Function Of Solution Chemistry Research Articles

Effect of solution chemistry on the sedimentation, dissolution, and aggregation of the bimetallic Fe/Cu nanoparticles pre- and post-grafted with carboxymethyl cellulose

The suitability of iron-based nanomaterials or composites for in-situ remediation hinges on their physicochemical stability. Introducing surface modifications like metal doping or polymer grafting can regulate interparticle forces, influencing particle stability. Thus, probing how grafting methods (i.e., pre- or post-grafting) tune material properties controlling interparticle forces, comprehend the synergistic effect of metal doping and polymer grafting, and evaluate stability under varying geochemical conditions are the way forward in designing sustainable remediation strategies. To this end, time-dependent sedimentation, dissolution, and aggregation of four synthesized iron-based nanoparticles (bare iron (Fe), copper doped bimetallic iron/copper (Fe/Cu), pre- and post-grafted Fe/Cu with carboxymethyl cellulose (CMC) – CMCpre-Fe/Cu and CMCpost-Fe/Cu, respectively) were carried out as a function of solution chemistry (i.e., pH – 5 to 10, ionic strength, IS – 0 to 100 mM NaCl, initial particle concentration, C0–20 to 200 mg.L−1) mimicking geoenvironmental conditions. CMCpre-Fe/Cu exhibited markedly higher particle availability (> 91 %) against sedimentation than others (bare Fe/Cu (11.28 %) > bare Fe (7.33 %) > CMCpost-Fe/Cu (6.09 %)) – suggesting the pivotal role of grafting method on particle stability. XDLVO energy profiles revealed pre-grafting altered magnetic properties favoring surface charge-driven electrostatic repulsion over magnetic attraction, thereby limiting aggregation-induced particle settling. In contrast, superior magnetic force overrides the electrostatic behavior for bare and post-grafted particles. Unlike bare and post-grafted nanoparticles, CMCpre-Fe/Cu aggregate size correlated positively with [H+] and IS, consistent with their settling behavior. Rise in C0 showed a visible negative effect on particle aggregation and, thereby, sedimentation except for CMCpre-Fe/Cu by facilitating particle collision through Brownian movement. Both acidic pH and copper doping promoted nanoparticle dissolution, whereas pre-grafting can provide a plausible solution against nanoparticle toxicity and loss of reactivity due to ionic release. To recapitulate, these findings are imperative in building a sustainable framework for environmental remediation application.

The effect of solution chemistry on elemental and isotopic fractionation of lithium during inorganic precipitation of calcite

The elemental and isotopic fractionation of lithium between inorganic calcite and aqueous solutions has been measured as a function of solution chemistry in mixed CaCl2-NaHCO3 aqueous solutions spanning a range of pH from ~6 to ~9.5 at room temperature. Vaterite added to the solution recrystallized to calcite with limited change in solution chemistry. With this experimental design solution pH and Na/Ca strongly correlate. The ratio of the concentration of Li in the calcite to that of the solution (Kd,Li = [Li]calcite/[Li]solution), increased by about an order of magnitude with increasing pH. The Li exchange coefficient (Kd,Li/Ca=([Li]calcite/[Ca]calcite)/([Li]solution/[Ca]solution) decreased by about four orders of magnitude, from ~10−2 to ~10−6 with increasing pH due to the large decrease in solution Ca content with increasing experiment pH. The Li isotope fractionation also changed systematically as the pH of the solution increased from 6.5 to 9.5 with the calcite ranging from about −6‰ to +2‰ relative to the solution it grew from. No change in either the elemental or isotopic partitioning was observed with either a change in the Li concentration of the solution or with a change in the experimental duration (5–400 days). To better understand the role of diffusive Li isotope fractionation a series of experiments were performed to determine the relative diffusivity of 6Li and 7Li in solutions with different pH. The results suggest little or no difference in relative diffusivity (D6Li ~ 0.998D7Li) across the range of solution chemistry considered (pH 4 to ~7.5). The change in Li partitioning between calcite and solution with changing solution chemistry is consistent with Li incorporation into calcite as LiHCO3 and hence a dependence on solution H+/Ca2+. If this interpretation is correct, it means that the concentration of Li in inorganic calcite precipitated from seawater at fixed Ca and Li concentrations will increase with decreasing pH. The change in the Li isotope fractionation factor with changing solution chemistry may either reflect a kinetic isotope fractionation associated with changing calcite growth rates or a change in the equilibrium fractionation factor due to changing Li speciation in the solution. These results, while not directly applicable to pristine biogenic calcite, have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification (e.g., all bulk carbonate Li isotope data). Diagenesis under variable conditions provides a plausible alternative explanation for Li isotope excursions in carbonate sediment sequences that have been interpreted as indicating changing seawater δ7Li that deserves further investigation.

Electrokinetic study of calcium carbonate and magnesium hydroxide particles in lime softening

Significant effort has been made to measure and understand the surface charge of CaCO3 and Mg(OH)2 particles. Many laboratory experiments have been reported on zeta potential of natural and prepared CaCO3 and a few have also been published for Mg(OH)2, however, there are very few reported measurements of zeta potential of CaCO3 and Mg(OH)2 particles at conditions relevant to lime softening, despite lime softening being a common and established process for water treatment. The present study aims to understand the interactions and electrokinetic properties of these two particles in lime softening. Effects of various experimental parameters such as pH, temperature, aging, inorganic carbon (CO32−/HCO3−), and divalent cations (Ca2+/Mg2+) on the electrokinetic properties (i.e. zeta potential) of CaCO3 and Mg(OH)2 particles were individually studied. The interactions between humic acid (mimicking natural organic matter), silicate (representing silica), clay (mimicking suspended solids) and CaCO3/Mg(OH)2 particles were studied, as well as the interactions between CaCO3 and Mg(OH)2. Thermodynamic modeling was used to predict precipitates as a function of solution chemistry and assist with data interpretation. The results provide considerable insight into factors that are of importance to lime softening.

Growth kinetics of siderite at 298.15 K and 1 bar

The widespread occurrence of siderite at the Earth’s surface has motivated investigations into the thermodynamic factors controlling its stability for most of the last century. However, despite a new appreciation for multiple Fe(II)-carbonate mineralisation pathways, the rates of siderite growth have not been accurately predicted as a function of solution chemistry. This has impeded a quantitative understanding of myriad geochemical systems, both modern and ancient. To address this issue, we investigated the growth kinetics of synthetic siderite seeds in anoxic closed-system conditions at 298.15 K and 1 bar. On the basis of monitoring the chemical evolution of the bulk solutions over the course of 46 days, our results demonstrate two distinct relationships between kinetic behaviour and solution saturation (Ω) with respect to siderite. These relationships are best explained by chemical affinity-based rate laws that have been extensively applied to the growth kinetics of calcite (a mineral isostructural with siderite). More specifically, the surface area-normalised siderite growth rates (mol·m−2·s−1) display a linear correlation with supersaturation (Ω-1) when Ω≳5, suggesting a growth rate controlled by the transport of ions to the mineral surface (rtr):rtr=10-12.60±0.16(Ω-1)1.057±0.112.At low solution saturation, growth rates are consistent with spiral mechanism-dominated surface reactions (rsr), exhibiting a parabolic correlation with (Ω-1):rsr=10-13.42±0.14(Ω-1)1.868±0.307.These data show that at comparable solution saturation at 25 °C, siderite growth is nearly 7-orders of magnitude slower than that of calcite. These kinetic behaviours imply that most examples of natural Fe(II) supply fluxes (e.g., dissimilatory iron reduction, hydrothermal activity, or dissolution of other Fe(II)-minerals) would only be balanced by siderite precipitation rates at very high supersaturation, or, if the barrier to Fe(II)-carbonate nucleation is surpassed, balanced by episodic fluctuations in precipitation rate in response to Fe(II) accumulation. In combination with kinetic barriers influencing nucleation, this slow growth rate provides a straightforward explanation for the common observation that many anoxic water bodies are persistently supersaturated with respect to siderite, and implies that the precipitation of siderite may be more dynamic than previously appreciated.

Pitting growth rate on Alloy 800 in chloride solutions containing thiosulphate: image analysis assessment

ABSTRACTThe growth rate of pits formed on Alloy 800 in chloride solutions containing various thiosulphate concentrations was assessed by potentiostatic polarisation, in situ corrosion image observation, and image analysis. The pitting growth rate was a function of solution chemistry: in chloride solution pits were small but numerous, in chloride + thiosulphate solution, the pits were large. Competitive adsorption on Alloy 800 of chloride and thiosulphate ions affected the pitting potential and therefore the pitting growth rate on the metal. The reduction of thiosulphate to elemental sulphur accelerated the pitting growth rate in chloride + thiosulphate solutions.

Utilization of chemical stability diagrams for improved understanding of electrochemical systems: evolution of solution chemistry towards equilibrium

Predicting the stability of chemical compounds as a function of solution chemistry is crucial towards understanding the electrochemical characteristics of materials in real-world applications. There are several commonly considered factors that affect the stability of a chemical compound, such as metal ion concentration, mixtures of ion concentrations, pH, buffering agents, complexation agents, and temperature. Chemical stability diagrams graphically describe the relative stabilities of chemical compounds, ions, and complexes of a single element as a function of bulk solution chemistry (pH and metal ion concentration) and also describe how solution chemistry changes upon the thermodynamically driven dissolution of a species into solution as the system progresses towards equilibrium. Herein, we set forth a framework for constructing chemical stability diagrams, as well as their application to Mg-based and Mg–Zn-based protective coatings and lightweight Mg–Li alloys. These systems are analyzed to demonstrate the effects of solution chemistry, alloy composition, and environmental conditions on the stability of chemical compounds pertinent to chemical protection. New expressions and procedures are developed for predicting the final thermodynamic equilibrium between dissolved metal ions, protons, hydroxyl ions and their oxides/hydroxides for metal-based aqueous systems, including those involving more than one element. The effect of initial solution chemistry, buffering agents, complexation agents, and binary alloy composition on the final equilibrium state of a dissolving system are described by mathematical expressions developed here. This work establishes a foundation for developing and using chemical stability diagrams for experimental design, data interpretation, and material development in corroding systems.

Equilibrium Denaturation and Preferential Interactions of an RNA Tetraloop with Urea

Urea is an important organic cosolute with implications in maintaining osmotic stress in cells and differentially stabilizing ensembles of folded biomolecules. We report an equilibrium study of urea-induced denaturation of a hyperstable RNA tetraloop through unbiased replica exchange molecular dynamics. We find that, in addition to destabilizing the folded state, urea smooths the RNA free energy landscape by destabilizing specific configurations, and forming favorable interactions with RNA nucleobases. A linear concentration-dependence of the free energy (m-value) is observed, in agreement with the results of other RNA hairpins and proteins. Additionally, analysis of the hydrogen-bonding and stacking interactions within RNA primarily show temperature-dependence, while interactions between RNA and urea primarily show concentration-dependence. Our findings provide valuable insight into the effects of urea on RNA folding and describe the thermodynamics of a basic RNA hairpin as a function of solution chemistry.

Adsorption of bisphenol A and ciprofloxacin by palygorskite-montmorillonite: Effect of granule size, solution chemistry and temperature

Adsorption and desorption studies of the endocrine-disrupting compound, bisphenol A (BPA), and the antibiotic ciprofloxacin (CIP) with palygorskite-montmorillonite (Pal-Mt) granules (0.3–0.6mm, 1.7–2.0mm and ≈2.8mm), were done as a function of solution chemistry, temperature, and particle size. The data best fit the Langmuir adsorption model with the smallest and the largest granule sizes, respectively, exhibiting the highest (in mgkg−1, qm=77.3 for BPA; qm=107,000 for CIP) and the lowest (qm=41.0 for BPA; qm=81,000 for CIP) adsorption capacities. CIP adsorption was strongly pH dependent, while BPA adsorption was slightly pH and ionic strength dependent. Thermodynamic parameters indicated BPA and CIP adsorption were spontaneous. Net-adsorption-desorption hysteresis were indicative of irreversible adsorption. FTIR and thermogravimetric analysis (TGA) data corroborate the mechanisms and the level of adsorption for CIP but not for BPA. The smallest and the medium granule sizes, respectively, are appropriate for potential BPA and CIP removal. Capsule abstractBPA and CIP adsorption-desorption data was granule size, experimental pH, ionic strength, and temperature dependent. Adsorption-desorption hysteresis indicated irreversible adsorption.

Nucleation of metastable aragonite CaCO3 in seawater

Predicting the conditions in which a compound adopts a metastable structure when it crystallizes out of solution is an unsolved and fundamental problem in materials synthesis, and one which, if understood and harnessed, could enable the rational design of synthesis pathways toward or away from metastable structures. Crystallization of metastable phases is particularly accessible via low-temperature solution-based routes, such as chimie douce and hydrothermal synthesis, but although the chemistry of the solution plays a crucial role in governing which polymorph forms, how it does so is poorly understood. Here, we demonstrate an ab initio technique to quantify thermodynamic parameters of surfaces and bulks in equilibrium with an aqueous environment, enabling the calculation of nucleation barriers of competing polymorphs as a function of solution chemistry, thereby predicting the solution conditions governing polymorph selection. We apply this approach to resolve the long-standing "calcite-aragonite problem"--the observation that calcium carbonate precipitates as the metastable aragonite polymorph in marine environments, rather than the stable phase calcite--which is of tremendous relevance to biomineralization, carbon sequestration, paleogeochemistry, and the vulnerability of marine life to ocean acidification. We identify a direct relationship between the calcite surface energy and solution Mg:Ca [corrected] ion concentrations, showing that the calcite nucleation barrier surpasses that of metastable aragonite in solutions with Mg:Ca ratios consistent with modern seawater, allowing aragonite to dominate the kinetics of nucleation. Our ability to quantify how solution parameters distinguish between polymorphs marks an important step toward the ab initio prediction of materials synthesis pathways in solution.

Surface complexation of antimony on kaolinite

Geochemical fate of antimony (Sb) – a similar oxyanion as arsenic (As) – in a variety of environment is largely unexplored. Kaolinite is an important, naturally occurring clay mineral in soils and aquifers and is known to control the fate of several contaminants via a multitude of geochemical processes, primarily adsorption. Here we report adsorption of antimony on kaolinite as a function of solution chemistry: initial antimony concentration, pH, ionic strength, and a competing anion. A surface complexation modeling (SCM) approach was undertaken to understand the potential mechanistic implications of sorption envelope data. In the SCM, a multicomponent additive approach, in which kaolinite is assumed to be a (1:1) mixture of quartz (SiOH) and gibbsite (AlOH), was tested. Results indicated that ionic strength has a minimal effect on antimony adsorption. For the lower initial antimony concentration (4.11μM), the additive model with binuclear surface complexes on quartz and gibbsite showed a better fit at pH<6, but somewhat under predicted the experimental data above pH 6. At the higher initial antimony concentration (41.1μM), the sorption envelope was of different shape than the lower load. The additive model, which considered binuclear surface complexes for quartz and gibbsite, resulted in over prediction of the adsorption data at pH>3.5. However, the additive model with binuclear surface complex on quartz and mononuclear surface complex on gibbsite showed an excellent fit of the data. Phosphate greatly influenced antimony adsorption on kaolinite at both low and high antimony loadings, indicating competition for available surface sites.

Adhesion of bacterial pathogens to soil colloidal particles: Influences of cell type, natural organic matter, and solution chemistry

Bacterial adhesion to granular soil particles is well studied; however, pathogen interactions with naturally occurring colloidal particles (<2 μm) in soil has not been investigated. This study was developed to identify the interaction mechanisms between model bacterial pathogens and soil colloids as a function of cell type, natural organic matter (NOM), and solution chemistry. Specifically, batch adhesion experiments were conducted using NOM-present, NOM-stripped soil colloids, Streptococcus suis SC05 and Escherichia coli WH09 over a wide range of solution pH (4.0–9.0) and ionic strength (IS, 1–100 mM KCl). Cell characterization techniques, Freundlich isotherm, and Derjaguin–Landau–Verwey–Overbeek (DLVO) theory (sphere–sphere model) were utilized to quantitatively determine the interactions between cells and colloids. The adhesion coefficients (Kf) of S. suis SC05 to NOM-present and NOM-stripped soil colloids were significantly higher than E. coli WH09, respectively. Similarly, Kf values of S. suis SC05 and E. coli WH09 adhesion to NOM-stripped soil colloids were greater than those colloids with NOM-present, respectively, suggesting NOM inhibits bacterial adhesion. Cell adhesion to soil colloids declined with increasing pH and enhanced with rising IS (1–50 mM). Interaction energy calculations indicate these adhesion trends can be explained by DLVO-type forces, with S. suis SC05 and E. coli WH09 being weakly adhered in shallow secondary energy minima via polymer bridging and charge heterogeneity. S. suis SC05 adhesion decreased at higher IS 100 mM, which is attributed to the change of hydrophobic effect and steric repulsion resulted from the greater presence of extracellular polymeric substances (EPS) on S. suis SC05 surface as compared to E. coli WH09. Hence, pathogen adhesion to the colloidal material is determined by a combination of DLVO, charge heterogeneity, hydrophobic and polymer interactions as a function of solution chemistry.

Dissolution of UK High‐Level Waste Glass Under Simulated Hyperalkaline Conditions of a Colocated Geological Disposal Facility

We report analysis of chemical durability of UK HLW MW+25% simulant glass under model hyperalkaline conditions of a colocated geological disposal facility. Glass powders and monoliths were dissolved for 168 days in saturated Ca(OH)2. Dissolution in the presence of high concentrations of Ca (&gt;200 mg/L) was an order of magnitude lower than dissolution in water. Dissolution of Si did not occur until a Ca:Si ratio of &lt;2 was achieved. The mechanism of dissolution involved the incorporation of Ca into the hydrated surface (initial, incubation regime), the precipitation of C‐S‐H phases, including a range of compositions in the C‐(N)‐(A)‐S‐H and M‐S‐H systems (intermediate regime), and the precipitation of C‐S‐H phases (the residual regime). Thermodynamic analysis and consideration of the CaO‐SiO2‐H2O phase diagram suggest that the rate‐limiting step of glass dissolution in Ca‐rich solutions is Ca‐Si equilibrium, involving the precipitation of C‐S‐H phases, which change in chemical composition as a function of solution chemistry. In low SA/V ratio experiments, the dissolution progressed only to the initial incubation regime, resulting from fewer surface sites for Ca incorporation. Overall, these results suggest that Ca and Si in solution play an important role in the long‐term durability of UK HLW in Ca‐rich solutions.

Removal of sulfamethoxazole by nanofiltration membrane

The retention of sulfamethoxazole (SMZ) by nanofiltration (NF) membranes is strongly influenced by the pH value of the solution. The retention of SMZ reaches its peak value when the solution pH rises above its pKa2 value as the compound transforms into a negatively charged species. Charge repulsion is the main mechanism involved in SMZ removal by NF membranes. In this study, the removal of SMZ by NF membranes, as a function of solution chemistry, was examined at pH 8.9 to investigate the effect of solution conditions on charge repulsion. The results show that the retention of negatively charged SMZ is relatively independent of SMZ concentration, and an increase in the ionic strength of the solution causes a relatively small reduction in retention. A small effect of humic acid (HA) on SMZ retention was noticed at pH 8.9, which can be explained by a small but insignificant improvement in the zeta potential of the membrane caused by HA at high pH values. However, it was found that SMZ concentration in the feed decreased significantly in solutions containing tannic acid (TA). The Adams-Bohart model was applied to our experimental data and was found to be suitable for describing the initial part of the breakthrough curves. The adsorptive parameters of the membrane were determined.

Localised corrosion of heat treated alloys Part 1 – repassivation potential of alloy 600 as function of solution chemistry and thermal aging

The crevice repassivation potential Ercrev of alloy 600 was measured as a function of chloride concentration, inhibitor concentration, temperature and degree of sensitisation (DOS) due to thermal aging. Electrochemical potentiokinetic reactivation tests were performed on thermally aged alloy 600 to correlate the DOS with Ercrev results. The electrochemical potentiokinetic reactivation results agreed well with the literature, and differed in the apparent sensitisation kinetics from Ercrev. The crevice repassivation potential was found to be inversely related to test temperature, chloride concentration, and DOS due to thermally aging the material. Additions of nitrate were found to significantly elevate the repassivation potential of both annealed and sensitised material. The results form the basis for establishing a model to predict the occurrence of localised corrosion for in service, real world materials used in the processing industry.

Effects of Solution Chemistry on Fibrin Nanomechanics

Fibrin, the polymerized protein that provides the structural scaffold of blood clots, is critical to hemostasis and wound healing. The complexity of the fibrin network has left many questions regarding its formation unanswered, including how temperature, metal concentration, and pH, factors known to mediate of fibrin polymerization, influence the mechanics of the interactions between fibrin monomers. Previously, we used the atomic force microscope to examine the mechanical properties of the ‘A–a’ knob-hole interaction, the most significant bond between fibrin monomers. Force applied to this bond was shown to cause stepwise unfolding of the hole-bearing region of fibrinogen as evidenced by force curves exhibiting a characteristic pattern of events. The dependence of environmental effects such as pH, temperature, ionic strength, and divalent ion (i.e., Ca2+ and Mg2+) concentration on this characteristic pattern remains unknown. Herein, we examine the force, spacing, and probabilities of each force event in the characteristic pattern as a function of solution chemistry. Calcium concentration significantly influenced incidence of the last event in the characteristic pattern, but otherwise had no effect on the knob-hole interaction. Such behavior, attributed to the high-affinity γ1 calcium-binding site, was found to be reversible and specific. The force data indicate that the γ1 site has no effect on the strength of the knob-hole bond prior to unfolding but makes the hole more resilient to unfolding. Our results may explain previous paradoxical findings that calcium had no affect on the affinity of knob ‘A’ for hole ‘a’, but was critical to ‘A-a’ driven fibrin polymerization. Likewise, understanding the dependence of fibrin mechanics on solution conditions may help resolve other questions surrounding the relationship between the fibrin polymerization and the local solution environment.

Copper and cadmium sorption onto kraft and organosolv lignins

The relative metal sorption ability of kraft and organosolv lignins was examined as a function of solution chemistry (pH, ionic strength ( I), sorbate-to-sorbent ratio) and reaction time. The surface charge characteristics and functional group composition of these lignins, especially kraft lignin, are favorable for metal sorption. Sorption of Cu and Cd increased with increasing pH and decreasing I. Description of sorption isotherms required the more complex Sips equation, as compared to the simpler Langmuir and Freundlich formulations, indicative of the presence of binding sites with varying affinities on these lignin biosorbents. Sorption capacity varied in the following order: softwood organosolv lignin < hardwood organosolv lignin < hardwood kraft lignin < softwood kraft lignin with sorption maximum of 21.5, 40, 66.7, and 80.6 μmol/g, respectively, for Cu, and 8.2, 18.3, 25.2, and 28.7 μmol/g for Cd. Both Cu and Cd sorption kinetics were rapid with equilibrium levels attained within 80 min and faster uptake was noticed for Cu. Strong competitive effects exhibited by H + and Na + in limiting Cu and Cd sorption are suggestive of the occurrence of weak ion-exchange type interactions involving the carboxylic and phenolic functional groups. Additional pretreatment and surface modifications of these biosorbents might be required to increase metal sorption capacity and enhance their use in waster/wastewater treatment.

Experimental study on the fractionation of yttrium from holmium during the coprecipitation with calcium carbonates in seawater solutions

The partitioning of Y and Ho between CaCO3 (calcite and aragonite respectively) and seawater was experimentally investigated at 25 degrees C and I atm. Both Y and Ho were observed to be strongly partitioned into the overgrowths of calcite or aragonite. Their partition coefficients, D-Y and D-Ho, were determined to be similar to 520-1400 and similar to 700-1900 in calcite, similar to 1200-2400 and similar to 2400-4300 in aragonite, respectively. Y fractionates from Ho during the coprecipitation with either calcite or aragonite. Within our experimental conditions, the fractionation factor, k = D-Y/D-Ho, was determined to be similar to 0.62-0.77 in calcite and similar to 0.50-0.57 in aragonite, respectively. The aqueous complexation of Y and Ho, which is a function of solution chemistry, probably plays an important role in both the partitioning and the fractionation. Further analyses suggest that the difference in covalency between Y and Ho associated with changes in their coordination environments is the determinant factor to the Y-Ho fractionation in the H2CO3-CaCO3 System.

Colloidal Deposition on Remotely Controlled Charged Micropatterned Surfaces in a Parallel-Plate Flow Chamber

This article describes a method to influence colloid deposition by varying the zeta potential at microelectrodes with remotely applied electric potentials. Deposition experiments were conducted in a parallel-plate flow chamber for bulk substrates of glass, indium tin oxide (ITO), and ITO-coated glass microelectrodes in 10 and 60 mM potassium chloride solutions. Colloid deposition was found to be a function of solution chemistry and the small locally delivered electric surface potentials. Electric fields and physical surface heterogeneity can be ruled out as cause of the observed deposition. Results are reported using experimentally determined Sherwood numbers and compared to the predictions of a previously developed patch model. Minor deviations between predicted and experimental Sherwood numbers imply that physical and chemical interactions occur. Specifically, we propose that colloidal particles respond to local variations in surface potential through electrostatic interactions, altering particle streamlines flowing along the surface and ultimately the extent of deposition.

Interaction of the Macrolide Antimicrobial Clarithromycin with Dissolved Humic Acid

The association of the cationic macrolide antimicrobial clarithromycin (CLA) with dissolved Elliot soil humic acid (ESHA) was investigated as a function of solution chemistry. CLA-ESHA association was strongly pH-dependent, reaching a maximum near pH 6.5, and was modeled successfully using FITEQL as a 1:1 complexation reaction between CLA+ and discreet deprotonated acidic functional groups with pKa values of 4 and 6. Approximate order of magnitude increases in ionic strength produced approximately 10-fold decreases in CLA+ -ESHA association. Coefficients for CLA+ -ESHA association were significantly smaller in the presence of K+ vs Na+. Sorption data were well-fit by the Freundlich model; the Freundlich exponent was <1, suggesting CLA+ interacted with sites having a range of binding energies. Sorption appeared largely reversible; little sorption-desorption hysteresis was observed. The affinities of erythromycin and CLA+ for ESHA association sites were indistinguishable, suggesting interaction with specific sorption sites. Comparison of experimentally determined CLA-ESHA association coefficients with those predicted from single-parameter linear free energy relationships based on log Kow suggested limited contribution of hydrophobic interactions to CLA-ESHA association at environmentally relevant pH values. CLA-ESHA association constants were similar in magnitude (10(3.9)-10(4.6) to those of many nonpolar organic contaminants, and macroscopic binding data were consistent with cation exchange dominating CLA+ -ESHA association.

Effect of New Photocatalytic Coagulant on NF Membrane Fouling

In this research, the results of the photocatalytic coagulation of humic acids (HAs) in aqueous solution were investigated, and the effect of the photocatalytic coagulation on the fouling of nanofiltration (NF) membrane was investigated. Nanoparticles of copper-doped titania were synthesized by a sol−gel method using titanium isopropoxide and cuperous chloride. The behavior of a thin-film composite NF membrane was investigated as a function of solution chemistry (ionic composition and strength and calcium and organic concentration). In the membrane fouling test, calcium bridges between the natural organic matter (NOM) and the membrane led to severe flux decline because of the development of densely compacted fouling layer on the membrane surface. The photocatalytic coagulation was applied to remove humic acids prior to membrane filtration. In the photocatalytic coagulation, the UV absorbance, chemical oxygen demand (COD), and cations decreased by 99%, 90%, and 10%, respectively, while anions and conductivity increased to 39% and 79%. The results emphasized that the photocatalytic coagulation could be an effective alternative solution for the elimination of the humic acids and for the prevention of the membrane fouling.