Colloidal Natural Organic Matter Research Articles

Microbial Life on the Surface of Microplastics in Natural Waters

Major water-polluting microplastics (for example, polyethylene, polypropylene and others) have lower density than water. Therefore, they are concentrated in the neustonic layer near the water-air interface altogether with dissolved or colloidal natural organic matter, hydrophobic cells and spores of bacteria. This can cause environmental and public health problems because the floating micro- and nanoparticles of plastics could be coated with biofilm of hydrophobic and often putative pathogenic bacteria. Biofilm-coated microplastics are more attractive for consumption by aquatic animals than pure microplastics, and that increases the negative impacts of microplastics. So, impacts of even small quantities of microplastics in aquatic environments must be accounted for considering their accumulation in the micro-layer of water-air interphase and its interaction with bacterioneuston. Microorganisms attached to the surface of microplastic particles could interact with them, use them as substrates for growth, to change properties and biodegrade. The study of microbial life on the surface of microplastic particles is one of the key topics to understanding their role in the environment.

Effects of concentration and chemical composition of natural organic matter on the aggregative behavior of silver nanoparticles

Silver nanoparticles (AgNPs) are incorporated in many consumer products and have a high-releasing potential in aquatic environments. In these environments, AgNPs could interact with colloidal natural organic matter (NOM) affecting their behavior and fate. Aggregation kinetics of AgNPs were conducted at 10 NaCl concentrations during 2 h with 2.5 or 25mgNOML−1. Four (4) colloidal NOM fractions were extracted from 50 m3 of St.-Lawrence estuarine waters: fulvic acids (SLFA), humic acids (SLHA), hydrophilic (SL4ILE), and hydrophobic (SL4OBE) fractions. The addition of NOM to AgNP dispersion in nanopure water induced a small increase in AgNP size depending on NOM concentration and fraction used. The presence of both salt and NOM significantly altered the size of silver aggregates. With added 2.5mgNOML−1, more than 90% of particles have a size smaller than 200 nm, independently of the NOM fraction used. At 25mgNOML−1, the particle size distribution was strongly affected by the nature of NOM fractions. Our results show that NOM with highly aromatic carbon content such as SLFA and SLHA promoted the formation of larger aggregates with a hydrodynamic diameter larger than 200 nm. A high content in non-polar aliphatic carbon as observed in SL4ILE and SL4OBE stabilized aggregates at a size below 200 nm. This study highlights the importance of the chemical composition of NOM fractions and their content in non-polar aliphatic and aromatic carbons as they are involved in hydrophobic effects and strongly influence the aggregate size distribution. Sulfur, also present at low concentration NOM fractions, appears to be involved in the aggregation of AgNPs, but its action mechanism was not determined.

Supramolecular aggregation of colloidal natural organic matter masks priority pollutants released in water from peat soil

Natural organic matter (NOM) from Sphagnum peat soil is extracted in water and subjected to several investigations to obtain structural and conformational information. Data show that the extracted NOM is self-organized in colloidal aggregates of variable sizes (from nano to micro scales, depending on the solvent composition, i.e., ultrapure water, solutions with denaturing agents, acetone, ethanol). Aggregates are formed by highly heterogeneous classes of organic compounds. According to the results of nuclear magnetic resonance and fluorescence measurements, the three-dimensional structure of aggregates, revealed by scanning electron microscope imaging, is supposed to be stabilized by the exposition of polar functional groups to the solvent, with consequent formation of hydrogen bonds, dipole-interactions and cation bridging. In contrast, the inner part of the aggregates displays hydrophobic features and is hypothesized to be further reinforced by the establishment of π-stacking interactions. The structure is assumed to be a supramolecular aggregation of small-medium oligomeric fragments (Max 750 Da) in which priority pollutants are entrapped by dispersive forces. The structures are shown to be nanosized spheroidal particles further aggregated to form higher dimension supra-structures. Carbohydrates play primary role, stabilizing the structure and giving marked hydrophilic properties to the aggregates.

Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes.

Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ199Hg and Δ200Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg0) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg0 through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50-70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg : C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

Static and dynamic removal of aquatic natural organic matter by carbon nanotubes

Carbon nanotubes (CNTs) were investigated for their capability and mechanisms to simultaneously remove colloidal natural organic matter (NOM) and humic substances from natural surface water. Static removal testing was conducted via adsorption experiments while dynamic removal was evaluated by layering CNTs onto substrate membranes and filtering natural water through the CNT-layered membranes. Analyses of treated water samples showed that removal of humic substances occurred via adsorption under both static and dynamic conditions. Removal of colloidal NOM occurred at a moderate level of 36–66% in static conditions, independent of the specific surface area (SSA) of CNTs. Dynamic removal of colloidal NOM increased from approximately 15% with the unmodified membrane to 80–100% with the CNT-modified membranes. Depth filtration played an important role in colloidal NOM removal. A comparison of the static and dynamic removal of humic substances showed that equilibrium static removal was higher than dynamic (p < 0.01), but there was also a significant linear relationship between static and dynamic removal (p < 0.05). Accounting for contact time of CNTs with NOM during filtration, it appeared that CNT mat structure was an important determinant of removal efficiencies for colloidal NOM and humic substances during CNT membrane filtration.

Preservation of NOM-metal complexes in a modern hyperalkaline stalagmite: Implications for speleothem trace element geochemistry

We report the first quantitative study of the capture of colloidal natural organic matter (NOM) and NOM-complexed trace metals (V, Co, Cu, Ni) in speleothems. This study combines published NOM–metal dripwater speciation measurements with high-resolution laser ablation ICPMS (LA-ICPMS) and sub-annual stable isotope ratio (δ18O and δ13C), fluorescence and total organic carbon (TOC) analyses of a fast-growing hyperalkaline stalagmite (pH ∼11) from Poole’s Cavern, Derbyshire UK, which formed between 1997 and 2008 AD. We suggest that the findings reported here elucidate trace element variations arising from colloidal transport and calcite precipitation rate changes observed in multiple, natural speleothems deposited at ca. pH 7–8. We find that NOM–metal(aq) complexes on the boundary between colloidal and dissolved (∼1nm diameter) show an annual cyclicity which is inversely correlated with the alkaline earth metals and is explained by calcite precipitation rate changes (as recorded by kinetically-fractionated stable isotopes). This relates to the strength of the NOM–metal complexation reaction, resulting in very strongly bound metals (Co in this system) essentially recording NOM co-precipitation (ternary complexation). More specifically, empirical partition coefficient (Kd) values between surface-reactive metals (V, Co, Cu, Ni) [expressed as ratio of trace element to Ca ratios in calcite and in solution] arise from variations in the ‘free’ fraction of total metal in aqueous solution (fm). Hence, differences in the preservation of each metal in calcite can be explained quantitatively by their complexation behaviour with aqueous NOM. Differences between inorganic Kd values and field measurements for metal partitioning into calcite occur where [free metal]≪[total metal] due to complexation reactions between metals and organic ligands (and potentially inorganic colloids). It follows that where fm≈0, apparent inorganic Kd app values are also ≈0, but the true partition coefficient (Kd actual) is significantly higher. Importantly, the Kd of NOM–metal complexes [organic carbon–metal ratio) approaches 1 for the most stable aqueous complexes, as is shown here for Co, but has values of 24–150 for V, Ni and Cu. This implies that ternary surface complexation (metal–ligand co-adsorption) can occur (as for NOM–Co), but is the exception rather than the rule. We also demonstrate the potential for trace metals to record information on NOM composition as expressed through changing NOM–metal complexation patterns in dripwaters. Therefore, a suite of trace metals in stalagmites show variations clearly attributable to changes in organic ligand concentration and composition, and which potentially reflect the state of overlying surface ecosystems.

Mechanisms of Membrane Fouling Control by Integrated Magnetic Ion Exchange and Coagulation

Colloidal natural organic matter (NOM) is an important foulant to low-pressure membranes (LPMs) employed in drinking water treatment. Removal of colloidal NOM by magnetic ion exchange (MIEX), coagulation, and integrated MIEX and coagulation was investigated in this study to determine the relationship between colloidal NOM removal and membrane fouling reduction. The results showed that coagulation did not selectively remove colloidal NOM and the optimal coagulant dose was primarily determined by the concentration of humic substances. Comparatively, MIEX pretreatment preferentially removed humic substances and reduced the coagulant dose needed for colloidal NOM removal as a result of coagulation stoichiometry. A matched-pair analysis showed that integrated MIEX and coagulation pretreatment at much lower coagulant doses was as effective as coagulation in reducing membrane fouling. It is concluded that integrated MIEX and coagulation is potentially a viable pretreatment approach to reduce membrane fouling and in general removal of colloidal NOM in feedwater is an effective approach for membrane fouling control and should be considered in the research, development, and application of novel LPM-based treatment processes.

From soil to cave: Transport of trace metals by natural organic matter in karst dripwaters

This paper aims to establish evidence for the widespread existence of metal binding and transport by natural organic matter (NOM) in karst dripwaters, the imprint of which in speleothems may have important climatic significance. We studied the concentration of trace metals and organic carbon (OC) in sequentially filtered dripwaters and soil leachates from three contrasting sites: Poole's Cavern (Derbyshire, UK), Lower Balls Green Mine (Gloucestershire, UK) and Grotta di Ernesto (Trentino, Italy). The size-distribution of metals in the three soils was highly similar, but distinct from that found in fractionated dripwaters: surface-reactive metals were concentrated in the coarse fraction (>100nm) of soils, but in the fine colloidal (<100nm) and nominally dissolved (<1nm) fractions of dripwaters. The concentration of Cu, Ni and Co in dripwater samples across all sites were well correlated (R2=0.84 and 0.70, Cu vs. Ni, Cu vs. Co, respectively), indicating a common association. Furthermore, metal ratios (Cu:Ni, Cu:Co) were consistent with NICA-Donnan n1 humic binding affinity ratios for these metals, consistent with a competitive hierarchy of binding affinity (Cu>Ni>Co) for sites in colloidal or dissolved NOM. Large shifts in Cu:Ni in dripwaters coincided with high fluxes of particulate OC (following peak infiltration) and showed increased similarity to ratios in soils, diagnostic of qualitative changes in NOM supply (i.e. fresh inputs of more aromatic/hydrophobic soil organic matter (SOM) with Cu outcompeting Ni for suitable binding sites). Results indicate that at high-flows (i.e. where fracture-fed flow dominates) particulates and colloids migrate at similar rates, whereas, in slow seepage-flow dripwaters, particulates (>1μm) and small colloids (1–100nm) decouple, resulting in two distinct modes of NOM–metal transport: high-flux and low-flux. At the hyperalkaline drip site PE1 (in Poole's Cavern), high-fluxes of metals (Cu, Ni, Zn, Ti, Mn, Fe) and particulate NOM occurred in rapid, short-lived pulses following peak infiltration events, whereas low-fluxes of metals (Co and V>Cu, Ni and Ti) and fluorescent NOM (< ca. 100nm) were offset from infiltration events, probably because small organic colloids (1–100nm) and solutes (<1nm) were slower to migate through the porous matrix than particulates. These results demonstrate the widespread occurrence of both colloidal and particulate NOM–metal transport in cave dripwaters and the importance of karst hydrology in affecting the breakthrough times of different species. Constraints imposed by soil processes (colloid/particle release), direct contributions of metals and NOM from rainfall, and flow-routing (colloid/particle migration) are expected to determine the strength of correlations between NOM-transported metals in speleothems and climatic signals. Changes in trace metal ratios (e.g. Cu:Ni) in speleothems may encode information on NOM composition, potentially aiding in targeting of compound-specific investigations and for the assessment of changes in the quality of soil organic matter.

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.

Fouling control mechanisms of demineralized water backwash: Reduction of charge screening and calcium bridging effects

This paper investigates the impact of the ionic environment on the charge of colloidal natural organic matter (NOM) and ultrafiltration (UF) membranes (charge screening effect) and the calcium adsorption/bridging on new and fouled membranes (calcium bridging effect) by measuring the zeta potentials of membranes and colloidal NOM. Fouling experiments were conducted with natural water to determine whether the reduction of the charge screening effect and/or calcium bridging effect by backwashing with demineralized water can explain the observed reduction in fouling. Results show that the charge of both membranes and NOM, as measured by the zeta potential, became more negative at a lower pH and a lower concentration of electrolytes, in particular, divalent electrolytes. In addition, calcium also adsorbed onto the membranes, and consequently bridged colloidal NOM and membranes via binding with functional groups. The charge screening effect could be eliminated by flushing NOM and membranes with demineralized water, since a cation-free environment was established. However, only a limited amount of the calcium bridging connection was removed with demineralized water backwashes, so the calcium bridging effect mostly could not be eliminated. As demineralized water backwash was found to be effective in fouling control, it can be concluded that the reduction of the charge screening is the dominant mechanism for this.

Optimized isolation procedure for obtaining strongly actinide binding exopolymeric substances (EPS) from two bacteria ( Sagittula stellata and Pseudomonas fluorescens Biovar II)

Different chemical extractants (NaCl, EDTA, HCl and NaOH) and physical methods (ultrasonication and heating) were examined by their efficacies of extracting “attached” exopolymeric substances (EPS) secreted by marine bacterium Sagittula stellata (SS) and terrestrial bacterium Pseudomonas fluorescens Biovar II (PF). Extraction by 0.5 N HCl for 3 h was best for SS while extraction by 0.05 N NaCl for 3–5 h was regarded as optimal for PF. Improvements in EPS purification included a pre-diafiltration step to remove the broth material and reduce the solution volume, thus the usage of ethanol, and time. The EPS harvested at the optimal time and purified by the improved method were enriched in polysaccharides, with smaller amounts of proteins, thus having amphiphilic properties. Isoelectric focusing of 234Th or 240Pu labeled EPS showed both actinides were strongly bound to macromolecules with low pI, similar to reported marine or soil colloidal natural organic matter (NOM).

Application of ceramic membranes with pre-ozonation for treatment of secondary wastewater effluent

Membrane fouling is an inevitable problem when microfiltration (MF) and ultrafiltraion (UF) are used to treat wastewater treatment plant (WWTP) effluent. While historically the use of MF/UF for water and wastewater treatment has been almost exclusively focused on polymeric membranes, new generation ceramic membranes were recently introduced in the market and they possess unique advantages over currently available polymeric membranes. Ceramic membranes are mechanically superior and are more resistant to severe chemical and thermal environments. Due to the robustness of ceramic membranes, strong oxidants such as ozone can be used as pretreatment to reduce the membrane fouling. This paper presents results of a pilot study designed to investigate the application of new generation ceramic membranes for WWTP effluent treatment. Ozonation and coagulation pretreatment were evaluated to optimize the membrane operation. The ceramic membrane demonstrated stable performance at a filtration flux of 100 gfd (170 LMH) at 20 °C with pretreatment using PACl (1 mg/L as Al) and ozone (4 mg/L). To understand the effects of ozone and coagulation pretreatment on organic foulants, natural organic matter (NOM) in four waters – raw, ozone treated, coagulation treated, and ozone followed by coagulation treated wastewaters – were characterized using high performance size exclusion chromatography (HPSEC). The HPSEC analysis demonstrated that ozone treatment is effective at degrading colloidal NOMs which are likely responsible for the majority of membrane fouling.

Direct-flow microfiltration of aquasols: II. On the role of colloidal natural organic matter

Natural organic matter (NOM) has been considered a major contributor to the fouling of microfiltration (MF) and ultrafiltration (UF) membranes employed in water treatment. However, the fouling potential of NOM has often been assessed in terms of its size or chemical composition. The colloid’s chemical properties have often been ignored. In this study, a chemical attachment-based (CAB) model established previously was used in conjunction with a variety of analytical techniques to investigate the existence of three major components of an aquatic NOM and their role in the fouling of a polyvinylidene fluoride MF membrane. The results suggest that colloidal NOM relevant to membrane fouling has a broader size distribution and variations in chemical properties than proposed previously. For the model aquatic NOM used in this research, fouling was primarily contributed by both non-humic and humic colloidal fractions. The non-humic colloids were larger in size and probably adhered to the membrane regardless of the solution chemistry, while humic colloids had variable size and stickiness depending on solution chemistry. The fouling caused by organic colloids was mostly hydraulically irreversible, as a consequence of favorable surface interactions. The CAB model provided a useful way to understand the role of organic colloids in membrane fouling.

Effect of membrane character and solution chemistry on microfiltration performance

To help understand and predict the role of natural organic matter (NOM) in the fouling of low-pressure membranes, experiments were carried out with an apparatus that incorporates automatic backwashing and long filtration runs. Three hollow fibre membranes of varying character were included in the study, and the filtration of two different surface waters was compared. The hydrophilic membrane had greater flux recovery after backwashing than the hydrophobic membranes, but the efficiency of backwashing decreased at extended filtration times. NOM concentration of these waters (7.9 and 9.1 mg/L) had little effect on the flux of the membranes at extended filtration times, as backwashing of the membrane restored the flux to similar values regardless of the NOM concentration. The solution pH also had little effect at extended filtration times. The backwashing efficiency of the hydrophilic membrane was dramatically different for the two waters, and the presence of colloid NOM alone could not explain these differences. It is proposed that colloidal NOM forms a filter cake on the surface of the membranes and that small molecular weight organics that have an adsorption peak at 220 nm but not 254 nm were responsible for “gluing” the colloids to the membrane surface. Alum coagulation improved membrane performance in all instances, and this was suggested to be because coagulation reduced the concentration of “glue” that holds the organic colloids to the membrane surface.

Colloidal fouling of RO membranes following MF/UF in the reclamation of municipal wastewater

Major municipal wastewater reclamation plants in California, USA, Singapore, and many built or planned for other regions of the world use the high pathogen reduction properties of microfiltration/ultrafiltration (MF/UF) membranes followed by reverse osmosis (RO) membranes. Operational experiences in these plants suggest that while MF and UF membranes encounter pore and cake fouling by colloidal particles (0.1 micron down to molecular sizes, ie. nanoparticles), a significant fraction of the smaller colloidal particles pass through the MF and UF membranes, and end up on RO membranes as cake-layer foulants. Autopsies, foulant analyses and cleaning studies performed on fouled RO membranes from the plants and pilot plants showed that colloidal natural organic matter, colloidal calcium phosphate and some times colloidal silicates are the main components. These colloidal particles have great affinity towards aggregation with each other. Due to incomplete removal by MF and even UF, fouling of RO membranes downstream becomes measurable by trend-charts of normalized values of permeate flow, differential pressure and salt passage. Normalized permeate flow is the most sensitive, and an early indicator of such fouling. In this paper we will provide some details of our studies and provide literature evidences that support the conclusion that calcium phosphate in foulants originated as nanoparticles in the wastewater.

Mechanisms of colloidal natural organic matter fouling in ultrafiltration

The role of membrane pore size and solution chemistry in ultrafiltration (UF) membrane fouling by colloidal natural organic matter (NOM) has been investigated. Fouling experiments were conducted with two laboratory-made cellulose acetate UF membranes, with estimated pore sizes of 2 and 10 nm, under identical hydrodynamic conditions. Flux decline with colloidal NOM was independent of pH and increased in the presence of calcium. The membrane pore size has been found to influence membrane flux decline. Permeate flux for the more permeable membrane (10 nm pore size) decreased faster than the flux of the tightest, less permeable membrane (2 nm pore size). The adhesion forces between the foulant (colloidal NOM) and the clean membrane, and between the foulant in bulk solution and foulant in the fouling layer, were determined by atomic force microscopy (AFM). Force measurements confirmed the fouling trends with calcium ions. For a given solution chemistry, foulant–foulant adhesion was identical for both membranes, indicating that the more pronounced flux decline observed for the largest pore-size membrane was not related to differences in intermolecular chemical interactions among NOM molecules. At the earlier stage of filtration, the dominant fouling mechanism was pore blocking for both membranes. For longer filtration times, there was a transition in the fouling mechanism from pore blocking to cake formation. This transition occurred first for the more permeable membrane, indicating that colloidal NOM penetrates more readily into the pores of this membrane and that the relative size of colloidal NOM and the membrane pore influences the rate of pore blocking. The results further suggest that the structure of the fouling layer is dependent on the operating pressure. At high pressure, the compactness and specific resistance of the colloidal NOM layer substantially increase, and thus, significantly affect the extent of flux decline.

The effect of colloid stability of wastewater treatment plants effluent on nanofiltration performance

Laboratory-scale filtration tests utilizing wastewater treatment plants (WWTP) effluent were conducted to investigate fouling and filtration behaviour, especially the influence of colloidal stability on nanofiltration performance. Acidification and coagulant dosage were used to create unstable colloidal conditions. Colloidal stability of the effluent was analysed on by zeta potential measurements. A statistical design method, full factorial design with blocking, has been used to account for effluent composition variations and to account for interaction between experimental parameters. The results show a high correlation between unstable colloidal natural organic matter and the formation of dense fouling layers. The reversibility of the fouling process was shown to be independent from colloidal stability. Irreversible fouling was show to be promoted by mild acidic (pH 5) effluent conditions, whereas coagulant addition showed an increase in the reversibility of the fouling.

Characterizations of the colloidal and microbial organic matters with respect to membrane foulants

Nanofiltration has been studied and operated as an advanced water treatment process with respect to the removal of natural organic matter (NOM) and micropollutants throughout the world. However, fouling is an inherent phenomenon that reduces membrane performance, such as the flux and removal efficiency. In this study, filtration with a semi-pilot type of nanofiltration was conducted with coagulation/sedimentation treated water for the removal of NOM at a drinking water treatment plant in Korea. However, severe flux decline and reduction of removal efficiency were observed after 3 months. Tested membranes were autopsied using cleaning agents, including pure water and caustic solutions. The foulants eluted with the cleaning agents exhibited relatively high molecular weight (MW) distributions, as determined by a high performance size exclusion chromatography method, and also contained polysaccharide and/or protein-like substances identified from IR spectra analysis. To identify the compounds with high MW values, a dialysis membrane bag, with MWCO range from 12,000 to 14,000 Da, was used to isolate the colloidal NOM from raw surface water from the Dongbok Lake in Korea. The isolated compounds were compared to various organic compounds, including algogenic organic matter (AOM), bovine serum albumin, humic acids from the Suwanee River and soluble microbial products (SMP), in terms of their size, structure, IR analysis and pyrolysis gas chromatography–mass spectrometry analysis. The colloidal NOM was highly hydrophilic, as identified from XAD resin fractionation method. The organic foulants from the tested NF membranes, with high MW and hydrophilicity, and the saccharides and/or proteins-like substances (i.e., colloidal NOM) had similar characteristics to microbial by-products, such as SMP and AOM.

Biofouling potential of various NF membranes with respect to bacteria and their soluble microbial products (SMP): Characterizations, flux decline, and transport parameters

A problem of considerable concern in membrane filtration processes is biofouling with microorganisms and their soluble microbial products (SMP). This work focused on NF membrane biofouling by either bacteria (biotic filtration/biofouling) or SMP (abiotic filtration/biofouling); the former was analyzed using NF membrane filtration, the concept of concentration polarization (CP), and the convection–diffusion–electrophoretic model (CDE), using Flavobacterium lutescens bacteria, with the latter investigated through NF membrane filtrations using SMP extracted from the bacteria. Four different NF membranes were tested: piperazine versus meta-phenylene diamine (MPD)-based polyamide thin-film composite (TFC) membranes. Through membrane characterizations, including contact angle, surface charge, SEM, and AFM, the MPD-based NF membranes exhibited relatively hydrophobic and higher roughness compared to the piperazine-based NF membranes. The convection–diffusion–electrophoretic migration model, in conjunction with the CP concept, could demonstrate the electrostatic interaction of bacteria against trans-membrane pressure during filtration. With increasing operating pressure, the CP thickness of the bacteria boundary layer decreased due to compaction of the CP layer. The CP layers of bacteria for the MPD-based membranes having relatively high negative surface charges, were less compacted in thickness due to electrostatic repulsion between the membrane surfaces and bacteria. All membranes exhibited high repulsive forces with the extended DLVO (XDLVO) approach, due to the high AB interaction energy in an aquatic system. To evaluate the interactions between the membrane surface and bacteria during filtration, the Hamaker constants of various NF membranes were also determined by measuring the contact angle between the membrane surface and bacteria. NF membranes with greater hydrophobicity and roughness exhibited higher biofouling potential in terms of membrane–bacteria interactions identified from the Hamaker constants estimation. From SMP (extracted from F. lutescens at a decay condition) characterizations, some portions of colloidal NOM (i.e., with higher molecular weights) were revealed to be comprised of SMP through the IR spectra analysis. Also, they were found to have high hydrophilicity and biodegradability from XAD resins and BDOC measurements, respectively. When the SMP were compared to other macromolecules (bovine serum albumin, colloidal NOM, and Suwannee River NOM) in terms of fouling through membrane fouling experiments, they provided high membrane fouling potential. Various transport parameters (mass transfer coefficient, membrane selectivity, and solute permeability) of the SMP were determined by irreversible thermodynamic models combined with the film model to quantitatively investigate the SMP behaviors in NF membranes [S. Lee, G. Amy, J. Cho, Sherwood correlations for natural organic matter (NOM) in nanofiltration (NF) membranes and its applicability, J. Membr. Sci., 240 (1–2) (2004) 49–65]. Relatively hydrophobic and rougher membranes (i.e., MPD-based NF membrane) exhibited higher selectivity, in terms of SMP, compared to relatively hydrophilic membranes.

Biodegradability, DBP Formation, and Membrane Fouling Potential of Natural Organic Matter: Characterization and Controllability

Various natural organic matter (NOM) constituents were evaluated in terms of their biodegradability, disinfection byproduct (DBP) formation potentials, and membrane fouling. The biodegradability of NOM was evaluated with respect to biodegradable dissolved organic carbon (BDOC) and its inhibition control. NOM was divided into (i) colloidal and noncolloidal NOM, using a dialysis membrane with a molecular weight cutoff of 3500 Da and (ii) hydrophobic, transphilic, and hydrophilic NOM constituents, using XAD-8/4 resins. The colloidal, and noncolloidal hydrophilic, NOM were identified as being more problematic than the other components, exhibiting relatively higher biodegradability and reactivity toward DBP formation potential. A higher biodegradability especially can provide a high risk of membrane biofouling, if a membrane is fouled by highly biodegradable NOM. Colloidal, and noncolloidal hydrophilic, NOM constituents were also shown as major foulants of negatively charged membranes due to their high neutral fractions. Filter adsorber (F/A) types of activated carbons were evaluated in terms of removals of NOM, DBP formation potential, and BDOC and were compared to conventional processes and a nanofiltration membrane. The F/A process exhibited a comparatively good efficiency, especially in DBP and BDOC control, but was not so good at removing NOM. This suggests that F/A could potentially be combined with a membrane process to minimize the DBP formation potential and bio-/organic-fouling (i.e., F/A process as a pretreatment for a membrane process).