That kind of digestion tends to foam due to the decomposition of H2O2 into water and oxygen gas, and potentially forming KO2. You should explore another alternative since foaming can hinder the digestion process and potentially lead to the loss of microplastics. I suggest you read the following articles:

Article Evaluation of existing methods to extract microplastics from...

Article Improving the Efficiency of Post-Digestion Method in Extract...

Sergio Valdivia Rivera, thanks for your answer, but the papers you suggested are not for water samples. Water used to have a lower biogenic load than to digest tissue. If the same process is replicated for the water samples, will it affect the integrity of plastic polymers?

Thanks in advance.

The cloud formation and subsequent precipitation during the digestion process of the water sample for microplastic quantification can also be explained in terms of colloidal dispersion and reaction by-products:

• KOH helps to digest organic materials by breaking them down into smaller components, which can then form colloidal suspensions in the solution. This is why the sample becomes white after the KOH is added.
• When you add H2O2, the oxidation reaction becomes more intense, leading to the further breakdown of the organic matter, especially any residual cellular material. The cloudiness likely comes from the formation of hydroperoxides and other by-products that are insoluble or poorly soluble in the solution, causing them to precipitate out as a fine suspension.

This is typical in chemical digestion processes where both alkaline (KOH) and oxidizing (H2O2) reagents are used together, causing a complex mix of organic and inorganic reactions that result in both the initial cloudiness and later precipitate formation.