This research presented six natural clay minerals (NCM) evaluated for the effectiveness of NH4+ adsorption from aqueous solution. For the first time, the NH4+ adsorption capacities of kaolinite, ...halloysite, montmorillonite, vermiculite, palygorskite, and sepiolite were examined and compared in the same study. All the NCM were fully characterized by XRD, SEM/EDS, XRF,FTIR, CEC, zeta potential and nitrogen adsorption-desorption isotherms to better understand the adsorption mechanism-property relationship. Adsorption kinetics showed that the adsorption behavior followed the pseudo-second-order kinetic model. The adsorption isotherms fitted by the Langmuir model illustrated that among all the NCM studied, vermiculite (50.06mg/g) and montmorillonite (40.84mg/g) showed the highest ammonium adsorption capacities. Our results revealed that the cation exchange is the main mechanism for the NH4+ adsorption. Additionally, negatively charged surface, water absorption process and surface morphology of NCM might also contribute to the high adsorption capacity for the NH4+. The maximum adsorption capacities for all NCM were rapidly obtained within 30min with a dosage of 0.3g/25mL at pH of 7. The results illustrated that the NCM have significant potential as economic, safe and effective adsorbent materials for the NH4+ adsorption from the aqueous solution.
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•Vermiculite and montmorillonite showed the highest ammonium adsorption capacities.•Cation exchange was the main mechanism onto NCM.•The structure and surface properties of NCM are the key for the highest adsorption.•Parameters such as contact time, dosage and pH affecting NH4+ adsorption•The adsorption isotherms and kinetic models were investigated.
The clay minerals, kaolinite and halloysite are dominant hosts for rare earth elements (REEs) in the weathered crust elution-deposited REE deposits. However, the accumulation of REEs on kaolinite and ...halloysite has not been systematically compared. In this study, the adsorption of complete REE series on kaolinite and halloysite was investigated under different pH levels and ionic strengths, which was linked to the enrichment and fractionation of REEs within the clay fraction of a typical weathered crust elution-deposited REE deposit. At low ionic strength, the adsorption of REEs on kaolinite and halloysite increased with increase in pH, with some noticeable fluctuation observed on halloysite at high pH. All the REEs were adsorbed to a similar extent without apparent fractionation, except for the slight enrichment of heavy REEs (HREEs) at high pH. At high ionic strength, REE adsorption exhibited a linear increase with the increase in pH, particularly at high pH, with HREEs being preferentially adsorbed. Compared to halloysite, kaolinite possessed a higher specific surface area (SSA) normalized adsorption capacity towards REEs. These observed adsorption characteristics could possibly explain the distribution of REEs in the clay fraction along the REE deposit. The decrease of ion-exchangeable REE content with depth was significant for kaolinite, while the REE fractionation was ascribed to the selective adsorption of HREEs on both kaolinite and halloysite. The enrichment and fractionation mechanism of REEs on kaolinite and halloysite were also discussed in terms of the surface chemistry and morphology of the clay minerals and the variations of chemical properties across the REE group.
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•REE adsorption on Kln and Hal was affected by pH and ionic strength (IS).•REE adsorption on tubular Hal showed conspicuous fluctuation at high pH and low IS.•HREEs were preferentially adsorbed on both Kln and Hal, especially at high IS.•Contribution of Kln on REE enrichment was comparable to Hal in field samples.
How clay delamination supports aseismic slip Zhou Huijun, Zhou Huijun; Chen Meng, Chen Meng; Zhu Runliang, Zhu Runliang ...
The American mineralogist,
01/2023, Letnik:
108, Številka:
1
Journal Article
Recenzirano
Aseismic slip is a stable fault slip, which allows strain to be relieved smoothly. Aseismic slip prevents the earthquake propagation, but it could nucleate an earthquake elsewhere. Understanding the ...mechanism of aseismic slip is promising in revealing the seismic cycle. Experimental evidence showed clay-rich fault gouge bears a low-friction strength, and the friction is strengthened with slip velocity (velocity-strengthening), which was thought to support aseismic slip. Clay minerals are comprised of platy crystalline layers with water intercalated between them, which may act as a lubricant. Sliding between clay layers was suspected to support aseismic slip but lacked a clarified mechanistic insight. We use non-equilibrium molecular dynamics simulations to show that shear-induced interlayer sliding is frictionally weak and velocity-strengthening, which evidences the role of clay minerals in aseismic slip. We find that interlayer water is a viscous fluid at most times, which explains the shear response of interlayer sliding. Depending on temperature and pressure conditions, intercalated water can be monolayer or bilayer, fluidic or ice like. Shear induces ice-like water to transform into fluidic water, which happens as a stick-slip phenomenon reflecting a first-order transition. Increased pore fluid pressure leads to the transformation from monolayer to bilayer intercalated water, resulting in a lower friction strength and enhanced velocity-strengthening behavior. Our work suggests that disclosing the hydration state of a clay mineral is preliminary when studying fault mechanics.
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Present study reports the successful development of a novel lanthanum (La)-based magnetic adsorbent and its use for phosphate removal from water. For its synthesis, natural magnetite ...(Mag), Fe3O4, was subjected to partial dissolution in HCl solution and the obtained suspension was mixed with an alkaline solution for in-situ synthesis of ferrihydrite (Fh)-coated Mag (Mag@Fh). Mag@Fh was then decorated with La (hydr)oxides followed by calcination to produce Fh-coated and La-decorated Mag (Mag@Fh-La). Obtained Mag@Fh-La represented high phosphate adsorption capacity (44.8 mg P/g at 15.7% La in its structure) and La usage efficiency. Moreover, Mag@Fh-La retained its high adsorption capacity (>35.0 mg P/g) over a wide range of equilibrium solution pH (3.2–10.7). The combination of FTIR, XPS analysis and adsorption experiments revealed that ligand exchange and electrostatic attraction were the main mechanisms that jointly facilitated the adsorption of phosphate. Adsorption-desorption cycle studies confirmed the well-retained adsorption efficiency of regenerated Mag@Fh-La for repeated applications. Final experiments with real domestic wastewater (initial phosphate concentration of 1.7 mg/L) revealed that 0.2 g/L Mag@Fh-La efficiently reduced the phosphate concentration to below 0.02 mg/L. Overall, this work clearly highlights that the synthesized novel adsorbent has promising applications in phosphate removal from real wastewater.
Kaolinization of 2:1 type clay minerals commonly occurs in the supergene environments of the Earth, which plays critical roles in many geochemical and environmental processes. However, the ...transformation mechanism involved and the specific behavior of 2:1 type swelling and non-swelling clay minerals during kaolinization remain poorly understood. In this study, laboratory experiments on the kaolinization of montmorillonite (swelling), illite (non-swelling), and rectorite (partially swelling) were carried out to investigate the kaolinization mechanism of 2:1 type clay minerals and to evaluate whether swelling and non-swelling layers of 2:1 type clay minerals perform differently or not in their kaolinization processes. The results show that montmorillonite, illite, and rectorite in acidic Al3+-containing solutions can be transformed into kaolinite, whereas such transformation is hard to take place in Al3+-free solutions. Part of the Al3+ in the solutions was exchanged into the interlayer spaces of swelling clay minerals at the early stage and resulted in the formation of hydroxy-aluminosilicate (HAS) interlayers, but they show no influence on the transformation process. Interstratified kaolinite-smectite (K-S), kaolinite-illite (K-I), and kaolinite-rectorite (K-R) formed as the intermediate phases during the transformations of the three different precursor minerals, respectively. The results obtained in this study demonstrate that 2:1 type clay minerals, including both swelling and non-swelling ones, can be transformed into kaolinite via a local dissolution-crystallization mechanism, which starts mainly from the layer edges rather than the basal surfaces. Due to different dissolution rates from domain to domain within a precursor mineral particle, the layers with a low dissolution rate become "splints," while the dissolved elements are concentrated between two "splints," leading to precipitation of kaolinite along the basal surfaces of precursor minerals. The size and stacking order of the newly formed kaolinite strongly depend on the morphology and property of the precursor minerals. These findings not only are of importance for better understanding the transformation procedures between different clay minerals and the mechanisms involved but also provide new insights for well understanding mineral-water interactions that are central to all geochemical processes.
Goethite and modified goethites have been found as good photocatalysts because their conduction band can mediate electron transfer in various redox processes. Many kinds of metal elements can be ...incorporated into the structure of goethite to form solid solutions in nature, but their optoelectronic properties have not been well disclosed. Mn-substituted goethite is one of the potential photocatalysts, which can exhibit high-photocatalytic activity in many Earth's surface processes. Based on the first-principles calculation, pairwise interaction energies and static lattice energies of goethitegroutite solid solution were computed, and the most thermodynamically stable configurations of Mn-substituted goethite were determined. The results indicate that Mn3+ ion tends to distribute within the cation layer parallel to the (001) plane. Phase relations of goethite-groutite solid solution were derived by subsequent configurational statistics with energies of all 232 configurations of a 2 × 1 × 4 supercell with 32 exchangeable cations. The phase diagram shows that no more than 3 mol% Fe of goethite can be substituted by Mn ions. Therefore, Mn-substituted goethite is thermodynamically metastable or bears groutite-like clusters/lamellae. Furthermore, the effects of Mn substitutions on the band gap were experimentally and theoretically investigated. It is found that a small amount of Mn-substitution can reduce the band gap of goethite significantly, and the decrease ceases when the Mn content is higher than 3-4 mol%. Such a decrease in band gap causes red-shift to the photo response wavelength of goethite and improves the responding capability. This improvement was confirmed in the experiments of photocatalytic degradation of methylene blue (MB). Such kind of photocatalytic reaction probably can happen widely in natural environments. Therefore, the contribution of photocatalysis of natural goethites to geochemical processes on Earth's surface should be considered.
Local structure determination of Zn-smectite Tao Qi, Tao Qi; Xing Chaogang, Xing Chaogang; Lee Seungyeol, Lee Seungyeol ...
The American mineralogist,
07/2023, Letnik:
108, Številka:
7
Journal Article
Recenzirano
Odprti dostop
An aluminum-free zinc-bearing smectite (Zn-smectite) was synthesized under hydrothermal conditions, together with its magnesium substituted products. Its layer charge calculated by cation exchange ...capacity (CEC) is 117.4 mmol/100 g. Powder X-ray diffraction (XRD) revealed turbostratic stacking and showed that the d06l value of the Zn-smectite was >1.525 A indicating that it is trioctahedral. Its d001 value increased from ca.12.8 A to ca. 16.0 A after ethylene glycol (EG) saturation. The Zn-smectite did not irreversibly collapse after heating the Li+-saturated sample to 300 °C, suggesting that its layer charge was generated from octahedral-site vacancies (defects). The Zn-smectite resembles zincsilite-like minerals with interlayer Na+ and Zn2+. The intralayer structure of zincsilite was confirmed by pair distribution function (PDF) analysis, and the whole crystal structure was built and optimized by DFT calculation in the CASTEP module of the Materials Studio software. Synthetic zincsilite is triclinic, space group P1, and its optimized unit-cell parameters are: a = 5.294 A, b = 9.162 A, c = 12.800 A, α = 90.788°, β = 98.345°, and γ = 90.399°.
The formation of heteroaggregates is critical to controlling the stabilization and transformation of nanominerals and mineral nanoparticles (NMMNs) in nature, but the underlying mechanisms remain to ...be deciphered. In this work, we study the effect of surface interactions between ferrihydrite (Fh) and montmorillonite (Mnt) within their heteroaggregates on the transformation behaviors of Fh. A series of heteroaggregates composed of Fh and Mnt were synthesized by modulating their mass ratios and synthesis methods, i.e., directly complexing Fh with Mnt (Fh-Mnt) or in situ growing Fh on Mnt (Fh/Mnt). Structural characterization using XRD, TG-DSC, TEM, and FTIR indicated that Fh particles coated more evenly on the Mnt surface within the heteroaggregates synthesized by in situ growing Fh on Mnt and with lower Fh to Mnt ratio, and accordingly these heteroaggregates showed stronger surface interactions between Fh and Mnt. The phase transformation of Fh to hematite (Hem) on the heteroaggregates can be significantly affected during the heating treatment. Compared with that of pure Fh, the transformation of Fh on all of the heteroaggregates was retarded (e.g., slower transformation rate and smaller produced Hem particles), particularly for the samples with stronger surface interactions (e.g., Fh/Mnt with lower Fh to Mnt ratio). Noticeably, the heated heteroaggregates may simultaneously contain pristine Fh, intermediate maghemite, and transformed Hem, showing a heterogeneous transformation behavior of Fh. The strong interactions between Fh and Mnt will enhance the dispersion of Fh and restrict the structural rearrangement of Fh (particularly those at the interface) during the phase transformation process, resulting in retarded and heterogenous transformation of Fh on these heteroaggregates. These findings not only enrich our knowledge of the phase transformation characteristics of Fh but also advance our understanding of the important role of mineral surface interactions in stabilizing NMMNs in nature.
Understanding clay mineral transformation is of fundamental importance to grasping phyllosilicate crystal chemistry and unraveling geochemical processes. In this study, hydrothermal experiments were ...conducted on lizardite and antigorite, to investigate the possibility of the transformation from serpentine to smectite, the effect of precursor minerals' structure on the transformation and the transformation mechanism involved. The reaction products were characterized using XRD, TG, HRTEM, and 27Al MAS NMR. The results show that both lizardite and antigorite can be converted to smectite, but such conversion is much more difficult than that of kaolinite group minerals. The successful transformation is mainly evidenced by the occurrence of the characteristic (001) reflection of smectite at 1.2-1.3 nm in the XRD patterns and smectite layers with a thickness of 1.2-1.3 nm in HRTEM images of hydro-thermal products as well as the dehydroxylation of the newly formed smectite at a higher temperature in comparison to that of the starting minerals. The difficulty for the transformation of serpentine to smectite may be due to the lack of enough available Al in the reaction system, in which the substitution of Al3+ for Si4+ in the neo-formed tetrahedral sheet is critical to control the size matching between the neo-formed tetrahedral sheet and octahedral sheet in starting minerals. Since the neighboring layers in antigorite are linked by the strong Si-O covalent bonds, the transformation only takes place at the edges of an antigorite layer rather than the whole layer, and the neo-formed smectite is non-swelling due to the inheritance of such Si-O covalent bonds. The conversion of lizardite to smectite is more feasible than that of antigorite, accompanied by exfoliation. This leads to a prominent decrease of the particle size in the hydrothermal products and the number of phyllosilicate layers contained therein. Two dominant pathways were observed for the transformation of lizardite and antigorite into smectite, i.e., conversion of one serpentine layer to one smectite layer via attachment of Si-O tetrahedra onto the octahedral sheet surface of the starting minerals and two adjacent serpentine layers merging into one smectite layer. In the case of the latter, dissolution of octahedra and inversion of tetrahedral sheets took place during the transformation. Besides these two dominant pathways, precipitation and epitaxial growth of smectite were also observed in the cases of lizardite and antigorite, respectively. The present study suggests that solid-state transformation is the main mechanism for conversion of serpentine minerals to smectite, similar to the transformation of kaolinite group minerals to beidellite.
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