Glass beads composed of soda lime glass are essential for providing retroreflectivity in many kinds of road surface markings.[1] Retroreflectivity occurs when incident light from vehicles is refracted within glass beads that are imbedded in road surface markings and then reflected back into the driver's field of view.[2] In North America, approximately 227 million kilograms of glass beads are used for road surface markings annually.[3] Roughly 520 kilograms of glass beads are used per mile during remarking of a five lane highway system,[4] and road remarking can occur every two to five years. In the United States, the massive demand for glass beads has led to importing from countries using outdated manufacturing regulations and techniques.
These techniques include the use of heavy metals such as arsenic, antimony, and lead during the manufacturing process as decolorizes and fining agents. It has been found that the heavy metals become incorporated into the bead's glass matrix and may leach under environmental conditions that roads experience.[5]
The synthesis of these beads begins when calcium carbonate is heated to anywhere from 800 to 1300
\circ
Similarly, sodium carbonate decomposes to sodium oxide and releases carbon dioxide gas.
Sodium oxide is then reacted with silica to produce sodium silicate liquid glass.
Lastly, to complete the general structure of the soda-lime glass, calcium oxide is dissolved in solution with sodium silicate glass, which ultimately reduces the softening temperature of the glass.[6] Additional metals and ions are added to this melted glass to improve its properties, and the compound is then sprayed and formed into beads using either the direct or indirect method.
Overall, the percent composition of major compounds found in the final glass bead product is shown below.
| Compound | % Composition | |
| 70-75% | ||
| 11-15% | ||
| 2-4% | ||
| 6-10% | ||
| 1-2% |
Antimony in the form of Sb2O5 performs a similar reaction as arsenic, oxidizing ferrous oxide to ferric oxide.
While these three heavy metals can typically be found in both domestic and imported glass beads, they vary in concentration. According to the US Environmental Protection Agency, the Resource Conservation and Recovery Act limits the levels of heavy metal content in accordance with their toxicity.[11] Due to increasing demands for marked roads, however, the majority of glass beads used in the U.S. are imported from countries with little to no regulation on heavy metal content. For example, beads obtained from North America contain approximately 15 mg of arsenic per kg of beads, while some from China have concentrations of up to 1000 mg/kg.[3] Imported bead concentrations of each of these metals are listed in the table below.
| Metal/Metalloid | Concentration (mg/kg) | |
| 103-683 | ||
| 23-179 | ||
| 62-187 |
Environmental conditions can cause degradation of glass beads, leading to release of incorporated heavy metals into the environment. While abrasion may dislodge these beads from the road marking itself, the reaction of these beads with an aqueous environment vastly accelerate their decomposition and heavy metal release.
There are three reactions involved in the corrosion of silicon dioxide. The first is an ion exchange reaction, in which mobile ions of a solution are exchanged for those of similar charge on the solid. Particularly, this reaction is involving cation exchange material, where a negatively charged structural backbone allows the replacement of positively charged cations.[12] This reaction involved in the degradation of soda lime beads shows various ions that are interaction with the silicon-oxygen network (e.g.
In addition to this reaction, a hydroxyl ion can attack the
As dissolution occurs, the non-bridging oxygen groups can abstract hydrogen ions from solution.
An increase in the concentration of hydroxyl ions comes with increased alkalinity of the aqueous solution. This increase in pH has shown, in varying column leaching studies, to increase the reduction potential and DOC (dissolved organic carbon) concentration of the solution. This ultimately leads to an increase in mobility of many metals including arsenic, copper, and nickel.
The mobility of these heavy metals are therefore affected by the presence of alkali oxides. The
During both routine road marking removal and harsh environmental conditions, these glass beads can degrade and leach incorporated heavy metals. Although the exact mechanism of heavy metal incorporation into the glass beads is unknown, current literature hypothesizes that the heavy metals are associated with alkali and alkali earth metals on the surface of glass beads. Environmental conditions relevant to road surfaces such as pH, different salts, and ionic strength strongly influence the leaching process. In particular, pH determines the speciation of the heavy metal which is critical for solubility in the aqueous phase. The following graphs show the speciation of heavy metals as a function of pH.