MATERIALS AND METHODS
4.4 Analytical Methods
4.4.1 Atomic Absorption Spectrophotometry (AAS)
Philips PU9200X atomic absorption spectrophotometry(AAS) was used in order to determine concentrations of cadmium, nickel and zinc in permeate and feed solutions.
For the analyses, acetylene-air flame AAS (FAAS) was used: Metal ions in the sample were atomized by nebulizer and air-acetylene flame. Background correction was achieved by means of deuterium lamp. At the beginning of each analysis, calibration of the instrument was performed by blank and standard solutions. Details of analysis procedure is given in Appendix A. Typical calibration curve for cadmium, zinc and nickel obtained during the analysis are depicted in Figure A.1, Figure A.2 and Figure A.3 in Appendix A.
In each measurement, samples were sucked three times and for each suction
absorbance signal and corresponding concentration value in calibration curve were obtained. Average of the three signals and corresponding values of concentration was recorded to be the measured concentration of the sample.
4.4.1.1 Analysis of Polymer Containing Solutions
In the previous studies, it was found that, presence of PEI results in decrease in atomic absorption signals and as a consequence increase in PEI concentration in samples causes a decrease in measured metal concentrations [276]. This adverse effect of PEI on absorption signals should be taken into account while analyzing the samples taken from the feed stream, since these samples contain free PEI and/or PEI-metal complexes. For the samples taken from the permeate stream no extra treatment was needed to get accurate signals, since TOC analysis showed that PEI can be completely retained by the membranes selected for this study and permeate streams were free from PEI macromolecules.
In order to get rid of the adverse effect of PEI present in the feed samples, following approach was recommended for the correction of analysis results and to get accurate concentration values [98, 276]. According to the proposed approach, in order to reduce the effect of PEI, its concentration in the samples should be reduced by dilution to acceptable limits for detection of metals by AAS. Then, standard solutions containing same amount of PEI as in the diluted samples should be prepared in addition to the preparation of standard solutions without PEI. Then, the standard solutions of metals containing PEI were calibrated and absorbance of samples containing PEI with unknown concentrations of metals were measured and the concentrations were determined using these calibration curves.
Another important point that should be considered for the analysis of samples containing high amounts of salt is, during the suction of samples through the flame by the nebulizer, excess salt may cause clogging in nebulizer unit and because of the clogging effect flame may extinguished. In order to minimize the negative effects of salt concentrations, samples were diluted to acceptable limits for detection of metals by AAS.
4.4.2 Conductivity Measurements
Conductivities of the samples were measured by Jenway 4020 Conductivity meter.
Prior to conductivity measurements, samples were nested in water bath which was adjusted to 25◦C. After temperature adjustment, conductivities of the samples taken from permeate and retentate streams were measured.
4.4.3 TOC Analysis
Total organic carbon (TOC) content of samples taken from feed and permeate streams were detected by Shimadzu 5000A TOC Analyzer in order to test the permeability of polymer through the membranes. In TOC measurements, high temperature burning method was applied to estimate total inorganic and organic carbon content of the samples.
4.4.4 Dynamic and Static Light Scattering Analysis
Stock solutions containing predetermined amounts of polymer and salt were prepared with Milli-Q water with R≈ % 18 MΩ·cm. The pH of all solutions was adjusted by adding a corresponding amount of a 0.1M HNO3solution or a 0.2M NaOH solution.
Prior to analysis, depending on the desired concentration for DLS/SLS experiments, samples were diluted and then filtered using a 0.4-mm filter, in sample cells.
Static and dynamic light scattering measurements were performed by ALV/CGS-3 Compact Goniometer System simultaneously. An argon ion laser operating at a 633-nm wavelength and 35-mW output power was used as a light source. The measurements were done at 10 different angles in the range of 60◦≤θ≤150◦and at 25◦C. For each angle three different measurements were performed and subsequently averaged. The data were normalized to absolute scale by using toluene as a reference material. For polyethleneimine (PEI) solutions the refractive index increment of the solution is taken as (dn/dc= 0.2998cm3/g) which was determined in previous studies [253].
Two different data file systems were used in ALV5000 and 60X0 software. Data
obtained during the DLS and SLS experiments were stored in two different files.
The correlation function and count rate trace data together with all relevant sample parameters can be stored as ASCII values, and the angular concentration dependent normalized mean values of the scattering intensity as well as diffusion coefficients derived by Cumulant analysis of the correlation functions are stored in DILS file format. An analysis of DLS/SLS data stored in DILS file format can easily be performed using the ALV/Static and Dynamic Fit and Plot program by creating Zimm, Guinier and Berry plots, plots of diffusion coefficients versus q2and form factor plots.
The average molecular weight, radius of gyration, hydrodynamic radius and second virial coefficient of the samples are calculated via Zimm plots where (q2+ Kc) is plotted versus Kc/R according to Equation 2.4.
Each fit at constant angle data is extrapolated to zero concentration, and each fit at constant concentration is extrapolated to zero angle, obtaining two lines. The average molecular weight (Mw) is calculated from the intercept of each extrapolated line with the vertical-axis. Radius of gyration (Rg) is calculated from the slope of the extrapolated line at zero concentration while the second virial coefficient (A2) is calculated using the slope of the extrapolated line at zero angle.