Eric M. V. Hoek

Limiting flux prediction remains a major challenge in protein ultrafiltration (UF) due to concentration polarization (CP), aggregation, and gel layer formation under practical operating conditions. In this study, crossflow ultrafiltration of lactalbumin solutions was systematically investigated at three ionic strengths (1 to 100 mM NaCl), five crossflow rates, and applied pressures up to 140 psi to evaluate the coupled effects of shear rate, aggregation and gel formation on permeate flux behavior. Dynamic light scattering measurements confirmed ionic-strength-dependent protein aggregation, with particle diameters increasing from 3 to 9 nm as electrostatic stabilization decreased. Channel-averaged flux data were analyzed using an analytical convection-diffusion model combined with both classical Brownian and osmotic-pressure-dependent collective diffusion coefficients. Brownian diffusion consistently underpredicted experimental limiting fluxes across all operating conditions. Incorporation of a collective diffusion coefficient significantly improved flux predictions in CP-dominated regimes, while diffusion coefficients derived from limiting-flux analysis provided the most accurate predictions in gel layer formation regimes. Allowing the membrane wall volume fraction to vary, rather than assuming a fixed maximum packing value, further enhanced model accuracy and indicated compressible cake behavior. Results demonstrate that ionic strength-induced aggregation and shear-dependent transport mechanisms strongly influence limiting flux behavior. The proposed modeling framework provides improved predictive capability for protein UF and offers guidance for optimizing operating conditions to mitigate fouling and maximize process performance.

Ultra-high-pressure reverse osmosis (UHPRO) delivers higher permeate flux and supports zero- and minimal-liquid-discharge desalination, but suffers from severe concentration polarization (CP) at the membrane surfaces. Conventional static spacers cannot generate the fluid mixing needed to transport the accumulated solute away from the membrane surfaces. Previous studies that used active approaches for CP mitigation imposed unsteadiness on the flow through pulsatile inlets, vibrating modules, or wall slip velocities. In this study, the spacers are fitted with rigid fins that oscillate across the feed channel. Time-resolved numerical simulations show that these spacers considerably reduce CP and enhance mass transfer.A parametric study that considers variations in fin length, oscillation amplitude, and cross-flow velocity was performed. At low fin length and amplitude, the fin performance remained essentially unchanged with respect to a static, no-fin baseline. At moderate amplitude, the Sherwood number increased more than fourfold. At the highest amplitude, the CP modulus was reduced to ∼ 1.06–1.13. These gains carry minimal steady hydraulic penalty: the cycle-averaged feed-channel pressure drop remains small and, for the largest amplitude fins, turns negative as the fins add energy to the feed channel flow. The limiting cost is instead the mechanical power supplied to the spacers, whose specific energy of actuation at 1 kHz is roughly a quarter to a third of the UHPRO desalination energy. Since this power scales steeply with frequency while the vortex shedding that drives the mixing does not, a lower operating frequency is expected to reduce the cost substantially.

A spinel phase of MnO2 (“λMnO2”) was synthesized and characterized as a sorbent for cobalt (Co) and 5 other metals common to Co-rich waste streams (Ba, Ca, Mg, Ni, and Sr). Sorption was characterized in both single and multi-metal solutions to understand potential applications for metal recovery under realistic conditions. The particles had a maximum Co uptake of 3.74 × 10−5 mol/g with 10 g/L particle loading and demonstrated significant selectiveness toward Co (separation factors between 2.5 and 190). Selectivity could be in part attributed to a redox exchange between Co and Mn, which is supported by an increase of Mn at a nearly 1:1 mol ratio to Co removed in metal solutions with initial concentrations above 0.0005 M. The sorption behavior toward Co was better described using the Freundlich isotherm (R2 = 0.973) than that of the Langmuir isotherm, indicating sorption site heterogeneity. The rate of sorption was generally well described with a first-order kinetic model (R2 for Co between 0.73 and 0.99). To study the regenerability of the λMnO2 sorbent, the particles were cycled between metal and nitric acid solutions. While Co sorption remained nearly constant over the 7 cycles tested, the uptake of other metals increased with each cycle, reducing the Co selectivity. Additionally, less than 70% of Co was recovered after each cycle. This, in combination with data indicating Mn/Co redox exchange, indicates that the λMnO2 particles are converted to a Co-enriched MnO2 spinel, which does not retain Co selectivity following the initial exchange. The irreversibility of the Co−Mn exchange under the conditions studied suggests that λMnO2 is not suitable for use as a regenerable Co-selective sorbent without potentially intensive metallurgical or hydrothermal regeneration. Importantly, because Co-enriched MnO2 spinel materials are used in electrodes and capacitors, λMnO2 may offer a pathway to generate valuable Co-enriched MnO2 spinel materials from complex metal waste streams.

This study investigates the chlorination resistance and performance of thin-film composite (TFC) membranes fabricated from a diverse set of amine monomers, including aromatic and aliphatic compounds. Using interfacial polymerization on polysulfone supports, membranes were synthesized from monomers such as m-phenylenediamine (MPD), cyclohexane-1,3-diamine (CHD), and others, followed by controlled exposure to chlorine (up to 12,000 ppmh). Membrane performance was evaluated via water permeability and salt rejection (NaCl and Na2SO4), revealing that aliphatic membranes (e.g., CHD, XCH) maintained performance with minimal degradation, while aromatic membranes (e.g., MPD, DAP, XDA) experienced increased water permeability and decreased salt rejection due to ring chlorination. XPS analysis confirmed chemical changes, particularly in oxygen-containing moieties, associated with ring oxidation rather than amide bond cleavage. A new mechanism is proposed in which ring chlorination and subsequent ring oxidation–not N-chlorination or Orton rearrangement–are the dominant degradation pathways under typical passive chlorination conditions (pH ~7). These findings suggest that chlorine-resistant membrane designs should prioritize reducing aromatic ring electron density or replacing aromatic structures with aliphatic, sp3-hybridized motifs to inhibit ring oxidation. The study offers a deeper mechanistic understanding of chlorine-induced degradation in polyamide and polyester membranes, challenging traditional assumptions and informing future TFC membrane design.

Electrodialysis with ion exchange (IEX) membranes offers a sustainable pathway for recovering critical minerals such as lithium from brines. However, existing membranes cannot efficiently separate lithium ions (Li⁺) from sodium ions (Na⁺), exhibiting a low Li⁺/Na⁺ selectivity. Here, we report a LiMn₂O₄-incorporated mixed matrix (MM) IEX membrane based on a quaternized poly(oxy-2,6-dimethyl-1,4-phenylene) matrix that achieves effectively near-infinite Li⁺/Na⁺ selectivity (Na⁺ was below the instrument detection limit) under electrodialysis by combining Donnan exclusion and size exclusion for competing cations with a percolating Li⁺-conducting network. Structural analyses of the MM membranes confirm a homogeneous LiMn₂O₄ distribution in the polymer matrix, spinel-phase retention, and superior thermal stability. Higher annealing temperatures reduced membrane water uptake/swelling and improved Li⁺/Na⁺ selectivity in diffusion dialysis; although in high concentration Na⁺ solutions with low(er) Li⁺ concentrations, the flux of Na⁺ exceeded that of Li⁺. In electrodialysis mode, there was no Na⁺ flux, while the Li⁺ flux reached as high as 46 ± 3 mmol·h^-1·m^-2 for Li⁺/Na⁺ ratios of 1 and 4 ± 3 mmol·h^-1·m^-2 for a Li⁺/Na⁺ ratio of 80. Hence, in cases of electrically driven ion transport, the MM membranes produced nearly infinite Li⁺ selectivity over Na⁺. No Mn leaching or membrane degradation was observed after repeated electrodialysis operating cycles, as confirmed by ICP and elemental mapping, suggesting good membrane stability. Given the global push toward electrification and the surging demand for lithium, these nearly infinitely Li-selective MM membranes address a critical need for a cost-effective, energy-efficient, and low-water-use method to separate Li⁺ from Na⁺-rich brines.

Conventional thin-film composite (TFC) reverse osmosis (RO) membranes experience irreversible performance loss at high temperatures, restricting their use in industries with high-temperature streams, including oil and gas, pharmaceuticals, electronics, power generation, food production, and hybrid desalination plants. However, the mechanisms driving the performance decline of TFC membranes at high temperatures remain poorly understood. Herein, we combine controlled experiments, molecular dynamics simulations, and micromechanical modeling to elucidate TFC failure mechanisms and to evaluate thermally resilient thin-film cross-linked (TFX) composite membrane. Upon exposure to elevated temperatures (>60°C), salt rejection of TFC dropped from ~99 to <90%, with irreversible structural damage in the polysulfone layer, confirmed by scanning electron microscopy. In contrast, the TFX membrane maintained ~99% salt rejection and showed no signs of physical degradation up to 80°C. Our combined analyses revealed that TFC membrane failure arises from irreversible pore expansion in the thermoplastic polysulfone support, leading to polyamide film rupture and delamination. TFX membranes resist thermal deformation, enabling ultrahigh-temperature RO desalination and water reuse.