The setup consisted of a homemade PEEK cell (Figure ) with a three-electrode configura�on using a glassy carbon rod as counter electrode (Sigradur grade G, HTW GmbH) and a leak-free Ag/AgCl/3.4 M Cl -(ET072, eDAQ) as reference electrode. The Ag/AgCl/3.4 M Cl -was calibrated versus reversible hydrogen electrode (RHE) via both a Hydroflex (Gaskatel) and a homemade Pt wire RHE. Fe in TAP 900@ Fe was used for online flow cell ICP-MS measurement to avoid interference from ArO + . The flow cell protocol and ICP-MS opera�on is detailed in the Supplementary Informa�on and Figure .