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.
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.
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.
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.
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.
While noble metals often occur as minor components in host minerals in various ore deposits, little theoretical assessment exists to predict the occurrence of these metals. Here, we probe the ...fundamental controls responsible for the occurrence of trace elements in host minerals through first-principles calculations. We apply the theoretical model to understanding the debated issues concerning the occurrence of gold (Au) in pyrite, in which the valence of Au is ascribed to either positive or negative values. Our results indicate that (1) both positive and negative valent Au may occur in pyrite and (2) higher sulfur fugacity and lower temperature lead to more Au+ occupying Fe sites in pyrite. These findings suggest that chemical states and speciation of the Au in host pyrite are ultimately controlled by temperature and sulfur fugacity, providing insight into the formation conditions of ore deposits and facilitating strategy design for beneficiation.
Trace elements in molybdenite can provide important information regarding the composition of ore-forming fluid and the evolution and genesis of ore deposits. However, the occurrence states and ...behavior of relatively incompatible trace elements (e.g., Pb and Os) in natural molybdenite remain ambiguous. Here, we report an abnormally high enrichment of Pb and layered PbS precipitate within molybdenite grains from the Huanglongpu carbonatite-hosted Mo-Pb deposit in the Qinling orogenic belt of Northern China. High-resolution transmission electron microscopy (HRTEM) and related nanobeam techniques were applied to characterize the occurrence states of Pb within molybdenite at the atomic scale. The results show that up to several weight percent of Pb can be incorporated into the molybdenite structure during initial crystallization, which can lead to the formation of screw dislocations and 3R/disordered stacking of S-Mo-S sandwich layers. Observations using a scanning transmission electron microscope also reveal that Pb diffuses from the host molybdenite into the layered PbS precipitates under prolonged electron beam irradiation. Pb-bearing molybdenite tends to transform into a Pb-poor ordered 2H1 polytype upon Pb exsolution during cooling. Pb preferentially exsolves along the (001) plane of molybdenite and is stored in structural defects (e.g., dislocation loops) and grain boundaries, resulting in nano-scale Pb heterogeneities in molybdenite. Further coarsening of the exsolved Pb results in the formation of layered PbS precipitates along the (001) plane of molybdenite. This study provides an example of the consequences of the incorporation and exsolution of incompatible trace elements in molybdenite and demonstrates that careful mineralogical examination is required to interpret geochemical data obtained by in situ analysis techniques.
The evolution of oxygenic photosynthesis is a pivotal event in Earth's history because the O
released fundamentally changed the planet's redox state and facilitated the emergence of multicellular ...life. An intriguing hypothesis proposes that hydrogen peroxide (H
O
) once acted as the electron donor prior to the evolution of oxygenic photosynthesis, but its abundance during the Archean would have been limited. Here, we report a previously unrecognized abiotic pathway for Archean H
O
production that involves the abrasion of quartz surfaces and the subsequent generation of surface-bound radicals that can efficiently oxidize H
O to H
O
and O
. We propose that in turbulent subaqueous environments, such as rivers, estuaries and deltas, this process could have provided a sufficient H
O
source that led to the generation of biogenic O
, creating an evolutionary impetus for the origin of oxygenic photosynthesis.
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