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The reaction of aluminium with silicic acid in acidic solution: An important mechanism in controlling the biological availability of aluminium?
Abstract
The reaction of aluminium (Al) with monomeric silicic acid (Si(OH)4) to form an hydroxyaluminosilicate (HAS) has been well documented over the past 40 or so years. The formation of an aluminium hydroxide template, upon which Si(OH)4 will condense in competition with Al, was demonstrated to be a prerequisite to HAS formation. This initial reaction results in the formation of a slowly aggregating HAS, with a Si:Al ratio of 0.5, in which silicon tetrahedra are bonded to Al octahedra through three SiOAl linkages. We have called this HASA. In solutions in which the concentration of Si(OH)4≥Al HASA acts as a template for the incorporation of further silicon tetrahedra to give a rapidly precipitating HAS (that we have called HASB), with a Si:Al ratio of 1.0, in which up to 50% of the constituent Al has adopted tetrahedral geometry. There are, at present, no reliable constants to describe either the formation or the solubility of these HAS. They are extremely insoluble and are likely to play an important role in the control of the release of Al from the edaphic to the aquatic environment. They may also have an important role in Al homeostasis in biota though the evidence to support this is more tentative.
... Amorphous Al [6] containing aluminosilicates such as allophanes or imogolites have been identified in a variety of environments , and are common in the weathering products of volcanic ash (Parfitt, 2009). Allophane-like phases have also been implicated in controlling the bioavailability of Al (Doucet et al., 2001; Exley et al., 2002). Al [4] containing aluminosilicate phases have been identified as the dominant phases in hydrothermal scales (Gallup, 1997; Nishida et al., 2009) siliceous hot spring deposits (Yokoyama et al., 2004), biogenic silica (Gehlen et al., 2002), and inorganic opals (Brown et al., 2003; Paris et al., 2007). ...
... Solid-state nuclear magnetic resonance spectroscopy has been applied in numerous studies to provide basic characterization of the phases formed during reactions between Al and silica. Basic differences in the Al and Si coordination environments in allophane, imogolite, and other hydrous aluminosilicates can all be identified from solid-state 27 Al and 29 Si NMR techniques (Barron et al., 1982; Idlefonse et al., 1994; Gallup, 1997; Exley et al., 2002; Yokoyama et al., 2002 Yokoyama et al., , 2004 Brown et al., 2003; Paris et al., 2007; Nishida et al., 2009). Advanced techniques which exploit the couplings between neighboring elements can provide more detailed structural determination of these Al containing phases. ...
... The previous model has suggested a multi-step process dependent on solution pH including Al [4] to Al [6] in the higher pH experiments of Houston et al. (2008) likely result from higher Al and Si concentrations present in their batch experiments driving the formation of the Al [4] silicates. The formation of metastable allophane-like aluminosilicate phases has been invoked as a possible controlling mechanism for Al solubility in surface waters (Doucet et al., 2001; Exley et al., 2002). The authors of these studies propose two phases which they term hydroxyl-aluminosilicate phases HAS A and HAS B . ...
We present the results of a series of experiments designed to probe the interactions between Al and the amorphous silica surface as a function of thermodynamic driving forces. The results from 27Al single pulse magic angle spinning (SP/MAS) and 27Al{1H} rotational echo double resonance (REDOR) allow us to identify the reaction products and constrain their structure. In all cases, despite low Al and Si concentrations we observe the formation of metastable aluminosilicates. Results from low temperature experiments indicate that despite thermodynamic driving forces for the formation of gibbsite we observe the precipitation of separate octahedrally coordinated Al (Al[6]) and tetrahedrally coordinated Al (Al[4]) silicate phases. At higher temperatures the Al[4] silicate phase dominates the speciation. Structural models derived from the NMR data are also proposed, and the results are discussed as they relate to previous work on Al/Si cycling.
... With this kinetic mechanism also recognised as a step during the synthesis of artificial aluminosilicate materials used in various commercial applications (Ryu et al., 2010). Additionally, the incipient nucleation of Al (oxy)hydroxide solids is now well documented as a trigger for the rapid precipitation of amorphous aluminosilicates at lower temperature (Exley and Birchall, 1993;Doucet et al., 2001a;Doucet et al., 2001b;Exley et al., 2002;Strekopytov and Exley, 2006;Exley, 2012;Leiviskä et al., 2014;Tokoro et al., 2014;Beardmore et al., 2016). Although the conditions of high temperature geothermal brines are markedly different to those found in soil pore fluids and shallow groundwaters, it is very possible that this trigger mechanism may also operate in hot geothermal brines. ...
... The Si/Al mole ratio of synthetic aluminosilicate produced from the individual test solutions was also observed to increase with reaction time for experiments in which multiple membrane filters were used. These features are consistent with those identified during the formation of hydroxyaluminosilicates (HAS) at low temperatures under acidic conditions (Exley and Birchall, 1993;Doucet et al., 2001a;Doucet et al., 2001b;Exley et al., 2002;Strekopytov and Exley, 2006;Exley, 2012;Leiviskä et al., 2014). The concentration of K in the synthetic solids was also observed to increase relative to Na with reaction time, in a similar fashion to Si/Al. ...
Amorphous aluminosilicate scales have been observed to form in geothermal pipelines at numerous locations internationally, with examples reported from geothermal fields in Iceland, El Salvador, Nicaragua, New Zealand, Japan, Indonesia, and the Philippines. These scales frequently accumulate at higher rates and temperatures than would be expected for pure amorphous silica scales, resulting in the unexpected fouling of surface infrastructure, and injectivity declines in geothermal brine reinjection wells; both of which present significant problems for the utilisation of geothermal energy. Yet, though this phenomenon has been studied for over 40 years, a mechanism responsible for the formation of aluminosilicate scales has yet to be unequivocally identified. In this work, aluminosilicate colloids were synthesised and captured under controlled conditions during time-constrained aqueous experiments. The rapid formation of aluminosilicate colloids was found to coincide with the supersaturation of aluminium (oxy)hydroxide, producing filterable aluminosilicate colloids within 120 seconds, with the initially Al-rich colloids becoming increasingly Si-rich with reaction time. Aluminosilicate colloids synthesised under neutral to alkaline conditions sequester both Na and K during their earliest formation, with Si/Al, K/Na, and Al/K+Na) mole ratios approaching those of real geothermal scales formed at the San Jacinto-Tizate field study site, as well as others reported in the literature. Similarly, these synthesized colloids demonstrate a near identical (K/Na)[solid]/(K/Na)[brine] enrichment to that of real geothermal aluminosilicate scales. The onset of aluminosilicate formation appears to be independent of aqueous silica concentration, and the saturation state of silica with respect to amorphous silica, although the solution Si/Al mole ratio does influence the stoichiometry of the colloids produced. These latter two features are congruent with a formation mechanism identified for amorphous aluminosilicates formed at low temperatures under acidic conditions within recent hydroxyaluminosilicate (HAS) research. Although these preliminary results require further testing, the successful synthesis of artificial geothermal aluminosilicate colloid analogues demonstrated by this work presents an invaluable opportunity to both better resolve geothermal aluminosilicate-scaling processes, and more rigorously test methods of aluminosilicate scale mitigation and prevention.
... For the complexation reaction, it was reported that silicic acid reacted with Al 3+ or Fe 3+ forming Si-O-Al (or Fe) [25][26][27]. According to Exley et al. [25], there are five possible reactions between Al 3+ and silicic acid, and silicon tetrahedra are bonded to Al octahedra through three Si-O-Al linkages with a Si:Al ratio of 0.5. ...
... For the complexation reaction, it was reported that silicic acid reacted with Al 3+ or Fe 3+ forming Si-O-Al (or Fe) [25][26][27]. According to Exley et al. [25], there are five possible reactions between Al 3+ and silicic acid, and silicon tetrahedra are bonded to Al octahedra through three Si-O-Al linkages with a Si:Al ratio of 0.5. Three types of Si-O-Fe bonds were reported by Swedlund et al. [27], which is monomer lingkage when the amount of H 4 SiO 4 is low (0.05), polymerization lingkage when the amount is high (0.20) and siloxane linkages when the amount increased from 0 to 0.027. ...
Silicic acid was prepared by acid hydrolysis of water-quenched calcium silicate, and Ca²⁺, Al³⁺, and Fe³⁺ were added to investigate the polymerization behavior. Effect order of the three ions on the polymerization was Fe³⁺ > Al³⁺ > Ca²⁺. Due to decreased polymerization rate caused by Fe³⁺ at pH < 3, average diameter of the colloidal particles at pH = 2 was larger than that at pH = 3. New infrared absorption peaks were found at 2360 cm⁻¹ which may be attributed to Ca–OH, Al–O–Si, and Fe–O–Si, respectively, and it was especially obvious for Fe³⁺. For three ions coexisting system, the new peak showed a slight shift to 2370 cm⁻¹. Preventing ability of Fe³⁺ on water binding is much stronger than promoting ability of Al³⁺. Dehydration process of bound water was hindered by adding the three ions, and due to higher bound water content, endothermic peak for Al³⁺ system was stronger. Crystallization process was hindered by Ca²⁺ and Fe³⁺.
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... Recent studies have reported that a high amount of Si was released from rice straw-derived biochar 33 . In addition, the formation of the alumina-silica compound in the solution and in the plant root tips has been reported [45][46][47][48] . Exley and coworkers first demonstrated that the formation of hydroxyaluminosilicate (HAS) limited the biological availability of Al 49 , and the inorganic chemistry of HAS and their role in the biogeochemical cycle of Al was excellently elucidated 47,48,50 . ...
... In addition, the formation of the alumina-silica compound in the solution and in the plant root tips has been reported [45][46][47][48] . Exley and coworkers first demonstrated that the formation of hydroxyaluminosilicate (HAS) limited the biological availability of Al 49 , and the inorganic chemistry of HAS and their role in the biogeochemical cycle of Al was excellently elucidated 47,48,50 . Similarly, the intracellular Si-Al biointeraction and nanometer-scale colocalized of Si and Al via elemental mapping were used to illustrate the avoidance of Al toxicity in freshwater snails 51 . ...
Replacing biosilicon and biocarbon in soil via biochar amendment is a novel approach for soil amelioration and pollution remediation. The unique roles of silicon (Si)-rich biochar in aluminum (Al) phytotoxicity alleviation have not been discovered. In this study, the alleviation of Al phytotoxicity to wheat plants (root tips cell death) by biochars fabricated from rice straw pyrolyzed at 400 and 700 °C (RS400 and RS700) and the feedstock (RS100) were studied using a slurry system containing typical acidic soils for a 15-day exposure experiment. The distributions of Al and Si in the slurry solution, soil and plant root tissue were monitored by staining methods, chemical extractions and SEM-EDS observations. We found that the biological sourced silicon in biochars served dual roles in Al phytotoxicity alleviation in acidic soil slurry. On one hand, the Si particles reduced the amount of soil exchangeable Al and prevented the migration of Al to the plant. More importantly, the Si released from biochars synchronously absorbed by the plants and coordinated with Al to form Al-Si compounds in the epidermis of wheat roots, which is a new mechanism for Al phytotoxicity alleviation in acidic soil slurry by biochar amendment. In addition, the steady release of Si from the rice straw-derived biochars was a sustainable Si source for aluminosilicate reconstruction in acidic soil.
... The influence of Si on the bioavailability and toxicity of Al to organisms has been extensively examined (Cocker et al., 1998;Exley et al., 2002). In the present study, addition of Na 2 SiO 3 (5 mg l À 1 ) had no effect on biosorbtion of 500 mg l À 1 Al (biosorption was not observed at 50 and 100 mg l À 1 Al) after 4 h and 16 days exposure but significantly reduced biosorption at 24 h. ...
... Table 2 Mean (mg g À 1 , n=5, 7 1SD) elemental concentrations in Daphnia-fed C. gigantea grown at high Al concentration (500 mg l À 1 ) for 16 days and in media containing no added Al (control). Binding of Al by Si can ameliorate the toxicity of the metal due to the formation of biologically unavailable compounds such as HAS (Exley et al., 2002). This reduction in toxicity to phytoplankton is, in part, due to the preferential binding to Al in competition with P (Martin, 1986;Exley et al., 1993). ...
Dissolved aluminium (Al) is generally at low concentrations in neutral freshwater due to its insolubility. However, a fall in pH resulting from acid deposition and mining alters the mobility of Al and so entry to adjacent neutral waters. The present study examines the environmental behaviour, cell-associated surface adsorption/absorption and toxicity of Al at neutral pH to the alga Chlamydomonas gigantea in the presence and absence of the key Al-binding ligand silica. We then examined transfer of Al from C. gigantea to the planktonic crustacean Daphnia pulex. Finally, the effect of Al on the elemental composition (and hence nutritional value) of the two organisms was compared to unexposed controls. C. gigantea increased the amount of Al in the algal culture medium. Binding of Al to extracellular glycoprotein is probably the reason why only one-third of the biosorbed Al was absorbed (accumulated) by C. gigantea. Aluminium concentrations between 50 and 500μgl−1 reduced growth of C. gigantea at 16 days exposure to the metal. Silica reduced biosorption, accumulation and toxicity of Al by C. gigantea. The concentration of Al in D. pulex fed Al-contaminated C. gigantea for 16 days did not differ from those fed alga grown in the absence of added Al. C. gigantea contaminated with Al contained less sulphur, magnesium, potassium and sodium although only sulphur fell in D. pulex subsequently fed the contaminated alga. Chloride, calcium, iron and silicon were significantly higher in D. pulex.
... Silicon is usually released in oxygenated forms such as silicate, which has been reported to be beneficial for the circulatory system and safer in terms of toxicity [36]. When silicon is present as silicic acid, Si(OH) 4 reacts with aluminum to form hydroxyaluminosilicate complexes, which results in the removal of harmful species from aqueous solutions, thus eliminating the biohazard of aluminum [37]. The aluminum contained in the glass filler particles replaces the silicon, forming Si-O-Al bonds, and this structure causes the formation of negative regions [38]. ...
Background
To predict the long-term performance of restorative materials in the oral environment, it is important to evaluate their resistance to chemical and mechanical degradation and to know the toxic potential of the type and amount of ions eluted from the filling material. In this study, home bleaching was applied to dental materials with different contents and it was aimed to determine the type and amount of ions released from these materials.
Methods
In this study, amalgam, posterior composite resin, anterior composite resin, bulk fill composite resin, indirect composite resin, hybrid ceramic and all-ceramic were used as restorative materials. 10 specimens of each material were prepared according to the manufacturer’s instructions. Each material group was divided into two subgroups as the bleached group and the control group. After bleaching, all specimens were stored in 1 ml of 75% ethanol/water solution. Solutions were renewed after 1, 14 and 28 days. The type and amount of ions released from the materials were determined using Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). Data were analyzed using the Friedman, Wilcoxon Signed Ranks, and Mann-Whitney U tests (α = 0.05).
Results
It was determined that the amount of ions release from the restorative materials decreased over time (p < 0.05). According to the results of the Mann-Whitney U test, there was no difference between the bleaching and control groups in most of the restorative materials (p > 0.05).
Conclusion
Within the limits of this study, home bleaching system does not have a significant effect on ion release from restorative materials.
... Traces of oligomeric silica in biological systems would profoundly affect the accumulation of aluminum. Exley et al. [20] showed hydroxyaluminosilicates (HAS) forms instantly in solutions with a considerable excess of Si, but when given more time, HAS continues to form, leading to colloid formation and subsequent precipitation, depending on the conditions. According to Guyonnet et al. [21], silica is probably the natural antidote of the aluminum and could play a benefit role by decreasing the biodisponibility of aluminum. ...
Aluminum and silicon are contaminants found in formulations used to prepare parenteral nutrition. Both elements are leached from glass containers, mainly during the heating cycle for sterilization. Insoluble and biologically inactive species of hydroxyaluminosilicates have been shown to form in solutions containing Al and Si. Therefore, this interaction may play an important role in protecting the body against Al toxicity. In this study, the bioavailability of Al in the presence of Si, calcium gluconate (Gluc.), and potassium phosphate (Phosf.) was investigated in rats. The rats were divided into 10 groups of 5 animals each: control, Al, Si, Al + Si, Gluc, Gluc + Al, Gluc + Al + Si, Phosf, Phosf + Al, and Phosf + Al + Si. The doses, consisting of 0.5 mg/kg/day Al and 2 mg/kg/day Si in the presence or absence of Gluc. or Phosf., were intraperitoneally administered for 3 months. Tissues were analyzed for Al and Si content. Al accumulated in the liver, kidneys, and bones, and the simultaneous administration of Si decreased Al accumulation in these tissues. The presence of Si reduced the amount of Al present by 72% in the liver, by 45% in the kidneys, and by 16% in bone. This effect was lees pronounced in the presence of parenteral nutrition compounds though. Si tissue accumulation was also observed, mainly when administered together with phosphate. These results suggest that Si may act as a protector against Al toxicity, by either reducing Al absorption or increasing its excretion, probably through hydroxyaluminosilicates formation. The presence of calcium gluconate and potassium phosphate decreases or inhibits this effect.
... This may be related to a lesser abundance of aluminol groups, as silanol groups do not participate in ligand exchange with organic functional groups (Pokrovski & Schott, 1998). On the other hand, silicic acid disturbs Al oxolation (Exley et al., 2002;Lenhardt et al., 2022), which may actually foster aluminol abundance. The formation of tetrahedral Al imparts negative charge on SROAS particles (Su et al., 1992). ...
The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorp-tion mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorp-tion of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3-8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octa-noic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectros-copy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm-1 , and symmetric stretching at 1430 cm-1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm-1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80-90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s-1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol-1. A slow process with a rate constant of 0.13 s-1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.
... Les concentrations moyennes d'Al dans les aliments consommés (mg/kg) présentées par ordre crescendo montrent les valeurs les plus élevées parmi les légumes (9,37), suivies par les sauces et les condiments (8,72), les bonbons(7,97), les plats cuisinés (7,58), les pommes de terre (7,28) et le groupe café, cacao et thé(6,36). Les quantités d'aliments ingérées quotidiennement les plus élevées (exprimées en g/j), comme précisé dans le Tableau IX ci-dessous, étaient les boissons gazeuses(102,70), les tomates fraîches(75,57), le thé(71,43), la crème glacée (74,07), le café turc(60,41) et les laitues (59,75). Les échantillons choisis et étudiés sont en contact direct avec l'Al principalement par l'intermédiaire des ustensiles de cuisine (9 échantillons), des emballages (63 échantillons) et des papiers en Al (5 échantillons). ...
Connu être un métal omniprésent dans la nature, l'homme est exposé à l'aluminium (Al) selon différentes sources induisant à des maladies émergentes. L’apport alimentaire joue un rôle majeur dans son accumulation organique. La prévalence élevée des maladies associatives avec le métal au Liban a été le point de départ de cette étude, afin d'explorer l'exposition alimentaire révélant toute association entre la consommation alimentaire, les maladies et les niveaux plasmatiques d'Al. Cette étude vise à estimer les taux de consommation alimentaire à base d'Al et à quantifier les taux d'Al dans différentes matrices alimentaires, révélant les principaux contributeurs de l'exposition à l'Al pour la population libanaise. Deux études transversales ont été menées à l'aide d'un questionnaire électronique personnalisé sur la fréquence alimentaire. La première étude ciblait des personnes âgées de 18 à 64 ans de différentes régions libanaises, réparties proportionnellement dans le but d'étudier la consommation alimentaire à base d'Al. La sélection des aliments était basée sur les résultats de l'étude française EAT2. La deuxième étude ciblait 100 participants, y compris des femmes enceintes, visant à corréler la consommation alimentaire avec les niveaux plasmatiques (e-FFQ) et à révéler toute association de maladie. Les niveaux d'Al ont été analysés par spectrométrie d'absorption atomique. L'analyse des données a été réalisée par le logiciel SPSS version 25.Le questionnaire a été complété par 167 participants. En outre, 97 aliments ont été étudiés en 2018. Les niveaux d'Al avaient une moyenne de 3,56 ± 2,08 mg / kg (allant de 0,14 à 9,37). Les niveaux d'Al les plus élevés ont été trouvés dans les légumes, suivis des sauces et condiments, des bonbons et des plats cuisinés. L'apport hebdomadaire tolérable provisoire (AHTP) d'Al a été fixé à 0,50 mg / kg du poids corporel (60 kg / personne). La deuxième étude a révélé une exposition positive aux aliments avec une corrélation de 32 % avec les niveaux plasmatiques moyens d'Al (atteignant 2,16 ± 1,30 μg / L); spécifiquement avec les légumes, les fruits, les pommes de terre, les légumineuses, le pain, la pâtisserie, les boissons gazeuses et les plats cuisinés. L'âge, l'état de grossesse et l'utilisation d'ustensiles de cuisine en Al étaient associés à des niveaux plasmatiques d'Al plus élevés, contrairement à la carence en vitamine D, à l'anémie, à l'arthrite et aux ulcères. La population libanaise, avec l'inclusion des femmes enceintes, est exposée à Al par l'ingestion d'aliments à base d'Al dans la limite des seuils internationalement établis d'apport tolérable (1 mg / kg / semaine), bien qu'elle reste protégée de certaines maladies liées au métal. Une recommandation nationale devrait être établie afin de maintenir des niveaux inférieurs de contamination des aliments par Al, limitant ainsi son augmentation dans l'organisme. Des recherches supplémentaires devraient être entreprises pour explorer la contamination par l'Al en tant que triade dans les aliments, l'eau et le sol.
... Concomitantly, absorption intensities at 690 relative to 590 cm − 1 decreased. These changes were more pronounced for initial Al:Si ratios of 2 and 3, indicating that the higher Si concentration impeded rearrangement of the chemical environment of Al (Exley et al., 2002;Luciuk and Huang, 1974). Vibrations of Si nuclei in Q 0 (3Al) Fig. 6. ...
Retention of dissolved organic matter (DOM) by short-range ordered aluminosilicates (SROAS) by adsorption and co-precipitation contributes to carbon accrual in soils and sediments. In this study, we investigated effects of SROAS composition on DOM adsorption, partitioning of carbon moieties by adsorption and co-precipitation, and the mineral structure of co-precipitates. We used four types of sorptive solutions, representing DOM collected in situ from topsoil and subsoil horizons of a Dystric Cambisol, and water-extracted DOM from beech and fir litter. We studied adsorption of soil DOM on three SROAS that structurally resemble proto-imogolites and Si-rich allophanes as a function of contact time (1–168 h) at initial pH 5. Co-precipitation of soil and litter-extracted DOM was quantified as a function of the molar Al:C ratio (0.3–1.4) and at two levels of Si concentration (molar Al:Si = 1 and 2). To resolve the impact of DOM on mineral structure, we first examined time-dependent structural evolution of SROAS within 1 to 72 h and subsequently investigated the effects of DOM interference in crystallization processes. Mineral structure of SROAS and co-precipitates was resolved by infrared, solid-state ²⁷Al- and ²⁹Si-NMR spectroscopy. Chemical composition of DOM prior to reaction with SROAS and in co-precipitates was analysed by solid-state ¹³C-NMR spectroscopy. Maximal C contents of adsorption complexes were 7.1 mg C g⁻¹ for Al-rich and 20.4 mg C g⁻¹ for Si-rich SROAS. We found selective adsorption of aromatic C and preferential exclusion of polysaccharide and alkyl C for both topsoil and subsoil DOM. Adsorption was larger for a Si-rich SROAS, since it exhibited a greater accessibility of aluminol groups. As a function of aromatic C content in initial DOM, 39.9 to 81% of C was retained by co-precipitation. Composition of co-precipitated organic matter was determined by C speciation in DOM supply, involving marked uptake of polysaccharides. In the absence of DOM, up to 50% of Si was present in imogolite-like configuration after 72 h, evidencing rapid development of short-range order. Complexation of Al by DOM during formation of SROAS caused partial exclusion of Si and slowed structural evolution, consequently enhancing abundance of ill-defied Si species in co-precipitates. Interactions of DOM with SROAS may cause selective accumulation of organic compounds and promote Si mobility in Andosols and Podzols.
... The separate role of airborne Atlantic Se, possibly upto 1/3 of Se fallout [12], could not be determined. Anyhow all atmospheric Se via (common) southwest wind could have had compensated the Se losses -better than by Si -of the hills, which are impoverishing by erosion Table 2. Low content of molybdate-reactive silicon (1 %) in biotite extracts by oxalate [7] can be dependent on aluminium-silicon interactions [29] in acid solution (pH 0.65). ...
... When silicon is present as silicic acid, Si(OH)4, it reacts with aluminum to form hydroxyaluminosilicate complexes that are condensed to form a polymeric structure. As a result, any potential harmful species are removed from aqueous solutions, and the biological hazard of aluminum is eliminated [99]. Aluminum from glass filler particles replaces in part the silicon to form Si-O-Al bonds. ...
Over the last years, diverse commercial resin-based composites have dominated as dental filling materials. The purpose of the present study was to determine organic and inorganic eluates from five restorative materials using GC/MS and ICP–OES and to compare the effect on cell survival of human gingival fibroblasts of a conventional and a bioactive resin. Five commercially available restorative materials were employed for this study: ActivaTM Bioactive Restorative, ENA HRi, Enamel plus HRi Biofunction, Fuji II LC Capsule, and Fuji IX Capsule. Disks that were polymerized with a curing LED light or left to set were immersed in: 1mL methanol or artificial saliva for GC/MS analysis, 5mL deionized water for ICP–OES, and 5mL of culture medium for cell viability. Cell viability was investigated with a modified staining sulforhodamine B assay.The following organic substances were detected: ACP, BHT, BPA, 1,4-BDDMA, CQ, DBP, DMABEE, HEMA, MCE, MeHQ, MOPA, MS, TMPTMA, and TPSb and the ions silicon, aluminum, calcium, sodium, and barium. Activa Bioactive Restorative was found to be biocompatible. Elution of organic substances depended on material’s composition, the nature of the solvent and the storage time. Ions’ release depended on material’s composition and storage time. The newly introduced bioactive restorative was found to be more biocompatible.
... The biochemistry of Al is exceptionally complex [5] and is not fully understood, probably because of co-occurring ions and a considerable range of polynuclear inorganic and organometallic complexes that occurs in naturally in soils and waters. Among various monomeric species of Al residing in the environment, the sulfate, phosphate, and silicate forms of Al are the major ones [6][7][8][9]. Aluminum is one of the most reactive elements, and its toxicity occurs when it comes in contact with the cell wall, plasma membrane, and symplasm of apical root cells of the plant [10] ( Figure 1). However, plants have evolved both internal and external mechanisms of Al tolerance ( Figure 2). ...
Aluminum (Al) precipitates in acidic soils having a pH < 5.5, in the form of conjugated organic and inorganic ions. Al-containing minerals solubilized in the soil solution cause several negative impacts in plants when taken up along with other nutrients. Moreover, a micromolar concentration of Al present in the soil is enough to induce several irreversible toxicity symptoms such as the rapid and transient over-generation of reactive oxygen species (ROS) such as superoxide anion (O2•−), hydrogen peroxide (H2O2), and hydroxyl radical (•OH), resulting in oxidative bursts. In addition, significant reductions in water and nutrient uptake occur which imposes severe stress in the plants. However, some plants have developed Al-tolerance by stimulating the secretion of organic acids like citrate, malate, and oxalate, from plant roots. Genes responsible for encoding such organic acids, play a critical role in Al tolerance. Several transporters involved in Al resistance mechanisms are members of the Aluminum-activated Malate Transporter (ALMT), Multidrug and Toxic compound Extrusion (MATE), ATP-Binding Cassette (ABC), Natural resistance-associated macrophage protein (Nramp), and aquaporin gene families. Therefore, in the present review, the discussion of the global extension and probable cause of Al in the environment and mechanisms of Al toxicity in plants are followed by detailed emphasis on tolerance mechanisms. We have also identified and categorized the important transporters that secrete organic acids and outlined their role in Al stress tolerance mechanisms in crop plants. The information provided here will be helpful for efficient exploration of the available knowledge to develop Al tolerant crop varieties.
... As is known to all, Al and Si will form HAS, which is generally considered to have no biological activity, at neutral and moderately acidic solution. Both Doucet et al. (2001) and Exley et al. (2002) proved that the formation of an Al hydroxide was a prerequisite for HAS formation, while the concentration of Al hydroxide is low in low pH solution, thus, the possibility of HAS formation is low in solution of pH 4.5. In addition, according to Kopittke and Blamey's research (2016), in our simple hydroponic culture system, Al remained soluble and the toxic Al 3+ ion was the dominant species. ...
Background and aims
Silicon (Si) is known to alleviate aluminum (Al) toxicity in a number of plant species; however, the mechanisms of Si-triggered Al detoxification have not been elucidated, especially in rice (Oryza sativa). We investigated the interactions between Si and Al in root and the role of cell wall polysaccharides in rice resistance to Al.
Methods
Rice seedlings were grown in 0.5 mM CaCl2 solution with or without 50 µM Al and 1.0 mM H4SiO4 for 24 h. The cell expansion and oxidative injury of root were measured by histochemical analyses; Al accumulation and distribution in root apices were determined by inductively coupled plasma mass spectrometry (ICP-MS); the degree of pectin methylesterification was analyzed by immunofluorescence, and the expression of genes (e.g. OsPME) related to Al resistance was measured by quantitative real-time PCR.
Results
Si relieved the inhibition of cell elongation and oxidative injury caused by Al and significantly reduced Al content (by about 41 %) in cell wall (CW). Meanwhile, Si inhibited pectin methylesterase (PME) activity and its gene expression, thereby increasing the degree of pectin methylesterification, and reduced the content of hemicellulose, which was the main binding site for Al in CW, under Al stress. Furthermore, Si decreased the non-exchangeable Al fraction of CW in root, but did not change the cation exchange capacity of CW.
Conclusions
Si reduces the deposition of Al in the cell wall of rice root apex by decreasing both the degree of pectin demethylesterification and the content of hemicellulose, thereby alleviating Al stress.
... Phosphate can also form complexes with Al, but concentrations are typically low enough in most freshwaters that it is of minor importance. Some evidence suggests that soluble Al complexing with silica can have a significant effect on Al speciation [8,9], although the importance of these complexes for Al toxicity is somewhat controversial [2,[10][11][12][13][14]. Carbonate complexes are not important for determining Al speciation, so to the extent that alkalinity correlates with Al bioavailability it is likely to be a result of covariation with pH and hardness. ...
Aluminum (Al) toxicity to aquatic organisms is strongly affected by water chemistry. Toxicity modifying factors such as pH, dissolved organic carbon (DOC), hardness, and temperature have a large impact on the bioavailability and toxicity of Al to aquatic organisms. The importance of water chemistry on the bioavailability and toxicity of Al suggests that interactions between Al and chemical constituents in exposures to aquatic organisms can affect the form and reactivity of Al thereby altering the extent to which it interacts with biological membranes. These types of interactions have previously been observed in the toxicity data for other metals where they have been well-described by the biotic ligand model (BLM) framework. In BLM applications to other metals (including cadmium, cobalt, copper, lead, nickel, silver, and zinc), these interactions have focused on dissolved metal. A review of Al toxicity data shows that concentrations of Al that cause toxicity are frequently in excess of solubility limitations. Aluminum solubility is strongly pH dependent, with a solubility minimum near pH 6 and increasing at both lower and higher pH values. For the Al BLM, the mechanistic framework has been extended to consider toxicity resulting from a combination of dissolved and precipitated Al to recognize the solubility limitation. The resulting model can effectively predict toxicity to fish, invertebrate, and algae over a wide range of conditions. This article is protected by copyright. All rights reserved.
... The transition could be due to modifications of the nucleation stage of the protoimogolite. Indeed, the precipitation of an amorphous phase of type HAS A (Exley et al., 2002) and its further internal restructuration could well produce proto-imogolites of various average sizes depending on the synthesis conditions. Concentration, pH and type of Si precursors seem to potentially control the formation of allophane or imogolite (Denaix et al., 1999). ...
Imogolite and allophane are two strongly curved nanominerals. Indeed, imogolite has a tubular shape with a diameter of only 2.5 nm. Allophane is often presented as a spherical nanostructure with a diameter of about 5 nm. The mechanism explaining the strong curvature of these two nanominerals has been extensively discussed. However, whether or not these two nanomaterials are related to each other is not clear and the mechanism responsible for the selection between the two different shapes is not well established. In this article, we propose that imogolite and allophane are nanopolymorphs of the imogolite local structure and that the transition from spherical to tubular shape occurs at an early stage of the precipitation because of edge stress in proto-imogolite. This hypothesis for the shape selection is supported by the use of a nanomechanical model tuned to mimic the main characteristics of imogolite-like nanomaterials.
... Probably, we may assume that the effect of silica depends on its form, applied concentration and route of entrance to organism. Exley group suggest that silicic acid in interaction with Al form hydroxyaluminosilicates, which could explain the influence of silicic acid on the absorption and urinary excretion of Al [41]. Their further results confirmed that regular drinking of silicic acid-rich potable waters may be an effective mechanism whereby Al might be purged from the body and the body burden of Al retained at a lowest possible level. ...
... Nevertheless, a recent theory suggests that its exclusion from successful biochemical pathways is mainly due to its very low natural availability [8,9]. Other theories are focused on the role of silicon: the formation of hydroxyl-alumino silicates would reduce the biological availability of aluminium itself [28,29]. ...
The aim of this chapter is to give a general view on the current status of the role of aluminium in human health and disease. The main aspects of aluminium metabolism in humans are covered, summarizing the state of knowledge on the absorption, transport, tissue distribution, and excretion of aluminium, giving particular emphasis to the main metabolic pathways of this metal ion in the bones and in the brain. Some features concerning the solution chemistry of aluminium are considered, giving special care to the speciation of aluminium hydroxides in municipal water and in water for dialysis. The function of different chelators utilized in clinical practice in the therapy of aluminium-depending diseases is discussed, providing some insight on the chelators recently proposed. As a last point the crystal structures of selected Al(III) complexes are presented and thoroughly discussed.
... In humans, silicic acid seems to reduce gastrointestinal absorption of Al(III) and to enhance its excretion through the kidneys [23,24]. Also few researches have reported that correlation of aluminium and Alzheimer's disease has led to the use of silicic acid in beverages [25], due to its abilities to both reduce aluminium uptake in the digestive system as well as cause renal excretion of aluminium. ...
Presence of fluorides in drinking water has become a public health problem. Aluminium compounds are popular for defluoridation of water owing to high affinity of fluoride toward aluminium. Use of these compounds may lead to high aluminium concentrations in drinking water. Aluminium is found to be a potential neurotoxicant. Synergistic associations of both aluminium and fluorides in the drinking water supply have been expounded by researchers. Aluminium–fluoride complexes also increase the risk of developing Alzheimer’s disease. Therefore, it is imperative to control the residual aluminium in the water. In the present work, the electrocoagulation process with aluminium electrodes has been used for defluoridation of water. In subsequent steps, activated silica sol has been used as a coagulant aid to remove aluminium from defluoridated water. Taguchi design has been used to develop a statistical model for aluminium removal. The experimental investigations revealed that activated silica sol reduces residual aluminium to a range of 0.003–0.034 mg/L.
... For waters with more residence time, such as groundwaters, waste rock pore waters, and tailings pore water, the attenuation of Al occurs at slightly lower pH values, closer to 4 than 5, and indicates a pH buffering reaction ( Blowes et al., 2013). The attenuation of Al in subsurface waters near pH 4 is correlated with some silica attenuation in the ARD near Questa, New Mexico (Nordstrom, 2008) and may be related to the formation of an insoluble aluminum-silica oxyhydroxide phase ( Hochella et al., 1999;Exley et al., 2002). Numerous applications of geochemical speciation models have provided insight into the precipitation, sorption, and reactive transport processes of acid mine drainage. ...
... For waters with more residence time, such as groundwaters, waste rock pore waters, and tailings pore water, the attenuation of Al occurs at slightly lower pH values, closer to 4 than 5, and indicates a pH buffering reaction . The attenuation of Al in subsurface waters near pH 4 is correlated with some silica attenuation in the ARD near Questa, New Mexico (Nordstrom, 2008) and may be related to the formation of an insoluble aluminum-silica oxyhydroxide phase (Hochella et al., 1999;Exley et al., 2002). Numerous applications of geochemical speciation models have provided insight into the precipitation, sorption, and reactive transport processes of acid mine drainage. ...
The extraction of mineral resources requires access to the ore body through underground workings, or open pit operations, or through drillholes for solution mining. Mining and mineral processing generates large quantities of waste, including mill tailings, waste rock and refinery wastes, leach pads, and slag. These activities expose large surface areas of sulfide minerals to oxygen and water, resulting in accelerated oxidation of the sulfide minerals and potential for the generation of low-quality drainage. Although acid mine drainage is the most prevalent and damaging environmental concern associated with mining activities, generation of saline, basic and neutral drainage containing elevated concentrations of dissolved metals, non-metals, and metalloids has recently been recognized as a potential environmental concern. The oxidation of sulfide minerals in mine wastes is accelerated by microbial catalysis of the oxidation of ferrous iron and sulfide minerals. These reactions can lead to extremely acidic drainage and very high concentrations of dissolved constituents. Acid neutralization reactions through the dissolution of carbonate minerals and secondary aluminum and ferric hydroxide phases can moderate the effects of acid generation and enhance the formation of secondary minerals which may lessen the concentration of dissolved metals. Numerical models provide powerful tools for assessing impacts of these reactions on water quality.
... Generally, coagulant mainly includes inorganic, organic and biologic type, in which inorganic Al(Ⅲ) and Fe(Ⅲ) salts have been used widely due to their low price and good performance [1][2][3] in treating various water samples, so extensive researches have been conducted in evaluating their performance [4,5]. However, more studies have shown that residual Al in finished water is harmful to human and animals [6][7][8][9][10]. By contrast, Fe is an essential element for the formation of hemoglobin in body, and 10mg of Fe is necessary for an adult every day. ...
Organic modifier (PSFD) of poly-silicic-ferric (PSF) coagulant was prepared by adding dimethyldiallylammonium chloride (DMDAAC) as additives in PSF. The size distribution of PSFD using Photon Correlation Spectroscopy (PCS) method was characterized, in comparison with that of PSF. The results show that the increasing of PSFD product’s size was far larger than that of PSF at lower level. The size of PSFD hydrolysis products tended to change continuously and small size of materials disappeared after its hydrolysis, while the size of PSF particles had very uneven distribution and there always existed small size of hydrolysis products. The disappearance of size change against level between PSFD and PSF gives PSFD a higher bridging ability than PSF in real wastewater treatment process.
... Precipitation of Al from solution provides ≡Al-OH surface sites which are known to be highly attractive for Si(OH) 4 to form Al-O-Si bonds (see Dietzel, 2002 and references therein). This process ultimately leads to the formation of crystalline silicate phases such as halloysite or kaolinite (Exley et al., 2002). Therefore the presence of Al (and other ions, see e.g. ...
Mass-dependent isotope fractionation occurring during precipitation of solids in low-temperature environments often depends on precipitation rate. Using a series of precipitation experiments in which continuous precipitation and dissolution of Si solids is forced by daily cyclic freezing (solid formation) and thawing (solid re-dissolution), we show this dependence. We conducted six Si precipitation experiments for about 120 days with initial dissolved Si concentration of 1.6 mmol/l Si, at pH values between 4.5 and 7, with additions of 0.1 - 1 mM of dissolved aluminium (Al), and in the absence of Al. During all experiments increasing amounts of an X-ray amorphous silica-containing solid are formed. No Si isotope fractionation occurs during formation of almost pure Si solids, interpreted as an absence of Si isotope fractionation during polymerization of silicic acid. Si isotope fractionation occurs only in the high-Al concentration experiments, characterized by an enrichment of the light Si isotopes in the solids formed early. With ongoing duration of the experiments, a re-dissolution of these solids is indicated as the Si isotope value of the complementary solution shifts to lighter values and eventually reaches near-initial compositions. Hence, our high-Al experiments are characterized by a gradual shift from a regime that is dominated by unidirectional kinetic isotope fractionation with solids formed that are up to ≈ 5 lighter in their 30Si/28Si ratio than the corresponding solution, to one of steady-state between dissolution and precipitation with the 30Si/28Si ratio of the solid being almost identical to the solution (Δsolid − solution ≈ 0). This suggests that the enrichment of light Si isotopes found in natural environments is caused exclusively by a unidirectional kinetic isotope effect during fast precipitation of solids, aided by co-precipitation with Al phases or other carrier phases (e.g. Fe(III)). By contrast, during slow precipitation, or in the absence of a carrier phase like Al, no Si isotope fractionation is expected and solids obtain the composition of the ambient fluid.
... Diatoms contribute 40% of the primary productivity of the oceans, thus significantly affecting the silicon and carbon global budgets (Nelson et al., 1995). Silica (SiO 2 ) or dissolved silicon can provide nutrition and internal rigidity to the organism as in the case of higher plants such as bamboo, wheat, oat, barley, and horse-tails; provide a physical defense mechanism as in stinging nettles; chemical resistivity to fungal diseases in plants; and play a role in controlling aluminum and heavy metal toxicity in plants and animals (Iler, 1979; Frausto da Silva and Williams, 1991; Epstein, 1993; Marschner, 1995; Hodgson and Sangster, 1999; Exley et al., 2002, Richmond and Sussman, 2003). Silicateorganic complexes have been postulated to play a role in the evolution of early life (Bernal, 1951; Pierson et al., 1993; Phoenix et al., 2001). ...
Serine- and polysaccharide-enriched organic matrix is associated with biogenic silica such as diatom tests, sponge spicules, and phytoliths. We have used molecular orbital theory to determine the relative stability and 29Si NMR shifts of direct Si-O-C ester-like bonds versus hydrogen bonds between the monomeric silicic acid and the alcohol group on aliphatic organics such as serine and threitol (a polyacohol as proxy for polysaccharides). Preliminary results suggest that at neutral pHs, H-bonds and ester-bonds of four-fold coordinated silicon are of comparable stability. Formation of ester-like bonds with five-fold coordinated silicon is endothermic at neutral pHs but is stabilized at higher pHs. 29Si shifts of the H-bonded and ester-bonded complexes of four-fold coordinated silicon range from −55 to −73 ppm similar to monomeric inorganic silicic acid but far more positive than the −92, −102, and −110 ppm values observed experimentally in biogenic silicas. The five-coordinated silicon complexes yield shifts of −96 to −107 ppm. The latter range is within the range of inorganic, polymerized silica. If five-fold coordinated Si with direct Si-O-C bonds is present as a precursor or intermediate or stable species in biogenic silica, it could have escaped detection due to overlap with inorganic polymerized silica. Thus, 29Si NMR shifts are not necessarily diagnostic of the presence or absence of Si-O-C bonds in biogenic silica.
... This potentially dangerous exposure is related to a great extent to atmospheric acidification due to acid rain, which is causing a progressive acidification of the soil, followed by a massive export of aluminium from the crust of the earth to surface waters, putting vegetables, animals and humans in contact with absorbable cationic aluminium species, probably for the first time in their history (4) . Owing to acid rain, numerous metal ions, including aluminium are escaping from mineral deposits where they had been stored for billions of years as hydroxy-aluminosilicates (HAS) (5,6) , increasing the biological availability of aluminium to living organisms. According to this hypothesis, acid rain is acting as a key to the lock for aluminium release, causing its appearance in polluted waters. ...
Abstract The aim of this review is to attempt to answer extremely important questions related to aluminium-related diseases. Starting from an overview on the main sources of aluminium exposure in everyday life, the principal aspects of aluminium metabolism in humans have been taken into consideration in an attempt to enlighten the main metabolic pathways utilised by trivalent metal ions in different organs. The second part of this review is focused on the available evidence concerning the pathogenetic consequences of aluminium overload in human health, with particular attention to its putative role in bone and neurodegenerative human diseases.
... Orthosilicic acid is stable in an acid environment, and it can be transported in the colloidal condition. When the geochemical condition turns to neutralization, orthosilicic acid will precipitate, and the accompanying dehydration leads to precipitation of amorphous quartz (Exley et al., 2002). In the case of ZK4353, the underlying Maokou Formation carbonate can form the geochemical barrier to block the downward transportation of liquid acid and orthosilicic acid colloids, and the neutralization leads to the precipitation of amorphous quartz; quartz veins that occur on the boundary between the bauxite layer and the underlying carbonate are the evidence of desilicification ( Fig. 4E and F). ...
Two types of bauxite deposits have been discovered in the Fusui area, Guangxi Province, South China. The original Permian bauxite occurs on the unconformity between the Middle Permian Maokou Formation and the Upper Permian Heshan Formation. The Salento-type (or karstic accumulational) bauxite is composed of the Quaternary incompact sediments that transformed from the original Permian bauxite. Samples were collected from two drilling cores and two profiles in study area, and field observations and mineralogical and geochemical analyses are integrated to reconstruct the metallogenic process from the original Permian bauxite to the Quaternary Salento-type bauxite. In the Permian lateritization and bauxitization, alkali metals and alkali earth metals are preferentially dissolved and removed from the parent rocks due to the intense chemical weathering; other major elements from the original profile are also depleted, as shown by the mass-change calculation result. Elements with a low mobility (e.g., Ti, Zr, Cr, Nb and V) are positively correlated with Al. When the bauxite deposit is affected by the modern groundwater system in the near-surface environment, the elements in the deposit display varied mobility due to the leaching intensity and the drainage conditions. After the bauxite ore horizons are exposed in the surface environment, impurities (primarily Fe and Si) are removed from the profiles, and Al is enriched; the clay minerals (kaolinite and chlorite) in the ores convert into aluminum minerals (boehmite and diaspore). When the exposed original bauxite orebodies break down and accumulate as bauxite gravel in the modern karstic depressions, the Salento-type bauxite deposits appear, and the ore quality is further improved. The intensity of the leaching process and the drainage conditions are the most important factors that control the ore quality during the ore-forming process that converts the original bauxite into the Salento-type bauxite.
... Silicon helps in the elimination of acute aluminium toxicity in fish when silicic acid reacts with aluminium to form hydroxialuminosilicates (HAS), one of the predominant forms of Al in acidified environment, which are extremely insoluble and play an important role in controlling the release of Al from soil to the aquatic environment (Exley et al., 1997;Exley et al., 2002). However, Camilleri et al. (2003), when tested the hypothesis that silica reduces the toxicity of Al, found no significant effects of silica on reducing toxicity of Al at pH 5.0 to a tropical freshwater fish (Mogurnda mogurnda) in Australia as <1 % of Al being complexed with silicate. ...
Aluminium (Al) is one of the important factors in the toxicity of acidified waters to freshwater fish species because low pH and high concentrations of Al have been of particular concern in affected waters. Al mobilization in its soluble forms from soil to aquatic ecosystems is an important consequence of acidification of lakes and steams. There are different soluble forms of Al toxic to aquatic biota that may pass onto wider food wed becoming potentially toxic to all living organisms including human through bioaccumulation and biomagnifications processes. Gills, skeleton, kidney, liver and muscles are the main target organs for Al toxicity; former three being more susceptible. The effects of pH and Al on fish vary not only from species to species but also among different life stages. Though relationship between Al and toxicity has been well understood, to know the mechanisms through which Al exerts its toxic properties needs more research. Liming, shell-sand filter and additions of seawater, uses of silicon are some of the important processes used for the detoxification of acidity and Al.
... In recent works Exley hypothesized why biochemical evolution has proceeded in absence of biological reactive forms of aluminum, especially remarking the role of silicon that reduces the biological availability of aluminum forming hydroxyl-alumino-silicates. [79][80][81] Toxicity of aluminum salts has been demonstrated at elevated concentrations. Its major effects are related to aluminum deposition in bone and to the action in the central nervous system. ...
Metal ions poisoning can result from environmental factors, intentional action, or disruption of homeostasis. Although the origin of toxicity may be different, the treatment is similar. Chelation therapy aims to remove the excess of metal ions from tissues to stop further damage of cells. For almost every metal ion, molecules that are able to bind it and remove from the human body are known. Over the years some new chelating agents were discovered and introduced into clinical treatment. In this paper we have focused on typical chelators for metal ions, both essential and toxic for humans. The treatment of poisoning caused by essential metal ions is hard due to the risk of removing them from the biologically relevant molecules (e.g. enzymes). Acute metal ions poisoning is rather rare, so the development of chelators for such cases are historical, but prolonged toxicity of, especially, essential metal ions is extensively studied.
Silicon (Si) participates in the alleviation of plants to aluminum (Al) toxicity. However, the mechanisms of Al detoxification by Si needs to be further clarified, especially in soybean. We investigated the roles of citrate secretion and cell wall polysaccharides in Si-triggered soybean resistance to Al stress. Results showed that applying 2 mM Si alleviated elongation inhibition of soybean roots exposed to 100 or 200 µM Al for 24 h. Si application increased citrate exudation from roots under Al exposure, thereby reducing Al accumulation. Furthermore, Si promoted citrate synthesis through activating citrate synthase and inhibiting aconitase under Al stress. Quantitative real-time PCR assay showed that Si enhanced the expression level of citrate transporter genes (GmMATE13 and GmMATE47) under Al stress. In addition, Si supply reduced Al content in cell wall and in cell wall polysaccharides (pectin and hemicellulose 1) under Al exposure. Meanwhile, Si addition decreased the contents of pectin and hemicellulose 1, which were the main binding sites for Al in cell wall, under Al exposure. Furthermore, Si increased the degree of pectin methylesterification by inhibiting pectin methylesterase activity under Al stress, thereby reducing the ability of pectin to bind Al. These results suggest that Si can promote citrate efflux by regulating the synthesis and transport of citrate, thus chelating the toxic Al cations and alleviating Al toxicity. Besides, Si can also regulate the cell wall properties to reduce Al deposition in the cell wall of root tip, thereby improving Al tolerance of soybean.
Aluminum and silicon are contaminants found in formulations used to prepare parenteral nutrition. Both elements are leached from glass containers, mainly during the heating cycle for sterilization. Insoluble and biologically inactive species of hydroxyaluminosilicates have been shown to form in solutions containing Al and Si. Therefore, this interaction may play an important role in protecting the body against Al toxicity. In this study, the bioavailability of Al in the presence of Si, calcium gluconate (Gluc.) and potassium phosphate (Phosf.) was investigated in rats. The rats were divided into 10 groups of 5 animals each: control, Al, Si, Al + Si, Gluc, Gluc + Al, Gluc + Al + Si, Phosf, Phosf + Al and Phosf + Al + Si. The doses, consisting of 0.5 mg/kg/day Al and 2 mg/kg/day Si in the presence ou absence of Gluc. or Phosf., were intraperitoneally administered for 3 months. Tissues were analyzed for Al and Si content. Al accumulated in the liver, kidneys, and bones, and the simultaneous administration of Si decreased Al accumulation in these tissues. The presence of Si reduced the amount of Al present by 72% in the liver, by 45% in the kidneys and by 16% in bone. This effect was lees pronounced in the presence of parenteral nutrition compounds though. Si tissue accumulation was also observed, mainly when administered together with phosphate. These results suggest that Si may act as a protector against Al toxicity, by either reducing Al absorption or increasing its excretion, probably through hydroxyaluminosilicates formation. The presence of calcium gluconate and potassium phosphate decrease or inhibit this effect.
This article surveys the coordination chemistry of the main group metals (s- and p-block metals) published during the period 2000–2021. Within the p-block, metallic elements include Al–Tl for group 13, Ge–Pb for group 14, Sb and Bi for group 15, and Te for group 16. The lighter elements in groups 15 (N, P, As) and 16 (O, S, Se) typically behave as donor atoms in ligands. However, the distinction is blurred, and As(III) is included as a metal center with, for example, S- and Se-donor ligands, and Al(III) and Ga(III) complexes containing Sb-donor ligands are included. The aim of the review is to provide as comprehensive as coverage as possible so as to demonstrate different bonding modes of ligands, and the range of metal centers to which they coordinate. At the same time, the number of structurally characterized main group metal complexes is so extensive that a totally comprehensive survey has not been possible within the page constraints.
Silicon was once considered a biologically inert element, but recent research has shown its value for human health.Soluble silicic acid is the available form of silicon in human body. This paper reviews the absorption, distribution and metabolic characteristics of dietary soluble silicic acid in human body, as well as its relationship with human health.Available data show that it has lots of supporting evidences that dietary soluble silicic acid can prevent osteoporosis, maintain vascular health, improve the symptoms of Alzheimer's disease and multiple sclerosis.Its unique crosslinking ability and antagonism to toxic aluminum play a crucial role. In the early stage of human life, there is a strong demand for silicon, and the level of silicon in the aged is generally reduced, suggesting that pregnant women and old people should pay attention to the intake of soluble silicic acid. It is suggested to strengthen the basic and applied research on dietary soluble silicic acid,and gradually establish the relevant nutrition and hygiene standards.
Plants can undergo different conditions of environmental stress. Two types of stress have been categorized: the abiotic stress produced by the physical environment to which plants are exposed (e.g. high and low temperatures, drought, UV radiation, metals) and the biotic stress generated by pathogens, herbivores, and parasites. Plants perceive different types of environmental stress as an external signal, and signal transduction routes may subsequently be activated. In recent years, various authors have proposed different mechanisms that are activated by an external signal. The knowledge achieved from these processes is relevant, because of the implications for biotechnological applications, for example to obtain plant resistant to different stress conditions or to improve the production of secondary metabolites. In this chapter, we discuss the mechanisms that are activated and related to the production of secondary metabolites under the effect of the abiotic stress related to aluminum and silicon.
Considering the immaturity of babies’ gastrointestinal system, particularly those of newborn infants, elevated aluminum (Al) concentrations can be toxic, and exposure to this contaminant due to the substitution of breast milk by infant formulae may represent a risk. The main sources of Al contamination in these products include the variability in the raw material (milk or soy), the formula composition, the contamination during processing and the presence of additives or mineral supplements. However, there are few recent reports in the literature on the occurrence of Al in milk-based or soy-based infant formulae. Thus, the aim of the present work was to study the presence of Al in samples of infant formulae acquired on the local market in Campinas, Sao Paulo, Brazil and estimate children exposure to this element. The Al contents were determined using optical emission spectrometry with an inductively coupled plasma source. The results obtained presented maximum values of 1.46, 5.94 and 4.49 mg kg⁻¹ for starter, follow-up and specialized formulae, respectively. As from these data, it was estimated that the consumption of infant formulae (0–6 months) could reach 22.4% of the Al tolerable weekly intake (TWI), whereas a maximum value of 29.4% was observed for children between 12–24 months. The results obtained in this study show that the Al levels in infant formulae could suggest a potential concern for infants, and should therefore be monitored.
Restrictions on the utilization of municipal sludge in agriculture are related to pathogenic microorganisms, aluminum (Al), and heavy metals (HMs) present in the sludge. On the other hand, organic sludge contains such macronutrients as nitrogen (N), phosphorus (P), potassium (K), and various micronutrients are important for the growth of agricultural plants. Special technologies for the treatment of organic sludge, based on the combined use of active forms of silicon (Si) (concentrated monosilicic acid) and quicklime, were tested. Quicklime eliminates the sludge pathogens in two ways: it increases the pH value and the temperature. Concentrated monosilicic acid detoxifies the active forms of Al and HMs through the formation of insoluble silicates. In greenhouse tests conducted with barley, the behavior of Al, Ca, Cd, K, Mg, N, and Pb in the soil-plant system as influenced by Ca- and Si- treated sewage sludge was studied. The results showed evidence that quicklime and monosilicic acid treatment of municipal sludge could be promising with regard to its utilization and use in agriculture, the greenery of large cities, covering mine tailings, and for other purposes.
Imogolite is not available in large quantities from natural deposits, and its extraction requires several purification steps. Even though some research studies still use purified natural imogolite, most research activities and actual industrial applications use synthetic imogolite, which has been available since it was devised by Farmer et al. (1977). The various synthesis pathways and the current understanding of the imogolite formation mechanism are reviewed in this chapter.
The main techniques needed to characterise imogolite are reviewed. The global quality of an imogolite sample can be assessed by a simple visual inspection. It should be transparent and should display some birefringence if sufficiently concentrated when observed between crossed polarisers. To gain a deep knowledge about a particular sample, the most often used techniques are infrared (IR) spectroscopy, transmission electronic microscopy (TEM) and X-ray diffraction. The main results are reviewed and some less commonly used techniques are also described. It is stressed that emerging techniques (namely, cryo-TEM and small-angle X-ray scattering) are particularly useful for assessing the global shape and the diameter of imogolite samples. Some characterisation developments are still needed, especially for obtaining reliable and fast measurements of the nanotube length distribution.
Partial chemical equilibrium plays an important role in geochemical systems. In the short turn-over time zone of hydrogeological systems, the conditions for the full chemical equilibrium amongst groundwaters, mineral phases and gases are rarely fulfilled. In these systems, partial chemical equilibrium is often responsible for controlling activity of solutes. Examples of partial chemical equilibrium with secondary phases in silicate systems of the weathered zone are given, and the role of metastable and stable secondary silicate solids is discussed.
Potential of geochemical modelling for groundwater systems, with the most important up-to-date literature was concisely described. Case studies considering different methods of geochemical modelling used for understanding of groundwater systems geochemistry are presented. Most of case studies concern modelling of groundwater in short turn-over time zone in the Sudetes Mts. (SW Poland). Present-day geochemical modelling state-of-art in Poland was discussed and evaluated.
Chemical composition of more than 600 fresh and mineral groundwater used for drinking and medicinal purposes from aquifers in the Sudetes Mountains (Poland) have been examined recently. The water chemistry was interpreted in terms of the solubility of mineral phases which can control silicon and aluminium activity, especially from the view-point of hydroxyaluminosilicate colloids (HAS). In some of studied aquifers, a partial chemical equilibrium with secondary aluminosilicates occurs and the conditions for formation the colloid HASB type are reached and maintained. Research on physical identification of the HASB colloid in groundwater is currently ongoing. Confirmation of the HAS presence in natural water systems will have important geochemical, mineralogical, environmental and ecotoxicological implications. Hydroxyaluminosilicate colloids might be helpful in understanding mineral transformations in soils and weatheredzone, especially information of purely ordered secondary aluminosilicate minerals. Formation of HAS colloids lowers the total aluminium concentration in drinking waters and reduces human exposure to bioavailable toxic aluminium.
In recent years, a number of investigations have suggested that a high intake of silicon (Si) might limit the aluminum (Al) absorption from dietary sources. The aim of the current study was to assess if a daily moderate beer intake, as a source of dietary Si, might reduce the gastrointestinal absorption of Al and brain accumulation in mice. Animals in 3 groups received Al(NO 3)3 in the drinking water (500 μg/ml) for 3 months. Mice in 1 of these groups were also administered by gavage 2.5 ml of a commercial beer (5.5% volume) per week, while mice in the second group received by gavage 2.5 ml of a solution of silicic acid per week at a concentration of 40 mg Si/1. The third group did receive neither beer nor Si (positive control group), while a group of untreated mice served as the negative control group. At the end of the period of treatment, mice were housed in individual plastic metabolic cages and urines and feces were collected (24 h) for Al and Si analyses. After this, brains were removed for Al and Si analyses. The results show that a moderate consumption of beer might have a protective role in Al absorption and retention. The Si content in beer would reduce the oral absorption of Al and would diminish its accumulation in brain, a target tissue of Al toxicity.
Zur Restaurierung eutrophierter Seen kommen unterschiedliche chemische Fällmittel zum Einsatz. Eine neuartige Fällmittelkombination von Aluminat und Calciumhydroxid wurde in einem whole-lake-Experiment zur Restaurierung des Tiefwarensees (Mecklenburg-Vorpommern) eingesetzt. Dabei wurden im Laufe der 5 Jahre währenden Fällmittelzugabe ca. 2 t P zusätzlich im Sediment festgelegt. Dadurch wechselte der vorher hocheutrophe See in den mesotrophen Zustand. Mit Sedimentuntersuchungen und einer modellhaften Prognose wird nachgewiesen, dass die P-Retention erhöht und damit eine nachhaltige Wirkung der Restaurierung erzielt wurde. Weiterhin wurde ein neuentwickelter Nitrat-Depotstoff in Labortests und einem Enclosureversuch im Dagowsee (Brandenburg) angewandt. Der Depotstoff stellte das Nitrat zeitverzögert über mehrere Wochen nach der Applikation an der Sedimentoberfläche zur Verfügung. Die erhöhten Nitratkonzentrationen bewirkten zusammen mit dem gleichzeitig eingebrachten P-Bindungspartner Fe eine Vergrößerung der P-Bindungskapazität der behandelten Sedimente. Dadurch wurde die P-Rücklösung aus den Sedimenten mindestens ein Jahr lang beinahe vollständig unterdrückt. Beide Fällmittelkombinationen förderten zunächst hauptsächlich die Bildung anorganischer partikulärer P-Verbindungen im Sediment. Die bakteriellen Mineralisationsprozesse wurden nur indirekt bzw. zeitlich versetzt beeinflusst. Die sich andeutenden Veränderungen in Art und Intensität der mikrobiellen Stoffumsetzungen werden als Folge der Entwicklung der Sedimentation sowie des Sedimentmilieus diskutiert. Um das Gefahrenpotenzial beim Einsatz aluminiumhaltiger Fällmittel in der Seenrestaurierung einschätzen zu können, wurde unter Berücksichtigung der relevanten Mechanismen der Al-Toxizität und der Al-Chemie ein geeignetes Analyse-Verfahren für die Anwendung in karbonatreichen Seen angepasst und damit erstmals die gelösten monomeren Al-Hydroxide, die als potenziell toxisch wirkende Al-Spezies beschrieben werden, vor und während einer Maßnahme analytisch überwacht. Das Risiko einer Al-Giftwirkung auf Organismen im behandelten Tiefwarensee wird als sehr gering eingestuft.
This chapter focuses on the solubility of hydroxyaluminosilicates (HAS(s)) and the biological availability of aluminum. HAS(s) are formed in solutions in which the precipitation of Al(OH)3(s) is approached from either acidic or basic conditions or where Si(OH)4(aq) is mixed with preformed Al(OH)3(s). The initial step in the formation of HAS(s) is the "competitive" condensation of Si(OH)4(aq) across adjacent hydroxyl groups on a framework of Al(OH)3(s). HASA(s) are composed of aluminum and silicon in the ratio of 2:1 and are the predominant form of HAS(s) in solutions in which the initial [Si(OH)4(aq)] is less than or equal to [AlT]. HASB(s) are composed of aluminum and silicon in the ratio of 1:1 and are the predominant form of HAS(s) in solutions in which the initial [Si(OH)4(aq)] is significantly in excess (more than twice) of [AlT]. The formation of HASA(s) is a prerequisite to the formation of HASB(s), irrespective of the initial solution ratio of Si(OH)4(aq) and AlT. Thus, HASB(s) are formed via the further condensation of Si(OH)4(aq) across adjacent hydroxyl groups on a framework of HASA(s). HAS(s) are effective in limiting the biological availability of aluminum because of their kinetic inertia. Under certain conditions, HAS(s) have been shown to aggregate much more slowly toward their critical size than Al(OH)3(s), and it is under such conditions that HAS(s) are less stable toward dissolution than Al(OH)3(s). Consequently, HAS(s) are the potent sources of biologically available aluminum.
The Freundlich model and the Constant Capacitance Model (CCM) were used to describe silicon (Si) and phosphorus (P) sorption, both individually and for binary P-Si systems, on two Andisols with different chemical properties: Freire soil (FS) and Piedras Negras soil (PNS). Silicon sorption kinetics were examined through the Elovich equation, revealing that the initial sorption rate was 16times greater in PNS. The Freundlich equation provides a good fit to the sorption data for both Andisols. When compared with FS, larger Si sorption capacity and lower Si affinity for the surface sites were observed in PNS; nevertheless, Si sorption decreased in both soils as P sorption increased. Slight reductions in P sorption capacity due to the presence of Si were found, whereas there was no apparent effect on P bonding intensity. The CCM was able to describe Si adsorption, and potentiometric titrations support that Si seems to be specifically sorbed mainly onto sites of negative charge. Comparable logKSi(int) values were obtained for both soils, indicating that Si was bound on similar sites. Phosphorus sorption was well described by the CCM, and logKP(int) denoted strong interactions of P with the surface sites. For binary systems, logKP(int) did not vary with increasing Si concentration; comparatively, logKSi(int) scarcely decreased with increasing P concentration in PNS, but a 28% reduction was found in FS at the highest initial P concentration.
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Silicon is the second most abundant element in nature behind oxygen. As a metalloid, silicon has been used in many industrial applications including use as an additive in the food and beverage industry. As a result, humans come into contact with silicon through both environmental exposures but also as a dietary component. Moreover, many forms of silicon, that is, Si bound to oxygen, are water-soluble, absorbable, and potentially bioavailable to humans presumably with biological activity. However, the specific biochemical or physiological functions of silicon, if any, are largely unknown although generally thought to exist. As a result, there is growing interest in the potential therapeutic effects of water-soluble silica on human health. For example, silicon has been suggested to exhibit roles in the structural integrity of nails, hair, and skin, overall collagen synthesis, bone mineralization, and bone health and reduced metal accumulation in Alzheimer's disease, immune system health, and reduction of the risk for atherosclerosis. Although emerging research is promising, much additional, corroborative research is needed particularly regarding speciation of health-promoting forms of silicon and its relative bioavailability. Orthosilicic acid is the major form of bioavailable silicon whereas thin fibrous crystalline asbestos is a health hazard promoting asbestosis and significant impairment of lung function and increased cancer risk. It has been proposed that relatively insoluble forms of silica can also release small but meaningful quantities of silicon into biological compartments. For example, colloidal silicic acid, silica gel, and zeolites, although relatively insoluble in water, can increase concentrations of water-soluble silica and are thought to rely on specific structural physicochemical characteristics. Collectively, the food supply contributes enough silicon in the forms aforementioned that could be absorbed and significantly improve overall human health despite the negative perception of silica as a health hazard. This review discusses the possible biological potential of the metalloid silicon as bioavailable orthosilicic acid and the potential beneficial effects on human health.
The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The
measurements were performed as potentiometric titrations using a hydrogen electrode with OH
− ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si
tot
] and [Al
t
ot], and −log[H
+] were varied within the limits 0.3 < [Si
tot
] < 2.5 mM, 0.5 < [Al
tot
] < 2.6 mM, and 2 ≤ -log[H
+] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH)
3
2+
complex with a formation constant log β1,1-1 = −2.75 ± 0.1 defined by the reaction
Al
3++Si (OH)4 ↔ AlOSi(OH)
3
2+
+H
+
An extrapolation of this value to I=0 gives log β1,1-1 = −2.30. The calculated value of log K (Al
3++SiO(OH)
3
−
↔ AlOSi(OH)
3
2+
) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF
2+ and AlOH
2+ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH
−>SiO(OH)
3
−
> F
−
A new composite polymer, poly-Si-Fe coagulant (PSF), was prepared by copolymerization. The influences of flocculation time, flocculation speed and settling time on the amount of residual particulates in treated water after coagulation using PSF as coagulant in treating various water samples were investigated in comparison with those of polyferric aluminum (PFA). Flocculation coefficient (KA) and destruction coefficient (KB) of PSF and PFA were calculated theoretically. The results show that a close collaboration between flocculation speed and flocculation time of PSF should be performed in order to achieve higher coagulation efficiency. A moderate flocculation speed and flocculation time should be applied in treating hight urbidity water, and a moderate flocculation speed and longer flocculation time should be applied to treat low turbidity water. For surface water, flocculation speed should not be too higher or too lower by using PSF. PFA floc scan be smashed easily than PSF with increasing the flocculation speed, and PSF flocs which have been smashed tends to be more easily re-coupled than PFA. KA of PSF is from 10 to10 in magnitudes, in comparison with KB from10 to 10 in magnitudes. For flocs whose size is smaller than 2 μm, KA of PSF is larger four magnitudes than that of PFA, and KB of PSF is almost the same as that of PFA in magnitudes. While for the other sizes of flocs, KA of PSF is larger two magnitudes than that of PFA, and KB of PSF is 10 times smaller than that of PFA. PSF flocs of smaller size can be connected easily, which favors the improvement of coagulation performance, and PSF flocs of larger size have great density and cannot be easily smashed by turbulent shear force.
Organic modifier (PSFD) of poly-silicic-ferric (PSF) coagulant was prepared by adding dimethyldiallylammonium chloride (DMDAAC) as additives in PSF, and was characterized by Scanning Electron Microscopy (SEM), Photon Correlation Spectroscopy (PCS) and Fourier Transform Infrared Spectroscopy (FT-IR). Color removal and floc change during coagulation process in treating a simulated dyeing wastewater by PSFD were investigated compared to those of PSF. The results show that a similar silicate of cross-linking matter was generated in PSFD, and PSFD has greater adsorbtion capacity than PSF. The size of PSFD hydrolysis products was larger than that of PSF over a given concentration range. The size of PSFD hydrolysis products tended to change continuously and the small size of products disappeared after its hydrolysis. PSFD has better color removal at given dosage range than PSF, with the largest difference of 30%. The floc growth and settling velocity of PSFD were larger than those of PSF. Larger flocs of PSFD were formed from its small initial flocs over a very short period of time. The floc density of PSFD was greater than that of PSF, leading to a much less amount of PSFD flocs in the finished water than that of PSF.
In several experiments albite, orthoclase, diopside, muscovite, sepiolite, magadiite, kenyaite, natrosilite, δ-Na2Si2O5, sodium metasilicate, kanemite, quartz, opal, amorphous silica, and silica gel were dissolved in sulfuric aqueous solutions at pH 3. The experimental results show that most of the solutions contain silicic acid as both polymeric and monomeric species. Polysilicic acid is measured as high- and as low-molecular-weight silica. The polymers may amount to ∼50 mol.% of total dissolved silica. The proportion of polymers varies with reaction time of the dissolution experiment. As a function of time, polymers decompose into monomeric species, which is the stable silica species at experimental conditions. Therefore, the transfer of polysilicic acid into the solution represents a transition stage during the dissolution of silicates. The pH, the temperature, and the kind of the dissolved components of most natural waters provide high depolymerisation capacities. Polysilicic acid decomposes to monomer within a few hours or days in river and seawater. Therefore, it is not surprising that in most natural waters silicic acid consists of monomeric species. However, in natural environments with acid solutions rich in bivalent cations, polysilicic acid may exist as a metastable component over several months.
AN increased level of aluminium in acidified natural waters is a primary cause of fish death from damage to gill epithelia and loss of osmoregulatory capacity1-4. Aluminium toxicity depends on the species of aluminium present (cationic, neutral or anionic) and hence is affected by pH and the presence of complexing ligands, such as fluoride, and organic material, such as humic acid, which may ameliorate aluminium toxicity5,6. But silicic acid, Si(OH) 4, present in natural waters as a consequence of the weathering of the aluminosilicates of rocks and soil minerals, has a strong and unique affinity for aluminium7, although its influence on toxicity has not been investigated. Here we show that, with an excess of Si over Al and with the formation of hydroxy-aluminosilicate species, the bioavailability of aluminium at pH 5 is reduced and acute toxicity is eliminated. Silicic acid concentration should therefore be considered as a key parameter in toxicity studies and could be important for the treatment of vulnerable waters.
Aluminium toxicity in dialysis patients is well described. Aluminium has a close chemical affinity with silicon. Silicon may have a role in protection against aluminium toxicity.
We measured serum aluminium and silicon levels from haemodialysis patients from four different centres.
Though no relationship was seen across all centres combined, in one centre there was a reciprocal relationship in patients on home haemodialysis (who did not require reverse osmosis). Median (range) aluminium levels were higher, 2.2 (0.4-9.6) micromol/l when serum silicon was less than 150 micromol/l, and lower, 1.1 (0.2-2.8) micromol/l when serum silicon levels were greater than 150 micromol/l (P = 0.03).
In patients treated by haemodialysis without reverse osmosis high serum silicon concentrations were associated with lower serum aluminium concentrations than those with low serum silicon. Further work needs to confirm a preventative role for silicon in the accumulation and subsequent toxicity of aluminium in dialysis patients.
To investigate the effect of aluminum and silica in drinking water on the risk of dementia and Alzheimer's disease, the authors analyzed data from a large prospective cohort (Paquid), including 3,777 subjects aged 65 years and over living at home in 75 civil parishes in Gironde and Dordogne in southwestern France in 1988-1989. The subjects were followed for up for 8 years with an active search for incident cases of dementia or Alzheimer's disease. Mean exposure to aluminum and silica in drinking water was estimated in each area. The sample studied included 2,698 nondemented subjects at baseline, for whom components of drinking water and covariates were available. A total of 253 incident cases of dementia (with 17 exposed to high levels of aluminum), including 182 Alzheimer's disease (with 13 exposed to high aluminum levels), were identified. The relative risk of dementia adjusted for age, gender, educational level, place of residence, and wine consumption was 1.99 (95 percent CI: 1.20, 3.28) for subjects exposed to an aluminum concentration greater than 0.1 mg/liter. This result was confirmed for Alzheimer's disease (adjusted relative risk = 2.14, 95 percent CI: 1.21, 3.80). However, no dose-response relation was found. Inversely, the adjusted relative risk of dementia for subjects exposed to silica (> or = 11.25 mg/liter) was 0.74 (95 percent CI: 0.58, 0.96). These findings support the hypothesis that a high concentration of aluminum in drinking water may be a risk factor for Alzheimer's disease.
The unique properties of imogolite are closely related to its structure, which is a tube of 23-27 A outer diameter and approx 10 A inner diameter, with an AlOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 A intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, film- and fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated. (Authors' abstract) -D.J.M.
source of aluminium exposure is from aluminiumcontaining phosphate binders as this may remain the Background. Aluminium toxicity in dialysis patients is well described. Aluminium has a close chemical aYnity only binder that can eVectively control hyperphosphataemia and consequent renal osteodystrophy. with silicon. Silicon may have a role in protection against aluminium toxicity. Silicon is the second most common element in the earth’s crust. Over the past 20 years its biological Methods. We measured serum aluminium and silicon levels from haemodialysis patients from four diVerent importance has become increasingly recognized and investigated. It is claimed to be required in bone,
The mechanisms and products of hydrolytic reactions of Al ions as influenced by Si(OH) 4 were studied under the experimental parameters of degere of hydrolysis of Al (OH/Al molar ratios,) Si/Al molar ratios, Al concentrations of 1.39 × 10 ‐3 M and 1.39 × 10 ‐4 M , and time.
As the degree of hydrolysis of Al increased, the amounts of Si along with Al initially precipitated and removed from the solution phase by ultrafiltration consistently increased. The coprecipitation data, infrared evidence, and stoichiometric considerations of Si(OH) 4 removal and proton release strongly indicate that the predominant coprecipitation of Si(OH) 4 with hydroxy‐Al ions is the condensation reaction involving the hydroxyl groups in hydroxy‐Al ions. At the OH/Al molar ratios of 1 and 2 and in the absence of Si(OH) 4 , precipitation of Al continued with aging; whereas at the Si/Al molar ratios of 0.5 and 1, the reaction products were metastable, and there was formation of a significant portion of nonextractable Al which was not removed by ultrafiltration.
In the absence of Si(OH) 4 , the product was crystalline Al(OH) 3 even at the OH/Al molar ratio of 1. Materials amorphous to X‐ray and electron diffraction were the only products at Si/Al molar ratios of 0.5 and 1, OH/Al molar ratios of 2 and 3, and both initial Al concentrations even after prolonged aging. Decreasing degree of hydrolysis of Al resulted in a distinct decrease in particle size for crystalline products, and in a decrease in the extent of aggregation for amorphous products. The thermal stability and Si/Al ratio of the reaction products were influenced by the Si/Al molar ratio of solution and the period of aging. The presence of Si(OH) 4 in solutions also resulted in a decreased Cl inclusion in the reaction products.
The results clearly show that the presence of Si(OH) 4 affects the hydrolytic reactions of Al at all degrees of hydrolysis of Al. This knowledge is of basic significance for a further understanding of pedogenic processes, and the fate of certain nutrients and environmental pollutants.
The extent of the reaction A13+ + Si(OH)4 → AlOSi(OH)32+ + H+ has been assessed by measuring the shift in log[H+] between solutions containing A1(ClO4)3, HClO4, and NaClO4 with and without added silicic acid. The observed shifts of 0.018–0.027 units are about one tenth of those predicted from the formation constant (pK110 = 1.07 ± 0.06) proposed for the monomeric aluminosilicate species by Browne and Driscoli (1992). An estimate of this formation constant on the basis of the shifts in log[H+] (pK110 = 2.50 ± 0.05) indicates that the concentration of aluminosilicate species can be neglected in many natural waters.
Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al13 that could be reached in waters in the absence of complexing ligands other than OH−. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al13 concentration in mildly acidic natural waters may not exceed a few μmol l−1 at AlT on the order of 10−4 mol l−1 and should be less than 1 μmol l−1 at AlT=10−5 mol l−1. Monomeric Al–Si species may not significantly interfere with the formation of Al13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al13 concentration. The maximum concentration of Al13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al13 may occur increases slightly with temperature and the most acidic pH value above which Al13 may be formed has been underestimated. At T=25°C, the Al13 polycation may be a significant Al species (4 to 5% of AlT) at pH < 4.5 if AlT > 10−4 mol l−1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al13 should be negligible in natural soil and surface waters and may not control either Al3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al13 to plants and aquatic organisms in natural conditions may be considered to be very low.
The essentiality of silicon in biota is described from the perspective of the interrelationships between geochemistry, biological evolution and biochemistry. A punctuated, as opposed to continuous, silicon bi-cycle implicates the environmental silicic acid concentration in such seemingly diverse processes as catchment acidification, global warming and chronic disease in man.
Mr Nakasone, Minister of State for Science and Technology, puts forward the view that the ``soft'' sciences will have to be encouraged more strongly in the future.
The unique properties of imogolite are closely related to its structure, which is a tube of 23–27 Å outer diameter and ∼10 Å inner diameter, with an AIOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively-charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively-charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 Å intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, film- and fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated.
Silicon is an essential element for growth in diatoms. It is, for example, used to build the silicieous frustule surrounding the diatom cell wall. Silicon starvation studies have also inferred that silicon is essential in the vital metabolic processes of DNA and chlorophyll synthesis. The mechanism of essentiality is, however, uncertain. In laboratory cultures at circumneutral pH and using a non-growth-limiting, but environmentally realistic, concentration of the essential nutrient phosphorus, we have demonstrated aluminium toxicity in the freshwater diatom Navicula pelliculosa and the amelioration of this toxicity with silicon, present in aqueous solution as silicic acid. The mechanism of aluminium toxicity was an aluminium-induced reduction in the biologically available phosphorus fraction and silicic acid protected against this effect by preferentially binding aluminium in competition with phosphorus. Silicon-stimulated growth in the presence of aluminium was also demonstrated in the non-silicon-dependent green alga, Chlorella vulgaris, in which, once again, the increase in growth rate upon the addition of silicon could be directly correlated with an increase in the biologically available phosphorus fraction. We contend that a possible hitherto unrecognized mechanism of silicon essentiality in biology is to increase the biological availability of phosphorus in the presence of aluminium.
Hydroxyaluminosilicates (HAS) are critical intermediates in the biogeochemical cycles of alumi-nium and silicon. To understand the extent of their role in controlling the solubility of Al in soil and surface waters, we need to know how they are formed and why they are formed in preference to other more soluble aluminous mineral phases. We have used a number of complementary analytical techniques to demonstrate the hitherto unproven mechanism of HAS formation of varying structure and stoichiometry. HAS were formed via the competitive condensation of silicic acid, Si(OH) 4 , at a hydroxyaluminum template (HAS A). Where Si(OH) 4 was present in excess, HAS A acted as a template for further reactions with Si(OH) 4 (HAS B). HAS A and HAS B had idealised Si : Al ratios of 0.5 and 1.0, respectively, and were representative of HAS found in soil horizons. This mechanism of formation of HAS could be used to both explain and predict the role of Si(OH) 4 in Al solubility control in the natural environment. Copyright © 2001 Elsevier Science Ltd
We examined the aluminium solubility in the upper B horizon of podzols and its relation to the solid phase of the soil in 60 samples covering a pH range from 3.8 to 5.1. Solid phases were characterized by extractions with acid oxalate and pyrophosphate (pH 10). The solubility of Al was studied in a batch experiment in which samples were equilibrated with 1 mm NaCl at 8°C for 5 days. We also monitored the dissolution kinetics of Al and Si, in some samples. The oxalate and pyrophosphate extractions suggested that secondary Al was mainly organically bound in most soils, and imogolite-type materials seemed to constitute much of inorganic secondary Al. No single gibbsite or imogolite equilibrium could explain Al3+ activities. In all samples Al solubility, defined as log{Al3+} + 1.65pH, was closely related to the molar ratio of aluminium to carbon in the pyrophosphate extracts (Alp/Cp). Solubility increased with the Alp/Cp ratio until the latter reached ≈ 0.1. This indicated that solubility was controlled by organic complexation, at least when Alp/Cp was small. Silica dissolved slowly in most soils used in the kinetic experiments. We conclude that imogolite-type materials in the upper B horizon dissolved slowly because of coating with humic substances or ageing or both.
1.(1) Silicic acid alone does not bind A13+ (10−3 M) at pH values below 6.8, but above this an association is formed which is not broken by addition of the dye chrome azurol S. The precipitate which forms slowly between silicic acid and Al3+ can be removed by centrifugation. At this concentration (10−3 M), Al3+ alone precipitates close to pH 7.2.(2) Phosvitin binds Al3+ at pH 5.0, with a KD of 10−8 M (E. Rowatt and R.J.P. Williams, J. Inorg. Biochem., 55 (1994) 249; T.P. Geladopoulos and T.G. Sotiroudis, J. Inorg. Biochem., 54 (1994) 247) with a ratio of Al3+ to phosvitin of 70. The complex precipitates when about half the Al3+ is added but redissolves at high concentrations of phosvitin.3.(3) Silicic acid does not remove Al3+ from phosvitin and phosvitin does not remove Al3+ from a mature precipitate of aluminosilicate, although it removes some Al3+ if it is added to aluminium newly associated with silicic acid.4.(4) The relationships between aluminium, phosvitin and silicic acid can only be explained by taking into account the kinetics of aging of aluminium-silicic acid precipitates.
The simple fractionation procedure membrane filtration was successfully applied to the identification of the formation of hydroxyaluminosilicates. The mechanism of formation was shown to proceed through the inhibition of the nucleation of aluminium hydroxide. The inhibition was the result of silicic acid replacing hydroxylated aluminium at growth sites on aluminium hydroxide lattices. The ability of silicic acid to poison the growth of aluminium hydroxide lattice was dependent upon solution pH and the silicic acid concentration. This may be the first fully supported observation of a reaction of this ilk involving the neutral silicic acid molecule.
The influence of different Al concentrations, (0, 60 and 120 M Al) on growth and internal concentrations of Al, Si and selected organic acids was analysed in plants of teosinte (Zea mays L. ssp. mexicana), a wild form of maize from acid soils from Mexico. The plants were grown in nutrient solutions (pH 4.0) with or without 4 M silicon. Analysis with the GEOCHEM speciation program did not reveal differences between free activities of Al3+ in solutions with and without 4 M Si, but solutions with Si yielded lower concentrations of monomeric Al species, [Al]mono, when analysed by a modified aluminon method. Plants grown on solutions with similar [Al]mono, but differing in silicon, showed highly significant differences in growth and tissue concentrations of Al and organic acids. Silicon prevented growth inhibition at [Al]mono concentrations as high as 35 M, while plants grown without Si suffered severe growth reductions with 33 M [Al]mono. In solutions with similar [Al]mono concentrations plants with Si had lower tissue Al concentrations and higher concentrations of malic acid than plants without Si. In view of both the significant influence of Si on the response of plants to Al toxicity and the fact that some soluble Si is always present in soil solutions, the addition of low Si concentrations to nutrient solutions used for Al-tolerance screening is recommended.
The influence of Si pretreatment on Al toxicity in an Al sensitive maize variety (Zea mays L. var. BR 201 F) was investigated using root elongation rates (RER) and hematoxylin staining as stress indicators. Plants pretreated with 1 mt M Si (+ Si) and then exposed for 24 h to Al in nutrient solution without concurrent Si supply in the rooting medium exhibited higher RER than plants that were not pretreated with Si (-Si). The ameliorative effect of Si was due to lower Al uptake and to the exclusion of Al from the root tips in + Si plants. Lower Al uptake in + Si plants was not a consequence of decreased Al availability in the bulk solution. The possible mechanisms of Si-induced increase of Al resistance are discussed
Aluminum(III) forms complexes with aqueous silicic acid in neutral or mildly acidic media, and these species are important in the protection of plant and animal life against aluminum toxicity. In highly alkaline media, surprisingly high concentrations of various aluminosilicate solute species can be achieved at least temporarily, the lifetime of the homogeneous solution depending on the pH, the nature of the cation(s) present, and the Al and Si concentrations. The longevity of aluminosilicate complexes in solution may have technological consequences — for example, in causing silica carryover in the Bayer process for aluminum production. The kinetics of silicate exchange on small, acyclic aluminosilicate solute species in alkaline solutions are much faster than on either the corresponding all-silicate ions or cyclic aluminosilicate species. The kinetic lability of small aluminosilicate species is attributable in part to the ability of AlIII to expand its coordination number easily from four to six in acyclic structures, but also to the availability of OH groups for condensation reactions on aluminate centers, even at high pH where SiOH functions become deprotonated. One implication is that, contrary to the traditional picture of zeolite formation from structured ‘secondary building units’ pre-existing in solution, cyclic and cage-like aluminosilicate solute species are not directly involved in the crystallization of solid aluminosilicates such as zeolites from aqueous solutions but simply serve as reservoirs for small, active, acyclic species responsible for crystal growth.
The discriminatory power of dialysis, ion exchange and membrane filtration techniques to identify hydroxyaluminosilicate (HAS) formation in solutions of near-physiological aluminium concentration has been tested. Results indicate the hitherto unreported formation of HAS in solutions of low total aluminium concentration. The identification of these species was dependent upon both the pH and the silicic acid concentration of the solution. It was also found that the method of preparation of solutions influenced the discriminatory power of the different techniques and this led us to suggest that the mechanism of HAS formation involved the poisoning of aluminium hydroxide polymerization by silicic acid.
Aluminosilicates have been identified at the core of senile plaques in Alzheimer's disease, and aluminum has been found within neurons bearing neurofibrillary tangles. Here we show that aluminum species interact with silicic acid, Si(OH)4--a normal component of plasma--to form aluminosilicate species solubilized by citrate. A switch in the binding of aluminum from silicate to phosphate at pH less than 6.6 calls attention to the strong binding of cationic aluminum species to proximate phosphate groups, as in the inositol phosphates, and to the potential effect on the activity of the phosphoinositide-derived intracellular messenger system. The chemistry may throw light on the debated relationship between aluminum and Alzheimer's disease.
We have used the HK/G6PDH coupled enzyme assay to determine the biological availability of aluminum in mixed-ligand media of biological interest. The biological availability of aluminum was measured as the inhibition of the activity rate of the assay and was shown to be dependent upon the equilibration state of the aluminum stock solutions (prior to their addition to the assay) and the comparative reaction kinetics of competitive aluminum equilibria in the assays. Aluminum was found to inhibit the assay, however, the inhibition by aluminum was abolished when silicic acid was present in both the aluminum stock solution and the assay medium. The assay is proposed as a model system for investigating the biological availability of aluminum in heterogenous media of biochemical significance.
The reported geographical association between Alzheimer's disease and levels of aluminium (Al) in water supplies may reflect the inverse relation between Al and silicon (Si) concentrations in water, and the potential for Si to reduce the bioavailability of the metal. We tested this hypothesis using isotopic 26Al tracer administered orally to five healthy volunteers in the presence and absence of Si. Dissolved Si, at a concentration found in some water supplies (100 mumol/L), reduced the peak plasma 26Al concentration to 15% of the value obtained in the absence of Si. The results indicate that dissolved Si is an important factor in limiting the absorption of dietary Al.
The chemical affinity of silicic acid for aluminium (AI) has been shown to reduce the bioavailability of AI in studies of human gastrointestinal (GI) absorption. Investigations were carried out to ascertain whether or not similar interactions may also enhance the renal excretion of AI by assessing the urinary output of both elements. Healthy individuals given monosilicic acid as naturally found in beer, excreted the majority of the silicic acid content (mean 56 percent) within 8 hours, concomitant with a significant increase in AI excretion (P < 0.05). Ingestion of increasing doses of silicic acid resulted in dose related increases in excretion of Si. Excretion of AI reached a maximum and then declined, consistent with depletion of AI body stores. This was confirmed using the 26AI isotope. The low serum but high urine concentration of Si suggests that if AI and Si interact to form an excretable species they do so in the kidney lumen such that Si limits the reabsorption of AI. Silicic acid's effect on the depletion of aluminium stores and reduced GI absorption suggest its addition to municipal water supplies may be a low risk public health measure to reduce the AI burden in the general population.
We studied the relation between silica and aluminum levels in drinking water and the risk of cognitive impairment using data from a population-based survey of 3,777 French subjects age 65 years and older. We also studied the effect of pH and the concentrations of calcium, magnesium, fluorine, zinc, copper, and iron. We used a mixed effects logistic regression adjusting for age, sex, educational level, and occupation of the subjects. We confirmed the inverse relation previously found between calcium level and cognitive impairment. We found no important association between cognitive impairment and fluorine, magnesium, iron, copper, or zinc. The association between cognitive impairment and aluminum depended on the pH and the concentration of silica: high levels of aluminum appeared to have a deleterious effect when the silica concentration was low, but there was a protective effect when the pH and the silica level were high. The threshold for an aluminum effect, however, was very low (3.5 micrograms per liter) and did not support the hypothesis of a deleterious effect for only high levels of aluminum.
The essentiality of silicon in biology might be explained in the terms of its chemistry with aluminium. In a previous study we demonstrated the elimination of acute aluminium toxicity in fish by silicon. We suggested that the reaction of silicic acid with aluminium to form hydroxyaluminosilicates reduced the biological availability, and hence toxicity, of aluminium. Though assumed in a burgeoning number of studies and contended in others this detoxification mechanism has remained unproven. Herein we have tested the toxicity of hydroxyaluminosilicates in fish and in doing so we have provided evidence which strongly supports a role for hydroxyaluminiosilicates in the elimination of acute aluminium toxicity in fish by silicon.Copyright 1997 Academic Press Limited Copyright 1997 Academic Press Limited
Morin-aluminium fluorescence and membrane filtration were successfully applied to the indirect identification of the formation of hydroxyaluminosilicates (HAS) in acidic solutions of varying pH and of known concentrations of aluminium (Al) and silicic acid (Si(OH)(4)). It was proven to be especially useful in providing evidence of the strong competition between Si(OH)(4) and Al(OH)(3) to condense with hydroxyaluminium templates to form HAS in preference to Al(OH)(3(s)). The aggregation and stability of HAS and Al(OH)(3(s)) were dependent upon both the pH and the [Al] of the solution. The applicability of these indirect techniques was confirmed using the direct observation of HAS in solution by atomic force microscopy (AFM). AFM was also a powerful tool in providing valuable information on the morphology of colloidal HAS of various structures and stoichiometries. The results have provided further confirmation of both the mechanism of HAS formation and the form and stability of HAS in solution. This information is essential to our understanding of the biological availability and hence toxicity of Al in biota, including man.
Silicon-29 NMR spectra of a neutral, dilute aqueous silicic
acid solution, with a pH and Si concentration typical of soil solutions,
reveal that a significant fraction of the silicon is incorporated in two
five-coordinated organosilicon complexes when sodium gluconate is
present.
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