Sorption and desorption behaviour of zinc in soils

Fourteen soils comprising A and B horizons, from Canterbury, New Zealand were examined for the sorption/desorption behaviour of added and native soil Zn. These soils were selected to represent a wide range of soil properties likely to be of major importance in controlling the sorption or desorption process of Zn from soils. The DTPA-extractable Zn (presumably available Zn) of these soils ranged from 0.12 to 15.98 µg g⁻¹ soil. The amounts and patterns of added and native soil Zn desorbed varied between the different soils studied. Greater concentrations of native Zn were desorbed after five consecutive desorption periods from the surface soils (from 0.33 /µg g⁻¹ to 2.09 /µg g⁻¹ soil) than from the sub-surface soils (from 0.11 /µg g-l to 0.57 µg g⁻¹ soil). Sorption of Zn was consistently lower in sub soil samples compared with surface soils. In contrast to native Zn desorption, the added Zn desorbed (%) was higher in sub surface soils than in surface soils. Cation exchange capacity and organic C were found to be the dominant soil variables contributing towards sorption or desorption of Zn. Cation exchange capacity itself accounted for most of the variation (48 to 62 %) in native Zn desorption, Zn sorption, and added Zn desorption. However, inclusion of clay and Mn oxides in the case of native Zn desorption; pH and Mn oxides in the case of Zn sorption; and clay, soil pH and amorphous Al oxides in the case of added Z desorption explained nearly 85 to 97 % of the variation between these Canterbury soils. Desorption of Zn was found to be reversible in soils having coarse texture and it closely followed the original sorption isotherm suggesting that in these soils, desorption reactions could be described by the sorption isotherm. However, for a soil with comparatively high clay content and a high CEC, desorption of Zn was only partially reversible and there was a marked hysteresis effect between sorbed and desorbed Zn. In the majority of soils studied, the longer the period of contact time of Zn with soil, the smaller was the Zn desorption (20 to 36.5 % reduction after 90 days). Zinc sorption/desorption varied widely depending on soil pH. Sorption of Zn increased with increase in pH and at pH near 6.5 most of the added Zn was sorbed by all the soils studied. Desorption of native and added Zn decreased with increase in pH and became very low as pH approached near neutral. The decrease, in both native Zn desorption as well as added Zn desorption, was larger in the pH range of 4.3 to 5.4 than in the range 5.4 to 6.5 in all the four soils studied. An examination of sorption/desorption isotherms indicated that the extent of reversibility decreased as the pH of soils increased. At pH near 4.3, Zn desorption closely followed the original sorption isotherm irrespective of the concentration of Zn added to the soils studied. As the pH of these soils was raised to 6.4 or above, there was a marked hysteresis effect between sorbed and desorbed Zn. The desorption of added Zn (5µg g⁻¹ soil) and native Zn from the soils over a range of pH values was not affected by the addition of fertilizer P (up to 50 kg ha⁻¹ ) suggesting that there is a low probability of P-Zn interaction in the soil system itself. Resin membranes appear to have considerable advantages for studying the kinetics of Zn desorption compared with chemical extractants such as 0.01 M Ca(NO₃)₂ or 0.005 MDTPA. The rates of native and added Zn desorption using both types Ion exchange and chelating of membranes at lower pH were rapid initially and gradually declined with time. The kinetics of native Zn desorption were best described by simultaneous first-order and pseudo first order models, and added Zn desorption by first-order and pseudo first order models. The parabolic diffusion model also gave reasonably good fits for both native and added Zn desorption. As the desorption data could be predicted by several different types of kinetic models, it is probable that in the heterogeneous soil system more than one type of mechanisms are likely to be involved. With increasing pH from 4.3 to 6.5, the Zn desorption rates in the soils were decreased. Increasing the length of contact period also substantially decreased the rates of Zn desorption. Irrespective of the length of contact time of Zn with soil, simultaneous first order, pseudo first order and parabolic diffusion gave good fits for the rate of Zn desorption

Nanocomposite of γ-Fe2O3 immobilized on graphene oxide for remediation of Ni(II) ions - Kinetics, isotherm and thermodynamics studies

Facile sonication method was used to immobilize γ-Fe2O3 nanoparticles (NPs) on graphene oxide (GO) to obtain environmentally stable γ-Fe2O3-GO nanocomposite (NC). Structure, surface morphology and composition of NC were thoroughly studied. Mössbauer analysis confirmed the presence of maghemite (γ-Fe2O3) as dominant phase of iron oxide. TEM images of NC revealed homogeneous distribution of γ-Fe2O3 NPs over the GO nanosheet. A comparative analysis of GO, γ-Fe2O3 NPs and NC for the removal of Ni(II) ions from water was carried out by batch method and adsorption kinetics, thermodynamics and isotherms were also studied. The adsorption data fitted better to Langmuir and Freundlich adsorption isotherms as compared to Dubinin Radushkevich (D-R) and Temkin adsorption isotherms. NC showed higher qmax value (615.0 mg/g) as compared to the pristine GO (403.7 mg/g) and γ-Fe2O3 NPs (572.4 mg/g). The adsorption kinetics followed pseudo-second-order model. NC displayed greater affinity for Ni(II) ions in comparison to pristine GO and γ-Fe2O3 NPs. The results suggested that the synthesized γ-Fe2O3-GO nanocomposite can be used as a promising novel material for the removal of Ni(II) from water due to its higher adsorption capacity, stability, convenient magnetic separation and regeneration

Synthesis of CaFe2O4-NGO Nanocomposite for Effective Removal of Heavy Metal Ion and Photocatalytic Degradation of Organic Pollutants

This paper reports the successful synthesis of magnetic nanocomposite of calcium ferrite with nitrogen doped graphene oxide (CaFe2O4-NGO) for the effective removal of Pb(II) ions and photocatalytic degradation of congo red and p-nitrophenol. X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), and scanning electron microscopy-energy dispersive X-ray (SEM-EDX) techniques confirmed the presence of NGO and CaFe2O4 in the nanocomposite. The Mössbauer studies depicted the presence of paramagnetic doublet and sextet due to presence of CaFe2O4 NPs in the nanocomposite. The higher BET surface area in case of CaFe2O4-NGO (52.86 m2/g) as compared to CaFe2O4 NPs (23.45 m2/g) was ascribed to the effective modulation of surface in the presence of NGO. Adsorption followed the Langmuir model with maximum adsorption capacity of 780.5 mg/g for Pb(II) ions. Photoluminescence spectrum of nanocomposite displayed four-fold decrease in the intensity, as compared to ferrite NPs, thus confirming its high light capturing potential and enhanced photocatalytic activity. The presence of NGO in nanocomposite offered an excellent visible light driven photocatalytic performance. The quenching experiments supported ●OH and O2●− radicals as the main reactive species involved in carrying out the catalytic system. The presence of Pb(II) had synergistic effect on photocatalytic degradation of pollutants. This study highlights the synthesis of CaFe2O4-NGO nanocomposite as an efficient adsorbent and photocatalyst for remediating pollutants

Hierarchical Nanoflowers of MgFe2O4, Bentonite and B-,P- Co-Doped Graphene Oxide as Adsorbent and Photocatalyst: Optimization of Parameters by Box–Behnken Methodology

In the present study, nanocomposites having hierarchical nanoflowers (HNFs) -like morphology were synthesized by ultra-sonication approach. HNFs were ternary composite of MgFe2O4 and bentonite with boron-, phosphorous- co-doped graphene oxide (BPGO). The HNFs were fully characterized using different analytical tools viz. X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersion spectroscopy, transmission electron microscopy, X-ray diffraction, vibrating sample magnetometry and Mössbauer analysis. Transmission electron micrographs showed that chiffon-like BPGO nanosheets were wrapped on the MgFe2O4-bentonite surface, resulting in a porous flower-like morphology. The red-shift in XPS binding energies of HNFs as compared to MgFe2O4-bentoniteand BPGO revealed the presence of strong interactions between the two materials. Box–Behnken statistical methodology was employed to optimize adsorptive and photocatalytic parameters using Pb(II) and malathion as model pollutants, respectively. HNFs exhibited excellent adsorption ability for Pb(II) ions, with the Langmuir adsorption capacity of 654 mg g−1 at optimized pH 6.0 and 96% photocatalytic degradation of malathion at pH 9.0 as compared to MgFe2O4-bentonite and BPGO. Results obtained in this study clearly indicate that HNFs are promising nanocomposite for the removal of inorganic and organic contaminants from the aqueous solutions