Leaching of Glass Waste – Structure and Humidity Cell Tests

Abstract: Glass production has historically occurred at around 50 glassworks in Sweden, in a region known as the Kingdom of Crystals (Glasriket). Today, most of these sites are no longer active and left behind is glass waste of different forms (both as fragments of finished glass as well as unrefined glass melts). Consequently, increased concentrations of different metals, especially arsenic, lead and cadmium, have been found around the sites, both in soil as well as in ground and surface water. Between 2016 and 2019, the Geological Survey of Sweden (SGU) assigned Golder Associates AB (Golder) to evaluate the environmental risks at three different glassworks: Flerohopp, Åryd and Alsterbro. The results, based on humidity cell tests (HCT) conducted on glass samples from each site, showed that glass itself leached to a surprisingly high extent. Based on this, the aim of this master thesis has been to explain trends in glass leaching by a thorough literature review and through the analysis of HCT data of glass samples. Additionally, the speciation of different metals in the leachate was investigated based on geochemical modelling using PHREEQC. Results from the literature review show that one of the possible mechanisms for the leaching of glass in contact with water is ion exchange, which occurs at the surface of the glass, namely between glass components and H+ ions in water. Additionally, the literature also argues that glass with higher silica content form a more resilient structure, in contrast to glass which contains a large amount of modifiers, such as Na and Ca. Researchers speculate that adding such modifiers to the glass mass opens up the structure, making it more vulnerable upon contact with water. Looking at the total concentration of elements from the three glassworks, the results show a variation in silica content in relation to other elements. In line with this hypothesis, the sample from Åryd, which contained a higher proportion of modifiers, showed a high leaching rate of both Na and Si. Furthermore, the result shows that the leaching of Na and As follows the same pattern over the HCT period for all glassworks. This is, to some extent, also the case for Pb although the correlation is not as significant. This could be explained by the result from geochemical modelling, showing that As tends to dissolve into the leachate while Pb is more prone to forming secondary minerals. Hence explaining their differences in leaching behavior. The result from this study showed no clear correlation between Ca and either As or Pb which could potentially be explained by the formation of precipitates. However, another approach to describe the difference in the behavior between Na and Ca is based on the glass structure itself as well as the hypothesis that Na+ participate in ion exchange to a larger extent than Ca2+. Consequently, the leaching of Na+ makes the surface structure more vulnerable, thereby promoting the leaching of other components such as As and Pb.