Cd(II) removal on surface-modified activated carbon: equilibrium, kinetics and mechanism

Removal of Cd(II) from Micro-Polluted Water by Magnetic Core-Shell Fe3O4@Prussian Blue

A novel core-shell magnetic Prussian blue-coated Fe3O4 composites (Fe3O4@PB) were designed and synthesized by in-situ replication and controlled etching of iron oxide (Fe3O4) to eliminate Cd (II) from micro-polluted water. The core-shell structure was confirmed by TEM, and the composites were characterized by XRD and FTIR. The pore diameter distribution from BET measurement revealed the micropore-dominated structure of Fe3O4@PB. The effects of adsorbents dosage, pH, and co-existing ions were investigated. Batch results revealed that the Cd (II) adsorption was very fast initially and reached equilibrium after 4 h. A pH of 6 was favorable for Cd (II) adsorption on Fe3O4@PB. The adsorption rate reached 98.78% at an initial Cd (II) concentration of 100 μg/L. The adsorption kinetics indicated that the pseudo-first-order and Elovich models could best describe the Cd (II) adsorption onto Fe3O4@PB, indicating that the sorption of Cd (II) ions on the binding sites of Fe3O4@PB was the main rate-limiting step of adsorption. The adsorption isotherm well fitted the Freundlich model with a maximum capacity of 9.25 mg·g-1 of Cd (II). The adsorption of Cd (II) on the Fe3O4@PB was affected by co-existing ions, including Cu (II), Ni (II), and Zn (II), due to the competitive effect of the co-adsorption of Cd (II) with other co-existing ions.

Remediation of lead polluted soil by active silicate material prepared from coal fly ash

To improve the effect of coal fly ash on the remediation of heavy metal polluted soils, the active silicate material (ASM) was prepared by coal fly ash and the remediation of lead polluted soils by ASM was investigated in this study. To study the reaction mechanism between ASM and Pb(II) in soil, the Pb(II) adsorption by ASM was investigated by a series of batch experiments. The result shows that the maximum adsorption capacity of ASM was 300.62 mg g^-1 according to the Langmuir isotherm model. The average adsorption energy obtained from the D-K model revealed that the adsorption process of ASM is the ion-exchange process. To apply the ASM to the remediation of lead polluted soils, the soil stabilization experiment and pot experiment were carried out. The results reveal that ASM can reduce the mobility and bioavailability of lead in the soils by transforming the lead from exchangeable fraction, carbonate fraction and reducible fraction to oxidizable fraction and residual fraction. Moreover, ASM can improve the growth of pakchoi by promoting the production of chlorophyll. Furthermore, ASM can reduce the Pb accumulation of pakchoi by inhibiting the absorption of lead in the roots. It is anticipated that this study can provide a novel active silicate material for the application of coal fly ash in heavy metal pollution treatment.

Adsorption, recovery, and regeneration of Cd by magnetic phosphate nanoparticles

Adsorption plays an important role in removing cadmium (Cd²⁺) from water, and magnetic adsorbents are increasingly being used due to their ease of separation and recovery. Magnetic Fe₃O₄-coated hydroxyapatite (HAP) nanoparticles (nHAP-Fe₃O₄) were developed by co-precipitation and then used for the removal of Cd²⁺ from water. The properties of these nanoparticles were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and magnetization curves. Experiments were conducted to investigate the effects of adsorption and mechanisms. Results illustrated that kinetic data were well fitted by a pseudo-second-order model. The adsorption capacity of nHAP-Fe₃O₄ was 62.14 mg/g. The mechanisms for the adsorption of Cd²⁺ on nHAP-Fe₃O₄ included rapid surface adsorption, intraparticle diffusion, and internal particle bonding, with the ion exchange with Ca²⁺ and chemical complexation being the most dominant. The regeneration efficiency and recovery rate of nHAP-Fe₃O₄ eluted by EDTA-Na₂ after the fifth cycle were 63.04% and 40.2%, respectively. Results revealed that the feasibility of nHAP-Fe₃O₄ as an adsorbent of Cd²⁺ and its environmental friendliness make it an ideal focus for future research.

Adsorption studies of cadmium onto magnetic Fe3O4@FePO4 and its preconcentration with detection by electrothermal atomic absorption spectrometry

Superparamagnetic Fe₃O₄@FePO₄ nanoparticles with core shell structure were prepared by coating iron phosphate on the surface of Fe₃O₄ nanoparticles by liquid phase deposition method. The prepared materials were characterized by vibrating sample magnetometer, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectrometer, nano Zetasizer, X-ray photoelectron spectroscopy and Raman spectrometer. These characterization methods were also used to describe the adsorption mechanism. The obtained composite material was used for the adsorption of a heavy metal element, cadmium. Its unique magnetic properties are favorable for rapid separation and preconcentration of trace cadmium from aqueous solutions. About 100% sorption was achieved at pH 7 for 1mL, 10μgL^-1 of cadmium. Batch adsorption experiments show that the adsorption fits Langmuir model, and a maximum adsorption capacity 13.51mgg^-1 is derived for Cd(II). The retained Cd(II) could be readily recovered by 200μL of HNO₃ (0.01molL^-1). The cadmium in the eluate is quantified with detection by electrothermal atomic absorption spectrometry (ETAAS). A sample volume of 2000μL creates an enrichment factor of 10.3, along with a detection limit of 0.021µgL^-1 (3σ, n=7) and a RSD of 1.3% (0.1µgL^-1, n=7) within a linear calibration range of 0.05-0.5µgL^-1. The practical applicability of this procedure was validated by analyzing cadmium contents in a certified reference material (GBW08608) and two environmental water samples.