Hydroxyaluminosilicate Research Articles

Precipitation of short-range order hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) at ambient temperature: Insights into mineral formation pathways, crystal chemistry and solubility-stability relationships

Chemical weathering of silicates on continents and the subsequent formation of clay minerals are important processes within the Earth's critical zone, controlling pH, water-holding capacity and ion exchange properties of soils. Short-range ordered (SRO) hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) phases, such as allophane (∼Al2O3(SiO2)1.3–2·2.5-3H2O) and hisingerite (∼Fe23+Si2O5(OH)4∙2H2O), are such common soil clays, but their crystal-chemical properties, solubilities and formation paths remain disputed. In this study, pure HAS and HFS phases were precipitated at molar [Al]aq/[Si]aq and [Fe]aq/[Si]aq ratios of 1.0, 1.3, 1.5 and 2.0 and ambient temperature using equilibrium-approaching experiments. The formation of HAS-HFS minerals was studied at [(Al + Fe)]aq/[Si]aq = 1 using replacement levels of [Fe]aq for [Al]aq of 10%, 25%, 50%, 75% and 90%. HAS, HFS and HAS-HFS minerals were formed at pH ∼3–6 through condensation of silica tetrahedrons onto Al/Fe-O-OH octahedral templates. The [Al]s/[Si]s, [Fe]s/[Si]s and [(Al + Fe)]s/[Si]s ratios of the precipitated SRO phases ranged from 0.7 for HAS and 0.7–1.0 for HAS-HFS to 1.0–1.3 for HFS minerals, and correlate linearly with the values of the solubility constants (pK) obtained herein and from literature as follows:pKHAS=2.9·Als/Sis+7.9r2=0.96n=6pKHAS−HFS=−23.2·Al+Fes/Sis+24.8r2=0.94n=5pKHFS=23.5·Fes/Sis–26.3r2=0.86n=4The faster formation kinetics and lower solubility of HFS phases (pK = −2.2 to 4.7) and HAS-HFS phases (pK = −1.0 to 6.0) compared to HAS phases (pK = 10.2 ± 0.3) suggests that hisingerite-like and Fe-substituted allophane-like minerals are probably more abundant in the Earth's critical zone than previously thought, thus providing highly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite group minerals.

Kaolinization of 2:1 type clay minerals with different swelling properties

Abstract Kaolinization of 2:1 type clay minerals commonly occurs in the supergene environments of the Earth, which plays critical roles in many geochemical and environmental processes. However, the transformation mechanism involved and the specific behavior of 2:1 type swelling and non-swelling clay minerals during kaolinization remain poorly understood. In this study, laboratory experiments on the kaolinization of montmorillonite (swelling), illite (non-swelling), and rectorite (partially swelling) were carried out to investigate the kaolinization mechanism of 2:1 type clay minerals and to evaluate whether swelling and non-swelling layers of 2:1 type clay minerals perform differently or not in their kaolinization processes. The results show that montmorillonite, illite, and rectorite in acidic Al3+-containing solutions can be transformed into kaolinite, whereas such transformation is hard to take place in Al3+-free solutions. Part of the Al3+ in the solutions was exchanged into the interlayer spaces of swelling clay minerals at the early stage and resulted in the formation of hydroxy-aluminosilicate (HAS) interlayers, but they show no influence on the transformation process. Interstratified kaolinite-smectite (K-S), kaolinite-illite (K-I), and kaolinite-rectorite (K-R) formed as the intermediate phases during the transformations of the three different precursor minerals, respectively. The results obtained in this study demonstrate that 2:1 type clay minerals, including both swelling and non-swelling ones, can be transformed into kaolinite via a local dissolution-crystallization mechanism, which starts mainly from the layer edges rather than the basal surfaces. Due to different dissolution rates from domain to domain within a precursor mineral particle, the layers with a low dissolution rate become “splints,” while the dissolved elements are concentrated between two “splints,” leading to precipitation of kaolinite along the basal surfaces of precursor minerals. The size and stacking order of the newly formed kaolinite strongly depend on the morphology and property of the precursor minerals. These findings not only are of importance for better understanding the transformation procedures between different clay minerals and the mechanisms involved but also provide new insights for well understanding mineral-water interactions that are central to all geochemical processes.

Aspalathus linearis(Rooibos tea) as potential phytoremediation agent: a review on tolerance mechanisms for aluminum uptake

Aspalathus linearis (Burm. F.) R. Dahlg., commonly referred to as Rooibos tea, grows naturally in nutrient-poor, sandy, acidic soils (pH 3–5.3) with high aluminum concentration ranging from 110 to 275 μg Al g−1in the Cederberg’s mountainous areas in South Africa. Earlier studies found significant differences in Al concentration in organs of A. linearis, with roots having higher amounts (1262–4078 μg Al g−1), suggesting that the plant is capable of accumulating excess Al in acidic soils. Identification of the mineralogical constituents of organs of A. linearis using X-ray diffraction (XRD) analysis revealed the presence of an Al–Si complex (aluminosilicate or hydroxyaluminosilicate (HAS) species) in the shoot and root, possibly to internally ameliorate Al toxicity. In addition, A. linearis has specialized cluster roots that exude Al-chelating organic acid ligands such as citric, malic, and malonic acids. Organic acids can bind strongly to Al in the plant and rhizosphere to reverse its phytotoxic effects to the plants. Field and glasshouse studies revealed significant differences in pH between rhizosphere and nonrhizosphere soils of A. linearis and also showed that roots of the plant release OH−and HCO3−anions to raise rhizosphere pH possibly to immobilize Al through complexation. Furthermore, A. linearis is easily infected by arbuscular mycorrhizae (AM) fungi, but mycorrhizal associations are known to inhibit transport of metallic cations into plant roots. These features of A. linearis are perceived as good indicators for bioremediation; and the plant could, therefore, be a suitable candidate for phytoremediation technologies such as phytoaccumulation, phytostabilization, and phytodegradation. The environmental and economic implications of the potential of A. linearis to bioremediate Al-contaminated soils are briefly discussed. Furthermore, this review briefly highlights future studies investigating the utilization of the shoot of A. linearis as adsorbent for the removal of trace and (or) heavy metal from aqueous solutions.

Biogenic and pedogenic controls on Si distributions and cycling in grasslands of the Santa Cruz soil chronosequence, California

Biogenic and pedogenic processes control silica cycling in grasslands growing on a soil chronosequence and dominated by strong seasonal variabilities of a Mediterranean climate. Shallow pore water Si, in spite of significant annual uptake and release by plant growth and dieback, exhibits only moderate seasonal fluctuations reflecting strong buffering from labile biogenic Si, dominated by phytoliths and by secondary pedogenic silicates. Long phytolith residence times (340–900yrs) reflect the seasonally dry climate and high solute Si concentrations. Water-extractable Si is closely associated with Al, indicating seasonal precipitation and dissolution of a highly labile 1:1 hydroxyaluminosilicate (HAS), probably allophane, which transforms in deeper soil into fine grained, poorly crystalline kaolinite. Shallow plant roots extract greater proportions of biogenic Si and deeper plant roots larger amounts pedogenic Si.High pore water Ge/Si in late winter and spring reflects the reinforcing effects of plant fractionation and concurrent dissolution of Ge-enriched HAS. The same processes produce pore waters with depleted 30Si/28Si. In the summer and fall, Ge/Si declines and 30Si/28Si increases, reflecting the cessation of plant uptake, continued dissolution of soil phytoliths and re-precipitation of less soluble HAS. Si inputs from weathering (2–90mmolm−2yr−1) and losses from pore water discharge (18–68mMm−2yr−1) are comparable for individual soils, decline with soil age and are significantly less than amounts of Si annual cycled through the vegetation (42–171mMm−2yr−1). Mobile Si is generally balanced in the soils with upward bio-pumping by the shallow-rooted grasses efficiently competing against downward leaching and pore water discharge. Small net annual increases in Si in the present day soils could not have been maintained over the time scale represented by the chronosequence (65–225yrs), implying past changes in environmental conditions.

SPECIATION OF ALUMINUM IN HYDROXYALUMINUM AND HYDROXYALUMINOSILICATE IONS FIXED BY MONTMORILLONITE, USING 27Al-NMR AND ICP-AES

We tried to elucidate the chemical species of hydroxyaluminum (HyA) and hydroxyaluminosilicate (HAS) ions intercalated into the Mt focusing on the Al by means of 27 Al-NMR, inductively coupled plasma atomic emission spectroscopy (ICP-AES), differential thermal analysis (DTA), x-ray diffraction (XRD), and fourier-transform infrared (FTIR). Using 27 Al-NMR and ICP-AES, the Al in a Al-OH-Si(OH) 4 mixture was separated into three fractions; Al 13 , Al SYM , and Al NON . The Al 13 and Al SYM were quantitatively determined by 27 Al-NMR at 62.5 and 0 ppm, respectively. The Al NON was defined as the Al which can be determined by ICP-AES but not by 27 Al-NMR. The Al SYM was attributed to electrically-symmetric octahedral Al including monomer (and dimer) HyA ions, and the Al NON was attributed to electrically-asymmetric Al including HAS and/or polymer HyA ions. Among the HAS and HyA solutions prepared, the proportion of Al 13 ion was higher in the smaller Si/Al ratio. The HAS solutions with the Si/Al ratio greater than 0.24 contained no Al 13 ion. As result of adsorption kinetics of Al 13 , Al SYM , Al NON , and orthosilicic acid in three solutions on the montmorillonite (Mt), most of the adsorption reaction occurred within I min., and Al 13 and HAS ions were selectively adsorbed by Mt compared with the Al SYM . The HyA-Mt complex showed a great expansion in the d-spacing compared with Mt, and the d-spacing of the HyA-Mt was 1.64 nm and its peak was broad. On the other hand, the strongest peak of the HAS-Mt was mostly at around 1.80 nm, irrespective of the Si/Al ratios of the HAS ions used. Intercalation of HAS ions may occur completely since the enlargement of the d-spacing was clearly seen by XRD and no specific absorption bands were not seen by FTIR. Most of the thermal characteristics of the Mt would be modified by the intercalated HAS ions with a higher Si/Al ratio greater than 0.05. These results indicate that, upon the intercalation of Mt, the outer surface properties detectable by the FTIR were not modified greatly, while the internal surface structure of the complex detected by XRD and DTA analyses was modified greatly. Al 1 3 ions could not be fixed in the interlayer space of the Mt, and most of the changes was brought by the fixation of HAS ions in the internal space of Mt.

Heavy Metal Removal from Water by Adsorption Using Pillared Montmorillonite

Removal of Cu2+, Cr3+ and Cd2+ from aqueous solutions by adsorption on montmorillonite modified by sodium dodecylsulfate (SDS) and hydroxy-alumino-silicate (HAS) was investigated. Experiments were carried out as a function of solution pH, solute concentration, and time. The Langmuir model was adopted to describe the single-solute adsorption isotherm, in which the Langmuir parameters were directly taken from those obtained in single-solute systems. The kinetics of metal ions adsorption was examined and the pseudo-first-order rate constant was finally evaluated.

Interaction of actinides(III) with aluminosilicate colloids: Part IV. Influence of humic acid

The present investigation deals with how aquatic hydroxy aluminosilicate (HAS) colloids are generated in nature via co-nucleation of Si and Al in the presence of humic acid and moreover how trivalent actinides are integrated into the process. For the experiment, 241Am (5 × 10 −8 M) is introduced into a colloid forming mother solution where competition takes place for the complexation of two trivalent metal ions, Al and Am, each with two different ligands: 14C-labelled humic acid and silicic acid. Colloids thus formed are separated from precipitate and solution by sequential filtrations at 450 and 1.5 nm pore size. Formation of the colloid-borne Al and Am species in the neutral pH range is ascertained radiometrically by determining the phase distribution of 14C and Am, respectively. Varying the pH as well as the concentration of reaction components involved, a broad screening experiment is further carried out. Replacing Am by Cm, the speciation of colloid-borne Cm is made by time-resolved laser fluorescence spectroscopy (TRLFS), exciting at two different wavelengths. A direct excitation is made at 382 nm, whereas an indirect excitation is followed at 370 nm to confirm the sensitized Cm excitation via Cm-borne humic acid. The result substantiates the formation of colloid-borne Cm in a mixed structure of HAS and humic acid aggregated. Affinity of Am and Al binding to silicic and humic acid, leading to incorporation into the colloidal phase, can be correlated with their different hydrolysis behaviour. The Am 3+ ion, less hydrolyzing than the Al 3+ ion, is favoured for complexation with humic acid and thus discriminated in the co-nucleation with silicic acid. In the pH range (≥6.6) where the Am 3+ ion becomes hydrolyzed, a synergic bond coupling of hydrolyzed Am species yields a mixed hybrid of HAS-humic colloid-borne Am. As measured by laser-induced breakdown detection (LIBD), colloids thus formed reveal an average particle size of about 15 ± 5 nm hard sphere diameter. The value is comparable to that of Am-HAS colloids and remains consistent for a 2-month period of observation. The present study shows that aquatic colloids composed of HAS and humic acid combine actinides via different but synergic mechanisms and hence enhance the stability of colloid-borne actinides.

Selenite Adsorption Mechanisms on Pure and Coated Montmorillonite: An EXAFS and XANES Spectroscopic Study

Selenite (SeO32−) is an oxyanion of environmental importance due to its toxicity to animals at higher concentrations, notably waterfowl and grazing animals. Sorption of SeO32− with mineral phases typically controls the movement and bioaccessibility of SeO32− in soils and sediments. Previous studies have successfully utilized synchrotron‐based Extended X‐ray Absorption Fine Structure (EXAFS) and X‐ray Absorption Near Edge Structure (XANES) spectroscopy to determine SeO32− bonding mechanisms on Fe and Mn oxides, but the direct evidence of SeO32− surface complexation mechanisms on important mineral phases such as Al hydroxide and aluminosilicate minerals is still lacking. In this study both EXAFS and XANES spectroscopy was conducted on aqueous SeO32− solutions and on a variety of Al‐bearing sorption samples at pH 4.5. The sorbents chosen were a hydroxyaluminosilicate (HAS) polymer, a hydroxyaluminum (HYA) polymer, montmorillonite, and both HYA and HAS coated montmorillonite. For SeO32− sorption on montmorillonite, only bidentate binuclear inner‐sphere complexation was observed. For the hydroxyaluminum and hydroxyaluminosilicate polymers, a mixture of outer‐sphere and bidentate binuclear inner‐sphere was observed. When montmorillonite was coated with either HYA or HAS polymers then adsorption behavior was intermediate between that of the mineral and the pure polymer. Since temperate soils often contain aluminum‐hydroxy and aluminosilicate coated minerals rather than discrete Al hydroxide minerals and pristine clay surfaces, the adsorption mechanisms observed on these coated surfaces are more realistic of the natural environment than sorption on pure minerals.

Sodium silicate as alternative to liming-reduced aluminium toxicity for Atlantic salmon ( Salmo salar L.) in unstable mixing zones

When acid aluminium (Al) rich water is limed, unstable mixing zones are formed until equilibrium is reached. In such mixing zones transient high molecular mass positively charged Al-species (HMM Al i) being extremely gill reactive are produced, causing toxic effects in fish. The transient HMM Al i-species are formed due to hydrolysis and polymerization of low molecular positively charged Al-species (LMM Al i), e.g. initiated by liming and the subsequent increase in pH. To counteract the toxicity of transient Al polymers in such mixing zones, sodium silicate, forming non-toxic hydroxyaluminosilicate (HAS) complexes, can be used as alternative to liming. In the present work the effect of sodium silicate on polymerization of LMM Al i in unstable mixing zones and subsequent gill reactivity and mortality of fish was compared to results obtained from liming. Diluted sodium silicate (< 1.5 g l − 1 ) and lime slurry (Ca(OH) 2), respectively, were continually added to acidified Al-rich water in six different channel-tank systems, to obtain mixing zones with pH 5.9, 6.0 and 6.4, respectively. Utilising in situ size and charge fractionation techniques and following the exposure of Atlantic presmolt ( Salmo salar L.) kept in cages at defined stations along the channel-tank systems, changes of Al-species in the mixing zones, the gill reactivity of Al-species and thus Al toxicity could be followed downstream the confluences (time of reaction after mixing: 1–100 min). By increasing the pH of the acid water to 6.0 or 6.4 by sodium silicate, the detoxification of Al was faster than using lime. Using sodium silicate, the transformation of LMM Al i, the formation of HMM Al i, the Al deposition in fish gills and fish mortality were lower than using lime. The formation of neutral LMM Al-species (Al o) was, however, higher and the formation of colloidal Al-species (Al c) lower in the presence of silicate compared to lime. Furthermore, the Al deposition in fish gills and fish mortality decreased by increasing concentration of sodium silicate dosed. Thus, sodium silicate is a good alternative to liming, and under certain circumstances when aging of water may represent a problem (e.g. aquaculture) sodium silicate should be the preferred agent.

Interaction of actinides(III) with aluminosilicate colloids in “statu nascendi”: Part III. Colloid formation from monosilanol and polysilanol

The chemical interaction of trivalent actinides, Am(III) and Cm(III), during the formation of hydroxy-aluminosilicate (HAS) colloids is investigated starting with polysilanol (polysilicic acid) in the near neutral pH range of 4–9. Polymerisation and depolymerisation of silanol (silicic acid) are also examined to ascertain the optimal condition of HAS colloid formation with polysilanol. The formation of colloid-borne Am(III) is investigated radiometrically by varying the Al concentration from 10 −7 to 10 −3 M, while keeping the Si concentration constant at ≥10 −2 M. Spectroscopic speciation is performed by time-resolved laser fluorescence spectroscopy (TRLFS) to characterise how trivalent actinides become chemically incorporated in HAS colloids. For this purpose optically sensitive Cm is used. An average size of HAS colloids, as determined by laser-induced breakdown detection (LIBD), is found to be in the range of 10–20 nm in diameter, increasing with lowering pH. The SEM-EDXS analysis results in an atomic Si/Al ratio of 1.23 ± 0.03. The TRLFS speciation shows the formation of three different colloid-borne Cm species: Cm-HAS(I), Cm-HAS(II) and Cm-HAS(III). These results are different from the HAS colloid formation with monosilanol, in which only two species – Cm-HAS(I) and Cm-HAS(II) – are observed. The number of hydration water molecules in the three colloid-borne Cm species, as confirmed by measuring the fluorescence lifetime with TRLFS, varies from 7 for Cm-HAS(I), to 6 for Cm-HAS(II) and to 0–1 for Cm-HAS(III), which infers bi-dentate and tri-dentate oxo-bridging and structural incorporation of Cm, respectively. The prevailing Cm-HAS(III) species at pH 9 converts by decreasing pH first to Cm-HAS(II) and then to Cm-HAS(I), revealing a gradual dissociation of Cm from HAS colloids upon acidification.

Kinetics of Selenite Adsorption on Hydroxyaluminum‐ and Hydroxyaluminosilicate‐Montmorillonite Complexes

A lack of understanding about the selenite adsorption behavior on hydroxyaluminum (HyA)‐ and hydroxyaluminosilicate (HAS)‐interlayered phyllosilicates led us to conduct the present study. The kinetics of selenite adsorption on montmorillonite (Mt), HyA(OH/Al = 2.0)‐Mt, HAS1(OH/Al = 2.0; Si/Al = 0.24)‐Mt, and HAS2(OH/Al = 2.0; Si/Al = 0.48)‐Mt were studied at pH 4.5, with an initial selenite concentration of 0.025 mM, a clay concentration of 0.5 g L−1, temperatures of 288, 298, 308, and 318 K, and background electrolyte concentration of 10−2 M NaNO3 Of the six different kinetic models tested, the second‐order rate equation best described the kinetic data obtained for the initial fast reaction (5–30 min) followed by a slow reaction (30–180 min) in the adsorption systems. Elevated temperatures brought about a substantial increase in the rate constants. Compared with Mt, different HyA/HAS‐Mts had 2 to 21 times higher rate constants for the fast reaction and up to five times higher rate constants for the slow reaction. Silication of HyA‐Mt to form HAS1‐Mt and HAS2‐Mt substantially lowered the rate constants for both the fast and slow reactions. For the fast reaction, Mt had the highest activation energy and HyA‐Mt had the lowest activation energy (around four times lower than Mt); silication increased the activation energy of selenite adsorption on the HAS‐Mts. The pre‐exponential factor, an index of the frequency of selenite collision with the clay surface, was remarkably lower for the HyA/HAS‐Mts in comparison with Mt. The data obtained in the present study are of fundamental significance in understanding the role of Al interlayering and coating and silication of Al polymers on expansible phyllosilicates in influencing the dynamics of Se in soil and related environments.

Desorption Behavior of Cd, Zn and Pb Sorbed on Hydroxyaluminum- and Hydroxyaluminosilicate-Montmorillonite Complexes

AbstractThere is a clear gap in the understanding of the desorption patterns of metals sorbed on soils and clays, despite their importance in the mobility, transport and fate of metals in natural environments. In this study, we investigated the desorption behavior of Cd, Zn and Pb sorbed on montmorillonite (Mt) and on hydroxyaluminum (HyA)- and hydroxyaluminosilicate (HAS)-Mt complexes. At pH 6.5, 2.5 g L–1 of HyA-Mt and HAS-Mt sorbed almost all of the 10–6 M Cd, Zn or Pb, while Mt under the same condition sorbed ~48, 49 and 55% of the added Cd, Zn and Pb, respectively. Based on pH50 values, the selectivity of metal sorption on Mt was Pb &gt; Zn &gt; Cd, and on the complexes, it was Pb ≫ Zn = Cd. In general, larger fractions of sorbed metals were remobilized from Mt than from the complexes. Again, in comparison with Pb, larger fractions of sorbed Cd and Zn were remobilized from different clays. Reducing the pHs of the equilibrium sorption systems from a fixed point (6.5) to different points (6.0, 5.5, 5.0, 4.5, and 4.0) and from different points (6.5, 6.0, 5.5, 5.0, and 4.5) to a fixed point (4.0) both yielded hysteretic metal desorption patterns. The fractions of Cd and Zn desorbed through Na and Cu exchange from the clays, especially from the complexes, were very different, indicating the existence of cation exchangeable metal sorption sites of weak and strong affinities on the complexes. Based on the EDTA-extractable fractions of Cd and Zn from HAS–Mt and HyA-Mt, it appeared that HyA–metal bonds are stronger than the HAS–metal bonds. Compared with other agents, acetic acid remobilized the highest fractions of all metals irrespective of the type of clays, with a concomitant release of Al or Al + Si. The Pb-HyA/HAS-Mt bonds were, however, still much too strong to be broken substantially by this mechanism. The results accomplished in this study suggest further attention to the fundamental understanding of the mobility, fate, bioavailability and toxicity of the concerned metals in soils and related environments.

Interaction of actinides with aluminosilicate colloids in statu nascendi

Abstract The chemical interaction of trivalent actinides, Am(III) and Cm(III) in the process of hydroxy aluminosilicate (HAS) colloid formation is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) with the assistance of radiometry. A screening experiment is performed at the beginning to ascertain under what conditions HAS colloids are formed favourably and incorporate trivalent actinides in their oxo-bridge structure. While keeping the Si concentration constant at 10−3 mol l−1, the Al concentration is varied from 10−7 to 10−3 mol l−1 in the pH range from 4 to 9. The electrolyte medium is made constant with 0.03 M NaCl. A trace amount of 241Am or 248Cm is introduced at a concentration of 4.9×10−8 mol l−1. The favourable condition of HAS colloid formation as ascertained by radiometric experiment is found to be at around pH 5 with 10−6–10−5 mol l−1 Al for a marginal amount and at pH≥7 with 10−6–10−4 mol l−1 Al for a major amount. Spectroscopic speciation is made on Cm, for its high fluorescence yield, in various experimental solutions for the appraisal of its chemical binding within oxo-bridges of HAS colloids. Speciation made by TRLFS shows two colloid-borne Cm species, Cm-HAS(I) and Cm-HAS(II), which have seven and six hydration water molecules, respectively. These results put forward a conclusion that the former undergoes a bidentate oxo-bridge binding and the latter a tridentate binding. In the absence of Al, Cm undergoes reaction with silanol moieties to form Cm–silicate complexes, which are found to be distributed in three phases: ionic phase, colloid and precipitate. However, the formation of Cm–silicate colloids is marginal due to a low Cm concentration. Applying laser-induced breakdown detection (LIBD), an average size of colloids is found to be 11–13 nm in hard sphere diameter at different pH.

The use of root growth and modelling data to investigate amelioration of aluminium toxicity by silicon in Picea abies seedlings

Three-week-old Picea abies seedlings were grown for 7 days in 100 μM aluminium (Al), combined with 1000 or 2000 μM silicon (Si). Solution pH was adjusted to 4.00, 4.25, 4.50, 4.75, or 5.00. In the absence of Si, solution pH had no effect on the decrease in root growth caused by 100 μM Al. Silicon did not ameliorate toxic effects of Al on root growth at pH 4.00, 4.25 and 4.50, whereas significant, and apparently complete, amelioration was found at pH 4.75 and 5.00. An equilibrium speciation model (EQ3NR), with a current thermodynamic database, was used to predict the behaviour of Al and Si in growth solutions. When Si was not present in the 100 μM Al solutions, Al 3+ declined from 92.4% of total Al at pH 4.00 to 54.6% at pH 5.00, and there was a concomitant increase in hydroxyaluminium species as pH increased. The addition of 1000 μM Si to the 100 μM Al solutions caused a reduction in Al 3+ content over the whole pH range: at pH 4.00 Al 3+ fell from 92.4 to 83.3% in the presence of Si; and at pH 5.00 the fall was from 54.6 to 17.7%. These falls were attributed to the formation of hydroxyaluminosilicate (HAS) species. Similar, but somewhat greater, changes were observed in solutions containing 2000 μM Si. The match between root growth observations and the modelling data was not very good. Modelling predicted that change in Al 3+ content with pH in the presence of Si was gradual, but root growth was markedly increased between pH 4.50 and 4.75. Differences between root growth and modelling data may be due to the model not correctly predicting solution chemistry or to in planta effects which override the influence of solution chemistry.

Simultaneous Adsorption of Cadmium, Zinc, and Lead on Hydroxyaluminum- and Hydroxyaluminosilicate-Montmorillonite Complexes

We examined the adsorption behavior of Cd, Zn, and Pb from mixed solutions on synthetically prepared Hydroxyaluminum (HyA)- and Hydroxyaluminosilicate (HAS)-montmorillonite (Mt) complexes within the pH range 4 to 8. Initial concentrations of each metal (MeI) of 1 × 10−6 M in binary systems and 1 × 10−6, 2 × 10−5, 5 × 10−5 M in ternary systems were used in a background electrolyte of 0.01 M NaClO4 The presence of HyA and HAS polymers on Mt greatly increased the adsorption of all three metals. Lead appeared to have the strongest affinity for adsorption on both Mt and on the complexes. The adsorption affinity sequence of Pb > Cd > Zn on Mt did not change with MeI and could be explained with hard-soft acid-base (HSAB) theory. On the complexes, the affinity sequences of Zn > Cd in binary systems and Pb ≫ Zn ≥ Cd in most ternary systems were consistent with predictions based on the first hydrolysis constants. We propose that HyA-Mt and HAS-Mt are composed of nonuniform metal adsorption sites. For a given type of site, metal selectivity was predominantly determined by the metal properties, softness and ease of hydrolysis, while ionic potential had a limited predictive index for metal adsorption. The overall adsorption behavior of the metals along with their extraction patterns showed that the strength of adsorption followed the order of Pb ≫ Zn > Cd among the metals and of HyA-Mt ≥HAS-Mt ≫ Mt among the adsorbents.

Simultaneous Adsorption of Cadmium, Zinc, and Lead on Hydroxyaluminum‐ and Hydroxyaluminosilicate‐Montmorillonite Complexes

We examined the adsorption behavior of Cd, Zn, and Pb from mixed solutions on synthetically prepared Hydroxyaluminum (HyA)‐ and Hydroxyaluminosilicate (HAS)‐montmorillonite (Mt) complexes within the pH range 4 to 8. Initial concentrations of each metal (MeI) of 1 × 10−6 M in binary systems and 1 × 10−6, 2 × 10−5, 5 × 10−5 M in ternary systems were used in a background electrolyte of 0.01 M NaClO4 The presence of HyA and HAS polymers on Mt greatly increased the adsorption of all three metals. Lead appeared to have the strongest affinity for adsorption on both Mt and on the complexes. The adsorption affinity sequence of Pb &gt; Cd &gt; Zn on Mt did not change with MeI and could be explained with hard‐soft acid‐base (HSAB) theory. On the complexes, the affinity sequences of Zn &gt; Cd in binary systems and Pb ≫ Zn ≥ Cd in most ternary systems were consistent with predictions based on the first hydrolysis constants. We propose that HyA‐Mt and HAS‐Mt are composed of nonuniform metal adsorption sites. For a given type of site, metal selectivity was predominantly determined by the metal properties, softness and ease of hydrolysis, while ionic potential had a limited predictive index for metal adsorption. The overall adsorption behavior of the metals along with their extraction patterns showed that the strength of adsorption followed the order of Pb ≫ Zn &gt; Cd among the metals and of HyA‐Mt ≥HAS‐Mt ≫ Mt among the adsorbents.

Role of hydroxy-aluminosilicate ions (proto-imogolite sol) in the formation of humic substances

Little is known of the role of solution species of soil aluminosilicates in the formation of humic substances. Catalysis by hydroxy-aluminosilicate (HAS) ions, one of the major forms of soluble aluminum and silicon and the precursor of noncrystalline aluminosilicates in acidic natural environments, in catechol humification was investigated in this study by using UV-visible, FTIR, and solid state 13C CP–MAS NMR spectroscopies, X-ray diffractometry, atomic force microscopy, and elemental analysis. Hydroxy-aluminosilicate ions substantially promoted the oxidative polymerization and ring cleavage of catechol, resulting in the formation of black precipitates of humic macromolecules. This abiotic catalysis by HAS ions merits close attention in understanding the formation and related geochemistry of humic substances especially in acidic environments in temperate and subtropical regions.

Speciation of hydroxyaluminosilicate and hydroxyaluminum ions as affected by the presence of montmorillonite: Extraction experiment with potassium chloride and speciation by 27Al-NMR

The changes of Al species in the presence of montmorillonite (Mt) with aging were investigated using 27Al-nuclear magnetic resonance and inductively coupled plasma atomic emission spectroscopy after extraction with 1 mol L-1 KCl. Composition of the Al species in a hydroxyaluminosilicate (HAS) solution with a Si/Al molar ratio of 0.63 without Mt was not appreciably affected by 42 d of aging. In the absence of Mt, the concentration of Al13 ([Al04Al12(OH)24- H2O)12]7+) in the HAS solution with a Si/Al molar ratio of 0.09 and hydroxy-aluminum (HyA) solution decreased during 42 d of aging, suggesting that degradation (or polymerization) of Al13 took place upon aging. In the presence of Mt, Al13 was adsorbed onto Mt from the HyA and HAS(0.09) solutions. The adsorbed Al13 was partly recovered by 1 mol L-1 KCl from HyA- and HAS(0.09)Mt complexes after 42 d of aging, suggesting that at least a part of the adsorbed Al13 was exchangeable and the rest was considerably stabilized by adsorption onto Mt. The desorption ratios of Al from the HyA- and HAS(O.09)Mt complexes accounted for 25 to 30% and 6 to 8% of total Al adsorbed, respectively. The species of Al desorbed from these complexes consisted mainly of Al13 and AlNON. The AlNON was attributed to electrically asymmetric Al including HAS and/or polymer HyA ions.

Adsorption Behavior of Cadmium, Zinc, and Lead on Hydroxyaluminum– and Hydroxyaluminosilicate–Montmorillonite Complexes

The current imperfect understanding about the adsorption behavior of heavy metals on hydroxyaluminum (HyA)‐ and hydroxyaluminosilicate (HAS)‐interlayered phyllosilicates led us to conduct this study. We examined the adsorption behavior of Cd, Zn, and Pb on synthetically prepared HyA– and HAS–montmorillonite (Mt) complexes in comparison with that on untreated Mt. A very dilute initial metal concentration of 10−6 M in 0.01 M NaClO4 background was used in all the adsorption systems. The presence of HyA and HAS polymers on Mt greatly promoted the adsorption of all three metals. Such promoting effects of HyA and HAS polymers on the metal adsorption were, however, not very different from each other. The observed adsorption selectivity sequences of Pb &gt; Zn &gt; Cd on Mt as well as Pb ≫ Zn ≥ Cd on the complexes resemble the reported metal selectivity sequences on amorphous Fe and Al hydroxides. At different pHs, partitioning the adsorbed metals into strongly and weakly held fractions indicated that specific adsorption rather than nonspecific adsorption might have largely controlled the metal selectivity, particularly on the complexes. This led us to assume a predominant involvement of interlayered HyA or HAS polymers in metal adsorption from such dilute solutions. On Mt, the metals were predominantly adsorbed on the permanent charge sites in an easily replaceable state. However, a substantial involvement of the edge OH− groups of Mt in specific adsorption of the metals was also evident, especially at higher pH. Obviously, on Mt and on the complexes, the relative abundance of each type of site and their affinity to heavy metals were substantially different.

Cadmium adsorption on hydroxyaluminosilicate-montmorillonite complex as influenced by oxalate and citrate

The hydroxyaluminosilicate (HAS)-montmorillonite (Mt) and hydroxyaluminum (HyA)-Mt complexes are important constituents of soil colloidal complexes in acidic environments. Oxalate and citrate are dominant organic acids in the root exudates of many plants. These acids, among other factors, strongly influence the adsorption behavior of HAS-Mt and HyA-Mt complexes. In this study, we investigated the adsorption phenomena of Cd on the HAS-Mt and HyA-Mt complexes as influenced by oxalate and citrate. Without addition of oxalate and citrate, the adsorption of Cd on Mt was easy. However, in the presence of oxalate and citrate, the adsorption was strongly inhibited. In contrast, the HAS-Mt and HyA-Mt complexes in the absence of oxalate and citrate hardly adsorbed Cd. In the presence of oxalate, the HAS-Mt and HyA-Mt complexes were able to adsorb Cd. Thus, optimal concentration of oxalate for Cd adsorption on both complexes could be determined. Similarly, the HyA-Mt complex showed an optimal concentration of citrate for Cd adsorption. However, the HAS-Mt complex did not adsorb Cd in the presence of citrate. We concluded that the optimal concentrations of oxalate and citrate for Cd adsorption depended on the form of Cd ions and their proportion in the solution. Based on the amount of Cd adsorbed and the distribution of Cd species in the solution, the adsorption of Cd on the HAS-Mt and HyA-Mt complexes took place in various chemical forms.