Flocculation of colloidal clay by bacterial polysaccharides: effect of macromolecule charge and structure

Re-entrant phase transitions in Laponite/Gum Arabic nanocomposites

A re-entrant gel transition is observed in a discotic clay, Laponite (Lap) aqueous suspensions with the addition of a complex polysaccharide, Gum Arabic (GA). For fixed concentrations of Lap (1-3 % w/v) and at low GA concentrations (CGA < 10 % w/v), the composites exhibit gel behaviour, while the suspensions undergo liquid phase separation for intermediate GA concentrations (10 % w/v < CGA < 20 % w/v). Gel behaviour is again observed in the samples at even higher GA concentrations (CGA > 30 % w/v). Here, we identify a thermodynamic phase transition in Lap/GA mixtures that is caused by a variation in GA content. The viscoelastic characteristics, phase transitions, and gelation kinetics of the Lap/GA mixtures have been studied by employing rheology, small-angle X-ray scattering, and optical investigations. Reduction of the negative values of zeta-potential and growth of the composite system's hydrodynamic size indicated the presence of interactions in Lap/GA mixtures. The phase diagram enables the apparent interactions and phase transitions between the nanoplatelets and the complex polysaccharides. Thus, our study provides new perspectives on a nanocomposite's tuneable rheological and structural features.

A Meta-Analysis to Revisit the Property-Aggregation Relationships of Carbon Nanomaterials: Experimental Observations versus Predictions of the DLVO Theory

Contradicting relationships between physicochemical properties of nanomaterials (e.g., size and ζ-potential) and their aggregation behavior have been constantly reported in previous literature, and such contradictions deviate from the predictions of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. To resolve such controversies, in this work, we employed a meta-analytic approach to synthesize the data from 46 individual studies reporting the critical coagulation concentration (CCC) of two carbon nanomaterials, namely, graphene oxide (GO) and carbon nanotube (CNT). The correlations between CCC and material physicochemical properties (i.e., size, ζ-potential, and surface functionalities) were examined and compared to the theoretical predictions. Results showed that the CCC of electrostatically stabilized carbon nanomaterials increased with decreasing nanomaterial size when their hydrodynamic sizes were smaller than ca. 200 nm. This is qualitatively consistent with the prediction of the DLVO theory but with a smaller threshold size than the predicted 2 μm. Above the threshold size, the material ζ-potential can be correlated to CCC for nanomaterials with moderate/low surface charge, in agreement with the DLVO theory. The correlation was not observed for highly charged nanomaterials because of their underestimated surface potential by the ζ-potential. Furthermore, a correlation between the C/O ratio and CCC was observed, where a lower C/O ratio resulted in a higher CCC. Overall, our findings rationalized the inconsistency between experimental observation and theoretical prediction and provided essential insights into the aggregation behavior of nanomaterials in water, which could facilitate their rational design.

Management of non-compressible hemorrhage and re-bleeding by a liquid hemostatic polysaccharide floccuronic acid

Effective management of excessive bleeding requires liquid hemostatic agents, especially in scenarios involving uncompressible and postoperative hemorrhage. This study introduces the microbial exopolysaccharide floccuronic acid (FA) as a liquid hemostatic agent, characterized by a high weight average molecular weight of 2.38 × 108 Da. The investigation focuses on the flocculation effect, hemostatic efficiency in both in vitro and in vivo settings, elucidating its hemostatic mechanism, and assessing its safety profile. Results reveal that FA solution significantly accelerates the coagulation process, leading to the formation of compact clots while specifically interfering with fibrin. Notably, FA demonstrates excellent hemostatic effects in animal liver models and a rat arterial rebleeding model. The biocompatible and biodegradable characteristics further underscore FA's potential as a valuable liquid hemostatic material, particularly suited for non-compressible and re-bleeding scenarios.

Preferential interactions of surface-bound engineered single stranded DNA with highly aromatic natural organic matter: Mechanistic insights and implications for optimizing practical aquatic applications

Engineered short-chain single stranded DNA (ssDNA) are emerging materials with various environmental applications, such as aptasensor, selective adsorbent, and hydrological tracer. However, the lack of fundamental understanding on the interactions of such materials with natural organic matter (NOM) hinders the improvement of their application performance in terms of sensitivity, selectivity, and stability. In this study, we investigated the interactions of ssDNA (four strands with systematically varied length and sequence) with two humic acids (Suwannee River humic acid (SRHA) and Aldrich humic acid (AHA)) and two humic-like NOM present in local aquatic matrices (ROM in river water and WOM in wastewater). Detailed, molecular level interaction mechanisms were obtained by probing the colloidal stability of the ssDNA-coated gold nanoparticles, coupled with product characterization using a suite of microscopic and spectroscopic techniques. Our study revealed that π-π interactions and divalent cation bridging were the major mechanisms for ssDNA-NOM interactions. ssDNA preferentially interacted with NOM with high aromaticity (AHA > SRHA/WOM/ROM). With divalent cations present (especially Ca²⁺), even a small amount of AHA could completely shield ssDNA, whereas the extent of shielding by SRHA/WOM/ROM depended on the relative content of ssDNA and NOM and whether bridges formed. The extent of shielding of ssDNA by NOM provides a potential answer to the reported conflicting effects of natural water matrices on the performance of DNA-based sensors. Taken together, our findings provide insights into the transformations of engineered ssDNA under environmentally relevant conditions as well as implications for their performance optimization in practical aquatic applications (e.g., from DNA design to pretreatment strategy).

Freshwater suspended particulate matter-Key components and processes in floc formation and dynamics

Freshwater suspended particulate matter (SPM) plays an important role in many biogeochemical cycles and serves multiple ecosystem functions. Most SPM is present as complex floc-like aggregate structures composed of various minerals and organic matter from the molecular to the organism level. Flocs provide habitat for microbes and feed for larger organisms. They constitute microbial bioreactors, with prominent roles in carbon and inorganic nutrient cycles, and transport nutrients as well as pollutants, affecting sediments, inundation zones, and the ocean. Composition, structure, size, and concentration of SPM flocs are subject to high spatiotemporal variability. Floc formation processes and compositional or morphological dynamics can be established around three functional components: phyllosilicates, iron oxides/(oxy)hydroxides (FeOx), and microbial extracellular polymeric substances (EPS). These components and their interactions increase heterogeneity in surface properties, enhancing flocculation. Phyllosilicates exhibit intrinsic heterogeneities in surface charge and hydrophobicity. They are preferential substrates for precipitation or attachment of reactive FeOx. FeOx form patchy coatings on minerals, especially on phyllosilicates, which increase surface charge heterogeneities. Both, phyllosilicates and FeOx strongly adsorb natural organic matter (NOM), preferentially certain EPS. EPS comprise various substances with heterogeneous properties that make them a sticky mixture, enhancing flocculation. Microbial metabolism, and thus EPS release, is supported by the high adsorption capacity and favorable nutrient composition of phyllosilicates, and FeOx supply essential Fe.

Silica-clay nanocomposites for the removal of antibiotics in the water usage cycle

The increasingly frequent detection of resistant organic micropollutants in waters calls for better treatment of these molecules that are recognized to be dangerous for human health and the environment. As an alternative to conventional adsorbent material such as activated carbon, silica-clay nanocomposites were synthesized for the removal of pharmaceuticals in contaminated water. Their efficiency with respect to carbamazepine, ciprofloxacin, danofloxacin, doxycycline, and sulfamethoxazole was assessed in model water and real groundwater spiked with the five contaminants. Results showed that the efficacy of contaminant removal depends on the chemical properties of the micropollutants. Among the adsorbents tested, the nanocomposite made of 95% clay and 5% SiO₂ NPs was the most efficient and was easily recovered from solution after treatment compared with pure clay, for example. The composite is thus a good candidate in terms of operating costs and environmental sustainability for the removal of organic contaminants.

Single versus multi-metal sulfide systems: The role of cysteine and complex environmental conditions

The presence of dissolved organic matter (DOM) can impact metal sulfide (MeS) precipitation and mobility. Thiol containing ligands such as cysteine have been shown to be effective capping agents in single metal MeS studies, allowing NPs to persist in oxic environments. In this study, both single (Cd or Zn) and multi-MeS (Cu, Pb, Cd, Zn, and As) nanoparticle (NP) formation was characterized to understand the impact of the thiol cysteine (CYS) on early stage (3 h) MeS NP behavior. Short duration single metal batch experiments, in the absence and presence of CYS, confirmed that MeS species readily formed solids with limited dissolved fraction; however, multi-metal systems exhibited divergent behavior reflecting a wider range of NP sizes and an increased dissolved concentration. Multi-metal batch experiments revealed that metals were generally sequestered into MeS solids in accordance with MeS solubility products (i.e., from least to most soluble: Cu > Pb ~ Cd > Zn). CYS concentrations in excess of sulfide (10:1 CYS:S ratio) stabilized MeS within the Small NP size fraction (3.2 nm < d < 43 nm) and limited Pb, Cd, and Zn dissolution compared to molar ratios of 1:1. In the combined presence of CYS and Ca²⁺, multi-MeS particle aggregation increased substantially compared to monovalent systems. Dissolution increased for Pb and Zn as a function of matrix ionic strength whereas dissolved Cu trends changed as a function of cation valence state (e.g., Na⁺ vs. Ca²⁺). Most noteworthy, single-metal Zn and Cd batch experiments demonstrated that single-metal studies can overestimate MeS NP resistance to oxidative dissolution compared to multi-metal counterparts. Thus, caution should be taken when broadly applying mechanisms and rates elucidated from single-metal systems.

Nanoscale Forces between Basal Mica Surfaces in Dicarboxylic Acid Solutions: Implications for Clay Aggregation in the Presence of Soluble Organic Acids

The stability of organomineral aggregates in soils has a key influence on nutrient cycling, erosion, and soil productivity. Both clay minerals with distinct basal and edge surfaces and organic molecules with reactive functional groups offer rich bonding environments. While clay edges often promote strong inner-sphere bonding of -COOH-laden organics, we explore typically weaker, outer-sphere bonding of such molecules onto basal planes and its significance in organomineral interactions. In this surface force apparatus study, we probed face-specific interactions of negatively charged mica basal surfaces in solutions containing carboxyl-bearing, low-molecular-weight dicarboxylic acids (DAs). Our experiments provide distance-resolved, nanometer-range measurements of forces acting between two (001) mica surfaces and simultaneously probe DA adsorption. We show that background inorganic ions display crucial importance in nanoscale forces acting between basal mica surfaces and in DA adsorption: Na⁺ contributes to strong repulsion and little binding of dicarboxylic anions, while small amounts of Ca²⁺ are sufficient to screen the basal surface charge of mica, facilitate strong adhesion, and enhance dicarboxylic anion adsorption by acting as cationic bridges. Despite reversible and weak adsorption of DAs, we resolve their multilayer binding via assembly of hydrophobic chains in the presence of Ca²⁺, pointing the importance of abundant, less reactive basal clay surfaces in organomineral interactions.

Application of Polysaccharide Biopolymer in Petroleum Recovery

Polysaccharide biopolymers are biomacromolecules derived from renewable resources with versatile functions including thickening, crosslinking, adsorption, etc. Possessing high efficiency and low cost, they have brought wide applications in all phases of petroleum recovery, from well drilling to wastewater treatment. The biopolymers are generally utilized as additives of fluids or plugging agents, to correct the fluid properties that affect the performance and cost of petroleum recovery. This review focuses on both the characteristics of biopolymers and their utilization in the petroleum recovery process. Research on the synthesis and characterization of polymers, as well as controlling their structures through modification, aims to develop novel recipes of biopolymer treatment with new application realms. The influences of biopolymer in many petroleum recovery cases were also evaluated to permit establishing the correlations between their physicochemical properties and performances. As their performance is heavily affected by the local environment, screening and testing polymers under controlled conditions is the necessary step to guarantee the efficiency and safety of biopolymer treatments.

Structure and molecular morphology of a novel moisturizing exopolysaccharide produced by Phyllobacterium sp. 921F

Bacterial exopolysaccharides (EPSs) are widely applied in food, cosmetic, and medical industries. The EPS produced by Phyllobacterium sp. 921F was a novel polysaccharide, which exhibits attractive characteristics of high yield, favorable rheological properties, and excellent moisture retention ability. Considering the complexity of polysaccharide structures, specific enzymatic hydrolysis was employed here to resolve the structure of the EPS. End-products including tetra-, hexa- and octa-saccharides were isolated. According to their mass spectroscopy (MS) and nuclear magnetic resonance (NMR) spectra, the backbone of the EPS was found to be mainly comprising a → 4)-β-d-Glcp-(1 → 3)-α-d-Galp(4,6-S-Pyr)-(1 → disaccharide repeating units. Based on atomic force microscopy results, EPS exhibited characteristics that were consistent with a stiff, elongated molecule with no branches. The length and height of the single molecular chain were approximately 600 and 0.7 nm, respectively. Our clarification of structure and molecular morphology of EPS from Phyllobacterium sp. 921F provide a foundation for the industrial application of this potential moisture-retaining material.

Hydrogels as a water bolus during hyperthermia treatment

The feasibility of using hydrogels as a water bolus during hyperthermia treatment was assessed. Three types of gels, high methoxyl (HM) pectin/alginate, xanthan/locust bean gum (LBG) and xanthan/LBG/agarose were evaluated based on their dielectric, rheological and mechanical properties. The most suitable, xanthan/LBG/agarose gel was further used as a water bolus in a hyperthermia array applicator. The gels composed of polysaccharides carrying low charge displayed dielectric properties close to those of water, while the dielectric properties of HM pectin/alginate gel was deemed unsuitable for the current application. The mechanical examination shows that the xanthan/LBG gel has a non-brittle behaviour at room temperature, in contrast to the agarose gel. The moduli of the xanthan/LBG gel weaken however considerably between the temperature range of 40 °C and 50 °C, reducing its potential to be used as water bolus. The ternary system of xanthan/LBG/agarose had advantageous behaviour as it was dominated by the thermal hysteresis typical of agarose upon temperature increase, but governed by the typical non-brittle behaviour of the xanthan/LBG at low temperatures. The final evaluation within the hyperthermia applicator showed excellent signal transmission from the antennas. The agarose/xanthan/LBG gel reduced the scattering of electromagnetic waves, enabled a tight closure between the body and the antennas, and offered a less bulky solution than the currently used water-filled plastic bags. The results presented here open up a new application area for hydrogels in improving heat delivery during hyperthermia treatment and other near-field microwave applications.

A review on the interactions between engineered nanoparticles with extracellular and intracellular polymeric substances from wastewater treatment aggregates

Engineered nanoparticles (ENPs) will inevitably enter wastewater treatment plants (WWTPs) due to their widespread application; thus, it is necessary to study the migration and transformation of nanoparticles in sewage treatment systems. Extracellular polymeric substances (EPSs) such as polysaccharides, proteins, nucleic acids, humic acids and other polymers are polymers released by microorganisms under certain conditions. Intracellular polymeric substances (IPSs) are microbial substances contained in the body with compositions similar to those of extracellular polymers. In this review, we summarize the characteristics of EPSs and IPSs from sewage-collecting microbial aggregates containing pure bacteria, activated sludge, granular sludge and biofilms. We also further investigate the dissolution, adsorption, aggregation, deposition, oxidation and other chemical transformation processes of nanoparticles, such as metals, metal oxides, and nonmetallic oxides. In particular, the review deeply analyzes the migration and transformation mechanisms of nanoparticles in EPS and IPS matrices, including physical, chemical, biological interactions mechanisms. Moreover, various factors, such as ionic strength, ionic valence, pH, light, oxidation-reduction potential and dissolved oxygen, influencing the interaction mechanisms are discussed. In recent years, studies on the interactions between EPSs/IPSs and nanoparticles have gradually increased, but the mechanisms of these interactions are seldom explored. Therefore, developing a systematic understanding of the migration and transformation mechanisms of ENPs is significant.

Enhanced transportability of zero valent iron nanoparticles in aquifer sediments: surface modifications, reactivity, and particle traveling distances

In this study, we assessed the transportability of zero valent iron nanoparticles (nano-Fe⁰) coated with different organics (carboxy methyl cellulose (CMC), poly acrylic acid (PAA), and xanthan gum) in standard porous sand and in real aquifer sediments. Our results suggest that the organic surface coatings optimized for nano-Fe⁰ in porous sand media do not necessarily reflect the same transportability in real field aquifer sediment. Xanthan gum-coated nano-Fe⁰ showed highest transportability in standard porous sand, but the performance was much lower in real aquifer sediment, whereas the PAA-coated nano-Fe⁰ particle showed better transportability both in aquifer sediment and in porous sand media. Nano-Fe⁰ without organic surface coating exhibited very low transportability and was largely retained by the porous medium. Our results suggest that the molecular weight and surface charge density of the organic may play a role in transportability of these nanoparticles. To assess the impact of organic coating on the nanoparticle reactivity with contaminants, we also conducted batch tests to follow TCE degradation using different surface coatings and found no significant difference albeit a minor delay in kinetics. Using theoretical calculations, we also estimated the potential distance traveled by nanoparticles in porous sand as well as in aquifer sediment. Our results suggest that using xanthan gum and PAA as surface coating, nano-Fe⁰ could travel up to 9.8 and 4.1 m, respectively, in the porous sand media as compared to 0.2 and 0.9 m in real aquifer sediment, respectively. Graphical abstract Nanoparticle mobility in porous sand vs and aquifer sediment.

Composition and copper binding properties of aquatic fulvic acids in eutrophic Taihu Lake, China

Fulvic acid (FA) plays a significant role in biogenic-elemental cycling in aquatic ecosystems which is highly dependent on their organic composition. In this study, the aquatic FA contents and binding properties during bloom and non-bloom periods in Taihu Lake were investigated by two-dimensional correlation spectroscopy Fourier transform infrared spectroscopy (2D-COS-FTIR), nuclear magnetic resonance (NMR) and elemental analysis. Compared with non-bloom FA, bloom FA was of lower nitrogen content and higher C/N ratio. It contained more carboxylic and aliphatic groups while less amide groups. 2D-COS-FTIR spectra evidenced the carboxyl groups in bloom FA had the fastest response to Cu(II) binding. Also, polysaccharide in bloom FA was more susceptive to Cu(II) concentrations than that in non-bloom FA. While comparing with bloom FA, the N-rich organic compounds in non-bloom FA exhibited faster binding sequence with Cu(II). A comprehensive scheme about the interaction process of FA-Cu(II) showed that both nitrogenous and oxygenic groups in FAs were active in binding to Cu(II). The alteration in binding behaviors of organic groups in FAs to Cu(II) may have been driven by algal products and microbial community variety in Taihu Lake. Our results here have the potential to contribute significantly to future studies of dissolved organic matter dynamic biogeochemistry processes and trace metal cycling processes in eutrophic lakes.

A network perspective reveals decreasing material diversity in studies on nanoparticle interactions with dissolved organic matter

Dissolved organic matter (DOM) strongly influences the properties and fate of engineered nanoparticles (ENPs) in aquatic environments. There is an extensive body of experiments on interactions between DOM and ENPs and also larger particles. [We denote particles on the nano- and micrometer scale as particulate matter (PM).] However, the experimental results are very heterogeneous, and a general mechanistic understanding of DOM-PM interactions is still missing. In this situation, recent reviews have called to expand the range of DOM and ENPs studied. Therefore, our work focuses on the diversity of the DOM and PM types investigated. Because the experimental results reported in the literature are highly disparate and difficult to structure, a new format of organizing, visualizing, and interpreting the results is needed. To this end, we perform a network analysis of 951 experimental results on DOM-PM interactions, which enabled us to analyze and quantify the diversity of the materials investigated. The diversity of the DOM-PM combinations studied has mostly been decreasing over the last 25 y, which is driven by an increasing focus on several frequently investigated materials, such as DOM isolated from fresh water, DOM in whole-water samples, and TiO₂ and silver PM. Furthermore, there is an underrepresentation of studies into the effect of particle coating on PM-DOM interactions. Finally, it is of great importance that the properties of DOM used in experiments with PM, in particular the molecular weight and the content of aromatic and aliphatic carbon, are reported more comprehensively and systematically.

Influence of extracellular polymeric substances on the aggregation kinetics of TiO2 nanoparticles

The early stage of aggregation of titanium oxide (TiO₂) nanoparticles was investigated in the presence of extracellular polymeric substance (EPS) constituents and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). The hydrodynamic diameter was measured and the subsequent aggregation kinetics and attachment efficiencies were calculated across a range of 1-500 mM NaCl and 0.05-40 mM CaCl₂ solutions. TiO₂ particles were significantly aggregated in the tested range of monovalent and divalent electrolyte concentrations. The aggregation behavior of TiO₂ particles in electrolyte solutions was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Divalent electrolytes were more efficient in destabilizing TiO₂ particles, as indicated by the considerably lower critical coagulation concentrations (CCC) (1.3 mM CaCl₂ vs 11 mM NaCl). The addition of EPS to the NaCl and low concentration CaCl₂ (0.05-10 mM) solutions resulted in a dramatic decrease in the aggregation rate and an increase in the CCC values. For solutions of 11 mM NaCl (the CCC values of TiO₂ in the absence of EPS) and above, the resulting attachment efficiency was less than one, suggesting that the adsorbed EPS on the TiO₂ nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. At high CaCl₂ concentrations (10-40 mM), however, the presence of EPS increased the aggregation rate. This is attributed to the aggregation of the dissolved extracellular polymeric macromolecules via intermolecular bridging, which in turn linked the TiO₂ nanoparticles and aggregates together, resulting in enhanced aggregate growth. These results have important implications for assessing the fate and transport of TiO₂ nanomaterials released in aquatic environments.

Guluronic acid content as a factor affecting turbidity removal potential of alginate

Alginates are natural polymers composed of mannuronic and guluronic acid residues. They are currently extracted from brown algae; however, alginate can also be synthesized by some species of Azotobacter and Pseudomonas. Alginates with different proportion of mannuronic and guluronic acids are known to have different characteristics and form gels at different extents in the presence of calcium ions. The aim of this work was to investigate the usefulness of alginate as a non-toxic coagulant used in purification of drinking water. This study utilized alginates from Azotobacter vinelandii having different guluronic acid levels. These were obtained partly by changing the cultivation parameters, partly by epimerizing a purified alginate sample in vitro using the A. vinelandii mannuronan C-5 epimerase AlgE1. The different alginates were then used for coagulation together with calcium. The results showed that turbidity removal capability was dependent on the content of guluronic acid residues. For the best performing samples, the turbidity decreased from 10 NTU to 1 NTU by the use of only 2 mg/L of alginate and 1.5 mM of calcium chloride.

Isolation, Identification, and Optimization of Culture Conditions of a Bioflocculant-Producing Bacterium Bacillus megaterium SP1 and Its Application in Aquaculture Wastewater Treatment

A bioflocculant-producing bacterium, Bacillus megaterium SP1, was isolated from biofloc in pond water and identified by using both 16S rDNA sequencing analysis and a Biolog GEN III MicroStation System. The optimal carbon and nitrogen sources for Bacillus megaterium SP1 were 20 g L⁻¹ of glucose and 0.5 g L⁻¹ of beef extract at 30°C and pH 7. The bioflocculant produced by strain SP1 under optimal culture conditions was applied into aquaculture wastewater treatment. The removal rates of chemical oxygen demand (COD), total ammonia nitrogen (TAN), and suspended solids (SS) in aquaculture wastewater reached 64, 63.61, and 83.8%, respectively. The volume of biofloc (FV) increased from 4.93 to 25.97 mL L⁻¹. The addition of Bacillus megaterium SP1 in aquaculture wastewater could effectively improve aquaculture water quality, promote the formation of biofloc, and then form an efficient and healthy aquaculture model based on biofloc technology.

Disaggregation of silver nanoparticle homoaggregates in a river water matrix

Silver nanoparticles (Ag NPs) could be found in aquatic systems in the near future. Although the interplay between aggregate formation and disaggregation is an important factor for mobility, bioavailability and toxicity of Ag NPs in surface waters, the factors controlling disaggregation of Ag NP homoaggregates are still unknown. In this study, we investigated the reversibility of homoaggregation of citrate coated Ag NPs in a Rhine River water matrix. We characterized the disaggregation of Ag NP homoaggregates by ionic strength reduction and addition of Suwannee River humic acid (SRHA) in the presence of strong and weak shear forces. In order to understand the disaggregation processes, we also studied the nature of homoaggregates and their formation dynamics under the influence of SRHA, Ca(2+) concentration and nanoparticle concentration. Even in the presence of SRHA and at low particle concentrations (10 μg L(-1)), aggregates formed rapidly in filtered Rhine water. The critical coagulation concentration (CCC) of Ca(2+) in reconstituted Rhine water was 1.5 mmol L(-1) and was shifted towards higher values in the presence of SRHA. Analysis of the attachment efficiency as a function of Ca(2+) concentration showed that SRHA induces electrosteric stabilization at low Ca(2+) concentrations and cation-bridging flocculation at high Ca(2+) concentrations. Shear forces in the form of mechanical shaking or ultrasound were necessary for breaking the aggregates. Without ultrasound, SRHA also induced disaggregation, but it required several days to reach a stable size of dense aggregates still larger than the primary particles. Citrate stabilized Ag NPs may be in the form of reaction limited aggregates in aquatic systems similar to the Rhine River. The size and the structure of these aggregates will be dynamic and be determined by the solution conditions. Seasonal variations in the chemical composition of natural waters can result in a sedimentation-release cycle of engineered nanoparticles.

Heteroaggregation of titanium dioxide nanoparticles with natural clay colloids

To better understand and predict the fate of engineered nanoparticles in the water column, we assessed the heteroaggregation of TiO2 nanoparticles with a smectite clay as analogues for natural colloids. Heteroaggregation was evaluated as a function of water salinity (10(-3) and 10(-1) M NaCl), pH (5 and 8), and selected nanoparticle concentration (0-4 mg/L). Time-resolved laser diffraction was used, coupled to an aggregation model, to identify the key mechanisms and variables that drive the heteroaggregation of the nanoparticles with colloids. Our data show that, at a relevant concentration, nanoparticle behavior is mainly driven by heteroaggregation with colloids, while homoaggregation remains negligible. The affinity of TiO2 nanoparticles for clay is driven by electrostatic interactions. Opposite surface charges and/or high ionic strength favored the formation of primary heteroaggregates via the attachment of nanoparticles to the clay. The initial shape and dispersion state of the clay as well as the nanoparticle/clay concentration ratio also affected the nature of the heteroaggregation mechanism. With dispersed clay platelets (10(-3) M NaCl), secondary heteroaggregation driven by bridging nanoparticles occurred at a nanoparticle/clay number ratio of greater than 0.5. In 10(-1) M NaCl, the clay was preaggregated into larger and more spherical units. This favored secondary heteroaggregation at lower nanoparticle concentration that correlated to the nanoparticle/clay surface area ratio. In this latter case, a nanoparticle to clay sticking efficiency could be determined.

Interactions of dissolved organic matter with natural and engineered inorganic colloids: a review

This contribution critically reviews the state of knowledge on interactions of natural colloids and engineered nanoparticles with natural dissolved organic materials (DOM). These interactions determine the behavior and impact of colloids in natural system. Humic substances, polysaccharides, and proteins present in natural waters adsorb onto the surface of most colloids. We outline major adsorption mechanisms and structures of adsorption layers reported in the literature and discuss their generality on the basis of particle type, DOM type, and media composition. Advanced characterization methods of both DOM and colloids are needed to address insufficiently understood aspects as DOM fractionation upon adsorption, adsorption reversibility, and effect of capping agent. Precise knowledge on adsorption layer helps in predicting the colloidal stability of the sorbent. While humic substances tend to decrease aggregation and deposition through electrostatic and steric effects, bridging-flocculation can occur in the presence of multivalent cations. In the presence of DOM, aggregation may become reversible and aggregate structure dynamic. Nonetheless, the role of shear forces is still poorly understood. If traditional approaches based on the DLVO-theory can be useful in specific cases, quantitative aggregation models taking into account DOM dynamics, bridging, and disaggregation are needed for a comprehensive modeling of colloids stability in natural media.

Coagulation of Na-montmorillonite by inorganic cations at neutral pH. A combined transmission X-ray microscopy, small angle and wide angle X-ray scattering study

The coagulation of sodium montmorillonite by inorganic salts (NaNO3, Ca(NO3)2 and La(NO3)3) was studied by combining classical turbidity measurements with wide-angle-X-ray scattering (WAXS), small-angle-X-ray scattering (SAXS), and transmission X-ray microscopy (TXM). Using size-selected samples, such a combination, associated with an original quantitative treatment of TXM images, provides a true multiscale investigation of the formed structures in a spatial range extending from a few ångstroms to a few micrometers. We then show that, at neutral pH and starting with fully Na-exchanged samples, coagulation proceeds via the formation of stacks of particles with a slight mismatch between layers. These stacks arrange themselves into larger porous anisotropic particles, the porosity of which depends on the valence of the cation used for coagulation experiments. Face-face coagulation is clearly dominant under those conditions, and no evidence for significant face-edge coagulation was found. These structures appear to arrange as larger clusters, the organization of which should control the mechanical properties of the flocs.

Aging of TiO(2) nanocomposites used in sunscreen. Dispersion and fate of the degradation products in aqueous environment

Aging in water of a TiO(2)-based nanocomposite used in sunscreen cosmetics has been studied as a function of light and time. It consisted initially in a TiO(2) core, coated with Al(OH)(3) and polydimethylsiloxane (PDMS) layers. Size measurement, coating alteration, and surface charge were followed by laser diffraction, TEM/EDS, ICP-AES and electrophoretic mobility measurement. The nanocomposite rapidly underwent progressive dispersion in the aqueous phase, enabled by the dissolution of the PDMS layer. A stable suspension of colloidal byproducts from 50 to 700nm in size was formed. Their positively charged Al(OH)(3) surface was evidenced with an isoelectric point around 7-8, controlling the dispersion stability. The critical coagulation concentrations measured with NaCl and CaCl(2) was 2 × 10(-2) and 8 × 10(-3)M respectively. The presence of natural organic matter affected the colloidal stability according to the NOM/byproduct ratio. A 2 wt% ratio favored bridging flocculation, whereas a 20 wt% ratio induced sterical stabilization.

Medium optimization for the production of a novel bioflocculant from Halomonas sp. V3a' using response surface methodology

The novel exopolysaccharide bioflocculant HBF-3 is produced by Halomonas sp. V3a', which is a mutant strain of the deep-sea bacterium Halomonas sp. V3a. Response surface methodology (RSM) was employed to optimize the production medium for increasing HBF-3 production. Using a Plackett-Burman experimental design to aid in the first step of optimization, edible glucose, MgSO(4) x 7 H(2)O, and NH(4)Cl were found to be significant factors affecting HBF-3 production. To determine the optimal concentration of each significant variable, a central composite design was employed. Based on response surface and canonical analysis, the optimum concentrations of the critical components were obtained as follows: edible glucose, 16.14 g/l; MgSO(4) x 7 H(2)O, 2.73 g/l; and NH(4)Cl, 1.97 g/l. HBF-3 production obtained by using the optimized medium was 4.52 g/l, which was in close agreement with the predicted value of 4.55 g/l. By scaling up fermentation from flask to fermenter, HBF-3 production was further increased to 5.58 g/l.

Microbial flocculation by Bacillus mucilaginosus: Applications and mechanisms

Microbiological flocculation in wastewater by a strain of Bacillus mucilaginosus was examined and the flocculation mechanism was investigated using Coomassie brilliant blue reaction, Anthrone reaction, thin-layer chromatography (TLC), scanning electronic microscope, and infrared spectroscopy. The GY03 strain, isolated from farmland soil, was identified as B. mucilaginosus by its morphology and 16S rDNA sequence. Cultivated in a nitrogen-free medium, the flocculation material (bacterium-mineral complex) was used to treat domestic, brewage, and pharmaceutical wastewater. On the basis of one-way experimental results, orthogonal experiments were carried out and the optimum processing conditions of the microbial flocculants were obtained. In the optimal conditions, the maximal removal ratio of COD in domestic, brewage, and pharmaceutical wastewater reached 74.6%, 70.5%, and 66.2%, respectively; the maximal removal ratio of BOD was 42.3%, 77.4%, and 41.7%, respectively; and the maximal removal of SS amounted 93.3%, 93.6%, and 88.4%, respectively. The treatment to kaolin suspended liquid by the GY03 strain may act as a model of studying microbial flocculation mechanisms in which bridging and charge neutralization hypothesis were proposed as the critical reasons based upon the experimental observations.