Introduction
What are micro plastics? Defined by the NOAA, micro plastics are plastics that are “less than five millimeters long which can be harmful to our ocean.” Micro plastics account for 92% of plastic debris on the ocean’s surface.
Common approaches to plastic recovery
- Floatation (separation by density)
- plastics float at different depths in the ocean but this makes recovery difficult
- Chemical Digestion (separation by biogenic material removal)
- chemical digestion works only for specific plastic types, of which there are many
- Sieving or filtering (separation by size)
- sieving and filtration has obvious physical limitations when it comes to scaling
- Visual identification (separation by manual sorting)
- same goes for visual identification
Attempts at separation by electromagnetic field 🧲
Electromagnetic Recovery Methods
Iron (Fe) Coating
“This method involves using coated [iron] nanoparticles to magnetize plastics, allowing magnetic extraction and the isolation of microplastics.
A key characteristic of this method is that the effectiveness is relative to the surface area to volume ratio; for smaller particles, more of the [iron] nanoparticles can bind per unit mass of plastic.
Thus, this method is particularly useful for small microplastics.”
from Grbic J, Nguyen B, Guo E, You JB, Sinton D, Rochman CM.
Procedure
Firstly, using a chemical called hexadecyltrimethoxysilane (HDTMS), we create hydrophobic iron particles.
Those particles attach to plastics due to hydrophobic interaction.Then, you can use magnets to pick them out since the magnetic force will now act on the particles.
from Grbic J, Nguyen B, Guo E, You JB, Sinton D, Rochman CM.
Results
Results may vary depending on conditions of water. For instance in sediments recovery could be lower due to soil particles impeding nanoparticles encountering plastic. Recoveries could be >100% due to plastics fragmenting during mixing and handling. Lipophilic substances or biota (e.g. fat in fish tissue) reduce effectiveness due to nonspecific binding.
average recovery rates
- 96 ± 6% for polypropylene (PP)
- 92 ± 7% for polyvinyl chloride (PVC)
- 105 ± 8% for polyurethane (PU)
- 96 ± 7% for high density polyethylene (HDPE)
- 74 ± 9% for polyethylene terephthalate (PET)
from Grbic J, Nguyen B, Guo E, You JB, Sinton D, Rochman CM.
Roller type corona field electro-separator
procedure: Using corona fields electrodes
- Step 1: Prep Materials (quartz, beach sand, freshwater matter & sediment)
- Sieve down to 20µm
- 20g of sieved material (freeze dried) with 100mL bidistilled water
- Sieved by 2000µm via sonication
- Centrifuged 2000-3000 x g
- Dried 105℃
- Step 2: Microplastic Spiking
- HDPE, LDPE, PP, PET, PS, PVC, PMMA, PLPA
- 0.85 g/cm^3 – 1.58 g/cm^3
- Produced via freeze-milling
- Sieved into three factions (63-200µm, 200-630µm, and 630-2000µm)
- Step 3: Recovery
- Add 10 plastic particles from each type of plastic and size to each sample material
- Run material through electro separator
- Step 4: Collect results
Methods of Determining Micro plastic Content in Soils: A Review
Factors in Micro plastics Analysis
A thorough and accurate analysis of microplastics in soils has currently not been reached due to the fact that soils naturally vary in composition in different climates and ecospheres.
Actual work on microplastic in soils is not common either, and because of the use of various collection, refining, and analytical techniques, current publications are still incomparable.
An appropriate and effective approach for standardized microplastic analysis is still to be be found, considering to the dynamic nature of soils.
Möller JN, Löder MGJ, Laforsch C.
Future Method Research
In the literature of microplastics research, some suggested analytical methods have been proven while others are a proof of concept and have not been applied to physical samples.
However for virtue of development, all possible strategies are being explored and tested to assess their possible applicability for various samples of soil.
To date, no established methods have been determined to be ideally suited for analyzing smaller microplastics, though minor adjustments and combinations of methods are being done in order to find one that is most effective.
Möller JN, Löder MGJ, Laforsch C.
Acknowledgements
Shoutout to Nathan Vasquez and Kris Erndt for providing assistance in researching this topic.
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