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

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.

Effects of Cu(II)-DOM complexation on DOM degradation: Insights from spectroscopic evidence

The fate of dissolved organic matter (DOM) is primarily governed by its sources, degradation, and transformation processes within the environment. However, the influence of metal-DOM complexation on DOM degradation remains ambiguous. In this study, controlled laboratory experiments were conducted using Cu(II) and natural water from the Duliujian River and the Beidagang Wetland to examine the effects of metal-DOM binding on the degradation pathway of DOM. Our results showed that Cu(II)-DOM complexation affected the distribution of DOM molecular weight with elevated Mw after complexed with Cu(II). Nevertheless, the concentration of DOM decreased over the incubation period due to degradation. In the absence of Cu(II) binding, both wetland and river DOM followed similar degradation pathways, transforming from high to low molecular weight with changes predominantly in the 1-10 kDa size-fraction during DOM degradation. In contrast, in the presence of Cu(II) and thus Cu(II)-DOM binding, the degradation of DOM was enhanced, resulting in higher kinetic rate constants for both wetland and river DOM. The results of differential spectra further confirmed the degradation of DOM with a decrease in bulk spectroscopic properties and an increase in the degree of DOM-Cu(II) complexation. These findings imply a mutually reinforcing relationship between metal-DOM complexation and the degradation of DOM in aquatic environments, providing new insights into the biogeochemical behavior and environmental fate of DOM.

Colloidal Chemistry in Water Treatment: The Effect of Ca2+ on the Interaction between Humic Acid and Poly(diallyldimethylammonium chloride) (PDADMAC)

The complexation of humic acid (HA), as a major component of natural organic matter (NOM) in raw water, with polycations is a key step in the water treatment process. At sufficiently high addition of a polycation, it leads to neutralization of the formed complexes and precipitation. In this work, we studied the effect of the presence of Ca2+ ions on this process, with poly(diallyldimethylammonium chloride) (PDADMAC) as a polycation. This was done by determining the phase behavior and characterizing the structures in solution by light scattering and small-angle neutron scattering (SANS). We observe that with increasing Ca2+ concentration, the phase boundaries of the precipitation region shift to a lower PDADMAC concentration, which coincides well with a shift of the ζ-potential of the aggregates in solution. Light scattering shows the formation of aggregates of a 120-150 nm radius, and SANS shows that Ca2+ addition promotes a compaction in the size range of 10-50 nm within these aggregates. This agrees well with the observation of more densely packed precipitates by confocal microscopy in the presence of Ca2+. Following the precipitation kinetics by turbidimetry shows a marked speeding up of the process already in the presence of rather small Ca2+ concentrations of 1 mg/L. It can be stated that the presence of Ca2+ during the complexation process of HA with a polycation has a marked effect on phase behavior and precipitation kinetics of the formed aggregates. In general, the presence of Ca2+ facilitates the process largely already at rather low concentrations, and this appears to be linked to a compaction of the formed structures in the mesoscopic size range of about 10-50 nm. These findings should be of significant importance for tailoring the flocculation process in water treatment, which is a highly important process in delivering drinking water of sufficient quality to humans.

Phytotoxicity of radionuclides: A review of sources, impacts and remediation strategies

Various anthropogenic activities and natural sources contribute to the presence of radioactive materials in the environment, posing a serious threat to phytotoxicity. Contamination of soil and water by radioactive isotopes degrades the environmental quality and biodiversity. They persist in soils for a considerable amount of time and disturb the fauna and flora of any affected area. Hence, their removal from the contaminated medium is inevitable to prevent their entry into the food chain and the organisms at higher levels of the food chain. Physicochemical methods for radioactive element remediation are effective; however, they are not eco-friendly, can be expensive and impractical for large-scale remediation. Contrastingly, different bioremediation approaches, such as phytoremediation using appropriate plant species for removing the radionuclides from the polluted sites, and microbe-based remediation, represent promising alternatives for cleanup. In this review, sources of radionuclides in soil as well as their hazardous impacts on plants are discussed. Moreover, various conventional physicochemical approaches used for remediation discussed in detail. Similarly, the effectiveness and superiority of various bioremediation approaches, such as phytoremediation and microbe-based remediation, over traditional approaches have been explained in detail. In the end, future perspectives related to enhancing the efficiency of the phytoremediation process have been elaborated.

Multi-scale modeling of natural organic matter-heavy metal cations interactions: Aggregation and stabilization mechanisms

Interaction between natural organic matters (NOM) and heavy metal cations in aqueous environment are of great significance for maintaining stability of organic carbon and restraining transport of heavy metal contaminants in (bio)geochemical processes. We systematically explore the aggregation process and complexation between NOM and heavy metal cations (Ag⁺, Cd²⁺, Pb²⁺, Zn²⁺, Eu³⁺) under different pH condition by molecular dynamics (MD) simulations, umbrella sampling method, and quantum chemistry calculations. The character of molecular structures NOM-heavy metal complexes and association are quantified. In acidic pH condition, aggregation proceeds via H-bonding and π-π interactions between NOM fragments. In neutral condition, Ag⁺, Cd²⁺, Pb²⁺, and Eu³⁺ can form inner-sphere complexes with the surface carboxylic groups and therefore reduce intermolecular charge repulsion, eventually leading to NOM aggregation, and it shows that even without direct binding, the outer-sphere adsorbed Zn²⁺ can also result in the formation of NOM assemble through H-bonding. Consequently, these heavy metals are capable of promoting NOM aggregation regardless of the complexing ways. Complexing free energy calculations characterized the dynamic processes of cations binding to the carboxylic groups of NOM fragment and the related energy landscape. This study provides quantitative insights for understanding the environmental processes of heavy metals and cycle of C in aquatic ecosystem, and contributes to developing environment-friendly strategies for controlling heavy metal contaminants.

Complexation modelling and oxidation mechanism of organic pollutants in cotton pulp black liquor during iron salt precipitation and electrochemical treatment

Removal behavior of organic pollutants such as lignin in cotton pulp black liquor (CPBL) was investigated in precipitation followed by electrochemical oxidation (EO) using FeCl₃, Fe₂(SO₄)₃, FeCl₂ and FeSO₄ as precipitants, electrolyte and catalysts. Based on comparison of precipitation efficacy of iron salts, spectroscopic techniques, thermodynamic equilibrium calculations and molecular dynamics (MD) simulations were used to provide insight into the interaction between iron cations and lignin. The results showed that FeCl₃ achieved the highest removal of chemical oxygen demand (COD, 76.05%), UV₂₅₄ (69.21%) and lignin (78.28%). Iron cationic complexation with lignin was identified as the key mechanism in precipitation. Fe³⁺ was more active in binding to organic ligands mainly due to charge effect compared to Fe²⁺. The strong Fe-sulphate coordination affected the complexation with lignin. MD simulations showed the formation of inner sphere complexes of iron cations with deprotonated carboxyl and hydroxyl groups via bidentate and monodentate coordination. The removal efficiency of electrochemical oxidation (EO) as a post-treatment of the precipitation was dependent on iron salts. Removals of COD, UV₂₅₄ and color can achieve 98.88%, 98.9% and 99.97% by FeCl₃ precipitation and EO processes. The effluent reached the primary discharge standard specified in Integrated Wastewater Discharge Standard of China (GB8978-1996). FeCl₃ demonstrated significant advantages in the removal of organic pollutants from cotton pulp black liquor in the combined process of precipitation and electrochemical treatment and may have practical application potential.

Understanding the Effect of pH on the Solubility and Aggregation Extent of Humic Acid in Solution by Combining Simulation and the Experiment

Molecular dynamics (MD) simulations were performed to investigate the dynamics of humic acid (HA) in an aqueous solution and the influence of pH, temperature, and HA concentration. The HA model employed in MD simulations was chosen and validated using experimental chemical composition data and Fourier transform infrared (FTIR) spectra. The simulations showed that the HA molecule has a strong propensity to adopt a compact conformation in water independent of pH, while the aggregation of HA was found to be pH-dependent. At high pH, the ionized HAs assembled into a thread-like structure, maximizing contact with water. At low pH, the neutral HAs formed a droplet-like aggregate, minimizing contact with the solvent. The simulation results are consistent with experimental data from dynamic light scattering (DLS) measurements and transmission electron microscopy (TEM) imaging. This work provides new insight into the folding and aggregation of HA as a function of pH and a molecular-level understanding of the relationship between the acidity and the structure, solubility, and aggregation of HA, with direct implications for HA-based remediation strategies of contaminated sites.

Insight into the adsorption of europium(III) on muscovite and phlogopite: Effects of pH, electrolytes, humic substances and mica structures

Europium(III), i.e., Eu(III), is chemically analogous to the trivalent lanthanides (Ln) and actinides (An). A good understanding of the adsorption behaviour of Eu(III) on mica group minerals is critical to the safety evaluation of the radioactive contamination. Nevertheless, the structural complexity of micaceous minerals makes it difficult to draw a consistent conclusion in the study of Eu(III) migration. In this work, we contrastively studied Eu(III) adsorption on dioctahedral muscovite and trioctahedral phlogopite as functions of pH, ionic strength, background electrolytes, interaction sequence, and fulvic acid (FA). Batch experiments showed that Eu(III) adsorption on both micas was strongly dependent on pH but quite independent on ionic strength that is determined by Na⁺. Planar sites are available on both muscovite and phlogopite while interlayer sites only on phlogopite under Na⁺ and Ca²⁺ electrolytes (not for K⁺ and Cs⁺). An interlayer expansion of phlogopite, as indicated by a newly appeared diffraction peak at ~6° 2-theta, occurred along with Eu(III) adsorption, which was also confirmed by transmission electron microscopy. Furthermore, the initial Eu(III) concentrations, the concentration ratios between Eu(III) and Cs⁺, and the reaction sequences of Eu(III)-electrolytes-FA affected both the adsorption behaviour of Eu(III) and reversely the structural alteration of phlogopite. The sequential extraction showed that the adsorbed Eu(III) was mainly in the ion-exchangeable form while the addition of FA could increase the portion of coordinative species. The currently proposed Eu(III) adsorption mechanism can shed new light on predicting the migration of Ln/An(III) at the mica-rich solid-liquid interface on a molecular scale.

Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a ⁶⁰Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50-100, 10-50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic -OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs⁺, Sr²⁺, and Eu³⁺ ions in the molecular-weight fractions of the metal-HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs⁺ ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr²⁺ and Eu³⁺ ions redistributed organic carbon toward the larger molecular-weight fractions.

Colloidal stability and correlated migration of illite in the aquatic environment: The roles of pH, temperature, multiple cations and humic acid

The mobility and environmental risk of colloids and associated pollutants are dependent on their dispersion stability under various conditions. In this work, the stability and correlated migration of illite colloids (IC) were systematically investigated over a wide range of aquatic chemistry conditions. The results showed that IC was aggregation favorable at low pH, low temperature and high ionic strength. The critical coagulation concentration (CCC) of IC increased exponentially with increasing values of r/Z³, following the Schulze-Hardy and Hofmeister series. Humic acid (HA) greatly mitigated colloid aggregation since the attachment of HA on IC surface increased the steric hindrance and electrostatic potential, and the enhancement of stability was linearly correlated with the HA concentration. The Derjaguin-Landau-Verwey-Overbeek (DLVO) model revealed that the interaction force deriving from van der Waals forces and electrostatic double-layer energy evolved as the aquatic chemistry varied, and the reduction in repulsion force between particles facilitated the colloid collision and then aggregation. The migration of IC in the porous sand column was highly correlated with the dispersion stability and filtration effect, the agglomerated colloids were redispersed and released when conditions favored dispersion. The illite colloids acted as efficient carriers for Eu(III) transport. These findings are essential for improving the understanding of the geological fate of environmental colloids and associated radionuclides.

Study on the co-effect of maifanite-based photocatalyst and humic acid in the photodegradation of organic pollutant

In this study, the co-effect of clay mineral-based photocatalyst and humic acid on the photodegradation of dye was revealed for the first time. The clay mineral-based photocatalyst, maifanite/g-C₃N₄, was prepared using the co-calcining method. The physical and chemical properties of the maifanite/g-C₃N₄ photocatalysts with various ratios were characterized by multiple characterization methods, including SEM, XPS, BET, UV-Vis, FTIR, contact angle, and XRD. The respective degradation experiment of humic acid and RhB was performed using maifanite/g-C₃N₄ photocatalysts. The degradation process of mixture solution of humic acid and RhB was measured using EEM and UV-vis. The result indicates that in the presence of humic acid, low ratio of maifanite/g-C₃N₄ inhibits the production of by-products derived from the interaction of humic acid and the degradation of RhB. However, high ratio of maifanite/g-C₃N₄ is not conducive to the degradation of RhB. The ratio of 1:3 for maifanite/g-C₃N₄ is optimal for the photodegradation of RhB in the presence of humic acid. This article provides a new perspective to develop the co-effect of clay mineral and humic acid in the photodegradation of organic pollutant.

Transport behaviors of Cs+ in granite porous media: Effects of mineral composition, HA, and coexisting cations

The transport of radiocesium (RCs) in granite has attracted great concerns for the consideration of a long-term safety assessment and performance evaluation of the nuclear waste disposal repository. In this study, the transport behaviors of Cs⁺ in granite were addressed and quantified by column experiments, sequential extraction, and a convection-dispersion equation model. The transport of Cs⁺ in granite experienced at least two stages including a rapid increase and a slow increase stages. The retardation of Cs⁺ in granite obviously became higher as biotite content increased. However, a consistent breakthrough plateau and almost overlapped breakthrough curves were observed under different feldspar contents, which suggested that the transport behaviors of Cs⁺ in granite was quite close to feldspar. Compared to Na⁺, K⁺ could effectively inhibit Cs⁺ adsorption and facilitate the mobility of Cs⁺ in granite column. In the presence of Sr²⁺, the transport of Cs⁺ was provoked in the granite column mainly due to the high competition effects. Humic acid (HA) did not obviously change the transport behaviors of Cs⁺ in granite column; however, HA could weakly change the adsorption species of Cs⁺ during Cs⁺ transport in granitic media. Both sequential extraction and two-site non-equilibrium model suggested that feldspar was the main contributor to the weak adsorption sites and biotite was responsible for the strong affinity sites for Cs⁺ in Beishan granite. The findings could provide important insights into RCs transport and fate in granitic media.

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.

Insight into the stability and correlated transport of kaolinite colloid: Effect of pH, electrolytes and humic substances

Environmental colloids play crucial roles in the transport of environmental pollutants in porous media by acting as pollutant carriers. In this work, the dispersion stability and correlated transport of kaolinite colloid were investigated as a function of solution pH, solution ionic strength, and concentration of humic acid (HA), the roles of kaolinite colloid in driving Eu(III) transport were discussed. The results showed that the dispersion of kaolinite colloid was favorable at alkaline and extremely acidic pH values, the trend of aggregation with varying pH was critically reversed at pH ∼3.2 due to the transformation of surface electrical properties. Cations with higher valence and mineral affinity showed a more significant contribution in inducing colloid aggregation, which was generally in accordance with the Schulze-Hardy rule and Hofmeister series. HA greatly increased the colloid stability by altering the surface electrostatic potential and steric effect. The Derjguin-Landau-Verwey-Overbeek (DLVO) model suggested that the electrostatic force between colloidal particles controlled the aggregation and destabilizing trend of colloid, and the theoretically calculated critical coagulation concentration was consistent with that determined from kinetic aggregation experiments. The roles of kaolinite colloid in driving Eu(III) transport varied under different conditions, and the transport behavior was highly correlated with the dispersion stability trend of colloid. These results can provide an enhanced understanding of the environmental fate of kaolinite colloid as well as commensal pollutants.

Macromolecular Structure of a Commercial Humic Acid Sample

The molecular structure of a commercial sample of humic acids (HA) was investigated by membrane dialysis experiments (MD) and low-pressure size-exclusion chromatography (LP-SEC). MD showed that HA molecules were retained by dialysis membrane with a cut-off of 6–8 kDa, independently from HA concentration (15 or 150 mg L−1), NaHCO3 concentration (0.005–2.0 mol L−1), and from propan 2-ol (0–5 v/v %). SEC experiments at low pressure gave chromatograms with a broad peak, with an elution volume between those of the globular proteins bovine serum albumin (molecular weight = 66.5 kDa) and lysozyme from egg (molecular weight = 14.4 kDa). The pattern of the chromatogram did not vary with HA concentration, and second-run chromatograms of single eluted fractions showed relatively sharp peaks. From these data, we reveal that the commercial HA sample analysed has a macromolecular structure rather than being a supramolecular aggregate of relatively small molecules, as recently proposed for some samples of HA obtained from different sources.

Advanced Gel Electrophoresis Techniques Reveal Heterogeneity of Humic Acids Based on Molecular Weight Distributions of Kinetically Inert Cu2+-Humate Complexes

Humic acids (HAs) play important roles for the fate of metal ions in the environment. Most chemical speciation models involving HAs assume heterogeneous metal ion binding. However, these models also assume that the binding affinities of metal ions with HAs are the same regardless of the molecular weight (MW) ranges of the HAs involved. Here, we develop new polyacrylamide gel electrophoresis (PAGE) techniques to investigate the MW distributions of HAs with strongly complexed Cu²⁺ ions. By combining contaminant metal-free and high-resolution PAGE for HAs, this work was able to provide accurate MW distributions for the complexed metal ions. The MW distribution of Cu²⁺ binding ability per quantity of HA indicates that strong metal-binding moieties in HAs are heterogeneous in terms of MW. Coupling of the PAGE techniques with UV-vis and excitation-emission matrix (EEM) spectrometry-parallel factor analysis (PARAFAC) methods revealed new insights into kinetically inert interactions between HAs and Cu²⁺ ions. By this method, we found that the protein-like fluorescence components in the high- and low-MW regions cooperatively responded through Cu²⁺ binding. Thus, the advanced gel electrophoresis techniques developed herein are able to shed new light on the heterogeneity of metal binding affinities of HAs in terms of MW.

Construction of novel graphene-based materials GO@SiO2@C@Ni for Cr(VI) removal from aqueous solution

A series of novel sandwich-like GO@SiO₂@C@Ni composites were developed. The morphologies and adsorption capacities of the materials sintered at different carbonization temperatures were investigated. The formed GO@SiO₂@C@Ni-400 possessed of wonderful dispersion, large surface area (229.88 m²/g) and high saturation magnetization. Batch experimental results revealed that maximum adsorption capacities of these materials towards Cr(VI) were as follows: GO@SiO₂@C@Ni-400 (299.20 mg/g) > GO@SiO₂@C@Ni-500 (244.05 mg/g) > GO (202.39 mg/g) > Graphene@C@Ni (188.80 mg/g) > GO@SiO₂@C@Ni-600 (165.51 mg/g) > GO@SiO₂@C@Ni-700 (93.36 mg/g). Moreover, the influence of hydrochemistry, such as contact time, pH, co-existing ions and solution temperature, on Cr(VI) adsorption was researched as well. It was demonstrated that GO@SiO₂@C@Ni-400 had remarkable adsorption capacity for Cr(VI) removal under the acidic condition, hardly disturbed by other anions, and showed better adsorption performance at 328 K. Besides, On the base of X-ray photoelectron spectroscopy analysis, mechanisms of adsorption could be explained that Cr(VI) was reduced to Cr(III) by nitrogen dopant, and the complexation was existed between Cr(VI) and oxygen-containing functional groups. Additionally, GO@SiO₂@C@Ni-400 could be easily separated under the external magnetic field and displayed outstanding reusability. Herein, GO@SiO₂@C@Ni-400 opens up the possibility of future practical applications.

Environmental fate and risk of ultraviolet- and visible-light-transformed graphene oxide: A comparative study

Currently, there is little comparative data on the colloidal stability and the toxicity of ultraviolet (UV)- and visible-light (VL)-transformed graphene oxide (GO). In order to identify this knowledge gap, the physicochemical properties of UV/VL-transformed GO are investigated in detail. Attempts are made to correlate the physicochemical alterations of UV/VL-transformed GO to the observed changes in its colloidal properties and toxicity. The results show that both UV and VL irradiations induce the significant change in the color, UV-vis absorbance, morphology, surface charge, size, oxygen containing functional groups, total of carbon, and photoluminescence properties of GO. The photo-reaction behavior of GO under UV exposure is different from that under VL irradiation in terms of reaction rate, order, and extent. Finally, the UV and VL irradiations show different effects not only on the colloidal stability of GO in the City water and Dongpu Lake water, but also on the toxicity of GO to Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria. This study clearly shows how the environmental fate and risk of GO are modified by UV and VL irradiations.

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.

Is the interaction between graphene oxide and minerals reversible?

The increased applications and production of graphene oxide (GO) make the necessity to study information on the interaction of GO with minerals. In this work, adsorption and desorption were used to study the reversibility of interaction between GO and goethite/kaolinite. Result showed that the pH value, ionic strength, and temperature had significant effects on the adsorption and desorption behavior of GO. Interaction force was stronger between GO and goethite than that of kaolinite. The interaction may be attributed to the electrostatic, hydrogen-bonding, and Lewis acid base interactions. The irreversible interaction between GO and minerals may be a main mechanism for the observed desorption hysteresis. These results are important for evaluating the fate and health risk of GO in the environment.

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.

Decontamination of U(VI) on graphene oxide/Al2O3 composites investigated by XRD, FT-IR and XPS techniques

The decontamination of U(VI) on graphene oxide/nano-alumina (GO/Al₂O₃) composites were investigated by batch, XRD, FT-IR and XPS techniques. The characterization results showed that GO/Al₂O₃ composites presented a variety of oxygen-containing functional groups, which provided the more surface reactive sites. The batch experiments indicated that sorption equilibrium of U(VI) on GO/Al₂O₃ composites was achieved within 30 min, and the maximum sorption capacity derived from Langmuir model was 142.8 mg/g at pH 6.5. In addition, the slight decrease of sorption capacity was observed even after fifth recycling times. These results indicated that GO/Al₂O₃ composites displayed the fast sorption rate, high sorption capacity and good regeneration performance. No effect of ionic strength revealed the inner-sphere surface complexation of U(VI) on GO/Al₂O₃ composites. FT-IR and XPS analysis demonstrated that the high adsorption of U(VI) on GO/Al₂O₃ was attributed to the various oxygen-bearing functional groups. In addition, the nano Al₂O₃ was transferred to amorphous AlO(OH) mineral phase by XRD pattern, which provided the additional reactive sorption sites. These observations indicated that GO-based composites can be regarded as a promising adsorbent for immobilization and pre-concentration of U(VI) from aqueous solutions in the environmental remediation.

K2Ti6O13 hybridized graphene oxide: Effective enhancement in photodegradation of RhB and photoreduction of U(VI)

The environmental pollutions by organic pollutants and radionuclides have aroused great concern. Developing highly efficient elimination methods becomes an imperious demand. In this study, a nanocomposite of K₂Ti₆O₁₃ (KTO) nanobelts hybridized graphene oxide (GO) nanosheets (GO/KTO) was used to photodegrade RhB (dye) and photoreduce U(VI) (radionuclide), which was synthesized by a facile hydrothermal method. The adsorption capacity and the slope (k) of the curve -ln(C/C) versus time in photodegradation of RhB by GO/KTO were higher than that by GO and KTO. In the presence of different free radical scavengers, superoxide radical (·O₂^-) was found to play the most significant role in the reaction. The XPS experiment indicates U(VI) was successfully photoreduced to less toxic U(IV). The pH dependent photocatalytic experiments on RhB and U(VI) both showed the best performance at neutral pH value (from pH 6 to pH 8). To investigate the reason for the enhanced photocatalysis of GO/KTO, the morphology/microstructure, optical and photo-electrochemical properties were examined. The enhanced abilities of separation of photo electrons and holes and the adsorption of GO/KTO were ascribed to the structure of KTO nanobelts laying on the surface of GO nanosheets, which may maximize the contacting area between KTO and GO, and thus greatly reduce the surface related oxygen defects to enhance the electron interface transfer between KTO and GO and decrease the recombination efficiency of electrons and holes. These results showed the GO/KTO has great application potential in environmental treatment of organic pollutants and high valent heavy/radionuclide ions at neutral condition.

Controls on rare-earth element transport in a river impacted by ion-adsorption rare-earth mining

Rare-earth elements (REEs) are known to be a group of emerging pollutants, but the geochemistry of REEs in river waters in ion-adsorption rare-earth mining areas has attracted little attention. In this study, samples of the <0.45 μm and 0.22-0.45 μm (large colloids) water fractions and acid-soluble particles (ASPs) were collected from a river impacted by ion-adsorption rare-earth mining activities. The roles of ligand complexation, colloid binding, and particle adsorption in REE transport and distribution were also investigated. Results showed higher concentrations of REEs in the <0.45 μm fraction of all sampling sites (3.30 × 10^-2-9.42 μM) compared with that in the control site (1.21 × 10^-3 μM); this fraction was also characterized by middle REE enrichment at upstream sites, where REEs are mainly controlled by the <0.22 μm fraction (55%-94% of the species found in the <0.45 μm fraction) and ligand complexation (REE³⁺, REE(SO₄)⁺, and REE(CO₃)⁺). At downstream sites, heavy REE enrichment was observed, which was largely determined by binding to large colloids (68%-83% of the species found in the <0.45 μm fraction) and adsorption to particles (>90% of the acidified bulk water). Furthermore, REE patterns indicated that the REE-associated large colloids were mineral or mixed mineral-organic matter (OM) at upstream sites and OM-dominated or functionalized at downstream sites. The particles were mainly coated by inorganic matter substances (e.g., Fe/Al oxyhydroxides). In summary, our results reveal that REE patterns provide a useful tool to study the fate of REEs in ion-adsorption rare-earth mining catchments.

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.

Recent advances in layered double hydroxide-based nanomaterials for the removal of radionuclides from aqueous solution

Layered double hydroxides (LDHs), one of the most important two-dimensional layered compounds, have enabled massive developments in effective pollution treatments. Their derivative materials have also attracted multidisciplinary attention owing to the intrinsic advantages of their moderate chemiostability, low cost and nontoxicity. Over the past few decades, significant advances have been made in the synthesis of novel LDH-based composites and the optimization of characterization techniques. In this review, we give an overview of the recent advances in LDH-based nanomaterials, from a brief introduction to their preparation and modification methods to an overview of their application in the removal of radionuclides and an exploration of their underlying adsorption mechanisms. In the end, a summary and outlook are also briefly addressed. This review intends to provide deep insight into the design of high-performance LDH-based materials for the potential elimination of radionuclides from aqueous solutions during environmental pollution cleanup.

Rationally designed core-shell and yolk-shell magnetic titanate nanosheets for efficient U(VI) adsorption performance

The hierarchical core-shell and yolk-shell magnetic titanate nanosheets (Fe₃O₄@TNS) were successfully synthesized by employing magnetic nanoparticles (NPs) as interior core and intercrossed titanate nanostructures (NSs) as exterior shell. The as-prepared magnetic Fe₃O₄@TNS nanosheets had high specific areas (114.9 m² g^-1 for core-shell Fe₃O₄@TNS and 130.1 m² g^-1 for yolk-shell Fe₃O₄@TNS). Taking advantage of the unique multilayer structure, the nanosheets were suitable for eliminating U(VI) from polluted water environment. The sorption was strongly affected by pH values and weakly influenced by ionic strength, suggesting that the sorption of U(VI) on Fe₃O₄@TNS was mainly dominated by ion exchange and outer-sphere surface complexion. The maximum sorption capacities (Q_max) calculated from the Langmuir model were 68.59, 121.36 and 264.55 mg g^-1 for core-shell Fe₃O₄@TNS and 82.85, 173.01 and 283.29 mg g^-1 for yolk-shell Fe₃O₄@TNS, at 298 K, 313 K and 328 K, respectively. Thermodynamic parameters (ΔH⁰, ΔS⁰ and ΔG⁰) demonstrated that the sorption process was endothermic and spontaneous. Based on X-ray photoelectron spectroscopy (XPS) analyses, the sorption mechanism was confirmed to be cation-exchange between interlayered Na⁺ and UO₂²⁺. The yolk-shell Fe₃O₄@TNS had more extraordinary sorption efficiency than core-shell Fe₃O₄@TNS since the yolk-shell structure provided internal void space inside the titanate shell to accommodate more exchangeable active sites. The flexible recollection and high efficient sorption capacity made core-shell and yolk-shell Fe₃O₄@TNS nanosheets promising materials to eliminate U(VI) or other actinides in wastewater cleanup applications.

Efficient removal of U(vi) from simulated seawater with hyperbranched polyethylenimine (HPEI) covalently modified SiO2 coated magnetic microspheres

Hyperbranched polyethylenimine (HPEI) covalently modified SiO₂ coated magnetic microspheres were prepared for the efficient U(vi) removal from simulated seawater.