Glass microspheres as a potential indicator of the Anthropocene: A first study in an urban environment

Abstract

Glass microspheres are spherical glass particles that have many applications including composite polymer material technology, medicine, analytical chemistry, abrasive blasting, paints, and coatings. They are widely used in the production of road and pavement marking materials, such as traffic paints, thermoplastics, and preformed markings. The abundance of glass microspheres in terrestrial deposits and possibly in marine sediments, as well as their physical and chemical stability, would make them a good indicator of the Anthropocene in geologic records. The Anthropocene as an epoch of the geologic timescale should be identified by the presence of specific signals in strata formed during this interval. An ideal geologic indicator of the Anthropocene should be ubiquitous in different environmental settings and should have a strong preservation potential in future strata. In this study, road dust and river sediment samples were collected in the city of Kielce (south-central Poland) and examined for the presence of glass microspheres. A large number of microspheres in the road dust samples (800–2700 microspheres/kg) suggests that the microspheres released by degradation of traffic paints may be effectively transported from the road dust to river sediments. This is evidenced by the highest number of glass microspheres found in a river sediment sample collected near street stormwater discharge (1500–9600 microspheres/kg). The occurrence of glass microspheres in the river sediments collected at about 500 m from the road where reflective traffic paints are used (200–500 microspheres/kg) suggests that these are likely to be subjected to fluvial transport.

Keywords

Anthropocene fluvial transport glass microspheres river sediments stratigraphy traffic paints

Introduction

Different stratigraphic indicators of the Anthropocene, the start of which broadly coincides with the mid-20th century ‘Great Acceleration’, have been proposed (Zalasiewicz et al., 2015). They include the first appearance or marked upturns in plastic products, long-living radionuclides, carbonaceous fly-ash particles, elemental aluminum, persistent organic pollutants, and various biological markers (Waters et al., 2016 and references therein). Of these indicators, the key ones are artificial radionuclides (Waters et al., 2015) and plastics (Zalasiewicz et al., 2016). A good stratigraphic marker of the Anthropocene should be ubiquitous in marine and terrestrial environments, have a high preservation potential, and be ideally synchronous. Glass microspheres, also known as glass microbeads (solid glass microspheres), microbubbles, or microbaloons (hollow glass microspheres), are particles varying from 1 to 1000 µm in diameter. They are produced in a multi-step high-temperature fluidized bed technique from soda-lime borosilicate, alumino-borosilicate glasses, or perlite. The glass microspheres are products of intentional human activities. Although natural materials, such as volcanic glass or pumice, can be used in production of glass microspheres, they represent mineral-like synthetic phases (Hazen et al., 2017) and do not have natural mineral equivalents. Production of solid microbeads started in New York City in 1914, whereas the first hollow microbubbles were manufactured in the 1950s. Since that time, their use in different fields has experienced a tremendous growth. Solid and hollow glass microspheres are currently used in many applications – from medicine (Atroshchenko and Sigaev, 2016) and analytical chemistry (Juang and Hsu, 2016), to several industrial applications (Budov, 1994; Budov and Egorova, 1993; Wood, 2008). They are typically used as inorganic fillers in composite polymer material manufacturing, abrasive blasting, production of reflective road and pavement marking materials, paints, and coatings. Important examples of glass microsphere applications are hydrogen storage material and their use in composite syntactic foams for special systems, such as deepwater buoyancy products. Some novel applications of glass microspheres have recently emerged as components of facades for urban heat island mitigation (Yuan et al., 2016) and greenhouse films for improving light and temperature conditions during plant cultivation. A dynamic growth of different applications of glass microspheres since the mid-20th century (Figure 1) has provided an opportunity to consider them as a stratigraphic indicator of the Anthropocene.

Figure 1.

Time trends in production of glass microspheres for different applications.

One of the earliest applications of glass microspheres was reflective traffic paints that improved nighttime road visibility. The first microbeads used in reflective paints were manufactured in the United States from scraps of window glass (Amos and Yalcin, 2015). Glass microbeads can be utilized on a wet paint surface or mixed with a paint during its manufacture. The paints are applied to mark road signs (center and roadside lines, pedestrian crossings, information about specific traffic regulations), spaces in parking lots, loading zones, airports, and so on. The typical durability of road marking paints is 6–24 months (Burghardt et al., 2016). However, the aging of road markings depends significantly on the material used, road surface properties, and traffic intensity. The most popular material is the paint that can be either mixed with glass microbeads or sprayed with them when the paint is wet. The second method of application is a potential source of glass microbeads because the excessive amount of glass microbeads, which is not bound to the paint, can be more easily distributed in the roadside environment. There are two types of paints used in the road marking systems, the water-based and the solvent-based (Asdrubali et al., 2013). The first ones have longer expected service life than the solvent-based paints, but they are less environmentally friendly because they contain high levels of volatile organic compounds that contribute to formation of ozone (Burghardt et al., 2016). The other road marking materials, such as thermoplastic materials which are a mixture of polymers, pigments, and glass microbeads, two-component resins, and tapes, are much more expensive and very durable (with more than 10 years of expected service life). After degradation of marking materials, the glass beads along with mineral and organic particles are windblown or washed away from road surfaces and transported in stormwater drains into river sediments. The scale of the use of glass microspheres in road surface marking is tremendous. In the late 1980s, the annual use in the United States was 130,000 tons (Dale, 1988). The current use of glass microspheres in road marking paints in the Canadian province of Quebec is about 2800 tons/yr (Tremblay, 2015).

In this article, we present the results of a preliminary study on the quantity of glass microspheres that originate from traffic paints and then undergo deposition in the road dust and river sediment samples. The occurrence of glass microspheres was confirmed using optical microscopy. This is the first study that reports the quantity of glass microspheres in river sediments.

Materials and methods

The study area is located in the city of Kielce, south-central Poland (Figure 2). Coordinates of sampling sites are given in Table 1. Two composite sediment samples were collected with a steel shovel from the Silnica River bed from a depth interval of 0–10 cm. Site S1 was located close to the stormwater discharge drain in the city center. Site S2 was situated about 5.5 km southwest of S1 and at least 500 m from the highway 762 (Figure 2). One road dust sample was collected with a plastic spatula from the roadside edge of one of the major roads in northeastern part of the city (shown as site R in Figure 2). Each collected sample weighed 200–300 g. Three subsamples weighing 10 g were taken from each of the air-dried samples for further investigation. The glass microbeads were examined in the subsamples under a stereoscopic microscope Leica M205A and were discriminated from quartz grains using a polarizing microscope Nikon Eclipse LV 100 Pol.

Figure 2.

Location of the investigation sites, S1, S2, and R.

Table 1.

The number of glass microspheres in sediment samples.

Sampling site	Coordinates of sampling sites	No. of glass microspheres in a 10-g sample			No. of glass microspheres in three 10-g subsamples (mean ± standard deviation)
S1	50°52′03.44″N 20°37′17.1″E	15	43	96	51 ± 41
S2	50°51′32.14″N 20°33′06.38″E	2	3	5	3 ± 2
R	50°53′32.24″N 20°40′36.67″E	27	8	9	15 ± 11

Results

The numbers of glass microspheres in collected subsamples are shown in Table 1. A few chips of traffic paint with embedded glass microspheres were recorded in the road dust samples (Figure 3). Glass microbeads were found in all subsamples representing river sediments (Figure 4a–d) and road dust (Figure 4e and f) although their numbers in each sample type are very variable. The microbeads form aggregates of different sizes and shapes (Figure 5).

Figure 3.

A chip of traffic paint separated from the road dust sample.

Figure 4.

Spheroidal (SGM) and ellipsoidal hollow glass microbeads (EGM) versus other particles (quartz grains (Q) and broken glass fragments of similar size) separated from samples collected at (a, b) site S1, (c, d) site S2, and (e, f) in road dust samples.

Figure 5.

Different forms and sizes of the glass microspheres used in traffic paints.

Some quartz grains are very similar to glass microbeads, but they are typically oval, subordinately spheroidal, and they are translucent. Some quartz grains comprise tiny mineral inclusions. The glass microspheres exhibit nearly a perfect spheroidal shape, in places forming composite clusters (Figure 5). The individual spherules are in the range of 0.1–1.0 mm in diameter. They are generally translucent, but some of them are opaque or show the presence of surface microcracks that may be induced by mechanical damage, for example, by heavy load traffic. Aside from the central hollow, a very characteristic feature of the glass microspheres examined is the presence of bubble inclusions. This feature is helpful in distinguishing the glass microspheres from quartz grains. Moreover, petrographic examinations have shown that the glass microspheres are optically isotropic, whereas quartz grains are anisotropic. Thus, the observation of samples at crossed nicols in transmitted and reflected light can easily discriminate the glass microspheres from the quartz grains (Figure 6).

Figure 6.

Isotropic glass microspheres (GM) and anisotropic quartz grains (Q): (a) transmitted light, crossed nicols and (b) reflected light, crossed nicols.

Discussion

Glass microbeads originating either from degraded traffic paints or as excess during applications of new paints are washed away by rainfall and carried into streams and rivers. They are also blown by wind and deposited at a different distance from roads because of snow plowing, dust removal, or other routine road use and road maintenance activities (Figure 7).

Figure 7.

Transport pathways of glass microspheres from traffic paints into the roadside environment: 1 – transport by water to stormwater discharge area and to adjacent surface waters; 2 – accumulation in the street dust; 3 – transport by wind and vehicles.

The occurrence of glass microspheres in road dust is not surprising and has already been reported by other authors. Marini (2003) studied glass microbeads in chips collected from road marking paints in Belgium, France, and United States. Zannoni et al. (2016) studied roadside dust retrieved from paved roads in urban, suburban, and rural areas of northern Italy, and in all the examined samples they found a large amount of fine glass microbeads in the range of 20–250 µm across. They also demonstrated that concentrations of glass microspheres in road dust were higher in samples collected near freshly painted roads. The presence of potentially toxic trace elements, such as arsenic, antimony, and lead in glass microspheres, has aroused an interest among environmental scientists (Dos Santos et al., 2013; Sandhu et al., 2013).

The general characterization of glass microspheres is summarized in Table 2. The typical feature of glass microspheres is their physical and chemical resistance, low density, and very high thermal stability. These properties contribute to the high persistence of glass microspheres in the environment. The ideal spheroidal shape of most particles inhibits corrosion. This seems very likely that the preservation potential of the glass microspheres is as high as that of microtektites found in the 3.4- to 2.5-Ga beds of South Africa and Australia (Koeberl, 2006).

Table 2.

Properties of glass microbeads.

Parameter	Values/features	Reference
Particle size (µm)	12–300 (hollow glass microspheres) 1–1000	Wood (2008) Marini (2003)
Shape	Spherical, splash form shapes (teardrop, pear, peanut, dumbbell), aggregates	Marini (2003)
Major components (wt%)	SiO₂ (55–90), B₂O₃ (1–30), Na₂O (2–25), CaO (0–25)	Budov (1994)
Major components (wt%)	Total Fe as FeO (<1), MgO (<5), Al₂O₃ (<3.5), Na₂O (9.5–17), CaO (>7.4)	Marini (2003)
Density (g/cm³)	0.15–0.70	Budov (1994)
Buoyancy^a (%)	89–99.9	Budov (1994)
Thermal conductivity (W/m K)	0.029–0.115	Budov (1994)

Buoyancy is the ratio of mass of microspheres that float in water to total mass of microspheres × 100%.

Eolian dispersal of glass microspheres will probably occur close to their source. Despite the similar size to spheroidal carbonaceous particles (SCPs) emitted from power plants, the examined glass microspheres (except for those used in high-building exterior paints) seem to be linked to low-altitude emissions. The wind carries them close to the ground, hence they will not be as widely dispersed as SCP (Rose, 2015). This may suggest that the main route of dispersal is more like microplastics through fluvial systems. However, the smallest glass microbeads of 1–6 µm in diameter are expected to be transported in the air over longer distances.

The high buoyancy of the glass microspheres (Table 2) allows us to presume that they behave similarly to plastic microbeads. Microplastics are transported through river systems out into the oceans and can be laid down in bottom sediments. The vertical transport of microplastics ingested by zooplankton is possible with fecal pellets. A wide application of glass microspheres in composite materials that show higher density than water could be another possibility of their transport and accumulation in marine sediments as a potentially important marine signal.

The behavior of glass microspheres in sediments probably resembles quartz grains of similar size. This is supported by their previous use as a proxy of quartz in the study of mechanical damage of quartz grains during eolian transport (Costa et al., 2013). The concentration of microbead particles found in the river sediments is relatively high. This may be the evidence for their effective transport by wind or water to the adjacent areas. It would be interesting to study the spatial distribution of glass microspheres at different distances from roads and to find possible temporal trends in their release from damaged paint covers. It is reasonable to assume that climatic factors play a critical role in traffic paint degradation. Especially during the winter season with low temperatures and in the summer with high ultraviolet (UV) irradiation, the number of glass microspheres released to soil and surface waters is probably higher than in other seasons. The amount of glass microbeads released from road marking signs is also influenced by the durability of the material used for road marking and the method of its application, traffic intensity, precipitation, and so on.

When considering glass microbeads as indicators of the Anthropocene, it is important to know that these particles can mimic some other natural materials. Marini (2003) showed that solid glass microbeads could easily be mistaken for natural microtektites. He documented that in some cases distinguishing of glass microspheres from microtektites required quantitative chemical analyses. The glass microbeads utilized in traffic paints contain some additives which improve their properties and can be used for discrimination of microbeads from microtektites. Lead oxides (PbO and Pb₃O₄) increase the refractive index of glass microspheres; arsenic and antimony oxides (As₂O₃ and Sb₂O₃) are, in turn, added as refining agents and decolorants (Dos Santos et al., 2013). However, it should be stressed that Pb, As and Sb are typical pollutants in the roadside environment and also originate from other potential pollution sources such as wearing of brakes and tires. Therefore, their elevated levels in the sediments or soils will not be indicative of glass microspheres as their source. The study of glass microspheres is in its initial phase, and before they can be seriously considered as a key signal for the Anthropocene, their occurrence and spatial and temporal distributions in stratified deposits should be examined in more detail at different locations throughout the world.

Conclusion

Glass microspheres are widely used, and their occurrence in surface deposits has been documented. Both the known and emerging applications of glass microbeads guarantee their input to terrestrial and marine depositional environments. Degradation of traffic paints releases a large amount of glass microbeads onto the road surface from which they are windblown to the neighboring soils or transported through stormwater drains to rivers. The number of glass microspheres in collected samples is very variable, but their average content is the highest in the river sediments close to stormwater discharges, lower in the road dust, and the lowest at several hundred meters from the nearest road with signs painted with reflective traffic paints. Further studies are needed to better understand spatial and temporal trends in dispersion of glass microbeads in different surface environments as well as to confirm the presence of glass microspheres in marine sediments.

Bearing in mind the gaps in our knowledge about global distribution, history, and fate of glass microspheres in sediments, it seems reasonable to consider them as a potential marker of the Anthropocene. The comparison of glass microspheres to other proposed stratigraphic indicators of the Anthropocene leads to the conclusion that they will be much more resistant to physical degradation and chemical changes than organic pollutants, plastics, organic fossils, elemental aluminum, and even long-living radioisotopes. Another advantage of glass microsphere studies is relatively simple sample preparation and observation procedures needed. The use of stereoscopic and petrographic microscopes is sufficient for distinguishing microbeads of different origin.

Footnotes

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Amos

Yalcin

(2015) Hollow Glass Microspheres for Plastics, Elastomers, and Adhesives Compounds. Oxford: Elsevier.

Asdrubali

Buratti

Moretti

et al . (2013) Assessment of the performance of road markings in urban areas: The outcomes of the CIVITAS Renaissance Project. The Open Transportation Journal 7: 7–19.

Atroshchenko

Sigaev

(2016) Glassy microspheres and their applications in nuclear medicine (review). Glass and Ceramics 11(72): 397–404.

Budov

(1994) Hollow glass microspheres. Use, properties, and technology (review). Glass and Ceramics 51(7): 230–235.

Budov

Egorova

(1993) Glass microbeads, application, properties, and technology (review). Glass and Ceramics 50(7): 275–279.

Burghardt

Pashkevich

Żakowska

(2016) Influence of volatile organic compounds emissions from road marking paints on ground-level ozone formation: Case study of Kraków, Poland. The Transportation Research Procedia 14: 714–723.

Costa

PJM

Andrade

Mahaney

et al . (2013) Aeolian microtextures in silica spheres induced in a wind tunnel experiment: Comparison with aeolian quartz. Geomorphology 180: 120–129.

Dale

(1988) Pavement Markings: Materials and Application for Extended Service Life. NCHRP Synthesis of Highway Practice 138. Washington, DC: Transportation Research Board, National Research Council.

Dos Santos

ÉJ

Herrmann

Prado

et al . (2013) Determination of toxic elements in glass beads used for pavement marking by ICP OES. Microchemical Journal 108: 233–238.

10.

Hazen

Grew

Origlieri

et al . (2017) On the mineralogy of the ‘Anthropocene Epoch’. American Mineralogist 102: 596–611.

11.

Juang

Hsu

(2016) Self-concentrating buoyant glass microbubbles for high sensitivity immunoassays. Lab on Chip 16(3): 459–464.

12.

Koeberl

(2006) Record of impact processes on the early Earth: A review of the first 2.5 billion years. In: Reimold

Gibson

(eds) Processes on the Early Earth. Boulder, CO: Geological Society of America (Geological Society of America Special Papers 405), pp. 1–22.

13.

Marini

(2003) Natural microtektites versus industrial glass beads: An appraisal of contamination problems. Journal of Non-Crystalline Solids 323(1): 104–110.

14.

Rose

(2015) Spheroidal carbonaceous fly ash particles provide a globally synchronous stratigraphic marker for the Anthropocene. Environmental Science & Technology 49(7): 4155–4162.

15.

Sandhu

Axe

Ndiba

et al . (2013) Metal and metalloid concentrations in domestic and imported glass beads used for highway marking. Environmental Engineering Science 30(7): 387–392.

16.

Tremblay

(2015) Pavement markings at the Ministère des Transports du Québec: Innovative measures to improve environmental performance. In: Getting You There Safely: 2015 Conference and Exhibition of the Transportation Association of Canada. Charlottetown, Prince Edward Island, Canada; 27–30 September.

17.

Waters

Syvitski

JPM

Gałuszka

et al . (2015) Can nuclear weapons fallout mark the beginning of the Anthropocene Epoch? Bulletin of the Atomic Scientists 71: 46–57.

18.

Waters

Zalasiewicz

Summerhayes

et al . (2016) The Anthropocene is functionally and stratigraphically distinct from the Holocene. Science 351: 137.

19.

Wood

(2008) Microspheres: Fillers filled with possibilities. CompositesWorld 14(2): 28–32.

20.

Yuan

Emura

Farnham

et al . (2016) Application of glass beads as retro-reflective facades for urban heat island mitigation: Experimental investigation and simulation analysis. Building and Environment 105: 140–152.

21.

Zalasiewicz

Waters

Ivar

Sul

et al . (2016) The geological cycle of plastics and their use as a stratigraphic indicator of the Anthropocene. Anthropocene 13: 4–17.

22.

Zalasiewicz

Waters

Williams

et al . (2015) When did the Anthropocene begin? A mid-twentieth century boundary is stratigraphically optimal. Quaternary International 383: 196–203.

23.

Zannoni

Valotto

Visin

et al . (2016) Sources and distribution of tracer elements in road dust: The Venice mainland case of study. Journal of Geochemical Exploration 166: 64–72.