Amorphous Silica Phase Research Articles

Near-IR study of water adsorption on acid-treated montmorillonite

The effect of degradation of montmorillonite structure on its hydration properties was investigated. Na-saturated <2μm fraction of bentonite from Kopernica deposit (Na-Kop) containing montmorillonite as the dominant component was dissolved in 6moldm−3 HCl at 95°C for 1–36h. The spectra in the mid-infrared (MIR) and near-infrared (NIR) regions confirmed gradual destruction of the montmorillonite structure upon acid treatment and creation of protonated amorphous silica. A NIR UpDRIFT accessory enabling the measurement of spectra directly in closed glass vials was used to study the effect of acid treatment and hydration on the strength of H-bonding between water molecules and to determine the amount of water adsorbed on the samples at 52% relative humidity (RH). The combination band of water near 5250cm−1 was shifted to lower wavenumbers with increasing H2O content indicating stronger H-bonds. In contrast, the upward shift of the band in the spectra of acid treated samples proved weak H-bonds between water molecules and amorphous silica phase. The presence of H-bonded SiOH groups in hydrated samples was demonstrated. Gravimetric analysis and NIR spectra showed that the hydration increased from Na-Kop to Na-Kop dissolved 5h, for which the maximum water content of 12.5 mass% and the highest (ν+δ)H2O band area were detected. Longer dissolution of montmorillonite produced samples with gradually decreasing ability to adsorb H2O. A good correlation of the results obtained by both methods indicates that the (ν+δ)H2O band area reasonably reflects the hydration of samples and can be used for the estimation of water content within sets of similar samples.

Reaction of water-saturated supercritical CO2 with forsterite: Evidence for magnesite formation at low temperatures

The nature of the reaction products that form on the surfaces of nanometer-sized forsterite particles during reaction with H2O-saturated supercritical CO2 (scCO2) at 35°C and 50°C were examined under in situ conditions and ex situ following reaction. The in situ analysis was conducted by X-ray diffraction (XRD). Ex situ analysis consisted of scanning electron microscopy (SEM) examination of the surface phases and chemical characterization of precipitates using a combination of confocal Raman spectroscopy, 13C and 29Si NMR spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). The results show that the forsterite surface is highly reactive with the primary reaction products being a mixture of nesquehonite (MgCO3·3H2O) and magnesite (MgCO3) at short reaction times (∼3–4days) and then magnesite (MgCO3) and a highly porous amorphous silica phase at longer reaction times (14days). After 14days of reaction most of the original forsterite transformed to reaction products. Importantly, the formation of magnesite was observed at temperatures much lower (35°C) than previously thought needed to overcome its well-known sluggish precipitation kinetics. The conversion of nesquehonite to magnesite liberates H2O which can potentially facilitate further metal carbonation, as postulated by previous investigators, based upon studies at higher temperature (80°C). The observation that magnesite can form at lower temperatures implies that water recycling may also be important in determining the rate and extent of mineral carbonation in a wide range of potential CO2 storage reservoirs.

Degradation of surfactant-modified montmorillonites in HCl

The effect of surfactant size on the extent of montmorillonites decomposition in HCl was investigated. Na-SAz montmorillonite and tetraalkylammonium salts of alkyl-chains length increasing from methyl- Me4N to pentyl- Pe4N were used for organo-montmorillonites preparation. Decreasing intensity of d001 diffraction proved destruction of montmorillonite structure connected with gradual surfactant release. A shift of the SiO stretching band to 1097 cm−1 confirmed formation of amorphous silica phase. A new band near 7315 cm−1 corresponding to SiOH overtone revealed creation of protonated silica. This band was observed in the spectra of all acid-treated samples, also in those with minor decomposition of the structure. The size of the cations significantly affected decomposition of montmorillonites in HCl. The less stable were Na-SAz and Me4N-SAz in which the content of octahedral atoms dropped to ∼5% of their original values upon 8 h treatments. Et4N-SAz and Pr4N-SAz were slightly more resistant mainly at short times. Bu4N-SAz and Pe4N-SAz showed the least structural modifications, only 50% and 35% of octahedral atoms, respectively, were released into solution within 8 h treatments. This observation proves that bulky alkylammonium cations covering the inner and outer surfaces of montmorillonite prevent effectively the access of protons to the layers protecting the mineral from degradation in the acid.

Reduction of a NiO thin film deposited by PLD on a single crystal YSZ (111) substrate

The NiO/YSZ interface prepared by depositing NiO on a single crystal YSZ (111) substrate has been investigated by transmission electron microscopy. As deposited, a very thin nickel layer ascribing to the nonstoichiometry at the very beginning growth of NiO and an amorphous silica phase resulting from silicon segregation were present at the interface. The orientational relationship of NiO \( (1\overline{1} 1) \)//Ni \( (1\overline{1} 1) \)//YSZ \( (1\overline{1} 1) \) with NiO [110]//Ni [110]//YSZ [110] was observed. The microstructural and chemical changes at the NiO/YSZ interface after being heated in vacuum and hydrogen indicated different reduction mechanisms. In vacuum, the reaction \( {\text{NiO}} \to {\text{Ni}} + 1/ 2 {\text{ O}}_{ 2} \left( {\text{g}} \right) \) was prevailing at the interface between NiO and pre-existing Ni, which led to the thickening of nickel layer. In hydrogen, the reduction initiated on the NiO surface was dominant, following the chemical equation H2 + OO (NiO) → H2O (g) + VO.. (NiO) + 2e (Ni).

PREPARATION AND CHARACTERIZATION OF HYDROXYAPATITE-SILICA COMPOSITE NANOPOWDERS

One of the most important objectives in the field of biomaterials science and engineering is development of new materials as bone substitutes. Silica ( SiO 2) has an important role in the biomineralization and biological responses. The aim of this research was to prepare and characterize hydroxyapatite-silica ( HA - SiO 2) composite nanopowder with different content of silica. Hydroxyapatite-silica composite nanopowders with 20 and 40 wt% silica were prepared using a sol–gel method at 600°C with phosphoric pentoxide and calcium nitrate tetrahydrate as a source of hydroxyapatite; also, tetraethylorthosilicate and methyltriethoxisilane as a source of silica. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) techniques were used for characterization and evaluation of the products. The results indicated the presence of nanocrystalline hydroxyapatite phase beside amorphous silica phase in prepared composite nanopowders. Moreover, by increasing the content of silica in composite nanopowders, the crystallinity will be decreased,and the ability of the product as a bone substitute material might be controlled by changing the content of the ingredients and subsequently its structure.

Fabrication of Silica Glass from Rice Husk Ash with Spodumene Additions

Silica glass is an interesting material due to its low thermal expansion coefficient, high chemical inertness, and transparency. In this study, low cost rice husk ash waste containing mainly amorphous silica phase was used as a starting raw material. Formation of cristrobalite caused product damage during cooling down was suppressed by an addition of some sintering aids. Spodumene was selected to use as the sintering aid due to its ability to promote the sinterability of the rice husk ash by liquid phase forming. Lithium carbonate and aluminum nitrate were mixed with the rice husk ash as starting chemicals for spodumene forming. To investigate the effects of spodumene addition on densification, physical properties and thermal expansion coefficient, 25 and 50 mass% of spdumene were added to the mixture. Mixed powders were dry pressed into pellet shapes and sintered at 1000-1250 °C for 30 min in an electric furnace. It was found that 50 mass% addition of spodumene enhanced the densification of the specimens sintered at the temperature higher than 1250 °C. Water absorption was reduced to 1.11 % with bulk density of 2.12 g/cm3. Low thermal expansion coefficient of 2.70×10−6 /°C was achieved with only the petalite phase detection.

Characterization of acid river dilution and associated trace element behavior through hydrogeochemical modeling: A case study of the Banyu Pahit River in East Java, Indonesia

Abstract Kawah Ijen volcano in East Java, Indonesia emits hyperacid (pH ≈ 0) brines rich in toxic elements including F, Al, Cd and Tl, which are transported downstream by the Banyu Pahit River, which is eventually used to irrigate farmland on the Asambagus Plain. The fate and behavior of major and trace elements are investigated, in the region of greatest change to the Banyu Pahit River, where thermal springs and the neutral Kali Sat and Kali Senggon Rivers increase its discharge 6-fold and pH increases from 1.9 to 4.5 with the development of abundant precipitates. The conservative behavior of Cl, F and SO 4 allows determination of the proportions of the mixing constituents at each confluence. Comparing a mass balance model based on the resulting proportions with measured concentrations demonstrates conservative behavior for most cations, with the exception of Si, Zr, Sn and Ba, which precipitate as amorphous silica, barite, and Zr and Sn phases, respectively. Iron is added as particulate Fe-(oxy)hydroxide in mixing with thermal spring outflow, and dissolves in the lower pH water. This also contributes Co, Cd, Mn, Y, and the rare earth elements, which are desorbed from the Fe-(oxy)hydroxide upon entering the acid water. Subsequent saturation in Fe-(oxy)hydroxide after mixing with the neutral rivers leads to a reverse response. Still, element behavior is close to conservative, which means that the volcanogenic toxic element load is almost entirely transferred to the Asambagus Plain with ensuing environmental and health impacts. This study shows that compositions of these acid waters can be understood and reproduced in a thermodynamic model, but only when fine-tuned using measured compositions and field observations. Therefore, the model’s utility in assessing the fate of toxic elements and in planning the environmental mitigation is limited.

Synthesis and characterization of praseodymium-containing ZrSiO 4 solid solutions from gels

The preparation and characterization of a series of praseodymium–zircon solid solution (Pr x –ZrSiO 4) materials with increasing nominal amounts of Pr is reported. Pr-doped zircon gels were prepared by gelling mixtures of zirconium n-propoxide, praseodymium acetylacetonate and tetraethylorthosilicate, and annealed over the range of temperature up to the formation of Pr–zircon solid solutions. The reaction sequence was followed by X-ray powder diffraction (XRD), ultraviolet–visible diffuse reflectance (DR) and infrared spectroscopy (IR). The first crystalline phase detected on annealing gels was a tetragonal praseodymium-containing ZrO 2 phase (t-Pr–ZrO 2). On further annealing, the subsequent transformation to the monoclinic form (m-Pr–ZrO 2) took place. The formation of final Pr–ZrSiO 4 solid solutions occurred by the reaction between m-Pr–ZrO 2 and amorphous silica phase. The mechanism of solid solution formation inferred from energy dispersive X-ray microanalysis (SEM/EDX) data, variation of lattice parameters and DR of final Pr–ZrSiO 4 solid solutions involved the replacement of Zr 4+ by Pr 4+ in dodecahedral sites of the zircon structure. DR revealed that a relatively small amount of Pr 3+ was still present in final Pr-containing ZrSiO 4 products. The estimated solubility of praseodymium in the zircon was around 0.067 mol of praseodymium per mol of zircon (∼11.5 wt% as Pr 2O 3). This study opens new perspectives to the development of more ecological zircon-based ceramic pigmenting systems by using mineralizer-free sol–gel synthetic techniques.

Fabrication and characterization of composite sol–gel coatings on porous ceramic substrate

Highly porous ceramic materials were coated using a composite sol–gel method. Alumina powder is dispersed in a silica sol–gel solution and then dip-coated on the substrate. The resulting coatings present a composite microstructure in which crystalline alumina grains are linked to each other by an amorphous silica phase. In this work, we show that, by accurately controlling the sol–gel parameters (water, solvent and silica precursor (TEOS) ratio, pH and ageing time of the sol) and also the powder grain size distribution it is possible to obtain crack-free thick films (more than 20 μm in one step). These coatings present good adherence to the substrate, decrease the roughness and also close the surface porosity of the substrate. Coating mechanical properties have been evaluated thanks to micro-indentation measurements and linked to coating structural evolution with the thermal treatment temperature.

Oxidation behavior of amorphous Si-(B)-C-N ceramic in ambient air

An exploratory investigation on the oxidation behavior of polymer derived Si-(B)-C-N ceramic in ambient air was conducted in this paper. The amorphous ceramic was oxidized at 1300 °C for 12 h, and the oxidation products were analyzed by employing XRD, FT-IR and TEM. The experimental results indicate that the amorphous Si-(B)-C-N ceramic undergo a rapid oxidation process, and the oxidation products are divided into two parts: amorphous silica and cristobalite phase silica. The formation mechanism of the oxidation products was analyzed on the basis of characterization results and collected DTA curve in ambient air up to 1100 °C. Furthermore, the oxidation mechanism of the amorphous Si-(B)-C-N ceramics was deduced based on analysis results and thermodynamic consideration.

Delamination of Layered Zeolite Precursors under Mild Conditions: Synthesis of UCB-1 via Fluoride/Chloride Anion-Promoted Exfoliation

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via (29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N(2) adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization.

Carbonation of Chrysotile under Subcritical Conditions

The carbonation of chrysotile [Mg3Si4O10(OH)4] under subcritical conditions was experimentally investigated in alkali solution, and the chemical composition as well as the morphological and structural changes were discussed. The starting material was hydrothermally treated by aqueous direct carbonation at a temperature of 100 � C and a CO2 partial pressure range of 0.5 MPa� 4 MPa at pH 13. Highly crystalline magnesite was synthesized under a CO2 partial pressure of 3 MPa. The carbonation rate increased at the proportional rate according to the applied CO2 pressure to approximately 57%. The surface morphology of chrysotile changed from the fibrous form to a round or oval shape at the initial stage and subsequently to magnesite with well-faceted rhombohedral planes. The dissolution rate of Mg was higher than Si, such that the Mg : Si ratio of chrysotile decreased from 1.56 to 0:4� 0:6 as the reaction time increased. The resultant silica-rich layer of the reaction product ultimately changed through the Mg-depleted skeletal phase to the amorphous silica phase. The experimental results suggest that carbonation in alkali solution under subcritical conditions is of great significance because an excessive amount of acid and alkali reagents can be eliminated in the carbonation process. [doi:10.2320/matertrans.M2011155]

Influence of the polymer architecture on the high temperature behavior of SiCO glasses: A comparison between linear- and cyclic-derived precursors

Two series of cross-linked polysiloxanes, precursors for silicon oxycarbide glasses, have been synthesized from a linear and a cyclic Si–H-containing siloxane having the same chemical formula (SiCOH 4). The crosslinking has been achieved by hydrosilylation reaction with various amounts of divinylbenzene (DVB). A detailed structural characterization has been performed by 29Si and 13C MAS NMR, FT-IR and chemical analysis. As a result, two different structural models have been proposed for the two series of resins. The two resins have been pyrolyzed at 1400 °C and the resulting SiCO ceramics characterized by X-ray diffraction. It has been shown that the stability of the amorphous silica phase present in the SiCO ceramics is strongly influenced by the molecular organization of the starting precursors. The presence of siloxane rings in the cyclic-derived polysiloxane decreases the stability of the amorphous SiO 2 and promotes the crystallization of cristobalite.

Mineralogy and technical properties of clayey diatomites from north and central Greece

Abstract Two bulk samples of clayey diatomite of Upper Miocene age originated from Western Macedonia, northern Greece and Thessaly central Greece were examined for their efficiency to be used as industrial absorbents. The samples were characterized using X-Ray Diffraction, Thermo-Gravimetric and Fourier Transform Spectroscopy, Scanning Electron Microscopy and ICP-MS analytical methods. The absorption capability of the clayey samples in oil and water were also examined. The mineralogy of both samples is predominated by the presence of clay minerals and amorphous silica. The clay minerals prevailed in the Klidi (KL) bulk sample, with muscovite being the dominant phase, and kaolinite and chlorite occurring in minor amounts. In the Drimos (DR) bulk sample, vermiculite was the predominant clay phase. Smectite was not found in either sample, whereas detrital quartz and feldspars were present in significant amounts. The amorphous silica phase (opal-A) occurs mainly with the form of disck-shaped diatom frustules. The chemistry of the samples is characterized by the predominance of silica, alumina, and iron, whereas all the other major and the trace elements are in low concentrations. Both clayey diatomite rocks exhibited sufficiently good oil and water absorption capacity, ranging between 70 to 79% in the clay-rich sample KL and 64 to 70% in the opal-A-rich sample DR. Comparing the properties of the rocks studied with other commercial absorbents, it is concluded that they may find applications as absorbents in industrial uses.

Texture and Microstructure of Thermally-Treated Acid-Leached Kaolinitic Clays

Natural and modified kaolinitic clays were investigated by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM and EDS) and N2 adsorption techniques. Disordered, poorly crystalline kaolinite, containing some quartz and very little mica, was calcined at different temperatures and different times and then leached with 25 wt% sulphuric acid for 1 h at 98 °C. Calcination led to a reduction in both the specific surface area and the total pore volume due to dehydroxylation of kaolinite to meta-kaolinite (MK). However, subsequent leaching of the meta-kaolinite had a striking effect on the microporosity of the modified products. Thus, heating at 886 °C at 2 h (to yield MK2 886°C) and subsequent leaching for 1 h produced porous silica with a specific surface area of 233 m2/g and a total pore volume of 0.22 m2/g. The increase in the specific surface area (SBET) of the porous silica was due to the formation of micropores of 1.2–1.8 nm diameter and mesopores of 4.0–5.5 nm diameter inside the porous structure. The variation in the specific surface area and the porosity of acid-leached meta-kaolinite was found to depend not only on the calcination temperatures but also on the calcination time. Increasing the calcination time up to 6 h had a negative effect on the microporosity, attributed to a condensation reaction. The characteristics of the hysteresis loops in the corresponding adsorption/desorption isotherms indicated the formation of mainly slit-shaped pores. A broad band at 2θ = 21.8° in the XRD patterns of the modified samples was assigned to amorphous silica. The infrared spectra of acid-leached meta-kaolinite showed bands at 1095 cm−1 and 795–802 cm−1, respectively, which could be assigned to a three-dimensional amorphous silica phase.

The impact of sulfuric acid on element migration and mineralogy of compacted clays: pH 1.0 to −3.0

Abstract The transport of H2SO4 (at pH = 1.0, −1.0 and −3.0) through two mineralogically different compacted clays (Kc and Km) was examined using single-reservoir diffusion cells with constant source concentrations. At the end of the 216 day test period, geochemical analyses indicated increased depth of acid diffusion with increased reservoir acidity for both Kc and Km cells. Elevated Ca, Al, Fe and Si concentrations were associated with decreased pH values in all cells. XRD results showed that these elevated concentrations corresponded to the loss of carbonate and montmorillonite peaks and decreased peak intensities for illite and kaolinite in the Kc and Km pH −1.0 and −3.0 Km cells. Moreover, Si X-ray absorption near-edge structure (XANES) indicated dissolution of the phyllosilicate phases, a relative increase in the amount of quartz, and the potential formation of an amorphous silica phase. The results of this study showed that, despite the extreme pH values considered, movement of H2SO4 solutions with pH

High pressure oxidative acid leaching of nickel smelter slag: Characterization of feed and residue

Discarded slags have potential adverse environmental impact and valuable metal contents. High pressure oxidative acid leaching (HPOXAL) was employed to clean a slow-cooled copper–nickel flash smelter slag containing about 1% Cu and 1% Ni and selectively leach the contained base metal values. Characterization of the slag and leach residues was carried out using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and particle size analysis (PSA). It was found that the copper–nickel smelter slag was composed of major crystalline fayalite and minor amorphous silica phases with trace amounts of magnetite and entrapped matte, existing as uneven and finely dispersed particles, which are responsible for the loss of base metals. Most of the copper in the slag appeared as sulfide, while most of the nickel existed in the oxide form with a minor portion as sulfide. Cobalt and zinc phases were rarely detected because of their low content. During high pressure oxidative acid leaching, temperature and acidity were the key variables controlling the leaching of the base metals and iron. At 250 °C and 20% acid addition, > 99% Ni, Co and Cu could be extracted into solution together with 94% Zn and < 2.2% Fe. Iron and silicon were transformed into crystalline hematite and amorphous silica in the residue. By adjusting the acidity to control the rate of fayalite dissolution and the growth of precipitates, flower-like hematite particles associated with amorphous silica were produced. An excellent slurry filtration rate of 0.36 m 3·m − 2 ·h − 1 was obtained.

Preparation of TiO2/SiO2 composite fiber by thermal decomposition of polycarbosilane–tetrabutyl titanate hybrid precursor

A hybrid polymer as the precursor for TiO2/SiO2 composite fibers was prepared by blending polycarbosilane (PCS) and tetrabutyl titanate, named as PCST, which contains unreacted organic tetrabutyl titanate compound. The PCST was characterized by FT-IR, TGA and NMR (1H, 13C and 29Si). The results showed that the PCST could be described as a part dispersion of Ti(OC4H9)4 groups in the PCS chain matrix. The TiO2/SiO2 composite fibers were manufactured by melt-spinning PCST, maturing and curing the resulting fibers in air, followed by a final pyrolysis at 1200 °C in air. Based on the XRD, EPMA, EDS, SEM and HRTEM measurements, the TiO2/SiO2 fibers were made up of anatase-TiO2 nanocrystallites with a mean size of ∼10 nm and in the amorphous silica phase.

Geochemical and mineralogical impacts of H2SO4 on clays between pH 5.0 and −3.0

Natural and constructed clay liners are routinely used to contain waste and wastewater. The impact of acidic solutions on the geochemistry and mineralogy of clays has been widely investigated in relation to acid mine drainage systems at pH > 1.0. The impact of H2SO4 leachate characterized by pH < 1.0 and potentially negative pH values on the geochemistry and mineralogy of clays is, however, not clear. Thus, laboratory batch experiments were conducted on three natural clay samples with different mass ratios of smectite, illite and kaolinite to investigate the impact of H2SO4 on the geochemistry and mineralogy of aluminosilicates from pH 5.0 to −3.0. Batch testing was conducted at seven pH treatments (5.0, 3.0, 1.0, 0.0, −1.0, −2.0 and −3.0) using standardized H2SO4 solutions for four exposure periods (14, 90, 180, and 365 d). Aqueous geochemical and XRD analyses showed: increased dissolution of aluminosilicates with decreasing pH and increasing exposure period, that smectite was more susceptible to dissolution than illite and kaolinite, precipitation of an amorphous silica phase occurred at pH ⩽ 0.0, and anhydrite precipitated in Ca-rich clays at pH ⩽ −1.0. In addition, global dissolution rates were calculated for the clays and showed good agreement to literature smectite, illite and kaolinite dissolution rates, which suggests global dissolution rates for complex clays could be determined from monomineralic studies. A stepwise conceptual model of the impact of H2SO4 on aluminosilicate geochemistry and mineralogy between pH 5.0 and −3.0 is proposed.

Interaction of aqueous acidic-fluoride waste with natural tunisian soil

AbstractClayey soils are essential materials used to reduce hydraulic conductivity and pollutant migration, common at sites of waste disposal. This study investigates the possible use of a Tunisian soil as a lining material for disposal sites for acidic-fluoride wastes. A permeability test on a waste-solution sample (pH = 2.7) obtained from a disposal site in southern Tunisia was conducted over a period of about 2 years. The test results show that the permeability decreased with time until stabilized at 8.33 × 10−11 m/s. After the permeability test, the samples retrieved from the permeameter show a degradation state which varied with the thickness of the specimen. These samples can be classified into three zones (Z1: unaffected, Z2: moderately affected; and Z3: extensively affected). Physicochemical characterization of the three samples (Z1, Z2, and Z3), and of the original argillaceous soil, was by X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal and thermal gravimetric analysis, 29Si and 27Al nuclear magnetic resonance, and N2-adsorption techniques. The original sample consists essentially of palygorskite, kaolinite, and quartz. Sample Z3 underwent complete dissolution of kaolinite which supports the precipitation of fluoroaluminate and the appearance of an X-ray amorphous silica phase. In samples Z1 and Z2, the soil adsorbs fluoride at a rate of ∼68.5 mg/g and is highly resistant to acidic attack.