalpy change between 40 and 120 kJ/mol, otherwise is physisorption. This change can be attributed to two major reasons: one was the increase of the hydrophilicity after adding nano-particles makes “similar phase absorption” greatly reduced between film and oil particles; the other one was interaction force gradually shifts from the original chemical bond force to van Edward force. In addition, the standard molar entropy change of this adsorption process on OM and MM were -143.8 J/mol·K and -140.2 J/mol·K, accordingly, reflecting the chaos of a system is greatly reduced caused by the concentration of oil droplet particles decreased sharply for the oil droplets adsorbed on the membrane surface in solvent dissociation events. 10


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Adsorption kinetics From previous experimental data, the adsorption capacity of the two membranes to oil beads were high. Therefore, it is necessary to study the adsorption kinetics of this process. In general, pH of oilfield raw wastewater was 10 and temperature of about 40 ℃, but after destabilization / coagulation-sedimentation / filtration etc. conventional pretreatment process, temperature often drops to about 30 ℃ and pH value between 6.8 to 7.2. Based on this, kinetic experiments were carried out with initial value of pH 6.8, temperature 30 ℃(303 K). Four kinetic models, such as: Elovich

equation,

intra-particle

diffusion

equation

pseudo-first-order

equation,

and

pseudo-second-order equation [38,39] were used to test the fit of experimental data obtained from batch o/w adsorption experiments.

Effect of initial concentration 20 mg/L, 50 mg /L and 200 mg/L as the initial concentration were used for elucidating the effect of initial concentration on the kinetic experiments with pH 6.8 and temperature 30 ℃. Although there is no such high oil concentration with 200 mg/L in the oilfield wastewater, the concentration of oil can be enriched along with ultrafiltration process, which, even go far beyond of it. This adsorption process needs about 48 h to reach equilibrium, which means the amount of adsorbed o/w did not change significantly after this enough long time. Then, Four types of kinetic models were used to simulate o/w adsorption rates which were obtained by the decrease of the oil concentration in liquid phase with contact time, the results are shown in Fig.4. Meanwhile, the correlation coefficients and parameters of the four kinetic equations are shown in Table 4. It can see from Fig. 4, under three different (high, middle, and low) initial concentrations, the adsorption amount of o/w on the membrane is no longer significant after 16 h, which can be 11


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deduced that the equilibrium adsorption time is about 16 h. Furthermore, both of adsorption kinetics curves of the two kinds membranes can be divided into two stages, one is initial rapid phase, which is fast and has a significant donation of equilibrium uptake; a relatively slower second phase with the total adsorption contribution is relatively small. At the meanwhile, the hither of the initial o/w concentration the later equilibrium occurs, which has an equilibrium time of about 5 h, 8 h, and 16 h, accordingly, and the capacity of o/w adsorption was about 6.6, 12.5, and 32.8 µg/cm2 OM and 4.9, 10.7, 27.8 µg/cm2 on MM; however, the equilibrium adsorption capacity has the contrary trend. This is due to efficient utilisation of the capacity of adsorbent, which is expected to the driving force caused by initial concentration [40]. It is obvious that the increase of concentration promoted the increase of adsorption capacity, but the increase rate slowed down greatly, mainly because the negative charge density of oil particles increases, increasing the repulsion force with negative charged membrane surface; while, the difference between oil particles concentration in the main solution and membrane surface increases greatly, leading the frequency of collision between particles and membrane surfaces. Obviously, this is the two opposite effect. Moreover, in order to achieve a enough adsorption equilibrium time, 16 h would be a better choice.

Effect of temperature Oil field wastewater usually has relatively high temperature, finding a suitable temperature will play an important role in the ultrafiltration process. 293K, 313K, 323K as operating temperature were studied for elucidating the effect of temperature on the kinetic experiments with pH 6.8 and concentration of 100mg/L, with results shown in Fig.5. Meanwhile, all the estimated parameters and R2 value are shown in Table 5. 12


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It can be seen from Fig.5 that the adsorption rate become slight lower at a higher temperature. Moreover, the majority of o/w adsorption amount was completed in about 8h. For instance, after 1 h, 2 h, and 8 h, the adsorption capacity of o/w on OM and MM can reach to about 50%, 67%, and 90% of the equilibrium adsorption capacity (13.5 µg/cm2), and 51%, 64%, and 92% of the equilibrium adsorption capacity (10.7 µg/cm2), correspondingly, under the temperature of 313 K. which can be further proved that the equilibrium time of the adsorption system is about 16 h. The reason can be ascribed to the increasing of o/w solubility in water with temperature, which means the affinity of o/w to water molecules is greatly enhanced [41]. Change perspective, the adsorption ability on the strong hydrophobic membrane surface decreased for hydrophobic increasing of o/w molecule along with the increase of temperature. Of cause, according to the description of characteristics of oil particles and Khelifa’s statement [42], which properties will change slightly in a relatively small temperature range, and even not changed when particles emulsified. Therefore, it is believed that the change of membrane surface characteristics were the decisive roles here, also the Similar compatibility principle was decisive theroy, determining adsorption type.

Analysis of dynamic model All the relevant parameters and regression coefficients of the four kinetic models of the adsorption system under different initial concentrations and temperatures have been solved. The results are shown in the following Tables. It can be seen from Table 4 and 5 that under the different temperature and concentration conditions, both of the fitting R2 values of the o/w adsorption processes on two membrane using pseudo-first-order kinetic model through multiple iterations and nonlinear regression, which are 13


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all higher than 0.98, indicating the adsorption process accords with this dynamic model. This result is almost [43] in accordance with the adsorption kinetics of some industrial dye on different adsorbents. While, the applicability of the two kinetic models for simulating this adsorption process is very poor, especially, the Intra-particle model, which is difficult to accurately describe the adsorption kinetics. The main reason is this adsorption process of o/w on the membrane is mainly controlled by external surface adsorption and transient adsorption, eg. although there is a certain repulsion between the oil particles and the membranes, the effect of intermolecular force, such as hydrogen bond, Fan Dehua force and covalent bond, still makes the oil molecules adsorb on the membrane surface, leading a certain adsorption pollution occured. In addition, the initial thermodynamic entropy and the interaction of free o/w particles in the solution and membrane surface are also the main factors in determining this process.

Effect of adsorption pollution on membrane flux In the light of flux measurement, the pure water flux of the membrane before and after soaking at 200 mg/L o/w with different TMP was investigated, and this pollution was described by the change of relative flux of pure water (J/J0) , which is shown in Fig.6. Moreover, the removal of oil was above 99.8% in our previous studies [44], thus, the retention characteristics were not did again, here. It can be seen from Fig.6 that，flux attenuation appears after o/w adsorbed on membranes, especially, the extent of flux attenuation increases with the increase of TMP. The results show that o/w particles are difficult to enter the membrane pores under low TMP with a slow flux attenuation rate, indicating the probability of membrane blockage is very low. In addition, the lower TMP is not enough to compress o/w gel layer on the membrane surface, has a large water 14


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flux. Because, with the increase of TMP, o/w particles migrates from surface to pores, resulting in the possibility of plugging the membrane pores, and then forming a dense gel layer on the surface of the membrane[45]. After TMP reaches a certain value (0.075 MPa), the thickness of the gel layer and pores blockage tending to a basic balance, leading to the membrane flux does not continue to decrease with the increase of TMP.

Microanalysis of membrane surface SEM of membranes surface adsorption Fig.7 shows the SEM images (with a magnification 1000) revealing characteristics difference of the two kinds of membrane surfaces after adsorption of o/w particles for 12h. It can be seen large amount of oil adsorbed on the surface of the membrane by the naked eyes, which is consistent the surface covered by a dense layer of oil particles observed by SEM. Although both of naked eyes and SEM cannot quantify the amount of oil attached to the membrane surface, it still gives us a clear visual and qualitative judgement. From the SEM photos Fig. 7(a) and (a’), it can be seen that the original surfaces of OM and MM are evenly distributed with honeycomb pores, while, the bulges and sags on MM surface are more obvious, indicating that the basic morphology of the membrane surface is not change obvious except roughness after the addition of TiO2 and Al2O3 nanoparticles, this find was identical with Li [46]. It was observed in Fig.7 (b) and (b’) that SEM photographs of surface morphology about this two kinds of membrane which adsorbing o/w, appears a thick layer of viscous material on the rough and porous original membrane surface, which must be a dense oil film constituted of oil molecules, moreover, the covering thickness of oil layer on OM is slightly higher than that of MM 15


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was also be observed. Thus, a preliminary conclusion was drawn based on this morphology analysis, that the decreasing in adsorption capacity of MM can be attributed to hydrophilic ability increasing by the introduction of nanoparticles, which is similar with the statements by Allen [47]. ATR-FTIR of membranes surface adsorption In order to understand the interaction between membrane and o/w particles clearly, ATR-FTIR spectra of membranes after immersing in different o/w concentrations were analyzed. The results were shown in Fig.8. As shown in Fig. 8, it is found that most of the characteristic absorption peaks of mebrane have not changed significantly when using infrared spectroscopy to carry out a quantitative analysis of the functional groups on the membrane surface. The main reasons can be drawn as: Firstly, the sensitivity of the crude oil to the infrared spectrum is poor. Secondly, oil layer attached to the membrane surface is laterally sliding in the process of ATR compression, due to the enormous external mechanical force, which makes the measured oil less measured. However, based on this analysis, if there is a certain change in the intensity of the functional group in the infrared spectrum, then the actual amount of the response should be considered to be magnified to a certain extent in theoretically. Based on this, fig.8 indicating that the near-infrared absorption peak in alpha crystalline phase[49] is 615cm-1, decreasing sharply compared with membrane without o/w covrage, even disappeared of the membrane after soaking in the high o/w concentration. This phenomenon manifested indirectly that o/w covered on the surface of the membrane and shielded the characteristic peaks of the membrane itself. In addition, the amount of o/w adsorbed on the surface of MM is slightly lower than that of OM by comparing the strength of the peaks in the two membranes. Based on this, it is safe to say the antipolluted performance of MM to o/w is higher 16


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than that of OM, obviously. Therefore, it is considered that the change of the strength of a certain functional group represents the situation of membrane fouling, then, it is safe to say that, the anti-fouling ability of MM has been improved on the point of view of the infrared spectrogram.

CONCLUSION The thermodynamic and kinetic characteristics of the adsorption system of o/w in the oil field wastewater and two kinds of UF membrane were investigated. The main conclusions are as follows: (1) O/w particles can be adsorbed on OM and MM for its strong affinity, which is a typical physico-chemical adsorption process belong to the Redlich-Peterson adsorption isotherms type. In addition, the adsorption system can be ascribed to the chemical-led non-spontaneous adsorption process from the calculated thermodynamic constants values. (2) The adsorption kinetics of o/w-membrane system shows that the adsorption equilibrium time was about 16 h, and the adsorption rate become slight lower with the increase of temperature. Moreover, the flux attenuation caused by adsorption-induced pollution was significantly lower than that of the original membrane. (3) The physical morphology of MM was not changed obviously, but the adsorption capacity of o/w decreased significantly from the change of SEM and ATR-FTIR spectrum , reflecting the hydrophilic of MM increased, verifying physical-chemical adsorption process occurred.

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ACKNOWLEDGEMENTS The authors gratefully acknowledge State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, for its support during the conduct of this research. Thanks are also extended to the financial support provided by Hainan University (Grant No. KYQD(ZR)1846); We also appreciate the editors’ valuable comments very much, which are helpful to improve the quality of our present study.

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Figure Captions:

Figure 1. Schematic diagram of experimental set-up

Figure 2. O/w adsorption isotherm on OM (a) and MM (b). pH 6.8; temperature 303K; contact time 48h

Figure 3. (a) Effect of temperature on o/w adsorption onto OM and MM; (b) Van’t Hoff plots of o/w adsorption

onto OM and MM

Figure 4. Kinetics curve of o/w adsorption on OM (a) and MM (b) with different initial concentration

Figure 5. Kinetics curve of o/w adsorption on OM (a) and MM (b) with different temperatures

Figure 6. Relative flux decline of pure water by adsorption of o/w vs. TMP

Figure 7. SEM photographs of the two membranes surface before and after adsorption of o/w

Figure 8. ATR-FTIR spectra of OM (a) and MM (b) with different o/w concentration

Table Captions:

Table 1. Characteristics of the two kinds membranes

Table 2. Estimated isotherm parameters for o/w adsorption on the two membranes

Table 3. Thermodynamic parameters of o/w adsorption onto OM and MM membranes

Table 4. Kinetic parameter of initial concentration for the adsorption of o/w on OM and MM membranes

Table 5. Kinetic parameter of temperature for the adsorption of o/w on OM and MM membranes

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Figures

3

4 7 6 1 5 8 9 10 Figure 1. Schematic diagram of experimental set-up

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Figure 2. O/w adsorption isotherm on OM (a) and MM (b). pH 6.8; temperature 303K; contact time 48h

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Figure 3. (a) Effect of temperature on o/w adsorption onto OM and MM; (b) Van’t Hoff plots of o/w adsorption onto OM and MM

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Figure 4. Kinetics curve of o/w adsorption on OM (a) and MM (b) with different initial concentration

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Figure 5. Kinetics curve of o/w adsorption on OM (a) and MM (b) with different temperatures

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Page 30 of 35

Figure 6. Relative flux decline of pure water by adsorption of o/w vs. TMP

（a）OM surface

（a’）MM surface

（b）fouled OM surface

（b’）fouled MM surface

Figure 7. SEM photographs of the two membranes surface before and after adsorption of o/w 30


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Figure 8. ATR-FTIR spectra of OM (a) and MM (b) with different o/w concentration

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Page 32 of 35

Tables

Table 1. Characteristics of the two kinds PVDF membranes Properties

PVDF membrane (PM)

Modified PVDF membrane (MPM)

Skin thickness (nm)

100nm

100nm

Membrane diameter (mm)

76

76

0.04

0.04

2

Effective area (m ) Contact angel (°)

65.2

48.4

Pore diameter

3-5 nm

3-5nm

MWCO

100 000Da

100 000Da

hydrophobic

hydrophilic property increased

250

360

Hydrophobic/ hydrophilic 2

Pure water flux (L/m h)

Table 2. Estimated isotherm parameters for o/w adsorption on the two membranes parameters

k

1/n

R

2

Freundlich q= kC

1/n

OM

3.3214

0.3522

0.9461

MM

2.9682

0.3355

0.8640

parameters

qm

b

R

OM

25.551

0.0101

0.9366

MM

20.058

0.0094

0.9998

parameters

a

b

R

OM

-9.1185

5.9040

0.9935

MM

-7.7956

4.9130

0.9596

parameters

a

b

n

R

Langmuir q= bqmC/(1+bC)

2

2

Temkin q= a + b lnC

Redlich-Peterson q= aC/(1 + bnC)

OM

0.5577

-0.1397

-0.1397

0.9682

MM

0.4615

-0.1405

-0.1405

0.9840

parameters

qm

b

1/n

R2

OM

12.8314

0.0213

0.779

0.9487

MM

35.1328

0.0202

0.5251

0.9720

Langmuir-Freundlich 1/n

1/n

q= bqmC /(1 + bC )

2

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Table 3 Thermodynamic parameters of o/w adsorption onto OM and MM membranes ∆ r Gmθ (KJ/mol)

o/w-membrane

∆ r H mθ

∆ r S mθ

293K

298K

303K

308K

313K

(KJ/mol)

(J/molK)

OM

0.81

0.96

2.00

2.88

3.84

-41.37

-143.8

MM

1.97

2.39

3.36

3.76

4.80

-39.22

-140.2

Table 4. Kinetic parameter of initial concentration for the adsorption of o/w on OM and MM membranes Pseudo-first-order model Model/factor

20 Initial 50

concentration (mg/L)

200

a

b

R

Pseudo-second-order model 2

1/h

qm

R2

OM

6.2433

0.5725

0.9881

0.1956

6.8928

0.9929

MM

4.6514

0.5810

0.9847

0.2641

5.1485

0.9796

OM

13.4786

0.9095

0.9907

0.05334

14.5684

0.9908

MM

10.2593

0.8119

0.9847

0.0803

11.1471

0.9827

OM

32.8599

0.4708

0.9826

0.0452

36.5126

0.9973

MM

26.6056

0.3306

0.9812

0.0873

26.6914

0.9740

Elovich model

Intra-particle model

Model/factor

20 Initial 50

concentration (mg/L)

200

α

β

R

OM

17.258

0.8891

MM

12.0877

2

2

Kt

C

R

0.9345

0.8824

2.0265

0.6093

1.1732

0.9125

0.6504

1.4989

0.5869

OM

154.022

0.5245

0.9158

1.6700

1.9367

0.4772

MM

73.1756

0.6386

0.9053

1.3168

4.1388

0.4999

OM

65.4652

0.1598

0.9572

4.8317

9.3660

0.6734

MM

36.0552

0.1892

0.9749

4.0335

6.0432

0.7558

Table 5. Kinetic parameter of temperature for the adsorption of o/w on OM and MM membranes Pseudo-first-order model Model/factor

293

T (K)

303

313

Pseudo-second-order model 2

b

R

1/h

qm

R

OM

22.0763

0.5746

0.9684

0.0552

24.4387

0.9952

MM

16.7847

0.6101

0.9829

0.0671

18.4665

0.9978

OM

17.4617

0.6229

0.9892

0.0617

19.0325

0.9807

MM

13.8610

0.6426

0.9834

0.0779

15.2511

0.9962

OM

13.6694

0.7704

0.9695

0.06257

14.8596

0.9904

MM

9.8160

0.6246

0.9601

0.1108

10.7730

0.9816

Elovich model Model/factor

293

T (K)

303

313

2

a

Intra-particle model

α

β

R2

Kt

C

R2

OM

63.6902

0.2526

0.9627

3.1818

7.1119

0.6592

MM

57.3797

0.3444

0.9508

2.3486

5.7078

0.6212

OM

39.6754

0.3408

0.9310

2.3598

6.18185

0.5718

MM

50.4962

0.4198

0.9479

1.9364

4.8056

0.6140

OM

98.0729

0.4818

0.9348

1.7664

5.4567

0.5329

MM

38.1149

0.6053

0.9284

1.3359

3.4727

0.5768

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Page 34 of 35

Table of Contents Graphic

To determination of isotherm parameters and kinetic parameters of oil/water (o/w) adsorption from aqueous solution on PVDF Ultrafiltration membrane (OM) and modified PVDF Ultrafiltration membrane (MM) is important in understanding adsorption mechanism in Ultrafiltration processes. Five different isotherm models and four different kinetic models were chosen to fit the experiment data to understand the mechanism of this adsorption process. The thermodynamic parameters were also calculated from the temperature dependence ( ∆ r Gmθ ,

∆ r H mθ , ∆ r S mθ ).

Moreover, SEM, and

ATR-FTIR spectrum were also investigated. The results show that the process of adsorption is not spontaneous and endothermic process.

34


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A Table of Contents Graphic To determination of isotherm parameters and kinetic parameters of oil/water (o/w) adsorption from aqueous solution on PVDF Ultrafiltration membrane (OM) and modified PVDF Ultrafiltration membrane (MM) is important in understanding adsorption mechanism in Ultrafiltration processes. Five different isotherm models and four different kinetic models were chosen to fit the experiment data to understand the mechanism of this adsorption process. The thermodynamic parameters were also calculated from the temperature dependence (  r Gm ,  r S m ).

 r H m ,

Moreover, SEM, and ATR-FTIR spectrum were also investigated. The results show that the

process of adsorption is not spontaneous and endothermic process.


Al2O3-PVDF ultrafiltration

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