Both ferric chloride and ...

Both ferric chloride and sulphate form good and large flocs which may work to sediment / precipitate micro / nano plastics.  Titration is the only way you are going to achieve success with varying concentrations.  Both are inexpensive flocculents.  The most important factor however is the starting pH.  It is perhaps best to start off with a buffered pH of 9 using calcium to raise the pH.  That will also help with the formation of large flocs.  Ferric is very acid so expect a finished pH of around 5.5 or even lower.