Dataset and detailed methodology for structure and performance characterization of modified polymeric membranes

The data contained in this publication refers to protocols adopted characterization of clay nanoparticles (CN) membranes with and without the use of polyethylene oxide (PEO) as pore former. The membrane casting solutions were produced by dissolving PS (18% w/w) in NMP with addition of CN (1–5% w/w CN/PS) and/or PEO (1–5% w/w PEO/PS) when applicable. Membranes with no CN or PEO were used as a control. Pure water permeability of cast membranes was determined using the cross-flow cell unit. Viscosity was measured for most casting solution compositions and contact angle was measured for all membranes. The control membrane was further compared in detail to the highest permeability membranes with only CN (1.5%), only PEO (5%), 1.5% CN and 5% PEO (combination of optimal individual permeabilities), and 4.5% CN and 5% PEO (optimal combined permeability) regarding thickness, porosity, rejection, fouling resistance, surface charge, and thermal/mechanical properties. The relevance of the data presented here is to show details about methods for characterizing membranes for future comparison of performance and eventual improvement of characterization methods.


Data
The data described include the evaluation of membrane permeability versus additive (nanoclay) content ( Fig. 1, Table 1), the changes in permeability by adding a second additive (PEO) for different concentrations of first additive (Fig. 2, Table 2), membrane porosity (Fig. 3, Table 3) and thickness (Fig. 4, Table 3) for different additive contents, membrane surface images (Fig. 5) and surface roughness (Fig. 6), the results of effluent quality in terms of NPOC, Turbidity, and UV (Fig. 7, Table 4), a correlation between surface roughness (Fig. 8, Table 5) and surface charges (Fig. 9, Table 5) versus organic matter desorption, and images of E. coli in the final effluent (Fig. 10) and efficiency in E. coli removal (Table 6). Data is complemented with tables containing results of Analysis of variance for permeability (Table 7), thickness and porosity (

Value of the Data
Why are these data useful? Because they can be used for establishing a standard data set for membranes' characterization and performance comparison. Who can benefit from these data? All the researchers who are involved on the field of polymeric membrane synthesis and characterization. How can these data be used for further insights and development of experiments? Researchers can use the data for understanding the process associated with membrane production and the effect of different additives on the produced membranes.

Solution viscosity
The casting solution viscosity was evaluated to assess the potential influence of additives on the phase inversion process. Viscosity is a qualitative measure of phase inversion kinetics as it is related to Fig. 1. DI permeability change with increasing clay content. Permeability reaches a best performance and after that, decreases. solvent-nonsolvent exchange rates [38,39]. Solution viscosity was measured using a medium range rotational viscometer (Brookfield RVDV-E). The casting solution was placed in a beaker and then slowly the rotational spindle is immersed to avoid bubble formation. The shear force is determined from the spindle geometry and the applied rotation rate according to equation (1): where h is the viscosity in poise, t is the shear stress in dynes cm À2 , and _ g is the rate of shear (sec À1 ).
The viscosity was measured at a constant shear rate of 30 RPM and temperature of 25 C (same temperature as membrane casting). At least two different casting solution samples of each composition were evaluated.

Estimation of the membrane surface pore size
The identification of membrane's molecular weight cutoff (MWCO) (Mn, in Dalton) can help to understand the real pore size. Assuming constant density of the contaminant molecules, the volume of a molecule will vary linearly with the molecular weight. Depending on the shape of the molecule, the diameter can be expressed by the equation below [3,4].
where d (nm) is the hydrodynamic diameter of the molecule and b (nm) is the proportionality constant.
n is a function of the molecular shape; and it varies from 0.33 for spheres to nearly 1.0 for linear longchains. The specific equation for PEG is: According to the results obtained ( Fig. 7), based on 90% efficiency of removal, membranes PS0 had a MWCO between 110-120 kg mol-1, while C1.5 and C1$5P5 had MWCO of 120e130 and 190e200 kg mol-1. Therefore, PS0 has a hydrodynamic diameter between 14.9 and 15.4 nm, C1.5 between 15.4 and 16nm and C1$5P5 between 18.9 and 19.3 nm.

Pure water permeability e cross-flow filtration
Ultrapure water permeability was determined using the cross-flow configuration. The experiments were conducted with a bench scale system using a commercial flat-sheet cross-flow filtration cell (Sterlitech CF042) with a 34 cm 2 filtration area. The feed solution was pumped (Micropump DJ604A) into the cross-flow filtration cell and the concentrate was recycled back to the feed reservoir. The feed solution temperature was in the range of 22e24 C. The feed flow rate and pressure were adjusted by a valve along the concentrate pipeline and recorded by a digital flow meter (Micro-Flow FTB321D) and pressure sensor (Omega PX482A-200GI). The permeate was collected for 1 h in a graduated cylinder to determine the permeate flow rate. The remaining permeate was recycled to the feed container to maintain a constant feed solution composition. The permeate tubing was open to the atmosphere and the permeate pressure was considered to be constant at 1 bar. Permeability was calculated using the following equation: where P is the permeability (L m À2 h À1 bar À1 ), V is the volume of permeate collected (L), A is the effective membrane area (m 2 ), Dt is the sampling time (h), and DP is the transmembrane pressure (bar).

Contact angle measurements
Contact angle measurements were determined with a goniometer (Ram e-Hart Instrument Co.; Model 190 CA) using the sessile drop technique. The membrane preparation and contact angle

Surface roughness
Surface roughness measurements were conducted using a Cypher Atomic Force Microscope (Asylum Research) operating in amplitude modulation with a silicon tip (Bruker OTESPA) at a resonance frequency of 340 kHz and nominal tip radius of 7 nm. Images of 10 Â 10 mm were acquired using 1 V amplitude and a 0.6 V set point amplitude at a scan rate of 1 Hz. The image refinement to create 3D surfaces was made using the software Argyle Light and the data extracted was analyzed with the R13.17.101 extension for IgorPro®. The images were flattened with order 1 (to minimize errors by curvature and slope) and the root mean squared (RMS) roughness was calculated using the deviation of data.

Morphology analysis
Membrane cross-sections images were obtained using a Quanta 600FEG Environmental Scanning Electron Microscope (ESEM) operated in secondary electron detection mode with a 10 kV accelerating voltage. All membrane samples were coated with a modular high vacuum coating system (BAL-TEC MED 020) resulting in an~10 nm platinum layer after 120e160 s of deposition at 43 mA current. To image cross-sections, the membrane samples were first immersed in liquid nitrogen for 30 s and then cleanly snapped.
Membrane porosity was determined following previously reported wet/dry weight methods. The wet weight was measured after removing the superficial water with two polyester/cellulose wipers (VWR International) and the dry weight was measured after drying the samples. The porosity was calculated using the following equation: where m 1 and m 2 (g) are the wet and dry weights, r w (g cm À3 ) is the density of water, V m (cm 3 ) is the membrane volume, and ε (%) is the bulk porosity. The volume, V m , was calculated by multiplying the sample area by its thickness, which was measured by a digital micrometer (Fowler Tools and Instruments; 1.27 to 25,400 mm).

Membrane molecular weight cut-off
To determine the molecular weight cut-off (MWCO), membranes were challenged in the same cross-flow cell used for ultrapure water permeability using PEG feed solutions of molecular weight 10, 50, 90, and 203 kg mol À1 . The feed and permeate PEG concentrations were determined using the nonpurgeable organic carbon (NPOC) method on a TOC analyzer (Shimadzu; TOC-VWS). The feed contained approximately 20 mgC L À1 of the chosen PEG (NPOC equivalent) in ultrapure water. Samples were collected after 1 h of operation for at least three membrane samples. Rejection (R) was defined by the following equation: where C p and C f are the NPOC concentrations in the permeate and feed, respectively.

Cross flow rejection
A similar procedure for MWCO evaluation was used to determine sodium alginate (10 mgC L À1 ) rejection with effluent measurements made after 2 hours of continuous operation.
To evaluate natural surface water treatment efficacy, the influent was used from Fresh Pond, a local drinking water reservoir, and permeate measurements were made after 2 hours of operation. NPOC, turbidity, and UV 254 of the feed and permeate were measured to evaluate the treatment efficiency. The turbidity was measured using a portable turbidimeter (Hach; Model 2100Q). The UV 254 absorption was measured using a UVeVisible spectrophotometer (Agilent; Model 8453).

Fouling evaluation
For evaluation of natural surface water fouling potential, the membranes were challenged for 8 hours at 1 bar transmembrane pressure and the permeability decrease was compared to initial    ultrapure water experiments. After fouling, the membranes were chemically cleaned using an alkaline (pH 11) feed solution for 30 minutes and subsequently rinsed with copious amounts of ultrapure water. After cleaning, the membranes were challenged again with ultrapure water to evaluate the flux recovery (as an estimative of irreversible fouling). At least three membrane samples were evaluated.
In order to evaluate the amount of organic matter adsorbed by the membrane surface, membrane samples were challenged with natural surface waters using the same conditions as the fouling potential measurement. After running for 8 hours, 9 cm 2 squares were cut, immersed in solutions at pH 2, 7, and 11 and kept in a rotating shaker (New Brunswick Scientific, E24) at 60 rpm for 6 hours. The NPOC of the solution containing these squares was measured to quantify the desorbed organic matter.

Negative surface charge evaluation
The membrane negative surface charge was quantified following a previously reported procedure [2]. Briefly, 4 Â 4 cm membrane samples were taped to a glass slide such that 9 cm 2 of the top surface of the membrane was exposed. The membrane surface was then coated with a thin film of 0.5 mM toluidine blue and 15 mM NaCl solution at pH 6e7 for~60 s. The dye solution was washed off with copious amounts of 15 mM NaCl and then the membrane samples were placed in a 15 mM NaCl water bath for 4 h to remove any weakly attached dye. The 9 cm 2 samples were then cut from the larger coupons and placed in a 20-mL glass vial containing 10 mL of 0.2 M NaCl at pH 2 (HCl). The vial was stirred in a rotating shaker at 50 rpm for 30 min at 35 C. The low pH protonates any negativelycharged surface groups and releases the electrostatically bound positively-charged dye, which was then quantified by UVevis spectroscopy (l max ¼ 634 nm; ε ¼ 45,200 cm À1 M À1 ). The negative surface charge density (n À ) in number per nm 2 was quantified using equation (5): where A is the absorption at 634 nm, V ¼ 0.01 L is the volume of extraction solution, N ¼ 6.022 Â 10 23 is Avogadro's number, and SA ¼ 9 Â 10 14 nm 2 is the surface area of the measured membrane sample. At least four samples of a selected membrane were evaluated for negative surface charge.

Thermal and mechanical properties
Thermogravimetric analysis (TGA) was completed using a TA Instruments Model TA 2950 TGA under nitrogen and oxygen flows at 10 mL min À1 each. Samples were placed on a platinum pan and the temperature was increased at 10 C min À1 from 25 to 750 C.
Strain/stress analysis was completed on a Shimadzu Compact Table-top Universal Testing Machine EZTest EZ-LX at 25 C with a crosshead speed of 5 mm min À1 . Samples had a cross section of thickness x 5 Â 15 mm. At least 3 samples were analyzed for average tensile strength, Young's modulus, and elongation at break. E. Coli (w3110) was used to measure bacterial removal efficiency. Bacteria were harvested at midexponential phase and then centrifuged and resuspended in 0.9% NaCl saline solution twice prior to addition to the feed solution. The bacteria concentration in the feed and permeate was determined using fluorescence microscopy. Briefly, the solution (1 mL for the feed and 20 mL for the permeate) was vacuum filtered onto a polycarbonate membrane (Sterlitech PCTE), and the bacteria were stained with 4',6-diamidino-2-phenylindole (DAPI) for 5 minutes and then analyzed (excitation/emission of 358/ 461 nm). The prepared filter was transferred to the fluorescence microscope and imaged at 40Â magnification. At least five random points on the filter were imaged (278 Â 200 mm) and analyzed for cell enumeration.