Coagulation kinetics of humic aggregates in mono- and di-valent electrolyte solutions

Optimizing extraction conditions to enhance the humification and soil remediation potential of compost-derived dissolved organic matter

Compost tea (CT), primarily composed of dissolved organic matter derived from compost, is widely used in various environmental and agricultural applications. Nevertheless, limited information is available regarding how extraction parameters influence the quality of CT and its efficacy in soil remediation. In this study, a multi-factor orthogonal design L16 (43) was employed to investigate the effects of compost-to-water ratio (CWR), extraction time (ET), and aeration pattern (AP) on nutrient extraction and humification of CT, aiming to optimize the extraction conditions. Results showed that N, P2O5, and K2O extraction efficiencies in all treatments ranged from 10 to 25 %, 10-20 %, and 50-85 %, respectively. The comprehensive humification score was in the range of 1.27-1.60. Among the three parameters, ET showed the most significant influence on CT quality. The optimal treatment for nutrient extraction was T15 (CWR 1:60, ET 48 h, and stirring), while T17 (CWR 1:30, ET 48 h, and aeration) exhibited superior performance on humification. Furthermore, the total Cd removal efficiency of T17 was 83.64 % after multiple washing cycles, which was attributed to an increased number of hydroxyl, carboxyl, and carbonyl functional groups that provided additional binding sites for Cd.

Environmental colloid behaviors of humic acid - Cadmium nanoparticles in aquatic environments

Humic acid (HA), a principal constituent of natural organic matter (NOM), manifests ubiquitously across diverse ecosystems and can significantly influence the environmental behaviors of Cd(II) in aquatic systems. Previous studies on NOM-Cd(II) interactions have primarily focused on the immobilization of Cd(II) solids, but little is known about the colloidal stability of organically complexed Cd(II) particles in the environment. In this study, we investigated the formation of HA-Cd(II) colloids and quantified their aggregation, stability, and transport behaviors in a saturated porous media representative of typical subsurface conditions. Results from batch experiments indicated that the relative quantity of HA-Cd(II) colloids increased with increasing C/Cd molar ratio and that the carboxyl functional groups of HA dominated the stability of HA-Cd(II) colloids. The results of correlation analysis between particle size, critical aggregation concentration (CCC), and zeta potential indicated that both Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions contributed to the enhanced colloidal stability of HA-Cd(II) colloids. Column results further confirmed that the stable HA-Cd(II) colloid can transport fast in a saturated media composed of clean sand. Together, this study provides new knowledge of the colloidal behaviors of NOM-Cd(II) nanoparticles, which is important for better understanding the ultimate cycling of Cd(II) in aquatic systems.

Regulation of the co-transport of toluene and dichloromethane by adsorbed phase humic acid under different hydro-chemical conditions

The transport behavior of combined organic pollutants in soil and groundwater has attracted significant attention in recent years. Research on the influence of humic acid (HA) on organic pollutant transport behavior mainly focuses on the study of the mobile phase HA, with less research on the adsorbed phase HA, especially regarding its interaction with combined pollutants. To enhance understanding of the regulation of co-transport and retention of combined pollutants by adsorbed phase HA, in this study, tests were conducted to investigate how toluene (TOL) and dichloromethane (DCM) are transported in the presence of adsorbed phase HA at different pH levels and ionic strengths. As the proportions of HA-coated sand increased, so did its adsorption capacity for TOL and DCM, which can be attributed to adsorbed phase HA providing more adsorption sites compared to plain sand, thereby reducing the transport potential of the pollutants. The presence of both TOL and DCM facilitated their mutual transportation due to competitive adsorption controlled by the adsorbed phase HA content in the porous medium. Furthermore, it was observed that pH levels influenced the transport behavior of TOL and DCM when adsorbed phase HA was present since adsorbed phase HA transformation into mobile phase was regulated by pH levels. The transport patterns can be effectively simulated using the chemical nonequilibrium two-site sorption model in HYDRUS-1D, accurately reflecting the retardation coefficients and transport distances based on model parameters. This work sheds new light on the regulatory role of adsorbed phase HA in TOL and DCM transport under diverse hydrochemical conditions, with implications for accurately depicting the behavior of combined pollutants, optimizing the remediation strategies and improving remediation efficiency in contaminated sites.

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.

Size, shape, and stability of organic particles formed during freeze-thaw cycles: Model experiments with tannic acid

HYPOTHESIS

Freeze-thaw cycles (FTC) in soils can cause the aggregation of dissolved organic matter but controlling factors are little understood.

EXPERIMENTS

In freeze-thaw experiments with tannic acid (TA) as model substance, we studied the effect of TA concentration, pH, electrolytes (NaCl, CaCl2, AlCl3), and number of FTC on particle formation. Tannic acid (0.005 to 10 g L-1) was exposed to 1-20 FTC at pH 3 and 6. The size and shape of particles was determined by confocal laser scanning microscopy. Particle stability was deduced from the equivalent circle diameter (ECD) obtained in dry state and the hydrodynamic diameter measured in thawing solutions.

FINDINGS

Tannic acid particles occurred as plates and veins, resembling the morphology of ice grain boundaries. Low pH and presence of electrolytes favored the formation of large particles. The freeze-concentration effect was most intense at low TA concentrations and increased with the number of FTC. While ECD of particles formed at low TA concentrations were smaller than at high concentrations, it was vice versa in the thawed state. At low TA concentrations, higher crystallization pressure of ice caused enhanced stability of large particles. We conclude that FTC can strongly alter the physical state of dissolved organic matter, with likely consequences for its bioavailability.

Co-transport behavior of Am(III) and natural colloids in the vadose zone sediments

Americium (III) (Am(III)) in the natural environment is considered immobile due to its low solubility, strong adsorption, and high affinity to solid surfaces. However, the presence of natural colloids may carry Am(III) transport for long distance. The individual and co-transport behaviors of Am(III) and natural colloids through the unsaturated packed columns were investigated under the influence of pH, electrolyte concentration, velocity, Am(III) concentration and natural colloids concentration. Under all experimental conditions, Am(III) individual transport construct sight breakthrough curves (BTCs, CAm/C0 < 3%), but the presence of natural colloids increased the BTCs plateau of Am(III) significantly (30% < CAm/C0 < 80%), indicating that the colloids were able to promote Am(III) transport in the unsaturated porous media. DLVO theoretical calculations reveal that the increased pH and decreased electrolyte concentration lead to a rase in electrostatic repulsion, and the natural colloids tend to be dispersed and stabilized, which facilitates elution. In addition to this, the increase of velocity and colloids concentration will lead to greater breakthrough of natural colloids. The non-equilibrium two-site model and the two-site kinetic retention model well-described the BTCs of Am(III) and natural colloids, respectively. This study provide new insights into the behavior of natural colloids carrying the Am(III) into aquifers through the vadose zone sediments.

Release of dissolved organic matter from wetland plants and its interaction with polycyclic aromatic hydrocarbons

Dissolved organic matter (DOM) derived from wetland plants played a critical role in CWs pollutant migration. This study investigated the character and release pattern of DOM derived from two wetland plants, Phragmites australis and Cladophora sp., and the interaction between DOM with phenanthrene (PHE), benzo(a)pyrene (Bap), and benzo [k]fluoranthene (BkF) under different physical conditions were also studied using spectroscopic techniques. DOM release was related to plant species and withering stage. Humic acid (HA)-like fractions (C3 and C5) were dominated in P. australis (52%) and completely withered Cladophora sp. groups (55%), while protein-like fractions (C1 and C2) dominated in early withered Cladophora sp. groups (52%). Due to the cell and tissue structure difference among plants and their withering stage, DOM derived from early withered P. australis revealed a two-stage slow-fast phase, while other groups were linearly released (R2 0.87207-0.97091). A strong correlation existed between HA-like fractions and water quality index, reflecting the critical influence of plant decay in CWs operation performance. The analysis with Stern-Volmer equation indicated that plant-based DOM interacted with PAHs to form ground state complexes with possible involvement of π-π interaction, hydrogen bonding and cation bridging effect. Aromatic, molecular weight, and hydrophilicity of both DOM and PAHs affected their binding with the interaction capability in the order of BKF > Bap > PHE and C3 > C5 > C2 > C1 > C4. Besides, alkaline environment and high DO condition was highly unsuitable for the combination. Scientific management and appropriate operating condition were important in optimizing operation performance and controlling pollutant migration in CWs.

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.

Ions play different roles in virus removal caused by different NOMs in UF process: Removal efficiency and mechanism analysis

In this study, we investigated the effect of different compositions of aquatic natural organic matter (NOM) and ions on virus removal by ultrafiltration (UF). MS2 bacteriophage was used as a surrogate. Humic acid (HA) improved the MS2 removal rate from 1.95 ± 0.09 LRV to 2.40 ± 0.03 LRV at the HA dosage of 9 mg/L through the combined mechanisms of size exclusion, electrostatic repulsion and hydrophobicity. MS2 removal rate further increased to 3.10 ± 0.05 LRV by 10 mmol/L Na⁺ dosage and 3.19 ± 0.12 LRV by Ca²⁺ 1 mmol/L in the HA-containing UF system. Size exclusion turned into the dominant virus removal mechanism according to the results of the fouling model fitting and the weakening of electrostatic repulsion and hydrophobicity. The complexation of Ca²⁺ also played a role in MS2 removal based on the analysis of interaction force. MS2 removal rate by bovine serum albumin (BSA) was poor, which was 2.07 ± 0.06 LRV at the BSA dosage of 9 mg/L. Hydrophobicity was greatly reduced and the dominant virus removal mechanisms were size exclusion and electrostatic repulsion. 10 mmol/L Na⁺ in the presence of BSA deteriorated MS2 removal rate to 2.02 ± 0.07 LRV by the weakening of electrostatic repulsion, hydrophobicity and size exclusion. Electrostatic repulsion severely decreased by 1 mmol/L Ca²⁺ and the enhanced adsorption barrier represented competitive adsorption of Ca²⁺ by BSA and MS2 contributed for MS2 removal further decline (1.99 ± 0.05 LRV). Complex components in water will have different effects on virus removal due to their properties and interactions. This study can provide references for selecting more efficient water treatment methods according to the different compositions of raw water in actual water treatment applications during the UF process. Moreover, the retention of virus by UF can be predicted based on our study results.

Impacts of Calcium Addition on Humic Acid Fouling and the Related Mechanism in Ultrafiltration Process for Water Treatment

Humic acid (HA) is a major natural organic pollutant widely coexisting with calcium ions (Ca²⁺) in natural water and wastewater bodies, and the coagulation-ultrafiltration process is the most typical solution for surface water treatment. However, little is known about the influences of Ca²⁺ on HA fouling in the ultrafiltration process. This study explored the roles of Ca²⁺ addition in HA fouling and the potential of Ca²⁺ addition for fouling mitigation in the coagulation-ultrafiltration process. It was found that the filtration flux of HA solution rose when Ca²⁺ concentration increased from 0 to 5.0 mM, corresponding to the reduction of the hydraulic filtration resistance. However, the proportion and contribution of each resistance component in the total hydraulic filtration resistance have different variation trends with Ca²⁺ concentration. An increase in Ca²⁺ addition (0 to 5.0 mM) weakened the role of internal blocking resistance (9.02% to 4.81%) and concentration polarization resistance (50.73% to 32.17%) in the total hydraulic resistance but enhanced membrane surface deposit resistance (33.93% to 44.32%). A series of characterizations and thermodynamic analyses consistently suggest that the enlarged particle size caused by the Ca²⁺ bridging effect was the main reason for the decreased filtration resistance of the HA solution. This work revealed the impacts of Ca²⁺ on HA fouling and demonstrated the feasibility to mitigate fouling by adding Ca²⁺ in the ultrafiltration process to treat HA pollutants.

Mercury Removal from Contaminated Water by Wood-Based Biochar Depends on Natural Organic Matter and Ionic Composition

Biochars can remove potentially toxic elements, such as inorganic mercury [Hg(II)] from contaminated waters. However, their performance in complex water matrices is rarely investigated, and the combined roles of natural organic matter (NOM) and ionic composition in the removal of Hg(II) by biochar remain unclear. Here, we investigate the influence of NOM and major ions such as chloride (Cl^-), nitrate (NO₃^-), calcium (Ca²⁺), and sodium (Na⁺) on Hg(II) removal by a wood-based biochar (SWP700). Multiple sorption sites containing sulfur (S) were located within the porous SWP700. In the absence of NOM, Hg(II) removal was driven by these sites. Ca²⁺ bridging was important in enhancing removal of negatively charged Hg(II)-chloro complexes. In the presence of NOM, formation of soluble Hg-NOM complexes (as seen from speciation calculations), which have limited access to biochar pores, suppressed Hg(II) removal, but Cl^- and Ca²⁺ could still facilitate it. The ability of Ca²⁺ to aggregate NOM, including Hg-NOM complexes, promoted Hg(II) removal from the dissolved fraction (<0.45 μm). Hg(II) removal in the presence of Cl^- followed a stepwise mechanism. Weakly bound oxygen functional groups in NOM were outcompeted by Cl^-, forming smaller-sized Hg(II)-chloro complexes, which could access additional intraparticle sorption sites. Therein, Cl^- was outcompeted by S, which finally immobilized Hg(II) in SWP700 as confirmed by extended X-ray absorption fine structure spectroscopy. We conclude that in NOM containing oxic waters, with relatively high molar ratios of Cl^-: NOM and Ca²⁺: NOM, Hg(II) removal can still be effective with SWP700.

Aqueous aggregation and deposition kinetics of fresh and carboxyl-modified nanoplastics in the presence of divalent heavy metals

The presence of heavy metals alters the colloidal stability and deposition of nanoplastics (NPs) in urban waters. Such processes are important to assess the mobility and fate of NPs and their associated heavy metals. Up to date, few studies have reported the impact of heavy metals on the colloidal behaviors of NPs and the involved mechanisms. In the study, time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation (QCM-D) methods were used to assess the aggregation and deposition kinetics of polystyrene nanospheres with divalent heavy metals. For comparison, carboxyl-modified polystyrene nanospheres were used. Results reveal that heavy metals destabilized NPs more significantly than calcium ions. Spectroscopy and transmission electron microscopy analysis propose that heavy metals destabilized NPs via inner-sphere coordination with carboxyl groups and cation-π interactions, further leading to the formation of different dimensional aggregates. QCM-D results suggest that the deposition rate, irreversibility, and film compactness of NPs on silica surfaces first increased but further decreased as heavy metal concentration increased. Such deposition behaviors depended on the bridging effects between NPs and silica and aggregation-induced diffusion limitation. In that case, the destabilization and retention ability of heavy metals for NPs were related to their electronegativity and hydration shell thickness. In urban waters, the presence of natural organic matter (NOM) decreased the destabilization and retention ability of heavy metals, whereas heavy metals with environmentally relevant concentrations still enhanced the aggregation and deposition of NPs compared with other environmental cations. This study highlights the impact of heavy metal property on the colloidal behaviors of NPs, thus deepening our understanding of the mobility and fate of NPs associated with heavy metals in urban waters.

A review on the generation, discharge, distribution, environmental behavior, and toxicity (especially to microbial aggregates) of nano-TiO2 in sewage and surface-water and related research prospects

This article reviews the nano-effects and applications of different crystalline nano‑titanium dioxide (nano-TiO₂), identifies their discharge, distribution, behavior, and toxicity to aquatic organisms (focusing on microbial aggregates) in sewage and surface-water, summarizes related toxicity mechanisms, and critically proposes future perspectives. The results show that: 1) based on crystal type, application boundaries of nano-TiO₂ have become clear, extending from traditional manufacturing to high-tech fields; 2) concentration of nano-TiO₂ in water is as high as hundreds of thousands of μg/L (sewage) or several to dozens of μg/L (surface-water) due to direct application or indirect release; 3) water environmental behaviors of nano-TiO₂ are mainly controlled by hydration conditions and particle characteristics; 4) aquatic toxicities of nano-TiO₂ are closely related to their water environmental behavior, in which crystal type and tested species (such as single species and microbial aggregates) also play the key role. Going forward, the exploration of the toxicity mechanism will surely become a hot topic in the aquatic-toxicology of nano-TiO₂, because most of the research so far has focused on the responses of biological indicators (such as metabolism and damage), while little is known about the stress imprint caused by the crystal structures of nano-TiO₂ in water environments. Additionally, the aging of nano-TiO₂ in a water environment should be heeded to because the continuously changing surface structure is bound to have a significant impact on its behavior and toxicity. Moreover, for microbial aggregates, comprehensive response analysis should be conducted in terms of the functional activity, surface features, composition structure, internal microenvironment, cellular and molecular level changes, etc., to find the key point of the interaction between nano-TiO₂ and microbial aggregates, and to take mitigation or beneficial measures to deal with the aquatic-toxicity of nano-TiO₂. In short, this article contributes by 1) reviewing the research status of nano-TiO₂ in all aspects: application and discharge, distribution and behavior, and its aquatic toxicity; 2) suggesting the response mechanism of microbial aggregates and putting forward the toxigenic mechanism of nanomaterial structure; 3) pointing out the future research direction of nano-TiO₂ in water environment.

Real-time changes of the adsorption process and conformation of marine dissolved organic matters on the solid-liquid interface

In this study, the adsorption characteristics of marine dissolved organic matters (MDOMs) on the solid-liquid interface in the coastal waters was investigated. The results showed that the organic macromolecules with adsorption ability in MDOMs are not rigid molecules. However, the macromolecules have viscoelasticity properties. At different dilution ratios, the MDOMs adsorption process includes rapid (0-200 s) and slow adsorption (200 s later) periods. MDOMs adsorption in the solid-liquid interface is a dynamic process in which adsorption and hydration occur simultaneously. MDOMs concentration is an important driving force for adsorption. The three macromolecules of acid polysaccharides, protein-like, and polycarboxylate-type humic acids in MDOMs are rich in functional groups and they have the ability to absorb to solid surface. Acidic polysaccharides exhibit a sustained adsorption ability, while the adsorption of other macromolecules occurred only in the initial rapid adsorption period. In addition, the acid polysaccharides show weak thixotropy during the adsorption process. It would cause the stretching of macromolecular structure of the adsorption layer, enhancing the hydration of the adsorption layer. The study shows the adsorption process of MDOMs at the solid-liquid interface and the structural characteristics of the adsorption layer. It can provide helpful information for the inhibition and removal of MDOMs pollution during the actual development of marine resources.

Mechanisms of bentonite colloid aggregation, retention, and release in saturated porous media: Role of counter ions and humic acid

In the subsurface environment, colloids play an important role in pollutant transport by acting as the carriers. Understanding colloid release, transport, and deposition in porous media is a prerequisite for evaluating the potential role of colloids in subsurface contaminant transport. In this work, the aggregation, retention, and release of bentonite colloid in saturated porous sand media were investigated by kinetic aggregation and column experiments, the correlation and mechanism of these processes were revealed by combining colloid filtration theory, interaction energy calculation and density functional theory. The results showed that the retention and release of colloids were closely related to the dispersion stability and filtration effect. Multivalent cations with higher mineral affinity reduced the colloid stability, and the dispersion stability and mobility of the colloid were greatly improved by humic acid due to the enhancement of electrostatic repulsion and steric hindrance effects. The primary minimum interaction was found to contribute more to irreversible colloid retention in a Ca²⁺ system, while the secondary energy minimum was found to be responsible for colloid release with the occurrence of transient solution chemistry. The deposited colloid aggregates could be redistributed and released when the solution chemistry became favorable towards dispersion. These findings provide essential insight into the environmental colloid fate as well as a vital reference for the risk of colloid-driven transport of contaminants in the subsurface aquifer environment.

Ion compositions in artificial media control the impact of humic acid on colloidal behaviour, dissolution and speciation of CuO-NP

The toxicity of copper oxide nanoparticles (CuO-NP) strongly depends on their interactions with the surrounding environment, impacting their dissolution and colloidal stability. This behaviour is studied quite extensively for simplified electrolytes, but information on the behaviour of CuO-NP in more complex artificial media are lacking. In our study, we analysed the colloidal behaviour and considered the speciation of CuO-NP in pure water and three artificial media of different complexity which are used in ecotoxicology. Measurements were done over 7 days in the absence and presence of humic acid (HA) as a model organic molecule. In pure water, the addition of HA lowered the zeta potential from +11 to -41 mV, while in all artificial media, it stayed constantly at about -20 mV. The hydrodynamic diameter of CuO-NP remained unaffected by HA in pure water and seawater, while in porewater and especially in freshwater, HA suppressed strong agglomeration. In pure water, HA strongly increased dissolution to the highest observed value (3% of total Cu), while HA reduced dissolution in all artificial media. Speciation calculations revealed that cations from the media competed with Cu from the NP surface for complexing sites of the HA. This competition may have caused the reduced dissolution in the presence of ions. Furthermore, speciation calculations also suggest that ion composition drove agglomeration behaviour rather than ion concentration: agglomeration was high when divalent cations where the major interaction partner and dominant in relative terms. HA may have reduced the relative dominance and thus altered the agglomeration, aligning it in all media. Summarizing, ion composition and the presence of HA strongly drive the dissolution and agglomeration of CuO-NP in artificial media, consequently, analysing complexation can help to predict environmental behaviour and toxicity.

Transformation of sulfadiazine in humic acid and polystyrene microplastics solution by horseradish peroxidase coupled with 1-hydroxybenzotriazole

Enzyme catalyzed coupling with redox mediators are considered as great interesting and viable technologies to transform antibiotics. This work demonstrated the horseradish peroxidase (HRP) was effective in transforming sulfadiazine (SDZ) transformation coupled with 1-hydroxybenzotriazole (HBT) at varying conditions. The removal of SDZ was independent of Na⁺ and its ionic strength, but Ca²⁺ could enhance transformation efficiency by increasing the enzyme activity of HRP. The presence of humic acid (HA) and polystyrene (PS) microplastics showed inhibition on the transformation of SDZ, and the transformation rate constants (k) decreased with the concentration of HA and PS particles increased. These primarily attributed to covalent coupling and electrostatic interaction between SDZ and HA, SDZ and PS, respectively, which reduced the concentration of free SDZ in the reaction solution. The presence of cation recovered the inhibition of SDZ transformation by HA and PS particles, which ascribed to compete between cation and SDZ. The divalent cations (Ca²⁺) showed more substantial competitiveness than mono (Na⁺) due to more carried charge. Eight possible transformation products were identified, and potential SDZ transformation pathways were proposed, which include δ-cleavage, γ-cleavage, carbonylation, hydroxylation, SO₂ extrusion and SO₃ extrusion. In addition, HA and PS particles couldn't affect the transformation pathways of SDZ. These findings provide novel understandings of the transformation and the fate of antibiotics in the natural environment by HRP coupled with redox mediators.

Effects of solution chemistry on conformation of self-aggregated tannic acid revealed by laser light scattering

Inorganic soil solution constituents can alter the charge, size, and conformation of dissolved organic molecules, thus affecting their environmental behavior. Here, we investigated how pH, cation valence and activities induce conformational changes and aggregation-sedimentation reactions of organic polyelectrolytes. For that we determined the hydrodynamic diameter of the model compound tannic acid by laser light scattering at concentrations of 1-30 g L^-1 in the pH range from 3 to 10 and with electrolyte additions of CaCl₂ and hydroxyl-Al cations. Charge properties were quantified by polyelectrolyte titration and zeta potential measurements. After dispersion by sonication, aggregation was determined in time sequences up to 60 min and suspension stability was traced in sedimentation experiments. Tannic acid was present in ultrapure water in a self-aggregated state. At pH <3 as well as >7.5, its hydrodynamic diameter increased. Whereas at high pH this behavior could be assigned to unfolding of molecular conformations, at low pH it is likely that charge neutralization decreased repulsive forces and facilitated aggregation. At pH 5 and ionic strengths of up to 5 mM, CaCl₂ did not affect aggregation state of tannic acid and results resembled those obtained in ultrapure water. Addition of hydroxyl-Al cations broke-up the self-aggregated tannic acid structures under formation of Al-organic coprecipitates. Strong aggregation only occurred at mixing ratios where opposite surface charges were completely balanced. Under natural conditions, self-aggregation of tannic acid can be expected only at higher solution concentrations. However, at acidic pH, hydroxyl-Al cations and tannic acid may form discrete colloidal particles already at low tannic acid concentrations, resulting in the destabilization of suspensions. Our data emphasize that the soil solution composition strongly modifies the physical state of tannic acid, and likely also of other biopolymers, and thus their interactions within environmental matrices.

Molecular Sorption Mechanisms of Cr(III) to Organo-Ferrihydrite Coprecipitates Using Synchrotron-Based EXAFS and STXM Techniques

Ubiquitous organo-ferrihydrite coprecipitates (OFC) significantly affect the mobility and availability of Cr in soil through sorption, but the underlying sorption mechanisms remain unclear at the molecular level. Due to the potential formation of OFC in agricultural soils with returned crop straws, we synthesized OFC with rice/rape straw-derived carbon (C) sources and different loadings. The molecular sorption mechanisms of Cr(III) to the synthesized OFC under different conditions were investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Cr(III) sorption by OFC decreased with increasing C loading and decreasing pH, regardless of C sources. Moreover, inhibition of Cr(III) sorption to OFC with high C loading occurred when ionic strength (IS) increased, suggesting the presence of outer-sphere complexed Cr(III). EXAFS analysis revealed that more Cr(III) were bound to ferrihydrite of the OFC at a relatively high pH, and organically bound Cr(III) enhanced when increasing C loading and decreasing IS. STXM analysis strongly suggested that C loading reduced Cr(III) sorption through blocking the binding sites on the ferrihydrite, which overwhelmed Cr(III) retention by the direct binding of Cr(III) to carboxyl of the particulate organic matter (OM) and OM coated on the Fh fractions of the OFC. These findings facilitated the comprehensive understanding of the sorption mechanisms of Cr(III) to OFC at the molecular level, which will assist the prediction of Cr(III) mobility in soils, particularly for Cr(III)-contaminated agricultural soils with the application of crop straws.

Opposite impacts of K+ and Ca2+ on membrane fouling by humic acid and cleaning process: Evaluation and mechanism investigation

Understanding the influences of cations on membrane fouling was important to improve the performance of membrane filtration system, however, opposite conclusions were made in different studies. Meanwhile, although the influences of cation concentration have been studied extensively, few attentions have been paid to the cation valence. To clarify it, the effects of typical cations on membrane fouling and cleaning, as well as the related mechanisms were investigated systemically in this study. K⁺ and Ca²⁺ were chosen as the representative cations, and humic acid (HA) was chosen as the membrane foulants. The results demonstrated Ca²⁺ promoted the formation of reversible fouling, meanwhile higher removal efficiency of HA could also be achieved with the assistance of filtration cake containing HA + Ca²⁺. However, K⁺ led to the formation of more recalcitrant irreversible fouling. By comparing the concentration of cations in feed and permeate, analyzing the influence of cations on size of HA flocs, and the detailed SEM, AFM and TEM observation, it could be found that different mechanisms dominated the interaction between cations and HA. The bridging effect induced by Ca²⁺ attributed to the extension of HA molecules, while the electrostatic shielding effect induced by K⁺ led to the compression of them. Moreover, the different characteristics of hydrated Ca²⁺ and K⁺ also contributed to the different structures of foulant layers formed by HA + Ca²⁺ and HA + K⁺. Given the abundance of K⁺ and Ca²⁺ in natural water, results of this study can provide valuable advice for practical membrane filtration process.

Electro-Oxidation of Humic Acids Using Platinum Electrodes: An Experimental Approach and Kinetic Modelling

Humic acids (HA) are a potential hazard to aquatic ecosystems and human health. Because biological treatment of contaminated water does not satisfactorily remove these pollutants, novel approaches are under evaluation. This work explores electrochemical oxidation of HA in aqueous solution in a lab-scale apparatus using platinum-coated titanium electrodes. We evaluated the effects of HA concentration, current density, chloride concentration and ionic strength on the rate of HA oxidation. The initial reaction rate method was used for determining the rate law of HA degradation. The results showed that the reaction rate was first-order relative to HA concentration, chloride concentration and current density. An appreciable effect of ionic strength was also observed, most likely due to the polyanionic character of HA. We propose a kinetic model that satisfactorily fits the experimental data.

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.

Colloidal Behaviors of Two-Dimensional Titanium Carbide in Natural Surface Waters: The Role of Solution Chemistry

Although two-dimensional titanium carbide (Ti₃C₂T_x MXene) has emerged as a shining star material in various communities, its environmental behaviors and fate remain unknown. Herein, the colloidal properties and stability of Ti₃C₂T_x MXene are explored in aquatic systems for the first time, considering the roles of solution chemistry conditions (e.g., pH, ionic types, and strength). It was found that pH had no effect on the stability of Ti₃C₂T_x in the range of 5.0-11.0, whereas ionic valence and concentrations displayed significant effects on the aggregation behavior of Ti₃C₂T_x. By employing time-resolved dynamic light scattering measurements, the critical coagulation concentration (CCC) value of Ti₃C₂T_x was determined to be 12 mM for NaCl. The divalent cations Ca²⁺ and Mg²⁺ exhibited higher destabilizing capacity to Ti₃C₂T_x, as evidenced by the lower CCC values (0.3 and 0.4 mM for CaCl₂ and MgCl₂, respectively) and faster coagulation rates. Long-term stability studies implied that Ti₃C₂T_x MXene was less likely to be transported over long distances in the synthetic or natural waters. These findings provided significant insights into the fate and transport of Ti₃C₂T_x in the aquatic environment.

Coagulation mechanisms of humic acid in metal ions solution under different pH conditions: A molecular dynamics simulation

Humic acid (HA) exerts a variety of significant environmental and geochemical influences on the soils, sediments and aqueous environments. The interaction with metal ions induces strong HA-metal complexation, thus effecting the transport of the toxic metals as well as the colloidal aggregation of HA. In the present work, we systematically report and analyze the aggregation mechanisms of HAs in solutions filled with different heavy cations (Ag⁺, Cd²⁺ and Cr³⁺) or common metal ions (Na⁺, Ca²⁺ and Al³⁺) under neutral and low pH conditions by using molecular dynamic simulations. We aim to explore the effects of pH, metal ions type and other possible weak interactions on the aggregation capabilities of HA. Scrutiny of the simulation results showed that the aggregation of HAs under neutral condition was driven by the HA-metal complexation which combined the effects of electrostatic attraction and inter-molecular bridging between cations and COO^- groups. Larger extent of aggregation was found in heavy metal ions compared with the common ones. On the other hand, under low pH condition, due to the protonation states of carboxyl and phenolic group, the aggregation of HAs was stabilized mainly by weak forces, such as hydrogen bonds between different functional groups. In addition, other weak interactions such as the hydrophilic and hydrophobic effects, the cation-π interactions have also been proposed to be progressive effects on the coagulation behavior. Our computational studies give supplement to the experimental observation and provide insights into the intrinsic mechanisms of the aggregation behavior of HAs and their complexation with metal ions. Such computational modelling supplied a highly effective tool for qualitatively evaluating their roles in environmental remediation.

pH dependence of the binding interactions between humic acids and bisphenol A - A thermodynamic perspective

The wide application of bisphenol A (BPA) leads to the emergence of BPA residuals in natural water environments. Dissolved organic matter (DOM) existed in water can bind with BPA, hence influencing the migration and transformation of BPA in aquatic environments. pH is a crucial factor governing the binding interactions between DOM and BPA. However, the mechanisms driven the binding process under different pH conditions are still unclear. In this study, the interactions between BPA and humic acids (HA), a primary component of DOM, are investigated over a wide pH range of 3-12 by integrating fluorescence quenching, dynamic light scattering and microcalorimetry. pH dependence of the binding interactions between HA and BPA are interpreted from a thermodynamic perspective. The results indicate that HA can spontaneously interact with BPA to form a stable HA-BPA complex. With the increasing pH, the binding interactions change from entropy driven to entropy-enthalpy co-driven. Hydrophobic force dominate the binding interactions under acidic condition. The synergy of hydrophobic force and hydrogen bond promotes the binding process under neutral condition. Under alkaline conditions, electrostatic repulsion participates the binding process in addition to hydrophobic force and hydrogen bond, weakening the binding strength. Therefore, neutral pH is favorable for HA to bind with BPA, consequently enhancing the dissolution of BPA in natural water bodies. The results are beneficial to better understand the pH dependent distribution of BPA in aquatic environments.

Insight into the interaction mechanism of iron ions with soil humic acids. The effect of the pH and chemical properties of humic acids

The main aim of this work was to study the mechanisms of interaction between iron(II) ions and humic acids as a function of pH, iron concentration and various humic acids chemical properties, including the degree of humification, elemental composition, aromaticity and content of acidic functional groups. The results indicated that iron was bound by humic acids at pH 7 in amounts ∼2 times higher than at pH 5 (averaged capacities: 117 and 57 cmol/kg, respectively). Iron binding at pH 7 increased with increasing the total carboxylic and phenolic groups content and the degree of humification of humic acids (R-coefficients: 0.99 and 0.95, respectively). The stability of humic acid-iron complexes at pH 7 were only slightly lower than at pH 5 due to iron hydroxides formed at pH > 5 (averaged stability constants: 5.18 and 5.26, respectively). Iron coordination mode varied depending on pH: at pH 5, the bidentate (chelate) mode dominated, whereas at pH 7 the bridging mode appeared prevalent. The total amount of bound iron was much smaller than the content of the carboxylic and phenolic groups in humic acids, on average by ∼80 (pH 7) and ∼90.1% (pH 5) indicating the occurrence of steric effects in humic acid structure i.e. the reduction of the complexation capacity of free functional groups by adjacent groups occupied by iron and/or the formation of intramolecular aggregates with iron hindering the access of further metal ions. At pH 5 the complexes were soluble in the iron concentration range positively correlated to carboxylic and phenolic groups content, showing the protective nature of negatively charged functional groups on the stability of the solution. At this pH, the destabilization of the system was governed by the neutralization of humic acid charged structures by metal cations and the compression of the double electric layer. At pH 7 the stability of the humic acid-iron solution was largely determined by the form of iron, mainly by the precipitation of metal hydroxides acting as a flocculant destabilizing the solution by co-precipitation of humic acid-iron complexes.

Effect of co-existing Co2+ ions on the aggregation of humic acid in aquatic environment: Aggregation kinetics, dynamic properties and fluorescence spectroscopic study

The fate and transport of humic substances in the aquatic environments depend significantly on their interactions with co-existing ions. Herein, we employed dynamic light scattering (DLS) measurement, molecular dynamic (MD) simulation and fluorescence spectrometry to investigate the aggregation of humic acid (HA) in the presence of Co²⁺ ions. The aggregation kinetics was depicted by hydrodynamic diameter (<D_h>) and the attachment efficiency (α) of HA aggregates. α increases gradually in the reaction-limited (slow) regime due to the decrease of the double layer repulsion, and the energy barrier is eliminated to a certain extent in the diffusion-limited reaction while α close to unity. The complexation between functional groups (i.e. carboxylic and phenolic groups) of HA and Co²⁺ ions contributes significantly to the aggregation process of HA. MD simulation and density functional theory (DFT) calculation demonstrate that the aggregation process of HA can be promoted by Co²⁺ through several inter- or intra-molecular interactions between HA and the Co²⁺ ions. The results provide a pathway for insight into the interactions between HA and metal ions, which is important for deeply understanding the environmental behaviors of HA in natural aqueous systems.

Strength of Humic Acid Aggregates: Effects of Divalent Cations and Solution pH

Aggregation-dispersion, charging, and aggregate strength of Leonardite humic acid (LHA) were investigated in CaCl₂ and MgCl₂ solutions as a function of pH and ionic strength (I). The strength or the withstanding force of aggregates of humic substances (HSs) against breakage is important because this force influences the transport and distribution of pollutants and nutrients along with HSs through the change in the size of HS aggregates as a transport unit. We observed the dominancy of aggregation of LHA at high pH than at low pH in every case of CaCl₂ and MgCl₂ solutions. This observation suggests the higher binding efficiency of these divalent ions at high pH, though there was no obvious relation with electrophoretic mobility and aggregation of LHA. Further, we first revealed the numerical value of the strength of HS aggregates by using a simple experimental setup of aggregate breakup under laminar converging flow through a capillary tube. The obtained values of the strength of LHA aggregates were higher in the presence of CaCl₂ solution than MgCl₂ solution, and the strength increased with pH. The highest strengths of LHA aggregates in 30 mM (I) CaCl₂ and MgCl₂ solutions were around 5.8 and 2.4 nN, respectively, at pH around 9.

Sorption of copper and norfloxacin onto humic acid: effects of pH, ionic strength, and foreign ions

Copper (Cu) and norfloxacin (Nor) are frequently used as feed additives for animal growth promotion, which results in a great probability of Cu²⁺ and Nor coexisting in animal excretion and in soils. Sorption of Cu²⁺ and Nor on soil organic matter (SOM) can markedly affect their environmental fate. Thus, humic acid (HA), a major fraction of SOM, was chosen to investigate the cosorption behaviors of Cu²⁺ and Nor on HA under different solution chemistry conditions (pHs, ionic strengths, and foreign ions). The addition of Nor decreased the maximum adsorption capacity (Q_m) of Cu²⁺ and an increasing effect was observed with increasing Nor concentration. Meanwhile, the addition of Cu²⁺ also markedly inhibited the sorption of Nor on HA. The Q_m of Cu²⁺ increased with increasing pH from 3.0 to 5.0 whether Nor was present or not, but more addition of Nor led to less increment in Q_m of Cu²⁺ at the same pH. The Q_m of Nor was observed at pH 4.0 without Cu²⁺, but that was found at pH 5.0 and 3.0 with the addition of 20 and 100 mg L^-1 Cu²⁺, respectively. The sorption of Cu²⁺ on HA decreased with increasing ionic strength and followed an order of NaH₂PO₄ > Na₂SO₄ ≈ NaNO₃ at pH 5.0 whether Nor was present or not. Additionally, the higher valence cation had a stronger inhibition effect on Cu²⁺ sorption. The competition between Cu²⁺ and Nor for sorption on HA under the same conditions indicated that the coexistence of Cu²⁺ and Nor may enhance the feasibility of their mobility and environmental risk.

Colloidal stability and aggregation kinetics of biochar colloids: Effects of pyrolysis temperature, cation type, and humic acid concentrations

An understanding of biochar colloid aggregation and stability in aqueous environments is critical for assessing biochar fate and mobility in the soil. The aggregation kinetics of wheat straw-derived biochar colloids pyrolyzed at two temperatures 300 and 600 °C (WB300 and WB600 colloids, respectively) were investigated in monovalent and divalent electrolyte solutions in absence/presence of humic acid (HA). Results show that the critical coagulation concentrations (CCCs) of WB300 colloids in NaCl and CaCl₂ solutions were 274 and 61.4 mM, which were higher than those (183 mM for NaCl and 38.1 mM for CaCl₂) of WB600 colloids. WB300 had more oxygen-containing functional groups than WB600, which induced more negative surface charge on WB300. HA of 5 mg L^-1 greatly increased the CCCs of WB300 and WB600 colloids to 1288 and 806 mM in NaCl solutions, but decreased the CCCs to 54.6 and 37.0 mM in CaCl₂ solutions because of strong bridging between HA and Ca²⁺. In CaCl₂ solutions with high salt concentrations (near to the CCCs), different HA concentrations caused distinct effects on the aggregation of biochar colloids. The aggregation of biochar colloids was accelerated by HA with the concentration higher than 5 mg L^-1 through cation-bridging while the aggregation was inhibited in the presence of <2.5 mg L^-1 HA. Our findings show that pyrolysis temperature used for biochar production had a large effect on the aggregation of biochar colloids in the aqueous environment and that cation type and dissolved natural organic matter are controlling variables.

Systematic studies on the binding of metal ions in aggregates of humic acid: Aggregation kinetics, spectroscopic analyses and MD simulations

The binding of metal ions with humic acid (HA) plays an important role in the aggregation of HA and the migration of metal ions in the environments. The effects of common cations (Na⁺, Mg²⁺, Ca²⁺ and Al³⁺) and heavy metal ions (Ag⁺, Cd²⁺, Cu²⁺, Cr³⁺ and Eu³⁺) on the aggregation of HA were investigated systematically by aggregation kinetics, spectroscopic techniques and molecular dynamic (MD) simulations. The critical coagulation concentration (CCC) of mono-, di- and trivalent cations could be predicted by the Schulze-Hardy rule. The aggregation of HA in the presence of Na⁺ and Ag⁺ was mainly due to the reduction of repulsive force and the hydrogen bonds between HA molecules. While the complexation of di- and trivalent cations with carboxylic/phenolic groups, or the cation-π interactions enhanced the intra- or inter-molecular bridges in HA and then contributed greatly to the aggregation of HA. Heavy metal ions could easily pass through the electric double-layer of HA compared with common cations. MD simulations further signified the strong aggregation ability of HA molecules in solutions containing high valence metal ions. These findings are important for understanding not only how the influence of metal ions on the aggregation of HA, but also the conditions which ions more efficient for aggregation.

Dynamic molecular size transformation of aquatic colloidal organic matter as a function of pH and cations

Knowledge of the dynamic changes in molecular size of natural colloidal organic matter (COM) along the aquatic continuum is of vital importance for a better understanding of the environmental fate and ecological role of dissolved organic matter and associated contaminants in aquatic systems. We report here the pH- and cation-dependent size variations of COMs with different sources (river and lake) quantified using flow field-flow fractionation (FIFFF), fluorescence spectroscopy and parallel factor analysis (PARAFAC), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and zeta potential analysis. Increasing pH caused a decline in molecular sizes and an obvious size transformation from the >10 kDa to 5-10 kDa and further to 1-5 kDa size fraction, whereas the opposite trend was observed for increasing cation (e.g., Ca²⁺ and Cu²⁺) abundance. Compared with lakewater COM, the riverwater COM exhibited a greater pH-dependent dispersion but less extent in cation-induced aggregation, demonstrating that the dispersion and aggregation dynamics were highly dependent on COM source and solution chemistry (e.g., pH and cations). Based on ATR-FTIR analysis, the extensive dissolution of C=O and C-O functional groups resulted in a greater pH-dependent dispersion for river COM. Fluorescence titration revealed that, despite their similar cation-induced aggregation behavior, the binding constants of all the PARAFAC-derived components for Cu²⁺ were 1-2 orders of magnitude higher than those for Ca²⁺ (logK_M: 4.54-5.45 vs. 3.35-3.70), indicating a heterogeneous nature in cation-DOM interactions. The greater extent of decline in zeta potential for lake COM suggested a Ca-induced charge neutralization and aggregation mechanism. However, for Cu-induced aggregation, chemical complexation was the predominant pathway for the river COM, with higher binding constants, while charge neutralization and chemical complexation co-induced the aggregation of lake COM. Thus, natural COMs may have different environmental behavior along the aquatic continuum and further affect the fate and transport of contaminants in aquatic environments.

Nanoparticle tracking analysis versus dynamic light scattering: Case study on the effect of Ca2+ and alginate on the aggregation of cerium oxide nanoparticles

The effect of Ca²⁺ and alginate on the stability of CeO₂ nanoparticles (NPs) was investigated using dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA); the two methods were then compared. DLS showed rapid aggregation of CeO₂ NPs in 8 mM Ca²⁺ solution; however, NTA showed that some primary aggregates (200-400 nm) still remained-a result that was obscured in DLS measurements. Aggregation of alginate molecules was also studied using DLS and NTA, where NTA particle concentration and video provided additional information on the alginate aggregation progress. Finally, DLS showed that in the presence of alginate, the aggregation rate and size of CeO₂ NPs increased. NTA intensity measurements provided insight into a heteroaggregation and bridging mechanism. NTA particle concentration and video also showed CeO₂ NPs were linked by alginate gel in high Ca²⁺ concentration (>4 mM). the DLS and NTA had different advantages in measuring particle size. DLS could better study the initial aggregation stage and large aggregates, while NTA could better detect small aggregates. NTA also measured particle number concentration, individual intensity, and particle motion video, which provided additional insight. Combining these two methods could help us to better understand the behavior and fate of NPs in water.

Effect of mono and divalent salts on the conformation and composition of a humic acid and on atrazine adsorption

We investigated the effects of sodium and calcium chlorides on the conformation and composition of a purified Aldrich humic acid (PAHA), as well as on the adsorption of atrazine. The PAHA was treated with 1, 10, and 100 mM NaCl, CaCl₂, or a mixture of NaCl and CaCl₂ (molar ratio 5:1) at pH 7.5 and 8.5. The conformation of treated PAHA was characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM) and spectral changes of functional groups of PAHA by Fourier transform infrared spectroscopy (FTIR). AFM and TEM images showed an increase in the aggregation of the PAHA as salinity increased. FTIR spectra revealed that changes in the aggregation of the PAHA were principally due to the formation of bridged interactions between calcium and carboxylate groups in the PAHA. The adsorption of atrazine on > 0.45 μm PAHA decreased as salt concentrations and pH increased. This reduction of atrazine adsorption was explained by the decrease in available adsorption sites due to agglomeration of PAHA.

Coagulation behavior of humic acid in aqueous solutions containing Cs+, Sr2+ and Eu3+: DLS, EEM and MD simulations

The coagulation behaviors of humic acid (HA) with Cs⁺ (10-500 mM), Sr²⁺ (0.8-10.0 mM) and Eu³⁺ (0.01-1.0 mM) at different pH values (2.8, 7.1 and 10.0) were acquired through a dynamic light scattering (DLS) technique combined with spectroscopic analysis and molecular dynamic (MD) simulations. The coagulation rate and the average hydrodynamic diameter (<D_h>) increased significantly as the concentration of nuclides increased. <D_h> could be scaled to time t as <D_h>∝ t^a at higher Sr²⁺ concentrations, which shows that HA coagulation is consistent with the diffusion-limited colloid aggregation (DLCA) model. Trivalent Eu³⁺ induced HA coagulation at a much lower concentration than bivalent Sr²⁺ and monovalent Cs⁺. The coagulation value ratio of Sr²⁺ and Eu³⁺ to Cs⁺ is almost proportional to Z^-6, indicating that the HA coagulation process is generally consistent with the Schulze-Hardy rule. Spectroscopic analysis indicated that the complexation between nuclides and carboxylic/phenolic groups of HA molecules played important roles in the coagulation of HA. MD modelling suggested that Sr²⁺ and Eu³⁺ ions increased the coagulation process through the formation of intra- or inter-molecular bridges between negatively charged HA molecules, whereas for Cs⁺, no inter-molecular bridges were formed. This work offers new insight into the interactions between HA and radionuclides and provides a prediction for the roles of HA in the transportation and elimination of radionuclides in severely polluted environments.

Real-time evaluation of natural organic matter deposition processes onto model environmental surfaces

Natural organic matter (NOM) is ubiquitous in aqueous systems and dynamically partitions onto/from environmental surfaces. However, such interfacial processes have not been uniformly quantified in situ and in real time. In this work, adsorption and deposition processes of Suwannee River humic acid (SRHA) and Suwannee River fulvic acid (SRFA), as model NOM, were evaluated for a series of environmentally relevant interfaces. Real-time, interfacial phenomenon, including deposition, release, and adlayer viscoelastic properties, were quantified over a variety of water chemistries via quartz crystal microbalance with dissipation monitoring (QCM-D). Specifically, adlayer mass and deposition rates of SRHA and SRFA were evaluated as a function of NOM concentration/molecular weight (fraction), pH, electrolyte composition (type and concentration), and collector surface type. For these, the adsorption of SRHA onto aluminum oxide (Al₂O₃) and polystyrene (PS) surfaces follows the Langmuir isotherm model. Rapid, near-monolayer formation of SRHA/SRFA adlayers were observed on Al₂O₃, hydroxyapatite (HAP), and poly (l-lysine) (PLL) surfaces, but not on PS or iron oxide (Fe₃O₄) surfaces. The presence of divalent cations (Ca²⁺/Mg²⁺) at relatively low concentrations (0.5-5.0 mM) significantly enhances the mass of SRHA/SRFA deposited onto the surfaces of silica (SiO₂), Al₂O₃, and PS. Viscoelastic properties of the adsorbed layer based on the ratio of dissipation to frequency revealed a relatively unique adlayer structure for SRHA in the presence of 5.0 mM Ca²⁺.

Water-based fractionation of a commercial humic acid. Solid-state and colloidal characterization of the solubility fractions

BACKGROUND AND HYPOTHESIS

Humic acid (HA) is of considerable environmental significance, being a major component of soil, as well as being considered for application in other technological areas. However, its structure and colloidal properties continue to be the subject of debate, largely owing to its molecular complexity and association with other humic substances and mineral matter. As a class, HA is considered to comprise supramolecular assemblies of heterogeneous species, and herein we consider a simple route for the separation of some HA sub-fractions.

EXPERIMENTS

A commercial HA sample from Sigma-Aldrich has been fractionated into two soluble (S1, S2) and two insoluble (I1, I2) fractions by successive dissolution in deionized water at near-neutral pH. These sub-fractions have been characterized by solution and solid-state approaches.

FINDINGS

Using this simple approach, the HA has been shown to contain non-covalently bonded species with different polarity and water solubility. The soluble and insoluble fractions have very different chemical structures, as revealed particularly by their solid-state properties (¹³C NMR and IR spectroscopy, and TGA); in particular, S1 and S2 are characterized by higher carbonyl and aromatic contents, compared with I1 and I2. As shown by solution SAXS measurements and AFM, the soluble fractions behave as hydrophilic colloidal aggregates of at least 50nm diameter.

Formation, Aggregation, and Deposition Dynamics of NOM-Iron Colloids at Anoxic-Oxic Interfaces

The important role of natural organic matter (NOM)-Fe colloids in influencing contaminant transport, and this role can be influenced by the formation, aggregation, and particle deposition dynamics of NOM-Fe colloids. In this work, NOM-Fe colloids at different C/Fe ratios were prepared by mixing different concentrations of humic acid (HA) with 10 mg/L Fe(II) under anoxic conditions. The colloids were characterized by an array of techniques and their aggregation and deposition behaviors were examined under both anoxic and oxic conditions. The colloids are composed of HA-Fe(II) at anoxic conditions, while they are made up of HA-Fe(III) at oxic conditions until the C/Fe molar ratio exceeds 1.6. For C/Fe molar ratios above 1.6, the aggregation and deposition kinetics of HA-Fe(II) colloids under anoxic conditions are slower than those of HA-Fe(III) colloids under oxic conditions. Further, the aggregation of HA-Fe colloids under both anoxic and oxic conditions decreases with increasing C/Fe molar ratio from 1.6 to 23.3. This study highlights the importance of the redox transformation of Fe(II) to Fe(III) and the C/Fe ratio for the formation and stability of NOM-Fe colloids that occur in subsurface environments with anoxic-oxic interfaces.

Aggregation behavior of TiO2 nanoparticles in municipal effluent: Influence of ionic strengthen and organic compounds

The influence of ionic strengthen and dissolved organic matter (DOM) on the aggregation of TiO₂ nanoparticles (NPs) in municipal effluent was investigated. The results demonstrated that DOM promoted the mobility of NPs in aquatic system by synergism between static repulsion and steric effect, while electrolytes were opposite by charge-neutralization. The physical-chemical characteristics of DOM played the major role on the mobility of NPs. Bovine serum albumin (BSA) showed the strongest enhancement on the mobility of TiO₂ NPs. High adsorption of BSA introduced vast negative charges on the TiO₂ NPs' surface, leading to static repulsion and neutralizing positive charges of electrolytes in surrounding as well. By contrast, another protein α-amylase retarded the aggregation rate of TiO₂ NPs through steric repulsion of the long-chain construction. Humic substances (Fulvic acid and alginate) also reflected the combination of static repulsion and steric effect. However, in the high electrolytes concentration (especially Ca²⁺), the long-chain aliphatic compounds were prone to form calcium bridge which increased the hydrodynamic diameter of TiO₂ aggregates consequently. Sodium dodecylbenzene sulfonate (SDBS) showed low adsorption capacity, while the unabsorbed SDBS retarded the aggregates caused by the changes of pH and electrolytes. These data indicated that decreasing of DOC concentration in aqueous system was important to reduce the mobility and potential risk of NPs in aqueous system.

Characterization and Cu sorption properties of humic acid from the decomposition of rice straw

Humic acid (HA) derived from rice straw decomposed for 1 (HA-1), 3 (HA-3), 6 (HA-6) and 12 (HA-12) months was characterized by potentiometric titration and solid-state cross-polarization magic-angle spinning ¹³C nuclear magnetic resonance spectroscopy (CPMAS ¹³C NMR). The sorption of Cu on examined HA was investigated using a combination of batch sorption, isothermal titration calorimetry (ITC) and sequential desorption. Results showed that the functional group content and the humification degree of HA tended to increase with increasing decomposition time especially in the latter stage of examined decomposition period. Cu sorption on HA was a rapid process that occurred within the first 1 h and the sorption capacity increased from 245.4 mmol kg^-1 on HA-1 to 294.6 mmol kg^-1 on HA-12. The sorption of Cu was endothermic, spontaneous and the randomness was increased during Cu sorption. Sorbed Cu on examined HA can be hardly released by NH₄Ac but nearly fully released by EDTA. Forming inner-sphere complexes was the main mechanism of Cu sorption on examined HA. This study could provide valuable information for a better understanding on the environmental impacts of the decomposition of organic waste.

Insights into key factors controlling GO stability in natural surface waters

The effects of pH, cations (Na⁺, K⁺, Mg²⁺, Ca²⁺ and Al³⁺), and anions (Cl^-, HCO₃^-, HPO₄^2- and SO₄^2-) on graphene oxide (GO) stability were investigated to address the current limitations in the knowledge regarding the stability of GO in natural surface water and its underlying mechanism. The threshold values of cations that destabilize GO were obtained and affected by both pH and anions. By employing elemental mapping and studying the effects of polyacrylic acid (PAA) on GO sedimentation and the re-dispersion of GO aggregates, we find that the GO aggregates induced by Na⁺ and K⁺ via electric double layer suppression and by Ca²⁺ and Al³⁺ via strong complxing are difficult to re-disperse completely. Specifically, more PAA is needed to re-disperse GO aggregates than to stabilize GO, which suggests that after GO binds with heavy metal ions. It is less likely to be transported over a long distance even in natural water that are rich in natural organic matter. Finally, we find that the key factor controlling GO sedimentation in natural surface waters is its binding with Mg²⁺ and Ca²⁺. This study is expected to provide critical knowledge to more accurately predict the fate of GO in natural surface aquatic environments.