British science has led to a use for waste glass that cannot be recycled that could help clean up polluted waterways by acting as an ion-exchange filter to remove lead, cadmium and other toxic metals.
Details are published in a forthcoming issue of the International Journal of Environment and Waste Management.
Only a fraction of waste glass bottles and jars can be recycled, partly because much of the glass is coloured, brown or green, and partly because the market sustains only a limited weight of recyclable glass. Millions of tonnes of waste container glass are generated across Europe. As such, large amounts of waste glass, purportedly for recycling, are shipped to China and elsewhere to be ground up and used as hardcore filling materials for road building.
Now, Nichola Coleman of the University of Greenwich, London, has developed a simple processing method for converting waste container glass, or cullet, into the mineral tobermorite. Tobermorite is hydrated calcium silicate, silicate being the main material that can be extracted from glass. In the form produced, the phase-pure 11-angstrom form -- the mineral can be used as an ion-exchange material that can extract toxic lead and cadmium ions from industrial effluent, waste water streams or contaminated groundwater.
To make the tobermorite, Coleman simply heats a mixture of ground cullet, lime (as a calcium source) and caustic soda (sodium hydroxide solution) to 100 Celsius in a sealed Teflon container. Initial tests show that uptake of lead and cadmium from solution are rather slow, so Coleman suggests that, at this stage of development, the synthetic mineral might best be used in the in situ remediation of groundwater rather than in industrial ex situ effluent filtration processes. The concept is now being extended to create other classes of ion exchange filter from unrecyclable and low-quality waste glass.
"The cullet-derived sorbent could be used in reactive barriers to prevent the lateral migration of pollutants in groundwater, rather than as a remediation material for waterways," says Coleman. "Heavy metal-contaminated land and groundwater are global problems, arising from industrial and military activities and also from the natural leaching of heavy metal-bearing minerals," she adds.
