Removal of radiocobalt from aqueous solution by different sized carbon nanotubes

Efficient adsorption of cesium cations and chromate anions by one-step process using surfactant-modified zeolite

Natural zeolite is organically modified with the surfactant cetyltrimethylammonium bromide (CTAB) and employed as a dual-function material for simultaneous adsorption of Cs+ cations and HCrO4- anions from aqueous solutions. Unmodified and modified zeolites are characterized by Fourier transform infrared (FTIR), dynamic light scattering (DLS), nitrogen adsorption-desorption isotherms, and X-ray diffraction (XRD). The results showed that CTAB-zeolite had the efficiency to simultaneously adsorb the concerned species in the pH range 2.5-4.2. The kinetic data showed that 90 and 300 min for Cs(I) and Cr(VI), respectively, were sufficient to attain equilibrium and the data are well-fitted by the double-exponential kinetic model. Of the studied adsorption isotherm models, Redlich-Peterson was the best one for describing the equilibrium adsorption isotherms. Values of ∆H°, ∆S°, and ∆G° for the present adsorption processes are estimated. CTAB-zeolite exhibited adsorption capacities of 0.713 and 1.216 mmol/g for Cs(I) and Cr(VI), respectively, which are comparable with the data reported in the literature. The adsorption mechanism of the concerned (radio)toxicants is proposed.

Efficient removal of radiocobalt and manganese from their binary aqueous solutions by batch adsorption process using PAN/HDTMA/KCuHCF composite

Simultaneous removal of radiocobalt and manganese by adsorption onto polyacrylonitrile/hexadecyltrimethylammonium bromide/potassium copper hexacyanoferrate (PAN/HDTMA/KCuHCF) composite was studied. The synthesized composite was characterized by Fourier-transformed infrared (FT-IR), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The influence of the solution pH was studied in the range 1.5–7.8 and the results showed the effectiveness of the synthesized composite for simultaneous adsorption of radiocobalt and manganese in the pH range 2.5–6 at an adsorbent mass of 4 g/L. Adsorption kinetic data of manganese at the studied concentrations were best fitted by pseudo-second-order kinetic model and the diffusion study showed that the adsorption process was controlled by film diffusion. Thermodynamic parameters (ΔGo, ΔHo and ΔSo) were estimated and the results indicated that adsorption processes of the concerned (radio)toxicants were spontaneous and endothermic in nature. Of the studied isotherm models, Freundlich and Langmuir were the best ones for describing the adsorption isotherm data of radiocobalt and manganese, respectively. The adsorption capacity of PAN/HDTMA/KCuHCF was found to be 23.629 (for radiocobalt) and 62.854 (for manganese). Desorption of Radiocobalt and manganese loaded onto PAN/HDTMA/KCuHCF composite was studied using various desorbing agents at different concentrations.

Size and shape dependences of the adsorption kinetics of malachite green on nano-MgO: a theoretical and experimental study

Compared with bulk materials, there is a considerable difference in the adsorption kinetics of nanoparticles, which mainly depend on particle size and shape. Herein, by introducing the shape factor, we have derived the relations between the kinetic parameters of adsorption and the particle size of nanoparticles of different shapes. Then, the influence of the regularities and mechanisms of particle size and shape on the kinetic parameters of adsorption were discussed. In the experiment, spherical nano-MgO and cubic nano-MgO of different particle sizes were synthesized via a sol-gel method, the kinetic parameters of the adsorption of malachite green on nano-MgO were determined, and the influence of the regularities of particle size and shape on the adsorption kinetic parameters were obtained. The experimental results show that shape and particle size may significantly influence the kinetic parameters of adsorption. For the adsorption of nano-MgO with the same equivalent particle diameter, compared to spherical shapes, the rate constant of adsorption k for cubic shapes is larger, while the apparent activation energy Ea and the pre-exponential factor A are smaller. For the adsorption of spherical or cubic nano-MgO, k increases with decreasing particle size, while Ea and A decrease, and there exist good linear relationships between ln k, Ea, ln A and the reciprocal of particle size. The experimental results are well in agreement with the theoretical relations. Furthermore, k is influenced by the shape factor, Ea by the shape factor and the specific surface enthalpy, and A by the shape factor and the temperature coefficient of surface tension. The kinetic theory of adsorption can quantitatively describe the influence of the regularities and mechanisms of particle size and shape on the adsorption kinetics of nanoparticles and provide significant guidance for the research and the application of nano-adsorptions in the related fields.

Organo-modification of montmorillonite for enhancing the adsorption efficiency of cobalt radionuclides from aqueous solutions

Montmorillonite clay was organically modified with thoron (TH) and was employed as an adsorbent for removal of cobalt(II) radionuclides from aqueous solutions. Batch adsorption experiments, under several operational parameters such as pH, contact time, initial adsorbate concentration, adsorbent dosage, ionic strength, and temperature, were conducted to determine the optimum conditions for efficient removal of cobalt(II) radionuclides. The obtained data showed that almost complete removals were achieved for cobalt(II) at pH values ≥ 3.5 using TH-modified montmorillonite (TMM), while only 63% were obtained by unmodified clay at pH ≥ 5.4. Adsorption kinetic data of cobalt(II) were better fitted by the pseudo-second order kinetic model and its adsorption rate was controlled by film diffusion. Both Langmuir and Freundlich models had the ability to well describe the equilibrium data of cobalt(II) radionuclides at the studied temperatures. The adsorption capacity of TMM (0.85 mmol/g) was found to be not only nine times that of unmodified montmorillonite (0.097 mmol/g), but also higher than those reported in literature using various unmodified and modified clays. Thermodynamic parameters (ΔH°, ΔS°, and ΔG°) were calculated. Among the examined desorbing agents, both Al³⁺ and EDTA were succeeded to desorb most of cobalt(II) radionuclides (desorption % ~ 90%) loaded onto TMM. The results of this study clarified that TMM can be considered as an effective adsorbent for removal of cobalt(II) radionuclides from aqueous solutions.

Adsorption of Gold(I) and Gold(III) Using Multiwalled Carbon Nanotubes

Carbon nanotubes are materials that have been investigated for diverse applications including the adsorption of metals. However, scarce literature has described their behavior in the case of the adsorption of precious metals. Thus, this work reports the efficient adsorption of gold from cyanide or chloride media on multiwalled carbon nanotubes (MWCNTs). In a cyanide medium, gold was adsorbed from alkaline pH values decreasing the adsorption as the pH values were increased to more acidic values. In a chloride medium, the MWCNTs were able to load the precious metal and an increased HCl concentration (0.1–10 M), in the aqueous solution, had no effect on the gold uptake onto the nanotubes. From both aqueous media, the metal adsorption was well represented by the pseudo-second order kinetic model. In the cyanide medium, the film-diffusion controlled process best fitted the rate law governing the adsorption of gold onto the nanotubes, whereas in the chloride medium, the adsorption of the metal onto the nanotubes is best represented, both at 20 °C and 60 °C, by the particle-diffusion controlled process. With respect to the elution step, in cyanide medium gold loaded onto the nanotubes can be eluted with acidic thiourea solutions, whereas in the chloride medium, and due to that the adsorption process involved the precipitation of zero valent gold onto the multiwalled carbon nanotubes, the elution has been considered as a leaching step with aqua regia. From the eluates, dissolved gold can be conveniently precipitated as zero valent gold nanoparticles.