Influence of the composition of natural organic matter on Pb bioavailability to microalgae

The current work examines the effects of model allochtonous (humic substances) and autochtonous (microbial polysaccharides) natural organic matter (NOM) on Pb speciation and bioaccumulation. The results demonstrated that polysaccharides, in particular alginic acid, had complexing properties and effects on Pb bioaccumulation by the green alga Chlorella kesslerii that were similar to equivalent complexing capacity of humic substances. Pb uptake decreased in the presence of humic, alginic, and polygalacturonic acids with respect to noncomplexed Pb, but accumulated Pb was higher than predicted from measured Pb2+ concentrations or from previous results obtained in the presence of simple synthetic ligands. An improved fit between experimental observations and Pb speciation was obtained by taking into account the formation of a ternary complex at the algal surface. The contribution of the ternary complexes to Pb bioaccumulation was dependent on the relative binding constants of the Pb to the NOM and to the binding sites on the biological surface. In the presence of the humic acid, a decreased surface charge and increased membrane permeability were considered to be of secondary importance to explain the observation of increased Pb uptake with respect to that predicted on the basis of [Pb2+]. The environmental implications of the results are discussed with respect to the development of site-specific water quality criteria.     

Depolymerization of chromophoric natural organic matter

Although the importance of natural organic matter (NOM) in the environment and in drinking water treatment is well-known, its structure is still ill-defined. The fragmentation patterns of NOM treated by irradiation (various wavelengths--185-400 nm), hydroxyl radicals, chlorine, ozone, and breakdown by a white rot fungus were studied to investigate the structure of chromophoric NOM molecules. Size exclusion chromatography was used to monitor the size distributions of NOM in two natural water waters and two NOM isolates. Three distinct fragmentation patterns were observed: ozone attack appeared to be nonsize specific, UV (> or = 254 nm) irradiation preferentially removed higher molecular weight chromophores, while processes involving hydroxyl radical showed intermediate size specificity. For the samples studied, the UV (> or = 254 nm) irradiation-induced fragmentation of NOM followed the patterns suggested by a simple trimer depolymerization model, supporting the viewpoint that NOM has repeating structural units joined by photolabile chemical bonds. The largest molecules reacted most rapidly, progressively fragmenting into slower reacting smaller molecules, which initially accumulated before breaking down to become nonchromophoric. This dependency of rate on molecular size appears to follow from the law of photochemistry which states the rate of reaction is proportional to the rate of light absorption: larger chromophores had higher molar absorptivities, absorbed more photons, and hence reacted faster than smaller chromophores.     

Adsorption of natural organic matter to air-water interfaces during transport through unsaturated porous media

To better understand how interactions with the air phase influence the movement of natural organic matter (NOM) through the vadose zone, we measured the transport of soil-humic acid (SHA) through laboratory columns packed with partially saturated sand. Our results demonstrate that sorptive reactions at air-water interfaces reduce SHA mobility and that the affinity of SHA for the air phase increases as the porewater pH declines from 8 to 3.9. SHA desorption from air-water interfaces is negligible for conditions of constant pH, but release of bound SHA occurs in response to perturbations in porewater pH. We analyzed the effluent samples collected from our laboratory columns using high-performance size-exclusion chromatography. The results of this analysis demonstrate that the SHA did not fractionate appreciably during transport through the columns, suggesting that the various components of the SHA pool (as distinguished on the basis of molecular weight) express an equal affinity for the air-water interfaces over the range of pH conditions tested. A mathematical model incorporating irreversible, second-order rate laws to simulate adsorption at air-water and solid-water interfaces closely describes the SHA breakthrough data. The mass-transfer parameters that govern this model vary in a discernible fashion with changes in porewater pH, and the parameter trends are consistent with published theories for SHA adsorption.     

Phenanthrene sorption by aliphatic-rich natural organic matter

Contaminant sorption, an important process that may limit bioavailability, hinder remediation, encourage environmental persistence, and control mobility in the environment, has been the focus of numerous studies. Despite these efforts, the fundamental understanding of sorptive processes in soil and sedimentary environments has not been resolved. For instance, many have suggested that contaminants, such as polycyclic aromatic hydrocarbons (PAHs), solely interact with aromatic domains of organic matter. Until now, studies have neglected the aliphatic components that are known to be a recalcitrant and significant part of soil and sedimentary organic matter (SOM). In this investigation, the sorption of phenanthrene to several aliphatic-rich SOM samples was measured. The samples included the following: SOM precursors (algae, degraded algae, cellulose, collagen, cuticle, and lignin), two kerogen samples, and a highly aromatic humic acid. All samples were characterized by cross polarization magic angle spinning carbon-13 (CPMAS 13C) NMR and carbon, hydrogen, and nitrogen analysis. Batch experiments demonstrated that the highest organic carbon normalized sorption coefficients (Koc values) were obtained with the Pula kerogen sample (log Koc = 4.88) that only contains 6.5% aromatic carbon. Other aliphatic-rich samples, namely the Green River kerogen, degraded algae, and collagen samples produced comparable log Koc values (4.64, 4.66, and 4.72, respectively) to that of the highly aromatic humic acid (log Koc = 4.67). Phenanthrene uptake was the least for cellulose and lignin, two major soil components. A comparison of phenanthrene Koc values and paraffinic carbon content revealed a positive correlation (Koc = 798 +/- 96.1 * paraffinic carbon (%), r2 = 0.56) and indicates that amorphous polymethylene carbon is an important consideration in phenanthrene sorption. This study establishes that aliphatic SOM domains have a strong affinity for phenanthrene and likely, other PAHs. Therefore, aliphatic structures, that are an important component of SOM, require more attention in the examination of sorption processes in terrestrial and sedimentary environments.     

Enhancement of hematite bioreduction by natural organic matter

The effects of natural organic matter (NOM), ferrozine, and AQDS (anthraquinone-2,6-disulfonate) on the reduction of hematite (alpha-Fe2O3) by Shewanella putrefaciens CN32 were studied. It has been proposed that NOM enhances the reduction of Fe(III) by means of electron shuttling or by Fe(II) complexation. Previously both mechanisms were studied separately using "functional analogues" (AQDS for electron shuttling and ferrozine for complexation) and are presently compared with seven different NOMs. AQDS enhanced hematite reduction within the first 24 h of incubation, and this had been ascribed to electron shuttling. Most of the NOMs enhanced hematite reduction after 1 day of incubation indicating that these materials could also serve as electron shuttles. The effect of ferrozine was linear with concentration, and all of the NOMs exhibited this behavior. Fe(II) complexation only enhanced hematite reduction after sufficient Fe(II) had accumulated in the system. Fe(II) complexation appeared to alleviate a suppression of the hematite reduction rate caused by accumulation of Fe(II) in the system. Addition of Fe(II) to the hematite suspension, prior to inoculation with CN32, significantly inhibited hematite reduction and greatly diminished the effects of all of the organic materials, although some enhancement was observed due to addition of anthroquinone-2,6-disulfonate. These results demonstrate that NOM can enhance iron reduction by electron shuttling and by complexation mechanisms.     

Modes of natural organic matter fouling during ultrafiltration

The fouling of ultrafiltration membranes by natural organic matter (NOM), isolated from a potable surface water source, was studied with an emphasis on elucidating fouling modes and the role of aggregates. NOM size was related to membrane pore sizes using parallel membrane fractionation and size exclusion chromatography, such analyses confirmed the predominance of low MW species and identified the presence of aggregates in concentrated NOM solutions. Cake formation was the dominant mode of fouling by the unfiltered feed, which contained aggregates. This was identified by a constant rate of increase in membrane resistance with permeate throughput and was independent of pore size over a 10-1000 kDa molecular weight cutoff (MWCO) range. Prefiltration (to remove aggregates) and dilution (to reduce aggregate concentration) reduced the rate of increase in membrane resistance for the low MWCO membranes but did not change the fouling mode. In contrast, such pretreatment prevented cake formation on the larger MWCO membranes and shifted the mode of fouling to pore blockage. The date lend support for the idea that an initial fouling layer of large aggregates can catalyze the fouling by lower MW species. The fouling layer could be removed from the large MWCO membranes by backwashing, but the lower MWCO membranes exhibited some irreversible fouling, suggesting that low MW species penetrated into the pore structure. A combined pore blockage-cake formation model described the data well and provided insight into how fouling modes evolve during filtration.     

Impact of natural organic matter on uranium transport through saturated geologic materials: from molecular to column scale

The risk stemming from human exposure to actinides via the groundwater track has motivated numerous studies on the transport of radionuclides within geologic environments; however, the effects of waterborne organic matter on radionuclide mobility are still poorly understood. In this study, we compared the abilities of three humic acids (HAs) (obtained through sequential extraction of a peat soil) to cotransport hexavalent uranium (U) within water-saturated sand columns. Relative breakthrough concentrations of U measured upon elution of 18 pore volumes increased from undetectable levels (<0.001) in an experiment without HAs to 0.17 to 0.55 in experiments with HAs. The strength of the HA effect on U mobility was positively correlated with the hydrophobicity of organic matter and NMR-detected content of alkyl carbon, which indicates the possible importance of hydrophobic organic matter in facilitating U transport. Carbon and uranium elemental maps collected with a scanning transmission X-ray microscope (STXM) revealed uneven microscale distribution of U. Such molecular- and column-scale data provide evidence for a critical role of hydrophobic organic matter in the association and cotransport of U by HAs. Therefore, evaluations of radionuclide transport within subsurface environments should consider the chemical characteristics of waterborne organic substances, especially hydrophobic organic matter.     

Effects of natural organic matter and ionic species on membrane surface charge

The surface charges of clean and natural organic matter (NOM)-adsorbed membrane surfaces of two different types of membranes (a UF and a NF membrane composed of the same material but having different pore sizes) were investigated. Concentrated NOM and its fractionated constituents were used as adsorbate and interacting macromolecules nearthe membrane surface. The zeta potential and the acidity of membranes were measured using electrophoresis and potentiometric titration methods, respectively, from the perspective of charge characterization, along with demonstration of ionic strength effects. The membrane surface was also characterized with attenuated total refractive Fourier transform infrared spectra to determine intrinsic functional groups and those changes before and after NOM adsorption. As a comparative study for the electrokinetic property of membrane, the zeta potentials for both examined polymeric membranes were determined by the electrophoresis and the streaming potential measurement methods as functions of ionic strength and the pH of measuring solution. Selectivity tests were performed to decide the relative importance of charge valence of cation in terms of the surface charge of membrane. It was demonstrated that divalent cations (Ca2+, Mg2+) increase zeta potentials relatively compared to monovalent cations (Na+, K+) because divalent cations have a greater potential in approaching membrane surfaces (i.e., inside the Stern layer). Thus, divalent cations can provide a greater double layer compaction and, when near the shear plane (available for both the zeta potential measurement methods), exist to a lesser extent than monovalent cations.     

Compound-specific factors influencing sorption nonlinearity in natural organic matter

Nonlinear sorption by natural organic matter may have a significant impact on the behavior of organic contaminants in soils and sediments. This study presents a molecular probe approach based on linear solvation energy relationships (LSERs) to identify and quantify the molecular interactions causing concentration-dependent sorption and proposes estimation methods for sorption nonlinearities. Sorption isotherms ranging over concentrations of more than 4 orders of magnitude were determined in batch systems for 23 and 16 chemically diverse probe compounds in a lignite sample and a peat soil, respectively. Each sorbent showed characteristic nonlinear sorption with Freundlich exponents (1/n) being 0.7-1. The LSER-based analysis revealed that the strength of nonspecific interactions did not vary with concentration for both sorbents. In lignite, specific interactions did not affect sorption nonlinearity either, suggesting that compound-independent factors of lignite were responsible for the nonlinear sorption. In the peat soil, by contrast, the specific interactions related to the solute polarizability/dipolarity parameter (S) decreased with increasing concentration. Consequently, compounds of higher S values were more susceptible to nonlinear sorption in the peat soil. Phenol probes have shown that hydrogen bond donating properties of sorbate compounds have a substantial impact on the overall strength of sorption with organic matter, but no significant influence on sorption nonlinearity. Heterocyclic aromatic compounds appear to undergo additional interactions that are not accounted for by the LSER. These additional interactions considerably enhance both sorption capacity and nonlinearity.     

Stoichiometry of removal of natural organic matter by ion exchange

Five anion exchange resins, including a magnetic ion exchange (MIEX) resin, were evaluated for removal of Suwannee River Fulvic Acid (SRFA) in the presence of bicarbonate and chloride. The charge density of SRFA, obtained by potentiometric titration, was used to perform charge balances for ion exchange reactions involving SRFA, bicarbonate, and chloride under different solution conditions. The results clearly show the equivalence of SRFA uptake and chloride release by ion exchange. Although the structure of the anion exchange resins did not affect the stoichiometry of the reaction, the polyacrylic resins did exhibit greater removal of SRFA than the polystyrene resins. The hindered removal of SRFA by the polystyrene resins was hypothesized to be a result of size exclusion. The MIEX resin, which has a polyacrylic structure, performed similarly to the other polyacrylic resins. For the MIEX resin, the separation factor for SRFA over chloride was approximately 8 times greater than for bicarbonate over chloride. This work provides an improved understanding of the interactions between natural organic matter (NOM), inorganic anions, and anion exchange resins, and should result in more effective applications of ion exchange for the removal of NOM in the treatment of drinking water.     

Interactions between natural organic matter and gold nanoparticles stabilized with different organic capping agents

The adsorption of natural organic matter (NOM) to the surfaces of natural colloids and engineered nanoparticles is known to strongly influence, and in some cases control, their surface properties and aggregation behavior. As a result, the understanding of nanoparticle fate, transport, and toxicity in natural systems must include a fundamental framework for predicting such behavior. Using a suite of gold nanoparticles (AuNPs) with different capping agents, the impact of surface functionality, presence of natural organic matter, and aqueous chemical composition (pH, ionic strength, and background electrolytes) on the surface charge and colloidal stability of each AuNP type was investigated. Capping agents used in this study were as follows: anionic (citrate and tannic acid), neutral (2,2,2-[mercaptoethoxy(ethoxy)]ethanol and polyvinylpyrrolidone), and cationic (mercaptopentyl(trimethylammonium)). Each AuNP type appeared to adsorb Suwannee River Humic Acid (SRHA) as evidenced by measurable decreases in zeta potential in the presence of 5 mg C L(-1) SRHA. It was found that 5 mg C L(-1) SRHA provided a stabilizing effect at low ionic strength and in the presence of only monovalent ions while elevated concentrations of divalent cations lead to enhanced aggregation. The colloidal stability of the NPs in the absence of NOM is a function of capping agent, pH, ionic strength, and electrolyte valence. In the presence of NOM at the conditions examined in this study, the capping agent is a less important determinant of stability, and the adsorption of NOM is a controlling factor.     

Effect of process variables and natural organic matter on removal of microcystin-LR by PAC-UF

The release of cyanobacterial toxins, such as microcystin-LR, in drinking water supplies is of increasing concern. In this study, we investigated the use of ultrafiltration (UF) combined with adsorption on powdered activated carbon (PAC) for the removal of microcystin-LR from drinking water. Process variables examined included PAC type, PAC dosage, membrane characteristics (material and pore size), and the presence of natural organic matter (NOM). Due to greater mesopore volume, wood-based activated carbon was up to 4-times more effective at removing microcystin-LR than coconut-based carbon, depending on contact time. Cellulose acetate (CA) membranes with a molecular weight cutoff (MWCO) of 20,000 Da did not reject or adsorb microcystin-LR. Membranes composed of polyethersulfone (PES) of similar pore size, on the other hand, adsorbed microcystin-LR presumably through hydrophobic interactions. A PES membrane with a MWCO of 5000 Da sorbed microcystin-LR, and also rejected 8.4% of the toxin through a size exclusion mechanism. When PAC was coupled to UF using PES membranes, greater removal of microcystin-LR occurred compared to when CA membranes were used due to sorption of the toxin to the PES membrane surface. The presence of Suwannee River fulvic acid (SRFA) reduced microcystin-LR removal by PAC-UF, primarily due to competition between SRFA and microcystin-LR for sites on the PAC surface.     

Nature and abundance of organic radicals in natural organic matter: effect of pH and irradiation

Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and biochemical processes. A major property, its redox behavior, can be attributed to the chinone building blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data indicating free radicals on solid NOM are sparse. Here we present EPR spectra of 23 NOM from European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM ranged from 5 x 10(15) to 1.84 x 10(17) spins g(-1), and g values ranged from 2.0031 to 2.0045. Number and type of organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR experiments indicate the presence of semiquinone-type radicals in coexistence with carbon-centered "aromatic" radicals, with the semiquinone-type radicals dominating at alkaline pH. Basically these processes are reversible. Organic radical concentrations in NOM adjusted to pH 6.5 before freeze-drying correlate with iron and aluminum contents. UV- and VIS-irradiation of solid NOM can lead to more than a 10-fold increase of the concentration of organic radicals. These radicals were long-lived and had the same g value as the original radical. Similar effects were not observed with isolated humic and fulvic acids, demonstrating the limited reflection of environmental properties of organic carbon by the classical isolation procedure.     

Impact of natural organic matter on floc size and structure effects in membrane filtration

Hematite (10 mg of Fe/L) floc-humic acid assemblages have been formed at pH 4 either by first aggregating hematite particles with salt (100 mM KCl) and then adding humic acid (salt-particle-organic or SPO assemblages) or by suspending the hematite particles in humic acid solutions and then adding salt to induce aggregation (organic-particle-salt or OPS assemblages). The behavior of these assemblages upon deposition on microfiltration (MF) membranes has then been investigated. In the OPS case, the fractal dimension (dF) of the assemblages formed varied dramatically depending upon the extent of charge neutralization by added fulvic acid with dF values typical of diffusion-limited cluster aggregates at low (0.1-0.2 mg/L) humic acid concentrations and dF values typical of reaction-limited cluster aggregates either in the absence of humic acid or concentrations greater than 0.4-0.6 mg/L. In the SPO case, dF values on the order of 2.1 were initially observed and were found to decrease to around 1.8-1.9 for humic acid concentrations greater than 0.6-0.8 mg/L. OPS assemblages with low fractal dimensions were found to be highly compressible once deposited on MF membranes with significantly higher specific cake resistances than was the case for SPO assemblages at transmembrane pressures of 50 kPa and above. These results highlight the importance of both the choice of coagulant (e.g., preformed vs formed in situ) and the transmembrane pressure to which a membrane filtration process might be allowed to rise prior to removal of the fouling layer.     

Breakage, regrowth, and fractal nature of natural organic matter flocs

The growth, breakage, regrowth, and fractal nature of flocs was investigated by use of a laser diffraction particle sizing device. A range of coagulants were investigated for the coagulation of natural organic matter (NOM) and compared to other coagulated systems. The results showed NOM floc structural characteristics varied in steady-state size depending upon which coagulant was used. When compared to other systems, the order of floc size was Fe precipitate > Fe-NOM > latex (in NaCl solution). Floc regrowth after exposure to high shear was limited for all of the flocs under investigation other than for latex in an inert electrolyte. This highlighted differences in the internal bonding structure of flocs, with the results suggesting that physical bonds have a capacity to re-form after breakage. Fractal dimension analysis by small-angle laser light scattering (SALLS) had limited applicability to large flocs that dominated all of the systems under investigation, but the degree of compaction increased as flocs were broken in high shear. This provided a possible mechanistic reason for the irreversible breakage seen.