Allophane Samples Research Articles

Phosphate Adsorption Capacity of Allophane from Two Volcanic Mountains in Indonesia

Allophane is known as clay mineral with high capacity of phosphate adsorption via ligand-exchange mechanism. This study aims to compare the phosphate adsorption characteristics by allophane from Mt. Merapi and Mt. Lawu in relation to its chemical and mineralogical properties. The results of X-Ray Flourescence analysis shows that both allophane samples from Merapi and Lawu have low Si/Al ratio, i.e. 1.18 and 1.16, respectively. Infrared spectral characteristics of the allophane materials indicated that the main adsorption bands appeared at the range of 2700-3700 cm-1 (due to stretching vibration of all hydroxyl (OH) groups), 1400 – 1800 cm-1 (vibration of HOH deformation), and 650 - 1200 cm-1 (vibration between the Si-O-Al). Adsorption experiment of phosphate on allophane samples were conducted at initial adsorbate concentration of up to 2.0 mM and at pH 4.0 and 8.0. Phosphate adsorption capacity of allophane shows that both allophane from Merapi and Lawu are categorized as very high in adsorbing phosphate and fit well with the Langmuir adsorption equation. Phosphate adsorption increases with decreasing pH due to the positive charge sites such as Al-OH2+ in the allophane structure increase. Another reason is the negative charge of phosphate gradually decreases from -2 to -1 with decreasing pH, and the repulsive force between the negatively charged Si-O- in the allophane structure and phosphate anions decreases.

Kinetics, adsorption and desorption of Cd(II) and Cu(II) on natural allophane: Effect of iron oxide coating

Volcanic soils are a potential source of abundant and low-cost adsorbent materials. Allophane and iron oxides are the main inorganic constituents of Andisols, soils derived from volcanic-ashes abundant in the central and southern continental Chilean territory. The present study aimed to elucidate the effect of free iron oxides on the adsorption of Cd(II) and Cu(II) from aqueous media on natural allophane (NA). Allophane samples consisted of natural allophane, obtained from a Chilean Andisol series, with (NA-FeOx) and without (NA) an iron oxide coating. Material characterisation showed that NA-FeOx was formed by nanometric (100–500nm) sized spheres coated with an iron oxide layer that most likely consisted of a ferrihydrite-like mineral. On the other hand, NA consisted of aggregates (with particles as big as 1.0μm) mostly constituted by allophane. The specific surface area of NA-FeOx and NA were ca. 150 and 17m2g−1, respectively, and presented variable surface charge with an isoelectric point of 10.3. and 5.4. From the kinetic studies, equilibrium adsorption was achieved within 60min and was almost independent of the presence of the iron oxides. Among the kinetic models evaluated, the pseudo-second order model presented a better correlation with the experimental data. The adsorption studies revealed that NA has a greater adsorption capacity of both Cd+2 and Cu+2 compared to NA-FeOx. Langmuir and Freundlich adsorption models fit the experimental data well, suggesting that the adsorption process is a combination of physical and chemical phenomena. The Freundlich constant (adsorption coefficient, KF) values obtained confirmed the superior affinity of NA active surface sites compared to NA-FeOx. Desorption studies demonstrated that the majority of adsorbed cations were retained by the adsorbent, indicating irreversible chemical adsorption on specific active sites of the adsorbents.

Improvement of the NH2OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan

We evaluated the validity of Tessier’s method as applied to the extraction of manganese (Mn) and iron (Fe) oxides in Japanese Andisols and other soil types in Japan. Using the original Tessier’s extractant mixture, 0.04 mol L−1 hydroxylamine hydrochloride in 25% acetic acid (0.04 mol L–1 NH2OH-HCl in 25% HOAc), we found that substantial amounts of short-range-ordered Fe oxides were not extracted from allophanic Andisol samples and that considerable amounts of total Fe oxides were not extracted from all soil types. Relatively high extraction pH and large amounts of short-range-ordered Fe oxides in the Andisol samples might be responsible for incomplete extraction. Stoichiometric calculation indicated that the concentration of NH2OH-HCl might be insufficient for complete extraction of Fe oxides. The extracted amounts of Mn and Fe increased with increasing concentration of NH2OH-HCl in the extractant, and most of the Mn and Fe oxides in the soil samples, including samples with as much as 5.6% Fe, were extracted with 0.6 mol L–1 NH2OH–HCl in 25% HOAc. As judged from the simultaneous dissolution of aluminum (Al) and silicon (Si) minerals, extraction selectivity of Fe oxides with 0.6 mol L–1 NH2OH-HCl in 25% HOAc was comparable to that of the original Tessier’s method and better than that of a modified Community Bureau of Reference (BCR) sequential extraction procedure or a method using an extractant consisting of a mixture of oxalate and ascorbate, especially for Andisol samples.

Extractability of manganese and iron oxides in typical Japanese soils by 0.5 mol L−1 hydroxylamine hydrochloride (pH 1.5)

We investigated the extractability of manganese (Mn) and iron (Fe) oxides from typical Japanese soils (Entisols, Inceptisols, and Andisols) by 0.5 mol L−1 hydroxylamine hydrochloride (NH2OH-HCl) extraction (pH 1.5; 16 h shaking at 25°C; soil:solution ratio 1:40), referred as to HHmBCR, which is Step 2 (used for the reducible fraction) of the modified BCR (Community Bureau of Reference) sequential extraction procedure. The HHmBCR procedure extracted almost all Mn oxides from the non-Andisol samples, but failed to extract a part of the Mn oxides from some Andisol samples. The procedure extracted most short-range ordered Fe oxides from non-Andisol samples, but it extracted only 7.5% and 13% of the short-range ordered Fe oxides from allophanic and non-allophanic Andisol samples, respectively. This remarkably low extractability of Fe oxides suggests that the HHmBCR method is not suitable for extracting oxide-occluded heavy metals from Andisols. Since the extraction rate of short-range ordered Fe oxides from various soils with the extractant was negatively correlated with the amounts of oxalate- and pyrophosphate-extractable Al even when the variability of the extraction pH was reduced by increasing the soil:solution ratio from 1:40 to 1:500, the extractability of Fe oxides would be negatively affected by the presence of active Al, including allophane/imogolite, amorphous Al, and Al-humus complexes. Because these Al constituents are abundant in Andisols, they would be at least partially responsible for the lower extractability of Fe oxides by HHmBCR from Andisols.

Catalytic wet peroxide oxidation of phenol over iron or copper oxide-supported allophane clay materials: Influence of catalyst SiO2/Al2O3 ratio

Allophane clay materials with SiO2/Al2O3 ratios of 1.0 (AlSi1) and 2.2 (AlSi2) were synthesized by a co-precipitation route and further impregnated with iron or copper species. The structure of the parent AlSi1 sample is similar to that of a typical Al-rich soil allophane, while the parent AlSi2 material resembles the structure of a hydrous feldspathoid with a large interspherule surface, thereby exhibiting a high surface area. The ability of the various iron or copper-based allophane samples to behave as efficient and stable catalysts in phenol oxidation using H2O2 was investigated in ambient conditions for the first time. Their structural and textural properties were determined by X-ray diffractometry, N2 adsorption–desorption at 77K, electrophoretic mobility measurements, infrared spectroscopy, transmission and scanning electron microscopy, as well as by thermo-gravimetric analysis. The catalytic activity of the iron or copper oxide-supported allophanes was markedly influenced by their SiO2/Al2O3 ratio and by their respective structure. The iron-based AlSi2 catalysts with tail-like structure and high surface area proved to be far more active than their corresponding AlSi1 counterparts. The highest catalytic efficiency in terms of total organic carbon abatement was obtained at 40°C for the calcined iron oxide-supported AlSi2 allophane sample, for which very low leaching level of iron species was noticed (0.37mgL−1). By contrast, large differences in terms of catalytic efficiency (conversion rates) and stability were observed for the copper-based counterparts, thereby indicating that the iron oxide-supported allophanes with a hydrous feldspathoid structure are highly active and stable in the catalytic wet peroxide oxidation of phenol.

Boron Adsorption on Allophane with Nano-ball Morphology.

Two types of reaction scheme were proposed for boron (B) adsorption at equilibrium B concentrations of less than 2 mM on nano-ball shaped allophane with its fundamental structure of aluminum orthosilicate. (i) Al-OH + B(OH) 3 → Al-O-B(OH) 2 + H 2 O (ii) Al-OH + B(OH) 4 - → Al-(OH)-B(OH) 3 + OH - With the aluminol groups (Al-OH) included in the structure, the reaction (i) occurs below equilibrium pH of about 10, whereas the reaction (ii) does at equilibrium pH of higher than about 8. Calculated Langmuir parameters suggested that the aluminol groups of nano-ball allophane had higher affinity for B(OH) 4 - than B(OH) 3 . Because the content of the aluminol groups increase with decreasing Si/Al ratio of allophane, B adsorption maxima were higher for KyP sample (Si/Al=1.34:2) than for KnP sample (Si/Al =1.98:2) irrespective of equilibrium pH. Release of Si from allophane samples during B adsorption on them was much smaller than the case of phosphate P) adsorption previously studied. This indicates that B has less ability than P for replacing Si in the structure of nanoball allophane and the main B adsorption site is the aluminol group at the pore of the allophane nano-ball.

CHANGE IN SURFACE CHARGE PROPERTIES OF NANO-BALL ALLOPHANE AS INFLUENCED BY SULFATE ADSORPTION

Change in surface charge of nano-ball allophane after sulfate adsorption was studied. Cation exchange capacity (CEC) was found to increase with increasing pH as well as after sulfate adsorption, with KnP (allophane sample with high Si/Al ratio) having higher CEC value as compared to KyP (low Si/Al ratio). The increase in CEC after adsorption for both samples was attributed to the charge carried by the anion. However, plotting the increase in CEC vs. the amount of sulfate adsorbed showed a slope of much less than 1, which showed that most of the sulfate adsorption occurred as bidentate reaction. This can be attributed to the adsorption mechanism with sulfate being adsorbed through ligand exchange aside from anion exchange reaction. Further, the adsorption of sulfate on allophane enhanced the deprotonation of the near silanol group, thus providing a new negative charge, thus an increase in the CEC. Molecular orbital calculation using MOPAC program affirmed these experimental results. The amount of positive charge or AEC on the other hand was found to decrease after sulfate adsorption for both KyP and KnP samples. The decrease in AEC was much lower than the sulfate adsorption, although more sulfate than AEC was adsorbed. This implies that some AEC site remains after much sulfate adsorption, providing nutrients such as Cl - and NO 3 - for plants.

ADSORPTION MECHANISMS OF COPPER AND ZINC ON NANO-BALL ALLOPHANE

Copper and zinc ions were strongly attracted to dissociated silanol groups at the inner surface of hollow spherical nano-ball allophane particle. Examination of adsorption isotherms of copper on allophane by the Langmuir theory indicated that the copper adsorption increased with the increasing bulk solution pH, and was higher for the allophane sample with a higher Si/Al ratio. The copper adsorption was greater than zinc adsorption previously reported with the same samples, and in both cases, solution pH decreased with the adsorptions. The copper and zinc adsorption on O atoms of the silanol groups are so strong that some of Si was released probably as Si-Cu compounds. Molecular orbital calculations with MOPAC AMI basis set indicated that the copper and zinc ions (Cu 2+ , CuOH + , Zn 2+ and ZnOH + ) could adsorb not only on the dissociated silanol groups, but also with the undissociated silanol groups. Bond distances between the heavy metal atoms and O atom of the silanol groups were shorter for copper than for zinc, in agreement with experimental results. The calculations also indicated that, when the heavy metal ions interacted with undissociated silanol groups, dissociation reaction of the silanol groups accelerated. These are due to bonding formation between heavy metal and O atom of the silanol groups, and coincide with the observed pH decrease.

Allophane compared with other sorbent minerals for the removal of fluoride from water with particular focus on a mineable Ecuadorian allophane

Worldwide F − contaminated groundwaters pose a serious health problem. For purification of these waters different adsorber materials are used such as activated aluminium oxide, iron oxyhydroxide (e.g. GEH®), or synthetic OH-Apatite (e.g. Fluorolith®). In some regions, however, inexpensive solutions are required motivating scientists to study the F − sorption of natural materials such as clays. The clay mineral allophane possesses a high affinity towards F −. In the present study different allophane samples are compared with other clay minerals as well as with Fluorolith®, a technically produced OH-apatite that is most commonly used in practice. Results obtained confirmed that allophane has the highest F − sorption capacity compared with kaolinite and goethite. The F − sorption capacity of some allophane samples is similar to the well-established F − adsorber material Fluorolith® and ranges from 3 to 5 mg F −/g. Sorption depends on the experimental conditions, particularly the pH value (present study: 8–9). The different sorption capacity of the different allophanes could be explained by the amount of aluminol groups, which is independent of the specific surface area. The comparison of the fluoride uptake plots of allophanes with those of Fluorolith®, however, proved that Fluorolith® is particularly advantageous at low F − concentrations suggesting a higher affinity of the Fluorolith® surface towards F −. Sorption kinetics, however, were similar. Fluorolith® adsorbs F − by exchange of the structural anion OH −. The comparison of the uptake plots, however, suggests that different reactions have to be considered in the case of allophane. The sorption mechanism of fluoride on allophane could not be characterized further.

Nanometer-scale chemical modification of nano-ball allophane

AbstractNano-ball allophane is a hydrous Al silicate with a hollow-sphere morphology that contains some defects or pores along the spherule walls. Enlargement of the pore openings by dilute alkali treatment was confirmed by cation exchange capacity determinations using various alkylammonium cations as replacement cations. An allophane sample with a low Si/Al ratio (0.67) was equilibrated with 10 mM CaCl2 (pH = 6.0) and the Ca2+ retained was extracted using aqueous 1 M NH4C1 or alkylammonium chloride salts. The Ca2+ extracted by NH4+ was 15.1 cmolc kg−1, but CH3NH3+${\rm{C}}{{\rm{H}}_3}{\rm{NH}}_3^ + $ (mean diameter = 0.38 nm) only extracted 7.9 cmolc kg−1 of Ca2+. After 10 mM NaOH treatment (0.25 g:100 mL) of the allophane, the Ca2+ extracted by NH4+${\rm{NH}}_4^ + $ was 29.7 cmolc kg−1, 29.6 cmolc kg−1 by CH3NH3+${\rm{C}}{{\rm{H}}_3}{\rm{NH}}_3^ + $, and 29.4 cmolc kg−1 by (CH3)2NH2+${{\rm{(C}}{{\rm{H}}_3}{\rm{)}}_2}{\rm{NH}}_2^ + $. The extraction of Ca2+ by the large C2H5NH3+${{\rm{C}}_2}{{\rm{H}}_5}{\rm{NH}}_3^ + $ cation (mean diameter = 0.46 nm) only decreased to 26.1 cmolc kg−1, indicating that pore diameters were enlarged from ∼0.35 to 0.45 nm. The significant increase in Ca2+ retention after NaOH treatment was attributed to the dissociation of increased numbers of newly exposed silanol groups in the enlarged pores. The low Si/Al ratio of the NaOH-dissolved material (0.35) and the decreased intensity of the 348 cm−1 IR band also suggested selective dissolution of the pore region. For allophane with a high Si/Al ratio (0.99) and much accessory polymeric Si, dissolution of polymeric Si and of the pore region occurred simultaneously. Alkali treatment produced a smaller increase in pore size and Ca2+ retention for allophanes with large Si/Al ratios than for allophanes with small Si/Al ratios. It was concluded that by altering the dilute alkali treatment conditions and varying the Si/Al ratio of allophane, the extent of structural modification or pore enlargement of the hollow spheres might be controlled.

Soil organic matter chemistry in allophanic soils: a pyrolysis‐GC/MS study of a Costa Rican Andosol catena

SummarySoil organic matter (SOM) in allophanic soils is supposed to accumulate due to protection caused by binding to allophane, aluminium and iron. We investigated a catena of allophanic and non‐allophanic soils in Costa Rica to determine the effect of such binding mechanisms on SOM chemistry. These soils contain no contribution of black carbon. Molecular characterization of litter, extractable and dispersed organic matter was done by Curie‐point pyrolysis‐GC/MS. The molecular chemistry of the organic fractions indicates a strong decomposition of plant‐derived organic matter and a strong contribution of microbial sugars and N‐compounds to SOM. Both the decomposition of plant‐derived SOM – including that of relatively recalcitrant compounds – and the relative contribution of microbial SOM were greater in allophanic samples than in non‐allophanic ones. This suggests that chemical protection does not act on primary OM, although it may influence the accumulation of secondary OM in these soils. The effect of allophane on SOM contents in such perhumid soils is probably through incorporation of decomposition products and microbial SOM in very fine aggregates that – in a perhumid environment – remain saturated with water during much of the year. Greater concentrations of aliphatics are found in allophanic residues, but there is no evidence of any specific mineral‐organic binding. The results do not support the existing theory of chemical protection of plant‐derived components through binding to allophane, iron and aluminium.

Application of the DLCA model to “natural” gels: The allophanic soils

Volcanic soil comprises weathering products such as allophane, originating from a leaching process of volcanic ashes and glasses. These soils are interesting in terms of mitigation of the greenhouse effect (C sequestration), because they are known for accumulating more C than non-volcanic soils. Allophanes are natural amorphous silicates and have physical features very close to those of synthetic gels. Knowledge of the allophanic soil structure is required to understand the sequestration mechanism. In this paper, nitrogen adsorption-desorption experiments, measured on allophanic soil samples, show that the hydraulic diameter (Dh) is shifted towards smaller size while the pore volume (Vp) and specific surface area (S) increase, when the allophane content of the soil increases. We introduce a numerical model to simulate the structure of this “natural gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation in which larger particles hinder the DLCA. As a function of the relative content of allophane (gel) and larger particles, the textural properties (Vp, S, Dh) of the different simulated structure are calculated using a simple triangulation method.

ADSORPTION OF WATER ON NANO-BALL ALLOPHANE AS AFFECTED BY HEAT TREATMENT

Adsorption of water is a one of important physicochemical properties of clays, and allophane has extremely high water adsorption capacity due to its high specific surface. The water vapor adsorption of two allophane samples with low and high Si/A1 ratios (0.67: KyP; 0.99: KnP), under various relative humidities (RHs), decreased with preheating treatment up to 400°C for 2 h. The decrease in water adsorption at monolayer level (RH = 0.45) was greater for KnP sample than for KyP sample, whereas the decrease in water adsorption due to capillary condensation between allophane unit particles (RH = 0.6) was greater for KyP sample. These indicate that allophane hollow spheric particles in KyP sample were directly connected each other with the preheating, but those in KnP sample were not. The difference in the way of bonding between unit particles in the two types of allophane was ascribed to the presence of polymerized silicate 'tails' exposed outside of hollow spheric allophane particles. In the KnP with higher Si/Al ratio, many 'tails' are exposed, and the 'tails' might prevented direct attachment between particles by forming connection between the 'tails'. The presence of the 'tails' in allophane with higher Si/Al ratio was also related to dispersion property of allophane of this type.

Mutual Interactions of Sulfate, Oxalate, Citrate, and Phosphate on Synthetic and Natural Allophanes

In soil environments, particularly at the soil–plant interface, organic and inorganic ligands may compete with each other for sorption sites onto soil components. The interaction between two or more ligands has a great importance for understanding the adsorption/desorption processes of a given ligand in soil. We studied the mutual interaction of sulfate (SO4), oxalate (OX), citrate (CIT), and phosphate (PO4) in binary and ternary systems on synthetic and natural allophanic samples at different pH values (4.0–7.0). The adsorption capacity of SO4, CIT, and OX (in the order listed) was much lower than that of PO4, but CIT decreased the zero point charge (ZPC) more than OX, meanwhile PO4 increased the ZPC, whereas SO4 did not change the ZPC of the sorbents. The adsorption of SO4 was strongly inhibited in the presence of equimolar amounts of OX, CIT, or PO4 even at low surface coverage of all the ligands. The lower the final pH of the binary systems, the greater the inhibition of SO4 adsorption. The capacity of CIT to prevent SO4 adsorption (ranging from 22–63%) was similar or slightly greater than that of PO4 (ranging from 25–48%) but clearly higher than OX (ranging from 20–42%). On the contrary, SO4 very poorly prevented the adsorption of the other ligands (usually lower than 10%). The effectiveness of the other ligands in retarding or inhibiting SO4 adsorption on an Andisol sample containing 40% allophane decreased with increases in the reaction time. However, even after 15 d of reaction CIT (38%) prevented SO4 adsorption more than PO4 (27%) or OX (12%) did. Our results seem to demonstrate that changes in the electric potential of the surfaces after the addition of anions, as well as competition for adsorption sites, affected the reduction in adsorption of SO4 and the other ligands. However, competition in adsorption with SO4 for common adsorption sites was greater for the organic ligands than for PO4, because the surface coverage of PO4 was particularly low with respect to that of SO4, CIT, and OX.

Structural Changes of Allophane During Purification Procedures as Determined by Solid-State 27Al and 29Si NMR

AbstractAllophanes are poorly crystalline and quasi-stable aluminosilicate minerals, the structures of which are sensitive to chemical treatment. In the present study, solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectra of allophane samples were monitored as they went through several purification procedures. It was confirmed that no significant structural changes were caused by boiling with 6% H2O2 to remove organic matter, by size fractionation (sonification), by sedimentation, by precipitation at pH 4.0, or by dithionite-citrate-bicarbonate treatment for the removal of Fe (hydr)oxides. Hot 5% Na2CO3 treatment for the removal of reactive silica-alumina gels and adsorbed citrate from allophane samples, however, decreased signal intensity corresponding to imogolite-like Si (Q33VIAl, −78 ppm in 29Si NMR) and increased signal intensities corresponding to IVAl (55 ppm in 27Al NMR) and possibly X-ray amorphous aluminosilicates (centered at −85 ppm in 29Si NMR). Cold (room temperature) 5% Na2CO3 treatment for 16 h proved to be effective in avoiding these structural changes.

Adsorption and discrimination of alanine and alanyl-alanine enantiomers by allophane

AbstractWe have investigated the adsorption of D- and L-alanine, and their respective dimers, by three allophanes with different Al/Si ratios. The Kanuma (Al/Si = 1.2) and Kitakami (Al/ Si = 1.5) allophanes were from Japan whereas the Te Kuiti (Al/Si = 1.6) sample came from New Zealand. The three allophanes differed in their capacity for adsorbing alanine but none of the samples showed a clear preference for either the D- or the L-form. In the case of alanyl-alanine both Kanuma and Kitakami allophanes gave a hint of preferring the L- to the D-enantiomer. On the other hand, the allophane sample from Te Kuiti showed a clear preference for L-alanyl-L-alanine. In keeping with this observation, circular dichroic spectrometry indicated that solutions of a racemic mixture of alanyl-alanine, after equilibration with Te Kuiti allophane, became relatively enriched in the Denantiomer. The size, intra-molecular charge separation, and surface orientation, of L-alanyl-Lalanine zwitterions apparently combine to confer ‘structural chirality’ to the complex (adduct) with Te Kuiti allophane. As a result, the mineral-organic complex develops a preference for the L-form of alanyl-alanine. This finding points to a possible role of certain allophanes in the origin of biochirality.

ADSORPTION OF SULFATE AND NITRATE ON NANO-BALL ALLOPHANE

The adsorption of sulfate and nitrate on two allophane samples, with high and low Si/Al ratio, was found to decrease with increasing pH. The highest adsorption of sulfate and nitrate was obtained at initial pH 3 for both the allophane samples. No adsorption of nitrate was observed at equilibrium pH above 7, however, sulfate was still adsorbed. The strong adsorption of sulfate at pH about below 7 could be attributed to ligand exchange reaction, whereas nitrate was non-specifically adsorbed throughout the pH examined. Allophane sample with low Si/Al ratio or KyP adsorbed more sulfate and nitrate than with the high Si/Al ratio sample or KnP. This is because KyP has high aluminol groups (Al-OH, Al-OH 2 ) per unit mass than KnP. The aluminol groups are believed to be responsible for the adsorption of sulfate and nitrate on nano-ball allophane. Both sulfate and nitrate adsorption results showed a better fitness with the Freundlich adsorption equation with sulfate having a higher k and n values as compared to nitrate which implies that sulfate was strongly adsorbed on nano-ball allophane as compared to nitrate.

Adsorptive mechanisms of orthosilicic acid on nano-ball allophane.

Adsorption of orthosilicic acid on two low Si/Al (KyP and KiP) and one high Si/Al (KnP) nano-ball allophane samples was found to be controlled by the species type of the acid. There seemed to be a higher preference for the anion species to the neutral species. Adsorption increased with increasing solution pH with the highest adsorption occurring at initial pH 10. Further increase in pH, however, led to reduced levels in adsorption, particularly in the KnP sample due to repulsion between the negatively charged silanol groups and the anion species of the acid. Adsorption was higher in the low Si/Al samples due to their relatively higher number of aluminol groups per unit mass. The estimated Zero Point of Silicon Adsorption, which was pH dependent, was found to be higher in the KnP sample due to its higher inherent Si content. Adsorption data conformed only to the Freundlich model with the Freundlich parameters increasing with increasing solution pH to a maximum at initial pH 10. Release of Al occurred only at pH 10 and was highly correlated with Si adsorption in the low Si/Al samples suggesting a partial replacement of Al with Si at the pore region.

Allophane-like materials in the weathered zones of Silurian phosphate-rich veins from Santa Creu d'Olorda (Barcelona, Spain)

AbstractAllophane-like materials occur in the weathered zones of the phosphate-rich veins hosted in Silurian metasediments of the Catalonian Coastal Ranges. These metasediments also host sulphide and phosphate sedimentary mineralizations. Mineralogical and geochemical investigations of the allophanic samples indicate that they comprise Si-rich allophane, with a molar SiO2/Al2O3 ratio ranging between 1.19 and 2.23, with amorphous Al-(Ca) phosphate and hydroxylapatite as major minerals, and minor goethite and quartz. It is assumed that allophane, amorphous Al-(Ca) phosphate and hydroxylapatite come from the reaction of acid solutions, released during the weathering of sulphide interbedded in black shales, with phosphate-rich veins and volcano- sedimentary host rocks. Silica-alumina gels were deposited in fissures and cavities left by a previous dissolution of the phosphate-rich veins. Later the phosphate minerals filled the conchoidal microfractures and shrinkage microcracks of the allophane.

Adsorption of molybdate on nano-ball allophane.

Adsorption behavior of Mo (Na 2 MoO 4 ) on three nano-ball shaped allophane samples was different between lower (up to 0.1 mM) and higher (0.2 to 1.6 mM) Mo concentrations. The difference may be due to chemical species of Mo in solution: at lower concentration, Mo exists as monomeric H 2 MoO 4 , HMoO 4 - and MoO 4 2- , and at higher concentrations as polymeric forms. The monomeric molybdate species, especially the anionic species, had very strong affinity for the allophane at lower pH where the allophane had much surface positive charge and essentially no negative charge. All of the molybdate added at equilibrium pH 3.7 to 3.9 and initial concentration up to 0.1 mM, 200 mmol kg -1 , was adsorbed on allophane samples with lower Si/Al ratio. With increasing pH, the affinity or amount of adsorption decreased markedly. At equilibrium pH near 5, adsorption isotherm was Langmuir type, whereas at higher pH such as 6 or 8 the isotherm was linear or Freundlich type. The strong interaction between molybdate anion and positively charged allophane was concluded as electrostatic one followed by ligand exchange reaction between Mo-O - and aluminol groups. At higher Mo concentrations, more than 0.2 mM, shape of the isotherms were linear indicating further polymerization reaction on the allophane surfaces, and partial destruction of allophane structure was observed as Si and Al release with the adsorption.