Kinetic Separations Research Articles

Performance evaluation of bimetallic ion exchanged clinoptilolite for potential equilibrium and kinetic adsorption separations of CO2 from CO2/CH4 mixture

The bimetallic ion exchanged clinoptilolites (M1xM2y-CPs, M1 = Li+, Na+ or Cs+, M2 = Li+ or Cs+, x, and y = the exchange degree of the first and second cation) were prepared. Their physic-chemical parameters were characterized using various methods. The results demonstrated that the relative crystallinity of M1xM2y-CPs gradually decreased with the increase of the secondary cationic introduction. Meanwhile, M1xM2y-CPs presented the surface fractal features, particularly, the values of M1xM2y-CPs were higher than that of single ion exchanged CPs, suggesting that the surface of M1xM2y-CPs was rougher and more disordered. The effects of the type, size, and distributions of the used various cations on the gas adsorption and separation properties were evaluated via their equilibrium adsorbed performances of CO2 and CH4 demonstrate. The results indicated that the CO2 uptake of NaxLiy-CP increased with the increase of Li+ exchange degree, in which, Na67.95Li17.15-CP presented the maximum adsorption capacity of 2.45 mmol/g for CO2. While, the CO2/CH4 selective factors of NaxCsy-CP were around 10.82–12.75 at 273 K and 9.92–12.44 at 298 K, much higher than that of other bimetallic ion exchanged samples. The breakthrough experiments displayed a longer breakthrough time and a stronger CO2 absorption capacity. The cycling tests indicated an excellent CO2 adsorption stability and reversibility. Furthermore, the kinetics adsorption of pseudo-first-order and intraparticle model were used to investigate their adsorption mechanism. Finally, the CO2 adsorption mechanism between the adsorption amount and the average total energy of the CO2 adsorption system was preliminary elucidated using Material Studio simulation software.

Tracking Molecular Transport Across Oil/Aqueous Interfaces: Insight into “Antagonistic” Binding in Solvent Extraction

Liquid/liquid (L/L) interfaces play a key, yet poorly understood, role in a range of complex chemical phenomena where time-evolving interfacial structures and transient supramolecular assemblies act as gatekeepers to function. Here, we employ surface-specific vibrational sum frequency generation combined with neutron and X-ray scattering methods to track the transport of dioctyl phosphoric acid (DOP) and di-(2-ethylhexyl) phosphoric acid (DEHPA) ligands used in solvent extraction at buried oil/aqueous interfaces away from equilibrium. Our results show evidence for a dynamic interfacial restructuring at low ligand concentrations in contrast to expectation. These time-varying interfaces arise from the transport of sparingly soluble interfacial ligands into the neighboring aqueous phase. These results support a proposed "antagonistic" role of ligand complexation in the aqueous phase that could serve as a holdback mechanism in kinetic liquid extractions. These findings provide new insights into interfacially controlled chemical transport at L/L interfaces and how these interfaces vary chemically, structurally, and temporally in a concentration-dependent manner and present potential avenues to design selective kinetic separations.

A study of kinetic air separation at low temperatures: Oxygen, nitrogen, and argon in carbon molecular sieve

Carbon molecular sieves (CMS) are among the most promising materials for industrial gas separations, including their utilization for the kinetical removal of O2 from N2 and Ar in commercial pressure swing adsorption processes. However, little is known about the characteristics of these kinetics processes at lower temperatures. In this article, a kinetic study was carried out at three temperatures (-25, 0, and 25 °C) for pure O2, N2, and Ar using a commercially available Shirasagi MSC-3K-172. A pressure-swing frequency response technique was applied to determine the dominating mass transfer resistances, as well as corresponding temperature dependences, revealing that a combined resistance model of surface barrier and micropore diffusion best describes the behavior of all three gases. In addition, this MSC-3K-172 exhibited higher kinetic rates but lower kinetic selectivities as compared to Shirasagi MSC-3R-162 and −170, which showed only surface barrier resistances for N2 and Ar. When the temperature was reduced from 25 °C to −25 °C, the equilibrium capacities for three gases increase similarly, but the kinetic rates decrease to varying extents, resulting in an unexpected increase in kinetic selectivity – the value near doubled (from 17 to 33) for O2/N2 and more than tripled (from 24 to 88) for O2/Ar. The observed kinetic selectivities were found to correlate well with the difference of the activation energies between the gas pairs. These activation energies are 36, 29, and 21 kJ/mol for Ar, N2, and O2, respectively, greater than the isosteric heats of adsorption, which follow the reverse order. Together, these results indicate a delicate balance among kinetic selectivities, controlling mass transfer resistances and rate parameters, and equilibrium capacities, and can be utilized for the design of kinetic separations with CMSs, especially when extended to sub-ambient temperatures.

Adsorption studies of a multi-metal system within acetate media, with a view to sustainable phosphate recovery from sewage sludge

Phosphate shortages and the ensuing pressures on food security have led to an interest in processed sewage sludge as a substitute for commercial fertilisers. The presence of heavy metals in this nutrient source causes concerns around environmental release and pollution. This work builds towards a resin-in-pulp sludge detoxification process. It showcases the kinetic and thermodynamic adsorption capabilities of the ion-exchange resins C107E (carboxylic acid functionality), MTS9301 (iminodiacetic acid) and TP214 (thiourea), with respect to Cu(II), Fe(II), Pb(II) and Zn(II), within a simulated sewage sludge weak acid (acetate) leachate. The isotherms produced in this complex system were quite different to those generated when single metals were investigated in isolation, with desorption of lower affinity species clearly observed at higher equilibrium concentration values. Mixed-metal isotherm data were fitted to common two-parameter isotherm models and also a novel modified Langmuir model, which better accounted for the effects of desorption and competition. Kinetic data were also fit to common two-parameter models; results suggesting the system was likely film diffusion-controlled and followed pseudo-2nd-order kinetics. C107E displayed rapid adsorption of lead (t1/2 = 26 ± 3min), and significant uptake of all metals. MTS9301 showed high affinity for copper ions, with concurrent desorption of all the other metals, and also displayed the fastest kinetics (t1/2 = 14.1 ± 0.9, 130 ± 20, 25 ± 5 and 49 ± 6 min for copper, iron(II), lead and zinc, respectively). C107E and MTS9301 showed far slower adsorption for iron(II) than the other three metals, which invited the possibility of kinetic separations. TP214 had reasonable effectiveness in removal of copper, but poor affinity for all other metals. The greatest difficulty in modelling the multi-metal system was the two-stage trends observed in equilibrium experiments, as metal-proton exchanges become metal-metal exchanges. While not having the highest capacity, MTS9301 was recommended as the most appropriate resin for rapid and efficient removal of Cu, Pb and Zn from the acetate medium.

Coordination Chemistry-Driven Approaches to Rare Earth Element Separations.

Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth's crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent-solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals.The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2-tBuNO)C6H4CH2}3N]3- (TriNOx3-) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SFNd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations.Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx3- ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SFEu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-defined speciation to afford purified RE materials. Recently, we determined that magnetic properties can be used to enhance both thermodynamic and kinetic RE separations processes to give an approximately 100% boost for pairs of paramagnetic/diamagnetic REs. These results have shown that both thermodynamic and kinetic RE separations were efficient for different selected RE binary pairs through coordination chemistry. The focus of this Account will detail the differences that are observed for RE separations when promoted by thermodynamic or kinetic factors. Overall, the development of rationally adjusted speciation of REs provides a basis for future industrial separations processes for technologies applied to ECEs derived from wind turbines, batteries for electric vehicles, and LEDs.

Molecular Simulations of CH4 and CO2 Diffusion in Rigid Nanoporous Amorphous Materials

Molecular diffusion in nanoporous materials is important in determining the rate of equilibration of various adsorption processes and plays a pivotal role in kinetic separations and membrane-based separations. Because generating realistic structures of amorphous nanoporous materials is difficult, far less is known about diffusion in amorphous nanoporous materials than in their crystalline counterparts. Here, we present molecular dynamics simulations assessing the room-temperature self-diffusion of CH4 and CO2 in a wide range of rigid amorphous nanoporous materials, including porous carbons, kerogens, polymers of intrinsic microporosity, and hyper-cross-linked polymers. Our results are the largest collection of molecular diffusivities in amorphous nanoporous materials to date. In each material, the diffusivity increases with the adsorbate concentration at low and moderate adsorbate concentrations, reaching a maximum before decreasing due to steric effects at higher concentrations. The observed diffusivities are much slower than that would be expected based on standard descriptions of Knudsen diffusivity. We show that the observed diffusivities are not correlated in a simple way with scalar descriptors of the pore structures such as the pore limiting diameter.

Configurations and Sensory Properties of the Stereoisomers of 2,6-Dimethyl-4-propyl-1,3-oxathiane and 2,4-Dimethyl-6-propyl-1,3-oxathiane.

The heterocyclic compounds 2,6-dimethyl-4-propyl-1,3-oxathiane 1 and 2,4-dimethyl-6-propyl-1,3-oxathiane 2 were obtained by condensing 4-mercapto-2-heptanol and 2-mercapto-4-heptanol, respectively, with acetaldehyde. For both, separation of the eight stereoisomers was achieved via capillary gas chromatography using heptakis(diethyl-tert-butyldimethylsilyl)-β-cyclodextrin as the chiral stationary phase. Their configurations were assigned by combinations of enzyme-catalyzed kinetic resolutions, HPLC separations, and assessments of NMR data. The odor thresholds and odor qualities of the stereoisomers were determined by capillary gas chromatography/olfactometry. The odor thresholds of the stereoisomers of 2 were generally higher than those of 1. For both oxathianes, the stereoisomers in which all substituents are in equatorial positions showed the highest odor thresholds. Most of the stereoisomers of 1 exhibited pleasant flowery, fruity, or sweet nuances; the stereoisomers of 2 were mainly characterized by descriptors, such as broth, mushroom, or pungent. The data demonstrate the impact of the positions of substituents and their spatial orientations on the sensory properties of 1,3-oxathianes.

Advanced pore characterization and adsorption of light gases over aerogel-derived activated carbon

Abstract Activated carbon produced by etching with CO2 is an attractive adsorbent material because of its ultrahigh surface area above 1000 m2/g and substantial pore volume. This being stated, because etched carbon was only reported recently, many of its properties, such as its textural properties, high-pressure adsorption capacities towards CO2, N2, H2, CO, and CH4, and dynamic performance, have never been thoroughly investigated. Accordingly, this study explored these characteristics for the first time. The textural and adsorptive properties were also assessed for zeolite 5A, a metal-organic framework (Ni MOF-74), and activated carbon provided by Ingevity corporation to allow for comparison to established benchmarks. The textural properties were assessed by a combination of N2 physisorption at 77 K and CO2 adsorption at 273 K using a combination of t-plot, Dubinin–Radushkevich, 2D-NLDFT, and Horvath–Kawazoe models. Therein, it was determined that the etched carbon had a hierarchical porosity, with pores appearing in both the ultramicro- and meso-porous ranges. From the adsorption isotherms at 298 K and 0–20 bar, the etched carbon exhibited adsorption capacities of 20, 9, 5, 4.5, and 1 mmol g−1 for CO2, CH4, CO, N2, and H2 respectively, indicating that the samples high (i.e., 95%) pore volume made it selective towards multiple species at high pressure. Having said this, when the material was examined for dynamic CO2 adsorption from N2, H2, and CH4 mixtures at 25 °C and 1 bar, it achieved CO2/CH4, CO2/H2, and CO2/N2 selectivities of 2.4, 15.4, and 6.6 mol/mol, respectively, meaning that it can be used for kinetic separations. The high dynamic selectivity was retained for CO2/H2 at 20 bar, however, the CO2/CH4 breakthrough fronts were much broader compared to the low-pressure experiments, meaning that the two species' diffusivities were competitive. As such, EC-RF was concluded to be effective for low-pressure CO2 separations as well as for high pressure CO2/H2 separations. More importantly, this study provides an in-depth characterization and better understanding of EC-RF's properties, which is imperative to its use as an adsorbent material.

Highlighting the Influence of Thermodynamic Coupling on Kinetic Separations with Microporous Crystalline Materials.

The main focus of this article is on mixture separations that are driven by differences in intracrystalline diffusivities of guest molecules in microporous crystalline adsorbent materials. Such “kinetic” separations serve to over-ride, and reverse, the selectivities dictated by mixture adsorption equilibrium. The Maxwell–Stefan formulation for the description of intracrystalline fluxes shows that the flux of each species is coupled with that of the partner species. For n-component mixtures, the coupling is quantified by a n × n dimensional matrix of thermodynamic correction factors with elements Γij; these elements can be determined from the model used to describe the mixture adsorption equilibrium. If the thermodynamic coupling effects are essentially ignored, i.e., the Γij is assumed to be equal to δij, the Kronecker delta, the Maxwell–Stefan formulation degenerates to yield uncoupled flux relations. The significance of thermodynamic coupling is highlighted by detailed analysis of separations of five different mixtures: N2/CH4, CO2/C2H6, O2/N2, C3H6/C3H8, and hexane isomers. In all cases, the productivity of the purified raffinate, containing the tardier species, is found to be significantly larger than that anticipated if the simplification Γij = δij is assumed. The reason for the strong influence of Γij on transient breakthroughs is traceable to the phenomenon of uphill intracrystalline diffusion of more mobile species. The major conclusion to emerge from this study is that modeling of kinetic separations needs to properly account for the thermodynamic coupling effects.

Cation exchange modification of clinoptilolite – Screening analysis for potential equilibrium and kinetic adsorption separations involving methane, nitrogen, and carbon dioxide

In this study, natural clinoptilolite was modified by cation exchange using alkali metal cations (Li+, Na+, K+, Rb+, Cs+), alkaline earth metal cations (Be2+, Mg2+, Ca2+, Sr2+, Ba2+), transition metal cations (Fe3+, Ni2+, Cu2+, Zn2+, Ce3+), and acid treatment (H+). The composition and structural properties of the modified samples were assessed using EDS and XRD analysis, and compared to the original clinoptilolite sample. Adsorption isotherms for CO2, N2, and CH4 gases were measured using a microgravimetric adsorption analyser. For all of the clinoptilolite samples, the adsorption equilibria and the diffusion rates were measured at 303 K over a pressure range from 0 to 8atm. The results of this screening analysis show that the cation exchanged clinoptilolites exhibit a wide range of adsorption characteristics, which make them suitable for a variety of gas separation applications by pressure swing adsorption. The high adsorption selectivity towards CO2 and CH4 found with the Cs+ exchanged clinoptilolite makes it advantageous for possible CH4/N2 and CO2/N2 equilibrium separations. Reduced CH4 equilibrium capacity resulting from pore blocking in the Ca2+ exchanged clinoptilolite leads to high selectivity for CO2/CH4 and N2/CH4 selective adsorbent. However, the potential of this material is limited due to increased microporous diffusion resistance. While both Li+ and Ni2+ clinoptilolites exhibit low CH4/N2 ideal selectivity, they present high N2/CH4 kinetic selectivity, thereby making them suitable for possible N2/CH4 kinetic separations.

Exploring the Framework Hydrophobicity and Flexibility of ZIF-8: From Biofuel Recovery to Hydrocarbon Separations

The framework hydrophobicity and flexibility of ZIF-8 are investigated by a detailed adsorption and diffusion study of a series of probe molecules including ethanol, 1-butanol, water, hexane isomers, xylene isomers, and 1,2,4-trimethylbenzene. The prospects for using ZIF-8 in biofuel recovery and hydrocarbon separations are discussed in terms of adsorption or kinetic selectivities. ZIF-8 shows extremely low water vapor uptakes and is especially suitable for vapor phase butanol-based biofuel recovery. The extraordinary framework flexibility of ZIF-8 is demonstrated by the adsorption of hydrocarbon molecules that are much larger than its nominal pore size, such as m-xylene, o-xylene and 1,2,4-trimethylbenzene. The calculation of corrected diffusion coefficients reveals an interesting spectrum of promising kinetic hydrocarbon separations by ZIF-8. These findings confirm that a molecular sieving effect tends to occur in the sorbate molecular size range of 4–6 Å rather than around the nominal ZIF-8 pore size of 3.4 Å, due to its surprising framework flexibility.

ChemInform Abstract: Simulating the Properties of Small Pore Silica Zeolites Using Interatomic Potentials

AbstractReview: 66 refs.

Simulating the properties of small pore silicazeolites using interatomic potentials

Despite the sustained use of forcefield methodologies to study SiO(2) polymorphs few reviews on the subject are available in the literature. The present study is an attempt to help fill this gap, focusing on classical forcefields used to reproduce and predict properties of pure silica zeolites (or zeosils) such as cell parameters, SiO distance and especially pore size. Instead of an exhaustive study we have focused on an application where diffusion of hydrocarbons makes important the use of pure silica zeolites. A particular area of interest is small pore zeosils containing 8-rings as the largest window, which are industrially interesting for their ability to perform kinetic separations of mixtures of C3 hydrocarbon molecules whose dimensions are of similar characteristics. A set of forcefields have been selected from the literature to analyze their accuracy and transferability when predicting structural, mechanical and dynamical properties of small pore pure silica zeolites and their performance at selective diffusion of C3 hydrocarbons.

High CO2 Selectivity of A Microporous Metal–Imidazolate Framework: A Molecular Simulation Study

Molecular simulations were used to investigate separation of CO2 from CH4 and N2 in a recently synthesized microporous metal–imidazolate framework (MMIF). Single component adsorption isotherms of CO2, CH4, and N2 in MMIF were computed using Grand Canonical Monte Carlo simulations, and a good agreement between simulations and experiments was found. Binary mixture adsorption isotherms were also computed and the validity of the ideal adsorbed solution theory was tested. Effects of bulk gas composition, temperature, pressure, and electrostatic interactions on the adsorption selectivity were investigated. MMIF outperformed many other metal organic frameworks and zeolites in adsorption-based CO2 separations. More interestingly, molecular dynamics simulations showed that diffusion of CO2 is several orders of magnitude larger than the diffusivity of CH4 in the pores of MMIF. This makes MMIF a very promising material for kinetic separations with an unprecedentedly high CO2/CH4 selectivity.

Molecular Sieve Separations

AbstractThe pore structures of adsorbents such as zeolites and metal‐organic frameworks comprise regular arrays of uniform channels of molecular dimensions, thus offering the possibility of size selective (molecular sieve) separations. Molecular sieve separations have traditionally been carried out in a cyclic batch adsorption/desorption process but, in principle, a steady state membrane process would be an attractive alternative. Considerable success has been achieved at the laboratory scale for several different molecular sieve separations but the difficulty of fabricating, on a large scale, a sufficiently coherent and robust zeolite membrane has largely prevented the commercial application of such processes. Selected examples of kinetic separations are presented in order to illustrate some of the features of such systems.

Efficient Calculation of Diffusion Limitations in Metal Organic Framework Materials: A Tool for Identifying Materials for Kinetic Separations

The very large number of distinct structures that are known for metal-organic frameworks (MOFs) and related materials presents both an opportunity and a challenge for identifying materials with useful properties for targeted applications. We show that efficient computational models can be used to evaluate large numbers of MOFs for kinetic separations of light gases based on finding materials with large differences between the diffusion coefficients of adsorbed gas species. We introduce a geometric approach that rapidly identifies the key features of a pore structure that control molecular diffusion and couple this with efficient molecular modeling calculations that predict the Henry's constant and diffusion activation energy for a range of spherical adsorbates. We demonstrate our approach for >500 MOFs and >160 silica zeolites. Our results indicate that many large pore MOFs will be of limited interest for separations based on kinetic effects, but we identify a significant number of materials that are predicted to have extraordinary properties for separation of gases such as CO(2), CH(4), and H(2).

Computational identification of a metal organic framework for high selectivity membrane-based CO2/CH4 separations: Cu(hfipbb)(H2hfipbb)0.5

The identification of membrane materials with high selectivity for CO(2)/CH(4) mixtures could revolutionize this industrially important separation. We predict using computational methods that a metal organic framework (MOF), Cu(hfipbb)(H(2)hfipbb)(0.5), has unprecedented selectivity for membrane-based separation of CO(2)/CH(4) mixtures. Our calculations combine molecular dynamics, transition state theory, and plane wave DFT calculations to assess the importance of framework flexibility in the MOF during molecular diffusion. This combination of methods should also make it possible to identify other MOFs with attractive properties for kinetic separations.

Molecular Modeling Study of the Permeability−Selectivity Trade-off in Polymeric and Microporous Membranes

A permeability−selectivity trade-off is observed in kinetic (diffusion-based) separations, such as oxygen/nitrogen, on polymeric and inorganic microporous membrane materials. Some materials are inherently better than others in the trade-off, but the molecular-level reasons are not clear. In this paper, we employed a molecular modeling approach to study the oxygen/nitrogen separation and provide insight into optimizing the pore characteristics (geometry, material of construction) for separation performance. The pore models were abstract, with features representative of different microporous or polymeric materials. They comprised three-dimensional atomistic models of unidirectional pores with diameters <5 A; various aspects of the pore geometry and material of construction were varied. Oxygen and nitrogen were modeled as rigid dimers, and the Lennard-Jones (LJ) potential was used to model the gas−pore site interactions. Free energy calculations and transition-state theory were used to predict the permeation...

Physical and kinetic speciation of copper and zinc in three geochemically contrasting marine estuaries.

The physical and kinetic speciation of Cu and Zn in three impacted marine estuaries was examined. Contrasts in sources of metal-binding ligands, solution chemistry, and hydrologic forcing between and withinthethree study systems (Cape Fear River Estuary, North Carolina; Norfolk-Hampton Roads-Elizabeth River, Virginia; San Diego Bay, California) were exploited to enhance our understanding of Cu and Zn speciation. Trace metal-optimized tangential-flow ultrafiltration at 1 kDa nominal molecular weight limit (NMWL) was used to fractionate <0.4 microm species into colloidal and "dissolved" pools. Colloidal species of dissolved organic matter (DOM) and copper were significant and often the dominant pools in each of the three study systems. Characteristic colloidal fractions of both DOM and Cu ranged from near 70% of <0.4 microm concentrations in Cape Fear to 50% in San Diego Bay. Colloidal Cu and DOM were strongly coupled, and variability in observed <0.4 microm Cu concentrations was closely related to the concentrations of colloidal-associated metal. Colloidal fractions were much smaller for Zn than that of Cu; ranging from 10-30% in Cape Fear to less than 5% in San Diego Bay, and no relationship to DOM was observed. Kinetic separations on Chelex resin revealed the presence of large nonlabile pools of Cu in each of the study systems, with the highest fractions (70-100%) in Cape Fear and Norfolk and lowest (30-50%) in San Diego Bay. A close relationship was observed between colloidal and nonlabile Cu species, implying slow reactivity of colloidal-bound Cu. The fraction of filterable Zn labile to Chelex averaged 97%, 85%, and 60% in San Diego, Norfolk, and Cape Fear, respectively. Anthropogenic Zn appeared almost exclusively in the <1 kDa fraction, while anthropogenic Cu was distributed between dissolved and colloidal pools. Copper particle-partition coefficients (Kd) followed the trend: San Diego >> Norfolk > Cape Fear and were inversely correlated with DOC concentrations. Colloid-based partition coefficients were significantly greater, in many cases an order of magnitude greater, than particle-based partition coefficients. The partitioning data suggest the presence of metal-enriched bacterial-derived exudates and/or discrete metal phases in colloidal-sized particles in impacted regions of these estuaries. The strong relationships observed between Cu and DOC indicate that Cu partitioning behavior over a range of estuarine environments may be modeled effectively with a limited set of coefficients. Our measurements of metal lability and size distribution imply that the fraction of <0.4 microm Zn that is likely to be bioavailable is greater than that for Cu, especially in impacted regions of the study systems.

Kinetic Gravity Separation

Separations by density, such as the separation of non‐ferrous scrap into light and heavy alloys, are often realized by means of heavy media. In principle, kinetic gravity separations in water can be faster and cheaper, because they do not rely on suspensions or salt solutions of which the density must be tightly controlled. Test results of a prototype kinetic gravity separator in combination with suitable size classification techniques show promising results for particle sizes between 2 and 10 mm.