Investigations on the conditional kinetic and thermodynamic stability of aquatic humic substance-metal complexes by means of EDTA exchange, ultrafiltration and atomic spectrometry

The conditional metal availability and the kinetic stability of humic substance-metal species in humic-rich waters (e.g. bog water) was characterized by means of EDTA exchange. For this purpose a combined procedure consisting of time-controlled ligand exchange by EDTA, species differentiation by a fast single-stage tangential-flow ultrafiltration (TF-UF) technique (cut-off 1 kDa) and sensitive atomic spectrometry methods (e.g. AAS, ICP-OES, TXRF) was developed. The kinetics and the yield of the EDTA exchange served as operational parameters for assessing the kinetic stability and EDTA availability of HS-metal species, respectively. Considerable fractions of natural HS-metal species studied were shown to be EDTA-inert (e.g. 31% of the total Fe, 44% of the total Al) even after long reaction times (48 h), in contrast to artificial ones formed in solutions of isolated HS. Moreover, the conditional thermodynamic stability of HS-metal complexes formed by successive loading of an aquatic reference HS (HO14) with a number of heavy metal ions (e.g. Cr(III), Cu(II), Fe(III), Mn(II), Zn(II)) was also evaluated discriminating the free metal concentrations by means of TF-UF. In addition, from the loading isotherms obtained conditional complexation capacities could be derived for the studied HS exhibiting the order Fe(III)>Cu(II)>Cr(III)>Co(II)>Mn(II).     

Tangential-flow ultrafiltration: a versatile methodology for determination of complexation parameters in refractory organic matter from Brazilian water and soil samples

In this work the copper(II) complexation parameters of aquatic organic matter, aquatic and soil humic substances from Brazilian were determined using a new versatile approach based on a single-stage tangential-flow ultrafiltration (TF-UF) technique (cut-off 1 kDa) and sensitive atomic spectrometry methods. The results regarding the copper(II) complexation capacity and conditional stability constants obtained for humic materials were compared with those obtained using direct potentiometry with a copper-ion-selective electrode. The analytical procedure based on ultrafiltration is a good alternative to determine the complexation parameters in natural organic material from aquatic and soil systems. This approach presents additional advantages such as better sensibility, applicability for multi-element capability, and its possible to be used under natural conditions when compared with the traditional ion-selective electrode.     

Ultrafiltration polyanionic poly (3-sulfopropyl methacrylate) membranes with enhanced antifouling and water flux

Polyethersulfone (PES) membranes are prevalent in the field of water treatment owing to their exceptional separation efficiency, robust mechanical properties, and resistance to chemical degradation. Nevertheless, these membranes are prone to fouling, resulting in a decrease in both flux and ultrafiltration efficiency. In the present study, PES membranes are blended with poly (3-Sulfopropyl Methacrylate) (PSPMA) in various weight percentages (0%–3%) to improve their antifouling and ultrafiltration properties. The physicochemical properties of the blended membranes, including surface morphology, contact angle, hydrophilicity and surface energy are evaluated. The findings indicate that incorporation PSPMA results in an enhancement of the hydrophilic properties and surface charge of the PES membranes, assessed by employing Bovine Serum Albumin (BSA) as a representative protein. Modified blended membranes display greater Flux Recovery Ratio (FRR%) and exhibit superior fouling resistance. Under the same experimental conditions (0.2 MPa applied pressure), a pure water flux of 154.18 L·m⁻²·h⁻¹ for PES/PSPMA membrane found substantially greater than pure PES membrane (103.52 L·m⁻²·h⁻¹) along with Total Fouling Ratio (TFR) of 36% and 64.9% respectively. Exceptional antimicrobial efficacy for modified membranes is revealed against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) using disc diffusion technique rendering them well-suited for water treatment applications.     

Effect of physicochemical conditions on the ultrafiltration of β-lactoglobulin: Fluorescence probing of induced structural changes

This work aims for determining the impact of different environmental conditions, such as pH, ionic strength (salt concentration) and the chemistry of the membrane surface (hydrophilic/hydrophobic character) on the structure of permeating proteins after ultrafiltration. In the permeation experiments reported in this paper, different solutions of a model protein – β-lactoglobulin – at pH 3, 5 and 8 and salt concentrations of 1, 10 and 100 mM were processed with membranes of different molecular weight cut-off (10 and 30 kDa) and materials (regenerated cellulose—RC, and polyethersulfone—PES).     

On-line fractionation and characterization of aquatic humic substances by means of sequential-stage ultrafiltration

A five-stage tangential-flow ultrafiltration (UF) device equipped with advanced membrane filters (molecular weight cut-off: 1, 5, 10, 50 and 100 kDalton) of the polyethersulfone type is described and applied for the analytical on-line fractionation of a series of aquatic humic substances (HS) originating from surface or groundwaters. Fractionation patterns of HS (6 fractions each) evaluated by this UF device exhibit their particular dependence on the HS concentration, the pH-value and the salt content of the sample (10 ml) to be analyzed. Fundamental parameters (e.g., washing volume) governing the molecular-size fractionation of HS by means of multistage UF are discussed, too. The fractionation of an aquatic reference HS (BOC 3/9.5) by means of the above UF procedure reveals considerable differences preferably characterized by the UV-VIS absorption ratio E₃₅₀/E₄₅₀ and metal complexing capacity (Cu(II)) of the produced fractions. Moreover, molecular spectroscopy investigations (FTIR, ¹H-NMR) of the fraction series of this HS indicate that carbohydrate substructures (preferably found in fractions >50 k Dalton) and aromatic ones (preferably in fractions <5 k Dalton) are unevenly distributed.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment. Received: 7 December 1998 / Revised: 25 March 1999 / Accepted: 27 March 1999     

Modification of polyethersulfone ultrafiltration membranes with phosphorylcholine copolymer can remarkably improve the antifouling and permeation properties

The synthesized phosphorylcholine copolymer composed of 2-methacryloyloxyethylphosphorylcholine (MPC) and n-butyl methacrylate (BMA), blended with polyethersulfone (PES), was used to fabricate antifouling ultrafiltration membranes. Water contact angle measurements confirmed that the hydrophilicity of the MPC-modified PES membranes was enhanced to certain extent. X-ray photoelectron spectroscopy (XPS) analysis verified the substantial enrichment of MPC at the surface of the MPC-modified PES membranes. The adsorption experiments indicated that the adsorption amounts of bovine serum albumin (BSA) on the MPC-modified PES membranes were dramatically decreased in comparison with the control PES membrane. Ultrafiltration experiments were carried out to investigate the effect of MPC modification on the antifouling and permeation properties of the PES membranes, it was found that the rejection ratio of BSA was decreased, the flux recovery ratio was remarkably increased, and the degree of irreversible fouling decreased from 0.46 to 0.09. In addition, the MPC-modified PES membranes could run several cycles without substantial flux loss.     

Preparation and performance evaluation of carboxylic acid containing polyamide incorporated microporous ultrafiltration PES membranes

Improved ultrafiltration membranes were prepared by the phase inversion technique via immersion precipitation of synthesized carboxylic acid containing polyamide (CPA) and polyethersulfone (PES) in dimethylacetamide. The CPA was synthesized and characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry analyses. Next, the influence of CPA adding and its different concentrations on the performances and membrane structure were investigated. The obtained membranes were characterized by means of FTIR in the attenuated total reflection mode, scanning electron microscopy, and contact angle. The membrane performance studies revealed that the presence of CPA in the membrane structure increased water permeability while reducing protein fouling. It turned out that the PES/CPA membranes had better porosity, more hydrophilic surface, and more vertically finger‐like pores in comparison with the bare PES membrane. When the CPA concentration in the blending solution reached 1 wt%, the water permeability increased from 7.3 to 153.6 L/m² h¹. The attenuated total reflection‐FTIR analysis confirmed that CPA was captured in the membrane matrix.     

Inclusion complexation of diclofenac with natural and modified cyclodextrins explored through phase solubility, 1H-NMR and molecular modeling studies

Guest–host interactions were examined for neutral diclofenac (Diclo) and Diclofenac sodium (Diclo sodium) with each of the cyclodextrin (CD) derivatives: α-CD, β-CD, γ-CD and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), all in 0.05 M aqueous phosphate buffer solution adjusted to 0.2 M ionic strength with NaCl at 20 °C, and with β-CD at different pHs and temperatures. The pH solubility profiles were measured to obtain the acid–base ionization constants (pK _as) for Diclo in the presence and absence of β-CD. Phase solubility diagrams (PSDs) were also measured and analyzed through rigorous procedures to obtain estimates of the complex formation constants for Diclo/CD and Diclo sodium/CD complexation in aqueous solutions. The results indicate that both Diclo and Diclo sodium form soluble 1:1 complexes with α-, β-, and HP-β-CD. In contrast, Diclo forms soluble 1:1 Diclo/γ-CD complexes, while Diclo sodium forms 1:1 and 2:1 Diclo/γ-CD, but the 1:1 complex saturates at 5.8 mM γ-CD with a solubility product constant (pK _sp = 5.5). Therefore, though overall complex stabilities were found to follow the decreasing order: γ-CD > HP-β-CD > β-CD > α-CD, some complex precipitation problems may be faced with aqueous formulations of Diclo sodium with γ-CD, where the overall concentration of the latter exceeds 5.8 mM γ-CD. Both ¹H-NMR spectroscopic and molecular mechanical modeling (MM⁺) studies of Diclo/β-CD indicate the possible formation of soluble isomeric 1:1 complexes in water.     

Tailoring polyethersulfone/quaternary ammonium polysulfone ultrafiltration membrane with positive charge for dye and salt selective separation

Polyethersulfone (PES)/quaternary ammonium polysulfone (QAPSf) blend ultrafiltration (UF) membranes with positive charge were fabricated by nonsolvent induced phase separation (NIPS) for use in dye and salt selective separation. QAPSf was synthesized by nucleophilic substitution with chloromethylated polysulfone (CMPSf). The effect of the PES/QAPSf mass ratio on the morphology and performance of blend UF membranes were studied. The membranes' zeta potentials gradually changed from negative to positive with decreases in the PES/QAPSf mass ratio. At PES/QAPSf mass ratios of 30:70 and 10:90, the zeta potentials of the membranes reached +1.8 mV and + 5.9 mV, respectively. Additionally, the contact angles of the membranes decreased from 74° to 52° as the QAPSf content increased from 0 wt% to 90 wt%. Furthermore, the membrane with a PES/QAPSf mass ratio of 30:70 showed a high water permeance (181.4 LMH bar⁻¹) and excellent dye and salt selective separation performance. The rejection ratios reached 99.1%, 87.8%, 99.6%, and 92.4% for dyes Congo red, methyl blue, Alixin blue 8GX, and basic blue 24, respectively, while those for salts Na₂SO₄, MgSO₄, MgCl₂, and NaCl were ≤ 10%. In addition, the PES/QAPSf membranes showed excellent antifouling performance and good operating stability with dye-salt mixtures of various pHs and salt concentrations.     

Preparation and characterization of polyethersulfone/silver nanocomposite ultrafiltration membrane for antibacterial applications

Antimicrobial ultrafiltration membranes were prepared by coating silver nanoparticles on the surface of polyethersulfone (PES) membranes which were fabricated via phase inversion induced by the immersion precipitation technique, and their morphology and performance were compared with the antimicrobial PES membranes synthesized by adding the silver nanoparticles into the casting solution during the phase inversion process. For this purpose, stable and uniform colloidal solutions of the silver nanoparticles were prepared by chemical reduction of silver salt using fructose and dimethylformamide as a reducing agent. The silver nanoparticles were characterized by ultraviolet–visible spectroscopy, X‐ray powder diffraction and dynamic light scattering analysis. The morphology and surface properties of the prepared membranes were examined by field emission scanning electron microscopy and atomic force microscopy analysis. Moreover, the separation properties, antimicrobial efficiency and amount of silver release from the PES nanocomposite membranes during the cross flow ultrafiltration were determined. The results indicated that the silver content of the coated PES membranes was greater than the membranes fabricated by the solution blending method. Also, the permeation flux of the silver‐coated membranes was similar to the neat PES membranes, while the membranes prepared by the second approach had less flux. The membranes synthesized by both coating and blending methods showed high antimicrobial and bactericidal activity against gram‐negative bacteria such as Escherichia coli and gram‐positive bacteria such as Staphylococcus aureus. Finally, the prepared antimicrobial membranes were successfully used for the ultrafiltration of raw milk to reduce the microbial load during the concentration process. Copyright © 2014 John Wiley & Sons, Ltd.     

Characterization of CO2 plasma treated polymeric membranes and quantification of flux enhancement

Low-temperature CO₂ plasma is used for the treatment of poly-ethersulfone (PES), polyamide (PA) and poly-phenylene ethersulfone (PPE) ultrafiltration membranes. This has led to significant enhancement of the wetting characteristics of the membrane surface as is shown by contact angle measurements and Fourier transform infrared (FTIR) spectrum analysis of the treated membranes. Changes in the physical characteristics of the surface, such as tensile property, surface roughness, etc. are quantified by tensile strength measurement and atomic force microscopy (AFM), respectively. An increase in the measured values of the di-electric constants further highlights the hydrophilic modification of the surface. A series of ultrafiltration experiments using a BSA solution of known concentration under different operating conditions is performed and the deposition thicknesses over the membrane surface during ultrafiltration are measured directly using image analyzing microscopy. The results clearly demonstrate that a plasma treated PES membrane is more hydrophilic with smoother surface and resists fouling leading to significant enhancement of permeate flux.     

Enhancing the antifouling property of polyethersulfone ultrafiltration membranes through surface adsorption-crosslinking of poly(vinyl alcohol)

An adsorption-crosslinking process of poly(vinyl alcohol) (PVA) was introduced to modify the surface of polyethersulfone (PES) ultrafiltration membranes for enhancement of their antifouling property. XPS and water contact angle measurement confirmed the obvious enhancement of surface hydrophilicity. Ultrafiltration results showed that the spreading of PVA chains over the hydrophobic membrane surface caused substantial but acceptable decrease on membrane flux. The fouling type analysis indicated that PVA adsorption effectively improved the antifouling property of PES membranes. With a PVA concentration of 0.5 wt% and three cycles of alternative adsorption-crosslinking, the total and irreversible fouling ratio of modified membranes were 0.38 and 0.22, respectively, much lower than those of control PES membrane (0.61 and 0.47), and the flux recovery ratio was increased accordingly. The long-term ultrafiltration experiment demonstrated the improvement of recycling property and the reliability of adsorption-crosslinking process.     

Validation of cross-flow ultrafiltration for sampling of colloidal particles from aquatic systems

The use of cross-flow ultrafiltration (CFF) for the isolation of colloids from the bulk aquatic matrices has been increasing over the last decade; however, a thorough validation of each CFF system is needed before its application to natural samples. Laboratory experiments have been conducted to validate a 1-kDa Millipore Pellicon 2 cartridge type CFF system, using a range of molecular probes spiked in natural waters. Using the separation data of these probes, key validation parameters have been calculated, including retention coefficient (RC), concentration factor (cf), colloid concentration, recovery, and mass balance. The permeation behaviour of these molecules has also been studied using a simple permeation model. Results show that good retention (>80%) of high molecular weight (HMW, >1 kDa) molecules and low retention (<20%) of low molecular weight (LMW, <1 kDa) molecules can be achieved at high cf values or over long timescales. In addition, permeation modelling and the calculation of colloid concentration demonstrate the same findings, again recommending high cf values or long timescales for ultrafiltration to minimise the retention of LMW molecules. The retention characteristics of 3-kDa dextran change slightly with salinity. Mass balance calculations show good recoveries for all of the molecules tested. The apparent membrane molecular cut-off of the CFF system has been estimated to be between 2.1 and 2.5 kDa, which is slightly greater than the manufacturer's nominal MW cut-off of 1 kDa.     

Multielement characterization of metal-humic substances complexation by size exclusion chromatography, asymmetrical flow field-flow fractionation, ultrafiltration and inductively coupled plasma-mass spectrometry detection: a comparative approach

The use of three different separation techniques, ultrafiltration (UF), high performance size exclusion chromatography (HPSEC) and asymmetrical flow field-flow fractionation (AsFlFFF), for the characterization of a compost leachate is described. The possible interaction of about 30 elements with different size fractions of humic substances (HS) has been investigated coupling these separation techniques with UV-vis absorption spectrophotometry and inductively coupled plasma-mass spectrometry (ICP-MS) as detection techniques. The organic matter is constituted by a polydisperse mixture of humic substances ranging from low molecular weights (around 1kDa) to significantly larger entities. Elements can be classified into three main groups with regard to their interaction with HS. The first group is constituted by primarily the monovalent alkaline metal ions and anionic species like B, W, Mo, As existing as oxyanions all being not significantly associated to HS. The second group consists of elements that are at least partly associated to a smaller HS size fraction (such as Ni, Cu, Cr and Co). A third group of mainly tri- and tetravalent metal ions like Al, Fe, the lanthanides, Sn and Th are rather associated to larger-sized HS fractions. The three separation techniques provide a fairly consistent size classification for most of the metal ions, even though slight disagreements were observed. The number-average molecular weight (Mn), the weight-average molecular weight (Mw) and the polydispersity (rho) parameters have been calculated both from AsFlFFF and HPSEC experiments and compared for HS and some metal-HS species. Differences in values can be partly explained by an overloading effect observed in the AsFlFFF experiments induced by electrostatic repulsion effects in the low ionic strength, high pH carrier solution. Size information obtained from ultrafiltration is not as resolved as for the other methods. Molecular weight cut-offs (MWCO) of the individual filter membranes refer to globular proteins and molecular weight information may therefore, deviate from that given by the other methods after calibration with polystyrene sulfonate (PSS) standards.     

Hydrophobic organic micropollutants in samples of coastal waters: efficiencies of solid-phase extraction in the presence of humic substances

Solid-phase extraction (SPE) has been used to enrich organic micropollutants (hydrophobic chlorinated and polycyclic aromatic hydrocarbons, CHC and PAH) from coastal water samples and to systematically study the influence of humic substances (HS) on SPE. A reversed phase (RP) system with high flow rates (rapid chromatography, RC) was used to show the basic adsorption principles and interaction processes which influence the enrichment of organic compounds. A model humic substance was found to hinder the enrichment of individual hydrophobic micropollutants (MP), depending on their octanol-water distribution coefficient P _OW. This effect was found to be lower with natural humic substances. For longer contact time between water sample and adsorption material, the pollutant/humic substance bonding proved to be reversible.     

Preparation and structure of poly(4,4′-oxydiphenylene)pyromellitimide asymmetric ultrafiltration membranes

A process was developed for preparing asymmetric ultrafiltration poly(4,4′-oxydiphenylene)pyromel litimide membranes, involving wet forming of the membranes from the prepolymer, followed by catalytic solidphase thermal transformation of the prepolymer into the polyimide. The influence exerted on the structure and characteristics of the membranes by the compositions of the forming solution and precipitation bath, and also by thermal imidization conditions was examined. The optimal conditions were determined for preparing asymmetric ultrafiltration poly(4,4′-oxydiphenylene)pyromellitimide membranes exhibiting the water permeability coefficient of (10−500) × 10⁻⁶ m³ m⁻² s⁻¹ atm⁻¹ and molecular weight cut-off in the range (5−100) × 10³ g mol⁻¹.     

Hydrophobic organic micropollutants in samples of coastal waters: efficiencies of solid-phase extraction in the presence of humic substances

Solid-phase extraction (SPE) has been used to enrich organic micropollutants (hydrophobic chlorinated and polycyclic aromatic hydrocarbons, CHC and PAH) from coastal water samples and to systematically study the influence of humic substances (HS) on SPE. A reversed phase (RP) system with high flow rates (rapid chromatography, RC) was used to show the basic adsorption principles and interaction processes which influence the enrichment of organic compounds. A model humic substance was found to hinder the enrichment of individual hydrophobic micropollutants (MP), depending on their octanol-water distribution coefficient P _OW. This effect was found to be lower with natural humic substances. For longer contact time between water sample and adsorption material, the pollutant/humic substance bonding proved to be reversible. Received: 8 December 1997 / Revised: 2 March 1998 / Accepted: 4 March 1998     

Cascade ultrafiltration and competing ligand exchange for kinetic speciation of aluminium, iron, and nickel in fresh water

Kinetic speciation of nickel, aluminium, and iron in fresh water has been investigated by cascade ultrafiltration followed by competing ligand exchange of the ultrafiltered fractions. Graphite furnace atomic absorption spectrometry was used to measure the kinetics of metal complex dissociation. Dissolved metal species were fractionated by cascade ultrafiltration. Metal speciation in each ultrafiltered fraction was then characterized as free metal ions, “labile” metal complexes (with dissociation rate constants ≥10⁻³ s⁻¹), “slowly labile” metal complexes (with dissociation rate constants >10⁻⁶ s⁻¹), and “inert” metal complexes (with dissociation rate constants <10⁻⁶ s⁻¹). The experimental results were compared with the predictions of a computer-based equilibrium speciation model, the Windermere humic aqueous model (WHAM) V. Cascade ultrafiltration coupled with kinetic speciation of the metal species in each molecular weight cut-off (MWCO) fraction provided a more comprehensive picture and insight into the physical and the chemical characteristics of the metal species than either ultrafiltration or measurement of dissociation kinetics alone.