INTRODUCTION

There have been few previous studies of silicon and aluminium in curative waters. Silicon (27.7% by weight) and aluminium (8.1%) are – after oxygen (46.6%) – the most abundant elements in the continental Earth’s crust. The hydrolytic decay of silicate minerals, which compose about 90% by weight of the Earth’s crust, is the primary mechanism of release of silicon and aluminium into natural waters. Aluminium occurs in the majority of silicate minerals, and consequently, their dissolution delivers both elements into solution.

The geochemistries of both elements are interrelated in the natural environment. Silicon and aluminium differ significantly in their chemical properties, and despite their often common origin, they also differ in their solubilities and solution behaviour during their migration in natural waters. For example, the silicon concentration of natural waters is always significantly higher than that of aluminium. In fresh groundwater of near-neutral pH, silicon usually occurs at concentrations of 0.1 mM to 0.3 mM, whereas aluminium concentrations are usually in the range 0.1 μM to 10 μM (1, 2).

The maximum admissible concentration of silicon in drinking water is not regulated. However, in some countries, e.g. Poland (3) silicon is regarded as a beneficial component which provides therapeutic benefits in groundwaters used in balneology. In Poland, the established threshold value for siliceous curative water is 70 mg/L H2SiO3 (≈0.9 mM). The predominant form of aqueous and biologically available silicon in natural waters and also in curative water, is orthosilicic acid, H4SiO4 (4). Silicic acid is the only form of silicon which is biologically available and in humans it is absorbed across the gut before being excreted via the kidneys in urine. While there is evidence that silicic acid is deposited as amorphous silica in various tissues, e.g. connective tissues and bone, in humans the mechanism whereby this takes place is unknown. Historically many studies have argued for beneficial, even essential, effects of silicon in humans. (e.g. 5, 6). For example, silicon is suggested to improve bone mineralisation; slow down bone decalcification and accelerate bone regeneration after injury; increase elasticity of blood vessels; reduce aluminium intake in healthy individuals and in patients with Alzheimer’s disease; improve the resistance of skin against mycosis and bacterial infections and to strengthen and recuperate skin, hair and nails (e.g. 7, 8, 9, 10, 11, 12). By way of contrast, aluminium is non-essential and a known toxicant and is commonly considered as an undesirable component in drinking waters and in relation to all forms of human exposure (13). The maximum admissible concentration of aluminium in drinking water in most countries, including Poland, is 0.200 mg/L Al. However, this limit was not established upon the grounds of human health but upon the aesthetic quality of potable waters treated with aluminium salts (14). Aluminium intake in drinking water is estimated to be less than 5% of total oral intake (14).

The solubility of silicon in water depends mainly on pH and temperature. It shows a rapid increase in solubility above pH ca 9.8, at which point silicic acid loses its first proton to form silicate species. High concentrations of silicon are also be found in thermal waters, and/or strongly acid groundwater, and/or in slightly acidified groundwater which occurs in bedrocks composed of easily weatherable silicate mineral phases, for example in modern or young volcanogenic rocks (15). Aluminium solubility depends on pH and the formation of stable, soluble complexes with various inorganic (mainly hydroxides, fluorides, sulphates, phosphates) and organic (e.g. citrates, oxalates) ligands.

Herein we present: 1 – fundamentals of the geochemical origin of silicon in groundwaters and in particular curative mineral waters; 2 – summaries of the main geochemical reactions which are responsible for silicon solubility control in curative waters from spas located in the Sudetes Mountains (SW Poland); 3 – some suggestions relating to the putative therapeutic role of silicon when taken with drinking water, for example, as curative water.

SILICON SOLUBILITY CONTROL IN CURATIVE WATERS OF THE SUDETES MOUNTAINS

In curative mineral waters from 33 intakes located in all spas in the Sudetes Mountains (SW Poland) (Fig. 1) the physico-chemical properties were measured in the field and waters were sampled for laboratory analyses. The chemical composition of waters was examined, including silicon and aluminium which were measured by ICP-MS. Speciation modelling was conducted by applying the PHREEQC code (16) with the LLNL thermodynamic database. The results of the speciation modelling were used for an interpretation of silicon activity and silicon-aluminium interactions in curative waters in terms of the solubility of mineral solid phases.

The concentration of dissolved silicon in water has previously been expressed in various ways. Usually it is given as the mass concentration of Si, or equivalent mass concentration of SiO2. In most waters, excluding strongly alkaline waters, orthosilicic acid (H4SiO4) is the predominant Si species. Sometimes the silicon content in curative waters is still expressed as a metasilicic acid (H2SiO3), but the presence of this form of dissolved silicon in natural waters is not confirmed by the present investigations.

In the Sudetes Mountains, two principal types of curative waters prevail: low-enthalpy CO2-rich (acidulous) waters and thermal waters. Cool acidulous waters are characterized by a varied cationic composition which is mainly dominated by Ca, Mg, Na, and Fe. Thermal waters are usually bicarbonate and/or sulphate waters which are rich in fluorides and contain hydrogen sulphide and/or radon.

Acidulous waters have pH ranging from 5.03 to 6.30 (average 5.70), temperatures between 8.8°C and 18.4°C (average 13.2°C) and redox potentials (expressed as a pe value), from +2.38 to +5.97 (average +4.41). Thermal waters have pH ranging from 6.63 to 9.33 (average 7.49), temperatures between 20.0°C and 58.8°C (average 30.6°C) and redox potentials (as pe) from -2.52 to +5.87 (average +0.44) (17).

In the curative waters of the Sudetes Mountains silicon concentrations range from 0.154 mM Si (Pieniawa Józefa 2 intake in Polanica Spa) to 1.555 mM Si (well K-200 in Kudowa Spa) (Tab. 1). In general, the lowest silicon concentration (ca 0.15-0.22 mM Si) in curative waters in the Sudetes Mountains occurs in waters of Polanica and Kudowa (except water from well K-200), whereas the highest silicon concentration was found in waters of Cieplice Śląskie, Świeradów and Czerniawa spas (about 1.1-1.4 mM Si). Aluminium concentrations range from 0.445 μM Al (well B-39 in Duszniki) to 53.407 μM Al (well 1-A in Świeradów).

The concentrations of components dissolved in groundwaters retained in bedrock are primarily affected by three main groups of processes: 1 – release into solution due to congruent or incongruent dissolution of mineral phases, 2 – removal of dissolved components by mineral precipitation, sorption, emission of gases, and/or with waters which outflow from the studied part of the hydrogeologic system, 3 – solubility control by solid phases (minerals, colloids).

The chemical composition of the studied curative waters has been interpreted in terms of the solubility of solids which potentially control silicon activities. The water chemistry was analysed from the point of view of: 1 – the solubility of hydroxyaluminosilicate colloid HASB (Al2Si2O5(OH)4); 2 – hypothetical incongruent reactions of aluminosilicate solid phases.

Dissolved components are transported in groundwaters as different phases and in different forms, which might also include colloids. Previously, numerous comprehensive studies have been devoted to hydroxyaluminosilicate colloids (called HAS colloids), though mainly concerning their synthesis, structure, and methods of identification (18, 19, 20, 21, 22, 23, 24). Two principal types of synthetic HAS colloids were identified, named colloid HASA and colloid HASB. The colloid of HASA type (of composition Al2SiO3(OH)4) forms as a result of the reaction of silicic acid (H4SiO4) with colloidal aluminium hydroxide, (Al(OH)3), in solutions where the silicon concentration is less than or equal to the concentration of aluminium. Some soil waters and in particular those which are heavily weathered and possibly acidic (pH<5.0) fulfil this condition. The colloid HASB (Al2Si2O5(OH)4) forms from the reaction of colloid HASA with silicic acid, so its formation is dependent upon the prior formation of HASA. The colloid HASB is formed when the concentration of silicic acid exceeds the concentration of aluminium. Such conditions occur in most ground and surface waters. Results of laboratory studies on HAS colloids indicate that they might be responsible for both silicon and aluminium solubility control in natural waters, like soil, ground and surface waters.

Previous interpretations of a wide set of hydrochemical data have suggested that in fresh groundwater and surface waters in the Sudetes Mountains there exist propitious conditions for the formation of HASB (25, 26). In the present study the method proposed by Schneider et al. (21) was used for testing the solubility of colloid HASB in curative mineral waters of the Sudetes Mountains.

The reaction quotient for colloid HASB (log IAQ HASB) was calculated based upon species’ (H+, Al3+, H4SiO40) activities in curative waters, which were earlier calculated by speciation modelling. The results are indicative of different geochemical conditions in acidulous water and in thermal water reservoirs. Semi-constant values of log IAQ HASB, for example, as in thermal waters (Fig. 2), could suggest the formation and subsequent stability of HASB colloids. In thermal waters, at pH above 6.5 and Al3+ activity below 10-11, one could expect conditions conducive to the formation and stability of the colloid HASB. The average value of log IAQ HASB in thermal waters is -45.38 (±1.27) (N=10; T≈31°C), which is close to the mean log IAQ HASB value estimated previously for fresh groundwater in the Sudetes Mountains (log IAQ HASB=-44.73 (±0.58) (N=188; T=7°C)) (26).

Numerous secondary minerals are formed in the weathered zone as a result of the hydrolytic decay of primary (alumino)silicate minerals. Those secondary minerals might also be subjected to transformation into the next mineral generation(s). Various components (mainly calcium, sodium, magnesium, potassium, and silicon) are released into the water due to these incongruent mineral transformations. The concentration of silicon in waters depends on the dissolution kinetics of the primary minerals, the intensity of the removal of dissolved components, and also on the mineral solubility and chemical equilibrium between dissolving and neo-forming minerals, i.e. on incongruent reactions (25, 27).

The chemistry of curative waters could also be interpreted in the terms of chemical equilibrium reactions of incongruently dissolving aluminosilicates. The set of hypothetical incongruent reactions between secondary aluminosilicate solids (minerals, colloids) and the formation of aluminium hydroxide have been analysed to find reactions which might control silicon concentration in curative waters (Tab. 2). Dissolution of aluminosilicates of Al2Si2O5(OH)4 composition (kaolinite, halloysite, colloid HASB), and of Al2SiO3(OH)4 composition (imogolite mineral and proto-imogolite colloid) with the formation of various forms of aluminium hydroxide (gibbsite, microcrystalline gibbsite, amorphous Al(OH)3) have been considered. Proto-imogolite is a colloidal solid equivalent to HASA colloid.

All reactions given in Table 2 lead to silicon, as a silicic acid, release into groundwater. If in the curative groundwater reservoirs partial chemical equilibrium with a particular reaction is reached and maintained, then the genuine silicon activity should correspond with, or at least be close to, the calculated theoretical silicon activity. The latter were calculated for fifteen possible chemical reactions, and two variants, i.e. for cool acidulous waters and thermal waters (Tab. 3). The genuine silicon activity has also been compared with the solubility of different silica (SiO2) minerals (i.e. quartz, chalcedony, amorphous silica) (Fig. 3). Comparison of the theoretical and genuine silicon activities has allowed us to verify hypotheses and to indicate reactions (and mineral phases) which are probably responsible for controlling silicic acid activity in these curative waters.

In curative waters from the Sudetes Mountains partial chemical equilibrium with (alumino)silicate mineral phases is likely to occur. In some of the cool acidulous waters the silicic acid activity might be controlled by incongruent dissolution reaction “halloysite – microcrystalline gibbsite” (Fig. 3). The distribution of silicic acid activity suggests that in some other acidulous waters silicic acid activity is best explained by the congruent dissolution of silica forms (chalcedony, or amorphous silica). The incongruent reaction of “colloid HASB – microcrystalline gibbsite” is less probable because HASB solubility conditions (Fig. 2) do not favour its formation. Silicic acid activity in thermal waters of the Sudetes Mountains is most likely controlled by the incongruent dissolution of halloysite with subsequent formation of microcrystalline gibbsite, or partly by the reaction of “colloid HASB – Al(OH)3 amorphous” (Fig. 3).

CURATIVE WATERS AND PAST, PRESENT AND FUTURE SILICON THERAPY

What did Louis Pasteur have in mind when he said that; “Effects of silicic acid are destined to play a great and major role in therapy”? Whatever it was in the mid-nineteenth century that heightened Pasteur’s interest in the therapeutic possibilities of silicic acid has today, remained largely unexplained though the therapeutic potential of silicic acid continues to be of wide and varied interest. Today, the fascination with silicic acid and health probably originates from the reputed health benefits of both bathing in and partaking of mineral or spa waters rich in silicic acid. These purported health benefits received scientific support in the latter half of the twentieth century from experiments which demonstrated the nutritional essentiality of silicon in laboratory animals (28, 4). Indeed research using both animals and plants has continued to this day to strongly support the notion that living things grow better and are healthier when their environment is replete with silicon. If it is generally agreed that silicon, as silicic acid, is beneficial then there is no such consensus concerning the mechanism of silicon’s essentiality. Myriad experiments over decades of research have failed to identify any silicon biochemistry. There are no silicon complexes in any biochemical system. There are no Si-C, Si-N, Si-O-C/N bonds in any biological process. Silicic acid is a neutral monomer, Si(OH)4, under almost all physiological conditions (29). How can such a molecule, whose closest analogue in biological systems is water, be beneficial to life (13)?

In the late 1980s, one of us (CE) made the serendipitous discovery that silicic acid protected salmon from the toxicity of aluminium in acid waters (30). It was already suspected that aluminium was the chief antagonist of the devastation which is wreaked on fish and forests by acid rain and while we were trying to understand the mechanism of toxicity of aluminium in fish we stumbled upon the protective effects of silicic acid. We were able to show that the benefits of silicic acid were accrued through its unique inorganic chemistry with aluminium, forming what we called hydroxyaluminosilicates (HAS). We went on to define silicic acid as a geochemical control of the biological availability of aluminium and to propose that the mechanism of silicon essentiality in life was through its exclusion of aluminium (20, 31). Put simply, we believe that many if not all of the symptoms of silicon deficiency can be explained by aluminium toxicity.

The ecotoxicology of aluminium is well known though our understanding of how exposure to aluminium impacts upon human health is less well understood (32, 33). Following our work on salmon we immediately hypothesised that silicic acid would act so as to reduce human exposure to aluminium. Evidence supported a role for silicic acid in reducing the gastrointestinal absorption of aluminium and, recently, for silicic acid to facilitate the urinary excretion of systemic aluminium. However, the chemistry which potentially underlies such effects, the formation of HAS, is not simple and most likely relies upon concentrations of silicic acid which are higher than are normally found in, for example, potable waters and, consequently, human blood and tissues. Indeed there is evidence that lower concentrations of silicic acid could increase the biological availability of aluminium which in humans could be manifested as greater absorption of aluminium across the gut (31). Epidemiological studies have, for example, suggested that drinking waters with concentrations of silicic acid above 0.2 mM show some protection against Alzheimer’s disease. We would suggest that any such protection afforded by silicic acid would be much greater for silicic acid concentrations in excess of 0.5 mM. We make this suggestion based upon our research with silicon-rich mineral waters in which it has become clear that such greatly facilitate the urinary excretion of aluminium and that the mechanism seems to involve a pulse of silicic acid at high concentration entering the blood and purging aluminium from the body via the kidney (11).

Many of mineral waters used for balneotherapy in spas of the Sudetes Mountains have silicic acid concentrations higher that 0.5 mM (Tab. 1) and we propose that they might also be successfully applied in therapy, e.g. aluminium-related diseases therapy. In siliceous curative waters, like in Świeradów or Cieplice Śląskie spas, the silicic acid concentration is at least twice the 0.5 mM limit that we are proposing.

As was mentioned previously, the closest chemical, and indeed biological, analogue to silicic acid is water and it is probably true to say that the body’s natural level of silicic acid, the concentration in plasma, is roughly in dynamic equilibrium with silicic acid in the environment. Populations living in geographical regions in which the environmental silicic acid concentrations in natural waters are low due to geological processes in the catchment will have lower ‘background’ levels of silicic acid in the body and this may increase their susceptibility to exposure to aluminium. They may be more prone to the absorption and retention of aluminium as well as the potential biological effects, toxicity, of systemic aluminium. We believe that any such enhanced exposure to aluminium could be countered by including silicic acid rich mineral waters in individual’s everyday diet.

While this is an obvious way to boost one’s protection against environmental aluminium it might be that herein also lies the secret of silicon-rich bathing waters in spas? We have stated in previous work that biology is permeable to silicic acid (29) and it would be a very interesting study to find out if regular bathing in silicon-rich waters resulted in higher blood levels of silicic acid and reduced susceptibility to aluminium-related disease?

CONCLUSIONS

Hydroxyaluminosilicate (HAS) colloids play an important role in controlling the biological availability of aluminium and its toxicity in living organisms. Our hitherto existing studies have suggested the likely presence of HAS colloids in reservoirs of curative mineral waters in the Sudetes Mountains An investigation to identify the presence HAS colloids in curative waters of the Sudetes is currently ongoing. Such a confirmation of the presence of HAS should have implications for past, present and future toxicological, environmental, geochemical, and mineralogical studies.

Biologically available silicon, as silicic acid, appears to play an essential role in promoting human health. Silicic acid rich mineral waters for drinking and for bathing may have hidden health benefits in protecting us against the chronic toxicity of our everyday exposure to aluminium. Those waters of the Sudetes Mountains which have silicic acid concentrations above 0.5 mM would be ideal for testing the health benefits of such waters to humans.

..............................................................................................................................................................

REFERENCES:

1.    Hem J.D.: Study and interpretation of the chemical characteristics of natural water. U.S. Geological Survey, Water-Supply Paper, 1989, 2254: 1-264.

2.    Svarcev S.L.: Gidrogeokhimia zony gipyergeneza. Nedra, Moskwa. 1998

3.    Rozporządzenie Ministra Zdrowia z dn. 13.04.2006 w sprawie zakresu badań niezbędnych do ustalenia właściwości leczniczych naturalnych surowców leczniczych i właściwości leczniczych klimatu, kryteriów ich oceny oraz wzoru świadectwa potwierdzającego te właściwości (Dz.U. 80, poz. 565).

4.    Exley C.: Silicon in life: A bioinorganic solution to bioorganic essentiality. Journal of Inorganic Biochemistry, 1998, 69: 139-144.

5.    Jugdaohsingh R., Anderson S.H.C., Tucker K.L., Elliott H., Kiel D.P., Thompson R.P.H., Powell J.J.: Dietary silicon intake and absorption. American Journal of Clinical Nutrition, 2002, 75: 887-893.

6.    Sripanyakorn S., Jugdaohsingh R., Dissayabutr W., Anderson S.H.C., Thompson R.P.H., Powell J.J.: The comparative absorption of silicon from different foods and food supplements. British Journal of Nutrition, 2009, 102(6): 825-834.

7.    Jugdaohsingh R.: Silicon and bone health. The Journal of Nutrition, Health and Aging, 2007, 11(2): 90-110.

8.    Maehira F., Iinuma, Y., Eguchi, Y., Miyagi, I., Teruya, S.: Effects of soluble silicon compound and deep-sea water on biochemical and mechanical properties of bone and the related gene expression in mice. Journal of Bone and Mineral Metabolism, 2008, 26(5): 446-455.

9.    Sripanyakorn S., Jugdaohsingh R., Thompson R.P.H., Powell J.J.: Dietary Silicon and bone health. Nutrition Bulletin, 2005, 30(3): 222-230.

10.    Schwarz K., Ricci B.A., Punsar S., Karvonen M.J.: Inverse relation of silicon in drinking water and atherosclerosis in Finland. Lancet, 1977, 8010: 538-539.

11.    Exley C., Korchazhkina O., Job D., Strekopytov S., Polwart A., Crome P.: Non-invasive therapy to reduce the body burden of aluminium in Alzheimer’s disease. Journal of Alzheimer’s Disease, 2006, 10: 17-24.

12.    Klempous J., Petruk I., Godziński J., Helemejko M., Pośpiech Z.: Wpływ kwasu ortokrzemowego na obrzęki pourazowe skóry. Doniesienie wstępne. Polimery w medycynie, 1998, 28(3-4): 71-74.

13.    Exley C.: Darwin, natural selection and the biological essentiality of aluminium and silicon. Trends in Biochemical Sciences, 2009, 34: 589-593.

14.    Guidelines for Drinking-water Quality. First Addendum to Third Edition. Vol. 1 – Recommendations. Geneva, World Health Organization, 2006, 1-515.

15.    Macioszczyk A., Dobrzyński D.: Hydrogeochemia strefy aktywnej wymiany wód podziemnych. Wydawnictwo Naukowe PWN, Warszawa, 2002.

16.    Parkhurst D.L., Appelo C.A.J.: User’s guide to PHREEQC (version 2) - A computer model for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. U.S. Geological Survey, WRI Report, 1999, 99-4259: 1-326.

17.    Dobrzyński D.: Rola częściowych równowag chemicznych w kontrolowaniu aktywności krzemu w sudeckich wodach leczniczych. Biuletyn PIG, 2009, 436: 87-94.

18.    Doucet F.J., Rotov M.E., Exley C.: Direct and indirect identification of the formation of hydroxyaluminosilicates in acidic solutions. Journal of Inorganic Biochemistry, 2001, 87: 71-79.

19.    Doucet F.J., Schneider C., Bones S.J., Kretchmer A., Moss I., Tekely P., Exley C.: The formation of hydroxyaluminosilicates of geochemical and biological significance. Geochimica et Cosmochimica Acta, 2001, 65: 2461-2467.

20.    Exley C., Schneider C., Doucet F.J.: The reaction of aluminium with silicic acid in acidic solution: an important mechanism in controlling the biological availability of aluminium? Coordination Chemistry Review, 2002, 228: 127-135.

21.    Schneider C., Doucet F., Strekopytov S., Exley C.: The solubility of an hydroxyaluminosilicate. Polyhedron, 2004, 23: 3185-3191.

22.    Strekopytov S., Exley C.: The formation, precipitation and structural characterisation of hydroxyaluminosilicates formed in the presence of fluoride and phosphate. Polyhedron, 2005, 24: 1585-1592.

23.    Strekopytov S., Exley C.: Thermal analyses of aluminium hydroxide and hydroxyaluminosilicates. Polyhedron, 2006, 25: 1707-1713.

24.    Strekopytov S., Jarry E., Exley C.: Further insight into the mechanism of formation of hydroxyaluminosilicates. Polyhedron, 2006, 25: 3399-3404.

25.    Dobrzyński D.: Silicon and aluminium in groundwater of the Kłodzko Region (the Sudetes, SW Poland) – partial geochemical equilibrium with secondary solid phases. Geological Quarterly, 2006, 50(3): 369-382.

26.    Dobrzyński D.: Chemistry of neutral and alkaline waters with low Al3+ activity against hydroxyaluminosilicate HASB solubility. The evidence from ground and surface waters of the Sudetes Mountains (SW Poland). Aquatic Geochemistry, 2007, 13(3): 197-210.

27.    Dobrzyński D.: Znaczenie częściowych równowag chemicznych w kształtowaniu chemizmu wód podziemnych w systemach krzemianowych strefy wietrzenia. Przegląd Geologiczny, 2007, 55(6): 460-466.

28.    Birchall J.D., Exley C.: Silicon and the bioavailability of aluminium. Metal Compounds in Environment and Life, 1992, 4: 411-418.

29.    Exley C.: Silicon in Life: Whither Biological Silicification? In: “Biosilica in Evolution, Morphogenesis, and Nanobiotechnology” (Eds. W.E.G. Müller, M.A. Grachev). Progress in Molecular and Subcellular Biology, Marine Molecular Biotechnology, 2009, 47: 173-184.

30.    Birchall J.D., Exley C., Chappell J.S., Phillips M.J.: Acute toxicity of aluminium to fish eliminated in silicon-rich acid waters. Nature, 1989, 338: 146-148.

31.    Exley C.: The solubility of hydroxyaluminosilicates and the biological availability of aluminium. In: “Thermodynamics, Solubility and Environmental Issues” (Ed. T.M. Letcher). Elsevier Science, Amsterdam, The Netherlands. 2007, 315-325.

32.    Exley C.: A biogeochemical cycle for aluminium ? Journal of Inorganic Biochemistry, 2003, 97: 1-7.

33.    Exley C.: Aluminium and Medicine. In: “Molecular and Supramolecular Bioinorganic Chemistry: Applications in Medical Sciences” (Eds. A.L.R. Merce, J. Felcman, M.A.L. Recio). Nova Science Publishers Inc. New York, 2009, 45-68.

..............................................................................................................................................................

Correspondence address:

Dariusz Dobrzyński
Department of Groundwater Geochemistry
Faculty of Geology
University of Warsaw
 Żwirki i Wigury 93
 02-089 Warsaw, Poland
dardob@uw.edu.pl

Artykuł nadesłano: 20.07.2010
Zaakceptowano do druku: 30.09.2010