Earth and Environmental Science 11–12 · Year 12
Finding microplastics in soil or sediment by density separation
Module 8: Resource Management
The idea
Plastic fragments less dense than saturated salt water float free of sand and mud, so a density separation reveals pollution the eye cannot see and a blank shows how much contamination the method itself adds.
What you need
- about 60 g of soil, pond mud or puddle sediment from each of two or three sites (beside a path, near a bin, a less-used control site), collected with gloves
- table salt and warm tap water to make saturated salt solution (salt left undissolved at the bottom), 300 mL per sample
- 500 mL beakers, funnel with filter paper or a coffee filter with 15 to 20 µm pores (Earthlearningidea), ×10 hand lens or dissecting microscope, balance to 0.1 g, 100 mL measuring cylinder
How to do it
- Weigh 60 g of each sample and record the site description.
- Make saturated salt solution and measure its density by weighing 100.0 mL.
- Stir each sample into 300 mL of salt solution for two minutes and leave it to settle for one hour.
- Pour the upper layer through the filter; run a blank of salt solution with no sample through a second filter at the same time.
- Dry the filters, examine them under the lens and count fibres and fragments by colour and shape; subtract the blank count.
- Report particles per 100 g of sample for each site and relate differences to nearby human activity.
What you should see
Salt dissolves to 36.0 g per 100 g of water at 25 °C (PubChem), so the saturated solution is 26.5 % salt by mass and denser than fresh water; the learner’s 100.0 mL weighing gives its density. Sand and most organic matter settle while low-density plastic fragments and fibres float and collect on the filter. Counts are reported per 100 g of sample after subtracting the blank, which counts fibres picked up from air and clothing during the method; plastics denser than the brine stay in the sediment, so the method gives a minimum count, a limitation the learner states.
What changes
- What you change
- sampling site
- What you measure
- microplastic particles per 100 g of sample (blank subtracted)
- What you keep the same
- same sample mass
- same brine volume and settling time
- same filter type
- blank run alongside
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- If no plastic can be seen, there is none.
- All plastics float.
- A result without a blank shows only the site’s pollution.
Safety card
Hazards
- soil micro-organisms and sharp litter at the sampling site
- broken glass
Controls
- gloves for sampling and a safe site chosen in advance
- wash hands after sampling and filtering
Note
Fieldwork: complete the school's excursion approval and a RiskAssess field risk assessment under NSW Department of Education procedures; any test reagents carried into the field fall under the Chemical Safety in Schools (CSIS) package, Section 1.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-15EES11/12-5EES11/12-6EES11/12-7
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-04
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
- www.earthlearningidea.com/PDF/467_Microplastics.pdf
- pubchem.ncbi.nlm.nih.gov/compound/Sodium-Chloride
- education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-11-12-curriculum-resources/earth-and-environmental-science-year-11-and-12