Mineral Halloysite Research Articles

Regulating the microenvironment of atomically dispersed cobalt to achieve the record-breaking mineralization and 100% removal of organic pollutant

Advanced-oxidation-process (AOPs) based on peroxymonosulfate (PMS) can provide strong reactive oxygen species (ROS) for the elimination of persistent-organic-pollutants (POPs), but the input of highly active environmentally friendly catalysts is the key. Here, confined cobalt was in-situ deposited on natural mineral halloysite nanotubes by Coulomb interaction, which further realizes the introduction of atomically-dispersed cobalt sites in nitrogen-doped hollow-carbon-nanotubes with large specific surface area. The constructed CoNC/NHCNTs-900 material can effectively activate PMS, resulting in the removal rate of tetracycline probe pollutants close to 100%, and the mineralization rate reached a record of 89.07%. Combined with density functional theory calculations, it was proved that the enrichment of graphite nitrogen is beneficial to regulate the microenvironment of Co and promote the adsorption and activation of PMS. In-depth tracking of the formation of ROS and the mechanism of tetracycline mineralization proved that CoNC/NHCNTs-900-driven AOPs degradation of POPs into a low-toxic species has reliability and practical significance.

An overview of halloysite mineral

Halloysite is a type of clay mineral found in the kaolin group together with kaolinite, dickite, and nacrite minerals. Kaolinite is the most common mineral in this group, while dickite, nacrite, and halloysite are rarer. Although halloysite is primarily used in ceramic production, application in other industries is increasing and gaining economic value due to its unique properties. The use of halloysite is determined by the degree of purity and the properties of the nano-sized tubular structure. Naturally obtained halloysite mineral has a key position in the field of nanotechnology due to its unique physicochemical properties originating from its tubular structure. Also, due to its reserves in many parts of the world, halloysite is a more economical material for the sectors compared to artificially produced nanomaterials. And, they find many different application areas because of these features (morphological and physicochemical). Halloysite reserves known in Türkiye are located in the Çanakkale and Balıkesir regions. Although there are many useful studies on halloysite in different, additional studies are needed to understand the nano-sized properties of halloysite ore and to ensure its use that will provide maximum benefit. This study aimed to contribute to the studies on halloysite in terms of literature by compiling studies on the characterization of halloysite minerals, reserve information, physicochemical properties, enrichment methods, and usage areas.

Efficiency of the complex mycotoxin adsorbent "Galluasorb" in combined mycotoxicosis of pigs

The paper presents the results of studying the efficiency of the method for preventing T-2, afla-, DON- and zearalenone toxicosis in pigs using the complex mycotoxin adsorbent “Galluasorb”. It is shown that as a result of including “Galluasorb” in pig feed, naturally contaminated with four mycotoxins at once, at the rate of 2.5 kg per ton of feed, a protective effect was observed, which was expressed in an increase in weight gain, feed consumption, a decrease in its conversion, normalization of the clinical status and morpho-biochemical blood parameters. Thus, on the 30th day of the experiment, the absolute increase in live weight in piglets receiving the main diet with mycotoxins with the addition of “Galluasorb” was 13.7 kg, while in piglets of the toxic control group - 11.8 kg. The average daily live weight gain per animal in the same groups was 456.7 g and 393.3 g, respectively. In the toxic control group, feed consumption was reduced by 7.4% compared to the group of animals receiving the main diet with mycotoxins with the addition of “Galluasorb”. A decrease in the feed conversion ratio was also noted in the preventive group. Thus, the use of the complex adsorbent had a protective effect in the natural contamination of feed in the T-2 farm with toxin, aflatoxin, DON and zearalenone. The developed adsorbent also opens up prospects for the use of the natural mineral halloysite in combination with an antioxidant, hepatoprotector and organic sorbent in pig farming.

Transform of Kaolinite in Iraqi Kaolin Clay to Halloysite by Intercalation Reaction

The kaolinite of the Duikhala Deposite in the Iraqi Western Desert underwent intercalation with potassium acetate (KAc) and was subsequently washing with water several times. More than 70% of intercalation was achieved and checked using the X-ray diffraction (XRD) technique by observing a decrease in the peak intensity 7.2 Å and the appearance of the peak intensity at 14 Å after two days of intercalation. The surface area and cation exchange capacity were increased in the treated sample compared to the untreated one. Transmission electron microscopy images of the treated sample of kaolinite, after 35 cycles of repeated intercalation and washing, showed that the kaolinite lamellar were wrapped along the main crystal directions and transformed into the nanotube shape halloysite mineral.

Double solvent synthesis of ultrafine Pt nanoparticles supported on halloysite nanotubes for chemoselective cinnamaldehyde hydrogenation.

The development of highly active, low cost and durable catalysts for selective hydrogenation of aldehydes is imperative and challenging. In this contribution, we rationally constructed ultrafine Pt nanoparticles (Pt NPs) supported on the internal and external surfaces of halloysite nanotubes (HNTs) by a facile double solvent strategy. The influence of Pt loading, HNTs surface properties, reaction temperature, reaction time, H2 pressure and solvents on the performance of cinnamaldehyde (CMA) hydrogenation was analyzed. The optimal catalysts with the Pt loading of 3.8 wt% and the average Pt particle size of 2.98 nm exhibited outstanding catalytic activity for the hydrogenation of CMA to cinnamyl alcohol (CMO) with 94.1% conversion of CMA and 95.1% selectivity to CMO. More impressively, the catalyst showed excellent stability during six cycles of use. The ultra-small size and high dispersion of Pt NPs, the negative charge on the outer surface of HNTs, the -OH on the inner surface of HNTs, and the polarity of anhydrous ethanol solvent account for the outstanding catalytic performance. This work offers a promising way to develop high-efficiency catalysts with high CMO selectivity and stability by combining clay mineral halloysite and ultrafine nanoparticles.

Unrolling the tubes of halloysite to form dickite and its application in heavy metal ions removal

Aluminol groups are effective for removing toxic metal ions. However, their use is significantly limited because they are usually on the inner surface of halloysite tubes. Herein, we realign and expose the abundant aluminol groups using a hydrothermal process. We found that dickite was gradually formed with increased sodium metasilicate proportion. Also, a uniform mushroom-like structure was formed, extending from the orifice to the middle of the hollow tubes and eventually forming a sphere. Further, part of the halloysite tube gradually broke, expanding into lamellar, eventually forming a small spheroid. The Al-O-Si groups were fractured by sodium metasilicate and aluminol groups on the inner surface of halloysite tubes, forming many aluminol groups. Moreover, the average pore size of the halloysite increased, indicating that numerous halloysite mesopores converted to dickite macropores. Thus, we inferred that the tubes gradually unfolded when the metasilicate first binds with the aluminol groups on the inner of the halloysite tubes. The modification enhanced the material's adsorption capacity for Pb2+, Cd2+, and Cr3+ nearly 20, 12, and 10 times, respectively, stronger than the original halloysite mineral. Consequently, exposing and realigning aluminol groups of halloysite significantly improved toxic metal ions' removal.

How herbicides like atrazine and diuron interact with the spiral halloysite structure

The spiral tubular morphology of the halloysite mineral presents an interesting opportunity for pollutant remediation: a large specific surface area, porosity, high adsorption capacity, bivalent adsorbancy, and colloidal behaviour. Two worldwide used herbicides are atrazine and diuron, which have shown harmful impacts on the environment and fauna, including humans. Therefore, both herbicides were studied to test their interactions on the halloysite tubular structure. Force field methods using the COMPASS force field were applied to perform the geometry optimisation and molecular dynamics on the atrazine/diuron-halloysite models. The models were prepared with different herbicide loading ratios on the inner and outer parts of the halloysite structure to find the preferable interacting surface. The simulation showed that both halloysite surfaces could interact with atrazine and diuron molecules. Further, the herbicides’ arrangement on the surfaces depends on the herbicide’s concentration and subsequently on their mutual interactions. The inner surface of the halloysite showed stronger interactions with both herbicides than the outer surface, and especially with diuron (e.g., binding energies per herbicide molecule for atrazine and diuron were –115 ± 2 and –130 ± 3 kJ·mol−1, respectively). Increasing herbicide concentration on the inner surface is limited by free space, and the interaction capacity becomes gradually saturated both for atrazine and for diuron.

Compressibility and permeability of weathered, sensitive volcanic ash (tephra) deposits at the Omokoroa flow slide, New Zealand

Volcanic ash (tephra) deposits from the Omokoroa flow slide, New Zealand, exhibit a high concentration of the secondary clay mineral halloysite being responsible for high sensitivities in these soils. Sensitivity is the ratio between intact and remoulded undrained strength at the same water content. So far, compressibility and permeability have not been studied for halloysite-rich sensitive soils. Here, we found that oedometer compression behaviour of intact tephras depended on the sensitivity. Compression behaviour of remoulded tephra seemed to be independent of sensitivity and halloysite morphology but varied with liquid limit. Compression lines of remoulded tephras were less steep and, thus, exhibited an overall smaller compressibility, then those determined from remoulded sedimentary clays. Permeabilities of remoulded tephra samples were generally smaller than permeabilities of remoulded sedimentary clays. For halloysite-rich tephra, it was found that permeabilities of remoulded samples were smaller than those of intact samples. A new parameter was introduced, the permeability sensitivity, which quantifies the difference in permeabilities between intact and remoulded tephras. We found a fairly strong positive relationship (R2 = 0.89) between the permeability sensitivity and the sensitivity. The halloysite morphology seemed to be moderately correlated with the permeability sensitivity (R2 = 0.45). Our findings have implications for progressive slope failure in sensitive soil but are limited by means of the indirect method of analysing time–settlement curves of step-loaded oedometer tests to derive permeability and re-orientation of samples to allow horizontal drainage in a conventional oedometer cell.

The Pedogenesis of Inceptisols on Southeast Toposequence of Mount Manglayang in West Java, Indonesia

DOI:10.17014/ijog.9.2.195-208The most potential soil order in Indonesia is dominated by Inceptisols, whereas the topography combined with a climatic factor are the main factors to regulate pedogenetic process. This research was intended to determine the pedogenetic process and soil development on various terrain positions at the southeast toposequence of Mount Manglayang areas that have hilly topography. The researched area was 28.83 ha. Based on those conditions, this research aims to study the pedogenetic process and soil development in the southeast slope toposequence of Mount Manglayang, the relationship between the physical, chemical, and mineralogical soil properties, the soil classification to family level based on soil taxonomy, National Soil Classification, and FAO soil classification systems. This research used survey, descriptive, and comparative methods. The result showed that the pedogenetic processes identified were the formation of B horizon through clay accumulation, soil colour, and soil structure development, and the formation of amorphous kaolinite and halloysite minerals. Based on soil taxonomy, the soil were classified as Fluventic Humudepts, coarse-loamy, kaolinitic, isohyperthermic at the upper slope and Fluventic Dystrudepts, fineloamy, kaolinitic, isohyperthermic at the middle and lower slopes. According to National Soil Classification, the soil is Humic Cambisol at the upper slope, Distric Cambisol at the middle slope, and Cromic Cambisol at the lower slope. FAO classified the soil as Umbric Cambisols at the upper slope and Dystric Cambisols at the middle and lower slopes.

Kaolinite nanoscroll significantly inhibiting polysulfide ions shuttle in lithium sulfur batteries

The shuttling effect of polysulfide ions is an important reason that hinders the commercial application of lithium‑sulfur batteries. The natural clay mineral halloysite has good adsorption performance and hollow tubular structure, which can absorb and accommodate enough sulfur, while the flake kaolinite can roll to form a halloysite-like structure. The effect of acid-modified halloysite and kaolinite nanoscrolls as sulfur carriers on the performance of lithium‑sulfur batteries was studied in this paper. As a result, both acid-modified halloysite and kaolinite nanoscrolls could effectively adsorb polysulfide ions, while kaolinite nanoscrolls were more conducive to the migration and diffusion of Li+ due to its thinner wall (about 5 nm), thus exhibiting faster reaction kinetics and higher sulfur utilization, and had better rate performance and cycle stability. The application of clay minerals in the field of energy has always been unpromising. This research work makes full use of the characteristics of clay minerals and broadens its application, providing a basis for the design of clay minerals and a possible modification direction for their application in lithium‑sulfur batteries.

Coupling Natural Halloysite Nanotubes and Bimetallic Pt-Au Alloy Nanoparticles for Highly Efficient and Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid.

The aerobic oxidation of 5-hydroxymethylfurfural (HMF), a key platform compound derived from biomass, to 2,5-furandicarboxylic acid (FDCA) is a highly important reaction in the production of green and sustainable chemicals. Here, we developed a highly efficient and stable halloysite-supported Pt-Au alloy catalyst for the selective oxidation of HMF to FDCA. The catalyst was synthesized through the organosilane functionalization of halloysite nanotubes, followed by the in situ formation and dispersion of Pt-Au alloy nanoparticles on the internal and external surfaces of nanotubes. The composition, morphology, and structure of the prepared catalyst were characterized. The catalyst with the optimal composition of Pt/Au molar ratio of 1/4 and metal loading of 1.5 wt % exhibited outstanding catalytic activity for the oxidation of HMF to FDCA using O2 as an oxidant with 100% conversion of HMF and 99% selectivity of FDCA. This excellent catalytic performance is mainly attributed to the high dispersion and alloying effects of bimetallic nanoparticles, which promoted the activation of reactants or intermediates and further improved FDCA selectivity. Furthermore, the halloysite-supported Pt/Au bimetallic catalyst showed high stability and reusability. This study provides a promising strategy by combining clay mineral halloysite and bimetallic alloys for developing efficient catalysts with high FDCA selectivity and stability for the oxidation of HMF to FDCA.

Structural Characterization and Adsorption Properties of Dunino Raw Halloysite Mineral for Dye Removal from Water.

In this work, raw halloysite mineral from Dunino (Poland) has been characterized and tested as an efficient and low-cost adsorbent for dye removal from water. The morphology and structure of this clay were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and the chemical composition was evaluated by means of X-ray fluorescence spectroscopy (XRF), energy dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS). The results showed that it is made up of both platy and tubular structures, mainly composed of Si, Al, and O. Iron oxide particles covering the platy structures were also observed. The surface charge of halloysite was measured by z-potential measurements and by the evaluation of the point of zero charge. The clay was tested as an adsorbent for the removal of positively and negatively charged dye molecules, i.e., methylene blue (MB) and methyl orange (MO), both separately and in a mixed-dye solution. Halloysite showed the ability to efficiently and selectively remove MB molecules by adsorption, both in a single-dye solution and in a mixed one. The adsorption of positive dyes on the clay surface mainly occurred through ion exchange at negatively charged sites on its surface. The possibility of regenerating the clay for further dye removal processes is also shown.

Highly photocatalytic activity of natural halloysite - based material for the treatment of dyes in wastewater

In this study, the halloysite nanotube material will be fabricated from a natural halloysite mineral and used as a support for the photocatalytic activity phase based on TiO2. The material is characterized by modern physicochemical methods such as XRD, SEM, BET, UV - vis spectrum, and EDX. Accordingly, the refined halloysite has a nanoscale with a length of about 1.3 μm and a capillary size of about 5 nm. After the deposition of Ag - TiO2 on the halloysite, the specific surface of the material measured by the BET method was about 60 m2/g, and the structure of the halloysite was intact. The band - gap energy of as - prepared materials is also significantly improved in comparison to pure TiO2, makes the material capable of absorbing longer wavelengths of light in the photocatalytic process. The Photocatalyst based on Halloysite and TiO2 showed very high efficiency, up to more than 95% in the decomposition of typical organic pollutant RR - 195. This result shows great potential in this novel material in environmental treatment applications.

One-step preparation of blue-emitting CsPbBr3 quantum dots loaded on natural mineral halloysite nanotube

All-inorganic cesium lead halide perovskite quantum dots (QDs) have attracted widespread attention due to unique photoelectric properties. However, blue-emitting (CsPbClxBr3−x, 0 < x ≤ 3) QDs have poor stability and low photoluminescence quantum yields (PLQY), which limits its application in perovskite based light-emitting diodes (LEDs). The chlorine-free CsPbBr3 QDs have fewer crystal defects and a more stable crystal structure. Here, we prepared CsPbBr3 QDs with adjustable photoluminescence (PL) (489–519 nm) by controlling the reaction temperature and composited it with natural mineral halloysite (Hal) nanotubes. The as-prepared CsPbBr3@Hal (100 °C) have high PL intensity, bright blue emission and enhanced stability. Under an open ambient condition, the CsPbBr3@Hal (100 °C) can still maintain 92.4% of original PL intensity after 60 days of storage. In addition, after 280 h of continuous irradiation with 365 nm UV lamp, its PL intensity still maintained 71%. Finally, a white LED (WLED) fabricated by incorporating the CsPbBr3@Hal (100 °C) and commercial red CaAlSiN3: Eu2+ phosphor on the 395 nm InGaN blue-violet chip showed the CIE chromaticity coordinates of (0.3358, 0.3331). This work provides a new way for the synthesis and application of blue-emitting CsPbBr3 perovskite materials in WLEDs and display devices.

Elektro Çekim Yöntemi ile Haloysit Katkılı Biyo-Bazlı Termoplastik Poliüretan Nanolif Üretimi ve Karakterizasyonu

In this study, it was aimed to produce biocomposite nanofibers by using electrospinning technique and to form biocomposite structure, bio-based thermoplastic polyurethane (BioTPU) and halloysite (HST) mineral obtained from natural sources were used. Electrospinning parameters have been optimized for the production of nanofibers with smooth morphology and the polymer solution with the most suitable parameter was determined. Different concentrations of HST filled BioTPU nanofibers were produced and the rheological behavior of the solutions was investigated with a rotational rheometer before electrospinning to observe the effects of halloysite on fiber morphology. Fourier transform infrared spectroscopy (FTIR) analysis was carried out to determine the chemical composition of acquired nanofibers, and scanning electron microscopy (SEM) was used to monitor surface morphologies. Contact angle measurements were carried out to observe the effects of halloysite on the hydrophilicity of nanofiber. According to rheology results, it has been found out that the solution viscosity, storage modulus (G') and loss modulus (G'') of halloysite increased up to a certain concentration (0.3 % HST), but later caused falls on viscosity. According to the results of FTIR analysis, there is no chemical bond between halloysite and BioTPU, but SEM images show that halloysite was added to the structure of nanofibers. It was also found that the halloysite added to the structure increased the fiber diameters and that the fiber cross-section was not uniformly distributed along the fiber axis. The results of contact angle analysis indicated that acquired nanofibers have hydrophobic surface and the added halloysite decreases contact angles of nanofibers.

Mineralogical and geochemical properties and genesis of kaolin and alunite deposits SE of Aksaray (Central Turkey)

The kaolin and alunite deposits, situated approximately 20 km southeast of Aksaray in Central Anatolia, were formed by hydrothermal alteration related to extensional tectonics from the Upper Miocene to Quaternary, mostly from rhyolitic volcanoclastic rocks. The mineralogical and geochemical properties of the deposits were investigated to explain the stages of hydrothermal alteration and the origin of the deposits. X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical and stable isotope (O, H, and S) studies were carried out on the alteration minerals (e.g., kaolinite, gypsum, alunite, native sulfur and smectite) of the deposits.Kaolinite/halloysite minerals were determined along with mostly alunite- and silica-group minerals (α-quartz, opal-CT, and tridymite), feldspar, native (elemental) sulfur, and partial hematite, goethite, gypsum, cinnabar and realgar. The δ34S isotope values for alunite, gypsum and native sulfur are in a fairly narrow range and vary from 5.03 to 6.62‰, 4.5–5.24‰ and 5.97–7.49‰, respectively. The δ18O values of the kaolinites and smectites vary from +1.8 to +13.3‰ and from −98 to −152‰, respectively, and their δD values range from −4.2 to 15.7‰ and from −93 to −191‰, respectively, showing a spread that is very close the primitive magmatic water line. From the δ18O values, the calculated formation temperatures of the kaolinites and smectites range from 38 to 135 °C and from 29 to 189 °C, respectively. The δ18O values of the alunites and amorphous silica vary from 4.4 to 8.9‰ and from −1.1 to 8.4‰, respectively, and the δD values vary from −72 to −87‰, and from −104 to −191‰, respectively. In addition, the formation temperatures of alunite and amorphous silica minerals range from 57 to 100 °C and from 139 to 281 °C, respectively. Hydrothermal alteration progressed in different stages between 29 and 281 °C according to the calculated modal formation temperatures. The isotopic mineral data suggest that the original hydrothermal waters were a mixture of magmatic and meteoric waters. Oxygen and deuterium analyses indicate that kaolinite and alunite formed at lower temperatures than smectite and silica minerals. The determined stable isotopic compositions of the minerals indicate that hydrothermal solutions at different compositions and temperatures played important roles in the formation of minerals. The determined neoformed minerals may have been formed by steam-heated hydrothermal solutions sourced from rhyolitic magma and are mostly of hypogene origin.

Bioformulation of Microbial Fertilizer Based on Clay and Alginate Encapsulation.

This study aims to develop new formulations for microbial fertilizers Pseudomonas fluorescens Ms-01 (Pf) and Azosprillum brasilense DSM1690 (Ab) using two kinds of clay minerals. The studied formulations were prepared as hybrid materials based on halloysite and alginate [Ha-Ag] or montmorillonite and alginate polymers [Mt-Ag] and were applied to the bacterial strains to develop low cost, efficient, and slow-release capsules. Their efficiency was evaluated in comparison with alginate [Ag] as the control. The produced capsules were spherical in shape and were chemically and physically characterized and further analyzed for their swelling ratios, soil biodegradability, release kinetics of microbial cells, and their survival stability over 3months of storage under different conditions (room temperature vs 4°C). The effect of the capsules on the growth of wheat plants was also investigated. Results showed that both formulations were able to preserve bacterial survival which reached 14.8log CFUg-1 after 3months storage in the halloysite formulation. The swelling ratios were ranged between 61.5 ± 1.35% and 36.5 ± 5% for the montmorillonite and the halloysite formulations, respectively. The release kinetics revealed the slow-release capacity of the capsules mainly with the halloysite formulation which significantly released bacterial cells after 15days of incubation in saline water (15.24log CFUmL-1). The application of the capsules to wheat plants significantly increased root and shoot biomasses and nitrogen content in the roots. In conclusion, halloysite minerals seem to be more adapted as additive to alginate in microbial encapsulation.

Mechanical and Bonding Behaviors Behind the Bending Mechanism of Kaolinite Clay Layers.

The density functional theory-based calculations were performed on stripe models of the single kaolinite layer. The calculations helped to explain why halloysite mineral, a member of the kaolinite group existing in a tubular form, has rolled tubes only in one way. In that form, aluminol octahedral sheet, terminated by surface hydroxyl groups, represents the inner surface of the nanotubes. The bending models with the inner surface formed by the SiO tetrahedral sheet showed significant structural instability with monotonically increasing strain energy as a function of the curvature. In contrast, for the bending models with the octahedral sheet as the inner surface, stabilization energetic minima were found at curvatures of about 10 nm. The calculations were also performed on the individual sheets (tetrahedral and octahedral) of the kaolinite layer to show their contribution to the bending strain. We found that the decrease of the bending energy and the layer stabilization with respect to the planar configuration for curvatures with radii RC > ∼5 nm can be attributed mainly to three factors—(i) better match between octahedral and tetrahedral sheets, (ii) local structural changes of the SiO and AlOH polyhedral units, and (iii) increasing effectivity of hydrogen bonding of the outer surface OH groups.

Modified halloysite minerals for radiation shielding purposes

ABSTRACTHalloysite is a clay mineral found in the natural environment and it has many applications in chemistry and medical fields. The linear attenuation coefficient (µ) for halloysite nanotubes m...

Crystal violet adsorption by oppositely twisted heat-treated halloysite and pecoraite nanoscrolls

Present study focuses on crystal violet adsorption by mineral halloysite and synthetic pecoraite nanoscrolls. Being scrolled in opposite directions, these two materials serve both as good adsorbents and model objects to understand the dye adsorption mechanism. Heat treatment of the nanoscrolls allowed us to track the adsorption process features by changing the structure and surface state of the particles. Structural transitions were observed by complementary techniques including X-ray diffraction, thermal analysis, electron microscopy, N2 adsorption and ς-potential measurements. By a fast UV/VIS study of adsorption kinetics, it became possible to take a closer look at the very initial steps of the process. Intraparticle diffusion governs the overall adsorption kinetics of the dye by halloysite and pecoraite nanoscrolls. The presence of OH-groups on the nanoscrolls' surface strongly facilitates the initial stage of adsorption. Structural transition at around 600 °C increases adsorption rate and performance of synthetic pecoraite due to ς-potential sign change.