Colloidal Size Range Research Articles

High-gradient magnetic separation of colloidal uranium oxide particles from soil components in aqueous suspensions

The separation of uranium oxide (UO2) particles from soil-surrogate particles in aqueous suspensions was achieved using filtration enhanced by a magnetic field. Enhanced attraction of paramagnetic UO2 colloids to a ferromagnetic stainless-steel filter placed in a strong magnetic field arises because of the positive magnetic susceptibility of the particles and the high-gradient field generated near ferromagnetic fibers. Enhanced uptake of smaller particles over larger ones occurs through Brownian motion that promotes the collision of particles with the ferromagnetic fibers of the filter. Hence, this work focused on UO2 particles in the colloidal size range. Experiments used a water-cooled electromagnet and an array of permanent magnets. Chemical analysis showed that the magnetic field increased the capture efficiency of uranium particles from a range of 27–53% with the magnet off up to 98% with the magnet on after a single pass of the suspension through the filter. The recovery of the UO2 particles from the filter, however, was more difficult to achieve. Small amounts of UO2, together with significant amounts of background SiO2 particles, were removed from the filter during a first flush with the magnetic field on. A much larger recovery of UO2 was not observed until a second out-of-field flush was performed, which also released some SiO2. The degree to which particle separation was enhanced through the use of multi-stage filtration compared to single pass-through filtration was also examined. A design was suggested that could be used to optimize the separation efficiency for a continuous process.

Phosphorus content in water extractable soil colloids over a 2000 years chronosequence of paddy-rice management in the Yangtze River Delta, China

Soil phosphorus (P) is usually hardly present in soluble forms as most of it is strongly bound to minerals. Therefore, transformations of soil colloidal P can play a key role in enhancing soil fertility. Here, we examined water extracts from a chronosequence of soils with 2000 years of paddy rice and 700 years of non-paddy cropping, and fractionated them into <1200 nm, <450 nm, and <3 kDa size fractions using sequential ultrafiltration. Asymmetric-flow field-flow fractionation (AF4) coupled to an organic carbon detector (OCD) and inductively coupled plasma mass spectrometry (ICP-MS) was used for organic carbon and elemental screening in the 3 kDa-450 nm colloidal size range. We found that P increased in all colloidal fractions both with long-term paddy rice and non-paddy managements. Furthermore, P in soil colloids of 3 kDa-450 nm size, as fractionated by AF4, peaked in three size ranges: nanoparticles (NP, ∼3 kDa-20 nm), fine colloids (FC, ∼20–225 nm), and medium-sized colloids (MC, ∼225–450 nm). The NP fraction originally contained mainly P and Ca for tidal wetland (TW), but turned into a complex of OC-Ca-P colloidal forms immediately after onset of arable management, with little temporal change thereafter. Fine colloidal P (FC-P) was detected only after > 50 years for the non-paddy and > 100 years under paddy management, associated with oxides, organic matter and fine clay fragments in the paddy soils and additionally Ca-bound P in the non-paddy ones, respectively. Increasing portions of medium-sized colloidal P appeared after 50 years paddy and non-paddy management, though not exceeding the concentrations of FC-P and with little changes during prolonged arable land use. Overall, after initial formation of FC-P during the first 50–100 years of management, longer term dynamics of colloidal P in these arable soils was fairly independent of paddy or non-paddy managements, but remained closely coupled to the increased presence or generation of fine colloidal clays, oxides and the associated P, thus potentially linking P fertility largely to the dynamics of fine colloids.

Dust discrimination in dynamic light scattering based on a quantile outliers detection method

Dynamic light scattering (DLS) is a popular method for measuring particle size in the colloidal size range. However, like all light scattering techniques, the results can be problematic when dust or large contaminant particles are present in the sample. Dust particles are usually much larger than the colloids or nanoparticles of interest and, consequently, scatter light very strongly when they pass through the laser beam in the scattering volume. This biases the measured particle size to much larger values than expected. However, it also suggests a means to detect and, potentially, reduce the effect of dust particles on the measurement results. If the high count-rate data associated with dust can be detected and removed during the DLS measurement, the particle size results will be more reliable. An a priori electronic dust filtering method based on the long-time count rate statistics is proposed and demonstrated. This allows the identification of data sets which have an abnormal number of high count-rate events which are, presumably, associated with scattering from dust particles. These data can be rejected and an autocorrelation function calculated using the retained counts after removing unwanted outlier data. The particle size distribution is then recovered. By measuring a standard polystyrene suspension containing “dust” particles, the results show that the method can effectively discriminate and eliminate data contaminated by dust, thereby improving the measurement results.

Quantification and molecular characterization of organo-mineral associations as influenced by redox oscillations

Organo-mineral association is one of the most important stabilization mechanisms of soil organic matter. However, few studies have been conducted to assess the retention, transformation, and transportation of colloids (1–1000 nm) and associated organic carbon (OC) in soil. Given the particularly significant role that wetland soils play in carbon storage and cycling, we quantified the dynamics of organo-mineral association within colloidal size range by conducting three consecutive 35-day redox (reduction-oxidation) oscillation experiments using a wetland soil. Molecular compositions of natural nanoparticle (NNP, 2.3–100 nm), fine colloid (100–450 nm), and particulate (450–1000 nm) fractions were measured using isotope ratio mass spectrometry (IRMS) and x-ray photoelectron spectroscopy (XPS). Results showed that NNP and fine colloids constituted up to 8.94 ± 0.50% and 22.19 ± 7.52% of bulk C concentration (2.3–1000 nm), respectively; indicating substantial contributions of these two fractions to the operationally defined “dissolved” (<450 nm) fraction. There was significant enrichment in heavier δ13C isotopes (p < 0.001) with size: NNP (−29.64 ± 0.32‰) < fine colloid (−28.81 ± 0.31‰) < particulate (−28.34 ± 0.25‰) fractions. NNP had the highest percentages of carbonyl/carboxyl C (C=O); while fine colloid and particulate fractions contained more reduced aromatic or aliphatic C (C–C, C=C, C–H). OC became more enriched (‰) in microbial-derived C (higher δ13C) with increasing particle size as well as with repeated redox oscillations. Our findings clearly demonstrate limitations of using the operationally defined “dissolved” fraction (<450 nm) to assess C cycling in ecosystems such as wetlands. Increase in colloid and OC concentrations and presence of more microbial-derived C in larger size fractions additionally imply that redox oscillations promote the formation of molecularly diverse sub-colloid sized organo-mineral associations. Being a composite unit of soil microaggregates, organic-mineral associations can thus influence the overall stability of OC in wetland soils that undergo frequent redox oscillations.

Colloidal Gelation through Thermally Triggered Surfactant Displacement.

Colloidal systems that undergo gelation attract much attention in both fundamental studies and practical applications. Rational tuning of interparticle interactions allows researchers to precisely engineer colloidal material properties and microstructures. Here, contrary to the traditional approaches where modulating attractive interactions is the major focus, we present a platform wherein colloidal gelation is controlled by tuning repulsive interactions. By including amphiphilic oligomers in colloidal suspensions, the ionic surfactants on the colloids are replaced by the nonionic oligomer surfactants at elevated temperatures, leading to a decrease in electrostatic repulsion. The mechanism is examined by carefully characterizing the colloids, and subsequently allowing the construction of interparticle potentials to capture the material behaviors. With the thermally triggered surfactant displacement, the dispersion assembles into a macroporous viscoelastic network and the gelling mechanism is robust over a wide range of compositions, colloid sizes, and component chemistries. This stimulus-responsive gelation platform is general and offers new strategies to engineer complex viscoelastic soft materials.

Heteroaggregation of an enveloped bacteriophage with colloidal sediments and effect on virus viability

Four sediments in the colloidal size range: goethite, montmorillonite, illite, and kaolinite, were suspended with the bacteriophage φ6, a model enveloped virus, to determine relative rates of heteroaggregation and the effect of aggregation on virus viability. Turbidity was measured on combinations of virus and each sediment type at low concentration to determine aggregation rates. Aggregation of sediment with virus occurred regardless of mineral type, and larger fraction of virus is expected to aggregate with increasing sediment concentration leading to higher deposition rates. The negatively charged sediments, aggregated with φ6 (also negatively charged at neutral pH) at a faster rate than the positively charged sediments, yielding turbidity slopes of 4.94 × 10−3 s−1 and 7.50 × 10−4 s−1 for φ6-montmorillonite and φ6-illite aggregates, respectively, and 2.98 × 10−5 s−1 and 2.84 × 10−5 s−1, for φ6-goethite and φ6-kaolinite, respectively. This indicates that the interaction between sediments and virus is hydrophobic, rather than electrostatic. Large numbers of virions remained viable post-aggregation, despite the fragility of the viral envelope, indicating that small-sized aggregates, which may travel more readily through porous media, may pose an infection risk. The fraction of φ6 that remained viable varied with sediment type, with montmorillonite-φ6 aggregates experiencing the greatest reduction in infectivity at 35%. TEM analyses reveal that in all sediment-φ6 combinations, infectivity loss was likely due to disassembly of the viral envelope as a result of aggregation.

Asymmetrical flow field flow fractionation methods to characterize submicron particles: application to carbon-based aggregates and nanoplastics.

In the last 10 years, asymmetrical flow field flow fractionation (AF4) has been one of the most promising approaches to characterize colloidal particles. Nevertheless, despite its potentialities, it is still considered a complex technique to set up, and the theory is difficult to apply for the characterization of complex samples containing submicron particles and nanoparticles. In the present work, we developed and propose a simple analytical strategy to rapidly determine the presence of several submicron populations in an unknown sample with one programmed AF4 method. To illustrate this method, we analyzed polystyrene particles and fullerene aggregates of size covering the whole colloidal size distribution. A global and fast AF4 method (method O) allowed us to screen the presence of particles with size ranging from 1 to 800 nm. By examination of the fractionating power F d, as proposed in the literature, convenient fractionation resolution was obtained for size ranging from 10 to 400 nm. The global F d values, as well as the steric inversion diameter, for the whole colloidal size distribution correspond to the predicted values obtained by model studies. On the basis of this method and without the channel components or mobile phase composition being changed, four isocratic subfraction methods were performed to achieve further high-resolution separation as a function of different size classes: 10-100 nm, 100-200 nm, 200-450 nm, and 450-800 nm in diameter. Finally, all the methods developed were applied in characterization of nanoplastics, which has received great attention in recent years. Graphical Absract Characterization of the nanoplastics by asymmetrical flow field flow fractionation within the colloidal size range.

Recycling of activated carbon filter backwash water using ultrafiltration: Membrane fouling caused by different dominant interfacial forces

The reuse of activated carbon filter backwash water was performed using an ultrafiltration (UF) system and the membrane fouling mechanism was investigated using a model involving XDLVO theory and hydrodynamic interaction. Pretreatment by micro-flocculation/sedimentation removed most particulate pollutants; thus, residual organic colloids determined the details of the UF membrane fouling. The membrane fouling was divided into two periods. Membrane fouling in the initial period was determined by interfacial forces between the membrane and colloids. The calculated interfacial forces showed that permeation drag (PD) force dominated the total interfacial force at long-range, while London-van der Waals and short-range Lewis acid-base (AB) forces governed membrane fouling at short-range (< 3nm). The electrostatic double layer force, reaching its energy peak at around 10nm, favored mitigation of membrane fouling. The initial membrane fouling could be alleviated by reducing the PD force, which was mainly associated with permeate flux. In the second membrane fouling period, a cake layer formed mostly of hydrophobic colloids retained at the membrane surface. The increasing AB force contributed mainly to the interfacial forces responsible for membrane fouling at short-range, greatly aggravating membrane fouling. The pre-ozonation of the UF influent was involved in mitigating membrane fouling in the second period. Pre-ozonation enhanced the hydrophilic properties of both cake layer and organic colloids, which clearly decreased the AB force. The pre-ozonation also narrowed the colloid size range and decreased the PD force. The use of interval pre-ozonation during the filtration process was a promising way to alleviate membrane fouling, and involved a chemical cleaning cycle extending from 15 to 18 days.

Improving the understanding of fullerene (nC60) aggregate structures: Fractal dimension characterization by static light scattering coupled to asymmetrical flow field flow fractionation

Fullerene (C60) aggregation mechanisms in aqueous media require considerable attention in the near future due to the heavy use and application of fullerene-based products within the context of nanotechnology. Such intensive development will result in the release of massive amounts of C60 in aqueous environmental systems in the aggregate form (nC60). In that sense, the aggregation mechanisms need to be fully determined to better evaluate the environmental fate and behavior of C60. To fulfil these needs, the aim of this work was to extensively characterize the aggregation mechanisms of fullerene aggregates in aqueous media by asymmetrical flow field fractionation (AF4) coupled to static light scattering (SLS). We developed a sequential ultrafiltration method that allows the fractionation of the whole nC60 size distribution into different size classes (1–100–200–450–800nm). Following a preliminary analysis by dynamic light scattering (DLS), we optimized several AF4 separation methods to allow screening of these colloidal size classes of nC60 with high efficiency and resolution. The fractal dimension (Df) of this entire size class was characterized directly on-line according to the radius of gyration through a combination of angle-dependent light scattering and fractal dimension analysis. We demonstrate the possible formation and persistence of colloidal populations of nC60 in aqueous media from a few nanometers up to 800nm. Df values ranging from 1.2 to 2.8, based on the nC60 colloidal size range, strongly depend on the method of the sample filtration.

Primitive electrolyte solutions around a charged colloid: density functional approach

ABSTRACTWe have developed the density functional theory, which is based both on the modified fundamental-measure theory for the hard spheres and on the weighted-density approximation for the electrostatic residual contribution, for studying the structure of the size-symmetric and asymmetric electrolyte solutions around a charged hard colloid. The calculation shows that the present theory provides an excellent description of the primitive electrolyte solutions and indicates the suitability of the approximations used in the formalism. The preset theory provides better the structural results than the density functional theories based on the excess free energy functional expansion with respect to the bulk ionic density and the hypernetted-chain equation based on the mean spherical approximation over a wide range of colloid sizes, ion concentrations and valence of a colloid. We have confirmed that the present theory is able to predict the phenomena of charge reversal and overcharging occurring near a colloidal interface, in agreement with previous primitive model Monte Carlo simulations and theoretical studies reported in the literature.

Contribution of Nano- to Microscale Roughness to Heterogeneity: Closing the Gap between Unfavorable and Favorable Colloid Attachment Conditions.

Surface roughness has been reported to both increase as well as decrease colloid retention. In order to better understand the boundaries within which roughness operates, attachment of a range of colloid sizes to glass with three levels of roughness was examined under both favorable (energy barrier absent) and unfavorable (energy barrier present) conditions in an impinging jet system. Smooth glass was found to provide the upper and lower bounds for attachment under favorable and unfavorable conditions, respectively. Surface roughness decreased, or even eliminated, the gap between favorable and unfavorable attachment and did so by two mechanisms: (1) under favorable conditions attachment decreased via increased hydrodynamic slip length and reduced attraction and (2) under unfavorable conditions attachment increased via reduced colloid-collector repulsion (reduced radius of curvature) and increased attraction (multiple points of contact, and possibly increased surface charge heterogeneity). Absence of a gap where these forces most strongly operate for smaller (<200 nm) and larger (>2 μm) colloids was observed and discussed. These observations elucidate the role of roughness in colloid attachment under both favorable and unfavorable conditions.

Distribution of trace metals (Cu, Pb, Ni, Zn) between particulate, colloidal and truly dissolved fractions in wastewater treatment

The distribution of Cu, Pb, Ni and Zn between particulate, colloidal and truly dissolved size fractions in wastewater from a trickling filter treatment plant was investigated. Samples of influent, primary effluent, humus effluent, final effluent and sludge holding tank returns were collected and separated into particulate (i.e. > 0.45 μm), colloidal (i.e. 1 kDa to 0.45 μm), and truly dissolved (i.e. < 1 kDa) fractions using membrane filters. In the influent, substantial proportions of Cu (60%), Pb (67%), and Zn (32%) were present in the particulate fraction which was removed in conjunction with suspended particles at the works in subsequent treatment stages. In final effluent, sizeable proportions of Cu (52%), Pb (32%), Ni (44%) and Zn (68%) were found within the colloidal size fraction. Calculated ratios of soluble metal to organic carbon suggest the metal to be adsorbed to or complexed with non-humic macromolecules typically found within the colloidal size range. These findings suggest that technologies capable of removing particles within the colloidal fraction have good potential to enhance metals removal from wastewater.

Ocular Disposition of ∆8-Tetrahydrocannabinol from Various Topical Ophthalmic Formulations.

The purposes of this project are to enhance the trans-membrane penetration of Δ8-Tetrahydrocannabinol (Δ8-THC) and to study the effect of various lipid based systems in delivering the compound, non-invasively, to anterior and posterior ocular chambers. Solid lipid nanoparticles (SLNs), fast gelling films were manufactured using high pressure homogenization and melt cast techniques, respectively. The formulations were characterized for drug content, entrapment efficiency, particle size and subsequently evaluated in vitro for trans-corneal permeation. In vivo, the drug disposition was tested via topical administration in albino rabbits. The eye globes were enucleated at the end of experiment and tissues were analyzed for drug content. All formulations showed favorable physicochemical characteristics in terms of particle size, entrapment efficiency, and drug content. In vitro, the formulations exhibited a transcorneal flux that depended on the formulation's drug load. An increase in drug load from 0.1 to 0.75% resulted in 12- to16-folds increase in permeation. In vivo, the film was able to deliver THC to all the tissues with high accumulations in cornea and sclera. The SLNs showed a greater ability in delivering THC to all the tissues, at a significantly lower drug load, due to their colloidal size range, which in turn enhanced corneal epithelial membrane penetration. The topical formulations evaluated in the present study were able to successfully deliver Δ8-THC in therapeutically meaningful concentrations (EC50 values for CB1: 6nM and CB2: 0.4nM) to all ocular tissues except the vitreous humor, with pronounced tissue penetration achieved using SLNs as a Δ8-THC delivery vehicle.

Preparation and investigation of solid lipid nanoparticles for drug delivery

Solid lipid nanoparticles (SLN), a promising drug delivery vehicle, offer an alternative system to traditional colloidal carriers. In our study 12 new SLN formulations were fabricated via the “microemulsion (ME) method”, each leading to a different SLN size and composition. This method is a relatively easy technique and involves biocompatible conditions. Stearic acid has been used as lipid material of which the ratio is kept under 4% to prevent particle growth. Particle size and surface properties of the synthesized SLNs were controlled using various combinations of emulsifiers such as lithocholic acid, Pluronic F127, Tween 20, lecithin and butanol. Furthermore, the mean size of the particles was adjusted by changing the ME:water ratio in the dilution step which is independent from composition. It was found that most of the SLNs were in the colloidal size range (below 100nm) and spherical in shape, which provides high surface area to exploit, as an alternative adsorptive drug carrier system. Also, the surface charge density values of SLNs were calculated by considering size and zeta potential values which then helps in understanding the surface potential of particles. MB was chosen as a model molecule and entrapped on the surface of SLNs after the preparation, to determine the loading capacities and release efficiencies. As such, SLNs have been successfully produced which have controllable drug entrapment efficiency and release capacity according to the chosen combination of emulsifiers and dilution ratio. Thus, this study contributes to the improvement of alternative drug delivery systems with biocompatible and stable newly formulated SLNs.

Americium and plutonium association with magnesium hydroxide colloids in alkaline nuclear industry process environments

The behaviours of Pu, Am and colloids in feed solutions to the Site Ion-exchange Effluent Plant (SIXEP) at the Sellafield nuclear reprocessing site in the U.K. have been studied. For both Pu and Am, fractions were found to be associated with material in the colloidal size range, with ∼50% of the Pu in the range 1–200 nm. The concentration of soluble Pu (<1 nm) was ∼1 nM, which is very similar to the solubility limit for Pu(V). The soluble Am concentration was of the order of 10−11 M, which was below the solubility limit of americium hydroxide. The size, morphology and elemental composition of the particulates and colloids in the feed solutions were investigated. Magnesium is homogeneously distributed throughout the particles, whereas U, Si, Fe, and Ca were present in localised areas only. Amongst some heterogeneous material, particles were identified that were consistent with hydrotalcite. The distribution of 241Am(III) on brucite (magnesium hydroxide) colloids of different sizes was studied under alkaline conditions representative of nuclear fuel storage pond and effluent feed solution conditions. The morphology of the brucite particles in the bulk material observed by ESEM was predominantly hexagonal, while that of the carbonated brucite consisted of hexagonal species mixed with platelets. The association of 241Am(III) with the brucite colloids was studied by ultrafiltration coupled with gamma ray-spectrometry. For carbonate concentrations up to 10−3 M, the 241Am(III) was mainly associated with larger colloids (>300 kDa), and there was a shift from the smaller size fractions to the larger over a period of 6 months. At higher carbonate concentrations (10−2 M), the Am was predominantly detected in the true solution fraction (<3 kDa) and in smaller size colloidal fractions, in the range 3–100 kDa.

Colloidal and spherical mesoporous silica particles: synthesis and new technologies for delivery applications

Continuous research in prominent fields such as biotechnology, biomedicine and nanopharmaceutics has brought the development of a widespread class of materials, and studies for mesoporous materials have been exponentially growing lately. The purpose of this review is to provide a useful guide for different materials, methods and configurations that have been reported in the last 5 years for the synthesis of spherical mesoporous silica particles (MSP), in the colloidal size range (1–1000 nm). MSP exhibit several limitations that must be overcome in order to enable their medical and clinical use. Surface modification of these particles will allow getting new promising characteristics of these materials, including better drug release control and biocompatibility improvement. These modified MSP could be potentially used in many biomedical applications, especially for drug delivery systems. Emphasis is made on the pore size, diameter and shape of the final particles since these parameters will establish key characteristics, i.e., drug delivery profile, loading capacity and efficiency. Spreading the use of mesoporous silica particles in biomedicine is possible by the improvement of its inner characteristics through surface modification. This may be done by chemical functionalization or by coating with macromolecular layers or brushes, thus creating novel responsive core–shell hybrid composites to be used as carriers for drug delivery applications.

Characterisation of algal organic matter produced by bloom-forming marine and freshwater algae

Algal blooms can seriously affect the operation of water treatment processes including low pressure (micro- and ultra-filtration) and high pressure (nanofiltration and reverse osmosis) membranes mainly due to accumulation of algal-derived organic matter (AOM). In this study, the different components of AOM extracted from three common species of bloom-forming algae (Alexandrium tamarense, Chaetoceros affinis and Microcystis sp.) were characterised employing various analytical techniques, such as liquid chromatography – organic carbon detection, fluorescence spectroscopy, fourier transform infrared spectroscopy, alcian blue staining and lectin staining coupled with laser scanning microscopy to indentify its composition and force measurement using atomic force microscopy to measure its stickiness. Batch culture monitoring of the three algal species illustrated varying characteristics in terms of growth pattern, cell concentration and AOM release. The AOM produced by the three algal species comprised mainly biopolymers (e.g., polysaccharides and proteins) but some refractory compounds (e.g., humic-like substances) and other low molecular weight acid and neutral compounds were also found. Biopolymers containing fucose and sulphated functional groups were found in all AOM samples while the presence of other functional groups varied between different species. A large majority (>80%) of the acidic polysaccharide components (in terms of transparent exopolymer particles) were found in the colloidal size range (<0.4 μm). The relative stickiness of AOM substantially varied between algal species and that the cohesion between AOM-coated surfaces was much stronger than the adhesion of AOM on AOM-free surfaces. Overall, the composition as well as the physico-chemical characteristics (e.g., stickiness) of AOM will likely dictate the severity of fouling in membrane systems during algal blooms.

COLLOIDS IN SEPTIC TANK EFFLUENT AND THEIR INFLUENCE ON FILTER PERMEABILITY

The aim of the study was to evaluate the content of colloids in septic tank effluent and their impact on textile filter permeability. Measurements were performed on septic tank effluent without suspended solids but containing colloids and without colloids – including only dissolved substances (filtered by micro-filters and centrifuged). The study was conducted on unclogged and clogged textile filter coupons. During the study the following measurements were conducted: turbidity, chemical oxygen demand and hydraulic conductivity of textile filter coupons. The colloid size range was assumed to be less than 1.2 microns according to the literature. Despite the relatively low content in the septic tank effluent the colloids played an important role in the clogging process. Both the filtering media, filled with low (unclogged) and high content of biomass (clogged) were sensitive to the clogging process acceleration due to the possibility of small diameter pore closure and oxygen access termination. Moreover, small size pores were probably sensitive to closing or bridging by small size colloidal particles.

Size characterization of dissolved metals and organic matter in source waters to streams in developed landscapes

Individual and mixed water samples from wastewater treatment plant effluents, stormwater runoff, streams from developed areas were characterized with respect to organic matter concentration and spectral properties and metal concentration and size distribution. In addition, asymmetric flow-field flow fractionation coupled to inductively coupled plasma mass spectrometry was used to measure concentration, size distribution and association of metals in the colloidal size range. Results reveal that Fe, Cu, Zn and Pb in the colloidal size range were mainly associated with the less than 5 nm, or less than 10 kDa size range. Cu was most strongly associated with organic matter, while Zn and Pb were mixed between Fe and organic matter. Effluent showed higher binding capacity for metals, while stormwater, even with higher organic matter concentrations showed more exchangeable metals. Upon mixing of source waters, colloidal metal concentrations and size distributions were conserved.

Colloidal size distribution of humic- and protein-like fluorescent organic matter in the northern Gulf of Mexico

The colloidal size distributions (0.5–400nm) of humic-like and protein-like fluorescent and UV-absorbing marine dissolved organic matter (DOM) have been determined using flow field-flow fractionation (FlFFF) coupled to UV-absorbance and fluorescence detectors. Surface seawater samples were taken monthly between November 2008 and September 2009, from eight stations along a transect across the Mississippi Sound and Mississippi Bight in the northern Gulf of Mexico. DOM was also isolated from cultures of two phytoplankton species. The concentration of dissolved organic carbon (DOC) in the study area was largely controlled by river inputs, but the specific UV-absorbance and fluorescence index indicated that the importance of marine DOM increased from nearshore to offshore stations. In the seawater samples, only a small fraction (3–16%) of the ‘humic-like’ fluorescent DOM (Ex/Em wavelengths at 240/440nm, 310/400nm, 320/440nm and 350/450nm) was found in the colloidal size range (>0.5nm), mainly associated with small colloids (mean hydrodynamic diameter 2–3nm). For the ‘protein-like fluorescent DOM’ (Ex/Em at 275/305nm and 275/340nm) a larger fraction (21–100%) was found in the colloidal size range, including both small (2–3nm) and larger (mean hydrodynamic diameter 6–7nm, ~75nm, and >400nm) colloids. A culture of the diatom Chaetoceros muelleri produced mainly humic-like fluorescent DOM, while the fluorescent DOM produced by the dinoflagellate Prorocentrum sp. was mostly protein-like. In both plankton cultures, the protein-like DOM was largely associated with colloids around 6nm, 12–14nm and larger, while only a small fraction of humic-like DOM was found in the colloidal size range. After five months of starving and aging of the cultures, both the humic-like and protein-like DOM became increasingly associated with small colloids having a mean hydrodynamic diameter of 2–4nm. We hypothesize that the larger (6–7nm, 12–14nm, ~75nm and >400nm) protein-like colloids in the seawater samples and phytoplankton cultures were freshly produced by phytoplankton or bacteria. The smaller (2–4nm) colloids in the aging phytoplankton cultures probably represented phytoplankton-derived organic material that had been transformed by microbial degradation, while the small (2–3nm) humic-like colloids in the seawater samples were most likely dominated by fulvic acids from terrestrial sources.