Fungal inspection

Two strains of fungi isolated from soil and a commercial white-rot fungus have been tested for the biodegradation of untreated, UV-, and heat-treated bisphenol A polycarbonate (PC) to see whether BPA release can be reduced. NMR and FTIR spectroscopy showed the formation of methyl groups due to the pre-treatment process, while EDAX analysis revealed surface oxidation of the PC.

Bisphenol A is a key monomer used in the manufacture of many polymer-based products including those made from polycarbonate and epoxy resins as well as a plasticizer in other polymers. It is, however, a known estrogenic compound, its chemistry mimicking certain aspects of the female hormone estrogen, and its potentially detrimental effects on ecological systems and human health have been a concern since its properties were first considered in the middle of the 1930s.

Environmental activists and pressure groups have brought concerns about BPA's biological effects to the attention of the news media increasingly in recent years and governments too have succumbed to pressure to force manufacturers to avoid the use of this compound in their products and for retailers to remove goods containing it from their shelves. Most recently, a 2010 report from the United States Food and Drug Administration (FDA) raised further concerns regarding exposure of young children and the unborn foetus to BPA and called for more research to determine absolute hazards.

Mukesh Doble and Trishul Artham of the Department of Biotechnology at the Indian Institute of Technology Madras, Chennai, India, point out that manufacturers produce about 2.7 million tonnes of plastic containing BPA every year. One of the major plastics that uses BPA as its monomer building block is polycarbonate, this extremely tough plastic is used in everything from flexible and fixed roofing materials to tool handles, plastic drinks bottles, baby-milk bottles, optical disks (CDs and DVDs) and even the lenses in a pair of "glasses". As such there is a substantial amount of BPA-containing PC to recycle and dispose of safely without releasing BPA into the environment at the end of a product's life. Intriguingly, in the US less than 5% of BPA manufactured is used in applications in which the material comes into contact with food.

"To enhance the degradation rate of polycarbonate, several pre-treatment strategies have been suggested," the team explains. "Photochemical and thermal treatments might increase the rate of biodegradation," they add. "These treatments generate free radicals, which are able to oxidize the polymer molecule resulting in the breakage of the chains."

In order to develop further the available methods for degrading BPA, the scientists pre-treated bisphenol A polycarbonate with ultraviolet light and heat and exposed it to three kinds of fungi, including the well-known white-rot fungus [Phanerochaete chrysosporium NCIM 1170 (SF2)], used commercially for bioremediation of recalcitrant pollutants. The other two fungi were isolated strains based on 18S rDNA analysis of Engyodontium album MTP091 (SF1) and Pencillium spp. MTP093 (SF3). They found that the fungi grew well on pre-treated plastic as they could utilise the BPA and other components as a carbon source only once the plastic had been degraded physically to some extent. Even after a year of fungal application there was no break down of plastic that had not been pre-treated.

Proton NMR spectra of the polymer were measured at room temperature with a 500 MHz Bruker Avance III spectrometer deuterated chloroform as solvent and tetramethylsilane as internal standard, the team reports. They found that the control PC sample displayed an aromatic A2B2 pattern at 7.16 and 7.24 ppm and a singlet peak corresponding to the gem-dimethyl at 1.69 ppm. In contrast, a new singlet peak appeared in photo-treated samples at delta 1.56 ppm, corresponding to a methyl proton. A triplet at delta 1.26 and a quartet at delta 3.73 ppm confirm previous research suggesting cleavage of the isopropylidene group and subsequent migration to the benzene ring.

The researchers saw no trace of bisphenol A monomer during the study of pre-treated PC exposed to the fungi, all of which bodes well of use of this approach in bioremediation of polycarbonate waste.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.