Coating of AFM probes with aquatic humic and non-humic NOM to study their adhesion properties

Aluminum substitution stabilizes organic matter in ferrihydrite transforming into hematite: A molecular analysis

The association of soil organic matter (SOM) with iron (Fe) oxyhydroxides, particularly ferrihydrite, plays a pivotal role in the biogeochemical cycling of carbon (C) in both terrestrial and aquatic environment. The aging of ferrihydrite to more crystalline phases can impact the stability of associated organic C, a process potentially influenced by aluminum (Al) substitution due to its abundance. However, the molecular mechanisms governing the temporal and spatial distribution of SOM during the aging process of Al-substituted Fe oxyhydroxides remain unclear. This study aims to bridge this knowledge gap through a comprehensive approach, utilizing batch experiments, solid characterization techniques, and atomic force microscopy (AFM) based peak-force quantitative nanomechanical mapping (PF-QNM). Batch experiments revealed that humic acid (HA) was released into the aqueous phase during aging, with Al inhibiting this release. Various solid characterization methods collectively suggested that Al hindered the crystalline transformation of ferrihydrite and significantly preserved HA on the surface of newly formed hematite, rather than it being occluded within the interior of the new minerals. Results from 3-Dimensional fluorescence spectroscopy (3D-EEM) and Fourier-transform infrared spectroscopy (FTIR) indicated that the structure of HA remained constant, with the carboxyl-rich and hydroxyl-rich portions of HA fixed at the mineral interface during the aging period. Furthermore, we developed AFM-based PF-QNM to both quantify and visualize the interactions between Fe oxyhydroxides and HA, demonstrating variations in HA affinity among different Fe oxyhydroxides and highlighting the influence of the Al substitution rate.

Molecular interaction mechanism for humic acids fouling resistance on charged, zwitterion-like and zwitterionic surfaces

Humic acids (HA) are ubiquitous in surface waters, leading to significant fouling challenges. While zwitterion-like and zwitterionic surfaces have emerged as promising candidates for antifouling, a quantitative understanding of molecular interaction mechanism, particularly at the nanoscale, still remains elusive. In this work, the intermolecular forces between HA and charged, zwitterion-like or zwitterionic monolayers in aqueous environments were quantified using atomic force microscope. Compared to cationic MTAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), which exhibited an adhesion energy of ∼1.342 mJ/m2 with HA due to the synergistic effect of electrostatic attraction and possible cation-π interaction, anionic SPMA (3-sulfopropyl methacrylate) showed a weaker adhesion energy (∼0.258 mJ/m2) attributed to the electrostatic repulsion. Zwitterion-like MTAC/SPMA mixture, driven by electrostatic attraction between opposite charges, formed a hydration layer that prevented the interaction with HA, thereby considerably reducing adhesion energy to ∼0.123 mJ/m2. In contrast, zwitterionic MPC (2-methacryloyloxyethyl phosphorylcholine) and DMAPS ([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) displayed ultralow adhesion energy (0.06-0.07 mJ/m2) with HA, arising from their strong dipole moments which could induce a tight hydration layer that effectively inhibited HA fouling. The pH-mediated electrostatic interaction resulted in the increased adhesion energy for MTAC but decreased adhesion energy for SPMA with elevated pH, while the adhesion energy for zwitterion-like and zwitterionic surfaces was independent of environmental pH. Density functional theory (DFT) simulation confirmed the strong binding capability of MPC and DMAPS with water molecules (∼-12 kcal mol-1). This work provides valuable insights into the molecular interaction mechanisms underlying humic-substance-fouling resistance of charged, zwitterion-like and zwitterionic materials at the nanoscale, shedding light on developing more effective strategy for HA antifouling in water treatment.

Nanoscale Insights into the Interaction Mechanism Underlying the Adsorption and Retention of Heavy Metal Ions by Humic Acid

The mobility and distribution of heavy metal ions (HMs) in aquatic environments are significantly influenced by humic acid (HA), which is ubiquitous. A quantitative understanding of the interaction mechanism underlying the adsorption and retention of HMs by HA is of vital significance but remains elusive. Herein, the interaction mechanism between HA and different types of HMs (i.e., Cd(II), Pb(II), arsenate, and chromate) was quantitatively investigated at the nanoscale. Based on quartz crystal microbalance with dissipation tests, the adsorption capacities of Pb(II), Cd(II), As(V), and Cr(VI) ionic species on the HA surface were measured as ∼0.40, ∼0.25, ∼0.12, and ∼0.02 nmol cm-2, respectively. Atomic force microscopy force results showed that the presence of Pb(II)/Cd(II) cations suppressed the electrostatic double-layer repulsion during the approach of two HA surfaces and the adhesion energy during separation was considerably enhanced from ∼2.18 to ∼5.05/∼4.18 mJ m-2. Such strong adhesion stems from the synergistic metal-HA complexation and cation-π interaction, as evidenced by spectroscopic analysis and theoretical simulation. In contrast, As(V)/Cr(VI) oxo-anions could form only weak hydrogen bonds with HA, resulting in similar adhesion energies for HA-HA (∼2.18 mJ m-2) and HA-As(V)/Cr(VI)-HA systems (∼2.26/∼1.96 mJ m-2). This work provides nanoscale insights into quantitative HM-HA interactions, improving the understanding of HMs biogeochemical cycling.

Enhanced antibacterial effect and biodegradation of coating via dual-in-situ growth based on carboxymethyl cellulose

The lack of antimicrobial effect of commercial paper coating for food packaging makes it difficult to prevent food spoilage and harms the environment by non-biodegradation. Herein, carboxymethyl cellulose (CMC) provides negatively charged sites for anchoring Ag⁺ and Zn²⁺ to grow AgNPs and ZIF-8 in situ on its molecular chains. The ZIF-8/AgNPs@CMC paper coating has excellent synergistic antibacterial activity to prolong the shelf-life of food. It not only has good thermal stability but binds closely to the paper and its adhesion force reaches 628.9 nN. Besides, the ZIF-8/AgNPs@CMC coated paper has better mechanical properties, water vapor barrier, and resists water solubility. Interestingly, due to the confinement effect of ZIF-8, the cumulative release of AgNPs after 168 h is only 2.66 % to avoid possible food safety risks. Especially, the coating can be almost biodegraded in the soil after 30 days, which provides the possibility to replace the non-biodegradable coatings in food packaging.

Influence of citrate/tartrate on chromite crystallization behavior and its potential environmental implications

The ferrite process has been developed to purify wastewater containing heavy metal ions and recycle valuable metals by forming chromium ferrite. However, organic matter has an important influence on the crystallization behavior and stability of chromite synthesized from chromium-containing wastewater. We focused on the influence and effect mechanism of two typical organic acid salts (citrate (CA) and tartrate (TA)) on the process of chromium mineralization. It was found that the presence of organic matter leads to the increase of the residual content of Cr in CA system (0.50 mmol/L) and TA system (0.61 mmol/L) in the solution, and the removal of chromium was mainly due to the surface adsorption of Fe(III) hydrolysate. The decreased crystallinity of mineralized products is ascribed to the completion of organic compounds with Fe(II) and Fe(III), which hinders the formation of ferrite precursors. There was bidentate and monodentate chelation between -COO- and metal ions in the CA system and TA system respectively, which resulted in a stronger affinity between CA and iron. This study provides the underlying mechanism for Cr(III) solid oxidation by the ferrite method in an organic matter environment and is of great significance to prevent and control chromium pollution in the environment.

Adsorption and interaction mechanisms of Chi-g-P(AM-DMDAAC) assisted settling of kaolinite in a two-step flocculation process

Flocculation has been widely employed in treatment of mineral tailings and water management. In this study, a chitosan-graft-poly(acrylamide-dimethyl diallyl ammonium chloride) (Chi-g-P(AM-DMDAAC)) was synthesized in-house. The adsorption and interaction mechanisms of Chi-g-P(AM-DMDAAC) and an anionic polyacrylamide (APAM) in a two-step flocculation process of kaolinite were explored using settlement tests, zeta potential measurement, quartz crystal micro-balance with dissipation (QCM-D) and atomic force microscopy (AFM) technique. The type of primary flocculant was critical for the two-step flocculation process. The treatment of the kaolinite suspension using 1 mg/L of Chi-g-P(AM-DMDAAC) followed by adding 2 mg/L of APAM displayed more efficient flocculation performance. QCM-D results showed that three dissipative layers were assembled on model kaolinite surface after sequentially injecting 3.5 mg/L of Chi-g-P(AM-DMDAAC), 0.05 wt% kaolinite suspension and 2.5 mg/L of APAM. The above total adsorption amount (Δf of -64.9 Hz) was much higher than that of using the two flocculants in reverse order (Δf of -23.1 Hz). This result indicated that the adsorption layer of the positively charged Chi-g-P(AM-DMDAAC) on kaolinite surface provided active adsorption sites for APAM. Further AFM measurement confirmed that the average adhesion between the silicon tip adsorbed Chi-g-P(AM-DMDAAC) and model kaolinite surface in 2.5 mg/L APAM solution increased from 0.25 ± 0.1 nN to 4.2 ± 0.3 nN with the effective interaction range of 700 nm, which was stronger than that measured between a bare silicon tip and silica substrate in single-component-flocculant solutions. The highly efficient two-step flocculation process could be ascribed to the strong electrostatic attraction between the kaolinite and the oppositely charged Chi-g-P(AM-DMDAAC) and APAM. Findings in this study will benefit the development of environmentally friendly flocculant for mineral tailings and water treatment.

Interactions between activated sludge extracellular polymeric substances and model carrier surfaces in WWTPs: A combination of QCM-D, AFM and XDLVO prediction

To understand the biofilm formation of biofilm-based processes in wastewater treatment plants (WWTPs), the interaction mechanisms between extracted extracellular polymeric substances (EPS) and three model carrier surfaces (i.e., negatively charged hydrophilic silica, positively charged hydrophilic alumina, and neutral charged hydrophobic polystyrene) were investigated employing a laboratory quartz crystal microbalance with dissipation monitoring equipment (QCM-D) and an atomic force microscope (AFM). The data suggested that surface charge and hydrophobicity of both EPS and carriers played significant roles in the interaction behaviors. Moreover, increases in ionic strength could lead to the increasing zeta potential and hydrophobicity of EPS. It is worth noting that long-range DLVO forces dominated the EPS deposition on carriers in lower ionic strength while short-range Lewis acid-base (AB) interaction controlled the adhesion behaviors in higher ionic strength. Besides, the presence of calcium ions contributed to the adhesion behaviors because of strong charge neutralization and hydrophobic effect. Bound EPS (BEPS) showed higher affinity to model carriers than dissolved EPS (DEPS), which conformed to XDLVO prediction rather than classical DLVO model. Overall, these results provide insights into the influence mechanisms of carrier characteristics, ionic strength, calcium ion and EPS components on the interaction between EPS and representative carriers, contributing to predict and regulate biofilm formation in biofilm-based processes.

Probing the intermolecular interaction mechanisms between humic acid and different substrates with implications for its adsorption and removal in water treatment

Humic substance is a ubiquitous class of natural organic matter (NOM) in soil and aquatic ecosystems, which severely affects the terrestrial and aquatic environments as well as water-based engineering systems by adsorption on solids (e.g., soil minerals, nanoparticles, membranes) via different interaction mechanisms. Herein, the chemical force microscopy (CFM) technique was employed to quantitatively probe the intermolecular forces of humic acid (HA, a representative humic substance) interacting with self-assembled monolayers (SAMs, i.e., OH-SAMs, CH₃-SAMs, NH₂-SAMs and COOH-SAMs) in various aqueous environments at the nanoscale. The interaction forces measured during approach could be well fitted by the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory by incorporating the hydrophobic interaction. The average adhesion energy followed the trend as: NH₂-SAMs (∼3.11 mJ/m²) > CH₃-SAMs (∼2.03 mJ/m²) > OH-SAMs (∼1.38 mJ/m²) > COOH-SAMs (∼0.52 mJ/m²) in 100 mM NaCl at pH 5.8, indicating the significant role of electrostatic attraction in contributing to the HA adhesion, followed by hydrophobic interaction and hydrogen bonding. The adhesion energy was found to be dependent on NaCl concentration, Ca²⁺ addition and pH. For the interaction between NH₂-SAMs and HA, their electrostatic attraction at pH 5.8 turned to repulsion under alkaline condition which led to the sudden drop of adhesion energy. Such results promised the adsorption and release of HA using the recyclable magnetic Fe₃O₄ nanoparticles coated with (3-aminopropyl)tiethoxysilane (APTES). This work provides quantitative information on the molecular interaction mechanism underlying the adsorption of HA on solids of varying surface chemistry at the nanoscale, with useful implications for developing effective chemical additives to remove HA in water treatment and many other engineering processes.

Electrostatically coupled SiO2 nanoparticles/poly (L-DOPA) antifouling coating on a nanofiltration membrane

In this work, the fouling resistance of TFC (thin film composite) nanofiltration membranes have been enhanced using an electrostatically coupled SiO₂ (silica dioxide) nanoparticles/poly(L-DOPA) (3-(3,4-dihydroxyphenyl)-l-alanine) antifouling coating. SiO₂ nanoparticles were synthesized in different size ranges and combined with L-DOPA; and then coated as an anti-fouling layer on the membrane surface by recirculated deposition. Membranes were coated with S-NP (silica nanoparticles) in small (19.8 nm), medium (31.6 nm) and large (110.1 nm) sizes. The zwitterionic compound L-DOPA in the form of self-polymerized poly(L-DOPA) (PDOPA) helped with the attachment of the S-NP to the membrane surface. It was confirmed by AFM (atomic force microscopy) measurement that coating of membranes led to an increase in hydrophilicity and reduction in surface roughness, which in turn led to a 60% reduction in the adhesion force of the foulant on the membrane as compared to the neat membrane. The modified membranes experienced almost no flux decline during the filtration experimental period, whereas the unmodified membrane showed a sharp flux decline. The best coating conditions of silica nanoparticles resulting in enhanced anti-fouling properties were identified. The biofouling film formation on the membranes was evaluated quantitatively using the flow cytometry method. The results indicated that the modified membranes had 50% lower microbial population growth in terms of total event count compared to the neat membrane. Overall, the experimental results have confirmed that the coating of electrostatically coupled SiO₂ nanoparticles and PDOPA (S-NP/PDOPA) on TFC-NF (nanofiltration) membrane surfaces is effective in improving the fouling resistance of the membranes. This result has positive implications for reducing membrane fouling in desalination and industrial wastewater treatment applications.

The characteristics of organic matter influence its interfacial interactions with MnO2 and catalytic oxidation processes

The influence of dissolved organic matter (DOM) properties on its interfacial interactions with MnO₂ and on catalytic oxidation processes was studied by Time-Resolved Dynamic Light Scattering (TR-DLS) and Atomic Force Microscopy (AFM) under varied solution conditions. Four DOM fractions of different characteristics (e.g., SUVA, hydrophobic character, structural properties) were selected. Bared-MnO₂ nanoparticles readily aggregated in NaCl and CaCl₂ solutions. Classic DLVO Theory successfully described critical coagulation concentrations and aggregation behaviors. In NaCl solution, DOM adsorbed on MnO₂ nanoparticles and provided electrosteric stabilization. The two DOM fractions of higher hydrophobic (HPO) character were more efficient in decreasing the aggregation rates. Enhanced MnO₂ aggregation was observed at high Ca²⁺ concentrations due to charge screening and cation bridging between carboxyl groups in DOM structures. The addition of oxidant (H₂O₂) induced a high aggregation of bared-MnO₂ nanoparticles, possibly due to the release of Mn²⁺ (i.e., complexation mechanisms) and generation of reactive species (O₂^-, HO₂^-, and H). Contrasted with their hydrophilic (HPI) counterparts, HPO isolates adsorbed on MnO₂ significantly decreased the catalytic oxidation processes between H₂O₂/MnO₂; suggesting a more efficient and stronger DOM coating. Interfacial forces measured by AFM, showed weaker interactions between HPI isolates and MnO₂; suggesting unfavorable polar interactions. Conversely, the high adhesion forces between MnO₂/HPO isolate would indicate stronger bonds and hydrophobic interactions. This study provided a nanoscale understanding of the impact of DOM characteristics on: a) performance of the MnO₂ coated ceramic membranes in water treatment, and b) biogeochemical cycle of Mn-oxides in the environmental.

Effect of alkaline precipitation on Cr species of Cr(III)-bearing complexes typically used in the tannery industry

Various organic compounds extensively used in the leather industry could influence the performance of alkaline precipitation with Cr(III). This study focused on two typical Cr(III)-bearing complexes (Cr(III)-collagen and Cr(III)-citrate) ubiquitous in tannery effluent yet with distinct treatment efficiencies, as Cr(III) was much more difficult to remove in the Cr(III)-citrate solution. Comprehensive analytical methods were employed to explore the intrinsic mechanism. It was found that a lower removal efficiency towards Cr(III) was significantly associated with higher oligomers. The molecular size of the Cr(III)-citrate complex continued to increase with rising pH, making it larger overall than Cr(III)-collagen species. The growing oligomer moiety of dissolved Cr(III)-complex species could persist in the stronger basic pH range, leading to the large amount of residual Cr(III) in the Cr(III)-citrate system. Combining this result with potentiometric titration and X-ray photoelectron spectroscopy data, it was believed that the polymeric species other than monomers facilitated resisting the attack from hydroxide ions, and the postulated Cr(III)-citrate species towards higher oligomers were discovered. Beyond that, both charge neutralization and sweeping effects were presented among the gradually emerging flocs in the Cr(III)-collagen system together with the electric double layer compression effect derived from salinity, thus resulting in a larger floc size and higher Cr(III) removal efficiency in saline solutions.

Shifts in pore connectivity from precipitation versus groundwater rewetting increases soil carbon loss after drought

Droughts and other extreme precipitation events are predicted to increase in intensity, duration, and extent, with uncertain implications for terrestrial carbon (C) sequestration. Soil wetting from above (precipitation) results in a characteristically different pattern of pore-filling than wetting from below (groundwater), with larger, well-connected pores filling before finer pore spaces, unlike groundwater rise in which capillary forces saturate the finest pores first. Here we demonstrate that pore-scale wetting patterns interact with antecedent soil moisture conditions to alter pore-scale, core-scale, and field-scale C dynamics. Drought legacy and wetting direction are perhaps more important determinants of short-term C mineralization than current soil moisture content in these soils. Our results highlight that microbial access to C is not solely limited by physical protection, but also by drought or wetting-induced shifts in hydrologic connectivity. We argue that models should treat soil moisture within a three-dimensional framework emphasizing hydrologic conduits for C and resource diffusion.

The impacts of top down (precipitation) and bottom-up (groundwater rise) wetting processes on carbon sequestration are poorly understood. Here, the authors use incubation experiments to show that drought legacy and pore-scale wetting patterns are important factors controlling short-term carbon dynamics.

Developing a molecular picture of soil organic matter-mineral interactions by quantifying organo-mineral binding

Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to physical isolation and chemical stabilization at the organic–mineral interface. Instrumentation for probing this interface is limited. As a result, much of the micron- and molecular-scale knowledge about organic–mineral interactions remains largely qualitative. Here we report the use of force spectroscopy to directly measure the binding between organic ligands with known chemical functionalities and soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that chemistry of both the organic ligand and mineral contribute to values of binding free energy and that changes in pH and ionic strength produce significant differences in binding energies. These direct measurements of molecular binding provide mechanistic insights into organo–mineral interactions, which could potentially inform land-carbon models that explicitly include mineral-bound C pools.

Most molecular scale knowledge on soil organo–mineral interactions remains qualitative due to instrument limitations. Here, the authors use force spectroscopy to directly measure free binding energy between organic ligands and minerals and find that both chemistry and environmental conditions affect binding.

Interfacial interactions between Skeletonema costatum extracellular organic matter and metal oxides: Implications for ceramic membrane filtration

In the current study, the interfacial interactions between the high molecular weight (HMW) compounds of Skeletonema costatum (SKC) extracellular organic matter (EOM) and ZrO₂ or Al₂O₃, were investigated by atomic force microscopy (AFM). HMW SKC-EOM was rigorously characterized and described as a hydrophilic organic compound mainly comprised of polysaccharide-like structures. Lipids and proteins were also observed, although in lower abundance. HMW SKC-EOM displayed attractive forces during approaching (i.e., leading to jump-to-contact events) and adhesion forces during retracting regime to both metal oxides at all solution conditions tested, where electrostatics and hydrogen bonding were suggested as dominant interacting mechanisms. However, the magnitude of these forces was significantly higher on ZrO₂ surfaces, irrespective of cation type (Na⁺ or Ca²⁺) or concentration. Interestingly, while HMW SKC-EOM interacting forces to Al₂O₃ were practically insensitive to solution chemistry, the interactions between ZrO₂ and HMW SKC-EOM increased with increasing cation concentration in solution. The structure, and lower charge, hydrophilicity, and density of hydroxyl groups on ZrO₂ surface would play a key role on favoring zirconia associations with HMW SKC-EOM. The current results contribute to advance our fundamental understanding of Algogenic Organic Matter (AOM) interfacial interactions with metal oxides (i.e., AOM membrane fouling), and would highly assist in the proper selection of membrane material during episodic algal blooms.

Higher Molecular Mass Organic Matter Molecules Compete with Orthophosphate for Adsorption to Iron (Oxy)hydroxide

The competition between orthophosphate and water-extractable organic matter (WEOM) for adsorption to iron (oxy)hydroxide mineral surfaces is an important factor in determining the plant bioavailability of P in soils. Chemical force spectroscopy was used to determine the binding force between orthophosphate and iron (oxy)hydroxide that was coated onto atomic force microscopy (AFM) tips and adsorbed with WEOM. The force measurements were conducted at pH 4.65 and 0.02 M ionic strength which are representative of typical acid soil solutions. The chemical composition of the WEOM was determined by ultrahigh resolution electrospray ionization Fourier transform ion cyclotron mass spectrometry. The results indicate a correlation between aromatic WEOM molecules that are greater than 600 Da and the reduced binding force of orthophosphate to WEOM-adsorbed iron (oxy)hydroxide AFM tips suggesting that the molecular mass of aromatic WEOM molecules plays a critical role in regulating the WEOM-P interactions with surface functional groups of minerals. Based on the results of this study, we show the importance of obtaining a detailed, molecular-scale understanding of soil processes that can help develop better management strategies to reduce waste of limited P resources and adverse environmental impacts. Specifically, soil amendments with greater content of high molecular mass aromatic components may positively affect dissolved P use efficiency in soils by maintaining P in soil solution.

Character of Humic Substances as a Predictor for Goethite Nanoparticle Reactivity and Aggregation

Natural organic matter (NOM) is ubiquitous in surface water and groundwater and interacts strongly with mineral surfaces. The details of these interactions, as well as their impacts on mineral surface reactivity, are not well understood. In this work, both the reactivity and aggregation of goethite (α-FeOOH) nanoparticles were quantified in the presence of well-characterized humic substances. Results from monitoring the kinetics of reductive degradation of 4-chloronitrobenzene (4-ClNB) by Fe(II) adsorbed onto the goethite nanoparticles with and without added humic substances demonstrates that, in all cases, humic substances suppressed Fe(II)-goethite reactivity. The ranking of the standards from the least to most inhibitive was Pahokee Peat humic acid, Elliot Soil humic acid, Suwannee River humic acid, Suwannee River NOM, Suwannee River fulvic acid I, Suwannee River fulvic acid II, and Pahokee Peat fulvic acid. Correlations between eight characteristics (molecular weight, carboxyl concentration, and carbon, oxygen, nitrogen, aliphatic, heteroaliphatic, and aromatic content) and 4-ClNB degradation rate constants were observed. Faster kinetic rates of reductive degradation were observed with increased molecular weight and nitrogen, carbon, and aromatic content, and slower rates were observed with increased carboxyl concentration and oxygen, heteroaliphatic, and aliphatic content. With these correlations, improved predictions of the reactivity of Fe(II)-goethite with pollutants based on properties of the humic substances are possible.

Synthesis and characterisation of non-ionic AB-diblock nanoparticles prepared by RAFT dispersion polymerization with polymerization-induced self-assembly

The RAFT-PISA synthesis and characterization of non-ionic soft matter nanoparticles is described.

Citrate-Coated Silver Nanoparticles Interactions with Effluent Organic Matter: Influence of Capping Agent and Solution Conditions

Fate and transport studies of silver nanoparticles (AgNPs) discharged from urban wastewaters containing effluent organic matter (EfOM) into natural waters represent a key knowledge gap. In this study, EfOM interfacial interactions with AgNPs, and their aggregation kinetics were investigated by atomic force microscopy (AFM) and time-resolved dynamic light scattering (TR-DLS), respectively. Two well-characterized EfOM isolates, i.e., wastewater humic (WW humic) and wastewater colloids (WW colloids, a complex mixture of polysaccharides-proteins-lipids), and a River humic isolate of different characteristics were selected. Citrate-coated AgNPs were selected as representative capped-AgNPs. Citrate-coated AgNPs showed a considerable stability in Na(+) solutions. However, Ca(2+) ions induced aggregation by cation bridging between carboxyl groups on citrate. Although the presence of River humic increased the stability of citrate-coated AgNPs in Na(+) solutions due to electrosteric effects, they aggregated in WW humic-containing solutions, indicating the importance of humics characteristics during interactions. Ca(2+) ions increased citrate-coated AgNPs aggregation rates in both humic solutions, suggesting cation bridging between carboxyl groups on their structures as a dominant interacting mechanism. Aggregation of citrate-coated AgNPs in WW colloids solutions was significantly faster than those in both humic solutions. Control experiments in urea solution indicated hydrogen bonding as the main interacting mechanism. During AFM experiments, citrate-coated AgNPs showed higher adhesion to WW humic than to River humic, evidencing a consistency between TR-DLS and AFM results. Ca(2+) ions increased citrate-coated AgNPs adhesion to both humic isolates. Interestingly, strong WW colloids interactions with citrate caused AFM probe contamination (nanoparticles adsorption) even at low Na(+) concentrations, indicating the impact of hydrogen bonding on adhesion. These results suggest the importance of solution conditions and capping agents on the stability of AgNPs in solution. However, the characteristics of organics would play a crucial role in the fate and transport of these nano contaminants in urban wastewaters and natural water systems.

Chemical Force Spectroscopy Evidence Supporting the Layer-by-Layer Model of Organic Matter Binding to Iron (oxy)Hydroxide Mineral Surfaces

The adsorption of dissolved organic matter (DOM) to metal (oxy)hydroxide mineral surfaces is a critical step for C sequestration in soils. Although equilibrium studies have described some of the factors controlling this process, the molecular-scale description of the adsorption process has been more limited. Chemical force spectroscopy revealed differing adhesion strengths of DOM extracted from three soils and a reference peat soil material to an iron (oxy)hydroxide mineral surface. The DOM was characterized using ultrahigh-resolution negative ion mode electrospray ionization Fourier Transform ion cyclotron resonance mass spectrometry. The results indicate that carboxyl-rich aromatic and N-containing aliphatic molecules of DOM are correlated with high adhesion forces. Increasing molecular mass was shown to decrease the adhesion force between the mineral surface and the DOM. Kendrick mass defect analysis suggests that mechanisms involving two carboxyl groups result in the most stable bond to the mineral surface. We conceptualize these results using a layer-by-layer "onion" model of organic matter stabilization on soil mineral surfaces.

Kinetic study of seawater reverse osmosis membrane fouling

Reverse osmosis (RO) membrane fouling is not a static state but a dynamic phenomenon. The investigation of fouling kinetics and dynamics of change in the composition of the foulant mass is essential to elucidate the mechanism of fouling and foulant-foulant interactions. The aim of this work was to study at a lab scale the fouling process with an emphasis on the changes in the relative composition of foulant material as a function of operating time. Fouled membrane samples were collected at 8 h, and 1, 2, and 4 weeks on a lab-scale RO unit operated in recirculation mode. Foulant characterization was performed by CLSM, AFM, ATR-FTIR, pyrolysis GC-MS, and ICP-MS techniques. Moreover, measurement of active biomass and analysis of microbial diversity were performed by ATP analysis and DNA extraction, followed by pyro-sequencing, respectively. A progressive increase in the abundance of almost all the foulant species was observed, but their relative proportion changed over the age of the fouling layer. Microbial population in all the membrane samples was dominated by specific groups/species belonging to Proteobacteria and Actinobacteria phyla; however, similar to abiotic foulant, their relative abundance also changed with the biofilm age.