Coagulation of Polyethylene Terephthalate Microplastics: Which Coagulant Works Best?

By Maya Honda-Granirer
Intermediate Category
Experiment | Environment

Microplastics are pieces of plastic less than 5mm in length, which are largely undetected in our current water treatment plants. Nowadays, they are everywhere – it was estimated in a 2015 study that there are between 15 and 51 trillion microplastics in the world’s oceans. 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). By optimum range, I am referring to the pH conditions of water in which the coagulants work most effectively.

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 (either iron (II) sulfate, iron (III) chloride or aluminum sulfate, 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 from those 5 measurements. To safely dispose of the microplastics once I was done, I used coffee filters and sealed the microplastics in a Ziploc bag.

After conducting my experiment, I found that my hypothesis was not supported. I hypothesized that the iron-based coagulants – iron (II) sulfate and iron (III) chloride – would coagulate more microplastics in the suspension than the aluminum-based coagulant – aluminum sulfate. 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 the results of my experiment, I would conclude that aluminum sulfate is the best coagulant to use. Although it does not have the highest rate of coagulation, it leaves 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!

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