Search

Abstract 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. Graphical abstract Image 1 Highlights • Hydrogen bonding interactions between HA molecules play important roles in the aggregation. • Multivalent cations affect the aggregation by forming intra- or inter-molecular bridges between HA molecules. • Heavy metal ions are more aggressively in HA aggregation than common cations. Different types of metal ions were considered to investigate the aggregation behaviors of HA colloids, which help us to better understand HA behaviors in the environment. [ABSTRACT FROM AUTHOR]

With the widespread use of nanomaterials, the impacts of nanoparticles on environment have attracted great concern. In this study, the effects of HA and Mg 2+ on the aggregation of silica aerogels were investigated for understanding the fate and transport of silica aerogels in aqueous environments. Dynamic light scattering (DLS) was used to monitor the size distribution of the silica aerogels. Zeta potentials of HA and silica aerogels both exhibit strong negative surface charge at pH 2 to 12 due to their surface functional groups. The reunition of silica aerogels is due to the electrostatic destabilization which could be explained by the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The combination of HA endows silica aerogels with a high critical coagulation concentration ( CCC ), which means that HA reduces the aggregation of silica aerogels at low Mg 2+ concentration due to the electrostatic repulsion and steric hindrance effects. Whereas, HA increases the aggregation of silica aerogels when the concentration of Mg 2+ ions is higher than CCC , which is because the HA-Mg 2+ chemical bonds, formed in the reaction process, act as bridge among silica aerogel aggregates. The colloid interface morphology of silica aerogel aggregates was characterized by transmission electron microscopy (TEM). The complexation between Mg 2+ ions and HA plays a critical role in silica aerogels' aggregation and is the main mechanism to increase the aggregation. These findings of the significant effects of HA and metal ions on the aggregation of silica aerogels are important for better understanding the transport and subsequent transformation of silica aerogels in aquatic environments. [ABSTRACT FROM AUTHOR]

The surface property of attapulgite was investigated by N2-BET surface area and zeta potential analysis in this paper. Solution pH had a remarkable effect on the sorption process, indicated an inner-sphere complexation. Humic acid (HA) in the solution enhanced U(VI) sorption significantly at pH < 5.0, while decreased U(VI) sorption obviously at pH > 9.0. The characteristic fluorescence changes of HA indicated that a strong chemical reaction occurred between the functional groups in HA and UO22+. The sorption was a spontaneous and endothermic process with increased entropy, and the increase in temperature would benefit the sorption. [ABSTRACT FROM AUTHOR]

The coagulation behaviors of humic acid (HA) with Cs + (10–500 mM), Sr 2+ (0.8–10.0 mM) and Eu 3+ (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 Sr 2+ concentrations, which shows that HA coagulation is consistent with the diffusion-limited colloid aggregation (DLCA) model. Trivalent Eu 3+ induced HA coagulation at a much lower concentration than bivalent Sr 2+ and monovalent Cs + . The coagulation value ratio of Sr 2+ and Eu 3+ 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 Sr 2+ and Eu 3+ 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. [ABSTRACT FROM AUTHOR]

This study explored the sorption behavior and mechanism of U(VI) on sericite. The surface properties of sericite were investigated, and the effects of several environmental factors on U(VI) sorption to sericite were evaluated. The experimental data were simulated by different isotherm models and kinetic models. Humic acids affects U(VI) sorption obviously while silicate promotes the process in the whole pH range. The sorption-desorption hysteresis suggests an irreversible sorption process. Spectroscopic analyses indicate that the −OH groups have an important effect in the U(VI) sorption. [ABSTRACT FROM AUTHOR]

The removal of Pb(II) from wastewater by Ca-rectorite was investigated as various environmental factors containing contact time, pH, ionic strength, solid content, coexisting ions, humic acid (HA), and temperature. The kinetic sorption of Pb(II) on Ca-rectorite was well described by the pseudo second-order model. The sorption process was strongly dependent on pH and ionic strength. The results indicated that the presence of HA and coexisting ions influenced the sorption of Pb(II) on Ca-rectorite obviously. Besides, the Langmuir and Freundlich models were employed to simulate the sorption isotherms. The thermodynamic parameters indicated that the sorption process was spontaneous and endothermic. It is possible to conclude that Ca-rectorite has a good potential for disposal of lead-contaminated wastewater. [ABSTRACT FROM PUBLISHER]

Abstract: In this study, nanocrystallined Mg/Al layered double hydroxides (LDH-CO3) and chloridion intercalated nanocrystallined Mg/Al LDHs (LDH-Cl) were synthesized and used for simultaneous removal of arsenic and natural organic matter (NOM) from contaminated groundwater. Humic acid (HA) was selected as a model compound of NOM. The maximum adsorption capacities of arsenate (As(V)) on LDH-CO3 and LDH-Cl are 44.66 and 88.30 mg/g, respectively, and those of HA on LDH-CO3 and LDH-Cl are 53.16 and 269.24 mg/g, respectively. It was found that more than 98% of arsenic and 94% of NOM were eliminated by LDH-Cl from both arsenic and NOM-rich groundwater, which is used as drinking water in Togtoh County, Inner Mongolia, China. The arsenic concentration declined from 231 to 4 μg/L, which meets the drinking water standard. The adsorption mechanisms were determined by using X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and extended X-ray absorption fine structure spectroscopy techniques (EXAFS). The results showed that the removal of HA was mainly via surface complexation as well as coagulation at the surface of LDHs, while the adsorption of As(V) was mainly via ion-exchange process. The presence of HA exhibited little inhibiting effect on As(V) adsorption by occupying partial binding sites on LDH surfaces. Nevertheless, it could not affect the ion-exchange process of As(V) with the interlayer anions of LDHs. The removal of As(V) and HA can be carried out independently due to the different adsorption mechanisms. By integrating the experimental results, it is clear that LDH-Cl can be potentially used as a cost-effective material for the purification of both arsenic and NOM contaminated groundwater. [Copyright &y& Elsevier]

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including cosorbing or complexing. In this work, the effects of pH, humic acid HA/Co(II) addition orders, ionic strength, concentration of HA, and foreign cations on the Co(II) sorption on γ-Al2O3 in the presence of HA were investigated. The sorption isotherms of Co(II) on γ-Al2O3 in the absence and presence HA were also studied and described by using S-type sorption model. The experimental results showed that the Co(II) sorption is strongly dependent on the pH values, concentration of HA, but independent of HA/Co(II) addition orders, ionic strength, and foreign cations in the presence of HA under our experimental conditions. The results also indicated that HA enhanced the Co(II) sorption at low pH, but reduced the Co(II) sorption at high pH. It was hypothesized that the significantly positive influence of HA at low pH on the Co(II) sorption on γ-Al2O3 was attributed to strong surface binding of HA on γ-Al2O3 and subsequently the formation of ternary surface complexes such as ≡S-OOC-R-(COO−) x Co2− x . Chemi-complexation may be the main mechanism of the Co(II) sorption on γ-Al2O3 in the presence of HA. [ABSTRACT FROM AUTHOR]

Abstract: Thorium is considered as a chemical analog of other tetravalent actinides. Herein, the sorption of Th(IV) on TiO2 in the presence or absence of soil fulvic acid (FA)/humic acid (HA) as a function of pH, ionic strength and FA/HA concentration has been studied by a batch method. The morphology was characterized by scanning electron microscopy (SEM). The results indicate that sorption of Th(IV) on TiO2 increases from 0% to ∼94% at pH 1∼4, and then maintains level with increasing pH values. Both FA and HA have a positive effect on Th(IV) sorption at low pH values and the contribution of FA on Th(IV) sorption is rather higher than that of HA at pH<4. The sorption is weakly dependent on the concentration of KNO3 in solution, but the cations K+, Na+ and Li+ influence Th(IV) sorption more obviously. The batch results indicate that the inner sphere complex formation is formed at bare surfaces or FA/HA-bound TiO2 particle surfaces. Results of SEM analysis show that the particle sizes of TiO2, Th–TiO2 and Th–HA–TiO2 colloids are quite different. Surface complexation may be considered as the main sorption mechanism. [Copyright &y& Elsevier]

Humic acid (HA) can markedly change the surface charge of clay minerals and influences the sorption of uranium(U) on clay minerals in aquatic environments. Furthermore, HA can potentially affect the mobility of U(VI) in the natural environment. The effect of HA on the surface charge, aggregation, sorption and bonding properties of attapulgite [(Mg,Al) 4 (Si) 8 (O,OH,H 2 O) 26 ·nH 2 O] was investigated using fluorescence spectroscopy technique in combination with other analytical tools. The UV–vis absorbance spectra and fluorescence peaks obtained in the experiments are attributed to a large number of electron donating substitutes, such as carboxylic acid-like and phenolic acid-like fluorophores, in HA. Electrostatic attraction and ligand exchanges are the primary interaction mechanisms between attapulgite and HA. A significant modification of excitation-emission matrix (EEM) spectra and emission intensity of HA are observed after the interaction, indicating a different binding complex of HA on attapulgite at the molecular level. The sorption kinetics indicate that strong chemical interactions and/or surface complexation primarily contributed to U(VI) sorption. An analysis of U(VI) sorption isotherms and model fitting reveals that the sorption of U(VI) onto attapulgite is a spontaneous and endothermic process. The presence of HA is an important factor that greatly influences the immobilization and removal of U(VI) ions from aqueous solutions. [ABSTRACT FROM AUTHOR]

To contribute to the comprehension of Eu(III) sorption properties in oxide-organic material systems, the influence of humic acid (HA) on Eu(III) sorption on bare and HA-bounded γ-Al 2 O 3 is studied using batch experiments and time resolved laser fluorescence spectroscopy (TRLFS). The effect of pH and addition sequences of Eu(III)/HA/Al 2 O 3 on Eu(III) sorption is carried out at different experimental conditions, and the results show that the sorption of Eu(III) is strongly dependent on pH and ionic strength. The sorption of Eu(III) is mainly dominated by ion exchange or outer-sphere surface complexation at low pH, and by inner-sphere surface complexation at high pH values. The presence of HA enhances Eu(III) sorption at low pH, but reduces Eu(III) sorption at high pH values. The addition sequences of Eu(III)/HA to alumina suspensions do not affect Eu(III) sorption, which is also evidenced by the TRLFS measurements. The surface complexes are characterized by their emission spectra [the ratio of emission intensities of 5 D 0 → 7 F 1 ( λ = 594 nm) and 5 D 0 → 7 F 2 ( λ = 619 nm) transitions] and their fluorescence lifetime. The TRLFS results show clearly that no effect of addition sequences on the sorption and speciation of Eu(III) in HA–Al 2 O 3 hybrids after equilibration. These findings on the effect of humic substances in the complexation properties of Eu(III) indicate that the clarification of the environmental behavior of humic substances is necessary to understand the physicochemical properties of Eu(III), or its analog trivalent lanthanide and actinide ions in the natural environment. [ABSTRACT FROM AUTHOR]

Understanding the rate and extent of 63 Ni(II) sorption and desorption on/from mineral surfaces are important to predict its mobility performance in the natural environment. We herein systematically investigated the effects of pH, electrolyte cations, humic acid (HA) and temperature on 63 Ni(II) sorption/desorption on/from Na-montmorillonite. The results indicated that the sorption/desorption of 63 Ni(II) on/from Na-montmorillonite were strongly dependent on pH and electrolyte cations. Desorption abilities of 63 Ni(II) were Na + ≈ K + > Ca 2+ ≈ Mg 2+ at the cationic concentration of 0.2 mol/L, which was associated with the clay tactoid size. The kinetic results suggested that 63 Ni(II) sorption/desorption initially increased rapidly and remained constant as time increasing. The presence of HA influenced 63 Ni(II) sorption clearly and promoted 63 Ni(II) desorption as a result of the dissolution of ternary surface complexes. Increasing temperature was favorable for the sorption reaction owing to the formation of strong inner-sphere complexes, however, hampered the desorption reaction because of the increased stability of inner-sphere complexes or the aggregation of montmorillonite. The results are crucial to understand the physicochemical behavior of radionickel in the natural environment. [ABSTRACT FROM AUTHOR]

Clay minerals have been studied extensively due to their strong sorption and complexation ability toward various environmental pollutants. In this study, the sorption of 90 Sr(II) on Na-montmorillonite was studied as a function of various environmental conditions such as pH, ionic strength, humic acid (HA) and temperature. The results indicated that the sorption of 90 Sr(II) on Na-montmorillonite was strongly dependent on pH and ionic strength. The experimental data of 90 Sr(II) sorption was simulated by the diffuse-layer model (DLM) well with the aid of Visual Minteq 3.0. At low pH, the sorption of 90 Sr(II) was dominated by outer-sphere surface complexation and ion exchange with Na + /H + on Na-montmorillonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA enhanced 90 Sr(II) sorption at pH < 7.0 but decreased 90 Sr(II) sorption at pH > 7.0. Langmuir and Freundlich models were used to simulate the sorption isotherms of 90 Sr(II) at three different temperatures of 303, 318 and 333 K. The thermodynamic parameters (Δ H , Δ S and Δ G ) calculated from the temperature-dependent sorption isotherms indicated that the sorption of 90 Sr(II) on Na-montmorillonite was an endothermic and spontaneous process. The thermodynamic parameters calculated from temperature-dependent sorption data were crucial to understand the interaction behavior of 90 Sr(II) with Na-montmorillonite. [ABSTRACT FROM AUTHOR]

Aluminum (hydr)oxides play an important role in the regulation of the composition of soil/water, sediment/water and other natural water systems. In this study, the interactions among Eu(III), humic acid (HA) and γ-Al2O3 were investigated using a combination of batch and extended X-ray absorption fine structure (EXAFS) techniques. Experiments were performed with varying contact times (2, 15, 60 and 180 d) at a pH of 6.5 for both the binary γ-Al2O3/Eu(III) and the ternary γ-Al2O3/HA/Eu(III) systems. In addition, two representative pH values (pH 6.5 for a near-neutral condition and pH 8.5 for an alkalescence condition) were selected to determine the sequestration mechanisms of Eu(III) in the ternary γ-Al2O3/HA/Eu(III) systems. To verify the specific binding modes and corresponding chemical species, a coordination geometry calculation and a quantitative comparison between the HA binding site concentration and the initial Eu(III) concentration were conducted along with EXAFS data analysis. The microstructure and thermodynamic stability of the formed Eu(III) species were dependent on various environmental parameters. For the binary γ-Al2O3/Eu(III) systems, quantitative analysis results of EXAFS spectra suggested the presence of two Eu(III) species within a contact time of 15 d. Using a coordination geometry calculation, the R Eu–Al values at ∼3.28Å and ∼3.99Å corresponded to the formation of edge-shared and corner-shared surface complexes, respectively. For samples reacted longer than 15 d, the appearance of an additional Eu–Eu shell at ∼3.50Å was indicative of a structural rearrangement process, leading to the formation of thermodynamically stable surface polynuclear complexes. For the ternary γ-Al2O3/HA/Eu(III) systems, the EXAFS-derived structural parameters indicated the formation of 1:1 type B ternary complexes and binary corner-shared complexes at pH 6.5 after 2 d. In contrast, the Eu(III) sequestration mechanisms at pH 8.5 were mainly attributed to the formation of 1:2 type A ternary complexes and binary edge-shared complexes. Considering the high proton dissociation constant of strong HA phenolic sites (8.8) and the high metal loading in the present study, the weak HA carboxylic sites are predominantly involved in Eu(III) complexation at pH 6.5 and 8.5. The time-dependent variation tendency of the Eu(III) chemical species formed in the ternary systems may arise from Eu(III)-induced HA agglomeration, binding of Eu(III) ions on stronger HA binding sites and migration of Eu(III) ions to less sterically accessible sites in the HA macromolecule structures. The adsorbed HA could accelerate Eu(III) immobilization at the γ-Al2O3/water interfaces and could enhance the thermodynamic stability of the formed chemical species. The findings presented in this study could provide important microcosmic information for the prediction of the long-term behaviors of Eu(III) and the relevant Ln/An(III) in a geological environment rich in aluminum hydr(oxides). [ABSTRACT FROM AUTHOR]

[Display omitted] • An available and facile way for the magnetic MOF-NH 2 was reported. • The adsorption efficiency of the magnetic MOFs towards U(VI) was improved dramatically. • The interactional mechanism between the tuned amine groups and U(VI) was revealed. A magnetic metal organic framework was finely tuned by abundant amine groups (Mag MOF-NH 2) for the purpose of improving the U(VI) removal from solution. The Mag MOF-NH 2 displays outstanding removal capacity (80 mg⋅g−1) to U(VI) (10 ppm, 400 mL) within 15 min. To illuminate the adsorption mechanism of Mag MOF-NH 2 , the microstructure, the magnetic property, chemical composition of Mag MOF-NH 2 and the reaction system were characterized. The adsorption mechanism was systematically investigated. Equilibrium adsorption experiments verifies the improved adsorption capability from nearly 0 for MIL-101 to 82 mg⋅g−1 for Mag MOF-NH 2 , which could be explained mainly by the interaction between the amine groups and targeted ions. The effect of humic acid (HA) and coexisting ions on the U(VI) adsorption were also investigated. The Ca2+ and Mg2+ ions have passive impact on the adsorption and the high pH value can facilitate the process. Mag MOF-NH 2 holds great potential for the practical application owing to the outstanding U(VI) removal and satisfied regeneration in simulative wastewaters. [ABSTRACT FROM AUTHOR]

S4B, and " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=2 d" and " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=180 d" as well as " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=15 d" and " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=60 d" from Fig. 4B spectrum " -Al SB 2 sb O SB 3 sb /Eu(III); t=60 d" appears similar to spectrum " -Al SB 2 sb O SB 3 sb /Eu(III); t=180 d", while in Fig. 10B spectrum " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=15 d" appears similar to spectra " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=60 d" and " -Al SB 2 sb O SB 3 sb /HA/Eu(III); t=180 d". [Extracted from the article]

In natural environments, the sequestration or release of U(VI) is usually affected by the coexistence of natural organic matters (NOM). Former works mainly concentrated on the design of novel adsorbents for the preconcentration of U(VI), while there is a knowledge gap between the coupled adsorption process in the mixed U(VI)-advanced materials-NOM system. Herein, the incorporation of multi-walled carbon nanotubes into zeolitic imidazolate framework-8 (denoted as MWCNTs/ZIF-8) was achieved and applied to sequester U(VI) and humic acids (HA) simultaneously from aquatic environments. The mutual effects of simultaneous U(VI) and HA adsorption affected by various water chemistry and the U(VI)/HA addition sequences were studied systematically. The saturated adsorption amounts were 200.77 and 55.68 mg/g for U(VI) and HA at pH 5.0, respectively, which are higher than most traditional adsorbents. U(VI) adsorption onto MWCNTs/ZIF-8 was pH-dependent but independent on ionic strength, revealing that the coordination between Zn atoms and U(VI) ions was the main reason for U(VI) sequestration, which was further evidenced by the X-ray photoelectron spectroscopy. U(VI) adsorption was improved significantly with the coexistence of HA owing to the appearance of HA-U(VI)-MWCNTs/ZIF-8 ternary complexes, but was independent on the addition sequences, disclosing that U(VI) ions preferred to bind directly onto MWCNTs/ZIF-8 instead of via HA. The addition sequence exerted an obvious effect on HA adsorption due to the different adsorption scenario. These findings are essential to predict U(VI) behaviors in geological environments. Both mechanistic and quantitative understanding on the mutual effects of U(VI) and humic acid in the binary U(VI)/humic acids-MWCNTs/ZIF-8 system and ternary U(VI)-humic acids-MWCNTs/ZIF-8 system was studied. Unlabelled Image • Hierarchical MWCNTs/ZIF-8 composites were prepared via precise stepwise control. • MWCNTs/ZIF-8 showed high adsorption properties for both U(VI) and HA in the binary systems. • U(VI) and HA adsorption in the coexisting ternary system was of mutual promotion. • U(VI) adsorption rate by MWCNTs/ZIF-8 was independent on the addition sequences. • The addition sequence exerted an obvious effect on HA adsorption onto MWCNTs/ZIF-8. [ABSTRACT FROM AUTHOR]