Abstract
The 3D printing technique is an innovative way to reduce natural resource consumption and the production of waste. However, the microplastic waste that is produced is both chemically and physically different to conventional polymers. Consequently, it is critical to explore the ways in which this new waste interacts with the environment. Furthermore, it is important to assess the validity of current microplastic sampling techniques when applied to 3D printed polymers. This study therefore aimed to investigate the relative metal adsorption behaviour of three common 3D printing polymers, namely polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG). The ability of these polymers to withstand a standard organic matter digestion protocol was also investigated.
To do this, microplastic fragments of PLA, ABS and PETG were 3D printed and then exposed to river water which had been impacted by acid mine drainage. Samples were drawn after 96-hour and 28-day exposure periods for heavy metal analysis. An additional group of microplastic fragments was exposed to 10% (w/w) KOH solution at 60˚C for 12 hours, as per the conventional digestion protocol. Scanning electron microscopy (SEM) imagery was taken after each treatment step to track structural changes.
It was found that after a 28-day exposure period, each polymer carried a small quantity of all the analysed metals. ABS carried significantly higher (p<0.05) concentrations of Co, Cr, Cu, Ti, U, and V than either PLA or PETG, potentially due to the more porous surface structure of this polymer. However, as multiple metals appear to have leached from the polymer particles into solution, it is perhaps the leaching of metal rich additives, rather than the polymer surface area, which explains this result. No differences in the adsorption performance of PLA and PETG were found. The digestion of organic matter in 10% (w/w) KOH solution at 60˚C for 12 hours caused the depolymerisation of PLA but did not greatly alter the structure of the ABS and PETG fragments. To preserve PLA in microplastic samples, digestion methods which do not require heat should be investigated, as it is likely that the elevated temperature was responsible for the depolymerisation process.