Silica in Natural Waters

Using the silicic acid dissociation conslant expressions for/ff, and / 2 for reactions (7.14) and (7.16) above, we can replace the last two terms in Eq. (7.17) with terms that contain only the species H4Si04. This leads to 

Factoring out H4Si04, we are left with the simple expression 

As shown previously, the dissolution of silica solids can be written 

Given the above solubility data for silica solids and values for K and K2, we can. substitute into expression (7.19) for different pH values to compute solubilities of the solids. At 25°C we will assume A jp(quartz) = 10 and plSiOjfam)] = 10. Substituting these values for H4Si05 in Eq. (7.19) and ignoring activity coefficients leads to the solubilities tabulated below, where concentrations have been converted from molal to ppm units. 

These values are plotted in Fig. 7.7. Consistent with the thermodynamic results, silica concentrations can be very high in evaporative alkali lakes. 

---

Geochemical data on the forms of transport and conditions of deposition of iron and silica in natural waters... 

In examining the particulars of the distribution, migration, and deposition of iron and silica in natural waters it is advisable to distinguish normal sedimentary processes and volcanic sedimentary processes. By normal sedimentary processes we mean the cycle weathered layer-transport-deposition in a sedimentary basin. The main distinguishing feature of volcanogenicsedimentary processes is the endogenetic source of the material and ex-ogenetic method of its deposition. 

The kinetics and temperature dependence of the solubility of silica were studied in [129] and the kinetics and ionic strength dependence in [130], in both cases at pH 2-10. The kinetics, pH dependence, and effect of alkali pretreatment were studied in [131]. Solubilities of 11 ppm for quartz and 116 ppm for amorphous silica are reported in [86]. The same study reports 10-80 ppm of silica in natural waters. 

This means that soil solutions in equilibrium with amorphous silica are oversaturated with respect to quartz. Nevertheless, because quartz crystallizes extremely slowly at ambient temperatures, the solubility of amorphous silica effectively sets the upper limit of dissolved silica in natural waters. 

Actually the results fiem the research confirm these expectations but the origin of the test water (natural or synthetic) also has a very important effect on aluminium migration. This effect has been explained by the presence of silica in natural water (see below). 

In 1955, Baumann (154) concluded that amorphous silica had a uniform solubility in the neutral pH region, that the dissolved species was almost entirely monomeric, and that in the pH range from 3 to 6 the rate of dissolution increased linearly with increasing pH. In the same year, Krauskopf (155) presented an excellent summary of previous studies of the dissolution and precipitation of silica at moderate temperatures, and emphasized the recognition of the differences between soluble, ionic, and colloidal silica, relating these to the solubility behavior of silica in natural waters. 

In view of the universal presence of at least some soluble silica in natural waters, it is not surprising that it is considered harmless in food and drink. Monomeric Si(OH)4 penetrates all body liquids and tissues at concentrations less than its solubility (0.01%) and is readily excreted (187a, 205). In human blood, in both corpuscles and plasma, the silica concentration is 1 ppm and in bovine blood 2.2 ppm. 

Various highly refined instrumental techniques such as X-ray diffraction, optical, electron optical, infrared spectroscopy, and thermal methods are currently used for the identification and determination of the crystalline polymorphs of silica. Efficient determination of the noncrystalline forms involves wet chemical methods similar to those used for the determination of silica in natural waters and in the soil solution. Numerous procedures have been proposed for the quantitative estimation of silica in solution, and their varying efficiency may account for some of the apparent discrepancies recorded in the literature concerning soluble silica. Colorimetric methods are favored for the determination of small amounts of silica and are based upon the formation and properties of a- and j8-silicomolybdates first studied by Strickland... 

Brombenztiazo (BBT) is known to be one of the best reagents for extraction-photometric determination of cadmium(II). The reagent also fonus complexes with Co(II), Cu(II), Fe(II), Ni(II), Zn(II). The aim of this work was to develop a solid-phase reagent on the base of BBT immobilized on silica gel for sorption-spectroscopic and visual test determination of Cadmium, and also for soi ption-atomic-adsoi ption determination of total heavy metals contents in natural waters. 

It has been shown elsewhere (26) that in natural waters the degree of enhancement of Mn(II) oxidation predicted on the basis of model calculations is as follows y-FeOOH > a-Fe00H > silica > alumina. It has also been shown that the rate of Mn(II) oxidation is strongly influenced by pH, y-FeOOH concentration, temperature and ionic strength. Depending on the conditions, the predicted half-life 1/2 = ln 2/ki ) f°r Mn(II) oxidation may vary from a few days to thousands of years. By way of example, at pH 8, p02 0.21 atm, 25°C in waters containing 4(iM y-FeOOH and 0.2uM Mn(II), the half-life for oxidation is about 30 days. 

Amorphous silica in nature may originate from aquatic organisms, secreted as amorphous solid in the form of shells, plates, or skeletons. Amorphous silica also is found in volcanic ash or in precipitated material from the hot supersaturated waters of hot springs. 

Ionic solids apart, the most important dissolved solid in natural waters is silica, SiC>2. The solubility of silica depends strongly on the solid-state form ... 

Such high values, however, do not exist in natural waters, except for areas of hydrothermal spring activities and in evaporating basins. The silica level in most rivers, lakes and in the ocean rarely exceeds a few ppm. Thus, for all practical purposes, the hydrosphere is unsaturated with respect to amorphous silica. 

Stober, W. Formation of silicic acid in aqueous suspensions of different silica modifications. In Equilibrium concepts in natural water systems (ed. R. F. Gould). Amer. Chem. Soc. 

Subramanian and coworkers developed polymeric sorbents using different support materials (such as Merrifield chloromethylated resin, Amberlite XAD 16) and complexing ligands (amides, phosphonic acids, TTA), and evaluated their binding affinity for U(VI) over other diverse ions, even under high acidities. The practical utility of these sorbents was demonstrated using simulated waste solutions (220-222). Shamsipur et al. reported the solid-phase extraction of ultra trace U(VI) in natural waters using octadecyl silica membrane disks modified by TOPO (223). The method was found satisfactory for the extraction and determination of uranium from different water samples. 

Shamsipur, M. Ghiasvand, A.R. Yamini, Y. Solid-phase extraction of ultratrace uranium(VI) in natural waters using octadecyl silica membrane disks modified by tri-N-octylphosphine oxide and its spectrophotometric determination with dibenzoylmethane, Anal. Chem. 65 (1999)4892 1895. 

Silica (SiOz) occurs in high abundance all over the earth. It occurs in the form of sand and quarts. It is also present in rocks and silicate minerals. It is found in natural waters at varying concentrations from 1 to 100 mg/L. 

Ohta, K. and Tanaka, K. (1999) Simultaneous Determination of Common Mono- and Divalent Cations in Natural Water Samples by Conductimetric Detention Ion Chromatography with an Unmodified Silica Gel Column and Oxalic Acid/18-crown-6 as Eluent, Anal. Chim. Acta. 381, 265-273. 

An alternative procedure for the study of neptunium oxidation states at trace concentrations has been described by Inoue and Tochiyama (1977). They showed that, in the pH range 6-7, Nplv may be quantitatively absorbed on silica gel whilst Npv remains in solution. In acid solution, however, a precipitate of barium sulfate selectively absorbs Nplv leaving the higher oxidation states in solution. The authors gave no environmental data for neptunium in their publication but Nelson and Orlandini (1979) subsequendy adapted the procedure to demonstrate that the dominant oxidised plutonium species in natural waters is Puv and not Puvl. 

West and co-workers11 achieved far greater sensitivity enhancement by trapping readily atomized elements, which had been nebulized conventionally for periods of up to a few minutes, onto the outer surface of a small bore, water-cooled quartz tube. If the water was then drained rapidly, the tube temperature quickly rose and the element was atomized off the surface. For cadmium in calcium chloride extracts of soil, for example, a detection limit of 4 ng (g soil) -1 was reported. A water-cooled double silica tube atom trap similarly has been very successfully employed for the determination of cadmium and lead in natural waters by flame AAS.12 Some further examples of applications of atom trapping are included in Chapter 7. 

Silicate minerals are more soluble in natural waters having high fluoride concentrations and low pH values than in other waters. High concentrations of dissolved silica may be maintained by the formation of hexafluorosilicic acid ... 

Roberson, C. E., and Barnes, R. B. Stability of fluoride complex with silica and its distribution in natural water systems. Chem. Geol. 21, 239-256 (1978). 

Conditions of deposition of silica from natural waters. At the present time it is considered to be firmly established that only biological extraction of silica from river and ocean waters sharply undersaturated in Si02 is possible (Strakhov, 1960, 1966). In the oceans this process takes place mainly in the uppermost layer, where photosynthesis occurs. Diatomaceous plankton sink down when they die and the silica gradually begins to dissolve (Bogoyavlenskiy, 1966). Despite strong solution of suspended silica during deposition, 0.01-0.1 of the silica reaches the surface layer of the bottom sediments. 

Ginzburg, I.l. and Kabanova, Ye.S., I960. Silica content in natural waters and forms of its occurrence. In Kora vyvetrivaniya (The Weathered Layer). Izd. Nauka, Moscow, 3 313-342 (in Russian). 

---