Mark M. Clark

Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z

The permeability and solute transport characteristics of amphiphilic triblock-polymer vesicles containing the bacterial water-channel protein Aquaporin Z (AqpZ) were investigated. The vesicles were made of a block copolymer with symmetric poly-(2-methyloxazoline)-poly-(dimethylsiloxane)-poly-(2-methyloxazoline) (PMOXA15-PDMS110-PMOXA15) repeat units. Light-scattering measurements on pure polymer vesicles subject to an outwardly directed salt gradient in a stopped-flow apparatus indicated that the polymer vesicles were highly impermeable. However, a large enhancement in water productivity (permeability per unit driving force) of up to ≈800 times that of pure polymer was observed when AqpZ was incorporated. The activation energy (Ea) of water transport for the protein-polymer vesicles (3.4 kcal/mol) corresponded to that reported for water-channel-mediated water transport in lipid membranes. The solute reflection coefficients of glucose, glycerol, salt, and urea were also calculated, and indicated that these solutes are completely rejected. The productivity of AqpZ-incorporated polymer membranes was at least an order of magnitude larger than values for existing salt-rejecting polymeric membranes. The approach followed here may lead to more productive and sustainable water treatment membranes, whereas the variable levels of permeability obtained with different concentrations of AqpZ may provide a key property for drug delivery applications.

Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes

Abstract The interaction between Suwannee River humic and fulvic acids and ultrafiltration membranes is characterized by direct adsorption measurements. The coated membranes are also characterized by the following parameters: hydrophobicity determined by contact angle measurements, pore zeta potential computed from streaming potential measurements, and surface chemical composition obtained from X-ray photoelectron spectroscopy (XPS). The results show that the adsorption capacity is smaller for fulvic acid than for humic acid. Fulvic acid competes with calcium phosphate compounds for adsorption sites on the membranes. Low pH and, in some cases, high calcium concentration increase the adsorption of humic substances on the membranes. The adsorption capacity decreases with decreasing (more negative) pore zeta potential and with increasing hydrophilicity. Upon adsorption of humic substances, the membranes become more hydrophilic, and the apparent pore charge becomes less negative. Our results support a model of humic substances adsorption in which pore adsorption sites are preferentially filled prior to other sites on the membrane surface.

The effect of CA membrane properties on adsorptive fouling by humic acid

Cellulose acetate membranes with varied charge, hydrophobicity, porosity, and pore size have been developed by annealing, hydrolysis and oxidation of a basic cellulose acetate membrane. The effects of these modifications were characterized by poly(ethylene glycol) retention, contact angle and streaming potential measurements, and atomic force microscopy. The behavior of the membranes during humic acid adsorption experiments has also been studied. The results show that humic acid adsorption occurs both inside the pore and on the membrane surface. Experiments at different pH show the importance of solution properties on humic acid adsorption through modification of membrane and humic acid charge. Although many membrane characteristics are modified by hydrolysis and oxidation, neither treatment prevented humic acid adsorption on the CA membranes. The most effective surface treatment was with an anionic polymer, which significantly reduced adsorption of humic acid.

Use of ATR/FTIR spectrometry to study fouling of microfiltration membranes by natural waters

Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectrometry provided insight into the chemical nature of deposits on polypropylene microfiltration membranes after filtration of two North American surface waters. The spectra of the foulants were easy to distinguish from the spectra of the membrane material. The results did not show strong evidence for the presence of carboxylic acid, carboxylate, phenolic, or hydroxyl functional groups in the foulants, although these functional groups are common in natural waters. ATR/FTIR also indicated the presence of inorganic foulants; the ratio of inorganic to organic foulants varied between the two water sources. The spectra of the foulants were significantly more distinct than spectra of other natural waters, suggesting that relatively few components present in Medina River and Beaver Lake surface waters may adsorb to this membrane material. ATR/FTIR appears to be a valuable tool for studying membrane fouling by natural waters.

The effects of pH and calcium on the diffusion coefficient of humic acid

Abstract Humic acid is a major component of natural organic matter in surface water and can cause serious fouling problems in membrane filtration processes for drinking water treatment. The transport of humic acid to the membrane surface and within membrane pores is related to its diffusivity. Thus, the diffusion coefficient of humic acid is an important mass transport parameter. Clark and Lucas [J. Membr. Sci. 143 (1998) 13–25] studied the diffusion and partitioning of humic acid into a porous ultrafiltration membrane and developed a model to predict how humic acid diffusivity changes under different pH and calcium concentration conditions. In this work, the diffusion coefficient of humic acid was directly measured and compared to the predictions of the Clark and Lucas model. The experiments were conducted in a two-chamber diffusion cell separated by a track-etched membrane. The results show that the diffusivity of humic acid increases with decreasing pH and increasing calcium concentration, which can be explained by the compaction of humic acid molecules at low pH and high ionic strength. The experimental measurements strongly support the predictions of the Clark and Lucas model.

Floc restructuring in varied turbulent mixing

Abstract Floc structure was examined after the following mixing program: (i) 1 min of intense initial mixing of latex spheres with an aluminum coagulant (characteristic velocity gradient, G i = 1500s −1 ), (ii) slow flocculation for 30 min at G f = 35s −1 , (iii) breakup for 1 min in the range G b = 150–1800s −1 , (iv) reflocculation for 30 min at G r = 35s −1 , and (v) quiescent settling (60 min). Depending upon the level of energy input during the short breakup step, significant alterations were found in the final floc characteristics. The density and fractal dimension of all floc subjected to the above mixing steps were greater than those for the standard mixing conditions, which did not include the breakup and reflocculation steps. However, floc properties did not vary monotonically with increasing breakup mixing intensity. For example, floc density passed through several minima and maxima over the range of G b examined. These and other measurements of the floc size distribution following breakup are shown to support the multilevel floc model.

Drop breakup in a turbulent flow-I. Conceptual and modeling considerations

Abstract This basic conceptual study of dilute second-phase drop breakup in turbulent mixing vessels includes examination of (1) local viscous effects in the breakup of droplets smaller than the Kolmogoroff microscale, and (2) inertial effects in the breakup of droplets larger than the Kolmogoroff microscale. Particular emphasis is placed on the evaluation of local and spatial variability in the disruptive forces, and the local time and space scales of interest. For the second mechanism, a two-dimensional linear drop oscillation model is developed. Simulations using the model suggest that the critical Weber number may be a function of drop size, interfacial tension, viscosity, and the magnitude and duration of the disruptive force.

Particle formation and growth in dilute aluminum(III) solutions. Characterization of particle size distributions at pH 5.5

Abstract Particle formation and growth over the 1–40 μm size range in dilute aluminum solutions (approx. 2 × 10 −4 M) have been studied using an electronic particle counter. Sulfate, fulvate and hydroxide ion accelerate the rate of particle formation and changes of the particle size distribution over time. Increasing ionic strength (inert electrolyte) produces similar but less dramatic effects. Combinations of sulfate and fulvic acid or sulfate and inert electrolyte further accelerate the rate of particle formation. Aluminum chloride solutions at moderate ionic strength are devoid of supramicron particles after several days. A conceptual pathway model is developed which suggests that two different solids are formed when aluminum is added to fulvic acid solutions: an aluminum-fulvate precipitate and Al(OH) 2 (s). The first solid dominates in fulvic acid solutions at pH ∼5.5.

A numerical model of steady-state permeate flux during cross-flow ultrafiltration

The mass transport mechanisms during cross-flow ultrafiltration (UF) are mathematically expressed using the two-dimensional convective diffusion equation where the axial diffusion term is neglected for an axial Peclet number much greater than the transverse Peclet number. A numerical scheme is presented to solve the steady-state two-dimensional convective diffusion equation for the case of known uniform permeate flux. However, in the actual cross-flow UF process, the permeate flux along the axial direction is unknown and usually decreases with axial distance. Therefore, an iterative algorithm is developed to predict the steady-state permeate flux based on the assumption that the concentration at membrane surface cannot exceed a certain limiting value. Using the numerical model with an effective diffusion coefficient, which is considered to be the sum of molecular diffusion and shear-induced hydrodynamic diffusion coefficients, the effects of particle size, feed concentration, and axial velocity on the steady-state permeate flux were investigated.

A dynamic model for predicting fouling effects during the ultrafiltration of a groundwater

The effects of fouling on the cross-flow ultrafiltration of a natural groundwater are modeled in both the constant pressure and constant flux operational modes. Semi-empirical equations are derived which accurately predict the various flows and pressures in an ultrafiltration system when used to treat both a raw groundwater and a groundwater pretreated with powdered activated carbon. The effects of both reversible and irreversible fouling are modeled, and the deviation between the predicted flows and pressures and the experimental results generally remained below 10%. The overall efficiency of water production is compared for the constant flux and constant pressure modes, and it is found that neither mode provides a higher efficiency. A method is also presented for the optimization of backflushing frequency in constant pressure ultrafiltration.