Extracting microplastics from soil typically involves two main steps: removing organic matter and separating the microplastics based on density.
Removing microplastics from soil is a multi-step process designed to isolate tiny plastic particles from the complex soil matrix. This involves eliminating organic components and then using differences in density to float or sink the plastics away from the heavier soil particles.
Steps for Extraction
The extraction process generally follows these stages:
- Sample Preparation: Drying and sieving the soil to remove larger debris and homogenize the sample.
- Organic Matter Removal: Breaking down organic material (like leaves, roots, and humus) that can interfere with separation.
- Density Separation: Using a liquid medium denser than water but less dense than most soil minerals to float the microplastics while the soil sinks.
Organic Matter Removal
Organic matter is a significant challenge as it can trap microplastics or interfere with density separation liquids. Various methods exist, but according to the provided reference, the optimal organic removal method was found to be hydrogen peroxide. Hydrogen peroxide ($\text{H}_2\text{O}_2$) is commonly used because it effectively oxidizes and breaks down organic compounds.
Density Separation
Once organic matter is reduced, density separation is performed. This involves adding a liquid that has a specific gravity between that of microplastics (typically around 0.9-1.4 g/cm³) and soil minerals (typically > 2 g/cm³). When the soil sample is mixed with this liquid, the lighter microplastics float, while the heavier soil particles settle at the bottom. The floating layer containing microplastics can then be collected.
- Common density separation liquids include:
- Sodium chloride (NaCl) solutions
- Sodium iodide (NaI) solutions
- Zinc chloride ($\text{ZnCl}_2$) solutions
- Sodium polytungstate solutions
Interestingly, the reference highlights that overall, canola oil was shown to be the optimal method for density separation in the context studied. This suggests that sometimes non-traditional or less common liquids can be effective, potentially offering advantages in cost or safety depending on the specific application.
Factors Affecting Efficiency
It's important to note that the efficiency of these methods can vary.
- Organic Matter: The reference states that the efficiency of canola oil for density separation was dependent on the amount of organic matter in the soil. High organic content can hinder the separation process even after initial treatment.
- Polymer Type: The recovery efficiency of microplastics was variable across polymer types. Different plastic types have different densities and surface properties, which can affect how well they separate and how resistant they are to the organic removal process.
Summary Table
| Step | Primary Goal | Common Method | Optimal Method (Reference) | Efficiency Note (Reference) |
|---|---|---|---|---|
| Organic Matter Removal | Eliminate interfering organics | Hydrogen peroxide | Hydrogen peroxide | |
| Density Separation | Separate plastics from soil | Salt/Chemical solutions | Canola oil | Dependent on organic matter; Variable by polymer type |
After separation, the collected microplastics are typically filtered, dried, and then analyzed using techniques like microscopy and spectroscopy (e.g., FTIR, Raman) to identify the types and sizes of plastic particles.
