'Electrolysed' tap water shows magnet-responding particles

A set of D-I-Y experiments electrolyse water and demonstrate the presence of black, magnet-responding particles. Is this graphene oxide? GO has been advanced as a component of water filtration systems in various countries, most recently the UK.

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https://www.notonthebeeb.co.uk/post/graphene-oxide-used-to-clean-our-water-supply