Effect Of Solution Ionic Strength Research Articles

The impact of ionic strength on the proton reactivity of clay minerals

The reactivity of clay mineral surfaces exerts a fundamental control on elemental cycling in environmental systems. However, the effect of solution ionic strength (IS) on their reactivity is poorly understood. Here we quantified the effect of changes in IS on clay mineral reactivity through the application of surface complexation modeling (SCM) based on acid-base titration data for three different clay minerals (kaolinite, illite, and montmorillonite). The results demonstrate that the acidity constants (Ka values) for proton interactions on the clay surface vary linearly as a logarithmic function of solution IS, and that proton binding ability is inversely related to solution IS. Two possible explanations for the observed relationship are proposed: (1) solution electrolytes (e.g., Na+) competitively bind to clay surface sites relative to protons, and (2) that the surface electrostatic field is attenuated by solution electrolytes. We conclude that the impact of increasing solution IS on clay surface reactivity is the result of a weakening negative surface electric field that impacts the surface potential. This mechanism ultimately leads to different adsorption behavior for cations and anions under changing IS conditions. The empirical relationship between K values and IS provided here can be applied to predict clay surface reactivity under varied environmental IS conditions and has specific implications for trace metal behavior in environments, such as estuaries, where changes in IS can be significant.

Co-transport and remobilization of Cu and Pb in quartz column by carbon dots

Carbon nanoparticles such graphene, carbon nanotubes, and carbon dots offer the potential to improve environmental treatment technologies due to their unique properties such as low toxicity and high metal sorption capacity. However, there are no studies on facilitated transport and remobilization of pre-sorbed metals by carbon dot (CD) nanoparticles in quartz sand columns. Here, we investigate the effects of solution ionic strength (IS; 1, 100 and 200 mM NaCl) and pH (Chen et al., 2017; Chen et al., 2010; Cornell and Schwertmann, 2006), initial CD concentration (200, 400, 600 and 800 mg L−1), and clay content (10, 20 and 30%w kaolinite) in quartz sand columns on the transport, retention and remobilization of Cu and Pb in saturated (upward flow) quartz porous media. Batch sorption experiments were employed to underpin the findings of the column transport experiments. Both CD and quartz adsorbed Cu and Pb from water, but adsorption was higher on CD than quartz.Co-transport experiment demonstrated the CD-facilitated transport of Cu and Pb. Sequential transport experiments (first three phases) demonstrated the retention of Cu and Pb in the quartz column, with higher retention of Pb compared to Cu. The Cu and Pb retention was attributed to their sorption on the quartz grains and precipitation under the experimental conditions investigated in this study. Cu retention increased with the increase in ionic strength, pH and clay content. Pb was nearly totally retained in the quartz column at all experimental conditions. The subsequent injection of CD resulted in Cu and Pb remobilization to different extents, except in the presence of high kaolinite concentration. CD is most efficient in remobilizing Cu and Pb at 400 mg L−1 CD concentration and under low ionic strength (ca. 1–100 mM), low pH (ca. 6) and in the absence of clays. Deviation from these conditions results in reduced remobilization of Cu and Pb.

Transport and retention of carbon dots (CDs) in saturated and unsaturated porous media: Role of ionic strength, pH, and collector grain size

Carbon-based engineered nanoparticles (ENPs) are widely used in consumer products due to their small size and unique physicochemical properties. Therefore, their release and distribution into the surface and subsurface environment is a subject of concern. Several studies have evaluated the transport and retention of carbon nanotubes and fullerenes, but none investigated the transport and retention of carbon dots (CDs). The aim of this research is to fill this knowledge gap by evaluating the transport and retention of CDs in saturated and unsaturated porous medium. Here, we investigate the effects of solution ionic strength (IS, 1–700 mM NaCl) and pH (4–9), the initial concentration of CDs (50–200 mg L−1), and porous media grain size (0.20–0.50 mm, 0.50–1 mm, 1–1.5 mm and 1.5–2 mm grain diameters) on the transport and retention of CDs in saturated (upward flow) and unsaturated (downward flow) quartz porous media. A mathematical model based on the advection-dispersion equation coupled with the second-order kinetics was used to fit the breakthrough curves and to calculate the attachment and straining rates under the different experimental conditions. These analyses were underpinned by characterization of CD surface functional groups, surface charge and aggregation under the different experimental conditions, calculation of CD-CD and CD-quartz sand interaction potential according to DLVO theory.Transport and retention of CDs in quartz porous media are consistent with those observed for other types of carbon-based ENPs such as fullerenes and carbon nanotubes. Mobility of CDs in both saturated and unsaturated porous media increases with the decrease in ionic strength, increase in pH, and increase in collector grain size. Retention of CDs increases with the increase in IS, decrease in pH and decrease in grain size. Generally, CDs mobility was higher under saturated than under unsaturated flow conditions, for the same experimental conditions. Overall, CDs tend to be highly mobile and could travel for long distances at a wide range of environmental conditions.

Effect of ionic strength on the aggregation kinetics of the amidated amyloid beta peptide Aβ (1-40) in aqueous solutions

In this work we study the effect of solution ionic strength on the structural evolution of amidated amyloid beta peptide Aβ (1-40) oligomers at the early stages of fibril formation. By light scattering, we follow the time evolution of the structure and short-time dynamics of peptide structures at low ionic strengths. Our results allow identifying initial oligomer structures as the effective building blocks in the amyloid fibrils formation and indicate that the oligomers growth pathway, from compact structures to flexible chain-like structures, becomes faster as the solution ionic strength is increased. Furthermore, we find no evidence of structural branching what suggests that elongation of amyloid fibrils is dominated by linear association. To describe our results we adapt a phenomenological model based on population balance equations and linear polymer growth, where the parameters required are obtained from the experiments. Model calculations are in good agreement with experimentally-obtained estimates for the radius of gyration of Aβ (1-40) oligomers, thus further supporting our findings. Additionally, we introduce a model for the effective interaction among initial Aβ structures that captures the dependence of the effective association rates on solution ionic strength.

Nanoparticle transport in water-unsaturated porous media: effects of solution ionic strength and flow rate

This paper presents the influence of ionic strength and flow on nanoparticle (NP) retention rate in an unsaturated calcareous medium, originating from a heterogeneous glaciofluvial deposit of the region of Lyon (France). Laboratory columns 10 cm in diameter and 30 cm in length were used. Silica nanoparticles (Au-SiO2-FluoNPs), with hydrodynamic diameter ranging from 50 to 60 nm and labeled with fluorescein derivatives, were used to simulate particle transport, and bromide was used to characterize flow. Three flow rates and five different ionic strengths were tested. The transfer model based on fractionation of water into mobile and immobile fractions was coupled with the attachment/detachment model to fit NPs breakthrough curves. The results show that increasing flow velocity induces a decrease in nanoparticle retention, probably as the result of several physical but also geochemical factors. The results show that NPs retention increases with ionic strength. However, an inversion of retention occurs for ionic strength >5.10−2 M, which has been scarcely observed in previous studies. The measure of zeta potential and DLVO calculations show that NPs may sorb on both solid-water and air-water interfaces. NPs size distribution shows the potential for nanoparticle agglomeration mostly at low pH, leading to entrapment in the soil pores. These mechanisms are highly sensitive to both hydrodynamic and geochemical conditions, which explains their high sensitivity to flow rates and ionic strength.

Effect of Solution Ionic Strength on the pKa of the Nitroxide pH EPR Probe 2,2,3,4,5,5-Hexamethylimidazolidin-1-oxyl.

Spin probe and spin labeling Electron Paramagnetic Resonance methods are indispensable research tools for solving a wide range of bioanalytical problems-from measuring microviscosity and polarity of phase-separated liquids to oxygen concentrations in tissues. One of the emerging uses of spin probes are the studies of proton transfer-related and surface electrostatic phenomena. The latter Electron Paramagnetic Resonance methods rely on molecular probes containing an additional functionality capable of reversible ionization (protonation, in particular) in the immediate proximity to an Electron Paramagnetic Resonance-active reporter group, such as (N-O•) for nitroxides. The consequent formation of protonated and nonprotonated nitroxide species with different magnetic parameters (A iso, g iso) could be readily distinguished by Electron Paramagnetic Resonance. Bioanalytical Electron Paramagnetic Resonance studies employing pH-sensitive paramagnetic probes typically involve determination of the equilibrium constant (pK a) between the protonated and nonprotonated forms of the nitroxide. However, any chemical equilibrium involving charged species, such as ionization of acids and bases, and so the reversible protonation of the nitroxide, is known to be affected by an ionic strength of the solution. Currently, only scarce data for the effect of the solution ionic strength on the experimental pK a's of the ionizable nitroxides can be found in the literature. Here we have carried out a series of Electron Paramagnetic Resonance titration experiments for aqueous solutions of 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl (HMI) nitroxide known for one of the largest differences in the isotropic nitrogen hyperfine coupling constant A iso between the protonated and nonprotonated forms. Electrolyte concentration was varied over an exceptionally large range (i.e., from 0.05 to 5.0 M) to elucidate the effect of ionic strength on the ionization constant of this pH-sensitive Electron Paramagnetic Resonance probe and the data were compared to the Debye-Hückel limiting law. Effects of the ionic strength on the magnetic parameters of the ionizable nitroxides are also discussed.

Effects of solution conditions on characteristics and size exclusion chromatography of pneumococcal polysaccharides and conjugate vaccines

Molecular properties of bacterial polysaccharides and protein-polysaccharide conjugates play an important role in the efficiency and immunogenicity of the final vaccine product. Size exclusion chromatography (SEC) is commonly used to analyze and characterize biopolymers, including capsular polysaccharides. The objective of this work was to determine the effects of solution ionic strength and pH on the SEC retention of several capsular polysaccharides from S. pneumoniae bacteria in their native and conjugated forms. The retention time of the charged polysaccharides increased with increasing ionic strength and decreasing pH due to compaction of the polysaccharides associated with a reduction in the intramolecular electrostatic interactions. The calculated radius of gyration was in good agreement with model calculations based on the worm-like chain model accounting for the increase in chain stiffness and excluded volume of the charged polysaccharide at low ionic strength. These results provide important insights into the effects of solution ionic strength on physical properties and SEC behavior of capsular polysaccharides and their corresponding conjugates.

Colloid release and clogging in porous media: Effects of solution ionic strength and flow velocity

The release and retention of in-situ colloids in aquifers play an important role in the sustainable operation of managed aquifer recharge (MAR) schemes. The processes of colloid release, retention, and associated permeability changes in consolidated aquifer sediments were studied by displacing native groundwater with reverse osmosis-treated (RO) water at various flow velocities. Significant amounts of colloid release occurred when: (i) the native groundwater was displaced by RO-water with a low ionic strength (IS), and (ii) the flow velocity was increased in a stepwise manner. The amount of colloid release and associated permeability reduction upon RO-water injection depended on the initial clay content of the core. The concentration of released colloids was relatively low and the permeability reduction was negligible for the core sample with a low clay content of about 1.3%. In contrast, core samples with about 6 and 7.5% clay content exhibited: (i) close to two orders of magnitude increase in effluent colloid concentration and (ii) more than 65% permeability reduction. Incremental improvement in the core permeability was achieved when the flow velocity increased, whereas a short flow interruption provided a considerable increase in the core permeability. This dependence of colloid release and permeability changes on flow velocity and colloid concentration was consistent with colloid retention and release at pore constrictions due to the mechanism of hydrodynamic bridging. A mathematical model was formulated to describe the processes of colloid release, transport, retention at pore constrictions, and subsequent permeability changes. Our experimental and modeling results indicated that only a small fraction of the in-situ colloids was released for any given change in the IS or flow velocity. Comparison of the fitted and experimentally measured effluent colloid concentrations and associated changes in the core permeability showed good agreement, indicating that the essential physics were accurately captured by the model.

Temperature-Regulated Fluorescence and Association of an Oligo(ethyleneglycol)methacrylate-Based Copolymer with a Conjugated Polyelectrolyte—The Effect of Solution Ionic Strength

Aqueous mixtures of a dye-labeled non-ionic thermoresponsive copolymer and a conjugated cationic polyelectrolyte are shown to exhibit characteristic changes in fluorescence properties in response to temperature and to the presence of salts, enabling a double-stimuli responsiveness. In such mixtures at room temperature, i.e., well below the lower critical solution temperature (LCST), the emission of the dye is strongly quenched due to energy transfer to the polycation, pointing to supramolecular interactions between the two macromolecules. Increasing the concentration of salts weakens the interpolymer interactions, the extent of which is simultaneously monitored from the change in the relative emission intensity of the components. When the mixture is heated above its LCST, the transfer efficiency is significantly reduced, signaling a structural reorganization process, however, surprisingly only if the mixture contains salt ions. To elucidate the reasons behind such thermo- and ion-sensitive fluorescence characteristics, we investigate the effect of salts of alkali chlorides, in particular of NaCl, on the association behavior of these macromolecules before and after the polymer phase transition by a combination of UV-vis, fluorescence, and (1)H NMR spectroscopy with light scattering and small-angle neutron scattering measurements.

Deposition and release kinetics of nano-TiO2 in saturated porous media: Effects of solution ionic strength and surfactants

The aggregation, transport and deposition kinetics (i.e. attachment and release) of TiO2 nanoparticles (nano-TiO2) were investigated as a function of ionic strength and the presence of anionic (sodium dodecylbenzene sulfonate, SDBS) and non-ionic (Triton X-100) surfactants in 100% critical micelle concentration (CMC). The electrolyte concentration of the suspensions dictated the kinetic stability of nano-TiO2 thus influencing the transport and retention of the nanoaggregates in the saturated porous medium. With increasing ionic strength, the interaction between approaching nano-TiO2 and nano-TiO2 already deposited onto collectors surfaces seemed to be more favorable than the interaction between approaching nano-TiO2 and bare collectors surfaces. The abrupt and gradual reduction in electrolyte concentration during the flushing cycles of the column experiments induced the release of previously deposited nano-TiO2 suggesting attachment of nano-TiO2 through secondary energy minimum.

Pore‐Scale Numerical and Experimental Investigation of Colloid Retention at the Secondary Energy Minimum

Mechanistic understanding of colloid retention and transport in porous media is important in many environmental processes and applications. In this study, we characterized and quantified colloid retention under unfavorable attachment conditions through modeling coupled with pore‐scale experiments, focusing on the effects of solution ionic strength and interstitial flow speed. A computational approach was developed to simulate the motion of colloids suspended in pore‐scale flow through a network of grain packing in a bounded channel. Simulation results showed that colloids could only be retained at the secondary energy minimum (SEmin) due to the presence of the high repulsive energy barrier (above 1500 kT). The fraction of colloids that can move into the SEmin well and be subsequently retained by the attractive van der Waals force is controlled by the competition of hydrodynamic transport along the streamline and Brownian shifting across the streamline. The tangential hydrodynamic force could slowly drive retained colloids toward the rear stagnation region along the surface, leading to accumulation of retained colloids. The retention at SEmin is dynamically irreversible when the SEmin depth reaches about −4 kT. These mechanistic insights explain well the dependence of the retention ratio on flow speed at a given ionic strength as well as the saturation of the retention ratio with ionic strength at a prescribed flow speed. Furthermore, for a given ionic strength, there is a critical flow speed below which Brownian motion dominates the colloid retention rate, leading to a very strong dependence of surface coverage on flow speed. These simulation results were confirmed by experimental confocal‐microscopy observations in capillary porous channels as well as by other published results, supporting the mechanistic findings.

Facilitated transport of Cu with hydroxyapatite nanoparticles in saturated sand: Effects of solution ionic strength and composition

Column experiments were conducted to investigate the facilitated transport of Cu in association with hydroxyapatite nanoparticles (nHAP) in water-saturated quartz sand at different solution concentrations of NaCl (0–100 mM) or CaCl 2 (0.1–1.0 mM). The experimental breakthrough curves and retention profiles of nHAP were well described using a mathematical model that accounted for two kinetic retention sites. The retention coefficients for both sites increased with the ionic strength (IS) of a particular salt. However, the amount of nHAP retention was more sensitive to increases in the concentration of divalent Ca 2+ than monovalent Na +. The effluent concentration of Cu that was associated with nHAP decreased significantly from 2.62 to 0.17 mg L −1 when NaCl increased from 0 to 100 mM, and from 1.58 to 0.16 mg L −1 when CaCl 2 increased from 0.1 to 1.0 mM. These trends were due to enhanced retention of nHAP with changes in IS and ionic composition (IC) due to compression of the double layer thickness and reduction of the magnitude of the zeta potentials. Results indicate that the IS and IC had a strong influence on the co-transport behavior of contaminants with nHAP nanoparticles.

Stabilization of Fe 0 nanoparticles with silica fume for enhanced transport and remediation of hexavalent chromium in water and soil

Effective in situ remediation of Cr(VI) in groundwater requires the successful delivery of reactive iron particles to the subsurface. Fe 0 nanoparticles (20–110 nm diameter) supported on silica fume were synthesized by borohydride reduction of an aqueous iron salt in the presence of a support material. The experimental result showed that attachment of Fe 0 nanoparticles on the commercial available sub-micrometer silica fume prevented them from aggregation while maintaining the particle reactivity. When the Fe 0 concentration was 0.4 g/L, 88.00% of 40 mg/L Cr(VI) was removed by silica fume-supported Fe 0 nanoparticles (SF-Fe 0) in 120 min, 22.55% higher than unsupported Fe 0. Furthermore, transport experiments confirmed that almost all unsupported Fe 0 was retained, whereas 51.50% and 38.29% of SF-Fe 0 were eluted from the vertical and horizontal sand column, respectively. Additionally, the effect of solution ionic strength on the transport ability of SF-Fe 0 was evaluated. The result showed that increase in the salt concentration led to a decrease in the mobility and also the divalent ion Ca 2+ had a greater effect than that of monovalent ion Na +.

Disruption of Supported Lipid Bilayers by Semihydrophobic Nanoparticles

Understanding the interaction between functional nanoparticles and cell membranes is critical to use nanomaterials for broad biomedical applications with minimal cytotoxicity. In this work, we have investigated the effect of adsorbed semihydrophobic nanoparticles (NPs) on the dynamics and morphology of model cell membranes. We have systematically varied the degree of surface hydrophobicity of carboxyl end-functionalized polystyrene NPs of varied size in buffer solutions with varied ionic strength. It is observed that semihydrophobic NPs can readily adsorb on neutral SLBs and drag lipids from SLBs to NP surfaces. Above a critical NP concentration, the disruption of SLBs is observed, accompanied with the formation and rapid growth of lipid-poor regions on NP-adsorbed SLBs. In the study of the effect of solution ionic strength on NP surface hydrophobic degree and the growth of lipid-poor regions, we have concluded that the hydrophobic interaction enhanced by screened electrostatic interaction underlies the envelopment of NPs by lipids that are attracted from SLBs to the surface of NPs or their aggregates. Hence, the formation and growth of lipid-poor regions, or vaguely referred as "pores" or "holes" in the literature, can be controlled by NP concentration, size, and surface hydrophobicity, which is critical to design functional nanomaterials for effective nanomedicine while minimizing possible cytotoxicity.

Layer-by-Layer Assemblies of Chitosan and Heparin: Effect of Solution Ionic Strength and pH

The growth of polysaccharide multilayers consisting of positively charged chitosan (CH) and negatively charged heparin (HEP) was monitored in situ by employing a quartz crystal microbalance (QCM-D) and dual-polarization interferometry (DPI). The main focus was on how the physicochemical properties of the solution affect the growth and structure of the resulting multilayer film. These results showed that when increasing the ionic strength of the polysaccharide solutions at a fixed pH, both the "dry" (optical) (DPI) mass and wet (QCM) mass of the adsorbed multilayer film increased. The same effect was found when increasing the pH while keeping the ionic strength constant. Furthermore, the growth of multilayers showed an exponential-like behavior independent of the solution conditions that were used in this study. It was also established that chitosan was the predominant species present in the chitosan-heparin multilayer film. We discuss the viscoelastic properties of the adsorbed layers and their variation during the multilayer buildup. Interestingly and contrary to common interpretation of the QCM-D results, we found that under one particular solution condition (pH 4.2 and 30 mM NaCl) the increase in the dissipation of oscillation energy from the adsorbed layer was a consequence of layer stiffening rather than indicating a more hydrated and viscous film. On the basis of the widely used Voigt viscoelastic model for an adsorbed layer, we show that it is the film viscosity and shear that define the layer viscoelasticity (structure) of the film and not the absolute value of energy dissipation, which in fact can be very misleading.

Evaluation of biofouling potential of microorganism using flow field-flow fractionation (Fl-FFF)

The adhesion property of Pseudomonas putida on reverse osmosis (RO) membranes was systematically investigated using the asymmetrical flow field-flow fractionation (AsFl-FFF). The adhesion of P. putida on two different RO membranes was investigated by varying the salt concentration of carrier solution to evaluate the effect of ionic strengths and membrane characteristics on the biofouling potential of RO membranes. The elution peak in terms of peak retention time and area obtained from AsFl-FFF analysis was used to evaluate the adhesion tendency of P. putida under different solution ionic conditions. Results showed that P. putida was favorably attached to RO membranes at higher ionic strengths. Hydrophobic RO membrane exhibited more adhesive property to P. putida compared to the tested hydrophilic membrane under the lower ionic strength condition. The effect of solution ionic strength on the adhesion tendency was more influential than membrane characteristics. In addition, the influence of ionic strength variation on adhesion tendency was more sensitive to hydrophilic membranes than hydrophobic membranes.

Adsorption of lysozyme, β-casein and their layer-by-layer formation on hydrophilic surfaces: Effect of ionic strength

The adsorbed amount and layer structure of lysozyme, β-casein and mixed layers of the two proteins were studied on hydrophilic silica and quartz surfaces using the following techniques: ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D) and total internal reflection fluorescence (TIRF). Particular emphasis was put on the effect of solution ionic strength on the layer formation. Both lysozyme and β-casein showed a higher affinity for the silica surface when adsorbed from a solution of low ionic strength even though β-casein and silica are negatively charged at the pH used. No β-casein remained adsorbed after rinsing with a 150 mM buffer solution. The adsorbed amount of lysozyme on silica exceeded a monolayer coverage irrespective of the solution conditions and displayed a rigid structure. β-Casein forms more than a single layer on pre-adsorbed lysozyme; an inner flat layer and an outer layer with an extended structure, which largely desorbs on rinsing. The build-up through sequential adsorption of lysozyme and β-casein is favoured at intermediate and high ionic strength. The total adsorbed amount increased slightly with each deposition cycle and the mixed lysozyme/β-casein layers contain higher amounts of protein compared to those of pure lysozyme or β-casein. Sequential adsorption gives rise to a proteinaceous layer consisting of both lysozyme and β-casein. The protein layers are probably highly interpenetrated with no clear separation between them.

Time-Resolved Studies of Adsorption and Transport of a Hydrophobic Ion at Escherichia coli Bacterial Membranes by Second Harmonic Generation

The adsorption and transport processes of a hydrophobic molecular ion Malachite Green (MW 329.4) at E.coli bacterial outer membrane and cytoplasmic membrane, respectively, have been characterized by using a nonlinear optical technique- Second Harmonic Generation. Adsorption isotherms of the MG ion to both membranes of the gram-negative bacteria E.coli cell have been measured for the determination of the maximum adsorption densities and adsorption equilibrium constants. In some of the experiments, the classical permeablizer EDTA was used to eliminate the bacterial outer membrane and enable independent determination for the cytoplasmic membrane. A pH-modified Langmuir adsorption model has been applied to analyze the isotherms obtained under physiological conditions. The effect of solution ionic strength on adsorption has been examined. The nonlinear optical signal also directly allows the observation, with real time resolution, of the transport of the molecular ions through the two membranes and the determination of the respective transport rates.

Polyelectrolyte Multilayer Assembly as a Function of pH and Ionic Strength Using the Polysaccharides Chitosan and Heparin

The goal of this work is to explore the effects of solution ionic strength and pH on polyelectrolyte multilayer (PEM) assembly, using biologically derived polysaccharides as the polyelectrolytes. We used the layer-by-layer (LBL) technique to assemble PEM of the polysaccharides heparin (a strong polyanion) and chitosan (a weak polycation) and characterized the sensitivity of the PEM composition and layer thickness to changes in processing parameters. Fourier-transform surface plasmon resonance (FT-SPR) and spectroscopic ellipsometry provided in situ and ex situ measurements of the PEM thickness, respectively. Vibrational spectroscopy and X-ray photoelectron spectroscopy (XPS) provided details of the chemistry (i.e., composition, electrostatic interactions) of the PEM. We found that when PEM were assembled from 0.2 M buffer, the PEM thickness could be increased from less than 2 nm per bilayer to greater than 4 nm per bilayer by changing the solution pH; higher and lower ionic strength buffer solutions resulted in narrower ranges of accessible thickness. Molar composition of the PEM was not very sensitive to solution pH or ionic strength, but pH did affect the interactions between the sulfonates in heparin and amines in chitosan when PEM were assembled from 0.2 M buffer. Changes in the PEM thickness with pH and ionic strength can be interpreted through descriptions of the charge density and conformation of the polyelectrolyte chains in solution.

Influence of solution ionic strength on the collision efficiency distribution and predicted transport distance of a Sphingomonas sp. flowing through porous media

The effects of solution ionic strength on the collision efficiency ( α) distribution of a Sphingomonas sp. were investigated using multiple sand columns of varying lengths and analyzing the bacteria clean-bed breakthrough concentrations using a distributed colloid filtration theory (D-CFT). Five different probability density functions (PDFs) were investigated and all accurately replicated the lab-scale experimental data, whereas a single α value could not. The α distribution shifted toward smaller values with decreasing ionic strength and the PDF parameters were strongly correlated to the Debye length, indicating that electrostatic interactions had a direct impact on the α distribution. The results indicate that while ionic strength has a large impact on bacterial transport distances for a concentration reduction of a few orders of magnitude, as occurs at the laboratory scale, due to the distributed nature of the collision efficiency, it has a minor effect on predicted transport distances required to achieve concentration reductions on the order of 10 6, which occurs at the field scale. Because of this, bacterial inactivation (e.g., death), rather than physically removing the bacteria from solution via filtration, is likely the key process impacting the transport of viable bacteria at the field scale. Overall, for systems with a distributed α, the results indicate that ionic strength has a strong influence on the transport of bacteria at the lab-scale (centimeters to one meter), both ionic strength and bacterial inactivation are important at the meso-scale (tens of meters), and inactivation becomes the dominant mechanism for reducing the transport of viable bacteria at the field scale (hundreds of meters).