| Iron oxides and phyllosilicate minerals are ubiquitous in natural environments. In soils and sediments they are often simultaneously present and tend to form associations that are cemented together by various interactions. The associations of iron oxides and phyllosilicate minerals are important constituents of soils and sediments and have a significant influence on the behavior and toxicity of heavy metals and organic contaminants. Moreover, the formation, structure and properties of aggregates in soils and sediments strongly depended on the associations of iron oxides and phyllosilicate minerals. In the present work, the formation and transformation of the associations containing iron oxides and phyllosilicate minerals as a function of pH, temperature and time will be investigated at different Fe(II)/Fe(III) molar ratios (R). The main aim is to gain a clear understanding of the formation processes of the associations containing crystalline iron oxides and phyllosilicate minerals. Furthermore, the associations of crystalline iron oxides, including goethite and hematite, and phyllosilicate minerals, including kaolinite and montmorillonite, were prepared in the presence of Fe(II), Fe(III) and phyllosilicate minerals. This study will do a first attempt to unravel the interaction mechanisms and micro structure of the associations containing crystalline iron oxides and phyllosilicate minerals prepared in this way by X-ray diffraction (XRD), scanning electron microscope and energy dispersive spectrometer (SEM-EDS), transmission electron microscopy (TEM), infrared spectra (IR), magic angle spinning nuclear magnetic resonance (MAS NMR), BET surface area, and porosity structure analysis. The microstructure and interaction mechanisms of the associations containing crystalline iron oxides and phyllosilicate minerals were compared with those of the corresponding iron oxides, phyllosilicate minerals, and the mixtures containing crystalline iron oxides and phyllosilicate minerals. The pH-dependent surface charges of the tested samples were determined by potentiometric proton titrations at various KCl concentrations, and the adsorption characteristics for Zn, phosphate, and humic by the samples were studied by batch adsorption experiments. The results obtained in this study are exhibited as follows:1. In the systems of the goethite and hematite syntheses (called hereafter as Fe(III) system), the formation of crystalline iron oxides was significantly inhibited due to the presence of kaolinite or montmorillonite. In a system with a total Fe(II) and Fe(III) concentration of 0.05 mol/L (called hereafter as Fe(II)-Fe(III) system), Fe(II) can accelerate the phase transformation of ferrihydrite into crystalline iron oxides at the molar ratios of Fe(II) to Fe(III) (R)> 0.02 and initial pH (pHi)> 6. The precipitation of goethite was accelerated by Fe(Ⅱ) at an initial Fe(Ⅱ)/Fe(Ⅲ) molar ratio R=0.04, initial pH (pHi) 6 and temperature 60℃; the formation of hematite was accelerated by Fe(Ⅱ) at R=0.04, pHi 7 and 80℃; magnetite was obtained at R=0.5, pHi 7 and 60℃.2. In the presence of Fe(Ⅱ), Fe(Ⅲ) and phyllosilicate minerals, an appropriate amount of Fe(Ⅱ) weakened the inhibition of the formation of crystalline iron oxides by phyllosilicate minerals to a certain extent. Alternatively, one could say that the presence of phyllosilicate minerals decreased the acceleration by Fe(Ⅱ). In a system with Fe(Ⅱ), Fe(Ⅲ) and kaonilite (called hereafter as K-Fe(Ⅱ)-Fe(Ⅲ) system), The formation of lepidocrocite- and goethite-kaolinite associations was accelerated by Fe(Ⅱ) at R=0.04-0.06, pHi 5-8 and 50-70℃; the formation of hematite-kaolinite association was accelerated by Fe(Ⅱ) at R=0.06, pHi 7-8 and 60-80℃; magnetite-kaolinite association was obtained at R=0.06, pHi 9 and 60℃or at R=0.1-0.5, pHi 7 and 60℃. In the system with Fe(Ⅱ), Fe(Ⅲ) and montmorillonite (called hereafter as Mt-Fe(Ⅱ)-Fe(Ⅲ) system), the different associations of iron oxides and montmorillonite could be formed at the R≥0.04, pHi≥6, and temperature≥50℃; at R=0.06, pHi 6-7 and 60℃, the products consisted of montmorillonite and goethite; at R=0.06, pHi 7 and 100℃, hematite-montmorillonite association was formed; at R=0.06 and pHi9 or R=0.5 and pHi 7, magnetite could be observed from the associations.3. The morphologies of crystalline iron oxides were different when they were formed in different systems. The goethite and hematite synthesized in the Fe(Ⅲ) system were needle-shaped and sphere-shaped respectively. In the Fe(Ⅱ)-Fe(Ⅲ) system, the lath-like lepidocrocite with a size range between hundreds of nanometers and several micron-meters, the rod-like or prismatic-shaped goethite particles with the length of 200-500 nm, and the sphere-like hematite with the particle diameter of 100-300 nm were obtained respectively. In the K-Fe(Ⅱ)-Fe(Ⅲ) system, the morphologies of crystalline iron oxides are strip-shaped lepidocrocite with a length of ca.500 nm, nanorod-like goethite with a 100-200 nm size, pseudo-cubic shaped hematite with the particle diameter of ca. 100 nm, respectively. In the Mt-Fe(Ⅱ)-Fe(Ⅲ) system, nanorod-like goethite with about 100 nm size, the sphere-like hematite particles with about 50 nm diameter can be detected in the iron oxide-montmorillonite associations.4. Compared to the average values of the corresponding iron oxides and phyllosilicate minerals, the associations containing iron oxides and phyllosilicate minerals possessed higher micropore volumes and lower pore diameter. However, the porosity parameters of the mixtures containing iron oxides and phyllosilicate minerals nearly remained unchanged. The surface fractal dimension D of both the mixtures and associations considerably increased; meanwhile the D values of the mixtures were higher than those of the corresponding associations. The IR spectra showed that the vibrational frequency of≡Fe-OH in the binary systems containing iron oxides and phyllosilicate minerals shifted to a higher wave number and those of=Al-OH, Al-O, Si-O and Fe-O moved to a lower. The MAS NMR spectroscopy demonstrated that the interactions between iron oxides and phyllosilicate minerals increased the chemical shifts of 29Si and 27Al. These results implied that iron oxides and phyllosilicate minerals not only interacted by electrostatic attraction, but also by (1) anion ligand exchange between the OH groups on the surfaces of iron oxides and the≡Si-O- or=Al-OH0.5- groups situated at the edges or on the hydroxyl-terminated planes of phyllosilicate minerals, (2) surface coordination between O atoms on the surfaces of phyllosilicate minerals and Fe atoms on the surfaces of iron oxides, and (3) hydrogen bonds.5. The SEM/EDS and TEM of the samples showed that, in the associations of iron oxides and phyllosilicate minerals, iron oxides coated the surfaces of phyllosilicate minerals well, whereas for the mixtures of iron oxides and phyllosilicate minerals the coating was weak; in the goethite-phyllosilicate associations a large amount of goethite particles coated the surfaces of phyllosilicate minerals, while the uncoated surfaces of phyllosilicate minerals were more visible in the hematite-phyllosilicate associations; in the binary systems containing iron oxides and montmorillonite, the iron oxide particles attached on the montmorillonite surfaces were much more than the binary systems of iron oxides and kaolinite. Although the IR and NMR parameters of the associations and mixtures displayed similar changes, the variation of the former was more significant. Furthermore, a cation exchange reaction occurred in the associations but not in the mixtures. In the goethite-phyllosilicate associations the variation of these spectra data was much more than those in the hematite-phyllosilicate associations, and in the iron oxide-montmorillonite associations the variation of these spectra data was much more than those in the iron oxide-kaolinite associations. These results showed that the interaction strength between iron oxides and phyllosilicate minerals followed the orders: the associations> the mixtures; goethite-phyllosilicate associations> hematite-phyllosilicate associations; and iron oxide-montmorillonite associations> iron oxide-kaolinite associations.6. At the same ionic strength, the total amount of H+ions consumed from pH 11.0 to 3.0 showed the order of goethite> montmorillonite> hematite> kaolinite. The amounts of H+ions consumed by the mixtures of iron oxides and phyllosilicate minerals were lower than the average values of the H+amounts consumed by the corresponding iron oxides and phyllosilicate minerals, and those consumed by the associations of iron oxides and phyllosilicate minerals were close to the corresponding average values. These indicated that the interactions between iron oxides and phyllosilicate minerals decreased the hydroxyl amounts and the proton charges of the samples. The pHPZC (pH of point of zero charge) of goethite, hematite, kaolinite, and montmorillonite appeared at about 8.2, 8.8,4.1, and 2.4 respectively. Compared to the pure phyllosilicate minerals, the pHPZC values of the binary systems containing iron oxides and phyllosilicate minerals increased considerably, meanwhile the pHPZC values of the associations containing iron oxides and phyllosilicate minerals were slightly higher than those of the mixtures.7. At pH 6.0, the maximum adsorption capacity (qmax) for Zn(II) by the pure samples followed the order of goethite> montmorillonite> hematite> kaolinite; at pH 5.0, the qmax for phosphate and humic showed the order:goethite> hematite> montmorillonite> kaolinite. The adsorption data of Zn(II), phosphate, and humic by the pure samples could be well fitted by one-site Langmuir and two-site Langmuir models. Compared to the average value of the corresponding iron oxide and phyllosilicate, the qmax of Zn(II) by the mixtures of iron oxides and phyllosilicate minerals nearly remained unchanged, and the qmax of phosphate and humic by the mixtures of iron oxides and phyllosilicate minerals were slightly increased, but the qmax of Zn(II), phosphate, and humic by the associations of iron oxides and phyllosilicate minerals were significantly increased. The two-site Langmuir model was suitable for describing the adsorption of Zn(II), phosphate, and humic by the associations of iron oxides and phyllosilicate minerals. |
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