[OneGuy123]

We just have to be careful so that "the cure doesn't become deadlier than the disease": in case some magnetic particles are left in it could cause greater issues perhaps than plastic. We can't just assume that "yeah don't worry bro the magnets will surely remove all magnetic particles".

[Xylakant]

The magnetic particles are iron oxide, it's fairly common in nature. Adding too much of it to water will turn it brown and cause issues, but small quantities shouldn't pose too much of a problem.

[flqn]

Excess iron in seawater can cause huge imbalalances by seeding algae blooms. Any large scale application of this would need to take that into account or risk disrupting ecosystems.

[rini17]

It is actually a fertilizer, algae needs it (of course, too much algae is a problem, too).