Natural Organic Matter Aggregation Research Articles

Linking water quality, fouling layer composition, and performance of reverse osmosis membranes

Fouling of polyamide membranes during reverse osmosis (RO) is a major challenge for adopting membrane technologies to treat highly contaminated waters, especially those containing organic foulants (e.g., natural organic matter (NOM), polysaccharides) and dominant cations (e.g., sodium, magnesium, calcium). This work combines bench-scale membrane fouling experiments with detailed characterization of feedwater chemistry and fouling layer composition/morphology to reveal fundamental mechanisms of (in)organic fouling during RO. Divalent cations are shown to promote fouling by hydrophobic NOM containing aromatic and carboxyl groups, while NOM fouling in the presence of a monovalent cation, sodium, occurs by smaller fulvic acids containing larger fractions of carboxyl groups and other oxygen-rich moieties. Calcium-carboxyl bridging occurs in solution and near the membrane surface to induce NOM aggregation on nanometer length scales. In complex waters containing foulant mixtures, co-fouling by calcium-carboxyl bridging and CaCO3 precipitation influence membrane performance at longer timeframes. However, the flux decline observed for the co-fouling mechanism was less significant than the sum of its parts, suggesting both synergistic and antagonistic fouling mechanisms should be considered in membrane design/operation. These results encourage the design of pretreatment processes to reduce concentrations of multivalent ions and hydrophobic NOM in RO feedwaters, and of membrane materials to limit attachment/deposition of aggregates to/on polyamide surfaces.

Undiscovered Multiple Roles of Multivalent Cations in the Pollutant Removal from Actual Water by Persulfate Activated by Carbon Materials

Enormous works have focused on the influence of natural organic matter (NOM) and anions (e.g., Cl– and HCO3–) on the pollutant removal efficiency by persulfate activation technologies. However, the impact of multivalent cations in actual water, e.g., Ca2+, on the pollutant removal has been overlooked in past research. In this work, we surprisingly found that pollutant degradation by the carbon materials activating persulfate in some actual water (e.g., groundwater) was higher than that in the buffer solution, and the bridging effect of multivalent cations in the actual water was the main reason. Through the bridging effect of multivalent cations, persulfate was bound on the surface of carbon materials to accelerate its decomposition. However, the promotion mechanism of multivalent cations on the radical and nonradical pathways of persulfate activation was different. Furthermore, the NOM aggregation was also promoted by multivalent cations, thereby inhibiting its quenching effect on the pollutant degradation. Besides that, these multivalent cations could lead to the coagulation of carbon materials, which was harmful for its catalytic performance. This work highlights the multiple roles of multivalent cations in the actual water in the persulfate activated by the carbon material process, and it was conducive to the practical application of Fenton-like technologies in actual water treatment.

Spatial Distribution and Location of Natural Organic Matter on Sediment Particles by Scanning Electron Microscopic Analysis and the Development of a New Persistent Organic Pollutant-Sediment Kinetic Desorption Model.

Natural organic matter (NOM) has long been shown to be the dominant factor in determining equilibrium and kinetic processes during sorption and desorption phenomena in sediment and soil experiments. Although several models have been suggested for predicting these processes, few offer mechanistic interpretations because the spatial location of organic matter on sediment particles is unknown. This investigation manually examined sediment particles from multiple locations, containing varying concentrations of NOM, using scanning electron microscopy with energy dispersive X-ray spectroscopy to determine the types of particles present by categorizing them as individual particles, aggregates, and "other" (detritus, algae, etc.). These types of particles were subsequently analyzed for their elemental composition, specifically the spatial location of carbon. By creating a carbon map of each particle, this investigation has determined that organic matter tends to occur in 2 forms: large aggregates or dispersed across individual sediment particles. These findings were then used to propose a more mechanistically sound mathematical model for pollutant desorption phenomena, assigning the traditional labile kinetic release component to the dispersed NOM spread randomly across sediment particles and the nonlabile kinetic release component to diffusion from densely packed NOM aggregates. Environ Toxicol Chem 2021;40:323-332. © 2020 SETAC.

A Mechanistic Exploration of Natural Organic Matter Aggregation and Surface Complexation in Smectite Mesopores

Soil minerals and organic matter play critical roles in nutrient cycling and other life-essential biogeochemical processes, yet the structural and dynamical details of natural organic matter (NOM) film formation on smectites are not fully understood on the molecular scale. XRD of Suwannee River NOM-hectorite (a smectite clay) complexes shows that the humic and fulvic components of NOM bind predominantly at the external surfaces of packets of smectite platelets rather than in the interlayer slit pores, suggesting that the key behavior governing smectite-NOM interactions takes place in mesopores between smectite particles. New molecular dynamics modeling of a ∼110 Å H2O-saturated smectite mesopore at near-neutral pH shows that model NOM molecules initially form small clusters of 2-3 NOM molecules near the center of the pore fluid. Formation of these clusters is driven by the hydrophobic mechanism, where aromatic/aliphatic regions associate with one another to minimize their interactions with H2O, and charge-balancing cations associated with the deprotonated carboxylate sites are located only at the outer surface of these clusters. Despite hydrophobicity driving the initial clustering, NOM clusters are formed more quickly when high-charge-density cations like Ca2+ are present vs low-charge-density cations like Cs+, as the former cations more effectively minimize the electrostatic repulsions between the negatively charged NOM molecules. Once the small hydrophobicity-driven NOM clusters form, the simulations show that Ca2+ promotes the aggregation of NOM clusters through tetradentate Ca2+ bridges involving carboxylate groups on two different NOM clusters. Importantly, our studies indicate that Ca2+ plays a crucial role in binding the NOM clusters to the smectite surface, which occurs through multiple quaternary complexes (Ob)-H2O-Ca2+-COO-NOM. In contrast, Cs+ never forms any coordination or acts like bridges between NOM molecules nor as ion bridges to the smectite surface. Additionally, we observe the formation of a metastable superaggregate involving all 16 NOM molecules several times in a Ca2+-bearing mesopore fluid. Superaggregates are never observed in the simulations involving Cs+. The modeling results are fully consistent with helium ion microscope images of NOM-hectorite complexes suggesting that NOM surface films develop when preformed NOM clusters interact with smectite surfaces. Overall, the binding of NOM clusters to the outer surfaces of smectite particles and the formation of large NOM aggregates at neutral pH occur through cation bridging, and cation bridging only occurs when high-charge-density cations like Ca2+ are present.

Partitioning of uranyl between ferrihydrite and humic substances at acidic and circum-neutral pH

As part of a larger study of the reactivity and mobility of uranyl (U(VI)O22+) cations in subsurface environments containing natural organic matter (NOM) and hydrous ferric oxides, we have examined the effect of reference humic and fulvic substances on the sorption of uranyl on 2-line ferrihydrite (Fh), a common, naturally occurring nano-Fe(III)-hydroxide. Uranyl was reacted with Fh at pH 4.6 and 7.0 in the presence and absence of Elliott Soil Humic Acid (ESHA) (0–835ppm) or Suwanee River Fulvic Acid (SRFA) (0–955ppm). No evidence was found for reduction of uranyl by either form of NOM after 24h of exposure. The following three size fractions were considered in this study: (1) ≥0.2μm (Fh–NOM aggregates), (2) 0.02–0.2μm (dispersed Fh nanoparticles and NOM macro-molecules), and (3) <0.02μm (dissolved). The extent to which U(VI) is sorbed in aggregates or dispersed as colloids was assessed by comparing U, Fe, and NOM concentrations in these three size fractions. Partitioning of uranyl between Fh and NOM was determined in size fraction (1) using X-ray absorption spectroscopy (XAS). Uranyl sorption on Fh–NOM aggregates was affected by the presence of NOM in different ways depending on pH and type of NOM (ESHA vs. SRFA). The presence of ESHA in the uranyl–Fh–NOM ternary system at pH 4.6 enhanced uranyl uptake more than the presence of SRFA. In contrast, neither form of NOM affected uranyl sorption at pH 7.0 over most of the NOM concentration range examined (0–500ppm); at the highest NOM concentrations (500–955ppm) uranyl uptake in the aggregates was slightly inhibited at pH 7.0, which is interpreted as being due to the dispersion of Fh aggregates. XAS at the U LIII–edge was used to characterize molecular-level changes in uranyl complexation as a result of sorption to the Fh–NOM aggregates. In the absence of NOM, uranyl formed dominantly inner–sphere, mononuclear, bidentate sorption complexes on Fh. However, when NOM concentration was increased at pH 4.6, the proportion of uranyl–Fh inner-sphere sorption complexes decreased relative to uranyl–ESHA or uranyl–SRFA complexes, which comprised up to ∼60% of the total uranyl in the systems studied. At pH 7.0, uranyl–NOM complexes were also present in the Fh–NOM aggregates in the concentration ranges of ESHA or SRFA considered; however, the proportion of these complexes was smaller at pH 7.0 than at pH 4.6 and did not increase significantly with increasing NOM concentration.

Size, speciation and lability of NOM–metal complexes in hyperalkaline cave dripwater

Transport of trace metals by natural organic matter (NOM) is potentially an important vector for trace metal incorporation in secondary cave precipitates [speleothems], yet little is known about the size distribution, speciation and metal binding properties of NOM in cave dripwaters. A hyperalkaline cave environment (ca. pH 11) was selected to provide information on colloid–metal interactions in cave waters, and to address the lack of high-pH data in natural systems in general. Colloidal (1 nm–1 μm) NOM in hyperalkaline cave dripwater from Poole’s Cavern, UK, was characterised by flow field–flow fractionation (FlFFF) coupled to UV and fluorescence detectors and transmission electron microscopy (TEM) coupled to X-ray energy-dispersive spectroscopy (X-EDS); trace-metal lability was examined by diffusive gradients in thin films (DGT). Colloidal aggregates and small particulates (>1 μm) imaged by TEM were morphologically heterogeneous with qualitative elemental compositions (X-EDS spectra; n = 41) consistent with NOM aggregates containing aluminosilicates, and iron and titanium oxides. Globular organic colloids, with diameters between ca. 1 and 10 nm were the most numerous colloidal class and exhibited high UV-absorbance (254 nm) and fluorescence intensity (320:400 nm excitation: emission) in optical regions characteristic of humic-like compounds. Metal binding with humic substances was modelled using the WHAM 6.1 (model VI) and visual MINTEQ 3.0 (NICA-Donnan) speciation codes. At pH 11, both models predicted dominant humic binding of Cu (ca. 100%) and minimal binding of Ni and Co (ca. <1–7%). A DGT depletion experiment (7 days duration) with the hyperalkaline dripwater showed that the available proportion of each metal was much lower than its total concentration. Metal availability for DGT in the initial stages (24 h) was consistent with weaker binding of alkaline earth metals by humic substances (Ba > Sr > V > Cu > Ni > Co), compared to the transition metals. Integrated over the entire experiment, the DGT-available proportion of transition metals (Ni > Cu & V >> Co) differed greatly from the expected hierarchy from WHAM and MINTEQ, indicating unusually strong complexation of Co. Total metal concentrations of Cu, Ni, and Co in raw and filtered PE1 dripwater samples ( n = 53) were well correlated (Cu vs. Ni, R 2 = 0.8; Cu vs. Co, R 2 = 0.5) and were strongly reduced (> ca. 50%) by filtration at ca. 100 and 1 nm, indicating a common colloidal association. Our results demonstrate that soil-derived colloids reach speleothems, despite transport through a karst zone with potential for adsorption, and that NOM is a dominant complexant of trace metals in high pH speleothem-forming groundwaters.

Molecular dynamics simulation of cationic complexation with natural organic matter

SummaryMolecular computer modelling of natural organic matter (NOM) and its interactions with metal cations in aqueous solution is a highly effective tool for helping to understand and quantitatively predict the molecular mechanisms of metal‐NOM complexation. This paper presents the results of molecular dynamics (MD) computations of the interaction of NOM with dissolved Na+, Cs+, Mg2+ and Ca2+. They show that Na+ forms only very weak outer‐sphere complexes with NOM, whereas Cs+ interacts somewhat more strongly, but also mainly via outer‐sphere association. Mg2+ interacts little with NOM due to its strongly held hydration shell. Ca2+ has the strongest association with NOM and forms inner‐sphere complexes with NOM carboxylate groups. This last result supports the idea of supramolecular, Ca‐mediated NOM aggregation. Cation‐NOM binding occurs principally with carboxylate groups, and to a lesser extent with phenolic and other –R‐OH groups. The contributions of other NOM functional groups are minimal. The diffusional mobility of NOM‐bound cations is ∼20% (NOM‐Na+ outer‐sphere complex) to ∼95% (NOM‐Ca2+ inner‐sphere complex) less than in aqueous solutions without NOM. The MD simulation results are in good agreement with NMR spectroscopic measurements for Cs‐NOM solutions.

Low-pressure membrane (MF/UF) fouling associated with allochthonous versus autochthonous natural organic matter

Natural organic matter (NOM) isolates/fractions; organic colloids, and hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) fractions; isolated from a natural surface water as an allochthonous source, and in the form of algal organic matter (AOM) derived from blue green algae as an autochthonous source, were investigated in low-pressure membrane filtration. The most significant flux decline was caused by organic colloids, with an intermediate flux decline caused by AOM derived (isolated) from ground and sonicated blue green algae. 3D fluorescence excitation–emission matrix (EEM) analyses revealed that colloids and AOM contain protein-like substances, and FTIR analyses showed overlapping peaks associated with the peptide bonds in proteins and alcohols in polysaccharides originating from extra- and/or intra-cellular materials. HP-SEC results also support a high content of apparently macromolecular compounds in the colloid fraction. The presence of a divalent cation (Ca 2+), hypothesized to enhance fouling by NOM acids by a reduction in molecular charge, showed little effect. Morphological analyses indicated that the surface topography of fouled UF membranes was elevated, presumably due to deposition of NOM on the membrane surface. The pores of MF membranes were reduced, suggesting pore blockage and/or constriction by NOM aggregates.

Conformations and aggregate structures of sorbed natural organic matter on muscovite and hematite

In-solution atomic-force microscopy was used to characterize molecular dimensions and aggregate structures of natural organic matter (NOM) sorbed to the basal-plane surfaces of muscovite and hematite as a function of pH (3–11), ionic strength (0.001–0.3 M), NOM concentration (20–100 mgC L −1), and cation electrolyte identity (Ca, Na, Li added as Cl-salts). Electrolyte identity and concentration exerted important effects on image stability, particle size, and sorption density. On mica, the presence of either Ca 2+ or Li + led to more stable images than Na +. For example, at pH 3, we were unable to obtain stable images of NOM on mica in NaCl. However, at pH 3 in CaCl 2 and LiCl solutions, we observed adsorbed spheres with diameters appropriate for single molecules. A higher apparent adsorption density was observed in CaCl 2 than in LiCl, consistent with previous reports that Ca enhances NOM adsorption. In LiCl, the spheres often were aggregated into small groups whereas in CaCl 2, they were mostly isolated. At intermediate pH and relatively low NOM concentrations on mica, larger spherical aggregates were observed, but at higher NOM concentrations, we observed ring structures with nanoporosity that could be important for partitioning of organic pollutants. At high pH on mica, we observed an highly ordered array which most probably indicates reordering of the mica surface structure as Si released by dissolution interacts with NOM. This indicates that NOM may play an important role in phyllosilicate diagenesis. On hematite at pH 4 and in the presence of high dissolved iron concentration, large spherical NOM aggregates were observed, consistent with observations by Myneni et al. (1999) using in solution X-ray imaging. Overall our results demonstrate that NOM sorbs in complex structures and aggregates. Therefore, current models of NOM sorption are likely overly simplistic and require further direct verification.

Dialysis Investigations of Atrazine-Organic Matter Interactions and the Role of a Divalent Metal

Contamination of surface and groundwaters by atrazine is a problem for water utilities in the United States and Europe. Though not removed by traditional liquid-solid separation, it is removed by nanofiltration. Retention of atrazine by a series of dialysis membranes was investigated over a range of solution chemistries with the goal of better understanding how the solution matrix may influence atrazine retention by membranes. Atrazine was significantly retained by membranes with molecular mass cut-offs less than 500 Da in the presence of natural organic matter (NOM), presumably by association with NOM. Atrazine retention was independent of the initial concentration. Solution chemistry was important in determining the extent of atrazine retention. Where NOM aggregation and hence NOM retention increased, atrazine retention decreased, and vice versa. Atrazine retention tended to decrease at higher ionic strengths. This effectwas significantly stronger with divalent calcium than with monovalent sodium. Partitioning coefficients calculated here are much higher than values reported from experiments of atrazine retention by soil organic matter. It is speculated that the most likely mechanism of atrazine retention by NOM is through association of atrazine with interior adsorption sites on the NOM molecule by transitory hydrogen bonding and subsequent physical entrapment.