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Toxicological Profile for Silica. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2019 Sep.
The synonyms, trade names, chemical formulas, and identification numbers of silica and selected forms of silica are provided in Table 4-1.
Chemical Identity of Silica and Compounds.
Information regarding the physical and chemical properties of selected c-silica, natural a-silica, and synthetic a-silica compounds is provided in Tables 4-2, 4-3, and 4-4, respectively.
Physical and Chemical Properties of Crystalline Silica Compounds.
Physical and Chemical Properties of Natural Amorphous Silica Compounds , .
Physical and Chemical Properties of Synthetic Amorphous Silica Compounds.
Silica occurs naturally in crystalline and amorphous (or non-crystalline) forms, herein referred to as c-silica and a-silica, respectively. Silica has one general Chemical Abstract Service registry number (CASRN 7631-86-9) and more specific CASRNs for individual silica forms and preparations. Both the crystalline and amorphous forms of silica are composed of a 1:2 net ratio of silicon atoms to oxygen atoms, corresponding to an empirical formula of SiO2 and the chemical name silicon dioxide (IARC 1997). All silica compounds are silicon dioxide. The internal chemical structure of most forms of silica consists of each silicon atom bonded to four oxygen atoms in a silicon and oxygen tetrahedral (SiO4) or pyramidal unit with four triangular sides. Crystalline forms of silica have regular, repeating three-dimensional patterns with internal oxygen atoms shared between two tetrahedral silicon atoms. Terminal oxygen atoms are negatively charged ions at environmentally relevant pH (OSHA 2013c). Amorphous forms of silica are composed of highly disordered, randomly linked silicon and oxygen tetrahedral units with no defined pattern. X-ray diffraction patterns distinguish crystalline polymorphs from each other and c-silica from a-silica.
The surface properties of silica compounds, even the same polymorph, vary. Both c- and a- forms of silica have surfaces composed of siloxane (covalently bonded silicon and oxygen; Si-O-Si) and silanol groups (Si-OH) (Rimola et al. 2013; Zhuravlev 2000). Exposure to water will break silicon-oxygen bonds on the surface of silica to form silanols. In contrast, heating silica results in condensation of pairs of silanols to form siloxane bridges. In general, c-silica surfaces tend to have more order, although some c-silica is found with an outer layer of a-silica. Naturally occurring a-silica may contain a c-silica component from exposure to high temperatures and pressures (e.g., flux calcination). Grinding silica results in either heterolytic cleavage or homolytic cleavage of silicon-oxygen bonds at the surface interfaces producing Si + and SiO − surface charges or surface radicals, respectively (Fubini et al. 1995). The total concentration and arrangement of silanol on the surface of c- and a-silica can vary greatly. Thus, for a single polymorph of c- or a-silica, surface chemistry of the compound may vary, depending upon production method and degree of hydration. As discussed in Sections 1.2 and 2.20.2, the biological activity of both c-silica and a-silica polymorphs is affected by surface chemistry of the silica particle (Donaldson and Borm 1998; Greenberg et al. 2007; Guthrie 1995; Mossman and Churg 1998; Mossman and Glenn 2013).
c-Silica is polymorphic, meaning that there are several distinctly different crystalline forms with the same chemical composition. c-Silica polymorphs have regular, repeating three-dimensional patterns with long-range order; however, discernable variations in tetrahedral orientation and crystal symmetry differentiate the polymorphs. c-Silica is often referred to as quartz. Quartz is the most common naturally occurring form of silica and is the second most common mineral in the world (USGS 1992). Other common forms of c-silica are tridymite and cristobalite, and less common forms of c-silica are keatite, coesite, stishovite, amethyst, and moganite (NIOSH 2002). Interconversion of the silica polymorphs occurs upon heating or cooling (see Section 5.4.2 for additional information).
a-Silica is composed of a random network of tetrahedral silica, and does not display long-range order. a-Silica forms are classified as natural or synthetic a-silica based on their origin. Natural a-silica, such as raw diatomaceous earth, contains small amounts of c-silica (mostly quartz); however, calcined and flux calcined diatomaceous earth can have cristobalite concentrations up to approximately 10 and 60%, respectively (IARC 1997). Sometimes, a-silica (silica fume, vitreous silica) is unintentionally formed during certain industrial processes, such as manufacture of ferrosilicon and silicons; these forms of a-silica are also often contaminated with c-silica (Arts et al. 2007; Fruijtier-Polloth 2012; IARC 1997). Vitreous silica can also be intentionally produced synthetically by melting c-silica and rapidly cooling to prevent recrystallization (Smith 2006). In general, other forms of synthetic a-silica are free of c-silica. They are further classified by their preparation method; there are wet process silica forms, which include precipitated silica, silica gels, and colloidal silica, and thermal process silica forms, including pyrogenic (or fumed) silica (Fruijtier-Polloth 2012; IARC 1997). Surface-modified silica is physically or chemically treated a-silica (IARC 1997).
Silica is a stable oxide of silicon. c-Silica does not readily react with most acids, but does react with hydrofluoric acid to produce silicon tetrafluoride gas (IARC 2012; OSHA 2013c). c-Silica also reacts with alkaline aqueous solutions and catechol (IARC 2012). a-Silica will react with mineral acids and alkaline solutions (OSHA 2013c).
In general, silica is considered poorly water soluble and chemically unreactive in the environment (EPA 1991; IARC 1997). The water solubility of silica has some variation due to differences in trace metal impurities and hydration (OSHA 2013c). Solubility is lower for c-silica polymorphs than for a-silica, and anhydrous a-silica dissolves less rapidly than hydrated a-silica (IARC 1997). a-Silica dissolves in water to form monosilicic acid (Waddell 2006). External conditions such as higher temperatures and pH increase the water solubility of silica. The hydrophilicity of c-silica particles increases in humid conditions because an external layer of hydroxylated silica (silanol; SiOH) forms on the surface of the particles. Fresh surfaces of silica exposed by fracture are highly reactive and have a propensity to produce surface radicals; however, the surface is inactivated once hydrated (Costa et al. 1991; Fubini et al. 1995). Aged quartz has an external amorphous layer, referred to as a Beilby layer. The Beilby layer is more water soluble than the underlying c-silica (IARC 1997; OSHA 2013c).
Particle size has also been found to influence the rate of solubility. Silica particulate surface areas and sizes are distinguishable based on their source. Ground vitreous silica and c-silica particles have acute edges and heterogeneous particle sizes; surface areas range from 0.1 and 10 to 15 m 2 /g (IARC 1997). Diatomaceous earth and cristobalite particles from diatomaceous earth are found in a variety of shapes and surface areas. Calcinated diatomaceous earth particles have surface areas that range from 2 to 20 m 2 /g. Pyrogenic a-silica particles are nonporous, smooth, round aggregates with surface areas that range from 50 to 400 m 2 /g. Precipitated a-silica particles have sizes and porous structures that vary in surface area from 50 to approximately 1,000 m 2 /g, depending on the procedure used in their preparation. Nanoscale forms of silica with a mean particle size in the nanoparticle range (≤100 nm) are not included in this profile. However, while synthetic a-silica compounds have initial particle sizes in the nanoparticle range, these particles covalently bond during the manufacturing process to form indivisible aggregates in the respirable range, which can further combine to form micron-sized agglomerates (Fruijtier-Polloth 2012, 2016; IARC 1997; Merget et al. 2002; Taeger et al. 2016; Waddell et al. 2006); see Table 4-5. Of the synthetic a-silica compounds, only colloidal dispersions have been shown to contain stable isolated nanoparticles in addition to aggregates in the respirable range (Fruijtier-Polloth 2012, 2016).
Particle Size Data for Synthetic Amorphous Silica Compounds.
