With the over-use of tetracycline (TC) and its ultimate accumulation in aquatic systems, the demand for TC removal from contaminated water is increasing due to its severe threat to public health. Clay minerals have attracted great attention as low-cost adsorbents for controlling water pollution. The objective of the present study was to measure the adsorption behavior and mechanisms of TC on allophane, a nanosized clay mineral with a hollow spherical structure; to highlight the advantage of the allophane nanostructure, a further objective was to compare allophane with halloysite and montmorillonite, which have nanostructures that differ from allophane. Structural features and surface physicochemical properties were characterized by transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), zeta potential, N2-physisorption, and acid–base titration. The adsorption data showed that TC adsorption followed the pseudo-second order and Langmuir models. The adsorption was pH dependent, as all three clay minerals performed better under neutral to weakly alkaline conditions and maintained high adsorption performance in the presence of co-existing Na+/K+/Ca2+/Mg2+ cations. Regeneration of the adsorbent was excellent, with efficiencies exceeding 75% after five recycles. By comparison, allophane always exhibited the greatest adsorption capacity, up to 796 mg g–1 at ~pH 9. The TC adsorption on allophane and halloysite was dominated by inner-sphere complexation, together with a small amount of electrostatic adsorption, while that on montmorillonite involved mainly interlayer cation exchange. The findings provide insights into the effects of nanostructures of clay minerals on their TC adsorption performance and highlight the huge potential of allophane as an efficient and inexpensive adsorbent for TC removal.

Differences were found in the differential thermal analysis curves and in the temperatures of new-phase development between allophanes of high (1.91–1.99) and low (1.47–1.53) SiO2/Al2O3 ratios. The endothermic peak due to continuous dehydration and dehydroxylation was at higher temperatures (153°-185°C) for allophanes with high SiO2/Al2O3 ratios and at lower temperatures (148°–165°C) for those with low SiO2/Al2O3 ratios. The temperature of the exothermic peak was lower and the height affected more by the exchangeable cation content for allophanes with high ratios than for those with low ratios. New phases did not develop in allophanes having high Si02/Al2O3 ratios even after they were heated to 1000°C, above the temperature of the exothermic peak. In contrast, a symptomatic development of new phases was noted in allophanes with low SiO2/Al2O3 ratios at 900°C, below the temperature of the exothermic peak. The effect of SiO2/Al2O3 ratio in the thermal behavior of allophane strongly suggests that differences in the structure are closely associated with the chemical composition of this material.

Samples containing allophane with molar Al/Si ratios from 1.0 to 2.0 have been examined by infrared spectroscopy, X-ray fluorescence, and phosphate adsorption methods. The infrared spectra of allophane with Al/Si ratios close to 2.0 showed that the wall of the allophane spherules is made up of imogolite structural units similar to “proto-imogolite.” X-ray fluorescence gave no clear evidence of Al in tetrahedral sites (AlIV), while pyridine adsorption results suggested that a small number of Bronsted acid sites (AlIVOH) are present in silica-rich allophanes. Lewis acid sites (AlH2O) are present in both silica-rich and alumina-rich allophanes. The results suggest that the framework for the allophane structure is an Al octahedral sheet.

Allophanes with Al/Si ratios close to 1.0 contain condensed silicate units either on the outside surface of the Al octahedral sheet, giving rise to a halloysite-like structure, or bonded on the inside surface of an imogolite-like structure. Allophanes with Al/Si ratios between 1.0 and 2.0 appear to be mixtures of the “proto-imogolite” structure and the allophane (Al/Si = 1.0) structure.

Phosphate in the form of organic compounds can be bound in soils containing the aluminosilicate allophane. A significant part of this phosphorus is believed to be present as nucleic acids. The interaction of yeast RNA with allophane was studied to further the understanding of the allophane/organic macro molecule interaction as well as the binding of organic phosphorus by allophanic soils. The extent of RNA adsorption on the allophane was dependent upon the pH, the charge and concentration of simple cations, the concentration of RNA, and the time of interaction. From a mixture containing 145 mg/liter RNA and 2.9 g/liter allophane in 10−2 M NaCl, the amount of RNA adsorbed increased from 6% at pH 10 to 98% at pH 3. The adsorption also increased as the concentration of added NaCl was increased from 10−4 M to 10−1 M, but only when the pH was greater than 5, i.e., above the isoelectric point of the clay. Mg2+ and Ca2+ were equally much more effective at promoting adsorption than Na+ at the same concentrations. There was no difference in the effectiveness of SO4−2, Cl−, or NO3− at pH 5 or higher. The adsorption isotherm at pH 7 can be described by the Langmuir equation; the apparent adsorption maximum was 38 mg/g. Van der Waals and simple electrostatic forces appear to dominate the interaction leading to the adsorption of RNA by allophane.

Allophane samples from soils, pumice, and stream beds have been studied by electron optical, infrared absorption, X-ray fluorescence, gas chromatography, and phosphate adsorption methods. The allophane particles were hollow spherules or polyhedra 35 and 50 Å in diameter with molar Al/Si ratios close to 2.0. The thickness of the wall of the spherules was estimated to be 7 Å. For the pumice allophanes, the wall was largely composed of imogolite structural units (OH)3Al2O3SiOH. Defects or pores were present in the wall and probably were the sites where phosphate was adsorbed. It is suggested that these allophanes with molar Al/Si ratios close to 2.0 should be called “proto-imogolite” allophane.

Two soil allophanes had a similar structure to the allophane from Stratford pumice, but small amounts of layer silicates, including halloysite, were also present in the soil samples, as indicated by infrared bands at 470, 1030, and 1100 cm−1. The allophane from the stream bed at Silica Springs had an infrared spectrum similar to feldspathoids, and it did not have the imogolite structure.

A procedure based on loss of weight after selective dissolution analysis (SDA) and washing with (NH4)2CO3 was developed for estimating the noncrystalline material content of soils derived from widely different parent materials. After extracting with 0.2 N ammonium-oxalate or boiling 0.5 N NaOH solutions, samples were washed with 1 N (NH4)2CO3 to remove excess dissolution agents and to prevent sample dispersion. The amount of noncrystalline material removed from the sample by the extracting solution was estimated by weighing the leached products dried to constant weight at 110°C. The results match closely with those obtained by chemical analyses of the dissolution product and assignment of the appropriate water. The proposed weight-loss method is less time-consuming than the chemical method, and no assumptions need be made concerning sample homogeneity or water content of the noncrystalline material.

Extractions of whole soil and dispersed clay fractions indicated that noncrystalline material determinations on the clay fractions underestimated the noncrystalline material content for whole soils from 0 to 34%. Acid ammonium oxalate was found to be a much more selective extractant for noncrystalline materials than NaOH.

Allophanes with SiO2/Al2O3 molar ratios from 1.38 to 1.92 were heated at temperatures up to 500°C, and the changes induced were investigated by means of infrared spectroscopy (IR) and X-ray powder diffraction (XRD). Heat treatment caused the IR absorption band near 1000 cm−1 due to Si-O stretching to shift towards higher frequencies, and the band near 450 cm−1 due to O-Si-O bending to increase markedly in intensity. These results are probably due to condensation of SiO tetrahedra, following breakdown of Si-O-Al linkages and dehydroxylation of SiOH groups. Concurrent intensity increase of the IR absorption at about 700 cm−1 suggested an increase in the amount of 4-coordinated Al in the heated materials. ‘Imogolite structures’ were decomposed progressively at higher temperatures and were almost absent at about 400°C, as indicated by the weakening and disappearance of the IR band at 348 cm−1 and of the XRD reflections at 2.25 and 1.40 Å. The XRD reflection at 3.3–3.45 Å shifted to about 3.6 Å, probably as a result of thermal condensation of the silica component. The observed thermal changes were much greater for samples with low SiO2Al2O3 ratios (1.38–1.51) than for those with high ratios (1.81–1.92), indicating a lower thermal stability for the former materials. Thus, the thermal stability of allophanes appears to be related to the content of ‘imogolite structures’ and to the polymerization status of the silica component.

The adsorption of sodium, chloride, and phosphate ions by allophane, imogolite, and halloysite has been studied in relation to the surface structure of the mineral samples. The high adsorption of phosphate (>200 μmole/g) and chloride (10–30 meq/100 g at pH 4) by allophane is ascribed to the small particle size of allophane, its high surface area (∼800 m2/g), and the presence at the surface of Al-OH-Al groups and defect sites. In contrast, halloysite has a relatively large particle size and a Si-O-Si surface. Accordingly, the adsorption of phosphate (5–10 μmole/g) and chloride (1 meq/100 g) by halloysite is very much lower as compared with allophane. Phosphate adsorption by halloysite is also related to particle morphology and the number of edge sites. Thus, a sample consisting entirely of spheroidal particles adsorbed only 5 μmole/g at a solution concentration of 1 × 10−4 M, whereas the tubular types of comparable surface area adsorbed 7–10 μmole/g at the same concentration. This is because spheroidal halloysite particles have few, if any, edge sites at which phosphate can adsorb. The relative degree of order and hydration of halloysite, as indicated by infrared spectroscopy, also affects phosphate adsorption. However, this factor is apparently less important than particle morphology and surface structure. Although imogolite also has an Al-OH-Al surface, it contains relatively few defect sites where phosphate can adsorb. Consequently, much less phosphate (120 μmole/g) was adsorbed as compared with allophane.

It is a high honor to be invited to give this first Brindley Memorial Lecture. I view it as taking the first step on a ladder, to be followed by suceeeding talks that climb higher to the pinnacle that George Brindley erected for us in clay mineralogy. If George were with us, he would be sitting on the front row, as usual, keeping the speakers and audience "honest" in our deliberations. In turn, I would be privileged to ask him personally to enlighten us with his valued opinion on the many questions I will be asking in this talk.

Small-angle X-ray powder diffraction analyses and high-resolution electron microscopy of allophane samples (SiO2/Al2O3 ratio, 1.12 to 1.68) showed that allophanes consist of nearly identical spherical particles with diameters of about 40 Å and retain their characteristic “hollow” spherical morphology at different ambient moisture and even after dehydroxylation by heating at 500° to 600°C. Unheated allophane samples gave another X-ray powder diffraction band whose maximum position varied from 12.3 to 14.5 A depending on their SiO2/Al2O3 ratio. The appearance of this band may denote some long-range ordering in the structure of allophane. Unlike the spherical particles of allophane, the tube unit of imogolite collapsed on dehydroxylation. This observation suggests that imogolite and allophane are different in their framework structures and that a Si- or Si(Al)-tetrahedral sheet rather than an Al-octahedral sheet constitutes the framework structure of allophane, irrespective of its SiO2/Al2O3 ratio.

Reactions between hydroxy-Al ions and orthosilicic acid as influenced by citric acid were studied at an initial Si concentration of 1.6 × 10−3 M, Si/Al molar ratios of 0.5 and 1.0, OH/Al molar ratios of 1.0-3.0, and citric acid/Al molar ratios of 0-0.3. In the absence of citric acid and at OH/A1 ratios of 1.0-2.8, imogolite (>0.01 μm) was dominant in the precipitates. At citric acid/Al ratios of 0.01-0.1, imogolite and/or pseudoboehmite were dominant in the precipitates at OH/Al ratios of 1.0 and 2.0, and imogolite and/or ill-defined aluminosilicate complexes at OH/Al ratio of 2.8. Instead of allophane or “proto-imogolite” allophane being the predominant species in the precipitates, the formation of ill-defined aluminosilicate complexes at OH/Al ratio of 3.0 was steadily promoted by increasing the solution citric acid/Al ratios from 0 to 0.3. The freeze-dried soluble products (<0.01 μ) ranged from silica gel to “proto-imogolite,” depending upon the basicity of Al and the level of citric acid of the parent solution. The amount of “proto-imogolite” increased with increasing citric acid/Al ratios from 0 to 0.1 in solution. Complexing low molecular weight organic acids, such as citric acid, impeded the formation of the short-range ordered aluminosilicates, allophanes and imogolite.

Adsorption of Cu2+ and Co2+ by synthetic imogolite, synthetic allophanes with a range of SiO2/ Al2O3 ratios, and allophanic clay fractions from volcanic ash soils was measured in an ionic medium of 0.05 M Ca(NO3)2. The effect of pH (and metal concentration) on adsorption was qualitatively similar for the synthetic and natural allophanes with relatively minor changes in behavior caused by variable SiO2/Al2O3 ratios. Cu and Co were chemisorbed by allophane at pH 5.0–5.5 and 6.9–7.2 (pH values for 50% adsorption level), respectively, with concomitant release of 1.6–1.9 protons/metal ion adsorbed. Quantitatively, adsorption by imogolite was less than that by the allophanes, presumably because of fewer sites available for chemisorption on the tubular structure of imogolite. Electron spin resonance studies of the imogolite and allophanes revealed that Cu2+ was adsorbed as a monomer on two types of surface sites. The preferred sites were likely adjacent AlOH groups binding Cu2+ by a binuclear mechanism; weaker bonding occurred at isolated AlOH or SiOH groups. These chemisorbed forms of Cu2+ were readily extracted by EDTA, CH3COOH, and metals capable of specific adsorption, but were not exchangeable. In addition, the H2O and/or OH− ligands of chemisorbed Cu2+ were readily displaced by NH3, with the formation of ternary Cu-ammonia-surface complexes.

Resilication of bauxite produced kaolin at and beneath an old erosion surface on bauxite at the Alabama Street Mine of ALCOA in Saline County, Arkansas. The transitional alteration can be traced in morphology by scanning electron microscopy, (SEM) and in Al:Si ratio by energy dispersive analysis. In one illustrated example, the sequence of resilication took place within 1 mm thickness; in another, across 80 mm. The first morphologic alteration of gibbsite (bauxite) appears to be to allophane that occurs in micrometer-size plates which show elongate cracking and/or straight to highly curved elongate edges. The next phase is kaolinite, first in micrometer-size flakes followed by coarser flakes that grade into a zone of typical stacked kaolinite, likewise identified by X-ray powder diffraction. Notably large stacks and small flakes of kaolinite are intimately mixed in the SEMs, thus suggesting that unequal sizes of kaolinite crystals can grow during one episode of an in-situ genesis.

Hollow spherical particles of aluminosilicate having diameters of 80 to 100 Å and high electric charge were synthesized by heating solutions containing Si and Al concentrations of about 2 mM (Si/Al molar ratio = 1.0–1.2; pH = 9.0) at 95°–100°C for 7 days. Electron microscopy, X-ray powder diffraction, and infrared spectroscopy suggested that the walls of the spherical particles consisted of monolayer aluminosilicate, most probably having a 1:1 layer structure. Chemical analysis supported this supposition but indicated modifications as shown in the formula (Si1.85Alo0.62)IV(Al1.38)VI(OH)4.10O4.96Na0.62, compared with (Si2)lv(Al2)VI(OH)4O5 for an ideal 1:1 layer silicate. High-resolution 29Si and 27A1 nuclear magnetic resonance spectroscopy indicated the substitution of Al for Si in the tetrahedral sheet, the presence of A1IV and A1VI, and the presence of Si bonded through oxygen to three Al atoms and one H. A natural counterpart of the synthesized aluminosilicate may be a precursor of halloysites formed by weathering of volcanic ash and pumice and have a close structural relation with allophane and imogolite

The size distribution of unit particles of two New Zealand allophanes (An and Rh), in dilute (0.8% w/v) aqueous suspensions, has been determined by small-angle neutron scattering (SANS). In addition, the specific surface area of the samples was measured by ethylene glycol retention, and their morphology examined by high-resolution transmission electron microscopy (HRTEM). The SANS data indicate that although both allophanes are somewhat polydisperse, the average diameter of their unit particles is significantly different, being 56 and 43 Å for allophane-An and allophane-Rh, respectively. Consistent with this observation, the specific surface area of allophane-Rh (897 mVg) is appreciably greater than that of allophane-An (638 m2/g). Under the electron microscope, both samples appear as aggregates of hollow spherules but HRTEM did not clearly distinguish between the two allophanes in that the largest population of spherules had diameters near 50 Å. Because of the assumptions and uncertainties involved in the SANS and surface area measurements, the data must be discussed in terms of their respective ratios. On this basis, the spherule diameter ratio is of the same order of magnitude as the inverse ratio of specific surface area. The latter value is also in reasonably good agreement with the corresponding ratios of phosphate adsorption capacity and BET nitrogen areas, derived from earlier studies.

Solutions containing AlCl3 and Si(OH)4 (concentrations ≤ 1.5 mM with molar Si:Al ratios of 1:2, 1:1 and 3:1) and FeCl2 (0, 0.5 and 1.0 mM) were adjusted to pH 8 with Ca(OH)2, and incubated at 23°C and 89°C without exclusion of air in the presence of CaCO3 for 8–12 weeks. The products were characterized by infrared spectroscopy and X-ray diffraction. Systems with 3:1 and 1:1 Si:Al ratios without Fe gave hydrous feldspathoids at 23° and 89°C. Systems with 3:1 Si:Al ratios containing Fe gave aluminous nontronites at 89°C and noncrystalline, nontronite-like products at 23°C. Systems with 1:1 Si:Al ratios with added Fe gave Fe(III)-substituted hydrous feldspathoids at 23°C. At 89°C, the system with 1:1 Si:Al ratios and 0.5 mM Fe produced a “protohalloysite,” while that with 1.0 mM Fe gave a poorly ordered nontronite-like layer silicate. In systems with 1:2 Si:Al ratios, the formation of “protoimogolite” at 23°C was little affected by additions of Fe. At 89°C, the “protoimogolite” decomposed to boehmite and poorly-ordered layer silicate phases. Inclusion of 1 mM MgCl2 in the above systems had no effect on the products at 23°C, but at 89°C produced saponites and a mixed layer saponite-chlorite in the 3:1 Si:Al systems, and saponite-like layer structures in the 1:1 and 1:2 Si:Al systems.

The negative surface charge of synthetic allophanes with a range of Si/Al ratios decreased and positive charge increased with increasing alumina content at a given pH. The phosphate adsorption capacity also increased with increasing Al content. That this relationship between composition and chemical reactivity was not found for the soil allophanes is attributed to the presence of specifically adsorbed organic or inorganic anions on the natural material. Both synthetic and natural imogolites had a much lower capacity to adsorb phosphate than the allophanes and adsorbed anomalously high amounts of Cl− and ClO4− at high pH. It is proposed that intercalation of salt occurs in imogolite, although electron spin resonance studies using spin probes failed to reveal the trapping of small organic molecules in imogolite tubes. These spin probes in the carboxylated form did, however, suggest an electrostatic retention of carboxylate by imogolite and a more specific adsorption by allophane involving ligand exchange of surface hydroxyl. The results illustrate the inherent differences in charge and surface properties of allophane and imogolite despite the common structural unit which the two minerals incorporate.

The surface and charge characteristics of 6 allophanic clays originating from a climatosequence of Andisols have been studied by titration, ion retention, surface area, and reactivity measurements. Several properties of these clays, such as the pH of zero charge, the magnitude of the surface area, and the reactivity of structural hydroxyl groups, appear to be governed by their chemical composition, especially by their silica content. A similar relationship between the above properties and silica content also exists for synthetic aluminosilicate gels (often quoted in the literature as possible models of allophanes), despite their different structure to the natural clays. An explanation is that as desilication proceeds, octahedral polymeric units of aluminum progressively individualize in both the synthetic and natural samples. The surface areas of the aUophanic clays, based on the slope of the titration curve at pH 7, are closely related to the amounts of structural hydroxyl that can react with fluoride at about the same pH. The permanent negative charges are always very low, even for silica-rich, allophane-bearing samples. It is therefore suggested that all of these clays should be virtually free of tetrahedral aluminum.