Finer filters from waste

A research group at the University of Granada is working on a project subsidised by the Spanish Ministry of Environmental Issues to make activated charcoal from polymeric waste.

Activated charcoal has a large active surface area due to the numerous nanoscale pores that run through it. A relatively small quantity of charcoal could have the same surface area as a football pitch, according to group coordinator Francisco Javier López Garzón.

Among other applications, activated charcoal is used to manufacture filters for cigarettes, in catalysis processes and to filter harmful substances form the air. The group has been working for four years to cheaply make activated charcoal with a controlled porosity to remove specific pollutants from waste gas.

Commercial activated charcoal is obtained from precursors like olive pits, almond shells or coconut shell. These natural products produce charcoal with varying porosity, which is not selective as to which pollutants it absorbs.

The group found that inorganic polymers could be used to make charcoal with a fixed pore size, but these precursors are usually very expensive. They decided to try using waste polyethylene terepthalate, commonly known as PET, a plastic used in the production of drink containers. Waste PET amounts to millions of tonnes across Europe each year, so it is an abundant raw material.

PET is made into active charcoal through a process known as pyrolysis in which the material is burnt in a special oven in the absence of oxygen so it does not react with charcoal. Using this method, the researchers obtained a highly porous, selective and uniform active charcoal. It has been proven through absorption tests carried out with molecules of different sizes, from nitrogen to organic vapours.

The team is now experimenting with other polymers. They also want to improve the production process as currently a significant proportion of the PET volatilises during the process and has to be recovered to turn it into charcoal.