Synergistic adsorption of Cd(II) with sulfate/phosphate on ferrihydrite: An in situ ATR-FTIR/2D-COS study

Elucidation of the co-adsorption characteristics of heavy metal cations and oxyanions on (oxyhydr)oxides can help to better understand their distribution and transformation in many geological settings. In this work, batch adsorption experiments in combination with in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to explore the interaction mechanisms of Cd(II) with sulfate or phosphate at the ferrihydrite (Fh)–water interface, and the two-dimensional correlation spectroscopic analysis (2D–COS) was used to enhance the resolution of ATR-FTIR bands and the accuracy of analysis. The batch adsorption experiments showed enhanced adsorption of both sulfate (S) and phosphate (P) on Fh when co-adsorbed with Cd(II); additionally, the desorbed percentages of Cd(II) were much lower in the P + Cd adsorption systems than those in the S + Cd adsorption systems. The spectroscopic results suggested that in the single adsorption systems, sulfate primarily adsorbed as outer-sphere complexes with a small amount of bidentate inner-sphere complexes, while the dominant adsorbed species of phosphate were largely the bidentate nonprotonated inner-sphere complexes, although there was significant pH-dependence. In the co-adsorption systems, the synergistic adsorption of Cd(II) and sulfate was dominantly attributed to the electrostatic interaction, as well as the formation of Fe–Cd–S (i.e., Cd-bridged) ternary complexes. In contrast, Fe–P–Cd (i.e., phosphate-bridged) ternary complexes were found in all of the co-adsorption systems of phosphate and Cd(II); furthermore, electrostatic interaction should also contribute to the co-adsorption process. Our results show that in situ ATR-FTIR in combination with 2D–COS can be an efficient tool in analyzing the co-adsorption mechanisms of anions and heavy metal cations on iron (oxyhydr)oxides in ternary adsorption systems. The co-existence of Cd(II) with sulfate or phosphate can be beneficial for their accumulations on Fh, and phosphate is more efficient than sulfate for the long-term immobilization of Cd(II)

Sorption of 2,4-Dichlorophenol onto Organobentonites: Influence of Organic Cation Structure and Bentonite Layer Charge

The influence of both the molecular structure of the organic cation and the layer charge on the bentonite on the sorption characteristics of organobentonites has been examined. The organic cations cetyltrimethylammonium bromide (CTMA), tetramethylammonium bromide (TMA) and Crystal Violet (CV) were selected for producing two bentonites with different layer charges. The results obtained showed that the adsorption capacities of the three organic cations towards bentonite were in the order CTMA TMA. TMA-bentonite had the smallest basal spacing values but the largest surface area, but both CV-bentonite and CTM-bentonite had small surface areas. The sorption capacity of 2,4-dichlorophenol onto CTMA-bentonite increased with increasing bentonite layer charge, while the reverse was observed with TMA-bentonite. With CV-bentonite, the sample with the lower layer charge showed only a limited capacity towards the sorption of 2,4-dichlorophenol, and exhibited a much smaller sorption capacity relative to CTMA-bentonite and TMA-bentonite. The results obtained in the present study indicate that CTMA aggregates during sorption and that siloxane surfaces provide the major sorption sites on CTMA-bentonite and TMA-bentonite, respectively. In contrast, both the organic cation and siloxane groups contributed weakly to the sorption of 2,4-dichlorophenol on CV-bentonite

Microstructure of organo-bentonites in water and the effect of steric hindrance on the uptake of organic compounds

To further elucidate adsorption-to-partition transitional mechanisms which have been proposed previously for organo-bentonites with different surfactant loadings, the structural characteristics of interlayer surfactant aggregates on organo-bentonite with different surfactant cetyltrimethylammonium bromide loading levels (0.20-2.56 times cation exchange capacity, CEC) have been investigated by in situ X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The sorption properties and the structure of the clay interlayers changed according to the type of surfactant, the surfactant loading level, and the state of hydration in the clays. Based on the sorption of nitrobenzene, phenol, and aniline to organobentonites, the contaminant sorption coefficients (K), normalized with the organic carbon content, show a remarkable dependence on surfactant loading levels. The K values first increased with surfactant loading until reaching a maximum at 1.0 to 1.2 times the CEC, and then decreased. According to the theoretical calculation of the volume fractions relating to the interlamellar space, the interlamellar microenvironment became a more hydrophobic medium, contributing to the dissolution of organic contaminants, as the surfactant loading increased from 0.20 to 2.56 times the CEC. However, the increase in packing density (ρ) for the intercalates, and induced steric hindrances both affect the result in terms of a reduction in the accessible free space where the organic contaminants can be located, which might be a negative factor in the sorption capacity

Efficient degradation of cefotaxime by a UV plus ferrihydrite/TiO2+H2O2 process: the important role of ferrihydrite in transferring photo-generated electrons from TiO2 to H2O2

Background Developing effective removal processes for antibiotics has attracted increasing interest recently. In this work, a novel strategy involving the combination of photocatalysis with Fenton reaction using ferrihydrite/TiO2 (Fh/TiO2) nanohybrids was developed to efficiently degrade the antibiotic cefotaxime. Fh/TiO2 nanohybrids were synthesized by simply growing Fh on the surface of commercially available TiO2. We expected that Fh of Fh/TiO2 could capture photo-generated electrons from TiO2, inhibiting the recombination of electron-hole pairs; also, by virtue of Fe(III)/Fe(II) cycle on Fh/TiO2, photo-generated electrons could be continually transferred to H2O2 to produce center dot OH. Accordingly, high degradation efficiency of cefotaxime could be achieved. Results With UV light and H2O2, Fh/TiO2 with a Fe/Ti molar radio of 7% showed high catalytic activity indeed, and its degradation rate for cefotaxime was nearly three times higher than that of TiO2. The decomposition of H2O2 and production of center dot OH in the UV+7%Fh/TiO2+H2O2 system were also increased markedly. The large amount of Fe(II) on 7%Fh/TiO2 determined in this system supported our hypothesis that Fh of 7%Fh/TiO2 could capture photo-generated electrons from TiO2. Although dissolved iron was observed, the contribution of Fenton reaction by dissolved iron was rather limited. After four consecutive cycles, 7%Fh/TiO2 still retained good stability. Conclusions The UV+7%Fh/TiO2+H2O2 system provides a potential alternative in degradation of cefotaxime for further practical application, and it has the following advantages: high catalytic activity, simple preparation method, good stability and low cost, as well as continued catalytic activity after total consumption of H2O2. (c) 2019 Society of Chemical Industr

Temperature-Dependent Structure and Dynamics of Water Intercalated in Layered Double Hydroxides with Different Hydration States

Properties of water confined in layered double hydroxides (LDH) are relevant with hydration, dehydration, and protonic conduction in interlayer galleries. Evolutions of structure and dynamics of water in LDHs (Mg2Al(OH)(6)Cl center dot mH(2)O) with temperature are disclosed through molecular dynamics simulations performed in the range from 300 to 430 K. LDHs with m = 0.78 and 1.44 which characterize two different hydration states are investigated. Water in the lower hydration state is characterized with higher ordered structure. Irrespective of water content, water becomes less hydrogen bonded and more disordered as temperature increases. This leads to a large decrement in dehydration enthalpy, which facilitates dehydration energetically. Irrespective of temperature or water content, water exhibits the preference for being fixed in hydroxyl sites and it diffuses through jumping between neighbor sites. Jump diffusion approximately exhibits an Arrhenius dependence on temperature. A jump is a collective process consisting of water translation and hydroxyl group reorientation, which is reflected in the high activation energy and low attempt frequency. Hydronium ions may be transported through jumping between neighbor sites, making a contribution to proton transfer in interlayer galleries

Sorption of 2,4-Dichlorophenol onto Organobentonites: Influence of Organic Cation Structure and Bentonite Layer Charge

The influence of both the molecular structure of the organic cation and the layer charge on the bentonite on the sorption characteristics of organobentonites has been examined. The organic cations cetyltrimethylammonium bromide (CTMA), tetramethylammonium bromide (TMA) and Crystal Violet (CV) were selected for producing two bentonites with different layer charges. The results obtained showed that the adsorption capacities of the three organic cations towards bentonite were in the order CTMA TMA. TMA-bentonite had the smallest basal spacing values but the largest surface area, but both CV-bentonite and CTM-bentonite had small surface areas. The sorption capacity of 2,4-dichlorophenol onto CTMA-bentonite increased with increasing bentonite layer charge, while the reverse was observed with TMA-bentonite. With CV-bentonite, the sample with the lower layer charge showed only a limited capacity towards the sorption of 2,4-dichlorophenol, and exhibited a much smaller sorption capacity relative to CTMA-bentonite and TMA-bentonite. The results obtained in the present study indicate that CTMA aggregates during sorption and that siloxane surfaces provide the major sorption sites on CTMA-bentonite and TMA-bentonite, respectively. In contrast, both the organic cation and siloxane groups contributed weakly to the sorption of 2,4-dichlorophenol on CV-bentonite

Special Issue Advances in Applied Clay Science Intercalated nanomaterials: From functional clays to advanced hybrid lamellar compounds Preface

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