Coagulation of Polyethylene Terephthalate (PET) Microplastics: Which Coagulant Works Best?
Maya Honda-Granirer
Sir Winston Churchill Secondary
Floor Location : M 067 F

Microplastics are pieces of plastic less than 5mm in length which are largely undetected in our current water treatment plants. There are growing concerns about the impacts of microplastics on marine wildlife, the environment, and our health. My experiment aims to investigate coagulation, which involves the addition of a coagulant to promote the clumping of fine particles which can then eventually settle, as a method to help remove microplastics from water. My research question is “Which coagulant consolidates the most polyethylene terephthalate (PET) microplastics from water?”

I hypothesized that if I use 3 different coagulants - aluminum sulfate, iron (III) chloride, and iron (II) sulfate - in a suspension of PET microplastics and tap water, then the iron-based coagulants (iron (III) chloride and iron (II) sulfate) would produce higher coagulation rates (meaning more microplastics consolidated at the bottom of the suspension) than the aluminum-based coagulant (aluminum sulfate) because the optimum pH range of iron-based coagulants (5-8.5) corresponds more to the pH level of tap water in Vancouver (7.5) than the optimum pH range of aluminum-based coagulants (5-7).

To test my hypothesis, I designed a procedure that consisted of four primary sections: preparation of the microplastic suspensions, coagulation, data collection (counting the microplastics), and disposal of microplastics. The coagulation procedure involved the addition of a coagulant solution - or in the case of the control group, nothing at all - to the microplastic suspension. There was a 2-minute mixing period and a 20-minute settling period. To count the microplastics afterward, I took 5 samples from each suspension and counted the number of plastic particles I saw under the microscope, then found the average.

After conducting my experiment, I found that my hypothesis was not supported. I hypothesized that the iron-based coagulants would coagulate more microplastics in the suspension than the aluminum-based coagulant. My results are as follows: the iron (II) sulfate had the highest coagulation rate (93.46%), the aluminum sulfate had the second-highest coagulation rate (88.32%) and the iron (III) chloride had the lowest coagulation rate among the experimental groups (77.57%). However, the aluminum sulfate left the water the cleanest compared to the other 2 coagulants, which left significant traces of themselves throughout the suspension even after coagulation. My control groups had a negative coagulation rate, of -42.06%, because, without the addition of a coagulant, many particles floated to the top of the suspension during the settling period.

Based on my experiment, I would conclude that aluminum sulfate is the best coagulant to use. Although it did not have the highest rate of coagulation, it left the water the cleanest out of all the coagulants I tested. Since my experiment only focused on one kind of plastic, though, the results I observed may not hold true for other kinds of microplastics. Finding a viable, affordable, and effective solution for microplastic removal will take many more innovations, methods, and experiments than I was able to achieve in one science fair project - the journey has just begun!