Experimental verification on real-time fouling analysis in crossflow UF of protein solutions by electrical impedance spectroscopy
Graphical abstract
Fouling development during crossflow UF of bovine serum albumin (BSA) using polyethersulfone and cellulose acetate membrane have been studied by two methods. The first method is based on the flux decline to calculate individual resistances caused by adsorption (Ra), cake formation (Rc), concentration polarization (Rp), and membrane itself (Rm). The second one applied electrical impedance spectroscopy (EIS) technique to model the equivalent circuit which infers the foulant layer. Both methods could be operated effectively to detect the time of fouling formation. However, the EIS technique presents more advantages such as simple, non-constructive, and time-saving. Especially, it can be applied to monitor the fouling development in real-time which is very important in UF operation.
Introduction
Membrane technology has been increasingly applied for water and wastewater treatment in recent decades [1,2]. In this technique, a membrane plays as a barrier separating two fractions from each other by selective transport of components through it. Membrane separations have been found to be spread application due to their many advantages such as cost-effectiveness, easy operation, no addition of chemicals, lower energy requirement, etc. [3,4]. These techniques can deal with a broad spectrum of pollutants. In a review, Carolin et al. [5] indicated that pressure-driven membrane filtration is one of the most efficient methods to remove varieties of heavy metals in wastewater. Microfiltration (MF), ultrafiltration (UF), nanofiltration, and reverse osmosis (RO) could be flexible used depending on the size of particles [6]. Pharmaceuticals can be well treated by membrane separations with high-quality permeate and no increase of its toxicity [7]. Membrane-based technologies also promise sustainable methods for the treatment of industrial effluents [8].
Although membrane filtration processes have been recorded to effectively remove a variety of pollutants, they still face a problem, that is membrane fouling [8]. Membrane fouling describes the accumulation of molecules or particles on the surface or in the pores of the membrane leading to the increase of membrane resistances and the restriction of the flow through the membrane. The fouling phenomenon can be caused by inorganic, organic, and biological substances [9], [10], [11]. Inorganic fouling starts with the deposition of particles on the surface or in the pores of a membrane and then forms a cake layer. Biological organisms interact complex with a membrane by adhesion to the membrane surface, growing to biofouling layer. Natural organic matters are often mentioned as the main source of organic fouling. Once happened, fouling will result in a decline of membrane flux and a higher pressure is always required pressure, which may cause the damage of the membranes in some cases depending on the nature of the scale and foulants [10,12]. Therefore, the fouling phenomenon of a membrane should be monitored during a filtration process.
Membrane fouling is commonly detected by changing the transmembrane pressure resulting in a decline of the flux [13], [14], [15]. During the filtration progress, particles or adsorbates accumulated on and/or within the membrane raise the filtration resistance and decrease the flux. Gradually, this layer becomes more complex and thicker and causes membrane fouling phenomenon. Based on the measurements of flux decline with time, the resistances of the membranes can be calculated, and the fouling is accordingly determined. However, the computation complexity of this method makes it impractical to real-time monitor the development of membrane fouling, which is important to observe and alert as early as possible.
Electrical impedance spectroscopy (EIS) has been applied as an effective tool to meet the above requirements, which is proven to be very sensitive to a change in the layer structure of the membrane [16], [17], [18]. This allows the researchers to determine conductance and capacitance. Be a non-destructive and real-time monitoring technique, EIS recently has been applied to in-situ analyze the structure of the membranes and the fouling behavior of the membranes [16,[18], [19], [20]] as well as the stability issues of supported liquid membranes [21,22]. For detecting fouling of RO membranes by EIS, Cen et al. [23] have indicated that EIS technique is more sensitive than flux decline measurements and is, hence, more potential to be applied in fouling observation. In the UF of oil emulsions using polyethersulfone membrane [24], EIS revealed a significant potential even though the flux decline was negligible. Moreover, Ahmed et al. [25] have indicated that EIS can provide feedback on fouling earlier than the performance in the flux decline of membrane distillation.
Membranes used in pressure-driven separation processes and electrodialysis are generally composed of a thin skin layer and a thick sublayer. Moreover, a diffusion polarization layer will be formed on the membrane surface during the separation process. Each of these layers will contribute to the conductance and capacitance to some extent. When alternating currents pass through such a membrane, we can measure the total conductance and total capacitance at different frequencies, and then calculate the conductance and capacitance within each layer. EIS has also been proven to be used to analyze the microstructure of UF and conductive membranes, including pore size and surface layer thickness [26], [27], [28]. This technique is able to be applied to analyze the fouling characteristics of RO membranes [29].
In the present work, we tried to monitor the fouling development of two common UF membranes by EIS in a crossflow mode. Bovine serum albumin (BSA) protein was selected as a model biological macromolecule. The EIS measurements can characterize the formation of electrically distinct layers during the filtration process. That is, each layer responds to a specific frequency, thereby monitoring the fouling formation. Another primary aim of this study was to verify the validity of real-time EIS analysis by determining the resistances based on a resistance-in-series model from flux decline measurements. By comparing the two techniques of EIS and flux decline analysis, we consequently assess the ability and validity to apply in-situ EIS in detecting and monitoring the formation and development of membrane fouling.
Section snippets
Impedance analysis: equivalent circuit model
The background of EIS has been reported previously [17,18,30]. Briefly, an alternating current, i=i0 sin(ωt) of known amplitude i0 and angular frequency ω = 2πf (f is frequency) is passed through the membrane system. The voltage response υ = υ0 sin(ωt‒θ), where υ0 and θ are the voltage amplitude and phase difference, respectively is then measured using digital technique. Based on this result, the capacitance and conductance can be inferred. The changes in the capacitance and conductance values
Reagents and solutions
Bovine serum albumin (BSA, 66.5 kDa, purity 99%), NaOH, and HCl were obtained from Sigma-Aldrich Co. (MO, USA). High-performance liquid chromatography (HPLC)-grade methanol and acetonitrile were supplied by J.T. Baker Co. (USA). Deionized (DI) water used in this study was from a Milli-Qsp system (Millipore, MA, USA). The pH value of the solution was adjusted by adding a small amount of 0.1 mol/L HCl or NaOH and measured by a pH meter (Horiba, Model F-22).
Membranes and experimental setup
Cellulose acetate (CA) and
Determination of various electrical impedances
To investigate the effects of applied pressure, initial feed BSA concentration, and solution pH on the electric impedance of the membranes, different pressures (68.9, 137.9, and 206.8 kPa) were applied. The EIS data was performed as Nyquist plots. Fig. 3 shows the typical results in crossflow UF of BSA solution using PES and CA membranes, respectively. More results were depicted in Figs. S1–S6 (see Supplementary material).
At low frequencies (for example, less than 10 kHz when the electrolyte
Conclusion
We have analyzed the variations of various electrical impedances with time based on a proposed equivalent circuit model by electrical impedance spectroscopy (EIS) and of various filtration resistances based on a series-in-series model from flux measurements in crossflow UF of BSA solutions using PES and CA membranes. It was found that the impedances due to the fouled layer RF is the most important part in total impedance for both PES and CA membranes. The value of RF depends on the applied
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Financial support for this work by a grant from the Ministry of Science and Technology, Taiwan (No. MOST 109-2221-E-182-024-MY3) is gratefully appreciated.
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