When the right combination of microorganisms are present, the dyes can be mineralised. This means that they are transformed to compounds which were naturally present on the earth. However, if only part of the degrading microorganisms are present, degradation will not only result in minerals but also in other end products.

Illustration of the mineralisation process

Illustration of the mineralisation process

There are many possible end products of the breakdown of the dyes. Ideally, an economic and sustainable application would be created for the breakdown products of the ink, and to utilise the waste stream rather than just detoxifying it. Many end products have potentials for use as a fuel, and they would be used as a mixture to increase the feasibility. Besides that, there are potential compounds in the mix which can be used for bioplastics. When the waste streams are used for fuel and bioplastic production, the degradation process might be longer profitable and therefore more attractive to apply.

Bioplastic and textile made from cow manure. Credit: Mestic.nl

Bioplastic and textile made from cow manure. Credit: Mestic.nl

 
 

In the decolourisation part, the dyes were decolourised by breaking the azo bond. However, for both mineralisation and degradation further breakdown of the possibly toxic intermediates is required. All of these intermediates have an aromatic ring.

 
Two possible intermediate products (toxic) from decoloursing the Reactive Black 5

Two possible intermediate products (toxic) from decoloursing the Reactive Black 5

 

Their breakdown is highly dependent on the functional groups which are attached to the aromatic ring. Therefore these functional groups will have to be degraded first. In case nitrogen or halogens are connected to the benzene ring, the ring is less reactive. This means that microorganisms first have to lower the energy barrier before they can remove these groups. Several species with ability of doing this were found. Bacteria have some difficulties removing sulphonate groups as molecules with these groups are not easily taken up by bacteria, but some candidates could still be found. Some of the intermediates had structures which resembled weedkillers. All in all, for all functional groups some bacteria exist which can remove the side groups from the ring. The last step is breaking the aromatic ring itself by bacteria. If they can use these aromatic rings for their own growth.