Analysis of particle fouling during microfiltration by use of blocking models

Particle fouling mechanisms in “dead-end” microfiltration is analyzed using blocking models. The blocking index and resistance coefficient of the models during microfiltration are calculated under various conditions. The major factors affecting these model parameters, such as the filtration rate, the amount of particles simultaneously arriving at the membrane surface and particle accumulation, are discussed thoroughly. Instead of the four different blocking models previously proposed, a membrane blocking chart is established for relating the blocking index, filtration rate, and particle accumulation. Blocking index variation during microfiltration can be interpreted using this chart. Membrane blocking occurs during the initial filtration periods until the condition reaches a critical value; then, the blocking index suddenly drops to zero by following up the cake filtration model. Once the normalized resistance coefficient is regressed to an exponential function of the blocking index under a wide range of conditions, the blocking models can be used to quantitatively explain filtration flux attenuation by solving a unitary mathematical equation. Comparing the experimental filtration rates obtained under different conditions with the simulated results reveals a good agreement between them and demonstrates the reliability of this analysis method.     

Properties of protein adsorption onto pore surface during microfiltration: Effects of solution environment and membrane hydrophobicity

Properties of bovine serum albumin (BSA) adsorption onto pore surface during the filtration of BSA containing solution with the Sirasu porous glass membrane with a pore size of 0.1 μm were studied. The effects of pH, ionic strength, and surface modification on the flux decline and breakthrough curves were observed. The adsorption properties of BSA were estimated quantitatively by using the internal fouling model, which relates the filtration performance to the adsorption interaction, the adsorption capacity, and the thickness of the adsorption layer. The electrostatic interaction between BSA and pore surface was estimated by the streaming potential measurement. The BSA adsorption involved a rapid adsorption in the early stage of filtration followed by a slow multilayer adsorption that dominates the long-term filtration performance. The electrostatic repulsive force reduced the overall adsorption interaction but the electrostatic attractive force did not affect the adsorption interaction. The effect of ionic strength on the BSA adsorption could be explained in terms of the shift of the IEP of BSA toward lower pH with the increase in ionic strength. The hydrophobicity of membrane did not affect the adsorption properties except for the adsorption interaction in the early stage of the filtration.     

Characterization and theoretical analysis of protein fouling of cellulose acetate membrane during constant flux dead-end microfiltration

A rapid characterization method was used to study protein fouling of cellulose acetate membrane during dead-end, in-line, constant flux microfiltration. Based on pressure-permeate volume profiles, two fouling phases could be identified and compared at different permeate fluxes. Using protein staining dyes, the model foulant (bovine serum albumin) was found to deposit on the upstream side of the membrane as a loose cake at its isoelectric point. The effects of solution pH on both the nature and extent of membrane fouling, and membrane cleaning were examined. To further understand and quantitatively analyze the fouling behavior, a combined mathematical model which took into account pore blocking, cake formation and pore constriction was developed based on existing fouling models. The data obtained by modeling was in good agreement with experimental fouling data. Theoretical analysis of data clearly indicated that cake formation was the main fouling mechanism. Using methods such as dynamic light scattering, the significant role of large protein aggregates in membrane fouling was confirmed. The dimer composition of protein did not change significantly during the fouling experiments, clearly indicating that smaller aggregates played less important role in membrane fouling.     

Fouling reduction in poly(acrylonitrile-co-acrylamide) ultrafiltration membranes

Ultrafiltration membranes with similar pore sizes were prepared from acrylonitrile homopolymer and copolymers with increasing acrylamide content. The membranes containing acrylamide were more hydrophilic, had a smaller dispersion force component of the surface energy, and a smaller negative zeta potential than those prepared from the homopolymer. The effect of the differing surface chemistry of these membranes with similar pore sizes was examined by studying the ultrafiltration of bovine serum albumin (BSA) as a function of feed pH. The hydrophilic membranes showed higher permeate fluxes and flux recoveries than the hydrophobic membrane, in spite of their reduced repulsive electrostatic interaction. With increasing pH, protein transmission increased markedly for the acrylamide containing membranes whereas the transmission through the hydrophobic membrane remained low. These rejection data are explained by the combined effects of the increased hydrophilicity, decreased dispersive surface energy and reduced electrostatic repulsion of the acrylamide containing membranes.     

Whey protein fouling of large pore-size ceramic microfiltration membranes at small cross-flow velocity

Microfiltration of whey protein solutions by tubular ceramic membranes, under constant cross-flow and trans-membrane pressure, with periodic backwashing, is investigated using a fully instrumented pilot unit. Relatively large nominal membrane pore size (0.8 μm) insures very high protein transmission, which is desirable in applications such as microbial load reduction. In the first of a sequence of three filtration-backwashing cycles, irreversible and reversible fouling are identified, over the tested pressure range of 5–17.5 psi. Early in the first cycle, especially at the higher pressures, a pore constriction/blocking mechanism appears to be responsible for the irreversible fouling. In the other two cycles only the reversible fouling is significant, possibly due to some kind of protein layer formation on the membrane surface. The permeate flux level tends to increase by increasing trans-membrane pressure up to a near-optimum value of 10 psi, beyond which pressure has a negative effect. This interesting trend is attributed to the interplay of cross-flow velocity, which tends to reduce fouling by promoting re-suspension and breakage of colloidal protein agglomerates, with the trans-membrane pressure (and related flux) which leads to protein layer formation on the membrane and may impose compressive stresses, thereby increasing its resistance to permeation.     

Chemical cleaning of oil contaminated polyethylene hollow fiber microfiltration membranes

The primary aim of this paper was to develop a more effective and economical procedure for cleaning polyethylene hollow fiber microfiltration membranes that have been used for removing oil from contaminated seawater. Alkaline cleaning showed higher recovery of operating cycle time but lower permeate flux recovery than acid cleaning. The combination of both alkaline and acid cleaning agents gave the best operating cycle time and flux recoveries (e.g. 96% and 94%, respectively). As the cleaning agent soaking time was reduced, the actual operating cycle time was reduced. However, the ratio of operating time/chemical cleaning time increased as the soaking time was reduced. The soaking time was recommended to be as short as possible (8–10 h) in the design of small capacity plants and 30 h or higher in case of large capacity plants. SEM analysis showed that in case of alkaline cleaning, most of the pores remained covered with a foulant layer, resulting in low flux recovery. The SEM results of acid cleaned membranes showed more complete removal of the foulant layer from the pores resulting in better flux recovery. Surface analysis of membranes cleaned with combined acid/base agents showed the best results. A membrane surface similar to the original one was obtained. The long-term objective is to increase the understanding of membrane fouling phenomena, preventive means and membrane cleaning processes as it applies to the clean-up and desalination of oil contaminated seawater.     

Influence of permeate flux on fouling during the microfiltration of β-lactoglobulin solutions under cross-flow conditions

Experiments to investigate the microfiltration fouling behaviour of a β-lactoglobulin solution were performed on a constant-flux, computer-controlled, cross-flow membrane rig equipped with zirconium oxide membranes. Fouling was dependent upon the permeate flux, being light at low flux (50 l/m² h) and severe at high flux (200 l/m² h). The fouling increased in severity as the flux was increased from 50 to 200 l/m² h. At 50 l/m² h, protein transmissions of>90% were observed. At higher fluxes, the protein transmission decreased with increasing fouling resistance. The relationship between fouling resistance and protein transmission was similar for 50 and 100 nm membranes and was independent of the starting permeate flux. Standard poreplugging and pore-narrowing models did not describe the observed behaviour. Development of a model to predict protein transmission from the fouling resistance indicated that fouling occurred only in a small part of the membrane pore, most likely at the pore entrance. It is proposed that the microfiltration pore acts in a way similar to a pressure-relief valve where shear-induced protein denaturation has been observed. Shear forces on the protein perhaps lead to protein denaturation and aggregation, and narrowing of the pore in the immediate vicinity of the pore entrance.     

Chemical and physical aspects of organic fouling of forward osmosis membranes

The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the demand for systematic research on FO membrane fouling. This study investigates the role of various physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial permeate flux, and membrane orientation, in organic fouling of forward osmosis membranes. Alginate, bovine serum albumin (BSA), and Aldrich humic acid (AHA) were chosen as model organic foulants. Atomic force microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correlation between organic fouling and intermolecular adhesion was observed, indicating that foulant–foulant interaction plays an important role in determining the rate and extent of organic fouling. The fouling data showed that FO fouling is governed by the coupled influence of chemical and hydrodynamic interactions. Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the development of a fouling layer on the membrane surface. However, the dominating factors controlling membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrodynamic conditions for favorable foulant deposition leading to cake formation are more readily attained. Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhesion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling behavior.     

Turbidity of pulpy fruit juice: A key factor for predicting cross-flow microfiltration performance

To determine the technical and economic feasibility of cross-flow microfiltration on an industrial scale, the expected decline of permeation flux must be predicted taking into account the variability of juice's fouling potential. However, the main difficulty is finding representative parameters. Two semi-empirical models – gel polarisation and mechanistic – were used to fit experimental permeation flows using initial juice turbidity as surrogate for the volumetric concentration of particles in the feed juice. The experimental data of different banana, pineapple, and blackberry juices fitted well in both models. Although the mechanistic model more accurately estimated the permeation flux density, for practical application, the simpler polarisation model was preferred. Because this method uses a factor (i.e. turbidity) that reflects juice's fouling potential, it allows the optimisation of processing parameters and the prediction of permeation flux range in real industrial conditions.     

The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration

Although macromolecular fouling of microfiltration membranes is one of the critical factors governing the performance of these filtration processes, there is still little fundamental understanding of the underlying phenomena that influence the initiation, rate, and extent of fouling. We have obtained experimental data for the flux decline during the stirred cell filtration of different commercial preparations of bovine serum albumin (BSA) through asymmetric polyethersulfone microfiltration membranes. The fouling characteristics of these commercial solutions varied substantially, with the flux decline directly related to the technique utilized to initially precipitate and prepare the BSA. Prefiltration of BSA solutions prior to microfiltration substantially reduced their fouling tendency, with the degree of improvement increasing as the prefiltration was performed through smaller molecular weight cut-off membranes. The protein solutions were also characterized using gel permeation chromatography (GPC), with the fouling tendency of the different BSA preparations highly correlated with the concentration of BSA dimers and other high molecular weight species present in these BSA solutions. These results suggest that BSA fouling of these microfiltration membranes is associated with the deposition of trace quantities of aggregated and/or denatured BSA, with these fouling species serving as initiation sites for the continued deposition of bulk protein.     

Characterization of floc size and structure under different monomer and polymer coagulants on microfiltration membrane fouling

The characteristic of aggregates pre-coagulated by inorganic monomer alum, polymer aluminium chlorohydrate(ACH) and polyaluminium chloride(PACl) coagulants impose major impact on the removal of humic acids (HAs) and the reduction of microfiltration (MF) membrane fouling. The fractal dimension of flocs formed by ACH and PACl is higher than that by monomer alum, indicating Keggin structure produced by polymer coagulants is much more compact compared with hexameric ring structure of alum hydrolysis species. Correspondingly, cake layer specific resistance is far higher and the MF membrane flux deteriorates much more severely when pre-coagulated by ACH and PACl than by alum. Moreover, the higher basicity contains in PACls, the cake layer fouling is more serious for producing more proportion of dense hydrolysis species Al₁₃. Thus, the polymer coagulant ACH and PACl seems not adapt to the pre-coagulation–MF process for cake layer resistance increase two to three times although they save 60–70% dose in comparison with alum for HAs removal. Additionally, for three Al-based coagulants under sweep coagulation condition, insufficient dose result in lower HAs removal and produce more small particles caused higher cake layer specific resistance according to Carman–Kozeny relationship. On the other hand, coagulant hydrolysis species as direct contaminant loading aggravated cake resistance on the MF membrane when overdosed. The optimum dose should keep the minimum to provide better HAs removal efficiency, and produce lower cake layer specific resistance and higher membrane filterability for pre-coagulation–MF hybrid process.     

A new method for the evaluation of the reversible and irreversible fouling propensity of MBR mixed liquor

In this paper, a new fouling measurement method is presented as a pragmatic approach to determine a mixed liquor's fouling propensity. The MBR-VFM (VITO Fouling Measurement) uses a specific measurement protocol consisting of alternating filtration and physical cleaning steps, which enables the calculation of both the reversible and the irreversible fouling resistances. The MBR-VFM principle, set-up and measurement protocol are described as well as the evaluation of the fouling measurement method. Finally, the MBR-VFM was validated by comparing the fouling propensity measured on-line by the MBR-VFM in a lab-scale MBR with the fouling of the MBR membranes themselves. Our experiments indicated that the MBR-VFM can accurately measure fouling and that it can even be detected earlier than can be seen from the on-line filtration data of the lab-scale system itself. Furthermore, the differences measured in reversible and irreversible fouling seemed to be related to the observed impact of physical and chemical cleaning respectively. Therefore, the application of the MBR-VFM as an on-line sensor in an advanced control system, enabling the deployment of the measured fouling data for the control of membrane cleaning, seems feasible and will be tested in the near future.     

A fluorescence method for quantitative measurements of specific protein at powder surfaces

A new method using fluorescence labelled proteins was developed to determine the quantitative amount of specific protein at the powder surface. The method is based on steady-state fluorescence measurements of pyrene labelled proteins with oxygen gas phase quenching at the powder surface. The surface load of protein was measured for spray-dried dextran powders containing bovine serum albumin (BSA). The results show a patchwise surface load of about 1.3 mg m⁻² at a concentration of 0.33% (dry weight) BSA in the powder. The patchwise surface load stays constant with increased BSA concentration in the powder.     

Separation of serum proteins with uncoupled microsphere particles in a stirred cell

Submicron microspheres were used directly without ligand coupling for the batch and continuous separations of proteins. In the batch experiments for separating BSA (bovine serum albumin) from BHb (bovine hemoglobin), introducing both hydrophobic effects for BSA and electrostatic repulsion for BHb (and vice versa) was required for high selectivity, and microspheres with low number density of surface groups were advantageous. For the continuous experiments, the utilization of a stirred cell was successful, where the microspheres were in the form of latex with good dispersion of particles. The flow rate without a pump was 0.5–1.3 ml min⁻¹, and the ratio of BSA and BHb was varied. In the experiments for eliminating BHb from BSA, elution curves of BHb corresponded to the single component breakthrough curves, while those for BSA did not. The latter is believed to be due to the interference by BHb in the adsorption of BSA.     

Applications of the MFI-UF to measure and predict particulate fouling in RO systems

The MFI-UF was developed to include smaller colloidal particles not measured in the existing Silt Density Index (SDI) and MFI_0.45 fouling indices. This research investigates the application of the MFI-UF to measure and predict the particulate fouling potential of reverse osmosis (RO) feedwater and also to assess pretreatment efficiency. MFI-UF measurements were carried out at the IJssel Lake and River Rhine RO pilot plants of the influent feedwater, RO concentrate and after pretreatment processes. Pretreatment efficiency was compared based on MFI-UF, MFI_0.45, and SDI measurements. The MFI-UF of the influent feedwater was approximately 700–2400 times higher than the corresponding MFI_0.45 and SDI, due to the retention of smaller particles. A pretreatment efficiency of ≥80%, was found by the MFI-UF at both pilot plants. For the larger particles the MFI_0.45 gave a 90–100% reduction. Minimum predicted run times for a 15% flux decline from MFI-UF measurements were shorter than that observed at the IJssel Lake pilot plant. This may be explained by problems with the ultrafiltration (UF) pretreatment at the time and/or almost negligible particle deposition in the RO pilot systems. Moreover, it was shown that cake resistance increased with ionic strength in MFI-UF tap water experiments and therefore, a correction of the MFI-UF index is required for salinity effects in RO concentrate.     

丝组二肽对牛血清白蛋白的切割作用研究

肽与蛋白质的切割作为生物化学中的热点之一 ,具有非常广泛的应用 .长期以来 ,天然存在的多种蛋白酶如胰蛋白酶 (Trypsin)、糜蛋白酶 (Chymotrypsin)由于其高活性及区域选择性 ,一直是蛋白切割的主要工具 .但是这些酶只能在某些特定的条件 (如 p H值和温度等 )下使用 ,而且由于蛋白酶对底物的特异性要求高 ,因而切割蛋白的断裂位点相对较少 ,有一定的局限性 .而现代生物化学及分子生物学的发展在蛋白切割方面提出了越来越多的要求 [1,2 ] ,如蛋白部分合成 ,大蛋白的序列测定 ,蛋白区域结构与功能的分析 ,蛋白与其它蛋白、核酸、膜形成复合…     

喹诺酮药物与血清蛋白相互作用的三维荧光光谱研究

应用三维荧光光谱和三维荧光偏振光谱研究了数种喹诺酮药物与牛血清白蛋白(BSA)分子间的相互作用。由三维荧光(偏振)光谱得到的指纹信息说明了喹诺酮药物与BSA结合反应对BSA分子构象的影响。通过研究喹诺酮药物发生相互作用前后BSA荧光偏振度及各向异性的变化,定量说明了喹诺酮药物-BSA所发生的结合反应。     

The preparation of bovine serum albumin surface-imprinted superparamagnetic polymer with the assistance of basic functional monomer and its application for protein separation

Currently, small proteins imprinting are more reported since large proteins molecular imprinting faces challenge due to their bulk size and complex structure. In this work, bovine serum albumin (BSA) surface-imprinted magnetic polymer was successfully synthesized based on atomic transfer radical polymerization (ATRP) method in the presence of common monomer (N-isopropylacrylamide) with the assistant of basic functional monomer (N-[3-(dimethylamino)propyl]-methacrylamide), which provides a achievable attempt for imprinting larger target proteins based on the ATPR with the mild reaction conditions. The BSA-imprinted polymer exhibited higher adsorption capacity and selectivity to BSA over the non-imprinted polymer. Competitive adsorption tests indicated the BSA-imprinted polymer had better selective adsorption and recognition properties to BSA in the mixture. The obtained BSA-imprinted polymer was applied to bovine serum, which also showed selectivity to BSA. In addition, a conventional aqueous two-phase solution of PEG/sulphate was used as elution for adsorbed BSA, which was compared with common NaCl elution.     

氯化血红素分子对牛血清白蛋白电子传输能级的调控

蛋白质分子的电子传输(ETp)性能,即导带(CB)和价带(VB)的能量差(带隙)是影响蛋白质电子器件性能的主要因素之一。因此,调控蛋白质ETp带隙是提高这些电子器件性能并扩展其应用领域的重要途径。本文报道一种通过外部分子结合调控蛋白质ETp带隙的方法。以氯化血红素(hemin)与牛血清白蛋白(BSA)结合为例,首先运用分子对接方法从理论上确定hemin分子能结合到BSA分子IIA域的疏水口袋中,位于Tpr213附近;然后实验(荧光光谱和吸收光谱)证实hemin与BSA结合后,能形成hemin-BSA复合物,并且没有改变BSA的原有结构;最后将hemin-BSA通过BSA分子表面Cys34的―SH固定在金电极表面,形成有序的分子层,研究其ETp性能;I–V结果表明,BSA表现出半导体的ETp特征,并且hemin的结合能使BSA的带隙由原来的~1.50±0.05e V降低到~0.93±0.05e V。本文的结果为调控蛋白质分子的ETp带隙提供了一种简单有效的方法,通过选择不同的结合分子能使蛋白质分子的带隙调控至所需要的范围,并且形成的蛋白质复合物还能用于各种电子器件的制作。     

D—,L—色氨酸与牛血清白蛋白立体专一性结合研究

对血清白蛋白(SA)的结合内源性化合物及许多药物的性质已进行了广泛研究,有关氨基酸与SA的结合还很少报道。氨基酸吸收路径几乎完全是血液,它们与SA的结合性质,无疑与其在体内的运输和代谢密切相关。色氨酸含有亲水性的NH_3~+及COO~-和疏水性的吲哚环,研究它与SA的结合性质对于全面阐明SA结合各种配体的规律和机理都有重要意义。本文采用离心超过滤法测定了25℃、pH 7.40时D-、L-、DL-色氨酸(Trp)与牛血清白蛋