One of our focuses in WP 3 has been to identify and quantify (micro-)plastics in organic waste resources
Simultaneous Thermal Analysis
NIBIO possesses a STA/FTIR/GC-MS (standing for Simultaneous Thermal Analysis/Fourier-Transform Infrared spectroscopy/Gas Chromatography-Mass Spectrometry), which can be used to quantify plastics in complex environmental samples, like organic waste resources and soils (Figure 1).
STA/FTIR/GC is an analytical line where the melting energy of plastics can be measured (by Differential scanning Calorimetry, DSC) and used for both quantification and identification, and where FTIR and CG-MS can be used to qualify the DSC results.
This method, combined with sample pre-treatments removing easily degradable organic matter, is developed and implemented as a tool in quality control of organic waste to be re-used as organic fertilizers and soil improvers.
The instrument has been used successfully to detect mixtures of plastics in compost, without sample pre-treatment.
An example of DSC analysis of compost spiked with low density polyethylene (PE-LD), polypropylene (PP) and polyethylene fibers (PE) is shown in Figure 2.
We have been testing the limit of detection of the instrument, using various temperature programmes and plastic types. Polypropylene in compost, for instance, can be detected at concentrations as low as 0.1 % by weight. In order to detect plastics at concentrations lower than 0.1 % w/w, various sample pre-treatments have been studied.
During this project, we also developed a database of DSC and FTIR spectra for a range of plastics from consumer products, which can be made of mixtures of polymers, including polymers of biological origin and biologically degradable polymers.
Microplastics in biogas digestate
During this project, we also studied microplastics present in biogas digestate.
Biogas digestate used as soil amendment was found to contain 0.2-1% of microplastics and their particle size was 0.2-3 mm.
Some of these plastic particles had a composition similar to that of the plastic bags used for food waste collection, but not all of them, indicating the presence of plastic waste in the food waste (incorrect waste sorting).
For more details, see the MSc thesis written by Ann-Katrin Dale “Utprøving av ny metode for kvantifisering av mikroplast i biogjødsel/Testing of new analytical method for quantifying microplastics in biofertilizer”
Pre-treatment of environmental samples
Separation of plastics from organic matter and sand can be done using solutions with various densities. Rubber granules from artificial football fields, for instance, were found in the soil surrounding the stadiums, and were separated from soil using saturated salt-sugar solution, as shown on Figure 3.
Oxidation of organic matter using Fenton’ reagent:
Another way of increasing the limit of detection of the STA-FTIR is to increase the fraction of plastics in the sample, by removing the organic matter. This can be done with Fenton’s reagent, which is made of H2O2 and ferrous iron solution at acidic pH. The oxidation was used on organic waste from biogas production plants and compost (Figure 4). It was also done directly on various plastic types, especially biodegradable plastics, to be able to account for the potential loss of plastic material during the oxidation.