Shitong Yang

Adsorption of Ni(II) on oxidized multi-walled carbon nanotubes: Effect of contact time, pH, foreign ions and PAA

The adsorption of Ni(II) on oxidized multi-walled carbon nanotubes (MWCNTs) as a function of contact time, pH and foreign ions in the absence and presence of polyacrylic acid (PAA) was studied using batch technique. The results indicated that adsorption of Ni(II) on oxidized MWCNTs increased from zero to approximately 99% at pH 2-9, and then maintained the high level with increasing pH. Kinetic data showed that the adsorption process achieved equilibrium within 2h and experimental data were fitted well by the pseudo-second-order equation. A positive effect of PAA on Ni(II) adsorption was found at pH<8, whereas a negative effect was observed at pH>8. The effect of addition sequences of PAA/Ni(II) on the adsorption of Ni(II) to PAA-MWCNT hybrids were also studied. The results indicated that the adsorption of Ni(II) was influenced by addition sequences obviously. The adsorption of Ni(II) on oxidized MWCNTs may be mainly attributed to surface complexation and ion exchange. Oxidized MWCNTs are suitable material in the solidification and pre-concentration of Ni(II) from aqueous solutions.

Sorption of Ni(II) on GMZ bentonite: Effects of pH, ionic strength, foreign ions, humic acid and temperature

Bentonite has been widely studied in nuclear waste management because of its special physicochemical properties. In this work, the sorption of Ni(II) from aqueous solution onto GMZ bentonite as a function of contact time, pH, ionic strength, foreign ions, humic acid (HA) and temperature was investigated under ambient conditions. The results indicated that the pseudo-second-order rate equation simulated the kinetic sorption process well. The sorption of Ni(II) on GMZ bentonite was strongly dependent on pH and on ionic strength. At low pH, the sorption of Ni(II) was dominated by outer-sphere surface complexation and ion exchange with Na(+)/H(+) on GMZ bentonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. A positive effect of HA on Ni(II) sorption was found at pH<8, whereas a negative effect was observed at pH>8. The Langmuir, Freundlich, and D-R models were used to simulate the sorption isotherms of Ni(II) at three different temperatures: 303.15, 318.15 and 333.15K. The thermodynamic parameters (DeltaH(0), DeltaS(0) and DeltaG(0)) of Ni(II) sorption on GMZ bentonite at the three different temperatures were calculated from the temperature-dependent sorption isotherms. The results indicated that the sorption process of Ni(II) on GMZ bentonite was endothermic and spontaneous. Experimental results indicate that GMZ bentonite is a suitable sorbent for pre-concentration and solidification of Ni(II) from large volume solutions.

Macroscopic and Microscopic Investigation of Ni(II) Sequestration on Diatomite by Batch, XPS, and EXAFS Techniques

Sequestration of Ni(II) on diatomite as a function of time, pH, and temperature was investigated by batch, XPS, and EXAFS techniques. The ionic strength-dependent sorption at pH < 7.0 was consistent with outer-sphere surface complexation, while the ionic strength-independent sorption at pH = 7.0-8.6 was indicative of inner-sphere surface complexation. EXAFS results indicated that the adsorbed Ni(II) consisted of ∼6 O at R(Ni-O) ≈ 2.05 Å. EXAFS analysis from the second shell suggested that three phenomena occurred at the diatomite/water interface: (1) outer-sphere and/or inner-sphere complexation; (2) dissolution of Si which is the rate limiting step during Ni uptake; and (3) extensive growth of surface (co)precipitates. Under acidic conditions, outer-sphere complexation is the main mechanism controlling Ni uptake, which is in good agreement with the macroscopic results. At contact time of 1 h or 1 day or pH = 7.0-8.0, surface coprecipitates occur concurrently with inner-sphere complexes on diatomite surface, whereas at contact time of 1 month or pH = 10.0, surface (co)precipitates dominate Ni uptake. Furthermore, surface loading increases with temperature increasing, and surface coprecipitates become the dominant mechanism at elevated temperature. The results are important to understand Ni interaction with minerals at the solid-water interface, which is helpful to evaluate the mobility of Ni(II) in the natural environment.

The removal of U(VI) from aqueous solution by oxidized multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) have exhibited high sorption capacity for radionuclides due to the unique hollow structure and large surface area. In this study, surface properties of oxidized MWCNTs were characterized by using XRD, SEM, FTIR and potentiometric acid-base titration. The sorption of U(VI) on oxidized MWCNTs as a function of contact time, U(VI) concentration, pH, ionic strength, humic acid/fulvic acid (HA/FA) and carbonate was investigated by using batch technique. The removal of U(VI) by oxidized MWCNTs was strongly dependent on pH and ionic strength. The presence of HA/FA enhanced U(VI) removal on oxidized MWCNTs at low pH while inhibited U(VI) sorption at high pH. The mechanism of U(VI) sorption on oxidized MWCNTs was assumed to be cation exchange/outer-sphere surface complexation in acidic pH and to form precipitation under circum neutral conditions. The oxidized MWCNTs exhibit higher sorption capacity and stronger chemical affinity than pristine MWCNTs.

Efficient removal of arsenate by versatile magnetic graphene oxide composites

The magnetic graphene oxide (MGO) composites were prepared by coprecipitation of FeCl3·6H2O and FeCl2·4H2O on graphene oxide (GO) nanosheets and characterized in detail. The Fe3O4 was uniformly deposited on the surface of GO. The synthesized MGO composites were used as a versatile adsorbent for As(V) removal from aqueous solutions. The results showed that the adsorption of As(V) on MGO is an endothermic process and the adsorption kinetic fitted the pseudo-second-order model well. The MGO composites had a good adsorption capability for As(V) removal and the adsorption isotherms were described by the Langmuir model better than by the Freundlich model. The adsorption of As(V) on MGO decreased with ascending pH due to the electrostatic interaction. In addition, the adsorption of As(V) on MGO was greatly affected by the nature and concentration of coexisting cations and anions. The presence of coexisting anions showed an inhibiting effect on As(V) adsorption, which was more efficient at low pH, whereas the presence of coexisting cations showed an enhancing effect on As(V) adsorption, which was more efficient at high pH. The results of this work indicated that the combination of the excellent adsorption capacity of GO and the magnetic properties of Fe3O4 nanoparticles is very important in drinking water treatment due to the easy magnetic separation of MGO from aqueous solutions.

Rapid and Highly Efficient Preconcentration of Eu(III) by Core–Shell Structured Fe3O4@Humic Acid Magnetic Nanoparticles

In this study, humic acid-coated Fe(3)O(4) magnetic nanoparticles (Fe(3)O(4)@HA MNPs) were synthesized using a chemical coprecipitation method and characterized in detail. The XRD analysis results showed that HA coating did not change the phase of Fe(3)O(4) cores. The TEM image suggested that Fe(3)O(4)@HA MNPs had nearly uniform size without the observation of aggregation. The Fe(3)O(4)@HA MNPs were stable in solution and could be easily separated from aqueous solution using a magnetic separation method. A batch technique was adopted to investigate the removal efficiency of Fe(3)O(4)@HA MNPs toward Eu(III) under various environmental conditions. The kinetic process of Eu(III) sorption on Fe(3)O(4)@HA MNPs reached equilibrium within <30 min. The fast sorption kinetics and high sorption amount were attributed to the plentiful surface sites provided by the surface-coated HA macromolecules. The Fe(3)O(4)@HA MNPs was able to remove ~99% of Eu(III) in aqueous solution at pH 8.5. Except for SO(4)(2-) anions, the coexisting electrolyte ions had no significant competition effects on the removal of Eu(III) by Fe(3)O(4)@HA MNPs. The obvious sorption-desorption hysteresis suggested that the removal of Eu(III) was dominated by inner-sphere surface complexation. The sorption isotherm agreed well with the Langmuir model, having a maximum sorption capacity of 6.95 × 10(-5) mol g(-1). The leaching test showed that the Eu(III)-loaded Fe(3)O(4)@HA colloids were capable to maintain high thermodynamic stability for long aging times. The findings herein suggested that Fe(3)O(4)@HA MNPs could be potentially used as a highly effective material for the enrichment and preconcentration of radionuclide Eu(III) or other trivalent lanthanides/actinides in geological repositories or in nuclear waste management.

Coexistence of adsorption and coagulation processes of both arsenate and NOM from contaminated groundwater by nanocrystallined Mg/Al layered double hydroxides.

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.

Impact of water quality parameters on the sorption of U(VI) onto hematite

In this study, the sorption of U(VI) from aqueous solution on hematite was studied as a function of various water quality parameters such as contact time, pH, ionic strength, soil humic acid (HA) or fulvic acid (FA), solid content and temperature by using a batch technique. The results demonstrated that the sorption of U(VI) was strongly dependent on ionic strength at pH<6.0, and outer-sphere surface complexation may be the main sorption mechanism. The sorption was independent of ionic strength at pH>6.0 and the sorption was mainly dominated by inner-sphere surface complexation. The presence of HA/FA increases U(VI) sorption at low pH, whereas decreases U(VI) sorption at high pH. The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) were calculated from the temperature dependent sorption isotherms, and the results suggested that U(VI) sorption was a spontaneous and endothermic process. The results might be important for the application of hematite in U(VI) pollution management.

Effect of humic acid, fulvic acid, pH, ionic strength and temperature on 63Ni(II) sorption to MnO2

Abstract The effects of pH, ionic strength, temperature, humic acid (HA) and fulvic acid (FA) on the sorption of radionuclide 63Ni(II) to MnO2 have been investigated by using batch techniques. The results indicated that the sorption of 63Ni(II) on MnO2 is obviously dependent on pH values but independent of ionic strength. The presence of HA/FA strongly enhances the sorption of 63Ni(II) on MnO2 at low pH values, whereas reduces 63Ni(II) sorption at high pH values. The sorption of 63Ni(II) on MnO2 is attributed to inner-sphere surface complexation rather than outer-sphere surface complexation or ion exchange. The diffuse layer model (DLM) is used to simulate the experimental data well with the aid of FITEQL 3.2. The thermodynamic parameters (ΔH0, ΔS0, ΔG0) are also calculated from the temperature dependent sorption isotherms, and the results suggest that the sorption of 63Ni(II) on MnO2 is a spontaneous and endothermic process.

Retention mechanisms and microstructure of Eu(III) on manganese dioxide studied by batch and high resolution EXAFS technique

Abstract The batch experiment and high resolution EXAFS technique with bent crystal analyzer were combined to study Eu(III) retention mechanisms on β-MnO2 as a function of various environmental factors. The results indicated that the sorption of Eu(III) onto β-MnO2 was obviously dependent on pH but independent of ionic strength, suggesting the formation of inner-sphere surface complexes. Results of high resolution EXAFS analysis showed that Eu was surrounded by 8 ± 1 O atoms in the first shell at REu−O ≈ 2.40 Å for all sorption samples. A second shell of Mn atoms at REu−Mn ≈ 3.78 Å was observed for all sorption samples, which was associated with the formation of a bidendate inner-sphere complex with chemical bonding via edge sharing to MnO6-octahedron. This study shows the utility of high resolution EXAFS technique with bent crystal analyzer to discriminate the fluorescence peak of Eu(III) from that of manganese hydroxides, whose fluorescence peaks are too close in energy to be discriminated by the conventional EXAFS technique with a low resolving solid-state detector. Both the macroscopic interaction data and the molecular level evidence of Eu(III) surface speciation at the oxide-water interface should be factored into better evaluation of Eu(III) and related radionuclide mobility in the natural environment.