Bulk Solution pH Research Articles

Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects

The liquid phase hydrodechlorination (HDC) of 2-chlorophenol (2-CP) and 2,4-dichlorophenol (2,4-DCP) has been studied over 1% (w/w) Pd/C and Pd/Al2O3 under conditions of minimal mass transport constraints. The HDC of 2,4-DCP generated HCl and 2-CP as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al2O3, but not over Pd/C, prior to complete dechlorination. Pd/Al2O3 is characterized (on the basis of TEM analysis) by a narrow distribution of smaller Pd particles to give a surface area weighted mean particle diameter = 2.4nm that is appreciably lower than the value of 13.2nm established for Pd/C, where the latter is characterized by a broader distribution of larger (spherical) particles. The addition of NaOH served to increase fractional dechlorination by suppressing HDC inhibition due to the HCl that is generated. Reuse of the catalysts revealed an appreciable deactivation of Pd/C and a limited loss of activity in the case of Pd/Al2O3. Deactivation of Pd/C can be linked to a decrease (up to ca. 60%) in the initial BET surface area allied to appreciable leaching (up to ca. 40%) of the starting Pd content through the corrosive action of HCl and, while the average Pd diameter is essentially unaffected, there is evidence of a preferred leaching of larger Pd particles. The stronger metal/support interactions prevalent in Pd/Al2O3 results in limited Pd leaching and comparable initial HDC activities during catalyst reuse with/without NaOH addition. Inclusion of HCl in the reaction mixture (pH 5–1.5) resulted in a marked decline in the initial HDC rate associated with Pd/Al2O3 and a lesser drop in HDC activity for Pd/C. The difference in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports.

Role of base addition in the liquid-phase hydrodechlorination of 2,4-dichlorophenol over Pd/Al 2O 3 and Pd/C

The aqueous-phase batch hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP), over 1 wt/wt% Pd/Al 2O 3 and Pd/C has been investigated with/without the addition of NaOH, NH 4OH, LiOH, KOH, RbOH, and CsOH; bulk solution pH spanned the range 1.5–13. The reaction was operated in the kinetic-controlled regime with 2-chlorophenol (2-CP) as the only intermediate partially dechlorinated product which reacts further to yield phenol; cyclohexanone was formed over Pd/Al 2O 3, but not Pd/C, prior to the completion of dechlorination. An increase in fractional dechlorination with the addition of base was observed and can be attributed to a suppression of HDC inhibition due to the HCl that is generated. The initial HDC activity and selectivity delivered by both catalysts were pH dependent and differences in response to bulk solution pH variations are discussed in terms of the nature of the reactive species in solution and the amphoteric behavior of the Pd supports. The Pd/Al 2O 3 catalyst is characterized by a high surface charge density while Pd/C bore a low basic functionality density on a high surface area carrier. A maximum initial HDC rate over Pd/Al 2O 3 was attained at pH 7–9 while a higher pH (⩾9) results in more effective HDC for Pd/C, effects that are linked to chloroarene dissociation and surface charge effects. In the case of Pd/C, the initial HDC rate with the addition of alkali metal hydroxide ( 0.074 mol dm −3 ) increased in the order Li +≈Na +<K +≈Rb +<Cs +, a sequence that matches the adsorption affinity on a negatively charged carbon surface; HDC over Pd/Al 2O 3 was insensitive to the nature of the alkali metal cation. The action of NH 4OH, as a weak base, is also considered.

Study of proton adsorption at heterogeneous oxide/electrolyte interface. Prediction of the surface potential using Monte Carlo simulations and 1-pK approach.

Adsorption of protons on a heterogeneous solid surface is modeled using the Monte Carlo (MC) simulation method. The surface of an oxide is assumed to consist of adsorption sites with pK assigned according to a quasi-Gaussian distribution. The influence of the electrostatic interactions combined with the energetic heterogeneity of the surface is examined and the MC results are compared with the predictions of the analytical 1-pK approach. The surface potential behavior is examined using both "experimental" MC results and "theoretical" results obtained from the application of 1-pK model. The results are compared qualitatively with experimental determination of the surface potential of metal oxide surfaces. They confirm that the relation between the surface potential and the pH of bulk solution should not be described by the Nernst equation but by the equation with the parameter linearly reducing Nerstian potential. The values of this parameter are examined with respect to degree of surface energetic heterogeneity and site density of the surface.

Effect of Premicellar Aggregation on the pKa of Fatty Acid Soap Solutions

Associative interactions of the various species found in the premicellar concentration region of aqueous fatty acid solutions have been investigated using acid−base titration. In previous studies, aqueous films of fatty acid salts were investigated at various bulk solution pH values. It was found that there exists a pH where minimum evaporation of water, maximum foamability, maximum foam stability, minimum contact angle on PMMA surface, maximum single-bubble stability, and maximum surface viscosity are observed. It was also found that this optimum pH value is near the measured pKa of the fatty acid, which in turn depends on the length of the fatty acid chains. Titration of the homologous series of C8−C12 fatty acids to determine the solution pKa has shown an increase in apparent pKa with concentration. The increase in pKa is maintained at concentrations well below the critical micelle concentration (cmc). Thus, similar to micelle formation, the submicellar aggregates must be responsible for the increase i...

Remediation of water contamination using catalytic technologies

Remediation of contaminated ground and underground water is becoming a critical issue in Europe and worldwide. We discuss here the role of catalysis in water remediation, with reference to two specific examples of catalytic water remediation technologies: (i) the elimination of nitrate and pesticides from water contaminated as a result of agricultural practices and (ii) the conversion of methyl tert-butyl ether (MTBE) in contaminated underground water. Of particular interest is a technology based on catalytic membranes for remediation of water contaminated by nitrate, which offers various advantages with respect to conventional technologies. Using a Pd-Cu-based catalytic membrane, a reaction temperature below 15°C, a mixed 4:1 CO2:H2 feed and controlling bulk solution pH by HCl addition, it is possible to obtain a nitrate conversion higher than 80% even with ammonium ion formation below 0.5ppm, i.e. the maximum concentration allowed to meet the requirements for drinking water quality. In MTBE conversion in contaminated underground water, acid zeolites with suitable pore structures (channel structure and pore openings) such as H-ZSM-5 and H-BEA can be used as catalytic permeable reactive barriers for in situ remediation. These zeolites not only act as adsorbents for both MTBE and its reaction products, but also effectively catalyze the hydrolysis of MTBE to t-butyl alcohol (TBA) and methanol (MeOH) which then can be rapidly biodegraded by indigenous microorganisms.

An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO 3 and MgCl 2

Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range −1 to 3 and in the presence of trace concentrations of aqueous NaPO 3 and MgCl 2. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl 2 at pH 1 and 3. The presence of trace quantities of NaPO 3, which dissociates in solution to Na + and H 2PO 4 −, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg 2+ may not adsorb, whereas aqueous phosphate, present as either the anion H 2PO 4 − or the neutral species H 3PO 4 0, readily adsorbs on calcite surfaces leading to significant dissolution inhibition.

Dissolution Kinetics, Step and Surface Morphology of Magnesite (104) Surfaces in Acidic Aqueous Solution at 60 °C by Atomic Force Microscopy under Defined Hydrodynamic Conditions

Dissolution of the (104) surface of magnesite (MgCO3) was studied as a function of bulk solution pH over the range 2.0 < pH < 5.0 at 60 °C using atomic force microscopy (AFM) with well-defined hydr...

Electrochemical Study of Arsenate and Water Reduction on Iron Media Used for Arsenic Removal from Potable Water

Zerovalent iron filings have been proposed as a filter medium for removing As(III) and As(V) compounds from potable water. The removal mechanism involves complex formation of arsenite and arsenate with the iron surface and with iron oxides produced from iron corrosion. There is conflicting evidence in the literature on whether As(V) can be reduced to As(III) by iron filter media. This research uses electrochemical methods to investigate the redox reactions that occur on the surface of zerovalent iron in arsenic solutions. The effect of arsenic on the corrosion rate of zerovalent iron was investigated by analysis of Tafel diagrams for iron wire electrodes in anaerobic solutions with As(V) concentrations between 100 and 20,000 microg/L. As(V) reduction in the absence of surface oxides was investigated by analysis of chronoamperometry profiles for iron wire electrodes in solutions with As(V) concentrations ranging from 10000 to 106 microg/L. The effect of pH on As(V) reduction was investigated by analyses of chronopotentiometry profiles for iron wire electrodes at pH values of 2, 6.5, and 11. For freely corroding iron, the presence of As(III) and As(V) decreased the iron corrosion rate by a factor of 5 as compared to that in a 3 mM CaSO4 blank electrolyte solution. The decrease in corrosion rate was independent of the arsenic concentration and was due to the blocking of cathodic sites for water reduction by arsenic compounds chemisorbed to the iron surface. The chronoamperometry and chronopotentiometry experiments showed that elevated pH and increased As(III) to As(V) ratios near the iron surface decreased the thermodynamic favorability for As(V) reduction. Therefore, reduction of As(V) occurred only at potentials that were significantly below the apparent equilibrium potentials based on bulk solution pH values and As(III) to As(V) ratios. The potentials required to reduce more than 1% of the As(V) to As(III) were below those that are obtainable in freely corroding iron media. This indicates that there will be minimal or no reduction of As(V) in iron media filters under conditions relevant to potable water treatment.

Effect of Microorganisms and Microbial Metabolites on Apatite Dissolution

The dissolution rate of apatite was determined in batch reactors in organic acid solutions and in microbial cultures. Inoculum for the cultures was from biotite plus apatite crystals from a granite weathering profile in South Eastern Australia. In both the biotic and the abiotic experiments, etching of the apatite surface leads to the formation of elongated spires parallel to the c axis. Apatite dissolution rates in the inorganic, acetate, and oxalate solutions increase as pH decreases from approximately 10 -11 mol/m -2 · s -1 at initial pH 5.5 to 10 -7 mol/m -2 · s -1 at initial pH 2. Under mildly acidic to near neutral pH conditions, both oxalate and acetate increased apatite dissolution by up to an order of magnitude compared to the inorganic conditions. Acetate catalyzed the reaction by forming complexes with Ca, either in solution or at the mineral surfaces. Oxalate forms complexes with Ca as well, and can also affect reaction rates and stoichiometry by forming Ca-oxalate precipitates, thus affecting solution saturation states. In all abiotic experiments, net phosphate release to solution approaches zero even when solutions are apparently undersaturated by several orders of magnitude with respect to the solubility of an ideal fluoroapatite mineral. In the microbial experiments, two enrichment cultures increased both apatite and biotite dissolution by producing organic acids, primarily pyruvate, fermentation products, and oxalate, and by lowering bulk solution pH to between 3 and 5. However, the microorganisms were also able to increase phosphate release from apatite (by two orders of magnitude) without lowering bulk solution pH by producing pyruvate and other compounds.

Electrochemical manipulation of localised pH: application to electroanalysis

The electrochemical manipulation of the local pH at a polymer functionalised electrode has been achieved in order to enhance the electrochemical response to cationic analytes. The changes in pH have been shown to provide a method for significantly enhancing the analytical signal towards the model compounds, dopamine and p-aminophenol. The procedure was found to operate irrespective of the electrical properties of the film. The main requirement for this electroanalytical system is that the film contains acidic groups within the polymer backbone. In the carboxylic acid functionalised polypyrrole film studied here, the performance was found to be greatest when the bulk solution pH was less than the p K a of the acid groups. The mechanism attributed to the enhanced response is elucidated and the limitations of the technique are assessed.

Crevice Corrosion Stabilization and Repassivation Behavior of Alloy 625 and Alloy 22

Abstract The crevice corrosion resistance of Alloy 625 (UNS N06625) and Alloy 22 (UNS N06022) was compared based on critical potentials for stabilization and repassivation as well as crevice stabilization rates derived from critical potential data. The effects of temperature and anion composition in 5 M lithium chloride (LiCl) brine-type electrolytes were examined. Repassivation potentials measured at 95°C were similar for both alloys and were not greatly influenced by bulk electrolyte composition, pH, or accumulated anodic charge associated with crevice attack. However, a decrease in temperature increased repassivation potentials to a greater extent for Alloy 22 in comparison to Alloy 625. Crevice corrosion stabilization properties of Alloy 625 and Alloy 22 were influenced by temperature and electrolyte composition but not bulk solution pH ranging from 2.75 to 7.75. Crevice stabilization occurred at more active critical potentials in 5 M LiCl electrolytes with a molar ratio of chloride ions to total oxya...

Molecular machines based on metal ion translocation.

Transition metal ions can be moved reversibly between the two coordinatively unequivalent compartments A and B of a ditopic ligand, using as an input the variation of a bulk solution parameter, either pH or redox potential. In a redox-driven translocation, the metal moves reversibly from A to B on cycling between two consecutive oxidation states (e.g., Cu(II)/Cu(I); Fe(III)/Fe(II)) by means of auxiliary oxidation and reduction reactions. In a pH-driven process, one compartment displays also acid-base properties (AH(n)() left arrow over right arrow A(n)(-) + nH(+)), and the M(n)(+) ion is translocated between B and A(n)(-) through consecutive addition of base and acid.

Effect of a natural contaminant on foam fractionation of bromelain.

Foam fractionation is a simple, inexpensive method for separating and purifying proteins. Typically, a dilute bromelain solution with a pH ranging from 2.0 to 7.0 foams very well when bubbles are introduced into a foam fractionation column. It was observed, however, that the dilute enzyme solution only foamed between approximately pH 2.0 and 3.0 when the inner wall of the fractionation column was coated with a natural contaminant (okra residue). We studied the separation ratio and the protein mass recovery to explore the effect of a natural antifoaming agent on the foam fractionation of a dilute bromelain solution. The control variables used in this process were the initial bulk solution pH, which ranged from 2.0 to 7.0, and the superficial air velocity, which varied between 1.7 and 6.2 cm/s.

Adsorption of Sodium Dodecyl Sulfate to Colloidal Titanium Dioxide: An Electrophoretic Fingerprinting Investigation

The adsorption of sodium dodecyl sulfate to colloidal titanium dioxide was investigated using the electrophoretic fingerprinting approach. An electrophoretic fingerprint is a contour diagram of the observed electrophoretic mobility as a function of the bulk solution pH and pλ, the log of the bulk solution conductivity. Surfactant adsorption was observed to be strong under acidic conditions, as illustrated in the dramatic changes in the electrophoretic fingerprints. Electrokinetic data were compared with adsorption isotherm data obtained by a depletion method and good qualitative agreement was found. The observed pH changes associated with surfactant adsorption suggested ligand exchange as a possible mechanism of adsorption. Electrophoretic fingerprinting was shown to be a powerful means of examining surfactant adsorption to colloidal particles.

Preparation and Characterization of Hyaluronate-Hydroxyethyl Acrylate Blend Hydrogel for Controlled Release Device.

Hyaluronate-hydroxyethyl acrylate blend hydrogels were investigated as matrices for controlled release devices. Glycidyl methacrylate (GMA) derivatized HA (GMA-HA) was synthesized by coupling of GMA to HA in the presence of a suitable catalyst. These hydrogels were prepared by a free radical copolymerization of GMA-HA and hydroxyethyl acrylate. The water content of these hydrogels at equilibrium swelling in water (Ww) was 0.978+/-0.0073 (n=18); however, these hydrogel was mechanically tough and could be used as disk shape. The hydrogels swelling were found to depend on ionic strength and pH. The dried hydrogels quickly regained their original condition in water, and they swelled to more than 90% of its initial water contents after 30 min. This swelling-deswelling behavior was reproducible. The release of chlorpromazine HCl as a model cationic drug from the gels was suppressed significantly in water. The release increased with increasing the ionic strength and decreasing pH of bulk solutions.

An experimental study of dolomite dissolution rates as a function of pH from −0.5 to 5 and temperature from 25 to 80°C

Steady-state dissolution rates of Haute Vallée de l'Aude dolomite were measured in mixed-flow reactors using rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range −0.39 to 4.44, and at temperatures of 25, 50 and 80°C. Dissolution rates were found to depend on disk rotation speed at all temperatures and pH. Interpretation of these rates using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disk. J. Chem. Soc., 33, 37–56] yields steady-state dissolution rates as a function of the solution pH adjacent to dolomite surface (pH surf). Rates at all temperatures and 1<pH surf<5 are found to be consistent with r= k 1 a n 1 H +,surf , where r refers to surface area normalized steady-state dissolution rates, k 1 stands for a rate constant, and a H +,surf designates hydrogen ion activity in the solution adjacent to the dolomite surface, and n 1 denotes a constant equal to 0.63, 0.73 and 0.80±0.05 at 25, 50 and 80°C, respectively. Rates tend towards pH independence at more acidic conditions suggesting the saturation of dolomite surfaces with rate controlling protonated species. Additional experiments performed to assess the effect of the presence of aqueous Ca 2+ and Mg 2+ demonstrate that these species have little to no effect on dolomite dissolution rates at pH=2 and 50°C.

Preserving the activity of cellulase in a batch foam fractionation process.

Foam fractionation is one of the low operating-cost techniques for removing proteins from a dilute solution. The initial bulk solution pH and air superficial velocity play an important role in the foam-fractionation process. Denaturation of proteins (enzymes) can occur, however, during the foam-fractionation process from the shear forces resulting from bursting air bubbles. At the extreme bulk solution pHs (lower than 3.0 and higher than 10.0), the enzymatic activity of cellulase in the foamate phase drops significantly. Within these two pH bounds an increase in the air superficial velocity, V0, and a decrease in the bulk solution pH leads to a decrease in the separation ratio (SR), defined as the ratio of the protein concentration in the foamate to the protein concentration in the residue. On the other hand, an increase in V0 provides a higher foamate-protein recovery. The process efficiency is defined as the product of foamate-protein recovery times the SR times the cellulase activity. The optimal operating condition of the cellulase foam-fractionation process is taken into account at the maximum value of the process efficiency. In this study, that optimal condition is at an air superficial velocity of 32 cm/min and a bulk-solution pH of 10.0. At this condition, the recovered foamate is about 80% of the original protein mass, the SR is about 12, and the enzymatic activity is about 60% of the original cellulase activity.

Partitioning invertase between a dilute water solution and generated droplets.

Water droplets or mist occur naturally in the air at seashores. These water droplets carry inorganic and organic substances from the sea to the land via the air, creating fertile land in sandy coastal areas (1). The same phenomenon occurs in an air-fluidized bed bioreactor (2). In an air-fluidized bed reactor, proteins can be transferred from the bioreactor semisolid bulk phase to an enriched droplet phase. This protein transfer process (droplet fractionation) can be experimentally simulated by shaking a separatory funnel containing a dilute solution of a given protein, which can be an enzyme like invertase. The created droplets become richer in invertase (protein) than that of the original dilute solution. The droplets can then be coalesced by trapping them and recovering the concentrated protein in the new liquid phase. Typically, in such a droplet fractionation process a collected enzyme can be degraded in its ability to catalyze a chemical reaction. In this article, we explore whether the initial solution pH control variable can be adjusted to minimize the decrease of enzyme activity in this process. The protein droplet recovery problem is one in which the recovered amount of desired protein (enzyme) in the droplet is maximized, subject to the minimization of the enzyme activity loss. The partition coefficient, which is the ratio between the protein concentration in the droplets and the residual solution, is maximized at approx 4.8 and occurs at pH 3.0. Here, the partition coefficient for invertase decreases as the initial solution pH increases, between pH 3.0 and 8.0. Interestingly, the initial solution surface tension seems to be inversely proportional to the partition coefficient. The partition coefficient reaches a maximum value at a surface tension value of approx 63 mN/m at pH 3.0. The enzymatic activity of the initial, the residual, and the droplet solutions all decrease as the bulk solution pH increases. A decrease of enzymatic activity was observed in the residual bulk solution when compared with that in the initial bulk solution at all pH levels. Also, up to 90% of the invertase activity was lost in the droplets when compared to the initial bulk solution.

Adsorption of cationic surfactants on hydrophilic silica: effects of surface energetic heterogeneity

The thermodynamics of adsorption of cationic surfactants on a hydrophilic silica surface is discussed. The adsorption isotherms of these surfactants on negatively charged silica surfaces exhibit a two-step character, as a rule. The heights of these steps and the concentration at which the second step begins depend strongly on the surface charge determined by the pH and indifferent electrolyte concentration in the equilibrium bulk solution. In our previous publication we have developed a theoretical description based on the model of the adsorbed phase being a mixture of monolayered and bilayered aggregates of different sizes. Only the “excluded area” interactions were taken into account. The potential energy per molecule was assumed to decrease linearly with the aggregate size. The theoretical expressions obtained for adsorption isotherms and heats of adsorption were next fitted to experimental data of cationic surfactants adsorption on precipitated silica. Although our model was able to reproduce well the two steps observed on the adsorption isotherms of these surfactants, it failed to predict the behavior of isosteric heats of adsorption at low surface coverages. In this paper our model is extended by taking into account the effects of surface heterogeneity on single monomer adsorption. This leads to a much better description of entropic effects accompanying surfactant adsorption in the low coverage region. It is therefore concluded that individual adsorption of surfactant ions, which is determined primarily by the specific solid–surfactant interactions, is strongly affected by the surface heterogeneity effects.

Membrane Surface-Charge Titration Probed by Gramicidin A Channel Conductance

We manipulate lipid bilayer surface charge and gauge its influence on gramicidin A channel conductance by two strategies: titration of the lipid charge through bulk solution pH and dilution of a charged lipid by neutral. Using diphytanoyl phosphatidylserine (PS) bilayers with CsCl aqueous solutions, we show that the effects of lipid charge titration on channel conductance are masked 1) by conductance saturation with Cs + ions in the neutral pH range and 2) by increased proton concentration when the bathing solution pH is less than 3. A smeared charge model permits us to separate different contributions to the channel conductance and to introduce a new method for “bilayer pKa” determination. We use the Gouy-Chapman expression for the charged surface potential to obtain equilibria of protons and cations with lipid charges. To calculate cation concentration at the channel mouth, we compare different models for the ion distribution, exact and linearized forms of the planar Poisson-Boltzmann equation, as well as the construction of a “Gibbs dividing surface” between salt bath and charged membrane. All approximations yield the intrinsic p K a in of PS lipid in 0.1 M CsCl to be in the range 2.5–3.0. By diluting PS surface charge at a fixed pH with admixed neutral diphytanoyl phosphatidylcholine (PC), we obtain a conductance decrease in magnitude greater than expected from the electrostatic model. This observation is in accord with the different conductance saturation values for PS and PC lipids reported earlier (Apell et al., 1979, Biochim. Biophys. Acta. 552:369–378) and verified in the present work for solvent-free membranes. In addition to electrostatic effects of surface charge, gramicidin A channel conductance is also influenced by lipid-dependent structural factors.