Specific ion effects on the aggregation behavior of aquatic natural organic matter

Molecular foundations for shear-induced dynamics of natural organic matter

The overall objective of the present work was to quantify how shear, coupled with varying salt concentration, affected the particle size distribution and relaxation/aggregation behavior for various organic sources of nonliving natural organic matter (NNOM) in surface water. NNOM has been implicated as a conditioning agent leading to the formation of biofilms such as algae. NNOM is also a responsible in surface waters for facilitated transport of a variety of anthropogenic pollutants. These are NNOM surface-related phenomena, yet the variable surface area and surface composition of NNOM, which can change dependent on shear rate, is not discussed in the literature. NNOM polymer-like dynamics can interact with stream water velocity differences to determine the process and result of aggregation. The fundamental role of post-shear NNOM molecular structure and dynamic aggregation (self-assembly) is examined here alongside fresh (hydrological) versus mined (terrestrial) NNOM. Shear rate can be seen as a change in the velocities of streamlines in hydrology. In this early work, the response to shear rate for three types of NNOM was measured using a stress-controlled rheometer under varying conditions of ionic strength. Samples were studied for rheological response after a variety of pre-shear conditions, and data then coupled with surface composition data from previously reported fluorescence studies. Interestingly, a size class of 5 μm aggregates disappeared when Aldrich humic acid samples were treated with 0.3 M Ca2+. Evidence is also presented that the environmental samples flocculated at shear rates up to 400 s-1, rather than exhibiting particle breakup, with implications for reducing NNOM surface area. Dynamic response of different NNOM sources was not the same, some sources showing evidence of self-assembly. The molecular response to shear may play an important role in understanding the surface area and composition of NNOM responsible for facilitated transport of pollutants and initiation of biofilms.

Photochemical behavior of dissolved organic matter in environmental surface waters: A review

As an important group of widespread organic substances in aquatic ecosystems, dissolved organic matter (DOM) plays an essential role in carbon recycling and transformation processes. The photochemical behavior of DOM is one of the main ways it participates in these processes, and it attracts extensive attention. However, due to a variety of sources and water conditions, including both freshwater and seawater environments, the photochemical properties of DOM exhibit great differences. Nowadays, a large number of studies have focused on the generation process of reactive species (RS) from sunlit DOM, while little effort has been made so far to provide a comprehensive summary of the photochemical behavior of DOM, especially in fresh and saline aquatic ecosystems. In this review, we analyzed the research hotspot on DOM photochemistry over the last 30 years, summarizing the generation of photoreactive species in natural water environments containing DOM (both freshwater and seawater) and listing the main factors affecting the rate, yield, and species of RS photoproduction. Compared with freshwater, seawater has unique characteristics such as high pH value, high ionic strength, and halide ions, which affect the photogeneration of RS, the photoconversion process, as well as the reaction pathways of various environmental substances. In general, DOM-induced surface water photochemistry has important impacts on the environmental transformation and toxic effects of aquatic pollutants and can even contribute significantly to the Earth's carbon cycle, which would have potential implications for both human and ecological health.

Participation of strong charge-assisted hydrogen bonds in interactions of dissolved organic matter represented by Suwannee River Humic Acid

Terrestrial dissolved organic matter (DOM) plays critical roles in many biotic and abiotic environmental reactions as well as in water treatment. Its structure is therefore of great interest. We examined dissolved Suwannee River Humic Acid (HA) to probe the potential participation of exceptionally strong, negative charge-assisted hydrogen bonds, (-)CAHB, in DOM cohesion and interaction with small weak acids using high performance size exclusion chromatography (HPSEC), transmission electron microscopy, zeta-pH curves, and pH drift experiments. The results support a previously proposed two-tier state of aggregation, in which tightly-knit primary particles (≤ ∼10 kDa) form larger secondary aggregates (up to micrometer in size). Evidence for (-)CAHB is gained through zeta potential changes and pH drift experiments. The primary particles interact with (-)CAHB-capable solutes (simple carboxylic acids and phosphate) but not (-)CAHB-incapable solutes. We identified disruption of intra-segmental and inter-molecular (-)CAHB leading to swelling and disaggregation, as well as formation of nouveau (-)CAHB with free groups on HA. The effects were solute-concentration dependent and greater at pH 5 than pH 6, consistent with CAHB theory. Phosphate induced the greatest shifts in the HPSEC molecular size distribution curves. The shifts were unaffected by prior stripping of innate polyvalent metals. We conclude that the (-)CAHB contributes to the cohesion of DOM, affecting its size and charge, and provides a means by which weak acid pollutants, nutrients, and natural compounds can interact with DOM. Such interactions have implications for the behavior of DOM in the environment, the fate and transport of anthropogenic pollutants, and the roles DOM play in water treatment technologies.

Photochemical Transformation of Ibuprofen and Chlorophene Induced by Dissolved Organic Matter

Both ibuprofen (IBP) and chlorophene (CP) are frequently detected contaminants in surface aqueous environment. Dissolved organic matter (DOM) is an important component in water with high photo-reactivity, playing an important role in the transformation processes of various organic pollutants. This study systematically studied the influence of DOM on the photochemical transformation of IBP and CP by using humic acid as model DOM. In addition, the effect of inorganic salts on this process is also considered due to the high salt content in the ocean. Further quenching experiments and reactive oxygen species (ROSs) detection were also conducted to explore the reactive species acting on the IBP and CP transformation. Based on the products analysis and theoretical calculation, we proposed the IBP and CP transformation mechanism. Overall, this study provides some new insights into the transformation of organic pollutants in natural surface water, which is significant for assessing the fate of pollutants.

Removal of strontium by nanofiltration: Role of complexation and speciation of strontium with organic matter

Strontium (Sr) removal from water is required because excessive naturally occurring Sr exposure is hazardous to human health. Climate and seasonal changes cause water quality variations, in particular quality and quantity of organic matter (OM) and pH, and such variations affect Sr removal by nanofiltration (NF). The mechanisms for such variations are not clear and thus OM complexation and speciation require attention. Sr removal by NF was investigated with emphasis on the role of OM (type and concentration) and pH (2-12) on possible removal mechanisms, specifically size and/or charge exclusion as well as solute-solute interactions. The filtration results show that the addition of various OM (10 types) and an increase of OM concentration (2-100 mgC.L-1) increased Sr removal by 10-15%. The Sr-OM interaction was enhanced with increasing OM concentration, implying enhanced size exclusion via Sr-OM interaction as the main mechanism. Such interactions were quantified by asymmetric flow field-flow fractionation (FFFF) coupled with an inductively coupled plasma mass spectrometer (ICP-MS). Both extremely low and high pH increased Sr removal due to the enhanced charge exclusion and Sr-OM interactions. This work elucidated and verified the mechanism of OM and pH on Sr removal by NF membranes.

The opposite influences of Cu and Cd cation bridges on sulfamethoxazole sorption on humic acids in wetting-drying cycles

Wetting-drying cycles in the environment could change the inner- or outer-sphere complexation of heavy metal cations on natural organic matter (NOM) and then influence ternary interactions with organic contaminants - a rarely-discussed essential geochemical process. In this work, the sorption of sulfamethoxazole (SMX) on humic acids (HAs) mediated by cations (Cu2+ and Cd2+) was investigated. Considering that outer-sphere complexation could be transformed into inner-sphere complexation during vacuum freeze-drying, the role of inner- or outer-sphere complexation on SMX sorption was explored. The experimental sorption results and density functional theory (DFT) calculations suggested that Cu2+ and Cd2+ sorption on HAs was mainly outer- and inner-sphere complexation, respectively. Cd2+ consistently promoted SMX sorption on HAs, while Cu2+ promoted and inhibited SMX sorption before and after freeze-drying. The structure of HA-Cu complexes with inner-sphere complexation was more compact than those with outer-sphere complexation, which reduced the accessibility of sorption sites for SMX on HA-Cu and inhibited SMX sorption. However, the greater number of coordination sites of Cd2+ may provide more sorption sites and the structure of HA-Cd was looser. These findings provide a groundbreaking understanding of the sorption of organics on natural adsorbents in the presence of cations.

Visual demonstration and prediction of the Hofmeister series based on a poly(ionic liquid) photonic array

The Hofmeister effect and associated Hofmeister series (HS) are ubiquitous in physicochemical phenomena and have demonstrated fundamental importance in a myriad of fields ranging from chemistry to biology. Visualization of the HS not only helps to straightforwardly understand the underpinning mechanism, but also enables the prediction of new ion positions in the HS and directs the applications of the Hofmeister effect. Owing to the difficulties of sensing and reporting complete multiple and subtle inter- and intramolecular interactions involved in the Hofmeister effect, facile and accurate visual demonstration and prediction of the HS remain highly challenging. Herein, a poly(ionic liquid) (PIL)-based photonic array containing 6 inverse opal microspheres was rationally constructed to efficiently sense and report the ion effects of the HS. The PILs can not only directly conjugate with HS ions due to their ion-exchange properties, but also provide sufficient noncovalent binding diversity with these ions. Meanwhile, subtle PIL-ion interactions can be sensitively amplified to optical signals owing to their photonic structures. Therefore, synergistic integration of PILs and photonic structures gives rise to accurate visualization of the ion effect of the HS, as demonstrated by correctly ranking 7 common anions. More importantly, assisted by principal component analysis (PCA), the developed PIL photonic array can serve as a general platform to facilely, accurately, and robustly predict the HS positions of an unprecedented amount of important and useful anions and cations. These findings indicate that the PIL photonic platform is very promising for addressing challenges in the visual demonstration and prediction of HS and promoting a molecular-level understanding of the Hoffmeister effect.

Reciprocal effects of NOM and solution electrolyte ions on aggregation of ferrihydrite nanoparticles

The effects of natural organic matter (NOM) types and electrolyte ions are crucial to the aggregation of ferrihydrite nanoparticles (Fh NPs) in the environment. Dynamic light scattering (DLS) was employed for the aggregation kinetics of Fh NPs (10 mg/L as Fe) in the present study. The critical coagulation concentration (CCC) values of Fh NPs aggregation in NaCl were obtained in the presence of 15 mg C/L NOM as SRHA (857.4 mM) > PPHA (752.3 mM) > SRFA > (420.1 mM) > ESHA (141.0 mM) > NOM-free (125.3 mM), indicating Fh NPs aggregation was inhibited as the above order. Comparatively in CaCl₂, the CCC values were measured in ESHA (0.9 mM), PPHA (2.7 mM), SRFA (3.6 mM), SRHA (5.9 mM), NOM-free (76.6 mM), implying NPs aggregation was enhanced following the order of ESHA > PPHA > SRFA > SRHA. To investigate the dominant mechanisms, the aggregation of Fh NPs was comprehensively studied under the effects of NOM types, concentrations (0-15 mg C/L) and electrolyte ions (NaCl/CaCl₂ beyond CCC). In NaCl/CaCl₂, the low concentration of NOM (<7.5 mg C/L) could accelerate NPs aggregation mainly due to patch-charge attraction. When NOM concentration was high (>7.5 mg C/L), the inhibition effect on NPs aggregation occurred in NaCl due to steric repulsion, whereas the enhancement effect in CaCl₂ of aggregation was dominated by the bridging effect. The results indicated that the effects of NOM types, concentration and electrolyte ions should be carefully considered for the environmental behavior of NPs.

Impacts of dissolved organic matter on the aggregation and photo-dissolution of cadmium pigment nanoparticles in aquatic systems

Cadmium pigments are a group of inorganic pigments used in consumer products. The aggregation behavior and photo-dissolution process of cadmium pigment nanoparticles in aquatic systems control their environmental fate, which is largely unknown. Herein, we investigated the influence of dissolved organic matter (DOM) on the aggregation behavior and photo-dissolution process of CdS nanoparticles in aquatic systems. Bare CdS nanoparticles are prone to aggregation in both NaCl and CaCl₂ solutions. DOM can remarkably increase the colloidal stability of CdS nanoparticles owing to the steric hindrance and enhanced electrostatic repulsion. With 10 mg/L Suwannee River natural organic matter (SRNOM), the colloidal stability of CdS nanoparticles is significantly enhanced in NaCl solutions (i.e., the critical coagulation concentration, CCC_Na, is 707.2 mM). Suwannee River humic acid (SRHA) has a stronger stabilization effect than SRNOM due to its higher molecular weight and aromaticity. The Ca²⁺ cations can induce charge neutralization and structural compacting of DOM corona, efficiently reducing the colloidal stability of CdS nanoparticles. The CCC_Ca is 10.8 mM and 14.9 mM with 10 mg/L SRNOM and SRHA, respectively. Upon solar irradiation, the presence of a low concentration of SRNOM (3 mg/L) can enhance the photo-dissolution of CdS nanoparticles and the consequent Cd²⁺ leaching. This is caused by the facilitated electron transfer from CdS nanoparticles to O₂ induced by SRNOM corona, leading to better electron-hole separation. However, a high concentration of SRNOM inhibited the photo-dissolution of CdS nanoparticles due to the strong inner filter effect and the scavenging of phototransients. The colloidal stability of SRNOM-coated CdS nanoparticles increases in NaCl but decreases in CaCl₂ solutions after irradiation owing to the oxidation of SRNOM corona. Our results highlight the decisive role of DOM in the environmental fate of cadmium pigments.

Effect of dissolved organic matter and its fractions on disinfection by-products formation upon karst surface water

In this study, disinfection by-products (DBP) formation from dissolved organic matter (DOM) and its fractions, including both hydrophilic and hydrophobic components, were investigated at a typical karst surface water. The subsequent DBP formation potential was evaluated by deducing chemical characteristics of DOM fractions and representative algal organic matter (Chlorella sp. AOM) under the influence of divalent ions (Ca²⁺ and Mg²⁺) via spectra analysis. Both terrigenous and autochthonous DOM performed as critical DBP precursors, and DBP formation patterns were tightly correlated to organic matter chemical variations. DBP formation was significantly higher in drought period compared to that in wet period (P < 0.05). Particularly, trichloromethane (TCM) and dichloroacetonitrile (DCAN) showed distinct formation patterns compared to the scenarios in non-karst water. For DOM fractions, hydrophobic components showed higher DBP formation compared to hydrophilic counterparts, hydrophilic neutral enriched more reactive organic nitrogen for N-DBPs production. It was preferable to enrich humic-like substances after Ca²⁺ and Mg²⁺complexation in Chlorella sp. AOM, TCM formation increased whereas DCAN production remained unchanged in the presence of divalent ions. This study innovatively provided a linkage between chemical characteristics of DOM and understanding of DBP formation in karst surface water.

Stability of Eu(III)-silicate colloids: Effect of Eu content, pH, electrolyte and fulvic acid

Dissolved silicic acid in the environment has strong affinity for actinides (An), but An(III)-silicate colloids have been scarcely investigated. In this study, Eu(III)-silicate colloids, an analogue to An(III)-silicate, were prepared and the aggregation kinetics of the colloids was investigated as a function of Eu content (Si/Eu molar ratio), pH, background electrolyte (NaCl, NaNO₃, NaClO₄, KCl and CsCl) and fulvic acid (FA). Results indicated that the colloids with higher Si/Eu molar ratio exhibited higher stability under the same conditions. The stability of the colloids increased with increasing aqueous pH (7.1-9.4) and decreasing ionic strength, and the inhibition effect of monovalent electrolytes on the colloid stability followed the order of Na⁺ < K⁺ < Cs⁺ and Cl^- < NO₃^- < ClO₄^-. In addition, the presence of FA significantly increased the stability of the colloids. The dependence of the stability on the chemical conditions in all cases could be illustrated by DLVO theory. Disaggregation kinetics showed that the aggregation process of the colloids was not fully reversible, because a time-dependent size memory effect led to a bigger mean size of disaggregated colloids as compared to the initial ones. The present work provides detailed insight in the formation and stability of An(III)-silicate colloids under the alkaline conditions relevant to geological disposal of radioactive waste, which is critical for understanding the behavior of this type of colloids in the environment.

Membrane Scaling and Wetting in Membrane Distillation: Mitigation Roles Played by Humic Substances

Membrane scaling and wetting severely hinder practical applications of membrane distillation (MD) for hypersaline water/wastewater treatment. In this regard, the effects of feedwater constituents are still not well understood. Herein, we investigated how humic acid (HA) influenced gypsum-induced membrane scaling and wetting during MD desalination. At low concentrations (5-20 mg L^-1), HA notably mitigated membrane scaling and wetting. The morphological characterization of scaled membranes revealed that the antiwetting behavior could be ascribed to the formation of a compact and protective gypsum/HA scale layer, which blocked the flow channel of scaling ions and suppressed the intrusion of scale particles into membrane pores. Based on the comprehensive analysis of the scaling process, the formation of the scale layer was related to the heterogeneous crystallization of gypsum on the membrane surface. Moreover, deprotonated HA interfered with the heterogeneous crystallization process by inhibiting the formation of gypsum nuclei and altering the orientation of crystal growth, thus delaying membrane scaling and altering the morphology of the scale layer. Thermodynamic and kinetic analyses further demonstrated the mitigation mechanism of HA. Furthermore, improved fouling reversibility and antiwetting ability in synthetic seawater treatment endowed by HA were observed. This study provides new insight into the roles played by the organic constituents of water/wastewater during membrane desalination, providing a valuable reference for developing novel strategies to improve the performance of MD.

High-Throughput Method for Natural Organic Matter Hydrophobicity Assessment Using an Ionic Liquid-Based Aqueous Two-Phase System

Hydrophobicity of natural organic matter (NOM) is one of its fundamental properties that influence the environmental fate of pollutants and the performance of many water treatment unit processes. In this study, a high-throughput method was developed for NOM hydrophobicity measurement based on the phase separation technique in the 96-well format. It measures the partition coefficients of NOM (K_ATPS,IL) in an ionic liquid (IL)-based aqueous two-phase system (ATPS). The ATPS was made of 1-butyl-3-methylimidazole bromide solution and a salt solution containing potassium phosphate monobasic and potassium phosphate dibasic. The partition of NOM in IL-based ATPS is mainly affected by its hydrophobicity. log K_ATPS,IL linearly correlated with the commonly used NOM hydrophobicity scales, including (O + N)/C, O/C, and aromatic carbons. K_ATPS,IL provided a more accurate assessment of NOM hydrophobicity than spectroscopic indices. Furthermore, K_ATPS,IL can predict the organic carbon-water partition coefficients for hydrophobic organic chemical sorption to NOM based on the two-phase system model. The high-throughput K_ATPS,IL measurement and the two-phase system model can be applied to real surface water samples. Our results suggest that the proposed high-throughput method has great potential to be applied to monitor NOM hydrophobicity for environmental risk assessment and water treatment purposes.

Ionic-strength-dependent effect of suspended sediment on the aggregation, dissolution and settling of silver nanoparticles

Suspended sediment (SS) is ubiquitous in natural waters and plays a key role in the fate of engineered nanomaterials. In this study, the effect of SS on the aggregation, settling, and dissolution of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) was investigated under environmentally relevant conditions. The heteroaggregation of AgNPs with SS was not observed at low ionic strength (≤0.01 M) due to high electrostatic repulsion and steric forces. At higher NaCl concentrations (0.1 and 0.3 M), PVP-AgNPs were found to attach onto the SS surface, and the formation of AgNP-SS heteroaggregates strongly promoted settling of PVP-AgNPs due to the overwhelming gravity force. PVP-AgNP dissolution was reduced after the addition of sediment to ultrapure water because the presence of sediment-associated dissolved organic matter (SS-DOM). The formation of an AgCl layer on PVP-AgNP surface in 0.01 M NaCl solution resulted in the minor effect of SS on AgNP dissolution. After addition of SS, the dissolved silver concentrations of PVP-AgNP increased in 0.1 and 0.3 M NaCl solution. The interactions of SS-DOM with AgNPs under different NaCl concentrations interfered the dissolution of AgNPs in sediment-laden water. This study provides new insight into the fate of AgNPs in sediment-laden water under various environmental conditions.

Influence of particle size on the aggregation behavior of nanoparticles: Role of structural hydration layer

More and more attention has been paid to the aggregation behavior of nanoparticles, but little research has been done on the effect of particle size. Therefore, this study systematically evaluated the aggregation behavior of nano-silica particles with diameter 130-480 nm at different initial particle concentration, pH, ionic strength, and ionic valence of electrolytes. The modified Smoluchowski theory failed to describe the aggregation kinetics for nano-silica particles with diameters less than 190 nm. Besides, ionic strength, cation species and pH all affected fast aggregation rate coefficients of 130 nm nanoparticles. Through incorporating structural hydration force into the modified Smoluchowski theory, it is found that the reason for all the anomalous aggregation behavior was the different structural hydration layer thickness of nanoparticles with various sizes. The thickness decreased with increasing of particle size, and remained basically unchanged for particles larger than 190 nm. Only when the distance at primary minimum was twice the thickness of structural hydration layer, the structural hydration force dominated, leading to the higher stability of nanoparticles. This study clearly clarified the unique aggregation mechanism of nanoparticles with smaller size, which provided reference for predicting transport and fate of nanoparticles and could help facilitate the evaluation of their environment risks.

Natural Organic Matter (NOM) Imparts Molecular-Weight-Dependent Steric Stabilization or Electrostatic Destabilization to Ferrihydrite Nanoparticles

Ferrihydrite nanoparticles (Fh NPs) are ubiquitous in natural environments. However, their colloidal stability, and fate and transport behavior are difficult to predict in the presence of heterogeneous natural organic matter (NOM) mixtures. Here, we investigated the adsorption and aggregation behavior of Fh NPs exposed to NOM fractions with different molecular weights (MW). The NOM fraction with MW < 3 kDa destabilized the NPs, resulting in accelerated aggregation even at high C/Fe mass ratios, whereas higher MW NOM fractions imparted better colloidal stability with increasing MW and C/Fe ratio. Despite differences in the functional group composition of the bulk (dissolved) NOM fractions, all NOM fractions produced similar adsorbed layer compositions on the NPs, suggesting minimal contribution of chemical properties to the distinctive aggregation behavior. Rather, the higher adsorbed mass and larger size of the higher MW fractions were key factors in stabilizing the NPs through steric repulsion, whereas the lowest MW fraction had low adsorbed mass and was unable to counter electrostatic patch-charge attraction when the NPs are positively charged. This mechanistic understanding helps us predict the transport and fate of Fh NPs and the associated contaminants in natural environments with varying NOM compositions.

Impact of origin and structure on the aggregation behavior of natural organic matter

The intermolecular interactions of natural organic matter (NOM) play a key role in the fate and transport of organic carbon and pollutants in environmental and engineered systems. In this study, the impact of origin and structure on the aggregation behavior of NOM was investigated in the presence of naturally abundant cations. The physicochemical properties of NOM were quantified using a range of indices. Thermodynamic analysis suggests that the colloidal stability of NOM was mainly determined by its hydrophobicity (i.e., Lewis acid-base interactions). All NOM can be coagulated by Ca²⁺ owing to the strong cation-NOM interactions, which lead to bridging effect and lower Lewis acid-base interactions. Terrestrial NOM can be coagulated by Mg²⁺ while aquatic NOM cannot, owing to their different hydrophobicity. The critical coagulation concentrations of tested terrestrial NOM in the presence of Ca²⁺ (CCC-Ca) were quite similar at 1.94-4.88 mM despite their different structural properties. The CCC-Ca of tested aquatic NOM varied significantly from 46.89 mM to 110.40 mM depending on their structure. The optical indices including E₂/E₃, FI, and HIX can be potentially used as convenient indicators for assessing the colloidal stability of aquatic NOM for water treatment and risk assessment purposes.